overview of different wind generators

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Published in IET Renewable Power GenerationReceived on 24th January 2007Revised on 23rd August 2007doi: 10.1049/iet-rpg:20070044

ISSN 1752-1416

Overview of different wind generator systemsand their comparisonsH. Li* Z. ChenInstitute of Energy Technology, Aalborg University, Aalborg East DK-9220, Denmark*H. Li is also with the College of Electrical Engineering, Chongqing University, Chongqing 400044, People’s Republic of ChinaE-mail: [email protected]

Abstract: With rapid development of wind power technologies and significant growth of wind power capacityinstalled worldwide, various wind turbine concepts have been developed. The wind energy conversion system isdemanded to be more cost-competitive, so that comparisons of different wind generator systems are necessary.An overview of different wind generator systems and their comparisons are presented. First, the contemporarywind turbines are classified with respect to both their control features and drive train types, and their strengthsand weaknesses are described. The promising permanent magnet generator types are also investigated. Then,the quantitative comparison and market penetration of different wind generator systems are presented. Finally,the developing trends of wind generator systems and appropriate comparison criteria are discussed. It is shownthat variable speed concepts with power electronics will continue to dominate and be very promisingtechnologies for large wind farms. The future success of different wind turbine concepts may stronglydepend on their ability of complying with both market expectations and the requirements of grid utilitycompanies.

1 IntroductionWind energy is the world’s fastest growing renewableenergy source. The average annual growth rate of windturbine installation is around 30% during last 10 years[1, 2]. At the end of 2006, the global wind electricity-generating capacity increased to 74 223 MW from59 091 MW in 2005 (Fig. 1). By the end of 2020, it isexpected that this figure will have increased to well over1 260 000 MW, which will be sufficient for 12% of theworld’s electricity consumption [3, 4]. Fig. 2 depicts thetotal wind power installed capacity for some countriesfrom 1985 to 2006. The countries with the highest totalinstalled capacity are Germany (20 622 MW), Spain(11 615 MW), the USA (11 603 MW), India(6270 MW) and Denmark (3136 MW). According toglobal wind energy council report [2], Europe continuesto lead the market with 48 545 MW of installedcapacity at the end of 2006, representing 65% of theglobal total, and the European wind energy associationhas set a target of satisfying 23% European electricityneeds with wind energy by 2030. It is clear that the

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global market for the electrical power produced bywind turbine generators has been increasing steadily,which directly pushes the wind technology into a morecompetitive area.

The development of modern wind power conversiontechnology has been going on since 1970s, and the rapiddevelopment has been seen from 1990s. Various windturbine concepts have been developed and differentwind generators have been built. Three types oftypical generator systems for large wind turbines exist[3, 5–7]. The first type is a fixed-speed wind turbinesystem using a multi-stage gearbox and a standardsquirrel-cage induction generator (SCIG), directlyconnected to the grid. The second type is a variablespeed wind turbine system with a multi-stage gearboxand a doubly fed induction generator (DFIG), wherethe power electronic converter feeding the rotorwinding has a power rating of �30% of the generatorcapacity; the stator winding of the DFIG is directlyconnected to the grid. The third type is also a variablespeed wind turbine, but it is a gearless wind

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turbine system with a direct-drive generator, normally alow-speed high-torque synchronous generator and afull-scale power electronic converter are used.Additionally, a variety of innovative concepts of windturbines appear, for example, an interesting alternativemay be a mixed solution with a gearbox and a smallerlow speed permanent magnet synchronous generator(PMSG) [7–9], because direct-drive wind generatorsare becoming larger and even more expensive forincreasing power levels and decreasing rotor speeds.

The main aim of this paper is to provide an overallperspective on various types of existing wind generatorsystems and possible generator configurations, andsome comparisons of different wind generator systemsin literatures and in the market. The paper is organisedas follows. First, it gives an overview of various windturbine concepts with respect to both their controlability and drive train types, including possible types ofdirect-drive permanent magnet (PM) machines. Thenthe quantitative comparisons of different windgenerator systems based on some available technicaldata from literatures are presented, including theirmarket penetration and share. Finally, the trends and

Figure 1 World cumulative wind power installed capacity(1980–2006)

Figure 2 Total cumulative wind power installed capacity fordifferent countries (1980–2006)

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developments of wind generator systems are presented,and suitable comparison criteria of different windgenerator systems are also discussed.

2 Wind turbine concepts andgenerator typesReferring to the rotation speed, wind turbine conceptscan be classified into fixed speed, limited variablespeed and variable speed. For variable speed windturbines, based on the rating of power converterrelated to the generator capacity, they can be furtherclassified into wind generator systems with a partial-scale and a full-scale power electronic converter. Inaddition, considering the drive train components, thewind turbine concepts can be classified into geared-drive and direct-drive wind turbines. In geared-drivewind turbines, one conventional configuration is amultiple-stage gear with a high-speed generator; theother one is the multibrid concept which has a single-stage gear and a low-speed generator [8]. In thissection, according to contemporary wind turbineconcepts, the basic configurations and characteristicsof different wind generator systems are described.

2.1 Fixed speed concept

The fixed speed wind generator systems have been usedwith a multiple-stage gearbox and a SCIG directlyconnected to the grid through a transformer asillustrated in Fig. 3. Because the SCIG operates only in anarrow range around the synchronous speed, the windturbine equipped with this type of generator is oftencalled the fixed-speed wind generator system. This isthe conventional concept applied by many Danish windturbine manufacturers during the 1980s and 1990s, thatis, an upwind, stall-regulated, three-bladed wind turbineconcept using an SCIG [3, 10], so that it is also referredto as ‘Danish concept’. Since the SCIG always drawsreactive power from the grid, during the 1980s thisconcept was extended with a capacitor bank for reactivepower compensation. Smoother grid connection wasalso achieved by incorporating a soft-starter.Furthermore, a pole-changeable SCIG has been used,which leads two rotation speeds. Some manufacturers,such as Micon (currently merged into Vestas), Bonus(currently Siemens), Made and Nordex, have productsbased on this concept.

