中文摘要：

多端柔性直流输电（voltagesourcedconverterbasedmulti．terminalhighvoltagedirectcurrent，VSC—MTDC）~安全运行至少应满足N-1法则，即当一个换流站由于故障或检修退出运行时，剩余系统可以恢复功率平衡而继续稳定运行，且暂态过电压不会超过设备绝缘裕度。为了维持VSC．MTDC直流电压稳定及整个网络功率平衡的站间协调控制，提出一种改进直流电压下垂控制策略，同时引入一个公共直流参考电压，参与下垂控制换流站的功率调整。在PSCAD／EMTDC中建立基于模块化多电平换流器的三端VSC．MTDC仿真模型，在稳态和暂态运行工况下对所提直流电压控制策略进行仿真验证。结果表明，所提策略可抑制换流站交流侧故障引起的直流侧功率振荡，进行换流站退出运行后系统功率的紧急输送，提高了VSC．MTDC系统的运行稳定性。

英文摘要：

The operation of voltage sourced converter based multi-terminal high voltage direct current （VSC-MTDC） system are required to meet the minimum N- 1 criterion, that is, when one converter is out of operation due to fault or maintenance, with the surviving converters a new power balance can be re-established and no voltage spikes, which are destructive to the power electronic switches, can be produced. In order to maintain the DC voltage and keep power balance, an improved DC voltage droop control strategy was proposed to control the DC voltage and schedule the DC power in the modular multilevel converter （MMC）-based VSC-MTDC system. A common DC voltage was introduced to participate in the DC voltage droop control. A MMC-HVDC simulation model including three stations was established on PSCAD/EMTDC. Simulations were done to validate the feasibility of the proposed improved droop control strategy under both the steady and transient states. The simulation results prove that the strategy can suppress the oscillation in the VSC-MTDC system caused by the AC side faults and the system could continue functioning even any one of the converters are tripped from the VSC-MTDC network.