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POWER QUALITY ASSESSMENT
BY
HUSSEIN MOHAMED EL-EISSAWI FATHI
A Thesis Submitted to the
Faculty of Engineering at Al-Azhar University
In Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
IN
ELECTRICAL POWER AND MACHINES
Under the Supervision of
Prof. M. Zaher Prof. N. Ayad Dr. G. Abdel Salam
Al-Azhar University Atomic Energy Authority Ministry of Electricity
FACULTY OF ENGINEERING
AL-AZHAR UNIVERSITY
CAIRO, EGYPT
2012
I
POWER QUALITY ASSESSMENT
BY
HUSSEIN MOHAMED ELEISSAWI FATHI
A Thesis Submitted to the
Faculty of Engineering at Al-Azhar University
In Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
IN
ELECTRICAL POWER AND MACHINES
Approved by the
Examining committee
Prof. Dr. Fahmy Metwally Ahmed Bendary (Member) ----------------------
Benha University
Prof. Dr. Mohamed Mohamed Ibrahim Al-Gazar (Member) ---------------------
Al-Azhar University
Prof. Dr. Mohamed Abdel-Moety Ragheb Zaher (Advisor) ----------------------
Al-Azhar University
Prof. Dr. Nabil Mohamed Abdel-Fatah Ayad (Advisor) ----------------------
Atomic Energy Authority
FACULTY OF ENGINEERING
AL-AZHAR UNIVERSITY
CAIRO, EGYPT
2012
II
TABLE OF CONTENTS
Page
CONTENTS II
LIST OF FIGURES V
LIST OF TABLES VII
LIST OF SYMBLOS VIII
ACKNOWLEDGMENT IX
ABSTRACT X
LIST OF PUBLICATIONS XII
CHAPTER (1)
INTRODUCTION 1
1.1 Background 1
1.2 Objectives – Scope of the thesis 2
1.3 Overview of the thesis 3
CHAPTER (2)
LITERATURE REVIEW 4
2.1 Power quality definition 4
2.2 Power disturbances 4
2.3 Types of electrical power disturbances 4
2.4 Power quality monitoring 14
2.5 Power quality standards 15
2.6 Power quality solutions 19
2.7 Power quality assessment procedure (PQAP) 22
2.8 Related studies done by other researchers 27
CHAPTER (3)
DESCRIPTION OF CASE STUDY AND MEASURING PROCESS 37
3.1 Description of the case study (ETRR-2) 37
3.2 Electrical system of ETRR-2 38
III
3.3 Electrical defects of ETRR-2 40
3.4 Measuring process 40
CHAPTER (4)
MONITORING IN ETRR-2 44
4.1 Introduction 44
4.2 Measurement results 44
4.3 Measurement results of the first feeder (source1) 44
4.4 Site appraisal of incoming feeder from substation1 (source 1) 53
4.5 Measurement results of the second feeder (source2) 54
4.6 Site appraisal of incoming feeder 2 (Source 2) 66
4.7 Invesigation results and mitigation techniques 67
CHAPTER (5)
MITIGATION APPLICATIONS 68
5.1 Introduction 68
5.2 Harmonic Filters 68
5.3 Dynamic Voltage Restorer (DVR) 76
5.4 Uninterruptible Power Supplies (UPS) 84
CHAPTER (6)
CONCLUSIONS AND RECOMMENDATIONS 87
6.1 Conclusions and recommendations 87
6.2 The most important points that have been reached 87
6.3 Future work 88
REFERENCES
89
APPENDIX A 95
Power quality standards
IV
APPENDIX B
Trend graphs
98
b.1 The trend graphs of incoming feeder 1 (Source 1) 99
b.2 The trend graphs of incoming feeder 2 (Source 2) 123
V
LIST OF FIGURES
Page
Figure (2-1) : The voltage sag scenario 5
Figure (2-2) : The voltage swell scenario 6
Figure (2-3) : The under voltage scenario 6
Figure (2-4) : The over voltage scenario 7
Figure (2-5) : Typical instantaneous flicker signal recorded Close
to an electric arc furnace
9
Figure (2-6) : Waveforms with harmonic and interharmonic
components
11
Figure (2-7) : Voltage notching due to power electronic devices 11
Figure (2-8) : Voltage noises 12
Figure (2-9) : power system with nonlinear load 13
Figure (2-10) : power quality assessment procedure (PQAP) 26
Figure (3-1) : ETRR-2 general view 37
Figure (3-2) : Single line diagram of the electrical system of ETRR-2 39
Figure (3-3) : The Unipower 900F front panel 42
Figure (3-4): Single line diagram of PCC and measuring points
43
Figure (4-1):
Figure (4-2):
VTHD of incoming feeder of Source 1
ITHD of incoming feeder of Source 1
45
45
Figure (4-3) : ITHD variations of source 1 at 2nd day 46
Figure (4-4) : power variations of source 1 at 2nd day 46
Figure (4-5) : Voltage swell on phase 3 49
Figure (4-6) : Overvoltage on phase 1 50
Figure (4-7) :
Figure (4-8):
Overvoltage on phase 2
Overvoltage on phase 3
50
51
Figure (4-9): Short time voltage flicker of Source 1 52
Figure (4-10) : Long time voltage flicker of source 1 52
Figure (4-11) : VTHD of incoming feeder of source 2 54
VI
Figure (4-12) :
Figure (4-13):
Figure (4-14):
Figure (4-15):
Figure (4-16):
Figure (4-17):
Figure (4-18):
VTHD variations of source 2 at 2nd day
Power variations of source 2 at 2nd day
ITHD of incoming feeder of source 2
ITHD variations of source 2 at 2nd day
Power variations of source 2 at 2nd day
ITHD variations of source 2 at 3rd day
Power variations of source 2 at 3rd day
55
55
56
57
57
58
58
Figure (4-19) : Voltage sag on phase 1 61
Figure (4-20) : Voltage sag on phase3 61
Figure (4-21) : Voltage swell on phase 3 62
Figure (4-22) : Voltage transient on the three phases 63
Figure (4-23) : Voltage transient on the three phases 63
Figure (4-24) : Voltage Outage 64
Figure (4-25) : Short time voltage flicker of source 2 65
Figure (4-26) : Long time voltage flicker of source 2 65
Figure (5-1) : Single tuned filter 70
Figure (5-2) : Typical schematic of a power system compensated by the
DVR.
77
Figure (5-3) :
Figure (5-4):
Basic Configuration of DVR
Effective transformer connection in stand-by and boost
modes
79
79
Figure (5-5) : General control system block diagram 81
Figure (5-6 ) : Schematic of double conversion on-line UPS 85
VII
LIST OF TABLES
Page
Table (2-1) : Summary of IEEE standard 1100-1992 21
Table (4-1) : VTHD and ITHD at max. and min.loads of source1 47
Table (4-2) : Voltage deviation at max. and min. loads of source1 47
Table (4-3) : Voltage unbalance at max. and min. loads of source1 48
Table (4-4) : VTHD and ITHD at max. and min. loads of source2 59
Table (4-5) : Voltage deviation at max. and min. loads of source2 59
Table (4-6) : Voltage unbalance at max.and min. loads of source2
60
Table (5-1):
Table (5-2):
calculations for 5th harmonic filter
IEEE Std 18-2002
75
75
Table (5-3) : The situation of developing and researching for DVR 77
VIII
LIST OF SYMBOLES
Pst Short time flicker severity
Plt Long time flicker severity
THD
V
I
D
Total harmonic distortion
Voltage
Current
Distortion power
Pfdisp Displacement power factor
Pfdistortion Distortion power factor
Wth Thermal power
Pav Average power
Pf total Total power factor
.hr. Harmonic order of resonant frequency
XC Capacitor reactance
XR Reactor reactance
STF Single tuned filter
DVR Dynamic voltage restorer
PCC
VPCC
Point of common coupling
Voltage at point of common coupling
S Apparent power
CDVR Cost of dynamic voltage restorer
CVS Cost of voltage sag
NVS Number of voltage sag per year
Tpayback Payback time
XF Filter reactance
IFF Filter fundamental current
QF Filter reactive power
XT Transformer reactance
IP Peak harmonic current
IX
ACKNOWLEDGMENT
The author wishes to express his sincere gratitude and appreciation first of all for his
parents, Prof. Dr. Mohamed Zaher; Al-Azhar University, Prof. Dr. Nabil Ayad;
Atomic Energy Authority and Dr. Gamal Abdel-Salam; Ministry of Electricity. For
their supervision and active guidance during the preparation of this thesis.
Thanks are also extended to Prof. Dr. Mohamed Al-Gazar; Al-Azhar University and
Prof. Dr. El-Metwally EL-Sherbiny; Atomic Energy Authority. For their great
assistance to complete this work.
And greeting to the spirit of the late Prof. Dr. Mohamed Askora; Al-Azhar University.
For his help and guidance during the work of this thesis.
X
ABSTRACT
The electrical power systems are exposed to different types of power quality
disturbances problems. Assessment of power quality is necessary for maintaining
accurate operation of sensitive equipments especially for nuclear installations, it also
ensures that unnecessary energy losses in a power system are kept at a minimum
which lead to more profits. With advanced in technology growing of industrial /
commercial facilities in many region. Power quality problems have been a major
concern among engineers; particularly in an industrial environment, where there are
many large-scale type of equipment. Thus, it would be useful to investigate and
mitigate the power quality problems. Assessment of Power quality requires the
identification of any anomalous behavior on a power system, which adversely affects
the normal operation of electrical or electronic equipment. The choice of monitoring
equipment in a survey is also important to ascertain a solution to these power quality
problems. A power quality assessment involves gathering data resources; analyzing
the data (with reference to power quality standards); then, if problems exist,
recommendation of mitigation techniques must be considered.
The main objective of the present work is to investigate and mitigate of power
quality problems in nuclear installations. Normally electrical power is supplied to the
installations via two sources to keep good reliability. Each source is designed to carry
the full load. The Assessment of power quality was performed at the nuclear
installations for both sources at different operation conditions.
The thesis begins with a discussion of power quality definitions and the results of
previous studies in power quality monitoring. The assessment determines that one
source of electricity was deemed to have relatively good power quality; there were
several disturbances, which exceeded the thresholds. Among of them are fifth
harmonic, voltage swell, overvoltage and flicker. While the second source has bad
power quality. There are several and regular disturbances, which exceeded the
thresholds. They were voltage sag, voltage swell, under-voltage, temporary outage,
voltage transient and flicker. Mitigation techniques were suggested to install passive
harmonic filter to mitigate harmonic distortion, install a dynamic voltage restorer
XI
(DVR) to mitigate voltage disturbances and isolate all the sensitive and critical loads
of nuclear installations and feed them via uninterruptible power supplies (UPS).
The thesis ends with the conclusion and recommendation of the effective/economic
mitigation techniques and the need for further studies in the discipline.
XII
LIST OF PUBLICATIONS
This thesis is based on the work reported in the following papers:
Paper 1: M. Zaher, M. Askora, N. Ayad, G. Abdel Salam, H. Eleissawi, "Power
Quality and Voltage Interruptions in Nuclear Research Reactors", Journal
of Al-Azhar University Engineering Sector, JAUES, 2012.
Paper 2: M. Zaher, N. Ayad, E. Elsherbiny , G. Abdel Salam, H. Eleissawi, "
Investigation and Mitigation Techniques of Power Quality Problems in
Nuclear Installations", Arab Journal of Nuclear Sciences and Applications,
2012.
١
CHAPTER (1)
INTRODUCTION
1.1- Background
The concept of power quality has often been misunderstood and oversimplified. But
in this day and age of sophisticated electronics, assessment of power quality has
become too important. The widespread use of high-tech devices has complicated all
aspect of electrical power. Not only are these devices more sensitive to the effects of
power quality, but they can also impact it negatively. Poor power quality can result in
less productivity, lost and corrupt data, damaged equipment and poor power
efficiency. “Power Quality” is a broad term used to describe the electrical power
performance.
The causes and solutions of power quality problems is site dependent, so the power
quality assessment is important for sensitive projects. Under normal (ideal sinusoidal,
balanced, and symmetric) conditions power quality is basically a loading problem.
But with the growth in the power electronics and control systems industry, the once
majority linear customer loads, are now being dominated by a majority of non-linear
customer loads. Such loads like: switch mode power supplies used in both industrial
and commercial computers / microprocessors; variable speed drives used in process
control; arcing device like welders and arc furnaces; silicon controlled rectifiers used
in air-conditioners; and basically any electronic device which draws current in pulses
are termed to be non-linear. So the power quality of a system is equally the
customers’ concern as much it is the supply authorities’ concern.
Power quality assessment has become a critical concern for virtually all electric
utilities through the world. It is primarily due to the fact that customer equipment has
become more sensitive and is now interconnected in extensive networks and
processes. The result is that variations in the power quality that were never a concern
can now be very expensive in terms of process shut-downs and equipments
malfunctions. The objectives of distribution system power quality assessment work
were to:
٢
1- Design a statistically valid power quality measurement program that would
enable assessment of power quality levels.
2- Perform measurements of power quality delivered to customers sufficient to
describe the transient, harmonics, short-and long- duration voltage variation, and
momentary interruption characteristics of present distribution supply system.
3- Perform analytical, modeling and simulation studies to verify and /or improve
analytical models by using the measured data.
4- Assess the limitations of a typical electric utility distribution system to supply
loads that degrade power quality.
5- Perform studies to identify measures to improve power quality from the supply
system perspective.
6- Determine the cumulative effect as the number of small (sensitive and /or
polluting) loads is increased.
7- Provide a rational basis for the development of a recommended practice for
providing solutions to power quality problems and improving system power quality
levels.
1.2- Objectives – Scope of the thesis:
The purpose of power quality investigation is to generate a data resource about a
power system where, following the analysis of that data resource, an appropriate
solution to the problem will be found. Due to the infancy of this technology and the
diversity of power systems, there is no set method for power quality surveys and
analysis. However there is one common goal of power quality assessment - to find a
solution. This analysis of the power system data was in accordance with the standards.
However, before proceeding directly to perform a site survey, some background
knowledge of the power quality area is essential. An understanding of the
characteristics of various faults, along with typical causes and prevention techniques
are suggested.
The aim of this thesis therefore is to firstly introduce the reader to common power
quality problems, their causes, and techniques or devices used to reduce their impact
on systems. An analysis of the results from the two incoming sources of nuclear
installation power quality studies is then presented. The investigation of the site
generated a data resource. If the data gathered from the site indicate deviations, which
٣
exceeds the limitation of the power quality standards, then logical explanations for
the disturbances should be suggested. Once this explanation is proposed, the next
stage of solving the problem can be initiated.
