Characteristics of Non-Insulated High-Temperature Superconductor Coils in Time-Varying Magnetic Fields

Non-insulated coils made of second-generation high-temperature superconductors (HTSs) have been developed to prevent the burning of superconducting wires. Although these coils perform reasonably well in a constant magnetic field, the induced current in the coil is a serious concern in a time-varying magnetic field. Hence, we aimed to determine the characteristics of non-insulated HTS coils in a time-varying magnetic field. We manufactured two HTS coils: 1) non-insulated coil and 2) metal-insulated coil with a higher turn-to-turn resistance. Experiments were performed under a time-varying field generated using the stator of a 1 MW rotating machine. The HTS coil in the rotating machine was severely affected by an asynchronous or transient magnetic field. In addition, we designed a special structure to simulate severe operating conditions. As a result, the magnetic field of the HTS coil decreased in the time-varying field. The metal-insulated coil with a higher turn-to-turn resistance exhibited a lower field reduction than the non-insulated coil. We also performed circuit analyses on the experimental results. Our findings suggest the feasibility of applying HTS coils without insulation in rotating machines.