Power Cable
HANDBOOK Harmonisation of Power Distribution Systems in the Lower Mekong Subregion 2011 Edition
Underground Power Power Cable Ca ble HANDBOOK
Copyright © 2010 International Copper Association Southeast Asia Ltd All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means, or store d in a database or retrieval system, without the prior written permission of the publisher. Printed in Singapore
Underground Power Power Cable Ca ble HANDBOOK
Copyright © 2010 International Copper Association Southeast Asia Ltd All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means, or store d in a database or retrieval system, without the prior written permission of the publisher. Printed in Singapore
Foreword Overhead power lines, a familiar sight in many old cities, are slowly slowly disappearing from city skylines. A noticeable trend towards towards underground underg round power cabling is gathering momentum around the world. world. And nowhere else is this more evident than tha n in Asia. City after af ter city, professionals professionals involved in city planning planni ng and development, from planners and architects architect s to consultants and engineers, engineer s, are deciding in favour of of underground power distribution, realising the immense benefits that it offers. Underground power cable systems offer far reaching benefits. Not only do these systems d ramatically improve the skyline of a city, they also result in better environment, lower power distribution costs, higher reliability and greate greaterr protection against hazards hazard s associated with overhead power power lines. This trend is a significant devel development opment especially for countries countr ies in the Lower Mekong Mekong Subregion (LMS) and for the copper industry industr y. Copper cables, due to much higher conductivity and other properties, are better suited for underground appli applications cations.. This means means relia reliability bility and quality of pow power er suppl supply y, critical factors in reducing technical losses in the electricity grids of Utilities in the LMS. In deciding on the choice of conductor for an underground cabling system, Utilities have to consider the basic properties of the conductor material - electrical resistivity, tensile strength, melting point and coefficient of therma thermall expansion. Other factors to consider include: Space
Underground cabling puts pressure to keep space requirements for trenches or ducts to the minimum. Conductivity, resistance and losses of the conductor in relation to its diameter will therefore determi ne its space efficiency. Allowable space must must also be provided for therm thermal al expansion of the conductor. Current Carrying Capacity
The higher the conductivity of the material, the higher is the current rating for the same overall diameter of the conductor. Ruggedness
Deterioration at cable joints and risk of mechanical damage can be minimised by the hardness of the conductor material. Its resistance to corrosion can protect joints against water penetrat penetration. ion. And
functional problems due to heating and developing hot spots will be less prone in conductors with the heating ability to withstand overloads/surges. Studies and experiences of Utilities have shown that transformers and power cables are the two largest ‘loss makers’ in the electricity grid. So, much can be done in these two areas to help reduce significant losses in the LMS power distribution systems. I am therefore pleased to note that the development of this underground power cable handbook is a progression of the power and distribution transformer handbooks. The development and harmonisation of technical specifications for transformers as well as power cables, in relation to international standards and best practices, can help to narrow the differences and gaps for LMS Utilities to work towards further reducing losses in their electricity grids. Development of handbooks is only an academic exercise. Reduction will only come when the guidelines and recommendations are followed and implemented by all associated with the design of the electricity grid, specifying the standards of equipment for procurement and subsequently operating or maintaining them.
Victor Zhou Director - China & Southeast Asia ICA Asia
Introduction The Lower Mekong Subregion (LMS) Harmonisation Programme Cambodia, Lao People’s Democratic Republic (Lao PDR), Thailand and Vietnam have achieved different levels of economic development. These countries in the Lower Mekong Subregion (LMS) have strong economic inter-dependence. Being developing countries, their power distribution systems, an essential infrastructure, play a significant role in the economic development. Energy end-users are dependent on the availability, reliability, and quality of electricity from the power distribution systems. The level of development and advancement of power distribution systems has direct impact on the developmental potential and economic growth, especially in urban cities. The power distribution systems in the urban areas of these LMS countries, however, do not have the same level of development. It is widely acknowledged that harmonisation in the development of power distribution systems can benefit these countries and accelerate their economic growth. In 2005, six power partners entered into a Memorandum of Understanding (MOU) to share the intent of working together towards harmonisation of power distribution systems in the following four LMS countries: Cambodia, Lao PDR, Thailand and Vietnam. The founding partners are:
•
Electricité du Cambodge (EDC), Cambodia
•
Electricité du Laos (EDL), Lao PDR
•
Ho Chi Minh City Power Company (HCMC PC), Vietnam
•
Hanoi Power Company (HNPC), Vietnam
•
Metropolitan Electricity Authority (MEA), Thailand
•
International Copper Association Southeast Asia (ICASEA) [formerly known as Copper
Development Centre • Southeast Asia]
This led to a study of power distribution systems of the power partners in Cambodia, Lao PDR and Vietnam; and the preparation of a regional cooperation roadmap and action plan. Building on the success of the first MOU, ICASEA and MEA inked a second MOU to continue their strategic partnership in conducting further studies and facilitating programmes as outlined in phase 2 of the road map and action plan. This impetus is to enable the LMS countries to make further progress towards harmonisation and the realisation of the objectives as set out in the MOU with all the power partners. The study of power distribution systems in the LMS countries under the first MOU had revealed that there are many differences in the power distribution systems in this region. The objective of this second MOU was to narrow down the differences in six key areas a nd enable the LMS countries to move towards greater harmonization of their power distribution systems. Joining this Harmonisation Programme in 2009 were: Danang Power Company (DNPC), Vietnam HaiPhong Power Company (HPPC), Vietnam And in 2010, Provincial Electricity Authority (PEA), Thailand Central Power Corporation (EVNCPC), Vietnam
Preface
Loss in the Power Distribution System is a common and pressing concern expressed by Utilities in the LMS. Reducing loss is the priority given the energy shortage arising from rapid economic growth and high oil prices. A Regional Loss Reduction Workshop for LMS Utilities was held in Phnom Penh, Cambodia on 18 & 19 March 2008. It concluded with a consensus to, amongst other areas of collaboration, reduce losses in the Power Distribution Systems of EDC, EDL, HCMC PC and HNPC by harmonising technical specifications and developing a best practices handbook for energy efficient equipment based on international standards. The views of and input from participating Utilities were crucial in the development of technical specifications for the harmonisation of power equipment in the LMS. Only with acceptance and implementation of the technical specifications can LMS Utilities reduce losses associated with inefficient power equipment. Hence, a 6-member Technical Working Group (TWG) comprising a senior technical representative from each Utility and ICASEA was formed to participate and contribute in discussions and meetings. The objective of this TWG was to start with the development of technical specifications to harmonise underground power cables in the LMS. This step-by-step approach was to enable the participating Utilities to review and evaluate the result of this Technical Working Group before collectively moving to the next step of harmonising other equipment. This handbook was developed to help LMS Utilities implement low loss power cables. Reduction will only come when the minimum performance guidelines are followed and implemented by all associated with the design of the electricity grid, specifying the standards of equipment for procurement and subsequently operating or maintaining them.
Members of the Technical Working Group for Power Cable: Chairman
Metropolitan Electricity Authority (MEA), Thailand
Mr.Asawin Rajakrom Mr. Werawat Buathong Director, Electrical Equipment and UG Cable Director, Electrical Engineering Division Installation Division Mr. Somchai Homklinkaew Metropolitan Electricity Authority, Thailand Asst Director, Power System Planning Division Electricite Du Cambodge (EDC), Cambodia
Mr.Preecha Tongkaewkerd
Mr. Lim Sisophuon
Senior Electrical Engineer, Power System
Deputy Chief, Dispatching Control Centre
Planning Division
Electricite Du Laos (EDL), Lao People’s Democratic Republic
Ms. Sasianong Vacharasikorn
Mr.Bounkheuth Vilayhak
Development Dept.
Deputy Chief, Technical Standards Office
International Copper Association Southeast Asia (ICASEA)
Ho Chi Minh City Power Corporation (HCMC PC), Vietnam
Senior Electrical Engineer, Research and
Mr. Louis Koh
Mr.Nguyen Huu Vinh
Project Leader, Power Distribution
Electrical Engineer, Technical Department
Mr. Piyadith Lamaisathien Country Manager, Thailand
Hanoi Power Corporation (HNPC), Vietnam
Mr.Dinh Tien Dung
MEA Project Support Team
Expert, Technical Department
Ms. Sutida Sindhvananda, Project Director, International Service Business Ms. Kunlathida Pongchavee, Executive Project Assistant, International Service Business Mr. Prawit Chaikaew, Electrical Engineer, International Service Business
Acknowledgements
The harmonisation of power distribution systems in the LMS will contribute to the expansion of the ASEAN Power Grid. However, harmonisation requires a robust partnership and sustained effort over many years. The harmonisation of technical specifications together with the development of this handbook is taking the process a step closer towards the realisation of the objectives as set out in the strategic roadmap for the harmonisation of power distribution systems in the LMS. Strengthening regional cooperation to build the capacity of both technical and functional staff would not have been possible without the endorsement and support of: Electricité du Cambodge, Cambodia
Hanoi Power Company, Vietnam
Mr. Keo Rottanak, Managing Director
Mr. Tran Duc Hung, Director
Mr. Chan Sodavath, Deputy Managing Director
Mr. Vu Quang Hung, Vice Director, Technical
Electricité du Laos, Lao People’s Democratic Republic
Mr. Nguyen Anh Tuan, Vice Director, Business Metropolitan Electricity Authority, Thailand
Mr. Khammany Inthirath, Managing Director Mr. Pornthape Thunyapongchai, Governor Mr. Sisavath Thiravong, Deputy Managing Mr. Danai Chitterapharb, Director, Business Director Investment Dept. Mr. Boun Oum Syvanpheng, Deputy Managing Director Ho Chi Minh City Power Corporation, Vietnam
Mr. Le Van Phuoc, Director Mr. Tran Khiem Tuan, Deputy Director
International Copper Association Southeast Asia
Mr. Steven Sim, Chief Executive Officer
Table of Contents Introduction
1
Chapter 1
1
1.1 Introduction
1
1.2 Objective
1
1.3 General Requirements 1.4 Principle Specifications
5
1.5 Existing Requirement for Electricite′ du Cambodge (EDC), Cambodia
10
1.6 Existing Requirement for Electricite′ du Laos (EDL), Lao People’s Democratic Republic
11
1.7 Existing Requirement for Ho Chi Minh City Power Corporation (HCMCPC), Vietnam.
11
1.8 Existing Requirement for Hanoi Power Corporation (HNPC) Vietnam
13
1.9 Additional Requirement for Metropolitan Electricity Authority (MEA), Thailand
14
1.10 Conclusion
16
Chapter 2
17
2.1 Introduction
17
2.2 Objective
17
2.3 Documents required for evaluation
17
2.4 Guideline for bid evaluation
22
2.5 Conclusion
24
Chapter 3
25
3.1 Introduction
25
3.2 Objective
25
3.3 Inspection Committee Management
25
3.4 Manufacturing Process Inspection
26
3.5 Factory acceptance tests
29
3.6 Conclusion
32
Chapter 4
34
4.1 Introduction
34
4.2 Objective
34
4.3 Acceptance Committee Management
34
4.4 Acceptance Process
35
4.5
39
Conclusion
Chapter 5
40
5.1 Introduction
40
5.2 Objective
40
5.3 Ampacity Calculation
40
5.4 Insulation and Sheath Thickness Calculation
55
5.5 Calculation on Cable Pulling Tension
58
5.6 Conclusion
71
Chapter 6
72
6.1
72
Introduction
6.2 Objective
72
6.3 Types of installation
72
6.4 Cable Laying Procedure
79
6.5 Installation Acceptance Process
88
6.6 Conclusion
89
Chapter 7
90
7.1 Introduction
90
7.2 Objective
90
7.3 Maintenance and Inspection
91
7.4 Field Tests on Cable
98
7.5 Cable Monitoring System
106
7.6 Conclusion
107
References
108
APPENDIX A : Specification of 8.7/15 kV & 12/20 kV XLPE COPPER CABLE
A-1
APPENDIX B : Specification of 69 & 115 kV XLPE COPPER CABLE
B-1
Figures Figure 1 : Cross-section of 69 & 115 kV PE Outer Sheath (Jacket)
10
Figure 2 : Cross-section of 69 & 115 kV PE Outer Sheath (Jacket)
15
Figure 3 : Cross-section of 69 & 115 kV Fire Retardant PVC Outer Sheath (Jacket)
16
Figure 4 : Cable Drawing
21
Figure 5 : Schematic diagram of Extrusion Line
28
Figure 6 : The geometric factor (G) and the Screening Factor
50
Figure 7 : Mutual Heating Effect
52
Figure 8 : Direct Burial Installation
73
Figure 9 : Semi-Direct Burial Installation
74
Figure 10 : Concrete Trough Installation
74
Figure 11 : Concrete Encased Installation
75
Figure 12 : Concrete Trench
76
Figure 13 : Horizontal Directional Drilling Construction Layout Crossing the River
77
Figure 14 : Cross Section of Concrete Pipe ID 1 m
78
Figure 15 : Pulling Eye
80
Figure 16 : Pulling Grips
80
Figure 17 : Direct Burial Cable Laying Procedure
82
Figure 18 : Cable Laying for Direct Burial
82
Figure 19 : Cable Laying Procedure for Duct Installation
83
Figure 20 : Test Rod or Dummy
84
Figure 21 : CCTV Camera for Checking Duct
84
Figure 22 : Checking Duct by Using CCTV Camera
84
Figure 23 : Method of Cable Pulling
84
Figure 24 : Cable Laying Procedure for Tunnel Installation
86
Figure 25 : Floor Mounting Rollers
87
Figure 26 : Cable Installation in Tunnel by Caterpillar Method
87
Figure 27 : Megger Test1 kV
88
Figure 28 : Insulation Testing
88
Figure 29: Murray Loop Bridge Method
95
Figure 30: Cable Fault Waveform Reflections
96
Figure 31 : Tan δ Test Results (K. Brown IEEE ICC Minutes Spring 2005)
104
Tables Table 1 :
Major characteristics of underground cable
5
Table 2 :
Nominal diameters of round armor wires
12
Table 3 :
Nominal thickness of armor tapes
12
Table 4:
Sample of Proposed Technical Data for High Voltage XLPE Copper Cable
20
Table 5 :
Partial Discharge Test
30
Table 6:
Voltage Test
30
Table 7 :
Routine and Special Tests Report
37
Table 8 :
Acceptance Tests Report
38
Table 9 :
Conductor AC Resistance
43
Table 10 : Dielectric losses of insulation
43
Table 11 : Sheath Resistance of each material
44
Table 12 : Thermal Resistances of each material
49
Table 13 : Nominal Thickness of PVC/B Insulation for Cable Rated Voltages
55
Table 14 : Nominal Thickness of Cross-lined Polyethylene (XLPE) Insulation for Cable Rated Voltages
55
Table 15 : Nominal Thickness of Ethylene Propylene Rubber (EPR) and Hard Ethylene Propylene Rubber (EPR) Insulation for Cable Rated Voltages from 6 kV (Um = 7.2 kV ) to 30 kV (Um = 36 kV)
56
Table 16 : Maximum Cable Cross-sectional Area as a Percentage of Internal Conduit or Duct Area (Refer to NEC)
58
Table 17 : Minimum Recommended Bending Radii for Unarmored Power Cables for Cables Rated
60
Table 18 : Cable Configurations in Conduit
61
Table 19 : Recommended Basic Dynamic Coefficient of Friction, Straight Pulls & Bearing Pressures
63
Table 20 : Recommended Maximum Pulling Tensions at Pulling Eyes
64
Table 21:
65
Recommended Maximum Pulling Tensions Copper and Aluminum Conductor Single
Table 22 : Definition of symbols
68
Table 23: Comparison of Different Methods of UG Cable Installation
79
Table 24: Value of Direct Voltage for Jacket Test
88
Table 25 : DC Test Voltage according to IEC 60502-2005
99
Table 26 : AC Test Voltage according to IEC 60840-2004
101
Table 27 : VLF Test Voltage according to IEEE 400.3-2006
102
Table 28: Summary of Different Tests
106
Table 29 : Methods for Monitoring Underground Cable System
107
Introduction This is the first ever handbook on underground cables. The objective of this handbook is to serve as a desk and field compendium for the utilities of Lower Mekong Sub-region (LMS) countries. These
utilities are Electricite′ du Cambodge (EDC), Cambodia; Electricite′ du Laos (EDL), Lao People’s Democratic Republic; Hanoi Power Corporation (HNPC); Ho Chi Minh City Power Corporation (HCMCPC), Vietnam; and the Metropolitan Electricity Authority (MEA), Thailand. This handbook describes all the important processes -- from procurement to final acceptance -involved in an underground power cable project. These processes include preparation of specifications, bidding evaluation, cable manufacturing inspection, contract acceptance, calculations on cable, cable installation, cable system operation, maintenance and testing. The handbook refers to the reputable reference books, the latest editions of international and national standards, and the current specifications of LMS utilities. Specifically, it covers underground cables with voltage rating from 22 kV (minimum) to 115 kV (maximum) and frequency rating of 50 Hz. The study of power distribution systems in the LMS countries reveals that there are many differences in underground cable system practice, hence, to promote the harmonization of the LMS practice, the common specification for underground cable of the LMS utilities shall be developed for concrete implementation in the future. The normative technical specification of underground power cable is then proposed in Appendices A and B for optional application in the LMS underground system.
Chapter 1
Preparation of National Normative Technical Specification of Underground Copper Cable 1.1 Introduction Underground power cables have different electrical characteristics from overhead lines. These differences must be taken into consideration during cable system planning, design and operation. This chapter describes descr ibes some of the most important requirements requi rements that should be considered by utilities while preparing specifications of underground cables.
1.2 Objective The objective is to provide general information on how to select, design, install and maintain a cable system effectively. And also to help cable engineers reduce cost, which tends to increase due to ineffective use and maintenance of underground cable. The information provided should also enable utilities to minimize minimi ze the need for new investments, learn about loss reduction programs and minimize min imize negative environmental impact. The secondary objectives include encouraging greater energy efficiency; developing new national standards; standa rds; star starting ting cost effective energy saving programs for both utilities and customers; reducing losses from utility-owned underground cables; and minimizing life cycle costs. Eventually, these programs would increase system capacity and decrease the cost of investment in constructing new distribution substations.
1.3 General Requirements This specification is for power utility companies in the Lower Mekong Sub-region (LMS). An underground cable shall be installed in a cable tray or in a duct under the ground grou nd and also by direct burial where where fault lev level el is up to 25 kA for MV system system and 40 kA for for HV system. Site and Service Condition: LMS utilities operate in a tropical climate. The altitude ranges from 0
meter to 1,800 meters above sea level, ambient temperature ra nges from 30ºC to 45ºC and relative
humidity measures measu res 84%. The cable shall be suitable for for continuous use at conductor temperature temperat ure of 90ºC for normal operation and 250ºC for short-circuit condition. Reference Standard: The International Electrotechnical Commission (IEC) is the common
reference standard for all LMS utilities as well as for a majority of countries cou ntries around arou nd the world. For MV underground undergrou nd cables, IEC 60502 series is the key reference. IEC 60840 is the reference for HV underground cable, and IEC 60228 serves ser ves as the reference for conductor. conductor. For fire retardant retarda nt cables, IEC 60332 and ISO 4589 shall be applied. Some utilities also refer to their own national standards, which are mostly equivalent to IEC except for some addition requirements due to their specific experience and local conditions. Test, Inspection I nspection and Test Report: There will be three main tests: Type test: The proposed cable should successfully pass all the type ty pe or design tests in accordance accorda nce with
the reference standards. In case the fire retardant jacket is required, the following tests shall be included:
•
The oxygen index of non-metallic sheath material shall be not less than 30 according to ISO 4589 458 9 or equivalent.
•
Testing on completed cable under fire condition according to IEC 60332-3-22 60332-3-22 or equivalent.
