Air trapped in Transformer

May be some air is trapped inside the tank. That is coming out through bucholz. Please do a vaccum oil filtration and you can solve it

You have not mentioned if the transformer has a silica gell breather or there is a nitrgen gas bag refil. If the Breather is not clogged there is no chance of air getting trapped inside the transformer.

Most probably Fult(s) inside the transformer generate the gas which is being accumulated in the relay.

Inspect the oil sample:

cloudiness in oil may be due to suspended moisture or suspended solid matter such as iron oxide or sludge, dark brown color indicates the presence of asphaltines, green color indicates presence of copper soaps, an acrid smell often indicates the presence of volatile acids which can cause corosion, a petrol like odour may indicate a low flash point.

If the color of the gas in the relay is brownish suspect decomposition of paper insulation, if it is grey there can be fault in the core laminations.

Essentially Carry Out: Oil Dielectric strength test, Meggar Test and polarization index test on the Transformer to confirm the health of the transformer.

(keeping in vew the intensity of the problem described by you I would suggest that the transformer should be Tested without further dealy)

@mountk2

-It is silica gell breather but how to check that it is clogged or not?

-I have also said that DGA test shows that only Nitrogen and Oxygen is more than normal. No eceytelen is there

I beg to disagree with Mountk2. It is unlikely that there is a fault in the transformer if the dissolved gas analysis is showing no unusual levels of combustible gases, but mainly N2 and O2. A fault would probably show considerable levels of acetylene, ethylene, etc.

Realize that when you drained off ~300 liters of oil, that space was filled by air. When you replaced that oil from the conservator, there are still areas at the top of the main tank where air bubbles can collect, and these would gradually migrate upwards, to collect in the Buchholz relay. The amount of gas collecting in the Buchholz could be measured in cu. cm, as compared to the amount of air that was in the tank and what could still be there. Unfortunately, it takes time for it to work its way up.

The first reply (vacuum filtration) would be the way to fill most of the tank, but it may be difficult to pull a vacuum on the transformer to get out a few liters or less of air, without drawing oil into the vacuum pump. You may just have to give it a few days for the last air to work its way up to the Buchholz and vent it periodically.

@PeterT

Suppose the had been remained in tank when we refilled the oil. But as I said before, the oil level was somewhat less than before. So there is little confusion is there. If air was there the level should be same or more than before.

Dear PeterT: I cannot totally disagree with your post and very much respect your openion but perhaps you did not take note of the fact that claim has been made for TRIPPING of transformer every 5~6 hours on Bucholz.

A standard Bucholz relay should have a float to activate an alarm only on slow gas/air accumulation while Tripping is activated by a seperate flap operated switch caused by considerable associated turbulance.

Although it is a remote possibility still I do not rule out wrong connections of gas actuated float switch but that can be checked the OP, if all is well with the wiring than in my openion any trapped dry air patches and slow accumulation of gas in bucholz simply cannot be the real issue.

As for my suggestion for the transformer tests, to the best of my knowledge those should have been carried out before putting back the unit in service after an overhaul as was carried out by the OP.