PERFORMANCE ANALYSIS OF WIND ENEGY
CONVERSION SYSTEM
A Thesis submitted in partial fulfilment of
the requirement for the award of the degree of
MASTER OF TECHNOLOGY
IN
ELECTRICAL ENGINEERING
(Specialisation: Electrical Systems)
Submitted by
NARAYANAREDDY BOMMAREDDY
(ROLL NO: 09/EE/402)
Under the guidance of
Dr. T.K.SAHA
DEPARTMENT OF ELECTRICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPUR-713 209
INDIA
JULY, 2011.
i
ACKNOWLEDGEMENT
First, I would like to thank my thesis supervisor, Dr.T.K.Saha, for providing me with the
right balance of guidance and independence in my research. I am greatly indebted to him for
his full support, constant encouragement and advice both in technical and non-technical
matters. His broad expertise and superb intuition have been a source of inspiration to me over
the past two years. His comments and criticisms have greatly influenced my technical
writing, and are reflected throughout the presentation of this dissertation.
I also wish to express my sincere and respectable thanks to our Head of the Department and
all faculty members of Electrical Engineering Department for consecutive suggestions and
valuable instruction for the execution of this project work.
I gratefully acknowledge to My classmates, Juniors and friends for their support, friendship,
help, and cheerfulness. I would also like to thank my good friends in other departments. In
addition, I gratefully acknowledge the financial support of our Electrical Engineering
department.
Above all, I am extremely grateful to my parents and other family members for their
unfailing support to me throughout my career. I owe everything I have achieved until now, to
my family.
NARAYANAREDDY BOMMAREDDY
09/EE/402
ii
NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPUR
DECLARATION
I hereby declare that this submission is my own work and that, to the best of my
knowledge and belief, it contains no material previously published or written by another
person nor material which has been accepted for the award of any other degree or diploma
of the university or other institute of higher learning, except where due acknowledgement has
been made in the text.
Place: N.I.T. Durgapur Signature :
Date: Name : NARAYANAREDDY BOMMAREDDY
Roll No. : 09/EE/402
iii
NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPUR
CERTIFICATE
This is to certify that NARAYANAREDDY BOMMAREDDY (Roll Number
09/EE/402), undergoing Master of Technology in Electrical Systems of Electrical
Engineering, has carried out the dissertation entitled “Performance Analysis of Wind
Energy Conversion System” and prepared the report under my guidance and supervision.
The dissertation is submitted as a partial fulfillment of the requirement for the award
of Master of Technology in Electrical Engineering with specialization in Electrical Systems
from National Institute of Technology, Durgapur.
To the best of my knowledge, the materials in this report have not been submitted
earlier as a part of any other academic programme.
Dr. T.K.Saha
Asst. Professor
Department of Electrical Engineering
National Institute of Technology, Durgapur
Dr. N.K.Roy
Professor and Head of Department
Department of Electrical Engineering
National Institute of Technology, Durgapur
iv
NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPUR
CERTIFICATE OF APPROVAL
The foregoing thesis entitled “Performance Analysis of Wind Energy Conversion
Sysytem” is hereby approved as a creditable study of an Engineering project carried out and
presented in a manner satisfactory to warrant its acceptance as prerequisite to the degree for
which it has been submitted. It is understood that by this approval, the undersigned do not
necessarily endorse any conclusion or opinion therein, but approved the thesis for the
purpose for which it is submitted.
……………………………………
Examiner
…………………………………...
Examiner
……………………………………
Examiner
v
ABSTRACT
In the past few decades wind power has become one of the most attractive solutions for clean
and renewable energy. Recent years have seen a huge application and improvement of wind
energy systems particularly with the improvement in power semiconductor technology. Squirrel
cage and wound rotor induction machines as well as synchronous machines have found
increasing application in the wind energy business which uses the wind turbines, specifically fast
wind turbines as their prime movers. The advancement in the embedded system technology has
extended a scope for improvement in the drives associated with the control of these Machines
particularly for the application on high ended algorithms in the control Aspects. Verification of
these algorithms necessitates the modeling of wind turbine in software due to huge size and wind
deficiency in the laboratories. Also real time verification of software results requires a turbine
model in real time capable of driving an asynchronous or a synchronous machine to generate
power without actually constructing one. Works available in literature show such attempts but all
of them lack proper modeling of the turbine drive train system.
The work reported in this thesis, therefore puts a effort towards the software modeling of a
pitch controlled horizontal axis wind turbine and later the turbine model is coupled to an existing
stand alone double output induction generator model in software. The thesis ends with hardware
description in which experimental setup is designed for the real time implementation of an open
loop control scheme for a chopper driven DC machine through DSP controller. This thesis makes
a way to real time emulation of wind turbine, by implementing the proposed model by
incorporating the chopper controlled dc machine at laboratory level which is the future goal of
this thesis work.
vi
List of Figures
1.1 Predicted fuel energy consumption as percentage of total in the year 2010………….. 2
1.2 Change in average cost of wind generated electricity………………………………….2
1.3 Structure of a typical wind energy system……………………………………………. 4
2.1 Lift and Drag forces of an aerofoil…………………………………………………….10
2.2 Tip speed ratio vs. power coefficient………………………………………………… 12
2.3 Tip speed ratio vs. torque coefficient………………………………………………….12
2.4 Spring-mass-damper model of the wind turbine……………………………………….13
2.5 Wind turbine connected to a grid connected squirrel cage induction generator……….13
2.6 Ideal wind machine block diagram…………………………………………………… 14
2.7 Simulation model block diagram………………………………………………...……...19
2.8 Actuator system block diagram………………………………………..………………..21
3.1 A grid connected DOIG……………………………………………………………… 24
3.2 DOIG in standalone mode…………………………………………………………….. 25
3.3 Block diagram for the simulation model of the DOIG………………………………... 27
3.4 The vector rotator block………………………………………………………………..28
3.5 The back to back connected PWM converters with the DC link capacitor…………… 29
3.6 Block diagram of for the rotor voltage derivation…………………………………… 32
3.7 Block diagram of the DC link voltage controller………………………………………33
3.8 Time vs. wind-gust speed………………………………………………………………36
3.9 Time vs. turbine speed………………………………………………………………….36
3.10 Time vs. generator speed……………………………………………………………...36
3.11 Time vs. turbine torque………………………………………………………………..36
3.12 Time vs. mechanical torque…………………………………………………………...37
3.13 Time vs. pitch angle…………………………………………………………………...37
3.14 Time vs. load voltage…...........………………………………………………………..37
3.15 Time vs. dc link voltage……………………………………………………………….37
3.16 Time vs. power………………………………………………………………………...38
3.17 Time vs. generator speed…………………………………………………………….. .39
vii
3.18 Time vs. mechanical torque…………………………………………………………. 39
3.19 Time vs. pitch angle…………………………………………………………………. 39
3.20 Time vs. output power………………………………………...................................... 39
3.21 Time vs. turbine speed………………………………………………………………. 40
3.22 Time vs. turbine torque………………………………………………………………. 40
3.23 Time vs. dc link voltage……………………………………………………………... 40
3.24 Time vs. load voltage……………………………………………………………… 40
3.25 Time vs. input wind velocity…………………………………………………………. 41
3.26 Time vs. turbine speed……………………………………………………………….. 41
3.27 Time vs. generator speed……………………………………………………………..42
3.28 Time vs. mechanical torque…………………………………………………………. 42
3.29 Time vs. turbine torque……………………………………………………………… 42
3.30 Time vs. pitch angle…………………………………………………………………. 42
3.31 Time vs. dc link voltage…………………………………………………………….. 43
3.32 Time vs. load voltage ………………………………………………………………43
3.33 Time vs. output power………………………………………………………………. 43
4.1 Block diagram of hardware………………………………………………………….. 45
4.2 Hardware setup……………………………………………………………………….. 46
4.3 Four quadrant chopper…………………………………………………………………47
4.4 Operating region of the four quadrant DC chopper……………………………………47
4.5 Power supply part of the interfacing gate signal……………………………………….51
4.6 (a) Interfacing part for the gate signal………………………………………………….51
4.6 (b) Interfacing part for the gate signal………………………………………………… 52
4.7 Gate Signal & Supply Voltage vs. Time (secs.)……………………………………… 53
4.8 Gate Signal & Input signal vs. Time (secs.)…………………………………………. 53
4.9 Gate Signal & Input signal vs. Time (secs.)……………………………………………54
4.10 Gate Signal & Input signal vs. Time (secs.)………………………………………… 54
4.11 Model of a separately excited DC motor……………………………………………. 55
4.12 Internal architecture and functional units of the DS1104 DSP board………………..57
4.13 Armature Voltage (V) vs. Time (secs.)……………………………………………… 60
4.14 Complementary Armature Voltage (V) vs. Time (secs.)……………………………. 60
viii
4.15 Armature Current (A) & Gate Signal vs. Time (secs.)……………………………….60
4.16 Field Voltage (V), Armature Voltage (V) & Gate Signal vs. Time (secs.)…………..61
4.17 Armature Current (A), Armature Voltage (V) & Gate Signal vs. Time (secs.)……... 61
4.18 Rotor Voltage vs Time (secs.)………………………………………………………. 61
4.19 Rotor Voltage vs Time (secs.)………………………………………………………………. 61
B.1 Diagram of The IGBT Modules Used……………………………………………….. 72
ix
List of Tables
1.1 Top 10 Wind Power Countries (February 2011)………………………………………... 3
2.1 Parameter Values Used In the Turbine Model………………………..…………………20
3.1 Machine Parameters Used In the Simulation……………………………………...…….34
x
CONTENTS
Acknowledgement i
Declaration ii
Certificate iii
Certificate of Approval iv
Abstract v
List of Figures vi
List of tables ix
Index x
CHAPTER 1
INTRODUCTION
1.1 Structure of Wind Energy Conversion Systems......................................................... 3
1.2 Motivation………………………………………………………………………….. 5
1.3 Literature Survey………………………………………………………………….... 5
1.4 Scope of the Work………………………………………………………………….. 7
1.5 Contribution……………………………………………………………………….....8
1.6 Thesis Layout………………………………………………………………………..8
CHAPTER 2
Modelling and Simulation of a Horizontal Axis Wind Turbine
2.1 Horizontal Axis Wind Turbine Structure…………………………………………… 9
2.1.1 Lift and Drag of Aerofoil………………………………………………… 9
2.1.2 Performance Characteristics of Horizontal Axis Wind Turbine…………. 10
2.2 Spring -Mass-Damper Model of Horizontal Axis Wind Turbine Drive Train………..12
2.2.1 Model of The Ideal Wind Machine………………………………………. 14
2.2.2 Model of The Blade……………………………………………………… 15
xi
2.2.3 Model of The Hub………………………………………………………... 16
2.2.4 Model of The Gear Box………………………………………………….. 17
2.2.5 Modelling of The Induction Generator Mechanical System……………… 18
2.3 Development of The Simulation Model In Simulink………………………………..18
2.3.1 Wind Turbine Characteristic………………………………………………19
2.3.2 Wind Turbine Drive Train…………………………………………………19
