Monitoring of Distribution System Power Quality

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Department ofComputer Science and Electrical Engineering

Monitoring of Distribution System

Power Quality

by

Jeff McGuireBachelor of Engineering - Honours

October 1999


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Abstract

i

AbstractPower Quality is a discipline still very much in its infancy. However, with the immense

growth of the power electronics and control systems industries currently being

experienced, its importance should not be underestimated. The termpower qualityis used

to describe any abnormal behaviour in a power system arising from voltage or current

variations which adversely affects the usual operation of electrical equipment. Power

quality disturbances can have catastrophic effects on systems. A major computer centre in

the United States reports that a two-second interruption in the power supply would result

in a $600,000 loss. Electrical utilities also share concerns about power quality issues.

With deregulation of the electricity industry, and hence growing competition between

utilities, the financial impact of lost customers becomes more significant.

A thorough understanding of power systems is imperative before any work in the area of

power quality is commenced. Knowledge of the varying types of loads in use, their

operation, and effect on nearby equipment should also be obtained. However, these skillsare only of use if the equipment used to conduct power quality surveys has better than

average capabilities.

This thesis report firstly introduces the reader to the fundamentals of power quality,

before proceeding to an analysis of three case studies. The first case study was conducted

at the Caltex Oil Refinery in the Brisbane suburb of Lytton for a duration of 17 days,

followed by a study in the Chemistry building at The University of Queensland for 11

days, before the final study in the laser laboratory within the Physics building also at The

University of Queensland is presented. A thorough description of the equipment used in

the surveys is also presented, as well as a description of the California Instruments AC

Power Source- another piece of equipment useful for studying power quality.

After careful analysis of the results obtained from each survey, it was found that the

quality of the power at each location was quite satisfactory, with the exception of a


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Abstract

ii

moderate degree of imbalance between the phases in each case. The most likely cause of

this imbalance experienced was found to be the operation of many non-linear loads in

each site with the exception of the Chemistry building, where the cause was probably dueto the majority of loads being single phase. The laser laboratory experienced two short

duration transient faults, which can be eliminated by the use of an AC power filter. A

description of the operation of such a filter is also presented. The thesis paper then

concludes with suggestions on how to approach future work in this area in order to gain

extra benefit from the experience.


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Acknowledgements

iii

Acknowledgements

In submitting this thesis project, there are a number of people who I would like to thankfor their help during the year. First of all a special thanks to my supervisors, Dr. Tapan

Saha and Dr. Allan Walton, who provided continual guidance and encouragement. Mr.

Adrian Mengede, the senior electrical engineer at The University of Queensland, whose

assistance and advice about the monitoring sites and their respective problems was much

appreciated. To Mr. Steve Wright, for his assistance with the setup and operation of the

monitoring equipment, and Mr Geoff Walker for his continual help with the numerous

problems which occurred during the year. Finally I would like to thank the following

group of people who went out of their way to contribute to my project:

Mr. Trevor Baque (Caltex Refineries QLD Ltd)

Mr. Peter Doyle (Caltex Refineries QLD Ltd)

Mr. Tom Glennon (Caltex Refineries QLD Ltd)

Mr. Patrick McGuinness (Caltex Refineries QLD Ltd)

Mr. Robert Trout (Caltex Refineries QLD Ltd)

Mr. Stefan Istratescu (Connell Wagner)

Mr. Ross Lawson (Connell Wagner)

Mr. Chris Dunn (Energex)

Mr. Andrew Meiklejohn (Energex)

Mr. David Simpson (Energex)


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Table of Contents

iv

Table of Contents Page

Abstract i

Acknowledgments iii

List of Illustrations viii

List of Tables xi

Chapter 1 : Introduction 1

1.1 Background 1

1.2 Objectives Scope of Project 2

1.3 Overview of Thesis Project 3

Chapter 2 : Fundamentals of Power Quality 5

2.1 Introduction 5

2.2 Classification of Power Quality Disturbances 5

2.2.1 Transients 5

2.2.2 Power Frequency Variations 6

2.2.3 Short-Term Voltage Variations 7

2.2.4 Long-Term Voltage Variations 8

2.2.5 Voltage Imbalance 9

2.2.6 Waveform Distortion 9


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Table of Contents

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Chapter 3 : Mitigation Devices for Power Quality Problems 13

3.1 Introduction 13

3.2 Effects of Voltage Sag in Industrial Distribution Systems, and Methods of Reducing their Impact 13

3.2.1 Uninterruptable Power Supplies 14

3.2.2 Dynamic Voltage Restorers (DVRs) 15

3.2.3 Motor-Generator (M-G) Sets 16

3.2.4 Ferro-resonant/Constant Voltage Transformers (CVTs) 17

3.2.5 Filters for surge protection 18

3.3 Elimination of Voltage Magnification Problems Due to Capacitor Switching 19

3.3.1 Pre insertion resistors 20

3.3.2 Pre insertion inductors 20

3.3.3 Controlled Closing Devices 20

3.4 Sources and Effects of Harmonic Distortion in Power Systems 21

3.4.1 Harmonic Control using Static Compensators(STATCOMs) 22

3.4.2 The Use of Filters for Harmonic Mitigation 23

Chapter 4 : The Monitoring Equipment - BMI 8010 PQNode 26

4.1 Introduction 26

4.2 The Hardware 26

4.3 The Software 284.3.1 PQNode Setups 28

4.3.2 Automatic Downloading of Data 29

4.3.3 Intuitive Data Presentation 30

4.3.4 Flexible Output 31


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Table of Contents

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Chapter 5 : California Instruments AC Power Source 32

5.1 Introduction 32

5.2 The Hardware 32

5.3 The Software 37

Chapter 6 : Monitoring at the Caltex Oil Refinery 40

6.1 Introduction and Background 40

6.2 Results 41

6.2.1 Transient Analysis 41

6.2.2 Short/Long Duration Variations: CBEMA Analysis 416.2.3 Voltage Imbalance 42

6.2.4 Waveform Distortion Analysis 45


6.2.6 Miscellaneous results 46

6.3 Tests Performed on the PQNode in the Laboratory 47

6.3.1 Voltage Sag Testing 47

6.3.2 Power Outage Testing 486.3.3 Frequency Testing 48

6.4 Site Appraisal and Mitigation Techniques 49

Chapter 7 : Monitoring in the Chemistry Building

The University of Queensland 50

7.1 Introduction and Background 50

7.2 Results 51


7.2.2 Short/Long Duration Variations: CBEMA Analysis 51






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Table of Contents

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Chapter 8 : Monitoring in the Physics Building Laser laboratory

The University of Queensland 58

8.1 Introduction and Background 588.2 Results 59


8.2.2 Short/Long Duration Variations: CBEMA Analysis 60





Chapter 9 : Discussion, Conclusion and Recommendations 64

9.1 Discussion 64

9.2 Conclusions 66

9.2.1 Caltex Oil Refinery 66

9.2.2 Chemistry Building 67

9.2.3 Laser Laboratory Physics Building 67

9.3 Recommendations

9.3.1 Monitoring Equipment 68

9.3.2 Future Works 68

Appendix A : Power Quality Standards 70

Appendix B : Transcript of Emails 72

Appendix C : Trends recorded in each survey 74

Appendix D : Thresholds used in each power quality survey 77

References 80


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List of Illustrations

viii

L ist of I l lustrations Page

Chapter 1

Figure 1-1 Protocol for power quality monitoring 2

Chapter 2

Figure 2-1 Impulsive transient waveform 5

Figure 2-2 Oscillatory transient waveform 6Figure 2-3 CBEMA curve 7

Figure 2-4 Voltage sag due to motor starting 8

Figure 2-5 Balanced and unbalanced voltages 9

Figure 2-6 Harmonic distortion 10

Figure 2-7 Formula for Total Harmonic Distortion 10

Figure 2.8 Notching caused by a three-phase power converter 11

Figure 2-9 Voltage flicker 12

Chapter 3

Figure 3-1 UPS Configuration 14

Figure 3-2 Connection of a DVR 15

Figure 3-3 Motor-Generator Set 16

Figure 3-4 Ferro-resonant/Constant voltage transformer 17

Figure 3-5 Surge/Spike protection filter 18

Figure 3-6 Voltage Waveform showing Effects of Capacitor Switching 19

Figure 3-7 Example of a STATCOM Connection 22

Figure 3-8 Current Waveforms of a three-phase diode rectifier

and STATCOM 23

Figure 3-9 Parallel Connected Resonant Filter 24


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Figure 3-10 Series Connected Resonant Filter 24

