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Substation Transformer Failures
2

Substation Transformer Failures

Substation Transformer Failures

(OP)
We have experienced 2 failures of substation transformers in the last 6 months.  We are stumped as to the root cause of these failures and I was hoping some of you tranformer experts out there might have some insight into this problem.

System description
The facility is a hydro generating plant.
The substations are double ended with 100 KVA transformers in each end.
The substations are fed via vacuum breakers at 13.8 KV
The substation secondary voltage is 480 volt.

Failure 1
Failure 1 occurred upon energizing the tranformer by closing the vacuum breaker upstream of the transformer.  The transformers are aluminum and have been in service for about 25 years.  This failure occured after a contractor had replaced the existing vacuum breakers with new breakers and was testing their new breakers by reenergizing the substation transformer.  These transformers were energized anywhere from 10 to 20 times a year but did not fail until the breaker was changed and closed.  There was no load on the transformer while it was energized.  Although there was some fingerpointing at the new breakers as the root cause of this failure there was no evidence to suggest that could be the cause of the failure.

Failure 2
Happened today ( the previous failure happened on a Monday also, not sure that's significant )  Identical situation.  The failure occurred when the contractor again came in to rack in his new replacement breakers and energized 1 of the transformers in a different substation.  This transformer failed too!  We plan on gathering some detailed forensics on this failure but again the fingerpointing is at the contractor/mfr of the breakers.  And again at this point we3 don't have anything to suggest that the root cause of the failure is the breaker.  Is it an incredible coincidence that we had the exact same failures in the exact same way twice?   Or is there something else going on here?   I should mention that after the first failure we added new snubbers to all of our transformers.

Some of the theories that have been tossed around are
- the new breakers seem to close faster & harder than our old ones.  Could that be affecting the impulse voltage seen by the transformers and causing these failures? Or  could there be pre-strike issues with the new breakers?

Thanks in advance for any insight.

RE: Substation Transformer Failures

Tuxedo

What is the type of new breakers.

The transformers are 100kVA and 25 years old. I assume that the transformer oil may not have been checked or replaced. Reclamping of transformers are necessary after 15 - 20 years. If the winding becomes loose, then you know the inrush can move the coils. You know the inrush is anyware between 12 to 15 times for a 100kVA transformer.

Interesting problem. I would like to have some feedback on the failure mode on inspection.

RE: Substation Transformer Failures

Tuxedo,

You are calling it transformer failure where actually it is breaker tripping.  Could you tell us:
1) what protective devices are operating
2) Transformer type
3) Breaker specs
4) Why did you change to new breakers.

Are you able to energize the transformer after the failures ?
Depending on your answer to above, the problem could be due to Oil or the selection of the breaker.

Best regards,
             AAK

RE: Substation Transformer Failures

Tuxedo,

can you give a little bit more information on the test sequence when testing the new breaker?

What type of failure did occur?
Did it occur during closing or opening the vacuum breaker? I guess at closing as you are talking about pre-strikes!

Best regards,

Uwe

RE: Substation Transformer Failures

Tuxedo

I may have misinterpreted as a failure. Confirm that only the breaker tripped and the transformer did not fail. If this is so then it is the inrush and the protection setting. You know the inrush sustains for about 300 milliseconds. Your protection co-ordination should withstand the inrush similar to a motor starting current and a fuse where the fuse is only a short circuit protection for the cable.

Regards

RE: Substation Transformer Failures

(OP)
Kantor -  The new breakers are Cutler-Hammer FC-VR a direct replacement for our Allis-Chalmers type FC breakers.  They are 1,000 KVA Dry type transformers the 3 coils stand side by side, they are delta connected with surge arrestors on the primary.

AAK - the breaker is opening when the transformer is failing.  Why did we change breakers?  Apparently it was due more to the existence of asbestos in the arc chutes rather than age or any other factor.

UweErnst -  Coil B, the center coil, has obvious scorching and arcing.  It appears that the arc followed a path from coil B to the High side bus travleed along the high side bus to the end of the busbars, The ends of the bus between are melted.

Thanks all for your interest & comments.  here's what I'm chewing on right now.  kantor makes a ggod observation - the xformers are 25 yrs old and perhaps things have loosened up a bit.  This could be a factor in the failure of these transformers.  But..  in my mind the breakers are contributing in some way to these failures.   I just don't havfe any evidence to prove this.  We are going to try to test the old breakers and the new and see if there are any major differences in closing times.  I will keep you posted as we follow this through.    Thanks again and please keep your comments coming.

