| Summary: | In asymmetric weak grid, there exists a complex coupling path between the positive- and negative-sequences of the impedance between a doubly-fed wind turbine (DFIG) and the grid, leading to intricate interactions. Since grid codes require wind turbines to provide positive- and negative-sequence dynamic reactive power support, improper settings of these currents may result in system instability and oscillations. Therefore, this paper first develops a sequence impedance model of the DFIG-grid system using a harmonic linearization method, revealing the effects of positive-sequence active current, positive-sequence reactive current, and negative-sequence reactive current on system stability, and analyzes the stability mechanism induced by inter-sequence coupling. Furthermore, considering stability constraints, grid codes, and converter capacity, it characterizes the stable operating domain of the DFIG. To solve the problem of insufficient stable operating range in weak grid, it proposes an adaptive oscillation suppression phase-locked loop (PLL) method, which adaptively suppresses the oscillatory component in the PLL input, thereby extending the stable operating domain of the DFIG. Finally, simulation results verify the correctness of the theoretical analysis and the effectiveness of the proposed control method.
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