The high voltage direct current transmission system using modular multilevel converter (MMC) can effectively reduce the output harmonics and switching losses, while it will bring the outstanding system stability problems. Impedance-based stability analysis method is an effective way to analyze the harmonic resonance of MMC system. The frequency coupling effect of positive and negative sequence impedance affects the impedance characteristics of MMC. Ignoring the frequency coupling effect, the stability analysis of the converter grid-connected system may lead to inaccurate analysis results. Firstly, this paper reveal the frequency coupling mechanism of MMC by analyzing the interaction between perturbation components and steady-state harmonics. Secondly, the harmonic transfer matrix is used to establish the MMC frequency coupling model under constant power control mode. The main influence factors of frequency coupling characteristics and the influence on system stability are also analyzed. Finally, the simulation results based on MATLAB/Simulink validate the accuracy of the frequency coupling model and the correctness of the system stability analysis results.

MMC, multiharmonic linearization, frequency coupling model, system stability analysis