Guidelines for Maintaining the Integrity of XLPE Cable Accessories...
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Guidelines for Maintaining the Integrity of XLPE Cable Accessories
Working Group B1.29
December 2013
Guidelines for maintaining the integrity of XLPE cable accessories
Guidelines for Maintaining the Integrity of XLPE Cable Accessories
WG B1.29
1
Guidelines for maintaining the integrity of XLPE cable accessories
Members
Eugene Bergin IE Convener, Caroline Bradley UK Secretary, Bart Mampaey BE, Jos Van Rossum NL, Sverre Hvidsten NO, Maria Dolores Lopez ES, Colin Peacock AU, Patrik Wicht CH, Walter Zenger US, Yoshitsugu Sudoh JP, Ray Awad (Martin Choquette) CA, Nirmal Singh US, Xialong Luo CN, Doc Shun Shin KR, Frederico Adamini IT, Jonathan Beneteau FR, Eric Dorison FR, Detlef Jegust DE
Copyright © 2013 “Ownership of a CIGRÉ publication, whether in paper form or on electronic support only infers right of use for personal purposes. Unless explicitly agreed by CIGRÉ in writing, total or partial reproduction of the publication and/or transfer to a third party is prohibited other than for personal use by CIGRÉ Individual Members or for use within CIGRÉ Collective Member organisations. Circulation on any intranet or other company network is forbidden for all persons. As an exception, CIGRÉ Collective Members are allowed to reproduce the publication only.
Disclaimer notice “CIGRÉ gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”.
ISBN : 978-2-85873-255-5
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Guidelines for maintaining the integrity of XLPE cable accessories
Guidelines for Maintaining the Integrity of XLPE Cable Accessories Table of Contents
Page
Executive Summary
8
1 Review Recent Experience with Failures of Outdoor and Oil Filled Terminations and Non-buried Joints
11 1.1 Review of Literature
11
1.1.1 CIGRÉ/Jicable
11
1.1.2 Statistics
11
1.1.3 Workmanship
13
1.2. Review the Consequences of Termination Failures for Cables within Substations and Outside.
14
1.2.1
CIGRÉ/Jicable
14
1.2.2
Statistics
14
1.2.3
Workmanship
15
1.3. Survey by B1-29
15
1.3.1 Survey on Terminations
15
1.3.2 Survey on Non- buried Joints
18
2. The Role of Improved Materials, Design, Assembly and Quality Control in Mitigating the Effects of Termination and Non-buried Joint Failures
4
21
Guidelines for maintaining the integrity of XLPE cable accessories 2.1 Survey Results
21
2.1.1 Terminations 2.1.1.1 Design
21
2.1.1.2 Manufacture
22
2.1.1.3 Workmanship
22
2.1.1.4 Overvoltage
23
2.1.1.5 Weather Effects
23
2.1.1.6 Bonding Problems
23
2.1.1.7 Fluid/Gas Problems
24
2.1.1.8 Others
24
2.1.2 Non-buried Joints
24
2.1.2.1 Design
24
2.1.2.2 Manufacture
25
2.1.2.3 Workmanship
25
2.1.2.4 Overvoltage
26
2.1.2.5 Weather Effects
26
2.2 Design and Materials
26
2.2.1 Air Insulated Terminations
26
2.2.1.1 Porcelain Insulators
26
2.2.1.2 Composite or Polymeric Insulators
27
2.2.1.3 Latest Developments
29
2.2.2 GIS and Oil Immersed Terminations
31
2.2.3 Insulation Medium
31
2.2.4 Connectors
31
2.2.4.1 Compression Connector
32
2.2.4.2 Cad Welding
32
2.2.4.3 Soldered or Brazed Connector
33
2.2.4.4 MIG or TIG welded connection
33
2.2.4.5 Plug-in Connector
34
2.2.4.6 Mechanical bolted connector (shear bolts)
34
2.2.4.7 Mechanical bolted connector
34
2.2.5 Non–buried Joints
35
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Guidelines for maintaining the integrity of XLPE cable accessories 2.3 Assembly
35
2.4 Quality Control
35
3. The Role of Testing (development, type, sample, routine & after-laying) and Condition Monitoring in Minimising the Incidence or Severity of Termination and Non-buried Joint Failures 37 3.1. Testing
37
3.1.1. General
37
3.1.2. Development Testing
37
3.1.2.1 Insulators
38
3.1.2.2 Connectors
38
3.1.2.3 Filling Fluids
39
3.1.3. Prequalification Test
39
3.1.4. Type Test
39
3.1.5 Short Circuit Tests
40
3.1.6. Sample Tests
40
3.1.7. Routine Tests
40
3.1.8. Test on Filling Materials
41
3.1.9. Commissioning Tests
41
3.2. Condition Monitoring
42
4 Recommendations
44
5 Conclusions
45
Appendix 1 Terms of Reference
47
Appendix 2 Bibliography/References
49
Appendix 3 TB 476 ‘Jointer Workmanship Technical Brochure’ - Contents Pages
52
Appendix 4 Short Circuit Tests
56
Appendix 5 Condition Monitoring for Terminations and Non-buried Joints
60
Table 1 Terminations installed on XLPE cables (including PE and EPR) in the period 2001-2005
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Table 2 Failure rates of terminations over the period 2000 to 2005
12
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Guidelines for maintaining the integrity of XLPE cable accessories Table 3 Failure rates by type of termination over the period 2000 to 2005
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Table 4 Average repair time for cables in days
15
Table 5 Comparison of Porcelain and Composite Insulators
28
Figure 1 Failure due to poor workmanship
15
Figure 2 50kV porcelain outdoor cable termination,
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Figure 3 Composite insulator filled with synthetic oil
27
Figure 4 Example of a 170kV composite cable termination
29
Figure 5 Example of a Self Supporting Fluidless Cable Termination
30
Figure 6 Example of a Dry Type Supported Termination
30
Figure 7 Compression connector
32
Figure 8 Examples of Cad Welding
32
F figure 9 Example of a MIG Weld
33
Figure 10 Welding of an aluminium conductor
33
Figure 11 Plug-in connector (male contact) on prepared cable end.
34
Figure 12 Example of a bolted connector
34
Figure 13 Example of non-buried joints: 145kV single core cable joints installed in a cable jointing chamber/manhole 35 Figure 14 Salt-fog test on insulator
38
Figure 15 Tests on connectors
39
Figure 16 Type Test loop of 400kV system
40
Figure 17 On site Commissioning Test (in this set up three mobile tests sets needed simultaneously, because of cable length) 41 Figure 18 Discharge tracks on cable PE outer serving due to a defect
42.
Figure 19 Example of condition monitoring technique:
43
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Guidelines for maintaining the integrity of XLPE cable accessories
EXECUTIVE SUMMARY This work was motivated by the occurrence of disruptive failures of cable terminations and the consequential risks. The original scope of the Working Group (WG) was limited to land XLPE cable systems 110 kV and above. Although priority was given to outdoor and oil-immersed terminations, joints that are not directly buried were also included. The Terms of Reference are attached as Appendix 1. Following discussions within the Working Group on the terms of reference, it was agreed that:
Bonding and earthing, including SVL failures, were, in the main, not to be included.
Any relevant learning points from PE cable accessories were to be included, although polyethylene (PE) cables are no longer installed.
There should be no time restriction on assets covered by the survey, as the relative newness of XLPE cable technology would naturally limit the scope.
The scope was extended to cover voltage ranges from 60kV and above, as relevant failures at these voltage levels have also occurred and designs are similar to those being used at higher voltages.
Priority was given to outdoor, oil-immersed and GIS terminations, but joints that are not directly buried were also to be considered.
Those items that needed to be considered and complied with to minimise the failure rate for terminations and non-buried joints are listed below, following detailed analysis by WG B1-29. Development, Prequalification and Type Tests The nature and scope of tests to be carried out when developing (new) cables and/or accessories have not been formally standardised and it has been left up to the individual producers /manufacturers to use their knowledge and philosophy to design such tests. However, in the early 1990’s the CIGRÉ Task Force 21.03 published comprehensive recommendations for development tests on extra high-voltage cables with extruded dielectric, including the associated accessories. It was recommended that development tests for accessories focus on the following aspects:
Analysis of chemical, electrical and mechanical behaviour of materials
Long-term voltage test under thermal load cycles
Impulse and/or AC step voltage tests, where appropriate, with maximum conductor temperature.
Short circuit/disruptive discharge tests
Type tests in IEC62067 and IEC 60840 focus mainly on the withstand levels of cables and accessories with respect to a.c. or impulse stresses. They do not supply much information on the long-term behaviour of components, as the longest voltage test in these standards is limited to 20 days or 20 cycles of heating and cooling. The issue of long term tests (typically 1 year) is dealt with in Prequalification Tests in IEC 60840 and is to be carried out if the electrical stresses at the design voltage Uo exceed 8.0 kV/mm at the conductor screen and 4.0 kV/mm at the insulation screen. Fluid leakage is a significant cause of termination breakdown and this concern has to be addressed e.g. through final examination, as in IEC 62067 and 60840 standards, which states:
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Guidelines for maintaining the integrity of XLPE cable accessories “Examination of the cable system with cable and accessories with unaided vision shall reveal no signs of deterioration (e.g. electrical degradation, moisture ingress, leakage, corrosion or harmful shrinkage) which could affect the system in service operation.” Factory Quality Control (QC) It is essential that full quality control is exercised in the manufacture and supply of terminations and joints. This applies to all the sub-components of each accessory e.g. stress cones, jointing material, compounds,etc. A full set of suitable tests e.g. dimensional checks, electrical tests, as appropriate, should be established and implemented. The different components of an accessory should be packaged in such a way as to avoid damage and moisture ingress during transport. Delicate components, such as stress cones, should be shipped in sealed plastic containers. A detailed list of these components should be included in each box together with a complete set of assembly instructions. Recommended handling, storage conditions and expiry dates for any components should also be provided. On Site Quality Control It is essential that full quality control is exercised on site with respect to the jointing area set-up, including the control of dust, humidity and temperature andthe use of the correct jointing tools in good condition. In addition it is essential that suitable jointing instructions and drawings are supplied and that checks are carried out to ensure that the proper jointing material is supplied to site, in good condition and not past it’s expiry date. Finally a proper check-off list (inspection /test plan) should be used to make sure the jointing is done properly and in accordance with instructions. Jointer Certification As the quality of cable preparation and accessory installation plays a significant part in the reliability of XLPE accessories, it is critical that cable jointers have sufficient knowledge and training to carry out the task. It is therefore important that jointers are continually assessed to ensure competence and to maintain a high standard of workmanship. These training records and an up-to-date CV of previous works can be requested for review. Jointers should have valid up-to-date certification, as contained in TB476, for the accessory they intend to assemble. Tools The minimum required tools are:
those found in a standard tool box, such as knives, screwdrivers, wrenches, spanners, etc.
specific tools for conductor jointing, insulation and semi-conducting screen preparation, installing premolded stress cones, metallic sheath, screen and armour connecting, inner and oversheath finishing.
