The Transformer oil helps cool the transformer. Because it also gives part of the electrical insulation among internal live parts, transformer oil must remain stable at high temperatures over an extended period. To better cooling of large power transformers, the oil-filled tank may have external radiators by which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of KVAs) might also have cooling fans, oil pumps, and even oil-to-water heat exchangers.