Capacitor Voltage Control and Harmonic Suppression of Modular Multilevel Converter

The Modular Multilevel Converter (MMC) utilizes cascaded submodules to form a converter.As the number of submodules increases, higher output voltages can be achieved, making them suitable for medium and high voltage applications.They offer advantages such as low losses, high transmission efficiency, low switching losses, and reduced output harmonics.This paper investigates an MMC based on half-bridge submodules.Capacitor voltage control utilizes a combination of nearest-level approximation modulation and capacitor voltage balancing, employing a bubble sorting algorithm.This PI controller based on a feedback mechanism, the problem of circulating currents was solved by removing the coupling between the two feedback loops and obtaining two separate closed-loop control systems, the external control loop and the internal control loop.The results show that the circulating current is more stable, the distortion of the bridge current is reduced from 10.75% to 3.77%, the double-frequency content is greatly reduced.The simulation model was built with MATLAB-Simulink.The circulating current suppression effect is good, and the capacitor voltage waveform is stable.After the circulating current is suppressed, the rated current capacity of the power switching device can be reduced, which can reduce system costs and better extend the life of the power switching device.