Why is mineral oil transformer fluid widely used in power transformers?

Mineral oil remains widely used because it offers a strong balance of dielectric performance, cooling capability, availability, and cost. It has good insulating strength when dry and clean, low viscosity for natural convection cooling, and a long history of field performance with well understood maintenance practices. Standardization is mature, with common specifications such as IEC 60296 defining key properties and acceptance criteria. Utilities also have established infrastructure for sampling, testing, filtration, and regeneration of mineral oil, making lifecycle management predictable and economical across large transformer fleets.