Frequency support from doubly fed induction generator wind turbines

An assessment on the capability of a doubly fed induction generator (DFIG) wind turbine for frequency regulation is presented. Detailed aerodynamic, structural and electrical dynamic models were used in this study. A control loop acting on the frequency deviation was added to the inertia contributing loop in order to enhance the inertia support from the DFIG wind turbine. The possibility of de-loading a wind turbine to provide primary and secondary frequency response was discussed. A frequency droop controller was examined where the droop is operating on the electronic torque set point below its maximum speed and is operating on the pitch demand at maximum speed. It is also shown that by reducing the generator torque set point the DFIG wind turbine can provide high frequency response     

Aggregated dynamic model for wind farms with doubly fed induction generator wind turbines

As a result of increasing wind farms penetration in power systems, the wind farms begin to influence power system, and thus the modelling of wind farms has become an interesting research topic. Nowadays, doubly fed induction generator based on wind turbine is the most widely used technology for wind farms due to its main advantages such as high-energy efficiency and controllability, and improved power quality. When the impact of a wind farm on power systems is studied, the behavior of the wind farm at the point common coupling to grid can be represented by an equivalent model derived from the aggregation of wind turbines into an equivalent wind turbine, instead of the complete model including the modelling of all the wind turbines. In this paper, a new equivalent model of wind farms with doubly fed induction generator wind turbines is proposed to represent the collective response of the wind farm by one single equivalent wind turbine, even although the aggregated wind turbines operate receiving different incoming winds. The effectiveness of the equivalent model to represent the collective response of the wind farm is demonstrated by comparing the simulation results of equivalent and complete models both during normal operation and grid disturbances.     

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.     

Evaluating reduced models of aggregated different doubly fed induction generator wind turbines for transient stabilities studies

For the representation of wind farms in transient stability studies of electrical power systems, reduced models based on aggregating identical wind turbines are commonly used. In the case of a wind farm with different wind turbines coupled to the same grid connection point, it is usual to aggregate identical wind turbines operating in similar conditions into an equivalent one. However, in the existing literature, there are not any references to the aggregation of different wind turbines (same wind turbine technology but different rated power or components) into a single one. This paper presents a comparative study of four reduced models for aggregating different DFIG wind turbines, experiencing different incoming winds, into an equivalent model. The first of them is the classical clustering model, in which each equivalent model experiences an equivalent wind. The other reduced models have the same equivalent generation system but different equivalent mechanical systems. Thus, the second and third ones are compound models with a clustering aggregated mechanical system and individual simplified models, respectively, to approximate the individual mechanical power according to the incoming wind speeds. The fourth is a mixed model that uses an equivalent wind speed, which is applied to an equivalent mechanical system (equivalent rotor and drive train) in order to approximate the mechanical power of the aggregated wind turbines. The equivalent models are validated by means of comparison with the complete model of the wind farm when simulated under wind fluctuations and grid disturbances. Finally, recommendations with regard to the applicability of models are established. Copyright © 2014 John Wiley & Sons, Ltd.     

An improved fault ride-through strategy for doubly fed induction generator-based wind turbines

Keeping the generators operating during transient grid faults becomes an obligation for the bulk wind generation units connected to the transmission network and it is highly desired for distribution wind generators. A proposed scheme is implemented to keep the wind-power DFIG operating during transient grid faults. Challenges imposed on the generator configuration and the control during the fault and recovering periods are presented. A comprehensive time domain model for the DFIG with the decoupled dq controller is implemented using Matlab/Simulink software. Intensive simulation results are discussed to ensure the validity and feasibility of the proposed fault ride through technique. The scheme protects the DFIG components, fulfills the grid code requirements and optimises the hardware added to the generator.     

Flicker study on variable speed wind turbines with doubly fed induction generators

Grid connected wind turbines may produce flicker during continuous operation. This paper presents a simulation model of a MW-level variable speed wind turbine with a doubly fed induction generator developed in the simulation tool of PSCAD/EMTDC. Flicker emission of variable speed wind turbines with doubly fed induction generators is investigated during continuous operation, and the dependence of flicker emission on mean wind speed, wind turbulence intensity, short circuit capacity of grid and grid impedance angle are analyzed. A comparison is done with the fixed speed wind turbine, which leads to a conclusion that the factors mentioned above have different influences on flicker emission compared with that in the case of the fixed speed wind turbine. Flicker mitigation is realized by output reactive power control of the variable speed wind turbine with doubly fed induction generator. Simulation results show the wind turbine output reactive power control provides an effective means for flicker mitigation regardless of mean wind speed, turbulence intensity and short circuit capacity ratio.     

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.     

