Stability Analysis and Oscillation Mechanism of the DFIG-Based Wind Power System

The interaction between the DFIG (doubly fed induction generator)-based wind farm and the ac grid makes the traditional positive-sequence subsystem and negative-sequence subsystem no longer decoupled, so the single-input single-out (SISO) characteristic in the frequency domain no longer holds. In this paper, the admittance models of the DFIG and ac grid are established by using the small-signal analysis method. On this basis, first the admittance relationship of the entire DFIG-based wind power system in polar coordinates is obtained. Second, the multi-input multi-output (MIMO) system is simplified into an SISO system by the matrix transformation. Then, the characteristic equation and the equivalent circuit of the DFIG-based wind power system are provided. In cases of a weak ac grid, low wind speed and a high proportional coefficient of the phase-locked loop (PLL) controller, the stability of the DFIG-based wind power system will be weakened, and the oscillation among different coupling frequencies will occur based on Nyquist criterion. Finally, the simulation results show the correctness of the theoretical analysis.