Coupling Characteristics of DFIG-based WT Considering Reactive Power Control and Its Impact on Phase/Amplitude Transient Stability in a Rotor Speed Control Timescale

The transient stability issues caused by doubly fed induction generator (DFIG)-based wind turbines (WTs) are receiving increasing attention. The q-axis reactive power control (QCtrl), as an essential part of DFIG-based WTs, has a significant impact on its transient response. In this paper, the impact of QCtrl on the phase/amplitude transient stability of a DFIG-based WT-dominated system is analyzed from the perspective of internal voltage amplitude-phase coupling characteristics. First, an amplitude/phase dynamic model of a DFIG-based WT in rotor speed control timescale (in seconds, corresponding to traditional electromechanical timescale) is developed. Then, in comparison with familiar synchronous generators (SGs), an inherently amplitude-phase characteristic of internal voltage for a DFIG-based WT is identified. Next, taking the DFIG-based WT-dominated system as an example, the impact of QCtrl on system transient stability via the internal coupling paths is analyzed. A novel phase-amplitude coupling instability mechanism is found, which is different from that in a traditional SG-dominated system. Finally, the effects of different QCtrl strategies on transient stability are discussed.