Cablewire 2008 Technical Papers

About IEEMA (Indan Electrcal and Electroncs Manufacturers' Assocaton)

» THE ASSOCIATION Founded in 1948, Indian Electrical and Electronics Manufacturers' Association (IEEMA) is the representative national organisation of manufacturers of electrical, professional electronics and allied equipment having over 550 members whose combined annual turnover is over Rs 1,00,000 crores i.e. US $ 22 billion.

Now in its 60th year of existence, IEEMA continues to provide unique services to its members. IEEMA undertakes various activities, major ones being dissemination of information of production statistics and government policy changes, representing the industry's views to the government, price variation clauses covering a wide range of products and evolving industry standards. Training for members and non-members on topical issues, library and business center facilities are among the other initiatives on offer.

IEEMA as the representative organisation for the industry is also a part of many councils and committees constituted by the Government.

IEEMA has the distinction of being the first association in India to achieve an ISO certification in January 1998 and successfully re-certified for the second time for ISO 9001:2000 in 2006.

» IEEMA VISION In consultation with its stakeholders and to cater to their emerging needs, IEEMA evolved a vision;

"Electricity for all and global excellence leading to human enrichment"

To realise the vision, IEEMA has taken a bold step to restructure itself and has drawn an ambitious medium term programme to provide value added services to its members and help facilitate their rapid expansion in both domestic and global business arena. IEEMA has realigned its structure and activities to successfully achieve the set vision objectives.

IEEMA's new vision is based on the five Building Blocks, which IEEMA members have short listed to be the most crucial for their success;

1. Credblty wth all stakeholders 2. Excellence 3. Global Presence 4. Enablng power to all 5. Eco-system focus

» IEEMA ACTIVITIES & INITIATIVES

Ï Voce of Industry IEEMA as the voice of electrical industry maintains a continuous dialogue with the Government and its various departments, utilities, other users, standardization bodies, educational institutions, research, development and testing as a major part of this goal.

Ï Intatves wth the Government Co-ordination with the Ministry of Power for successful implementation of Accelerated Power Development & Reforms Programme (APDRP) and rural electrification under the Rajiv Gandhi Grameen Vidyut Vitran Yojna or RGVVY. Support to Bureau of Energy Efficiency for standards and labeling, Programme on Energy Efficient Products, interface with standards and testing Institutions, organizing DRUM training programmes with Ministry of Power etc.

Ï Internatonal Co-operaton Networking with overseas counterpart associations from many countries for exchange of information, assistance to membership and other joint programmes aimed at enhancing business co-operation opportunities. MOUs with a number of countries like China, Korea, Spain, Taiwan and Malaysia. IEEMA is also one of the founder members of FAEMA i.e. Federation of Asian Electrical Manufacturers' Associations.

Ï SME Focus IEEMA has added this activity solely to facilitate the betterment and up gradation of SMEs to globally excellent levels and has initiated action.

Ï Corporate Socal Responsblty

IEEMA on its part in a small manner has launched a media awareness campaign to Save Electricity and save the environment, using print, voice and electronic media. 10,000 secondary level school children too are being exposed to this campaign through presentations, posters and brochures.The campaign is being carried forward

Ï Cross Sectoral Networkng

IEEMA is networking with other sectoral and apex associations and chambers. Building up Industry academia relations and assisting ministry of power to draw up sustainable power solution models for rural India.

Ï Commtment to Qualty and Benchmarkng

* Standardzaton : Formation of industry standards, operation and maintenance guidelines to serve specific need of members and the user Industries.

About IEEMA

* Qualty : Promotion of product and system quality through training, awareness programmes and consultancy

* Benchmarkng : CRISIL- LLYOD rating for Meters is now in second stage while, for Cables and Distribution transformers is under planning, Export division too is considering rating of Exporters.

Ï Energy Conservaton Intatves

Promotion of energy conservation through promotion, manufacture and usage of energy efficient products through external media and IEEMA journal

Ï Informaton Dssemnaton * Informaton Crculars : Circulation of information

about procedural and policy changes made by the government in direct and indirect taxation, import-export policy, industrial regulations as well as tender information, business opportunities, standards and other matters of interest to the industry.

* Publcatons: Publication of IEEMA JOURNAL and IEEMAIL on monthly basis and IEEMA News & Views on every fortnight covering technical and techno-commercial articles, industry information, statistics, business opportunities, IEEMA activities and more. IEEMA Journal with a subscription of 10,000 celebrated its Silver Jubilee in the year 2005-06.

* Drectory of Members: Publication of directory of members, i.e. IEEMA Directory containing exhaustive information about its members and the industry.

* Specal Servces: Statistical Information - Circulation of monthly production and import- export statistics covering various segments of the industry.

Ï IEEMA Journal

Today a synonym for the Indian electrical industry, IEEMA Journal was started with the intent of keeping its members aware of technological and related developments in the local as well as international arena. And what started as a small journal has today evolved into a full-fledged magazine that signifies a fine example of professionalism in the domain of trade publications. With an Audit Bureau of Circulation (ABC) certification for 10,000 copies, it is also the only trade journal in India that enjoys a readership of well over 50,000.

Ï Prce Varaton Clauses

Evolution and operation of equitable Price Variation Clauses, covering a wide range of products, being used both by purchasers and suppliers. Circulation of basic

prices and indices to operate these clauses on monthly basis.

Ï Commercal Terms Formation of standard terms and conditions for contracts.

Ï IEEMA Webstes The IEEMA websites contain updated information about IEEMA, its members, the industry and various services offered by the Association.

Separate web site is available only for members for information dissemination.ELECRAMA website caters to the ELECRAMA participants.

Ï Export Promoton Organization of high-level delegation visits and participation in exhibitions abroad for export promotion MADE IN INDIA brand.

Ï IEEMA Tranng Programmes

As a result of globalisation, the market conditions have become fiercely competitive. Under such circumstances, quality human resources emerge as the most vital factor for effective operation of the industry. IEEMA, in its constant endeavour to find new and innovative ways towards improvement of its services, plans to put focused efforts on training activity catering to the needs of Indian industry.

Ï IEEMA Events

Under the aegis of IEEMA Events, the activities held will be more of interactive series, promotions, seminars and exchange-of-ideas forums. The focal point of all these activities will revolve around bringing together professionals across borders for a common vision.

All in the interest of taking the industry to a new level. Giving India its much-deserved place in the world.

Ï IEEMAGINE Semnars

Every year IEEMA organises IEEMAGINE, a discussion platform to bring forth the issues pertaining to the industry.

Ï ELECRAMA Exhbtons

Started way back in 1990 with 283 exhibitors spread over an area of 12,500 square metres, ELECRAMA has become the largest international exhibition of electrical and industrial electronics industry in Asia, Middle East and Africa.

Since then there was no looking back. ELECRAMA saw a tremendous growth of 1086 exhibitors spanning an area of 40,000 square metres in the year 2006, breaking all past records. And this is just the beginning.

About IEEMA

Organising Committee

ChairmanVijay Karia

Ravin Cables Limited

Technical Committee

D. Guha Reliance Energy Limited

U.S. Bapat Tata Power Company Limited

Ashok Harane IL & FS Infrastructure Dev. Corp. Ltd.

S.K. Dutta Cable Corporation of India Limited

R.C. Agrawal Universal Cables Limited

C.B. Pundlik Finolex Cables Limited

H.H. Goenka Universal Cables Limited

G.P. Saxena Hindusthan Vidyut Ptroducts Limited

S.N. Pagay Polycab Wires Private Limited

Organising Secretary

Swapna Naik

i IEEMA Executive Council

THE COUNCILMembers of IEEMA EXECUTIVE COUNCIL

for the year 2007-2008

PresidentMr. S.C. Bhargaa

Exe.Vice President –Electrical Sector & Member of Divisional Board

Larsen & Toubro Limited

Vice PresidentMr. P.P. Gupta

Managing DirectorTechno Electric & Engg. Co. Ltd.

Vice PresidentMr. Murali Venkatraman

Vice Chairman & Managing DirectorW.S. Industries (India) Limited

Immediate Past President

Mr. D.J. RameshChairman & Managing Director

Vijai Electricals Limited

Elected Members

Mr. A.K. AgrawalGeneral Manager

Vam Electro Devices Pvt. Ltd.

Mr. Vishnu AgarwalManaging Director

Technical Associates Ltd.

Mr. A.K. Banerjee President (Swg)

Vijai Electricals Ltd.

Mr. Aaditya R. DhootJt. Managing DirectorIMP Powers Limited

Mr. Madha M. DigraskarPresidentABB Ltd.

Mr. Raj H. Eswaran Director

Easun Reyrolle Limited

Mr. P.V. KrishnaHead – Power Plant Sales &

Head – Western RegionWartsila India Ltd.

Mr. R. N. KhannaChairman & Managing DirectorControls & Switchgear Co. Ltd.

Mr. J. G. KulkarniVice President – CG Power (Asia)

Crompton Greaves Limited

Mr. D.K. MajumdarChief Executive - Operation

Electroteknica Switchgears Pvt. Ltd.

Mr. Vimal MahendruPresident – Corporate Affairs

Indo Asian Fusegear Ltd.

Mr. Jitendra U. MamtoraChairman & Managing Director

Transformers & Rectifiers (India) Ltd.

Mr. D.R. Venkatesha MurthyAdvisor

Kirloskar Electric Co. Ltd.

Mr. Vijay ParanjapeDirector, Member Managing Board

Siemens Ltd.

Mr. Anil SabooManaging Director

Elektrolites (Power) Pvt. Ltd.

Mr. Sanjee SardanaManaging Director

Yamuna Power & Infrastructure Limited

Dr. (Ms) Jaya SatheManaging Director

Gilbert & Maxwell Electricals Pvt. Ltd.

ii Executive Council IEEMA

Co-opted Members

Mr. R.D. ChandakManaging Director

KEC International Ltd.

Mr. A.N. ChaudhuriDirector

Modern Malleables Limited

Capt. V.W. KatreDirector

Aplab Limited

Ms. Indra Prem MenonPresident

Lakshmanan Isola Pvt. Limited

Standing Initees

Mr. S. K. DattaChief (Electrical)

Biecco Lawrie Ltd.

Mr. Rajesh S. JainChairman & Managing Director

Emco Limited

Mr. Premchand GoliyaChairman & Managing Director

Meco Instruments Pvt. Ltd.

Mr. A.K. SinghDirector

Electrical Research & Development Association (ERDA)

Mr. A.K. TripathyDirector General

Central Power Research Institute

Counsellors

Mr. A. K. Dhagat Mr. P. KrishnakumarDirector & CEO

Reliance Engineers Ltd.

Mr. V.P. MahendruChairman and Managing Director

Indo Asian Fusegear Ltd.

Mr. R.N. Mukhija President (Operations)

Electrical & Electronics Div. (EBG)Larsen & Toubro Limited

Mr. S. Ramaswamy

Chairmen of Diisions

Mr. Vijay P. KariaCables

Mr. Mustafa WajidCapacitors

Ms. Indra Prem MenonElectrical Insulating Materials

Mr. Sanjee SardanaExports

Mr. P. SridharanInsulators

Mr. S.C. Sarkar Meters

Mr. D.R. Venkatesha MurthyRotating Machines

Mr. S.B. GupteSwitchgear & Controlgear

Mr. Akella S.S. SarmaSurge Arresters

Mr. Mohan GuptaStamping & Laminations

Mr. Jitendra U. MamtoraTransformer

Mr. A.S. ChouhanTransmission & Distribution Projects

Mr. Nikhil SanghiWinding Wire

Chairmen of Committees & Cells

Mr. Rajesh Jain & Mr. S. Ramaswamy

Energy Conservation Cell

Mr. Cadaasal S. KumarQuality Cell

Chairmen - Regional Committees

Mr. D.R. Venkatesha MurthyMember & Chairman

Southern Region

Mr. Vimal MahendruMember & Chairman

Northern Region

Mr. S.K. DattaChairman

Eastern Region

Mr. Madha M. DigraskarMember & Chairman

Western Region

iii

Index of Papers to be presented at CABLEWIRE 2008

NO Title of Paper Name of Author/s Page No.

DAY 1 - 17TH JANUARY 2008

SESSION : I

1 EHV Cables Manufacturing - A study in Contamination Control and Monitoring - Cable Corporation of India

S. G. Hoskote, N. Bidarkar 3

2 Overview of Advanced Medium Voltage Cable Materials Technology - The Dow Chemicals Company, USA

S. Ram Ramachandran, Arnab Majumdar

7

SESSION : II

3 Design Criteria of an Optical Fiber with Improved bend performances - Prysmian Cables & Systems, Italy

D. Cuomo, M. Ruzzier, L. Terruzzi

15

4 Technical Advancements in Manufacturing of EHV XLPE Cables - Universal Cables Ltd.

Amitava Bose 19

5 Advanced Online Condition Assessment Technique for Cables - ERDA

N. R. Pandya, P. A. Krishnamoorthy, A. K. Singh

30

SESSION : III

6 Development of Outdoor Terminations for XLPE Cable - VISCAS Corporation, Japan

Hiroshi Niinobe, Shozo Kobayashi, Koichi Ono, Nobuyuki Shinagawa

35

7 Spectroscopic Studies to Assess the Effect of Shrink Temperature and Duration for Heat Shrink Materials - CPRI

S. Ganga, V. Asai Thambi, P. Sadasiva Murthy, S. Vynatheya, A. Sudhindra

40

8 Optimisation in Cable Sizing and Laying Philosophy - NTPC Sanjay Seth, K. Nagesh 44

SESSION : IV

9 Efficient Fault Management using Fault Indicators - Nortroll AS, Norway

Eilert Bjerkan, Runar Myhre 49

10 Cold Shrinkable Technology for Medium Voltage Cable Accessories and its Application to 3-Core Systems - Nexans Network Solutions N.V., Belgium

F. Claeys 53

11 Qualification of Cables for Nuclear Power Plants - NPCIL G. Sanjeev, S. P. Panda, M. L. Jadhav, S. B. Agarkar

59

Index Chronological

ix

DAY 2 - 18TH JANUARY 2008

SESSION : I & II

12 Flame Retardant Cable Materials Technology Advances - The Dow Chemicals International Pvt. Ltd.

Arnab Majumdar, Dr. Jeffrey M. Cogen, Dr. Scott H. Wasserman

65

13 Novel Technology for Insulating MV and HV XLPE Cables - Maillefer Extrusion Oy, Finland

Pekka Huotari 70

14 Dielectric Response Measurements to Assess the Condition of MV XLPE Power Cables - CPRI

P. K. Poovamma, K. Mallikarjunappa, A. Sudhindra, B. S. Manjunath, Thirumurthy

74

15 An overview of Engineering Reforms to Design 11 kV Heat Shrinkable Cable joints - Reliance Energy Ltd.

Ravi Agrawal, T. Swaminathan 80

16 Development of Factory- Expanded Cold-Shrinkable Joint upto 400 KV XLPE Cables - VISCAS Corporation, Japan

Shozo Kobayashi, Hiroshi Niinobe, Nobuyuki Shinagawa, Masahiro Suetsugu

84

SESSION : III

17 Advances in Fluid Silicone Jointing Technology for Medium Voltage Power Cable - Lovink Enertech B.V., Netherlands

Ralf Meier, Jeroen Krak 93

18 New Generation Electron Beam Cross Linked Building Wires - NICCO Corporation Ltd.

Rupa Bhattacharyya, Nazreen Akhtar, Gopal C Dhara, Nilambar Mongal

98

19 Loading of Flexible Wires in Conduit Pipes - CPRI S. Ramprasath, A. Sudhindra, K. P. Meena, Thirumurthy, G. K. Raja

102

SESSION : IV

20 New Range of Special Alloy Conductors: Advantages over Conventional ACSR and AAAC Conductors - Sterlite Technologies Ltd.

G. L. Prasad 109

21 New Developments in International Standards for Winding Wires - Elantas Beck India Ltd

Shirish Gokhale 111

22 Development of Water-soluble Polyester Enamel for Winding Wire - ERDA & M. S. University of Baroda.

C. N. Murthy, Uma Jejurkar, V. Shrinet, R. C. Jain, A. K. Singh

117

Chronological Index

x

Subject-wise index of Papers to be presented at CABLEWIRE 2008

NO Title of Paper Name of Author/s Page No.

Improed technology trends in manufacturing & Deelopement in raw material for cables

A EHV Cables Manufacturing - A study in Contamination Control and Monitoring - Cable Corporation of India

S. G. Hoskote, N. Bidarkar 3

B Overview of Advanced Medium Voltage Cable Materials Technology - The Dow Chemicals Company, USA

S. Ram Ramachandran, Arnab Majumdar

7

C Design Criteria of an Optical Fiber with Improved bend performances - Prysmian Cables & Systems, Italy

D. Cuomo, M. Ruzzier, L. Terruzzi

15

D Technical Advancements in Manufacturing of EHV XLPE Cables - Universal Cables Ltd.

Amitava Bose 19

E Flame Retardant Cable Materials Technology Advances - The Dow Chemicals International Pvt. Ltd.

Arnab Majumdar, Dr. Jeffrey M. Cogen, Dr. Scott H. Wasserman

65

F Novel Technology for Insulating MV and HV XLPE Cables - Maillefer Extrusion Oy, Finland

Pekka Huotari 70

G New Generation Electron Beam Cross Linked Building Wires - NICCO Corporation Ltd.

Rupa Bhattacharyya, Nazreen Akhtar, Gopal C Dhara, Nilambar Mongal

98

H New Range of Special Alloy Conductors: Advantages over Conventional ACSR and AAAC Conductors - Sterlite Technologies Ltd.

G. L. Prasad 109

Condition monitoring & online assessment

A Advanced Online Condition Assessment Technique for Cables - ERDA

N. R. Pandya, P. A. Krishnamoorthy, A. K. Singh

30

B Dielectric Response Measurements to Assess the Condition of MV XLPE Power Cables - CPRI

P. K. Poovamma, K. Mallikarjunappa, A. Sudhindra, B. S. Manjunath, Thirumurthy

74

New techniques for installation & maintenance

A Efficient Fault Management using Fault Indicators - Nortroll AS, Norway

Eilert Bjerkan, Runar Myhre 49

B Loading of Flexible Wires in Conduit Pipes - CPRI S. Ramprasath, A. Sudhindra, K. P. Meena, Thirumurthy, G. K. Raja

102

Index Subject - wise

xi Subject - wise Index

Cable accessories - deelopment

A Development of Outdoor Terminations for XLPE Cable - VISCAS Corporation, Japan

Hiroshi Niinobe, Shozo Kobayashi, Koichi Ono, Nobuyuki Shinagawa

35

B Spectroscopic Studies to Assess the Effect of Shrink Temperature and Duration for Heat Shrink Materials - CPRI

S. Ganga, V. Asai Thambi, P. Sadasiva Murthy, S. Vynatheya, A. Sudhindra

40

C Cold Shrinkable Technology for Medium Voltage Cable Accessories and its Application to 3-Core Systems - Nexans Network Solutions N.V., Belgium

F. Claeys 53

D An overview of Engineering Reforms to Design 11 kV Heat Shrinkable Cable joints - Reliance Energy Ltd.

Ravi Agrawal, T. Swaminathan 80

E Development of Factory- Expanded Cold-Shrinkable Joint upto 400 KV XLPE Cables - VISCAS Corporation, Japan

Shozo Kobayashi, Hiroshi Niinobe, Nobuyuki Shinagawa, Masahiro Suetsugu

84

F Advances in Fluid Silicone Jointing Technology for Medium Voltage Power Cable - Lovink Enertech B.V., Netherlands

Ralf Meier, Jeroen Krak 93

Users’ expectations

A Optimisation in Cable Sizing and Laying Philosophy - NTPC Sanjay Seth, K. Nagesh 44

B Qualification of Cables for Nuclear Power Plants - NPCIL G. Sanjeev, S. P. Panda, M. L. Jadhav, S. B. Agarkar

59

Winding wires

A New Developments in International Standards for Winding Wires - Elantas Beck India Ltd.

Shirish Gokhale 111

B Development of Water-soluble Polyester Enamel for Winding Wire - ERDA & M. S. University of Baroda.

C. N. Murthy, Uma Jejurkar, V. Shrinet, R. C. Jain, A. K. Singh

117

POLYCAB WIRES PVT. LTD. E - m a i l : i n f o @ p o l y c a b . c o m W e b s i t e : w w w . p o l y c a b . c o m

Contributing to India’s Growth Story, Fundamentally.

Polycab-1220 (Elecrama Full page2 2 1/3/08 5:20:08 PM

xx Index Advertisements

Index to advertisers

ELANTAS Beck India Ltd. ---------------Front Cover Gatefold

Finolex Cables Limited ----------------------------------------------- iv

Indo Asian Fusegear Ltd. --------------------------------------xii, xiii

KEI Industries Limited -------------------------------------- xix, 122

KSH International Private Limited --------------------------- xviii

NICCO Corporation Limited --------------------------------------xvii

Plaza General Cable Energy Pvt. Ltd. ---------------- 62, 120

Polycab Wires Pvt. Ltd. --------------------------xvi, 34, 64, 108

Ravin Cables Limited ---------------------------------- xv, 2, 48, 90

Supreme & Co. ----------------------------------------------------------xiv

Universal Cables Limited ------------------------------------14, 92

Day Thursday, January 7, 2008

Session I

Day 1 - Session I Technical Papers

IntroductionThe global growth of electricity demand will continuously increase the need for Medium voltage, High voltage and Extra high voltage power cables in transmission and distribution.

The use of Underground Power cables to carry electricity power is proving more and more acceptable.

Capital costs of over Overhead Transmission, are offset by better availability, lower losses, reduced environmental impact and easier acceptance of under ground Cables. This change in cost perspective and the Growing need for Reliable power has fuelled the increasing need for power cables.

This raises the major challenge of manufacturing the increased amounts of high quality Power Cables with the manufacturing infrastructure that exists today. Recently the available manufacturing technologies employed for XLPE Power Cables have been upgraded to enhance the productivity and performance of XLPE reliability of Cable is directly proportional to Contamination in Insulation. The sources of contamination are a) Basic Raw material, b) Ingress of particles in Handling and Extrusion process.

Contamination definition and standardsWhile Compound manufacturers define their products with so much size of particles (undesired) parts per million. Cleanliness is mainly defined for environment.

ISO 14644-1 and -2 have defined cleanliness. They have been made more stringent than earlier ones, shift paradigm from o.5 micron particles to 0.1-micron particles. These standards have been predominantly for Bio/pharmaceutical and electronic Industry, are applicable to Cable Industry too. Perhaps in Future IEEE or other organization should formulate their own standards for Cable Industry

EHV Cable Manufacturing A Study in Contamination Control and Monitoring

SG Hoskote, N Bidarkar Cable Corporation of India Ltd., Mumbai, India

The cleanliness classification levels defined by FS209E and ISO 14644-1 are approximately equal, except the new ISO standard uses new class designations, a metric measure of air volume and adds three additional classes - two cleaner than Class 10 and one beyond than Class 100,000. The second new ISO standard, ISO 14644-2, gives requirements for monitoring a clean room or clean zone to provide evidence of its continued compliance with ISO 14644-1.

The following table compares FED STD 209E to the new ISO 14644-1 classifications.

Airborne Particulate Cleanliness Class Comparison

ISO 14644-1 FED STD 209E

ISO Class English Metric

1

2

3 1 M1.5

4 10 M2.5

5 100 M3.5

6 1,000 M4.5

7 10,000 M5.5

8 100,000 M6.5

9

The ISO standard also requires fewer sample locations, especially as the clean room size increases; however, the ISO standard does require minimum one-minute samples, whereas the Federal Standard allows shorter samples, especially at smaller particle sizes.

ISO 14644-1 requires 3 sample locations, 19.6 liter minimum sample volume (0.85 cuff ), but also a minimum

EHV Cable Manufacturing - A Study in Contamination Control and Monitoring

sample time of one minute yielding three samples of one

cubic foot. This yields a total sample time of 180 seconds

and three equipment moves.

The precise count levels required by ISO 14644-1 for each

classification, by particle size, are given below.

Airborne Particulate Cleanliness Classes (by cubic meter)

CLA CLASS Number of Particles per Cubic Meter by Micrometer Size

0.1 um 0.2 um 0.3 um 0.5 um 1 um 5 um

ISO 1 ISO 1 10 2

ISO 2 ISO 2 100 24 10 4

ISO 3 ISO 3 1,000 237 35 8

ISO 4 ISO 4 10,000 2,370 1,020 352 83

ISO 5 ISO 5 100,000 23,700 10,200 3,520 832 29

ISO 6 ISO 6 1,000,000 237,000 102,000 35,200 8,320 293

ISO 7 ISO 7 352,000 83,200 2,930

ISO 8 ISO 8 3,520,000 832,000 29,300

ISO 9 ISO 9 35,200,000 8,320,000 293,000

ISO 14644-2 determines the type and frequency of testing

required conforming to the standard. The following tables

indicate which tests are mandatory and which tests are

optional.

Required Testing (ISO 16-2)

Schedule of Tests to Demonstrate Continuing

Compliance

Test Parameter

Class Maximum Time

Interval

Test Procedure

Particle Count Test

<= ISO 5 6 Months ISO 14644-1 Annex A

> ISO 5 12 Months

Air Pressure Difference

All Classes 12 Months ISO 14644-1 Annex B5

Airflow All Classes 12 Months ISO 14644-1 Annex B4

Testing (ISO 16-2)

Schedule of Additional Optional Tests

Test Parameter

Class Maximum Time Interval

Test Procedure

Installed Filter Leakage


Containment Leakage


Recovery All Classes 24 Months ISO 14644-3 Annex B13

Airflow Visualization


The major constraint for Reliability of Cables is the Stresses created due to contamination in Insulation material.

Hence it is very important to look into contamination in Insulation from all angles. Right from insulation material manufacturing, Transportation, Storage, Material-handling Cable manufacturing, Cable laying and jointing. At each stage the approach and methodology for contamination control and monitoring is different.

This Paper focuses on Contamination controlling in Material handling within Manufacturing plant, Manufacturing process and Cable Jointing.

Material HandlingThe manufacturer specifies the cleanliness of material and container. The containers are very special, sometimes filled with Nitrogen or pressurized air with minimum pressure. The Boxes or containers are to be opened by special method, taking care of external dirt/dust. The container needs to pass through three zones to cool-down and get clean from outside at least up to 1000 class, before being opened. It is preferred that bare hands don’t come in contact with Containers.

A good practice is to move material boxes in clean room cooling area. Allow the system to clean the external of boxes. Then move then to waiting area and finally to area where boxes will be opened with utmost care.

Thus material handling will be three staged, the boxes will pass through successive cleanliness standards.

Clean Room Area Entire machine from Bare conductor to endof Gradiant cooling to be totally closed area called Clean room. It is established practice that the conductor be taped with paper tape and removed just before the extrusion.

The entire clean room area is very specialized field and to be designed by specialist. The main specifications are; class of clean room, Static and Dynamic recovery, which


decides number of Air changes, Use of HEPA or ULPA filters. These Filters are also to be checked for its efficacy periodically and certified by authorities.

Clean room accessories like, air showers, curtains, furniture, hoods, lights, hard walls, Doors etc are to be specially designed and selected, so that use of these equipments itself do not add in contamination.

General Requirements Location and surroundings: - The factory buildings (s) for manufacture of cables shall be so situated and shall have such measures as to avoid risk of contamination from external environment emissions.

Buildings and premises: - The buildings (s) used for the factory shall be designed, constructed, adapted and maintained to suit the manufacturing operations so as to permit production of cables under hygienic conditions.

Designed / constructed / maintained to prevent entry of insects, pests, birds, vermins and rodents. Interior surface (walls, floors and ceilings) shall be smooth and free from cracks, and permit easy cleaning, painting and disinfection.

The walls and floor of the areas where manufacture of cables is carried out shall be free from cracks and open joints to avoid accumulation of dust, these shall be smooth washable, coved and shall permit easy and effective cleaning and disinfection, and the interior surfaces shall not shed particles. A periodical record of cleaning and painting of the premises shall be maintained.

Hopper Area This is most crucial part of clean room material handling. The material passes from Container Through suction pipes, through Fine removals to Hopper. The material comes in contact with various elements starting from steel, PE hoses, and gasket materials of valves, glass and Conveying air itself. None of this material can be abrasive. The system to be designed by Experts and special materials are employed to eliminate dust generation.

Extrusion Area Due to over acquaintance of these parts they are most neglected in contamination monitoring. These parts should be subjected to same attention as of Operation theatre instruments of Doctors. Since they are in direct contact with material, they can carry contamination from outside due to insufficient cleaning or may generate dust themselves. Hence special monitoring techniques are required. They require very strict and disciplined military aptitude to maintain cleanliness.

Special materials are used to clean extruder screws and barrels. Cleaning area to be flood lighted so that mirror finish cleaning can be checked.

Garments - Personnel in the manufacturing and filling section shall wear suitable single-piece garment made out of non-shedding, tight weave material. Personnel in support areas shall wear clean factory uniforms. Gloves made of suitable material having no interaction with the propellants shall be used by the operators and filling areas. Preferably, disposable gloves shall be used.

Suitable departmental-specific personnel protective equipment like footwear and safety glasses shall be used .

Environmental Conditions - Where products or clean components are exposed, the area shall be supplied with filtered air of minimum class ISO 5.

The temperature and humidity and air flow in the manufacturing area shall be controlled with modern equipments. facility. Other support areas shall have comfort levels of temperature and humidity.

There shall be a difference in room pressure between the manufacturing areas and the support areas and the differential pressure shall be not less than 15 Pascals, (0.06 inches or 1.5 mm Water guage).

Monitoring of CleanlinessThere shall be written schedule for the monitoring of environmental conditions. Temperature and humidity and shall be monitored daily.

Cleanliness is measured for Raw material. It is done by set methods. The Sheet of specific thickness and size is extruded and Under High powered Laser microscope the contaminates are scattered on Graph and measured. The Manufacturers normally guarantee these specs.

The cleanliness of extruded insulation material can be controlled online by means of an optical cleanliness scanning system (CSS). This is installed between insulation extruder and the cross-head and inspects 100% of insulation material. The polymer flows between two glass inspection windows. Since molten LDPE is transparent, any foreign bodies can be seen and recorded. For large particles size and shape is recorded and for small, number of particles per category is indicated.

But at there manufacturing sites. These need to be measured before extrusion and confirm.

Online Material supervision is possible with the help of Ultra high tech instruments. These Instruments are installed in line of extrusion. The instrument checks continuously particles in melted material and maps them.

Cleanliness of Environment is to be measured for airborne particles. Specific and dedicated Instruments are available, which are approved by accrediting Agency.

EHV Cable Manufacturing - A Study in Contamination Control and Monitoring

6

There are agencies, Specialized in Independently checking Cleanliness. This can become a part of Quality Plan

Some of the Quality parameters is specified under ISO 14644-2.

Raw materials It shall be ensured that all the containers of raw materials are placed on the raised platforms/racks and not placed directly on the Maintenance workshops shall be separate and away from production areas. Whenever spares, changed parts and tools are stored in the production area, these shall be kept in dedicated rooms or lockers. Tools and spare parts for use in sterile areas shall be thoroughly cleaned before these are carried inside the production areas.

Cable JointingThe Cable insulation is exposed to atmosphere for Jointing

or termination. Hence the area needs to be as clean as at

the time of manufacture. Special Portable clean rooms

are commercially available, which need to be properly

identified and employed. The personnel working

As Jointer to follow methodology to avoid any dusting. The

Jointing material to be cleaned with Specified material.

Conclusion

Cleanliness required is not only for Product, but is

essential from Environmental and Economic (Reliability)

needs in Manufacturing of EHV Cables. The cleanliness to

be maintained is required to the standards of Operation

Theatres or Bio/Pharmaceutical Industry. In India since HV

and EHV manufacturing is increasing , It is time the clean

room


AbstractState-of-the-art materials technology available for medium voltage (11-33 kV) cables is reviewed. This ‘system’ of semiconductive shields, insulation, and jacketing has a demonstrated field performance for many years. The importance of accelerated wet aging tests to assure underground field performance is highlighted. This paper outlines the growing global trends in shifting to higher performance medium voltage cables.

IntroductionReliable power delivery is important for residential, commercial and industrial sectors. Over the years, the distribution cable systems (11-33 kV) have advanced and now allow electric utilities to install cables underground without concern for traditional water tree electrical degradation, while enhancing aesthetics, safety, reliability and extended cable life. In India, infrastructure improvements are critical to keep up with growth, while optimizing total owned or life cycle costs. Polymeric materials to help achieve this have been in use in developed economies for nearly 25 years and gaining recognition globally.

Underground CablesFor underground cables, design, materials, cable quality, and pre-qualification or type tests are critical to ensure performance in the field. This is well recognized for high voltage (110+ kV) cable classes. However, similar focus is not always extended to medium voltage (11-33 kV) cables. With increasing volumes of cables being installed underground, it is imperative that rigorous qualification testing, optimum cable design and state-of-the-art materials be considered since the penalty for premature failures is heavy (system reliability, reinstallation costs & revenue losses).

Overview of Advanced Medium Voltage Cable Materials Technology

S. Ram Ramachandran, Arnab MajumdarThe Dow Chemical Company, USA

Medium Voltage Cable DesignsNorth America: Simplified design (Fig. 1) with stranded or solid aluminum conductors, inner semicons, ‘water tree retardant insulation, strippable outer semicons, concentric copper wire neutrals and an encapsulated polyethylene jacket is common. Filled conductor strands, while common, is not mandatory.

Western Europe: In contrast to North American practice of using ‘strippable’ outer or insulation shield, many European designs call for ‘bonded’ shields. Use of co-polymer insulation is also common, though there are exceptions.

Asia: Asian designs have used strippable outer semicon shields and standard XLPE insulation. Use of PVC ‘overlay’ jackets is quite common, while polyethylene jackets are also prevalent.

Test StandardsNorth America: ANSI / ICEA medium voltage specification standard has been used for many years (1). The most significant requirement is the use of AWTT (accelerated water treeing) test of ‘wet-aging’ of cable cores for one year to ensure cables meet minimum performance standards to be qualified.

Europe: European Union countries have adopted CENELEC HD620 qualification requirements, involving 2 year ‘wet-aging’ of cable cores and meeting minimum electrical strength requirements (2). Germany requires higher minimum performance requirements with this protocol.

Asia: Many Asian countries follow IEC standards for medium voltage cables, with exceptions. Japan uses JIS Standards, Korean Standard is similar to North American AWTT protocol and, as of December 1, 2007, Chinese Utility Industry Standard will include TR-XLPE performance standards based on AWTT protocols. Current Indian standards are based on IEC, with no wet aging tests.


Material Advancements

Typical Cable DefectsMaterial cleanliness and consistency affect performance of underground cables, with ‘water treeing’ in the insulation being the most common dielectric degradation phenomenon. Fig 1 illustrates such defects. Materials advancements over the past several years have minimized these.

Semiconductive ShieldsModern extruded dielectric cables for medium and high voltage applications have one or more conductors in a cable core that is surrounded by several layers of polymeric materials.

Figure 1 shows typical construction of North American concentric MV cables. Copper or aluminum conductors are covered with semiconductive conductor shield to provide uniform electrical field gradient to the dielectric insulation. The insulation is layered between the conductor shield and the insulation shield. An insulation shield protects the insulation from the damaging effects of ionization at the outside of insulation surface and provides a permanent ground contact around the insulation.

