Experimental validation of model‐based blade pitch controller design for floating wind turbines: system identification approach

This paper discusses the model‐based design of a blade pitch controller for a floating offshore wind turbine (FOWT) scale model. A mathematical model of the FOWT is constructed from an input–output measurement in an experiment using system identification. The blade pitch controller is designed by an control method, and the effectiveness of the controller is evaluated by means of a basin experiment using the FOWT scale model. The results show that the blade pitch controller is effective in reducing platform pitch motion and rotor speed fluctuation. Copyright © 2017 John Wiley & Sons, Ltd.     

The Design of closed loop controllers for wind turbines

This article reviews the design of algorithms for wind turbine pitch control and also for generator torque control in the case of variable speed turbines. Some recent and possible future developments are discussed. Although pitch control is used primarily to limit power in high winds, it also has a significant effect on various loads. Particularly as turbines become larger, there is increasing interest in designing controllers to mitigate loads as far as possible. Torque control in variable speed turbines is used primarily to maximize energy capture below rated wind speed and to limit the torque above rated. Once again there are opportunities for designing these controllers so as to mitigate certain loads. In addition to improving the design of the control algorithms, it is also possible to use additional sensors to help the controller to achieve its objectives more effectively. The use of additional actuators in the form of individual pitch controllers for each blade is also discussed. It is important to be able to quantify the benefits of any new controller. Although computer simulations are useful, field trials are also vital. The variability of the real wind means that particular care is needed in the design of the trials. Copyright © 2001 John Wiley & Sons, Ltd.     

Load reduction on a clipper liberty wind turbine with linear parameter‐varying individual blade pitch control

The increasing size of modern wind turbines also increases the structural loads caused by effects such as turbulence or asymmetries in the inflowing wind field. Consequently, the use of advanced control algorithms for active load reduction has become a relevant part of current wind turbine control systems. In this paper, an individual blade pitch control law is designed using multivariable linear parameter‐varying control techniques. It reduces the structural loads both on the rotating and non‐rotating parts of the turbine. Classical individual blade pitch control strategies rely on single‐control loops with low bandwidth. The proposed approach makes it possible to use a higher bandwidth since it accounts for coupling at higher frequencies. A controller is designed for the utility‐scale 2.5 MW Liberty research turbine operated by the University of Minnesota. Stability and performance are verified using the high‐fidelity nonlinear simulation and baseline controllers that were directly obtained from the manufacturer. Copyright © 2017 John Wiley & Sons, Ltd.     

Experimental study of closed‐loop thermosyphon with a different evaporator geometry

In this study, the effect of evaporator geometry on the loop thermosyphon's heat transfer coefficient is experimentally verified by using water as a working fluid with three filling ratios (50%, 70%, 90%), constant heat input (185 W), and condenser cooling water flow rate remaining constant at 2 Lpm. Three evaporator pipes are used (I: straight; II: helical coil evaporator with a diameter of 100‐mm coil and two turns; III: helical coil evaporator with a diameter of 50‐mm coil and four turns). From the experimental results, it can be observed that the performance of evaporator III is higher than the two other forms. A greater heat transfer coefficient value is found in case of type III evaporator and is equivalent to 2456 W/m²·°C. The maximum thermal resistance reduction occurs in the type III evaporator (37.32%), and the highest effective thermal conductivity for the same type is 6.123e + 05 W/m·°C. The experimental results demonstrate good agreement with the empirical equations.     

A simple robust controller for power maximization of a variable‐speed wind turbine

This paper presents a high‐order sliding mode control strategy that aims to optimize the power conversion efficiency of a wind energy conversion system within the partial load zone of operation. The main challenges of this control problem are related to the random variations of the wind speed, the nonlinear nature of the whole system, usual model uncertainties and external disturbances. For all these reasons, the robustness, simplicity and low computational burden of the proposed super‐twisting algorithm result very attractive in this context. Simulation results that show the achievement of the desired characteristics are provided. Copyright © 2009 John Wiley & Sons, Ltd.     

