现有的二、三电平变换器容量有限，而模块化多电平换流器(modular multilevel converter，MMC)独特的级联扩展能力，使得其满足大功率、高电压光伏发电场所的要求，MMC结合到光伏发电系统中成为一种新趋势。将光伏组件直接并在MMC子模块中的新拓扑相较于两级式二、三电平光伏逆变系统少了DC/DC环节，大幅度提高了并网系统容量和电压，但是系统的稳定性大大降低，特别是光伏组件在局部阴影条件下的最大功率点电压不均衡以及MMC子模块电压不稳定成为一个难题，针对这个问题进行子模块的调制策略研究，提出一种不受MMC输出影响的独立调制策略。分析子模块电容电压与MMC输出的关系，将子模块电压稳定在光伏组件的最大功率点电压处。最后通过仿真验证该调制策略的可行性，结果证明该控制策略不仅提高了光伏利用率，还能对MMC子模块起到稳压保护作用，整个光伏并网系统对电网的谐波污染也更少。

The capacity of existing two-level and three-level converters is limited, and the unique cascading expansion ability of modular multilevel converter (MMC) makes it meet the requirements of high-power and high-voltage photovoltaic power generation sites. The combination of MMC into photovoltaic power generation system has become a new trend. Compared with the new topology of the two-stage two-level and three-level photovoltaic inverter system, the new topology of the photovoltaic module directly combined with the MMC sub-module has less DC/DC link, which greatly improves the capacity and voltage of the grid-connected system, but the stability of the system is greatly reduced. In particular, the voltage imbalance of the maximum power point and the voltage instability of the MMC submodule in the condition of local shadow become a difficult problem. To solve this problem, the modulation strategy of the submodule is studied, and an independent modulation strategy that is not affected by the MMC output is proposed. The relationship between the capacitor voltage of the submodule and the output of the MMC is analyzed, and the voltage of the submodule is stabilized at the maximum power point voltage of the PV module. Finally, the simulation verifies that the modulation strategy is feasible. The results show that the control strategy not only improves the photovoltaic utilization rate, but also plays a role in voltage regulation protection for the MMC sub-module, and the whole photovoltaic grid-connected system has less harmonic pollution to the power grid.