560
Guidelines for Maintaining the Integrity of
XLPE Cable Accessories
Working Group
B1.29
1
Guidelines for Maintaining
the Integrity of XLPE
Cable Accessories
2
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”.
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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 21
5
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 W elding 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 346
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 12 Table 2 Failure rates of terminations over the period 2000 to 2005 12
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Table 3 Failure rates by type of termination over the period 2000 to 2005 13 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, 17 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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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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“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 pre-molded 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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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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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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VOLTAGE RANGE YEAR OF INSTALL ATION kV Outdoor Terminati on - Fluid filled - Porcelain Outdoor Terminati on - Fluid filled - Composit e insulator Outdoor Terminati on - Dry - Porcelain Outdoor Terminati on - Dry - Composit e insulator GIS or Transfor mer Terminati on - Fluid filled GIS or Transfor mer Terminati on - Dry 60 to 109 2001 531 27 12 75 0 311 2002 753 15 27 69 6 296 2003 513 21 15 96 5 225 2004 483 24 24 186 2 190 2005 600 21 51 138 3 225 110 to 219 2001 267 131 159 32 116 394 2002 282 128 216 35 77 565 2003 546 163 51 83 130 447 2004 226 190 63 32 98 366 2005 187 285 162 41 106 389 220 to 314 2001 135 0 0 0 54 135 2002 63 0 0 0 30 12 2003 102 6 0 0 0 42 2004 66 9 0 0 3 27 2005 60 3 0 12 3 42 315 to 500 2001 12 0 0 0 0 0 2002 0 0 0 0 0 0 2003 0 0 0 0 0 12 2004 0 0 0 36 0 0 2005 28 12 0 0 12 0 > 500 2001 0 0 0 0 0 0 2002 0 0 0 0 0 0 2003 0 0 0 0 0 0 2004 0 0 0 0 0 0 2005 0 0 0 0 0 0 ac Accessories installed 2000 to 2005 AC ACCESSORIESExtruded cables (EPR, PE or XLPE)
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):
60-219kV 220-500kV ALL VOLTAGES Failure rate [fail./yr 100 comp.] 60-219kV 220-500kV ALL VOLTAGES Failure rate [fail./yr 100 comp.] 60-219kV 220-500kV ALL VOLTAGES Failure rate [fail./yr 100 comp.] Termination 0,006 0,032 0,007 0,005 0,018 0,006
FAILURE RATES BASED ON ALL REPLIES
XLPE CABLES (AC)
A. Failure Rate - Internal Origin Failures
B. Failure Rate - External Origin Failures
C. Failure Rate - All Failures
Termination
Termination 0,011 0,050 0,013
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Internal External Unknown Outdoor Termination - Fluid filled - Porcelain 46226 15 0,007 0,003 0,003 0,001
Outdoor Termination - Fluid filled - Composite insulator 2619 2 0,019 0,019 0,000 0,000
Outdoor Termination - Dry - Porcelain 1954 2 0,024 0,024 0,000 0,000
Outdoor Termination - Dry - Composite insulator 1353 1 0,020 0,000 0,020 0,000
Outdoor Termination - Type not specified 0 17
Outdoor Terminations - Total 52152 37 0,015 0,007 0,006 0,002
GIS or Transformer Termination - Fluid filled 4222 0 0,000 0,000 0,000 0,000
GIS or Transformer Termination - Dry 20771 19 0,019 0,015 0,002 0,002
Outdoor Termination - Fluid filled - Porcelain 1493 5 0,075 0,030 0,045 0,000
Outdoor Termination - Fluid filled - Composite insulator 61 0 0,000 0,000 0,000 0,000
Outdoor Termination - Dry - Porcelain 0 0 0,000 0,000 0,000 0,000
Outdoor Termination - Dry - Composite insulator 53 0 0,000 0,000 0,000 0,000
Outdoor Termination - Type not specified 0 18
Outdoor Terminations - Total 1607 23 0,330 0,215 0,086 0,029
GIS or Transformer Termination - Fluid filled 2447 2 0,016 0,016 0,000 0,000
GIS or Transformer Termination - Dry 637 2 0,071 0,071 0,000 0,000
220 to 500 Extruded (XLPE, PE or EPR) 60 to 219 Extruded (XLPE, PE or EPR) Total number of faults Failure rates Total failure rate Cause of failure Voltage range
kV Cable type Accessory tyoe
Total number of accessories in
2005
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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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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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
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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 DesignMoisture ingress due to inadequate sealing. Pre-molded component breakdown.
