AckermannTh. (ed) Wind Power in Power Systems

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It is cheap and reliable, but it consumes reactive power and produces current harmonics

that are difficult to filter out. Typical self-commutated converters consist of either GTO

thyristors or transistors. Self-commutated converters are interesting because they have

high switching frequencies. Harmonics can be filtered out more easily and thus their

disturbances to the network can be reduced to low levels. Today the most common

transistor is the IGBT. As illustrated in Table 4.3, the typical switching frequency of an

IGBT lies in the range of 2 to 20 kHz. In contrast, GTO converters cannot reach switching

frequencies higher than about 1 kHz; therefore, they are not an option for the future.

Self-commutated converters are either voltage source converters (VSCs) or current source

converters (CSCs; see Figure 4.2). They can control both the frequency and the voltage.

Table 4.3 Switches: maximum ratings and characteristics

Switch type

GTO IGCT BJT MOSFET IGBT

Voltage

(V) 6000 6000 1700 1000 6000

Current

(A) 4000 2000 1000 28 1200

Switching frequency

(kHz) 0.2–1 1–3 0.5–5 5–100 2–20

Drive requirements High Low Medium Low Low

Maximum output rating.

Operational range.

Note: GTO ¼gate turn-off thyristor; IGCT ¼integrated gate commutated thyristor;

BJT ¼bipolar junction transistor; MOSFET ¼metal oxide semiconductor field effect transistor;

IGBT ¼insulated gate bipolar transistor.

Source: L. H. Hansen et al., 2001.

(a) (b)

(grid)

(turbine)

Inverter

Rectifier

Inverter

(grid)

(turbine)

Rectifier

Inverter

Figure 4.2 Types of self-commutated power converters for wind turbines: (a) a current source

converter and (b) a voltage source converter. Reproduced from S. Heier, 1998, Grid Integration of

Wind Energy Conversion Systems, by permission of John Wiley & Sons Ltd

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VSCs a nd CSCs supply a relat ively well-defined switched voltage waveform and a

current waveform, respectively, at the terminals of the generator and the grid. In the

case of a VSC, the voltage in the energy storage (the DC bus) is kept constant by a large

capacitor. In a CSC, it is just the opposite; the current in the energy storage (the DC bus)

is kept co nstant by a large inductor . It has to be stressed that voltage source conversion

and current source conversion are different concepts. They can be implemented in

several ways: six-step, pulse amplitude modulated (PAM) or pulse width modulated

(PWM). By using the PWM technique, the low-frequency harmonics are eliminated and

the frequency of the first higher-order harmonics lie at about the switching frequency of

the inverter or rectifier.

4.2.5 State-of-the-art market penetration

Table 4.4 contains a list of the world’s top-10 wind turbine suppliers for the year 2002

(BTM Consults Aps., 2003), with respect to installed power. The table includes the two

largest (i.e. latest) wind turbines produced by each of the top-10 manufacturers. The

applied configuration, control concept, generator type, generator voltage and generator

or rotor speed range for each wind turbine have been evaluated by using data publicly

available on the Internet or based on email correspondence with the manufacturers.

The Danish company Vestas Wind Systems A/S is the largest manufacturer of wind

turbines in the world, followed by the German manufacturer Enercon. Danish NEG

Micon and Spanish Gamesa are in third and fourth positions, respectively.

All top-10 manufacturers produce wind turbines in the megawatt range. For the time

being, the most attractive concept seems to be the variable-speed wind turbine with pitch

control. Of the top-10 suppliers, only the Danish manufacturer Bonus consistently uses

only the active-stall fixed-speed concept. All the other manufacturers produce at least

one of their two largest wind turbines based on the variable-speed concept. The most

commonly used generator type is the induction generator (WRIG and SCIG). Only two

manufacturers, Enercon and Made, use synchronous generators. All top-10 manufac-

turers use a step-up transformer for coupling the generator to the grid. Only one,

Enercon, offers a gearless variable-speed wind turbine.

