Closed-Loop Distribution Network by a Midvoltage Flexible HTS DC System

A scheme of closed-loop distribution network by a midvoltage flexible high-temperature superconducting (HTS) dc system is proposed and analyzed. This midvoltage (10-kV) scheme is compared, in terms of capital costs, with that of the closed-loop distribution network by a high-voltage (110-kV) flexible conventional dc system. Based on the HTS cables that are made of the first-generation Bi2223 tapes with a length of 2.5 km, the total investment cost of the 10-kV flexible superconducting dc system with a power capacity of 42 MVA is estimated about 1% lower than that of the 110-kV flexible conventional dc system with the same power capacity, owing to the substantial cost reduction of the voltage-source converter stations with the reduced voltage level. According to the “N - 1” principle of distribution networks, with the interconnecting flexible HTS dc system of 42 MVA, the total power to supply loads of the connected substations can be increased by the same power capacity of the interconnecting system. The simulation results of a three-phase short-circuit fault reveal that the voltage recovery time of the critical loads in the closed-loop distribution network can be reduced to less than 0.1 s, which is much faster than that in open-loop distribution networks.