1.3- Overview of the thesis
This thesis discusses power quality assessment conducted with in a nuclear
installation. These surveys present a modular format for the analysis and
classification of power quality and power system problems.
This thesis consists of six chapters:
Chapter one presents the introduction, the objective and the overview of the thesis.
Chapter two contains all the terms and definitions of power quality, it describes the
different types of power disturbances, power quality monitoring, power quality
standards, solutions of power quality problems and power quality assessment
procedure. It contains also a review of studies conducted by other researchers.
Chapter three presents a general description of electrical system of nuclear
installation, electrical defects and measuring process.
Chapter four discusses the results, analysis and discussion of the power quality
investigation conducted at the case study.
Chapter five contains the mitigation techniques. The thesis document concludes with
chapter six, which presents conclusions, recommendations and future work in this
area.
٤
CHAPTER (2)
LITERATURE REVIEW
2.1 - power quality definition [1]
Power quality problem is any power problems manifested in voltage, current, or
frequency deviations that result in failure or missed operation of utility or end user
equipment.
2.2 -Power disturbances [2], [3] and [4]
The electrical power system is exposed to different types of disturbances leading to
power quality problems. These problems such as; voltage, current or frequency
deviation may result in failure or misoperation of customer equipment.
2.2.1 - Sources of disturbances
Electrical power quality is mainly affected by disturbances in feeding systems.
The sources of these disturbances are: -
1- Disturbances originating from utility feeding system due to faults which disturb the
source voltage waves and can be isolated within two seconds to one minute by
protection systems, or supply interruption will occur.
2- Disturbances originating from consumers’ networks and devices is mainly due to
three-phase loads, connection unbalance, absence of appropriate neutral wire,
absence of earthing system or low circuit breaker rating.
3- Non-linear characteristics of loads and devices or unsuitable line sites are other
sources of disturbances.
2.3- Types of electrical power disturbances:-
2.3.1- Voltage spikes and surges
It is a short duration from microsecond to millisecond voltage increase; it occurs due
to lightning, switching of heavy loads and power system faults. It leads to equipment
failure, system lock-up, data corruption and data loss. Solutions to voltage spikes and
٥
surges problems include equipment such as surge arresters, filters and isolation
transformer.
2.3.2-Voltage sag (dip)
It is a reduction in voltage outside the normal tolerance for a short time less than few
seconds. The magnitude of the reduction is between 10 percent and 90 percent of the
normal (rms) voltage. It occurs due to starting of heavy loads and power system
faults. It reduces the energy being delivered to the end user and causes computers to
fail, adjustable-speed drive to shut down and motors to stall and over heat.
Figure (2-1) shows the voltage sag scenario. Solutions to voltage sag problems
include equipment such as ferroresonant transformer, energy storage technologies,
uninterruptible power supply (UPS) and dynamic voltage restorer (DVR).
Figure (2- 1) the voltage sag scenario
٦
2.3.3-Voltage swell
It is a momentary increase in voltage outside the normal tolerance. The (rms.) voltage
variations that exceed 110 percent. Faults and turning off heavy electrical equipment
cause voltage swell. The increased energy from a voltage swell often overheats
equipment and reduces its life. Figure (2-2) shows the voltage swell scenario. Voltage
regulator, motor-generator set and uninterruptible power supply can mitigate the
voltage swell effects.
Figure (2-2) the voltage swell scenario.
2.3.4-Under voltage
As shown in figure (2-3) it is a decrease in the rms ac voltage to less than 90% at the
power frequency. A load switching on or a capacitor bank switching off can cause an
under voltage until the voltage regulation equipment in the system can bring the
voltage back within tolerances. Overloaded circuits and the loss of major
transmission support can also result in under voltages. It can cause sensitive computer
equipment to read data incorrectly and motor to stall and operate inefficiently. Utility
can prevent under voltage by building more generation and transmission lines.
Figure (2-3) the under voltage scenario.
٧
2.3.5- Over voltage
As shown in figure (2-4) it is an increase in the rms ac voltage greater than 110% at
the power frequency. Over-voltage is usually the result of the switching-off of a large
load, or the energizing of a capacitor bank. Over-voltages occur either because the
system is too weak for the desired voltage regulation or the voltage controls are
inadequate. An incorrect tap setting in transformers is one example.
Figure (2-4) the over voltage scenario
2.3.6- Voltage modulation
It is a periodic increase and decrease of amplitude. Periodic loads cause it. It leads to
poor power quality.
2.3.7- Voltage imbalance (IEEE Std. 1159) or unbalance
It can be defined as the maximum deviation from the average of the three-phase
voltages, divided by the average of the three-phase voltages and expressed in
percentage points. Imbalance can also be defined using symmetrical components. The
ratio of either the negative or zero sequence components to the positive sequence
component can be used to specify the percent imbalance. The primary source of
voltage imbalance (typically less than 2%) is the unequal distribution of single-phase
loads in a three-phase circuit. Voltage imbalance can also be the result of blown fuses
in one phase of a three-phase capacitor bank. Severe voltage imbalance (greater than
5%) can result from single-phasing conditions. Voltage imbalance causes motors and
transformers to overheat. This is because the current imbalance in an induction
device.
٨
2.3.8- Phase angle imbalance
It is the deviation from the normal 120 or 240 degree between three-phase voltages.
Phase angle imbalance can be caused by the uneven distribution of loads among the
phases.
2.3.9-Voltage fluctuations (flicker)
As shown in figure (2-5) they are cyclical variations in the voltage rms values or a
series of random voltage changes, whose magnitude does not normally exceed
voltage ranges of 0.9 p.u. to 1.1 p.u. A common phenomenon of voltage fluctuations
is the voltage flicker. Loads, which can exhibit continuous, rapid variations in the
load current magnitude, can cause voltage fluctuations or flickers. The present
industry practice is to characterize the severity of a voltage flicker with respect to the
sensitivity of the human visual perception. Typically, magnitudes as low as 0.5% can
result in a perceptible lamp flicker. Arc furnace and welders are the most common
causes of voltage fluctuations in utility transmission and distribution systems. Other
sources of voltage fluctuation include lumber mills, draglines, and rock crushing
machines. Voltage fluctuations can be considered as repetitive random voltage sags
and swells. Therefore, voltage fluctuations have propagation characteristics similar to
those of sags. They can be assessed using steady-state power system models. A strong
supply system can greatly reduce severity of voltage fluctuation. Voltage fluctuations
can cause incandescent and fluorescent lights to blink rapidly. It can also cause
sensitive equipment to malfunction. Static VAR system can mitigate the flicker
effects [2]. As the annoyance created by flicker is a function of both the intensity of
perception and the duration of exposure, according to IEC the severity of the
disturbance is described by two parameters: the short term severity (Pst) measured
over a period of ten minutes. And the long term severity (Plt) calculated from a
sequence of 12 Pst - values over a two hours interval, according to the following
expression:
312
1
3
12∑ =
=i
sti
lt
pp (2-1)
٩
Figure (2-5) typical instantaneous flicker signal recorded
Close to an electric arc furnace.
2.3.10- Outage
It is a complete loss of power, it is caused by faults and accidents. Solutions to outage
problems including equipment such as standby engine generator.
2.3.11- Power frequency variations
They are the deviations in the power system fundamental frequency from its normal
value. The power system frequency is directly related to the rotational speed of the
generators supplying the system. Slight variations in frequency occur as the dynamic
balance between load and generation changes. The size of the frequency shift and its
duration depend on the load characteristics and the response of the generation control
system to load changes. In modern interconnected power systems, significant
frequency variations are rare. Frequency variations of consequence are much more
likely to occur for loads that are supplied by generators isolated from the utility
system. In such cases, the generator response to abrupt loads changes may not be
adequate to regulate the frequency within the narrow bandwidth required by
frequency sensitive equipment. [1]
١٠
2.3.12- Waveform distortion [1], [5] and [6]
It is defined as a steady-state deviation from an ideal sinusoidal wave of power
frequency. It is characterized by the spectral content of the deviation. Primary types of
waveform distortion are harmonics, notching, interharmonics, DC offset and noise.
Figure (2 –6) indicates waveforms with harmonic and interharmonic components.
- Harmonics are sinusoidal voltages or currents having frequencies that are integer
multiples of the fundamental frequency. Distorted waveforms can be decomposed into
the sum of the fundamental frequency and the harmonic components. Harmonic
distortion levels are described by the complete harmonic spectrum with magnitudes
and phase angles for each individual harmonic component. It is also common to use a
single quantity, the Total Harmonic Distortion (THD), as a measure of the effective
value of harmonic distortion. Harmonic distortion originates in the nonlinear
characteristics of devices and loads in the power system. Typical harmonic sources
are variable speed drives and other power electronics based equipment. One of the
major problems related to harmonic disturbances is harmonic resonance. The
resonance can magnify harmonic distortions to a level that can damage the equipment
or cause equipment malfunction. Power factor correction capacitors in distribution
system are the main cause of harmonic resonance. Other effects of harmonics are
equipment overloading, increased losses and sometimes equipment malfunction.
The most commonly used harmonics index is:-
THDV = 1
2
2
V
Vh
h∑∞
= * 100 % and THDI = 1
2
2
I
Ih
h∑∞
= * 100 % (2-2)
This is defined as the ratio of the rms value of the harmonic components to the rms
value of the fundamental component and usually expressed in percent. This index is
used to measure the deviation of a periodic waveform containing harmonics from a
perfect sine wave. For a perfect sine wave at fundamental frequency, the THD is zero.
Similarly, the measures of individual harmonic distortion for voltage and current at h-
th order are defined as Vh/V1 and Ih/I1, respectively.
١١
-Interharmonics are voltages or currents having frequency components that are not
integer multiples of the fundamental frequency. They can appear as discrete
frequencies or as a wide-band spectrum. Cyclo-converters are one of the sources of
interharmonics. It must be noted that due to the limitations of power quality
instruments time-varying harmonics can be recorded as interharmonics. Technically
sound methods for the accurate recording of the interharmonics have yet to be
developed. The effects of interharmonics are not known well. They have been shown
to affect power line carrier signaling.
Figure (2 –6) waveforms with Harmonic and Interharmonic Components
-Notching, as shown in figure (2-7). It is a periodic voltage disturbance caused by the
normal operation of power electronics devices when current is commutated from one
phase to another. Notching can be characterized through the harmonic spectrum of the
affected voltage. Although notching is a special case of voltage harmonics, it is
generally treated as an independent disturbance. The frequency components
associated with notching can be quite high and may not be readily measured with
equipment normally used for harmonic analysis.
Figure (2 –7) voltage notching due to power electronic devices.
١٢
-Noise, as shown in figure (2-8). It 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 in neutral conductors or signal lines.
Power electronic devices can cause noise in power systems, control circuits, and
arcing equipment. Improper grounding that fails to conduct noise away from the
power system often exacerbates noise problems. Basically, noise consists of any
unwanted distortion of the power signal that cannot be classified as harmonic
distortion or a transient. Noise disturbs electronic devices such as microcomputers and
programmable controllers.
Figure (2 –8) voltage noises
-DC Offset refers to the presence of a dc voltage or current in an ac power system.
This phenomenon can occur as the result of a geomagnetic disturbance or due to the
effect of half-wave rectification. Direct current in alternating-current networks can
have a detrimental effect by biasing transformer core fluxes. Transformers can
become saturated even in normal operation. This causes additional heating, loss of
transformer life, and the production of harmonics. DC offset may also cause the
electrolytic erosion of grounding electrodes and other connectors.
2.3.13- Distortion power factor [7]
Example for the non-sinusoidal situations is shown in figure (2-9). An expression for
distortion power factor can be arrived from current and voltage harmonic distortion
factors. From [7] and the definition of these factors, rms harmonic voltages and
currents can be written as
١٣
Figure (2-9): power system with nonlinear load
2
)( )100(1 Vrmshrms THDVV += (2-3)
2
)( )100(1 Irmshrms THDII += (2-4)
Therefore, the total power factor is
22)()( )100(1)100(1 IVrmsrmshrmshrmstotal
total
THDTHDIV
P
IV
P
S
Ppf
++=== (2-5)
Where, 222 DQPS total ++= , D is the distortion power , (2-6)
Neglecting the power contributed by harmonics and also voltage distortion, as it is
generally small
distortiontdisplacmen
I
total pfpfTHD
pf •=+
−=2
11
)100(1
1)cos( θδ (2-7)
Because displacement power factor (pfdisp) can never be greater than unity, then the
true power factor in non-sinusoidal situations has the upper bound
distortiontotal pfpf ≤ . (2-8)
It is important to point out that it cannot be, in general, compensate for poor distortion
power factor by adding shunt capacitors. Only the displacement power factor can be
improved with capacitors. This fact is especially important in load areas that are
dominated by single-phase power electronic loads, which tend to have high
displacement power factors but low distortion power factors. In these instances, the
١٤
addition of shunt capacitors will likely worsen the power factor by inducing resonance
and higher harmonic levels. A better solution is to add passive or active filters to
remove the harmonics produced by the nonlinear loads, or to utilize low distortion
power electronic loads.
2.4- Power quality monitoring: [8] and [9]
Power quality monitoring is needed to determine what type of power quality
disturbance is present, as disturbances are not always obvious. Many types of power
quality monitors are available. It measures and records all energy data such as
voltage, current, average power factor, instantaneous power factor, active power,
reactive power, frequency and all the disturbances.
Monitoring power quality instruments are connected to the network through
transducers and transducers should be selected according to the system. This will
require current transformers for current measurement on low voltage. Higher voltages
require correctly installed current and voltage transformers for connection of
monitoring equipment. Transducers should be selected based on the frequency
response required.
There are some basic instruments specifications such as the following: -
- Appropriate variable threshold for all required power quality disturbances.
- Range of disturbance types recorded.
- High crest value needed to avoid clipping and modifying over voltage in the
monitored waveform.
- Can ride through disturbances.
- Type and amount of data stored – raw data or parameters, sufficient for the desired
logging period.
- Number of channels, sampling rate and accuracy: 7 channels are necessary for the
recording of three phase voltage, current and neutral current as is desirable for
investigation of plant problems. Another channel for measuring neutral-earth voltage
can also be useful.
- Good reporting software.
- Associated large database capabilities.
- Easy to use.
