Study on Thermal and Electromagnetic Properties of Superconducting Wires for AC Use

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Superconducting wires with a CuNi matrix for a.c. applications are not fully stabilized and normal transition is easily induced by various disturbances because the CuNi matrix has a high electrical resistivity and a very low thermal conductivity comparing with the properties of copper. Therefore, the thermal and the electromagnetic behavior of such wires during quench is apparently different from that of conventional copper-stabilized composites. To investigate these properties, we consider the effects of temperature gradient in radial direction of wire cross-section and the redistribution of currents at normal front. We measured longitudinal normal-zone propagation velocities of NbTi/CuNi composites wires which have different cooling condition and compared them with analytical results by the three-dimensional finite element method taking into account the redistribution of currents; influence of the transverse electrical conductance on the current sharing in a composite. The analytical … More results agree well with experiments. And in a.c. use of superconductivity we also need to know the thermal and electromagnetic behaviors during such abrupt increase of transport currents as short-circuit current in superconducting transformers, current changing in super-conducting current limiters and current redistribution during quench process in multi-strand cables. We measured the quench current of NbTi/CuNi superconducting wires as a function of a external d.c. magnetic field, the current sweep rate in the range of 10A/s to 100KA/s and the initial current, which is a transport current just before current changing. Experimental data is compared with analytical results by the finite element method taking the magnetic and the thermal diffusion in radial direction of wire cross-section into account. From these results, the mechanism of normal-zone propagation, the quench current degradation and influence of these phenomena on the abnormal quench process with apparent high normal-zone propagation velocity in multi-strand superconducting cables were discussed. Less