What type of oil is used in transformer cooling and insulation?

The most common is mineral insulating oil specified for transformers, but natural ester, synthetic ester, and silicone oils are also used depending on fire safety and environmental needs. The selected oil must provide dielectric strength, low moisture content, stable chemistry, and suitable viscosity for the cooling design. Oil choice affects temperature rise, aging rate, and safety classification. Compliance with relevant IEC or ASTM specifications and verified batch testing is essential to ensure reliable insulation and cooling performance in service.