Doubly fed induction generator‐based variable‐speed wind turbine: Proposal of a simplified model under a faulty grid with short‐duration faults

The aim of this study is to provide a simplified model of a variable‐speed wind turbine (VSWT) with the technology of a doubly fed induction generator (DFIG), which operates under faulty grid conditions. A simplified model is proposed, which consists of a set of electrical and mechanical equations that can be easily modeled as simplistic electrical circuits. It makes it an excellent tool to achieve fault ride‐through capability of grid‐connected VSWT with DFIGs. Both symmetrical and unsymmetrical grid faults, which cause symmetrical and unsymmetrical voltage sags, have been applied to the system in order to validate the model. The proposed simplified model has been compared with the traditional full‐order model under multiple sags (different durations and depths), and the results reveal that both models present similar accuracy. As the idea is to reduce the computational time required to simulate the machine behavior under faulty grid conditions, the proposed model becomes suitable for that purpose. The analytical study has been validated by simulations carried out with MATLAB .     

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.     

Coordinated control for unbalanced operation of stand‐alone doubly fed induction generator

This paper proposes a coordinated control scheme of a stand‐alone doubly fed induction generator (DFIG)‐based wind energy conversion system to improve the operation performance under unbalanced load conditions. To provide excellent voltage profile for load, a direct stator flux control scheme based on auto‐disturbance rejection control (ADRC) is applied, and less current sensors are required. Due to the virtues of ADRC, the controller has good disturbance rejection capability and is robust to parameter variation. In the case of unbalanced loads, the electromagnetic torque pulsations at double synchronous frequency will exist. To eliminate the undesired effect, the stator‐side converter (SSC) is used to provide the negative sequence current components for the unbalanced load. Usually, proportional integral controllers in a synchronous reference frame are used to control SSC. To simplify the algorithm, an improved proportional resonant (PR) control is proposed and used in the current loop without involving positive and negative sequence decomposition. The improved PR provides more degree of freedom which could be used to improve the performance. The effectiveness of the proposed control scheme has been validated by the simulation and experimental results. Copyright © 2012 John Wiley & Sons, Ltd.     

Co‐ordinated voltage control of DFIG wind turbines in uninterrupted operation during grid faults

Emphasis in this article is on the design of a co‐ordinated voltage control strategy for doubly fed induction generator (DFIG) wind turbines that enhances their capability to provide grid support during grid faults. In contrast to its very good performance in normal operation, the DFIG wind turbine concept is quite sensitive to grid faults and requires special power converter protection. The fault ride‐through and grid support capabilities of the DFIG address therefore primarily the design of DFIG wind turbine control with special focus on power converter protection and voltage control issues. A voltage control strategy is designed and implemented in this article, based on the idea that both converters of the DFIG (i.e. rotor‐side converter and grid‐side converter) participate in the grid voltage control in a co‐ordinated manner. By default the grid voltage is controlled by the rotor‐side converter as long as it is not blocked by the protection system, otherwise the grid‐side converter takes over the voltage control. Moreover, the article presents a DFIG wind farm model equipped with a grid fault protection system and the described co‐ordinated voltage control. The whole DFIG wind farm model is implemented in the power system simulation toolbox PowerFactory DIgSILENT. The DFIG wind farm ride‐through capability and contribution to voltage control in the power system are assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a DFIG wind farm equipped with voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride‐through control strategy in the active stall wind farm. Copyright © 2006 John Wiley &Sons, Ltd.     