Figure 3 Scheme of a fixed speed concept with SCIG system

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The well-known advantages of SCIG are it is robust,easy and relatively cheap for mass production. Inaddition, it enables stall-regulated machines to operateat a constant speed when it is connected to a largegrid, which provides a stable control frequency.Although the stall control method is usually used incombination with the fixed speed SCIG for powercontrol, the active stall control or pitch control havealso been applied.

The disadvantages of SCIG for the fixed speed windturbine concept are as follows [3, 10–15].

† The speed is not controllable and variable only overa very narrow range, in which only speeds higher thanthe synchronous speed are possible for generatoroperation. Because a higher slip means a higherdissipation of electrical energy in the rotor bars, forexample, the slip is normally not higher than 1% for1 MW wind turbine [11]. Additionally, the fixedspeed concept means that wind speed fluctuations aredirectly translated into electromechanical torquevariations, this causes high mechanical and fatiguestresses on the system (turbine blades, gearbox andgenerator) and may result in swing oscillationsbetween turbine and generator shaft. Also theperiodical torque dips because of the tower shadowand shear effect are not damped by speed variationsand result in higher flicker. Furthermore, the turbinespeed cannot be adjusted with the wind speed tooptimise the aerodynamic efficiency. Although apole-changeable SCIG has been used in somecommercial wind turbines, it does not providecontinuous speed variations.

† A three-stage gearbox in the drive train is necessaryfor this wind turbine concept. Gearboxes represent alarge mass in the nacelle, and also a large fraction ofthe investment costs.

† It is necessary to obtain the excitation current fromthe stator terminal of SCIG. This makes it impossibleto support grid voltage control. In most cases,capacitors are connected in parallel to the generatorto compensate for the reactive power consumption.

2.2 Limited variable speed concept

The limited variable speed concept with a multiple-stagegearbox is also known as the Optislip concept, which hasbeen applied by the Danish manufacturer Vestas sincethe mid 1990s [10, 11]. This wind turbine conceptuses a wound rotor induction generator (WRIG) withvariable rotor resistance by means of a powerelectronic converter and the pitch control method, asshown in Fig. 4. Presently, manufacturers of Vestasand Suzlon have products based on this concept.



The stator of WRIG is directly connected to the grid,whereas the rotor winding is connected in series with acontrolled resistor. Variable-speed operation can beachieved by controlling the energy extracted from theWRIG rotor; however, this power must be dissipatedin the external resistor. With the increase in variablespeed range, a higher slip means a high powerextracted by the rotor, and the lower generatorefficiency, so that the rating of the resistor must alsobe higher. Therefore the dynamic speed control rangedepends on the size of the variable rotor resistance,and the energy extracted from the external resistor isalso dumped as heat loss in the controllable rotorresistance. A typical limited variable speed range isless than 10% above the synchronous speed [3, 10,12]. Additionally, the slip rings may be avoided, forexample, the wind turbine manufacturer Vestas builtthe power converter and resistor on the rotor,the control signals are transmitted to the rotatingelectronics by an optical coupling. Furthermore,reactive power compensation and a soft-starter arealso required for this concept.

2.3 Variable speed concept with apartial-scale power converter

This configuration is known as the DFIG concept, whichcorresponds to a variable speed wind turbine with aWRIG and a partial-scale power converter on therotor circuit, as illustrated in Fig. 5. The stator isdirectly connected to the grid, whereas the rotor isconnected through a power electronic converter. Thepower converter controls the rotor frequency and thusthe rotor speed. This concept supports a wide speedrange operation, depending on the size of thefrequency converter. Typically, the variable speedrange is +30% around the synchronous speed [3,10–13]. The rating of the power electronic converteris only 25–30% of the generator capacity, whichmakes this concept attractive and popular from aneconomic point of view. There are manymanufacturers, such as Vestas, Gamesa, Repower,

Figure 5 Scheme of a variable speed concept with DFIGsystem

Figure 4 Scheme of a limited variable speed concept withWRIG system (Optislip)

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Nordex, using this concept in the market. The largestcapacity for the commercial wind turbine productwith DFIG has been up to 5 MW from Repower.

Compared with the Optislip concept, the rotorenergy, instead of being dissipated, can be fed into thegrid by the power electronic converter. Moreover, thepower converter system can perform reactive powercompensation and smooth grid connection, forexample, the grid-side converter can control itsreactive power, independently of the generatoroperation; this allows the performance of voltagesupport towards the grid. However, the DFIG systemhas the following disadvantages [3, 10, 12, 15].

† A multi-stage gearbox is still necessary in the drivetrain because the speed range for DFIG is far from acommon turbine speed of 10–25 rpm. A gearbox isinevitable to have some drawbacks, such as heatdissipation from friction, regular maintenance andaudible noise.

† The slip ring is used to transfer the rotor power bymeans of a partial-scale converter, which requires aregular maintenance, and maybe result in machinefailures and electrical losses.

† Under grid fault conditions, on the one hand, largestator currents result in large rotor currents, so thatthe power electronic converter needs to be protectedfrom destroy; on the other hand, large stator peakcurrents may cause high torque loads on the drivetrain of wind turbines.

† According to grid connection requirements for windturbines, in case of grid disturbances, a ride-throughcapability of DFIG is also required, so that thecorresponding control strategies may be complicated.

