Effects of nanoparticles on reducing partial discharge induced degradation of polyimide/Al2O3 nanocomposites

Polyimide (PI) films, which are widely used in turn-to-turn insulation of inverter-fed motors, are generally degraded by bipolar continuous square impulse voltage (BCSIV) induced partial discharge (PD). In this paper, Al2O3nanoparticles were dispersed into PI (with 0, 2, 5 and 8 wt.%) to prepare PI/Al2O3nanocomposites. In order to understand the effects of nanoparticles on reducing the degradation of PI/Al2O3nanocomposites from the physical, chemical and electrical point of view, PD aging characteristics, micromorphology, bonding structure, surface conductivity and trap level distribution of PI and PI/Al2O3nanocomposites under different PD aging time were studied. The PD aging results showed that the lifetime of NC8 (PI/Al2O3with 8 wt.% of Al2O3) increased by 215.8% (from 19.6 to 61.9 min) compared with that of PI. The micromorphology results indicated that nanoparticles could prevent the same local point of PI from being eroded continuously during PD aging. Based on the bonding structure analysis, it was found that new bonds formed in the nanoparticle/polyimide interfacial region would be destroyed firstly, which helped reduce dissociations of other chemical bonds of polyimide molecules, by absorbing PD energy. It was also found that the conductive acids generated during the degradation process were responsible for the increase of surface conductivity. Besides, the trap level distribution showed that both trap density and energy in PI/Al2O3nanocomposites decreased as degradation, while the trend of PI was opposite. It indicated the degradation of PI/Al2O3nanocomposites was mitigated compared with PI, which could be attributed to the improved electrical characteristics of PI/Al2O3nanocomposites.