基于分级控制的模块化多电平换流器电容电压平衡控制

子模块电容电压的平衡关乎模块化多电平换流器(modular multilevel converter，MMC)能否安全稳定工作。根据MMC结构特点，将MMC电容电压的平衡划分为相间电容电压平衡和相内电容电压平衡2部分，并基于此划分提出一种基于分级控制的MMC子模块电容电压平衡方法：首先分析MMC相间电容电压平衡控制，在对MMC各相子模块电容电压平均值进行独立控制的基础上，实现相间电容电压平衡；然后，分析MMC子模块电容电压波动规律，通过对各桥臂调制波叠加平衡控制量，实现单相内部上、下桥臂间电容电压平衡和桥臂内子模块间电容电压平衡；最后，对所提控制策略在PSCAD/EMTDC软件中进行仿真验证。仿真结果表明该方法正确、有效，有利于MMC的电压稳定和安全运行。

Capacitor Voltage Balancing Control of Modular Multilevel Converter Based on Hierarchical Control

Capacitor voltage balancing is critical for modular multilevel converters (MMC). This paper presents a voltage balance method based on hierarchical control. Firstly, voltage control for one phase of MMC is established; to realize voltage balancing among different phases, average voltage of sub-modules (SM) in every phase is set to the same reference. Secondly, fluctuation law of capacitor voltage is analyzed, to suppress voltage imbalance between positive arm and negative arm, as well as voltage imbalance among SMs in an arm, voltage balancing method for one phase is proposed. At last, simulation for the proposed method is carried out using PSCAD/EMTDC, which confirms the validity of this method.