TEPCO and Furukawa have been developing compact 66-kV HTS power cables that can fit into 150-mm ducts. To realize these compact and highly efficient cables, decreasing AC losses in the cables is important. An effective method for reducing the AC losses is to suppress the electromagnetic coupling between layers and between filaments: equalizing the impedance of each layer suppresses the electromagnetic coupling between layers; and using twisted filaments in Bi-2223 tape suppresses the electromagnetic coupling between filaments. An HTS conductor was fabricated using Ag-Mg sheathed tapes that had twisted filaments, and the AC losses in the conductor were measured. The AC losses were lower than that of non-twisted filamentary conductors. Furthermore, the AC losses in an HTS cable were measured at temperatures ranging from 67 K to 77 K, using a device that can cool liquid nitrogen to less than 77 K. As a result of twisting the filaments in the tape and uniforming current distribution for the conductor, the AC losses were reduced to the lowest levels ever achieved.
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