Feedback–feedforward individual pitch control for wind turbine load reduction

Abstract

SUMMARYThis paper focuses on the problem of wind turbine fatigue load reduction by means of individual pitch control (IPC). The control approach has a two-degree-of-freedom structure, consisting of an optimal multivariable LQG controller and a feedforward disturbance rejection controller based on estimated wind speed signals. To make the control design problem time invariant, all signals are transformed to the non-rotating reference frame using the Coleman transformation. In the Coleman domain, the LQG control objective is minimization of the rotor tilt and yaw moments, whereas the feedforward controller tries to achieve even further improvement by rejecting the influence of the low-frequency components of the wind on the rotor moments. To this end, the tilt-and yaw-oriented components of the blade-effective wind speeds are approximated using stochastic random walk models, the states of which are then augmented with the turbine states and estimated using a Kalman filter. The effects of these (estimated) disturbances on the controlled outputs are then reduced using stable dynamic model inversion. The approach is tested and compared with the conventional IPC method in simulation studies with models of different complexities. The results demonstrate very good load reduction at not only low frequencies (1p blade fatigue load reduction) but also at the 3p frequency, giving rise to fatigue load reduction of the non-rotating turbine components.

Fundamental performance similarities between individual pitch control strategies for wind turbines

“…In practice, this is difficult to achieve owing to the limitations of the CPC, in addition to the coupling between CPC and IPC through the tower dynamics (Selvam et al, 2009). This challenge to maintaining fixed rotor speed can clearly be seen in Figure 6(d) for the case without IPC, where changes in rotor speed are causing the CPC to continuously adjust the blade pitch angle.…”

“…If the turbine dynamics are linear, or can be approximated as such, then conventional tools of linear and time-invariant (LTI) control system design can further be applied to design controllers to attenuate the unsteady loads upon the non-rotating turbine structures. This is the main reason why the majority of IPC studies have employed the Coleman Transform (Bossanyi, 2003(Bossanyi, , 2005Bossanyi & Wright, 2009;Engels, Subhani, Zafar, & Savenije, 2014;Geyler & Caselitz, 2008;Lackner & van Kuik, 2010;Lu et al, 2014;Plumley, Leithead, Jamieson, Bossanyi, & Graham, 2014;Selvam, Kanev, van Wingerden, van Engelen, & Verhaegen, 2009;Stol, Moll, Bir, & Namik, 2009;van Engelen, 2006;van Engelen & van der Hooft, 2005).…”

Analysis and design of a tower motion estimator for wind turbines

“…This work implicitly assumes the tower is a prismatic beam so that the ratio between rotation and displacement is 2 3h where h ∈ R is the height of the tower (e.g. [4], [3]). Thus, the fore-aft rotational velocity of the tower-top can be approximated asφ fa (t) ≈ 2 3hẋ fa (t).…”

“…For brevity, a constant wind-induced disturbance model is assumed (e.g. [4]):ḋ Thus, the augmented model is described as follows:…”

“…For example, wind speeds across the turbine rotor were estimated based on an aerodynamic turbine structural model [4], [7]. In addition, an observer was employed to estimate the unknown system states for state-feedback control strategies [8].…”