A trap regulated space charge suppression model for LDPE based nanocomposites by simulation and experiment

Abstract

Space charge suppression is one of the key goals for the development of HVDC cable insulation. This study intends to reveal space charge suppression mechanism in LDPE based nanocomposites by numerical simulation and experiments in terms of the effect of trap characteristics. The simulation model takes into account bipolar charge injection, transport, charge trapping, recombination and extraction. Both the experiments and simulations clearly indicate that a dramatic increase in the deep trap density can result in remarkable bulk space charge suppression. The large number of deep traps introduced in the LDPE based nanocomposite by nanofillers should be responsible for its excellent space charge suppression effect. The simulation results indicate that the increase in the trap density shows much more remarkable space charge suppression performance as compared to the trap depth, while the increase in both the trap density and trap depth shows the most remarkable space charge suppression performance. A trap regulated space charge suppression model has been proposed indicating that the trap density and depth dominate space charge behavior via regulating the balance between electrode injection current and bulk conduction current due to its control on the rates of charge trapping and detrapping processes, control on the number and location of the trapped charges. The dramatic increase in the trap density and depth results in extremely low bulk conductivity and carrier mobility. A large number of the injected charges will be trapped immediately around electrode interface forming a thin trapped charge layer. The trapped charge layer will produce a strong opposite electric field and enhance the potential for charge injection, leading to a significant decrease in the injection current and finally a balance between the injection current and the bulk conduction current at a lower current level, resulting in remarkable bulk space charge suppression. In addition, high bulk conductivity due to lower trap density and lower trap depth can also contribute to space charge suppression due to the quick relaxation of the bulk space charges, though it will increase the leakage current.