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EEE Control Systems Magazine welcomes suggestions for books to be reviewed in this column. Please contact either Michael Polis or Zongli Lin, associate editors for book reviews.
Optimal Control of Wind Energy Systems: Towards a Global Approach by IULIAN MUNTEANU, ATONETA I. BRATCU, NICOLAOS-ANTONIO CUTULULIS, and EMIL CEANGÃ Reviewed by Jason H. Laks and Lucy Y. Pao
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ind is recognized worldwide as a cost-effective, environmentally friendly solution to energy shortages, and wind energy is currently t he fa ste st- g row i ng e nerg y source in the world. Wind power investment worldwide is expected to expand threefold in the next decade, from about $18 billion in 2006 to $60 billion in 2016 [1]. While the cost of wind energy is already very competitive with energy from coal and natural gas, there are still many unsolved challenges in expanding wind power. From bearings under constant friction to blades that must be able to handle gusts, lightning, and constantly changing wind conditions, today’s wind turbines need increasingly sophisticated component designs, sensors, and control systems. Further, better wind forecasting methods are needed to improve site selection, optimize operations, and mitigate load fluctuations on the power grid. Although the United States receives only about 1% of its electrical energy from wind [2], the corresponding figure in Denmark is more than 15% [3]. The integration of more than 20% penetration of wind energy into the grid will require modifications of the grid design and operation, with the possible addition of new transmission lines and energy storage systems. Despite the amazing growth in the installed capacity of wind turbines in recent years, engineering and science challenges still exist. These large, flexible structures operate in uncertain environments and lend themselves nicely to advanced control solutions. Advanced Springer-Verlag, 2008 ISBN 978-1-84800-079-7, US$149, 283 pages.
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controllers can help achieve the overall goal of decreasing the cost of wind energy by increasing the efficiency, and thus the energy capture, or by reducing structural loading and increasing the lifetimes of the components and turbine structures. Wind energy literature has been similarly expanding with the growing interest in the area. Several tomes cover the subject from a broad perspective, and these tend to blend reference and tutorial material in their presentation [4], [5]. As such they contain large sections that essentially serve as introductions to atmospheric sciences, aerodynamics, structural dynamics, electrical power systems, generator dynamics, power electronics, control systems, and the economics of the power industry. That is the nature of the beast for any book that undertakes the ambitious goal of providing a technical foundation for those interested in the wind industry. Unfortunately, for the controls engineer, the research literature tends to be an expanse of narrowly focused studies with only a few that are tutorial in nature. Further, research is typically focused on control of wind turbines in a particular operating region such as maximizing power in partial load conditions, mitigating loads in above-rated wind conditions, or investigating the interplay between generator control and torque transients. Other areas slightly more removed from the turbine structure involve investigations of power quality and generation. The book under review provides technical background on wind turbines specifically tailored for the controls engineer, and thus the target audience for this text is members of the control research community who are interested in wind energy applications. The authors of the book are active wind energy researchers [6]–[9], having developed optimal and nonlinear controllers for wind energy systems, and the book highlights several of these methods. Overall, the book provides a reasonably good overview of wind energy systems for control engineers, but being a monograph as such, it is not comprehensive enough to be the sole source of information for control researchers wanting to move into the wind energy area. The phrasing and usage of the English language are awkward at times, making some sections difficult to read. Further, there are some errors in equations and references to variables that can sometimes be misleading; an example being that the “tip speed ratio” is often referred to as the “tip speed.” Specific terminology (for example, “U/f control”) is occasionally not defined well, and the authors sometimes assume that readers have a good background in electrical power systems. JUNE 2009
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Investigations of the performance and response of a control system for wind turbines through the transitions between operating regions are not common, and it is this kind of perspective that we might expect a global approach to encompass. Or it could be that a global approach would encompass the aerodynamic loading on one end, to the electromagnetic torque loads on the other and perhaps even include the effects of electrical load disturbances. It is this latter approach that Optimal Control of Wind Energy Systems: Towards a Global Approach at first seems to espouse. In this respect, the book is required to include material on stochastic wind modeling, aerodynamics and blade element theory, control of electrical machinery (such as generators), and even some power electronics—not to mention structural dynamics. The book contains a wealth of information (even an exposition on the theory of feedback linearization); it transitions from a wind turbine controls handbook including case studies, to one that presents a methodology that facilitates designing for all regions of operation. Ultimately, since pitch control is not included consistently as an integral part of the control strategy, this book is about optimizing power capture in partial load while mitigating torque transients. This objective is explored in case studies, and it is where the level of detail is sufficient for the reader, with some effort, to design controllers using the author’s methodology.
