Post-Arc Dielectric Recovery Characteristics of Liquid-Nitrogen Arcs in Cryogenic Hybrid DC Circuit Breakers

Liquid nitrogen has demonstrated promising DC interruption capability under cryogenic conditions, which may facilitate the realization of cryogenic hybrid DC circuit breaker. During interruptions, the liquid nitrogen gap must withstand the transient recovery voltage to prevent arc restrikes. However, existing studies mainly focus on gaseous media and vacuum. Dielectric strength recovery of liquid-nitrogen gaps remains absent. This paper aims to characterize the dielectric strength recovery behavior of liquid nitrogen gaps under different arc currents, arc duration and transverse magnetic fields. The results show that after the free burning arc extinguished, the dielectric strength of the liquid nitrogen gap follows a logarithmic regression function. Under a transverse magnetic field, the dielectric strength exhibits a two-stage logarithmic growth. The transverse magnetic field significantly enhances the dielectric strength recovery level of the liquid nitrogen gap. A 50 mT transverse magnetic field increases the withstand voltage by an average of 60% at 20 ms after arc extinction. However, the recovery rate and final dielectric strength level of the electrode gap do not increase with increasing magnetic field strength. The findings of this paper may provide support for insulation design of a hybrid DC circuit breakers employing liquid nitrogen as the arc-quenching medium, contributing to the feasibility of fault-protection strategies for superconducting DC electric-propulsion systems.