W26IEEE power & energy magazine november/december 2003WIND POWER IS THE MOST RAPIDLY GROWING TECHNOLOGY FOR RENEWABLEpower generation. However, fundamental differences exist between conventional thermal,hydro, and nuclear generation and wind power. These differences are reflected in the specificinteraction of wind turbines with the power system. Further, there are differences between thevarious wind turbine types, which also affect their system interaction. In this article, first thecurrent status and the technology of wind power are briefly discussed. The general workingprinciples are explained and the different wind turbine types are described. Then, the differencesbetween wind power and conventional power generation are highlighted as well as their conse-quences for interaction with the power system, both locally and on a system level.Is the Answer Blo© 1998 CORBIS CORP.J.G. Slootweg and W.L. Kling1540-7977/03/$17.00©2003 IEEEnovember/december 2003 IEEE power & energy magazineStatus of Wind PowerAs a result of increasing environmental concern, the impactof conventional electricity generation on the environment isbeing minimized and efforts are beingmade to generate electricity from renew-able sources. One possibility is to usewind turbines that convert the energycontained in flowing air into electricity.During the last decade, the installedwind power capacity has grown rapidly.In Figure 1, the installed capacity in theUnited States, Europe, and the world isdepicted. As can be seen, growth is notevenly distributed throughout the world, with Europeaccounting for the largest part of it. Afurther detailing of the figures wouldshow that, within Europe, growth isconcentrated in a number of countrieswith both a good wind resource andfavorable legislative arrangements. Incountries where these two precondi-tions are not met, hardly any windpower is installed and growth of theinstalled capacity is rather low.During recent years, a substantialscaling up has taken place in thewind power area. This applies bothto the size of the individual turbineand to the scale of the typical proj-ect. For modern wind turbines of themulti-MW class, both the nacelleheight and the rotor diameter are inthe order of 100 m. Thus, at the ver-tical position, the blade tip can reachup to heights of 150 m. The develop-ment of the size and power of newwind turbines introduced on the mar-ket is depicted in Figure 2. Figure 3shows the German Enercon E-112wind turbine, which, with a rotordiameter of 112 m and a nominalpower of 4.5 MW, is currently thelargest wind turbine.Further, in order to use good windlocations effectively and to geographically concentrate thevisual impact of wind turbines at certain locations, a tendencyto group wind turbines in wind parks, or wind farms, can beobserved. Rather than individual turbines or small groups,wind farms with tens or even hundreds of turbines are erect-ed, leading to a substantial increase in the scale of the typicalwind power project. Figure 4 shows the 278.2-MW KingMountain Wind Ranch.In densely populated countries adjacent to shallowwaters, such as many countries in Northwest Europe, con-struction of offshore wind farms is considered a promisingoption. The advantages of offshore wind power are reducedvisibility and noise problems and steadier winds with higheraverage speeds, resulting in a higher energy yield. The disad-vantage is the cost increase when compared to onshore tur-bines, caused by the additional cost of constructing offshoreand the longer distance that must be covered for connectingto the grid. Figure 5 depicts the Utgrunden offshore windfarm for the Swedish coast, which consists of seven 1.5-MWwind turbines.Generating SystemsThe working principle of a wind turbine encompasses twoconversion processes, which are carried out by its main com-ponents: the rotor that extracts kinetic energy from the windand converts it into generator torque and the generator thatconverts this torque into electricity and feeds it into the grid.This general working principle is depicted in Figure 6.Although this sounds rather straightforward, a wind turbine isa complex system in which knowledge from the areas ofaerodynamics and mechanical, electrical, and control engi-neering is applied. Currently, there are three main wind turbine types avail-able. The main differences between these concepts concern27The Current Status of Wind as aRenewable Energy Source and ItsPower System Integration Issueswing in the Wind?the generating system and the way in which the aerodynamicefficiency of the rotor is limited during wind speeds abovethe nominal value in order to prevent overloading.As for the generating system, nearly all wind turbines cur-rently installed use either one of the following systems (seeFigure 7):✔ squirrel-cage induction generator✔ doubly fed (wound rotor) induction generator✔ direct-drive synchronous generator. The first generating system is the oldest one. It consistsof a conventional, directly grid-coupled squirrel-cage induc-tion generator. The slip, and hence the rotor speed, of asquirrel-cage induction generator varies with the amount ofpower generated. The rotor speed variations are, however,small: approximately 1 to 2%. Therefore, this type is nor-mally referred to as a constant speed or fixed speed turbine.It should be mentioned that squirrel-cage induction genera-tors used in wind turbines can often run at two different (butconstant) speeds by changing the number of poles of the sta-tor winding. A squirrel-cage induction gener-ator always consumes reactivepower. In most cases, this is unde-sirable, particularly in the case oflarge turbines and weak grids.Therefore, the reactive power con-sumption of the squirrel-cage induc-tion generator is nearly alwayspartly or fully compensated bycapacitors in order to achieve apower factor close to one.The other two generating sys-tems depicted in Figure 7 are vari-able-speed systems. These are usedin variable-speed turbines. Toenable variable-speed operation, themechanical rotor speed and theelectrical grid frequency must bedecoupled. To this end, power elec-tronics are used. In the doubly fed induction generator, aback-to-back voltage source converter feeds the three-phaserotor winding. In this way, the mechanical and electrical rotorfrequency are decoupled and the electrical stator and rotorfrequency can match, independently of the mechanical rotorspeed. In the direct-drive synchronous generator, the genera-tor is