2.3.3 Induction Generator………………………………………………………..20
2.3.4 Speed Controller…………………………………………………………...20
2.3.5 Actuator……………………………………………………………………21
2.4 Chapter Summary…………………………………………………………………...22
CHAPTER 3
Simulation of a DOIG Driven by Wind Turbine
3.1 DOIG in Standalone Mode……………………………………………………….. 23
3.1.1 Grid Connected Systems……………………………………………….. 24
3.1.2 Isolated Systems………………………………………………………….25
3.2 Interconnected Model of The HAWT With The DOIG……………………………. 26
3.2.1 Development of The DOIG Model……………………………………….26
3.2.1.1 Vector Rotator Block……………………………………………..28
3.2.1.2 Converter Block………………………………………………….29
3.2.1.3 Flux Estimation and Vector Rotator Block……………………...30
3.2.1.4 Stator Voltage and Rotor Current Control Block………………..32
3.2.1.5 DC Link Voltage Control Block………………………………....33
3.2.1.6 Load and Filter Block…………………………………………....33
3.3 Simulation of The Interconnected Model…………………………………………...34
3.4 Simulation results……………………………………………………………………35
3.4.1 Case Study 1……………………………………………………………….35
3.4.1.1 Discussion of Results……………………………………………...38
3.4.2 Case Study 2…………………………………………………………….. 38
xii
3.4.2.1 Discussion of Results……………………………………………. 40
3.4.3 Case Study 3……………..………………………………………………..40
3.4.3.1 Discussion of Results…………………………..………………. 43
3.5 Chapter Summary…………………………………………………………..………....44
CHAPTER 4
4 Hardware Description
4.1 Four Quadrant Chopper ……………………………………………………………... 47
4.1.1 Semikron Power Electronics Converters………………………......………48
4.1.1.1 The IGBT Module……………………………………………… 48
4.1.1.2 The Bridge Module ………………………………………………49
4.1.1.3 Gate Driver………………………………………………………..49
4.1.1.4 Heat Sink and Fan………………………………………………..49
4.1.1.5 DC Capacitor Bank and Snubber Capacitor……………………. 50
4.1.1.6 Temperature Protection…………………………………………..50
4.2 Interfacing Part for the Gate Signal…………………………………………………...50
4.2.1 Testing of Power Converters……………………………………………...53
4.3 Separately Excited DC Motor………………………………………………………...54
4.4 DSP Board and Interfacing Hardware………………………………………………..56
4.5 Experimental Results…………………………………………………………………58
4.5.1 Considerations……………………………………………………………59
4.5.2 Discussion of Results…………………………………………………….62
4.6 Chapter Summary……………………………………………………………………..62
xiii
CHAPTER 5
Conclusions
Future scope of work……………………………………………………………………….63
Appendix A
Determination of Per Unit Turbine Parameters For Simulation Model
A.1 Determination of Turbine Inertia……………………………………………………65
A.2 Determination of the Induction Machine Inertia…………………………………....67
A.3 Determination of the Compliance Between The Generator and The Gear………….67
A.4 Determination of the Compliance Between The Blade and The Hub……………….68
A.5 Determination of the Damping Coefficient of The Blade…………………………..68
A.6 Determination of the Damping Coefficient of The Induction Generator…………...68
A.7 Determination of the Damping Coefficient of The Hub…………………………….69
Appendix B
Details of the Equipments and Accessories Used
Bibliography
List of Principal Symbols
lift coefficient
drag coefficient
P output power of wind turbine
torque of wind turbine
power coefficient of wind turbine
torque coefficient of wind turbine
angular velocity of wind turbine
pitch angle
furling wind velocity
inertia of element k
D damping coefficient
K spring constant
per phase stator resistance
per phase rotor resistance
per phase stator inductance
per phase rotor inductance
magnetizing inductance
per phase value of the resistance used in the filter circuit
per phase value of the inductance used in the filter circuit
filter capacitance
per phase load resistance
per phase load inductance
C dc link capacitor
f frequency in cycles/sec
stator current vector
rotor current vector
d-axis component of stator circuit
q-axis component of stator circuit
d-axis component of rotor circuit
q-axis component of rotor circuit
d-axis component of stator voltage
q-axis component of stator voltage
d-axis component of rotor voltage
q-axis component of rotor voltage
stator voltage vector
rotor voltage vector
d-axis component of stator flux
q-axis component of stator flux
d-axis component of rotor flux
q-axis component of rotor flux
F value of stator flux at steady state
frequency of stator flux in rad./sec
electrical speed of rotor in rad./sec
slip frequency in rad./sec
rotor angle in elect.rad.
P time derivative operator
angle between the synchronously rotating and stationary
reference frames
s per unit slip of the rotor
d-axis component of the load current
q-axis component of the load current
load current vector
internal power factor angle
load power factor angle including the filter
d-axis component of the inverter current
q-axis component of the inverter current
voltage across the dc link capacitor
dc link currents
line current of the stator a-phase
line current of the stator b-phase
line current of the stator c-phase
line current of the rotor a-phase
line current of the rotor b-phase
line current of the rotor c-phase
line voltage of the stator a-phase
line voltage of the stator b-phase
line voltage of the stator c-phase
line voltage of the rotor a-phase
line voltage of the rotor b-phase
line voltage of the rotor c-phase
stator side converter line currents
rotor side converter line currents
SW1 switch of the filter circuit
SW2 switch of the load circuit
line currents in the filter circuit
line currents in the load circuit
voltage across the filter capacitors
current through the filter capacitors
propotional controller gain for the d-axis rotor current
propotional controller gain for the q-axis rotor current
integral controller gain for the d-axis rotor current
integral controller gain for the q-axis rotor current
propotional controller gain for the dc link voltage Controller
integral controller gain for the dc link voltage controller
modulating waveforms to the stator side converter
modulating waveforms to the rotor side converter
dc machine armature current
dc machine armature voltage
dc machine armature resistance
dc machine torque constant
Note:
All quantities with prime are those referred to the stator side
Subscript ‘s’ corresponds to stator quantities
Subscript ‘r’ corresponds to rotor quantities
Superscript ‘s’ corresponds to stator reference frame quantities
Superscript ‘r’ corresponds to rotor reference frame quantities
Superscript ‘e’ corresponds to synchronously rotating reference frame quantities
Superscript ‘ ’ corresponds to set/reference quantities
corresponds to 3 phase quantity , , ( f is either V or I )
corresponds to 3 phase quantity , , ( f is either V or I )
corresponds to 2 phase quantity , ( f is either V or I )
corresponds to 2 phase quantity , ( f is either V or I )
Introduction
1
CHAPTER-1
INTRODUCTION
The wind turbine industry has recently gained increased interest as the demand for cheap
renewable energy has grown. With increasing global concern about environmental pollution and
increasing fossil fuel cost, research initiatives for clean and renewable energy sources have gained
momentum. The continued growth and expansion of the wind power industry in the face of a
global recession and a financial crisis is a testament to the inherent attractiveness of the
technology. Wind power is clean, reliable, and quick to install; it’s the leading electricity
generation technology in the fight against climate change, enhancing energy security, stabilizing
electricity prices, cleaning up our air and creating thousands of quality jobs in the manufacturing
sector when they’re particularly hard to come by. Figure 1.1 shows the predicted percentage of
renewable energy in the year 2010. Wind power has emerged as the most attractive renewable
option in economic terms in the recent years. Due to rapid advancement of aerodynamics and
mechanical drive train design with the associated breakthrough in power semiconductor
technology during last two decades , the cost of energy generation from wind has come down to
the competitive level, which is supported by Figure 1.2. Table 1.1 shows the total installed
capacity of wind power plants for some leading countries indicating that the use of wind energy
has increased significantly contributing an increased percentage of the total global energy
generation.
Introduction
2
Figure 1.1 Predicted fuel energy consumption as percentage of total in the year 2010
(Source: U.S. Energy Information Administration)
Avergae Cost Per Kilowatt-Hour of Wind-Generated Electricity, 1982-2002, with
Projection to 2020 :
Figure 1.2 Change in average cost of wind generated electricity (Source: EPI from NREL,EWEA)
Introduction
3
Table 1.1
Top 10 Wind Power Countries (February 2011)
[Source: Wikipedia]
Country Wind Power Capacity (MW)
China 44,733
United States 40,180
Germany 27,215
Spain 20,676
India 13,066
Italy 5,797
France 5,666
United Kingdom 5,204
Canada 4,008
Denmark 3,734
India rank 5th in all over global market of wind energy and there are many number of
installations are there for India in 2011.
1.1 STRUCTURE OF WIND ENERGY CONVERSION SYSTEMS
The major components of a typical wind energy conversion system include a wind turbine,
generator, interconnection apparatus and control systems, as shown in Figure 1.3. Wind turbines
can be classified into the vertical axis type and the horizontal axis type. Most modern wind
Introduction
4
turbines use a horizontal axis configuration with two or three blades, operating either down-wind
or up-wind. A wind turbine can be designed for a constant speed or variable speed operation.
Variable speed wind turbines can produce 8% to 15% more energy output as compared to their
constant speed counterparts, however, they necessitate power electronic converters to provide a
fixed frequency and fixed voltage power to their loads. Most turbine manufacturers’ have opted
for reduction gears between the low speed turbine rotor and the high speed three-phase
generators. Direct drive configuration, where a generator is coupled to the rotor of a wind turbine
directly, offers high reliability, low maintenance, and possibly low cost for certain turbines.
Several manufacturers have opted for the direct drive configuration in the recent turbine designs.
At the present time and in the near future, generators for wind turbines will be synchronous
generators, permanent magnet synchronous generators, and induction generators, including the
squirrel cage type and wound rotor type. For small to medium power wind turbines, permanent
magnet generators and squirrel cage induction generators are often used because of their
reliability and cost advantages. Induction generators, permanent magnet synchronous generators
and wound field synchronous generators are currently used in various high power wind turbines.
Figure 1.3 Structure of a typical wind energy conversion system
Introduction
5
Interconnection apparatuses are devices to achieve power control, soft start and
interconnection functions. Very often, power electronic converters are used as such devices.
Most modern turbine inverters are forced commutated PWM inverters to provide a fixed voltage
and fixed frequency output with a high power quality. Both voltage source voltage controlled
inverters and voltage source current controlled inverters have been applied in wind turbines. For
certain high power wind turbines, effective power control can be achieved with double PWM
(pulse width modulation) converters which provide a bi-directional power flow between the
turbine generator and the utility grid.
