Circulating current and DC current ripple control in MMC under unbalanced grid voltage

This paper presents a control method for MMC under unbalanced grid voltage condition. The control method regulates the power, line current, and circulating current of a MMC. Under the unbalanced condition, the objective of control is to reduce the negative-sequence line current. Furthermore, the circulating current of a MMC consists not only double-line negative-sequence component, which appears under the balanced condition, but also positive- and zero-sequence double-line components. Thus the controller should be able to eliminate all those components of the circulating current. A proportional-resonant controller (PR controller) is applied to regulate the positive- and negative-sequence components of circulating current. In addition, the zero-sequence component of circulating current is controlled by a dc current controller. The proposed control method is verified in MATLAB/SimPowerSystem.