Assessing the energy capture capability of a MW scale wind turbine during below rated operation

The operational strategy in below rated operation of a variable speed wind turbine is frequently chosen to maximise the energy capture. To do so the operating state of the wind turbine is caused to track the maximum aerodynamic efficiency curve, the so-called Cp-max curve. Whether it does so accurately depends on the wind turbine controller. It is sometimes claimed that the energy capture can be increased by better design of this controller. The purpose of this paper is to examine this claim. The energy capture per year achieved by different control algorithms and strategies is assessed as a percentage of the total energy available with an ideal strategy. These estimates are obtained using a Simulink simulation of a large wind turbine that enables the latter to be determined precisely. The results from the Simulink model suggest that the energy capture is very weakly dependent on the control algorithm. There is more dependence on the choice of control strategy but even here the potential gains are limited. To support these conclusions, it is demonstrated, by comparison to the full wind turbine aero-elastic simulation, Bladed, that the Simulink simulation provides an accurate guide to tuning the controller algorithm for below rated operation.