Wind Energy Systems for Electric Power Generation Green Energy and Technology

Wind Energy Systems for Electric Power Generation Green Energy and Technology

(Parte 1 de 5)

Green Energy and Technology Green Energy and Technology

Manfred Stiebler

Wind Energy Systems for Electric Power Generation


Prof. Dr. Manfred Stiebler Technical University of Berlin Faculty of Electrical Engineering and Computer Science Inst. of Energy and Automation Technology Einsteinufer 1, D-10587 Berlin Germany manfred.stiebler@iee.tu

Springer Series in Green Energy and Technology ISSN 1865-3529 Library of Congress Control Number: 2008929626 c© 2008 Springer-Verlag Berlin Heidelberg

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law.

The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

Cover design: WMX Design, Heidelberg Printed on acid-free paper



Among renewable sources wind power systems have developed to prominent suppliers of electrical energy. Since the 1980s they have seen an exponential increase, both in unit power ratings and overall capacity. While most of the systems are found on dry land, preferably in coastal regions, off-shore wind parks are expected to add significantly to wind energy conversion in the future.

The theory of modern wind turbines has not been established before the 20th century. Currently wind turbines with three blades and horizontal shaft prevail. The driven electric generators are of the asynchronous or synchronous type, with or without interposed gearbox. Modern systems are designed for variable speed operation which make power electronic devices play an important part in wind energy conversion. Manufacturing has reached the state of a high-tech industry.

Countries prominent for the amount of installed wind turbine systems feeding into the grid are in Europe Denmark, Germany and Spain. Outside Europe it is the United States of America and India who stand out with large rates of increase. The market and the degree of contribution to the energy consumption in a country has been strongly influenced by National support schemes, such as guaranteed feed-in tariffs or tax credits.

Due to the personal background of the author, the view is mainly directed on

Europe, and many examples are taken from the German scene. However, the situation in other continents, especially North America and Asia is also considered.

This book was written from the standpoint of electrical engineering. It is meant to provide basic knowledge on wind energy systems for graduate students of technical disciplines, and for engineers who seek overview information apart from their own special field. The intention is to convey the properties and performance of wind rotors and of the electrical components both to the electrical and the mechanical engineer.

The author wishes to extend special thanks to his colleagues Prof. Dr. R. Gasch and Prof. Dr. J. Twele to whom he owes valuable communication during a yearlong cooperation at TU Berlin in common projects pertaining to wind energy. Also, thanks are due to Springer-Verlag for their care during the book production.

Berlin, Germany M. Stiebler

1 Role of Wind as a Renewable Energy1
1.1 Renewable Energies and Their Application1
1.1.1 Sources of Renewable Energy1
1.1.2 Sources of Electrical Energy Production2
1.1.3 Wind and Solar Energy3
1.2 Wind Energy Contribution to Electrical Supply3
1.2.1 Installed Power3
1.2.2 Technical Standardization and Local issues7
1.2.3 Governmental Regulations8
2 Wind Turbines1
2.1 General1
2.2 Basics of Wind Energy Conversion1
2.2.1 Power Conversion and Power Coefficient1
2.2.2 Forces and Torque14
2.3 Wind Regime and Utilization17
2.3.1 Wind Velocity Distribution17
2.3.2 Power Distribution and Energy18
2.3.3 Power and Torque Characteristics19
2.4 Power Characteristics and Energy Yield20
2.4.1 Control and Power Limitation20
2.4.2 Wind Classes2
2.4.3 System Power Characteristics23
2.4.4 Annual Energy Yield26
3 Generators29
3.1 General29
3.2 Asynchronous Machines30
3.2.1 Principles of Operation30
3.2.2 Performance Equations and Equivalent Circuits31