Figure 3-11 Zig-Zag Filter 25

Chapter 4

Figure 4-1 8010 PQNode 27

Figure 4-2 PASS setup main menu 28

Figure 4-3 PASS menu used to establish download schedule 29

Figure 4-4 Disturbance roll for the High Voltage Laboratory 30

Figure 4-5 A transient fault recorded in the High Voltage Laboratory 30

Chapter 5

Figure 5-1 California Instruments 1251 P Series AC Power Source 32

Figure 5-2 Front panel display of power source 33

Figure 5-3 Main window of PGUI software 37

Figure 5-4 Transient programming window 38

Chapter 6Figure 6-1 Electrical location of the PQNode at the Caltex Oil Refinery 41

Figure 6-2 An RMS variation recorded at the Caltex Oil Refinery 42

Figure 6-3 The RMS current trends for each of the phases 43

Figure 6-4 Percentage imbalance for each phase to phase voltage 44

Figure 6-5 Voltage waveform recorded at the Caltex Oil Refinery 45

Figure 6-6 Voltage sag produced by the AC power source 47

Chapter 7

Figure 7-1 Electrical location of the monitoring equipment in the Chemistry

building 50

Figure 7-2 An RMS variation recorded in the Chemistry building 52

Figure 7-3 RMS variations with CBEMA curve overlay 52

Figure 7-4 Current trends for each phase conductor and neutral conductor 53


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Figure 7-5 Percentage voltage imbalance for each phase 55

Figure 7-6 Waveform distortion as recorded in the Chemistry building 56

Chapter 8

Figure 8-1 Voltage impulses recorded in the laser laboratory 59

Figure 8-2 RMS variations in Physics building with CBEMA curve overlay 60

Figure 8-3 Percentage voltage imbalance for each phase to phase voltage 61


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List of Tables

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L ist of Tables Page

Chapter 5

Table 5-1 California Instruments 1251 P Series Power

Source Specifications 36

Appendix A

Table A-1 IEEE 1159 Standard 70

Table A-2 AS 2279 Part 2 71


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Chapter 1- Introduction

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Chapter 1

Introduction

1.1 Background

Electrical power would have to be considered one of the most critical requirements for

companies today. Without it, entire systems cease to operate. When power fails, the cost

is often measured in millions of dollars per minute, and in some cases, it is immeasurable.

Maintaining continuous operation of these systems is no longer considered a goal, but

rather a necessity for conducting business in todays global environment.

Power quality is defined as the degree to which the supply system is free from major

distortions and fluctuations in supply voltage and frequency, and free from interruptions

to supply [27]. Currently there is an increasing concern about the standard of the

electrical power being delivered. There are a number of reasons for these concerns. First

of all, an increasing amount of load equipment now contains microprocessor based

controls and power electronic devices, which are becoming extremely sensitive to a wide

range of disturbances. Secondly, with the advent of new technologies, and the growth of

the control systems and power electronics industries, customer loads are exhibiting a

more non-linear behaviour than previously experienced. Non-linear loads are ones in

which current is drawn in pulses, as opposed to being drawn continuously. These non-

linear loads inject harmonic currents into the system, which can have harmful effects on

loads connected elsewhere.

The principal reason for the concern about power quality though, is for the financial

impact of disturbances. As mentioned earlier, many industries now rely on electronic

equipment which is much more sensitive to electrical disturbances than its


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electromechanical predecessors. The financial difficulties are not only due to

maintenance costs associated with disturbances, but also the revenue lost during the time

in which repairs were taking place. A prime example of this is a major computer centre inthe United States, which estimates that a two-second outage would result in a $600,000

loss.

Electrical utilities also share the concerns about power quality issues. With deregulation

of the electricity industry, and hence growing competition between utilities, the financial

impact of lost customers becomes more significant.

1.2 Objectives Scope of Project

The purpose of any power quality monitoring project is to collect information on faults

which have occurred in a system, and after careful analysis of this information, to

determine the cause of the faults, and finally suggest various mitigation techniques to

prevent any possible re-occurrences of the problem.

Even though this technology is still in its infancy, a set of protocols has been developed

for conducting power quality studies. First of all, background research should be

conducted into the electrical location of the customer disturbance report, such as past

history of power quality problems, and types of equipment connected to the supply and

nearby. The next step is to collect information by monitoring the power at one or more

locations, and then analyse this data with the appropriate power quality standards. If

required, some form of mitigation technique should then be recommended. The power

should then be monitored once any mitigation techniques or devices have been

implemented to determine their effectiveness. A summary of this monitoring process can

be seen in figure 1-1.

Figure 1-1: Protocol for power quality monitoring


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However, before proceeding directly to performing 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 project 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 three separate power quality studies is then

presented, before an appraisal of each site is given, and recommendations for future work

in this area.

1.3 Overview of Thesis Project

Chapter 2 contains all the terms and definitions encompassing the area of power quality.

A description is given of terms such as transients, power frequency variations, short and

long term voltage variations, voltage imbalance, and the various classes of waveform

distortions.

Chapter 3 is a more thorough review of some of the most common power quality

problems faced, and various devices and techniques to reduce their impact on systems.

Chapter 4 is a brief analysis of the equipment used to conduct the power quality surveys

the BMI 8010 PQNode. A complete description of the hardware is presented, along

with the software required to analyse the results.

Chapter 5 contains an analysis of another valuable piece of equipment in conducting

power quality studies the California Instruments 1251P Series AC Power Source.

This piece of equipment can be used for simulating common power quality problems

such as voltage sag and power frequency variations in the laboratory in order to

determine how other equipment will react to these disturbances. A complete description

of the hardware is presented, along with the additional software required to simulate the

specific power quality problem.

PQNode is a registered trademark of Basic Measuring Instruments (BMI), Santa Clara, California

AC Power Source is a registered trademark of California Instruments, San Diego, California


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Chapter 6 contains the results, analysis and discussion of a power quality study conducted

at the Caltex Oil Refinery in the Brisbane bayside suburb of Lytton. During this survey, a

few inconsistencies were detected with the monitoring equipment, therefore a descriptionof calibration tests carried out, and the results obtained are also contained in this chapter.

Chapter 7 contains the results, analysis and discussion of a power quality study conducted

in the Chemistry building at The University of Queensland.

Chapter 8 contains the results, analysis and discussion of the final power quality study

conducted in the laser laboratory within the Physics building again at The University of

Queensland.

This thesis report concludes with chapter 9, which presents a discussion of the progress of

the project throughout the year, as well as an appraisal of each site which was monitored.

The chapter concludes with recommendations for future work in this area.


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Chapter 2- Fundamentals of Power Quality

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Chapter 2

Fundamentals of Power Quali ty

2.1 Introduction

Prior to conducting a power quality survey, an extensive knowledge of power system

theory is required. These theories and the equivalent knowledge must then be able to be

applied to specific situations to perform an accurate analysis of the data attained during

the survey.

The following sections of this chapter provide the reader with an analysis of various

power quality faults, and the associated standards used to classify such disturbances.

