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.     

Comparison and testing of power reserve control strategies for grid‐connected wind turbines

The stability of the electrical grid depends on enough generators being able to provide appropriate responses to sudden losses in generation capacity, increases in power demand or similar events. Within the United States, wind turbines largely do not provide such generation support, which has been acceptable because the penetration of wind energy into the grid has been relatively low. However, frequency support capabilities may need to be built into future generations of wind turbines to enable high penetration levels over approximately 20%. In this paper, we describe control strategies that can enable power reserve by leaving some wind energy uncaptured. Our focus is on the control strategies used by an operating turbine, where the turbine is asked to track a power reference signal supplied by the wind farm operator. We compare the strategies in terms of their control performance as well as their effects on the turbine itself, such as the possibility for increased loads on turbine components. It is assumed that the wind farm operator has access to the necessary grid information to generate the power reference provided to the turbine, and we do not simulate the electrical interaction between the turbine and the utility grid. Copyright © 2013 John Wiley & Sons, Ltd.     

厄立特里亚风能资源、电力需求及其适宜的风力机组

分析了厄立特利亚东部红海低地、中部山区高原和西部低地三个地区的风能资源.采用当地的全年风速数据比较了各个地区的风能潜力.根据目前和未来若干年的用电状况预测了各地区的电力需求.也对厄立特里亚政府的风能鼓励政策和与文化背景有关风险提示作了介绍.研究表明,在厄立特里亚东南部地区存在可观的风能资源并且当地也具有电力需求.估计年发电量在2 GWh,相当于全年满负荷运行2 500 h,当地可以安装大量风力机组,全部发电量将超出厄立特里亚全国在可预见的将来的用电量.风速8m·S-1的低速机组是最适合该地区的风电机组.     

Power correlation for vertical axis wind turbines with varying geometries

In this paper, a new predictive model that can forecast the performance of a vertical axis wind turbine (VAWT) is presented. The new model includes four primary variables (rotor velocity, wind velocity, air density, and turbine power output) as well as five geometrical variables (rotor radius, turbine height, turbine width, stator spacing, and stator angle). These variables are reduced to include the power coefficient (C_p) and tip speed ratio (TSR). A power coefficient correlation for a novel VAWT (called a Zephyr Vertical axis Wind Turbine (ZVWT)) is developed. The turbine is an adaptation of the Savonius design. The new correlation can predict the turbine's performance for altered stator geometry and varying operating conditions. Numerical simulations with a rotating reference frame are used to predict the operating performance for various turbine geometries. The case study includes 16 different geometries for three different wind directions. The resulting 48 data points provide detailed insight into the turbine performance to develop a general correlation. The model was able to predict the power coefficient with changes in TSR, rotor length, stator spacing, and stator angle, to within 4.4% of the numerical prediction. Furthermore, the power coefficient was predicted with changes in rotor length, stator spacing, and stator angle, to within 3.0% of the numerical simulations. This correlation provides a useful new design tool for improving the ZVWT in the specific conditions and operating requirements specific to this type of wind turbine. Also, the new model can be extended to other conditions that include different VAWT designs. Copyright © 2010 John Wiley & Sons, Ltd.     

永磁直驱风力发电系统MPPT控制的研究

文章首先介绍了风力机模型。然后介绍了一种改进型的变步长爬山算法,通过该算法改变直驱风力发电系统三重交错并联Boost变换电路的占空比,从而实现最大功率跟踪,获取最大风能。最后,利用MATLAB/SIMULINK建立直驱永磁风力发电系统仿真模型并进行研究。试验结果表明,改进型变步长爬山算法比传统爬山算法能更快跟踪最大功率点,控制系统具有较好的控制精度和稳定性。     

