Control strategy for seamless transfer between island and grid-connected operation for a dual-mode photovoltaic inverter

The dual-mode photovoltaic inverter is capable of operating either in grid-connected mode or island mode, acting as a current source for the ac grid in the former and a voltage source for the load in the latter. Transitioning from one mode to the next is non-trivial and can cause large deviations in voltage, current, and frequency because a mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. This paper presents a model predictive control (MPC) for a single phase, grid-connected voltage source inverter (VSI) to support dual-mode operation and seamless transfer between the two. This technique uses decoupled power control in grid-connected mode and voltage control in island mode respectively. Seamless transfer is implemented through synchronization algorithm and smoothly changing weighting factor for the cost function of the MPC. Compared with PI based controller, the MPC controller is simpler to implement since there is only one parameter to tune, the weighting factor. Combined with second order generalized integrator (SOGI), the synchronization algorithm is used to achieve fast phase detection and improved disturbance rejection. The theoretical analysis, simulation and experimental results validate the effectiveness of the proposed control strategy.