The semiconductive shields are polymer composites, filled with carbon black to achieve good electrical conductivity. Most standards require the conductivity of conductor shield in terms of cable volume resistivity to be below 100,000 ohm-cm at 90 oC maximum continuous operation temperature and 1300C emergency operations. For insulation shield, the volume resistivity should not exceed 50,000 ohm-cm at 900C /1100C.

Carbon BlackCarbon blacks in shields should be very clean and conductive in comparison to carbon black in rubber and reinforcement applications. Two types of carbon black have been widely used for cables (10). Furnace black is produced by partial oxidation of hydrocarbon oil or gas. Acetylene black is produced by continuous thermal decomposition of acetylene gas. Due to the nature of feedstock, acetylene black has higher purity than furnace black. Shields based on furnace black are called conventional shields, while those based on acetylene black are referred to as supersmooth shields.

Figure 2 depicts the stress enhancement factors for various cable voltages, estimated from Mason’s equation (11-12) As the protrusion tip radius gets smaller or sharper, the stress enhancement increases for the same insulation thickness, which can induce more damage to the insulation. At higher voltages, this factor increases dramatically. Hence it is critical to control the interfacial stress risers by minimizing protrusions and protrusion density.

Smoothness: Acetylene black has lower grit and ionic impurities levels than furnace black, resulting in smoother interfaces than conventional shields.

Micro-protrusion analysis with a laser-scanning instrument reveals that size and shape of protrusions of supersmooth shields are quite distinguishable from those of conventional shields (Fig.4) Supersmooth shields have not only an order of magnitude lower protrusion density at a given micro-protrusion height, but also have lower aspect ratio (ratio

Fig.1: Typical defects encountered in medium voltage cables


Water Treeing Phenomenon & ControlWater treeing, an electrical degradation process in polymers occurring in the presence of electrical stress and moisture, has been identified as the leading cause of non-mechanical insulation degradation and consequent loss of dielectric strength. This phenomenon has been extensively studied (4-9). In medium voltage cables, water-treeing phenomenon can be considered in three phases:

Initiation - due to uneven stress at interface (vented tree) and /or contamination in insulation (bow-tie tree)

Growth- along the electrical field in presence of water

Bridging - As trees get to be long and /or have high tree density, this could lead to an electrical tree and cable failure.

Controlling the water trees can be accomplished by:

Improved smoothness and cleanliness of shields and interfaces

Maximizing cleanliness of the insulation

Minimizing ingress of moisture into the insulation

Retarding the growth of trees in insulation once initiated

Over the years, developments have addressed all these steps. On simplified cable designs indicated above (without water swellable tapes etc.), material quality has been the focus.

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of protrusion height over the protrusion base-width) indicating much flatter protrusions than conventional shields. This induces lower localized electrical stress on the insulation. As voltage ratings go up, especially for High and Extra High Voltage Cables, Supersmooth is the preferred shield choice.

Cleanliness: The water-soluble ionic impurities in shield compounds affect the number and size of water trees developed in the insulation, which can lead to dielectric failure in power cables. Elemental analysis by induction coupled plasma (ICP) spectroscopy (Figure 5), demonstrates that supersmooth shields contain much lower level of ionic impurities and sulfur.

InsulationThe water treeing phenomenon identified with early distribution cables led to the development and introduction of ‘tree retardant cross linked polyethylene’

Fig.2. Stress enhancement factor

Fig.3. Micrograph of smooth interface between conductor shield and insulation by atomic force microscopy (13).

Fig. 4.Surface smoothness data of shields from surface smoothness analyzer

Fig. 5.Contamination levels of shields


or “TR-XLPE” as is widely known, in 1982-83. Around the same time, to combat unacceptable cable failures in Germany, researchers found that blends of polyethylene and ethylene acrylate copolymer (“Copolymer XLPE”) improved the resistance to electrical breakdown after aging in water. Both these materials have demonstrated excellent field performance. For the purposes of this paper, we will focus our attention on the ‘additive’ tree retardant insulation, its performance in laboratory aging tests, its proven performance in the field and growing recognition.

“TR-XLPE” Compound has a very small amount of ‘additive’ that prevents the ‘growth’ of water trees, while maintaining the excellent electrical properties of XLPE. The compounded TR-XLPE, including the peroxide, is still ~97% all polyethylene by weight. It also processes in conventional extrusion equipment at similar rates and is compatible with standard cable accessories commonly used with XLPE cables.

Water Treeing EvaluationTR-XLPE materials can be tested in the laboratory according to the ASTM water treeing test, D6097-97 (14). In this test, the growth rate of water trees initiated from an induced defect in the insulation material in a conductive solution is monitored over time. This test allows comparison of materials in a relatively short time (30-90 days) for compound development.

10

Cable Aging TestsCurrent IEC specification does not call for cable evaluations under conditions relatively close to field operation, restricting testing under ‘dry’ conditions. The North American utility group, AEIC (Association of Edison Illuminating Companies), working in conjunction with cable manufacturers (ICEA - Insulated Cable Engineers Association), developed a Qualification test in the mid 80s that has since been refined, and recently has become the national standard, ICEA S-94-679-2004. (1). In this test, standard 15 kV, 53 sq. mm. cable cores (no jacket) are wet aged up to one year, submersed in water in PVC conduits, while injecting water through cable strands, referred to as AWTT test (Accelerated Water Treeing Test). Following dry thermal load cycling to drive off extrusion by-products, 3 cable samples each are aged for 120, 180 and 360 days of aging in water at 3Vo and load cycling. Each 8 hour ‘ON’ cycle must result in the insulation shield temperature of 45 C. At the end of each time period, three samples are ‘broken’ down for AC Breakdown as well as other diagnostic tests. The purpose is to evaluate cable performance under conditions close to field conditions, while ‘accelerating’ induced degradation of the insulation, with water being forced to enter from both outside the cable core and from the conductor strands.

Accelerated Wet Aging Test ResultsNorth America. Figure 6, is illustrative of typical AWTT test results from 6 or 7 cable manufacturers’ qualification tests and represents an independent body of results. The ACBD values are plotted as a function of time and it is evident that TR-XLPE cables retain much higher aged electrical strength than standard XLPE cables under severe wet aging.

of this 2 year test, where the 2 VDE models for minimum

performance are indicated. In both, TR-XLPE retains a

higher electrical strength. In fact, regular XLPE may not

consistently meet the minimum requirements.

Figure 6: AWTT Test Results

Figure 7: Early German VDE test: TR-XLPE vs. XLPE cables (15)

Fig 8. TR-XLPE & Standard XLPE Compared in VDE Performance

commercial cables made in PRC with: North American TR-XLPE, North American XLPE and locally produced PRC XLPE compounds, with corresponding semiconductive shields (16). Figure 3 corroborates what we have outlined earlier, that TR-XLPE cables maintain a higher electrical breakdown strength thru the one year wet aging test. Cables of local XLPE compounds all electrically failed before 140 days and displayed significant water treeing by that time, while TR-XLPE did not.

Field Performance

The performance of TR-XLPE in the field has been well documented.

Alabama Power, a major North American utility, compared


Europe: Current CENELEC and VDE (German) requirements call for a 2 year wet accelerated cable aging protocol. Figure 7 compares typical results of TR-XLPE and standard XLPE (15). Figure 8 compares some most recent results

China: Wuhan High Voltage Research Institute undertook comparative AWTT performance evaluation of three

11

the field aging performance of their 35 kV cables operating through 17 years in the field. For purposes of this paper, we illustrate the comparative performance in Figure 5. While both TR-XLPE and EPR cables demonstrate relative stability of breakdown strength after 17 years of operation, TR-XLPE maintains higher breakdown strength. The project entailed more diagnostics as well, detailed in the reference (17).

The most recent survey of the major Investor Owned

field performance has also been summarized elsewhere (20).

Value to UtilitiesPostponing cable replacement by 20-25 years or more improves ‘system’ value significantly. Increasingly, utilities are focused on this comprehensive analysis, since it addresses both the long term system costs as well as improved reliability of the delivery system.

Global use of TR-XLPEIn addition to North America, Mexico, Brazil, Chile, UK, Germany, Italy, Russia, Saudi Arabia, UAE, Hong Kong, New Zealand, South Korea and Philippines have either used TR-XLPE or are considering its use. Use of TR-XLPE is predominant in USA, Canada and South Korea. European countries use both the water tree retardant ‘Copolymer XLPE’ and additive TR-XLPE as a means to extend cable life

ConclusionsWith increasing demand for quality and sustainable power delivery, global utilities are focused on improving their underground cable delivery system reliability, while optimizing system life cycle costs. Use of state-of-the-art semiconductive shields and TR-XLPE insulation has demonstrated field performance for nearly 25 years. Many global regions, especially developing countries, are moving to such ‘performance’ based system.

REFERENCES1. ICEA S-94-649-2004, Standard for Concentric Neutral Cables Rat-

ed 5 Through 46 kV, Insulated Cable Engineers Association, Inc, 2004.

2. CENELEC

3. Eichhorn, R. M., “Treeing In Solid Extruded Electrical Insulation”, IEEE Trans. Elec. Insul. 12, 2-18, 1977.

4. Lawson, J., Vahlstrom, W., “Investigation of Insulation Deterioration in 15 kV and 22 kV Polyethylene Cables Removed from Service - Part 2”, IEEE Trans.Pwr.App.Sys., 92,,824-835, 1973.

5. Shaw, M. T., Shaw, S.H., “Water Treeing In Solid Dielectrics”, IEEE Trans. Elec. Insul., 19, 419 452,1984.

6. Bahder, G., Katz, C., Lawson, J., and Vahlstrom, W., “Electrical and Electrochemical Treeing Effect in Polyethylene and Cross-linked Polyethylene Cables”, IEEE Trans. Pwr. App. Syst., 93, 977-990, 1974.

7. Nunes, S. L., Shaw, M. T., “Water Treeing In Polyethylene - A Review Of Mechanisms”, IEEE Trans. Elec. Insul. 15, 437-450, 1980.

8. Ross, R., “Current State of Water Tree Material Tests”, Proceedings of the 1994 IEEE 4th International Conference on Properties and Applications of Dielectric Materials. Part 1, p 416-419, 1994.

9. Jow, J., and Eichorn, R. M., “Water Treeing”, Wiley Encyclopedia of Electrical and Electronics Engineering, John G. Webster, Editor, John Wiley & Sons, Inc., 1999

10. N. M. Burns, et al., “Stress Controlling Semiconductive Shields in

Figure 9: Wuhan Institute AWTT Data (16)

Figure 10: 35 kV Field Aged Performance, Alabama Power & Houston Lighting & Power

Utilities (IOUs) in North America was presented at the Fall ICC Meeting in 2004 (18). Among other findings, the survey illustrated the predominance of TR-XLPE for 15-35 kV cables among the utilities in USA.

The latest EPRI (Electric Power Research Institute) Cable Research Digest states that “Based on research results and field data, it appears that the service life of both of these insulations will be greater than 40 years when incorporated into a cable when all of the suggestions in this Digest are followed” (19). The performance of TR-XLPE under a host of other accelerated aging tests and


12

Medium Voltage Power Distribution Cables,” IEEE Electrical Insula-tion Magazine, vol. 8, No. 5, pp. 8-24, September/October 1992.

11. J. H. Mason, Proc. Instn. Elec. Engers., vol. 98, pp. 45-59, 1951.

12. J. H. Mason, “In Progress in Dielectrics”, vol. 1. J. S. Birks, and J. H. Shulman, editors, London: Heywood and Co., 1959.

13. Suh Joon Han et al., “Overview of Semiconductive Shield Technology in Power Distribution Cables”, T & D Conference, 2006

14. ASTM Standard, D6097-97, 1997

15. H. Schadlich., “Comparative Wet Aging Test Of Medium Voltage XLPE Cables”, pp 51-59, Fall ICC, 1998.

16. A. Mendelsohn et al, “Evaluation of Tree Retardant XLPE, TR-XLPE and EPR Insulated 35 kV Cables After 17 Years of Field Service”, Jicable Versailles, France, pp, 556-561, 2003.

17. A. Mendelsohn et al., “Cable Aging Study of Medium Voltage Ma-terials at Wuhan High Voltage Research Institute”, Wire China, Shanghai, September 2004.


18. J. Dudas et al., “Underground Cable Specification Advances and

Installation Practices of the Largest Investor Owned Utilities”, IEEE

/ PES / ICC, Fall Meeting.

19. EPRI Distribution Cable Research Digest, 2000.

20. S. Ramachandran et al, “TR-XLPE Cables for Utility Power Distribu-

tion: 20 Years of Field Proven, Value Added Performance”, Rural

Electric Power Conference, Raleigh-Durham, North Carolina, May

2003.

Acknowledgements

The authors would like to acknowledge The Dow Chemical

Company for the opportunity to present this paper and their

colleagues Joon Han, Paul Caronia, Al Mendelsohn and

Jerker Kjelquist for useful discussions and information.

1

Day 1 Thursday, January 1, 200

Session II

Day 1 - Session II Technical Papers

15

Design Criteria of an Optical Fiber with improved bend performances

D.Cuomo FOS/Prysmian, Battipaglia, Italy

Introduction

Telecommunication optical cables are being deployed closer and closer to the final user, in order to meet the increasing demand for larger bandwidth resulting from the diffusion of Internet. From the installation viewpoint, this is creating new problems, since congested urban areas make it difficult to find space for new cables and

connecting devices. This was a challenge for optical fibre manufacturers, encouraged to develop more robust products for distribution or access networks in metropolitan areas. The main parameter to improve is the bending loss performance of the fibres.

A further improvement consists in downsizing the fibre (hence the cable): this cannot be done on the active glass portion where the light propagates, since all tools, equipment and devices for optical transmissions are adapted to a standard value: nevertheless, the protectivecoating around the fibre (250 mm in diameter) can be optimized and reduced.

This paper gives a brief overview of a possible approach to the design of a bend optimized fibre, as described in the new ITU-T Recommendation G.657 [1].

BackgroundOptical fibres for telecommunications are made by an inner core manufactured with GeO

2 -doped SiO

2 (silica),

and a surrounding cladding made by pure silica. The amount and radial distribution of the GeO

2 defines the

“refractive index profile”, the more common being the so-called step index type (Figure 1), with a constant amount of dopant from the axis to a radius of approximately 4 microns (the cladding radius being set by international standards to 62.5 mm). The light propagates along the fibre with minimal losses (attenuation), the order of 0.20 dB/km, with the shape of a (quasi) Gaussian curve centred on the fibre axis and extending slightly outside the core: these “tails” are more likely lost in bends, and

must be therefore minimized for optimal fibre performances.

Figure 1: a step index profile in an optical fibre

Figure 2: fibre section (not in scale)

Fibre Development

The reduction of the ratio between the mfd and the core

diameter can be obtained by reducing the width and/or by

M. Ruzzier, L.TerruzziPrysmian Cables & Systems, Milano, Italy


A section of a step index optical fibre looks like Figure 2, and the light spot size is known as “Mode Field Diameter” or mfd.

16

increasing the height of the ”step” in the radial refractive index profile. This is not straightforward however for a few reasons:

- an excessive reduction of the core radius, hence of the mfd would create problems in connecting the fibre with apparatus or with other fibres (connector or splice loss);

- to increase the height of the step an increase in the GeO2

content is required, but this can result into a corresponding increase of fibre attenuation;

- the actual profile is not as regular as the ideal step, and can change along the fibre during the manufacturing process: proper secondorder tuning is necessary to define a profile sufficiently stable during the manufacturing; this is particularly relevant for metropolitan area networks, where the fibre is likely cut in small sections to reach many different users.

Together with profile optimization, coating material studies have been carried out, in order to reduce the coating diameter: basic materials, UV curable acrylic resins, as well as the application process required an optimization in order to keep as constant as possible the protection provided to the glass part of the fibre.

From the macroscopic view point, the following modifications to fibre characteristics have been obtained:

1. mfd @1310 nm from 9.2 µm to 8.8 µm (nominal values, with a tolerance of + 0.4 µm)

2. coating diameter from 245 µm to 200 µm

Note that the modifications are not necessarily coupled, and can be adopted separately for different applications.

Fibre CharacterizationExtensive studies have been presented in literature[2],[3],[4] on fibre bend loss; the reader is referred to scientific papers for a complete analysis of the phenomenon and of some lateral effects, such as oscillations in bend losses as a function of wavelengths, temperature, mandrel radius.

As anticipated, in this paper we focus on some specific aspects of fibre performances, bending losses. We distinguish between micro and macro bending, by the value of the bending radius:

when it is the order of the relevant parameter of an optical fibre from the propagation viewpoint, i.e. the mfd, approximately 10 µm or less, we talk of microbending; when it is much higher (two orders of magnitude, 100 times) then the mode field diameter, i.e. 1 mm or more, we talk of macrobending. Performances of fibres vs. the

first aspect are more relevant in handling and cabling, for the second aspect are more relevant for installation and deployment.

MicrobendingMicrobending can be characterized by a few methods, as described in international standards[5]. We adopt the “expandable drum” method, consisting in winding a 200 m sample around an expandable bobbin covered with sand paper of a know roughness, then measuring the transmitted power while slowly expanding the drum. This method simulates indirectly the perturbations which can be introduced by a nonaccurate cabling process or by improper lateral pressures to which the fibre can be exposed during or after the installation. Concerns areobvious about performances of small-coating (200 µm, PrimaLight™ in our catalogue) fibres vs. this test, and experiments have been carried out to optimize the structure of the coating system to reduce fibre sensitivity. Good results have been achieved, as shown in Figure 3, where the microbending sensitivity at 1550 nm is shown for standard fibres (green dots) and PrimaLightÔ (red dots). As expected the last is more sensible to lateral pressure, but still behaves almost the same as a standard product. Please note that the glass part of fibres in the figure is not a bend improved (low mfd) profile.

Figure 3: microbending sensitivity of standard and small coating fi-bres


MacrobendingMacrobending performances of optical fibres are typically characterized by the “mandrel winding” method, consisting in bending the sample in one or more coils around a suitable cylindrical support of known radius[6].

Standard products in long distance networks are supposed to be bent on radii not smaller then 25 mm in splice closures or in cabinets. For 100 turns around such a mandrel, the loss is kept below 0.03 dB at 1550 nm. However, much smaller radii are required for metropolitan applications: the minimum value is still under debate and study, but at least fibres are required to exhibit reduced losses when bent down to 10 mm radius (one turn only).

17

Standard fibres exhibit losses as high as 1 dB or more in this condition, which is not acceptable from the power budget viewpoint. Our bend optimized fibre, CasaLightÔ, has been developed in order to reduce this loss to a few tenths of dB, achieving a 10-times improvement, 0.1 dB vs. 1 dB max loss. Experiments have been carried out in laboratory to simulate field conditions. Figures 4 to 6 below show respectively:

- the open air turn of 10 mm radius to which fibre samples have been subjected during experiments; please note that losses are roughly proportional to the number of turns, in first-order approximation; - performances of a standard product in this condition, as measured with the backscattering method, OTDR (Optical Time Domain Reflectometry): the vertical scale is fixed at 0.5 dB per divisionand as expected the measured loss is approx. 0.5 dB (1 dB for a complete loop);

- performances of a bend optimized fibre; from the OTDR trace the reduction of bend loss well below 0.1 dB is evident.

Splices and ConnectionsA negative side effect of mfd reduction, as already mentioned, is the possible increase of connection losses: fibres are necessarily coupled both with other fibres and with transmitter/ receivers (transceivers). A large spot size makes it easier to align the fibres in mechanical, nonpermanent connections and in particular in fusion splices. A trade-off between mfd value and splice loss must be therefore established, also considering the compatibility with legacy and long distance networks. Our final choice for CasaLight™ target value of mfd was fixed at 8.8 µm, vs. 9.2 µm of the standard product. With this choice, splice results shown in the following table have been achieved.

G.652.D Casa Light

G.652.D 0.02 0.03

Case Light 0.03 0.02

Table 1: showing average splice losses

among standard and bend optimized fibres

ConclusionsWe have described two possible approaches to fibre deployment in congested metropolitan areas:

1. adoption of a small coating diameter product; although the fibre sensitivity to handling and mechanical pressures is slightly increased, its use consents an increase of fibre count in standard cables (from 12 to 16 per tube) or a reduction of cable size (hence occupancy) of 10 to 20%;

2. adoption of a small mfd product, which allows to reduce the deployment bends from 25 mm to 10 mm radii, while keeping connection losses to an excellent value, comparable to standard products.

The two approaches can also be combined, in order to provide a more robust and smaller product, easy to handle and install.

Figure 6 - OTDR trace of a bend optimized fibre in bend loss analysis

Figure 4 - sample deployment in bend loss analysis

Figure 5 - OTDR trace of a standard fibre in bend loss analysis


18

AcknowledgmentsThe Authors wish to gratefully acknowledge Prysmian Management for their support in developing new products and for the kind permission to publish this paper, and F. Catalano and G. Colella from FOS/Prysmian for their support in experiments.

REFERENCES

[1] ITU-T Rec. G.657, Characteristics of a bending loss insensitive single mode optical fibre and cable for the access network, Dec. 2006


[2] W.A. Gambling et al.: “Radiation from curved single mode fibres”, Electron.Lett., 12, 567 (1976)

[3] Q. Wang et al., “Theoretical and experimental investigations of macro-bend losses for standard single mode fibers”, Opt. Express, 13, 4476 (2005)

[4] Q. Wang et al., “Polarization dependence of bend loss for a standard single mode fiber”, Opt. Express, 15, 4909 (2007)

[5] IEC 62221 TR3: Microbending sensitivity

[6] IEC 60793-1-47: Macrobending loss

19

IntroductionHistorically, the metropolitan cities and the other populous towns in India have largely been dependent on power supplies through over Head Transmission Lines. However, the situation is dramatically changing due to the insatiable demand of power from the metro cities and the rapidly urbanizing towns, where it is practically impossible to transmit power through Over Head Transmission Lines due to restricted right-of-way for construction of Over Head Lines. This scenario has led to a spurt in demand for underground transmission system using Extra High Voltage (EHV) Cables. Years back the same situation prevailed in the developed countries, where large network of EHV Underground Transmission had to be installed to keep pace with the increasing load requirement.

In future, for new and extended cities, Town Planners would need to put in place an adequate plan for EHV Underground Transmission network, apart from other utilities such as, Telecom, Power Distribution and Water & Gas Supplies Networks.

World-wide, EHV XLPE cables have rapidly replaced the conventional Oil-Impregnated-Paper insulated cables and Gas-Filled cables upto 500kV for the advantages as higher power transmission capacity, lower dielectric losses, maintenance freedom, independence of route profile and terrain, easier jointing and termination, lower installation cost, no environmental hazard.

The design & manufacturing technology of EHV XLPE Cables have undergone major technical advancements since its inception in the sixties. The paper attempts to cover the areas of advancements in design and manufacture.

Evolution of TechnologyXLPE insulated cables were introduced in the World in the 60s. In India, XLPE cable manufacturing plants started

Technical Advancements in Manufacturing of EHV XLPE Cables

Amitava BoseUniversal Cables Limited, Satna, India

operations during the late 70s. The initial crosslinking technology adapted Worldwide was the steam curing process. However, it was realized that microvoids were the inevitable outcome from the steam curing process caused by permeation of moisture into the insulation. In order to overcome this shortcoming, the dry-cure process was introduced. This process dramatically reduced the void content in the insulation and also improved the rate of production. Seeing these advantages manufacturers all over the world gradually switched over to the dry cure technology.

In India, EHV - XLPE cables of 66 kV, 110 kV, 132 kV and 220 KV have been installed for short and medium distance underground transmission lines.

Worldwide the technological progress during the past two decades is presented in Fig (1).

Fig (1)

EHV XLPE TechnologyEssentially, EHV Cable is to be engineered to operate under higher working stress level, as the insulation thickness has


20

to be restricted considering the following:

Operational limits of the extrusion lines.

Maximization of production lengths (Less requirement of cable Joints during installation).

Reduction of the core diameter & perfecting the geometry of the core.

Reduction in weight and cost.

In order to achieve higher working stress on the insulation, design and manufacturing process becomes more critical than the conventional process for Medium Voltage (MV) Cables.

The advancement of technology is centered critically on the following areas

Extrusion Process (Triple extrusion using single cross-head)

Cross-linking Method ( Dry Cure Dry Cooled system)

Configuration of the XLPE Manufacturing Line (Vertical Continuous Vulcanizing).

Clean Material Handling Technology (Class-1000 cleanliness level)

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The above stated areas are a means to achieve “defect

control” for achieving higher insulation breakdown stress (BDS) levels.

The requirements Fig.(2) are imperative in the manufacturing process of EHV XLPE Cables.

Basic Components The basic components of EHV Cables are:

Conductor

Insulation

Conductor and Insulation shields

Water Blocking Tapes

Metallic Sheath

Jacket

Conductor

Hypothetically, it is desirable to have a conductor with a perfectly smooth surface similar to a rod or a tube with uniform curvature to maintain a uniform electrical stress. However, for practical reasons a conductor has to be composed of wires to impart flexibility. As a result the surface is not perfectly smooth.

In EHV cables, the conductor designing and manufacturing is a critical aspect. Copper and Aluminium are both used, however, copper is preferable to gain higher ampacity and lowering the diameter of the cable. This also largely offsets the cost impact of copper. In design and manufacturing the following features are to be achieved

Circular and Smooth surface—for uniform electrical stress.

Low AC/DC ratio – reduction in skin and proximity effect & maximizing the ampacity.

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Fig. (2)


Fig. (3)

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Types of EHV Cable ConductorsCircular compacted conductor: is applicable upto a size of 1000 mm2 for copper and aluminium conductors. The conductor consists of annealed copper or hard drawn aluminium wires arranged in a circular configuration with a high degree of compaction.

Milliken Conductors: when conductor sizes exceed 1000 mm2, Milliken or Segmental conductors are recommended in order to reduce skin and proximity effects.

Rac

= Rdc

(1+Ys+Y

p) R

ac = AC resistance of conductor

Rdc

= DC resistance of conductor

Ys = skin effect factor

Yp = proximity effect factor

Calculation methods indicated in IEC: 287

The phenomena of skin and proximity effect is responsible for the increase in the AC resistance of the conductor. The skin effect is due to self- induction causing a higher current density towards the outer surface of the conductor whereas the proximity effect is due to mutual induction between conductors of adjacent cables causing a higher current density along one side of the conductor.

The Milliken conductor usually comprises of 4 to 6 segments, usually 5 segments Fig. (3) is a common practice. The segments are stranded together in a manner that over a complete lay length; the average spacing of each segment from the adjacent conductor is equal, thus virtually eliminating proximity effect. The segments are insulated from each other, thereby dramatically reducing the skin effect.

Skin effect is significant for large conductors at power frequencies. The AC resistance (R

ac) of 2000 mm2

conductor increases by almost 20% due to skin effect.

InsulationThis is the most critical part in terms of design, material and manufacturing technology.

In context to manufacturing of extra high voltage cables, especially in the voltage levels of 132kV and above where the insulation is subjected to higher stress levels the following imperatives need to be addressed.

Material Selection: It is essential to use super clean XLPE compounds from selected suppliers for manufacturing EHV cables above 66 KV. Also, the quality of semi conducting compound for conductor and insulation shielding is extremely critical for the performance of the cable. For EHV cables of 220 KV & above, super smooth, extra clean compound made with scorch-resistant peroxide is essential.

House Keeping & Cleanliness Levels: High standard of house keeping is of utmost importance, as themanufacturing environment has to be clinically clean. Different levels of cleanliness needs to be maintained at various stages when the compounds are handled and processed i.e. compound storage, material handling system, clean room environment and extrusion platform. The compound feeding to the extrusion must be performed in a close system.

Designing Insulation Thickness: There are no International specifications, which recommend the insulation thickness for EHV cables; therefore the insulation thickness varies considerably between manufactures.

With improvements in quality of materials, production processes e.g. VCV technology, quality control procedures & cleanliness levels, it is possible to reduce the insulation thickness by increasing the working stress gradient.

The statistical Method using Weibull Distribution Method on breakdown studies is the most widely accepted method for determining the insulation thickness for EHV XLPE Cables. The concept of the Weibull Statistical Method considers the reliability of the system based on the “weakest link”.

The insulation thickness of XLPE is calculated from the basic equation:

t = V

(eqn.1)

E

where,

V=AC (Vac) or Impulse V(imp)

withstand voltage(kV)

E=AC (Eac) or Impulse E (imp)

design stress (kV/ mm)

t = insulation thickness (mm)

The E (ac

)& E (imp

) are determined as follows:

E (ac

) = (V0/ 3). K

1. K

2 . K

3

E (imp

) = BIL .K4 .K

5. K

6

where,

V0 = Maximum Line to Line System Voltage

BIL= Basic Impulse Level

K1 & K

4 = Temperature Coefficient

K2 & K

5 = Deterioration Coefficient

K3 & K

6 = Indeterminate / Aging Coefficient

Temperature Coefficient:Temperature influences the AC dielectric breakdown strength of Polymeric materials. With the increase in temperature the AC breakdown strength (BDS) drops.

The probable cause is due to the structural change of polymeric materials.


22

The Impulse breakdown strength is more sensitive to the influence of temperature. The Impulse breakdown strength (BDS) of XLPE drops rapidly at temperatures above 900C.

Typically,

For AC Breakdown

BDS room Temp = 1.06

BDS 900C

For Impulse Breakdown

BDS room Temp = 1.19

BDS 900C

The Deterioration Coefficient K2 for AC Voltage is

determined as follows:-

K2 = (T/t)1/n

where,

T = Guaranteed Life (considered 30 years)

n = Life exponent obtained from V-t characteristics

K2 = (24 x 365 x 30/t)1/n

Determination of n : The nth Power Law, also know as

Life Law is based on the equation

Vnt = constant

log V = K – (1/n). log t

Where K = log (const), n = life exponent

Gradient = - (1/n)

From the line it is possible to determine n and predict the breakdown stress time, thus the life expectancy

In an updated Technology as VCV process the values of n can be safely considered as 15. Hence the value of K

2 =

2.3 (with 1 hour test)

The values of E (ac) & E (imp ) is arrived at by experimental breakdown studies on model cables using the Weibull distribution function (Fig 5 ).

Fig 4

which is establish through extrapolation of experimental results on model cables (of lower insulation thickness say 6.0mm) which are manufactured using the same material and same controlled production method as would be applicable in the actual EHV cables.

Fig(5)


F(E)

= 1 – exp – (E-EL)b / E

0

Where, F(E)

= Breakdown voltage,

E0 = Scale parameter, b = Shape parameter

EL = Locational parameter (lowest BDS)

For VCV line the typical values are

E (ac

) = 40 kV /mm, E (imp

) = 80 kV/mm

Substituting values of E (ac

) & E (imp

) in equ.(1) and taking the highest value of t derived from either case, we can determine the insulation thickness ‘t’.

Manufacturing TechnologiesManufacturers have adopted the following manufacturing technologies:

Continuous Catenary Vulcanization: The insulation is extruded onto the conductor and then in a continuous process is cross-linked (cured) and cooled in a long tube having a catenary profile. This technology suffers from the following limiting factors:

Size of conductor and insulation thickness- over-weight per unit length can lead to the core touching the heated pressurized catenary tube.

When the ratio of insulation thickness to the conductor diameter is large the molten insulation tends to droop under the gravitational force to give the core a “pear shape”. Although to reduce this effect special heat treatment, rotation of cores during extrusion, high viscosity XLPE compounds are recommended, nevertheless perfecting of the geometry of the core till has its limitation.

23

Long Land Die Method: In this method the insulation is extruded into a very long horizontal die in which it is heated to achieve cross-linking and cooled to the temperature of solidification prior to leaving the die.

Additional pressure is not applied, because the extruded core fully fills the tube. High temperature fluid lubricants are used to reduce friction between the core and the die. However, the gravitational force causes heavy conductors to sag through the molten XLPE insulation.

Presently, it is recognized that the VCV technology overcomes the limitations of CCV and Horizontal Long Land Die technology, especially for manufacturing of EHV XLPE cables mainly where the conductor sizes are generally large and the insulation thickness is high.

Vertical Continuous Vulcanizing (VCV) Line The Vertical Continuous Vulcanizing (VCV) Technology is best suited to achieving the above requirements. In the VCV line, the insulation is extruded simultaneously vertically downwards and undergoes the cross-linking process in the vertical vulcanizing tube. The vertical extrusion and cross-linking (VCV), unlike the classical CCV technology, avoids the gravitational force acting in he radial direction of the insulation thereby eliminating the “drooping effect” of the insulation. The gravitational force cannot impinge ovality and eccentricity in the insulation. This process enables perfect geometry of the insulation (Fig.6).

Completely closed Material Handling System:

The XLPE compound in the supplier’s package container

is stored specially to avoid external pollution and

contamination.

Fig (6)

Fig.(7)

Fig. ( 8 ) Vertical Continuous Vulcanization (VCV) Line


Universal Cables Limited has installed the latest VCV technology for manufacturing XLPE Cable upto 400 kV. This line, the first of its kind in India, having the following special features See Fig. ( 8 ).

View of VCV Plant of Universal Cables Ltd

24

In the manufacturing process XLPE compound containers are positioned above the insulation Extruder which enables the XLPE granules to fall vertically downwards simply under gravitational force, thus eliminating long conveying pipes through which the XLPE compounds need to be fed by air suction method to the extruder, as required in the classical CCV process. The vertical XLPE

compound feeding prevents ultra fine foreign particle pollution e.g. dust from the ambient air or metallic particles from abrasion of conveying pipes.

The entire extrusion process takes place in a clean room having Class 1000 cleanliness level. This necessitates the area to be air-conditioned, use of air-filters and slightly over-pressurized ambient.

True Triple ExtrusionThe insulation comprising of conductor screen of semiconducting XLPE compound, super clean XLPE insulation and the insulation screen of semiconducting XLPE compound are extruded simultaneously in a single (common) crosshead.

This technique imparts the following essential properties:

Uniform three-layer insulation structure with perfect interface bonding between the semiconducting layers and the insulation.

No irregularities e.g., protrusions, disruptions, in the inner and outer semi-conducting layers.

Free from micro-voids.

As a result a higher insulation breakdownstrength is achieved.

Controls with Visual DisplayThe entire process is computerized with an automatic control system. This includes X-ray monitor for automatic diameter control of the insulated core.

Programmable Logic Control for the process control for monitoring temperature control of the extruders and temperature profile control of the vertical vulcanizing tube.

Cure calculation programme.

Line synchronization with cure control.

Alarm system.

Historical trending – by storing the running values for analysis of production parameters. This helps in repeatability as required for achieving better quality control.

l

l

l

l

l

l

l

l

l

Vertical Vulcanization Process The vertical vulcanizing tube has a completely dry curing and dry cooling features in pressurized nitrogen gas. The heating/cooling profile and gas pressure is automatically controlled by the PLC. The completely dry cure system ensures void and moisture free insulation. This feature is essential to prevent formation of electrical and water trees during service life of the cable especially when subjected to extra high voltages. Most importantly, the vertical line off-sets the gravitational force to act in the radial direction of the insulation thus eliminating the “drooping effect” of the insulation and also does not involve the conductor sag phenomenon which causes the conductor to sag through the molten insulation during extrusion due to gravity.

In extra high voltage cables, where the insulation thickness is very high, with the VCV Process it is possible to maintain perfect geometry of the insulation.

Permissible Contamination, Void & Protrusion LevelThe manufacturing process has to ensure that the contamination, void and protrusion levels are within the specified level, as the predominant reason for insulation breakdown are due to insulation voids, impurities and protrusions.(Table-1 )

Table-1Permissible Contamination, Void & Protrusion Level in XLPE insulation:

Fig. ( 9) On Line Control – Perfect Geometry


25

Voltage class (kV)

Void(µm) Contamination (µm)

Protrusions as Semi-

conducting layer (µm)

66 60 250 150

110 50 250 100

132 50 250 100

220 40 200 100

Metallic SheathIt is mandatory to provide a metallic sheath on EHV cables 66kV and above.This sheath should be hermetically sealed to eliminate the ingression of moisture as the presence of water with the insulation under high electrical stress initiates the water-treeing phenomenon.