١٥
2.5- Power quality standards: [10], [11] and [12]
The purpose of power quality standards is to protect utility and end-user equipment
from failing or misoperation when the voltage, current or frequency deviates from
normal values. Power quality standards provide this protection by setting measurable
limits as to how for the voltage, current or frequency can deviate from normal values,
by setting these limits.
Power quality standards help utilities and customers to gain agreement to what are
acceptable and unacceptable levels of service. The customer should have level of
service corresponding to their devices, it is impossible to obtain a pure voltage wave
with fixed amplitude at each customer terminals. So, acceptable and allowed
disturbance levels should be specified in networks at different levels. There are many
standard organizations provide the acceptable levels. Such as Institute of Electrical
and Electronics Engineers (IEEE), European Union Standards organization
(CENELEC), International Electrotechnical Commission (IEC), American National
Standard Institute (ANSI), National Electrical Manufactures Association (NEMA),
Underwriters Laboratories (UL), National Fire Protection Association (NFPA) and
Semiconductor Equipment and Material International (SEMI).
2.5.1- Standard levels in power quality: [13] and [14]
- IEEE Standard 141-1993, Recommended Practice for Electric Power Distribution
for Industrial Plants. A thorough analysis of the basic electrical-system
considerations are presented. Guidance is provided in design, construction, and
continuity of an overall system to achieve safety of life and preservation of property;
reliability; simplicity of operation; voltage regulation in the utilization of equipment
within the tolerance limits under all load conditions; care and maintenance; and
flexibility to permit development and expansion.
- IEEE Standard 142-1991, Recommended Practice for Grounding of Industrial and
Commercial Power Systems. Presents a thorough investigation of the problems of
grounding and the methods for solving these problems .
- IEEE Standard 242-2001, Recommended Practice for Protection and Coordination
of Industrial and Commercial Power Systems. Deals with the proper election
١٦
application and coordination of the components which constitute system protection
for industrial plants and commercial buildings.
- IEEE Standard 446-1995, Recommended Practice for Emergency and Standby
Power Systems for Industrial and Commercial Applications. Recommended
engineering practices for the selection and application of emergency and standby
power systems. It provides facility designers, operators and owners with guidelines
for assuring uninterrupted power, virtually free of frequency excursions and voltage
dips, surges, and transients.
- IEEE Standard 493-1995, Recommended Practice for Design of Reliable Industrial
and Commercial Power Systems, the fundamentals of reliability analysis as it applies
to the planning and design of industrial and commercial electric power distribution
systems are presented. Included are basic concepts of reliability analysis by
probability methods, fundamentals of power system reliability evaluation, economic
evaluation of reliability, cost of power outage data, equipment reliability data, and
examples of reliability analysis. Emergency and standby power, electrical preventive
maintenance, and evaluating and improving reliability of the existing plant are also
addressed.
- IEEE Standard 519-1992, Recommended Practices and Requirements for
Harmonic Control in Electrical Power Systems .This guide applies to all types of
static power converters used in industrial and commercial power systems. The
problems involved in the harmonic control and reactive compensation of such
converters are addressed, and an application guide is provided. Limits of disturbances
to the AC power distribution system that affect other equipment and communications
are recommended. This guide is not intended to cover the effect of radio frequency
interference.
- IEEE Standard 929-2000, Recommended Practice for Utility Interface of
Photovoltaic (PV) Systems.
- IEEE Standard 1100-2005, Recommended Practice for Powering and Grounding
Sensitive Electronic Equipment. Recommended design, installation, and maintenance
practices for electrical power and grounding (including both power-related and signal-
١٧
related noise control) of sensitive electronic processing equipment used in commercial
and industrial applications.
- IEEE Standard 1159.3-2003, Recommended practice for the transfer of power
quality data.
- IEEE Standard 1250-1995, Guide for Service to Equipment Sensitive to
Momentary Voltage Disturbances. Computers, computer-like products, and
equipment using solid-state power conversion have created entirely new areas of
power quality considerations. There is an increasing awareness that much of this new
user equipment is not designed to withstand the surges, faults, and reclosing duty
present on typical distribution systems. Momentary voltage disturbances occurring in
AC power distribution and utilization systems, their potential effects on this new
sensitive, user equipment and guidance toward mitigation of these effects are
described. Harmonic distortion limits are also discussed.
- IEEE Standard 1346-1998, Recommended Practice for Evaluating Electric Power
System Compatibility with Electronic Process Equipment. A standard methodology
for the technical and financial analysis of voltage sag compatibility between process
equipment and electric power systems is recommended. The methodology presented
is intended to be used as a planning tool to quantify the voltage sag environment and
process sensitivity. It shows how technical and financial alternatives can be evaluated.
Performance limits for utility systems, power distribution systems, or electronic
process equipment is not included.
- IEEE Standard 18-2002, Standard for Shunt Power Capacitor.
- IEEE Standard 1453-2004, Recommended Practice for Measurement and Limits
of Voltage Fluctuations and Associated Light Flicker on AC Power System.
- IEEE Standard 1159-2009, Recommended Practice for Monitoring Electric Power
Quality. Monitoring of electric power quality of AC power systems, definitions of
power quality terminology, impact of poor power quality on utility and customer
equipment, and the measurements of electromagnetic phenomena are covered.
١٨
- SEMI E-10-1999, Standard for Definition and Measurement of Equipment
Reliability, Availability, and Maintainability, it defines the sag ride through
capability.
- SEMI F-42-1999 ،Test Method for Semiconductor Processing Equipment Voltage
Sag Immunity, it defines the test methodology to confirm compliance to the standard.
- UL 1449 Second Edition, 1998. Standard 1449 is a Safety, Construction, and
Performance Standard for Transient Voltage Surge Suppressors. The second edition
revision includes withstand for loss of neutral condition without damage to the
suppressor.
- ANSI C84.1-1995, Electric Power Systems and Equipment - Voltage Ratings.
Published by NEMA, National Electrical Manufacturers Association, This voluntary
standard was first approved in 1954 as a joint effort by the Edison Electric Institute
and the NEMA to recommend voltage ratings for both electric systems and equipment
to promote compatibility. ANSI standard establishes the steady state voltage delivery
window of +/- 5% at the point of delivery. It also recommends a tolerance window of
+6% and -13% for end use equipment. The standard also establishes a tolerance
window for voltage unbalance of +/-3%.
- EMA MG 1- 1998, Motors and Generators National Electrical Manufacturers
Association. This standard gives technical specifications used by manufacturers.
Power quality concerns that can be referenced include voltage and current unbalance
tolerance, over and under voltage tolerance, electrical starting characteristics, and
insulation values.
- NEMA Standards Publication No. LS1-1992, Low Voltage Surge Protective
Devices. This is a good standard for quality construction of the device.
-EN50160 indicates Voltage Characteristics of Electricity Supplied by Public
Distribution Systems. [11]
- IEC61000-4-15, Flicker meter- Functional and Design Specifications. [12]
١٩
2.6- Power quality solutions: [1]
There are four ways to solve power quality problems:
1- Design equipment and electrical systems to prevent electrical disturbances from
causing equipment or systems to malfunction. Where, manufactures of sensitive
equipment can reduce or eliminate the effect of power quality problems by designing
their equipment to be less sensitive to disturbances. They can add some devices to
their equipment according to situation, for instance a capacitor to provide temporary
energy storage when the voltage sags are too low. They can also alter their equipment
to desensitize it to power quality problem for example; they can design special K-
factor transformers that tolerate harmonics.
2- Analyze the symptoms of power quality problems to determine its cause and
solution. It is important to determine source and type of power quality problems, the
type of power quality problem and its cause often determine the solution.
3-Identify the medium that is transmitting the electrical disturbances and reduce or
eliminate the effect of that medium.
4- Treat the symptoms of the power quality problems by use of power conditioning
equipment. It provides essential protection against disturbances. Power conditioning
equipment include devices that reduce or eliminate the effect of a power quality
disturbance. It can be used to condition the source, the transmitter, or the receiver of
the power quality problems. The equipment can be divided into ten categories, surge
suppressors, noise filter, isolation transformer, low-voltage line reactors, various line-
voltage regulators, motor-generator sets, dual feeders with static transfer,
uninterruptible power supplies, harmonic filters and Dynamic voltage restorer (DVR).
2.6.1-Selection of appropriate power conditioning equipment:
End user should implement the following steps before selecting the appropriate power
conditioning equipment to mitigate their problem:
· Determine the power quality problem.
· Correct wiring and grounding and faulty equipment problems before purchasing
power conditioning equipment.
Recent surveys by ERRI and others indicate that improper grounding and wiring
٢٠
cause 80 to 90 percent of the power quality problems. However, many end users
overlook improper grounding and wiring in their facilities. They should always
investigate the wiring and grounding in their facilities before purchasing and
installing expensive power conditioning equipment.
· Evaluate alternative power conditioning solutions.
· Develop a power-conditioning plane.
· Determine if the utility source is compatible with the load.
· Select and install power conditioning equipment.
· Operate and maintain power-conditioning equipment
Table (2-1) from IEEE standard 1100-1992. Indicates how to select the appropriate
power conditioning technology to match the power quality problem.
٢١
Table (2-1) Summary of IEEE std.1100-1992
The condition should be corrected by the indicated power-conditioning
technology.
There is a significant variation in product performance.
The condition may or may not be fully correctable by the technology.
٢٢
2.7- Power quality assessment procedure (PQAP): [6] and [15]
Power quality assessment procedure is based on a variety of different power quality
concerns that can exist and focuses on a combination of monitoring and analysis to
characterize these concerns. Once the power quality concerns are characterized, the
analysis procedures developed can be used to evaluate possible solutions to the power
quality problems. These solutions must then be evaluated from both a technical and an
economic perspective.
There are a number of important areas that must be addressed in the power quality
assessment procedure. These include:
- Data collection requirements.
- Important power quality concerns as a function of the type of customer.
- Equipment sensitivity.
- Important parameters of the power quality concerns.
- Roles of measurements and simulations in evaluating the concerns.
- Implementation of possible solutions to solve the problem.
Power quality assessment procedure provides a general framework that contains all
the possible elements that may be needed for power quality study. The following
sections summarize the general steps involved in the procedure.
2.7.1-Identify power quality concerns
The specific power quality concerns that need to be evaluated will be different from
customer to customer. A review of the types of equipment used by the customer,
process requirements and economic impacts of problems will lead to a list of concerns
that need to be studied. They can include possible problems with both the utility
distribution system and the customer facilities. Possible power quality problem
categories to be evaluated include the following:
- Voltage transients caused by circuit switching and load switching within the
customer facility.
٢٣
- Harmonic distortion from nonlinear loads.
-Transformer heating caused by harmonic current levels.
-Transient magnification at low voltage capacitor banks.
-Transients and notching associated with power electronics equipment operation.
-Neutral conductor overloading due to harmonic producing loads.
-Voltage flicker from arc furnace loads and arc welding loads.
-Voltage sags due to faults on parallel circuits on the same distribution system or faults
on the transmission system.
-Momentary interruptions at industrial and commercial installations due to recloser
operations on feeder circuit breakers.
-Coupled voltage at customer facilities due to lightning transients on the primary
distribution system.
Identification of particular concerns involved for an installation provides a focus
for the study. Development of a model for analysis of the problem is dependent on the
frequency range of the power quality variations that need to be studied. The model
can be for computer simulations, hand calculations, or application of simple rules.
2.7.2-Collect data
A representation of the customer system and important parts of the utility system
should be developed for preliminary analysis. In cooperation with the customer, the
data is collected and compiled into a database for convenient reference during the
analysis. Important information includes:
- load characteristics
motors
power electronics
process control
computers
٢٤
adjustable speed drives
lighting
- Transformer sizes/ ratings
- Conductor lengths, characteristics
- Customer capacitor sizes, location and switching procedures
- Customer equipment and circuit switching
- Power conditioning equipment
surge suppressors
isolation transformers
constant voltage transformers
U.P.S system
harmonic filters
- Distribution system characteristics
primary voltage
underground / overhead
protection practices and switching procedures
capacitor applications (locations, sizes and switching)
2.7.3-Develop measurement program and perform monitoring
The utility and customer systems being evaluated should be monitored to
characterize the power quality variations. The measurement program should be
designed based on the particular sensitive loads existing at the customer facilities.
Monitoring will typically be performed at the customer service entrance and close to
particular sensitive loads in order to characterize disturbances coming from the utility
system and disturbances which are localized at the sensitive loads. A measurement
program plan should be developed which specifies the following:
٢٥
- Quantities to monitor.
- Monitoring durations.
- Threshold levels which will trigger recording of disturbances.
- Waveform sampling and data storage requirements.
- Analysis procedure and data presentation formats.
The monitoring process requires close cooperation between the customer and utility
personnel. Monitoring sites and instrumentation should be selected based on the
particular concerns being characterized. The duration of monitoring will depend on
the parameters which can affect the power quality concerns.
2.7.4-Evaluate measurement results and develop solutions
The measurement results are analyzed. The initial measurements and the site survey
are used to identify the phenomena involved and the important parameters. This
information is used for possible solutions to the power quality problem.
Once the range of technical solutions is identified, economic analysis need to be
performed to evaluate the possible alternatives for solving customer power quality
problems. These alternatives will generally include the following options:
- Power conditioning and / or filtering at the sensitive loads.
- Central power conditioning and /or filtering at the customer service entrance.
- Changing operating procedures or system design on the utility distribution
system.
- Modification to the design of sensitive loads to make them less sensitive to power
quality variations.
The requirements for each of these options will be developed and the analysis of
measurements results will be performed.
Figure (2-10) indicates a summary of power quality assessment procedure (PQAP)
٢٦
Implement Solution
Identify Power Quality Concerns
Voltage Sags
Momentary Interruptions
Capacitor Switching Transients
Lightning
Harmonic Distortion
Neutral Conductor Overloading
Transformer Heating
Voltage Flicker
Voltage Notching
Circuit/Load Switching Transients
Collect Data
Utility System Data
Customer System Data
Equipment Characteristics
Develop Measurement Program/
Perform Monitoring
Location(s) to Monitor
Quantities to Monitor
Instrumentation Requirements
Thresholds
Analysis Requirementst
Customer Participation
Evaluate Measurement Results/Develop Solutions
Identification of Cause of Problem
Evaluate Solutions
Economic Analysis of Solutions
Power Conditioning
Utility System Modification
Equipment Design Modifications
Figure (2-10) power quality assessment procedure (PQAP)
٢٧
2.8- Related studies done by other researchers:
This part presents preliminary results from different researchers in the power quality
assessment on electric distribution system. These researches were done in various
locations around the world.
