Distributed Coordination Control for Multiple Bidirectional Power Converters in a Hybrid AC/DC Microgrid

This paper proposes a distributed coordination control method for multiple bidirectional power converters (BPCs) in a hybrid ac/dc microgrid. The researched hybrid ac/dc microgrid is composed of both ac and dc subgrids connected by multiple parallel BPCs, and only the ac subgrid contains storages for the sake of economy. The proposed control method can realize the appropriate power interaction between the two subgrids and make the two subgrids to support each other, so the hybrid microgrid can operate well in both the grid-connected and islanded modes. Concretely, there are three improvements in this proposed control methods. First, a d-q-0 three-axis control strategy instead of the conventional d-q two-axis control strategy is adopted to suppress the circulating current on the ac side generated by the multiple parallel BPCs. And then the square of the dc voltage rather than the dc voltage itself is controlled, which can make the control of the dc voltage linearized and can make the BPCs provide well voltage support for the dc subgrid. Second, the dc droop control is adopted for the dc current sharing. Also, in order to relieve the storages from generating too much reactive power if the ac load reactive power is very large in the ac subgrid, the reactive power droop control is adopted to make the BPCs share some reactive power. Third, taking the capacities and load types of the two subgrids into consideration, a coordinated control strategy based on the dc voltage versus the frequency droop is designed to realize the power interaction between the two subgrids. Furthermore, the stability analysis of the proposed control method is conducted. All the conclusions are verified by the real-time hardware-in-loop tests.