Substation Transformer Failures 2

[Tuxedo]

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.

 

[Tuxedo]

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.

 

[jghrist]

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?

 

[CarlPugh]

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

 

[Shortstub]

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?

 

[rashieda]

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?

 

[jghrist]

"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."

 

[busbar]

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

 

[jghrist]

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

 

[jbartos]

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.

 

[lukedric]

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.

 

[RRaghunath]

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.