Construction, Laying, Installation of XLPE Oil Fluid Cable

194

CONSTRUCTION, LAYING AND

INSTALLATION TECHNIQUES FOR

EXTRUDED AND SELF CONTAINED

FLUID FILLED CABLE SYSTEMS

Working Group

21.17

October 2001

/ 145

STUDY COMMITTEE 21: HV INSULATED CABLES

CONSTRUCTION, LAYING AND INSTALLATION TECHNIQUESFOR EXTRUDED AND

SELF CONTAINED FLUID FILLED CABLE SYSTEMS

TECHNICAL BROCHURE

Picture 1 : Direct burial

WG21-17 August 2001

WG 21-17 Technical Brochure Page 2 / 145

MEMBERSHIP LIST OF WG 21 – 17

J.P.M. ANTONISSEN (The Netherlands),P. ARGAUT (France),R. AWAD (Canada),B. DRUGGE replaced by F. RÜTER (Sweden),T. FAGERENG (Norway),M. GENOVESI replaced by F. MAGNANI (Italy),A. GILLE (Belgium),P. HUDSON (United Kingdom) (Secretary),R. JOHNSTON (Australia),T. KARASAKI replaced by G. KATSUTA and after by T. SASAKI (Japan),K. LAGERSTEDT (Denmark),H.S. LEE (South Korea),Y. MAUGAIN1 (France) (Convenor),M. PORTILLO (Spain),T. J. RODENBAUGH (United States),R. SAMICO (Brazil),R. SCHROTH (Germany).

1 EDF – RTE, 34, 40 rue Henri Régnault F-92400 COURBEVOIE - France


LAYING AND INSTALLATION TECHNIQUES USEDFOR EXTRUDED AND SELF CONTAINED FLUID FILLED CABLES

TECHNICAL BROCHURE

WORKING GROUP 21-17

TABLE OF CONTENTS

1. INTRODUCTION 9

1.1 Terms of reference 9

1.2 Scope of work 101.2.1 What is the difference between “construction techniques” and “installation techniques” ? 111.2.2 What is an innovative construction technique ? 111.2.3 How is it possible for a newcomer in the cable world to design an underground link ? 13

2. DESCRIPTION OF THE CABLE SYSTEM 14

2.1 Description of the cable 14

2.2 Main cable systems configurations 142.2.1 Meshed underground network 142.2.2 Siphon 152.2.3 Substation entrance 152.2.4 Power generator output 162.2.5 Auxiliary supply 16

2.3 Cable 172.3.1 Extruded -dielectric cables : 17

2.3.1.1 Cable description 182.3.2 Cables with lapped insulation 18

2.3.2.1 Cable Description : 182.3.2.2 Self-Contained Fluid Filled Cable : SCFF 192.3.2.3 Impregnated Paper Characteristics : 19

2.4 Accessories 192.4.1 General 192.4.2 Accessory types 20

2.4.2.1 Types of joints 202.4.2.2 Types of terminations 20

2.4.3 Compatibility of the accessory with the cable 212.4.3.1 Number of cable cores 212.4.3.2 Cable constructional details 212.4.3.3 Conductor area and diameter 222.4.3.4 Operating temperature of the cable conductor and sheath 222.4.3.5 Compatibility of the accessory with the type of cable insulation and semi-conducting screens 222.4.3.6 Cable electrical design stresses to be withstood by the accessory 232.4.3.7 Mechanical forces and movements generated by the cable on the accessory 232.4.3.8 Short circuit forces 23

2.4.4 Compatibility of the accessory performance with that of the cable system 24


2.4.4.1 Circuit performance parameters 242.4.4.2 Circuit life required 242.4.4.3 Metallic screen bonding requirements 242.4.4.4 Earth fault requirements 24

2.4.5 Compatibility of the accessory with the cable system design and operating conditions 252.4.5.1 Type of cable installation design 252.4.5.2 Standard dimensions for cable termination 252.4.5.3 Types of accessory installations 252.4.5.4 Jointing limitations in restricted installation locations 252.4.5.5 Mechanical forces applied to the accessory 252.4.5.6 Climatic conditions 262.4.5.7 Type of accessory outer protection required 262.4.5.8 Situations requiring special accessory protection 262.4.5.9 Quality Assurance scheme for accessory installation 262.4.5.10 Training of Personnel 272.4.5.11 Assembly instructions 272.4.5.12 Special assembly tools 282.4.5.13 Preparation of the assembly environment 28

2.4.6 Compatibility of the accessory with specified after laying tests 282.4.6.1 Voltage test on main insulation 282.4.6.2 Partial discharge detection 282.4.6.3 Voltage withstand test on the cable over sheath and joint protection 292.4.6.4 Current balance test on the cable sheath and screening wires 29

2.4.7 Maintenance requirements of the accessory 292.4.7.1 Monitoring of fluid insulation 292.4.7.2 Voltage withstand tests on the over sheath and joint protection 292.4.7.3 Shelf life of accessories for emergency spares 292.4.7.4 Availability of accessory kits for emergency spares 29

2.4.8 Economics of accessory selection 292.4.8.1 Cost of the accessory complete with all components 302.4.8.2 Cost of guarantee and insurance 302.4.8.3 Cost of assembly time 302.4.8.4 Cost of preparing the installation environment for the accessory 302.4.8.5 Cost of safe working conditions 302.4.8.6 Cost of special jointing tools 302.4.8.7 Cost of training 302.4.8.8 Comparative cost of cable and accessories 302.4.8.9 Cost of verification of accessory performance 30

3. CONSTRUCTION TECHNIQUES 31

3.1 Definition of the main technical terms 31

3.2 Description of traditional techniques 313.2.1 Ducts 31

3.2.1.1 Description of the technique 313.2.1.2 Limits of the technique 323.2.1.3 Adaptation of the technique to the cable system design 34

3.2.2 Direct burial 353.2.2.1 Description of the technique 353.2.2.2 Limits of the technique 36

3.2.3 Tunnels 393.2.3.1 Description of the technique 393.2.3.2 Limits of the technique 423.2.3.3 Adaptation of the technique to the cable system design 44

3.2.4 Troughs 443.2.4.1 Description of the technique 443.2.4.2 Existing installation techniques 44


3.2.4.3 Installation methods 463.2.4.4 Limits of the technique for buried troughs 473.2.4.5 Limits of the technique for surface troughs 47

3.3 Description of innovative techniques 473.3.1 Bridges 48

3.3.1.1 Description of the technique 483.3.1.2 Limits of the technique 48

3.3.2 Shafts 493.3.2.1 Description of the technique 493.3.2.2 Limits of the technique 49

3.3.3 Horizontal drilling 513.3.3.1 Description of the technique 513.3.3.2 Limits of the technique 533.3.3.3 Adaptation of the technique to the cable system design 54

3.3.4 Pipe jacking 553.3.4.1 Description of the technique 553.3.4.2 Limits of the technique 573.3.4.3 Adaptation of the technique to the cable system design 60

3.3.5 Microtunnels 603.3.5.1 Description of the technique 613.3.5.2 Limits of the technique 633.3.5.3 Adaptation of the technique to the cable system design 65

3.3.6 Mechanical laying 663.3.6.1 Description of the technique 663.3.6.2 Limits of the technique 68

3.3.7 Embedding 693.3.7.1 Description of the technique 693.3.7.2 Limits of the technique 69

3.3.8 Use of existing structures 723.3.8.1 Description of the technique 723.3.8.2 Limits of the technique 733.3.8.3 Adaptation of the technique to the cable system design 73

4. CABLE INSTALLATION DESIGN AND LAYING TECHNIQUES 75

4.1 Cable installation design 754.1.1 Installation design in air 75

4.1.1.1 Rigid systems 754.1.1.2 Flexible systems (Western approach) 794.1.1.3 Flexible systems (Japanese approach) 834.1.1.4 Cable in ducts 86

4.1.2 Installation design for buried cables 874.1.2.1 Backfill 874.1.2.2 Cooling systems 87

4.1.3 Transition between different installation types 874.1.3.1 Transition between ducts and manholes (open air) 884.1.3.2 Transition between flexible and rigid systems (open air) 904.1.3.3 Transition between flexible and rigid systems (buried) 90

4.2 Cable laying and installation techniques 914.2.1 Cable pulling calculations 91

4.2.1.1 Clearance in ducts 914.2.1.2 Pulling tension 914.2.1.3 Side wall pressure 94

4.2.2 Installation Methods 954.2.2.1 Introduction 954.2.2.2 Nose pulling 95


4.2.2.3 Synchronised power drive rollers 964.2.2.4 Caterpillar or hauling machine 964.2.2.5 Bond Pulling 964.2.2.6 Mechanical laying 964.2.2.7 Other installation methods in tunnel 96

4.2.3 Installation process 984.2.3.1 Transportation of cable to site 984.2.3.2 Cable Bending Radius 994.2.3.3 Cable Temperature 994.2.3.4 Pulling Length 994.2.3.5 Route Profile 994.2.3.6 Obstacles 1004.2.3.7 Setting Up 1004.2.3.8 Installation of Cable 1004.2.3.9 Final Installation Stages 1004.2.3.10 Site Quality Assurance 1004.2.3.11 After Laying Tests 101

4.2.4 Adaptation of the Cable System Design to the Technique/Environment 1014.2.4.1 Adaptation of the Cable System Design to the Technique 1014.2.4.2 Adaptation of the Cable System Design to the Environment 106

5. EXTERNAL ASPECTS 110

5.1 Location (Urban vs. Rural) 110

5.2 Right of way 110

5.3 Magnetic fields 1105.3.1 Flat arrangement 1105.3.2 Trefoil arrangement 1145.3.3 Vertical arrangement 1175.3.4 Comparison between overhead lines and buried links 1195.3.5 Conclusion 120

5.4 Existing services 120

5.5 Legal aspects 122

5.6 Safety aspects 1235.6.1 Protection of the link from external damage 1235.6.2 Protection of the environment from a system fault 1245.6.3 Protection of the workers 1245.6.4 Protection of the public 1255.6.5 Safety of the different laying techniques 125

5.7 Environment 125

6. DESIGN OF A LINK 127

6.1 Methodology 127

6.2 Study cases 133

7. GLOSSARY 139

8. BIBLIOGRAPHY 142


LIST OF FIGURES page

Figure 1 : Percentage of use for the different techniques 12Figure 2 : Percentage of use for the different techniques 12Figure 3 : Percentage of techniques used 13Figure 4 : Meshed underground network 15Figure 5 : Siphon, an underground cable between 2 overhead lines 15Figure 6 : Underground substation entrance 16Figure 7 : Power generator output 16Figure 8 : Auxiliary transformer supply 17Figure 9 : Tunnel boring methods 40Figure 10 : Shield machine 41Figure 11 : Cooling system in tunnel 42Figure 12 : Filled troughs 45Figure 13 : Unfilled troughs 45Figure 14 : Unfilled troughs in air 46Figure 15 : Mechanical Laying 68Figure 16 : Maximum external cable diameter in terms of internal pipe diameter and clearance 74Figure 17 : Cable cleated with movement in a vertical plan 79Figure 18 : Plan view of cables installed with movement in a horizontal plan 81Figure 19 : Horizontal snaking 83Figure 20 : Vertical snaking 84Figure 21 : Shape of bend part 90Figure 22 : Horizontal bend 92Figure 23 : Vertical bend (pulling up) 92Figure 24 : Vertical bend (pulling down) 93Figure 25 : Upward slope 93Figure 26 : Downward slope 94Figure 27 : Cable installation in tunnel 97Figure 28 : Magnetic belt pulling machine 97Figure 29 : Flat arrangement, 1 circuit 111Figure 30 : Brms profiles with various s 111Figure 31 : Brms profiles with various d 112Figure 32 : Flat arrangement, 2 circuits 112Figure 33 : Brms profiles for two cable system configurations with various h 113Figure 34 : Brms profiles for two cable system configurations with various g 114Figure 35 : Trefoil arrangement, 1 circuit 114Figure 36 : Brms profiles for both flat and trefoil formations with various s flat and s trefoil 115Figure 37 : Brms profiles with various d for both flat and trefoil formations 115Figure 38 : Trefoil arrangement, 2 circuits 116Figure 39 : Brms profiles for two cable system configurations with various h 116Figure 40 : Brms profiles for two cable system configurations with various g 117Figure 41 : Vertical arrangement, 1 circuit 117Figure 42 : Brms profiles for flat, trefoil and vertical formations with various s flat, s trefoil and s vertical 118Figure 43 : Vertical arrangement, 2 circuits 118Figure 44 : Brms profiles for two cable system configurations with fixed h, d, g and s = s t = sv = 0.3m 119Figure 45 : Stage 1 128Figure 46 : Stage 2 129Figure 47 : Stage 3 130Figure 48 : Stage 4 131Figure 49 : Stage 5 132Figure 50 : Possible routes 134


LIST OF TABLES page

Table 1 : Horizontal drilling references 52Table 2 : Pipe jacking figures 57Table 3 : Horizontal snaking calculations 84Table 4 : Vertical snaking calculations 85Table 5 : Vertical cable installation at shafts 86Table 6 : Offset calculations 89Table 7 : Route cost 138

LIST OF PICTURES

Picture 1 : Direct burial 1Picture 2 : 400 kV XLPE cable 17Picture 3 : PVC ducts – double circuit 31Picture 4 : Direct burial 35Picture 5 : Open cut gallery 40Picture 6 : Cables in trough 45Picture 7 : Unfilled troughs in air 46Picture 8 : Dedicated tunnel for cables 48Picture 9 : Pipe Jacking 55Picture 10 : Microtunnelling 61Picture 11 : Mechanical laying 66Picture 12 : Embedding 70Picture 13 : ROV machine 72Picture 14 : Snaking in a tunnel 83Picture 15 : Cable pulling in duct 95Picture 16 : Cable installation in tunnel 96Picture 17 : Cable laying Locomotive undergoing trials. 98Picture 18 : Cable reel 99


1. INTRODUCTION

1.1 Terms of reference

GENERAL

The following terms of reference had been established by S. SIN (France), P. COUNESON(Belgium), W-D. SCHUPPE (Germany) and S.G. SWINGLER (United Kingdom).

They were accepted by SC 21 on August 1996 meeting.

INITIAL TITLE OF THE WORKING GROUP

The name of this new group is " Laying and Installation Techniques for High Voltage Cable Systems ".

INITIAL TERMS OF REFERENCE

To review existing and innovative methods for HV cable installation. The review should include cableinstalled in trenches, ducts and tunnels.

To compare the relative merits of the installation methods and to give recommendations for theirapplication.

Starting from the studies of the previous working group 21-01, it is anticipated that the method ofworking will be :

- Remind existing practices for cable installation and identify the factors responsible for the choiceof a particular practice.

- Review possible innovations, improvements and alternatives in the light of increasing economic andenvironmental pressures.

- Give recommendations for the application of new installation technologies to high voltage cablesystems.

In reviewing the achievements of WG 21-01 and the existing information available in their reports, theTask Force noted the need for a document summarising methods for design calculations. The workrequired is to :

- Review the calculations and parameters necessary to perform design calculations for cableinstallation (including for example, on the one hand, pulling tension during installation, and on theother hand requirements for installations in tunnels, ducts, manholes and towers).

- Compare theoretical productions with the results of engineering trials.

- Recommend simplified methods for the calculation of design parameters for cable laying.

The Task Force evaluated the work required and the skills necessary for its rapid and effectivecompletion. The results are necessary for the main task of the proposed working group in order toevaluate the optimum installation techniques taking into account network conditions, regulation, cables


type, etc. It is, however, unlikely that the main working group would have the necessary skills andresources to complete this task. It is therefore recommended that the Working Group establishes asubsidiary Task Force to advise and report on methods of calculation within timescales acceptable tothe main Working Group.

REVISIONS

1. A first revision was accepted in 1997 by the CIGRE Study Committee 21 on the limitation of theterms of reference.

It impacts the type of cable studied. The scope of work was limited to land extruded cables assubmarine ones are studied in other Working Groups and as technical brochures are published on theseitems. Nevertheless, an extension to LP SCFF (Low Pressure Self Contained Fluid Filled cable) hasbeen asked.

2. A second one was decided in 1999 by adding the review of the link safety with respect to theenvironment.

It has been decided that this Group will focus on what is under the soil, the upper part being treated bythe Group 21-19 "Technical and environmental issues regarding integration of underground cablesystems".

3. A third one was asked in 2000 by the SC on the term “Laying”.

This word is usually understood all around the world more as the pulling than the civil works prior to thepulling. As an example, we can refer to the concept “After laying test” which is well known by thecable industry. As so, it was considered that the word “Construction” should be added for a bettercomprehension in the title of the Working Group and in some chapters of the Technical brochure toexplain the civil works that are necessary to build an underground link.

The name of the group is now " Construction, Laying and Installation Techniques for High VoltageCable Systems ".

MEMBERSHIP

The membership of the Working Group should largely be made up of representatives from utilities withsignificant experience of cable installation. The subsidiary Task Force will require representatives fromcable manufacturers and construction companies.

TIME SCHEDULE

The Working Group should start their work before the end of 1996 and produce a final report inadvance of the Study Committee 21 meeting in September 2000.

RECOMMENDATIONS

The WG will review existing and innovative methods for HV cable installation and giverecommendations for their optimum implementation. The final report of the WG will be available inadvance of the 2000 meeting of Study Committee 21.

1.2 Scope of workThe group was composed of permanent and corresponding members, but all members were asked tocontribute, either by writing part of the document or by checking it. In its final version, the technical


brochure represents a comprehensive state of art shared by the technical cable systems communitythroughout the world.None of the members were implicated in the writing of the terms of reference. As so, the first taskwas to go through them to be sure to have a good understanding.Two questionnaires were prepared and sent in December 1997 to Utilities (46 replies from 22countries) and Cable manufacturers (27 replies from 16 countries) in order to collect the domesticpractice of the different countries.The first one dealt with laying and installation techniques and the second one with design calculations.Based on the replies and the technical knowledge of the writers, the technical brochure was thenestablished.Two task forces were then created, one with utilities and the other with cable manufacturers. A personfrom each task force was included in the other to guarantee an homogeneous work.As milestones of the Working Group's work, two technical papers were published :- a first paper in Jicable (June 99, paper A4.4) which was also published in REE special report n°4(1999), in REE (May 2000),- a second one in CIGRE session 2000 (paper 21-202),These two papers are a summary of the main results obtained from the completed questionnaires.In addition, a session concerning "Trends in high voltage cable laying and installation techniques" waschaired in the 1999 ICC-CIGRE Colloquium.

Throughout the life of the Working Group life, there was continuing discussion about :- What is the difference between “construction techniques” and “installation techniques” ?- What is an innovative construction technique ?- How is it possible for a newcomer in the cable world to design an underground link ?

Finally, the twelve existing construction techniques (traditional and innovative) are reported andexplained. A hypothetical case study is presented in Chapter 6.2 in order to demonstrate the way acomparative evaluation could be carried out. Cable engineers should apply the methodology to theiractual projects at the earliest possible stage. Estimated installation cost and anticipated environmentalconstraints should be used in order to compare these techniques and choose the optimal ones.Installation cost depends on many factors such as location, local regulations, etc… and will greatly varyfrom one project to another.

1.2.1 What is the difference between “construction techniques” and “installationtechniques” ?At the beginning of the Working Group's work, the difference was not very clear with the both wordsbeing used to define the same processes in a number of countries..Throughout this brochure, the terms have to be considered as follows: The term “constructiontechniques ” is considered as relating to the techniques used to create the cable route, mainly coveringthe civil works such as trenching. Likewise the term “installation techniques” is considered to relate tothe cable system design and cable installation methods.Cable design issues associated with the laying and installation techniques have also been consideredunder the general subject of "Installation Techniques".The cable installation was then the rest : the pulling and backfilling, the fixing when laid in open air.

1.2.2 What is an innovative construction technique ?Twelve different existing laying techniques were identified : they are detailed in the correspondingsections. Among them, only three are commonly used , (i.e. mentioned by more than 50% of thecompanies which replied to questionnaire N° 1 concerning utilities). These are : trenches (direct burial),ducts and tunnels.


To be more precise, the Working Group detailed the analysis on 2 voltage ranges : 60-170 kV,corresponding to HV and 220-500 kV corresponding to EHV. Therefore, 24 laying techniques can beconsidered, 12 for HV and 12 for EHV.

Figure 1 : Percentage of use for the different techniques

Figure 2 : Percentage of use for the different techniques

6 out of 46 companies have already used 50% of the different techniques (among the 24) and only 1out of 46 has used 90% of them.

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Figure 3 : Percentage of techniques used

To conclude, it was evident that a technique used by more than 50 % of the companies may beconsidered as a traditional technique and the others may be considered as innovative even though theyare already in use in other countries.

1.2.3 How is it possible for a newcomer in the cable world to design an underground link?This lead to a lot of discussions among the members of the Working Group, however all agreed on theprinciple that : a reliable link is based on a reliable cable design and manufacture, a reliable cablesystem design and reliable construction and installation techniques.It therefore appears necessary to not only give the description of the different techniques, but also togive guidance on the overall design process. For this, it was decided that the best approach would be todefine the process from the beginning to allow a complete understanding of what is needed to ensure areliable project..


2. DESCRIPTION OF THE CABLE SYSTEMThe purpose of this chapter is to give a quick overview on the different cable system components, todraw attention to the fact that the cable design is usually dependant upon the construction andinstallation techniques.An understanding of the cost can be obtained by taking into consideration the cost of the differentcomponents and the cost of their installation. The optimum costs can be developed by selectingdifferent solutions depending upon each of the cable system sections along the route. This will bedeveloped in chapter 6.

2.1 Description of the cableUnderground power transmission lines in the voltage range 60 kV and above make use of one of thefollowing types of cable systems :

• Extruded-dielectric insulated cables.• Self-contained medium or low pressure Fluid-filled cables (SCFF).

Where cables interconnect with other circuits, the transition is achieved through a termination. Thelength of a continuous section of cable is often limited by the size or weight of the cable reel that canbe transported to the installation site, sometimes by the safe pulling tension that can be applied to thecable, or by the maximum induced voltage on the metallic screen of the cable. The lengths are thenconnected in joint-bays. This is achieved through joints (or splices).Joints and terminations are the main components of equipment called cable accessories.

2.2 Main cable systems configurationsVarious configurations such as single circuit, double circuit and triple circuit lines with differentarrangements of transformer and generator connections are in use.

Many types of connections comprising overhead lines, underground cables or both are possible and canbe found. The length of such transmission lines and cables can vary significantly.

For load reasons, one circuit can consist of several cable systems. Note that in the subsequent figureseach cable can consist of several cable systems

Main configurations, given below, are representative of the most common practical situations.

2.2.1 Meshed underground networkSome parts of a HV network may be entirely underground as can be often seen in large towns whereurbanisation prevents the construction of overhead lines. Cables connect the busbars in the system, asindicated in Figure 4.


Undergroundcables

Substation 3

Substation 2 Substation 1

Figure 4 : Meshed underground network

2.2.2 SiphonA siphon is an underground cable connected between two overhead lines. It is assumed that noswitching device is located between line and cable. This configuration allows a HV/EHV link to passthrough areas too wide for an overhead line span such as rivers or small lakes. The configuration mayalso permit the transmission line to pass through or near a protected site or an urbanised area.

Underground cable

Overhead lineOverhead line

Figure 5 : Siphon, an underground cable between 2 overhead lines

2.2.3 Substation entranceAn underground cable is often used as the interface between an overhead line and a substation,especially when it is a gas insulated station. This configuration allows the design of more compactstations, in particularly when there is a large number of incoming overhead lines.


Overhead line

Undergroundcable

Substation

Figure 6 : Underground substation entrance

2.2.4 Power generator outputAn underground cable may be used to carry power from an inaccessible generator to a busbar. In thiscase, there is not room enough to put a breaker between the generator and the cable. In many hydropower stations the generator is located inside a mountain. In order to save space the generator isconnected directly to the step-up transformer, without usage of a circuit breaker. The secondary sideof the transformer is connected to an outdoor substation via cable which may have a length up toseveral kilometres. The substation (air insulated or gas insulated) is connected to one or more overheadlines.

Busbar

Generator

Overheadline

Underground cable

Figure 7 : Power generator output

2.2.5 Auxiliary supply

In this configuration, a cable is connected between a high power busbar and the auxiliary transformerof a power unit. The cable is usually short.


BusbarGenerator Transformer

Auxiliarytransformer

Undergroundcable

Overheadline

Figure 8 : Auxiliary transformer supply

2.3 CableAlthough the present work is focusing on construction and installation techniques of extruded and self-contained fluid-filled cables, it seems useful to give a brief overall view of the different types of cablesin service at the present time. These cables belong to two main families:• cables with extruded insulation : extruded – dielectric cables• cables with lapped insulation : SCFF, HPFF, HPGF. but SCFF are only considered here.In this document, only extruded and SCFF cables are considered.

2.3.1 Extruded -dielectric cables :

Extruded-dielectric cables, also known as solid-dielectric cables, have been introduced for mediumvoltage cables in the fifties. The first high voltagecables with extruded insulation on 110 kV systemswhere installed in the 1960'. Insulation materials areeither Ethylene-Propylene Rubber (EPR), low/high-density polyethylene (LDPE/HDPE) or crosslinkedpolyethylene (XLPE). EPR, XLPE, LDPE and HDPEhave been in use for many years. XLPE becomes apredominant choice for high voltage cables up to 500kV level.Maximum conductor temperature in normal operationis depending on the insulation material: 70 °C forLDPE, 80°C for HDPE and 90°C for EPR and XLPE.

Picture 2 : 400 kV XLPE cable


2.3.1.1 Cable description

The conductor is in most cases stranded copper or aluminium, sometimes solid aluminium. For sizesgenerally equal to or larger than 1200 mm² for copper and 1600 mm² for aluminium, the conductor issegmented to reduce the ac/dc resistance ratio.

A semi-conducting bedding tape is sometimes wrapped over the conductor before extrusion. Thisprevents the inner semi-conducting layer from entering the strand interstices during the extrusionprocess and, in turn, facilitates removal for splicing and terminating.

The inner semi-conducting layer is extruded over the conductor or semi-conducting bedding tape. Itspurpose is to provide a smooth interface between the conductor and the insulation, and an uniformelectric field. It avoids the presence of air between metallic and insulation materials (no partialdischarge) and constitutes a thermal barrier in short-circuit conditions.

The insulation and outer semi-conducting layer are the other parts of the dielectric which arepreferably applied by triple extrusion process. Indeed, the simultaneous extrusion of the semi-conducting layers and the insulation through a common (triple) cross-head is the best solution toeliminate protrusions at the interfaces which are sources of high voltage stress points.

A metallic screen made with copper or aluminium wires and/or a metallic sheath carries thecapacitive current and the fault current of a specified magnitude and duration before reaching aspecified temperature.A metallic sheath is normally applied to prevent the ingress of moisture. Its design must take intoaccount thermal and mechanical considerations. Since extruded dielectric materials have significantlyhigher coefficients of expansion than metals, the radial volumetric expansion can be quite large. Thesheath must remain in good contact with the outer semi-conducting layer during heating and cooling.

A jacket or outer covering or oversheath (made of PE or PVC) prevents the corrosion of the metallicsheath and isolates it from the ground. It is also required to protect the cable during handling and pullingoperations.

2.3.2 Cables with lapped insulationImpregnated paper cables, widely used from the beginning of the last century, made possibleunderground power transmission up to highest voltages. Many grids are still fitted out to a large extentwith these cables, even if they are replaced by extruded-dielectric cables to an ever-increasing extent.2.3.2.1 Cable Description :

A conductor screen, containing carbon black or acetylene black, or a metallised paper, is lapped aroundthe conductor to provide a smooth interface between conductor and insulation and an uniform electricfield.

The insulation consists of either a pure cellulose material, a high-quality kraft paper or, more recently, alaminated paper-polypropylene. Many individual crossed layers of tape (width 10 to 30 mm, thickness0,06 to 0,15 mm) are helically applied to the thickness required for the rated voltage. According todifferent methods, the cable is first dried in a tank and then impregnated with a degassed and driedimpregnating compound.

An insulation shield has the same function as the conductor shield on the outer side of the insulation.


Every type of cable is subjected to load variations. The temperature cycling during operation leads tothermal expansion and contraction of both the conductors and insulation materials. Small cavities mayappear in the insulation under the metallic sheath. The service life may be reduced by partial dischargeunder high electric field. Therefore, a pressurising fluid with high-quality dielectric characteristics isused to impregnate the insulation and fill the cable core. It increases dielectric strength, suppressesionisation in the insulation and delays moisture ingress in case of sheath leaking.2.3.2.2 Self-Contained Fluid Filled Cable : SCFF

A self-contained fluid filled cable is internally pressurised with low viscosity dielectric fluid. Eachindividual phase is contained within a hermetically sealed metallic sheath, typically extruded lead orcorrugated aluminium.

A central hollow core in the conductor provides a passage for dielectric fluid. The oil pressurenecessary to prevent from ionisation is 1 to 3 bar, but recent developments allow operation until 15 barhigh pressure.

For three core cables, the phases are generally contained within a common sealed metallic sheath,again typically extruded lead or aluminium. Ducts located between the phase conductors provide forpassage of the dielectric fluid.2.3.2.3 Impregnated Paper Characteristics :

Both kraft-paper and laminated paper-polypropylene insulations have normal operating temperatureslimits of 85°C, and allowable maximum emergency operating temperatures of 105°C.

The hydraulic system design must take into account the cable route and elevation differences to ensurethat all parts of the cable route are maintained at a pressure above atmospheric under all operatingconditions. In addition, the design must ensure that the pressure limits are not exceeded.

To achieve this, it is normal for longer routes to be divided into a number of hydraulically separatesections by using stop joints which maintain electrical continuity but isolate adjacent cable sectionshydraulically.

In some applications, the cable is impregnated with a special non-draining compound.

2.4 AccessoriesAs a general note, we only discuss extruded cable in this section with no reference whatsoever toSCFF accessory design issues.

2.4.1 GeneralThe reliability and performance of a cable circuit is dependent in equal measures on the designs of thecable and accessory and on the skill and experience of the person who is assembling the accessory.The cable insulation is extruded or lapped in the factory under controlled process conditions usingselected materials of high quality. It is equally important that the same quality measures are employedfor the manufacture of the accessories in the factory and for their assembly on site onto the speciallyprepared cable.

It is essential to select the design of accessory to be compatible with the particular cable type and theparticular service application. Compatibility should be validated and be supported by appropriate tests,or service experience. In particular the performance of the accessory is dependent on the quality, skilland training of the jointing personnel in the installation conditions and on the use of the specialised toolsrequired for a particular accessory.


The itemised sub-headings below form the basis of the information that is needed by the manufacturerand installer of the cable and accessories. For many applications the cable manufacturer alsomanufactures, supplies and installs the accessories as part of the complete cable circuit, thus theinformation is immediately available in-house. In the event that the user purchases the accessoriesseparately from the cable, then the following items form the basis of the questions that should be askedto the manufacturers of the cable and accessories to ensure that the accessories are suitable. As thedesign of the cable can depend upon the construction and installation technique, theaccessory design or selection must be made accordingly.

2.4.2 Accessory types

2.4.2.1 Types of joints

A joint is the insulated and fully protected connection between two or sometimes more cables. It is alsotermed “splice”. The following types exist :

• Straight joint,• Transition joint,• Screen interruption joint,• Y branch joint. For SCFF systems, an additional joint is required to isolate adjacent hydraulic sections of the cableroute to ensure the system hydraulic pressure limits are not exceeded. This is referred as a stop joint. The design requirements common to each type of joint are : • A high current connection between conductors,• Joint insulation which meets the same performance standard as the cable,• A high current connection to permit the flow of short circuit current between the two cable sheaths

or screen wires,• A metallic joint shell or screen wire connection electrically insulated from earth potential to match

the insulating integrity of the cable oversheath,• Protection of the joint and cable insulation against the ingress of water,• Protection of the joint metalwork against corrosion,• A tough protective sleeve against mechanical aggressions,• A heat dissipating device to ensure that the joint is not a hot spot along the power link.

