Theoretical Analysis of Additional AC Loss Properties of Two-Strand Transposed Parallel Conductors Composed of REBCO Superconducting Tapes

Superconducting tapes made of REBa2Cu3Oy (REBCO; RE: rare earth, i.e., Y, Gd, or Eu) exhibit superior performance at the critical current density, even at liquid-nitrogen temperatures. However, the current capacity of these tapes must be enhanced for use in large-scale applications. We have proposed a method of constructing parallel conductors, which must be insulated and transposed to obtain uniform current sharing and low ac loss. If the transposition points deviate from the optimal ones, a shielding current is induced to maintain a constant interlinkage magnetic flux, which produces an additional ac loss. In our previous studies, the theoretical expressions for the additional ac loss were derived on the basis of a critical state model. In this study, by using an n-value model to express the I-V characteristics of REBCO superconducting tapes, we theoretically investigated the additional ac loss properties of two-strand transposed parallel conductors wound into solenoid coils. The results showed that under the nonsaturation condition, the additional ac loss increases with decreasing n value. This suggests that in parallel conductors composed of REBCO tapes that operate at liquid-nitrogen temperatures, transposition must be performed with high accuracy at the optimal points. To avoid increasing the additional ac loss, the n value of the tapes must be consistently larger than approximately 10, and the deviation of the transposition point should remain below a few percent of the conductor length.