IMPLEMENTATION OF LONG AC HV AND EHV CABLE SYSTEMS

680
IMPLEMENTATION OF LONG AC HV
AND EHV CABLE SYSTEMS
WORKING GROUP
B1.47
MARCH 2017

Members
K. Barber, Convenor AU G. Aanhaanen, Secretary NL
S. Lauria IT F. Waite GB
S. Kobayashi JP H. Suyama JP
V. Werle DE H. Orton CA
J. Kim KR C. Akerwall SE
F. Renaudin NO J. Domingo ES
F. Lesur FR M. Boedec FR
N. Rahman AU U. Gudmundsdottir DK
P. Morgen IE Y. Wang CN
P. Bracher CH D. Lindsay US
S. Dambone Sessa IT S.K. Ghosh IN
M. Soga JP T. Yamamoto JP
WG B1.47
Copyright © 2017
“All rights to this Technical Brochure are retained by CIGRE. It is strictly prohibited to reproduce or provide this publication in
any form or by any means to any third party. Only CIGRE Collective Members companies are allowed to store their copy on
their internal intranet or other company network provided access is restricted to their own employees. No part of this
publication may be reproduced or utilized without permission from CIGRE”.
Disclaimer notice
“CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the
accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent
permitted by law”.
WG XX.XXpany network provided access is restricted to their own employees. No part of this publication may be
reproduced or utilized without permission from CIGRE”.
Disclaimer notice
“CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the
IMPLEMENTATION OF LONG AC HV
AND EHV CABLE SYSTEMS
ISBN : 978-2-85873-383-5

IMPLEMENTATION OF LONG AC HV AND EHV CABLE SYSTEMS
3
EXECUTIVE SUMMARY
Introduction
The aim of this CIGRE WG is to prepare a comprehensive Technical Brochure which it is hoped will be
a valuable document for reference by any Utility, Government agency or Investor looking to put in an
underground system in lieu of an overhead line, or a long length of submarine cable, particularly in
terms of appreciating what can be done or has been done.
Background
The power transmission network has been developed during the last decades based on the use of
overhead lines (OHL). EHV underground cables systems have been available since a long time, but
their development has been limited by large capacitance and dielectric losses as well as a relatively
low current rating compared to OHL. However, with the use of new materials and processing
technology, the situation has changed significantly, so that the constraints on maximum length and
power transfer have largely been overcome.
The difficulties in installing new overhead lines are making it essential to consider the use of longer
underground cable links, as demonstrated by the increasing numbers of long AC cable projects. At the
same time the development of off-shore facilities has created a demand for long submarine cables.
There are still however technical challenges to consider whilst planning such cable installations. The
most sensitive topics being those concerning reliability, impact on the transmission grid and
installation.
A Cigre Working Group has prepared a Technical Brochure TB 556 “Power System Technical
Performance Issues Related to the Application of Long HVAC Cables” [1.2] which covers many of the
technical aspects relating long AC cable links; this document therefore addresses some of the other
aspects, with the emphasis on implementation issues.
Definition
The definition that has been chosen for long length of HVAC cables for this document is: -
“A long length of insulated cable is one where the load due to the capacitive current needs to be
considered in the system design. Typically, this would be 40 km for voltages less than 220 kV and 20
km for 220 kV or greater”
Given the scope of work, this definition is slightly different to that of TB 556 and that used by other
documents. It was selected to be able to draw on the experience gained from approximately one
hundred current and future projects.
Scope
The aim of the WG is to create a Technical Brochure which covers the practical issues relating to the
System Design, installation and monitoring of long HVAC Cables. Focus is made on:
1. Current state of development (SCFF cable vs XLPE cable, Surge arrestors, Reactive
compensation and issues relating to reliability of supply)
2. Challenges for Implementation (Matching power rating by Hybrid circuits, controlling EMF)
3. System Design (Amount of reactive compensation, Losses and Sheath bonding)
4. Installation (Construction, Horizontal directional Drilling, Right of way)
5. Monitoring (Temperature monitoring, control of route condition)
6. Maintenance (Fault Location, access to route information)
Practical Experience & reference documents
1. Examples of significant projects from different countries.
2. Table of projects undertaken or being undertaken as covered by the above definition
3. System design and Reference papers on this subject.

4

5
Content
EXECUTIVE SUMMARY ............................................................................................................................... 3
1. CURRENT STATE OF DEVELOPMENT..........................................................................................14
1.1 REASONS FOR GROWTH..............................................................................................................................................14
1.1.1 Overall system description....................................................................................................................................14
1.2 CABLE DESIGN TRENDS ..................................................................................................................................................15
1.2.1 Insulation thickness...................................................................................................................................................16
1.3 CABLE TYPES......................................................................................................................................................................18
1.3.1 SCFF, HPOF, cable designs...................................................................................................................................18
1.3.2 XLPE Cable designs.................................................................................................................................................20
1.4 NEW INSTALLATION TRENDS........................................................................................................................................21
1.5 ASSOCIATED EQUIPMENT..............................................................................................................................................21
1.5.1 Joints and Terminations ..........................................................................................................................................21
1.5.2 Surge Arrestors to protect cable and accessories............................................................................................25
1.5.3 Reactive compensation to offset cable capacitance .......................................................................................25
1.5.4 Harmonic Filters and Resonance Mitigation Techniques..................................................................................27
1.6 RELIABILITY OF SUPPLY...................................................................................................................................................28
2. CHALLENGES FOR IMPLEMENTATION ......................................................................................30
2.1 INTRODUCTION................................................................................................................................................................30
2.2 EFFECT ON THE GRID......................................................................................................................................................30
2.2.1 Matching cable and OHL ratings.........................................................................................................................31
2.2.2 Practical example matching cable and overhead line rating.......................................................................32
2.2.3 Dynamic cable rating .............................................................................................................................................33
2.3 PROTECTION SYSTEMS...................................................................................................................................................34
2.3.1 Power Cable Protection and protection of Hybrid links.................................................................................34
2.3.2 Auto re-closure and lock-out system....................................................................................................................34
2.4 VOLTAGE EFFECT (FERRANTI EFFECT) .........................................................................................................................35
2.4.1 HVAC test after the installation ...........................................................................................................................37
2.4.2 Under no or low load operation..........................................................................................................................37
2.5 ZERO MISS PHENOMENON...........................................................................................................................................37
2.6 SWITCHING OFF CAPACITIVE CURRENTS.................................................................................................................37
2.7 HARMONIC RESONANCE...............................................................................................................................................38
2.8 MITIGATION OF MAGNETIC FIELDS............................................................................................................................41
2.9 LIFE TIME EXPECTANCY...................................................................................................................................................41
2.9.1 Cable design and manufacturing ........................................................................................................................41
2.9.2 Cable route planning and installation ................................................................................................................42
2.9.3 In Service maintenance...........................................................................................................................................42
2.10 TESTING .........................................................................................................................................................................42
3. SYSTEM DESIGN............................................................................................................................46
3.1 TRANSMISSION SYSTEM – COMPARISON OF AC OR DC VOLTAGE ................................................................46
3.2 REACTIVE COMPENSATION...........................................................................................................................................46
3.3 CABLE SHEATH BONDING .............................................................................................................................................50
3.3.1 Introduction ...............................................................................................................................................................50
3.3.2 Cross-bonding system.............................................................................................................................................51

