Training Report

On

220 kV Substation, Vasant Kunj

Submitted By: Vishal Kaushik (8EE131)

Yogendra Krishan (8EE133)

Sec. B

7th Semester

Jun-Jul 11

Dept. of Elec. & Elect. Engg.

Lingaya’s Instt. Of Mgmt. & Tech.

ACKNOWLEDGEMENT

I would like to take this opportunity to express my profound sense of gratitude and respect to all those who helped me throughout the duration of my training. I acknowledge the efforts of those who have contributed significantly to my training. I feel privileged to offer my sincere thanks and deep sense of gratitude to Mr. Sameer Bidani, Astt. Manager, 220 kV substation, Vasant Kunj, Delhi Transco Limited for helping and guiding me throughout my training period.

ABSTRACT

A well planned, systematically executed industrial training helps a great deal in inculcating a good work culture. It provides a linkage between students and the industry in order to develop awareness of the industrial approach to problem solving based on broad understanding of operations of the industrial organizations.

During my training in NHPC I learned a lot about the various aspects of power generation and came to know about the techniques used practically for generating the electrical energy.

My training has been an enriching experience for me in the field of power generation and I learned a lot about the practical application of my

knowledge of various subjects of electrical engineering in industry.

INDEXIntroduction to electricity..……………………………………………………………………………………………………….…………1

1. Introduction To DTL…………………………………………………………………………………………………………..……2

1.1DTL at a glance…………………………………………………………………………………………………………….…………2

1.2Quality Policy…………………………………………………………………………………………………………………….…..3

1.3Quality Objective…………………………………………………………………………………………………………………...3

1.4Transmission Network of DTL.…………………………………………………………………………………………………3

1.5Proposed Substations………………………………………………………………………………………..…………………..6

2. Transmission and distribution in India…………………………………………………………………………………….7

2.1Transmission……………………………………………………………………………………………………………….…….…..7

2.2Distribution…………………………………………………………………………………………………………………………….8

2.3Mission-Power for all by 2012……………………………………………………………………………………….……….9

2.3.1 Objectives…………………………………………………………………………………………………………………………9

2.3.2 Strategies…………………………………………………………………………………………………………………………9

3. 220 kV Substation Vasant Kunj………………………………………………………………………………………..……11

3.1Salient Features…………………………………………………………………………………………………………………….11

3.2Technical Specifications………………………………………………………………………………………………………..12

3.3Layout Plan………………………………………………………………………………………………………………………..…15

4. Equipments of Substation..………………………………………………………………………..…………………………16

4.1Main Transformer.………………………………………………………………………………………………..………………16

4.2Lightning Arrestor…………………………………………………………………………………………………………………23

4.3Isolator……………………………………………………………………………………………………………………………...…27

4.4Circuit Breaker……………………………………………………………………………………………………………….…….28

4.5Current Transformer……………………………………………………………………………………………………….…….37

4.6Wavetrap………………………………………………………………………………………………………………………………42

4.7Power Transformer & Capacitive Voltage Transformer………………………………………………….……..43

4.8Busbar…………………………………………………………………………………………………………………………….……46

4.10 Battery Charger & Battery Bank…………………………………………………………………………………………..47

INTRODUCTION

“Electricity” means only one thing: it’s the electrons and protons, the electric charge as described by the scientists.

The first usage of the word electricity is ascribed to Sir Thomas Browne in his 1646 work, Pseudodoxia Epidemica. The word electricity was derived from a latin word electricus which itself was derived from a word elektron(a greek word for “amber”).

But electricity in everyday life can be described as the electromagnetic field energy sent out by batteries and generators or we can say it is just a directional flow of electrons…or simply electrical energy.

This movement of electrons is used by us as a source of energy in a large number of ways like-

1) These electrons can produce heat and light while passing through tungsten wires.

2) These electrons can run electric motors using the principle of electro-magnetics which are used in fans, water pumps, lifts, etc.

3) These moving electrons can turn on various LED’s, processors, etc. which are used in various appliances ranging from sign boards to TV’s to computers.

So we need to produce electricity for our daily requirement of energy.

This electricity is generally produced by the principle of electromagnetism in which some mechanical energy (which may be obtained from steam, running water, wind, etc.) is converted into electrical energy with the help of generators. These generating units can’t be constructed just with the load always due to many reasons like safety or sources providing mechanical energy are not available there or there may be many other reasons. So to distribute the electrical energy in an economic and efficient manner we always need a well developed transmission and distribution system.

This electrical energy is supplied to the load end from the generating unit with the help of conductors. To reduce the transmission losses, the electrical energy is sent at very high values from the generating units and then again these voltage levels are stepped down to lower voltages at the consumer end. Transformers are used for this purpose.

The substations are constructed at different places to check the proper functioning of such transmission and distribution networks.

1. Introduction to DTL

1.1 DTL at a glance

Delhi Transco Limited is the State Transmission Utility for the National Capital Territory of Delhi. It is responsible for the transmission of power at 220 kV and 400 kV level and for upgrading, operating and maintaining the high voltage network.

On July 1, 2002, The Delhi Vidyut Board (DVB) was unbundled into six successor companies: Delhi Power Supply Company Limited (DPCL) - Holding Company; Delhi Transco Limited (DTL) - TRANSCO; Indraprastha Power Generation Company Limited (IPGCL) - GENCO; BSES Rajdhani Power Limited (BRPL) - DISCOM; BSES Yamuna Power Limited (BYPL) - DISCOM; North Delhi Power Limited (NDPL) - DISCOM. DTL and IPGCL, are wholly owned by the Delhi Government. Delhi Transco Limited is a 'State Transmission Utility of the National Capital of Delhi.

Over the years, DTL has evolved as a most dynamic performer, keeping pace with the many-fold challenges that confront the ever increasing demand-supply-power-situation and achieving functional superiority on all fronts. The Transmission losses have been brought down from 3.84% in 2002-03 to 0.83% in 2006-07, and are the lowest in the country. Delhi, being the capital of India and the hub of commercial activities in the Northern Region, coupled with the prosperity of population, the load requirement has been growing at a much faster pace. Added to that, being the focus of socio-economic and political life of India, Delhi is assuming increasing eminence among the great cities of the world. Plus the vision-2021, aiming to make Delhi a global Metropolitan and world class city demands greater infrastructure to enrich many services of infrastructure development.

DTL, has been responsibly playing its role in establishing, upgrading, operating and maintaining the EHV (Extra High Voltage) network. DTL has

also been assigned the responsibility of running the State Load Dispatch Centre which is an apex body to ensure integrated operations of power systems in Delhi.

Care for Environment

DTL operates its obligations in a clean, green pollution free environment and, has been providing more green coverage to the National Capital. It is spreading awareness among the masses to use eco-friendly electrical appliances. DTL is also introducing Energy Conservation Building Code in Delhi to maximize the use of natural resources and minimize the use, of electricity. Its proposed corporate office at 400 KV Sub Station Maharani Bagh will be a Green Building. The building is aimed to be a Platinum Rated Green Building. More than one lakh sq meter land has been earmarked for plantation in the ensuing year.

1.2 Quality Policy

Delhi Transco Limited is committed to:

• Establish and maintain an efficient, effective and reliable EHV Grid network for Transmission of power in Delhi to the satisfaction of Licensees and stake holders.

• Continual improvement in capacity, performance and availability of the system.

• Employing advance technology and management practices in a cost effective manner with due social concern.

• Ensuring quality standards of the work and in conformity to be applicable Statutory and Regulatory requirements.

1.3 Quality Objective

Delhi Transco Limited has set following quality objectives:

• To ensure grid safety and stability.• To enhance capacity and performance of the system.• To improve availability of the system.• To ensure the quality standard of the work.• To enhance competency and productivity of employees.

• To maximize stake holders’ delight.

1.4 Transmission Network of DTL

Existing Transmission Network

The existing network of DTL consists of a 400KV ring around the periphery of Delhi interlinked with the 220KV network spread all over the city. Summary of Transmission of DTL is as given below.

Parameters400KV Level

220KV Level

No. of Substations 3 26

Transmission Capacity (in MVA) 3465 7860

Transmission Lines (length in Ckt. Km.)

227574.2 + 40.206 (underground)

Power Arrangements of Delhi

DTL had been arranging power from various sources for all the five distribution licensees since 1 July 2002. Keeping in mind the Commonwealth Games 2010 it had signed Power Purchase Agreements for more than 9000 MW of power. This arrangement continued till 31 March 2007. From 1 April 2007 onwards all the distribution agencies are directly purchasing power and all the long and short term Power Purchase Agreements have been transferred to these agencies by Delhi Electricity Regulatory Commission (DERC) on the basis of their consumption. Now it is the responsibility of the distribution companies to arrange power for their respective areas. However a Power Procurement Group has been formed to coordinate the procurement and sale of power which is headed by a DTL Officer. Now DTL is responsible only for efficient transmission of power.

