An Improved Grid Current and DC Capacitor Voltage Balancing Method for Three-Terminal Hybrid AC/DC Microgrid

In this paper, a three-terminal ac/dc hybrid microgrid with two dc terminals and one ac terminal is proposed. The proposed system consists of cascaded H-bridge (CHB) converters based ac grid interface and two dual active bridge (DAB) converters based dc subgrid interface that connects two isolated dc buses. In order to reduce the number of power conversion stages and power devices, the DAB converters are directly connected to CHB dc rails according to the system operation requirement. To overcome the imbalanced grid currents and dc rail voltages issues caused by this modified system configuration with only two power conversion stages, an improved method is proposed through the zero-sequence voltage injection in the CHB converters. In addition, to avoid the conflicts between zero-sequence voltage injection and the voltage/current regulation of the system, the impacts of the control parameters to the system stability and dynamic response are investigated. Evaluation results from both three-terminal and five-terminal hybrid ac/dc microgrids show that the generalized effectiveness of the proposed three-phase ac current and dc rail voltage balancing method.