6
3.3.3 Single point bonding system .................................................................................................................................55
3.3.4 Solid bonding system..............................................................................................................................................58
3.3.5 Submarine cables bonding techniques................................................................................................................59
3.4 THERMO MECHANICAL FORCES ..................................................................................................................................61
3.4.1 Introduction ...............................................................................................................................................................61
3.4.2 Calculation of cable thrust force..........................................................................................................................61
3.4.3 Route alignment .......................................................................................................................................................62
3.4.4 Geometry of the snaking arrangement..............................................................................................................62
3.4.5 Other considerations...............................................................................................................................................64
3.5 EMF ......................................................................................................................................................................................64
3.6 MAINTAINING CIRCUIT RATING...................................................................................................................................65
3.7 LIMITING INDUCED VOLTAGES....................................................................................................................................66
3.7.1 Introduction ...............................................................................................................................................................66
3.7.2 Induced voltage level.............................................................................................................................................67
3.8 FUTURE SYSTEMS .............................................................................................................................................................67
3.8.1 Reduction in frequency...........................................................................................................................................67
3.8.2 New Cable insulation materials ...........................................................................................................................69
4. INSTALLATION................................................................................................................................70
4.1 SELECTION OF BEST CABLE DESIGN DEPENDING ON INSTALLATION METHOD.............................................70
4.2 ROUTES AND RIGHTS OF WAY ...................................................................................................................................70
4.3 ROUTE PLANNING - TRAFFIC MANAGEMENT AND SECURITY.............................................................................70
4.4 INSTALLATION METHODS AND DEFINITIONS...........................................................................................................70
4.4.1 Direct Buried and Ploughing – Rigid Constrained............................................................................................70
4.4.2 Ducts – Conduits and HDD –Semi Flexible Constrained .................................................................................71
4.4.3 In Air – Flexible Constrained ................................................................................................................................77
4.5 TRANSPORTATION...........................................................................................................................................................79
4.5.1 Sea transportation ..................................................................................................................................................79
4.5.2 Inland waterways....................................................................................................................................................80
4.5.3 Railway transportation...........................................................................................................................................80
4.5.4 Road transportation................................................................................................................................................82
4.5.5 Access to the joint bays..........................................................................................................................................82
4.5.6 Examples of transportation of very long lengths.............................................................................................83
4.5.7 Example of a standard transportation on land................................................................................................86
4.5.8 Example of transportation of submarine cables ..............................................................................................86
4.6 TESTING AFTER INSTALLATION.....................................................................................................................................87
4.6.1 Outer Sheath DC Voltage Withstand.................................................................................................................88
4.6.2 SVL Verification .......................................................................................................................................................88
4.6.3 Phase Identification.................................................................................................................................................88
4.6.4 Conductor Insulation Resistance Measurement..................................................................................................88
4.6.5 Insulation Capacitance Measurement .................................................................................................................89
4.6.6 Conductor Resistance Measurement....................................................................................................................89
4.6.7 Zero-Sequence Impedance Measurement..........................................................................................................89
4.6.8 Positive / Negative-Sequence Impedance Measurement ..............................................................................89
4.6.9 Cross Bonding Verification ....................................................................................................................................90
4.6.10 High Voltage AC Test ........................................................................................................................................90
4.6.11 Partial Discharge Measurement on Accessories...........................................................................................91
4.6.12 Link Contact Resistance Measurement............................................................................................................92
4.6.13 Measurement of Earth Resistance....................................................................................................................92
4.6.14 TDR trace..............................................................................................................................................................92
4.6.15 FO telecom and DTS fibre OTDR attenuation tests.....................................................................................92
4.6.16 The Sequence of Testing....................................................................................................................................92
4.7 QUALITY ASSURANCE.....................................................................................................................................................92
5. MONITORING................................................................................................................................94

7
5.1 INTRODUCTION MONITORING (PD, TEMPERATURE, STRAIN) ..............................................................................94
5.2 TEMPERATURE MONITORING........................................................................................................................................94
5.2.1 Measurement principle...........................................................................................................................................94
5.2.2 Basic Layout Options ..............................................................................................................................................95
5.2.3 Positioning the fibre................................................................................................................................................96
5.2.4 Cable Types with Optical Fibres .........................................................................................................................98
5.2.5 Data Analyses and Interpretation .................................................................................................................... 101
5.3 STRAIN MEASUREMENT ............................................................................................................................................... 102
5.4 MONITORING OF SVL’S - INSPECTION .................................................................................................................. 102
5.5 MONITORING OF THE SHEATH CONDITION......................................................................................................... 102
5.5.1 Underground Cables ........................................................................................................................................... 102
5.5.2 Submarine Cables................................................................................................................................................ 102
5.6 MONITORING PD.......................................................................................................................................................... 102
5.7 TDR MEASUREMENT...................................................................................................................................................... 103
5.8 OTHER MEASUREMENT SYSTEMS.............................................................................................................................. 104
6. MAINTENANCE............................................................................................................................ 106
6.1 LAND CABLE ................................................................................................................................................................... 106
6.1.1 Maintenance guidelines ...................................................................................................................................... 106
6.1.2 Fault Location ........................................................................................................................................................ 107
6.1.3 Access to route information ................................................................................................................................ 110
6.1.4 Typical time durations for repair works.......................................................................................................... 111
6.1.5 Rapid Response Repair Options ....................................................................................................................... 115
6.2 SUBMARINE CABLE........................................................................................................................................................ 117
6.2.1 Maintenance guidelines ...................................................................................................................................... 117
6.2.2 Origin and nature of cable failures................................................................................................................. 117
6.2.3 Fault detection and location............................................................................................................................... 118
6.2.4 Planning of the repair operations..................................................................................................................... 118
6.2.5 Repair execution and operations...................................................................................................................... 119
6.2.6 Typical time durations for repair works.......................................................................................................... 122
6.2.7 Conclusions about maintenance of submarine cables................................................................................... 123
7. EXAMPLES OF WORLD WIDE EXPERIENCE........................................................................... 124
7.1 AUSTRALIA - 220 KV LONG HVAC CABLE CIRCUIT IN MELBOURNE............................................................... 124
7.2 CANADA - 525 KV SUBMARINE CABLE SYSTEM TO VANCOUVER ISLAND................................................... 128
7.3 CHINA - HAINAN CONNECTION .............................................................................................................................. 130
7.4 DENMARK - THE LAND CABLE PROJECT OF THE ANHOLT WIND FARM......................................................... 131
7.5 ITALY - SORGENTE – RIZZICONI TRANSMISSION LINE ....................................................................................... 134
7.6 JAPAN - 500 KV SHIN-TOYOSU LINE AND 275KV SOUTH ROUTE ............................................................... 138
7.7 GERMANY – RIFFGAT 155 KV – 80 KM 113 MVA CONNECTION.................................................................. 141
7.8 NETHERLANDS – 220 KV GEMINI WIND FARM CONNECTION........................................................................ 144
7.9 NORWAY - OSLO FJORD AND KOLLSNES – MONGSTAD PROJECT ............................................................. 147
7.10 SWEDEN – BORNHOLM AND ALAND CABLE.................................................................................................... 151
7.11 U.K. - ELSTREE TO ST JOHN’S WOOD TUNNEL................................................................................................ 153
8. STATISTICS OF LONG LENGTH HVAC CABLE PROJECTS.................................................. 156
8.1 LONG HVAC POWER CABLE PROJECTS IN HISTORY.......................................................................................... 156
8.2 GEOGRAPHICAL REPARTITION OF PROJECTS ...................................................................................................... 156
8.3 LENGTH OF LONG HVAC POWER CABLE PROJECTS PER VOLTAGE LEVEL.................................................. 157

8
8.4 VOLTAGE VERSUS PROJECT LENGTH...................................................................................................................... 157
8.5 LENGTH OF LONG HVAC POWER CABLE PROJECTS PER POWER LEVEL...................................................... 158
9. CONCLUSION............................................................................................................................. 160
10. BIBLIOGRAPHY/REFERENCES................................................................................................... 162
APPENDIX A. LISTING OF LONG HV AC PROJECTS – LANDSCAPE ANNEXES........................ 166
APPENDIX B. LONG HVAC LINE LOAD FLOW ................................................................................ 177
B.1. ENERGY TRANSPORTATION THROUGH THE LONG AC POWER LINE AND EHV CABLE............................ 177
B.2. APPLICATION TO THE VICTORIAN DESALINATION PLANT PROJECT............................................................... 179
B2.1 Introduction of the VDP project......................................................................................................................... 179
B2.2 The chain-matrixes of the power-line elements............................................................................................. 179
B2.3 Solving the model to reach the nominal steady-state.................................................................................. 180
APPENDIX C. EXAMPLE OF A LONG HVAC LINE - LOAD FLOW ................................................ 183
C.1. THE PHASE VOLTAGE/CURRENT REPARTITION ALONG THE POWER LINK............................................... 183
APPENDIX D. EXAMPLE OF EFFICIENCY OF A 400KV UG POWER TRANSMISSION CABLE
VERSUS CIRCUIT LENGTH .................................................................................................................... 185
D.1. EFFICIENCY OF THE POWER LINK WITHOUT REACTIVE COMPENSATION ............................................... 185
D.2. THE REACTIVE COMPENSATION BY SHUNT REACTOR/INDUCTOR............................................................. 187
D2.1 A single shunt reactor/inductor at any location............................................................................................. 187
Figures and Illustrations
Figure 1.1 275 kV XLPE cable configuration....................................................................................17
Figure 1.2 Capacitance and charging current per unit length in each phase......................................17
Figure 1.3 Inductive reactance required and dielectric loss per unit length in each phase..................17
Figure 1.4 HV XLPE Cable with corrugated Aluminium sheath..........................................................20
Figure 1.5 Typical HV termination with composite type insulator (Source: CIGRE WG B1.29) ............22
Figure 1.6 Typical EHV GIS with GIS Terminations installed in horizontal position (Picture by courtesy:
Nexans).......................................................................................................................................22
Figure 1.7 Typical non-buried joints installed in a cable chamber/manhole (CIGRE WG B 1.29).........23
Figure 1.8 Typical Cross-bonding cabinet equipped with SVL (Picture by courtesy: Nexans) ..............24
Figure 1.9 345 kV PPLP Pipe Type cable terminations, cable route length 30 km ..............................25
Figure 1.10 Typical Shunt reactor for reactive power compensation.................................................26
Figure 2.1 Series reactor in 380 kV hybrid line to balance the current distribution, 8.5 Ohms, pass
through current 4 kA ....................................................................................................................30
Figure 2.2 Emergency cable rating matching post-fault overhead line ..............................................32
Figure 2.3 Existing 150 kV cables equipped with PT100 elements at the 'hot-spot'............................33
Figure 2.4 Relative voltage increase due to Ferranti effect at different voltage levels and conductor
cross sections...............................................................................................................................36
Figure 2.5 Resonance frequencies for different cable lengths and short circuit power .......................38
Figure 2.6 220 kV AC offshore grid connected, length of each cable 65 km, connected by transformers
to the 380 kV grid on land ............................................................................................................39
Figure 2.7 Impedance curves 380 kV grid without (red) and with additional offshore cables (blue)....39
Figure 2.8 Simple presentation of the grid for calculation amplification harmonic voltages ................40