Power Supply Position of Delhi Over the Years

There has been considerable improvement in the power supply position at the end of five years of restructuring of power sector of Delhi. The peak demand is increasing every year while the load shedding has reduced tremendously.

Parameters2002-

032003-

042004-

052005-

062006-

072007-

082008-09 2009-10

Peak Demand met in MW

3097 3289 3490 3626 3736 4030 4034 4408

Energy consumption in MUs

19686 20385 20810 21184 21977 22372 22006 23349

Shedding, in MUs

450 229 176 322 411 136 128 185

Shedding as %age of Energy Consumption

2.29% 1.12% 0.84% 1.50% 1.87% 0.61% 0.58% 0.80%

Transmission losses (in %age)

3.84 %1.69% 1.30% 0.72% 0.83% 0.95%Data not available

Data not available

Revenue Statistics

Year2004-

052005-

062006-

072007-

082008-09

Revenue (INR billions) 46.61 52.10 57.39 2.45 3.92

Profit after tax (INR billions)

(9.55) 0.94 0.35 0.52 0.63

Transmission Load Forecast for 11th Plan

As per the projections made by Central Electricity Authority in its 17th EPS report and the recommendations for the transmission requirements for Delhi systems in the 11th Plan System Studies report of Central Electricity Authority for the period 2007-2012.

Transmission Load Forecast for Delhi

As per Statistics published on DTL's website:

Year

2008-09

(MW)

2009-10

(MW)

2010-11

(MW)

2011-12

(MW)

Anticipated Demand as per 17th Electric

Power Survey of CEA

4877 5253 5657 6092

Anticipated Demand calculated as per CEA’s Grid Monitoring

Methodology on the basis of previous years Data

4034 4425 4626 4837

Anticipated Demand calculated as per the Trend of

Actual of last 10 years

4034 4255 4489 4735

Anticipated Demand as per 17th Electric Power survey (MW)

The year wise anticipated demand of Delhi as per 17th Electric Power survey of CEA is indicated in the above mentioned table from which it is evident that there is a 7 to 8 % increase yearly in the demand of power in Delhi during 2008-12.

Existing Substations

Bawana 400kV Mundka 400kV Bamnauli 400kV Narela 220kV Rohini 220kV Subzi Mandi 220kV

Ridge Valley 220kV Najafgarh 220kV Sarita Vihar 220kV Geeta Colony 220kV DAIL 220kV Mehrauli 220kV Vasant Kunj 220kV Pappankalan - I 220kV IP Estate 220kV Gazipur 220kV Lodhi Road 220kV Patparganj 220kV South of Wazirabad 220kV Kashmere Gate 220kV AIIMS 220kV Okhla 220kV Naraina 220kV DSIDC Bawana 220kV Gopalpur 220kV Pappankalan - II 220kV Parkstreet 220kV Shalimar Bagh 220kV Kanjhwala 220kV

1.5 Proposed Substations

East of Loni (Harsh Vihar) 400 kV Electric Lane 220 kV Masjid Moth 220 kV Peeragarhi 220 kV Wazirpur 220 kV Rohini 220 kV

2. Transmission and Distribution in India

2.1 Transmission

Transmission of electricity is defined as bulk transfer of power over a long distance at high voltage, generally of 132kV and above. In India bulk transmission has increased from 3,708ckm in 1950 to more than 165,000ckm today(as stated by Power Grid Corporation of India). The entire country has been divided into five regions for transmission systems, namely, Northern Region, North Eastern Region, Eastern Region, Southern Region and Western Region. The Interconnected transmission system within each region is also called the regional grid.

The transmission system planning in the country, in the past, had traditionally been linked to generation projects as part of the evacuation system. Ability of the power system to safely withstand a contingency without generation rescheduling or load-shedding was the main criteria for planning the transmission system. However, due to various reasons such as spatial development of load in the network, non-commissioning of load center generating units originally planned and deficit in reactive compensation, certain pockets in the power system could not safely operate even under normal conditions. This had necessitated backing down of generation and operating at a lower load generation balance in the past. Transmission planning has therefore moved away from the earlier generation evacuation system planning to integrate system planning.

While the predominant technology for electricity transmission and distribution has been Alternating Current (AC) technology, High Voltage Direct Current (HVDC) technology has also been used for interconnection of all regional grids across the country and for bulk transmission of power over long distances.

Certain provisions in the Electricity Act 2003 such as open access to the transmission and distribution network, recognition of power trading as a distinct activity, the liberal definition of a captive generating plant and

provision for supply in rural areas are expected to introduce and encourage competition in the electricity sector. It is expected that all the above measures on the generation, transmission and distribution front would result in formation of a robust electricity grid in the country.

2.2 Distribution

The total installed generating capacity in the country is over 148,700MW and the total number of consumers is over 144 million. Apart from an extensive transmission system network at 500kV HVDC, 400kV, 220kV, 132kV and 66kV which has developed to transmit the power from generating station to the grid substations, a vast network of sub transmission in distribution system has also come up for utilization of the power by the ultimate consumers.

However, due to lack of adequate investment on transmission and distribution (T&D) works, the T&D losses have been consistently on higher side, and reached to the level of 32.86% in the year 2000-01.The reduction of these losses was essential to bring economic viability to the State Utilities.

As the T&D loss was not able to capture all the losses in the net work, concept of Aggregate Technical and Commercial (AT&C) loss was introduced. AT&C loss captures technical as well as commercial losses in the network and is a true indicator of total losses in the system.

High technical losses in the system are primarily due to inadequate investments over the years for system improvement works, which has resulted in unplanned extensions of the distribution lines, overloading of the system elements like transformers and conductors, and lack of adequate reactive power support.

The commercial losses are mainly due to low metering efficiency, theft & pilferages. This may be eliminated by improving metering efficiency, proper energy accounting & auditing and improved billing & collection efficiency. Fixing of accountability of the personnel / feeder managers may help considerably in reduction of AT&C loss.

With the initiative of the Government of India and of the States, the Accelerated Power Development & Reform Programme (APDRP) was launched in 2001, for the strengthening of Sub – Transmission and Distribution network and reduction in AT&C losses.

The main objective of the programme was to bring Aggregate Technical & Commercial (AT&C) losses below 15% in five years in urban and in high-density areas. The programme, along with other initiatives of the Government of India and of the States, has led to reduction in the overall AT&C loss from 38.86% in 2001-02 to 34.54% in 2005-06. The commercial loss of the State Power Utilities reduced significantly during this period from Rs. 29331 Crore to Rs. 19546 Crore. The loss as percentage of turnover was reduced from 33% in 2000-01 to 16.60% in 2005-06.

The APDRP programme is being restructured by the Government of India, so that the desired level of 15% AT&C loss could be achieved by the end of 11th plan.

2.3 Mission- Power for ALL by 2012

The Government of India has an ambitious mission of POWER FOR ALL BY 2012. This mission would require that the installed generation capacity should be at least 200,000 MW by 2012 from the present level of 144,564.97 MW. Power requirement will double by 2020 to 400,000MW.

2.3.1 Objectives

• Sufficient power to achieve GDP growth rate of 8% • Reliable power • Quality power • Optimum power cost • Commercial viability of power industry • Power for all

2.3.2 Strategies

• Power Generation Strategy with focus on low cost generation, optimization of capacity utilization, controlling the input cost, optimization of fuel mix, Technology up gradation and utilization of Non Conventional energy sources.

• Transmission Strategy with focus on development of National Grid including Interstate connections, Technology up gradation & optimization of transmission cost.

• Distribution strategy to achieve Distribution Reforms with focus on System up gradation, loss reduction, theft control, consumer service

orientation, quality power supply commercialization, Decentralized distributed generation and supply for rural areas.

• Regulation Strategy aimed at protecting Consumer interests and making the sector commercially viable.

• Financing Strategy to generate resources for required growth of the power sector.

• Conservation Strategy to optimize the utilization of electricity with focus on Demand Side management, Load management and Technology up gradation to provide energy efficient equipment / gadgets.

• Communication Strategy for political consensus with media support to enhance the general public awareness.

3. 220 kV Substation Vasant Kunj

3.1 Salient Features

3.2 Technical Specifications

3.2.1 Transformers

Sub-Station transformer is used in sub stations to transfer the incoming voltage to the next voltage level. It can be system or auto transformer with two/three windings. In general it is equipped with On load tap changers and are connected to transmission grids by bushings and cables.