2.4.2.2 Types of terminations

A termination is the connection between a cable and other electrical equipment. It is also termedpothead. The following types exist :

• Metal enclosed GIS termination, (GIS: Gas Insulated Switchgear)• Oil immersed transformer termination,• Outdoor termination,• Indoor termination,• Temporary termination.

The design requirements common to each type of termination are :


• A high current connection between the cable conductor and an external busbar,• Insulation which meets the same performance standard as the cable,• A high current connection to permit the flow of short circuit current from the cable metallic sheath

or screen wires via a bonding lead to the system earth,• A connection to the cable metallic sheath or earth wires which is electrically insulated from earth

potential to match the insulating integrity of the cable oversheath,• Protection of the cable insulation against the ingress of water and the ingress of pressurised

dielectric fluid from adjacent metalclad busbar trunking,• Protection of metalwork against corrosion,• Provision of support to the cable,• Ability to withstand cable thermomechanical loads and external forces such as wind, ice and busbar

loading.

2.4.3 Compatibility of the accessory with the cable

2.4.3.1 Number of cable cores

The user should determine whether the cable construction is of single, three core or triplex construction(i.e. three single core cables twisted together). The design of the accessory and the method ofassembly is dependent upon the number of cable cores; however it is unusual for three core extrudedcables to be employed above 60 kV.

2.4.3.2 Cable constructional details

For satisfactory service performance it is most important that the correct size of accessory is selectedto suit the particular cable. The outer diameter of the cable insulation, its tolerance and shape areparticularly important in the selection of an accessory employing a premoulded component, such as anelastomeric stress cone or an elastomeric joint moulding. Such components are designed to fit aspecific range of diameters of prepared cable insulation, (that is with the insulation screen removed andthe insulation smoothed and shaped). The components must not be used outside this range. Theminimum diameter is determined by the need to achieve sufficient pressure to eliminate voids at theinterface with the cable insulation. The maximum diameter is determined by such considerations as a)preventing damage by over stretching during assembly and b) limiting the maximum pressure at theinterface such that compression set of the cable insulation and moulded insulation is minimised.

The diameter and tolerance of the conductor and of its compaction (the radio of the effective crosssectional area of the metal to the total area occupied) are needed in selecting a connector that willexhibit stable conductivity and high mechanical strength.

The diameters and tolerances of the cable metallic barrier and over sheath are needed to ensure thataccessory metallic flanges and other components can be passed back over the cable during assembly.

The following dimensional and constructional details should be obtained by the user to ensurecompatibility of the accessory with the cable :

The detailed cable construction should be obtained from the cable manufacturer, which includes thefollowing information as a minimum requirement. Diameters, maximum and minimum tolerances,eccentricity dimensions, construction and material need to be obtained for each of the following cablecomponents :

• conductor and special features (e.g. water blocking), if any


• conductor screen• insulation (ovality and eccentricity dimensions are required)• insulation screen• screen wires, if any• longitudinal water blocking, if any• metallic barrier, if any, for example whether an extruded sheath, a welded sheath, or a laminated foil

barrier. Also whether of cylindrical or corrugated form• over sheath• armour, if any• special features (e.g. presence of optical fibre or pilot wires).

2.4.3.3 Conductor area and diameter

The user should ensure that the accessory has been designed and tested for the particular cableconductor size. The electrical performance of an accessory design can become critical on largeconductor cables because of the high cable insulation screen stress.

The user should ensure that the conductor connections in the complete kit of components are suppliedto suit the particular conductor construction. The conductor connection must be capable of carrying thesame current as the cable conductor and must be capable of withstanding the cable longitudinalthermomechanical forces, depending on the installation design, these being proportional to thecross sectional area.

2.4.3.4 Operating temperature of the cable conductor and sheath

The operating temperature of the cable conductor and sheath under continuous, short term overloadand short circuit current loading have to be taken into account properly.The materials of the accessory must be capable of operating satisfactorily at the operatingtemperatures specified for the cable. IEC 61443 Standard may be taken as a reference. The short termoverload temperatures depend upon the type of cable and application. The temperature of theconductor under short circuit is typically taken as 250°C for XLPE and 160°C for paper insulatedcable. The permitted short circuit temperature of the cable extruded metallic sheath or screen wires isdetermined by the type of metallic sheath and thermoplastic over sheath, this temperature usually beingsignificantly less than that of the cable insulation.

2.4.3.5 Compatibility of the accessory with the type of cable insulation and semi-conducting screens

• Physical compatibility with the extruded cable

The insulation of the polymeric cable must be identified by the user. There are significant differencesbetween the electrical and mechanical characteristics of extruded insulation. The usual insulants forextruded polymeric cables in the voltage class of 60 kV and above being XLPE (crosslinkedpolyethylene), LDPE (low density polyethylene), HDPE (high density polyethylene) and EPR (ethylenepropylene rubber).

• Chemical compatibility with the extruded cable

The type of insulating liquid or lubricant used in joints and terminations should be identified to ensurethat these do not affect the properties of the polymeric insulation and semi-conducting screensemployed in the cable and accessories. For example a) hydrocarbon liquids at elevated temperaturecan cause swelling of XLPE and EPR insulation and reduction of the conducting properties of screensand b) silicone liquids can have an effect on silicone rubber components.


• Compatibility with the paper insulated lapped cable

In the case of transition joints between polymeric cable and paper insulated cable it is important toestablish whether the cable is of the internally or externally pressurised type and whether the fluiddielectric is a gas or a liquid; these details will determine the performance requirements of the barrierplate that segregates the two cables. In the case of mass impregnated non pressurised cables it isimportant to determine the type of impregnating compound and whether it is of the liquid type or of thenon draining type; these details will determine the chemical suitability of the materials employed withinthe joint to segregate the impregnating fluid from the insulation of the polymeric cable and joint.Penetration of a hydrocarbon impregnating fluid into the polymeric cable can result in swelling andmodification of the electrical characteristics of the semi-conducting screens and insulation of both thecable and accessory components, thereby reducing their electrical performance. Loss of theimpregnating fluid into the polymeric cable can result in eventual electrical failure of the paper cable.

2.4.3.6 Cable electrical design stresses to be withstood by the accessory

The user is advised to obtain the magnitude of the cable stresses at the conductor and insulationscreens, or obtain the dimensions of the cable, thereby permitting the stresses to be calculated. The unitof stress is kV/mm calculated at U0 voltage. There are significant differences in the magnitude of theelectrical design stress employed in cables, these being dependent upon the type and thickness ofinsulation, the conductor size, the system voltage and the lightning impulse voltage. It is essential thatthe accessory has been designed and tested to operate at the particular cable design stress.The stress at the cable insulation screen is of particular significance because this normally determinesthe maximum design stress in the accessory. The insulation screen stress is usually of higher magnitudein those cables designed for high system voltages and large conductor diameters.

2.4.3.7 Mechanical forces and movements generated by the cable on the accessory

The magnitude of the forces and movements generated by the cable on the accessory depends uponthe cable materials, the method of cable manufacture and the type of cable installation design (i.e. rigidor flexible installation).

The following mechanical strains are dependent on the cable construction :

• insulation retraction (shrink back) (extruded insulation),• insulation radial thermal expansion,• oversheath retraction (shrink back). The following forces are dependent upon the cable construction, current loading, operating temperature,method and type of cable constraint and accessory design : • conductor thermomechanical thrust and retraction,• sheath thermomechanical thrust and retraction.

2.4.3.8 Short circuit forces

Electromagnetic forces are present during a short circuit between the individual conducting componentsof the accessory and between the adjacent cables and the accessory. The following information isapplicable :

• method of restraint of the accessory and cable,


• method of restraint and the spacing of adjacent cables.

2.4.4 Compatibility of the accessory performance with that of the cable system

2.4.4.1 Circuit performance parameters

The current rating and optimum circuit economics are dictated by the cable conductor size, cablematerial costs and the method of installation. To achieve the optimum economical solution it isimportant that the accessory design is not allowed to limit the performance of the cable. The accessorymust therefore match the following cable performance :

• Rated voltages(Nominal system voltage U and maximum Um), • Current rating (Current magnitude), • Continuous, cyclic and short time overload (Current magnitude, time and temperature), • Short circuit rating, “ phase to earth ” and “ phase to phase ” (Current magnitude, asymmetry, time

and temperature), • Basic impulse level (Withstand voltages for lightning impulse and switching surge), (Flash over

voltage for the system insulation co-ordination of outdoor terminations, if specified ).

2.4.4.2 Circuit life required

The accessory should match the design life specified for the particular cable circuit. This is typicallyrequested to be from 20 to 40 years, however some cable circuits are installed as temporary links, forexample in an overhead line circuit. Such accessories may be designed to be suitable for quickassembly with a reduction in performance and service life.

2.4.4.3 Metallic screen bonding requirements

The following information is required on a) the type of bonding leads, (concentric or single conductors)and their conductor size and overall dimensions and b) the type of cable bonding scheme, for examplesolidly earthed or specially bonded metallic screens.

- Magnitude of induced sheath or screen wire voltage under normal and short circuit current,- Magnitude of circulating sheath or screen wire current under normal loading,- Magnitude of short circuit current,- Magnitude of specified over sheath lightning withstand voltage and dc withstand voltage.

It is important that the accessory design incorporates means of connecting the cable screen wires,metallic tapes or sheath and joint shell to the insulation screen.

2.4.4.4 Earth fault requirements

Some Utilities require that short circuit currents be returned within the cable system. The user shouldensure that the accessory is also able to withstand this current.


2.4.5 Compatibility of the accessory with the cable system design and operatingconditionsThe user is advised to ensure that accessory design is a) compatible with the particular cableinstallation design, as this determines the mechanical loading applied, b) capable of being assembled inthe site environmental conditions and c) capable of a satisfactory service performance under adverseclimatic conditions.

2.4.5.1 Type of cable installation design

• Rigidly constrained (cable laid direct in the ground or close cleated)• Flexible unconstrained (cable horizontally snaked or vertically waved)• Semi-flexible (cable constrained, but permitted to exhibit a controlled deflection, for example at a

bridge crossing or adjacent to gas immersed switch gear)• Unfilled duct.

2.4.5.2 Standard dimensions for cable termination

The user is advised to ensure the following dimensional compliance :

• Outdoor and indoor termination : Harmonisation with existing equipment of the overall height of the off-going bus bar connector and ofthe bottom metalwork fixing arrangements to the support structure. • GIS and transformer termination :Harmonisation of the cable termination with both the design of the metal clad switch gear (internaldiameter, overall length, off-going bus bar connector, bottom metalwork sealing arrangements andpressure) and the design of the support structure (fixing arrangements for the particular cableconstraint selected).2.4.5.3 Types of accessory installations

• Buried in the ground (laid direct)• Jointing chamber,• Tunnel,• Above ground,• Bridge,• Tower,• Shaft.

2.4.5.4 Jointing limitations in restricted installation locations

• Space limitations,• Time limitations (for example arising from road or rail traffic influences),• Tolerance limitations of assembly personnel (for example arising from extremes of temperature,

humidity, vibration, noise and induced voltage).

2.4.5.5 Mechanical forces applied to the accessory

• Thermomechanical forces• Earthquake,• Vibration,


• Off-going bus bar at terminations,• Wind loading on bus bars at terminations,• Ice loading on bus bars at terminations,• Short circuit loading on bus bars at terminations,• GIS pressure,• Angle of installation of terminations,• Hydraulic or pneumatic pressure forces at transition joints.

2.4.5.6 Climatic conditions

Accessories require to be suitable for the extremes of climatic conditions expected both in service andduring assembly. Some types of accessories are required to be assembled under controlledenvironmental conditions.

• Altitude (reduction of electrical strength of air),• Air pollution (reduction of electrical strength of outdoor insulator surface),• Precipitation (reduction in electrical strength of air and outdoor insulator surface),• Salt fog (reduction in electrical strength of outdoor insulator surface),• Moisture condensation (reduction in electrical strength of insulator surface),• Temperature,• Atmospheric humidity.

2.4.5.7 Type of accessory outer protection required

The accessory protection is required to provide corrosion protection and, for a specially bonded cablecircuit, insulation from ground.

• Joint box (laid direct in the ground or in air),• Pedestal insulator (in air),• Moulded sheet insulation (in air, to protect personnel against electric shock),• Metallic fences or screens (in air, to protect personnel against electric shock).

2.4.5.8 Situations requiring special accessory protection

• Submerged under water,• Fire risk,• Termite infestation.

2.4.5.9 Quality Assurance scheme for accessory installation

Assembly of the accessories onto cable with extruded insulation is the most vulnerable part of a projectinvolving the manufacture. Accessories and cables are manufactured and tested under controlledfactory conditions, whereas the in-service performance of the accessory is dependent upon the training,skill and reliability of the personnel, who are often required to work under adverse site conditions.

For many project applications one company will manufacture the cable and accessories and undertaketo complete the installation of the circuit. In other applications the installer may complete the circuitusing cable and accessories supplied by different manufacturers. In some applications the installer mayonly assemble the accessories. For each application the requirements of the QA system are equallyrigorous :


• Quality Assurance approval for installation

The user should ensure that the installer provides evidence of an approved quality assurance system forinstallation to an internationally recognised standard.

• Quality Plan

The installer is required to produce a Quality Plan for each project, this includes the project timeschedule together with the requirements for suitably qualified personnel, training, on-site storage ofcomponents and accessories, tools, testing equipment, constructing materials, assembly instructions,preparation of the jointing environment and records of the assembly work. It is important that therecords of assembly are traceable to the location of each accessory in the cable circuit. If purchasingseparately, the user is advised to ensure that, for the purposes of traceability, the quality systems of thecable manufacturer, accessory manufacturer and installer are compatible.

2.4.5.10 Training of Personnel

When selecting the designs of accessories the user should ensure that training courses are available forthe jointing and supervisory personnel. It is strongly advised that personnel receive training on theparticular designs of accessories and cable.

Examples of the elements of a training course for assembly personnel are :

• General training at specific system voltages with the standard range of accessories required by theuser

• Repeat training after a defined period for those personnel who have completed general training • Specified training on a new accessory or cable design for those personnel who have completed

general training.

At the end of the training course the proficiency of the assembly personnel is normally assessed, forexample, by a verbal or written examination, by a practical test and preferably by performing on theassembled accessories an electrical partial discharge test and voltage withstand test.

Proficiency is recognised at the completion of training by the issue of a certificate, which should bechecked by the user as part of the quality plan for a specific project. In many instances a kit of generaljointing tools and a set of general assembly instructions is also issued to the personnel followingsatisfactory completion of training.

2.4.5.11 Assembly instructions

The accessory manufacturer is required to supply a complete set of assembly instructions together withdrawings of the particular accessory.

The instructions should also include lists of the specified assembly tools, the specified consumablematerials and the health and safety precautions. Recommendations for the preparation of the assemblyenvironment should also be given.

It is important that the user studies the instructions before work begins to ensure that the workplace iscorrectly prepared and that all the tools and consumable materials are available.


2.4.5.12 Special assembly tools

Most designs of accessories, particularly those operating at higher system voltages, require special toolswhich are purchased or hired from the accessory manufacturer. The user should ensure that fullinstructions are provided and that the personnel are trained in their use. These tools may take the form,for example, of a) hydraulic compression presses or welding equipment for connecting the conductors,b) cutting equipment to remove the insulation screen and to shape the cable insulation c) assemblymachines which stretch and position pre moulded elastomeric components, d) taping machines thatapply tape and e) heated mould tools and mobile extruders for field moulded joints.

2.4.5.13 Preparation of the assembly environment

It is strongly recommended that the assembly area for both joints and termination to be enclosed withina tent or temporary building, with the objective of providing a clean and dry environment. The enclosureshould be a) well lit to facilitate accurate preparation of the cable insulation, b) provided with a soundfloor and c) lined with sealed materials to facilitate cleanliness. In extremes of climate it is goodpractice to provide control of temperature and humidity to ensure a) consistent performance of thepersonnel and b) consistent properties of the polymeric materials.

• Joint assembly :

• An appropriately sized joint bay or chamber.• The provision of a temporary and/or permanent support for the completed joint.

• Termination assembly :

• A permanent support structure.• A temporary weatherproof structure during assembly.• Means of lifting the cable and insulator into position.

2.4.6 Compatibility of the accessory with specified after laying testsWhen the installation of the cable and accessories has been completed it is standard practice toperform electrical tests to demonstrate that the assembly of the accessories is of satisfactory qualityand that mechanical damage to the cable and accessories has not occurred during installation.The following tests can be performed. It is important to ensure that the accessory design is suitable forthe particular test :

2.4.6.1 Voltage test on main insulation

DC tests have been traditionally applied to transmission circuits, however their use on cable withextruded polymeric insulation is not recommended. Experience has shown that the dc voltage test is notalways sufficiently sensitive to detect damaged cable insulation or incorrectly assembled accessoriesand hence prevent them from entering service. In particular the electrical stress distribution under dcvoltage in an accessory is usually significantly different from that under ac voltage in normal service.The application of an ac voltage is now under evaluation as an after laying test, either by the applicationof service voltage from the transmission system or by the application of test voltage from mobile testequipment.

2.4.6.2 Partial discharge detection


Partial discharge detection techniques are at present being developed for some cable and accessoryapplications to check for the absence of damage to the cable during installation and incorrect assemblyof the accessories. Methods are not yet available for this to be done in a simple manner as a routinecommissioning test on normal cable circuits.

2.4.6.3 Voltage withstand test on the cable over sheath and joint protection

It is usual for specially bonded cable systems, including their accessories, to be subjected to an afterlaying test comprised of the application of a dc withstand voltage applied to the metallic sheath orscreen wires.

2.4.6.4 Current balance test on the cable sheath and screening wires

This test is performed on cross bonded cable systems at or adjacent to accessory positions to confirmthat a) the bonding connections of the accessory are correct and b) the cable lengths and spacing aresymmetrical, such that the magnitude of residual circulating current is of an acceptably low magnitude.

2.4.7 Maintenance requirements of the accessoryThe user should ensure that adequate maintenance tests and checks have been recommended by thecable and accessory suppliers, for example :

2.4.7.1 Monitoring of fluid insulation

Liquid and gas levels : some types of termination, straight joints and transition joints are filled withinsulating liquid or gas and may require to be regularly inspected or monitored in service to ensure thatneither the liquid or gas have escaped.

2.4.7.2 Voltage withstand tests on the over sheath and joint protection

These tests are similar to the after laying tests, but are usually performed at reduced voltage levels.

2.4.7.3 Shelf life of accessories for emergency spares

The user should ensure that information is provided on the shelf life of the components in an accessoryfor long term storage as these may vary according to the type of material, the way they are packed andthe appropriate temperature and humidity conditions of storage.

2.4.7.4 Availability of accessory kits for emergency spares

The user is recommended to obtain either a sufficient stock of spare accessories or to have anagreement with the manufacturer to supply accessories at short notice. The design of an accessory foremergency use may be different from that installed.

2.4.8 Economics of accessory selectionA comparison of the relative costs of different designs of accessory kits should not be undertakenwithout giving due consideration to the total costs of installation and assembly. The following are themain items of cost :


2.4.8.1 Cost of the accessory complete with all components

The accessory design should be checked to ensure that it is a complete kit and will be supplied with allthe components and assembly instructions for the particular application. Some components that may notnecessarily be supplied by all accessory manufacturers are for example a) conductor connections andanti-corrosion protection for joints and b) bus bar take-off connectors and support metalwork fortermination.2.4.8.2 Cost of guarantee and insurance

At the higher system voltages it is more usual for the cable and accessories to be supplied, installed andguaranteed as a “ turn-key ” project. Under such circumstances the guarantee will usually extend to aspecified number of years in service. If the user decides to divide the supply and installation ofaccessories between companies, it is recommended that the cost of financial self insurance beconsidered, because the responsibility for an accessory failure in service can be difficult to apportionbetween the accessory manufacturer, the cable manufacturer and the installer.2.4.8.3 Cost of assembly time

The jointing time required to assemble accessories can differ dependent on their design. Similarly thetime required to assemble the anti-corrosion protection and the final mechanical support to theaccessory can be the over-riding factors in determining the jointing time.2.4.8.4 Cost of preparing the installation environment for the accessory

Accessories require the provision of a weatherproof enclosure together with the environmentalconditions necessary for jointing (e.g. good lighting, cleanliness and, when necessary, air conditioning.The supply of electricity and gas may be required).2.4.8.5 Cost of safe working conditions

In addition to the cost of constructing the installation environment to comply with the regulations forsafe working practices , the provision may be required for temporary and permanent protection to a)the installer's personnel from electric shock during assembly and b) the user's personnel when theaccessory is in service.2.4.8.6 Cost of special jointing tools

There may be significant differences in purchase cost and hiring charges of the tools required fordifferent accessories.2.4.8.7 Cost of training

Qualified jointers who are trained to assemble the particular accessory should always be employed.The user should decide whether it will be more cost effective to a) employ qualified and experiencedpersonnel to assemble the accessories, or b) employ qualified and experienced personnel to install thecable and assemble the accessories as part of a turn-key contract, or c) incur the on-going costs oftraining and regular repeat training for his own personnel.2.4.8.8 Comparative cost of cable and accessories

The design of the cable can influence the cost of the accessory design. Thus a reduction in the cost ofthe cable construction may result in an increase in the cost of the accessories. Similarly an increasein the cost of installation by laying longer lengths of cable may achieve a reduction in overallcosts by requiring fewer joints.2.4.8.9 Cost of verification of accessory performance

If a type test report is not available for the particular cable and accessory in combination then the useris advised to allow for the cost of performing a type approval test. This cost may be born by thesupplier, in the case of a turn-key project, but this is less usually so in the case of separately suppliedcable and accessories.


3. CONSTRUCTION TECHNIQUESTwelve high voltage cable construction techniques have been identified and are reported in this section.Four are considered traditional while eight are labeled innovative. They are being used to a varyingdegree by different companies around the world.

3.1 Definition of the main technical termsSee the glossary, page 139.

3.2 Description of traditional techniquesAmong the twelve identified techniques, four are categorized as traditional. This is mainly becausemost or all companies around the world consistently use one or more of them in laying high voltagecables. They have been successfully used for many decades due to their simplicity, relatively low costand the availability of materials and equipment as well as qualified entrepreneurs to execute thenecessary work.

3.2.1 Ducts

3.2.1.1 Description of the technique

Ducts are normally used jointly with manholes in a system that isfavored in urban areas of major cities for its convenience. It offers thepossibility of carrying out the civil work independently from theelectrical work. Also, the flexibility of cable maintenance orreplacement with minimum disturbance to local traffic and economicactivities are considered advantageous. In less congested areas, jointbays would replace manholes to reduce cost.

Picture 3 : PVC ducts – double circuit

Three or more ducts having the proper diameter and wall thickness are placed in a trench at the pre-determined depth and configuration. A layer of special bedding material having low thermal resistivity isplaced on the bottom of the trench prior to placing of ducts. Thinner wall ducts could be encased inconcrete to form a duct bank. Ducts could also be stacked in two or more layers to accommodate therequired number of cables to be installed. Special spacers are used to ensure the exact configurationand to allow concrete to flow between ducts.Reinforcing steel rods should be used in special cases such as crossing under railways in order toincrease the rigidity of the duct bank .In some cable sections, the space between cables and ducts could be filled using special materials toenhance cable current carrying capacity or restrict its movement. This is recommended in excessivelydeep installation or when difference in elevation between manholes is substantial.

Manholes are underground chambers built to house the joints and other auxiliary equipment such asfluid feeding tanks, sheath cross bonding cables and sheath protection surge arresters. Access tocables and joints is easy using fixed or removable ladders installed in two or more chimneys dependingon manhole design.

Manhole dimensions depend on the number of cables to be jointed as well as the circuit voltage.Metallic structures are usually used inside the manholes to support cables and joints. All metallic steelmembers inside manholes should be properly connected to a solid ground rod or bare ground cable loop.


Joints should be protected from mechanical forces due to cable expansion under load cycles either byexpansion loops or by a rigid clamping system. Manhole dimensions could be reduced if joints andcables are rigidly clamped.Manholes should be designed in accordance with local standards to withstand normal road traffic loads.Most manholes are built in place using reinforced concrete. However recent development in pre castconcrete made it possible to use high quality prefabricated manholes. This would reduce the timeneeded for assembling the factory pre cast concrete slabs forming manhole walls and the roof. Floorsare still poured in place to allow for proper ground leveling and ground water drainage.Joint bays offer a more economic way to house and mechanically protect the joints. They could beregarded as the lower half of manholes. They could also be built in place or assembled using pre castconcrete slabs Temporary shelter should always be used during jointing operations to protect workers,cables and joints from the elements and ensure a clean environment for jointing operations. Once thejoints are completed , the joint bay is filled with thermal back filling material and top slabs placed overthe entire length. A warning tape is usually placed about 30 cm below grade level.Joints are not accessible in joint bays. Sheath testing is possible using link boxes located either aboveground or in below ground accessible pits where cross bonding cable leads are connected.

High Voltage cable circuits are normally installed in dedicated duct banks, often one cable per duct.However, for economical reasons, three cables could be installed in the same duct in case of lowervoltages. Also two circuits (six cables ) could be installed in the same duct bank. It is not recommendedto install more than two circuits in order to reduce the risk of cable damage due to accidentalexcavation. This would enhance underground system availability as well as maintain a reasonable cableload rating.Laying cables in ducts is considered one of the safest type of installation regarding safety in case ofshort circuit. It should be noted that a good earth cover over the duct bank is necessary to ensurepublic safety. It is also worth mentioning that manholes could present a safety hazard in case of cableor joint explosion.Empty ducts could be used for a reserve cable provided that sheath bonding is designed accordingly.Fiber optic communication cables could also be installed in the same duct bank .

3.2.1.2 Limits of the technique

• Civil workCivil work includes excavation of trenches and shoring them if necessary, relocation of existingservices, placing of ducts and spacers, pouring of concrete to form duct banks and covering them withthe proper back filling materials as well as reinstating of all surfaces to their original conditions. It isrecommended that construction of necessary manholes or assembling prefabricated ones is oftencarried out after ducts have been securely placed. Compacting of back filling materials as well as ofthe soil layers is essential in order to obtain a low thermal resistivity.

Long cable lengths could be pulled through straight duct sections provided that cable reels could betransported to site. However, due to factors related to cable route that have to follow existing road andstreet network, land topography and existing subterranean services, almost all cable routes includebends and offsets that would increase the required cable pulling tensions and thus limit distancesbetween manholes. Cables installed in ducts rarely exceed 800 meters. In major cities the maximumlength of open trenches at any given time may be limited by local authorities to a few hundred meters.

• Drying of the soilOver the years, soil drying may occur due to change in back filling materials properties, presence oftree roots or higher than normal cable operating temperatures. This could be avoided by using a proper


back fill, compacting of different layers during installation, keeping a tree-free zone along cable routeand ultimately by monitoring of cable's temperature or that of the surrounding soil.Use of thermocouples or fiber optic cables particularly during peak load periods and hot and dryweather spells would ensure this feed back.

• Water drainageWater table level varies with location. In some areas, abundant surface water could hinder civil workprogress. Water seeping through the ground during construction should be pumped out, usingappropriate equipment, to ensure personnel safety as well as quality of work.

Although the presence of water around cables and accessories could be considered somewhatbeneficial, many utilities do not allow it to accumulate in ducts or manholes to prevent possiblepremature deterioration of cables and accessories. Ducts would be installed with a continuous slightslope towards manholes. Manholes would be connected to city sewage or storm draining systemsthrough an anti pollution arrangement particularly in the case of fluid filled cables. Local regulationsshould be followed and authorization should be obtained for these connections.

• Temperature of the soil/environmentDucts could be installed in soils that are naturally warm provided that some forced cooling arrangementis foreseen.

• Hardness of the soilIn hard rocky soils it would be advantageous to consider alternative techniques to install cables such asmicro tunneling described in this document. Technical and economic studies should be carried out inorder to compare different viable alternatives.

• Stability of the soilDifferent soil formation could exist along any cable route. Soil should be tested and its propertiesinvestigated by carrying out on-site and laboratory tests. Soil stability should be ensured prior toinstallation of ducts or duct banks.

• Thermal resistivity of the soilSoil resistivity should be measured along cable route using appropriate instruments to determine theneed for replacing native soil by special thermal back filling. Some laboratory measurements could alsobe useful in establishing the maximum thermal resistivity and percentage of water content by weight ofsoil samples.

Back filling materials having higher thermal resistivities than that assumed in cable design calculationscould lead to higher cable operating temperatures, soil drying out and eventually dielectric breakdowndue to thermal runaway. Back filling of trenches should be done in layers that are properly compacted.Local regulations could influence the choice of back filling materials.

• SeismicityDucts could be used in seismic risk areas provided that they have been designed to withstand theexpected earth tremors. Both rigid and flexible designs would be acceptable. Some experimental workon a model are advisable.

• FrostFrost and ground freezing occurs for short or long duration in many countries. Ducts and duct banksshould be placed below the expected frost line in order to avoid damage due to ground movementcaused by (severe and frequent) freezing and thaw cycles.


In extreme cases where ground is permanently frozen, special arrangement such as placing insulatingmaterials underneath the duct bank is recommended. Non insulated duct installation would risk beingdamaged due to soil instability caused by heat dissipated from cables. Cable failure might result.

• ArchaeologySensitive archaeological areas should be avoided when cable route is selected. However shouldarchaeological finds be encountered during excavation, work should be immediately stopped and localauthorities advised .Depending on the importance of the findings, some countries would allow work tocontinue after proper investigations and documentation are completed. In other cases ,an alternativecable route might have to be chosen.

• Presence of termitesCables should be designed to have an anti-termite protection and ducts should be blocked using aproper sealing material such as" ductseal".

• Laying in National ParkLocal authorities should be consulted and if necessary, alternative techniques such as directional drillingbe used to minimize digging in sensitive areas of national parks.

• Duration of the workCivil work duration depends on many factors. The major factors to be considered are, access to site,the nature of soil, depth of excavation, presence of underground services, type of equipment used,weather conditions and restrictions imposed by local authorities.Average construction duration vary also according to the size of the project. Some values, includingcable installation, were reported in CIGRE joint working group 21/22-01 report issued in may 1996.

• Maintenance and repairing processManholes should be periodically inspected to ensure their structural integrity.Although cables installed in ducts are inaccessible, joints could be inspected at manholes. Visualinspection of joints and cables could be done after pumping out any water from manholes. At joint baylocations, only sheaths transposition cables could be reached through hand-holes.Periodical jacket testing could be performed from these points. Insulation or jacket faults could belocalized using different techniques. Repair should be carried out. This work would require someexcavation at fault location.In case of major problems, an existing cable section, between two manholes, could be replaced withoutany excavation.

• Cable removal after operationWith the introduction of new international standards for environment protection cables would have tobe removed at the end of their useful service life and their components disposed of and recycled. It isusually possible to remove cables from ducts without excavation. However, some sections might provedifficult or impossible to remove due to cable snaking, accumulation of dirt, deterioration of ducts andground up-heaving. In these cases new excavation permits would have to obtained to gain access tocables at locations between existing manholes.Structural integrity of empty manholes should be investigated. Local authorities could impose thedemolition of manholes for safety reasons.

3.2.1.3 Adaptation of the technique to the cable system design

Duct and manhole system is well suited for cable installation in congested city core areas. In designingcable systems to be installed in ducts many electromechanical factors should be carefully consideredtogether with civil engineering aspects, such as :


• proper cable size for the required load (larger cables are required for duct installations as comparedto directly buried or in air)

• maximum pulling tensions required for cable installation• metallic and non metallic sheaths for cable protection• maximum induced sheath voltage and allowable sheath currents• clamping of cables and joints in manholes if necessary• sheath permutation and protection schemes• grounding in manholes• size and location of manholes• size and type of ducts

3.2.2 Direct burial


This method consists of digging atrench and directly placing the cablesin it.