9
Figure 2.9 Filter arrangements applied in High Voltage grids ...........................................................40
Figure 2.10 HV resonance test ......................................................................................................43
Figure 3.1 Comparison of losses for 100km long underground transmission line with 135MVA load
transfer with 132kV AC, 220 kV AC or ± 150 kV DC........................................................................46
Figure 3.2 Reactive compensator for 30 km 345 kV PPLP pipe type cable, USA ................................47
Figure 3.3 Reactive power compensation of 380kV cables at 50kV level...........................................48
Figure 3.4 Reactors as part of the connection itself ........................................................................49
Figure 3.5 Shield break over-voltage signal at joint shield break during transient conditions..............50
Figure 3.6 Normal cross-bonding...................................................................................................51
Figure 3.7 The metal sheath of the cable is interrupted at the cross-bonding points .........................52
Figure 3.8 Kirke-Searing cross-bonding (the high voltage cables are transposed) .............................52
Figure 3.9 Detail of the cross-bonding in a Kirke-Searing cross-bonding system ...............................52
Figure 3.10 Screen induced voltage profile along the major section .................................................53
Figure 3.11 Kirke-Searing cross-bonding with midsection transposition ............................................53
Figure 3.12 Screen induced voltage profile along the major section .................................................53
Figure 3.13 Cross-bonding with 4 minor sections, used in case of span length issues........................54
Figure 3.14 Interconnection without and with transposition ............................................................54
Figure 3.15 Direct cross-bonding system (between C-D and D-X) ....................................................55
Figure 3.16 Cabinet with SVL’s connection .....................................................................................55
Figure 3.17 Single point bonded system with Earth Continuity Conductor (ECC) ...............................56
Figure 3.18 Cables in spaced trefoil formation with ECC cable .........................................................56
Figure 3.19 ECC transposed among the cable.................................................................................56
Figure 3.20 ECC is optimal transposed among the cable .................................................................57
Figure 3.21 ECC is optimal transposed among the cables in flat formation .......................................57
Figure 3.22 Solid bonding method (earthed at both side only).........................................................58
Figure 3.23 Reduction of sheath losses in a solid bonded system by transposition of the high voltage
cables itself..................................................................................................................................58
Figure 3.24 Screen induced voltage magnitudes in SB and in SB with short-circuits between screen
and armour..................................................................................................................................59
Figure 3.25 Screen induced current magnitudes in SB and in SB with short-circuits between screen
and armour..................................................................................................................................60
Figure 3.26 Multiple grounding systems in one and the same cable connection ................................60
Figure 3.27 Example horizontal snaking .........................................................................................62
Figure 3.28 General installation of horizontal snaking .....................................................................63
Figure 3.29 Example vertical snaking.............................................................................................63
Figure 3.30 General installation of vertical snaking .........................................................................63
Figure 3.31 Snaking of cables inside the joint bay ..........................................................................64
Figure 3.32 Lay-out of low frequency AC transmission system.........................................................68
Figure 3.33 Comparison of 50 Hz and 16.7 Hz 220 kV HVAC cable (3-core 1200 Cu) ........................68
Figure 4.1 Cables installed in open cut trench and direct buried ......................................................71
Figure 4.2 Ducts installation..........................................................................................................72
Figure 4.3 Double-circuit trench once the first layer of concrete has been poured.............................72
Figure 4.4 The two red ducts are the uppermost ones of the two circuits (the other four are under the
first layer of concrete). The green small ducts on both sides of each circuit are intended to house the
earth continuity conductor (ECC)...................................................................................................73
Figure 4.5 Ducts for communication cables on top of the HV circuits and third (and last) layer of
concrete ......................................................................................................................................73
Figure 4.6 Directional bore across intersection ...............................................................................74
Figure 4.7 HDD machine...............................................................................................................74
Figure 4.8 Micro Tunnelling / Sleeve Bore ......................................................................................75
Figure 4.9 Regular HDD with 4 tubes in one hole ...........................................................................75
Figure 4.10 HDD for 380 kV cable circuit with 6 single drillings........................................................75
Figure 4.11 The spacers are usually shaped to fix not only the HV cable ducts, but also, the earth
continuity conductor (ECC) ducts and communication cable ducts ...................................................76
Figure 4.12 Drawing of a typical single-circuit HV cable trench ........................................................76
Figure 4.13 Outline of infrastructure facilities under the road ..........................................................76
Figure 4.14 Duct pipes installed in open cut trench under the road..................................................76
Figure 4.15 Micro Tunnelling / Reinforced concrete sleeve ..............................................................77

10
Figure 4.16 Examples of flexible constrained, snaked & cleated cable installation arrangements........78
Figure 4.17 Cable Installation work in the tunnel by motor-driven rollers .........................................79
Figure 4.18 Cable installation work by a hauling machine................................................................79
Figure 4.19 Barge going down the Seine River in France.................................................................80
Figure 4.20 Typical shapes of railway loading gauges .....................................................................81
Figure 4.21 Cable drum trailer with two escort vehicles in a rural road in Provence, France (Boutre-
Trans link project) ........................................................................................................................82
Figure 4.22 Steel plates pathway leading to a joint bay of the Anholt land cable link, in Djursland
(Denmark) ...................................................................................................................................83
Figure 4.23 Water transportation of a cable by a dedicated carrier vessel in a river ..........................84
Figure 4.24 Land transportation by a dedicated carrier vehicle in a power station yard (1) (2) ..........84
Figure 4.25 Cable installation base in a power station yard (1) (2)...................................................84
Figure 4.26 Long cable transportation vehicle.................................................................................85
Figure 4.27 Preparation of a cable transport in Spain......................................................................86
Figure 4.28 Turn table on a cable vessel........................................................................................87
Figure 4.29 Loading on the turn table............................................................................................87
Figure 4.30 Testing the SVL by a DC supply ...................................................................................88
Figure 4.31 Measuring the zero-sequence impedance .....................................................................89
Figure 4.32 Measurement of the direct sequence impedance...........................................................90
Figure 4.33 Qatar, 2009, commissioning 16 km of 400 kV cable length at 275 kV for one hour .........91
Figure 4.34 Scotland, 2015, commissioning 45 km of 220 kV cable length at 180 kV for one hour .....91
Figure 5.1 Basic principle of Distributed Temperature Sensing (DTS) ...............................................94
Figure 5.2 Spectrum of backscattered light in three components .....................................................95
Figure 5.3 Example of DTS measurement system schematic for long cable circuit ............................96
Figure 5.4 Temperature profile in- and outside the cable (Source: book Pirelli, E. Peschke, R. von
Olshausen) ..................................................................................................................................96
Figure 5.5 Finite element analysis of the heat transfer in a cable and local environment ...................97
Figure 5.6 Setup and result of a cable temperature monitoring demonstration .................................97
Figure 5.7 shows the effect of cyclic loading regarding location of the fibre (The internal fibre unit is
incorporated with screen wires and under the metallic sheath, the external fibre is lashed to the outer
sheath of the cable) .....................................................................................................................98
Figure 5.8 Fibre in Metallic Tube (FIMT) ........................................................................................99
Figure 5.9 FO-cable inside the wire screen.....................................................................................99
Figure 5.10 Low profile OF unit .....................................................................................................99
Figure 5.11 Examples of using different types of FO designs depending on sheath bonding
arrangement arrangement ............................................................................................................99
Figure 5.12 FO-cable inside the bedding ......................................................................................100
Figure 5.13 Three core cable with FO-cables in the interstices between cores ................................100
Figure 5.14 Hot spot along a cable route .....................................................................................101
Figure 5.15 Equivalent circuit of PD measurement system using foil-electrode sensors ...................103
Figure 5.16 TDR traces after installation and after a cable fault.....................................................104
Figure 6.1 Sheath Fault Location HV underground cable by pulse current technique .......................109
Figure 6.2 Case of severe sheath fault .........................................................................................109
Figure 6.3 Example of extreme failure in a cable joint...................................................................110
Figure 6.4 Preparation of Site for Cable Joint Repair .....................................................................113
Figure 6.5 Example of Flow Diagram of Repair Works...................................................................116
Figure 6.6 Deck layout of a cable repair vessel.............................................................................119
Figure 6.7 Cable cut near the fault, after visual assessment or acoustic detection of the exact fault
location......................................................................................................................................120
Figure 6.8 ROV mounted saw......................................................................................................120
Figure 6.9 DTR is used to determine the fault on which side of the cut..........................................121
Figure 6.10 Stiff repair joint 3x400 kV (Photo ABB).......................................................................121
Figure 6.11 Repair joint being lowered down................................................................................121
Figure 6.12 Insertion of the second joint......................................................................................122
Figure 6.13 Cable on the quadrant ..............................................................................................122
Figure 6.14 Cable is laid back in an omega shape.........................................................................122
Figure 7.1 Aerial view of Wonthaggi desalination plant .................................................................124
Figure 7.2 88km route map from Cranbourne to Wonthaggi..........................................................125

