Electrical Power Systems Quality

525 Pages·2008·4.44 MB·english

by  Surya Santoso

Electrical Power Systems Quality, Second Edition

CHAPTER 1: INTRODUCTION
What is Power Quality?
Power Quality -- Voltage Quality
Why Are We Concerned About Power Quality?
The Power Quality Evaluation Procedure
Who Should Use This Book
Overview of the Contents

CHAPTER 2: TERMS AND DEFINITIONS
Need for a Consistent Vocabulary
General Classes of Power Quality Problems
Transients
Long-Duration Voltage Variations
Short-Duration Voltage Variations
Voltage Imbalance
Waveform Distortion
Voltage Fluctuation
Power Frequency Variations
Power Quality Terms
Ambiguous Terms
CBEMA and ITI Curves
References

CHAPTER 3: VOLTAGE SAGS AND INTERRUPTIONS
Sources of Sags and Interruptions
Estimating Voltage Sag Performance
Fundamental Principles of Protection
Solutions at the End-User Level
Evaluating the Economics of Different Ride-Through Alternatives
Motor-Starting Sags
Utility System Fault-Clearing Issues
References

CHAPTER 4: TRANSIENT OVERVOLTAGES
Sources of Transient Overvoltages
Principles of Overvoltage Protection
Devices for Overvoltage Protection
Utility Capacitor-Switching Transients
Utility System Lightning Protection
Managing Ferroresonance
Switching Transient Problems with Loads
Computer Tools for Transients Analysis
References

CHAPTER 5: FUNDAMENTALS OF HARMONICS
Harmonic Distortion
Voltage versus Current Distortion
Harmonics versus Transients
Harmonic Indexes
Harmonic Sources from Commercial Loads
Harmonic Sources from Industrial Loads
Locating Harmonic Sources
System Response Characteristics
Effects of Harmonic Distortion
Interharmonics
References
Bibliography

CHAPTER 6: APPLIED HARMONICS
Harmonic Distortion Evaluations
Principles for Controlling Harmonics
Where to Control Harmonics
Harmonic Studies
Devices for Controlling Harmonic Distortion
Harmonic Filter Design: A Case Study
Case Studies
Standards of Harmonics
References
Bibliography

CHAPTER 7: LONG-DURATION VOLTAGE VARIATIONS
Principles of Regulating the Voltage
Devices for Voltage Regulation
Utility Voltage Regulator Application
Capacitors for Voltage Regulation
End-User Capacitor Application
Regulating Utility Voltage with Distributed Resources
Flicker
References
Bibliography

CHAPTER 8: POWER QUALITY BENCHMARKING
Introduction
Benchmarking Process
RMS Voltage Variation Indices
Harmonics Indices
Power Quality Contracts
Power Quality Insurance
Power Quality State Estimation
Including Power Quality in Distribution Planning
References
Bibliography

CHAPTER 9: DISTRIBUTED GENERATION AND POWER QUALITY
Resurgence of DG
DG Technologies
Interface to the Utility System
Power Quality Issues
Operating Conflicts
DG on Distribution Networks
Siting DGDistributed Generation
Interconnection Standards
Summary
References
Bibliography

CHAPTER 10: WIRING AND GROUNDING
Resources
Definitions
Reasons for Grounding
Typical Wiring and Grounding Problems
Solutions to Wiring and Grounding Problems
Bibliography

