Thermal Runaway and Propagation Process of Defects from Local Defects in the Superconducting Cylinder for Magnetic Shielding Fault Current Limiters

Superconducting fault current limiters have been researched to protect electric power systems. We have developed a superconducting fault current limiter. As the active element, we choose Bi-2212 thick film for the superconductor cylinder. In order to attain practical use of the superconducting fault current limiter, larger scale superconducting cylinders must be developed. Processing energy increases as the size of the limiter is increased. If there are local defects in the superconducting cylinder and the processing energy is concentrated on them, there is a possibility that the cylinder will be damaged. We therefore investigated the thermal runaway propagation of the Bi-2212 thick film from a local heat point and how local defects affect the current-carrying capacity. The result is that the velocity of the thermal runaway propagation in the thick film from the local heat point is very slow. Thermal runaway doesn't propagate within a few milliseconds. If there are local defects such as cracks in the superconducting cylinder, the current will concentrate at the tip of the defect and the Joule temperature rises. The cracks then grow as the result of thermal stress. When a crack runs from end to end on the cylinder, superconductor on the cylinder melts due to arc discharge.