11
Figure 7.3 500kV submarine cable design ....................................................................................130
Figure 7.4 Cable route Grenaa shunt reactor station to Trige substation ........................................131
Figure 7.5 Electrical losses along the cable circuit.........................................................................132
Figure 7.6 Capitalisation of losses................................................................................................132
Figure 7.7 The "Sorgente-Rizziconi "mixed line (on the left) and Submarine part of the "Scilla-
Villafranca" double-circuit link (on the right).................................................................................134
Figure 7.8 Design of 1500 mm2 armoured submarine single-core cable (Paper-Polypropylene
insulated) (on the left) and 2500 mm2 land single-core cable (XLPE-insulated) (on the right) .........135
Figure 7.9 Scilla-Villafranca land+submarine double-circuit single-core cables (on the left). View into
the vertical tunnel (of the land cable system) during construction (on the right).............................137
Figure 7.10 Cable bonding arranagement.....................................................................................137
Figure 7.11 Calculated screen induced voltages (on the left) and current magnitudes (on the right)
along the whole link with receiving-end complex power equal to 1160 MW+j 235 Mvar..................137
Figure 7.12 Transmission system for the Tokyo area ....................................................................138
Figure 7.13 Previous cable installation method with long drum......................................................138
Figure 7.14 Traverse Cable Pay-Out method ................................................................................139
Figure 7.15 Transmission system for the Nagoya area [3].............................................................139
Figure 7.16 Testing equipment [3] ..............................................................................................140
Figure 7.17 cable route...............................................................................................................141
Figure 7.18 Cross bonding scheme of the land cable section .........................................................142
Figure 7.19 220 KV Gemini wind farm connection.........................................................................144
Figure 7.20 Cable construction (picture by NKT)...........................................................................145
Figure 7.21 Overboarding of inline offshore joint ..........................................................................145
Figure 7.22 Cable route - The Outer Oslo Fjord Project.................................................................147
Figure 7.23 Cable cross sectional drawing - The Outer Oslo Fjord Project ......................................148
Figure 7.24 Cable route - Kollsnes –Mongstad project...................................................................149
Figure 7.25 Cable cross sectional drawing - Kollsnes –Mongstad project ........................................150
Figure 7.26 Cable route Bornholm cable.......................................................................................151
Figure 7.27 Cable route Aland cable ............................................................................................152
Figure 7.28 Tunnel design...........................................................................................................153
Tables
Table 1.1 Comparison XLPE versus Fluid Filled Insulated Cables......................................................18
Table 1.2 Reactive Compensation – Current State Development......................................................26
Table 1.3 Typical Reactive Compensation Requirements for situation of no-load...............................26
Table 1.4 Harmonic Filters and Mitigation Techniques – Current State Development.........................27
Table 2.1 Requirements under different conditions .........................................................................33
Table 2.2 Characteristics extruded cables.......................................................................................36
Table 2.3 Indicative figures charging current..................................................................................37
Table 3.1 Comparison of different Technologies for Reactive Power Absorption................................49
Table 3.2 Mitigation Method Summary...........................................................................................65
Table 4.1 Maximum AC cable length in consideration of transportation method in Japan...................83
Table 4.2 Comparison of a case of long cable transportation with standard transportation in Japan...85
Table 4.3 List of possible commissioning tests................................................................................87
Table 6.1 Cable fault location methods (ref. ICC Spring 2016 C11W).............................................108
Table 6.2 Indicative duration of installation works of high voltage joints and terminations ..............111
Table 6.3 Duration of repair works in case of cable insulation failure .............................................112
Table 6.4 Duration of repair works in case of cable termination failure ..........................................112
Table 6.5 Duration of repair works in case of cable joint failure.....................................................113
Table 6.6 Duration of repair works in case of cable sheath fault ....................................................114
Table 6.7 Duration of repair works in case of link box fault ...........................................................114
Table 6.8 Typical duration of repair works in a three-core submarine cable....................................123
Table 7.1 Dimensions and related information for the two cable designs........................................129
Table 7.2 Characteristics of the submarine cable ..........................................................................135
Table 7.3 Characteristics of the land cable ...................................................................................136
Table 7.4 Lengths of each single-core submarine cables ...............................................................136

12
Table 7.5 Lengths of each single-core land cables ........................................................................136
App Table D.3 Document version information...............................................................................194
Equations
Equation 1.1 ................................................................................................................................16
Equation 2.1 ................................................................................................................................35
Equation 2.2 ................................................................................................................................36
Equation 2.3 ................................................................................................................................36
Equation 2.4 ................................................................................................................................36
Equation 2.5 ................................................................................................................................38
Equation 2.6 ................................................................................................................................43
Equation 3.1 ................................................................................................................................46
Equation 3.2 ................................................................................................................................54
Equation 3.3 ................................................................................................................................57
Equation 3.4 ................................................................................................................................57
Equation 3.5 ................................................................................................................................58
Equation 3.6 ................................................................................................................................58
Equation 3.7 ................................................................................................................................58
Equation 3.8 ................................................................................................................................61
Equation 3.9 ................................................................................................................................62
Equation 3.10...............................................................................................................................67
Equation 4.1 ................................................................................................................................90

13

14
1. CURRENT STATE OF DEVELOPMENT
1.1 REASONS FOR GROWTH
There are several reasons for the very significant growth in use of long lengths of HV & EHV insulated
cables operating at AC. Whilst HV and EHV AC cables have been available for many years their use for
long length applications has often been limited by cable capacitance, dielectric losses, current carrying
capacity and high installation costs. Now with, cable designs using new materials, modern processing
techniques, the development of accessories, associated equipment and installation techniques, the
performance has very much improved. At the same time the cost of the supply and installation of
underground cables has been significantly lowered.
Until recently power transmissions networks have been principally designed based on overhead lines.
Firstly, because of costs but secondly on technical grounds because need to compensate for the cable
capacitance and higher installation costs due to environmental conditions.
Today there are often demands for transfer of power from renewable energy sources to the grid or
the need to provide electric power to remotely located plants as quickly as possible. In many
countries, the process of getting environmental approvals to build an overhead line may take many
years whereas the process of obtaining approval for installation of underground cables in public areas
such as roads and road reserves may be much quicker. The net result is that an AC cable link may
often be built in a relatively shorter time and this alone can be sometimes being the reason to justify
the AC cable link due to the quicker return on the investment.
In the case of connections for offshore winds farms the use of AC cable may often provide a lower
cost solution than DC cable when considering factors such as convertor costs, space requirements and
overall system losses. At the same time, there is also an interest to reduce transmission losses and
gain community support for improved public safety with a supply that is not effected by extreme
weather conditions, which appear to be more common these days.
When all these factors are considered it can sometimes be shown that there is a real advantage in
adopting the insulated cable transmission solution. [1]
1.1.1 Overall system description
High voltage cables form an integral part of electricity transmission and distribution network. The
system may consist of different types of cables and a wide range of necessary accessories. For long
cable links is it also necessary to compensate for the reactive power generation by installing large
reactors at appropriate locations. The design of a high voltage system is therefore extremely
important and requires highest quality of all components. A high voltage cable system for a long-
distance link must be custom-designed to consider power demand and installation conditions [2]. As
such it requires the highest competence from system planning through to final testing. [3]. Some
cables, have an increased insulation thicknesses to reduce the effect of reactive power on the cable
system.
An HVAC cable system consists of cables, terminations, joints, link boxes, earthing system, remote
monitoring systems and compensation reactors. For land and submarine applications cables are
available in both three and single core designs but typical for long lengths on land, single core cables
are used and for long lengths of submarine 3 core cables are used. In both cases, optical fibre
elements are included in the cable to measure the operating temperature as the conditions along the
route may be subject to may vary. Because of transportation restrictions a land cable is composed of
several lengths (each up to 2000m) which are installed and jointed together using pre-moulded joints.
In the case of a submarine system, since jointing small lengths together at sea is often not a feasible
solution, the cables are manufactured in one single length of over 100km. In the factory lengths (up
to10 - 20 km) are produced and jointed together using a flexible factory joint which has the same
dimension and properties as the cable. The transport capacities of cable installation vessels are in
thousands of tons.
Submarine cables are used for 3 main reasons: -

