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DECI.ARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT

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Title

MOHAMMAD AZRUL BIN ARIS

19 JUNE 1985

MODELLING OT' VOLTAGE SAG GEi\IERATOR

Acodemic Session 2008n049

I declore thct this thesis is clossified os:

I ocknowledged thot UniversitiTeknologiMoloysio reserves the right os follows:

l. The thesis is the property of UniversitiTeknologi Moloysio.2. The Librory of UniversitiTeknologi Moloysio hos lhe right to moke copies for

the purpose of reseorch only.3. The Librory hos the right to mqke copies of the ihesis for ocodemic

exchonge.

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{NEW rC NO. /PASSPORT NO.}

Dcte: 7 MAY 2009

{Conioins confidentiol informotion under the OfficiolSecret Act 19721*

{Contoins restricted informotion os specified by theorgonisotion where reseorch wos done)"

logree thot my thesis to be published os online openoccess {fulltext}

DR- AHMAD SAFAWI BIN MOKIITARNAME OF SUPERVISOR

Dote: 7 MAY 2009

Certified by:

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lf the thesis is CONFIDENTIAL or RESTRICTED, pleose ottoch with theletter from ihe orgonisotion with period ond reosons for confidentiolityor restriction.

NOTES :

"I declare that I have read this project report and in my opinion

this project report is adequate in tenn of scope and quality for the purpose of

awarding a Bachelor's degree of Electical Engineering"

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Supervisor's Name

Date

MODELLING OF VOLTAGE SAG GENERATOR


A report submitted in partial fulfillment of the requirement for the award of the

degree of Bachelor in Electrical Engineering

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

MAY 2009

"I hereby declared that the following thesis entitled

'Modelling of Voltage Sag Generator' is the result of my own effort except

as cited in the references"

Signature

Name of Author

Date

' \ " "


iii

Dedicated, in thankful appreciation for support, encouragement and

understandings to my beloved mother, father, brothers and sisters.

iv

ACKNOWLEDGEMENT

Alhamdulillah, with bless from Allah s.w.t finally I have completed my final

year project. I would like first of all to thank our creator for giving me strength and

courage to end up my project successfully.

I would like to express my gratitude to my supervising lecturer, Dr. Ahmad

Safawi bin Mokhtar for his support, help, and guidance. I have benefited

tremendously from his knowledge and experience in the fields of power.

I am extremely grateful to my beloved family for their continuous support

and supplication.

I also wish to thank my friends and individual who have offered help, support

and suggestion, contributing towards the successful completion of this project.

Without the involvement and support of many people in my studies, it would not

have been possible for me to complete this work.

v

ABSTRACT

Modern power systems are becoming more and more sensitive to the quality

of supplied power. As one of the most common power disturbances, voltage sag

typically happens randomly and usually lasts only a few cycles. In order to identify

the responses of modern power system such as electrical and electronics equipments

to such voltage disturbance, a signal generator that can produce voltage sags of

desired characteristic is needed. A modelling of Voltage Sag Generator (VSG) is

developed to fulfill this requirement. The VSG is a kind of device which can supply

reliable voltage sags to measure equipment susceptibility to the voltage sag. Some

standard methodologies have been proposed to construct the model of VSG. The

simulation is carried out in order to get the desired output with specific voltage sag

magnitudes and durations. The influence of various parameters of components,

different circuit topologies and different parameters control are investigated and

discussed. The main results of VSG model and simulation are illustrated graphically.

The PSCAD software environment is used in all VSG modelling and simulation.

vi

ABSTRAK

Sistem kuasa moden pada masa sekarang menjadi semakin sensitif terhadap

kualiti bekalan kuasa. Sebagai salah satu gangguan kuasa yang lazim, lendutan

voltan khasnya berlaku secara rambang dan kebiasaannnya bertahan hanya beberapa

kitaran. Dalam mengenal pasti sambutan sistem kuasa moden seperti peralatan

elektrik dan elektronik terhadap gangguan voltan, satu penjana isyarat yang boleh

menghasilkan lendutan voltan dengan sifat yang dikehendaki adalah diperlukan.

Permodelan Penjana Lendutan Voltan (PLV) dibangunkan untuk memenuhi

permintaan ini. PLV ialah satu alat yang mana boleh membekalkan lendutan voltan

yang baik untuk mengukur tahap kelemahan peralatan terhadap lendutan voltan.

Beberapa metodologi dicadangkan untuk membangunkan model PLV tersebut.

Simulasi dijalankan untuk mendapatkan keluaran yang dikehendaki dengan magnitud

dan tempoh lendutan voltan yang khusus. Pengaruh kepelbagaian parameter dalam

komponen, kaedah litar yang berbeza dan kawalan parameter yang berlainan turut

diselidiki dan dibincangkan. Hasil utama model PLV dan simulasi digambarkan

secara grafik. Perisian PSCAD digunakan dalam semua permodelan dan simulasi

PLV.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION OF THESIS ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF ABBREVIATIONS AND SYMBOLS xiii

LIST OF APPENDICES xiv

1 INTRODUCTION 1

1.1 General Background 1

1.2 Objective of Project 2

1.3 Scope of Project 3

1.4 Thesis Organization 3

2 LITERATURE REVIEW 4

2.1 Introduction 4

2.2 Voltage Sags 4

2.3 Sensitivity of Voltage Sags 6

2.4 Causes of Voltage Sags 7

2.5 Effects of Voltage Sags 7

2.6 Characterization of Voltage Sags 8

2.7 Phase Shift 9

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2.8 Voltage Sag Generator 10

2.8.1 Voltage Sag Generators Control 11

2.8.2 Structure of Voltage Sag Generator 12

2.8.3 Three Phase Bridge Rectifier 13

2.8.4 Three Phase Inverter 15

2.8.5 Third Order Output Filter 16

2.9 PSCAD Version 4.1 17

2.10 Fast Fourier Transform (FFT) 17

2.11 RMS – Root Mean Square

2.12 Summary

18

18

3 METHODOLOGY 19

3.1 Introduction 19

3.2 Voltage Sag Generator Structure Implementation 19

3.2.1 Rectifier 20

3.2.2 Inverter and Drive Circuit 21

3.2.3 Output Filter 22

3.2.4 Two Input Selector with Timer 23

3.3 Variation of Voltage Sag Parameters 26

3.4 Summary 27

4 RESULT AND DISCUSSION 28

4.1 Introduction 28

4.2 Model of Voltage Sag Generator 28

4.2.1 Three-Phase Full Bridge Diode Rectifier

and Inverter

29

4.2.2 Third Order Low Pass Filter 31

4.2.3 Input Selector with Timer 33

4.3 Simulation of Voltage Sag Parameters 36

4.3.1 Sag Magnitude 36

4.3.2 Sag Duration 40

4.3.3 Phase Shift 44

4.4 Summary 46

ix

5 CONCLUSION AND RECOMMENDATION 47

5.1 Conclusion 47

5.2 Recommendation 48

REFERENCES 49

APPENDICES 51

x

LIST OF TABLES

TABLE NO. TITLE PAGE

4.1 Peak and RMS value of voltage sag 40

4.2 Voltage sag phase shift 45

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Voltage sag definition in term of its parameters 5

