×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Can energizing a transformer cause the ground fault to trip

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

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.

RE: Can energizing a transformer cause the ground fault to trip

(OP)
The breaker is a molded case breaker with built in LSG trips

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

Was the neutral sensor installed?

RE: Can energizing a transformer cause the ground fault to trip

(OP)
I was going to gradually increase the time delay upto 0.5 seconds.
No there was no seperate neutral sensor

RE: Can energizing a transformer cause the ground fault to trip

No neutral sensor - there would be your problem. With a wye/wye transformer, you need the neutral sensor.

RE: Can energizing a transformer cause the ground fault to trip

(OP)
How would a neutral sensor help? It would also see the imbalance and any third harmonics

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

(OP)
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

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to 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.  

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

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.

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

Thanks jghrist.

 

RE: Can energizing a transformer cause the ground fault to trip

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.
 

RE: Can energizing a transformer cause the ground fault to trip

(OP)
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

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

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.
 

RE: Can energizing a transformer cause the ground fault to trip

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    
 

RE: Can energizing a transformer cause the ground fault to trip

(OP)
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

RE: Can energizing a transformer cause the ground fault to trip

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.  

RE: Can energizing a transformer cause the ground fault to trip

NKE:

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

RE: Can energizing a transformer cause the ground fault to trip

(OP)
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?

RE: Can energizing a transformer cause the ground fault to trip

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.

RE: Can energizing a transformer cause the ground fault to trip

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

RE: Can energizing a transformer cause the ground fault to trip

(OP)
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?

RE: Can energizing a transformer cause the ground fault to trip

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.  

RE: Can energizing a transformer cause the ground fault to trip

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.  

RE: Can energizing a transformer cause the ground fault to trip

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.  

RE: Can energizing a transformer cause the ground fault to trip

(OP)
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

RE: Can energizing a transformer cause the ground fault to trip

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.  

RE: Can energizing a transformer cause the ground fault to trip

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources