Research on DC Voltage Level Selection For Modular Multilevel Converters in Grid-forming Operation

The modular multilevel converter (MMC) has many advantages in grid-forming operation, and in island power supply scenarios the MMC is an effective solution. However, the volatility of the loads and the filters will influence the circuit parameters design (includes DC voltage). This paper analyzes the influence of the loads on MMC in isolated island power supply scenarios, and build a precise mathematical models to reflect the influence of loads on MMC’s modulation ratio. And this model finds the accurate modulation ratio of MMC can be calculated by the conventional modulation ratio and a proportional coefficient, after that the relationship between DC voltage and loads can be found, then the DC voltage can be selected accurately to meet the reliability and stability requirements of the system. Finally, the studies are verified by simulation.