Resistive superconducting fault current limiter AC loss measurements and analysis

Resistive superconducting fault current limiters (SFCL’s) offer the advantages of low weight and compact structure. Multi-strand Magnesium Diboride (MgB2) wire can be used in SFCL coil design to increase the transport current capacity. A monofilament 0.36 mm MgB2 wire with a stainless steel sheath was used to build three SFCL coils with 3 strands, 16 (9+7) strands and 50 (28+22) strands of the MgB2 wire. The quench current level and AC losses in the MgB2 wire are critical design parameters for a resistive SFCL. The experimental results showed the measured quench current densities reduced as the strand number increased and the AC losses increased as the strand number increased. An axisymmetric 2D finite element (FE) model therefore was built to analyze the current distribution and the AC losses in the coil. The multi-stranded coil FE model showed that proximity effect can modify the current distribution in the strands. This not only reduces the current carrying ability, but also increases the AC losses non-linearly. The FE model confirmed the issues highlighted by the experimental testing. Finally a winding method for multi-strand coil has been proposed to reduce the impact of these effects.