- Barker P. P [16], this work done in mid 1989, the Niagara Mohawk Power
Corporation’s Research and Development Department sponsored a major power
quality study of two distribution feeders in the Buffalo, New York region. The aims of
this two-year study were:
i) Assist Niagara Mohawk in identifying and correcting abnormal conditions resulting
from possible poor power quality in a residential area, served by the Shawnee
Substation, and referred to as Wurlitzer Park.
ii) Create a database of power quality measurements, which could be used for system-
wide comparison and analysis.
The results of the study proved that majority of the problems were actually caused by
the customer’s owned equipment.
– Dabbs W. et al [17], in 1990, the Electric Power Research Institute (EPRI) has
decided to corporate with Electrotek Concepts to conduct a study of the state of power
on the various power distribution systems in the United State. The purpose of this
survey is to develop a statistical representation of the power system, the disturbances
being measured, the protective relaying devices and other site characteristics. The
distribution power quality project was planned which involves the monitoring and
simulation of power quality phenomena on the distribution systems.
The project was starting in June 1992, over a period of two years, quite a large
number of PQ nodes were places on a total of about 300 monitoring points. The
results of these studies have shown that the most common type of faults found in the
power disturbances were voltage sag, transients, harmonic distortions and momentary
interruptions.
- Dorr D. S. et al. [18], in 1991, the Canadian Electrical Association (CEA) started a
three year long power quality survey. With twenty-two utilities involved on a total
550 site, the monitored activities went on for 25 days per site. The data collected were
٢٨
from different groups, such as industrial, commercial and residential. The main
objective was to obtain an indication of power quality on the utility’s distribution
system in Canada. In order to avoid getting readings influenced by individual loads or
wiring error in the utilities, the sites being monitored were at the customer's service
entrance panel. The gathered results were then served as a baseline for further
comparisons of newly founded results in the near future.
- M. McGranahan [15], in 1991, the Electric Power Research Institute (EPRI)
published procedure for assessment the power quality. This report presents the results
of an effort sponsored by the Electric Power Research Institute (EPRI) and Pacific Gas
and Electric Company (PG E) to develop a standardized approach for dealing with
power quality problems. The approach taken here is to develop an understanding for
the full range of possible power quality concerns. A set of definitions for different
power quality categories is presented. Under each category, important characteristics
of the concern are identified, possible causes of the power quality variations are
presented, and possible solutions are outlined. After developing an understanding for
the various phenomena, a standard procedure for evaluating power quality problems is
presented. This Power Quality Assessment Procedure (PQAP) involves a combination
of measurements and simulations and emphasizes on the required cooperation between
all the parties involved. A power quality survey was also performed to assist in
identifying the most important concerns reported by customers on the PG E system.
- D. D. Sabin et al[19], this work done in 1996, the Electric Power Research Institute
(EPRI) published a technical report "An assessment of distribution system power
quality”, this report provides a comprehensive statistical database of power quality
measurements collected during the EPRI distribution power quality projects as well as
guidelines for monitoring and modeling power quality phenomena on distribution
system. Presented are triggering methods, characterization algorithms, and statistical
analysis for voltage disturbances.
- Chan, V.K.K [20], this work presents power quality survey at University of
Queensland, St. Lucica campus was done during a thesis study. The monitoring was
carried out in three different locations within the University of Queensland. Two
transformers that were monitored one with a 1000kVA rating and the other one with
٢٩
750kVA rating. The result of this case study has concluded that, the most of the
transients occurred between 5:00 – 7:00AM. These disturbances were probably
caused by the switching of the controlled capacitor banks.
- Tapan Saha et al [21], this paper presents an investigation of power quality
problems in a remote gold mine site in Papua New Guinea and was done during thesis
work at the University of Queensland. The results has concluded that, the common
voltage sag phenomena was observed and a dynamic voltage restorer (DVR) was used
as a solution of this problem.
- Eloi Ngandui [22], this work discusses power quality monitoring and analysis of a
university distribution system. The analysis of the recorded data at the point of
common coupling of the University of Quebec at Trois-Rivieres yield the following
results: - the voltage THD is below the 5 % limit set by the IEEE 519-1992 - the
current injected has a TDD below the 5 % limit set by the IEEE 519-1992. 99% of the
time the TDD is below 2.04 %. The information provided in this analysis gives a
reference level for future study and extension of the electrical installation of the
University of Quebec at Trois-Rivieres.
- M. Izhar et al [23], In 2003, an investigation on power quality at electrical and
electronic engineering department building in the University of science Malaysia for
three phase four-wire system was done. The survey was conducted through out a day
during peak and off peak hours. The harmonic level, total harmonic distortion, neutral
current and power associated such as power consumption, true power factor, etc. for
each line were measured and analyzed. The analysis of data from the measured
distribution system showed that the harmonic level was highest at odd harmonic
compared to even harmonic. The results also reveal that power consumption and
instantaneous power increased during peak hours. The power factor correction
mechanism too was functioning well and approaching unity. One very interesting
factor is that the neutral currents and total harmonic distortion were excessive in the
line neutral distribution system and would affect the distribution system performance.
Ideally, it should be zero.
٣٠
-Wes Sunderman et al [24], this paper presents the findings of a follow-on project,
referred to as DPQ 11, which was conducted in 2001 and 2002 in U.S. This project
resulted in characterizing power quality in terms of short-duration variations such as
voltage sags, voltage swells, and voltage interruptions. The characterization was
based on analysis of data from 480 power quality monitors at different locations in a
power system spanning a date range from August 30, 1993, through December 12,
2002. The results of the analysis that are presented in this paper provide a unique
opportunity to understand the electrical environment in terms of short-duration
variations and further validate the findings of DPQ 1.
- Jose C.C. Costa et al [25], this work presents an example of power quality
assessment study in an electrical panel that supplies investigation laboratories, office
rooms and an electronics workshop, using a developed power quality monitor. The
results of this study are analyzed using international standards as reference in order to
determine the quality of the supplied energy.
-A.M. El-Zonkoly [26], the paper presents an intelligent system for power quality
assessment application. This system is used for power system model validation. A
genetic algorithm (GA) based system for validating the power system model in
capacitor switching studies has been developed. The problem formulation and the
proposed solution are illustrated. The feasibility of the developed system for practical
applications is demonstrated by evaluation studies.
- M.H.J. Bo1 ten et al [27], the work presents model has been incorporated in the
method for reliability/power quality analysis of industrial power systems, Simulations
are presented to show the influence of the post-fault voltage sag due to motor re-
acceleration on the reliability/power quality of the supply. The simulations result in a
table with the expected number of interruptions of plant operation for different load
sensitivities.
-Surya Santoso et al [28], the paper presents an approach to detect, localize, and
investigate the feasibility of classifying various types of power quality disturbances.
The approach is based on wavelet transform analysis, particddy the dyadic-
٣١
orthonormal wavelet transform. The key idea underlying the approach is to
decompose a given disturbance signal into other signals which represent a smoothed
version and a detailed version of the original signal. The decomposition is performed
using multiresolution signal decomposition techniques. They demonstrate and test
their proposed technique to detect and localize disturbances with actual power line
disturbances. In order to enhance the detection outcomes, they utilize the squared
wavelet trans-form coefficients of the analyzed power line signal. Based on the results
of the detection and localization, they carryout an initial investigation of the ability to
uniquely characterize various types of power quality disturbances. This investigation
is based on characterizing the uniqueness of the squared wavelet transform
coefficients for each power quality disturbance.
- G. T. Heydt et al [29], the paper discusses the application of the Windowed Fast
Fourier Transform to electric power quality assessment. The WFFT is a time
windowed version of the Discrete Time Fourier Transform. The window width may
be adjusted and shifted to scan through large volumes of power quality data. Narrow
window widths are used for detailed analyses, and wide window widths are used to
move rapidly across archived power quality data measurements. The mathematics of
the method is discussed and applications are illustrated.
- A. P. Salas Meliopoulos et al [30], the work represents a statistical method to
power quality assessment. The method is based on Monte Carlo simulation of the
integrated utility system/customer system. The proposed integrated model is based on
the physical design parameters of the system. This approach makes it possible to
relate the design parameters of a system to the statistical power quality level of the
system at the customer site. Thus the proposed model is very fusel for meaningful
improvements of the system to maximize power quality.
- E.F. EL-SaadanyY et al [31], the paper focuses on capacitor switching events,
capacitor switching transient depending on capacitor location, load type, load level,
voltage level and instant of switching. The effect of these factors on capacitor
transient was examined by using Wavelet and Fast Fourier Transforms.
٣٢
-T. K. Abdel-Galil et al [32], the paper discusses a new monitoring strategy for
power quality events. This new strategy will be implemented using a distributed
power quality monitoring nodes, which will be connected via Ethernet connection to a
central diagnosis unit.
-Mihaela Albu et al [33], the paper presents using the root mean square, a
mathematical operation in power quality assessment. The root mean square
mathematical operation is widely used in power engineering. The process has a
frequency response characteristic and an associated time constant which is important
especially for short term signals. Potential problem areas in using RMS values in
power quality assessment are identified and discussed.
- Abd-Elmoneim Moussa et al [34], the paper discusses a proposed work to
introduce a new concept of advanced power quality assessment. The introduced
system is implemented using applications of a set of powerful software algorithms
and a digital signal processor based hardware data acquisition system. The suggested
scheme is mainly to construct a system for real time detection and identification of
different types of power quality disturbances that produce a sudden change in the
power quality levels. A new mitigation technique through generating feedback
correction signals for disturbance compensation is addressed. The performance of the
suggested system is tested and verified through real test examples. The obtained
results reveal that, the introduced system detects fast and accurately most of the power
quality disturbance events and introduce new indicative factors estimating the
performance of any supply system subjected to a set number of disturbance events.
- Lucian Mandache et al [35], the paper discusses a new and accurate method of
harmonic analysis that permits to mitigate most of power quality related problems.
The principle is to estimate intermediate points between the initial samples given by
the available data acquisition system; therefore, the Fourier coefficients are estimated
more precisely using the Fast Fourier Transform. As interpolation technique they
chose the reconstruction of the analog signal using an ideal low pass filter. The
excellent results are validated on a pair of synthesized signals having known harmonic
spectrum.
٣٣
- Ivan J. Rivera et al [36], the paper represents a work to develop a time frequency
signal analysis system to detect and recognize different kinds of power quality events
or disturbances. To achieve their goal, design of computational signal processing
methods is addressed using developing time-frequency tools based on signal algebra
operators. The use of signal algebra operators permits the formulation of time-
frequency algorithms in a computational framework setting, allowing the search for
efficient hardware implementations. Time-frequency formulations are implemented
for embedded system applications using Digital Signal Processing (DSP) and Field
Programmable Gate Array (FPGA) units. Voltage disturbances are responsible for
many disruptions in industrial, commercial and residential power supply systems,
causing time and monetary losses. It is necessary to identify fast methods of
determining when these disturbances are occurring in order to make correct power
quality decisions. Automated information processing systems are needed to assess
existing problems. Although a lot of work has been done on power line disturbance
assessment, new techniques are desired to address new challenging power quality
issues, especially in the area of physical security. Time-frequency signal analysis is a
well known tool used in fields such as speech, sonar, and radar processing and is now
finding its being in power quality assessment. A time frequency representation is a
two-dimensional representation that shows how the spectral content of a given signal
changes with time. There is a genuine interest in this kind of representations for the
field of power quality signal analysis systems since they provide more information
than the typical one dimensional analysis. To achieve the goals of this work an
environment is created using MATLAB to simulate power line signal disturbances
such as swells, sags, harmonic distortions, and outages. This environment computes
desired time-frequency representations of selected signals to produce two-dimensional
time frequency characterizations of the disturbances. Time-frequency tools such as
the Discrete Short Time Fourier Transform (DSFT), the Discrete Ambiguity Function
(DAF), and the Discrete Wigner Distribution (DWD) are being used in the MATLAB
environment. These tools are characterizing the voltage disturbances, since the time-
frequency representations reveal patterns or properties that are not readily perceptible
in one dimension.
٣٤
-A. K. Al-Othman et al [37], the paper discusses a new digital approach for the
measurements of voltage flicker and its frequency using Particle Swarm Optimization
technique (PSO). The problem is formulated as a dynamic estimation problem. The
goal is to minimize the error of the estimated coefficients via a deigned fitness
function. The method is tested using simulated case study. The algorithm is tested
using simulated data. Effects of number of samples, sampling frequency and the
sample window size are studied. Results are reported and discussed.
- Cheng-Ting Hsu et al [38], the work presents the power quality assessment of
large synchronous motor starting and loading in the integrated steel-making
cogeneration facility. To execute the transient stability analysis, the proper
mathematical models, and the accurate parameters of the cogeneration units,
excitation systems, governor systems, load, and Static Var Compensators (SVCs) are
investigated in detail. Four case studies with or without considering the connection of
the power grid, the installation of autotransformer (AT) starter, and SVC are
performed to demonstrate the dynamic responses of the system frequency, voltage,
and cogeneration units due to motor starting and loading. Also, the voltage sag ride-
through curve of sensitivity load has been included, and a Power Quality Index (PQI)
due to voltage variation in the assessment period has been proposed to find the impact
of motor starting and loading on the power quality of the cogeneration system. It is
concluded that the system dynamic responses and PQI values have better performance
if the AT starter is applied with either the regulation of the SVC system or connection
to the bulk power grid.
- Surajit Chattopadhyay et al [39], the paper discusses an approach for assessment
of power quality parameters using analysis of fundamental and harmonic voltage and
current waveforms. Park transformation technique has been utilized for the analysis in
three-phase system, which has reduced the computational effort to a great extent.
Contributions of fundamental and harmonic components in power system voltage and
current signals have been assessed separately. An algorithm has been developed to
calculate the power quality parameters from online signals. This algorithm has been
simulated for a radial system, and the results have been compared with that obtained
٣٥
from a standard FFT-based system. The results are seen to be in good agreement with
that of the standard system.
- Sudipta Nath [40], this paper presents the application of continuous wavelet
transform to detect power quality disturbances. A power system network revealing
power quality disturbances has been simulated using Electromagnetic Transient
Program. Then continuous wavelet transform has been applied for feature extraction.
