Transformer oil

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 transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant. 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 transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant. Transformer oil is most often based on mineral oil, but alternative formulations with better engineering or environmental properties are growing in popularity. Transformer oil's primary functions are to insulate and cool a transformer. It must therefore have high dielectric strength, thermal conductivity, and chemical stability, and must keep these properties when held at high temperatures for extended periods. Typical specifications are: flash point 140 °C or greater, pour point −30 °C or lower, dielectric breakdown voltage 28 kV (RMS) or greater. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Power transformers with capacities of thousands of kVA may also have cooling fans, oil pumps, and even oil-to-water heat exchangers. Power transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the insulating oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load. Oil filled transformers with a conservator (oil reservoir) may have a gas detector relay (Buchholz relay). These safety devices detect the buildup of gas inside the transformer due to corona discharge, overheating, or an internal electric arc. On a slow accumulation of gas, or rapid pressure rise, these devices can trip a protective circuit breaker 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. Mineral oil is generally effective as a transformer oil, but it has some serious disadvantages, of which the worst is its high flammability. If a transformer leaks mineral oil, it can easily start a fire. Fire codes often require that transformers inside buildings use a less flammable liquid, or the use of dry-type transformers with no liquid at all. Mineral oil is also an environmental contaminant, and its insulating properties are rapidly degraded by even small amounts of water. Pentaerythritol tetra fatty acid natural and synthetic esters have emerged as an increasingly common mineral oil alternative, especially in high-fire-risk applications such as indoors or offshore, due to their low volatility and high fire point, which can be over 300 °C. They also have a lower pour point, greater moisture tolerance, and improved function at high temperatures, and they are non-toxic and readily biodegradable. Silicone or fluorocarbon-based oils, which are even less flammable, are also used, but they are more expensive than esters, and less biodegradable. Researchers are experimenting with vegetable-based formulations, using coconut oil for instance. As yet these are unsuitable for use in cold climates or for voltages over 230 kV. Researchers are also investigating nanofluids for transformer use; these would be used as additives to improve the stability and thermal and electrical properties of the oil.