A Novel Graded Pressure Equalizing Control Strategy for MMC Submodule

The Modular Multilevel Converter (MMC) has advantages such as low switching frequency, adjustable system output voltage and power level, and modular design, which are attributed to the cascaded structure of its sub-modules. When there is a large fluctuation in the capacitor voltages of the sub-modules and energy distribution among them is uneven, it can cause distortion in the output waveform. Therefore, balancing control of MMC sub-module capacitor voltages becomes crucial. This paper proposes a hierarchical voltage equalization control strategy that achieves balance by superimposing correction quantities on the capacitor voltages. Detailed analysis and explanations are provided for each component, and simulation results validate the effectiveness of this control strategy in achieving capacitor voltage balance.