大型风力发电机组调向功率的探讨

针对风力发电机组调向对风机构工作性能直接关系到机组的整体性能，从研究风力发电机组调向阻力矩入手，提出了确定机组调向功率需考虑的一些因素和方法。     

Low wind speed turbines and wind power potential in Minnesota, USA

Wind power development in Minnesota largely has been focused in the “windy” southwestern part of the state. This research evaluates the additional power that potentially could be generated via low wind speed turbines, particularly for areas of the state where there has been comparatively little wind energy investment. Data consist of 3 years (2002–2004) of wind speed measurements at 70–75 m above ground level, at four sites representing the range of wind speed regimes (Classes 2–5) found in Minnesota. Power estimates use three configurations of the General Electric 1.5-MW series turbine that vary in rotor diameter and in cut-in, cut-out, and rated speeds. Results show that lower cut-in, cut-out, and rated speeds, and especially the larger rotor diameters, yield increases of 15–30% in wind power potential at these sites. Gains are largest at low wind speed (Class 2) sites and during the summer months at all four sites. Total annual wind power at each site shows some year-to-year variability, with peaks at some sites partially compensating for lulls at others. Such compensation does not occur equally in all years: when large-scale atmospheric circulation patterns are strong (e.g., 2002), the four sites show similar patterns of above- and below-average wind power, somewhat reducing the ability of geographic dispersion to mitigate the effects of wind speed variability.     

Simulation of wind-power impact on the transient fault behaviour of grid-connected wind turbines

This paper deals with the analysis of the possible adoption of a static synchronous compensator (STATCOM) with grid-connected constant-speed wind turbines. Three different cases are simulated using Matlab/Simulink for investigating wind-power impact on a power grid connected to wind turbines. The simulations yield information on (i) how the faults impact on the wind turbines and (ii) how the STATCOM influences the post-fault behaviour of the power system. In this paper, an attempt is made to compare the impact, in terms of voltages and active and reactive powers, of adding wind turbines and STATCOM to an electrical power grid. The simulations show that the goal of keeping turbines operational can be achieved.     

Research and development for shrouded wind turbines

In order to exploit wind power as economically as possible, it was suggested that the wind turbine should be enclosed inside a specially designed shroud. Various geometries are discussed; it is shown that with a fairly compact shroud a significant power augmentation can be achieved. For improving the shroud performance, the use of a ring-shaped flap or boundary layer control technique is introduced. It is shown that up to 80% improvement in the shroud power augmentation can be obtained by the use of an appropriate ring-shaped flap while proper bleeding of the shroud's external flow into its inner rear part will increase its power augmentation by about 25%. The present review also discussed the design and performance of an axial flow turbine which is the most suitable for the proposed shrouds. It is shown that such a turbine produces a fairly stable output for varying wind speeds while exhibiting a fairly high efficiency. Based on the reported research with shrouds, a pilot plant producing 1 hp at 5 m/s with a 3 m dia. turbine was built. Its design and preliminary field test results are also included in the review.     

A stochastic power curve for wind turbines with reduced variability using conditional copula

It has been observed that a large variability exists between wind speed and wind power in real metrological conditions. To reduce this substantial variability, this study developed a stochastic wind turbine power curve by incorporating various exogenous factors. Four measurements, namely, wind azimuth, wind elevation, air density and solar radiation are chosen as exogenous influence factors. A recursive formula based on conditional copulas is used to capture the complex dependency structure between wind speed and wind power with reduced variability. A procedure of selecting a proper form for each factor and its corresponding copula models is given. Through a case study on the small wind turbine located in southeast of Edmonton, Alberta, Canada, we demonstrate that the variability can be reduced significantly by incorporating these influence factors. Wind turbine operators can apply the method reported in this study to construct a stochastic power curve for local wind farms and use it to achieve more accurate power forecasting and health condition monitoring of the turbine. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel adaptative maximum power point tracking algorithm for small wind turbines

A novel maximum power point tracking algorithm for small wind turbines is proposed. The solution presented here is an adaptive intelligent algorithm that uses a new advanced perturb and observe method to search for the optimum relationship of the system for tracking the maximum power point even under variable wind conditions. The validity of the proposed algorithm is analysed and the design procedure is presented. Its main virtue resides in its capability to adapt to changes in the turbine and in the surrounding environment, even under variable wind conditions, improving the efficiency of the system. The experimental results confirm the validity of the proposed algorithm.     

