A novel control strategy for smooth transition between stand-alone and grid-connected operations of distributed generation

This paper proposes a novel control strategy for smooth transition between stand-alone and grid-connected modes of distributed generation (DG). The proposed control scheme is designed in stationary (α-β) reference frame consisted of outer grid current control loop using a proportional resonant (PR) controller and the inner load voltage control loop developed with a repetitive controller (RC). The proposed control strategy is able to produce a sinusoidal load voltage in stand-alone mode and a sinusoidal grid current in grid-connected operation of the DG despite the presence of nonlinear local load. One unified control scheme is developed for both stand-alone and grid-connected operations so that the DG does not need to change control scheme and voltage or current spikes can be effectively prevented during transition period. Since the control system is developed in the α-β reference frame, coordinate transformations are not required, which can significantly reduce the complexity and improve the accuracy of the DG control system. The feasibility of the proposed method is validated through selected simulated and experimental results.