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Can energizing a transformer cause the ground fault to trip

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NKE007

Electrical
Jul 1, 2008
13
When we energize a 1500KVA, 480V, Wye/Wye transformer the upstream breaker trips on ground fault. The ground fault is set at 800A, 0.15 secs. Could it be because of the unbalance between phases on the magnetizing current plus the presence of third harmonic current, a typical compoment of the magnetizing current
 
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Many things are possible but you provided no information how the GF current is sensed and where.

Have you tried it after increasing the GF time delay? 0.15s or 9 to 10 cycles may not be long enough time for inrush transient to die out in some cases.
 
The breaker is a molded case breaker with built in LSG trips
 
Try upping the time delay to 0.5s and post back, if there are no other issues observed and you are sure that there is no actual ground fault.
 
I was going to gradually increase the time delay upto 0.5 seconds.
No there was no seperate neutral sensor
 
No neutral sensor - there would be your problem. With a wye/wye transformer, you need the neutral sensor.
 
How would a neutral sensor help? It would also see the imbalance and any third harmonics
 
david and alehman:

May be I am missing something (which is possible, its very late in the day here). Where would you install that N sensor? The xmr needs only 3 wire input and most likely the Wye pts are connected together and grounded. So even if there is a neutral conductor installed in primary(which is not required) it would not sense correctly as any N current will be shared by ground.

Netural sensor is required for 4 wire loads for which the netural is not grounded downstream of the feeder breaker.
 
Good point. If the neutrals are grounded, then the source neutral should not be connected to the transformer and no neutral CT needed. If the neutral is connected, that could explain the problem.
 
Perhaps I should have explained the circit a bit better. The 480V switchboard has a 3000A main fed from a 13.8KV/480V utility transformer. This main has LSIG trips. The G trip is set at 1200A with a delay of 0.3 secs. We used a 2000A breaker on the bus to connect to the new 1500KVA transformer. The Ho and Xo terminals of the new transformer are connected together and then to ground. The neutral of the swbd is also bonded to ground. I will lift the xformer ground and connect the xformer neutral to the swbd neutral and then keep the common bond to ground. Thus there will be only 1 neutral to ground bond.Certainly with2 grounding sources now the ground fault will split between the xformers in proportion to their impedances.
My real question is: will the unbalance in the xfrmer magenetizing current cause the ground fault to trip if set low

Many thanks to all so far
 
I understand that a 2000A molded case breaker feeds a 2000 KVA transformer? Are you sure it trips on ground fault? I think it trips because of the inrush. Even if it is LSG, a fixed Instantaneous might cause the trip.
 
I had an almost similar problem when a genset breaker tripped on energizing a transformer. I think it may be inrush since I dilayed the operation on inrush a few more cycles and the problem was solved.
 
NK:

It can if the high inrush currets saturate the sensors and they may "read" the currents incorrectly.

I am not sure but some more theoratically competent people can chime in here if the grounded netural could cause certain type of currents to flow thru netural/ground and back to the source which could be seen as ground fault by the trip unit.

I would still repeat what I said in my earlier posts. If there is no actual garound fault evidence, its very easy to find out if the GF is a malfuction. Try by either raising the time delay or defeat the GF and test.

The grounding connection revision you are propsing sounds good. But it would not be the cause of your problem.
 
As per unclebob, 2000A is probably too small for 1500kVA @ 480V. You should look at the breaker TCC vs. a typical inrush value at 12x full load current for 0.1 sec and maybe 25x for 0.01 sec.

I second rbulsara's suggestion.

If you lift the transformer ground, you will have created an ungrounded system on the secondary. This may not be desired.

With YG-YG, if the you do not carry the neutral to the primary, any zero sequence current (and triplen harmonics) will appear on the ground. If you do carry the neutral, secondary neutral current will be reflected on the primary neutral. If there is a 3rd delta winding, which may be buried, secondary neutral current appears as unbalanced primary phases, but negligible primary neutral current so no primary neutral is needed.
 
Sorry, I missed the bit on transferring the neutral to the source swbd neutral. The secondary would not be ungrounded, but secondary neutral current would be reflected at the primary (assuming no delta tertiary).

Also, I'm not sure about the unbalanced inrush question. I suppose it's possible.
 

Try check your Y-ground point of the transformer, it should not be grounded or tapped to the body of the transformer, it should have an independent earthings.
 

If its possible with the situation, try a no load energization of the transformer.
 
See thread238-206517 In oscilloagraphy from power transformers during inrush, I have found that there is no zero-sequence current.
 
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