The testing shall be done by a reputable independent testing agency or an agency acceptable to LMS utilities. Cable manufacturers who do not have a type test report for the proposed cable shall alternatively submit a type test report within the range of type approval as specified in IEC standard. Routine tests: Routine tests shall be made on each reel of the finished cables in accordance with the
reference standards. At minimum, the following tests shall be included: a) Measurement of electrical resistance of conductors b) Partial discharge test c) High voltage test Special tests: The special tests shall be made on one length from each manufactured lot of the cables
of the same type and a nd size. But these tests shall be limited lim ited to not more than 10% of of the number of cable
lengths in the contract, contract , rounded to the upper unity un ity.. Special tests, conducted in accordance with the reference standards, will at least include the following items: a) Conductor examinat examination ion b) Checking of dimensions including measurement of externa externall diameter c) Electrical test d) Hot set test for XLPE insulation Special and routine tests shall be carried out to determine whether the cable complies with the specifications. For For any additional tests required as per the mutual agreement between the purchaser and the manufacturer, the test method shall be proposed by the manufacturer and approved by the purchaser bef before ore proceeding proceeding with the the testing. testing. Type tests help to validate design, raw material, workmanship and quality control during the manufacturi manufact uring ng process. Routine tests, as the name implies, implies, are tests that are routinely performed on each drum dr um of cable cable to assure that cables are good quality and made according to requi red specification before befo re it leav leaves es the factory factory.. The utility reserves the right to send representati representatives ves to witness all the required tests at the factory. Routine and Special Test Report: Prior to shipment, the supplier shall submit to the utility six (6)
complete and certified sets of all test reports. The test reports shall include all the data required for their complete interpretation, e.g., diagrams, methods, instruments, constants and values used in the tests and the results obtained. Drawings and Instruction: The supplier shall fur furnish nish six (6) sets of documents covering all
the significant details of the t he underground cable to the utility for approval within a stipulated timeframe. To protect prote ct mutual mut ual interest in cases of delayed or late submission, s ubmission, compensation terms shall be be specified in the contract. Special installation instructions and precautions, characteristic curves, installation instructions inst ructions and instr instruction uction manuals with the contract number marked thereon in the metric metr ic system - shall be machine printed or typed t yped and delivered prior with the first shipment. Rating and Features: The major characteristics characterist ics of the underground cable must be properly specified,
such as shown in the t he following table:
Type
Solid Extruded Dielectric
(MV) 15, 22, 24, 35 kV System voltage level (kV) (HV) 69, 110, 115 kV (MV) 12.7/22, 12/20, 21/35 kV Uº /U (kV)
(HV) 36, 64 kV
Frequency ( Hz )
50 (MV) 70, 150, 240, 400
Conductor Size (sq.mm.) (HV) 400, 800, 1000, 1200 Insulation
XLPE
Metallic Screen Non Metallic Outer Sheath
COPPER WIRE PVC or PE or fire retardant PVC
Operating Temperature (ºC)
90
Table 1 : Major characteristics of underground cable
1.4 Principle Specifications The following specifications apply to underground cables for LMS utilities. 1.4.1 Medium Voltage (MV) Cable
The MV underground cable shall be extruded-dielectric XLPE type manufactured by d ry curing process only. The cables shall be installed direct burial, in ducts, trays. The requirement of each layer shall be as follows. Conductor: The conductor type shall be plain annealed copper and the construction shall be
compact round concentric lay stranded. Conductor Screen: The conductor screen is a conducting material by the triple extrusion
with the insulation over the surface of the conductor. The thickness of the conductor screen for each utility shall be between 0.0635 and 1.0 mm. The extruded conductor screen shall have resistivity in accordance with the reference standards. Semi-conductive tape may be applied between conductor and conductor screen. Insulation: The insulation shall be cross-linked polyethylene (XLPE) and simultaneously
extruded with the semi-conductive conductor screen and insulation screen layer. Electrical,
mechanical, and other properties shall comply with IEC 60502-2 or equivalent. The dry curing process is required. Conventional steam or hot water curing processes are not acceptable. The average thickness of insulation shall not be less than the nominal value specified in the reference standards. The minimum thickness shall not fall below the nominal value by more than 0.1 mm + 10% of the nominal value, i.e.:
t m ≥ t n – (0.1+0.1 t n) Where,
t m is the minimum thickness t n is the nominal thickness
Insulation Screen: The insulation screen shall consist of a nonmetallic covering directly over
the insulation in combination with metallic screen. Nonmetallic covering having maximum volume resistivity at rated temperature shall be applied over the insulation in one or more layers. Nonmetallic screen may consist of a conducting tape or a layer of conducting compound having thickness between 0.0635 and 1.0 mm. Metallic Screen: Metallic screen shall consist of nonmagnetic metal component applied over
the nonmetallic covering. The metallic screen shall be made of copper. Copper screen shall consist of plain annealed copper flat or round wires applied helically over the nonmetallic covering. The wires shall be electr ically continuous and bonded together throughout the cable length with copper contact tape. The total cross-sectional area of the screen and minimum number of wire shall be not less than the specified value in the contract. Outer Sheath: The properties of outer sheath shall comply with mechanical requirements
specified in IEC 60502-2 or equivalent. It shall also be suitable for use with the cable having maximum conductor temperature of 90ºC. The material of outer sheath shall be black PVC or black PE (ST7). If the fire retardant outer sheath is specified in the contract, the sheath shall be black f lame retardant PVC. The oxygen index of outer sheath material shall be not less than 30 as measured
according to ISO 4589 or equivalent. A certified test report from the raw material manufacturer or an accepted reputable independent institution shall be submitted for approval. The f lame retardant outer sheath shall be able to stop flame propagation along vertical or horizontal cable ways and delay damage to cables. Test on completed cable under fire condition according to IEC 60332-3-22 or equivalent shall be done by reputable independent testing institution or at the factory test station witnessed by utility’s representative. The test report shall be submitted before shipment. Fire retardant is used only in confined area such as substation building or tunnel. Marking: The outer sheath shall be marked on the surface with, at minimum, cable
description, manufacturer’s name or symbol, and date of manufacturing. The details of the marking shall be specified in the contract. Continuous marking on the sheath along the whole cable length shall also be provided at 1 meter interval. Cable End Sealing: Immediately after the factory test, the cable inner end shall be covered
with an end cap, and the cable outer end shall be con nected to a moisture-proof pulling eye of sufficient strength. Cable rib shall be removed before sealing. The material of cable end sealing shall be a metal cap or a heat shrinkable cap. 1.4.2 High Voltage (HV) Cable
The HV underground cable shall be extruded-dielectric XLPE type, manufactured by dry curing process only. The cables shall be installed by direct burial in ducts or in trays where they are immersed in the water all the time. The detailed requirements for each layer are as follows: Conductor: The conductor type shall be plain annealed copper. The construction shall be
compact round concentric lay stranded or compact segmental stranded for cross-section area less than 1,000 mm. It will be compact segmental stranded for cross-section area 1,000 mm and above. Conductor Screen: The conductor screen shall be semi-conductive cross-linked polyethylene.
The conductor screen is a conducting material by the triple extrusion with the insulation over the surface of the conductor. The thickness of the conductor screen for each utility shall be between 0.8 to 1.5 mm. The extruded conductor screen shall have resistivity in accordance with the reference standards.
Semi-conductive tape may be applied between conductor and conductor screen. Insulation: The insulation shall be cross-linked polyethylene (XLPE) simultaneously extruded
with the semi-conductive conductor screen and insulation screen layer. Mechanical, electrical and other properties shall comply with IEC 60840 or equivalent. The dry curing process is required. Conventional steam or hot water curing processes are not acceptable. The minimum thickness of the insulation shall not be less than 90 per cent of the nominal value specified, and additionally it should satisfy:
ℎ − ℎ ℎ
≤0.15
Where, maximum thickness and minimum thickness are the measured values at one and the same cross-section of the insulation. Insulation Screen: The conductor screen shall be semi-conductive cross-linked polyethylene.
The conductor screen is a conducting material applied by triple extrusion with the insulation over the surface of the conductor. The thickness of the conductor screen for each utility shall be between 0.8 to 1.5 mm. The extruded conductor screen shall have resistivity in accordance with the reference standards. Synthetic Water Blocking Layer: A semi-conductive, non-biodegradable water blocking
layer shall be provided under the metallic screen to provide a continuous longitudinal watertight barrier throughout the cable length. This layer shall be compatible with other cable materials and shall not corrode adjacent metal layers during heat aging of the cable. Metallic Screen (Grounding Screen): The metallic screen shall be a concentric layer of
copper wires, which is electrically continuous and bonded together th roughout the cable length with copper contact tape. The total cross-sectional area and minimum number of wires of the metallic screen shall not be less than the value specified in the contract.
Synthetic Water Blocking and Cushioning Tape: A non-conductive, non-biodegradable
water blocking tape shall be applied over the metallic screen to provide a continuous longitudinal watertight barrier throughout the cable length. The tape shall have sufficient thickness to perform well as a thermal stress relief layer and to provide for cushioning and bedding. The tape shall be compatible with other cable materials and shall not corrode adjacent metal layers during heat aging of the cable. Radial Water Barrier: As a protection against formation of water trees in the insulation, a
traverse water barrier consisting of laminated aluminum tape coated on both sides with an ethylene acrylic acid adhesive co-polymer or polyethylene shall be incorporated under the nonmetallic sheath. The average thickness of aluminum tape shall not be less than 0.19 mm. Outer Sheath: The outer sheath shall be PVC or compound black polyethylene (PE) ST7. It
should be suitable for use with the cable having maximum conductor temperature of 90ºC and 130ºC under normal and emergency condition respectively. The mechanical properties shall be in accordance with reference standard. If the fire retardant outer sheath is specified in the contract, the sheath shall be black f lame retardant PVC. The oxygen index of outer sheath material shall be not less than 30 as measured according to ISO 4589 or equivalent. A certified test report from the raw material manufacturer or an accepted reputable independent institution, shall be submitted for approval. The flame retardant outer sheath shall be able to stop flame propagation along vertical or horizontal cable ways and delay damage to cables. Test on completed cable under fire condition according to IEC 60332-3-22 or equivalent shall be done by reputable independent testing institution or at the factory test station witnessed by utility’s representative, the test report shall be submitted before shipment. Fire retardant is used only in confined area such as substation building or tunnel. Additional requirement for 69 & 115 kV PE outer sheath for the purpose of electrical protection and ease of pulling, the sheath shall be ribbed type having crest width and depth of approximately 2.5 mm and the center to center distance between crests shall be approx. 7 mm, except for length marking. The crest width shall be approximately 10 mm. See Figure 1.
10 mm (APPROX.) FOR LENGTH MARKING 2.5 mm (APPROX.)
7 m m ( A P P R O X . )
Figure 1 : Cross-section of 69 & 115 kV PE Outer Sheath (Jacket) Marking: Manufacturer’s name or trade name, year of manufacturing and contract number
shall be provided at appropriate interval throughout the cable length. This will be done on the outer longitudinal water blocking or on the outer sheath by inser ting identification tape between radial water barrier layer and outer longitudinal water blocking layer. Cable End Sealing: Immediately after the factory test, the cable inner end shall be covered
with an end cap, and the cable outer end shall be connected to a moisture-proof pulling eye of sufficient strength. Cable rib shall be removed before sealing. The material of cable end sealing shall be a metal cap or a heat shrinkable cap.
1.5 Existing Requirement for Electricite` du Cambodge (EDC), Cambodia The additional specifications for MV underground cables required by Cambodia’s EDC utility are as follows:
•
Metallic screen shall be copper tape. The minimum thickness shall not be less than 0.2 mm.
•
For 3 cores cable only, the cables shall include an armor layer, which will be double tape type.
•
Water blocking is required by using swelling material in conductor strand.
HV underground cable specification is not currently employed.
1.6 Existing Exist ing Requirement for Electr Electricite icite`` du Laos (E (EDL), DL), Lao People’s Democratic \ Republic The existing specification for MV underground cables required by EDL utility is as follows:
• Metallic screen shall be copper tape. HV underground cable specification is not currently employed.
1.7 Existing Exist ing Requirement for Ho Chi Minh City Pow Power er Corporation (HCMCPC), Vietnam. The existing specifications required by HCMCPC utility are as follows: 1.7.1 1.7 .1 MV underground cables
•
The metallic screen shall sha ll be double copper tape having maximum ma ximum thickness th ickness of 0.127 0.127 mm and minimum width of 12.5 mm.
•
Inner covering and fillers are required for multi-cores cables. The cables shall have an inner covering over the laid-up cores. The inner in ner coverings and fillers shall be of suitable materials. An open helix of suitable tape is permitted as a binder before applying an extruded inner covering. The material used for inner coverings and fillers shall be suitable for the operating temperature of the cable and compatible with the insulating material. The thickness of extruded inner i nner coverings shall be specified in accordance with the reference standards.
•
The cables shall have separation sheath. When the metal screen and armor are made of different metals, metals, these shall be separated by an imper vious extruded sheath. This may be instead of, or in addition to, an inner covering. The separation sheath shall be thermoplastic compound (PVC, PE or similar materials) or vulcanized elastomeric compound (polyethylene or similar materials). The quality of the material used for the separation sheath shall be suitable for the operating temperature of the cable. The nominal thickness of this sheath, rounded to the nearest 0.1 0.1 mm, shall be derived from the formula: Ts = 0.02D + 0.6 mm
Where: D is the fictitious diameter under the sheath. The smallest nominal thickness shall be 1.2 mm. The minimum thickness at any point shall not fall below 80% of the nominal value by more than 0.2 mm.
•
For multi-cores cables, the cables shall consist of armor layer. The armor shall be flat or round wire (galvanized (galvan ized Fe, Pb-coated Fe, Al or Al alloy) or double tape (Fe., galvanize galvanized d Fe, Al or Al alloy). Fictitious diameter under the armor [mm] Up to and including
Nominal diameter of armor wire [mm]
15
0.8
15
25
1.6
25
35
2.0
35
60
2.5
Above
60
3.15
Table 2 : Nominal diameters of round armor wires
The dimension of round armor wires shall not fall below the nominal value by more than 5%. Flat armor wires: For fictitious diameters under the armor greater than 15 mm, the nominal thickness of the flat steel wire shall be 0.8 mm. The dimension of flat armor wires shall not fall below bel ow the nomin nominal al val value ue by mor moree than than 8%.
Fict Fi ctiiti tio ous di diam ameeter un und der the ar arm mor [mm]
Nomi min nal thi hicckn knes esss of ta tap pe [mm]
Up to and including
Steel or galvanized steel
Aluminum or aluminum alloy
30
0.2
0.5
30
70
0.5
0.5
70
-
0.8
0.8
Above
Table 3 : Nominal thickness of armor tapes The dimension of armor tapes shall not fall below the nominal value by more than 10%.
1.7.2 1.7 .2 HV underground cables
The metallic screen (sheath) shall be of aluminum laminated or corrugated aluminum and complying with the following requirements: a. Laminated aluminum
The metallic screen shall consist of laminated aluminum. This screen shall be a combination of a copper wire and a layer of aluminum tape. The cross-section of the copper screen shall have sufficient area to withstand the thermal and dynamic effect of a single-phase to ground short circuit current cur rent of 31.5 31.5 kA for 3 seconds. The bidder bid der shall shall submi submitt the calc calculati ulation on for for determining determining the cross-s cross-sectio ectional nal area area of of the copper wire screen. b. Corrugated aluminum sheath
The metal sheath shall consist of of a tube of corr corrugated ugated aluminum. The thickness th ickness of the corr corrugated ugated aluminum sheath shall be sufficient to withstand the thermal and dynamic effect of a single phasee to phas to ground ground sho short rt circui circuitt current current of 31.5 kA for for 3 secon seconds. ds. The bidder shall submit the calculations for determining the sheath thickness. The sheath shall be designed and manufactured manufactu red as a homogeneous construction with the following characteristics: uniform thickness, close fitting, seamless and free from defects, porosity and inter-crystalline fracture. The sheath corrugation corr ugation shall be of of annular ring or helix construction constr uction designed to minimize the ingress of moisture even when the serving is damaged.
1.8
Existi ng Requirement for Hanoi Pow Existing Power er Corporat Corporation ion (HNPC) Vietn Vietnam am •
The existing specifications of MV and HV Underground Cable required by HNPC utility are as follows: follows: Metallic screen shall be copper wires. The total cross-sectional cross-sect ional area of the copper wires screen shall be specified in the contract.
•
For multi-cores cable, the cables shall consist of armor layer. The armor shall be steel tape type.
•
Swelling material in conductor strand, longitudinal water blocking over copper wire screen and radial water blocking below outer sheath are required.
1.9 Additional Requirement for Metropolitan Electricity Authority (MEA), Thailand In addition to 1.4 Principle specifications, the following specifications should be employed to enhance safety, reliability, performance and maintenance: 1.9.1 MV underground cables
•
The tension necessary to remove an extruded insulation screen from cable at room temperature shall not be less than 13.3 N. Copper wire screen will consist of plain annealed copper fate or round wires applied helically over the nonmetallic covering. The wires shall be electrically continuous and bonded together throughout the cable length with copper contact tape. The total crosssectional area of the screen and minimum number of wire shall be not less than the specified value in the contract.
•
If PVC fire retardant outer sheath is specified in the contract, the sheath shall be black, flame retardant PVC. The oxygen index of outer sheath material shall be not less tha n 30 as measured according to ISO 4589 or equivalent. A certified test report from the raw material manufacturer or a reputable independent institution, which is acceptable to MEA, shall be submitted for approval. The flame retardant outer sheath shall be able to stop f lame propagation along vertical or horizontal cable ways and delay damage to cables. Test on completed cable under fire condition according to IEC 60332-3-22 or equivalent shall be done by reputable independent testing institution or at the factory test station witnessed by MEA’s representative. The test report shall be submitted before shipment.
1.9.2 HV underground cables.
•
If a fire retardant outer sheath is specified in the contract, the sheath shall be black, flame retardant, and made of PVC. The oxygen index of non-metallic sheath material shall be not less than 30 as measured according to ISO 4589 or equivalent. A certified test
report from the raw material manufacturer or a reputable independent institution, which is acceptable to MEA, shall be submitted for approval. The flame retardant non-metallic sheath shall be able to stop flame propagation along vertical or horizontal cable ways and delay damage to cables. Testing on the completed cable under fire condition according to IEC 60332-3-22 or equivalent shall be done by a reputable independent testing institution or at the factory (to be witnessed by MEA’s representative). And the test report shall be submitted before shipment.
•
Additional requirement for 69 & 115 kV PE outer sheath for MEA: The sheath shall be ribbed type having crest width and depth of approximately 2.5 mm and the center to center distance between crests shall be approx. 7 mm, except for length marking. T he crest width shall be approximately 10 mm. See Figure 2. 10 mm (APPROX.) FOR LENGTH MARKING 2.5 mm (APPROX.)
7 m m ( A P P R O X . )
Figure 2 : Cross-section of 69 & 115 kV PE Outer Sheath (Jacket)
•
Additional requirement for 69 & 115 kV fire retardant PVC sheathed for MEA: The sheath shall be ribbed type having crest width and depth of approx. 2.5 mm and the center to center distance between crests shall be approx. 7 mm. T he crest width at the quarters shall be approximately 5 mm, and the crest width for length marking shall be approximately 10 mm. See Figure 3.
10 mm (APPROX.) FOR LENGTH MARKING 2.5 mm (APPROX.)
7 m m ( A P P R O X . ) 5 m m ( A P P R O X . )
Figure 3 : Cross-section of 69 & 115 kV Fire Retardant PVC Outer Sheath (Jacket)
1.10 Conclusion From this chapter cable engineers can learn the specif ications of other power power utilities and compare them with their own specifications. This will enable them to know the advantages and disadvantages of different types of cable design and improve their specifications. The cable constructions for each power pow er utility utility may be different different because of its unique installatio installation n requirements requirements.. It It is is recommended recommended that power utilities review their their specifications for the cable insulation material from the point point of view of of the cost as well as the losses. However, in order to promote the harmonization of the LMS practice, the common specification for underground cable of the LMS utilities shall be developed for concrete implementation in the fut future. ure. The normative technical specification specificat ion of underg underground round power cable is then proposed in Appendices A and B for optional application in the LMS underground system.
Chapter 2
Bidding Evaluation 2.1 Introduction Bidding evaluation is one of of the most important processes to ensure e nsure that a utility utilit y gets good quality cables on time. And to support bid evaluation, utilities must instruct suppliers to submit all the necessary documents. Inefficient documents may lead to poor quality of cables. The specification should clearly specify the necessary documents required to be submitted by the suppliers. Suppliers should fill-in all tables and forms as required in the bid documents.
2.2 Objective The objective of of this chapter is to guide g uide and assist the utilities to learn about documents, data and tables t ables required for evaluation. If a bidding bidding document is complete, complete, its evaluation will be easy and efficient. If not, more documents will be requested and submitted in a limited period, otherwise the bid shall not be considere considered. d. Only underground underground cables cables shall be discussed discussed in this chapter chapter..
2.3 2. 3 Documents Required for Evaluation Evaluation Usually, there are five main documents required for evaluation: (i) type test report, (ii) proposed technical data, (iii) deviation form, (iv) detail drawing and (v) reference list of supply. Additional documents or samples may be required depending on special requirements of each utility. Bids should be evaluated by an Evaluation Committee whose members are selected from related departments. It is important not to get bids evaluated by only a single person or department. The requirements and instructions for evaluating each of the above-mentioned five bid documents are as follow. 2.3.1 Type test report
The proposed cables should pass all the type tests according to the reference standard specified in the specifications. The tests shall be conducted by a reputable independent testing agency
acceptable to the utility. All test reports shall be submitted with the offer otherwise otherw ise such offer will not be considered. Cable manufacturers who do not have type test reports of the proposed cables can submit a type test report repor t of the cable with with the range of type approval specified in reference standard for consideration or bigger sizes of conductor but same ratings and construction design. In the case of fire retardant retard ant cable, the test for for vertical flame f lame spread of vertically-mounted vertically-mounted bunched cables can be carried out after the award of contract but before shipment or else the right will be reserved to purchase from the second lowest bidder with penalty to compensate for the balance in order to keep delivery schedule. 2.3.2 Proposed technical data
Proposed technical data is important for bid evaluation as it provides information for the Evaluation Committee to assess whether the proposed cables conform to the specifications. Normall No rmally y, the the standard standard pro provide videss guidel guidelines ines for the techni technical cal data requir required ed to to be filled up in the inquiry and order section. Any other additional requirements for technical data should also be provide pro vided d in in this this secti section. on. Bidders are requested to fill in all the blank spaces in the technical proposal data form and return retu rn with the bid. Failure to submit the form or incomplete forms may render the bid invalid and constitute a sufficient case for bid rejection. The sample of technical proposal data form is shown in Table 4. 2.3.3 Deviation form form
Bidders must clearly indicate all deviations from the specifications fill in the Deviation Form and attach it with the bid. If there is no deviation stated in the Deviation Form, the character istics of the proposed cables shall be considered to be in complete compliance with specifications. However, if the delivered cable is found not in compliance with the specification, it will be rejected.
