MODELING AND SIMULATION
OF WIND TURBINES
A Thesis Submitted to the
Graduate School of Natural and Applied Sciences of
Dokuz Eylül University
In Partial Fulfillment of the Requirements for
the Degree of Master of Science in Electrical & Electronics Engineering,
Electrical & Electronics Engineering Program
by
Osman Oral KIVRAK
February, 2003
IZMIR
M.Sc. THESIS EXAMINATION RESULT FORM
We certify that we have read this thesis and “MODELING AND
SIMULATION OF WIND TURBINES” completed by OSMAN ORAL
KIVRAK under supervision of PROF. DR. MUSTAFA GÜNDÜZALP and that in
our opinion it is fully adequate, in scope and in quality, as a thesis for the degree of
Master of Science.
Prof. Dr. Mustafa GÜNDÜZALP
Supervisor
(Committee Member) (Committee Member)
Approved by the
Graduate School of Natural and Applied Sciences
Prof. Dr. Cahit HELVACI Director
I
ACKNOWLEDGMENTS
I wish to thank to my supervisor Prof. Dr. Mustafa GÜNDÜZALP for his
guidance and understanding throughout my project.
I wish also thank to Prof. Dr. Eyüp AKPINAR for his support on critical points.
I am also grateful to my family and colleagues for their advices.
Osman Oral KIVRAK
II
ABSTRACT
Increasing worldwide energy deficiency causes raising importance of
development of new energy resources. It is foreseen that new energy resources
should not harm environment and natural life beside meeting present and future
energy demand. Accordingly, a great tendency towards renewable energy resources
took place in the market.
Wind energy has become the most popular resource in the last decade by its purity
and sustainability. Wind energy conversion systems convert the aerodynamic power
in an air stream into the electric power. Principally, a wind energy conversion system
consists of blade(s), which captures the aerodynamic power in the wind, shaft,
which transfers the torque created by the turning action of blade(s) and generator,
which converts this torque into electric power.
Unlike other energy production systems, wind, as a source of energy for wind
energy conversion systems, has a structure of showing sudden changes depending on
climatic conditions. These sudden changes in wind speed may cause some unwanted
mechanical or electrical damages, therefore it is necessary to supervise produced
power curve continuously. Several power control methods are developed for this
purpose. Pitch control – opening and closing of blades along their longitudinal axes -
is the most efficient and popular power control method especially for variable-speed
wind turbines.
In this project, status and importance of wind energy conversion systems
throughout the world, the energy conversion operation in wind turbines and
components of them are investigated. Then, wind turbines are classified according to
different categories. At final, a megawatt size, variable-speed wind turbine is
modeled and its operation is observed by using MATLAB v5.2 – SIMULINK
III
software. Output power curve regulation is carried out by ‘pitch control’ method.
The prototype for the simulation is VESTAS V80 – 2.0 MW model wind turbine.
Keywords: Wind energy, renewable, turbine, variable speed, pitch control,
energy conversion, MATLAB.
IV
ÖZET
Enerji açiginin her geçen gün arttigi dünyamizda, yeni enerji kaynaklari
gelistirmenin önemi de her geçen gün artmaktadir. Olusturulacak yeni enerji
kaynaklarinin, mevcut ve gelecekteki enerji ihtiyacini karsilamasi ile birlikte, çevreyi
ve dogal yasami da olumsuz yönde etkilememesi öngörülmektedir. Bu dogrultuda,
enerji sektöründe yenilenebilir enerji kaynaklarina yönelim artmaktadir.
Rüzgar enerjisi, temizligi ve sürekliligi ile, son 10 yilda en popüler kaynak
olmustur. Rüzgar enerjisi dönüsüm sistemleri, rüzgarin içinde bulundurdugu
aerodinamik gücü elektriksel güce dönüstürürler. Bir rüzgar enerjisi dönüsüm
sistemi, prensip olarak, rüzgardaki aerodinamik gücü yakalayan kanat(lar), kanatlarin
dönme hareketi ile olusan torku ileten saft ve bu mekanik torku elektriksel güce
çeviren jeneratörden olusmaktadir.
Diger enerji üretim sistemlerinden farkli olarak, rüzga r enerjisi dönüsüm
sistemlerinde enerji kaynagi olarak kullanilan rüzgar, iklim kosullarina bagli olarak
ani degisimler gösterebilen bir yapidadir. Bu ani degisimler, sistemde mekaniki ve
elektriki birçok hasara yol açabileceginden, üretilen güç egrisinin sürekli denetim
altinda bulundurulmasi gerekmektedir. Bu amaçla, çesitli güç kontrol yöntemleri
gelistirilmistir. Pitch kontrolü – türbin kanatlarinin kendi dikey eksenlerinde açilip
kapatilmasi -, özellikle degisken hizlarda çalisan rüzgar türbinleri için en verimli ve
popüler güç kontrolü yöntemidir.
Bu projede, rüzgar enerjisi dönüsüm sistemlerinin önemi ve dünyadaki durumu,
rüzgar türbinlerinde gerçeklesen enerji dönüsüm islemi ve türbin aksamlari
incelenmistir. Daha sonra rüzgar türbinleri çesitli kategorilere göre siniflandirilmistir.
Son olarak, MATLAB v5.2 – SIMULINK yazilimi kullanilarak, degisken hizlarda
çalisan megawatt boyutunda bir rüzgar türbini modellenerek çalismasi gözlenmistir.
V
Çikis gücü ayari ‘pitch control’ yöntemiyle gerçeklestirilmistir. Modelde prototip
olarak VESTAS V80 – 2.0 MW model rüzgar türbini alinmistir.
Anahtar Kelimeler: Rüzgar enerjisi, yenilenebilir, türbin, degisken hizli, açi
kontrolü.
VI
CONTENTS
Page
Contents………………………………………………………………………... VI
List of Tables…………………………………………………………………... X
List of Figures...……………………………………………………………….. XI
Chapter One
INTRODUCTION
1.1 Historical Background…………………...………………………………....... 4
1.2 Functional Structure of Wind Turbines….………………………………....... 6
Chapter Two
COMPONENTS OF WIND TURBINES
2.1 Common Components……………………...……………………………..... 8
2.1.1 Nacelle……………………..………………………………………........ 8
2.1.2 Blade……………..……………………...…………………………........ 8
2.1.3 Low Speed Shaft………………..….………………………………........ 11
2.1.4 High Speed Shaft…………..………………………………………........ 11
2.1.5 Disc Brake……………………………...….………………………........ 11
2.1.6 Generator……….……………………….…………………………........ 12
2.1.7 Tower……………………..………..………………………………........ 12
2.2 Optional Components……………………………………………………..... 13
VII
2.2.1 Gear Box……………..…………….………………………………..... 13
2.2.2 V / Hz Converter………………………..…………………………..... 13
2.2.3 Yaw Assembly………………………………………….…………..... 14
2.2.4 Pitch Control Mechanism……………...……………………………... 14
2.2.5 Electronic Controller…………………...…………………………...... 15
Chapter Three
ELECTROMECHANICAL ENERGY CONVERSION
3.1 Aerodynamics of Wind Turbines………...………………………………....... 18
3.1.1 Aerodynamic Forces………..……...………………………………........ 18
3.1.1.1 Drag Forces……………….......………………………………........ 19
3.1.1.2 Lift Forces……………………………………….……………........ 19
3.1.2 Aero-Foils…………………………..………...……………………........ 20
3.2 Energy and Power in The Wind………….………………………………....... 22
3.2.1 Power Coefficient ……………………..…………………..………........ 25
3.2.2 Tip Speed Ratio………………………………………………................ 27
3.2.3 Effect of The Number of Blades……...................................................... 28
3.3 Generator Theory………………………...………………………………....... 33
3.3.1 DC Machines……..……………………………………………….......... 33
3.3.1.1 Theory…………………………...……………………………........ 33
3.3.1.2 DC Generator Applications in Wind Turbines…………………….. 36
3.3.2 Synchronous AC Machines (Alternators)………………………………. 36
3.3.2.1 Theory…………………………………………………................... 37
3.3.2.2 The Rotation Speed of a Synchronous Generator…………………. 39
3.3.2.3 Internal Voltage of a Synchronous Generator……………………... 40
3.3.2.4 The Equivalent Circuit of an Alternator…………………………… 42
3.3.3 Asynchronous (Induction) AC Machines………………………………. 44
3.3.3.1 Equivalent Circuit of an Induction Machine………………………. 46
3.3.3.1.1 Rotor Circuit Model………………………………………...... 48
3.3.3.1.2 Final Equivalent Circuit……………………………………….50
VIII
3.3.4 Recent Developments in Generators for Wind Turbines……………….. 56
3.3.4.1 Dual Generators……………………………………………………. 56
3.3.4.2 Direct-Drive Generators…………………………………………… 57
3.4 Grid Integration……………………………………………………………..... 58
3.4.1 Frequency Converter Systems………………………………………...... 59
3.4.1.1 Power Semiconductors for Frequency Converters………………… 63
3.4.1.1.1 Semiconductor Diodes……………………………………...... 64
3.4.1.1.2 Thyristors…………………………………………………...... 65
3.4.1.1.3 Transistors…............................................................................. 65
3.4.1.2 Characteristics of Power Converters………………………………. 67
Chapter Four
CLASSIFICATION OF WIND TURBINES
4.1 Classification by Axis of Rotation……………………...………………......... 69
4.1.1 Horizontal Axis Wind Turbines (HAWT)…………………………........ 70
4.1.2 Vertical Axis Wind Turbines (VAWT)……………………………........ 71
4.2 Classification by Rotor Speed……………………………………………....... 72
4.2.1 Variable Rotor Speed…………..….………………………………........ 73
4.2.2 Constant Rotor Speed.…………………..…………………………........ 74
4.3 Classification by Power Control…………………………………………...… 75
4.3.1 Pitch Control……………………………………………………………. 80
4.3.2 Stall Control…………………………………………………………….. 81
4.4 Classification by Location of Installation…………………………………..... 83
4.4.1 On-Shore Wind Turbines……………………………………………….. 83
4.4.2 Off-Shore Wind Turbines………………………………………………. 84
IX
Chapter Five
EXPERIMENTAL WORK
5.1 Sub-Systems in The Model……………………………...………………........ 89
5.1.1 Yaw Control Block………………………...………………………........ 89
5.1.2 Turbine Efficiency Block…………….……………………………........ 90
5.1.3 Pitch Control Block…………………………………………………...... 91
5.1.4 Angular Speed Calculation Block…........................................................ 93
5.1.5 Cp – ? Selection Block………………………………………………….. 95
5.2 Simulation Results…………………………………………………………… 95
Chapter Six
CONCLUSIONS
6.1 Future Prospects………………………………………...……………….........106
References………...………………………………………...………………....... 108
Appendices….…………………………………………………………………... 110
Appendix A – Flowchart of The Simulated System………………………..... A
Appendix B – VESTAS V80 – 2.0 MW Wind Turbine…………………....... B
X
LIST OF TABLES
Page
Table 1.1 World Electricity Consumption with Estimations………...………... 2
Table 1.2 Wind Power Installations Worldwide…..…………………………... 3
Table 1.3 Wind Energy Capacity Leaders Worldwide by End 2001....……….. 4
Table 2.1 Number of Blades for Commercial Wind Turbine Designs………… 11
Table 3.1 Speed Definitions…………………………………………………… 27
Table 3.2 Common Synchronous Speeds for Generators……………………... 55
Table 3.3 Characteristics and Maximum Ratings of Switchable Power
Semiconductors………………………………….………………….. 67
Table 4.1 Descriptions of Operational Regions for a Typical Wind Turbine…. 77
Table 4.2 Pitch vs. Stall Issues………………………………………………… 82
Table 5.1 Modelled Wind Turbine Simulation Results……….......................... 103
XI
LIST OF FIGURES
Page
Figure 1.1 World electricity consumption with estimations ..……………….. 1
Figure 1.2 Wind power installations worldwide…..…………………............. 2
Figure 1.3 Power transfer in a wind energy converter…………….................. 6
Figure 2.1 Wind turbine types by rotor assemblies………………………….. 7
Figure 2.2 Nacelle………...………………………………………….............. 8
Figure 2.3 Horizontal axis wind turbines according to number of blades…… 10
Figure 2.4 A typical gear…………………………………………………….. 13
Figure 2.5 AC – AC signal conversion………………………………............. 14
Figure 2.6 A typical wind turbine in detail (VESTAS V27 / 225 kW)...……. 16
Figure 3.1 A typical wind turbine showing all components…………………. 17
Figure 3.2 Lift and drag forces acting on rotor blade…………………........... 19
Figure 3.3 Components of wind power acting on rotor blade……………….. 21
Figure 3.4 Cylindrical volume of air passing at velocity V (10 m/s) through
a ring enclosing an area, ‘A’, each second……………………….. 23
Figure 3.5 Wind flow through a wind turbine……………………………….. 25
Figure 3.6 Power coefficient versus tip speed ratio for a constant speed wind
turbine…………………………………………………………….. 31
Figure 3.7 Power coefficient versus tip speed ratio for a variable speed wind
turbine for different pitch angles from 0 to 15 degrees by 0.5
degree increments…………….…………………………………... 32
Figure 3.8 The equivalent circuit for DC motors……………………….……. 34
Figure 3.9 A salient six-pole rotor for a synchronous machine……………… 38
Figure 3.10 A non-salient two-pole rotor for a synchronous machine………... 39
XII
Figure 3.11 a. Plot of flux vs. field current for synchronous generators
b. The magnetization curve for synchronous generators………….
41
Figure 3.12 A simple circuit for alternators…………………………………… 42
Figure 3.13 The per-phase equivalent circuit for synchronous generators……. 43
Figure 3.14 Cutaway diagram for a wound-rotor induction machine…………. 45
Figure 3.15 Cutaway diagram for a squirrel-cage induction machine………… 45
Figure 3.16 Transformer model for an induction machine……………………. 47
Figure 3.17 Magnetization curve for an induction machine compared to that
for a transformer………………………………………………….. 47
Figure 3.18 The rotor circuit model for induction machines………………….. 49
Figure 3.19 The rotor circuit model with all the frequency (slip) effects
concentrated in resistor RR ………………………..……………... 49
Figure 3.20 The per-phase equivalent circuit for induction machines………… 51
Figure 3.21 Torque-Speed curve for a MW-size induction machine………….. 52
Figure 3.22 Electrical energy conversion by power converters……………….. 60
Figure 3.23 Basic wiring diagram for direct frequency converters…………… 62
Figure 3.24 Indirect frequency converters…………………………………….. 63
Figure 4.1 Horizontal and vertical axis wind turbines……………………….. 70
Figure 4.2 Horizontal axis wind turbine configurations……………………... 71
Figure 4.3 Vertical axis wind turbine configurations………………………... 72
Figure 4.4 Operating regions of a typical wind turbine……………………… 76
Figure 4.5 Rotor diameter vs. power output…………………………………. 78
Figure 4.6 Swept area by rotor blades……………………………………….. 79
Figure 4.7 Pitch Control……………………………………………………… 81
Figure 4.8 Stall Control………………………………………………………. 81
Figure 4.9 Stall & Pitch controlled power schemes………………………….. 83
Figure 5.1 Overview of the wind turbine simulation…...……………………. 88
Figure 5.2 Yaw control block……………………………………………....... 90
Figure 5.3 Turbine efficiency block..........………………………………….... 90
Figure 5.4 Turbine efficiency characteristics correspond ing to wind speed.... 91
Figure 5.5 Graphical demonstrations for the response of pitch control
mechanism....................................................................................... 92
XIII
Figure 5.6 Pitch control block with 0-15 degrees adjustment interval………. 93
Figure 5.7 Angular speed calculation block..................................................... 94
Figure 5.8 Wind speed values filtered by yaw control block………………... 96
Figure 5.9 Aerodynamic power in the wind…………………………………. 96
Figure 5.10 Captured wind power by the turbine (Input power to generator)… 97
Figure 5.11 Angular speed variation of the turbine in respect of each wind
speed change (Change of input torque)…………………………... 97
Figure 5.12 Angular shaft speed of the turbine………………………………... 98
Figure 5.13 Rotational speed of turbine shaft before gearbox………………… 98
Figure 5.14 Rotational speed of turbine shaft after gearbox (Rotational speed
of generator rotor)………………………………………………… 99
Figure 5.15 Tip speed ratio…...……………………………………………….. 99
Figure 5.16 Blade pitch angle (a)………………...…………………………… 100
Figure 5.17 Power coefficient (Cp)……………………………………………. 100
Figure 5.18 Tip speed ratio vs. power coefficient…………….........…………. 101
Figure 5.19 Turbine wind speed – power characteristics…………………....... 101
Figure 5.20 Turbine efficiency vs. wind speed………………………………... 102
1
CHAPTER ONE
INTRODUCTION
World electrical energy consumption gets higher as the technology being
developed and the human life’s dependency on electricity is growing. Predictions
say that world electrical energy demand will continue to increase in the following 20
years period as shown in Figure 1.1. So, electrical energy supplies will be
insufficient to respond this demand. Therefore, new and cost-reduced energy
supplies must be introduced into the market.
