A Repetitive Control Scheme for Circulating Current Suppression in Parallel Three-Level T-Type Inverters Under Unbalanced Conditions

Parallel three-level T-type inverters (3LT(2)Is) can increase power rating and reliability, but the circulating current problem will arise. Unbalanced conditions including unbalanced filter inductances and unbalanced grid voltage have negative effects on the operation of parallel 3LT(2)Is. This paper presents an improved repetitive control scheme for circulating current suppression in parallel 3LT(2)Is under unbalanced conditions. An average model of parallel 3LT(2)Is in dq0 reference frame is derived to analyze the circulating current. It can be seen that unbalanced filter parameters generate additional disturbance in zero-axis current system. A repetitive controller is designed to mitigate the disturbance caused by unbalanced filter parameters, and feed-forward terms are introduced to eliminate the remaining disturbances. The proposed control scheme can effectively suppress the circulating current in parallel 3LT(2)Is under unbalanced conditions. Experimental results are presented to demonstrate the effectiveness of the proposed scheme.