Quench current degradation in superconducting coil for 6.6 kV/2 kA fault current limiter

A 6.6 kV/2 kA class fault current limiter that consists of noninductive superconducting windings was developed and tested. The limiter can deliver continuous power at 2 kArms and limit fault current of more than 20 kA to 4 kA in 6 kV. The limiter recovers to the superconducting state within a few seconds and can perform a fault current limiting operation in 60 seconds after the last limiting operation. After high-voltage operation, however, the quench current of the limiter dropped significantly. This quench current degradation is related to the coil bobbin strain due to helium pressure raised by large ohmic heating in quenching. The degradation is a fatal problem for a fault current limited if the quench current falls below the operating current because the limiter cannot revert to the normal operation. High Young's modulus bobbin is effective to remove the coil degradation. The authors have obtained a fault current limiter without any degradation after repeated limiting operations.