Case 2 - High Voltage Cable Joint Failure On 11kV Trifurcating Transition Joint

By Chris Dodds on 14th August, 2013

HV Heat Shrink Cable Joints & Cable Terminations. Fault Analysis.

T&D have an obligation to investigate any cable joint or cable termination product failure, but over the years we have been asked more and more to get involved in general cable failure analysis for high voltage HV networks.

This Blog is Part 2 in a series of 5 covering Cable Fault Analysis linked to the cable jointing and terminating of high voltage power cables.

T&D undertake to make an un-biased investigation into any failure involving high voltage cable joints and cable terminations regardless of manufacturers or whether heat shrink or cold shrink type.

Case 1 - An 11kV 3 Core Transition Cable Joint Which Was Audibly Discharging

Case 3 - An 11kV Heat Shrink Transition Cable Joint Failure

Case 4 - Problems Of Cable Joint Earth Continuity On Low Voltage Feeder Cable

Case 5 - "Buzzing" 11kV Cable Box Terminations On XLPE 3 Core Cables

Case 2

We received a report of a high voltage cable joint failure on an 11kV trifurcating transition joint (3 core pilc to 3 x single core xlpe cables). The cable joint was cut open along its length where some initial investigation had taken place. The external heat shrink tubing shows no signs of any piercing damage which may have been caused by bad backfill materials during re-instatement of the cable trench following jointing

Cable joint shows signs of moisture ingress due to what appears to be a bad seal on the PILC end. Copper wire screens show signs of oxidisation due to moisture ingress.

Seal on the PILC cable end could be better, P.V.C. tape applied round the dirty outer serving of the cable mastics then applied on top of this, no attempt to clean cable. We would expect to see cable cleaned down to the hessian or armours to enable mastic to stick and provide a good moisture seal onto the high voltage cable.

A cage has been added to the cable joint for mechanical protection - the connection of the cage would not be satisfactory enough for the passage of fault current. The cage extends past the earthing connection point on the 11kV PILC cable end limiting the outer heat shrink tubes ability to provide a good seal onto the lead and earthing connection point.

Initial Thoughts on 11kV Cable Joint Failure

The 11kV joint has failed in the crutch area of the PILC side. Initial indications shows that grey mastic has been applied and shows no sign of moisture ingress through the earth braids or breakout itself. Further comment to be made later upon breakdown of this area.

Removal Of External Heat Shrinks & Cage

As you can see from the photograph, the cage has restricted the recovery of the heat shrink tube onto areas were we would want it to obtain a good seal onto the cable joint.

On the 11kV XLPE single core end a non-adhesive lined breakout has been used, although the jointer has tried to add extra moisture blocking by adding additional Raychem WCSM tubes also the breakout is not under the outer shrink as it should be. This may be because the cable jointer has used a semi conductive breakout and is trying to avoid earth contact in some way.

Removal of heat shrink over the sheath cut on the XLPE single core end shows the cage tightened down onto nothing therefore it is very loose, the whole area is covered in P.V.C. tape removing an adhesive area of nearly 150mm. No abrasion on the 11kV single core outer jackets. Some black mastic has been applied around the high voltage power cables but none in the voids between cores.

Braids

Braiding attached at the 11kV PILC cable end with a wire binder. We would expect to see these tied off - binder is introducing a piercing risk into the cable joint. Braids could have been taken in an open weave up to the earth connection on the PILC end.

Braid Removed

Most heat shrink tubes appear to be shrunk correctly except the stress controls on all phases and the dual wall heat shrink on yellow phase. Incorrect selection of stress control single core diameter is 22mm and this tubing will recover to 25mm.

Tube Removal


Red Phase

Dual wall heat shrink tube - tube shrunk OK
Insulation heat shrink tube - tube shrunk OK
Stress control heat shrink tube - incorrect size for 11kV polymeric single core

Yellow Phase

Dual wall heat shrink tube - not enough heat applied all the way round not fully recovered
Insulation heat shrink tube - shrunk OK
Stress control heat shrink - incorrect size for 11kV polymeric single core

Blue Phase

Dual wall heat shrink tube - shrunk OK
Insulation heat shrink tube - shrunk OK
Stress control heat shrink tube - incorrect size for 11kV polymeric single core

Burning can be seen on all three phases, extra heat has been applied to get the heat shrink shrink to recover more than it’s capable of.

11kV Polymeric Screen Removal

Red Phase - Screen removal not bad, finish quite ribbed but acceptable, jointers tool set quite deep a lot of insulation removed.
Yellow Phase - Screen removal OK
Blue Phase - Screen removal OK

Stress tape applied at the polymeric screen cut and measurements adhered to. Taping over the 11kV connectors is good although has encroached onto the insulation by more than the 5mm suggested, would not be detrimental to the lifespan of the high voltage cable joint.

PILC Crutch Preparation

Sufficient grey mastic appears to have been applied under the breakout and there is evidence of a crutch wedge being installed. Belting papers have been terminated at the same distance as the insulation papers, 28mm from lead cut. Lead removal has left a jagged edge but has been “belled”.

Stress mastic only taken 20mm onto the lead sheath. Burning of braid, roll spring and lead sheath due to passage of fault current. Roll spring not tight enough and no PVC tape applied to keep it under tension also no copper mesh has been applied under the roll spring. We would have expected to see outer serving removed further back and the lead cleaned up for a longer length also jute removed from lead sheath.

This all aids in the adhesion of the outer shrink and obtaining a good moisture seal.

PILC Crutch

Fault has occurred between red and blue phase in the crutch at the 11kV PILC end upon removal of the conductive tube on yellow phase it is apparent that the oil barrier tube is 15 mm up from the insulation papers. It should be as tight to the papers as possible to protect them from damage when they are spread for breakout fitting and jointing.

Oil barrier tubes on red and blue phase are 20mm up from the insulation papers.

Conclusion

The cause of this particular 11kV cable fault is damage to the papers on red and blue phase - this has probably happened when the breakout has been installed or when setting the cores for cable jointing.

Having fitted the oil barrier tubes a good distance from the crutch the papers in this area have not had sufficient protection when being manipulated, fracturing a few layers of papers in this highly stressed area will have a significant effect on the life time of the cable joint.


It is also apparent that a mix of 11kV and 24kV cable joint kits have been used for the completion of this joint combining material from one and measurements from another which is extremely bad practice.

Correct joint for correct cable and voltage range should be used at all times instructions for these should be followed and not deviated from.

 

 

 


Category:  HV Cable Joints & Terminations

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