Research on Grid-Connected and Off-Grid Control Strategy for Bidirectional Energy Storage Inverter

Bidirectional energy storage inverters serve as crucial devices connecting distributed energy resources within microgrids to external large-scale power grids. Due to the disruptive impacts arising during the transition between grid-connected and islanded modes in bidirectional energy storage inverters, this paper proposes a smooth switching strategy based on droop control to mitigate such impacts. Firstly, this paper introduces the principle of droop control under inductive line impedance conditions, elaborating on the relevant components and the design rationale and parameter selection for the control components adopted in this study. During the transition from grid-connected to islanded mode, an improved Active Frequency Drift with Positive Feedback (AFDPF) method is introduced. Building upon the traditional AFDPF, this method optimizes the chopping coefficient by incorporating a positive feedback coefficient and a piecewise function, and proposes a tracking step size with the cube root of the frequency difference, thereby enhancing the efficiency of islanding detection while ensuring harmonic quality in the system. Conversely, during the transition from islanded to grid-connected mode, this paper proposes a composite pre-synchronization control strategy based on droop control, which enables precise tracking of the phase, amplitude, and frequency of the output voltage of the bidirectional energy storage inverter relative to the grid voltage. Finally, simulation models are established using MATLAB/Simulink(MathWorks, R2022b, U.S.), and an experimental platform with the TMS320F28378DPTPS controller built for verification. The results demonstrate that the proposed switching strategy exhibits excellent stability and achieves smooth transitions between grid-connected and islanded modes, outperforming traditional control strategies.