BREAKDOWN STRENGTH OF IMPREGNATED POLYPROPYLENE FILMS AGED UNDER HIGH A.C. FILEDS

Electrical breakdown measurements were performed on polypropylene films impregnated with benzyltoluene and aged for different electric field values (100 Vrmsμm−1< Ea< 300 Vrmsμm−1). Two films of 13.6 μm thickness were sandwiched between two plane electrodes and immersed in the liquid at 80°C and with substantial 02concentrations. For this arrangement, the threshold voltage discharge (Vth) was ca6–7 kVrms. The results show that for a given ageing time the films breakdown strength dependence on the ageing voltage Vais different for Va< Vthand Va>Vth. This result is a confirmation of the occurrence of electrical discharges in the capacitor for Va>Vth. The area of degradated film is also different in both cases. For Va< Vththe region with lower breakdown strength is restricted to the part of the film sandwiched between the electrodes. For Va>Vththe region with lower breakdown voltage is larger (about 1 mm) than the electrode diameter (8 mm). This is the result of the radial propagation of discharges or streamers. The influence of an epoxy additive which is an effective stabiliser of impregnated PP films was also investigated. © 1997 Published by Elsevier Science Ltd