CONTENTS The book is organized into eight chapters and three appendices. The first three chapters introduce wind energy and discuss modeling of both the wind and wind turbines, while the next three chapters discuss basic and advanced controllers for wind turbines. The last two chapters outline experimental systems for validating control algorithms and give some general conclusions. The appendices provide the wind turbine parameters considered in particular case studies, give an overview of the main ideas behind three advanced control methodologies, and present some photos and diagrams of experimental evaluation testbeds. Chapter 1 provides an overview of the wind industry and how different types of wind technology are situated within the market. The end of this chapter provides an outline of the rest of the book. Chapter 2 is an overview of wind turbine operating environments and technology and introduces the reader to the elements of the conversion chain that are developed in more detail within the following chapter on modeling. In Chapter 3, detailed models for wind disturbances and wind turbines are developed. This chapter suggests that the strategy of the book is to investigate control all the way from wind models to generator models and perhaps grid connection as well. It therefore seems a little surprising that models 106 IEEE CONTROL SYSTEMS MAGAZINE
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are not developed for the dynamics of the turbine structure, for example, models for the blades and tower. Further, some assumptions are made about the nature of the wind across the rotor disk that may not be adequate for larger turbines having blade lengths on the order of tens of meters. Surprisingly detailed models for the generator are provided, though the development of these models tends to be more summary in nature than tutorial. The modeling chapter concludes with sections on linearization, eigenanalysis, and a case study. These sections tend to be condensed summaries making them difficult to follow in some instances. References to “well-known linearization techniques” and some of the results presented and used in later chapters depend heavily on a Ph.D. thesis [10], which is not readily available. The concluding case study is interesting though because it shows that including a generator model can reveal significant high-frequency torque transients where many turbine studies simply assume that the torque command couples instantaneously into the system through a static gain. From a controls perspective, Chapter 4 could appear before the chapter on modeling. The reader is walked through the control objectives and transitions between operating regions of a wind turbine. The majority of this presentation is accomplished without relying on the models previously developed. Except for a few organizational issues (for example, a discussion of blade feathering in a section on fixed-pitch operation), this chapter provides a good orientation for the controls engineer. The section on generator motion control could serve to motivate the models developed in Chapter 3 since the discussion relies very little on them, and the block diagrams provided indicate, from a systems perspective, how d-q coordinates come into play. Finally, this chapter includes a section on grid connection and power quality, which is narrative, and dynamical models are not involved. This last section is informative for controls engineers without exposure to the power industry or power electronics. Chapter 5, which focuses on advanced control techniques for optimizing power capture, is largely narrative as well but with enough detail to inform the reader of the complexities involved. Chapter 5 also presents most of the case studies contained in the book. The level of detail is considerable, but in many cases probably not sufficient for the uninitiated reader to utilize the methodology without further research. Each section considers a different control method and presents at least one case study in which it is employed. Technical details related to the methodologies are pushed back to the book’s appendices, and it becomes clear that the intention is not to teach the various control techniques but instead to summarize and demonstrate their effectiveness. The chapter includes maximum power point tracking, on-off control, sliding-mode control, feedback linearization, quantitative feedback theory (QFT), as well as standard PI/PID loops. Results are generally presented consistently in terms of each system’s trajectory in the wind-power-versus-rotor-speed plane with the locus
of optimal operation overlaid It is this metric that facilitates comparison of the results between the various approaches since the authors do not compare them directly. The chapter concludes with a summary of comments regarding each approach and the corresponding case studies. In Chapter 6, the authors introduce linear-quadratic (LQ) optimal control and then present their control design methodology based on partitioning the objectives into what are described as low- and high-frequency loops. The low-frequency loop typically incorporates the nonlinear nature of the turbine and is tasked with tracking the operating point through the regions of operation. The high-frequency loop utilizes what might be described as small-signal perturbations about the operating point as determined by the low-frequency loop and captures the transient behavior excited by the higher frequency, turbulent