2.4 Variable speed direct-drive conceptwith a full-scale power converter

This configuration may correspond to a variable speedwind turbine with a direct-drive generator connectedto the grid through a full-scale power converter. Themost important difference between geared drive windturbines and direct-drive types is the generator rotorspeed. The direct-drive generator rotates at a lowspeed, because the generator rotor is directlyconnected on the hub of the turbine rotor. To delivera certain power, the lower speed makes it necessary toproduce a higher torque. A higher torque means alarger size of the generator. Therefore for direct-drivegenerators, the low speed and high torque operationrequire multi-poles, which demand a larger diameterfor implementation of large number of poles with areasonable pitch. Moreover, for a larger direct-drivegenerator, considering on the current loading and gap



flux density limitations, a higher torque also requiresa larger machine’s volume, so that the torque densitycould not be further significantly increased. Toincrease the efficiency, to reduce the weight of theactive parts and to keep the end winding losses small,direct-drive generators are usually designed with alarge diameter and small pole pitch [16, 17]. Inaddition, the advantages of direct-drive wind turbinesare the simplified drive train, the high overallefficiency, the high reliability and availability byomitting the gearbox.

Compared with the variable speed concept with apartial-scale power converter, the full-scale powerconverter can perform smooth grid connection overthe entire speed range. However, it has a higher costand a higher power loss in the power electronics,since all the generated power has to pass through thepower converter.

Basically, types of direct-drive generators used in themarket can be classified into the electrically excitedsynchronous generator (EESG) and the PMSG. Themain features of EESG are described in Section 2.4.1.The features of different topologies of PMSG arepresented in Section 2.4.2.

2.4.1 Electrically excited synchronous generator:The EESG is usually built with a rotor carrying thefield system provided with a DC excitation. Thestator carries a three-phase winding quite similar tothat of the induction machine. The rotor may havesalient poles or may be cylindrical. Salient poles aremore usual in low-speed machines and may be themost useful version for application to direct-drivewind turbines. A grid connection scheme of EESG fordirect-drive wind turbines is shown in Fig. 6. Theamplitude and frequency of the voltage can befully controlled by the power electronic at thegenerator side, so that the generator speed is fullycontrollable over a wide range, even to very lowspeeds. In addition, the EESG has the opportunities ofcontrolling the flux for a minimised loss in differentpower ranges, because the excitation current can becontrolled by means of the power converter in therotor side. Moreover, it does not require the use ofPMs, which would represent a large fraction of thegenerator costs, and might suffer from performanceloss in harsh atmospheric conditions. Therefore it isthe mostly used direct-drive generator type in the

Figure 6 Scheme of a direct-drive EESG system



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current market [14]. The typical manufacturer isEnercon, the largest capacity of the direct-drive EESGhas been up to 4.5 MW (E-112).

Including the disadvantages of direct-drive windturbines compared with geared-drive wind turbines,some disadvantages of direct-drive EESG systems canbe summarised as follows [16, 17].

† In order to arrange space for excitation windings andpole shoes, the pole pitch has to be large enough for thelarge diameter-specific design, so a larger number ofparts and windings probably make it a heavy weightand expensive solution.

† It is necessary to excite the rotor winding with DC,using slip rings and brushes, or brushless exciter,employing a rotating rectifier, and the field losses areinevitable.

2.4.2 PM synchronous generator: The scheme of agrid-connected PMSG for direct-drive wind turbines isshown in Fig. 7.

The advantages of PM machines over electricallyexcited machines can be summarised as followsaccording to literatures [16–23]:

† higher efficiency and energy yield,

† no additional power supply for the magnet fieldexcitation,

† improvement in the thermal characteristics of thePM machine due to the absence of the field losses,

† higher reliability due to the absence of mechanicalcomponents such as slip rings,

† lighter and therefore higher power to weight ratio.

However, PM machines have some disadvantages,which can be summarised as follows:

† high cost of PM material,

† difficulties to handle in manufacture,

† demagnetisation of PM at high temperature.

Figure 7 Scheme of a direct-drive PMSG system



In recent years, the use of PMs is more attractive thanbefore, because the performance of PMs is improvingand the cost of PM is decreasing. The trends makePM machines with a full-scale power converter moreattractive for direct-drive wind turbines. Currently,Zephyros (currently Harakosan) and Mitsubishi areusing this concept in 2 MW wind turbines in themarket.

PM machines are not standard off-the-shelf machinesand they allow a great deal of flexibility in theirgeometry, so that various topologies may be used. PMmachines can be classified into the following types:radial flux, the axial flux and the transversal flux,based on the direction of flux penetration. Some basicstructures and features from literatures [16–28] arebriefly described and summarised as follows.

Radial-flux PM machines: The PMs of radial-flux machinesare radically oriented. When using radial-fluxPM (RFPM) machines for direct-drive wind turbines,the wind generator system can operate with agood performance over a wide range of speeds. Inmanufacture, the simple way of constructing themachine with high number of poles is gluing PMs onthe rotor surface. In RFPM machines, the length ofthe machine and the air-gap diameter can be chosenindependently. If necessary, the radial-flux machinecan be made with a small diameter by using a longmachine. RFPM machines have advantages as a bettertorque density than the EESG, so that some types ofRFPM machines have been discussed in a number ofliteratures.

Two types of RFPM machines, the slotted surface-mounted PM machine and the slotted flux-concentrating PM machine, have been mostly discussedin references [16, 23]. One rotor design with surface-mounted magnets and one rotor design with fluxconcentration are shown in Fig. 8. Compared with theflux concentration, magnets on the rotor surface haveto have a remanent flux density higher than therequired air-gap flux density, this leads to a very simplerotor design with a low weight.

