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STUDY OF UPQC FOR POWER
QUALITY IMPROVEMENT
Thesis Submitted in the partial fulfillment of the requirements for the degree of
MASTER OF ELECTRICAL ENGINEERING
MIHIR HEMBRAM
Examination Roll Number: M4ELE14-01
Registration No: 97117of 2006-07
Under the Guidance of
Mr. Ayan Kumar Tudu
Electrical Engineering Department
Faculty of Engineering & Technology
JADAVPUR UNIVERSITY
KOLKATA- 700032
2014
Faculty of Engineering & Technology
JADAVPUR UNIVERSITY
KOLKATA- 700032
Certificate of Recommendation
This is to certify that Shri. Mihir Hembram has completed his project work entitled “Study of
UPQC for Power Quality Improvement” under my direct supervision and guidance. This
thesis is submitted in partial fulfilment of the requirements for the award of degree of Master
of Electrical Engineering during the academic year 2012-2014.
Mr. Ayan Kumar Tudu
Assistant Professor
Electrical Engineering Department
Jadavpur University, Kolkata – 700032
Forwarded by:
Prof.(Dr.) Samar Bhattacharya
Prof. Sivaji Bandyopadhyay
Head
Dean
Electrical Engineering Department
Faculty of Engineering & Technology
Jadavpur University, Kolkata – 700032
Jadavpur University, Kolkata – 700032
i
Faculty of Engineering & Technology
JADAVPUR UNIVERSITY
KOLKATA- 700032
Certificate of Approval *
The foregoing thesis is hereby approved as a creditable study of Master of
Electrical Engineering and presented in a manner satisfactory to warrant its
acceptance as a prerequisite to the degree for which it has been submitted. It is understood
that by this approval the undersigned do not necessarily endorse or approve any statement
made, opinion expressed or conclusion therein but approve this thesis only for the
purpose for which it is submitted.
Final Examination for
Evaluation of the Thesis
Signature of Examiners
* Only in case the thesis is approved
ii
Declaration of Originality and Compliance of Academic Ethics
I hereby declare that this thesis contains literature survey and original research work by
the undersigned candidate, as part of Master of Electrical Engineering studies.
All information in this
document
have been obtained
and
presented in
accordance with academic rules and ethical conduct.
I also declare that, as required by these rules and conduct, I have fully cited and
referenced all material and results that are not original to this work.
Name
:
MIHIR HEMBRAM
Exam Roll Number
: M4ELE14-01
Thesis Title
: STUDY OF UPQC FOR POWER
QUALITY IMPROVEMENT
Signature with Date :
iii
ACKNOWLEDGEMENTS
It is a pleasant task to express my gratitude to all those people who have
accompanied and helped me in my project work.
First and foremost, I really take this opportunity to express my deep sense of gratitude
to my guide, Mr. Ayan Kumar Tudu, Assistant Professor, Department of Electrical
Engineering, Jadavpur University, Kolkata, for his invaluable guidance, suggestions and
encouragement throughout the project, which helped me a lot to improve this project work. It
has been very nice to be under his guidance. His appreciation during the good times has been
boosting my morals and confidence.
I am also indebted to Prof. Samar Bhattacharyya, Head, Department of Electrical
Engineering, Jadavpur University, for his kind help during this thesis work. And I am also
thankful to Prof. Sivaji Bandyopadhyay, Dean of Faculty of Engineering and Technology for
his kind help and co-operation during this thesis work.
I am also indebted to my parents who encouraged me to give my best effort during
this thesis work.
Last, but not the least, I would like to thank my batch-mates and seniors, who have
directly or indirectly helped me in this work.
Mihir Hembram
Electrical Engineering Department
Jadavpur University
Kolkata-700032
May,2014
iv
CONTENTS
Title
Page. No.
Certificate of Recommendation
i
Certificate of Approval
ii
Declaration of Originality and Compliance of Academic Ethics
iii
Acknowledgements
iv
Contents
v
List of Figures
vii
List of Abbreviation
ix
List of Symbols
x
CHAPTER-1: INTRODUCTION
1-6
1.1 Overview
1
1.2 Literature Survey
2-5
1.3 Objective of Work
5
1.4 Organization of Thesis
6
CHAPTER-2: POWER QUALITY
7-13
2.1 Introduction
7
2.2 Major Power Quality Problems
7-12
2.2.1 Short Duration Voltage Variation
7-8
2.2.2 Long Duration Voltage Variation
9
2.2.3 Transients
9-10
2.2.4 Voltage Fluctuations
10
2.2.5 Voltage Unbalance
11
2.2.6 Waveform Distortion
11-12
2.3 Solution to Power Quality Problems
12-13
CHAPTER-3: UNIFIED POWER QUALITY CONDITIONER
3.1 Introduction
14-30
14
3.2 Basic Configuration of UPQC
14-15
3.3 Operation of the UPQC
16-17
3.4 UPQC Configurations
18
3.5 Steady-State Power Flow Analysis of UPQC
18-24
3.6 Control Scheme for Series Active Power Filter
24-25
v
3.7 Control Scheme for Shunt Active Power Filter
25-28
3.8 Hysteresis Controller
29-30
3.8.1 Hysteresis Voltage Controller
29
3.8.2 Hysteresis Current Controller
29-30
3.9 Conclusion
30
CHAPTER-4: RESULTS AND DISCUSSION
4.1 Introduction
31-46
31
4.2 Simulation Results of Shunt APF
31-34
4.3 Simulation Results of Series APF
34-38
4.4 Simulation Results of UPQC
38-45
I. Current Harmonic Compensation
39-40
II. Voltage and Current Harmonic Compensation
40-42
III. Voltage Sag and Current Harmonic Compensation
42-43
IV. Voltage Swell and Current Harmonic Compensation
43-44
V. Single Phase Voltage Sag and Current Harmonic Compensation 45
4.5 Conclusion
46
CHAPTER-5: CONCLUSION AND FUTURE WORK
47-48
5.1 Conclusion
47
5.2 Future Work
48
REFERENCES
49-51
APPENDIX
51
vi
LIST OF FIGURES
Fig. No.
Title
Page No.
