Calculation of Corona-Starting Voltage in Air-Solid Dielectric Systems [includes discussion]

Abstract

field remains sufficiently uniform. The extent of uniformity will be discussed more fully later.Using the predictions of Dubsky, Hall, and Russek that measurements of breakbown voltage in a uniform field are the same as corona-starting voltage, it should be possible to substitute the spark breakdown voltage V, at given gap distances for ",. Thus the equation becomes Synopsis: A method is described for determining the corona-starting voltage in air-solid dielectric systems when the thickness and the dielectric constants are known. The method is derived by applying data from Paschen's law to the theoretical distribution of a voltage potential between air and a solid insulation. Two procedures for verifying the method and a determination of the accuracy are described. The extent of error due to field distortion and other factors is discussed. Examples showiiig how the method may be applied practically for air-solid dielectric systems containing more than one solid dielectric and the expected accuracy are included. SINCE the development of the resonant circuit method'-3 for detecting the presence of corona in electric systems, and the sudden realization that corona was present in many relatively low-voltage as well as high-voltage apparatus, a great deal of emphasis has been placed on damage to insulation by corona. Engineers designing insulation systems wanted to avoid corona if possible. The problem has been to find a convenient way, other than by trial and error, for determining whether or not corona might be present. This has stimulated projects4 for conceiving a method or methods for calculating the coronastarting voltage in electrical apparatus.A few simple experiments, such as placing flat sheets of insulation between planeparallel electrodes and determining the vToltage point at which corona has just started, indicated that as the thickness of the same type of insulation was increased, the corona-starting voltage increased. The experiments also indicated the coronastarting voltage to be different for the same thickness of different insulation materials.Dubsky,5 and Hall and Russek,6 have shown that measurements of breakdown voltage in a uniform field over a given range of gap distances and pressures in air and a number of other pure gases are the same as the corona-starting voltage. They further state that for atmospheric pressures, the given range of air-gap distances extends from approximately 0.005 to 0.075 inch. It has been demonstrated that the second Townsend coefficient or the number of new electrons produced at the cathode by secondary processes per avalanche crossing the gap is the same to within 5 per cent (%) for brass, lime glass, or polyethylene electrodes.Von Hippel7 has shown in his work on determining the breakdown voltage in solids that corona in solids is many times higher than in gases. He further states that in solids as in gases the start of cumulative ionization is the beginning of dielectric breakdown.On the basis of these findings and if it is assumed that the insulation is homogeneo...

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