transformer damage?

[drbond24]

Yesterday there was a phase to phase fault on a pole outside our manufacturing facility. The main breaker tripped on our switchgear and isolated the building, but the transformer outside was left to sit and cook until the POCO showed up to disconnect it. In that time (approximately 30 minutes) the transformer exterior reached 560 degrees F and was smoking quite a bit. Oil leaked from every available orifice (bayonet fuses, temperature gauge, etc.) and covered the inside of the transformer. After the POCO fixed their problem, they turned our transformer back on and today it seems to be operating normally. This is the first time I've been in this situation, so I'm looking for advice. What steps should we take to determine the damage done to our transformer, if any, during this event? Should we test the oil, the windings, replace the whole thing, all of the above...?

 

[nakulak]

call the poco and tell them that their fault cooked your transformer, when will they be replacing it ?

 

[drbond24]

We'll certainly pin whatever cost is incurred on the POCO, but question #1 is what needs done? Does the transformer need replaced? I want to go ahead and just replace it to be safe, but it isn't my money either. We need to prove that a replacement is necessary. We could send an oil sample for testing, but that takes time and in the meantime we'll operate the plant as usual which could be unsafe if transformer failure is imminent.

I guess I'm just not sure how worried to be about our transformer at this point.

 

[nakulak]

you could certainly get someone to analyze the transformer, test the fluids, estimate the life expectancy, etc etc (there are companies that do this). but imshio, why not just tell the poco that your transformer was n years old, and in your opinion the life expectance based on the accident is now possibly zero. let them do the tests if they want to prove to you its ok, but why sell yourself short. convince them it will be cheaper for them to give you a new one.

 

[zog]

Yesterday there was a phase to phase fault on a pole outside our manufacturing facility. The main breaker tripped on our switchgear and isolated the building, but the transformer outside was left to sit and cook until the POCO showed up to disconnect it. In that time (approximately 30 minutes) the transformer exterior reached 560 degrees F and was smoking quite a bit. Oil leaked from every available orifice (bayonet fuses, temperature gauge, etc.) and covered the inside of the transformer. After the POCO fixed their problem, they turned our transformer back on and today it seems to be operating normally. This is the first time I've been in this situation, so I'm looking for advice. What steps should we take to determine the damage done to our transformer, if any, during this event? Should we test the oil, the windings, replace the whole thing, all of the above...?

Have a DGA oil sample drawn and analyised for sure, should only cost a couple hundred bucks for someone to come out and take one, DGA will tell you if any damage occured.

If you are really concerned (or if the utillity is paying for it) have a testing company come out and do a Pf test, a TTR, a DAR, and winding resistamce tests.

 

[mivey]

...convince them it will be cheaper for them to give you a new one.

Would not work here unless you could prove real negligence on the part of the poco. The customer is responsible for protecting their own equipment.

The poco does not guarantee power and is not liable for damage caused by service interruptions or for damage caused by acts of nature, equipment failure, or the normal operation & maintenance of the system.

 

[wirenut1980]

The transformer should be replaced. Doing an oil sample will most likely confirm this is if there was oil leaking out. I am very surprised it is working at all. I am curious though, if the transformer is yours, that must mean that you are primary metered (on the high side of the transformer). How did a fault on the utility primary fail your transformer and cause your main breaker to trip?

Is your transformer a pole mount cluster type or padmount? Wye-delta?

Or was it a fault on the secondary between your main switchgear and the transformer? If so, why did your main breaker trip? It seems that for the main breaker to trip, the fault would have to be on the load side of it.

 

[kingpb]

If the low voltage side of the transformer opended, then there would be no current flow through the transformer. I think you have a bigger issue on your hands, unless addtional info is available that has not been furnished.

The situation as described does not seem plausible.

 

[bob]

I am curious though, if the transformer is yours, that must mean that you are primary metered (on the high side of the transformer). How did a fault on the utility primary fail your transformer and cause your main breaker to trip?

Is your transformer a pole mount cluster type or padmount? Wye-delta?

Or was it a fault on the secondary between your main switchgear and the transformer? If so, why did your main breaker trip? It seems that for the main breaker to trip, the fault would have to be on the load side of it.

I'm with wirenut. I am having trouble visualizing how an overhead primary phase to phase fault damage your transformer. Can you give more details?

 

[mivey]

What about the possibility of grounding through the highside or some kind of current circulating through the high side?

 

[drbond24]

Have a DGA oil sample drawn and analyised for sure, should only cost a couple hundred bucks for someone to come out and take one, DGA will tell you if any damage occured.

This is all I can get management to agree to do for now. They will test the oil and base any further decisions upon those test results.

I am curious though, if the transformer is yours, that must mean that you are primary metered (on the high side of the transformer). How did a fault on the utility primary fail your transformer and cause your main breaker to trip?

Is your transformer a pole mount cluster type or padmount? Wye-delta?

Or was it a fault on the secondary between your main switchgear and the transformer? If so, why did your main breaker trip? It seems that for the main breaker to trip, the fault would have to be on the load side of it.

The fault was on the next pole upstream from our meter. I don't really know how it happened, only that A and B phases ended up touching. Transformer is pad-mounted 34.5 kV ground wye to 480/277 wye. Our main breaker is set to trip after a few seconds of a voltage unbalance. When Vab went to zero, the breaker tripped after the programmed delay had expired.

