中文摘要：

为使模块化多电平换流器（modular multilevel converter,MMC）的调制比保持在合适的区间内,提出了一种附加直流电压控制策略,可保证换流器输出全电平数的交流电压阶梯波形且稳定运行。首先,通过分析正弦脉宽的调制原理,得到满足换流器输出全电平电压波形且稳定运行的调制比最小限值和最大限值。然后,根据直流电压与调制比的非线性关系,设计了附加直流电压控制器,保证直流系统内所有换流器的调制比均运行于合适区间内。最后,采用11电平的双端MMC系统对所提策略进行验证。仿真结果显示,MMC-HVDC系统总能保持完整电平数的输出波形,并且总能够在不同的工况下寻找到一个新的稳态平衡点运行,验证了所提策略的可行性。

英文摘要：

An additional DC voltage control strategy is proposed to ensure the modulation index of a modular multi-level converter within a proper range and guarantee the converter to operate in steady state. With the proposed control strategy, the AC stepwise waveform generated by the converter could keep full voltage levels. Firstly, through analyzing the theory of sinusoidal pulse width modulation, the minimum and maximum limits of the modulation index is proposed. Secondly, the additional DC voltage controller is designed with a non-linear relationship between the DC voltage and the modulation index. Finally, a two-terminal MMC-HVDC system with 11 voltage levels is used for validating the proposed additional DC voltage control strategy. Simulation results show that the stepwise waveforms of the converter could keep full voltage levels and the MMC-HVDC system is able to reach a new steady state when operation point of the system changes. The proposed control strategy is proved effective and flexible.