Numerical Simulation of Trap Modulated Multi-Mobility Charge Transport Under AC Field

Based on the characteristics of charge distribution under ac electric field with a wide frequency range, this article focuses on the applicability of the traditional bipolar charge transport model under ac field in the oil-impregnated paper (OP) insulation by simulation. Three physical processes of charge injection, migration, and trapping/detrapping are discussed. In reaction to the influence of amplitude variation of the ac field on the injection rate, the charge injection process is described by a concatenation of the ohmic mechanism and the Schottky mechanism. On account of the correspondence between the frequency and carrier mobility, the single-mobility mode is modified to the multi-mobility mode, which exhibits the effect of localized state differences in materials on charge migration. The charge hysteresis under ac field is to do with the trapped charges, so the multi-energy trap distribution model is adopted to reveal the matching relationship between the period of the electric field and charge residence time in deep traps. By comparing the simulation results and experimental phenomena, the modified model achieves space charge accumulation under a wide frequency range, and the shape of charge distribution is similar to the experiments, which proves the validity of the model. Furthermore, the simulation results demonstrate that the net charge accumulation decreases with the increase in the electric field frequency, while the total charge accumulation shows an inverse trend with frequency, which is related to the coexistence of positive and negative charges.