Modelling and control of variable speed wind turbines for power system studies

Modern wind turbines are predominantly variable speed wind turbines with power electronic interface. Emphasis in this paper is therefore on the modelling and control issues of these wind turbine concepts and especially on their impact on the power system. The models and control are developed and implemented in the power system simulation tool DIgSILENT. Important issues like the fault ride‐through and grid support capabilities of these wind turbine concepts are addressed. The paper reveals that advanced control of variable speed wind turbines can improve power system stability. Finally, it will be shown in the paper that wind parks consisting of variable speed wind turbines can help nearby connected fixed speed wind turbines to ride‐through grid faults. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamic modelling and control of fully rated converter wind turbines

Today, many countries are integrating large amount of wind energy into the grid and many more are expected to follow. The expected increase of wind energy integration is therefore a concern particularly to transmission grid operators. Based on the past experience, some of the relevant concerns when connecting significant amount of wind energy into the existing grid are: fault ride through requirement to keep wind turbines on the grid during faults and wind turbines have to provide ancillary services like voltage and frequency control with particular regard to island operation.While there are still a number of wind turbines based on fixed speed induction generators (FSIG) currently running, majority of wind turbines that are planned to be erected are of variable speed configurations. The reason for this is that FSIG are not capable of addressing the concern mentioned above. Thus, existing researches in wind turbines are now widely directed into variable speed configurations. This is because apart from optimum energy capture and reduction of mechanical stress, preference of these types is also due to the fact that it can support the network such as its reactive power and frequency regulation. Variable wind turbines are doubly fed induction generator wind turbines and full converters wind turbines which are based on synchronous or induction generators.This paper describes the steady state and dynamic models and control strategies of wind turbine generators. The dynamic models are presented in the dq frame of reference. Different control strategies in the generator side converter and in the grid side converter for fault ride through requirement and active power/frequency and reactive/voltage control are presented for variable speed wind turbines.     

Modelling and control of synchronous generators for wide‐range variable‐speed wind turbines

The modelling and control of a wide‐range variable speed wind turbine based on a synchronous generator are presented. Two different methods to control the operation of the synchronous generator are investigated, i.e. load angle control and instantaneous vector control. The dynamic performance characteristics of these control strategies are evaluated and compared using three model representations of the generator: a non‐reduced order model including both stator and rotor transients, a reduced order model with stator transients neglected, and a steady‐state model that neglects generator electrical dynamics. Assessment on the performance of grid‐side controller is shown during network fault and frequency variation. A simplified wind turbine model representation is also developed and proposed for large‐scale power system studies. Simulation results in Matlab/Simulink are presented and discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

利用回热抽汽控制参与风电系统调频方法研究

随着风力发电在电力系统中发电比例的提升,风速的波动性所带来的风功率的随机波动性给电力系统调频带来了新的挑战,在不增加系统装机容量的前提下,如何提高现有火电机组的调频能力应对大规模风电并网具有较重要的意义。本文通过对再热式汽轮机的并网运行进行了仿真研究,证明了汽轮机在带负荷运行中,回热系统的抽汽量变化会影响汽轮机的功率,回热系统的抽汽效应对汽轮机频率调节有显著影响。并且,本文还重点讨论了低风速风电功率欠发时,火电机组处于高负荷运行工况下,回热系统抽汽效应对汽轮机频率调节的影响。因此,在高渗透率风电的电网系统中,通过调整回热抽汽量是一种能够提高汽轮机调峰调频出力的可行方法。     

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.     

变速恒频风力发电机组动态模型及并网研究

分析了风力发电系统的特点,着重介绍了变速恒频（Variable Speed Constant Frequency,VSCF）风力发电机组运行的基本原理．在此基础上,探讨了风力发电机组各部分的仿真建模,并分析了目前适用于不同条件下的双馈感应式异步发电机（Double—Fed Induction Generator,DFIG）变速恒频（AC—Exited Variable Speed Constant Frequency,AEVSCF）风力发电机的数学模型。对含变速恒频风机电网系统的故障扰动过程进行仿真分析．结果验证了建模的正确性。在故障扰动的最后对未来风电机组建模的研究重点提出了一些建议。     

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.     

