7489_chap02

HANDBOOK OF RENEWABLE ENERGY TECHNOLOGY © World Scientific Publishing Co. Pte. Ltd.http://www.worldscibooks.com/environsci/7489.html

November 26, 2010 19:32 9.75in x 6.5in b1032-ch02 Handbook of Renewable Energy Technology FA

Chapter 2

Wind Turbine Systems: History, Structure,and Dynamic Model

S. Masoud Barakati

Faculty of Electrical and Computer Engineering,University of Sistan and Baluchestan

Zahedan, Iran

[email protected]

This chapter focuses on wind turbine structure and modeling. First, a brief historicalbackground on the wind will be presented. Then classification of the wind turbinebased on generators, power electronic converters, and connecting to the grid will bediscussed. The overall dynamic model of the wind turbine system will be explainedin the end of the chapter.

2.1 Wind Energy Conversion System (WECS)

A wind energy conversion system (WECS) is composed of blades, an electricgenerator, a power electronic converter, and a control system, as shown in Fig. 2.1.The WECS can be classified in different types, but the functional objective of thesesystems is the same: converting the wind kinetic energy into electric power andinjecting this electric power into the electrical load or the utility grid.

2.1.1 History of using wind energy in generating electricity

History of wind energy usage for the generation of electricity dates back to the 19thcentury, but at that time the low price of fossil fuels made wind energy economicallyunattractive.1 The research on modern Wind Energy Conversion Systems (WECS)was put into action again in 1973 because of the oil crisis. Earlier research was onmaking high power modern wind turbines, which need enormous electrical gener-ators. At that time, because of technical problems and high cost of manufacturing,making huge turbines was hindered.1,2 So research on the wind turbine turned to

21



22 S. M. Barakati

Blades

Wind

Machine Converter(not always)

Primary ConversionSecondaryConversion

Gearbox(not always)

Electrical Grid

Fig. 2.1. Block diagram of a WECS.

making low-price turbines, which composed of a small turbine, an induction gen-erator, a gearbox and a mechanical simple control method. The turbines had ratingsof at least several tens of kilowatts, with three fixed blades. In this kind of system,the shaft of the turbine rotates at a constant speed. The asynchronous generator isa proper choice for this system. These low-cost and small-sized components madethe price reasonable even for individuals to purchase.3

As a result of successful research on wind energy conversion systems, a newgeneration of wind energy systems was developed on a larger scale. During the lasttwo decades, as the industry gained experience, the production of wind turbines hasgrown in size and power rating. It means that the rotor diameter, generator rating,and tower height have all increased. During the early 1980s, wind turbines with rotorspans of about 10 to 15 meters, and generators rated at 10 to 65 kW, were installed.By the mid-to late 1980s, turbines began appearing with rotor diameters of about15 to 25 meters and generators rated up to 200 kW. Today, wind energy developersare installing turbines rated at 200 kW to 2 MW with rotor spans of about 47 to80 meters. According to the American Wind Energy Association (AWEA), today’slarge wind turbines produce as much as 120 times more electricity than early turbinedesigns, with Operation and Maintenance (O&M) costs only modestly higher, thusdramatically cutting O&M costs per kWh. Large turbines do not turn as fast, andproduce less noise in comparison to small wind turbines.4

Another modification has been the introduction of new types of generators inwind systems. Since 1993, a few manufacturers have replaced the traditional asyn-chronous generator in their wind turbine designs with a synchronous generator,while other manufacturers have used doubly-fed asynchronous generators.

In addition to the above advances in wind turbine systems, new electrical con-verters and control methods were developed and tested. Electrical developmentsinclude using advanced power electronics in the wind generator system design, andintroducing the new concept, namely variable speed. Due to the rapid advancementof power electronics, offering both higher power handling capability and lowerprice/kW,5 the application of power electronics in wind turbines is expected to



Wind Turbine Systems: History, Structure, and Dynamic Model 23

increase further. Also, some control methods were developed for big turbines withthe variable-pitch blades in order to control the speed of the turbine shaft. The pitchcontrol concept has been applied during the last fourteen years.

A lot of effort has been dedicated to comparison of different structures for windenergy systems, as well as their mechanical, electrical and economical aspects.A good example is the comparison of variable-speed against constant-speed windturbine systems. In terms of energy capture, all studies come to the same result thatvariable speed turbines will produce more energy than constant speed turbines.6

Specifically, using variable-speed approach increases the energy output up to 20%in a typical wind turbine system.7 The use of pitch angle control has been shown toresult in increasing captured power and stability against wind gusts.

For operating the wind turbine in variable speed mode, different schemes havebeen proposed. For example, some schemes are based on estimating the wind speedin order to optimize wind turbine operation.8 Other controllers find the maximumpower for a given wind operation by employing an elaborate searching method.9−11

In order to perform speed control of the turbine shaft, in an attempt to achievemaximum power, different control methods such as field-oriented control and con-stant Voltage/frequency (V/f ) have been used.12−15

As mentioned in the previous section, in the last 25 years, four or five generationsof wind turbine systems have been developed.16 These different generations aredistinguished based on the use of different types of wind turbine rotors, generators,control methods and power electronic converters. In the following sections, a briefexplanation of these components is presented.

2.1.2 Classification of wind turbine rotors

Wind turbines are usually classified into two categories, according to the orientationof the axis of rotation with respect to the direction of wind, as shown in Fig. 2.217,18:

• Vertical-axis turbines• Horizontal-axis turbines.

2.1.2.1 Vertical-axis wind turbine (VAWT)

The first windmills were built based on the vertical-axis structure. This type has onlybeen incorporated in small-scale installations. Typical VAWTs include the Darriusrotor, as shown in Fig. 2.2(a). Advantages of the VAWT20,21 are:

• Easy maintenance for ground mounted generator and gearbox,• Receive wind from any direction (no yaw control required), and• Simple blade design and low cost of fabrication.



24 S. M. Barakati

(a) (b)

Nacellec

Drive Train

Generator

Tower

Rotor

Hub

Blade

Gearbox

Fig. 2.2. (a) A typical vertical-axis turbine (the Darrius rotor),19 (b) a horizontal-axis windturbine.1

Disadvantages of a vertical-axis wind turbine are:

• Not self starting, thus, require generator to run in motor mode at start,• Lower efficiency (the blades lose energy as they turn out of the wind),• Difficulty in controlling blade over-speed, and• Oscillatory component in the aerodynamic torque is high.

2.1.2.2 Horizontal-axis wind turbines (HAWT)

The most common design of modern turbines is based on the horizontal-axisstructure. Horizontal-axis wind turbines are mounted on towers as shown inFig. 2.2(b). The tower’s role is to raise the wind turbine above the ground to interceptstronger winds in order to harness more energy.

Advantages of the HAWT:

• Higher efficiency,• Ability to turn the blades, and• Lower cost-to-power ratio.




(a)

Yawmechanism

Wind

(b)

Wind

Fig. 2.3. (a) Upwind structure, (b) downwind structure.1

Disadvantages of the horizontal-axis:

• Generator and gearbox should be mounted on a tower, thus restricting servicing,and

• More complex design required due to the need for yaw or tail drive.

The HAWT can be classified as upwind and downwind turbines based on thedirection of receiving the wind, as shown in Fig. 2.3.22,23 In the upwind structurethe rotor faces the wind directly, while in downwind structure, the rotor is placedon the lee side of the tower. The upwind structure does not have the tower shadowproblem because the wind stream hits the rotor first. However, the upwind needs ayaw control mechanism to keep the rotor always facing the wind. On the contrary,the downwind may be built without a yaw mechanism. However, the drawback isthe fluctuations due to the tower shadow.

2.1.3 Common generator types in wind turbines

The function of an electrical generator is providing a means for energy conversionbetween the mechanical torque from the wind rotor turbine, as the prime mover,and the local load or the electric grid. Different types of generators are being usedwith wind turbines. Small wind turbines are equipped with DC generators of upto a few kilowatts in capacity. Modern wind turbine systems use three-phase ACgenerators.21 The common types of AC generator that are possible candidates inmodern wind turbine systems are as follows:

• Squirrel-Cage rotor Induction Generator (SCIG),• Wound-Rotor Induction Generator (WRIG),



26 S. M. Barakati

• Doubly-Fed Induction Generator (DFIG),• Synchronous Generator (with external field excitation), and• Permanent Magnet Synchronous Generator (PMSG).

