The charge transport in polypropylene was studied under DC electric fields at different temperatures. By the experimental measurement and simulation of the BCT model, we studied conduction currents, breakdown strength, and space charge distribution. In particular, the conduction characteristics under high temperature and high field, especially the conduction characteristics before the breakdown, were studied by systematic experiments, and the conduction characteristics and the breakdown mechanism were further studied by simulation. The results show that in the process of measuring conduction currents until breakdown, both high temperature and high electric field will promote charge transport. However, the free volume will increase at high temperature, which will easily lead to faster charge transport and breakdown. In the breakdown process at different temperatures, there are different breakdown mechanisms. At 20–60 °C, the electric breakdown process has mainly occurred in polypropylene film, and the breakdown strength is almost unchanged. At 80 °C, electric breakdown and thermal breakdown act together, and the charge transport is faster, and the breakdown field becomes smaller. Finally, we conclude that thermal stress plays a very important role in charge transport. In a high-temperature environment, the volume expansion of polypropylene will promote charge transport, and the insulation of polypropylene capacitor films will be damaged.

The insulating properties of polypropylene (PP) film play a very important role in the operating status of direct current (DC) support capacitors. Charging and discharging currents in PP film under high DC electric fields and temperatures correspond to charge transportation and accumulation, which significantly influence the electrical insulating properties of PP. In this paper, we have comprehensively studied the dependence of charging/discharging currents in PP film on time, electric field (150–670 kV/mm), and temperature (40–120 °C). The results showed that the charging current increased by almost an order of magnitude from 150 kV/mm to 670 kV/mm and exhibits a steep increase with temperature above 80 °C. The discharging currents are about 10 times less than the corresponding charging currents. Carrier mobility varies little with the electric field and becomes slightly larger with an increase in temperature. The quantity of the accumulated charges was calculated by the integral of the charging and discharging current differentials and showed a significant increase with the electric field and temperature. The corresponding electric field distortion becomes larger above 80 °C compared to 20–60 °C. Both electric field and temperature have an important effect on PP film and capacitors based on charge transport and accumulation and their electric field distortion. This study is innovative in that it combines the operating status of DC support capacitors with traditional methods to research synthetically charged transport mechanisms of PP film. The findings are meaningful for understanding the insulation failure mechanisms of PP film and capacitors under complex stresses.

Charging and discharging current behavior under high DC electric field in polypropylene (PP) film is closely related to the charge transport and accumulation process, which has an important effect on the electrical insulating properties of PP. In this paper, the dependence of the charging and discharging current of polypropylene films on time and electric field has been comprehensively studied. The results showed that the transient and steady current values of the charging and discharging process increase with the increase of electric field. Dependence of the charging current on the electric field conformed well to the space charge limited current (SCLC) theory with a transition electric field of 270 kV/mm, at which the charge transport changed from ohmic conduction to SCLC conduction. The carrier mobility derived from the discharging current became significantly smaller with increase of the charging electric field. The charge accumulation after discharging was derived from the integration of the difference of the charging and discharging current and it showed an increase with the electric field and increased sharply above a certain threshold electric field (the same as the transition electric field in SCLC theory). It was proved that the conduction current and charge accumulation evolution and dependence on the electric field were mainly determined by the balance between the electrode charge injection process and the bulk conduction process.