Discharges in asymmetric cavities under AC stresses

Failures of paper-insulated underground cables are often attributed to the action of electrical discharges in cavities in the insulation, in particular those next to the conductor or under the screen. These are bounded on one side by paper and on the other by a conducting material. The sequences of discharges that occur in such cavities under AC stresses are characterised so that their presence in cables can be recognised leading to estimates of the failure probability. The results of experiments on model cavities, and analyses of discharge size distributions, indicate that the sizes and frequencies of the discharges are governed by the interaction between random ionising events and the sinusoidal variation of the applied voltage. It is suggested that the presence of asymmetric cavities can be inferred from an inequality in the apparent charge per cycle transferred by discharges of opposite polarities, and that, where such cavities are present, their dimensions may be approximately related to the maximum discharge size observed.