2.2 Voltage sag definition based on IEEE Standard 5

2.3 Information Technology Industry Council (ITIC) curves 6

2.4 Voltage sag characteristic 9

2.5 Voltage sag phase shift 10

2.6 Devices used in the VSG 12

2.7 Structure of VSG 13

2.8 Uncontrolled three phase rectifier 14

2.9 Input voltage and output voltage of rectifier 14

2.10 Three phase inverter 15

2.11 Drive circuit pulses and inverter output waveforms 16

3.1 VSG operation block diagram 20

3.2 Construction of rectifier using PSCAD 21

3.3 Construction of inverter using PSCAD 22

3.4 Construction of output filter using PSCAD 23

3.5 Construction of Two Input Selector with timer using PSCAD 24

3.6 Two Input Selector 25

3.7 Timer configuration 25

3.8 Parameters controlled in simulation of VSG model 26

4.1 Model of rectifier and inverter 29

4.2 Rectifier output waveform 30

4.3 Inverter output waveform 30

4.4 Model of output filter 31

4.5 Pre sag voltage waveforms (a) peak value (b) RMS value 32

4.6 Models of Two Input selectors with timers 33

xii

4.7 AM/FM/PM Function waveforms (a) peak value (b) RMS value 34

4.8 Voltage sag waveforms (a) peak value (b) RMS value 35

4.9 10% voltage sag waveform (peak) 37

4.10 10% voltage sag waveform (RMS) 37





4.15 Sag duration waveform (0.02s) (peak) 41

4.16 Sag duration waveform (0.02s) (RMS) 41





4.21 Phase shift waveform (Phase A) 44

4.22 Phase shift waveform (Phase B) 44

4.23 Phase shift waveform (Phase C) 45

xiii

LIST OF ABBREVIATIONS AND SYMBOLS

AC - Alternating Current

AM - Amplitude Modulation

Ctrl - Control

DC - Direct Current

FACTS - Flexible AC Transmission Systems

FFT - Fast Fourier Transform

FM - Frequency Modulation

H - Henry

Hz - Hertz

IEEE - Institution of Electrical Engineering

kV - kilo Volts

L-L - Line to Line

pF - Pico Farad

PM - Phase Modulation

PLV - Penjana Lendutan Voltan

PSCAD - Power Systems Computer Aided Design

RMS - Root Means Square

s - Seconds

SLG - Single Line to Ground

µF - micro Farad

V - Volts

Ω - Ohm

xiv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A The Model of Voltage Sag Generator 51

CHAPTER 1

INTRODUCTION

1.1 General Background

A power distribution system is similar to a vast network of rivers. It is

important to remove any system faults so that the rest of the power distribution

service is not interrupted or damaged. When a fault occurs somewhere in a power

distribution system, the voltage is affected throughout the power system [1]. Modern

power systems are becoming more and more sensitive to the quality of supplied

power.

The reason is that not only does modern equipment include a vast variety of

electronic components which can be very vulnerable to power disturbance, but also

the customers become more susceptible to the losses produced by equipment

malfunction.

Among various power quality problems, the major event that usually occurs

is voltage sag. Voltage sags is one of the power quality problems affecting industry

and they often cause serious power interruptions. The causes of voltage sags are

associated with faults within the power distribution system. A voltage sag condition

implies that the voltage on one or more phases drops below the specified tolerance

for a short period of time. Sags account for the vast majority of power problems

experienced by end users.

2

As one of the most common power disturbances, voltage sag typically

happens randomly and usually lasts only a few cycles [1]. However, sensitive

equipment often trips or shuts down for those sags, even if nominal voltage returns in

just a few cycles. For sensitive loads, even a voltage sag of short duration can cause

serious problems in the manufacturing process. Normally, a voltage interruption

triggers a protection device, which causes the entire branch of the system to shut

down. Thus, voltage sag brings the greatest financial loss compared with most other

kinds of power disturbances.

In order to test the sensitivity of electrical equipment to such momentary

voltage disturbance or voltage sag, a particular device is needed. To this goal, it is

necessary to have a voltage sag generator, that is, a device or equipment capable of

generating the suitable voltage-time profiles. Voltage sag generator (VSG) is a signal

generator that can produce voltage sags of desired characteristics in order to test and

identify equipment responses to such voltage disturbances.

Generally, current power quality standards define and describe voltage sags

by only two parameters which are magnitude and duration. All these voltage sag

characteristics introduced by the VSG should be fully controlled and easily repeated

in systematic experiments. The influence of these voltage sag parameters on certain

equipment can be significant.

1.2 Objective of Project

The objective of this project can be specified as follows:

1) To model a three phase voltage sag generator (VSG) which is can

produce such voltage sag signal. The voltage sag generator then can be

applied in the power system design and electrical equipment test.

2) To get a variation of waveform of voltage sag by simulating the

parameters of obtained model in term of its magnitude, duration and

phase shift of voltage sag signal.

3

1.3 Scope of Project

In order to achieve the objective of the project, there are several scopes that

had been outlined. The scope of this project can be specified as follows:

1) Study of the structure of voltage sag generator and the characteristic of

voltage sag and how it can be used in the simulation to obtain voltage sag

waveform.

2) Modelling of the voltage sag generator using PSCAD software version

4.1.

3) Simulation of the model of voltage sag generator and the variation of the

voltage sag parameters to get a desired waveform.

1.4 Thesis Organization

This thesis is organized in six (6) chapters and is structured in the following

manner. Chapter 1 includes introduction of this thesis such as objective and scope of

this project. Chapter 2 provides a brief review on voltage sag theory, the

characteristics of voltage sag and voltage sag structures. In Chapter 3, the

methodology of modelling of voltage sag generator as well as its simulation using

PSCAD are presented.

