The Impedance Modeling And Stability Analysis Of Doubly Fed Induction Generator System

In order to dealing with the ongoing energy crisis and increasing environmental problems, wind energy, as a clear and sustainable energy source, is widely utilized all over the world. The massive injections of wind power to the grid, however, also bring new problems. The power electronic equipment coupling with the weak grid which is usually appeared in remote areas where wind generators are mostly assembled at, may cause resonances and harmonics in the interconnected systems, threatening the power quality. For these unstable problems which are aroused by system interconnection, impedance stability analysis method can accurately reveal the causation of the instability and the frequency range of the possible resonance, and provide a solution. The Doubly Fed Induction Generator (DFIG) based wind power generation system is the most popular wind system type, which contains two typical grid-connected structures in one system:the stator-side grid-connected structure and the gird-side inverter structure. Based on impedance stability analysis method, focusing on Doubly Fed Induction Generator (DFIG), this dissertation builds the model of DFIG’s output analytical impedance and analyzes the stability of the DFIG-Grid system.1. Using harmonic linearization method, the impedance analytical models of grid-side converter is developed in positive and negative sequences. The circuit topology, rated operating point, phase locked loop, current regulator, and the possible unbalance of filter inductance and common coupling voltage are all taken into account in this model and the effects of these factors in the impedances are also thoroughly evaluated.2. The impedance model of stator-side of DFIG is built, considering the motor structure, operating point, phase locked loop, and current regulator. The output impedance of the whole system is also deduced and verified. 3. According to the impedance stability analysis method and DFIG’s output impedance model, the stabilities of the inverter-grid system, the DFIG stator-grid system, and the DFIG-grid system are investigated, during which the effects of different grid impedances and different system control parameters to system stability are discussed as well. Then experiments were conducted to verify the generalized Nyquist stability analysis method in unbalanced grid condition and the effects of unbalanced factors to inverter-grid system stability are discussed.