A circulating current suppression method with adaptive virtual impedance for multi-bidirectional power converters under unbalanced conditions

Abstract

Multi-paralleled bidirectional power converters (BPCs) are usually used to improve the power capacity and reliability in an AC/DC hybrid microgrid. However, circulating current among multi-BPCs has been one of the challenges and it can be aggravated in the presence of non-ideal operating conditions, such as unbalanced AC voltages, mismatch of hardware parameters. In order to suppress the circulating current, this paper proposes a distributed method based on adaptive virtual impedance, which also employs positive sequence power droop control and voltage deviation compensation control. The traditional positive sequence power droop control is adopted to only regulate the positive components of BPCs output voltage. The negative sequence power term is fed to adaptive virtual impedance generator to modify the damping characteristics of BPCs. Also, an adaptive virtual impedance based voltage deviation compensation method is proposed to recover fluctuated output voltage of BPCs due to droop action and the power fluctuations. The fully distributed regulation of adaptive virtual impedance enables the load power to be shared accurately among BPCs modules and thus the circulating current can be effectively suppressed. The proposed control strategy does not require an additional communication system and the precise parameters of hardware equipment and line impedance. Furthermore, the effectiveness of the proposed method is verified by the experimental results.