Chapter 4 presents the results of modelling and simulation. The explanations

and discussions on results obtained also be presented in this chapter. Chapter 5

concludes the work presented in this thesis. This chapter also briefs several

suggestions for this project for future work and research.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter includes the study voltage sag characteristics, the causes and

effects of voltage sag and also the voltage sag generator includes its structures and

parameters which are controlled in producing voltage sag waveform. It also brief

discusses about PSCAD software which was used as a platform of the modelling of

VSG.

2.2 Voltage Sags

Voltage disturbances can occur anywhere in the power system and within an

electric customer’s facility. Voltage sags is one of the power quality problems

affecting industry. It is a momentary disturbance that can cause a failure to electrical

equipments operation. Among various types of power quality disturbances in a

power system, voltage sags are particularly troublesome since they occur rather

randomly and their characteristics are difficult to predict.

Voltage sags are short duration reductions in RMS voltage which is caused

by faults in the electric supply system and the starting of large loads, such as motors.

The IEC 61000-4-30 defines the voltage dip (sag) as “a temporary reduction of the

voltage at a point of the electrical system below a threshold”. In IEEE Std. 1159-

5

1995 a voltage sag is defined as “an RMS variation with a magnitude between 10%

and 90% of nominal voltage and a duration between 0.5 cycles and one minute” [2].

Figure 2.1 and Figure 2.2 show the definition of voltage sag graphically.

Figure 2.1 Voltage sag definition in term of its parameters [4]

Figure 2.2 Voltage sag definition based on IEEE Standard [5]

6

2.3 Sensitivity of Voltage Sags

Figure 2.3 shows the Information Technology Industry Council (ITIC) curve

that has been introduced to suggest a guideline for voltage quality in power

distribution systems serving main computers, and it has become an industry

reference for acceptable voltage tolerance. This curve specifies the voltage dip

magnitude and the duration of the voltage sag for 120 V single-phase applications

[3].

The curve shows that a 10% voltage deviation is acceptable even if the

voltage sag or swell remains for a long time, but a 30% voltage drop for a time

period longer than 0.5 second is not acceptable. This curve is useful for providing

general insight into acceptable voltage quality. The SEMI F47 specifies the

requirement of voltage quality for the voltage sag immunity of semiconductor

manufacturing processing [3].

Figure 2.3 Information Technology Industry Council (ITIC) curves [3]

7

2.4 Causes of Voltage Sags

Disruptive voltage sags are usually caused by fault conditions on the utility

transmission and distribution systems or within a customer’s facility. Voltage sags

are generally created on the electric system when faults occur due to lightning,

accidental shorting of the phases by trees, animals, birds, human error such as

digging underground lines or automobiles hitting electric poles, and failure of

electrical equipment.

In the case of a short-circuit fault, the utility system would detect the

resulting over-current, and perform a feeder breaker trip for disconnecting the

downstream loads from the system, followed, if it is possible, by a re-closure

operation for clearing the fault and therefore maintain the service continuity of the

electric supply for the majority of its customers.

Faults in the distribution or transmission line can be classified as single-line-

to-ground (SLG), and line-to-line (L-L) faults. SLG faults often result from severe

weather conditions such as lightning, ice, and wind. Animal or human activity such

as construction or accidents also causes SLG faults. Lightning may cause flashover

across conductor insulators and is the major source of SLG faults [3].

Sags also may be produced when large motor loads are started, or due to

operation of certain types of electrical equipment such as welders, arc furnaces and

smelters. Motors starting within the customer facilities can also result in voltage sags

for neighborhood customers. The characteristics of these voltage sags are predictable

and can be prevented.

The duration of the sag caused by motor starting is generally longer, but the

voltage drops are usually small and do not cause serious problems at the customer

locations. In case of starting large motors, the voltage sags are usually shallow and

last a relatively long time.

8

2.5 Effects of Voltage Sags

The interests in voltage sags are increasing because they cause the

detrimental effects on several sensitive equipments such as adjustable-speed drives,

process-control equipments, and computers. Some pieces of equipments trip when

the RMS voltage drops below 90% for longer than one or two cycles [4]. Although a

voltage sag is not as damaging to customers as an interruption, the total damage due

to sags is still larger than that of interruptions because there are far more voltage sags

than interruptions.

Whether or not a voltage sag causes a problem will depend on the magnitude

and duration of the sag and on the sensitivity of your equipment. Many types of

electronic equipment are sensitive to voltage sags, including variable speed drive

controls, motor starter contactors, robotics, programmable logic controllers,

controller power supplies, and control relays.

Much of this equipment is used in applications that are critical to an overall

process, which can lead to very expensive downtime when voltage sags occur.

Therefore it is important to assess the effects of the voltage sags correctly.

2.6 Characterization of Voltage Sags

Voltage sags are characterized by its magnitude and duration as shown as

Figure 2.4. The magnitude is defined as the percentage of the remaining voltage

during the sag and the duration is defined as the time between the sag

commencement and clearing. This characterization is fine for single phase systems

and three-phase balanced faults [5].

However for three-phase unbalanced sags the three individual phases would

be affected differently leading to a case where we have three different magnitudes

9

and three different durations. In this instance the most affected phase is taken as sag

magnitude and the duration is the longest of the three durations [5].

However, several studies have shown that some other characteristics

associated with sags, such as phase-angle jump, point-on-wave of initiation and

recovery, waveform distortion and phase unbalance, may also cause problems for

sensitive equipment.

Figure 2.4 Voltage sag characteristic [5]

2.7 Phase Shift

The term ‘during-sag phase shift’ will be used to denote all changes in the

phase angles of the sagged or un-sagged phase voltages (phase-to-neutral or line-to-

neutral voltages) that are present during the sag. A during-sag phase shift is assumed

to be a continuous function of time, expressed as the difference between the phase

angles of the pre-sag and during-sag instantaneous voltages [6].

In the general case, different phases may experience different during-sag

phase shifts, and a per-phase representation should be used to describe the

characteristics of multi phase sags. The difference between the pre-fault and post

fault phase angles (for example, between the pre-sag and post-sag phase angles) will

10

be denoted in this study as the ‘post-sag phase shift’. Phase shift during the voltage

sag is from 0° to +180° [6]. The illustration of voltage sag phase shift is shown in

Figure 2.5.

Figure 2.5 Voltage sag phase shift [6]

2.8 Voltage Sag Generator

Voltage sag generator (VSG) is a signal generator that can produce voltage

sags of desired characteristics in order to test and identify equipment responses to

such voltage disturbances. There are two common types of the VSG; variable

transformer-switch type, and power amplifier type [7]. Both types usually have data

acquisition system attached, number of digital and analogue input/output ports and

controllers for efficient supervision and regulation of operations.

