Stability analysis of modular multilevel converter based on harmonic state-space theory

Modular multilevel converter (MMC) is regarded as one of the most potential topologies for high-voltage direct current transmission system. However, the multi-frequency behaviour and complex control structure of MMC bright great challenges to the stability analysis, identification of factors affecting stability, and controller design. To address this problem, the harmonic state-space (HSS) theory is applied to MMC in this study. First, the small-signal HSS model is established to describe the multi-frequency behaviour of MMC. Capacitor voltage control, circulating current control, and output current control are also taken into account in the modelling. On this basis, the stability analysis is performed by evaluating the eigenvalues of the HSS model, and participation factor is introduced to identify the factors affecting stability. In addition, the proposed approach reveals the influence of different controllers on the system stability, providing guidance for the tuning of multiple controller parameters. Finally, the effectiveness and accuracy of the proposed approach are verified by simulation and experiment results.