Breakdown of polypropylene films under DC stress

We set out to recognize the various factors involved in the long term breakdown of polypropylene films used in high voltage power capacitors. Variations of dielectric strength and current under DC voltage (ramp or step voltages) are reported, the temperature varying in the range - 20°C to 100°C. The breakdown voltage was averaged out by using a cell containing 18 pairs of spherical electrodes. Some of the films were impregnated and others were not. They were first tested in air or immersed in either an unswelling liquid or in the actual impregnating one.