Variable transformer-switch type is usually realized as a combination of

transformers (for adjustment of both pre-sag voltage and sag voltage magnitudes)

and appropriates witching devices (for switching from pre-sag voltage to sag voltage

and, eventually, for adjustment of phase shift and points on wave of sag initiation

11

and recovery). In its simplest configuration, this type of the VSG consists of two

single phase transformers (per phase, in the case of three-phase VSG). One

transformer is used for pre-sag voltage magnitude setting and the other for voltage

sag magnitude setting. Both magnitudes are adjusted manually [7].

The power amplifier type of the VSG usually uses a waveform generator for

definition of the voltage sag waveforms with desirable characteristics. After defining

the related signal it is passed to power amplifier, at which outputs adequate voltage

current levels of the voltage sag are produced. This configuration is more convenient

than variable transformer-switch type, because it enables more precise control of all

voltage sag characteristics and also allows testing of equipment in context of

frequency variations and harmonic distortions.

Regarding the requirements related to full control of complex outputs of the

VSG, conversion of fixed magnitude 50Hz AC mains supply (primary energy

source) to a variable magnitude variable-phase 50Hz AC voltage (output of the VSG)

in simulations is carried out in two stages [7]. The AC mains voltage was first

rectified to create the DC link voltage, and then converted back to the AC voltage

using the DC/AC inverter.

Output waveforms of the inverter were filtered, in order to obtain accurate

reproduction of desired waveforms. The use of inputs selector is found to be

significant since it is the main part for selecting either two input with different

magnitude.

2.8.1 Voltage Sag Generators Control

Following parameters of voltage sag were controlled in simulations:

1. Voltage sag magnitude (from 10% of nominal voltage to 90% of nominal

voltage)

2. Duration of the voltage sag (from half of a cycle to a few seconds)

3. Phase shift during the voltage sag (from 0° to +180°)

12

4. Point on wave of voltage sag initiation and point on wave of voltage recovery

(from 0° to 360°) [7].

2.8.2 Structure of Voltage Sag Generator

Figure 2.6 shows the main components can be used in order to model the VSG.

In all models used in simulations, following four main parts of the VSG can be

distinguished [7]:

1. The DC voltage supply system

2. The DC/AC voltage inverter (with drive circuit)

3. The output filter

4. The input selector

Figure 2.6 Devices used in the VSG [6]

The first part of the VSG which is the DC voltage supply system includes the

three phase AC source as the input of the rectifier. It uses three phase diode rectifier

to convert from AC voltage to DC voltage. The second part of the VSG, the DC/AC

inverter, is simulated as a full bridge, three-phase inverter made of six Insulated Gate

Bipolar Transistors (IGBTs) controlled by pulse drive circuit.

13

In performed simulations, the IGBT was selected as switching component

because it is voltage controlled and easy to drive, with relatively low on-state voltage

drop. Moreover, by the use of IGBTs for switching and precise controls of all output

voltage waveform characteristics is enabled (phase shift and points on wave of

initiation and recovery can be controlled in full range which are from 0° to 360°) [7].

The third part of the VSG is an output filter. The purpose of adding a filter to

the output of the DC/AC inverter is to improve its voltage output. The last part is

inputs selector which is drive by timer to select either one of its two inputs. The input

consists of three phase AC voltage from main circuit and also the voltage signal from

voltage signal generator. The part of structures of VSG is shown as Figure 2.7.

Figure 2.7 Structure of VSG [6]

2.8.3 Three Phase Bridge Rectifier

This type of rectifier is used to convert an AC voltage into a DC link voltage.

It used six diodes as the devices as shown as Figure 2.8. There are two main group of

rectifier operation. On the top group, diode with its anode at the highest potential will

conduct and the other two will be reversed. While on the bottom group, diode with

the cathode at the lowest potential will conduct and the other two will be reversed.

14

For example, if D1 (of the top group) conducts, Vp is connected to Van. If D6

(of the bottom group) conduct, Vn connects to Vbn and all other diodes are off. The

resulting output waveform is given as: Vo=Vp-Vn while for peak of the output voltage

is equal to the peak of the line to line voltage Vab as shown as Figure 2.9.

Figure 2.8 Uncontrolled three phase rectifier

Figure 2.9 Input voltage and output voltage of rectifier

15

2.8.4 Three Phase Inverter

An inverter is an electrical or electro-mechanical device that converts direct

current (DC) to alternating current (AC). The resulting AC can be at any required

voltage and frequency with the use of appropriate transformers, switching, and

control circuits. Three-phase inverters are used for variable-frequency drive

applications and for high power applications such as HVDC power transmission. A

basic three-phase inverter consists of three single-phase inverter switches each

connected to one of the three load terminals as Figure 2.10.

For the most basic control scheme, the operation of the three switches is

coordinated so that one switch operates at each 60 degree point of the fundamental

output waveform. This creates a line-to-line output waveform that has six steps. The

six-step waveform has a zero-voltage step between the positive and negative sections

of the square-wave such that the harmonics that are multiples of three are eliminated

as described above.

When carrier-based PWM techniques are applied to six-step waveforms as

shown as Figure 2.11, the basic overall shape, or envelope, of the waveform is

retained so that the third harmonic and its multiples are cancelled.

Figure 2.10 Three phase inverter

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/Alternating_current

http://en.wikipedia.org/wiki/Transformer

http://en.wikipedia.org/wiki/Three-phase_electric_power

http://en.wikipedia.org/wiki/Variable-frequency_drive

http://en.wikipedia.org/wiki/High-voltage_direct_current

16

(a) (b)

Figure 2.11 Drive circuit pulses (a) and inverter output waveforms (b)

2.8.5 Third Order Output Filter

The purpose of adding a filter to the output of the DC/AC inverter is to

improve its voltage output. This output is influenced by PWM switching technique

and that is why it needs additional filtering. For each filter configuration and each

filter order, two different topologies are considered: even (with inductance at input

side of the filter) and odd (with capacitance at input side of the filter).

Some filter topologies (which end with capacitance: second order even, third

order odd and fourth order even) produce smaller THD of load current than other

filter topologies (which end with inductance: second order odd, third order even and

fourth order odd). The cut-off frequency of all simulated low-pass filter

configurations was 50Hz. However, it is found that fourth order even filter

introduces instability in output voltage of the VSG. Therefore, third order odd filter

is chosen as optimal filter configuration [7].

