AC loss analysis of a HTS coil with parallel superconducting tapes of unbalanced current distribution

Considering the current capacities of the HTS tapes recently being developed, it is inevitable to use these HTS tapes in parallel in order to apply them to power transformers. But non-uniform distributions of current and large AC losses are major problems in a HTS coil with parallel superconducting tapes. Many parallel HTS tapes have been fabricated and tested so far. The results showed that we had to transpose each tape in order to reduce AC losses and have uniform distributions of current. But the measurement of the current distribution process has been quite difficult not only in the case of the parallel HTS tapes but also in the case of the coil with parallel HTS tapes. In our study, we investigated the effect of current distribution on the AC losses in parallel HTS tapes and coils with parallel superconducting tapes. The current distribution in parallel HTS tapes could be measured with small Hall sensors, the dependence of AC losses and critical current on the current distribution was analyzed. We fabricated a parallel HTS tape and HTS coil with the transposed parallel HTS tapes for an application of a power transformer. The performance of non-uniform current distribution in the parallel HTS wires is worse than that of a uniform one.