15
 Connecting two power grids
 Transport of offshore generated power
 Connection of an offshore platform
Compensation systems take up valuable space on off-shore platforms, so in the case of transport of
offshore generated power and connection of these offshore platforms, the compensation is therefore
usually done at the onshore ends of the cable where practical.
A submarine cable system is often designed to allow for redundancy because if a cable is damaged
and out of service, the economic consequences can be very high. For example, if a cable connecting
an offshore wind farm is out of service, the power cannot be transferred to land and the loss of
income for the owner is very significant. In addition, the time to carry out a repair operation offshore
can be quite long. Hence the solution is often to have two cables installed in parallel. Thus, if one
cable fails, the other one can still be operational for half of the power transfer capacity.
For submarine cables both ends of the cable are earthed, in strict contrast with and cables where
single core cables tend to be more common for long length and high power transfer requirements,
special sheath bonding arrangements are required. This so called, cross-bonded system provides the
advantage of increased rating. The disadvantage is that sectionalized joints (more expensive) are
required together with special link boxes which must be installed adjacent to the joint pits/manholes.
For submarine cables the joints are like the vulcanised factory joint and for repair vulcanized or
prefabricated joints are used. Typical maximum lengths for HV AC submarine Links may be up to 100
km for 400 kV or 200 km for 132 kV. For Land cables, longer lengths are possible by the addition of
reactive compensation along the route.
1.2 CABLE DESIGN TRENDS
Traditionally HV & EHV cable were made with paper insulation impregnated with oil under pressure.
There have been several designs and significant improvements made to increase the operating
temperature and ultimate ratings of such cables which have an excellent service record. However as
will be explained later there are several technical, installation and service limitations with these
designs, such that today there are very few plants worldwide that are producing AC cables using this
technology.
In the last two decades, we have seen a very rapid development of HV & EHV cables made using
Cross-linked polyethylene (XLPE) as an insulating medium. As an example, modern XLPE cables have
a lower dielectric constant and higher operating temperatures so they are many times more efficient
than the very early paper insulated oil impregnated cables. Whilst the more modern pressurized oil
filled cables have higher operating temperatures, the easier manufacturing process for XLPE cables
has led to a dramatic increase in the supply and use of HV AC cables.
In China for instance, there are now many factories and more than 75 insulation lines capable of
making HV cables. In 2014 more than 5800km of 110kV, 1100km of 220 kV and 100km of 500 kV
cable were produced in China and during the past 10-15 years more than 100,000km of HV & EHV
cables have been installed in the country. Currently world-wide there are now more than ten fully
qualified manufacturers of XLPE insulated AC cable rated at 500 kV and the demand for such cables is
growing significantly due to the growth of major cities.
Most of the HVAC submarine cables are 3 cores cables. Each core is composed of a conductor of
copper or aluminium. Often submarine cables have a copper conductor, the reason being that whilst it
is more expensive, its electrical resistance is lower, thus the required cross section is lower than
aluminium and hence less material is required for the outer layers. Furthermore, it was often argued
that the corrosion resistance properties of copper are better than aluminium, especially in a marine
environment, but this has little relevance, as a well-designed cable conductor will never experience
contact with sea water.
Hence, aluminium is now becoming more widely accepted because of its, lower cost, low weight and
better, strength to weight, mechanical properties. This is specially the case for deep installation and
dynamic situations.

16
For submarine cables the insulation system can be XLPE or Self Contained Fluid Filled. However today,
except for very high voltages, most of the HVAC submarine cables are made with XLPE insulation.
The insulation system is protected from the water by a metallic layer such as lead alloy or a welded
metallic sheath which is also used as electrical screen and a PE layer is extruded to protect this metal
sheath. The 3 phases are laid up together and optical fibre elements are often laid in the interstices
between the cores as well as some other materials e.g. PP ropes or PE profiles. The bundle is then
protected against mechanical damage by metallic armour made of steel wires. An outer protective
covering is often made of PP yarns applied outside the armouring.
For land situations, different types of cables LPOF (Low Pressure Oil Filled), HPGF (High Pressure Gas
Filled), HPFF (High Pressure Fluid Filled), EPR, PE, XLPE, GIL (Gas Insulated Lines-SF6) and
Superconducting Cables are available. However as mentioned above except in some countries where
there is considerable experience with fluid filled cable systems most new long length AC cable links
are being supplied with single core XLPE cables. Where there are very high current carrying capacity
requirements copper conductors are still specified. Due to lighter weight and requirement for longer
lengths Aluminium conductor cables are now becoming far more common. The trend to long lengths
of cable with large conductors means that very careful consideration needs to be given to the system
design to consider the mechanical forces exerted due to thermal expansion under load. [4]
1.2.1 Insulation thickness
The insulation thickness of XLPE cables is mainly determined by the withstand voltage. In case of EHV
cables of long length the insulation thickness will also influence the reactive power produced by the
cable. The formula below shows that the reactive power will mostly depend on voltage, but also on
the capacitance and frequency.
= 2. . . .
Equation 1.1
Where:
Qcable is the reactive power in Var
f is the power frequency in Hz
C is the cable capacitance in Farad
V is the line voltage in Volt
Reactive compensation is usually carried out by the installation of shunt reactors. They make the
system more complex, due to electrical and spatial issues, additional losses and need for redundancy.
Therefore, a reduction of produced reactive power may be of interest. This can be done by increasing
the insulation thickness or by decreasing the conductor size. The latter however, often not being
practical.
A thicker insulation results in less capacitance, which will be translated in less reactive power
compensation and in less dielectric loss and charging current. However, increasing the insulation
thickness of a XLPE cable has some negative consequences. Most important of which being the
maintenance of the quality of the extrusion process when processing very long runs of HV cable.
Whilst the insulation thickness may only be increased by a few millimetres this could still be
considered of benefit for long lengths of EHV cables. Some of the additional costs of a cable system
with more insulation can be recouped by less investment in reactive compensation and lifetime lower
system losses.
Figure 1.1 shows an example of a 275 kV XLPE cable with an insulation thickness of 27 mm.

17
Figure 1.1 275 kV XLPE cable configuration
Figure 1.2 indicates the relationship between the insulation thickness and the capacitance and
charging current for the cable as shown above.
Figure 1.2 Capacitance and charging current per unit length in each phase
Figure 1.3 indicates calculation results on relationship between the insulation thickness and the
inductive reactance required or the dielectric loss for the cable as shown above
Figure 1.3 Inductive reactance required and dielectric loss per unit length in each phase
These effects are even more significant for higher voltage cables such as 400 kV and 500 kV.
XLPE
27 mm