World Electricity Consumption
0
6000
12000
18000
24000
1990 1995 2000 2005 2010 2015 2020
Years
Net
Ele
ctri
cal E
ner
gy
Co
nsu
mp
tion
(G
Wh)
Figure 1.1 World electricity consumption with estimations
2
Table 1.1 World Electricity Consumption with Estimations
World Electricity Consumption Annual Consumption (GWh)
1990 10,549
1998 12,725
1999 12,833
2005* 15,182
2010* 17,380
2015* 19,835
2020* 22,407
* Estimated values.
Wind energy offers the potential to generate substantial amounts of electricity
without the pollution problems of most conventional forms of electricity generation.
The scale of its development will depend critically on the care with which wind
turbines are selected and sited. (Boyle, 1996, p.267)
Figure 1.2 shows that, for about 10 years, generating electricity from wind sites is
one of the most popular methods to provide demanded electricity of the world.
Wind Power Installation History 1991 - 2002
0
4000
8000
12000
16000
20000
24000
28000
32000
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
Year
Inst
alle
d M
W
Annual Installation
Cumulative Installation
Figure 1.2 Wind power installations worldwide
3
Table 1.2 Wind Power Installations Worldwide
WECS
Installations Annual Installation (MW) Cumulative Installation (MW)
1991 2,223
1992 338 2,561
1993 480 3,041
1994 730 3,771
1995 1,290 5,061
1996 1,292 6,353
1997 1,568 7,921
1998 2,597 10,518
1999 3,922 14,440
2000 4,495 18,935
2001 6,824 25,759
2002* 6,000 31,759
* Estimated value.
Since 1996, global wind power capacity has continued to grow at an annual
cumulative rate close to 40%. Over the past decade, installations have roughly
doubled every two and a half years. During 2001 alone, close to 6,800 MW of new
capacity was added to the electricity grid worldwide. (EWEA, European Wind
Energy Association, 2002, p.11)
By the end of 2001, global wind power installed had reached a level of almost
25,000 MW. This is enough power to satisfy the needs of around 14 million
households, over 35 million people. Europe accounts for around 70% of this
capacity, and for two-thirds of the growth during 2001. But other regions are
beginning to emerge as substantial markets for the wind industry. Over 45 countries
around the world now contribute to the global total, and the number of people
employed by the industry world-wide is estimated to be around 70,000. (EWEA,
European Wind Energy Association, 2002, p.11)
4
Table 1.3 Wind Energy Capacity Leaders Worldwide by End 2001
COUNTRY Installed MW
Germany 8,734
USA 4,245
Spain 3,550
Denmark 2,456
India 1,456
Italy 700
UK 525
China 406
Greece 358
Japan 357
Turkey 19
Others 2,121
TOTAL 24,927
1.1 HISTORICAL BACKGROUND
Wind energy has been used for thousands of years for milling grain, pumping
water, and other mechanical power applications. Today there are over one million
windmills in operation around the world; these are used principally for water
pumping. Whilst the wind will continue to be used for this purpose, it is the use of
wind energy as a pollution-free means of generating electricity on a potentially
significant scale that is attracting most current interest in the subject. Strictly
speaking, a windmill is used for milling grain, so modern ‘windmills’ tend to be
called wind turbines, partly because of their functional similarity to other types of
turbines that are used to generate electricity. They are also sometimes referred to as
wind energy conversion systems (WECS) and those used to generate electricity are
sometimes described as wind generators or aero-generators. For utility-scale sources
of wind energy, a large number of wind turbines are usually built close together to
form a wind plant.
5
Attempts to generate electricity from wind energy have been made (with various
degrees of success) since the end of the nineteenth century. Small wind machines for
charging batteries have been manufactured since the 1940s. It is, however, only since
the 1980s that the technology has become sufficiently mature. An extensive range of
commercial wind turbines is currently available from over 30 manufacturers around
the world. Several electricity providers today use wind plants to supply power to
their customers. (Boyle, 1996, p.267)
Wind turbines, like windmills, are mounted on a tower to capture the most energy.
At 30 meters or more above ground, they can take the advantage of faster and less
turbulent wind. Turbines catch the wind’s energy with their propeller- like blades.
Usually, two or three blades are mounted on a shaft to form a rotor.
A blade acts much like an airplane wing. As wind blows, a pocket of low-pressure
air forms on the downwind side of the blade. The low-pressure air pocket then pulls
the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is
actually much stronger than the wind's force against the front side of the blade,
which is called drag. The combination of lift and drag causes the rotor to spin like a
propeller, and the turning shaft spins a generator to make electricity.
Wind turbines can be used in stand-alone applications, or they can be connected to
a utility power grid or even combined with a photovoltaic (solar cell) system. Stand-
alone wind turbines are typically used for water pumping or communications.
However, homeowners or farmers in windy areas can also use wind turbines as a way
to cut their electric bills.
The cost of wind energy equipment fell steadily between the early 1980s and the
early 1990s. The technology is continually being improved to make it both cheaper
and more reliable, so it can be expected that wind energy will tend to become more
economically competitive over the coming decades.
6
An understanding of machines that extract energy from the wind involves many
fields of knowledge, including meteorology, aerodynamics, electricity and planning
control, as well as structural, civil and mechanical engineering.
1.2 FUNCTIONAL STRUCTURE OF WIND TURBINES
Figure 1.3 Power transfer in a wind energy converter
As shown in Figure 1.3, blades of a wind turbine rotor extract some of the flow
energy from air in motion, convert it into rotational energy then deliver it via a
mechanical drive unit (shafts, clutches and gears) to the rotor of a generator and
thence to the stator of the same by mechanical-electrical conversion. The electrical
energy from the generator is fed via a system of switching and protection devices,
leads and any necessary transformers to the mains, to the end user or to some means
of storage. (Heier, 1998, p.21)
7
CHAPTER TWO
COMPONENTS OF WIND TURBINES
A wind turbine converts the kinetic energy of the wind firstly to the rotational
mechanical energy then to the electrical energy. All of these duties are carried out by
special components.
The rotor assembly may be placed either;
1. Upwind of the tower and nacelle, so receiving wind unperturbed by the tower
itself or,
2. Downwind of the tower, which enables self alignment of the rotor with the
wind direction (yawing), but causes the wind to be deflected and made
turbulent by the tower before arriving at the rotor (tower shadow).
Figure 2.1 Wind turbine types by rotor assemblies
8
The lifetime of a rotor is related to variable loads and environmental conditions
that it experiences during service. Therefore, the rotor's inherent mechanical
properties and design will affect its useful service life.
2.1. COMMON COMPONENTS
2.1.1. NACELLE
Nacelle contains the key components of a wind turbine, including the gearbox,
and electrical generator. Service personnel may enter the nacelle from the tower of
the turbine in order to make maintenances. Towards the other side of the nacelle,
there is wind turbine rotor, i.e. rotor blades and the hub.
Figure 2.2 Nacelle
2.1.2. BLADE
Rotor blade design has advanced with knowledge from wing technology, and
utilizes the aerodynamic lift forces that an airfoil experiences in a moving stream of
air. The shape of the blade and its angle in relation to the relative wind direction both
affect its aerodynamic performance.
9
The materials used in modern wind turbine blade construction may be grouped
into three main classes;
• Wood (including laminated wood composites)
• Synthetic composites (a polyester or epoxy matrix reinforced by glass fibers)
• Metals (predominantly steel or aluminum alloys)
Rotor blades should have the optimum design in order to capture maximum
amount of wind and so to provide maximum rotation of the shaft. Wind turbines can
have different number of rotor blades. The principle rule is; the lower the number of
rotor blades the faster turns the rotor. The measure for this is called tip speed ratio, λ,
which is defined as rotor tip speed divided by the wind velocity. If λ = 1, the blade
tip velocity is as high as the wind speed. Rotors of wind turbines should have
rotational speeds as high as possible to reduce the masses of gearboxes and
generators. So, the number of rotor blades is low and in general not more than three.
Most of today’s wind turbines have blade tip speeds of less than 65 m/s. In the old
prototypes of large wind turbines, designers tried to increase the blade tip speed more
and more because the shaft torque reduces with increasing rotational speed, but high
blade tip speeds have the disadvantage of high noise emissions and physical damages
of the rotor.
3-bladed rotors are the most common ones all over the world. The main reason to
use 3 blades is the constant inertia moment of the rotor for all circumferential
azimuth angles in relation to operational motions around the longitudinal axis of the
tower. (German Wind Energy Institute - DEWI, 1998, p.40)
2-bladed rotor offered the chance to reduce the cost for the rotor, but
unfortunately the dynamic behaviour of the 2-bladed rotor caused additional efforts
that increase again the overall cost. (German Wind Energy Institute - DEWI, 1998,
p.41)
10
As compared to 3-bladed rotors, 1-bladed rotors have tip speed two times that of
3-bladed ones. This means a 1-bladed wind turbine is several times noisier than a 3-
bladed one. Additionally, the rotor blade can be fixed to the hub by a single hinge
that allows for a movement that reduces structural loads on the blade. On the other
hand, 1-bladed rotors principally have an aerodynamic unbalance, which introduces
additional motions, causes loads and needs complicated hub constructions to keep
the movements under control. (German Wind Energy Institute - DEWI, 1998, p.41)
a. One-Bladed b. Two-Bladed c. Three-Bladed
Figure 2.3 Horizontal axis wind turbines according to number of blades
If 1, 2 or 3 bladed rotors are designed for similar tip speeds (as they have not been
in the past but would require to be in the future for European land based applications
subject to current sound limits), then the blades of the 3-bladed rotor are more highly
stressed than for the 2 or 1 bladed system and thus rotor blade costs will be high for
the 3 bladed system.
Table 2.1 illustrates the relative proportion of 1, 2 and 3 bladed designs among
present commercially available wind turbines of over 30 kW rated output. If the data
were presented as the proportion of operational machines the dominance of the 3-
11
bladed designs would be still more pronounced. (European Commission Directorate-
General for Energy, 1997, pp.5-6)
Table 2.1 Number of Blades for Commercial Wind Turbine Designs
Number of Blades % of Designs
1 2
2 24
3 74
Conventional wisdom holds that three-bladed machines will deliver more energy
and operate more smoothly than either one or two bladed turbines. They will also
incur higher blade and transmission costs as a result. Some experiments say that
rotors with three blades can capture 5% more energy than two-bladed turbines while
encountering less cyclical loads than one and two bladed turbines.
2.1.3. LOW SPEED SHAFT
While transferring the primary torque to the gear train from the rotor assembly,
the main shaft is usually supported on journal bearings. Due to its high torque
loadings, the main shaft is susceptible to fatigue failure. Thus, effective pre-service
non-destructive testing procedures are advisable for this component.
2.1.4. HIGH SPEED SHAFT
The high-speed shaft rotates with over 1,000 revolutions per minute (rpm) and
drives the electrical generator. It is equipped with an emergency mechanical disc
brake.
2.1.5. DISC BRAKE
This may be situated either on the main shaft before the gearbox, or on the high-
speed shaft after the gearbox. The latter arrangement requires a smaller (and cheaper)
12
brake assembly in order to supply the necessary torque to slow down the rotor.
However, this arrangement does not provide the most immediate control of the rotor,
and in the event of a gearbox failure, braking control of the rotor is lost.
2.1.6. GENERATOR
The generator converts the mechanical energy of the input shaft to electrical
energy. It must be compatible at input with the rotor and gearbox assemblies, but at
output with the utility's power distribution (if connected to a grid) or to local power
requirements (if the turbine is part of a stand alone system).
The generator can be either DC, synchronous or induction (asynchronous). DC
machines are used for stand alone systems such as battery charging which do not
need to produce grid compatible electricity. Synchronous machines are generally
used for high synchronous speeds, but induction machines can be used for low
variable speeds. Generally for wind turbines, induction generators are used for the
opportunity of controlling the system under different wind speeds. This situation is
the result of unstable wind speeds. In some systems, permanent magnet generators
can also be used.
2.1.7. TOWER
The tower of a wind turbine carries the nacelle and the rotor. Generally, it is an
advantage to have a high tower, since wind speeds increase farther away from the
ground. For example, a typical modern 600 kW turbine will have a tower of 40 to 60
metres (the height of a 13-20 story building).
Towers may be either of tubular or lattice types. Tubular towers are safer for the
personnel that have to maintain the turbines, as they may use an inside ladder to get
to the top of the turbine. The advantage of lattice towers is primarily that they are
cheaper.
13
2.2. OPTIONAL COMPONENTS
2.2.1. GEAR BOX
Gearboxes are used for non-direct drive designs. In general, the transmission gear
is used to adapt WECS to low wind speeds in order to help the rotational speed
getting close to the frequency of the grid system. But, this adaptation brings the
addition of mechanical machinery parts (Large gearboxes, coupling elements etc.) to
be installed.
Figure 2.4 A typical gear
Gearboxes are not intrinsic to wind turbines. Designers use them only because
they need to increase the speed of the slow-running main shaft to the speed required
by mass-produced generators. Manufacturers can produce for special purpose, slow-
speed generators and drive them directly without using a transmission. For this
reason, specially designed permanent-magnet alternators have revolutionized the
reliability and serviceability of small wind turbines.
2.2.2. V / Hz CONVERTER
The AC-AC converter includes a rectifier and an inverter to control the frequency.
Its aim is to keep the generated system voltage near grid frequency (50 or 60 Hz). A
controlled rectifier-inverter group converts the generated AC voltage to a DC signal
and then again to an AC signal. The controlling principle is based on the controlling
of the inverter elements (IGBTs, thyristors etc.).