The system/auto transformer is built in core form. HV/LV windings are galvanically separated for system transformer while they are Auto connected for auto transformer.

Type of Transformer used: Auto Transformer

Power: 160MVA

Type of Connection: Primary winding- Star connection

Secondary winding- Star connection

Tertiary winding- Delta connection

Percentage impedance: Tap no. 1- 10.47%

Tap no. 5- 11.79% (Used most widely)

Tap no. 17- 16.5%

Capacity: ONAN ONAF OFAF

96000MVA 128000MVA 160000MVA

Vector Symbol: YNynod11

Manufactured by: Bharat Bijlee Ltd.

Protection System: Nitrogen Injection Fire Protection System

Supply Voltage: 230V AC

Cylinder Capacity: 10 m3

Drain Pipe Size: DN 125

Manufactured by: CTR MFG. IND. LTD.

3.2.2 Lightning arrestor

A lightning arrestor (or surge protector) is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold.

Type of LA used: Metal Oxide (Zinc Oxide)

Ratings: 198 kV, 10 kA for 220 MVA

60 kV, 10 kA for 66 MVA

Winding Voltage: ONAN ONAF OFAF

HV Side- 252.0 336.0 420.0

LV Side- 839.7 1139.7 1399.6

Tertiary Side- 2799.3 2799.3 2799.3

3.2.3 Isolator

High-voltage isolators are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance. The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device.

Type of Isolator used: HLM (In both 220 and 66 MVA)

SMC (In 66 MVA)

Type of Break used: Central Break (In 220 MVA)

Twin Break (In 66 MVA)

3.2.4 Circuit Breaker

Type of CB used: Sulphor Hexafluoride

Rated Voltage: 245 kV

Rated Lightning Impulse Withstand Voltage: 1050 kV

Rated Breaking Current: 40 kA

Gas Pressure: 7 kg/cm2 at 20°C

Rated Normal Current: 3150 A

3.2.5 Current Transformer

A current transformer (CT) is used for measurement of electric currents. Current transformer is also known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

Type of CT used: Dead Tank CT

Number of Cores: 5 Cores

Quantity of Oil used: 240 L

Rated Current: 800 A

3.3 Layout Plan

4. Equipments of Substation

4.1 Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding.

Autotransformer

An autotransformer (sometimes called autoformer) is an

electrical transformer with only one winding. The auto prefix refers to the

single coil acting on itself rather than any automatic mechanism. In an

autotransformer portions of the same winding act as both

the primary and secondary. The winding has at least three taps where

electrical connections are made. An autotransformer can be smaller, lighter

and cheaper than a standard dual-winding transformer however the

autotransformer does not provide electrical isolation.

Autotransformers are often used to step up or down between voltages in the

110-117-120 volt range and voltages in the 220-230-240 volt range.

An autotransformer has a single winding with two end terminals, and one or

more terminals at intermediate tap points. The primary voltage is applied

across two of the terminals, and the secondary voltage taken from two

terminals, almost always having one terminal in common with the primary

voltage. The primary and secondary circuits therefore have a number of

windings turns in common. Since the volts-per-turn is the same in both

windings, each develops a voltage in proportion to its number of turns. In an

autotransformer part of the current flows directly from the input to the

output, and only part is transferred inductively, allowing a smaller, lighter,

cheaper core to be used as well as requiring only a single winding.

One end of the winding is usually connected in common to both the voltage

source and the electrical load. The other end of the source and load are

connected to taps along the winding. Different taps on the winding

correspond to different voltages, measured from the common end. In a step-

down transformer the source is usually connected across the entire winding

while the load is connected by a tap across only a portion of the winding. In a

step-up transformer, conversely, the load is attached across the full winding

while the source is connected to a tap across a portion of the winding.

Advantages of autotransformer

• Saving of conductor material if autotransformer is used, K times saving with respect to two-winding transformer.

• The losses are reduced by (1-K) factor.

• Superior voltage reduction and has lower value of leakage impedance.

• Rating is increased by 1/ (1-K) times.

• Impedance is decreased by (1-K) factor.

Short circuit test

The purpose of short circuit test is to determine the series branch

parameters of the equivalent circuit. As the name suggests, in this test

primary applied voltage, the current and power input are measured keeping

the secondary terminals short circuited. Let these values be Vsc, Isc and

Wsc respectively. The supply voltage required to circulate rated current

through the transformer is usually very small and is of the order of a few

percent of the nominal voltage. The excitation current which is only 1% or

less even at rated voltage becomes negligibly small during this test and

hence is neglected. The shunt branch is thus assumed to be absent. Wsc is

the sum of the copper losses in primary and secondary put together.

The reactive power consumed is that absorbed by the leakage reactance of

the two windings.

For carrying short circuit test on power transformer:

Isolate the power transformer from service.

Remove HV/LV jumps and disconnect neutral from earth/ground.

Short LV phases and connect these short circuited terminals to neutral.

Energize HV side by LV supply.

Measure current in neutral, LV line voltages, HV voltage and HV line

currents.

If neutral current is near to zero transformer windings are operational.

If neutral current is higher or equal to line current between LV phases one of

the winding is open.

Open circuit test

The Open circuit test, or "no-load test", is one of the methods used in electrical engineering to determine the impedance in the excitation branch of a real transformer.

The secondary windings of the transformer are left open-circuited while a

full-rated voltage is applied to the primary winding. A wattmeter is

connected to the primary. An ammeter is connected in series with the

primary winding.

Since the secondary of the transformer is open, the primary draws only no

load current. This no load current is negligible. As the copper losses depend

on current they can be neglected. But as the rated voltage is applied, the

iron loss will be present and is maximum. Since the impedance of

the series winding of the transformer is very small compared to that of the

excitation branch, all of the input voltage is dropped across the excitation

branch. Thus the wattmeter measures only the iron loss.

Current, voltage and power are measured at the primary winding to

ascertain the admittance and power factor angle.

Tan delta test

The tan delta test is a diagnostic procedure to assess the deterioration of the

insulation of a medium- or high-voltage cable. Due to the well known water-

tree effect the conductivity of the insulation increases, this reflects in an

increase of tan delta values. So, the interpretation of tan delta test results

gives an idea about the aging process in the cable-insulation and hence,

allows an assessment of the operational reliability of the cable. The test

engineer is able to distinguish between new, strongly aged and faulty cables

and appropriate maintenance and repair measures may be planned. When a

voltage is applied, any cable with a perfect insulation works as a capacitor

without loss! Theoretically there is an angle of 90° between voltage and

current. If however the insulation deteriorates due to moisture the current

will also show a resistive component, and the angle between voltage and

current will decrease. By a highly accurate measurement of the phase lag

between current and voltage the dissipation factor tan d can be determined.

The dissipation factor tan delta is defined as the ratio between active current

and ideal capacitive current. As a low frequency voltage causes higher

values of capacitive reactance leading to a lower power requirement during

the test, a tan delta test is performed using 0.1 Hz very low

frequency sinusoidal voltages. That prevents the device under test from

damages during the tan delta test and guarantees damage-free

measurement.

The tan delta test procedure comprises measurement at nominal voltage Vo

and measurement at twice the nominal voltage 2 Vo. A drastic increase of the

dissipation factor at increasing measurement voltages indicates a strong

ageing of the cable insulation. The tan delta test gives an indication of the

general condition of the insulation for the entire length of the measured

cable, but it will not detect much localized problems in a cable system.

Therefore, like many other diagnostic tests, a single tan delta test will not

provide a good indication of whether a cable is good or bad unless the

condition of the cable is really very bad. Usually the measurements have to

be repeated at different times to determine if there is a trend of

deterioration.

Comparison of the gathered measurement values with the data of the same

cable collected during the acceptance-test helps to classify the dielectric

condition (new, strongly aged, faulty) of the cable.

Oil testing

The insulation oil of voltage- and current-transformers fulfills the purpose of

insulating as well as cooling. Thus, the dielectric quality of transformer oil is

a matter of secure operation of a transformer.

Since transformer oil deteriorates in its isolation and cooling behavior due to

ageing and pollution by dust particles or humidity, and due to its vital role,

transformer oil must be subject to oil tests on a regular basis.

In most countries such tests are even mandatory. Transformer oil testing

sequences and procedures are defined by various international standards.

Periodic execution of transformer oil testing is as well in the very interest of

energy supplying companies, as potential damage to the transformer

insulation can be avoided by well timed substitution of the transformer oil.