1.2 MOTIVATION
In practice, synchronous generators and induction generators are used for the generation of
electricity from wind energy. Both types of induction generators namely the squirrel cage
(SQIM) and the wound rotor (WRIM) find their application in wind power generation. Doubly
Fed Induction Generators (DFIG) are widely used for this purpose in both grid connected and
isolated wind power generation systems for economic reasons. However, further investigation is
necessary to optimize the design and operation of such systems as well as to increase their
reliability. But in practice it is not possible to install a wind turbine in the laboratory due to its
huge size and insufficiency of wind to carry out laboratory experiments on wind power
generation. As a solution turbine models are used in software, which does not give a real time
solution to the problem. As a first step the turbine including the drive train (i.e. blade, hub etc)
has been modelled and simulated using MATLAB; SIMULINK platform. The turbine model is
then coupled to a Double Output Induction Generator model and its performance is analyzed. In
this work an attempt is made to implement a chopper controlled D.C machine whose speed is
controlled by a dSPACE DSP 1104 processor i.e. open loop control.
1.3 LITERATURE SURVEY
Many works related to wind power generation systems have been reported in the past decades.
Various schemes and control strategies have been proposed which lead towards the Variable
Speed Constant Frequency (VSCF) generation system. In our work we have concentrated
towards the power generation scheme using the induction generators and mainly the wound rotor
type induction generators. Leithead et. al. [1] have presented the modelling and control of a
horizontal axis wind turbine. In this work the turbine is modelled as simply consisting of a three
Introduction
6
blade rotor with rigid hub and gearbox. The generator is directly connected to the grid operating
in the constant speed mode. [2] shows a schematic controller design methodology for variable
speed wind turbines. The dependence of the effectiveness of the pitch regulation systems on
various turbine design parameters is quantified in [3]. Here the effectiveness of the pitch
regulation system is analyzed by varying the number of blades and the blade pitch span. In [4] a
variable speed wind turbine is investigated using pitch regulation and generator reaction torque
regulation to control the rotor speed and gearbox loads. [5] shows a comparison between the
wind, hydro and steam turbines. It also refers to the general control requirement and the
interaction between the adjacent wind turbines in a wind farm. Dynamic interaction of wind
turbine driven generators on electric utility networks is simulated in [6]. Here it is shown that a
high performance of the blade pitch control loop can reduce the mechanical as well as electrical
stresses on the system. A digital computer modelling and simulation of wind turbine-generator
system is described in [7] along with their equations. The dynamic stability of the system is
shown for the variation in wind velocity.
From the literature survey it is seen that the turbine models available so far have lacked
detailed modelling of its drive train. A detailed modelling of a horizontal axis wind turbine
along with its pitch controller and drive train dynamics is presented in [8]. The performance
of the simulated turbine speed controller is also analyzed by integrating it with a grid
connected squirrel cage induction machine model.
Pena et. al [9],[10] have designed a back to back voltage source PWM converters for doubly
fed induction generators to have independent control of the active and reactive power drawn
from the supply, while ensuring sinusoidal supply currents. Vector control scheme is
embedded in the control loops which enable optimum speed tracking for maximum energy
capture from the wind [9]. Later they expanded their work for the standalone system [10].
For experimental verification of their scheme they coupled their system to a dc machine,
which runs the induction machine in both sub- and super- synchronous speeds. But in their
model they have not given enough stress on the wind turbine modelling.
For the analysis and control of induction machines the field oriented control strategies
are adopted. [11]-[13] present the basics of field orientation and vector control operation for
the induction generators. [14]-[23] describe the operation of self excited induction generators
Introduction
7
for both the isolated and grid connected configurations. The induction generator performance
and some control strategies are discussed there. The voltage build up procedure in the self
excited generators and the requirement for capacitor in the isolated self excited system is also
discussed. [24]-[26] show the analysis of the induction generators for variable speed operation
in isolated mode.
Analysis and design of doubly fed induction machine drives by vector control method
requires the theoretical framework of reference frame analysis, which is given in [27]. [28]
shows the state variable model and the dynamic response for the doubly fed induction
machine whereas [29] shows a mathematical model for the same. [30]-[34] are concerned with
variable speed operation of doubly fed induction generators.
1.4 SCOPE OF THE WORK
Literature survey shows that significant amount of work has been done in past in the area
of wind turbine modelling. However, in most of the works reported so far, particularly
those, involving hardware emulation, the dynamics of the drive train is not given enough
attention. In this work an attempt is made to implement a chopper controlled D.C machine
whose speed is controlled by a dSPACE DSP 1104 processor i.e. open loop control.
As a first step, the wind turbine along with the drive train dynamics is modelled in details.
The performance of the wind turbine model is analyzed by simulation with pitch control. All
the simulation results are verified on this hardware platform.
The wind turbine model is then coupled with an existing DFIG based VSCF generation
system model. The complete model is run in closed loop pitch controlled mode as an
isolated wind power generation system. The induction generator supplies a load where the
load active power can change unpredictably. This load demand has to be met under randomly
varying wind speed condition.
A Hardware kit is designed for the real time implementation of a chopper driven separately
excited DC machine through DSP controller. And by controlling the duty ratio of the chopper
through DSP processor, the armature voltage varies hence the speed changes. Thus, open loop
control is performed.
Introduction
8
1.5 CONTRIBUTION
The following are the main contributions of this thesis.
• Derivation of the speed torque characteristic of a horizontal axis wind turbine
taking into account the effect of pitch angle variation.
• Modelling of a horizontal axis wind turbine along with its pitch control
system and drive train dynamics.
• Simulation of the interconnected turbine model with a standalone doubly
fed induction generator.
• Implementation of a open loop contro l scheme for separately excit ed
DC machine in real time by a chopper controlled using dSPACE DS1104.
1.6 THESIS LAYOUT
The thesis has been organized in five main chapters. The work has been divided into three
main phases such as, modelling of the wind turbine along with its pitch controller and
drive train dynamics, interconnection of the turbine model with a standalone doubly fed
induction generator and real time implementation of a chopper controlled dc machine.
Chapter 1 presents the introduction, motivation and the scope of the work. A brief
literature review on the topic is also given.
Chapter 2 describes the derivation of the turbine speed torque characteristics and turbine
modelling.
Chapter 3 presents the simulation results from the interconnected turbine model with a
standalone doubly fed induction generator.
Chapter 4 gives the details of the experimental set up for real time implementation of a open
loop control scheme for separately excited dc motor controlled by a chopper.
Chapter 5 draws the final conclusions.
Modelling and Simulation of a Horizontal Axis Wind Turbine
9
CHAPTER-2
MODELLING AND SIMULATION OF A HORIZONTAL
AXIS WIND TURBINE
Wind turbines are installed on towers to extract kinetic energy from wind. As flowing air
approaches the turbine blades the mass of air, which passes through the turbine rotor disc slows
down. Different types of wind turbines (horizontal axis and vertical axis, slow and fast etc.) exist
nowadays, but main concern in this thesis work will be on fast Horizontal Axis Wind Turbine
commonly known as HAWT.
2.1 HORIZONTAL AXIS WIND TURBINE
Horizontal Axis Wind Turbines have their rotor aligned horizontally whereas the blades
rotate on a vertical plane. For slow turbines the number of blades can go up to 8 to 24 whereas
for fast wind turbines this number is limited to 2 to 4. For fast turbines with the increase in the
number of blades the interaction between blades increases along with the inertia of the rotor and
cost of the blades. A three bladed turbine will be considered for our discussion throughout the
thesis. The characteristics of the wind turbine depends on the blade profile which basically
determines the drag and lift coefficients of the aerofoil.
2.1.1 LIFT AND DRAG OF AEROFOIL
Airflow over a stationary aerofoil produces two force, a lift force perpendicular to the airflow
and a drag force in the direction of the airflow. This is shown in Figure 2.1. The existence of the
lift force depends on the laminar flow over the aerofoil, which means that the airflows smoothly
over both sides of the aerofoil. If turbulent flow exists rather than laminar flow, there will be
little or no lift force. The air flowing over the top of the aerofoil has to speed up because of
greater distance to travel and this increase in speed causes a slight decrease in pressure. This
pressure difference across the aerofoil yields the lift force, which is perpendicular to the direction
of airflow.
Modelling and Simulation of a Horizontal Axis Wind Turbine
10
Figure 2.1 Lift and Drag forces of an aerofoil
Let the plane area of the aerofoil (or wing area) is s, the wind velocity (or true airspeed) actually
passing through the turbine rotor is V and the density of the air is .Then the lift (L) and drag
(D) of an aerofoil can be expressed as follows:
(2.1)
(2.2)
The symbols and represent the lift coefficient and drag coefficient respectively. They
depend on the shape of the aerofoil and will later with changes in the angle of attack and other
wing appurtenances.
The characteristics of any particular aerofoil section can conveniently be represented by
graphs showing the amount of lift and drag obtained at various angles of attack, the lift-drag
ratio, and the movement of the center of the pressure. The coefficients of lift, drag also depend
upon the Mach number and the Reynolds number.
2.1.2 PERFORMANCE CHARACTERISTICS OF HORIZONTAL AXIS
WIND TURBINE
The performance characteristics of a wind turbine express the power and torque output of the
turbine with wind speed variation. The power and torque output of a wind turbine is a
nonlinear function of the tip speed ratio and the pitch angle of the turbine blades. The output
power (P) and torque (T) of a wind turbine can be expressed as
Negative direction
Of Blade
Translation Relative
wind
Undistributed Wind
Drag
Lift
Direction of
Translation Of Blade
Modelling and Simulation of a Horizontal Axis Wind Turbine
11
(2.3)
and
(2.4)
Where, is the Power coefficient and is the Torque coefficient of the turbine.
R is the radial distance of the turbine blade tip from hub. S is the swept area of the turbine
blades.
The Tip Speed Ratio of a wind turbine refers to the ratio of the turbine speed at the blade
tip and the wind velocity. This can be expressed as
(2.5)
Where, is the tip-speed ratio and is the angular velocity of the turbine.
It is a common practice to express the turbine power and torque characteristics in the form of
vs and vs curves.
And a generic equation is used to model Cp (λ , ). This equation, based on the modelling turbine
characteristics of
(2.6)
With
(2.7)
Where, is the pitch angle.
The coefficients to are: = 0.5176, = 116, =0.4, =5, =21, = 0.0068.
The above equation is taken from the wind turbine block set in the Matlab library.
Also,
(2.8)
Modelling and Simulation of a Horizontal Axis Wind Turbine
12
In order to obtain the vs and vs curves of the wind turbine a Matlab program
is written. The tip speed ratio is varied in steps of 0.1 from 0.1 to 13. The range of pitch angle
taken is 0° to 40°.The vs and vs curves obtained from the Matlab program are
shown in Figure 2.2 and Figure 2.3:
Figure 2.2 Tip speed ratio vs. power coefficient Figure 2.3 Tip speed ratio vs. torque coefficient
From the above figures it is seen that with the increase in the pitch angle the power
coefficient and torque coefficient decreases. At minimum pitch angle i.e. when the pitch angle is
0°, the power and torque output from the turbine is maximum. It is also observed that the output
power maximum is occurring at a tip speed ratio of around eight whereas the output torque is
maximum at a tip speed ratio of around seven. Also at very high and very low values of tip
speed ratio the output power and torque decreases.
