Transformers are filled with oil for a number of reasons, most important of which is insulation. Additionally, the oil is used as a coolant and prevents arcing, the electrical breakdown of gases accompanied by the discharge and resulting ionization known as corona. Transformer oil is not just for transformers; it is also used in fluorescent light bulbs, capacitors and high-voltage switches.
Back in the 1970s, transformers mounted indoors used polychlorinated biphenyl, or PCB liquid, for cooling purposes. It comprises several chlorine atoms bonded with benzine rings, the latter of which is a suspected carcinogen. Large pieces of equipment still used PCBs until December 2000. It made for an ideal cooling agent in enclosed transformers due to its high boiling point, effective insulation properties and chemical stability. According to the Environmental Protection Agency, PCBs were banned in the United States in 1979.
Modern Transformer Oil
Today's transformer oil is ASTM D3487 standard mineral oil. There are two types of oil used: Type I and Type II. Type I oil is used in equipment that does not require much oxidation resistance; Type II oil offers greater protection against oxidation.
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Type II Mineral Oil Standards
According to the American Society for Testing and Materials, Type II oils can have no more than 0.3 percent oxidation inhibitors. Their pour points cannot be higher than -40 degrees Fahrenheit, and they cannot have aniline points lower than 76 degrees Celsius. The minimum flash point, or the temperature at which a liquid can vaporize into a combustible form, is 294.99 degrees Fahrenheit. It must have a dielectric strength of at least 29.9 KVA.
Type I Mineral Oil Standards
Type I oil is similar across many measures as Type II oil. The biggest difference is in oxidation inhibitor content. Type I oil cannot have more than 0.08 percent of the inhibiting substance, whereas Type II oils can have up to 0.3 percent. Type I oil can have up to 0.3 percent sludge by mass while Type II oil can only have up to 0.2 percent.