Novel Circulating Current Suppression Strategy for MMC Based on Quasi-PR Controller

According to the advantages and disadvantages of the traditional control strategy used in modular multilevel converter (MMC), an improved control method is discussed in this paper. In the proposed method, the outer loop of traditional control strategy is still in use to balance submodule capacitor voltage. However, the dc component extraction and quasi-proportional resonant controller (quasi-PR) controller are added to modify the inner loop to eliminate second-order circulating harmonic current. Besides, design principles of parameters related to the improved part are given in discrete domain and the stability analysis are also discussed. Finally, experiment results under traditional and improved control strategy are analyzed and compared, which indicate that the improved control method has the merit of capacitor voltage balance and performs well in suppressing circulating harmonic current.