References [16, 17, 20, 27] discussed RFPM machineswith surface-mounted magnet, which seems to be a goodchoice for the design of large-scale direct-drive windturbines [16, 17]. RFPM machines with fluxconcentration have been discussed and compared withsurface-mounted RFPM machines in [26, 29]. Inaddition, Chen et al. [20] have presented an outer rotordesign for this type of generator in stand-aloneapplications. Several advantages of the outer-rotorRFPM machine were identified in this reference; forexample, compared with the inner-rotor construction,the multi-pole structure can be easily accommodatedbecause of the enlarged periphery of the outer-rotor

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Figure 8 Basic configurations of RFPM machines

a Surface mountedb Flux concentrating

drum, and therefore the total length of the magnetic pathcan be reduced. As the rotor is directly exposed to thewind, the cooling condition can be improved for themagnets so that the resistance to temperaturedemagnetisation is enhanced. Moreover, Chen et al. [19]have also made a comparison of different PM windgenerator topologies. In addition, Hanitsch and Korouji[21] have designed a rare-earth RFPM wind-energygenerator with a new topology, which is constructedfrom two rotors and one stator with short end windings.It can improve the performance of the machine byreducing the weight, increasing the efficiency andreducing the cost of active materials.

Axial-flux PM machines: The axial-flux PM (AFPM)machine is a machine producing magnetic flux in theaxial direction, instead of the radial direction. Twotypes of AFPM machines, the slotless and slottedsurface-mounted PM, have been mostly discussed inreferences. Compared with RFPM machines, theadvantages of AFPM machines can be summarised asfollows:

† simple winding,

† low cogging torque and noise (in slotless machine),

† short axial length,

† higher torque/volume ratio.

However, the disadvantages of AFPM machines incomparison with RFPM machines are as follows[23, 27]:

† lower torque/mass ratio

† larger outer diameter, large amount of PM andstructural instability (in slotless machine)

† difficulty to maintain air gap in large diameter (inslotted machine)

† difficulty in production of stator core (in slottedmachine).



The possibility and potential of AFPM machines forlarge-scale direct-drive wind turbines have beendiscussed, and some different structures of AFPMmachines with surface-mounted PM have also beenpresented in some references [19, 22, 26].

The slotless single stator double rotor is a typicalstructure of slotless AFPM machines, which is oftenreferred to as a Torus machine, as shown in Fig. 9[22]. The two rotor discs are made of mild steel andhave surface-mounted PM to produce an axiallydirected magnetic field in the machine air gaps. Themachine stator comprises a slotless toroidally woundstrip-iron core that carries a three-phase winding in atoroidal fashion by means of concentrated coils.The slotless, toroidal-stator AFPM generator has beenalso discussed with several advantages, such asthe lightness, the compactness, the short axial length,the suitable integration with the engine and othersby Spooner and Chalmers [30] and Wu et al.[31, 32], the machine’s short axial length tends to giveit a high power to weight ratio. Parviainen [26] haspresented an analytical method to perform thepreliminary design of a surface-mounted, low-speed,slotted AFPM machine with one-rotor two-statorconfiguration, as shown in Fig. 10. The performanceand construction between the low-speed radial fluxand axial flux PM machine were also compared in thepower range from 10 to 500 kW at 150–600 rpm [26].

Moreover, five different topologies of AFPMmachines, a double-stator slotted type, a double-rotorslotted type, a single-sided AFPM with stator balance,

Figure 9 Slotless single-stator double-rotor AFPMconfiguration [22]



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a single-sided AFPM with rotor balance and a slotlesssingle-stator double-rotor (Torus machine), have beeninvestigated and compared with RFPM machines byChen et al. [19]. According to [19], the two-sidedAFPM machine is superior to the one-sided AFPMmachine; however, one-sided constructions use lesscopper and have a lower conduction loss. The Torusconstruction is simple; however, it requires moremagnet weight because of the presence of theadditional air gap for accommodating stator windings.As the power rating increases, both the air gap andair gap reluctance become larger, so that thisconstruction is more suitable for low powerrating wind generators. In addition, the potentialapplication of soft magnetic composite (SMC) materialapplied to the low speed, direct-drive axial flux PMwind generator was also discussed by Chen et al.Comparative design studies were conducted ondifferent configuration PM generators with bothlamination core and SMC core [33].

Transversal-flux PM machines: The transverse-fluxprinciple means that the path of the magnetic flux isperpendicular to the direction of the rotor rotation.There are also some different rotor structures for thistechnology, such as the rotor with single-sided surfacemagnets with single-sided flux concentration and withdouble-sided flux concentration. Fig. 11 shows theconfiguration of a surface-mounted transverse-fluxPMSG [12]. A transverse flux PM (TFPM) machine isa synchronous machine in nature, and it will functionin a manner similar to any other PMSG in principle.Compared with longitudinal machines, TFPMmachines have some advantages, such as higher forcedensity, considerably low copper losses and simplewinding. However, the force density of TFPMmachines with large air gap may be a little high oreven low depending on the outside diameter [16, 23].The construction of TFPM machines is much morecomplicated than longitudinal flux machines.

Compared with RFPM or AFPM machines, a majordifference is that TFPM machines allow an increase inthe space for the simple windings without decreasingthe available space for the main flux, and so that the

Figure 10 Slotted double-stator single-rotor AFPMconfiguration [26]



machines have very low copper losses. TFPMmachines can also be made with a very small polepitch; however, the electromagnetic structure is muchmore complicated. TFPM machines have also the lowpower factor, which leads to an increase in thenecessary rating of the power electronic converter.

TFPM machines have been discussed in a number ofreferences [16, 23, 27, 34]. References [16, 27] showthat the weight of a 55 kW TFPM machine is abouthalf of the total weight of an asynchronous machinewith a gearbox. TFPM machines seem to be suitablefor direct-drive applications because of the highspecific torque, although special methods ofmanufacturing and assembly are required [23]. Harriset al. [34] compared the advantages and disadvantagesof three different TFPM machine topologies, whichinclude a single-sided surface-mounted PM machine, asingle-sided surface-mounted PM machine with statorbridges and a double-sided flux concentrating PMmachine [34].

