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Abstract-Loss of synchronization between wind farm and power grid during severe grid faults would cause wind farm tripping. In this paper, the mechanism of grid-synchronization is uncovered, described as motion of an autonomous nonlinear differential equation with specific initial states. The revealed mechanism indicates that even though steady state working point exists, improper initial states and poor system dynamic properties could lead to synchronization instability. In order to keep wind farm synchronous with the power grid during severe grid faults, special requirements on system dynamic properties are stated. Moreover, to satisfy all the requirements, a current injecting method is proposed. By adjusting active and reactive output currents of the wind farm, the proposed method could ensure system synchronization stability during severe grid faults. Implementation of the proposed method on PMSG and DFIG based wind farm is illustrated. Simulation results validate the analysis and the control method.

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For doubly-fed induction generator (DFIG)-based wind energy conversion systems (WECS), large electromotive force (EMF) will be induced in the rotor circuit during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from over-currents, which may even destroy the rotor side converter (RSC). To mitigate this problem, a new flux linkage tracking-based low voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5 MW DFIG-based WECS is carried out by simulation using the full order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.Index Terms-Doubly-fed induction generator (DFIG), flux linkage tracking, low voltage ride-through (LVRT), rotor side converter.