The Probable Breakdown Voltage of Paper Dielectric Capacitors

The frequency of occurrence of defects in electrical insulation can be shown to follow the Poisson law of probability. One of the more important defects in thin paper insulation is conducting particles. In this paper expressions are derived for the probable breakdown voltage as a function of conducting particle occurrence. It is shown by statistical evidence that large conducting particles exist in the insulating paper and bridge one or more layers of paper in the finished capacitor through chance reorientation caused by manufacturing processes and operation. The degree and frequency of reorientation of particles with a typical grade of paper is determined experimentally, and the probable insulation thickness and consequent voltage strength of various size capacitors is calculated. A method of determining the optimum voltage strength of this insulation is illustrated. It is concluded that conducting particles are a predominant factor in determining the voltage strength of present day capacitors and that one of the greatest opportunities for improvement lies in the reduction of the conducting particle content in the paper.