Mineral oil is the most commonly
used insulating liquid for transformer
applications, and an extensive database
of knowledge has been accumulated
over the years. Increased environmental,
safety, performance and economic
have led to the consideration of other
oils for use as transformer insulating fluids.
Vegetable oils or esters have been used
as dielectric liquids since the invention
of oil-filled transformers in the late 1880s.
The earliest natural esters were found
to be incompatible with free breathing
equipment, because of their oxidation
characteristics, and were gradual l y
replaced by mineral oils [1].