For assessing the type of generator in WECS, criteria such as operationalcharacteristics, weight of active materials, price, maintenance aspects and the appro-priate type of power electronic converter, are used.

Historically, the induction generator (IG) has been extensively used in com-mercial wind turbine units. Asynchronous operation of induction generators is con-sidered an advantage for application in wind turbine systems, because it providessome degree of flexibility when the wind speed is fluctuating.

There are two main types of induction machines: squirrel-cage (SC), and wound-rotor (WR). Another category of induction generator is the DFIG; the DFIG may bebased on the squirrel-cage or wound-rotor induction generator.

The induction generator based on SCIG is a very popular machine because of itslow price, mechanical simplicity, robust structure, and resistance against disturbanceand vibration.

The wound-rotor is suitable for speed control purposes. By changing the rotorresistance, the output of the generator can be controlled and also speed control ofthe generator is possible. Although the WRIG has the advantage described above, itis more expensive than a squirrel-cage rotor.

The DFIG is a kind of induction machine in which both the stator windings andthe rotor windings are connected to the source. The rotating winding is connectedto the stationary supply circuits via power electronic converter. The advantage ofconnecting the converter to the rotor is that variable-speed operation of the turbineis possible with a much smaller, and therefore much cheaper converter. The powerrating of the converter is often about 1/3 the generator rating.24

Another type of generator that has been proposed for wind turbines in severalresearch articles is a synchronous generator.25−27 This type of generator has thecapability of direct connection (direct-drive) to wind turbines, with no gearbox.This advantage is favorable with respect to lifetime and maintenance. Syn-chronous machines can use either electrically excited or permanent magnet (PM)rotor.

The PM and electrically-excited synchronous generators differ from theinduction generator in that the magnetization is provided by a Permanent Magnetpole system or a dc supply on the rotor, featuring providing self-excitation property.Self-excitation allows operation at high power factors and high efficiencies for thePM synchronous.

It is worth mentioning that induction generators are the most common type ofgenerator use in modern wind turbine systems.5




2.1.3.1 Mechanical gearbox

The mechanical connection between an electrical generator and the turbine rotormay be direct or through a gearbox. In fact, the gearbox allows the matching of thegenerator speed to that of the turbine. The use of gearbox is dependent on the kindof electrical generator used in WECS. However, disadvantages of using a gearboxare reductions in the efficiency and, in some cases, reliability of the system.

2.1.3.2 Control method

With the evolution of WECS during the last decade, many different control methodshave been developed. The control methods developed for WECS are usually dividedinto the following two major categories:

• Constant-speed methods, and• Variable-speed methods.

2.1.3.2.1 Variable-speed turbine versus constant-speed turbine

In constant-speed turbines, there is no control on the turbine shaft speed. Constantspeed control is an easy and low-cost method, but variable speed brings the followingadvantages:

• Maximum power tracking for harnessing the highest possible energy from thewind,

• Lower mechanical stress,• Less variations in electrical power, and• Reduced acoustical noise at lower wind speeds.

In the following, these advantages will be briefly explained.Using shaft speed control, higher energy will be obtained. Reference 28 com-

pares the power extracted for a real variable-speed wind turbine system, with a34-m-diameter rotor, against a constant-speed wind turbine at different wind speeds.The results are illustrated in Fig. 2.4. The figure shows that a variable-speed systemoutputs more energy than the constant-speed system. For example, with a fixed-speed system, for an average annual wind speed of 7 m/s, the energy produced is54.6 MWh, while the variable-speed system can produce up to 75.8 MWh, underthe same conditions. During turbine operation, there are some fluctuations relatedto mechanical or electrical components. The fluctuations related to the mechanicalparts include current fluctuations caused by the blades passing the tower and variouscurrent amplitudes caused by variable wind speeds. The fluctuations related to theelectrical parts, such as voltage harmonics, is caused by the electrical converter.The electrical harmonics can be conquered by choosing the proper electrical filter.



28 S. M. Barakati

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

Variable speed

Constant speed

Wind speed [m/s]

TU

RB

INE

PO

WE

R [K

wat

]

Fig. 2.4. Comparison of power produced by a variable-speed wind turbine and a constant-speed wind turbine at different wind speeds.

However, because of the large time constant of the fluctuations in mechanical com-ponents, they cannot be canceled by electrical components. One solution that canlargely reduce the disturbance related to mechanical parts is using a variable-speedwind turbine. References 6 and 28 compare the power output disturbance of a typicalwind turbine with the constant-speed and variable-speed methods, as shown inFig. 2.5. The figure illustrates the ability of the variable-speed system to reduceor increase the shaft speed in case of torque variation. It is important to note that thedisturbance of the rotor is related also to the mechanical inertia of the rotor.

Fig. 2.5. Power output disturbance of a typical wind turbine with constant-speed methodand variable-speed methods.1,5,27




Although a variable-speed operation is adopted in modern wind turbines, thismethod has some disadvantages, such as additional cost for extra components andcomplex control methods.9,30

2.1.4 Power electronic converter

The power electronic (PE) converter has an important role in modern WECS withthe variable-speed control method. The constant-speed systems hardly include a PEconverter, except for compensation of reactive power. The important challenges forthe PE converter and its control strategy in a variable-speed WECS are31:

• Attain maximum power transfer from the wind, as the wind speed varies, bycontrolling the turbine rotor speed, and

• Change the resulting variable-frequency and variable-magnitude AC output fromthe electrical generator into a constant-frequency and constant-magnitude supplywhich can be fed into an electrical grid.

As a result of rapid developments in power electronics, semiconductor devicesare gaining higher current and voltage ratings, less power losses, higher reliability, aswell as lower prices per kVA. Therefore, PE converters are becoming more attractivein improving the performance of wind turbine generation systems. It is worth men-tioning that the power passing through the PE converter (that determines the capacitythe PE converter) is dependent on the configuration of WECS. In some applications,the whole power captured by a generator passes through the PE converter, while inother categories only a fraction of this power passes through the PE converter.

The most common converter configuration in variable-speed wind turbine systemis the rectifier-inverter pair. A matrix converter, as a direct AC/AC converter, haspotential for replacing the rectifier-inverter pair structure.

2.1.4.1 Back-to-back rectifier-inverter pair

The back-to-back rectifier-inverter pair is a bidirectional power converter consistingof two conventional pulse-width modulated (PWM) voltage-source converters(VSC), as shown in Fig. 2.6. One of the converters operates in the rectifyingmode, while the other converter operates in the inverting mode. These two con-verters are connected together via a dc-link consisting of a capacitor. The dc-linkvoltage will be maintained at a level higher than the amplitude of the grid line-to-line voltage, to achieve full control of the current injected into the grid. Considera wind turbine system including the back-to-back PWM VSC, where the rectifierand inverter are connected to the generator and the electrical grid, respectively. Thepower flow is controlled by the grid-side converter (GSC) in order to keep the dc-link



30 S. M. Barakati

)(tarv

)(tbrv

iar(t)

ibr(t)ear(t)

ebr(t)

ecr(t)

eai(t)

ebi(t)

eci(t)icr(t)

iai(t)

ibi(t)

ici(t)

)(taiv

)(tbiv

)(tciv)(tcrv

LS

RL

LSRdcIdc Rdc

RL

Cdc

4 6 7 4 6 2

1 35 1 3 5

Fig. 2.6. The back-to-back rectifier-inverter converter.

voltage constant, while the generator-side converter is responsible for excitation ofthe generator (in the case of squirrel-cage induction generator) and control of thegenerator in order to allow for maximum wind power to be directed towards thedc bus.31 The control details of the back-to-back PWM VSC structure in the windturbine applications has been described in several papers.32−35

Among the three-phase AC/AC converters, the rectifier-inverter pair structureis the most commonly used, and thus, the most well-known and well-established.Due to the fact that many semiconductor device manufacturers produce compactmodules for this type of converter, the component cost has gone down. The dc-link energy-storage element provides decoupling between the rectifier and inverter.However, in several papers, the presence of the dc-link capacitor has been consideredas a disadvantage. The dc-link capacitor is heavy and bulky, increases the cost, andreduces the overall lifetime of the system.36−39

2.1.4.2 Matrix converter

Matrix converter (MC) is a one-stage AC/AC converter that is composed of an arrayof nine bidirectional semiconductor switches, connecting each phase of the input toeach phase of the output. This structure is shown in Fig. 2.7.

