MODELING AND SIMULATION OF WIND TURBINES
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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
41
b. The magnetization curve for synchronous generators…………. 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 (C p )……………………………………………. 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 Net Electrical Energy Consumption (GWh)
24000
18000
12000
6000
0 1990
1995
2000
2005
2010
2015
Years
Figure 1.1 World electricity consumption with estimations
2020
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 32000
Installed MW
28000 24000 20000 16000 12000 8000 4000 0
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
Year
Annual Installation Cumulative Installation
Figure 1.2 Wind power installations worldwide
3
Table 1.2 Wind Power Installations Worldwide WECS
Annual Installation (MW)
Cumulative Installation (MW)
Installations 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. Standalone 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 3bladed 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 DirectorateGeneral 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 highspeed 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.
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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, slowspeed 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 ACAC converter elements (i.e. firing angles of thyristors). At this point, electronic controller takes on the frequency synchronization duty between ge nerated signal and grid.
Figure 2.6 A typical wind turbine in detail (VESTAS V27 / 225 kW)
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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
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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.
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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
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situatio n, 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)
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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);
CL =
2⋅L ? ⋅ V2 ⋅ AL
(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);
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CD =
2⋅D ? ⋅ V2 ⋅ AD
(3.2)
where CD ρ
: Drag coefficient : 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;
1 W =1 j s
(3.3)
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The energy contained in the wind is its kinetic energy; E = 12 ⋅ m ⋅ V 2
(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.
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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 m= ?⋅A⋅V
(3.5)
where ρ : Air density (kg/m3 ) A : Rotor disk Area (m2 ) V : Wind velocity (m/s)
Consequently the kinetic energy formula becomes;
E = 12 ⋅ ? ⋅ A ⋅ V 3
(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;
P = 12 ⋅ ? ⋅ A ⋅ V 3
(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.
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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);
P=
? ⋅ C ⋅ η ⋅ A ⋅ V13 2 p
(3.8)
26
where
?
: Air density (kg/m3 )
Cp
:
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; P = 2 ⋅ ? ⋅ η ⋅ A ⋅ V13 ⋅ a ⋅ (1 − a 2 )
(3.9)
where "a" is the dimensionless axial interference factor.
Thus, by substitution, the power co-efficient Cp may be defined as; C p = 4 ⋅ a ⋅ (1 − a 2 )
(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.
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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
Rotatio nal Speed
nr
rpm
Angular Speed
?r
rad/s
1 rpm =
2⋅ π rad/s = 0.10472 rad/s 60
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;
U=
2⋅ π⋅ r ⋅ n r 60
(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.
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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;
λ=
Blade Tip Speed ωr ⋅ r U = = Wind Speed V V
(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);
tb =
2⋅π n ⋅ ωr
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
(3.13)
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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);
tw =
d V
(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;
n ⋅ ωr 2 ⋅ π ≈ V d
(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);
λ0 ≈
2⋅π r ⋅ n d
(3.16)
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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);
λ0 ≈
4⋅ π n
(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 Universityhttp://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 (C p ) 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
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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 s peed 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;
Pt arg et
? = ⋅ C p ⋅ η ⋅ A ⋅ C p ,t arget 2
r ⋅ λ t arg et
3
⋅ ω 3r
(3.18)
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where : Air density (kg/m3 )
? Cp
target
: 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
0,45 0,40 0,35
Cp
0,30 0,25 0,20 0,15 0,10 0,05 0,00 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
TSR
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
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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 sho uld 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)
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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);
EA =
Z⋅P ⋅ Φ ⋅ω 2⋅π⋅ a
(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;
EA = K ⋅ Φ ⋅ ω
where K is a constant representing the construction of the machine.
(3.20)
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The induced torque developed by the machine is given by; Τind = K ⋅ Φ ⋅ I A
(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 behavio ur 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);
VR =
Vnl − Vfl × 100% Vfl
(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 ma chine
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;
fe =
nm ⋅ p 120
(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;
E A = 2 ⋅ π ⋅ NC ⋅ Φ ⋅ f
(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;
EA = K ⋅ Φ ⋅ ω
(3.25)
where K is a constant representing the construction of the machine. If ? is expressed in radians per second, then
K=
NC ⋅ p 2
(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;
VΦ = E A − j ⋅ X ⋅ I A
(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;
VΦ = E A − j ⋅ X ⋅ IA − j ⋅ XA ⋅ I A − R A ⋅ I A
(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; XS = X + XA
(3.29)
43
Therefore, the final equation describing VF is; VΦ = EA − j ⋅ XS ⋅ I A − R A ⋅ IA
(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 shortcircuited 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 selfexcited 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 forceflux 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); E R = s ⋅ E R0
(3.31)
and the frequency of induced voltage at any slip will be given by Equation (3.32); fr = s ⋅ fe
(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
X R = ωr ⋅ LR = 2 ⋅ π ⋅ f r ⋅ LR
(3.33)
Substituting Equation (3.32) into Equation (3.33); X R = 2 ⋅ π ⋅ s ⋅ f e ⋅ LR X R = s ⋅ (2 ⋅ π ⋅ f e ⋅ L R ) X R = s ⋅ X R0
(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
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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 trans former, 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: ′ Vp = Vs = a ⋅ Vs ′ 1 I p = I s = ⋅ Is a ′ Zs = a 2 ⋅ Zs
(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; E1 = E′R = a eff ⋅ E R 0
(3.36)
and the rotor current becomes;
I2 =
and the rotor impedance becomes
IR a eff
(3.37)
51
R 2 Z2 = a eff ⋅ R + jX R 0 s
(3.38)
so R 2 = a 2eff ⋅ R R 2 X 2 = a eff ⋅ X R0
(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=
(n s − n r ) ns
where ns
: Electrical speed of the magnetic field (or stator speed) (rpm)
nr
: Rotor mechanical speed (rpm)
(3.40)
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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 ge nerator. 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)
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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 constantspeed 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 constantvoltage, 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 gridconnected turbines use induction AC ge nerators, 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 variablespeed 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.
n = s
120 ⋅ f p
(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 1000rpm 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 noncontrollable,
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)
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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 directcurrent systems.