Comparing Table 4.4 with the analysis made by L. H. Hansen et al. (2001), there is an

obvious trend towards the configuration using a DFIG (Type C1) with variable speed

and variable pitch control. We wanted to illustrate this trend by looking at specific

market shares and have therefore carried out detailed market research regarding the

market penetration of the different wind turbine concepts from 1998 to 2002. The

analysis is based on the suppliers’ market data provided by BTM Consults Aps. and

on evaluat ing the concept of each individual wind turbine type sold by the top-10

suppliers over the five years considered. This information was gathered from the Inter-

net. The investigation processed information on a total of approximately 90 wind

turbine types from 13 different manufacturers that were among the top-10 suppliers of

wind turbines between 1998 and 2002: Vestas (Denmark), Gamesa (Spain), Enercon

(Germany), NEG Micon (Denmark), Bonus (Denmark), Nordex (Germany and

Denmark), GE-Wind/Enron (USA), Ecotechnia (Spain), Suzlon (India), Dewind

(Germany), Repower (Germany), Mitsubishi (Japan) and Made (Spain). Table 4.5 presents

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Table 4.4 Concepts applied by the top-10 wind turbine manufacturers (with respect to installed

power in 2002); including the two largest (i.e. latest) wind turbines from each manufacturer

Turbine, by

manufacturer

Concept

Power and speed

control features

Comments

Vestas, Denmark:

V80, 2.0 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator voltage: 690 V

Generator speed range: 905–1915 rpm

Rotor speed range: 9–19 rpm

V80, 1.8 MW Type B1 Pitch Limited

variable speed

WRIG Generator voltage: 690 V

Generator speed range: 1800–1980 rpm

Rotor speed range: 15.3–16.8 rpm

Enercon, Germany:

E112, 4.5 MW Type D1 Pitch Full

variable speed

Multiple WRIG Generator voltage: 440 V

Generator and rotor speed range: 8–13 rpm

E66, 2 MW Type D1 Pitch Full

variable speed

Multiple WRIG Generator voltage: 440 V

Generator and rotor speed range: 10–22 rpm

NEG Micon, Denmark:

NM80, 2.75 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator stator/rotor

voltage: 960 V/690 V

Generator speed range: 756–1103 rpm

Rotor speed range: 12–17.5 rpm

NM72, 2 MW Type A2 Active stall

Fixed speed

SCIG Generator voltage: 960 V

Two generator speeds: 1002.4 rpm

and 1503.6 rpm

Two rotor speeds: 12 rpm and 18 rpm

Gamesa, Spain:

G83, 2.0 MW Type C1 Pitch Limited

variable speed

WRIG Generator voltage: 690 V

Generator speed range: 900–1900 rpm

Rotor speed range: 9–19 rpm

G80, 1.8 MW Type B1 Pitch Limited

variable speed

WRIG (OptiSlipÒ concept)

Generator voltage: 690 V

Generator speed range: 1818–1944 rpm

Rotor speed range: 15.1–16.1 rpm

GE Wind, USA:

GE 104, 3.2 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator stator/rotor voltage: 3.3 kV/690 V

Rotor speed range: 7.5–13.5 rpm

Generator speed range: 1000–1800 rpm

GE 77, 1.5 MW Type C1 Pitch Limited

variable speed

WRIG Generator voltage: 690 V

Rotor speed range: 10.1–20.4 rpm

Generator speed range: 1000–2000 rpm

Bonus, Denmark:

Bonus 82, 2.3 MW Type A2 Active stall

Fixed speed

SCIG Generator voltage: 690 V

Two generator speeds: 1000 rpm and

1500 rpm

Two rotor speeds: 11 rpm and 17 rpm

Bonus 76, 2 MW Type A2 Active stall

Fixed speed

SCIG Generator voltage: 690 V

Two generator speeds: 1000 rpm and

1500 rpm

Two rotor speeds: 11 rpm and 17 rpm

(continued overleaf )

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the results, which show that the capacity installed by these 13 suppliers in each year

corresponds to more than 90 % of the total capacity installed during these years. This

makes this an alysis highly credible. These data cover approximately 76 % of the accu-

mulated worldwide capacity installed at the end of 2002. They are therefore very

comprehensive and illustrative.

