Research on Smooth Grid-Connected and Islanding Transitions Control Strategy for Microgrid Inverters Based on State Machine Theory

With the evolving demands of renewable energy power systems and the progressive establishment of industry standards, grid-connected inverters are required to possess enhanced functionalities. In microgrid systems, inverters must support grid stabilization through flexible charging/discharging during grid-connected operation, while maintaining autonomous islanded operation with load-bearing capability under specific conditions. These requirements present significant challenges in control strategy design. To address this, this study proposes an innovative mode-switching control strategy based on state machine theory. By introducing transition modes and optimizing state machine switching logic, the proposed strategy effectively suppresses voltage/current fluctuations, significantly improving system stability, dynamic response characteristics, and power quality. Experimental results validate the feasibility of the strategy, demonstrating active grid-to-island transition in 100 ms (50 % faster than the national standard requirement of 200 ms), active island-to-grid transition in 150 ms (25 % improvement over the 200 ms national standard), and passive grid-to-island transition within 120 ms (achieving a 94 % reduction compared to the 2 s national standard). The developed methodology ensures compliance with contemporary grid code specifications while exhibiting superior transient performance.