Inverters convert direct-current into alternating or three-phase current. The energy flows into the alternating-current side.
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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.
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Figure 3.23 Basic wiring diagram for direct frequency converters
The conversion of grid frequency f 1 into machine frequency f 2 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)
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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. Noncontrollable 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)
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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 socalled 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
1000
3000
4500
28
300
4000
Symbol
Voltage (V) Current (A) Output (kVA) Turn-Off Time (µs) Frequency (kHz) Drive Requirement
1200
800
1700 (3300) 600 (1200)
480
360
14
450
4500
15 - 25
1–4
0.3 - 0.5
5 – 10
10 - 25
0.5 – 5
2 – 20
5 – 100
1–3
0.2 – 1
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.
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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. Selfcommutated 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)
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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 na me.
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)
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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.
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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”
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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.
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• 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.
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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 extractio n. • 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 behavio ur 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 efficienc y 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
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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.
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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)
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Table 4.1 Descriptions of Operational Regions for a Typical Wind Turbine Operating
Operational Description:
Region
Power Output vs. Wind Speed
Region - I -
Region - II -
Region - III -
Wind Speed Range
Wind speeds too low to produce
0 to cut- in wind speed;
usable electric power.
0 to 4 m/s.
Production of electric power
Cut- in to rated wind speed;
increasing with wind speed.
4 to 13 m/s.
Production of electric power at
Rated wind speed to cut-
constant, rated power level. Wind
out wind speed;
turbine blades purposely made less
13 m/s to 25 m/s.
efficient as wind speed increases.
Region - IV -
No electric power output. Winds
Cut-out wind speed to
too energetic to justify added
survival wind speed; 25
strength and cost for the small
m/s to rated survival wind
number of hours per year beyond
speed.
cut-out 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;
A = π ⋅ r2 where r is the rotor radius.
(4.1)
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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.
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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)
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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 cur ve 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 lo wer turbine masses.
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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
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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
Difficult in high wind speeds
Fixed Speed Control With Variable Speed
Safety
Generally satisfactory, although design uncertain
Better power quality, lower drive train loads
Requires proving
than any stall option 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
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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.
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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 offshore 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 offshore 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.
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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)
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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:
Pcap = 0.5 ⋅ ρ ⋅ η ⋅ C p ⋅ A ⋅ V 3
(5.1)
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π ⋅ (λ − 3) C p = (0.44 − 0.0167 ⋅ α) ⋅ Sin − 0.00184 ⋅ (λ − 3) ⋅ α 15 − (0.3 ⋅ α )
λ=
r ⋅ ωr V
Pcap(t +1) = Pcap(t ) + ωr (t ) ⋅ J ⋅
(5.3)
dωr ( t ) dt
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 )
(5.2)
(5.4)
Figure 5.1 Overview of the wind turbine simulation
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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.
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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.
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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 equa l to 0.05 second.
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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.
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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;
τ ( t +1) = τ( t ) + J ⋅
dω r (t ) dt
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;
(5.5)
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Pcap( t +1) = Pcap(t ) + ω r( t ) ⋅ J ⋅
dωr ( t )
(5.6)
dt
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);
dωr ( t ) dt
= ∆ωr =
Pcap( t +1) − Pcap( t ) ω r( t ) ⋅ J
⇒ ∆ωr =
∆Pcap ω r (t ) ⋅ J
(5.7)
Consequently, this speed difference (indicating acceleration, deceleration or constant speed operation) is added to the speed value at time (t);
ωr ( t +1) = ωr ( t ) + ∆ω r
(5.8)
The resultant angular speed can be used to find tip speed ratio (?), power coefficient (C p ) and the power input to the generator (Pcap), respectively.
Figure 5.7 Angular speed calculation block
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5.1.5 Cp – ? SELECTION BLOCK
After the system decides pitch angle in degrees, power coefficient (C p ) 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 (C p ). 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.
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Figure 5.8 Wind speed values filtered by yaw control block
Figure 5.9 Aerodynamic power in the wind
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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)
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Figure 5.12 Angular shaft speed of the turbine
Figure 5.13 Rotational speed of turbine shaft before gearbox
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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)
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Figure 5.18 Tip speed ratio vs. power coefficient
Figure 5.19 Turbine wind speed – power characteristics
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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 (P w) is still increasing.
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Table 5.1 Modelled Wind Turbine Simulation Results V
Pw
(m/s)
(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
?
a (degrees)
Cp
Pcap (kW)
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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
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• 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.
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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.
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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.
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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/
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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.
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APPENDICES
A
Get wind data (V) Rotor radius (r) Gear ratio
V=0
4.5 < V < 20 m/s No
Yes
Calculate turbine efficiency (?) (Look-up table)
Calculate aerodynamic wind power ( Pw = 0.5 ⋅ ρ ⋅ A ⋅ V3 )
Mechanical power ( Pm = Pw ⋅ Cp )
Calculate captured power (Generator input power) ( Pcap = Pm ⋅ η )
Calculate pitch angle (a)
Calculate angular speed (? r) Tip speed ratio (?) Calculate power coefficient (C p)
- FLOWCHART OF THE SIMULATED SYSTEM -
B
C
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