An oil-immersed transformer, also called Oil filled transformer, is a kind of voltage transformation device utilizing the oil cooling method to reduce the transformer temperature. Unlike the dry type transformer, the body of the Oil-immersed transformer is installed in the welded steel oil tank filled with insulation oil. When an Oil-immersed transformer in operation, the heat of the coil and iron core firstly is transformed into the insulation oil and then to the cooling medium. And according to the capacity sizes, it can be divided into an immersed natural cooling transformer and an immersed forced air cooling transformer.