component of the wind. It is an intriguing idea that one would be able to systematically couple the load mitigation feedback with the operating point determined by another loop that is tracking longer term operating conditions. The low-frequency loop might use any of the previously studied power optimization strategies, while the high-frequency loop is designed based on the LQ techniques introduced at the beginning of the chapter. As in previous chapters, case studies present results using various power point tracking approaches. The chapter concludes with several sections discussing the extension of the approach to larger scale turbines. This extension, which is referred to by the authors as a “global” approach or “global” optimization, seems to imply that the structural dynamics should be incorporated into the objectives that have to this point been exclusively concerned with operating point and torque transients in the drive train. These last sections are largely suggestive in style, and discuss results and approaches from other studies. Chapter 7 is of interest to anyone considering a laboratory environment utilizing hardware for emulation of a turbine. As the authors explain, emulation is attractive to the extent that the torque disturbances generated in different wind conditions can be generated by an electromechanical system. The material is largely a review of literature and seemingly an exposition of the techniques employed by the authors to set up such a system. Specific examples deal with the partitioning of models developed earlier between software and laboratory hardware for “hardware-in-the-loop” simulation. Here again, it can be observed that this approach is most valuable for exploring turbine response at the back end of wind energy conversion systems (WECSs) (for example, generator control or power quality at the grid). The authors also discuss issues related to interfacing with real-time simulations and models for parts of the turbine that are not part of the laboratory hardware, which is intriguing when considering an interface to higher fidelity codes for the turbine structure incorporating aeroelastic computations
The authors provide a reasonable introduction to the field, covering necessary objectives, operating environments, and systems modeling.
such as those implemented in the FAST code from the U.S. National Renewable Energy Laboratory [11]. The book concludes, in Chapter 8, with a short discussion reviewing the book’s contents. This chapter observes that WECS control systems are still evolving and that in the variable-speed application no single approach has become prevalent to the extent that it would be considered “classical.” This final, short chapter is followed by appendices covering tabulated features of WECSs used in case studies, elements of theoretical background for the more technically complex controllers, and then photos and diagrams related to the hardware emulation discussed in Chapter 7 and also used in some of the case studies.
CONCLUSIONS The authors provide a reasonable introduction to the field, covering necessary objectives, operating environments, and systems modeling. However, dynamic models for the turbine structure (tower and blades) are not incorporated in their studies and subsequently there is little elaboration on issues that may be related. If a “global approach” is going to address wind turbine dynamics from front to back, then structural loads would certainly need to be included. On the other hand, if the objective is toward the development of a unified approach that encompasses all regions of turbine operation, then in light of the material in Chapter 6, this book is a solid first step. As the authors discuss, it is not difficult to imagine that the approach described might be extended to include structural dynamics and multi-input, multi-output systems—a characteristic feature likely to be integral to controlling large scale turbines. The reader would benefit from a clear definition of what the authors intend by “global” and also from a baseline turbine configuration used with each of the controllers presented. Realistically, it is impossible to have everything that a reader would like in a single book, much less a monograph. The authors have collected together in one place a fairly wide array of advanced techniques for torque control and an extensive list of references and material on hardware emulation that appears to be unique in the available literature. Except for general information on turbine operation, the particular methodologies presented in this text complement those JUNE 2009
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covered in [12] and 1[3]. For more details explaining the introductory material, readers can refer to [4] and [5]. The book has supporting Matlab files that are well organized, documented, and the reader can easily reproduce the author’s results, but in some instances certain toolboxes are required (for example, the system identification toolbox). In this respect, Optimal Control of Wind Energy Systems: Towards a Global Approach can serve as an excellent reference for researchers in the field pursuing one of the methodologies covered in the text. Overall, by providing a broad overview of many of the considerations and complexities involved in the control of wind turbines, the book demonstrates a few advanced control approaches with enough detail for the interested reader to embark on research using the methods presented in the text.
