LIQUIDS AS INSULATORS

Liquid dielectrics, because of their inherent properties, appear as though they would
be more useful as insulating materials than either solids or gases. This is because both
liquids and solids are usually 103 times denser than gases and hence, from Paschen's
law it should follow that they possess much higher dielectric strength of the order of
107 V/cm. Also, liquids, like gases, fill the complete volume to be insulated and
simultaneously will dissipate heat by convection. Oil is about 10 times more efficient
than air or nitrogen in its heat transfer capability when used in transformers. Although
liquids are expected to give very high dielectric strength of the order of 10 MV/cm,
in actual practice the strengths obtained are only of the order of 100 kV/cm.
Liquid dielectrics are used mainly as impregnants in high voltage cables and
capacitors, and for filling up of transformers, circuit breakers etc. Liquid dielectrics
also act as heat transfer agents in transformers and as arc quenching media in circuit
breakers. Petroleum oils (Transformer oil) are the most commonly used liquid
dielectrics. Synthetic hydrocarbons and halogenated hydrocarbons are also used for
certain applications. For very high temperature application, silicone oils and
fluorinated hydrocarbons are also employed. In recent times, certain vegetable oils
and esters are also being tried. However, it may be mentioned that some of the isomers
of poly-chlorinated diphenyls (generally called askerels) have been found to be very
toxic and poisonous, and hence, their use has been almost stopped. In recent years, a
synthetic ester fluid with the trade name 'Midel' has been developed as a replacement
for askerels.
Liquid dielectrics normally are mixtures of hydrocarbons and are weakly
polarised. When used for electrical insulation purposes they should be free from
moisture, products of oxidation and other contaminants. The most important factor
that affects the electrical strength of an insulating oil is the presence of water in the
form of fine droplets suspended in the oil. The presence of even 0.01% water in
transformer oil reduces its electrical strength to 20% of the dry oil value. The
dielectric strength of oil reduces more sharply, if it contains fibrous impurities in
addition to water.