Behavior of Second Generation HighTemperature Superconductor Tapes in Fault Current Limiting Systems

Abstract

Optimization of high-temperature superconductor (HTS) coated conductor for standalone superconductor fault current limiting (FCL) systems and for inherently fault current limiting cables is quite different. To minimize the amount of conductor, and so to lower cost, a high conductor critical current per unit width I _c,w is desirable to minimize the number of parallel conductors required to meet operating current requirements. In standalone systems, a high normal state resistivity and thin conductor are also desirable to minimize conductor length. But in FCL cables, conductor length is fixed by the application. Limits on I _c,w, resistivity, and thickness also come from the necessity to prevent local overheating during a fault or, in a cable, bubbling of liquid nitrogen which could precipitate a dielectric failure. A critical insight is that even for low-voltage faults, hot spots can arise in the conductor, and heating within these hot spots must also be controlled. A simple model is introduced to describe this phenomenon and estimate hot-spot heating. Tolerable hot-spot temperature rise in FCL cables is drastically less than in standalone FCLs. Implications for design of FCL cables are discussed and their viability for applications such as linking substations at the distribution level is demonstrated.