Contents vii

3.2.3 Reactive Power Compensation40
3.2.4 Self-Excited Operation41
3.3 Synchronous Machines43
3.3.1 Principles of Operation43
3.3.2 Performance Equations and Equivalent Circuits4
3.3.3 Unconventional Machine Types48
3.4 Generator Comparison52
4 Electrical Equipment5
4.1 General5
4.2 Conventional Electrical Equipment5
4.3 Power Electronic Converters5
4.3.1 General5
4.3.2 External-Commutated Inverters57
4.3.3 Self-Commutated Inverters61
4.3.4 Converters with Intermediate Circuits67
4.3.5 D.c./d.c. Choppers67
4.3.6 A.c. Power Controllers70
4.4 Energy Storage Devices71
4.4.1 General71
4.4.2 Electrochemical Energy Storage71
4.4.3 Electrical Energy Storage75
4.4.4 Mechanical Energy Storage76
5 Wind Energy Systems81
5.1 General81
5.2 Systems Overview81
5.2.1 General81
5.2.2 Systems Feeding into the Grid82
5.2.3 Systems for Island Supply83
5.3 Systems for Feeding into the Grid84
5.3.1 General84
5.3.2 Induction Generators for Direct Grid Coupling85
5.3.3 Asynchronous Generators in Static Cascades86
5.3.4 Synchronous Generators95
5.3.5 Examples of Commercial Systems103
5.4 Systems for Island Operation103
5.4.1 Systems in Combined Generation103
5.4.2 Stand-Alone Systems106
6 Performance and Operation Management115
6.1 General115
6.2 System Component Models115

viii Contents 5.2.4 Wind Pumping Systems with Electrical Power Transmission 84 6.2.1 Model Representation . ............................... 115

6.2.2 Asynchronous Machine Models120
6.2.3 Synchronous Machine Models127
6.2.4 Converter Modeling133
6.2.5 Modeling the Drive Train135
6.3 System Control138
6.3.1 General138
6.3.2 Control of Systems Feeding into the Grid139
6.4 Basics of Operation Management143
6.4.1 General143
6.4.2 States of Operation143
6.4.3 Grid Fault Reaction144
7 Grid Integration and Power Quality147
7.1 Basics of Grid Connection147
7.1.1 General147
7.1.2 Permissible Power Ratings for Grid Connection147
7.1.3 Power Variation and Grid Reaction150
7.2 Standard Requirements152
7.2.1 Safety-Relevant Set Values152
7.2.2 Reactive Power Compensation152
7.2.3 Lightning Protection152
7.3 System Operator Regulations154
7.3.1 General154
7.3.2 Active Power and Frequency155
7.3.3 Reactive Power and Voltage156
7.3.4 Short-Circuit and Fault Ride-Through158
7.4 Power Quality158
7.4.1 Harmonics158
7.4.2 Voltage Deviations and Flicker159
7.4.3 Audio Frequency Transmission Compatibility165
7.5 Noise Emission166
7.5.1 General166
7.5.2 Sound Emission by WES167
8 Future of Wind Energy171
8.1 Off-Shore Wind Energy Generation171
8.1.1 General171
8.1.2 Foundation171
8.1.3 Connection172
8.1.4 Specific Issues and Concerns174
8.2 Power Integration and Outlook177
8.2.1 Wind Energy in Power Generation Mix177
8.2.2 Integration in Supranational Grids177
Annex A – List of Symbols179
Annex B – List of Abbreviations183

List of Figures

source, in TWh, (b) visualization of components2
(a) installed capacity, in MW; (b) electric output; in GWh4
1.3 Installed capacity as total and new in 2006 in top 10 countries5
1.4 Concentration of installed wind power in the EU6
1.5 Average installed power per unit in Germany, in kW6
1.6 Schemes of renewable energy support in the EU 259
2.1 Idealized fluid model for a wind rotor (Betz)12
2.3 Typical torque coefficients of different rotor with hotizontal shaft14

1.1 World net generation of electricity (2004) (a) per world part and 1.2 Wind energy installed power ratings and annual output (2002). 2.2 Typical power coefficients of different rotor types over tip-speed ratio 13

specific profiles15
2.5 Wind speeds and forces acting on the blade15

2.4 Coefficients cA(α) of lift and cW(α) of drag over blade angle of

three-blade rotor16
2.7 Curve of drag (drag) coefficient cS(λ)16

2.6 Curves of power coefficient cp(λ) and torque coeffient cT(λ) of a 2.8 Representation of wind velocity distribution. (a) example

function for k = 2)18

histogram; (b) approximation by Weibull-functions (Raileigh- 2.9 Histograms of wind velocity distribution and normalized energy yield 18