Design of a wind turbine rotor for maximum aerodynamic efficiency

The design of a three‐bladed wind turbine rotor is described, where the main focus has been highest possible mechanical power coefficient, CP, at a single operational condition. Structural, as well as off‐design, issues are not considered, leading to a purely theoretical design for investigating maximum aerodynamic efficiency. The rotor is designed assuming constant induction for most of the blade span, but near the tip region, a constant load is assumed instead. The rotor design is obtained using an actuator disc model, and is subsequently verified using both a free‐wake lifting line method and a full three‐dimensional Navier–Stokes solver. Excellent agreement is obtained using the three models. Global CP reaches a value of slightly above 0.51, while global thrust coefficient CT is 0.87. The local power coefficient C_p increases to slightly above the Betz limit on the inner part of the rotor; the local thrust coefficient C_t increases to a value above 1.1. This agrees well with the theory of de Vries, which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation, both C_p and C_t will increase towards the root. Towards the tip, both C_p and C_t decrease due to tip corrections as well as drag. Copyright © 2008 John Wiley & Sons, Ltd.     

Adjustment of wind farm power output through flexible turbine operation using wind farm control

When the installed capacity of wind power becomes high, the power generated by wind farms can no longer simply be that dictated by the wind speed. With sufficiently high penetration, it will be necessary for wind farms to provide assistance with supply‐demand matching. The work presented here introduces a wind farm controller that regulates the power generated by the wind farm to match the grid requirements by causing the power generated by each turbine to be adjusted. Further, benefits include fast response to reach the wind farm power demanded, flexibility, little fluctuation in the wind farm power output and provision of synthetic inertia. Copyright © 2015 John Wiley & Sons, Ltd.     

Novel power capture optimization based sensorless maximum power point tracking strategy and internal model controller for wind turbines systems driven SCIG

Under the trends to using renewable energy sources as alternatives to the traditional ones, it is important to contribute to the fast growing development of these sources by using powerful soft computing methods. In this context, this paper introduces a novel structure to optimize and control the energy produced from a variable speed wind turbine which is based on a squirrel cage induction generator (SCIG) and connected to the grid. The optimization strategy of the harvested power from the wind is realized by a maximum power point tracking (MPPT) algorithm based on fuzzy logic, and the control strategy of the generator is implemented by means of an internal model (IM) controller. Three IM controllers are incorporated in the vector control technique, as an alternative to the proportional integral (PI) controller, to implement the proposed optimization strategy. The MPPT in conjunction with the IM controller is proposed as an alternative to the traditional tip speed ratio (TSR) technique, to avoid any disturbance such as wind speed measurement and wind turbine (WT) characteristic uncertainties. Based on the simulation results of a six KW-WECS model in Matlab/Simulink, the presented control system topology is reliable and keeps the system operation around the desired response.     

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.     

Virtual inertia for variable speed wind turbines

Inertia provision for frequency control is among the ancillary services that different national grid codes will likely require to be provided by future wind turbines. The aim of this paper is analysing how the inertia response support from a variable speed wind turbine (VSWT) to the primary frequency control of a power system can be enhanced. Unlike fixed speed wind turbines, VSWTs do not inherently contribute to system inertia, as they are decoupled from the power system through electronic converters. Emphasis in this paper is on how to emulate VSWTs inertia using control of the power electronic converter and on its impact on the primary frequency response of a power system. An additional control for the power electronics is implemented to give VSWTs a virtual inertia, referring to the kinetic energy stored in the rotating masses, which can be released initially to support the system's inertia. A simple Matlab/Simulink model and control of a VSWT and of a generic power system are developed to analyse the primary frequency response following different generation losses in a system comprising VSWTs provided with virtual inertia. The possibility of substituting a 50% share of conventional power with wind is also assessed and investigated. The intrinsic problems related to the implementation of virtual inertia are illustrated, addressing their origin in the action of pitch and power control. A solution is proposed, which aims at obtaining the same response as for the system with only conventional generation. The range of wind speeds near the power limitation zone seems to be the most critical from a primary response point of view. The theoretical reasons behind this are elucidated in the paper. Copyright © 2012 John Wiley & Sons, Ltd.     

