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Tri-axial cable has been rapidly developed recently because of the advantages such as more compact structure, small leakage field, and low heat and AC losses. One the other hand, it causes an inherent imbalanced three-phase currents distribution due to the different radii of concentric layers. In our previous research, it is demonstrated that a proposed tri-axial cable composed of two longitudinal sections with adjusted twist pitches can transmit balanced three-phase currents. However, before the cable can be installed in a real grid, the heat reliability of the cable in steady-state operation and fault condition should be ensured. The heat balance calculation of the tri-axial cable in steady state under the balanced current distribution allows us to estimate thermal parameters of the cable such as inlet and outlet temperatures, pressure drop and operation temperature. When a phase current suddenly changes under fault conditions, the remaining phase currents become large because of mutual inductances between fault and sound phases. Simulation of the typical fault suggests the copper stabilizer amount on the HTS tape for stability and safety. It is found that copper of 2 mm in thickness keeps the cable temperature under 110 K.Index Terms-Fault current, HTS power cable, stabilizer, triaxial cable.

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SUMMARYHigh Temperature Superconducting (HTS) cables have been studied because of their low loss and compactness compared with conventional copper cables. Threephase 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 threephase 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.