When the water content in the oil in the Oil rises up to or more than 2% than it is needed to replace the transformer oil or filter it.
Transformer oil is pure insulating oil.
That depends on the model of the transformer. I have seen transformers that required 7500 liters and some that needed 2,5 liters.
to cool a winding of transformer and it is a insulating material and transformer oil is a purifing oil
Nytro libra is an uninhibited transformer oil
An oil transformer is filled with PCB oil to better transfer heat from the windings to the outside case (and into the air).
To cool and to insulate the transformer. Transformer oil has a higher specific heat capacity than air, so it is more efficient at removing energy through heat transfer, and limiting the windiings' temperature rise. Transformer oil has a much greater dielectric strength than air, which means that components at different potentials can be placed closer together, so the overall volume of the transformer can be made smaller, giving it a smaller 'footprint'.
transformer consists of polychlorinated biphenyls,...etc
It is better to check the BDV (Break down voltage) of the oil to decide whether the said oil is usable for 11KV application, instead of depending upon its "expiry" date.
As well as providing a cooling medium, transformer oil is also used to insulate a transformer. As its dielectric strength (how well it insulates) is much higher than air, it means that components within a transformer that are at different potentials can be placed much closer together if they are immersed in oil. So, operating an oil-filled transformer without oil will likely cause insulation breakdown within the transformer.
convection currents
minarel or power oil
Its "Mineral Oil".