Impedance Modeling of Modular Multilevel Converters by Harmonic Linearization

The impedance modeling of modular multilevel converters (MMCs) is the prerequisite for applying the impedance-based stability analysis method to MMC-based power electronic systems. MMC has complex internal dynamics, e.g., harmonic circulating currents and capacitor voltage ripples, which might have a significant impact on operation stability of the interconnect system based on MMC. The harmonic linearization method, which is able to capture the internal dynamic behaviors of an MMC, is used to develop the ac-side small-signal impedance models of the MMC in this paper. An inherent low-frequency resonance phenomenon in the ac-side impedance of the MMC is revealed. Furthermore, the influence of the closed-loop control on the ac-side impedance of the MMC is also examined. Finally, the analytical impedance models are validated by the measured results in the time domain simulation by MATLAB/Simulink.