Individual Pitch Control Research Articles

Experimental wind tunnel testing of linear individual pitch control for two-bladed wind turbines

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 high- bandwidth 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.

Three Dimensional Dynamic Model Based Wind Field Reconstruction from Lidar Data

Using the inflowing horizontal and vertical wind shears for individual pitch controller is a promising method if blade bending measurements are not available. Due to the limited information provided by a lidar system the reconstruction of shears in real-time is a challenging task especially for the horizontal shear in the presence of changing wind direction. The internal model principle has shown to be a promising approach to estimate the shears and directions in 10 minutes averages with real measurement data. The static model based wind vector field reconstruction is extended in this work taking into account a dynamic reconstruction model based on Taylor's Frozen Turbulence Hypothesis. The presented method provides time series over several seconds of the wind speed, shears and direction, which can be directly used in advanced optimal preview control. Therefore, this work is an important step towards the application of preview individual blade pitch control under realistic wind conditions.The method is tested using a turbulent wind field and a detailed lidar simulator. For the simulation, the turbulent wind field structure is flowing towards the lidar system and is continuously misaligned with respect to the horizontal axis of the wind turbine. Taylor's Frozen Turbulence Hypothesis is taken into account to model the wind evolution. For the reconstruction, the structure is discretized into several stages where each stage is reduced to an effective wind speed, superposed with a linear horizontal and vertical wind shear. Previous lidar measurements are shifted using again Taylor's Hypothesis. The wind field reconstruction problem is then formulated as a nonlinear optimization problem, which minimizes the residual between the assumed wind model and the lidar measurements to obtain the misalignment angle and the effective wind speed and the wind shears for each stage. This method shows good results in reconstructing the wind characteristics of a three dimensional turbulent wind field in real-time, scanned by a lidar system with an optimized trajectory.

Design, manufacturing and characterization of aero-elastically scaled wind turbine blades for testing active and passive load alleviation techniques within a ABL wind tunnel

In the research described in this paper, a scaled wind turbine model featuring individual pitch control (IPC) capabilities, and equipped with aero-elastically scaled blades featuring passive load reduction capabilities (bend-twist coupling, BTC), was constructed to investigate, by means of wind tunnel testing, the load alleviation potential of BTC and its synergy with active load reduction techniques. The paper mainly focus on the design of the aero-elastic blades and their dynamic and static structural characterization. The experimental results highlight that manufactured blades show desired bend-twist coupling behavior and are a first milestone toward their testing in the wind tunnel.

Cyclic pitch control for the reduction of ultimate loads on wind turbines

In this paper we study the use of individual blade pitch control as a way to reduce ultimate loads. This load alleviation strategy exploits the fact that cyclic pitching of the blades induces in general a reduction of the average loading of a wind turbine, at least for some components as the main bearing, the yaw bearing, or the tower.When ultimate loads are generated during shutdowns, the effect of the use of cyclic pitch results in reduced peak loads. In fact, as the machine starts from a less stressed condition, the response to an extreme gust or other event will result in reduced loading on its components. This form of load mitigation can be seen as a preventative load mitigation strategy: the effect on load reduction is obtained without the need to detect and react to an extreme event, but by simply unloading the machine so that, in case an extreme event happens, the result will be less severe.The effect of peak load mitigation by preventative cyclic pitch is investigated with reference to a multi-MW wind turbine, by using high-fidelity aeroelastic simulations in a variety of operating conditions.

Analysis and design of Coleman transform‐based individual pitch controllers for wind‐turbine load reduction

