A fault current limiter in toroidal form to maximise effective Jc

The performance and characteristics of a new form of inductively coupled fault current limiter (FCL) are described. It has a toroidal geometry and the prototype windings comprise 20 series connected primary coils interleaved with 20 independent superconducting secondary coils. By reducing leakage fields around the windings, the interleaving reduces the leakage impedance of the unit by an order of magnitude and significantly increases the effective critical current density of the superconductor. These effects are examined theoretically and experimentally and the benefits of reduced leakage fields on the performance of the FCL are assessed. It is shown that the benefits of symmetrically positioned superconducting coils which ensure that current is shared equally enable thick film materials to be used effectively for high currents, and that switching from superconducting to nonsuperconducting conditions is not a slow thermal effect, but the result of increasing magnetic field on the critical current density. To investigate the effect of current distribution across the width of the superconducting coils, the single turn versions were replaced by 3-turn, double-sided, interconnected spirals of equivalent conductor dimensions, which demonstrate superior performance, in terms of higher resistivity under nonsuperconducting conditions. The toroidal FCL is shown to be a useful vehicle with which to demonstrate the effect of magnitude and direction of the magnetic field on FCL performance. It has benefits in terms of leakage impedance and effective use of superconductor material and resistivity.