Breakdown of insulating material.
2)
ManufacturePoor 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)
WorkmanshipDamage to primary insulation during jointing. Poor fluid treatment prior to filling.
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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)
OverloadNo cases reported in the returned survey results.
5)
OvervoltageFour cases due to switching/lightning surge.
6)
AnimalsNo cases reported in the returned survey results.
7)
Weather EffectsNo cases reported in the returned survey results.
8)
Cable Insulation InadequaciesTwo cases, no details supplied.
9)
Bonding ProblemsThermal 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 ProblemsPartial 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 /Sabotage18
12)
OthersFailure 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 DesignMethod for earthing of sheath improved Change in specifications for pre-molded parts
2)
New TestsNo new tests were specified in the returned surveys.
3)
New Installation SpecificationImproved 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 ManagementOn-Line PD tests introduced.
Exclusion zones set up around termination, including screening walls.
5)
Repair/Corrective Actioni. Changed whole joint/ termination.
ii. Changed stress cone only. All faults required some form of repair or corrective action to be taken.
6)
Preventative ActionIn 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.
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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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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
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Chapter 2 The Role of Improved Materials, Design, Assembly and
Quality Control in Mitigating the Effects of Termination and
Non-buried 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.
Degradation of components in stress cone.
Change components or design Use appropriate materials and enhance the interface design Consider extended Prequalification Tests.
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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. 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.
Follow Appendix 3
Consider use of inspection test plans (ITP’s)
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Cause Mitigation
Void generation at cable/stress cone interface by conductor centering error, when conductor sleeves were compressed
Wrong insert position
Follow Appendix 3
Consider use of inspection test plans (ITP’s)
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 Carry out checks during commissioning 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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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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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.
Water penetration, metallic contaminants, wrong inset position.
Follow Appendix 3
Consider use of inspection test plans (ITP’s)
Poor adhesion of stress cone Follow Appendix3
Consider use of inspection test plans (ITP’s)
Metallic contaminants in the insulation tape. Void generation with poor tape shaping. Contaminants.
External damage by jointing tool, when connection box was assembled.
Follow Appendix3
Consider use of inspection test plans (ITP’s)
Fibrous contaminant in extruded insulation.
Clamping of screen wires caused damage of outer semi-conducting layer
Follow Appendix3
Consider use of inspection test plans (ITP’s)
Loose flakes of applied semiconducting coatings in joint assembly.
Follow Appendix 3
Ensure proper procedures followed, adequate drying time and care in positioning of the joint body.
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2.1.2.4 Overvoltage
Cause Mitigation
In only one case was joint damage attributed to possible lightning strike
Ensure appropriate lightning protection is used.
2.1.1.5 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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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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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
Porcelain Insulators
Composite Insulators
Environmental Can shatter
Periodic cleaning required Poor pollution performance It’s earthquake performance is not so good
Impermeable to animal attack even when unenergised
Safe/ Inert
Limited cleaning required High performance in polluted areas
Good earthquake performance Possible attack by animals during storage and while unenergised
Chemical Not hydrophobic
Compatibility with SF6
by-products and oil
Hydrophobic
Compatibility of filling material to be checked
Mechanical Can shatter under fault conditions
High weight
Vulnerable to vandalism
No moisture ingress through the insulator from outside.1
1 Note for both types of insulators there may still be some moisture ingress through the top and bottom metal components or gaskets
Will not shatter but may split Low weight
Less susceptible to vandalism Possible moisture ingress through the insulator from outside.1
Rating Performance No practical temperature limit (temperature limits exceed those of other components)
Temperature limits of -55 to +110 oC
Other Properties Lot of experience, but relatively long manufacturing time Because of its weight it’s not so
Limited service experience Because of its weight its relatively
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Element
Porcelain Insulators
Composite Insulators
easy to handle and install. Heavy manual handling or mechanical assistance required
Can be damaged (cracked or chipped) by handling and installation. Small damage can be repaired in-situ.
easy to handle and install
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.
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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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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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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.
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2.2.4.3 Soldered or Brazed Connector
Soldered connectors are used with small conductor cross sections (below 630mm2) 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
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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
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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.
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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.
