Impedance modeling and stability analysis of PV grid-connected inverter systems considering frequency coupling

Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems. However, the existing impedance modeling of a grid-connected photovoltaic inverter usually only considers the effect of a single perturbation frequency, ignoring the coupling frequency response between the internal control loops of a grid-connected inverter, which severely affects the accuracy of the stability analysis. Hence, a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper. First, the generation mechanism of frequency coupling in grid-connected photovoltaic inverters, and the relationship between the coupling frequency and perturbation frequency are analyzed. Secondly, a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method. The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated, and the impact of a coupling frequency term on system stability is quantitatively analyzed. Finally, the advantages of the proposed method are verified by several simulations. The results show that the proposed impedance model can accurately predict the potential resonance points of the system, and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.