Fault ride-through capability of DFIG wind turbines

This paper concentrates on the fault ride-through capability of doubly fed induction generator (DFIG) wind turbines. The main attention in the paper is, therefore, drawn to the control of the DFIG wind turbine and of its power converter and to the ability to protect itself without disconnection during grid faults. The paper provides also an overview on the interaction between variable-speed DFIG wind turbines and the power system subjected to disturbances, such as short circuit faults. The dynamic model of DFIG wind turbine includes models for both mechanical components as well as for all electrical components, controllers and for the protection device of DFIG necessary during grid faults. The viewpoint of the paper is to carry out different simulations to provide insight and understanding of the grid fault impact on both DFIG wind turbines and on the power system itself. The dynamic behaviour of DFIG wind turbines during grid faults is simulated and assessed by using a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk in the power system simulation toolbox PowerFactory DIgSILENT. The data for the wind turbines are not linked to a specific manufacturer, but are representative for the turbine and generator type used in variable-speed DFIG wind turbines with pitch control.     

Optimization of wind turbine energy and power factor with an evolutionary computation algorithm

An evolutionary computation approach for optimization of power factor and power output of wind turbines is discussed. Data-mining algorithms capture the relationships among the power output, power factor, and controllable and non-controllable variables of a 1.5 MW wind turbine. An evolutionary strategy algorithm solves the data-derived optimization model and determines optimal control settings. Computational experience has demonstrated opportunities to improve the power factor and the power output by optimizing set points of blade pitch angle and generator torque. It is shown that the pitch angle and the generator torque can be controlled to maximize the energy capture from the wind and enhance the quality of the power produced by the wind turbine with a DFIG generator. These improvements are in the presence of reactive power remedies used in modern wind turbines. The concepts proposed in this paper are illustrated with the data collected at an industrial wind farm.     

A novel method for reliability and risk evaluation of wind energy conversion systems considering wind speed correlation

This paper investigates an analytical approach for the reliability modeling of doubly fed induction generator (DFIG) wind turbines. At present, to the best of the authors’ knowledge, wind speed and wind turbine generator outage have not been addressed simultaneously. In this paper, a novel methodology based on the Weibull- Markov method is proposed for evaluating the probabilistic reliability of the bulk electric power systems, including DFIG wind turbines, considering wind speed and wind turbine generator outage. The proposed model is presented in terms of appropriate wind speed modeling as well as capacity outage probability table (COPT), considering component failures of the wind turbine generators. Based on the proposed method, the COPT of the wind farm has been developed and utilized on the IEEE RBTS to estimate the well-known reliability and sensitive indices. The simulation results reveal the importance of inclusion of wind turbine generator outage as well as wind speed in the reliability assessment of the wind farms. Moreover, the proposed method reduces the complexity of using analytical methods and provides an accurate reliability model for the wind turbines. Furthermore, several case studies are considered to demonstrate the effectiveness of the proposed method in practical applications.     

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.     

Fundamental time–domain wind turbine models for wind power studies

One critical task in any wind power interconnection study involves the modelling of wind turbines. This paper provides the most basic yet comprehensive time–domain wind turbine model upon which more sophisticated models along with their power and speed control mechanisms, can be developed. For this reason, this paper concentrates on the modelling of a fixed-speed wind turbine. The model includes turbine's aerodynamic, mechanical, and electrical components. Data for the rotor, drive-train, and electrical generator are given to allow replication of the model in its entirety. Each of the component-blocks of the wind turbine is modelled separately so that one can easily expand the model to simulate variable-speed wind turbines or customise the model to suit their needs. Then, an aggregate wind turbine model, or wind farm, is developed. This is followed by four case studies to demonstrate how the models can be used to study wind turbine operation and power grid integration issues. Results obtained from the case studies show that the models perform as expected.     