This technique is extensively usedworld-wide for extruded cable as wellas for fluid filled cable. Indeed, in the60 to 170 kV range it comes secondonly to laying in ducts, whereas forthe voltages between 200 to 500 kV itranks just after the laying in tunnelsand the laying in ducts.

Picture 4 : Direct burial

This solution is particularly interesting economically, since apart from digging and backfilling the trenchno other heavy works are necessary. This is why the technique is used in urban as well as in ruralareas. HV cables are usually installed along the public ways. As far as possible installation in privateground is avoided.

An advantage of this method is that the route of the link can easily be deflected to avoid unforeseenobstacles.

The depth of the trench is such that in most cases the cables have an earth cover at least one metrethick (this often is a legal requirement or this can also depend on the short-circuit levels).

Cables are usually laid in trefoil formation. Every metre an adequate non-corrodable clad or rope iswrapped around the cables to keep the trefoil formation during the backfilling of the trench. The othertype of laying configuration is the flat formation which is used mainly for cables in the 220 to 500 kVrange (depending on the carrying capacity).

Trench width obviously varies according to the type of formation and the voltage level of the cables :• width <0.8 m (60 to 170 kV) and close to 1.0 m (220 to 500 kV) in trefoil formation;• width close to 1.0 m (60 to 170 kV) and >1.0 m (220 to 500 kV) in flat formation.

Over the backfilling material cable-protective slabs are placed.


Above these slabs the telecommunication cables are usually placed (running mostly in ducts).


• Civil workThe civil works are identical to those required for duct laying, except that the cables are laid directly inthe trench on a bottom layer of materials intended to protect them from any sharp rocks likely to bepresent in the bottom of the trench.The backfilling materials used to fill the trench are composed, starting with the protective bottom layerreferred to above, of sand, special backfill or possibly lean concrete. It is not so frequent that theexcavated soil or concrete are used for backfilling.Weak mix may be used instead of the normal backfill to increase the mechanical protection around thecables.In many countries, a special backfill (so-called controlled backfill) is used in order to create a lowthermal resistivity environment to dissipate the heat released by the power cables (this greaterdissipation allowing an increased power rating of the link).In this respect, the use of fibre-optic cable or of optical fibres in HV cable, although not yetgeneralised, brings certainly advantages in the future as it will make possible permanent thermalmonitoring of the link (giving precise knowledge about the thermal environment of the cables).Civil works include also the excavation of joint pits. The size of these pits is naturally larger than thetrench (width and depth) itself, and may vary according to the voltage level, the type of joint and layout(in parallel or longitudinally).The length of the trenches is often defined by the size of the cable drums, drum-size itself being oftendependent on the means and possibilities of transport and handling (but the length can also depend onthe calculated pulling tensions if they exceed the limits, environmental aspects, …). Furthermore, inurban situations, especially taking into account of the traffic, opening a trench several hundreds ofmetres long may give rise to problems. Accordingly, local authorities may restrict the length of opentrenches (with increase of joint's number), restricting the periods of the year or week and sometimesthe hours of day during which work may be done.This problem is the more critical, in terms of site planning and organisation, when controlled backfill isbeing used as this material must be very carefully applied (degree of humidity, of compaction) andclosely inspected by a laboratory.The length of the trench may be some hundreds of metres, hardly ever exceeding 800 metres.

• Drying of the soilSoil drying can occur in the immediate vicinity of cables in service, due to migration of the humidityfrom the hot zone to the cold zone (so increasing the thermal resistivity of the dry zone).This phenomenon can also be observed when the link runs parallel to certain types of vegetation (treesthat have deep roots, …).In certain regions the soil may be permanently dry naturally.Various solutions can be considered :• use of controlled backfilling material;• creation of a ‘root-free’ corridor;• a different configuration for the cables (in flat formation instead of trefoil formation) to improve the

rate of heat dissipation of each cable.These points can have influence on the type or the width of the trench but don’t really change themanner of working.

• Water drainageConversely to the soil drying phenomenon, wet soil may seem more favourable to laying of HV cablesbecause it constitutes a natural cooling system.


If the water can flow out, there is nevertheless a risk of erosion of the soil and the materials embeddingthe cables.The execution of trenches in wet soil often calls for installing substantial means of drainage (pumps,etc.) in order to avoid erosion of the trench edges leading to collapse or "flooding".

• Temperature of the soil/environmentConstruction of trenches in a soil that is naturally or artificially (old mining areas) hot does notapparently cause any particular problems.However, it remains obvious that too hot a soil will significantly reduce the power transit capacity ofthe link.

• Hardness of the soilLaying of cables in particularly rocky soil, although technically feasible, is not advisable, as excavatingthe trench will require costly heavy equipment (and entails risk of equipment damage).Work planning has to make due allowance for the difficult conditions, to avoid falling undue delay.

• Stability of the soilConstruction of trenches in unstable soil such as in marshlands implies that the trench must becompletely shored (with sheet piling if necessary) and that drainage systems must be installed.Considerable cost savings are possible by carrying out a survey of the subsoil along the intended route,so that the costly or difficult route can be avoided by selecting another route for the trench.

• Thermal resistivity of the soilSoil resistivity considerations have little influence on the choice of the construction method. However, itdoes have a considerable impact on the power-transit capacity of the link.Accordingly, soil samples have to be taken for analysis well in advance of the start of the works inorder to determine the characteristics of the cables (cross-section, material, …), their configuration(trefoil or flat formation) and decide whether or not to use controlled backfilling. For a selected backfillto achieve an average resistivity, the size of the trench can also be influenced by the results of theanalysis.

• SeismicityCables laid in plain soil incur the stresses generated by soil movements. However, the soil movementsdo not normally seriously affect the cables as the cable components have a certain elasticity.Nevertheless, it is obvious that a large crack in the soil at right angles to the link could seriously damagethe cables.

• FrostFrozen soil renders trench digging difficult (or impossible) due to the hardness of the soil. Theconditions are also arduous for the workers who build the trenches, as they have to work in subzerotemperatures and for longer times at the site as work progress is slower in hard soil.In particularly cold regions there may be prolonged periods of the year where work is not reasonablypossible.It can also be mentioned that the mix of frost and water during the thaw can swell the soil (withpossibly the collapse of the trench).

• ArchaeologyIf during route selection for the link it is not possible to avoid a potentially archaeological area, it mustbe borne in mind that, due to local authorities, any finds made during excavation will make itmandatory to stop the work, at least in the portion where the find has been made.Work at the site is then delayed until the archaeologists have investigated the find. The length of theportion affected and the duration of the stoppage depend on what further discoveries are made.


• Presence of termitesIn soil infected with termites there are two possible ways of protecting the cables :either• by placing an anti-termite chemical in the outer covering of the cables, or wrapping an anti-termite

cloth ribbon under the outer covering. However, international environmental guidelines increasinglyforbid these options.

or• by inserting each cable in a steel pipe (these being different techniques from direct burial) or

providing a steel covering around all the cables.

For other pests it may be effective to lay the cables at a depth where these pests are not usually active(for instance, rodents do not normally stray below a depth of 80 cm).

• Laying in National ParkCertain local or national authorities may make mandatory a different laying technique in order topreserve the natural environment (for example, directional drilling instead of direct burial).Assuming digging a trench is allowed, a number of particular recommendations or stipulations will haveto be complied with restoration of the soil and the vegetation.

• Duration of the workWhen the duration of the work would otherwise be too long, a different technique may be imposed.This may be the result of the already being other rights of soil occupancy (e.g. utilities, telecom) or oflocal circumstances (important road crossings, residential areas) where the local authorities wouldimpose different techniques (for instance, laying in ducts or directional drilling).

• Maintenance and repairing processOnce the link has been built with the direct burial method, the only points where access remainspossible are the extremities where the cables emerge from the soil for connection to terminals, andpossibly the cable shields at the joints between cables. These are the only places where direct visualinspection is possible.Nevertheless the link operator can still perform tests on the outer covering (generally a DC. test).Defect location and repair will always necessitate some excavation work. If the cable defect is asubstantial one, the civil works will also be substantial, as the deteriorated length of cable will have tobe replaced, and two new joints made.

• Cable removal after operationCable removal at the end of his life has been definitively stopped represents a huge amount of workand cost, because the trenches will have to be completely reopened. This is the reason why these daysthe disused links are usually abandoned in place.However, this may result in environmental concern if the cables are the fluid-insulated type. The fluidshould be regularly drained by pumping it from the central channel so as to avoid the risk of fluidleaking into the soil.New developments to remove the cables with a trenchless method are at the present time underinvestigation.

• Adaptation of the technique to the cable system designFor a well-defined cable system the choice made concerning the conductor materials has a significanteffect on the link construction method and cable laying technique, on account for instance of thedifference in weight between copper and aluminium.

The lighter cables (aluminium) allow to have drums with longer length of cable (but with a differentcarrying capacity for the same size in copper), in turn allowing the laying of longer lengths.


During laying of lighter cable the drawing strain is applied to the entire cable by using stockings,whereas for the heavier cable the drawing strain is applied to the conductor itself by means of aspecially designed pulling grip.

3.2.3 Tunnels

The reason for selecting the tunnel instead of ducts.The tunnel is usually used when a lot of cables will be laid out in that particular section, which results indifficulty to ensure the transmission capacity required.

Shield method

Shield is a kind of tunnelling method designed to operate even in poor subsoil. Tunnels are excavated bya tunnel driving machine known as a "shield machine" and tunnel wall is constructed by fixing a pre-fabricated circular pre-cast members called "segments" against each other using bolts.By using the Shield method, circular tunnels with diameters from Ø‘1800mm up to Ø‘14000mm couldbe bored.


A tunnel is used for cable accommodation when a lot of circuits must be installed along one particularroute, when it is difficult to secure the required transmission capacity using direct burial or ducts.It is also used within urban areas where the logistics of using other techniques at ground level areinsurmountable.

A tunnel is constructed by open-cut method, shield method, or pipe jacking method. Pipe jackingmethod is described in Section 3.3.4. Shield method and pipe jacking method are similar in their shapes.The difference of them is just construction method and only shield method is described here.

a) Open Cut MethodOpen cut is a method of constructing a tunnel. First, excavate from the ground surface and then buildthe tunnel in required location and restore the ground surface by the backfill.The most common method is generally the full face one.

b) Shield MethodWhen Open-cut method cannot be used, the Shield method should be used. It may be applied wherethe road traffic is too heavy or the tunnel to be constructed too deep to excavate from ground surfacebecause of keeping away from other underground equipment such as telephone cables, gas pipes,water pipes, sewage pipes, subways, etc.Shield method can be used when the subsoil is poor. A shield tunnel is excavated by a tunnel drivingmachine known as a "shield machine" and tunnel walls are constructed by fixing pre-fabricated circularpre-cast members called "segment" against each other using bolts. Circular tunnels with diameters fromØ 800 mm up to Ø 4000 mm have been constructed in Japan.


Picture 5 : Open cut gallery

Figure 9 : Tunnel boring methods


Figure 10 : Shield machine

Ventilation is generally used in tunnels for human safety. Ventilation also dissipates the heat generatedby the cables, thus increasing the transmission capacity compared to direct burial or ducts. When largertransmission capacity is required, cooling system may be applied.


Figure 11 : Cooling system in tunnel

Cables in tunnels may be installed in a rigid configuration (normally when there is not much spaceavailable) or, more commonly, in a flexible configuration. In the latter case, both vertical and horizontalsnaking are used, depending on practical considerations. Unfilled troughs with horizontal snaking mayalso be adopted.3.2.3.2 Limits of the technique

• Civil WorkSince tunnel construction method is much more expensive than construction of ducts by open cutmethod, it is necessary to carefully evaluate the construction cost. Construction of a tunnel iseconomically unfavourable when there are only a few circuits to be installed. At the time ofconstructing shield tunnel, all the route need not be excavated but land for shafts is necessary. Land forventilating facility is necessary for both open cut tunnel and shield tunnel.


• Drying of the SoilWhen the tunnel temperature is low enough for human beings (e.g. 40 degree C), drying of the soilneed not be considered.

• Water DrainageTunnel is designed as an almost waterproof structure. Seepage water into the tunnel is drained into apit and pumped to the outside.

• Temperature of the Soil / EnvironmentTransmission capacity is decreased if the surrounding soil temperature is high or if the air temperaturewithin the tunnel is high.

• Hardness of the SoilMuch power is required for excavation in case the soil is hard. But it is not a fatal problem.

• Stability of the SoilStability of the ground is fundamental to the construction of the tunnel since any settlement must notcompromise the integrity of the tunnel and shafts.

• Thermal Resistivity of the SoilCompared to direct burial or ducts, thermal resistivity of the soil does not have as much influence onthe transmission capacity because heat from the cables is mainly transferred to the air within thetunnel.

• SeismicityIn Japan, a tunnel is designed to have stability against an earthquake with an acceleration of 0.3G andwith safety margin for earthquake is more than 2. According to the past experience, it can be said thata tunnel has enough strength against earthquakes.

• FrostNo need to be considered.

• Archaeology (prehistoric sites)If prehistoric ruins are found, it should be reported to relevant authority and site investigation should becarried out before the construction.

• Presence of TermitesCables should be protected with anti-termite sheath.

• Laying in National ParkIt is necessary to get permission from relevant authorities.

• Duration of the WorkRequired construction period is as follows.Shaft : 6-9 months (depth 30m)Shield Driving : 10 - 15 m/dayInvert concrete, Cable Supporting Material, Lighting : 15-20 m/day

• Maintenance and Repairing ProcessBy monitoring for cracks produced in the tunnel wall, erosion rate may be estimated and the mostsuitable repair method determined. Repairing method varies from simple repair like filling the cracks tolarge construction projects such as building a steel reinforcement to support the tunnel itself frominside.

• Cable Removal after OperationWhen the flexible installation is applied, it is rather easy to remove the cables.



• PlanningAt the time of the planning, various items should be considered such as, number of circuits, supportingmaterial, ventilation, cooling system, working space, road condition, countermeasure for fire,environmental impact since the construction of a tunnel may involve major earth movements, etc.These items affect one another and should be considered systematically.

• Basic DesignThe height of the tunnel needs to be such as to allow adequate space for the installation andmaintenance work. Joints are generally positioned within the tunnel with the distance between jointsbeing as long as is possible based on the longest length of cable that can be transported to site andinstalled. Ventilation is provided by shafts along the route as needed to satisfy the ventilationrequirements for personnel access and safety requirements and to ensure the capacity of the link.

• Snaking DesignIt is very important to evaluate the thermal expansion of the cables. To cope with thermal expansionand contraction of single-core cables installed on shelves in tunnels, pits, etc., a snaking installationtechnique is generally used.This technique enables thermal cable expansion and contraction to be absorbed by lateraldisplacements of the cable initially laid in waves at a certain pitch and width. There are horizontal andvertical snaking installations.Selection between them is made depending on site conditions, available space, economy, etc. Horizontalsnaking installations are widely used for fluid-filled cables and both snaking installations are used forXLPE cables.

3.2.4 Troughs


A trough is a generally prefabricated U-shaped covered housing which is used to protect the installedcable from mechanical damage.The trough can be cast in place as a single element composed of precast sections of approximately onemeter long installed end to end or by means of continuous concrete casting process with the top of thesides permitting a structural cover such as concrete or steel or fibre reinforced plastic, to be used toprotect the installed cable. The troughs are generally filled with special backfill in the form of selectedsand or weak mix mortar to aid heat dissipation.Once the trough path has been assembled, the cables may be installed as in an open trench, either by apulling or a laying process from joint to joint or from joint to termination. Then covers are placed.

3.2.4.2 Existing installation techniques

There are three types of cable installations in troughs :1) Direct buried troughs2) Filled/unfilled surface troughs3) Unfilled troughs in air (in tunnel)

(1) Direct buried troughsCables are laid in reinforced concrete troughs which are installed in a trench.The troughs are filled by sand and then backfilled completely.Internal dimensions of the trough must be such that enough space exists between cable(s) and internalwalls of the trough element :


Bottom : ≅ 1 cm, Side walls : ≅ 1 cm, Cover : ≅ 4 cm

Picture 6 : Cables in trough

(2) Filled/unfilled surface troughs

Reinforced concrete troughs are installed at the surface of ground as shown in the drawing below andcables are installed in the troughs.In the case of filled troughs (sand filling in the troughs), the most likely movement of cable forthermomechanical behaviour is in the vertical direction where there is least resistant and lifting oftrough lids can occur.Care is therefore necessary to ensure that the trough lids are either heavy enough or sufficiently wellfixed to the trough to prevent movement.In the case of unfilled troughs, cables are necessary to be snaked and fixed with cable cleats to caterfor thermomechanical behaviour same as the unfilled trough in air of class (3) described below.This surface trough type has been used running along side railways and in substations.

Figure 12 : Filled troughs Figure 13 : Unfilled troughs

(3) Unfilled troughs in air (tunnel)

In the case of the reduced transmission efficiency of many cable circuits installation, cables are laid intunnel and fixed with cleats on hangers.To limit the extension of fire or prevent from external damage, cables could be laid in closed FRPtroughs as shown below.


Figure 14 : Unfilled troughs in air Picture 7 : Unfilled troughs in air

3.2.4.3 Installation methods

Cable installation using troughs is classified with regard to installation method : namely direct buriedtrough and filled surface trough are of the rigid type, while unfilled surface trough and unfilled trough inair are of the flexible type.

(1) Rigid type

The bottom of the trough is filled with a layer of thermally suitable sand backfill or weak mix mortarbefore laying the cable.If the cables are in trefoil, then after laying the bottom cable, the trough is backfilled to the top of theinstalled cable to eliminate the air remaining in empty spaces and in preparation of the next cable to beinstalled.After installation of all three cables, the trough is completely filled with sand, then covers are installed,sealed and eventually fixed.For buried troughs, backfilling is achieved in several successive layers carefully compacted.

(2) Flexible type

When single core extruded cable is installed in the straight line in unfilled surface trough or in unfilledtrough in air, irregular thermal cable movement occurs due to longitudinal thermal expansion. So,snaking installation, where cables are laid in waves at a certain pitch and width, is applied to absorbthermal expansion and contraction. The dimension of snake is determined by considering the cableoccupied space, axial force at the end of snake section, workability of snaking and so on.

• Kinds of snake

There are two type of snaking, one is horizontal snake and the other is vertical snake. Horizontal snakeis applied in cable installation into the trough.

• Sheath distortionSheath distortion of cable with metallic sheath is in most cases sufficiently less than the permissiblevalue, when the above mentioned parameters are adopted. This is confirmed theoretically and


practically. Whenever necessary, precise calculations can be performed according to the design criteriadescribed in chapter 4.1.1.

• OtherThere are some cases that the cables are bound together at regular interval against electrodynamicforce at the occurrence of short circuit.

3.2.4.4 Limits of the technique for buried troughs

• Civil workFor buried troughs, civil work include excavation of a trench and shoring when necessary. Afterinstallation of the cable as described before, the trench is backfilled and different layers of the nativesoil are compacted. Limits are the same as for directly buried cables with an additional limit concerningbending radius which is generally 70 times the cable outside diameter.

• Drying of the soilThe buried troughs method is better than direct buried method because of the improved heat flowprovided by the use of concrete trough

• Hardness of the soilSame limits as for directly buried technique

• Stability of the soilIf the soil is not stable, it is necessary to anchor the troughs on a concrete sole.

• Thermal resistivity of the soilTroughs material and backfilling inside and outside the troughs can be selected to take account of thethermal resistivity of he soil.3.2.4.5 Limits of the technique for surface troughs

The use of this technique is strictly limited to these cases where the right of way is the utility property.

3.3 Description of innovative techniquesThese techniques have been developed more recently, mainly to reduce cost and to accommodate theincreasing demand to transmit more power using high voltage cables. They are categorized asinnovative as very few companies have used them to date. Some special applications such asunderground hydroelectric power stations required the use of shafts to house high voltage cables.Lately, the horizontal drilling technique was borrowed from the oil and gas industry and used as atrenchless method to lay cables. It is mostly suited for environmentally sensitive locations as well asriver crossings. Mechanical cable laying has also been developed to install high voltage cables quicklyand economically over long distances. It is expected that more innovative techniques will be developedin the future to meet the increasing demand for underground high voltage cable laying.


3.3.1 Bridges


It is common to use existing bridges where the cable route iscrossing rivers, railways, road junctions etc. Some bridgeshave a natural space for placing cables, either inside thebridge or in the sidewalk. On concrete bridges the cables areplaced in precast troughs in the sidewalk where the cables aredirectly laid. Single core cables are laid in trefoil to reduce themagnetic field (reduction of the circulating current in thesheath and then of the losses). The trough is filled withcement bound sand which has a low thermal resistivity. Thespace available is often limited and does not allow makingjoint pits.

Picture 8 : Dedicated tunnel for cables

On steel profile bridges, the cables can be laid in steel profiles or on cable ladders. In this caseadditional protection is needed, especially at the piers.The cables can also be directly cleated on the bridge or installed into ducts.Before deciding to use an existing bridge as a crossing, a careful study should be made. The designerhas to take into consideration the dynamic mechanical stress caused by its vibrations, elongation andbending at junctions and the environmental stress such as sunlight heat and wind pressure.When constructing new bridges there should always be a design with a space for possible futurecables. Cast-in ducts make it easy to pull the cables through the bridge. But again the offset of thecables at the piers is very important.

Vibration

If cables, that have an extruded metallic sheath, are installed in bridges the vibration generated byautomobiles and trains may introduce strain into the sheath, which could lead to fatigue. To reduce thisstrain to an acceptable value it is necessary to design the cable supporting method and cable supportingintervals with regard to their resonance frequency.


• Civil workThe civil work will normally be to make modifications or extensions of the existing bridge structures.This should be done in close co-operation with the owner of the bridge to avoid reducing themechanical properties of the bridge. The transition zone at the piers is the most critical points. Here thecable needs to be installed with an offset to compensate the thermal movement of the bridge. It shouldalso be considered if the magnetic field or the increased temperature may affect the lifetime of thebridge. A research made in Norway in 1995 show that the risk is low. The cable racks/ trays and sunshielding should allow the maintenance of the bridge. If the cables are laid in unfilled troughs, a drainage system should be provided.

• Temperature of the soil/environment


The temperature variations have to be considered when calculating the ampacity of the cable. Thetemperature is significant in assessing the thermal expansion of the bridge and hence the cable offset tocater for this expansion.

• SeismicityIn areas with seismic activities, the cables should be laid with an larger offset at the transition zones

• FrostIf the cables are laid in unfilled troughs, a drainage system should prevent ice in the through.

• Presence of termitesThe cable designer should take care of the protection against termites. Tin-bronze tapes are widelyused together with a PVC protective covering with an anti-termite repellent additive.

• Maintenance and repairing processThe space for repair is often very limited. If a damage occurs, replacement of the cable on the wholelength of the bridge may be needed.

• Cable removal after operationIf cables are laid along a bridge it is normally easy to remove the cables if they have to be replaced.

3.3.2 Shafts


Shafts are generally used in hydraulic generation plants where the power generated from theunderground equipment have to be brought up to the beginning of the aerial lines.Shafts may also be part of cable routes in cities where the cables are running in deep tunnels and mustbe connected to aerial lines or substations.

Cables may be fixed with clamps at the shaft walls or to metallic structures.Several circuits may be installed in the same shaft; in this case walls or special structures are used toreduce the possible damages in case of problems of one of the circuits.

Joints, if present, are normally installed in horizontal configuration in special chambers to be purposelycreated.

Sometimes the shafts are used as vent of the production plant, and the air temperature during normaloperating conditions has to be considered when designing the installation layout.


In case of FF cables the limits of this technique are represented by the high internal fluid pressurevalues inside of the cable. Stop joints will have to be used to reduce this pressure, and the installationtechnique of the joints must be carefully evaluated. Accessories design or the hydraulic pressure limitthe maximum cable length that can be installed in the shaft between two stop joints.Also the supporting structures along the cable route and at the terminations may have an impact on thedesign of the system.

In case of extruded cables attention must be paid to the significant expansion coefficient of the cables,limiting the restraining force that each clamp may transfer to the cable and hence requiring special carewhile designing the supporting structures.Special clamps will have to be used in this case.


For both types of cables, but especially for FF cables, the laying operations, requires the use of specialprocedures and tools to be adopted. The design of the installation shall carefully consider the layingaspects in terms of space and sufficient working areas.Laying of the cables is normally carried out from the top of the shaft, and enough space for reel andcable laying equipment handling has to be present. Transportation of the cables inside the terminalstation may be also a critical aspect if for example the station is completely underground.

In shaft the safety aspects have a significant impact, because short circuits or explosions may lead tothe complete failure of the circuits.Special structures or precautions shall be taken to minimise the effect of fire inside the tunnel.

Special laying tools and ancillaries structures will have to be in place and available during the wholeservice life to allow the recovery and replacement of a faulty phase.

• Civil workShafts are often designed as part of the power plant or tunnel structures and the cable installationdesign shall consider the existing facilities. Sometime calculation have to be made to be sure that theexisting facilities can withstand the weight of the cable and the relevant structures.In case of FF cables, following the maximum allowable cable length, joint chambers will have to be builton purpose along the shaft. The size of the chambers shall take into account the size of the joints, thenumber of cables and the structures to be installed.

• Water drainageWater may permeate through the shaft walls, and heavy moisture may condense over the cables.Corrosion problems has to be carefully considered when selecting the materials for the supportingstructures.

• Temperature of the soil/environmentThe design of the whole system requires the knowledge of the shaft temperature in operatingconditions and the annual excursions to evaluate the thrust developing in the cables.

• Thermal resistivity of the soilIn shafts the cables are installed in air that can circulate from the bottom to the top of the shaft.Generally no cooling problems are present

• SeismicityAccelerations imposed by earthquakes may have to be considered when designing supportingstructures. Being the shaft part of the station or tunnels civil works, the civil structures are alreadydesigned considering these accelerations.

• Duration of the workAs the laying and jointing operations are carried out in substations and in the shaft itself, the operationscan be planned quite easily.On the other side, installation works may last more than usual, due to the particular care to be taken forcable clamping.

• Maintenance and repairing processMaintenance for cable in shaft may consist of a periodical visual inspection on:cable sheathsupporting structures and fixing devicesjoints

In shaft the repair of a faulty phase will involve the recovery of the faulty cable and the lay of a newcable. Repair of an existing cable putting joints in the middle of a cable length is generally not possible.


• Cable removal after operationCable removal is possible but with difficulty due to access restrictions caused by access stairways andother features installed after the original cable installation was undertaken. Removal is in the samemanner as installation but precautions must be taken to ensure personnel safety and that no damagecan occur to the remaining cables in the shaft.

3.3.3 Horizontal drilling


• IntroductionAugers, vibratory pipe reamers, micro-tunnellers, directional drilling, pneumatic moles; all of these aredevices for installing underground facilities with a minimum of digging required.

Many of the techniques and equipment have been around for many decades, however the mostinteresting and versatile trenchless technique, in use, has increased exponentially over recent years,guided horizontal drilling.

• The technique

Guided horizontal drilling, sometimes referred to as guided boring, is a construction technique thatprovides for faster product placement with less disruption to the surrounding urban and suburbanneighbourhood.Drilling can be initiated on the job either by being placed into a pre-dug launching pit, or starting fromthe road or soil surface, commonly called surface launch. The following outlines the procedurecommonly followed to install a conduit or small conduit bundle:

Step One: Lay out of entrance and exit pits for determining drilling path and segment lengths.(highly dependent on machine size and capability)

Step Two: Pilot hole drilling between the pits at the proper depth to avoid "frac-outs" or situationswhere the drilling fluids used might bubble to the surface.

Step Three: With the drill steel left in the ground, a back reamer is attached to the exit end of thesteel to widen the hole to the proper dimensions for the conduit.

Step Four: Repeat step three as often as is necessary to obtain the desired hole diameter. On thelast back ream, a high tensile strength swivel and packer will be added to pull in the conduit or conduitbundle behind the reaming system.

In all jobs, the proper drill heads, reamers, fluid handling systems and pumps must be carefully selectedin order to have an efficient and cost effective product installation. Drilling fluids, sometimes referred toas "mud", must be mixed using proper densities for the job at hand. Fluids normally are comprised ofwater, bentonite and sometimes a polymer oxide additive to provide for better performance. Thebentonite "mud" or slurry, serves various purposes; it is the medium that provides a path for rockcuttings and surrounding soil to flow back to the launch pit, it provides cooling of the drill head andlubrication when drilling into rock substrata and it stabilises the hole.For long lengths, an installation to the recycling of the mud is to be considered.

• The equipment


Horizontal drilling rigs can be classified into four equipment sizes based on there capability : servicetools, mini-rigs, midi-rigs and river crossing units.

Depending on the installation, the size of the tool needs to be chosen carefully for maximum economicbenefit.

Service tools are used for very small installations of gas and underground residential distribution (URD)services. These are easily transportable and require very little set-up space. Service tools can drill smalldiameter holes for about 60 m.(200 ft) effectively. Many are "dry borers", meaning that they use noslurry to act as a coolant or for hole stabilisation.

Mini-rigs, currently represent the largest market segment for drill rigs. These are small versatile toolsthat employ a minimum amount of slurry for drilling in relatively easy to moderate soil conditions. Mini-rigs come in various sizes and capabilities just as back hoes. In many respects these small machinesare the equivalent of backhoes while midi-rigs, which are approximately the same physical size, areequivalent to excavators. The Table 1 below shows the performance ranges to the four categories ofdrilling units.

Table 1 : Horizontal drilling references

Rig Type : Service Mini Midi RiverRange : m (ft) : 60 (200) 180 (600) 900 (3000) 1800 (6000+)Hole Size : cm (in): 10 (4) 30 (12) 61 (24) 132 (52)Pull Back kN : (klbf) 17-31 (4-7) 44-178 (10-40) 222-400 (50-90) 534-3337 (120-750)Torque : kN-m (ft-klbfs) 0.6-0.9 (0.5-0.64) 0.8-4 (0.6-3) 13.5-27(10-20) 34-122 (25-90)Crew Size (#) : 2 3-4 4-8 6-12

• Benefits

Safety - Open trenches endanger pedestrians and traffic; including cave-ins, trench debris, and failed orimproperly installed cover platesConvenience - Businesses, homes, and commuters are less inconvenienced by traffic backups, dust anduneven pavement due to removal.Productivity - Installed lengths can exceed open trenching by as much as 5 times on a per day basis.This translates into less traffic disruption, noise and reduces the construction times in any one location.However, in many countries, the horizontal drilling is mainly used to cross obstacles and not to replacethe open trench where it is possible to work with direct burial.Conflict Reduction - Increasingly congested utility corridors and easements make it very difficult toplace cable or conduit. Directional drilling can be a solution to this dilemma through the use ofmeasure-while-drilling, MWD, electronic tracking systems accurately drill beneath existing utilities.Route Selection - Drilling may allow for different or shorter routes to be taken for the installation of acable circuit. This is because in many cities there are moratoriums on cutting open recently paved orrefurbished road surfaces. The local governments in many cases make allowances for the use ofdrilling instead of trenching.Reduced Environmental Issues - Run-off from job-site excavation is minimised as is the risk ofexcavating and disposing of soils that may be contaminated. Regulatory restrictions related toexcavations in wetlands and other sensitive areas will be reduced.Cost Savings – Faster installation time, less backfill materials used, traffic control issues, pavementremoval, separation and disposal / refurbishment effectively eliminated, reduced spoil handling and


trucking cost; all of these provide for significant savings when guided horizontal drilling is effectivelyemployed.


Although the use of guided horizontal drilling can be a faster more efficient means of installing services,there are cases where technical limitations out weigh the potential benefits. In those situations hybridinstallation practices should be employed, i.e. trenching and drilling used in the proper locations.

• Civil workCivil limitations to drilling are fairly obvious. The following limit the effectiveness or are the cause ofproblems during job performance :The necessary hole diameter exceeds the capability of the available equipmentThe depth of installation doesn’t allow enough cover to prevent the drilling fluids from percolating to thestreet surface.Segment lengths between pit locations are too long to be done in a cost effective manner.The underground environment contains flowing sands with high water table, or contains a rockyenvironment not conducive for the proper directional controls needed.Attempting to drill around tight bends or intersection corners in a street.