Specific tools and consumables shall be specified by the cable and accessory supplier/s. Jointing Instructions and Drawings Jointing instructions and drawings should be part of the quality assurance system. This is particularly crucial where accessories and cables are supplied by different providers. It is essential that the correct and suitable jointing instructions and drawings are used and that they are delivered with the accessory. Site Testing It is strongly recommended that an AC voltage test should be carried out on the insulation of the cable system in accordance with IEC Standards. Maintenance and Condition Monitoring In order to reduce the likelihood of failure of a termination or a non-buried joint, an inspection and test regime is recommended to monitor the condition of accessories. Many techniques are available to assess the condition of XLPE cable accessories. However, these techniques vary significantly with regards to practicality, availability of test equipment and the level of expertise required. The condition monitoring techniques employed should generally be assessed on a case by case basis and assessed against the
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Guidelines for maintaining the integrity of XLPE cable accessories requirements and cost of monitoring compared to the consequence of a failure. A list of the currently available techniques is contained in Appendix 4. In the event of oil or compound leakage or other incipient failure mechanism, a risk assessment should be carried out and corrective action taken if necessary. Risk Assessment The continued use of any accessory should be based on:
Public and employee safety The criticality of the circuit The history of the circuit and its accessories The potential repair time The potential cost of an outage to complete the repair The potential cost of an outage, if a failure occurs Potential damage from the failure Potential cost of the damage Effect on reputation, licence compliance and potential for prosecution Effectiveness of any monitoring system adopted Availability of monitoring tools and trained personnel The cost of monitoring Potential for damage of the accessory due to external factors
In case of a failure in service the first step is to verify if the cable systems (cable and accessories) has been subjected to the tests (development, prequalification, type, sample, routine), as requested by the relevant IEC standards or CIGRE recommendations.Following that one should investgate manufacture, delivery, installation and operation to determine the source of the fault. In the case of new cable systems, utilities should try to adopt designs that either do not experience disruptive discharge and/or have been tested to ensure the impact is kept to a minimum.
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Guidelines for maintaining the integrity of XLPE cable accessories
Chapter 1 Review of Recent Experience with Failures of Outdoor and Filled Terminations and Non-buried Joints The Working Group carried out a review of published literature on the subject and also carried out a survey of the experience of the Working Group members’ and Study Committee B1 members’.
1.1. Review of Literature The first step taken was to review existing literature and determine what was relevant to the study of accessory failures. It was agreed reviews should be short and take the following format:
Cause of defect Consequence of the defect Corrective steps taken
1.1.1, CIGRÉ, Jicable and Other Technical Literature Nothing of particular relevance was found in the published CIGRÉ literature. A recent paper for Jicable 2011 (A.5.4) described a failure in an XLPE cable termination installed in a 400kV GIS substation and the remedial actions taken. Another Jicable 2011 paper (A.3.7) summarised the experiences of three European TSO's. It showed that only a small part of the total cable circuit outage time is due to the actual repair time. More time was spent on other aspects, such as approvals to enter the premises, arranging the proper permissions to start repair works, cleaning the area and getting the necessary parts to site. The relevant literature is listed in Appendix 2.
1.1.2 Statistics TB 379 ‘Update of Service Experience of HV Underground and Submarine Cable Systems’ supplied the statistics in Table 1 below regarding XLPE terminations. There is no information in TB 379 for non-buried joints. The table below gives an overview of the number of terminations installed on XLPE cables (including PE and EPR) in the period 2001-2005. Later statistics are not available in a TB, but the WG addressed this in Section 1.3 below by gathering up-to-date experience from those 14 countries that responded to the WG survey enquiry.
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Guidelines for maintaining the integrity of XLPE cable accessories
ac Accessories installed 2000 to 2005 AC ACCESSORIES YEAR OF VOLTAGE INSTALL RANGE ATION
kV
60 to 109
110 to 219
220 to 314
315 to 500
> 500
2001 2002 2003 2004 2005 2001 2002 2003 2004 2005 2001 2002 2003 2004 2005 2001 2002 2003 2004 2005 2001 2002 2003 2004 2005
Extruded cables (EPR, PE or XLPE)
Outdoor Terminati on - Fluid filled Porcelain 531 753 513 483 600 267 282 546 226 187 135 63 102 66 60 12 0 0 0 28 0 0 0 0 0
Outdoor Terminati on - Fluid filled Composit e insulator 27 15 21 24 21 131 128 163 190 285 0 0 6 9 3 0 0 0 0 12 0 0 0 0 0
Outdoor Terminati on - Dry Porcelain 12 27 15 24 51 159 216 51 63 162 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
GIS or Outdoor Terminati Transfor GIS or on - Dry mer Transfor Composit Terminati mer e on - Fluid Terminati insulator filled on - Dry 75 0 311 69 6 296 96 5 225 186 2 190 138 3 225 32 116 394 35 77 565 83 130 447 32 98 366 41 106 389 0 54 135 0 30 12 0 0 42 0 3 27 12 3 42 0 0 0 0 0 0 0 0 12 36 0 0 0 12 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Table 1 Terminations installed on XLPE cables (including PE and EPR) in the period 2001-2005
The table below indicates the failure rates over the same time period (2000 to 2005):
FAILURE RATES BASED ON ALL REPLIES XLPE CABLES (AC) A. Failure Rate - Internal Origin Failures Failure rate Termination [fail./yr 100 comp.] B. Failure Rate - External Origin Failures Failure rate Termination [fail./yr 100 comp.] C. Failure Rate - All Failures Failure rate Termination [fail./yr 100 comp.]
60-219kV
220-500kV
ALL VOLTAGES
0,006
0,032
0,007
60-219kV
220-500kV
ALL VOLTAGES
0,005
0,018
0,006
60-219kV
220-500kV
ALL VOLTAGES
0,011
0,050
0,013
Table 2 Failure rates of terminations over the period 2000 to 2005
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Guidelines for maintaining the integrity of XLPE cable accessories Voltage range kV
60 to 219
220 to 500
Cable type
Accessory tyoe
Outdoor Termination - Fluid filled - Porcelain Outdoor Termination - Fluid filled - Composite insulator Outdoor Termination - Dry - Porcelain Extruded (XLPE, Outdoor Termination - Dry - Composite insulator PE or EPR) Outdoor Termination - Type not specified Outdoor Terminations - Total GIS or Transformer Termination - Fluid filled GIS or Transformer Termination - Dry Outdoor Termination - Fluid filled - Porcelain Outdoor Termination - Fluid filled - Composite insulator Outdoor Termination - Dry - Porcelain Extruded (XLPE, Outdoor Termination - Dry - Composite insulator PE or EPR) Outdoor Termination - Type not specified Outdoor Terminations - Total GIS or Transformer Termination - Fluid filled GIS or Transformer Termination - Dry
Total number of accessories in 2005 46226 2619 1954 1353 0 52152 4222 20771 1493 61 0 53 0 1607 2447 637
Total number of faults 15 2 2 1 17 37 0 19 5 0 0 0 18 23 2 2
Failure rates Cause of failure
Total failure rate 0,007 0,019 0,024 0,020
Internal 0,003 0,019 0,024 0,000
0,015 0,000 0,019 0,075 0,000 0,000 0,000
0,007 0,000 0,015 0,030 0,000 0,000 0,000
0,006 0,000 0,002 0,045 0,000 0,000 0,000
0,002 0,000 0,002 0,000 0,000 0,000 0,000
0,330 0,016 0,071
0,215 0,016 0,071
0,086 0,000 0,000
0,029 0,000 0,000
External Unknown 0,003 0,001 0,000 0,000 0,000 0,000 0,020 0,000
Table 3 Failure rates by type of termination over the period 2000 to 2005 In Table 1, for the period 2001-2005, we can see that for the HV cable systems (60 to 219kV) the use of outdoor composite insulators is already a commonly used technology. For EHV (above 219kV) this technology is only starting. The same findings are made with regard to the use of dry type GIS terminations. From Table 2 we can see that the failure rate on terminations for EHV cable systems (above 219kV) is around 5 times higher than that for the HV cable systems (60-219kV). Table 3 gives indicates the failure rate per type of termination and is grouped for the voltage levels 60-219 and 220-600kV. For a relatively high number of failures on terminations, the type of the terminations was not specified. As a result, the reader must be careful when comparing the different types of terminations. The information as shown in Tables 1 to 3 is based upon replies received by WG B1-10 to their questionnaire. For further information regarding these statistics we refer to CIGRÉ Technical Brochure 379.
1.1.3 Workmanship CIGRÉ Technical Brochure 476 ‘Cable Accessory Workmanship on Extruded High Voltage Cables’ was published in October 2011. This section 1.1.3 is substantially reproduced from that Technical Brochure. TB 476 covers workmanship associated with the jointing and terminating of AC land cables, incorporating extruded dielectrics for the voltage range above 30kV (Um=36kV) and up to 500kV (Um=550kV). This brochure is a complement of TB177. A short chapter covers general risks and skills, but the bulk of the document focusses on the specific technical risks and the associated skills needed to mitigate these risks. This is done for each phase of the installation. This Technical Brochure is not an Instruction Manual, but rather gives guidance to the reader on which aspects need to be carefully considered in evaluating the execution of the work at hand. High voltage cable accessories are manufactured using high quality materials and very sophisticated production equipment. Recent technical and technological developments in the field of their design, manufacturing and testing have made it possible to have pre-molded joints and stress cones for terminations up to 500kV, as well as cold shrink joints up to 400 kV. One of the conclusions of TB 476 is that internal failure rates of accessories, particularly on XLPE cable, are higher than other components and are of great concern due the larger impact of a failure. Therefore the focus on quality control during jointing operations must be maintained. Many utilities have adopted the “system approach” by purchasing the cables as well as the major accessories from the same supplier. Some utilities also request that the link should be installed by the supplier or by a contractor under the supplier’s supervision in a “turnkey” fashion. The main advantage of this approach is that the entire responsibility for the materials and workmanship is clearly the supplier’s. Some customers have adopted the component approach by purchasing cables and taccessories from different suppliers and entrusting the installation to a third party. In all cases, it is imperative that the
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Guidelines for maintaining the integrity of XLPE cable accessories installation be carried out by qualified jointers, who follow the jointing instructions provided by the accessory supplier. International standards such as IEC and IEEE provide the necessary guidelines concerning the interface between cables and accessories. However, it is strongly recommended that the responsible engineer should verify the compatibility of the different components of the link. It is of vital importance to manage the interface between the cables and the accessories in order to reduce the potential technical risk, e.g. cables and pre-molded accessories having non-compatible diameters or other non-compatible dimensions or characteristics. One of the international trends in cable technology has been the reduction of the cable insulation thickness and the corresponding increase in electrical stress. This tendency is based on better knowledge, increased quality of the insulating material and improvements in the extrusion process. Cables and accessory components are made under well-defined factory conditions and their quality and reliability are assured by adherence to well defined specifications. However, the accessories are assembled on site and, notwithstanding that this job is carried out by skilled and trained jointers, it is often performed in more delicate and less controlled conditions than in the factory. This means that correct assembly is even more important, because, with the increased stress level due to the reduced insulation thickness, bad workmanship will, sooner or later, lead to a breakdown of the accessory. It is noted that the majority of the new HV cable links being considered will use XLPE insulated cables. TB 476 captured the state of the art of jointing and is considered the best practice internationally. It is acknowledged that other practices, which are not explicitly covered in this brochure, are not necessarily bad practices. Great care should be exercised and the approach agreed when departing from practices recommended in TB 476. While TB476 does not directly refer to failures or the consequences of failures, it is a comprehensive document on the assembly of cable accessories. If used properly it can provide vital advice on the avoidance of failures due to bad workmanship.
1.2. Review the Consequences of Termination Failures for Cables within Substations and Outside. 1.2.1 CIGRÉ, Jicable and Other Technical Literature In the case of CIGRÉ the only consequences are the repair times that are covered in 1.2.2 below.
1.2.2 Statistics From TB 379, average repair times in days for XLPE systems are set out in the Table 4 below. This average repair time was calculated for all the reported failures on extruded cables for the corresponding voltage levels. No separate values were calculated for specific types of accessory. The definition of repair time as used in the questionnaire by B1-10 is the following: Repair time is the cumulative period of time required to mobilize resources, locate and repair the failure. The repair time associated with a failure is of fundamental importance since the summation of repair times is required to obtain a measure of non-availability, which from a reliability viewpoint is of greater significance than fault rate.
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Guidelines for maintaining the integrity of XLPE cable accessories
60 to 219kV
15 days
220 to 500kV
25 days
Table 4 Average repair time for cables in days -
1.2.3 Workmanship TB 476 does not specifically refer to the consequences of failures, except to indicate the potential damage in the area, the very serious transmission system consequences with potential safety implications, loss of load, loss of customers, poor public relations and potential loss of revenue and additional costs.
