Start-up Control Strategy of Flexible Low Frequency AC Transmission System Based on Y-MMC

Due to the Y-shaped circuit structure of modular multilevel converter in Y configuration (Y-MMC), the pre charging and grid connection processes are more complex, and the general start-up control strategy based on circulating current control is no longer applicable. To address the above issues, a progressive pre charging method and an undisturbed grid connection method based on model predictive control are designed, and a model predictive progressive start-up control strategy is proposed. Firstly, establish a predictive model for the current and capacitor voltage of each branch, and use the model predictive control to change the charging current circuit, amplitude, and phase, achieving the maximum active power input of controllable charging and gradually charging each branch. Secondly, flexibly switch submodules based on current error, gradually balance the potential difference of the charging circuit, and assist model predictive control to suppress impulse current. Then, coordinate the outer loop proportional integral control and the inner loop model predictive control, flexibly switch the outer loop power and voltage control modes, and realize the undisturbed grid connection process of the flexible low frequency AC transmission system based on Y-MMC. Finally, a two terminal flexible low frequency AC transmission system was built in Matlab/Simulink for simulation analysis. The results of the simulation show that the proposed strategy ensured low current impact and high charging speed during the startup process, enabling the system to enter a stable operating state smoothly and quickly.