What types of oils are used as transformer cooling oil?

Transformer cooling oils include mineral insulating oils, natural esters, synthetic esters, and silicone fluids. Mineral oil is most common for cost and performance. Ester oils provide higher fire point and biodegradability, often preferred in sensitive locations. Silicone oils offer strong fire safety and thermal stability in special applications. The choice depends on voltage, fire risk, environmental requirements, and compatibility with insulation materials and standards.