Variable structure strategy to avoid amplitude and rate saturation in pitch control of a wind turbine

This work proposes the application of a recent compensation technique for input constraints avoidance to the pitch control of a wind turbine. The pitch angle actuators commonly present a hard limit on their rate of change together with the natural amplitude saturation, and a dynamics during their unconstrained operation that can be modeled as a first-order linear system. This dynamic behavior of the pitch actuator requires a particular design of the compensation method, which is based on variable structure systems to avoid both amplitude and rate input saturation by means of an auxiliary loop. The developed methodology reduces the pitch actuator activity necessary to regulate the generated power around its nominal value when facing sudden wind gusts. Another interesting feature of the proposal is that it allows the operator to fix conservative bounds for the actuator speed operation in order to increment the structural robustness of the wind turbine and to extend in this way the service life of the energy system. The effectiveness of the proposed strategy is evaluated by simulation results in an autonomous wind energy conversion system for water pumping with a brushless double feed induction generator (BDFIG).     

风力发电机组的自适应转矩控制及载荷优化

针对变速变桨风力发电机组(variable speed variable pitch,VSVP)如何在低风速时最大限度捕获风能以及在额定风速以上降低传动链载荷进行研究。低风速时在研究了传统风能追踪控制策略的基础上,文中提出通过改变最优增益系数来追踪最佳风能利用系数的自适应转矩控制策略。同时针对风力发电机组传动链的扭转振动,提出了基于发电机转速反馈滤波的转矩纹波控制方式。以2MW变速变桨风力发电机组为验证对象,基于Blade软件平台对所采用的控制策略进行仿真研究。结果表明：所提出的自适应转矩控制策略能够更好的追踪最大功率点,同时采用转矩纹波能够降低传动链载荷     

Modelling–based approach for digital control design for nonlinear WECS in the power system

The case has been established that the wind power plant must be treated as an integral part of the electric system, thereby constituting the wind energy conversion system. Recent advancement in size and technology of wind turbines requires sophisticated control systems to effectively optimize energy conversion and enhance grid integration. As a first step toward controller design, modelling has become a prerequisite. This paper explores controller design based on modelling the wind speed as a stochastic process, and the wind turbine as a multi‐mass system with a soft shaft linking the turbine with the doubly fed induction generator. A control strategy incorporating a linear quadratic Gaussian (LQG) that relies on state estimation for full‐state feedback is proposed to augment a linear controller for generator torque control. The control objectives are to reduce stresses on the drivetrain and to ensure operation geared toward optimal power conversion. This study focuses on above‐rated wind speeds, and the LQG's main purpose is to add damping to the drivetrain, thereby minimizing cyclic fatigue, while a pitch control mechanism prevents rotor overspeed, thereby maintaining rated power. Simulations show the efficacy of the proposed paradigm in meeting the control objectives. Copyright © 2009 John Wiley & Sons, Ltd.     

An Autonomous System Supplied Only by a Pitch-Controlled Variable-Speed Wind Turbine

This paper presents a control strategy for a variable-speed pitch-controlled wind turbine (WT) generation scheme for the supply of an autonomous system with no energy storage units. The synchronous generator includes two three-phase stator windings displaced by 30deg that are connected to the transformer load through two dc links with voltage source inverters (VSI). Following priority rules, the load is divided into steps. Each load step can be supplied by the WT when the wind speed varies between two predefined speed levels. The first goal of the WT control system is to supply the load with constant real power under constant voltage as the wind speed varies between two levels and the second is to operate smoothly interchanging the load steps when the wind speed breaks through a speed level. There are two controllers: the inverter controller that keeps the load voltage constant and the pitch controller acting on the blade's angle. Using simulation techniques, the operation of the WT system and the efficiency of the proposed control strategy are demonstrated for a wide range of wind speeds.     

Variable-speed wind power generation using doubly fed wound rotor induction machine-a comparison with alternative schemes

In this paper, a wind energy conversion system (WECS) using a grid-connected wound rotor induction machine controlled from the rotor side is compared with both fixed speed and variable speed systems using a cage rotor induction machine. The comparison is done on, the basis of: (1) major hardware components required; (2) operating region; and (3) energy output due to a defined wind function using the characteristics of a practical wind turbine. Although a fixed speed system is more simple and reliable, it severely limits the energy output of a wind turbine. In case of variable speed systems, comparison shows that using a wound rotor induction machine of similar rating can significantly enhance energy capture. This comes about due to the ability to operate with rated torque even at supersynchronous speeds; power is then generated out of the rotor as well as the stator. Moreover, with rotor side control, the voltage rating of the power devices and DC bus capacitor bank is reduced. The size of the line side inductor also decreased. Results are presented to show the substantial advantages of the doubly fed system.     

