Space charge behavior evolution of thermal-aged double-layered polyimide films

Polyimide film (PI) is widely used as dry-type smoothing reactor insulation for ultra-high voltage direct current (UHVDC) projects. In practice, the temperature of a central conductor can reach 80°C to 100°C at the rated power flow. Thus, the inner insulation composed of PI sustains these high temperatures. In this study, the space charge characteristics of PI at different temperatures (40°C, 60°C, and 80°C) and thermal aging degrees (336, 500, 1000, and 1500 h) were examined with an improved pulsed electro-acoustic (PEA) measurement system to investigate the effects of temperature and thermal aging on dielectric performance. The apparent trap depths corresponding to various aging degrees were obtained according to the space charge decay dynamics at the interface. Results indicate that positive charges likely accumulate within the bulk of the material because of the thermal aging of homogeneous materials. The polarity of space charge at the interface also changes from positive to negative as aging degree increases. The space charge decay dynamics at the interface further shows that the trap energy increases as the thermal aging degree increases. The amount of residual space charge also increases remarkably as the electric strength and aging degree increase. The electric field distortion is closely related to temperature, thermal aging degree, and electric strength.