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The stability of Modular multilevel converter (MMC) itself is the premise of analyzing the stability of MMC cascading with other cells, so this paper addressed the small-signal stability of MMC with dc voltage control mode which is the common operation mode of grid-side MMC in high-voltage direct current (HVDC) systems. First, the small signal model of MMC with open-loop control mode and dc voltage control mode while considering the dc voltage controller and current controller are established with the harmonic state-space (HSS) modeling method. Subsequently, eigenvalues and participation factors are analyzed to estimate the stability of MMC system and the influence of controller parameters, especially the proportional coefficients of dual-loop controllers, which can improve the design efficiency of control system. Finally, time-domain simulations in MATLAB/Simulink are provided to verify the accuracy of HSS small signal model and the effectiveness of the theoretical stability analysis.