Theory of discharges at the foil edge in capacitors

There is a high field concentration at the electrode edge in rolled-paper or film capacitors, and if the edge is squarely cut, the field at the corner tends to infinity. Breakdown in the dielectric fluid surrounding the edge constitutes a partial discharge, but a breakdown field, and hence the discharge-inception voltage, cannot be simply derived with an infinite field at the corner and a highly divergent field. lt.is argued that a discharge occurs when the voltage gradient along a line of maximum field exceeds a certain critical value over a certain length. The applied voltage necessary to reach this condition is calculated from an approximate analytical treatment of the potential configuration. The resulting formula is verified as regards the dependence of discharge-inception voltage on the dielectric and electrode thickness, using castor-oil- impregnated paper capacitors. Capacitors constructed with longitudinally folded foil electrodes show a markedly increased inception voltage. An analysis of the field for a rounded-edge profile is carried out to a first approximation. The experimental results for the two types of edge profile enable the disposable constants in the theory to be evaluated.