D-C breakdown strength of air and of Freon in a uniform field at high pressures

Abstract

D-c breakdown studies of air in a uniform field have been extended to 1,000 kv. D-c studies of Freon (CCl <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) have been extended to over 350 kv and to 135 pounds per square inch absolute. The departure from Paschen's law at the higher values of pressure times gap is noted. The mechanisms which account for the higher insulating strength of such gaseous compounds as CCl <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> are outlined and their possibilities and limitations in the insulation of high-voltage apparatus are briefly discussed. Introduction It has long been recognized <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> that the insulating strength of air and other gases in a uniform field increases with the pressure. The classical analysis of the voltage-breakdown mechanism and its dependence on pressure and other properties of the gas was made by Townsend in 1903. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> This explanation was based on the ionization of the interelectrode gas by both electron and positive-ion collision — a process which Townsend showed would become cumulative when the positive ions produced by the electrons moving toward the anode under the influence of the field become capable, by their ionizing collisions, of producing an even greater number of new electrons. Since that time, Townsend's second coefficient β has been reinterpreted to include various other mechanisms which contribute regeneratively to the charged particles in the interelectrode space. Among these are photoelectric emission, secondary electron emission due to positive-ion impact at the cathode, photo-ionization in the gas, and ionization by excited atoms and by multiple collisions. In addition, it has become evident that any explanation of high-pressure breakdown must also take into account the several processes of ion removal and the important influence of space charge and the myriads of tiny self-healing electron avalanches in modifying the electric field. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Recognition of these various ion-producing, ion-removing, and field-modifying mechanisms has clarified the general picture of breakdown in gases but at the same time has shown it to be too intricate for full quantitative description. Because of this complexity, knowledge of gaseous insulation continues to depend to a large extent on the experimental determination of the breakdown strength and pre-breakdown behavior of the various gases as a function of pressure and electrode geometry, and on the investigation — both under simplified and under practical conditions — of the relative importance of the several mechanisms involved in gaseous breakdown. This paper extends d-c break-down studies in air and in Freon 12 (dichlorodifluoromethane) at high pressures and in a uniform field to higher voltages than have been reported previously, and indicates reasons for the observed departures from Paschen's law and for the relatively high insulating strength of the Freon.