In the new power system

studying the system oscillation mechanism brought about by the large-scale grid integration of distributed energy resources is particularly important. To study the impedance characteristics and main influencing factors of grid-forming control in modular multilevel converter (MMC) systems

an integral modeling method is adopted under the complete virtual synchronous generator (VSG) control structure

and the influence of active power-frequency control loop

reactive power-voltage control loop

and non-ideal dq coordinate transformation is fully considered to establish an accurate MMC impedance model. Based on this grid-connected model

the impedance characteristics of MMCs are further analyzed

and the influence of control parameters and power grid strength on system stability is explored. The relationship between the control parameters is revealed. The results show that the increase of the modified damping coefficient and the decrease of the virtual rotational inertia or reactive power adjustment coefficient are beneficial to improve the system's stability. The correctness of the theoretical analysis and the adaptability of VSG control to a weak power grid are verified by simulation in MATLAB/Simulink. This paper provides a theoretical basis for addressing the MMC oscillation problem in weak power grids and the regulation and optimization of system parameters.