The basic idea behind the matrix converter is that a desired output frequency,output voltage and input displacement angle can be obtained by properly operatingthe switches that connect the output terminals of the converter to its input terminals.The development of MC configuration with high-frequency control was first intro-duced in the work of Venturini and Alesina in 1980.40,41 They presented a static fre-quency changer with nine bidirectional switches arranged as a 3 × 3 array and namedit a matrix converter. They also explained the low-frequency modulation method anddirect transfer function approach through a precise mathematical analysis. In thismethod, known as direct method, the output voltages are obtained from multipli-cation of the modulation transfer matrix by input voltages.42 Since then, the research




inputvoltagesource

a

b

c

OutputA CB

Fig. 2.7. Matrix converter structure, the back-to-back rectifier-inverter converter.

on the MC has concentrated on the implementation of bidirectional switches, as wellas modulation techniques.

As in case of comparison MC with the rectifier-inverter pair under PWMswitching strategy, MC provides low-distortion sinusoidal input and output wave-forms, bi-directional power flow, and controllable input power factor.43 The mainadvantage of the MC is in its compact design which makes it suitable for applicationswhere size and weight matter, such as in aerospace applications.44

The following drawbacks have been attributed to matrix converters: The mag-nitude of the MC output voltage can only reach 0.866 times than that of the inputvoltage, input filter design for MC is complex, and because of an absence of a dc-link capacitor in the MC structure the decoupling between input and output andride-through capability do not exist, limiting the use of MC.45

2.1.5 Different configurations for connecting wind turbines to the grid

The connection of the wind turbine to the grid depends on the type of electrical gen-erator and power electronic converter used. Based on the application of PE convertersin the WECS, the wind turbine configurations can be divided into three topologies:directly connected to the grid without any PE converter, connected via full-scalethe PE converter, and connected via partially-rated PE converter. In the following,the generator and power electronic converter configurations most commonly usedin wind turbine systems are discussed.

As a simple, robust and relatively low-cost system, a squirrel-cage inductiongenerator (SCIG), as an asynchronous machine, is connected directly to the grid,as depicted in Fig. 2.8. For an induction generator, using a gearbox is necessaryin order to interface the generator speed and turbine speed. The capacitor bank(for reactive power compensation) and soft-starter (for smooth grid connection) arealso required. The speed and power are limited aerodynamically by stall or pitchcontrol. The variation of slip is in the range of 1–2%, but there are some wind



32 S. M. Barakati

GearBox

ReactiveCompensator

GridSCIG

Fig. 2.8. Wind turbine system with SCIG.

turbines based on SCIG in industry with increased rotor resistance and, therefore,increased slip (2–3%). This scheme is used to allow a little bit of speeding up duringwind gusts in order to reduce the mechanical stresses. However, this configurationbased on an almost fixed speed is not proper for a wind turbine in a higher powerrange and also for locations with widely varying wind velocity.5,46

Three wind turbine systems based on induction generators, with the capabilityof variable-speed operation are shown in Fig. 2.9.5,16 The wind turbine system inFig. 2.9(a) uses a wound-rotor induction generator (WRIG). The idea of this model

GearBox

GearBox

GearBox

ReactiveCompensator

Grid

Grid

Grid

WRIG

Resistancecontrol by PEC

(a)

DFIG

(b)

_~

_~

BDFIG

(c)

_~

_~

Fig. 2.9. Wind turbine systems based on the induction generator with capability of variable-speed operation: (a) Wound-Rotor, (b) Doubly-Fed, and (c) Brushless Doubly-Fed inductiongenerators.




is that the rotor resistance can be varied electronically using a variable externalrotor resistance and a PE converter. By controlling the rotor resistance, the slip ofthe machine will be changed over a 10% range (speed range 2–4%).5 In normaloperation, the rotor resistance is low, associated with low slip, but during wind guststhe rotor resistance is increased to allow speeding up.

Figure 2.9(b) shows a configuration employing a Doubly-Fed Induction Gen-erator (DFIG) and a power electronic converter that connects the rotor winding tothe grid directly. With this configuration, it is possible to extend the speed rangefurther without affecting the efficiency. The reason for speed control without lossof efficiency is that slip power can be fed back to the grid by the converter insteadof being wasted in the rotor resistance. Note that the power rating of the powerconverter is sPnom, where “s” is the maximum possible slip and Pnom is the nominalpower of the machine. The rotor slip (s) can be positive or negative because therotor power can be positive or negative, due to the bidirectional nature of the powerelectronic converter. For example, if the power rating of the converter is 10% ofthe power rating of the generator, the speed control range is from 90% to 110% ofthe synchronous speed. It means that at 110% speed, s = −0.1 and power is fedfrom the rotor to the grid, whereas at 90% speed, the slip is s = +0.1, and 10% of thepower is fed from the grid to the rotor through the converter. With these attributes,i.e., a larger control range and smaller losses, the configuration in Fig. 2.9(b) is moreattractive than the configuration in Fig. 2.9(a).

In the configurations shown in Figs. 2.9(a) and 2.9(b), with the wound-rotorinduction generator, the access to the rotor is possible through the slip rings andbrushes. Slip rings and brushes cause mechanical problems and electrical losses. Inorder to solve the problems of using slip rings and brushes, one alternative is byusing the Brushless Doubly-Fed induction generator (BDFIG), shown in Fig. 2.9(c).In this scheme, the stator windings (main winding) are directly connected to the grid,while the three-phase auxiliary winding is connected to the electrical grid through aPE converter. By using the appropriate control in the auxiliary winding, it is possibleto control the induction machine at almost any speed. Also, in this configuration, afraction of the generator power is processed in the converter.

In the third category, the electrical machine is connected to the electrical gridvia a fully-rated converter. It means that the whole power interchanged betweenthe wind turbine and the electrical grid must be passed through a PE converter.This implies extra losses in the power conversion. However this configuration willimprove the technical performance. In this configuration, as an electrical machine,it is possible to use an induction machine or synchronous machine, as shown inFig. 2.10.5,16,31 Note that the system of Fig. 2.10(a) uses a gearbox together with aSCIG. The systems of Figs. 2.10(b) and 2.10(c) use synchronous generators withouta gear box.



34 S. M. Barakati

GearBox

Grid

IG

Multi-pole SG

_

~

(c)

(b)

(a)

~~

__

Grid

~_

Pref Qref

Pref Qref

Pref Qref

~

_

Multi-pole PM-SG

_

Grid

~_

~

Fig. 2.10. Wind turbine systems with a fully-rated power converter between generatorterminals and the grid: (a) induction generator with gearbox, (b) synchronous and (c) PMsynchronous.

In the configuration in Fig. 2.10(b), the synchronous generator needs a smallpower electronic converter for field excitation, and slip rings. An advantage of usingthe synchronous generator is the possibility of eliminating the gearbox in the windturbine (direct-drive wind turbine). Direct drive generators essentially have a largediameter because of the high torque. In gearless drives, induction machines cannot beused because of the extreme excitation losses in these large machines due to the largeair gap. However, synchronous machines can be used in direct-drive wind turbines,with either electrically excited or permanent-magnet rotor structures (Fig. 2.10(c)).Direct-drive systems with permanent magnet excitation are more expensive, becauseof the high price of magnets, but have lower losses. Nowadays, the price of permanentmagnets is decreasing dramatically. Another disadvantage of using the permanentmagnet synchronous machine is the uncontrollability of its excitation.

