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Wind turbines: New challenges and advanced control solutions

After more than a decade with a quasi-exponential growing, wind energy has tripled its worldwide generation capacity in the last five years, being once again the fastest growing energy source in the world, with about 100GW installed by 2008. Moreover, the current European and American expectations for the near future suggest a significant upward trend of about 170GW installed by the end of 2010. The currently installed wind power capacity generates 200 TWh per year, equaling 1.3% of the global electricity consumption. The wind industry employs today 350 000 people worldwide. Simultaneously, the average power output of a wind turbine unit has increased significantly, going from 300 kW (30m rotor diameter) some years ago, to 750 kW (48m rotor diameter) by 1999, 1.5MW (70m rotor diameter) by 2003, 2MW (85m rotor diameter) by 2006, till the large 3–6MW range (up to 130m rotor diameter) that most of the major wind turbine manufacturers are developing nowadays. Wind turbines are complex systems, with large flexible structures working under very turbulent and unpredictable environmental conditions, and subject to a variable and demanding electrical grid. The efficiency and reliability of a wind turbine strongly depend on the applied control strategy. Large nonlinear characteristics and high model uncertainty due to the interaction of the aerodynamic, mechanical and electrical subsystems, stability problems, maximization of wind energy conversion matters, load reduction strategies, mechanical fatigue minimization problems, reliability issues, availability aspects, costs per kWh reduction strategies, etc. make it necessary to design powerful control systems to regulate in a coordinated way many variables such as pitch, torque, power, rotor speed, yaw orientation, temperatures, currents, voltages, power factors, etc. Furthermore, increasing level of wind energy penetration in the grid (nowadays wind power accounts for approximately 19% of electricity production in Denmark, 10% in Spain, 6% in Germany, etc.) has revealed new critical challenges and problems: response to grid voltage dips, frequency and active power control loops, voltage and reactive power regulation, restoration of grid services after power outages, wind prediction, etc. They have resulted in an urgent need for new control solutions, combining nonlinear control, adaptive techniques, robust methodologies, load sharing aspects, predictive laws, multivariable control, etc.