2.1
Most encountered power system problems
8
2.2
Impulsive transient
9
2.3
Oscillatory transient
10
2.4
Voltage fluctuation or flicker
10
2.5
Notching
12
3.1
Detailed configuration of UPQC
15
3.2
Right shunt UPQC
18
3.3
Left shunt UPQC
18
3.4
Equivalent circuit of a UPQC
19
3.5 (a) – (b)
Reactive power Flow
20
3.6
Active power flow during voltage sag condition
20
3.7
Active power flow during voltage swell condition
21
3.8
Active power flow during normal working condition
22
3.9 (a) – (d)
Phasor representation of all possible conditions
23
3.10
Control scheme of series APF
24
3.11
Control block diagram of shunt APF
27
3.12
Voltage harmonics filtering block diagram
28
3.13
Simplified model for fixed hysteresis-band voltage control
29
3.14
Simulink model of the hysteresis voltage control
29
3.15
Simulink model of hysteresis current control
30
4.1
Simulink model of shunt APF
32
4.2
Simulink model of control scheme for shunt APF
32
4.3
Three phase load voltage
32
4.4
Load current
33
4.5
Source current before and after compensation
33
4.6
Compensation current
33
4.7
Capacitor voltage
33
4.8
Load current THD
34
4.9
Source current THD (with shunt APF)
34
4.10
Simulink model of series APF
35
vii
LIST OF FIGURES
4.11
Simulink model of control scheme for series APF
35
4.12
Source voltage
36
4.13
Load voltage
36
4.14
Injected voltage
36
4.15
Distorted source voltage
37
4.16
Load voltage before and after compensation
37
4.17
Injected voltage
37
4.18
Source voltage THD
37
4.19
Load voltage THD (with Series APF)
38
4.20
Simulink model of UPQC
38
4.21
Simulated results of UPQC
39
4.22
THD
40
4.23
Simulated results of UPQC
41
4.24
THD
42
4.25
Simulated results of UPQC
43
4.26
Simulated results of UPQC
44
4.27
Simulated results of UPQC
45
viii
LIST OF ABBREVIATIONS
APF
Active Power Filter
ASD
Adjustable Speed Drive
DFT
Discrete Fourier Transform
DSTATCOM
Distribution Static Compensator
DVR
Dynamic Voltage Restorer
FACTS
Flexible AC Transmission System
HB
Hysteresis Band
IEEE
Institute of Electrical and Electronics Engineers
LPF
Low Pass Filter
PCC
Point of Common Coupling
PE
Power Electronics
PLC
Programmable Logic Controller
PLL
Phase Locked Loop
PQ
Power Quality
PWM
Pulse Width Modulation
SMPS
Switch Mode Power Supply
SPLL
Software phase Locked Loop
SSC
Static Series Compensator
STATCOM
Static Compensator
STS
State Transfer Switch
SVC
Static VAR Compensator
THD
Total Harmonic Distortion
TSC
Thysristor Switched Capacitor
TSR
Thyristor Switched Reactor
UPQC
Unified Power Quality Conditioner
UVT
Unit Vector Template
VSI
Voltage Source Inverter
ix
LIST OF SYMBOLS
A
Ampere
C
DC link capacitor
,
,
Load current components in 0
,
,
coordinate
Compensating reference currents in
,
coordinate
Compensating reference currents for phase-a, phase-b, phase-c
iC
Current injected by shunt inverter
iC *
Reference compensating current
if
Shunt inverter current
iL
Load current
iS
Source current
Hz
Hertz
L
Inductance
LS
Source inductance
F
Micro farad
mH
Milli henry
min
Minute
PL
Active power demand of load
PS
Active power supplied by source
PSh
Active power handled by shunt inverter
PSr
Active power handled by series inverter
pu
Per unit
QL
Reactive power demand of load
QS
Reactive power supplied by source
QSh
Reactive power handled by shunt inverter
RS
Source resistance
sec
Second
x
LIST OF SYMBOLS
V
Volt
,
,
Source voltage components in 0
coordinate
Voltage injected by series APF
,
Source voltage components in d-q coordinate
Reference compensating voltage
,
,
Vdc
Sensed APF output voltage for phase-a, phase-b, phase-c
DC link voltage
Series inverter output voltage
Load voltage
,
,
Reference load voltages for phase-a, phase-b, phase-c
Desired peak value of PCC phase voltage
Source voltage
,
,
Source voltages for phase-a, phase-b, phase-c
Terminal voltage at PCC load
ZL
Load impedance
xi
CHAPTER 1
INTRODUCTION
1.1 Overview
Power Quality (PQ) has become an important issue to electricity consumers at all levels of
usage. The PQ issue is defined as “Any power problem manifested in voltage, current, or
frequency deviations that results in failure or misoperation of customer equipment.” The
development of power electronic based equipment has a significant impact on quality of
electric power supply. The switch mode power supplies (SMPS), dimmers, current regulator,
frequency converters, low power consumption lamps, arc welding machines, etc, are some
out of the many vast applications of power electronics based devices. The operation of these
loads/equipments generates harmonics and thus, pollutes the modern distribution system. The
growing interest in the utilization of renewable energy resources for electric power generation
is making the electric power distribution network more susceptible to power quality
problems. In such conditions both electric utilities and end users of electric power are
increasingly concerned about the quality of electric power.
Many efforts have been taken by utilities to fulfill consumer requirement, some consumers
require a higher level of power quality than the level provided by modern electric networks.
This implies that some measures must be taken so that higher levels of Power Quality can be
obtained.
Active power filters (APF) have been proposed as efficient tools for power quality
improvement. Active power filters can be classified as series or shunt according to
their
system configuration. The series APF generally takes care of the voltage based
distortions, while shunt APF mitigates current based distortions. The combination of series
and shunt active power filter is called the unified power-quality compensator (UPQC).
UPQC mitigates the voltage and current based distortion simultaneously as well as
independently. In this thesis the main focus is on UPQC.
1
1.2 Literature Survey
Today power quality [1-3] has become the most important factor for both power suppliers
and customers due to the deregulation of the electric power energy market. Efforts are being
made to improve the power quality. The acceptable values of harmonic contamination are
specified in IEEE standard in terms of total harmonic distortion [4]. The concept of custom
power was introduced by N.G.Hingorani [5]. Power electronic valves are the basis of those
custom power devices such as the state transfer switch (STS), active filters and converterbased devices [6]. The active filter technology is now mature for providing compensation for
harmonics, reactive power, and/or neutral current in ac networks. AF’s are also used to
eliminate voltage harmonics, regulate terminal voltage, suppress voltage flicker, and improve
voltage balance in three-phase systems. This wide range of objectives is achieved either
individually or in combination, depending upon the requirements and control strategy and
configuration which have to be selected appropriately [7]
Akagi et al. [8] proposed reactive power control theory for three-phase systems with or
without neutral wire, and it is valid for both steady state and transients. In [9] the authors also
explained the physical meaning of instantaneous reactive power and proposed instantaneous
reactive power compensator comprising switching device without energy storage was
proposed.
Edson H. Watanabe et al. [10] presented a detailed analysis of the instantaneous reactive
power theory in systems with non-linear loads.
Changjiang Zhan et al. [11] presented the analytical and practical design issues of a
software phase-locked loop (SPLL) for dynamic voltage restorer (DVR). A SPLL model that
uses a lag/lead loop controller is derived in order to analyse the system performance and
filtering characteristic by the use of bode diagrams and root-locus methods. The practical
aspect of the SPLL implementation has also been discussed. The operating principle and
design considerations of a SPLL under practical condition such as: voltage unbalance,
voltage harmonics, frequency change, phase jumps and sampling delay have been analysed
and discussed.
S. R. Naidu et al. [12] described a software phase-locked loop (SPLL) for custom power
devices. The phase angle and frequency of the estimated positive sequence component are
tracked. Performance of a dynamic voltage restorer incorporating the proposed phase locked
loop (PLL) has been presented under unbalanced and distorted utility conditions.
Juan W. Dixon et al. [13] presented a series active power filter working as a sinusoidal
current source, which is in phase with the mains voltage. The amplitude of the fundamental
2
current in the series filter is controlled with the help of error signal generated between the
load voltage and a pre established reference. The control provides the effective correction of
power factor, harmonic distortion, and load voltage regulation.
Chellali Benachaiba et al. [14] described DVR principles and voltage restoration methods at
the point of common coupling (PCC) and analysed different voltage injection method.
F.A.L Jowder [15] proposed a Dynamic voltage controller with hysteresis controller. The
work presented discrete Fourier transform (DFT) scheme for sag/swell detection. In [16] the
author also proposed a Dynamic voltage restorer (DVR) with capability of compensating
voltage harmonic and deep sag. The work presented DFT scheme for harmonic detection and
sag/swell detection technique with PLL. Then for compensating this non-ideal part of
voltage, hysteresis voltage controller was used for generating gate pulses to inverter and
voltage source inverter parameters calculation methods have been shown.
Tarek I. El-Shennawy et al. [17] proposed a simple DVR, which utilized the classical
Fourier Transform for sag detection and quantification. A controller based on feed-forward
technique utilized the error signal (difference between the reference voltage and actual
measured voltage) to trigger the switches of an inverter using a Pulse Width Modulation
(PWM) scheme. The proposed DVR utilized energy from other available feeder or from an
energy storage unit through a rectifier.
Bhim Singh et al. [18] suggested a simple control algorithm for the dynamic voltage restorer
(DVR) to mitigate the power quality problems in terminal voltage. Two PI (proportionalintegral) controllers are used each to regulate the dc bus voltage of DVR and the load
terminal voltage respectively. The fundamental component of the terminal voltage is
extracted using the synchronous reference frame theory. The control signal for the series
connected DVR is obtained indirectly from the extracted reference load terminal voltage.
M. A. Chaudhari et al. [19] presented a simplified control algorithm of a three-phase Series
Active Power Filter as Power Quality Conditioner. SAPF compensates supply voltage
unbalance and harmonics in such a way that they do not reach the load end resulting in low
THD at the load voltage. The series APF is realized using a three phase, three leg voltage
source inverter (VSI).
Shazly A. Mohammed et al. [20] describes the DVR, its functions, configurations,
components, operating modes, voltage injection methods and closed-loop control of the DVR
output voltage along with the device capabilities and limitations.
A. Banerji et al. [21] discusses the various control algorithms of DSTATCOM. The manner
in which the power quality issues are mitigated by the converter are also explored.