- Sutherland, P. E et al [41], the paper discusses a project to assess the specific
transformer grounding and connection methods that are used at the subject utility for
distributed generators (DGs) and provide a risk assessment in terms of potential
impact, ways to minimize the impact, and site specific screening criteria for additional
protection that may be required for DG interconnection. The utility has standardized
on a grounded wye-grounded wye transformer connection for customers connected to
their distribution system. When a backup generator exists in the customer's facility
that could be paralleled with the utility system, a contactor is installed on the neutral
to ground connection on the generator. This contactor is opened at any time the
generator is paralleled. The utility is considering remotely dispatching, these
distributed generation assets in times of peak load in the future. This will require the
generators to run paralleled for extended periods of time. Grounding practices for
auxiliary generators were evaluated, and simulations of both steady-state and fault
conditions were conducted. The most critical factors were found to be assuring that
the generator ground switch was closed during islanding and that the over-voltage
protection operates correctly to isolate the generator during fault conditions.
Recommendations for over-voltage protection and testing were developed based upon
the results.
- Legarreta, Andres E. et al [42], the work describes some of the constrictions and
requirements assumed for the design of the high performance power quality data
logger PQ1000, taking into account the international standards IEC 61000-4-30 Class
A and the IEC 61000-4-15 published in August 2010. Trough it, the most important
demands of the IEC 61000-4-30 class A instruments are exposed. Details of the
hardware components are also shown, and the most important points of the signal
٣٦
processing path are explained. The performance in the RMS values determination is
given, also the frequency response for harmonics measurement are shown, and a
detailed analysis of fulfillment of the tests given in the IEC 61000-4-15 are given in
the final section.
-Souza, L.F.W. et al [43], this paper discusses methods and criteria for the
assessment of the Low Voltage Ride-Through (LVRT) needs of the power grid.
Simulation techniques are used to investigate both how voltage dips caused by short-
circuits spread over the grid and what impact do they cause in the operation
performance of power systems with wind generation. The analysis is performed for
Wind Trbines (WT) with different LVRT characteristics. It is shown that, depending
on system's topology characteristics and the voltage level of WT's point of
connection; ride-through needs may be different throughout power systems, especially
those of continental countries as Brazil. As a conclusion, it is possible to have less
severe LVRT requirements without risking system operation security.
-Su, H.J. et al [44], This paper presents simulations for numerical models of two
wind turbine schemes, fixed and variable speed types, by using Matlab/Simulink,
where simplified analytical model of wind turbines for voltage spectral analysis are
illustrated for the power quality assessment.
٣٧
CHAPTER (3)
DESCRIPTION OF THE CASE STUDY
AND MEASURING PROCESS
3.1- Description of the case study [45]:
The case study is the Egypt 2nd Testing Research Reactor (ETRR-2), also known as
the Multipurpose Nuclear Reactor (MPR), it is an open pool type reactor, 22 MW
thermal power purchased from INVAP Argentina. The reactor is a powerful tool for
various researchs and applications. Several experimental and production facilities are
installed to meet the requirements of various utilization groups including universities,
research institutes, industry, and medical organizations. Figure (3-1) shows ETRR-2
general view.
Figure (3-1) ETRR-2 general view
The Egyptian 2nd Testing Research Reactor (ETRR-2) has the following benefits:-
- Economic Benefits:
• Production of radioisotopes to be used in different fields (such as, medicine,
agriculture, industry...etc.).
٣٨
• Production of radioactive cobalt-60 radiation sources with applications in food
preservation, oil logging and medicine.
• Production of silicon ingots, doped using Neutron Transmutation Doping
(NTD) techniques.
• Gem stones irradiation.
• Neutron radiography for industrial utilization.
Using Neutron activation analysis for environmental studies, geological mapping and
medical applications.
-Technical Benefits:
• Material testing research.
• Reactors physics research.
• Reactors and thermal engineering research.
• The reactor will serve as a training school to qualify staff on reactor operation
and maintenance.
-National Benefits:
• Development of highly qualified and skilled personnel in the field of research
reactor design and operation.
• Increasing the role of Egypt in the field of nuclear industries.
3.2- Electrical system of ETRR-2 [46]
The electrical loads of the reactors have been classified according to the following
categories:
- Class ‘A’ loads: are those loads essential from a safety point of view, they required
uninterruptible AC power. The capacity of UPS is 15 KVA. This capacity meets all
class ‘A’ required load demands and conditions ( including duty cycles , electrical
transients ) with autonomy of 30 minutes.
- Class ‘B’ loads: are loads whose reconnection to the system is convenient in order
to increase their availability after interruption of electrical supply from the external
lines. Class ‘B’ loads are fed by two sources, the normal power supply and the power
plant. The power plant has two diesel generators design to furnish AC power
٣٩
adequate for supplying class ‘B’ and the uninterruptible power system in case the
external lines are unavilable. The capacity of plant is two generators with 300 KVA
for each.
- Class ‘C’ loads: they admit interruption the supply for definite time. They fed from
the normal power supply. Power is brought to the ETRR-2 substation by two
independent sources at 11kV line. Source1 feeds transformer T1 , and Source 2
feeds transformer T2 . Transformer T1 feeds the left busbars and transformer T2
feeds the right busbars , both the busbars class ‘C’ loads left and right have
redundant full capacity. The load categories and the single line diagram are shown in
figure (3-2).
The normal power supply is capable of starting and operating all required loads and
the transformers are identical, each of them (primary voltage 11kV, 50Hz, secondary
voltage 0.4/0.231kV, connected group Dy 11) has 100 % of the total sum of
individual maximum demands. Each transformer has a capacity of 2000 kVA, which
is the power required for the ETRR-2.
Figure (3-2) single line diagram of the electrical system of ETRR-2
T2 Power Transformer
Source 1 ( 11 Kv)
T1 Power Transformer
2000 KVA
CB1 CB2
CB3
2000 KVA
Class " C" Loads Class " C" Loads
UPS
G G
CB 4 CB 5
CB 6 CB 7CB 8
CB 9
Class A LoadsClass B Loads Class B Loads
Source 2 ( 11 Kv)
٤٠
3.3- Electrical defects of ETRR-2
Because of a lot of electrical power disturbances, The ETRR-2 is affected by
electrical defects such as: -
- Failure of some component such as electronic cards, capacitors, etc.
- Phantom tripping of breakers, noisy bus ducts and distribution equipment.
- Overheating at low loads, noisy and failures of transformers.
- Overheating, excessive vibration, noise and winding burning out of motors.
- A lot of lamps and ballasts failing at abnormal rate.
- Erratic operation of sensitive equipment.
- Loss of power and hence shutdown the reactor.
- Malfunction and error signals lead to shut down the reactor.
- Bad performance of the electrical system.
All the problems described above, reflect poor power quality of electrical system of
ETRR-2. These problems lead to increase of operating costs and decrease the useful
life of the system component. Where, any disturbance leads to shut down the reactor
and then loss of money. The amount of money loss is depending on the cost and the
state of the reactor fuel. Normally the reactor needs about one hour to restart after
scram, but need about 50 hours to restart if it was at the end of fuel cycle due to the
reactivity. From experience the disturbance cost of ETRR-2 is approximately (5000 $)
at normal state.
3.4- Measuring process:
A general rule, it is necessary to test each location for at least one week, unless results
definity indicate power quality issues at location that could causing problems. In such
case the interval could be shorted [10]. The electrical power of the nuclear research
reactor is supplied to the installation via two incoming feeders from two different
substations to keep good reliability. Each feeder is designed to carry the full load. The
monitoring investigations were performed at the nuclear research reactor for both
feeders at different operation conditions. Measurements were carried out with power
analyzer instrument on the medium voltage side for each source separately.
٤١
3.4.1- power quality analyzer (UNIPOWER 900F) [47]
Network optimization and preventive measures increase in actuality when the
demands from power quality are increased. Long term measurements are required in
order to demonstrate that delivered electric energy maintains a promised quality.
The network analyzer Unipower 900F from Unipower is an excellent aid to this type
of weekly or monthly measurements. So, it is chosen as a power quality analyzer
instrument.
- Main functions of Unipower 900F:
-Automatic transducer identification
- General purpose transmitters 0-20 and 4-20mA
-Measures Quantities:
All units measured simultaneously and the Unipower 900F has the following
measurements: -
- Quantities and units like V, A, W, VA, VAr, kWh, kVArh, PF, CosΦ, Hz, °C,
IFL, PST, PLT
- Transients
-Voltage sag (dips) and swell.
- Flicker (one and three phase)
-Harmonics. Odd, even, inter harmonics and power harmonics
- Three-phase voltage unbalance
- Frequency deviations
- In-Rush current
- Temperature
٤٢
-Performance of Unipower 900F:
Unipower 900F is an eight-channel network analyzer with its own hard disc and
battery operation for up to fifteen minutes of standby operation. The unit can store
multiple files in sequences without previous dumping. Useful if it is necessary to
measure on several points in the same facility and collect an amount of measuring
files. Figure (3-3) shows the front panel of the Analyzer (Unipower 900F).
Figure (3-3) the Unipower 900F front panel
- Real-time measurements:
The network analyzer Unipower 900F allows measurements to take place in real-time
for all units simultaneously. The oscilloscope waveforms for currents and voltages
can be studied as well as the vector diagram phase positions. The real-time window
allows studying of waveforms, diagram for harmonics, flicker, etc. With the
disturbance analyzer it is easy to capture transients, sags and swells, and also voltage
and current levels. Simply adjust nominal voltage level with permitted discrepancy
and instruct Unipower 900F to monitor the waveforms for all the phases. If and when
a disturbance appears it will be captured by the analyzer. The waveforms are stored
on the hard disc and the disturbances can then be shown on a viewing screen.
-Evaluation of Unipower 900F
- Power Profile shows data and waveforms in a graphic mode, easy to print and to
copy.
- Unipower Report for power quality assurance in relation to engineering standard
and standard specifications e.g. EN 50160 including automatic report generation.
٤٣
- Possibility to export data and measured files to other software, for instance spread
sheets. The Unipower 900F complies with the international standard specifications.
3.4.2-Measuring Points:
Measurments were carried out on the medium voltage side of the following:-
1- Incoming feeder from substation1 (source1) for one week.
2- Incoming feeder from substation2 (source2) for one week .
As shown in figure (3-4) When the measurments are carried out of source1 from
substation1 , the distribution center (PCC) feeds through panel (Pn2) and panels no.
(1,3,4) were switched off. And when the measurments are carried out of source2
from substation2 , the distribution center feeds through panel (Pn3) and panels no.
(4,1,2) were switched off.
Measurments were carried out under different operation conditions of Egypt 2nd
Testing Research Reactor (ETRR-2) as indicated in the following chapter.
Source 1 Source 2
Pn1 Pn2 Pn3 Pn4
Pn5,6
Pn7 8 9 10 11 12 13 14
Senstive Senstive
load1 load2
2 MVA2 MVAOther Loads
Other Loads
11 KV 11 KV
Points of Measurment
PCC 11 KV
Figure (3-4) Single line diagram of distribution center and measuring points
٤٤
CHAPTER (4)
MONITORING IN ETRR-2
4.1- Introduction :
This chapter illustrates the analysis and discussion of the results collected by power
analyzer and evaluating the power quality in accordance with the international
standard specifications. The monitoring process was carried out in the Egypt 2nd
Testing Research Reactor (ETRR-2) at two ponits as the following:
- Incoming feeder from substation1 (source1) for a period of one week.
- Incoming feeder from substation2 (source2) for a period of one week.
4.2- Measurement results:
The block diagram in figure (3-4) shows the single line diagram of distribution center
and measuring points.
After gathering data, an analysis using MS Excel was done. And all the daily trend
graphs are shown in appendex B. The discussion of the results and analysis are
shown at the following:-
4.3-Measurement results of the first feeder (source1):
4.3.1- Harmonics:
During the measurements period, it is observed that all the three phases are combined
into one single plot for comparisons. As shown in figure (4-1) the Voltage Total
Harmonics Distortion (VTHD) levels do not exceed the acceptable tolerance of 5 % set
by the IEEE standard, but from the recorded measurements data the 5th harmonic
exceeds the acceptable tolerance of 3 %. And as shown in figure (4-2) the Current Total
Harmonics Distortion (ITHD) levels are not exceeding the acceptable tolerance of 15 %
set by the IEEE standard. However there is an immense increase in the ITHD levels at
one point and referring to figures (4-3) and (4-4). This point is light load, wherever the
power at this point is approximately 180 kVA.
٤٥
Figure (4-1) VTHD of incoming feeder of Source 1
Figure (4-2) ITHD of incoming feeder of Source 1
٤٦
Figure (4-3) ITHD variations of source 1 at 2nd day
Figure (4-4) power variations of source 1 at 2nd day
٤٧
From the analysis of measured data, the values of VTHD and ITHD at maximum and
minimum loads are indicated in table (4-1). It is obvious that the THD at minimum
load is greater than at maximum load.
Table(4-1) VTHD and ITHD at max. and min.loads of source1
VTHD(%) ITHD(%)
R
S
T
IEEE
limit
R
S
T
IEEE
limit
Max
. lo
ad
1.45
1.63
1.64
5
1.7
2.1
1.91
15
Min
. lo
ad
2.6
2.95
2.8
5
6.44
5.45
7.64
15
4.3.2- Voltage Deviation
As shown in table (4-2) the percentage of voltage deviation does not exceed the IEEE
limit which is 5%.
Table (4-2) Voltage deviation at max. and min. loads of source1
Percentage of Voltage deviation
At max.load At min.load Accepted limit
1.65 - 1.72 4.1 - 4.45 5
٤٨
4.3.3- Voltage Unbalance
The percentage of voltage unbalance does not exceed the accepted IEEE limit of 2 %,
which most equipment can tolerate. The percentage of voltage unbalances values at
maximum and minimum loads are indicated in table (4-3).
Table(4-3) Voltage unbalance at max. and min. loads of source1
Percentage of voltage unbalance
At max.load At min.load Accepted limit
0.062 0.06 2
4.3.4- Voltage swell:
Occurrence of voltage swell on phase3 is being observed; it was occurred on fourth
day at (10:00:22 AM). As shown in figure (4-5). Percentage voltage increasing is 25%
and lasted for 150 ms. Referring to the trend graphs, the system is stable and the
power is about 200 kW and there was no capacitor switched on. The disturbance is
occurred at phase 3 only due to network transient. This indicated that this disturbance
is imported from the substation.
٤٩
Time
Vo
lta
ge
Figure (4-5) Voltage swell on phase 3
4.3.5- Overvoltage:
During the monitoring priod, an over-voltage of about 10% of the nominal value was
occurred on 6th day at (06:09:03 AM); and it was lasted for 1 minute. This disturbance
was occurred on three phases as shown in figures (4-6), (4-7) and (4-8) respectively.
