Multiphase AC loss mechanisms in HTS prototype multistrand conductors

We report on multiphase AC losses in four-layer prototype multi-strand conductors (PMCs) wound from high temperature superconducting (HTS) tape provided by American Superconductor Corporation, "Two-phase" losses are induced with no current flowing in the PMC but with an AC magnetic field generated by currents flowing in the two normal conductors arranged at the remaining corners of an equilateral triangle forming a three-phase configuration. This is a typical configuration that a power cable of the "warm dielectric" design could have. The losses were measured at 65 to 76 K, in a frequency range from 10 to 180 Hz, and for currents from 600 to 1600 Arms. We compare the losses for two PMCs, one wound conventionally with equal pitch angles for all layers and the second wound to achieve uniform current distribution (UCD) among the layers. The UCD method results in reduced single-phase losses at currents greater than about 1/2 of the critical current. However, the two-phase losses are somewhat larger for the PMC wound by the UCD method. We investigated this difference empirically and theoretically.