Wake redirecting using feedback control to improve the power output of wind farms

Abstract

Abstract-In future, a wind turbine will not only be seen as a single systems operating independently, but also as a component of a larger interacting system, the wind farm. To increase the efficiency of a wind farm, two main concepts have been proposed: axial induction control and wake redirecting. This contribution focuses on the latter. Remote sensing technologies in wind energy applications have opened new ways to control wind turbines. In this contribution, a further step is taken by using a remote sensing device for redirecting the wake of a wind turbine. A controller is proposed which uses the yaw actuator of a wind turbine to steer the wake center of the turbine to a desired position. The wake propagation from the wind turbine to the measurement location is modeled with a time delay. This forms a challenging problem for controller design. The controller follows the idea of the internal model principle and uses a model to predict the system behavior avoiding an overestimation of the error. Further, an adaptive filter is proposed in order to filter uncontrollable frequencies from the wake center estimation. The estimation from lidar measurement data is assumed to be perfect. Closed-loop simulations are conducted using the nominal system and a wind farm simulation tool, which was adapted to the scenario. The results are compared to the uncontrolled baseline case and a statically applied yaw offset. They show an increase in the total power output of the wind farm. Together with wake tracking methods, the approach can be considered as a promising step towards closed-loop wind farm control.

Field test of wake steering at an offshore wind farm

“…The present method can be regarded as open loop because the actual wake to be controlled is never observed or estimated by the controller. Research that uses lidar to track and control wakes (Raach et al, 2016) or uses estimation techniques (Doekemeijer, 2016) may very well improve upon these first results. This is also the subject of ongoing multiyear research projects in the United States (A2e, 2016) and Europe (CLWindcon, 2016).…”

Lidar-based wake tracking for closed-loop wind farm control

“…This work focuses mainly on the measurement task but also gives a summary of a solution to the control task, which was presented in Raach et al (2016). Figure 1 presents the general concept of closed-loop wake redirection and the link between the measurement and control task.…”

“…The following closed-loop controller was first presented in Raach et al (2016) and is recapped here. See also Raach et al (2017), in which a H ∞ controller design for closedloop wake redirection with defined performance margins is used.…”

“…(2) There is no observation of whether the wake is being redirected correctly. The concept of closed-loop wake redirection, which was introduced in Raach et al (2016) can help to overcome the drawbacks.…”

Robust lidar-based closed-loop wake redirection for wind farm control

“…A nominal H ∞ controller is designed like presented in Raach et al (2016a) using the performance weights as defined in (19). The controller design achieves a controller resulting in ||N|| ∞ = 0.99 .…”