2.3.4 Drawing
The drawing of the proposed cables is also important for bid evaluation. It should show all important details of the proposed cables, including cable construction, dimension, material, etc. All information shall be in English or the country’s official language, machine printed or typed. Information on drawing shall be in engineering lettering. All measurements and quantities shall be expressed in the units of metric system. If they are expressed in other unit systems, the metric equivalent shall also be indicated. Figure 4 shows a sample of detailed drawing of a cable. 2.3.5 Reference list of supply and field experience
A reference list of bidder’s supply and field experience shall also be taken into consideration when evaluating bids. Bidders should attach a reference list of supply and field experience in the same design of cables as proposed with the quotation. A reference list of previous supply projects is particularly important for evaluation in case the cable(s) offered are of new manufacturers. This is to ensure that the manufacturer is qualified for supplying cables as per the specifications and associated standards.
Material Code
204-6500
Manufacturer
ABC
Country
THAILAND
Applied standard, publication number and year
IEC 60840
Rated voltage
115 kV
Outline drawing number (to be attached)
HVMAC-O5-155
Conrm to attach type test reports of the cable with similar design (yes or no)
Conrm to attach the detail of water penetration test (yes or no)
YES YES
Copper conductor Applied standard, publication number and year
IEC 60228
Volume conductivity at 20ºC, minimum IACS
100 %
Number of wires
53 (minimum)
Number of layers
4
Wire diameter (with tolerance)
4.47 ± 1% mm
Conductor temper (anneal, hard-drawn, etc.)
ANNEALED
Material Code
204-6500
Lay ratio of the outer layer
10
Direction of lay of the outer layer
LEFT- HAND
Nominal cross-sectional area
788 mm2
Stranding (concentric, compress or compact)
COMPACT
Overall diameter
34 mm
Tolerance of overall diameter
1%
Weight
14,000 kg/km
Maximum dc resistance at 20ºC
0.221Ω/km
Conductor screen Material
SEMI-CONDUCTIVE
Volume resistivity, maximum At room temperature, .........ºC
1*104Ω.cm
At 90ºC
1*105Ω.cm
Thickness Average
1.5 mm
Minimum
1.2 mm
Table 4: Sample of Proposed Technical Data for High Voltage XLPE Copper Cable
Figure 4 : Cable Drawing
2.4 Guideline for Bid Evaluation The bid evaluation process is very important. All bids shall be scrutinized by the Evaluation Committee. Only those bids which are complete in terms of data and information shall be evaluated. This process takes a lot of time. It is recommended to make a request for more information in a limited period. so that it will not delay the delivery schedule and the whole project. Usually, bids are evaluated based on the following criteria and priorities. 2.4.1 Type Test report
As the top priority, type test report should be assessed first. The proposed cable should pass all the type tests according to the reference standard as specified in the specifications. Otherwise, the bid shall be rejected immediately, and there is no need to review other documents. Manufacturers may not usually have the type test report for cables of the same size, design and rating as the proposed cables. There is, however, no need to do type test for all sizes and ratings of the cables and in such a case, the type test report of an identical cable may be acceptable on the following conditions. Once the type tests on cable(s) of specific cross-section(s), rated voltage and construction have been successfully performed and “cleared/passed”, the same type approval shall be considered valid for cables of other cross-sections, rated voltages and constructions as long as all of the following conditions are met: 1.
The voltage group is not higher than that of the tested cable(s).
2.
The conductor cross-section is not larger than that of the tested cable(s).
3.
The cable has the same or similar construction to that of the tested cable(s).
4.
The calculated nominal electrical stress at cable conductor screen does not exceed the electrical stress at cable conductor screen of the tested cable(s) by more than 10%.
5.
The calculated nominal electrical stress at cable insulation screen does not exceed the electrical stress at cable insulation screen of the tested cable(s).
The type test on cables of different voltage ratings and conductor cross-sectional areas are required if these cables are of different materials and/or have been produced using different manufacturing processes.
Repetition of ageing test on pieces of a complete cable to check the compatibility of materials may be required in the following condition: The combination of materials applied over the screened core is different from that of the cable on which the type tests have been successfully carried out. 2.4.2 Proposed technical data
After the type test report has been scrutinized and found in compliance with the specifications, the next step is to review the proposed technical data provided by the supplier. This data is also important because it provides the key details of the proposed cable, that are cable construction, physical & electrical characteristics, materials applied and packing details. If the proposed technical data is not in compliance with the specifications, the bid will be rejected unless explanation is given in a deviation form. In case, some of proposed technical data has not been asked or specified clearly in the specifications, it could be accepted, provided that it does not have implications on the installation, rating and life time of the cable. 2.4.3 Deviation form
The Deviation Form clearly describes the characteristics of the proposed cable that are different from the specifications. If there are any major deviations, the proposed cable shall be rejected. In the event of minor deviations, the decision shall be made based on certain key factors, such as effect on installation, rating and life span of the cable. 2.4.4 Drawings
Drawings are required to allow the committee members understand the detailed construction and characteristics of the proposed cable. Drawings containing all information required shall be attached with the bid. Drawings and the proposed technical data should be evaluated together. If there are any major deviations, the proposed cable shall be rejected. In the event of minor deviation(s), the related departments shall be consulted and the decision shall be made based on the likely impact on installation, capacity and life span of the cable.
2.4.5 Reference list of supply and field experience
Sometimes, one supplier offers two options of cables, one from a reputable manufacturer and the other from an inexperienced manufacturer whose price is lower than that of the former manufacturer. In this case, a reference list of supply field experience is needed. Some utilities may make a trip to visit the factory of the new manufacturers to make sure they are qualified. If necessary, it is recommended that utilities make a special requirement for reference list of supply field experience of the proposed cable manufacturer in their specification; for example, a specific number and year of supply of the proposed cable(s) to other countries. Another solution to purchase from the new manufacturer is called tr ial contract which the utility can specify in the tender’s condition to purchase not more than 10% of the tender quantity and to purchase the balanced quantity from the fully comply tender’s condition bidder.
2.5 Conclusion The bid evaluation process and documents required of each LMS utility may be similar. However, it takes a lot of time to get through the process. To complete the evaluation process in a short period of time, all required documents and data to be included in the bid shall be clearly specified in the specifications. Also, the Evaluation Committee shall be represented by qualified personnel. It is also recommended to perform separate evaluation if proposed cables have different construction characteristics and/or made of different materials.
Chapter 3
Cable Manufacturing Inspection 3.1 Introduction After the procurement contract for underground cables is signed, several processes need to be verified to ensure that cables comply with the specifications and conditions specif ied in the contract. Drawings shall be provided for approval. If necessary, the test method shall be, by the agreement between the purchaser and the manufacturer, proposed by the manufacturer and approved by the utility. Representatives from the utility, who are appointed based on experience in various aspects, such as specification handling, installat ion, testing, maintenance and purchasing, shall inspect the production of cables at the factory.
3.2 Objective The inspection of cable manufacturing processes is required to ensure all processes run according to contractual requirements. If any processes are found not conforming to the reference standard and specifications, the Inspection Committee shall reserve the right to suspend the production and resolve all issues before restarting the production line. This chapter describes the cable manufacturing processes and testing, which includes partial discharge test and high voltage test. A diligent inspection of cable manufacturing processes is fundamental to ensuring satisfactory quality of cables and operating life of at least 25 years.
3.3 Inspection Committee Management Each utility may have different practices and regulations depending on their own policies. This proven practice is very useful as it can prevent introduction of poor cables in the service. It also has a track record of success in a utility that is responsible for distributing electricity in capital and other main cities. After the contract is signed, an Inspection Committee should be approved by the top management. The committee members will be selected from the related departments, such as:
•
Electrical Engineering Department: The department responsible for specification or term of reference (TOR).
•
Installation and Construction Department: The department with field experience in installation and construction of the cables and their accessories.
•
Testing Division or Maintenance Department: The departments with experience in testing and preventive maintenance of cables.
•
Purchasing or Contract Department: The department that controls all the related documents -quotation, technical and commercial condition agreement and the approval drawings.
All engineers and technicians represented in the production Inspection Committee will be given the approval drawings and the related correspondence for their references before they go witnessing the cable production line and testing. The supplier shall provide free access to the production facilities and shall satisfy the representatives that the material and equipment are in accordance with the specifications and the contract. In the event of a disagreement or dispute, for example, if either the contract details are not clear or the supplier would like an exception to be made, the issues should be discussed with the top management. The meeting with the top management should keep the interests of the utility as the main focal point while deciding on such disputed matters. This should be considered as a standard practice.
3.4 Manufacturing Process Inspection This section describes manufactu ring process of extruded-dielectric cables, which are used nowadays. Once the production of cables starts, the Manufacturing Inspection Committee will be requested to visit the factory and inspect the production line of cables. This is to make sure that manufacturing is according to the specifications in the contract. The manufacturing process begins by making copper wires and then stranding the conductor. Stranding is performed using the conventional method regardless of whether the conductor is concentric, round , compressed, compact, or segmental, and with or without strand blocking. The stranded conductor is reeled onto a drum, which is placed into its position on the extrusion line. Then the drum becomes the payoff reel or the first subsystem in a series of subsystems that comprise the extrusion process.
Other subsystems (not all required for all insulation types) are:
•
Accumulator: Provides in-line conductor time for changing reels and welding the conductor for continuous extruder operation.
•
Conductor preheating: It shortens the cu ring time by 20-60% depending on conductor size, consequently increasing the line speed and plant production.
•
Compound Handling Subsystem; Stores, dries, and feeds the compound into the extruders and, in some cases, incorporates in-line inspection.
•
Extruder: The polymer materials are supplied mixed with additives and cross-linking catalysts, and are heated to a plastic state. The extruder screw compresses the material and forces it through a fine mesh screen into the crosshead through which the conductor travels.
•
Crosshead: In a true-triple extrusion process, one crosshead contains the extruder for the insulation and the two semiconducting layers. The three layers are formed onto the conductor simultaneously through their respective dies. This special process will prevent the impurity such as moisture, dust or any pollution particles from penetrating the insulation layer
•
Vulcanizing Tube: It provides the pressure and temperature required for the cross-linking process used for XLPE and EPR (Ethylene Propylene Rubber) insulations.
•
Cooling Tubes: It provide a carefully controlled cooling zone following the cross-lin king process for XLPE or EPR.
•
Traction Units: The capstan and caterpillar maintain the proper line tension required for catenary vulcanizing lines.
•
Control Unit: It monitors and controls the temperature and pressure, and provides synchronized operation between the line speed and the extruder screw speed.
•
Take-Up: This final step reels completed cable core onto a drum and hauls the drum away for subsequent processes in cable assembly.
Take up (Copper wire) Pay off Cleaning
Drawing
Reducing
Copper Wire Making Process Take up (Conductor)
Pay off Cleaning
Stranding 1 Stranding 2
Stranding 3 Tractive
Conductor Making Process
Take up
Compound Pay off Dio monitor
Preheat
Corss Head for 3 layer extrusion
Cooling
Monitor Electrotechnical Length monitor Counter
Extrusion Line
Figure 5 : Schematic Diagram of Extrusion Line Of the three insulation types, only PE doesn’t require curing tube since there is no cross-linking of the polymer. After the insulation and extrusion of the two semi-conducting layers onto the conductor, the line passes through a controlled cooling chamber (frequently a closed, pressurized tube) before being reeled on a take-up drum. Dry-Cure Systems: The radiant dry-cure system, as opposed to the steam-cure system, features
independent control of pressure and temperature. This system uses an inert gas, such as nitrogen, as the pressurizing medium. Pressure prevents the premature release of the volatile curing agents. The temperature is maintained by an electrically heated curing tube. Heat is transferred from the tube to the cable by radiation. In the gas-cure system, both pressure and temperature are provided by a high-pressure, nitrogen circulating system. This system circulates nitrogen through a curing tube, then through a heat exchanger and finally back to the curing tube. In the long-land die-curing system, the extruder die is 50-66 ft (15-20 m) long and the required temperature and pressure are maintained within the die, which serves as the curing tube. Because the die is full and under high pressure, there is little opportunity for gravity to pull the extruder off the center.
Cooling: Opinions vary as to which is the best method of cooling. Some manufactures believe
that even the driest cables have some residual moisture that cannot be reduced. They claim that the moisture is not from the cooling water, but rather is a by-product of the curing process. Others claim that water cooling shocks the insulation and sets up mechanical stresses which, in turn, intensify the shrink-back phenomenon. Dry-cooling methods use gas or silicone-oil circulat ing systems. It is in the cooling system that cables release volatile by-products of cross-linking, which must be carried away. The cable continues to de-gas for about three weeks. Without de-gassing, cable outer sheath may swell when subject to high temperature in tropical climate of LMS, then the manufacturer or contractor should take response to release all generated gas by pumping machine until the cable resume as standard specification requirement before installation. Hermetic Sealing: The industry practice is that if the cable is to be installed in a wet environment
where moisture ingress through the jacket is probable, the cable should be hermetically sealed. This is to prevent moisture ingress and initiated treeing under voltage stress. The types of hermetic sealing in common use are lead sheath, corrugated aluminum or copper sheath, and metal laminates.
3.5 Factory Acceptance Tests When the production is finished, the cable is ready to undergo routine and special tests required by the customer -- the utility. The routine and special tests witnessed by the Inspection Committee are called “Factory Acceptance Tests” or FAT. The Inspection Committee will witness at least the following tests. Testing Procedure Routine tests 1.
Partial discharge test: The partial discharge test shall be carried out in accordance with
IEC 60885-3, except that the sensitivity as defined in IEC 60885-3 shall be 10 pC or less. The test voltage shall be raised gradually to hold at 1.75 Uo for 10s and then slowly reduced to 1.5 Uo. The magnitude of the discharge at 1.5 Uo shall not exceed 10 pC. Values of the test voltage for the standard rated voltages are given below.
Rated voltage U kV
22 - 24
45 - 47
60 - 69
110 - 115
132 - 138
150 - 161
Rated voltage Uo kV
12
26
36
64
76
87
First raised voltage 1.75 Uo kV
21
45.5
63
112
133
152.3
Test voltage 1.5 Uo kV
18
39
54
96
114
131
Table 5 Partial Discharge Test 2.
Voltage test: The voltage test shall be made at ambient temperature using an alternating
test voltage at power frequency. The test voltage -- between the conductor and metallic screen/sheath -- shall be raised gradually to the specified value and held there for 30 minutes. The test voltage shall be 2.5 Uo. There will be no breakdown of the insulation. The voltage test values are shown below.
Rated voltage U kV
22 - 24
45 - 47
60 - 69
110 - 115
132 - 138
150 - 161
Rated voltage Uo kV
12
26
36
64
76
87
Test voltage 2.5 Uo kV
30
65
90
160
190
218
Table 6: Voltage Test
3.
Electrical test on non-metallic sheath
If required in contract, the non-metallic sheath shall be subject to the routine electrical test as specified in IEC 60229. Special tests
Special tests shall be done on the cables for about 10% of total drums. 1. Construction and dimension check: Construction and dimension of each layer shall be checked. The test method shall be in accordance with clause 8 of IEC 60811-1-1.
Requirement for insulation
The lowest measured thickness at any point shall not fall below 90% of the nominal thickness:
Tmin ≥ 0.9 Tn (Tmax- Tmin)/ Tmax ≤ 0.15
Additionally: Where
Tmax : The maximum thickness (mm) Tmin : The minimum thickness (mm) Tn
: The nominal thickness (mm)
Note: Tmax and Tmin are measured at the same cross-section of the sample. Thickness of the semi-conducting screen on the conductor and over the insulation shall not be included in the thickness of the insulation. Requirement for the non-metallic sheath
The lowest measured thickness shall not fall below 85 % of the nominal thickness by more than 0.1 mm.
Tmin ≥ 0.85 Tn- 0.1 Where Tmin : The minimum thickness (mm)
Tn
: The nominal thickness (mm)
2. Conductor resistance test: The complete cable length, or a sample thereof, shall be placed in a test room, which shall be maintained at a reasonably constant temperature for at least 12 hours before the test. In case of a doubt that the conductor temperature is not the same as the room temperature, the resistance shall be measured afte r the cable has been in the test room for 24 hours. Alternatively, the resistance can be measured on a sample of conductor conditioned for at least 1 hour in a temperature-controlled liquid bath. The D.C. resistance of the conductor shall be corrected to a temperature of 20°C and 1 km length in accordance with the formulae and factors given in IEC 60228. The D.C. resistance of each conductor at 20°C shall not exceed the appropriate maxi mum value specified in IEC 60228, if applicable.
For example, the maximum D.C. resistance of conductor at 20°C for stranded copper conductor size 70, 150, 240, 400, 800, 1000 and 1200 mm 2 are 0.268, 0.124, 0.0754,
0.0470, 0.0221, 0.0176 and 0.0151 Ω/km respectively. 3. Hot set test of insulation: The test piece shall be suspended in an oven and weights attached to the bottom jaws to exert a force as specified in the applicable standard. Af ter 15 min in the oven at the specified temperature, the dista nce between the marker lines shall be measured and the percentage elongation shall be calculated. If the oven does not have a window and the oven door has to be opened to make the measurement, the measurement shall be made not more than 30 seconds after opening the door. In case of a dispute, the test shall be car ried out in an oven with a window and the measurement made without opening the door. The tensile force shall then be removed from the test piece (by cutting the test piece at the lower grip), and the cable piece shall be left to recover for 5 minutes at the specified temperature. The test piece shall then be removed from the oven and allowed to cool slowly to the ambient temperature, after which, the distance between the marker-lines shall be measured again. For the evaluation of results, the med ian value of the elongation -- der ived after 15 minutes at the specified temperature with the weight attached -- shall not exceed the value specified in the standard. And the median value of the distance between the marker lines -- after removing test piece from the oven and allowing it to get cool -- should not increase compared to the value before inserting the piece in the oven by more than the percentage specified in the standard. 4. Capacitance test: The capacitance shall be measured between conductor and metallic screen/sheath. The measured value shall not exceed (usually by more than 8%) the nominal value declared by the manufacturer.
3.6 Conclusion Usually, utilities have their own inspection committee teams to inspect the production line, and witness routine and special tests at the factory. In case the factory is located outside the country, utility may send a representative of the Inspection Committee to the factory or hire a third party inspector.
Some utilities may have their own inspection forms or special documents to process the comments and some corrections during the inspection, such as witness tests, material and const ruction check, discussion, etc. In case of no inspection form, it is necessary for inspection committee to investigate the production process and quality control before signing in every document which is usually prepared by the factory and make a request for a copy for their reference and every comment to the factor y should be sent in official paper.
Chapter 4
Contract Acceptance 4.1 Introduction The purchasing contract includes all commercial conditions and technical requirements. After the contract is signed, contract acceptance has to be done to ensure that the quality of the cables complies with the contract requirements. This process is also important for utilities to ensure cables are delivered as per the contractual delivery period. The contract acceptance becomes a problem when the delivered cables do not conform to the contract requirements, such as construction, physical and electrical characteristics, or when the cables are damaged during tra nsportation. All such problems shall be settled before cables are accepted.
4.2 Objective Usually, the contract acceptance is done by an Acceptance Committee and not by one person or a department. The contract acceptance process and procedure may be different for each utility. The objective of this chapter is to share the experience in preventing and solving problems faced during contract acceptance process. Various scenarios have been explained as a guide to solving problem.
4.3 Acceptance Committee Management This follows the same procedure as mentioned for the Production Inspection Comm ittee in Chapter 3. The Acceptance Committee should be approved by the upper management after the contract is signed. The members will be selected from the related departments, such as: Maintenance Depar tment which is responsible for cable maintenance and repair.
•
Installation and Construction Department that has field experience in installation and construction of the cables and their accessories.
•
Testing Division which is experienced in testing and preventive maintenance of cables.
•
Purchasing or Contract Depar tment that deals with all the related document like quotation, technical and commercial agreement, and approval drawings.
All engineers and technicians who are members of the Acceptance Committee will be given the approval drawings and the related correspondence for their reference. They will use these documents during sampling of the delivered cables for testing. The supplier shall submit the routine test report and special report of all cables to the utility before the shipment of the cables. After that the routine tests and special reports will be sent to the Acceptance Committee for approval. In the event of a disagreement or dispute, for example, if the contract details are not clear or the supplier would like an exception to be made, the issue should be presented at a meeting with the top management. During discussions on the dispute, the meeting participants should address the disadvantages and advantages to the utility as the main focal point. This convention should be considered as a standard practice.
4.4 Acceptance Process Practically, the contract acceptance process comprises three steps. The first step is visual inspection. The second step is routine and special reports verification and the third is acceptance test or sampling test by utility. Routine and special reports and acceptance test report shall be reviewed by the Acceptance Committee. The detailed specification with approval drawings shall be used as a reference. Here is a practical guideline for contract acceptance process. The delivered cables shall be accepted provided that all three following conditions are met: 4.4.1 Visual Inspection
After the delivery of cables, the Acceptance Committee will conduct a visual i nspection at the site to check the quantity of the cables (according to the invoice of the supplier) and any damages that may have occurred during transportation. If any damaged cable is found, the Committee will ask the supplier to replace it with a new one. During visual inspection, the Committee will also randomly select the quantity of cables as stated in the contract for acceptance tests by utility itself. The quantity typically does not exceed 3 meters per drum and not in exceed of 3 drums due to the constraints of cost, time and laboratory capacity.