The influence of output filter on the voltage sag generator performances is

found to be significant. Generally, with increasing the filter order output voltage

waveform is less distorted. However, higher order filters are more “load dependant”,

more expensive and more difficult to construct, and they introduce larger phase shift

and delay in filtered output signal

17

2.9 PSCAD Version 4.1

The modelling of voltage sag generator circuit is performed using a

professional design tool set named PSCAD. PSCAD (Power Systems Computer

Aided Design) is a powerful and flexible graphical user interface to the world-

renowned. PSCAD enables the user to schematically construct a circuit, run a

simulation, analyze the results, and manage the data in a completely integrated,

graphical environment.

Online plotting functions, controls and meters are also included, so that the

user canalter system parameters during a simulation run, and view the results

directly. PSCAD comes complete with a library of pre-programmed and tested

models, ranging from simple passive elements and control functions, to more

complex models, such as electric machines, FACTS devices, transmission lines and

cables. If a particular model does not exist, PSCAD provides the flexibility of

building custom models, either by assembling those graphically using existing

models, or by utilizing an intuitively designed Design Editor.

2.10 Fast Fourier Transform (FFT)

The Fast Fourier Transform (FFT) is the DFT’s computational efficient

implementation, its fast computation is considered as an advantage. The model of

FFT can be obtained in PSCAD software master library. Fourier analysis is used to

convert time domain waveforms into their frequency components and vice versa.

When the waveform is periodical, the Fourier series can be used to calculate the

magnitudes and phases of the fundamental and its harmonic components. The

Fourier series therefore represents the special case of the Fourier Transform applied

to a periodic signal.

FFT can determine the harmonic magnitude and phase of the input signal as a

function of time. The input signals first sampled before they are decomposed into

18

harmonic constituents. Options are provided to use one, two or three inputs. In the

case of three inputs, the component can provide output in the form of sequence

components. In practice data are always available in the form of a sampled time

function, represented by a time series of amplitudes, separated by fixed time intervals

of limited duration [5].

With this tool it is possible to have an estimation of the fundamental

amplitude and its harmonics with reasonable approximation. FFT performs well for

estimation of periodic signals in stationary state; however it does not perform well

for detection of sudden changes in waveform such as transients or voltage sags.

2.11 RMS – Root Mean Square

The root mean square (RMS) voltage or current value is the one which is

applied most broadly in power system monitoring and measurement. A great

advantage of this method is its simplicity, speed of calculation and less requirement

of memory, because RMS can be stored periodically instead of sample per sample.

However, its dependency on window length is considered a disadvantage; one

cycle window length will give better results in terms of profile smoothness than a

half cycle window at the cost of lower time resolution. Moreover RMS does not

distinguish between fundamental frequencies, harmonics or noise components,

therefore the accuracy will depend on the harmonics and noise content. When using

RMS technique phase angle information is lost.

2.12 Summary

In this chapter, the basic concept of voltage sags and voltage sag generator

had been explained. The method of implementing the model of VSG will be

explained in the next chapter.

CHAPTER 3

METHODOLOGY

3.1 Introduction

Based on the review of voltage sag characteristics and voltage sag generator

structures in previous chapter, the construction methods of the model of voltage sag

generator are introduced in this chapter. Modelling of voltage sag generator had been

developed using PSCAD V4.1 software. Both of modelling and simulation will be

then implemented in this software.

3.2 Voltage Sag Generator Structure Implementation

Figure 3.1 shows the structure of voltage sag generator (VSG) that has been

proposed. Based on its operation, AC signal from AC supply is first converted to DC

using three phase full bridge diode rectifier to get a DC link voltage. The DC voltage

is converted back to AC using three phase full bridge inverter and is filtered to get

improved voltage output. Based on timer operation, the Two Input Selector will

select its inputs which are from the filtered voltage and from the signal generator to

get sag voltage signal. The Two Input Selector, timer and signal generator are the

models found in PSCAD master library. The signal generator is represented as

AM/FM/PM Function model in PSCAD.

20

Figure 3.1 VSG operation block diagram

3.2.1 Rectifier

The three phase AC mains supply is modeled as the ideal AC voltage source

(balanced three-phase voltage 230 kV in RMS value, 50 Hz, ideal sine wave). This

means that there is no unbalance, harmonics and other voltage magnitude and

waveform distortions. The uncontrolled three-phase diode rectifier was selected as

optimal solution for the DC voltage supply system.

The obtained DC voltage is then flows through DC link inductance and

capacitance. In the cases when DC voltage is obtained from rectifier, attached DC

link capacitance at the output of rectifier was varied in the range from 500pF to

500000pF [7]. With good and stable inverter driving circuit, output of the VSG is

practically constant for the whole range of investigated frequencies. This means that

there is no need for a large DC link capacitance. In this project, the inductance value

is 1µH while the capacitance value is 1µF. Figure 3.2 shows the construction of

rectifier using PSCAD.

21

Figure 3.2 Construction of rectifier using PSCAD

3.2.2 Inverter and Drive Circuit

The DC/AC inverter is simulated as a full bridge, three-phase inverter made

of six Insulated Gate Bipolar Transistors (IGBTs) controlled by drive circuit. Each

IGBT is installed with the snubber circuit. For the drive circuit, six signal generators

50Hz are used which initial phases are 0°, 180°, 120°, -60°, -120°, 60° respectively.

The generators signals are delayed 1 milliseconds to avoid shoot-through faults i.e.

short circuit across the DC rail. The pulses are then used to drive the IGBTs of the

inverter where DC voltage is converted to three phase AC voltage. Figure 3.3 shows

the construction of inverter using PSCAD.

DC link capacitance

Three phase rectifier

22

Figure 3.3 Construction of inverter using PSCAD

3.2.3 Output Filter

Output filter which is used for filtering is simulated as third order low pass

filter. The filter is put at each phase of the output of the inverter. The resistance,

inductance and capacitance values for this filter are 0.8Ω, 0.004897H, and 2448uF

respectively while loads are valued by 29Ω [7]. Voltage signal for each phase is

taken after filter for measurement and for output channel display. This three phase

voltage signal is simulated as pre sag voltage. Figure 3.4 shows the construction of

output filter using PSCAD.

Inverter drive circuit

Three phase inverter

23

Figure 3.4 Construction of output filter using PSCAD

3.2.4 Two Input Selector with Timer

A model in PSACD software named Two Input Selector is used to select

either input from main circuit or input from sinusoidal waveform generator. The

output of this model will be either the signal from A, or the signal from B, depending

on the value of ‘Ctrl’ function (see Figure 3.6). In this case, the value of ‘Ctrl’ is

controlled by Timer which produces binary pulse. The initial value of the pulse is

zero and the duration for the pulse indicates to HIGH depends on the ‘Duration ON’

of the Timer while the starting point for the pulse to get HIGH is depends on its

‘Delay until ON’ as shown in Figure 3.7. When the pulse is LOW, the selector

selects B as input and A when the pulse is HIGH.

