Theoretical study on ac loss properties of REBCO superconducting two-strand parallel conductors exposed to a non-uniform magnetic field

RE1Ba2Cu3O7-δ(RE: Rare Earth, Y Gd and so on, REBCO) superconducting tapes have great performance in critical current density, J_c, property even at liquid nitrogen temperature. However, it is necessary to enhance a current capacity for practical use. We proposed the configuration of parallel conductors and adopted it to several pieces of test transformers, e.g. a 66kV-2MVA one for a power grid, 25kV-4MVA one for a Shinkansen rolling stock, and so on. The constituent strands of parallel conductors need to be insulated and transposed for the sake of uniform current distribution and low ac loss. In case the transposition points deviate from the optimum ones, shielding current is induced according to the interlinkage magnetic flux of the twisted loop enclosed by the insulated strands and the contact resistances at the terminals. It produces an additional ac loss. Up to now, we have studied in the simple situation where parallel conductors are exposed to a uniform ac magnetic field. In this study, we studied the additional ac losses in the case that two-strand parallel conductors are located in non-uniform magnetic field. We derived theoretical expressions of the additional ac losses and discussed the dependences of them on the non-uniformity in magnetic field, the deviation length of the transposition point from the optimum one, and so on.