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A Review on the Development of Wind Turbine Generators

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  Int. J. Dynam. Control (2013) 1:192–202DOI 10.1007/s40435-013-0016-y A review on the development of wind turbine generatorsacross the world N. Goudarzi  ·  W. D. Zhu Received: 23 May 2013 / Accepted: 27 May 2013 / Published online: 20 June 2013© Springer-Verlag Berlin Heidelberg 2013 Abstract  Wind power as a source of green and abundantenergy is proposed as one of the main new world powersourcesandhasacquiredagreatmomentumacrosstheworld.In the last few decades, wind turbines with different gener-ators have been developed to increase the maximum powercapture, minimize the cost, and expand the use of the windturbinesinbothonshoreandoffshoreapplications.Thispaperreviews the development of different types of wind turbinegenerator technologies and discusses advantages and disad-vantages of each type. In addition, a comparison of differentgenerator designs based on the technical data and markettrends is provided. To better understand the development of generator concepts on the market, the market trends of cur-rent large generators with a capacity of 2.5MW and aboveacross the world are evaluated. Keywords  Induction generators  ·  Synchronousgenerators  ·  Permanent magnet generators  · High-temperature-superconducting generators  · Wind power generation  ·  Onshore/offshore wind turbines  · Power electronics  ·  Power quality  ·  Wind farms 1 Introduction Inrecentyears,implementingrenewableenergieshasmoder-ate to strong support across the world. Some types of renew-able energies such as hydropower are considered to be fully N. Goudarzi  ·  W. D. Zhu ( B )Department of Mechanical Engineering, University of Maryland,Baltimore County, Baltimore, MD 21250, USAe-mail: wzhu@umbc.eduN. Goudarzie-mail: navid2@umbc.edu developed, and others such as solar power are limited tospecific regions [1]. Wind power has been used for morethan two thousand years; windmills were capturing windpower since 200  b c using a constant speed rotor assembly[2]. Wind power as a free, abundant, globally available, andgreen energy source is an obvious choice among all renew-able energy sources for generation of electricity [3].Figure1showstheworld’stotalcumulativeinstalledwindpower capacity between 1991 and 2012, and Fig. 2 showsthe shares of five continents in the total installed wind powercapacitybetween2002and2012[1,4–7].Theaverageannual growth in the total installed wind power capacity in the lastten years has been 25% per year, and it is expected thatthe cumulative installed wind power capacity would pass400GWby2015.Itisalsoanticipatedthat12%oftheworld’selectricity consumption will be provided by wind power by2020 [7]. Europehasthemostshareofthetotalinstalledwindpowercapacity since 1997, with an increase from 4.6GW in 1997[1] to more than 102GW in 2012. While Oceania and SouthAmerica have the minimum wind power capacity increasefrom 2002 to 2012, the total installed capacity in Asia hasa remarkable growth from 1.1GW in 1997 [1] to more than100GW in 2012 that will make it the pioneer in the totalinstalled wind power capacity, ahead of Europe in the fol-lowing years.Figure 3 shows the top 12 countries in the world with thelargest wind power capacities among more than 97 countriesthat use wind power on a commercial basis as of 2012 [7–9]. The significant growth in the total installed wind powercapacity in Asia is mainly due to the rapid increase in thewind power capacity in China, which had an increase from5.9GWin2007toalmost76GWin2012,withaconsiderabledifference from the second ranked country, the United Statesof America (USA). While Germany still has the largest wind  1 3  A review on the development of wind turbine generators across the world 193 Fig. 1  World’s cumulativeinstalled wind power capacityduring 1991–2012 Fig. 2  Continents’ shares of wind power capacity during2002–2012 power capacity in Europe, there has been a very low rate of installedwindpowercapacityincreasesince2007,comparedwith China and the USA, and it became the country with thethird largest annual wind power capacity.Awindturbineconvertsthecapturedkineticenergy inthewindtoelectricalenergybymeansofagenerator.Generatorswith more reliable, efficient, and compact designs should beused in wind turbines to maximize the wind power captureand produce a higher quality output power. To determine theappropriate generator designs for onshore and offshore windturbines, different types of wind turbine generators that havebeen studied in the literature are discussed in this paper, withthe criteria based on the speed range, cost, weight, size, andpower quality at the grid connection. Also, the wind turbinegenerators are compared, and advantages and disadvantagesof different designs are summarized with moreextensive cri-teria. Consequently, a generator concept that has a greaterperformance over all other generators can be obtained. Inaddition,toevaluatemarketsharesofdifferentgeneratorcon-ceptsthatareusedinlargecapacitywindturbines,themarketandtechnologytrendsoflargewindturbinegeneratorswithacapacityof2.5MWandaboveacrosstheworldareprovided. 2 Electric machines Electric generators can be classified based on the applicationdomain that implies the power level and principles of oper-ation [10]. This work categorizes generators based on theprinciplesofoperation.Figure4showsvarioustypesofelec-tric machines, ranging from DC electric generators to newstate-of-the-art high-temperature-superconducting (HTS)generators [10–12]. The main focus of this paper is on the AC poly-phase group, which is the main concept used in thewind power industry.  1 3  194 N. Goudarzi, W. D. Zhu Fig. 3  Top 12 countries withthe largest annual wind powercapacities during 2007–2012 Fig. 4  Electric machine diagram 2.1 Induction generator 2.1.1 Squirrel-cage induction generator (SCIG) A SCIG has been the most popular generator type of fixedspeed stall control wind turbines for a long time [13]. TheadvantagesofaSCIGareinexpensivemassproductionofthegenerators with a robust and easy technology and its directconnectiontothegrid[13–15].However,thespeedofaSCIG has a limited range of variation and is not controllable. TheSCIG has a multiple-stage gearbox, and it always consumesreactive power that is not controllable [13]. 