partial discharge

[SilverfoxUK]

On with some maintenance on an 11kV board.
Knowns - we know the utility supply cable is in poor state but that's their cables.
our incoming switch is out of the board.
The bushings have been examined and are as new.
Connections at the back are shrouded but appear good and have not been touched.

The utility supply has been restored but our breaker is still out of the board.
A hissing noise can be heard in the panel
Using a TEV PD tester the dB reading is off scale on the yellow / L2.
We know the cable fault is on red L1.
We know there is a buried joint about 20m away.
The feeder cable is about 1700m long and is clearly jointed underground.

Question - If we had a fault on the cable about 20m from the board would that be picked up on L2 or because its hissing would that suggest its at the board.

 

[RRaghunath]

A ground fault in L2 phase results in L1 & L3 phase-to-ground voltages to go up. This can go as high as 1.9 times the rated voltage depending on the type of power system earthing.
Hissing noise from L2 bushing in the switchgear panel suggests that there is partial discharge due to excessive voltage.
If there is no such hissing noise from L3 phase bushing, it could mean that the L2 bushing is unclean/dirty etc.

R Raghunath