2.10 Power and torque characteristics vs. rotational speed (vN = 12m/s) .1 9 2.1 Power, torque and drag coefficients over tip speed ratio with pitch

angle as parameter21
2.12 Sketch of a blade with laminar and turbulent air flow21
2.13 Illustration of stall, active-stall and pitch effects2
2.15 Sketch of a the measuring setup24
2.16 Power curve of a system specified for 1800 kW24

2.14 Typical power curves for pitch-controlled and stall-controlled systems 23 2.17 Rotor diameters and rotation speeds of systems of 850kW and above 25 xii List of Figures

systems 2000 kW25
wind speed26

2.18 Specific power of systems > 850kW and reference energy yield of 2.19 Full-load equivalent annual energy yield over ratio rated/average 2.20 Energy yield kWh/m2 per month as recorded in Germany and Austria 26

3.1 Diagram of three-phase induction machine with wound rotor30
3.2 Asynchronous machine T-model circuit31
3.3 T-model for asynchronous machine connected to the grid32

3.4 T-model variants containing loss resistors

(b) resistor Rp representing constant losses3

(a) conventional iron loss resistor RFe; 3.5 Principal characteristics of current and torque at constant flux linkage 34

diagramme); (b) vector diagramme (Generator operation)35
(b) Current and torque vs. speed and slip36
(b) generator operation37
3.9 Flux and inductance under main field saturation39
3.1 Induction machine capacitive self-excitation41
diagram; (b) Equivalent circuit with ohmic load only42
at C = const, speed n as parameter43

3.6 Performance in steady-state (a) current locus (Ossanna’s circle 3.7 Operation at rated voltage (a) Performance characteristics; 3.8 Sankey diagrams of induction machines (a) motor operation; 3.10 Compensation device concepts (a) basic circuits; (b) admittance 3.12 Self-excited induction generator with passive load (a) Circuit 3.13 Load characteristics of a laboratory setup with SEIG and ohmic load (a) curves at n = const, capacitance C as parameter; (b) curves 3.14 Diagram of three-phase synchronous machine with separate excitation 43 3.15 Equivalent circuits of the turbo-type synchronous machine (a) with

voltage source Up;( b) with current source If4

3.16 Steady-state characteristics of a synchronous machine in

grid-operation (a) Current locus diagram; (b) vector diagram45

3.17 Torque characteristic in grid operation. (a) Turbo-type machine;

(b) Salient polemachine, Xq < Xd46

3.18 Operation with passive R, L load (a) Equivalent circuit for

turbo-type machine; (b) load curves vs. normalized frequency47
3.19 Output power characteristic for ohmic load47
3.20 Principal types of axial field machines49
3.21 Flux path sketch in axial field machines49
3.2 Small axial generator with air-gap ring winding50
3.23 Sketch of modular axial field machine50

3.24 Principle of transversal flux machines (a) single-sided component of polyphase machine; (b) Double-sided with intermediate rotor . . . . 51

4.1 Power electronics applications56
4.2 Six-pulse bridge (B6) thyristor rectifier circuit57
controlled (0 < α < π/2), example59
inverter, α = 142◦59

List of Figures xiii 4.3 Simplified waveforms of B6 circuit under load (a) Rectifier operation, uncontrolled (α = 0) or diode; (b) Rectifier operation, 4.4 Waveforms of M3 circuit under load, (a) rectifier, α = 2◦,( b)a .c 4.5 Circle diagram showing the relation between d.c voltage and

overlap angles)60
(b) voltage and currents (example)61
4.7 Full-wave bridge voltage-source inverter61
4.8 Voltage waveforms in six-step operation62
4.9 PWM bridge (B6) inverter circuit63
4.10 Voltage waveforms in PWM operation with sinusoidal modulation63
4.1 Model of generator, coupling inductor and inverter64
4.12 Phasor diagram of a.c. fundamentals in different cases of operation65
inverter (VSI); (b) current-source inverter (CSI)67
(buck) converter; (b) Step-up (boost) converter68