REVIEWER INFORMATION Jason H. Laks (
[email protected]) received B.S. degrees in electrical engineering and mathematics from the University of Minnesota, and is currently a doctoral candidate in the Electrical and Computer Engineering Department at the University of Colorado at Boulder. Previously, he worked at Honeywell’s Systems Research Center, 3M, and Maxtor. Lucy Y. Pao (
[email protected]) received the B.S., M.S., and Ph.D. degrees in electrical engineering from Stanford University, and she is currently a professor in the Electrical and Computer Engineering Department at the University of Colorado at Boulder. She is leading the development of the Center for Research and Education in Wind (CREW), a multi-institutional wind energy center involving the University of Colorado at Boulder, the National Renewable
Introduction to Discrete Event Systems, Second Edition by CHRISTOS G. CASSANDRAS and STÉPHANE LAFORTUNE Reviewed by Andrea Paoli and N. Eva Wu
Springer Science + Business Media LLC, 2008 ISBN 978-0-387-33332-8, US$89.95, 769 pages
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ntroduction to Discrete Event Systems (DES) covers, in a progressively refined manner, concepts, theories, and methods for modeling, control, and performance analysis of
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Energy Laboratory, Colorado School of Mines, and Colorado State University, in partnership with the National Center for Atmospheric Research and the National Oceanic and Atmospheric Administration. Her interests are in the areas of control systems, multisensor data fusion, and haptic and multimodal visual/haptic/audio interfaces.
REFERENCES [1] Global Wind Energy Council. (2009, Jan. 24). [Online]. Available: http:// www.gwec.net/ [2] American Wind Energy Association. (2009, Jan. 24). Wind Energy Fast Facts [Online]. Available: http://www.awea.org/newsroom/pdf/ Fast_Facts.pdf [3] L. R. Brown. (2009, Jan. 24). Wind Electric Generation Soaring [Online]. Available: http://www.earthpolicy.org/Indicators/indicator10.htm [4] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Energy Explained: Theory, Design, and Application. New York: Wiley, 2002. [5] T. Burton, D. Sharpe, N. Jenkins, and E. Bossanyi, Wind Energy Handbook. New York: Wiley, 2001. [6] N. A. Cutululis, “Contributions to control strategies synthesis of renewable power systems with hybrid structures,” Ph.D. dissertation, Univ. of Galati, Romania, 2005. [7] I. Munteanu, N. A. Cutululis, A. I. Bratcu, and E. Ceanga, “Optimization of variable speed wind power systems based on a LQG approach,” Control Eng. Pract., vol. 13, no. 7, pp. 903–912, July 2005. [8] I. Munteanu, “Contributions to the optimal control of wind energy conversion systems,” Ph.D. dissertation, Univ. of Galati, Romania, 2006. [9] N. A. Cutululis, E. Ceanga, A. D. Hansen, and P. Sorensen, “Robust multi-model control of an autonomous wind power system,” Wind Energy, vol. 9, no. 5, pp. 399–419, Sept./Oct. 2006. [10] T. Ekelund, “Modeling and linear quadratic optimal control of wind turbines,” Ph.D. dissertation, Chalmers Univ. of Goteborg, Sweden, 1997. [11] J. Jonkman and M. L. Buhl, “FAST User’s guide,” U.S. National Renewable Energy Laboratory, NREL/EL-500-38230, 2005. [12] F. D. Bianchi, H. De Battista, and R. J. Mantz, Wind Turbine Control Systems. New York: Springer-Verlag, 2007. [13] K. E. Johnson, “Review of wind turbine control systems,” IEEE Control Syst. Mag., vol. 27, no. 5, pp. 124–126, Oct. 2007.
discrete event systems. The book contains two complete instructional paths: chapters 2–5 for deterministic discrete event systems and chapters 6–11 for stochastic discrete event systems. A common need in many complex technological systems is to integrate their computation, control, communication, and information management functionalities. Examples of such systems include automated manufacturing systems, sensor networks, and transportation systems. Primarily event driven, as opposed to time driven, transitions among discrete states are a characteristic feature of these systems. A substantial portion of the second edition of this book is a revision based on Discrete Event Systems: Modeling and Performance Analysis, by Christos G. Cassandras (1993), which was awarded the 1999 Harold Chestnut Prize as the best control engineering textbook by the International Federation of Automatic Control. This book was the first textbook that integrated modeling, design, and analysis techniques 1066-033X/09/$25.00©2009IEEE
