Effective PLL-Based Control Technique for Seamless Transition from Islanded Mode to Grid-Connected Mode

This study presents a practical phase-locked loop (PLL) based control system that uses minimal PI Controllers to ensure a smooth transition from islanding mode to grid-connected mode for an AC microgrid. The developed controls are specifically designed for converters operating in droop control mode during islanded operation and grid following control mode when connected to the grid. The system also effectively manages transient issues on the connected load during the island-to-grid synchronization process. The paper provides a comprehensive design approach for the developed controls and demonstrates their practicality and effectiveness through simulation studies and validation using Control hardware in a loop system comprising a 500 kW Grid Forming converter and a 500 kW Grid Following Converter operating in parallel.