Stability Analysis and Robust Parameter Design of DC-Voltage Loop for Three-Phase Grid-Connected PV Inverter Under Weak Grid Condition

In the grid-connected inverter, both the phase-locked loop (PLL) and dc-voltage loop (DVL) can lead to the frequency coupling in the weak grid. Instabilities caused by PLL frequency coupling have been widely discussed. For DVL, past studies have analyzed the effect of DVL parameters on the degree of frequency coupling, but it is quite unclear how DVL parameters affect the inverter stability and how DVL can be designed under the weak grid. Hence, by fully considering the abovementioned frequency coupling, an accurate admittance model of photovoltaic grid-connected inverter is established in this article. By deriving the modified admittance matrix, the effects of grid impedance and DVL parameters on the frequency coupling are clearly studied. In addition, by modeling the equivalent inverter output impedance, the impact of DVL on the inverter stability under the weak grid is revealed. As proved, the increase of DVL bandwidth <inline-formula><tex-math notation="LaTeX">${f}_{vbw}$</tex-math></inline-formula> and grid impedance will aggravate the frequency coupling, but the current distortion does not necessarily increase. Meanwhile, the increase of <inline-formula><tex-math notation="LaTeX">${f}_{vbw}$</tex-math></inline-formula> helps to improve the inverter stability under the weak grid. Accordingly, a novel design for DVL in the weak grid is proposed. Experiments verify the correctness of the analysis and design.