PLL-Free Active Synchronous Grid-Connected Control Strategy of Distributed Energy Storage

To achieve a smooth transition from the off-grid mode to the grid-connected mode for plug-and-play distributed energy storage (DES), the synchronization of the output voltage of DES and the grid voltage should be primarily considered. In this paper, an optimized active synchronization grid-connected strategy for DES is proposed. The impact of the grid-connected switching of distributed energy storage is analyzed. Considering that the product of two voltage vectors contains the information of both frequency and phase, a single frequency phase synchronization controller is designed which can rapidly achieve active synchronization of DES. The proposed PLL-free technology can avoid the accuracy and response speed problems that may be caused by the PLL during grid connection, and can also reduce the complexity of the control system. The simulation results show that the proposed control strategy can realize grid voltage quick tracking and smooth gird-connecting. The effectiveness of this method is proved.