Changes in Loss Tangent of Polythene subjected to Partial Discharge

Abstract

IN determining the resistance of materials to partial discharges, two distinct types of measurement tend to be used. In one, the time to breakdown is measured for film or sheet material mounted between rod/plane, needle/plane or hemisphere/plane electrodes. In the other, relatively large areas of sheet are subjected to discharge between plane/plane electrodes so that degradation after periods of exposure can be measured by following changes in conductivity, loss tangent, mechanical strength, etc. Because these properties change progressively with duration of exposure, the impression is being gained that the changes in the material which cause them are the same as those taking place in the breakdown process. This assumption leads to suggestions like that of Boeck¹ that impinging electrons become trapped in the material, forming a space charge and enhance the conductivity, the layer becoming ever deeper as fresh charge carriers are supplied.