What type of oil is used in electric transformer insulation systems?

Most electric transformers use mineral insulating oil, especially for conventional outdoor distribution and power transformers due to cost and proven performance. Increasingly, natural ester fluids and synthetic esters are used where fire safety and environmental performance are priorities. Silicone fluids are used in special fire safe applications. The chosen oil must meet the unit’s design approvals, dielectric requirements, thermal limits, materials compatibility, and regulatory constraints. Utilities also consider climate, maintenance strategy, and containment requirements when selecting the insulation liquid.