Effect of additives on the electrical aging of impregnated polypropylene films

In relation to the aging of power capacitors, the authors study the degradation of polypropylene (PP) films impregnated with an aromatic liquid at high AC electric fields. An investigation at 135 V_rms μm^-1, 80°C, and with O₂-saturated benzyltoluene as impregnant has been conducted. By measuring the threshold voltage for discharge detection in a capacitor model the authors have verified that the aging field was about 75% lower than the discharge threshold. In these conditions the model lifetime is reduced to about 200 hr. PP degradation has been evaluated based on film breakdown voltage and Fourier-transform infrared microspectrometry results. Various additives to the liquid have been tested in order to determine the degradation mechanism. Without any detected discharges, a large degradation under AC high electric fields is observed. The decrease of breakdown voltage is more than 25%. Oxygen increases the degradation kinetics, but the main degradation factor is the high electric field