2.2 SPRING-MASS-DAMPER MODEL OF HORIZONTAL
AXIS WIND TURBINE DRIVE TRAIN
The various parts of a wind turbine like the blade, hub etc. can be represented by an
equivalent spring-mass-damper model. The model used for this work is shown in Figure 2.4.
Modelling and Simulation of a Horizontal Axis Wind Turbine
13
Figure 2.4 Spring-mass-damper model of the wind turbine
Figure 2.5 Wind turbine connected to squirrel cage induction generator
Figure 2.5 shows the wind turbine connected with a squirrel cage induction generator. The
turbine is driving the generator, which produces electrical energy to supply the load. The
‘K’ terms in the model represents the springs and the ‘D’ terms represents dampers in the
Generator Electrical
System
V
Alpha
Blade Inertia
Hub Inertia
Gear
1:n
Generator
Inertia
Ideal Wind Machine
Wind Turbine Drive Train Arrangement
Wind Turbine
Gear Box
Induction Generator
LOAD
Modelling and Simulation of a Horizontal Axis Wind Turbine
14
model. Modelling of the different blocks such as blade, hub, gear etc. are given below.
2.2.1 MODEL OF THE IDEAL WIND MACHINE
The Ideal Wind Machine computes the torque speed characteristics of the wind turbine
for various wind speed and pitch angle. The Matlab program from the previous section was
run to determine the wind turbine Cm − λ characteristics and the output values were
stored in a look up table. The ideal wind machine basically contains this look up table.
The inputs to this block were the wind speed, the pitch angle and the turbine speed and
the output is the turbine torque. The ideal wind machine is shown in Figure 2.6.
Figure 2.6 Ideal wind machine block diagram
The equation incorporated in the block is the turbine torque equation
(2.9)
Where, is the turbine torque.
Modelling and Simulation of a Horizontal Axis Wind Turbine
15
In the model all the equations are converted to per unit form for generalization. To convert
the equations in per unit some base quantities are defined. They are:
= Base torque of the turbine.
Furling wind velocity.
Then,
(2.10)
Now the above equation can be written as:
(2.11)
Or,
(2.12)
where and
2.2.2 MODEL OF THE BLADE
The mechanical equation for the blade can be written as
(2.13)
Here we have assumed that
Therefore,
(2.14)
This assumption will be followed through this thesis.
Modelling and Simulation of a Horizontal Axis Wind Turbine
16
Apart from the base quantities define above, some other base quantities are defined below:
= Base Inertia = =
= Base Angle = Angle traversed per second in radians at base speed
= Base value of damping coefficient = = .
= Base value of spring coefficient = = .
Now the above equation becomes
(2.15)
Or
(2.16)
2.2.3 MODEL OF THE HUB
From the assumptions made in the previous section the mechanical equation for the hub
can be written as
(2.17)
Where, = reflected machine torque on the low speed side of the gear box.
Modelling and Simulation of a Horizontal Axis Wind Turbine
17
To convert the equation in to per unit form the base quantities are taken as in the previous
section. With those base quantities the equation can be written as:
(2.18)
Or,
(2.19)
2.2.4 MODEL OF THE GEAR BOX
The mechanical equation for the gear box can be written as
= + (2.20)
Here n is the gear ratio i.e. the mechanical speed at the generator side of the gear is n times
the mechanical speed at the turbine side of the gear. is taken as gear efficiency i.e. the ratio of
the output and input power of the gear box.
According to previous assumptions the equation can be written as
+
(2.21)
This equation can be rewritten as
= + (2.22)
Modelling and Simulation of a Horizontal Axis Wind Turbine
18
2.2.5 MODEL OF THE INDUCTION GENERATOR MECHANICAL
2SYSTEM
The generator mechanical part (i.e. the rotor) is connected to the high speed side of the gear
box. The mechanical equation for the generator can be written as
(2.23)
Here is the electromagnetic torque generated by the machine. The above equation is
converted into per unit form using the same base values as used in the previous section as given
below.
(2.24)
Now this equation can be written as
(2.25)
2.3 DEVELOPMENT OF THE SIMULATION MODEL IN
SIMULINK
The turbine model is developed in Simulink to analyze its performance under different
operating conditions. The simulation model block diagram is shown in Figure 2.7.
Modelling and Simulation of a Horizontal Axis Wind Turbine
19
Figure 2.7 Simulation model block diagram
In the simulation model the wind turbine is connected to a grid connected squirrel cage
induction generator. The wind turbine acts as a prime mover for the generator, which
generates electrical power to supply the utility grid. The simulation model is made to run in
closed loop where a suitable external speed command on the generator side can be achieved.
The different parts of the simulation model are described below.
2.3.1 WIND TURBINE CHARACTERISTIC
The wind turbine torque speed characteristic is obtained by running a Matlab program as
described in section 2.2. The values of the torque coefficient for( ) different values of
tip speed ratio ( λ ) and pitch angle ( α ) are stored in the form of a look up table. The inputs
to this block are tip speed ratio, wind velocity and pitch angle. The output of this block is turbine
torque in per unit form.
2.3.2 WIND TURBINE DRIVE TRAIN
The wind turbine drive train consists of the blade, hub and the gearbox. The dynamic
equations for all these are shown in section 2.3.Those equations are all implemented in the
drive train blocks. The turbine torque coming out of the wind turbine characteristic block is the
input to the drive train and its output is the torque on the high speed side of the gear. This
torque acts as the prime mover torque for the next section i.e. the squirrel cage induction
generator. Table 2.1 shows all the parameter values used in the turbine model. The
Modelling and Simulation of a Horizontal Axis Wind Turbine
20
conversion of the turbine parameters in per unit values is shown in Appendix A.
Table 2.1
Parameter Values Used In The Turbine Model
Parameter Name Symbol Value in p.u.
Inertia of Blade
0.18 p.u
Inertia of Hub
0.02 p.u
Inertia of Generator
6e-5 p.u
Damping coefficient of Blade
0.05 p.u
Damping coefficient of Hub
0.02 p.u
Damping coefficient of Generator
6.25e-6 p.u
Compliance between blade &
Hub
4.6 p.u
Compliance between Generator
& Gearbox
13.75 p.u
2.3.3 INDUCTION GENERATOR
The induction generator is connected to the high speed end of the gearbox. Modelling
of its mechanical and electrical system is shown separately in section 2.4.2. All those equations
are implemented in the model. The input to the induction machine is the torque coming from
the gear and the line voltage. The output is the speed and the active power (all in per unit).
2.3.4 SPEED CONTROLLER
The speed controller generates the pitch angle command for the actuator. There is one error
block in the speed controller, which has the external speed command and the actual machine
speed as its inputs and the output of it is the speed error. The speed error then passes through a
proportional integral controller, which generates the required pitch angle command for the
Modelling and Simulation of a Horizontal Axis Wind Turbine
21
actuator to attain the speed.
2.3.5 ACTUATOR
The actuator use in the simulation assumed to be a small dc machine, which has inertia and friction
coefficient . The block diagram of the actuator system is shown in Figure 2.8.
Figure 2.8 Actuator system block diagram
The actuator accepts the pitch angle command as its input and it generates the torque
required to rotate the turbine blade at its output. There are speed and torque limits in the
actuator. The speed limit for the actuator is degrees/second i.e. 0.17 rad./second.
The torque limit is set at times the torque produced by the friction of the actuator.
The friction coefficient of the actuator is determined from the assumption that the
torque produced by the actuator at its maximum speed is 0.001% of the rated torque
produced by the turbine. So we can write
Or,
Integrator
Speed Limit Torque Limit
Actual Pitch Angle
Pitch Angle
Command
Modelling and Simulation of a Horizontal Axis Wind Turbine
22
=
A time constant of 30 milliseconds has been assumed for the actuator. So, we can write
Or,
Now the torque limit is
=0.7242 N – m
2.4 CHAPTER SUMMARY
This chapter presents the derivation of the torque-speed characteristics of a horizontal axis
turbine. The effect of pitch angle variation on the characteristics is also included. This chapter
also presents the modelling technique of a horizontal axis wind turbine using the spring damper
model equivalent model.
Simulation of a DOIG Driven by Wind Turbine
23
CHAPTER-3
SIMULATION OF A DOIG DRIVEN BY WIND TURBINE
The induction machine has extensive applications for wind power generation. The Squirrel
Cage Induction Machines also known as SQIMs are widely used for this purpose but in the last
few years the Wound Rotor induction machines have become popular due to their property of
having all the three stator as well as the rotor terminals available to the user. This property
enables the connection of two power electronic converters, one on the stator side and the other
on the rotor side, with the machine to give a better control over the machine dynamics and
enhanced power output capability. The wound rotor machine can deliver power to the load both
from its stator and rotor terminals justifying the name Double Output Induction Generator
(DOIG). The two converters connected to the Stator and Rotor side give full control over active
and reactive power flow in the machine and give rise to the possibility of having twice the power
output as compared to a SQIM of same electrical rating. The double power comes from the
capability of the machine to run at twice the synchronous speed at rated torque that is at rated
current. The ruggedness of SQIM is sacrificed though and problem appears with the slip ring
and brush arrangement of the wound rotor machine.
3.1 DOIG IN STANDALONE MODE
A wound rotor induction machine can be used to generate power in both grid connected and
isolated modes. The advantage of this type of machine is that the slip power becomes available
for speed control of the machine. The slip power being a small fraction of the total powers of the
machine the converter rating and hence the cost becomes substantially reduced making the drive
viable for high power applications. Slip power controlled machines are used in variable speed
constant frequency (VSCF) generation systems where the mechanical energy from a variable
speed shaft is converted to fixed frequency, fixed voltage power supply.
Simulation of a DOIG Driven by Wind Turbine
24
3.1.1 GRID CONNECTED SYSTEMS
In the grid connected mode the wound rotor induction machine is operated directly from the
line voltage thus running at a nearly constant flux. A wound rotor induction machine
mechanically coupled to a wind turbine with its stator connected to the grid and its rotor supplied
from a variable frequency source can provide power to the grid over a wide speed range (both
sub synchronous and super synchronous). The arrangement of a grid connected DOIG system is
shown in Figure 3.1. Here the DOIG supplies the energy to the grid and the power flow for sub
synchronous and super synchronous operations are shown in the diagram.
Figure 3.1 A grid connected DOIG
Grid
LC filter
Sub-Synchronous < 0
Super-Synchronous > 0
Gear Box
WRIG
Blades
Stator Circuit
Sub-Synchronous > 0
Super-Synchronous < 0
Rotor Circuit
AC
DC
DC
AC
Rotor-Side Converter
Grid-Side Converter
DC-Link
Simulation of a DOIG Driven by Wind Turbine
25
3.1.2 ISOLATED SYSTEMS
In the isolated mode the wound rotor induction machine can be used to generate power at
constant voltage and constant frequency to supply isolated loads while the rotor speed varies.