The functions of the metallic sheath are:

To prevent ingression of moisture.

To protect the cores from possible mechanical damage.

To function as an earthed metallic screen (shield) and therefore capable of carrying the specified earth fault current.

Types of Metallic SheathAluminium Foil Fig. ( 10 ) : This is composed of an aluminium foil coated with polyethylene. Poly-Al is mainly used as a moisture barrier for Jelly Filled Telephone cables. However, this application has been extended to EHV Power cables. The genesis of this design was linked with overhead to underground transition power transmission lines – when it was found necessary to connect a light weighted underground EHV cable to an overhead line at the top of a transmission tower. The main advantage of being a lightweight cable consequently finds its use also on steep slopes and in vertical shafts.

Since the Poly-Al foil by itself is incapable of serving as an earthed metallic screen for carrying the earth fault current, it is necessary to use copper wires over the core beneath the Poly-Al.

This construction suffers some disadvantages.

The Poly-Al foil by itself does not provide a mechanical protection as it has poor mechanical strength – there is evidence of cracking of Poly-Al sheath resulting from repeated cyclic elongation due to expansion of the XLPE insulation.

The Poly-Al construction suffers another major disadvantage – during overload condition the XLPE, which

l

l

l

has a high coefficient of expansion causes radial pressure on the adjacent copper screen wires leading to formation of indentations on the XLPE core.

Seamless Lead Sheath Fig.(11) : This is the most popular metallic sheath used Worldwide as well as in India. Pure lead is prone to inter-crystalline cracking particularly with effects of vibration. It is for this reason Lead Alloy ‘E’, which has higher fatigue endurance limit, is preferred. The specified composition of Lead Alloy ‘E’ is as follows:

Elements % by weight

Antimony 0.15 – 0.25

Tin 0.35 – 0.45

Tellurium 0.005 max.

Silver 0.005 max.

Copper 0.06 max.

Bismuth 0.05 max.

Zinc 0.002 max.

The Lead Alloy ‘E’ sheath provides mechanical protection to the core. This construction does not require additional armouring. Over and above, lead Alloy serves as an earthed metallic screen and is capable of carrying the specified earth fault current. Generally, when the earth fault current level is high it is preferred not to increase the Lead Alloy sheath thickness for carrying the earth fault current as this undesirably increases the weight of the cable. In place of increasing the Lead Alloy sheath, Copper wires bound with a counter helix copper tape are applied below or over the lead Alloy Sheath, to share the earth fault current.

Fig. (10) EHV XLPE Cable with Al Foil

Fig. (11) EHV XLPE Cable with Lead sheath


26

For application of Lead Alloy Sheath, it is imperative to use a continuous lead extruder. Discontinuous-type lead press is not recommended as stoppages of the press during each discharge cycle undesirably subjects the XLPE insulation to localized heating.

The minimum bending radius of Lead Alloy Sheath is considered 20 times the diameter over the sheath.

Corrugated Aluminium Sheath: This is another alternative method. The advantage of corrugated aluminium sheath is of its being lighter in weight. The corrugation should be spiral in nature, which imparts a longer creepage path in case of entry of water due to any damage in the aluminium sheath. Aluminium has good mechanical strength; as such additional armouring is not necessary.

The corrugation provides the flexibility, almost similar to lead Alloy Sheath cables, the minimum bending radius is 20 times the diameter of the aluminium sheath. Aluminium sheath serves as an earthed metallic screen and is capable of carrying the specified earth fault current. However, major disadvantages of the aluminum sheath is because of its lower resistance compared to lead Alloy Sheath, it has higher sheath losses and results to lowering the current rating of the cable. The voids of the corrugation contain air and act as an thermal insulation disrupting the heat transfer to the surrounding soil causing high cable core temperatures and subsequently the possible derating of current carrying capacity of the cable. Apart from these factors, there is a concern regarding the possible corrosion of aluminium sheath. The use of high-grade quality PVC and Polyethylene jackets over the aluminium sheath does not serve as a complete corrosion protection. Polymers e.g. Polyethylene are not completely impermeable to water. With the passage of time extending to 10 to 15 Years, it has been found that water passes between the molecules of the Polyethylene thereby causing corrosion on the Aluminium Sheath. Use of anticorrosive material on the Aluminium Sheath is essential especially in tropical conditions.

Considering the above alternative of Metallic Sheaths, it has been recognized that Lead sheathing has no complete substitute and provides the best metallic protection from water ingression. Lead sheathing is preferred in routes having corrosive soil and in water submersed areas.

Radial Water Blocking Features: In order to arrest the ingression of water resulting from a possible breakage in the metallic sheath, water blocking tape is provided below the metallic sheath. When moisture is in contact with the tape, it causes the tape to swell blocking the path of the moisture.

JacketingThe jacking materials for EHV cables generally consist of PVC or Heavy Duty Polyethylene (HDPE). Some utilities

specify anti-termite PVC compound. PVC compound is blended with Lead Napthanate or Copper Napthanate, which are termite repellents. With growing concern over pollution some countries debar manufacturers from using such anti-repellent chemicals. In such cases, Nylon Sheath, which has an extremely hard physical character, is gaining importance. As Nylon has poor fire resistance the sheath is extruded in 3 layers with the inner sheath consisting of PVC followed by Nylon Sheath and finally an outer layer of PVC or HDPE.

TestingFor EHV cables of 170 kV & above voltage grade IEC 62067 is followed. The specification has been extended the Type testing on cable system (including joints).

IEC: 60840 covers the testing of EHV XLPE cables upto 150 kV. The sequence of test is as mentioned in Table 2.

Table –2 Electrical Type Tests

Tests Parameters >66-132 kV >220 -400 kV

Requirements Test on Cable Cable System

Bending test Plain Aluminium Sheath

36(d+D)+5% 36(d+D)+5%

Lead/Lead Alloy/Corrugated Aluminium Sheath

25(d+D)+5% 25(d+D)+5%

Partial Discharge Test

At Ambient temp. & At conductor temp. 900C+ (5 to10)0C for 2 hrs.

1.75 Uo for 10 s & then reduced to 1.5 Uo

1.75 Uo for 10 s & thenreduced to 1.5 Uo

Tan δ Measurement

At conductor temp. 900C+ (5 to10)0C

Power frequency

Power frequency

Heating Cycle Voltage Test

At conductor temp. 900C+ (5 to 10)0C

Voltage of 2Uo 20 Load Cycle 8/16 Hrs: Heating/Cooling

Voltage of 2Uo 20 Load Cycle 8/16 Hrs: Heating/Cooling

Switching Impulse Test (Applicable above 220 kV)


Values Table- 3 10+ve & 10-ve impulses

Lightning Impulse Test




HV Test Ambient temp. 2.5 Uo for 15 min. Values in Table- 3

2 Uo for 15 min. Values in Table- 3


27

Table-3 Test Voltages

Rated voltage

Value of Uo for determi-nation of test voltages

Partial disch-arge test

Heating cycle voltage

Lightning Impulse voltage tests

Voltage test after Impulse voltage tests

Switching Impulse voltage test

kV kV kV kV kVp kVp kV

66 36 54 72 325 90 -

110 64 96 128 550 160 -

132 76 114 152 650 190 -

220 127 190 254 1050 254 -

400 220 330 440 1425 440 1050

Installation – Sheath Bonding Method During installation, one has to consider the sheath bonding method, which has a critical factor from the point of view of sheath losses.

In case the sheaths are not bonded together, then open-circuit voltages will be set-up, which result to sparking, pitting, corrosion due to electrolysis and possibility of accidental contact.

Consider the example Fig(12 ) of two cables laid side by side. This is equivalent to that of an aircored transformer with the primary (conductor) and secondary (sheath) having one-turn(Fig 13), In case the sheaths are bonded together at each end, it will result to a circulating current in the sheath and give rise to sheath losses.

In case the sheaths are bonded together at one end only, the induced voltage at the open end will rise.

In view of the above phenomena, the following sheath bonding methods are followed.

Single Point Bonding

This is a simplest bonding method. The sheaths of the 3 cables are grounded at one end only along with their length. As a consequence, induced voltage develops between the sheath and ground having maximum voltage at farthest end. Therefore, the sheaths have to be adequately insulated from the ground. The open ends of the sheath are connected to the ground through a Link Box having Surge Voltage Limiter (SVL) to restrict the induced voltage rise during fault condition or in the event of any surge voltage in the system.

Mid Point Bonding

This is a modified version of the Single Point Bonding system. At the mid point of cable section the sheath is grounded through a Link Box without SVL with both the ends of sheath current to the ground through a Link Box having SVL. In this method, the length of cable section can be doubled with same rise of sheath induced voltage at the open end as compared to Single Point Bonding System.

Both End Bonding

This is also a common installation practice where the sheaths are bonded to ground through a Link Box without SVL at both the ends. In this method there is no induced sheath voltage. However, circulating current develops which contributes to sheath losses (Equ.1)

Cross Bonding

The popular practice is to use sectional cross bonding. The cable route is sectionalized into major & minor sections. The major sections having both the ends of the sheath grounded. The major section comprises of 3 minor sections of equal lengths where the sheaths of these minor sections are linked in a transposed manner and connected to the ground through a Link Box having SVL. In this method there is practically no, induced sheath voltage and circulating sheath current.

Site Installation Test: will comprise of the following Test:

DC Voltage Test of the outer sheath (Jacket). DC voltage shall be applied between each metal sheath or concentric wires or tapes and the ground.

AC Voltage Test of the insulation

The voltage applied shall be:

1.7 Uo for 1 hour alternatively

Uo for 24 hours

l

l

l

l

(Fig.12)


28

The sheath losses are related to the resistance of the sheathing material.

Ratio of Sheath Losses to Conductor Losses.From Fig(13)

R’ = R+ Rs . ω2 M2 /( Rs2+ω2 M2)

L’ = L- M . ω2 M2 /( Rs2+ω2 M2)

Conductor Losses/ Core = I2 R

Combined core & Sheath Loss = I2R’

Sheath Loss per core = I2(R’- R)

Sheath Loss/ Conductor Loss= I2 Rs ω2 M2 /( Rs2+ω2 M2)/ I2 R

= Rs ω2 M2 / R ( Rs2+ω2 M2) …………..(Equ.1)

R = Conductor Resistance

Rs = Resistance of Sheath

ω = 2 πf

M = Mutual Inductance

L = Self Inductance

Comparison of Bonding Systems

Single Point / Mid Point Bonding System

Both End Bonding System

Cross-Bonding System

Advantage Simple bonding system

Simple & Lower cost

Low circulating current – Low sheath losses

No circulating sheath current – no sheath losses

No sheath induced voltage caused by conductor current

No need of earth continuity wire

Can be used for larger cables having higher current carrying capacity.

No need of earth continuity wire

Can be used for larger cables having higher current carrying capacity.

Disadvantage Sheath induced voltage caused by conductor current becomes higher in proportion to length of bonding span.

Large circulating current & sheath losses

Comply bonding system Higher cost.

Earth continuity wire may be required to be installed in parallel for minimizing interference voltage to telecommu nication cables during power cable fault.

Suitable for lower size cables with smaller current carrying capacity.

Highest cost. Sheath induced voltage caused by conductor current increases in proportion to bonding span

Recomme ndation

Induced Sheath Voltage should be restricted to 65V.

In case, circuit length is long, Sectionalized Cross-Bonding is to be followed

ConclusionEHV underground power transmission is the only solution for meeting today’s growing power demand of the power starved metro cities and towns. Underground power transmission requires the highest reliability to ensure uninterrupted power supplies. Cable manufactured with the proven Vertical Continuous Vulcanizing (VCV) technology with simultaneous triple extrusion in single cross-head technique, completely dry-cure-dry-cooled process, on line X-ray monitoring system and computerized control in production process, class-1000 contamination free manufacturing environment unarguably ensures highest reliability and efficient performance of the cable throughout the service life. In India, EHV-XLPE cables for underground power transmission would definitely play an increasing role in future. Although, underground transmission is approximately 12 times costlier than overhead power transmission, the cost of land occupied by overhead transmission lines largely offsets the investment cost in underground transmission, apart from the feasibility of constructing overhead lines in metro cities and satellite towns. The future challenges would focus on development of low loss EHV XLPE cable for bulk power transmission for ever higher efficiency.


(Fig.13)

29

References: Electra No.151 December 11993 --- Working Gradient of HV and

EHV cables with Extruded Insulation and its effect.

Electra No.151 December 1993 – Recommendations for Electrical Tests Pre-qualification and Development on Extruded Cables and Accessories at voltage > 150 ( 170) KV and 400 (420) KV.

S.Y.King, N.A. Halfter, Underground Power Cables.

D.Mac Allister – Electric Cables Handbook.

Siemens – Power Cables and their Application.

Michel Pays – New Developments in the field of High- Voltage and Extra High-Voltage Cables “The user’s point of view” – IEEE Trans-actions on Power Delivery, Vol.5, No.2, April 1990


30

AbstractGood health of cables in service is important for better reliability and safety of the service area over the entire designed life span. During service, these cables are exposed to various stresses in addition to natural aging, which are responsible for degradation of insulating material and ultimate cable failure. Condition monitoring of cables in service helps the utility to identify the aged cables and take necessary remedial action to prevent premature failure. This necessitates the assessment of aging condition of installed cables. Various condition-monitoring techniques are in force for assessment of aging condition of cables in service. Some of techniques like Insulation Resistance measurement; Tan delta & capacitance measurement; Partial discharge measurement etc. are most widely followed for condition assessment of cables. All these techniques are OFFLINE, and hence have limitations.

To ensure system reliability through a minimum shut down period, an ONLINE method is the need of the hour. For an ONLINE method in the field, only cable outer surface is accessible for any measurement. There is an online method known as Indenter Polymer Ageing Monitor (IPAM). Through IPAM the surface degradation due to aging of the cable is evaluated. A detailed study using this new ONLINE condition monitoring technique was undertaken at authors’ laboratory. Four different types of cables were subjected to laboratory simulated accelerated ageing at three different temperatures and evaluated periodically for their physical properties and testing using IPAM. As per standard guidelines available for insulation materials, the elongation at break has been considered as the end point criteria and the corresponding compressive modulus obtained using IPAM.

Attempt has been made to establish a correlation between the two. The results are discussed in this paper.

Introduction

Advanced Online Condition Assessment Technique for Cables

N. R. Pandya, P.A. Krishnamoorthy, A.K.Singh Electrical Research and Development Association, Vadodara, India

In spite of being on backside of the ‘curtain’; cables have established their importance in routine life of mankind. In general, after installation, a cable draws attention only when it fails and the forced outage due to a cable failure disturbs the whole system as well as results in major loss of revenue and sometimes lives too. During service, these cables are exposed to different stresses like electrical, thermal, moisture, vibration, chemical etc.[1]. Because of these stresses and natural aging process, degradation of cable insulation takes place and brings about physico-chemical changes in the cable material. This results in hardening and embrittlement of the insulation, thereby leading to micro cracks. Over a period of time, due to thermal expansion and contraction, the micro cracks grow to form major pathways for leakage current. Ultimately, loss of dielectric strength - that finally leads to failure of cable and that too without any prior alarm [2]. This makes it important to maintain a high level of reliability and safety of a system by maintaining good health of the cables throughout their designed life span. The utility technocrats are adopting various measures for ensuring that cables- particularly for critical operations do not fail prematurely during service. Efforts have been directed to evaluate the degradation level of installed cable and thereby estimate its residual life by way of condition monitoring [3].

Condition monitoring is an advanced technique adopted today for a cable to monitor its health over a designed life span. It gives aging curve of a cable in addition to indication of any abnormalities developed in the system at its initial level. This helps in timely replacement of aged cable to prevent any sudden and major damage due to cable fault as well as in avoiding any unnecessary replacement of cable. This ultimately increases the level of reliability and safety. Partial Discharge measurement, Insulation Resistance measurement, Tan delta and Capacitance measurement, Polarization Index etc. are some of the techniques [4] utility people are adopting for condition


31

monitoring of installed cables. Efforts are also made using other techniques such as Infrared spectroscopy, Plasticizer content, Time Domain Reflectometry (TDR), Time Domain Spectrometry (TDS) etc. Apart from other limitations and effectiveness, these methods are OFFLINE. The system requires shut down for measurements which is generally not favorable to the users. Further, during measurement at site, the site conditions affect the measurement and thus make interpretation of results very difficult. All these factors have lead to development of ONLINE measurement technique for cables.

The major ONLINE condition monitoring methods for cables under development are Sonic Velocity method, Near Infrared Reflectance measurement, Torque test and Indenter [5]. At author’s laboratory, a study is conducted on Indenter technique using IPAM - Indenter Polymer Ageing Monitor. In this technique, in principle, the surface degradation in terms of compressive modulus is measured. It is well known that, due to aging the cable insulation looses its flexibility, surface becomes hard and this change in hardness reflected in compressive modulus when measured using IPAM. The measurement is conducted on the surface of a cable and therefore there is no need to shut down the system i.e. measurement can be made ONLINE. For a cable, measuring compressive modulus at regular interval and at a particular location, the change in compressive modulus with time is plotted. By comparing the data at a time t1 with the base line t

0,

the instantaneous condition of a cable can be predicted. Further, by having the end point value for a particular type of cable, from the value at time t

1, cable residual life can

be also predicted.

ExperimentalFour cables of different types of insulation / sheath and construction were taken for the study. Cables were designated as A ( HR PVC insulation / PVC Outer sheath), B (PVC insulation / PVC FRLS Outer sheath) and C (Silicon insulation / FS Outer sheath). To achieve the degradation of outer surface of cables, they were subjected to laboratory simulated accelerated aging at 1000C, 1100C and 1200C following guidelines given in IEC 60216 [6]. As per the specification, elongation at break was considered as the parameter for determining the end point in thermal ageing. The compressive modulus using IPAM and elongation at break was considered as performance tests of a cable. At regular intervals, test specimens were taken out from the aging oven and above parameters were measured. The experiment was continued until the endpoint criteria, i.e. 50% absolute value of elongation was reached. Efforts were made to establish correlation between degradation in physical property (elongation at break) due to conventional aging and compressive modulus (using IPAM) by plotting the aging curve.

Results & DiscussionFigure 1, 2 and 3 shows the trend observed for compressive modulus and elongation with thermal aging for sample A. Similar trends are also observed for other samples B and C. As it shows, the modulus is increasing and elongation at break is decreasing with ageing, which is due to hardening

Fig.1: Aging of Sample A at 1000C


of the cable outer surface with ageing, as expected. With

increased temperature, the degradation becomes faster

and with reduced temperature, it takes longer to reach the

same end point criterion. In other words, theoretically, the

compression modulus corresponding to the end point are

expected to be same or within a very narrow range at all

temperatures.


Advanced Online Condition Assessment Technique for Cables

32

Considering the 50% absolute value of elongation as an end point, the corresponding compressive modulus value can be derived from the curve. In the field, the compressive modulus is to measured using IPAM. When the value reaches to corresponding 50% elongation value, the cable can be considered to be degraded.

The results suggest that the IPAM can be used as an ONLINE condition-monitoring tool for cables in service. However, with the available results of limited number of cables and of limited type, it is too early to apply to different cables to predict the life /aging during service. This is purely a characteristic of the material and therefore results obtained for a cable may not be hold good for another cable having different material / construction. It is, therefore essential to conduct measurements on different types and number of cables covering the wide application, for generating norms for condition assessment of cables.

ConclusionThe condition of cables can be assessed ONLINE with the help of IPAM, provided authentic correlation factor is established for the particular material. In view of limited cable samples studied in this project, it is too early to suggest an absolute end point criterion for IPAM. However, as a thumb rule, it can be stated that if the modulus of any PVC cable reaches a value of 100 and above, it needs

attention. More detailed studies are required on different types of materials and in a random variable theory and reliability settings. in order to establish the ultimate cable life prediction technique.

AcknowledgementThe authors are thankful to ERDA management for permission to present this paper.

REFERENCE

1. Dr. S R Ayodhya et. al; “ Concept of Multifactor Ageing and its effect on life of cables, an approach by CCI” - 6th International Seminar on Electrical & Electronic Insulating Materials & Systems, INSULEC-2000.

2. Banford, H.M. et. al.; “ Nuclear Technology and aging’, IEEE Elec-trical Insulation Magazine, Sept./Oct. 1999, Vol.15, No.5, pp. 19-27.

3. P K Ramteke et. al; ‘ Condition Monitoring methodology for predeic-tion of Cables’ service life in Nuclear Power Plants’ - 5th In-ternational Seminar on Power / Telecom Cables, Conductors and winding wires, January 2002, Mumbai.

4. Anandakumaran K et.al; ‘ Condition Assesment of Cable Insulation Systems in Operating Nuclear Power Plants’, IEEE Transaction on Dielectrics and Electrical Insulation, Vol.6 No.3, June 1999, pp 376-384.

5. IEC 61244-3, ‘Long-term radiation aging in polymers - procedures for in-service monitoring of low voltage cable materials.’

6. IEC 216-Pt.2, ‘Guide for the determination of thermal endurance properties of electrical insulating materials.’


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Session III

Day 1 - Session III Technical Papers

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Development of Outdoor Terminations for XLPE Cable

Hiroshi Niinobe, Shozo Kobayashi, Koichi Ono, Nobuyuki ShinagawaVISCAS Corporation, Japan

IntroductionOutdoor termination for XLPE cable is generally composed of porcelain bushing enclosed oil or gas insulation and insulating part. In Japan the prefabricated type terminations are usually used up to 275kV system. The same insulation structure must be supposed to be able to apply up to 400kV. But there is an epoxy insulation and spring unit to press the stress cone. We have another type of terminations which have no epoxy and spring unit. This type of termination should be kept the interfacial pressure between rubber unit and cable insulation appropriately pressure value by rubber self elastic force only to secure the electrical strength.

In case of enclosing insulating oil in the bushing, oil pressure should be control properly to prevent oil leakage. We have been studying the oil pressure change along oil temperature and oil sealing property, providing that heating cycle test was executed in the actual sample. Thus, oil leakage issue has not been occurred for long working.

But, recently customer requirement increasingly tends to be environmental-friendly composite, reduction of the risk of both harming people and severe destructions caused by breakdown. We have been developing dry outdoor terminations to cope with such needs.

This paper reports the developments for both of 110kV synthetic type and rubber unit type simplified termination. This synthetic termination consists of only rubber bushing. Its feature is lighter than that of the existing termination and simple structure to minimize the number of parts. The all rubber shed is molded at one time without separate area, therefore the bending performance is excellent electrically when the termination is set on the utility pole etc. and also long term characteristic for interfacial electrical strength was confirmed by imitating dry interfacial condition.

As several qualification tests were conducted for two type terminations to evaluate its performance, the results are presented the below.

110kV dry outdoor terminationStructure and feature

Fig. 1 Structure of dry-type outdoor termination for 66-110kV class cables

Fig. 2 Potential distribution for outdoor termination


Figure 1 shows the structure of the dry outdoor termination for 110kV class cable, which has the following features.

1. Completely dry-type solid insulation structure

2. Easy handling and lightweight (about 40kg)

3. Free installation angle and environment-friendly

4. No need for special tools or skills

Electrical designThe insulation of the outdoor termination was designed in accordance with JEC3408 for 77kV XLPE cable and IEC60840 for 110kV XLPE cable. Figure 2 shows an example of the equivalent potential distribution. The electric field control mechanism was optimized by the electric field design in the deformation of expanding rubber bushing on the cable. Main portions in this design

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are style of semi-conductive stress cone and the area of air on the surface of rubber casing. The relationship between the shape of silicone rubber sheds and electrical strength is important to control the flash over phenomenon. We carried out many impulse voltage tests by using porcelain or polymeric bushing to study the electrical stress on the bushing surface so that the permissible electrical strength was acquired. Our experience gives better course to get the best design.

The creepage distance is approximately 4200mm long. It is longer than distance of very heavy condition in IEC pollution 138kV level.

Assembly proceduresThe procedure for assembling the dry outdoor termination is the follows as;

(1) Cable bending (annealing)

(2) Cable screen removing

(3) Compression of connecting rod

(4) Wrapping insulating tape

(5) Applying lubricant on cable surface and rubber inside

(6) Inserting rubber casing

(7) Forming Water proof layer

The inserting force is about 160kg so that jointer can slide on cable by proper lubricant.

The testing sample pulled up is shown in Figure 3.

performance test was conducted with approximately 45 degree by mechanical bending as shown in Figure 4.

Table 1 Results of initial performance test

Requirement Bending

AC160kV 30min PD96kVNo detection for 110kV class

45deg.5cycles 133 k V P D No detection

+550kV 10shots for 110kV class Good

Break down phenomenon was not occurred in the insulating material. The flash over at Impulse 700kV was appeared on silicone casing.

The starting point in which the flash over occurred was at the position of maximum electrical stress on the surface of silicone rubber casing. The calculated peak stress was nearly equal to the break down stress of air. Therefore, the design method was verified to be reliable.

F igure. 3 Impulse vol tage tes t for 110 kV s y nthet ic termination

This type termination can be incomplete to stand straight longitudinally. Therefore, we have to check the bending and electrical performance to verify satisfactory property in actual use.

Electrical performance test(Initial performance)

At first stage, we conducted several initial performance tests by using prototype rubber casing. This termination is made of silicone rubber and consists of an insulated part with a high resistance to tracking, vulcanized to a semi-conducting part which forms the field controlling stress cone. One of the

Figure.4 Impulse voltage test for prototype 110kV synthetic termination

The other sample of the termination was tested to check its initial electrical performance based on JEC3408 and IEC60840. The outdoor termination was connected with the 1000mm2 XLPE cable. Table 1 shows the test results. The result of the initial characteristics test was excellent in terms of the performance required of IEC 110kV class and JEC 77kV class terminations.


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Table 2 Result of initial characteristics tests

Items Condition Results

AC withstand voltage test

150kV/1hr (JEC3408)

Good

160kV/30minutes (IEC60840)

Good

AC breakdown Voltage test

250kV↑

Lightning impulse withstand voltage test

+550kV/3-shots (JEC3408)

Good

+550kV/10-shots (IEC60840)

Good

Lightning impulse breakdown voltage test

+650kV↑

(Long-term performance)A long-term loading cycle test was carried out during 20 days applying 128kV based on IEC60840 to confirm the long-term stability. Two outdoor terminations were arranged in the long-term test circuit. The loading condition was controlled at the conductor temperature of a dummy XLPE cable. Long-term test for 110kV class was successful in accordance with IEC 60840.

Table 3 Conditions for long-term test for 110kV class

Items Condition

Applied voltage 128kVAC

Applied current cycle condition Conductor temperature 95-100°C_20 cycles

Loading cycle 20 cycles

Development for 138kV - 154kV terminationThe outdoor termination is typically consisted of rubber cone and bushing providing oil insulation in the bushing as shown in Figure 5. The conventional prefabricated type termination has epoxy insulation and sprig unit to press the stress cone. But, the simplified termination should be kept the interfacial pressure between rubber unit and cable insulation appropriately pressure value by rubber self elastic force only to secure the electrical strength.

Thus, EPR unit must have good elastic property especially for relaxation as shown in Figure 6. This rubber is the same as one piece joint having many supply records for both of 110kV and 220kV class. The relaxation after 30 years is expected 80% of initial value.

Fig. 5 Termination for 138-154kV XLPE cable

Fig. 6 Relaxation property for EPDM

Electrical performance testThe requirement based on IEC60840 is shown in Table 4.

Table 4 The Requirement

IEC60840

110kV Class 138kV Class

AC 160kV x 30min 190kV x 30min

Imp. +550kVx 10shots +650kV x 10shots

Long term Test 128kV x 20days 152kV x 20 days

Loading (conductor temperature 95)

Test results for new simplified termination were indicated on Table 5.

This new simplified termination had enough property for 154kV XLPE cable.

Table 5 Test results

Result

AC 160kV x 30min OK 190kV x 30min OK 210kV x 1hr OK(*1) 295kV_1hrOK(*2)

Imp. +550kV_10shotsOK +650kV_10shotsOK +760kV_3shotsOK -950kV_3shotsOK -1000kV_1shot Flash Over

(*1)110kV and (*2) 154kV are Specification in JEC3408

Expansion method for rubber unitDue to the diameter difference with cable insulation, the conventional stress relief cone can be inserted on the cable by human hand strength easily so far.


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However, in order to maintain the interfacial side pressure, the expansion of inner diameter of rubber unit by about 1.4 times to install the inside diameter of rubber on the cable is needed. Figure 7 shows method to install the rubber unit on the cable. The expanding method that easily installed the rubber stress unit on the cable was newly adopted with a simple insertion tool. The cleat is set on the cable. There is the flange put on the rubber stress unit is used in with the treatment device of the lever block etc.

We have many supply records for this type oil sealing layer.

Table 6 Result of pressure test

Temperature Result

Room 150C 0.5MPa 1H No leakage of gas

Room ⇔90C 0.3MPa 5cycles No leakage of gas

0.4MPa 5cycles No leakage of gas

0.5MPa 10cycles No leakage of gas

Long - term testThe long term test circuit was constructed by two different type bushing. One porcelain and another composite type bushings were applied for 1800sqmm conductor insulating 15mm thickness cable as shown in Figure 9.

Fig. 7 Insertion tool

Oil sealing layerTo control an excessive oil pressure rise, the air l space has been installed up. The oil sealing layer is made by sealing tape and PE tube. The property of oil sealing has been tested by some sample as shown in Figure8. Table 6 shows the result of hot and pressure test. It was confirmed that more than 0.5MPa can be permitted to stop the oil leakage.

Figure 8 Pressure test

Fig. 9 Test circuit for 154kV termination

The test condition is shown in Table 7.

Table 7 Test condition

Requirement

IEC60840 (138kV) Loading for Type Test 152kV x 20days

JEC2408 (154kV) 130kV x 180days 900C150cycles/1050C30cycles

Table 8 shows the result of the residual examination after a long term had been completed in both simplified termination samples. The oil analysis and the observation of the surface of insulation and the oil sealing layer were


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executed by the dismantlement investigation, and an especially abnormal appearance was not admitted.

Table 8 Residual test results

Result

Porcelain &

Composite

Impulse Test (Hot) +550kV x 10shotsOK →+650kV x 10shots OK AC Test (room temperature) 160kV x 30min →190kV x 30minOK → dismantlement investigation Good

Development for up to 400kV terminationThe simplified termination without epoxy insulation is also applied up to 400kV class as shown in Figure 10. These terminations have also self-oil pressure control without pressure tank or special system. The Prequalification test

400kV class termination having the similar structure compared with 230kV termination was tested as shown in Figure 12.

The initial electrical performance of this termination was satisfactory to IEC 500kV requirement. We are planning the long term test for 400kV simplified termination to connect the cable of 2500sqmm conductor.

ConclusionThe authors developed dry outdoor termination for 110kV XLPE cables, in which no porcelain bushing or insulating oil is used. The electrical and mechanical test was conducted for 110kV synthetic termination. Its performance was excellent to use in actual setting situation and satisfactory according to IEC 60840. .

The simplified terminations up to 400kV XLEP cable also was designed and evaluated. The long term tests for both of 138kV -154kV and 230kV class were executed. The electrical and heat cycling performance was verified to be steady and have enough margin for power transmission.

The authors can respond on customer requirement with enough satisfaction because we have appropriate solution by two kind of termination.

REFERENCES[1] H.Iizuka et al., “Development of simplified outdoor termination for

110-138kV XLPE cable” Power&Energy Society in Japanese IEEJ 2002, No357.

[2] A.Watanabe et al., “Single - piece Joint for 230kV XLPE Cable” in Fujikura Technical Review, 2001

based on IEC62067 for 230 kV new simplified termination was executed during 1 year applying AC 216kV as shown in Figure 11.

Figure.10 Construction of Simplified Outdoor Termination


Figure.11 View of outdoor terminations in long-term tests for 230kV XLPE cable

Figure.12 View of outdoor termination for 400kV XLPE cable

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Spectroscopic Studies to Assess the Effect of Shrink Temperature and Duration for Heat Shrink Materials

S.Ganga, V.Asai Thambi, P.Sadasiva Murthy, S.Vynatheya and A.SudhindraCentral Power Research Institute, Bangalore, India

Introduction

Over the years there has been a continuous improvement in design and development of heat shrinkable material for cable joints and terminations. However literature survey indicates that the percentage of failure in accessories is more than that of cable itself [1]. The commonly known causes leading to failure of a joint and termination are a) over heating of connector due to higher loads in cable b) incompatibility between conductor and connector c) usage of improper crimping tools d) improper practices followed while jointing and terminating e) attempts to hasten the installation process etc.

Heat shrink process is an irreversible process where heat shrinkable material shrinks to its original size (unilateral direction) when heated. Precisely, in the shrinking process, the material is heated above the crystalline melt temperature and is mechanically stretched which is then rapidly cooled. The product is shipped in its stretched state.To form a tight seal around cable, heat shrink tubing is heated again by the end-user. The heating activates the shrink mechanism, in other words, reheating the tubing melts the crystalline regions again and causes it to shrink.The stretching temperature has significant influence in the shrink temperature [2].

Improper selction of temperature and uneven heating might leave few of the crytalline region as it is or partially melted in its recovered state leading to incomplete recovery which is not desirable. In general, heating loosens the linkage bonds within the material. Depending on the recovery (shrink) temperature the heat shrinkable material might shrink differently for different shrink temperatures and also on the duration of heat application. The question arises as to whether any test parameter or analytical technique could be arrived at to quantitatively and/or qualitatively reveal the presence of inherent change taken place due to improper selection of temperature and

time. Since failure of heat shrinkable tubing in service can lead to serious consequences, it is desirable to know the effect of improper shrink temperature on the material.

In the recent past, tubing has been engineered to offer a low shrink temperature. This allows the tubing to shrink rapidly thereby minimizing heat exposure. The number of joint and termination suppliers from national and international market is on the rise, which necessitated a check for quality & investigation at macro or microscopic level. A reseach study, at microscopic level indicated that improper supply of heat and duration of its application results in incomplete recovery of the polymeric material on the surface over which it is shrunk. Also there is formation of internal defects viz., micro voids, clevages, surface brittleness, formation of weak links and poor interfaces between conductor and polymeric material. These defects could prove deleterious during service. The paper discusses the effect of improper selection of temperature and its duration of application employing spectroscopic studies.

EXPERIMENTAL INVESTIGATION A polyolefin heat shrinkable material in tube form (HST) was used in the current investigation. The temperature was varied in steps of +50C, +100C and +150C from the prescribed temperature (110(C) and time, (5min from the prescribed duration (15 min) for recovery. Five specimens were prepared for a pair of temperature and time combination. Various properties, physical, mechanical and electrical were determined as per specification [3]. Further, the samples were studied for structural and morphological changes using FTIR Spectrometer and Scanning Electron Microscope respectively.

Although there are prescribed tests and requirements in National and International standards to assess the heat shrinkable material, no direct test parameter is available to check whether complete recovery has taken place or


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not, except possibly the wall thickness ratio.

The results are summarized in the following section.

Results and DiscussionThe objective of this study was to check the variations in shrinkage brought about by different temperatures and durations employed for recovery. The results pertaining to samples shrunk at temperature 100C below & above the prescribed recovery temperature and time at 10 & 15 minutes are discussed. The enunciation of shrunk temperature & duration along with sample names are given in Table 1.