14
Figure 2.5 AC – AC signal conversion
2.2.3. YAW ASSEMBLY
It is necessary for the rotor axis to be aligned with the wind direction in order to
extract as much of the wind's kinetic energy as possible. The smallest upwind
machines (up to 25 kW) most commonly use tail vanes to keep the machine aligned
with the wind. However, larger wind turbines with upwind rotors require active yaw
control to align the machine with the wind. To enable this, when a change in wind
direction occurs, sensors activate the yaw control motor, which rotates the nacelle
and rotor assembly until the turbine is properly aligned.
Downwind machines of all sizes may possess passive yaw control, which means
that they can self-align with the wind direction without the need for or a tail vane or
yaw drive.
Yaw system can also be used to shut down the wind turbine in order to save it
from the physical effects of very high wind speeds.
2.2.4. PITCH CONTROL MECHANISM
This mechanism is used on wind turbines for active power control. At a
sufficiently high level of wind, a blade pitch adjuster ensures that the turbine speed is
kept roughly constant by altering the blade angle.
15
For reasons of stability and to reduce the component loading, this mechanism
changes the blade pitch angle along its longitudinal axis to limit the input torque
loading to turbine blades.
A simple pitch control design can be achieved by using a hydraulic or mechanical
centrifugal governor.
2.2.5. ELECTRONIC CONTROLLER
It contains a computer, which continuous ly monitors the condition of the wind
turbine and controls the pitch and yaw mechanisms. In case of any malfunction, (e.g.
overheating of the gearbox or the generator), it automatically stops the wind turbine
and calls the turbine operator's computer via a telephone modem link.
Another important characteristic of the electronic controller is to control the AC-
AC converter elements (i.e. firing angles of thyristors). At this point, electronic
controller takes on the frequency synchronization duty between generated signal and
grid.
16
Figu
re 2
.6 A
typi
cal w
ind
turb
ine
in d
etai
l (V
EST
AS
V27
/ 22
5 kW
)
17
CHAPTER THREE
ELECTROMECHANICAL ENERGY
CONVERSION
Electromechanical energy conversion is carried out by the full operation of wind
turbine. In case of any component’s failure, either the complete energy conversion
stopped or some losses must be taken into account.
Figure 3.1 A typical wind turbine showing all components
18
As shown in Figure 3.1, the wind blade(s) is able to capture the wind energy and
rotates itself. This rotation of the blade is transferred to the generator shaft or namely
to the rotor by an optional gearbox. This box increases the rotational speed of the
shaft, which provides more electrical energy production. The high-speed generator
(asynchronous or synchronous) is connected to the V/Hz converter to keep the
frequency of the generated voltage in the order of the grid frequency.
The sequence of events in the generation and transmission of wind power can be
summarized as follows:
1. A torque is produced as the wind interacts with the rotor,
2. The relatively low rotational frequency of the rotor is increased via a gearbox,
3. The gearbox output shaft turns a generator,
4. The electricity produced by the generator passes through the turbine controller
and circuit breakers and is stepped up to an intermediate voltage level
(generally 690 V) by the turbine transformer,
5. The site cabling system delivers the electricity to the site transformer via the
site control and circuit breaker system,
6. The site transformer steps up the voltage to the grid value,
7. The grid system transmits the electricity to the locality of its end use,
8. Transformer substations reduce the voltage to domestic or industrial values,
9. Local low voltage networks transmit the electricity to homes, offices and
factories.
3.1. AERODYNAMICS OF WIND TURBINES
3.1.1. AERODYNAMIC FORCES
An object in an air stream experiences a force that is imparted from the air stream
to that object. This force can be considered to be equivalent to two component
forces, acting in perpendicular directions, known as the drag force and the lift force.
19
The magnitudes of drag and lift forces depend on the shape of the object, its
orientation to the direction of the air stream, and the velocity of the air stream.
Figure 3.2 Lift and drag forces acting on rotor blade
3.1.1.1. DRAG FORCES
Drag forces are in line with the direction of the air stream. For example, a flat
plate in an air stream experiences maximum drag forces when the direction of the air
flow is perpendicular to the flat side of the plate. When the direction of the air stream
is in line with the flat side of the plate, the drag forces are at a minimum. (Boyle,
1996, p.284)
For wind turbine blades, the objective is to minimize drag forces.
3.1.1.2. LIFT FORCES
Lift forces are perpendicular to the direction of the air stream. They are termed
‘lift’ because they are the forces that enable aero planes to lift off the ground and fly.
Lift forces acting on a flat plate are smallest when the direction of the air stream is at
a zero angle to the flat surface of the plate.
At small angles relative to the direction of the air stream (that is, when the so
called angle of attack is small), a low pressure region is created on the downstream
side of the plate as a result of an increase in the air velocity on that side. In this
20
situation, there is a direct relationship between air velocity and pressure: The faster
the air flow, the lower the pressure. This phenomenon is known as the Bernoulli’s
Effect. The lift force thus acts as a ‘suction’ or ‘pulling’ force on the object. Lift
forces are the principal that cause a modern wind turbine to operate. (Boyle, 1996,
p.284)
3.1.2. AERO-FOILS
The angle that an object makes with the direction of an air flow, measured against
a reference line in the object, is called the angle of attack or angle of incidence. The
reference line on an aero-foil section is usually referred to as the chord line . Arching
or cambering a flat plate will cause it to induce higher lift forces for given angle of
attack, but the use of so-called aero-foil sections is even more effective. When
employed as the profile of a wing, these sections accelerate the air flow over the
upper surface. The high air speed thus induced results in a large reduction in pressure
over the upper surface relative to the lower surface. (Boyle, 1996, p.284)
21
Figure 3.3 Components of wind power acting on rotor blade
The lift force, in a direction at right angles to the air stream, is described by the
lift coefficient CL, and is defined by Equation (3.1);
L
2L AV?L2
C⋅⋅
⋅= (3.1)
where
CL : Lift coefficient
ρ : Air density (kg/m2)
AL : Area of aero-foil in plan (m2)
V : Wind speed (m/s)
L : Lift force (N)
Similarly, the drag force is described by the drag coefficient CD by Equation (3.2);
22
D
2D AV?D2
C⋅⋅
⋅= (3.2)
where
CD : Drag coefficient
ρ : Air density (kg/m2)
AD : Area of aero-foil in plan (m2)
V : Wind speed (m/s)
D : Lift force (N)
Horizontal and vertical axis wind turbines both make use of the aerodynamic
forces generated by aero-foils in order to extract power from the wind, but each
harnesses these forces in a different way.
In a fixed pitch horizontal axis wind turbine, the angle of attack at a given position
on the rotor blade stays constant throughout its rotation cycle.
In a vertical axis wind turbine, the angle of attack at a given position on the rotor
blade is constantly varying throughout its rotation cycle.
3.2 ENERGY AND POWER IN THE WIND
A wind turbine obtains its power input by converting the force of the wind into
torque (turning force) that is acting on the rotor blades. The amount of energy which
the wind transfers to the rotor depends on the density of the air, the rotor area, and
the wind speed.
Power can be defined as the rate at which energy is used or converted and it can
therefore be expressed as energy per unit of time;
sj1W1 = (3.3)
23
The energy contained in the wind is its kinetic energy;
2Vm21E ⋅⋅= (3.4)
where m is the mass and V is the velocity with which this mass is moving.
It can be considered that the air is passing through a circular ring (enclosing a
circular area, say 100 m2) at a velocity V (say 10 m/s) as shown in Figure 3.4;
Figure 3.4 Cylindrical volume of air passing at velocity V (10 m/s) through a
ring enclosing an area, ‘A’, each second
As the air is moving at a velocity of 10 m/s, a cylinder of air with a length of 10 m
will pass through the ring each second. Therefore, a vo lume of air equal to
100x10=1000 cubic meters will pass through the ring each second. By multiplying
this volume by the air density, the mass of the air moving through the ring each
second can be obtained.
24
In other words;
Mass of air per second = air density x volume of air passing each second
= air density x area x length of cylinder of air
passing each second
= air density x area x velocity
VA ⋅⋅= ?m (3.5)
where ρ : Air density (kg/m3)
A : Rotor disk Area (m2)
V : Wind velocity (m/s)
Consequently the kinetic energy formula becomes;
3VA21E ⋅⋅⋅= ? (3.6)
However, energy per unit of time is equal to power (1 W = 1 j/s), so above
formula is also the expression for the power in the wind;
3VA21P ⋅⋅⋅= ? (3.7)
An airstream moving through a turbine rotor disc cannot give up all of its energy
to the blades because some kinetic energy must be retained in order to move the
airstream away from the disc area after interaction. In addition, there are frictional
effects, which produce heat losses. Thus, a turbine rotor will never extract 100 % of
the wind's energy.
There are some new parameters to be introduced into calculations in order to
express the system efficiency.
25
3.2.1. POWER COEFFICIENT
The ability of a turbine rotor to extract the wind's power depends upon its
"efficiency". Thus, to express the power output of the turbine, a non-dimensional
power co-efficient Cp is included.
Also, rotors reduce the wind velocity from the undisturbed wind speed V1 far in
front of the rotor to a reduced air stream velocity V2 behind the rotor as shown in
Figure 3.5;
Figure 3.5 Wind flow through a wind turbine
The difference in the wind velocity is a measure for the extracted kinetic energy
which turns the rotor and at the opposite end of the drive train, the connected
electrical generator.
By including the losses, the power theoretically extracted by the wind turbine can
be described by Equation (3.8);
31VApC
2P ⋅⋅η⋅⋅=
? (3.8)
26
where
? : Air density (kg/m3)
pC : Non-dimensional power coefficient
η : Mechanical / Electrical efficiency
A : Rotor disk area (m2)
V1 : Undisturbed wind velocity in front of the rotor (m/s)
This describes the fraction of the wind's power per unit area extracted by the rotor,
governed by the aerodynamic characteristics of the rotor and its number of blades.
As the air stream interacts with the rotor disc and power is extracted, the air
stream speed is reduced by an amount described by the axial interference factor, a.
This is the ratio of the upstream to the downstream wind speed. Equation (3.9)
expresses the power using the axial interference factor;
)a1(aVA2P 231 −⋅⋅⋅⋅η⋅⋅= ? (3.9)
where "a" is the dimensionless axial interference factor.
Thus, by substitution, the power co-efficient Cp may be defined as;
)a1(a4C 2p −⋅⋅= (3.10)
By differentiating (3.10) with respect to a, the maximum value of Cp occurs when
a = 0.33. Thus, Cpmax = 16/27 = 0.593.
27
3.2.2. TIP SPEED RATIO
The speed of rotation of a wind turbine is usually given in either revolutions per
minute (rpm) or radians per second (rad/s). The rotation speed in rpm is usually
symbolized by nr and the angular velocity in rad/s is by ? r.
Table 3.1 Speed Definitions
Definition Symbol Unit
Rotational Speed nr rpm
Angular Speed ? r rad/s
1 rpm = 60
2 π⋅ rad/s = 0.10472 rad/s
Another measure of a wind turbine’s speed is its tip speed, U, which is the
tangential velocity of the rotor at the tip of blades, measured in meters per second. It
is the product of the angular velocity, ? r, of the rotor and the tip radius, r.
Alternatively, it can be defined as;
60
nr2U r⋅⋅π⋅
= (3.11)
By dividing the tip speed, U, by the undisturbed wind velocity, V, at the upstream
of the rotor, the very useful non-dimensional ratio known as the tip speed ratio,
which is usually symbolized by λ is obtained. This ratio provides us with a useful
measure with which to compare wind turbines of different characteristics. (Boyle,
1996, p.283)
If a rotor turns very slowly, it will allow wind to pass unperturbed through the
gaps between the blades. Likewise, a rotor turning very rapidly will appear as a solid
wall to the wind. Therefore, it is necessary to match the angular velocity of the rotor
to the wind speed in order to obtain maximum efficiency.
28
The relationship between the wind speed and the rate of rotation of the rotor is
characterized by a non-dimensional factor, known as the tip speed ratio, λ , given by
Equation (3.12). Note that this factor arises from the full aerodynamic theory of wind
power extraction;
VU
V
r
SpeedWindSpeedTipBlade r =
⋅ω==λ (3.12)
where
r : Rotor radius measured at the blade tip (m)
? r : Angular speed of the blade tip (rad/s)
U : Blade tip speed (m/s)
V : Wind Speed (m/s)
3.2.3. EFFECT OF THE NUMBER OF BLADES
The optimum tip speed ratio may be inferred however by relating the time taken
for the disturbed wind to re-establish itself tw, to the time taken for a blade of
rotational frequency omega to move into the position occupied by its predecessor tb.
For an n-bladed rotor, the time period for the blade to move to its predecessor's
position is given by Equation (3.13);
r
b n2
tω⋅π⋅
= (3.13)
where
tb : Time period for the blade to move its predecessor’s position (sec)
? r : Angular speed of the blade tip (rad/s)
n : Number of blades
29
If the length of the strongly disturbed airstream upwind and downwind of the
rotor is d, then the time for the wind to return to normal is given by Equation (3.14);
Vd
tw = (3.14)
where
tw : Time period for the wind to return to normal (sec)
d : Length of disturbed air stream (m)
V : Wind Velocity (m/s)
Maximum power extraction occurs when these time periods are equal (If tb
exceeds tw, then some wind is unaffected. If tw exceeds tb, then some wind is not
allowed to move through the rotor). For this case, Equation (3.15) applies;
d
2V
n r π⋅≈
ω⋅ (3.15)
where
? r : Angular speed of the blade tip (rad/s)
n : Number of blades
d : Length of disturbed air stream (m)
V : Wind velocity (m/s)
Therefore, for optimum power extraction, the rotor must turn at a frequency which
is related to the speed of the oncoming wind. This rotor frequency decreases as the
radius of the rotor increases, and may be characterized by calculating the optimum
tip speed ratio by Equation (3.16);
⋅
π⋅≈λ
dr
n2
0 (3.16)
30
where
λ0 : Optimum tip speed ratio
r : Blade tip radius of rotation (m)
n : Number of blades
d : Length of disturbed air stream (m)
If we substitute a constant k for the term (r/d), which practical results have shown
to be approximately 2 for an n bladed machine, then the optimum tip speed ratio is
defined by Equation (3.17);
n
40
π⋅≈λ (3.17)
Thus, for a two-bladed rotor, the maximum power extracted from the wind (at
Cpmax) occurs at a tip speed ratio of about 6, and for a four-bladed machine at a tip
apeed ratio of about 3. If the aerofoil is carefully designed, the optimum tip speed
ratios may be about 30% above these values. (De Montfort University-
http://www.iesd.dmu.ac.uk/wind_energy/m32extex.html, 1996).
Most modern horizontal axis wind turbine rotors consist of two or three thin
blades. These are known as "low solidity" rotors, due to the low fraction of the swept
area which is solid. This arrangement gives a relatively high tip speed ratio in
comparison to rotors with a high number of blades (such as those used in water
pumps, which require a high starting torque), and gives an optimum match to the
frequency requirements of modern electricity generators. This minimizes the size of
the gearbox required and increases efficiency.
Figure 3.6 shows the relationship between rotor efficiency (Cp) and the tip speed
ratio for a typical wind turbine; as wind speed increases, it is necessary for the rotor
to speed up in order to remain near the optimum tip speed ratio. However, this is in
conflict with the requirements of most generating systems, which require a constant
generator frequency in order to supply electricity of a fixed frequency. Thus, the
31
wind turbine which has a generator directly coupled to the grid operates for much of
the time with a tip speed ratio which is not optimized.