For sinusoidal waveform generator, a model in PSCAD software named

AM/FM/PM Function is used to generate other signal with lower magnitude for

voltage sag. This is because the model can also be used simple as a sinusoidal

waveform generator. One AM/FM/PM Function model is used for each Two Input

Load

Output filter

24

Selector model where the magnitude should be lower than the waveform taken from

main circuit and the frequency is fixed by 50Hz. An output channel is connected to

the output of the Two Input Selector for output waveform display.

The output from each Two Input Selector is observed by peak and RMS value

from its output channel. For RMS value, the Fast Fourier Transform (FFT) model is

used where the output from the Two Input Selector is connected to FFT as its input

and the output of FFT can be observed in magnitude and phase form. Figure 3.5

shows the construction of input selector with timer using PSCAD. The model

obtained is used for one phase only. Three sets of the same model are needed to

obtain a three phase voltage sag waveform except the phase of each AM/FM/PM

Function which is differed by 120°.

Figure 3.5 Construction of input selector with timer using PSCAD

AM/FM/PM Function

FFT

Two input selectorTimer circuit

25

Figure 3.6 Two Input Selector

Figure 3.7 Timer configuration

Control functionInput A and B

26

3.3 Variation of Voltage Sag Parameters.

In order to identify the variation results of the voltage sag, some particular

parameters are controlled or varied. The parameters that are controlled to get desired

output are voltage sag magnitude, duration and phase shift.

For magnitude and phase shift control, the model used to vary the parameters

is the AM/FM/PM Function as shown as Figure 3.5. The control of these parameters

involves all three phase of voltages .The frequency is fixed at 50 Hz, while the

magnitude is varied by 10% to 90% of nominal voltage since the characteristic of the

voltage sag shows that the voltage sag occurs in this range of voltage. The phase shift

of voltage sag is controlled by changing the phase of the each input selector where

each phase is differed by 120°.

For timer control, the voltage sag duration is observed by changing the

‘Duration ON’ configuration of the Timer (see Figure 3.7). The effects of all

parameters change are then recorded graphically in the output channel form.

Figure 3.8 Parameters controlled in simulation of VSG model

‘Duration ON’ of Timer

Magnitude of waveform

Initial phase of waveform

27

3.4 Summary

In this chapter, the method for modelling of voltage sag generator using

PSCAD V4.1 is presented. The value of parameters and the structure of voltage sag

generator circuit are also presented as well as the method on how to control or vary

the voltage sag parameters. Next chapter will explain the results and discussions for

the modelling and simulation process.

CHAPTER 4

RESULT AND DISCUSSION

4.1 Introduction

This chapter will show the results obtained from the modelling and simulation

of the voltage sag generator which was modeled in PSCAD. Based on the parts of the

obtained model of VSG, the waveforms of each part including the input and output

waveforms will be graphically shown in this chapter. The various results for control

of VSG will be discussed as well.

4.2 Model of Voltage Sag Generator

Based on the structure of voltage sag generator obtained by using the

particular method of modelling, a model of voltage sag generator is presented. The

final model of voltage sag generator consists of three-phase AC supply, three-phase

full bridge diode rectifier, three-phase inverter with drive circuit, third order low pas

filter, Two Input Selector, timer with counter, AM/FM/PM Function and Fast Fourier

Transform (FFT). Full model of voltage sag generator using PSCAD is shown in

Appendix A.

29

4.2.1 Three-Phase Full Bridge Diode Rectifier and Inverter

A 240kV RMS AC supply is first rectified to DC voltage to be the input of the

inverter. From the model shown as Figure 4.1, a DC waveform is obtained by using

three phase diode rectifier and the magnitude of the DC waveform obtained after

rectification is about 150V as shown as Figure 4.2. The waveform of DC voltage is

obtained from the output of Eb as shown in Figure 4.1. An ideal DC voltage is

obtained after using the Fast Fourier Transform for output display where the input

signal first sampled before it is decomposed into harmonic constituents. All outputs

are displayed through the output channels as shown in Figure 4.1.

Figure 4.1 Model of rectifier and inverter

Rectifier output

Output channels

Inverter output

30

Figure 4.2 Rectifier output waveform

Figure 4.3 Inverter output waveform

By inverting the DC voltage using three-phase inverter, a three-phase AC

waveform is obtained as shown in Figure 4.3. The waveform is obtained from the

inverter outputs which are va, vb and vc as shown in Figure 4.1. The peak value of

the AC voltage is simulated as 180V but it is not a smooth three-phase AC waveform

because of the operation of the inverter itself since it uses six step operations. This

waveform represents the pre sag voltage or the nominal voltage for the in term of

voltage sags characteristic but the magnitude and the shape of the waveform itself is

needed to be fixed to meet the requirement of 325V of peak voltage. A smooth

waveform can be obtained by using output filter after the operation of inverter.

31

4.2.2 Third Order Low Pass Filter

From the model obtained as shown as Figure 4.4, the previous three phase

AC waveform is filtered to get a smooth three phase sinusoidal waveform. The

filtering operation is performed using a third order low pass filter, connected to each

of the inverter output. The output for display is taken at point va, vb and va for peak

value of the waveform and then each output is connected to FFT to obtained the

RMS value of output waveform. All outputs are displayed through the output

channels.

Figure 4.4 Model of output filter

Output taken for display

Outputfilter

FFTFFT FFT

Output channels

32

(a)

(b)

Figure 4.5 Pre sag voltage waveforms (a) peak value (b) RMS value

From Figure 4.5 (a), the waveform obtained is a three phase sinusoidal

waveform with a magnitude of 325V represents a peak voltage of nominal voltage or

pre sag voltage. Figure 4.5 (b) shows the waveform obtained in RMS value after

using the FFT. The magnitude of this waveform is simulated as 240V. The peak

value of sinusoidal waveform is increased from 180V to 325V after the filtering

operation to meet the output requirement of 240V of RMS voltage for any equipment

testing purpose.

