2001 Tb 194 Construction, Laying and Installation t

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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


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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. RTER (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. MAUGAIN 1 (France) (Convenor),M. PORTILLO (Spain),T. J. RODENBAUGH (United States),R. SAMICO (Brazil),R. SCHROTH (Germany).

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


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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 23

2.4.3.8 Short circuit forces 232.4.4 Compatibility of the accessory performance with that of the cable system 24


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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 313.2.1.1 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


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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 483.3.2 Shafts 49

3.3.2.1 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 72

3.3.8.2 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 87

4.1.2.1 Backfill 874.1.2.2 Cooling systems 874.1.3 Transition between different installation types 87

4.1.3.1 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


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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 101

4.2.4.1 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 117

5.3.4 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


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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) 93

Figure 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 : Brm s profiles with various s 111Figure 31 : Brm s profiles with various d 112Figure 32 : Flat arrangement, 2 circuits 112Figure 33 : Brm s profiles for two cable system configurations with various h 113Figure 34 : Brm s profiles for two cable system configurations with various g 114Figure 35 : Trefoil arrangement, 1 circuit 114Figure 36 : Brm s profiles for both flat and trefoil formations with various sflat and strefoil 115

Figure 37 : Brm s profiles with various d for both flat and trefoil formations 115Figure 38 : Trefoil arrangement, 2 circuits 116Figure 39 : Brm s profiles for two cable system configurations with various h 116Figure 40 : Brm s profiles for two cable system configurations with various g 117Figure 41 : Vertical arrangement, 1 circuit 117Figure 42 : Brm s profiles for flat, trefoil and vertical formations with various sflat, strefoil and svertical 118Figure 43 : Vertical arrangement, 2 circuits 118Figure 44 : Brms profiles for two cable system configurations with fixed h, d, g and s = st = 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


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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


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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 their application.

Starting from the studies of the previous working group 21-01, it is anticipated that the method of working 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 work required 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 effective

completion. 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


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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) has been 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 the pulling. 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 better comprehension 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 work

The 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


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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 task was 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 22

countries) and Cable manufacturers (27 replies from 16 countries) in order to collect the domestic practice 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 n4(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 a

comparative evaluation could be carried out. Cable engineers should apply the methodology to their actual 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 installation techniques ? At the beginning of the Working Group's work, the difference was not very clear with the both words being 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 i s an i nnovative construction techni que ? 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 the

companies which replied to questionnaire N 1 concerning utilities). These are : trenches (direct burial),ducts and tunnels.


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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 H ow is it possibl e for a newcomer in the cable world to design an un derground li nk ? This lead to a lot of discussions among the members of the Working Group, however all agreed on the principle 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..


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2. DESCRIPTI ON OF TH E CABL E SYSTEM The 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 atermination . Thelength of a continuous section of cable is often limited by the size or weight of the cable reel that can be 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 join ts (or splices).Joints and terminations are the main components of equipment calledcable accessor ies .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 can be 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 M eshed underground network Some 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.


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Undergroundcables

Substation 3

Substation 2 Substation 1

Figure 4 : Meshed underground network

2.2.2 Siphon A 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 may

also 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 entr ance An 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.


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Overhead line

Undergroundcable

Substation

Figure 6 : Underground substation entrance

2.2.4 Power generator output An 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 hydro

power 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 Auxi li ary supply

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


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BusbarGeneratorTransformer

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 dielectr ic cables cables with lapped insulation :SCFF , H PFF , H PGF. but SCFF are only considered here.In this document, only extruded and SCFF cables are considered.

2.3.1 Extruded -dielectri c 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 crosslinked polyethylene (XLPE). EPR, XLPE, LDPE and HDPEhave been in use for many years. XLPE becomes a

predominant choice for high voltage cables up to 500kV level.Maximum conductor temperature in normal operationis depending on the insulation material: 70 C for LDPE, 80C for HDPE and 90C for EPR and XLPE.

Picture 2 : 400 kV XLPE cable


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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 mm for copper and 1600 mm for aluminium, the conductor issegmented to reduce the ac/dc resistance ratio.