2.5 Variable speed single-stage gearedconcept with a full-scale power converter

In this scheme, a variable speed pitch control windturbine is connected to a single-stage planetarygearbox that increases the speed by a factor of roughly10 and a low-speed permanent-magnet generator. Thegrid connection scheme of this concept is shown inFig. 12. This concept, which was introduced as theMultibrid, has gained the attention because it has theadvantages of a higher speed than the direct-driveconcept and a lower mechanical component thanthe multiple-stage gearbox concept. Wind turbine

Figure 11 Surface-mounted transverse-flux PM structure[16]

Figure 12 Scheme of a single-stage drive PMSG systemwith a full-scale converter

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manufacturers, such as Multibrid and WinWind, haveproducts based on this concept in the market.

Furthermore, the Clipper system, a single-stagegearbox with multiple output shafts that drive anumber of medium speed, medium-torque PMSGs,has also been introduced. Each of the generatoroutputs is connected to a dedicated power electronicconverter. Currently, the Clipper system concept isused in the market with the rated power of 2.5 MW(four 660 kW PMSGs) [35]

2.6 Variable speed multiple-stage gearedconcept with a full-scale power converter

2.6.1 PMSG system: A PMSG system with a multiple-gearbox is used in order to reduce the generator’svolume and improve the generator efficiency invariable speed wind turbine concepts with a full-scalepower converter. Fig. 13 shows the grid connectionscheme of this concept.

Compared with the DFIG system, this windgenerator system has the following advantages.

† The generator has a better efficiency.

† The generator can be brushless.

† The grid-fault ride-through capability is lesscomplex.

And the following disadvantages:

† larger, more expensive converter (100% of ratedpower instead of 30%),

† The losses in the converter are higher because allpowers are processed by the power electronic converter.

In the market, this configuration has been used in GEmulti-megawatt series.

2.6.2 SCIG system: In order to fulfill the variablespeed operation with an SCIG, an alternativegenerator system that might replace the capacitorbank and soft-starter of ‘Danish concept’ is a variablespeed multiple-stage geared SCIG with a full-scaleconverter, as shown in Fig. 14.

Figure 13 Scheme of a multiple-stage geared PMSG systemwith a full-scale converter



Compared with ‘Danish concept’ as mentionedabove, this concept has advantages of the flexiblecontrol with a full-scale power, such as variable speedoperation, better performances of reactive powercompensation and smooth grid connection. However,its disadvantage is the high cost and losses of the full-scale converter, the efficiency of the total system(gearbox induction generator and converter) may below. Presently, Siemens is using this concept with therated power of 3.6 MW (Bonus 107) in the market,and the generator speed range is designed to be 595–1547 rpm.

The decreasing cost of power electronics (roughly afactor of 10 over the past 10 years) and the absence ofbrushes may make variable speed multiple-stagegeared concepts (both PMSG and SCIG as mentionedabove) more attractive.

2.7 Other potential generator types fordifferent wind turbine concepts

Many other types of wind generators are also mentionedin literatures, such as linear induction generators [36],switched reluctance generators [37], claw-polegenerators [36]; brushless DFIGs (BDFIGs) [36, 38].Among them, the BDFIG may be one of the mostinnovative types. A grid connection scheme of thewind turbine concept with BDFIG is shown in Fig. 15.

For this configuration, the output of the inductiongenerator is directly connected to the grid, and thusthe generator output frequency must be equal to thegrid frequency. The BDFIG does not need the slipring; however, it requires double stator windings, withdifferent number of poles in both stator layers. Thesecond stator layer generally has lower copper mass,because only a part of the generator nominal currentflows in the second winding. This second statorwinding is connected through a power electronic

Figure 14 Scheme of a multiple-stage geared SCIG systemwith a full-scale converter

Figure 15 Scheme of a BDFIG system with a partial-scaleconverter



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converter, which is rated at only a fraction of the windturbine rating.

The BDFIG system has the capability of realisingthe variable speed operation and independentlycontrols the stator active and reactive power.Compared with the DFIG system, this concept doesnot require slip rings; however, the machine operationprinciple and its assembly are relatively complex.

3 Comparison of different windgenerator systemsIn this section, a survey of quantitative comparisons ofdifferent wind generator systems is performed, whichincludes the performance comparison and the marketpenetration share.

3.1 Performance comparison of differentwind generator systems

Some comparisons of different wind generator systemhave been conducted by some researchers [8–13, 17,19, 24, 26, 39–41]. Grauers [17] has presented aquantitative comparison between the variablespeed direct-drive concept of the RFPM generatorsystem with a forced-commutated rectifier and thecommercial product of the fixed-speed concept withSCIG. Some main parameter comparisons for tworated power levels of 500 kW and 3 MW arerespectively, shown in Table 1.

According to [17], the outer diameter of the direct-drive PMSG is almost two times of the conventionalgeared-drive SCIG; however, the total length is two tothree times shorter than that of SCIG systemincluding the length of high-speed shaft. Additionly,the direct-drive PMSG system has its averageefficiency of 2.3% and 1.6% higher than the fixedspeed SCIG system at the 500 kW and 3 MW ratedpower, respectively. Because of the variable speedoperation, the direct-drive PMSG system can produce5–10% more energy than the fixed two-speed

Table 1 Comparison of the direct-drive PMSG and the fixed-speed concept of SCIG system [17]

Generators concepts PMSG SCIG PMSG SCIG

rated power, kW 500 500 3000 3000

outer diameter ofgenerator, m

2.7 1.5 5 2.5

length of system (incl.high-speed shaft inSCIG)

1.2 3 2 6

average efficiency, % 90.7 88.4 91.6 90.0

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concept, or 10–15% more than the fixed single-speedconcept.

