Oil-immersed transformers,oil is the primary insulation material，which also use oil as a cooling medium for forced oil circulation, oil-immersed air cooling, oil-immersed water cooling, and oil-immersed self-cooling. Iron core, winding, fuel tank, oil pillow, breathing device, explosion-proof tube (pressure relief valve), radiator, insulation sleeve, tap changer, gas relay, thermometer, oil purifier, and so forth are the essential parts of the transformer.