Reffering to the measured data and the trend graphs, the power is constant at 400 kW
and there is no capacitor bank switching on. This disturbance was probably caused by
the switching of large loads.
٥٠
Time
Vo
lta
ge
Figure (4-6) Overvoltage on phase 1
Time
Voltage
Figure (4-7) Overvoltage that occur on phase 2
٥١
Time
Vo
lta
ge
Figure (4-8) Overvoltage that occur on phase 3
4.3.6- Short Time Flicker
During the measurements period, and as shown in figure (4-9) it is observed that, the
short time flicker (Pst) exceeds the acceptable limits. The short term severity (Pst) is
measured over a period of ten minutes. Short time flicker (Pst) is indicated daily. It is
imported from the substation because it is feeding industrial loads and these loads are
considered sources of voltage flicker.
4.3.7- Long time flicker
The long time flicker (Plt) exceeds the limit as shown in figure (4-10). It is indicated
daily. The long term severity (Plt) is calculated from a sequence of 12 Pst - values over
a two hours interval, according to the following expression:
3
12
1
3
12∑ =
=i
sti
lt
pp
٥٢
Fig. (4-9) Short time voltage flicker of Source 1
Fig (4-10) Long time voltage flicker of source 1
٥٣
4.3.8- Power frequency variations
Throughout the whole survey, the power frequency variations do not exceed the IEEE
limit, which is ±1%.
4.4- Site appraisal of incoming feeder from substation1 (source 1):
Generally, the analysis of all the data attained during the survey compared with the
power system disturbance suggests that the power quality is relatively good at source
1, with the exception of a few irregular occurrences. The results of monitoring are:-
- THD does not exceed the IEEE limits, but the fifth harmonic exceeds 3%.
- Voltage deviation does not exceed the IEEE limits.
- Voltage imbalance does not exceed the IEEE limits.
- Power frequency variations don not exceed the IEEE limits.
- Flicker exceeds the IEEE limits.
- Occurrence of few irregular disturbances (voltage swell, over-voltage and flicker).
The power disturbance encountered during the period of the investigation were due to
the natural causes of the disturbances.
٥٤
4.5- Measurement results of the second feeder (source2):
4.5.1- Harmonics:
During the monitoring period, it is observed that all three phases are combined into
one single plot for comparisons. As shown in figure (4-11) generally the Voltage
Total Harmonics Distortion (VTHD) levels do not exceed the acceptable tolerance of
5% set by the IEEE standard. However, there is an immense increase in the VTHD
levels at one point. And referring to figures (4-12) and (4-13) the power is
approximately 150 kVA, so, this point is considered light load.
Voltage Total Harmonic Distortion variation
0
5
10
15
20
25
30
35
40
14:05 21:17 04:29 11:41 18:53 02:05 09:17 16:29 23:41 06:53 14:05 21:17 04:29 11:41
Time
VTHD
VTHD1 VTHD2 VTHD3
Figure (4-11) VTHD of incoming feeder of source 2
٥٥
Figure (4-12) VTHD variations of source 2 at 2nd day
Figure (4-13) power variations of source 2 at 2nd day
٥٦
And as shown in figure (4-14) the Current Total Harmonics Distortion (ITHD) levels
are close to the acceptable tolerance of 15% set by the IEEE standard. But there are
increasing in the ITHD levels at three points. Referring to figures (4-15), (4-16), (4-
17) and (4-18), the power is very low at these points, hence these points are
considered light load.
Current Total Harmonic Distortion Variation
0
10
20
30
40
50
60
70
80
90
100
14:05 21:17 04:29 11:41 18:53 02:05 09:17 16:29 23:41 06:53 14:05 21:17 04:29 11:41
Time
ITHD
ITHD1 ITHD2 ITHD3
IEEE Limit
Figure (4-14) ITHD of incoming feeder of source 2
٥٧
Figure (4-15) ITHD variations of source 2 at 2nd day
Figure (4-16) power variations of source 2 at 2nd day
٥٨
Figure (4-17) ITHD variations of source 2 at 3rd day
Figure (4-18) power variations of source 2 at 3rd day
٥٩
And the values of VTHD and ITHD at maximum and minimum loads are indicated in
table (4-4). It is obvious that the THD at minimum load is greater than at maximum
load.
Table (4-4) VTHD and ITHD at max. and min. loads of source2
VTHD(%) ITHD(%)
R
S
T
IEEE
limit
R
S
T
IEEE
limit
Max
. lo
ad
1.18
1.26
1.22
5
1.92
1.93
2.02
15
Min
. lo
ad
1.43
1.45
1.41
5
9.91
10.44
9.98
15
4.5.2- Voltage deviation
As shown in table (4-5) that the percentage of voltage deviation does not exceed the
limit of IEEE standard which is 5%.
Table (4-5). Voltage deviation at max. and min. loads of source2
Percentage of Voltage deviation
At max.load At min.load Accepted limit
3.31 – 3.42 -.096 - -.099 5
٦٠
4.5.3- Voltage Unbalance
The percentage of voltage unbalance does not exceed the IEEE accepted limit of 2%,
which the most equipment can tolerate. The percentage of voltage unbalance values at
maximum and minimum loads are indicated in table (4-6).
Table(4-6) Voltage unbalance at max.and min. loads of source2
Percentage of voltage unbalance
At max.load At min.load Accepted limit
0.083 0.28 2
4.5.4- Undervoltage
An undervoltage of about 20% of the nominal value was occurred on 3rd day at (10:15
AM); and it was lasted for 1.1 min, and from the recorded measurments data the
power is 450 kW and there is no capacitor bank switching off. And the disturbance
has a long time. So this disturbance is imported from the utility.
4.5.5- voltage sag
It’s observed the occurrence of voltage sag on phase 1. As shown in figure (4-19),
percentage voltage reduction is 35%, for duration of 250 ms. And also, on phase 3 as
shown in figure (4-20) that percentage voltage reduction is 30%, for duration of 250
ms. This disturbance occurred on 4th day at (17:48:40 PM). Referring to the trend
graphs, the power is about 250 kW and there is no starting or short circuit at the
electrical system of ETRR-2. So, this disturbance is imported from the substation.
٦١
Time
Vo
lta
ge
Figure (4-19) Voltage sag on phase 1
Time
Vo
lta
ge
Figure (4-20) Voltage sag on phase3
٦٢
4.5.6- Voltage swell
It’s observed the occurrence of voltage swell on phase 3. As shown in figure (4-21),
the percentage voltage increasing is 25%, for duration of 200 ms. This disturbance
was occurred on 4th day at (17:49:10 PM). Referring to the trend graph, the system is
stable and the power is about 250 kW and there is no capacitor switched on. The
disturbance is occurred at phase 3 only due to transient network. This disturbance is
imported from the substation.
Time
Vo
lta
ge
Figure ( 4-21) Voltage swell on phase 3
4.5.7- Voltage Transient
There was a transient voltage observed during the period of monitoring, it’s observed
the occurrence of voltage transient on the three phases as shown in figures (4-22) and
(4-23). These disturbances were occurred on 4th day at (08:07:39 AM) and (16:25:03
PM) respectively. Referring to the power trend graphs and the recorded data the
ETRR-2 load is stable; there is no starting or any variation. So, this disturbance is
imported from the substation also. This disturbance was probably caused by
switching or short circuit.
٦٣
Time
Voltage
Figure (4-22) Voltage transient on the three phases
Time
Vo
lta
ge
Figure (4-23) Voltage transient on the three phases
٦٤
4.5.8- Outage
Throughout the whole survey, there was one outage occurred and lasted for 500 ms;
It was occurred on 4th day at (09:03:02 AM). As shown in fig. (4-24). There is no
faults or accidents in the electrical system of ETRR-2, so this outage occurred from
the substaion.
Time
Voltage
0
Figure (4-24) Voltage Outage.
4.5.9- Short Time Flicker
During the period of monitoring the short time flicker (Pst) exceeds the limit as
shown in figure (4-25). It is indicated daily. Because of the substation is feeding
industrial loads and these loads including arc furnace and welders. These loads are
considered sources of voltage flicker, so this disturbance is imported from the
substation.
4.5.10- Long Time Flicker
The long time flicker (Plt) exceeds the IEEE limit as shown in figure (4-26). It is
indicated daily. The long time flicker is calculated from a sequence 12 Pst – values
over a two hours interval.
٦٥
Figure ( 4-25) Short time voltage flicker of source 2
Figure ( 4-26) Long time voltage flicker
٦٦
4.5.11- Power frequency variations
Throughout the whole survey, the power frequency variations did not exceed the
IEEE limit, which is ±1%.
4.6 - Site appraisal of incoming feeder 2 (Source 2):
Generally, the analysis of all the data attained during the survey compared with the
power system disturbance suggests that the power quality is bad at the incoming
feedre of substation2 (Source 2), there are many irregular occurrences, like
(undrevoltage, voltage sag, voltage swell, voltage transients, temporary outage, and
flicker ). The results are:-
- THD does not exceed the IEEE limits.
- Voltage deviation does not exceed the IEEE limits.
- Voltage imbalance does not exceed the IEEE limits.
- Power frequency variations does not exceed the IEEE limits
- Flicker exceeds the IEEE limits.
- Occurrence of many irregular disturbances (under-voltage, voltage sag, voltage
swell, voltage transients and flicker).
From experience, most of the electrical defects of ETRR-2 occurred when ETRR-2
was fed through the incoming feeder of substation 2. This substation is pollutant
because it is feeding industrial and non-linear loads which are considered as source of
power disturbances.
٦٧
4.7- Invesigation results and mitigation techniques:
Generally, the analysis on all the information gathered during the investigation period
done in ETRR-2, there are many irregular disturbamces such as :-
- Fifth harmonic.
- Voltage flicker.
- Voltage sags/swells.
- Under/over voltage.
- Transients.
- Temporary outage (500 ms).
Referring to the results of power quality monitoring of the electrical system of
ETRR-2, and according to the power quality solutions, mitigation techniques are
suggested as the following: -
- Install Passive Filters at low voltage side.
- Install Dynamic Voltage Restorer (DVR) at medium voltage side.
- All sensitive and critical loads should be isolated and fed through
Uninterruptible Power Supply (UPS).
The following chapter will indicate the mitigation applications required for
elimination of the power disturbances of the electical system of ETRR-2.
٦٨
CHAPTER (5)
MITIGATION APPLICATIONS
5.1- Introduction
Disturbance mitigation can be done through changes in the power system
configuration, increasing equipment immunity or the use of mitigation devices. It is
well known that the delta connection of transformer prevents the flow of triplen
harmonics. Other changes in the system configuration that play a role in the
mitigation of power quality problems, especially voltage sags are in the reduction of
the number of faults, reducing fault-clearing time or designing parallel feeders. [21].
Referring to the results of monitoring of power quality of the electrical system of
ETRR-2 and according to the power quality solutions, mitigation techniques are
suggested as the following: -
- Install Passive Filters at low voltage side.
- Install Dynamic Voltage Restorer (DVR) at medium voltage side.
- All sensitive and critical loads should be fed through Uninterruptible Power
Supply (UPS).
Passive filters are the most common method used to control the flow of harmonic
currents. The most common design is a single-tuned filter.
DVR is used to protect voltage sags on lines feeding sensitive / critical equipment, the
DVR is specifically designed for large loads served at distribution voltage. [48]
Normally, UPS are used to interface critical loads such as computers and
communication systems to the utility systems. The most common design of UPS is
The double conversion mode.
5.2- Harmonic filters:
Passive filters at suitable locations, preferably close to the source of harmonic
generation can be provided so that, the harmonic currents are trapped at the source
and the currents propagated in the system are reduced. The active filtering techniques,
generally incorporated with the harmonic producing equipment itself can reduce the
٦٩
harmonic generation at the source. Hybrid combinations of active and passive filters
are also a possibility.
5.2.1- Shunt filters [49] and [50]:
Shunt filters are the most common method used to control the flow of harmonic
currents. They are designed as a combination series of reactors and capacitors. They
are referred to as “tuned filters” or “traps” because they absorb the harmonic current
to which they are tuned. The most common design of shunt filter is a Single Tuned
Filter (STF). As shown in figure (5-1) it is connected as shunt element in parallel with
the source of a certain harmonic considered. This harmonic current is shorted to
ground with the filter and prevented from entering the system. The filter may have a
series resistance either added to it or found inherently in its inductance. The resonance
frequency is given by the following expression:
L
C
rX
Xf
LCf 0
2
1==
π (5-1)
Where:
fr : Resonance frequency.
L : The inductance of the filter.
C : the capacitance of the filter.
f0 : the fundamental frequency.
XL : Inductive reactance at fundamental frequency = 2Лf0L.
XC : Capacitive reactance at fundamental frequency =1/(2Лf0C).
Also the following parameters are defined to filter:
R : Series resistance.
Q : Quality factor of the filter , it is equal to the ratio of the inductive or capacitive
reactance at resonance, to the resistance. It ranges from 15 to 80 for the filters used in
٧٠
low voltages in industrial and commercial applications. It determines the sharpness of
tuning and, it determines the fundamental frequency losses.
Figure (5-1) single tuned filter
-Steps of filter design [51] and [52]:
1- Determine the capacitance needed to improve the power factor to eliminate any
penalty. This usually is about 0.95
2- Select the reactor to series tune the capacitor to the desired harmonic order. The
tuned frequency is taken slightly less than the filter harmonic order (3-10 %) to
reduce the stresses on the filter and to avoid parallel resonance at less harmonic order.
3- Calculate the voltage and current stresses on the capacitors and inductors of the
filter.
i- Capacitor duties
1. rms current.
2. Harmonic voltage.
3. Fundamental voltage.
4. Maximum peak voltage.
5. Maximum peak current.
6. Maximum reactive power.
ii- Inductor duties
1- Fundamental current.
2- Filter rms current.
٧١
3- Harmonic current.
4- Choose standard components to the duty that is placed upon them.
5-2-2- Design of fifth harmonic passive filter: [53]
Referring to the results of monitoring in ETRR-2, the Total Harmonic Distortion
(THD) does not exceed the IEEE limits, but the fifth harmonic level increases 3%. It
is considered a common feature in the ETRR-2 system, and must be mitigated.
According to the power quality solutions, mitigation techniques are suggested
installing passive filter at low voltage side. The most cost-effective filter is generally a
single- tuned passive filter and this will be applicable for the majority of the cases.
Filters must be carefully designed to avoid unexpected interactions with the system.