Many of the above can be overcome using lesser known drilling methods, however in some cases thebetter approach to take would be to use microtunnelling.

• Temperature of the soil/environmentDepending on the depth of drilling, temperature of the ambient soil or rock where the cable circuitwould eventual reside, can actual be more constant and at a lower temperature than for shallowerinstallations done using open cut methods. Because of the greater installation depths, the circuitampacity may need a de-rating. This disadvantage can be overcome through the use of dynamicmonitoring on the circuit or through the use of a larger conductor cable.

• Hardness of the soilAs previously mentioned under civil limitations, the hardness of soil can affect the drilling effectiveness.

• Stability of the soilUnstable soil conditions such as flowable sands and cobble mixed with softer soil materials areprobably the worse conditions for drilling operations to be effective. In the case of flowing sands, orvery sandy soil conditions where the bentonite slurry cannot stabilise and maintain the drill hole, awashover technique can be performed (for small diameters). This is where sections of a largerdiameter pipe are connected and pushed into the bore hole during the pilot drilling operation. Once thedrill rod has gone the desired length, it can be removed from the washover pipe and also be used to pullin the cable. The washover pipe is sacrificial and it stays in the ground to act as a conduit. For largerdiameter holes, a technique known as forward reaming is used. During the pilot hole drilling, reamersare attached every so often onto the pilot drill rod faced in the forward direction (smaller reamers first,eventually having the last reamer at the desired diameter). Behind the last reamer is attached a sleeveor large pipe, which is pushed into the large hole in parallel with the pilot hole drilling and reamingoperations. This sleeve or large pipe provides a path for the cuttings and slurry back to the drill rig. Thefact that it is pushed or spin into the hole during the drilling results in a stable hole. This however doeslimit the advance rate of the drilling and as a result dramatically increases to cost. This same methodmay also be needed where cobbles and covered river bottom rock is encountered.

• Thermal resistivity of the soilInstallation of circuits at greater depths tend to reside in surround soil and or rock that have betternative thermal resistances than the soils closer to the surface that contain higher organic content. Thusthe thermal resistivity of the native environment is not a limitation on the guided horizontal drilling


technique. However, it is more careful to take soil samples for analysis before the start of the works(to avoid hot spots). The only limitation is on the hole size and the proper sizing of the hole with thespacing of the conduits that the cables would eventually be pulled into. In this case the interstitialregions between the conduit walls and the hole wall might need to be filled with a good thermal groutsimilar to flowable backfill used in trenching.

• ArchaeologyTrenchless techniques in general are preferred methods for installing services where archaeologicalsites have been noted. This is because less surrounding soil is removed during drilling and the drill pathcan be planned to go deeper under these sites.

• Laying in National ParkNational Parks, golf courses, old growth forested areas where services may need to be installed, havemuch less change of having environmental damage if guided drilling is use over open cut methods.

• Duration of the workGuided horizontal drilling when properly planned and deployed to the field can be 5 times or more fasterin placing product into the ground over open cut. This means that in most situations the duration of theproject will be greatly reduced. Even in hard rock situations, drilling is a much faster process than jackhammering, explosives with backhoe, or carbide toothed saws.

• Maintenance and repairing processThe preferred practice for installing cable is in a conduit. This means that maintenance and repairsissues would be similar to shallower installations. The only situation where there are significantlimitations on repair capability is where the cable system is direct buried. If direct buried design for thecircuit are used, this technique from a repair and maintenance standpoint is probably not a good choiceto use.

• Cable removal after operationShould direct buried installations be the preferred method, then the use of guided horizontal drilling is anexcellent choice for placement, but is much more expensive and difficult when it comes to repairoperations where a segment of cable has to be removed. Because there is a minimum depth requiredfor drilling that is based primarily on the hole diameter, most drilled-in installations are generally deeperthan their open cut counterparts. Repair and removal is a limitation for this technology. The generalpractice in the United States is of course to always place the cable phases in a conduit. If conduits areinstalled, then removal is no more of a limiting factor than in the open cut case.3.3.3.3 Adaptation of the technique to the cable system design

The use of any trenchless technique will more than likely dictate a change in the cable design in orderto get closer to the equivalent open cut / shallow installation ampacity. Guided horizontal drillingcontractors are also not used to installing facilities where heat transfer and product spacing is such abig issue. Most directionally drilled facilities are still water pipes and gas distribution lines. In theUnited States, there have been many efforts done to installing sewers by drilling. This is usually the onearea where micro-tunnelling has seen dominance.

In adapting the cable circuit design to the drilling installation case, the following need to be looked at indetail before the total design is selected;

How deep will the cable be installed ?This affects ampacity and may dictate a larger conductor for the cable.

Will the cables and/or conduits be spaced ?This again affects the cable ampacity and also the isolation of the phases to improve reliability and helpprevent a phase-to-phase fault condition.

Will the bore hole be sleeved ?


HDPE is used by drillers. This material has a thermal resistivity of about 4 degrees Kelvin-m/watt, alarge thermal resistance over the native soil contact. Sleeving will most likely dictate a larger bore hole,filled with a good thermal grout and conduits spaced equally, if good ampacity ratings are to be had. Alarger conductor design will help as well.

Adaptation of cable design and trade-off analysis is complicated with a myriad of civil parametersversus thermal performance and cable design. More and more engineering design firms, consultantsare doing detailed analyses for clients. When done properly, application of guided horizontal drilling tothe electric industry can save significant civil / construction costs. These savings by far are larger thanany increase cable cost caused by larger conductors.

3.3.4 Pipe jacking


There are three different pipe jacking techniques :i) Jacking by beating or pneumatic rocketii) Jacking by rotationiii) Jacking by thrust

The last technique will be described in great detail as it is the most common used.Pipe jacking can be considered as an environmental balanced installation technique, as it does noteffect the surroundings. The surplus soil (equal to the volume of the pipe) is removed from the groundimplying that displacement of the in situ and settled soil can be avoided.

As a rule of thumb, the following minimum depth to the upper pipe surface could be considered :

Øpipe = 200 – 500 mm 2mØpipe = 500 – 1500mm 3mØpipe > 1500mm 4m

Picture 9 : Pipe Jacking


Pipe jacking is not applicable in hard soil as limestone, sandy shale or granite. Pipe jacking is notrecommended in heterogeneous soil with blocks (clay with flint, sandstoned sand).

The nature and composition of the soil must be investigated before the pipe jacking method is selected.Without this study, the pipe jacking may be halted due to a block, which will impose a manual and timeconsuming action at the front of the first pipe when the block shall be cut to pieces. Such action is notpossible with pipe diameters less than 800 mm. Only excavation from the surface or use of explosivewithin the pipe (if possible) can prevent that the installation must be given up. In such a case a newpipe jacking operation must be started at a new depth or location.Cathodic protection shall be considered if the pipe jacking is done with steel pipes.

i) Jacking by Beating or by Pneumatic RocketThis system consists of beating horizontally steel tubes. The technique is characterised by the speed inthe progression in non consolidated and homogenous soil (clay, silt, sand).

The excavated soil is taken out of the tube by compressed air or by using a flush.

The saturated steel tubes can have a diameter ranging from 100 to 500 mm. The penetration speed canreach 3 to 10 m/hour in unconsolidated soil. The pipe jacking lengths in these soils are between 30 and40 m but this technique by beating is not very accurate.

ii) Jacking by RotationThe pipe jacking installation is performed from a work shaft and consists of pushing into the soil steel orconcrete bore tube with a drill inside that turns the drill bit. The function of the rotating drill is totransport the soil from the first pipe backward to the work shaft and not to perform excavation in thenature soil. If the soil is removed by the drill at small depths and in loose soil a risk of undermining thesurface layers exist. Such undermining shall be avoided since it can cause damage on constructionssituated on the ground surface or reduce the mechanical stability of roads, railways, etc.

When a tube enters the soil the subsequent pipe is electrically soldered on to it.

This technique can only be contemplated in homogenous and soft soils (clay, silt, sand, etc.) withdiameters ranging from 400 to 800 mm and with 40 to 50 m lengths. It is simple and quick, useful whenthe soil is suitable and there is no need for great precision (about 0.5m for works of 40 to 50 m).

iii) Jacking by ThrustThe technique consists of pushing into the soil prefabricated tubes having the exact diameter of thefinal tube. The tubes are pushed from the work shaft. This shaft’s walls will be mainly shored up withoverlapping planks adapted to the subsoil and have a concrete slab which evacuate rain andunderground water to a draining well. It will also have a support system capable of receiving andtransmitting to the subsoil the jacking thrust, which can sometime be very strong.

As the pipe jacking progresses the earth works are done, either manually or mechanically, according tothe requested diameter. The first tube is equipped with a steel drum curb, which bites into the subsoilwhile protecting the workers cleaning the earth.

The drum curb is equipped with correcting screw jacks that direct the unit of assembled tubes.Topological measurements are done with a theodolite or more usually with a laser. The extracted soil atthe working face is taken to the thrust by a winching tip truck.


When the first tube is completely pushed in the subsoil the second is taken down into the shaft and issoldered to it. The thrust and extracting then continues.

When the thrust linked to the friction load becomes too high, it is possible to resort to intermediarystations. During the pipe jacking, bentonite is injected between the soil and the tube in order to reducethe friction coefficient. The substance is injected into the tube and goes out through holes on the side ofthe tube. Towards the end of the pipe jacking the bentonite is replaced by cement grout to "solder" thetube to the subsoil and spread the earth’s thrusts.

When the pipe jacking is finished, the drum curb is retrieved in the exit shaft. The screw jacks at theintermediary station are dismantled.

This technique is applicable for pipe diameters between 1000 and 3200 mm.

The thrust station at the work shaft is equipped with 4 to 6 screw jacks which each are capable ofdeveloping 1000 to 3000 kN. The average friction of the pipe surface / soil is approximately 1.2 kN/m2

of the external surface.

The maximum permissible thrust for a standard 2 m reinforced concrete pipe is:

Table 2 : Pipe jacking figures

Diameter [mm] Maximum permissible thrust (kN)1000 20001200 26001400 35001600 52001800 70002500 10500


• Civil workPipe jacking is possible up to 100 m length without intermediary stations. With intermediary stationslengths up to 500-600 m may be installed, but the work is limited by the time consumption fortransporting of the soil backward in the pipeline. Health and safety concern for the workmen might alsogive reason to limit the length of the pipe. However the length can be doubled if it is possible toexcavate a central receiving shaft and perform two pipe jacking installations (one from each side).

Pipe jacking precision : 50 mm around the theoretical axe.Speed of progress : 4,5 m/day if the soil is removed by a mechanised system.

2,5-3 m/day if the soil is removed manually.

Pipe jacking by thrust is not applicable for rocky soil. The technical limit of the cutting head is 300 kN.

For pebbled soil situated below the ground water (sand and gravel, sandy silt) measure must be takento lower the groundwater level and the pipe jacking shall be done by compressed air.


• Drying of the soilThe design of the pipe jacking installation shall consider the possibility of drying out the soil at theexternal pipe surface which might result in a thermal run away causing an insulation cable failure if thesystem is heavily and continuously loaded.

Drying out of the soil is not of any importance if the permanent groundwater level is above the pipeinstallation. Therefore the groundwater level must be verified at the geological survey before thedetailed design of the pipe installation is completed. Additionally the soil characteristics (thermalresistivity and drying out performance) must also be clarified at the geological survey.

Drying out of the soil can be expected to start at a continuous temperature of 50 oC depending on thesoil characteristics. (In sand material the drying out phenomena will most properly start at a lowertemperature).

Cable installation with pipe jacking will always result in that all three phase conductor cores are pulledthrough the same concrete or steel pipe. The pipe surface temperature varies (among others) with thecurrent, phase distance, depth of the pipe, and the outer diameter of the pipe.

Drying out of the soil must be considered in particular for cable systems designed for a continuouslyload and with a conductor size being equal along the whole cable route. In such a case a large depth(above 3m) might result in a bottleneck for the current capacity of the whole link, since availablemeasures against the possibilities of the drying out of the soil is limited. Spreading out of the cablesbeyond the inner pipe diameter is impossible. Improving the soil characteristics by substitution of soilwith better thermal performance is not possible. Injection of a "specific developed fluid with excellentthermal properties" within the pipes will only have a minor effect, since the major temperature raiseappears in the soil.Above mentioned heating problem can be prevented if a cooling arrangement is applied. Natural orforced air circulation in the cable pipes can remove the heat generated by the cable cores.Alternatively a water cooling system can be adopted. A cooling system will result in additionalinstallation costs and require supervision and maintenance on a larger scale than the cable system,which is practically maintenance free.

• Water drainageWater drainage shall be considered for the implementation period. Permanent water drainage is notrecommended.

• Temperature of the soil/environmentPipe jacking will be performed at a depth more than 2,5 meter in almost every installation. Thetemperature at the upper surface layers (depths 0,2-1,0 m) varies during the day and week dependingon the sun heating, wind, and air temperature. This fluctuation in the soil temperature is less distinct at1-3 m depth and vanishes at a larger depth. The soil temperature at large depth is almost constant andequal to the yearly average air temperature at the particular location. This implies that a different soiltemperature can be applied for the calculation of the cable temperatures.

• Hardness of the soilPipe jacking in soil consisting of rock, granite or similar soil can not be performed.

• Thermal resistivity of the soilPipe jacking with three conductors within one pipe is not recommended in soils with high thermalresistivity unless a cooling system is applied.

• FrostSince pipe jacking is performed at depths larger than 1.5 m frost does not affect the performance ofthe pipe jacking operation.


• ArchaeologyPipe jacking can be applied close to archaeology sites within the cities. If large depths is required andthe available area for a sloping horizontal drilling to the maximum depth is insufficient a pipe jacking canbe performed.

• Presence of termitesIntrusion of termites and other vermin must be prevented if an air installation is chosen for large pipediameters.

• Laying in National ParkPipe jacking requires a large working area at the working shaft and the receiving shaft compared withthe relative short length. If a cable route shall cross a national park or a similar area with sensitiveenvironment, it is unlikely that pipe jacking is a feasible trench less installation compared with thehorizontal drilling with PE-pipes.

• Duration of the workThe implementation of the work will follow the phases mentioned below :

Soil investigation and report 1 weekTendering/Contract 2..3 weeksSite mobilisation 1 weekExcavation of shafts 1 weekPipe Jacking 1-3 weeksAttachment of cable pipe ½ weekInjection of bentonite & concrete slurry ½ weekReestablishment ½ weekDemobilisation ½ week

Above mentioned duration is of course dependent on the complexity of the installation (pipe diameter,length and dept of installation).

• Maintenance and repairing processCable pipe with bentoniteNo maintenance is required for the cable installation if the plastic pipes and the concrete/steel pipes areinjected with bentonite. In case of cable failure within the pipe installation the bentonite can be flushedout and the cable core redrawn from the pipe.Cable suspended in airIf the pipe installation can be accessed a regular visual inspection (each second year) can berecommended in order to detect any deterioration of the cable suspension arrangement or to inspect ifobjects or animals has intruded the installation and caused any damage.

• Cable removal after operationCable pipe with bentoniteThe bentonite can be flushed out and the cable core redrawn from the pipe when the cable system istaken out of operation.Cable suspended in airCables suspended in air to the inner surface of the pipe by mean of applicable cable clamps are easilyremoved when the system fails.



• Installation practice

Depending on the size of the concrete/steel pipe two different installation methods can be applied:

• Air installationIf the concrete pipe is large (above 1,5m inner diameter) the cables can be suspended at the inner pipesurface either as a flexible or rigid installation in air. In this case the cable installation design practiceused for cables in tunnels can be applied taking into consideration the mechanical forces during shortcircuit and the thermal design (heating up of the soil and the cable insulation). A proper sealing of thepipe must be considered in order to prevent intrusion of water, insects etc.

• Cables laid in pipesIf a small pipe diameter is designed the cables must be laid in cable plastic pipes (made of PE or PVC)in order to avoid damage of the cable sheath during the pulling operation. Depending on the dimensionsand weight of the cables one or three cable pipes can be used. Small cable sizes can be pulled out inone pulling operation if the three phase conductors are attached together with a suitable tape.

Unintended overheating of cables must be prevented by substitution of the residual air volume with amaterial with improved thermal characteristics. Bentonite or a similar pumpable mixture is injectedbetween the cable and the plastic pipe. A slurry mixture with a thermal resistivity less than 1 Km/W indry condition is normally selected for use between the plastic pipes and the concrete pipe.

If a trefoil formation is essential for the cable system design all three plastic pipes must be attachedfirmly in trefoil and pulled through the large pipe in one operation. Increasing the phase distance to themaximum possible can reduce the thermal mutual heating between the phase conductors. In such ainstallation (with pipe diameter above 1.5 m) it is necessary to adapt clearance wedges between thecable plastic pipes.

It is not mandatory that the cable cores are located in a true trefoil formation. The current and voltageof the screen depend on it’s bonding, the location of each phase conductor and the length of the wholecable system. If close trefoil is applied in the standard trench a short length with flat formation willresult in an unbalanced screen bonding system. The screen voltage at the opened end for single pointbonded systems and the circulating current for cross bonded & two point bonded systems will increasea little compared with a true balanced system. This however is of negligible importance if the totallength of pipe/cable installation is relatively short compared with the whole cable system length.

If the space is available more than one cable system consisting of three phase conductors can beinstalled in the same concrete pipe. However consideration related to the mutual heating and thepossibility of a cable failure caused by one system to the other system must be taken.

3.3.5 MicrotunnelsMicrotunnels are one of the installation techniques which are adopted where open ditches are notpossible, e.g. crossing of obstacles like railways, rivers, duct banks, motorways etc.

Their diameters are typically between 300 and 1200 mm and their lengths 200 m maximum. In contraryto larger tunnels they are not accessible by man and mostly contain only one 3-phase power cablesystem.



Similar to the pipe jacking method this technique also consists of pushing prefabricated tubes in thesubsoil. The earth works at the front of the tubes, however, are systematically mechanised using a socalled microtunneller, i.e. a steerable drilling device, which allows to penetrate even harder subsoilsthan with simple pipe jacking.

The machine is driven electrically or hydraulically and remote controlled and can dig horizontal holes ofup to 1200 mm diameter over a length of appr. 150 m (max. 200 m).

As the pipe jacking or large tunnel techniques the microtunnelling, too, starts and ends in vertical shafts,which have to be prepared in advance to provide the necessary space for the tunnelling and thrustingequipment.

The clearing of the dug out earth can be done in three different ways via tube systems to the decantingbox

- removal by an endless screw (earth pressure)- hydraulic removal (mud pressure)- pneumatic removal (air pressure)

Picture 10 : Microtunnelling

For the main tubes of the tunnel two different techniques exist:- pipe jacking the final tubes directly- pipe jacking temporary tubes, which will be replaced by the final tubes once the exit shaft has

been reached.

Advantages of using temporary tubes are:- a rapid execution due to simplicity of assembling the tubes by bolt-connections


- a lesser risk of damaging the final tubes, as the second thrust is generally reduced to a third ofthe initial thrust

- in the event of being stuck in the excavations the possibility of bringing it back to the work shaftby pulling on the temporary tubes.

Inconveniences of using temporary tubes are:- longer completion time- extra cost of temporary tubes- a more complicated installation- the need for more room on site to store the temporary tubes

The temporary and the final tubes have the same diameter. The latter are fitted into each other and canalso be soldered (when made of steel).

The main components of the microtunnel technique are:

• on the surface, near to the entrance work shaft:- a control system, where all controls are automatic and are backed up by manual controls- a decanting box for the excavated mud- a hydraulic pump or electric generator for the thrusting station

• the entrance work shaft, which houses- a thrusting station and the screw jacks to push the prefabricated tubes into the pre-drilled subsoil- a complete control system for the microtunneller- the various materials actually needed during the process

• the microtunneller, which is composed of the following elements- a steerable drum curb fixed to the tunneller by guiding jack screws- a steel covering, which comprises the tunneller’s body and the tubes- a pre-drilling wheel and its kinematics, which is adjusted according to the soil properties- an electric or hydraulic engine, which drives the wheel- a laser system to control the bearings

• a train of different tubes which comprises- the main tunnel tubes, either steel or concrete- a circuit to bring the mud- the sludge process to remove the earth- a lubricant circuit to reduce the external friction of the tunnel tubes against the earth (injection

of bentonite)- the passage for the laser beam- the passage for control cables

• the exit work shaft, which allows to remove at the end- the microtunneller equipment- temporary tubes (if applied)

If the drilling length becomes too long, it can be divided into two with a central work shaft (providedthat space is available) and two lateral exit shafts. In this case, the thrusting station must be turnedround.



• Civil work

Microtunnels are constructed where open trenches are not possible. Such technology of digging withoutusing a ditch requires excellent geological knowledge of the lay of the land. A geological study and adetailed survey of the character of the subsoil including the position (or non-existence) of the groundwater table are necessary before defining the civil work to be done.

These data will determine the technological choice of the earth removal method by screw, hydraulicallyor pneumatically, the type of drill head, closed for hard soil, half closed for less hard, open for soft soil,and also the lubrication, (bentonite, polymeric mixture, foam etc.).

The possible risk of an incomplete examination and the use of an inappropriate drill head can lead to themicrotunneller being blocked. If this happens there are several systems that allow part of the equipmentto be recuperated, but in most cases it will be lost.

It is therefore necessary to take any precautions possible, as the cost of a microtunneller is in the orderof several hundreds of thousands of US $.

Apart from the boring of the underground tunnel the civil work is more or less restricted to a fewlimited areas, i.e. mainly the work shafts at the start and the exit of the tunnel and their surroundings.

The shafts have to be carefully designed and executed according to size of equipment to be installedtherein, to depth, kind of soil, ground water table, thrust pressure etc. They must be kept dry from rainand ground water. Their typical dimensions are 4 m x 2.5 m or a diameter of 3.5 m for the entranceshaft and 2.5 m x 2.5 m for the exit shaft. Around these shafts a certain surface area must be availablefor storage of material, decanting of mud, machines, cranes, control and other equipment.Typical dimensions are 40 m x 3 m (max. 150 m²) at work site of entrance shaft.

• Drying of the soil

The thermal situation of a power cable system within a microtunnel is determined by a large number ofdifferent components:

- cables will be laid in filled or unfilled PE or PVC ducts- these ducts are installed inside the common tunnel tubes, either steel or concrete- interstices between ducts and tunnel tubes are filled with special slurry- tunnel tubes are ”soldered” to surrounding subsoil by special fillers (bentonite, cement grout)- surrounding subsoil can be of great variety with regard to geological composition, equality along

the route, homogeneity and, last but not least, thermal resistivity- the position of the microtunnel in the ground will be different with each installation, especially

with regard to depth, ground water table, distance to foreign heat sources etc. The calculationof admissible thermal ratings of the cable system has to adequately consider these parametersto guarantee for stable thermal conditions rather than thermal runaway and drying out of soil.

• Water drainageThe robotisation of the mechanism allows work to be carried out in the groundwater table withoutlowering the water level. This is possible up to 20 m water pressure but only for microtunnellers whichremove the earth hydraulically. The only problems can arise at the work shafts, but there are methodsto seal these watertightly.


• Temperature of the soil/environmentAs microtunnels typically will be positioned in greater depths the average yearly ground temperature ofthe soil will be dominant. This will be especially true, when the tunnel is below the ground water table.If drying out of soil is avoided (see clause b) stable soil temperatures can be assumed.

• Hardness of the soilThe microtunneller does not have problems in hard soils because its tools can be adjusted to the type ofsoil.Of course, there are limits to the hardness and/or the abrasiveness of the tools, e.g. with extremelyhard rocks. Screw machines are well adapted to clay conditions and therefore are used in any sort ofsoil from sand to soft rocks.Hydraulic and pneumatic earth removal machines are efficient in groundwater tables and very stickysoils.

• Stability of the soilThe microtunneller is badly adapted for heterogeneous soils. Once the microtunneller type and thedrilling head are chosen, a change is very difficult and costly.

Blocs or obstacles that exceed about 30 % of the drill’s diameter can not be dealt with and it istherefore impossible to use this method in soils containing large blocks.

Certain clay soils, which are too sticky, should be avoided either.

• Thermal resistivity of the soilThe detailed knowledge of the thermal characteristics, especially the thermal resistivity of the soil alongthe microtunnel is a must before such a cable system can be designed. If changes of these parameterscannot be excluded over the life time of the cable system a real time temperature monitoring systemwith temperature sensors along the cables can be installed to identify changes and to adjust cableratings accordingly.

• SeismicityThe tunnel tubes, whether steel or concrete, provide a certain mechanical protection for the cablesystem inside. The degree of seismic impact, which can be withstood without damage is hard to define.

• FrostSince microtunnels are positioned in larger depth, frost does not influence its performance.

• ArchaeologyAs microtunnelling avoids opening of trenches, it could be a favourable technique to keeparchaeological sites of limited area (< 200 m) undisturbed.

• Presence of termitesThe part of a longer cable link, which is laid inside the microtunnel, will be much more protected againsttermite attacks than the remainder outside.

• Laying in National ParkAlthough microtunnelling avoids open trenches, it does not seem to be a favourite for application inNational Parks, as their standard lengths are limited to 100-200 m. It might be too short for extendedareas and the impact of the construction work at the shafts on the environment could be considered toohigh.

• Duration of workThe duration of the installation of a cable system in a microtunnel is hard to estimate, as it depends on anumber of individual parameters, the most important of which are:


- geological study, either conventional with drilling and core sampling or advanced with georadaror seismic examination

- definition and approval of tunnel route and working areas- erection of shafts, depending on width, depth, kind of soil, ground water table etc.- diameter and length of tunnel, kind of tubes- progress by thrust is about 10 m/day, it can vary from 30 m/day in very soft soils (chalk) to 4 to

5 m/day for harder soils (clay, hard marl), if temporary tubes had been used, the later laying ofthe final tubes is about 50 m/day

- installing of ducts inside tunnel including slurry to fill the interstices- laying and connecting of cables inside and outside the microtunnel- reestablishment of the shaft and the working area

Altogether a duration of 2 to 3 months seems to be realistic.

• Maintenance and repairing processOnce the microtunnel and the cable installation is completed no maintenance is required.As the tunnel tubes provide a strong mechanical protection, damages from outside are very unlikely,thus excluding the need of mechanical repairs. If the cable should fail internally it can easily be pulledout of the duct for exchange. Repair by cable joints in the section of the tunnel is not possible due to thelimited diameter of the ducts.

• Cable removal after operationAs the cables are laid in ducts inside the microtunnel they can easily be removed, provided that thefilling is removable (e.g. bentonite).


The microtunnel provides the mechanically well protected tube for the cable system. Typically onePVC or PE duct for each cable phase is either thrust or pulled into the tunnel and fixed by drowning ininjected cement (slurry). A fourth duct can be laid and used as a reserve in the event of a problem withone of the other phases. The position of the ducts is maintained by clearance wedges.

Putting in place the slurry is a delicate operation. This material must be spread in all the volume in orderto avoid the presence of air bubbles, which could create hot spots. The injection of the slurry must bedone without damaging the ducts.

To avoid ovalisation during injection of the slurry it is recommended to strengthen the plastic ducts byair, water or helium pressure.

Lastly, the slurry should have a low and consistent thermal resistivity of preferably 1 Km/W.

Once this installation of ducts and filling is completed, the three single core cables can be pulled into theducts within the microtunnel. The ducts can be left unfilled or be filled with e.g. bentonite to improvethermal heat dissipation.


3.3.6 Mechanical laying

Picture 11 : Mechanical laying


This technique, coming from the traditional trench technique is suitable for buried cables and consists ofopening the excavation and simultaneously laying the three phases, and possibly earthing cable andtelecommunication cable as well as their backfilling. When combined with the use of weak-mix mortarit offers:

good cable protection from external damagegood control of the direct heat environment of the cablegood protection of the environment in case of short-circuitreduction of the size of the trench compared with conventional techniquereduction of work duration

Laying principle ( HV cable systems)

Cable laying conditionsThe cables are laid in a trefoil or flat position. The depth is 1.30 m to the bottom of the excavation,which on the one hand enables any effects of a zero phase-sequence short circuit to be controlled, andon the other hand, it protects cables from third party damage.

The thickness of the mortar around the cables and in particular under the cables (raft) must be at least50 mm.

For trefoil laying, fastening must be used if the cable guides cannot maintain the trefoil position until thecovering is in place.

A telecommunication cable may be laid if necessary in a separate duct above the power cables (on theweak mix mortar) and directly laid in the soil.

A warning plastic netting is laid on top. Then the trench is backfilled and the soil is compacted.

The width of the trench which takes into account the diameter of the cables is between 300 and 400mm for trefoil cables and 450 and 500 mm for cables laid in a flat position (H V Cables).


Cable laying :The cables can be laid using two different methods :

- simultaneously : the cables are laid with the help of cable laying machines or a cable drum carrier(Figure 15, •1) which is a few metres in front of the trenchdigger,

- beforehand : the three phases are laid beside each other along the future route. The cables must notcome into direct contact with the ground. They can be laid on rollers or on a polyane sheet.

Joints :Depending on the voltage and the accessory technology, joints might not be laid by machine. Indeed,their preparation and completion time might not be compatible with the works progress. In this case,they are placed in special excavations (joint chambers) that are then filled once the joints have beenmade.

Trenchdigger (Figure 15, •2) :The trenchdigger has to dig the trenches at the dimensions indicated previously and must be equippedwith a cable guiding system to maintain the cables above the trenchdigger. Different kind of trenchingmachines can be found :with sawing wheel or sawing chain.

This cable guiding system is necessary to obtain the correct positioning of the cables at the entrance tothe cubicle tray . If the cable guides are not able to maintain the cables in a trefoil position until thecovering is in place, a fastening system becomes necessary.

Cubicle Tray (Figure 15, • 3):The cubicle tray which follow the digger must carry out the following tasks :- Positioning the cables in the excavation,- Vibrating the weak mix mortar as necessary,- Covering the cables with weak mix mortar,- Compacting the covering.

The equipment must allow a permissible route curve radius of approximately 12 to 15 m and must becapable of being dismantled when necessary.

Mortar hoppers (Figure 15, • 4) :Although they are not specific to this technique, the mortar hoppers carry out the following tasks :- Carrying to the works site the weak mortar mix necessary for the covering of the cables. One cubicmeter of mix allows a trench length of about ten meters,

- Pouring the mortar into the cubicle tray while operating the trenchdigger.

Backfill (Figure 15, •5) :The backfill is identical to that used for conventional methods. In order to reduce the works time, itssynchronisation is made with the progress of trenching.

Equipment :All the above equipment is part of a laying "train" about 50 m long, an example of which can be seen inFigure 15.


Figure 15 : Mechanical Laying

6) Examples of utilisation of this method

Since August 1993 many sites have used this technique in Europe. Thus we can consider that thistechnique has moved from the experimental to the industrial stage in the HV cables range3.3.6.2 Limits of the technique

• Civil workThis technique imposes the use of dedicated equipment which are expensive and heavy . Thetransportation of the equipment from one site to another one must be carefully considered (cost ,duration.) . The ground occupied by the works site is larger than for a conventional site for severalreasons:

- the width must include excavated earth,- digging equipment,- pathway for the mortar hoppers.

Consequently, an access strip of about 7 m is necessary. Nevertheless , a local complement ofbackfilling is still possible.

On the other hand, advantages of this technique compared to conventional open trench are :

- important reduction of time (time duration divided by 2 or 3)- less cable handling and consequently reduction of the risk of damaging the cables.

For good achievement a few points have to be carefully taken in consideration :

- management of the supply of weak mortar mix which is conditioned by three parameters :the distance,climaticprogress

- management of the quality of the weak mortar mix.

A few conditions are necessary to envisage the use of this technique :- rural type lands or along roads provided only a few obstacles are present- route more than a few hundred meters long .- slopes with less than 25 % .


• Drying of the soilAs the heat is first dissipated through the weak-mortar mix, thermal resistivity of the soil as well asdrying of the soil are not so critical. The use of specially formulated weak-mix mortar can be a solutionfor local problems.

