AC loss reduction of a 6.6 kV superconducting fault current limiter

The authors have been investigating a 6.6 kV superconducting fault current limiter (SCFCL) for use in power distribution systems. For practical application, heat load at 4.2 K must be reduced to a level which can be compensated by a small refrigerator. This paper describes AC loss reduction of a SCFCL coil, which is comprised of a conductor and junctions. The conductor is a cable of double-twisted multi-filament NbTi strands in a high-resistivity CuNi matrix. The filament diameter in strands and the filament twist pitch were reduced for AC loss reduction. Current junctions made of copper generate eddy current loss because an alternating magnetic field is applied on them by the conductor. It was confirmed experimentally that cylindrical-shaped junctions effectively reduced the loss. A total coil loss of 0.8 W at 1 kA has been achieved, which value is sufficiently low for the development of a closed cryostat of superconducting fault current limiter compensated by a small refrigerator