Control of parallel bidirectional converters under unbalanced conditions in an AC/DC hybrid microgrid

The parallel operation of bidirectional converters is a promising way to increase the power rating, efficiency and reliability, and maintain the power balance in an AC/DC hybrid microgrid. However, zero-sequence circulating current (ZSCC) and active-reactive disturbance current will be generated under the unbalanced conditions of the DC side and AC side. This paper aims to study the performance characteristics of the parallel converters and propose an effective control scheme to control the ZSCC and active-reactive disturbance current. Firstly, the generation mechanism of ZSCC and active-reactive disturbance current are analyzed based on an averaged phaseleg model. The coupling relationship is derived and two categories of disturbance can be found: the primary disturbance caused by the unbalanced condition of one side, and the secondary disturbance generated by the primary disturbance and the unbalanced condition of the other side. On this basis, a decentralized control scheme is proposed to suppress the primary disturbance, thereby eliminating the ZSCC and active power oscillation without the distortion of output current. Finally, simulation cases in PSCAD/EMTDC verify the correctness of the theoretical analysis and the validity of the proposed control scheme.