Conduction in polythene with strong electric fields and the effect of prestressing on the electric strength
There has been speculation in the past about the extent to which space charge formed by the flow of prebreakdown current affects the electric strength of polythene. In the present work, conduction and absorption-current measurements up to about half the breakdown field have given clear evidence of space-charge effects. Further evidence was obtained from measurements of the apparent ‘intrinsic’ electric strength under various modes of stressing. It was found that the electric strength of polythene measured with direct voltage below about 30°C exceeded the corresponding value measured with impulse voltages by up to about 25% The impulse electric strength could be increased to equal the direct voltage by prestressing each specimen for 4min with direct voltage of the same polarity as the impulse voltage. However, when the impulse was of opposite polarity to the direct prestressing voltage, the electric strength decreased with increase in the prestressing field. The effect was dramatic, for the impulse electric strength changed from the normal value of about 6MV cm−1 to less than 2MV cm−1 as the prestressing field was increased to about 75% of the electric stength measured with direct voltage. Experiments in which the breakdown impulse was applied some time after removal of the prestressing field showed that the impulse electric strength at 20°C recovered in about 10−2s to the value obtained with impulses without prestressing. Thus the prestressing caused no permanent damage. The observed effects are attributed to space-charge injection from the electrodes. The observations are not consistent with the concept that breakdown occurs when the field at a point within the specimen exceeds a critical value, the intrinsic electric strength of polythene.