2.2 Classification of Power Quality Disturbances

2.2.1 Transients

The nature of a transient fault can be classified as either impulsive or oscillatory. An

impulsive transient is defined as, a sudden, non-power frequency change in the steady-

state condition of voltage, current, or both, that is unidirectional in polarity [14].

Impulsive transients are generally caused by lightning strikes [36].

Figure 2-1: An Impulsive Transient


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Oscillatory transients on the other hand, are characterised by fluctuations in the measured

quantity at very high frequencies. These can be classed even further according to their

frequency as either high, medium or low, with the cause of the fault dependent upon thefrequency. Utility switching and capacitor energisation are just two causes of oscillatory

transient faults.

Figure 2-2 :An Oscillatory Transient

2.2.2 Power Frequency Variations

The frequency of supply in an electrical power system is determined by the rotational

speed of prime movers in the synchronous generators which supply electrical power.

Power frequency variations rarely occur on large networks in developed countries, yet

they are quite common in under-developed countries. The main cause of these variations

is a severe mismatch between available generation and the connected load, however the

removal or addition of large loads is also known to produce power frequency variations.Energex will allow frequency deviations of +0.3 Hz for the system to still be considered

synchronous.

Energex is the electricity distribution authority for south-east Queensland


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2.2.3 Short-Term Voltage Variations

Short-term voltage variations is the name used to describe faults such as voltage drops

(sag), voltage rises (swells), or complete loss of voltage (interruptions) in the electrical

supply. The thresholds used to classify these disturbances are contained within the

Institute of Electrical and Electronic Engineers (IEEE) 1159 standard (see Appendix A).

Another useful tool in classifying short-term voltage variations is the Computer Business

Equipment Manufacturers Association (CBEMA) curve (see figure 2-3). The CBEMA

curve is a plot of voltage magnitude versus duration, which contains a tolerance curve for

which faults, when plotted, must lie to the left of to be considered harmless. A drawbackof the CBEMA curve is that it was designed primarily with mainframe computer loads in

mind, therefore should not be used independently to classify disturbances.

Figure 2-3: CBEMACurve


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Voltage sags are generally associated with faults within a power system, such as single

line to ground faults, although they are also caused by the starting of large motors, or

energisation of heavy loads. Figure 2-4 shows a typical voltage sag caused by motorstarting.

Figure 2-4 :Voltage Sag due to Motor Starting

Voltage swells are not as common as sags, however they are usually caused also by

system faults. The removal of large loads, or the energisation of capacitor banks used to

correct the power factor are two system faults known to cause swells.

An interruption in the supply voltage, as defined by the IEEE 1159 standard, occurs

when the magnitude of the measured quantity falls below 0.1 per unit (pu) for a period of

time less than 1 minute. An interruption of more than 1 minute in duration is classified as

a long-term voltage variation which is defined in the following section.

2.2.4 Long-Term Voltage Variations

Long term voltage variations are similar to short term voltage variations except, as the

name suggests, the duration of the fault is longer. A sag, swell or interruption of longer

than 1 minute is classified as a long term variation by the IEEE 1159 standard. Long-term

sags and swells are referred to as undervoltages and overvoltages respectively.


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2.2.5 Voltage Imbalance

A voltage imbalance in a three-phase system is characterised by differences in the

magnitudes and/or angles between the R.M.S. phase voltages. A term known as the

Voltage Unbalance Factor is used to determine the extent of the imbalance, and is defined

as, The ratio of the negative or zero phase sequence component of the voltage to the

positive phase sequence component of the voltage [27]. Known causes of voltage

imbalance are non-linear loads, or a blown fuse in a three-phase capacitor bank.

The IEEE 1159 standard states that the Voltage Unbalance Factor should remain less than

2% for the system to remain balanced.

Figure 2-5 :Balanced & Unbalanced Voltages

2.2.6 Waveform Distortion

Waveform distortion is defined as, steady state deviation from an ideal sinusoid [36].

Distortion of waveforms can be classed into six separate categories:

Harmonic

Inter-harmonic

DC Offset Notching

Noise

Flicker


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The harmonic components of a waveform are sinusoidal in shape, with a frequency equal

to an integer multiple of the fundamental frequency, e.g. the 3rdharmonic has a frequency

of 150Hz (3 x 50Hz).

Figure 2-6 :Harmonic Distortion

Harmonic frequencies are caused by non-linear devices such as variable speed drives

(VSDs) and Silicon Controlled Rectifiers (SCRs) or thyristors. Another cause of

harmonics is due to devices which contain a steel core, such as transformers and

induction motors, and this is due to the non-linear magnetising characteristics of the steel.

The amount of harmonic distortion can be measured by a factor known as the Total

Harmonic Distortion (THD).

Figure 2-7 : Formula for Total Harmonic Distortion


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Inter-harmonics are identical in nature to harmonics except that their frequencies are not

integer multiples of the fundamental frequency. The limits on harmonic distortion for a

system can be found in the AS 2279 (see Appendix A)

A DC offset component will generally be present in the neutral conductor of the supply

system due to loads with non-linear characteristics. Two consequences of DC offsets in

the distribution system are increased transformer saturation and enhanced insulation

stress.

Three phase power converters cause what is commonly known as notching in a power

system. During an AC cycle of a three phase power converter, the current is repeatedly

switched from one phase to another, which results in a momentary phase to phase short

circuit, followed immediately by restoration of the voltage (see figure 2-8).

Figure 2-8 :Notching caused by a three phase power converter

The IEEE defines noise as, unwanted electrical signals superimposed upon the power

system voltage or current in phase conductors, neutral conductors, or signal lines [36].

Electrical noise can be caused by a number of different factors ranging from nearby

magnetic fields, to poor grounding of equipment.

Voltage fluctuation, or flicker is caused by the rapid variation of the current drawn by a

load, in particular reactive current. Arc furnaces are believed to be the most common


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cause of voltage flicker in power systems. Figure 2-9 shows an example of voltage

flicker.

Figure 2-9 :Voltage Flicker


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Chapter 3- Mitigation Devices for Power Quality Problems

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Chapter 3

Mitigation Devices for Power Quali ty Problems

3.1 Introduction

This chapter is devoted to looking at common problems in the area of power quality. A

number of various problems such as voltage sag, capacitor switching, and harmonic

distortion are considered, with likely causes of each fault presented. Mitigation

techniques for each fault are then evaluated.

3.2 Effects of Voltage Sag in Industrial Distribution Systems,

and Methods of Reducing Their Impact [15] [16] [17]

Voltage sag is one of the most important problems facing industrial and large commercialcustomers [15]. In recent years, utilities have been faced with an increasing number of

complaints about voltage sag. The most common cause of voltage sag is power system

faults, although lightning strikes and motor starts are also causes of this problem [16].

Single line-to-ground faults are responsible for the majority of incidents of voltage sag on

the system, and are capable of producing a reduction to 33% of the nominal voltage [16].

Three-phase faults are less common, however they are associated with more severe

problems. Another factor which influences the magnitude of the sag is the location of the

fault. A customer situated in close proximity to a fault will experience a more severe

voltage sag than a customer located at a greater distance from the fault [17].

Equipment such as computers, process controllers, and power electronic devices are

notorious for their sensitivity to power quality disturbances. DC drives and chiller


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controls are also quite sensitive, and can trip on a reduction in the voltage as small as

10%.

Voltage sag is extremely difficult, if not impossible to prevent, although it is possible to

lessen the effect on equipment. Devices such as Uninterruptable Power Supplies (UPS),

Dynamic Voltage Restorers (DVRs), Motor-Generator sets, and ferro-resonant/constant

voltage transformers can be used to combat voltage sag, and will consequently be

analysed in detail in the following sections.

3.2.1 Uninterruptable Power Supplies [14] [18]One solution to the problem of voltage sag is the installation of Uninterruptable Power

Supplies (UPS). A UPS operates by rectifying the incoming AC power into DC power,

which charges a battery bank. The DC power is then inverted back into AC power to

supply the load [18] (see figure 3-1).

