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

By Chris Dodds on 3rd February, 2015

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

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 1 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 2 - High Voltage Cable Joint Failure On 11kV Trifurcating Transition Joint

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 1

We were asked to make a report on an 11kV 3 core transition joint which was audibly discharging. The yellow stress relief mastic was applied correctly, to the ends of the extruded screen of the XLPE cables (Fig 1), and the applied screens of the PILC cable (Fig 2). These tapes should have been covered by the dual wall core heat shrink insulation tube. Unfortunately this was not the case (Fig 3 and 4).

HV Joints Fig1

3 Core Cable Joints Fig 2

Heat Shrink Cable Joints Fig 3

Transition Cable Joints Fig 4

Conclusion

It can sometimes be difficult to be 100% accurate when attributing cause for any breakdown in high voltage joints and terminations. However, in this case we can confidently state that the primary cause was the incorrect installation.

The high voltage cables being jointed were 185-240sqmm and the cable jointer had used the stripping dimensions for that range as per the installation instructions provided within the cable joint kit. Unfortunately, the actual cable joint kit used was designed for cables ranged 35-95sqmm. The dimensional difference accounts for the dual wall tubing “floating” between the core screens.

Items within the cable joint kit such as constant force springs, under armour support rings, tubings and mastics, would have been sized to suit the smaller cables for which this heat shrink cable joint was designed.

Invitation

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Discussion subjects include cable installations, cable jointing, electrical substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.

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Category:  HV Cable Jointing & Terminating

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