narayana thesis

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

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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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

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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

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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.

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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.

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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.


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


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)


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.


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


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.


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.


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.


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)


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.


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


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


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


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.


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.


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


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


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)


27

which in the steady state gives

= and = F

Figure 3.3 Block diagram for the simulation model of the DOIG

(3.6)


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


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)


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)


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)


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)


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)


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)


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.


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


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


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


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


40


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


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


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


43


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


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


46

Figure 4.2 Hardware setup

DC

Machine

DS 1104 combo

pack

Control

Desk

DSP

Interfacing

Cable

Power

Converter

Gate Interfacing

Signal


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-


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,


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.


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.


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


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

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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,)


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


Time (secs,)


Time (secs,)

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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


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.


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

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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

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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

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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.)


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.)


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.

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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.

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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


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


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

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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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