Control for grid-connected and stand-alone operations of three-phase grid-connected inverter

This paper describes a simple grid current control method for the grid-connected operation, and inverter voltage control method based on the phase locked loop (PLL) for the intentional islanding operation at the three-phase grid-connected inverter. The PLL controller based on the pq theory with a simple P-controller is used to synchronize the phase of inverter output voltage with a grid voltage at the grid-connected operation or generate a desired inverter output voltage at the islanding operation. The outputs of current controller are connected together to those of voltage controller, in order to prevent a sudden change of the outputs of both controllers during the transfer instant. The simulation and experimental results are carried out to verify the effectiveness of the proposed control strategies.