Abstract
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.
Highlights
New sources of low-carbon, clean and sustainable energy have received widespread attention worldwide
Wind farms usually cover a relatively wide area, and the wind power connects to the AC grid through long-distance outgoing lines, so that the AC grid strength decreases with the increasing length of the outgoing line
To more accurately study the dynamic characteristics of the wind power system, the rotor-side converter (RSC) and grid-side converter (GSC) controller of the DFIG are first modeled in detail
Summary
New sources of low-carbon, clean and sustainable energy have received widespread attention worldwide. To more accurately study the dynamic characteristics of the wind power system, the RSC and GSC controller of the DFIG are first modeled in detail. A. ADMITTANCE MODELING OF THE DFIG REGARDLESS OF THE OUTER LOOP CONTROL In the dq coordinate system, the stator and rotor voltage equations of the DFIG are written as shown in (5) and (6), VOLUME 7, 2019 respectively. B. STABILITY CRITERION AND EQUIVALENT CIRCUIT The dynamic model of the DFIG-based wind farm and AC grid can be written in (23). The characteristic equation of the wind power system is given by 1 + Yeq. The characteristic equation of the wind power system corresponds to the series-parallel circuit, which is composed of the equivalent impedance of the DFIG and AC grid in the polar coordinate system. The series-parallel resonance occurs in the system equivalent circuit
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