Visual examination was carried out on the recovered samples. In all the samples, no deformities, splits and pin holes were observed. Besides no color change was seen. The surface did not show any chalking or grainy type appearance, bulging and signs of deformation. It appeared from visual analysis that the structural integrity must be intact.

The variation in dimensional analysis indicated that, axial shrinkage was slightly inadequate for a few pairs of temperature- time combinations. The recovered wall thickness of the tubing is proportional to the degree of recovery. As such for a few pairs of temperature-time combinations, full recovery would not have taken place. However, for other samples shrink ratio was almost 2:1.

Variation in wall thickness was less in few samples. However, shrink concentricity was greater than 90% meeting the requirement of the specification [3]. Water absorption was less than 1% in all the samples. Tensile strength and elongation percentage after recovery was meeting the requirement of the specification for all samples. After thermal ageing @ 1350C for 168 hours, the change in properties allowed as per standard [3] is 30% and the same was met by all samples.

Table 1 presents volume resistivity @ 500V DC, dielectric constant & dissipation factor @ 50Hz, 500V and dielectric strength at room temperature for heat shrink tubing shrunk at different temperatures and durations.

It could be inferred from Table 1 that, electrical parameters volume resistivity & dielectric Strength were meeting the requirements of the specification [3].

The morphologies of all samples were obtained using Scanning electron microscope, Leica S440i and are depicted in Figures 1-6 @ 4KX magnification. EDAX analysis was also performed for all the samples.

It is apparent from these figures that there is noticeable variation in the pattern which in turn indicates that there is significant variation in the shrink mechanism and that could be due to difference in temperature-time

combination employed for recovering each sample. The presence of particles seen in Figure.1 appears to be fillers added to achieve specific property in heat shrinkable material.

Figure.1 SEM Picture of Sample A (100, 10)

Comparing Figures.1 and 2, it appears that shrinking process would have just initiated in case of sample A and partially taken place in Sample B indicating that the temperature-time pair selected might not have been sufficient for full recovery.

Table 1: Variation in Dielectric properties of HST at different shrink temperature and duration

Sl. No Shrunk Temp (0C), Duration (min)

Volume Resistivity Ω-cm

Dielectric constant

Dissipation factor

Dielectric strength kV/mm

1 Unshrunk 4.94 x 1013 3.23 0.060 10.7

2 Sample A (100, 10)

4.31 x 1013 3.78 0.041 11.5

3 Sample B (100, 15)

5.08 x 1013 3.80 0.027 11.0

4 Sample C (110, 10)

5.42 x 1013 3.82 0.025 12.3

5 Sample D (110, 15)

5.25 x 1013 3.73 0.023 12.0

6 Sample E (120, 10)

5.44 x 1013 3.68 0.029 12.9

7 Sample F (120, 15)

5.82 x 1013 3.77 0.020 12.0

Figure.2 SEM Picture of Sample B (100, 15)

Spectroscopic Studies for Heat Shrink Materials

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Presence of micro voids are apparent in Figure.2. A similar inference i.e. formation of shrinkage voids due to insufficient time and pressure in the injection molding process is reported. These voids have caused failure of injection molded parts [4].

No surface brittleness, cracks or voids are noticed in Figures.4 and 5. The shrinkage appears to be uniform

Figure.3 SEM Picture of Sample C (110, 10)

Figure.3 appears to indicate that sample has almost shrunk to its initial state, perhaps a little more duration would have resulted in full recovery. Polyolefin is semi crystalline in nature and hence SEM picture is likely to project both crystalline and/or amorphous regions. SEM pictures at different locations for a sample shrunk at 1100C for 15 minutes are depicted in Figures.4 and 5.

Figure.5 SEM Picture of Sample D (110, 15)

Figure.4 SEM Picture of Sample D (110, 15)

Figure.6 SEM Picture of Sample E (120, 10)

Figure.7 SEM Picture of Sample F (120, 15)

be inferred that the selection of higher temperature and time employed for Sample E and Sample F has resulted in formation of defects afore mentioned.


as there are insignificant undulations on the surface when compared to other figures. All the above inferences

possibly indicate that the pair (110, 15) could be the right choice for recovery.

Figures.6 and 7 illustrate the presence of micro voids, cleavages and surface unevenness and brittleness. It could

Figure.8 SEM Picture of Sample D (110,15)

A few samples were prepared, employing most probable temperature-time pair as inferred from this study, for full

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recovery. The Figure.8 depicts a micro graph at a random location of a typical sample.

It is apparent from Figures.5 & 8 that the textures of surfaces appear alike. The presence of additive/filler is seen in both figures. The above inferences are likely to be indicative of full recovery. Subsequently tracking and erosion test was carried out on few other samples and observed that there was no tracking but slight erosion was seen after test duration of 3hours and 20 minutes.

FTIR spectra were obtained for these samples in order to assess structural changes undergone, if any, during Inclined plane tracking (IPT).Typical FTIR spectra of sample before and after IPT are presented in Figures.9 and 10 respectively. It can be inferred that the prominent peaks have not shifted after IPT indicating that no structural changes have occurred during IPT. It is imperative that during IPT test, sample experiences high temperature. As there was no dimensional change, it could be inferred that additional shrinkage had not taken place during IPT test. The sample met the requirements of heat shock, low

1997 [3]. The combination of temperature-time pair arrived from this study appears satisfactory for full recovery.

Conclusions The effect of temperature and time is not imperative from physical, mechanical and thermaltests.

It is apparent from SEM pictures that there is deleterious effect of improper temperature and duration employed during heat shrink process i.e. recovery.

Variation in patterns as seen in SEM pictures indicates that there is change in shrinkage qualitatively due to various combination of temperature-time pair employed for recovery.

A temperature below actual recovery temperature gives rise to micro voids.

A temperature above actual recovery temperature incorporates micro voids and cleavages and surface brittleness during recovery due to over shrinking.

In view of the above conclusions, the temperature and duration could be printed on the heat shrinkable component besides providing brief booklet with the kit.

Acknowledgement

The authors wish to thank the management of Central Power Research Institute, Bangalore for permitting to publish this paper.

REFERENCES

[1] H.A. Kohnakdar et al. “Thermal and Shrinkage behaviour of stretched peroxide- cross linked high density polyethylene” , European Poly-mer Journal vol. 39, Issue 8, August 2003, pp. 1729-1734.

[2] Deepak T. More, et al. “Analysis of Failures of MV Cables and Ac-cessories in the NDPL Network” National Power Seminar on Power Cable Technology, 2006, pp. 235-243.

[3] Electricity Association - Technical Specification 09-13 -1997.

[4] Vishu Shah, “Handbook of Plastics Testing Technology”, A Wiley-Interscience

Publication, John Wiley & Sons.

temperature flexibility, retention percentage of tensile strength and elongation besides other tests as per EATS-

Spectroscopic Studies for Heat Shrink Materials

Figure.9 FTIR Spectra of Sample D (110, 15)

Figure.10 FTIR Spectra after IPT of Sample D (110, 15)

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BACKGROUNDNTPC is procuring HT cables, LT Cables and control cables for setting up several power stations in India. It is imperative to absorb the latest technology alongwith its economic aspect. The initial cost as well as long term cost has been considered. The guidelines and checklists formulated for cables sizing and philosophy is enumerated in this paper. Operational experience is available with this methodology adopted for various project sites. The benefits of outlining the philosophy are:

Optimised cost

Improved reliability

Reduced engineering man hours.

Sizing OptimisationsState of Art Software is available for Cable Sizing. The criteria for cable size selection are:

- Working Voltage & current

- Running and motor starting voltage drop

- Laying conditions

- Ambient temp

- Laying in air/ ground/ ducts

- Grouping

- Short Circuit current, its duration and primary protection

Optimisation of Engg. ManpowerIntegrated Cable Schedule Software ensures:

- Cable listing & Cable Scheduling

- Auto generation of interconnection Diagrams

- Automatic Cable segregation Separation

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Optimisation in Cable Sizing and Laying Philosophy

Sanjay Seth, K. NageshNTPC Ltd., Noida, India

- Tray loading under control

- Large data base for BOQ estimation

Cable Cost Reduction - Use of unarmoured cables in plant areas.

- Use of earthed cables for Medium voltage systems for effectively earthed system (Earth fault current 300A) inline with provisions if IS-7098 (II). Exception has been made for cables for fire water system and cables which are buried and cables routed beyond the plant boundary.

- For single core armoured cables, armouring has been considered part of metallic screen. The armour contribution to short circuit current shall reduce the copper component in screen and hence reduction in cost of cables.

- Cable scheduling software developed in-house gives shortest length.

CABLE LAYING PHILOSOPHY - Cable laying philosophy intends to reduce the

operation and maintenance cost, improve reliability and increase flexibility. Cables are run in cable trays mounted horizontally or vertically. Cable tray support system used is flexible bolted type. FRP cable trays are used in plant areas in the vicinity of sea.

- Cables in outdoor areas are generally routed in trestles. Cables trench/duct banks are avoided to the extent possible due to possibility of water ingress.

- Cable trays are generally laid in horizontal tiers. However, cable trays in Boiler, ESP, CHP areas are laid in vertical formation to minimize coal/ash/dust accumulation. Cables are not routed thorough coal conveyor galleries.


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Reliability ImprovementReliability Improvement is achieved by Separation and Segregation.

Separation:

At least 300mm clearance is provided between:

HY power &LT power cables

LT power & LT Control/Instrumentation cables.

Segregation:

Segregation means physical isolation to prevent fire jumping.

All cables associated with the unit are segregated from cables of other units.

Power and control cables for AC drives and corresponding emergency AC or DC drives shall be laid in a separate/segregated route.

Two separate cable routes are provided for each boiler unit. Cables for one set of auxiliaries such as ID, FD, PA fan & half of the coal mills, shall be routed in one route & for other set of auxiliaries through other route.

Two separate cable routes are provided for each boiler unit. Cables for one set of auxiliaries through other route.

Not more than half the unit auxiliaries are lost in case of single event of fire.

For interplant cabling of station auxiliaries such as DM plant. Air compressor, Ash handling & Coal handling etc. minimum two segregated cables routes shall be provided such that in a single incident:

No more than half the pumps are lost in a station in case of CW pumps.

All the streams are lost in case of DM plant, compressor house, etc.

Both the streams of conveyors are not lost in case of Coal And Ash Handling.

Frls Properties - Oxygen Index (IS10810 Part-58) more than 29

- Smoke density (ASTMD 2843) less than 60%

- Acid gas generation (IEC-754-1) less than 20%

Fire Risk Prevention - Cable and electrical plant fire risk prevention is

achieved by physical segregation of cables and fire protection system.

- All cables are flame retardant low smoke (FRLS) type.

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- Directly buries cables are armoured type.

- All cables in Coal Handling Area are armoured type for improved mechanical strength. Cable feeding Fire water transformers are armoured, unearthed grade and routed separately from other cables.

Fire Proof Cable Penetration (FPCP) Sealing SystemFire proof Cable Penetration (FPCP) Sealing System is used to prevent spreading of fire in cable beyond the seal in case of fire. A minimum one hour fire resistance rating system is installed. The general requirements are high integrity, mechanical strength, and thermal stability, non-toxic and should be non-reactive to cable components. The issue of Fire Sealing have scope of improvement are lack of testing facilities, shelf life and controlling testing parameters like air temp and density.

In NTPC two types of Fire sealing material are being used- Type ‘A’ and Type ‘B’.

TYPE ‘A’:

Silicon RTV foam or equivalent foam system or suitable Block system using individual polymer blocks for each cable along with suitable frame work. Type-A is used for sealing of floor opening below C&I panels, control panels, UCB etc. in CER &UCR. Silicon foam has self expansion feather which block inter cable voids effectively.

TYPE ‘B’:

Used for sealing of floor openings below HT/LT Switchgear, distribution boards, wall & floor crossings of cable/cable trays in main plant area. Type-B is any proven fire sealing system having one hour fire rating. Generally Mineral wool or Mortar based systems are provided.

Fire Detection and Protection SystemThe area wise application of fire detection and protections are listed in Table-1

Experience

The guidelines and checklists formulated for cable sizing and philosophy have been used for detail engineering of projects and site feedback is incorporated on continual basis. Further, the layout of plant and equipment when taken care at initial planning state can result in enormous savings in cable lengths and therefore costs. The experience of layout engineer has a major impact in reducing the costs and improving operational reliability.

Optimisation in Cable Sizing and Laying Philosophy

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Table-1

Fire Detection and Protection System

Area Detection System Protection System

Cable vaults and spreader room in main plant bldg., ESP,CHP & Swyd. areas

a) Isolation type smoke detector*

b) hotoelectric type smoke detector*

c) Linear cable sensor (Digital)

Medium velocity spray system

Control room, control equipment room, inverter room & DAS room

a) Ionisation type smoke detector*

b) Photoelectric type smoke detector*

a) inert gas b) Portable fire extinguishers

CHP Control room, switchyard control room ESP control room

Ionisation type smoke detector* Portable fire extinguishers

Switchyard room location inside power house building

Ionisation type smoke detector* Portable fire extinguishers

*Analog addressable intelligent type


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Session IV

Day 1 - Session IV Technical Papers

49

Efficient Fault Management using Fault Indicators

Eilert Bjerkan, Runar Myhre Nortroll AS, Norway

AbstractThe paper contains a brief historical review of fault passage indicators (FPIs also called FCIs: Faulted Circuit Indicators), both in terms of development and also technology behind the detection. Limitations and advantages are elucidated both technically and economically. Local vs. remote indication (with communications) is discussed. The paper concludes that FPIs is a cost-efficient measure in order to increase operational efficiency during Fault Management in MV distribution systems.

AbbreviationsQoS Quality of Supply

FPI Fault Passage Indicator

CENS Cost for Energy Not Supplied

ENS Energy Not Supplied

PCC Point of Common Coupling

DG Distributed Generation

IntroductionRequirements to QoS (Quality of Supply) are increasing in all industrialized and developing countries. This enables the use of FPIs as cost-reducing measures in all MV distribution networks since it reduce duration of outages and thus increase QoS.

Electro-mechanical FPIs have been used for more than 30 years with little or no success because of the limited possibilities of adjusting the units to the specific networks.

The first fully electronic FPI was released in 1976 [1] . This was a major breakthrough in fault management since the device was programmable and led to a big reduction of the time spent for fault location.

The first units were line-mounted and suitable for detecting over-current in most overhead line, high voltage distribution systems. Similar FPIs for underground cable networks followed shortly after. During the 80’s, new electronic FPIs enabled detection of earth faults in both overhead line and underground cable systems with isolated and resistance-grounded neutral increasing the operational value of FPIs during fault management.

Non-directional threshold-sensing FPIs have been widely used during the last decades to track down faults as fast and efficient as possible, reducing usage of man-hours and number of trial connections. Trial connections often lead to switching surges that may lead to damage of customer equipment and installations.

While FPIs have been used for many years, utilities have in certain cases been disappointed, due to:

Incorrect settings, resulting in false or missed tripping.

Inflexibility of products designed to order for specific applications and networks.

Incorrect installation

Poor product quality.

By investing in state-of-the art microprocessor based FPIs, utilities can avoid these problems and then dramatically reduce average outage time.

Distributed Generation (DG) is being implemented on a large scale in some countries. DGs connected to the MV grid support the fault current and directional FPIs will therefore be usual in near future. Technology exists for directional detection of both earth faults and short circuits. The directionality is added by adding voltage measurements or comparative analysis (several feeders) to traditional FPIs.

l

l

l

l


50

Efficient fault management:The Scandinavian countries were among the first in the world to deregulate the power markets and to develop penalty-schemes for the utilities depending on severity and duration of outages. These regulations were enforced to encourage new investments to reduce the number of faults. Remote control systems integrated with fault locating equipment such as fault indicators with communication have proved to be a cost-efficient and time-saving solution for all utilities operating under such penalty-schemes. The trend is that all industrialized countries adopt these regulatory frameworks [1] , since customer requirements to quality of supply are increasing rapidly. The penalty-scheme in Norway is based on CENS (Cost of Energy Not Supplied) with differentiated costs for different customers (Industry, households etc.)

Many studies have been performed to investigate outage-costs in different types of networks, and most of these studies conclude with the fact that investments in remote control equipment and fault locating equipment such as fault indicators have very short pay-back time (in some cases less than one year [2] ).

Example: Cost-Benefit Analysis of FPIs and remote control investments

An example of cost-benefit analysis using FPIs is given in [3] , and major results are given below:

The example uses a common MV (22kV) overhead distribution network to illustrate cost-benefit of different actions:

FPIs with local indication

FPIs with communication (remote FPIs)

Remote controlled sectionalizing switches.

The network layout of one radial is given below with 4 sections and 4 PCCs (Point of Common Coupling/loads):

l

l

l

Table 1: Loading and no. of customers of the network given in the example

The reliability data for this network is taken from Norwegian fault statistics [3] . Sections are between 1-3 km, and failure rate for sections 1-4 is 0,1 faults per km and year. For sections a-d the corresponding value is 0,2. Average duration of each outage is 4 hours per fault for sections 1-4 and 2 hours for sections a-d.

Estimated time for preliminary sectionalizing (without FPIs and remote control) is 30 minutes and counts for 20% of the total outage duration. Number of outages for this particular example is calculated to 2,2 faults per year (average).

The yearly ENS due to sectionalizing time without outage-reducing measures is calculated to 10,1 MWh/year in [3] . This is then used to calculate the reduced time spent for sectionalizing and thus the reduction of ENS (Energy Not Supplied) as seen from table 2

An average CENS of 17 NOK/kWh is used in the following analysis due to the mix of households and industry in this particular network. A social economic point of view is taken into consideration where the total reduction of outage-costs consists of both the cost-reduction for the utility and the customer, as seen from table 3 on the next page.

Actions: Reduction in time spent for sectionalizing

Reduction of ENS [MW/year]

Total investment: (NOK, 1997)


20 % 2 35 000

FPIs with remote communication

45 % 4,5 126 000

Remote control, day-time crew

only

50 % 5,1 533 750

Remote control with 24h crew

available

90 % 9,1 536 250*

Table 2: Costs and benefits of investing in FPIs and remote control

* The increased man-hour costs of operating a 24h operating centre is not included

In the right column of table 3, the marginal CENS is shown. This means the CENS where the different actions start to become profitable. Taking into account the switching surges applied to customer installations during trial and error connections, a combination of remote controlled switches and FPIs with remote communication would be preferable.

Figure 1: Example network for Cost-Benefit Analysis of FPIs and remote control

Loading and number of customers are given in table 1:

PCC No. Customers Maximum Load MW]

A 1000 7,5

B 800 6

C 700 4,5

D 500 3


51

In this particular example, all actions are profitable. Profitability is very dependent on the CENS used in the analysis. In developing countries with low labour cost, low focus on QoS and thus a low CENS, only investments related to FPIs will be profitable.

Indian CENSIndian CENS [4] is calculated to be 3,4 NOKs making actions 1 & 2 (local and remote fault indicators) profitable. This number will increase rapidly as the country becomes more and more industrialized, and requirements to QoS increases.

Actions: Total cost-

reduction [NOK/ year]

Annuity of invest-

ments [NOK/ year]

Payback- time

[years]

Benefit/Cost-Ratio

Marginal CENS [NOK/ kWh]


34 900 3 850 1 9,1 1,9

FPIs with remote communication

78 530 13 860 1,6 5,7 3,1

Remote control, day-time centre only

88 970 58 740 6 1,5 11,5

Remote control, 24h operating centre

158 770 59 070 3,4* 2,7 6,4

Table 3: Profitability of different actions to reduce outage time during preliminary sectionalizing

New developments on FPIs:Earth fault detection in coil-compensated networks:

During the last 20 years, many utilities have converted to coil-compensated networks leading to lack of methods for detecting earth-faults by FPIs. The Peterson-coil is mainly used to compensate for the capacitive fault current during a phase to ground fault in order to suppress the arc and leave the customers undisturbed during temporary faults.

The traditional threshold sensing fault indicators can only be used for detection of earth faults on relatively short braches since the steady state fault current is very low. The development of a new generation of fault indicators that use the fault-initiated transients for directional detection was started during the late 90’s [5] .

Detection of downed and broken conductors:Downed and broken conductors are not always detected and disconnected by the principal relay-protections. This situation leads to very dangerous situations for human beings, animals and assets. Recently, distributed systems are able to detect all kinds of broken and downed conductors, including back-feed faults, broken loop and even blown HV-fuse. Such systems have been tested several times with success in real MV distribution systems [6] .

Figure 2: Directional detection of Phase to ground and phase to phase faults using colors


52

When conductor breaks and falls down on the load side rather than the supply side, a back-fed situation occurs. The faulty phase is energized via the distribution transformer depending on the connection group of the transformer and the degree of secondary loading of the transformer.

RF

Feeder equv.

MV/LV

Transformer LV Load

works (In Norwegian)”, Master’s Thesis, 1997,Norwegian University of Science and Technology

[4] “Defining an integrated energy strategy for India: Ensuring security, sufficiency, and sustainability”, TERI, 2002, ISBN: 81-7993-008-4

[5] Bjerkan, E., Venseth, T., “Locating Earth-Faults in Compensated Distribution Networks by means of Fault Indicators”, Presented at the 6th Int. Conf. on Power System Transients (IPST05), June 19-23, 2005, Montréal, Canada

[6] Bjerkan, E., Høidalen, H.K., Hernes, J., “Efficient and Secure detec-tion of Downed and Broken Power Lines”, Presented at: CIRED 2007, Vienna

Equivalent fault resistance (in series with the distribution transformer) can easily reach values above 100 kOhms, making such faults undetectable by the relayprotection, but for units measuring on the low voltage side of a set of distribution transformers in the network as shown in figure 4, the fault will clearly be visible for units outside the fault side due to the voltage imbalance. This makes dangerous and difficult fault types detectable.

References:

[1] Gjermstad, J., “Fault indicators on overhead distribution lines are cost effective”, IEE with reprint in Modern Power Systems, August 1987

[2] Bjerkan, E., Svendsen, G., “Efficiency improvements by systematic investments in remote control systems”, Presented at: NORDAC 2006, Stockholm, Sweden, 21-23 August 2006

[3] Foshaug, A., “Cost-Benefit Analysis of using FPIs and Remote control to reduce duration of outages in Medium Voltage distribution net-


Figure 3: Back-fed earth fault due to broken and downed conductor

Figure 4: Downed conductor detection by distributed LV measure-ments

53

Cold-shrinkable Technology for Medium Voltage Cable Accessories and its Application to 3-Core Systems

F. ClaeysNexans-Euromold, Belgium

AbstractAfter maturing for over 10 years on the European markets, our cold-shrinkable technology is now making its entry in the Asian markets. A description of the concept of the cold-shrinkable technology is presented. This paper focuses on the cold-shrinkable joint and its application on 3-core cables, describing from the concept to the manufacturing the main orientation chosen to warranty the reliability of the product in service. Some results concerning the product (short and long duration tests) are presented and main advantages given to the user are pointed out.

I IntroductionIn the medium voltage cable accessories field, there is a concept that presents numerous advantages. The so-called ‘cold-shrinkable technology’ provides non-size sensitive accessories able to cover a wide range of cable sections, without requiring any special tools or heat. The principle of this technology relies on the fact that the product is made of elastomers with very high elastic features. Pre-fabricated cold-shrinkable accessories are stretched over a support tube several times larger in diameter than the product itself. During installation, the accessory on support is slid over the cable, and positioned. Then the support is removed and the elastomeric product retracts and comes in direct contact with the cable providing the required interference pressure for voltage withstand. This concept offers the users a simple design and an

easy to install product, minimising installation errors. In this paper, we will focus on the concept, development and evaluation of a cold-shrinkable joint and its application on 3-core systems.

II Cold-shrinkable ConceptThe objective was to develop a pre-fabricated one-piece product to limit the operations in the field as much as possible. The concept is based on a plastic support tube wrapped in a thin film of very low coefficient of friction for easy gliding. On that film, various types of materials, such as mastics, metal braids, and rubbers under stretched conditions are placed. This allows pre-installing on one support all functions required for cable accessories (i.e. field control, sealing, and protective functions), as shown in Fig. 1.

Fig. 1: All functions pre-installed on support tube

Two supporting tubes are placed opposite under the stretched and assembled accessory. During the installation, the assembly is slid over the prepared cable, and positioned by anchoring one of the tubes to the cable. The other tube is pulled out along the cable allowing half the accessory to immediately shrink on the cable, as shown in Fig. 2.

Then the first tube is untied and removed in a similar way. A low coefficient of friction film that is removed with the tubes facilitates easy removal of the supporting tubes

Cold-shrinkable Technology for Medium Voltage Cable Accessories

54

from the accessory. It is an installation aid and one person without any tools can carry out such an installation. Single piece housing with built in functional features, minimising installation errors, and ease of installation are the main advantages of this system.

Based on the above concept, cold-shrinkable terminations and joints have been developed. In this paper we will present the main aspects of design and evaluation of such a joint. The development of cold-shrinkable terminations has been described in a previous paper [1].

III. Cold-shrinkable Joint

III-1 DesignThe cold-shrinkable joint was first developed for extruded polymeric cables (24 kV class) to cover cable cross sections from 95 mm2 up to 300 mm2, covering a minimum cable insulation diameter of 23 mm and a maximum cable outer sheath diameter (or insulation diameter for unsheathed cable) of 46 mm.

A. Complete joint on support tube:Basically, the accessory is composed of:

-a triple layer body, for the dielectric continuity of the cable: high K layer, insulation and semi-conductive layer,

-a metallic stocking to connect the metallic cable screens,

-an outer protection with mastics at the ends to ensure water sealing and mechanical protection.

All these components are pre-installed one upon the other, on the support tubes described above, and can be stored as such up to two years. For this product the film is free to turn around the tube, like a driving belt, and doesn’t get in between the shrinking joint and cable during installation. A hook allows to pull out the tube during the extraction. After installation, each support tube, made of two half shells, can be removed from the cable.

B. Installation of cold-shrinkable joint on cableThe installation sequence of the cold-shrinkable joint on the cable is as follows: the one-piece accessory is slipped over the prepared cable. After connecting the conductors, a double layer plate is wrapped around the metallic contact. This is a non-size sensitive plate that

has been specifically designed for the product. It is made out of a semi-conductive cured EPDM layer, on which a high K mastic layer -slightly larger than the first layer- is superimposed.

The semi-conductive side is placed on the metallic contact, overlapping each cable insulation end, in order to obtain a ‘Faraday cage’ in the connection area. This plate can overlap any contact: crimped (with either deep indent or hexagonal crimping) or non-size sensitive bolted contact and allows the installer to have a visual control of its positioning. In case of mistake during installation, the plate can be unrolled and repositioned.

After this first operation, the one piece assembly is centred on the contact area. Installation of the complete joint (body, metallic stocking, outer protection and sealing mastics) is done by extraction of the support tubes, one after the other, in a very simple translation movement. During the removal of the first tube, the second tube is anchored onto the cable with a plastic strap.

Fig. 2: Principle of installation

C. After installationBy shrinking the product, all required parts are correctly positioned on the cable, ensuring all functions of the joint (field control, earth screen continuity, sealing and protective functions), as shown in Fig.3. Main components are:

-joint body extending from the semi-conductive screen of one cable to the other,

-metallic stocking connecting the metallic screens of each cable,

-outer protection (over sealing mastics at ends) extended over the cable outer sheath and ensuring no water penetration within the joint.

The mastic layer that was installed in the conductor connection area has a high dielectric constant that refracts the equipotential lines at semi-conductive edges, reducing at these points the electrical field value. High K mastic bands may be placed also at the edge of the semi-conductive screen of the cable, for the same reason. An electrical field calculation has been done taking into account all material properties and joint design. The


Fig 3 Design

55

results presented in table 1 shows that the field control is properly done inside the joint.

Measure of gradient AC at 12 kV/mm

Impulse at 125 kV/mm

E1

in joint screen end1.1 11.5

E2

in joint at semi-conductive plate end

2 20.6

Et

tangential gradient at cable/joint interface

0.96 10

Table 1: Gradient calculation in the joint

The continuity of the cable screen is maintained by the radial and longitudinal pressure of the elastomeric outer protection on the metallic stocking placed at each end of the cable screen. The flexibility of the elastomeric housing ensures a stable connection during service with long heat cycles.

The cold shrinkable joint concept, developed for extruded cables, has been successfully adapted to paper cables, both draining and non-draining. In this case a barrier is shrunk over the prepared cable prior to joint installation. Its specific properties combined with the pressure applied by the shrunk joint avoid oil migration from the paper cable to the EPDM joint body.

III-2 ProcessLike the design, main concern in the production was to ensure a perfect reliability of the product. By transferring the operations from the field to the production site, this new technology required from the manufacturer unprecedented skills. The choice of stable process associated with quality control at each manufacturing step provides a guarantee to the reliability of the cold-shrinkable joint.

The triple layer EPDM body extruded like a cable, and cured under nitrogen atmosphere, makes sure that the behaviour of the joint is as close as possible to the cable itself. The extruded cable drum is then fully electrically tested to detect possible defects.

Before extrusion, three materials constituting the body, high K, insulation and semi-conductive, are tested to check the accordance with the required specifications.

After extrusion, the joints are cut to the right dimensions and expanded on the support tubes that can be 3 times larger in diameter than the joint itself. Samples are taken regularly to check that nothing is damaged during the expansion phase. In the same way, main polymeric components of the cold-shrinkable joint are extruded (mastics, outer protection) and tested after production. Once the body has been installed on the film wrapped

plastic tubes, the metallic stocking is put over it, then mastics are rolled at the tube ends and finally the outer protection is expanded over the assembly. The shrinkage rate is then tested by sampling.

III-3 Results

A. MaterialsMore than any other technology, the cold shrink relies on perfect mastering of materials. Thus, in addition to the severe electrical properties required for all medium voltage products, the materials must have high mechanical properties combining antagonistic features such as high elongation at break, low set, low modulus and high tear resistance. This development was made successfully by extensive research on the materials for cold-shrinkable joints. Here some key results obtained during material evaluation under mechanical and electrical ageing are presented. More detailed information concerning these materials has been published earlier [2].

Amongst the important set of requirements the product must meet, the most specific one is the ability to function in a wide range of cable sizes, by applying a pressure sufficient to withstand high voltages. This requires a quick elastic recovery of the material, after long duration strain sustained under various atmospheric conditions during storage. More precisely, the product must be storable under radial expansion of 250% during 2 years, and shrink on the smallest cable section (diameter over insulation is 23 mm).

The elastic features of the insulation material used in the triple layer cold-shrinkable joint were evaluated on compression plates, according to an internal test specification. Each sample was stretched to 250% level during various periods of time at room temperature. The residual set was measured at room temperature 15 minutes after strain removal. The results are plotted in Fig. 4 as a function of expansion period.

As shown by these results, the evolution of the set after shrinking seems to follow a logarithmic law. The expected


Fig. 4: Residual set of insulation material, after 15’.

56

set after 2 years of storage at room temperature (22°C) is 23%, 15 minutes after strain removal. Of course set continues to decrease further with time. The permanent set (set after infinite relaxation time) has been extrapolated by a mathematical model to be about 3%. Electrical stability of the material properties have been checked under very critical conditions.

for up to 200 kV (start 125 kV with increments of 5 kV, 10 shots +/- per step).

Test description Standard Result

Partial discharge at 24 kV

CENELEC HD 629 <5 pC

Impulse at 125 kV 10 positive shots 10 negative shots

CENELEC HD 629 Passed

Load cycles at 30 kV 10 cycles 4hrs heating/ 4 hrs cooling 120°C in the core

C 33-001 C33-050/A1

Passed

Load cycles at 30 kV 123 cycles 4 hrs heating/ 4 hrs cooling 100°C in the core

CENELECHD 629 Passed

DC test 72 kV 1 hour CENELEC HD 629 Passed

AC step test 4 hrs step from 30 up to 50 kV

C 33-001 C33-050/A1

Passed

Sealing test C 33-001 C33-050/A1

Passed

Short circuit in screen 2 overloading 3 kA, 1s

IEC 20-24 Passed

Table 2: Short duration tests

Long duration tests were performed on joints under various conditions. They have been installed on French and German 24 kV class XLPE cables with a cross section of 150mm_. The different loops have been placed in various thermal dissipation conditions, and submitted to an AC voltage of 36 kV:

-in air (8 hours heating to 120°C - 16 hours cooling),

-in water (4 hours heating to 100°C - 4 hours cooling),

-buried (8 hours heating to 120°C - 16 hours cooling).

These tests have accumulated more than 3000 hours at the beginning of 1996, without accidents.

In addition, several samples have been immersed in water at 95°C and are submitted to a DC voltage at 70 kV. No incidents were noticed after 2500 hours.

As an example, stability of dielectric properties of insulation material after immersion in water at 95°C is shown on Fig. 5. After 1000 hours of trial, no degradation in electrical properties was noticed, which exhibits the hydrophobic behaviour of the material even at high temperature.

B. Product qualificationThe product has been exhaustively tested on each cable section in the range to be covered. Evaluations have been carried out successfully in line with IEC60502-4 and according to main European standards: C 33-001 and C 33-050 (France), VDE 0278 (Germany), IEC 20-24 (Italy) including also some tests required by CENELEC HD 629 and internal test specifications. Main results obtained during short duration tests are presented in table 2. At the end of the complete test sequence, the AC breakdown voltage achieved was at or above 100 kV (start 55 kV, 1 hour steps with increment of 5 kV). In the same way, no failures occurred during impulse test at room temperature

IV. Application to 3-core systems

A. Designs for straight joints for XLPE cablesOnce the cold-shrinkable body had proven its functionality for a joint for single core cables, the extension of the use for 3-core cables was investigated. Obviously, the cold-


Fig. 5: Test in water at 95°C.

57

shrinkable protection and the metal stocking should be modified for this application. Therefore the supporting tubes could be shorter and made asymmetrical. However, the space to pull out the supporting tubes needs to be foreseen in the design.

Once the three joint bodies are installed, the continuation of the metal screens need to be made. Note that with the cold shrink technique, the thickness of the insulation is always guaranteed, independent of the accessibility of the joint bodies. When using armoured cables, a galvanic connection must be made between the armoured wires or tapes and the cable screen. Different systems exist on the market, depending on the requested fault current carrying capacity of the screens.

A key issue in 3-core jointing is to make the joint watertight. Therefore sufficient moisture barrier mastic is foreseen at the crutch of the cables and around the metallic connector. These mastics ensure that no water can enter the joint of a damaged cable and oxidise the contact, increasing its resistance.

accredited labs according to the CENELEC HD 629 (and IEC 60504 standards) for 12 and 24 kV networks.

Heat cycles in air and in water are included in these qualification tests in order to prove the suitability of the water sealing of the design.

Test reports are available for:

-12 kV straight joints between 3-core XLPE cables in a resin box (/PBR).

-24 kV straight joint between 3-core XLPE cables with tape injected resin (/TIR).

There are different techniques to restore the mechanical outer protection of a 3-core joint. The first is the tape injected resin method (/TIR). In this method several layers of porous tape are applied over the 3 phases of the joint. Above this tape, a reinforcing tape is applied. Some valves are folded between these last tapes, through which resin can be injected. After injection, the resin polymerises and becomes hard. The advantage of this technique is that no mould is needed. The amount of resin used is reduced and the joint is quite slim.

A second method is to use a protection box around the 3 phases in which the resin can be poured (/PBR). A third technique is to use a heat shrink protection cover with moisture protection glue integrated in the tubes. This system is useful in not too humid environments.