CHAPTER 11: POWER QUALITY MONITORING
Monitoring Considerations
Historical Perspective of Power Quality Measuring Instruments
Power Quality Measurement Equipment
Assessment of Power Quality Measurement Data
Application of Intelligent Systems
Power Quality Monitoring Standards
References
Index
Source: Electrical Power Systems Quality
Chapter
1
Introduction
Both electric utilities and end users of electric power are becoming
increasingly concerned about the quality of electric power. The term
power quality has become one of the most prolific buzzwords in the
power industry since the late 1980s. It is an umbrella concept for a mul-
titude of individual types of power system disturbances. The issues
that fall under this umbrella are not necessarily new. What is new is
that engineers are now attempting to deal with these issues using a
system approach rather than handling them as individual problems.
There are four major reasons for the increased concern:
1. Newer-generation load equipment, with microprocessor-based con-
trols and power electronic devices, is more sensitive to power qual-
ity variations than was equipment used in the past.
2. The increasing emphasis on overall power system efficiency has
resulted in continued growth in the application of devices such as
high-efficiency, adjustable-speed motor drives and shunt capacitors
for power factor correction to reduce losses. This is resulting in
increasing harmonic levels on power systems and has many people
concerned about the future impact on system capabilities.
3. End users have an increased awareness of power quality issues.
Utility customers are becoming better informed about such issues as
interruptions, sags, and switching transients and are challenging
the utilities to improve the quality of power delivered.
4. Many things are now interconnected in a network. Integrated
processes mean that the failure of any component has much more
important consequences.
1
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Introduction
2 Chapter One
The common thread running though all these reasons for increased
concern about the quality of electric power is the continued push for
increasing productivity for all utility customers. Manufacturers want
faster, more productive, more efficient machinery. Utilities encourage
this effort because it helps their customers become more profitable and
also helps defer large investments in substations and generation by
using more efficient load equipment. Interestingly, the equipment
installed to increase the productivity is also often the equipment that
suffers the most from common power disruptions. And the equipment
is sometimes the source of additional power quality problems. When
entire processes are automated, the efficient operation of machines and
their controls becomes increasingly dependent on quality power.
Since the first edition of this book was published, there have been
some developments that have had an impact on power quality:
1. Throughout the world, many governments have revised their laws
regulating electric utilities with the intent of achieving more cost-com-
petitive sources of electric energy. Deregulation of utilities has compli-
cated the power quality problem. In many geographic areas there is no
longer tightly coordinated control of the power from generation
through end-use load. While regulatory agencies can change the laws
regarding the flow of money, the physical laws of power flow cannot be
altered. In order to avoid deterioration of the quality of power supplied
to customers, regulators are going to have to expand their thinking
beyond traditional reliability indices and address the need for power
quality reporting and incentives for the transmission and distribution
companies.
2. There has been a substantial increase of interest in distributed
generation (DG), that is, generation of power dispersed throughout the
power system. There are a number of important power quality issues
that must be addressed as part of the overall interconnection evalua-
tion for DG. Therefore, we have added a chapter on DG.
3. The globalization of industry has heightened awareness of defi-
ciencies in power quality around the world. Companies building facto-
ries in new areas are suddenly faced with unanticipated problems with
the electricity supply due to weaker systems or a different climate.
There have been several efforts to benchmark power quality in one part
of the world against other areas.
4. Indices have been developed to help benchmark the various
aspects of power quality. Regulatory agencies have become involved in
performance-based rate-making (PBR), which addresses a particular
aspect, reliability, which is associated with interruptions. Some cus-
tomers have established contracts with utilities for meeting a certain
quality of power delivery. We have added a new chapter on this subject.
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Introduction
Introduction 3
1.1 What Is Power Quality?
There can be completely different definitions for power quality, depend-
ing on one’s frame of reference. For example, a utility may define power
quality as reliability and show statistics demonstrating that its system
is 99.98 percent reliable. Criteria established by regulatory agencies
are usually in this vein. Amanufacturer of load equipment may define
power quality as those characteristics of the power supply that enable
the equipment to work properly. These characteristics can be very dif-
ferent for different criteria.
Power quality is ultimately a consumer-driven issue, and the end
user’s point of reference takes precedence. Therefore, the following def-
inition of a power quality problem is used in this book:
Any power problem manifested in voltage, current, or frequency devia-