Performance evaluation of pairing between sites and wind turbines

This paper presents a simple method for determination of pairing between sites and wind generators. It requires six parameters to describe the matching between turbine models and site characteristics, and the energy output performance can thus be easily estimated and used as the index of pairing effectiveness. To describe a Weibull model of wind speed distribution, the shape parameter and the scale parameter are necessarily required. Besides, four other parameters are chosen to specify the characteristics of the power curve of a wind generator: the cut-in speed, the rated speed, the cut-off speed and the nominal power. By combining these six parameters, the average power output of some particular wind turbine at a specific site can be practically and quickly approximated as a reference for turbine siting consideration. An example is also shown to demonstrate the utilization of the proposed method to choose between a group of wind sites and a list of commercial wind turbines.     

Dynamic characteristics of autonomous wind–diesel systems

In this paper the dynamics of a small autonomous system, comprising a diesel generator and a wind turbine, are investigated. The analysis is performed both in the frequency and time domain, using simplified models of the system components and taking into account the diesel engine speed governor and the wind turbine pitch controller (for pitch regulated machines). The investigation is extended to include different types of wind turbines, equipped with induction or synchronous generator and using pitch or stall regulation, as well as operation of the wind turbine in an autonomous or infinite system. The objective is to determine the main factors affecting the behaviour of the system and to illustrate the effect of the speed governor and pitch controller settings on the expected performance. Particular emphasis is placed on identifying the main modes of the system and determining their dependence on the controllers' parameters. A comparative assessment of the dynamic characteristics of different types of wind turbines is also included and the operation of the wind turbine in a small system and against an infinite bus is addressed and discussed.     

Wind turbine dynamic response?difference between connection to large utility network and isolated diesel micro-grid

The difference in dynamic behaviour of wind turbines connected to different size networks is analysed for synchronous and induction generators. Modal analysis has been applied to equivalent mechanical models of the wind turbine generator and the grid connection to show the difference in dynamics for a wind turbine generator connected to a large (interconnected) utility network or an (isolated) diesel micro-grid. Frequency domain analysis of these models has been used to quantify the effect of system parameters, such as enhanced drive train damping, diesel governor action and generator slip, on dynamic interaction. It has been shown that for wind turbines connected to a large grid an induction generator can reduce dynamic interaction. In contrast, for an isolated diesel micro-grid the diesel governor response and the compliance of the diesel generator dominate the system's dynamics, and the choice of either generator option for the wind turbine is dictated by other factors.     

Structural integrity of a direct-drive generator for a floating wind turbine

In this work, the suitability of a direct-drive radial flux permanent magnet generator is examined as a probable drive-train candidate for a stepped-spar floating wind turbine system that supports a 2 MW downwind turbine. The suitability of the generator is assessed based on the structural integrity of its design (i.e., the stability of the air-gap between the rotor and stator) in response to the nacelle motions and its possible design implications on the overall system. Air gap deflections due to structural deflection and bearing tolerances were examined independently. The nacelle motions are obtained from experimental and numerical investigations on a 1:100 scale model. ANSYS suite is used to estimate the structural deformations of the generator and the changes in the air-gap distribution. Also, a simplified analytical model is used to compute the resulting changes in flux density and force distribution along the rotor periphery. The analytical model is also validated by 2D magneto-static simulations by utilising Finite Element Methods Magnetics software (FEMM).Preliminary results suggest that, if the nacelle accelerations are limited to 0.3 times the acceleration due to gravity (g) and the motion response cycles are below the fatigue limit, the air-gap stability of the generator is more sensitive to magnetic forces. Contributions to air-gap eccentricity from shaft displacements can be limited if the bearing supports can be designed for high stiffness. This also confirmed the adequacy of the platform design. The results also emphasise the need for air-gap management when designing direct-drive generators for floating wind turbines. Two methods are investigated as potential solutions to limit the maximum air-gap deflection to 10% level. The method of increasing structural stiffness led to a structurally unfavourable design that could potentially affect the stability and resonance properties of the system. The method of increasing the design air-gap led to a structurally more favourable design, although this meant an increase in magnetic material and hence the costs. Thus, implementing direct-drive radial flux permanent magnet generators for floating wind turbines is challenged by the difficulty in achieving optimal weight and costs at acceptable performance without compromising the air-gap tolerances. There is a need for an amendment to design standards to recognise the design challenges of Floating wind turbines.     

