A high temperature superconducting (HTS) triaxial cable fits long-distance power transmission compared to the three-in-one superconducting (SC) cable, since the HTS triaxial cable has lower ac loss and larger cooling channel, and the HTS tape length used for the HTS triaxial cable is much shorter than that of the three-in-one SC cable. We investigated a suitable cable structure of the HTS triaxial cable, which has the limitation of the outer diameter (150 mm), cooled by counter-flow cooling for long-distance power transmission using the numerical analysis that considered the heat transfer from the cable outside and the heat generation caused by the electrical loss and the frictional loss of a coolant inside the cable. Our results show that it is very important to expand an outside channel and increase the flow rate in order to operate the HTS triaxial cable for a long-distance. The suitable flow rate for increasing the cable length that each cooling station can cool the HTS cable at 77 K or lower is about 50 L/min. The maximum cable length is 1.7 km at the conductor diameter of 88 mm. This cable length of 1.7 km is larger than that of the three-in-one cable; the maximum cable length of the three-in-one cable is less than 1 km. Therefore, the number of cooling stations can be reduced by adopting HTS triaxial cable.
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