3
Mehmet Ucar et al. [22] proposed instantaneous reactive power theory, also known as p–q
theory based on which a new control algorithm is proposed for 3-phase 4-wire and 4-leg
shunt active power filter (APF) to suppress harmonic currents, compensate reactive power
and neutral line current and balance the load currents under unbalanced non-linear load and
non-ideal mains voltage conditions.
V.Khadkikar et al. [23] proposed a control technique for Unified Power Quality Conditioner
(UPQC), which is a combination of series APF and shunt APF. A control strategy based on
unit vector template generation is discussed in this paper with the focus on the mitigation of
voltage harmonics present in the utility voltage. In [24] the authors present the steady state
analysis of unified power quality conditioner (UPQC). The mathematical analysis is based on
active and reactive power flow through the shunt and series APF, wherein series APF can
absorb or deliver the active power whereas the reactive power requirement is totally handle
by shunt APF alone during all conditions. The derived relationship between source current
and percentage of sag/swell variation shows shunt APF playing an important role in
maintaining the overall power balance in the entire network. The digital simulation is carried
out to verify the analysis done. . This analysis can be very useful for selection of device
ratings for both shunt and series APFs.
Yash Pal et al. [25] presents a control strategy for a three-phase four-wire Unified Power
Quality (UPQC) for an improvement of different power quality (PQ) problems. The UPQC is
realized by integration of series and shunt active power filters (APFs) sharing a common dc
bus capacitor. The shunt APF is realized using a three-phase, four leg voltage source inverter
(VSI) and the series APF is realized using a three-phase, three legs VSI. A control technique
based on unit vector template technique is used to get the reference signals for series APF,
while Ic
implemented control algorithm is evaluated in terms of power-factor correction; load
balancing, neutral source current mitigation and mitigation of voltage and current harmonics,
voltage sag and swell and voltage dips in a three-phase four-wire distribution system for
different combination of linear and non-linear loads. In this control scheme, the
current/voltage control is applied over the fundamental supply currents/voltages instead of
fast changing APFs currents/voltages, thereby reducing the computational delay and the
required sensors.
Metin Kesler et al. [26] suggested a new control method to compensate the power quality
problems through a three-phase unified power quality conditioner (UPQC) under non-ideal
mains voltage and unbalanced load conditions. The performance of proposed control system
4
was analyzed. The proposed UPQC system can improve the power quality at the point of
common coupling (PCC) on power distribution system under non-ideal mains voltage and
unbalanced load conditions.
Sai Shankar et al. [27] modelled a UPQC for both active and reactive power compensation
using different control strategies. The behaviour of UPQC has been analyzed with sudden
switching of R-L loads, and R-C loads as well as occurrences of different shunt faults.
The control scheme has been devised using PI controller in UPQC for real and reactive power
control, and operation in case of switching and faults in transmission systems.
1.3 Objective of Work
This dissertation proposes the MATLAB/SIMULINK model of unified power quality
conditioner
which is used for the improvement of power quality at distribution level. The
major objectives are summarized as follows:
Study the model of UPQC
Investigating the performance of Unified Power Quality Conditioner (UPQC) using
the hysteresis control for Non-ideal Voltage and non-linear load.
5
1.4 Organization of Thesis
This thesis is compiled in five chapters as per the details given below:
The Chapter 1 highlights the brief introduction, summary of work carried out by various
researchers. The scope of the work is also identified and the outline of the thesis is also given
in this chapter.
The Chapter 2 explains the power quality and different kinds of power quality problems and
the various solutions that can be implemented to improve the quality of power in distribution
networks.
The Chapter 3 discusses the UPQC basics and steady state power flow analysis in detail and
outlines the control technique used in the simulation of Unified power quality conditioner.
The Chapter 4 presents the results for various operating conditions.
The Chapter 5 Conclusions and the scope of further work are presented.
6
CHAPTER 2
POWER QUALITY
2.1 Introduction
The widespread use of power electronic (PE) equipments, such as adjustable speed drives
(ASD), programmable logic controllers (PLC), energy-efficient lighting, led to a complete
change of nature of electric loads. These loads are simultaneously major cause of power
quality problems and the major victims of that. Due to their non-linearity, all these loads
cause disturbances in the voltage waveform. The Power Quality (PQ) problem can be
detected from one of the following several symptoms depending on the types of issue
involved.
Lamp flicker
Frequent blackouts
Sensitive-equipment frequent dropouts
Voltage to ground in unexpected
Locations
Communications interference
Overheated elements and equipment.
Sensitivity of each PE equipment to various PQ problems depends on the type of both the
equipment and the disturbance. Furthermore, the effect of PQ on electric power systems, due
to the presence of PE equipments, depends on the type of PE equipment utilized. The
maximum acceptable values of harmonic contamination are specified in IEEE519-1992
standard in terms of total harmonic distortion. [4]
2.2 Major Power Quality Problems
2.2.1 Short Duration Voltage Variation
Depending on the fault location and the system conditions, the fault can cause either
temporary voltage drops (sags), voltage rises (swells), or a complete loss of voltage
(interruptions). The duration of short voltage variations is less than 1min. These variations
are caused by fault conditions, the energization of large loads which require high starting
currents, or intermittent loose connections in power wiring [1].
7
(i) Voltage Sag: voltage sag (also called a “dip”) is a brief decrease in the rms line voltage of
10 to 90 percent of the nominal line-voltage. The duration of a sag is 0.5 cycle to 1 min.
Common sources that contribute to voltage sags are the starting of large induction motors and
utility faults.
(ii) Voltage Swell: A swell is a brief increase in the rms line-voltage of 1.1 to 1.8 percent of
the nominal line-voltage for duration of 0.5 cycle to 1 min. Swells can be caused by
switching off a large load or energizing a large capacitor bank.
(iii) Interruption: An interruption is defined as a reduction in line-voltage or current to less
than 0.1 pu of the nominal, for a period of time not exceeding 1 min. Interruptions can occur
due to power system faults, equipment failures and control malfunctions.
Fig. 2.1: Most encountered power system problems (a) Voltage swells. (b) Voltage sags.
(c) Voltage interruption. (d) Frequency variation. (e) Voltage unbalance. (f) Harmonics.
8
2.2.2 Long-Duration Voltage Variation
Long-duration variations can be categorized as over voltages, under voltages or sustained
interruptions.
(i) Overvoltage: An overvoltage is an increase in the rms ac voltage greater than 110 percent
at the power frequency for duration longer than 1 min. Over voltages are usually the results
of load switching or incorrect tap settings on transformers.
(ii) Under Voltage: An under voltage is decreases in the rms ac voltage to less than 90
percent at the power frequency for duration longer than 1 min. A load switching on or a
capacitor bank switching off can cause an under voltage until voltage regulation equipment
on the system can restore the voltage back to within tolerance limits. Also overloaded circuits
can result in under voltage.
(iii) Sustained Interruptions: When the supply voltage has been zero for a period of time in
excess of 1 min the long-duration voltage variation is considered a sustained interruption.
2.2.3 Transients
(i) Impulsive Transient: An impulsive transient is a brief, unidirectional variation in voltage,
current, or both on a power line. Lightning strikes, switching of inductive loads, or switching
in the power distribution system are the most common causes of impulsive transients. The
effects of transients can be mitigated by the use of transient voltage suppressors such as zener
diode.
Time
Fig. 2.2: Impulsive transient
9
(ii) Oscillatory Transient: An oscillatory transient is a brief, bidirectional variation in
voltage, current, or both on a power line. These are caused due to the switching of power
factor correction capacitors.
Time
Fig. 2.3: Oscillatory transient
2.2.4 Voltage Fluctuations
Voltage fluctuations are relatively small (less than 5 percent) variations in the rms line
voltage. Cycloconverters, arc furnaces, and other systems that draw current not in
synchronization with the line frequency are the main contributors of these variations.
Fig. 2.4: Voltage fluctuation or flicker
10
2.2.5 Voltage Unbalance
A voltage unbalance is a variation in the amplitudes of three-phase voltages, relative to one
another. Voltage imbalance can be the result of different loads on the phases, resulting in
different voltage drops through the phase-line impedances.
2.2.6 Waveform Distortion
Waveform distortion is defined as a steady-state deviation from an ideal power frequency
sine wave principally characterized by the spectral content of the deviation.
