Droop Control of Three- and Four-Leg Hybrid Parallel Inverters Based on Three-Phase Generalized Coordinate Transformation

In recent years, widespread adoption of three-leg inverters has been observed. However, there is often a need for three-phase four-wire inverters to provide a neutral connection for asymmetrical loads within microgrid contexts. This article proposes a three- and four-leg hybrid inverter parallel structure to address the above issues. The aim is to fulfill the four-wire requirement under asymmetrical load conditions and mitigate the voltage unbalance problem. The article begins by investigating the output voltage balance control of three- and four-leg inverters and the control techniques of the hybrid parallel system. Deficiencies in traditional power droop control and virtual impedance control under unbalanced conditions are identified by calculating the inverter's output power under asymmetric load conditions. To address these issues, the article adopts average power droop and proposes a novel virtual impedance method. This method considers the impact of line impedance differences on the distribution accuracy of active power and unbalanced load current, effectively improving their distribution accuracy. Connecting three- and four-leg inverters in parallel, the four-leg inverter is capable of significantly reducing the zero-sequence voltage of the common load terminal and enhances three-phase voltage balance. Finally, experimental results validate the feasibility and effectiveness of the improved virtual impedance control.