control-reactive power (u0 values indicate initial commutation 4.6 Reactive power inverter with inductive storage element (a) circuit; 4.13 Power locus diagram in consumer (motor) coordinates, for lossless 4.14 Reactive power inverter with capacitive storage element, in square-wave operation (a) circuit; (b) voltage and currents (example) 6 4.15 Converter schemes with intermediate circuits. (a) voltage-source 4.16 D.c./d.c. chopper with inductor storage element. (a) Step-down 4.17 Step-up converter, continuous and discontinuous conduction. (a) bondary case of cont./discont. conduction; (b) boundary curves;

Iref = UoT/L69
4.18 Step-up converter, control characteristics69
load; (b) Control characteristic of r.m.s. current70
4.20 Ragone diagram of energy storage devices72
4.21 Cell voltage U over degree of charge p72
(b) Battery voltage during discharging73
4.23 Charging method using I/U control (example)73
4.24 Battery equivalent circuit74
4.25 Concept of a CAES plant (Huntorf/Germany)78

4.19 Three-phase power controlers (a) Circuit with ohmic-onductive 4.2 Characteristic of a conventional battery (a) Available capacity;


5.1 Typical concepts for generating electrical power. – using induction generator; (a) direct coupling, (b) fully-fed, (c) doubly fed, – using synchronous generator, fully fed; (d) electrical excitation, (e)P M 5.2 Common concepts of systems feeding into the grid (Legend see text) 83

5.3 Shares of type groups installed in 2004 in Germany85
fixed blades86
5.5 Power curve of a stall turbine system87
5.6 Principle diagram of a constant speed system8
5.8 L-model for asynchronous wound-rotor machines89
5.9 Static Kramer system, basic circuit diagram90

xiv List of Figures 5.4 Normalized power and torque characteristics of a wind turbine with 5.7 Operation and control of a wind energy system. Sketch of data acquisition (left) and block diagram of operation management (right)8 9 5.10 Steady state operation of the wound rotor induction machine

example); (b) phasor diagram for operation as a generator91
(s < 0)92

(a) complex current locus for constant rotor voltage (s0 = ±0,1a s 5.1 Visualization of Kramer cascade operation (a) Diagram for slip power recovery (losses neglected); (b) Sankey diagrams; left: motor operation (s > 0); right: generator operation 5.12 Torque and power characteristics of the cascade system (a) Torque

characteristics; (b) Power characteristics, example for k2 = 0,193
5.13 Doubly-fed induction generator with rotor-side converter93
(a) current loci; (b) variable speed operation (legend see text)94
5.15 Synchronous generator with full-load converter (legend see text)95
5.16 Synchronous generator with slipringless excitation96
5.17 Demagnetization curves of different magnetic materials98
5.18 Demagnetization curves of aNdFeB magnets material [VAC]9
5.19 PMSM characteristics (a) Magnetization; (b) field line pattern9
5.20 Concept of an island grid with combined generation and storage104
5.21 Variants of frequency control105
5.23 Characteristics of a battery loader (example)107
5.24 Sketch of furling action108
small WES108
5.26 Typical inverter circuits for small ratings109
5.27 Circuit of a system with step-up inverter110
(example) (a–b) voltages, currents; (c) duty ratio; (d) input power110
5.30 Self-excited induction generator with phase control112

5.14 Steady-state performance of a doubly-fed induction machine 5.2 Concept of an autonomous island system with renewable energy 5.25 Turbine diameters and maximum speed of commercially available 5.28 Performance of a system with step-up inverter and battery storage 5.29 Stand-alone system with synchronous generator and battery storage . 1 5.31 Stand-alone system with cage induction machine and battery storage 112

windings in stator and rotor; (b) different frames120
6.2 Induction machine model in Clarke components121

List of Figures xv

(b) block diagram125
(b) Transformed windings arrangement128
arrangement in d,q components; (b) equivalent circuit model132
6.7 Four-pole equivalent circuit model in hybrid form134
6.8 Model of intermediate circuit voltage source converter135
B gear box; G generator136

