Assessment study of superconducting fault-current limiters operating at 77 K

The possible impact of nitrogen-cooled superconductors on the design and cost of superconducting fault current limiters is assessed by considering the technical specifications which such devices must meet, and by comparing material properties of 77 K and 4 K superconductors. The main advantage of operating superconductors at 77 K is that the refrigeration operating cost is reduced by a factor of up to 25, and the refrigeration capital cost is reduced by a factor of up to 10. The heat capacity at 77 K is several orders of magnitude larger than at 4 K. This phenomenon increases conductor stability against flux jumps but makes switching from the superconducting to normal state slow and difficult. Consequently, a high critical current density, probably at least 10/sup 5/ A/cm/sup 2/, is required.<>