Balancing energy capture and structural loads on variable speed wind turbines

A variable speed wind turbine requires a controller that matches the rotor speed to the wind speed in order to maintain a constant tip speed ratio and maximize energy capture. As the wind gusts and lulls, the controller must adjust the rotor speed accordingly. If energy capture is to be maximized, then every change in the wind speed must be tracked by the rotor speed. However, to track every change in wind speed would introduce excessive torque transients into the drive train. Therefore, it is necessary to determine an appropriate tradeoff between energy capture and fatigue loads. This paper presents a quantitative method for determining an appropriate tracking error in the variable speed controller. An opportunity is identified for finding an optimal operating state of the controller in which fatigue damage is minimized and energy capture is nearly maximized.