6.4 Rotor model of induction machine (a) Space-vector diagram; 6.5 Synchronous machine model (a) Salient pole three-phase machine; 6.6 Model of the synchronous machine with five windings (a) windings 6.9 Three-mass drive train model of a wind system T wind turbine; 6.10 Resonance curves of displacement angle Φ (left) and shaft torque

6.1 Analogy of PV- and wind energy systems139
6.12 Control scheme of a system for constant speed operation140
6.13 Control scheme of a system for variable speed operation141
6.14 Control scheme of a WES with doubly-fed induction generator141
6.15 Waveforms of the WES quantities during an example wind regime142
voltage variation149
7.2 Example of measured power variations150
with interharmonics; (8) voltage with notches151
7.4 Reactive power distribution example of a 50MW wind park153
7.5 Lightning protection concept153
7.6 Quadrant definition in consumer system155
7.8 Reactive power supply requirements157
7.9 Power factor assigned to grid voltage area157
7.1 Limiting curve of voltage variations per minute160
7.12 Block diagram of a flickermeter161
7.13 Typical signal waveforms in a flickermeter161

7.1 Model of short-circuit impedance between generator and grid and 7.3 Periodic and non-periodic voltage distortions. (1) oscillatory transient; (2) voltage sag; (3) voltage swell; (4) momentary interruption; (5) voltage flicker; (6) harmonic distortion; (7) voltage 7.7 Required active power capability of WES supplying a high-voltage 7.10 Limiting voltage/time area excluding automatic tripping of wind 7.14 Example from an arc furnace of a cumulative flicker power curve

to determine Plt163
7.15 Flicker-relevant voltage drop on a short-circuit impedance165
7.16 Model of capacitive compensator with series inductor166
7.17 Relative impedance of series resonant circuit167
7.18 Measured sound vs. wind velocity168
7.19 Sound power levels of wind energy systems169

7.20 Curves of equal sound pressure level in the vicinity of a WES . . . . . . 169

8.1 Foundation structures172
8.2 Cable Π-model and limiting length173
8.3 Principal comparison of HVAC and HVDC connection cost174
(a) AC connection; (b) DC connection175
1.1 Sources of renewable energy2
1.2 Installed wind capacity in Europe, in MW (2006)5
2.1 IEC type classes2
4.1 Properties of accumulators75
4.2 Main properties of CAES plant Huntorf78
5.1 Properties of selected wind energy systems100
5.1 (continued)101
5.1 (continued)102
5.2 Comparison of inverter circuits in Fig. 5.26109
6.1 Transformation matrices in the power-variant form I119
6.2 Transformation matrices in the power variant form I119
6.3 Properties of preferred transformations124
7.1 Permissible harmonic currents at connecting point159
from tests164
7.3 Noise limits established by TA Laerm169
8.1 Basic comparsion of AC and DC connections176
8.2 HVDC design features as compared with the classical concept176

List of Tables 7.2 Characteristics of selected systems (see Table 5.1) as determined xvii

Chapter 1 Role of Wind as a Renewable Energy

1.1 Renewable Energies and Their Application

1.1.1 Sources of Renewable Energy

It is commonly accepted that the earth’s fossil energy resources are limited, and the global oil, gas and coal production will come beyond their peak in the next decades, and price rises will continue. At the same time there is strong political opposition against strengthening nuclear power in many parts of the world. In this scenario renewable energies will have to contribute more and more to the world’s ever rising need of energy in the future [Bul01]. Renewables are climate-friendly forms of energy, due to the absence of emissions detrimental to the environment. The savings especially in carbon-dioxide and sulphur dioxide emissions are a significant advantage over fossil power stations. Hence a main role is assigned to renewable energy in the proclaimed fight against Climate Change.

The major source of renewable energies is the sun, with some forms also attributed to the earth and the moon. Table 1.1 lists the primary sources, the natural ways of conversion and the technically used conversion methods. Notable for their contribution to the current energy demand are water, wind, solar energy and biomass. Utilization of renewables is mostly with conversion into electrical energy.