Figure 3.6 Power coefficient versus tip speed ratio for
a constant speed wind turbine
The alternative is to decouple the generator from the grid by an intermediate
system which facilitates variable speed operation. Some manufactures are producing
variable speed turbines (where the rotor speeds up with the wind velocity), in order
to maintain a tip speed ratio near the optimum. These turbines utilize electronic
inverter/rectifier based control systems to stabilize the fluctuating voltage from the
turbine before feeding into the grid supply.
For a variable-speed turbine, the objective is to operate near maximum efficiency,
where the resulting target power can be expressed as;
3r
3
etargtetargt,petargt
rCApC
2P ω⋅
λ⋅⋅⋅η⋅⋅=
? (3.18)
32
where
? : Air density (kg/m3)
pCtarget : Power coefficient target
η : Mechanical / Electrical efficiency
A : Rotor disk area (m2)
r : Rotor radius measured at the blade tip (m)
? r : Angular speed of the blade tip (rad/s)
λtarget : Tip speed ratio target
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
TSR
Cp
Figure 3.7 Power coefficient versus tip speed ratio for a variable speed wind
turbine for different pitch angles from 0 to 15 degrees by 0.5 degree increments
Figure 3.7 illustrates the Cp-λ relationship for a variable-speed wind turbine at
different pitch angles. For constant-speed turbines, only one of the curves will be
valid and an attempt is made to design the rotor blades to operate near maximum
efficiency (Cpmax) at wind speeds that occur most frequently at the design site. The
rotor speed varies by only a few percent, but the wind speed varies over a wide
range. Therefore, the operating point is rarely, and randomly, at λ for Cpmax. It is
apparent from Equation (3.18) and Figure 3.7 that the power at any wind speed is
33
maximized by operating near the tip-speed ratio which results in the maximum
power coefficient. For a variable-speed turbine, this means that as the wind speed
changes, the rotor speed should be adjusted proportionally.
3.3. GENERATOR THEORY
All generators produce electricity by Faraday Law of electromagnetic induction:
A magnetic field cuts a wire with a relative velocity, so inducing an electric potential
difference in the wire. If this wire forms a circuit, then an electrical current is
produced. The magnitude of the current is being increased with the strength of the
field, the length of wire cut by the field and the relative velocity.
Of the wind turbine systems currently being manufactured, their generating
systems may be classed as follows;
3.3.1. D.C. GENERATORS
3.3.1.1. THEORY:
DC machines convert mechanical power to dc electric power, and vice versa.
Most dc machines are like ac machines in that they have ac voltages and currents
within them – dc machines have a dc output only because a mechanism exists that
converts the internal ac voltages to dc voltages at their terminals. Since this
mechanism is called commutator, dc machinery is also known as commutating
machinery.
DC generators are dc machines used as generators. There is no real difference
between a generator and a motor except for the direction of power flow. (Chapman,
1999, p.566)
34
Figure 3.8 The equivalent circuit for DC motors
In Figure 3.8, the armature circuit is represented by an ideal voltage source EA
and a resistor RA. This representation is really the Thevenin equivalent of the entire
rotor structure, including rotor coils, interpoles and compensating windings, if
present. The brush voltage drop is represented by a small battery Vbrush opposing the
direction of current flow in the machine. The field coils, which produce the magnetic
flux in the generator, are represented by inductor LF and resistor RF. The separate
resistor Radj represents an external variable resistor used to control the amount of
current in the field circuit. (Chapman, 1999, p.508)
The internal generated voltage in a DC machine is given by Equation (3.19);
ω⋅Φ⋅⋅π⋅
⋅=
a2PZ
EA (3.19)
where ‘Z’ is the total number of conductors and ‘a’ is the number of current paths
in the machine. This equation is sometimes rewritten in a simpler form that
emphasizes the quantities that are variable during machine operation. This simpler
form is;
ω⋅Φ⋅= KEA (3.20)
where K is a constant representing the construction of the machine.
35
The induced torque developed by the machine is given by;
Aind IK ⋅Φ⋅=Τ (3.21)
Equations (3.20) and (3.21), the Kirchhoff’s Voltage Law equation of the
armature circuit and the machine’s magnetization curve, are all the tools necessary to
analyze the behaviour and performance of a dc motor. (Chapman, 1999, p.508)
There are five major types of dc generators, classified according to the manner in
which their field flux is produced:
1. Separately Excited Generator: In a separately excited generator, the field flux
is derived from a separate power source independent of the generator itself.
2. Shunt Generator: In a shunt generator, the field flux is derived by connecting
the field circuit directly across the terminals of the generator.
3. Series Generator: In a series generator, the field flux is produced by
connecting the field circuit in series with the armature of the generator.
4. Cumulatively Compounded Generator: In a cumulatively compounded
generator, both a shunt and a series field are present, and their effects are
additive.
5. Differentially Compounded Generator: In a differentially compounded
generator, both a shunt and a series field are present, but their effects are
subtractive.
These various types of dc generators differ in their terminal (voltage-current)
characteristics, and therefore in the applications to which they are suited. DC
generators are compared by their voltages, power ratings, efficiencies, and voltage
regulations. Voltage regulation (VR) is defined by Equation (3.22);
%100V
VVVR
fl
flnl ×−
=
(3.22)
36
where Vnl is the no- load terminal voltage of the generator and Vfl is the full- load
terminal voltage of the generator. It is a rough measure of the shape of the generator's
voltage-current characteristic—a positive voltage regulation means a drooping
characteristic, and a negative voltage regulation means a rising characteristic.
All generators are driven by a source of mechanical power, which is usually called
the prime mover of the generator. A prime mover for a dc generator may be a wind
or steam turbine, a diesel engine, or even an electric motor. Since the speed of the
prime mover affects the output voltage of a generator, and since prime movers can
vary widely in their speed characteristics, it is customary to compare the voltage
regulation and output characteristics of different generators, assuming constant-speed
prime movers. (Chapman, 1999, pp.566-567)
3.3.1.2. DC GENERATOR APPLICATIONS IN WIND TURBINES
Small scale stand-alone wind turbines are the most commonly used to charge
batteries at relatively low voltages. They use simple DC generators. In these systems,
the rotating generator shaft (connected to the turbine blades either directly or through
a gearbox) turns the rotor within a magnetic field produced by either the field coil
windings or by an arrangement of permanent magnets on the armature. The rotation
causes an electric current to be set up in the rotor windings as the coils of wire cut
through the magnetic field. This current (whose magnitude depends upon the number
of turns in the windings, the strength of the magnetic field and the speed of rotation)
is drawn off from the commutator through graphite brushes and fed directly to the
battery, sometimes via a voltage regulator which smoothes out fluctuations in the
generated voltage.
3.3.2. SYNCHRONOUS AC MACHINES (ALTERNATORS)
AC generators employ a rotary magnetic field, known as a rotary field. This may
be obtained by the use of a rotating permanent magnet or by rotary excitation using a
current fed via so-called brushes and slip-rings. In stationary conductors—the stator
37
windings of the generator—such rotary fields excite electric currents that vary with
the frequency of rotation. In these synchronous generators, coils are set (spatially) at
e.g. 120° intervals or an integral multiple thereof. The voltage is dependent on the
construction of the generator, the speed of rotation of the rotary field, the excitation
and the load characteristics, and in isolated and stand-alone operation can be
regulated by varying the excitation. When connected to the public supply, both
voltage and frequency are dictated by the grid.
If the three-phase alternating current stator of a generator is supplied with
alternating current from the grid, it also sets up a rotary field. This excites currents in
the rotor windings of the generator, which vary with a frequency corresponding to
the difference between the field rotation frequency and the mechanical speed of
rotation. These currents cause torques on the rotor, which, in synchronous machines,
have a damping effect.
3.3.2.1. THEORY
A synchronous generator or alternator is a device for converting mechanical
power from a prime mover to AC electric power at a specific voltage and frequency.
The term synchronous refers to the fact that this machine's electrical frequency is
locked in or synchronization with its mechanical rate of shaft rotation. The
synchronous generator is used to produce the vast majority of electric power used
throughout the world. (Chapman, 1999, p.316)
In a synchronous generator, a dc current is applied to the rotor winding, which
produces a rotor magnetic field. The rotor of the generator is then turned by a prime
mover, producing a rotating magnetic field within the machine. This rotating
magnetic field induces a three-phase set of voltages within the stator windings of the
generator.
Two terms commonly used to describe the windings on a machine are field
windings and armature windings. In general, the term "field windings" applies to
38
the windings that produce the main magnetic field in a machine, and the term
"armature windings" applies to the windings where the main voltage is induced. For
synchronous machines, the field windings are on the rotor, so the terms "rotor
windings" and "field wind ings" are used interchangeably. Similarly, the terms "stator
windings" and "armature windings" are used interchangeably.
The rotor of a synchronous generator is essentially a large electromagnet. The
magnetic poles on the rotor can be of either salient or non-salient construction. The
term salient means "protruding" or "sticking out" and a salient pole is a magnetic
pole that sticks out from the surface of the rotor. On the other hand, a non-salient
pole is a magnetic pole constructed flush with the surface of the rotor. Non-salient
pole rotors are normally used for two- and four-pole rotors, while salient-pole rotors
are normally used for rotors with four or more poles. (Chapman, 1999, pp.250-252)
Figure 3.9 A salient six-pole rotor for a synchronous machine
39
Figure 3.10 A non-salient two -pole rotor for a synchronous machine
A DC current must be supplied to the field circuit on the rotor. Since the rotor is
rotating, a special arrangement is required to get the DC power to its field windings.
There are two common approaches for supplying this DC power;
1. Supply the DC power from an external DC source to the rotor by means of slip
rings and brushes.
2. Supply the DC power from a special DC power source mounted directly on the
shaft of the synchronous generator.
3.3.2.2. THE ROTATION SPEED OF A SYNCHRONOUS GENERATOR
Synchronous generators are by definition synchronous, meaning that the electrical
frequency produced is locked in or synchronized with the mechanical rate of rotation
of the generator. A synchronous generator’s rotor consists of an electromagnet to
which direct current is supplied. The rotor magnetic field points in whatever
direction the rotor is turned. Now, the rate of rotation of the magnetic fields in the
machine is related to the stator electrical frequency by;
120
pnf m
e⋅
= (3.23)
40
where
fe : Electrical frequency (Hz)
nm : Mechanical speed of the magnetic field (rpm)
(equals the speed of the rotor for synchronous machines)
p : Number of poles
Since the rotor turns at the same speed as the magnetic field, this equation relates
the speed of the rotor rotation to the resulting electrical frequency. (Chapman, 1999,
pp.254-255)
3.3.2.3. INTERNAL VOLTAGE OF A SYNCHRONOUS GENERATOR
The magnitude of the voltage induced in a given stator phase is;
fN2E CA ⋅Φ⋅⋅π⋅= (3.24)
In solving problems with synchronous machines, this equation is sometimes
rewritten in a simpler form that emphasizes the quantities that are variable during
machine operation. This simpler form is;
ω⋅Φ⋅= KEA (3.25)
where K is a constant representing the construction of the machine. If ? is
expressed in radians per second, then
2
pNK C ⋅
= (3.26)
The internal generated voltage EA is directly proportional to the flux and to the
speed, but the flux itself depends on the current flowing in the rotor field circuit. The
field current IF is related to the flux in the manner shown in Figure 3.11 (a). Since EA
is directly proportional to the flux, the internal generated voltage EA is related to the
41
field current as shown in Figure 3.11 (b). This plot is called the magnetization curve
or the open-circuit characteristic of the machine.
Figure 3.11 a. Plot of flux vs. field current for synchronous generators
b. The magnetization curve for synchronous generators
The voltage EA is the internal generated voltage produced in one phase of a
synchronous generator. However, this voltage EA is not usually the voltage that
appears at the terminals of the generator. In fact, the only time the internal voltage EA
is the same as the output voltage VF of a phase is when there is no armature current
flowing in the machine. (Chapman, 1999, pp.255-256)
There are number of factors that cause the difference between EA and VF ;
1. The distortion of the air-gap magnetic field by the current flowing in the stator,
called armature reaction
2. The self inductance of armature coils
3. The resistance of armature coils
4. The effect of salient-pole rotor shapes
42
3.3.2.4. THE EQUIVALENT CIRCUIT OF AN ALTERNATOR
Figure 3.12 A simple circuit for alternators
The armature reaction voltage on a phase is;
AA IXjEV ⋅⋅−=Φ (3.27)
In addition to the effects of armature reaction, the stator coils have a self
inductance and resistance. If the stator self inductance is called LA (and its
corresponding reactance is called XA) while the stator resistance is called RA, then
the total difference between EA and VF is given by;
AAAAAA IRIXjIXjEV ⋅−⋅⋅−⋅⋅−=Φ (3.28)
The armature reaction effects and the self inductance in the machine are both
represented by reactances, and it is customary to combine them into a single
reactance, called the synchronous reactance of the machine;
AS XXX += (3.29)
43
Therefore, the final equation describing VF is;
AAASA IRIXjEV ⋅−⋅⋅−=Φ (3.30)
Figure 3.13 The per-phase equivalent circuit for synchronous generators
The way in which a synchronous generator operates in a real power system
depends on the constraints on it. When a generator operates alone, the real and
reactive powers that must be supplied are determined by the load attached to it, and
the governor set points and field current control the frequency and terminal voltage,
respectively. When the generator is connected to an infinite bus, its frequency and
voltage are fixed, so the governor set points and field current control the real and
reactive power flow from the generator. In real systems containing generators of
approximately equal size, the governor set points affect both frequency and power
flow, and the field current affects both terminal voltage and reactive power flow.
A synchronous generator's ability to produce electric power is primarily limited
by heating within the machine. When the generator's windings overheat, the life of
the machine can be severely shortened. Since here are two different windings
(armature and field), there are two separate constraints on the generator. The
maximum allowable heating in the armature windings sets the maximum
kilovoltamperes allowable from the machine, and the maximum allowable heating in
the field windings sets the maximum size of EA. The maximum size of EA and the
maximum size of IA together set the rated power factor of the generator. (Chapman,
1999, p.316)
44
Early alternators, which produce an AC voltage, were developed as a replacement
for DC generators. Alternators have a number of advantages. They are generally
cheaper and more durable, due to the use of slip rings rather than commutators. A
further design improvement is their incorporation of the armature windings in the
stator, whilst the rotor provides the magnetic field. If permanent magnets are used,
the power is drawn from the alternator through fixed contacts and wear due to the
passage of high currents through moving contacts is eliminated. In excited field
alternators, the magnetic field is provided by a supply of relatively low current to the
field windings, via slip rings.
Thus, in order to be compatible with a utility's grid supply, the machine must be
driven at a constant speed by turbine rotors, to produce power which is in phase with
grid supply. In practice, this may be achieved by altering the pitch of the turbine
rotor blades to alter their lift coefficient as the wind speed varies. More commonly,
however, the generator output is small enough in relation to that of the utility supply
to allow it to "lock-on" to the grid frequency, ensuring a grid-compatible output
frequency despite small variations in wind speed.