When a bidirectional converter is used in the rotor circuit, the speed range can be extended to
both sides of the synchronous speed and power can be generated both from the stator and the
rotor. This type of DOIG has the advantage that the converters need only be rated for a fraction
of the total output power, the fraction being dependent on the allowable sub and super
synchronous speed range. This system finds its applications for small domestic or industrial
loads and also where the wind site is far away from national grid. Figure 3.2 shows the
arrangement for such an isolated wind power generation system. Here the back to back converter
configuration is shown where the converters have the capability to operate in all the four
quadrants.
Figure 3.2 DOIG in standalone mode
Stator side
converter
Rotor side
converter
Gear Wind
Turbine
Induction machine
Ir Is
Vdc
Ii
Load and Filter
I load
Simulation of a DOIG Driven by Wind Turbine
26
3.2 INTERCONNECTED MODEL OF THE HAWT WITH THE
DOIG
To analyze the performance of a DOIG in standalone mode the DOIG model from [37] is
taken and interconnected with the HAWT model described in chapter 2. The interconnected
model is then run for different wind speeds and load active power demands.
3.2.1 DEVELOPMENT OF THE DOIG MODEL
The DOIG model is developed in Simulink. A synchronously rotating reference frame
aligned with stator flux linkage space vector is considered for modeling. The machine equations
in this reference frame are [37]
Where, and superscript “e” refers to synchronously rotating reference frame.
The rotor side converters are current controlled with the reference coming from the closed
loop dc link voltage controller. is a free variable which is utilized to optimally distribute
the machine and the load reactive power demand between the stator and the rotor side converters
in order to achieve minimum machine loss.
The stator side converters are voltage controlled to control the machine flux and hence,
indirectly the output voltage and frequency of the rotor side. The governing equations are
(3.1)
(3.2)
(3.3)
(3.4)
(3.5)
Simulation of a DOIG Driven by Wind Turbine
27
which in the steady state gives
= and = F
Figure 3.3 Block diagram for the simulation model of the DOIG
(3.6)
Simulation of a DOIG Driven by Wind Turbine
28
3.2.1.1 VECTOR ROTATOR BLOCK
The vector rotator block between the machine and the controller is shown in the below Figure
3.4.
Figure 3.4 The vector rotator block
The rotor position information required for these calculations is obtained from a separate
discrete system where is computed by discrete integration of the rotor speed .The output
voltage from the inverter block are transformed into
stationary reference frame d-q axes voltages and fed to the
machine block. The below equations show how the three phase machine voltages are
transformed into two phase.
Inverter Block
Vector Rotator
Block
Machine
Model in
reference
frame
Simulation of a DOIG Driven by Wind Turbine
29
3.2.1.2 CONVERTER BLOCK
Figure 3.5 The back to back connected PWM converters with the DC link capacitor
The above Figure 3.5 shows the two back to back connected converter blocks along with
the dc link capacitor and the PWM control block. In simulation the converters blocks are
considered to be consisting of ideal on/off switches instead of real power switches. Both the
converters are controlled by SPWM modulation technique. The control voltage
are compared with a triangle wave of 5 Volts peak value
and 5KHz frequency. The stator side converter currents can be written as
(3.7)
(3.8)
(3.9)
(3.10)
Simulation of a DOIG Driven by Wind Turbine
30
Similarly the rotor side converter currents can be written as
are obtained from the vector rotator block and
are obtained from the RL load block. The output voltages of the stator side converters
are fed to the stator of the induction machine and the output voltage of the rotor side
converters are fed to the rotor of the induction machine. For the inverters, equating the
instantaneous power input equal to the instantaneous power output, we have
The dc link voltage dynamics can be written as
3.2.1.3 FLUX ESTIMATION AND VECTOR ROTATOR BLOCK
The stator flux magnitude and orientation angle with respect to the stator flux axis
are computed in this block. Taking a balanced three phase system the stator and rotor
currents can be computed as
These currents are then transformed into stator reference frame as
(3.11)
(3.12)
(3.13)
(3.14)
(3.15)
(3.16)
(3.17)
(3.18)
(3.19)
Simulation of a DOIG Driven by Wind Turbine
31
The magnitude and orientation angle of the stator flux in stator reference frame can
be calculated as
=
=
And
The stator and rotor currents obtained in the stator reference frame are transformed to
synchronously rotating reference frame by the following equation
The d-q axes stator and rotor voltage commands are calculated in the
synchronously rotating reference frame in the stator voltage and rotor current control blocks
and are transformed into the stationary reference frame variables and then to the respective
three phase quantities. The stator and rotor voltage commands are used to generate the
control voltages. They are governed by the following equations
(3.20)
(3.21)
(3.22)
(3.23)
(3.24)
(3.25)
(3.26)
(3.27)
(3.28)
Simulation of a DOIG Driven by Wind Turbine
32
3.2.1.4 STATOR VOLTAGE AND ROTOR CURRENT CONTROL BLOCK
Figure 3.6 Block diagram of the rotor voltage derivation
The stator side converter is voltage controlled with
Where and are the measured values of d-q axes rotor currents in synchronously rotating
reference frame. Here F and re the command variables which are kept constant. Rotor side
currents are controlled by the PI controllers with back emf compensation. Now
(3.28)
(3.29)
(3.30)
(3.31)
(3.32)
(3.33)
(3.34)
(3.35)
Simulation of a DOIG Driven by Wind Turbine
33
Any error in the actual value of rotor currents will produce rotor voltages to control the
switching of the rotor side converter to keep the currents equal to their reference values.
Figure 3.6 shows the derivation of the rotor voltage. From the figure we get
Similarly,
3.2.1.5 DC LINK VOLTAGE CONTROL BLOCK
Figure 3.7 Block Diagram of DC link voltage controller
The above Figure 3.7 shows the voltage control block. The closed loop dc link voltage
controller consists of a PI controller, a current limiter block and the calculation block. is
generated from the PI controller and is then fed to calculation block through a limiter. In the
calculation block, is using Pmech, Ploss and Pload values and is then fed back to have closed
loop control.
3.2.1.6 LOAD AND FILTER BLOCK
After the dc link voltage build up and stabilization the load along with the filter is switched
on. The load block consists of an LC filter and RL load. A small value (0.1Ω) of resistance is
also included in the filter circuit. Figure 3.3 shows the load and filter block. The load and filter
Calculation
block
(3.34)
(3.35)
(3.35) (3.36)
(3.37)
Simulation of a DOIG Driven by Wind Turbine
34
system equations are
We can also write
3.3 SIMULATION OF THE INTERCONNECTED MODEL
The DOIG-HAWT interconnected model is simulated in the Simulink environment. The
DOIG parameter values are shown in Table 3.1.
Table 3.1
Machine Parameters Used In the Simulation
Parameter Name Symbol Used Value
Turbine Power Output
45 KW
Induction Machine Rating
5.6 KW
Furling Wind Velocity
15 m/s
System Rotational Speed Base
75 r.p.m
Induction Machine Synchronous
Speed
750 r.p.m
(3.38)
(3.39)
(3.40)
(3.41)
(3.42)
(3.43)
Simulation of a DOIG Driven by Wind Turbine
35
Number of Poles P 8
Gear Ratio n 20
Gear Efficiency
95%
DC Link Voltage
500 Volts
Machine Stator Resistance
0.872 ohms
Machine Rotor Resistance
0.8635 ohms
Machine Stator Self Inductance
0.03946 Henry
Machine Rotor Self Inductance
0.03946 Henry
Machine Magnetizing Inductance
0.0359 Henry
Stator Voltage(Line to Line r.m.s)
220 Volts
Rotor Voltage(Line to Line r.m.s)
300 Volts
Rated Stator Current (r.m.s)
22 A
Rated Rotor Current (r.m.s)
9.1 A
3.4 SIMULATION RESULTS
3.4.1 CASE STUDY 1: A gust wind speed varying from 0.77 p.u. to 0.83 p.u is given as
input. The load active power demand is varying and initially the load demand is at 0.2 p.u. and
at 50 seconds the load demand is changed to 0.6 p.u. With these conditions the interconnected
model is run for 100 seconds.
Simulation of a DOIG Driven by Wind Turbine
36
Figure 3.8 Time vs. wind-gust speed Figure 3.9 Time vs. turbine speed
Figure 3.10 Time vs. generator speed Figure 3.11 Time vs. turbine torque
Simulation of a DOIG Driven by Wind Turbine
37
Figure 3.12 Time vs. mechanical torque Figure 3.13 Time vs. Pitch angle
Figure 3.14 Time vs. load voltage Figure 3.15 Time vs. DC link voltage
Simulation of a DOIG Driven by Wind Turbine
38
3.4.1.1 DISCUSSION OF RESULTS
Figure 3.8 shows the gust wind velocity variation with time. The load power demand is
changed in steps from 0.2 p.u. to 0.6 p.u at 50 seconds. The DOIG supplies the demanded load
with a time delay. Figure 3.9 shows the turbine speed, which during the load transient
are turbine speed changes. Figure 3.14 and Figure 3.15 shows the load voltage magnitude
and the dc l ink vo lt age respectively during the simulation period. The dc link voltage
dips during the load transient but that dip is observed to be within tolerable limit. The load
voltage is dropped after 50 seconds due to the drop in the line. The actual machine speed seems
to follow the speed command. Figure 3.13 shows the change in the pitch angle. The pitch
angle at first droops from 20 but after that it rises again and then falls. This is due to
the wavy nature of the characteristics at very low values of Tip-Speed ratio. For
the first 50 seconds the pitch angle is lower in order to extract more power from the wind to
follow the speed command. After first 50 seconds the load power demand is increased and the
pitch angle is reduced. Figure 3.16 shows the variation of load power and the power increases as
the load increases.
3.4.2 CASE STUDY 2: In this case the wind speed is held constant at a value of 0.8 p.u.
The load active power demand is changed in steps. Initially there is no load and at 20 seconds
Figure 3.16 Time vs. Power
Simulation of a DOIG Driven by Wind Turbine
39
0.3 p.u. load is applied and further the load demand is increased to 0.6 p.u. at 30 seconds and
further increased to 0.9 p.u at 60 seconds respectively. With these conditions the interconnected
model is run for 80 seconds.
Figure 3.17 Time vs. generator speed Figure 3.18 Time vs. mechanical torque
Figure 3.19 Time vs. pitch angle Figure 3.20 Time vs. output power
Simulation of a DOIG Driven by Wind Turbine
40
3.4.2.1 DISCUSSION OF RESULTS
A constant wind velocity of 0.8 p.u. is given as input . The load power demand is changed
Figure 3.21 Time vs. turbine speed Figure 3.22 Time vs. turbine torque
Figure 3.23 Time vs. dc link voltage Figure 3.24 Time vs. load voltage
Simulation of a DOIG Driven by Wind Turbine
41
in steps from 0 p.u. to 0.3 p.u at 20 seconds and further increased to 0.6 p.u. at 30 seconds and
further increased to 0.9 p.u. at 60ato seconds. The DOIG supplies the demanded load with a
time delay. Figure 3.21 shows the turbine speed, which during the load transient are
turbine speed changes. Figure 3.23 and Figure 3.24 shows the load voltage magnitude and
the dc l ink vo lt age respectively during the simulation period. The dc link voltage dips
during the load transient but that dip is observed to be within tolerable limit. And the load
voltage decreases as the load increases as the drop in the line increases. The actual machine
speed seems to follow the speed command. Figure 3.19 shows the change in the pitch angle.
