Wind turbine control systems. Principles, modelling and gain scheduling design. Fernando D. Bianchi, Hernán De Battista and Ricardo J. Mantz, Springer, London, 2006. No. of pages: XIX + 207. Price: $119

The control of wind energy conversion systems (WECS) must face a multitude of challenges; among them, the most important is to integrate a profoundly erratic source of energy—the wind—into electrical energy grids required to meet tough quality standards. The multitude of architectures encountered nowadays in WECS practically renders impossible the associated control systems being standardized. Moreover, with the emergence of variable-speed wind energy systems, new control issues have been formulated. Thus, even if recording a significant technological advance, most of commercial wind energy systems are still equipped only with primary control functions. In the frame of increasing attention paid to alternative sources of energy, the design of WECS control systems is an open issue, raising a vivid interest especially from the academic research community. A significant literature dealing with wind energy systems is already available, but most of the books are focused on technological aspects (aerodynamics, energy generation, etc.), while only briefing the main control aspects. In spite of their apparent marginal interest, there are important problems that claim the use of control systems: the grid integration 1, operation in various regimes and under various wind conditions 2, supervision, etc. The book in my hand is the first—as to my knowledge—completely dedicated to the control of wind energy systems; more precisely, it addresses a specific control design approach, the gain scheduling. As the WECS control task is not trivial, such a sophisticated approach might be a solution in the future. This monograph is addressed mainly to scientists from academia, but the reader can perceive its utility for industrials as well. WECS are profoundly nonlinear and parameter-varying systems that are required to fulfil quite ambitious performances. Among these is the power regulation in full load, efficiency maximization below rated power, fatigue alleviation for improving the overall reliability, extreme operation handling and grid integration. These aspects, the stochasticity of the main exogenous signal, the wind, and the uncertainties inherent to complex systems result in major difficulties related to any control approach of WECS. When the wind speed varies over the entire operating range, then elaborated control strategies are necessary. The gain scheduling approach is one that complies with such requirements. The book deals with both essential WECS control approaches, namely power regulation by pitch control and variable-speed control, dedicated to induction-machine-based generation configurations. The proposed gain-scheduling philosophy, framed in the context of linear parameter varying (LPV) systems, is intended to unify the design procedures for the entire range of wind speed variation. Topicwise, the book can be structured into three parts, as follows. In the first part, starting from wind energy conversion principles, the authors have given the basics of WECS modelling. Generally, the presented models are control oriented and result from a thorough and quite complete analysis. The presented modelling elements denote the interest focused on large WECS, although not explicitly stated. The interaction between the wind flow and the turbine is gradually described. Starting from basic modelling—actuator disc and blade element theories—passing through the widely used torque/power aerodynamic characteristics, the authors complete the analysis with the various supplementary effects, such as influence of the tower shadow, wind shear and rotational sampling effect 2. Whereas the aerodynamics is extensively dealt with, modelling of the generation subsystem is briefed and is rather descriptive. No dynamic model can be found because the induction generator torque control allows the electric subsystem dynamics to be neglected. The structural dynamics analysis, the model of the collective pitch subsystem and drive train modelling are described well enough with respect to the stated goal of the book. The second part is dedicated to a detailed presentation of the WECS control fundamentals. The most common strategies related to the adopted generation structure are extensively laid on. The control requirements and strategies envisage both the partial-load and the full-load operating regimes and are supported by both variable-pitch rotors and variable-speed generators. The control objectives, modes of operation and power limitation fundamentals, as stated in the beginning, sketch the framework for introducing some more recent advanced control strategies, consistent with authoritative works in the literature. Among others, the speed-assisted stall regulation concept is assessed, as compared with the well-known passive stall regulation. Some modified speed-assisted pitch-to-feather control approaches aimed at improving the system behaviour around rated wind speed are also presented. This matches some approaches 2, 3, where a third region of constant speed operation, placed just below the rated wind speed, is separately dealt with. The interference between the pitch controller and the generator controller for different operating conditions is also analysed. The third part is oriented to the design and analysis of the WECS controllers using gain-scheduling techniques. Comprising Chapters 5 and 6, this part is dedicated to applying the theory of LPV systems 4 to variable-speed WECS, having fixed pitch and variable pitch, respectively. The gain-scheduling control can thus be systematically designed, while ensuring stability and performance features over the entire operating range. Indeed, this approach allows what the authors call the 'control strategy locus' being unitarily defined as a 3D trajectory in the wind speed—rotational speed—pitch angle space, over the whole domain of wind speed variation. The control objectives are not defined for each operating regime anymore, but there is a single goal expressing the trade-off between tracking the control strategy locus and minimizing the control efforts. A third goal—mitigation of cyclic loads inside the drive train—is separately fulfilled by using a sort of notch filtering (damping injection). The authors note that the power conditioning, essentially consisting in reactive power control, can also be independently achieved. The entire theoretical background and development, based on frequency methods, are more than sufficiently reported in appendices, which suggest important computation effort being necessary. But as long as this effort is performed off-line, this approach is a promising one for real-world application. I found the appendices too extended; the information provided within them can certainly be found in the rich bibliography provided by authors. The reader may find the first two parts easy to read, well explained and application oriented. The third part is well formalized, but the reader might find it difficult to understand some practical aspects concerning the control design. Without an implementation guide, dedicated to a system integrator, the implementation of a certain real application is not immediate. The conclusion of the third part, as well as the general conclusions, is missing, which may lead to difficulties in properly integrating the information contained in the book. Nevertheless, I feel that this book has achieved what the authors set out to do, namely the application of an advanced control method to some complex nonlinear industrial systems, such as wind energy systems. The strength of the book lies in proposing a WECS control approach that ensures unitary performances for the entire operating range by combining variable-speed and pitch-control actions.