Electric Power Distribution Reliability

Electric PowerDistribution Reliability

Second Edition

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© 2009 by Taylor & Francis Group, LLC

POWER ENGINEERING

Series EditorH. Lee Willis

Quanta TechnologyRaleigh, North Carolina

Advisory EditorMuhammad H. RashidUniversity of West Florida

Pensacola, Florida

1. Power Distribution Planning Reference Book, H. Lee Willis2. Transmission Network Protection: Theory and Practice,

Y. G. Paithankar3. Electrical Insulation in Power Systems, N. H. Malik,

A. A. Al-Arainy, and M. I. Qureshi4. Electrical Power Equipment Maintenance and Testing,

Paul Gill5. Protective Relaying: Principles and Applications,

Second Edition, J. Lewis Blackburn6. Understanding Electric Utilities and De-Regulation,

Lorrin Philipson and H. Lee Willis7. Electrical Power Cable Engineering, William A. Thue8. Electric Systems, Dynamics, and Stability with Artificial

Intelligence Applications, James A. Momoh and Mohamed E. El-Hawary

9. Insulation Coordination for Power Systems, Andrew R. Hileman

10. Distributed Power Generation: Planning and Evaluation, H. Lee Willis and Walter G. Scott

11. Electric Power System Applications of Optimization, James A. Momoh

12. Aging Power Delivery Infrastructures, H. Lee Willis,Gregory V. Welch, and Randall R. Schrieber

13. Restructured Electrical Power Systems: Operation, Trading,and Volatility, Mohammad Shahidehpour and Muwaffaq Alomoush

14. Electric Power Distribution Reliability, Richard E. Brown15. Computer-Aided Power System Analysis,

Ramasamy Natarajan

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16. Power System Analysis: Short-Circuit Load Flow and Harmonics, J. C. Das

17. Power Transformers: Principles and Applications, John J. Winders, Jr.

18. Spatial Electric Load Forecasting: Second Edition, Revised and Expanded, H. Lee Willis

19. Dielectrics in Electric Fields, Gorur G. Raju20. Protection Devices and Systems for High-Voltage

Applications, Vladimir Gurevich21. Electrical Power Cable Engineering, Second Edition,

William Thue22. Vehicular Electric Power Systems: Land, Sea, Air,

and Space Vehicles, Ali Emadi, Mehrdad Ehsani, and John Miller

23. Power Distribution Planning Reference Book, Second Edition, H. Lee Willis

24. Power System State Estimation: Theory and Implementation, Ali Abur

25. Transformer Engineering: Design and Practice, S.V. Kulkarni and S. A. Khaparde

26. Power System Capacitors, Ramasamy Natarajan27. Understanding Electric Utilities and De-regulation:

Second Edition, Lorrin Philipson and H. Lee Willis28. Control and Automation of Electric Power Distribution

Systems, James Northcote-Green and Robert G. Wilson29. Protective Relaying for Power Generation Systems,

Donald Reimert30. Protective Relaying: Principles and Applications,

Third Edition, J. Lewis Blackburn and Thomas J. Domin31. Electric Power Distribution Reliability, Second Edition,

Richard E. Brown

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Electric PowerDistributionReliabilitySecond Edition

Richard E. Brown

CRC Press is an imprint of theTaylor & Francis Group, an informa business

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© 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business

