Electrical Damage Characteristics of Polyimide Film Caused by Surface Discharge

Polyimide(PI) film is an important type of insulating material used in the inverter-fed motors.Partial discharge(PD) under a sequence of high-frequency square impulses is one of the key factors which lead to premature failure of insulation systems of inverter-fed motors.In order to explore the damage mechanism of PI film caused by discharge,an aging system of surface discharge under bipolar continuous square impulse voltage(BCSIV) was designed based on the ASTM 2275 01 standard and the PI film was tested.The surface conductivity of PI film increases because of the carbonized surface layer caused by surface charge,which would affect the characteristic of surface discharge.The micro-morphology and structure change of PI film aged above partial discharge inception voltage(PDIV) was investigated by Scanning Electron Microscopy(SEM).The degradation path of PI film is initiated from surface and then gradually extends to the interior with continuous aging.The chemical bonds of PI chain were analyzed through Fourier Transform Infrared Spectroscopy(FTIR).The results show that the degradation mechanism of PI film is the fracture of chemical bonds,such as ether bond(C-O-C) and imide ring(C-N-C),caused by the physical and chemical erosion of surface discharge.