All these outer protection systems have to pass two design criteria that are contradictory. On one side, the cover must be strong enough to provide a mechanical protection; on the other side it needs to be flexible to allow the cable to expand and shrink during heat cycles. All types of joints have been tested during the qualification tests in

B. Application for transition jointsWith transition joints, two additional components are needed to seal the cables and to prevent oil leakages. To cover the crutch, a breakout (a glove with three fingers) is needed. For 12 kV belted cable, stress control mastic without air pockets needs to be applied under this breakout to insulate the cable phases from each other. 24 kV paper cables are shielded, with individual metallic perforated papers and a common lead sheet (Höchstädter cable), or with individual lead sheets. Here, no mastic is needed under the breakout. The second element is an oil barrier sleeve, to be applied over every core to protect, which goes over the metallic conductor contact to prevent oil leakages.

These two additional components can also cold-shrinkable components, delivered on support tubes.

On these joints, qualification tests were also performed in accredited labs. Test reports are available for:

-12 kV straight joints between 3-core 12 kV belted paper and a 3-core XLPE or 3 single core XLPE cables.

-24 kV transition joints between 3-core paper cable and 3 single core XLPE cables.

Also hybrid types of transition joints with heat shrink protection covers and oil barrier sleeves are developed. Due to the excellent electrical properties, described in part III, the joint bodies are always of the cold-shrinkable


58

type. For the French market, a complete cold-shrinkable transition joint is released since 2004 [3].

C. ConclusionA complete pre-fabricated cold-shrinkable joint has been developed, that can cover a wide range of medium voltage cable sections. Many possibilities given by this new technology were investigated in order to built a very reliable product from production up to installation. The concept was developed based on a very original support tube, allowing integrating all functions required to ensure the continuity of the cable and an easy installation in one operation. With the new design concept, stable production process and simplified installation, it is considered that this new generation of joints will improve service reliability.

Based on this concept for jointing single core cables, products were developed for 3-core cables for the


international market. The existing designs for protection covers were used to develop a new generation of 3-core joints. The products were tested according to international standards covering IEC 60502-4

REFERENCES

[1] C. Brackeniers, A. Chéenne, ‘A new family of terminations’, Third international conference on Power cables and Accessories 10 kV-500 kV, IEE, 23-25 November 1993.

[2] A. Chéenne, S. Chatterjee, I. De Schrijver, P. Mirebeau, B. Aladenize, ‘Material development for the new cold shrink technology’, Fourth international conference on insulated power cables, Jicable, 25-29 June 1995.

[3] S. Chatterjee, E. De Ridder, J. Cardinaels, ‘New design for MV acces-sories: Cold-shrinkable transition joints and evolution of separable connectors’, International conference on insulated power cables,

Jicable ‘03.

59

Qualification of Cables for Nuclear Power Plants

G Sajeev, S P Panda, M L Jadhav,S B AgarkarNuclear Power Corporation of India Ltd., Mumbai, India

IntroductionIn each Nuclear Power Plant more than 1500 Km of power and control cables are used for feeding power and control of non-safety and safety related equipment. The equipment are required to function during normal and off normal conditions. The environmental conditions include exposure to radiation, temperature and humidity during normal operation and other Design Basis Event (DBE) like Loss Of Coolant Accident (LOCA) and Main Steam Line Break (MSLB). To qualify the cables and to meet this onerous requirement, specific testing procedure and acceptance criteria are developed and specified. As the cables, including some of which are installed in inaccessible areas have to perform for more than 40 years of service life, the effects of various parameters have to be carefully evaluated to ensure cable integrity and availability. These requirements are specific to Nuclear Power Plant applications.

In addition to regular electrical and mechanical parameter measurements, cables are subjected to accelerated thermal ageing, accelerated radiation ageing and LOCA simulation tests as per various national and international standards. This paper describes a step-by-step method evolved and adopted by NPCIL for cable qualification. Selection of representative samples, establishing Arrhenius curve for insulating material and arriving at thermal ageing parameters, LOCA simulation programme and test facilities available in India etc are covered in brief. The findings of some of the earlier tests performed are also discussed.

The sizes of cables used in Nuclear power plant range from 0.75 mm2 used in Instrumentation & Control circuits to 1000 mm2 used in power circuits. Cables are used in system voltage ranging from 24 V DC to 6.6 kV AC. Power and I & C cables catering to safety functions are expected to be functional during and after LOCA.

Qualification Procedure

Pre Qualification TestsSample selected should be a representative of the particular type of cable with respect to material used, design and manufacturing process followed. The sample length should be sufficient to permit reliable test readings and evaluation consistent with good testing practice. Single conductor cables should be tested with its insulation exposed to the test environment. However medium voltage single conductor cables and multi-conductor cables should be tested as a complete assembly of conductor, insulation, fillers, binding material, sheath etc. Inspection shall ensure that the cable is not damaged and as a pre-qualification check, base line functional tests like high voltage test, insulation resistance measurement should be performed as per relevant standards. Values should meet the requirements as per relevant standards.

Aging TestsNormal operating condition may enhance or degrade the withstanding properties of cable prior to the occurrence of LOCA, so both aged and un-aged samples are subjected to LOCA conditions. Aging tests pertains to the application of temperature and exposure to radiation sequentially or simultaneously (subject to availability

of such facility) in an accelerated manner to match the actual installation condition.

Thermal Aging TestArrhenius method is used to find out the equivalent qualified life and temperature for accelerated thermal aging simulation. Cable insulation samples (dumbbells) are thermally aged in ovens set at different temperatures. Samples are removed at regular interval and acceptance parameters are measured. Acceptance criterion of 65 percentage of the original value is adopted. Graphs


60

are plotted using time duration against percentage degradation for different temperature settings. Typical graph depicting the degradation of tensile strength on thermal aging is shown in figure-1

simulated in a test chamber. One un-aged cable sample and one more sample, which is already subjected to thermal and radiation aging, are subjected to LOCA. A typical LOCA test programme is shown in figure-3

Figure-1

Further an Arrhenius curve is plotted using the data obtained from the above graphs. From this Arrhenius curve, the temperature and the duration are selected for the accelerated thermal aging test. Typical Arrhenius curve is shown in figure-2

Figure-2

Radiation Aging TestThermally aged cable samples are subjected to radiation aging. The dose rate and duration would depend upon the radiation source strength. The samples should be subjected to a cumulative radiation dose of 100 MRads.

LOCA Exposure Test During the postulated accident, LOCA, the temperature, pressure, humidity and radiation levels in Reactor Building would rapidly increase for a short period. These parameters would return to normal values with time. The rate of increase, peak values and rate of decrease of above parameters is specific for a particular design and rating of nuclear reactor.

The environmental conditions prevailing during LOCA is

After completion of LOCA test, the cable samples are straightened and recoiled with an inside diameter of 40D (D is the cable diameter) and condition of cables are to be visually checked.

Acceptance CriterionAfter LOCA test, Insulation Resistance Test and High Voltage Test are carried out. The sample should have IR more than the minimum value specified for un-aged sample as per relevant standard. The sample should also withstand H V test voltage of minimum 50 % of the value applied on un-aged sample.

Test ResultsQualification as per the above procedure was carried out on XLPE insulated 6.6 kV and 415 V power cables. Samples were subjected to Visual inspection, HV test and IR measurement after each stage. Some of the test results along with observations are indicated in Table-1. LOCA simulation is done as per pressure and temperature transients given below:

Hot water impingement at 950C for 15 minutes.

Steam impingement at 2 kg/sq. cm for 15 minutes.

1.4 kg/sq.cm. & 1270C (saturated steam) for 20 minutes.

1.0 kg/sq.cm. & 1230C (saturated steam) for 40 minutes

0.5 kg/sq.cm. & 1110C (saturated steam) for 60 minutes.

Exposure to saturated air at 600C for 1 week.

Test Facilities Some of the test facilities in India are listed below. They differ in size of the samples, which they can accommodate for testing.

For thermal aging test and LOCA test

Bhabha Atomic Research Centre (BARC), Trombay,

Figure-3


61

Mumbai.

Electrical Research and Development Association (ERDA), Vadodra.

For Radiation aging test

Board of Radiation and Isotope technology (BRIT) facilities located at Trombay, Mumbai and Vashi, Navi-Mumbai

Conclusion The cables used in Reactor building of Nuclear power plants, shall have radiation withstand capabilities. Special type tests are to be conducted to assess the performance of cable during normal and accident conditions. Temperature and duration of thermal aging is found out by


TABLE-1

Sl. No. Test Test Conditions and Observations for different Type & Size of Copper Power Cables

6.6 kV XLPE, FRLS Sheath UA 3C X 150 Sq. mm. 415 V, XLPE, EVA Sheath,Armoured, 3C X 240 Sq. mm.

1 Pre-qualification tests

1.1 Visual Inspection OK OK

1.2 Volume resistivity 6.87 x 1015 W cm 4.7 x 1016 W cm

1.3 HV test Withstood 17 kV for 5 minutes Withstood 3 kV for 5 minutes

2 Thermal aging 120 deg. C for a period of 3 months 140 deg. C for a period of 3 months

2.1 Visual Inspection OK OK


2.3 HV test Withstood 8.5 kV for 5 minutes Withstood 3 kV for 5 minutes

3 Radiation aging Cumulative dose of 100 MRad @ 0.2 MRad/hr Cumulative dose of 100 MRad @ 0.2 MRad/hr

3.1 Visual Inspection OK Outer Sheath became brittle


3.3 HV test Withstood 8.5 kV for 5 minutes Withstood 3 kV for 5 minutes

4 LOCA Test

4.1 Visual Inspection 6.6 kV cables are not rquired to be qualified for LOCA test

Outer Sheath became brittle

4.2 Volume resistivity 9.48 x 1015 W cm

4.3 HV test Withstood 3 kV for 5 minutes

plotting Arrhenius curve for insulating material. Test results have proved that cable samples, which have undergone thermal aging, radiation aging and LOCA tests, have met the acceptance criteria.

REFERENCES1. IEC 216 Guide for the determination of thermal endurance proper-

ties of electrical insulating materials.

2. IEEE standard for Qualifying Class IE Equipment for Nuclear Power Generating Stations. IEEE, std 323

3. IEEE standard for Type Test of Class IE Electric Cables, Field Splices and connections for Nuclear Power Generating Stations ANSI/IEEE, std 383

4. Modern power station practice. Vol. D.

5. Test report on power cables qualification.

63

Day 2 Friday, January 18, 2008

Session I & II

Day 2 - Session I & II Technical Papers

65

Flame Retardant Cable Materials Technology Advances

Mr. Arnab Majumdar,Dow Chemical International Pvt. Ltd, Pune, India

Dr. Jeffrey M. Cogen, Dr. Scott H. Wasserman,The Dow Chemical Company, USA

IntroductionIn each critical end use application segment, materials technology continues to advance. The recent trend is definitely is the development of flame retardant compounds. Zero-halogen low smoke (LS0H) FR compounds are being widely accepted due to environmental concerns associated with halogenated compounds. Low-smoke zero-halogen (LS0H) polyolefin compounds typically require high levels of additives, which significantly impact mechanical properties and processability. This paper presents recent developments in LS0H polyolefin technology, including novel nanoclay flame retardants, coupling technology & metal hydroxide structure-property relationships. These developments provide new tools for creation of novel LS0H materials.

BackgroundPlastic articles used in enclosed spaces usually must possess flame retardant (FR) properties to provide resistance to fire. Though halogenated and halogen free flame retardants have both traditionally been used, there is a steady global trend toward the selection of halogen free flame retardants, due to lower smoke, toxicity, and corrosivity, as well as finished article end of life and recycling considerations. and processability.

The most widely used FR technology for polyolefins is metal hydroxides, particularly aluminum trihydroxide (ATH) and magnesium dihydroxide (MDH), which function in part by endothermically releasing water vapor during combustion. Most high volume applications require 50 - 70% percent by weight of metal hydroxide to achieve the necessary level of flame retardancy. This high loading of inorganic additive causes a significant decrease in mechanical properties and a dramatic increase in viscosity.

Current research is focused on understanding and controlling the chemistry in LS0H systems to minimize

the negative impact caused by these high levels of metal hydroxide. Identification of additives that work synergistically with metal hydroxides enables a reduced level of inorganic additives required to achieve a given level of flame retardancy, minimizing the negative impact of high FR loadings. Application of surface treatments, coupling agents, and compatibilizers minimizes the negative impact imparted by a given level of FR agents. Furthermore, understanding of structure-property relationships among metal hydroxides synthesized by various processes enables selection of the best candidates for new compositions. In addition, assessment of rheology of LS0H compounds requires special attention if these materials are to be fabricated into useful articles in an economic manner. And small scale FR testing, a topic of active research, is necessary to avoid the need for frequent iterative fabrication and FR testing during the material development process.

These developments represent new tools available for creating novel materials for LS0H polyolefin applications, including layers and components for wires, cables, and other electronic articles [1].

Experimental DetailsThe following instruments were used for the study.

1.Compounds : Brabender(r) mixed and processed two-roll milled. Tests were done on compression molded plaques.

2.. LOI : Redcroft LOI instrument.

3. UL-94 vertical flame testing was carried out on 5” x 0.5” x 0.125” chamber.

4.. Cone calorimetry was run on a Fire Testing Technology cone calorimeter per the ASTM E1534 standard using 4.0 x 4.0” plaques.

5. Tensile strength and elongation were ASTM D 638 using Type IV .


66

6. Capillary rheology studies : Rosand capillary extrusion rheometer.

7. Polymer-filler coupling study was carried out in Instron capillary rheometer at 230°C.

Results and Discussion

New Co-Additives to Improve Efficiency of Metal HydroxidesAs discussed, metal hydroxides although meeting the required performance properties, impart negative impact on mechanical and rheological properties. Figure 1 depicts how Nanocomposites have attracted recent attention in this regard, and LS0H systems using organically modified natural Montmorillonite in combination with metal hydroxides have been studied extensively [2,3]. improved performance can be utilized to lower the total

FR loading at a constant level of FR performance (see Figure 1 above). Further improvements in efficiency can be envisioned by identifying lower cost additives to replace all or some of the metal hydroxide. It was hypothesized that the tortuous path provided by a nanocomposite might facilitate new chemistries to achieve such a goal.

Table 1 shows interesting results obtained in nanocomposites similar to those discussed above, wherein half of the metal hydroxide was replaced with calcium carbonate. Interestingly, the data suggest that there is a significant synergy between metal hydroxide, calcium carbonate, and nanoclay. In this case, using the UL-94 vertical burn test, the highest rating could be achieved using the three component synergy, even though neither of the two component systems containing calcium carbonate were able to achieve a rating in this test. It is proposed that this synergy results from reaction of acetic acid, formed from decomposition of EVA during combustion, with calcium carbonate, which is more efficient in the presence of nanoclay, which slows the escape of acetic acid from the composition due to a tortuous path.

Table 1. Ternary blend nanocomposites

Component A B C D E F G

Magnesium hydroxide

65 32.5 31

Aluminum trihydroxide

65 32.5 31

Calcium carbonate 65 32.5 32.5 31 31

Organo-montmorillonite

3 3

EVA/Additive blend 35 35 35 35 35 35 35

UL-94 Vertical burn rating (3.2mm)

V0 V0 No rating

No rating

No rating

V0 V0

In addition to identification of co-additives to enable more efficient utilization of metal hydroxides, selection

Fig. 1: Improved balance of properties obtained with nanocomposites

A novel synthetic nanoclay has been prepared which is a layered sodium silicate. Partial exchange of the sodium with quaternary ammonium affords synthetic clay that can be intimately mixed with polymers such as ethylene-vinyl acetate (EVA), resulting in a nanocomposite having an intercalated structure, i.e. a structure in which the EVA has inserted between individual clay layers, causing a detectable increase in average spacing between clay layers.

Cone calorimetry data of the nanocomposites and control sample are shown in Figure 2. The nanocomposites achieve a 40-50% reduction in peak heat release rate. Additionally, the valley between the two peaks in the cone calorimetry curves for the nanocomposites is indicative of significant char formation. The results with the synthetic clay are comparable to those obtained with the organo-Montmorillonite. However, the synthetic clay offers potential advantages over natural Montmorillonite, such as purity, color, and reproducibility of composition.

Although the present work focused on demonstrating better FR performance at total additive level of 60%, the


Figure 2. Heat release rate data from cone calorimeter (heat flux = 35 kW/m2, 1.9 mm, no grid)

67

of the proper grade of metal hydroxide can itself have a major impact on the delicate balance between cost and performance. Therefore, a study of structure-property relationships of various metal hydroxides was undertaken.

Structure Property Relationships in The Selec-tion of an Ideal Metal HydroxideMagnesium hydroxide (Mg(OH)

2) and aluminum

trihydroxide (Al(OH)3) are the most commonly used metal

hydroxides for LS0H applications. The performance properties are affected by the particle characteristics (such as size, shape, size distribution, and surface area) which are influenced by the raw material source and manufacturing process [3-6].

To assess the differences between metal hydroxides made using various processes, five grades were evaluated (Table 2). Crosslinkable compounds were prepared using the metal hydroxides in ethylene-vinyl acetate copolymer (EVA) with 2,2’-bis(t-butylperoxy) diisopropylbenzene as the crosslinking agent.

Table 2: Average Particle Size and Surface Area for Metal Hydroxides

Metal Hydroxide Grades

Mg(OH)2 A

Mg(OH)2 B

Mg(OH)2 C

Mg(OH)2 D

ATH

Average particle size (micron)

1.4 1.4 1.2 1.5 1.8

Surface area (m2/g)

5 9 15 13 4

tensile strength and elongation even though MgH-C and MgH-D have similar average particle sizes and surface areas (Figure 3). The sample containing MgH-A has the lowest tensile strength even though the average particle size of MgH-A is similar to the average particle sizes of the other magnesium hydroxides. The observed differences in tensile properties most likely are due to differences in the particle morphology and particle size distribution which can ultimately affect the polymer-particle interface properties.

Figure 4 compares LOI and peak heat release rate (PHRR) for samples containing different grades of magnesium hydroxides. The sample containing MgH-B has the highest LOI while the sample containing MgH-D has the lowest LOI. On the other hand, the sample containing MgH-A has the highest PHRR while all other samples show similar PHRR. There is no correlation between LOI and PHRR for the above magnesium hydroxide grades.

The samples containing MgH-B and MgH-C have similar

tensile strength and elongation although MgH-B and

MgH-C have different particle sizes and surface areas.

The sample containing MgH-D exhibits an intermediate

Figure 4. Comparison of LOI and Peak Heat Release Rate for Different Grades of Magnesium Hydroxides (1.9 mm, with grid).

The above differences in mechanical properties and flame resistance are most likely influenced by different characteristics of magnesium hydroxide particles, and highlight the importance of consideration of structure property relationships in selection of the flame retardant.

Surface Treatment and Coupling to Improve Performance of Metal Hydroxide-containing CompoundsTo improve performance properties in a polymer composite, the polymer-filler interface properties can be modified to improve compatibility between the polymer and filler particles [7,8]. The most commonly used approaches for surface modification or coupling of metal hydroxides in polymer composites include fatty acids, alkoxysilanes, and maleic anhydride-grafted polymers.


Figure 3. Tensile Properties for Samples Containing Four Grades of Magnesium Hydroxides.

68

Fatty acids or salts of fatty acids are commonly used for treating metal hydroxides [9-10]. Silane treatment of magnesium hydroxide in an EVA composite has been found to increase tensile strength and elongation relative to an untreated composite [11]. Maleic anhydride-grafted polyethylenes or polypropylenes are also used to help improve adhesion between polyolefins and fillers [12-15].

Considerations in Assessing Rheology of Com-pounds Containing Metal HydroxidesIncorporation of metal hydroxides and other functional fillers results in changes to the rheological characteristics of the polymer due to breadth of morphologies and different surface energies.

The first application of rheology to LS0H product development is the identification of optimal processing flame retardant fillers for wire and cable applications where loading levels of about 60%, by weight, are required to achieve acceptable flame retardance. Relative viscosity measurements at multiple loadings were measured and the data modeled to obtain the maximum filler packing fraction. Fillers with a higher maximum packing fraction have been identified that provide the basis of compounds with high levels of flame retardance and excellent processability.

The second application of rheology to wire and cable product development is the optimization of coupling agent resin content. Rheological data was obtained for filled polymer melts at various loading levels with and without coupling agent. These studies help define the optimal amount of coupling agent resin that minimizes changes to the viscosity profile.

Polymer-Filler CouplingCoupling agents, such as maleic anhydride-grafted polyolefins discussed above, can have a significant effect on the rheological properties of the filled polymer system. In filled systems, significant relative viscosity increases are observed as the filler content approaches the maximum particle packing fraction. In systems where the polymer is covalently bonded to the filler, additional relative viscosity increases are observed as the polymer shells around the filler particles interact with one another forming filler agglomerates bridged by polymer [16].

Considerations for Laboratory Assessment of Flame Retardancy of Systems Containing Metal HydroxidesAssessment of flame retardancy in LS0H compounds has been done using a variety of methods. In end-use applications, such as Wire and Cable, the ultimate test relies on performance in full scale burning of actual fabricated article. LOI remains one of the most popular flammability test methods. Despite its ease of use and

continued popularity, correlation of LOI with real life performance is limited, and application to predict burn behavior in actual fires is uncertain [17].

Cone calorimetry offers another potential approach to raw material testing, and can be run on raw materials as well as coated wires and finished cables. A cone calorimeter experiment offers a wealth of data, and thorough data analysis takes longer than the experiment itself. While cone calorimetry is steadily gaining in popularity, its use outside of research remains limited due to a number of factors, including the high cost and complexity of cone calorimeters and the resources required to operate and maintain them. Additionally, there is no widespread agreement in the industry about which parameters are most meaningful.

SummaryDevelopment of low-smoke zero-halogen polyolefin compounds having a good balance of properties requires a range of technologies. Co-additives to work in conjunction with metal hydroxides to reduce FR loadings, coupling agents and surface modifiers, and metal hydroxides with a variety of morphologies are important tools to fine tune compound performance. Careful consideration of rheological properties and FR testing are critical to success in LS0H product development.

AcknowledgementsThe authors would like to thank the following people: Rich Prow for his rheology measurements; Don McDaniel, Ron Patrick, Patrick Smith, Scott Gulliford, Fred McHenry for compounding and property testing; Joseph Harris for TEM analysis; Alex Morgan and Juan Garcés for technical contributions on the nanocomposite work; and Kurt Bolz and Manuel Alsina for their technical contributions.

REFERENCES

[1] Portions of the work discussed in this paper, and additional details, can be found in the following publications: (a) J.M. Cogen, T.S. Lin, A.B. Morgan, and J.M. Garcés, Novel Synthetic Nanocomposite Materials and Their Application in Polyolefin-Based Wire and Cable Compounds, Proceedings of the 52nd IWCS/Focus, 638 (2003); (b) J.M. Cogen, P.D. Whaley, T.S. Lin, K. Bolz, Assessment of Flame Re-tardancy in Polyolefin-Based Non-Halogen FR Compounds, Proceed-ings of the 53rd IWCS/Focus, 185 (2004); (c) T.S. Lin, S.P. Bunker, P.D. Whaley, J.M. Cogen, K.A. Bolz, M.F. Alsina, Evaluation of Metal Hydroxides and Coupling Agents for Flame Resistant Industrial Cable Applications, Proceedings of the 54th IWCS/Focus, 229 (2005); P.D. Whaley and S.J. Han, Rheology of Highly Filled Compounds for Wire and Cable Applications, Proceedings of the 54th IWCS/Focus, 285 (2005); (d) J.M. Cogen and T.S. Lin, Fire Retardant Composition, patent application WO2004/074361; (e) J.M. Cogen, T.S. Lin, and A.B. Morgan, Flame Retardant Composition, patent application WO2004/111118.

[2] M. Alexandre and P. Dubois, Polymer-Layered Silicate Nanocompos-ites: Preparation, Properties, and Uses of a New Class of Materials, Materials Science and Engineering: R: Reports, 28, 1 (2002).


69

[3] G. Beyer, Nanocomposites as a New Class of Flame Retardant, Proceedings of the 51st International Wire and Cable Symposium, 584 (2002).

[4] M. Alexandre, G. Beyer, C. Henrist, R. Cloots, A. Rulmont, R. Jerome, and P. Dubois, Preparation and Properties of Layered Silicate Nano-composite Based on Ethylene Vinyl Acetate Copolymer, Macromol. Rapid Communication, 22(8), 643 (2001).

[5] J.W. Gilman, T. Kashiwagi, A.B. Morgan, R.H. Harris, L.D. Brassell, M. VanLandingham, and C.L. Jackson, Flammability of Polymer Clay Nanocomposites Consortium: Year One Annual Report, National Institute of Standards and Technology Internal Report (NISTIR) 6531, July 2000.

[6] J.M. Garcés, D.J. Moll, J. Bicerano, R. Fibiger, and D.G. McLeod, Polymeric Nanocomposites for Automotive Applications, Advanced Materials 12(23), 1835 (2000).

[7] R.W. Grimshaw, The Chemistry and Physics of Clays and Other Ceramic Material, 4th ed., Benn, London, (1980).

[8] K. Kamena, An Emerging Family of Nanomer Nano-clays for The Plas-tic Industry, SPE 5th Annual Recycling Conference, 281 (1998).

[9] J.T. Kloprogge, S. Komarneni, and J.E. Amonette, Synthesis of Smectite Minerals: a Critical Review, Clays & Clay Minerals, 47(5), 529 (1999).

[10] R.J. Vogels, M.J. Kerckhoffs, and J.W. Geus, Non-Hydrothermal Synthesis, “Characterization and Catalytic Properties of Saponite


Clays, Studies in Surface Science and Catalysis (Preparation of Catalysts VI), 91, 1153 (1995).

[11] Z. Wang and T.J. Pinnavaia, New Directions in Polymer-Clay Nano-composite Formation, Polymeric Materials: Science and Engineering, 82, 274 (2000).

[12] J.W. Gilman, C.L. Jackson, A.B. Morgan, R.H. Harris, E. Manias, E.P. Giannelis, M. Wuthenow, D. Hilton, and S. Phillips, Flammability Properties of Polymer-Layered-Silicate Nanocomposites, Polypropyl-ene and Polystyrene Nanocomposites, Chem. Mater. 12(7), 1866 (2000).

[13] J.W. Gilman, Flammability and Thermal Stability Studies of Poly-mer Layered-Silicate (Clay) Nanocomposites, App. Clay Sci. 15, 31 (1999).

[14] A.B. Morgan and J.W. Gilman, Characterization of Polymer-Layered Silicate (Clay) Nanocomposites by Transmission Electron Microscopy and X-ray Diffraction: A Comparative Study, J. App. Polym. Sci. 87(8), 1329 (2003).

[15] A. Riva, M. Zanetti, M. Braglia, G. Camino, and L. Falqui, Thermal Degradation and Rheological Behaviour of EVA/Montmorillonite Nanocomposites, Poly. Deg. and Stab. 77, 299 (2002).

[16] Larson, R. G., Chapter 6 in The Structure and Rheology of Complex Fluids, Oxford University Press, New York, (1999).

[17] A. Tewarson, Flammability, Chapter 42 in Physical Properties of Polymers Handbook, ed. J.E. Mark, AIP Press, NY (1996).

70

Novel Technology for Insulating MV and HV XLPE Cables

Pekka Huotari, Dr. Tech.Maillefer Extrusion Oy, Finland

IntroductionMedium voltage (MV), high voltage (HV) and extra high voltage (EHV) underground cables today are generally plastic-insulated and cross-linked. Cross-linking is performed together with insulation on catenary CV-lines (CCV), vertical CV-lines (VCV) or, rarely, on so-called Mitsubishi-Dainitchi-lines (MDCV). Cross-linking offers higher current and thermal loads without softening the layers of insulation. Most new CV-lines are radiant curing lines comprising a nitrogen atmosphere inside a water- or gas-cooled curing tube. The radiant curing process is already 30 years old and many technical developments has been made to improve both the efficiency of the CV-line and product quality.

Qualitatively speaking, the most important achievements have been the triple extrusion cross-head and x-ray measurement for layer thickness and centricity. Both also improve efficiency since the triple cross-head makes it possible to use effective inductive conductor pre-heating while x-ray measurement reduces start scrap and allows smaller layer dimensional tolerances. In some cases, gravimetric control is used to maintain layer thicknesses within tolerance.

Purity plays an important role in HV and EHV cable insulation. Cleanliness is typically ensured by careful material handling prior to extrusion, clean room conditions and extruder screens. Granule purity can also be scanned prior to extrusion and insulation purity form melt prior to cross-head.

Heavy wall cables used to be insulated on either VCV- or MDCV-lines, but for the last decade this has also been possible on CCV-lines. For EHV cable insulation, VCV-lines are still often preferred and the line speed of VCV-lines has been increased by using inductive conductor heating, known as postheating, after the cross-head.

CV-line automation typically consists of a distributed system with PLCs, operator panels and process interface connected together via a field bus. The Curing Calculation program, which numerically optimizes the cross-linking process, still plays a vital role since it is not possible to measure cross-linking or core temperature inside CV-tubes.

Increasing Vcv-line Productivity VCV-lines (fig 1) are used for insulating and cross-linking HV and EHV cores. Since CV-tower construction costs are

remarkably high, the height of the tower and, consequently, line speed are limited. As extruders can normally satisfy VCV-line output needs, the problem is mainly limited to curing and cooling capacity.

To fully exploit the layout possibilities, pressurized turn pulleys are normally used on VCV-lines to extend cooling length. To improve cross-linking, inductive conductor heating can be effectively implemented by using what is known

as a postheater. Typical preheating temperatures on VCV-lines today range from 60 to 1000C, limited, for instance, due to copper oxidation, conductor tape deformation or cross-head overheating. The post-heater (fig 2) is located

Figure 1 Vertical CV-line (Maillefer)


71

after the cross-head in the pressurized tube where the conductor can be heated without these limitations. P o s t h e a t i n g temperatures of up to 180-200 (C can be used. This generates asignificant increase in line speed, between 20-45 %, depending on the core and the CV-line layout. Part of the heating length which would be used

without postheating for core heating can now be used for cooling. In CCV-lines, postheating cannot be used for HV cables due to insulation drooping.

Producing Hv & Ehv Cores On Ccv-linesRoundness and concentricity of the insulated core are

Cv-process ControlThere can be significant differences in the output of two identical CV-lines. To effectively use a CV-line, one should

Figure 2 Postheater (Maillefer) in VCV-line

Figure 4 EHT (Maillefer) on CCV-line

Figure 3 CCV-line (Maillefer)

essential quality parameters for MV, HV and EHV power cables with XLPE insulation. Specifications for roundness and concentricity have generally become stricter. With round and concentric layers, significant amounts of insulation and semiconductive material are saved and subsequent processing phases, including jointing and terminating during installation, are made easier. Insulation drooping has limited the production of HV and EHV cores in inert gas catenary CV-lines (fig. 3), since nitrogen does not provide any buoyancy. Generally speaking, VCV-lines give better roundness compared with inert gas CCV-lines, though processing costs are high. Drooping on an inert gas CCV-line can be reduced by using rotating caterpillars before the cross-head and after the end seal or by using what is known as the Entry Heat Treatment method (EHT, see fig. 4) explained here. In most cases, the conductors used for HV and EHV cables are taped and with large conductor sizes of Milliken type and thus sensitive and thus sensitive to torsional forces generated by rotating caterpillars.

Round and concentric HV and EHV cores can also be produced with moderate rotation. In recent years, a significant number of CCV-lines have been fitted with the Entry Heat Treatment (EHT) system and now produce heavy wall cores with excellent results. EHT is a nitrogen cooling section immediately after the cross-head and before the curing section. Cold nitrogen shrinks the core surface and forces core into a round shape. Cross-head off-centering can be used to eliminate eccentricity.


72

be familiar with the performance and limits of the CV-process. Even today, core temperature and cross-linking cannot be measured on-line. Numerical simulation and process optimization, based on thorough process know-how and seamlessly connected to CV-line automation, are of vital importance.

Simulation opens a window onto the CV-process, as it indicates both core temperature and cross-linking, as shown in fig. 5.

Curing Calculation consists a simulation and an optimization section. The optimization section uses simulation to iteratively maximize line speed within given process constraints. Curing Calculation is typically used as a recipe generator for CV-line automation systems.

Today’s CV-line automation system comprises PLCs (programmable logic controllers) with distributed process interfaces, a PC-based Process Supervision Unit as the operator interface and Curing Calculation as the recipe generator. The modern CV-line may include several independent PLCs for separate units or functions, such as line equipment and tube heating, connected with Profibus, Interbus and the like.

The system is installed immediately after the cross-head and it measures the diameter of the conductor and the thicknesses of all the layers in the two directions parallel to the cross-head centering directions.

Gravimetric control can also generate significant material savings. The system is installed on the extruder hopper and gravimetrically measures granular flow and controls screw speed. The X- ray system is of vital importance for centering and start-up, but for controlling longitudinal variations, gravimetric control can also be considered, especially as regards insulation flow.

Figure 5 Curing Calculation (Maillefer NCC) view of the CV-process

Savings on Raw MaterialsSavings on materials include both reduced scrap during start-up and minimized layer thicknesses during stable production. Great attention is paid to both due to the reduced margins in the cable business. During CV-line start-up, it is important to achieve acceptable process conditions and core quality as fast as possible. For core dimensions this means using an X-ray centering system. In the production phase, it is possible to use either an X-ray system or a gravimetric method, or both. The X-ray system (see fig. 6) is included in more or less all new CV-lines.

Hv & Ehv Core Insulation PurityThe dielectric strength of XLPE insulated cables mainly depends on the smoothness of the insulation-

Figure 6 X-ray system (Sikora X-ray) for core dimensional control

Figure 7 Cleanliness scanning system (CSS by Furukawa/Sikora/Maillefer)


73

semiconductive layer interface, as well as on insulation purity and integrity. These depend on the cleanliness, handling and extrusion of the materials. The cleanliness of extruded insulation material can be controlled in-line by means of an optical cleanliness scanning system (CSS, see fig. 7). This is installed between the main extruder and the cross-head and inspects 100% of the insulation material. The polymer flows between two glass inspection windows. Since molten LDPE is transparent, any foreign bodies can be seen and recorded. For large contaminants, size and shape is reported, while for small ones, only the number of particles per category is indicated.

Granule purity can also be checked just before entering the extruder. This optical system removes contaminated pellets.


ConclusionBasic CV-line solutions have become stable. Single-screw extruders, triple cross-heads, radiant heating and water or nitrogen cooling are now used.

Several improvements have taken place. HV and EHV cores can be insulated and cross-linked on catenary CV-lines. Extended conductor heating is used to improve CV-line efficiency on vertical lines. Curing Calculation and automation have been further developed in order to fully exploit CV-line capacity.

Instruments, such as X-ray and gravimetric systems, are used to save on materials and reduce scrap and on-line insulation cleanliness scanning is also available.

74

Dielectric Response Measurements to Assess the Condition of MV XLPE Power Cables

P.K. Poovamma, K. Mallikarjunappa, A. Sudhindra, B.S. Manjunath, Thirumurthy Central Power Research Institute, Bangalore, India

Abstract The paper presents the findings of dielectric response measurements in frequency domain carried out on in-service MV Power cables, as well as the laboratory measurements carried out on new and aged representative Power cable samples. The analysis of the data has helped to catergorise and bench mark the dielectric responses due to insulation degradation; presence of water trees in the cable insulation; influence of accessories and surface leakage currents. Based on the analysis of the data it was possible to evaluate the extent of degradation of the insulation system and also identify the type of degradation, influence of accessories and rank the cables based on their extent of deterioration. The method adopted has helped to categorize the cables based on the extent of degradation.

