To analyze the inf luence of±400 k V Qinghai-Tibet HVDC transmission system on transmissionline protections in Qinghai AC power system,a closed-loop simulation system was constructed by combing Hyper Sim system with HVDC control protection devices.Various faults on double-circuit 750 k V and multicircuit 330 k V AC transmission lines in Qinghai power system were simulated.The impedance characteristics and harmonic components at Qinghai side of Qing-Tibet DC transmission line were analyzed.The harmonic proportion in voltages and currents were studied for faults that took place at different locations near the DC system.The inf lence of Qing-Tibet DC system on the directional components of protections,differential protections and distance protections of AC transmission lines was discussed and drew the conclusions that the DC sytem had little inf luence on differential protections,while had great inf lence on directional components and distance protection.The conclusions can provide reference for studying the interaction between AC and DC systems.
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