Lifetime of plant can be substantially increased and the requirement for new

investment may be delayed. To assess the insulating property of dielectric

transformer oil, a sample of the transformer oil is taken and its breakdown

voltage is measured.

The transformer oil is filled in the vessel of the testing device. Two

standard-compliant test electrodes with a typical clearance of 2.5 mm are

surrounded by the dielectric oil.

A test voltage is applied to the electrodes and is continuously

increased up to the breakdown voltage with a constant, standard-

compliant slew rate of e.g. 2 kV/s.

At a certain voltage level breakdown occurs in an electric arc, leading

to a collapse of the test voltage.

An instant after ignition of the arc, the test voltage is switched off

automatically by the testing device. Ultra fast switch off is highly

desirable, as the carbonization due to the electric are must be limited to

keep the additional pollution as low as possible.

The transformer oil testing device measures and reports the root mean

square value of the breakdown voltage.

After the transformer oil test is completed, the insulation oil is stirred

automatically and the test sequence is performed repeatedly. (Typically 5

Repetitions, depending on the standard)

As a result the breakdown voltage is calculated as mean value of the

individual measurements.

Recently time consuming testing procedures in test labs have been replaced

by on-site oil testing procedures. There are various manufacturers of

portable oil testers.

With low weight devices in the range of 20 to 40 kg tests up to 100 kV rms

can be performed and reported on-site automatically. Some of them are

even battery-powered and come with all sorts of accessories.

Results

• Breakdown voltage should be equal to 60 kV.

• Water content should be less than 10 ppm.

Dissolved gas analysis

Dissolved Gas Analysis or DGA is the study of dissolved gases in transformer

oil. It is the most sensitive and reliable technique which gives an early

indication of abnormal behavior of a transformer.DGA is advanced tool to

diagnose the health of a transformer under Preventive

Maintenance Programme.

DGA consists of three steps. They are, sampling of transformer oil in an

airtight glass tube, complete extraction of gases from the sample and

subsequent analysis of the extracted gases for their quantity and

combination. Transformer oil is used as a coolant and insulator in a

transformer. It baths every internal component and contains a lot of

diagnostic information in the form of dissolved gases. Since these gases

reveal the faults of a transformer, they are known as Fault Gases. They are

formed in transformer oil, due to natural ageing and as a result of faults

inside the transformer. Formation of fault gases is due

to oxidation, vaporization, insulation decomposition, oil breakdown and

electrolytic action. Oil sample tube is used to draw, retain and transport the

oil sample of transformer oil in the same condition as it is inside a

transformer with all fault gases dissolved in it. It is a gas tight borosilicate

glass tube of capacity 150 ml or 250 ml, having two airtight Teflon valves on

both the ends. The outlets of these valves have been provided with a screw

thread which helps in convenient connection of synthetic tubes while

drawing sample from transformer. Also this provision is useful in transferring

the oil into Sample oil burette of the Multiple Gas Extractor without any

exposure to atmosphere, thereby retaining all its dissolved and evolved fault

gases contents. It has got a septum arrangement on one side of the tube for

drawing sample oil to test its moisture content. Thermo foam boxes are used

to transport the above Oil Sample Tubes without any exposure to sunlight.

Complete extraction of fault gases from transformer oil is achieved by

Multiple Gas Extractor. This is a unique glass apparatus designed by Central

Power Research Institute, Bangalore, India and developed by Dakshin Lab

Agencies, Bangalore. In this apparatus, the same sample oil is exposed to

high vacuum many times until there is no further increase in the volume of

extracted fault gases. The entire extraction takes place at very high vacuum

under ambient temperature, without any escape of fault gases in to

atmosphere. A fixed volume of sample oil is directly drawn from sample tube

into degassing vessel under high vacuum, where the gases are released.

These gases are isolated using a mercury piston to measure its volume at

atmospheric pressure (Total Gas Content) and subsequent transfer to Gas

Chromatograph using gas tight syringe or auto sampler. The fault gases are

measured, in milliliter of gases per milliliter of transformer oil and converted

into parts per million. Moreover, in this method small traces of incipient fault

gases are detected at very early stage. This method alone, provides the

repeated accurate results for Total Gas Content.

Atmospheric Gases:

• Hydrogen, Oxygen & Nitrogen

• Carbon Monoxide

• Carbon dioxide

• Acetylene

• Ethylene

• Methane

• Ethane

The gases extracted from the sample oil are injected into Gas

Chromatograph where the columns separate gases. The separated gases are

detected by Thermal Conductivity Detector for atmospheric gases, by Flame

Ionization Detector for hydro carbons and oxides of carbon. Methanator is

used to detect oxides of carbon, when they are in very low concentration.

When transformer is overloaded it generates more heat and deteriorates the

cellulose insulation. In this case DGA results show high carbon monoxide and

high carbon dioxide. In extreme cases methane and ethylene are at higher

levels. The overheating of insulation liquid results in breakdown of liquid by

heat and formation of high thermal gases. They are methane, ethane and

ethylene.

In a new transformer the levels of hydrocarbons in transformer oil after

vacuum filtration shall be 5 ppm. After commissioning a new transformer

DGA shall be done every month or earlier depending on the DGA results

observed.

In an overhauled and repaired transformer, DGA is to be done a week after

re-commission. Subsequently DGA is required every month or earlier

depending upon the DGA results.

In interpretation of the results obtained for a particular transformer, due

regard should be given to the following factors before arriving at a specific

conclusion:

Date of commissioning of the transformer

Loading cycle of the transformer

Date on which the oil was last filtered

Buchholz relay

In the field of electric power distribution and transmission, a Buchholz

relay is a safety device mounted on some oil-filled power transformers

and reactors, equipped with an external overhead oil reservoir called

a conservator. The Buchholz Relay is used as a protective device sensitive to

the effects of dielectric failure inside the equipment.

Depending on the model, the relay has multiple methods to detect a failing

transformer. On a slow accumulation of gas, due perhaps to slight overload,

gas produced by decomposition of insulating oil accumulates in the top of

the relay and forces the oil level down. A float switch in the relay is used to

initiate an alarm signal. Depending on design, a second float may also serve

to detect slow oil leaks.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the

conservator. This flow of oil operates a switch attached to a vane located in

the path of the moving oil. This switch normally will operate a circuit

breaker to isolate the apparatus before the fault causes additional damage.

Buchholz relays have a test port to allow the accumulated gas to be

withdrawn for testing. Flammable gas found in the relay indicates some

internal fault such as overheating or arcing, whereas air found in the relay

may only indicate low oil level or a leak.

Buchholz relays have been applied to large power transformers at least since

the 1940s. The relay was first developed by Max Buchholz (1875–1956) in

1921.

Names like Beachwood relay or beech relay are an indication of incorrectly

translated German language manuals.

Types of relay

• Voltage selection relay

• Trip circuit supervision relay

• Alarm annun relay

• Bus bar protection trip relay

• Protection supply supervision relay

• IDMT open circuit relay

• IDMT earth fault relay

• Master trip relay

• Local breaker backup protection relay

• Differential protection relay

• Restricted earth fault protection relay

• Over flux protection relay

4.2 Lightning Arrestor

A lightning arrester is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal oxide varistors (MOVs) have been used for power system protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth.

There are several types of lightning arresters in general use. They differ only in constructional details but operate on the same principle viz, providing low resistance path for the surges to the round. Following are the different types of lightning relays:

1. Rod arrester

2. Horn gap arrester

3. Multigap arrester

4. Expulsion type lightning arrester

5. Valve type lightning arrester

Zinc Oxide Surge ArrestorZinc Oxide Surge Arrester can be made of semiconductor ceramic materials composed mainly of zinc oxides. It has excellent non-linearity coefficient,

great withstanding surge current, low clamping voltage, non-renewal of flow and more function. the MYG11 voltage dependent resistor is recognized as an advanced over voltage protector which, due to the inherent superior nonlinear V-A characteristics and steep wave response of zinc oxide varistor, can suppress over voltages to a low level thus providing a reliable for insulation of electrical apparatus.Zinc Oxide Surge Arrester mainly use as surge protection of communication, measuring or controller instrument and transient voltage surge suppressor units, protection of railway automatic signals. Compared with the other lightning protection devices, it can be used in outdoors.Ratings

For 220 MVA – 198 kV and 10 kA

For 66 MVA – 60 kV 10 kA

Leakage current should be less than 50 µa.

Corona discharge

In electricity, a corona discharge is an electrical discharge brought on by

the ionization of a fluid surrounding a conductor that is carrying a current.

The discharge will occur when the strength (potential gradient) of the electric

field produced by the current is high enough, but not high enough to

cause electrical breakdown or arcing.