All configurations shown in Fig. 2.10 have the same control characteristics sincethe power converter between the generator and the grid enables fast control of




active and reactive power. Also, the generator is isolated from the grid by a dc-linkcapacitor. But, using a fully-rated power electronic converter is the disadvantage ofthese configurations.

Different wind turbine manufacturers produce different configurations. Com-paring different systems from different points of view shows a trade-off betweencost and performance.

2.1.6 Starting and disconnecting from electrical grid

When wind velocities reach approximately 7 miles per hour, the wind turbine’sblades typically start rotating, but at 9 to 10 mph, they will start generating elec-tricity. To avoid damage, most turbines automatically shut themselves down whenwind speeds exceed 55 to 65 mph. When the wind turbines are connected to or dis-connected from the grid, voltage fluctuation and transient currents can occur. Thehigh current can be limited using a soft-start circuit.20

2.2 Overall Dynamic Model of the Wind Turbine System and SmallSignal Analysis

2.2.1 Dynamic model of the wind turbine system

In this section, a nonlinear dynamic model of a grid-connected wind-energy con-version system is developed in qdo reference frame. Dynamic models of themechanical aerodynamic conversion, drive train, electrical generator, and powerelectronic converter are presented.

Different components of a wind turbine system model and the interactions amongthem are illustrated in Fig. 2.11.47 The figure shows model blocks for wind speed,the aerodynamic wind turbine, mechanical components, electrical generator, powerelectronic converter, and utility grid. The system may also contain some mechanicalparts for blades angle control. In the following sections, detailed discussions of

PowerCoefficient

AerodynamicTorque

WindSpeedModel

Bladeangle

Control

β

Aerodynamic Model

WV

g

Qg

Generator Model

MechanicalModel

UtilityGrid

TT

ωTω

Tshaft

PowerElectronicConverter

Model

Pgrid

Qgrid

PG

Cp ( , )

β

λ

λ

Fig. 2.11. Block diagram of the overall wind turbine system model.



36 S. M. Barakati

the building blocks of the overall model are presented. Note that, in the modeling,a wind turbine system with constant blade angle, without blade angle control, isconsidered.

2.2.1.1 Aerodynamic model

As illustrated in Fig. 2.11, the output of the aerodynamic model block is themechanical torque on the wind-turbine shaft, that is a function of the wind-turbinecharacteristics, wind speed, shaft speed, and the blade angle. In the following, aformula for the turbine output power and torques will be introduced.

2.2.1.2 Wind turbine output torque

As the wind blows, it turns the wind turbine’s blades, which turns the generatorrotor to produce electricity. The output power of the wind turbine is related to twoparameters: wind speed and rotor size. This power is proportional to the cubic windspeed, when all other parameters are assumed constant. Thus, the output power ofwind turbines will increase significantly as the wind speed increases. In addition,larger rotors allow turbines to intercept more wind, increasing their output power.The reason is that the rotors sweep a circular surface whose area is a function of thesquare of the blade length. Thus, a small increase in blade length leads to a largeincrease in the swept area and energy capture. But, for economical and technicalreasons, the size of the blades in wind turbines has limitations.

The mechanical power and mechanical torque on the wind turbine rotor shaftare given by Eqs. (2.1) and (2.2), respectively.60−63

PT = 1

2ρArCp(β, λ)V 3

w, (2.1)

TT = 1

2ωT

ρArCp(β, λ)V 3w, (2.2)

where

PT = mechanical power extracted from turbine rotor,

TT = mechanical torque extracted from turbine rotor,

Ar = area covered by the rotor = �R2 where R is turbine rotor radius [m],

VW = velocity of the wind [m/s],

Cp = performance coefficient (or power coefficient),

ρ = air density [kg/m3],

λ = tip-speed-ratio (TSR),

β = rotor blade pitch angle [rad.],

ωT = angular speed of the turbine shaft [rad/s].




6 7 8 9 10 11 12

0.25

0.3

0.35

0.4

PO

WE

R C

OE

FF

ICIE

NT

Cp

(λ,

β)

Tip-Speed-Ratio λ

3

2

11β β2 β3< < β

β

β

Fig. 2.12. A typical Cp versus λ curve.

The blade tip-speed-ratio is defined as follows:

λ = blade tip speed

wind speed= ωT × R

Vw

. (2.3)

The power coefficient Cp is related to the tip-speed-ratio λ, and rotor blade pitchangle, β. Figure 2.12 shows a typical Cp versus tip-speed-ratio curve. Cp changeswith different values of the pitch angle, but the best efficiency is obtained for β = 0.18

In the study, it is assumed that the rotor pitch angle is fixed and equal to zero.The power coefficient curve has been described by different fitted equations in the

literature.9,18,63 In this study, the Cp curve is approximated analytically accordingto:61,62

Cp(λ, β) = (0.44 − 0.0167β) sin

[π(−3 + λ)

15 − 0.3β

]− 0.00184(−3 + λ)β. (2.4)

The theoretical upper limit for Cp is 0.59 according to Betz’s Law, but its practicalrange of variation is 0.2–0.4.18,64

Equations (2.1)–(2.4) give a model for the transfer of wind kinetic energy tomechanical energy on the shaft of wind turbine. The block diagram of this model isshown in Fig. 2.13.

2.2.1.3 Tower-shadow effect

The tower-shadow effect is caused by the periodical passing by of the wind turbineblades past the wind tower.65,66 This gives a drop in the mechanical torque whichis transferred to the generator shaft and subsequently felt as a drop in the output



38 S. M. Barakati

T R

RVW

VWCp Cp

Ptp

ωTω

1( , )

2p WA V 3r pC λβ

(λ,β)

1

2p

WA V 3rpC

Tω

λ

β

(β,λ)

TTp

TT + TTP+

+

Fig. 2.13. Block diagram of the aerodynamic wind turbine model.

voltage. Usually the tower-shadow effect has a frequency proportional to the numberof blades, for example, three per revolution for a three blade turbines.

To account for the tower-shadow effect, a periodic torque pulse with frequencyfTP is added to the output torque of the aerodynamic model. The frequency of theperiodic torque is:46

fTP = N × fr, (2.5)

where N is the number of blades and fr the rotor angular speed (in Hz).The magnitude of the torque depends on the type of wind turbine. As mentioned,

based on the direction of wind received by the wind turbine, there are two structures:upwind and downwind. The tower-shadow effect is more significant in the downwindturbine. For this case, as a rule of thumb, the magnitude of this torque pulse equals0.1 p.u., based on the rated torque of the wind turbine. The magnitude of the torquepulse for the upwind rotor is smaller in comparison with that for the downwindrotor.20,21 The tower-shadow torque should be considered as a disturbance at theoutput of block diagram Fig. 2.13.

2.2.1.4 Mechanical model

In this subsection, a complete mechanical model for the wind turbine shaft dynamicsis presented. Since the time constants of some mechanical parts are large in com-parison with those of the electrical components, and detailed information on allmechanical parameters is not available,67 the mechanical model has been developedbased on reasonable time constant values and the data available. The model of awind turbine drive train is fundamentally a three-mass model corresponding to alarge mass for the wind turbine rotor, a mass for the gearbox and a mass for the gen-erator. The moments of inertia of the shafts and gearbox can be neglected becausethey are small compared with the moments of inertia of the wind turbine and thegenerator.68,69 Therefore, the mechanical model is essentially a two-mass model ofrotor dynamics, consisting of a large mass and a small mass, corresponding to thewind turbine rotor inertia JT and generator rotor inertia JG, respectively,8,63,66−70

as shown in Fig. 2.14. The low-speed shaft is modeled as an inertia, a spring with




eT

GJTT

TJ

sK

BT g

1: gearn

Aer

odyn

amic

Low-speedshaft

Gearbox

High-speedShaft

Generator

ω ω

Fig. 2.14. A complete mechanical model of the wind turbine shaft.

Table 2.1. Mechanical model parameters.

