Economically Viable Fault Current Limiters using YBCO coated conductors

Resistive type superconducting fault current limiters (FCL) utilize a current-driven transition from the superconducting state to the normal state to limit short circuit currents in electric power grids. The FCL needs no active triggering at inception of a short-circuit and recovers automatically after the short circuit has been opened. The technical performance of superconducting fault current limiters has been demonstrated by numerous successful projects worldwide. Since the advent of commercial second generation (2G) high temperature superconductor wires (HTS) based on YBCO thin films, a cost-effective commercial design is becoming feasible. We have investigated the fault current limiting performance of 2G HTS wire stabilized with stainless steel tapes. Bifilar coils have been manufactured and tested with a typical limitation period of 50 ms under stepwise increasing voltage loads to determine the maximum temperature the wires can withstand without degradation. Several coils have been assembled into a limiter model to demonstrate uniform tripping of the individual coils and fast recovery within a few seconds. The FCL assembly planned for grid application consists of a switching module of HTS coils in parallel with an inductor made of normal conductor. It is designed to limit only a part of the total fault current. This permits use of existing circuit breakers and time dependent protection system. During normal operation, the effective impedance of the FCL assembly is essentially zero whereas during a fault, the HTS module transitions to the normal state and the fault current is limited by the inductor.