Capacitor Voltage Ripple Suppression of Modular Multilevel Converters Based on Improved High-Frequency Injection Method

The main challenge of the medium-voltage drive with modular multilevel converters (MMCs) is the capacitor voltage ripple, and the existing suppression method inevitably injects high-frequency common-mode voltage and circulating current into MMCs. However, circulating current errors occur with the increase of common-mode voltage frequency and lead to failure of high-frequency injection methods. This paper proposes an improved high-frequency injection method, which can suppress the capacitor voltage ripple under circulating current errors through compensating the circulating current reference. The proposed improved high-frequency injection method can reduce the peak value of arm current and regulate the compensation parameter automatically. Simulation studies with the professional tool PLECS are conducted and results confirm the effectiveness of the proposed improved high-frequency injection method.