18
1.3 CABLE TYPES
1.3.1 SCFF, HPOF, cable designs
Fluid filled cables
These cables were developed over 100 years ago, based on the available technologies of the time.
Early Mass impregnated paper insulated cable designs at higher electrical stresses failed due to
discharges in voids formed in the butt-gap spaces between paper tapes because of expansion of the
impregnating compound on heating followed by insufficient contraction to fill the insulation completely
when the cable was cooled. In the early 1920’s pressurised fluid filled cables were introduced to
eliminate this problem by maintaining the liquid impregnant at a positive pressure inside the metal
sheath. Fluid filled cables are manufactured with a reinforced metallic sheath for self-contained fluid
filled cables or the cores are put inside a steel pipe to contain the oil for pipe type cables. During cable
operation, the surplus oil due to expansion with the increase in temperature is collected in the
pressure tanks and then pumped back into the cable system when the cable cools.
With the requirement to maintain a positive oil pressure in the cable system, the fluid filled cables
present installation difficulties and higher maintenance costs. For self-contained cables, ducts for oil
flow must be incorporated in the cable design. For single core cables, typically the oil duct is in the
middle of the conductor and in the three core cables the ducts are incorporated in the interstices
between the cores.
As mentioned with the Introduction of the XLPE insulated cables, fluid filled cables are slowly being
replaced with XLPE cables. As such the demand for new fluid filled cables has significantly decreased
however there are still many existing fluid filled cable circuits in operation and in the USA high
pressure pipe type cable systems are still being installed.
Comparison of fluid filled cables and XLPE cables
The construction of the XLPE cable compared to fluid filled cables is simpler. At higher voltages, XLPE
cable designs will generally still have a metal or metal laminate sheath, e.g. aluminium, copper,
stainless steel, lead alloy etc. to prevent water vapour passing into the cable insulation. Table 1.1
gives an overview:
Table 1.1 Comparison XLPE versus Fluid Filled Insulated Cables
Characteristic XLPE insulated Fluid filled insulated Remarks/Comments
Dielectric losses Low e.g.
Relative
Permittivity 2.5
DLA = 0.001
Paper high e.g. Relative
Permittivity 3.5 DLA = 0.0028
PPL medium e.g. Relative
Permittivity 2.8 DLA = 0.0014
Dielectric losses in FF cables are
higher compared to XLPE cables
Metal sheath or
pipe
Welded, laminated
or extruded is
required to prevent
moisture ingress
Extruded metal sheath or
steel pipes required
Alternative XLPE cable sheath
designs can lead to lighter and
less costly cables
Sheath and over-
sheath reliability
Sheath failure will
allow moisture
ingress and may
lead to insulation
failure.
Over-sheath
damage can lead to
sheath corrosion.
Pressurised sheath leads to
increased likelihood of
sheath failure compared with
XLPE. Sheath failure will lead
to leaks rather than
insulation failure.
For pipe type cables
corrosion is a problem
Regular voltage test of the over-
sheath required for both self-
contained technologies.
Cathodic protection can be used
for pipe type cables

19
Need to maintain
pressure
Not required Need for an oil pressure
feeding system including oil
treatment plant, pumps, oil
pressure tanks, gauges, and
oil piping work in joint bays.
Often oil feeding is done at
locations in the middle of the
route (For self-contained
cables typically every 3 km)
XLPE cables do not incur this
expense and risk of failure of any
of these components.
For submarine routes, feeding
points in the middle of the route
difficult.
Need for pressure
monitoring
Not required Need for pressure
monitoring and alarms at
each oil feeding over the
length of the cable.
XLPE cables do not incur this
expense
Maintenance
requirements
Generally, over-
sheath voltage
tests, sheath
bonding and
earthing
maintenance, and
visual inspections
Additionally, regular
maintenance to check oil
feeding equipment and the
quality of oil.
Fluid filled cables require
additional maintenance
Problems when
there is a
difference in level
of the electrical
circuit
No problem Need positive oil pressure in
the cable system (cable and
accessories) at the highest
point along the cable route.
For self-contained cables
requires stop-joints when the
difference in height level is
too high
Self-contained cables may incur
additional cost and maintenance
for routes with differences in
level
Problems with oil
leakage
No problem Oil leakage must be
detected, located and
repaired.
Oil leakage can be difficult to find
and may create environmental
problems.
Manufacturing
The primary aspect of a cable is the insulation. XLPE cables are manufactured by a triple extrusion
process. Fluid filled cables are manufactured with wrapped paper tapes or poly propylene paper
laminate tapes wound around the conductor, which then are impregnated with fluids.
Installation
Installation of the fluid filled cables is more difficult compared to XLPE cables. For self-contained fluid
filled cables during transportation & cable pulling positive pressure needs to be maintained within the
cable, which is done by utilising an oil tank always connected to the cable. It is difficult to manage this
tank during transportation and installation. Special consideration is given when designing the hydraulic
system to maintain positive oil pressure in the cable system, depending on the hydraulic section
considering height variations, stop joints and feed joints. Pressure tanks and gauges are utilised when
finalising the hydraulic cable system design to maintain positive oil pressure in the cable. This makes
the system design more complex by having an electrical and a hydraulic system both of which need to
operate simultaneously and at all times. This is more expensive when compared to the XLPE cable
system, where there is no hydraulic component.
Jointing
In paper insulated fluid filled cables the insulation around the conductor joint is built layer by layer
with paper. For self-contained fluid filled cables during jointing works positive oil pressure is
maintained within the cable system to avoid trapping any air within the paper layers.
For XLPE cables prefabricated joints are available.

20
Jointing of fluid filled cables involves a much greater number of activities and is therefore more
difficult and time consuming when compared to XLPE cables. Availability of experienced staff is
becoming an issue as the use of fluid filled cables is decreasing, experienced staff are retiring and
replacements are hard to find.
1.3.2 XLPE Cable designs
Increasing operation and maintenance costs for fluid-filled cables prompted the search for other
materials. Polyethylene PE was originally chosen but the low operational temperature was the main
limitation. The solution was - XLPE - cross-linked polyethylene which is now one of the most common
and well established insulation materials in modern extruded high voltage cable design. Since the
1970’s, the fluid-filled cables have therefore gradually been replaced by extruded dielectric cables
(Figure 1.4).
XLPE cables have today a rapidly growing market share for both on land and for submarine
applications. A major reason for the XLPE success is the excellent electrical, mechanical and thermal
properties of the material. The most advantageous features are the low dielectric losses, the low
dissipation factor, the high electrical breakdown strength, the high modulus of elasticity and the high
tensile strength. Low operating and low maintenance costs, combined with good system availability,
results in a low life time cost for the XLPE cable system.
Significant technological advancements have been made to the XLPE material itself, as well as the
manufacturing methods since the introduction in the mid 1960’s. Very early it was understood the
importance of quality in manufacturing as well as of the purity of the raw material, therefore the triple
extrusion and dry curing techniques were introduced into the XLPE manufacture.
The insulation material is vulcanised under pure nitrogen pressure after the triple extrusion process,
e.g. the inner semi-conductive screen, the XLPE insulation, and the insulation screen are all extruded
simultaneously. This takes place in a closed environment, ensuring the extra high cleanliness and
quality of the XLPE insulation system.
XLPE - cross-linked polyethylene - is a polymeric insulation material based on pure polyethylene, PE.
In the vulcanisation process after the extrusion of the material, the peroxides in the compound at
elevated temperature and pressure form an intricate and complex cross-linked (i.e. cross-bonded)
mesh. It is the breakdown of the peroxides into free radicals which react with hydrogen in the
polyethylene chains that enable the polyethylene chains to react with each other to form the cross-
linked structure. The difference between PE and XLPE is considerable, both in mechanical and
chemical terms, and the characteristics of the XLPE are very well suited for electrical applications.
XLPE was, in fact, developed because of its excellent electrical properties, especially regarding losses.
The low dielectric constant and the negligible dissipation factor are the reasons for the very low losses
in the XLPE cable.
XLPE is a suitable insulating material for conductor temperatures up to 90 °C which is the normal
operating temperature for XLPE cables. The cables can however withstand up to 250 °C under short-
circuit conditions. Consequently, there is both a high over-load potential and a high safety margin in
the cables.
Figure 1.4 HV XLPE Cable with corrugated Aluminium sheath

