Influence of dielectric constant on dielectric strength by defect discharge and molecular polarization in solid insulation materials

Abstract

Dielectric constant and dielectric strength are two intrinsic electrical parameters of solid insulating materials. In order to finally understand the relationship between them, the one-sided influence of dielectric constant on dielectric strength is investigated. For theoretical analysis, we propose that the influence is realized indirectly by two mechanisms: defect discharge in the micro level and molecular polarization in the nanoscale. The transition phase is composed of the field enhancement related to permittivity and the breakdown of defect molecular bond. While the local field around the bond is always higher than the cavity interior field, the breakdown field is a competition between the two mechanisms. According to the simulation model of electrostatic field, the electric field around the cavity is significantly enhanced with a larger dielectric constant. In view of the simulation result of partial discharge, the discharge intensity of dielectric and gas breakdown in the cavity is promoted by the increase of material permittivity. To confirm the defect distribution and quantity, several prepared samples are scanned with the ultrasonic microscope and the cavities are measured via image software. Based on the published experiment data from oxide films and polymer bulks, a revised relationship is plotted and fitted for the dielectric strength and the dielectric constant. As molecular polarization is applicable to oxide films with few defects and high permittivities, defect discharge is more effective for polymer bulks with generous cavities and low permittivities. Since molecular polarization leads to breakdown by enhancing the local electric field, cavity discharge is mainly due to the lower breakdown threshold of defect.