风电场中多台风力机的数值模拟

将NREL 5 MW风力机作为基本机型,使用致动线模型和大涡模拟相结合的数值方法,在中性大气边界层中模拟含有多台风力机的风电场。为了模拟风电场的复杂入流条件,首先模拟体积为3000 m(长)×3000 m(宽)×1000m(高)的大气边界层,并对模拟结果进行验证,结果表明:在覆盖逆温层以下,不同高度处的位温不变,平均风速满足剪切特性,脉动风速满足湍流谱特性;然后,分析了致动线模型中风轮直径上的网格节点数量(N)和高斯分布因子(ε)的取值规律,发现ε以网格尺度(η)为自变量取值时,N越大,η的系数越大,当N取63时,η的系数可取2或3,但N取25时,η只能取1.2;最后,使用致动线模型在大气边界层中布置8台风力机,模拟风电场,并对风力机间的相互干扰进行分析,发现第一排风力机功率明显大于其他风力机功率输出,占风场总功率输出的40.3%。     

An up to date review of continuously variable speed wind turbines with mechatronic variable transmissions

As a promising and potential alternative to conventional fixed or variable speed wind turbines, continuously variable speed wind turbines (CVSWTs) with variable transmissions offer improved power efficiency and enhanced power control capabilities. The CVSWTs can be generally achieved by adapting mechatronic variable transmissions in the turbine drive train for continuously variable speed operations for wind turbines. Therefore, this paper serves to provide an up to date and exhaustive review of the CVSWTs with mechatronic variable transmissions such as mechanical variable transmission, electrical variable transmission, and power splitting transmission. In this paper, the analysis of CVSWTs with different mechatronic transmission topologies is performed regarding basic configurations, dynamic characteristics, control principles, and experimental or simulation results. Review results indicate the feasibility of applying CVSWTs with such mechatronic transmissions and highlight superiorities of the CVSWTs with power splitting transmission. The CVSWT with power splitting transmission will be particularly suitable for megawatt‐scale turbine systems and will hence increase the economic competitiveness of these turbines due to its large power capacity and high reliability. The directions or challenges for future investigations of CVSWTs with such mechatronic transmissions are also presented to foster in‐depth understanding of such CVSWTs and their control strategies.     

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.     

Simulation and wake analysis of a single vertical axis wind turbine

Because of several design advantages and operational characteristics, particularly in offshore farms, vertical axis wind turbines (VAWTs) are being reconsidered as a complementary technology to horizontal axial turbines. However, considerable gaps remain in our understanding of VAWT performance since cross‐flow rotor configurations have been significantly less studied than axial turbines. This study examines the wakes of VAWTs and how their evolution is influenced by turbine design parameters. An actuator line model is implemented in an atmospheric boundary layer large eddy simulation code, with offline coupling to a high‐resolution blade‐scale unsteady Reynolds‐averaged Navier–Stokes model. The large eddy simulation captures the turbine‐to‐farm scale dynamics, while the unsteady Reynolds‐averaged Navier–Stokes captures the blade‐to‐turbine scale flow. The simulation results are found to be in good agreement with three existing experimental datasets. Subsequently, a parametric study of the flow over an isolated VAWT, carried out by varying solidities, height‐to‐diameter aspect ratios and tip speed ratios, is conducted. The analyses of the wake area and velocity and power deficits yield an improved understanding of the downstream evolution of VAWT wakes, which in turn enables a more informed selection of turbine designs for wind farms. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact of wind power in autonomous power systems—power fluctuations—modelling and control issues

This paper describes a detailed modelling approach to study the impact of wind power fluctuations on the frequency control in a non‐interconnected system with large‐scale wind power. The approach includes models for wind speed fluctuations, wind farm technologies, conventional generation technologies, power system protection and load. Analytical models for wind farms with three different wind turbine technologies, namely Doubly Fed Induction Generator, Permanent Magnet Synchronous Generator and Active Stall Induction Generator‐based wind turbines, are included. Likewise, analytical models for diesel and steam generation plants are applied. The power grid, including speed governors, automatic voltage regulators, protection system and loads is modelled in the same platform. Results for different load and wind profile cases are being presented for the case study of the island Rhodes, in Greece. The scenarios studied correspond to reference year of study 2012. The effect of wind fluctuations in the system frequency is studied for the different load cases, and comments on the penetration limits are being made based on the results. Copyright © 2010 John Wiley & Sons, Ltd.     

