Modified sequence domain impedance modelling of the modular multilevel converter
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The accurate sequence impedance model of the modular multilevel converter (MMC) is essential for the precise stability analysis of various MMC-based interconnected systems. This paper presents a modified sequence impedance model for MMC considering off-diagonal coupling effects. Instead of simplifying the MMC as the single-phase model, the three-phase harmonic state-space model of the MMC is established based on harmonic state space theory. Then, the linear symmetric transformation is applied to transform the MMC's three-phase model into the modified sequence impedance model. The accuracy of the proposed model is validated by comparing with corresponding frequency scanning results in Matlab/Simulink. The research shows that there exist strong couplings between the positive and negative sequence of the MMC which cannot be ignored.