High-field dielectric properties of polypropylene films in high-temperature region

Abstract

Nonpolar polymers are used widely as electrical insulation materials, and their operating conditions sometimes become high-electric field and high temperature. Under these conditions, the conduction loss is assumed to affect the dielectric properties and to make the ac dissipation factor (tan δ) increase. Therefore the high-field dielectric properties of polymeric materials in the high-temperature region become very important. This paper describes the high-field dielectric properties of polypropylene films measured at 50 and 400 Hz above room temperature with the new electrodes developed by authors. At low temperature, the high-field tan δ is independent of the frequency of the applied field. However, at high-temperature, the high-field tan δ becomes almost inversely proportional to the frequency, namely, the conductivity is independent of the frequency. The theoretical analysis shows that the high-field dielectric loss is governed by two processes: one is the loss which appears only under ac field; and the other is conduction loss which appears also under dc field. The latter loss becomes prominent under high-electric field at high temperature.