What type of oil is in a transformer and why is it selected?

Common oils include mineral transformer oil, natural ester, synthetic ester, and silicone fluid. Mineral oil is selected for proven dielectric and cooling performance and cost effectiveness. Ester fluids are selected for higher fire safety and biodegradability, and silicone for very high fire resistance in special applications. Selection depends on voltage rating, cooling design, ambient temperature range, fire and environmental constraints, compatibility with insulation materials, and utility specifications. The supplier must provide compliance documentation and test results to confirm properties such as BDV, moisture, acidity, and flash point.