Fig 1 Failure due to poor workmanship (surface scratch due to bad workmanship)
1.3
Survey by B1-29
The Working Group compiled a survey to be completed by all members of the WG and SC B1 members, whose country were not represented on the Working Group. The survey was split into the voltage ranges recommended by CIGRÉ below:
50-109kV
110-219kV
220-314kV
315-500kV
Replies were received from 14 countries. Terminations and non-buried joints were dealt with separately. The survey results may be summarised as follows:-
1.3.1 Survey on Terminations a) A total of 61 failures were reported b) Most of the installations were inside substations with only 6 being in a public area
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Guidelines for maintaining the integrity of XLPE cable accessories c) The voltage range was from 51 to 400kV, with the main installations being in the 50-150kV range d) The installation year varied from 1972 to 2010 e) The year of failure varied from 1988 to 2010 f)
Most installations had commissioning tests and, in most cases, voltage tests were carried out as part of commissioning
g) Most installations were outdoor (37) h) The outdoor housings were generally filled with silicon oil or polybutene and the GIS (Gas Insulated Substation) housings were mainly unfilled i)
Most AIS (Air Insulated Substations) installations had composite or polymeric outer housings – 18 had porcelain housings. However it should be noted that failures in porcelain housings are likely to be more serious in view of the shards that are created during the fault
j)
The terminations were mainly installed by a manufacturer, with only 15 being installed by a utility or contractor
k) The conductor sizes varied from 100 to 2500 sq mm and were both copper and aluminium l)
The metallic shield varied from lead to aluminium foil to copper wires
m) In nearly all cases the cable and termination were from the same manufacturer n) In most cases prequalification test had not been completed o) Nearly all termination designs had undergone type tests p) In only a few cases were maintenance test carried out – varying from a serving test, DC test and thermovision tests q) The pollution design ranged varied from normal to serious r)
The causes of failure were listed as:
1) Termination Design Moisture ingress due to inadequate sealing. Pre-molded component breakdown. Breakdown of insulating material.
2) Manufacture Poor adherence of pre-molded components within stress cone Rough surface of metallic parts leading to Partial Discharge In one case manufacture was identified, but a reason was not given. Poor fluid quality leading to internal discharges.
3) Workmanship Damage to primary insulation during jointing. Poor fluid treatment prior to filling. Poor XLPE surface preparation.
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Guidelines for maintaining the integrity of XLPE cable accessories Poor preparation of the outer semi-conducting layer. Copper particles between cable and stress cone. XLPE shavings left in position between cable and stress cone. Incorrect application of stress cone. Cable not sufficiently straightened prior to jointing.
4) Overload No cases reported in the returned survey results.
5) Overvoltage Four cases due to switching/lightning surge.
6) Animals No cases reported in the returned survey results.
7) Weather Effects No cases reported in the returned survey results.
8) Cable Insulation Inadequacies Two cases, no details supplied.
9) Bonding Problems Thermal runaway due to a metal sheath being solidly bonded during installation. This was not in accordance with the specified bonding design, which was based on single point bonding. Poor earth connection due to mechanical movement causing flash-over.
10) Fluid/Gas Problems Partial discharge caused by solidifying silicon oil. Multiple failures due to leaks of insulating oil.
Fig 2 50kV porcelain outdoor cable termination, leaking high viscous insulating oil at bottom flange
11) External Damage /Sabotage No cases reported in the returned survey results.
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Guidelines for maintaining the integrity of XLPE cable accessories
12) Others Failure of pressure relief system, leading to loss of insulating fluid.
s) Consequences of Failure – fire, outage time, collateral damage, reputation Most cases resulted in a disruptive failure and some collateral damage that required a lengthy repair outage. t)
Actions Taken
1) New Design Method for earthing of sheath improved Change in specifications for pre-molded parts
2) New Tests No new tests were specified in the returned surveys.
3) New Installation Specification Improved termination fluid filling and treatment processes Changes made to compounds used during jointing and methods for handling compounds Suitable hold and witness points introduced during jointing New XLPE shaping techniques implemented Improvements made to Jointing Instructions
4) Risk Management On-Line PD tests introduced. Exclusion zones set up around termination, including screening walls.
5) Repair/Corrective Action i. Changed whole joint/ termination. ii.
Changed stress cone only. All faults required some form of repair or corrective action to be taken.
6) Preventative Action In many cases sealing ends that were leaking insulating fluid were replaced or repaired before an electrical failure occurred.
1.3.2 Survey on Non-buried Joints a)
27 failures were reported: 12 of the failures in premolded joints and 11 in taped joints. The remaining four failures being EMJ (extruded molded) or transition joints.
b) The location of the joints was generally not stated. c) The voltage range was 50 to 314kV, but the taped joints were in the lower voltage range. d) Core sizes varied from 400 to 2000 sq mm with both copper and aluminium conductors. e) Most joint casings were unfilled. f)
The installations were mainly carried out by the manufacturer.
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Guidelines for maintaining the integrity of XLPE cable accessories g) It was not clear if the joints and cables were from the same manufacturer h) In general the joints were type tested. i)
Most joints were commissioned with DC voltage tests (both insulation and serving).
j)
There was no maintenance testing before failure.
k) Many joints failed within 1-2 years of commissioning. l)
The causes of failure were attributed as follows:1) Joint Design Incorrect stress cone internal diameter. Incorrectly shaped embedded electrode. Poor tape design. 2) Manufacture Defective manufacture of stress cone that contained voids. Poor quality stress cone material. Water penetration via a crack, due to a manufacturing defect within the metallic casing. 3) Installation Damaged insulation during jointing. Poor shaping of XLPE. Voids created, due to poor shaping of insulating tapes. Incorrect positioning of stress cones. Cable inadequately plugged into joint body. Metallic particle contamination. Loss of earthing connection to screen wires, due to poor soldering. Racking or tray system that permitted joint movement. 4) Overload No cases of failure were attributed to overload. 5) Overvoltage One reported case was attributed to a possible lightning strike. 6) Animals There were no failures attributed to animals. 7) Weather Effects In only two cases failures were attributed to weather effects, namely water penetration. The water penetration in joints may be a design/material/workmanship issue 8) Unknown One case was listed as unknown.
m) Consequences of Failure
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Guidelines for maintaining the integrity of XLPE cable accessories No consequences were provided in the survey replies. n) Actions Taken 1) New Design In most cases where joint design was identified as the cause of failure, the joint was redesigned. 2) New Tests Post-installation PD testing of joints was introduced in many cases.
3) New Installation Specification Hold and witness points were introduced including photographic records. New guidance on joint protection and waterproofing was introduced. Clean room conditions introduced to joint bays. Improvements were made to jointing instructions. 4) Risk Management Joints identified as potential failure candidates were replaced with either joints of a different design from the same manufacturer or joints from a different manufacturer. Inspection, partial discharge testing and X-Raying of all joints installed from the same manufacturer were carried out. 5) Repair/Corrective Action In most cases the affected joints were removed, which required the insertion of a new piece of cable and 2 joints and the joint bay was extended to fit the new joints 6) Other A new reinforced racking design was introduced
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Guidelines for maintaining the integrity of XLPE cable accessories
Chapter 2 The Role of Improved Materials, Design, Assembly and Quality Control in Mitigating the Effects of Termination and Nonburied Joint Failures This section examines how matters may be improved with respect to materials, design, assembly and quality control in preventing termination and non-buried joint failures and mitigating their effects. As part of this process, the results of the survey are reviewed to identify the causes of faults and steps identified that could be taken to ensure these faults did not occur. It should be noted that some of the measures identified in the Survey Results Section 2.1 below may be repeated to some extent in the Sections 2.2 to 2.4 dealing with Materials, Design, etc. This was done to ensure the Technical Brochure is as complete as possible.
2.1 Survey Results It is of considerable importance that the results of the survey in section 1.3 are taken into account and that, where causes were identified, these are acknowledged and steps are taken to avoid these causes in the future. The causes and recommended mitigations are listed below:-
2.1.1 Terminations 2.1.1.1 Design Cause
Mitigation
Unsuitable top O ring seal used leading to moisture ingress
Use appropriate O ring and fit properly
Powder separation of chemical mixture.
Ensure correct compounds are used and installed correctly
Earthing conductors slipping off metal sheath in termination by sliding over PE sheath.
Ensure correct installation. Use checklist for installation.
Circulating current flowing through insulator screen causing overheating and damage.
Ensure the correct bonding design is installed
Pre-molded insulation degradation at extremely low temperatures
Ensure design suitable for operating temperatures high and low
Damage due to thermal cycling.
Design and test for heat conditions. (Snaking cable before terminating to minimise conductor expansion into the termination )
Interface design.
Change components or design
Degradation of components in stress cone.
Use appropriate materials and enhance the interface design Consider extended Prequalification Tests.
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Guidelines for maintaining the integrity of XLPE cable accessories Cause
Mitigation
GIS copper corona shield with thin layer having whiskers, leading to PD and breakdown.
Design corona shield materials for use in GIS cable termination box. Inspect all components prior to fitting.
Stress cone interface contaminants
Jointer trained on fitting accessory, as recommended in Appendix 3 Ensure clean conditions when jointing
2.1.1.2 Manufacture One case was identified but no details were supplied – no additional mitigation proposed.
2.1.1.3 Workmanship Cause
Mitigation
Jointer damaged insulation
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Poor XLPE surface shaping - copper contaminants between cable and stress cone-contaminants invasion of oil
Follow Appendix3 Consider use of inspection test plans (ITP’s)
Shavings of copper contamination during the insertion of pre-molded insulation
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Poor surface of outer semi conducting layer-defective position of compression device
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Void generation between epoxy and stress cone
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Plastic wrap is used for protection during construction.
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Void generation at cable/stress cone interface by overbending of cable and shaving cable insulation too much. Generation of crack in epoxy insulator by stressing it more than it was designed. Overbending of cable.
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Guidelines for maintaining the integrity of XLPE cable accessories Cause
Mitigation
Void generation at cable/stress cone interface by conductor centering error, when conductor sleeves were compressed
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Wrong insert position
2.1.1.4 Overvoltage Cause
Mitigation
One case due to switching/lightning surge
Ensure appropriate design and installation of lightning protection, when required.
2.1.1.5 Weather Effects Cause
Mitigation
Lightning
Ensure lightning protection used, when needed
Water entry
Follow Appendix 3 and use proper O ring and fit it properly (it could be a design/material problem)
Connection broken, due to mechanical overload
Ensure that not overbend
Jointing with high relative humidity
Use of an enclosed air conditioned work environment Follow Appendix 3
2.1.1.6 Bonding Problems Cause
Mitigation
Metal sheath incorrectly bonded on a single core cable, resulting in a sheath circulating current that overheated and damaged the termination
Ensure bonding design is followed
Bad connections; poor design of wiping gland leading to mechanical movement, sparking and failure
Ensure design suitable for operating temperatures high and low and installed properly.
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Carry out checks during commissioning
Guidelines for maintaining the integrity of XLPE cable accessories
2.1.1.7 Fluid/Gas Problems Cause
Mitigation
Partial discharge in fluid
Ensure correct fluid is used and that fluid is properly treated and tested and that it is at the right level.
Leaking fluid or gas
Check where fluid or gas is leaking from, repair if necessary, and top up. Replace termination or component causing the leak.