AbstractAs the size of wind turbines increases, the effects of dynamic loading on the turbine structures become increasingly significant. There is therefore a growing demand for turbine control systems to alleviate these unsteady structural loads in addition to maintaining basic requirements such as power and speed regulation. This has motivated the development of blade individual pitch control (IPC) methodologies, many of which employ the Coleman transformation to simplify the controller design process. However, and as is shown in this paper, the Coleman transformation significantly alters the rotational system dynamics when these are referred to the non‐rotating frame of reference, introducing tilt–yaw coupling in the process. Unless this transformation is explicitly included in the model employed for IPC design, then the resulting controllers can yield poor performance. Therefore, in this paper, we show how to model the Coleman transformation in a form that is amenable to IPC analysis and synthesis. This enables us to explain why traditional design parameters of gain and phase margin are poor indicators of robust stability and hence motivate the need for a multivariable design approach. The robust multivariable IPC approach advocated in this paper is based upon loop shaping and has numerous desirable properties, including reliable stability margins, improved tilt–yaw decoupling and simultaneous rejection of disturbance loads over a range of frequencies. The design of a robust multivariable IPC is discussed, and simulation results are presented that demonstrate the efficacy of this controller, in terms of load reduction on both rotating and non‐rotating turbine parts. Copyright © 2014 John Wiley & Sons, Ltd.

Iterative Learning Control for Improved Aerodynamic Load Performance of Wind Turbines With Smart Rotors

Currently, there is significant research into the inclusion of smart devices in wind turbine rotor blades, with the aim, in conjunction with collective and individual pitch control, of improving the aerodynamic performance of the rotors. The main objective is to reduce fatigue loads, although mitigating the effects of extreme loads is also of interest. The aerodynamic loads on a wind turbine blade have periodic and nonperiodic components, and the nature of these strongly suggests the application of iterative learning control. This paper employs a simple computational fluid dynamics model to represent flow past an airfoil and uses this to undertake a detailed investigation into the level of control possible by, as in other areas, combining iterative learning control with classical control action with emphasis on how performance can be effectively measured.

A novel individual pitch control algorithm based on μ-synthesis for wind turbines

In this paper, we present a novel individual pitch control algorithm based on μ-synthesis for blade load reduction for wind turbines. Our algorithm is different from the conventional individual pitch control in the sense that the multi-blade coordinate transformation is not used at all. Instead, the wind turbine model coefficients that depend on the azimuth angle are regarded as structured uncertainties. The control objective is formulated with weighting functions on the disturbance input and performance output in the framework of the μ-synthesis approach. Our control algorithm can be added on to the collective pitch control as an on-off mechanism with minimal effect on it. Simulation results show that the proposed controller achieves very good robust stability and performance and can significantly reduce blade bending moments.

Flicker Mitigation by Individual Pitch Control of Variable Speed Wind Turbines With DFIG

Due to the wind speed variation, wind shear and tower shadow effects, grid connected wind turbines are the sources of power fluctuations which may produce flicker during continuous operation. This paper presents a model of an MW-level variable-speed wind turbine with a doubly fed induction generatorto investigate the flicker emission and mitigation issues. An individual pitch control (IPC) strategy is proposed to reduce the flicker emission at different wind speed conditions. The IPC scheme is proposed and the individual pitch controller is designed according to the generator active power and the azimuth angle of the wind turbine. The simulations are performed on the NREL (National Renewable Energy Laboratory) 1.5-MW upwind reference wind turbine model. Simulation results show that damping the generator active power by IPC is an effective means for flicker mitigation of variable speed wind turbines during continuous operation.

Linear individual pitch control design for two‐bladed wind turbines

AbstractIn 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. Copyright © 2014 John Wiley & Sons, Ltd.

Wind tunnel testing of scaled wind turbine models: Beyond aerodynamics

An aeroelastically scaled model of a wind turbine is described, featuring active individual blade pitch and torque control. The model, governed by supervision and control systems similar to those of a real wind turbine, is capable of simulating steady conditions and transient maneuvers in the boundary layer test section of the wind tunnel of the Politecnico di Milano. Expanding the classical scope of wind tunnel models, the present experimental facility enables applications ranging from aerodynamics to aeroelasticity and control.After a description of the model design and of its main characteristics, several applications are presented. Results are shown for the validation of a wind misalignment observer, for the optimization of the open-loop pitch profile used during emergency shutdowns, for the control in wake interference conditions of two models, and for active load alleviation by higher harmonic individual blade pitch control. Results demonstrate the potential of the proposed experimental facility to enable non-standard observations in the controlled environment of the wind tunnel, beyond the classical purely aerodynamic ones.