21
1.4 NEW INSTALLATION TRENDS
In the past when it came to crossing difficult terrain, rivers, roads and train tracks etc. overhead lines
appeared the only solution, but today we have directional drilling techniques with a capability of
drilling of up to 2.5 km in length. Whilst there has always been the option to use multipurpose tunnels
newer construction techniques using drilling machines or precast assemblies have made such
structures more cost effective. Similarly, where there are open areas, mechanised cable laying
systems like those used for the installation of pipe lines, are becoming practical for cable laying. Also,
the ability to install submarine cables has advanced considerably during the past 20 years. The net
result of all these factors is making a reduction in the overall cost of long length AC cable links.
1.5 ASSOCIATED EQUIPMENT
Electrical fields in high voltage Terminations and Joints must be controlled so that the electrical
strength of the insulation of the surrounding material is not put at risk. Depending on the voltage
level, different methods exist, e.g. geometrical, refractive or resistive field control. Geometrical field
control is mostly achieved with pre-moulded stress cones and splicing blocks.
The outer part of a cable termination may consist of a porcelain or composite insulator with sheds
depending on the environment. The internal field control component is normally a pre-moulded stress
cone and the internal space may be dry or filled with insulating oil. Different types of terminations
exist such as those for outdoor installations or indoor GIS installation.
Several different types of joint exist for land as well as for submarine applications. Many joints have a
pre-moulded one-piece joint body and joints are available either with or without screen interruption
depending on the sheath bonding methods for the system. These prefabricated parts and materials
for accessories comprise stress cones for field control, insulators and housings. The materials used are
EPR or EPDM, Silicone, Epoxy, Porcelain, Paper and special insulating fluids. The installation of these
terminations and joints requires very skilled well trained personnel.
1.5.1 Joints and Terminations
Prefabrication means lower risk of failures due to installation by making the process of installation
easier and hence providing better system reliability. The reliability and performance of a HV/EHV cable
circuit depends on the quality of the cable, the accessories as well as on the quality of the installation
of the system components on site.
The cables are produced in the factory under a controlled process, using selected and clean material
of highest quality and are submitted to severe routine tests before delivery.
It is important that the same quality standards are applied to all components of the accessories during
their manufacturing and in particular during their assembly on site. [5]
Today’s HV/EHV cable accessories are manufactured by using high quality materials and sophisticated
production equipment. Recent technical and technological developments in design, manufacturing and
testing have made it possible to have pre-moulded joints and stress cones for terminations up to 550
kV as well as cold shrink joints for up to 420 kV. To avoid, that internal failure rate of accessories on
XLPE cables become higher than other system equipment, the focus on quality control during
manufacturing and assembly of the HV accessories is of importance. It is vital, to manage the
interface between the cables and the accessories to reduce the potential technical risk. Usually
HV/EHV cables and accessories used for important HV Cable circuits have passed a system type test.
[6]
System approach versus Component approach
Some Utilities adopt the “system approach” by purchasing the cables as well as the major accessories
from same supplier. Some of these Utilities would also request that the link should be installed by the
supplier or by a contractor under the supplier’s supervision in a “Turn Key” fashion. The main
advantage of this approach is that the entire responsibility for the materials and workmanship is
clearly defined.
Some utilities adopt the “component approach” by purchasing the cables and the accessories from
different suppliers and to entrust the installation to a third party.

22
However, in all cases, it is imperative, that the installation be carried out by qualified accessories
installation teams, who follow the assembly instructions provided by the supplier and that the
procedures for the reinstatement of the cable and accessories are strictly observed.
Major Accessories needed for a complete HV/EHV Cable Circuit using XLPE insulated
cables: -
Air Insulated Terminations (Outdoor Terminations)
Air insulated terminations are generally used outdoor, to terminate the HV/EHV cables in air insulated
substations or on poles. They may have porcelain or composite insulators (Figure 1.5) and may be
fluid filled or unfilled (dry). The design adopted may depend on the local environment with respect to
required BIL, maintenance requirements, pollution, reliability and altitude. More and more composite
insulators are in use for extra high voltages. The technical and economic advantages are of
significance and lie in their low weight, ease of handling, safety in case of explosion, intrinsic
hydrophobic characteristics and reliability under exceptional events such as earthquakes, system faults
and vandalism. [7]
Figure 1.5 Typical HV termination with composite type insulator (Source: CIGRE WG B1.29)
GIS and Oil Immersed Terminations (Transformer Terminations)
EHV and HV Cables may also be directly terminated in SF6 insulated switchgear (GIS) (Figure 1.6) and
transformer to eliminate air-insulated interfaces.
GIS and oil immersed terminations (transformer terminations) have very similar construction. The
electrical stress control for GIS and oil immersed terminations follows the same approach usually
employed for outdoor sealing ends i.e. it uses a pre-moulded elastomeric stress relief cone.
Proven dry versions of HV/EHV GIS and transformer terminations are available, compared to fluid
filled types they have decisive advantages such as no risk of leak, maintenance free, simplified
installation (plug-in types) and safety in case of failure. IEC 62271-209 defines the interface between
HV/EHV Cable and Switchgear. [8]
Figure 1.6 Typical EHV GIS with GIS Terminations installed in horizontal position (Picture by
courtesy: Nexans)

23
Joints
Today’s HV/EHV voltage joints usually have pre-moulded elastomeric joint bodies with additional
covering against moisture and mechanical damage. The additional covering could be heat shrink tubes
or metal housings with additional insulation housings/coffins. The joint protection should be chosen in
view of the type of cable sheath and in particular in view of the location of the joints. Non-buried
joints may be in tunnels (Figure 1.7), on bridges, in underground chambers or similar enclosures. [9]
Figure 1.7 Typical non-buried joints installed in a cable chamber/manhole (CIGRE WG B 1.29)
The design of the cable joint needs to consider the sheath bonding method and must be able to
withstand sheath voltages under fault conditions. For various sheath bonding systems, accessories
suppliers offer various types of joints and their associated hardware: -
 shield break or cross-bonding joints
 earthing joints
 straight joints
Their application is illustrated in chapter 3.3 Sheath Voltages of the present Brochure.
Various accessories suppliers recommend various methods allowing a proper positioning of the pre-
moulded or cold shrink joint body and offer the adequate special installation tools.
The magnitudes of the forces and movements generated by the cable on the joints (and
terminations), must be taken into consideration by the system designer. This includes considering the
civil work aspects of the HV/EHV circuit. The accessories must be positioned and supported. Similarly,
the cable must be laid and fixed in such a manner that avoids exposing the cable accessories to
inappropriate mechanical stress. [10]. See also Chapter 3.4 'Thermo-. Mechanical Forces' of the
present Brochure.
Equipment for earthing, bonding and screen disconnection
Most of the HV cable accessory suppliers as well as various specialized companies offer the equipment
for an efficient and reliable earthing, bonding (Figure 1.8) and disconnection of cable screens
depending on the design of the circuit and described in chapter 3.3 Cable Sheath Bonding of the
present Brochure.

24
Figure 1.8 Typical Cross-bonding cabinet equipped with SVL (Picture by courtesy: Nexans)
Handling of Optical Fibres
In case of use of HV/EHV cables with integrated optical fibres for DTS (Distributed Temperature
Sensing) and other applications, the HV/EHV accessories suppliers offer specific solutions for a proper
handling of the fibre and fibre splice box at the location of the HV/EHV joints (and terminations).
Quality Management
Joints and terminations are delivered on site as kits, which in turn are made up by numerous
components. In order to guarantee a high reliability of the accessories used in a cable circuit it is
essential that: -
 full quality control is exercised in the manufacture of terminations and joints and their sub-
components
 full quality control is exercised on site with respect to the jointing area set up and assembly of
the accessories
 HV/EHV jointers in charge of the assembly have sufficient knowledge and training, are using
the adequate installation tools and are following strictly the assembly instruction of the
accessories supplier. [11]
Commissioning voltage test
Commissioning voltage tests which are usually applied to complete HV/EHV cable circuits are of a
particular importance for the numerous cable accessories and their reliability.
PD (Partial Discharge) Measurement/Monitoring
PD Measurement / Monitoring is becoming more common on some HV/EHV cable circuits (see chapter
5.5 Monitoring PD of the present Brochure). Accessories suppliers offer joints and sealing ends with
various types of sensors allowing an efficient and reliable measurement/monitoring. The object being
to be able to locate any partial discharges coming from installation errors or a deterioration of the
quality of the insulation part of the accessories in service over the years. [12]
Maintenance
Modern HV/EHV Cable Accessories for XLPE insulated cables are very nearly maintenance free. In
polluted areas insulators of outdoor terminations need to be cleaned following standard procedures
and frequencies applicable for all types of HV Outdoor Insulators.
Spare Accessories / Components
For the maintenance of important HV/EHV cable circuit’s (Figure 1.9) end users usually maintain a
number of accessories for eventual repair work and to reduce the time duration for outages. Other
critical points in regards to spares are discussed in chapter 6.5 Rapid Response Repair Options of the
present Brochure. Spare accessories and components must be packed by the supplier for long-term
storage and kept in stock by the end user under controlled conditions.