A corona is a process by which a current, perhaps sustained, develops from

an electrode with a high potential in a neutral fluid, usually air, by

ionizing that fluid so as to create a plasma around the electrode. The ions

generated eventually pass charge to nearby areas of lower potential, or

recombine to form neutral gas molecules. When the potential gradient is

large enough at a point in the fluid, the fluid at that point ionizes and it

becomes conductive. If a charged object has a sharp point, the air around

that point will be at a much higher gradient than elsewhere. Air near the

electrode can become ionized (partially conductive), while regions more

distant do not. When the air near the point becomes conductive, it has the

effect of increasing the apparent size of the conductor. Since the new

conductive region is less sharp, the ionization may not extend past this local

region. Outside this region of ionization and conductivity, the charged

particles slowly find their way to an oppositely charged object and are

neutralized.

If the geometry and gradient are such that the ionized region continues to

grow instead of stopping at a certain radius, a completely conductive path

may be formed, resulting in a momentary spark, or a continuous arc.

Corona discharge usually involves two asymmetric electrodes; one highly

curved (such as the tip of a needle, or a small diameter wire) and one of low

curvature (such as a plate, or the ground). The high curvature ensures a

high potential gradient around one electrode, for the generation of a plasma.

Coronas may be positive or negative. This is determined by the polarity of

the voltage on the highly-curved electrode. If the curved electrode is positive

with respect to the flat electrode we say we have a positive corona, if

negative we say we have a negative corona. (See below for more details.)

The physics of positive and negative coronas are strikingly different. This

asymmetry is a result of the great difference in mass between electrons and

positively charged ions, with only the electron having the ability to undergo a

significant degree of ionizing inelastic collision at common temperatures and

pressures.

An important reason for considering coronas is the production

of ozone around conductors undergoing corona processes. A negative corona

generates much more ozone than the corresponding positive corona.

Corona discharge has a number of commercial and industrial applications:

Drag reduction over a flat surface.

Removal of unwanted electric charges from the surface of aircraft in

flight and thus avoiding the detrimental effect of uncontrolled electrical

discharge pulses on the performance of avionic systems.

Manufacture of ozone.

Sanitization of pool water.

Scrubbing particles from air in air-conditioning

systems (see electrostatic precipitator).

Removal of unwanted volatile organics, such as chemical pesticides,

solvents, or chemical weapons agents, from the atmosphere.

Improvement of wet ability or 'surface tension energy' of polymer films

to improve compatibility with adhesives or printing inks.

Photocopying.

Air ionizers.

Production of photons for Kirlian photography to expose photographic

film.

EHD thrusters, Lifters, and other ionic wind devices.

Nitrogen laser.

Surface treatment for tissue culture (polystyrene).

Ionization of a gaseous sample for subsequent analysis in a mass

spectrometer or an ion mobility spectrometer.

Solid-state cooling components for computer chips (see solid-state

fan).

Coronas can be used to generate charged surfaces, which is an effect used

in electrostatic copying (photocopying). They can also be used to remove

particulate matter from air streams by first charging the air, and then

passing the charged stream through a comb of alternating polarity, to

deposit the charged particles onto oppositely charged plates.

The free-radicals and ions generated in corona reactions can be used to

scrub the air of certain noxious products, through chemical reactions, and

can be used to produce ozone.

Coronas can generate audible and radio-frequency noise, particularly

near electric power transmission lines. They also represent a power loss, and

their action on atmospheric particulates, along with associated ozone

and NOx production, can also be disadvantageous to human health where

power lines run through built-up areas. Therefore, power transmission

equipment is designed to minimize the formation of corona discharge.

Corona discharge is generally undesirable in:

Electric power transmission, where it causes:

Power loss

Audible noise

Electromagnetic interference

Purple glow

Ozone production

Insulation damage

Inside electrical components such as transformers, capacitors, electric

motors and generators. Corona progressively damages the insulation

inside these devices, leading to premature equipment failure. One form of

attack is ozone cracking of elastomer items like O-rings.

Situations where high voltages are in use, but ozone production are to

be minimized.

Static electricity discharge.

Lightning.

Coronas can be suppressed by corona rings, toroidal devices that serve to

spread the electric field over larger area and decrease the field gradient

below the corona threshold.

4.3 Isolator

In electrical engineering, a disconnector or isolator switch is used to make

sure that an electrical circuit can be completely de-energized for service or

maintenance. Such switches are often found in electrical

distribution and industrial applications where machinery must have its

source of driving power removed for adjustment or repair. High-voltage

isolation switches are used in electrical substations to allow isolation of

apparatus such as circuit breakers and transformers, and transmission lines,

for maintenance. Isolating switches are commonly fitted to domestic

extractor fans when used in bathrooms in the UK. Often the isolation switch

is not intended for normal control of the circuit and is only used for isolation.

Isolator switches have provisions for a padlock so that inadvertent operation

is not possible (see: Lockout-Tag out). In high voltage or complex systems,

these padlocks may be part of a trapped-key interlock system to ensure

proper sequence of operation. In some designs the isolator switch has the

additional ability to earth the isolated circuit thereby providing additional

safety. Such an arrangement would apply to circuits which inter-connect

power distribution systems where both end of the circuit need to be isolated.

The major difference between an isolator and a circuit breaker is that an

isolator is an off-load device intended to be opened only after current has

been interrupted by some other control device. Safety regulations of the

utility must prevent any attempt to open the disconnector while it supplies a

circuit.

Standards in some countries for safety may require either local motor

isolators or lockable overloads (which can be padlocked).

Types

• Central break for 220 MVA.

• Twin break for 66 MVA.

DC contact resistance should be less than 100 mΩ.

It is measured by CRM kit.

4.4 Circuit Breaker

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Types of circuit breaker

Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, and include:

• MCB (Miniature Circuit Breaker)- rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

• MCCB (Molded Case Circuit Breaker)- rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

• Low voltage power circuit breakers can be mounted in multi-tiers in low-voltage switchboards or switchgear cabinets.

The characteristics of Low Voltage circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear. Large low-voltage molded case and power circuit breakers may have electrical motor operators, allowing them to be tripped (opened) and closed under remote control. These may form part of an automatic transfer switch system for standby power. Low-voltage circuit breakers are also made for direct-current (DC) applications, for example DC supplied for subway lines. Special breakers are required for direct current because the arc does not have a natural tendency to go out on each half cycle as for alternating current. A direct current circuit breaker will have blow-out coils which generate a magnetic field that rapidly stretches the arc when interrupting direct current. Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components:

Actuator lever

Actuator mechanism

Contacts

Terminals

Bimetallic strip

Calibration screw

Solenoid

Arc divider/extinguisher

Magnetic circuit breaker

Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.

Thermal magnetic circuit breaker

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions. The thermal portion of the circuit breaker provides an "inverse time" response feature which provides faster or slower response for larger or smaller over currents respectively.

Common trip breakers

Three pole common trip breaker for supplying a three-phase device. This breaker has a 2 A rating. When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120/240 volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.

Two and four pole breakers are used when there is a need to disconnect the neutral wire, to be sure that no current can flow back through the neutral wire from other loads connected to the same network when people need to touch the wires for maintenance. Separate circuit breakers must never be used for disconnecting live and neutral, because if the neutral gets disconnected while the live conductor stays connected, a dangerous condition arises: the circuit will appear de-energized (appliances will not work), but wires will stay live and RCDs will not trip if someone touches the live wire (because RCDs need power to trip). This is why only common trip breakers must be used when switching of the neutral wire is needed.

Medium-voltage circuit breakers

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:

• Vacuum circuit breaker- With rated current up to 3000 A, these breakers interrupts the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

• Air circuit breaker- Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

• SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing the breaker to be removed without disturbing the power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure.

High-voltage circuit breakers

Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault. High-voltage breakers are broadly classified by the medium used to extinguish the arc:

• Bulk oil • Minimum oil • Air blast • Vacuum • SF6

Some of the manufacturers are ABB, GE (General Electric) , Tavrida Electric, Alstom, Mitsubishi Electric, Pennsylvania Breaker, Siemens, Toshiba, Končar HVS, BHEL, CGL, Square D (Schneider Electric).

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1200KV breakers are likely to come into market very soon.

High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.