F igure 3-1 :UPS Configuration

A UPS can be designed to operate in on-line mode, standby mode, or hybrid mode. In

on-line mode, the load is always fed through the UPS, however in standby mode, the

utility supply is connected to the load under normal conditions, and when a disturbance is

detected, a switch transfers the load to the UPS output. In the hybrid mode, a voltage

regulator is connected to the output of the UPS to provide voltage regulation when the

transfer from normal to UPS mode is made [14].


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A UPS is not suitable for the cancellation of voltage sag in every situation in which it

occurs. For example, variable speed drives are too large to be fed from a UPS, thereforecomputers and computer-based equipment are generally the main users of UPS.

3.2.2 Dynamic Voltage Restorers (DVRs) [1] [2] [3]

The DVR is one device in a class known as Active Power Line Conditioners (APLCs),

and is also referred to as a series type APLC. APLCs are a well-developed technology

for regulating terminal voltage, and compensation of reactive power [3].

Figure 3-2 :Connection of a DVR

As can be seen from figure 3-2, a DVR consists of a transformer in series with the feeder

cable. Under normal operating conditions, the voltage across the transformer is very

small, however when a fault condition occurs, the power converter connected to the

transformer generates a voltage across the transformer that is in quadrature with the

current in the feeder. The transformer then appears as a variable impedance, which can

add or subtract from the line impedance depending on the fault situation.

In the situation when the supply voltage is depressed, the transformer can produce a

capacitive voltage, which will counter some of the voltage drop in the line. Should a

voltage surge occur due to capacitor switching or some other means, inductive voltage

can be produced to reduce this increase in voltage [2].


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In the process of achieving voltage control, DVRs are controlled so as not to supply real

power to the system.

3.2.3 Motor-Generator (M-G) Sets [13][15]

M-G Sets are used to completely decouple the load from the utility supply system. This

technology is capable of providing 100 percent output voltage to the rated load for up to

15 seconds with zero voltage on the input [13]. A diagram of an M-G set is shown in

figure 3-3.

Figure 3-3 :Motor-Generator Set

As can be seen from figure 3-3, the induction motor is fed from the utility supply. The

rotor of the induction motor drives a flywheel situated on a common shaft with the

generator. Should a voltage sag or complete outage occur on the utility system, the torque

produced by the induction motor is reduced. However, due to inertia, or mechanical

energy storage, the flywheel keeps the shaft rotating at a constant speed for a short period

of time. This ensures rated voltage is continually supplied to the load [15].

If the duration of the sag or outage is long enough such that the energy stored in the

flywheel is no longer capable of maintaining constant speed, then the motor should trip

once the voltage begins to drop below a specified value. This can be achieved by placing

undervoltage relays on the output of the generator.


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3.2.4 Ferro-resonant/Constant Voltage Transformers (CVTs) [15] -

[16]

CVTs are another device that can be used to improve voltage sag ride-through

capability. CVTs are basically transformers with a 1:1 turns ratio, yet they are excited

high on their magnetic saturation curves, hence providing output voltages which are not

significantly affected by variations in the input voltage [15]. A typical CVT circuit is

shown in figure 3-4.

Figure 3-4 :Ferro-resonant/Constant voltage transformer

CVTs operate in exactly the same manner as regular transformers, however should a

voltage sag occur on the primary side, a CVT has the capability of maintaining the

output/secondary voltage at a constant level. If the transformer is fully loaded, the output

voltage can be maintained for a sag which causes the primary voltage to drop by 30%,

while if it is only loaded to of its rating, it can maintain the secondary voltage for a

reduction of 70% in the primary voltage [16].


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3.2.5 Filters for Surge Protection [28]

This type of filter is known as a surge/spike protection filter. As the name suggests, the

purpose of this type of filter is to protect sensitive loads from short duration impulsive

and oscillatory transient faults. A diagram of this type of filter is shown in figure 3-5.

Figure 3-5: Surge/spike protection filter

Should any surges occur, they would first encounter the surge reactor L1, which

immediately provides protection by limiting the rate of increase of the current. L1 in

conjunction with C2 restricts the incoming voltage slew rate depending upon the values

chosen for the components. Should an unusually large surge occur such that L1 and C2

can no longer act as an effective clamp, the combination of D1 and C3 will operate,

which further reduce the slew rate of the incoming surge.

Two crowbar circuits are included to cope with high energy surges, which consist of a

Silicon Controlled Rectifier (SCR) switch, an inductor (not shown in diagram), and a

capacitor. If a surge is large enough to generate a particular slew rate across C3, the first

crowbar circuit (C4) will neutralise the surge. If the surge is still too large, the second

crowbar circuit (C5) will activate to provide a final measure of protection. Filters such as

these are readily available from most power supply manufacturers in a compact unit at a

reasonable price.


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3.3 Elimination of Voltage Magnification Problems Due to

Capacitor Switching [7] [8] [9] [19]

Electrical utilities worldwide use capacitor banks to improve the power factor. However,

these capacitor switching schemes give rise to what is known as voltage magnification,

which, as the name suggests, is a transient overvoltage at the instant of switching.

Voltage magnification is caused by the inrush of current into the capacitor bank at the

instant of switching.

Energisation of a capacitor bank results in an immediate drop in the system voltage

toward zero, followed by a fast overshoot, and finally an oscillating transient voltage

superimposed on the 50 Hz waveform [19]. An example of this can be seen in figure 3-6.

Voltage magnification occurs due to the capacitor bank exciting a series resonance

between itself and nearby step-down transformers.

Figure 3-6 :Voltage Waveform showing Effects of Capacitor Switching

Transient overvoltages due to capacitor switching can cause a wide range of problems,such as tripping of variable speed drives (VSDs) [9], and tripping of power supplies [7].

In order to reduce the magnitude of these overvoltages, three different methods are

generally employed:

Circuit breakers with pre-insertion resistors

Circuit switchers with pre-insertion inductors, and

Controlled closing devices.


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3.3.1 Pre-insertion resistors

The use of pre-insertion resistors involves inserting resistors into the capacitor

energisation circuit prior to the closure of the main set of contacts. This is done in order

to reduce the magnitude of the initial inrush current into the capacitor bank. The resistors

are kept in place for a duration of about 20 ms once the main switch is closed, at which

time they are shorted out of the circuit. This is to prevent an undesired voltage drop

across the resistors once steady state is achieved.

3.3.2 Pre-insertion inductors

The use of pre-insertion inductors operates in a similar manner, except the inductors are

not switched out of the circuit once the transient is completed. Considering that the

impedance of inductors is frequency dependent, then during initial inrush of current into

the bank, the frequency is quite high, and hence the impedance is quite high as well.

When the system returns to steady state, the frequency is lower, and hence the effective

impedance is reduced significantly, therefore the inductors do not interfere significantly

with the operation of the circuit.

3.3.3 Controlled Closing Devices

Controlled closing devices are usually high speed vacuum switches, or SF6 circuit

breakers. The switch is closed as close as possible to zero voltage to minimise the inrush

current transients. This is not the preferred method of reducing transient overvoltages due

to the difficulty in switching at such a precise instant of the waveform [9].

In deciding between the use of pre-insertion resistors or pre-insertion inductors, the

system under consideration should be modelled under both solutions using a computer-based simulation. This is due to the fact that parameters such as line length, system

strength, and capacitor size might indicate that the use of pre-insertion inductors is ideal

for a particular system, yet in another system with different parameter values, pre-

insertion inductors are more beneficial [8]. In fact, the incorrect choice of mitigation

device could accentuate the problems caused by voltage magnification. The

Electromagnetic Transients Program (EMTP) is an ideal choice for analysis of a system

[8].


