Modeling and control of a wind turbine driven doubly fed induction generator

This paper presents the simulation results of a grid-connected wind driven doubly fed induction machine (DFIM) together with some real machine performance results. The modeling of the machine considers operating conditions below and above synchronous speed, which are actually achieved by means of a double-sided PWM converter joining the machine rotor to the grid. In order to decouple the active and reactive powers generated by the machine, stator-flux-oriented vector control is applied. The wind generator mathematical model developed in this paper is used to show how such a control strategy offers the possibility of controlling the power factor of the energy to be generated.     

Controller design for an induction generator driven by a variable-speed wind turbine

This paper presents the modeling, controller design and a steady-state analysis algorithm for a wind-driven induction generator system. An output feedback linear quadratic controller is designed for the static synchronous compensator (STATCOM) and the variable blade pitch in a wind energy conversion system (WECS) in order to reach the voltage and mechanical power control under both grid-connection and islanding conditions. A two-reference-frame model is proposed to decouple the STATCOM real and reactive power control loops for the output feedback controller. To ensure zero steady-state voltage errors for the output feedback controller, the integrals of load bus voltage deviation and dc-capacitor voltage deviation are employed as the additional state variables. Pole-placement technique is used to determine a proper weighting matrix for the linear quadratic controller such that satisfactory damping characteristics can be achieved for the closed-loop system. Effects of various system disturbances on the dynamic performance have been simulated, and the results reveal that the proposed controller is effective in regulating the load voltage and stabilizing the generator rotating speed for the WECS either connected with or disconnected from the power grid. In addition, proper steady-state operating points for an isolated induction generator can be determined by the proposed steady-state analysis algorithm. Constant output frequency control using the derived steady-state characteristics of the isolated induction generator is then demonstrated in this paper.     

孤岛双馈异步风力发电机组自抗扰控制研究

针对孤岛运行双馈异步风力发电机输出电压易受负载切换影响、PI控制器对电机参数依赖性高等问题,建立了包含励磁电流动态过程与电机参数误差的数学模型。将电机参数误差和励磁电流动态过程分别视为系统的内扰与外扰,应用分离性原理和零极点配置方法设计双闭环自抗扰控制器替代PI控制器,扩张状态观测器可实时估计系统总扰动,并通过前馈进行补偿。建立兆瓦级DFIG孤岛运行仿真模型进行验证,结果表明文章控制方法能够有效抑制负载切换引起的电压扰动和电机参数变化的影响,提高了系统抗扰动性能。     

A variable speed wind turbine power control

To optimize the power in a wind turbine, the speed of the turbine should be able to vary with the wind speed. A simple control scheme is proposed that will allow an induction motor to run a turbine at its maximum power coefficient. The control uses a standard V/Hz converter and controls the frequency to achieve the desired power at a given turbine speed     

变风速下双馈异步风机在微电网中的电压频率协调控制

微电网线路阻抗比值较大,当微电网孤岛运行时,负荷频繁投切导致微电网电压、频率波动较大。文章对双馈感应风电机组与柴油机协调控制共同为微电网提供电压、频率支撑,提出了DFIG在微电网中的电压、频率协调控制策略。DFIG有功功率控制采用虚拟惯量控制与超速减载控制,并采用f-P下垂控制与柴油机配合对微电网频率进行调节。通过控制DFIG转子侧换流器的无功功率,并采用V-Q下垂控制与柴油机配合对微电网电压进行控制。最后在DIgSILENT中搭建了风光柴中压微电网模型,仿真结果验证了文章所提控制策略的有效性。     

Transient and dynamic control of a variable speed wind turbine with synchronous generator

In this article, a controller for dynamic and transient control of a variable speed wind turbine with a full‐scale converter‐connected high‐speed synchronous generator is presented. First, the phenomenon of drive train oscillations in wind turbines with full‐scale converter‐connected generators is discussed. Based on this discussion, a controller is presented that dampens these oscillations without impacting on the power that the wind turbine injects into the grid. Since wind turbines are increasingly demanded to take over power system stabilizing and control tasks, the presented wind turbine design is further enhanced to support the grid in transient grid events. A controller is designed that allows the wind turbine to ride through transient grid faults. Since such faults often cause power system oscillations, another controller is added that enables the turbine to participate in the damping of such oscillations. It is concluded that the controllers presented keep the wind turbine stable under any operating conditions, and that they are capable of adding substantial damping to the power system. Copyright © 2007 John Wiley & Sons, Ltd.     