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.     

Condition monitoring and fault diagnosis of a wind turbine synchronous generator drive train

Some large grid connected wind turbines use a low-speed synchronous generator, directly coupled to the turbine, and a fully rated converter to transform power from the turbine to mains electricity. The condition monitoring and diagnosis of mechanical and electrical faults in such a machine are considered, bearing in mind that it has a slow variable speed and is subject to the stochastic, aerodynamic effects of the wind. The application of wavelet transforms is investigated in the light of the disadvantages of spectral analysis in processing signals subject to such stochastic effects. The technique can be used to monitor generator electrical and drive train mechanical faults. It is validated experimentally on a wind turbine condition monitoring test rig using a threephase, permanent-magnet, slow-speed, synchronous generator, driven by a motor controlled by a model representing the aerodynamic forces from a wind turbine. The possibility of detecting mechanical and electrical faults in wind turbines by electrical signal and particularly power analysis is heralded.     

Modelling and ride‐through capability of variable speed wind turbines with permanent magnet generators

A model for a variable speed wind turbine with a permanent magnet, multipole, synchronous generator is developed and implemented in the simulation tool PSS/E as a user‐written model. The model contains representations of the permanent magnet generator, the frequency converter system with control, the aerodynamic rotor and a lumped mass representation of the shaft system. This model complexity is needed for investigations of the short‐term voltage stability and ride‐through capability of such wind turbines. Ride‐through capability is a major issue and, for the given concept, can be achieved by applying blocking and restart sequences to the frequency converter at the voltage drop in the power grid. Copyright © 2005 John Wiley & Sons, Ltd.     

并网双馈风电机组低电压穿越能力研究

详细分析了双馈风电机组LVRT功能的实现原理,并在电力系统仿真分析软件PSASP中建立双馈风电机组的LVRT功能模型,采用地理接线图直观地表示风电场外部系统发生短路故障瞬间对风电机组端电压的影响．并以我国某地区电网为例来分析在风电场接入方式不同的情况下系统短路故障对风电机组的影响。根据仿真结果给出风电机组LVRT能力的最低电压限值要求。最后提出了利用串联制动电阻来提高风电机组的LVRT能力的新方法。分析结果表明,串联制动电阻能够可观地提高风电机组的低电压穿越能力。具有较高LVRT能力的风电机组。可以节省一定的投资费用,在一定程度上降低了风电的上网电价。     

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.     

Modelling and control of variable‐speed multi‐pole permanent magnet synchronous generator wind turbine

Emphasis of this article is on variable‐speed pitch‐controlled wind turbines with multi‐pole permanent magnet synchronous generator (PMSG) and on their extremely soft drive‐train shafts. A model and a control strategy for a full back‐to‐back converter wind turbine with multi‐pole PMSG are described. The model comprises submodels of the aerodynamic rotor, the drive‐train by a two‐mass model, the permanent magnet generator and the full‐scale converter system. The control strategy, which embraces both the wind turbine control itself and the control of the full‐scale converter, has tasks to control independently the active and reactive powers, to assist the power system and to ensure a stable normal operation of the wind turbine itself. A multi‐pole PMSG connected to the grid through a full‐scale converter has no inherent damping, and therefore, such configuration can become practically unstable, if no damping by means of external measures is applied. In this work, the frequency converter is designed to damp actively the drive‐train oscillations, thus ensuring stable operation. The dynamic performance of the presented model and control strategy is assessed and emphasized in normal operation conditions by means of simulations in the power system simulation tool DIgSILENT. Copyright © 2008 John Wiley & Sons, Ltd.     

