cable termination failure

[zaza123]

We had a failure of 34.5 KV cable termination in 34.5 KV/13.8 KV Transformer during switching of 25.4 MW motor (through Softstarter). The concerned motor & Transformer are in different parts of plant.Is there any contact between switching & termination failure.

 

[electricpete]

It is probably not coincidence that the failure happened at the time of switching. Some possible ties, roughly in order of likelihood imo:

1 - switching can produce transient voltages exceeding normal voltages which might push a weakly insulated termination over the edge.
2 - High-resistance contacts will be most challenged during starting. We had a motor which tripped on current unbalance within seconds of DOL start, and we found an open circuit at the T-lead lug crimp Presumably it was a bad connection that degraded due to 6*FLA starting current which means 36*normal I^2*R in a high-resistance connection. Should not be as much a factor for soft start.
3 - prior to starting, terminations are at room temperature and there may be condensation/moisture present. Once the motor is running, the heat from conducting current in the cable tends to drive moisture away. Of course the terminations at that voltage are sealed and it shouldn't make a difference, but in real world it may not be the case.

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(2B)+(2B)' ?

 

[prc]

pete, is there a possibilty of transient overvoltages with soft starting? I thought otherwise.

 

[zaza123]

Thanks, please note that the Transformer was energised & running before start of the Motor. The Transformer & motor are from two different plants with power coming from common 34.5 KV bus. If we energise the Transformer & then there is a Termination failure of Transformer HV cable it is sure this is because of swicthing. But starting of Motor in another plant & that too on Sofstarter,how can it affect the cable termination of Transformer in another plant.

 

[allan789]

starting of such a big motor, even with a soft starter would drop your bus voltage abit. your other transformer might experience greater current depending on its load, which might contribute to the fault.

seems like a transient voltage or sheer luck would be the more probable causes