Referring to the electrical system of ETRR-2,
Harmonic order = 5
Three-phase capacitor bank rating = 500 kVAr
Capacity rating = 500 kVAr, 400 V
Nominal bus voltage = 400 V
Transformer rating = 2000 kVA, 6%
5th harmonic current = 40% of fundamental current (assumed)
Filter tuning harmonic = 4.7
Utility harmonic voltage source = 1%
The design steps as the following: -
Capacitor F.L current
IF.L = Q / ( 3 *V) = 500 / ( 3 *0.4) = 721.68 A (5-2)
Capacitor reactance
Q = V2 / XC, hence, XC = (0.4)2 / 0.5 = 0.32 Ω (5-3)
٧٢
Reactor reactance
XR = XC / h2 = 0.32 / (4.7)2 = 0.01448 Ω (5-4)
Filter reactance at fundamental frequency
XF = XC –XR = -0.32 + 0.01448 = -0.305 Ω (5-5)
Filter fundamental current
IFF = V / (XF* 3 ) = 400 / (0.305* 3 ) = 757.18 A (5-6)
Filter reactive power
QF = 3 * V* IF = 3 * 0.4*757.18 = 524.59 kVAr (5-7)
Transformer reactance at 5th harmonic
XT5 = 5*0.06*V2 / S = 5*0.06*(0.4)2 / 2 = 0.024 Ω (5-8)
Filter reactance at 5th harmonic
XF5 = (h*XR)-(XC/h) = (5*0.01448)-(0.32/5) = 0.0084 Ω (5-9)
Total harmonic reactance
XTOTAL = XT5+XF5 = 0.024+0.0084 = 0.0324 Ω (5-10)
Filter utility harmonic current
IF5 = (0.01*V) / ( 3 *XTOTAL) = (0.01*400) / ( 3 *0.0324) = 71.277 A (5-11)
Peak harmonic current
IP5 = (IFL*0.4) + IF5 = (721.68*0.4) + 71.277 = 359.9 A (5-12)
٧٣
Max r.m.s current
Irms = 2
5
2 )()( PFF II + = 22 )9.359()18.757( + = 838.38 A (5-13)
Fifth harmonic voltage (line voltage)
VC5 = IP5*(XC/5)* 3 = 359.9*(0.32/5)* 3 =39.89 V (5-14)
Fundamental capacitor voltage (line voltage)
VCF = ICF*XC* 3 = 757.18*0.32* 3 = 419.67 V (5-15)
Max capacitor voltage
Vcmax = VCF + VC5 = 419.67 + 39.89 = 459.56 V (5-16)
Max filter current
IFmax= IFL + IP5 = 757.18 + 359.9 = 1117.08 A (5-17)
Capacitor r.m.s voltage
VCrms = 2
5
2 )()( CCF VV + = 22 )89.39()67.419( + = 421.56 V (5-18)
Max capacitor reactive power
Qmax = 3 * Vcrms*Irms (5-19)
Qmax =0.001* 3 * 421.56 * 838.38 = 612.15 kVAr
-Capacitor limits ( IEEE Std 18-2002) [54]
Peak voltage: Vcmax / 400 = 459.56 / 400 = 114.8 % < 120 %
Maximum r.m.s current: Irms / Icfl = 838.38 / 757.18 = 110.72 % < 135 %
Capacitor r.m.s voltage: VCrms / 400 = 421.56 /400 = 105.39 % < 110 %
٧٤
Maximum reactive power: Qmax. / 500 = 612.15 /500= 122.43 % < 135%
Hence, all the values are accepted and the design is correct.
5.2.3- Cost of Filter:
The cost of fully automatic system is 60 $/ kVAr [55]
For 500 kVAr, this is required for the case study, then,
Cost = 500 * 60 = 30000 $.
And it is very low compared with its benefits; it can protect the electrical system
equipment from the harmonics effects.
5.2.4- Harmonic Filter Calculation Spreadsheet:
The harmonic filter calculations spreadsheet provides a convenient method for
determining low voltage filter component values and duties. A computer program
designs it for uses with Microsoft excel.
The design of this spreadsheet is depend on the previous equations, which used to
design the 5th harmonic passive filter.
This spreadsheet provides the user with a convenient method for entering the required
data. The entered specified data includes:
- filter tuning specification
- capacitor bank rating, voltage and frequency
- nominal bus rating
- transformer name plate rating and impedance
- total harmonic load
- utility background harmonic voltage distortion
Table (5-1) shows design example of the calculations for the 5th harmonic filter.
Calculations including capacitor derating, filter component values and capacitor duty
with respect to standard. And table (5-2) shows IEEE Std 18-2002.
٧٥
Table (5-1) calculations for 5th harmonic filter
Table (5-2) IEEE Std 18-2002
CAPACITOR LIMITS:(IEEE Std 18-2002)
Limits Actual value %
Peak Voltage 120% 458.8600254 114.7150064
RMS Current 135% 837.2054316 110.7550444
KVAR 135% 523.7079184 122.0567296
RMS Voltage 110% 420.8613741 105.2153435
٧٦
5.3- Dynamic Voltage Restorer (DVR) [56]
The proliferation of voltage sensitive equipment in industrial sector has made
industrial processes more vulnerable to supply voltage deviations. Such voltage
deviations in the form of voltage sag, swell or temporary outage cause severe process
disruptions resulting in millions of dollars of loss of revenue.
Therefore, power supply authorities as well as customers have been desperately
looking for a cost-effective solution currently to ride through momentary power
supply disturbances. As such, the proposition of a novel custom power device called
Dynamic Voltage Restorer (DVR) for compensating voltage disturbances in
distribution systems has generated a great deal of interest recently.
Apart from the DVR, some researchers have proposed several other devices to
mitigate momentary disturbances. Among those, static voltage booster and unified
voltage controller have been noteworthy.
The DVR is the most economic and effective means in improving the voltage relative
power quality problems. The DVR is power electronics based solution that employs
series voltage boost technology for compensating voltage sags / swells.
The DVR usually consists of an injection transformer, which is connected in series
with the distribution line, a voltage sourced PWM inverter bridge which is connected
to the secondary of the injection transformer and an energy storage device (batteries,
capacitors…etc.) connected at the dc-link of the inverter bridge.
A typical schematic of the DVR is shown in Fig. (5-2). The inverter bridge output is
filtered before being fed to the injection transformer in order to nullify switching
frequency harmonics. The series injected voltage with a variable amplitude, phase and
frequency of the DVR is synthesized by modulating pulse widths of the inverter
bridge switches. The injection of an appropriate series voltage component in the face
of a voltage disturbance requires a certain amount of real and reactive power supply
by the DVR. The real and reactive power supplied by the DVR however depend on
the type of voltage disturbance experienced, as well as the direction of the DVR
injected voltage component with reference to pre-sag voltage. The idea of advancing
the injected voltage in order to minimize the real power supplied by the DVR has
generated a great deal of research interest recently.
٧٧
Many companies have begun to customize the special monographic study of the
electric power technology, and have introduced commercialization DVR devices.
Table (5-3) shows the developing and demonstration projects situation of the ABB,
Siemens, and American Superconductor for the DVR installations. [57]
Fig. (5-2): Typical schematic of a power system compensated by the DVR
Table (5-3): The situation of developing and researching for DVR
٧٨
5.3.1-- Operating principle of DVR: [58] and [61]
When we talk about maintaining a certain voltage level on a bus, usually we think
first of capacitors and Static Var Compensators (SVC). However, they only control of
the voltage indirectly is through altering the system’s effective impedance. To cope
with voltage sags caused by faults in the transmission or distribution system, the
injection of voltage combined with the supply of active power is necessary, which is
the distinguishing feature of a DVR as outlined in the following. As shown in figure
(5-3) the basic idea of the DVR is to inject a dynamically control1ed voltage VG (t)
generated by a forced-commutated converter in series to the bus voltage by means of
a booster transformer. The momentary amplitudes of the three injected phase voltages
are controlled such as to eliminate any detrimental effects of a bus fault to the load
voltage VL(t). This means that, any differential voltages caused by transient
disturbances in the AC feeder will be compensated by an equivalent voltage generated
by the converter and injected on the medium voltage level through the booster
transformer T1. This principle works independently of the type of the AC fault
provided that the whole system remains connected to the supply grid, i.e.; the line
breaker does not trip. For most practical cases, a more economical design can be
achieved by only compensating the positive and negative sequence components of the
voltage disturbance seen at the input of the DVR. This option is reasonable because
for a typical distribution bus configuration, the zero sequence part of a disturbance
will not pass through the step down transformers because of infinite impedance for
this component. This zero sequence blocking function is achieved by the delta
winding of the load-side step down transformer TL. The DC capacitor between the
charger and the converter serves as the energy buffer to the DVR, generating and
absorbing power during voltage sags and voltage swells, respectively. For most of the
time, the DVR has virtually “nothing to do” except monitoring the bus voltage, that
means it does not inject any voltage ( VG = 0) independently of the load current.
Therefore, it is suggested to particularly focus on the losses of a DVR during regular
hot standby operation. Two specific features addressing this loss issue have been
implemented in this concept, which are the transformer design with low impedance,
and the semiconductor devices employed.
٧٩
Figure (5-3): Basic Configuration of DVR
The booster transformer's low voltage winding is shorted through the converter in the
DVR's standby mode (left side in Figure (5-4). No switching of semiconductors
occurs in this mode of operation, because the individual converter legs are triggered
such as to establish a short-circuit path for the transformer connection. Therefore,
only the comparatively low conduction losses of the semiconductors in this current
loop contribute to the losses.
Figure (5-4): Effective transformer connection in stand-by and boost modes
٨٠
As shown in Figure (5-4), the DVR transformer in short circuit mode (SCO)
resembles a shorted current transformer. In this mode the booster transformer works
like a secondary shorted current transformer - that means the injected voltage and the
magnetic flux are virtually zero. It should be noted that the converter does not switch
during SCO-mode. Only the semiconductors' conduction losses appear, which are
very low due to the advanced IGCT (Integrated Gate Commutated Thyristor)
technology. Since the DVR will be most of the time in this particular standby mode,
conduction losses will account for the bulk of converter losses during operation.
5.3.2- Control and Protection of DVR
The basic functions of a controller in a DVR are the following:
- Detection of voltage sag/swell events in the system.
- Computation of the correcting voltage.
- Generating of trigger pulses to the sinusoidal PWM based DC-AC inverter.
- Correction of any anomalous in the series voltage injection.
- Termination of the trigger pulses when the system has passed.
The controller may also be used to shift the DC-AC inverter into rectifier mode to
charge the capacitors in the DC energy link in the absence of voltage sag. Figure (5-5)
shows a general control block diagram of DVR.
To maximize dynamic performance, a direct feed-forward-type control architecture
should be applied in the control concept of the DVR. With this concept a fast response
time (approximately 1ms) can be achieved to compensate voltage sags.
All protective functions of the DVR should be implemented in the software.
Differential current protection of the transformer, or short circuit current on the
customer load side are only two examples of many protection functions possibility.
Depending on the particular fault condition, the fast control and protection may
switch the DVR into bypass if it becomes inoperable, thus secure an uninterrupted
energy flow to the customer’s plant. A modified industrial PC could serve as an event
recorder to log and display the various steps during operation of the DVR.
٨١
Figure (5-5) General control system block diagram
Pulse logic shall establish the interface between the electrical switching orders supplied
by the modulator, and the input / outputs of the Gate Units (GU) of the converters. In
particular they can have the following basic:
- Signal conversion electrical ↔ optical
- Maintaining of timing constraints (min. On-time ; min. Off-time ; Overcurrent
delay) required by the Voltage Source Converter ( VSC)
- Control of Short Circuit Operation (SCO)
- Supervision and signaling of correct operation of the IGCT’s.
5.3.3- Losses of DVR:
As the DVR most of the time is in standby operation, the standby losses must be very
small. Any switching of semiconductors or other operations of the DVR should be
avoided during the standby time. A concept that does not require the DVR converter
to maintain and control the DC-voltage is preferred, in order to avoid switching and
the thereto-related losses. The function of the DVR should be insensitive to minor
variations in the DC voltage. The evaluation and specification of the losses should be
related to the operation and during standby also, considering all auxiliaries, which are
in operation. This means the transformer losses, converter losses, DC-link losses and
auxiliaries' losses. Another loss cut results from the IGCT technology (Integrated
٨٢
Gate Commutated Thyristor) utilized in this converter. These novel semiconductor
devices combine the low conducting losses of a GTO (Gate Turn Off) thyristor with a
switching performance similar to a transistor. With turn-off times in the range of a
few microseconds, high-power IGCT type converters show approximately 30 percent
lower losses than conventional GTO-type systems. The efficiency should be related to
the definition of the DVR, which is the load to be protected (for example 2MVA) and
the standby losses. This definition will give a possibility to compare different DVR
solutions. Typically an efficiency of >98.5% can be achieved.
5.3.4-- Harmonics of DVR:
In standby mode, the DVR should not actively generate any harmonics, the valves
have to be in SCO mode, i.e., establishing only the short-circuit path for the booster
transformer's secondary winding. Under boost mode, the DVR will generate
harmonics, which are almost negligible with a 3-level 12-pulse converter
configuration.
5.3.5- Availability of DVR:
The reliability and the availability are very important items in the design of a DVR.
Usually, the DVR operates in a standby mode and is waiting for sag; therefore it is
very crucial that the equipment has a high reliability and availability.
A high availability can be achieved by reducing the number of components, which
are used in the DVR. The design of the parts itself should have enough safety so that,
for example a short over-current (e.g. motor start-up) does not bring the DVR in a
bypass mode.
Today, an availability of >98.5% can be achieved with the use of IGCT’s. As there is
the capability of operating in direct coupling to the energy storage, with only one
energy conversion, which does increase the availability of the equipment.
٨٣
5.3.6- DVR capacity and specification: [57] and [58]
Referring to the electrical system of ETRR-2:
- Vpcc = 11 kV
- S = 2000 kVA , PF = 0.9
- Response time = ¼ cycle
- Max three phase voltage sag = 60 %
- Duration of sag to protect = 500 ms
- Rated of DVR (kVA) =?
- Energy (kJ) =?
PCC (11 kV)
DVR Sensitive load (ETRR-2)
It is recommended to adopt DVR technology to compensate the bus voltage sag and
restore to 100 % of the rated value.
When the sag depth is lower than 60 %
Therefore, the compensating voltage of DVR = 0.7 p.u.
By taking into consideration of peak load (2000 kVA) with power factor 0.9
The compensating power = 0.7*2000 = 1400 kVA
Energy = power * time (5-20)
For duration time 0.5 sec.
