In 400 MVA transformer we get every night first stage alarm at , we noticed that the alarm comes when the ambient temperature is low and unit load is low. we susbect air ingress during low temperature time when both oil and air temperature are at thier lowest value.

we performed DGA for the oil and the result was as follow

Gas                 ppm
 H2                    82
 O2                  5980
 N2                  91848
CO2                    8199
C2H4 Ethylene           38
C2H6 Ethane             185

C2H2  Acetylene          0

C3H8 & C3H6
Propane & Propene     183

CH4 Methane            149

CO Carbon Monoxide      1337

we are searching for any source of air ingress through leaking gaskets on the oil pipes and coolers.

Any feed back experience please. any available procedure.

Low conservator oil level can cause this problem if the level drops so far that the Buchholz relay becomes empty. Thermal expansion can cause the level change, especially on a load which cycles.

DGA levels are of far greater use when considered as a trend than as a spot value. That said, some of the values are unusually high - methane, ethane, hydrogen certainly are. What does the trend information tell you? anything rising rapidly? When was the oil last processed?/Chemical pumps

Trust the conservator -breather is an open breathing type and not with air cell.Some of the occasions that  I noticed breather pipe blocking were 1) Compacting of gel in breather and thereby air blocking 2) Insects building up in oil cup inlet 3) Simple errors in erection-closing the pipe with a full  dia gasket at one of the pipe joints etc.

I dont think the problem is due to air sucking by oil pump.If so it should have happened always.

As you may be knowing transformer oil can contain air  up to 8-10 % of its volume.For every 10 C fall in oil temperature,oil volume shrinks by 0.8%.So if breather pipe is closed, during temperature drop,a vacuum will be created over the oil level in conservator which in turn allows the dissolved air in oil  to escape out as bubbles.

To your question, the situation will not cause a trip.But it is not a good condition to  allow to continue.Bubbling can happen between two winding discs which can trigger a voltgae breakdown. I have seen such failures.

Best way to check whether breathing is happening freely or not is to  watch the oil cup during a temperature change.You must find air bubbling in or out through the oil seal.If it is not happening,check for air blocking  anywhere in pipe by disassembling and blowing air.

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