Boiling and Breakdown Characteristics of Liquid Nitrogen During Quench-recovery-requench Process of Resistive Superconducting Fault Current Limiters

Resistive-type superconducting fault current limiters (R-SFCLs) can limit short-circuit fault currents and have good prospects in HVDC systems. Superconducting tape in the R-SFCLs gradually returns to the superconducting state after it quenches, and the system can be reclosed at this time. If the short-circuit fault is permanent, the R-SFCLs will be quenched for the second time after the system is reclosed. The bubbles generated by the tape after the second quench may be different from the first quench. Since the bubbles are closely related to the decrease of the breakdown strength of liquid nitrogen, the effect of the bubbles after the second quench on the internal insulation of the R-SFCLs may also be different. To obtain the effect of quenched bubbles during the quench-recovery-requench process of high-temperature superconducting (HTS) tapes on the insulating properties of liquid nitrogen in R-SFCLs, this paper uses the method of two impulse currents to study the recovery characteristics of the HTS tape after two quenches and tested the breakdown characteristics of the liquid nitrogen/bubbles composite system. The results show that when the HTS tape is in the “critical recovery” state, the recovery time for the second quench is three times that of the first quench. The positive and negative 50% breakdown voltage dropped from 75 kV and 62.5 kV to 65 kV and 50 kV after the second quench, respectively. The internal insulation design of R-SFCLs should make the field strength less than 28.3 kV/cm to leave a margin for safety.