Some comparisons between the direct-drive PMSGand the geared-drive traditional SCIG of commercial500 kW wind turbines have been performed byAnnon. [39]. The detailed results are given in Table 2.

As it can be observed in Table 2, the annual energyproduction of the direct-drive PMSG is higher thanthat of the geared-drive conventional SCIG. Althoughthe wind turbine rotor diameter of the direct-drivePMSG is greater than that of the geared-drive SCIG,the total weight of the rotor and nacelle is lower; itseems realistic to conclude that the total weight of thetwo alternative systems will be of the same order.

A 1.5 MW direct-drive wind turbine system withEESG has been compared with the DFIG system witha multi-stage gearbox by Siegfriedsen and Bohmeke [8]and Bohmeke et al. [40]. They concluded that thedirect-drive system would be more expensive andheavier than the DFIG wind turbines. In addition, thecomparison between the direct-drive PMSG and EESGshows the cost for active material of PMSG is lower.This is mainly due to the reduced pole pitch ofPMSG, and the increased number of poles can be setfor a given diameter [16, 17, 23]. Recently, Polinderet al. [9] have also presented a detailed comparison offive 3 MW different generator systems for variablespeed wind turbine concepts, which are a DFIGsystem with three-stage gearbox (DFIG 3G), a direct-

Table 2 Main comparison of two commercial 500 kW windturbines with the direct-drive PMSG and the fixed-speedSCIG system [39]

Generators concepts PMSG SCIG

speed of wind turbines rotor, rpm 18–38 30

speed of generator rotor, rpm 18–38 1500

annual energy production at meanwind speed, kWh

5 m/s 615 528

10 m/s 2350 2189

wind turbine rotor diameter, m 40.3 38.2

wind turbine weight, ton

rotor, including hub 20.5 9.2

nacelle 5.6 19.9

rotorþ nacelle 26.1 29.0

tower 34.0 27.8

total 60.1 56.9

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drive EESG system (EESG DD), a PM excited direct-drive synchronous generator system (PMSG DD), aPM excited synchronous generator system withsingle-stage gearbox (PMSG 1G) and a DFIG systemwith single-stage gearbox (DFIG 1G). Approximateddesigns of the five different generator systems havebeen presented for a given wind turbine. Maindimensions and performances of the comparison arepresented in Table 3.

Based on the survey, the following conclusions can besummarised.

† From the aspects of size and weight, the outerdiameter of the direct-drive wind generator is usuallylarger than the geared-drive generator, but the totallength is shorter. Considering the parts of windturbine blade, the total weight of wind turbinesystems may have no big difference between a three-stage geared-drive configuration and a direct-drivePMSG solution.

† DFIG 3G is the lightest and low-cost solution withstandard components according to [9].

† For direct-drive wind turbine topologies, PMSG DDhas the highest energy yield, EESG DD appears to be theheaviest and the most expensive solution.



† A solution with a one-stage gearbox together with amulti-pole generator may be an interesting solution. Forexample, DFIG 1G seems to be the most interestingchoice because of the highest annual energy yielddivided by cost and the lowest generator system cost[9]. PMSG 1G has a better performance than PMSGDD with respect to the energy yield per cost.

3.2 Market penetration of different windturbine concepts

Various types of wind turbines have been on the marketwith different power levels. In order to present thetrends of different wind generator systems on themarket, Table 4 shows some wind turbines with arated power over 2 MW from different manufactures,such as Vestas, Gamesa, GE wind, Repower, Nordexand so on, where the wind turbine concept, generatortype, rated power and turbine rotor speed areobtained from manufacturers’ websites [42–54].

As it can be seen, most manufactures are usinggeared-drive wind turbine concepts. The windturbines produced by Vestas, Gamesa, GE wind,Repower, Nordex and Ecotecnia are using DFIG witha multiple-stage gearbox. According to this survey, itis clear that the wind market is still dominated byDFIG with a multiple-stage gearbox, and the mostlyused generator type is still the induction generator

Table 3 Comparisons of five different wind generator systems [9]

Generators concepts DFIG 3G EESG DD PMSG DD PMSG 1G DFIG 1G

Stator air-gap diameter, m 0.84 5 5 3.6 3.6

Stack length, m 0.75 1.2 1.2 0.4 0.6

active material weight, ton

iron 4.03 32.5 18.1 4.37 8.65

copper 1.21 12.6 4.3 1.33 2.72

PM — — 1.7 0.41 —

total cost, kEuro 5.25 45.1 24.1 6.11 11.37

generator active material 30 287 162 43 67

generator construction 30 160 150 50 60

gearbox 220 — — 120 120

converter 40 120 120 120 40

sum of generator system cost 320 567 432 333 287

total cost (incl. margin for company costs) kWh/Euro 1870 2117 1982 1883 1837

annual energy yield, MW h 7690 7740 7890 7700 7760

annual energy yield/total cost, kW h/Euro 4.11 3.67 3.98 4.09 4.22



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Table 4 Large wind turbine concepts on the market over 2 MW