Parameter Description Parameter Description

JT Wind turbine inertia [kg.m2] ωT Wind turbine shaft speed [rad/s]JG Generator inertia [kg.m2] ωg Generator shaft speed [rad/s]Ks Stiffness coefficient θT Wind turbine shaft angle [rad]

[N.m/rad]B Damper coefficient θg Generator shaft angle [rad]

[N.m/rad./s]TT Wind turbine torque [N.m] 1:ngear Gear ratioTe Generator electromechanical

torque [N.m]

stiffness coefficient Ks, and a damper with damping coefficient B. An ideal gearbox with the gear ratio 1 : ngear is included between the low-speed and high-speedshafts. Also, the parameters of the mechanical model are defined in Table 2.1.

The drive train converts the aerodynamic torque TT on the low-speed shaft to thetorque on the high-speed shaft Te. The dynamics of the drive train are described bythe following three differential equations:

d

dtωT = 1

JT

[TT − (Ksδθ + Bδω)], (2.6)

d

dt(δθ) = δω, (2.7)

d

dtωg = 1

JT

[1

ηgear(Ksδθ + Bδω) − Te

], (2.8)

where δθ = θT − θg/ngear, δω = ωT − ωg/ngear, TT is the turbine mechanicaltorque from Eq. (2.2) and Te is the generator electromechanical torque which willbe introduced in the next section.



40 S. M. Barakati

It is worth mentioning that as a simple dynamic model, one can consider asingle mass model, i.e., one lumped mass accounting for all the rotating parts ofthe wind turbine. In fact, the stiffness and damping of shaft are used for the sakeof completeness and can be removed in case they are not important in a specificapplication. This removal simplifies the dynamic model and reduces system order,but the completeness of the dynamic model will be compromised.

2.2.1.5 Induction machine model

Figure 2.15 shows an idealized three-phase induction machine consisting of a statorand a rotor.71,72 Each phase in stator and rotor windings has a concentrated coilstructure. The balanced three-phase ac voltages in the stator induce current in theshort-circuited rotor windings by induction or transformer action. It can be shownthat the stator current establishes a spatially sinusoidal flux density wave in the airgap which rotates at synchronous speed given by:

ωs = 2

Pωe, (2.9)

where ωs is the synchronous speed in rad/sec, ωe stator angular electrical frequencyin rad/sec, and P the number of poles. If the mechanical shaft speed of the machineis defined as ωr (in rad/sec), at any speed ωs, the speed difference ωs − ωr createsslip (s). The slip is defined as follows:

s = ωs − ωr

ω. (2.10)

as

bs

cs

arbr

cr

c r

a r b r

c s

a s

b s

Statoras axis

Rotorar axis

r

Rotor

Stator

Rotor

r = ωrtθ

Fig. 2.15. Equivalent circuit for the induction machine.69




rs Lls

LM

Llr

rr

s

VsIr

Fig. 2.16. A per-phase equivalent circuit for induction machine.

In the induction generator, at steady-state operating point, ωr (= ωg) is slightlyhigher than ωs (i.e., s < 0), while in induction motor, ωr is slightly lower than ωs

(i.e., s > 0).A transformer-like per-phase equivalent circuit for induction machine, in steady-

state, is shown in Fig. 2.16.In this equivalent circuit, rs is the stator resistance, Lls the stator inductance, LM

the magnetizing inductance, Llr the rotor inductance (referred to stator circuit) and rr

the rotor resistance (referred to stator circuit). In the generator mode, the resistancerr/s has a negative value. This negative resistance implies the existence of a sourceand therefore, the direction of power in the generator mode is from the rotor circuitto the stator circuit.

In steady-state, the electromechanical torque on the shaft is a function of therotor current, rotor resistance and slip, as expressed by Eq. (2.11).71,72

Te = 3

ωs

I2r

Tr

s. (2.11)

If the terminal voltage and frequency are constant, Te can be calculated as afunction of slip (s) from Eq. (2.11). Figure 2.17 shows the torque-speed curve,where the value of slip is extended beyond the region 0 ≤ s ≤ 2. In Fig. 2.17two distinct zones can be identified: generating mode (s < 0) and motoring mode(0 ≤ s ≤ 1). The sign of the torque in the motoring and generating regions has beenspecified based on the convention that: Tmotor > 0 and Tgenerator < 0. The magnitudeof the counter torque that is developed in the induction generator as a result of theload connected at the machine’s stator terminals is then Tc = −Te. The theoreticalrange of operation in the generator mode is limited between the synchronous angularspeed ωs and the ωr corresponding to the pushover torque.

It is worth noting that, as shown in the equivalent circuit of Fig. 2.17, the inductionmachines have inductive nature, and therefore, the induction generator (similar toinduction motor) absorbs reactive power from its terminals. The reactive poweressentially sustains the rotating magnetic field in the air gap between the cage rotorand the stator winding. This reactive power should be supplied by the grid, in grid-connected mode, or by the capacitor-bank that is connected at the stator terminals,in stand-alone mode. Moreover, it is possible to add a power electronic converter,acting as a dynamic Var compensation device, at the stator terminals, for additionaland smoother Var control.73



42 S. M. Barakati

Motoring generating

Synchronous Speed

1

0Slip, s

puSpeed

10

ωs

ωr

ωsωr

ws

ωr

Fig. 2.17. Torque-speed curve of induction machine.69,70

The output voltage of the generator, in stand-alone operation, can be estimatedfrom the intersection point of the magnetization curve of the machine and theimpedance line of the capacitor. This intersection point defines the operating point.Also the output frequency, in a grid connection, is dictated by the grid, while in stand-alone operation, it is a function of the load, rotor speed and excitation capacitance.74

2.2.1.5.1 Dynamic model of the induction machine

A commonly-used induction machine model is based on the flux linkages.72 Thedynamic equivalent circuit of the induction machine in qdo frame is illustrated inRefs. 72 and 75.

Note that in the qdo-equivalent circuit all the rotor parameters are transferredto the stator side. The machine is described by four differential equations based onflux linkage in the qdo frame and one differential equation based on rotor electricalangular speed, as follows:

dψqs

dt= C1ψqs − ωeψds + C2ψqr + ωbvqs, (2.12)

dψds

dt= ωeψqs + C1ψds + C2ψdr + ωbvds, (2.13)

dψqr

dt= C3ψqs + C4ψqr − (ωe − ωre)ψdr, (2.14)




sr lsL

LM

lsL (ωe ωre ) dr rr

qsi+

-

qrVqri+

-

sr lsL

L

lrL qr rr

dsi+

-dsV drV

dri+

-

qrqr

b=

qsqs

bω=

dsb

ds = drdr

b=

qsV

Ψ

Ψ

Ψ

Ψ

Ψ

Ψ

Ψ

Y

ϕ ϕ

M

−

(ωe ωre )−

ωe ds

Ψωe qs

ϕ ϕ

ω

ωω

Fig. 2.18. Qdo-equivalent circuit of an induction machine.70,73

.

dψdr

dt= C3ψds + (ωe − ωre)ψqr + C4ψdr, (2.15)

dωre

dt=

( p

2J

)(Te − TL), (2.16)

where

C1 = ωb

rs

xls

(x*

ml

xls

− 1

), C2 = ωb

rs

xls

x*ml

xlr

, C3 = ωb

rr

xlr

x*ml

xls

,

C4 = ωb

rr

xlr

(x*

ml

xlr

− 1

)x*

ml = (x−1

m + x−1ls + x−1

lr

)−1,

ψds, ψqs, ψdr, and ψqr: d-axis and q-axis stator and rotor flux linkages,rr and rs: rotor and stator resistances, Xls = ωeLls and Xlr = ωeLlr: stator and

rotor leakage reactances, Xm = ωeLml : magnetization reactance, ωe, ωb: statorand base electrical angular speeds, ωre : rotor electrical angular speed, vqs, vds:q and d-axis stator voltages, vqr, vdr: q and d-axis rotor voltages, Te and TL: elec-tromechanical and load torque.

The stator and rotor currents, in the qdo-equivalent circuit of Fig. 2.18, can befound as follows:

iqs = 1

χls

(ψqs − ψmq), (2.17)

ids = 1

χls

(ψds − ψmd), (2.18)

iqr = 1

χlr

(ψqr − ψmq), (2.19)

idr = 1

χlr

(ψdr − ψmd), (2.20)



44 S. M. Barakati

where

ψmq = x*ml

[ψqs

χls

+ ψqr

χlr

]and ψmd = x*

ml

[ψds

xls

+ ψdr

xlr

].

