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.

Thermally conductive epoxy resin composites are widely used as electrical equipment insulation and package materials to enhance heat dissipation. It is important to explore the dielectric properties of the composites at high temperatures for the safe operation of the equipment. This paper investigated the charge transport behavior of an epoxy/nano−SiO₂/micro−BN composite at varied temperatures by combined analysis of the TSDC (thermally stimulated current), conduction current, complex permittivity and space charge distribution between 40 and 200 °C. The results show that ionic space charge accumulation was significantly suppressed in the composite at high temperatures. The conduction current increased gradually with temperature and manifested a remarkable shift from electron charge transport to ion charge transport near the glass transition temperature (T_g). The real and imaginary permittivity showed an enormous increase above T_g for both the epoxy resin and the composite. The conduction current and permittivity of the composite were remarkably reduced in comparison to the epoxy resin. Therefore, the ionic process dominated the high temperature dielectric properties of the epoxy resin and the composite. The nano–micro fillers in the composite can significantly inhibit ion transport and accumulation, which can significantly enhance the dielectric properties of epoxy resin. Thus, the nano–micro composite has a strong potential application as a package material and insulation material for electronic devices and electrical equipment operated at high temperatures.

Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (E_t) from 0.80 to 1.20 eV for the trap densities (N_t) of 10 × 10¹⁹ and 80 × 10¹⁹ m⁻³, and become more serious with the increase in N_t from 10 × 10¹⁹ to 1000 × 10¹⁹ m⁻³ for E_t = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as E_t = 0.80 eV, N_t = 10 × 10¹⁹ m⁻³) or increasing trap depth and trap density (such as E_t = 1.20 eV, N_t = 1000 × 10¹⁹ m⁻³), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.

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.

The evolution of partial discharge (PD) with time can provide a deep understanding on the insulation status of power cables. It is of great significance for intelligent operation and maintenance of power cables. In this paper, the PD pulse signal in the 10kV cable joint during accelerated electrical aging under 20kV AC voltage was acquired in the real-time for about 160h. The characteristic parameters of partial discharge-pulse number, average voltage, maximum voltage and energy per second were extracted. The results show that the phase of partial discharge is mainly concentrated at 30°-90° and 200°-270°, which can be characterized as internal discharge. PD characteristic parameters gradually increased after 50h. The pulse number, energy per second and the average voltage of PD pulse reached a peak between 60-80h. Then these parameters reached a steady state between 80-130h and showed a steep rise after 130h. The maximum voltage of PD pulse shows a steep rise at about 70h from 0.1V to 0.3V. It rises sharply from 0.3V to 0.5V after about 120h and then enters a relatively stable oscillation stage. The evolution rules of the PD characteristic parameters comply well with the electrical tree growth states-initiation period, lag period and rapid growth period. Furthermore, model for predicting and evaluating the insulation state based on BP neural network are established and the prediction accuracy is verified. The proposed models can provide early warning for the cable joint before the insulation failure, so as to ensure timely maintenance or replacement.

Due to frequent multiple harmonic overvoltages of the contact wire, the arresters in electric multiple units (EMUs) suffer from accelerated impulse deterioration and serious lifethreatening. Thus, this article proposes two novel kinds of arresters. One is a compound arrester with parallel gap and zinc oxide (ZnO) varistors (CA-PGZR), and the other is a compound arrester with parallel gap and linear resistors (CA-PGLR). Simultaneously, their electrical characteristics are verified by theoretical simulation and experiment. The results demonstrate that: 1) their chopping overvoltage is lower than that of the arrester with the series gap (ASG) under the operating overvoltage(the chopping overvoltage of CA-PGZR, CA-PGLR, and ASG are 75.4kV, 54.3kV, 142.0kV, respectively); 2) their impulse discharge voltage does not exceed the impulse residual voltage of the arrester body under the standard impulse current; and 3) under the increased power frequency voltage or harmonic overvoltage, their stable temperature rise are reduced by more than 50% compared with the gapless arrester (Under the harmonic voltage of 47.5 kV and 2050 Hz, the stable temperature rise of CA-PGZR, CA-PGLR, and the gapless arrester are 63.4°C, 45.4°C, 126°C, respectively). The two proposed arresters in this article show superior protection and tolerance characteristics. They can effectively mitigate the damage caused by harmonic overvoltage to the arrester body and can be used to replace the gapless arrester in EMUs.