There are five primary types of waveform distortion:
(i) DC offset: The presence of a dc voltage or current in an ac power system is termed dc
offset. This can occur as the result of a geomagnetic disturbance or asymmetry of electronic
power converters
(ii) Harmonics: Harmonics are sinusoidal voltages or currents having frequencies that are
integer multiples of the frequency at which the supply system is designed to operate, and that
is known as the fundamental frequency which is usually 50 or 60 Hz.
The harmonic distortion originating in the nonlinear Harmonic distortion levels can be
described by the calculating total harmonic distortion (THD) which measures the complete
harmonic spectrum with magnitudes and phase angles of each individual harmonic
component.
Voltage THD is
V
=
(1.1)
Where V1 is the rms magnitude of the fundamental component, and Vn is the rms magnitude
of component n where n = 2,....., .
(iii) Interharmonics: Voltages or currents having frequency components that are not integer
multiples of the frequency at which the supply system is designed to operate (50 or 60 Hz)
are called interharmonics. Interharmonics can appear as discrete frequencies or as a wideband
spectrum. The main sources of inter-harmonic waveform distortion are static frequency
converters, induction furnaces, cycloconverters and arcing devices. Power line carrier signals
can also be considered as inter-harmonics.
11
(iv) Notching: Notching is a periodic voltage disturbance caused by the normal operation of
power electronic devices when current is commutated from one phase to another.
Time
Fig.2.5: Notching
(v) Noise: Noise is defined as unwanted electrical signals with broadband spectral content
lower than 200 kHz superimposed upon the power system voltage or current in phase
conductors, or found on neutral conductors or signal lines.
2.3 Solution to Power Quality Problems
The power electronics based devices/equipments have become key components in today's
modern power distribution system. In spite of the vast advantages offered by utilizing the
power electronics based equipment for power processing, the operation of these devices gives
rise to some serious drawbacks in terms of power quality. These devices generate harmonics
polluting the power distribution system, and demand reactive power. In order to provide
technical solutions to the new challenges imposed on the power systems, the concept of
flexible AC transmission systems (FACTS) was introduced in the late 1980s. The FACTS
devices incorporate power electronics based controllers to enhance the controllability and to
increase power transfer capability of the transmission system. There are two approaches for
the realization of power electronics based compensators: one employs conventional thyristorswitched capacitors (TSC) and reactors (TSR), and the other uses self-commutated switching
converters. Both the schemes help to efficiently control the real and reactive power, but only
the second one can be used to compensate current and voltage harmonics. Moreover, selfcommutated switching converters present a better response time and more compensation
flexibility.
The static VAR compensators (SVC) are used to control AC voltage by generating or
absorbing the reactive power by means of passive elements. A SVC consists of an anti
12
parallel thyristors and passive elements such as a capacitor (TSC) or a reactor (TCR). The
effective value of the capacitor or inductor reactance is changed continuously by controlling
the firing angle of the thyristors. A major drawback in the use of SVC is that the reactive
power handled by the SVC system is limited by the size of passive elements. One of the most
versatile FACTS devices is the STATCOM. It consists of a voltage source converter (VSC)/
voltage source inverter (VSI) with pulse width modulation (PWM) and has a faster speed of
response. In the transmission system, it can be used to improve the system stability and
damping or to support the voltage profile. The same structure at the distribution level, known
as D-STATCOM, can be used for reactive power support or for voltage regulation. The static
series compensator (SSC) or dynamic voltage restorer (DVR) is a VSI connected in series
with the supply line and acts as a controlled voltage source to obtain the desired load voltage.
When an external DC voltage source is utilized for VSI, the SSC/ DVR can be used to
compensate harmonics in the voltage, to regulate load voltage, and to compensate voltage
unbalance, sag and flicker.
Another device, the active power filter (APF) is the most promising solution to mitigate some
of the major power quality problems at the distribution level. They can be classified as shunt
APFs, series APFs, hybrid APFs, and unified power quality conditioner (UPQC). The UPQC
is one of the most versatile power quality enhancement devices which offer advantages of
both the shunt and series APFs, simultaneously. The series APF is connected in series with
the ac line and shunt APF is connected in shunt with the same ac line. These two are
connected back to back with each other though a DC link. The series component of the
UPQC inserts voltage so as to maintain the voltage at the load terminals balanced and free of
distortion. Additionally, shunt component maintains the DC link voltage within reference
value. Simultaneously, the shunt component of the UPQC injects current in the ac system
such that the currents entering the bus to which the UPQC is connected are balanced
sinusoids. A detailed operating principle and the vast capabilities of UPQC are discussed in
CHAPTER 3.
13
CHAPTER 3
UNIFIED POWER QUALITY CONDITIONER
3.1 Introduction
Unified Power Quality Conditioner (UPQC) is a multifunction power conditioner that can be
used to compensate various voltage disturbance of the power supply, to correct voltage
fluctuation, and to prevent harmonic load current from entering the power system. It is a
custom power device designed to mitigate the disturbances that affect the performance of
sensitive and/or critical loads. UPQC has shunt and series compensation capabilities for
(voltage and current) harmonics, reactive power, voltage disturbances (including sag, swell,
flicker etc.), and power-flow control. Normally, a UPQC consists of two voltage-source
inverters with a common dc link designed in single-phase, three-phase three-wire, or threephase four-wire configurations. One inverter is controlled as a variable voltage source in the
series active power filter (APF). The other inverter is controlled as a variable current source
in the shunt active power filter (APF). The series APF compensates for voltage supply
disturbances (e.g., including harmonics, imbalances, negative and zero sequence components,
sag, swell, and flickers). The shunt APF converter compensates for load current distortions
(e.g., caused by harmonics, imbalances) and reactive power, and perform the dc link voltage
regulation [2, 26].
3.2 Basic Configuration of UPQC
Fig. 3.1 shows system configuration of a three-phase UPQC. The key components of UPQC
are as follows:
Series inverter: It is a voltage-source inverter connected in series with AC line
through a series transformer and acts as a voltage source to mitigate voltage
distortions. It eliminates supply voltage flickers and imbalances from the load
terminal voltage. Control of the series inverter output is performed by using pulse
width modulation (PWM). Among the various PWM technique, the hysteresis band
PWM is frequently used because of its ease of implementation. Also, besides fast
response, the method does not need any knowledge of system parameters. In this
work hysteresis band PWM is used for the control of inverters. The details of the
hysteresis control technique are analysed in the subsequent sections.
14
Shunt inverter: It is a voltage-source inverter connected in shunt with the same AC
line which acts to cancel current distortions, compensate reactive current of the load
and improve the power factor of the system. It also performs the DC-link voltage
regulation, resulting in a significant reduction of the DC capacitor rating. The output
current of shunt converter is adjusted using a dynamic hysteresis band by controlling
the status of the semiconductor switches such that output current follows the reference
signal and remains in a predetermined hysteresis band.
DC link capacitor: the two VSIs are connected back to back with each other through
this capacitor. The voltage across this capacitor provides the self-supporting DC
voltage for proper operation of both the inverters. With proper control, the DC link
voltage acts as a source of active as well as reactive power and thus eliminates the
need of external DC source like battery.
Low-pass filter is used to attenuate high-frequency components of the voltages at
the output of the series converter that are generated by high-frequency
switching of VSI.
High-pass filter is installed at the output of shunt converter to absorb ripples
produced due to current switching[2].
Series transformer: The necessary voltage generated by the series inverter to
maintain a pure sinusoidal load voltage and at the desired value is injected in to the
line through these series transformers. A suitable turns ratio is often considered to
reduce the current flowing through the series inverter.
iS
vL
vC
Series
Transformer
iC
C
Series
VSI
Shunt
VSI
L
L
Sensitive
Nonlinear
Vdc
Load
vf
PWM
vC * iC *
PWM
voltage
current
control
control
if
Low-pass
filter
High-pass
vS
UPQC control
iL
system
Fig. 3.1: Detailed configuration of UPQC
15
filter
3.3 Operation of the UPQC
,
As shown in Fig. 3.1
, ,
are the supply voltage, series compensation voltage, shunt
compensation current and load voltage respectively. The source voltage may contain
negative, zero as well as harmonic component. The system (utility) voltage at pint S can be
expressed as:
=
( )+
( )+
( )
(3.1)
Equation (3.1) can also be written as:
=
sin
Where
+
sin(
+
)+
+
(
+
)
(3.2)
is the fundamental frequency positive sequence components,
is the
fundamental frequency negative sequence components respectively. The last term of equation
represents the harmonic content in the voltage and
1p
,
1n
and
k
are the corresponding
voltage phase angles.
