An increasing number of wind turbines implement individual blade pitch control (IPC) to reduce turbine dynamic loading, and thereby, to reduce the capital and operational costs associated with energy production. The aim of this paper is to demonstrate IPC on a wind turbine prototype, in a model-free data-driven manner and with reduced pitch activity. For this, subspace predictive repetitive control (RC) is used, which combines online system identification with the continuous implementation of RC to form a fully adaptive control law. The controller is tested on a scaled two-bladed wind turbine with active pitchable blades, placed in an open-jet wind tunnel. Substantial load reductions to an extent of 68% are observed, and strict control over actuator signal frequency content is achieved. The control law also demonstrates the ability to adjust to changes in system dynamics while maintaining a high degree of load alleviation.Index Terms-Adaptive control, individual pitch control (IPC), load alleviation, repetitive control (RC), subspace identification, wind turbine.

In this article, the conventional individual pitch control (IPC) strategy for wind turbines is reviewed, and a linear IPC strategy for two-bladed wind turbines is proposed. The typical approach of IPC for three-bladed rotors involves a multi-blade coordinate (MBC) transformation, which transforms measured blade load signals, i.e., signals measured in a rotating frame of reference, to signals in a fixed non-rotating frame of reference. The fixed non-rotating signals, in the so-called yaw and tilt direction, are decoupled by the MBC transformation, such that single-input single-output (SISO) control design is possible. Then, SISO controllers designed for the yaw and tilt directions provide pitch signals in the non-rotating frame of reference, which are then reverse transformed to the rotating frame of reference so as to obtain the desired pitch actuator signals. For three-bladed rotors, the aforementioned method is a proven strategy to significantly reduce fatigue loadings on pitch controlled wind turbines. The same MBC transformation and approach can be applied to two-bladed rotors, which also results in significant load reductions. However, for two-bladed rotors, this MBC transformation is singular and therefore, not uniquely defined. For that reason, a linear non-singular coordinate transformation is proposed for IPC of two-bladed wind turbines. This transformation only requires a single control loop to reduce the once-per-revolution rotating blade loads ('1P' loads). Moreover, all harmonics (2P, 3P, etc.) in the rotating blade loads can be accounted for with only two control loops. As in the case of the MBC transformation, also the linear coordinate transformation decouples the control loops to allow for SISO control design. High fidelity simulation studies on a two-bladed wind turbine without a teetering hub prove the effectiveness of the concept. The simulation study indicates that IPC based on the linear coordinate transformation provides similar load reductions and requires similar pitch actuation compared with the conventional IPC approach.

SUMMARYThis paper aims to introduce a new approach to optimize the tunable controller parameters of linear parameterizable controllers. The presented approach is frequency-domain based and can therefore directly be used to tune, among others, proportional integral derivative controllers, low/high-pass filters, and notch filters, using a Frequency Response Function of the plant. The approach taken in this paper is to extract the tunable controller parameters into a diagonal matrix gain and absorb the remainder of the controller in the plant. Then, the generalized Nyquist stability criterion is exploited so as to impose stability and H 1 performance specifications on the closed-loop system. It is shown that the approach results in a convex feasibility problem for certain controller cases and can be reformulated such that it can also be used for grey-box system identification. Simulation and experimental examples demonstrate the efficacy of the approach.

Wind turbine load alleviation has traditionally been addressed in the literature using either full-span pitch control, which has limited bandwidth, or trailing-edge flap control, which typically shows low control authority due to actuation constraints. This paper combines both methods and demonstrates the feasibility and advantages of such a combined control strategy on a scaled prototype in a series of wind tunnel tests. The pitchable blades of the test turbine are instrumented with free-floating flaps close to the tip, designed such that they aerodynamically magnify the low stroke of high-bandwidth actuators. The additional degree of freedom leads to aeroelastic coupling with the blade flexible modes. The inertia of the flaps was tuned such that instability occurs just beyond the operational envelope of the wind turbine; the system can however be stabilised using collocated closed-loop control. A feedforward controller is shown to be capable of significant reduction of the deterministic loads of the turbine. Iterative feedforward tuning, in combination with a stabilising feedback controller, is used to optimise the controller online in an automated manner, to maximise load reduction. Since the system is non-linear, the controller gains vary with wind speed; this paper also shows that iterative feedforward tuning is capable of generating the optimal gain schedule online.Published by Copernicus Publications on behalf of the European Academy of Wind Energy e.V.

Abstract. In this paper Linear Individual Pitch Control (LIPC) is applied to an experimental small-scale two-bladed wind turbine. LIPC is a recently introduced Individual Pitch Control (IPC) strategy specifically intended for two-bladed wind turbines. The LIPC approach is based on a linear coordinate transformation, with the special property that only two control loops are required to potentially reduce all periodic blade loads. In this study we apply LIPC to a control-oriented small-scale two-bladed wind turbine, equipped with, among others, two highbandwidth servomotors to regulate the blade pitch angles and strain gauges to measure the blade moments. Experimental results are presented that indicate the effectiveness of LIPC.