双馈风力发电系统最大风能追踪控制

在分析风力机功率特性和DFIG运行特性的基础上,通过对双馈机转速控制进行最大风能追踪具体过程的深入研究,提出一种基于最大风能追踪的双馈电机有功、无功功率的解耦控制方法。建立了基于发电机定子磁链定向矢量控制的双馈风力发电系统最大风能追踪系统模型,并利用PSCAD／EMTDC对其进行仿真,结果验证了控制策略的正确性。     

双馈风力发电系统最大风能追踪控制的研究

在分析了风力机功率特性和DFIG运行特性的基础上,通过对双馈机转速控制进行最大风能追踪具体过程的深入研究,提出了一种基于最大风能追踪的双馈电机有功、无功功率的解耦控制方法。建立了基于发电机定子磁链定向矢量控制的双馈风力发电系统最大风能追踪系统模型,并利用PSCAD/EMTDC对其进行仿真,结果验证了控制策略的正确性。     

大型风力发电机组独立桨叶控制系统

设计出独立桨叶控制系统的机构方案，依据空气动力学分析，提出模糊控制结合以桨叶空间方位角作为主体因素的加权系数的控制策略，建立了系统模型，仿真结果表明，在风速高于额定风速时，作用在桨叶上的负载波动大为减小，输出功率维持在额定功率附近。     

载荷优化的海上漂浮式风电机组二阶滑模变桨控制

针对强非线性、强耦合的海上漂浮式风电机组动力学系统,提出一种基于二阶滑模的统一变桨控制策略,解决受海浪风速等随机干扰引起浮式支撑平台运动而产生的疲劳结构载荷及功率波动问题。构建漂浮式风电机组的不确定仿射非线性模型,基于风电机组“额定转速”设计积分滑模面,此“额定转速”不再是恒定值,而是取决于平台纵摇速度的变量,基于超螺旋算法实现二阶滑模变桨控制律。采用FAST和Matlab/Simulink联合仿真,所提出的方案与传统PI控制相比,对稳定高风速时风力发电机功率,抑制浮式支撑平台运动及减少叶根载荷具有更好的控制作用,对塔基也有较好的减载作用。     

基于CFD的二维垂直轴风力机性能计算

采用计算流体力学方法(CFD)针对垂直轴风力发电机,开展简化的二维绕流特性研究。首先,基于开放型转子和增强型转子,研究网格节点数和壁面y+、计算时间步长和湍流模型等的变化对计算结果的影响,对计算模型和方法进行确认。随后,计算分析增强型垂直轴风力机与开放型垂直轴风力机的特性。结果表明,与开放性垂直轴风力发电机相比,增强型垂直轴风力发电机的功率系数和转矩系数有明显增加,且达到最大值的位置向叶尖速比增大的方向移动。然后对增强型垂直轴风力机发电机在不同来流风速下进行计算,发现增强型垂直轴风力发电机的转子转矩随来流风速增加,而转矩系数和功率系数与来流风速无关。最后,针对定子叶片在不同的方向开展计算研究。结果表明,定子叶片在不同方向时,增强型垂直轴风力机的转子转矩不同,且转矩到达峰值的位置也不同;在当前3个方向角中,叶片处于0°方向角时风力机具有最高的转矩系数,即具有最佳的功率系数。     

Multivariable control strategy for variable speed, variable pitch wind turbines

Reliable and powerful control strategies are needed for wind energy conversion systems to achieve maximum performance. A new control strategy for a variable speed, variable pitch wind turbine is proposed in this paper for the above-rated power operating condition. This multivariable control strategy is realized by combining a nonlinear dynamic state feedback torque control strategy with a linear control strategy for blade pitch angle. A comparison with existing strategies, PID and LQG controllers, is performed. The proposed approach results in better power regulation. The new control strategy has been validated using an aeroelastic wind turbine simulator developed by NREL for a high turbulence wind condition.     

Robust direct power control based on the Lyapunov theory of a grid-connected brushless doubly fed induction generator

This paper deals with robust direct power control of a grid-connected brushless doubly-fed induction generator(BDFIG). Using a nonlinear feedback linearization strategy, an attempt is made to improve the desired performances by controlling the generated stator active and reactive power in a linear and decoupled manner. Therefore, to achieve this objective, the Lyapunov approach is used associated with a sliding mode control to guarantee the global asymptotical stability. Thus, an optimal operation of the BDFIG in sub-synchronous operation is obtained as well as the stator power flows with the possibility of keeping stator power factor at a unity. The proposed method is tested with the Matlab/Simulink software. Simulation results illustrate the performances and the feasibility of the designed control.     

双馈风力发电系统的最大风能控制策略

在分析变速恒频风力发电系统最大风能捕获策略的基础上,提出了一种通过直接控制双馈发电机转子电流就可实现系统低于额定风速下的最大风能捕获又可使得电机铜耗最小化运行的控制策略.首先,在考虑风力机特性和双馈发电机基本电磁关系的基础上,分别推导了双馈发电机定子铜耗最小化运行和最大风能捕获的数学模型.其次,根据双馈发电机最优转子电流的数学模型,建立了双馈风力发电机系统最大风能捕获的控制策略.最后,利用Matlab/Simulink对不同风速下双馈发电机系统的运行性能进行了分析和比较,结果验证了该控制策略的正确性和可行性.     

