Mineral Oil
Mineral oil surrounding a transformer core-coil assembly
enhances the dielectric strength of the winding and prevents oxidation of the
core. Dielectric improvement occurs because oil has a greater electrical
withstand than air and because the dielectric constant of oil (2.2) is closer
to that of the insulation.
As a result, the stress on the insulation is lessened when
oil replaces air in a dielectric system. Oil also picks up heat while it is in
contact with the conductors and carries the heat out to the tank surface by
selfconvection. Thus a transformer immersed in oil can have smaller electrical
clearances and smaller conductors for the same voltage and kVA ratings.
Askarels
Beginning about 1932, a class of liquids called askarels or
polychlorinated biphenyls (PCB) was used as a substitute for mineral oil where
flammability was a major concern. Askarel-filled transformers could be placed
inside or next to a building where only dry types were used previously.
Although these coolants were considered nonflammable, as
used in electrical equipment they could decompose when exposed to electric arcs
or fires to form hydrochloric acid and toxic furans and dioxins. The compounds
were further undesirable because of their persistence in the environment and
their ability to accumulate in higher animals, including humans.
Testing by the U.S. Environmental Protection Agency has shown
that PCBs can cause cancer in animals and cause other noncancer health effects.
Studies in humans provide supportive evidence for potential carcinogenic and
noncarcinogenic effects of PCBs.
The use of askarels in new transformers was outlawed in 1977
(Claiborne, 1999). Work still continues to retire and properly dispose of
transformers containing askarels or askarel-contaminated mineral oil. Current
ANSI/IEEE standards require transformer manufacturers to state on the nameplate
that new equipment left the factory with less than 2 ppm PCBs in the oil (IEEE,
2000).
High-Temperature Hydrocarbons
Among the coolants used to take the place of askarels in
distribution transformers are high-temperature hydrocarbons (HTHC), also called
high-molecular-weight hydrocarbons. These coolants are classified by the
National Electric Code as “less flammable” if they have a fire point above
300˚C.
The disadvantages of HTHCs include increased cost and a
diminished cooling capacity from the higher viscosity that accompanies the
higher molecular weight.
Silicones
Another coolant that meets the National Electric Code
requirements for a less-flammable liquid is a silicone, chemically known as
polydimethylsiloxane. Silicones are only occasionally used because they exhibit
biological persistence if spilled and are more expensive than mineral oil or
HTHCs.
Halogenated Fluids
Mixtures of tetrachloroethane and mineral oil were tried as
an oil substitute for a few years. This and other chlorine-based compounds are
no longer used because of a lack of biodegradability, the tendency to produce
toxic by-products, and possible effects on the Earth’s ozone layer.
Esters
Synthetic esters are being used in Europe, where
high-temperature capability and biodegradability are most important and their
high cost can be justified, for example, in traction (railroad) transformers.
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