Prospects for the employment of superconductors in alternating magnetic fields

Because of their complete diamagnetism, soft superconductors can only be used, whether with alternating or direct current, as boundaries to magnetic flux. This prevents their application wherever the magnetic flux-density, in which they are immersed or to which they give rise, exceeds a characteristic critical level. The highest level occurs with niobium at about 1500 G. But the windings of conventional transformers already work with densities of 2600 G at their rated load and materially exceed this under fault conditions. The same difficulty applies to the conductors of cables used in high-power transmission. The provision of tandem transformer windings, in which niobium would deal with rated conditions, and auxiliary copper windings with fault current is shown to be thermally unsatisfactory, in that it would lead to outage periods following a fault that are far too long to be acceptable to a user. Hard superconductors carry direct or alternating current in depth and need not be diamagnetic, but ac conduction in depth cannot avoid losses. With alternating current there is an essential loss per cycle that is seen to accompany penetration. This loss is not dependent on coil current, for the whole winding conductor is at all times fully occupied by current flowing in opposite directions, and in almost cancelling amounts, so that the net current is small compared with the conductor-section capability. The loss is a function of ambient leakage flux-density and of the conductor thickness. Loss is reduced with high saturation density, only because volume is reduced, so this cannot lessen the embarrassingly high rate of winding temperature-rise during an overload. Loss expressions are presented and the discussion is conducted in terms of a 570-MVA generator transformer and of a 760-MVA 275-kV 3-phase cable feeder. The paper concludes that no economic case can be made for the employment of superconductive materials in either transformers or cables.