Effects of Direct Fluorination on Space Charge and Trap Distribution of PI Film in LN2

In recent years, high-temperature superconducting (HTS) cables are quickly being developed because of high efficiency and high current transportation capability. Compared with the HTS ac cable, the HTS dc cable has lower loss and stable properties. Under dc environment, the space charge formed in the polyimide (PI) film would distort the local electric field and further influence the characteristics of partial discharge and accelerate the insulation aging. Direct fluorination, as a solution to change the chemical component, can alter the electrical properties of the polymer surface. This paper concentrates on the effect of the fluorination on the space charge and trap distribution of the PI film in liquid nitrogen (LN2). The polarization and leakage current of the PI film in LN2are measured under dc voltage. And then, the space charge mobility, amount, and the conductivity are calculated. Moreover, the trap distribution was obtained by the modified isothermal discharge current test. The results indicated that the fluorination improved the space charge mobility and conductivity. The appropriate fluorination suppressed the space charge accumulation in the PI film. However, excessive fluorination brought in more deep traps leading to significant amount of space charge accumulation in the bulk of polymer. As a consequence, the appropriate fluorination has a potential application for insulation in the HTS dc cable.