Subspace predictive repetitive control to mitigate periodic loads on large scale wind turbines

Manufacturing and maintenance costs arising out of wind turbine dynamic loading are one of the largest bottlenecks in the roll-out of wind energy. Individual Pitch Control (IPC) is being researched for cost reduction through load alleviation; it poses a challenging mechatronic problem due to its multi-input, multi-output (MIMO) nature and actuation constraints related to the wear of pitch bearings. To address these issues, Subspace Predictive Repetitive Control (SPRC), a novel repetitive control strategy based on the subspace identification paradigm, is presented. First, the Markov parameters of the system are identified online in a recursive manner. These parameters are used to build up the lifted matrices needed to predict the output over the next period. From these matrices an adaptive repetitive control law is derived. To account for actuator limitations, the known shape of wind-induced disturbances is exploited to perform repetitive control in a reduced-dimension basis function subspace. The SPRC methodology is implemented on a high-fidelity numerical aeroelastic environment for wind turbines. Load reductions are achieved similar to those obtained with classical IPC approaches, while considerably limiting the frequency content of the actuator signals.

풍력발전시스템 개별피치제어설계 및 피로해석에 관한 연구

Structural loading on a wind turbine is due to cyclic loads acting on the blades under turbulence and periodic wind field. The structural loading generates fatigue damage and fatigue failure of the wind turbine. The individual pitch control(IPC) is an efficient control method for reducing structural loading. In this paper, we present an IPC design method using Decentralized LQR(DLQR) and Disturbance accommodating control(DAC). DLQR is used for regulating rotor speed and DAC is used for canceling out disturbances. The performance of the proposed IPC is compared with CPC, which was designed with a gain-scheduled PI controller. We confirm the effect of fatigue load reduction with the use of damage equivalent load(DEL).

Wind and Wave Disturbances Compensation to Floating Offshore Wind Turbine Using Improved Individual Pitch Control Based on Fuzzy Control Strategy

Due to the rich and high quality of offshore wind resources, floating offshore wind turbine (FOWT) arouses the attentions of many researchers. But on a floating platform, the wave and wind induced loads can significantly affect power regulation and vibration of the structure. Therefore, reducing these loads becomes a challenging part of the design of the floating system. To better alleviate these fatigue loads, a control system making compensations to these disturbances is proposed. In this paper an individual pitch control (IPC) system integrated with disturbance accommodating control (DAC) and model prediction control (MPC) through fuzzy control is developed to alleviate the fatigue loads. DAC is mainly used to mitigate the effects of wind disturbance and MPC counteracts the effects of wave on the structure. The new individual pitch controller is tested on the NREL offshore 5 MW wind turbine mounted on a barge with a spread-mooring system, running in FAST, operating above-rated condition. Compared to the original baseline collective pitch control (CPC) (Jonkman et al., 2007), the IPC system shows a better performance in reducing fatigue loads and is robust to complex wind and wave disturbances as well.

Subspace Predictive Repetitive Control with Lifted Domain Identification for Wind Turbine Individual Pitch Control

Individual pitch control (IPC) is gaining increasing acceptance as a method for mitigation of periodic disturbances in wind turbines. This paper aims at formulating a repetitive control (RC) methodology capable of adapting online to changing turbine dynamics. This is achieved by performing system identification in a reduced-dimensional space using basis functions and using the identified parameters to synthesise an RC law to reject periodic disturbances: this methodology is termed Subspace Predictive Repetitive Control (SPRC). The method is tested on an industrial simulation test bench and is able to identify and perform load control on the turbine without affecting its power production.

Research on Joint Power and Loads Control for Large Scale Directly Driven Wind Turbines

Emphasis of this article is on the dynamic characteristics analysis of individual pitch control for MW scale directly driven wind turbines with permanent magnet synchronous generator (PMSG). The pitch control objectives were analyzed and the objective expressions were deduced, including power expression, loads expression, and vibration expressions of blade and tower. Then, both the collective pitch control aiming at power control and the individual pitch control strategy aiming at joint power and loads control were analyzed, too. The blade root bending moments and the actual capture power of wind rotor were employed to be the control variables. The power was calculated based on the conventional measured parameters of wind turbines. In order to reflect the difference between the pitch angle command value and the actual value, the pitch actuator dynamic model was used. The research results show that both the collective pitch control strategy and the proposed individual pitch control strategy can effectively control the power injected into grid; moreover, the individual pitch control can reduce fatigue loads; while in the process of individual pitch control, the actual variation of blade pitch angle is closely related to not only the inflow speed but also the blade azimuth angle; individual pitch control strategy can reduce the variation amplitude of flapwise moments, but has little influence on the edgewise moments.

