What is oil filled self cooled type transformer?

Transformers like other electrical equipment, produce heat. The heat has to be removed to keep the temperatures below maximum levels. There are various methods to do this. Using water is a efficient method; but cannot be done directly as water can be conductive. In the 'Oil immersed water cooled method', the entire transofmer is immersed in a tank filled with insulating mineral oil. Heat from the windings and core is transfered to the insulating oil and therefore heats up the oil. Water pipes run along the tank, in contact with the oil. Heat from the oil is now transfered to the water. This is a very efficient method of cooling especially when the voltages involved are high.