Usually, the voltage at the load at point of common coupling (PCC) is expected to be
sinusoidal with fixed amplitude VL:
vL
VL sin( t
1p
(3.3)
)
Hence the series inverter will need to compensate for the following components of voltage:
=(
)sin(
+
)
( )
( )
(3.4)
In the subsequent sections, it will be shown how series-APF can be designed to operate as a
controlled voltage source whose output voltage would be automatically controlled using the
above described logic [2, 18].
To
provide load reactive power demand and compensation of the load harmonic and
negative sequence currents, the shunt-APF acts as a controlled current source and its output
component should include harmonic and negative sequence components in order to
compensate these quantities in the load current [2,18].The distorted non-linear load current
can be expressed as:
=
sin
+
+
( )+
( )
16
(3.5)
It is usually desired to have a certain phase angle (displacement power factor angle),
,
between the positive sequence voltage and current at the load terminal[15]:
=
=
Or,
+
(3.6)
Substituting equation (3.6) into equation (3.5) and simplifying yields
=
sin
+
cos(
)+
cos (
+
)sin (
)+
( )+
(t)
(3.7)
In order to compensate harmonic current and reactive power demand, the shunt active filter
should produce the following current:
=
cos (
+
)sin (
)+
+
(3.8)
Then the harmonic, reactive and negative sequence current will not flow into power source.
Hence, the current from the source terminal will be:
=
=
sin
+
cos(
)
(3.9)
There are also some switching losses in the converter, and hence the utility must supply a
small overhead for the capacitor leakage and converter switching losses in addition to the real
power of the load. The total current supplied by the source is therefore
=
Where
+
(3.10)
is the current drawn due to switching loss.
Hence, for accurate and instantaneous compensation of reactive and harmonic power it is
necessary to estimate the harmonic component of the load current as the reference current of
shunt APF.
17
3.4 UPQC Configurations
There are two possible ways of connecting the unit to the terminal voltage ( vt ) at PCC:
Right-shunt UPQC (Fig.3.2), where the shunt compensator ( iC ) is placed at the right
side of the series compensator ( vC ).
Left-shunt UPQC (Fig.3.3), where the shunt compensator ( iC ) is placed at the left
side of the series compensator ( vC ).
vC
iS
iL
PCC
Non-Sinusoidal
Utility
vS
vL
iC
vt
Voltage
Fig. 3.2: Right shunt UPQC
vC
iS
iL
PCC
Non-Sinusoidal
Utility
vS
iC
vt
vL
Voltage
Fig. 3.3: Left shunt UPQC
As shown in [2] out of these two structures the overall characteristics of the right shunt
UPQC are found to superior (achieving unity power factor at load terminals, and full
reactive power compensation). For this work right shunt UPQC is used.
3.5 Steady-State Power Flow Analysis of UPQC
The powers due to harmonic quantities are negligible as compared to the power at
fundamental component, therefore, the harmonic power is neglected and the steady state
operating analysis is done on the basis of fundamental frequency component only. The
UPQC is controlled in such a way that the voltage at load bus is always sinusoidal and at
18
desired magnitude. Therefore the voltage injected by series APF must be equal to the
difference between the supply voltage and the ideal load voltage. Thus the series APF
acts as controlled voltage source. The function of shunt APF is to maintain the dc link
voltage at constant level. In addition to this the shunt APF provides the VAR required by
the load, such that the input power factor will be unity and only fundamental active power
will be supplied by the source. The equivalent circuit of UPQC was shown in the Fig.
3.4[24].
vC
vt
vL
iL
iS
vS
iC
Fig. 3.4: equivalent circuit of a UPQC
Where, vS = source voltage
vt = terminal voltage at PCC load
vL = load voltage
iS = source current
iL = load current
vC = voltage injected by series APF
iC = current injected by shunt APF
k = fluctuation of source voltage i.e. k=
vt vL
vL
Case I
The reactive power flow during the normal working condition when UPQC is not connected
in the circuit is shown in the Fig. 3.5(a). In this condition the reactive power required by the
load (QL) is completely supplied by the source only. When the UPQC is connected in the
network and the shunt APF is put into the operation, the reactive power required by the load
19
is now provided by the shunt APF alone; such that no reactive power burden is put on the
mains. So as long as the shunt APF is ON, it is handling all the reactive power even during
voltage sag, voltage swell and voltage harmonic compensation. The series APF does not take
any active part in supplying the load reactive power demand. The reactive power flow during
the entire operation of UPQC is shown in the Fig. 3.5 (b) [24].
QL
QS
QL
QSh
(b) Shunt APF ON
(a) No UPQC
Fig. 3.5: (a) – (b) Reactive power flow
QS=Source reactive power
QL=Load reactive power
QSh= Shunt APF reactive power
Case II
If k < 0, i.e. vt < vL, series injected power (Psr) will be positive, means series APF supplies
the active power to the load. This condition is possible during the utility voltage sag
condition, IS will be more than the normal rated current. Thus we can say that the required
active power is taken from the utility itself by taking more current so as to maintain the
power balance in the network and to keep the dc link voltage at desired level [24].
20
PS'
PS'
PL
PSh '
Supply
PSr '
Fig. 3.6: Active power flow during voltage sag condition
PS ' = Power Supplied by the source to the load during voltage sag condition
PSr ' = Power Injected by Series APF in such way that sum PSr ' + PS ' will be the required load
power during normal working condition i.e. PL
PSh ' = Power absorbed by shunt APF during voltage sag condition
PSr ' = PSh '
This active power flows from the source to shunt APF, from shunt APF to series APF via dc
link and finally from series APF to the load. Thus the load would get the desired power even
during voltage sag condition. Therefore in such cases the active power absorbed by shunt
APF from the source is equal to the active power supplied by the series APF to the load. The
overall active power flow is shown in Fig. 3.6.
Case III
If k > 0, i.e. vt > vL, Psr will be negative, this means series APF is absorbing the extra real
power from the source. This is possible during the voltage swell condition, iS will be less than
the normal rated current. Since vs is increased, the dc link voltage can increase. To maintain
the dc link voltage at constant level the shunt APF controller reduces the current drawn from
the supply. In other words we can say that the UPQC feeds back the extra power to the
supply system. The overall active power flow is shown in Fig. 3.7 [24].
21
PS''
PL
PSr ''
PSh ''
Fig. 3.7: Active power flow during voltage swell condition
PS'' = Power Supplied by the source to the load during voltage swell condition
PSr '' = Power Injected by Series APF in such way that sum PS'' - PSr '' will be the required load
power during normal working condition
PSh '' = Power delivered by shunt APF during voltage swell condition
PSr '' = PSh ''
Case IV
If k = 0, i.e. vL =vt, then there will not be any real power exchange though UPQC. This is the
normal operating condition. The overall active power flow is shown in Fig. 3.8.
PS
PL
PS
Supply
Load
PSh
PSr
UPQC
DC Link
Fig. 3.8: Active power flow during normal working condition
22
The phasor representations of the above discussed conditions are shown in the Fig. 3.9 (a) –
(d). Fig. 3.9(a) represents the normal working condition, considering load voltage vL as a
L
is lagging power factor angle of the load. During this condition iS will
be exactly equal to the iL since no compensation is provided. When shunt APF is put into the
operation, it supplies the required load VARs by injecting the leading current such that the
source current will be in phase with the terminal voltage. The phasor representing this is
shown in Fig. 3.9 (b). The phasor representations during voltage sag and voltage swell
condition on the system are shown in the Fig. 3.9 (c) and Fig. 3.9 (d) respectively. The
deviation of shunt compensating current phasor from quadrature relationship with terminal
voltage suggests that there is some active power flowing through the shunt APF during these
conditions [24].
IC
IS
C
VL ,Vt
0
VL , Vt
0
L
L
IL , IS
IL
Fig. (a): Normal working condition,
without any compensation
IC
Fig. (b): Shunt APF ON
IC
C
0
VC
IS
Vt
VL
L
IS
C
VC
0
VL
Vt
L
IL
IL
Fig. (c): UPQC ON, voltage sag
Fig. (d): UPQC ON, voltage swell
Fig. 3.9 (a) – (d): Phasor representation of all possible onditions
In normal operating condition, the shunt APF provides the load VAR, whereas, series APF
handles no active or reactive power, so in this case the rating of series APF should be small
fraction of load rating. The shunt APF rating mainly depends on the compensating current
provided by it, which depends on the load power factor or load VAR requirement. Lower the
23
load power factor or higher the load VAR demand, higher would be the shunt APF rating. For
the series APF rating depends on two factors; source current iS and factor k.