A novel power splitting drive train for variable speed wind power generators

In this paper a novel electrically controlled power splitting drive train for variable speed wind turbines is presented. A variable speed wind turbine has many advantages, mainly it can increase the power yield from the wind, alleviate the load peak in the electrical-mechanical drive train, and posses a long life time, also, it can offer the possibility to store the briefly timely wind-conditioned power fluctuations in the wind rotor, in which the rotary masses are used as storages of kinetic energy, consequently, the variable speed wind turbines are utilized in the wind power industry widely. In this work, on the basis of a planetary transmission a new kind of drive train for the variable speed wind turbines is proposed. The new drive train consists of wind rotor, three-shafted planetary gear set, generator and servo motor. The wind rotor is coupled with the planet carrier of the planetary transmission, the generator is connected with the ring gear through an adjustment gear pair, and the servo motor is fixed to the sun gear. By controlling the electromagnetic torque or speed of the servo motor, the variable speed operation of the wind rotor and the constant speed operation of the generator are realized, therefore, the generator can be coupled with the grid directly. At the nominal operation point, about 80% of the rotor power flow through the generator directly and 20% through the servo motor and a small power electronics system into the grid. As a result, the disadvantages in the traditional wind turbines, e.g. high price of power electronics system, much power loss, strong reaction from the grid and large crash load in the drive train will be avoided.     

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.     

One‐way mesoscale‐microscale coupling for simulating a wind farm in North Texas: Assessment against SCADA and LiDAR data

One‐way nested mesoscale to microscale simulations of an onshore wind farm have been performed nesting the Weather Research and Forecasting (WRF) model and our in‐house high‐resolution large‐eddy simulation code (UTD‐WF). Each simulation contains five nested WRF domains, with the largest domain spanning the north Texas Panhandle region with a 4 km resolution, while the highest resolution (50 m) nest simulates microscale wind fluctuations and turbine wakes within a single wind farm. The finest WRF domain in turn drives the UTD‐WF LES higher‐resolution domain for a subset of six turbines at a resolution of ～5 m. The wind speed, direction, and boundary layer profiles from WRF are compared against measurements obtained with a met‐tower and a scanning Doppler wind LiDAR located within the wind farm. Additionally, power production obtained from WRF and UTD‐WF are assessed against supervisory control and data acquisition (SCADA) system data. Numerical results agree well with the experimental measurements of the wind speed, direction, and power production of the turbines. UTD‐WF high‐resolution domain improves significantly the agreement of the turbulence intensity at the turbines location compared with that of WRF. Velocity spectra have been computed to assess how the nesting allows resolving a wide range of scales at a reasonable computational cost. A domain sensitivity analysis has been performed. Velocity spectra indicate that placing the inlet too close to the first row of turbines results in an unrealistic peak of energy at the rotational frequency of the turbines. Spectra of the power production of a single turbine and of the cumulative power of the array have been compared with analytical models.     

Integrated design of downwind land‐based wind turbines using analytic gradients

Wind turbines are complex systems where component‐level changes can have significant system‐level effects. Effective wind turbine optimization generally requires an integrated analysis approach with a large number of design variables. Optimizing across large variable sets is orders of magnitude more efficient with gradient‐based methods as compared with gradient‐free method, particularly when using exact gradients. We have developed a wind turbine analysis set of over 100 components where 90% of the models provide numerically exact gradients through symbolic differentiation, automatic differentiation, and adjoint methods. This framework is applied to a specific design study focused on downwind land‐based wind turbines. Downwind machines are of potential interest for large wind turbines where the blades are often constrained by the stiffness required to prevent a tower strike. The mass of these rotor blades may be reduced by utilizing a downwind configuration where the constraints on tower strike are less restrictive. The large turbines of this study range in power rating from 5–7MW and in diameter from 105m to 175m. The changes in blade mass and power production have important effects on the rest of the system, and thus the nacelle and tower systems are also optimized. For high‐speed wind sites, downwind configurations do not appear advantageous. The decrease in blade mass (10%) is offset by increases in tower mass caused by the bending moment from the rotor‐nacelle‐assembly. For low‐wind speed sites, the decrease in blade mass is more significant (25–30%) and shows potential for modest decreases in overall cost of energy (around 1–2%). Copyright © 2016 John Wiley & Sons, Ltd.     

Site‐specific Design Optimization of Wind Turbines

This article reports results from a European project, where site characteristics were incorporated into the design process of wind turbines, to enable site‐specific design. Two wind turbines of different concept were investigated at six different sites comprising normal flat terrain, offshore and complex terrain wind farms. Design tools based on numerical optimization and aeroelastic calculations were combined with a cost model to allow optimization for minimum cost of energy. Different scenarios were optimized ranging from modifications of selected individual components to the complete design of a new wind turbine. Both annual energy yield and design‐determining loads depended on site characteristics, and this represented a potential for site‐specific design. The maximum variation in annual energy yield was 37% and the maximum variation in blade root fatigue loads was 62%. Optimized site‐specific designs showed reductions in cost of energy by up to 15% achieved from an increase in annual energy yield and a reduction in manufacturing costs. The greatest benefits were found at sites with low mean wind speed and low turbulence. Site‐specific design was not able to offset the intrinsic economic advantage of high‐wind‐speed sites. It was not possible to design a single wind turbine for all wind climates investigated, since the differences in the design loads were too large. Multiple‐site wind turbines should be designed for generic wind conditions, which cover wind parameters encountered at flat terrain sites with a high mean wind speed. Site‐specific wind turbines should be designed for low‐mean‐wind‐speed sites and complex terrain. Copyright © 2002 John Wiley & Sons, Ltd.     