2.1.1.8 Others Cause
Mitigation
Unknown - breakdown just above stress cone
Ensure design is suitable for high and low operating temperatures
Contaminants noticed at the cable stress cone interface
Remove Follow Appendix 3
Moving cables after installation
Ensure cables do not exceed their thermomechanical design limits, are properly clamped and are not physically disturbed
2.1.2 Non-buried Joints 2.1.2.1 Design Cause
Mitigation
Stress cone with incorrect inner diameter
Ensure joint is suitable for use on specified cable after cable is prepared
Shape of embedded electrode not right
Ensure design is compatible Ensure adequate Prequalification and Type Tests are carried out
Poor tape design
Ensure material used has the right properties and installation instructions. Consider Prequalification Testing
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Guidelines for maintaining the integrity of XLPE cable accessories
2.1.2.2 Manufacture Cause
Mitigation
Defective manufacture of stress cone (voids)
Ensure manufacturer’s QC system is adequate Consider Prequalification testing
Poor material quality
Ensure manufacturer’s QC system for materials is adequate Consider Prequalification testing
Water penetration from a crack, because of manufacture problem with metallic sheath
Ensure manufacturer’s QC system is adequate
2.1.2.3 Workmanship Cause
Mitigation
Jointer mistakes causing damage to insulation and poor insulation shield shaping.
Follow Appendix 3 Consider use of inspection test plans (ITP’s)
Water penetration, metallic contaminants, wrong inset position. Poor adhesion of stress cone
Follow Appendix3 Consider use of inspection test plans (ITP’s)
Metallic contaminants in the insulation tape.
Follow Appendix3 Consider use of inspection test plans (ITP’s)
Void generation with poor tape shaping. Contaminants. External damage by jointing tool, when connection box was assembled. Fibrous contaminant in extruded insulation. Clamping of screen wires caused damage of outer semiconducting layer Loose flakes of applied semiconducting coatings in joint assembly.
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Follow Appendix3 Consider use of inspection test plans (ITP’s) Follow Appendix 3 Ensure proper procedures followed, adequate drying time and care in positioning of the joint body.
Guidelines for maintaining the integrity of XLPE cable accessories
2.1.2.4 Overvoltage Cause
Mitigation
In only one case was joint damage attributed to possible lightning strike
2.1.1.5
Ensure appropriate lightning protection is used.
Weather Effects Cause
Mitigation
In only two cases was failure attributed to weather effects, namely water penetration.
Follow Appendix 3 Consider use of inspection test plans (ITP’s). Adequately designed casing (coffin) filled with waterproof compound.
2.2. Design and Materials In considering the design of terminations and joints it is necessary to consider the materials to be used, the pressures in different parts of the accessory assembly, the different electrical characteristics, etc
2.2.1 Air Insulated Terminations Air Insulated Terminations are generally used outdoor to terminate cables in air insulated substations. They may have porcelain or composite insulators and may be filled or unfilled. The design adopted may depend on the local environment with respect to the required basic impulse level voltage (BIL), maintenance requirements, pollution (industrial and ocean), reliability and altitude. Surface creepage distances may need to be increased in areas of high pollution, excessive sea spray or at high altitudes.
2.2.1.1 Porcelain Insulators Glazed electrical grade porcelain is the most common and widely installed insulator. It has high reliability in terms of electrical and mechanical performance. It requires periodic maintenance (cleaning) to remove pollution deposits from the insulator surface (sheds). It has high resistance to surface tracking. Porcelain production is a mature technology and can be provided for MV to EHV cable terminations and for both AC and DC application. However, porcelain can be susceptible to external mechanical damage and to electrical failure (internal or external). It can shatter on termination failure with pieces of glazed porcelain and other debris projected over the surrounding area by the force of the failure. The potential for injury or damage to adjacent equipment in the surrounding area is high.
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Guidelines for maintaining the integrity of XLPE cable accessories
2.2.1.2 Composite or Polymeric Insulators.
Fig 3 Composite insulator filled with synthetic oil
There are many types of composite insulators available on the market. The most common design consists of a fibreglass tube covered by elastomeric sheds (silicone). This solution is much lighter than a porcelain insulator and is normally much easier to handle during installation. However, the bond between silicon rubber and the epoxy glass fibre pipe must be certified as this can be a weak point. Composite insulators are available up to EHV applications, even though at this stage there is no long term operational experience at EHV levels. Composite insulators have many advantages. In particular they have proven to be reliable even under exceptional events such as earthquakes, system faults and vandalism. They also provide good insulation performance due to their silicone housing and the intrinsic hydrophobic characteristic of this material. Well designed composite insulators have limited ageing. They give satisfactory performance in heavily polluted areas, where no cleaning or special maintenance is necessary and this can provide important economic savings. Their technical and economic advantages are of particular significance in the EHV and UHV range of accessories. This is because of their design flexibility (single pieces of 10 m or more may be manufactured), relative low weight (10-30% of a corresponding porcelain insulator), ease of handling for manufacturing and installation and their ability to withstand stresses, such as seismic events and high levels of pollution. From the point of view of end-users, a very important feature of composite insulators is safety. They reduce the potential for manual handling injury during delivery and installation. Since they are not brittle, the risk following an internal fault, with the associated projection of material, is greatly reduced compared with porcelain. The satisfactory long term performance of composite insulators is directly related to electrical and mechanical design, good selection of the material, good manufacturing processes and quality control. Environmental constraints of the installation site such as the required BIL, temperature, barometric pressure (for high altitude), presence of aggressive gases, pollution, and humidity should be taken into account in the design. Qualification procedures can help to qualify the technology and the materials and assure the performance during the required life time of the insulator and these are dealt with in detail in TB455 ‘Aspects for the Application of Composite Insulators to High Voltage (≥72kV) Apparatus’. A range of biological growths have been reported on composite insulators leading to a reduction of the hydrophobicity. However, the overall performance of the composite insulator design generally remains satisfactory. Bird attacks have also been reported, but this appears to be a problem related to insulators in some countries and usually only happens when de-energised or before the insulators are put into service
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Guidelines for maintaining the integrity of XLPE cable accessories Another consideration is whether vapour could permeate directly through the sheds and walls of the housing (polymeric materials are generally slightly permeable for vapour) or through the bonding area between flanges and fibre-reinforced plastic (FRP) tube. Investigations and service experience indicate that the amount of moisture ingress due to these mechanisms is below the quantities which can pass through a good sealing system. Quantities can easily be controlled by internal desiccants as is usual practice for much of the HV apparatus in the electric power system. In the case of terminations/sealing ends this is often accomplished by using filling compounds. Nevertheless research continues in an attempt to better understand these mechanisms and to derive minimum design requirements on composite hollow core insulators used for HV apparatus applications. Most damage in composite insulators can be attribute to errors during transport, un-packing, re-packing, manipulation and storage of the insulators. These aspects are dealt in detail in TB 455 ‘Aspects for the application of Composite Insulators to High Voltage (>=72 kV) Apparatus’, Chapter 9 ‘Handling and Maintenance’. In this chapter, procedures and rules are given for: unpacking, repacking, storage, handling and cleaning. A composite termination has the advantages of a simple structure. Its anti-pollution capacity depends mainly on the number of sheds and their size and orientation.The terminal must be installed upright. - it cannot be installed inclined or curved. Porcelain and composite terminations are compared in the Table 5 below
Element Environmental
Chemical
Mechanical
Porcelain Insulators
Composite Insulators
Can shatter
Safe/ Inert
Periodic cleaning required
Limited cleaning required
Poor pollution performance
High performance in polluted areas
It’s earthquake performance is not so good
Good earthquake performance
Impermeable to animal attack even when unenergised
Possible attack by animals during storage and while unenergised
Not hydrophobic
Hydrophobic
Compatibility with SF6 byproducts and oil
Compatibility of filling material to be checked
Can shatter under fault conditions
Will not shatter but may split
High weight
Low weight
Vulnerable to vandalism
Less susceptible to vandalism
No moisture ingress through the 1 insulator from outside.
Possible moisture ingress through 1 the insulator from outside.
1
Note for both types of insulators there may still be some moisture ingress through the top and bottom metal components or gaskets
Rating Performance
Other Properties
No practical temperature limit (temperature limits exceed those of other components)
Temperature limits of o -55 to +110 C
Lot of experience, but relatively long manufacturing time
Limited service experience
Because of its weight it’s not so
Because of its weight its relatively
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Guidelines for maintaining the integrity of XLPE cable accessories
Element
Porcelain Insulators
Composite Insulators
easy to handle and install. Heavy manual handling or mechanical assistance required
easy to handle and install
Can be damaged (cracked or chipped) by handling and installation. Small damage can be repaired in-situ.
Not so likely to be damaged
Table 5 Comparison of Porcelain and Composite Insulators
It can be seen that each outer housing material has its advantages and disadvantages. The selection of the appropriate termination body depends on the particular installation conditions. The satisfactory performance of composite terminations is dependent on the inner electrodes and the electric field distribution within and along the termination. This, in turn, depends on the top electrodes, the insulator material, the inner electrodes, non-linear coatings, cable make-up; etc All of these components must be designed, manufactured and installed to control the operating electrical stresses.
Fig 4 Example of a 170kV composite cable termination 2.2.1.3 Latest Developments The latest developments on the market provide two alternative solutions:1) Self Supporting Terminations a) A termination filled with silicon based leak-proof gel that replaces the traditional liquid fluids. This solution has been tested up to EHV, but service experience is available only up to 132kV. The filling procedure has to be strictly controlled to ensure proper filling. b) A fully dry termination, where no liquid or filling is used
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Guidelines for maintaining the integrity of XLPE cable accessories
Fig 5 Example of a Self Supporting Fluidless Cable Termination
2) Supported or Flexible Type A Prefabricated Outdoor Termination This type of termination has elastomeric sheds and an external stress cone. The stress cone and the sheds form one single factory-tested premolded component and they are widely used in the voltage class up to 150kV. With this termination type a completely “dry” design is obtained. Note this termination is not self supporting and must be connected to an overhead conductor or to another component e.g. a surge arrester, able to support the termination.
Fig 6 Example of a Dry Type Supported Termination 3) Disruptive–proof Outdoor Terminations i.e. terminations that are designed to limit the consequence of an internal power arc, etc.
One must also bear in mind the effect of insulation retraction on the termination. Retraction is a result of the mechanical stress formed in the insulation during the manufacturing process. When the cable is cut, in order to install the accessory, the insulation may retract on the accessory and lead to a failure. This must be taken into account in the accessory design.
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Guidelines for maintaining the integrity of XLPE cable accessories
2.2.2 GIS and Oil Immersed Terminations EHV and HV cables may also be directly terminated in SF6 insulated switchgear (GIS) and transformers to eliminate air-insulated interfaces. This solution has the significant advantage of markedly reducing substation area requirements and costs in urban, suburban and industrial plant locations. It also eliminates insulation contamination from pollutant deposits and reduces exposure to lightning and vandalism. GIS and oil immersed terminations have similar construction, except for the use of a larger top corona shield on the termination in order to reduce the top-end stress. The electrical stress control for GIS and oil immersed terminations follows the same approach usually employed for outdoor terminations i.e. it uses a premolded stress relief cone, which is fitted over the cable insulation. The cable is then accommodated inside a cast epoxy resin bushing which separates the cable from the pressurised SF6 or the oil in the termination end box. The space inside the epoxy bushing can be filled with insulating fluid or SF6 gas. In order to eliminate any risk of leakage of this fluid or gas from inside the epoxy bushing, a new generation of dry type SF6 and oil immersed terminations have been developed. In these dry terminations there is no insulating fluid or gas between the epoxy insulator and the stress cone, because the latter is in intimate contact with the inner surface of the bushing; the pressure of the stress-cone at the cable core interface as well as at the inner epoxy insulator surface can be obtained by means of compression devices such as springs or by special design of the polymeric part. It should be noted that currently there is a Joint Working Group B1/B3.33 examining the ‘Feasibility of a common, dry type interface for GIS and Power cables of 52 kV and above’ (2009 – 2012) and a Technical Brochure is expected to issued by this WG by the end of 2013. 2.2.3 Insulation Medium Terminations are generally filled with a dielectric fluid, usually a synthetic (polybutene or silicone based) insulating liquid, at or slightly above atmospheric pressure. The type and quantity of the fluid depends on the specific design of the termination. Poor quality of the liquid or contamination, due to external factors (humidity, water ingress, metallic or other polluting particles, etc), can reduce the electrical performance of the fluid and result in termination failure.One of the most common issues with the use of fluid is the risk of leakage through the sealing point areas, typically the weld/plumbing between the cable metallic screen and the bottom part of the termination or the mechanical seal onto the stress cone. A well-made seal depends mostly on the skill of the jointers. There are also designs that use SF6 gas as the insulation medium, but this solution has to bear in mind the environmental concerns of using SF6 gas.
