Motivated by the increasing concerns over environmental challenges such as global warming and exhaustion of fossil-fuel reserves, the renewable energy industry has become the most sought-after source of electrical energy production worldwide. In this context, wind energy conversion systems (WECS) are one of the most dominant and fastest-growing technologies, playing an increasingly vital role in renewable power generation. To meet this growing demand and mitigate the vulnerability of WECS to various sets of internal/external faults, the cost-effectiveness and efficient power production of WECS must be ensured, which highlights the critical role of the control system. This topic has been intensively studied in the literature, and many control approaches have been developed to deal with the simultaneous enhancement of efficiency and reliability of WECS. However, sliding mode control (SMC) has proved its reliable and superior performance among most control strategies due to its inherent robustness to parametric uncertainties and disturbances and ease of design and implementation. Accordingly, this paper provides a comprehensive survey of existing literature on the application of SMC and its emerging modifications to address different control design problems for WECS. 

Sliding mode control of wind energy conversion systems: Trends and applications

Floating offshore wind growth places the industry development close to commercial scale. However, the harsh environment with strong winds, waves, and currents, together with the increasing size of wind turbines and floater motions could affect power production and reduce system lifetime due to fatigue loads. In any case, operating the wind turbine close to the optimum efficiency value guaranteeing its reliability through fatigue load reduction could be achieved with a proper control strategy. This paper proposes a critical review of the state-of-art of floating wind turbine control technologies, discussing the advantages and drawbacks of the most used control algorithms and classifying them, to summarize the future trends of the research. 

Review of control technologies for floating offshore wind turbines

This paper investigates the effects power oscillation damping (POD) controller could have on a wind turbine structural system. Most of the published work in this area has been done using relatively simple aerodynamic and structural models of a wind turbine which cannot be used to investigate the detailed interactions between electrical and mechanical components of the wind turbine. Therefore, a detailed model that combines electrical, structural and aerodynamic characteristics of a grid-connected Doubly Fed Induction Generator (DFIG) based wind turbine has been developed by adapting the NREL (National Renewable Energy Laboratory) 5 MW wind turbine model within FAST (Fatigue, Aerodynamics, Structures, and Turbulence) code. This detailed model is used to evaluate the effects of POD controller on the wind turbine system. The results appear to indicate that the effects of POD control on the WT structural system are comparable or less significant as those caused by wind speed variations. Furthermore, the results also reveal that the effects of a transient three-phase short circuit fault on the WT structural system are much larger than those caused by the POD controller.

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Effects of POD Control on a DFIG Wind Turbine Structural System

Wind energy takes an important role in the transformation of the global energy system towards clean and sustainable sources. The main development of wind energy technology in recent decades is the growth of wind turbine size motivated by economic factors. The larger turbine size helps increase power output and energy efficiency, however, it leads to challenges in wind turbine operation and maintenance. To further reduce the cost of wind energy, advanced control approaches are developed focusing on power maximization, structural load mitigation, lifetime extension, and reliability improvement. This multi-objective problem is difficult to solve due to design conflicts. The optimal trade-off between goals is varying and depends on actual operating situations such as on-site wind characteristics, system aging, and grid requirements. Modern utility-scale wind turbines are equipped with numerous sensors providing useful information about turbine components’ operation status. With the development of computation capability and big data analytics techniques, the turbine performance and state-of-health (SoH) information could be obtained and evaluated through historical logged data using Prognostics and Health Management (PHM) systems. This information aids the optimal operation and maintenance of wind energy systems. The health state of a system has significant effects on its performance, reliability, and remaining useful life. So it is crucial to consider SoH when designing controllers for optimal operations. In recent years, the integration of SoH information into the closed-loop control system has begun to attract the attention of the wind energy researcher community. Controllers have been adapted based on current and future aging behaviors optimizing the trade-off between service life expansion and power production maximization. This paper provides a review of integrated prognostics and health management control (IPHMC) systems for the optimal operation and maintenance of wind turbines and wind farms reducing the cost of wind energy. The review focuses on the combination of real-time PHM and advanced control for wind turbines. The most recent developments, generalization, classification, and comparison of IPHMC approaches for wind energy systems are given. Integrated PHM control concept has the potential to improve the reliability of wind turbines, however, further research on real-time RUL prognostic and reliability evaluation techniques is required for the effective implementation of the concept.