Standard current ratings

International Standard IEC 60898-1 and European Standard EN 60898-1 define the rated current In of a circuit breaker for low voltage distribution applications as the current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The commonly-available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A and 100 A (Reynard, slightly modified to include current limit of British BS 1363 sockets). The circuit breaker is labeled with the rated current in amperes, but without the unit symbol "A". Instead, the ampere figure is preceded by a letter "B", "C" or "D" that indicates the instantaneous tripping current, that is the minimum value of current that causes the circuit-breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of In:

Type

Instantaneous tripping current

B above 3 In up to and including 5 In

C above 5 In up to and including 10 In

D above 10 In up to and including 20 In

Kabove 8 In up to and including 12 In

For the protection of loads that cause frequent short duration (approximately 400 ms to 2 s) current peaks in normal operation.

Z

Above 2 In up to and including 3 In for periods in the order of tens of seconds.

For the protection of loads such as semiconductor devices or measuring circuits using current transformers.

Sulphor Hexafluoride (SF6) High Voltage Circuit Breakers

High-voltage circuit-breakers have greatly changed since they were first introduced about 40 years ago, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy. These breakers are available for indoor or outdoor applications, the latter being in the form of breaker poles housed in ceramic insulators mounted on a structure.

Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.

Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulphor hexafluoride is generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV).

Into the 1980s, the pressure necessary to blast the arc was generated mostly by gas heating using arc energy. It is now possible to use low energy spring-loaded mechanisms to drive high-voltage circuit-breakers up to 800 kV.

Several characteristics of SF6 circuit breakers can explain their success:

• Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber; • Autonomy provided by the puffer technique; • The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers; • Short break time of 2 to 2.5 cycles; • High electrical endurance, allowing at least 25 years of operation without reconditioning; • Possible compact solutions when used for gas insulated switchgear or hybrid switchgear; • Integrated closing resistors or synchronized operations to reduce switching over-voltages; • Reliability and availability; • Low noise levels.

Thermal blast chambers

New types of SF6 breaking chambers, which implement innovative interrupting principles, have been developed over the past 30 years, with the objective of reducing the operating energy of the circuit-breaker. One aim of this evolution was to further increase the reliability by reducing the dynamic forces in the pole. Developments since 1980 have seen the use of the self-blast technique of interruption for SF6 interrupting chambers.

These developments have been facilitated by the progress made in digital simulations that were widely used to optimize the geometry of the interrupting chamber and the linkage between the poles and the mechanism.

This technique has proved to be very efficient and has been widely applied for high voltage circuit breakers up to 550 kV. It has allowed the development of new ranges of circuit breakers operated by low energy spring-operated mechanisms.

The reduction of operating energy was mainly achieved by the lowering energy used for gas compression and by making increased use of arc energy to produce the pressure necessary to quench the arc and obtain current interruption. Low current interruption, up to about 30% of rated short-circuit current, is obtained by a puffer blast.

Self-blast chambers

Further development in the thermal blast technique was made by the introduction of a valve between the expansion and compression volumes. When interrupting low currents the valve opens under the effect of the overpressure generated in the compression volume. The blow-out of the arc is made as in a puffer circuit breaker thanks to the compression of the gas obtained by the piston action. In the case of high currents interruption, the arc energy produces a high overpressure in the expansion volume, which leads to the closure of the valve and thus isolating the expansion volume from the compression volume. The overpressure necessary for breaking is obtained by the optimal use of the thermal effect and of the nozzle clogging effect produced whenever the cross-section of the arc significantly reduces the exhaust of gas in the nozzle. In order to avoid excessive energy consumption by gas compression, a valve is fitted on the piston in order to limit the overpressure in the compression to a value necessary for the interruption of low short circuit currents.

Self-blast circuit breaker chamber (1) closed, (2) interrupting low current, (3) interrupting high current, and (4) open.

This technique, known as “self-blast” has now been used extensively since 1980 for the development of many types of interrupting chambers. The increased understanding of arc interruption obtained by digital simulations and validation through breaking tests, contribute to a higher reliability of these self-blast circuit breakers. In addition the reduction in operating energy, allowed by the self blast technique, leads to longer service life.

An important decrease in operating energy can also be obtained by reducing the kinetic energy consumed during the tripping operation. One way is to displace the two arcing contacts in opposite directions so that the arc speed is half that of a conventional layout with a single mobile contact.

The thermal and self blast principles have enabled the use of low energy spring mechanisms for the operation of high voltage circuit breakers. They progressively replaced the puffer technique in the 1980s; first in 72.5 kV breakers, and then from 145 kV to 800 kV.

The double motion technique halves the tripping speed of the moving part. In principle, the kinetic energy could be quartered if the total moving mass was not increased. However, as the total moving mass is increased, the practical reduction in kinetic energy is closer to 60%.The total tripping energy also includes the compression energy, which is almost the same for both techniques. Thus, the reduction of the total tripping energy is lower, about 30%, although the exact value depends on the application and the operating mechanism. Depending on the specific case, either the double motion or the single motion technique can be cheaper. Other considerations, such as rationalization of the circuit-breaker range, can also influence the cost.

In this interruption principle arc energy is used, on the one hand to generate the blast by thermal expansion and, on the other hand, to accelerate the moving part of the circuit breaker when interrupting high currents. The overpressure produced by the arc energy downstream of the interruption zone is applied on an auxiliary piston linked with the moving part. The resulting force accelerates the moving part, thus increasing the energy available for tripping.

With this interrupting principle it is possible, during high-current interruptions, to increase by about 30% the tripping energy delivered by the operating mechanism and to maintain the opening speed independently of the current. It is obviously better suited to circuit-breakers with high breaking currents such as Generator circuit-breakers.

Circuit Breaker Contact Resistance Measurement Test

Stationary and moving contacts are built from alloys that are formulated to endure the abuse of electrical arcing. However, if contacts are not maintained on a regular basis, their electrical resistance due to repeated arcing builds up. This resistance build-up results in a significant decrease in the contact’s ability to carry current. Excessive corrosion of contacts is detrimental to the breaker performance. One way to check contacts is to apply dc current and measure the contact resistance or voltage drop across the closed contacts. The breaker contact resistance should be measured from bushing terminal to bushing terminal with the breaker in the closed position. It is recommended that for medium and high voltages the resistance test be made with 100-Amps (or higher) direct current. The use of a higher current value gives more reliable results than using lower current

values. The resistance value is usually measured in micro-ohms. The average resistance value for 15-kV-class air circuit breakers is approximately between 200-250 micro-ohms. The evaluation should be made on comparing resistance values of the three phases with each other, or with resistance values of similar breakers. A difference of more than 50 percent in resistance values among the three phases of a breaker should warrant further investigation.

The contacts in the circuit breaker need to be checked periodically to ensure that the breaker is healthy and functional. Poorly maintained or damaged contacts can cause arcing, single phasing, and even fire. The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact measurement check. The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.

The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument.

Then, using Ohm's law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records. Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in damaged conditions. Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.

Opening and closing time

For 220 kV it should be less than 30 millisecond.

Difference between any two poles should be less than 5 sec.

For 66 kV it should be less than 40 millisecond.

Closing time should be less than 100 millisecond for both.

4.5 Current Transformer

In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

Like any other transformer, a current transformer has a primary winding,

a magnetic core, and a secondary winding. The alternating current flowing in

the primary produces a magnetic field in the core, which then induces a

current in the secondary winding circuit. A primary objective of current

transformer design is to ensure that the primary and secondary circuits are

efficiently coupled, so that the secondary current bears an accurate

relationship to the primary current.

The most common design of CT consists of a length of wire wrapped many

times around a silicon steel ring passed over the circuit being measured. The

CT's primary circuit therefore consists of a single 'turn' of conductor, with a

secondary of many hundreds of turns. The primary winding may be a

permanent part of the current transformer, with a heavy copper bar to carry

current through the magnetic core. Window-type current transformers are

also common, which can have circuit cables run through the middle of an

opening in the core to provide a single-turn primary winding. When

conductors passing through a CT are not centered in the circular (or oval)

opening, slight inaccuracies may occur.

Shapes and sizes can vary depending on the end user or switchgear

manufacturer. Typical examples of low voltage single ratio metering current

transformers are either ring type or plastic molded case. High-voltage

current transformers are mounted on porcelain bushings to insulate them

from ground. Some CT configurations slip around the bushing of a high-

voltage transformer or circuit breaker, which automatically centers the

conductor inside the CT window.

The primary circuit is largely unaffected by the insertion of the CT. The rated

secondary current is commonly standardized at 1 or 5 amperes. For

example, a 4000:5 CT would provide an output current of 5 amperes when

the primary was passing 4000 amperes. The secondary winding can be

single ratio or multi ratio, with five taps being common for multi ratio CTs.

The load, or burden, of the CT should be of low resistance. If the voltage time

integral area is higher than the core's design rating, the core goes

into saturation towards the end of each cycle, distorting the waveform and

affecting accuracy.

