Balanced three-phase distribution experiment of a triaxial HTS cable

High Temperature Superconducting (HTS) cables have been studied because of their low loss and compactness compared with conventional copper cables. Three-phase cables are usually composed of three single-phase coaxial cables. Recently, triaxial cables, composed of three concentric phases, have been intensively developed, because they have advantages such as reduced amount of HTS tape, small leakage fields, and small heat loss in leaks, compared with three single-phase cables. However, there is an inherent imbalance in the three-phase currents in the triaxial cable due to differences in the radii of the three-phase current layers. The imbalance of currents causes additional losses and large leakage field in the cable, and also degrades the electric power quality. Therefore, we propose a new model, a triaxial cable composed of two longitudinal sections with different twist pitches to obtain the solutions of the balanced three-phase currents and the homogeneous current distribution in each phase of the triaxial cable. We derive a general equation satisfying both the balanced three-phase currents and homogeneous current distribution, as functions of the winding pitches, and finally apply it to the simplest cable. We fabricated and tested a 1-m HTS cable in order to verify that the proposed theory can satisfy the balanced distribution. The results demonstrate the validity of the theory. We also investigated the current distributions along a long triaxial cable considering the capacitances between the layers in the triaxial cable.