• Temperature of the soil/environmentAs for any laying operation, special care should be given to cable temperature during laying.( minimumas well as maximum temperature, sun beams protection )

• Hardness of the soilThis technique is available for a rocky soil but the time is increased consequently, tractors withcaterpillar tracks should be used in marshy areas for better carrying performance.

• SeismicityThe use of weak-mortar mix seems to be a good improvement in case of seismicity.

• ArchaeologyThe technique is not suitable for archaeological sites.

• Presence of termitesThe technique is similar to other buried techniques.

• Laying in National ParkThis technique could be interesting in some cases (re-use of native soil, duration of the work )

• Duration of the work:A speed of 50 m/day (rocky soil ) to 300 m/day can be expected.

3.3.7 Embedding


This technique consists of excavating a riverbed from a barge or with an amphibious machine, buryinga tube or cables and filling up the trench.Burying of cables presents the effective, definitive protection against mechanical damage.


• Civil workThe methods that can be used to bury cables in riverbeds vary widely; the choice depends on suchfactors as river-bed conditions, operating depth, route obstructions, depth of burial desired or required,total length to be installed, cable size, and tools available.

Trenching can be performed by:

a) Dredgingb) Blastingc) Jetting operated by divers

Usually, the use of these methods is limited to rather shallow waters, practically up to depths of about20 m, which is usually enough for river crossings.


As the technical brochure is limited to land cables, the equipment dedicated for submarine cables, suchas submersible equipment which can operate down to a water depth of 1000 m and even more is notdescribed in this document.

Picture 12 : Embedding

• Method of operation:

Pre-trenchingPost-embeddingSimultaneous laying and embedding

Plugging or cutting of cable trenches before laying the cables requires precision laying of the cable andis usually limited to a depth where divers can work for some time and where the river is comparativelycalm.

• Method of excavation:

a) Static ploughb) Static plough water jets (injectors)c) Water jets (fluidisers)d) Suction-pumpse) Cutting-chainf) Cutting-wheelg) Mechanical disintegratorsh) Various combinations of the above

Methods a), b), c), and d) may only be used where the riverbed is soft, i.e. sand, shingle or clay.


• Propulsion:

a) Towed from the surface

b) Self propelled:

• Operators:

a) Operated from the surfaceassisted by diversnot assisted by divers

b) Operated by diversat bottom pressureat atmospheric pressure

It is to be pointed out that if the riverbed changes its morphology along the route, different types ofequipment might be needed.

• Hardness of the soil

Depending of the type of soil vehicles can accommodate three distinct cable-trenching tools:

A rock wheel cutter which require a cable route over the top of the vehicle will create a trench 1.2meters deep in any riverbed up to quite strong rock. This is a robust device, but the work rate can below with wear rate and consequently is time consuming.

A chain cutter can provide trenches more than 2 meters deep in quite hard material, but is subject tosignificant wear, with low work-rates in difficult conditions.

A powerful jet tool creating trench up to 2 meters. It can provide high work-rates in sandy riverbeds.

• Maintenance and repairing process

Due to the recent technological progress in the field of embedding machines and other ancillaryequipment (remote operated vehicles (ROV), etc.), cable burial is presently possible, with variousmethods, up to a considerable depth and practically in every kind of riverbed. However in many casesthe cost of the embedment is very high therefore the right way to proceed is to limit it to the sectionswhere the risk of a cable damage is so high that it offsets the embedment cost.

In case of limited risk, and where the power availability of the link is high, the possibility of a cablerepair has to be considered as preferable, its cost will be paid only if, and when, the cable will bedamaged.

Of course the cost of a repair, weighed with its probability, has to be compared with the cost ofprotection. The result of this approach could be different case by case: for example in a shortconnection where extensive human activities (shipping, with anchoring) are present. A total embeddingmay be preferable, whilst in a long connection with limited local activity, an exposed cable (exceptlimited portions with special protection) may be much more economic, even taking into account the costof a possible repair.


Picture 13 : ROV machine

• EnvironmentWhen crossing navigable waterways, this method implies that river traffic be stopped or deviatedduring the excavation and laying operations.

3.3.8 Use of existing structures


Finding new layouts for high tension lines is more and more difficult, especially in urban or conservationareas. The use of existing structures is very attractive to solve integration problems in the landscapeand may lead to drastic reductions in cost and start-up delays. The cohabitation of energy lines withrailway and road constructions is considered in previous items (tunnels and bridges). This section isdedicated to existing pipes, including an application to pipe-type cables retrofitting.

Pipe-type cables are the most commonly used in the United States to transmit power at high voltages.Three phase conductors are insulated with layers of fluid-impregnated paper and housed in a coatedsteel pipe. The free area in the pipe is pressurised with a dielectric fluid (oil or gas filled) to increasethe dielectric strength of the system, to suppress ionisation in the insulation, and to defer moistureingress in the event of a leak in the pipe.

This mode of installation offers several advantages : the pipe itself is very tough and can be installedwith short and narrow roadway openings, minimising traffic disturbances. When the pipe sections arewelded together, the cables may be pulled at a later date, and the maintenance requirements are lowcompared with self-contained fluid-filled cables.


The first pipe-type cable system was installed as far back as 1932. Retrofitting of systems is planned,and steel pipes are very suitable for the replacement of old cables or to increase the cable size. Existingpipes can stay on site, only the cables have to be changed by pulling.

By adopting another technology, utilities reduce their environmental exposure to fluid leaks. An idea isto replace the old fluid-impregnated paper tapes cables by cables with extruded dielectric insulationsuch as polyethylene. Due to electrical stress design considerations, the outer diameter of extrudedcables may be larger than for the previous fluid-impregnated cables. Therefore, the substitution is notalways feasible because of the minimum clearance between the top of the upper cable and the pipe.

With extruded cables designed with a moisture barrier as a thin metallic sheath, no pressuriseddielectric fluid is required. The technological change affects cable ratings because insulation and thefree area in the pipe are modified.


• Civil workThe use of existing structures offers the advantage of reduced civil work operations, without trenchopening or disturbance. Nevertheless it may be important to empty the pressurised dielectric fluid inaddition to removing the existing cables, if it is to be feared a risk of chemical incompatibility betweenthe remaining fluid and the replacing cables.

If a grout is injected after the pulling of the new cables, some vents have to be placed along the link.

• Drying of the soilThe air gap issued from the lack of pressurised fluid is prejudicial to the efficient heat flow dissipationfrom conductors towards the surrounding soil. Special injection grouts, with low well characterisedthermal resistivity, can decrease the risk of overheating and thermal instability due to moisturemigration.

• Duration of the workSince no civil work is involved, the use of existing structures is very favourable to shorten the siteduration. Retrofitting operations can be anticipated and planned to optimise installation.

• Cable removal after operationAny operation to have access to cables after laying is similar to ducts configuration.


The electrical stress design is the first element to design extruded cables to be pulled in place ofexisting insulated conductors in steel pipes. A second design point of view concerns ampacities and isof great interest. It rules the performance and the final acceptance of the proposed new solution.

Fluid-filled cables have been designed for the voltage stress at lightning impulse. The main insulation isthe fluid which fully impregnates the cable. Its breakdown strength in the butt gaps between the papertapes determines the insulation thickness.

The critical design parameter for extruded insulation cables is generally not the lightning impulsevoltage but the maximum stress at the alternating current operating voltage to achieve an expectednominal lifetime of more than 30 years. Historically, the ageing parameters were not accuratelyestablished. Low insulation design stress levels resulted in high insulation thickness and large cablediameters.


Today, the improvements of design, materials and manufacturing techniques led to cables withsynthetic extruded insulation for higher voltages up to 500 kV. The insulation thickness has beenreduced for high performance materials, and the maximum continuous operating temperature of 90°Cof XLPE permits to be competitive with fluid filled paper cables, and with polypropylene paper laminatecables to a certain extent.

Improved ampacities are not the only consequence of the external cable diameter reduction. Longershipping lengths are achieved, and the overall system cost may be positively affected.

Cable clearance :A critical parameter for pipe-type cable design is the clearance between the cables and pipe to ensurethat the cables can be pulled through the conduit. A minimum clearance of about 0,5 in. (12,7 mm) isrecommended by most utilities for straight pulls.

For three single-core touching cables in trefoil formation :

( ) ( )C D D D D Dd e d d e= − + + −

12

1 3 2

where :De = external diameter of cable (mm),Dd = internal diameter of duct or pipe (mm),C = clearance between the cables and the pipe (mm).

The value of the external cable diameter De can be increased by a few per cent to allow for variationsin cable and pipe dimensions or ovality at bends.

Reciprocally, the maximum external cable diameter for a given clearance value is :

( ) ( )[ ] ( )D D C D C De d d d=+

+ + − + +

1

2 2 34 1 3 3 2 1 3 3

20

40

60

80

100

120

140

100 120 140 160 180 200 220 240 260 280 300 320internal diameter of pipe Dd (mm)

De

(mm

)

C=1/4 in. C=1/2 in. C=1 in.

4"5"

6"

8"

10"

12"

Figure 16 : Maximum external cable diameterin terms of internal pipe diameter and clearance

Jam ratio :

When the ratio of the internal diameter of the duct or pipe to the cable external diameter is higher than3.0, one of the cables in a group of three or four may slip between two other cables, causing the cablesto jam in the conduit. The limit on jam ratio should be modified to take into account variations in cableor conduit diameter and ovality in conduit diameter at bends.


4. CABLE INSTALLATION DESIGN AND LAYING TECHNIQUES

4.1 Cable installation design

4.1.1 Installation design in airBasically two types of cable installation must be considered.In the first type, the cable is rigidly supported and restrained from any movement due to thermalexpansion or contraction. This is the type of support provided by closely spaced cleats in air. This typeof installation is described in Chapter 4.1.1.1.

The second type of support includes all systems in which the cable is free to move as a result of itsthermal expansion or contraction. The cable may be supported in cleats with a spacing wide enough toallow it to deflect vertically or horizontally as it expands or contracts. The cleats are usually supportedfrom below, but can also be suspended from above, depending on the local situations. This type ofinstallation is described in Chapter 4.1.1.2 or 4.1.1.3.

There is another situation where the cables are in air, being installed in pipes not filled with solidmaterial. From the thermo-mechanical point of view the cable may be rigid or flexible, depending on thespecific installation design, as described in Chapter 4.1.1.4.

The different types of support give rise to very different mechanical stresses and strains within thecomponents of the cable and the design procedures are therefore quite different. In general it ispreferable that any given cable system should be designed throughout its length on the basis of eitherrigid support or flexible support. If for any reason it becomes essential to mix the two types of supportwithin a single cable route, special precautions must be taken at the interface between the differentsystems, as described in Chapter 4.1.3.4.1.1.1 Rigid systems

When a length of cable is subjected to a temperature change, each component attempts to expand orcontract by an amount corresponding to its temperature change and its coefficient of expansion. Whenthe cable is installed in a rigidly restrained environment, no longitudinal expansion or contraction canoccur and the cable therefore develops a thrust when heated and its components are subjected to acorresponding compressive strain. The conductor and sheath need be considered in practice whencalculating this thrust, if the sheath is not present only the conductor must be considered. Experimentsshow that the value of the thrust developed on heating depends on the cable size and design, thetemperature rise and the rate of temperature rise, the slower the rate the more the cable elements willrelax and reduce the actual thrust.

It is generally assumed that at the time of installation the cable is in a stress free condition so that if thecable is laid at a ground temperature (or ambient temperature for cables in air) below the maximumdesign ambient temperature on which rating calculations are based it will develop a thrust when theambient temperature increases to the design value.This temperature increase is likely to be very slow however and hence allows a greater relaxation tooccur. The rate of temperature increase from the design ambient temperature to the maximumoperating temperature depends upon the cable environment and the rate of load increase.A buried cable cannot increase in temperature rapidly because of the thermal capacity of itssurrounding. A cable in air can rise in temperature more rapidly but it is most unusual to require anewly installed cable to carry full load immediately, load growth is usually gradual and cyclic so thatsome opportunity for relaxation occurs. To allow for these effects it is necessary to include relaxationfactors in the calculation of total cable thrust.


• Calculation of cable thrust

The evaluation of the cable thrust is essential when dealing with rigidly installed cables, or at theinterface points where flexible and rigid systems meet.

Generally speaking the total thrust of a cable can be calculated as:

C= C1 + C2 + C3 + C4 (kg)

where:C1 = conductor thrust due to loadC2 = conductor thrust due to ambient changeC3 = sheath thrust due to loadC4 = sheath thrust due to ambient change

C1 is given by:

C1 = K1. αc

. ∆Tc1. Ec

. Ac (kg)

where α is the coefficient of thermal expansion of the conductor metal

αc = 17.10-6 for copper (1/K)αc = 24.10-6 for aluminium (1/K)∆ Tc1 = conductor temperature rise from the maximum ambient temperature to the maximumconductor temperature (K)Ac = conductor cross section (total cross section for three-core cables)(mm2)K1 is the relaxation coefficient, which is of the order of 0.75 for load temperature variations,depending on cable constructionsEc is the equivalent Young’s modulus for the conductor which depends upon its construction andmaterials and on the constraint provided by the insulation surrounding the conductor. Experimentalmeasurements are necessary to obtain accurate results.

C2 is given by:

C2 = K2. αc

. ∆Tc2. Ec

. Ac (kg)

where the symbols have the same values as above, but K2 (the relaxation factor) is of the order of 0.45for ambient, temperature variations, depending on cable construction.

∆Tc2 is the conductor temperature rise from the laying temperature to the maximum design ambienttemperature (since the laying temperature is not usually known at the design stage the minimumambient temperature may be assumed). (K)

For the metallic sheath the thrust is given by:

C3 = K3. αg

. ∆Tg1. Eg

. Ag (kg)

where αg is the coefficient of thermal expansion of the sheath metal


αg = 28.10-6 for lead sheaths (1/K)αg = 24.10-6 for aluminium sheaths (1/K)

∆Tg1 is the sheath temperature rise from the maximum ambient temperature to the maximum sheathoperating temperature (K)

Ag = sheath cross section (mm2)K3 is of the order of 0.30 for lead sheaths and of 0.65 for aluminium sheathsEg is the equivalent Young’s modulus for the sheath in compression and must be found

experimentally for reasons similar to those given for the value of Ec (kg/mm2)

gggg AETKC ⋅⋅∆⋅⋅= 244 α (kg)

where the symbols have the same value as above, but K4 may be taken as 0.1 for lead sheaths and0.45 for aluminium sheaths, again depending on cable constructions

∆Tg 2 is the sheath temperature rise from the laying temperature to the maximum design ambient

temperature (K)

• Spacing and cleating

For rigidly restrained system the spacing and cleating evaluations must be done considering severalparameters:

- At curves the restraining elements must be capable of withstanding the radial force given by

F= C/R (kg/m)

where C is the cable thrust and R the curve radius

- the radial pressure at bends due to maximum conductor thrust must be compatible with the insulationmaterial

- cleat spacing must be calculated considering that the cable thrust must be less than the critical loadfor instability ( crC )For cables with thick aluminium sheath the critical load may be calculated as follows:

2

2

lJE

Ccr⋅⋅

=π

whilst for cables with lead or thin aluminium sheath:

2

22l

JECcr

⋅⋅⋅=

π

This difference of behaviour, which is shown in experimental tests, may be explained by the fact thatthe more rigid aluminium sheathed cables behaves as though the cleat acts as a hinge, whilst for theless rigid cables the restraint normally appears as midway between a hinge and a rigidly fixed beam.

- sheath strain must be checked if daily temperature changes are significant (> 35°)


- At bends the cleat spacing is reduced by half compared with the straight sections.

• Short circuit forces in rigidly restrained cables

Short circuit forces may be significant in the case of rigidly restrained cables cleated in air. In this casethe cable between cleats will already be in compression due to its temperature rise under normal loadand the electrodynamic effect of the short circuit will result in the addition of a uniformly distributedside loading to the original compression load which, assuming a phase/phase short circuit, is given by:

FI

S=

⋅⋅ ⋅ ⋅

µπ

02

2 9 81.(kg/m)

where:µo = magnetic permeability of air, 1256.10-6 (H/m)I = short circuit current, rms (A)S = cable spacing (m)

These forces result in a bending moment in the sheath which is a maximum adjacent to the cleat andhas a value:

MF l

=⋅ 2

12(kg.m)

where:l = cleat spacing (m)

This equation is valid for the normal case where the thrust C existing in the cable prior to the shortcircuit is less than 10% of Ccr the critical thrust causing deflection, where:

( )2

*2* 4

lEJ

C erπ⋅

= (kg)

(EJ)* is the flexural rigidity of the cable based on the short term properties of the sheath.

If the thrust existing in the cable before the short circuit exceeds 0.1. for a lead sheath cable or if thesheath is of aluminium, a more elaborate calculation must be used.

Having calculated the bending moment M, the sheath strain ε is given by:

( )*

3

210

EJDM s

⋅⋅⋅

=−

ε

where:Ds = outside diameter of cable sheath (mm)

To avoid noticeable permanent deformation of the cable the maximum sheath strain ε should be limitedto an acceptably low level.


4.1.1.2 Flexible systems (Western approach)

Flexible types of cable support are those systems which allow the cable to expand in length and todeflect laterally to accommodate this expansion when the cable is heated and to return to the originalformation on cooling. In order to control the movement of the cable within pre-determined limits it isusually installed initially in an approximately sinusoidal formation with cleats at appropriate intervals sothat expansion takes place by an increase in the amplitude of the sine wave.Because the flexible system allows cable expansion to take place it is not characterised by the highvalues of thrust which occur in the rigidly restrained system.

• Cables cleated with movement in a vertical plane

The cable is held in widely spaced cleats with an initial sag between cleats which increases withtemperature rise. Figure 17 illustrates a system of this type.

Figure 17 : Cable cleated with movement in a vertical plan

The spacing of the cleats is not critical and within the limits given below can be chosen to suit thefixings available.

The weight of the cable is supported by the cleat and if the cleat spacing is too large the side pressureon the cable at the cleat will become excessive and there will be a tendency to concentrate bending atthe edge of the cleat. On the assumption that the cleat length is approximately equal to the cablediameter and has suitably rounded edges the following practical rule is suggested

lDe

W≤

⋅

2

65(m)

where:l = cleat spacing (m)W = cable weight (kg/m)De = cable outside diameter (mm)

Similarly, to avoid concentrated bending at the edge of the cleat the cable deflection δ due to its ownweight should be at least five time less than the initial sag between cleats fo required to ensuresatisfactory expansion and contraction movement. It is therefore necessary to make an initial estimateof cleat spacing. The following criteria for δ and fo may be followed

( )δ =

⋅⋅

≤W l

EJf o

4

384 5(m)

where:δ = cable deflection due to its own weight (m)


(EJ) = flexural rigidity of the cable (kg.m2)fo = initial sag (m)W = cable weight (kg/m)l = cleat spacing (m)

Having determined the cleat spacing it is necessary to fix the value of fo, the initial sag between cleats.This sag should not normally be less than 2 De but it may be necessary to increase it beyond this valuein order to ensure that the change of strain in the sheath due to thermal movements does not exceedthe maximum imposed by the fatigue properties of the sheath.

To simplify the calculation of the sheath strain it is assumed that the longitudinal expansion of thecomplete cable follows the expansion of the conductor.The total sheath strain is then the sum of the absolute values of the strain due to the movement of thecable together with the strain due to the differential expansion of the conductor and the sheath.

On this basis it can be shown that the maximum sheath strain change ∆εmax will not be exceededprovided:

ggcc

scc

TTDT

f∆⋅−∆⋅−∆

⋅⋅∆⋅⋅≥

−

ααεα

max

3

0102

(m)

where:αc = coefficient of thermal expansion of the conductor (l/K)∆Tc = daily temperature rise of the conductor (K)αg = coefficient of the thermal expansion of the sheath (l/K)∆Tg = daily temperature rise of the sheath (K)

Ds = outside diameter of the metal sheath (or average outside diameter for a corrugated sheath)(mm)

∆ε max = maximum allowable sheath strain change due to daily load cycles.

For a typical system designed for a life of 30, 40 years the standard values of 0.1% for lead and 0.25%for aluminium sheathed cables are normally adopted, particularly for fluid filled cables.

Taking into account the excellent experience during many years, however, slightly less conservativevalues such as 0.12% for lead and 0.35% for aluminium can also be considered, particularly forextruded cables.

The system described above is suitable for straight or gently curved cable routes. If it becomesnecessary to install the cable around a small radius bend in the route it should be supported on ahorizontal plane within the bend and with suitable means of minimising friction as the cable moves dueto thermal changes.

Flexible system with cable movement in a horizontal plane

In this type of installation the cables are arranged in a sinusoidal formation in a horizontal plane withcleats fixed at the points of flexure of these sinusoids, as shown inFigure 18.


Figure 18 : Plan view of cables installed with movement in a horizontal plan

Swivelling cleats may be used, capable of rotating on a vertical axis as the cable moves, but it ispreferred to use fixed cleats with a length approximately equal to the cable diameter and with a rubberlining of 3 to 5 mm thickness.These cleats must be installed at an appropriate angle.The movement of the cable due to thermal cycles will be largely influenced by the friction between theoutside surface of the cable and the support between cleats. It is essential that the cable should besupported so that it moves only in the horizontal plane using a low friction support and allowingadequate air movement around the cable to avoid de-rating.As a practical rule the cleat spacing should be:

lDe=20

(m)

where:De = outside diameter of the cable (mm)

The initial deflection of the cable fo should be fixed following the same rules as given in paragraph forcable moving in a vertical plane.

• Calculation of cable thrust

As already mentioned the cable thermal expansion in a flexible configuration give rise to small axialthrust, while the initial sag is increased.

Simple formulae can be used to calculate these parameters, assuming that the initial configuration is asinusoid.

The sag is given by the formula:

2

22

0

4π

α lTff cc ⋅∆⋅⋅

+=

Where the symbols are the same used as before.

The axial thrust is given by the formula:

fff

lJE

C 02

24 −⋅

⋅⋅⋅=

π


It may be easily verified that the axial thrust in a flexible system is much lower than in a rigid systemand may be practically neglected in most applications.

• Short circuit forces in flexible type cable installation

Short circuit forces are of much greater significance in cable installations of the flexible type becauseof the wider cleat spacing used compared with rigidly restrained cables.It is normally necessary to provide straps around the three cables at intervals between the cleats tohold the cables together during a short circuit. It therefore becomes necessary to consider the spacingof these straps and the strength of the strap necessary to withstand the forces involved.

As in the case of rigidly restrained cables, the length of cable between restraints will be subjected to auniformly distributed side loading and assuming a phase/phase short circuit this is given by:

Fl

So=

⋅⋅ ⋅ ⋅

µπ

2

2 9 81.(kg/m)

where:µo = magnetic permeability of air = 1256.10-6 (H/m)I = short circuit current (rms) (A)S = cable spacing (m)

This force results in a bending moment in the sheath adjacent to the restraint of:

MF l

=⋅ 1

2

12(kg.m)

where:l1 = distance between restraints (m)

Since it is usually necessary to fit at least one restraining clamp around the cables between the cleats, l1corresponds to the distance between these clamps or between a cleat and a clamp.

Hence, as before, the maximum sheath strain is given by:

( )*2

10 3

EJ

DM s

⋅

⋅⋅=

−

ε

where the symbols have the same meaning as before and to avoid noticeable permanent deformation ofthe cable the value of the sheath strain ε must be limited to an acceptably low level.

The strength of the strap can be calculated from the equation for F above but since this ignores theinstantaneous value of current, which may substantially exceed the rms value and also ignoresresonance effects which may occur, a factor of safety of 2 should be allowed so that the minimumstrength of the strap is given by:

2.F.l1 (kg)


Using these equations the number of strap within a span of cable between cleats may be calculated.The same equations are valid for both types of flexible installation with movement in the vertical orhorizontal planes.4.1.1.3 Flexible systems (Japanese approach)

Generally, there are uniform basics for the snaking design, as described below.

Horizontal snaking installation

1) Initial snaking width : 1 De or more

Figure 19 : Horizontal snaking Picture 14 : Snaking in a tunnel

2) Pitch (2 L): 6-9 m3) Occupied width (W): W = D + B + n + σWhere D = Cable occupied width (2De De = outside diameter of the cable when trefoil installation)B = Initial snake widthn = Lateral snake displacementσ = Tolerance

4) Lateral displacement (n): n = B− 0.8 x L.m2 + B² Where: m = Cable expansion = α.t.Lα = Coefficient of linear cable expansiont = Temperature rise5) Formulas for axial tensions generated (Fa):Depending on types with or without metal sheaths, the formulas in Table 3 are used.


Table 3 : Horizontal snaking calculations

Formulas for Calculating Axial Snaking Tensions Generated (Horizontal Snake)

Metal Sheath Low Temperature High TemperatureWith Metal Sheath µ. . ²w L

B2x 0.8 −

8. .²

E IB

.α. t2

−+

8. .( )²

E IB n

.α. t2

−µ

+. . ²

( )w LB n2

x 0.8

Without Metal Sheath−

8. .²

E IB

.α. t2

+µ. . ²

.w L

B2x0.8 −

+8. .

( )²E I

B n.α. t2

−µ

+. . ²

( )w LB n2

x 0.8

Note: + tension; - : compression

EI= Cable bending rigidityW = Unit cable weightµ = Coefficient of friction between cable and installation surface

6) End section of snaking installationThe necessary number (N) of fastening cleats is generally determined as follows:

N = Fa/F + 1 (or Fa/F × Sf)Where: F = Restraining force of terminal fastening cleatsSf = Safety factor

7) Middle section of snaking installationThe snake formation is fastened at inflection points with intermediate cleats at every or several pitches.

Vertical snaking installation

Figure 20 : Vertical snaking


Initial snake width (B) : 1 De or morePitch (2 L) : snake pitchBending distortion : 1.5% or lessRadial surface pressure : 3.33 kg/cm² or less

Lateral displacement (n) : n = 0.8 x L.m2 + B² - BFormulas for axial tensions generated (Fa):

From the same viewpoint as for horizontal snaking installations, the formulas in Table 4 are used withor without metal sheaths.

Table 4 : Vertical snaking calculations

Formulas for Calculating Axial Snake Tensions Generated (Vertical Snake)

Metal Sheath Low Temperature High TemperatureWith Metal Sheath w L

B. ².2

x0.8 −8. .

²E IB

.α. t2

−+

8. .( )²

E IB n

.α. t2

−+

w LB n. ²

( )2x 0.8

Without Metal Sheath−

8. .²

E IB

.α. t2

+w L

B. ².2

x0.8 −+

8. .( )²

E IB n

.α. t2

−+

w LB n. ²

( )2 x 0.8

End section of snaking installationThe necessary number (N) of terminal fastening cleats is generally determined as follows:

N = Fa/F + 1 (or Fa/F x Sf)Where: F = Restraining force of terminal fastening cleatsSf = Safety factor

Middle section of snaking installationThe cable is supported by direct cable rests at crests of the vertical snaking. In some installations,restraining cleats are used at every several pitches.

Vertical Installation DesignFluid-Filled cables involve fluid pressure rises due to their vertical installation. XLPE cables do notinvolve such difficulties and are easy to install upward on a tower. Vertical installations used on towers,in vertical tunnels, etc. can generally be classified as follows:


Table 5 : Vertical cable installation at shafts

Item Method Triplex cable - Straight installation

- Fastened with cleats at several m intervals Single-core cable

Shaft height6-10m or less - Straight installation

- Fastened with cleats at several m intervals Shaft height6-10m or more - Snaking installation (6-8m pitch)

- Fastened with cleats at snake inflection points (For some sizes, movable cleat supports are also used at snake crests.)

Shafts where cleatscan not be used

- One-point fastening using tension member cable - Steadying cleats are used (in special installations case).

4.1.1.4 Cable in ducts

The ducts may be filled with solid material such as Bentonite or not filled.The first solution is often preferred for relatively short ducts (normally less then 100 m) used to crossroads, railways or other obstacles, inserted in sections where the cable is directly buried. Anymovement of thermal origin is completely prevented and the cable behaves as if it were directly buried.

In other situations according to different practices or due to other constraints, the ducts are not filledand three different design concepts may be adopted.

a) Large diameter duct, cable blocked at the extremities.If the inner diameter of the duct is significantly larger than that of the cable (typically 1.5 to 2 times),the thermal elongation results in cable snaking. The geometrical configuration is similar to a sinusoid ora helix with a certain pitch and amplitude, depending on duct and cable diameters, weight, axial rigidity,flexural rigidity of the cable, friction coefficient between cable and duct, temperature variation. Thanksto the snaking effect, the axial thrust is drastically reduced with respect to the thrust developed by arigidly restrained cable. As a consequence of the thermal cycling, the amplitude of the deformationvaries cyclically and the resistance of the cable to fatigue phenomena must be considered.

b) Small diameter duct, cable blocked at the extremities.If the diameter of the duct is only slightly larger than the cable (minimum clearance to allow the cablepulling), the very limited snaking is not sufficient to reduce the axial thrust and the thermal movementsare negligible. In practice this case may be considered a rigid installation.

c) Small diameter duct, cable free to move at the extremities.In this type of installation there is a certain movement of the cable from the duct towards the manholesas a consequence of the thermal expansion. In the manholes the cable is installed in a snakingconfiguration properly designed in order to maintain the cyclic bending of the cable within acceptablelimits. This situation is dealt with as a transition in Chapter 4.1.3.


4.1.2 Installation design for buried cables

4.1.2.1 Backfill

To improve heat dissipation, only sand or special backfill shall be used around cables or ducts.Sand :There are specifications available how to select sand for cable trenches. During performance of thework the constructor shall take random samples for clay tests and sieve tests. The thermal resistivitycharacteristics of the sand shall also be verified by testing. During installation of the cable theexcavated trench shall be kept in dry condition by dewatering until cable laying and backfilling arecompleted. Backfill material shall be placed in uniform layers and compacted. Moisture content ofbackfill material shall be adjusted as required to obtain the specified density. To protect the cable shallbe covered by concrete tiles or plastic sheets. On top of the tiles native backfill may be used.

• Special backfillThe thermal resistivity of a carefully selected Cement Bound Sand (CBS) for electrical cables hasseveral advantages as a good thermal backfill material. It will eliminate the risk of voids due to watererosion or movement caused by thermal expansion. CBS will also keep the thermal resistance of thebackfill surrounding the cable at a very stable level.The thermal resistivity of a good backfill for electrical cables when hardened is 0.35 K.m/W or less inmoist condition and 1 K.m/W in its totally dried out condition. Testing of the thermal resistivity andcompressive strength has to be performed to be sure it reaches the specified values.The thermal backfill shall be composed of fine and coarse aggregates, cement and fluidising agent. Thefluidising agent consists normally of fly ash and water.The thermal backfill should be installed by pouring it into the trench or by use of grout pumps.Natural backfill should not be placed in the trench until one day after pouring and inspection.4.1.2.2 Cooling systems

To increase the capacity of a cable circuit forced cooling can be used for the direct buried cables andcables in duct banks. There are two main systems that have been used. External cooling, where coolingwater runs in four pipes separated from the three single core cables and laid parallel to them. The othersystem is surface cooling where water is in direct contact with the outside surface of the cable. Eachcable and its cooling water being contained in a pipe. Cooling stations are normally placed at the endsof the cable route. The cooling station consists of water pumps, water storage and expansion tank, anda heat exchanger for cooling the water circulated round the cable route. The cooling stations can beoperational from either local or remote positions.To increase the load capacity of cables installed in tunnels and shafts, forced ventilation can be used.Temperature sensors are monitoring the surface temperature of the cables in various places along theroute. The ventilation fans starts at pre set levels to cool the cables during the high load periods.

A range of articles describing design and calculation of forced cooling of cable installations areavailable in Electra and IEEE.

