Application of sinter-forged Bi-2223 bars to 1500-A a.c. power utility service as high-frequency current leads in a 77-4 K temperature gradient

Two assemblies of Pb-BSCCO 2223 superconductor bars were produced for use in ac connections between utility system lines at room temperature and a fault-current limiter operating at 4 K. Each assembly, consisting of four parallel bars arranged within a 100 mm-diameter boundary, delivered 1500 A (peak), 50-60 Hz ac through the 77-4 K range of the temperature gradient while dissipating < 0.3 W. The sinter-forged bars displayed dc critical current densities of 950-1300 A cm^-2 at 77 K and > 5000 A cm^-2 at 4 K; magnetic field sensitivity was relatively small. Although thermal conductivity tests showed values much higher than those found in the literature for polycrystalline Pb-BSCCO 2223 made by other processes, this undesirable result was counterbalanced by the relatively high values of critical current, which enabled the use of bars with smaller cross-sectional areas. Typical 50-Hz ac power losses at 77 K in each bar (with a cross-sectional area of 0.54 cm²) were 1 mW cm^-1 of length at 310 A and 1.75 mW cm^-1 at 375 A. Losses were much smaller at 4 K. Bars were 25 cm long, including ends that carried silver contacts sinter forged to the ceramic, with a 21 cm silver-free length in the thermal gradient between the fixed-temperature terminals. Thermal efficiency of the assemblies was assessed by helium boiloff tests.