2.2.4 Connectors The connector electrically and mechanically joins the conductors of two cables or the cable and the top connector of a termination. Thus the connector must exhibit good electrical conductivity to avoid temperatures higher than that of the conductor in any operating condition and also present sufficiently high mechanical pull-out (tensile) strength to withstand thermo mechanical stresses during operation. It should be noted that TFB1.46 is currently working on Conductor Connectors (Mechanical and Electrical Testing). The following types of connectors are used for extruded cable connections:-
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Guidelines for maintaining the integrity of XLPE cable accessories 2.2.4.1 Compression Connector This connector includes a tube of the same material as the cable conductor into which the conductors to be joined are inserted. The tube is then compressed by a hydraulic press. The compression connector is the most commonly used type, because it is easy to install and does not require heat. The cross section of the connector is at least equal to the cross section of the conductors to be joined. When the connector is exposed to an electric field, as in taped joints, it is necessary to provide suitable chamfers at both ends to minimize the effects of longitudinal electrical stresses.
Fig 7 Compression connector
A special bimetallic connector is used when it is necessary to join a copper conductor to an aluminium conductor. These connectors are half copper and half aluminium. The two connector halves are joined in the factory by friction welding. Some companies use a copper alloy connector for both copper and aluminium conductors. 2.2.4.2 Cad Welding Another way is to make a connection of copper and aluminium conductors by Cad-welding on site, though Cad welding is not used that often for aluminium. This is an exothermic welding process in which metal and metal oxide powders are placed in a special crucible mold around the parts to be welded. This mixture is ignited resulting in a short high temperature reaction,causing the flow of molten metals to form a localised solid connection.
Fig 8 Examples of Cad Welding
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Guidelines for maintaining the integrity of XLPE cable accessories 2.2.4.3 Soldered or Brazed Connector 2
Soldered connectors are used with small conductor cross sections (below 630mm ) and with cables having a short circuit current temperature below 160 °C, b ecause the solder can become soft during the cable system operation. Brazed connectors do not present this problem, but are more difficult to make. 2.2.4.4 MIG or TIG welded connection The two conductors are fused together by the application of molten metal. A Metal Inert Gas (MIG) or Tungsten Inert Gas (TIG) welding process is applied in this case. Due to the high temperature developed during the process, air or water cooling clamps are required on both sides of the weld, in order not to damage the cable insulation The welding process is used for large aluminium conductors and for insulated wire copper conductors; in the latter the burning of the wire insulation, if necessary, ensures a good contact between strands. This technology requires an operator with a very high skill level and is time consuming. This weld provides a connection with an electrical conductivity, which is equivalent to that of the conductor itself. The connection is not subject to instability due to decrease of contact pressure as a result of load cycling. However the tensile strength of the welded connector is significantly (50 to 60 %) lower than the ultimate tensile strength of the conductor, due to the annealing of the conductor near the weld. If necessary, for submarine cables, the tensile strength can be improved by round compressing the conductor and the weld (hardening process).
Fig 9 Example of a MIG Weld
Fig 10 Welding of an aluminium conductor
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Guidelines for maintaining the integrity of XLPE cable accessories 2.2.4.5 Plug-in Connector Two metal connectors, that terminate the conductor, are jointed through elastic or multi contact spring loaded contacts that are able to carry the current. Locking pins can be used to anchor the two parts together. Plug-in connectors can easily join conductors of different materials and cross section.
Fig 11 Plug-in connector (male contact) on prepared cable end. One of the advantages of a plug-in connection is the shorter length of the joint.
2.2.4.6 Mechanical bolted connector (shear bolts) With these connectors compression of the conductors inside a ferrule is made by tightening threaded bolts. The bolts shear off at a predetermined torque and are then finished flush with the surface of the connector. These connectors are extensively used in MV accessories, and may also be used in HV joints or terminations, subject to checking their short circuit current and current loading capacity. The compatibility of these connectors with the termination or joint design must be checked. These connectors have a diameter larger than the compressed connectors and care must be taken to ensure there are no bits of bolt protruding above the connector surface. Before using shear connectors consideration must be given to tensile strength during load cycling and pull out. 2.2.4.7 Mechanical bolted connector With these connectors compression of the conductors inside a ferrule is made by tightening threaded bolts. These connectors are extensively used in MV accessories, and may also be used in HV joints or terminations, subject to checking their short circuit current and current loading capacity. The compatibility of these connectors with the termination or joint design must be checked. These connectors have a diameter larger than the compressed connectors and care must be taken to ensure there are no bits of bolt protruding above the connector surface
Fig 12 Example of a bolted connector
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Guidelines for maintaining the integrity of XLPE cable accessories
2.2.5 Non-buried Joints Non-buried joints locations may be in tunnels, on bridges, in underground chambers or similar enclosures. Non-buried joints for XLPE cables usually have premolded joint bodies with additional covering for protection against moisture and mechanical damage. The additional covering could be heat shrink tubes or metal housings with additional insulating housings/coffins. Transition joints for XLPE to oil filled cable are often installed as non-buried joints in underground chambers. They use metal-tubes combined with epoxy insulators as a barrier between the different insulating materials - XLPE and fluid impregnated paper. In the case of transition joints full quality control must take into account electrical and mechanical stresses for both sides of the joint and any interface locations. Water can seep into a non buried joint, if any earth or bonding wire connections to the joint are not sealed properly.
Fig 13 Example of non-buried joints : 145kV single core cable joints installed in a cable jointing chamber/manhole
2.3 Assembly TB 476 is a comprehensive document on assembly and quality control of XLPE accessories and the contents pages are attached as Appendix 3. It gives guidance on aspects of cable accessory workmanship that need to be carefully considered in evaluating the execution of the work, including the specific technical risks and the associated skills needed to mitigate them. Where a termination is to be filled with compound, the manufacturers filling instruction should be followed. Filling compounds may be such items as polybutene, silicon oil or other dielectric fluid or gas.
2.4 Quality Control Joints and terminations are delivered to site as kits, which in turn are made up of many components It is vital to have quality control on all components. The main insulation is either the premolded joint body or premolded stress-cone, and the testing requirements for these are as defined in IEC60840 and IEC62067. The manufacturer shall demonstrate or guarantee that the components forming the accessory are the same as those tested to IEC standards.
35
Guidelines for maintaining the integrity of XLPE cable accessories Each component has a specific function, whether it is secondary insulation, oil, gas or air tightness, mechanical protection, conductor or sheath connection, etc. It is essential that the manufacturer has in place quality control plans that define the tests to be carried out and their frequency and these should be related to the function of the component. The inspection or testing may include visual, dimensional, mechanical, dielectric, pressure, whether as an incoming control from sub-suppliers or as final control as semi-finished products (insulators for example). Components must be inspected according to drawings and specifications with given tolerances, and there must be no deviations outside the given tolerances. Final checking must be done on delivery to site to ensure the right quantity and quality of materials has been delivered. Of course the QC aspects with respect to jointing, as set out in TB 479, must also be followed. This applies in particular to the certification/approval for the jointers and the site conditions.
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Guidelines for maintaining the integrity of XLPE cable accessories
Chapter 3 The Role of Testing and Condition Monitoring in Minimising the Incidence or Severity of Termination and Non-buried Joint Failures 3.1. Testing 3.1.1 General In order to prove that a cable system meets the expectations of the customer the role of testing at all stages of design, supply and in-service is clearly important for both the supplier and end-user. In addition, once a cable system is in service, it may be beneficial to carry out in-service testing to assess the condition of the system and its components. This section will examine the types of testing and condition monitoring that may be carried out, when assessing a cable system. This is not intended to be exhaustive, but to provide guidance on the areas that should be considered. The level of testing required for a cable system should be decided on by the customer, based on risk and performance requirements. International standards for underground cable systems generally provide design rules and testing procedures to assess a cable system and to ensure it meets the requirements for reliable operation during its design life. These generally focus on prevention of failure, rather than actions that can be taken to mitigate the consequences of a fault. Some National Standards or individual utility specifications have introduced fault simulation testing and specify requirements for the performance of the system under these conditions e.g. an internal arc test is carried out by some utilities to evaluate the consequence of an internal fault - there is a requirement for this within IEC 62271 requirements for switchgear testing. It should be noted that a cable system incorporates the cable, terminations, joints, internal terminations and joint components, filling media, connectors, screen connections, bonding etc, and great care must be exercised in testing to ensure that all of the components are properly represented and identified in testing regimes.
3.1.2. Development Testing Development testing is carried out by the cable accessory supplier during the design of a new accessory. The results of these tests may indicate to the manufacturer and, where required, the customer, any changes and improvements that can be made to a cable accessory. An example of development tests are the environmental tests including salt/fog, rain and pollution tests, carried out on composite insulators, which are not covered by cable international standards. These tests are carried out by manufacturers to demonstrate the long term performance of their products and are carried out to in-house test specifications. IEC61462 ed 1.0 covers the test procedures for Composite Insulators for AC Overhead Line with Nominal Voltage greater than 1000 volts. Results of development testing are generally not specified by customers, but may help to inform a decision on the suitability of a cable termination or joint for use for a particular application or in a particular location, for example the suitability of terminations for use in areas of high pollution.
Development tests are performed by the manufacturer during the development of a new accessory and are intended to ensure the accessories long term performance and to assess safety margins. The tests include:
Analysis of electrical, mechanical and material compatibility
Electrical tests up to breakdown and mechanical and thermal tests on prototypes
Wet and pollution test on outdoor terminations
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Guidelines for maintaining the integrity of XLPE cable accessories
Electrical and thermal tests of connectors
Mechanical tests on premolded components (on the insulators and connectors)
Fire and disruptive failure performance, including Internal Power Arc test on terminations in accordance with Appendix 4
3.1.2.1 Insulators IEC 61462 ‘Composite hollow insulators –pressurised and unpressurised insulators for use in electrical equipment with rated voltage greater than 1000 V’ specifies both design and type test requirements for self supporting composite insulators. The tests in this IEC standard are designed to provide information on material selection, manufacturing processes, material thickness and adhesion and end fitting material selection an attachment. To complete the project of developing a new accessory, construction drawings shall be prepared of all components and a full size prototype shall be manufactured and subjected to tests. If the prototype includes specific components such as premolded parts, composite and epoxy resin insulators, it is necessary to develop the technology to produce these components The tests should show the limit in the performance of the accessory and guarantee a proper safety margin with respect to test values stated in the relevant IEC standard. Tests carried out must ensure that the entire family of accessories is able to withstand the stresses, which they may be subjected to in their operational life.
Fig 14 Salt-fog test on insulator
The termination may be exposed to a saline solution of a different concentration depending on the level of pollution it will experience. In this condition it is then subjected to an AC voltage test. For composite insulators with a polymeric coating, which are subject to aging of the surface, the pollution test is performed after an aging of 1000 hours in saline fog or an electrical cycle-environmental of 5000 hours (see IEC 62 217) 3.1.2.2 Connectors Development testing may also be done for connectors. Thermal cycles are performed on connectors and contacts used in the accessories following the standards of IEC 61238-1, currently restricted to medium voltage. During the test, measurements of temperature and electric resistance as a function of time are taken. Short circuit current tests are also performed on the connectors.
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Guidelines for maintaining the integrity of XLPE cable accessories
Fig 15 Tests on connectors 3.1.2.3 Filling Fluids Before using any type of oil or fluid within a specific housing material, equipment manufacturers should have verified its full compatibility with materials and assembly processes, including health and safety. This is especially of interest where new types of fluids or other fillers are considered. Some manufacturers have developed their own qualification procedures, specifying test conditions in terms of temperature, duration, safety and final acceptance criteria. This forms part of the development tests.