Wind turbine concept Generatortype

Power/rotor diameter/speed

Manufacturer

variable speed multiple-stage concept with partial-scalepower converter

DFIG 4.5 MW/120 m/14.9 rpm

Vestas

2 MW/90 m/19 rpm Gamesa

3.6 MW/104 m/15.3 rpm

GE Wind

5 MW/126 m/12.1 rpm Repower

2.5 MW/90 m/14.85 rpm

Nordex

3 MW/100 m/14.25 rpm

Ecotecnia

limited variable speed with multiple-stage gearbox WRIG 2 MW/88 m/17 rpm Suzlon

variable speed multiple-stage gearbox with full-scalepower converter

SCIG 3.6 MW/107 m/13 rpm Siemens WindPower

PMSG 2 MW/88 m/16.5 rpm GE Wind

variable speed single-stage gearbox with full-scale powerconverter

PMSG 5 MW/116 m/14.8 rpm Multibrid

3 MW/90 m/16 rpm Winwind

2.5 MW/93 m/15.5 rpm

Clipper Windpower

variable speed direct-drive with full-scale powerconverter

EESG 4.5 MW/114 m/13 rpm Enercon

PMSG 2 MW/71 m/23 rpm Zephyros

(DFIG, SCIG and WRIG). Two companies, Multibridand WinWind, use PMSG with a single-stage gearbox.Direct-drive wind turbines are used in Enercon andZephyros. Enercon have applied EESG, and Zepyroshave applied PMSG. According to [3], Vestasmanufacturer maintains its position as the word’slargest manufacturer, followed by the Gamesa,Enercon and GE Wind. The world market share atthe end of 2004 for each company is 34%, 17%, 15%and 11%.

Fig. 16 depicts the market penetration and share ofdifferent wind generator systems based on therecorded world suppliers’ market data over a 10-yearperiod (1995–2004) [3, 55]. As it can be seen, thefixed-speed SCIG system has decreased about 3-foldover 10 years, from almost 70% in 1995 to almost25% in 2004. Market penetration of the Optislipconcept (WRIG in Fig. 4) has declined since 1997 infavour of the more attractive variable speed concept(DFIG). The trend depicted in Fig. 16 clearly indicatesthat the WRIG type is being phased out of themarket. The DFIG wind turbines have increased from0% to almost 55% of the yearly installed wind powerover 10 years, and it clearly becomes the most



dominant concept at the end of 2004. Marketpenetration of the SG concept (EESG or PMSG) hasaltered little over 10 years, with no such dramaticchanges as observed for SCIG, WRIG and DFIG.There is, however, a slight increasing trend over thelast 3 years (2002–2004). During the 10 years, thedirect-drive SG (EESG and PMSG) has ranked thirdor fourth (Fig. 16).

Figure 16 World share of yearly installed power fordifferent wind generator systems

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4 Trends discussionWith rapid development of wind turbine technologies,future trends in the wind turbine industry willprobably be focused on the gradual improvement ofalready known technologies, which can be summarisedas follows [3, 15, 56, 57].

† The power level of a single wind turbine willcontinue to increase, because this reduces the cost ofplacing wind turbines, especially for offshore windfarms.

† Offshore wind energy is more attractive, because ofhigher wind speed and more space than on shore windenergy.

† An increasing trend is to remove dispersed singlewind turbine in favour of concentrated wind turbinesin large wind farms.

† An increasing trend in the penetration of wind powerinto the power system.

4.1 Grid connection requirements

The penetration of wind power into the power systemcontinues to increase, which implies the situation ofthe large wind farms is changing from being simpleenergy sources to having power plant status with gridsupport characteristics. One major challenge in thepresent and coming years is the connection andoptimised integration of large wind farms intoelectrical grids [3]. With increased wind powercapacity, transmission system operators (TSOs) havebecome concerned about the impact of high levels ofwind power generation on power systems. To handlelarge-scale integration of wind power, TSOs haveissued grid codes and grid requirements for windturbines connection and operation. The main issues ofgrid codes can be summarised as follows [3, 10, 11]:

† active power control,

† reactive power control,

† voltage and frequency control,

† power quality, for example, flickers and harmonics,

† fault ride-through capability.

As mentioned above, the power-control capabilityand the fault ride-through capability are mainlyconcerned by some TSOs. Wind farms are requiredto behave as conventional power plants in powersystems, such as regulating active and reactive powerand performing frequency and voltage control. And



the fault ride-through capability is required to avoidsignificant loss of wind power production in the eventof grid faults. This means wind turbines should stayconnected and contribute to the grid in case of adisturbance such as a voltage dip. They shouldimmediately supply active and reactive power forfrequency and voltage recovery after the fault hasbeen cleared. As an example, the requirementsconcerning immunity to voltage dips as prescribed byE.On Netz, a grid operator in Northern Germany, isshown in Fig. 17. Only when the grid voltage dropsbelow the curve (in duration or voltage level), theturbine is allowed to be disconnected. When thevoltage is in the shaded area, the turbine should alsosupply reactive power to the grid in order to supportgrid restoration [11].

4.2 Trends of wind generator systems

According to the survey of different wind generatorsystems and considering the grid connectionrequirements on wind turbines, the developing trendsof wind generator systems may be summarised asfollows.

† Variable speed concept

Variable speed operation is very attractive for anumber of reasons, including reduced mechanicalstress and increased power capture. As mentioned, themarket share of the fixed speed concept has decreasedslightly, whereas variable speed wind turbineincreases. For various variable speed wind turbineconcepts, a multiple-stage geared-drive DFIG with apartial-scale power converter is still dominant in thecurrent market. Compared with other variable speedconcepts with a full-scale power converter, the mainadvantage of this concept is only 30% of the generatedpower passing through the power converter, so that itmay have substantial cost advantages even with low-cost power electronics in the future. However, fromthe viewpoint of the fault ride-through capability, the

Figure 17 Voltage dip that wind turbines should be able tohandle without disconnection (E.On Netz)



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DFIG system has to endure large peak currents duringgrid faults, an advanced protection system may berequired. On the contrary, variable speed windturbines with a full-scale power converter may bemore effective and less complicated to deal with grid-related problems. Therefore variable speed windturbine concepts with a full-scale power converter willbecome more attractive.

