500 V/200 A fault current limiter modules made of large-area MOD-YBa₂Cu₃O₇thin films with high-resistivity Au–Ag alloy shunt layers

We developed 500 V rms / 200 A rms superconducting thin-film fault current limiter (FCL) modules that can withstand high electric fields ( E >30 V rms cm −1 ) by using large-area YBa 2 Cu 3 O 7 (YBCO) thin films with high-resistivity Au–Ag alloy shunt layers. Au–Ag alloy films about 60 nm thick were sputter-deposited on YBCO/CeO 2 /sapphire films (2.7 cm × 20 cm) prepared using a fluorine-free MOD method. Each 20 cm long Au–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 effective length of 17 cm. The 500 V/200 A FCL modules were then fabricated by first connecting two of the segmented films in parallel using Ag-sheathed Bi-2223 superconducting tapes and then connecting in parallel an external resistor and a capacitor for each segment to protect the Au–Ag/YBCO film from hot spots. Switching tests using a short-circuit generator revealed that all the modules carried a superconducting ac current of ≥237 A rms and that modules prepared with YBCO films having a relatively homogeneous critical current I c distribution successfully withstood ≥515 V rms for five cycles without any damage. These results demonstrate that (a) the FCL modules fabricated here successfully achieved the rated current of 200 A rms and rated voltage of 500 V rms 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 FCL modules that use gold shunt layers, leading to the significantly reduced cost of thin-film FCLs. Film damage due to hot spots depended on the difference in I c between the two parallel-connected films and on the inhomogeneity of the I c distribution in the film, and is most probably due to nonlinear current flows at the moment of quenching that cause local overheating.