Small-Signal Modeling and Analysis of MMC Under Unbalanced Grid Conditions Based on Linear Time-Periodic (LTP) Method

As in grid-tied Modular Multilevel Converters (MMC), the steady-state operation under asymmetric grid conditions is rather general, especially for the overhead line of the AC network. Therefore, the small-signal dynamics of MMC under unbalanced grid conditions have become a great important research topic. In this paper, the inherent periodic time-varying characteristic of MMC's original relationships is given priority consideration. Based on the linear time-periodic (LTP) method, a general small-signal LTP model under unbalanced grid conditions is built considering the internal dynamic of submodules (SMs). Then based on the eigenvalue locus analysis method from Floquet- Lyapunov theory, the effects of controller parameters and interactions between different controllers on small-signal dynamics are studied, respectively. Finally, the results from the detailed nonlinear MMC model are provided to verify the effectiveness of the proposed analysis.