Bucholtz Trip 2

[haisa]

Hello Everybody
We have a 30MVA, 132/33KV transformer that is tripping on bucholtz. We carried out Insulation resistance, magnitisation and short circuit tests which all shows a satisfactory result. The magnitisation and short circuit tests were carried out using 415V and the Insulation resistance test using 5KV Megger. We dont have the facility for anlysing the gas being evolved but the gas coming out is colourless. What could be the problem with this transformer?

 

[jghrist]

Faulty bucholtz. Bucholtz problems have been discussed before on this forum. Try a search.

 

[haisa]

We also suspected the relay to be faulty and changed it, but the transformer is still tripping.

 

[ScottyUK]

If the Buchholz is collecting gas then why would you suspect the relay? It sounds like it is doing its job.

Collect the gas if possible and get it analysed. Definitely get an oil sample and send it for DGA at a reputable oil lab. If your transformer is gassing then you have a fault within the tank. The DGA results should give some idea what type of fault you are looking at, although trending the DGA results would be even better. As your transformer appears to be sick, sample at least monthly and trend the results. If the results show a deteriorating condition then plan to take it out of service for an internal inspection and repair before it takes itself out of service on a more permanent basis.

You should find a link under an external cover which allows you to separate the core from the tank earth. Megger the insulation at the manufacturer's recommended voltage - 500V should be enough. The core should not be earthed to the tank with the link removed.

A 30MVA transformer is probably just large enough to enter without detanking the core if you remove the oil, otherwise it will likely be a detanking job. Either way hire in some expertise to assist you with the inspection.


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If we learn from our mistakes I'm getting a great education!

 

[Marmite]

I agree with Scotty in terms of the diagnostics, however I'm alarmed that you appear to have repeatedly energised the transformer after a buchholz trip. Normally Buchholz relays alarm on gas and trip for a surge. You are risking causing further damage to the transformer and potentially a catastrophic explosion and fire.
Does the tapchanger have a separate buchholz?
Is there a history of Buchholz gas alarms? Is the gassing load related?
Regards
Marmite

 

[ScottyUK]

Please take heed of Marmite's warning about catastrophic failure and explosion. I lost three friends when a transformer of roughly this size blew up: you have the chance to avoid, or at least minimise, the possibility of that happening at your site.


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If we learn from our mistakes I'm getting a great education!

 

[panter]

Haisa,
I agree with Marmite and Scotty that you are playing with fire because you’re Bucholz, after replacing, again tripping transformer. In the past, I made simple test with lighter. If gas from rele burning it is sign of carbonmonoxid or severe problem with transformer and you have to overhaul the transformer.
Good luck!

 

[isquaredr]

This may seem silly and could be overlooked if conservator site glasses are dirty, have you checked there is oil in the conservator tank?

 

[haisa]

Thanks to you all.
Isquaredr we are sure of the oil level.
However I obsered that, when meggering the HV side the insulation resistance will drop at a certain stage and then rise again.

 

[ScottyUK]

That's fairly normal: it occurs as the insulation material polarises. This phenomenon is used as the basis of the polarisation index test or PI test which takes the reading at 1 minute and the reading at 10 minutes and calculates the ratio. Google has loads of hits on this test if you want to read more.


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If we learn from our mistakes I'm getting a great education!

 

[edison123]

Do a surge comparison test (Baker instrument) on all three phases of HV and LV windings to detect turn shorts.

Measure the winding reistances (micro-ohmmeter) of HV winding (on all taps) and the LV winding to detect loose connections.

I wouldn't take any Buchholz trip lightly. I have seen enough disastrous transformer failures to be afraid of the silent beast.

 

[electricuwe]

I just can confirm what other posters already wrote: Trust the Buchholtz

 

[slavag]

Of course I agree with all other posters.
Buchholtz relay operated---isn't joke.

But.. as Marmite wrote, are you have some history of Buchholtz alarm? Are trips was together with Buchholtz alarms?
Are you see Buchholtz trip on the relay and de-latch it, or see only on the some panel indication?

 

[capuchi]

Wow, you are having a Buchholtz tripping with that size of transformer. Act now, bring the experts in who have the capability and right test equipment to analyze the gas which a Buchholtz relay is essentially intended for. They may also do transformer turns ratio (TTR) test and oil dielectric strength test before it is too late.

Good luck

 

[PolyMike]

I will be probably late, but here I go with my advice.

I think the insulation resistance drop might indicate some kind of incipient damage. While you are testing insulation resistance, the HV isolation of the xformer acts like a big condenser charging, so, the more time you apply Vcd, the higher insulation resistance will become.

It seems you have a hot spot inside the xformer, so ask for technical assistance in order to check resistance of the windings, transformer ratio, power factor, partial discharge test and DGA to confirm the diagnosis.

Regards.