Circulating currents control method for paralleled three-level NPC inverters

Parallel multilevel inverter becoming a very attractive structure for medium- and high-power/voltage applications. This is mainly due to the fact that it can achieve high-power level and overcome voltage /current limitations of switching devices. However due to the differences in hardware parameters, the circulating current appears in this structure and degrades its performance. This paper presents a circulating current control method for paralleled three-level neutral point clamped (NPC) inverter. The analytical model that describes the circulating current generation and behaviors is demonstrated, and then using this model, a control method to eliminate the circulating current is proposed. The proposed strategy is realized by introducing a control variable adjusting the application times of the redundant vectors of the three-level space vector modulation (SVPWM). In addition, a PI-controller based on closed loop control is synthetized to determine automatically the appropriate adjustable duration, during which the redundant vectors will be applied to eliminate the circulating current among the paralleled inverter. Finally, the effectiveness of the proposed circulating current control method under different operating conditions is confirmed through experimental validation.