Strange fault event - 3 phase fault, Single phase fault or others 3

[BINGMD]

All,

The following link contains a fault event graph. It shows that there was an event that ALL three phase currents were increasing for 4 ~ 5 cycles, however all three phase currents are in phase. During this event, C phase voltage is lower at 0.3 per unit. So, is it a single phase fault or three phase fault or others?

What if there are multiple ground points at CT secondary and there was a nearby single phase fault, will the relay (SEL-351) see something like this?

Thanks for your opinion.
Regards,
Bing

 

[davidbeach]

Single-phase to ground fault on c-phase. Study your one-line of the local area and you'll find the transformer connections that make that possible. What you've got is just zero sequence in all three phases.

 

[jghrist]

Single-phase to ground fault on c-phase. Study your one-line of the local area and you'll find the transformer connections that make that possible. What you've got is just zero sequence in all three phases.

One place you'll see this is the line currents of a grounding transformer (zig zag or grd wye - delta) during a ground fault.

 

[ijl]

Interesting curves! The attached picture tries to illustrate what davidbeach and jghrist said. There is a fault in phase a in this case.

 

[BINGMD]

The transformer is Yg-Yg-D connection, and it is a wind farm. SEL-351 relay is at 115kV side and generation level is very low, about 1MVA. So, the Yg-Yg-D xmer is function as a grounding transformer for a fault at 115kV transmission line, so all three phases contains only zero sequence current.

Great! Thank you all for your help!