System identification and controller design for individual pitch and trailing edge flap control on upscaled wind turbines

Active load reduction strategies such as individual pitch control (IPC) and trailing edge flap (TEF) actuation present ways of reducing the fatigue loads on the blades of wind turbines. This may enable development of lighter blades, improving the performance, cost effectiveness and viability of future multi‐megawatt turbine designs. Previous investigations into the use of IPC and TEFs have been limited to turbines with ratings up to 5 MW and typically investigate the use of these load reduction strategies on a single turbine only. This paper extends the design, implementation and analysis of individual pitch and TEFs to a range of classically scaled turbines between 5 and 20 MW. In order to avoid designing controllers which favour a particular scale, identical scale‐invariant system identification and controller design processes are applied to each of the turbines studied. Gain‐scheduled optimal output feedback controllers are designed using identified models to target blade root load fluctuations at the first and second multiples of the rotational frequency using IPC and TEFs respectively. The use of IPC and TEFs is shown in simulations to provide significant reductions in fatigue loads at the blade root. Fatigue loads on non‐rotating components such as the yaw bearing and tower root (yaw moment) are also reduced with the use of TEFs. Individual pitch performance is seen to be slightly lower on larger turbines, potentially due to a combination of reduced actuator bandwidth and movement of the rotational frequency of larger turbines into a more energetic part of the turbulent spectrum. However, TEF performance is consistent irrespective of scale. Copyright © 2015 John Wiley & Sons, Ltd.     

IPC-based robust disturbance accommodating control for load mitigation and speed regulation of wind turbines

Over the past few decades, global demand for renewable energy has been rising steadily. To meet this demand, there has been an exponential growth in size of wind turbines (WTs) to capture more energy from wind. Consequent increase in weight and flexibility of WT components has led to increased structural loading, affecting reliability of these wind energy conversion systems. Spatio-temporal variation of rotor effective wind field acts as a disturbance to a WT system, hence, necessitating controllers that can cancel this disturbance. Additionally, assumptions made in extracting linear models for controller design lead to modeling errors resulting from changing operating conditions. Previous attempts have proposed robust controllers incorporating wind disturbance models. However, these controllers have been evaluated on smaller WTs, which experience lower structural loading than larger ones. Additionally, a majority these controllers are based on collective pitch control (CPC), hence do not address loading in the blades. To address these challenges, this contribution proposes an independent pitch-based robust disturbance accommodating controller (IPC-RDAC) for reducing structural loads and regulating generator speed in utility-scale WTs. The proposed controller is designed using $$-synthesis approach and is evaluated on the 5 MW National Renewable Energy Laboratory (NREL) reference WT. Its performance is evaluated against a gain-scheduled proportional integral (GSPI)-based reference open-source controller (ROSCO) and a CPC-based RDAC (CPC-RDAC) controller, developed previously by the authors. Simulation results for various wind conditions show that the proposed controller offers improved performance in blade and tower load mitigation, as well a generator speed regulation.     

A fuzzy logic pitch angle controller for power system stabilization

In this article the design of a fuzzy logic pitch angle controller for a fixed speed, active‐stall wind turbine, which is used for power system stabilization, is presented. The system to be controlled, which is the wind turbine and the power system to which the turbine is connected, is described. The advantages of fuzzy logic control when applied to large‐signal control of active‐stall wind turbines are outlined. The general steps of the design process for a fuzzy logic controller, including definition of the controller inputs, set‐up of the fuzzy rules and the method of defuzzification, are described. The performance of the controller is assessed by simulation, where the wind turbine's task is to dampen power system oscillations. In the scenario simulated for this work, the wind turbine has to ride through a transient short‐circuit fault and subsequently contribute to the damping of the grid frequency oscillations that are caused by the transient fault. It is concluded that the fuzzy logic controller enables the wind turbine to dampen power system oscillations. It is also concluded that, owing to the inherent non‐linearities in a wind turbine and the unpredictability of the whole system, the fuzzy logic controller is very suitable for this application. Copyright © 2006 John Wiley &Sons, Ltd.     