3.1.3. Prequalification Test Prequalification testing, as in IEC 62067 & 60840, is only specified for cable systems above 150kV or where the conductor screen stress is designed to be greater than 8kV/mm or the insulation screen stress is designed to be greater than 4kV/mm, Prequalification tests are long term tests that are carried to assess the performance of a cable system and attempt to replicate in-service duty. The test arrangement should be representative of installed conditions, e.g. fixed and flexible sections and contain both joints and terminations to give a true replication of the cable system. These tests are intended to verify the thermo-mechanical and electrical behaviour of the cable and accessories. In some local standards it is also a requirement to monitor and record the pressure of any insulating mediums used in order to assess the robustness of any sealing arrangements. After testing, all accessories are to be examined to check for any changes or deterioration that might affect the performance. 3.1.4. Type Test Type tests are carried out on the complete cable system and are required for all voltages and design stresses. These tests provide a minimum requirement to show specific cables and accessories are fit for a specific purpose. Type tests, as specified in IEC 60840 & IEC 62067, focus mainly on the cable system short-term voltage withstand performance. They include AC, over-voltage and lightning transients combined with material aging effects. Following completion of these tests, the cable system must be shown to be partial discharge free or to have a level of discharge below a certain requirement. If any partial discharge is present, even below the level specified, it may be prudent to identify the cause of this discharge. Once tests are completed it is important to disassemble all accessories and closely inspect them for any signs of electrical activity or physical changes, which may not have caused an electrical discharge, but may cause mechanical or operational problems. The interpretation shall be based on the previous experience with development, prequalification and other type tests.
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Guidelines for maintaining the integrity of XLPE cable accessories
Fig 16 Type Test loop of 400kV system
3.1.5 Short Circuit Tests The WG identified that short-circuit behaviour was not addressed by any IEC standard relating to HV cable systems. Several utilities have independently taken the step of specifying an additional type test to check the behaviour of terminations (especially those containing insulating fluid) when they are subjected to short circuits. Two cases need to be considered 1) A low energy external fault. In this case the fault current passes though the conductor. The fault is external to the accessory. 2) A high energy internal fault. In this case the fault is the result of component failure or arcing inside the accessory. Consideration, depending on the design and installation, should be given to whether it is necessary to do one or both of the above tests to cover the worst case condition. These tests are detailed in Appendix 4.
3.1.6. Sample Tests Sample test requirements are outlined in IEC 60840 and 62067. These tests are to be carried out on a specified number of components and complete accessories during a production run. For accessories, where the main insulation cannot be routine tested, IEC 60840 states that a partial discharge and an AC voltage test shall be carried out on a fully assembled accessory. For individual components the characteristics of each component shall be verified in accordance with the specifications of the accessories’ manufacturer, either through test reports from the supplier of a given component or through internal tests. Also the components shall be inspected against their drawings and there shall be no deviation outside the declared tolerances.
3.1.7. Routine Tests Routine tests are carried out on some accessory components to be supplied. These tests should form part of a robust quality control regime and provide confidence in accessories’ quality. As part of these tests, the main insulation of prefabricated accessory designs is required to undergo AC voltage and partial discharge tests. Finally each component should be visually inspected for defects. Insulators filled with oil, gas, or other material should also undergo a pressure test before delivery.
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Guidelines for maintaining the integrity of XLPE cable accessories
3.1.8 Tests on Filling Materials Filling materials, like polybutene or synthetic oil, are selected based on the material parameters and characteristics and they are approved during the development, prequalification and type tests. – specification IEC 60836 covers silicon oil. It is recommended that a ‘finger print’ of the filling material be determined after delivery, as this ‘finger print’ might be useful during condition assessment programs or failure analysis. Well established material ‘finger print’ techniques are
AC electrical strength Dielectric dissipation factor Fourier transform infrared spectroscopy (FTIR) Thermal gravimetric analysis (TGA)
3.1.9 Commissioning Tests Commissioning tests are carried out on the assembled cables, joints terminations, bonding and earthing once the installation is completed. They are the final tests performed on the cable system prior to energising and provide the final check that the system has been correctly designed and installed. The requirements for commissioning tests will vary depending on the type of circuit installed and the consequences of failure. There are very few tests that can be carried out that will prove the long term life of cable, joints and terminations. However, it is recommended that an AC insulation test is carried out with partial discharge monitoring, if possible, of all joints and terminations. Ideally this is carried out using a resonant test voltage generator. This allows the cable system to be energised off-line and at low energy and so there is a minimised risk of a disruptive accessory failure during the test. The tests may give an early warning of potential failure points, before a later breakdown of the complete cable system in service leads to bigger problems. The commissioning tests should be performed according to the relevant IEC standard. It is possible to carry out an AC test by energising the termination with system voltage (soak test) and using on-line partial discharge monitoring. This is not ideal, as noise from the system can mask discharge activity occurring within the accessory. In addition, if a breakdown does occur this will lead to a disruptive failure of the joint or termination (as the full system short circuit current is available to flow through the failed accessory) and may lead to an outage and power disruption. Such a failure presents both a safety risk on site and introduces a significant delay to commissioning of the circuit while the affected components are replaced.
Fig 17 On site Commissioning Test (in this set up three mobile tests sets needed simultaneously, because of cable length) A DC oversheath test should also be carried out to ensure the cable system and its accessories are insulated from earth
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Guidelines for maintaining the integrity of XLPE cable accessories
. Fig 18 Discharge tracks on cable PE outer serving due to a defect. The discharge tracks are a consequence of fault localisation pulses
3.2. Condition Monitoring As indicated in TB420 Generic Guidelines for Life Time Condition Assessment of HV Assets and Related Knowledge Rules, it is recommended that a good database of information is established for each piece of equipment as it ages. Useful information on the aging process during the full service life includes loading, maintenance test results, fault history, general ambient and environmental conditions and details of any site incidents. To effectively manage the aging of HV cable accessories, a structured methodology to analyse and prevent in-service failures is recommended. A suggestion for such methodology is given in Cigré TB420, clause 4.2. The final step in this methodology is to gather the outputs from this process into a management strategy which can be used for: (a) preventative maintenance, (b) decisions on equipment change-out (c) improvement in the specification, design or manufacture of new equipment. Regarding (a) preventative maintenance, there are many possible approaches to monitoring the condition of terminations and non-buried joints. These vary from visual inspection to on-line monitoring or regular testing while out of service, etc. The monitoring to be carried out depends on:i. ii. iii. iv. v. vi. vii. viii. ix. x.
The importance of the circuit The history of the circuit and its accessories The potential repair time The potential cost of the outage Potential cost of the damage Effect on reputation Potential damage from the failure Effectiveness of the monitoring system adopted Availability of monitoring tools and trained personnel Cost of monitoring
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Guidelines for maintaining the integrity of XLPE cable accessories
Fig 19 Example of condition monitoring technique: The X-ray photo of cable outdoor termination used to check any internal displacement of the top-connector A list of current Condition Monitoring Tools is detailed in Appendix 5. To assist in the selection of a monitoring tool, each tool is described under a number of headings including:•
experience - the level of working experience of each condition monitoring tool is categorized as either well established (‘W’) or under development (‘D’).
•
effectiveness - one diagnostic monitoring tool may be considered (based on costs, time and results) as more effective than another in finding damages or degradations that will lead eventually to system failure ; categorized here as useful (‘U’) and less useful (‘L’).
•
level of expertise required - whether high or low level expertise is required i.e. a technician/engineer trained in the particular tool being used or is a general operative sufficient to operate the tool.
•
cost
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Guidelines for maintaining the integrity of XLPE cable accessories
Chapter 4 Recommendations The aim of the WG has been to produce a Technical Brochure that could be used by designers, manufacturers, contractors and utilities to increase the integrity of terminations and non-buried joints. Many approaches to this subject are possible, depending on the factors outlined in Section 3.2 above. Two cases need to be considered:a) where the accessories are on an existing cable circuit b) where the accessories are to be installed on a new cable circuit
4.1 Existing Circuits For existing circuits the following considerations apply:i. ii. iii. iv. v. vi. vii. viii. ix. x.
The importance of the circuit The history of the circuit and its accessories The potential repair time The potential cost of the outage Potential damage from the failure Potential cost of the damage Effect on reputation Effectiveness of the monitoring system adopted Availability of monitoring tools and trained personnel Cost of monitoring
4.2 New Circuits If a new circuit is being installed then it seems appropriate to use proven composite terminations (unfilled, if possible) and proven joints. The designs should comply with IEC 60840 and 62067 as far as PQ and Type testing, Routine and Site Test are concerned. There should be a full QC system in the factory for both cables and accessories. Of course both joints and terminations should be installed fully in accordance with the manufacturer’s instructions, and in accordance with TB 476. When new accessories are being installed a decision will have to be made on what condition monitoring, if any, is necessary. Refer to recommendations of Section 3.2.
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Guidelines for maintaining the integrity of XLPE cable accessories
Chapter 5 Conclusions The following conclusions resulted from the work carried out by this working group: 1. The survey completed by this WG has shown that disruptive discharge has been experienced in terminations and non-buried joints. 2. Utilities are concerned about these discharges.
3. In the case of installing new cable systems, utilities should try to adopt designs that either do not experience disruptive discharge and/or that have been tested to ensure the impact is kept to a minimum. 4. Full quality control procedures should be employed during the manufacture, delivery, storage and the installation process.
5. Jointers should be fully certified, have experience of the accessory to be installed and their work should be checked/monitored/inspected. 6. All materials and jointing tools used should be appropriate for the work, be in good condition, have been correctly stored and be within their expiry dates.
7. The site conditions should be suitable with respect to space, safety, dust, pollution, humidity and temperature. 8. On-site testing at an elevated voltage level, as prescribed in the IEC standards, is strongly recommended during commissioning.
9. A risk analysis should be done to determine the corrective actions required for existing accessories, which have experienced disruptive discharge or it is suspected they may do so in the future. This can vary from leaving the accessory in service to partial or full replacement. Whether it is decided to go for full or partial replacement, steps 3 to 8 above should be followed. 10. If it is decided to do condition monitoring on existing or new circuits, then the following items need to be considered a) The importance of the circuit b) The history of the circuit and its accessories c) The potential repair time
45
Guidelines for maintaining the integrity of XLPE cable accessories d) The potential cost of the outage e) Potential cost of the damage f)
Effect on reputation
g) Potential damage from the failure h) Effectiveness of the monitoring system adopted i)
Availability of monitoring tools and trained personnel
j)
Cost of monitoring
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Guidelines for maintaining the integrity of XLPE cable accessories
Appendix 1 Terms of Reference
Study Committee No: B1 WORKING BODY FORM
Group No : WG B1.29
Name of Convener : Eugene Bergin (Irl)
TITLE of the Working Group : Guidelines for maintaining the integrity of XLPE transmission cable accessories Background: The work is motivated by the occurrence of disruptive failures of cable end terminations, with consequent risks for personal and material loss and damage. Terms of Reference: The scope shall be limited to land XLPE cable systems at 110 kV and above. Priority shall be given to outdoor and oil-immersed terminations, but also joints (that are not directly buried) shall be considered. The work shall concentrate on recent incidents, but near misses shall also be included in the analysis. The WG shall: • • • •
Review recent experience with failures of outdoor and oil-filled terminations Review the consequences of termination failures for cables within substations and outside. Examine the role of design, assembly and quality control in mitigating the effects of termination failures Examine the role of testing (development, type, routine & after-laying) and condition monitoring in minimising the incidence or severity of termination failures At the SC B1 meeting in 2010, the WG shall provide recommendations on possible extensions of work into joints (not directly buried), and accessories for oil-filled cable. The full report shall be made available for final review at the B1 annual meeting in 2011.
