Stability enhancement control strategy for grid-connected wind power system based on composite feedforward damping.

• Considering the effects of frequency coupling and cascade transformer, this paper establishes the frequency coupling impedance model of GCWPS and applies the generalized Nyquist criterion to analyze the stability of the whole system as the grid strength, PLL bandwidth and output power change. • Aiming at the power oscillation problem of GCWPS under weak grid, a composite feedforward damping oscillation suppression strategy is proposed, and the output impedance characteristic of GCWPS is reconstructed. • The frequency coupling impedance model of GCWPS adopting the oscillation suppression strategy proposed in this paper is constructed, and a comparison between its stability and that of the GCWPS adopting the traditional control strategy is made. It is proved that the system has stronger adaptability to the increase of output power and PLL bandwidth under weak grid when adopting the proposed oscillation suppression strategy. The stability of grid-connected wind power system (GCWPS) is prone to deteriorate due to the impedance interaction between wind turbines and the weak grid. For purpose of finding out the cause of power oscillation and effectively improving the stability of GCWPS under weak grid, firstly of all, a frequency coupling impedance model (FCIM) for GCWPS is established. Through analyzing the impedance characteristic of GCWPS, a significant coupling effect between GCWPS impedance and grid impedance in the middle-low frequency region under weak grid is observed, which is easy to cause power oscillation. For purpose of restraining the power oscillation of GCWPS under weak grid, this paper proposes an oscillation suppression strategy based on composite feedforward damping. Virtual damping is incorporated into the feedforward loop to rectify the impedance characteristic curve of GCWPS. Then, a FCIM of GCWPS that adopts the proposed oscillation suppression strategy is established, and the stability analysis demonstrates that the proposed strategy effectively enhances the GCWPS stability under weak grid. Additionally, compared with conventional control strategy, GCWPS exhibits enhanced adaptability to the increased output power and phase-locked loop (PLL) bandwidth under weak grid. Finally, experiments are undertaken to evidence the feasibility of the proposed oscillation suppression strategy. [ABSTRACT FROM AUTHOR]