In addition, the electromechanical torque of the machine can be written asfollows:

Te = 3

2

(p

2

) 1

ωb

(ψdsiqs − ψqsids) = C5(ψdrψqs − ψqrψds), (2.21)

where C5 = 32

P2

1ωb

x*ml

xlsxlr.

2.2.1.5.2 Constant V/f speed control method

To avoid saturation of the induction machine when the stator frequency changes, thestator terminal voltage is also adjusted using a constant V/f strategy. This method iswell known for the induction machine speed control.76 A power electronic convertershould be employed at the terminals of the induction generator to implement theconstant V/f strategy. In the study, this strategy is implemented for adjusting thespeed of the turbine shaft to achieve maximum power point tracking.

2.2.1.6 Gearbox model

The duty of a mechanical gearbox is transforming the mechanical power from theslow turning rotor shaft to a fast-turning shaft, which drives the generator. Thegearbox is mostly used in the wind turbines with induction generators. The needfor this transmission arises from the problem that an induction generator cannot bebuilt for very low speeds with good efficiency.

In order to model the gearbox, it is only needed to consider that the generatortorque can simply be transferred to the low speed shaft by a multiplication. Forexample, for the gearbox of Fig. 2.14, one can write77:

TT

Te

= ωg

ωT

= ηgear. (2.22)

Note that for a non ideal gearbox, the efficiency of the gearbox should be con-sidered in the model.

2.2.1.7 Grid model

The grid model consists of an infinite bus. The infinite-bus model can be used whenthe grid power capacity is sufficiently large such that the action of any one useror generator will not affect the operation of the power grid. In an infinite bus, thesystem frequency and voltage are constant, independent of active and reactive powerflows.




2.2.1.8 Wind speed model

Although the wind model is not part of the wind turbine model, the output powercalculation in the wind turbine rotor requires the knowledge of instantaneous windspeed.

Wind is very difficult to model because of its highly-variable behavior bothin location and time. Wind speed has persistent variations over a long-term scale.However, surface conditions such as buildings, trees, and areas of water affect theshort-term behaviour of the wind and introduce fluctuations in the flow, i.e., windspeed turbulence.

A brief review of the literature reveals different wind speed models. For example,a wind model based on superposition of components is proposed in Ref. 78. In thismethod, the wind speed is modeled by four components: mean wind speed, rampwind component, gust wind component and noise wind component. However, deter-mining all four components is a difficult task.

In this study, wind speed is modeled with a random process. The model is basedon Van Der Hoven and Von Karman’s models.60,79 The instantaneous value of windspeed, vW(t), can be described as the wind speed average value plus fluctuations inthe wind speed, as follows:

vw(t) = VWM +N∑

i=1

Ai cos(ωit + ψi), (2.23)

where VWM is the mean value of wind speed, typically determined as a 10-minuteaverage value, Ai is the amplitude of the wind speed fluctuation at discrete frequencyof wi (i = [1, N]), N is the number of samples, and ψi is a random phase angle witha uniform distribution in the interval [−π, π].

The amplitudes Ai are based on a spectral density function S(ω) that is empir-ically fit to wind turbulence. The function S(ω) can be determined using Van DerHoven’s spectral model.79 The independence of the model from the mean windspeed is a drawback of the model. Therefore, it cannot model the low frequencycomponents, and it is not proper for a complete description of the wind speed over ashort time scale, i.e., seconds, minuets, or hours.79 Von-Karman’s distribution givenby Eq. (2.24),79 a commonly-used turbulence spectral density function, is a solutionto this problem.

S(w) = 0.475σ2 LVWM[

1 +(

ωLVWM

)2]5/6 . (2.24)



46 S. M. Barakati

0 5 10 15 20 25 30 35 408

8.5

9

9.5

10

10.5

11

11.5

12Wind Speed

Win

d S

peed

[m/s

]

t[Sec]

Unfiltreed

Low-pass filtered

Fig. 2.19. Wind speed fluctuation: Unfiltered and low pass filtered.

In Eq. (2.24), σ is the standard deviation of the wind speed, and L is the turbulencelength scale [m]. The parameter L equals:

{20h, if h ≤ 30 m600, if h > 30 m

(2.25)

where h is the height at which the wind speed signal is of interest [m], whichnormally equals the height of the wind turbine shaft.

The amplitude of the ith harmonic, Ai, based on the spectral density function ofEq. (2.24), can be defined as:

Ai(ωi) = 2

π

√1

2[S(ωi) + S(ωi+1)](ωi+1 − ωi). (2.26)

Figure 2.19 shows a spectral density function based on Eq. (2.24). The parameterschosen for the simulation were: VWM = 10 [m/s], L = 180 [m], σ = 2, N = 55.The instantaneous wind speed fluctuation, based on Von-Karman’s spectral densityover time is shown in Fig. 2.20.

2.2.1.8.1 High-frequency damping effect

For wind power calculations, the instantaneous wind speed model should beaugmented with complex wind effects on the wind turbine blades, including




0 5 10 15 20 25 30 35 408

8.5

9

9.5

10

10.5

11

11.5

12

Win

d S

peed

[m/s

]

t[Sec]

Fig. 2.20. Instantaneous wind speed as a function of time.

high-frequency damping effects and tower-shadow effects. In this section, the high-frequency damping effect is discussed.

The phenomenon of damping the high-frequency wind speed variations over theblades surface, namely high-frequency damping effect, should be included in theaerodynamic model of the wind turbine.63 To approximate this effect, a low-passfilter with the following transfer function is employed.

H(s) = 1

1 + τs. (2.27)

The filter time constant τ depends on the turbine radius, average wind speed athub height, and the intensity of wind turbulence. Figure 2.19 shows the instantaneouswind speed and corresponding low-pass filtered signal.

It is worth mentioning that the low-pass filtered wind speed data can be savedin a memory and used later for simulation, instead of using the instantaneous windspeed data and the low-pass filter dynamic equation.

References

1. S. Masoud Barakati, Applications of Matrix Converters for Wind Turbine Systems (VDM VerlagDr. Muller, Germany, 2008).

2. S. Krohn, “The wind energy pioneer — Poul la Cour,” The Danish Wind Turbine ManufacturersAssociation, http://webasp.ac-aix-marseille.fr/rsi/bilan/action 0405/13LduRempart/Doc/Docpage/ windpower.pdf (2008).

3. R. Hoffmann and P. Mutschler, “The influence of control strategies on the energy capture of windturbines,” Application of Electrical Energy (2000), pp. 886–893.



48 S. M. Barakati

4. P. Migliore, J. van Dam and A. Huskey, “Acoustic Tests of Small Wind Turbinesat,” http://www.wind.appstate.edu/reports/NRELAcousticTestsofSmallWindTurbines.pdf (2007).

5. L.H. Hansen, L. Helle, F. Blaabjerg, E. Ritchie, S. Munk-Nielsen, H. Bindner, P. Sørensen andB. Bak-Jensen, “Conceptual survey of generators and power electronics for wind turbines,” RisøNational Laboratory, Roskilde, Denmark, December 2001.

6. O. Carlson, J. Hylander and K. Thorborg, “Survey of variable speed operation of wind turbines,”European Union Wind Energy Conf., Goeteborg, Sweden, May 1996, pp. 406–409.

7. M. Idan, D. Lior and G. Shaviv, “A robust controller for a novel variable speed wind turbinetransmission,” J. Solar Energy Engineering 120 (1998) 247–252.

8. S. Bhowmik and R. Spee, “Wind speed estimation based variable speed wind power generation,”Proc. 24th Annual Conf. of the IEEE, vol. 2, 31 August–4 September 1998, pp. 596–601.

9. M.G. Simoes, B.K. Bose and R.J. Spiegel, “Fuzzy logic based intelligent control of a variablespeed cage machine wind generation system,” IEEE Tran. Power Electronics 12 (1997) 87–95.