Control of a Doubly Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions

Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three-phase stator voltages cause a number of problems, such as overcurrent, unbalanced currents, reactive power pulsations, and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance, induction wind generators are switched out of the network. This can further weaken the grid. In doubly fed induction generators (DFIGs), control of the rotor currents allows for adjustable speed operation and reactive power control. This paper presents a DFIG control strategy that enhances the standard speed and reactive power control with controllers that can compensate for the problems caused by an unbalanced grid by balancing the stator currents and eliminating torque and reactive power pulsations     

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.     

Analysis of reactive power strategies in HVDC‐connected wind power plant clusters

Offshore wind power plants (WPPs) built near each other but far from shore usually connect to the main grid by a common high‐voltage DC (HVDC) transmission system. In the resulting decoupled offshore grid, the wind turbine converters and the high‐voltage DC voltage‐source converter share the ability to inject or absorb reactive power. The overall reactive power control dispatch influences the power flows in the grid and hence the associated power losses. This paper evaluates the respective power losses in HVDC‐connected WPP clusters when applying 5 different reactive power control strategies. The case study is made for a 1.2‐GW–rated cluster comprising 3 WPP and is implemented in a combined load flow and converter loss model. A large set of feasible operating points for the system is analyzed for each strategy. The results show that a selection of simulations with equal wind speeds is sufficient for the annual energy production comparison. It is found that the continuous operation of the WPPs with unity power factor has a superior performance with low communication requirements compared with the other conventional strategies. The optimization‐based strategy, which is developed in this article, allows a further reduction of losses mainly because of the higher offshore grid voltage level imposed by the high‐voltage DC voltage‐source converter. Reactive power control in HVDC‐connected WPP clusters change significantly the overall power losses of the system, which depend rather on the total sum of the injected active power than on the variance of wind speeds inside the cluster.     

A robust power control strategy to enhance LVRT capability of grid‐connected DFIG‐based wind energy systems

This paper presents a new robust and effective control strategy to mitigate symmetrical voltage dips in a grid‐connected doubly fed induction generator (DFIG) wind energy conversion system without any additional hardware in the system. The aim is to control the power transmitted to the grid so as to keep the electrical and mechanical quantities above their threshold protection values during a voltage dip transient. To achieve this, the references of the powers are readjusted to adapt the wind energy conversion system to the fault conditions. Robust control strategies, combining the merits of sliding mode theory and fuzzy logic, are then proposed in this paper. These controllers are derived from the dynamic model of the DFIG considering the variations in the stator flux generated by the voltage drop. This approach is found to yield better performance than other control design methods which assume the flux in the stator to remain constant in amplitude. This control scheme is compliant with the fault‐ride‐through grid codes which require the wind turbine generator to remain connected during voltage dips. A series of simulation scenarios are carried out on a 3‐MW wind turbine system to demonstrate the effectiveness of the proposed control schemes under voltage dips and parameter uncertainty conditions.     

Large‐eddy simulations with extremum‐seeking control for individual wind turbine power optimization

Large‐eddy simulations of the flow past an array of three aligned turbines have been performed. The study is focused on below rated (Region 2) wind speeds. The turbines are controlled through the generator torque gain, as usually done in Region 2. Two operating strategies are considered: (i) preset individual optimum torque gain based on a model for the power coefficient (baseline case) and (ii) real‐time optimization of torque gain for maximizing each individual turbine power capture during operation. The real‐time optimization is carried out through a model‐free approach, namely, extremum‐seeking control. It is shown that ESC is capable of increasing the power production of the array by 6.5% relative to the baseline case. The extremum‐seeking control reduces the torque gain of the downstream turbines, thus increasing the angular speed of the blades. This results in improved aerodynamics near the tip of the blade that is the portion contributing mostly to the torque and power. In addition, an increase in angular speed leads to a larger entrainment in the wake, which also contributes to provide additional available power downstream. It is also shown that the tip speed ratio may not be a reliable performance indicator when the turbines are in waked conditions. This may be a concern when using optimal parameter settings, determined from isolated turbine models, in applications with waked turbines. Copyright © 2017 John Wiley & Sons, Ltd.     

Performance of PI controller for control of active and reactive power in DFIG operating in a grid-connected variable speed wind energy conversion system

Due to several factors, wind energy becomes an essential type of electricity generation. The share of this type of energy in the network is becoming increasingly important. The objective of this work is to present the modeling and control strategy of a grid connected wind power generation scheme using a doubly fed induction generator (DFIG) driven by the rotor. This paper is to present the complete modeling and simulation of a wind turbine driven DFIG in the second mode of operating (the wind turbine pitch control is deactivated). It will introduce the vector control, which makes it possible to control independently the active and reactive power exchanged between the stator of the generator and the grid, based on vector control concept (with stator flux or voltage orientation) with classical PI controllers. Various simulation tests are conducted to observe the system behavior and evaluate the performance of the control for some optimization criteria (energy efficiency and the robustness of the control). It is also interesting to play on the quality of electric power by controlling the reactive power exchanged with the grid, which will facilitate making a local correction of power factor.