Some Factors Preventing the Attainment of Intrinsic Electric Strength in Polymeric Insulations

While the intrinsic electric strength of polymeric insulating materials is of the order of hundreds of kilovolts per millimeter, the service stress in actual insulations is lower by approximately two orders of magnitude. The objective of the work described in this paper is to establish how much the electric strength of polymers is affected by the following factors, in which there is a difference between measurement of intrinsic electric strength on one side and stressing of the insulation under service conditions on the other: stressing with ac or impulse voltage instead of dc, increasing the size of the insulation, and increasing the duration of stressing, all in the absence of internal corona. It is found that even with intrinsic test specimens, in which there are believed to be no discharges, the electric strength decreases with the area of the test specimen and with the duration of stress. The formulas of the statistical theory of extreme values are applied to experimental results obtained on some polymers. If these are used for extrapolation to insulation sizes and lives typical of actual insulations, a considerable reduction of the breakdown stress results even in the absence of internal corona.