10. Q. Wang and L. Chang, “An intelligent maximum power extraction algorithm for inverter-basedvariable speed wind turbine systems,” IEEE Trans. Power Electronics 19 (2004) 1242–1249.

11. A. Miller, E. Muljadi and D.S. Zinger, “A variable speed wind turbine power control,” IEEETrans. Energy Conversion 12 (1997) 181–186.

12. L. Xu and C. Wei, “Torque and reactive power control of a doubly fed induction machine byposition sensorless scheme,” IEEE Trans. Industry Applications 31 (1995) 636–642.

13. W. Lu and B.T. Ooi, “Multiterminal LVDC system for optimal acquisition of power in wind-farmusing induction generators,” IEEE Trans. Power Electronics 17 (2002) 558–563.

14. J.L. Rodriguez-Amenedo, S. Arnalte and J.C. Burgos, “Automatic generation control of a windfarm with variable speed wind turbines,” IEEE Trans. Energy Conversion 17 (2002) 279–284.

15. D. Seyoum and M.F. Rahman, “The dynamic characteristics of an isolated self-excited inductiongenerator driven by a wind turbine,” Rec. in Proc. IAS Conf., Industry Applications Conf., AnnualMeeting, vol. 2, October 2002, pp. 731–738.

16. F. Blaabjerg, Z. Chen and S.B. Kjaer, “Power electronics as efficient interface in dispersed powergeneration systems,” IEEE Trans. Power Electronics 19 (2004) 1184–1194.

17. L.L. Freries, Wind Energy Conversion Systems (Prentice Hall, 1990).18. S. Heier, Grid Integration of Wind Energy Conversion Systems, Chap. 1 (John Wiley & Sons Ltd,

1998).19. Live Journal, www.windturbine.livejournal.com.20. J.F. Manwell, J.G. McGowan and A.L. Rogers, Wind Energy Explained, Theory, Design and

Application (Wiley, 2002).21. S. Mathew, Wind Energy Fundamentals, Resource Analysis, and Economics (Springer, 2006).22. P.G. Casielles, J. Sanz and J. Pascual, “Control system for a wind turbine driven self excited

asynchronous generator,” Proc. Mediterranean Electrotechnical Integrating Research, Industryand Education in Energy and Communication Engineering, MELECON’89 Conf., April 1989,pp. 95–98.

23. D.A. Torrey, “Variable-reluctance generators in wind-energy systems,” Rec. in IEEE PESC ’93,Power Electronics Specialists Conf., June 1993, pp. 561–567.

24. J. Matevosyan, “Wind power in areas with limited export capability,” Licentiate Thesis RoyalInstitute of Technology Department of Electrical Engineering Stockholm (2004).

25. A.J.G. Westlake, J.R. Bumby and E. Spooner, “Damping the power-angle oscillations of apermanent-magnet synchronous generator with particular reference to wind turbine applica-tions,” IEE Proc. Electric Power Applications, vol. 143, May 1996, pp. 269–280.

26. S. Grabic, N. Celanovic and V. Katie, “Series converter stabilized wind turbine with permanentmagnet synchronous generator,” IEEE Conf. PESC’04, Power Electronics Specialists Conf.,vol. 1, June 2004, pp. 464–468.




27. L. Dambrosio and B. Fortunato, “One step ahead adaptive control technique for a wind turbine-synchronous generator system,” Proc. IECEC-97, Intersociety Energy Conversion EngineeringConf., vol. 3, July 1997, pp. 1970–1975.

28. D.S. Zinger and E. Muljadi, “Annualized wind energy improvement using variable speeds,” IEEETrans. Industry Applications 33 (1997) 1444–1447.

29. P. Bauer, S.W.H. De Haan, and M.R.Dubois, “Wind energy and offshore windparks: State of theart and trends”, EPE-PEMC Dubrovnik & Cavat (2002), pp 1–15.

30. P. Bauer, S.W.H. De Haan, C.R. Meyl and J.T.G. Pierik, “Evaluation of electrical systemsfor offshore wind farms,” Rec. in IEEE Industry Applications Conf., vol. 3, October 2000,pp. 1416–1423.

31. R. Teodorescu and F. Blaabjerg, “Flexible control of small wind turbines with grid failuredetection operating in stand-alone and grid-connected mode,” IEEE Trans. Power Electronics19 (2004) 1323–1332.

32. E. Bogalecka and Z. Krzeminski, “Control systems of doubly-fed induction machine suppliedby current controlled voltage source inverter,” 6th Int. Conf. on Electrical Machines and Drives,Conf. Publ. No. 376, September 1993, pp. 168–172.

33. C. Knowles-Spittle, B.A.T. Al Zahawi and N.D. MacIsaac, “Simulation and analysis of 1.4 MWstatic Scherbius drive with sinusoidal current converters in the rotor circuit,” Proc. IEE 7th Int.Conf. on Power Electronics and Variable Speed Drives, Conf. Publ. No. 456, September 1998,pp. 617–621.

34. R. Pena, J.C. Clare and G.M. Asher, “A doubly fed induction generator using back-to-backPWM converters supplying an isolated load from a variable speed wind turbine,” Electric PowerApplications, IEE Proc., vol. 143, September 1996, pp. 380–387.

35. T. Yifan and L. Xu, “A flexible active and reactive power control strategy for a variable speedconstant frequency generating system,” IEEE Trans. Power Electronics 10 (1995) 472–478.

36. W.-S. Chienand and Y.-Y. Tzou, “Analysis and design on the reduction of DC-link electrolyticcapacitor for AC/DC/AC converter applied to AC motor drives,” Proc. IEEE PESC 98, PowerElectronics Specialists Conf., vol. 1, May 1998, pp. 275–279.

37. J.S. Kim and S.K. Sul, “New control scheme for AC-DC-AC converter without DC link elec-trolytic capacitor,” IEEE Proc. PESC’93, Power Electronics Specialists Conf., 1993, June 1993,pp. 300–306.

38. K. Siyoung, S. Seung-Ki and T.A. Lipo, “AC to AC power conversion based on matrix convertertopology with unidirectional switches,” Proc. APEC’98, Applied Power Electronics Conf. andExposition, 1998, vol. 1, February 1998, pp. 301–307.

39. M. Salo and H. Tuusa, “A high performance PWM current source inverter fed induction motordrive with a novel motor current control method,” Proc. IEEE PESC’99, Power ElectronicsSpecialists Conf., 1999, vol. 1, 27 June–1 July 1999, pp. 506–511.

40. M. Venturini, “A new sine wave in sine wave out, conversion technique which eliminates reactiveelements,” Proc. POWERCON 7 (1980), pp. E3 1–E3 15.

41. M. Venturini and A. Alesina, “The generalized transformer: A new bidirectional sinusoidalwaveform frequency converter with continuously adjustable input power factor,” Proc. IEEEPESC’80 (1980), pp. 242–252.

42. J. Rodriguez, “A new control technique for AC–AC converters,” Proc. IFAC Control in PowerElectronics and Electrical Drives Conf., Lausanne, Switzerland (1983), pp. 203–208.

43. L. Helle, K.B. Larsen, A.H. Jorgensen, S. Munk-Nielsen and F. Blaabjerg, “Evaluation of mod-ulation schemes for three-phase to three-phase matrix converters,” IEEE Trans. Industrial Elec-tronics 51 (2004) 158–171.

44. M. Aten, C. Whitley, G. Towers, P. Wheeler, J. Clare and K. Bradley, “Dynamic performanceof a matrix converter driven electro-mechanical actuator for an aircraft rudder,” Proc. 2nd Int.Conf. Power Electronics, Machines and Drives, PEMD 2004, vol. 1, April 2004, pp. 326–331.



50 S. M. Barakati

45. B. Ooi and M. Kazerani, “Elimination of the waveform distortions in the voltage-source-convertertype matrix converter,” Conf. Rec. IEEE-IAS Annu. Meeting, vol. 3 (1995), pp. 2500–2504.