Table 4.5 presents a detailed overview of the market share of each wind turbine concept

from 1998 to 2002. It becomes obvious that the market share of the fixed-speed wind turbine

concept (Type A) decreased during this period, with the DFIG concept (Type C) becoming

Table 4.4 (continued)

Turbine, by

manufacturer

Concept

Power and speed

control features

Comments

Nordex, Germany:

N80, 2.5 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator voltage: 660 V

Generator speed range: 700–1300 rpm

Rotor speed range: 10.9–19.1 rpm

S77, 1.5 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator voltage: 690 V

Generator speed range: 1000–1800 rpm

Rotor speed range: 9.9–17.3 rpm

Made, Spain:

Made AE-90, 2 MW Type D1 Pitch Full

variable speed

WRSG Generator voltage: 1000 V

Generator speed range: 747–1495 rpm

Rotor speed range: 7.4–14.8 rpm

Made AE-61, 1.32 MW Type A0 Stall Fixed speed SCIG Generator voltage: 690 V

Two generator speeds: 1010 rpm and

1519 rpm

Two rotor speeds: 12.5 rpm and 18.8 rpm

Repower, Germany:

MM 82, 2 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator voltage: 690 V

Generator speed range: 900–1800 rpm

Rotor speed range: 10–20 rpm

MD 77, 1.5 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator voltage: 690 V

Generator speed range: 1000–1800 rpm

Rotor speed range: 9.6–17.3 rpm

Ecotecnia, Spain:

Ecotecnia 74, 1.67 MW Type C1 Pitch Limited

variable speed

WRIG (DFIG concept)

Generator voltage: 690 V

Generator speed range: 1000–1950 rpm

Rotor speed range: 10–19 rpm

Ecotecnia 62, 1.25 MW Type A0 Stall Fixed speed SCIG Generator voltage: 690 V

Two generator speeds: 1012 rpm and

1518 rpm

Two rotor speeds: 12.4 rpm and 18.6 rpm

For definitions of concept types, see Table 4.1 and Section 4.2.3 in text.

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the dominant concept. The full variable-speed concept (Type D) slightly increased its market

share, thus becoming the third most important concept in 2001 and 2002.

The market position of the OptiSlip concept (Type B) was almost constant during the

first three years of the period analysed, but during the last two years it decreased, ceding

its market share to the Type C concept. From the trend depicted in Table 4.5, Type B is

likely to lose its market altogether. This is mainly because of the variable speed range of

Type B, which is much more limited than is the variable speed range of Type C.

Table 4.5 also provides information on the total capacity installed annually by the 13

suppliers. It shows that the total install ed capacity in 2002 was three times that in 1998,

primarily because of the increased rated power of the wind turbines.

4.3 Generator Concepts

Basically, a wind turbine can be equipped with any type of three-phase generator. Today,

the demand for grid-compatible electric current can be met by connecting frequency

converters, even if the generator supplies alternating current (AC) of variable frequency

or direct current (DC). Several generic types of generators may be used in wind turbines:

Asynchronous (induction) generator:

– squirrel cage induction generator (SCIG);

– wound rotor induction generator (WRIG):

OptiSlip induction generator (OSIG),

Doubly-fed induction generator (DFIG).

Synchronous generator:

– wound rotor generator (WRSG);

– permanent magnet generator (PMSG).

Table 4.5 World market share of wind turbine concepts between 1998 and 2002

Concept

Share (%)

1998 1999 2000 2001 2002

Type A 39.6 40.8 39.0 31.1 27.8

Type B 17.8 17.1 17.2 15.4 5.1

Type C 26.5 28.1 28.2 36.3 46.8

Type D 16.1 14.0 15.6 17.2 20.3

Total installed power

(MW) 2349 3788 4381 7058 7248

Percentage share of the world market (supply)

92.4 90.1 94.7 97.6 97.5

For definitions of concept types, see Table 4.1 and Section 4.2.3 in text.

For the 13 suppliers studied (Vestas, Gamesa, Enercon, NEG Micon, Bonus, Nordex, GE Wind/

Enron, Ecotechnia, Suzlon, Dewind, Repower, Mitsubishi and Made).

Source: Internet survey.

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Other types of potential interest:

– high-voltage generator (HVG);

– switch reluctance generator (SRG);

– transverse flux generator (TFG).

In this section, we will summarise the essential properties of these generic generator

types. For a detailed analysis of generator types, see the standard literature on this field

(Heier, 1998; Krause, Wasynczuk and Sudhoff, 2002).