† Direct-drive concept

Compared with geared-drive wind generatorsystems, the main advantages of direct-drive windgenerator systems are higher overall efficiency,reliability and availability because of omitting thegearbox. Although the size of direct-drive generatorsis usually larger, it may not be a serious disadvantagefor the offshore wind energy.

† PM excited generator type

PM machines are more attractive and superior withhigher efficiency and energy yield, higher reliabilityand power to weight ratio compared with electricity-excited machines. According to the above survey ofRFPM, AFPM and TFPM machines, RFPM machineswith surface-mounted PM may be more suitable fordirect-drive PM generator types, because of allowingthe simple generator structure, good utilisation of theactive materials and also allowing the relatively smalldiameter in comparison with AFPM and TRPMmachines. In the case of AFPM machines, thedisadvantages as described in Section 2.4.2, whichmake the machine cost increase and manufacturingdifficult, must be solved or improved significantly.Although TFPM machines have some advantages, suchas high-force density and simple winding with lowcopper losses, the disadvantages, such as low-forcedensity in large air gap, complicated construction inmanufacturing and low power factor may be obvious.However, TFPM still have potential to be used as adirect-drive PM generator with new topology design,since the machines are more flexible for new topologies.

Considering the performance of PMs is improvingand the cost of PM is decreasing in recent years, inaddition to that the cost of power electronics isdecreasing, variable speed direct-drive PM machineswith a full-scale power converter become moreattractive for offshore wind powers. On the otherhand, variable speed concepts with a full-scale powerconverter and a single- or multiple-stage gearboxdrive train may be interesting solutions not only inrespect to the annual energy yield per cost but also inrespect to the total weight. For example, the marketinterest of PMSG system with a multiple-stagegearbox or a single-stage gearbox is increasing.

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Although the market share in the fixed-speed windturbine concept has decreased, the market interest inSCIG based on wind turbines may increase [3], if it isdemonstrated that High Voltage Direct CurrentTransmission (HVDC) technologies-based wind farmsconsisting of such SCIG are robust to grid faults.Because HVDC can enhance the ability against powersystem faults, consequently, the generators, which aresensitive to power system faults such as SCIG, can beused without the problem of ride through powersystem faults. Furthermore, a variable speed SCIGsystem with a full-scale power converter has beenused for over 3 MW wind turbines, such as Bonus107 model 3.6 MW of Siemens wind power.

It is clear that power electronics will continue to playan important role in the integration of future large windfarms and design of wind generator systems.

4.3 Discussions of comparison criteria

Various criteria may be used for comparing differentwind generator systems, including the torque density,the cost per torque, the efficiency, the active materialweight, the outer diameter, the total length, the totalvolume, the total generator cost, the annual energyyield, the energy yield per cost, the cost of energyand so on [8, 9, 12, 13, 16, 17, 21, 25]. However,with the increase in wind energy penetration intogrids and the development of grid connectionrequirements, overall qualitative comparison criteriaconsidering wind power quality and wind energy yieldmay be worthwhile for consideration.

† Current trends of research and development of windturbine concepts are mostly related to offshore windenergy. The most important difference between therequirements for onshore and offshore wind energytechnologies is that it is much more importantfor offshore turbines to be robust and maintenance-free [58, 59], because it is extremely expensiveand difficult and even impossible to do offshoremaintenance and reparations under some weatherconditions, so that the reliability and availability oflarge wind generator systems may be more importantaspects to be taken into consideration.

† With the increasing penetration of wind energy intothe grid, some performances related with gridconnection requirements may need to be considered inthe quantitative comparison. For example, the solutionof the flicker problem may yield an extra costdepending on the types of wind generator systems.The fault ride-through capability is also stronglyrelated to the type of the wind generator systems.

† Some performance indexes referred to wind turbinesmay have important effects on the annual energy yield of

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wind generator systems. For example, the cut-in andcut-off wind speeds need to be taken intoconsideration for comparison of different windgenerator systems, because they can influence theannual energy output and the available operationaltime of wind turbines, with variation of generatortypes [60].

The further development of variable speed windturbine concepts would be focused on optimisedturbines and thus moving towards more cost-effectivemachines. An overall and practical comparison ofdifferent wind generator systems, including techniques,economy, control function, availability and reliability,may require to be further investigated.

5 ConclusionsThe paper provides an overview of different windturbine concepts and possible generator types. Thebasic configurations and characteristics of variouswind generator systems based on contemporary windturbine concepts are described with their advantagesand disadvantages. The promising direct-drive PMmachines, such as AFPM, RFPM and TFPMmachines, have been surveyed. A detailed analysishas been performed based on the survey of thequantitative comparison of different wind generatorsystems as well as their market penetration. Thedeveloping trends of wind generator systems havebeen presented, and some comparison criteria havealso been discussed.

The multiple-stage geared drive DFIG concept is stilldominant in the current market. Additionally, themarket shows interest in the direct-drive or geared-drive concepts with a full-scale power electronicconverter. Current developments of wind turbineconcepts are mostly related to offshore wind energy;variable speed concepts with power electronics willcontinue to dominate and be very promisingtechnologies for large wind farms.

The performance of PMs is improving and the cost ofPMs is decreasing in recent years, which make variablespeed direct-drive PM machines with a full-scale powerconverter more attractive for offshore wind powergenerations.

With the increasing levels of wind turbine penetrationin modern power systems, grid connection issues haveposed several new challenges to wind turbine designand development. The future success of different windturbine concepts will strongly depend on their ability ofcomplying with both market expectations and therequirements of grid utility companies.



6 AcknowledgmentThe research was supported by a grant from the EU 6thframework program UP-WIND project. The authorsare grateful for the support.

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