Types:

• Live tank

This type of tank is mounted on towers along the transmission line. High level current passes through them so that direct contact cannot be made.

• Dead tank

This type of tank is situated on the ground. Wires are brought from the transmission lines to the ground. It is properly earthed so that it can be touched directly. However this leads to power loss and increase in amount of core material.

TECHNICAL DETAILS:

1.

Class of Accuracy 0.5

2.

Rated Burden 5.00 VA

3.

Power Frequency Withstand Voltage

3KV

4.

Highest System Voltage 433 V

5.

Nominal System Voltage 400 V

6.

Frequency 50 Hz

7.

Supply System 3 Ph. Solidly grounded Neutral System

Transformation ratio shall be specified from the following standard ratings as per requirement:

Ratio 50/5 150/5 300/5 400/5 1000/5

(Secondary with 1 A may be specified by the utility incase the same is desired.)

Bore diameter of the CT shall not be less than 40 mm. Ring type CTs shall have suitable clamp to fix the CT to panel Board, wherever required.

The limits of current error and phase angle displacement as per IS:2705 at several defined percentage of rated current are:

Accuracy Class

% Ratio error at % of rated current

Phase displacement in minutes at % of rated current

5 20 100 120 5 20 100 120

0.5 1.5 0.75

0.5 0.5 90 45 30 30

Current error and phase displacement at rated frequency is required to be as above when the secondary burden from 25% to 100% of the rated burden i.e. 50 V A.

Rated extended primary current shall be 120% of rated primary Current in accordance with IS: 2705 Pt-II.

Rated ISF (Instrument Security Factor) shall be declared by the manufacturer & marked on the CT.CT’s shall be made with good engineering practices. Core winding shall evenly spread stress & avoid stress concentration at any one point. Cast resin CT’s shall be processed by hot curing method under controlled vacuum conditions.The base shall be of hot dip galvanized steel.

The accuracy of a CT is directly related to a number of factors including:

Burden

Burden class/saturation class

Rating factor

Load

External electromagnetic fields

Temperature and

Physical configuration.

The selected tap, for multi-ratio CTs

For the IEC standard, accuracy classes for various types of measurement are

set out in IEC 60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class

designation is an approximate measure of the CT's accuracy. The ratio

(primary to secondary current) error of a Class 1 CT is 1% at rated current;

the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also

important especially in power measuring circuits and each class has an

allowable maximum phase error for specified load impedance. Current

transformers used for protective relaying also have accuracy requirements at

overload currents in excess of the normal rating to ensure accurate

performance of relays during system faults.

The load, or burden, in a CT metering circuit is the

(largely resistive) impedance presented to its secondary winding. Typical

burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30

VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-

1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can

tolerate up to 0.2 Ω of impedance in the metering circuit before its output

current is no longer a fixed ratio to the primary current. Items that contribute

to the burden of a current measurement circuit are switch-blocks, meters

and intermediate conductors. The most common source of excess burden in

a current measurement circuit is the conductor between the meter and the

CT. Often, substation meters are located significant distances from the meter

cabinets and the excessive length of small gauge conductor creates a large

resistance. This problem can be solved by using CT with 1 ampere

secondaries which will produce less voltage drop between a CT and its

metering devices.

The knee-point voltage of a current transformer is the magnitude of the

secondary voltage after which the output current ceases to follow the input

current. This means that the one-to-one or proportional relationship between

the input and output is no longer within rated accuracy. The output current

increases abruptly even with small increment in the input, if the voltage

across the secondary terminals exceeds the knee-point voltage. The knee-

point voltage is not applicable for metering current transformers; the

concept of knee point voltage is pertinent to protect current transformers

only since they are necessarily exposed to high currents during faults.

Rating factor is a factor by which the nominal full load current of a CT can be

multiplied to determine its absolute maximum measurable primary current.

Conversely, the minimum primary current a CT can accurately measure is

"light load," or 10% of the nominal current (there are, however, special CTs

designed to measure accurately currents as small as 2% of the nominal

current). The rating factor of a CT is largely dependent upon ambient

temperature. Most CTs have rating factors for 35 degrees Celsius and 55

degrees Celsius. It is important to be mindful of ambient temperatures and

resultant rating factors when CTs are installed inside pad-mounted

transformers or poorly ventilated mechanical rooms. Recently,

manufacturers have been moving towards lower nominal primary currents

with greater rating factors. This is made possible by the development of

more efficient ferrites and their corresponding hysteresis curves. This is a

distinct advantage over previous CTs because it increases their range of

accuracy, since the CTs are most accurate between their rated current and

rating factor. The accuracy of the ct is changed if the temperature of the

surroundings is exceeded.

Usage

Current transformers are used extensively for measuring current and

monitoring the operation of the power grid. Along with voltage leads,

revenue-grade CTs drive the electrical utility's watt-hour meter on virtually

every building with three-phase service and single-phase services greater

than 200 amp.

The CT is typically described by its current ratio from primary to secondary.

Often, multiple CTs are installed as a "stack" for various uses. For example,

protection devices and revenue metering may use separate CTs to provide

isolation between metering and protection circuits, and allows current

transformers with different characteristics (accuracy, overload performance)

to be used.

Tan Delt a testing

Tan Delta is a a diagnostic test conducted on the insulation of cables and

windings. It is used to measure the deterioration in the cable. It also gives an

idea of the aging process in the cable and enables us to predict the

remaining life of the cable. It is alternatively known as the loss angle test or

the dissipation factor test.

The Tan Delta test works on the principle that any insulation in its pure state

acts as a capacitor. The test involves applying a very low frequency AC

voltage. The voltage is generally double the rated voltage of the cable or

winding.

A low frequency causes a higher value of capacitive reactance which leads to

lesser power requirement during the test. Besides, the currents will be

limited enabling easier measurement.

In a pure capacitor, the current is ahead of the voltage by 90 degrees. The

insulation, in a pure condition, will behave similarly. However, if the

insulation has deteriorated due to the entry of dirt and moisture. The current

which flows through the insulation will also have a resistive component.

This will cause the angle of the current to be less than 90 degrees. This

difference in the angle is known as the loss angle. The tangent of the

angle which is Ir/Ic (opposite/adjacent) gives us an indication of the

condition of the insulation. A higher value for the loss angle indicates a high

degree of contamination of the insulation.

The cable or winding whose insulation is to be tested is first disconnected

and isolated. The test voltage is applied from the Very Low Frequency

power source and the Tan delta controller takes the measurements. The

test voltage is increased in steps up to the rated voltage of the cable. The

readings are plotted in a graph against the applied voltage and the trend is

studied. A healthy insulation would produce a straight line.

The test should be continued only if the graph is a straight line. A rising trend

would indicate weak insulation which may fail if the test voltage is increased

beyond the rated voltage of the cable.

There are not standard formulae or benchmarks to ascertain the success of a

tan delta test. The health of the insulation which is measured is obtained

by observing the nature of the trend which is plotted. A steady, straight

trend would indicate a healthy insulation, while a rising trend would indicate

an insulation that has been contaminated with water and other impurities.

4.6 Wavetrap

Wavetrap traps the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals. Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU). The Line trap offers high

impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation busbars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.

4.7 Power Transformer & Capacitive Voltage Transformer

Capacitive Voltage transformers

A capacitor voltage transformer (CVT), or capacitance coupled voltage

transformer (CCVT) is a transformer used in power systems to step

down extra high voltage signals and provide a low voltage signal, for

measurement or to operate a protective relay. In its most basic form the

device consists of three parts: two capacitors across which the transmission

line signal is split, an inductive element to tune the device to the line

frequency, and a transformer to isolate and further step down the voltage for

the instrumentation or protective relay. The device has at least four

terminals: a terminal for connection to the high voltage signal, a ground

terminal, and two secondary terminals which connect to the instrumentation

or protective relay. CVTs are typically single-phase devices used for

measuring voltages in excess of one hundred kilovolts where the use of

voltage transformers would be uneconomical. In practice, capacitor C1 is

often constructed as a stack of smaller capacitors connected in series. This

provides a large voltage drop across C1 and a relatively small voltage drop

across C2.

The CVT is also useful in communication systems. CVTs in combination with

wave traps are used for filtering high frequency communication signals from

power frequency. This forms a carrier communication network throughout

the transmission network.

Voltage transformers

Voltage transformers (VT) or potential transformers (PT) are another type of

instrument transformer, used for metering and protection in high-voltage

circuits. They are designed to present negligible load to the supply being

measured and to have a precise voltage ratio to accurately step down high

voltages so that metering and protective relay equipment can be operated at

a lower potential. Typically the secondary of a voltage transformer is rated

for 69 V or 120 V at rated primary voltage, to match the input ratings of

protective relays.