33

4.2.3 Two Input Selector with Timer

A model of input selectors with timers is obtained as shown as Figure 4.6. It

consists of four main models which are AM/FM/PM Functions, timers with counters,

Two Input Selectors and FFTs. A Two Input Selectors is used for each phase to

obtain three phase pre sag and sag voltage. The operation of timer for selector drive

has been discussed in previous chapter. Two input selector will select either input A

or B depends on Timer output pulse as the operation of obtaining voltage sag

waveform. Input B represents nominal voltage or pre sag voltage which is obtained

from va, vb and vc (as shown in Figure 4.4) while input A represents sag voltage

which is obtained from AM/FM/PM Function (as labeled as 1 in Figure 4.6).

\

Figure 4.6 Models of Two Input Selectors with Timers

1

1

1

2

2

2

3

33

LEGEND:

1. AM/FM/PM Function 5. AM/FM/PM Function output2. Timer 6. Two Input Selector output3. Two Input Selector 7. FFT magnitude output4. FFT

4 4 4

5

5

5

6

6

6

777

Phase

Magnitude

Frequency

34

(a)

(b)

Figure 4.7 AM/FM/PM Function waveforms (a) peak value (b) RMS value

Figure 4.7 (a) shows the waveform obtained from AM/FM/PM Function

outputs which is labeled as 5 in Figure 4.6. The three phase waveform is simulated as

162.5V of its peak value which is lower than nominal voltage to make it voltage sag

signal. The magnitude of the waveform is fixed at 50% of the nominal voltage for

clear display of the waveform of the voltage sag that will be obtained at the Two

Input Selector output. For obtaining RMS waveform as shown as Figure 4.7 (b), all

outputs from AM/FM/PM Function are connected to FFTs and the RMS value is

simulated as 115V.

.

35

(a)

(b)

Figure 4.8 Voltage sag waveforms (a) peak value (b) RMS value

Figure 4.8 (a) and (b) show the voltage sag waveforms which obtained by the

operation of the Two Input Selector based on Timer circuit. The waveforms are taken

from the output channels which are labeled as 7 in Figure 4.7. From Figure 4.8 (a),

the pre sag waveform which is 325V of peak voltage drops to 162.5V at the time of

0.25s and lasts within 0.1s before rising again to its original magnitude. The

magnitude of sag voltage is fixed at 50% of its nominal voltage while the sag

duration is fixed at 0.1 seconds for clear display.

36

4.3 Simulation of Voltage Sag Parameters

There are three parameters that can be controlled or varied to get desired

waveform of voltage sag which are magnitude, duration and phase shift. This desired

waveform is then be used in the future work such as for electrical equipment test

depends on level of response of those equipment to such voltage disturbance.

4.3.1 Sag Magnitude

For magnitude control, several changes of voltage magnitude are made to get

the variation of voltage sag magnitude. The magnitude of sag voltage is controlled by

changing the magnitude of AM/FM/PM Function. The magnitude of input selector is

controlled depends on the desired output.

In this case, only three variations of voltage sag magnitude are made to get the

different between each magnitude which is 10%, 50% and 90% of nominal voltage

while the duration of sag is fixed at 0.1 seconds. The value of nominal voltage is

325V (peak) and 230V (RMS). The results are observed in peak voltage and RMS

voltage of the sag magnitude as shown as Table 4.1 and the waveforms of voltage sag

are recorded graphically as shown in Figure 4.9 until Figure 4.14.

37

Figure 4.9 10% voltage sag waveform (peak)

Figure 4.10 10% voltage sag waveform (RMS)

Figure 4.9 and Figure 4.10 show the waveform obtained from the output of

Two Input Selector as labeled as 6 in Figure 4.6 (page 33). From the both

waveforms, the sag voltages are simulated as 10% of its nominal voltage. The sag

voltage is obtained by changing the magnitude of AM/FM/PM Function as shown in

Figure 3.8 (page 26) to 32.5V. The duration of voltage sag is fixed at 0.1s as only

magnitude is varied. From Figure 4.10, the RMS voltage drops extremely from 240V

to 23V at 0.25s until 0.35s before rising to its original magnitude. The 10% voltage

sag is defined to be a bad voltage drop in a power system application.

38




Two Input Selector as labeled as 6 in Figure 4.6 (page 33). From the both



Figure 3.8 (page 26) to 162.5V. From Figure 4.12, the RMS voltage drops from

240V to 115V at 0.25s until 0.35s before rising to its original magnitude. The range

of 50% voltage sag is a common voltage drop that happens in power system

application.

39




Two Input Selector as labeled as 6 in Figure 4.6 (page 33).. From the both



Figure 3.8 (page 26) to 292.5V. From Figure 4.14, the RMS voltage drops slightly

from 240V to 207V at 0.25s until 0.35s before rising to its original magnitude.

40

Table 4.1 Peak and RMS value of voltage sag

Voltage sag magnitude

(%)

Sag Peak Voltage

(V)

Sag RMS Voltage

(V)

10 32.5 23

50 162.5 115

90 292.5 207

Results in Table 4.1 show that there are three voltage sag magnitudes

variation obtained. The exact value of desired sag magnitudes can easily obtained by

changing the magnitude of AM/FM/PM Function depends on the percentage of

voltage sag which is wanted to be used in other purpose such as electrical and

electronic equipment sensitivity test.

4.3.2 Sag Duration

Voltage sag commonly occurs between half of cycle or 0.01 seconds and one

minute. For this simulation, the magnitude of voltage sag is fixed at 50 percent of its

nominal magnitude because only duration of sag is observed. Only three variations of

voltage sag duration are made to get the different between each variation which is

0.02 seconds, 0.1 seconds and 0.15 seconds. All the waveforms of voltage sag in term

of duration are recorded graphically as shown in Figure 4.15 until Figure 4.20.

From the figures shown, there are three variations voltage sag duration that

are obtained. The exact value of desired sag duration can easily obtained by changing

the ‘Duration ON’ configuration of the Timer as shown in Figure 3.7 (page 25).

41

Figure 4.15 Sag duration waveform (0.02s) (peak)

Figure 4.16 Sag duration waveform (0.02s) (RMS)

Figure 4.15 and Figure 4.16 show the waveform of voltage sag obtained from

the output of Two Input Selector. The duration of the voltage sag is obtained by

changing the ‘Duration ON’ of the Timer as shown in Figure 3.7 (page 25) in

previous chapter. In this case, the ‘Duration ON’ of the Timer is set to be 0.02s.

From Figure 4.16, the RMS voltage drops from 240V to 115V at 0.25s until 0.27s

before rising to its original magnitude. This is a short and minimum duration of

voltage sag that can occur in power system application.