Direct power control of wind‐turbine‐driven DFIG during transient grid voltage unbalance

In this paper, a direct power control (DPC) of a wind‐turbine‐driven doubly fed induction generators (DFIGs) under unbalanced network voltage conditions is studied. Variations of the stator output active, reactive and electromagnetic powers are fully deduced in the presence of negative sequence supply voltage. The rotor side converter is controlled on the basis of DPC to eliminate the electromagnetic torque oscillations at double supply frequency under unbalanced stator supply. The rotor voltage references estimation requires only simple calculations without any integral operation. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. The performance of the proposed and conventional DPC schemes is compared under the same operating conditions. Simulation results using Matlab/Simulink are carried out for a 1.5 MW DFIG wind generation system to show the validity of the proposed scheme during unbalanced voltage supply. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Modelling analysis of transient stability simulation with high penetration of grid‐connected wind farms of DFIG type

This paper shows a model of a doubly fed induction generator (DFIG) including a simplified model of a wind turbine for the purpose of transient stability analysis of large‐scale power systems with great wind farms penetration. The wind turbine model and the DFIG model are systematically deducted in this paper. Specially, the improved model of rotor‐side converter and the simplified grid‐side converter model are considered in our work. The corresponding machine–network interface solution based on the synchronously rotating common x‐y reference frame is elaborately issued in this paper. Furthermore, a method is proposed to calculate the DFIG initial conditions as well. A simplified model of the turbine is used excluding among other components the pitch control. Copyright © 2007 John Wiley & Sons, Ltd.     

Power system frequency response from fixed speed and doubly fed induction generator‐based wind turbines

Throughout Europe there is an increasing trend of connecting high penetrations of wind turbines to the transmission networks. This has resulted in transmission system operators revising their grid code documents for the connection of large wind farms. These specifications require large MW capacity wind farms to have the ability to assist in some of the power system control services currently carried out by conventional synchronous generation. These services include voltage and frequency control. It is now recognized that much of this new wind generation plant will use either fixed speed induction generator (FSIG)‐ or doubly fed induction generator (DFIG)‐based wind turbines. The addition of a control loop to synthesize inertia in the DFIG wind turbine using the power electronic control system has been described. The possibility of deloading wind turbines for frequency response using blade pitch angle control is discussed. A pitch control scheme to provide frequency response from FSIG and DFIG wind turbines is also described. A case study of an FSIG wind turbine with frequency response capabilities is investigated. Copyright © 2004 John Wiley & Sons, Ltd.     

New management structure of active and reactive power of a large wind farm based on multilevel converter

This paper proposes a system of supervision and operation of a new structure wherein a large wind farm is connected to an electrical grid. The farm is managed in such a manner that it can produce the power needed by the grid system. The supervision algorithm is used to distribute the active and reactive power references to the wind turbines proportionally. Based on the aerodynamic power and wind speed of each turbine, the active and reactive power references are produced individually. By using the vector field oriented control, each doubly fed induction generator is controlled through the rotor, which is connected to the two-level pulse width modulation converter. The close loop control is used to provide a constant DC voltage using a five-level neutral point clamped converter. The five-level neutral point clamped converter allows also the adaptation of the voltage level to the electrical grid with better resolution waveform. The analysis of the simulation results shows the effectiveness of the proposed system.     

双馈风力发电用交直交变流器控制策略的研究

概述了变速恒频双馈风力发电用交直交变流器的工作原理,转子侧变流器采用基于最大功率点跟踪的并网发电解耦控制策略,网侧变流器采用基于固定开关频率与电网电动势前馈相结合的双闭环控制策略,构建了110kW变速恒频双馈风力发电模拟平台,经过实验结果分析验证了上述控制策略的有效性和可行性。     

A novel line drop secondary voltage control algorithm for variable speed wind turbines

This paper addresses the design and implementation of the line drop secondary voltage control (LDSVC) for the doubly fed induction generator‐wind turbine (DFIG‐WT) complemented with reactive power allocation algorithm to achieve more efficient voltage regulation, reactive power compensation and to enhance the transient stability margin of the electric power system. The LDSVC is used to generate the local voltage reference, providing an improvement for overall voltage profile. The paper presents the influence of the integration of variable speed wind turbines‐based doubly fed induction generator (DFIG) while employing LDSVC for increasing the transient stability margin. This paper proposes an improved voltage control scheme, based on a secondary voltage controller complemented with an automatic gain controller (AGC). The scheme is applied to a wind energy system incorporating DFIG‐based wind turbines. The controller structure is developed and the performance of the self‐tuning AGC scheme is developed and analysed. The proposed controller is tested in response to system contingencies for different short circuit ratios. The performance of the secondary voltage control without and with AGC is verified. The influence of the AGC in improving the transient response and damping of voltage oscillations is verified. Copyright © 2010 John Wiley & Sons, Ltd.     

