Fault Current Mostly Exist at Positive Value

I think you are correct. Phase A ground fault current seems to be having a very large
DC componenet. That is why it hardly passes the zero axis.
Reason should be very high X/R ratio.
Is the measurement correct?
Was it happened near a generator?

The lower the R in the fault path, the longer the decay period.

Bill
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"Why not the best?"
Jimmy Carter

Is it a fault or was it simply that an A-phase transformer was energized?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

Not that I know what this is, but this could be a ground fault on phase A.

Years ago I was a utility rep. for a customer project at a plant (I don't need to discuss which one).
The plant was adding a new fault reduction scheme that inserted a diode and resister in parallel, between the transformer X0 bushing and ground.
So I can't help but wonder, but it's not likely.

thanks all for your responds, additional information:
[ul]
[li]the breaker fed 34.5 kV subsea cable[/li]
[li]the cable was unloaded, the end breaker was open[/li]
[li]the cable is confirmed faulty, near cable end[/li]
[li]cable length is 108600 ft [/li]
[li]the CT's are 800/5 C100 [/li]
[/ul]
answering the questions:

Kiribanda said:

I think you are correct. Phase A ground fault current seems to be having a very large
DC componenet. That is why it hardly passes the zero axis.
Reason should be very high X/R ratio.
Is the measurement correct?
Was it happened near a generator?

Click to expand...

I'm not sure the measurement is correct, but the main protection work properly (87L). It happened quite far from generator 333600 ft (electrically) from generator bus .

davidbeach said:

Is it a fault or was it simply that an A-phase transformer was energized?

Click to expand...

confirmed single phase to ground fault on the cable.

How to calculate the X/R of system?