State-of-the-art in integrated prognostics and health management control for utility-scale wind turbines

Modelling of anisotropic beam for rotating composite wind turbine blade by using finite-difference time-domain (FDTD) method

On-line fatigue alleviation for wind turbines by a robust control approach

This work is part of a research project funded by the Italian Ministry of the University and Research (MIUR), under the call for "National Interest Research Projects 2015 (PRIN 2015)", titled "Smart Optimized Fault Tolerant WIND turbines (SOFTWIND)". Within this project, the research unit of the University of Perugia (UniPG) aims to develop dynamic modeling and simulation methodologies and fatigue behavior evaluation ones for wind turbine as a whole. The development of these methodologies will be aimed at predicting the life of generic wind turbines, also providing important and fundamental parameters for optimizing their control, aimed at reducing the failures of these machines. In the present paper, a small turbine, developed at the Department of Engineering of the University of Perugia, will be analyzed. The multibody modeling technique adopted and the experimental activity conducted in the wind tunnel of UniPG, needed for the tuning of the model, will be described. The analysis of both model behavior and experimental data has allowed for the definition of a robust multibody modeling technique that adopts a freeware code (NREL - FAST), universally considered to be a reference in this field. The goodness of the model guarantees the capabilities of the simulation environment to analyze the real load scenario and the fatigue behavior of this kind of device.

Dynamic modeling of wind turbines. Experimental tuning of a multibody model

The evaluation of fatigue behavior of wind turbines, that is of supporting structures, blades or gear boxes, is always performed off-line, by post processing experimental acquisitions or simulation results. Moreover, the evaluation of potentiality of smart controls, that have the aim to avoid failures by reducing loads and consequently fatigue stresses, is performed in the same way. In this paper is presented a tool that allows to on-line evaluate and foresight fatigue potential damage by simply on time processing reference signals such as tower top acceleration (typical experimental acquisition) or tower base bending moment (typical numerical measure). This evaluation technique is converted into a well know numerical code, oriented to control systems (Simulink), to be used into multibody simulation by co-simulation approach. This step allowed to verify its capabilities and the possibility to realize its physical prototype and to use its results as input variable for active control strategies oriented to minimize damage. As test case a standard 5 MW wind turbine and a classical control logic were used.

Dynamic behavior of wind turbines. An on-board evaluation technique to monitor fatigue

This work is part of a research activity inserted into “Smart Optimazed Fault Tolerant WIND Turbines (SOFTWIND)” project of PRIN 2015, funded by the Italian Ministry of the University and Research (MIUR). The need to define a robust multibody modelling procedure to realistically characterize the dynamical behavior of a generic wind turbine and to have a reduced computational burden has pushed the authors to adopt a freeware software called Nrel-FAST, that is universally considered to be a reference in the field of aeroelastic wind turbine simulations. The lightness of this software is paid in terms of modelling simplicity, which makes the modelling of wind turbines with unconventional support structures (i.e. that con not directly outlined as a fixed-beam) difficult. In this paper, some methodologies to overcome this obstacle are presented, including the use of a more powerful multibody software which, on the other hand, entails higher simulation times. In particular, the authors present a methodology based on structure stiffness-matrix reconstruction that allows, under appropriate hypothesis, to reduce a complex wind turbine support frame to a simple fixed beam so that the simulations can be done directly in FAST environment, with low computational times. The results obtained from these different approaches are compared using as test-case a small wind turbine property of University of Perugia (UniPG).

A. Cetrini

Papers

On-line fatigue alleviation for wind turbines by a robust control approach

Dynamic modeling of wind turbines. Experimental tuning of a multibody model

Dynamic behavior of wind turbines. An on-board evaluation technique to monitor fatigue

Dynamic modeling of wind turbines. How to model flexibility into multibody modelling