Circulating Current Analysis and the Improved Virtual Impedance Droop Control Strategy Based on SMC for Parallel Three-Phase Inverter Systems

An improved virtual impedance droop control (VIDC) strategy based on sliding mode control (SMC) is proposed in this paper to effectively suppress the inter-phase circulating current of parallel three-phase grid-connected inverters. Unlike the traditional VIDC method, the proposed strategy is discussed and improved in four aspects: bus voltage stability, Q-U loop performance, P-f loop performance, and virtual impedance regulation. The generation mechanism of the inter-phase circulating current is emphatically analyzed, and the output voltage difference and power distribution are studied and integrated with the improved VIDC strategy to suppress the circulating current. The simulations are carried out to demonstrate that the improved VIDC strategy is effective in restraining the circulating current in the system while also significantly improving the system's response speed and output accuracy.