Phase shifts of parallel currents in a single-Layer model of Superconducting cables

To reach large current carrying capability and mechanical flexibility, superconducting cables consist of many single conductors connected in parallel. Distribution of ac current into these parallel paths is controlled by their impedances, which often are not identical. We have found that, even in the case of a cable model made from straight Bi-2223 tapes placed in just one single layer, the nonuniformity of contact resistances causes a dispersion in the amplitudes and phases of ACs in individual tapes. Two models, one analytical and other a Finite Element Model, were used to predict the influence of phase shifts on ac loss and its measurement by an electrical method. Due to phase shifts of local magnetic fields, the purely inductive component of voltage taken by an arbitrary pair of taps is not exactly out-of-phase regarding the total cable current. Then, its product with the cable current is evaluated as a false loss-contributing component. Fortunately, with the use of voltage taps and wires regularly distributed around the cable, true value of ac loss can be recovered.