Stability of DC-link voltage control for paralleled DFIG-based wind turbines connected to Weak AC grids

Dynamic stability issues of power converter interfaced devices integrating into the power grid have drawn increased attention. This paper studies DC-link voltage control stability for a weak-ac-grid connected wind farm, in which doubly-fed induction generator (DFIG) based wind turbines (WTs) operate in parallel. First, a reduced analytical model is proposed of separated dynamics for different time scales. Operating points of WTs, grid strengths and DC-link voltage control system dynamics are included in the model. Eigenvalue analysis for the proposed model shows that the mode, usually called common mode which represents the interconnection behavior between the overall wind farm and the grid, determines system stability. Damping of this common mode turns weak as grid strength decreases, and even negative thus leading system instability. Eigenvalue locus shows that tuning of phase-locked loop (PLL) has noticeable effect on this mode under weak ac grid, and larger PLL gain can effectively improve system damping. Time domain simulation with corresponding detailed model verifies the analysis.