Two improved control strategies for rotor speed protection and recovery in the frequency support process of grid-forming wind turbines

Traditional grid-following virtual inertia control wind turbines (WTs) typically achieve rotor speed protection (RSP) and recovery (RSR) through adjusting coefficients. However, for grid-forming (GFM) virtual synchronous generator (VSG) controlled WTs, due to the coupling between parameters and damping characteristics, the adjustment range is limited, making it difficult to achieve RSP and RSR by parameter tuning. Furthermore, under frequency support conditions, these turbines are prone to external environmental influences, leading to significant rotor speed fluctuations and instability. To address this issue, this paper proposes two improved control strategies for GFM WTs: the improved VSG control based on VSG control, which sets the power reference in three modes according to the rotor speed, and the adaptive configurable natural droop (CND) control based on CND control, which adjusts the droop coefficient in three modes according to the rotor speed. Both strategies not only support system frequency but also ensure a stable intersection between the WT and the mechanical dynamic curve under varying external conditions, thereby achieving the RSP function. Furthermore, after disturbances, the rotor speed returns to the optimal speed, increasing the turbine's captured power, improving the system's steady-state frequency, and realizing the RSR function.