IntroductionThe distribution cable network in Indian scenario is an ageing one. It’s performance has been a cause of concern demanding enormous and expensive efforts to maintain continuity of power supply to consumers. A solution that progressively and expediously improves the performance of the network is the need of the hour. An appropriate suggestion, in addition to reliability will also reduce the corrective maintenance cost.

Condition monitoring of in - service Power Cables has been a continuous process and has seen many improvements over the years. Although several diagnostic tests are available, interpretation of the data still appears to be a challenging task. Each diagnostic test becomes significant only when they are sensitive to the changes in the dielectric properties of the system. The correlation and proper analysis of the monitored data from each test is of prime importance to evaluate appropriately the status of the insulation.

As on today, there are no well laid diagnostic methodologies available for reliable assessment of healthiness of Power Cables in-service. CPRI is in the process of evolving appropriate methodology for condition assessment of Power Cables by adopting different testing techniques.

One such measurement method which is called Frequency Domain spectroscopy, involves application of AC voltage at variable frequency and monitoring of various dielectric parameters. Interpretation of the changes in the dielectric response patterns in frequency domain is a complex task.

Common type of failure of Power cables are mainly due to degradation of the insulation, defective terminations or joints. Hence it is essential to understand the spectral behaviour of dielectric parameters in frequency domain due to the influence of accessories and degradation of insulation.

Dielectric response measurements were carried out on in-service MV Power cables in the field and also on new and service aged representative Power cable samples in the laboratory.

From the findings of the laboratory investigations, as well as field measurements, it was possible to categorise the dielectric responses due to insulation degradation, influence of accessories and surface leakages, presence of water trees in the insulation of the cable. From the exhaustive analysis it was possible to evaluate the extent of degradation of the insulation system and also identify the type of degradation.

The findings may help to suggest appropriate remedial measures to plan maintenance strategy and also to reduce unscheduled cost.

Monitoring of various dielectric parameters in Frequency domain has been found to be one of the sensitive diagnostic tool to assess the condition of in-service Power cables.


75

MethodologyMeasurements were carried out on 3 core 11kV XLPE power cables using commercially available Dielectric Spectroscopy. Measurements were performed in frequency range varying from 1000 Hz to 0.0001Hz at different voltages. The highest voltage level used was 6 kV, which is equal to the service phase voltage level (Uo), normally used. Measurements were carried out on cable samples at different sweep voltage levels of 1.5kV, 3.0 kV, 4.5 kV, 6.0 kV, 4.5 kV and 3.0 kV.

During analysis, both the real part of the permittivity, (‘ and the imaginary loss component, (“ are used for the assessment of the insulation system. From the measured complex impedance, Dielectric loss, Power Factor, Capacitance & Permittivity are obtained as a function of frequency. The changes in the response patterns of the dielectric properties as a function of frequency are evaluated to assess the condition of the insulation system.

Results & DiscussionsLaboratory Investigations

(i) Responses from a new and aged cable sample

Fig. 1 depicts dielectric loss in frequency domain for a new 11kV XLPE , 3 core AL conductor cable sample, of length 10m with Heat shrinkable termination and with one joint in the length of the cable. The spectra indicate very low values of dielectric loss. It is seen from the figure that the dielectric loss spectra are independent of both voltage and frequency. The analysis also revealed that the permittivity did not depend on the applied voltage and frequency.

Resistance measurements indicated high values of Insulation Resistance. Such spectral behaviour with high values of insulation resistance and low values of dielectric loss indicate healthy condition of the cable insulation as well as accessories.

Fig.2, shows the responses of dielectric loss of a 11kV XLPE cable representative sample of 6m with heat shrinkable termination. The cable length had one joint. The figure also illustrates the loss responses of the same cable sample after 5000 hrs. of electrical ageing at 19kV.

Figure. 1 : Dielectric loss of new 11kV XLPE cable sample at different voltages upto U0

Fig.2.: Response of an un-aged & 5000 hrs of electrically aged XLPE cable sample

It is observed that the dielectric losses increased with ageing and are more pronounced at lower frequency range. The responses also exhibited frequency dependence, in the low frequency range. The relatively higher dielectric losses may be attributed to the ageing of the insulation.

Higher dielectric losses in the lower frequency range may be due the conductivity related losses due to terminations.

(ii) Responses from water treed cable sample

The response from the water treed XLPE cable sample as reported in [3] is presented in Fig.3.

Figure. 3 : Response of an water-treed XLPE cable sample at different voltages


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The figure depicts increase in dielectric loss values with increase in voltage. As seen the loss spectra are highly voltage dependent and are almost independent of frequency.

Such type of dielectric response behaviour with increase in losses with voltages indicate presence of water trees in the cable insulation.

(iii) Responses due to influence of accessories

Cable terminations are mounted at both ends to control the electrical field distribution. Otherwise the electric stress along the surface close to the insulation screen becomes too high. Terminations also protects the cable from different environments.

A cable joint connects two cables /pieces of cable and hence a cable joint is a continuation of the cable itself. The joint therefore should be able to withstand the operational conditions and expected service life of the cable.

During jointing, the conductor is jointed and the conductor joint is covered with a conducting screen, an insulation material and an insulation screen. In some joint designs, a stress-grading material is put along the cable insulation surface making contact with both the conductor and insulation screen.

The materials used in the accessories may influence the dielectric response measured in a cable circuit. When testing cables in the field, especially low-loss PE and XLPE cables, understanding of the influence of the accessories is very important for accurate analysis of the condition of the cable system.

During measurements at site on in-service Cables, the cable circuit has to be disconnected from the network at both ends to avoid contribution of surface leakage current. These leakage currents can easily contribute to the bulk of the measured losses. The influence of the terminations can be minimized by cleaning their surfaces.

During field measurements, basic knowledge of joint and termination designs are essential to evaluate the influence of accessories that may be significant.

Fig.4 depicts the dielectric response of 11kV XLPE 3 core , AL conductor XLPE cable sample of length 10m. The length of the cable had no joint and terminations. Measurements were carried out at voltage levels of 1.5kV, 3.0 kV, 4.5 kV, 6.0 kV, 4.5 kV and 3.0 kV. The spectral behaviour indicates increase in dielectric losses at low frequency range. As seen the dielectric loss responses indicate dominant frequency dependence in low frequency and slightly voltage dependence. As depicted, these dependencies are more profound in lower frequency ranges less than 1.0 Hz

Fig. 5 presents the dielectric loss of an XLPE cable due to termination failure. The sample during electrical ageing at 19kV developed arcing at one of the termination. On evaluation the responses indicate frequency dependence. High dielectric losses and increase in permittivity values at low frequency range were observed. The changes in the responses are more significant at low frequency range.

Similar work carried by Dr. Bolarin Oyegoke et al has been reported in [4]. As reported in the literature, damage in the form of a knife cut was introduced mainly to simulate the effect of poor workmanship on termination. It is reported that the dielectric responses due to the influence

Fig. 4 : The un-aged 11kV XLPE cable sample at different voltages upto U0


Fig. 5 : Dielectric loss response of the failed termination of Electrically aged 11kV XLPE cable sample at different voltages upto U0

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of termination were more sensitive in the lower frequency range ~ less than 1.0Hz.

Field Measurements & Condition Assessment of In-service Power Cables A number of in-service MV Power cables at different networks were monitored using Dielectric spectroscopy to assess their condition. Measurements on numerous MV XLPE Power cable systems have helped to categorize the type of responses and formulate guidelines to identify the cause of degradation; extent of deterioration and also identify the influence of accessories.

Case Study 1

Fig. 6 depicts the dielectric loss response of an 11kV, XLPE, 3 core, Al conductor of length 2050m with 25 joints and in service operating for 13 years. As seen in the figure the dielectric losses are low and varies from 0.1437% at 1.0 Hz to 0.1332% at 0.1 Hz and indicate good condition of the insulation. The resistance measurement indicated high values of In-

sulation Resistance and Polarisation index. These results reveal good condition of the cable insulation.

Though the spectra revealed low losses, the non-linear behaviour of the spectra can be attributed to the influence of surface condition of the termination due to contaminants.

Case Study 3The dielectric loss response of 11kV XLPE Cable of 300 sq.mm, of AL conductor with Heat Shrinkable termination is shown in Fig.8. The cable had 12 no. of joints in 1785m length and is in service operating since ~10 years. The dielectric loss values as seen in the figure are low, of the order of 10-4. These values indicate healthy condition of the cable insulation.

Fig. 6: Variation of tan delta with frequency at different voltages

Dielectric loss response spectra as seen characterise non - dependence of voltage and frequency. The resistance measurements also indicated a high value of Insulation resistance. These results indicate healthy condition of the cable insulation.

Case Study 2 The dielectric loss pattern of a 11kV XLPE 300 sq.mm AL cable, with Heat Shrinkable termination of length 575m is shown in Fig.7. The cable length had 4 no. of joints and is in service since 8 years. As it can been seen dielectric loss responses indicate very low values of the order of 10-

4 at all voltages. The spectral behaviour also exhibit slight Voltage dependence & is independent of Frequency.




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The spectra exhibits dominant frequency dependence. The spectral behaviour also shows non-linearity and marginal voltage dependence, indicating the influence of accessories.

Case Study 4The dielectric loss response of 11kV XLPE Cable of 300 sq.mm, AL conductor with Heat Shrinkable termination & joints of length of 800m is shown in Fig.9. The cable had 10 no. of joints, and is in service operating since ~8 years. The spectral behaviour exhibit both Voltage & Frequency dependence. The dielectric loss values are moderately high indicating 0.155% at 0.1Hz. Further the measurement of Insulation Resistance & Polarisation index values indicated low values.

The above results indicate initiation of water trees in the cable insulation.

Case Study 6 Fig. 11 presents the Dielectric loss pattern of 11kV XLPE cable of 313m long with two joints in the length of the cable, with Heat shrinkable terminations.


Case Study 5The dielectric loss response of 11kV XLPE Cable of 300 sq.mm, of AL conductor with Heat Shrinkable termination is shown in Fig.10. The cable had 3 no. of joints in 611m length and is in service operating since ~10 years. As seen the spectra exhibit dominant frequency dependence and also reveals high tan delta values of 4.6243% at 1.0Hz. The higher values of tan delta indicate considerably degradation of the cable insulation. The frequency dependence characteristics may be attributed to the influence of accessories.

The response shows dominant Frequency and Voltage dependence characteristics. They also indicate high values of dielectric losses. The tan delta at 0.1 Hz is 8.3913% which is much higher value for an XLPE insulation.

As observed the responses are characterized by (i) significant effect at low voltage level, (ii) increase in losses with increase in the test voltage levels, & also (iii) dominant




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frequency dependence. The resistance measurements also indicated low values of Insulation resistance as well as Polarisation Index.

Such type of responses exhibiting dominant voltage and frequency dependence, with high dielectric loss and low resistance values may characterize considerable degradation of the cable insulation due to water trees.

ConclusionMeasurements of various dielectric parameters in frequency domain has found to be a powerful diagnostic tool to assess the condition of the cable and influence of accessories. The spectral analysis can reveal the extent of degradation of the insulation; presence of water trees in cable insulation and influence of accessories.

Such an exhaustive information, immensely guide to suggest appropriate maintenance strategy and to reduce


unscheduled outages and cost.

REFERENCES

1. P. K. Poovamma et al, 2005,”Evaluation of MV Power Cables in sevice by various diagnostic Techniques”, Asia Pacific Conference on MV Power Cable Technology, Malaysia

2. Martin SCHNEIDER et al, 2003, “The Combination of the Diagnostics of Paper-insulated Cables by the Partial Discharge mapping with the determination of the Moisture contents in the Cable insulation” CIRED 17th International Conference on Electricity Distribution

3. Peter Werelius, 1999, “Power Cable Diagnostics by Dielectric Spectroscopy”, IEEE/PES T&D Conference

4. Peter Werelius et al, 2001, “Dielectric Spectroscopy for Diagnostics of Water Tree Deteriorated XLPE Cables” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 8, Issue 1, Page(s):27 - 42

5. Bolarin Oyegoke, et al, 2007 “The Effect of Cable Terminations on dielectric Response Measurements” Int. Conf. on Solid Dielectrics

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An Overview of Engineering Reforms to Design 11 kV Heat Shrinkable Cable Joints

(Past & Present Design Review)

Ravi Agrawal, T. SwaminathanReliance Energy Ltd., Mumbai. India

Executive SummaryWhile analyzing the cable faults, it was found that faults could be attributed to three reasons

a) Age of cable

b) External damage

c) Joint failures.

The first is a natural phenomenon and we need to plan for gradual replacement.

For the second category stronger external vigil is necessary.

In the third category we found that there were joint failures within months of being installed. After critically analyzing the faults the reasons could be attributed to

a) Weak spots in material specifications which needed to be reinforced

b) Mistakes in jointing for which upgrading the skills of the jointers was necessary

c) Usage of improper tools, for which procurement of the proper tools with selection guides, was necessary.

The old specification was analyzed critically. The weak spots were identified and adequately reinforced to have the correct specification of the material involved, in line with the best practices.

The kit component was standardized in terms of dimension and material, so that it becomes vendor independent. A jointer should be able to carry out the jointing work without taking into consideration the ‘make’ of the jointing accessory.

Extensive training program was organized by sending the jointers to the vendor’s supervisor.

Finally a jointing school was set up for imparting regular training to jointers.

Good quality crimping tools have been procured with the correct set of dies, to ensure excellent crimping. A die selection guide has been prepared, laminated and made available with each crimping tool box.

With the above interventions, the joints were tracked for failure in the last one & half year.

It is heartening to note that no joints with the new design have failed till now.

IntroductionIn the wake of an alarming rate of failures of 11 KV cable accessories, significant Engineering reforms were introduced for cable accessories in early 2006. These are summarized below:

i) Adoption of internationally proven designs for Straight and Transition Joints.

ii) Vendor-independent cable stripping dimensions; common to all vendors for specific type and size of accessory.

iii) Introduction of world-class crimping tools.

iv) Standardized connectors (for conductor continuity) common to all vendors and to a group of related sizes of accessories.

v) Periodic training and refreshers for cable jointers and supervisors in the jointing school

vi) Regular field (On-the-job) training to the cable jointers by vendor experts.

Past designs, their shortcomings, areas of weaknesses and improvements implemented by drastic reforms are elaborated below.

Additionally, analysis of failed cable accessories brought to light several shortcomings in design of accessories.


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Old Jointsa) Joint Design

The old design of 11KV, Straight (PILC-PILC) and Transition (PILC-XLPE) joints (failure rate of which have been the highest) required a manual filling of air space inside the joint. It is done by using several short strips (at least 50 no) of a synthetic rubber based, black insulation supplied in a mastic form. This procedure was laborious, time consuming and person-dependent. Failure to fill the joint properly led to formation of air voids, resulting in premature failures.

b) Diversity in connector dimensions

Connectors are vital components for conductor continuity. The connectors supplied in the jointing kits by different vendors for a specific type and sizes of kit were different in dimensions. No die selection guide was available to the jointers on selection of proper die for each size of connector. Poorly made crimped conductor connection with improper dies resulted in overheating of the connectors and subsequent burn-out of the insulation and failure. Majority of failures analyzed were indicating this point.

c) Quality or crimping tool

A sound conductor bond with the connector depends upon good quality hydraulic crimping tool. Available hydraulic crimping tools suffer from frequent breakdowns due to poor quality of vital components and most of the hydraulic tools were out of order. Jointers resorted to an inferior alternative of mechanical crimping tool for jointing. Mechanical tools require a long leverage for proper crimping. Due to space constraint in the trench proper crimping could not be achieved. This resulted in many failures due to overheating. Further, shape of the crimps provided by these mechanical tools deviated considerably from an ideal hexagonal shape. Excess metal flow on opposite sides caused occurrence of high electrical stresses and eventual insulation puncture at these points.

d) Diversity (Vendor-specific) in cable Stripping dimensions

Cable stripping dimensions for a specific type and size of joint were specific to each vendor. Most jointers have limited educational level and they are required to install joints of different vendors. Diversity in stripping dimensions from vendor to vendor for a specific type and size of joint let to serious errors in cable stripping. This was evident from analyses of several joints in which the cores were over stripped, thus exposing paper insulation to ingress of water leading to failures.

e) Lack of Training

There was no regular training programmes and refresher courses for the jointers and supervisors. Various failure analysis clearly reflected the mistakes made during cable preparation (before making joint / termination), which ultimately led to joint failure.

Due to all the above mentioned problems, pre-mature failure rate of joints was very high.

New Jointsa) Joint Design: Following are the highlights of the new

design, taking care above issues:

i) PILC cables are very sensitive to water ingress, as the paper insulation is highly hygroscopic.

Providing a water-tight covering over each core and sealing-off the crutch makes the cables imperious to water ingress. By making these coverings in the form of semi-conductive tubes, a screen is provided over each core which results in a radial electric field.

ii) The crutch, which is the most sensitive part, is filled with superior mastic and sealed-off by means of a semi-conductive breakout which extends over the lead sheath. Zero potential of the lead sheath is maintained all round the cores.

iii) The PILC cable is now equivalent to a screened XLPE cable.

iv) When jointed either to another PILC cable or to an XLPE cable, the joint does not need filling mastic in the interstices along the length and remains air-filled. The air inside is at zero potential.

v) We have also introduced water seals at critical areas inside the joint.

vi) Insulation over the connector is re-built by layers of insulation tubes of high electric strength.

vii) Occurrences of high electrical stresses are addressed to by stress control tube and stress grading mastic.

An Overview of Engineering Reforms to Design of 11 Kv Heat Shrinkable Cable Joints

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This above design, referred to as ‘Totally Screened Design’ has long been adopted globally by other manufacturers of heat-shrink accessories.

The above design has been standardized for all vendors now.

b) Vendor-independent, stripping dimensions for cable preparation

Cable stripping dimensions which are followed globally were reviewed by us and wherever felt necessary, revised in consultation with experienced vendors. Dimensions are now common to all vendors for a specific type and size of joint. The stripping dimensions for each type and size of joint were standardized. These have liberated the jointers from being prone to making errors when making joints of different vendors. In addition the cables are now procured with freely strippable semiconductor layers. This has totally avoided removal of the screens through mechanical

means, which depended on the jointer’s skills.

c) Introduction of World-class crimping tools

Many failures of joints occurred due to poor quality of crimps provided for conductor continuity, leading to overheating, insulation burn-out and failure. We have now acquired world-class crimping tools designed to provide sound conductor continuity.

d) Providing identical connectors to groups of closely related joint sizes

A unique reform implemented was to form groups of joints related closely in sizes and make barrel diameter of the connector identical for the group. This has minimized the range of dies required.

Standardization of in Line connectors(ferrules) dimensions

Barrel Diameter

Bore Diameter

Length

l

l

l

l

(Above sketch is only for reference)

e) Die selection chart

To facilitate proper selection of dies, a die selection chart has been prepared. (Laminated Photo copies of this chart have been distributed to the jointers and supervisors for ready reference. A copy of the chart is also kept inside the crimping tool box. )

Jointing Training School Periodic training and refresher courses for cable jointers and supervisors are held at our Technical Training Centre. Induction of supervisors in the training and refresher courses enlighten the supervisors in proper jointing procedures and enable them to keep tabs on proper jointing.


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Improvement in Cable Designa) Introduction of Freely Strippable semi- c o n d u c t i n g

screen: It minimizes the chances of damage to XLPE insulation during cable preparation for jointing / termination & thereby enhancing reliability of cable accessories. It also minimizes jointing /termination time and down time of the feeder.

b) Water swellable tape on each core (below the copper

An Overview of Engineering Reforms to Design of 11 Kv Heat Shrinkable Cable Joints

tape screen) of three core cable for longitudinal water protection is added in HT cable construction.

ConclusionThe results of above exercise are quite encouraging. There are no reported failures of accessories of the new design, introduced in the past one & half year.

With the above mentioned engineering reforms, failure of joints has substantially reduced.

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Development of Factory-expanded Cold-shrinkable Joint upto 400kV XLPE Cables

Shozo Kobayashi, Hiroshi Niinobe, Nobuyuki Shinagawa, Masahiro SuetsuguVISCAS Corporation, Tokyo, JAPAN

IntroductionThe ideal application of one-piece premolded joints up to 400kV XLPE cables has been pursued. The quality function deployment of EHV XLPE cable joints was studied and the factory-expanded cold-shrinkable technology was found to provide an ideal solution for such a joint.

The installation process, interfacial property, insulating property, elastic property, and thermal behavior of the joint were studied. And by using silicone rubber, which has superior elasticity and good insulating properties, applying cold-shrinkable technology to premolded joints up to 400kV was succeeded. With cold-shrinkable technology, a premolded rubber unit can be shipped expanded onto the carrier pipe. The carrier pipe is made of a plastic string and can be removed easily by hand. The user therefore requires no tools for assembly at the jointing site and simply has to pull out the carrier pipes. The required insulating properties of the rubber unit can be tested in the factory, and the expansion process of rubber unit is carried out in the clean, controlled conditions of the factory. The installation process can thus maintain the high reliability of the insulating properties of the joint.

In a one-piece premolded joint, the interfacial property is important as a guarantee of quality because the insulating property inside the rubber unit can be tested in the factory. Interfacial pressure is an important parameter of the interfacial insulating property. The interfacial pressure between a one-piece premolded joint and the cable insulation is caused by the expansion of the rubber. Therefore the interfacial insulating property and elastic property of the rubber were studied and silicone rubber was found to give excellent results in terms of both properties, enabling the rubber unit to be expanded up to 300% and to maintain the expanded state for over three years. Because the rubber unit can be expanded up to 300%, the inside diameter of the carrier pipe can be larger

than the cable jacket. The overall size of the joint including a protection box can be minimized, because additional removal of the cable jacket can be avoided. One rubber unit is also applicable to various cable sizes, allowing it to be used to joint cables of different sizes (e.g., 1800 mm2 - 800 mm2).

The insulating properties of the rubber were also studied. AC voltage tests, impulse voltage tests, and long-term tests were carried out, and the silicone rubber was found to possess excellent electrical properties for cable accessories. The electrical stress of the rubber unit has been optimized by field analysis with respect to deformation of the unit.

IEC PQ tests up to the 400 kV class have already completed without encountering any problems.

Cold-shrinkable JointThe cold-shrinkable joint (CSJ), which is shipped from the factory site with its moulded rubber unit expanded, has significant advantages over other premoulded joints [1, 2]. Assembly of the rubber unit as shown in Figure 1 is easier because the only work required is to “pull out” the string of the carrier pipe. Moreover, since the inside diameter of the carrier pipe is larger than outside diameter of the cable jacket even in case of metallic corrugated sheath, it is possible to avoid removing too much of the sheath.

Moreover, dust-proof packing at the factory site facilitates quality control at the jointing site and reduces the risk of contaminants at the interface.

Electrical Design of CSJElectrical property of the silicone rubber

The electrical property of the silicone rubber was investigated by using recessed specimens. An AC voltage test, impulse voltage test, and long-term test were carried out [2]. The results of these tests showed that the silicone


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rubber selected for the CSJ has excellent electrical properties for a cable accessory.

Electrical design of rubber unitThe electrical field in the moulded rubber unit of the CSJ was optimised by means of computer calculation. The stress-relief configuration was thus optimally designed and the size of the moulded rubber unit was minimized. The design points of the CSJ were limited to the four

major points (p1-p4) as shown in Figure 2. An example of electrical field calculation for premoulded rubber unit is shown in Figure 3.

Figure 1 Assembly of Cold-Shrinkable Joint

Figure 2 Electrical Design of Rubber Unit of CSJ

Interfacial DesignoOf CSJInterfacial design is the most important aspect of any cable joint. It is well known that the stability of interfacial dielectric performance in premoulded joints is mainly affected by interfacial pressure. Therefore, the insulation property of the interface, the elastic property of the silicone rubber, and the interfacial pressure characteristics of the CSJ were studied.

Insulation property of the interface The insulation property of the interface between XLPE and silicone rubber was studied using the models shown in Figures 4 to 7.

Interface model testsUsing the models shown in Figures4 and 5, electrical break down tests even at lower interfacial pressures well below 0.02 MPa were performed [2]. The structures of the specimens were designed by field calculation as shown in Figure 5, allowing us to simulate electric stress distribution similar to that of cable joints in these models. The XLPE sheets applied to these models had a smooth surface or scrubbed rough surface to simulate actual cable joints. The tests were also carried out with and without oil, which

Figure 3 Example of Electrical Field Calculation for Premoulded Rub-ber Unit


Figure 4 Sample Setup Modelling Electric Stress Distribution at ®4

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would not be contained in either the XLPE or silicone rubber at the interface, to elucidate the effect on the electrical properties. As a result of these tests, it was found that a satisfactory interfacial property of almost double the required value for 400 kV cable could be obtained even at the lower interfacial pressure and without the use of oil. These results were obtained for both the AC and lightning impulse tests.

66 kV-class CSJ model testsThe dependence of interfacial pressure on the electrical properties was also confirmed using the 66 kV-class CSJ models shown in Figures 6 and 7. This time, several XLPE cables having different outside diameters were applied to simulate the interfacial pressure range from 0.04 MPa to 0.18 MPa.

Figure 5 Sample Setup Modelling Electric Stress Distribution at ®3

Figure 6 66 kV-Class CSJ ®3 Model to Evaluate the Dielectric Strength of ®3

Figure 7 66 kV-Class CSJ ®4 Model to Evaluate the Dielectric Strength of ®4

Figure 11 Impulse Voltage Test Results for 66 kV-Class CSJ ®4 Model

Figure 8 AC Voltage Test Results for 66 kV-Class CSJ ®3 Model

Figure 9 Impulse Voltage Test Results for 66 kV-Class CSJ ®3 Model

Figure 10 AC Voltage Test Results for 66 kV-Class CSJ ®4 Model


The model in Figure 6 enhances the interfacial properties between XLPE cable and the moulded rubber unit. The model in Figure 7 represents the properties at the starting point of the stress relief cone. The results of both AC and lightning impulse tests were shown in Figures 8 to 11. The results showed that even at the lower interfacial pressure of 0.04 MPa, the breakdown stress had a high value equivalent to that at the higher interfacial pressure of 0.18 MPa.

Overall, we reached the conclusion that the lower elasticity of silicone rubber can provide a better interfacial insulation

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property even in the case of lower interfacial pressure, and that a CSJ with a moulded rubber unit consisting of silicone rubber will not require an interfacial pressure of more than 0.04 MPa for application to the EHV XLPE cable.

Elastic property of silicone rubberThe elastic property of silicone rubber was studied to understand the long-term interfacial pressure property of the CSJ using a sheet sample. Figure 12 shows an example of the expansion rate calculation for the CSJ. To realize a factory-expanded cold-shrinkable joint, the rubber must have an excellent elastic property that can maintain the interfacial pressure after several years of expansion over a carrier pipe at a higher expansion rate.

Stress vs. strain propertyTo estimate the interfacial pressure of the CSJ, it is very important to understand the stress vs. strain property of silicone rubber because the CSJ is subjected to a high expansion rate for a long period. Therefore, the stress vs. strain property of silicone rubber was studied carefully. After numerous measurements, the dependency of the stress vs. strain property on the history of expansion was elucidated.

Stress relaxation propertyThe stress relaxation property is indicated by the permanent set and the change in the stress vs. strain property. After measuring many cases of stress relaxation property for long periods, the selected silicone rubber shows an excellent elastic property for a factory-expanded cold-shrinkable joint.

Electrical property of the silicone rubberThe actual interfacial pressure of the CSJ was measured in several cases using cables on which pressure sensors had been installed. In each case, the measured values were almost equal to the estimated values.

The interfacial pressure characteristics of the CSJ immediately after the installation and during a heating cycle test were also studied.

Interfacial pressure immediately after installationThe interfacial pressure immediately after installation was measured. Figure 14 shows the interfacial pressure behaviour of a 110 kV-class CSJ immediately after installation. This 110 kV-class rubber unit was expanded for 1 year and installed on a test cable. The interfacial pressure reached saturation within 5 minutes and become almost stable after 2 hours. The value of the interfacial pressure was almost the same as that calculated from the permanent set and elasticity properties of silicone rubber.

Permanent set propertyThe long-term permanent set property was studied and the master curve of permanent set was obtained. If the permanent set property is unsatisfactory, the inside diameter of the rubber unit becomes larger and sufficient interfacial pressure cannot be obtained. Figure 13 shows an example of the excellent permanent set property of silicone rubber. Using the master curve, it is possible to calculate the permanent set of a certain expansion rate and temperature.

Figure 12 Example of Expansion Rate Calculation

Figure 13 Example of Permanent Set Property of Silicone Rubber

(expansion rate: 250%, temperature: 25°C)

Figure 14 Interfacial Pressure Behaviour of 110 kV-Class CSJ Immedi-ately after Installation

Interfacial pressure during heating cycle test

By passing an alternating current through the cable

conductor, the conductor temperature was raised to


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90°C. Heating was applied for 8 hours followed by 16 hours of natural cooling. Figure 15 shows the conductor temperature and the corresponding interfacial pressure of a 66 kV-class CSJ during the heating cycle test. From the figure, it can be seen that the interfacial pressure increases with increasing conductor temperature. It was assumed that the increase in interfacial pressure was caused by thermal expansion of the rubber and waterproof compound inside the protection box.

Figure 16 Long-Term Trend of Interfacial Pressure of 66 kV-Class CSJ

Figure 15 Conductor Temperature and Corresponding Interfacial Pres-sure of 66 kV-Class CSJ during Heating Cycle Test

Long-term characteristics of interfacial pressureFigure 16 shows the long-term trend of the interfacial pressure of the 66 kV-class CSJ subjected to the heating cycle. The drop in interfacial pressure is very small. It can be estimated that 30 years after the installation of a rubber unit, the interfacial pressure will drop by about 20% from that at the early stage of installation.

Observation of the interface conditionThe contact interface between cable surface and the rubber unit was observed using the transparent pipe, which is simulating the grooved surface of the actual cable. The simulated grooved shape of the cable surface was

shown in Figure 17. The photographs of the observation

using the transparent pipe and small CCD camera were

shown in Figure 18. Observed images of the interface

were shown in Figure 19. Air voids were observed right

after the installation of the rubber unit. Although, the air

voids were disappeared in a short time, because of the

better elastic property of the silicone rubber. Finally all the

air voids were disappeared in about 10 minutes.

Figure 17 Simulated grooved shape of the cable surface

Figure 18 Photographs of the observation using the transparent pipe and CCD camera

Figure 19 Observed images of the contact interface between the rubber unit and the transparent pipe, which is simulating the grooved cable surface


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Figure 20 Assembly of Premoulded Rubber Unit for 400 kV-class CSJ

Development of 400kV-class CSJIEC PQ & type tests up to the 230 kV-class have already completed without encountering any problems. And then, 400 kV-class CSJ has been developed and confirmed to have enough electrical properties for an AC and impulse voltage tests required for IEC spec. And IEC PQ tests up to the 400 kV class have already completed without encountering any problems. Figure 20 shows the assembly of premoulded rubber unit for 400 kV-class CSJ.

Jointing Cables of Different SizesThe CSJ interfacial design technique makes it possible to apply one rubber unit to various cable sizes, allowing it to be used to joint cables of different sizes.

A load cycling test of the CSJ jointing 154 kV 1800 mm2 and 800 mm2 cables, including thermo mechanical tests was carried out to reveal the stability of the CSJ. Figure 21 show a view of 154 kV-class CSJ and the test circuit for the load cycling and thermo-mechanical tests.

The thermo-mechanical behaviours applied to the CSJ simulated the yearly and daily displacements of a cable system. The displacements were applied by power cylinders and were superposed on the load cycling test.

All of the tests were completed without encountering any problems.

ConclusionWe are the first manufacturer in the world to have successfully developed cold-shrinkable joints_with a silicone rubber unit for EHV XLPE cables. The interfacial design has been studied carefully, and the high performance of the CSJ has been revealed.

We have already supplied about a thousand of 220kV-class CSJ. We have already completed IEC PQ tests up to the 400 kV-class without encountering any problems and we are about to supply over a hundred of 400kV-class CSJ.

REFERENCES[1] H. Kurihara et al; p.47, vol.20, Furukawa Electric Review, (2001)

[2] D. Muto. et al; 26-6, IEEE/PES T&D 2002 Asia Pacific

Figure 21 View of 154 kV-class CSJ and the test circuit for the load cycling and thermo-mechanical tests


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Session III


93

Advances in Fluid Silicone Jointing Technology for Medium Voltage Power Cable

Ralf Meier, Jeroen Krak Lovink Enertech b.v., Netherlands

Introduction This paper describes new applications that were developed over the last five years based on fluid silicone jointing technology for medium voltage power cable. This jointing technology is relatively new to distribution network operators outside Europe, however it has a reputable track record for nearly 20 years in northwest Europe and more specifically in the Netherlands [1]. In its own right, this technology has contributed to make the vast underground cable network in the Netherlands one of the most reliable in the world.

the Netherlands’ land area is below sea level, where other parts of the country are relatively dry. Flooding of major parts of the country has been a regular phenomenon since long. In 1906, 1916 and 1953 major flooding disasters happened, in which large parts of village population died. Since these events took place, major engineering projects have been undertaken to prevent the Netherlands from being further affected by water disasters.

In the designing of underground power cable accessories, always a lot of attention has been paid to the specific requirements for the different soil conditions, and also the different power cable types utilized. Besides mass-impregnated cable, other varieties like the oil-filled cable and today’s single and three-phase extruded polymeric cable have been incorporated in the Dutch distribution networks ever since they were invented.

Through NEC, the National Electrotechnical Commission established in 1911 as the Dutch branch of IEC, important contributions have been made by the Dutch cable industry to the drawing up of international specifications for power cables and their accessories. Aside from the usual requirements, to ensure a lifetime of 40 years in special soil conditions, Dutch network operators today demand testing of power cable accessories with a water pressure of 1 or 2 bar, equivalent to a water table of 10 or 20 meters, which is 10 to 20 times higher than the minimum international standard [2].

Fluid Silicone Jointing Technology Local product development resulted in a new hybrid jointing concept, meeting all major requirements and more. The concept practically consists of an inner electrical insulation part and an outer protection shell, together creating an almost indestructible joint.

Electrical insulation build-up

The insulation part is made of a glass fiber reinforced polyester shell with a high mechanical strength. Inside,

Fig. 1 In the Netherlands water is a phenomenon that still influences the choices of technology applied.

Underground cable infrastructure in the Netherlands has been in place since early 1900. An important role in the development of this underground cable infrastructure was played by the Dutch cable industry, including today’s worldwide highly acclaimed testing institute KEMA (established 1927). Dutch company Lovink has been involved in the development and manufacturing of underground power cable accessories since 1919.

Background : Soil Conditions The Netherlands historically lies on the delta of three major rivers, amongst which the river Rhine. Approximately half of


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all elements responsible for electric field grading are contained, regardless of the type of connector being used. Both mechanical and press connectors are accepted in the jointing concept.

The first part of the primary insulation around the connectors is realized with a pre-moulded polymeric tube-set, consisting of two spacers and three individual tubes, one for each conductor. The main function of the tube-set is to create sufficient distance between the conductors.

The second part of the primary insulation is realized by filling the polyester shell with a high-grade fluid silicone compound. Because of the dielectric strength of this material, flashover between conductors is further prevented.

Additional to the electrical insulation characteristics of the fluid silicone compound, it has special sealing (skin forming) properties. When the fluid silicone comes in contact with water or humid air, the material reacts to form a soft and perfectly insulating silicone elastomer. Through this effect, water is blocked by the silicone elastomer, thus creating a seal preventing further ingress.

