One-Dimensional Modeling of Charge Transport in Epoxy for DC-GIS Insulating Spacer

This study focuses on charge transport in epoxy bulk, an insulating spacer material. We discuss the relationship between the space charge behavior and the external current determined by conduction and displacement currents. One-dimensional space charge behavior in epoxy is simulated using a bipolar charge transport model that considers charge trapping, de-trapping and recombination. The simulated results show that the charge accumulation in epoxy is successfully modeled by considering high-density deep traps at the epoxy/electrode interface and optimizing the parameters, whereas the simulated external current is inconsistent with the experimental results where the external current decreases in power-law behavior. In the simulation, the external current gradually increases and reaches a steady state as the charge accumulation gets saturated. Charge accumulation does not contribute to the decrease in the external current, which is contrary to the prediction based on the experimental results. We then show that the power-law decrease of the external current is due to the presence of bipolar initial charges before voltage application and multi-depth traps in epoxy. The results strongly indicate that multi-depth trapping/de-trapping, like hopping transfer, rather than single-depth trapping/de-trapping should be taken into account when modeling charge transport in epoxy.