Abstract: When an asymmetric fault occurs in the weak grid, there are coupling paths of the positive- and negative-sequences between the doubly-fed wind turbine (DFIG) and the grid impedance. And the interaction is very complicated. Considering that the grid codes require the wind turbines to provide positive- and negative-sequence dynamic reactive currents to the faulty grid, if the currents are not set reasonably, the system will have the risk of instability. In this paper, the harmonic linearization method is used to establish the sequence-impedance model of the grid-connected DFIG system, and the influence law of the positive-sequence active and reactive current, and the negative-sequence reactive current on system stability is explained. And the inter-sequence coupling stabilization mechanism is analyzed as well. Furthermore, considering the stability constraints, the grid codes constraints and converters’ capacity constraints, the operation domain of DFIG is characterized. To solve the problem of insufficient stable operation domain of DFIG under weak grid, an adaptive phase-locked method is proposed to realize the suppression of the oscillation component, which in turn extends the stable operation domain of the system. Finally, the simulation examples verify the correctness of the theoretical analysis and the effectiveness of the proposed control method.

Key words: weak grid, doubly-fed induction generator (DFIG), stability analysis, inter-sequence coupling