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3.4 Sources and Effects of Harmonic Distortion in Power

Systems [14] [20] [21]

With the advent of power electronic devices in rectifiers, motor drives, and power

supplies, the increasing levels of harmonics has become quite a concern for power system

engineers. Harmonic distortion is caused by non-linear loads in a power system where the

current is not proportional to the applied voltage [20]. A common example of this is in

Variable Speed Drives (VSDs), where the diode rectifier module within the VSD is

responsible for injecting harmonic currents into the system.

Research has shown that non-linear loads inject harmonic CURRENTS into the system,

therefore the harmonic producing loads can be treated as current sources. However, these

harmonic currents which pass through the system cause a voltage drop for each harmonic.

This results in voltage distortion at the load bus, as well as current distortion.

A particular group of harmonics that deserve special mention is the triplen harmonics,

which are odd multiples of the third harmonic. This is due to the fact that, unlike the

fundamental frequency components which cancel in the neutral conductor, triplen

harmonics coincide in phase and time, and thus produce a third harmonic current in the

neutral that is 300% of the phase current [14]. Triplens can be eliminated by the use of a

delta-wye transformer, with the source of harmonics connected to the wye side. The

triplens will flow into the neutral conductor, yet on the delta side, they are trapped in the

delta winding, and hence do not show up in the line currents on this side.

The most notable effects that harmonics have on a power system are the impact on

motors and transformers. In transformers, harmonic currents cause the rms current to be

greater than its capacity, leading to increased conductor loss and heating. In motors,

decreased efficiency, excessive heating, vibration, and high-pitched noises are symptoms

of harmonic voltage distortion. Nuisance tripping of protective relaying, telephone

interference, and false meter reading are other consequences of harmonics in power

systems [21].


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3.4.1 Harmonic Control using Static Compensators (STATCOMs) [3]

[5] [6]

STATCOMs are another class of Active Power Line Conditioners (APLCs), also

referred to as shunt/parallel-type APLCs. This is due to the power converter unit being

connected in parallel with the feeder cable.

Figure 3-7 :Example of a STATCOM Connection

The main purpose of STATCOMs is to eliminate current harmonics, reactive power

compensation, and balance unbalanced currents [3]. They are generally connected at the

load end of the system, since harmonic currents are injected by non-linear loads. The

harmonics are cancelled by the injected current being in anti-phase with the harmonic

currents.

To demonstrate the principles of STATCOMs, consider a load consisting of a three-

phase diode rectifier. The current drawn by the load (IL) is shown in figure 3-8. In order

to shape the source current (IS) into a sinusoid to avoid causing disturbances to other

equipment connected to the same source, the STATCOM injects a compensating current

(IC) into the line to cancel the harmonics [5].


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Figure 3-8 :Current Waveforms of a three-phase diode rectifier and STATCOM

Further to the capabilities mentioned above, STATCOMs also have the ability to

regulate real power of the line [6]. This is due to the presence of an energy storage system

connected to the DC side of the power converter seen in figure 3-7.

3.4.2 The Use of Filters for Harmonic Mitigation. [22] [23]

The use of filters is a common method of reducing harmonic related losses and protection

of power systems. However, in achieving the desired results, the filter type, location,

load, and source characteristics are all critical factors [22].

The first type of filter to be considered is a Parallel Connected Resonant Filter (PCRF)

(see figure 3-9). A PCRF is connected as close as possible to the source of the harmonics.

The impedance of the resonant branch approaches zero at the resonant frequency, which

is found at fr= 1/ LrCr. Therefore the filters shunt the harmonic current from the load,

and reduce the amount injected into the source [23].


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Figure 3-9 :Parallel Connected Resonant Filter

The next type of filter is a Series Connected Resonant Filter (SCRF). A SCRF is also

connected as close as possible to the source of the harmonics, yet now the impedance of

the filter approaches infinity at the resonant frequency. Instead of shunting the harmoniccurrents, this filter simply blocks their path [23]. A number of SCRFs can be connected

together to block any number of harmonic currents (see figure 3-10).

Figure 3-10 :Series Connected Resonant Filter

The following filter is connected to the busbars from which the harmonic producing

equipment is connected, and is known as a Zig-Zag Filter (ZZF). The ZZF is a three-

phase auto-transformer which is generally used to cancel triplen harmonic currents from

single phase loads. This is done by providing a zero sequence current path to trap andcancel the triplen harmonics. A diagram of a ZZF is shown in figure 3-11.


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Figure 3-11 :Zig-Zag Filter


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Chapter 4- The Monitoring Equipment

26

Chapter 4

The Monitori ng Equipment -

BM I 8010 PQNode

4.1 Introduction

The BMI 8010 PQNode was developed in cooperation with the Electric Power Research

Institute (EPRI), and is a distributed hardware and software system used for continuous,

long-term monitoring of power quality, energy and harmonics. The Windows based

PQNode Application and System Software (PASS) allows the user to remotely set

thresholds used to classify power quality disturbances, download data, and create reports

on voltage and current behaviour from a PC. The unit is housed in a weatherproof

enclosure designed for permanent installation if required.

4.2 The Hardware

The main attributes of the 8010 PQNode are:

Eight input channels (4 voltage/4 current),

1 Megabyte of data storage,

Harmonic analysis through 127th(voltage) and 63rd(current), Serial port and modem communications, and

Internal UPS.

The PQNode has the capability of capturing the most common disturbances which are

likely to impact on a power distribution system. These common disturbances range from

voltage sags and swells through to harmonic distortion and transients.


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Information on voltage is acquired through four probes. The probes are generally

connected to Voltage Transformers (VTs) used purely for metering purposes. Currentcharacteristics are captured through four clip-on current transformers (CTs), which have

a turns ratio of 1000/1. The CTs are simply clipped onto the busbars of interest.

The information which the PQNode acquires is downloaded onto a PC via either an

RS-232 serial connection or a modem. Both methods were used to download data for the

power quality surveys conducted in this thesis project.

Figure 4-1 :8010 PQNode


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4.3 The Software

The PQNode Application and System Software (PASS) is the interface between the user

and the PQNode. PASS is a Windows based program, and is used for control and data

display purposes. The main attributes of the PASS software are discussed in the

subsequent sections.

4.3.1 PQNode Setups

This property allows the user to define the thresholds for each of the fault conditions.

Along with this, full-scale voltage and current, modem information, location, and

frequency of operation are just a few other parameters which are defined. The setupinformation for the PQNode is prepared off-line, and is sent to the device once all the

parameters have been set.

Figure 4-2: PASS setup main menu


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4.3.2Automatic Downloading of Data

In order to prevent the memory of the PQNode from becoming full, a download schedule

is established, in which the PQNode is contacted regularly for retrieval of the data. Theinformation is erased from the memory of the PQNode following a download. This

process is completely automatic, and requires no operator intervention. It is important to

note that a download requires exclusive use of the PCs Central Processing Unit (CPU),

therefore no other programs should be in use at the time of a scheduled download.

It is also significant to mention the function of the PQNode Data Server (PNDS). The

PNDS is started automatically by the PASS software, and is required for providing data

to the program. It is recommended that the PNDS is left running, even if the PASS

software is not, as without it, scheduled downloading of data from PQNodes is not

possible.

Figure 4-3: PASS menu used to establish download schedule


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4.3.3 Intuitive Data Presentation

The data that has been collected by the PQNode is displayed in what is known as a

disturbance roll. The disturbance roll only contains information on the location, date &time, and type of fault for each disturbance. If required, additional information on a

particular disturbance can be obtained simply by double-clicking on the fault of interest.

Figure 4-4: Disturbance roll for the High Voltage Laboratory

Figure 4-5: A transient fault recorded in the High Voltage Laboratory


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4.3.4 Flexible Output

The various graphs and displays generated by the PASS software can be copied to otherWindows programs for inclusion in reports. This is achieved by the clipboard function of

Windows programs.


