Single and multi-phase AC losses in HTS prototype power transmission conductors

AC losses in several 1 m long high temperature superconductor (HTS) prototype multi-strand conductors (PMCs) have been measured with a temperature-difference calorimeter. Both single-phase and three-phase losses were examined with AC currents up to 1000 A rms in the PMC. In addition, "two-phase" losses, with current flowing in the two normal conductor phases but not in the PMC, were also investigated. The calorimeter, designed specifically for these measurements in the liquid nitrogen temperature range, has a precision of 1 mW. The PMCs consist of two layers of helically-wound Bi-2223/Ag sheathed HTS tape, each layer wrapped with opposite pitch. Losses were found to depend on about the third power of the current-a result in agreement with the Bean-Norris model adapted to the double-helix configuration. Increasing either the critical current I/sub c/ of the tapes or decreasing the temperature of a PMC at fixed current resulted in decreased single-phase losses, in accord with the model. The three-phase losses indicate a significant interaction between phases and a smaller dependence on tape I/sub c/ than expected.