The small-signal impedance modeling of modular multilevel converters(MMCs)is the key for the resonance and stability analysis of MMC-based power systems. MMC is a converter system featuring multi-frequency

nonlinearity and periodically time-varying operation trajectory due to its complex internal dynamics. Hence

traditional small-signal methods cannot be directly applied to MMC. The AC-side small-signal impedance of MMC is modeled by the harmonic linearization method

which has taken into consideration the harmonic circulating current and capacitor voltage steady-state ripples. Furthermore

the influence of the circulating current control on the AC-side impedance of MMC is analyzed. A detailed time-domain simulation model of a three-phase MMC is built by using MATLAB/Simulink and the analytical models are validated by simulation results. This work is supported by National Key Research and Development Program of China(No. 2016YFB0900901).