Why is synthetic ester oil used in transformers instead of mineral oil?

Synthetic esters are chosen for improved fire safety due to higher fire points, and for environmental benefits such as better biodegradability compared with mineral oil. They also can tolerate higher moisture in the oil without losing dielectric margin as quickly, which can reduce paper aging rate in some operating profiles. The tradeoffs are higher fluid cost, different viscosity behavior at low temperatures depending on product, and the need to confirm compatibility with gaskets, paints, and retrofill procedures. Utilities select them where risk and environmental constraints justify the cost.