4.3.1 Asynchronous (induction) generator

The most common generator used in wind turbines is the induction generator. It has

several advantages, such as robustness and mechanical simplicity and, as it is produced

in large series, it also has a low price. The major disadvantage is that the stator needs a

reactive magnetising current. The asyn chronous generator does not contain permanent

magnets and is not separately excited. Therefore, it has to receive its exciting current

from another source and consumes reactive power. The reactive power may be supplied

by the grid or by a power electronic system. The generator’s magnetic field is established

only if it is connected to the grid.

In the case of AC excitation, the created magnetic field rotates at a speed determined

jointly by the number of poles in the wind ing and the frequency of the current, the

synchronous speed. Thus, if the rotor rotates at a speed that exceeds the synchro nous

speed, an electric field is induced between the rotor and the rotating stator field by a

relative motion (slip), which causes a current in the rotor windings. The inter action of

the associated magnetic field of the rotor with the stator field results in the torque acting

on the rotor.

The rotor of an induction generator can be designed as a so-called short-circuit rotor

(squirrel cage rotor) or as a wound rotor.

4.3.1.1 Squirrel cage induction generator

So far, the SCIG has been the prevalent choice because of its mechani cal simplicity, high

efficiency and low maintenance requirements (for a list of literature related to SCIGs,

see L. H. Hansen et al., 2001).

As illustrated in Figure 4.1, the SCIG of the configuration Type A is directly grid—

coupled. The SCIG speed changes by only a few percent because of the generator slip

caused by changes in wind speed. Therefore, this generator is used for constant-speed

wind turbines (Type A). The generator and the wind turbine rotor are coupled through a

gearbox, as the optimal rotor and generator speed ranges are different.

Wind turbines based on a SCIG are typically equipped with a soft-starter mechanism

and an installation for reactive power compensation, as SCIGs consume reactive power.

SCIGs have a steep torque speed characteristic and therefore fluctuations in wind power

are transmitted directly to the grid. These transients are especially critical during the

grid connection of the wind turbine, where the in-r ush current can be up to 7–8 times the

rated current. In a weak grid, this high in-rush current can cause severe volta ge

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disturbances. Therefore, the connection of the SCIG to the grid should be made

gradually in order to limit the in-rush current.

During normal operation and direct connection to a stiff AC grid, the SCIG is very

robust and stable. The slip varies and increases with increasing load. The major problem

is that, because of the magnetising current supplied from the grid to the stator winding,

the full load power factor is relatively low. This has to be put in relation to the fact that

most power distribution utilities penalise industrial customers that load with low power

factors. Clearl y, generation at a low power factor cannot be permitted here either. Too

low a power factor is compensated by connecting capacitors in parallel to the generator.

In SCIGs there is a unique relation between active power, reactive power, terminal

voltage and rotor speed. This means that in high winds the wind turbine ca n produce

more active power only if the generator draws more reactive power. For a SCIG, the

amount of consumed reactive power is uncontrollable because it varies with wind

conditions. Without any electrical components to supply the reactive power , the reactive

power for the generator must be taken directly from the grid. React ive power supplied

by the grid causes additional trans mission losses and in certain situations, can make the

grid unstable. Capacitor banks or modern power electronic converters can be used to

reduce the reactive power consumption. The main disadvantage is that the electrical

transients occur during switching-in.

In the case of a fault, SCIGs without any reactive power compensation system can lead

to voltage instability on the grid (Van Custem and Vournas, 1998). The wind turbine rotor

may speed up (slip increases), for instance, when a fault occurs, owing to the imbalance

between the electrical and mechanical torque. Thus, when the fault is cleared, SCIGs draw a

large amount of reactive power from the grid, which leads to a further decrease in voltage.

SCIGs can be used both in fixed-speed wind turbines (Type A) and in full variable-

speed wind turbines (Type D). In the latter case, the variable frequency power of the

machine is converted to fixed-frequency power by using a bidirectional full-load back-

to-back power converter.

4.3.1.2 Wound rotor induction generator

In the case of a WRIG, the electrical characteristics of the rotor can be controlled from the

outside, and thereby a rotor voltage can be impressed. The windings of the wound rotor

can be externally connected through slip rings and brushes or by means of power electronic

equipment, which may or may not require slip rings and brushes. By using power electro-

nics, the power can be extracted or impressed to the rotor circuit and the generator can be

magnetised from either the stator circuit or the rotor circuit. It is thus also possible to

recover slip energy from the rotor circuit and feed it into the output of the stator. The

disadvantage of the WRIG is that it is more expensive and not as robust as the SCIG.