42





changing the ‘Duration ON’ of the Timer to 0.1s. From Figure 4.18, the RMS

voltage drops from 240V to 115V at 0.25s until 0.35s before rising to its original

magnitude.

43





changing the ‘Duration ON’ of the Timer to 0.15s. From Figure 4.20, the RMS

voltage drops from 240V to 115V at 0.25s until 0.4s before rising to its original

magnitude. The longer duration of voltage sag can be obtained but it must not longer

than one minute because of the definition of voltage sag itself which occur between

0.5 cycles and one minute.

44

4.3.3 Phase Shift

Voltage sag must have its phase shift. Phase shift of voltage sag is defined as

the phase difference between nominal pre sag waveform and phase sag waveform.

The phase shift of voltage sag can be controlled by changing the initial phase of

AM/FM/PM Function as shown in Figure 3.8 (page 26). In this simulation, the phase

for each AM/FM/PM Function are valued by -160° for phase A, -40° for phase B and

80° for phase C. Figure 4.21 until Figure 4.23 show the phase shift waveforms for

each phase. The phase shift of the voltage sag is recorded in Table 4.2.

Figure 4.21 Phase shift waveform (Phase A)

Figure 4.22 Phase shift waveform (Phase B)

45

Figure 4.23 Phase shift waveform (Phase C)

Table 4.2 Voltage sag phase shift

Phase Pre Sag Phase

(Degree)

Sag Phase

(Degree)

Phase Shift

(Degree)

A -115.0 -160.0 45.0

B 4.9 -40.0 44.9

C 124.9 80.0 44.9

The result in Table 4.2 shows that the phase for each phase is about 45°. To

get higher value of phase shift, the initial phase (pre sag phase) of each AM/FM/PM

Function must be increased and vice versa as long as each phase is differed by 120°.

46

4.4 Summary

This chapter has presented the result of all modelling and simulation for

voltage sag generator including the results for variation of voltage sag parameters. All

of the modelling and simulation are done by using PSCAD V4.1 software. The

voltage sag parameters can easily controlled by changing the value of particular

parameters of the models in voltage sag generator. The next chapter will conclude the

overall project outcome and come out with several recommendations for future work

and research.

CHAPTER 5

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

As one of the concerns in power quality, voltage sag brings lots of troubles to

the performance of electrical equipment. The VSG is widely used to evaluate

equipment susceptibility to voltage sag. In the current market, there are some VSG

products which are successful but are very expensive. An amplifier type of VSG is

presented in this project, and it is easy to build and simulated.

The purpose of this project is to develop a model of a voltage sag generator

using PSCAD software which to be applied in the power system design and can be

used for electrical equipment test to identity its responses to such voltage

disturbance. Computer simulations illustrating design and operation of three-phase

voltage sag generator are presented in this project.

The scheme consisting of an uncontrolled 3 phase diode rectifier, a full bridge

3 phase inverter, a third order low pass filter output filter, Fast Fourier Transform

(FFT), inputs selector and timer have been proposed. The use of inputs selector with

timer drive component is found to be significant because it is the main part for

producing voltage sag signal which is by selecting either main voltage signal or

voltage generator signal with lower magnitude.

The influence of various design parameters, and diverse control strategies are

investigated and main results are graphically illustrated. The software capability and

48

the method are further demonstrated by identifying relevant voltage sag

characteristics from the recorded voltage waveforms.

The operation results showed that the designed VSG is capable of controlling

all required sag parameters effectively includes the magnitude and duration of

nominal voltage and sag voltage, and the starting point and ending point and phase

shift of the sag voltage.

5.2 Recommendation

For future work and research, there are several recommendations that can be

carried out by using this project as a platform. The recommendations are as below:

1. Another tool sets for modelling and simulation may be used to develop this

type of voltage sag generator such as MATLAB, P-Spice and Multism

soffware.

2. With appropriate control of its parameters i.e. magnitude and duration, this

voltage sag generator can also work as voltage swell generator and voltage

interruption generator.

3. This voltage sag generator parameters also can be controlled by using PWM

technique where the inverter will be drive by using PWM drive circuit. The

magnitude and duration of voltage sag can be controlled by changing the

frequency and magnitude of sinusoidal and triangular signal of PWM.

REFERENCES

1. Yan Ma. Karady, G.G. A Single Phase Voltage Sag Generator for Testing

Electrical Equipments. Transmission and Distribution Conference and

Exposition. April 21-24, 2008. Department of Electrical. Engineering, Arizona

State University: IEEE. 2008. 1-5.

2. Felce, A. Matas, G and Da Silva, Y. Voltage Sag Analysis and Solution for

Industrial Plant with Embedded Induction Motors. Industry Applications

Conference. October 3-7, 2004. Caracas, Venezuela: IEEE. 2004. Vol. 4. 2573-

2578.

3. Dong-Myung Lee. A Voltage Sag Supporter Utilizing a PWM-Switched

Autotransformer. Ph. D. Thesis. School of Electrical & Computer Engineering,

Georgia Institute of Technology, Atlanta; 2004.

4. Dong-Jun Won, Seon-Ju Ahn, Yop Chung, Joong-Moon Kim, and Seung-Moon.

A New Definition of Voltage Sag Duration Considering The Voltage Tolerance

Curve. IEEE Bologna PowerTech Conference. June 23-26, 2003. Bologpa, Italy:

IEEE. 2003. Vol. 3. 5 pp.

5. Readlay Makaliki. Voltage Sag Source Location in Power Systems. Master

Thesis. Institutionen för Energi och Miljö, Chalmers Tekniska Högskola,

Göteborg, Sweden; 2006.

6. Djokic, S. Z. and Milanovic, J. V. Advanced Voltage Sag characterisation. Part I:

Phase Shift. Generation, Transmission and Distribution, IEE Proceedings. July

50

7. 13, 2006. Institution of Engineering and Technology UK: IEEE. 2006. Vol. 153.

423-425.

8. Djokic, S. Z. and Milanovic, J. V. and Charalambous, K. A. Computer

Simulation of Voltage Sag Generator. 10th International Conference on

Harmonics and Quality of Power. 6-9 October, 2002. Institution of Engineering

and Technology UK: IEEE. 2002. Vol. 2. 649-654.

51

APPENDIX A

MODEL OF VOLTAGE SAG GENERATOR

Rec

tifie

rT

hree

Pha

se In

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Inve

rter

dri

ve c

ircu

it

FFT

FFT

FFT

FFT

Tim

er

AM

/FM

/PM

Fun

ctio

nT

wo

Inpu

t Sel

ecto

r

Out

put G

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