Resistive Superconducting Fault Current Limiter AC Loss Measurements and Analysis

Resistive superconducting fault current limiters (SFCLs) offer the advantages of low weight and compact structure. Multistrand magnesium diboride (MgB₂) wire can be used in the 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 that the measured quench current densities reduced as the strand number increased and the ac losses increased as the strand number increased. An axisymmetric 2-D finite-element (FE) model therefore was built to analyze the current distribution and the ac losses in the coil. The multistranded 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 nonlinearly. The FE model confirmed the issues highlighted by the experimental testing. Finally, a winding method for the multistrand coil has been proposed to reduce the impact of these effects.