Published: Jan 1994
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Dielectric liquids have served as insulators or dielectrics in electrical equipment for over one hundred years already. Equally long in duration is the history of research on the electrical conduction of these liquids. The results are not too impressive if we compare the state of knowledge with that which has been achieved for gases or solids. This situation is due to the fact that in most applications of insulating liquids in electrotechnology, mixtures of liquids are being used which in addition are in contact with a variety of materials. Transformer oil contains well over one hundred chemical compounds. In a transformer it is in contact with organic and inorganic materials. Traces of these materials will leak into the liquid, and it is often that the presence of these traces determines the overall properties of the liquid insulation in the particular apparatus. This complex situation impeded the progress in the understanding of the basic conduction mechanisms in dielectric liquids because in fundamental studies it was attempted to incorporate as many conditions as possible under which actual liquid insulations were employed in industry. The result is an enormous amount of empirical information which is difficult to understand or to put into a proper perspective. Many books or reviews published during the last fifty years reflect this situation [1-7]. This lack of understanding of the fundamental properties of liquid insulations may be one of the reasons for their gradual replacement in electrotechnical apparatus. Large power transformers, railway transformers, power capacitors, and paper/oil insulated high voltage cables are nowadays the major electrotechnical devices in which mineral oil and other liquids are used as insulants and coolants. Judging from the developments in gaseous and solid state electronics, it can be predicted that a better understanding of the basic conduction processes in liquids will also lead to new applications.
Hahn-Meitner-Institut Berlin, Abteilung Strahlenchemie, Berlin 39,