Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled , some types of , fluorescent lamp , and some types of high voltage and . Its functions are to , suppress and , and to serve as a .
The oil helps cool the transformer. Because it also provides part of the between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external through which the oil circulates by natural . Very large or high-power transformers (with capacities of thousands of ) may also have , oil pumps, and even oil-to-water .
Large, high voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a , or both to ensure that the transformer is completely free of before the cooling oil is introduced. This helps prevent formation and subsequent under load.
Oil filled transformers with a conservator may have a gas detector relay (). These safety devices detect the buildup of gas inside the transformer due to discharge, overheating, or an internal . On a slow accumulation of gas, or rapid pressure rise, these devices can trip a protective to remove power from the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.
The (min) and (max) are 140 °C and −6 °C respectively. The of new untreated oil is 12 MV/m (RMS) and after treatment it should be >24 MV/m (RMS).
Large transformers for indoor use must either be of the dry type, that is, containing no liquid, or use a less-flammable liquid.
Recently, research is underway in making transformer oil by mixing insulating or semiconducting particles with transformer oil to enhance its thermal conductivity and electrical breakdown strength.