Advanced Control of Wind Turbines

Abstract

Wind energy technology has experienced huge progress during the last decade. This was encouraged by the need to develop ambient friendly clean and renewable forms of energy and the continuously rise of oil price. Sophisticated designs of wind turbines were performed. Large-size wind turbine farms are nowadays producing electricity at great scale throughout the world. Wind energy represents actually the most growing renewable energy; the rate of growth reaches actually 30% in Europe. The cost of wind energy was not always cheaper than that of the other energy resources if the impact on environment and the risk linked to the classical forms of energy is not considered. The cost has however experienced a regular drop since the early 1970s. Cost reduction continues to constitute a main concern in the field of wind energy and research and development programs are considering it as a top priority. The objective is to extract optimal electric energy from wind with high quality specifications and with reduced installation and servicing expenses (Ackermann & Soder, 2002; Gardner et al., 2003; Sahin, 2004; AWEA, 2005). Among the most important issues that allow to deal with cost reduction and its stabilisation in the field of wind turbines, one finds controller design for these installations. The objective is to deserve better use of the available energy in wind by providing through intelligent control its optimal extraction. Three main goals are generally pursued during designing of wind turbine controllers. The first one is to optimize use of the wind turbine capacity (optimal extraction of electric energy from the kinetic energy contained in the incident wind). The second is to alleviate mechanical loads in order to increase life of wind turbine components (fatigue loads should be reduced during operation). The third one is to improve power quality to approach the habitual performances met in the classical forms of energy, this is to assess compatibility of wind energy with the common standards about consumption of electricity (Ackermann, 2005). Control must take into account variability of wind resource and should also cope with the intermittent nature of wind energy. The idea is to exploit optimally the wind resource when it is available and to limit overloading at risky high wind speeds. For this raison modern wind turbines are variable speed. They function by seeking optimal orientation of the wind turbine rotor and by pitching the blades to limit the captured energy from wind when wind speed exceeds the cut-off limit. Knowing that if the electric generator is directly connected to the grid, then only one rotational speed can be used in order to synchronize with the grid frequency, modern wind