Analysis of the Efficiency Improvement in Small Wind Turbines when Speed Is Controlled F. Martínez Rodrigo, L.C. Herrero de Lucas, Santiago de Pablo Gómez, J.M. González de la Fuente Departamento de Tecnología Electrónica University of Valladolid Valladolid, Spain Email: fer_mart@tele.uva.es, lcherrer@tele.uva.es, sanpab@eis.uva.es, j_m_gonz@tele.uva.es Abstract— This paper analyzes the efficiency of three small commercial wind turbines, from different manufacturers and with different technology. Their actual energy production is compared with the case where they use turbine speed control to follow the wind speed variations. The simulation uses a Weibull probability distribution for the wind speed. A fixed pitch turbine with permanent magnets generator is simulated to demonstrate the wind turbine efficiency reduction when the speed is not controlled. NOMENCLATURE A rotor area P C power coefficient S I stator current expressed in the stator reference frame S L self-inductance of the stator P number of pole-pairs R rotor radius S R resistance of the stator TSR tip speed ratio S U stator voltage expressed in the stator reference frame V wind speed air density turbine speed R rotor angular speed multiplied by the number of pole pairs F rotor flux I. INTRODUCTION Wind generators may be classified by the presence or not of a system to control the generated power. The control may be done in two ways, by the modification of the blades pitch angle or by changing the rotor speed. Only great power turbines use whatever of them (more than 100 kW). Small wind turbines don’t use, usually, control systems to improve the generated energy when wind speed changes. These turbines work in open loop, and they don’t use the wind speed as feedback. When this kind of turbines work in isolated applications, e.g. battery charging, we must over-scale the application to have enough energy in small wind periods, so, it may not be important to optimize the turbine energy production. But, in the other applications, the best solution is to optimize energy production. Some small wind turbines use passive systems to change the blades pitch angle to follow the wind variations, but it is unusual. Power control systems for small wind turbines are directed to introduce a dc/dc converter in order to modify the generator speed, when it is a permanent magnet generator [1][2][3][4]. When wind speed changes, the control modifies the dc converter voltage to change the generator speed to follow the wind speed. When wind speed is constant, the power extracted from the wind depends on the turbine speed [5]. In Fig. 1, it is represented the mechanical power versus turbine speed for two different wind speeds, V 1 and V 2 , for a fixed pitch turbine. When turbine speed is fixed, the maximum power is produced only for one unique wind speed. For example, if turbine speed is 1 , the power is the maximum when wind speed is V 1 (point A). If wind speed goes to V 2 and turbine speed stays at 1 , then the work point is B’ and the power is not the maximum. That is why, the speed must be increased to 2 (point B). V1 V2 Turbine speed Mechanical power 1 2 A A’ B’ B Fig. 1. Power curves vs. turbine speed The objective of the paper is to show the performance improvement when small wind turbines use speed control systems. Three small turbines, from different manufacturers, are analyzed. The energy production of the commercial turbines is going to be compared, for different wind speeds, with the production of the turbines if they were speed controlled. The paper has the following parts. First, it is demonstrated that a fixed pitch turbine that uses a permanent magnet generator can’t extract the maximum power from every wind speed. Afterwards, some concepts about wind energy and wind 437 1-4244-0755-9/07/$20.00 '2007 IEEE