Methods to Increase Current Capacity of Superconducting Thin-Film Fault Current Limiter Using Au-Ag Alloy Shunt Layers

We have proposed a superconducting thin-film fault current limiter (FCL) that uses high-resistivity Au-Ag alloy shunt layers for the protection against ldquohot spotsrdquo. Its design enables reduction of the total length of FCL elements that can withstand high electric fields E > 40 V_peak/cm, thus greatly reducing the cost of FCLs. In this paper, two methods to increase current capacity of the FCLs were demonstrated. The first method protects the films from hot spots by using capacitors in addition to external resistors. The capacitors decrease rapid voltage rise in the films at the moment of quenching. The second method maximizes the rated current of two parallel-connected thin films by using parallel connections via superconducting tapes, even if the critical currents of these films are largely different. We then successfully tested two parallel-connected YBCO films prepared by a low-cost, metal organic deposition (MOD) method. Over-current tests showed that (a) superconducting transport currents up to 380 A_peak flowed 5 cycles without quenching, which is more than 1.3 times larger than the peak value of the 200 A_rms rated current and (b) after switching, E = 45.7 V_peak/cm was achieved without film degradation.