Effect of Surface State on DC Breakdown Characteristics of Thermally Aged Double-Layered Polyester Films

The surface state of polymer films has a significant influence on direct current (DC) breakdown characteristics, but its mechanism needs further investigation. In this paper, DC breakdown strength of thermally aged two-layer polyester films is studied. The charge trap and their space distribution characteristics are analyzed by an isothermal relaxation current and the pulsed electro-acoustic method. Furthermore, a bipolar charge transport model suitable for two-layer dielectrics is established, and the correlation between surface state and DC breakdown performance is dynamically simulated. The experimental results show that as the aging time increases, DC breakdown strength first increases and then decreases. The trap depth and its density first increase and then decrease, and the change mainly comes from dielectric surface area. The simulation results show that through modulating the injection and accumulation behaviors of space charges, the change of trap depth, density and extended depth of surface state have an important effect on the space charge and electric field distribution at the pre-breakdown time.