4.1.3 Transition between different installation typesAlong the route of an underground link, different installation techniques may be used. If all thetechniques are all rigid or all flexible, the behaviour of the link seems to be homogeneous, but even inthis case, problems may appear around the jointing areas.If they are different, i.e. unfilled duct and direct burial or filled trough and manhole, damages canhappen on the link if transitions between flexible and rigid installations and of course around the jointingare not treated or badly treated.


4.1.3.1 Transition between ducts and manholes (open air)

Cable thermal expansion of the span appears in the manhole by duct method.Therefore the “Offset” part is made, and it must absorb thermal expansion so that the thermalexpansion does not have a bad influence on the cable and joint part.Offset design of the XLPE cable in the manhole uses a geometrical analytic technique so that the cablebend radius do not to become less than allowable radius when the cable come out from duct face.And, the consideration is necessary about the amount of occurrence sheath distortion in the case of thecable that has a metal sheath.So, allowable amplitude distortion has only to find amplitude condition by the day data, from the S-Ncurve (amplitude Strain - Number of times to the destruction), because the thermal expansion numberof times of the year is smaller than the thermal expansion number of times of the day.

1) Design of straight offsetThe experiment was done about movement of jointbox offset form that absorbed thermal expansion ofone core XLPE cable, based on the technique of SCFF cable experiment in the beginning.The cable spiral deformation occurred, and the inclination of the jointbox and direction movement in thecore occurred as a result of this examination.Therefore, it confirmed that the rigid of the jointbox was necessary, because the deformation isconcentrated on one bend part and an allowable radius can not secure it.Such an experiment is continued, and an examination is done on the jointbox fixed condition, and thedesign technique of the following 3 forms is applicable at present :

a) Equal arcs range offset typeb) Long offset typec) Three equal arcs with pendency type

Each calculation technique is shown in Table 6.

a) Calculation technique of “Equal arcs range offset type”This form is the way of designing that suppose to absorb thermal expansion by two circular arcswithout change the turning point.The attention is needed with this form that the cable contact to the other cables and the other jointboxbecause of spiral deformation offset by thermal expansion.

b) Calculation technique of “Long offset type”This form has straight line part in the offset section.It is the way of designing that suppose to absorb thermal expansion by forms three circular arcs bystraight line part's being curved.

c) Calculation technique of “Three equal arcs with pendency type”Large conductor size cable hangs down greatly between support points at the time of the thermalexpansion, the part where a cable in the neighbourhood of the bottom hung down point can not satisfythe allowable radius.So, this form takes that problem into consideration.This form does not contain a straight line in the offset section in the same way as “Equal arcs rangeoffset type”.It is the way of designing that supposes to absorb thermal expansion by three circular arcs three-dimensionally.


Table 6 : Offset calculations

Equal arcs range offset type Long offset typeThree equal arcs with pendency

type

Bend radius R at the time of thermalexpansion

Where,R: Bend radius at the thermalexpansion length of the annualmaximum (mm)L: Offset length (mm)F: Offset width (mm)θ : Centre angle at the time ofthermal expansion (rad)

Bend radius R is approximated bythree arcs.

Where,R: Bend radius at the thermalexpansion length of the annualmaximum (mm)L: Offset length of arc part (mm)l : Offset length of straight partF: Offset width (mm)θ : Centre angle at the time of thermalexpansion (rad)

Where,R1:Radius decided more than theinterior radius rate of both endssupport hardwareR2:Bend radius at the thermalexpansion length of the annualmaximum d : The amount of pendenta1,a2:The position to the bottompoint

As for the details, it is omitted.

d) Method of supporting cable in the manholeThe cable guide is set up at duct face and joint side to avoid bad influence for cable against thermalexpansion in the all forms.Support method for the cable that can be put between the cableguide, varies in the kind of the eachform.In the case of “Equal arcs range offset type”, It is supported with the cable guide in the duct face andthe jointbox side, and the cable between that is not supported.In the case of “Long offset type”, It is supported the middle point for the prevention of hang down thecable, because the offset length sometimes becomes long by the existence of the straight line part atthe time of thermal expansion.And, In the case of “Three equal arcs with pendency type”, It is supported with the suitable supporthardware to keep the cable allowable radius in the pendent part at the time of the thermal expansion.On the other hand, the Triplex XLPE cable of the horizontal part, is supported with the pillowsmanufactured by the porcelain material, and so on in the interval of about 1-1.5m.

2) Offset design of the bend partThis form is the way of designing that supposes to so that the degree of radius might become aconstant, as shown in the bottom figure.The bend radius changes in "R" from "R0" by thermal expansion quantity "m", then the straight line partwhich is equal on both sides of the bend part is established.

Joint box

Joint box

Pendent part at the time ofthermal expansion

Plane distance between the support points

Initial form

2sin2

tan

tan21

22

122

122

θθ

θ

=+

++

++=

−

−

LF

LF

FLF

m

LF

FLF

mR

θ

θ

θ

2sin

2sin

2

)(

2sin

221

221

22

22

221

22

=+

++

+−+

+

++

=

−

−

l

l

l

LFLF

FLF

FmL

LFLF

FLF

R


Figure 21 : Shape of bend part

4.1.3.2 Transition between flexible and rigid systems (open air)

As already stated it is desirable that any given cable installation should be designed as a wholly rigid orwholly flexible system. It may sometimes be necessary, however, to mix the two above designs withina single cable route and special consideration must be given to the interface between the two systems.

Rigidly restrained cable systems are characterised by the presence of a substantial mechanical thrust inthe conductor when the cable is heated, whilst flexible systems have a low value of conductor thrust.

At the interface between these systems the conductor will tend to move from the rigidly restrainedsection into the flexible section. The amount of the movement depends on the cable characteristics andparticularly on internal friction between the conductor and the other cable components. The movementmay extend over a few meters on both sides of the transition section and may cause damage ordisturbance to the insulation and unacceptable sheath strains, if appropriate precautions are not taken.

In order to reduce the movement and its effect on cable integrity, it is good practice to install the cablein a series of rigidly fixed curves at the extremity of the buried section, in order to provide a highfrictional resistance to the movement of the conductor within the cable.

If a joint is installed at the transition section, the behaviour of the joint itself must be carefullyconsidered in relation with the above mentioned phenomena of movement and axial thrust of theconductor.

In the most common joint design there is no mechanical restriction to the conductor movements,whereas in other designs a mechanical block of the conductor is provided.

It should be verified that the movements or the mechanical thrust do not exceed acceptable limits.4.1.3.3 Transition between flexible and rigid systems (buried)

This case can be found where cables are partly laid in ducts and partly directly buried or laid in filledtroughs.As already stated before, at the interface between rigid and flexible systems, the conductor will tend tomove from the rigidly restrained section into the flexible section ; Ducts of appropriate size are thenrequired in order to allow a kind of snaking inside the conduit.One must be aware that the movement of the whole cable inside the duct may extend on both sides ofthe duct section. If a joint is installed at the transition section, the behaviour of the joint must be

θθ −−=

)2/tan(20m

RR Straight part

Straight part


carefully considered in relation with the above mentioned phenomena of possible movement and axialthrust of the cable. Even when the joint is buried, additional fixing of the joint is required, for exampleby the use of weak mix or by appropriate cleating.

4.2 Cable laying and installation techniques

4.2.1 Cable pulling calculations

The basic calculations relevant to cable pulling are reported hereunder.4.2.1.1 Clearance in ducts

Cables pulling in ducts or pipes requires that the duct or the pipe have an internal diameter in excess ofthe cable diameter to allow for a safe operation.The free space between the cable and the duct inner size is called clearance.The clearance to be considered is not the result of the implementation of precise formulae, but theresult of the practical experiences.For single core cable the inner size of the ducts should be normally at least 1.5 times the cable size,particularly with long ducts with some bends along the route.For three cables pulling in the same duct, the duct size should not have a ratio with the cable size ofless than 2.8 to 3, because jamming may take place at bends.According to a different industrial practice a standard clearance of 30 mm is adopted. Even smallerclearance may be adopted for straight pulls.

4.2.1.2 Pulling tension

The main parameter to be evaluated when assessing the cable laying aspects is the cable pullingtension.The knowledge of the pulling tension is not only essential to plan the actual lay, but also to assess thesuitability of cable design / route design / laying methodologies.

The following equations are applicable to single cables, nose pulled into trenches or into long ducts orpipes.The route should be first divided into specific sections of straight, curved, uphill slope and downhillslope. The pulling tension required for each section is then calculated, starting at the drum and takingthe exit tension for each section as the entry tension for the next. The formulae are as follows:

Straight pull

T T W K L2 1= + ⋅ ⋅ (kg)

where:T2 = exit tension (kg)T1 = entry tension (kg)W = cable weight (kg/m)L = length of section (m)K = coefficient of friction for that section (m)


Horizontal bend

T1

T2

R

ϑ

Figure 22 : Horizontal bend

( )22112 sinhcosh RWTKKTT ⋅++= θθ (kg)

where:θ = angle subtended by the bend (radians)R = bend radius (m)

Vertical bend

Pulling up the bend

T3

R

ϑ

T1

T2Rϑ

Figure 23 : Vertical bend (pulling up)

( )( )[ ]θθ θθ cos1sin21

.22

.12 −−−⋅⋅

+⋅

−⋅= KK eKKKRW

eTT (kg)


( )( )[ ]θϑθθ cos1121

.22

.23 ⋅−−+⋅⋅

+⋅

+⋅= KKK eKeKKRW

eTT (kg)

Pulling down the bend

T3

R

ϑ

T1

T2

Rϑ

Figure 24 : Vertical bend (pulling down)

( )( )[ ]T TeW R

KK K eK K

2 1 22

12 1= +

⋅+

⋅ ⋅ − − −⋅θ θθ θsin cos. (kg)

( )( )[ ]T T eW R

KK e K eK K K

3 2 22

12 1 1= −

⋅+

⋅ ⋅ ⋅ + − − ⋅⋅ ⋅. sin cosθ θ θθ θ (kg)

Upward slope

L

ϑ

T1

T2

Figure 25 : Upward slope

( )T T W L K2 1= + ⋅ + ⋅sin cosθ θ (kg)


Downward slope

L

ϑ

T1

T2

Figure 26 : Downward slope

( )T T W L K2 1= − ⋅ − ⋅sin cosθ θ (kg)

In the formulae given above the value of K, the coefficient of friction, for the part of the route inquestion will depend on the material of the cable outer sheath and the surface with which it is incontact.It is essential to have good reference values for the friction coefficient to have reliable values, whilesimplified formulae can be used to calculate the pulling tension.

Having established the pulling tension required it must be checked that this tension is within theacceptable limits for the cable.To avoid relative movement between conductor and sheath with possible disturbance of the insulation itis essential to fit a cable pulling grip which is anchored to the conductor or conductors and to the sheathat the leading end of the cable. A pulling grip is also fitted at the trailing end of aluminium sheathcables. However it is assumed that the tension is withstood by the conductor.As reference the following values could be considered:

single core cables - copper conductors 6 kg/mm2

single core cables - aluminium conductors 3 kg/mm2

3 core cables - copper conductors 5 kg/mm2

3 core cables - aluminium conductors 3 kg/mm2

but alternative values could be considered.

For example, France considers- for aluminium single core cables, 5 kg/ mm2,- for all single core cables, a limitation on the pulling tension of 4000 kg.

In any case, the maximum permitted levels of conductor tension have to be checked with the cablesupplier.

4.2.1.3 Side wall pressure

Having established the pulling tension required it must be checked that this tension is within theacceptable limit for the cable and that the side pressure on the cable at bends is also acceptable.


At bends in the route the compression force between the roller and the cable is given by:

FT d

R=

⋅

(kg)

where:F = compression force on roller (kg)T = tension in cable (kg)R = bend radius (m)d = distance between rollers (m)

If iron skid plates are used at the bend, the compression force between cable and skid plate is given by

FTR

=(kg/m)

The maximum permissible values of F are different and dependant on the type of sheath and insulation.These values have to be checked with the cable manufacturers.

4.2.2 Installation Methods

4.2.2.1 Introduction

The following five techniques are now used: Nose pulling by winch, synchronised power drive rollers,caterpillars, mechanical laying and bond pulling and the most common is nose pulling followed by powerrollers, caterpillars , bond pulling and finally mechanical laying.

Picture 15 : Cable pulling in duct

A brief description of each of the five techniques is given below:

4.2.2.2 Nose pulling

With this technique the cable is installed by using winch with a pulling hawser directly connected to thecable end, or "nose".In this case the tension required to install the cable is taken by the cable itself and hence it is importantthat the pulling tensions are calculated beforehand to ensure the design limits are not exceeded.


4.2.2.3 Synchronised power drive rollers

This technique relies on the use of multiple powered rollers positioned at regular intervals along thecable route to install the cable. The frequency of the rollers is dependent upon the cable constructionand the route itself.Since each roller has to provide an equal force they require to be synchronised to operate effectivelyand to avoid any damage to the cable due to compressive forces.Normally a winch and hawser are used to supplement the rollers by nose pulling but the tension on thecable end is very low due to the effects of the powered rollers.

4.2.2.4 Caterpillar or hauling machine

Caterpillars apply a pushing force directly onto the cable outer sheath and can be used to install thecable directly or in conjunction with power rollers or winches.

4.2.2.5 Bond Pulling

With this technique the pulling tension applied by the winch is taken by a wire bond to which the cableis tied at regular intervals.At bends the bond is passed through a snatch block and the ties attaching the cable are removed beforethe bend and reapplied after the bend in a continuous operation.The tension required to install the cable is therefore distributed along its full length and sidewallpressure at bends is reduced to a minimum.

4.2.2.6 Mechanical laying

There are three ways of organising the mechanical laying site:- mechanically excavated narrow trench, and separate laying of the cables: laying and backfilling isdone by traditional methods after the trench has been mechanically excavated;- trench excavation and cable laying both mechanical : trench excavation, cable laying and sometimesthe backfilling are performed by a machine;- trench excavation, cable laying, backfilling all continuous and mechanised : with this method, trenchexcavation, cable laying and sometimes trench backfilling can all be done simultaneously in acontinuous process over the full length of a homogeneous portion of the link (the joints have to beprepared beforehand).This technique is only used for voltages under 170 kV.The cables are usually buried directly in trefoil formation with a minimum cover of one metre.4.2.2.7 Other installation methods in tunnel

• By magnetic beltsWhen laying a cable in a tunnel, many electric poweredcaterpillars are placed in the tunnel. Caterpillars are operatedsynchronously to pull the cable in the tunnel. Recently, in order toshorten the construction period and lower the cost by decreasingthe number of joints, the cable span becomes longer and longer.For quick and steady cable drawing of such long cables, a cabletransfer system with magnetic belts may be used.

Picture 16 : Cable installationin tunnel


Figure 27 : Cable installation in tunnel

Figure 28 : Magnetic belt pulling machine

• By locomotive and trolley systemAn innovative technique is being developed for the installation of high voltage cables in a 3m diameter10km long tunnel beneath Auckland (New Zealand). The cables in 1350m long sections are to belocated on racks at varying height on either side of the tunnel. The tunnel has a 710mm gaugeconventional light rail track on the floor.

Position of the settingmachine in the tunnel Driving gear

(Magnetic belt)Rail

Guide rollerCable


The installation is to be achieved by lowering 30 tonne drums into the cable shafts where an hydraulicCaterpillar Cable Pusher feeds the cable onto 700 custom-made trolleys attached to a wire rope. Thetrolleys are guided by one of the rails and support and tow the cable along the tunnel into position. Theempty trolleys are parked on one rail and travel around to a turntable near the Caterpillar where theyaccept the cable from the drum. A diesel hydraulic locomotive (tow-shoe unit), driving on solid rubbertyres and guided by the light rail track tows the cable and trolleys from the turntable along the floor intoposition. The locomotive uncouples from the trolleys and reverses over them while simultaneouslylifting from the trolleys up to the cable brackets. The cable is snaked before being lowered onto thewall brackets.

The Locomotive has a maximum draw bar pulling capacity of 30 kN and can lay cable at 2 km/h, witha top travelling speed when not working of 4 km/h. The maximum tensile load imposed on the cableduring handling is only about 1 kN allowing for lightweight cable brackets to be utilised.

Picture 17 : Cable laying Locomotive undergoing trials.

The photo shows the Locomotive with trolleys in left foreground. The simulated tunnel and bracketsare to the right.

4.2.3 Installation process

4.2.3.1 Transportation of cable to site

Cables are traditionally transported to site on cylindrical drums. The size of these drums beingdetermined by the length of cable to be delivered, the minimum internal diameter of the drum requiredto satisfy the cable minimum bending radius, the maximum size and weights allowed under existing


transport legislation, handling limitations in the cable makers facilities and specific loading and sizelimitations relevant to the particular project forwhich the cable is supplied.For HV and EHV cables this normally limits themaximum lengths of cables that can be deliveredon drums and by road to between 1500 metresfor HV and 1000 metres for EHV cables.

Where access to the installation site is possible bysea then the cable can be delivered on turntablesor very large drums. In these circumstances, thecable length is only limited by the cable makers’factory facilities or by the cable system design.

Picture 18 : Cable reel

4.2.3.2 Cable Bending Radius

The bending radius of the cable both on the despatch drum and especially during the installation andfinal positioning has to be controlled to avoid damage to the cable during transport and installation toensure the long term reliability of the cable circuit.The minimum bend radius of the cable is normally specified by the cable manufacturer and varies in asite environment dependent upon whether the cable is bent in a controlled manner or not.The minimum bend radius when the cable is bent around a former or by using a formed support isgenerally significantly smaller than when the bend is formed naturally by applying lateral force to thecable.During the installation phase the minimum bend radius is also dependent upon the need to limit the cablesidewall bearing pressures to within acceptable limits as discussed previously and the cable installationdesign needs to take all these aspects into consideration.4.2.3.3 Cable Temperature

The range of acceptable temperature for cable installation is generally defined by the properties of thematerial used for the cable outer sheath.This range is more restricted when PVC is used for the cable outer sheath than when PE is used.Although not normally a significant issue the scheduling of installation activities and selection of theinstallation techniques used need to take this into account in countries where temperature extremes areexperienced.4.2.3.4 Pulling Length

The maximum pulling length that can be achieved when using nose pulling is fundamentally a functionof the allowable pulling tension and the maximum sidewall pressure (see chapter 4.2.1.3) that the cablecan withstand.For other pulling techniques the maximum pulling length is more often determined by either transport orhandling difficulties, than the maximum size or weight of the cable drum, or the system design – sectionlengths for special bonded circuits for example.4.2.3.5 Route Profile

The route profile is significant as it effects the magnitude of the forces needed to install the cable.It is important that a full route survey is available to the installation system designer at the early stagesof the project to enable the profile to be taken into account since this could effect the maximumallowable pulling lengths of the cable sections being installed.


4.2.3.6 Obstacles

In most situations the cable route selected will encounter obstacles of one kind or another along theroute.Various installation techniques have been developed to allow all eventualities to be overcome in acontrolled manner. It is normal therefore to find a number of installation techniques used along a givencable route.Again it is important that all obstacles along the proposed route are identified and known at the earlystages of the design phase since they may effect both the cable design and the selection of theinstallation and pulling techniques4.2.3.7 Setting Up

To ensure the successful implementation of the installation activities the logistics and facilities requiredneed to be carefully considered.Adequate provisions need to be made for the storage and handling of cable drums generally weighingbetween 15 tonnes and 30 tonnes.Access to the site by heavy transporters, availability of lifting equipment and suitable hard standing forstorage and during cable installation needs to be carefully considered and the decision on pullingdirection and method must take all these issues into consideration.Prior to actually installing the cable the necessary installation equipment needs to be set up along thecable route.The actual equipment needed is dependent upon the installation technique used but must in all cases bepositioned to satisfy the installation design criteria and must be supported and fixed in a manner thatcan withstand the mechanical forces generated during the installation activity.

4.2.3.8 Installation of Cable

The cable drum is moved into position and mounted on a purpose made stand that allows the drum torotate. This stand is often motorised to overcome the forces required to turn the drum, henceminimising the "backtension" on the cable, and also to allow controlled braking of the drum.A braking facility must always be available to ensure the drum only rotates at the required speed.The cable is then installed along the route in accordance with the selected method under closesupervision at all times to guarantee that the process runs smoothly and that the design criteria are notexceeded.Upon completion of this stage of the installation and where the cable is accessible, such as in troughs,trenches and tunnels the cable is carefully positioned into its final position in the trench or trough or onits support systems, as determined during the system design phase. For flexible systems the cable isoffset either manually or by use of jigs at this stage.On successful installation of the first cable the equipment is moved as necessary for the next cable andthe process is repeated until all cables are successfully installed.4.2.3.9 Final Installation Stages

On completion of the installation process for all cables the final stages of the installation are carried out.For open trench and trough type installation this requires the installation of stabilised thermal backfillaround and above the cables, the application of protective mechanical barriers and warning tapes andfinally the reinstatement of the upper layers of the excavation in accordance with local requirements.For other types of installations this requires the installation of mechanical restraints such as cleats andshort circuit straps, filling and sealing of ducts etc.4.2.3.10 Site Quality Assurance


The quality of the materials used in manufacturing the cable and accessories and the quality of themanufacturing process itself can be closely monitored and is assured by rigorous type and routinetesting of the product prior to delivery to site.Long term reliability of the cable circuit can only be guaranteed if the same attention to quality istransferred into the installation phase.It is therefore essential that the installation design and the laying and installation techniques areengineered correctly such that the laid down criteria are complied with. During the actual installationphase it is important that these criteria are complied with. This requires the correct use and positioningof all equipment, appropriate site controls and monitoring throughout the process to record and checkthat the design standards are complied with.4.2.3.11 After Laying Tests

Before the cable circuit is connected to the power transmission system it is normal to carry out a seriesof tests to confirm the integrity of the cable system including the cable and accessories.The tests vary across the industry but generally include HV tests on the cable outer protection(outersheath) and on the primary insulation. Further tests are carried out such as conductor resistance,bonding and earthing tests.Historically the HV tests have been DC tests and whilst DC testing is still used for SCFF cables andfor the outersheath tests it has been recognised that DC testing is not suitable for extruded cables beingboth unable to consistently detect defects in the system and potentially causing damage to the cableunder test.With the availability of mobile on-site AC test sets, of the resonant frequency type, there is a generalmove to testing extruded cable on site by AC only.Further developments in partial discharge testing have shown that the combination of a PD test with ahigh voltage AC test is the most reliable means of confirming the integrity of extruded cables prior toconnection to the power transmission system.

4.2.4 Adaptation of the Cable System Design to the Technique/Environment

4.2.4.1 Adaptation of the Cable System Design to the Technique

• DuctsFactors that need to be considered for the cable system design and cable design :

1. Flexible or Rigid System

With the cable installed in ducts the system design depends upon whether the ducts are filled orunfilled.If the ducts are filled, usually with bentonite, the cable is effectively restrained and the systemdesign is considered as a rigid system.If the ducts are unfilled then the cable can move to an extent, dependant upon the relativeproportions of the cable and the duct. The system is therefore generally described as a flexiblesystem however where the movement of the cable is limited by the size of the duct then it isimportant to be aware that the cable will develop thrust due to thermomechanical stress.

2. Pulling tension

Since the only practical method of installing cables through a fully ducted system is by “nose”pulling it is essential that the necessary design studies are completed to calculate the pullingtensions that will be required to install the cable and to check that the cable limits are notexceeded. If necessary the route and system will have to be modified to ensure the pullingtensions are within the cable design limits.


3. Thrust in Manholes

It must be recognised that cables installed in ducted systems will develop thrust. In manholesthe system design needs to take this into consideration to avoid problems with the accessories.

4. Cross bonding

Transposing of cables in a ducted system is more difficult to execute than for other layingtechniques and the system design needs to provide for this.

5. Conductor cross sectional area

Due to the poorer thermal performance of the unfilled duct the cable rating will be lower andtherefore it may be necessary to increase the cross sectional area of the cable conductor tocarry the required current.

6. Metallic Sheath

With the cable unrestrained the sheath fatigue performance over the life of the cable needs tobe carefully considered and the cable system design and cable design need to be reviewed toensure the integrity of the metallic sheath.

7. Cable Oversheath

It is essential that the installation method avoids damage to the cable oversheath and as anadded precaution it is normal for a more robust material to be used such as MDPE rather thanPVC. Dependant upon the cable route and type of ducts it may be necessary to increase thethickness of the cable oversheath to provide greater protection during the installation phase.

• Direct BurialDirect burial is the most commonly used cable laying technique and since the cable is restrainedthroughout the route the system is always a rigid system.

Factors that need to be considered for the cable system design and cable design: -1. Route details

Careful planning of the route is required to ensure that the rating and long-term performance ofthe cable circuit can be assured. Details of any obstacles along the route need to be provided toallow the system to be designed to avoid these.The route details will allow an assessment to be made of the positioning of joint bays andlocation of installation equipment, drums etc to allow the optimum solution to be engineered.

2. EnvironmentKnowledge of the environment through which the route is passing is essential. The thermalresistivity and make up of the indigenous soil should be understood to allow the cable crosssection, cable spacing, depth of laying, backfill requirements and bonding arrangements to bedefined to achieve the required rating.

3. Cable OversheathThe cable oversheath acts as a corrosion barrier for the cable metallic sheath. Depending uponthe location and environment additional precautions may be necessary to provide an oversheaththat is resistant to local ground contaminants or lifeforms such as termites and rodents thatcould damage the normal oversheath materials.


4. Duration of the worksTrench have to stay open between two joints up to the cable pulling. This can lead localauthorities to ban this technique in urban areas to avoid having trenches open too long.

• TunnelsBy their nature, tunnels allow the cable system and cable design to be optimised and enables thedesigner to adopt the most cost effective form of design for the support systems and the use of longcable lengths to minimise the number of joints within the system.

In all cases it is essential that the long term performance of the system is not compromised and therisks associated with each stage of the process must be fully assessed and understood.

Factors that need to be taken into consideration for the system and cable designs :

1. Flexible or rigidTunnel installations are normally installed as flexible systems although in some circumstancesthe systems may be installed in troughing or cement bound sand surround making a rigidsystem.

2. Support System3. Cable Lengths4. Sheath Voltages5. Bonding6. Metallic Sheath7. Oversheath

It is common for cables installed within tunnels to be required to have enhanced fireperformance capability. This can be provided by low flame type materials or by the addition ofaddition flame retardant coatings applied after installation.

8. Fire Performance of the systemThe possibility of fire within a tunnel is extremely serious and the system design andcomponents of the system need to be assessed to minimise risk to personnel and assets in theevent of a breakdown within the cable system.This is a significant issue for SCFF cables installed within a tunnel and often such cables aresurrounded by cement bound sand to reduce the risk of damage which could lead to cablefailure.

• Troughs

This installation technique is fundamentally identical to that of direct burial and as such the system andcable designs are the same as for the direct burial technique.

• Bridges


1. Flexible or RigidCables installed in or on bridges may be installed as flexible or rigid systems in general thesystem design is dependent upon the particular requirements of the route.

2. Transition DesignCareful consideration has to be given to the design and installation of facilities within the routeto cater for the movement of the bridge structure due to thermal expansion or other possiblemovement. There is generally a need to design a transition area between the fixed portion of


the route and the route on the bridge to allow for the inevitable movement between the twosystems.

3. Oversheath4. Fire Performance of the system

• Shafts

Installation of cables in shafts introduces the problem of potential differential movement between thecable core and metallic sheath. The severity of the problem being directly related to the depth of theshaft.

Cable can be installed in either flexible or rigid systems although generally the cables are rigidly fixed atthe top and bottom of the shaft.

In either case particular attention needs to be paid to ensuring the clamping system is designed toprevent any slippage of the core within the cable sheath.

The system and cable design is more complicated for the fluid filled cable since the hydraulic systemdesign needs to be taken into account. Again dependent upon the depth of the shaft it may benecessary to include special reinforcing of the cable metallic sheath to withstand the hydraulicpressures or to introduce stop joints within the shaft to ensure the maximum acceptable hydraulicpressures are not exceeded.


1. Flexible or Rigid2. Cleating3. Metallic Sheath4. Oversheath

• Horizontal Drilling

For this technique the cables are usually installed within ducts that are installed during the drillingprocess.

In this instance the system design and cable design adaptations are as described for the ductedinstallation technique (see above).

• Pipe Jacking

The installation technique adopted is dependent upon the diameter of the pipe jack and can be any of anumber of alternative techniques.

For example the cable may be installed in ducts pulled trough the pipe jack after completion of the pipejacking operation, alternatively the cables may be installed in air on steelwork installed after the pipejacking process is complete.

The system design chosen may therefore be flexible or rigid with the decision being based upon theinstallation technique selected for the particular application.

• Microtunnels


Microtunnels can be treated in the same manner as pipejacks except that generally the diameter doesnot allow personnel access into the microtunnel and therefore the installation of support steelwork is notpossible.

The installation technique adopted is therefore generally that of ducts pulled into the microtunnel aftercompletion of the tunnelling process

• Mechanical Laying

For this technique the cable is installed in a direct buried environment and therefore the system is arigid system.

This technique is best suited to light cables which allow longer lengths to be installed.

Whilst it is possible to surround the cable with cement bound sand the process is not as controllable asother techniques, such as direct burial and therefore the cable design needs to take this into account.This may effect the sizing of the conductor due to a degree of uncertainty regarding the thermalresistivity of the backfill material.

In addition the metallic sheath and cable oversheath design may be adapted to provide a light cabledesign which will allow longer cable lengths to be transported and installed but which will be robustenough to withstand the rigours of this installation technique without effecting the performance of thesystem in the long term.

Factors that need to be taken into consideration for system and cable design :

1. lightweight construction2. oversheath3. rating4. bonding/sheath voltages.

• Embedding

For this technique the cables are usually installed in ducts which are embedded into the ground duringthe embedding process.

Generally therefore the system design and cable design are as for the ducted technique.

• Use of Existing Structures


1. Flexible or Rigid2. Thrust at transitions and joint positions3. Duct Sealing4. Cleaning and Assessment of asset5. Pulling Tensions6. Overall dimensions7. Lengths8. Oversheath


4.2.4.2 Adaptation of the Cable System Design to the Environment

• Drying of Soil

For highly loaded cables drying of the surrounding soil is a strong possibility and the cable ratingcalculations need to take this into account with the conductor cross sectional area being selected on thisbasis. The difference between a fully dry backfill and the same material with even a very smallmoisture content is dramatic. For example research has shown that a 2% moisture content reduces thethermal resistivity of normal backfill such as sand or cement bound sand by 50%.

Failure to recognise this possibility will result in the cable exceeding its design temperature limits due tothe dramatic increase in the thermal resistivity of the surrounding material. This will lead to eventualfailure of the cable.


1. Depth of laying and separation of cables2. Special backfill requirements

• Water Drainage

Water drainage may have a number of effects upon the cable system. It is possible that over timewater draining into the cable route could wash away the cable system backfill material compromisingthe cable rating.

Inadequate drainage could lead to the cable and accessories being immersed continuously in water andspecial precautions need to be taken to ensure that adequate sealing is provided to prevent moistureingress into the cable and accessories such as joints and link boxes.

Factors that need to be taken into consideration for system and cable design :1. Special backfill requirements2. Sealing for joints and other accessories

• Temperature of the Soil/Environment

The temperature of the medium surrounding the cable circuit is a fundamental factor in determining therating of the cable system.

It is essential therefore that the temperature profile is known throughout the route.

Under extreme conditions of either high or more normally low temperature installation of the cable maynot be possible and work may have to be planned at a time when the ambient environmenttemperatures are within acceptable limits.

Factors that need to be taken into consideration for the cable system design:

1. Cable cross section and spacings2. Depth of laying3. Special backfill4. Oversheath material


• Hardness of the Soil

The hardness of the soil in the main effects the construction of the route and may influence the routeplan and selection of installation technique.

• Stability of the Soil

Where unstable soil conditions are expected then the installation design needs to allow for this. This cantake the form of the use of civil construction techniques to stabilise the soil in the vicinity of the cablecircuit.

Where settlement of the route is expected to take place then the installation system design can makeprovision for this along the route. This is especially important at points of known discontinuity wherethere is the potential for shear stress to be imposed on the cable system.

