HV Cable Terminations – A Report Into 11kV Heat Shrink Termination Failure
Published 02 Jan 2018
Contributed By: Watkins and Jacomb Construction Power & Consultancy
Uploaded By: Chris Dodds – Thorne & Derrick: Distributors of LV, MV & HV Cable Jointing, Cable Terminations, Substation & Electrical Eqpt
WJCPC are specialist LV HV Cable Termination & Jointing contractors with over 20 years experience in the High Voltage Electrical Industry covering the utilities, renewable energy, rail, data centre and general industry sectors – UKPN competent for LV & HV Termination and Jointing Of Cables, Confined Spaces, Substation Entry and Cable Fault Finding.
In the following post Ben Jacomb highlights 5 serious shortcomings across cable terminations installations on the same site – this installation investigation underlines the scope for jointer error at various key stages including critical cable preparation mistakes prior to installing the HV cable terminations.
Clearly under-trained, unskilled labour has been used to perform work normally carried out by highly-skilled, experienced and certified cable jointers.
- Steel Wire Armour – poor termination of the 11kV wire amours (SWA)
- Semicon Screen – failure to observe correct measurement and use correct jointing tools
- XLPE Insulation – undue care and attention by jointer when working on 11kV insulation
- Disregard Of Cable Termination Instruction – failed to apply the void filler correctly
- Cable Stripping – insufficient cable insulation on 11kV cable due to overstripping
“On behalf of one of our clients we were asked to complete several heat shrink terminations onto high voltage 11kV 3 core XLPE SWA power cables. After the first inspection of the HV cable terminations I was not satisfied with the cable jointers technique undertaken to remove the semi conductive layer, which had been done by using a semicon cable stripping tool. The high voltage semi conductive layer is semi bonded type so the stripping of the layer should be stripped by using a depth knife and the ringing of the stress control point on the 11kV cable by a soft file.
Any marks left in the XLPE insulation of the 11kV cable will influence the magnetic fields inside the power cable and should be removed by emery cloth to leave a smooth finish eliminating any voids. If the heat shrink tube is shrunk on top of a void there will be moisture and where there is moisture there will be arcing, burning and then potential failure of the 11kV cable. If the semi conductive layer is not removed correctly at the crucial point then discharges arise causing damage to the cable.
♦ Further Reading: The Semicon Screen – A Most Critical Point In Any MV Cable Joint, Termination Or Connector.
Below are pictures and comments on several findings from recent 11kV cable termination inspections.
Steel Wire Armour
When terminating 11kV cables with heat shrink cable terminations there is a requirement to ensure safe earthing and termination of the armour wires (SWA steel wire armour) using cable glands, commonly referred to as “top-hat” cable glands.
In most 11kV cable installations the cable gland body can be installed after completion of the 11kV terminations. Completion of the final 11kV connections should be done before bolting the cable gland into place on the 11kV cable box.
Note, the correct jointing procedure for termination of the wire armour is to open-up or splay the SWA armours and bend back over onto the cable gland body and using worm drive clamps to connect the armour and screen earth braids to the gland body.
Before applying heat with a gas torch to the heat shrink tube, which is used to shroud and provide environmental and corrosion protection to HV cable terminations, the jointer can abrade the cable sheath and then using cable cleaning tissues thoroughly degrease the 11kV cable.
Insulated 11kV cable glands are also available for industrial medium/high voltage power systems with steel wire (SWA) or aluminium wire (AWA) cables.
The semi conductive layer cut back should be 20mm and not over according to the HV cable terminations instruction – on this cable installation the dimensional error was consistent on all 5 cable termination breakdowns with semicon screen layers averaging between 25mm – 40mm.
Here, the semi conductive layer has been left on the 11kV XLPE insulation which can cause surface tracking and eventual flash over. This occurred on 2 out 5 cable termination breakdowns.
Disregard Of Cable Termination Instruction
Here, the yellow mastic void filler has not been applied correctly by the cable jointer. It clearly states in the heat shrink termination kit instructions that 5mm of yellow mastic is to be installed on the copper tape screen and to continue over the semicon layer and then finish 10mm onto the cable installation. The yellow mastic is used to fill voids – the mastic provides stress relief from electrical fields so it is essential this is installed correctly.
Score marks on insulation – this will more then certainly cause flash overs as previously mentioned in this report.
The image above shows the XLPE cable insulation cut for a cable lug on one cable termination – the required insulation measurement is 5mm.
On all the cables that have been stripped using a stripping tool the cable jointer has left less then the minimum requirement of insulation needed around the conductor of the 11kV cable. The cable manufacturer has confirmed that the finished XLPE insulation around a conductor should be 3.4mm and any part of the insulation under the minimum of 2.95mm is a failure.
After breaking down 5 cable terminations to find 5 different failures on each one this is very concerning. I would not energise any of the 11kV cable terminations until further investigation.”
HV Cable Terminations
Thorne & Derrick distribute the most extensive range of HV Cable Terminations & Joints to suit 11kV/33kV medium and high voltage power cables, including indoor cold shrink and heat shrink terminations, outdoor pole-mounted terminations or separable connectors for gas insulated equipment applications.
THORNE & DERRICK are national distributors of Cable Installation, Jointing, Substation & Electrical Safety Equipment MV HV – we service UK and global businesses involved in cable installations, cable jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.
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