One type of transformer is an oil-immersed transformer. Transformers often operate in high-energy, high-heat situations. An oil-immersed transformer suspends the transformer of a steel tank filled with oil. The oil cools and insulates the transformer. The device uses convection to move the oil around and through the transformer, cooling it off.
To avoid oil deterioration, transformer oil must be kept at an operating temperature of less than 85°C. For the transformer to run correctly and to prevent excessive oil deterioration, the daily average operating temperature should be around 30°C.