Mineral or synthetic oil in power transformers acts as both an electrical insulator and a cooling medium. The oil fills voids around windings, increases dielectric strength, and allows high voltage clearances to be reduced compared to air. It also transfers heat from the windings and core to the tank and radiators, where it is dissipated to ambient air or water. The combination of good dielectric properties, thermal capacity and circulation makes oil a very effective medium for high voltage, high MVA transformers.