Instantaneous Complex Power Controller for a Grid-Tied Inverter Connected to an Unknown Grid

This paper proposes a control strategy for injecting complex instantaneous power from a grid-feeding converter into a grid of unknown impedance. Before designing the controller, the steady-state stability and safe operation limits of injecting complex instantaneous power to a grid of unknown impedance are analyzed. This analysis is independent of the control strategy, and applies to all power injection schemes. Then, the proposed control strategy is presented, designed measuring the Point of Common Coupling (PCC) voltage and assuming a low impedance grid, as it is typically done in this kind of controllers. The transient stability of the proposal is then analyzed when in presence of an unknown grid impedance, and it is found that filtering of the PCC voltage is required to guarantee the transient stability of the system. The use of a resonant filter is proposed to this end, and a criterion to select the gains of the filter and the controller is derived. Finally, simulation results are presented for validating the proposed method.