The pitch angle at first droops from 20 but after that it rises again and then falls. This
is due to the wavy nature of the characteristics at very low values of Tip-Speed
ratio. For the first 20 seconds the pitch angle is lower in order to extract more power from the
wind to follow the speed command. After first 20 seconds the load power demand is increased
and the pitch angle is reduced and once again reduced at 30 seconds and at 60 seconds because
load power demand is increased. Figure 3.20 shows the output power variation.
3.4.3 CASE STUDY 3: The wind speed is varied with a ramp of 0.01 /sec. Initially the wind
speed was 0.8 p.u. Then it is changed to 0.9 p.u. and after 20 seconds once again it is changed to
0.8 p.u. And the load demand is kept constant at 0.2 p.u.. With these conditions the
interconnected model is run for 80 seconds.
Figure 3.25 Time vs. input wind velocity Figure 3.26 Time vs. turbine speed
Simulation of a DOIG Driven by Wind Turbine
42
Figure 3.27 Time vs. generator speed Figure 3.28 Time vs. mechanical torque
Figure 3.29 Time vs. turbine torque Figure 3.30 Time vs. pitchangle
Simulation of a DOIG Driven by Wind Turbine
43
3.4.3.1 DISCUSSION OF RESULTS
A varying wind profile is given as input and Figure 3.25 shows the wind velocity profile.
Figure 3.31 Time vs. dc link voltage Figure 3.32 Time vs. load voltage
voltage
Figure 3.33 Time vs. output power
Simulation of a DOIG Driven by Wind Turbine
44
And the load power demand is kept constant at a value o f 0.2 p.u. . The DOIG
supplies the demanded load with a time delay. Figure 3.26 shows the turbine speed, which
during the load transient are turbine speed changes. Figure 3.31 and Figure 3.32 shows
the dc link voltage and output voltage magnitude during the simulation period. The dc link
voltage dips during the load transient but that dip is observed to be within tolerable limit. The
actual machine speed seems to follow the speed command. Figure 3.30 shows the change in
the pitch angle. The pitch angle at first droops from 20 but after that it rises again and
then falls. This is due to the wavy nature of the characteristics at very low
values of Tip-Speed ratio. Figure 3.27 shows the generator speed going to reference speed after
some time limit.
3.5 CHAPTER SUMMARY
This chapter presents the modeling of a standalone DOIG system and its interconnection with
a HAWT. The interconnected model is then simulated under various wind speed, load active
power demand and hence the simulation results are observed.
Hardware Description
45
CHAPTER-4
HARDWARE DESCRIPTION
The thesis is aimed to have the real time emulation of the wind turbine by a chopper driven
DC machine. The experimental set up consists of a four quadrant dc chopper made of IGBTs, the
gate driver card, a separately excited dc machine, and the DS1104 DSP board and associated
interface circuitry. Figure 4.1 shows the block diagram of the hardware. The actual setup
diagram is shown in Figure 4.2. Details of the different parts of the hardware are discussed next.
Figure 4.1 Block diagram of hardware
220
VOLT
DC SUPPLY
+
-
GATE DRIVE
DATA
LINES
DEDICATED HARDWARE
FOR CHOPPER DRIVE
SIGNAL
PROCESSOR
(DS1104) PWM
SIGNALS
+ -
F FF
220 VOLT
DC SUPPLY
HOST
COMPUTER
C
E
+15
V GND
G
Va
+
-
A
AA
FOUR QUADRANT CHOPPER
GATE
DRIVE
Hardware Description
46
Figure 4.2 Hardware setup
DC
Machine
DS 1104 combo
pack
Control
Desk
DSP
Interfacing
Cable
Power
Converter
Gate Interfacing
Signal
Hardware Description
47
4.1 FOUR QUADRANT CHOPPER
The Four quadrant chopper is shown in Figure 4.3.
Figure 4.3 Four quadrant DC chopper
The four-quadrant chopper is connected to the armature of the separately excited dc machine
to control the armature voltage. The chopper can apply both positive and negative voltages
across the armature and allows bidirectional flow of current. The four quadrant operating region
in the V-I plane is shown in Fig. 4.4. The chopper circuit, made of 1200 volts, 75 amperes,
SKM75GAR123D and SKM75GAL123D IGBT modules, are supplied from a 400 volts DC
supply. A450 volts, 3300 µF dc link capacitor is provided for absorbing harmonic current
generated by the chopper. The chopper circuit is hardware protected against over-current, shoot
through and dc bus over-voltages.
Figure 4.4 Operating region of the four quadrant DC chopper
DC Link Capacitor
To DC Machine Armature
G1
G3
G2
G4
I+
I-
V+ V-
Hardware Description
48
4.1.1 SEMIKRON POWER ELECTRONICS CONVERTER
Semikron’s Power Converter system consists of 3-phase uncontrolled rectifier and 3-phase
IGBT based controlled inverters. A 3-phase 415 V input is applied to the uncontrolled rectifier
(MD8TU100/16) using an Autotransformer (variac).The DC output of the rectifier is fed to the
inverter as source to the in inverter. Driver is the interface unit between the power module and
controller. Each Driver drives 2 switches in a Module. +15 V/0 V supply is given to Vs and
GND. Alternate ON/OFF pulses of 15 V are given to Vin1 and Vin2. Vin1 corresponds to TOP
IGBT and Vin2 corresponds to BOTTOM IGBT.
Semikron’s Power Converter kit consists of
IGBT module SKM75GB123D (3 no’s)
Diode Bridge MD8TU100/16 (1 no)
IGBT drivers SKYPER 32 R (3 no’s)
Heat Sink MDP3/250mm (1 no)
DC link Capacitors Semikron make 3300 µF/450 V (2 no’s)
Fan HICOOL Make (1 no)
Thermal trip 80 Deg C (1 no)
All the above components are encapsulated in Acrylic case for protection from electrical
shock.
4.1.1 .1 THE IGBT MODULE: SKM75GB123 D
MOS input (voltage controlled)
Low inductance case
Very low tail current with temperature dependence.
High short circuit capability, self limiting ti 6 * Icnom
Latch-up free
Fast & soft inverse CAL diodes
Isolated copper base plate using DCB Direct Copper Bonding Technology
Non Punch Through type of IGBT with low Vce (sat) which reduces conduction losses,
Hardware Description
49
Eon & Eoff and switching losses which is specially advantageous for high switching
frequency.
Each of these Modules is an Inverter leg & is made up of 2 IGBT with an antiparallel
diode.
The IGBT is triggered by charging the gate, which is done by applying voltage across the
gate and the emitter.
4.1.1.2 THE BRIDGE MODULE: MD8TU100/16
Three phase bridge rectifier
Blocking voltage of 1600 V
High surge current carrying capability
Large slated base plate & Easy mounting
Typical applications in power supplied, variable frequency drives, battery charger
rectifiers etc.
4.1.1.3 GATE DRIVER: SKYPER 32R
It interfaces and isolates the Control Unit/Primary Circuit from the secondary which is
directly connected to the high power.
Gate Driver controls the IGBT’s dynamic behavior and its short circuit protection.
Input signal level is 0/15 V.
Interlocking time between the input signals is 3 µs.
It monitors the errors : power supply under-voltage (below 13.5 V), short-circuit between
Collector and Emitter. The error rest time is typically 9 µs.
On detection of error/fault, the Gate driver switched off t he IGBT.
The IGBT switching speed is fixed by the resistors and .
4.1.1.4 HEAT SINK AND FAN
The stack assembly is provided with forced air cooling.
IGBT modules are mounted on 250 mm heat sink (extruded type).
Axial fan is connected to the heat sink to dissipate the heat generated by the IGBTS.
Hardware Description
50
Air flow from fan is at speed of 3 m/s.
Separate Power supply of 1-Φ, 230 V A.C to be provided for the fan.
4.1.1.5 DC CAPACITOR BANK AND SNUBBER CAPACITOR
Rectified DC input is given to electrolytic filtering capacitors.
Each capacitor is 3300 µF / 450 V.
2 capacitors are connected in series to have equivalent capacitance of 1650 µF / 900V.
Resistors of value 27 kΩ / 20 W are connected across each capacitor for voltage
balancing.
Snubber Capacitors of 0.22 µF / 1500 V dc (3 no’s) are connected across the dc link for
voltage overshoot protection.
The snubbers limit the over-voltages during switch off and as a consequence reduce the
losses.
They are kept very close to the device to reduce the inductance between the switches and
the capacitors.
4.1.1.6 TEMPERATURE PROTECTION
Normally Closed Thermal contact switch is used for protection against thermal runaway.
The position of the thermal switch normally closed when its temperature is below the
threshold temperature (80 deg C) & it opens above 80 deg C.
After cooling down, it again retains it normally closed position.
Thermal switch is placed at the warmest point on the heat sink.
It is recommended to take the feedback of the thermal trip output to the controller.
4.2 INTERFACING PART FOR THE GATE SIGNAL
In the power supply part, as shown in Figure 4.5, the 230 volts ac supply is stepped down to
18 volts by a transformer and are then rectified by a full bridge diode rectifier. The rectified
signals are the inputs to LM7815, LM7915 regulator IC’s respectively. These IC’s generate
regulated +15 volts, -15 volts respectively to supply different IC’s for the rest of the circuitry.
Hardware Description
51
3LM7815
1
U6
C2
1000uF,50v
D4
1N400712
GN
D
D1
1N400712
2
2
1
-18V
D3
1N400712
LM7915
V0
VI
C4
22uF,50v
C6
22uF,25v
+18V
+15V
-15V
0
D2
1N400712
0
C1
1000uF,50v
VI
C3
0.22uF,50v
V0
GN
D
C5
1uF,25v
U7
3
Figure 4.5 Power supply part of the interfacing gate signal
INTERFACING CIRCUIT FOR GATE SIGNAL OF ONE SWITCH
R3
1k
11
TL084
2
3
-
+
TL084
-
+
R8
1k
C12
1n
R7
1k
R1
1k
R6
1k
SIGNAL FROM DSPTO LM339
4
R2
1k
-15V R
R9
1k
+15V R
R4
1k
C11
1n
Figure 4.6 (a) Interfacing part for the gate signal
Hardware Description
52
D1
Q2
C20
1n
C13
1n
D2
C17
1n
C15
1n
C191n
R4
1kR5
45
D3
C211n
R3
2.2k
C22
1n
U1
LM7815
3 1
2
OUT IN
GN
D12
C161n
D4
1 2
Q1
R1
2.2k
C181n
FROM TL084
C141n
R22.2k, 0.25
LM339
4
5
-
+
R1
2.2k
TO IGBT GATE
3
U2 LM7815
3 2
1
OUT IN
GN
D
Figure 4.6 (b) Interfacing part for the gate signal
The gate driver circuit is shown in Figure 4.6. The PWM signals coming from the DSP
processor connects to the input terminals of the TL084. The high pulse is connected to the
inverting terminal of the first opamp and the low pulse is connected to the non-inverting
terminal. And the output of the first opamp is phase shifted with respect to the input signal. The
output phase shifted signal is once again fed to inverting opamp to get the original signal.