3.3.3. ASYNCHRONOUS (INDUCTION) AC MACHINES
An induction generator differs from a synchronous generator in that its rotor
consists in its simplest form of an iron cylinder with slots on its periphery that carry
insulated copper bars. These are short-circuited by rings which are positioned on the
flat faces of the cylinder. The currents that produce the magnetic field are in short-
circuited loops. If positioned on the stator, the field current in these loops is induced
from currents in the stator windings, and vice versa. In operational terms, power
generation can only occur when the induced closed- loop field currents have been
initiated and maintained. This is facilitated in one of three ways;
• Reactive power is drawn from the live grid, to which the generator is
connected,
45
• Capacitors connected between the output and the earth enable autonomous self-
excited generation (some residual magnetism in the system is necessary),
• A small synchronous generator may be run in parallel, which may (if diesel,
fuelled, for example) then provide power at times of inadequate wind.
Figure 3.14 Cutaway diagram for a wound-rotor induction machine
Figure 3.15 Cutaway diagram for a squirrel-cage induction machine
3.3.3.1. EQUIVALENT CIRCUIT OF AN INDUCTION MACHINE
An induction machine relies for its operation on the induction of voltages and
currents in its rotor circuit from the stator circuit (transformer action). Because the
induction of voltages and currents in the rotor circuit of an induction machine is
46
essentially a transformer operation, the equivalent circuit of an induction machine
will turn out to be very similar to the equivalent circuit of a transformer. An
induction machine is called a singly excited machine (as opposed to a doubly excited
synchronous machine), since power is supplied to only the stator circuit. Because an
induction machine does not have an independent field circuit, its model will not
contain an internal voltage source such as the internal generated voltage EA in a
synchronous machine.
It is possible to derive the equivalent circuit of an induction machine from the
knowledge of transformers and the variation of rotor frequency with speed in
induction machines. (Chapman, 1999, p.365)
A transformer per-phase equivalent circuit, representing the operation of an
induction machine, is shown in Figure 3.16. Like any transformer, there is a certain
resistance and self- inductance in the primary (stator) windings, which must be
represented in the equivalent circuit of the machine. The stator resistance will be
called as R1 and the stator leakage reactance will be called as X1. These two
components appear right at the input to the machine model. Also, like any
transformer with an iron core, the flux in the machine is related to the integral of the
applied voltage E1. The curve of magnetomotive force versus flux (magnetization
curve) for this machine is compared to a similar curve for a power transformer in
Figure 3.17. Notice that the slope of the induction machine's magnetomotive force-
flux curve is much shallower than the curve of a good transformer. This is because
there must be an air gap in an induction machine, which greatly increases the
reluctance of the flux path and therefore reduces the coupling between primary and
secondary windings. The higher reluctance caused by the air gap means that a higher
magnetizing current is required to obtain a given flux level. Therefore, the
magnetizing reactance Xm in the equivalent circuit will have a much smaller value
(or the susceptance Bm will have a much larger value) than it would in an ordinary
transformer.
47
Figure 3.16 Transformer model for an induction machine
The primary internal stator voltage E1 is coupled to the secondary ER by an ideal
transformer with an effective turns ratio aeff.
The voltage ER produced in the rotor in turn produces a current flow in the shorted
rotor (or secondary) circuit of the machine.
Figure 3.17 Magnetization curve for an induction machine compared to that for
a transformer
The primary impedances and the magnetization current of the induction machine
are similar to the corresponding components in a transformer equivalent circuit. An
induction machine equivalent circuit differs from a transformer equivalent circuit
48
primarily in the effects of varying rotor frequency on the rotor voltage ER and the
rotor impedances RR and jXR. (Chapman, 1999, pp.366-367)
3.3.3.1.1. ROTOR CIRCUIT MODEL
In an induction machine, when the voltage is applied to the stator windings, a
voltage is induced in the rotor windings of the machine. In general, the greater the
relative motion between the rotor and the stator magnetic fields, the greater the
resulting rotor voltage and rotor frequency. The largest relative motion occurs when
the rotor is stationary, called the locked-rotor or blocked-rotor condition, so the
largest voltage and rotor frequency are induced in the rotor at that condition. The
smallest voltage (0 V) and frequency (0 Hz) occur when the rotor moves at the same
speed as the stator magnetic field, resulting in no relative motion. The magnitude and
frequency of the voltage induced in the rotor at any speed between these extremes is
directly proportional to the slip of the rotor. Therefore, if the magnitude of the
induced rotor voltage at locked-rotor conditions is called ER0, the magnitude of the
induced voltage at any slip will be given by Equation (3.31);
0RR EsE ⋅= (3.31)
and the frequency of induced voltage at any slip will be given by Equation (3.32);
er fsf ⋅= (3.32)
This voltage is induced in a rotor containing both resistance and reactance. The
rotor resistance RR is a constant (except for the skin effect), independent of slip,
while the rotor reactance XR is affected in a more complicated way by slip.
(Chapman, 1999, p.367)
The reactance of an induction machine rotor depends on the inductance of the
rotor and the frequency of the voltage and current in the rotor. With a rotor
inductance of LR, the rotor reactance is given by;
49
RrRrR Lf2LX ⋅⋅π⋅=⋅ω= (3.33)
Substituting Equation (3.32) into Equation (3.33);
( )0RR
ReR
ReR
XsXLf2sXLfs2X
⋅=⋅⋅π⋅⋅=
⋅⋅⋅π⋅= (3.34)
where XR0 is the blocked-rotor rotor reactance.
Figure 3.18 The rotor circuit model for induction machines
Figure 3.19 The rotor circuit model with all the frequency (slip) effects
concentrated in resistor RR
50
3.3.3.1.2. FINAL EQUIVALENT CIRCUIT
To produce the final per-phase equivalent circuit for an induction machine, it is
necessary to refer the rotor part of the model over to the stator side. The rotor circuit
model that will be referred to the stator side is shown in Figure 3.19, which has all
the speed variation effects concentrated in the impedance term.
In an ordinary transformer, the voltages, currents and the impedances on the
secondary side of the device can be referred to the primary side by means of the turns
ratio of the transformer:
s2
s
ssp
ssp
ZaZ
Ia1
II
VaVV
⋅=′
⋅=′=
⋅=′=
(3.35)
where the prime refers to the referred values of voltage, current and impedance.
Exactly the same sort of transformation can be done for the induction machine’s
rotor circuit. If the effective turns ratio of an induction machine is aeff, then the
transformed rotor voltage becomes;
0ReffR1 EaEE ⋅=′= (3.36)
and the rotor current becomes;
eff
R2 a
II = (3.37)
and the rotor impedance becomes
51
+⋅= 0RR2
eff2 jXs
RaZ (3.38)
so
0R
2eff2
R2eff2
XaX
RaR
⋅=
⋅= (3.39)
Figure 3.20 The per-phase equivalent circuit for induction machines
In wind energy conversion systems, depending on the speed of the wind, the
generator may act either as a generator, supplying power to the grid, or as a motor
(acting as a sink of power from the grid). In either case, there will be a difference in
speed between the shaft speed nr and the output ns. This is known as generator slip,
and may be expressed as;
s
rs
n)nn(
s−
= (3.40)
where
ns : Electrical speed of the magnetic field (or stator speed) (rpm)
nr : Rotor mechanical speed (rpm)
52
The slip is defined as negative when the machine is acting as a generator, and
positive when acting as a motor. (Chapman, 1999, pp.369-370)
Figure 3.21 Torque-Speed curve for a MW-size induction machine
The torque-speed characteristic curve in Figure 3.21 shows that, if an induction
motor is driven at a speed greater than synchronous speed by an external effect (i.e.
wind), the direction of its induced torque will reverse and it will act as a generator.
As the torque applied to its shaft increases, the amount of power produced by that
generator increases. There is a maximum possible induced torque in the generator
mode of operation. This torque is known as the pushover torque of the generator. If
a torque is applied to the shaft of the induction generator which is greater than the
pushover torque, the generator will over-speed. (Chapman, 1999, p.436)
53
As a generator, an induction machine has severe limitations. Because it lacks a
separate field circuit, an induction generator cannot produce reactive power. In fact,
it consumes reactive power, and an external source of reactive power must be
connected to it at all times to maintain its stator magnetic field. This external source
of reactive power must also control the terminal voltage of the generator—with no
field current, an induction generator cannot control its own output voltage. Normally,
the generator's voltage is maintained by the external power system to which it is
connected.
The one great advantage of an induction generator is its simplicity. An induction
generator does not need a separate field circuit and does not have to be driven
continuously at a fixed speed. As long as the machine's speed is some value greater
than synchronous speed for the power system to which it is connected, it will
function as a generator. The greater the torque applied to its shaft (up to a certain
point), the greater its resulting output power. The fact that no fancy regulation is
required makes this generator a good choice for windmills, heat recovery systems,
and similar supplementary power sources attached to an existing power system. In
such applications, power-factor correction can be provided by capacitors, and the
generator's terminal voltage can be controlled by the external power system.
(Chapman, 1999, p.437)
Wind machines driving electrical generators operate at either variable or constant
speed. In variable-speed operation, rotor speed varies with wind speed. In constant-
speed machines, rotor speed remains relatively constant, despite changes in wind
speed. (Gipe, 1995, p.211)
Small wind turbines typically operate at variable speed. This simplifies the
turbine’s controls while improving aerodynamic performance. When these small
wind machines drive an induction generator, both the voltage and frequency vary
with wind speed. The electricity they produce is incompatible with the constant-
voltage, constant- frequency alternating current (AC) produced by the utility, but can
54
be used as is for resistive heating or pumping water at variable rates, or it can be
rectified to direct current (DC) for charging batteries.
If a grid-connected turbine is fitted with an AC generator, this must produce
power that is in phase with the utility's grid supply. Many commercial grid-
connected turbines use induction AC generators, whose magnetizing current is drawn
from the grid, ensuring that the generator's output frequency is locked to that of the
utility and so controlling the rotor speed within limits. Synchronous generators
produce electricity in synchronization with the generator's rotating shaft frequency.
Thus, the rotor speed of grid-connected turbines must exactly match the utility
supply frequency.
To generate utility-compatible electricity, the output from a variable-speed
generator must be conditioned. Although it is possible to use rotary inverters for this
task, variable-speed turbines typically use a form of synchronous inverter to produce
constant-voltage 50 or 60 Hz AC like that of the utility. Most of these inverters use
the utility’s alternating current as a signal to trigger electronic switches that transfer
the variable-frequency electricity at just the right moment to deliver 50 or 60 Hz AC
at the proper voltage.
Although some manufacturers of medium-sized wind turbines build variable-
speed turbines, most operate the rotor at or near constant speed. These machines
produce utility-compatible power directly via induction (asynchronous) generators.
Induction generators have two advantages over alternators;
• They are inexpensive.
• They can supply utility-compatible electricity without complicated controls.
For AC generators, a critical design factor, that is synchronous speed, must be
considered. AC generators produce alternating current, the frequency of which varies
directly with the speed of the rotor and indirectly with the number of poles in the
55
generator. For a given number of poles, frequency increases with increasing
generator speed.
p
f120sn
⋅= (3.41)
where
ns : Synchronous or stator speed (rpm)
f : Grid frequency (Hz)
p : Number of poles
Manufacturers should decide the number of poles of the generator (for either
synchronous or asynchronous) for optimum conditions.
Table 3.2 Common Synchronous Speeds for Generators
Pole Number Europe (50 Hz) North America (60 Hz)
4-pole 1500 rpm 1800 rpm
6-pole 1000 rpm 1200 rpm
An induction generator begins producing electricity when it is driven above its
synchronous speed which is generally 1000 or 1500 rpm in Europe (1200 or 1800
rpm in North America). Induction generators are not true constant-speed machines.
As torque increases, generator speed increases 2 to 5 %, or 20 to 50 rpm on a 1000-
rpm generator. This increase of 1 to 3 rpm in rotor speed is imperceptible in a wind
turbine operating at a nominal speed of 50 rpm. As torque increases, the magnetic
field in the induction generator also increases. This continues until the generator
reaches its limit, which is about 5 % greater than its synchronous speed. Induction
generators are readily available in a range of sizes and are easily interconnected with
the utility. Medium-sized wind turbines use induction generators almost exclusively.
56
3.3.4. RECENT DEVELOPMENTS IN GENERATORS FOR WIND
TURBINES
As well as applying to the basic process of energy conversion, technological
development also relates to the design and size of machines used for the generation
of electric power from wind energy. Whilst the induction machine is now well
established as the most popular generator for reliable, efficient, low-cost power
production from the wind, other designs of machines are used and there are several
"drivers" for change.
The 'traditional' Danish design of wind turbine is fixed-speed, using an induction
generator. Variations on this theme which are now appearing include;
• Multiple or dual (two speed) generators,
• Induction machines with variable generator rotor resistance.
3.3.4.1. DUAL GENERATORS
Generators operate inefficiently at partial loads. For example, in a 500-kW wind
turbine, where the generator is designed to reach its rated capacity at a wind speed of
16 m/s, the generator operates at partial load much of the time. At a site with an
average wind speed of 7 m/s, the generator will operate 97 % of the time at less than
rated capacity and about half the time at less than 100 kW. (Gipe, 1995, pp.212-213)
Efficiency drops off rapidly when the generator is operated at less than one-third
its rated value. For example, the efficiency falls nearly 15 % (from 95 % at rated
output) when a 500-kW wind turbine is operated at 100 kW. To avoid this, designers
of constant-speed wind turbines often use dual generators or dual windings: One
main generator and a small generator having the capacity from one-fifth to one-third
of the main generator. The small generator operates at nearly full load in low to
moderate winds. When the wind speed reaches the rated wind speed of the small
generator, it switches off and the main generator switches on instead. Thus both
57
generators operate more efficiently then either one alone. At many sites, the small
generator will operate more than 50 % of the total generating time, although it
delivers less than half the total generation.
The two generators may be in tandem and driven by the same shaft or they can be
side by side, with the small generator driven by belts from the main generator.
During the mid-1990s, most new constant-speed turbines used one generator with
dual windings. The generator operates on 6 poles during light winds and uses 4 poles
in higher winds.
The use of dual generators permits the turbine to operate at two speeds, enables
designers to drive the rotor at a higher aerodynamic efficiency over a broader range
of wind speeds than with only one generator. Dual-speed wind turbines, while
incapable of taking the full advantage of the optimum tip-speed ratio over the entire
operating range, can capture most of the efficiency advantages of variable-speed
turbines, at only a small increase in cost for the extra windings. (Gipe, 1995, p.213)
The advantage of one single generator with dual windings becomes problematical
as turbines grow ever more powerful. Because a generator’s power is proportional to
its volume, while losses are proportional to its surface area, larger generators are also
more efficient than smaller ones. This could add perceptibly to the improved
performance of larger turbines over that of their smaller predecessors. (Gipe, 1995,
p.214)
3.3.4.2. DIRECT-DRIVE GENERATORS
In fact, the gearbox is needed for the generator frequency to catch grid frequency
for grid-connected systems. As turbine size increases, the relative cost of the gearbox
becomes more important. Removing the gearbox could save not only cost, but also
mass, losses, acoustic noise and reliability problems. For a doubling of wind turbine
diameter, rated power will quadruple, and rotor torque, which is closely related to
58
gearbox cost, will increase by a factor of eight. Another important issue is the
integration of the generator into overall nacelle design.
