Circulating Current Injection Method for the Modular Multilevel Converter

The circulating current of modular multilevel converter (MMC) is an inherent phenomenon. The existing circulating current analysis methods have mainly considered the power losses and the submodule (SM) capacitor voltage. Under low-frequency operation, MMC SM has a larger capacitor voltage ripple, and this paper proposes a novel circulating current injection method to reduce the SM capacitor voltage ripple. The method is based on injecting a circulating current, which is easily derived by the instantaneous information of MMC, within the phase to attenuate the lower frequency arm powers. And, it omits using an extensive lookup table and injecting a common-mode voltage, thus simplifies the control and avoids causing over-modulation. In addition, a mathematical analysis model is built and it reveals the impact on the SM capacitor voltage ripple. The effectiveness of the introduced circulating current injection method is demonstrated by the experimental results.