The transformer winding high-voltage connection points are typically labeled

as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2 and

sometimes an X3 tap may be present. Sometimes a second isolated winding

(Y1, Y2, Y3) may also be available on the same voltage transformer. The high

side (primary) may be connected phase to ground or phase to phase. The

low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity.

This applies to current transformers as well. At any instant terminals with the

same suffix numeral have the same polarity and phase. Correct identification

of terminals and wiring is essential for proper operation of metering and

protective relays.

Some meters operate directly on the secondary service voltages at or below

600 V. VTs are typically used for higher voltages (for example, 765 kV

for power transmission) , or where isolation is desired between the meter

and the measured circuit.

Difference between PT & CVT

PT uses windings for voltage step down transformer high voltage to lower

voltage whereas CVT uses two capacitors in series to drop voltage or split AC

like a filter circuit. It has a tuned 60 cycle resonant cap & coil circuit on

primary side like a voltage divider circuit but uses capacitors combined for

protection relaying work fault trip zone protection function circuit. It’s also

for phase shift, there is really a coupling capacitor voltage splitter circuit

centered tap to a reactor coil on primary side input of trans CCVT which is

dropped to around 6kV pre input, the center tap of these series caps feed

coil and coil feeds trans pre one cap is connected to ground one cap goes to

HV line. But multi caps are used in series for the incoming HV line 165kv

transmission line , not just two caps by example to make it simple and there

is a iron choke coil called a reactor coil it cancels out the caps at 60 cycles

for phase shift on output from primary to secondary of CVT transformer there

is also a suppression circuit on CVT secondary side for discharge of a

potential high voltage spike or back feed kick protection when a major fault

occurs on HV transmission line voltage drops off sharply. But the whole

circuit works in a delay of false tripping by capacitance stored energy in the

caps to hold voltage on primary this controls the tripping time in seconds

before a trip of differential relay coils circuits on secondary side which is 115

volts to supply differ relay coils as stated and to control all this with total CVT

package circuit and must be included in explaining the difference of both PT

and CVT.

A PT, potential transformer, can be thought of as a pure transformer with

primary and secondary windings; PT's are sometimes referred to as magnetic

transformers due to the fact that their mode of operation is purely magnetic.

It is used to step-down the input voltage from a power line to a voltage level

that can be processed by metering devices and protection relays in a

substation. CVT or CCVT, capacitor-coupled voltage transformer, is made

with two capacitor sets acting as a voltage divider that brings the line

(actually the phase) voltage down to around 12Kv then this voltage is fed to

a relatively small transformer for the voltage signal to be processed. CVT is

rated for extremely high voltage levels above 230KV, while PT's aren't

designed for such large values. CVT's offer the advantage that the voltage

divider capacitor, being itself relatively smaller and lighter, configuration

makes the transformer's iron core much smaller in size, and hence more

economical, versus what it would be if a pure magnetic transformer would be

used. Also the CVT's can be tuned to the fundamental frequency of the line,

and the capacitance prevents the inductive "fire-back" of the coils in the

transformer when a breaker trips. PT's can't provide such advantage. Some

CVT's are also used to tune to PLCC, Programmable Logic Controller Carrier

frequency, which is a signal transmitted over power lines providing inter-

PLCC communication.

Power line carrier communication

Power line carrier communication (PLCC) is mainly used

for telecommunication, tele-protection and tele-monitoring

between electrical substations through power lines at high voltages, such as

110 kV, 220 kV, 400 kV. PLCC integrates the transmission of communication

signal and 50/60 Hz power signal through the same electric power cable. The

major benefit is the union of two important applications in a single system.

In a PLCC system the communication is established through the power line.

The audio frequency is carried by a carrier frequency and the range of carrier

frequency is from 50 kHz to 500 kHz. The modulation generally used in these

systems is amplitude modulation. The carrier frequency range is allocated to

include the audio signal, protection and the pilot frequency. The pilot

frequency is a signal in the audio range that is transmitted continuously for

failure detection.

The voice signal is converted/compressed into the 300 Hz to 4000 Hz range,

and this audio frequency is mixed with the carrier frequency. The carrier

frequency is again filtered, amplified and transmitted. The transmission of

these HF carrier frequencies will be in the range of 0 to +32db. This range is

set according to the distance between substations.

PLCC can be used for interconnecting PBXs. The electricity boards in India

have an internal network PLCC between PBXs.

Wavetrap is connected in series with the power (transmission) line. It blocks

the high frequency carrier waves (24 KHz to 500 KHz) and let power waves

(50 Hz - 60 Hz) to pass through. It is basically an inductor of rating in

MilliHenry (approx 1 milliHenry for 220 kV 1250 Amp.).

Line Trap has three main components:

1. Main coil.

2. Tuning Device.

3. Lightning Arrestor

It provides low impedance path for carrier energy to HV line and blocks the

power frequency circuit by being a high impedance path.

A power line carrier using a power line as transmission media needs to

change its transmission system from analog to digital to address rapid

diffusion of IP devices and digital telecommunication devices. With this view,

digital power line carrier (DPLC) was developed featuring several

technological measures which enable digital transmission via power lines

and performed a field evaluation test. As a result, DPLC has the required

quality of bit error rate characteristics and transmission ability such as

transmitting information from monitored electric-supply stations and images.

4.8 BusbarIn electrical power distribution, a busbar is a strip

of copper or aluminium that conducts electricity within

a switchboard, distribution board, substation or other electrical apparatus.

The size of the busbar determines the maximum amount of current that can

be safely carried. Busbars can have a cross-sectional area of as little as

10 mm² but electrical substations may use metal tubes of 50 mm in diameter

(1,963 mm²) or more as busbars, and an aluminium smelter will have very

large busbars used to carry tens of thousands of amperes to

the electrochemical cells that produce aluminum from molten salts.

Busbars are typically either flat strips or hollow tubes as these shapes

allow heat to dissipate more efficiently due to their high surface

area to cross-sectional area ratio. The skin effect makes 50–60

Hz AC busbars more than about 8 mm (1/3 in) thick inefficient, so hollow or

flat shapes are prevalent in higher current applications. A hollow section has

higher stiffness than a solid rod of equivalent current-carrying capacity,

which allows a greater span between busbar supports in outdoor

switchyards.

A busbar may either be supported on insulators, or else insulation may

completely surround it. Busbars are protected from accidental contact either

by a metal earthed enclosure or by elevation out of normal

reach. Neutral busbars may also be insulated. Earth busbars are typically

bolted directly onto any metal chassis of their enclosure. Busbars may be

enclosed in a metal housing, in the form of bus duct or bus way, segregated-

phase bus, or isolated-phase bus.

Busbars may be connected to each other and to electrical apparatus by

bolted or clamp connections. Often joints between high-current bus sections

have matching surfaces that are silver-plated to reduce the contact

resistance. At extra-high voltages (more than 300 kV) in outdoor

buses, corona around the connections becomes a source of radio-frequency

interference and power loss, so connection fittings designed for these

voltages are used.

Busbars are typically contained inside of either a distribution board or bus

way. Distribution boards split the electrical supply into separate circuits at

one location.

Bus ways, or bus ducts, are long busbars with a protective cover. Rather

than branching the main supply at one location, they allow new circuits to

branch off anywhere along the route of the bus way.

4.9 Battery RoomA battery room is a room in a facility used to house batteries for large-scale

custom-built backup or uninterruptible power systems providing electric

power for telecommunication and computing equipment in datacenters,

telephone company central office facilities, and remote telecommunications

stations. The batteries provide direct current (DC) electricity primarily for

uninterruptible power supply (UPS) equipment, which in turn provides

continuous, uninterrupted alternating current (AC) power for the facility. The

batteries may provide power for minutes, hours or days depending on the

electrical system design, although most commonly the batteries power the

UPS during brief electric utility outages lasting only seconds.

Battery rooms are also found in electric power plants and substations where

reliable power is required for operation of switchgear, critical standby

systems, and possibly black start of the station. Often batteries for large

switchgear line-ups are 125 V or 250 V nominal systems, and feature

redundant battery chargers with independent power sources. Separate

battery rooms may be provided to protect against loss of the station due to a

fire in a battery bank. For stations that are capable of black start, power from

the battery system may be required for many purposes including switchgear

operations.

The world's largest battery is in Fairbanks, Alaska, composed of Ni-

Cd cells. Sodium-Sulphor batteries are being used to store wind power.