Experiments with a high voltage (40 kV) superconducting fault current limiter

Fault current limiters are devices that restrict fault currents without appreciable delay, so that they can protect electrical grids and devices more efficiently than classical circuit-breakers. Extremely low response times (a few microseconds) are provided by low- T c superconducting conductors, which combine a high critical current density and a high normal-state resistivity, with a vanishing heat capacity. Due to the risks of dielectric breakdown and overheating, the rated voltages have been limited to a few kV up to now. A significant step forward was recently attained with the successful tests of a 40 kV rms prototype; the device accepts rated currents of 315 A rms, and protects the line from any current over 1100 A peak (less than one tenth of the ordinary fault current). Its design and test results are presented here, with a tentative extrapolation to higher-rated currents and voltages.