On mid-1990s, some manufacturers successfully developed gearless wind
turbines. Instead of using a gear with a high transmission ratio, they use low speed
multi-pole generators directly connected to the blade shaft. The large dimensions of
these multi-pole generators lead to a certain transportation disadvantage especially in
the megawatt class.
As rotor diameter increases, rotor speed decreases. So, lower rotor speeds make
the design of direct-drive generators problematic, requiring large-diameter ring
generators with numerous poles. For example, an existing Darrieus type turbine uses
a 162-pole synchronous generator coupled directly to the vertical axis turbine’s
torque tube.
Direct designs have the maintenance and operation advantage as compared to the
usage of gearboxes.
3.4. GRID INTEGRATION
With regard to the transfer of energy to electrical supply installations, we must
differentiate between;
• Systems with limited supply options, that either operate in isolation or supply
weak grids,
• Unlimited capacity connection with the rigid grid.
Wind energy converters should give reliable operation in both operations.
Due to its very high output capacity (in comparison with the nominal values of the
consumers connected to it), the so-called rigid combined grid can be regarded both as
an infinitely rich source of active and reactive current and, for the low-level energy
59
supply devices that wind power plants usually represent, as a sink of unlimited
capacity and constant voltage and frequency.
Unlike thermal power plants, wind turbines are usually installed at remote sites
with limited supply options. Therefore a weak grid connection is often made using
stub cables, which are sometimes long. In large wind energy converters and wind
parks, supply power can reach the same order of magnitude as grid transfer power, or
even approach its level, which means that mutual influences must be taken into
account. (Heier, 1998, p.181)
There is currently a clear trend in favor of robust single systems, mainly
characterized by stall-controlled turbines with asynchronous generators and direct
connection to the grid, rather than more expensive units. However, synchronous
machines are also popular, often based on gearless, ring-type designs with non-
controllable, controlled or machine-commutated rectifiers, direct-current
intermediate circuits and grid- or self-commutated inverters. The increased cost of
such systems is justified if, by adjusting the turbine speed to the prevailing wind
speed, the compatibility of the plant to the environment and the grid can be
improved, leading to a higher energy output and reduced drive-train loading.
This type of system also requires a frequency-converter system that is capable of
supplying the variable-frequency electrical energy from the turbine generator to a
grid of (almost) constant frequency and voltage. (Heier, 1998, p.183)
3.4.1. FREQUENCY CONVERTER SYSTEMS
Electronic power frequency converters, so-called power converters, are the most
common solution for the conversion and control of electrical energy. They are also
used to an increasing degree in wind energy converters to adjust the generator
frequency and voltage to those of the grid, particularly in variable-speed systems.
(Heier, 1998, p.183)
60
Power converters have significant advantages over the rotating transformers based
on groups of mechanical components and the mechanical commutators that were
common in the past, namely;
• Low-loss energy conversion
• Rapid engagement and high dynamic ratio
• Wear-free operation
• Low maintenance requirement
• Low volume and weight
Figure 3.22 Electrical energy conversion by power converters
Rectifiers convert alternating or three-phase current into direct-current, with the
electrical energy flowing from alternating or three-phase current systems into direct-
current systems.
Inverters convert direct-current into alternating or three-phase current. The
energy flows into the alternating-current side.
61
Direct-current conversion is the conversion of direct-current with a given
voltage and polarity for use in a direct-current system with a different voltage and
possibly reversed polarity.
In alternating-current conversion, alternating-current of a given voltage,
frequency and number of phases is converted for use in an alternating-current system
with a different voltage, frequency and possibly a different number of phases.
The main components of current-conversion systems are the power section, with
so-called power converter valves, which carries the electrical power, and an
electronic signal processing unit, which performs numerous control, protective and
regulating tasks.
As wind power plants are almost always fitted with three-phase current
generators, only three-phase current converters are relevant for power conditioning.
Here, it must be differentiated that;
• Direct frequency converters,
• Intermediate circuit frequency converters.
Direct frequency converters are used particularly for the reduction of frequency.
In the case of supply from or to a 50 Hz grid, the operating range 0-25 Hz is
preferred. Direct frequency converters require two complete anti-parallel power
conversion bridges per phase to operate the consumer and supply systems. This
results in high costs for power gates and control elements.
62
Figure 3.23 Basic wiring diagram for direct frequency converters
The conversion of grid frequency f1 into machine frequency f2 or vice versa, in a
direct frequency converter takes place by the selection of voltage sections from the
three phases and by triggering the power converter such that the voltage path after
smoothing has the amplitude, phase position and frequency required by the machine.
(Heier, 1998, p.185)
Indirect frequency converters consist of a rectifier, direct current or direct voltage
intermediate circuit and an inverter. A frequency converter with a direct current
intermediate circuit will be referred to as an I frequency converter, and one with a
direct voltage intermediate circuit as a U frequency converter. (Heier, 1998, p.186)
63
a. I frequency converter b. U frequency converter
Figure 3.24 Indirect frequency converters
Particular characteristics of the intermediate circuit are;
• The inductor for current smoothing in the I frequency converter,
• The capacitor for voltage smoothing in the U frequency converter.
Indirect frequency converters have achieved a clear dominance in energy
conversion and the connection of variable speed wind power plants to the grid.
Direct frequency converters were only used in individual cases to supply the rotor
circuit of double-fed asynchronous generators.
3.4.1.1. POWER SEMICONDUCTORS FOR FREQUENCY
CONVERTERS
So-called power converter valves are the main components of the power section
of frequency converters. They consist of one or more power semiconductors, and
64
conduct electrical current in one direction only. These valves generally alternate
periodically between the electrically conductive and non-conductive states, and
therefore function primarily as switches. As there is no need to operate any
mechanical contacts, these can initiate and/or terminate current conduction very
rapidly (i.e. in the microsecond range).
Power converter valves can be either controllable or non-controllable. Non-
controllable valves (diodes for example) conduct in the forward direction and block
in the reverse direction. Controllable valves permit the selection of the moment at
which conductivity in the forward direction begins. Thyristors can be switched on by
their gate and block if the direction of the current is reversed. Switchable thyristors
and transistors, on the other hand, can be switched on by one gate electrode and off
by a second (or the same) gate. (Heier, 1998, pp.186-187)
3.4.1.1.1. SEMICONDUCTOR DIODES
Diodes consist of positively (p) and negatively (n) doped semiconductor material
with a barrier layer between them that ensures current can flow in one direction only.
This is possible in the case of positive diode voltages. If the current direction and
voltage are reversed, the diode becomes non-conducting and blocks the flow of
current. Its application is thus limited to use in uncontrolled rectifiers and for
protective and back-up functions, for example as a recovery diode in direct-current
circuits or similar circuit elements.
In addition to limit values for current and voltage in the forward and reverse
directions, and thermal behaviour, another determining variable is conducting- state
dynamic behaviour, particularly for protective functions. For the effective protection
of semiconductor components, so-called fast-recovery diodes with low storage
charges are necessary to protect power converter valves from destruction by
overvoltage. (Heier, 1998, p.187)
65
3.4.1.1.2. THYRISTORS
Thyristors are semiconductor components with four differently (p and n) doped
layers. Conventional thyristors, GTO thyristors and MCTs are the main types used in
frequency converters.
Thyristors, unlike diodes, do not automatically go into a conducting state when an
adjoining positive anode-cathode voltage is present. The transition from blocking to
conducting state is initiated by the supply of a power impulse to the gate, and is
known as the firing of the thyristor. Once triggered, thyristors behave like diodes.
They remain in the conducting state as long as a current flows in the positive
direction and the current does not fall below the component's minimum value, the so-
called holding current. If a thyristor is in off-state, it can be fired by a new current
impulse or periodic impulse sequences at the gate.
However, in conventional thyristors, it is not possible to interrupt the current by
intervention at the gate. Switchable thyristors do permit this. The best known type is
the Gate-Turn-Off, or GTO thyristor. With these types of thyristors, uninterrupted
current requires a free-wheeling arm.
The metal-oxide-semiconductor controlled thyristor, abbreviated to MCT,
behaves in a similar manner to the GTO thyristor. The MCT can be switched on
almost without power by a negative voltage (in relation to the anode) at the gate. A
positive gate voltage switches it off, and at null current it automatically switches to
blocking operation. (Heier, 1998, p.187)
3.4.1.1.3. TRANSISTORS
Transistors are semiconductor components with three differently (p and n) doped
layers. Mainly bipolar, MOSFET and IGBT transistors are used in frequency
converters. As valve components they function exclusively as switches.
66
Bipolar transistors (BPT), in their function as power semiconductors, are usually
used in emitter mode. This allows a high level of power amplification to be achieved.
Almost like switches, they become conductive when a control current is passed
through the base electrode. When switched off, the on-state of the transistor is
terminated and the flow of current blocked. In order to achieve low on-state voltage,
and thus low losses, transistors are operated with a relatively high base current. The
transistors therefore operate in the so-called saturation range.
Much smaller control currents are needed for metal-oxide-semiconductor field
effect transistors than those for bipolar transistors. These MOSFETs can be switched
almost without power, by voltage control at the gate. This, however, requires that the
internal capacities of the transistor to be reloaded. Increasing the switching frequency
causes increased currents and thus higher losses in the drive level. MOSFETs are
used in the lower-output range at high switching frequencies for combinational
circuit components and frequency converters, and have advantages over bipolar
transistors and IGBTs, particularly at high switching frequencies.
IGBTs (insulated gate bipolar transistors) combine the advantageous
characteristics of MOSFETs and bipolar power transistors. The field-effect transistor
at the control input facilitates rapid switching at very low driving power. IGBTs
automatically limit current increases at the output. This results in good excess current
and short-circuit behaviour. Integrated free-wheeling diodes protect the transistor in
the off-state direction. Different types of IGBTs are used as individual transistors or
are connected together in modules of two to six transistors to form bridge
connections. In more recent developments, transistors are built into modules with
driver switches, protective switches and potential divisions. IGBTs can be connected
in parallel. However, this requires that all transistors exhibit the same thermal
behaviour.
The development and availability of new power electronic semiconductor
components has given a new impetus to power converter technology and its
application in the field of drive and energy engineering. Particularly in the small and
67
medium output range, new components have largely pushed transistors and GTOs
out of the market. (Heier, 1998, p.188)
Table 3.3 shows symbols, maximum ratings and characteristics of power
semiconductors;
Table 3.3 Characteristics and Maximum Ratings of Switchable Power
Semiconductors
Component Rating
BPT IGBT MOSFET MCT GTO
Symbol
Voltage
(V) 1200
1700
(3300) 1000 3000 4500
Current
(A) 800
600
(1200) 28 300 4000
Output
(kVA) 480 360 14 450 4500
Turn-Off Time
(µs) 15 - 25 1 – 4 0.3 - 0.5 5 – 10 10 - 25
Frequency
(kHz) 0.5 – 5 2 – 20 5 – 100 1 – 3 0.2 – 1
Drive
Requirement Medium Low Low Low High
3.4.1.2. CHARACTERISTICS OF POWER CONVERTERS
The main components of power converters are the power converter valves and
their electrical connections and trigger equipment. Also necessary are circuit
elements, energy storages, auxiliary devices and devices for commutation, filtering,
cooling and protection, and usually also transformers.
68
Power converters must be run at their voltage and timed according to frequency.
The origin of the commutation voltage and commutation reactive power at the
conductive connection to another valve is decisive for current carrying. Externally
commutated power converters operate using natural commutation. They require a
grid, load or machine that specifies the voltage and can supply reactive power. Self-
commutated converters, on the other hand, operate with forced commutation. The
required reactive power is provided by capacitors.
The internal function of power converters must also be differentiated with regard
to the origin of the elementary frequency. Externally clocked power converters take
their control pulse from the system that they work in parallel with. Line clocking is
the adjustment of the zero-crossings or phase intersections to the grid voltage. Thus
the load- or machine-clocked power converter orientates itself to the load or machine
voltage. Self-clocked power converters have an internal clock generator and are thus
not dependent upon external frequency information.
As well as the commutation voltage and elementary frequency, the so-called pulse
number, the number of non-simultaneous conductive connections (commutations)
from one valve to another within one cycle, is an important parameter of power
converter circuits. Three and six, as well as twelve, pulse connections are normal for
three-phase current systems. The pulse number is characterized by the number of
sine peaks (pulses) of the unsmoothed direct-current. (Heier, 1998, p.190)
Commutation, the transfer of current between the individual valves, can occur in
different ways. If the live valve is turned off before the next valve is fired then the
connection becomes temporarily dead. As ripples occur in direct-current, this process
is known as intermittent flow. In contrast, it is possible to fire a second valve while
the valve to be turned off is still live. This creates a temporary short-circuit between
two alternating-current lines. The current in the valve to be turned off is quickly
forced to be under its holding point. This interrupts the short circuit before the
operating current is exceeded. This changeover is known as commutating
operation. (Heier, 1998, p.191)
69
CHAPTER FOUR
CLASSIFICATION OF WIND TURBINES
Wind turbines can be classified in several ways due to there are more than one
design criteria which affects turbine performance. Classification categories can be
arranged as;
• Classification by axis of rotation
• Classification by rotor speed
• Classification by power control
• Classification by location of installation
4.1. CLASSIFICATION BY AXIS OF ROTATION
As mentioned before, modern windmills are usually referred to as wind turbines
or wind energy conversion systems to distinguish them from their traditional name.
Apart from a few innovative designs, modern wind turbines come in two basic
configurations:
1. Horizontal Axis Wind Turbines
2. Vertical Axis Wind Turbines
The majority of modern wind turbines are electricity-generating devices. They
range from small turbines that produce a few tens or hundreds of watts of power to
relatively large turbines that produce 2 MW or more. (Boyle, 1996, p.280)
70
Figure 4.1 Horizontal and vertical axis wind turbines
4.1.1. HORIZONTAL AXIS WIND TURBINES (HAWT)
Modern low-solidity horizontal axis wind turbines evolved from traditional
windmills and are by far the most common wind turbines manufactured today. They
have a clean, streamlined appearance; due to wind turbine designers’ improved
understanding of aerodynamics, derived largely from developments in aircraft wing
and propeller design. They are almost universally employed to generate electricity.
(Boyle, 1996, p.280)
They generally have either two or three blades or else a large number of blades,
although only one is necessary. Wind turbines with large numbers of blades have
what appears to be virtually a solid disc covered by solid blades and are described as
high solidity devices. These include the multi-blade wind turbines used for water
pumping on farms. In contrast, the swept area of wind turbines with few blades is
largely void and only a very small fraction appears to be ‘solid’. These are referred to
as low solidity devices.
71
The rotor axis of conventional wind turbines is seldom truly horizontal. Designers
tilt the rotor axis slightly to provide more clearance between the blades and tower
than with a truly horizontal driveline (i.e. 6°). (Gipe, 1995, p.175)
Figure 4.2 Horizontal axis wind turbine configurations
4.1.2. VERTICAL AXIS WIND TURBINES (VAWT)
Vertical axis wind turbines have an axis of rotation that is vertical, and so, unlike
their horizontal counterparts, they can harness wind from any direction without the
need to reposition the rotor when the wind direction changes. (Boyle, 1996, p.280)
D.G.M. Darrieus invented the modern vertical axis wind turbine in the 1920s. The
French engineer’s name has become synonymous with the “φ” or “eggbeater”
72
configuration, although he experimented with several designs, including a
conventional two-bladed turbine. (Gipe, 1995, p.171)
Figure 4.3 Vertical axis wind turbine configurations
Vertical axis designs have an advantage of rotational symmetry that obviates any
need for a yaw system. It was often a claimed advantage that all the drive train and
power conversion equipment can be at ground level, but it was found that this
implied a long and heavy torque tube for the main shaft and various designs
compromised with gear boxes at the top of the main shaft. The overriding
disadvantages, however, of the vertical axis design compared to horizontal axis are:
• Inherently lower aerodynamic efficiency because the drive torque varies
strongly with blade position in the rotor circle (and may even be negative in
some positions)
• Substantial passive support structure in the rotor system with an associated cost
penalty
• At the present time, VAWTs are not economically competitive with HAWTs.