Model‐based control addition to prescribe DFIG wind turbine fast frequency response

This paper investigates the physical capability of double‐fed induction generator (DFIG) wind turbines for inertial support of frequency response. Frequency stability is modeled using the DFIG electromechanical and generator controller dynamics, and a destabilizing effect is demonstrated in low‐inertia systems. To improve response, a synchronous reference frame DFIG controller is proposed that acts by following low‐frequency grid dynamics and adds a fast acting proportional plus integral (PI)‐controlled frequency‐responsive component to existing qd current commands. The proposed controller is derived in a straightforward manner using only the DFIG dynamic equations and is designed using pole/zero placement techniques. Laboratory experiments using a micro‐scale DFIG wind turbine with hub‐emulating flywheel prove better capability for transient frequency regulation even under extreme load change. The result is a DFIG controller that balances the appearance of transients in electrical and mechanical systems. Value is achieved in providing immediate continuous inertial response to support load change. The proposed frequency response can improve the use of existing physical inertia from wind turbines.     

Reliability analysis for wind turbines

Modern wind turbines are complex aerodynamic, mechanical and electrical machines incorporating sophisticated control systems. Wind turbines have been erected in increasing numbers in Europe, the USA and elsewhere. In Europe, Germany and Denmark have played a particularly prominent part in developing the technology, and both countries have installed large numbers of turbines. This article is concerned with understanding the historic reliability of modern wind turbines. The prime objective of the work is to extract information from existing data so that the reliability of large wind turbines can be predicted, particularly when installed offshore in the future. The article uses data collected from the Windstats survey to analyse the reliability of wind turbine components from historic German and Danish data. Windstats data have characteristics common to practical reliability surveys; for example, the number of failures is collected for each interval but the number of turbines varies in each interval. In this article, the authors use reliability analysis methods which are not only applicable to wind turbines but relate to any repairable system. Particular care is taken to compare results from the two populations to consider the validity of the data. The main purpose of the article is to discuss the practical methods of predicting large‐wind‐turbine reliability using grouped survey data from Windstats and to show how turbine design, turbine configuration, time, weather and possibly maintenance can affect the extracted results. Copyright © 2006 John Wiley &Sons, Ltd.     

基于运行数据的风电机组转矩控制性能评估

风电机组的性能评估方法具有多样性及复杂性的特点,基于风电场SCADA系统中采集的大量风电机组运行数据,对风电机组转矩控制的性能评估方法进行了研究。在深入分析风电机组中发电机转速与发电机转矩关系的基础上,提出了风电机组在最佳风能利用系数Cp(max)跟踪区内的转矩优化控制的性能评估方法。通过筛选有效数据,拟合计算出风电机组的实际运行转矩增益系数;再通过与理论最优转矩增益系数进行对比,找出风能捕获能力较弱的风电机组,进而采取措施提高其发电量。通过软件仿真及案例分析表明,该方法在不增加设备及成本的情况下,可有效识别因转矩控制的性能差而影响发电量的风电机组,以便及时进行控制策略调校,维护风电场的利益。     

考虑叶片旋转及土-结构相互作用对风电结构动力特性影响的研究

风力发电建设场地土特性及叶片转速变化会引起风力发电机动力特性及响应发生变化。基于西北地区某风电场的2 MW三桨叶水平轴风力发电机,利用ANSYS软件建立风力发电机整机模型,考虑叶片旋转及土-结构相互作用对风力发电机动力特性的影响,研究风力发电机在不同叶片转速、不同场地条件及两者共同作用下的动力特性变化规律,并利用坎贝尔图分析上述工况下风力发电机的共振特性。结果表明:叶片转速增加使风力发电机的自振频率逐渐增大,塔筒与叶片的耦合变形使频率增长加快;考虑土-结构相互作用时,风力发电机频率降低,且场地土越软风力发电机频率越小;当两种工况共同作用时,叶片旋转只对一阶频率产生影响,对高阶频率影响甚微,且风力发电机频率降低;考虑叶片旋转时风力发电机在1P转频内不产生共振点,在3P转频内一阶频率的共振点为14 r/min。此外在不同场地土下风力发电机均会产生共振点,其中Ⅲ类土的共振频率最远离额定转频。因此在风电场的场地选择、结构设计及日常运行时应注意其动力特性变化,使其频率远离共振区域,为风力发电机安全运行提供保障。