Suppression of Circulating Currents for Paralleled Three-Level T-Type Inverters Under Unbalanced Operating Conditions

Paralleled three-level T-type inverters (3LT2Is) have the merits of high power rating, improved reliability and convenient maintenance. However, there exists circulating current which will lead to current distortion and increased system losses. Especially in unbalanced operating conditions, the circulating current is even more serious, and the conventional control methods are difficult to reduce the circulating currents. To overcome this drawback, a generalized model for analyzing circulating currents in parallel 3LT2Is under unbalanced conditions including unequal grid voltage amplitude and inductance imbalance is developed, and a novel deadbeat control scheme is proposed for the circulating current suppression with the advantages of easy to implement and no inter-module communication. Compared to the conventional methods, controller proposed by this paper is simpler in design and parallel expansion is easier while the influence of unbalanced operating conditions is eliminated. The effectiveness of the proposed method is verified by simulation and experiment results.