Estimation of partial wake loads for wind farm control design

Model-based design of wind farm controllers is usually performed on the basis of simple surrogate farm models, which take the wake interaction among turbines into account and allow the controller development at reasonable computational time. These surrogate models need to receive as input information the wind turbine characterization (at least, thrust and power factor, also loads if applicable) at all the possible operating points so that the farm controller can optimize the overall farm performance within this operational space. If the turbine characterization is provided by pre-calculated simulation databases, these must then sweep the allowed operational space arising from the combination of relevant inflow conditions (wind speed, turbulence intensity, etc.) and operational control handles (yaw variations for wake steering and/or derating set points). For calculations corresponding to real farm conditions the number of combinations may easily explode to tens of thousands.

When dealing with loads pre-calculation, it is intuitive to think that loads for turbines unaffected by wakes should differ from partial wake impingement in the rotor. If all combinations of wake impingement should be considered additionally, then the pre-calculation would become intractable.

The present paper proposes a new way of dealing with the problem by estimating the Damage Equivalent Loads (DEL) corresponding to partial wake impingement based on the databases of an unaffected turbine. In this way, the computational cost is reduced in comparison to the complete simulation, and it allows the use of the estimator in an optimization code for controller development.

First, an analysis of the need for such a correction is made. Second, development of the estimation is performed. Finally, results coming from simulations for different partial wake impingements, by varying wake deficit and wake impact zone on the rotor, are shown to validate the approach. The implementation and validation of the new estimation method has been applied to the INNWIND.EU 10 MW wind turbine, and the study has focused on blade root flapwise bending moment.