Grid-Forming Control Based on Capacitor Voltage Synchronization for Modular Multilevel Converter Under Weak Grid Condition

Considering the voltage source characteristics, grid-forming (GFM) control is regarded as an effective method to enhance the voltage support capability of inverters, especially under weak grid condition. However, relatively mature grid-forming control methods such as virtual synchronous generator control typically assume a constant voltage source on the dc side, which is not suitable for the modular multilevel converter (MMC) that controls the dc voltage. To address this issue, a GFM control strategy based on the decoupled control is proposed. This strategy achieves grid synchronization based on the dynamic variation of submodule capacitor voltage rather than the dc voltage, by linking the swing equation of the synchronous generator with the energy dynamic equation of the MMC. Additionally, the energy stored in submodule capacitors can be flexibly adjusted to provide inertial responses for the connected grid. With the proposed GFM control, the MMC can provide voltage and reactive power support under weak grid condition. Meanwhile, the dc voltage can be regulated independently through the decoupled control of MMC. The effectiveness of the proposed control method is verified through the simulation results in MATLAB/Simulink.