The wind turbine industry uses most commonly the following WRIG configurations: (1)

the OptiSlip

induction generator (OSIG), used in the Type B concept and (2) the d oubly-fed

induction generator (DFIG) concept, used in the Type C configuration (see Figure 4.1).

OptiSlip induction generator

The OptiSlip

feature was introduced by the Danish manufacturer Vestas in order to

minimise the load on the wind turbine during gusts. The OptiSlip

feature allows the

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generator to have a variable slip (narrow range) and to choose the optimum slip,

resulting in smaller fluctuations in the drive train torque and in the power output. The

variable slip is a very simple, reliable and co st-effective way to achieve load reductions

compared with more complex solutions such as full variable-speed wind turbines using

full-scale converters.

OSIGs are WRIGs with a variable external rotor resistance attached to the rotor

windings (see Figure 4.1). The slip of the generator is changed by modifying the total

rotor resistance by means of a converter, mounted on the rotor shaft. The converter is

optically controlled, which means that no slip rings are necessa ry. The stator of the

generator is connected directly to the grid.

The advantages of this generator concept are a simple circuit topology, no need for

slip rings and an improved operating speed range compared with the SCIG. To a certain

extend, this concept can reduce the mechanical loads and power fluctuations caused by

gusts. However, it still requires a reactive power compensation system. The disadvan-

tages are: (1) the speed range is typically limited to 0–10 %, as it is dependent on the size

of the variable rotor resistance; (2) only poor control of active and reactive power is

achieved; and (3) the slip power is dissipated in the variable resi stance as losses.

Doubly-fed induction generator

As depicted in Table 4.5, the concept of the DFIG is an interesting option with a

growing market. The DFIG consists of a WRIG with the stator windings directly

connected to the constant-frequency three-phase grid and with the rotor windings

mounted to a bidirectional back-to-back IGBT voltage source converter.

The term ‘doubly fed’ refers to the fact that the voltage on the stator is applied from

the grid and the voltage on the rotor is induced by the power converter. This system

allows a variable-speed operation over a large, but restricted, range. The converter

compensates the difference between the mechanical and electrical frequency by injecting

a rotor current with a variable frequency. Both during normal operation and faults the

behaviour of the generator is thus governed by the power converter and its controllers.

The power converter consists of two converters, the rotor-side converter and grid-side

converter, which are controlled independently of each other. It is beyond the scope of this

chapter to go into detail regarding the control of the converters (for more detail, see

Leonhard, 1980; Mohan, Undeland and Robbins, 1989; Pena, Clare and Asher, 1996).

The main idea is that the rotor-side converter controls the active and reactive power by

controlling the rotor current components, while the line-side converter controls the DC-link

voltage and ensures a converter operation at unity power factor (i.e. zero reactive power).

Depending on the operating condition of the drive, power is fed into or out of the

rotor: in an oversynchronous situation, it flows from the rotor via the converter to the

grid, whereas it flows in the opposite direction in a subsynchronous situation. In both

cases – subsynchronous and oversynchronous – the stator feeds energy into the grid.

The DFIG has several advantages. It has the ability to control reactive power and to

decouple acti ve and reactive power control by independently controlling the rotor

excitation current. The DFIG has not necessarily to be magnetised from the power

grid, it can be magnetised from the rotor circuit, too. It is also capable of generating

reactive power that can be delivered to the stator by the grid-side converter. However,

the grid-side converter normally ope rates at unity power factor and is not involved in

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the reactive power exchange between the turbine and the grid. In the case of a weak grid,

where the voltage may fluctuate, the DFIG may be ordered to produce or absorb an

amount of reactive power to or from the grid, with the purpose of voltage control.

The size of the converter is not related to the total generator power but to the selected

speed range and hence to the slip power. Thus the cost of the converter increases when

the speed range around the synchronous speed becomes wider. The selection of the

speed range is therefore based on the economic optimisation of investment costs and on

increased efficiency. A drawback of the DFIG is the inevitable need for slip rings.

4.3.2 The synchronous generator

The synchronous generator is much more expensive and mechanically more complicated

than an induction generat or of a similar size. However, it has one clear advantage

compared with the induction generator, namely, that it does not need a reactive

magnetising current.

