Augmented Control for Flexible Operation of Wind Turbines

Stock, Adam and Leithead, Bill and Yue, Hong (2015) Augmented Control for Flexible Operation of Wind Turbines. PhD thesis, University Of Strathclyde.

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In this thesis a novel controller for providing greater flexibility of operation of wind turbines known as the Power Adjusting Controller (PAC) is presented. The controller takes the form of an augmentation to a wind turbine’s full envelope controller, allowing it to be applied to any horizontal axis, pitch regulated, variable speed wind turbine. Conventional wind turbine control seeks to maximise the power output of a wind turbine whilst minimising the loads on the turbine, controlling on the error in generator speed via demands to the blade pitch actuator and generator torque actuator. The PAC uses additions to the full envelope controller inputs and outputs to alter the power output of the turbine by an additional input value ΔP. It is ensured that the operation of the full envelope controller is not compromised by the PAC. Testing of the PAC using lumped parameter models of wind turbines and full aero-elastic models makes clear a requirement for a wind speed estimator within the PAC that incorporates the effects of dynamic inflow. A novel wind speed estimator that accounts for dynamic inflow by redefining blade element momentum theory solely in terms of the dynamics at the rotor is therefore developed and incorporated into the PAC. Limits are designed to ensure that the operating point of a wind turbine with the PAC is kept within a safe operational envelope, and a system of flags and sub-flags is developed to allow easy integration of the PAC into a hierarchical wind farm control structure. The effect of using the PAC on the wind turbine loads is investigated, with the ultimate loads introduced by operation of the PAC found to be within the range of normal operating loads and the impact of prolonged reduction of the power output found to reduce the lifetime damage equivalent loads in most cases. Two applications of the PAC, namely providing synthetic inertia and providing droop control, are presented, with the PAC shown to be able to match the performance of conventional synchronous plant in both cases. It is shown that neither application causes ultimate loads outside of the range of normal operation and that providing droop control reduces the lifetime damage equivalent loads.