Evaluation of different power tracking operating strategies considering turbine loading and power dynamics

This work focuses on the design, implementation, and implications of different operational strategies for wind turbines when providing a power tracking functionality. Power tracking is necessary for the contribution to stabilization of the electrical grid. Specifically, two different operational strategies are used as the foundation for a model-based control design that allows the turbine to follow a given power demand. The first relies on keeping a constant rotational speed while varying the generator torque to match the power demand. The second approach varies both the generator torque and the rotational speed of the turbine equally to yield the desired power output. In the power reduction mode, both operational strategies employ the pitch to maintain the desired rotational speed of the turbine. The attainable power dynamics of the two closed-loop systems with varying power demands are analyzed and compared. Reduced-order models formulated as transfer functions and suitable for integration into an upper-level control design are proposed. It is found that the first strategy involving only the generator torque while keeping a constant rotational speed provides significantly faster power control authority. Further, the resulting fatigue loading in turbulent wind conditions is discussed for the two operational strategies, where constant operational storage is emulated to enable a bidirectional variation in the power output. Without any additional load-reducing control loops, the results suggest that the first operational strategy involving variation in the generator torque only is more favorable with regard to the resulting loading of the turbine structure. The simulation studies are conducted for NREL's 5 MW reference turbine using FAST.