双馈感应风力发电系统电流控制策略研究

由双馈发电机q-d坐标系下基本数学模型,推导出双馈发电机的转子电流和转子电压、磁链之间的关系模型。文章研究了双馈风力发电系统转子侧电流控制策略框图,计算选择PI控制器参数。利用MATLAB/SIMULINK平台自行建立了1.5 MW风力发电系统整体仿真模型。运行该系统模型,得到了双馈发电机转子电流、电压,滤波器输出电流及定子电压等信号波形曲线。仿真结果验证模型及控制器的正确性,证明PI控制器能够满足双馈风力发电系统的稳、准、快等性能指标。     

变速变桨风力发电机组的优化控制

提出了大型变速变桨风力发电机组在不同控制阶段的优化控制策略。在低风速时,采用自适应转矩控制方式,实现机组的变速运行,追踪最佳风能利用系数。在额定风速以上时,为了解决传统桨距控制方式系统超调量大的问题,提出了一种新型气动转矩观测器,并将气动转矩与发电机转矩偏差输入控制器。通过Bladed外部控制器模块编程并进行仿真,结果表明,所提出的控制策略能够更好地追踪最大功率点,并改善桨距控制效果,稳定功率输出。     

A sensorless control for a variable speed wind turbine operating at partial load

This paper presents a sensorless control for a variable speed wind turbine (WT) operating at partial load in order to eliminate the direct measurement of the wind speed. In this proposal, the estimated aerodynamic torque is used to determine the optimal reference of the speed control for maximum energy conversion. The maximization of the efficiency on energy conversion and the minimization of detrimental dynamical loads are control trade-offs considered in the design of an optimal discrete-time feedback LQG/LTR controller for the Wind Energy Conversion System (WECS), which is based on the optimization of a quadratic cost function. The performance of the proposed control when the WT is submitted to a gust or step variation on wind speed is evaluated from computational simulations. It is also presented some proposals for sensorless control of the electrical generator.     

Voltage-frequency control of a self-excited induction generator

A new strategy for controlling voltage and frequency of a self excited induction generator (SEIG) is presented. The SEIG operates in the linear region of the core magnetizing curve, so that efficiency and performance are upgraded. An external excitation circuit, comprising permanently connected capacitors and electronically switched inductances is used. The external circuit allows to compensate for the generator reactive demand. A detailed analysis is performed, showing some salient aspects related to the connection of the external excitation circuit on the control performance. Asynchronous switching is used, but some important considerations must be taken into account related to the instantaneous phase angle between stator voltage and external inductor current at the switching instant, if good transient response is desired. Sliding mode controllers are proposed, showing good dynamic response and robust behavior upon changes in load and generator parameters. Computer simulations are used to demonstrate the validity of the proposed scheme     

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.     

Selection of capacitance for self-excited six-phase induction generator for stand-alone renewable energy generation

This paper presents a simple method for finding the suitable value of shunt and series capacitance necessary to initiate self excitation and self-regulation (voltage regulation) in a self-excited six-phase induction generator (SPSEIG) for stand-alone renewable energy generation in conjunction with the hydropower. The problem is formulated as multivariable unconstrained nonlinear optimization problem. The admittance of the equivalent circuit of SPSEIG is taken as an objective function. Frequency and magnetic reactance or speed and magnetic reactance or frequency and capacitive reactance are selected as an independent variables depending upon the operational condition of the machine. Fmincon method is used to solve the optimization problem. Computed results were experimentally verified to validate the analytical approach presented in the paper.     

Modeling and experimental analysis of a self-excited six-phase induction generator for stand-alone renewable energy generation

This paper presents a simple d–q model of a saturated multi-phase (six-phase) self-excited induction generator (SP-SEIG). Performance equations for this machine are given which utilize the saturated magnetizing inductance L_m=(λ_m/i_m) and its derivative (dL_m/di_m) rather than dynamic inductance L=(dλ_m/di_m). In the analytical model, the effects of common mutual leakage inductance between the two three-phase winding sets have been included. A detailed experimental investigation about the voltage build-up, collapse of voltage, and various performance including loading and unloading characteristic, power capability and reliability of six-phase self-excited induction generator is also presented in the paper. Experimental results are recorded: (a) with capacitor bank connected across both the three-phase winding sets, and (b) with capacitor bank connected across only one three-phase winding set. Loading and unloading transients are recorded with independent three-phase resistive loads at each of the two three-phase winding sets, and measured steady-state characteristics for various load and/or capacitor bank configurations. Experimentations were also carried out to judge the performance of the SP-SEIG when three-phase load was connected via an interposed Y−Δ/Y six-phase to three-phase transformer.     

