Circulating Current Suppression for Parallel Three-Level Inverters Under Unbalanced Operating Conditions

The parallel-operated three-level T-type inverters (3LT2Is) are increasingly used for large capacity renewable energy sources, which can increase power rating, efficiency, and reliability. However, the circulating current problem arises. Circulating current inevitably distorts output currents, increases power losses, and reduces the efficiency. Unbalanced operating conditions, including both unbalanced grid voltage and unbalanced filter parameters, cause adverse effects on the operation of parallel 3LT2Is. In this paper, a hybrid control scheme is developed to suppress the circulating current for parallel 3LT2Is under unbalanced operating conditions. An average model is derived and analyzed. Two quasi-resonant controllers are added to the conventional PI controller, and feed-forward control terms are introduced to eliminate the effects of disturbances in a zeroaxis current system. Circulating current suppression is realized by real-time adjusting dwell times of small vectors in space vector modulation technique. A proportional controller is adopted to balance the neutral point voltage. Simulation and experimental results validate theoretical analysis and the effectiveness of the proposed scheme.