中文摘要：

为预测基于模块化多电平换流器的高压直流输电系统（MMC-HVDC）电容预充电的准确时间并实现子模块电容的额定充电,考虑冗余子模块在内,提出了一种启动预充电和停机过程都适用的协同调制策略。首先,建立了模块化多电平启动预充电不可控阶段的动态数学模型,并据此修正了不可控阶段直流电压的表达式以及启动限流电阻选取的表达式。其次,分析了换流器解锁瞬间存在冲击电流的原因,调整了限流电阻的切除时间,从而较好地抑制了解锁时刻的冲击电流。再次,针对换流器子模块级联的特点,提出了一种模块协同直流电压的调制策略,该策略解决了换流站直流电压和子模块电容电压的匹配问题。最后,在MATLAB/Simulink中搭建了MMC-HVDC预充电仿真模型,对所提调制算法进行了仿真验证,结果表明,随着直流电压从Up开始升至Udc0期间逐渐将每相投入子模块数目从T（每桥臂子模块数目）下降至N（额定投入运行数）,可以将子模块电容电压充电至额定值。

英文摘要：

To accurately forecast the capacitor＇s pre-charging time of modular multilevel converter based high-voltage direct current （MMC-HVDC） and to realize nominal pre-charging of the sub-niodules, we proposed a synergy modulation strategy which suits both start and shutdown procedures after taking redundant sub-modules into consideration. Firstly, we established a dynamic mathematical model of modular multilevel converter（MMC） in the uncontrollable pre-charging stage, by which we modified the expressions of DC voltage and current suppression resistance. Secondly, we analyzed the reason of impulse current at the moment of MMC deblocking and adjusted the cut-off time of current limiting resistance to restrain the impulse current. Thirdly, according to the cascaded feature of MMC converter, we proposed a cooperative modulation control strategy to match the DC voltage of converter station to the capacitor voltage of sub-modules. Finally, we established a numerical model of MMC-HVDC pre-charging in MATLAB/Simulink to test the proposed modulation strategy. The results show that, by using the proposed strategy, the capacitor voltage is successfully charged to its nominal level in the progress of increasing the DC voltage from Up to Udco and decreasing the number of operating sub-modules from the full set to the rated number.