Transient Stability Analysis Between Grid-Following DFIG-WT and Grid-Forming Converter in Electromechanical Timescale

Due to the transient control switch, the electromechanical dynamic of grid-following-based doubly fed induction generator-based wind turbine (GFL-DFIG) would be excited and significantly interact with grid-forming-based voltage-source converter (GFM-VSC). So that new electromechanical synchronous issues would occur in the becoming power-electronized system. The coupling characteristic between mechanical and electrical phases, which reflects the relationship between unbalance active power and internal voltage phase of apparatus, is crucial for understanding the synchronous process. This paper reveals the nonlinear mechanical-electrical coupling and analyzes the transient synchronous stability between GFL-DFIG and GFM-VSC. Firstly, the analytical model is proposed from the perspective of unbalanced power excitation and internal voltage response. Then, the mechanical-electrical coupling characteristic between virtual rotor position and relative internal voltage phase is quantitatively analyzed by employing concept of ‘flexible coupling’. Finally, the transient synchronization mechanism between GFL-DFIG and GFM-VSC is analyzed and a novel transient instability constraint is found. The loss-of-synchronization phenomenon and influence principles of key factors are also discussed.