Fractional‐order sliding mode control for damping of subsynchronous control interaction in DFIG‐based wind farms

This paper proposes a fractional‐order sliding mode control (FOSMC) based on feedback linearization (FL) technique to mitigate subsynchronous control interaction (SSCI) in doubly fed induction generator (DFIG)–based wind farms connected to series‐compensated transmission lines. A linearized form of the studied system is obtained with the use of FL, which leads to reduced system order and small computational burden. Then the FOSMC is designed for grid‐side converter (GSC) to stabilize SSCI and to provide a considerable robustness against external disturbances and parameter uncertainties. For FOSMC parameter tuning, genetic algorithm (GA) is performed through MATLAB/SIMULINK. Time‐domain simulation are carried out to evaluate the effectiveness of the FOSMC in mitigating SSCI at varied operating conditions, and the superior performance of the proposed control is demonstrated as compared with conventional vector control (VC), feedback linearization sliding mode control (FLSMC), high‐order sliding mode control (HOSMC).     

5 MW双馈风电机组低电压穿越的仿真分析

针对海上风力发电机组安全可靠运行要求的发展趋势,本文在阐述双馈风电机组控制原理的基础上,建立了双馈发电机及其变流器的控制模型。其次,在分析电力系统对并网风电机组低电压穿越原理基础上,比较分析了双馈风电机组低电压穿越的各种控制技术方案。最后,结合海上用5.0 MW双馈风力发电机组电气参数,对2种典型低电压穿越的转子电路保护措施进行了仿真比较。分析结果表明,采用二极管整流桥加IGBT和保护电阻构成斩波器的措施具有较好的暂态控制效果。     

Grid Synchronization of Doubly-fed Induction Generator Using Integral Variable Structure Control

This paper proposes a novel direct voltage control scheme, using integral variable structure control to synchronize a doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) to the grid. The proposed scheme directly controls the stator terminal voltage of the DFIG to track the grid voltage without current control loop; hence, the structure of controller is simplified. The control scheme includes parametric uncertainty and external disturbances into the formed design procedure; hence, the proposed scheme has better robustness than existing synchronization methods. Both computer simulation and hardware implementation results are presented to demonstrate the advantages of the proposed scheme.      

A novel method for obtaining virtual inertial response of DFIG‐based wind turbines

This paper investigates virtual inertia control of doubly fed induction generator (DFIG)‐based wind turbines to provide dynamic frequency support in the event of sudden power change. The relationships among DFIGs' virtual inertia, rotor speed and network frequency variation are analysed, and a novel virtual inertia control strategy is proposed. The proposed control strategy shifts the maximum power point tracking (MPPT) curve to the virtual inertia control curves according to the frequency deviation so as to release the ‘hidden’ kinetic energy and provide dynamic frequency support to the grid. The calculation of the virtual inertia and its control curves are also presented. Compared with a PD regulator‐based inertial controller, the proposed virtual inertia control scheme not only provides fast inertial response in the event of sudden power change but also achieves a smoother recovery to the MPPT operation. A four‐machine system with 30% of wind penetration is simulated to validate the proposed control strategy. Simulation results show that DFIG‐based wind farms can provide rapid response to the frequency deviation using the proposed control strategy. Therefore, the dynamic frequency response of the power grid with high wind power penetration can be significantly improved. Copyright © 2015 John Wiley & Sons, Ltd.     