• Thermal Resistivity of the Soil

The thermal resistivity of the soil surrounding the cable circuit has a direct effect upon the rating of thecable circuit.

It is therefore important that this information is available to allow the system design to proceed.

Where the surrounding soil is found to have a high thermal resistivity then it may be necessary toexcavate beyond the normal area required to install the cable and replace with material with a moresuitable thermal resistivity.


1. Cable cross section2. Depth of laying3. Cable separation4. Special backfill

• Seismicity

Where seismic activity is expected, it is possible to accommodate possible ground movement byadopting the same techniques as would be used for unstable ground conditions.

• Frost

In general high voltage cables are installed at depths which are not normally effected by frost. Duringoperation frost will have little effect on the cable circuit although frosts occurring during period of de-energisation could lead to cracking and disturbance of the backfill surrounding the cable. This couldlead to voids being generated which would effect the thermal performance of the cable surround in adirect buried situation.

Frost during installation may mean that the ambient temperature is below the minimum installationtemperature. In which case installation of the cable will have to be delayed until a temperature increaseoccurs.



1. Depth of laying2. Backfill3. Oversheath material

• Archaeology

Evidence of archaeological remains along a planned cable route would not influence the cable designbut would influence the technique used for constructing the route.

This could in turn effect the cable design in line with comments previously made depending upon theinstallation technique selected.

• Presence of Termites

Termites will attack the outer sheath covering and compromise the corrosion protection system for themetallic sheath.

The outer sheath material needs to be impervious to termite attack or provided with suitable chemicaldeterrents if possible. Otherwise an alternative installation techniques such as ducted techniques will berequired.

• Laying in National Park

The cable system design is influenced by the installation technique used to overcome any restrictionsplaced as a prerequisite to approval of a cable route through the National Park.

The factors that need to be taken into consideration are dependent upon the technique selected asindicated before.

• Duration of the Work

The duration of work has no influence upon the cable system design but may influence the choice ofinstallation technique.

SCFF cables require longer to install since the hydraulic procedures are an additional complexity whencompared with extruded cables. However selection of the type of cable to be used is not normallyinfluenced by this factor.

The factors that need to be taken into consideration are dependent upon the technique selected asindicated before.

• Maintenance and Repairing Process

Cable systems are designed for 40 years operation and are generally very reliable. Maintenanceprocedures for extruded cable systems are generally limited to inspection of the cable and associatedequipment and periodic checking of the integrity of the cable oversheath and bonding systems. Theintroduction of partial discharge monitoring techniques should allow the condition of the cable system tobe assessed over time.


Maintenance for SCFF cable systems is more complex due to the periodic checks required on thehydraulic system and components. Routine sampling of the dielectric fluid and analysis of the dissolvedgases within the fluid allow a degree of condition assessment to be undertaken and comparisons madeover time.

Considerations need to be made at the design stage as to how the system will be maintained and toensure access and provisions are made such that the maintenance regime can be carried out.

Although cable systems are very reliable the need for repair cannot be discounted and again this needsto be considered at the design stage. Whilst not directly effecting the cable design or installation systemdesign the need to cater for a future repair will influence the choice of installation technique with theresulting effect on the cable system design as mentioned earlier.

• Cable Removal after Operation.The factors associated with removal are very similar to the issue of cable repair noted above.


5. EXTERNAL ASPECTS

5.1 Location (Urban vs. Rural)The high-voltage cable installation methods are obviously adapted depending on the location of thelaying site so as to take into account the local and environmental limitations and constraints.Therefore, not surprisingly, in urban areas the installation in ducts is the most frequent method,followed, in this order by :• conventional installation in open trench;• visually unobtrusive methods (tunnels, microtunnels and, to a lesser extent, pipe jacking and

horizontal drilling);• utilisation of existing structures, e.g. bridges.

In rural areas, however, the constraints regarding time-limits on disruption due to execution of theworks, and with respect to available space, are much less important than in urban areas.Therefore, in rural areas the more traditional laying methods are most frequently applied, such as ductsand direct burial (of which costs are lower than those used in urban areas).At special crossings, placing on bridges and directional drilling methods may be applied, though speciallaying methods are not the general philosophy for cable laying in rural areas.As regards laying depth there is not much difference between cable laying in urban or in rural areas,since a minimum depth is usually set by regulations, as we will see later (chapter 5.5.).

5.2 Right of wayThe rights of way are usually settled by joint agreement between the utility and a private owner or theutility and one or several public authorities (Roads, Railways, Bridges, …).When using a public authority property, the construction techniques used are generally agreed with thepartners, before beginning the works. Then, it has to be decided under whose responsibility the workshave to be done, the utility or the public authority.

5.3 Magnetic fieldsReference: "Magnetic field in HV cable systems: systems without ferromagnetic component" – ElectraCIGRE – Technical Brochure n°104 - Joint Task Force 36.01/21 - June 1996.Although the delicate question of magnetic fields is usually discussed regarding overhead power lines,increasingly attention is being paid to magnetic fields when selecting the configuration of the cables andthe routes of buried links.Indeed, in more and more countries now exist recommendations, limits and possibly even standards asto the level of magnetic fields. These concerns may eventually dictate changes in planned routes, but,above all, they may increase the burial depth or implicate some precaution disposition.We shall bear in mind that buried cables (contrary to overhead lines) do not generate electric fieldsoutside their metallic screen. As such sheaths are earthed, an electric field only exists between theconductor and the sheath.Several three phase single core cable configurations can be considered.A lot of factors have a influence on the magnetic field, e.g. phase spacing, burial depth, load currentamplitude, phase arrangement in systems of several three-phase circuits, distance between them andinduced currents in the sheaths (which are strongly affected by a lot of factors).

5.3.1 Flat arrangementA system of three single core cables in flat formation is first considered. It is characterised bygeometrical parameters which are phase spacing (s) and burial depth (d). Height considered forcalculations above ground (h) is also defined.


Figure 29 : Flat arrangement, 1 circuit

The currents in cables are assumed to be balanced, i.e. IA = I < 0°, IB = I < -120 °,IC = I < -240 °, and the frequency is 50 Hz. The current will be fixed to a reference value of I =1000A.The two diagrams below represent the magnetic flux density along a horizontal line at 1 metre abovethe ground surface, considering various burial-depths and spacings of the phases.

Figure 30 : Brms profiles with various s


Figure 31 : Brms profiles with various d

The highest magnetic field value immediately occurs above the cables. The distance from the cables (h+ d) as well as the phase spacing appear to have an important influence on the flux density whosevalues are higher for low burial depths and high phase spacings. Moreover, it particularly appears that,for a fixed phase spacing, burial depth has no effect on the flux density at horizontal distances from thesystem centre line that are greater than several times this depth. Further, the reduction of magneticfield away from the centre line is higher for a low burial depth.Two systems of three single core cables in flat formation can also be considered, assuming the samecurrent of 1000 A in both circuits. Their geometrical parameters are phase spacing (s), burial depth (h)and distance between systems (g). Height above ground (h) is also defined.

Figure 32 : Flat arrangement, 2 circuits

The hypotheses are identical to those referred to above. Furthermore, two configurations have beenretained : ABC-ABC (same order of phases) and ABC-CBA (inverted order of phases).The figure below illustrates the evolution of the magnetic flux field considering various heights abovethe ground, at the set parameters g, d and s.


Figure 33 : Brms profiles for two cable system configurationswith various h

The ABC-ABC configuration appears to give a lower magnetic flux density near the cables than theABC-CBA configuration. However this last configuration gives the lowest magnetic flux density froma certain distance from the cables (breakpoint for h = 1 m). Such a breakpoint distance depends on gand s.The next figure shows profiles of magnetic flux density along a horizontal line one meter above groundfor both configurations and for several system spacings.


Figure 34 : Brms profiles for two cable system configurationswith various g

It can be seen that increasing system spacing respectively decreases or increases the magnetic fieldfor ABC-ABC and ABC-CBA configurations.We must also mention that the magnetic field is often higher at locations where junctions are made, i.e.where connections are made between power cable screens and the ground wires (if any), especially ifthese connections are made in an aboveground junction box (for paralleling of the screens).Judicious connection of the screens and ground wires (connection between portions of the buried linkmade underground instead of in an aboveground junction box) or connection made in a buried junctionbox can significantly reduce the value of the magnetic field.

5.3.2 Trefoil arrangementA system of three single core cables in trefoil formation is now considered. Its geometrical parametersare phase spacing (s) and burial depth (d). Height above ground (h) is also defined.

Figure 35 : Trefoil arrangement, 1 circuit

In fact, usually, the three cables touch each other and variations of phase spacing allow to considercables of several outer diameters.The first of the two next figures compares the magnetic flux density profiles along a horizontal line onemeter above ground with a burial depth of one meter for both flat and trefoil formations with severalphase spacings. The second one compares the magnetic flux density profiles along a horizontal line onemeter above ground for several burial depths and a fixed phase spacing for both formations.


Figure 36 : Brms profiles for both flat and trefoil formationswith various sflat and strefoil

Figure 37 : Brms profiles with various dfor both flat and trefoil formations


The trefoil formation gives clearly the lowest magnetic field which is more than 30 % lower than theone of the flat formation whatever phase spacing is.

Two systems of three single core cables in trefoil formation are also considered.

Figure 38 : Trefoil arrangement, 2 circuits

The hypothesis are the same as those made in paragraph 5.3.1.

The next figure shows again the profiles of the magnetic fields for the configurations ABC-ABC andABC-CBA considering various heights above the cables, at the set parameters g, d and s.

Figure 39 : Brms profiles for two cable system configurationswith various h

The ABC-CBA configuration appears to give a lower magnetic flux density then the ABC-ABCconfiguration.

The next figure shows again the profiles of the magnetic fields along a horizontal line 1 metre aboveground level, for the two configurations and with various spacings.


Figure 40 : Brms profiles for two cable system configurationswith various g

Unlike in the horizontal arrangement, here we can see that the increase of spacing between the twosystems results in a lower magnetic field in the two configurations ABC-ABC and ABC-CBA.

5.3.3 Vertical arrangementA system of three single core cables in vertical formation can also be considered. Its geometricalparameters are phase spacing (s) and burial depth (d). Height above ground is also defined (h) :

Line Xhd

Sv

Sv

Figure 41 : Vertical arrangement, 1 circuit

This configuration is in fact an artifice considered for the purpose of enabling to compare the magneticfield values of this configuration with those of the two configurations discussed above (trefoil, flat). Inreality a vertical configuration is only adopted in tunnels, bridges or other structures, practically neverfor buried links.The next figure shows the magnetic flux density profiles along a horizontal line 1 metre above ground,with a burial depth of one metre for flat, trefoil and vertical configurations and several phase spacings.


Figure 42 : Brms profiles for flat, trefoil and vertical formations with various sflat, s trefoil andsvertical

The values of magnetic field for the vertical configuration are lower but close to the flat configuration.They rapidly match when departing from the vertical axis.It results that the trefoil formation clearly remains the most advantageous option with respect tomagnetic field.Two systems of three single-core cables in vertical formation are also considered.

Line X

dh

g

Sv

Sv

Figure 43 : Vertical arrangement, 2 circuits

The hypotheses are identical to those made in paragraph 5.3.1.The last figure below shows the magnetic field profiles for the ABC-ABC and ABC-CBAconfigurations for vertical, trefoil and flat formations.

h=1m; d=1ms=Phase spacing (flat)st=Phase spacing (trefoil)sv=Phase spacing (vertical)

0

5

10

15

20

25

30

-10 -5 0 5 10

Distance from center line (x) [m]

Mag

net

ic f

lux

den

sity

(B

rms)

[10

e-6T

/kA

] st=0.08m

st=0.12m

s=0.12m

s=0.3m

sv=0.12m

sv=0.3m


Figure 44 : Brms profiles for two cable system configurations with fixed h, d,g and s = s t = sv = 0.3m

It appears that in the ABC-ABC assumption the magnetic field of the vertical formation is higher in themiddle of the 2 systems that it is with the flat formation.Conversely, for the ABC-CBA configuration, the magnetic field is clearly less in vertical formationthan in flat and trefoil formations. At higher distances of the centre line, it appears that the decreasingof the values of magnetic field for the trefoil configuration is slower.

5.3.4 Comparison between overhead lines and buried linksReference: "Magnetic field in HV cable systems: systems with ferromagnetic component" – ElectraCIGRE – Technical Brochure n°104 - Joint Task Force 36.01/21 - June 1996.In addition, it is important to remind that an electric field is present around overhead lines, whereas incables, the electric field is completely confined inside the electric screen.The assumptions regarding the types of cable are the same as in the previous paragraphs, and we haveconsidered three configurations : trefoil, flat and vertical.As regards the overhead line we considered a line composed of one circuit and an earth wire. Themagnetic field calculations were performed for a distance of 10 m between ground level and theconductor and with a base current of I = 1000A.

In order to establish a parallel between the buried links and the overhead lines we imagined the threeoverhead line conductors to be in vertical and flat formations.

h=1m; d=0.5m;g=0.7ms=Phase spacing (flat)st=Phase spacing (trefoil)sv=Phase spacing (vertical)

010203040506070

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from center line (x) [m]

Mag

net

ic f

lux

den

sity

(b

rms)

[10

e-6T

/kA

]

ABC ABC ; st=0.3m

ABC CBA ; st=0.3m

ABC ABC ; s=0.3m

ABC CBA ; s=0.3m

ABC ABC ; sv=0.3m

ABC CBA ; sv=0.3m


Figure 39 : Brms profiles for flat, trefoil and vertical formations (buried links)

Brms profiles for flat and vertical formations (overhead lines)

The graph clearly indicates that the magnetic field of overhead line and buried link are of the sameorder on the axis, the difference being that the magnetic field of the overhead line diminishes muchmore slowly than that of the buried link. With the buried link the magnetic field becomes very low atonly a few metres apart from the link axis.

It also appears that for a given formation the magnetic field values of a buried link may be higher in aburied link compared to an overhead line.

However, the information supplied by the graph cannot be considered totally reliable, because magneticfields are known to be sensitive to many parameters (order of the phases, number of circuits, current inthe screens or groundwire(s), configuration of the cables, …) which may significantly affect the values.For example, if the value of h decreases, the value of the magnetic field will be increase forunderground cables and decrease for overhead lines.

5.3.5 ConclusionAs a conclusion it must be said that, whatever the cables formation, the magnetic fields induced byburied links are lower than those specified in the national or international recommendations generallyaccepted.Also, there are now several ways in which the magnetic fields of buried links can be further reduced,for instance by placing steel or aluminium sheets around the cables in the trench, or by placing thecables in steel pipes.In this respect, we can mention the work done by the Joint Task Force 36-01/21 "Magnetic fieldcalculation in underground cable systems with ferromagnetic components" (Electra n° 174,October 1994).

5.4 Existing servicesThe proximity of buried power lines to other services in ducts, sewers, cables and other utilities'networks is these days practically unavoidable, particularly in urban areas.

h=1m; d=1mh overhead line=10ms=Phase spacing (flat)st=Phase spacing (trefoil)sv=Phase spacing (vertical)

0

5

10

15

20

25

30

-10 -5 0 5 10Distance from center line (x) [m]

Mag

netic

flux

den

sity

(B

rms)

[10e

-6T/

kA]

st=0.12m

sv=0.12m

s=0.12m

s=0.3m

line v-config

line h-config


Generally, power cables are laid as much as possible at sufficient distance from other services in orderto prevent the damage of existing installations during the laying of HV cables. In urban areas thisbecomes increasingly difficult.The effects of a short-circuit of a phase on its environment (gas, telecommunications, …) arediscussed in paragraph 5.6.

• GasThe clearances to be observed between gas pipes and HV cables, and to a lesser extent the layingtechniques, are generally imposed by the gas utility. The gas utility’s stipulations naturally differdepending on the type of gas transmitted, the pipe diameter and the gas pressure in the pipe.Using ducts in which later the cables should be laid, is strongly advised against in the vicinity of gaspipes, because the former ducts may be a source of accumulation of gas in the event of a gas pipe leak(hence a risk of explosion).Problem of parallelism of a gas pipe to HV cables : see l Telecommunications

• Electrical cablesThe proximity of various electrical links can have both an electric impact (in case of defects) and athermal impact. They contribute to soil heating and in this way they reduce the carrying capacity of theelectrical links.A deeper investigation of these situations is highly recommended prior to laying the cables.

• District heatingLike in the above case, the thermal impact of the steam pipes on the carrying capacity of the buriedpower line must be investigated.

• TelecommunicationsTelecommunication cables (like gas pipes) are a typical and frequent example of a system affected bythe HV cables that run parallel to them.Indeed, any electric current transiting in a conductor generates a magnetic field around that conductor.If the current is the alternating type this magnetic field will in turn induce a potential rise between theextremities of an open circuit surrounding the conductor, or the circulation of an induced current in aclosed circuit surrounding the conductor.However, we shall bear in mind that the most critical situation arises in the event of a fault. If theconditions are acceptable during a fault situation, they are obviously acceptable during a normalsituation.When installing a buried HV link, this impact can be first reduced by the presence of the cables’ metalscreens which are earthed at either ends (circulation of a screen current) and also by placing groundwires connected to the earthing network of the line, so facilitating the returning of a fault current to thesource, this reducing the magnetic field perceived by the world outside the cables.World-wide, protection of telecommunication infrastructure is one of the main concerns of electricalutilities.

• WaterWater pipes do not present a particular risk, except when leaks are sprung.However, even if the protective screen of the HV cables is damaged, most modern cables havesufficient radial and longitudinal leak-tightness to protect them (but the cable has even so to be repairedif damaged).In turn, the erosion of the backfill following an accidental leak of water presents a certain risk(particularly if it is a special backfill that is being washed away by the leak).

• SewersSewers do not present a specific risk except possibly in relation to mechanical damage during work ator around sewers on cables that have been laid too close to the sewers.


It is worthwhile to mention also that especially main (large diameter) sewers may release quite someheat, which may negatively affect the carrying capacity of the cables.

• TreesTrees and cables may have an impact on each other.If cables have to be laid very close to threes, excavation must be extremely careful (at extra cost andtime) so as not to damage the root systems of the trees, and, laterwhen the buried power link is in service, the drying of the soil may affect the growth of the trees (andpossibly eventually kill the trees);the drying of the soil may also negatively affect the carrying capacity of the cables.Also, the root systems of the tree may get entwined around the cables, making difficult laterintervention on the line.Accordingly it is recommended (or imposed by local authorities) that sufficient distance be adopted (atleast 2.5 m) between the link and the trees, or that the cables be placed in ducts.

• RailwaysThe phenomenon most feared in the vicinity of railway or tramway substations is that of corrosion ofthe metal screens around the buried power line (which corrosion may be incurred even at considerabledistance from these substations).Corrosion develops when a direct current strays from the screen and flows through the soil to a direct-current source.In order to protect a metal screen like a gas pipe against this type of corrosion, the utilities installcathodic protection stations to bring the ducts to a sufficiently negative potential compared to the soil,so that no current can escape from it. The direct connection of a steel structure to a cathodicprotection station protects this structure against any electrochemical corrosion.Although in the past the metal screens of HV lines have been connected to such cathodic protectionstations but the method has the disadvantage that the electrical utilities can depend on other companies’installations.Moreover, a very simple passive protection method of the metal screens exists, which consists of makeuse of the normally existing plastic outer sheath (medium or high-density polyethylene). This sheaththanks to its high electric resistivity impedes the leakage of any currents to the soil.However, it will be able to provide this protection only as long as the sheath is not damaged. Indeed if adefect appears, the density of DC current through the outer sheath defect could reach very high valuesand causes very rapidly important damage to the metallic sheath. This implies regular inspection (atleast annually) of the dielectric strength of this sheath.

5.5 Legal aspectsAmong the great choice of laying techniques, there are none that are systematically forbidden by thelocal authorities.The two techniques that cause the most controversy are the laying in trenches, which naturally causesquite some local disturbance, and the placing of cables on bridges (which may be historic orarchitectural monuments, bridges that require constant maintenance work, …).It can be observed that national authorities or the utilities hardly ever forbid one or the other method,unless there are very particular reasons.It must be borne in mind that in certain countries the law is such that the owner of the land also ownsthe subsoil under that land. During link construction requiring for instance tunnel jacking or horizontaldrilling, it is in those cases absolutely necessary to secure the authorisation from the person that ownsthe land under which the link construction will take place.An alternative option may be to build the link under the public roadway, in which case onlyauthorisation by the public authorities is necessary (but this may entail a longer route for the link).


Also, as said before, magnetic field calculations more and more dictate, through guidelines, or eventhrough standards, the choice of the route and, above all, the depth at which the cables will have to beburied.The usually regulatory-imposed minimum depth is 1 metre. In turn, there are hardly ever any legalstipulations regarding the width of the trench.Finally, it should also be borne in mind that, world-wide, there are more and more legal restrictionsregarding the duration of opening of the trench, this resulting in the necessity to adopt shorter lengths,and restrictions concerning the periods of the year or day during which civil works and cable-layingwork can be done.

5.6 Safety aspectsSafety aspects must be taken into account as soon as at the early stages of link design. A risk analysisbecomes increasingly necessary that clearly identifies the potential risks.A first step in this analysis consists of collecting all information about the various utility networks in thevicinity of the link, and evaluating them with respect to possible risks.Safety implies that precautions be taken in order to protect :• the HV link;• the other links or networks in the vicinity;• the workers working near the HV links or other networks.• the publicThe safety aspects relating to terminations are considered by Cigre Working Group 21-19.

5.6.1 Protection of the link from external damageApart from being at a depth that gives them some protection, the HV cables are usually protected allalong their route by a cover of durable and mechanically resistant materials that protect them againstdamage from excavation tools.This cover extends on either sides above the cables and may consist of concrete or polyethylene orother slabs.Furthermore, the cables are often signalled by non-corrodable markers placed above the line. This mayconsist of coloured plastic strip showing the voltage level and the name of the utility, sufficientlyresistant not to be ripped by a mechanical excavator.It is clear that in particular cases additional safety devices may be installed like, for instance:• placing of steel plates above the protection slabs when the minimum regulatory depth is impossible

to comply with;• placing of warning panels above ground level on a bridge or on the river banks (embedding, pipe

jacking, …);• simply placing the cables in strong ducts or troughs.

Lateral protections are rarely used because the depth of the cables is considered to be (nearly) enoughlarge to avoid aggressions due to works carried by the most other Utilities.Joint pits when in plain soil are protected similarly to the cables, but their protection extends at leastover the entire surface of the joints.When joints are made for instance in prefabricated pits or in tunnels, these joints are naturally protectedby these structures that house them. Only the access to these structures needs to be designed againstintrusion by unauthorised persons.Also, in certain cases, particularly safety measures may be required (such as lining) in order to protecta link situated in a tunnel from other cables or pipes running in that tunnel.

It is clear that placing slabs or markers is not possible when using trenchless laying techniques.Moreover, it is not easy to give the exact route in the x, y, z axis. Nevertheless, it has to be noticed that


with these techniques, cables are usually laid at great depth and so are protected from usual externaldamage.

5.6.2 Protection of the environment from a system faultIn the event of a short-circuit in a cable the best protections against an explosion remain the adequatedepth at which the cables are buried and the presence of protection slabs.Moreover, in the case of a phase-to-earth fault (e.g. in the event the cable is punctured by amechanical excavator), the surrounding soil and the installations of third parties situated in the closevicinity of the HV line will incur a rise in potential.Most often the installations of third parties are grounded or connected to a cathodic protection system.Accordingly it is the external protection cover of the installation, i.e. the layer that insulates the metalpart (screen or pipe) from the soil that will have to sustain the fault voltage.Generally these protections are not particularly designed to have high dielectric strength, as they areprimarily designed for protection against electrochemical corrosion by the soil. Accordingly the possiblelocal rise in potential has to be reduced to an acceptable value.The only component on which it is possible to act in this respect is the linear resistance of the screen,as it can be demonstrated that the maximum value of the fault voltage is as follows :

Vf = 4

LR If

where R = linear resistance of the screenL = length of the linkIf = fault current

This being, the value of R can be reduced by two methods, which may both be applied simultaneously :• paralleling of the HV cable screens between each other,• paralleling of all the HV cable screens with the ground wire.

Such paralleling may be done in aboveground or in buried joint boxes.

5.6.3 Protection of the workersWorkers are particularly exposed to mechanical hazards (deep trenches, …) and electrical hazards(voltage step, …).During work in plain soil, the depth of the trench (for instance > 1.5 m) can be such that it needsstabilising (lining). Furthermore, the workers must wear their individual protective equipment (hard hats,gloves, …).For other laying techniques such as in tunnel, bridge or shaft, various philosophies may be envisaged:• bar access and install fixed or mobile CCTV cameras inside, in order to verify the proper condition

of the cables. Once the link installed, human interventions will be few and far between (only in caseof repair or other).

• authorise access, either after putting the HV link(s) out of service or if the links are left live,restricting the access to authorised persons only.In this case, particular systems have to be installed in the tunnel (ventilation, emergency hatches orexits, ..).

In the event where there are other users of the tunnel, bridge, shaft who installed cables or pipelines init, other collective protection equipment may be necessary depending on the type of cables andproducts involved. For example, steel sheets may be placed around the cables in order to protect themfrom external aggression (piercing by screwdrivers, …) or the cables may be placed inside ducts orconcrete troughs in order to limit the impact of a defect on personnel or the installations of otherutilities.Also, the case of prefabricated joint pits deep in the ground is similar to that in tunnels, except that thespace is less. Therefore, particular measures for venting and extraction of possible toxic gases may benecessary, as well as provision of an extra exit hatch as a backup.


The electrical risks must be considered also.We can mention the case of laying of pipe or an other underground power link parallel to buried powerlinks without any particular protective measures. When the cables of a portion of link are placed oninsulating supports prior to backfilling the trench, there may appear a considerable voltage on the pipewhich is dangerous to individuals working on the pipe or the new link. Local grounding during the workon the pipe is effective for protecting the workers from that type of impact.It should be also noted the accessible parts of pipes of third parties (valve control stations, cathodicprotection stations, measurement stations, …) situated near HV links must be earthed in order to limitthe induced voltages and so protect their personnel.Interventions in joints boxes must also be carefully considered on account of the considerable currentsthat may flow in these.

5.6.4 Protection of the publicAs already mentioned in paragraphs 5.6.1. and 5.6.2., the cables are usually buried at such depth that adefect arising in them is not noticeable at the ground surface, except perhaps for a slight noise.At the joint boxes, an adequate and possibly strengthened grounding circuit enables to eliminate anyelectrical risk (step voltage, …) to the public.Although the joint boxes limit the mechanical impact of a defect on the outside, there remains the factthat a short-circuit may provoke an explosion generating such a pressure that no aboveground or buriedboxes can resist.It may therefore be highly advisable that joint boxes be placed in a concrete or steel containment.

5.6.5 Safety of the different laying techniquesAmong the twelve laying techniques described in this brochure, some are more safe than othersdepending on which aspect the designer looks at.Globally, techniques where cables and joints are laid at a sufficient depth in the soil and completelyprotected (i.e. cable in duct, joints in jointbays) are the best ones. As soon as cables or joints are inopen air, (i.e. cables laid in tunnel without protection, joints in manholes), the security of the public orthe workers is at a lower level.More details are given in the previous chapters.

5.7 EnvironmentInstallation of power cables entails environmental impacts which depend on which installation method isapplied.

For instance, the results of techniques such as horizontal drilling or tunnel thrust jacking are practically‘invisible’ to the outside world.

Conversely, when trenches are dug (direct burial, ducts, tunnels built with the open cut method, etc.)the natural environment may be substantially altered.

It is important not to underestimate the environmental impact during the construction itself, andtherefore it is wise to consider it already at the preliminary study stage, before it becomes the mainvisible point for the population.

The ultimate objective is to restore an environment identical to what it was before the installation of theHV link.

In order to analyse the environmental effects arising from the construction and operation of a powerlink, a distinction is made, following the usual practice in this kind of studies, between the physicalmedium (soil, water, air), the biological medium (flora, fauna), the social medium (population, economicsectors, …) and the landscape.


The potential alterations which may be attributed to the construction and operation of a power link,classified according to the element of the impacted, are as follows :• Soil- direct damage due to the excavations- deposition in the water (embedding) and/or land medium of the materials excavated from the

trench- movements of plant and machinery on the banks of a river, for example for embedding and

horizontal drilling- possible contamination of the soil (Fluid Filled cables) • Water- alteration of the water quality by materials or products during the construction work (embedding)- alteration of the water quality by pollutants (embedding, bentonite injected during horizontal

drilling)

• Air- release of pollutants to atmosphere during the construction work- noise generated by plant and machinery during the work- vibrations generated by plant and machinery during the work (tunnel, …)- generation of magnetic fields

• Flora- direct destruction of the vegetation cover- indirect destruction of the nearby plant communities- damage to unique or interesting species

• Fauna (embedding)- direct disturbances to benthic communities- indirect damage arising from changes to the water ecosystem

• Socio-economic aspects- difficulties caused to parking and access to shops etc. during the work (direct burial, …)- temporary effects on tourism trade during the construction work- rights of way affected during the construction work- effects on fisheries (embedding)

• Landscape- Alteration of the landscape during the construction phase.

It clearly appears that most of the disturbances can be prevented or mitigated at the outset, i.e. duringthe studies. The use of particular installation techniques can also harm to the environment bypropagation of polluted materials (for example, horizontal drilling going through a gas or oil pipe) but themajor impact arises most of the time during the construction phase and can be mitigated by using, forinstance, less disturbing plant and equipment (low noise, low vibration equipment or measures duringdigging of shafts) and by informing and consulting with the local authorities and population.

However, it should be borne in mind that using SCFF cables may under certain accidentalcircumstances (leaks) cause some impact to the environment by the fluid leaking into the subsoil.Finally, it is worth noting that more and more international or local guidelines stipulate that the cables beremoved at the end of their service life and that the materials be recycled.


6. DESIGN OF A LINK

When an engineer is at the beginning of a new project, the problem is always : how could it bemanaged in order to be the most effective on the technical and economical points of view.The following chart will help the inexperienced engineer in the management of his project. If you are inan organisation accustomed with the underground cable system project management, it is clear thatsome stages have to be jumped over.It can be seen that the exercise is very much an integrated process with the impact of the variousstages being considered and steps taken to modify previous and subsequent stages of the process toachieve an optimised end result.

6.1 Methodology


Cable selection among existing cables

Accessories selection among existing accessories

Preliminary design of cable cross-section

Design of earthing (grounding) method

Determination of number of cables per phase

STAGE 1 : Preliminary design of the cable system

Operation voltage Ampacity (normal operation, emergency)

Load curveCable aim temperature during operation

Short circuit level and durationImpulse levelsTouch voltage

Cost of kWh, cost of lossesEstimated length of the link

Type of soilSoil temperature

Maximum allowed temperatureat soil-cable contact

Soil resistivityFrost depth

Environmental hazards (earthquake, flood,...)

Need of a cooling system ?

No

Stage 2

New cable design

Yes

Figure 45 : Stage 1


Figure 46 : Stage 2

STAGE 2 : Preliminary cable route design

Identification of obstacles to cross :

Roads, railways,rivers,National parks, archeological sites,

Thermal proximities (steam,...)Electrical proximities

ServicesTrees

Division of the global studied areain sections

In each section

Identification of the possible civilwork techniques in each section

In the global studied area

Choice of optionsRoute section / Possible techniques

Stage 3

Loca

l and

nat

iona

l reg

ulat

ions Allowed civil work techniques

Allowed time of trench openingLocation

Right of way

Available civil work techniques inthe country

Soil stabilitySoil hardnessSoil resistivitySoil seismicity


Stage 3 : Checking of the cable design

Stage 1 Stage 2

Checking of the cable design

Is the ampacity stillgood ?

