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 LN
₂
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.