A new circulating-current restraining method for modular multilevel converter

The common mode current of upper arm and lower arm in a modular multilevel converter (MMC) can be divided into two parts: a DC component and an AC component which is called the circulating current. The circulating current will increase power losses and may threaten the safe operation of the power devices and capacitors. To solve this question, this paper analyses the harmonics of the circulating current and makes the conclusion that the main components of circulating current are even and low order harmonics. Based on this conclusion, a new control method with parallel PI regulators is proposed. At last, experiment results demonstrate the correctness of the theoretical derivation and the effectiveness of the proposed control scheme.