Parallel operation of bi-directional interfacing converters in a hybrid AC/DC microgrid under unbalanced grid conditions

In order to integrate renewable energy into the utility grid with high penetration, AC/DC hybrid microgrids have obtained much attentions in recent years since they feature both advantages of AC microgrids and DC microgrids. In an AC/DC hybrid microgrid, the parallel-operated AC/DC bi-directional interfacing converters (IFCs) are key elements to connect AC bus and DC bus. When unbalanced grid faults occur at the AC bus, the active power transferred by the parallel-operated AC/DC interfacing converters is desired to be maintained constant and oscillation-free to ensure the stable operation of DC bus. However, with conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs will be affected due to the current rating limitations. Therefore, the parallel operation of IFCs in a hybrid microgrid under unbalanced AC grid conditions is studied, and a control strategy, which enhances the active power transfer capability with zero active power oscillation, is proposed. Adjustable reference current coefficients for parallel IFCs are introduced in the proposed control strategy to enable the flexible current sharing among IFCs. By utilizing the proposed control strategy, only one IFC needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power. Simulations and experiments have verified the feasibility and effectiveness of the proposed strategy.