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U NIVERSITI TEKNOTOGI MALAYSIA
DECI.ARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT
Author's full nome
Dote of birfh
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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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.
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"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"
Signature
Supervisor's Name
Date
MODELLING OF VOLTAGE SAG GENERATOR
MOHAMMAD AZRUL BIN ARIS
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
' \ " "
MOHAMMAD AZRUL BIN ARIS
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.
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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
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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
4.11 50% voltage sag waveform (peak) 38
4.12 50% voltage sag waveform (RMS) 38
4.13 90% voltage sag waveform (peak) 39
4.14 90% voltage sag waveform (RMS) 39
4.15 Sag duration waveform (0.02s) (peak) 41
4.16 Sag duration waveform (0.02s) (RMS) 41
4.17 Sag duration waveform (0.1s) (peak) 42
4.18 Sag duration waveform (0.1s) (RMS) 42
4.19 Sag duration waveform (0.15s) (peak) 43
4.20 Sag duration waveform (0.15s) (RMS) 43
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
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
Figure 4.11 50% voltage sag waveform (peak)
Figure 4.12 50% voltage sag waveform (RMS)
Figure 4.11 and Figure 4.12 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 50% 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 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
Figure 4.13 90% voltage sag waveform (peak)
Figure 4.14 90% voltage sag waveform (RMS)
Figure 4.13 and Figure 4.14 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 90% 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 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
Figure 4.17 Sag duration waveform (0.1s) (peak)
Figure 4.18 Sag duration waveform (0.1s) (RMS)
Figure 4.17 and Figure 4.18 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 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
Figure 4.19 Sag duration waveform (0.15s) (peak)
Figure 4.20 Sag duration waveform (0.15s) (RMS)
Figure 4.19 and Figure 4.20 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 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
vert
erFi
lter
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
raph