Transformer gas analysis 1

[eeprom]

I have some transformer gas results which show large increases in H2 (>300%), CO(15%), CH4(200%), C2H6(120%), C2H4(30%). Tests were taken 3 months apart. There is not enough motor load on this transformer to overload the transformer. The transformer is 5MVA. The largest single load is a fan of about 1.25 MVA. Between take of oil test 1 and oil test 2, the large fan was balanced. And in doing so they had some problems, so the fan was allowed to start many times in the course of an afternoon. I don't know how many times, but I'm sure they didn't stick with the 2 starts per hour rule. The fan takes close to 40 seconds to start. Of course the starting current is some 600% of the normal current. I don't know if any other loads were active when the fan was being balanced.

Does anyone know if multiple starts of this large fan would cause enough internal heating to result in such a large change in gases?

thanks
EE

 

[prc]

The term no-load tap changer is ( was) popular in US and Japan. In Japan they were calling it NRA ( no-load ratio adjuster).In field, many times accidents happened due to this misleading term.So people started calling it Off-circuit tap -changer(OCTC).Then engineers thought it better to term it De-energised Tap-changer(DETC)making it clear that this shall be operated only in de-energised condition. IEC on transformer 60076-1(2011)use the term DETC.

Hydrogen is produced in oil from low energy faults like partial discharges in paper insulation or corona discharges from live electrodes. Such discharges breakdown the C-H bonds of oil by ionisation reaction,releasing out hydrogen.In the reported case only H2 has gone up substantially and faulty gases( methane,ethane and ethylene) has increased modestly.So I cocluded there is no local heating. There is no increase CO, ruling out any chance of heating,involving paper insulation.

H2 can also be released in transformers due to rusting or other chemical reactions involving steel( esp stainless steel or Zinc coating)alkyde paints and overheated oil films in between core laminations.Interestingly this H2 can continue to increase even without energising the transformer.This is called stray gassing in oil.IEC 60599(2007) DGA caluse 4.3. More can be learnt on this by IEEE papers of TV Oommen( ABB USA).In the reported case, chances for this is little as the increase started only recently.

 

[eeprom]

PRC,
Thank you for the detailed explanation. But I don't agree with your assessment that there is no localized heating based on the fact that the volatile gases have gone up modestly. Methane has doubled, and ethane has nearly doubled. Ehtylene and acetylene have been stable. I see this is as moderate temperature heating. Any kind of significant arcing would likely produce more temperature, thereby releasing the higher energy gases.

Gas analysis is definitely more an art than a science.

 

[ScottyUK]

'De-energised tap changer' makes a lot of sense, certainly removes any ambiguity. I hadn't met the DETC acronym before - thanks gents.


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

 

[stevenal]

I reviewed C57.104, and see that every method of interpretation leads to partial discharge and not to heating. Whether artist or scientist, prc has it right.

 

[DTR2011]

Then why not move on to determining where the problem is. There are several ways of locating PD in a transformer.

I am guessing that this is a smaller unit, so connecting to the bushings would not be practical. There are several acoustical methods available which can use triangulation to help identify where the problem is.