500 V/200 A fault current limiter modules made of large-area MOD-YBa2Cu3O7 thin films with high-resistivity Au–Ag alloy shunt layers

Abstract

We developed 500 Vrms/ 200 Arms superconducting thin-film fault current limiter (FCL) modules that can withstand high electric fields (E>30 Vrms cm−1) by usinglarge-area YBa2Cu3O7 (YBCO) thin films with high-resistivity Au–Ag alloy shunt layers.Au–Ag alloy films about 60 nm thick were sputter-deposited onYBCO/CeO2/sapphire films(2.7 cm × 20 cm) prepared using a fluorine-free MOD method. Each 20 cm longAu–Ag/YBCO film was then divided into three segments (each∼5.7 cm long) by four Ag electrodes deposited on the Au–Ag layer, resulting in an effectivelength of 17 cm. The 500 V/200 A FCL modules were then fabricated by first connecting twoof the segmented films in parallel using Ag-sheathed Bi-2223 superconducting tapes andthen connecting in parallel an external resistor and a capacitor for each segment toprotect the Au–Ag/YBCO film from hot spots. Switching tests using a short-circuitgenerator revealed that all the modules carried a superconducting ac current of≥237 Arms and that modules prepared with YBCO films having a relatively homogeneous critical currentIc distributionsuccessfully withstood ≥515 Vrms for five cycles without any damage. These results demonstrate that (a) theFCL modules fabricated here successfully achieved the rated current of200 Arms and ratedvoltage of 500 Vrms and (b) total area of the YBCO films on sapphire substrates required for the 500 V/200 A(100 kV A) module was less than one-third that for conventional thin-film FCLmodules that use gold shunt layers, leading to the significantly reduced cost ofthin-film FCLs. Film damage due to hot spots depended on the difference inIc between the two parallel-connected films and on the inhomogeneity of theIc distribution in the film, and is most probably due to nonlinear current flows at the momentof quenching that cause local overheating.