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Abstract
Abstract. 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.
Highlights
The increasing size and flexibility of wind turbines demand that attention be devoted to the active and passive control of rotor loads in order to limit the costs related to both the construction as well as maintenance of the turbine blades and the support structure
One of the most interesting and readily accessible methods of blade load control are individual pitch control (IPC) (Bossanyi, 2003), whereby each blade is pitched along its longitudinal axis independently to counteract the variation in wind loading
Previous experimental studies conducted by the authors (Navalkar et al, 2015) show that in a controlled, wind tunnel environment, wind turbine blade load reductions of up to 70 % can be reached, since the blade loading under these circumstances is almost entirely deterministic
Summary
The increasing size and flexibility of wind turbines demand that attention be devoted to the active and passive control of rotor loads in order to limit the costs related to both the construction as well as maintenance of the turbine blades and the support structure. In an effort to reduce pitch actuator duty, target higher frequencies in the load spectrum and address localised disturbances in the wind loading, recent literature has explored the concept of the “smart” rotor (Lackner and Van Kuik, 2010), i.e. a rotor where the blades are instrumented with sensors and flow-modifying actuators at various radial locations Reviews of such rotors (Barlas and Van Kuik, 2010; Bernhammer et al, 2012) invariably conclude that trailing-edge flaps give the best control authority for load alleviation. Numerical and experimental investigations of the freefloating flap concept (Heinze and Karpel, 2006; Bernhammer et al, 2013) have shown that the additional degree of freedom adds a rigid-body mode to the system, the dynamics of which are strongly dependent on the total air speed at operation Aeroelastic coupling of this mode with the flexibleblade mode induces flutter at low wind speeds, an instability that can lead to dangerously high vibrations and even structural failure.
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