Numerical and experimental study of a thermosyphon closed‐loop system for domestic applications

A numerical and experimental study has been conducted to enhance the thermal performance of the thermosyphon system. The enhancement response focused on the temperature of both the working fluid within the system loop and water inside the tank. To achieve this, three models were investigated to increase the surface area of the riser pipe without changing the amount of the working fluid. The first one (model A) involved increasing the diameter of the riser pipe and inserting a closed tube inside it to maintain the same amount of working fluid. The second method (model B) involved adding toroidal fins around the riser pipe. However, the third model (model C) combined both models (A and B). The thermal performance of the thermosyphon system for the conventional model has been compared experimentally. Furthermore, numerical simulations for all cases have been done using commercial computational fluid dynamics, ANSYS R 19.3 software. The results show that there is good agreement between the experimental and numerical results. Furthermore, it is found that the thermal responses of models A and B are approximately equal and both are higher than that of the traditional model. Moreover, the thermal performance of model C is found to be higher than those of all the other models under study.     

Response analysis and comparison of a spar‐type floating offshore wind turbine and an onshore wind turbine under blade pitch controller faults

This paper analyses the effects of three pitch system faults on two classes of wind turbines, one is an onshore type and the other a floating offshore spar‐type wind turbine. A stuck blade pitch actuator, a fixed value fault and a bias fault in the blade pitch sensor are considered. The effects of these faults are investigated using short‐term extreme response analysis with the HAWC2 simulation tool. The main objectives of the paper are to investigate how the different faults affect the performance of wind turbines and which differences exist in the structural responses between onshore and floating offshore wind turbines. Several load cases are covered in a statistical analysis to show the effects of faults at different wind speeds and fault amplitudes. The severity of individual faults is categorized by the extreme values the faults have on structural loads. A pitch sensor stuck is determined as being the most severe case. Comparison between the effects on floating offshore and onshore wind turbines show that in the onshore case the tower, the yaw bearing and the shaft are subjected to the highest risk, whereas in the offshore case, the shaft is in this position. Copyright © 2014 John Wiley & Sons, Ltd.     

风电机组模糊PI控制与前馈控制相结合的变桨控制

针对经典PID变桨距控制器超调量大、波动剧烈等缺陷,提出了一种采用模糊PI控制与前馈控制相结合的控制器来控制浆距角,进而使机组功率稳定在额定功率附近的控制思路。通过对某1.5MW风力发电机组进行仿真,结果表明,该控制器控制效果优于经典PID变桨距控制器。     

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

As 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.     

Flexure pitch bearing concept for individual pitch control of wind turbines

The excessive use of individual pitch control (IPC) for fatigue load reduction is accompanied by the uncertainty of potential bearing failures. This problem, which is due to the small swivel angles associated with IPC, arises because of the rolling and sliding contacts that occur with the rolling element bearings that are typically used. The use of a flexure bearing is proposed as a way of bypassing this issue. The flexure bearing enables a certain range of motion to be exclusively provided by elastic deformation. This article presents a novel bearing concept that is based on the hypothesis that such a flexure bearing can handle the unfavorable load conditions associated with IPC better than a rolling element bearing. Methods for the dimensioning of the aforementioned flexure bearing are therefore presented. The loads, particularly the required elastic rotation angle of the flexure bearing, are determined first. A promising design for the flexure bearing itself is then chosen and adapted to meet the specific requirements of IPC. These methods are applied to develop an initial conceptual design of the novel bearing unit for a 3‐bladed wind turbine of about 3.6 MW. The result demonstrates the feasibility of the concept, and a final discussion presents further opportunities of the design that will make this concept satisfy the special requirements of IPC.     