25
Figure 1.9 345 kV PPLP Pipe Type cable terminations, cable route length 30 km
1.5.2 Surge Arrestors to protect cable and accessories
Whilst it is important to install surge arrestors at locations where an overhead line enters a substation
to protect the substation equipment from the effects of lightning, there are several views regarding
the need to use surge arrestors to protect cables which are connected to overhead lines.
Where a long cable circuit is directly connected to the substation in an outdoor switchyard then surge
arrestors are typically used to protect the substation equipment from a phase to earth overvoltage as
for an overhead line. Where there is a short section of cable inserted in an overhead line circuit
(siphon) then a similar arrangement is also provided. It is however questionable how effective this will
be on a long cable link due to the high impedance of the cable which may reduce this transient effect.
To protect cables sheaths from phase to earth faults, surge protection devices, Sheath Voltage limiters
(SVL’s) are used in the link boxes and at termination structures. Where EHV cables are connected to
GIS terminations within a substation special surge protection is provided directly at the GIS.
1.5.3 Reactive compensation to offset cable capacitance
HV & EHV cables are capacitive in nature and this capacitance raises the voltage, so if
uncompensated, the voltage at the receiving end an EHV cable will be considerably higher than the
voltage at the sending end.
The capacitance of the cable also reduces the percentage of real or usable power arriving at the load
because of the capacitive charging currents. The cable should be rated for the total current and thus
the ability of the cable to transfer real power is reduced unless there is reactive compensation.
Reactive power compensation is provided by supplying inductive power which acts in the opposite way
to capacitive power and consequently cancels it out. Small amounts of capacitive reactance can be
compensated by generators or a section of overhead line in the circuit, but for the surplus of reactive
power produced by HV and EHV cables, additional devices are necessary. Reactive power
compensation is usually applied in form of passive compensation (shunt reactors) (Figure 1.10) and
sometimes by active compensation FACTS (Flexible AC Transmission Systems), like SVC and
STATCOM.
It should be noted that even by compensating for the capacitance at each end of the cable, the
voltage somewhere in the centre of the cable can become unacceptably high and may prematurely
age the cable insulation unless this is considered at the cable design stage.
The effect can be most noticeable at times of low load when the capacitive reactance is greater than
the load related voltage drop.

26
Figure 1.10 Typical Shunt reactor for reactive power compensation
FACTS are costly and mainly applied if there are more system issues to solve than reactive power
solely. FACTS can be a solution in case of voltage instability, unbalanced power distribution or phase
angle problems. The current state of reactive compensation technology is summarised in table 1.2.
Table 1.2 Reactive Compensation – Current State Development
Type of
technology
Technology Technical Maturity
Passive
Shunt Reactor (Air Core) Mature technology, also known as dry-type.
Shunt Reactor (Oil Core)
Mature technology. Available in two design
configurations; coreless and iron core (and either self-
cooled or force cooled).
Both types can be constructed as single phase or three
phase units and are similar in appearance to conventional
power transformers.
TRACOM
Switchable device, functioning as regular transformer
during day hours and as a reactor in weak loading
situations.
Active
Static VAR Compensation (SVC)
The SVC is a shunt connected device which can be used
for the voltage control by using reactive power
compensation. It is a mature technology with many
installed projects. The main disadvantage is the
production of harmonics, which requires a series of filters
to remove or to reduce the harmonic distortion when
implementing a SVC.
Static Synchronous Compensators
(STATCOM)
The STATCOM (or SVC light/plus) is the most mature
second generation technology, with a significant number
of installed projects. The STATCOM offers continuous and
dynamic voltage support, has a faster response,
introduces less harmonic distortion and has a smaller
footprint (40-60%) of that of a similar sized SVC. The main
drawback is that it is more expensive (1.2 to 1.8 times)
than the SVC.
The size of reactors depends on reactive power generation in the cables (Table 1.3) which in turn
depends on the voltage, capacitance and length. As a guide situation of no-load:
Table 1.3 Typical Reactive Compensation Requirements for situation of no-load

27
Voltage
[kV]
Conductor cross
section
[mm
2
]
Capacitance
[µF/km]
Reactive power @ 50Hz
[Mvar/km cable circuit]
Reactive power @ 60Hz
[Mvar/km cable circuit]
132 1.200 0,29 1,6 1,9
132 2.000 0,34 1,9 2,2
220 1.200 0,21 3,2 3,8
220 2.000 0,24 3,6 4,4
400 1.200 0,17 8,5 10,3
400 2.000 0,19 9,5 11,5
1.5.4 Harmonic Filters and Resonance Mitigation Techniques
EHV cables have high levels of capacitance across the insulation, which is between the conductor and
the external sheath. This capacitance will resonate with the inductance of the external system at a
particular frequency. In the power system, not only the power frequency is present but also higher
harmonics from power electronics by end users, HVDC connections and wind farms. Harmonics and
inter-harmonics could also be from switching actions, especially transformers and cables. Adding a
long AC cable to an existing network can also amplify present higher harmonics due to resonance.
Resonance causes damage to components of the grid and must be avoided. This is mainly done by
passive filters or in special cases by means of active filters.
The current state of harmonic filter mitigation techniques is summarised in Table 1.4 below. Chapter 3
'System Design' discusses the capability of each of the compensation technologies.
Table 1.4 Harmonic Filters and Mitigation Techniques – Current State Development
Technology Technical Maturity
Single Tuned Filters (STFs)
Conversion of Mechanically Switched Capacitors (MSCs) to
STFs to filter a single harmonic component or alternatively
design from new. STF’s offer harmonic mitigation and
voltage support. A mature technology.
Double Tuned Filters (DTs)
Filters two harmonic components. DT’s offer harmonic
mitigation, voltage support and a smaller space
requirement. A mature technology.
C-Type Filter
For filtering, higher order harmonics. C-Type filters have the
benefit of second order filter but negligible losses at
fundamental frequency. A mature technology.
Series connected Filter
Blocks harmonic current flow by introducing high
impedance. Series connected Filters are not commonly used
and if used tend to be in conjunction with shunt filters. A
mature technology.
Distributed Power Flow Controller (DPFC)
DPFC are like a unified power flow controller (UPFC). Korea
Electric Power Corporation (KEPCO) has an installed device.
This is a new and novel technology.
Current Source Converter (CSC) based shunt
Active Power Filter (APF)
This is a Current Source Converter (CSC) based shunt Active
Power Filter (APF) using IGBT devices. A prototype has been
operated at 31.5kV PCC Denizli-2-TS distribution substation
in Turkey.
Replace MSC by Converter Based STATCOM
STATCOM will not add to system capacitance in problematic
areas.

28
1.6 RELIABILITY OF SUPPLY
Due to the high cost of the AC Link and the fact that unlike an overhead line any fault in a cable is not
visible, reliability of supply is therefore an issue of major importance when considering a long cable
link.
All new designs of cable and accessories are required to be fully Type Tested to IEC, AEIC and other
standards. Most suppliers then carry out long term pre-qualification test of the cables together with
accessories to the requirements of these standards to prove the technology. For any long length cable
project, it is certainly most advisable to ensure that such data is available.
Cable manufacturers ensure that every drum length of cable is fully tested to comply with routine test
requirements and similarly the cable accessory manufacturers carry out test on the HV stress control
components of all HV and EHV cable accessories before dispatch.
Long length AC cable connections on land require numerous joints, also for long submarine cables
there may be quite several factory joints so that this becomes a significant issue in terms of the
system reliability. As will be discussed later special consideration needs to be given to all the aspects
that can provide the best possible system reliability.
Some aspects that should be considered in respect to reliability of the AC cable link, whether it is on
land in water depends are:
1. Selection of the most appropriate cable and accessory design for the particular section of the
circuit.
 A robust cable design – different cables are often required on sections of long links.
 A cable design with reduced stress – see section 1.2.
 Best possible accessories.
2. Suitable system design engineering to ensure cable and components will not be installed or
operated beyond their design limits.
 Making longest possible lengths to reduce number of joints.
 Appropriate route selection and design to guarantee reliable life-time operation in that
environment. see section 2.
3. Have an experienced installation team to avoid any damage to the cable or other components
during installation.
 Fully qualified and experienced personnel carrying out and supervising the installation of
cables and accessories.
 Appropriate quality assurance systems to ensure the correct use of cable constraints,
backfilling, reinstatement and protection.
4. Appropriate on-site testing and commissioning which does not damage cable or components.
 Carry out commissioning tests – HV AC and PD detection during commissioning wherever
possible.
5. Include real time temperature monitoring of the cable and accessories.
6. Implement a rapid fault location and repair procedure - see section 6.1.
7. Carrying out routine maintenance on certain components.
 Regular checking for changes in the environment which might affect the operating life-time of
the circuit.
8. Consider having two cable circuits in parallel to provide an N + 1 capability.
In the following chapters, it is intended to address all of these issues and to provide some examples
of projects which are operating with good system reliability.

29

IMPLEMENTATION OF LONG AC HV AND EHV CABLE SYSTEMS

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to IMPLEMENTATION OF LONG AC HV AND EHV CABLE SYSTEMS

Similar to IMPLEMENTATION OF LONG AC HV AND EHV CABLE SYSTEMS (20)

More from Power System Operation

More from Power System Operation (20)

Recently uploaded

Recently uploaded (20)

IMPLEMENTATION OF LONG AC HV AND EHV CABLE SYSTEMS