Although the primary water seal is formed by the outer protection shell, the sealing properties of the fluid silicone provide extra security.

Research of the parameters defining the optimum shape of the stress grading elements has resulted in superb characteristics of these elements with a radius of 20 mm or more [4].

Fig. 2 Skin forming properties fluid silicone compound in contact with water.

Furthermore, when applied to polymeric cable, because the dielectric constant of the silicone compound is significantly higher than the polymeric cable insulation (2.9 versus 2.3), the electric field will be more contained in the cable insulation. When applied to paper insulated cable, the silicone compound keeps the paper impregnated, does not react with cable grease and has an even better insulating function. The latter guarantees the long-term quality of the connection, while minimizing the risk of discharge due to dried out insulating paper.

Finally, to deal with the specific requirements of polymeric cable, additionally pre-moulded capacitive stress grading elements are applied to the transition between the semi-conductive layer and the bare core insulation [3]. Fig. 4 Lightweight polypropylene outer protection shells.


Fig. 3 Typical examples of optimized stress grading cones.

Mechanical protection build-up To provide sufficient mechanical strength and long-term moisture protection, the inner electrical insulation part is mounted into a outer protection shell, which is then filled with a two-component polyurethane resin.

The outer protection shell is made up of two lightweight but impact resistant polypropylene halves, which are connected to one another using socket-head bolts. The design of the outer protection shell mirrors the contours of the inner electrical insulation part, thus ensuring sufficient polyurethane resin coverage at all places.

The polyurethane resin forms a watertight barrier between the insulation part and the soil. As mentioned earlier, the electric heat cycling test was carried out with a water pressure of 2 bar which was easily passed by the fluid silicone joint.

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Open and modular technology

Another design parameter was to achieve rapid, simple and foolproof application. During all critical stages of the jointing process, full view on the pre-moulded mounting elements should be ensured. Build up as a modular system, a variety of cable depending modules define the final construction of the fluid silicone joint. The main modules required are the same for every type of joint. Big advantage is that low inventory levels can be supported through this.

The fluid silicone jointing technology does not involve any heating (no cable damage), which makes for shorter installation times, limited use of tools and greater ease of use. On average the full installation of this joint by trained jointers [5] requires 2,5 hours, regardless whether it concerns a straight or transition joint. Because of the design of the fluid silicone joint, immediately after installation the trench can be backfilled, tested be put into service.

Practical data reported back by long time users show very low failure rates of the universal fluid silicone joints. A figure below 0,5‰ (less than 5 per 10.000) has been achieved easily over the years.

Fig. 5 Inside view of a 11kV 3-core fluid silicone transition joint

Development of New ApplicationsDue to the reliability of the fluid silicone jointing technology, in the last five years demand emerged in the market to apply this technology to higher voltage levels and other applications. Where initially the jointing technology was only available for 12kV underground power cable, now also solutions are available for 24kV and 36kV. Furthermore, besides the original straight and transition joints, today also branch and loop joints are available, as well as stop ends (or pot heads).

With the branch joints, the possibility exists to eliminate for instance a separate station. Relative to the traditional network configuration, there is the disadvantage of not being able to switch over in the event of a failure. However, it is a big saving and with a highly reliable technology like fluid silicone the risk of failure is very minimal. Branch

Fig. 6 Fluid silicone branch joint after installation

Fig. 7 Inside view of a 33kV 3-core fluid silicone transition joint

joints also appear to be very popular with wind generator parks where the energy generated is supplied to the network through branch joints.

To elevate the fluid silicone jointing technology to the 36kV level, changes had to be made to the field grading system. With the lower voltages (12kV and 24kV) the use of a pre-moulded tube set with spacers was sufficient to deal with the electrical field around the conductors. For the 36kV version, a newly designed stress grading element made of insulating and semi-conductive silicone is used. The 36kV technology is available for both single core as well as three core transition joints.


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Recent International Case Studies: Fluid Sili-cone is Technology of Choice In 2006, Siemens Westinghouse has chosen fluid silicone jointing technology for the revision of the medium-voltage network at the Shell Bukom refinery plant in Singapore.

The choice was determined by the fact that this jointing technology does not need external heat sources and the design proved to be superior in terms of resistance to polluted soil. The fluid silicone accessories were used to connect 24kV XLPE cable at the Shell Bukom plant.

In 2007, the largest Malaysian Distribution Network Operator decided to switch to fluid silicone jointing technology for all transitions from paper-insulated cable to polymeric cable, for both 12kV and 24kV systems, approximately 2.500 joints annually. Utilization of other technologies did not lead to satisfying long term results regarding network reliability.

in different areas of the country. At this moment further studies are carried out to see if the technology could be implemented for other connections in the distribution network as well.

More International Case StudiesAlso in 2007, the power supply and distribution systems for the Neihu Line in Taipei, Taiwan, will be installed with fluid silicone jointing technology. The Neihu Line – an extension of the existing Muzha Line – will incorporate 14.8 kilometres of dual lane guide way and is an Automated Rapid Transit System, demanding a very high reliability.


The adoption of the new jointing technology followed extensive (two year) evaluation by the DNO’s Engineering and Research departments, including broad field trials

Closer to home, in 2007 a large UK Distribution Network Operator renewed their decision to continue with the fluid silicone jointing technology for all their 12kV underground cable connections. After the initial adoption of this technology in 2002, the price/performance ratio during the initial five years has been of such a level that this DNO decided to continue for potentially another five year period.

The total distribution network of this company comprises over 30.000 kilometres of buried 12kV power cable. Due to regular flooding in the DNO’s territory, dealing with high water tables is essential.

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Similar to other long time users of this jointing technology in the Netherlands and Germany, the UK network operator engineering staff have rated the fluid silicone technology as exceptionally reliable. Partly as a result of this, the UK company has been able to drastically reduce the customer minutes lost over consecutive years. In 2005 and 2006, industry regulator OFGEM awarded a bonus for this achievement.

Conclusions Cable jointing technologies like oil-insulated, cast-resin, heat-shrink, cold-shrink and push-on already have been applied to cable networks for many years. Very seldom new technologies surface on the international cable jointing platforms that can add new dimensions to the existing knowledge.

Since the introduction nearly 20 years ago of the fluid silicone jointing technology in the Netherlands, many network operators throughout Europe have decided in favour of this technology. It is believed that for other parts in the world the fluid silicone technology offers also concrete opportunities to further increase network reliability.


In this paper it has been demonstrated that the relatively unknown fluid silicone jointing technology is designed to meet a wide range of requirements, both relative to different cable types and soil conditions.

Operators that run networks containing large parts of paper-insulated cable and who are located in areas with dry and wet soil, can benefit tremendously from the special characteristics of the fluid silicone jointing technology. Reduction of customer minutes lost can be easily achieved through application of this technology.

REFERENCES

[1] KEMA Transmission & Distribution Power, 2001, “KEMA course Power Cables”, 6-4/6-10.

[2] Cenelec, 1996/1997, “Test requirements on accessories for use on power cables of rated voltage from 3,6/6(7,2)kV up to 20,8/36(42)kV”, HD 629.1 S1:1996, HD 629.2 S1:1997, paragraphs 4.7.

[3] F .H. Krueger, 1991, “Industrial High Voltage”, 151-163.

[4] B . Brus, F. Hartwig, 2004, “Cleverly designed cable accessories”, Energietechniek, vol. 12 Dec. 2004, 32-35.

[5] B. Brus, J. Krak, 2004, “Reliable power cable networks”, MENA Power & Water, vol. Q3/4 2004, 42-43.

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New Generation Electron Beam Cross Linked Building Wires

Rupa Bhattacharyya, Nazreen Akhtar, Gopal C Dhara, Nilambar MongalNicco Corporation Ltd., West Bengal, India

Abstract Safety begins at home. Each and every constructed building constitutes a large number of cables for power supply. These cables which consists of the insulating and sheathing material are mainly halogenated hydrocarbon in nature and result in the release of halogen gases and toxic fumes when subjected to fire conditions. However, the nature of these fumes depend on the compounds used for insulation and sheathing. Nicco has introduced halogen free, low toxic, low smoke building wire for the ultimate safety of human lives and property. The operating temperature of these building wires are about 1200C and the process involves cross linking technology by electron beam irradiation.

Conventional PVC wiring was initially replaced by PVC-FRLS (fire resistant low smoke) and subsequently by HFFR (halogen free fire retardant) cables but in a more advanced development, zero halogen electron beam crosslinked building wires were found to be much superior in quality aspects.

Zero halogen EBXL building wires possesses some advantageous properties over thermoplastic HFFR cables. The former produces better physico-mechanical and ageing resistance properties. They also exhibit higher dynamic cut through resistance at 200C and 900C. Scrap abrasion at 200C exceeded 1000 cycles and heat shock properties were found to be superior. The halogen content being zero in this case, the generation of acid gases were found to be absent and it also exhibited higher volume resistivity compared to that of halogen free fire retardant thermoplastic cables.

Building wires of cross linked zero halogen insulation with electron beam cross linking (EBXL route) has been employed for 1200C operating temperature. The process

of cross linking raises the operating temperature from 900C to 1200C, thus preventing dripping of the compound at elevated temperature and subsequent exposure of the conductor leading to short curcuit and accidents.

IntroductionGenerally, building wires are made up of poly(vinyl chloride) (PVC) insulated copper wires of different cross section. Since the commercial use of PVC started in this area, many changes in the formulation have taken place over the years for improving the performance of PVC as insulating material for building wire. The operating temperature of these cables have improved from 700C in earlier days to 900C and further development was made to raise the temperature to 1050C. However, PVC is vulnerable to environmental and toxic hazard point of view in case of fire, which may have caused due to short curcuit or current overload. PVC, when burns, emit 350 mg of hydrochloric acid fumes approx. per gram of PVC. One coil of 100 meters of cable contains around 600 grams of PVC. In order to eliminate such toxic situations and fire hazards, an attempt has been made to replace PVC building wires by non halogenated wires involving cross linking technology which would produce cables of higher operating temperature compared to that of normal thermoplastics like PVC.

The benefits of cross linking are to change thermoplastic material into thermosets by the process of curing. When the polymeric material is cross linked, the molecular movement is severely impeded and the form is stable against heat. This locking together of molecules is the origin of all the benefits of cross linking including increased tensile strength and form stability, resistance to deformation, abrasion,solvents, stress cracking and others.


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Thus, the need of the situation was to develop fire resistant, self extinguishable or non flammable, low smoke, non toxic infusible cables.

Electron Beam Cross Linking Technology The electron beam cross linking technique which is applied in this development has significant advantages over other methods of cross linking. Cables are generally cross linked by chemical cross linking technique. The greatest advantage over EBXL technique is to get a very uniform three dimensional network and high degree of cross linking (up to 85%) resulting in superior mechanical properties compared to conventional process. Electron beam cross linking process has the advantage of producing the same level of cross link density irrespective of the polymer type. The cross linking of polymers with Electron Beam (EB) is an innovative process, which improves the physico-mechanical properties of polymers significantly compared to chemical cross linking.

EB cross linking is effected by the interaction of high energy electrons with the polymeric material resulting in a three dimensional network structure. EB Cross linking leads to enhanced thermal, mechanical and physical properties, while the electrical properties remains almost unchanged.

Electrons accelerated under a voltage of 3 MeV, travel at 99.6% of the speed of light. A huge numbers of such high energy electrons penetrates the surface of polymeric system and initiate chemical changes leading to a three dimensional cross linked structure. This process of cross linking are highly efficient compared to chemical, thermal or photon initiated processes.

The Accelerator works on the same principal of television tube. Electrons are originated from a super heated metal filament. A voltage gradient draws them away from the filament and accelerates through the vacuum tube. As the high energy beam of electrons passes from the beam tube and through the scan magnet, an oscillating magnetic field sweeps the beam back and forth across the scan window. The electrons bundle into a 3 to 5 cm diameter beam to irradiate on the desired target or object.

Comparison of technical properties of electron beam cross linking technology and normal wire technology

Table 1

Electron beam cross linking

Normal wire technology

Process - Specially formulated compounds resulting in a very strong short circuit free insulation

Normal extrusion and no cross-linking

No risk of deformation above short circuit temperature

Insulation melts and flows causing short circuit.

Rugged Mechanical properties - Superior Abrasion / Scrap & Cut-thru’ resistance

Sharp edges / surfaces may damage insulation during wiring

Higher current capacity due to higher operating temperature (120oC) for a specific circuit load, reduction in size/ no. of wires - space saving, lighter weight

Lower operating temperature 70oC and hence, current ratings much lower

Environment friendly and easy to dispose

Due to presence of halogen, it is not environment friendly

ExperimentalMaterials

A combination of ethylene vinyl acetate and copolymers of ethylene was used in the process. A required dose of antioxidant (hindered phenol type) coupled with adequate quantity of flame retardant filler and a combination of processing aid was added to the polymers. To such a combination , an optimum dosage of sensitizer was added which acts as an agent for cross linking the polymeric chains when electron beam is incident on the insulated wires.

Methods

The building wires produced by Nicco Corporation Ltd. was made on a 30 mm screw diameter extruder within a temperature range of 1100C - 1500C during extrusion with a line speed of 40m/min. The wires were manufactured in two sizes of 1 and 1.5 sq. mm and of two colours viz. red and black.

The product features :-

Conductor :- High purity electrolytic grade annealed copper conductor was made in bunching process.

Insulation :- The wires were insulated with a superior grade non halogenated fire retardant EBXL compound that is specially formulated and manufactured in-house.

Electron beam curing :- The insulated core was subjected to curing in an offline process with optimum dosage by high frequency electron beam.

Printing :- The wires were printed with brand name, size in sq. mm and voltage grade.

The particulars of the different sizes are as follows :-

Size :- 1 sq. mm

Conductor :- 14/0.3 mm ABC

Insulation :- EBXL HFFR

Cable diameter (approx) :- 2.65 mm.

Size :- 1.5 sq. mm

Conductor :- 22/0.3 mm ABC

Insulation :- EBXL HFFR

Cable diameter (approx) :- 3 mm.


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Characterization

The samples were characterized subjected to the following

tests and the results so obtained are as mentioned.

Table 2

Sl No.

Tests Test methods Specified limit

1 Tensile strength (N/mm2)

IS : 10810 - 1984 Part - 7

12.5

2 Elongation at break (%) IS : 10810 - 1984 Part - 7

125

3 Air oven ageing at 1500C for 10 days Tensile strength variation Elongation at break variation

IS : 10810 - 1984 Part - 11 + 30

+ 30

4 Hot set at 2000C , 15 min , 20N/mm2 a) Elongation under load(%) b) Permanent elongation after cooling (%)

IS : 10810 - 1984 Part - 30

175%

15%

5 Volume resistivity i) at 270C, cm ii) at 900C, cm

IS : 10810 - 1984 Part - 43

1x1013 1x1010

6 High voltage test at 3kV Ac for 5 minutes

IS : 10810 - 1984 Part - 45

Should withstand without break

7 Dynamic cut through resistance (kg)

TDE/76/P/16 (para - 27) British railway board spec.

Min. 10 kg for sizes <= 2.5 mm2at RT

8 Scrap abrasion resistance (cycles)

TDE/76/P/16 (para - 28)

1000 DS

9 Hot deformation at 1200C for 4 hrs (%)

IS : 10810 - 1984 Part - 15

Indentation max. 50%

10 Heat shock at 1500C for 1 hour

IS : 10810 - 1984 Part - 14

No cracks

11 Shrinkage (%) at 1500C for 15 minutes

IS : 10810 - 1984 Part - 12

4

12 Flammability (period of burning)

IS : 694 60 sec max.

13 Acid gas generation (%) IEC : 754-1 2% max.

14 Smoke density rating (%) ASTM D - 2843

20% max.

15 Oxygen index (%) ASTM D - 2863

29 min.

16 Temperature index (0C) ASTM D - 2863

250°C min.

17 Strippability by manual stripper

---- Should be easily strippable

New Generation EBXL Building Wire Comparison

Table 3

Tests EBXL HFFR TP FRLS-PVC

Before Ageing :

TS, Mpa 13 11.60 14.5

EB, % 200 125.00 175

Aging, 150 deg.C / 10 days

100 deg C/ 7 days

80 deg C/ 7 days

TS % variation +10 +10 +10

EB % variation +15 +15 +15

Hot Set at 200 0C / 15 min / 20N/cm2

Elongation under load, %

175 N A N A

Permanent set after cooling

15.00 N A N A

Smoke Density as per ASTM D 2863, % light transmission

98.00 95.00 40

LOI, % as per ASTM D2863

34.00 29.00 31

Temp. Index, deg.C

285.00 240.00 250

Dynamic Cut through resistance, kg as per TDE/76/P/16

At 20 deg.C 30.8 18.00 21

At 90 deg.C 13.00 3.00 2

Scrap abrasion at 20 deg.C

> 1000 cycles. >500 >500

Hot Deformation 120 deg.C / 4 hrs

80 deg C/4 hrs 80 deg C/ 4 hrs

% indentation 33.00 35.00 30

Heat Shock test 150 deg.C / 4 hr.

150 deg.C / 1 hr.

150 deg.C / 1 hr.

Observation No cracks No cracks No cracks

Acid gas generation, %

Nil Nil 20

Vol. Resistivity, ohm-cm

At 27 deg.C 1.92X10^14 5.5X 10^13 5.0 X 10^13

At 90 deg.C 3.45X10^12 2.5 X 10^11 2.3 X 10^11

Results and Discussion

The results obtained for EBXL building wires display overall superior properties compared to that of


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other types. The former type withstands ageing at a higher temperature for a larger time period thus offering a higher operating temperature (1200C). The hot set test as applicable in this case due to cross linking prevents the problem of dripping which may have been caused in other cases due to short circuit or current overload. Thus the problem of cable deformation or fusibility can be eliminated to a great extent. It also generates less smoke, higher limiting oxygen index and temperature oxygen index as compared to the other types. Acid gas generation is nil in this case thus producing a non-toxic, non -hazardous environment. It also exhibits higher volume resistivity, scrap abrasion resistance, dynamic cut through resistance and shock absorption.

FRLS-PVC cables have the greatest disadvantage of being halogenated in nature which emits toxic halogen fumes. Halogen free fire retardant thermoplastic building wires

are non cross-linked and the problem of dripping, if higher temperature is reached, cannot be eliminated in this case. EBXL building wires thus provide the greatest range of advantages over normal FRLS-PVC or thermoplastic HFFR type cables.

Summary and ConclusionThe new generation building wires thus developed with the latest technology possesses such distinctive and unique properties over the other similar products available in the market. Owing to the improved mechanical properties, high limiting oxygen index and specially high operating temperature of 1200C, such wires can be used for speciality applications for a safe and hazard free environment. The process of manufacturing such cables also possesses the unique technology of electron beam cross linking which makes it more strong and durable. The normal problem of dripping is also prevented in this case to a great extent.


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Loading of Flexible Wires in Conduit Pipes

S.Ramaprasath, A.Sudhindra, K.P.Meena, Thirumurthy, G.K.RajaCentral Power Research Institute, Bangalore, India

IntroductionThe house wiring cables plays a vital role in the total reliability of electrical installation. When short circuit takes plays, the house wiring cables can lead to a fire & thus cause havoc.

Copper is the next best conductor of electricity after silver. Copper is used as a conductor for manufacturing of PVC wire & cables, winding cables, flat cables & Power cables. In house wiring applications copper wires insulated with PVC is widely used. The different sizes of wires that are normally used are 1sq mm, 1.5sq mm, 2.5sq mm & 4.0sq mm.

The ampacity i.e. current ratings of the house wire cables varies depending upon the size of the conductor. The smaller sizes of wires have higher current density ie, Amps /sq. mm, however as the size of the conductor increases, the current comes down due to the skin effect. The wires are normally housed in PVC insulated conduit pipes.

In this paper experimental studies have been carried out on the cable wires of sizes 1.0, 1,5, 2.5, & 4.0sq mm by using two different sizes of PVC insulated conduit pipes. Various conditions has been used to decide on the safe operating conditions of the wires .This paper covers different types of PVC wires that are normally used for the house wiring purposes. In addition to the general purpose PVC grade for 700C for continuos conductor operation, the heat resisting PVC grade (HRPVC)for 850C & the special heat resisting PVC grade for 1050C has been covered. The current carrying capacity plays a vital role for selection of wires for various electrical equipment and in house wiring applications.

House Wiring Cables House wiring cables consist of copper conductor over which the PVC insulation is extruded.

Copper conductor

Copper is used as the conductor for housing wiring application as it has very good electrical & mechanical properties.

Properties of copper

The properties of copper can be summarized broadly as follows:

a) Good electrical conductivity of copper ensures higher current carrying capacity and thus makes it most efficient electrical conductor for house wiring applications.

b) The electrical resistivity of copper is 0.01724 ohms mm2 /meter as compared to aluminum 0.028264 ohms mm2 /meter

c) High melting point.

d) Good thermal conductivity.

e) Very good mechanical strength, highly ductile,

malleable & good annealing property.

Conductor sizes

Copper conductor used for house wiring the legendary 1/18, 3/20 & 7/20 are the rigid types of wires whose flexible equivalents are 14/0.3, 28/0.3 and 65/0.3 respectively. By the use of thinner wires, the no. of wires used for a cross section will be increased which gives an increased surface area for the current to flow . Mechanically too, more number of wires are bunched to form a rope, thus increasing its strength.

As on date people prefer the flexible wires for house wiring, as the chances of the wires getting cut are minimum. Whereas when one strand of 3/20 gets cut , the cross section reduces by one third & thus leads to the failure, due to the over heating of the conductor.


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Pvc InsulationOver the copper conductor extrusion is done using PVC granules for manufacturing PVC insulated wires. Different PVC compounds are available both of inferior and superior quality. The quality of wire depends on the PVC insulation. Short circuit causes fire leading to extensive damage. Hence it is required to add fire retardant materials along with PVC insulation, so that propagation of flame, emission of smoke is reduced at the instant of fire. To check the quality of PVC insulation it is essential to check for the insulation resistance test at operating temperature.

The insulation used for house wiring are generally thermoplastic materials. They are of the following types:

1. General purpose Poly Vinyl Chloride (PVC) 700C max conductor temperature & max short circuit conductor temperature 160 0C.

2. Hard grade Poly Vinyl Chloride (PVC) max conductor temperature 700C & max short circuit conductor temperature 1600C.

3. Heat resisting Poly Vinyl Chloride (HRPVC) 850C max conductor temp. & max short circuit conductor temperature 1600C.

4. Special heat resisting Poly Vinyl Chloride max conductor temperature 850C & max short circuit conductor temperature 1600C.

InstallationThe PVC conduit pipes, which are embedded in the wall/ceilings of the building, enclose the house wire. The sizes of the wire to be used depends on the type of load accordingly the conductor size can be selected. It is a general practice to have two wires or more than two wires within the PVC conduit pipe. The conductor size that is used for house wiring application are 1.0 sq.mm,1.5 sq.mm,2.5 sq.mm & 4.0 sq.mm .The size of the PVC conduit pipes that are normally used are 20mm and 25 mm outer diameter.

Heat DissipationWhen the current flows in a conductor, the heat is radially dissipated from the conductor. The dissipation of heat through the insulation depends on the thermal resistivity of the material. Lower the thermal resistivity, higher the heat dissipation & thus increases the ampacity of the cable.

In case of PVC wires housed in PVC conduit pipes, the first component for thermal circuit is the PVC insulation, and then air is the medium and above which is the PVC conduit pipe. Outside the PVC conduit pipe is air, if the pipe is laid in air and the thermal circuit is air. If the PVC conduit pipe is embedded in the wall, then the thermal resistivity of concrete comes into play.

The thermal resistivity of various components can be represented as shown in Fig 1.

1: Conductor

2: Insulation

3: Air inside the PVC conduit pipe

4: PVC Conduit pipe

5: Air outside the PVC conduit pipe / Concrete surface outside the PVC conduit pipe

Fig. 1 Cross section of a wire in a conduit pipe

The thermal resistivity of various materials in the circuit included is as follows:

Material Thermal resistivity Km / W

PVC 5.0

Concrete 1.0

The current rating of the wire depends upon not only on the thermal resistivity of the insulation in contact with the conductor, but also depends on the thermal resistivity of the material surrounding it. If the PVC conduit pipes are in contact with each other, the de-rating factor has to be taken into account.

The current ratings are based on the ambient temperature & if the ambient temperature is different then the temperature derating factors had to be applied. Derating factors comes down (<1.00) if the ambient temp is higher than 300C, as the reference temperature is 300C for derating factor calculations.


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Current Carrying CapacityExperimental setup

General purpose PVC

To verify the current carrying capacity of PVC wires the wires were inserted into a PVC conduit pipe of length three meters & both the ends were sealed with a suitable thermal insulating material so that no heat is radiated out from the ends of the PVC conduit pipe. The temperature of the conductor was checked by removing the insulation over a small portion of the conductor & thermocouple was fixed on to it. One thermocouple was inserted into the duct to measure the duct ambient. The ac current was passed through the conductor so that the conductor temperature was steady. The current was increased after steady state at every step of current until the temperature of the conductor was maintained at 700C for general-purpose PVC grade. This was repeated for wires of sizes 1.0,1.5,2.5 & 4.0 sq. mm. The results are tabulated in graph I for general purpose PVC.

Graph 1. Current density Versus Cross Sectional Area (CSA)

Though the 1.0-sq. mm. conductor size can carry current density of 20 Amps/sq. mm, for higher sizes, the current density comes down due to skin effect.

Heat resisting PVC

The general purpose PVC is used for maximum conductor temperature of 700C. For higher temperature up to 850C heat resisting PVC is used and for conductor temperature of 105 0C special heat resisting PVC is used.

The results are as shown in graph 2 for special heat resisting PVC.

Analysis of results

The current density for 1.5 sq. mm for general purpose PVC is 16.25 Amps/sq. mm whereas for special heat resisting PVC (for 1050 C applications) the corresponding value is 23.5 Amps/sq. mm. For application where the wires are to be used for higher conductor temperatures

Graph 2. Conductor Temperature Versus Current

Graph 3. Conductor Temperature Versus Duct Temperature


without damaging the insulation, special heat resisting wires can be used.

Duct temperature

The effect of duct temperature for two different conductor sizes was carried out separately.

Conductor size 4.0 sq.mm

When higher currents are required (>40 Amps) 4.0sq-mm wire is commonly used. Generally for single phase loading two wires are required to carry the load current.

To check the effect of duct temperature within the conduit pipe two 4.0sq mm was inserted into a 25mm diameter PVC conduit pipe & two ends were thermally sealed. It was found that for a current density of 11.25 A/sq. mm for a conductor temperature of 910C the duct temperature inside the PVC conduit was observed to be 810C. Whereas when only one wire was loaded (i.e. the other wire was kept open at the other end), for a conductor temperature of 880C the duct temperature was observed to be 450C (Graph 3).

The above experiments were repeated for a current density of 10 A/ sq. mm for 4 sq. mm PVC wire. For a conductor temperature of 72 0C the duct temperature was observed

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Graph 4. Conductor Temperature ( in deg.C) Vs Duct Temperature (In deg C)


to be 640C, when two wires were loaded. Whereas for single wire loading for a conductor temperature of 630C the duct temperature was observed to be 390C.

It is important to note that PVC conduit pipe generally used are of general-purpose grade for a maximum temperature of 700C . For higher temperature of duct it is required to use Heat resisting conduit pipes which can withstand high temperatures without damaging the PVC wires enclosed within the conduit pipe.

Conductor size 1.5 sq.mm.

If we study the effect of current density on 1.5sq mm conductor size the increase in duct temperature will be very high at higher current density. For a current density of 10 A/sq. mm, the duct temperature is only 340C for a conductor temperature of 460C. If we study the same at a current density of 16.7 A/sq. mm which is the value for a general purpose PVC wires, the duct temperature is 550C for a conductor temperature of 720C. At 550C duct temperature the wire which is enclosed within the duct, are not subjected to ageing compared to higher duct temperatures.

When the current density is raised to 20 A/sq. mm, then the duct temperature will raise. For example the duct temp. is 730C for a conductor temperature of 1050C as shown in graph 4.

If the temperature in the duct is high around 60-650C then the ageing of the wire takes place. This becomes the prevalent ambient temperature for the wire running in this duct which leads to accelerated ageing of the insulation of these wires. It is preferable to use HRPVC wires wherever the temperature of the duct are high, as HRPVC has got better thermal properties compared to the general purpose PVC insulation. Or it is preferable to decrease the current density or to use higher cross section of wires to reduce the ageing effect.

Conduit PipeIn the actual installation different sizes of PVC conduit

pipes are used. The most commonly used sizes are the pipes with outer diameter of 20mm & 25mm.

It is opined that for a higher diameter conduit pipe the temperature of the conductor will be lower compared to a conduit pipe of lower diameter. But in experimental results the temperature of the conductor enclosed in a conduit pipe of higher diameter is higher than the same conductor placed in a lower diameter conduit pipe.

The above fact is justified because the thermal resistivity of air is lower. The ambient air, which is at low temperature, is cooling the outer surface of the PVC conduit pipe. For a 4.0sq mm conductor wire for a current of 50 Amps, the temperature of the conductor is 730C for 20mm diameter conduit pipe whereas the temperature is 780C for 25mm diameter conduit pipe. This is shown in graph 5.

Protection Suitable protection for the circuits is to be used in conjunction with the wires. The protection devices can be either rewireable fuses or MCB. The maintenance of rewireable fuses or the HRC fuses are time consuming and as on date it has been replaced by MCB’s as they are fast operating devices and the characteristics can be selected depending upon the requirements.

For a current of two times the rated current (2 In ) MCB

will operate in 20 seconds. For a current of three times the rated current (3 I

n ) MCB will operate in 8 seconds and

for 6 In it operates in 2 seconds. By taking into account the characteristic of MCB’s the short circuit faults can be cleared faster.

ConclusionThe current carrying capacity for house wiring PVC insulated wires are arrived at by selecting the appropriate

Graph 5. Conductor Temperature ( in deg.C) Vs Current (In Amps)

106 Day 2 - Session III Technical Papers

standard sizes of the wires. The type of insulation in contact with the conductor decides on the maximum temperature of the conductor to which the wires can be used.

For general purpose PVC for a conductor temperature of 700C the current density for 1.0sq mm, 1.5sq mm, 2.5sq mm & 4.0sq mm are 20 A/sq. mm,16.7A/sq mm, 13.2 A/sq. mm & 10.75 A/sq. mm respectively, for safe operating conditions. For 700C maximum conductor temperature the general purpose PVC wire has to be selected. In applications where the conductor temperature is 850C and 1050C heat resisting PVC and special heat resisting PVC are to be selected respectively.

For laying the wires for house wiring application, these wires has to be enclosed in PVC conduit pipe. In addition

the appropriate sizes of the conduit pipes needs to be selected. For a lower conductor temperature, a 20mm conduit PVC pipe should be used instead of 25mm conduit PVC pipes. In order to reduce the failure rate the next higher conductor size should be selected so that the ageing of the wire is minimized.

In case of essential services and not to cause any damage to wires only one wire housed in smaller PVC conduit wires improves the reliability of the system, though it involves higher installation cost.

AcknowledgementThe authors wish to thank the management of Central Power Research Institute, Bangalore for permitting to publish this paper.

107


Session IV


Reaching the Moon and Beyond ...

POLYCAB WIRES PVT. LTD. Regd. & Head off ice : Polycab House, 771, Pandit Satwalekar Marg, Mahim (W), Mumbai 400 016.Tel. : 91-22-2432 7070 - 4, 2436 2199, 2432 9118 • Fax : 91-22-2432 7075 • E-mail : [email protected] • Website : www.polycab.com


POLYCAB, India’s largest Wires & Cables Company takes pride in fuelling some of the finest projects in India & World over.

With a production in Power & Control Cables alone reaching over 7,93,063 kms.+ in last five year stretch.It is like touching the Moon twice or encircling the World about 20 times over.

*The production figure does not include the innumerable length of wires produced till date.


109

New Range of Special Alloy Conductors: Advantages over Conventional ACSR and AAAC Conductors

G. L. PrasadSterlite Technologies Limited, Pune, India

IntroductionFor several years, distribution and transmission lines have been designed using aluminum conductors steel reinforced (ACSR) or in some countries all aluminum alloy conductors (AAAC). Both types normally have a conductivity calculated on the total area of 53-54% copper (IACS).

Since the mid-1970s the cost of producing electric energy has grown rapidly, leading to an increase in the cost of losses. An attempt to find new conductor material producing fewer losses in the network was started in Sweden at the beginning of the 1970s and led in 1979 to the release of a new conductor standard called Al- 59, where 59 stands for its conductivity (IACS).

HistoryAt the beginning of 70’s AB Electrokopper in Helinsburg developed an alloy with the trade name DUCTALEX. It has a better conductivity than ordinary alloys used for conductors but a slightly lower mechanical strength.

This new conductor alloy was laboratory tested at the Swedish state power board with respect to creep, corrosion resistance, self-damping and fatigue strength. All tests showed the same or, in some aspects better than the ordinary alloys need. Tests were made on actual distribution lines starting in 1975.

As only satisfactory results were obtained, the new alloy conductors were installed in 400 KV lines. The measurement of sag after five years showed a high degree of agreement with what was predicted. Concurrent with all trials the Swedish standardization committee worked out and issued the standards SS4240813 for Al 59 wires and SS4240814 for Al 59 Alloy Conductors.

Application of Al-59 Alloy ConductorsAl-59 Alloy Conductors are used in power transmission

and distribution lines for a wide voltage range - low voltage to Ultra high Voltage.

Compliance with StandardsAl-59 alloy conductor complies with the standard SS4240814 and Al-59 alloy wires comply with standard SS4240813. These standards specify the limits for conductivity, strength and creep irrespective of the chemical composition. Other properties are similar to conventional AAAC conductors.

Material PropertiesSize of the Conductor

Al-59 conductors ranging from Al-59 31Sq.mm (7/2.38 mm) to Al-59 910 Sq.mm (61/4.36mm)

Strength of the wire

For diameter less than 3.5 mm – 250 MPA

For diameter less than 4 mm – 240 MPA

For diameter less than 4.5 mm – 230 MPa

Resistivity of the wire

Individual resistivity – 29.30 n m

Average resistivity – 29.08 n m

Creep of the conductor

Maximum conductor Creep at 230 Celcius, 40 %

of rated tensile strength and Figure 1 (Original photo at-tached)


110

1500 Hours are as follows:

7 strands: 350 mm/Km

19 and 37 strands: 400 mm/Km

61 strands: 450 mm/Km

Comparision with Conventional ACSR

and AAACFor comparison based on the current carrying capacity, Power transfer capacity, Strength of the conductor and Sag calculated after 10 years from the creep value, please refer Table 1.

Table 1

PROPERTIES ACSR MOOSE

AAAC MOOSE 6201 Alloy

Al59 593 Sq.mm

Cross sectional Area Sq.mm

597 604 593

Current carrying capacity - Amperes

Operating Temp 65Deg.Cel 838

Operating Temp 85 Deg.Cel 1233 1307

Power Transfer Capacity - Mwh

Twin Bundle 901 1326 1405

Quad Bundle 1802 2652 2811

Sag after 10 Years - Mtrs

Sag 10.33 9.3 9.83

Strength of the conductor - MPA

UTS 270.02 264.57 240.86

Mass of the conductor – Kg/Km

Mass 2004 1666 1640

Note: CCC calculation based on wind velocity of 1 m/sec and solar absorption coefficient is 0.6 Even though strength of the Al59 conductor is less compared to conventional AAAC. But strung at the same tension as like conventional AAAC.

