Flexible Grid-Connected/Off-Grid Switching Control Strategy for Storage Inverter

Taking a dual-mode grid-connected/off-grid storage inverter as the research subject, control models for both grid-connected and off-grid operation modes were established. For the grid-to-off-grid transition, an improved adaptive active frequency drift islanding detection algorithm was proposed, which employs a cubic power-based detection method when frequency deviation is small to reduce positive feedback speed, and a parabola-based detection method when frequency deviation is large to enhance positive feedback speed. Compared with the traditional active frequency drift islanding detection algorithm, the proposed method can ensure islanding detection speed while effectively reducing the current total harmonic distortion during grid-connected operation. Experiments conducted on a storage inverter prototype demonstrated stable operation in both grid-connected and off-grid modes. The results indicate that the proposed control strategy enables rapid identification of operating conditions and mode switching, significantly improving the stability and reliability of the inverter during transition, thus laying a foundation for the autonomous operation of dynamic microgrids.