Stability and protection of coils wound with YBCO bundle conductor

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of a low-temperature superconducting (LTS) coil. A HTS conductor has a greater volumetric heat capacity at the operating temperature envisaged for practical applications. Therefore, a HTS coil is much less likely to be quenched than a LTS coil by mechanical disturbances such as the heat generated by the cracking of the impregnation material or by the friction resulting from wire movements. However, the HTS conductor is cyclically subjected to tensile strain because electrical charging and discharging are repeated in real applications involving the Superconducting Magnetic Energy Storage (SMES) system. The superconducting characteristics may locally deteriorate due to this cyclic strain. Therefore, to enhance the reliability and safety of the HTS coil, a quench protection scheme is needed. Because the normal-zone propagation velocity is quite low, detecting a non-recovering normal zone is difficult in HTS coils, and quenching produces excessive overheating that may cause the conductor to melt. In this study, we focus on a coil wound with a YBCO bundle conductor used in SMES applications and investigate the redistribution characteristics of the transport current in and the thermal behavior of the coil during a quench; we use a newly developed computer code based on the finite element method (FEM) and an equivalent circuit. We also discuss a protection scheme to dump the magnetic energy stored in the coils on an external resistance connected in parallel.