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vii

Contents

Series Introduction xi

Preface xiii

Author xvii

1. DISTRIBUTION SYSTEMS 1

1.1. Generation, Transmission, and Distribution 1

1.2. Distribution Substations 8

1.3. Primary Distribution Systems 15

1.4. Secondary Distribution Systems 26

1.5. Load Characteristics 28

1.6. Distribution Operations 33

1.7. Study Questions 38

References 38

2. RELIABILITY METRICS AND INDICES 41

2.1. Power Quality, Reliability, and Availability 41

2.2. Reliability Indices 51

2.3. Customer Cost of Reliability 82

2.4. Reliability Targets 90

2.5. History of Reliability Indices 97


References 102


viii Contents

3. INTERRUPTION CAUSES 107

3.1. Equipment Failures 107

3.2. Animals 127

3.3. Severe Weather 133

3.4. Trees 150

3.5. Human Factors 155

3.6. Most Common Causes 157


References 159

4. COMPONENT MODELING 163

4.1. Component Reliability Parameters 163

4.2. Failure Rates and Bathtub Curves 165

4.3. Probability Distribution Functions 167

4.4. Fitting Curves to Measured Data 176

4.5. Component Reliability Data 182


References 188

5. SYSTEM MODELING 191

5.1. System Events and System States 192

5.2. Event Independence 195

5.3. Network Modeling 196

5.4. Markov Modeling 200

5.5. Analytical Simulation for Radial Systems 206

5.6. Analytical Simulation for Network Systems 232

5.7. Monte Carlo Simulation 241

5.8. Other Methodologies 258


References 262

6. SYSTEM ANALYSIS 265

6.1. Model Reduction 265

6.2. System Calibration 272

6.3. System Analysis 277

6.4. Improving Reliability 285

6.5. Storm Hardening 301

6.6. Conversion of Overhead to Underground 307

6.7. Economic Analysis 317

6.8. Marginal Benefit-to-Cost Analysis 325

6.9. Comprehensive Example 333


References 357


Contents ix

7. SYSTEM OPTIMIZATION 361

7.1. Overview of Optimization 361

7.2. Discrete Optimization Methods 371

7.3. Knowledge-Based Systems 385

7.4. Optimization Applications 392

7.5. Final Thoughts on Optimization 418


References 422

8. AGING INFRASTRUCTURE 425

8.1. Equipment Aging 425

8.2. Equipment Age Profiles 426

8.3. Population Aging Behavior 428

8.4. Age and Increasing Failure Rates 432

8.5. Inspection, Repair, and Replacement 438

8.6. State of the Industry 441

8.7. Final Thoughts 450


References 452


xi

Series Introduction

Power engineering is the oldest and most traditional of the various areas within

electrical engineering, yet no other facet of modern technology is currently un-

dergoing a more dramatic revolution in technology or business structure. Perhaps

the most fundamental change taking place in the electric utility industry is the

move toward a quantitative basis for the management of service reliability. Tra-

ditionally, electric utilities achieved satisfactory customer service quality through

the use of more or less “one size fits all situations” standards and criteria that

experience had shown would lead to no more than an acceptable level of trouble

on their system. Tried and true, these methods succeeded in achieving acceptable

service quality.

But evolving industry requirements changed the relevance of these methods

in two ways. First, the needs of modern electric energy consumers changed.

Even into the early 1980s, very short (less than 10 second) interruptions of

power had minimal impact on most consumers. Then, utilities routinely per-

formed field switching of feeders in the early morning hours, creating 10-second

interruptions of power flow that most consumers would not even notice. But

where the synchronous-motor alarm clocks of the 1960s and 1970s would just

fall a few seconds behind during such interruptions, modern digital clocks, mi-

croelectronic equipment and computers cease working altogether. Homeowners

of the 1970s woke up the next morning—not even knowing or caring—that their

alarm clocks were a few seconds behind. Homeowners today wake up minutes or

hours late, to blinking digital displays throughout their home. In this and in many


Series Introduction

xii

other ways, the widespread use of digital equipment and automated processes

has redefined the term “acceptable service quality” and has particularly in-

creased the importance of interruption frequency as a measure of utility perform-

ance.

Second, while the traditional standards-driven paradigm did achieve satisfac-

tory service quality in most cases, it did not do so at the lowest possible cost. In

addition, it had no mechanism for achieving reliability targets in a demonstrated

least-cost manner. As a result, in the late 20th century, electric utility manage-

ment, public utility regulators, and energy consumers alike realized there had to

be a more economically effective way to achieve satisfactory reliability levels of

electric service. This was to engineer the system to provide the type of reliability

needed at the lowest possible cost, creating a need for rigorous, quantitative reli-

ability analysis and engineering methods—techniques capable of “engineering

reliability into a system” in the same way that capacity or voltage regulation tar-

gets had traditionally been targeted and designed to.

Many people throughout the industry contributed to the development of what

are today the accepted methods of reliability analysis and predictive design. But

none contributed as much to either theory, or practice, as Richard Brown. His

work is the foundation of modern power distribution reliability engineering. It is

therefore with great pride that I welcome this book as the newest addition to the

CRC Press series on Power Engineering. This is all the more rewarding to me

because for the past decade Richard Brown has been one of my most trusted co-

workers and research collaborators, and a good friend.

