Leakage From Transformer Secondary Bushing

sir,

Bushing level is 10'' lower to oil level, i mean bushing is in oil level

Okay, how about the other two questions?

I wouldn't advise that. Oil level may go below the windings and might cause an insulation breakdown or even a fire.

I don't know to whom you are responding. The best solution is probably to power down the transformer and repair/replace the leaking bushing. But in the meantime, it is logical to keep the oil topped up, with frequent monitoring.

For low oil level in transformer/bushing, if you have a thermograph, do thermal image scans of the transformer radiators while it is in operation. The mages will show dim for region without oil and brighter image is the areas with oil.

Perform a temperature rise test on the transformer with an infrared camera. The surface temperature of the tank must not be more than 20°C higher than the top oil temperature of the transformer.

Infrared Temperature Criteria-NASA

This may assist your decision on the level of maintenance on the transformer.

As is rightly advised the best thing is to take a shut down and rectify the leakage to avoid risk to the transformer.But then the risk does depend upon the extent of the leakage.The essential condition is that the oil level must be all the time visible on the gauge fixed to the conservator tank of the transformer. If the leakage is very small, you may continue to run the transformer by keeping a constant watch on the conservator level gauge.It would not be advisable to run the transformer once the level disappears from the conservator.