Control Method for Improved Energy Capture Below Rated Power

To maximize energy capture, a variable-speed wind turbine should operate continuously at the tip-speed-ratio that results in the maximum power coefficient (Cpo) and, therefore, extracts the maximum energy from the wind. This is the main idea behind improved energy capture from variable-speed operation. However, this goal is only partially achievable due to rapid variations in wind speed and the inertia of the wind turbine rotor. Although it is not possible to operate continuously at maximum efficiency, improvements in energy capture during variable-speed operation can be gained by improved tracking of Cpo. In this paper the aerodynamic torque, estimated by an observer, and rotor speed are used to improve the energy capture of a variable-speed turbine. Two methods are used. The first method uses the torque error for control. The second method is formulated such that the estimated percent power loss is used directly for control. Also, the use of blade pitch below rated power is investigated. A small improvement in energy capture is realized by use of the described control methods. For turbines with a sharp Cp peak or slower time constant, greater improvement would be observed.