An Improved Steady-state Analysis Model for Nine-arm Modular Multilevel Converter with Higher Accuracy and Versatility

As a novel high-voltage and high-power multilevel topology with two sets of AC terminals, nine-arm modular multilevel converter (9A-MMC) can effectively reduce system volume, weight, and cost by multiplexing the middle arm. Steady-state analysis for 9A-MMC is crucial to its circuit parameters design, semiconductor devices selection, and operation performance evaluation, however, the existing steady-state analysis model of 9A-MMC is oversimplified and can only realize qualitative analysis. This paper proposes an improved steady-state analysis model by adopting more accurate modulation references and considering circulating current components in arm currents. Firstly, the fundamental circuit equations are established according to Kirchhoff’s laws of voltage and current. Then, the mathematical relationships among internal electrical parameters are derived from the internal circular interaction path. Next, according to the dynamic power equilibrium relationships, a set of equilibrium equations is constituted to solve the unknown variables in modulation references and arm currents, followed by all electrical parameters being obtained accurately. Finally, the feasibility and accuracy of the proposed analysis model are verified by both simulation and experimental results.