RE: Substation Transformer Failures

I don't think the new C/Bs are responsible, but:
What is the distance between the transformers and the C/Bs?
Are there any capacitors in this system?

...and:
 
How long were the transformers in a de-energized state before they were re-energized?
What is the location and ambient temperature and humidity where the transformers are located?
When was the last time the transformers were cleaned and tested?

RE: Substation Transformer Failures

Tuxedo

Have you considered the possibility that the fault is occurring as the breaker trips?  By this I mean that the new breaker protection is causing it to trip on overcurrent (due to incorrectly set protection not allowing for transformer inrush current).  The rapid breaking of current to the transformer may be the cause of flashover.

Cast resin (or dry) type transformers tend to be wound with aluminium tape rather than circular cross section copper wire - this GENERALLY causes them to have a significant capacitance to earth, and between adjacent windings on the same core.  I have heard of problems like this when using dry transformers with large variable speed drives, due to the switching action of the drive.  I believe this is less of a problem with newer-generation dry transformers.

It is possible, I guess, that if your circuit breaker trips during 'start up' of the transformer, you will be breaking quite a large current, and hence causing voltage surges accross the transformer windings.  The new breakers may be capable of extinquishing fault current much faster than the old type (as you would expect with newer technology).

What sort of trip does the breaker show?  If it is instantaneous overcurrent (and not a ground fault), then this could be the case.  I would expect a ground fault if the arcing you describe is where the fault initiates, rather than the circuit breaker protection.

RE: Substation Transformer Failures

I also wonder if transformer damage is occuring on circuit breaker opening. The new circuit breakers could be causing current chopping due to arc extiction occuring before a current zero. This would result in voltage spikes. Are the surge diverters connected on the transformer side of the circuit breaker, and are they effective?

RE: Substation Transformer Failures

This could be just lamination failure due to age.

Mike

RE: Substation Transformer Failures

PLEASE CHECK WHETHER THE NEW BREAKER HAS ANY CONTACT BOUNCING PROBLEM IN ANY OF THE POLES. IN CASE OF CONTACT BOUNCING, THE SURGES ARISING OUT THERE OF MAY BE CAUSING INSULATION FAILURE OF THE TRAFOS

RE: Substation Transformer Failures

Tuxedo,

   Was the transformer OK with the old breaker ?  (I mean right before changing to new breaker)... What I'm trying to get at is if the transformer didn't have any problems before, then there is a big chance that the problem is with the new breaker or it's protection settings.  One more issue, if the new breaker is OK, then during the change to new breaker while the xfr was shut down, was there any maintenance activity such as testing which could've damaged the 25yr old tranformer especially if the allowed test values were exceeded.
What protection does the breaker trip on ?  This can calrify things as mentioned by ColinR's comment above.

Best regards,
             AAK

RE: Substation Transformer Failures

(OP)

AAK - Yes the transformer was OK with the old breaker.  The only thing that had changed was the breaker.  The old breakers are an air type of breaker.  The new are vacuum type.  I gave ColinR a star because his post is tracking what we are hearing from some "experts" regarding these failures.  

Here's what we have so far:
The new vacuum breakers have a higher BIL rating than the the tranformers.  95 versus 50
The new breakers have a higher "surge" voltage than the old breakers.
The tranformers had station class surge arrestors installed phase to ground. ( no phase to phase arrestors or caps)
The transformer failed in coil B, the center coil,  on the high side.  The second group of windings have several windings welded together.  Fairly close to the high side connection.

Where we are at is that we seem to have a system that worked well until we changed 1 part and now that change is forcing us to add to our protection of the transformer or to change the transformers out to one with a higher BIL rating.  Thanks for you comments.

RE: Substation Transformer Failures

A friend of mine just dealt with a problem that sounded very similar to yours.  I cannot guarantee that it was in fact the identical problem, but they sure sound very similar.

He had brand new TX's burning up whenever the input breaker was closed.  They burnt up quite a few, which greatly upset many people.

It finally turned out that the high frequencies generated when the switch closed were resonating with the transformer.  The solution was to provide inductance (I forget if that meant additional cable or a discrete choke) between the switch and TX.  No problems since.