tions that results in failure or misoperation of customer equipment.
There are many misunderstandings regarding the causes of power
quality problems. The charts in Fig. 1.1 show the results of one survey
conducted by the Georgia Power Company in which both utility per-
sonnel and customers were polled about what causes power quality
problems. While surveys of other market sectors might indicate differ-
ent splits between the categories, these charts clearly illustrate one
common theme that arises repeatedly in such surveys: The utility’s and
customer’s perspectives are often much different. While both tend to
blame about two-thirds of the events on natural phenomena (e.g., light-
ning), customers, much more frequently than utility personnel, think
that the utility is at fault.
When there is a power problem with a piece of equipment, end users
may be quick to complain to the utility of an “outage” or “glitch” that has
caused the problem. However, the utility records may indicate no abnor-
mal events on the feed to the customer. We recently investigated a case
where the end-use equipment was knocked off line 30 times in 9 months,
but there were only five operations on the utility substation breaker. It
must be realized that there are many events resulting in end-user prob-
lems that never show up in the utility statistics. One example is capaci-
tor switching, which is quite common and normal on the utility system,
but can cause transient overvoltages that disrupt manufacturing
machinery. Another example is a momentary fault elsewhere in the sys-
tem that causes the voltage to sag briefly at the location of the customer
in question. This might cause an adjustable-speed drive or a distributed
generator to trip off, but the utility will have no indication that anything
was amiss on the feeder unless it has a power quality monitor installed.
In addition to real power quality problems, there are also perceived
power quality problems that may actually be related to hardware, soft-
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Introduction
4 Chapter One
Customer Perception
Natural
60%
Other
3%
Neighbor
8%
Customer
12%
Utility
17%
Utility Perception
Natural
66%
Other
0%
Neighbor
8%
Customer
25%
Utility
1%
Figure 1.1 Results of a survey on the causes of power quality
problems. (Courtesy of Georgia Power Co.)
ware, or control system malfunctions. Electronic components can
degrade over time due to repeated transient voltages and eventually
fail due to a relatively low magnitude event. Thus, it is sometimes dif-
ficult to associate a failure with a specific cause. It is becoming more
common that designers of control software for microprocessor-based
equipment have an incomplete knowledge of how power systems oper-
ate and do not anticipate all types of malfunction events. Thus, a device
can misbehave because of a deficiency in the embedded software. This
is particularly common with early versions of new computer-controlled
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Introduction
Introduction 5
load equipment. One of the main objectives of this book is to educate
utilities, end users, and equipment suppliers alike to reduce the fre-
quency of malfunctions caused by software deficiencies.
In response to this growing concern for power quality, electric utilities
have programs that help them respond to customer concerns. The phi-
losophy of these programs ranges from reactive, where the utility
responds to customer complaints, to proactive, where the utility is
involved in educating the customer and promoting services that can
help develop solutions to power quality problems. The regulatory issues
facing utilities may play an important role in how their programs are
structured. Since power quality problems often involve interactions
between the supply system and the customer facility and equipment,
regulators should make sure that distribution companies have incen-
tives to work with customers and help customers solve these problems.
The economics involved in solving a power quality problem must also
be included in the analysis. It is not always economical to eliminate
power quality variations on the supply side. In many cases, the optimal
solution to a problem may involve making a particular piece of sensi-
tive equipment less sensitive to power quality variations. The level of
power quality required is that level which will result in proper opera-
tion of the equipment at a particular facility.
Power quality, like quality in other goods and services, is difficult to
quantify. There is no single accepted definition of quality power. There
are standards for voltage and other technical criteria that may be mea-
sured, but the ultimate measure of power quality is determined by the
performance and productivity of end-user equipment. If the electric
power is inadequate for those needs, then the “quality” is lacking.
Perhaps nothing has been more symbolic of a mismatch in the power
delivery system and consumer technology than the “blinking clock”
phenomenon. Clock designers created the blinking display of a digital
clock to warn of possible incorrect time after loss of power and inad-
vertently created one of the first power quality monitors. It has made
the homeowner aware that there are numerous minor disturbances
occurring throughout the power delivery system that may have no ill
effects other than to be detected by a clock. Many appliances now have
a built-in clock, so the average household may have about a dozen
clocks that must be reset when there is a brief interruption. Older-tech-
nology motor-driven clocks would simply lose a few seconds during
minor disturbances and then promptly come back into synchronism.
1.2 Power Quality (cid:1)Voltage Quality