46. J. Wu, P. Dong, J.M. Yang and Y.R. Chen, “A novel model of wind energy conversion system,”Proc. DRPT2004, IEEE Int. Conf. on Electric Utility Deregulation and Power Technologies,April 2004, Hong Kong, pp. 804–809.

47. S.M. Barakati, M. Kazerani and J.D. Aplevich, “A dynamic model for a wind turbine systemincluding a matrix converter,” Proc. IASTED, PES2007, January 2006, pp. 1–8.

48. T. Satish, K.K. Mohapatra and N. Mohan, “Speed sensorless direct power control of a matrixconverter fed induction generator for variable speed wind turbines,” Proc. Power Electronics,Drives and Energy Systems, PEDES’06, December 2006, pp. 1–6.

49. S.F. Pinto, L.Aparicio and P. Esteves, “Direct controlled matrix converters in variable speed windenergy generation systems power engineering,” Proc. Energy and Electrical Drives, POWERENG2007, April 2007, pp. 654–659.

50. K. Huang and Y. He, “Investigation of a matrix converter-excited brushless doubly-fed machinewind-power generation system,” Proc. Power Electronics and Drive Systems, PEDS 2003,November 2003, vol. 1, pp. 743–748.

51. Q. Wang, X. Chen and Y. Ji, “Control for maximal wind energy tracing in matrix converter ACexcited brushless doubly-fed wind power generation system,” Proc. IEEE Industrial Electronics,IECON 2006, November 2006, pp. 718–723.

52. L. Zhang and C. Watthanasarn, “A matrix converter excited doubly-fed induction machine as awind power generator,” Proc. Power Electronics and Variable Speed Drives, September 1998,pp. 532–537.

53. D. Aouzellag, K. Ghedamsi and E.M. Berkouk, “Network power flow control of variable speedwind turbine,” Proc. Power Engineering, Energy and Electrical Drives, POWERENG 2007,April 2007, pp. 435–439.

54. L. Zhang, C. Watthanasarn and W. Shepherd, “Application of a matrix converter for the powercontrol of a variable-speed wind-turbine driving a doubly-fed induction generator,” Proc. Indus-trial Electronics, Control and Instrumentation, IECON’97, vol. 2, November 1997, pp. 906–911.

55. M. Kazerani, “Dynamic matrix converter theory development and application,” Ph.D. thesis,Department of Electrical Eng., McGill University, Canada (1995).

56. B.T. Ooi and M. Kazerani, “Application of dyadic matrix converter theory in conceptual designof dual field vector and displacement factor controls,” Proc. IEEE Industry Applications SocietyAnnual Meeting, vol. 2, October 1994, pp. 903–910.

57. H. Nikkhajoei, A. Tabesh and R. Iravani, “Dynamic model of a matrix converter for controllerdesign and system studies,” IEEE Trans. Power Delivery 21 (2006) 744–754.

58. A. Alesina and M. Venturini, “Solid-state power conversion: A Fourier analysis approach togeneralized transformer synthesis,” IEEE Trans. Circuits and Systems 28 (1981) 319–330.

59. S.M. Barakati, M. Kazerani and J.D. Aplevich, “An overall model for a matrix converter,” IEEEInt. Symp. Industrial Electronics, ISIE08, June 2008.

60. S.M. Barakati, M. Kazerani and J.D. Aplevich, “A dynamic model for a wind turbine systemincluding a matrix converter,” Proc. IASTED, PES2007, January 2006, pp. 1–8.

61. S.M. Barakati, M. Kazerani and X. Chen, “A new wind turbine generation system based onmatrix converter,” Proc. IEEE PES General Meeting, June 2005, pp. 2083–2089.

62. E.S. Adin and W. Xu, “Control design and dynamic performance analysis of a wind turbine-induction genrator unit,” IEEE Trans. Energy Conversion 15 (2000) 91–96.

63. J.G. Slootweg, H. Polinder and W.L. Kling, “Representing wind turbine electrical generatingsystems in fundamental frequency simulations,” IEEE Trans. Energy Conversion 18 (2003)516–524.

64. L.S.T. Ackermann, “Wind energy technology and current status. A review,” Renewable andSustainable Energy Review 4 (2000) 315–375.




65. E. Muljadi and C.P. Butterfield, “Power quality aspects in a wind power plant,” IEEE PowerEngineering Society General Meeting Montreal, Quebec, Canada, June 2006.

66. R. Grunbaum, “Voltage and power quality control in wind power,” ABB Power Systems AB,http://library.abb.com/GLOBAL/SCOT/scot221.nsf/VerityDisplay/6B5A44E46BAD5D3DC1256FDA003B4D11/$File/PowerGenEurope2001.pdf (2008).

67. S. Kim, S.-K. Sul and T.A. Lipo, “AC/AC power conversion based on matrix converter topologywith unidirectional switches,” IEEE Tran. Industry Applications 36 (2000) 139–145.

68. A.D. Hansen, C. Jauch, P. Sørensen, F. Iov and F. Blaabjerg, “Dynamic wind turbine models inpower system simulation tool DIgSILENT,” Risø National Laboratory, Roskilde, http://www.digsilent.de/Software/Application Examples/ris-r-1400.pdf (2003).

69. S. Seman, F. Iov, J. Niiranen and A. Arkkio, “Comparison of simulators for variable speed windturbine transient analysis,” Int. J. Energy Research 30 (2006) 713–728.

70. L. Mihet-Popa, F. Blaabjerg and I. Boldea, “Wind turbine Generator modeling and simulationwhere rotational speed is the controlled variable,” IEEE Trans. Industry Application 40 (2004)3–10.

71. B.K. Bose, Power Electronics And Ac Drives (Prentice-Hall, 1986).72. P.C. Krause, O. Wasynczuk and S.D. Sudhoff, Analysis of Electric Machinery (IEEE Press, 1994).73. T.F. Chan and L.L. Lai, “Capacitance requirements of a three-phase induction generator self-

excited with a single capacitance and supplying a single-phase load,” IEEE Trans. Energy Con-version 17 (2002) 90–94.

74. Electric Systems Consulting ABB Inc., “Integration of wind energy into the alberta electricsystem — stages 2 & 3: Planning and Interconnection Criteria,” Alberta Electric System OperatorReport Number: 2004-10803-2.R01.4.

75. B. Ozpineci and L.M. Tolbert, “Simulink implementation of induction machine model — amodular approach,” Proc. Electric Machines and Drives Conference, IEMDC’03, vol. 2, June2003, pp. 728–734.

76. A. Munoz-Garcıa, T.A. Lipo and D.W. Novotny, “A new induction motor V/f control methodcapable of high-performance regulation at low speeds,” IEEE Tran. Industry Applications 34(1998) 813–821.

77. Powersim Technologies INC, Psim user manual, version 4.1, Set. 1999, http://www.powersimtech.com (2008).

78. P.M. Anderson and A. Bose, “Stability simulation of wind turbine systems,” IEEE Trans. PowerApparatus and system PAS-102 (1983) 3791–3795.

79. C. Nichita, D. Luca, B. Dakyo and E. Ceanga, “Large band simulation of the wind speed for realtime wind turbine simulators,” IEEE Trans. Energy Conversion 17 (2002) 523–529.

80. Starting (and stopping) a turbine, http://www.windpower.org/en/tour/wtrb /electric.htm (2008).81. L. Neft and C.D. Shauder, “Theory and design of a 30-hp matrix converter,” IEEE Trans. Indus-

trial Applications 28 (1992) 546–551.82. P. Wheeler and D. Grant, “Optimized input filter design and low loss switching techniques for a

practical matrix converter,” Proc. of Industrial Electronics, vol. 144, January 1997, pp. 53–60.83. D.G. Holmes and T.A. Lipo, “Implementation of a controlled rectifier using AC-AC matrix

converter theory,” Conf. Rec. PESC’89, Power Electronics Specialists Conf., June 1989, vol. 1,pp. 353–359.

84. H.W. van der Broeck, H.-C. Skudelny and G.V.Stanke, “Analysis and realization of a pulsewidthmodulator based on voltage space vectors,” IEEE Tran. Industry Applications 24 (1988) 142–150.

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