Nonlinear Stability Analysis for Three-Phase Grid-Connected PV Generators

Numerous emerging oscillatory stability issues have recently arisen as renewable energy sources (RESs) are integrated into the power system. Some of the existing literature attempts to explain these issues through the corresponding simplification, which usually omits the influence of the nonlinear discontinuous elements in the system. Hence, the obtained results are incomplete. To fill this gap, this article proposes a stability analysis method based on the describing function (DF) method to explore the influence of nonlinear discontinuous elements in the system. Without loss of generality, taking the three-phase grid-connected photovoltaic (PV) generator as the research object, the perturbation and observation (P&O)-based power control, which is a typical nonlinear discontinuous element, was carefully studied. First, the complete mathematical model of the three-phase grid-connected PV generator was established, including the outermost P&O-based power control. Then, according to the DF method, the stability of the PV generator was analyzed, and the related influence factors were studied in detail. It was found that the nonlinear discontinuous P&O-based power control has a substantial influence on the stability of the PV generator and can result in the oscillation. At the same time, the DF method and the conventional method were compared, which shows that the proposed DF method can enhance the accuracy of the stability assessment. Especially regarding critical stability, the oscillation magnitude and frequency can be calculated accurately. All the theoretical analyses were verified by the real-time hardware-in-loop (HIL) tests.