Stability analysis of MMC grid-connected systems based on Floquet theory

When considering the internal dynamics of the modular multilevel converter, the multi-frequency frequency coupling problem cannot be ignored, which complicates the modeling process. Compared with other modeling methods, the method using linear time-periodic modeling can preserve each frequency component and take into account the frequency coupling effect, and its stability can be discriminated by Floquet theory. In this paper, the linear time-periodic model of modular multilevel converter grid-connected system is established, and a selection method of period parameters when finding the Monodromy matrix is given. The analysis results show that increasing the short-circuit ratio, decreasing the delay time and the phase-locked loop integration factor can make the system stable when considering the time delay, while for the phase-locked loop scale factor and the current loop scale factor both large or small may make the system unstable. Finally, a simulation model is built in MATLAB/Simulink to verify the analysis results.