Reactive power capability of a wind turbine with doubly fed induction generator

The aim of the work is to derive a steady state PQ‐diagram for a variable speed wind turbine equipped with a Doubly Fed Induction Generator. Firstly, the dependency between optimal rotor speed and wind speed is presented. Secondly, the limitations in reactive power production, caused by the rotor current, the rotor voltage and the stator current are derived. Thirdly, the influence of switching from Δ to Y coupling of the stator is investigated. Finally, a complete PQ diagram for a wind turbine is plotted. It is concluded that the limiting factor regarding reactive power production will typically be the rotor current limit, and that the limit for reactive power absorption will be the stator current limit. Further, it is concluded that the rotor voltage will only have a limiting effect at high positive and negative slips, but near the limitation, the reactive power capability is very sensitive to small changes in the slip. Copyright © 2007 John Wiley & Sons, Ltd.     

Reduced models of doubly fed induction generator system for wind turbine simulations

This article compares three reduced models with a detailed model of a doubly fed induction generator system for wind turbine applications. The comparisons are based on simulations only. The main idea is to provide reduced generator models which are appropriate to simulate normal wind turbine operation in aeroelastic wind turbine models, e.g. for control system design or structural design of the wind turbine. The electrical behaviour such as grid influence will therefore not be considered. The work presented in this article shows that with an ideal, undisturbed grid the dynamics of the doubly fed induction generator system is very well represented by the dynamics due to the generator inertia and the generator control system, whereas the electromagnetic characteristics of the generator can be represented by the steady state relations. The parameters for the proposed models are derived from parameters typically available from the generator data sheet and from the controller settings. Thus the models are simple to apply in any case where the generator data sheet is available. Copyright © 2005 John Wiley & Sons, Ltd.     

Unbalanced voltage faults: the impact on structural loads of doubly fed asynchronous generator wind turbines

This paper investigates the impact that unbalanced voltage faults have on wind turbine structural loads. In such cases, electromagnetic torque oscillations occur at two times the supply voltage frequency. The objectives of this work are to quantify wind turbine structural loads induced by unbalanced voltage faults relative to those during normal operation; and to evaluate the potential for reducing structural loads with the control of the generator. The method applied is integrated dynamic analysis. Namely, dynamic analysis with models that consider the most important aeroelastic, electrical, and control dynamics in an integrated simulation environment based on an aeroelastic code (HAWC2) and software for control design (Matlab/Simulink). In the present analysis, 1 Hz equivalent loads are used to compare fatigue loads, whereas maximum–minimum values are used to compare extreme loads. A control concept based on resonant filters demonstrates reduction of the structural loads (shaft torsion and tower top side‐to‐side moment) induced by an unbalanced voltage fault.Copyright © 2013 John Wiley & Sons, Ltd.     

Mechanical stresses in fixed-speed wind turbines due to networkdisturbances

Electrical faults and other network disturbances generate transients of the generator electromagnetic torque which result in significant stresses for the wind turbine (WT) mechanical system and may also have a detrimental effect on the fatigue life of important drive train components, such as the gearbox. In this paper, the effect of faults and other disturbances, common in rural medium voltage (MV) distribution systems, is analyzed for a typical fixed speed WT, equipped with an induction generator. The resulting torque transients are evaluated and their propagation in the drive train is examined. Important conclusions are drawn with respect to the type of electrical faults to be considered in the design and certification of WTs according to the current IEC 61400-1 Standard. Although the emphasis is placed on the developed torque transients, the methodology presented is also suitable for analyzing electrical stresses in the generator, due to overcurrents or overvoltages     

Comparative study on the performance of control systems for doubly fed induction generator (DFIG) wind turbines operating with power regulation

As a result of the increasing wind power penetration on power systems, the wind farms are today required to participate actively in grid operation by an appropriate generation control. This paper presents a comparative study on the performance of three control strategies for DFIG wind turbines. The study focuses on the regulation of the active and reactive power to a set point ordered by the wind farm control system. Two of them (control systems 1 and 2) are based on existing strategies, whereas the third control system (control system 3) presents a novel control strategy, which is actually a variation of the control system 2. The control strategies are evaluated through simulations of DFIG wind turbines, under normal operating conditions, integrated in a wind farm with centralized control system controlling the wind farm generation at the connection point and computing the power reference for each wind turbine according to a proportional distribution of the available power. The three control systems present similar performance when they operate with power optimization and power limitation strategies. However, the control system 3 with down power regulation presents a better response with respect to the reactive power production, achieving a higher available reactive power as compared with the other two. This is a very important aspect to maintain an appropriate voltage control at the wind farm bus.     

