Long term breakdown of polypropylene films

Breakdown (BKD) is a very complex problem - and often a confusing one -. A great number of possible mechanisms of various origins (electrical, thermal, mechanical, etc …) has been proposed. In certain solids materials (for instance, semi-conductors) theoretical predictions are confirmed by experimental results and intrinsic BKD fields (independent of electrodes, gap, voltage shape and duration) can be evaluated. In solid insulators, it is difficult to assure that any single mechanism is uniquely responsible, especially in “long term” BKD (i.e. time duration extending from say 1s to months or years). There are the major problems of selection and engineering of insulators for long term use. The purpose of our work is to evaluate the physicochemical factors involved in such conditions. We used mainly polypropylene (PP) films dry, wetted or impregnated with different liquids, and life tests (voltage vs time: V-t curves) were performed under DC and AC voltage.