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Chapter 5- California Instruments AC Power Source

32

Chapter 5

Cali fornia I nstruments AC Power Source1251 P Series

5.1 Introduction

The 1251P series AC power source was developed by California Instruments, and is a

hardware and software system which can be used for a wide range of applications. The

Windows based P Series Graphical User Interface Software (PGUI) allows the user to

control the frequency and voltage, as well as set current limit levels from a PC. However,

perhaps the most beneficial function is the ability to program voltage and frequency

transients in order to simulate common power quality faults in the laboratory such as

motor starting. The following sections outline the hardware, software, and interfacing

capabilities of the 1251P Series AC Power Source.

5.2 The Hardware

The 1251P series AC power source is contained in a compact, portable aluminium chassis

as shown in figure 5-1. It has an important list of specifications detailed in table 5-1

which make it an ideal choice for power quality simulation tests.

Figure 5-1 : California Instruments 1251 P Series AC Power Source


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Figure 5-2: Front panel display of power source

The front panel of the device as seen in figure 5-2 can be divided into a number of

functional areas:

Output sockets

Status indicator lights

Shuttle knobs

LCD display

Button controls

Output Sockets:

The output sockets are located on the right side of the front panel and provide a

connection to the load. The low voltage socket (0-135 VRMS) will only be active if the

low voltage range is selected, likewise for the high voltage socket (0-270 VRMS).


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Status Indicator Lights:

1. TheRemoteLED indicates that the unit is operating in remote control mode.

In this mode the user controls all the various parameters via a computerconnected to the power source via an RS-232 cable.

2. The Fault LED indicates that a fault condition has occurred. The conditions

which cause this LED to be lit are overvoltage and overtemperature.

3. The Output LED indicates whether a voltage is present at the output socket. If

this LED is not lit, then no voltage is present, regardless of the voltage setting.

4. The Range LED indicates which output range has been selected. When it is

illuminated, it indicates that the high voltage range has been selected.

5. The Frequency LED is lit when the LCD display shows the programmed

frequency, as opposed to the current limit value.

6. The CurrentLED is illuminated when the LCD display shows the current limit

value or the measured current value, as opposed to the programmed frequency

value.

Shuttle Knobs:

There are two shuttle knobs located on the front panel of the power source. The left

shuttle knob is used to control the voltage level, while the right shuttle knob controls

either the frequency or the current limit, depending on the status of the mode button. It

is important to remember that these shuttle knobs can only be operated when the unit is

not operating in remote mode, i.e. not being controlled from a computer.

LCD Display:

The LCD display consists of two 4 digit, 7 segment displays. The voltage display shows

the programmed level of the voltage, while the frequency/current display shows either the

programmed frequency, or the current limit value, depending on the status of the mode

button.


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ParameterParameter SpecificationSpecificationOutputOutput

Power 1250 VA

Voltage Low range 0-135 VRMS

High range 0-270 VRMS

Resolution 0.1 V

Distortion < 0.55% @ 50/60Hz

Noise < 0.1V typical

Frequency Range 16-500 Hz

Accuracy 0.02%

Resolution 0.1 Hz from 16 to 99.9 Hz

1 Hz from 100 to 500 Hz

Max. RMS current Low range 9.2 ARMS

High range 4.6 ARMS

Max. Peak Current Low range 27 A

High range 13 A

InputInputVoltage 100-240 + 10% V

Frequency 47-63 Hz

Hold up time 20ms

ProtectionProtectionOver current

Over temperature

Over voltage

MeasurementsMeasurementsCurrent Range 0.0-10.0 ARMS

Accuracy + 0.2 ARMS

Resolution 0.1 ARMS

Voltage (with RS-232option only)

range 278 V

MechanicalMechanicalDimensions 8.75 x 8.5 x 17.5 inches

223 x 216 x 445 mm

Weight 30 lb

13 kgOperating

temperature0-40

oC

Air intake/exhaust rear/sides

Table 5-1: California Instruments 1251P AC Power Source Specifications


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5.3 The Software

The P Series Graphical User Interface (PGUI) software is used to control all aspects of

the AC power source over an RS-232C cable. This allows full control of the unit from a

PC, without the need to use the front panel.

Each time the PGUI program is started, it will automatically try and locate the power

source through the serial port of the PC. If it is unsuccessful, the program will launch in

simulation mode. The main program window is shown in figure 5-3.

Figure 5-3: Main window of PGUI software

As can be seen from figure 5-3, the voltage, frequency, and current limit values can be set

via the respective slider bars, or can be entered directly in the text boxes to the right of

each slider bar.

In the lower left corner of the window is the front panel lock check box. This box allows

the unit to operate in local or remote modes, however when operating in remote mode, it


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is recommended to check this box to prevent any accidental changes to the settings via

the front panel.

The Save and Recall boxes in the Registers sub-menu allow the user to specify a

number of different values for all the various parameters when the unit is initially

powered up.

Perhaps the most powerful feature of the PGUI software however, is the ability to design

transient programs. Transient programs are useful in simulating specific faults in the

laboratory in order to observe how particular pieces of equipment will react to the faults.

A typical transient program is shown in figure 5-4.

Figure 5-4: Transient programming window


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The transients which are available for programming are as follows:

Voltage:

Drop Step

Surge

Sag

Sweep

Frequency :

Surge

Sag

Sweep

Voltage and Frequency combination :

Step

Sweep


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Chapter 6- Monitoring at the Caltex Oil Refinery

41

Figure 6-1: Electrical location of the PQNode at the Caltex Oil Refinery

6.2 Results

Over the 17 day monitoring period at the Caltex Oil Refinery (12/7/99 29/7/99), a total

of 1608 disturbances were recorded 397 wave faults and 1211 RMS variations. Due to

the extreme number of faults recorded it appears that the thresholds used to classify

power quality disturbances were far too narrow. The actual thresholds used in monitoring

the power at the refinery are displayed in appendix D.

6.2.1 Transient Analysis

There were no impulsive or oscillatory transients recorded for the duration of this survey.

6.2.2 Short/Long Duration Variations: CBEMA Analysis

The voltage level in any distribution system fluctuates around the nominal value,

therefore on occasions when the level dropped below the lower threshold only

momentarily, the PQNode would record that an RMS variation had occurred. This is the

situation which occurred for every RMS variation that was recorded. An example of this

is shown in figure 6-2.


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Figure 6-2: An RMS variation recorded at the Caltex Oil Refinery


When a power system is unbalanced, the electrical current will exhibit abnormal

behaviour, such as a significant current in the neutral conductor. The graphs of the

current trends for each of phases at the refinery are shown in figure 6-3.


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Figure 6-3: The RMS current trends for each of the phases.

The sudden increase in current seen in each phase was the result of a voltage sag

experienced at the refinery on the 16thJuly. The feeder cables supplying electrical power

to one of the Energex transformers were severed due to a small explosion in the

transformer cable box. The increase in current was caused by the load from the faulty

transformer being transferred to the two remaining healthy transformers. Once the

transformer was repaired and returned to service a few days later, the current levels

returned to a value close to prior to the fault.

The voltage imbalance was calculated in MS Excel for each of the phase to phase

voltages and is shown in Figure 6-4. As can be clearly seen in each case, the limit for

voltage imbalance of 2% specified by the IEEE 1159 standard is exceeded on numerous

occasions. Due to the reasonably even distribution of imbalances, it is unlikely that a


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power system fault such as a line to ground fault was the cause. The most likely

explanation is that the imbalance was caused by the many non-linear loads such as VSDs

which operate at the refinery.

Figure 6-4: Percentage imbalance for each phase to phase voltage


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6.2.4 Waveform Distortion Analysis

The wave faults recorded in this survey occurred exactly every hour. This is due to the

fact that 1 hour was specified in the setup as the time interval between samples of all the

various parameters. Due to an internal hardware error, the period of the waveforms was

recorded as approximately 12 ms, or a frequency of 83 Hz, with an extremely large value

of the second harmonic. Since the frequency of supply was specified as 50 Hz in the

setup, a fault was recorded every time a sample was taken. This phenomenon also

explains the extremely large value of Total Harmonic Distortion (THD) recorded in each

case. An example of this problem is shown in figure 6-5. The PQNode will be returned to

the manufacturers in Melbourne to rectify this problem.