Comprehensive control strategy for a variable speed cage machine wind generation unit

A comprehensive control strategy, that addresses all three control objectives in a wind generation system, i.e. control of the local bus voltage to avoid voltage rise, capture of the maximum power in the wind and minimization of the power loss in the induction generator is proposed. The control signals are the desired current wave shapes (instantaneous three-phase currents) of the rectifier and the inverter in a double-sided PWM converter system connected between the wind generating unit and the grid. Studies performed on a complete model for a variable speed cage machine wind generation unit, including wind profile, wind turbine, induction generator, PWM converter, local load and transmission line, show that even as the wind speed changes randomly, the proposed control strategy leads the system to the optimum operating conditions.     

Performance evaluation of series compensated self-excited six-phase induction generator for stand-alone renewable energy generation

This paper presents the steady-state behavior of a series compensated (short-shunt) self-excited six-phase induction generator (SPSEIG) configured to operate as stand-alone electric energy source in conjunction with a hydro power plant. A purely experimental treatment is provided with the emphasis placed on operating regimes that illustrate the advantages of using SPSEIG. In particular, it is shown that the SPSEIG can operate with a single three-phase capacitor bank, so that the loss of excitation or fault at one winding does not lead to the system shutdown. The generator can also supply two separate three-phase loads, which represent an additional advantage. Experimental results include loading transients with independent three-phase resistive and resistive–inductive load at each of the two three-phase winding sets, and measured steady-state characteristics for various load and/or capacitor bank configurations. Practical results for long-shunt configuration are also given for comparative performance evaluation of series compensated SPSEIG.     

Review of contribution to frequency control through variable speed wind turbine

The increasing wind penetration in today’s power grids has led to growing interest in the frequency control capabilities of wind generation. Several publications have proposed a variety of methods both on the levels of a single turbine and of a wind farm. This paper focuses on the role of wind generation in a system’s primary frequency control. Wind turbine control methods that enable frequency support and control are presented. The advantages and disadvantages of each method are discussed.     

Interfacing a fixed-pitch-angle wind turbine with a double-output induction generator

We describe the performance of a variable-speed, constant-frequency, double-output induction generator (VSCF—DOIG) that is being driven by a fixed-pitch angle wind turbine. The induction generator is run according to a control strategy that forces the generator to extract more than its rated stator power output from the wind without being overheated. Since, in the steady state, the mechanical power developed by the wind turbine is balanced by the electromechanical power developed by the generator, the interfacing problem may be solved by representing the wind turbine performance curve C_p = f(λ) as a polynomial function of the induction generator slip and wind speed. Using this approach, the induction generator outputs that correspond to a given wind speed can easily be calculated. This procedure is superior to previously used trial and error methods.     

Experimental study of variable speed wind generator contribution to primary frequency control

The work presented in this paper analyses, with the help of experiments on a 2.2 kW test bench, the possibility to participate in the primary frequency control with a variable speed wind generator. A power reserve is obtained with the help of the generator torque control by following a power reference value lower than the maximum power which must be extracted from the wind. This approach allows also using a part of the kinetic energy in the blades inertia to contribute to this reserve.The dynamic tests carried out on the test bench, by using medium and high variable wind speeds, confirm the capacity of the wind turbine generator (WTG) to participate in the primary frequency control.     

DFIG sliding mode control fed by back-to-back PWM converter with DC-link voltage control for variable speed wind turbine

This paper proposes an indirect power control of doubly fed induction generator (DFIG) with the rotor connected to the electric grid through a back-to-back pulse width modulation (PWM) converter for variable speed wind power generation. Appropriate state space model of the DFIG is deduced. An original control strategy based on a variable structure control theory, also called sliding mode control, is applied to achieve the control of the active and reactive power exchanged between the stator of the DFIG and the grid. A proportional-integral-(PI) controller is used to keep the DC-link voltage constant for a back-to-back PWM converter. Simulations are conducted for validation of the digital controller operation using Matlab/Simulink software.     

Design of a three-phase self-excited induction generator

The main goals in the design of a self-excited induction generator (SEIG) are minimizing the rotor resistance and increasing the flux density until the magnetic circuit of the generator saturates. In this paper, a computer design package was developed in order to investigate the best way to obtain these goals. By reducing the stator core length by 40%, the frequency regulation and the voltage drop were reduced. The frequency regulation decreased from 10% to 4% and the voltage drop decreased from 30% to 6%. In addition, voltage and frequency regulations in the standard ranges were obtained in the present design without any regulation devices