MPC framework for constrained wind turbine individual pitch control

The reduction of structural loads is becoming an important objective for the wind turbine control system due to the ever‐increasing specifications/demands on wind turbine rated power and related growth of turbine dimensions. Among various control algorithms that have been researched in recent years, the individual pitch control has demonstrated its effectiveness in wind turbine load reduction. Since the individual pitch control, like other load reduction algorithms, requires higher levels of actuator activity, one must take actuator constraints into account when designing the controller. This paper presents a method for the inclusion of such constraints into a predictive wind turbine controller. It is shown that the direct inclusion of constraints would result in a control problem that is nonconvex and difficult to solve. Therefore, a modification of the constraints is proposed that ensures the convexity of the control problem. Simulation results show that the developed predictive control algorithm achieves individual pitch control objectives while satisfying all imposed constraints.     

Linear individual pitch control design for two‐bladed wind turbines

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. Copyright © 2014 John Wiley & Sons, Ltd.     

Airfoil optimisation for vertical‐axis wind turbines with variable pitch

To advance the design of a multimegawatt vertical‐axis wind turbine (VAWT), application‐specific airfoils need to be developed. In this research, airfoils are tailored for a VAWT with variable pitch. A genetic algorithm is used to optimise the airfoil shape considering a balance between the aerodynamic and structural performance of airfoils. At rotor scale, the aerodynamic objective aims to create the required optimal loading while minimising losses. The structural objective focusses on maximising the bending stiffness. Three airfoils from the Pareto front are selected and analysed using the actuator cylinder model and a prescribed‐wake vortex code. The optimal pitch schedule is determined, and the loadings and power performance are studied for different tip‐speed ratios and solidities. The comparison of the optimised airfoils with similar airfoils from the first generation shows a significant improvement in performance, and this proves the necessity to properly select the airfoil shape.     

New design of robust PID controller based on meta‐heuristic algorithms for wind energy conversion system

This paper proposes a new robust control method for a wind energy conversion system. The suggested method can damp the deviations in the generator speed because of the penetration of wind speed and load demand fluctuations in the electrical grid. Furthermore, it can overcome the uncertainties of the plant parameters because of load demand fluctuations and the errors of the implementation. The new method has been built based on new simple frequency‐domain conditions and the whale optimization algorithm (WOA). This method is utilized to design a robust proportional‐integral‐derivative (PID) controller based on the WOA in order to enhance the damping characteristics of the wind energy conversion system. Simulation results confirm the superiority and robustness of the proposed technique against the wind speed fluctuations and the plant parameters uncertainties compared with other meta‐heuristic algorithms.     

Simulating Feedback Linearization control of wind turbines using high‐order models

As wind turbines are becoming larger, wind turbine control must now encompass load control objectives as well as power and speed control to achieve a low cost of energy. Due to the inherent non‐linearities in a wind turbine system, the use of non‐linear model‐based controllers has the potential to increase control performance. A non‐linear feedback linearization controller with an Extended Kalman Filter is successfully used to control a FAST model of the controls advanced research turbine with active blade, tower and drive‐train dynamics in above rated wind conditions. The controller exhibits reductions in low speed shaft fatigue damage equivalent loads, power regulation and speed regulation when compared to a Gain Scheduled Proportional Integral controller, designed for speed regulation alone. The feedback linearization controller shows better rotor speed regulation than a Linear Quadratic Regulator (LQR) at close to rated wind speeds, but poorer rotor speed regulation at higher wind speeds. This is due to modeling inaccuracies and the addition of unmodeled dynamics during simulation. Similar performance between the feedback linearization controller and the LQR in reducing drive‐train fatigue damage and power regulation is observed. Improvements in control performance may be achieved through increasing the accuracy of the non‐linear model used for controller design. Copyright © 2009 John Wiley & Sons, Ltd.     

基于模糊控制的风电机组独立变桨距控制

在额定风速以上时,通常采用变桨距控制技术调节大型风电机组来稳定其输出功率.由于风力发电系统的数学模型具有高度非线性、多变量、强耦合的特点,风速又具有多变性,因此文章在分析传统的PID变桨距控制技术优缺点的基础上,提出了基于三维模糊自适应PID控制的独立变桨距控制技术,并且引入风速的模糊前馈控制技术.对1 MW风电机组进行仿真,结果表明,在额定风速以上时,该方法不仅能稳定风电机组的输出功率,而且可以减小桨叶的拍打振动.