Abstract
High-temperature superconducting (HTS) cables have been studied because of their low loss and compact properties as compared to conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, a tri-axial cable composed of three concentric phases has been intensively developed, because it has advantages such as reduced HTS tape, small leakage field and small leakage heat loss as compared to three single-phase cables. However, there is an inherent imbalance in the three-phase currents in tri-axial cables due to the differences in the radii of the three-phase current layers. The imbalance of the currents causes additional loss and a large leakage field in the cable, and deteriorates the electric power quality. Therefore, a model composed with two longitudinal sections is proposed. This model allows us to determine cable construction parameters such as winding pitches and the radii of the three concentric phase layers, thereby satisfying both conditions: three balanced concentric phase currents and a homogeneous current distribution in each phase of the tri-axial cable. We formulate and derive general equations for the three-phase current distribution in the tri-axial cable as functions of the winding pitches of the three concentric phase layers. The equations are applied to tri-axial cables composed of one or two layers per phase.