• •
Deliverables: • • •
An Executive Summary article for Electra A full report to be published as a Technical Brochure A Tutorial
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Guidelines for maintaining the integrity of XLPE cable accessories
Created: 2008
Duration: 3 years
Convener e-mail:
[email protected]
WG members from: AU, BE, BR, CA, FR, DE, IN, IT, JP, KR, NL, NO, ES, CH, UK, US
Other stakeholding SC’s: B2, B3, C3
Approval by TC Chairman :
Date :
48
2008
Guidelines for maintaining the integrity of XLPE cable accessories
Appendix 2 Bibliography/References IEC Standards
1) IEC 60840 Ed 3 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
2) IEC 62067 Ed 2 Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) – Test methods and requirements
3) IEC 62217 Ed. 1: Polymeric insulators for indoor and outdoor use with a nominal voltage greater than 1 000 V —General definitions, test methods and acceptance criteria.
4) IEC 61462 Ed. 1.0: Composite insulators - Hollow pressurized and unpressurized insulators for use in electrical equipment with rated voltage greater than 1000V - Definitions, test methods, acceptance criteria and design recommendations
5) IEC 62271:High voltage switchgear and control gear – Part 209: Cable connections for gasinsulated metal-enclosed switchgear for rated voltages above 52kV – Fluid-filled and extruded insulation cables – Fluid-filled and dry-type cable-terminations
6) IEC 61039: General Classification of insulating liquids
7) IEC 60815-1 TS Ed. 1.0: Selection and dimensioning of high-voltage insulators for polluted conditions - Part 1: Definitions, information and general principles
8) IEC 60836 Ed 2.0 b 2005 Specification for unused silicon insulating liquids for electrotechnical purposes.
9) IEC 61109 Ed 2 Insulators for overhead lines - Composite suspension and tension insulators for AC. systems with a nominal voltage greater than 1 000 V - Definitions, test methods and acceptance criteria
49
Guidelines for maintaining the integrity of XLPE cable accessories CIGRE
Electra no
Title of Electra Paper
10) 243
Update of Service experience of HV underground and submarine cable systems
11) 235
Statistics on AC underground cables in power networks
12) 210
Current cable practises in Power Utilities (A report on the recent AORC Panel Regional Workshop in Malaysia)
13) 204
General overview on experience feedback methods
14) 141.1
Service experience of cables with laminated protective covering.
15) 137
Survey of the service performance on HV AC cables.
16) 212
Thermal ratings of HV cable accessories
17) 203
Interfaces between HV extruded cables and accessories
TB no
Title of Technical Brochure
18) 502
High Voltage On Site Testing with Partial Discharge Measurement
19) 476
Cable Accessory Workmanship on Extruded High Voltage Cables
20) 455
Aspects for the Application of Composite Insulators
21) 420
Generic Guidelines for Life Time Condition Assessment of HV Assets and Related Knowledge Rules
22) 379
Update of Service experience of HV underground and submarine cable systems
23) 338
Statistics on AC underground cables in power networks
24) 303
Revision of Qualification Procedures for HV and EHV AC Extruded Underground Cable Systems
25) 279 Maintenance of HV Cables and Accessories
26) 211
Preparation of guidelines for collection and handling of reliability data
27) 210
Interfaces between HV extruded cables and accessories
28) 177
Accessories for HV cables with extruded insulation Accessories for HV extruded cable. Types of accessories and terminology
50
Guidelines for maintaining the integrity of XLPE cable accessories
Session Paper No.
Title of Session Paper
29) 21-01
Studies of Impurities and Voids in Cross-linked Polyethylene Insulated Cables. Prefabricated Terminations.
30) 21-02
Plastic insulated cable with voltage dependent core screen.
Jicable
31) Jicable 2011 paper A.3.7 “Return of Experience of 380 kV Power Cable Failures” from Sander MEIJER (TenneT TSO), Johan SMIT, Xiaolin CHEN (Delft University of Technology), Wilfried FISCHER (50 Hertz Transmission GmbH), Luigi COLLA (Terna S.p.A.) 32) Jicable 2011 paper A.5.4 “Remedial action and further quality assuring measures after a failure in a 400 kV GIS cable termination” from Frank JAKOB, Frank KOWALOWSKI, Claus KUHN, Wilfried FISCHER (50 Hertz Transmission GmbH), Sigurdur A. HANSEN (Südkabel GmbH) 33) Jicable 2011 paper A.5.3 “Dry terminations for high voltage cable systems” from Pascal STREIT (NEXANS)
34) Jicable 2003 paper A.6.2 “Anti-explosion protection for HV porcelain and composite terminations” from Gahungu, Cardinaels, Streit, Rollier (Nexans) 35) Jicable 2003 paper A.6.4 “New dry outdoor terminations for HV extruded cables” from DEJEAN (PIRELLI France), QUAGGIA, PARMIGIANI (PIRELLI Italy), GOEHLICH (Technical University of Berlin);.
36) Jicable 1999 paper A.5.4 “Development of synthetic and composite terminations for HV and EHV extruded cables” (LE PURIANS from EDF R&D and JUNG from EDF CNIR – RTE 37) Jicable 1995 paper A.3.2 “Composite EHV terminations for extruded cables” (ARGAUT, LUTON from SILEC and JOULIE, PARRAUD from SEDIVER.
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Guidelines for maintaining the integrity of XLPE cable accessories
Appendix 3
TB 476 Cable Accessory Workmanship on Extruded High Voltage Cables Oct 2011
TABLE OF CONTENTS 1 Summary ............................................................................................
4
2 Introduction......................................................................................................
4
3 Scope ..............................................................................................................
6
3.1 Inclusions................................................................................................... …
6
3.2 Exclusions .....................................................................................................
6
4 Related Literature and Terminology ................................................................ 6
5 General risks and skills..................................................................................... 8
6 Technical risks and required specific skills .................................................. 10 6.1 Conductors ..................................................................................................
10
6.1.1 Conductor preparation ..........................................................................
10
6.1.2 Compression.........................................................................................
11
6.1.3 MIG/TIG Welding ..................................................................................
12
6.1.4 Thermit Weld.........................................................................................
12
6.1.5 Mechanical Connection.........................................................................
13
6.2 Insulation Preparation..............................................................................
15
6.2.1 Straightening.........................................................................................
15
6.2.2 Stripping of insulation screen ................................................................
16
6.2.3 Preparing the end of the insulation screen............................................
18
6.2.4 Smoothening the insulation surface ......................................................
19
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Guidelines for maintaining the integrity of XLPE cable accessories 6.2.5 Cleaning of insulation............................................................................
20
6.2.6 Shrinkage..............................................................................................
21
6.2.7 Lubrication ............................................................................................
21
6.3 Metallic sheath.........................................................................................
22
6.3.1 Welded Aluminium Sheath (WAS) ........................................................
22
6.3.2 Corrugated Sheaths: Aluminium (CAS); Copper (CCS); Stainless Steel (CSS).............................................................................................................
25
6.3.3 Lead Sheath..........................................................................................
28
6.3.4 Laminated sheaths: Aluminium Polyethylene Laminate (APL); Copper Polyethylene Laminate (CPL)........................................................................
30
6.4 Oversheath..............................................................................................
32
6.4.1 Case of graphite coating .......................................................................
32
6.4.2 Case of extruded and bonded semi-conducting layer ...........................
32
6.4.3 Low Smoke, Zero Halogen, Enhanced Flame Performance Sheaths ...
32
6.5 Installation of joint electric field control components................................
33
6.5.1 Slip on prefabricated joint......................................................................
34
6.5.2 Expansion joints....................................................................................
37
6.5.3 Field Taped Joints.................................................................................
40
6.5.4 Field Molded Joints (Extruded or taped) .............................................
41
6.5.5 Heatshrink sleeve joint ..........................................................................
41
6.5.6 Prefabricated composite type joint ........................................................
42
6.5.7 Plug-in joint ...........................................................................................
43
6.5.8 Pre-molded three piece joint ...............................................................
44
6.6 Installation of termination electric field control components.....................
45
6.6.1 Slip-on prefabricated field control components .....................................
45
6.6.2 Plug-in terminations ..............................................................................
45
6.6.3 Taped Terminations ..............................................................................
47
6.6.4 Heatshrink sleeve insulated terminations..............................................
48
6.6.5 Prefabricated composite dry terminations.............................................
48
6.7 Outer Protection of Joints ........................................................................
49
6.7.1 Polymeric outer protection by taping and/or heatshrink tubes...............
49
6.7.2 Outer Protection Assembly ...................................................................
50
53
Guidelines for maintaining the integrity of XLPE cable accessories 6.7.3 Filling compounds for joint protections (joint boxes) .............................
51
6.8 Filling of Terminations..............................................................................
52
6.9 Handling of Accessories ..........................................................................
53
6.9.1 Supporting of accessory........................................................................
53
6.9.2 Lifting of accessories.............................................................................
54
6.9.3 Special bonding configurations and link box installation .......................
56
6.9.4 Sensor connections...............................................................................
56
6.9.5 Fibre optics ...........................................................................................
57
7 Skills Assessment..........................................................................................
58
7.1 Aspects to be tested ................................................................................
58
7.2 Methods of qualification...........................................................................
58
7.2.1 Theoretical ............................................................................................
58
7.2.2 Training on the job and observation......................................................
58
7.2.3 Testing – Electrical & Mechanical .........................................................
59
7.3 Certification..............................................................................................
59
7.4 Duration of certification............................................................................
60
7.5 Upskilling .................................................................................................
60
7.6 New Accessory type ................................................................................
60
8 Set Up ...........................................................................................................
61
8.1 Organisation of jointing location...............................................................
61
8.2 Positioning of Joint ..................................................................................
61
8.3 Environmental Conditions........................................................................
61
8.4 Cable End Inspection ..............................................................................
61
8.5 Verification of Each Step .........................................................................
62
8.6 Measuring of Diameters, Ovality, Concentricity, Position ........................
62
8.7 Safety and Health ....................................................................................
62
8.8 Environmental Aspects...........................................................................
62
8.9 Quality Insurance.....................................................................................
62
9 Bibliography...................................................................................................
54
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Guidelines for maintaining the integrity of XLPE cable accessories
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Guidelines for maintaining the integrity of XLPE cable accessories
Appendix 4 Short Circuit Tests The possibility of two types of fault has to be considered - a fault external to the accessory and a fault inside the accessory. Both faults will have very different impacts on the accessory. The external fault may cause rapid heating of the conductor and result in a build up of pressure, if there is fluid or gas present in the accessory. The internal fault results in fault current flowing through the insulation of the accessory with high energy being dissipated in the insulation and filling medium and this may cause large thermomechanical and pressure changes inside the accessory. 1) Low energy external fault (through-fault i.e. breakdown outside the accessory) In the case of a system fault in another part of the electrical system external to the accessory, the fault current passes through the conductor of the termination or joint. Testing for such a case is carried out on terminations, joints (buried and non-buried) installed as in service. The test installation shall be in accordance the requirements of the specification and rules of each System Operator. This test should be performed on terminations and joints connected by the specified cables, which have either already gone through a type test or have gone through at least ten thermal cycles. The aim is to study the effects of a simulated external fault on the accessories, including a check that pressure relief devices in terminations do not break during an external short-circuit. Simulation of the fault The accessory shall be installed in a suitable circuit to permit the fault current to flow through the accessory. Position of the Fault The fault shall be external to the accessory being tested External Fault Withstand Test The test is performed with AC. In order to prevent fade-out of the electrical arcing, the test will be performed with a symmetrical start-up on a voltage crest. The current is injected from the cable to the accessory. The test voltage shall be at least 20 kV. Examples are given in the table 1 and each country will have its own set of values depending on system configurations and fault conditions. FRANCE Voltage
Short-Circuit Parameters
kV U (U m)
Three-phase Short-circuit Intensity and duration
Single-Phase Short-circuit Intensity, duration,
63 (72,5)
a) b)
20 kA - 1 s 31,5 kA - 0,5 s
8 kA - 1,7 s – 0,2 s
90 (100)
a) b)
20 kA - 1 s 31,5 kA - 0,5 s
10,3 kA - 1,7 s – 0,2 s
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Guidelines for maintaining the integrity of XLPE cable accessories 225 (245) 400 (420)
a) b)
31,5 kA - 0,5 s
31,5 kA - 0,5 s - 0,16 s
63 kA - 0,5 s 40 kA - 0,5 s
a) 63 kA - 0,5 s – 0,07 s b) 40 kA - 0,5 s – 0,06 s
NOTE - Cases a) and b) depend on the grid characteristics and short-circuit power of the grid.
