First... I wish a happy Christmas...
There is a problem that intrigues me.
There is a lot power transformers (12,5 to 25 kV <---> 380 V, from 50 to 300 kVA) that works like that: 4 to 5 months working and the rest of the year is stopped.
When the power transformers will be starting to working again, after the stooped time, some of transformers got broken in the time that they are energized. In two situation that I was there, I could hear the oil inside of it bubbling and after it the fuse of high voltage side opened. In other situation it work for 10 to 15 minutes and it broken.
What could be done to avoid it?
Experience guys over here, said that this happen because the oil is stopped in the lower part of the transformer then it lower the isolation and the problem happen. It make sense to me. If it´s the reason then before the restart of the transformer, the oil should be moved inside of it. Comments? Experience?