What type of oil is used in distribution transformer oil systems?

Distribution transformers most commonly use mineral insulating oil, either inhibited or uninhibited, due to its cost effectiveness, good dielectric strength, and thermal performance. In environmentally sensitive or urban areas, natural ester or synthetic ester fluids are increasingly adopted because of higher fire point and biodegradability. The selected oil must be compatible with transformer design, sealing system, and local standards. Oil choice also considers ambient temperature, loading profile, fire safety regulations, and maintenance philosophy of the utility or distribution network operator.