Can energizing a transformer cause the ground fault to trip
Can energizing a transformer cause the ground fault to trip
(OP)
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






RE: Can energizing a transformer cause the ground fault to trip
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.
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
No there was no seperate neutral sensor
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
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.
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
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
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
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.
RE: Can energizing a transformer cause the ground fault to trip
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.
RE: Can energizing a transformer cause the ground fault to trip
Also, I'm not sure about the unbalanced inrush question. I suppose it's possible.
RE: Can energizing a transformer cause the ground fault to trip
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.
RE: Can energizing a transformer cause the ground fault to trip
If its possible with the situation, try a no load energization of the transformer.
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
Also make sure that breaker and trip units are tested good.
RE: Can energizing a transformer cause the ground fault to trip
The 2000A breaker is rated for 100% of its rating continously. There is no instantaneous setting on it.
The Ho/Xo terminal on the transformer may either be grounded at the xformer, or a cable run from it to the swbd neutral which is grounded. This way it is a single point grounding.
The xfromer magnetizing current, as I understand, is primarily third harmonic which will find its way to the neutral. The imbalance, if any of other harmonics and any fundamental, may be caused by unequal impedances of the windings, or a breaker pole closing slightly behind the others which will cause a huge imbalnce, albeit for a short time.
My quest is to find out if there is anything seriously wrong here
THANKS
RE: Can energizing a transformer cause the ground fault to trip
Repeatedly asking the same question would not get any different answer, unless you present any different facts.
RE: Can energizing a transformer cause the ground fault to trip
The issue as I understand is that the 1500kVA, 13.8kV/480V Transformer trips on earh fault when energised. The transformer is with YNyn0 vector group.
- Are you energising the transformer with with load connected on secondary!
- Could the load be unbalanced (likely if the distribution is 3-phase, 4-wire)!
If yes, for the above queries, then you have an answer to the trip issue.
Another aspect is - check whether the transformer is being energise with an uncleared earth fault downstream of the 480V switchboard.
I find one thing strange in the information is that the setiing of earth fault protection you mentioned as 800A, 0.15sec. For a 1500kVA trafo,
- why the earth fault setting on 13.8kV side should be so high.
- Could it be that what you informed the forum is not 13.8kV side setting but that on 480V side.
Verifying the same and looking at the actual earth fault protection setting on 13.8kV side could help further in solving the issue.
Trust the above is helpful.
RE: Can energizing a transformer cause the ground fault to trip
OP is talking about 480:480V wye-wye xfmr. See his Jul 2 post.
NKE007:
Does the switchboard has zone interlocking on GF? That will make the feeser GF instantaneous, regardless of its setting.
RE: Can energizing a transformer cause the ground fault to trip
NKE007, the answer of your "real question" if "the unbalance in the xfrmer magenetizing current cause the ground fault to trip if set low" is YES.
The transformer inrush is an unbalanced phenomenon and as it cause different currents in each phase, the resultant sum of these currents will flow in the transformer neutral.
If a system has a relatively large series resistance, when a transformer is energized nearbay other transformers, the inrush current will decay slowly. In this case, the neutral overcurrent relay may operate if it is not adjusted for this condition. This phenomenon is known as "sympathetic interation".
I would suggest you to register the current during the transformer energizing to see if there is any problem with the transformer, anyway.
Best regards,
Herivelto Bronzeado
RE: Can energizing a transformer cause the ground fault to trip
We were able to energize the xformer with the 3000A main breaker GF set at 1200A with a delay of 0.5 secs, and the 2000A xformer feeder breaker set at 1000A with a delay of 0.4 secs. Interestingly when we lowered the 3000A main breaker GF delay to 0.4 secs, it tripped but the 2000A feeder did not.The 3000A brkr was last tested when it was installed 10 years ago, and maybe tripping faster than it should.We are planning to test it
Interestingly after we energized the 1500 KVA xfrormer successfuly, we tried to energize a 2.75MVA transformer connected to te 1500KVA xformer (long story on why we have a larger xfromer), the 3000A main tripped again on GF but the 2000A feeder breaker with a lower faster setting did not.Seems like more evidence that we need to test the 3000A breaker. Incidentally it a CH Type SPB breaker
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
Thank you for the feedback. I am glad things worked out for you.
RE: Can energizing a transformer cause the ground fault to trip
It does have a Digitrip unit. Can this be tested and repaired at the site, or does the entire breaker have to go to the shop? Also is it possible to swap just the trip unit with another tested, or new, unit in the field?
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
A ground/neutral current sensor would be installed between the H0 terminal and the ground connection.
Setting the time delay to bypass the inrush may be the easiest fix.
Remember that inrush intensity and duration are influenced by the system. A system upgrade may have caused both the intensity and duration of the inrush to increase.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
Can be done on site via primary injection, secondary injection only tests the trip unit 9And only then, just some of the circuitry), you may have a bad CT (Not likely, but possible) so do a primary injection test.
RE: Can energizing a transformer cause the ground fault to trip
Yes, the Digitrip trip unit can be replaced by another one easy. You will need to put the original rating plug in the new trip unit.
SPB breakers usually come with POW-R-7 trip units. They can be replaced with digitrip 510.
RE: Can energizing a transformer cause the ground fault to trip
Swapping trip units is easy, I have hundreds of spare Digitrips out back.
RE: Can energizing a transformer cause the ground fault to trip
Primary injection would require removal of this fixed mounted breaker and taking it to the shop. The plant would be down for a long day. Do you have a drg that shows the polarity orientation of the neutral CT for the Digitrip 520 unit? Thanks
RE: Can energizing a transformer cause the ground fault to trip
RE: Can energizing a transformer cause the ground fault to trip
Does your Zero Sequence CT have multiple taps? Could be problem, ny best guess is still the trip unit itself, premature trips are good indicator of trip unit failures.