REF and bucholtz
REF and bucholtz
(OP)
We have a 90MVA, 330/132KV, Y/Y transformer that is tripping on REF on the secondary and bucholtz but when re-energised it remain in service. We carried out magnitization, ratio, short circuit and insulation resistance tests which show the transformer to be healthy. What other tests can we perform? What can be the nature of the fault?






RE: REF and bucholtz
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RE: REF and bucholtz
RE: REF and bucholtz
You might want to investigate an FRA test on the transformer. This test uses a frequency sweep to obtain a 'signature' for the transformer. It is better if you have a signature from the transformer when it is healthy against which to compare the results of the test, but you may show up an anomaly which helps locate your problem.
I agree with the others - a 90MVA transformer is an expensive piece of hardware to risk: take it out of service and test further. You might find the problem with an internal inspection, but many low level faults don't leave much in the way of evidence until they become terminally bad.
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RE: REF and bucholtz
RE: REF and bucholtz
RE: REF and bucholtz
Could you clarify something for me: the saturated NCT causes gassing, or the effects of a through fault on a damaged transformer causes gassing? I've never heard of a CT causing gassing as a result of saturation before - maybe it's just my interpretation of your words.
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RE: REF and bucholtz
Anyone interested can follow it (maybe different comments / opinions) at:
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I totally forgot a gas analysis. Nice point, Steve
Regards
Ralph
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
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RE: REF and bucholtz
Though i believe most likely there is an internal fault. Gas analysis should reveal C2H2 if Bucholz operated due to arcing. Also wouldnt hurt to test the winding resistance.
The transformers are designed to withstand short circuits both meachanically & thermally. But there are times when one of the thru fault gets the transformer and starts causing problems in the unit. Repeated winding movements over the units age and loosening of winding clamping pressure (also with age)can surely be a killer.
I meant to ask Khadisa - How was the impedence test preformed and how much did the results deviate from previously measurement?
RE: REF and bucholtz
This problem normally comes when there is a through fault or a fault on one of the 330KV lines emanating from the same swithchyard.
RE: REF and bucholtz
The first question you should probably ask is: are the two trips actually related? Can you prove the veracity of the relay scheme? If your relay scheme is proven to be working correctly, and the Buccholz trips at the same time, then the balnce of probability is that your transformer has problems. Given the amount of work involved in an internal investigation, I would consider proving everything you can prove before embarking on such major work - you may save a lot of money and effort, and even if you have to do the inspection after the other work, you have usefully proved that the auxiliaries are correctly operating.
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I don't suffer from insanity. I enjoy it...
RE: REF and bucholtz
In addition, The transformer should be tested periodically and the test should be performed exactly in the same manner everytime, to monitor any changes (by keeping the measurement error the same.)Impedence test is very effective, if performed correctly.
RE: REF and bucholtz
Is the REF(or more appropriately balanced earth fault protection) on ungrounded star winding? If yes than one cannot rule out a possibilty of REF maloperation on capacitive discharge current due to transmission line faults in the vicinity.