The current iS increases during voltage sag condition whereas decreases during voltage swell
condition. Therefore the rating of series APF is considerably affected by the % of sag needed
to be compensated. Since during voltage sag condition the increased source current flows
through shunt APF, increasing the shunt APF rating too. Moreover, the shunt APF rating
further affected during voltage sag / swell compensation, since it has to maintain the dc link
voltage at constant level, which is done by taking requisite amount of active power from the
source. A compromise can be made while considering shunt and series APF device ratings,
which directly affects the sag/swell compensation capability of UPQC.
3.6 Control Scheme for Series Active Power Filter
A simple algorithm is developed to control the series filters. The control strategy is based on
the extraction of Unit Vector Templates (UVT) from the distorted supply. The series filter is
,
controlled such that it injects voltages(
,
,
unbalance present in the supply voltages(
terminal (
,
,
) , which cancel out the distortions and/or
,
), thus making the voltages at the load
) perfectly balanced and sinusoidal with the desired amplitude. In other
words, the sum of the supply voltage and the injected series filter voltage makes the desired
voltage at the load terminals. The control strategy for the series APF is shown in Fig. 3.10.
Since the supply voltage is unbalanced and or distorted, a phase locked loop (PLL) is used to
achieve synchronization with the supply. Three phase distorted/unbalanced supply voltages
are sensed and given to the PLL which generates angle ( t ) varying between 0 and 2*
radian, synchronized on zero crossings of the fundamental (positive-sequence) of phase A.
The sensed supply voltage is multiplied with a suitable value of gain before being given as an
input to the PLL. The angle ( t ) output from the PLL is used to compute the supply in
phase, 1200 displaced three unit vectors (
=
;
= sin(
120 ) ;
,
,
= sin (
) using equation (3.11).
+ 120 )
(3.11)
The computed three in-phase unit vectors are then multiplied with the desired peak value of
the PCC phase voltage(
), which becomes the three-phase reference load voltages as from
equation (3.12).
24
vSa
K
vSb
K
vSc
t
PLL
sin( t 2 / 3)
Ub
Gating
vCa *
Ua
sin( t )
vLa *
Signal
to
Inverter
vCb *
Hysteresis
Controller
vLb *
K
sin( t 2 / 3)
vCc *
UC
K=1/Vm
vLc *
vCa
vLm *
Series APF
Output
Voltage
vCb
vCc
Fig. 3.10: Control scheme of series APF
= [
]
(3.12)
In order to have distortion less load voltage, the load voltage must be equal to the computed
reference voltages from equation (3.12).To generate injected voltages, supply voltage signals
are compared with these reference signals and these signals are then given to the hysteresis
controller along with the sensed series APF output voltages. The output of the hysteresis
controller controls the six switches of the VSI of the series APF. The hysteresis controller
generates the switching signals such that the voltage at the load becomes the desired
sinusoidal reference voltage. Therefore, the injected voltage across the series transformer
through the ripple filter cancels out the harmonics and unbalance present in the supply
voltage.
3.7 Control Scheme for Shunt Active Power Filter
The instantaneous reactive power (p-q) theory is used to generate reference signal for shunt
APF. The control block diagram of shunt active filter is given in Fig.3.11. In this theory, the
instantaneous three-phase currents and voltages are transformed to
-ß-0 coordinates as
shown in equation (3.13) and (3.14).
=
1
(3.13)
0
25
1
=
ß
(3.14)
0
Equation (3.15) shows calculation of instantaneous real power (p), imaginary power (q) and
zero sequence power (p0) components drawn by the load
p
0
=
=
Where
0
0
+
0
(3.15)
ß
ß
=
;
+
(3.16)
Where, the ˜ sign points to the alternating term and the - sign points to the direct component
of each active and reactive power. In general, each one of the active and reactive
instantaneous power contains a direct component and an alternating component. The direct
component of each presents the power of the fundamentals of current and voltage. The
alternating term is the power of the harmonics of currents and voltages.
For harmonic and reactive power compensation the direct and alternating components of the
imaginary power ( ,
components) and harmonic component ( ) of the real power is
selected as compensation power references and compensation current reference is calculated
using equation (3.17). There will be no zero sequence power (p0) as the load is balanced.
=
The signal
+
ß
(3.17)
ß
is used as an average real power and is obtained from the voltage regulator.
DC-link voltage regulator is designed to give both good compensation and an excellent
transient response. The actual DC-link capacitor voltage is compared by a reference value
and the error is processed in a proportional-integral (PI) controller, which is employed for the
voltage control loop since it acts in order to minimize the steady-state error of the DC-link
voltage to zero.
Equation (3.17) represents the required compensating current references (
,
) in –
coordinates to match the power demand of the load. Equation (3.18) is used to obtain the
compensating phase currents (
,
,
) in the a–b–c axis in terms of the compensating
currents in the – coordinates:
26
1
iC a *
2
3
iC b *
iC b *
0
1
2
3
2
1
2
iC *
(3.18)
iC *
3
2
Vdc
Vdc_ref
vSa
v
vSb
vSc
v
p
v0
calc.
p-q
iC * iC *
instant.
current
power
iLa
i
iLb
iLc
calc.
i
q
-1
calc.
i0
iCa * iCb *
iCc *
ref .
current
calc.
ref .
calc.
iCabc *
Fig. 3.11: Control block diagram of shunt APF
The p–q theory is suitable for ideal 3-phase systems but is inadequate under non-ideal mains
voltage cases. In fact, under non-ideal mains voltage conditions, the sum of components
+
will not be constant and alternating values of the instantaneous real and imaginary
power have current harmonics and voltage harmonics. Consequently, the shunt APF does not
generate compensation current equal to current harmonics. To overcome these limitations,
instantaneous reactive and active powers have to be calculated after mains voltages have been
filtered [22].
Since the mains voltages, applied to control algorithm is to be balanced and sinusoidal,
voltage harmonics filter block diagram is used as shown in Fig. 3.12.
In this method, instantaneous voltages are first converted to synchronous d–q coordinates
(Park transformation) as equation (3.19).
=
sin (
cos (
)
)
sin (
cos (
)
)
27
sin (
cos (
+ )
+ )
(3.19)
The produced d–q components of voltages are filtered by using the 5th order low-pass filters
(LPF) with a cut-off frequency at 50 Hz. These filtered d–q components of voltages are then
–
–
voltages are used in conventional instantaneous reactive power theory. Hence, the non-ideal
mains voltages are converted to ideal sinusoidal shape by using LPF in d–q coordinate.
=
vSa
vd
sin (
cos (
vq
) cos (
) sin (
)
)
vd
(3.20)
vd
Calculation
v
v
v
Calculation
vSb
vq
vq
vSc
v
Fig. 3.12: Voltage harmonics filtering block diagram
The reference currents as calculated by the control algorithm are supplied to the power
system by controlling the switching action of the IGBT of inverters. The switching pattern is
generated by instantaneous current control of the shunt APF currents. The actual APF line
currents are monitored instantaneously, and then compared to the reference currents
generated by the control algorithm. A hysteresis–band PWM current control is then
implemented to generate the switching pattern of the VSI.
28
3.8 Hysteresis Controller
The basic implementation of hysteresis control is based on deriving the switching signals
from the comparison of the voltage or current error with a fixed tolerance band. This control
is based on the comparison of the actual phase voltage or current with the tolerance band
around the reference voltage or current associated with that phase.
3.8.1 Hysteresis Voltage Controller
Hysteresis voltage controller governs the inverter switching pattern in such a manner as to
control the output voltage of series APF. The principal diagram of fixed hysteresis band (HB)
voltage control is shown in Fig.3.13. The instantaneous value of the output voltage is
compared with the reference voltage(
), when the sensed output signal deviates from the
reference by more than a prescribed value; the inverter is operated to reduce the deviation.
This means that the switching occurs whenever the output voltage crosses the value of HB.
The output voltage signal of the Series APF is given by:
=
=
+ HB in rising case.