Sizing and control of trailing edge flaps on a smart rotor for maximum power generation in low fatigue wind regimes

An extension of the spectrum of applicability of rotors with active aerodynamic devices is presented in this paper. Besides the classical purpose of load alleviation, a secondary objective is established: optimization of power capture. As a first step, wind speed regions that contribute little to fatigue damage have been identified. In these regions, the turbine energy output can be increased by deflecting the trailing edge (TE) flap in order to track the maximum power coefficient as a function of local, instantaneous speed ratios. For this purpose, the TE flap configuration for maximum power generation has been using blade element momentum theory. As a first step, the operation in non‐uniform wind field conditions was analysed. Firstly, the deterministic fluctuation in local tip speed ratio due to wind shear was evaluated. The second effect is associated with time delays in adapting the rotor speed to inflow fluctuations caused by atmospheric turbulence. The increase in power generation obtained by accounting for wind shear has been demonstrated with an increase in energy production of 1%. Finally, a control logic based on inflow wind speeds has been devised, and the potential of enhanced power generation has been shown by time‐domain simulations. Copyright © 2015 John Wiley & Sons, Ltd.     

风电机组模糊PI控制与前馈控制相结合的变桨控制

针对经典PID变桨距控制器超调量大、波动剧烈等缺陷,提出了一种采用模糊PI控制与前馈控制相结合的控制器来控制浆距角,进而使机组功率稳定在额定功率附近的控制思路。通过对某1.5MW风力发电机组进行仿真,结果表明,该控制器控制效果优于经典PID变桨距控制器。     

Horizontal axis wind turbine working at maximum power coefficient continuously

The performance of a horizontal axis wind turbine continuously operating at its maximum power coefficient was evaluated by a calculation code based on Blade Element Momentum (BEM) theory. It was then evaluated for performance and Annual Energy Production (AEP) at a constant standard rotational velocity as well as at a variable velocity but at its maximum power coefficient.The mathematical code produced a power coefficiency curve which showed that notwithstanding further increases in rotational velocity a constant maximum power value was reached even as wind velocity increased.This means that as wind velocity varies there will always be a rotational velocity of the turbine which maximises its coefficient. It would be sufficient therefore to formulate the law governing the variation in rotational velocity as it varied with wind velocity to arrive at a power coefficient that is always the same and its maximum.This work demonstrates the methodology for determining the law governing the rotational velocity of the rotor and it highlights the advantages of a wind turbine whose power coefficient is always at maximum rather than very variable in line with the variation of wind velocity.     

Development of low-noise drag-type vertical wind turbines

In this study, the aerodynamic noise characteristics of Savonius wind turbines were investigated using hybrid computational aero-acoustics techniques, and low-noise designs were proposed based on the understanding of the noise generation mechanism. First, the flow field around the turbine was analyzed in detail by solving three-dimensional unsteady incompressible Reynolds-averaged Navier–Stokes equations using computational fluid dynamics techniques. Then, the aerodynamic noise radiating from the wind turbine was predicted using the Ffowcs Williams and Hawkings equation with the obtained flow field information. Two distinct harmonic noise components—the blade passing frequency (BPF) and harmonics with a fundamental frequency that is much higher than the BPF—were identified in the predicted noise spectrum. The origin of the higher harmonic components was found to be related to vortex shedding from the rotating turbine. Based on this finding, the proposed low-noise design for Savonius wind turbines uses S-shaped blades. S-shaped blades were found to reduce the noise levels of Savonius wind turbines by up to 2.7 dB.