The Important factor for choosing tension from the vibrating point of view is tension divided by the conductor mass. This value is independent of Ultimate tensile strength.

Advantages Over ACSR and AAACHigher current carrying capacity with lesser cross sectional area

Higher power transfer capacity with lesser cross sectional area

Lower Sag

Commercial benefits due to lower power loss and higher power transfer capacity

l

l

l

l

l

l

l

Reference of use of AL-59 Alloy ConductorsAl-59 Alloy Conductors have been used extensively by Swedish Electricity, Swedish Railways and Norway Electricity.

Conclusion“Power for all by the year 2012” is the vision of the Ministry of Power, Government of India. The Government of India’s Transmission Perspective Plan focuses on the creation of a ‘National Grid’ in a phased manner by adding over 60,000 km of Transmission Network by 2012.

Such an integrated grid is envisaged to evacuate additional 1, 00,000 MW by the year 2012 and carry 60% of the power generated in the country. The existing inter-regional power transfer capacity is 9,000 MW, which is to be further enhanced to 30,000 MW by 2012 through creation of “Transmission Super Highways”.

On a global level, ABS Research, UK reports that the global market for power transmission conductors was valued at about US$12.3 Billion in 2006 and has a CAGR of 7% from 2004 through 2006. ABS Research also anticipates a stable demand growth in the global market from 2007 through 2010 at a CAGR of about 7%.

In view of development of new power transmission and distribution grids by global power incumbents, Al-59 Alloy Conductors would have a special significance while designing transmission line networks, as the properties of these conductors enable superior power evacuation while optimizing the cost of the entire grid.

Reference StandardsSwedish Standards:

SS4240814: Aluminium alloy stranded conductors for overhead lines – Al 59 Conductors.

SS4240813: Aluminium alloy wire for stranded conductors for overhead lines – Al 59 Wires.


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New Developments in International Standards for Winding Wires

Shirish GokhaleELANTAS Beck India Ltd., Pune, India

IntroductionThe work of up gradation or revision of the specifications is an on going task to maintain the standards more consumer oriented thus reflecting upon the needs & trends in the user industry. The undersigned has presented a paper on the subject in Cablewire 2002. This work is in continuation with that paper.

The International Electro-technical Commission’s (IEC) committee IEC TC 55 undertakes the preparation & revision of international standards for Winding Wires. Presently important objective is to rationalize methods of test between IEC, JIS & NEMA, standards.

Based on the work done by WG1 of IEC TC 55 & the discussions therein, this paper attempts to discuss various test methods being re-looked for changes in IEC standards. This will help Indian industry & users of winding wire standards to know likely changes in future, in the Indian Standards.

DimensionsThe work is continued to study measurement of dimensions by varying anvil diameters & force on the anvil. It is observed that the NEMA measuring force on tape wrapped wire is much more severe (11 to 13 N) than IEC (4 to 10 N). It is proposed that the anvil diameter be larger than the wire specimen and that pressure applied on tape wrapped wire be based on mm2 of wire. The work is not concluded.

Chemical propertiesIEC standards are now more focused on environmental aspects in formulating the tests & test methods. Refrigerants have always been a matter of attention to have some control over its environmental impact due to its ozone depleting properties. It was decided that a survey is needed to determine the most common refrigerants,

other than R-22, and a round robin needs to be conducted to standardize these new refrigerants as required. For this, a team of experts form the participating members is formed. India is given the responsibility of compiling the world wide information on use of refrigerants. Experts from Italy, Japan & USA are other members of the group. The team will present its report in forth coming IEC TC 55, WG1 meeting to be held in October 2007 in France.

Japan has conducted a survey on the various grades of soldering material, particularly lead free type compositions again due to its impact on environment. They are able to identify roughly 20 nos. of lead-free solder compositions being in use by the industry.

High voltage continuity of covering testIt was felt by a member from USA that the present test method needs to be reviewed so as to make it giving more reliable test results. The draft presented is as follows:

High-voltage continuity (nom. conductor dia. over 0,050 mm up to and incl. 1.6 mm)

PrincipleA wire specimen with the conductor earthed is pulled over a “V” grooved electrode (pulley) or through a graphite brush electrode at a constant speed. A d.c. test voltage is applied between the electrode and earth. Any faults in the insulation of the wire are detected and recorded on a counter. The result is listed in faults per 30 m.

EquipmentThe following equipment shall be used:

- high-voltage power supply providing a smooth filtered d.c. voltage with a ripple content less than 5 %, with an open-circuit test voltage adjustable from 350 V to 3 000 V with a short-circuit current limited by internal series resistance to (25 + 5) (A at any test voltage and


112

with not more than 75% drop in voltage in case of a 50 M fault resistance;

- fault detection circuit, which operates at a fault current as shown in Table 4 with a speed of response of (5 + 1) ms and with a fault counter repeating at a rate of (500 + 25) counts per minute when a bare wire is tested;

- dual high-voltage electrode pulleys according to Figure 5 made of stainless steel and providing a wire contact length of 25+0, -2.5 mm on each pulley;

– high-voltage electrode pulley according to Figure 6 made of stainless steel and providing a wire contact length of 25 mm to 30 mm;

– graphite fibre brush electrode assembly according to Figure 7, constructed so that the conductive brushes completely surround and contact the wire surface for a length of (25+ 2,5) mm (See figure 5). The graphite fibre brush electrode shall be inspected, cleaned, or replaced if excessive wear or accumulation of foreign material is present. The graphite brush electrode assembly shall be electrically isolated for the duration of the test to prevent false readings at the specified voltages.

- earthed guide pulleys having an outside diameter of (50+0.25) mm and root diameter of (40+0,25) mm and spaced (140 + 2) mm apart;

- surge damping resistor of 4,7 M + 10% installed in the high-voltage line.

NOTE The earth insulation for the high-voltage electrode should be a high-Resistivity material, non-hygroscopic, non-tracking and easily cleaned, having a clearance for maintaining a continuous voltage of 3 000 V. No shielding should be used on the high-voltage lead since a minimum capacitance to ground is required during switching and counting events. The drive motor should be the brushless type and should have sufficient power to maintain the required speed to pull 1,600 mm wire.

Table 4 - Fault currents

Test voltage (d.c.) V Fault current µA

2 000 12

1 500 10

1 000 8

750 7

500 6

350 5

Figure 1 on Page 115 shows Graphite fibre brush electrode assembly.

Di-electric dissipation factor measurement test methods:In the earlier annexure published by IEC, the method of test for Die electric dissipation factor measurement contained only the test method having curve plotted with increasing temperature of the molten metal bath. However, in Indian subcontinent, the industry has standardized for tan delta curve drawn during cooling cycle of the molten metal bath. The difference in test methods was known to show difference in observations. Hence, India has made representation in IEC meeting to incorporate cooling curve method also appropriate method for measurement. Now IEC has agreed on this the cooling curve method. All the method test are described as below for information.

A.1 Tangent delta - Intersection point A number of methods are available in order to check the repeatability of curing. These two methods are included as examples.

The principle is as follows: A specimen of enamelled wire is treated as a capacitor, using the conductor as one electrode and as the other electrode either a coating of dried film of graphite, or a bath of molten metal. The temperature of the specimen is raised at a controlled and uniform rate and the dissipation factor (δ) is determined and plotted to produce a graph of dissipation factor (tangent delta) vs. temperature. Interpretation of the curve allows a value of temperature to be obtained which relates directly to the degree of cure of the enamel film. Alternative methods are in use, in which the specimen is cooled from a higher to a lower temperature.

A.1.1.1 Using molten metal alloy with decreas-ing temperatureAn electronic bridge allowing the value of δ to be determined directly shall be used.

An enamelled wire specimen shall be wiped clean with a soft cloth and assembled onto the fixture. The wire specimen with fixture shall be immersed for 30 s in a molten liquid metal bath pre-adjusted at the highest temperature. The specimen shall then be removed and shaken to remove excess molten alloy, cooled for approximately 10 s at room temperature, then immersed again. The specimen shall be connected to the bridge with the conductor as the one electrode and the molten liquid metal as the other. The temperature of the assembly shall be steadily decreased to give a clearly defined curve of dielectric dissipation factor vs. temperature. One test shall be conducted.

Readings of tangent delta and temperature shall be taken regularly and the results plotted in a graph with temperature on the X-axis (linear) and dielectric dissipation factor (tangent delta) on the Y-axis (log). Because the readings can vary quickly, it is preferable to


113

take the readings automatically onto a chart recorder or computer system. The use of automatic recording allows the test to be performed with a more rapid temperature rise although great care should be taken to ensure that there is no significant lag between the reading and the actual temperature. The actual equipment, temperature rise and interpretation should be agreed upon between customer and supplier.

Note: The highest temperature of molten alloy bath at which the wire specimen is inserted and tan-delta plotted on the cooling curve depends on the type of insulation and the glass-transition temperature (t

g) of the enamel.

This can be determined by pre-testing of unknown wire enamel.

A.1.2 Method B - Wire coated with a conduc-tive filmAn electronic bridge allowing the value of d to be determined directly shall be used.

The specimen shall be connected to the bridge with the conductor as the one electrode and the graphite coating as the other.

The temperature of the assembly shall be increased at a steady rate from ambient temperature to a temperature to give a clearly defined curve. The temperature shall be taken through a detector in contact with the specimen. The position of the temperature detector and the type of contact can influence the reading and different devices can give different results. Readings of tangent delta and temperature are taken regularly and the results are plotted in a graph with linear axis for temperature and logarithmic or linear axis for tangent delta. Because the readings can vary quickly it is preferable to take the readings automatically onto a chart recorder or computer system. The use of automatic recording allows the test to be performed with a more rapid temperature rise although great care should be taken to ensure that there is no significant lag between the reading and the actual temperature. The actual equipment, temperature rise and interpretation should be agreed between customer and supplier.

A.2 Interpretation of resultsThe tangent delta curve can be presented in two ways in the resulting graph. The δ value can be presented on either a linear or a logarithmic Y-axis. The calculation of the tan ( value is made in different ways for the two methods. Distinction shall be made when presenting the results as to which method has been used. The following graphs are only to be used to understand the methods and do not represent any specific requirements for materials.

A.2.1 Linear methodA tangent is drawn to the steepest part of the first ascent with rising temperature of the tangent delta versus

temperature curve. A horizontal line is drawn through a point on the curve corresponding to a temperature to be agreed between customer and supplier. The temperature corresponding to the point where this line crosses the aforesaid tangent is determined. The value is presented as tan δ = xxx °C (lin).

A.2.2 Logarithmic methodIn the case of increasing temperature, two horizontal lines are drawn from the Y- axis at values agreed between customer and supplier. A line is drawn through the intersections of these points and the curve, and extended to cross a horizontal line through the minimum value on the curve.

The temperature corresponding to the latter crossing point is determined. The value is presented as tan δ = xxx °C (log).

Figure A.2.2 - Example of logarithmic method

Figure A.2.1 - Example of linear method


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ºC + 3 ºC for 10 min (see note 1 below) (if not otherwise specified in the relevant specification).

After this heat treatment, without any bending or stretching (see note 2 below), the specimen after cooling to room temperature shall be immersed in an electrolytic solution of sodium chloride (2 g/l) added with a proper quantity of phenolphthalein alcohol solution (30 g/l) for the easy evidence of any pin holes (typically pink streams in the solution), with the conductor of the wire and the solution connected to an electrical circuit with an open-circuit d.c. test voltage of (12 + 2) V.

The voltage shall be applied for 1 min with the specimen as negative electrode relative to the solution and, in order to avoid excessive heating, the short-circuit current shall be limited to 500 mA.

The number of observed pin holes, without magnification (normal vision), shall be reported.

NOTE 1 Without heat treatment the results cannot be significant.

NOTE 2 Elongation of the wire may lead to the creation of pin holes in the electrolytic solution.

NOTE 3 Because this test is done in an aqueous solution, misleading results may be found for specific enamel types, which show crazing behaviour in water.

New workDue to very stringent requirements of the industry using enamelled wires for winding of electric machines including high frequency application, a new work for the development of ‘EXTRUDED INSULATED AND FULLY INSULATED WINDING WIRES’ was taken up in Germany. The group on fully insulated wires finalized its work at the end of 2005, drafting a specification and test method for zero-defect polyurethane class H wires. This has become a DIN pre-standard in July 2006, An English translation could be available around 2007. There are plans to propose electrical tests for TC 55 to consider. Details of the general and particular specifications and relevant electrical test methods are being reviewed.

Summary This paper has attempted to make a note of the rationalization work between IEC, NEMA & JIS standards for winding wires. Presently this is the main work with the IEC technical committee for Winding Wires. Lots of efforts by various bodies are taken to arrive at common test methods. It will be extremely helpful for the users of the specifications world over, when only one harmonized international standard is available.

Annexure A gives an updated list of IEC standards.

References

1. Minutes of meeting of IEC TC 55/WG 1.

In the case of decreasing temperature, the tangent shall be drawn to the first steep rise of the curve (see figure A.2).

Pin hole testDuring last meeting, it was decided to incorporate pin hole test as presently was described in JIS In the harmonization work, this method was found to be effective & hence it was decided to incorporate the same in IEC standards. The method described is as follows:

The intent of this test is to find insulation defects after treatment with a salt water solution. The objective of this test is similar to that of the high-voltage continuity test.

A wire specimen approximately 1,5 m in length is taken for conductors of nominal diameter less than 0,07 mm, and approximately 6 m in length for conductors of nominal diameter equal to 0,07 mm or more.

For a nominal diameter less than 0,07 mm, 1 m + 0,05 m of wire shall be wound in a round shape with a diameter of 100 mm + 50 mm.

For a nominal diameter of 0,07 mm or more, 5 m + 0,2 m of wire shall be wound in a round shape with a diameter of 300 mm + 100 mm.

The specimen is placed in an air circulation oven at 125

Figure A.2 - Example cooling curve


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fig.1 Graphite bursh assembly

Annexure A

IEC Standards for Winding Wires Status: September 2007

Table 1: Test Methods

60851-1 Am.#1 General

60851-2 Am.#2 Ed.2.1 Determination of dimensions

60851-3 Am.#2 Ed.2.1 Mechanical properties

60851-4 Ed.2.2 Chemical properties

60851-5 Ed.3.2 Electrical properties

60851-6 Am.#2 Ed.2 Thermal properties

Table 2: IEC specifications - General requirements

60317-0-1 Ed.2.2 Enamelled round copper wires

60317-0-2 Ed.2.2 Enamelled rectangular copper wire

6 0 3 1 7 -0-3

Ed.2.2 Enamelled round aluminium wires

6 0 3 1 7 -0-4

Ed.2.2 Glass-fibre wound bare or enamelled rectangular wires

6 0 3 1 7 -0-5

Glass-fibre braided enamelled rectangular wires

6 0 3 1 7 -0-6

Am.#1 Ed.1.1 Glass fibre wound, resin or varnish impregnated, bare or enamelled round copper

Table 3: IEC specifications for enamelled round copper wires (Non-solderable)

60317-1 Ed.3.2 Class 105, Polyvinyl-acetate

60317-3 Class 155, Polyester

60317-7 Am.# 2 Ed.3.2

Class 220 Poly imide

60317-8 Am.# 2 Ed.3.2

Class 180, Polyesterimide

60317-10A Am.# 1 Ed.1 TI 180, for use in refrigeration

60317-12 Am.# 2 Class 120, Polyvinyl-acetate

60317-13 Am.# 2 Ed.2.2

Class 200 Polyester (imide) + Polyamide- imide

60317-22 Class 180 PEI + Polyamide (nylon)

60317-26 Am.# 1 Class 200 Polyamide- imide

60317-34 Class 130L, Polyester

60317-37 Ed.1.2 Class 180, PEI + Bonding

60317-38 Ed.1.2 Class 200 PE(I) + PAI + Bond coat

60317-42 Class 200 Polyester-Amide-Imide

60317-45 Ed.1 Class 130, Polyester

60317-46 Class 240 Aromatic Poly imide

60317-54 Class 155L, Polyester


116

Table 4: IEC specifications for enamelled Round Copper Wires (Solderable)

60317-2 Ed. 3.2 Class 130 Polyurethane + Bonding

60317-4 Ed. 3.2 Class 130 Polyurethane

60317-19 Ed. 2.2 Class 130 PUR + Poly amide (Nylon)

60317-20 Ed. 2.2 Class 155 Polyurethane

60317-21 Ed. 2.2 Class 155 PUR + Polyamide (Nylon)

60317-23 Ed. 2.2 Class 180 PEI

60317-35 Ed. 1.2 Class 155 PUR + Bonding layer

60317-36 Ed. 1.2 Class 180 PEI + Bonding layer

60317-51 Class 180 PUR

Table 6: IEC Specifications for enamelled Rectangular Copper Wires

60317-16 Am.# 1 Class 155 Polyester

60317-17 Am.# 2 Class 105 Polyvinyl- acetate

60317-18 Class 120 Polyvinyl- acetate

60317-28 Am.# 2 Class 180 Polyesterimide

60317-29 Am.# 1 Class 200 Polyester (imide) + PAI

60317-30 Am.# 2 Class 220 Polyimide

60317-47 Class 240, Aromatic Poly-imide

Table 7: IEC Specification for enamelled Round Aluminium Wires

60317-15 Class 180, Polyesterimide

60317-25 Am.# 2 Ed. 2.2

Class 200, Polyester (imide) + PAI

Table 8: IEC Specifications for Insulated Winding Wires

60317-27 Am.#1corr.1 Paper covered

60317-31 Am.# 2Ed. 1.1

Class 180 Glass-fibre wound polyester or polyester imide varnish, silicone varnish treated

60317-32 Am.# 2Ed. 1.1

TI 155 Glass-fibre Covered polyester or polyester imide varnish treated bare or enamelled

60317-33 Am.# 2Ed. 1.1

TI 200 Glass-fibre wound silicone varnish treated bare or enamelled rectangular copper wire

60317-39 Am.# 2 TI 180 Glass-fibre braided, PE(I) varnish treated bare/enamelled rectangular wire

60317-40 Am.# 2 TI 200 Glass-fibre braided, PE(I) varnish treated bare/enamelled rectangular wire

60317-43 Class 240 Polyimide tape wrapped rect. wire

60317-44 Class 240 Aromatic Poly imide tape wrapped rect. wire

60317-48 TI 155 Glass fibre wound resin or varnish impregnated bare or enamelled round copper



60317-52 Class 220 Polyimide tape wrapped round wire

60317-53 Class 220 Polyimide tape wrapped rect. wire


117

Development of Water-soluble Polyester Enamel for Winding Wire

Uma Jejurkar, V. Shrinet, R. C. Jain, A. K. SinghElectrical Research and Development Association, Vadodara,

India

AbstractPolyester enamel available in the market has organic solvents. These solvents are not eco-friendly. Efforts are being made worldwide to reduce the volatile organic solvent content. In this study an attempt has been made to replace organic solvent by water by synthesizing water-soluble polyester enamel. Detailed experiments were carried out for different combinations of monomers such as trimellitic anhydride, phthalic anhydride, diethylene glycol, ethylene glycol, glycerol, etc. with an objective to reduce VOC content and making environment friendly material.

Winding wire was prepared by coating newly developed water-soluble polyester on bare copper wire at laboratory. The enamel was cured in the temperature range of 200-2150C for a time period of 4-5 hours. The magnet wire was evaluated as per IS-13730/ IEC-60851 for properties such as abrasion resistance, peel strength, cut through, mandrel winding, etc. It was observed that this water-soluble enamel complies with requirements of all tests.

IntroductionA large number of organic solvents are widely used in manufacturing enamels, varnishes paints, inks, coating compositions, adhesives textile auxiliaries, paper products, etc. Wire enamels manufactured for continuous operation at temperatures above 1500C require aromatic solvents such as cresol, cresylic acid, phenol, hydrocarbon diluents, etc. which are very harmful in nature. Recently, it has become very difficult to use a large amount of organic solvents in view of saving oil resources and preventing environmental pollution. These solvents also pose a disposal problem apart from fire and health hazards. Therefore, alternatives to these solvent based magnet wire coating compositions are being sought which will reduce the emission of volatile organic compounds (VOCs) in the environment and at the same time provide good electrical

and mechanical properties. Some of the alternatives to solvent-based coating system are high-solid coatings, non-solvent coatings, aqueous dispersions, and water-soluble coatings.

The conventional resins used for these compositions are mostly hydrophobic so, it is difficult to disperse them into water or to make them compatible.

The work mentioned here is on synthesis of water-soluble polyester resin, its application on magnet wires and evaluation of properties as per IS:13730.

Alternatives to Solvent Based Coatings The conventional wire enamels and insulating varnishes contain solvents like xylene, m-cresol, and phenol to the extent of 50-70 %. These solvents get evaporated completely during application process causing atmospheric pollution, health hazards, toxicity and economic losses. Therefore, many industries are diverting their research to find suitable alternatives for solvent-based systems. There are attempts to modify existing products or produce new ones that are solvent-free, or reduced volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Waterborne systems reduce the emissions of VOCs while powder coatings eliminate the use of solvents that are recycled. High solids coatings utilize the existing production lines for application and reduce the emission of VOCs. Therefore, powder coatings, water-borne coatings and high solid coatings are some of the prominent areas to carry development in the field of insulating systems.

1) High Solid Coatings - High solids coatings contain less solvent than conventional solvent borne coatings. These polymers are, generally, low molecular weight than their conventional solvent borne coatings.

2) Powder Coatings - Powder coatings generally consist of binders in solid form along with pigments, curing agents and additives.

C. N. Murthy,The M. S. University of Baroda, Vadodara, India


118

3) Water-borne Coatings - Water-borne system is another class of resin composition where solvents are totally replaced by water.

Generally, water-borne varnishes are of two types -

(1) Water Soluble

(2) Water Dispersible

Water-based polymers used in electrical insulation applications require heat cure to cross-link the polymer and remove the last traces of residual water. Water-based varnishes are used in impregnating stators and transformers. These products not only offer workers safety and low VOC’s, but also give excellent electrical and mechanical properties.

Preparation of ResinSynthesis of water-soluble polyester resin was carried out by condensation polymerization reaction of anhydrides such as trimellitic anhydride, phthalic anhydride, and glycols of the type diethylene glycol, ethylene glycol, glycerol, etc. The anhydride content was varied in the range of 1.0-1.5 moles whereas the glycol content was varied in the range of 1.0-1.8 to obtain the most suitable composition. The resin was rendered water-soluble by addition of ammonia at a suitable acid value.

A coating formulation of this resin was prepared by diluting the resin with distilled water till a viscosity of about 40-43 seconds was obtained using Ford Cup Viscometer (No.4). Further, the coating was formulated by addition of different additives to achieve the desired properties. This developed water-soluble polyester was then coated on annealed copper wire of 0.315 + 0.004 mm diameter and cured in the temperature range of 400-415 0C at the wire drawing speed of 15 m/min. After seven passes, the magnet wire obtained was evaluated as per IS-13730/ IEC-60851 for properties such as break down voltage, abrasion resistance, cut through, mandrel winding, heat shock, etc. and the results are reported in the Table 1.

Results and DiscussionThe water-soluble polyester obtained from this process is a completely transparent, dilutable with water, does not smell of ammonia and shows superior properties as compared to amine terminated water based resins. Moreover, these resins are cheaper and have negligible VOC level. These resins are applied to the copper wire by conventional coating systems and prove to be comparable to the conventional solvent based varnish.

The thermal class of these coatings was determined using DiCerbo and Toop Methods and it was observed that the water-soluble polyester belongs to the Class-F (1550C) of the electrical insulation.

As it can be seen from the Table 1, the water-soluble

polyester resin conforms to the standard IS:13730 Part-3 and Part-34 for the properties such as break down voltage, jerk test, heat shock test, cut through test, abrasion resistance test and mandrel winding test.

ConclusionThe water-soluble polyester resins were formulated to make wire enamels for electrical applications. These products were tested for break down voltage, cut-through, abrasion resistance, jerk test, mandrel winding, etc. as per IS-13730. The test results confirm that these materials meet the required standard. Therefore, they are suitable for the magnet wire application. The insulating varnish provides a baked coating that has electrical, mechanical and thermal properties comparable to those of the conventional solvent-type varnishes.

Further, as described above, the insulating varnish uses water as a thinner, which is inexpensive and is free from air pollution which is generally caused by the solvent vapours or harmful gases at the time of baking. Use of these coatings also prevents from the danger of fire or explosion.

AcknowledgementAuthors are grateful to Shri Dilip Oswal of M/s Micro Inks Limited, Vapi for his kind support in preparing enameled wire using water-soluble polyester at their plant for this study.

REFERENCES1) Kotera et. al., U. S. Patent - 4340519, 1982.

2) Dettling CJ, U.S. Patent - 2974060, 1961.

3) Gemmer E, U.S. Patent - 2974059, 1961.

4) Ishizuka T, and Miwa M, U.S. Patent - 3936404, 1976.

5) Raghavan D, Paint India, 142, Sept. 2005.

6) Shiro M, and Hiroshi S, U. S. Patent - 4576990, 1986.

7) Baraskar NV, Paintindia, 73, Jan. 2007.

8) Shenoy MA, Therattil JJ, and ParikhPF, ELROMA-04, Session-4, Paper-4, IV-23-26, 2004.

9) MICRO-DISPERSION TM A NEW WATERBORNE TECHNOLOGY, Inter-national Waterborne, High-Solids, and Powder Coatings Symposium, Feb. 26-28, 2003.

10) Finch CA, Industrial Water Soluble Polymers, The Royal Society of Chemistry, 1996.

11) Jejurkar U, Shrinet V, Murthy CN, Jain RC, Singh AK, Ramamoorty M, J. Appl. Polym. Sci., 104(5), 3309, 2007.


119

Table 1. Properties of Water-Soluble Polyester Enamel on Copper Wire

Sr. No. Clause No. Tests/ Properties Requirements as per IS: 13730 (Part-34)

Requirements as per IS: 13730 (Part-3)

Obtained Values Remarks

1 4 Dimensions

4.1 Conductor diameter, mm

0.315 + 0.004 0.315+ 0.004 0.314 conforms

4.4 Overall diameter, mm [For Grade-I of Polyester Enamel] 0.349 (min.)

0.349 (min.) 0.369 (max.)

0.369 (max.) 0.340 conferm

2 6 Elongation at fracture, %

23 (min.) 23 (min.) 26 Conforms

3 8 Flexibility and adherence

8.1 Mandrel winding test

No cracks visible to naked eyes


No cracks visible to naked eyes Conforms

8.3 Jerk test No cracks visible to naked eyes


No cracks visible to naked eyes Conforms

4 9 Heat shock No cracks visible to naked eyes at

155 0C

No cracks visible to naked eyes at

175 0C

No cracks visible to naked eyes at

1750C

Conforms

5 10 Cut through test at 240 (C

Should withstand 4.5N load, 100V for two minutes

Should withstand 4.5N load, 100V for two minutes

Pass Conforms

6 11 Resistance to abrasion

Force for failure, N 2.65 (Min.) 3.15 (Avg.)

2.65 (Min.) 3.15 (Avg.)

5.8 Conforms

7 13 Breakdown Voltage, Volts-rms

(a) At Room Temperature

2200 2200 3800 Conforms

(b) At Class Temperature

1700 at 1300C 1700 at 1550C 2100 at 1550C Conforms


c m y k

105

Graph 4. Conductor Temperature ( in deg.C) Vs Duct Temperature (In deg C)


to be 640C, when two wires were loaded. Whereas for single wire loading for a conductor temperature of 630Cthe duct temperature was observed to be 390C.

It is important to note that PVC conduit pipe generally used are of general-purpose grade for a maximum temperature of 700C . For higher temperature of duct it is required to use Heat resisting conduit pipes which can withstand high temperatures without damaging the PVC wires enclosed within the conduit pipe.

Conductor size 1.5 sq.mm.

If we study the effect of current density on 1.5sq mm conductor size the increase in duct temperature will be very high at higher current density. For a current density of 10 A/sq. mm, the duct temperature is only 340C for a conductor temperature of 460C. If we study the same at a current density of 16.7 A/sq. mm which is the value for a general purpose PVC wires, the duct temperature is 550C for a conductor temperature of 720C. At 550Cduct temperature the wire which is enclosed within the duct, are not subjected to ageing compared to higher duct temperatures.

When the current density is raised to 20 A/sq. mm, then the duct temperature will raise. For example the duct temp. is 730C for a conductor temperature of 1050C as shown in graph 4.

If the temperature in the duct is high around 60-650Cthen the ageing of the wire takes place. This becomes the prevalent ambient temperature for the wire running in this duct which leads to accelerated ageing of the insulation of these wires. It is preferable to use HRPVC wires wherever the temperature of the duct are high, as HRPVC has got better thermal properties compared to the general purpose PVC insulation. Or it is preferable to decrease the current density or to use higher cross section of wires to reduce the ageing effect.

Conduit PipeIn the actual installation different sizes of PVC conduit

pipes are used. The most commonly used sizes are the pipes with outer diameter of 20mm & 25mm.

It is opined that for a higher diameter conduit pipe the temperature of the conductor will be lower compared to a conduit pipe of lower diameter. But in experimental results the temperature of the conductor enclosed in a conduit pipe of higher diameter is higher than the same conductor placed in a lower diameter conduit pipe.

The above fact is justified because the thermal resistivity of air is lower. The ambient air, which is at low temperature, is cooling the outer surface of the PVC conduit pipe. For a 4.0sq mm conductor wire for a current of 50 Amps, the temperature of the conductor is 730C for 20mm diameter conduit pipe whereas the temperature is 780C for 25mm diameter conduit pipe. This is shown in graph 5.

Protection Suitable protection for the circuits is to be used in conjunction with the wires. The protection devices can be either rewireable fuses or MCB. The maintenance of rewireable fuses or the HRC fuses are time consuming and as on date it has been replaced by MCB’s as they are fast operating devices and the characteristics can be selected depending upon the requirements.

For a current of two times the rated current (2 In ) MCB

will operate in 20 seconds. For a current of three times the rated current (3 I

n ) MCB will operate in 8 seconds and

for 6 In it operates in 2 seconds. By taking into account the characteristic of MCB’s the short circuit faults can be cleared faster.

ConclusionThe current carrying capacity for house wiring PVC insulated wires are arrived at by selecting the appropriate

Graph 5. Conductor Temperature ( in deg.C) Vs Current (In Amps)

113

take the readings automatically onto a chart recorder or computer system. The use of automatic recording allows the test to be performed with a more rapid temperature rise although great care should be taken to ensure that there is no significant lag between the reading and the actual temperature. The actual equipment, temperature rise and interpretation should be agreed upon between customer and supplier.

Note: The highest temperature of molten alloy bath at which the wire specimen is inserted and tan-delta plotted on the cooling curve depends on the type of insulation and the glass-transition temperature (tg) of the enamel. This can be determined by pre-testing of unknown wire enamel.

A.1.2 Method B - Wire coated with a conduc-tive filmAn electronic bridge allowing the value of d to be determined directly shall be used.

The specimen shall be connected to the bridge with the conductor as the one electrode and the graphite coating as the other.

The temperature of the assembly shall be increased at a steady rate from ambient temperature to a temperature to give a clearly defined curve. The temperature shall be taken through a detector in contact with the specimen. The position of the temperature detector and the type of contact can influence the reading and different devices can give different results. Readings of tangent delta and temperature are taken regularly and the results are plotted in a graph with linear axis for temperature and logarithmic or linear axis for tangent delta. Because the readings can vary quickly it is preferable to take the readings automatically onto a chart recorder or computer system. The use of automatic recording allows the test to be performed with a more rapid temperature rise although great care should be taken to ensure that there is no significant lag between the reading and the actual temperature. The actual equipment, temperature rise and interpretation should be agreed between customer and supplier.

A.2 Interpretation of resultsThe tangent delta curve can be presented in two ways in the resulting graph. The value can be presented on either a linear or a logarithmic Y-axis. The calculation of the tan ( value is made in different ways for the two methods. Distinction shall be made when presenting the results as to which method has been used. The following graphs are only to be used to understand the methods and do not represent any specific requirements for materials.

A.2.1 Linear methodA tangent is drawn to the steepest part of the first ascent with rising temperature of the tangent delta versus

temperature curve. A horizontal line is drawn through a point on the curve corresponding to a temperature to be agreed between customer and supplier. The temperature corresponding to the point where this line crosses the aforesaid tangent is determined. The value is presented as tan = xxx °C (lin).

A.2.2 Logarithmic methodIn the case of increasing temperature, two horizontal lines are drawn from the Y- axis at values agreed between customer and supplier. A line is drawn through the intersections of these points and the curve, and extended to cross a horizontal line through the minimum value on the curve.

The temperature corresponding to the latter crossing point is determined. The value is presented as tan = xxx °C (log).

Figure A.2.2 - Example of logarithmic method

Figure A.2.1 - Example of linear method

New Developments in International Standards for Winding Wires70


Pekka Huotari, Dr. Tech.Maillefer Extrusion Oy, Finland

IntroductionMedium voltage (MV), high voltage (HV) and extra high voltage (EHV) underground cables today are generally plastic-insulated and cross-linked. Cross-linking is performed together with insulation on catenary CV-lines (CCV), vertical CV-lines (VCV) or, rarely, on so-called Mitsubishi-Dainitchi-lines (MDCV). Cross-linking offers higher current and thermal loads without softening the layers of insulation. Most new CV-lines are radiant curing lines comprising a nitrogen atmosphere inside a water- or gas-cooled curing tube. The radiant curing process is already 30 years old and many technical developments has been made to improve both the efficiency of the CV-line and product quality.

Qualitatively speaking, the most important achievements have been the triple extrusion cross-head and x-ray measurement for layer thickness and centricity. Both also improve efficiency since the triple cross-head makes it possible to use effective inductive conductor pre-heating while x-ray measurement reduces start scrap and allows smaller layer dimensional tolerances. In some cases, gravimetric control is used to maintain layer thicknesses within tolerance.

Purity plays an important role in HV and EHV cable insulation. Cleanliness is typically ensured by careful material handling prior to extrusion, clean room conditions and extruder screens. Granule purity can also be scanned prior to extrusion and insulation purity form melt prior to cross-head.

Heavy wall cables used to be insulated on either VCV- or MDCV-lines, but for the last decade this has also been possible on CCV-lines. For EHV cable insulation, VCV-lines are still often preferred and the line speed of VCV-lines has been increased by using inductive conductor heating, known as postheating, after the cross-head.

CV-line automation typically consists of a distributed system with PLCs, operator panels and process interface connected together via a field bus. The Curing Calculation program, which numerically optimizes the cross-linking process, still plays a vital role since it is not possible to measure cross-linking or core temperature inside CV-tubes.

Increasing Vcv-line ProductivityVCV-lines (fig 1) are used for insulating and cross-linking HV and EHV cores. Since CV-tower construction costs are

remarkably high, the height of the tower and, consequently, line speed are limited. As extruders can normally satisfy VCV-line output needs, the problem is mainly limited to curing and cooling capacity.

To fully exploit the layout possibilities, pressurized turn pulleys are normally used on VCV-lines to extend cooling length. To improve cross-linking, inductive conductor heating can be effectively implemented by using what is known

as a postheater. Typical preheating temperatures on VCV-lines today range from 60 to 100

0C, limited, for instance,

due to copper oxidation, conductor tape deformation or cross-head overheating. The post-heater (fig 2) is located

Figure 1 Vertical CV-line (Maillefer)


From

3 Back

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Name : Mr. Shirish B. Agarkar

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Cablewire 2008 Technical Papers

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