Which oil is filled in transformer insulation systems?

Most transformer insulation systems use mineral insulating oil, selected for its dielectric strength, thermal conductivity, availability, and cost. In applications requiring higher fire safety or environmental protection, natural ester, synthetic ester, or silicone oils are used. The oil must be compatible with cellulose insulation, gaskets, and metals, and meet relevant IEC or ASTM standards. Selection depends on voltage level, location, fire risk, environmental sensitivity, and lifecycle cost considerations defined in transformer specifications.