Dr. Brown’s book lays out the rules and structure for modern power distribu-

tion reliability engineering in a rigorous yet accessible manner. While scrupu-

lously correct in theory and mathematics, his book provides a wealth of practical

experience and useful knowledge that can be applied by any electric power engi-

neer to improve power distribution reliability performance. Thus, Electric Power

Distribution Reliability fits particularly well into the theme of the Power Engi-

neering series, which focuses on providing modern power technology in a con-

text of proven, practical application—books useful as references as well as for

self-study and classroom use. I have no doubt that this book will be the reference

in power delivery reliability engineering for years to come.

Good work, Richard.

H. Lee Willis


xiii

Preface

Distribution reliability is one of the most important topics in the electric power

industry due to its high impact on the cost of electricity and its high correlation

with customer satisfaction. The breadth and depth of issues relating to this sub-

ject span nearly every distribution company department including procurement,

operations, engineering, planning, rate making, customer relations, and regula-

tory. Due in large part to its all-encompassing nature, distribution reliability has

been difficult for utilities to address in a holistic manner. Most departments, if

they address reliability at all, do so in isolation without considering how their

actions may relate to those in different parts of the company—an understandable

situation since there has been no single reference that covers all related issues

and explains their interrelationships. This book is an attempt to fill this void by

serving as a comprehensive tutorial and reference book covering all major topics

related to distribution reliability. Each subject has been extensively researched

and referenced with the intent of presenting a balance of theory, practical knowl-

edge, and practical applications. After reading this book, readers will have a

basic understanding of distribution reliability issues and will know how these

issues have affected typical utilities in the past. Further, readers will be knowl-

edgeable about techniques capable of addressing reliability issues and will have

a basic feel for the results that can be expected from their proper application.

Electric Power Distribution Reliability is intended for engineering profes-

sionals interested in the topic described by its title. Utility distribution planners


Preface

xiv

and reliability engineers will find it of greatest use, but it also contains valuable

information for design engineers, dispatchers, operations personnel, and mainte-

nance personnel. Because of its breadth, this book may also find use with

distribution company directors and executives, as well as with state regulatory

authorities. It is intended to be a scholarly work and is suitable for use with sen-

ior or graduate level instruction as well as for self-instruction.

This book is divided into eight chapters. Although each is a self-contained

topic, the book is written so that each chapter builds upon the knowledge of prior

chapters. As such, this book should be read through sequentially upon first en-

counter. Terminology and context introduced in prior chapters are required

knowledge to fully comprehend and assimilate subsequent topics. After an initial

reading, this book will serve well as a refresher and reference volume and has a

detailed index to facilitate the quick location of specific material.

The first chapter, “Distribution Systems,” presents fundamental concepts,

terminology, and symbology that serve as a foundation of knowledge for reliabil-

ity-specific topics. It begins by describing the function of distribution systems in

the overall electric power system. It continues by describing the component and

system characteristics of substations, feeders, and secondary systems. The chap-

ter concludes by discussing issues associated with load characteristics and

distribution operations.

The second chapter, “Reliability Metrics and Indices,” discusses the various

aspects of distribution reliability and defines terms that are frequently used later

in the book. It begins at a high level by discussing power quality and its relation-

ship to reliability. Standard reliability indices are then presented along with

benchmark data and a discussion of their benefits and drawbacks. The chapter

continues by discussing reliability from the customer perspective including the

customer cost of interrupted electrical service and the customer surveys used to

obtain this information. The chapter ends with a discussion of reliability targets

and the industry trend towards performance-based rates, reliability guarantees,

and customer choice.

Remembering that reliability problems are caused by real events, Chapter 3

provides a comprehensive discussion of all major causes of customer interrup-

tions. It begins by describing the most common types of equipment failures and

their associated failure modes, incipient failure detection possibilities, and fail-

ure prevention strategies. It then discusses reliability issues associated with

animals, presents animal data associated with reliability, and offers recommenda-

tions to mitigate and prevent animal problems. The chapter continues by

discussing severe weather including wind, lightning, ice storms, heat storms,

earthquakes, and fires. Human causes are the last interruption category ad-

dressed, including operating errors, vehicular accidents, dig-ins, and vandalism.


Preface

xv

To place all of this information in perspective, the chapter concludes by discuss-

ing the most common interruption causes experienced by typical utilities.

The analytical section of this book begins in Chapter 4, “Component Model-

ing.” The chapter starts by defining the component reliability parameters that

form the basis of all reliability models. It then discusses basic modeling concepts

such as hazard functions, probability distribution functions, and statistics. It ends

by providing component reliability data for a wide variety of distribution equip-

ment, which can be used both as a benchmark for custom data or as generic data

in lieu of custom data.

