Modelling and Transient Synchronization Stability Analysis for PLL-Based Renewable Energy Generator Considering Sequential Switching Schemes

The synchronization characteristics of phase-locked loop (PLL)-based renewable energy generators (REG) are considerably sensitive to the grid condition and the inner sequential switching actions of their control system, especially suffered from grid faults. In this study, the general output characteristics of REG systems are investigated considering the nonlinear behaviour of PLL. In addition, general sequential switching control schemes for the entire grid fault process are introduced. Then, to physically determine and theoretically analyse the transient synchronization stability of REG systems, the synchronization model of a REG system for different fault stages is built and presented in the form of rotor swing equations, which are similar to those of a synchronous generator (SG). The proposed model was able to deduce the characteristics of virtual torque, virtual inertia, and virtual damping coefficient of the REG system, and identify the coupling relationship between the angular frequency/magnitude states of the terminal voltage of the REG system during the synchronization process. Thus, the synchronization stability criteria were proposed based on those of the SG, in which the transient instability phenomenon and mechanism for different fault stages is physically explained by the deduced equal area criteria (EAC). Finally, the analysis was verified by simulations and experiments.