From the point of view of molecular arrangements, liquids can be described as 'highly compressed gases ' in which the molecules are very closely arranged. This is known as kinetic model of the liquid structure. However, the movement of charged particles, their microscopic streams and interface conditions with other materials cause a distortion in the otherwise undisturbed molecular structure of the liquids. The well known terminology describing the breakdown mechanisms in gaseous dielectrics, such as, impact ionization, mean free path, electron drift etc. is, therefore, also applicable for liquid dielectrics.

Liquid dielectrics are accordingly classified in between the two states of matter, that is gaseous and solid insulating materials. This intermediate position of liquid dielectrics is also characterized by its wide range of application in power apparatus. Insulating oils are used in power and instrument transformers, power cables, circuit breakers, power capacitors etc. Here they perform a number of functions simultaneously, namely:

• insulation between the parts carrying voltage and the grounded container, as in transformers.

• impregnation of insulation provided in thin layers of paper or other materials, as in Transformers, cables and capacitors, where oils or impregnating compounds are used.

• cooling action by convection in transformers and oil filled cables through circulation.

• filling up of the voids to form an electrically stronger integral part of a composite dielectric.

• arc extinction in oil circuit breakers.

• high capacitance provided by liquid dielectrics with high permittivity to power capacitors.

A large number of natural and synthetic liquids are available which can be used as dielectrics. These possess a very high electric strength and their viscosity and permittivity vary in a wide range. The appropriate application of a liquid dielectric in an apparatus is determined by its physical, chemical and electrical properties on one hand, and on the other, it depends upon the requirements of the functions to be performed.

Dielectric materials can be divided into two broad classifications: organic and inorganic. Organic dielectrics are basically chemical compounds containing carbon. Earlier, under organic chemistry only those compounds which were derived from either plant or animal organism were considered. This concept underwent changes with concurrent developments in chemical technology. Carbon compounds, in general, are now called organics. Among the main natural insulating materials of this type are: petroleum products and mineral oils. The most important and widely used organic liquid dielectrics for electrical power equipment are mineral oils. The other natural organic insulating materials are asphalt, vegetable oils, wax, natural resins, wood and fibre plants (fibrins).

A large number. of synthetic organic insulating materials are also produced. These are nothing but substituents of hydrocarbons in gaseous or liquid forms. In gaseous forms are fluorinated and chlorinated carbon compounds. Their liquid forms are chlorinated diphenyles, besides some nonchlorinated synthetic hydrocarbons. The chlorodiphenyles, although possessing some special properties, are not widely used because of being unfriendly to human beings and very costly.

Among halogenfree synthetic oils are polymerization products, the polyisobutylenes and the siliconpolycondensates. Polyisobutylene offers, better dielectric and thermal properties than mineral oils for its application in power cables and capacitors, but it is many times more expensive. Silicon oils are top grade, halogenfree synthetic insulating liquids. They have excellent stable properties, but because of being costly, have so far found limited application in power apparatus for special purpose.

Among inorganic liquid insulating materials, highly purified water, liquid nitrogen, oxygen, argon, sulphurhexafluoride, and helium etc. have been investigated for possible use as dielectrics. Liquified gases, having high electric strength, are more frequently used in cryogenic applications, whereas, because of its high relative permittivity, low cost, easy handling and disposal, Water and water mixtures are being actively investigated for use as dielectrics (for example, in pulse power capacitors and pulsed power modulators etc. ).

To summarize the classification of liquid dielectrics explained above, a schematic is drawn in the following diagram. The insulating liquids, commonly applied in high voltage apparatus, are classified as illustrated in the following diagram.