The topic of component reliability modeling leads naturally into the next

chapter, “System Modeling.” This chapter begins with a tutorial on basic system

analysis concepts such as states, Venn diagrams, network modeling, and Markov

modeling. The bulk of the chapter focuses on analytical and Monte Carlo simula-

tion methods, which are the recommended approaches for most distribution

system reliability assessment needs. Algorithms are presented with detail suffi-

cient for the reader to implement models in computer software, and reflect all of

the major system issues associated with distribution reliability. For complete-

ness, the chapter concludes by presenting reliability analysis techniques

commonly used in other fields and discusses their applicability to distribution

systems.

The sixth chapter, “System Analysis,” focuses on how to use the modeling

concepts developed in the previous two chapters to improve system reliability. It

begins with the practical issues of actually creating a system model, populating it

with default data and calibrating it to historical data. It then presents techniques

to analyze the system model including visualization, risk analysis, sensitivity

analyses, root-cause analysis, and loading analysis. One of the most important

topics of the book comes next: strategies to improve reliability and how to quan-

tify their impact by incorporating them into component and system models. This

includes the nontraditional topics of underground conversion and storm harden-

ing. The chapter then discusses how to view reliability improvement projects

from a value perspective by presenting the basics of economic analysis and the

prioritization method of marginal benefit-to-cost analysis. The chapter concludes

with a comprehensive example that shows how system analysis techniques can

be applied to improve the reliability of an actual distribution system.

Since most distribution companies would like to optimize the reliability of

their distribution system, this book continues with a chapter on system optimiza-

tion. It begins by discussing common misconceptions about optimization and

continues by showing how to properly formulate an optimization problem. It

then presents several optimization methods that are particularly suitable for dis-

tribution system reliability. Finally, the chapter presents several practical

applications of reliability optimization and discusses potential barriers that might


Preface

xvi

be encountered when attempting to implement a reliability optimization initiative

that spans many distribution company departments and budgets.

This book concludes with a chapter on aging infrastructure and the impact of

aging infrastructure on reliability. It begins by discussing equipment and popula-

tion aging, and when age can be used as a reasonable proxy for equipment

condition. The chapter continues by discussing how failure rates increase as a

function of age. This includes techniques to develop age-versus-failure models

using data available at most utilities. The book concludes by presenting the state

of the industry in terms of equipment age; US distribution systems are surpris-

ingly old, are getting older, and will become less reliability as a result. As such,

the topics covered in this book will become increasingly important in the next

decade. Utilities will have to spend increasingly more money just to keep reli-

ability from getting worse. Using the techniques described in this book, utilities

can ensure that this reliability spending is done so that the highest level of reli-

ability can be attained for the lowest possible cost.

The second edition of Electric Power Distribution Reliability is the product

of approximately fifteen years of effort in various aspects of electric power dis-

tribution reliability. I would like to thank the following people for teaching,

collaborating, and supporting me during this time. In the academic world, I

would like to thank Dr. Mani Venkata, Dr. Richard Christie, and Dr. Anil Pahwa

for their insight, guidance and support. In industry, I would like to acknowledge

the contributions and suggestions of my co-workers at with special thanks to Dr.

Damir Novosel, Mr. Lee Willis, and Mr. Jim Burke, all IEEE Fellows. Last, I

would like to offer special thanks to my wife Christelle and to our four children

for providing the inspiration and support without which this book would not be

possible.

Richard E. Brown


xvii

Author Richard E. Brown is the Vice President of Operations and co-founder for Quanta

Technology, a firm specializing in technical and management consulting for

electric utilities. He has previously worked at Jacobs Engineering, ABB, and

KEMA. During his career, Dr. Brown has developed several generations of reli-

ability assessment software programs, has provided consulting services to most

major utilities in the United States and many around the world, and has pub-

lished more than 90 technical papers. In 2007, Dr. Brown was made an IEEE

Fellow for “contributions to distribution system reliability and risk assessment.”

He earned his BSEE, MSEE, and PhD degrees from the University of Washing-

ton in Seattle, and his MBA from the University of North Carolina at Chapel

Hill. He is a registered professional engineer.

Dr. Brown lives in Cary, North Carolina with his wife and four children.


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Electric Power Distribution Reliability