Electrical ageing of impregnated polypropylene

In previous accelerated life testing of liquid-impregnated polypropylene capacitors, it appeared that the breakdown voltage of aged films was significantly lower than that of new films, but only in places (Weibull probability <5%). The causes of such an aging process are still unknown. This problem was investigated by testing model capacitors at high electric stress (150 V_RMS/μm) and temperature (80°C) for different aging conditions (liquid conductivity, water content, and additive concentration). The aging of the model capacitors was found to be similar to that of full-size capacitors with respect to the lowering of breakdown voltage and the influence of epoxides. A mechanism other than partial discharges in a gaseous phase at the electrode edges seems to be responsible for the film degradation. No discharges were detected with a tuned circuit; however, tiny electrical discharges, such as those measured at a tip in a liquid but for much higher fields (about 700 V/μm in cyclohexane), may exist in the liquid. It is stressed that the mechanisms may be electrochemical, since degradation is enhanced when aging is performed with a more conductive liquid. Both mechanisms are enhanced by the use of thicker liquid layers. In the first case the liquid film allows discharge growth, and in the second case it supplies with ions at the electrodes the double layers necessary for electrochemical reactions