Stability Analysis of Modular Multilevel Converter Using Nearest Level Modulation

Nearest Level Modulation (NLM) is a kind of staircase wave modulation suitable for MMC with massive submodules. There are several implementations of NLM for different application scenarios and operation conditions. In order to discuss the feasibility of different implementations and the inner stability of MMC, a state-space system is established firstly to represent MMC internal dynamics. Based on these results, Lyapunov theory is introduced for stability analysis. Optimized designs to improve stability are also discussed. The analysis of this paper shows that the inner stability of MMC depends on whether measured value or reference value of submodules voltage and dc-bus voltage are taken in NLM calculation. By designing common-mode voltage controller carefully, or low-pass filtering measured dc-bus voltage, inner stability of MMC can be effectively improved. Simulation results in PSCAD/EMTDC verify the validity of stability analysis of this paper.