4.2. CLASSIFICATION BY ROTOR SPEED
Modern wind turbines have two types of electrical connections to the grid:
• With the simple direct synchronization of an induction generator, the rotor
operates with nearly constant speed because the strong grid keeps generator’s
frequency. The only rotational speed variation is given by the slip range of the
generator.
73
• With the help of an inverter system between the wind turbine generator and the
grid, the turbine is decoupled from the grid frequency and is able to rotate at
variable speeds. For a long period, directly grid coupled wind turbines
dominated the world market due to their technical simplicity. But several
positive aspects of variable speed turbines changed the current development
situation. (German Wind Energy Institute, DEWI, 1998, p.48)
4.2.1. VARIABLE ROTOR SPEED
The aerodynamically optimized lay out of wind turbines is based on a fixed
relationship between wind and rotor tip speed, the so-called tip speed ratio. To keep
the maximum efficiency, the rotor must change its rotational speed according to the
wind speed, in other words, low winds with low rotor speeds, high winds with high
rotor speeds. (German Wind Energy Institute, DEWI, 1998, p.48)
Variable speed is attractive because it enables designer to gain greater rotor
efficiencies by allowing rotor speed to vary with wind speed. There may be
additional benefits as well. Slower rotor speeds in light winds lower noise emissions
just when the aerodynamic noise of the blades is most noticeable. Variable-speed
operation may also reduce dynamic loads on the turbine’s drive train, thus extending
turbine life. When operating at variable speed, the rotor stores the energy of gusty
winds as inertia as its speed increases, rather than forcing the drive train to absorb the
increased torque instantaneously.
Due to their ability to operate at tip speed ratios closer to the optimum value,
variable speed machines can be more efficient than fixed speed systems. However,
modification of both the generator and the intermediate electronic control systems
are necessary in order to provide a grid-compatible supply. One of the main factors
favoring this route is the requirement of some utilities for very smooth output power.
74
Variable rotor speeds normally are combined with a “pitch angle control system”.
They have various operational advantages in comparison with constant rotor speed
machines;
• Higher energy extraction.
• Very low power fluctuations during rated power operation.
• Lower rotor loads due to rotor speed yielding in gusts.
• Low blade pitch change rates possible.
• Low rotor speed at low wind conditions reduces the noise emission
considerably.
High power variable speed drives are now being designed into turbines and with
them a new set of engineering aspects need to be considered, including;
• Fault level of network.
• Voltage regulation.
• Electromagnetic compatibility.
• Electrical system behaviour during gusting conditions.
• Power converter efficiency.
For variable speed turbines, relatively complex power converter hardware is
necessary. The power conversion equipment must provide low harmonics and unity
power factor control of the current delivered to the network.
4.2.2. CONSTANT ROTOR SPEED
Constant rotor speed is the simplest way of operating a wind turbine because the
rotor speed is guided by the frequency of a strong grid. The tip speed ratio cannot be
maintained constant during operation that means the efficiency reaches its optimum
only with one wind speed, which is the design wind speed of the rotor blade. During
all other wind velocities, the efficiency is smaller than maximum. To better adapt the
rotor operation to the aerodynamic design point, the manufacturers often use two
75
speed induction generators which allow changing the rotor speed in two steps: At
low wind speeds; generator operates with a low rotational speed (higher number of
poles) and at high wind speeds; with a high rotational speed (lower number of poles).
Constant one or two steps rotor speed operation is the simplest way of rotor speed
control, because the strong grid takes over the speed guidance;
• No rotor speed control system is necessary.
• Simple rotor speed regulation by the strong grid.
• Only rotor speed monitoring is necessary.
• Low cost design.
Due to stiff grid coupling, the rated power fluctuations reach higher values than
variable speed designs.
4.3. CLASSIFICATION BY POWER CONTROL
Wind turbines can be classified into 3 groups as “small scale”, “medium scale”
and “large scale” in terms of their power output capacity. Wind turbines with power
ratings lower than 100 kW are called as small scale where the turbines with power
ratings between 100 and 700 kW are called as medium scale.The large scale wind
turbines have the power output capacity of greater than 700 kW.
76
Figure 4.4 Operating regions of a typical wind turbine
The maximum power which can be produced by a wind turbine is the rated
power of it, and the wind speed at which the turbine reaches rated power output is
called as the rated wind speed. Above this, there is a maximum wind speed, called
as cut-out wind speed, at which the turbine is designed to shut down in order to save
mechanical parts of the wind turbine from harmful effects of high wind speed. The
lowest wind speed at which a wind turbine will operate is known as the cut-in wind
speed. At or above the rated wind speed, the power output remains constant
whatever the wind speed (below the cut-out wind speed), but below the rated wind
speed the output power varies with the wind speed. (Boyle, 1996, pp.268-269)
77
Table 4.1 Descriptions of Operational Regions for a Typical Wind Turbine
Operating
Region
Operational Description:
Power Output vs. Wind Speed Wind Speed Range
Region - I - Wind speeds too low to produce
usable electric power.
0 to cut- in wind speed;
0 to 4 m/s.
Region - II - Production of electric power
increasing with wind speed.
Cut- in to rated wind speed;
4 to 13 m/s.
Region - III -
Production of electric power at
constant, rated power level. Wind
turbine blades purposely made less
efficient as wind speed increases.
Rated wind speed to cut-
out wind speed;
13 m/s to 25 m/s.
Region - IV -
No electric power output. Winds
too energetic to justify added
strength and cost for the small
number of hours per year beyond
cut-out wind speed.
Cut-out wind speed to
survival wind speed; 25
m/s to rated survival wind
speed.
As the blades of the wind turbine rotate through circular path, they sweep through
a disc- like area which is referred to as the swept area. This value can be normally
calculated by area formula for circles;
2rA ⋅π= (4.1)
where r is the rotor radius.
78
Figure 4.5 Rotor diameter vs. power output
The power that a wind turbine can extract from the wind at a given wind speed is
directly proportional to its rotor’s swept area. It is extremely important that the
maximum swept area is presented to the wind and this is achieved by making sure
that the rotor’s axis is aligned with the direction from which the wind is blowing. As
the wind does not always blow from the same direction, a mechanism of some kind
is needed to realign the rotor axis in response to changes in wind direction. This
aligning or slewing action, about a vertical axis that passes through the center of the
tower, is known as yawing.
A wind turbine blade has a distinctive curved cross-sectional shape, which is
rounded at one end and sharp at the other. The shape of the blade’s cross-section is
the key how modern wind turbines extract energy from the wind. This special profile
is known as an aerofoil section and is already familiar as the cross-sectional shape of
aeroplane wings.
79
Figure 4.6 Swept area by rotor blades
Due to the aerodynamic forces on rotor blades, a wind turbine converts the kinetic
energy of wind flow into rotational mechanical energy. These driving aerodynamic
forces are generated along the rotor blades, which need specially shaped profiles that
are very similar to those, used for wings or aeroplanes. With increasing airflow
speed, the aerodynamic lift forces grow with the second power and the extracted
energy of the turbine with the third power of the wind speed, a situation which needs
a very effective, fast acting power control of the rotor to avoid mechanical and
electrical overloading in the wind turbine’s energy transmission system.
Modern wind turbines use two different aerodynamic control principles to limit
the power extraction to the nominal power of the generator. The most passive one is
the so-called stall control, the active one pitch control. Stall control is a traditional
way and has restrictions. Pitch control is more flexible and has opportunities to
influence the operation of the wind turbine. (German Wind Energy Institute, DEWI,
1998, p.44)
80
4.3.1. PITCH CONTROL
Pitch control is an active control system, which normally needs an input signal
from the generator power. Always when the generator’s rated power is exceeded due
to increasing wind speeds, the rotor blades will be turned along their longitudinal
axis (pitch axis), or in other words, change their pitch angle to reduce the angle of
attack of incoming air flow. Under all wind conditions, the flow around the profiles
of the rotor blade is well attached to the surface, thus producing aerodynamic lift
under very small drag forces. Therefore, turbine blades reach the optimum pitch
angle, at which it will produce the maximum power at that wind speed.
Pitch controlled turbines are more sophisticated than fixed pitch stall controlled
turbines, because they need a pitch changing system. (German Wind Energy
Institute, DEWI, 1998, p.45)
The advantages of the pitch controlled wind turbines are;
• Allow for active power control under all wind conditions, also at partial power.
• Straight power curve at high wind speeds.
• They reach rated power even under low air density conditions (high site
elevations, high temperatures).
• Higher energy production under the same conditions (no efficiency reducing
stall adaptation of the blade).
• Simple start-up of the rotor by simple pitch change.
• No need of strong brakes for emergency rotor stops.
• Decreasing rotor blade loads with increasing wind above rated power.
• Feathering position of rotor blades for low loads at extreme winds.
• Lower rotor blade masses lead to lower turbine masses.
81
Figure 4.7 Pitch Control
4.3.2. STALL CONTROL
Stall control is a passive control system, which reacts on the wind speed. The
rotor blades are fixed in their pitch angle, and cannot be turned along their
longitudinal axis. Their pitch angle is chosen in a way that for winds higher than
rated wind speed the flow around the rotor blade profile separates from the blade
surface (stall). This reduces the driving lift forces and increases the drag. Lower lift
and higher rotational drag act against a further increase of rotor power. (German
Wind Energy Institute, DEWI, 1998, p.44)
The advantages of stall controlled wind turbines are;
• No pitch control system.
• Simple rotor hub structure.
• Less maintenance due to fewer moving machinery parts.
• High reliability of power control.
Figure 4.8 Stall Control
82
In last years, a mixture of pitch and stall control is appeared, the so-called active
stall. In that case the rotor blade pitch is turned in direction towards stall and not
towards feathering position (lower lift) as it is done in normal pitch systems.
The advantages of this system are;
• Very small pitch angle changes necessary.
• Power control under partial power conditions (low winds) is possible.
• Feathering position of rotor blades for low loads at extreme winds.
The main issues in deciding between pitch and stall control are listed in Table 4.2.
Table 4.2 Pitch vs. Stall Issues
Issues Pitch Stall
Energy Capture Better in principle Compromised power curve
Control With
Fixed Speed Difficult in high wind speeds
Generally satisfactory,
although design uncertain
Control With
Variable Speed
Better power quality,
lower drive train loads
than any stall option
Requires proving
Safety Complete rotor protection Needs auxiliary systems for
over-speed protection
Cost More cost in rotor systems Less cost in rotor, but more
in braking system
Large wind turbines almost exclusively use pitch or stall control. In a few
instances, yawing out of wind is used as a back up safety procedure or as
contributory to control.
Recently, some manufacturers have used stall in conjunction with variable speed
operation. The one configuration that has now been unanimously rejected is fixed
speed pitch control. This combination produced very large transients in the power
83
output when controlling power. This rejection is, however, rather interesting since it
was, in the early days, a popular choice.
Figure 4.9 Stall & Pitch controlled power schemes
As shown in Figure 4.9, pitch controlled power scheme results almost zero
oscillations. Beside, stall control scheme shows some unwanted fluctuations causing
power losses.
4.4. CLASSIFICATION BY LOCATION OF INSTALLATION
Wind turbines are installed either on the land or on the sea level by some
additional equipment. They are classified as on-shore and off-shore wind turbines.
4.4.1 ON-SHORE WIND TURBINES
In order to get the best efficiency from wind turbine operation and provide
sustainable electricity to consumers, wind turbines should be erected in windy areas.
For this purpose, locations with continuous and fast wind should be selected.
84
Wind turbines on the land are called as on-shore wind turbines. In order to benefit
from wind speed as much as possible, windy and smooth areas such as lowlands, sea
coasts, large farms are selected for siting.
4.4.2 OFF-SHORE WIND TURBINES
Off-shore wind turbines are installed on sea up to some depths. It is a fact that,
there is a noteworthy difference of available wind speeds between on-shore and off-
shore locations. It is possible to obtain higher output power levels for off-shore
designs than the same turbines designed for on-shore.
The next great leap for the wind energy industry will be in the area of offshore
development. The potential for this technology is vast and it requires, and deserves
sustained and substantial research and development support. (European Commission
Directorate-General for Energy, 1997, p.10)
Most turbines operate with a blade tip speed less than 65 m/s principally in order
to contain sound emission within acceptable limits. It has been recognized that if off-
shore wind turbines are remote from the coast and can be allowed increased sound
emission, then there is considerable scope for reduction of the weight and cost of the
turbines themselves. A tip speed of 100 m/s may be acceptable for offshore wind
turbines. As with sound, if there is some relaxation in concern about the near field
visual effect for offshore wind farms, there is added potential for cost reduction in
support structures and greater tolerance of more unusual design configurations that
may have economic merit.
Thus the general view is that, if higher tip speeds can be exploited, the cost of the
wind turbine component of the offshore system can be significantly reduced
compared to land based designs. Obviously this is very desirable to help offset the
increased costs of foundations and electrical transmission associated with offshore
projects.
85
A key objective for the design of cost effective offshore wind turbines will be that
inspection and maintenance requirements are reduced to a minimum. Design for high
reliability will be an important priority with an emphasis on minimising long term
operation and maintenance costs, possibly at the expense of a somewhat higher wind
turbine capital cost. (European Commission Directorate-General for Energy, 1997,
p.11)
86
CHAPTER FIVE
EXPERIMENTAL WORK
In this chapter, a wind turbine is modelled by MATLAB v5.2 - SIMULINK
software. The prototype chosen for the simulation is VESTAS V80 – 2.0 MW wind
turbine.
The characteristics of the modelled wind turbine are;
Rated Mechanical Power (Pcap) : 2 MW
Rated Wind Speed : 12.5 m/s
Cut- in Wind Speed : 4.5 m/s
Cut-out Wind Speed : 20 m/s
Power Regulation Method : Pitch Control (0-15 degrees)
Rotor Diameter (2.r) : 74 m
Disc Swept Area (A) : 4300.84 m2
Air Density (?) : 1.225 kg/m3
Moment of Inertia (J) : 1000 t.m2
Gear Ratio : 38
Rotational Speed (nrlow) : 20 – 28.5 rpm
Generator Rotor Speed (nrhigh) : 760 – 1083 rpm
While constructing the closed- loop model, some mathematical expressions
describing the power output and rotational motion of the turbine are used.
