Possibilities for using space charge quantities as aging markers and as tool for material ranking

Space charge is often brought in connection with electrical aging: in some cases as a cause and in some as a consequence of aging, sometimes even both. This makes space charge related quantities possibly useful for different purposes; as aging markers or for research on the influence of fillers on the electrical performance of insulating materials or for material ranking. We studied epoxy and polycarbonate (PC) (with and without filler), both new and long-term stressed, and examined all three mentioned aspects of possible applications of space charge related quantities. Breakdown tests on new and previously stressed test specimens were used to check if long-term stress triggered any changes (electrical aging) in the materials. We show that space charge quantities can be used as aging markers in the case of PC and PC with TiO₂-filler (PCTiO₂) which were stressed at 100 kV/mm for 850 hours. There was no aging of the polycarbonates at 60 kV/mm according to breakdown tests which was confirmed by space charge measurements. According to breakdown test there was also no aging of the epoxy test specimens which had been stressed with 60 kV/mm for 860 hours. The space charge measurements however gave some indication of a very early stage of aging. Material ranking was only partly successful: it was concluded that proper material ranking is only possible if the applied poling fields for space charge measurements are higher than electrical threshold for space charge accumulation.