Therefore, the TL084 IC acts like an isolating and buffering circuit. And from then the signal is
connected to the non inverting terminal of the LM 339 IC. And the inverting terminal of the LM
339 IC is always at around 3.56 Volts. When the high pulse comes to the Pin 5, this gives +15
Hardware Description
53
Volts as the output and it is fed to the gates of the Push Pull amplifier. This makes Q1 on and Q2
off. The point G being connected as a common emitter load to the transistor passes +15 Volts to
the gate terminal of the IGBT. When the low pulse comes to the Pin 5 of LM 399, this gives -15
Volts at the output and it is fed to the gates of the Push Pull amplifier. This makes Q1 off and Q2
on. The point G being connected as a common emitter load to the transistor passes -15 Volts to
the gate terminal of the IGBT. The IGBTs thus receive ±15 Volts at their input terminals i.e.
between gate and source.
4.2.1 TESTING OF POWER CONVERTER
Figure 4.8 Gate signal & Input signal vs.
Time (secs,)
Figure 4.7 Gate signal & Supply voltage
vs. Time (secs,)
Hardware Description
54
The ± 15 V signal generated is then fed to one IGBT of one leg of the semikron’s power
converter. The ± 15 V pulse is connected to the Vin1 pin and + 15 V is connected to Vs pin and
ground of the interfacing circuit is connected to GND pin of the power converter. And the power
converter is tested by giving those signals and the some waveforms are observed. Figure 4.7
shows the supply voltage of 15 V and input signal i.e. 0 to 5 V pulse coming from dSPACE 1104
DSP processor. Figure 4.8 shows the Gate signal of one IGBT i.e. of ± 15 V pulse and input
signal. Figure 4.9 & 4.10 shows the variation of input signal of the gate interfacing circuit and
the output signal of the gate interfacing circuit. In order to avoid the short circuit of two IGBT’s
in same leg, a dead band of 5 µsec is given. And from the waveform it is clearly observed there
is a delay of 5 µsec between the input and the output signal.
4.3 SEPARATELY EXCITED DC MOTOR
Figure 4.11 shows a model of separately excited DC motor. When a separately excited DC
motor is excited by a field current of and an armature current of flows in the circuit, the
motor develops a back EMF and a torque to balance the load torque at particular speed. The is
independent of the . Each winding are supplied separately. Any change in the armature current
Figure 4.10 Gate signal & Input signal vs.
Time (secs,)
Figure 4.9 Gate signal & Input signal vs.
Time (secs,)
Hardware Description
55
ha no effect on the field current. The is normally much less than The relationship of the
field and armature are shown in below equations.
Figure 4.11 Model of a separately excited DC motor
Instantaneous field current
Where and are the field resistor and inductor respectively.
Instantaneous armature current
Where and are the armature resistor and inductor respectively.
The Motor back EMF which is also known as speed voltage is expressed as
Where is the Motor Constant (V/A-rad/s) and is the motor sped (rad/s).
The torque developed by the motor is
Where ( ) is the torque constant (in V/A-rad/s)
+
-
J
Hardware Description
56
For normal operation, the developed torque must equal to the load torque plus the friction and
inertia i.e.
Where B = Viscous friction constant (Nm/rad/s)
= Load Torque (Nm)
J = Inertia of the Motor (kg.m2)
Under steady state operation, a time derivative is zero. Assuming the motor is not saturated
For field circuit,
The back emf is given by
The armature circuit equation is,
The Motor speed can be easily derived
If is a small value (which is usual), or when the motor is lightly loaded, i.e., is small
That is the if field current is kept constant, the speed of the motor depends on the supply voltage.
These observations lead to the application of variable DC voltage to control the speed and torque
of DC motor.
4.4 DSP BOARD AND INTERFACING HARDWARE
DSPACE DS1104 is a controller board installed in the PCI slot of the PC. It contains two
processors. The main processor is a MPC8240 PowerPC with a clock speed of 250MHz and 32
kB internal cache memory. It acts as the master processor with TMS320F240 DSP as the slave
containing 4 K Word of dual port ram. Figure 3.15 shows the internal architecture and functional
units of the DSP 1104.
Hardware Description
57
Figure 4.12 Internal architecture and functional units of the DS1104 DSP board (Source: DS1104
Features)
The master PowerPC consists of an interrupt controller, a synchronous DRAM controller, a
PCI interface (5 Volts, 32 bit, 33MHz) and 6 timer devices. It allows the control of some
standard I/O features i.e. ADCs, DACs, Bit I/Os and Serial Interfaces. The ADC unit consists of
two different types of A/D converters, one multiplexed to four channels and four parallel A/D
converters. The multiplexed A/D converters have 16 bit resolution Volts input voltage
range. The parallel A/D converters have 12 bit resolution 10 Volts input voltage range. The
converters provide an interrupt at the end of the A/D conversion. Starting A/D conversion can be
synchronized with PWM signal generation or an external trigger source. The signal conditioning
for the ADCs is already discussed in the sensing and protection part.
The DAC unit consists of eight parallel DAC channels each of 16 bit resolution and +10
Volts output voltage range. There are 20 digital Bit I/Os present in the master PPC with a
Hardware Description
58
selectable direction for each individual pin. They have TTL voltage range for input and output
and +5 mA maximum outputs current.
The master PPC provides two incremental encoder interfaces supported for both single
ended TTL and differential RS422 signals. The encoder interface has 24 bit position counter
and 1.65 MHz maximum encoder line count frequency is supported. The encoder interfaces
take two quadrature axis pulses and one index pulse all with their corresponding
complementary pulses.
The slave DSP is a TMS320F240 floating point DSP. It has got a clock frequency of 20
MHz’s 4K*16 Bit dual port memory is used for communication with the master PPC. The
slave DSP features 14 bit digital I/O, timing I/O and Serial Peripheral Interface. The timing I/O
unit can be used to generate and measure PWM and square wave signals. There are four
single phase PWM signals with variable polarity, frequency and duty ratio. Apart from this
there are inverted and non inverted outputs for 3 phase PWM signal generation.
Programmable dead bands are also provided for the digital PWMs. The buffering of the
output PWM signals from the DSP is already discussed in the sensing and protection part.
4.5 EXPERIMENTAL RESULTS
A 3-phase 415 V input is applied to the uncontrolled rectifier using an Autotransformer
(variac). By varying the autotransformer, the dc bus voltage varies and finally the voltage is
maintained at 220 V. This dc bus voltage acts as the input to the inverter circuit which consists
of three phases. And the armature terminals of the DC motor are connected between R and B
phases. On the other hand, the field winding is supplied from a single phase auto transformer
and then rectified to DC through a diode bridge rectifier and some capacitors are provided to
filter out the harmonics and the field voltage is maintained at 220 V and the field current is 0.7A
respectively. And the gate interfacing signals are connected to the skyper circuit of the power
converter. An interfacing cable is connected between the DSP combo pack and the hardware
circuit, which acts as a data line.
Regarding the gate signals, a ± 15 V pulse is connected to the top IGBT in R-phase and to the
bottom IGBT in the B-phase and whenever + 15 V appears the armature is supplied with positive
voltage. A complementary ± 15 V pulse is connected to the bottom IGBT in R-phase and
Hardware Description
59
similarly to the top IGBT in B-phase and whenever + 15 V of the complementary signal appears
the armature is supplied with negative voltage. From the DSP processor, the duty ratio of the
pulses can be varied from -1 to 1 so that the armature is supplied with variable DC voltage i.e.
-220 V to +220 V. Thus the open loop speed control of the DC machine is achieved. But, this
thesis aim is to have the closed loop control of DC motor, which will be possible with some
current and speed feedback signals. Due to time limitation, this work cannot be completed and
will remain as the scope for future work.
4.5.1 CONSIDERATIONS
The field voltage is maintained at 200 Volts D.C and current of 0.8 A is flowing in the field
winding. The dc link voltage is maintained at 220 Volts, which is the input to the inverter circuit
of the power converter. And the four quadrant chopper operation is limited to only two quadrant
i.e. voltage can be both positive and negative but current flow is unidirectional only. With a duty
ratio of 0.5, the voltage across the armature is a symmetric signal of ± 200 Volts which
corresponds to the average value is zero. As the average value of the armature voltage is zero the
motor is at standstill condition and its value depends on the duty ratio which can be controlled
through dSPACE 1104 DSP processor. If the duty ratio is 0.5, the average armature voltage is
zero and the dc motor is in standstill condition. If the duty ratio is varying from 0.5 to one, the
average armature voltage varies from 0 V to 200 V and the motor starts rotating. If the duty ratio
is varying from -0.5 to one, negative voltage i.e. the average armature voltage varies from 0 V to
-220 V respectively and the motor starts rotating in opposite direction. A small AC voltage (15
Volt, 50 Hz) is applied to the stator terminals of the 3-phase induction machine and the variation
of frequency of rotor voltage is observed with respect to the change in speed. Here, initially the
machine is running at 610 rpm which corresponds to duty ratio of 0.464 and then the speed is
increased to 900 rpm which corresponds to duty ratio of 0.682 suddenly and the change in
frequency of the rotor induced voltage is observed. The induction machine rated speed is 750
rpm. At duty ratio of 0.464, the induction machine is running at sub-synchronous speed i.e. 600
rpm. Suddenly, the duty ratio is increased to 0.682 and the speed goes to 900 rpm which will be
the super synchronous speed to the induction machine. Here, the pattern in which frequency of
the rotor voltage is varying from sub-synchronous speed to synchronous speed and further to
Hardware Description
60
super-synchronous speed has been observed. All the above considerations are observed and the
waveforms are shown below.
Figure 4.13 Armature Voltage (V) vs. Time
(secs.) Figure 4.14 Complementary Armature Voltage (V)
vs. Time (secs.)
Figure 4.15 Armature Current (A) & Gate Signal vs. Time (secs.)
Hardware Description
61
Figure 4.16 Field Voltage (V), Armature Voltage (V) & Gate Signal vs. Time (secs.)
Figure 4.17 Armature Current (A), Armature Voltage (V) & Gate Signal vs. Time (secs.)
Figure 4.18 Rotor Voltage vs Time (secs.) Figure 4.19 Rotor Voltage vs Time (secs.)