4.4.2 Routine and Special Tests Verification
The routine and special tests shall be carried out in order to determine whether the cable complies with the specification. The specification includes the required tests. The routine and special test reports shall be submitted to the utility before shipment. If the test result of any cable does not comply with the specification, the cable shall be rejecte d. Samples of routine and special tests report are shown in Table 7. As minimum, the following acceptance tests should be done at the utility’s laboratory. Special tests a) Construction and dimension check b) Conductor resistance test c) Hot set test of insulation d) Capacitance test Routine test a) Partial discharge test b) Voltage test c) Electrical test on non-metallic sheath
Routine and Special Tests Report Customer:
Drum No.:
Cable Type: 115 kV XLPE Copper cable, 800 mm2
Sample Condition:
Contract No.:
Ambient Temp.:
Reference Standard:
Manufacturer:
Test Items
Unit
Specification
Test Results
pC -
10 (Max) No breakdown No breakdown
0.5 No breakdown No breakdown
mm
Plain annealed copper Compact circular strand 53 (Min) 34.0± 1%
Plain annealed copper Compact circular strand 61 33.9
-
Semi-conductive tape
Semi-conductive tape
3. Conductor screen - Material - Average thickness - Minimum thickness
mm mm
Semi-conductive XLPE 1.5 1.2
Semi-conductive XLPE 1.9 1.7
4. Insulation - Material - Average thickness - Minimum thickness - Diameter over insulation
mm mm mm
XLPE 16 14.4 69-72
XLPE 16.9 16.2 70
5. Hot set test - Elongation under load - Elongation after cooling
% %
175 (Max) 15 (Max)
75 0.8
mm mm
Semi-conductive XLPE 1.5 1.2
Semi-conductive XLPE 1.8 1.7
mm
Semi-conductive 0.45 40
Semi-conductive 0.45 40
Routine tests 1. Partial discharge test at 96 kV 2. AC. High Voltage Test at 160 kV for 30 minutes 3. Electrical test on non-metallic sheath at 15 kV
Special tests 1. Conductor examination & check of dimensions - Material - Design type - Number of wires - Diameter of conductor 2. Separator tape - Material
6. Insulation screen - Material - Average thickness - Minimum thickness 7. Synthetic water blocking tape - Material - Thickness - Width
Table 7 : Routine and Special Tests Report 4.4.3 Acceptance Tests
It is recommended that even though the supplier performs the entire routine and special tests on the cables, the utility should also perform sample tests for its own verification before acceptance. The utility should implement a detailed step by step acceptance test
protocol to verify that the delivered cables conform to the specification, are of good quality and condition. In case the utility doesn’t have its own laboratory to perform acceptance tests because of technical and/or monetary reasons, it is recommended that a third party shall be employed to witness the tests and approve all contract documents at the factory. This may require a monetary investment, but the value outweighs the cost due to the high cost of owning testing equipment. As minimum, the following acceptance tests should be done at the utility’s laboratory. a) Construction check b) Dimension check c) Conductor resistance d) Cross-section area A sample of acceptance tests performed by the utility is shown in Table 8. Acceptance Tests Report — By Testing Division Cable type: 115 kV XLPE Copper Cable
Customer:
Contract No.:
Date of receipt:
Manufacturer:
Date of test:
Test Items
Units
Specification
Test Results
Nominal cross-sectional area
mm 2
800
798
53
61
Minimum number of wires in the conductor Diameter of conductor
mm
34
33.9
Thickness of conductor screen
mm
1.5
1.7
Thickness of insulation
mm
16
16.7
Range of diameter over insulation
mm
69-72
70
Thickness of insulation screen
mm
1.5
1.68
Total cross-sectional area of copper wire screen (minimum)
mm 2
120
120
Minimum number of screen wires
wire
70
70
Average thickness of aluminum tape in radial water barrier (minimum)
mm
0.19
0.2
Thickness of non-metallic sheath (excluding rib)
mm
3.5
3.6
Range of diameter over rib-bottom of the sheath
mm
86-91
89
Ω/km
0.0221
0.022
Maximum dc resistance of conductor at 20ºC
Table 8 : Acceptance Test Report
4.5 Conclusion The purpose of this chapter is to guide the utilities to learn and share experience in solving these problems in contract acceptance process. The process and solutions mentioned in this chapter have been used by some utilities and proven successful. More steps may be required, if necessary. In reality, there may be many different opinions or arguments due to different policies of each utility. In case a utility does not have its own laboratory, it is recommended that the utility should engage an independent testing company to conduct contract acceptance tests. This may require a monetar y investment, but the value outweighs the cost due to the high cost of testing equipment as well as cables.
Chapter 5
Calculations on Cable 5.1 Introduction This chapter will explain the important principles and calculations for cable ampacity, cable sheath thickness, and cable pulling tension.
5.2 Objective This chapter will enable the readers to understand the principles, methods, parameters and formulae for calculating cable ampacity, cable sheath thickness and cable pulling tension.
5.3 Ampacity Calculation Underground cables are far more expensive to install and maintain than overhead lines. The major cost of underground installation comes from the cables itself; labor; and the time required to manufacture the cables, excavate, backfill the trench, a nd to install the cables. Most underground installations are constructed in congested urban areas and also as leads f rom generating plants to substations. The cable must carry the load currents without overheating and also without producing excessive voltage drop. This voltage is known as IZ drop after the formula used to determine it, but in underground systems, this is ra rely a limiting factor. In addition to the normal loads, a transmission system is customarily designed to car ry overloads. These overloads may happen due to equipment and line outages or other abnormal system conditions for limited periods of time. These overload periods could often be 10 hours or even longer. Cables are permitted to operate at higher than normal temperatures dur ing these overload periods, and the response of the cable system to these overloads is evaluated in transient rating computations.
The ampere rating of an electric power cable will depend on its construction and the method of installation. There is a great variety of cable constructions currently used around the world. Also, installation conditions vary widely.
Cable Ampacity Calculation Sheets 5.3.1 General Data
f (Hz)
= system frequency
U (V)
= cable operating voltage (phase-to-phase)
LF
θt, θs, θa θamb
= daily load factor = temperature of tape, sheath and armor respectively = ambient temperature
5.3.2 Cable Parameters Conductor
S (mm2)
= cross-sectional area of conductor
De (mm)
= external diameter of cable or equivalent diameter
De* (m)
= external diameter of cable or equivalent diameter of cable (cables in air)
dc (mm)
= external diameter of conductor
dc′ (mm)
= conductor diameter of equivalent solid conductor having the same central oil duct
di (mm)
= conductor inside diameter
n
= number of conductors in a cable
Insulation
Di (mm)
= diameter over insulation
t 1 (mm)
= insulation thickness between conductor and sheath
ρ (K ∙ m/W) = thermal resistivity of the material * * The same symbol is used for thermal resistivity of various materials. The appropriate numerical value taken from clause 5.3.1 will correspond to the material considered.
Sheath
Ds(mm)
= sheath diameter
d (mm)
= sheath mean diameter
p2, q2
ζ (Ω ∙ m)
= ratios of minor section lengths where minor section lengths are a, p2a, q2a and a is the shortest section = electrical resistivity of sheath material at operating temperature
Armor or Reinforcement
A (mm2)
= cross-sectional area of the armor
Da (mm)
= external diameter of armor
da (mm)
= mean diameter of armor
df (mm)
= diameter of armor wires
d2 (mm)
= mean diameter of reinforcement
na
= number of armor wires
nt
= number of tapes
ℓa (mm)
= length of lay of a steel wire along a cable
ℓT (mm)
= length of lay of a tape
tt (mm) Wt (mm)
β
= thickness of tape = width of tape
= angle between axis of armor wire and axis of cable
Jacket/serving
tJ (mm)
= thickness of the jacket
t3 (mm)
= thickness of the serving
5.3.3 Cable parameters – installation conditions Duct Bank/Thermal Backfill
LG (mm)
= distance from the soil surface to the center of a duct bank
x, y (mm)
= sides of duct bank/backfill (y > x)
N
= number of loaded cables in duct bank/backfill
ρc (K · m/W) = thermal resistivity of concrete used for a duct bank or of the backfill
ρe (K · m/W) = thermal resistivity of earth surrounding a duct bank/ backfill
Cables in Ducts
Dd (mm)
= internal diameter of the duct
Do (mm)
= external diameter of the duct
θm (ºC)
= mean temperature of duct filling medium
5.3.4 Conductor AC Resistance Material
(ρ20) Resistivity −8 · 10 Ω ∙ m at 20ºC
Temperature Coefficient (α20) · 10 −3 per K at 20ºC
Copper
1.7241
3.93
Aluminum
2.8264
4.03
Table 9 : Conductor AC Resistance
R’ = 1.02 · 106 ρ20 S R’ =
[1 + α20 (θ – 20)] Ω · m
5.3.5 Dielectric Losses
ε
tan δ
Butyl rubber
4
0.050
EPR – up to 18/30 kV
3
0.020
EPR – above 18/30 kV
3
0.005
PVC
8
0.1
PE (HD and LD)
2.3
0.001
XLPE up to and including 18/30 (36)
2.5
0.004
kV – unfilled
2.5
0.001
3
0.005
2.8
0.001
Type of Cable
Cable with other kinds of insulation
XLPE above 18/30 (36) kV – unfilled XLPE above 18/30 (36) kV – filled Paper-polypropylene-paper (PPL) Table 10 : Dielectric Losses of Insulation
= 2 ∙ ∙ 20 ∙ tan =
W/m
5.3.6 Sheath Loss factor Sheath Resistance Resistivity (20) 10−8 Ω ∙ m at 20ºC
Temperature Coefficient (20) · 10−3 per K at 20ºC
Lead or lead alloy
21.4
4.0
Steel
13.8
4.5
Bronze
3.5
3.0
Stainless Steel
70
Negligible
2.84
4.03
Material
Aluminum Table 11 : Sheath Resistance of Each Material
=
20 ∙ 10−6 ∙ ∙
[1+20(−20)]
=
Ω/m
For lead sheath reinforced with nonmagnetic tapes :
20∙10−6 1+ = ∙∙ ℓ
[ ( ) ] [1+
t =
If
2
(−20)]
20
Ω/m
ℓ ≥0.44, shall be multiplied by 2.
To calculate sheath losses, use the combined resistance of sheath and reinforcement.
=
+
Substitute for in what follows.
5.3.7 Sheath resistance
For single conductors in flat formation -- regularly transposed and sheaths bonded at both ends:
1 = 4 ∙ 10 ∙ ln [2 ∙ √2 ( −7
3
1=
s d
)]
Ω/m
Single-conductor cables
(1) Sheath bonded both ends – triangular configuration:
1′=
1
∙ 1+ (
1
2
)
1′= ______ 1′′= 0 (2) Sheath bonded both ends – flat configuration, regular transposition:
1′=
1
∙ 1+ (
2
)
1′= ______ 1′′= 0 Large Segmental Conductors
When conductor proximity effect is reduced, for example, by large conductor having insulated segments, 1′′ cannot be ignored and is calculated as follows:
== = + =
− 3
Cable in flat formation with equidistant spacing:
422+(+2) = 4(2+1)(2+1) 1′′ is calculated by multiplying the value of the eddy current sheath loss factor calculated below by .
Sheaths Single-point Bonded or Cross Bonded
Lead-sheathed cables
1=0 =1 For corrugated sheaths, the mean outside diameter shall be used.
1=
√
4 107
=2 1.74 =1+ ( ) (1 ∙ 10−3 − 1.6) The frequency-to-resistance ratio () can be calculated by the following equation.
=
2 −7 ∙ 10
If ≤ 0.1, ∆1=0, ∆2=0 Three Single-conductor Cables in Flat Configuration
(1) Center cable:
2 2 0 = 6( 2 ) 1+2 0= 1.4+0.7 Δ1 = 0.863.08 ( 2 ) Δ1= Δ2= 0
(2) Outer cable leading phase:
2 2 0 = 1.5( 2 ) 1+2 0= 0.16+2 Δ1 = 4.70.7 ( 2 ) Δ1= 1.47+5.06 Δ2 = 213.3 ( 2 ) Δ2 =
(3) Outer cable lagging phase:
2 2 0 = 1.5( 2 ) 1+2 0= 0.74 (+2) 0.5 +1 Δ1 = 2 + (−0.3)2 ( 2 ) Δ1= +2 Δ2 = 0.923.7 ( ) 2
Δ2 = (1)4 0(1+Δ1+Δ2)+ ∙10−12] 1′′ = [ 12 1′′= 5.3.8 Sheaths Cross Bonded
The ideal cross-bonded system will have equal lengths and spacing in each of the three sections. If the section lengths are different, the induced voltages will not sum up to zero, and as a result, circulating currents will be present. These circulating
currents are t aken into account by calculating current loss factor
1′, assuming the
cables were not cross bonded, and multiplying this value by a factor to take into account the variations in length. T his factor
=
Where:
[
2+2−2 2+2+1
is given by
2
]
2 = length of the longest section 2 = length of the second longest section
= length of the shortest section
This formula deals only with t he differences in the length of minor sections. Any deviations in spacing must also be taken into account. Where the lengths of the minor sections are not known, IEC 287-2-1 (1994) recommends that the value for on experience with carefully installed circuit be
1′ = 0.03 for cables laid directly in the ground 1′ = 0.05 for cables installed in ducts 5.3.9 Total Sheath Loss Factor
1 = 1′ + 1′′ 5.3.10 Armor Loss factor
For cables without armor, the armor loss factor ( 2) is equal to zero.
1′ based
5.3.11 Thermal Resistances
Material
Thermal Resistivity ( ) ( ∙/ )
Thermal Capacity ( ∙10−6 ) [ /(3 ∙ )]
PE
3.5
2.4
XLPE
3.5
2.4
Polyvinyl chloride up to and including 3 kV cables greater than 3 kV cables
5.0 6.0
1.7 1.7
EPR up to and including 3 kV cables great than 3 kV cables
3.5 5.0
2.0 2.0
Butyl rubber
5.0
2.0
Rubber
5.0
2.0
Paper-polypropylene – paper (PPL)
6.5
2.0
Compounded jute and brous materials
6.0
2.0
Rubber sandwich protection
6.0
2.0
Polychloroprene
5.5
2.0
PVC up to and including 35 kV cables greater than 35 kV cables
5.0 6.0
1.7 1.7
PVC/bitumen on corrugated aluminum sheaths
6.0
1.7
PE
3.5
2.4
Concrete
1.0
2.3
Fiber
4.8
2.0
Asbestos
2.0
2.0
Earthenware
1.2
1.8
PVC
6.0
1.7
PE
3.5
2.4
Insulating materials*
Protective coverings
Materials for duct installations
Table 12 : Thermal Resistances of Each Material * For the purpose of current rating computations, the semiconducting screening materials are assumed to have the same thermal properties as the adjacent dielectric materials.
5.3.12 Insulation Thermal Resistance Single-conductor Cables
1 =
2t ln(1+ 1) 2 dc
1= ____________________ K ∙ m/W Three-conductor Shielded Cables
(1) With round or oval conductors:
1=/2 1= ____________________ mm The geometric factor (G) and the screening factor (K) are obtained from the following figures.
Figure 6 : The geometric factor (G) and the screening factor
1=
2
1= ____________________ K ∙ m/W 5.3.12.1 Jacket Thermal Resistance
2 =
2t ln(1+ J ) 2 DS
2= ____________________ K ∙ m/W 5.3.12.2 Serving Thermal Resistance
3 =
2t ln(1+ 3 ) 2 Da
3= ____________________ K ∙ m/W 5.3.12.3 External Thermal Resistance of Buried Cables
For buried cables, two values of the external thermal resistance are calculated: T 4, which corresponds to dielectric losses (100% load factor), and T 4µ , which is the thermal resistance corresponding to the joule losses, where allowance is made for the daily load factor ( ) and the corresponding loss factor µ
=0.3∙()+0.7∙()2 = __________________ The effect of the loss factor is considered to start outside a diameter D x, defined as
=61200 √ ( length of cycle in hours) , where is soil diffusivity (m 2/h). For a daily load cycle and typical value of soil diffusivity of 0.5·10 −6 m 2/s, D x is equal to 211 mm (or 8.3 in). The value of Dx is valid even when the dia meter of the cable or pipe is greater than D x. Mutual Heating Effect
A factor
accounts for the mutual heating effect of the other cables or cable pipes
in a system of equally loaded, identical cables or cable pipes. The distances nee ded to compute factor are defined in the following diagram. These are center-to-center distances.
Figure 7 : Mutual Heating Effect
For cable p:
=
′1 1
′2 … 2
′k … ′q k q
( )( ) ( ) ( ) = __________________
There are (q – 1) terms, with term d’ pp/d pp excluded. The rating of the cable system is determined by the rating of the hottest cable or cable pipe, usually the cable with the largest ratio L/D o . For a single isolated cable or cable pipe, F = 1. Single-conductor Cables
When the losses in the sheaths of single-core cables laid in a horizontal plane are appreciable, and the sheaths are laid without transposition and/or the sheaths are bonded at all joints, the inequality of losses affects the exter nal thermal resistance of the cables. In such cases, the value of the factor F used to calculate T4µ is modified by first computing the sheath factor ( ):
=
1 + 0.5 (′11+′12) 1+′
= __________________
Then calculate:
′=() ′= __________________ (1) Equally loaded similar cables:
Directly buried cables. For cables in pipes, use
4 =
in place of in the following formulas.
4∙ ln 2
4= __________________ K ∙ m/W 4 =
4∙ ln + ∙ ln 2
(
)
4= __________________ K ∙ m/W (2) Cables in ducts: Installation Condition
In metallic conduit
5.2
1.4
0.011
In ber duct in air In ber duct in concrete
5.2
0.83
0.006
5.2
0.91
0.010
In asbestos cement duct in air duct in concrete
5.2 5.2
1.2 1.1
0.006 0.011
Earthenware ducts
1.87
0.28
0.0036
′4 =
1+0.1(+)
′4= __________________ K ∙ m/W ′′4 = 2 ln ′′4= __________________ K ∙ m/W is the ther mal resistivity of duct material. For metal ducts, 4′′=0.
The equivalent radius of the envelope () is obtained by the following equation
2 4 x − 1+ ln In+ ( ) ( ) 2 y 2 2 = __________________ mm
ln =
1
= = __________________ = ln[+√2−1] =_________________ 4∙ ln ( − ) + 2 2 4′′′= _________________ K ∙ m/W
4′′′ =
4′′′=
2
Dx
(ln Do + ln
4∙ ( − ) + x ) 2
4′′′′ = __________________ K ∙ m/W 4 = 4′+4′′+4′′′ 4 = __________________ K ∙ m/W 4 = 4′+4′′+4′′′ 4 = __________________ K ∙ m/W 5.3.13 AMPACITY
Buried Cables
=
∆−[ 0.5,1+(2+3+,4)]−∆
[ +(1+ ) +(1+ + ) ( + ] 1
1
2
1
2
3
0.5
4 )
= __________________ A Temperature Rise of Cable Components (buried cable)
Δθ = Δ+{[(1+1+2)(3+4)]+(3+4)} Δ= __________________ ºC
5.4 Insulation and Sheath Thickness Calculation 5.4.1 Insulation Thickness
IEC 60502 specifies the nominal insulation thickness for cables of rated voltages from 6 kV (Um = 7.2 kV) to 30 kV (Um = 36 kV) in Tables 13 to 15. The thickness of any separator or semi-conduct ing screen on the conductor or over the insulation shall not be included in the thickness of the insulation. Nominal cross-sectional area of conductor mm2
Nominal thickness of insulation at rated voltage 3.6/6 (7.2) kV mm
10 to 1600
3.4
Table 13 : Nominal Thickness of PVC/B Insulation for Cable Rated Voltages from 6 kV (Um = 7.2 kV) to 30 kV (Um = 36 kV) Nominal crosssectional area of conductor mm2
Nominal thickness of insulation at rated voltage U0 /U (Um) 3.6/7 (7.2)kV
6/10 (12)kV
8.7/15 (17.5)kV
12/20 (24)kV
18/30 (36)kV
mm
mm
mm
mm
mm
10
2.5
-
-
-
-
16
2.5
3.4
-
-
-
25
2.5
3.4
4.5
-
-
35
2.5
3.4
4.5
5.5
-
50 to 185
2.5
3.4
4.5
5.5
8.0
240
2.6
3.4
4.5
5.5
8.0
300
2.8
3.4
4.5
5.5
8.0
400
3.0
3.4
4.5
5.5
8.0
500 to 1600
3.2
3.4
4.5
5.5
8.0
Table 14 : Nominal Thickness of Cross-lined Polyethylene (XLPE) Insulation for Cable Rated Voltages from 6 kV (U m = 7.2 kV) to 30 kV (Um = 36 kV)
Nominal crosssectional area of conductor mm2
Nominal thickness of insulation at rated voltage U0 /U (Um) 3.6/7 (7.2)kV 6/10 (12)kV
8.7/15 (17.5)
12/20 (24)kV
18/30 (36)kV
mm
mm
-
-
-
3.4
-
-
-
2.5
3.4
4.5
-
-
3.0
2.5
3.4
4.5
5.5
-
50 to 185
3.0
2.5
3.4
4.5
5.5
8.0
240
3.0
2.6
3.4
4.5
5.5
8.0
300
3.0
2.8
3.4
4.5
5.5
8.0
400
3.0
3.0
3.4
4.5
5.5
8.0
500 to 1600
3.2
3.2
3.4
4.5
5.5
8.0
Unscreened
Screened
mm
mm
10
3.0
2.5
-
16
3.0
2.5
25
3.0
35
mm
kV mm
Table 15 : Nominal Thickness of Ethylene Propylene Rubber (EPR) and Hard Ethylene Propylene Rubber (EPR) Insulation for Cable Rated Voltages from 6 kV (U m = 7.2 kV) to 30 kV (Um = 36 kV) 5.4.2 Sheath Thickness
Unless otherwise specified, the nominal thickness t s expressed in millimeters shall be calculated by the following formula:
=0.035+1.0 where D is the fictitious diameter (in millimeters) immediately under the oversheath. (See IEC 60502-1 and 60502-2 for calculation details) The value resulting from the formula shall be rounded off to the nearest 0.1 mm. For unarmored cables and cables with oversheath not applied directly over the armor, metallic screen or concentric conductor, the nominal thickness shall be not less than 1.4 mm for single-core cables and 1.8 mm for three-core cables. For cable with oversheath applied directly over the armor, metallic screen or concentr ic conductor, the nominal thickness shall be not less than 1.8 mm.
