False feeder trip

[Golfex]

Dear All,
Could you please mentioned the actual reason of the following false tripping of one healthy feeder due to a single line to ground fault on the adjacent feeder.
Details are as follows:
Both feeders are being fed from the same bus 33KV feeding each one one string of an offshore wind farm. They are adjacent to each other like feeder 1 and 2. Feeder 2 had SLG fault and it caused feeder 1 also tripped on the same time.

Faulty Feeder 2 : Offshore wind farm string with 7 turbines of 7MW each, 33 KV, total string cable length 10KM (Underground cable 3x1x800 Sq.mm CU/XLPE/STA/PVC), CT 1000/1 with Micom P139 relay.
Protection settings:

Healthy Feeder 1 which also tripped : Offshore wind farm string with 7 turbines of 7MW each, 33 KV, total string cable length 10KM (Underground cable 3x1x800 Sq.mm CU/XLPE/STA/PVC), CT 1000/1 with Micom P139 relay.
Protection settings:

To note that in another feeder it is connected a 315 kVA Y-Y earthing/auxiliary transformer with both neutral points directly earthed.

Could you please explain the actual reason and measures to be taken to avoid such false tripping. Could it be due to a sympathetic tripping event?
Best Regards.

 

[davidbeach]

Directional or non-directional overcurrent elements? If non-directional it's very likely that turbines on the non-faulted feeder supplied enough fault current that their feeder also tripped.

 

[RRaghunath]

Feeders 1&2 both are source feeders. If the fault is in feeder-2, the fault current flows in to fault from the generators connected to feeder-1 to 33kV bus to fault in feeder-2.
So, the relays in both feeders see the fault. If the settings are identical (as it shows) and if the protection is non-directional (presumably), the protection in both feeders operate simultaneously and isolate both feeders.
Solution is to directionalise the protection. This applies to phase overcurrent protection as well.

 

[Golfex]

Hi,

Thank for your replies.

If it is the case RRaghunath describes, why a third string feeder, Feeder 3, connected to the same 33 kV bus didn't had same behaviour than Feeder 1 and didn't trip?

 

[RRaghunath]

I would expect Feeder-3 which is identical to feeder-1&2 also to trip.
The reasons for not tripping (though I am sure you are happy that it has not tripped) could be that the
[ul]
[li]contribution from that feeder was lower than the pickup threshold set in the protection (may be no. of generators working are less) or[/li]
[li]there is some problem with the relay (relay needs to be tested) or[/li]
[li]problem in the protection DC system in the feeder.[/li]
[/ul]