Emergency Blackout Operation of Cryogenic System for Hybrid SFCL

Operation scheme of cryogenic system is investigated in preparation for temporary loss of cryocooler power in hybrid SFCL. Under normal operation, HTS trigger elements are cooled in liquid nitrogen pool at 77 K and 300 kPa by a GM cryocooler. A step-by-step procedure is devised to maintain the HTS elements at or below 78 K during blackout and recover period. On detecting the onset of temperature rise followed by power outage, a vent valve is opened to atmosphere so that thermal load can be covered by evaporation of liquid nitrogen at 77.4 K. When the cooling power is restored, liquid is pressurized by helium gas back to 300 kPa. The instrumentation and control parts needed for unmanned operation are designed and installed on a prototype of 22.9 kV/ 630 A SFCL. The emergency operation is successfully rehearsed on scheduled outage of 10 minutes, 1 hour, and 4 hours, respectively.