which type of oil is used as a transformer oil?

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which type of oil is used as a transformer oil?..

Answer / ankamsreekar

Transformer oil, or insulating 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.

Well into the 1970s, polychlorinated biphenyls (PCB)s were
often used as a dielectric fluid since they are not
flammable. They are toxic, and under incomplete combustion,
can form highly toxic products such as furan. Starting in
the early 1970s, concerns about the toxicity of PCBs have
led to their banning in many countries.

Today, non-toxic, stable silicon-based or fluorinated
hydrocarbons are used, where the added expense of a
fire-resistant liquid offsets additional building cost for a
transformer vault. Combustion-resistant vegetable oil-based
dielectric coolants and synthetic pentaerythritol tetra
fatty acid (C7, C8) esters are also becoming increasingly
common as alternatives to naphthenic mineral oil. Esters are
non-toxic to aquatic life, readily biodegradable, and have a
lower volatility and a higher flash points than mineral oil.