Control stratories for grid-connected and island dualmode operated inverter under unbalanced grid voltage conditions

The grid-connected inverter works as a controlled current source in grid-connected mode, while operates as a controlled voltage source in island mode. The grid-connected inverter should continue to operate normally under unbalanced grid voltage. In case of utility serious faults or intentional islanding, the inverter should be able to seamlessly transfer from grid-connected mode to island mode. In order to achieve these aims, this paper proposed a new feedback multi-loop control strategy with capacitor current, capacitor voltage and grid-side current in αβ reference frame, in which a proportional resonant (PR) regulator is adopted. Compared with the dual-PI control scheme in dq reference frame, the PR controller does not require decomposition of positive-and negative-sequence grid side currents, the transient performance will be improved and low voltage ride through (LVRT) capability of the grid-connected inverters during unbalanced grid voltage condition. The proposed scheme can realize the adjustable power factor, and produce symmetrical and balanced currents in case of unbalanced voltage dips. With this control method, seamless transfer can be achieved between island and grid-connected mode, and load voltage quality during the transition is improved with the proposed control method. Simulations are carried out to verify the proposed controllers and algorithms.