Figure 6-5 : Voltage waveform recorded from the Caltex Oil Refinery.


Knowing that the frequency of supply in an electrical power system is 50Hz, the results

which show the power frequency as approximately 83 Hz can be attributed to an internal

error with the PQNode. Taking this problem into account when analysing the results, the

frequency does not deviate from this value by any amount which could create problems

within a power system.


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6.2.6 Miscellaneous results

On 16/7/99, a problem occurred with one of the transformers within the Energex

substation inside the refinery which caused the cable box enclosing the incoming feeder

cables to explode, severing the electricity supply cables to the transformer. A voltage sag

was experienced while the load from the faulty transformer was transferred to the other

two healthy transformers not only within the refinery, but also at a chemicals factory

electrically upstream from the refinery. The PQNode did not detect this voltage sag.

Information on the characteristics of the voltage sag was obtained from Energex, who

have an EDMI Power Quality Meter located within their substation in the refinery. A

transcript of the email detailing the fault is shown in appendix B. The sag occurred at

9:00:57.12 hours on the 16/7/99, with a duration of 0.27 seconds. The EDMI meters have

been set to record a voltage sag should the voltage drop below 80% of nominal voltage,

however the exact magnitude of the sag is unknown.

Of greater concern however, is a complete power outage that occurred on the 19/7/99.

The details of the outage were again obtained from Energex, who recorded the duration

as 47 minutes, beginning at 05:47:37.00 hours. The cause is believed to be a traffic

accident, which tripped an entire Energex 110/33 kV bulk supply substation. The UPS

within the PQNode did not operate when the outage occurred, hence no details of this

fault were recorded.

Once it was realised the PQNode had not recorded these faults, the equipment was

returned to the university laboratory in order to determine the cause of this. Although it

was desirable to monitor the power at the refinery for a longer period of time, no

guarantee could be given that the PQNode would not miss any future events, therefore it

was decided that this was the most appropriate course of action given the problems faced.


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6.3 Tests Performed on the PQNode in the Laboratory

6.3.1 Voltage Sag Testing:

The first test performed was to determine whether a voltage sag produced by the AC

power source described in chapter 5 would be recorded by the PQNode. A voltage sag

with characteristics identical to the one which was not recorded by the PQNode was

simulated using the information obtained from Energex. With a nominal output voltage of

110 volts from the power source, the voltage was decreased to 88 volts (a 20% reduction

from nominal) for 0.27 seconds. The only difference between the actual sag and the

simulated sag is that the actual sag occurred on a phase to phase voltage, while the

simulated sag occurred on a phase to neutral voltage. This is due to the physical

construction of the power source. In this instance, the PQNode DID record a disturbance,

as shown diagrammatically in figure 6-6.

Figure 6-6: Voltage Sag produced by the AC Power Source

Several possibilities exist for why the PQNode did not record the voltage sag experienced

at the refinery on the 16thJuly. The first explanation is that the memory of the PQNode

was at full capacity at the time of the fault. If this were the case, then no record would

exist of any faults occurring after the memory had reached full capacity. The PQNode

contains 1 Megabyte of storage capacity, which should have been sufficient considering

data on the recorded faults was downloaded from the PQNode by the PC every 12 hours.

It is unlikely then that this was the cause of the PQNode not recording the faults,

nevertheless, it cannot be completely ruled out as a possibility.


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The other possibility is a hardware problem with the PQNode itself. The PQNode

contains a card which tells it whether to record the voltage as phase to neutral, or phase to

phase, depending on the orientation of the card when inserted into the PQNode. Allprevious monitoring studies using the PQNode had recorded phase to neutral voltages,

however at the refinery it was only possible to monitor phase to phase voltages. It is

conceivable then that there is an error on the phase to phase card.

6.3.2 Power Outage Testing :

Once the voltage sag testing in the laboratory was complete, the PQNode was taken to the

university electronics workshop, as it was believed an internal hardware problem causedthe UPS not to operate when the power outage occurred. It was found that although the

PQNode does contain a UPS, there is no form of energy storage within the PQNode to

supply power during an outage, therefore a battery must be connected externally. A

battery capable of supplying 32 volts DC is required. There is no reference made

regarding this requirement in the documentation for the PQNode.

6.3.3 Frequency Testing :The final testing which was carried out dealt with the frequency of supply, and any

variations which may occur. Small variations in the frequency of supply are common in a

power system, and are caused by loads continually being added or removed. However,

the PQNode did not record any instances of power frequency variations. In order to

confirm these results, the frequency of the voltage supply was varied in order to

determine whether or not the PQNode would record any disturbances.

The test involved increasing the frequency to 75 Hz for 1 second using the AC power

source. The PQNode did not record a fault in this instance, therefore it is quite likely that

the PQNode contains an error which prevents it from accurately recording frequency

measurements.


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6.4 Site Appraisal and Mitigation Techniques

After careful analysis of the data recorded from the Caltex Oil Refinery, the quality of the

electrical power appears to be quite satisfactory. However, due to problems with the

PQNode, it is quite possible that other problems which exist have gone unnoticed.

Assuming the results are in fact accurate, there doesnt seem to be any problem with

transients or voltage variations. Although numerous minor voltage variations were

recorded, none were considered significant enough by the CBEMA curve to affect the

power quality of the refinery.

The limits specified by the IEEE 1159 standard for voltage imbalance were exceeded on

numerous occasions, sometimes even by as much as 200% of the maximum

recommended value. An attempt was made to correlate any system faults with major

occurrences of voltage imbalance, however this proved unsuccessful. The most likely

explanation is that the imbalance was caused by the many non-linear devices in operation

at the refinery, such as VSDs and induction motors.

Although there was no record of any waveform distortion apart from the fault which

records the frequency as approximately 83Hz, it is quite likely that at sections of the

electrical reticulation system operating at lower voltage levels within the refinery, there is

bound to be some waveform distortion. This is due to the presence of VSDs, which are

commonly associated with harmonic distortion. Since harmonic currents originate from

the load, and travel toward the source, they will only be present on the side of the

transformer which supplies the harmonic producing load, and will not be visible in the

line currents on the primary side, hence their existence only at lower voltage levels in the

reticulation system. One possible solution to this problem is to supply all the harmonic

producing loads from a dedicated substation. A transformer connected in delta-wye with

a 1:1 turns ratio would be required to supply the substation. This configuration would not

eliminate harmonics, however it would reduce their effect on neighboring equipment.

This method is not often implemented though due to the substantial costs involved.


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Chapter 7- Monitoring in the Chemistry Building

50

Chapter 7

Monitor ing in the Chemistry Bui lding

Results, Analysis and Discussion

7.1 Introduction and Background

This power quality survey was conducted in the Chemistry building at The University of

Queensland. The Chemistry building has a wide range of commercial loads such as

computers, fluorescent lights and General Purpose Outlets (GPOs), not to mention

industrial loads such as air-conditioners, elevators, and scientific laboratories. The

location that the monitoring of the power quality was conducted was on the secondary

side of transformer #2 as shown in figure 7-1.

Figure 7-1: Electrical location of the monitoring equipment at the Chemistry building


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7.2 Results

The quality of the power supplied to the Chemistry building was monitored for 11 days

(31/8/99 10/9/99). During this period, there were a total of 1032 faults recorded, of

which 255 were wave faults, and 777 were RMS variations.

There were no complaints lodged regarding equipment malfunctions due to poor power

quality in the Chemistry building during this period, therefore similar to the situation at

the refinery, it appears that the thresholds used in this survey were far too tight. The

actual thresholds used are displayed in appendix D.