If the low voltage side of the transformer opended, then there would be no current flow through the transformer. I think you have a bigger issue on your hands, unless addtional info is available that has not been furnished.

You are correct; there was no current flow through the transformer. The breaker tripped because of the voltage unbalance caused by the phase-to-phase fault. I can't explain why everything happened, I'm only relaying what I saw with my own eyes. After the fault, our meter on the secondary side of the transformer showed that Vab was zero. At that time, the transformer outside was buzzing extremely loudly. After a few minutes we noticed smoke coming out from under it, so I got out the thermal camera and started taking pictures. Like I said, it increased to as much as 560 degrees F on the OUTSIDE of the transformer. I can only assume that it would have been even more than that on the inside. We have no way of disconnecting our transformer. We had to wait for the POCO to come and disconnect it from the pole.

 

[kingpb]

You need to adivse management that simply testing the oil may not show the true nature of the damage, or condition. If this transformer is critical to the operation of the facility, then this is not time to cinch out on an appropriate course of action.

 

[mivey]

You need to adivse management that simply testing the oil may not show the true nature of the damage, or condition. If this transformer is critical to the operation of the facility, then this is not time to cinch out on an appropriate course of action.

I would agree. I'd be willing to bet the oil test will returned "cooked".

I've had transformers with oil temperature peaks a lot lower than this be called "cooked" when we yanked it and sent it to the lab for testing. They were still working when we took them out of service. I would not even care to venture a guess as to what this temperature has done to the life of the internal insulation. You may be living on borrowed time. I would not feel comfortable keeping it in service unless a long outage is not a big deal. Who knows what else will fail when this transformer eventually goes out?

Since you have a thermal camera, it sounds like they are in the outage avoidance business so the admin response just sounds odd.

 

[jim dungar]

The OP said the oil expanded enough to leak out of every possible location. When the oil cooled and contracted it would have created a vacuum and drawn air and moisture back in to the transformer. So now there is less cooling fluid in the transformer and it is contaminated.

 

[wirenut1980]

The fault was on the next pole upstream from our meter. I don't really know how it happened, only that A and B phases ended up touching. Transformer is pad-mounted 34.5 kV ground wye to 480/277 wye. Our main breaker is set to trip after a few seconds of a voltage unbalance. When Vab went to zero, the breaker tripped after the programmed delay had expired.

Ok this explains why your breaker tripped. As to why the transformer cooked, I haven't a clue. Either somehow it was in the fault current path and the fuses did not operate, or it somehow experienced a rise in voltage and insulation broke down causing an internal fault. Neither of these options makes much sense in your case.

If it was a phase to ground fault, I might have a theory that makes some sense, but not for a phase to phase fault...

 

[zog]

You need to adivse management that simply testing the oil may not show the true nature of the damage, or condition. If this transformer is critical to the operation of the facility, then this is not time to cinch out on an appropriate course of action.

I have to disagree with that, a DGA analysis can determine alot about the nature of any damage, it can tell you the max winding and oil temo reached from a fault, if the celuose has broken down, if any arcing occured, etc..

Problem is, most people dont do a DGA sample and/or it is improperly drawn.

 

[zog]

However, I want to add to my last statement, from the OP description of the events, you might as well just order a replacement transformer, I would not count on the DGA coming back normal.

 

[kingpb]

I agree that testing the oil can determine many things, but to fully analyze the transformer, isn't it necessary to check all the failure modes, which, includes mechanical, electrical, and dielectric; both internal and external.

Looking at the oil only, may give some indication of the trouble ahead, and this may be enough by itself to make a determination on whether to replace immediately. However, say the decision was that based on the oil test it could last a while longer. Then, unexpectedly the transformer fails and it is because the whole transfomer wasn't tested, of which the other tests would have given a better understanding of the actual situation.

Based on this, oil tests alone may lead you to a conclusion that gives a false sense of security, which ultimately could lead to a unscheduled and undesired outage.

 

[drbond24]

My next problem will be, knowing those to whom I report, if the oil comes back bad they will just change the oil and keep going. Sad but true.

I'm having a little trouble figuring out what happened myself. As soon as the main breaker on the building trippped, there was no load on the transformer. On the other hand, for it to sit and cook like it did, there would have to be current flowing, right? It was humming extrememly loudly indicating opposing magnetic fields, but the magnetic fields are created by current, not voltage. What path was the current flowing through? It had to have been inside the transformer, which means there has to be a dead short in there somewhere. If that is the case, how the heck is it working now? We've only loaded it to 50% so far. Is it boing to blow if we go higher?

Where is the fault in my logic? I've got to be missing something.

 

[djohns6]

Good luck getting the power company to replace your transformer . Unless you have some kind of fault event data to back up your claim , they are going to tell you that it's YOUR responsibility to protect YOUR transformer .
What ARE you using to protect the transformer and isolate the primary side during a fault ?
A DGA test will tell you a lot about the internal condition of the transformer , but it should have had other testing done prior to energizing .
You rolled the dice when you re energized . That thing could have been full of all sorts of combustible gases .