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All concepts for fault current limiters employing high temperature superconductors (SCFL) have the disadvantage of significant AC losses caused by the superconductor carrying a continuous load current. Therefore, in the presented paper, it is investigated what electrical and thermodynamic requirements have to be fulfilled by a superconductor if the fault current limiter is of a hybrid type. Assuming the availability of a suitable design for a superconductor used as a delayed reacting resistive limiting element in parallel with a fast acting load switch, the concept of a hybrid fault current limiter is introduced. The concept is based on a novel fast acting mechanical switch for several kA at medium voltage level to commutate the rising fault current to the superconducting element within several hundred microseconds after fault detection. It is found that a critical current density not higher than 1.5 kA per centimeter square is adequate for the proposed system. The superconductor carries only the fault current during switching and thus the total losses of the hybrid system are only 4 percent of the losses of a conventional SCFL, which offers high economical benefits. Cooling of the superconductor by gaseous nitrogen is feasible, and presents simplicity and flexibility of operation.
Date of Publication: March 2000