The PQNode did not record any transient faults during this monitoring period. Low

frequency oscillatory transients arising from utility capacitor switching have been

observed within the Chemistry building in the past, however they were not detected on

this occasion [30]. The previous monitoring location was further downstream from the

monitoring location in this survey, therefore if oscillatory transients were recorded at this

lower voltage level, then it is reasonable to expect to observe them on the incoming

feeders as well.

7.2.2 Short/Long Duration Variations : CBEMA Analysis

Every RMS variation which was recorded was due to the level of the voltage dropping

below the lower threshold only momentarily as shown in figure 7-2. None of these

recordings were significant enough in magnitude or duration to be classified by the

CBEMA curve as having an impact significant enough to effect the power quality of the

system.


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Figure 7-2: An RMS variation recorded in the Chemistry building

Figure 7-3: RMS variations with CBEMA curve overlay

There were no undervoltages or overvoltages as classified by the IEEE 1159 standard

recorded in the Chemistry building for the duration of this survey.

7.2.3 Voltage ImbalanceA typical indication of any imbalance within a power system is abnormal current trends,

such as a non-zero current in the neutral conductor [14]. The graphs of the current trends

for each of phases as well as the neutral conductor is shown in figure 7-4.


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Figure 7-4: Current trends for each phase conductor and neutral conductor


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From figure 7-4, it can clearly be seen that the current in the neutral has a significant non-

zero value. This is partly due to the fact that the majority of loads in the Chemistrybuilding are single phase, and hence an unbalanced system is the result. Another cause of

this non-zero reading could be due to the presence of triplen harmonics. Triplen

harmonic currents coincide in phase and time for each phase, and hence add in the neutral

conductor, producing an electrical current. In order to determine whether this non-zero

value of neutral current is caused by a power system imbalance, or by triplen harmonics,

a more detailed analysis will be performed using MS Excel.

An analysis using MS Excelproduced the graphs in figure 7-5 showing the percentage

imbalance for each phase. As can be clearly seen, the maximum recommended value

specified by the IEEE 1159 standard of 2% is exceeded quite often. These results indicate

that the neutral current is most likely caused by a power system imbalance.


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Figure 7-5: Percentage voltage imbalance for each phase

7.2.4 Waveform Distortion Analysis

The same waveform distortion problem was experienced in the Chemistry building as

what was seen in the results from the refinery, in that the period was recorded as

approximately 12ms. This phenomenon was the cause of every instance of waveform

distortion that was recorded during the survey. Once again this also explains the

unusually high value of Total Harmonic Distortion (THD) which the PQNode recorded.

The Australian Standard 2279 (AS 2279) specifies that the maximum allowable value of

THD is 5 %, and as can be seen from figure 7-6, on this occasion the PQNode recorded a

value of 361.5%. If this were a true reading, then the consequences would be

catastrophic, and since no complaints were lodged regarding power quality disturbances

during the survey, it can be concluded that the error lies within the PQNode.


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Figure 7-6: Waveform distortion as recorded in the Chemistry building

It is interesting to note that in this survey there was no waveform distortion recorded due

to mains signalling [27]. Mains signalling is the process whereby a sinewave with a

magnitude of 2-3 % of the nominal supply voltage, and of frequency 1050 Hz (21 x 50

Hz) is superimposed upon the power frequency signal and transmitted along the power

lines. Mains signalling is used to control loads such as special tariffs for customer hot

water systems. The signal is injected over the supply system up to 20 times per day, and

can last in duration up to three minutes. Power quality monitoring studies conducted in

the Chemistry building in 1998 detected this mains signalling component, therefore it is

reasonable to expect the PQNode to record it again.


Apart from the fault that causes the power frequency to be recorded as approximately 83

Hz, no significant variations from this value were experienced in the Chemistry building.

However it is important to remember that frequency variations were not recorded by the

PQNode in the tests described in chapter 6, therefore it is possible that frequency

deviations did occur, but were not recorded.


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7.3 Site Appraisal and Mitigation Techniques

An accurate evaluation of the quality of the power in the Chemistry building is difficult

given the problems experienced with the monitoring equipment, however, apart from a

reasonable degree of imbalance seen in each phase voltage, the power quality appears

quite satisfactory.

The voltage imbalance appearsto be caused due to the majority of loads in the Chemistry

building being single phase. However, substation 10 which distributes power to the

Chemistry building, also distributes power to the neighbouring Computer Science

building, which has many non-linear loads connected. The combination of these two

factors appears to be the most likely cause of the voltage imbalance.

The voltage level was quite stable during the survey, as there were no recordings of any

short or long duration variations. Although there were also no recordings of any power

frequency variations, this does not suggest they did not occur, as the PQNode did not

behave as expected with regard to frequency measurements. The fact that the power

frequency was measured as approximately 12 ms confirms this conclusion.

The analysis of the data attained from this survey suggests that apart from the voltage

imbalance, the power quality can be deemed satisfactory within the Chemistry building

given the problems with the PQNode. The data obtained is mostly within the

recommended thresholds and complies with all the relevant standards with the exception

of the voltage imbalance measurements.


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Chapter 8- Monitoring in the Physics Laser Laboratory

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Chapter 8

Monitoring in the Physics Building Laser Laboratory

The University of Queensland

Results, Analysis and Discussion

8.1 Introduction and Background

This power quality survey was conducted in the laser laboratory within the Physics

building at The University of Queensland. The Physics building is supplied from

substation 2 (SUB 2), which contains a Tyree 1000kVA and an English Electric 750kVA

transformers.

The laboratory in which this power quality study was conducted contains a very

sophisticated Innova Sabre Argon Ion Laser, manufactured by Coherent systems. During

operation of the laser, the power supply failed, and after consultation with the

manufacturer by staff in the Physics department, it was believed that fluctuations, or a

spike in the voltage supply was the cause. An attempt was made to obtain the power

supply requirements from the manufacturer, however a reply was not received from the

initial email (see appendix B). The PQNode monitored the voltage in the survey from the

same socket that the laser was supplied from.


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8.2 Results

The power quality survey in the laser laboratory was conducted for a period of 14 days

(14/9/99 27/9/99). During this time, a total of 692 faults were recorded, of which 621

were wave faults, and 71 were RMS variations.


There were no impulsive transients recorded during this survey, however there were two

recordings of oscillatory transients (see figure 8-1). Both transients occurred on the

neutral voltage within one second of each other.

Figure 8-1: Voltage impulses recorded in the laser laboratory

An attempt was made to correlate these faults with any system faults occurring at the

same time, however this was unsuccessful. Due to the time of occurrence of these faults,


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it suggests that the cause was not due to other customer activity, but rather other

equipment connected nearby.

8.2.2 Short/Long Duration Variations : CBEMA Analysis

The laser laboratory experienced a number of voltage variations, however as can be seen

from figure 8-2, none were considered to have a magnitude or duration significant

enough to affect the power quality of the system. The voltage swells enclosed by the box

and denoted by a 1 in the figure are on the border of the CBEMA curve, however there

were no reports of any equipment malfunctions at this time.

Figure 8-2: RMS variations in Physics building with CBEMA curve overlay.


Figure 8-3 shows the results of an analysis performed in MS Excel to determine the

percentage imbalance for each of the phase to phase voltages. As can be seen from the

diagrams, the recommended maximum of 2% is frequently exceeded. This imbalance in

the power system is possibly due to non-linear loads in the Physics building, such as

other lasers containing AC to DC power supplies, which are commonly associated with

harmonic distortion and other power quality problems. It is unlikely that the imbalance

was caused by a system fault due to the regular distribution of imbalance measurements.

Non-linear loads are therefore the most likely contributing factor to the voltage

imbalance.


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Figure 8-3: Percentage voltage imbalance for each phase to phase voltage


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Chapter 8-

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Monitoring of Distribution System Power Quality