Which type of oil is commonly used in power transformers?

The most common oil in power transformers is mineral insulating oil, selected for its balance of dielectric strength, cooling performance, availability, and cost. Many utilities specify mineral oil meeting IEC 60296 or an equivalent ASTM standard, with inhibited or uninhibited formulations depending on asset strategy. In locations with high fire safety requirements or environmental sensitivity, natural ester, synthetic ester, or silicone fluids may be specified. The correct choice must match transformer design approvals and maintenance practices.