Currently, grid-following (GFL) control is widely adopted in direct-drive wind farms. Its external impedance characteristic exhibits negative resistance and capacitive reactance, frequently inducing sub/super-synchronous oscillations in the direct-drive wind farm and weak grid interactive system. The positive resistance characteristic of grid-forming (GFM) control can, to a certain extent, improve the impedance characteristic of wind farms and enhance the system stability margin. However, the influence of the proportion and deployment location of GFM control within a wind farm on the mitigation of sub/super-synchronous oscillations merits further exploration. First, this paper establishes the sequence impedance models for both GFL and GFM control, analyzes the underlying causes of system oscillations from an impedance perspective, and proposes a method for calculating the stability margin of a grid-connected direct-drive wind farm system that comprehensively accounts for the generalized short-circuit ratio, the critical short-circuit ratio of the equipment, and the steady-state operational constraints of the system. Subsequently, the mitigation effects of the connection location and capacity proportion of GFM wind turbines on sub/super-synchronous oscillations are quantitatively assessed, yielding feasible ranges of the short-circuit ratio under various operating conditions that ensure stable operation of the direct-drive wind farm. The system stability is further examined via Nyquist curve analysis. Finally, the effectiveness of the proposed method is validated by electromagnetic transient simulations in MATLAB/Simulink.
Wang et al. (Sun,) studied this question.