HB in decreasing case.
vC *
vC
Fig. 3.13: Simplified model for fixed hysteresis-band voltage control
Fig. 3.14: Simulink model of the hysteresis voltage control
29
3.8.2 Hysteresis Current Controller
The hysteresis band current control scheme used for the control of shunt active power filter
line current comprises a hysteresis band around it. The reference line current of the active
power filter is referred to as
as
and actual line current of the active power filter is referred to
. The hysteresis band current controller decides the switching patterns of the devices in
the shunt APF. The switching logic is formulated
<(
as if
OFF and lower switch is ON in leg “a” of the APF; if
>(
HB) upper switch is
HB) upper switch is ON
and lower switch is OFF in leg “a” of the APF. Similarly, the switches in the legs “b” and “c”
are activated. Here, hb is the width of the hysteresis band around which the reference
currents. In this fashion, the supply currents are regulated within the hysteresis band of their
respective reference values. Fig. 3.15 shows the implementation of hysteresis current control
using simulink blocks.
Fig. 3.15: Simulink model of hysteresis current control
3.9 Conclusions
In this chapter unified power quality conditioner (UPQC) has been discussed in details.
UPQC consists of both series active power filter and shunt active power filter has the
capability to compensate the current and voltage related disturbances simultaneously.
Detailed discussion of operating principle and control scheme for UPQC has been carried out.
To realize how active and reactive power flows in the presence of UPQC, steady state power
flow analysis has been presented. A simple hysteresis voltage control with the principle for
extraction of unit vector template is proposed to control the series APF. Control scheme
based on the instantaneous reactive power theory for the shunt APF is presented and
hysteresis current controller model is also shown. The simulink model of series APF, shunt
APF, UPQC and results thereof are shown in the next chapter.
30
CHAPTER 4
RESULTS AND DISCUSSION
4.1 Introduction
This chapter discusses the simulation results of shunt active power filter (APF), series active
power (APF) filter and the unified power quality conditioner to evaluate the proposed control
strategy. The simulation models have been developed in MATLAB/SIMULINK
environment. The models have been operated for non-linear load. In order to introduce nonlinear load a three phase diode bridge with RL load on dc side is used. The simulation results
under voltage sag and swell condition are presented. Additionally, the simulation result under
distorted voltage condition is also presented. First the simulation analysis of shunt APF is
presented then that of series APF is presented and finally the simulation results for UPQC is
presented. The parameters used for the simulation model are shown in appendix.
4.2 Simulation Results of Shunt APF
In this section the simulation results of shunt APF are shown. The developed model of a
shunt APF and its control scheme in MATLAB/SIMULINK environment are shown in
Fig.4.1 and Fig. 4.2 respectively. Due to the non-linear load connected to the system,
harmonics are produced in load current waveform as shown in Fig. 4.4. At 0.1 sec, shunt
APF is put in operation for compensating current harmonics. As soon as the shunt APF is
turned ON, the feedback PI controller acts immediately forcing the DC link voltage (Fig. 4.7)
to settle down at reference value, here 700V. At the same time, the shunt APF also starts
compensating the current harmonics generated by non-linear load. The shunt APF injects a
current (Fig. 4.6) in such a way that the source current becomes sinusoidal. The improved
source current profile can be noticed from Fig. 4.5. Fig. 4.3 and Fig. 4.4 show that the load
voltage and current remain unaffected throughout the operation.
31
Fig. 4.1: Simulink model of shunt APF
Fig.4.2: Simulink model of control scheme for shunt APF
Fig. 4.3: Three phase load voltage
32
Fig. 4.4: Load current
Fig. 4.5: Source current before and after compensation
Fig. 4.6: Compensation current
Fig. 4.7: Capacitor voltage
The harmonic spectrum of load current and source current after compensation are shown in
Fig. 4.8 and 4.9 respectively for phase-a. The load current has Total harmonic distortion
(THD) of 25.74%.With shunt inverter in operation there is a considerable reduction in THD
at source side current, from 25.74% to 2.53%. Shunt inverter is able to prevent the
penetration of current harmonics from the load side to the source side.
33
Fig. 4.8: Load current THD
Fig. 4.9: Source current THD (with shunt APF)
4.3 Simulation Results of Series APF
In this section, the simulation result of series APF is shown. The developed model of series
APF and its control scheme are shown in Fig. 4.10 and Fig. 4.11 respectively. To analyze the
performance of series APF during voltage sag conditions, the voltage source is assumed
sinusoidal and contains no harmonics as shown in Fig. 4.12. At instant 0.1 sec. a sag(25%) is
introduced to the supply voltage .This sag lasts till the instant 0.3 sec. During the voltage sag
condition, the series inverter injects an in-phase voltage (25%) equals to the difference
between the desired load voltage and actual source voltage, as seen from Fig. 4.14. Thus, it
34
helps to maintain the load voltage profile (Fig. 4.13) at desired level such that the sag in
source voltage does not appear at the load terminal. The desired load voltage ( vLm ) is
assumed to be 310.2867 volts which is calculated as:
=
Where,
2/3 ;
is the line voltage equals to 380 volts.
Fig. 4.10: Simulink model of series APF
Fig. 4.11: Simulink model of control scheme for series APF
35
Fig. 4.12: Source voltage
Fig. 4.13: Load voltage
Fig. 4.14: Injected voltage
To analyze the harmonic compensating capability of series APF, the distortion in utility
voltages are introduced deliberately by injecting a 5th (20%) and 7th (15%) order voltage
harmonics. The resultant highly distorted source voltage waveform shown in Fig. 4.15 has
THD of 25% as the THD information of source voltage for phase-a is shown in Fig. 4.18.
Such a highly distorted voltage may be problematic for many sensitive loads. The series APF
is put into operation at 0.1 sec. The series APF starts compensating voltage harmonics
immediately by injecting sum of 5th and 7th harmonics, hence giving distortion free load
voltage (Fig. 4.16). The voltage injected by series APF is shown in Fig. 4.17. Here load
voltage THD is improved from 25 % to 1.82% as THD information of load voltage for phasea is shown in Fig. 4.19.
36
Fig. 4.15: Distorted source voltage
Fig. 4.16: Load voltage before and after compensation
Fig. 4.17: Injected voltage
Fig. 4.18: Source voltage THD
37
Fig. 4.19: Load voltage THD (with Series APF)
4.4 Simulation Results of UPQC
In this section the simulation analysis of UPQC is described. In this two filters are used i.e.
shunt active power filter and series active power filter. The developed model of UPQC in
MATLAB/SIMULINK environment is shown in Fig. 4.20. The control circuits for this model
are same as shown in Fig. 4.2 and Fig. 4.11. The shunt active power filter compensates
current disturbances and also maintains the dc link voltage to reference value. While series
active power filter compensates voltage related problems for maintaining required load
voltage.
Fig. 4.20: Simulink model of UPQC
38
I .Current Harmonic Compensation
Fig. 4.21 shows the simulation results for UPQC working as current harmonics compensator.
In this case, the terminal voltages are assumed pure sinusoidal, the UPQC is put into the
operation at instant 0.1sec. Within the very short time period shunt APF maintains the dc link
voltage at constant level as shown in Fig. 4.21(d). In addition to this the shunt APF also helps
in compensating the current harmonics generated by the non-linear load. The load current is
shown in Fig. 4.21(a). The shunt APF injects a current (Fig. 4.21(c)) in such a manner that
the source current becomes sinusoidal. At the same time, the shunt APF compensates for the
reactive current of the load and improves power factor. The improved source current profile
is shown in Fig. 4.21(b).
Fig. 4.21: Simulated results of UPQC (a) Load current (b) Source current (c) Shunt APF
current (d) capacitor voltage
Fig 4.22(a-b) shows the harmonic spectrum of load current and source current for phase-a
after shunt APF is put in operation. THD of load current is 25.84%. With shunt APF in
operation there is a significant reduction in THD at source side current, from 25.84 % to 2.61
%. Shunt inverter is able to reduce the current harmonics entering into the source side.
