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Distributions of the velocity, the temperature and the pressure of a liquid nitrogen (LN2) flow are analyzed at hydraulic cross-sections on two types of superconducting power transmission lines (SC PT). One is on a DC-SC PT, the other is on a three phase AC-SC PT. Inflow heat from the pipe wall and the cables to LN2 are evaluated, however the heat flux from the DC cable is zero. The channels are straight, and hydraulic cross-sections are 3.77 times 10-3 m2 and 4.27 times 10-3 m2 on the DC and AC-SC PT lines, respectively. The increased temperature and the pressure drops on the DC-SC PT lines are lower than those of the AC-SC PT. The fluid velocity dependence on the distance between two cooling stations and on the pump power are estimated. When cooling stations are installed every 10 km, the flow velocity of the DC and AC system are 0.10 and 0.25 m/s, respectively. The pump power of the DC system is dramatically lower compared with the AC system. Therefore, minimizing the SC PT by using DC systems is much more practical.