The required energy = (kVA * PF) * time = (1400*0.9)*0.5 = 630 kJ. (5-21)
For more reliability and availability, DVR with (2 MVA, 900 kJ) is selected. And it
should be installed in the 11 kV side of the system.
٨٤
5.3.7- Cost and Payback Time: [55] and [59]
Assume, Cost of DVR: C DVR
Cost of sag: C VS
Number of sags: NVS, (number per year)
Payback time: T, (year)
Then, C DVR = C VS * NVS * T (5-22)
The Cost of DVR = 300$/KVA + 5% (maintenance and running)
The Cost of DVR= 2000*300*1.05 = 630000 $
According to disturbance cost of ETRR-2 at normal mode, the cost of voltage sag is
(5000 $), and assume the number of voltage sag occurrence is 52/ year (one sag every
week)
Then, T = 630000/ (5000*52) = 2.42
T payback= 2.5 year
Hint: this solution is economic because the life time of DVR is about 15 years.
5.4- Uninterruptible Power Supplies (UPS):
Uninterruptible power supplies in nuclear reactors are designed to provide a stable,
and uninterruptible vital AC power to the safety related, non-safety related
instrumentation, and control systems. Uninterruptible power supplies are used to
provide a reliable uninterruptible source of voltage and frequency regulated AC
power to the vital loads required to shutdown the reactor and maintain in a safe
condition after an anticipated operational occurrence or a postulated design basis
accident. There are many types of UPSs, but the on-line double conversion
technology offers highest security. Only this design offers absolutely uninterruptible
power supply and equalizes all power disturbances such as voltage fluctuations,
distorted voltage waveform, frequency fluctuations, voltage transients, short
٨٥
interruptions and long power outage. Critical loads will be supplied with a high
quality voltage waveform, even in the case of heavy non-linear loads drawing a non-
sinusoidal high crest factor current.
5.4.1- The Operational Principle of on-line double conversion UPS: [60]
This is the most common type of UPS above 10 kVA. The block diagram of the
Double Conversion On-Line UPS is shown in Figure (5-6), it is the same as the
Standby, except that the primary power path is the inverter instead of the AC main.
Figure (5-6): Schematic of Double Conversion On-Line UPS
It is the ultimate in UPS protection because the utility supply power does not flow
directly to the load like the off-line UPS. Instead, the power flows continuously
through a charger/rectifier that feeds both a storage battery and an inverter. The
inverter generates AC power to the load being protected. In the event of a power
failure, the battery feeds the inverter. Since the power flows through the rectifier and
inverter before reaching the load, most power disturbances are eliminated through
constant filtering. Therefore, an on-line UPS is a good idea for any system which is
sensitive to transients, noise, and/or cannot tolerate any power interruption.
5.4.2- UPS capacity and specification:
All the critical and sensitive loads of ETRR-2 = 45 kVA
The grand load of UPS = total loads * 1.2 (5-23)
Wherever: (The factor 1.2, may be dynamic load at the system)
٨٦
The grand load of UPS = 45 * 1.2 = 54 kVA
Hence, UPS with 60 kVA rated power was selected.
5.4.3- Cost of UPS:
According to [55] the cost of UPS is (500$ / kVA)
The cost of UPS = (rated kVA) * 500 + 5% (running and maintenance)
The cost of UPS = (60 * 500) * 1.05 = 31500 $.
And it is very low compared with its benefits; however, it is related to the safety of
nuclear research reactor.
٨٧
CHAPTER (6)
CONCLUSIONS AND RECOMENDATIONS
6.1- Conclusion and recommendations
This work presents the assessment of power quality problems on the behavior of the
electrical system of nuclear installations. The analysis of the recorded data at the point
of common coupling yields that, fifth harmonics, flickers, voltage sags/swells,
under/over voltage, transients and temporary outage (500 ms) are the most severe
events and should be taken in consideration for any evaluation. It is recommended
that, mitigation techniques should be done to keep good performance of the electrical
system and then avoid operation problems of the nuclear installations. The Passive
Filters, Dynamic Voltage Restorer (DVR) and Uninterruptible Power Supply (UPS)
are most economic and effective solutions to mitigate the power quality problems.
6-2- The most important points that have been reached
1-Power quality of supply has a direct impact on the electrical equipments and various
systems because the disturbances lead to damage of equipments and bad performance
of the system.
2-In any project – especially projects with high sensitivity equipments, power quality
assessment should be done regularly to keep good performance.
3-The thesis considers a guide of power quality assessment study by using data
recorded, analysis according to standards specifications and developing solutions to
improve the power quality.
4-In the projects that contain high level sources of harmonics, it is not preferred
adding capacitors to improve power factor, but the best solution is using filters to
eliminate harmonics and also improve the power factor.
5-Power quality assessment should be considered as one of the important steps in
determining the site for sensitive projects to avoid the bad impact of power
disturbances.
٨٨
6.3-Future work
Power quality assessment should be continued regularly at the sensitive projects. And
it is strongly recommended that for future work on this topic, the time allocated for
monitoring should be longer, it should be more than seven days, and this to ensure
that, more detailed and complete data could be obtained. Thus, the analysis of the data
would be more precise. It is also suggested that, more monitoring points to be
allocated in the site, so that more data could be gathered on the distribution system.
This would allow a more detailed analysis on the affected areas and other locations on
the same distribution system. It is strongly recommended that, for important and
sensitive projects, the assessment of power quality should be done as part of the site
selection procedure. It is strongly recommended that, for future work, solutions of
power quality problems can done by load reconfiguration. And also for future works,
power quality aspects of smart grids should be involved.
٨٩
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٩٥
APPENDIX (A)
POWER QUALITY STANDARDS
Power quality standards set voltage and current limits that sensitive electronic
equipment can tolerate from electrical disturbances. Utilities need standards that set
limits on the amount of voltage distortion their power systems can tolerate from
disturbances produced by their customers with nonlinear loads. End users need
standards that set limits not only for electrical disturbances produced by utilities but
also for disturbances generated by other end users. Several national and international
organizations have developed power quality standards. There is a confusing number
of different organizations that set power quality standards. The following tables show
some international standards.
Table (A-1) IEEE1159-1995 standards.
٩٦
Table (A-2) Voltage distorsion limites
Note – High voltage systems can have up to 2.0 % THD where the cause is an HVDC
terminal that will attenuate by the time it is tapped for user.
Table (A-3) Current distortion limits for general distribution systems
(120 V through 69000 V)
All power generation equipment is limited to these values of current distortion, regardless
of actual ISC/IL.
Note: - Even harmonics are limited to 25 % of the odd harmonic limits above. Current
distortions that result in a direct current offset, e.g., half-wave converters are not
allowed.
Where ISC is the maximum short-circuit current at PCC, IL is the maximum demand
load current (fundamental frequency component) at PCC .
٩٧
It is recommended that the load current IL be calculated as the average current flow
during the maximum demand for the preceding twelve months.
Table (A-4) Power system disturbance classification to EN 50160
٩٨
APPENDIX (B)
TREND GRAPHS
This appendex illustrates the the trend graphs of the analysis. The monitoring was
carried out in the case study at two ponits as the following:
- Incoming feeder 1 (source1) for a period of one week.
- Incoming feeder 2 (source 2) for a period of one week.
The trend graphs show the analysis of voltage variations, current variations, power
factor variations, power variations , voltage total harmonics distortion, current total
harmonics distortions, short time flicker and long time flicker.
All the daily trend graphs of the monitoring are illustrated as the following: -
- Part (B-1) shows the Trend graphs of incoming feeder1 (source1)
- Part (B-2) shows the trend graphs of incoming feeder 2 (source 2)
٩٩
B.1- The Trend graphs of incoming feeder1 (source1)
B.1.1- The Trend graph of 1st day
Figure (b-1) voltage variations of source 1 at 1st day
Figure (b-2) current variations of source 1 at 1st day
١٠٠
Figure (b-3) power factor variations of source 1 at 1st day
Figure (b-4) power variations of source 1 at 1st day
١٠١
Figure (b-5) VTHD variations of source 1 at 1st day
Figure (b-6) ITHD variations of source 1 at 1st day
١٠٢
Figure (b-7) short time voltage flicker of source 1 at 1st day
Figure (b-8) long time voltage flicker of source 1 at 1st day
١٠٣
B.1.2- The trend graphs of 2nd
day
Figure (b-9) voltage variations of source 1 at 2nd day
Figure (b-10) current variations of source 1 at 2nd day
١٠٤
Figure (b-11) power factor variations of source 1 at 2nd day
Figure (b-12) power variations of source 1 at 2nd day
١٠٥
Figure (b-13) VTHD variations of source 1 at 2nd day
Figure (b-14) ITHD variations of source 1 at 2nd day
١٠٦
Figure (b-15) short time voltage flicker of source 1 at 2nd day
Figure (b-16) long time voltage flicker of source 1 at 2nd day
١٠٧
B.1.3- The trend graphs of 3rd
day
Figure (b-17) voltage variations of source 1 at 3rd day
Figure (b-18) current variations of source 1 at 3rd day
١٠٨
Figure (b-19) power factor variations of source 1 at 3rd day
Figure (b-20) power variations of source 1 at 3rd day
١٠٩
Figure (b-21) VTHD variations of source 1 at 3rd day
Figure (b-22) ITHD variations of source 1 at 3rd day
١١٠
Figure (b-23) short time voltage flicker of source 1 at 3rd day
Figure (b-24) long time voltage flicker of source 1 at 3rd day
١١١
B.1.4- The trend graphs of 4th day
Figure (b-25) voltage variations of source 1 at 4th day
Figure (b-26) current variations of source 1 at 4th day
١١٢
Figure (b-27) power factor variations of source 1 at 4th day
Figure (b-28) power variations of source 1 at 4th day
١١٣
Figure (b-29) VTHD variations of source 1 at 4th day
Figure (b-30) ITHD variations of source 1 at 4th day
١١٤
Figure (b-31) short time voltage flicker of source 1 at 4th day
Figure (b-32) long time voltage flicker of source 1 at 4th day
١١٥
B.1.5- The trend graphs of 5th day
Figure (b-33) voltage variations of source 1 at 5th day
Figure (b-34) current variations of source 1 at 5th day
١١٦
Figure (b-35) power factor variations of source 1 at 5th day
Figure (b-36) power variations of source 1 at 5th day
١١٧
Figure (b-37) VTHD variations of source 1 at 5th day
Figure (b-38) ITHD variations of source 1 at 5th day
١١٨
Figure (b-39) short time voltage flicker of source 1 at 5th day
Figure (b-40) long time voltage flicker of source 1 at 5th day
١١٩
B.1.6- The trend graphs of 6th day
Figure (b-41) voltage variations of source 1 at 6th day
Figure (b-42) current variations of source 1 at 6th day
١٢٠
Figure (b-43) power factor variations of source 1 at 6th day
Figure (b-44) power variations of source 1 at 6th day
١٢١
Figure (b-45) VTHD variations of source 1 at 6th day
Figure (b-46) ITHD variations of source 1 at 6th day
١٢٢
Figure (b-47) short time voltage flicker of source 1 at 6th day
Figure (b-48) long time voltage flicker of source 1 at 6th day
١٢٣
B.2- The trend graphs of incoming feeder 2 (Source 2)
B.2.1- The trend graphs of 1st day
Figure (b-49) voltage variations of source 2 at 1st day
Figure (b-50) current variations of source 2 at 1st day
١٢٤
Figure (b-51) power factor variations of source 2 at 1st day
Figure (b-52) power variations of source 2 at 1st day
١٢٥
Figure (b-53) VTHD variations of source 2 at 1st day
Figure (b-54) ITHD variations of source 2 at 1st day
١٢٦
Figure (b-55) short time voltage flicker of source 2 at 1st day
Figure (b-56) long time voltage flicker of source 2 at 1st day
١٢٧
B.2.2- The trend graphs of 2nd
day
Figure (b-57) voltage variations of source 2 at 2nd day
Figure (b-58) current variations of source 2 at 2nd day
١٢٨
Figure (b-59) power factor variations of source 2 at 2nd day
Figure (b-60) power variations of source 2 at 2nd day
١٢٩
Figure (b-61) VTHD variations of source 2 at 2nd day
Figure (b-62) ITHD variations of source 2 at 2nd day
١٣٠
Figure (b-63) short time voltage flicker of source 2 at 2nd day
Figure (b-64) long time voltage flicker of source 2 at 2nd day
١٣١
B.2.3- The trend graphs of 3rd
day
Figure (b-65) voltage variations of source 2 at 3rd day
Figure (b-66) current variations of source 2 at 3rd day
١٣٢
Figure (b-67) power factor variations of source 2 at 3rd day
Figure (b-68) power variations of source 2 at 3rd day
١٣٣
Figure (b-69) VTHD variations of source 2 at 3rd day
Figure (b-70) ITHD variations of source 2 at 3rd day
١٣٤
Figure (b-71) short time voltage flicker of source 2 at 3rd day
Figure (b-72) long time voltage flicker of source 2 at 3rd day
١٣٥
B.2.4- The trend graphs of 4th day
Figure (b-73) voltage variations of source 2 at 4th day
Figure (b-74) current variations of source 2 at 4th day
١٣٦
Figure (b-75) power factor variations of source 2 at 4th day
Figure (b-76) power variations of source 2 at 4th day
١٣٧
Figure (b-77) VTHD variations of source 2 at 4th day
Figure (b-78) ITHD variations of source 2 at 4th day
١٣٨
B.2.5- The trend graphs of 5th day
Figure (b-79) voltage variations of source 2 at 5th day
Figure (b-80) current variations of source 2 at 5th day
١٣٩
Figure (b-81) power factor variations of source 2 at 5th day
Figure (b-82) power variations of source 2 at 5th day
١٤٠
Figure (b-83) VTHD variations of source 2 at 5th day
Figure (b-84) ITHD variations of source 2 at 5th day
١٤١
Figure (b-85) short time voltage flicker of source 2 at 5th day
Figure (b-86) long time voltage flicker of source 2 at 5th day
١٤٢
B.2.6 – The trend graphs of 6th day
Figure (b-87) voltage variations of source 2 at 6th day
Figure (b-88) current variations of source 2 at 6th day
١٤٣
Figure (b-89) power factor variations of source 2 at 6th day
Figure (b-90) power variations of source 2 at 6th day
١٤٤
Figure (b-91) VTHD variations of source 2 at 6th day
Figure (b-92) ITHD variations of source 2 at 6th day
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