Mechanism Of The Impact Of DFIGs On Power System Stability

With the increasing interconnection of Doubly-fed induction generators (DFIGs), the impact of DFIG-based wind power on power system stability becomes more and more prominent. DFIGs are very different from synchronous generators in infrastructure and response characteristics, which brings new challenges to understand and analyze the mechanism of power system stability. Thus, revealing the effect of DFIGs on the power system stability has great theoretical significance and engineering value. In this dessertation, the mathematical model of DFIG system with the dynamic of phase-locked loop (PLL) considered is established in detail, and the impact of PLL on DFIG system small-signal stability is analyzed. On these basis, methods used to analyze the interaction mechanism between DFIG and synchronous generator is put forword, and the mechanism that how DFIGs and their frequency response controllers affect the small-signal stability of power system is revealed. Meanwhile, the mechanism of the impact of DFIGs on transient stability is analyzed based on the power-angle characteristic equation. The main research work and novel achievements are as follows:The detailed mathematical model of DFIG system with the PLL dynamic considered is established, and the multi-time scale and coupling properties of the system is analyzed. Based on the time scale difference and approximate decoupling characteristics between the mechanical and rotor electrical dynamic, a reduced-order model based modes estimation method is proposed, and the correctness of the method is verified by singular perturbation theory.The similarity between the small-signal stability of DFIG and that of synchronous generator are analyzed, based on the comparison between the dynamic equations of PLL in DFIG and the dynamic equations of rotor motion in synchronous generator. The power angle of DFIG is defined based on the knowledge of synchronous generator power angle. And then, the small-signal stability of DFIG is revealed based on the analysis of the relationship among power angle and space voltage vectors. Furthermore, the similarity between DFIG and synchronous generator in small-signal stability is presented based on the similarity in small-signal model and modal analysis. The analysis laid the foundation for the study of the effect of DFIGs integration on system small-signal stability.Methods based on dynamic response and coupling state matrix are proposed to analyze the interaction mechanism between DFIG and synchronous generator. The second order non-homogeneous differential equations with constant coefficients, which reflects the interaction between the dynamics of PLL output and synchronous generator power angle, is deduced in a extended single machine infinite bus (SMIB) system. On the basis, the mechanism analysis method based on the dynamic response is put forward. Meanwhile, the interaction mechanism difference between the strong coupling and the weak coupling of the state equations of PLL and synchronous generator power angle is analyzed.The mechanism that how frequency response controllers affect the small-signal stability is revealed based on the torque analysis method and the state matrix method, which are put forward in this disertation. The relationship between the frequencies of DFIG access point and synchronous generators are derived firstly. And then, the effects of parameters of droop control and virtual inertial control, locations of DFIG and inertias of the synchronous machine on the oscillation modes of the extended two machine system and the multi-machine power system are analyzed, respectively.An equivalent model for DFIG considered its output power characteristic is presented. Based on the equivalent model, the power angle characteristic equation of the synchronous machine affected by DFIG in a extended single machine infinite bus system is derived, which reveals the effect of DFIG-based wind power penetration on the system transient stability. Then, the effects of wind power added to the system additionally and replacing the synchronous generator active power with same capacity on transient stability are deeply studied based on the equal-area criterion (EAC). The study can provide a good direction and reference for the transient stability analysis of multi-machine power systems with DFIG integrated.