An Improved PLL-Based Control Technique for Seamless Transition Between Grid-Connected and Islanding Modes

This paper proposes an improved practical phase-locked loop (PLL) based control with minimum PI Controllers for a seamless transition between grid-connected and islanding modes and vice versa for an AC microgrid. The developed controls are suitable for converters in droop control mode of operation in islanded mode and grid following control mode for grid-connected mode. Further, it also addresses issues of transients on connected load during the initialization of the island to the grid synchronization process. A detailed design approach for developed controls is presented. The proposed system controls have been verified by simulation studies and validated for Control hardware in a loop system consisting of one 500 kW Grid Forming converter and a 500 kW Grid Following Converter operating in parallel.