Five-leg converter topology for wind energy conversion system with doubly fed induction generator

In this paper, application of a five-leg converter in Doubly Fed Induction Generator (DFIG) for Wind Energy Conversion Systems (WECS) is investigated. The five-leg structure and its PWM control are studied and performances are compared with the classical six-leg topology. The main drawback of five-leg converter with respect to the six-leg back-to-back converter is the need to increase the dc-link voltage for the same operation point, i.e. the same powers in case of WECS. So, different methods for the reduction of the required dc-link voltage in the five-leg case are studied. The five-leg converter is used to replace the conventional six-leg one, with the same ability. For the performance evaluation of this structure and its fully digital controller in a more realistic and experimental manner, Hardware in the Loop experiments is carried out. It is shown that efficient control of active and reactive powers and dc-link voltage is performed. Hardware in the Loop results demonstrate the high performance of the proposed fully digital control which is implemented on an Altera FPGA target.     

适合于变速恒频双馈感应发电机的Crowbar对比分析

电网运行新规则要求变速恒频双馈感应发电机在电网电压跌落的情况下仍与电网相连接，为做到这一点需要安装低压旁路系统。综合分析了低压带来的负面影响以及低压旁路的具体实现方法；对比了多种Crowbar电路各自的优缺点；最后介绍了变速恒频双馈发电机相关保护控制策略和新型旁路系统。     

Compensation of network voltage unbalance using doubly fed induction generator-based wind farms

A control strategy for compensating AC network voltage unbalance using doubly fed induction generator (DFIG)-based wind farms is presented. A complete DFIG dynamic model containing both the rotor and grid side converters is used to accurately describe the average and ripple components of active/reactive power, electromagnetic torque and DC bus voltage, under unbalanced conditions. The principle of using DFIG systems to compensate grid voltage unbalance by injecting negative sequence current into the AC system is described. The injected negative sequence current can be provided by either the grid side or the rotor side converters. Various methods for coordinating these two converters are discussed and their respective impacts on power and torque oscillations are described. The validity of the proposed control strategy is demonstrated by simulations on a 30 MW DFIG-based wind farm using Matlab/Simulink during 2 and 4% voltage unbalances. The proposed compensation strategy can not only ensure reliable operation of the wind generators by restricting torque, DC link voltage and power oscillations, but also enable DFIG-based wind farms to contribute to rebalancing the connected network.     

Control of a doubly fed induction generator in a wind turbine during grid fault ride-through

This paper analyzes the ability of a doubly fed induction generator (DFIG) in a wind turbine to ride through a grid fault and the limitations to its performance. The fundamental difficulty for the DFIG in ride-through is the electromotive force (EMF) induced in the machine rotor during the fault, which depends on the dc and negative sequence components in the stator-flux linkage and the rotor speed. The investigation develops a control method to increase the probability of successful grid fault ride-through, given the current and voltage capabilities of the rotor-side converter. A time-domain computer simulation model is developed and laboratory experiments are conducted to verify the model and a control method is proposed. Case studies are then performed on a representatively sized system to define the feasibility regions of successful ride-through for different types of grid faults.     

双馈风电机组网侧变换器L_2增益干扰抑制控制

为减小外界干扰对直流母线电压的影响,实现双馈风电机组网侧变换器的控制目标,文章设计了网侧变换器L2增益干扰抑制控制律。该控制策略以广义Hamilton理论为基础,避免了求解HJI偏微分不等式,减小了建模误差对控制性能的影响。仿真结果表明,文章设计的控制律能够实现系统渐进稳定控制,对外界干扰有较好的抑制能力,改善了控制系统的性能。     

双馈风力发电机无速度传感器矢量控制

针对风力发电系统中的双馈电机提出一种转子感应电势定向矢量控制方法。通过调节双馈电机转子侧的瞬时有功电流和无功电流,实现对电机力矩和转子侧励磁电流的调节,进而实现双馈电机无功功率调节。在控制过程中只需检测交流侧电流电压,不需要位置传感器,所以可以应用无速度传感器。最终通过仿真试验证明该方法的正确性和实用性。     

Dynamic participation of doubly fed induction generator in automatic generation control

Increasing levels of wind generation have resulted in an urgent need for the assessment of their impact on frequency control of power systems. The displacement of conventional generation with wind generation will result in erosion of system frequency. The paper analyzed the dynamic participation of doubly fed induction generator (DFIG) to system frequency responses of two-area interconnected power system having variety of conventional generating units. Frequency control support function responding proportionally to frequency deviation is proposed to take out the kinetic energy of turbine blades in order to improve the frequency response of the system. Impacts of different wind penetrations in the system and varying active power support from wind farm on frequency control have been investigated. Integral gains of AGC loop are optimized through craziness-based particle swarm optimization (CRPSO) in order to have optimal transient responses of area frequencies, tie-line power deviation and DFIG parameters.