IRELAND Voltage
Short-Circuit Parameters
kV U (U m)
Three-phase Short-circuit Intensity and duration
110 (123)
a) b)
31.5 kA - 1.0 s 40.0 kA – 1.0s
Single-Phase Short-circuit Intensity, duration, a) b)
31.5 kA - 1.0 s 40.0 kA – 1.0s
220 (245)
40 kA – 1.0 s
40 kA – 1.0 s
400 (420)
50kA – 1.0 s
50kA – 1.0 s
NOTE - Cases
a) outside Dublin
b) in Dublin
NETHERLANDS
Voltage kV U (Um) 50 (72.5)
110 (123)
150 (170)
Short circuit parameters Three-phase Short –circuit Intensity and duration
Single-phase Short-Circuit Intensity, duration
9 kA – 0.5 sec
9 kA – 0.5 sec
15 kA – 1.0 sec
12.5 kA – 1.0 sec
25 kA – 1.0 sec
15 kA – 1.0 sec
30 kA – 0.5 sec
25 kA – 0.5 sec
40 kA – 1.0 sec
25 kA – 1.0 sec
30 kA – 0.5 sec
15 kA – 0.5 sec
40 kA – 1.0 sec
30 kA – 1.0 sec
50 kA – 1.0 sec
40 kA – 1. 0 sec
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Guidelines for maintaining the integrity of XLPE cable accessories 220 (245)
40 kA – 1.0 sec
27 kA – 1.0 sec
380 (420)
50 kA – 0.5 sec
50 kA – 0.5 sec
50 kA – 1.0 sec
50 kA – 1.0 sec
63 kA – 0.5 sec
63 kA – 1.0 sec
63 kA – 1.0 sec The short-circuit levels are depending on the protection settings, imposed by the grid owner, and the position of the cable system in the grid: close to a power plant or more remote.
Table 1 Short Circuit Levels at Different Operating Voltages Due to safety regulations, testing terminations that contain SF6 gas is no longer allowed, as some gas byproducts that may be generated by internal arcing are harmful. Replacing SF6 with air (or nitrogen) has to be carefully considered, since there are a lot of differences between arcs in SF6 and air. WG A3.20 is currently carrying out studies on this question. Requirements On completion of the test, the pressure relief shall be observed to have operated correctly. The whole test shall be recorded and filmed with a high-speed camera (at least 1000 images per second) in order to witness and analyse the behaviour and reaction of the termination and fixing installation devices.
2)
High energy internal fault (internal fault i.e. breakdown inside the accessory)
The test is carried out on a termination or joint installed as in service. The installation shall be in accordance with the requirements of the specification and rules of each System Operator. The aim is to study the external effects generated by the accessory during the simulation of an internal arc fault. This test is intended to check that the accessory does not disruptively eject components that might cause external damage.
Simulation of the fault An internal fault is initiated by drilling a hole in the main insulation of the cable within the termination or joint. A 1.5 mm² copper wire shall connect the conductor to the metallic screen/sheath or to a metallic piece itself connected to the screen/sheath. Position of the Fault In the case of a termination or joint having a stress cone, the fault is initiated by drilling a hole at the top of the stress cone to the conductor in order to connect the 1.5mm² copper wire. Internal Fault Withstand Test The test is performed with AC. In order to prevent from the fade-out of the electrical arcing, the test will be performed with a symmetrical start-up on a voltage crest. The rms. value and duration of the phase-to-earth short-circuit are given in the table above .The current is injected from the cable to the termination or joint. The test voltage shall at least 20 kV. Due to safety regulations, testing accessories which contain SF6 is not allowed any more, as some by-products that may be generated in case of arcing are considered harmful.
58
Guidelines for maintaining the integrity of XLPE cable accessories Replacing SF6 by air (or nitrogen) has to be considered carefully, since there are a lot of differences between arcs in SF6 and air. Requirements – On completion of the test, no solid debris shall be observed at a distance of more than 3 metres from the termination or joint. The whole test shall be recorded and filmed with a high-speed camera (at least 1000 images per second) in order to witness and analyse the behaviour and reaction of the termination or joint and fixing installation devices.
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Guidelines for maintaining the integrity of XLPE cable accessories
Appendix 5 Condition Monitoring Techniques for Terminations and Non-buried Joints
Condition monitoring techniques for terminations and associated auxiliary components are summarized in the tables in this Appendix under the following headings: Condition monitoring tool
details the specific diagnostic tool for monitoring the termination/auxiliary components
Component
Identifies the component to which the monitoring tool can be used on.
Event/Cause detected
application of the ‘condition monitoring tool’ reduce the probability of the here mentioned event that cause the cable system failure, damage or degradation
On/off-line
Condition monitoring techniques are categorized as capable of being done either on-line (cable system in service) or off-line (cable system must be switched out)
Experience
The level of working experience of each condition monitoring tool is categorized as either well established (‘W’) or under development (‘D’).
Effectiveness
One diagnostic monitoring tool is considered as more effective in finding damages or degradations that will lead eventually to system failure than other tools, considering costs and time versus result, categorized here as useful (‘U’) and less useful (‘L’).
Frequency
Suggested interval of application of the monitoring tool versus the cable life time cycle. Please note that most monitoring tools will be selected based on the service experience of the termination type and hence the frequency.
Primary / secondary test
Primary tests are considered as the minimal test one shall perform on a cable system after it has been put into service. Secondary tests will be selected to monitor or discriminate terminations with (suspected) specific defects, based on (service) experience.
Cost
Indication of cost per test.Range: minor costs < +, ++, +++ > considerable costs. These costs are for the test only cost and do not include cost of preparatory work, outages and other associated expenses.
Expertise
Indication of required skills to perform the test. Range: less skilled personal < +, ++, +++ > skilled personal
Reference.
reference source of the monitoring technique
60
Guidelines for maintaining the integrity of XLPE cable accessories
Condition monitoring techniques
No.
Condition Monitoring Tool
Component
Event or Cause Detected
On/OffLine
Experience
Effectiveness
Cost
Expertise
Reference.
Annual
P
+
+
CIGRÉ TB 279, table 6.4, item 2
1
Serving test (DC test)
Terminations, non buried joints, cable
Pollution on support insulators or screen separations
Off-line
W
2
Dielectric loss angle test 1
Terminations, non buried joints
Ingress of water in insulation area
Off-line
D
L
Depending on service experience
S
+++
+++
CIGRÉ TB 279, table 6.4, item 3
3
PD testing
Terminations, non buried joints
Detecting assembly errors, low contact pressure at interface, shrink back of cable insulation, contamination of internal insulation fluid and/or gas due to aging or leaking, insulator tracking,
Off-line
W
U
Depending on service experience
P
++ (on-line)
+++
CIGRÉ TB 279, table 6.4, item 5
Contamination of internal insulating fluid.
Off-line
+++
CIGRÉ TB 279, table 6.4, item 6
(various methods, such as: acoustic, UHF, Radio interference, voltage test) 4
1
Chemical and Physical analysis of insulating fluid, such as: DGA, Tan delta, Water content, Particles etc.
Terminations, fluid filled non buried joints
U
Primary / Secondary Test
Frequency
/ On-line
D
+++ (off-line)
W
U
Annual
P
+ (N.B. does not include the costs of taking the sample)
Distinction between cable and termination might be a problem
61
Guidelines for maintaining the integrity of XLPE cable accessories No.
Condition Monitoring Tool
Component
Event or Cause Detected
On/OffLine
Experience
Effectiveness
Frequency
Primary / Secondary Test
Cost
Expertise
Reference.
5
X-ray
Terminations, non buried joints
Movement of cable due to thermal cycling or poor clamping
Off-line
W
L
Depending on service experience
S
++
++
CIGRÉ TB 279, table 6.4, item 7
6
Visual inspection with visible or UV light
Terminations, non buried joints
Surface pollution, mechanical damage, uncontrolled movement of cable, cable clamping, tracking marks on outdoor insulators, ferrule retraction, leakages, corrosion, animal attack, vandalism.
On-line
W
U
Annual
P
+
++
CIGRÉ TB 279, table 6.4, item 8
7
Visual inspection with IR on current carrying components
Terminations, non buried joints
As with Item 6 above plus detecting possible hotspots on top-connector and earthing circuit
On-line
W
U
5 yearly
S
+
++
CIGRÉ TB 279, table 6.4, item 8
8
Leakage current measurement
Terminations
Insulator surface pollution, surface tracking or damage
On-line
W
L
Depending on service experience
S
+++
++
CIGRÉ TB 279, table 5.4, item 25
9
Test at elevated voltage: AC, VLF, DC with or without DLA (Item 2) and/or PD (Item 3).
Terminations, non buried joints
Main insulation / stress cone/ interface defects
Off-line
W
U
depending on service experience
P
+++
+++
IEEE St 48, clause 8.6 (DC), IEC 60840, IEC62067
62
Guidelines for maintaining the integrity of XLPE cable accessories No.
Condition Monitoring Tool
Component
Event or Cause Detected
On/OffLine
Experience
Effectiveness
Frequency
Primary / Secondary Test
Cost
Expertise
Reference.
10
Surface wetting characteristics (STRI method)2
Terminations
Extrinsic surface pollution on outdoor polymeric insulators
Off-line
W
L
Depending on location
S
+
++
CIGRÉ TB 279, table 5.4, item 26
11
Continuous measurement of fluid or gas pressures and/or low pressure alarms
Auxiliary
Indication of falling fluid / gas pressure
On-line
W
U
Continuous
S
+
+
CIGRÉ TB 279, table 5.4, item 23
12
Regular gauge maintenance and calibration.
Auxiliary
Leakage of internal insulating fluid / gas from termination
On-line
W
L
Annual
S
+
+
CIGRÉ TB 279, table 5.4, item 23
13
SF6 sniffers or cameras
Auxiliary
Leakage of internal insulating fluid/gas from termination
On-line
W
L
Annual
S
++
+
CIGRÉ TB 279, table 5.4, item 23
14
Testing of fluid/gas monitoring equipment
Auxiliary
Testing alarm settings and signals for fluid/gas pressure monitoring
Off-line
W
L
Annual
P
+
++
CIGRÉ TB 279, table 5.4, item 31
15
Visual inspection
Earthing and cross bonding boxes.
Water ingress in link box, Condition of any insulating compounds, Link arrangement
Off-line
W
L
P
+
+
CIGRÉ TB 279, table 5.4, item 32
2
Annual
STRI hydrophobicity classification guide provides a coarse value of the wetting status, reference is made here to IEC TS 62073
63
Guidelines for maintaining the integrity of XLPE cable accessories No.
Condition Monitoring Tool
Component
Event or Cause Detected
On/OffLine
Experience
Effectiveness
Frequency
Primary / Secondary Test
Cost
Expertise
Reference. CIGRÉ TB 279, table 6.4, item 10
16
Voltage test on SVL
Earthing and cross bonding boxes.
Failure of SVL to operate at rated voltage
Off-line
W
L
Depending on service experience (usually done on same outage as serving test)
S
+
+
17
Continuous SVL monitoring
Earthing and cross bonding boxes.
Failure of SVL to operate at rated voltage
On-line
D
L
Continuous
S
+++
+++
-
18
Measurement of earthing system electrical resistance
Auxiliary
Integrity of earthing circuit
Off-line
W
L
As per 16
P
+
+
-
19
Optical fibre
Cable and joint
Excessive temperature rise
Off or on line
W
U
Continuous
S
++
+++
64
TB 247
