Can energizing a transformer cause the ground fault to trip

[NKE007]

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

 

[rbulsara]

I also agree that 2000A breaker is rather small for a 1500kVA xfmr but not end of the world. But I am assuming that NKE007 has verified that it a GF trip and not overcurrent trip.

Also make sure that breaker and trip units are tested good.

 

[NKE007]

Many thanks to all who have responded. I will clarify some points raised:

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

 

[rbulsara]

I don't think there is anything serious there. You are not even doing a simple check as advised. 100% rated breaker is still a 2000A breaker which could be found underrated. But I do not suspect that is your problem.

Repeatedly asking the same question would not get any different answer, unless you present any different facts.

 

[RRaghunath]

NKE007,

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.

 

[rbulsara]

raghun:

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.

 

[Bronzeado]

Hi folks,

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

 

[NKE007]

Raghun. I am talking about the 480V side

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

 

[Zogzog]

That probally has a digitrip trip unit, those failsafe to the lowest setpoints when something fails in the trip unit circutry, we repair a ton of those. Sometimes dirt gets in the dials and causes this type of failure.

 

[rbulsara]

NKE:

Thank you for the feedback. I am glad things worked out for you.

 

[NKE007]

Zogzog
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?

 

[rbulsara]

The class/rating of CTs could be different on the two breakers. One saturates sooner than other? GF trips because sensors are misreading the high inrush currents.

 

[waross]

With a wye/wye bank the primary neutral must be conneted back to the system neutral. On a wye/delta transformer the heavy inrush current of one phase passes through the other two phase windings. This current is limited somewhat by the impedance of the other windings. The vector sum of the currents is zero. With a wye/wye transformer the inrush of the saturated winding(s) returns on the neutral. The vector sum of the three phases plus the neutral currents is zero. If you neglect the neutral current you cannot expect the phase currents to sum to zero.
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

 

[NKE007]

Many thanks Waross. Icannot install a seperate CT as the main 3000A breaker is equipped with its own CT for ground sensing. I beleive it is the zero-sequence type. By code we cannot set it to more than 1200A and 0.5 secs. Is this delay not sufficient for the magnetizing current to decay?

 

[Zogzog]

NKE007,

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.

 

[unclebob]

NKE007,

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.

 

[Zogzog]

Why? Rating plug could be bad too, needs to be fully tested to determine what is cause of failure.

Swapping trip units is easy, I have hundreds of spare Digitrips out back.

 

[NKE007]

Zogzog
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

 

[Zogzog]

Thats why you always purchase draw out, fixed costs more in the long run. The drawing is in the 520 inst book. I guess you either pull it out and test it (Should only be 2 hours total downtime) or do a secondary injection test.

 

[Zogzog]

Usually the polarity mark should face the line side of the conductor, assuming it is wired correctly. But this should have been tested during acceptance testing.

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.