Calculation Example
Determining sheath thickness of 240 mm 2 12/20 kV XLPE cable
•
Find (fictitious diameter of conductor) from IEC 60502-2
=17.5 mm •
Calculate (fictitious diameter of any core) from IEC 60502-2
= +2 =17.5 + 2(5.5)=28.5 mm
•
Where, = insulation thickness (XLPE) from Table 7
Calculate (fictitious diameter over laid-up cores) from IEC 60502-2
= =1×28.5=28.5 mm
•
Where, = assembly coefficient is equal to 1 for single-core cable
Calculate (fictitious diameter over inner covering) from IEC 60502-2
= +2 =28.5 + 2(0.4)=29.3 mm
•
Where,
=0.4 when ≤40 mm
Find increase of diameter for concentric conductors and metallic screens from Table A.2 in IEC 60502-2
= + increment number =29.3+5.0=34.3 mm •
Calculate sheath thickness from equation in clause 5.4.2
=0.035+1.0 =0.035(34.3)+1.0=2.2 mm So sheath thickness of 240 mm 2 12/20 kV XLPE cable is 2.2 mm.
5.5 Calculation on Cable Pulling Tension 5.5.1
Technical Parameter
Cable Diameters & Weights
Cable diameters and weights listed in manufacturers’ catalogs and specification sheets are generally approximate and subject to normal manufacturi ng tolerances. Possible variations in cable diameters are taken into consideration in the formulae for cable clearance and jam r atio. Catalog weights are generally adequate except for marginal cable pulls, for which more accu rate weights should be requested f rom the cable manufacturer. Determining Conduit Size
The National Electric Code (NEC) specifies limitations with regard to cable and conduit size for installations under its jurisdiction. As stated in the code, the crosssectional area of cables shall not be larger than certain percentages of the duct, as shown in Table 16. Number of Cables
Types of Cable
1
2
3
4
Over 4
Cable (not lead-covered)
53%
31%
40%
40%
40%
Cable Lead Covered
55%
30%
40%
38%
35%
Table 16 : Maximum Cable Cross-sectional Area as a Percentage of Internal Conduit or Duct Area (Refer to NEC)
Cable Clearance
In applications where the NEC limits do not apply, it is necessary to calculate the clearance between the cable(s) and conduit to ensure that the cables can be pulled through the conduit. It is recommended that the calculated clearance should not be less than 0.5 inches. A lesser clearance, as low as 0.25 inches, may be acceptable for essentially straight pulls. The clearance should also be adequate to accommodate the pulling eye or cable grip, which will be employed for the cable pull.
a) Single Cable Pull
=−
b) Three Cable Pull (Based on Triangular Configuration)
=
2
− 1.366+
1 2
√
2 1− [ ] −
(−)
c) Four Cable Pull (based on Diamond Configuration with Where:
/ ≤ 3)
= conduit I.D. (inches) = 1.05 x nominal cable O.D. (inches) = clearance (inches)
To account for variations in cable and conduit dimensions, and the ovality of conduit at bends, the nominal cable diameter has to be increased by 5 percent for use in the above formulae. Jam Ratio
When the ratio of the inside diameter of the duct to the cable diameter is equal to 3.0, one of the cables in a group of three or four cable pull may slip between two other cables causing the cables to jam in the conduit. This is most likely to occur when the cables are pulled around a bend rather than in a straig ht pull. The following guidelines are suggested to minimize the r isk of such and occurrence during cable installation in conduit. The limits on Jam Ratio indicated herein recognize variations in cable and conduit diameter and ovality in conduit diameter at bends:
1.05 <2.9
1.03d
<3.1
Where:
= conduit I.D (inches) = nominal cable O.D. (inches) To conform to these guidelines one of the above two expressions should be satisfied.
Minimum Bending Radius of Cables
The minimum recommended bending radii for various types of cables are presented in Table 17. These limits should be observed during planning and installation to avoid damage to the cable structure. 1. Single and multiple conductors, unshielded cables with or without lead sheath. Overall Diameter of Cable (inches) Thickness of Cable Insulation (mils*)
1.000 & less
1.001 to 2.000
2.001 & over
Min. Bending Radius as a Multiple of Cable Diameter 155 & less
4
5
6
170 – 310
5
6
7
325 & over
***
7
8
*1 mil = 1/1000 inch
Table 17 : Minimum Recommended Bending Radii for Unarmored Power Cables for Cables Rated up to and including 35 kV (ICEA Standard) 2. Single and multiple conductor, wire shielded cables: Same as above. 3. Single and multiple conductor tape shielded cable: Twelve times the overall diameter of the completed cable. Cable Configuration in Conduit
The relative position of cables in a conduit in a multiple cable pull is important in that it affects the weight distribution of cables and hence the normal force between the cable(s) and the conduit. In case of a three cable pull, when the ratio of inside diameter of the conduit to the nominal diameter of the cable is greater than 2.4 and less than 3.0 , the cables can form a cradled or triangular configuration along straight sections of the duct (see Table 18). At a bend the cables are usually pulled into a cradled formation at these ratios. At ratios greater than 3.0, the cables will assume a cradled formation. In case of four cable pulls with the ratio of conduit inside diameter to the nominal cable diameter is less than 3, the cables will tend to align in a diamond configuration (see Figure 18).
Single Cable
Three Cables
Three Cables
Four Cables
Triangular
Cradled
Diamond
Table 18 : Cable Congurations in Conduit Calculation of Weight Correction Factor
The Weight Correction Factor accounts for the weight distribution of single cables in a multiple cable pull. The factor depends on the relative position of cables in a conduit, which produces a greater normal force between cables and conduit than the force in case of a single cable. This can be viewed as an effective increase in the weight of the cable(s) and leads to the development of the Weight Correction Factor (W c) Wc for a single cable pull is unity. Wc for three cable pulls in cradled and triangular configurations and for four cable pulls in a diamond configuration can be calculated as shown below: a) Three Single Cables in Cradled Configuration: 2 = 1+ [ ] 3 (−)
4
b) Three Single Cables in Triangular Conf iguration:
=
1
√
2 1− [ (−) ]
c) Four Single Cables in Diamond Configuration: 2 = 1+2 [ (−) ]
Where:
= weight correction factor (dimensionless) = inside diameter of conduit (inches) = nominal outside diameter of a single conductor cable (inches)
Coefficient of Friction Effective Coefficient of Friction The effective coefficient of friction for three or
more conductors is a function of the basic coefficient of friction and the relative occupancy of the conductors in a pipe. This is because when th ree or more conductors are pulled into a duct, wedging action exists, which increases t he effective pressure between the cables and the duct. The effect of this action is taken into consideration by use of the Weight Correction Factor ( ). The effective coefficient of friction ( ) is, therefore, expressed as the product of the basic coefficient of friction ( ) and Weight Correction Factor ( ).
= Dynamic Coefficient of Friction The dynamic coefficient of friction is defined as
the factor which is multiplied by the nor mal force exerted by the cable on a conduit by virtue of its weight and the weight of other motion. The basic dynamic coeff icient of friction is a function of the materials that are in contact with each other and the pulling lubricant that is employed. It does not vary signif icantly with the speed of pulling. It is influenced to some extent by the ambient temperat ure.
Duct Material
Cable Outer Covering
One Cable Per Duct
Three Cables Per Duct
75ºF
20ºF
75ºF
PVC
XLPE PE PVC N CN Pb
0.40 0.40 0.50 0.90 0.40 0.25
0.40 0.35 0.25 0.55 0.40 0.25
0.60 0.45 0.60 1.50 -
PE
XLPE PE PVC N CN Pb
0.45 0.25 0.30 0.65 0.20 0.20
0.35 0.20 0.20 0.45 0.20 0.25
0.55 0.85 0.45 -
FIBRE
XLPE PE PVC N CN Pb
0.30 0.25 0.40 0.40 0.40 -
0.20 0.35 0.20 0.30 0.35 -
0.65 0.60 0.45 0.55 -
CONCRETE
XLPE PE PVC N CN Pb
0.30 0.35 0.55 0.50 0.55
-
-
Table 19 : Recommended Basic Dynamic Coefcient of Friction, Straight Pulls & Bearing Pressures Less than 150 lbs/ft (Soap & Water Base Lubricants)
Start-up and Surging The coefficient of friction of the pulling line affects the
maximum tension in the early stages of the pull. And therefore, it governs the required capacity of the pulling equipment. Stopping and starting during the pull and surging phenomena are complicated transient conditions, which can result in higher levels of pulling tension than what would be expected from the analysis of a steady pulling condition. Pulling Tension Limits for Pulling Eyes and Bolts
The maximum recommended pulling tensions when employing pulling eyes and pulling bolts are listed in Table 20. The values listed are for standard compression type
aluminum pulling eyes. It should be noted that higher pulling tension are attainable when aluminum epoxy filled eyes are used with aluminum conductor cables and when solder type cups are used with copper conductor cables.
Conductor – Metal & Type
Maximum Tension - psi Al. Compression Eye1
Epoxy Filled Eye
Copper (annealed)
14,000
-
Aluminum Solid (1/2 thru Full Hard) Stranded (3/4 & Full Hard)
8,000 10,000
10,000 14,000
Table 20 : Recommended Maximum Pulling Tensions at Pulling Eyes
1) When the strength of the compression eye is limiting the length of the pull, the use of a solder type copper pulling eye will permit pulling tensions up to 16,000 psi. For three single conductor cables in parallel config uration, the allowable conductor stress should be based on two cables sharing the load. Pulling Tension Limits for Pulling Grips
The maximum recommended pulling tensions when using steel wire basket grip are given in Table 21. The four inches of cable immediately under the back of the grip should be cleaned and wrapped with two half lapped layers of cloth friction tape. The back end of the grip should be secured with a punch lock type band to aid in initially seating the grip and to prevent it from loosening in case the pulling tension is relaxed during the pull.
CABLE CONSTRUCTION TYPE
Maximum Tension, lbs1 Single Cable
Multi Cable
XLPE Insulation/Jacket – 600 V Cable
2,000
2,000
EPR – Neoprene – 600 Cable
2,000
2,000
PE & XLPE insulation, concentric wire shield, with & without encapsulating jacket
10,000
5,000
8,000 4,000
4,000 2,500
PE & XLPE insulation, concentric wire or tape shield, LDPE & PVC sleeved jackets
10,000
5,000
EPR, insulation, concentric wire or tape shield, LDPE & PVC sleeved jackets
10,000
10,000
16,0002 8,000
16,0002 8,000
18,000
9,000
PE & XLPE insulation, LC shield, LDPE jacket 15, 25 & 35 kV Cable 69 & 138 kV Cable
Lead sheathed cable, with & without jacket XLPE insulation EPR insulation XLPE insulation, copper wire or ribbon shield, MDPD jacket
Table 21: Recommended Maximum Pulling Tensions Copper and Aluminum Conductor Single and Multi Cables per Pull, Pulling Grips
1) When considering use of the above pulling grip tensions, the stress on the cable conductor should not exceed the following values : 16,000 psi for copper conductor (annealed) 14,000 psi for stranded aluminum conductor (1/2 thru Full Hard) 10,000 psi for solid aluminum conductor (3/4 & Full Hard) For three single conductor cables in par allel configuration, the allowable conductor stress should be based on two cables sharing the load. 2) The values are the maximum stress in psi computed on the total cross-sectional area of the lead sheath. It should be noted that the maximum tension may be reduced when three cables are pulled in one grip because the g rip in most instances will not grab t he cables as effectively as is the case when a single cable is pulled.
For lead sheathed cables the maximum lim it is based on the stress computed on the area of the lead sheath. Maximum Sidewall Bearing Pressure (SWBP)
IEEE 690 recommends that the maximum side wall bearing pressure (SWBP) is 500 lb/ft. For single cable pulls the SWBP is the ratio of the pulling tension to the inside radius of the duct bend (SWBP = T/R). The inside radius of the duct bend should be used when calculating the SWBP. For specially fabricated elbows, the inside radius should be measured or calculated using the following formula:
=
(−0.5) 12
Where:
= inside radius of bend (feet) = centerline radius of bend (inches) = duct diameter (inches) The cable sidewall bearing pressure is the radial pressure experienced by the cable as it is pulled through a curved section. The pressure is caused by the tension and weight of the cable, which presses it against the conduit wall. The parameters that influence the SWBP are cable(s) tension, weight, and inside radius of the bend. For practical purposes the weight of a cable(s) is negligible compared to the resultant radial force. Consequently, the cable (s) weight has been omitted from the general SWBP formulae included herein. For a single cable in conduit pulls:
=
The SWBP for three cable pulls should be calculated for the cable that presses hardest against the conduit. In cradled formation, the center cable presses hardest against the conduit and the SWBP for that cable is expressed as follows:
=
(3−2) 3R
In triangular formation, the bottom two cables bear the load equally and experience the greatest SWBP. For this condition, the SWBP equation is as follows:
= 2 For four cable pulls in a diamond configuration, the bottom most cable will be subjected to the greatest crushing force as it is being pulled around a bend. The recommended SWBP equation for this formation is as follows:
=
(−1)
Where:
= sidewall bearing pressure on cable with greatest radial load
5.5.2
=
maximum combined tension of cables for multiple cable pulls or tension on one cable for single cable pulls when exiting the bend
=
inner radius of conduit bend
=
weight correction factor for multiple cable pulls
Calculation of Pulling Tensions
The following formulae can be used to determine pulling tensions for a cable installation. Each equation applies to a specif ic conduit configuration. In order to use the formulae, the cable pull should be subdivided into specific sections. The configuration of each section should be identical with one of the graphical depictions accompanying the equations. The mathematical expression associated with each of the a ccompanying sketches will yield the cumulative tension (T 2) on the leading end of the cable as it exits from a specified section. And T 1 is the tension in the cable entering that section. The maximum tension obtained when pulling in one direction often differs from that obtained when pulling in the opposite directions due to the location of the bends and the slope of the pull. Therefore, the required pulling tension should be calculated for both directions. A listing of the symbols employed and their definitions are as follows:
DEFINITION OF SYMBOLS Symbol
1 2
Denition
Section incoming cable tension
Pounds
Section outgoing cable tension
Pounds
Inside radius of conduit bend Total weight of cables in conduit
Feet Pounds/foot
Angle subtended by bend for curved sections or Angle of slope measured from horizontal for inclined planes
Radians
Offset angle from vertical axis
Radians
Total angle from vertical axis
Radians
′
Effective coefcient of friction
2
-
Actual length of cable in section
Feet
Depth of dip from horizontal axis
Feet
Horizontal length of dip section
Feet
Table 22 : Definition of symbols
5.5. 3
Units
Pulling Tension Formulae for Cable in Conduit
STRAIGHT PULL
2 = 1 + KL HORIZONTAL BEND PULL
2 = 1 cosh + √ 12+()2 sinh SLOPE – UPWARD PULL
Note: Angle measured from horizontal axis
2 = 1+(sin + cos) SLOPE – DOWNWARD PULL
Note: Angle measured from horizontal axis
2 = 1−(sin − cos)
VERTICAL DIP PULL – SMALL ANGLE
Where ′ is small compared to (i.e., tan/2=sin/2=′/)
=
2′
2 = 4′ = 1 + [−1] (Use coefficient of friction corresponding to SWBP < 150 lbs/ft in the equation above) For >
2=14+[4−23+2−1] For ≤ 2=1+2S CONVEX BEND – UPWARD PULL
A. For angle measured from vertical axis
2=1+
[2 sin+(1−2)(1−cos)] 2 1+
B. For angle offset from vertical axis by angle
(derived from (A) above)
=− = 1+
[2 sin+(1−2)(1− cos)] 2 1+
b−
[2a sina+(1−2)(1−a cosa)] 2 1+ a
2 =
2 = −+1 CONVEX BEND – DOWNWARD PULL
A. For angle measured from vertical axis
2 = 1 +
[2 sin− (1−2)( − cos)] 2 1+
B. For angle offset from vertical axis by angle
(derive from (A) above)
=− a = 1a −
[2 sina−(1−2)(a − cosa)] 2 1+
b = 1 +
[2 sinb−(1−2)(b − cosb )] 2 1+ 2 = − + 1
CONCAVE BEND – UPWARD PULL
A. For angle measured from vertical axis
2 = 1 −
[2sin− 2 sin (1−2)( − cos)] 1+2
B. For angle offset from vertical axis by angle
(derive from (A) above)
=− a = 1a −
[2 sina−(1−2)(a − cosa)] 2 1+
b = 1b −
[2 sinb−(1−2)(b − cosb)] 2 1+ 2 = −+1
CONCAVE BEND – DOWNWARD PULL A. For angle measured from vertical axis
2 = 1−
[2 sin+ (1−2)(1− cos)] 2 1+
B. For angle offset from vertical axis by angle (derived from (A) above)
=− = 1−
[2 sin+(1−2)(1− cos)] 2 1+
b+
[2a sina+(1−2)(1−a cosa)] 2 1+ a
2 =
5.6 Conclusion Cable ampacity is an important consideration for the design of an underground cable system. It helps the designer to decide on cable configu ration and selection of suitable cable size for particular requirements. Cable sheath is important for the protection against mechanical damage that could happen both during installation and operation. Therefore, the identification of suitable cable sheath thickness is necessary for minimizing damage to the cables. The calculation for cable pulling tension that occurs d uring installation will help the designer to check if it is higher than the maximum tensile strength of cable. The calculation also enables the designer to come up with the proper design of cable installation to avoid damage to the cables during installation. The thorough and precise calculations of each aspect are important for the most efficient and long working life of the cables. It will also reduce cable maintenance costs.
Chapter 6
Installation 6.1 Introduction Cable installation is one of the costliest items in an underground cable system project. The underground cable shall be installed with proper methods and equipment to minimize cable damage during installation which may cause higher installation and maintenance cost. Although some utilities have their own installation crews, many utilities rely on contractors to install cables. In either case, a properly prepared instruction manuals is essential for a quality installation.
Chapter 6 is devoted to the specications and procedure for installing underground cables.
6.2 Objective This chapter describes underground cable installation methods, cable laying procedure, and installation acceptance process. There are mainly three types of installations: direct burial, duct installation and tunnel installation. This chapter can be used as a guide to learn about the cable laying procedure and also about equipment for cable pulling.
6.3 Types of Installation This chapter presents the typical methods of installation of underground cable, which are typically used
by power utilities around the world. The three main installations will be classied into eight installation methods are discussed in this chapter. 6.3.1 Direct Burial
A direct burial installation is defined as installing cable without pulling into a pipe or duct, but by pulling or laying cables in an open trench and having the ear th in direct contact with the cable jacket or sheath after backfilling, as shown in Figure 8. To avoid the excavation by a stranger, it is recommended that utility apply this method only in the specific areas, e.g., in substation.
Figure 8 : Direct Burial Installation Advantages
Disadvantages
• •
No mechanical protection of the cables and therefore lower reliability.
Low construction cost Quick construction
Insufcient protection in case of short circuit. Maintenance is difcult. Restrictions in place because there is no protection to the specic area. Since the cables are touching each other and surrounded by normal ground with uncontrolled thermal resistivity, the allowable ampacity may be reduced. During installation, the trench has to be opened for the entire cable
route, which can impact trafc on site. 6.3.2 Semi-Direct Burial
Semi-direct burial installation is similar to di rect burial but the cables are pulled into the conduits. Conduits provide mechanical protection to cables as shown in Figure 9. To avoid the excavation by a stranger, some utilities apply this method in only the specific areas, e.g., in substation and foot path.
Figure 9 : Semi-Direct Burial Installation Advantages
Disadvantages
• • •
•
Restrictions in place because there is no
•
During installation, the trench has to be opened for the entire cable route, which can impact
Low construction cost Quick construction Easy maintenance. (especially cable replacement)
protection to the specic area. trafc on site.
6.3.3 Concrete Troughs
Concrete troughs installation is similar to direct burial but the cables are pulled into the concrete troughs. Concrete troughs provide mechanical protection to cables. The following Figure 10 shows a typical cross-section of a concrete trough.
Figure 10 : Typical Cable Laying in Concrete Trough
Advantages
Disadvantages
•
Limited installation cost due to width reduction of cable trench.
•
•
Very good control of cable during pulling all along the cable route.
Cables touch each other, which leads to mutual heating and can result in damage to all three phases in case of a fault on any one cable.
•
The trench has to be opened only the section of installation and resumed back to public utilization before starting the next section until
•
No stress on the cable head or conductor during pulling due to the use of motorized rollers.
•
Use of prefabricated elements possible.
the installation is nished.
6.3.4 Concrete Encased
The cables are laid in conduits which are arranged in rectangular formation and covered by reinforced concrete to be named as double protection. A group of conduits can also be called “Duct Bank”. Conduit spacing shall be 25 cm. Nowadays, this method is commonly selected for construction as main line because it can prevent damage to cable from unintentional excavation by manual or machine due to double protection, but it has an effect on the environmental, especially the traff ic jam in urban area. Figure 11 shows the construction of concrete encased ductbank.
