Flexible Power Control of PV Grid-connected Quasi-Z Source Inverter under Unbalanced Faults

Under unbalanced grid voltages, the currents injected into the grid become non-sinusoidal and unbalanced. The unbalanced grid current and voltage may lead to double-frequency oscillations in the active and reactive power. This paper proposes a flexible power control method for the quasi-Z source inverter in PV grid-connected systems under unbalanced conditions. The proposed control strategy can inject a sinusoidal current and mitigate the double-frequency oscillation of the active or reactive power delivered to the network. Furthermore, an easy-to-implement current limitation strategy is proposed to protect the power converter and deliver maximum power to the grid. A non-MPPT mode is triggered to handle the injected current under severe faults. Finally, simulation results are presented to validate the theoretical analysis and controller design.