39
Fig. (a)
Fig. (b)
Fig.4.22: THD (a) distorted source current (b) compensated source current
II. Voltage and Current Harmonic Compensation
Fig. 4.23 shows the simulation result of UPQC for voltage and current harmonic mitigation
simultaneously. To study the harmonic compensating capability of UPQC, the distortion in
utility voltages are introduced deliberately by injecting a 5th (20%) and 7th (20%) order
voltage harmonics as shown in Fig. 4.23(a). The series APF injects a out-of phase voltage
40
with 5th and 7th harmonic which is the difference between the desired load voltage and actual
supply voltage. The injected voltage profile can be observed in Fig. 4.23(b). The load voltage
profile can be observed in Fig. 4.23(c) which is free from supply voltage distortion. The
source current (4.23(f)) is also free from any distortion as shunt APF compensates load
current (Fig. 4.23(d)) by injecting required compensation current (Fig. 4.23(e)) to make
source current sinusoidal. Fig. 4.23(g) shows the dc link voltage which is maintain around
reference by PI controller.
Time (sec)
Fig. 4.23: Simulated results of UPQC (a) Source voltage (b) series injected voltage (c) Load
voltage (d)Load current (e) Injected current (f) source current (g) DC capacitance voltage
Fig. 4.24(a-b) shows the harmonic spectrum of source voltage and load voltage for phase-a
after the UPQC is put in operation. THD of source voltage is 27.37%. With the UPQC in
operation there is a significant reduction in THD at load side voltage, from 27.37 % to 1.74
%. Series APF is able to prevent the harmonics from disturbing load voltage.
41
Fig. 4.24: THD (a) distorted source voltage (b) load voltage
III. Voltage Sag and Current Harmonic Compensation
The simulation result of voltage sag compensation is shown in Fig. 4.25. The UPQC is put in
operation at 0.05 sec. A sag (20%) is introduced to the supply voltage at 0.1sec. and lasts till
the instant 0.25 sec. as shown in Fig. 4.25(a). During the voltage sag condition, the series
APF injects an in-phase voltage (20%) equals to the difference between the desired load
voltage and actual source voltage, as seen from Fig. 4.25(b). Thus, UPQC helps to maintain
the load voltage profile (Fig. 4.25(c)) at desired level such that the sag in source voltage does
not appear at the load terminals. As discussed in CHAPTER 3, in order to inject the in-phase
voltage the UPQC requires certain amount of active power. This active power comes from
the source, extracted by shunt inverter, by taking extra fundamental current component to
42
maintain the DC link voltage (Fig. 4.25(g)) at constant level. If it is not maintained, the DC
link voltage will drop to very low value in very few cycles. The fundamental current drawn
by shunt inverter can be noticed from Fig. 4.25(e) (between time 0.1 sec. and 0.25 sec.), and
therefore, the source current magnitude also increases accordingly (Fig. 4.25(f)).
Time (sec)
Fig. 4.25: Simulated results of UPQC (a) Source voltage (b) series injected voltage (c) Load
voltage (d)Load current (e) Injected current (f) source current (g) DC capacitance voltage
IV. Voltage Swell and Current Harmonic Compensation
The simulation result of voltage swell compensation is shown in Fig. 4.26. The UPQC is put
in operation at 0.05 sec. A swell (20%) is introduced to the supply voltage at 0.1sec. and lasts
till the instant 0.25 sec as shown in Fig. 4.26(a). The voltage swell phenomenon is exact
opposite to the voltage sag condition. Therefore, during a swell on the source side voltage,
the series APF now injects out-of phase voltage (20%) equals to the difference between the
desired load voltage and actual source voltage (Fig. 4.26(b)). Thus, the UPQC cancels the
43
increased source voltage that may appear at the load side and maintains the load voltage
profile (Fig. 4.26(c)) at desired level. The rise in source voltage means the utility is supplying
some extra power to the load. This may damage equipments and loads due to the increase in
the current drawn by them. At the same time, the rise in source voltage also causes the DC
link voltage to increase. Under such condition, the shunt APF injects fundamental out-of
phase current component (Fig. 4.26(e)), between instants 0.1sec. and 0.25 sec. to maintain the
DC link voltage at constant level. Therefore, the source current magnitude decreases (Fig.
4.26(f)).
Time (sec)
Fig. 4.26: Simulated results of UPQC (a) Source voltage (b) series injected voltage (c) Load
voltage (d)Load current (e) Injected current (f) source current (g) DC capacitance voltage
44
V. Single Phase Voltage Sag and Current Harmonic Mitigation
The simulation result of single phase voltage sag compensation is shown in Fig. 4.27. The
UPQC is put in operation at 0.05 sec. A sag (40%) is introduced at instant 0.1 sec. in phase-a.
and it lasts till 0.3 sec. In Fig. 4.27(a) it can be clearly seen that phase-a has dropped sharply.
From the Fig. 4.27(b), it can be observed that during the voltage sag condition, the series APF
injects an in-phase voltage (40%) equals to the difference between the desired load voltage
and actual source voltage for phase-a. This difference is zero for the other two healthy
phases. The load voltage is free from source side disturbance which can be seen in Fig.
4.27(c). In Fig. 4.27(e) (between time 0.1 sec. and 0.3 sec), it can be observed that the shunt
APF draws extra fundamental current and therefore source current (Fig. 4.27(f)) magnitude
also increases accordingly as discussed in section III.
Fig. 4.27: Simulated results of UPQC (a) Source voltage (b) series injected voltage (c) Load
voltage (d)Load current (e) Injected current (f) source current (g) DC capacitance voltage
45
4.5 Conclusion
This chapter presents simulation results of shunt APF, series APF and UPQC. The
performance of the model has been studied under non-linear load. The performances of the
models have been observed to be satisfactory for load harmonic and reactive current
compensation, mitigation of voltage sag and swells, mitigation voltage harmonic and
mitigation of single phase sag. It is observed that source current and load voltage THD levels
are maintained below 5 % , the THD limit imposed by IEEE 519-1992. The conclusion of the
thesis is drawn in the next chapter and the future scope is proposed in the same.
46
CHAPTER 5
CONCLUSION AND FUTURE WORK
5.1 Conclusion
In this thesis, a Unified Power Quality Conditioner (UPQC) has been investigated for power
quality enhancement. UPQC is an advanced hybrid filter that consists of a series active filter
(APF) for compensating voltage disturbances and shunt active power filter (APF) for
eliminating current distortions. Different power quality problems, their causes and
consequences and the available solution have been discussed briefly. UPQC system
configuration is discussed in detail. A conceptual analysis to understand the active and
reactive power flow between source and load under different operating condition is carried
out.
The modelling of series APF, shunt APF and the UPQC has been carried out. A simple
control technique, extraction of unit vector template has been used to model the control
scheme for series APF. This scheme utilizes phase locked loop (PLL) and a hysteresis band
controller to generate the reference signals for series APF. The instantaneous reactive power
theory has been used to model the control scheme for shunt APF. The series and shunt APF
models are combined to configure the UPQC model.
Using hysteresis band controller the model has been developed in MATLAB/SIMULINK
environment. It is found from the simulation results that UPQC improves power quality of
power system by compensating harmonic and reactive current of load current which makes
source current sinusoidal and it also makes load voltage sinusoidal at required voltage level
by compensating with series APF. The THD of the source current and load voltage is below
the harmonics limit imposed by IEEE standard 519-1992.
47
5.2 Future Work
The UPQC model as developed can be modified to be more effective in eliminating power
quality related problems in power system. The various paths in which the presented work can
be extended are listed below:
A laboratory prototype can be made for the developed model.
The control strategy used here can be modified for three-phase four-wire system
under unbalance load.
The model has been developed for right shunt UPQC configuration. The model can be
modified for left shunt UPQC.
Nowadays, generation of electricity from renewable sources has improved very much.
Utilizing wind energy and solar energy as a renewable source to generate electricity
has developed rapidly. UPQC can be combined with one or several distribution
generation (DG) system to provide good quality power to the consumers. Power
generated by wind or solar energy can be fed to the DC link through converter to
make the UPQC more effective during severe system conditions.
48
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APPENDIX:
The system parameters used are as follows:
Supply:
Voltage and frequency Vs= 380 Vrms, f= 50 Hz,
Load:
3 phase ac line inductance LLabc = 2 mH
3 phase dc inductance and resistor Ldc3=10mH, Rdc3=30 ohm
DC Link:
Voltage, Vdc = 700 V; Capacitor , C = 1100 µF
Transformer : 1MVA, 240 V/120 V
Shunt APF:
Filter resistor and capacitor: RCabc =5 ohm, CCcabc=10 µF
Ac line inductance: LCabc= 3.5 mH
Series APF:
Filter resistor and capacitor: RTabc =5 ohm, CTabc=20 µF
Ac line inductance: LTabc = 1.5 mH
51