Transformer Ground Over Current

[mariner]

I have a new transformer, 3 phase 3 winding. 115KV/13.8kv/13.8kv, high side wye, low side delta. It recently tripped on high side winding ground time over current. The system operators claim there was a spike on the system around the time the transformer tripped. I also have another identical transformer connected to the same line that showed no faults. What causes high transformer ground current? Did this trip most likely come from the system, the transformer, or my plant?

 

[dpc]

If both of the 13.8 kV windings are really delta, then I don't think the 115kv ground current had anything to do with your plant.

And since you are still operating it, I doubt if it came from the transformer winding.

As a best guess, I'd say there was a 115 kV ground fault somewhere on the utility system. The fact that it tripped on time overcurrent, not instantaneous would make me believe it was a real ground fault. Your transformer will act as a ground source for any 115 kV faults.

This leaves unexplained the lack of fault operation for the identical transformer. I don't have an explanation other than the possibility that your relays are not set the same.

 

[stevenal]

Any generation on the 13.8 side? Motor loads? Do transformers feed the same loads in parallel? Or do they serve similar loads? The fact that it is a new transformer leads me to suspect a CT wiring error.

 

[busbar]

With the dual-delta windings, the unit makes a nice zero-sequence source. It can be spooky re-energizing a transformer when the trip cause is not apparent. You've probably tested relays and CTs after the outage. That's fairly routine when the cause of a trip is not completely understood. I'd make darn sure any differential protection is doing its job. Does the relay generate any fault records?

 

[mariner]

thank-you for all of your quick responses. To answer all your questions: there was no generation on the 13.8 side. The only loads were station loads, which on that transformer is only transformer auxiliaries. The other transformer supplies station auxiliaries from both windings when we are not generating. The transformer that tripped only uses 2 out of the three windings, and doesn't supply any station service loads.

Wouldn't a CT wiring problem cause problems during high loads and not low loads?

yes, I have fault records generated from the multilyn 745. This didn't happen during an outage. The transformer has been energized for 9 months with no problems and we start and stop on a daily basis.

 

[stevenal]

If there are no sources or spinning loads on the 13.8 side, fault was sourced by the transmission system through your transformer as dpc said. Your identical transformers may not have identical zero sequence impedances. If you look at fault records for the other relay, you may find that the 51G picked up and never timed out. If fault was remote enough, no local outage would occur. Have you asked your supplier if they had a disturbance?