Just curious, when you replaced the switches, did you move them any closer to the TX's?

RE: Substation Transformer Failures

Can you confirm that all three phases are closing?  If one or two phases fail to close, and if the connection between the breaker and the transformers is cable, you would have a high probability of ferroresonance.  This would cause a high voltage and possibly arcing faults.  What is the arrester rating?

RE: Substation Transformer Failures

(OP)
peebee - we can't discount anythingat this point but the possibility of the issue of ferroresonance being the issue was raised.   That seemed like less of a possibility when we learned that the conductors from brkr to xfrmr on the first failure was about 50 feet.   The conductor length for the 2nd failure was about 200 feet.  So there is no consistency with cable lenths.   The arrestor ratings are 95.  They were connected phase to grnd.  The arrestors were tested and all 3 seem fine.

RE: Substation Transformer Failures

Tuxedo - let me ask again:

How long were the transformers in a de-energized state before they were re-energized?
What is the location and ambient temperature and humidity where the transformers are located?
When was the last time the transformers were cleaned and tested?

Since only my first two questions were answered. Thanks.

RE: Substation Transformer Failures

(OP)
DanDel - The transformers were likely never cleaned to our knowledge.  However, the environment is ideal in terms of humidity and temperature.  The transformer was extremely clean upon inspection.  Some background - These tranformers are in the double ended substations and are energized continuously.

RE: Substation Transformer Failures

Thanks for the (partial) response.

I'm trying to ascertain whether there was:

1. Any resonant/ferroresonant conditions present(hence my first questions about length of cable and capacitors).

2. If the transformers had cooled off completely during this shutdown and picked up moisture(my next 2 questions).

3. If you have any test results over the service life of the transformers which may indicate deterioration.

When you say "ideal in terms of humidity and temperature", what do you mean?

Just trying to help.

RE: Substation Transformer Failures

Also:

Did you determine what relay function opened the C/Bs, and which phases tripped?

Did you say that the secondary windings of the transformer had welded together? Does this appear to be caused by a primary failure or not?

Did anyone touch anything on the secondary Main C/B or switchboard before re-energization?

Was the secondary Main C/B and feeders open or closed?

RE: Substation Transformer Failures

(OP)
DanDel - Had flags on the 50/51 Relays Phases A & C.

No damage whatsoever on the secondaries.  I was probably not clear on that.  Failure was definitely on the high side.

The trades that work on this were very systematic and conservative about how these new breakers were installed.  First the loads on the load side of the transformer were shut down and not operating, minimal load.  Second the breaker on the load side of the transformer was "Opened".  Third - The tie breaker was activated to pick up the minimal loads.  Fourth the new breaker to the transformer was racked in.  Fifth - after racking in the new breaker was closed and BANG the transformer failed.

The transformer was sitting there with it's secondary breaker "Open" when the feeder breaker to the transformer was energized.

RE: Substation Transformer Failures

My concern about the arrester was that if there was a ferroresonant overvoltage, the arresters should have protected the transformer or failed in the attempt because they are not suited for a sustained low frequency overvoltage.  An arrester rating of 95 does not make sense.  I would expect a 9 or 10 kV arrester rating which would provide adequate protection for 95 or 110 kV BIL but maybe not for 50 kV BIL.  50 kV BIL is low which might point to an overvoltage being the problem.  If there is a ferroresonance problem, disconnecting the load before energizing will make things worse.

If all three phases are opening, then ferroresonance is not likely.  Closing the breaker quickly should not cause a problem.  Maybe ColinR is right and the transformers are failing when the breakers open for some other reason.  There could be current chopping with transient overvoltages.  Do the new breakers have different relay settings?  If they are faster, they may be tripping on inrush.

You mentioned replacing snubbers.  I am not familiar with the use of snubbers on MV transformers; what are they?

RE: Substation Transformer Failures

Vacuum type breakers open the circuit extremely fast. The di/dt can be extreme. This is what is causing the transformer failures. The vacuum breaker will give a very high voltage when it opens, and on close in the contacts will bounce. The close in may be even worse than when opening as the inrush current will be opened.
Dry type transformers generally have lower BIL ratings than liquid insulated transformers and require more care with BIL protection.
You should get some equipment to monitor the transient voltages when the contactor is closed and opened. Guessing what is happening will lead you to disaster.
I have worked for and used equipment manufactured by Ross Engineering Corporation phone (408) 377-4621. There are other manufactures of this type of equipment, however Ross Engineering Corporiation is the only company I am familiar with.
Good Luck

RE: Substation Transformer Failures

Tuxedo,

1) Is 13.8 kV system grounded?  If so, how?