基于双环控制的双馈风力发电机组控制策略

在分析了双馈风力发电机组运行特性的基础上,提出一种基于双环控制的变换器控制策略。在转子侧,变换器采用定子磁链定向矢量控制技术,推导出了用转子有功电流和无功电流独立解耦控制有功功率和无功功率的策略,并实现了风能的最大跟踪;在电网侧,变换器采用电网电压定向矢量控制技术,构建了电流内环、电压外环的双闭环PI控制系统。利用PSCAD/EMTDC软件,构建了双馈风力发电机组仿真模型。仿真结果验证了所提控制策略的有效性和合理性     

DFIG-DVR系统在不平衡电网电压下的运行特性

针对双馈风电机组(DFIG)在电网电压不平衡时,二倍频扰动分量会造成定转子过电流、功率脉动、转矩脉动等一系列电气和机械的问题,提出了新型DFIG-DVR系统,即串联DVR始终维持DFIG定子端电压恒定,从根源上隔离电网不平衡故障的影响,从而在整个故障运行过程中,DFIG仍可以实现转子侧变换器功率解耦控制和网侧变换器维持直流电压恒定的目标。采用PSCAD/EMTDC建立DFIG-DVR系统模型,对比分析了电网电压不平衡时DVR的不投切与投切对DFIG的影响。结果表明,在电网电压不平衡条件下,所提控制方案可以实现DFIG的平衡运行。     

An input‐output linearization–based control strategy for wind energy conversion system to enhance stability

This paper proposes a novel control strategy for doubly fed induction generator (DFIG)‐based wind energy conversion system to investigate the potential of enhancing the stability of wind energy transmission system, a synchronous generator weakly integrated to a power system with a DFIG‐based wind farm. The proposed approach uses state feedback to exactly linearize the nonlinear wind energy transmission system from control actions (active power and reactive power control order of DFIG) to selected outputs (power angle and voltage behind transient resistance of synchronous generator) at first. Then, on account of the linearized subsystem, the stability enhancement controller is designed based on linear quadratic regulator algorithm to contribute adequate damping characteristics to oscillations of the synchronous generator system under various operation points. The proposed control strategy successfully deals with the nonlinear behaviors exist from the inputs to outputs and improve the robustness with respect to the variation of system operation points. Furthermore, not only the rotor angle stability but also the voltage stability is enhanced by using the proposed control strategy. The simulation results carried on the studied system verify the effectiveness of the proposed control strategy of wind energy conversion system for system stability enhancement and the robustness against various system operation points.     

Initialization of DFIG wind turbines with a phasor‐based approach

The objective of this paper is to propose a simple approach to solve the steady state of a wind turbine (WT) equipped with a doubly fed induction generator (DFIG), which can be used to initialize dynamic studies of the machine. The idea is to model the rotor‐side converter (RSC) as a constant current source connected to the rotor of the DFIG. The resulting equivalent circuit consists of a voltage source in series with a reactance, which makes it possible to obtain simple phasor expressions that can be used to obtain the Park components of the variables. The proposed method is compared with the traditional Newton–Raphson algorithm, showing that it is easier and faster to implement, as it makes use of the phasor expressions and it does not require an iterative process to obtain the final solution. Finally, the results of the proposed method are used to simulate a 2‐MW DFIG‐based WT under three‐phase faults, considering three different WT‐operating points. In these simulations, the idea of constant rotor current is extrapolated to the entire event. The simulated results show that both current at torque peaks are reduced. The analytical study and the simulations have been carried out in Matlab ?.     

SSCI mitigation of grid‐connected DFIG wind turbines with fractional‐order sliding mode controller

To mitigate subsynchronous control interaction (SSCI) in doubly fed induction generator (DFIG)‐based wind farm, this paper proposes a robust controller for rotor‐side converter (RSC) using fractional‐order sliding mode controller (FOSMC). The proposed FOSMC can improve robustness and convergence properties of the controlled system, thus achieving SSCI damping under various operating conditions. Impedance‐based analysis and time‐domain simulation are performed to check the capability of the designed FOSMC as compared with conventional sliding mode control (SMC) and subsynchronous damping control (SSDC). Simulation results demonstrate that FOSMC can mitigate SSCI within shorter time and effectively reduce the fluctuation range of system transient responses under various operating conditions of wind speeds and compensation levels. Moreover, FOSMC also improves system robustness against parameter uncertainties and external disturbances, which is important for safe operation of realistic wind farms.