The oil can only be filled in the transformer which satisfies the standard specifications mentioned by the manufacturer of transformer. Normally Transformer is supplied with gas filled i.e. the Main Tank of Transformer is filled with Nitrogen gas under some specified pressure to prevent any degradation of the insulation of core and winding. But we cannot keep Transformer Main Tank filled with nitrogen for a period more than three months and hence oil filling becomes important. Oil filling in Transformer also becomes important if we want to store the Transformer for more than six months.
The oil filling is done in the main tank under vacuum. The large Transformers are generally designed to withstand the full vacuum for long periods. Oil filling and filtration of oil is carried out simultaneously. Oil sampling is done during the process of oil filtration and if sample result for Dielectric Strength and moisture comes under limit then filtration of oil is stopped. Figure below shows Transformer Oil Filtration unit.
During oil filling in the main tank of the transformer, it is preferable to connect the inlet hose pipe at the bottom of the tank. After filling the oil up to the top of the transformer core, maximum suitable vacuum is maintained above the oil level in the tank to minimize the hazard of bubbles lodging in the inaccessible corners of winding.
It should be noted that condenser bushing, diaphragm, Buchholz relay, tap changer board, conservator tank, radiators units etc are not designed for withstanding vacuum hence they should not be subjected to vacuum. That means vacuum processes only to be done in main tank without connecting or with blocking all the above-mentioned components of the transformer.
You may like to read
If we not doing vacuuming then problem is that, it will create bubble in oil which is very dangerous for Transformer operation at high voltage. If there is bubble in oil then at the time of application of high voltage it offers less dielectric strength than oil. It results in partial breakdown of an insulating material which further result in breakdown of whole oil insulation.