Applying power quality characteristics of wind turbines for assessing impact on voltage quality

Injection of wind power into an electric grid affects the voltage quality. As the voltage quality must be within certain limits to comply with utility requirements, the effect should be assessed prior to installation. To assess the effect, knowledge about the electrical characteristics of the wind turbines is needed or else the result could easily be an inappropriate design of the grid connection. The electrical characteristics of wind turbines are manufacturer‐specific but not site‐specific. This means that, having the actual parameter values for a specific wind turbine, the expected impact of the wind turbine type on voltage quality when deployed at a specific site, possibly as a group of wind turbines, can be calculated. The methodology for this is explained and illustrated by case studies considering a 5 × 750 kW wind farm on a 22 kV distribution feeder. The detailed analysis suggests that the wind farm capacity can be operated at the grid without causing unacceptable voltage quality. For comparison, a simplified design criterion is considered assuming that the wind farm is only allowed to cause a voltage increment of 1%. According to this criterion, only a very limited wind power capacity would be allowed. Measurements confirm, however, the suggestion of the detailed analysis, and it is concluded that a simplified design criterion such as the ‘1% rule’ should not be used for dimensioning the grid connection of wind farms. Rather, this article suggests a systematic approach including assessment of slow voltage variations, flicker, voltage dips and harmonics, possibly supported by more detailed analyses, e.g. system stability if the wind farm is large or the grid is very weak, and impact on grid frequency in systems where wind power covers a high fraction of the load, i.e. most relevant for isolated systems. Copyright © 2002 John Wiley & Sons, Ltd.     

High‐throughput computation and the applicability of Monte Carlo integration in fatigue load estimation of floating offshore wind turbines

Long‐term fatigue loads for floating offshore wind turbines are hard to estimate because they require the evaluation of the integral of a highly nonlinear function over a wide variety of wind and wave conditions. Current design standards involve scanning over a uniform rectangular grid of metocean inputs (e.g., wind speed and direction and wave height and period), which becomes intractable in high dimensions as the number of required evaluations grows exponentially with dimension. Monte Carlo integration offers a potentially efficient alternative because it has theoretical convergence proportional to the inverse of the square root of the number of samples, which is independent of dimension. In this paper, we first report on the integration of the aeroelastic code FAST into NREL's systems engineering tool, WISDEM, and the development of a high‐throughput pipeline capable of sampling from arbitrary distributions, running FAST on a large scale, and postprocessing the results into estimates of fatigue loads. Second, we use this tool to run a variety of studies aimed at comparing grid‐based and Monte Carlo‐based approaches with calculating long‐term fatigue loads. We observe that for more than a few dimensions, the Monte Carlo approach can represent a large improvement in computational efficiency, but that as nonlinearity increases, the effectiveness of Monte Carlo is correspondingly reduced. The present work sets the stage for future research focusing on using advanced statistical methods for analysis of wind turbine fatigue as well as extreme loads. Copyright © 2015 John Wiley & Sons, Ltd.     

Exploring the benefits of vertically staggered wind farms: Understanding the power generation mechanisms of turbines operating at different scales

Wind farms are known to modulate large scale structures in and around the wake regions of the turbines. The potential benefits of placing small hub height, small rotor turbines in between the large turbines in a wind farm to take advantage of such modulated large‐scale eddies are explored using large eddy simulation (LES). The study has been carried out in an infinite wind farm framework invoking an asymptotic limit, and the wind turbines are modeled using an actuator line model. The vertically staggered wind turbine arrangements that are studied in the present work consist of rows of large wind turbines, with rows of smaller wind turbines (ie, smaller rotor size and shorter hub height) placed in between the rows of large turbines. The influence of the hub height of the small turbines, in particular, how it affects the interactions between the large and small turbines and consequently their power, along with the multiscale dynamics involved, has been assessed in the current study. It was found that, in the multiscale layouts, the small turbines at lower hub heights operate more efficiently than their homogeneous single‐scale counterparts. In contrast, the small turbines with higher hub heights incur a loss of power compared with the corresponding single‐scale arrangements.