Transformer oil

Transformer oil is usually a highly-refined mineral 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 and arcing, and to serve as a coolant.

Explanation

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

Large, high-voltage 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 cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.

Oil filled transformers with conservators (an oil tank above the transformer) tend to be equipped with Buchholz relays. These are safety devices that can sense gas buildup inside the transformer (a side effect of corona or an electric arc inside the windings) and then switch off the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.

The flash point(min) and pour point(max) are 140 degree Celsius and -6 degree Celsius respectively. The dielectric strength of new untreated oil is 30kV(rms) and after treatment it should be 60kV(rms).

Oil alternatives

Large transformers to be used indoors must use a less-flammable liquid or be Dry Type, i.e., having no fluid.

Prior to about 1970, polychlorinated biphenyl (PCB) was often used as a dielectric fluid since it was not flammable. However, they are toxic, and, under incomplete combustion, PCBs can form highly toxic products, furans, etc. Due to the stability of PCB and its environmental accumulation, it has not been permitted in new equipment since late 1960's in the United States.

Today, nontoxic, stable silicone-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Natural or synthetic esters are becoming increasingly common as alternative, to naphthenic mineral oil too. Esters are non toxic, readily biodegradable, and have higher flash points than mineral oil.