Effect of DC-Voltage Loop on Grid-Connected PV Inverter Stability under Weak Grid

In the grid-connected inverter, both PLL (phase-locked loop) and the DC-voltage loop will lead to frequency coupling. For the frequency coupling caused by PLL, many literatures have discussed its effect on the stability, and propose PLL improvement strategies to improve the inverter stability under weak grid. For the frequency coupling problem caused by DC-voltage loop, literatures do have analyzed the effect of DC-voltage loop parameters on the degree of frequency coupling. However, it is still unclear how the parameters and bandwidth of the DC-voltage loop affect the inverter stability and how the parameters should be selected under weak grid. In this paper, for the grid-connected PV inverter, the sequence admittance matrix is established by fully considering the frequency coupling caused by PLL and DC-voltage loop, and the equivalent output impedance is derived. Based on the impedance stability criteria, the effect of DC-voltage loop bandwidth on the stability of grid-connected PV inverters under weak grid is analyzed. The results of stability analysis show that the certain increase of the DC-voltage loop bandwidth is beneficial to the stability of the grid-connected PV inverter under weak grid. Finally, the correctness of the analysis in this paper is verified.