Partial discharge characteristics of direct-fluorinated polyimide films

Due to the excellent electrical properties, such as low dielectric loss, extremely low conductivity and high breakdown strength, polyimide film is widely applied as a kind of engineering plastic film and a type of insulating material in industry. However, with the disadvantage of organic polymer, its corona-resistance property is not good enough. During service operation, partial discharge (PD) may be appeared in the insulation systems, which makes insulation early failures occur frequently. By changing the chemical component in surface layer of polymers, fluorination will give rise to the corresponding change in electrical properties of the surface layer. Thus the charge injection from electrode is influenced when the fluorinated polymers are used as insulator. Samples were surface fluorinated in a laboratory vessel at about 328K (55 °C) using a F₂/N₂ mixture with 20% F₂ by volume and 0.05MPa (500 mbar) for respectively 15, 30, 45 and 60 minutes. This paper presents a study aimed at clarifying the effect of fluorination time on surface partial charge characteristics of polyimide film subjected to pulse stimulus. Both of the original and fluorinated polyimide films were subjected to the pulse stimulus at predefined voltage parameters and were exposed to partial discharges action. A wideband detection system and a video recorder were employed to detect PD current and record discharges phenomenon in a shielded darkroom. Obtained results show the dependence of the partial discharge intensity and the corona resistant time upon the fluorination time of samples. It is suggested that fluorination can significantly improve the corona resistant time and reduce the partial discharge intensity of polyimide films.