System Equation Set:
3pcap VAC5.0P ⋅⋅⋅η⋅ρ⋅= (5.1)
87
( )( )
( ) α⋅−λ⋅−
α⋅−
−λ⋅π⋅α⋅−= 300184.0
3.0153
Sin)0167.044.0(Cp (5.2)
V
r rω⋅=λ (5.3)
dt
dJPP )t(r
)t(r)t(cap)1t(cap
ω⋅⋅ω+=+ (5.4)
where
Pcap : Captured power by the turbine (input to the generator) (W)
? : Air density (kg/m3)
? : Turbine mechanical efficiency
Cp : Power coefficient
A : Swept area by rotor blades (m2)
V : Wind speed (m/s)
a : Blade pitch angle (degree)
? : Tip speed ratio
r : Rotor radius (m)
? r : Angular shaft speed (rad/s)
J : Moment of inertia (kg.m2)
88
Fig
ure
5.1
Ove
rvie
w o
f the
win
d tu
rbin
e si
mul
atio
n
89
The aim of the simulation is to observe system output power curve versus wind
input that changes with time. The captured power is used to calculate shaft speed
variation corresponding torque change. For example, when input wind power
increases, input torque to the turbine increases as well. Then, acceleration on the
turbine shaft will be observed.
5.1 SUB-SYSTEMS IN THE MODEL
5.1.1 YAW CONTROL BLOCK
Yaw mechanism should be adapted to all wind turbines to avoid two unwanted
effects;
1. Physical damage of turbine machinery parts due to extremely high wind
speeds; occurs when the wind speed is as high as unacceptable over the rated
value. This causes teetering effects on turbine tower and over-speed of
generator rotor. Manufacturers should take into account the upper damage limit
to keep turbine in service. This limit is called cut-out wind speed.
2. Motoring operation of the turbine generator due to very low wind speeds
because of insufficient starting torque; a specific wind speed occurs as the
lower limit to enable starting of generator mode of the machine. The specific
lower limit of the wind speed is called cut-in wind speed.
Another usage purpose of the yaw system is aligning the turbine in line with the
wind direction in order to allow the turbine to absorb maximum energy from the
wind.
In the studied model, 4.5 m/s is defined as cut- in and 20 m/s as cut-out wind
speeds. Any wind data outside the 4.5 – 20 m/s interval is neglected to make system
efficient.
90
Figure 5.2 Yaw control block
5.1.2 TURBINE EFFICIENCY BLOCK
At each wind speed, the mechanical torque input onto turbine shaft changes and
mechanical efficiency also changes due to friction and heating. So, it may be stated
that, turbine mechanical efficiency is directly proportional to the wind speed.
Figure 5.3 Turbine efficiency block
An efficiency curve is constituted for the model by using the operating values of
different turbines present in the market.
91
Figure 5.4 Turbine efficiency characteristics corresponding to wind speed
5.1.3 PITCH CONTROL BLOCK
Pitch control mechanism allows turbine blades to turn along their longitudinal
axes. As any blade moved to increase the pitch angle, its capacity of absorbing wind
power will decrease.
In the studied system, when the absorbed wind power exceeds 2 MW, pitch
control mechanism will be activated. After the power curve decreases below 2 MW,
blade pitch angle will begin to decrease. To make power curve smooth while pitch
control is activated, blade response time to any increment or decrement command is
tried to be minimized. For this purpose, linear interpolation is applied to input wind
speed data. By this way, present 137 wind inputs are raised to 2740 data with sample
time equal to 0.05 second.
92
Figure 5.5 Graphical demonstrations for the response of pitch control
mechanism
As seen from Figure 5.5, when the captured power exceeds 2 MW level at time
70.57 seconds, pitch mechanism is activated at time 70.60 seconds and the power
curve is corrupted at time 70.60 sec. approximately at 2.0135 MW. The
corresponding pitch mechanism response time is approximately 30 milliseconds.
After the blade opening command is received by pitch control mechanism, the
time required for the output power curve to recover itself to 2 MW level is about 10
milliseconds as shown in Figure 5.5.
93
Figure 5.6 Pitch control block with 0-15 degrees adjustment interval
5.1.4 ANGULAR SPEED CALCULATION BLOCK
This block is a key for turbine performance. By using the advantage of taken
samples of captured power in narrow time intervals (sample time=0.05 sec.), shaft
angular speed variation corresponding to changing input torque at each step is
calculated accurately in this block. Then, obtained angular speed value is used to
calculate tip speed ratio.
The general mechanical rotational motion equation is used to define acceleration,
deceleration or constant speed operations by wind speed changes;
dt
dJ )t(r
)t()1t(
ω⋅+τ=τ + (5.5)
where
t (t+1 ) : New captured mechanical torque input to the shaft (N.m)
t (t) : Existing mechanical torque on the shaft (N.m)
J : Moment of inertia (kg.m2)
? r(t) : Angular shaft speed (rad/s)
This equation can be modified to provide system compatibility;
94
dt
dJPP )t(r
)t(r)t(cap)1t(cap
ω⋅⋅ω+=+ (5.6)
where
Pcap(t+1) : New captured mechanical power input to the shaft (W)
Pcap(t) : Existing mechanical power on the shaft (W)
Here, derivative term states the speed variation between times (t) and (t+1). This
value is added to the speed value at time (t) to find the new speed value at time (t+1);
J
P
J
PP
dt
d
)t(r
capr
)t(r
)t(cap)1t(capr
)t(r
⋅ω
∆=ω∆⇒
⋅ω
−=ω∆=
ω + (5.7)
Consequently, this speed difference (indicating acceleration, deceleration or
constant speed operation) is added to the speed value at time (t);
r)t(r)1t(r ω∆+ω=ω + (5.8)
The resultant angular speed can be used to find tip speed ratio (?), power
coefficient (Cp) and the power input to the generator (Pcap), respectively.
Figure 5.7 Angular speed calculation block
95
5.1.5 Cp – ? SELECTION BLOCK
After the system decides pitch angle in degrees, power coefficient (Cp) can be
found by using its characteristic equation depending on tip speed ratio (?) and pitch
angle (a).
Cp – ? selection block has two inputs (?, a), and one output (Cp). Block has a
Cp=f(?, a) function for each a input (Equation 5.2).
Multiport selection block inside the sub-system decides the function to be used.
After the output Cp is found, it is fed back to power calculation block to determine
the captured power of the turbine. This power is also the input mechanical power to
the generator.
At the end of simulation, output power graph says that pitch control is a very
useful way to control system output whatever the wind power. Pitch control allows
user to control the power absorbing capacity of the turbine.
5.2 SIMULATION RESULTS
Simulation takes 137 seconds. Input wind data is interpolated by the system with
0.05 second sample time. Totally, simulation includes 20 x 137 = 2740 steps.
Small sample time enables system to be stable and captured power to be kept
around the rated value. Note from Figure 5.10 that, output power fluctuations can be
kept in 200 kW tolerances.
All graphical results of the simulation are shown below.
96
Figure 5.8 Wind speed values filtered by yaw control block
Figure 5.9 Aerodynamic power in the wind
97
Figure 5.10 Captured wind power by the turbine (Input power to generator)
Figure 5.11 Angular speed variation of the turbine in respect of each wind speed
change (Change of input torque)
98
Figure 5.12 Angular shaft speed of the turbine
Figure 5.13 Rotational speed of turbine shaft before gearbox
99
Figure 5.14 Rotational speed of turbine shaft after gearbox
(Rotational speed of generator rotor)
Figure 5.15 Tip speed ratio
100
Figure 5.16 Blade pitch angle (a)
Figure 5.17 Power coefficient (Cp)
101
Figure 5.18 Tip speed ratio vs. power coefficient
Figure 5.19 Turbine wind speed – power characteristics
102
Figure 5.20 Turbine efficiency vs. wind speed
In Table 5.1, variations of all parameters of the wind turbine can be observed
corresponding to each available wind speed value. Note that, until wind speed (V)
reaches the rated value, pitch angle (a) kept at zero by the system, and after the rated
wind speed occurred, pitch angle is started to increase in order to allow keeping the
output power (Pcap) around rated value At the same time, the available aerodynamic
wind power (Pw) is still increasing.
103
Table 5.1 Modelled Wind Turbine Simulation Results
V
(m/s)
Pw
(kW) ?
a
(degrees) Cp
Pcap
(kW)
5 330 15.7 0 0.21 51
6 570 13.4 0 0.36 165
7 904 11.9 0 0.42 323
8 1,345 10.8 0 0.44 514
9 1,922 10 0 0.44 753
10 2,632 9.4 0 0.43 1,031
11 3,505 9 0 0.42 1,360
12 4,551 8.6 0 0.41 1,732
13 5,788 8.3 2 0.35 1,925
14 7,225 7.1 2.5 0.30 2,020
15 8,890 6.7 4.5 0.24 1,999
16 10,790 6.35 6 0.21 2,052
17 12,938 5.9 7 0.16 1,869
18 18,466 5.6 8.5 0.14 1,847
104
CHAPTER SIX
CONCLUSIONS
Wind power is a deceptively simple technology. Behind the tall, slender towers
and gently turning blades lie a complex interplay of lightweight materials,
aerodynamic design and computerized electronic control.
Although a number of variations continue to be explored, the most common
configuration has become the horizontal three bladed turbine with its rotor positioned
upwind on the windy side of the tower. With this broad envelope, continuing
improvements are being made in the ability of the machines to capture as much
energy as possible from the wind. These include more powerful rotors, larger blades,
improved power electronics, better use of composite materials and taller towers.
The most dramatic improvement has been in the increasing size and performance
of wind turbines. From machines of just 25 kW twenty years ago, the typical size
being sold today is up to 2500 kW.
Today’s wind turbines include properties of modern technology. They are
modular and very quick to install and commission.
Advantages of using wind energy conversion systems instead of other energy
production systems are;
• Environmental protection (No CO2 emission)
• Low-cost. Wind can be competitive with nuclear, coal and gas
• Diversity and security of supply
• Rapid deployment. Modular and quick to install
105
• Fuel is abundant, free and inexhaustible
• Costs are predictable and not influenced by fuel price fluctuations
• Land-friendly. Agricultural / industrial activity can continue around it
Power control of the studied horizontal axis, variable speed wind turbine is made
by pitch angle adjustment. This seems as the most efficient method to supply 3-phase
utility grids. As the number of wind speed samples increases, the pitch control
mechanism works more efficiently, in other words; the oscillations around rated
power line can be minimized above rated wind speeds.
Moment of inertia, rotor diameter and gear ratio are three critical parameters for a
variable speed wind turbine and must be selected carefully by manufacturers while
designation.
Moment of inertia is the rotational mass of the turbine rotating parts. The
constructing material of blades and other rotating masses should be selected optimum
to verify the minimum cut- in wind speed. This means minimum starting torque and
maximum usage of the wind power.
Rotor diameter is directly specifies the swept area and so captured power from
the wind. It should be selected carefully to ensure reaching rated power output level
and allowing minimum cut- in wind speed. For this purpose, long time wind speed
measurements should be made and then it will be possible to investigate the optimum
wind speed interval to allow maximum overall energy capturing.
Gear ratio is the adjustment location of induction machine generator region. For
example, in the studied system, 20-28.5 rpm operating interval of low-speed shaft is
modified into 760-1083 rpm region for a 750 rpm synchronous speed asynchronous
machine with the gear ratio of 38.
106
Although tip speed ratio values seem acceptable in both raising and falling regions
of ?–Cp curve, allowing tip speed ratio to exceed 10 causes the over-speed of
generator rotor, resulting in the physical damage of machinery parts.
Figure 6.1 ?–Cp curve indicating operating regions of the generator
6.1 FUTURE PROSPECTS
In the future, even larger turbines than today’s 2500 kW will be produced to
service the new offshore market. Machines in a range from 3000 kW up to 5000 kW
are currently under development. In 2002, the German company Enercon is
scheduled to erect the first prototype of its 4500 kW turbine with a rotor diameter of
112 meters. (EWEA, European Wind Energy Association, 2002, p.13)
European Wind Energy Association (EWEA) which is the international voice of
the wind industry located in the center of Europe has launched an industrial blueprint
including the targets to be reached by 2020.
107
The main objectives of this study are;
• Supplying 12 % of global electricity demand, assuming that global demand
doubles by then
• Creation of 1475 million recruitments
• Cumulative CO2 savings of 11,768 million tones
• 1,261,000 MW wind energy capacity installed generating 3093 TWh,
equivalent to the current electricity use of all Europe
This study demonstrates that there are no technical, economic or resource
limitations to achieve this goal, but the political and policy changes are required in
order for the wind industry to reach its full potential.
108
REFERENCES
American Wind Energy Association. (2002). Global wind energy market report.
URL: http://www.awea.org/pubs/documents/
Boyle, G. (1996). Renewable energy: Power for a sustainable future. Oxford
University Press.
Chapman, Stephen J. (1999). Electric machinery fundamentals. (3rd ed). Melbourne:
McGraw-Hill International Editions Electric Machinery Series.
Chen, Z., & Spooner, E. (2001). Grid power quality with variable speed wind
turbines. IEEE Transactions on Energy Conversion, 16, 148-153
Çam, E. (1999). Yeni tip kanat modeli ile rüzgardan elektrik eldesi. Bornova, Izmir.
Aegean University.
Danish Wind Turbine Manufacturers Association. (2001). Guided tour on wind
energy.
URL: http://www.windpower.org/download/
De Montfort University. (1998). Wind energy training course.
URL: http://www.iesd.dmu.ac.uk/wind_energy/index.html
European Commission Directorate-General for Energy. (1997). Wind energy - The
facts.
URL: http://www.ewea.org/doc/
109
European Wind energy Association. (2002). Wind energy – Clean power for
generations.
URL: http://www.ewea.org/doc/
European Wind energy Association. (2002). Wind force 12.
URL: http://www.ewea.org/doc/
European Wind Energy Association. (2002). Wind force 12, The new global
challange. Wind Directions, XXI - 4, 16-19
URL: http://www.ewea.org/doc/
German Wind Energy Institute. (1998). Wind Energy Information Brochure.
Gipe, J. (1995). Wind energy: Comes of age. John Wiley & Sons Inc.
Heier, S. (1998). Grid integration of wind energy conversion systems.
(Waddington R.). Swadlincote, UK: John Wiley & Sons Inc. (Original book
published 1996).
Muljadi, E., & Butterfield, C.P. (2000). Pitch-controlled variable-speed wind
turbine generation. Phoenix, Arizona, USA: 1999 IEEE Industrial Applications
Society Annual Meeting, October 3-7, 1999.
Ramage, J. (1983). Energy – A guidebook. Oxford University Press.
Shaltout, A. A. (1994). Analysis of torsional torques in starting of large squirrel
cage induction motors. IEEE Transactions on Energy Conversion, 9, 135-141
Wang, Q., & Chang, L. (1999). An independent maximum power extraction
strategy for wind energy conversion systems. Shaw Conference Center,
Edmonton, Alberta, Canada May 9-12 1999: Proceedings of the 1999 IEEE
Canadian Conference on Electrical and Computer Engineering.
110
APPENDICES
A
Get wind data (V) Rotor radius (r)
Gear ratio
4.5 < V < 20 m/s
Calculate aerodynamic wind power
( 3w VA5.0P ⋅⋅ρ⋅= )
Mechanical power
( pwm CPP ⋅= )
Calculate captured power (Generator input power)
( η⋅= mcap PP )
Calculate angular speed
(? r)
Tip speed ratio (?)
Calculate turbine efficiency
(?) (Look-up table)
Calculate pitch angle
(a)
Calculate power coefficient
(Cp)
V = 0
Yes
No
- FLOWCHART OF THE SIMULATED SYSTEM -
B
C
