A new superconducting switch

The sharp, reversible transition of a superconductor from the normal to the superconducting state gives it obvious switching properties. One use for such a switch would be the protection of power-system components, especially superconducting cables and generators, which are now being developed. The bulk of the paper is concerned with a preliminary design study of such a device.

The required lengths of superconductor are such (similar 1 km) that burn-out at regions of low critical current (I_c) would be probable before the rising fault current in the whole superconductor reaches the average I_c and switching occurs. The proposed switch avoids this difficulty by backing the superconductor with a semiconductor which has a negative temperature coefficient of resistance in the relevant temperature range, thus preventing the catastrophic temperature rise that would occur in the normal superconductor alone because of its positive temperature coefficient. The crucial requirement of the semiconductor is that it should have, just below its melting point, the correct intrinsic resistivity, for which an expression is derived.

An illustrative design for a high-power generator-protection switch is worked out, and problems and incentives are outlined.