IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 16, NO. 3, SEPTEMBER 2001 283 Normalized Power Curves as a Tool for Identification of Optimum Wind Turbine Generator Parameters Suresh H. Jangamshetti, Student Member, IEEE and V. Guruprasada Rau, Senior Member, IEEE Abstract—This paper presents a novel method of matching wind turbine generators to a site using normalized power and capacity factor curves. The site matching is based on identifying optimum turbine speed parameters from turbine performance index curve, which is obtained from the normalized curves, so as to yield higher energy production at higher capacity factor. The wind speeds are parameterized using cubic mean cuberoot and statistically modeled using Weibull probability density function. An expression for normalized power and capacity factor, expressed entirely in normalized rated speed, is derived. Wind Turbine Performance Index, a new ranking parameter, is defined to optimally match turbines to a potential wind site. The plots of normalized power, capacity factor and turbine performance index versus normalized rated wind speed are drawn for a known value of Weibull shape parameter of a site. Usefulness of these normalized curves for identifying optimum wind turbine generator parameters for a site is presented by means of two illustrative case studies. The generalized curves, if used at the planning and development stages of wind power stations, will serve as useful tool to make a judicious choice of a wind turbine generator that yields higher energy at higher capacity factor. Index Terms—Capacity factors, normalized power curves, normalized rated wind speed, turbine performance index, Weibull probability density function, wind turbine generator. I. INTRODUCTION G ENERATION of electrical power by a wind turbine gen- erator system at a specific site depends upon many fac- tors. Significant among them being the mean wind speed of the site and more significantly the speed characteristics of the wind turbine itself, namely cut-in velocity ( ), rated velocity ( ) and furling or cut-out velocity ( ) including the hub height. There is a need for developing a method to identify optimum values of these speed parameters such that the wind turbine de- livers higher average power at higher capacity factor. It has been established that for a realistic assessment of power in the wind, the wind speeds are parameterized using cubic mean cuberoot and statistically modeled using Weibull proba- bility density function [1]. For a given wind regime with known values of Weibull scale parameter and shape parameter , the values of , and can be so selected as to maximize the average power output, and thereby maximize the energy pro- duction. However, the relationships among the wind speed char- acteristics have significant effect on energy production. Of the three speed parameters, rated wind speed, is most important Manuscript received August 13, 1999. This work was financially supported by the Council for Scientific and Industrial Research (CSIR), New Delhi, and All India Council for Technical Education (AICTE), New Delhi. The authors are with the Department of Electrical Engineering, Indian Insti- tute of Technology, Kharagpur, India. Publisher Item Identifier S 0885-8969(01)07335-1. for wind turbine design. If the rated speed is chosen too low, too much energy will be lost in the higher-speed winds. On the con- trary, if the rated speed is too high, the turbine seldom operates at its capacity and will loose too much energy at lower-speed winds. This means that the rated speed has to be selected such that the turbine yields higher energy at higher capacity factors. Usually the cut-in velocity is roughly chosen as half the rated velocity and cut-out velocity as nearly twice the rated speed [2]. However, this is not always strictly followed as can be seen from Table I, which summarizes all the commercially available wind turbines in India. This paper addresses the problem of site matching of wind turbine generators by selecting optimum speed characteristics of wind turbine generators from a set of normalized curves so as to yield higher energy production at higher capacity factor. An expression for normalized power and capacity factor, ex- pressed entirely in terms of normalized rated speed, is derived. Plots of normalized power and capacity factors versus normal- ized rated wind speed are drawn for various values of Weibull shape parameter. Application of these normalized curves for identifying op- timum wind turbine generator parameters for a site is presented by means of two illustrative case studies. In the first case study, an existing wind power station is used to compare the analyt- ical results obtained from the curves with the actual data from the site. In the second case study, a site with no wind turbine in- stallation is used. Purpose of second case study is to show that the generalized curves, if used at the planning and development stages of wind power stations, will serve as useful tool to make a judicious choice of a wind turbine generator that yields higher energy at higher capacity factor. Wind Turbine Performance Index (TPI) is a newly introduced concept in this paper. It is shown that there exists a unique TPI curve for every site from which speed parameters of a turbine that will optimally match a site can be obtained. TPI curve is obtained from normalized power and capacity factor curves and is drawn on the common axis of normalized rated speed. II. NORMALIZED POWER EXPRESSION The average electrical power output of a wind turbine gener- ator system is [1]: (1) where is the rated power output at rated wind speed given by [1]: W (2) 0885–8969/01$10.00 © 2001 IEEE