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

The inner semi-conducting l ayer is extruded over the conductor or semi-conducting bedding tape. Its purpose 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 are preferably 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 metall ic screen made with copper or alumi ni um wi res and/or a metall ic 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. The

sheath must remain in good contact with the outer semi-conducting layer during heating and cooling.A j ack et 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 in sul ation Impregnated 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.


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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, suppresses

ionisation 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 or corrugated 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 bar high 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 for passage of the dielectric fluid.2.3.2.3 Impregnated Paper Characteristics :

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

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 General The 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 specially prepared cable.

It is essential to select the design of accessory to be compatible with the particular cable type and the particular 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.


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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 pressuriseddielectric 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 Compatibil ity of th e accessory wi th the cable

2.4.3.1 Number of cable coresThe 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 of assembly 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 are particularly 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


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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 250C for XLPE and 160C 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 cableThe insulation of the polymeric cable must be identified by the user. There are significant differences between the electrical and mechanical characteristics of extruded insulation. The usual insulants for extruded polymeric cables in the voltage class of 60 kV and above being XLPE (crosslinked polyethylene), LDPE (low density polyethylene), HDPE (high density polyethylene) and EPR (ethylene propylene rubber).

Chemical compatibility with the extruded cableThe 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.


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Compatibility with the paper insulated lapped cableIn 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 barrier plate 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 of insulation, 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,


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method of restraint and the spacing of adjacent cables.

2.4.4 Compatibi li ty of the accessory perf ormance 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, timeand 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, for example in an overhead line circuit. Such accessories may be designed to be suitable for quick

assembly 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.


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2.4.5 Compatibility of the accessory with the cable system design and operating conditions The 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 of the 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 and

pressure) 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,


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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 installationAssembly 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 equally

rigorous :


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Quality Assurance approval for installationThe user should ensure that the installer provides evidence of an approved quality assurance system for installation to an internationally recognised standard.

Quality PlanThe 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 of components 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 PersonnelWhen selecting the designs of accessories the user should ensure that training courses are available for the jointing and supervisory personnel. It is strongly advised that personnel receive training on the particular 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, for example, 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 general jointing 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.


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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 good practice to provide control of temperature and humidity to ensure a) consistent performance of the personnel 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 Compatibil i ty of the accessory with specif ied after laying tests When the installation of the cable and accessories has been completed it is standard practice to perform 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 for the 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


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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 after laying test comprised of the application of a dc withstand voltage applied to the metallic sheath or screen 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 M ain tenance requi rements of the accessory The 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 and

the 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 for emergency use may be different from that installed.

2.4.8 Economics of accessory selection A 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 the

main items of cost :


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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 for termination.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 of accessories 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 apportion between 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 for safe 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 for different 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 experienced personnel 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 of training 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 of the 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 overall costs 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 user is 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.


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3. CONSTRUCTI ON TECHNI QUES Twelve 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 the possibility of carrying out the civil work independently from theelectrical work. Also, the flexibility of cable maintenance or replacement with minimum disturbance to local traffic and economicactivities are considered advantageous. In less congested areas, joint bays 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 is placed 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 to

enhance 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.


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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 cast

concrete 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 the joints are completed , the joint bay is filled with thermal back filling material and top slabs placed over the 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 lower voltages. 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 of short circuit. It should be noted that a good earth cover over the duct bank is necessary to ensure public 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 work Civil 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 of

the 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 include bends and offsets that would increase the required cable pulling tensions and thus limit distances between 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 of tree roots or higher than normal cable operating temperatures. This could be avoided by using a proper


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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 work progress. 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 somewhat beneficial, many utilities do not allow it to accumulate in ducts or manholes to prevent possible premature 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 properties

investigated 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 also be useful in establishing the maximum thermal resistivity and percentage of water content by weight of soil 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 work on 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.


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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 a proper sealing material such as" ductseal".

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

Duration of the work Civil 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 to be removed at the end of their useful service life and their components disposed of and recycled. It is

usually 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 :


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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 Di rect bur ial

3.2.2.1 Description of the technique

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 for the 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 other type 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 1.0 m (220 to 500 kV) in flat formation.

Over the backfilling material cable-protective slabs are placed.


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

3.2.2.2 Limits of the technique

Civil work The 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 be present in the bottom of the trench.The backfilling materials used to fill the trench are composed, starting with the protective bottom layer referred 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 use

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