Impedance-Based Characterization of Positive–Negative Sequence Separation and Its Effects on MMC-HVDC Stability

Impedance models of modular multilevel converter (MMC) are required to analyze the stability of the MMC high-voltage direct current (HVDC) system. In practical engineering, the positive–negative sequence separation algorithm is the basis of positive and negative sequence current control of MMC. Therefore, the influence of sequence separation on the impedance models and stability of MMC system needs to be further studied. This article develops complete impedance models of the MMC with consideration of all control loops and positive–negative sequence separation and comprehensively analyzes the impact of sequence separation on impedances and stability of the MMC-HVDC. It is found that the positive–negative sequence separation results in periodic fluctuations in the middle- and high-frequency regions of ac impedance and distinct fluctuations in the middle-frequency region of dc impedance. Meanwhile, the difference in the parameters of the positive and negative sequence current controllers will greatly aggravate the fluctuations of both ac and dc impedances and increase negative damping of the system. Moreover, the analysis reveals that the sequence separation is a key factor weakening the stability of the MMC-HVDC. Final, the correctness of the impedance models and the theoretical analysis are verified by the experimental results.