The Research of Model-Free Adaptive Control for Large Scale Wind Turbine

Dynamic load is a key consideration in large scale wind turbine design. It is approved that the performance of controller can distinguish impact wind turbine loads. For strong external disturbances and inaccurately modeled of large wind turbines, In this paper, we propose model-free adaptive (Model Free Adapt, MFA) individual pitch control algorithms. The controller was developed in to mitigate the rotor unbalance structural load for variable speed wind turbine. The controller is designed from a nonlinear model of the system which takes into account the blades, shaft and tower flexibilities. Bladed software was used for the control strategy and traditional PID control strategy simulation comparison. The results show that the new control strategy can effectively stabilize wind turbine power output and reduce aerodynamic loads.

Aerodynamic Modeling Research of Wind Turbine Suitable for Individual Pitch Control

Momentum theory model which was widely used in pitch control cant calculate wind turbine load, so it can't meet pitch control research needs of large-scale wind turbine. In this paper, the traditional model based on momentum theory firstly was improved to be able to calculate load under some hypothesis. Then the wind turbine model suitable for individual pitch control was built based on blade element theory. And wind shear and tower shadow on load of wind turbine was calculated and analyzed. Finally, the model was simulated in the turbulent flow conditions and load was analyzed by Bladed. Simulations indicate that the model built in the paper can be used in simulation and verification of individual pitch control, and the conclusions drawn by load analysis can provide theoretic basis and the reference standard for individual pitch control strategy.

Individual Pitch Control for Load Reduction Based on Norm Theory

As the capacity of wind turbine increases, unsteady blade loads and performance caused by asymmetrical effects like turbulence, wind shear and tower shadow lead to fatigue loads which seriously affect wind turbine life cycle. This paper focused on wind turbine fatigue load response characteristics, built the multiple input multiple output linearized matrix model, introduced individual pitch control algorithm for optimized disturbance control based on norm theory. Modeling, simulation and verification of the control algorithm are conducted in Matlab. The investigation is conducted based on a 2MW doubly-fed wind turbine. By comparing the results of wind turbine deflection rate and angle deflection rate to collective pitch control method, it shows the individual pitch control algorithm can significantly reduce wind turbine fatigue load, and the control effect is superior to traditional control algorithm.

Individual pitch control of wind turbines: Model-based approach

Significant fore-and-aft tower vibrations of large wind turbines due to wind shear can be reduced using individual pitch control, in which the pitch angle of each blade is adjusted individually. A short survey of the existing individual pitch control strategies is presented, and a new architecture with preview measurements of the wind speeds at different heights is proposed in this article for the turbine tilt moment reduction. The approach includes look-ahead calculations of the desired blade loads and pitch angles as well as preprocessing algorithms for calculation of the derivative of the desired pitch angle. Wind speed measurement records, performed at two different heights with Risoe P2546 cup anemometer, are directly used in the turbine simulations. It is shown that the variations of tilt moment can be essentially reduced using the approach described in this article.

$\ell_{1}$-Optimal Control of Large Wind Turbines

We present an l1-control scheme for multivariable pitch control in full load region. Two decoupled linear time-invariant models are derived using Coleman transformation and gain scheduling to design collective and individual pitch controllers independently. Individual pitch control is used to decrease the blade root bending moment. A new l1-control setup for collective pitch control taking into account the collective bending moment in Coleman mode is presented. This further decreases the blade root bending moment while rotor speed is maintained constant above rated wind conditions. Simulations with a full nonlinear aeroelastic model over the whole load region show the applicability of the proposed control strategy and lifetime weighted damage equivalent loads are computed. Compared to classical collective and individual pitch control, a significant load reduction is achieved without losses in energy production.