Effect of direct fluorination on surface partial discharge characteristics of polyimide films

Surface charge accumulation induced by partial discharge has a great effect on breakdown characteristics of polymer insulation. By changing the chemical component in the surface layer of insulation material, fluorination will cause the corresponding change in electrical property. This paper presents a study aimed at clarifying the effect of fluorination time on the partial discharge of fluorinated polyimide (PI) film. PI samples were fluorinated in a reaction kettle at about 328 K (55 °C) using a F2/N2 blend with volume fraction of 20% nitrogen and 0.05 MPa (500 mbar) for respectively 15, 30, 45, 60 and 90 min. The partial discharge experiment was tested at room temperature with the relative humidity of ~ 40%. A wideband detection system and a video recorder were employed to detect the partial discharge current and record discharge images in a shielding darkroom. Obtained results show that the conductivity of polyimide film becomes large after the fluorination. The corona resistance time increases with the fluorination time increases from 0 to 45 min, while it decreases when the fluorination time increases from 45 to 90 min. Recurrent plot (RP) analysis of discharge current shows that the change of chemical component in surface layer of polyimide film by fluorination makes the partial discharge become more stable and regular.