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Trap Distribution and Breakdown Characteristics of Direct-Fluorinated PI Film for DC-HTSFCL

High-temperature superconducting fault current limiter (HTSFCL) is an important application in the power industry. In high-voltage dc system, a polyimide (PI) is used as superconducting winding insulation in HTSFCL also withstands pulse voltage of lightning and operating voltage caused by line fault and switching. The combined dc-pulse voltage makes the electric field more deformed, which induces partial discharge and accelerates insulation failure. Direct fluorination is a good means of regulating the molecular structure and electrical properties of polymers. This paper studies the trap distribution and breakdown characteristics of direct-fluorinated PI film. The corona charging was implemented in an environment with a relative humidity of 40%, using a combined dc-pulse voltage. To calculate the trap distribution, the surface potential was obtained. The breakdown voltage was also measured in LN2 under different voltages. The experimental results indicate that moderate fluorination can effectively reduce the depth of trap level, accelerate charge detrapping, and improve the breakdown voltage of the PI film.