Figure 11 : Concrete Encased Installation Advantages
Disadvantages
• The phase spacing and cable conguration can
• Due to small spacing of the cables, the sheath
be adjusted easily.
voltage or current is higher than in the case of troughs. • During installation, the pulling force on the cable end can be very high, depending on the cable weight and cable route. • A visual control of the cables after pulling is not possible. • The ducts have to be inspected by a camera before cable pulling.
6.3.5 Trenches
Trenches are commonly used in substations. The trenches are generally accessible from the surface for the ease of extension or maintenan ce. Cables are laid in reinforced concrete trench and backf illed with sand. The trench can be easily accessed by opening the top cover. Some utilities apply this method only in the specific areas, e.g., in substation. Typical drawing of cable trench is illustrated in Figure 12.
Figure 12 : Concrete Trench
Trenches are only used in substations or restricted areas because they do not provide sufficient protection against mechanical aggression. Furthermore, due to the limited air volume compared to tunnels and galleries, the ventilation may not be sufficient and the allowable ampacity may be reduced significantly. Since the trenches are covered by steel or concrete covers, the impact in case of a cable fault can lead to danger, even in substations. Advantages
Disadvantages
• • • •
•
Low construction cost Quick construction Easy maintenance Use of prefabricated concrete trench
• •
Restrictions in place because there is no protection to the
specic area. Fewer circuits can be installed. The trench has to be opened only the section of installation and resumed back to public utilization before
starting the next section until the installation is nished.
6.3.6 Horizontal Directional Drilling (HDD)
HDD process begins with boring a small, horizontal hole (pilot hole) under the obstacle (e.g., a highway or river) that needs to be crossed. This is done with a continuous string of steel dr ill rod. When t he bore head and rod emerge on the opposite side of the crossing, a special cutter called a back reamer will be attached and pulled back through the pilot hole. The reamer bores through the pilot hole, enlarging it so that a pipe can be pulled through. The pipe is usually pulled through from the side of the crossing opposite to the drill rig. Utilities usually apply this method for crossing obstacles such as a river or a road. The following figure shows the horizontal directional drilling construction layout.
Figure 13 : Horizontal Directional Drilling Construction Layout Crossing the River. Advantages
Disadvantages
• Quick construction • Minimal impact to trafc • Able to avoid obstacles • Low Construction cost
• •
No mechanical protection for cables. When more pipelines have to be constructed, it should be well planned of the spacing of the ducts group , level, size, etc.
6.3.7 Pipe Jacking
Pipe Jacking is a method of constructing a pipeline under the ground. T he technique involves pushing a pipe through the ground with t he thrust provided by hydraulic jacks. The excavation of soil takes place at the same time the pipe moves into the ground. This provides a structurally flexible and watertight finished conduit. This method is commonly adopted for underground cables. Nowa days, it is the main method used for lay ing pip e to preser ve the natur al environment and maintain pleasant living conditions. Some utilities use reinforced concrete pipes with internal diameter from 1.0- 1.8 m as a jacking pipe with small conduits arranged inside (see Figure 14).
Figure 14 : Cross Section of Concrete Pipe ID 1 m
This is the preferred choice for constructing the main line because it helps to minimize environmental effects. Advantages
Disadvantages
• Quick construction • Minimal impact to trafc • Many circuits can be installed
• • •
Very high construction cost. To avoid obstacles, the pipeline should be constructed quite deep from the ground level. Due to its construction at depth, the current ampacity of the cables is relatively lower.
6.3.8 Tunnels
Burial installation has the advantage of relatively low construction and installation cost due to the limited civil works. However, direct burial is not realistic in case of a high number of circuits (20 or more) or circuits with high tra nsmission capacities or in countries with high average temperatures and dr y soil, the installation of direct burial is not realistic any more. With burial installation, big conductor cross-section would be required because of higher temperature compared with installation in t unnel. That means more expensive cable. In addition, cables installed in t unnels can be inspected and monitored easier. For all these reasons, more and more utilities opt to install cables in tunnel. In some cases cables are buried on a long section and installed in a tunnel, for example at the entrance of substation where there are many joints or splicing.
Advantages
Disadvantages
• No environmental impact • Minimal impact to trafc • Multiple circuits feasible • Less tension for pulling the cable
• • • •
• • • • •
because the
duct is straight Easy control of temperature and cooling by means of ventilation or other means, if necessary Excellent protection of the cables from mechanical aggression Cable pulling without excessive force on the cable end Easy monitoring of cable status and access for maintenances (oil leakage) Easy for future cable laying
Very high construction cost Higher technology Longer duration of construction. Higher impact on surroundings due to relatively bigger dimensions.
Cost of construction
Impact to trafc
Cable protection
Direct Burial
low
high
poor
Semi-direct Burial
low
high
fair
Concrete Troughs
fair
high
fair
easy
Concrete Encased
high
high
good
fair
Trenches
low
high
fair
easy
HDD
high
low
fair
very high
fair
excellent
fair
extremely high
fair
excellent
easy
Norm
Pipe Jacking Tunnels
Maintenance
difcult difcult
difcult
Table 23: Comparison of Different Methods of UG Cable Installation
6.4 Cable Laying Procedure The cable laying procedure depends on the cable type, installi ng method and the condition of the cable route. Particularly huge equipment is required for installing la rge-sized extra high tension cables for long distance circuits. In addition, various new machines suitable for the road and traff ic conditions have also been developed. 6.4.1 Preparations for Cable Laying Work Starting Procedure
It is necessary to fully examine the work specifications and plans before starting with cable laying. The following applications must be completed before starting the work:
• Application for approval by the concerned authorities • Application for private use of road (to road managing authorities) • Items specified by the road coordinating council • Application for permission for railway crossing (to railway companies) • Inquiries on underground installations for utilities, such as gas, service water, telephone, etc.
• Start of road excavation notification • Application for permission for using roads (to the police) Preparations for Cable Laying
The following preparations must be made before starting cable laying work:
• Examination of Work Specifications and work Plans, and determination of installation method
• Coordination of work schedules with other projects if they are existing on the route.
• Checking and adjustment of tools and equipment used for cable pulling • Establishment of measures for safety and prevention of environmental pollution 6.4.2 Cable Laying Work
Each cable is usually cut to the required length at the factory. One end of the cable is made moisture proof (through ter mination processing) before it is wound on a cable drum. The pulling end of the cable is provided with a pulling eye as shown in Figure 15. However, solid cables, such as X LPE cables, are sometimes provided only with termination processing at both the ends. A net-like pulling grip is used as shown in Figure 16 when no pulling eye is used. Such a net is sometimes used as a pulling aid by attaching it at middle of a cable.
Figure 15: Pulling Eye
Figure 16: Pulling Grips
Some important items to be considered for pulling works are as follows:
• Site considerations • Measures for prevention of environment ( such as pollution, noises and vibrations etc.)
• Measures for prevention of traffic troubles • Measures for prevention of working temperature, moisture and ga s absorption in pulling the cable
• Confirmation of safety of equipment in manhole • Measures for human safety 6.4.3 Cable Laying for Direct Burial Cable laying procedure
The cable laying procedure for direct burial is as shown in Figure 17. Laying method
An example of ordinary cable laying method is illustrated in Figure 18. When the pulling length is long or the route involves bends, caterpillars may be used in the middle of the route, and cable laying vehicles may be used when drums are large. After cable pulling is over, cut off the excessive cable and apply caps on both ends of the cable for moisture protection until jointing.
Route excavation Trough arrangement Roller stationin
Drum stationin
Winch stationin
Small scant plate removal Pulling wire feeding
Swivel fixing
Pullin e e-fixin
Connection of the wire to the ullin e e Cable ullin
Sand filling in trough
Sand filling in trough
Coverin Sand burying Protection late la in Backfillin Figure 17 : Direct Burial Cable Laying Procedure
Figure 18 : Cable Laying for Direct Burial
6.4.4 Cable Pulling for Duct Installation Cable laying procedure
The cable laying procedure for duct installation is as shown in Figure 19. Leading wire pulling
Passing of cable piece or duct check by video camera is carried out to check the duct further depending on the result
Test rod passing Pullin wire feedin
Drum stationin
Laying vehicle stationing
Jack u
Winch stationin Recorder and other instrument installation
Small scant plate removal
Snatch block fixing Preparations in manhole
Cable releasin Pulling eye fixing
Swivel fixing at the head of wire
Roller positioning in manhole
Connection of pulling eye and wire Cable pulling
The oil tank must be switched for oil fill cables
Fixing of waterproofing device to duct opening Cable end processing and cable protection Removal of pulling equipment
Figure 19 : Cable Laying Procedure for DUct Installation
Test of the internal condition of duct
To test the internal condition of a duct before cable pulling, a test rod passing is carried out. An example of the test rod is shown in Figure 20.
Figure 20: Test Rod or Dummy
Figure 21: CCTV Camera for Checking Duct
When external scars are observed on the dummy or when bumps are suspected in the duct, a short cable of about 5 meter length is pulled into the duct. This is to check the duct in more detail by observing the appearance of the pulled test cable. CCTV camera is pulled into the duct, if necessary, to visually check specific obstacles in the duct. A method of checking duct by using a CCTV camera is il lustrated in Figure 22.
Figure 22 : Checking Duct by Using CCTV Camera Method of cable pulling
Single or three cables are pulled into a duct. The general machine arr angement for each case is shown in Figure 23
Figure 23 : Method of Cable Pulling
At the end of cable pulling, workers have to check the protective covering by using a Megger test for proper insulation resistance; then cut the excessive cable as necessary; cover the cable ends with some protective material; and support the cable in manhole to avoid excessive bending. Make sure to attach a water-proofing device to the duct opening. 6.4.5 Cable Pulling for Tunnel Installation Winch method
In the winch method, guide rollers are fixed in the tu nnel, and the cable is pulled on the guide rollers as in the case of the duct installation. A wire is attached to the leading end of the cable through a swivel, and the cable is pulled on the guide rollers by pulling the wire. Cable laying procedure
The cable laying procedure by winch method is shown in Figure 24. Roller installation
Equipment to be installed on the ground, such as the winch and drums, can be positioned in the same manner as that for cable pulling for duct installation. But cable rollers are installed in the tunnel. There are three ways for roller installation in the tunnel: Rack mounting: Each roller is mounted on a rack. Wall mounting: Each roller is mounted on wall using a vertical metal support. Floor mounting: Rollers as shown in Figure 25 are mounted on the floor.
Safety trough installation Arrangement in tunnel Pulling wire feeding
Drum positioning
Laying vehicle stationing
Winch positioning
Jack up Recorder setting Small scant plate removal Preparations in manhole Cable releasing Pulling eye fitting
Preparations in manhole
Snatch block fixing
Connection of pulling eye and wire Cable pulling Cable installation in trough Cable end processing and cable protection
Safety trough covering Safety trough sanding
Fire detector installation
Removal of Pulling equipment Figure 24 : Cable Laying Procedure for Tunnel Installation
Figure 25 : Floor Mounting Rollers
Caterpillar method
Electric caterpillars or powered rollers are used in the caterpillar method. The cater pillar method has following advantages over the winch method:
•
No pulling eye is required.
•
The cable pulling can be car ried out with relative ease even when there are many bends along the route.
•
It allows installation of long-span cables.
•
It allows the snake installation easily.
•
A number of cables can be pulled in one preparation.
The caterpillar method is illustrated in Figure 26.
Figure 26 : Cable Installation in Tunnel by Caterpillar Method
6.5 Installation Acceptance Process Usually, utilities hire contractors to install cables. Therefore, installation acceptance is needed to complete the contract. Only an electrical test after installation is necessary. The test procedure is described below. Test procedure: Apply direct voltage as per Table 23 between the metallic sheath or concentric wires or tapes and the outer electrode or jacket for a period of 1 min. Value of direct voltage for test (kV) Rated voltage U0 / U (kV)
Size of cable 70 mm2
240 mm2
400 mm2
800 mm2
8.7 / 15
7.2
8.0
9.2
-
12 / 20
7.2
8.0
9.2
10.0
Table 24: Value of Direct Voltage for Jacket Test
Considerations: The outer electrode or jacket of cable must not breakdown. Megger test of 1 kV is employed as shown in Figure 27 to ensure the integrity of the cables after laying. Test process: Megger test (Figure 28) is used to examine the insulation resistance between: Conductor – Shielding Wires Conductor – Ground Shielding Wires – Ground Considerations:
Resistance should not be less than 2000 MΩ.
Figure 27 : Megger Test1 kV
Figure 28 : Insulation Testing
6.6 Conclusion Utilities select the method of underground cable system construction based on construction cost, impact
on trafc, ease of maintenance and the capacity as shown in Table 23. In normal cases, duct bank is selected. Pipe jacking method is the rst choice for construction of main line to minimize environmental effects. Utilities usually employ horizontal directional drill for river and road crossing situations. Tunnels are selected only in case of a huge power project requiring very high capacity to accommodate a number of main lines. The method of cable laying depends on the budget, cable type, the installation method and the condition of the cable route. Usually, underground system construction and cable laying are carried by contractors.
However, some utilities have their own workers. In both cases, qualied and trained workers are required, especially for cable laying.
Chapter 7
Operation, Maintenance and Testing 7.1 Introduction Whenever a cable system is put into operation, it is comes under the following four types of stresses:
•
Electrical stress
•
Thermal stress
•
Mechanical stress
•
Radiation environmental stress
These stresses can harm the integrity of cable insulation. If these stresses are controlled within design limits, the cables are affected only by the natural ageing process and can perform satisfactorily over their full life expectancy which is 50 years well proved in MEA. if the stresses are higher than the acceptable levels, ageing process will be accelerated. The ageing of solid insulation materials is the process of irreversible changes that adversely affect performance and shorten useful life. The degree and the rate of aging of insulation depend on:
•
The physical and chemical properties of the material;
•
Material processing and treatment during manufacturing, and subsequent use in equipment;
•
Defects that exist in insulation body; and
•
The degree and duration of induced stresses.
If the ageing process happens fast, the insulation may fail even if the cable is new. The full useful life of a cable can be achieved by preventing or reducing the abnormal ageing process of cable insulation. This can be done through proper cable loading and health monitoring as well as appropriate testing and maintenance methodologies.
7.2 Objective Power cable shall be operated within its designed criteria in friendly environment. Violation of operating condition leads to premature degradation of cable, or if critically overstressed the cable is subject to fail.
However, failure of cable due to premature ageing can be alleviated by appropriate maintenance strategy. This handbook discusses the basic principle on failure mechanism and detection of defects existed in solid insulation (XLPE) of cable and provides some guidelines for inspection, testing and maintenance of underground cable system. For informative purpose, this handbook also provides introductory information on the cable monitoring system.
7.3 Maintenance and Inspection After manufacturing, cables are tested at well above their working voltage. And so, if the cables are installed correctly, there is a low risk of cable failure. Similarly, with modern accessories and welltrained construction personnel, the risk of failure is limited. Furthermore, before putting the cables into service, their condition can be checked by conducting an acceptance test. So within the normal operating conditions, good health of a cable system is assured throughout its design working life. However, no one can guarantee that cable would not be exposed to harmful stresses or other problems during the operating life span. Therefore, proper maintenance processes should be in place to ensure smooth and
efcient cable operation. 7.3.1 Cable System Inspection
Unlike overhead lines, transformers, switchgears or other above ground facilities where visual inspection can be easily performed, the inspection of underground cables requires special technique and tools. 1) Frequency of underground system inspections The frequency of inspection of a cable system is largely determined by the importance of the equipment or facility it serves. Inspections can vary in frequency from 6 months to 5 years, but a 2-year cycle of inspection is recommended. The records of each inspection should be maintained. 2) Structural inspections Inspect structures and check their cleanliness and physical condition -- cracking of walls, roofs or floor slabs; spalling of concrete; and the condition of frames and covers. Inspect for corrosion of pulling eyes; collapsed grounds; and other miscellaneous fixtures, such as cable racks, arms, and insulators.
3) Cable inspections Patrol the route of underground direct-burial cable circuits to inspect for potentially harmful conditions. Changes in grade caused by washouts can expose cables to adverse conditions. Adjacent new construction should be closely monitored. Examine connections to equipment terminals or cable terminations, both in the structure and above-ground. In structures, check duct entrances, fireproofing, splices, cable tags, and ground connections to cable shielding and sheathes. Look for par tial discharge noise or signs of traction on cable terminations or direct-burial cable, which may be a result of the expansion and contraction of the cable. a. Cable supports. Check mountings and supports to ensure they are secure. Remove rust and corrosion, and clean and repaint supports with corrosion-resistant paint. b. Duct entrances. End bells are usually used to prevent cable damage at duct entrances. If they were not installed or are damaged, str ips of hard rubber or similar material should be used to protect the cable at the duct entrance. c. Testing. Cable insulation integrity can not be visually checked; it requires testing, which will be elaborated in section 7.4. d. Cable faults. Inspection alone may reveal the location of a cable fault or it may require complicated process using test equipment. Visual check and test procedures are covered in section 7.3.2. 4) What to look for during u nderground cable system inspections? Here are some guidelines for personnel who perform visual inspection on underground facilities. a. Above Ground: Make sure the cover is in good physical condition and fits the opening properly. If a vented cover is used, verify that the vents are clear of debris. Also ensure that the cover is at grade. b. Water and Debris: Look for water and debris in the structure. Does the structure need to be pumped or cleared before inspection? Are the cables or equipment submerged? Are the ducts free of debris? Verify the operation of a sump pump
if it is present. (This can usually be accomplished by lifting the f loat switch and observing if the pump starts.) c. Structural Conditions: Check the overall cleanliness and physical condition of the structure. Look for cracks, spalling or exposed reinforced bar. Check the conditions of the floor, roof and walls. If cover tethers are used, verify that they are installed properly and appear to be in good physical condition. d. Racks and Saddles: Do the cable racks, saddle, and other structu ral components appear to be in good working order? Are any saddles missing their porcelain inserts? e. Ducts: Are the duct entrances chipped or cracked and in need of grouting? Are the ducts properly sealed? f. Cables, services, and other conductors: Inspect all cables for insulation wear or abrasion. Be especially mindful of exposed conductors and visible burnouts. Is the insulation swollen, damaged, peeling, cracked, or burnt? Do the cables show signs of excessive heating? Inspect for leaking cables. g. Joints: Look for leaking, swollen, imploded, or otherwise deformed joints. Do any joints show signs of excessive heating? What about burning or arcing? h. Neutral cable and connections: Do the neutral conductors and connections appear to be in good working condition? Is any bond broken? Is the neutral bus appearing to be in good working condition? i. Transformers and other equipment: Does all equipment appear to be in good working condition? Are there any leaks? Are there any cracked or damaged bushings? Does any equipment show signs of arcing, burning, or excessive heating? 7.3.2 Cable Fault and Fault Locations
Use fault locating equipment when a check of associated equipment and lines confirms that the fault is actually in the cable, and visual methods fail to locate the fault. Since no single test will locate all types of faults, the type of fault must be determined in order to use the most suitable test method to locate it. To determine the type of fault, a
portable insulation testing set, such as Megger is most commonly employed. For safety purpose, the section of cable under test must be disconnected from feeders, buses, and equipment. 1) Types of faults: Cable insulation failures result i n low- or high-resistance faults, because one or combinations of the following conditions occur: a. Ground fault: One or more of the conductors may be grounded. b. Short circuit: Two or more conductors may be short circuited. c. Open circuit: One or more conductors may be open circuited. d. Combination of above. 2) Checking for fault types In case of a cable fault, the information about the fault, such as type of fault, distance to fault, is made available from substation relay. If such information is not available, supplementary procedures are required to attain the necessary fault information. a. Grounded conductor: In checking for a g rounded conductor, the insulation tester is successively connected between each conductor and ground with the far end of the cable open circuited. A good conductor will indicate a resistance commensurate with that of its insulation. A grounded conductor will show a very low resistance. b. Short circuit: In checking for a short circuit, the insulation tester is successively connected between each possible combination of conductors. Far ends of the cable must be open-circuited. A low reading indicates a short circuit between the conductors under testing. c. Open circuit: The continuity of the conductors is determined by grounding the conductors at the far end and then testing between each conductor and ground. If the conductors are continuous, the resistance reads low. If an open circuit exists, the tester will indicate a very high resistance.
3) Fault location The method that generally applied for locating the cable fault comprises the followings. a. The halving method: The procedure consists of isolating the fault by progressively limiting it to one half of the previously considered length of cable. Start cutting the cable halfway down and megger both sections to identify which of two sections contains the fault. Next, cut the faulty section halfway and megger both sub-sections. Continue until a small faulty section is isolated. It is time consuming and costly. Preferably, modern methods utilizing sophisticated signaling instru ments should be applied, if feasible. b. Mur ray loop resistance bridge method: To u se this method , the grounded conductor must be continuous at the fault and a continuous u ngrounded conductor in the faulted cable must be available. The accuracy of this method is directly related to the accuracy of the plans showing cable routing. The fault is located in terms of the distance from its cable terminal by measuring and comparing electrical characteristics of the faulted cable and an unfaulted conductor. It is essentially a Wheatstone bridge of the slide-wire type. When the bridge is balanced, the fault distance is found as indicated in Figure 29. Instructions for use, including applicable mathematical formulas, should be supplied with the instrument.
Figure 29: Murray Loop Bridge Method
c. Time domain reflectometer (TDR) method: This method is based upon the measurement of the time t it takes a generated pulse to reach the fault and be ref lected back. The fault distance d equals the cable propagation velocity v multiplied by t and divided by two, which results in the equation below.
