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.

 

[neps3]

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.

 

[AAK]

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

 

[UweErnst]

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

 

[neps3]

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

 

[Tuxedo]

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.

 

[DanDel]

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?

 

[ColinR]

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.

 

[gradon]

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?

 

[mpparent]

This could be just lamination failure due to age.

Mike

 

[ELECTRICALTESTING]

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

 

[AAK]

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

 

[Tuxedo]

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.

 

[peebee]

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?

 

[jghrist]

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?

 

[Tuxedo]

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.

 

[DanDel]

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.

 

[Tuxedo]

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.

 

[DanDel]

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.

 

[DanDel]

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?