2.1.2 Doubly-fed induction generator (DFIG) Theincreaseinthecapturedpowerlevel,thegridrequirementfor good power quality, and the requirement for reducingthe noise level and the mechanical stress of the drivetrainresulted in initiating the variable speed operation in the late1990’s and introducing DFIGs [2]. The operation principle  1 3  A review on the development of wind turbine generators across the world 195 of a DFIG is the same as that of a SCIG, but a DFIG rotorhas a three-phase winding instead of a squirrel-cage rotorin the SCIG. The DFIG has a sufficient range of variablespeed by controlling the active power flow direction, and thereactive power can be controlled by the rotor current in theconverter [14]. Brushes and the multiple-stage gearbox in aDFIGrequireregularmaintenanceandhaveapotentialcauseofmachinefailure.Itsoutputisconnectedtothegridthroughexpensive power electronic converters that are very sensitiveto over currents. In addition, the control strategies in case of grid disturbances are very complex [14,16,17]. 2.2 Synchronous generator (SG)SGswithnewstate-of-the-artdesignsarecomingbacktothemarketandbecomeacompetitivedesignforinductiongener-ators[18–20].FixedspeedSGshavebeenusedinsomewind turbines [21]. Multi-phase SGs with different designs, suchas DC current excited rotors, claw-pole electrically excitedrotors, permanent magnet (PM) rotors, and variable reluc-tance rotors, have been developed [10]. SGs with a DC cur- rent excited rotor and a PM rotor are described here.Variable speed gearless wind turbines usually use lowspeed,hightorqueSGs.Therotorofadirectdrive(DD)gen-eratorisdirectlyconnectedtothehuboftherotorblades[15].Thus, the DD generator rotates at a lower rotational speedthat makes it necessary to produce a higher torque comparedwith a regular generator connected to a gearbox system. Toeliminate the need for a gearbox, a DD machine has beenmade with a large number of poles to control the generatorspeed over a wide range, even at very low speeds, whichresults in a heavier and larger diameter generator. However,themanufacturingcostandthegeneratednoiseandpollutionarereduced,andtherequiredregularmaintenancebesidesthepotential cause of mechanical failure of the gearbox is elimi-nated[22–29].Thus,theDDmachinebecomesacompetitive solutiondespiteahigherstart-upcost.Notethateitheramin-imum size atthe rated power factor withthe given efficiency,or a maximum efficiency at the rated power factor with thegiven size are desired in designing a generator with givenmechanical specifications [30]. DD SGs are usually equipped with a three-phase windingon the stator; they have either a rotor winding supplied by aDC current from a separate circuit, which are called electri-callyexcited(orwoundfield)SGs(EESGs),orPMsattachedto the rotor, which are called PM SGs (PMSGs). 2.2.1 EESG TheEESGwasfirstintroducedwitha500kWvariablespeedDD wind turbine by Enercon of Germany in 1992 [31]. TheEESG stator is the same as that of the induction machinecarrying a three-phase winding; its rotor may have salientpoles that are usually used in low speed machines or maybe cylindrical [32]. The amplitude and frequency of the volt-age at the generator side of an EESG converter can be fullycontrolled by the converter, independently of the grid char-acteristics [14]. Compared with a PMSG, an EESG does notrequire PMs; it will reduce a large fraction of the generatorcost and makes it the most used generator type of large DDwind turbines [14]. One drawback of an EESG is the neces-sity of exciting its rotor winding by a DC current sourceusing brushes and slip rings or a rotating rectifier withoutusing brushes. Expensive power electronic components andthe requirement of intensive cooling are among other draw-backs of an EESG. An EESG usually has a large rotor diam-eter. For instance, the Enercon E-126 wind turbine, which isthe largest onshore wind turbine (7.58MW) built to date, hasarotordiameterof126m.Theverylargediametermakesthetransportation and installation difficult. 2.2.2 PMSG The most promising PM machine category deals with DDwind turbines [33]. General Electric (GE), Zephyros, Mit-subishi, and Siemens are among manufacturers that imple-ment PMSGs in their wind turbine designs. The rotor in aPMSG is replaced with PMs; the excitation is provided byPMsinsteadofthefieldwinding,butthestatorwindingisthesameasthatofathree-phaseEESG[28].PMSGshaveseveral advantages over EESGs, including smaller rotor diameters,higher reliability due to elimination of different mechanicalcomponents such as slip rings,better thermal characteristics,and lower mass per kilowatt output power [12,31,34,35]. Lower weight in PMSGs is achieved by such modificationsas eliminating the gearbox, and providing a larger airgapthat reduces flux linkage and results in a smaller generatorsizewithrespecttothewindturbinepowerrating[31].Other advantagesofDDPMSGsarediscussedintheliterature[23–25,28,36–38]. High cost of PMs, manufacturing and assem- bling, and demagnetization of PMs at high temperatures areamong main drawbacks of a PMSG [12,31,33,39,40]. In recent years, improvements in the PM performance and thecost reduction have made them more attractive than before.Inthecase ofaconverter faultinaPMmachine, themachinecounterelectromagneticforcefeedsthefaultandcausesdan-geroustorquepulsation[41].Tomaintainthetransientstabil-ity of a PMSG and avoid the risk of wind turbine mechanicalrunaway during fault conditions, more robust control strate-gies are recommended [12].Figure 5 shows the design options for PMSGs used in DDwind turbines, which can be categorized into three groups:axial flux (disk type), transverse flux, and radial flux (drumtype) [12,14]. The radial flux design is usually used for DD PMSGs, transverse flux machines are not available on themarket, and the axial flux design has only been used in low  1 3

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