= 2
The TDR analyzer measures the ref lection time and the fault distance is automatically calculated based on the entered velocity of the pulse travel, which is usually the ratio of the cable’s propagation factor to the speed of light (a value of less than one). The analyzer can determine whether the fault is open-circuited or short circuited based on waveform reflections as shown in Figure 30.
Figure 30: Cable Fault Waveform Reections 4) Selection of cable fault locating methods The fault locating method differs dependent upon the way the cable is inst alled. a. Duct line: Pinpointing the fault between structures, e.g., manholes, is unnecessary since the entire length between structures of a faulty cable must be replaced. Normal tracing methods (e.g., gas sniffer) or visual signs (e.g., manhole cover blown out) may be implemented. Murray loop resistance bridge method can be employed as supplementary to the previous mentioned method.
b. Direct burial: The fault-locating method for direct-burial installations must pinpoint the fault, so that the repairs can be made at the point of failure. Generally, such faults can be best located with impulse equipment such as with the TDR method. Faults can also be located by patrolling the cable and listening for the noise of an impulse discharge. On longer cables it may be preferable to use some other means, such as a Murray Loop resistant bridge method, to obtain an approximate fault location. In the absence of audible noise, test holes must be dug so that detector tests can be carried out using a tracer method. 7.3.3 Cable Repair Procedure
After the location of a fault has been pinpointed, the cable needs to be repaired to restore the power. While repair methods described below are basically the same for any underground cable, there may be some variations depending on the installation conditions. 1) Direct burial: While there may be splice boxes, normally there is no structure to consider, and a hole will have to be dug to repair the cable. This could well be a test hole used earlier to pinpoint the fault location. Such access may have to be enlarged if the repair involves an appreciable length of cable. The major challenge in repairing a direct-burial cable is ensuring dr y environment for the repair work. A temporary shelter may be required. 2) Duct line: If the faulty cable length is in a duct line between structures (e.g., manholes), there are several possible repair methods. Usually, only one circuit is installed in a duct line in order to avoid the derating of cable capacity. a. Spare duct: If there is a spare duct available, the simplest solution may be to pull a new length of a cable into this duct and connect it to the good ends of the faulty cable. Then pull out the faulty cable, if possible, to provide a spare duct. At the very least, put a tag at both ends to indicate that the cable is faulty and has been abandoned. . b. No spare duct: The faulty cable should be pulled out and replaced by a new cable. If this is not possible because of duct damage, a new duct must be installed. Alternately, it may be faster and more economical to open up the duct line at the
point of fault. If the cable can be repaired without a splice, the duct line can be closed again. It may be necessary to build one or more new structures at the fault location to house any new cable by splicing into the existing cable. The method to be used is largely a matter of judgment based on all the factors k nown at the time.
7.4 Field Tests on Cable 7.4.1 Need for Field Testing
Although medium and high voltage power cables are carefully tested by manufactu rers before shipment, some defects may still not be detected. More li kely, da mage during shipment, storage, or installation may occur. Additional testing of completed installations including all the joints and ter minations may be conducted prior to being put in service. Many users observe that as the time passes, these cable systems degrade and service failures become more frequent. Cable users may perform periodic tests to reduce or eliminate failures. Fur ther, cable users need special diagnostic tests as an aid to determine the most optimum economic replacement interval for deteriorated cables. 7.4.2 Definitions 7.4.2.1 Installation Tests: Field tests that are conducted after cable installation but
before jointing (splicing) or ter minating. The test is intended to detect any damages occurred during shipping, storage, or installation. 7.4.2.2 Acceptance Test: A field test made after cable system installation but before it is
placed in normal service. It includes testing terminations and joints.. The test is intended to further detect any damages during installation and also to show any gross defects or errors in the installation of other system components. 7.4.2.3 Maintenance Test: A field test made during the operating life of a cable system.
It is intended to detect deterioration of the system and to check the serviceability so that suitable maintenance procedures can be initiated. 7.4.2.4 Type 1 Field Test: A test intended to detect defects in the insulation of a cable
system in order to improve the service reliability after the defective part is removed and appropriate repairs are performed. These tests are usually achieved by application of
moderately increased voltages across the insulation for a prescribed duration. Such tests may be categorized as pass/fail or go/no go or withstand/ destructive test. 7.4.2.5 Type 2 Field Test: A diagnostic test intended to provide indications of insulation
deterioration. Some of these tests will show the overall condition of a cable system, while others will indicate the locations of discrete defects, which may become the sites of future failures. Both varieties of such tests are usually performed by means of moderately increased voltages applied for relatively short duration, or by means of low voltages. Due to the relatively low level of harmful stress involved, this test is also called diagnostic or non-destructive test. 7.4.3 Methods of Field Test on Cable 7.4.3.1 DC High-Potential Test The DC hi-potential withstand test is a Pass/Fail test that has been applied to all types of cables and accessories. The advantage of this test lies in its relatively simple and lightweight, portable test equipment. The test involves the measurement of leakage current when a high voltage (above nominal) is applied to the conductor while the metallic shield of the cable is grounded. The behavioral characteristics of the leakage current are evaluated
to determine the condition of the cable, specically the insulation. To perform the test on distribution cables, the DC test voltage equal to 4 U 0 shall be applied for 15 min.
Rated voltage U0/U (Um) (kV)
3.6/6 (7.2)
6/10 (12)
8.7/15 (17.5)
12/20 (24)
18/30 (36)
Test voltage (kV)
14.4
24
34.8
48
72
Table 25 : DC Test Voltage according to IEC 60502-2005 Although IEC 60502-2005 still allows DC withstand test on MV XLPE cable, the results from various studies and utility experiences have shown that DC testing mostly
nds conductive type gross workmanship errors in extruded dielectric cable systems. It hardly detects insulation defects -- even a massive one -- in extruded dielectric insulation. Consequently, DC hi-potential testing is recommended only for paper insulated cable systems and for performing a safety check before switching an extruded cable system into service (to prove that the system is not grounded).
Furthermore, DC hi-potential testing of extruded insulation cables that have been serviceaged in a wet environment at the currently recommended DC voltage levels may cause the cables to fail after they are returned to service. Space charges (trapped charge) that can occur under elevated DC voltages at the sites of these water trees or other defect sites in insulation can result in localized stress enhancements. With the reapplication of normal AC power to the cable, these localized stressed areas at the water tree sites can ultimately lead to an electrical tree or trees. Once an electrical tree has been initiated, complete cable failure is normally imminent and inevitable. Such failures would not have occurred at that point in time if the cables had remained in service and not been tested with DC. 7.4.3.2 AC Voltage Withstand Test Alternating voltage tests at power line frequencies stress the insulation in a manner similar to normal operation. The test is similar to that used in the factory on new reels of cable. A serious disadvantage of power frequency AC tests at increased voltage levels is the requirement for heavy, bulky, and expensive test transformers that may not be readily
transportable to a eld site. And the additional disadvantage is the high power consumption of test unit. Thus the AC test voltage to be applied shall be subject to an agreement between the supplier and the user. For distribution cable, an AC withstand voltage test at power frequency can be performed as follows: a) Test for 5 min with the phase-to-phase voltage of the system applied between the conductor and the metallic screen/sheath; or b) Test for 24 hours with the normal operating voltage of the system. For transmission cable, the waveform of AC test voltage shall be substantially sinusoidal and the frequency shall be between 20 Hz and 300 Hz. Alternatively, the AC voltage test at power frequency may be conducted for 24 hours with the normal operating voltage of the system applied to the primary insulation.
Rated voltage
Highest voltage for equipment
Value of U0 for Determination of test voltages
Voltage test after installation
U (kV)
Um (kV)
U0(kV)
(kV)
45 to 47
52
26
52
60 to 69
72.5
36
72
110 to 115
123
64
128
132 to 138
145
76
132
150 to 161
170
87
150
Table 26 : AC Test Voltage according to IEC 60840-2004
7.4.3.3 Very Low Frequency Test
In order to leverage the benets of both DC and AC test philosophy, a very low frequency (VLF) testing incorporates the application of an AC voltage at a low frequency (in the range of 0.01 to 1.00 Hz). The typical frequency applied is 0.1 Hz. VLF testing methods can be categorized as withstand (Type 1) or diagnostic (Type 2). In withstand testing, insulation defects are caused to break down (fault) at the time of testing. Faults are repaired, and the insulation is retested until it passes the withstand test. The withstand test is considered to be a destructive test.
Diagnostic testing allows the identication of the condition of a degraded cable system and establishes, by comparison with gures of merit, if a cable system can or cannot continue operation. Diagnostic testing is considered nondestructive. The recommended test voltage is 2.0 to 3.0 times the cables’ normal line to ground voltage (2U0 – 3U 0). The recommended test duration is 15 to 60 minutes. The test voltage and time are dependent on the type of test being performed (i.e., installation, acceptance, or maintenance). A properly implemented VLF test will not cause damage to good insulation, but will reveal any cable system defects during the test.
Cable Rating phase to phase (rms)
Installation Test phase to ground
Acceptance Test phase to ground
Maintenance Test phase to ground
kVrms (or peak)
kVrms (or peak)
kVrms (or peak)
kVrms (or peak)
5
9 (13)
10 (14)
7 (10)
8
11 (16)
13 (18)
10 (14)
15
18 (25)
20 (28)
16 (22)
25
27 (38)
31 (44)
23 (33)
35
39 (55)
44 (62)
33 (47)
Table 27 : VLF Test Voltage according to IEEE 400.2-2004
7.4.3.4 Partial Discharge Test Partial Discharge (PD) testing is a diagnostic (Type 2) test that analyzes cable systems for defects occurred during manufacturing, transportation, and installation. A partial discharge is a localized electrical breakdown in the electrical insulation system under voltage stress that only partially bridged the insulation between conductors. It may or may not occur
adjacent to a conductor. The discharge may occur in a gas-lled void within the extruded cable insulation. It can occur at the interface between a shield protrusion and the insulation, at a shield skip, at the boundaries of a contaminant, or at the tip of a well-developed water tree. PD can also occur in a cable termination, in a joint, in air, or within a cable. PD
testing can be implemented online or ofine and is the only test that can detect, locate, and characterize defects in cable insulation. Both methods – online and ofine -- have advantages and disadvantages. There are also different types of partial discharge detection methods, one using the time domain and the other the frequency domain. Both types can locate the source of the partial discharge. Online methods do not require the cable system to be de-energized. It typically employs high frequency current transformers (CTs) or capacitively coupled voltage sensors to detect transient signals from discharges. Acoustic PD measurement techniques could potentially be applied to parts of the cable system that allow direct contact. No external voltage source is needed as the online technique provides testing under normal operating conditions.
Ofine techniques energize the cable system with an external voltage source. The source could be a power frequency sinusoidal source or alternative voltage sources that are non-
sinusoidal and/or having frequencies other than power frequency. The latter alternative voltage sources may include VLF, damp alternating voltage, or impulse voltage. An elevated AC voltage, about 1.5-1.7U 0, is applied between the conductor and metallic shield of the cable under test. An oscilloscope and/or proprietary digital signal analysis platform is used to detect transient microvolt or microampere level signals that are generated at the discharge site and travel through the cable to the detection equipment. Partial discharge threshold levels have been established for factory testing of terminations, joints, connectors and cable. Comparison of these values provides excellent reference for the condition of the cable system. 7.4.3.5 Dissipation Factor Test
Dissipation factor (DF) or tan delta (Tan δ) testing is a diagnostic test that provides a means of measuring the AC dielectric losses of the insulation and then making a determination of the condition of the cable based on this information. In theory, a shielded cable, which is a dielectric sandwich between a central conductor and a surrounding metallic earth conductor, is a perfect capacitor, if insulation is a perfect dielectric. However, such perfection may not be obtained. This is because defects, such as water trees, electrical trees, moisture and air pockets, are always existing in insulation body. As the cable ages, the dielectric loss increases, and therefore, it is an important indication of the dielectric quality. In a perfect capacitor, the voltage and current have the phase difference of 90 degrees, and the current through the insulation is capacitive. If there are impurities in the insulation, like those mentioned above, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current
and voltage will no longer have 90 degrees phase difference. It will be something, say δ degrees, less than 90 degrees. This will indicate the level of resistance in the insulation. By measuring and comparing resistive current over capacitive current (IR/IC), we can determine the quality of the cable insulation. In a perfect cable, the angle would be nearly zero. An increasing angle indicates an increase in the resistive current through the insulation, meaning insulation contamination or deterioration. The greater is the angle, the worse is the cable.
The cable to be tested must be de-energized and each end isolated. Using a VLF or other
AC voltage sources, the test voltage is applied to the cable while the tan δ controller takes measurements. Typically, the applied test voltage is raised in steps, with measurements rst taken up to 1U0, or normal line to ground operating voltage. If the tan δ numbers indicate good cable insulation, the test voltage is raised up to 1.5 – 2U 0. The tan δ numbers at the higher voltages are compared to those at lower voltages and an analysis is done. However,
there are not standard formulae or benchmarks to ascertain the success of a tan δ test. The health of the insulation is obtained by observing the nature of the trend, which is plotted. A steady, straight trend would indicate a healthy insulation, while a rising trend would indicate an insulation that has been contaminated with water and other impurities. Figure 31 shows an example of test results, which can be interpreted as New, Aged or Highly Degraded cable.
Figure 31 : Tan δ Test Results (K. Brown IEEE ICC Minutes Spring 2005) 7.4.3.6 Test on Cable Jacket For the tests stated in clauses 7.4.3.1 – 7.4.3.5, the test voltage is applied between cable conductor and metallic earth sheath. These tests prove the insulation integrity (Type 1) or help to diagnose the deterioration of insulation (Type 2). The cable outer jacket (oversheath), which protects metallic earth sheath against aggression from external
environment, also requires proving its integrity. Damaged cable jacket can lead to water ingress into inner metallic layer, resulting in its corrosion. Breaks in the metallic shield on polymeric cables may lead to points of high electrical stress, which, in turn, may lead to local partial discharge and eventual failure. Oversheath integrity can be tested during installations tests. The test voltage shall be applied
between each metal sheath and metallic screen and the ground. As specied in IEC 60229, a direct voltage of 4 kV per millimeter of specied thickness of extruded oversheath shall be applied (with a maximum of 10 kV) between the metallic sheath or concentric wires or tapes and the outer electrode for a period of 1 min. For the test to be effective, it is necessary that the ground makes good contact with the complete outer surface of the oversheath. A
conductive layer on the oversheath, which may be moist backll or a graphite layer, can serve as an outer “electrode”. 7.4.3.7 Selection of Test Methods
Each utility may select test methods according to its specic objectives, test objects and the prevailing environment. Some utilities may still prefer DC hi-potential test because of its light weight and ease of use. They may use this test to prove the gross conductive defect occurred during installation. Some utilities, however, may not allow DC hi-potential test but employ 24-hour cable energization instead. They may take this approach believing that the installation process is strictly controlled and DC hi-potential test is harmful to the cable.
Since AC hi-pot test with xed frequency at 50/60 Hz is comparatively heavy and costly, VLF, resonance or damped AC voltage tester might be a compromised solution if an AC hi-pot test is required. Along with the operation of a cable system, maintenance testing is required to detect deterioration and defective site development. In such cases the diagnostic methods of
partial discharge and tan δ are the most appropriate options. Table 28 summarizes the ageing mechanisms, their causes and the most suitable diagnostic test methods that best detect such imperfection.
Imperfection/Ageing Mechanisms
Cause
Diagnostic Test Method
Water treeing (WT)
Localized defects (LD), Water, Voltage
Dissipation Factor(DF)
Electrical treeing (ET)
LD, WT, Voltage
PD
Partial discharges (PD)
LD, ET, Voltage
PD
Tracking
Surface contamination, Interfacial voids, Voltage
PD, visual
Intrinsic breakdown
Lightning
none
Chemical changes, e.g., oxidation, hydrolysis
High temperature or direct contact with aggressive liquids
DF
Thermomechanical
Current overloading
Visual, PD (if voids form due to mechanical deformation)
Dielectric heating, thermal runaway, hot spots
Current overloading,
Visual (signs of overheating) DF
Hardening/softening of insulation/shields/jackets
High temperatures, exposure to solvents, etc.
Visual, PD if voids form, otherwise none
Concentric neutral corrosion
Water, aggressive chemicals
Time domain reectometry (TDR), resistance measurement
Weak spots
Manufacturing, installation
Hi-potential test: AC, VLF
Conductive type defects
Workmanship errors
Hi-potential test: DC, AC, VLF
Table 28: Summary of Different Tests
7.5 Cable Monitoring System Utilities need to ensure the efcient and reliable operation of their underground cable system. This objective can only be achieved through continuous monitoring and analysis of the condition of the
cable. It also gives the owner and operator condence to make correct decisions regarding the available capacity in the cable system. On the other hand, the main objectives of cable monitoring are preventing cable failure by early detection of any deterioration; improving the operational availability; reducing downtime of the system; and achieving loading optimization. Table 29 lists the methods for monitoring an underground cable system.
Measured Parameter
Objectives
Operational current and voltage
• • • • • • •
Axial distribution of cable temperature Partial discharge monitoring
Water permeability under the cable shield SF6 monitoring of gas sealing end
• • • • • • • •
Loading optimization for the network Short circuit current Temporary overvoltage Hot spot recognition Prediction of temperature trend due to overload Optimized thermal loading Early recognition, localization and assessment of cable failure including its accessories Damage limitation Scheduling for shutdown and repairs Localization of jacket damage Prevention of water treeing and cable shield corrosion Supplement to cable sheath test Early recognition and assessment of leakage at cable termination in gas insulated switchgear Damage limitation Scheduling for shutdown and repairs
Table 29 : Methods for Monitoring Underground Cable System
7.6 Conclusion In order to maintain the reliability of power supplied by an underground cable system, one has to understand the degradation and failure mechanisms of power cables. The most conservative approach is to strictly operate the cable at less than its ampacity limit. However, this is not an option for utilities anymore; utilities are now required to extract the best performance out of their cable system. A utility has to cautiously manage its underground cable network. The cable testing as well as cable monitoring system can help to identify the possible defects and prevent future failures. It is already known that the failure of an underground cable results in relatively longer periods of power breakdown. Hence, the implementation of appropriate maintenance and inspection procedures is extremely important.
References [1]
Department of the Army, the Navy and the Air force: Technical Manual for Facilities Engineering - Electrical Exterior Facilities
[2]
Electric Power Research Institute, Utility Line Inspections and Audits: A Power Quality and Reliability Guidebook: EPRI, Palo Alto, CA, 2007.
[3]
IEEE Std 400 (2001): IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems
[4]
IEEE 400.2 (2004) : Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)
[5]
IEC 60229 (1982): Tests on cable oversheaths which have a special protective function and are applied by extrusion
[6]
IEC 60502-2 (2005): Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) – Part 2: Cables for rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV)
[7]
IEC 60840 (2004): Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) – Test methods and requirements
[8]
Prysmian’s Wire and Cable Engineering Guide
[9]
Online Partial Discharge Monitoring and Diagnosis at Power Cable, 2009 Doble Engineering Company – 76th Annual International Dobel Client Conference
[10]
Cable Condition Monitoring to Improve Reliability, Olex Australia
APPENDIX A
SPECIFICATION NO. 100 8.7/15 kV & 12/20 kV XLPE COPPER CABLE 100A Scope
This specication describes the requirements with which the manufacturer shall comply in order to supply single conductor, copper cable, cross-linked polyethylene insulated, copper wire screen,
polyethylene sheathed and/or re retardant PVC jacket, for 12 kV & 24 kV, 3 phase, 50 Hz, solidly grounded neutral system, to Lower Mekong Sub-region utilities ( LMS). 100B Site and Service Condition The cable shall be suitable for use in ducts, trays and for direct burial in ground. The cable is subject to immerse in water all the time. 100C Reference Standards
Except otherwise specied elsewhere in the specication, the cable required by LMS shall be manufactured and tested in conformity with the latest revision of the standards listed below : International Electrotechnical Commission (IEC) Publication 60228(1978-01)
:
Conductors of Insulated Cables.
Publication 60502-2 (1997-04)
:
Power cables with extruded insulation and their accessories for rated voltages from 1 kV(Um=1.2 kV) up to 30 kV (Um=36 kV) Part 2 : Cable for rated voltage from 6 kV (Um=7.2 kV) up to 30 kV (Um=36 kV)
Publication 60332-3-22 (2000)
:
Test for vertical ame spread of verticallymounted bunched wires-Category A.
International Standard (ISO) Publication 4589 (1996)
:
Determination of burning behavior by oxygen index
If there are any minor parts deviated from the standards, they shall be clearly mentioned in the “DEVIATION FROM MEA’S SPECIFICATION FORM” attached herewith.
The cable conforming to other national standards having similar characteristics and providing
equal performance and/or quality to those specied may be proposed. In this case, the complete ENGLISH language copies of the standards shall be submitted with the quotation, otherwise such offer may not be considered. 100D Test, Inspection and Test Report 100D1 The proposed cable shall have successfully passed all the type tests or design tests in
accordance with the reference standards except for re retardant property as specied in clause 100I3. The test shall be done by reputable independent test station or test station accepted by LMS. The test reports of the proposed cables are required. Test reports of identical units are acceptable.
All test reports or a letter of conrmation that test report will be submitted to LMS within 30 (thirty) days after opening date shall be attached to the bid. If the type test reports are not delivered to LMS within 30 (thirty) days after the opening date, LMS will not consider such offer. 100D2
The following routine tests shall be made on each reel of all nished cables in
accordance with the reference standards. a) Measurement of electrical resistance of conductors. b) Partial discharge test. c) High voltage test. 100D3 The following special tests shall be made in accordance with the reference standards.
a) Conductor examination. b) Check of dimension including measurement of external diameter. c) Electrical test. d) Hot set test for XLPE insulation.