While water power has been used in electrical power stations and pumped storage systems since many decades, the use of wind power conversion in larger ratings has begun only in the 1980s. Backed by intense technical development, unit ratings have grown fast into the MW range, and wind parks were erected in large numbers with considerable increase rates.

Solar energy is applied both by direct conversion in photovoltaic generators, and via thermal collectors and steam production. The energy from renewable sources is partly already competitive in price, and partly supported by state legislative to promote their share in the market. Wind energy systems are about to reach the competitiveness before long, while photovoltaic energy production is still expensive and will require further support on their way to market relevance.

M. Stiebler, Wind Energy Systems for Electric Power Generation. Green Energy 1 and Technology, c© Springer-Verlag Berlin Heidelberg 2008

2 1 Role of Wind as a Renewable Energy

Table 1.1 Sources of renewable energy Primary source Medium Natural conversion Technical conversion

Sun Water Evaporation, precipitation, melting Water power plants

Wind Atmospheric airflow Wind energy conversion

Wave movement Wave power plant

Solar energy Ocean current Ocean power plant

Heating earth surface and atmosphere

Thermal power units, heat pumps

Solar radiation Heliothermal conversion,

Photovoltaic conversion

Biomass Biomass production Co-generation plants

Earth Isotop decay Geothermal heat Co-generation plants Moon Gravitation Tides Tide power plants

1.1.2 Sources of Electrical Energy Production

Looking at the sources of energy currently applied world-wide for generating electricity, it can be seen that fossil fuels in form of coal, oil and natural gas prevail (65%). Nuclear energy (16%) and hydro energy (17%) follow with almost same percentage. In the representation of Fig. 1.1 the part indicated as Renewables (2%)

Fig. 1.1 World net generation of electricity (2004) (a) per world part and source, in TWh, (b) visualization of components

1.2 Wind Energy Contribution to Electrical Supply 3 covers mainly wind, biomass and solar energy, but is shown separately from hydro energy, while it is understood that the latter is the energy form contributing the best part among to the useful renewables [WEC04].

1.1.3 Wind and Solar Energy

It may be worth noting that the per-area power densities offered by kinetic wind energy and solar radiation are in the same order of magnitude, when considering exploitable values in specific regions. As an example, the wind exerts at 20m/s on a vertical plane 1,04kW/m2, while the solar radiant flux density on a horizontal

conversion is 4050% for wind systems, and 12 ... 18% for photovoltaic genera-

plane is e.g. (on 21st June at noon, latitude 50 north) 1,05kW/m2. Both forms of renewable energy are characterized by non-steady regime. The efficiency of power tion with current commercially available silicon cells.

1000kWh/(m2 a) for European in-land wind parks (at 5,5m/s average wind

The order of the annual reference energy yield per swept area is roughly 800 velocity, 1700 h annual full-load hours and 0,45 best point efficiency). For pho-

tovoltaic systems 700900kWh/kWp are relevant values, e.g. for Northern

Germany. Assuming a specific PV-generator area of 5m2/kWp, this corresponds to 160kWh/(m2 a) yield per PV converter area. In these values the system losses up to the generator or converter terminal output, respectively, are taken into account. Admittedly the reported magnitudes are relatively low when compared with coal, oil and gas fueled power stations, while obviously a fair per area comparison cannot be made.

1.2 Wind Energy Contribution to Electrical Supply

1.2.1 Installed Power

By the end of 2002 the rated installed power in wind farms was roughly 32GW worldwide. Statistical values of regional distribution are given in Fig. 1.2 [eia]. It is obvious that Europe has been the leader in wind power utilization, contributing 76% of the total power. North America which follows by 16% has since increased its percentage considerably.

As by 2006 roughly 65GW of rated power were installed in wind farms worldwide, of which more than 47GW located in the countries of the European Community, and more than 11GW in the United States. The Wind capacity installed in 2007 was almost as high as 20GW [GWEC, 08 update], so that for the the end of 2007 a cumulative 94GW were reported, where the EU stands for 56,5GW and the USA contribute 16,8GW. The exorbitant progress since the 1980s was accompanied by a significant decrease in cost per kWh, due to technical development

(Parte 1 de 5)