中文摘要：

不对称交流电网下的功率波动将引起模块化多电平换流器子模块能量的不平衡，进而影响模块化多电平变流器型高压直流输电（modular multilevel converter based HVDC,MMC-HVDC）的动态性能。基于不对称交流电网下MMC桥臂瞬时功率的分析，确定换流器内部子模块电容电压及桥臂环流的控制目标。在此基础上，提出一种基于子模块电容电压预估的最近电平调制和基于桥臂环流预估的直接环流控制，两者相结合的复合控制策略。不论交流系统对称与否，在所提出的控制策略下，均能保证换流器上下桥臂间，三相间以及总子模块电容电压的相对平衡，实现对基频及二倍频谐波环流的抑制。基于PSCAD／EMTDC，建立两端MMC—HVDC仿真模型，分别在有功功率和直流电压控制站进行不对称交流电网的仿真验证。仿真结果表明，所提出的控制策略能够保证故障期间子模块电容电压平均值保持恒定，直流电压不会由于二倍频零序瞬时功率出现二倍频波动，系统故障穿越能力得以提升。

英文摘要：

The power fluctuation under asymmetric AC voltages caused the sub-module energy unbalance for modular multilevel converters, which thereby affected the dynamic characteristics of the MMC-HVDC system. By analyzing the arm instantaneous power of MMC, the control targets of the sub-module capacitor voltage and the circulating current were determined. On this basis, a control strategy composing of the sub-module capacitor voltage predicted based the nearest level modulation and circulating current predicted based direct circulating current control was presented. Whether the AC system is symmetrical or not, under the proposed control strategy, the voltages of sub-module capacitor each ann were kept balanced relatively, and the fundamental frequency and the second harmonic of the circulating current were suppressed. A MMC-HVDC simulation model of two ends was established based on PSCAD/EMTDC. The simulations under the asymmetric AC grid for the active power and the DC voltage control station were conducted respectively. Simulation results show that the proposed control strategy could keep the average value of the sub-module capacitor voltage remain constant during the AC system asymmetric fault, and avoid the DC voltage fluctuation caused by the zero sequence instantaneous power, which thereby improves the fault ride-through capability of the system.