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Describing Function Method Based Power Oscillation Analysis of LCL-Filtered Single-Stage PV Generators Connected to Weak Grid

For the LCL-Filtered single-stage photovoltaic (PV) generators, the low-frequency power oscillation (<50 Hz) occurs frequently when they are connected to the weak grid, whose oscillation mechanism has been a hot topic recently. However, the existing stability analysis methods rarely take the complete model of the PV generators into consideration, thus the accuracy and completeness of the corresponding conclusions are influenced. In this paper, a novel stability analysis method is proposed to obtain the more accurate and complete results for the LCL-Filtered single-stage PV generators. First, the complete model of the PV generators is established including the perturbation and observation (P&O) based PV power loop and dc voltage loop, which are rarely considered by the existing stability analysis methods. Especially, the P&O based PV power loop is nonlinear and discontinuous, and the conventional small-signal modeling is inapplicable. Hence, its influence is completely ignored in the existing stability analysis methods. Second, considering the nonlinear discontinuous link, the describing function method is adopted to analyze the whole system stability based on the established complete model. In this way, the accuracy and completeness of the stability analysis are enhanced. Furthermore, for the critically stable state, both the oscillation amplitude and frequency can be calculated accurately. At the same time, different influence factors are analyzed quantitatively including the operation points, grid strength, and the interaction of multiple control loops. It is first revealed that the grid impedance has different influence on the system high-frequency and low-frequency stability. Also, the proposed analysis method and the conventional method are compared, which presents the advantages of the proposed analysis method. Finally, all the theoretical analyses are verified by the real-time hardware-in-loop tests.