The magnetic field in the synchronous generator can be created by using permanent

magnets or with a conventional field winding. If the synchronous generator has a

suitable num ber of poles (a multipole WRSG or a multipole PMSG), it can be used

for direct-drive applications without an y gearbox.

As a synchronous machine, it is probably most suited for full power control as it is

connected to the grid through a power electronic converter. The converter has two

primary goals: (1) to act as an energy buffer for the power fluctuations caused by an

inherently gusting wind energy and for the transients coming from the net side, and (2) to

control the magnetisation and to avoid problems by remaining synchronous with the grid

frequency. Applying such a generator allows a variable-speed operation of wind turbines.

Two classical types of synchronous generators have often been used in the wind

turbine industry: (1) the wound rotor synchronous generator (WRSG) and (2) the

permanent magnet syn chronous generator (PMSG).

4.3.2.1 Wound rotor synchronous generator

The WRSG is the workhorse of the electrical power industry. Both the steady -state

performance and the fault performance have been well-documented in a multit ude of

research papers over the years, (see L. H. Hansen et al., 2001).

The stator windings of WRSGs are connected directly to the grid and hence the

rotational speed is strictly fixed by the frequency of the supply grid. The rotor winding is

excited with direct current using slip rings and brushes or with a brushless exciter with a

rotating rectifier. Unlike the induction generator, the synchronous generator does not

need any further reactive power compensation system. The rotor winding, through

which direct current flows, generates the exciter field, which rotates with synchronous

speed. The speed of the synchronous generator is determined by the frequency of the

rotating field and by the number of pole pairs of the rotor.

The wind turbine manufacturers Enercon and Lagerwey use the wind turbine concept

Type D (see Figure 4.1) with a multipole (low-speed) WRSG and no gearbox. It has the

advantage that it does not need a gearbox. But the price that has to be paid for such

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a gearless design is a large and heavy generator and a full-scale power converter that has

to handle the full power of the system. The wind turbine manufacturer Made also

applies the wind turbine concept Type D, but with a four-pole (high-speed) WRSG

and a gearbox (see Table 4.4).

4.3.2.2 Permanent magnet synchronous generator

Many research articles have suggested the application of PMSGs in wind turbines

because of their property of self-excitation, which allows an operation at a high power

factor and a high efficiency, (see Alatalo, 1996).

In the permanent magnet (PM) machine, the efficiency is higher than in the induction

machine, as the excitation is provided without any energy supply. However, the mater-

ials used for producing permanent magnets are expensive, and they are difficult to work

during manufacturing. Additionally, the use of PM excitation requires the use of a full-

scale power converter in order to adjust the voltage and frequency of generation to the

voltage and the frequency of transmission, respectively. This is an added expense.

However, the benefit is that power can be generated at any speed so as to fit the current

conditions. The stator of PMSGs is wound, and the rotor is provided with a permanent

magnet pole system and may have salient poles or may be cylindrical. Salient poles are

more common in slow-speed machines and may be the most useful version for an

application for wind generators. Typical low-speed synchronous machines are of the

salient-pole type and the type with many poles.

There are different topologies of PM machines presented in the literature. The most

common types are the radial flux machine, the axial flux machine and the transversal

flux machine. A detailed description of all these types is given in Alatalo (1996).

The synchronous nature of the PMSG may cause problems during startup, synchron-

isation and voltage regulation. It does not readily provide a constant voltage (Mitcham

and Grum, 1998). The synchronous operation causes also a very stiff performance in the

case of an external short circuit, and if the wind speed is unsteady. Another disadvan-

tage of PMSGs is that the magnetic materials are sensitive to temperature; for instance,

the magnet can lose its magnetic qualities at high temperatures, during a fault, for

example. Therefore, the rotor temperature of a PMSG must be supervised and a cooling

system is required.

Examples of wind turbine manufacturers that use configuration Type D with PMSGs

are Lagerwey, WinWind and Multibrid.

4.3.3 Other types of generators

In the following, we will briefly present other types of generators that are possible future

candidates for the wind turbine industry.

4.3.3.1 Highvoltage generator

Most commonly, wind turbine generators are operated at 690 V (see Table 4.4) and they

therefore require a transformer in the nacelle or at the bottom of the tower. The main

motivation for increasing the voltage of the generator is to reduce the current and

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