The common term for describing the subject of this book is powerqual-
ity; however, it is actually the quality of the voltage that is being
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Introduction
6 Chapter One
addressed in most cases. Technically, in engineering terms, power is
the rate of energy delivery and is proportional to the product of the volt-
age and current. It would be difficult to define the quality of this quan-
tity in any meaningful manner. The power supply system can only
control the quality of the voltage; it has no control over the currents
that particular loads might draw. Therefore, the standards in the
power quality area are devoted to maintaining the supply voltage
within certain limits.
AC power systems are designed to operate at a sinusoidal voltage of
a given frequency [typically 50 or 60 hertz (Hz)] and magnitude. Any
significant deviation in the waveform magnitude, frequency, or purity
is a potential power quality problem.
Of course, there is always a close relationship between voltage and
current in any practical power system. Although the generators may
provide a near-perfect sine-wave voltage, the current passing through
the impedance of the system can cause a variety of disturbances to the
voltage. For example,
1. The current resulting from a short circuit causes the voltage to sag
or disappear completely, as the case may be.
2. Currents from lightning strokes passing through the power system
cause high-impulse voltages that frequently flash over insulation
and lead to other phenomena, such as short circuits.
3. Distorted currents from harmonic-producing loads also distort the
voltage as they pass through the system impedance. Thus a dis-
torted voltage is presented to other end users.
Therefore, while it is the voltage with which we are ultimately con-
cerned, we must also address phenomena in the current to understand
the basis of many power quality problems.
1.3 Why Are We Concerned about Power
Quality?
The ultimate reason that we are interested in power quality is eco-
nomic value. There are economic impacts on utilities, their customers,
and suppliers of load equipment.
The quality of power can have a direct economic impact on many
industrial consumers. There has recently been a great emphasis on
revitalizing industry with more automation and more modern equip-
ment. This usually means electronically controlled, energy-efficient
equipment that is often much more sensitive to deviations in the sup-
ply voltage than were its electromechanical predecessors. Thus, like
the blinking clock in residences, industrial customers are now more
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Introduction
Introduction 7
acutely aware of minor disturbances in the power system. There is big
money associated with these disturbances. It is not uncommon for a
single, commonplace, momentary utility breaker operation to result in
a $10,000 loss to an average-sized industrial concern by shutting down
a production line that requires 4 hours to restart. In the semiconductor
manufacturing industry, the economic impacts associated with equip-
ment sensitivity to momentary voltage sags resulted in the develop-
ment of a whole new standard for equipment ride-through (SEMI
Standard F-47, Specification for Semiconductor Process Equipment
Voltage Sag Immunity).
The electric utility is concerned about power quality issues as well.
Meeting customer expectations and maintaining customer confidence
are strong motivators. With today’s movement toward deregulation
and competition between utilities, they are more important than ever.
The loss of a disgruntled customer to a competing power supplier can
have a very significant impact financially on a utility.
Besides the obvious financial impacts on both utilities and industrial
customers, there are numerous indirect and intangible costs associated
with power quality problems. Residential customers typically do not
suffer direct financial loss or the inability to earn income as a result of
most power quality problems, but they can be a potent force when they
perceive that the utility is providing poor service. Home computer
usage has increased considerably in the last few years and more trans-
actions are being done over the Internet. Users become more sensitive
to interruptions when they are reliant on this technology. The sheer
number of complaints require utilities to provide staffing to handle
them. Also, public interest groups frequently intervene with public ser-
vice commissions, requiring the utilities to expend financial resources
on lawyers, consultants, studies, and the like to counter the interven-
tion. While all this is certainly not the result of power quality problems,
a reputation for providing poor quality service does not help matters.
Load equipment suppliers generally find themselves in a very com-
petitive market with most customers buying on lowest cost. Thus, there
is a general disincentive to add features to the equipment to withstand
common disturbances unless the customer specifies otherwise. Many
manufacturers are also unaware of the types of disturbances that can
occur on power systems. The primary responsibility for correcting inad-
equacies in load equipment ultimately lies with the end user who must
purchase and operate it. Specifications must include power perfor-
mance criteria. Since many end users are also unaware of the pitfalls,
one useful service that utilities can provide is dissemination of infor-
mation on power quality and the requirements of load equipment to
properly operate in the real world. For instance, the SEMI F-47 stan-
dard previously referenced was developed through joint task forces
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