The number of test sample shall be in accordance with clause 17.2 of IEC Publication 60502-2 or equal. 100D4 LMS reserves the right to send the representative at his expense to inspect and witness
tests of the material and equipment during manufacturing, at the time of shipment or at any time he deems necessary. The supplier shall provide free access to the facilities where the equipment is being manufactured and shall satisfy the representatives that the material and equipment are in
accordance with this specication and purchase contract.
The number of samples to be witnessed shall be as follows :Numbers to be witnessed
For routine tests as specied in clause 100D2 For special tests as specied in clause 100D3 100D5
At least 10% of the total test samples All test samples
Prior to the shipment, the supplier shall submit to LMS the complete and certied reports
of all tests made in 6 (six) copies. The test reports shall contain all data required for their complete understanding such as; diagrams, methods, instruments, constants and values used in the tests and the results obtained. 100E Conductor The conductor shall be plain annealed copper and shall be compact round concentric lay stranded construction conformable to IEC Publication 60228-1978 or equal or compact segmental stranded. 100F Conductor Screen 100F1 The conductor screen is a conducting material of at least 0.0635 mm thick applied by the
tandem extruded with the insulation over the surface of the conductor. 100F2
The extruded conductor screen shall have a maximum volume resistivity of 5,000 Ω.cm
at room temperature, and 50,000 Ω.cm at 90°C, and have 100 per cent minimum elongation after air oven test at 121°C for 7 (seven)days. 100G Insulation 100G1 The insulation shall be unlled, no carbon black, cross-linked polyethylene (XLPE),
simultaneously extruded with the semi-conductive conductor screen and insulation screen layer,
whose properties meet electrical requirements specied in column 5 of Table 15, mechanical requirements in column 6 of Table 17 and particular requirements in column 5 of Table 21 of IEC Publication 60502-2 or equal. 100G2 Only the dry curing process is required. Conventional steam or hot water curing processes
are not accepted.
100G3
The average thickness of insulation shall not be less than the nominal value specied in
the table of clause 100N. 100G4
The minimum thickness shall not be less than 90 per cent of the nominal value specied
in the table of clause 100N. 100H Insulation Screen The insulation screen shall consist of nonmetallic covering directly over the insulation and nonmagnetic metal component directly over the nonmetallic covering. Nonmagnetic metal component shall be copper wire only. 100H1
Nonmetallic covering having maximum volume resistivity of 50,000 Ω.cm at room
temperature and rated temperature shall be applied over the insulation in one or more layers in
direct contact and shall be plainly identied as being conducting. Nonmetallic screen may consist of a conducting tape or a layer of conducting compound having minimum thickness of 0.0635 mm. If an extruded covering is used, the tension necessary to remove an extruded covering from cable at room temperature shall not be less than 13.3 N. 100H2
Copper wire screen shall consist of plain annealed copper at or round wires applied
helically over the nonmetallic covering. The wires shall be electrically continuous and bonded together throughout the cable length with copper contact tape. The total cross-sectional area of the screen and minimum number of wire shall be not less than
the specied value in the table of clause 100N. 100I Nonmetallic Sheath 100I1 Over the copper ground screen, a fabric or mylar tape shall be applied for the manner of
separator under nonmetallic sheath. 100I2 For PE sheathed copper cable, the sheath shall be ST7 compound black polyethylene
whose properties meet mechanical requirements specified in column 6 of Table 18 of IEC Publication 60502-2 or equal and shall be suitable for use with the cable having maximum
conductor temperature of 90°C.
100I3
For re retardant PVC sheathed copper cable, the sheath shall be black, ame retardant,
PVC. The oxygen Index of non metallic sheath material shall be not less than 30 measured
according to ISO 4589 or equivalent, certied test report from raw material manufacturer or reputable independent institution accepted by LMS shall be submitted for approval. The ame retardant non-metallic sheath shall be able to stop ame propagation along vertical or horizontal cable ways and delay damage to cables. Test on completed cable under re condition according to IEC 60332-3-22 or equivalent by reputable independent institution accepted by LMS or LMS ‘s approved test station witnessed by LMS ‘s representative shall be submitted before shipment. 100I4
The average thickness of sheath shall not be less than the nominal value specied in the
table of clause 100N. 100I5
The minimum thickness shall not be less than 80 per cent of the nominal value specied
in the table of clause 100N. 100I6 The sheath shall be resistant to petrol, oil, acids and alkalis.
100J Marking 100J1 On the surface of the sheath, it shall have the following information marked at the interval
of not more than 50 cm. For PE sheathed copper cable LMS underground cable for A kV system size B mm2, Contract No. C : D. Where A : system voltage. B : the nominal cross-sectional area of conductor. C : the Purchase Contract Number. D : Manufacturer’s name or symbol. The colour of making shall be white.
For re retardant PVC sheathed copper cable LMS re retardant PVC sheathed underground cable for A No. C : D.
kV system size B mm2, Contract
Where A : system voltage. B : the nominal cross-sectional area of conductor. C : the Purchase Contract Number. D : Manufacturer’s name or symbol. The colour of making shall be yellow. 100J2
Continuous reel length marking(in gure) shall be made on the sheath at every 1 meter
starting from “0” 100K Additional requirements for Packing 100K1 The cable shall be delivered on returnable metallic drums having preferable dimensions
as followings :Unit
70-240 mm2
400 mm2
800 mm2
Flange diameter
cm
160
180
220
Drum diameter
cm
80
80
120
Outer width
cm
95
95
95
100K2 The standard length of cable per reel shall be 500 m with the tolerance of 5 per cent. The
length of cable per reel less than the specied standard length shall not be accepted. 100K3 An amount not exceeding 10 per cent of the total may be delivered in random length, the
said lengths are to be not less than 50 per cent of the standard length. 100K4 Reel shall be lagged with suitable lagging of wood thickness not less than 23 mm or
equivalent lagging. A steel plate of 7 mm minimum thickness (10 mm thickness preferred) shall
be xed at an arbor hole of each outer face of the reel. The steel plate shall be drilled for insertion of temporary axle. Arbor hole diameter shall be between 70-110 mm. 100K5 The reel shall be marked with at least the following :-
a) Conductor type and size. b) The system voltage c) Manufacturer’s name or symbol. d) Contract number and year of production. e) Length of cable. f)
Gross and Net weight.
g) The wording “Returnable Drum” on both anges of reel.
100K6 LMS will not be responsible for the payment of the supplied cables excessive from the
length specied in the contract, and on the contrary if the supplied cables are shorter than the length required in the contract, LMS will pay according to the actual length supplied only. 100L Return of Cable Drum 100L1 LMS will inform the supplier to bring back the empty drum within 540 days after the cable
has been arrived and accepted by LMS. If the supplier does not bring the drums to his belonging within 30 days after being informed, LMS reserves the right to take action at LMS’s discretion. 100L2 LMS will return the drum at normal wear and tear conditions. If the drum requires to be
refurbished such as repainting, it shall be borne by the supplier. 100M Additional Requirement for Responsibility If the cable is damaged within the guaranteed period, the manufacturer shall promptly investigate, repair or replace it if the damaged is cause by the cable itself. The replacement shall be submitted
to LMS within 60 days after being rst informed otherwise the performance security shall be forfeited. 100N Attached Table Table of 8.7/15 kV and 12/20 kV XLPE copper cable are as follow:Nominal cross-sectional area of conductor
mm2
Min. number of wires in conductor
70
240
400
12
34
53
Diameter of conductor
mm
9.73 ± 1%
18.47 ± 1%
23.39 ± 1%
Thickness of conductor screen, minimum
mm
0.0635
0.0635
0.0635
Thickness of insulation
mm
4.5
4.5
4.5
Range of diameter over insulation
mm
19.7-21.7
28.5-31.4
33.4-36.7
Thickness of insulation screen, minimum
mm
0.0635
0.0635
0.0635
20
30
30
Number of wire screen, minimum Total cross-sectional area of copper wire screen, minimum
mm2
10
25
25
Thickness of non-metallic sheath
mm
1.8
2.0
2.3
Range of overall diameter
mm
26.0-27.1
37.5-40.0
43.0-45.5
Ω /km
0.268
0.0754
0.0470
Max. dc resistance of conductor at 20°C
Nominal crosssectional area of conductor
mm2
Min. number of wires in conductor
70
120
240
400
800
12
18
34
53
53
Diameter of conductor
mm
9.73 ± 1%
12.95 ± 1%
18.47 ± 1%
23.39 ± 1%
34.00 ± 1%
Thickness of conductor screen, minimum
mm
0.0635
0.0635
0.0635
0.0635
0.0635
Thickness of insulation
mm
5.5
5.5
5.5
5.5
5.5
Range of diameter over insulation
mm
21.7-23.9
25.0-27.4
30.5-33.5
35.4-38.9
46.0-50.6
Thickness of insulation screen, minimum
mm
0.0635
0.0635
0.0635
0.0635
0.0635
20
20
30
30
35
mm2
10
10
25
25
25
mm
1.8
2.0
2.1
2.3
2.6
mm
28.0-30.0
31.0-35.0
39.0-42.2
44.5-48.0
57.5-61.0
0.268
0.153
0.0754
0.0470
0.0221
Number of wire screen, minimum Total cross-sectional area of copper wire screen, minimum Thickness of non-metallic sheath Range of overall diameter Max. dc resistance of
Ω/km
conductor at 20ºC
Table of 12/20 kV XLPE Copper Cable
APPENDIX B
SPECIFICATION NO. 200 69 & 115 kV XLPE COPPER CABLE 200A Scope
This specication describes the requirements with which the manufacturer shall comply in order to supply 69 kV and 115 kV XLPE copper cable, single conductor, cross-linked polyethylene
insulated, copper wire screen, polyethylene sheathed and/or re retardant PVC sheated for 69 kV and 115 kV solidly grounded neutral system to Lower Mekong Sub-region utilities(LMS). 200B Site and Service Conditions
The cable shall be suitable for use in ducts, trays and direct burial in ground, subjected to immerse in water all the time. The cable shall be suitable for use at conductor temperature of 90ºC continuously for normal operation, 130ºC for emergency overload condition and 250ºC for short-circuit condition. 200C Reference Standards
Except otherwise specied elsewhere in the specication, the cable required by LMS shall be manufactured and tested in conformity with the latest revision of the standards listed below : International Electrotechnical Commission (IEC) Publication 60228 (1978-01)
:
Conductors of Insulated Cables
Publication 60840 (1999-02)
:
Tests methods and requirements for Power cables with extruded insulation for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV)
Publication 60332-3-22 (2000)
:
Test for vertical ame spread of vertically-mounted bunched wires-Category A
International Standard (ISO) Publication 4589 (1996)
:
Determination of burning behavior by oxygen index
If there are any minor parts deviated from the standards, they shall be clearly mentioned in the “DEVIATION FROM MEA’S SPECIFICATION FORM” attached herewith. The cable conforming to other national standards having similar characteristics and providing
equal performance and/or quality to those specied may be proposed. In this case, the complete ENGLISH language copies of the standards shall be submit-ted with the quotation, otherwise such offer may not be considered. 200D Test, Inspection and Test Report 200D1 The proposed cable shall have successfully passed all the type tests ac-cording to the
reference standard except for re retardant property as specied in clause 200N2. The test shall be done by reputable independent test station accepted by LMS. All test reports or a letter of
conrmation that test report will be submitted to LMS within 15 (fteen) days after opening date shall be attached to the bid. If the documents are not de-livered to LMS within 15 (fteen) days after the opening date, LMS will not consider such offer. 200D2 Cable manufacturer, who have no type test report of the proposed cable, can submit a
type test report of the higher voltage XLPE Cable with the same or larger conductor size for consideration, without the need to do the type test after contract. In case the cable components from test report are different from the proposed cable, the type test reports for the cable components
including re retardant property (if applicable) shall be done be-fore shipment. 200D3 The following special tests and routine test shall be carried out in order to determine
whether the cable comply with the specication. If any tests required the agreement between the purchaser and the manufacturer, the test method shall be proposed by the manufacturer and
approved by LMS be-fore such tests can be proceeded. Where test voltages are specied in this specication as multiples of the rated voltage U o, the value of Uo for the determination of the test voltages shall be as specied in table I.
Table I
Rated nominal voltage
Value of Uo for determination of test voltages
(kV)
(kV)
69
36
115
64
200D4 Special tests
The special tests shall be made on one length from each manufacturing se-ries of the same type and size of cable, but shall be limited to not more than 10% of the number of lengths in the contract, rounded to the upper unity. a) Conductor examination and check of dimensions b) Measurement of electrical resistance of conductor c) Hot set test d) Measurement of capacitance
Note : The test in 200D4 b) and d) may be made on complete drum length of cable, taken to represent batches. 200D5 Routine tests
a) Partial discharge test b) Voltage test c) Electrical test on non-metallic sheath 200D6 LMS reserves the right to send the representative at his expense to inspect and witness
tests of the material and equipment during manufacturing, at the time of shipment or at any time he deems necessary. The supplier shall provide free access to the facilities where the equipment is being manufactured and shall satisfy the representatives that the material and equipment are in
accordance with this specication and purchase contract. The number of samples to be witnessed shall be as follows : Special test
• • •
Numbers of sample to be witnessed
Conductor examination and check of dimensions Measurement of electrical resistance of conductor
1
Hot set test
Table II Routine test
• • •
Numbers of sample to be witnessed
Partial discharge test Voltage test Electrical test on non-metallic sheath
Table III
2
200D7 Prior to the shipment, the supplier shall submit to LMS the complete and certied reports
of all tests made in 6 (six) copies. The test reports shall contain all data required for their complete understanding such as; diagrams, methods, instruments, constants and values used in the tests and the results obtained. 200E Conductor
The conductor shall be plain annealed copper and shall be compact round concentric lay stranded construction conformable to IEC Publication 60228 or compact segmental stranded. 200F Conductor Screen 200F1 Over the conductor, semi-conductive cross-linked polyethylene shall be extruded as
conductor screen layer. 200F2 The average thickness of the conductor screen shall not be less than the nominal value
specied in table V of clause 200S. 200F3 The minimum thickness of the conductor screen shall not be less than 80 per cent of the
nominal value specied in table V of clause 200S. 200G Insulation 200G1
The insulation shall be unlled, no carbon black, cross-linked polyethylene (XLPE),
simultaneously extruded with the semi-conductive conductor screen layer and insulation screen layer. 200G2 Only the dry curing process is required. Conventional steam or hot water curing processes
are not accepted. 200G3
The average thickness of the insulation shall not be less than the nominal value specied
in table V of clause 200S. 200G4 The minimum thickness of the insulation shall not be less than 90 per cent of the nominal
value specied in table V of clause 200S and additionally : maximum thickness - minimum thickness maximum thickness
≤ 0.12
where maximum thickness and minimum thickness are the measured values at one and the same
cross-section of the insulation. 200H Insulation Screen 200H1 Over the XLPE insulation, semi-conductive cross-linked polyethylene shall be extruded
as insulation screen layer. 200H2 The average thickness of the insulation screen shall not be less than the nominal value
specied in table V of clause 200S. 200H3 The minimum thickness of the insulation screen shall not be less than 80 per cent of the
nominal value specied in table V of clause 200S. 200I Synthetic Water Blocking Layer 200I1 A semi-conductive non-biodegradable water blocking layer shall be provided under the
metallic screen to provide a continuous longitudinal watertight barrier throughout the cable length. 200I2 This layer shall be compatible with other cable materials and shall not effect corroding
acting on adjacent metal layers during heat aging of the cable. 200J Metallic Screen (Grounding Screen) 200J1 The metallic screen shall be a concentric layer of copper wires which is electrically
continuous and bonded together throughout the cable length with copper contact tape. 200J2 The total cross-sectional area and minimum number of wires of the metallic screen shall
not be less than the value specied in table V of clause 200S. 200K Synthetic Water Blocking and Cushioning Tape 200K1 A non-conductive non-biodegradable water blocking tape shall be applied over the
metallic screen to provide a continuous longitudinal watertight barrier throughout the cable length. 200K2
The tape shall have sufcient thickness to perform well as a thermal stress relief layer and
to provide for cushioning and bedding.
200K3 The tape shall be compatible with other cable materials and shall not effect corroding
acting on adjacent metal layers during heat aging of the cable. 200L Radial Water Barrier
As a protection against formation of water trees in the insulation, a traverse water barrier consisting of laminated aluminum tape having average thickness at least 0.19 mm coated on both sides with an ethylene acrylic acid adhesive co-polymer or polyethylene shall be incorporated under the non-metallic sheath. 200M Marking
Manufacturer’s name or trade name, year of manufacture, and contract number at appropriate
interval shall be provided throughout the cable by inserting identication tape between radial water barrier layer and outer longitudinal water blocking layer, or on the outer longitudinal water blocking. 200N Non-metallic Sheath 200N1 The sheath shall be ST7 compound black polyethylene suitable for use with the cable
having maximum conductor temperature of 90ºC and 130ºC under normal and emergency
condition respectively unless other-wise specied. 200N2
For re retardant cable, the sheath shall be black, ame retardant, PVC. The oxygen
Index of non metallic sheath material shall be not less than 30 measured according to ISO 4589
or equivalent, certied test report from raw material manufacturer or reputable independent institution accepted by LMS shall be submitted for approval. The ame retardant non-metallic sheath shall be able to stop ame propagation along vertical or horizontal cable ways and delay damage to cables. Test on completed cable under re condition according to IEC 60332-3-22 or equivalent by reputable independent institution accepted by LMS shall be submitted before shipment. 200N3 For 69 & 115 kV PE sheathed, the sheath shall be of ribbed type having crest width and
depth of approx. 2.5 mm each and center to center distance between crests shall be approx. 7 mm.
see gure 1
Figure 1
200N4
For 69 & 115kV re retardant PVC sheathed, the sheath shall be of ribbed type same as
clause 200N3 except the crest width at the quarters four shall be approx. 5 mm. instead of 2.5
mm. see gure 2
Figure 2
200N5 The average thickness of the sheath (excluding rib) shall not be less than the nominal
value specied in table V of clause 200S. 200N6 The minimum thickness of the sheath (excluding rib) shall not be less than 85 per cent of
the nominal value specied in table V of clause 200S. 200O Cable End Sealing
Immediately after factory test, the cable inner end shall be greased by silicone paste and covered by PVC end cap, and the cable outer end shall be connected with mois-ture-proof pulling eye of
sufcient strength. Cable rib shall be removed before seal-ing.
200P Additional Requirements for Packing 200P1 The cable shall be delivered on returnable metallic drums having maxi-mum dimensions
as follows:Unit
69 & 115 kV 800 mm2
Flange diameter
cm
300
Drum diameter
cm
170
Outer width
cm
170
200P2 Length of 69 & 115 kV 800 mm2 cable per drum shall be 500 + 5 m and maximum weight
shall be approximately 8500 kg. 200P3
Cable length of the last drum shall be adjustable to meet the length specied in the
purchase contract but not less than 50 per cent of the length of cable per drum. 200P4 The cable drums shall be arranged to take a round spindle and to be lagged with strong,
closely tting battens so as to prevent damage to the cable. Each drum shall bear a distinguishing number, either branded or neatly chiseled on the outside of one ange. Manufacturer name, particulars of the cables, i.e. voltage, length, conductor size, number of cores, gross and net
weights, and the wording “Returnable Drum” shall be shown clearly on both anges of the drum. The direction of rolling shall be indicated by an arrow. 200P5
A steel plate, round, square or hexagonal in shape of suitable thickness shall be xed
at arbor hole of each outer face of the drum. The steel plate shall be drilled for insertion of temporary axle. Diameter of hole shall be 13 cm. 200P6 LMS will not be responsible for the payment of the supplied cables excessive from the
length specied in the contract, and on the contrary if the supplied cables are shorter than the length required in the contract, LMS will pay according to the actual length supplied only. 200Q Return of Cable Drum
200Q1 LMS will inform the supplier to bring back the empty drum within 540 days after the cable has been arrived and accepted by LMS. If the supplier does not bring the drums to his belonging within 30 days after being in-formed, LMS reserves the right to take action at LMS’s discretion.
200Q2 LMS will return the drum at normal wear and tear conditions. If the drum requires to be
refurbished such as repainting, it shall be borne by the supplier. 200R Additional Requirement for Responsibility
If the cable is damaged within the guaranteed period, the manufacturer shall promptly investigate, repair or replace it if the damaged is cause by the cable itself. The replacement shall be submitted to LMS within 60 days after being fi rst in-formed otherwise the performance security shall be forfeited. 200S Attached Table
Table of 69 and 115 kV XLPE copper cable is as follows:Table V 69 kV and 115 kV XLPE Copper Cable Rated nominal voltage Nominal cross-sectional area
kV
69
115
mm2
800
800
53
53
Minimum number of wires in the conductor Diameter of conductor
mm
34
34
Thickness of conductor screen
mm
1.5
1.5
Thickness of insulation
mm
11
16
Range of diameter over insulation
mm
59-62
69-72
Thickness of insulation screen
mm
1.5
1.5
Total cross-sectional area of copper wire screen, minimum
mm2
120
120
50
70
Minimum number of screen wires Average thickness of aluminum tape in radial water barrier, minimum
mm
0.19
0.19
Thickness of non-metallic sheath (excluding rib)
mm
3.5
3.5
Range of diameter over rib-bottom of the sheath
mm
76-81
86-91
Ω/km
0.0221
0.0221
Maximum dc resistance of conductor at 20ºC
Notes
