Elementary processes in the development of the electrical breakdown of liquids (a review)

The investigation of electric breakdown phenomena in liquids has been a continuing effort since the early years of this century. An immense amount of information (both published in journals and books and unpublished, resting in the archives of industry or research laboratories) has been accumulated — but our understanding of the phenomenon remains rudimentary when compared to our state of knowledge with respect to the breakdown of gases or solids. While their theoretical concepts have been developed which incorporate the physical elementary processes of charge carrier generation and transport (e.g. for gases the Towns-end and streamer theory, and for solids the Landau-Zener theory and the electrothermal breakdown theory) a similar high level status of theoretical penetration is missing for liquids. The main reasons for this poor performance are the following: a) the understanding of the physical properties of the liquid state is less well developed as compared to gases and solids; b) the experimental information on liquid breakdown accumulated in the literature to a large extent does not lend itself suitable for generalization or as a starting point for theoretical considerations. Hundreds of papers and reports are, for instance, available on transformer oil which actually is an illdefined heterogeneous system comprised of different liquids, solid materials, and gases. From the measurements point of view many papers lack sufficiently well controlled experimental conditions so that the data cannot be used as points of reference for theories. c) the number of liquids of possible interest to electrical engineering applications is in the range of ten to hundred and these liquids exhibit a wide spectrum of physical and chemical properties. Time and space preclude an extensive treatment of all the various aspects of the breakdown phenomenon. Some topics omitted here may be found in other talks of this symposium or in some recent reviews of this field [1-3]. In this review we shall limit ourselves mainly to pure non-polar liquids and we shall consider only two extreme cases of voltage application: step-impulse voltages and voltages which vary slowly in time (DC, line frequency AC).