Simplified Frequency-Domain Model of Modular Multilevel Converter

The dynamics of circulating current (CC) of Modular Multilevel Converters (MMCs) can be proved to affect the dynamics of fundamental frequency output, but the strong coupling and complex signaling relationship makes the affect paths fuzzy, which will be inconvenient for various oscillation analysis and optimization control design. Thus, a simplified method for frequency-domain MMC model is proposed in this paper, which considers the dynamics of every signal paths when a small disturbance occurs and neglects the insensitive signal paths in the same node. Due to the sensitivity of signal path relates with the operating point, two typical operating conditions are chosen for extracting the sensitive signal paths. The first is dominated by the active current, and the second is changed to the reactive current. Integrating the main affect paths, the simplified frequency-domain MMC model can be obtained. It is found that the coupling between the active and reactive currents is inevitable, which is related with the physical parameters and operating conditions, and the frequency characteristic below 100 Hz is matched well from the comparisons by frequency-domain bode graph. Finally, the obtained simplified model is verified through MATLAB simulations in time-domain.