In this article, the mechanism of stable levitation by bulk high-temperature superconductors is presented. Superconductor levitation is fascinating and thus displayed in various demonstrations. In general, superconductor levitation is explained in terms of the Meissner effect, which is not applicable in the case of levitating a permanent magnet by Y-Ba-Cu-O cooled with liquid nitrogen. For stable levitation in free space, the restoring forces must act on the levitated object in three rectangular directions, which cannot be fulfilled by simple diamagnetic levitation like that found with the Meissner effect. A pinning effect that fixes quantized fluxoids is the source of such restoring forces, and therefore, superconductor levitation is stabilized by flux pinning.
Using a high-temperature superconductor, we constructed and tested a model superconducting fault current limiter (SFCL). The superconductor and vacuum interrupter as the commutation switch were connected in parallel using a bypass coil. When the fault current flows in this equipment, the superconductor is quenched and the current is then transferred to the parallel coil due to the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter and the current in the superconductor is broken. Using this equipment, the current flow time in the superconductor can be easily minimized. On the other hand, the fault current is also easily limited by large reactance of the parallel coil. In this paper, we describe the current limiting module using four serial and two parallel-connected YBCO thin films.