REF and bucholtz

[khadisa]

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?

 

[RalphChristie]

Buchholtz and REF? If both are tripping there have to be a fault inside the transformer. My bet would be a fault very near the neutral point on the secondary side of the transformer. Where are your neutral CT (for REF) located? Inside the neutral bushing?



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[dpc]

I'm with Ralph. The REF and the Bucholz unit obviously work on two completely different principals. If both are operating, this points very strongly towards a fault within the transformer. I certainly would not put the transformer back in service. A winding fault near the neutral point seems the most likely candidate.

 

[ScottyUK]

khadisa,

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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[stevenal]

Gas in oil testing should be done.

 

[amps21]

Was there any thru faults where in the NCT would saturate, it can also cause buchholz. But most likely what all the others have contributed. Winding resistance / Gas analysis ASAP

 

[ScottyUK]

amps21,

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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[RalphChristie]

This question is also asked in the power forum at

http://www.iee.org:

Anyone interested can follow it (maybe different comments / opinions) at:

http://www.iee.org/forums/forum/messageview.cfm?catid=226&threadid=9217&enterthread=y

I totally forgot a gas analysis. Nice point, Steve

Regards
Ralph


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[amps21]

I would be wrong if i meant that - gassing due to neutral CT saturation. I meant either misoperation due to NCT saturation and at the same time sudden winding stress / vibration caused by the thru fault causing the buchholz relay to operate (cause some older buchholz relays used mercury switches.)

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?

 

[khadisa]

Regarding the NCT, it was on the neutral bushing but changed by mounting an external one when this problem started.
This problem normally comes when there is a through fault or a fault on one of the 330KV lines emanating from the same swithchyard.

 

[ScottyUK]

When the neutral CT was changed from a bushing type to an external type, was care taken to match the characteristics of the new external CT to those of the internal one? If your REF scheme is a high impedance design, imperfect CT matching could be the source of the trip under through fault conditions, as could NCT polarity. These problems have been discussed previously on Eng-Tips.

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...

 

[amps21]

Scotty is abs. right. As u know the whole reason behind REF is to be insensitive to thru faults. I would think that this should be proven before hand by doing thru fault tests and checking current phasors @ the relay.

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.

 

[HiSet]

Khadisa,
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.