Breakdown Characteristics of Direct-fluorinated Polyimide Film in LN2 under Combined DC-Pulse Voltage

With the development of high voltage flexible DC transmission technology, the high voltage (HV) superconducting fault current limiter (SFCL) is of great significance for the protection of the equipment such as the converter valve. Polyimide film (PI) used as the cryogenic insulation of superconducting windings in SFCL has to bear the common effect of DC and pulse overvoltage in the HVDC transmission system. Fluorination is an effective method of polymers surface modification which can improve the properties of original material, such as chemical stability and electrical properties. This paper presents a research to clarify the effect of fluorination time on the breakdown characteristics of fluorinated PI films in LN
₂
under combined DC-pulse voltage. PI specimens were fluorinated in a reaction kettle at 55 °C using a F
₂
/N
₂
blend with volume fraction of 20% nitrogen and 0.05 MPa for respectively 15, 30, 45 and 60 min. The experimental results indicate that the fluorination can increase the breakdown voltage of the PI film in LN
₂
, which provides reference for the insulation design of superconducting winding in the superconducting current limiter.