Stage 4

Modification of cable cross sectionModification of earthing method

Modification of cable architecture

Yes

No

Identification of the sizing point(thermally speaking)

Figure 47 : Stage 3


Stage 4 : Choice of options Route section / Civil work technique

Site "prejudice"Site duration

Site working time (24 h/24 h possible)Magnetic fieldLaying depth

Maintenance and repair processCable removal

Construction costSite prejudice cost

Maintenance and repair costOperation cost

Link non availibility costRepair cost

Choice of a technique for each sectionwith cost and technique aspects

Stage 5

Figure 48 : Stage 4


Stage 5 : Cable installation

Laying techniqueInstallation technique : rigid or flexible

Pulling methodPulling tension

Sidewall pressureDrum transportation

Choice of the cableChoice of the accessories

Choice of the length on each drum

Cable system performanceThermomechanical performance

Good

Commissioning

Final design of the cableFinal design of the civil worksFinal design of the installation

Writing of link specifications

Works

Final ampacity, according to works

Administrative authorizations

Stage 1

modification of the design

Figure 49 : Stage 5


6.2 Study cases

hill

river

A

bridgesnationalpark

floodedarea

B

R1C1

L1

L2

L3

L4

L5

C2

C3

C4

C5

C6

C7

R2

R3

R4

R5


Figure 50 : Possible routes

Let us assume that we have a new project to design : Join substation A to substation B.We will go through the flow chart to optimise the project.

Stage 1 : Preliminary design of the cable system

Electrical data collection :

Operation voltage : 225 kV,Ampacity : Normal : 800 A in winter and in summer,

Emergency : 1000 A during 6 hours in winter and in summer,Cable aim temperature during

normal operation : 90 °C, emergency operation : 100 °C,

Short circuit level : 50 kA, 0.3 second,Impulse level : 1050 kVc,Estimated length of the link : 6 km,Allowed sheath voltage : 400 V. This information is only useful to determine how the link will beearthed : solid, single point or cross bonding.

Civil work data collection :

Soil : mainly sandy clay,Soil resistivity : native soil : 1.2 K.m/W in winter, 1.6 K.m/W in summer,Available special backfill : 0.7 K.m/W in winter, 1.0 K.m/W in summerSoil temperature : 15 °C in winter, 25 °C in summer,Maximum allowed temperature at soil-cable contact : 55 °C,Maximum allowed air temperature in bridge or in tunnel : 20 °C in winter, 30 °C in summer,Frost depth : 0.8 m,Environmental hazards : flood along the river,

Cable selection :From the cable temperatures, we have to choose an extruded cable and premoulded joints.Preliminary design of cross-section cable : 1600 mm² Al, 1 cable per phase with special backfill or 1600mm² Cu, 1 cable per phase, with native soil.No need of a cooling systemEarthing method : cross bonding, coming from the allowed sheath voltage.

Stage 2 : Preliminary cable route design

Three main routes were found by the design team.They will be called Left (L), Central (C), Right (R) in all the study.

Identification of obstacles to cross :

Left :Private land,a river, 50 m wide and an area liable to flooding, 150 m wide,a national park,


Central :Private streets,Narrow streets with low traffic,electrical cables in a street, crossing of a 225 kV existing link laid at 1.8 m deep, cross bonded,1000 mm² Cu, which ampacity is 900 A in winter, 770 A in summer,a bridge,a sloping hill,Public rural road,

Right :Public streets, but with heavy traffic,numerous services in the urban streets but not in the rural road,a bridge,Public rural road,

Available civil works techniques in the country where the link has to be built : all,Trench opening : 300 m at the same time in the city (urban streets),Working hours : not allowed at night in the city (urban streets).

Civil work techniques selection :

From the collected data, some techniques are of non interest :Troughs and direct burial in urban areas, where the maximum allowed length to be opened is 300 m.The length between two joints is then too short to be cost-efficient.

Division of the route in sections :

The different routes can be divided in seven sections : urban streets, river, national park, hill, bridge,land, rural road.The following table identifies the possible civil work techniques in each section :

Street River National park Hill Bridge Land RoadDuct Y N Y Y Y Y YDirect burial N N Y Y N Y YTunnel Y Y Y Y N Y YTrough N N Y N Y N YBridge N Y N N Y N NShaft Y Y Y Y N Y YHorizontal drilling Y Y Y Y N Y YPipe jacking Y Y Y Y N Y YMicrotunnel Y N Y Y N Y YMechanical laying N N Y Y N Y YEmbedding N Y N N N N NUse of existing structures Y N Y Y Y Y Y

Stage 3 : Checking of the cable design

At this stage of the study, it is necessary to identify the sizing point. According to the collected data, thesizing point is the native soil with the bad resistivity. The cross section necessary to meet the ampacityrequirement is 1600 mm² Cu.


This cable being of current manufacturing, nothing has to be changed in the cable architecture or in thetype of soil to be used for the backfill.If it was decided to change the native soil by a special backfill, the sizing point will also change andbecome the two 225 kV links crossing.

Stage 4 : Choice of options route section / civil work technique

The laying technique to adopt in each section depends on the cost structure in the country where thelink has to be laid. Nevertheless, we propose an a priori choice without knowing in which country weare.

• Private streets on route C : Duct filled with air, laid in special backfill at 2 m deep (bottom of thetrench). It will be necessary to have a reinforced block to cross the 225 kV link, as the depth is lessthan the normally allowed one.

• Public streets on route L : Tunnel. It was not allowed by the local authority to open trench in thesestreets, due to heavy traffic,

• River : Horizontal drilling with native soil and water inside the duct. Max depth : 7 m under the riverbed,

• National park : Mechanical laying with native soil. Trench bottom at 1.7 m deep,• Hill : Direct burial with native soil. Trench bottom at 1.50 m deep,• Bridge : Ducts filled with air,• Land : duct laid in special backfill at 2 m deep as in the streets.• Rural road : Direct burial with special backfill. Trench bottom at 1.50 m deep.

It must be noted that in case of native soil surrounding, the very vicinity of the cable is made of sand orspecial backfill to avoid direct contact with stones or other materials that could hurt the cable.

The cables are systematically laid in trefoil formation, one cable per duct when any, except at the 225kV link crossing where cables are in flat formation, to improve the heat diffusion, taking into accountthe mutual heating of the two links..

Comments :

In the sloping hill, direct burial could be preferable to duct as the installation will be considered as rigidand so, the creeping problem can be avoided.The width of the area liable to flooding is 150 m large. Microtunnel is not the best technique in this caseas the maximum length is considered by experts to be around 100 m on the present site.

The land section, in accordance with the private land owners, will be done with ducts.In private streets, the same technique will be applied (ducts), the only difference will be in the finallayer, good soil for land and asphalt for streets.In the public streets, the tunnel has been chosen after discussions between utility and local authorities inorder to limit traffic problems.In the national park, an important environmental impact study was realised by independent specialistsand it concluded that it was possible to cross it. According to the type of soil, land, animals, flowersfound in this park, the best solution was to use mechanical laying at a precise period (Fall for example).The main criteria that convinced the specialists were : speed of the works, narrow width, no big soilmovements.


With the different laying techniques encountered along the routes, transitions between rigid and flexibleinstallation have to be designed.In particular, it could be reasonable in the bridge to fill the ducts with bentonite to avoid to change theinstallation technique from rigid, as the contiguous sections are envisaged with direct burial, to flexiblewith the unfilled ducts and so to keep the rigid one on three sections.

Cable checking :

Street or land River National park Hill Bridge Tunnel Road

Cable 1600 Al 1600 Cu 1600 Cu 1600 Cu 800 Al 800 Al 1600 Al

It is important to state that, along the route, we can have changes in the cable cross section, and sooptimise the cable section, but it is not easy to joint two cable sections if the difference between crosssections is too important.As the length of our project is rather short, we decide to homogenise the cross section all along theroute, with 1600 mm² Cu.This approach would make the impact of the cables an invariant, but in real projects it could be a wayto reduce the cost of links. A full optimisation can give a reduction that could be up to 10 %.

Time and period to complete the site

An other important aspect of the project is the time required to complete it. Of course, theadministrative and design items need time but do not affect the inhabitants living along the routes. Onthe contrary, the required time to complete the civil works, the jointing and the period during when theworks will be realised is of first importance for them.Some local authorities have strong demand on this specific point, so strong that it could determine theroute or the laying technique that will be finally chosen by the project manager.On the left route, the specialists propose to cross the national park during the Fall season. We can alsoimagine that works on the central route can only be done during the Summer season when most of theinhabitants are on holidays, that is to say away from their houses, to be sure that the noise coming fromthe works will not hurt their ears.

Cost of the project, comparison of the different routes :

After the cable checking, you have to choose the final route among the three identified ones whichrespective lengths are 6 km for left route, 5 km for central route and 7 km for right route and thenreview the installation of the cable.For a better understanding, lengths of the different routes could be identical, but it is rarely the case inan actual project.As soon as you know the costs of the different items, you can calculate the overall cost and choose thebest route. All the discussions with the local authorities are not recorded here, but are necessary toofficially finalise the route.It is necessary at this stage of the project to check the completion time. A line is dedicated to thisspecific point in the table so that the right decision could be taken before writing the tender.

Each of you can fill up Table 7 with the real costs of his country.


With this example, we can see that it is not easy to say which route is the best one, according to thedifferent criteria that can be used to establish the ranking.

An analysis was done with French costs. Regarding this criteria, the left and the central routes are atthe same price, slightly in favour of the central one, and the right one not far from twice. If we had thesite completion time, the most favourable route becomes the left one.This analysis is only valid for France.

It is of evidence that limitations due to administrative regulations, unknown in this study, can upset thetechnical and economical ranking shown here.To optimise the project, it is obvious that we won’t change the cable section at each new route section.One or two changes are possible but not more. Depending on the route that will be chosen, anoptimisation is then necessary, the place of the manholes being for example one of the problems.

As a conclusion, we have to underline that :• It is necessary to follow up the different stages of the process as described in the flow chart, (see

chapter 6.1),• It is important to be open minded at the beginning of a new project to study all possible solutions. It

is clear that some will fall as soon as the designer gets new constraints.• The shortest route is not always the cheapest route and that savings can be done by using other

routes or innovative laying techniques.

Section Length (m) Cable Laying technique Total cost

TypeLinear cost

Section cost

TypeLinear cost

Section cost

Land L1-L2 450 1 600 Cu DuctLand L4-L5 400 1 600 Cu DuctRiver L2-L3 150 1 600 Cu Horiz.DrillingNational Park L3-L4 5 000 1 600 Cu Mecha.Laying

Left route cost

Private street C1-C2 800 1 600 Cu DuctHill C5-C6 2 500 1 600 Cu Direct burialBridge C3-C4 100 1 600 Cu DuctRoad C2-C3 400 1 600 Cu Direct burialRoad C4-C5 200 1 600 Cu Direct burialRoad C6-C7 1 000 1 600 Cu Direct burial

Central route cost

Public Street R1-R2 1 200 1 600 Cu TunnelBridge R3-R4 200 1 600 Cu DuctRoad R2-R3 600 1 600 Cu Direct burialRoad R4-R5 5 000 1 600 Cu Direct burial

Right route cost

Route Left Central Right

Site duration (Months)

Total length (m) 6 000 5 000 7 000

Total route cost

Table 7 : Route cost


7. GLOSSARYThroughout this brochure, the terms have to be considered as follows: The term “constructiontechniques ” is considered as relating to the techniques used to create the cable route, mainly coveringthe civil works such as trenching. Likewise the term “installation techniques” is considered to relate tothe cable system design and cable installation methods.Cable design issues associated with the laying and installation techniques have also been consideredunder the general subject of "Installation Techniques".The cable installation was then the rest : the pulling and backfilling, the fixing when laid in open air.

PIPE JACKING :This technique consists in pushing into the soil prefabricated tubes having the exact diameter of thefinal tube. The tubes are pushed from a work shaft. As the pipe jacking progresses the earth works aredone, either manually or mechanically, according to the requested diameter. The first tube is equippedwith a steel drum curb which bites into the subsoil while protecting the site workers clearing the earth.This technique concerns diameters between 1000 and 3200 mm.

MICROTUNNEL :This technique consists in pushing in the subsoil prefabricated tubes having the exact diameter of thefinal tube. They are pushed from a work shaft. The earth works are systematically mechanised : amicrotunneller is put in front of the tubes. This remote controlled machine can dig small diameter pipejacking horizontally. By using the microtunneller tubes of a diameter between 300 mm and 1200 mmcan be put in place.

SHAFT :vertical circular or rectangular excavation from which the tubes are pushed.

HORIZONTAL DRILLING :Directly issued from oil drilling techniques, horizontal drilling is carried under rivers beds, railway tracks,motorways,..., and is composed of four phases : drilling of the pilot hole from the bank for rivers orfrom one side for motorways or railways tracks, casing the pilot hole, boring and pulling and laying ofthe final tubes. Direct drilling is an other word that is used in some countries to design the sametechnique.

EMBEDDING :This technique consists of excavating a river bed from a barge or with an amphibious machine, buryinga tube or cables and filling up the trench.

USE OF EXISTING STRUCTURES :sometimes, it is possible to use some existing structures, like water, gas or old fluid-fluid tubes, to putHigh Voltage extruded cables inside of them.

BRIDGES :The cables can be put in rail or road infrastructures. They can be placed in or outside bridgesstructures. This avoids other techniques which remain costly and can be difficult to accomplish.

MECHANICAL LAYING :This technique, entirely mechanical, consists in excavating a trench, and burying the cablessimultaneously in a continuous progression.


TRENCH : Excavation in which the cables are directly buried.

Backfill

Bedding material

Trench

Cable or duct

BEDDING MATERIAL :Material which can be put at the bottom of the trench under the cables, the troughs or the ducts.

BACKFILL :Material used to fill up the trench.

DUCT :Tube in PVC, PE, concrete, steel,... in which high voltage cables are pulled.

PIPE :Tube used for some laying techniques, i.e. pipe jacking, microtunnelling,…to open a hole in the soil forthe underground link. Cables are not pulled directly in the pipes ; Usually, ducts are pulled in the pipes,and cables pulled in the ducts.

TROUGH :Prefabricated concrete element, placed at the bottom of the trench, in which high voltage cables arelaid.

CABLE REMOVAL :Action of removing the cables at the end of their operation.

RIGHT OF WAY :High voltage cables are installed in Public or private areas. In these two cases, the electrical companyhas to obtain a "right of way" which allows to excavate a trench, to bury high voltage cables andoperate the underground line.

RIGID INSTALLATION :in a rigid system, the cable is held in such a manner that virtually no lateral movement occurs and thecable absorbs the thermal expansion by developing a high internal compressive force. To ensure asatisfactory performance, the cable must not buckle under this force giving rise to severe local sheathstrains.


FLEXIBLE INSTALLATION :in a flexible system, the cable is held in such a manner that the expansion movement does not causeexcessive strain in any of the cable components and hence a short fatigue life. The basic principle offlexible systems is that the thermal expansion and contraction are absorbed by movements of the cableat right angles to the longitudinal axis of the cable.

CATERPILLAR OR HAULING MACHINE :pulling or pushing machine used for the installation of cables. The cable passes between twocaterpillars and is belt-driven or braked by rubbing. They are frequently used in conjunction with cablerollers for the installation for cables in trenches, troughs and on bridges. Motorised rollers can be usedto assist in the installation of cables around bends. When installing a cable down a steep tunnel, abraking caterpillar is used to prevent the cable from running away down the slope. In addition, brakingrollers can be used to hold back the cable from running away on steep inclines.

cable

caterpillar

MANHOLEVisitable bay where joints are laid. Usually, a manhole has two covers that allow the workers to enterin.JOINT BAYNon visitable bay where joints are laid. Usually, these bays are backfilled after joint completion.


8. BIBLIOGRAPHY

Number, Authors, Title, Date, Source, Page number, Key-word

1, C.S. SCHRIFREEN, Thermal expansion effects in power cables, 1951, Proceedings AIEE,Vol. 70 -Paper 51-22, Installation

2,J. Banks et F.O Wollaston, Le comportement mécanique des câbles à très haute tension en conduitssouterrains, 1960, CIGRE, Installation

3,Bror Hansson, Bengt Bjurstrom,Une installation de câble 380 kV en Suède, 1960, CIGRE, Installation4,,Theoritical aspect of thermomechanical forces in power cables, 1969, Proceedings AIEE, Vol. 116 -

Paper 58.95, Installation5,J.D. Endacoot & H.W. Flack - H.W. Holdup & F.J. Miranda,Paramètre thermiques adoptés en

grande bretagne pour l'étude des projets de circuits de câbles à très haute tension, 1970, CIGRE 21-03, Forced Cooling

6,N, Palmiere et G.M Lanfranconi, Câbles à huile fluide à très haute tension (de 300 à 400 kV), 1972,CIGRE 21-06, Vertical shaft

7,G.M. Lanfranconi - G. Gualtieri - M. Cavalli, 330 kV Oil Filled cables laid in 1600 ft vertical shaft atKafue Gorge Hydroelectric plant., 1973, IEEE PES Winter Meeting,Paper T 73 126-0, Verticalshaft

8,Cortaillod et Cossonay,Dilatation des câbles, 1974, Bulletin des câbleries de Brugg,,Mechanicalproperties

9,J. Lepers,Contraintes thermomécaniques de système de câbles isolés au papier imprégné d'huile àpression interne et de câble isolés au polyéthylène - incidence sur les modes de pose,1975,NoteEDF,D. 6070-291,Installation

10,K. Lizuka, T. Kasahara, T. Hirai, T. Ueno, The design of snake installation for bulk size aluminiumsheated cables, 1975, IEEE PES Winter Meeting, Installation

11,A. Brauch, Ludwigshafen, Kurt Fertig und Rolf-Dieter steckel, Verlegevarianten von einadrigenKunststoffabeln fur mittelspannung,1978, Elektrizitatswirtschaft jg 77, Installation

12,D. Wenzel.,Projektierun und Bu Einer 380 kV Kabel verbindung mit IndrekterKuhlung.,1979,Elektrizitatswirtschaft jg 78, Forced Cooling

13,M.Hattori, S. Shimizu, S.Chiba, H. Saito,500 kV corrugated aluminium sheated, high pressure oil-filled cables installed in incline tunnel, 1979, Hitacho Review vol 28,N05, Incline tunnel

14,H. Nakawaga - K. Tanizawa, The Tokyo electric Power Co., IncM.Ono - H. Takehana, Fujikura Ltd.,Installation of 275 kV XLPE cables in the long and steep slope

tunnel, 1983, IEEE, Incline tunnel15,T. Aabo, J. A. moran, JR. John F. Shimshock, Thermo-mecanical bending effects in EHV Pipe-

Type cables,1984,IEEE,,Installation16,E.H. Ball, HW. Holdup, Pirelli General plcD.J. Skipper, B. Vecellio Société Cavi Pirelli,Développement de l'isolation au PRC pour câbles à haute

tension, 1984, CIGRE,,Mechanical properties17,H. Nakagawa, T. Nakabasami, K. Sugiyama, A. Shimada,Dévelopment of various snaking

installation methods of cables in Japan, 1984, JICABLE, Rapport B 7.5, Installation18,Y. Césard, La technique du forage horizontal dirigé, ses principes, ses applications à l'installation de

conduits de grande longueur sous des obstacles,1987,Journée SEE Pose des câbles souterrains etsous-marins, C6, Horizontal drilling

19,P. Dejean - Filergie,Pose des câbles PRC en "Snaking" - Choix de la méthode de pose et réalisation,1987, Journée SEE Pose des câbles souterrains et sous-marins, Installation

20,E. Dorison, M. D'Haussy, P. Ruaud, EDF, Evolution des méthodes de pose des câbles souterrainsHaute Tension, 1987, Journée SEE Pose des câbles souterrains et sous-marins, Installation

21,Klevjer - Lervik,Contraintes électromagnétiques et méthodes d'installation pour câbles unipolaires,1987, JICABLE, Rapport B. 4.2, Mechanical properties


22,Leufkens, Willems, Thrusts and short circuit phenomena in extruded HV cable and methods tocontrol them, 1987, JICABLE, Rapport B. 6.3, Short-circuit

23,G. Gasparini, CEAT CAVI, Torino, Italie, D. Roy - Filergie, Recommandations pour l'installation descâbles électriques compte tenu des contraintes apportées par les opérations de pose, 1987,JICABLE, Rapport B. 6.4, Cable pulling

24,B. Kern, Electricité de Strasbourg, Expérience de pose mécanisée des câbles souterrains en zonerurale, 1987, JICABLE, Rapport B. 7.1, Mechanized laying

25,Bennet, Gibbons,Cables 275 kV au PR pour l'installation en tunnel ou en puits, 1987, JICABLE,Rapport B. 7.2, Tunnel

26,Watanabe, Yagisawa, Hiyama, Nishinoma, Comportement thermomécanique d'un câble 275 kV entunnel, 1987, JICABLE, Rapport B. 7.3, Tunnel

27,H. Auclair, E. Favrie, B. Dhuuicq, SILEC MontereauL. Dheeman, T. Nadu, Electricity board, India, Installation de câble 225 kV à isolation synthétique dans

des puits de grande profondeur,1987,JICABLE,Rapport B. 7.4,Vertical shaft28,J.G. Head, A. Crockett, T. Taylor and A. Wilson,Thermo-mechanical characteristics of XLPE H.V.

cable insulation, 1988, IEE 5th Conference on dielectrics materials, Mechanical properties29,Y.H. Hahm, H. Takehana, Y. Ikegay, S. Okuyama, Underground 161 kV XLPE power cable

project for Iowa Power, 1988, Fujikura Technical Review,,Installation30,Iordanescu, IREQ,analysis of thermomechanical bending of self-contained oil-filled cables in

ducts,1988, Mechanical properties31,Ruger, Mechanical Short-circuit effects of single-core cables, 1989, IEEE, Short-circuit32,H. Kent, N. Barden, Méthodes de pose et techniques d'installation des câbles HT récemment

adoptés en Australie, 1990, CIGRE 21-201, Installation33,L. Aanerud, G. Balog, K. Bjorlow-Larsen, Acatel STK, Norvege, Vue d'ensemble des techniques

actuellement utilisées pour l'installation et la réparation des câbles sous-marins, 1990, CIGRE,21.202, Submarine cable

34,K. Hänninen, Imatran voima Oy,FinlandeA.R, Pettersson, Swedish State Power Board, Suède,G. Hjalmarsson, ABB Cables, J.ELarsen, Alcatel STK, Norvège,Installation du câble Fenno-Skan

entre la Suède et la Finlande, 1990, CIGRE, 21.203, Submarine cable35,B. Dellby, ABB cables, M. Dam-Andersen, Copenhagen lightning Dept, J. Jorgensen, NKT Power

cables, Installation et surveillance des câbles à 145 kV dans la ville de Copenhague, 1990, CIGRE,21.204, Submarine cable

36,P. Schoonakker, P, HMJ Willems, NKF KABEL BV., Comportement thermomécanique des câblesHT et THT à conducteurs en cuivre, 1991, JICABLE,Rapport A 4.3, Mechanical properties

37,JG Head, AE Crockett, A. Wilson, D.E. Williams,Comportement thermomécanique des câbles PRen régime permanent et de court-circuit., 1991, JICABLE,Rapport A 4.4, Mechanical properties

38,Ishii, Iwata, Inoue,Méthodes de dimensionnement et analyse du comportement thermomécanique decâbles PR 275 kV, 1991, JICABLE, Rapport A 4.5, Mechanical properties

39,Sin, Barsacq, Saint-Martin, Latarges, Méthodes de pose de câbles de tension spécifiée 225 kV dansle cas de traversées sous-fluviales.,1991,JICABLE,Rapport A 5.5,Sub-fluviale crossing

40,Fotys, Gupta, Horrocks, Hufel,Installation de câbles 230 kV à isolant PR à Ontario Hydro, 1991,JICABLE, B 2.2, Installation

41,N. Bell, Q. Bui-Van, P. Meyere, D. Couderc, G. Ludasi, C. Picard, Hydro-Quebec, traversée sous-fluviale à 450 kV CC du fleuve Saint-Laurent intégrée à une ligne aérienne de 1500 km reliant 5terminaux, 1992, CIGRE, 21.305, Sub-fluviale crossing

42,U. Arnaud, A. Bolza, F. Magnani, E. Occhini, Pirelli, Italie, Traversée en câble sous-marin 345 kV,750 MVA du détroit de Long Island, 1992, CIGRE, 21.306, Sub-fluviale crossing

43,M.T. O'Brien, Transpower N.Z Ltd.J.E. Larsen, Alcatel Kabel, Norvège


G. Hjalmarsson, ABB HV cables, Suède,Installation de câbles électriques sous-marins à courantcontinu de 350 kV pour améliorer le réseau à CCHT en Nouvelle-Zélande, 1992, CIGRE, 21.308,Submarine cable

44,P.Boniface, S. Sin, EDF, Guides techniques des chantiers de pose mécanisée des câbles HTA etHTB, 1993, Journée SEE Pose Mécanisée, Mechanized laying

45,D. Paulin, JL. Legue, E. Vasseur, JM. Coquet, Alcatel Câble, France, Câble 550 kV en tunnel pourla station de pompage de Guangzhou, 1995, JICABLE, Rapport A 1.4, Tunnel

46,G. Klvjer, H. Kulbotten, J.K. Lervik, EFI, Norvège,Comportement mécanique des câbles unipolairesposés en trèfle, 1995, JICABLE, Rapport A 2.5, Installation

47,P. Andersen, ELSAMM. Dam-Andersen, L.Lorensen, NESAT. Tanabe, S. Suzuki, The Fukurawa Electric CO. Ltd, Mise en place d'un câble de 420 kV au PR pour

le projet d'énérgie de la ville de Copenhague, 1996, CIGRE, 21.201, Installation48,F. Donnazi, R. Gaspari, Pielli Cavi SpAB.A. Cauzillo, L. Lagostena, ENEL,O. Bosoti, W. Mosca, CESI,Etude de la performance du réseau en câbles extrudés de 400 kV dans les

conditions de court-circuit, 1996, CIGRE, 21-205, Short-circuit49,W. Holdup, E. Occhini, D.J Skipper,Thermo-mécanical behaviour of large conductor cables, jul-67,

IEEE Summer Power Meeting,,Mechanical properties50,G. Palante, Comportement de conducteurs rigides et de leurs supports dans les conditions de court-

circuit, comparaisons ente valeurs calculées et mesurées, 23/10/76, CIGRE 23-10, Mechanicalproperties

51,K. Ohhata, H.Ohno, T. Matsuike, O. Yoneyama, I. Oguma, T. Keishi.,Study on the method ofinstallation of class 66 kV cables, déc-78, Sumitomo Electric Technical review n°18, Installation

52,A.Bonamy,Comportement suivant le type de pose des câbles HT lors de courts-circuitsinternes,03/05/79, Note EDF, HM-62/3649, Short-circuit

53,G. Smith David,A general method for the calculation of pipe pulling forces, jan-81,I EEE, Vol. PAS100 n°1, Cable pulling

54,R.D. Marquez, O.R Ledezma, N. Noda, H. Kuki, 230 kV self-contained oil filled cable line installedunderneath a bridge located in maracaibo, Venezuela, jul-81, IEEE Transaction on Power apparatusand systems,Vol. PAS 100 n°7, Bridge crossing

55,S. Yoshikawa, A Fujimori, S. shimada, M. Kubo, T. Fukui,Installation method for 77 kV XLPEcables., jan-83, Sumitomo Electric Technical review n°22, Installation

56,K. Zbinden, M. Stalder, (Brugg), 150/110 kV seekabelanlage Morcotte-Brusino : Installation sous-lacustre de câbles 150 et 110 kV entre Morcotte et Brusino, mai-86, Kabel Cables n°22, Sub-fluviale crossing

57,D. Minakuchi, M. Fujii, H. Inoguchi, H. Uno, The Kansai electric Power Co., Inc.T. Matsui, S. Toyoda, I. Gessei, S. Asai, Sumitomo electric Industries, Ltd.,Installation of 187 kV

XLPE Power Cable along the Ohnaruto suspension bridge, 03/07/87,IEEE Transaction on PowerDelivery, Vol. PWRD-2, n°3, Franchissement de pont

58,Y. Watanabe, K. Kanazawa, T. Sasaki,Measures against Creepage of 275 kV HPFF cable.,jan-89,IEEE Transaction on Power Delivery, Vol. 4, n°1, Mechanical properties

59,E. Dorison ,Méthodes d'installation des câbles en galerie, 14/01/92, Note EDF,HM-62/6913,Installation

60,Y. Maugain,Compte rendu de la visite du chantier 400 kV à Copenhague (Danemark) le 30 Avril1997,11/09/97, Note EDF,D-5730.06.78/97.045, Installation

61,J. Tarnowski, M. Iordanescu, R. Awad, C. Royer (Hydro-Québec, IREQ), Thermomecanicalmodelling of 345 kV XLPE cables in duct, 1999, Jicable'99, 121-125, Modeling, ducts62,T. Brincourt, E. Dorison (EDF), Thermomecanical behaviour of 400 kV synthetic cables, 1999,

Jicable'99, 126-131, backfills, ducts,


63,T. Brincourt (EDF), J-C. Robin (Marais), M. Boucher, Rimaudière (Mios), E. Leflaive, J. Puig(Ingénieurs Conseil), New mechanised laying techniques of MV cables with geosyntheticprotection, 1999, Jicable'99, 132-136, geosynthetic protection, laying machine

64,Y. Maugain (EDF), P. Hudson (Pirelli), Cable laying : state of the art of HV and EHV cablesystems, 1999, Jicable'99, 137-141, Laying techniques

65,J. Berdala, P-M. Dejean, A. Maxan, J. Mansour (Pirelli), Installation of a "new technical step" 90kV cable system, A real experience on the French network,1999, Jicable'99, 142-147, mechanicallaying, ducts

66,B. Gregory, J. Monteys (BICC), S. Barris (Enher-Hec), J-M. Mendez, Installation of 220 kV XLPEcables in a tunnel, whilst minimising electromagnetic induction in communication cables, 1999,Jicable'99, 148-153, tunnels

67,P. Lavantureux, P. Deguines, F. Gahungu (Alcatel Cables), T. Todorov (Nationalna ElektrichesckaKompania Bulgaria), 400 kV insulated cable link installation carried out at the Chaira pumpedstorage power plant in Bulgaria, 1999, Jicable'99, 154-159,tunnels

70,C. Figaret (EEE), The mechanical laying of underground high voltage cables (63 and 90 kV) inFrance, 1999, Jicable'99, 160-161, mechanical laying

71,Y. Maugain, P. Hudson in name of WG 21-17, Cable installation: State of the art for the installationdesign of HV and EHV cable systems, Cigre 2000, paper 21-202

Le CIGRÉ a apporté le plus grand soin à la réalisation de cette brochure thématique numérique afin de vous fournir une information complète et fiable. Cependant, le CIGRÉ ne pourra en aucun cas être tenu responsable des préjudices ou dommages de quelque nature que ce soit pouvant résulter d’une mauvaise utilisation des informations contenues dans cette brochure. Publié par le CIGRÉ 21, rue d’Artois FR-75 008 PARIS Tél. : +33 1 53 89 12 90 Fax : +33 1 53 89 12 99 Copyright © 2000 Tous droits de diffusion, de traduction et de reproduction réservés pour tous pays. Toute reproduction, même partielle, par quelque procédé que ce soit, est interdite sans autorisation préalable. Cette interdiction ne peut s’appliquer à l’utilisateur personne physique ayant acheté ce document pour l’impression dudit document à des fins strictement personnelles. Pour toute utilisation collective, prière de nous contacter à [email protected]

The greatest care has been taken by CIGRE to produce this digital technical brochure so as to provide you with full and reliable information. However, CIGRE could in any case be held responsible for any damage resulting from any misuse of the information contained therein. Published by CIGRE 21, rue d’Artois FR-75 008 PARIS Tel : +33 1 53 89 12 90 Fax : +33 1 53 89 12 99 Copyright © 2000 All rights of circulation, translation and reproduction reserved for all countries. No part of this publication may be produced or transmitted, in any form or by any means, without prior permission of the publisher. This measure will not apply in the case of printing off of this document by any individual having purchased it for personal purposes. For any collective use, please contact us at [email protected]

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