A Single-Cell-Based Injection Method for Circulating Current Control in MMC

Modular multilevel converters are becoming more and more attractive for many high-voltage high-power applications. However, due to their topology and operation they present technical challenges in the implementation of the control system, such as balancing of the submodule capacitor voltages and suppressing the circulating currents. The circulating current introduces additional power losses, increases the current stress on power devices and reduces their lifetime. The aim of this paper is to propose an innovative control technique for the elimination of the circulating current by injecting a low-frequency alternating signal into a single submodule of each converter arm, to also achieve a reduction of the voltage ripple across capacitor, beneficial to extend their lifetime. Experimental results performed on a seven-levels converter are presented to validate the proposed technique and assess its performance in comparison with state-of-the-art approaches.