A Novel Two-Degrees-of-Freedom Current Controller Based on Virtual Impedance for Grid-Connected Inverters under Unbalanced Conditions

This paper proposes a novel current controller for three-phase grid-connected inverters under unbalanced conditions, which is based on the series virtual impedance algorithm. Unlike the conventional methods, the transformation of unbalanced current to positive and negative sequence components is not required. Thus, the current controllers are only two, considering the controller is applied in the d-q synchronous reference frame (SRF). Here, the current controller regards unbalance as disturbance, so current reference and disturbance suppression responses can be separated. By doing separation, the controller has two degrees of freedom, meaning the suppression of disturbance does not affect the reference response. Simulation and experiments have been done to justify the effectiveness of the proposed method, which can suppress the current unbalance factor from 23.33 % to 0.13 % and current THD from 8.9 % to 3 %.