Hardware Description
62
4.5.2 DISCUSSION OF RESULTS
Figure 4.13 shows the armature voltage and Figure 4.14 shows the complementary armature
voltage. Figure 4.15 shows the armature current of 2 A magnitude and gate signal of ± 15 V
magnitude. Figure 4.16 shows the field voltage of 200 V DC line and armature voltage of ±200V
amplitude and the gate signal of ± 15 V magnitudes. Figure 4.16 shows the combination of
armature current, armature voltage and the gate signal. From the waveform, whenever the gate
signal is + 15 V, positive voltage is applied to the armature and when the gate signal is – 15 V,
negative voltage is applied. And the average value of the armature voltage depends on the duty
ratio. Figure 4.17 shows the induced voltage in the rotor circuit ant its variation with respect to
the change in speed. A small AC voltage (15 Volt, 50 Hz) is applied to the stator terminals of the
3-phase induction machine and the variation of the rotor voltage is observed with respect to the
change in speed. Here, initially the machine is running at 610 rpm which corresponds to duty
ratio of 0.464 and then the speed is increased to 900 rpm which corresponds to duty ratio of
0.682 suddenly and the change in frequency of the rotor induced voltage is observed. Figure 4.18
gives the clear incremental change in frequency of the rotor voltage.
4.6 CHAPTER SUMMARY
This chapter describes some hardware description that is mainly the real time implementation
of a chopper driven DC machine and its speed control through DSP 1104 dSPACE. In this
chapter, some experimental results are taken and the results demonstrate the satisfactory
performance of the DC machine.
Conclusions
63
CHAPTER 5
CONCLUSIONS
The work reported in this thesis is concerned with the modelling, simulation of a pitch
controlled Horizontal Axis Wind Turbine. The model has been coupled to an isolated Double
Output Induction Machine (DOIM) to verify the performance of the interconnected system. And
an experimental set up is designed for the real time implementation of a chopper driven DC
machine through DSP controller.
The thesis starts with a detailed derivation of the torque coefficient vs. Tip speed ratio and
power coefficient vs. Tip speed ratio characteristics of a horizontal axis fast wind turbine
involving the effect of pitch angle variation. The turbine model has been developed as an
equivalent spring mass damper system in simulink, where the turbine generated torque is
calculated using the above mentioned torque coefficient vs. Tip speed ratio characteristics.
The turbine model is then integrated with an existing isolated DOIG model and the combined
system is run in simulink. The interconnected system is run under different wind speeds and load
active power demands and the simulation results are observed.
Finally, the thesis ends with hardware description in which experimental setup is designed for
the real time implementation of a chopper driven DC machine through DSP controller. This
thesis makes a way to real time emulation of wind turbine, by implementing the proposed model
by incorporating the chopper controlled dc machine at laboratory level which is the future goal
of this thesis work.
5.1 FUTURE SCOPE OF WORK
The work represented in this thesis represents the modelling, simulation of a pitch controlled
horizontal axis wind turbine and some hardware description. There are several refinements that
can be incorporated in the turbine model in future to make it more realistic. The length of the
turbine blade causes a difference in the wind speed faced by different parts of the blade itself i.e.
Conclusions
64
when the blade in the extreme top or bottom position. This is known as “Wind Shear” and it
changes the turbine torque speed characteristics significantly. Also the tower shadow effect
introduces some harmonics in the turbine torque speed profile. The turbine Yaw control action
enables the wind turbine to track the variation in the direction of the wind velocity. All these
features can be implemented in the turbine model in future.
And further regarding with the hardware description, by adding some current sensors and
some feedback signals the horizontal axis wind turbine has been emulated in real time by a
chopper driven separately excited DC machine. This also remains as a scope for future work.
Appendix A
65
APPENDIX A
DETERMINATION OF PER UNIT TURBINE
PARAMETERS FOR SIMULATION MODEL
The turbine parameters (i.e. compliance and damping coefficients of the blade, hub etc.) are
determined for the simulation model. All the parameters are converted to per unit values to make
the simulation model a general one. The parameter values are all taken from [6] and converted to
the system base used for the simulation. For the simulation the base quantities taken are
= Base power of the turbine = 45 Kw.
= Rated turbine speed = 75 r.p.m.
= = Rated turbine torque.
To convert the turbine parameters of [6] to the system base the following parameters are defined
=Base Power of bigger turbine, =Base Power of smaller turbine.
=Base Torque of bigger turbine, =Base Torque of smaller turbine.
=Base Speed of bigger turbine, =Base Speed of smaller turbine.
=Radius of bigger turbine, =Radius of smaller turbine.
=Inertia of bigger turbine, =Inertia of smaller turbine.
Determinations of the parameters are shown below:
A.1 DETERMINATION OF TURBINE INERTIA
For the large and small turbines, we can write
Appendix A
66
Here we have assumed .
Again,
where, mass of the bigger turbine, mss of the smaller turbine and m
Now,
From [6] we have in machine base. We want to convert it to the system turbine
base. The induction machine rating is 5.6 Kw and the turbine rating in the referred paper is 1
Mw.
So, we can write
Where, Per unit inertia in machine base.
The turbine inertia can be determined as
.
Appendix A
67
Now our equivalent turbine inertia is 0.2 p.u, which we will divide in 9:1 ratio between the blade
and the turbine hub.
So, we get 0.18 p.u and 0.02 p.u.
A.2 DETERMINATION OF THE INDUCTION MACHINE
INERTIA
From the retardation test of the induction machine we found its inertia to be 0.04366NW-m/Sec2.
Now we convert it to the turbine base by our definition:
So, p.u.
A.3 DETERMINATION OF THE COMPLIANCE BETWEEN
THE GENERATOR AND THE GEAR
From [6] we have K = 70 p.u torque/electrical rad. in machine base.
Now the basic relationship is
where T is the torque applied, i the small change in mechanical angle, K is the compliance.
We know,
where The electrical angle and P = The number of poles = 8 here.
We know that
Appendix A
68
S0,
A.4 DETERMINATION OF THE COMPLIANCE BETWEEN
THE BLADE AND THE HUB
The compliance between the blade and the hub is taken to be one third of the compliance
between generator and gear and is taken to be 4.6 p.u approximately. So,
A.5 DETERMINATION OF THE DAMPING COEFFICIENT OF
THE BLADE
We assume that the power loss in the blade is 5% of the turbine base power. So, we get
A.6 DETERMINATION OF THE DAMPING COEFFICIENT OF
THE INDUCTION GENERATOR
We have assumed that at rated machine speed the power loss is 2% of the rated power.
So, we can write,
Appendix A
69
0.02
So, we get
A.7 DETERMINATION OF THE DAMPING COEFFICIENT OF
THE HUB
The power loss at the hub is taken to be 2% of the rated turbine power.
So, we get
Appendix B
70
APPENDIX B
DETERMINATION OF EQUIPMENT AND
ACCESSORIES USED
B.1 SPECIFICATIONS OF THE IGBT MODULES USED (Source:
Semikron IGBT Datasheet)
Modules Used: SKM 75123D
Features
MOS input (voltage controlled)
Very low tail current with low temperature dependence
High short circuit capability, self limiting to 6
Latch-up free
Fast & soft inverse CAL diodes
Isolated copper base plate using DCB Direct Copper Bonding Technology
Large clearance (10 mm) and creepage distance (20 mm)
Absolute Maximum Ratings , unless otherwise specified
Symbol Conditions Values Units
IGBT
1200 V
75 A
60 A
150 A
V
10
Appendix B
71
Inverse Diode
75 A
50 A
150 A
480 A
Freewheeling Diode
95 A
65 A
200 A
720 A
Module
200 A
-40...+150
-40...+125
AC, 1 min. 2500 V
Characteristics , unless otherwise specified
Symbol Conditions min. typ. max. Units
IGBT
4,5 5,5 6,5 V
0,1 0,3 mA
1,4 1,6 V
1,6 1,8 V
Appendix B
72
22 28 m
30 38 m
2,5 3 V
3,3 4,4 nF
f = 1 MHz 0,5 0,6 nF
0,22 0,3 nF
500 nC
5
44 100 ns
56 100 ns
8 mJ
380 500 ns
70 100 ns
5 mJ
per IGBT 0,27 K/W
Figure B.1 Diagram of the IGBT modules used
Appendix B
73
B.2 MACHINES USED FOR THE EXPERIMENTAL STUDY
B.2.1 INDUCTION MACHINE NAME PLATE DETAILS
Output Power: KW, Speed: 750 R.P.M, Connection: Delta/Star
Stator:
Voltage:220 V , Current: 22 A
Rotor:
Voltage:300 V , Current: 9.1 A
B.2.2 DC MACHINE NAME PLATE DETAILS
Output Power: 2 HP, Speed: 1500 R.P.M
Armature Voltage: 220V, Current: 8A
Excitation Voltage 220V, Current: 0.8A
B.2.2.1 MEASUREMENT OF ARMATURE RESISTANCE
Voltage (V) Current (A) Resistance (ohms)
28.8 7.8 3.69
25.7 7 3.67
22.6 6.1 3.70
18.80 5 3.76
15.3 4 3.82
Appendix B
74
11.5 3.1 3.70
7.7 2.0 3.85
Avg. Value = 3.74Ω
B.2.2.2 MEASUREMENT OF ARMATURE INDUCTANCE
Voltage (V) Current (A) Impedance (ohms)
30 0.9 33.33
40 1..0 40
60 1.6 37.5
70 2.2 31.81
Avg. Value = 35.66Ω
Armature Inductance =
B.2.2.3 MEASUREMENT OF FIELD RESISTANCE
Voltage (V) Current (A) Resistance (ohms)
100 0.11 909.09
Appendix B
75
120 0.14 857.14
140 0.16 875
160 0.2 800
180 0.22 818.18
200 0.24 833.33
220 0.28 785.71
Avg. Value = 839.77Ω
B.2.2.4 MEASUREMENT OF FIELD INDUCTANCE
Voltage (V) Current (A) Impedance (ohms)
100 0.25 m 400 K
120 0.275 m 436.36 K
140 0.3 m 466.66 K
160 0.32 m 500 K
180 0.34 m 529.411 K
200 0.351 m 569.8 K
220 0.369 m 596.20 K
B.2.2.5 RETARDATION TEST ON DC MACHINE
On no load
Appendix B
76
Ni (rpm) Nf (rpm) Vi –Vf (volts) If (Amp.) Time taken in
secs.
750 408 100 – 56 0.8 2.84
750 402 100 – 54 0.8 2.65
750 378 100 – 50 0.8 250
750 366 100 – 49 0.8 2.47
750 300 100 - 40.7 0.8 2.40
750 246 100 – 34 0.8 2.33
750 230 100 - 32.9 0.8 2.04
Ni ----- Initial speed of the set in rpm.
Nf ---- Final speed
Vi ---- Initial Armature voltage of DC machine
Vf ---- Final Armature Voltage of Dc machine
The expression for final voltage is,
Where mechanical time constant of the set =
By using the above expression and making the calculations, average .
Viscous Coefficient (B) is calculated from the Speed-Torque characteristics of the DC machine:
B =
And the Moment of inertia (J) can be calculated as
77
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