2) Did failure occur while system was operating from generator(s)?

3) Or, was 13.8 kV system being fed from transmission link?

4) Or, both?

5)When xfmr feeder air-breakers were replaced with vacuum-breakers, were others in switchgear also replaced?

6) Do you know approximate length of 13.8 kV cables connected to xfmr side of bus at time of failure?

RE: Substation Transformer Failures

Tuxedo, was any electrical testing done on the transformer prior to re-energization, and what were the tests done and the results? Were the results deemed acceptable for energising the transformer?

Were new 50/51 relays used after the breaker retro-fit, or were the original 50/51 relays used? What were the settings of the old and new 50/51 relays that tripped in terms of primary amps? Did the INST flag or did the TIME flag of the 50/51 relay operate during the incident? Both the INST and the TIME settings should permit the flow of transformer inrush current, without tripping. If the relay is correctly set, and it operated, then it responded to currents exceeding the inrush current, which means that insulation failure occured first, followed by relay action to open the breaker.

What were the rated contact opening and closing times for the old and new breakers?

RE: Substation Transformer Failures

"A Comparison of Vacuum and SF6 Technologies At 5kV Through 38kV", a paper presented at IEEE Petroleum and Chemical Conference, Denver, Colorado, Sept. 14, 1983, by C.L. Swindler, Square D Company, gives some comments that may be relevant to your situation.  This paper may be available from Square D (Groupe Schneider); it is in their Power Systems Engineering Data Volume 3, Number 12.

Current chopping, a sudden reduction of current to zero prior to natural current zero, can occur with vacuum breakers.  This can cause a high transient overvoltage because of L di/dt.  According to the paper, "Surge capacitors are usually recommended for vacuum interrupters when using lower BIL rated equipment such as some dry transformers and rotating machinery.  This technique lowers the rate of rise (di/dt)..."

Also, "Metal oxide arresters are used to limit the voltage to within the BIL rating of the equipment but cannot modify the rate of rise of current (di/dt).  Also, they are always connected line to ground, thus not really connected to the source of the overvoltage which, in the case of switching transient, are more related to line to line phenomena.  For thses reasons metal oxide surge protection is not consedered to be adequate for all applications."

This would indicate the problem to be during breaker opening, not closing, but also:
"Prestrike interruptions are very similar in nature to restrike only in this case-they occur during contact closing. ... it is necessary that vacuum contactors have very little or no contact bounce in order to reduce such phenomenon to a minumum."

RE: Substation Transformer Failures


50kVBIL on 13.8kV windings?  Oops.  Someone seems to have goofed 25 years ago.  
   

RE: Substation Transformer Failures

Standard BIL for 13.8 kV dry type transformers is 60 kV per C57.12.01.

RE: Substation Transformer Failures

Suggestion: I just would like to elaborate on the CarlPugh (Electrical) Mar 2, 2003 posting by considering a combination of that posting explanation with the age of the transformer and its potentially deteriorated insulation as a possible overall culprit.

RE: Substation Transformer Failures

My company has the same problem. We replaced an oil filled transformer 5 years ago with a cast-coil type:

34.5kV Class
27.6-4.16kV step down, 750kVA, 150kV BIL
outdoor environment- -20C when it failed.

replacement cast coil flashed over to ground last spring,
replaced core/coil assy and arced over phase to phase few weeks ago. fuses & upstream breakers operated.

Tuxedo, can you let us know the results of the forensic tests of your transformer? Have you isolated the problem?

Thanks in advance.  

RE: Substation Transformer Failures

After going through the above posts, I feel it is an application / Insulation coordination problem.

The vaccuum breaker coupled with appropriately sized surge arrestor (and may be surge suppression capacitors too) should have been the correct solution. The cast resin transformers are similar to motors in that their with stand capability for voltage surges is limited.

If it is true that the surge arrestor rating is 95kV, there is problem there too and if my guess is right, the type of arrestor is unlikely to be gap-less type (that would have given closer protection and with more predictable discharge characteristic). Age / health of the arrestor also has a role to play.

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