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Study on the characteristics of AC/DC hybrid system under UHVDC hierarchical connection mode

Ultra High Voltage Direct Current (UHVDC) is the technical foundation of Global Energy Internet. The lacking of supporting ability of receiving system brought by multi-infeed DC can be solved by a new pattern called UHVDC hierarchical connection (HC) during the transition period of weak AC system with strong DC power input, which was put forward by Chinese researchers from grid structure. The characteristics of receiving hybrid system are in need of overall and in-depth research under this mode. Based on an actual project, the equivalent system model under HC mode was established firstly, and the relevant stability evaluation indexes were put forward or improved; the absorbing ability of receiving AC system and the reactive power coupling characteristics were analyzed based on the improved indexes; at last, a method was presented to judge the possibility of commutation failure of both high-end and low-end converters. The validities of the indexes and the method was verified by PSCAD through dynamic time domain simulations. The research work of this paper can provide technical references for the safe and stable operation of actual projects.