Electric Power and Energy Distribution Systems E-Book

Subrahmanyam S. Venkata

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley - IEEE
• Sprache: Englisch

Seitenzahl: 505

Veröffentlichungsjahr: 2022

Beschreibung

Electric Power and Energy Distribution Systems

Provides a comprehensive introduction to today’s electric power distribution systems, perfect for advanced students and industry professionals

Due to growth of renewable resources and advances in information technology, electric power distribution systems have undergone significant changes over the past fifteen years. The expansion of technologies such as consumer rooftop solar panels, electric vehicles, smart energy storage, and automated metering infrastructure make planning and operating power distribution systems challenging. Integration of advanced technologies at the distribution level is critical for realizing higher efficiency, reliability, resiliency, and flexibility.

Electric Power and Energy Distribution Systems: Models, Methods, and Applications provides comprehensive coverage of the key aspects of conventional and emerging distribution systems, including modeling, methodologies, analysis, planning, economics, distribution automation, reliability, grounding, protection, power quality, and distributed energy resources. Written by experts with decades of experience in academia and industry, this textbook integrates theory and practice to present a well-balanced treatment of topics relevant to modern electric power distribution systems. Detailed chapters address modeling of distribution system components, load characteristics and optimal selection of devices, microgrids and other types of energy resources, the challenges associated with the planning and operation of distribution systems, and more.

• Covers a wide range of both legacy and contemporary issues supported by rigorous analysis and practical insights
• Provides in-depth examination of outage management, voltage control, system restoration, and other operational functions
• Features real-world case studies of distribution automation functions in urban and rural power systems
• Discusses technologies for distributed energy resources (DER) with a focus on wind, solar, and battery storage
• Describes fundamental economics in the context of power distribution systems, such as the impact of tariffs on selling electricity to consumers of different types
• Explains the architecture of distribution system protection, including fuses, reclosers, overcurrent relays, and grounding practices

The ideal textbook for advanced undergraduate and first-year graduate courses, Electric Power and Energy Distribution Systems: Models, Methods, and Applications is also an excellent reference for professionals with limited prior knowledge about distribution systems.

Leseprobe

Table of Contents

Cover

Title Page

Copyright

Dedication

Biography

Preface

Organization of the Book

Acknowledgments

About the Companion Website

1 Introduction

1.1 Prologue

1.2 The Past

1.3 The Present

1.4 The Future

1.5 New Developments

1.6 Epilogue

1.7 The Electric Power System

1.8 Distribution System Devices

1.9 Frequently Asked Questions on Distribution Systems [1]

Reference

2 Distribution System Transformers

2.1 Definition

2.2 Types of Distribution Transformers

2.3 Standards

2.4 Single‐Phase Transformer

2.5 Distribution Transformer Connections

2.6 Three‐Phase Transformer Connections

Problems

References

3 Distribution Line Models

3.1 Overview

3.2 Conductor Types and Sizes

3.3 Generalized Carson's Models

3.4 Series Impedance Models of Overhead Lines

3.5 Series Impedance Models of Underground Lines

Problems

References

4 Distribution System Analysis

4.1 Introduction

4.2 Modeling of Source Impedance

4.3 Load Models

4.4 Distributed Energy Resources (DERs)

4.5 Power Flow Studies

4.6 Voltage Regulation

4.7 Fault Calculations

Problems

References

5 Distribution System Planning

5.1 Introduction

5.2 Traditional vs. Modern Approaches to Planning

5.3 Long‐term Load Forecasting

5.4 Load Characteristics

5.5 Design Criteria and Standards

5.6 Distribution System Design

5.7 Cold Load Pickup (CLPU)

5.8 Asset Management

Problems

References

6 Economics of Distribution Systems

6.1 Introduction

6.2 Basic Concepts

6.3 Selection of Devices: Conductors and Transformers

6.4 Tariffs and Pricing

Problems

References

7 Distribution System Operation and Automation

7.1 Introduction

7.2 Distribution Automation

7.3 Communication Infrastructure

7.4 Distribution Automation Functions

7.5 Cost–Benefit of Distribution Automation

7.6 Cost–Benefit Case Studies

References

8 Analysis of Distribution System Operation Functions

8.1 Introduction

8.2 Outage Management

8.3 Voltage and var Control

8.4 Distribution System Reconfiguration

8.5 Distribution System Restoration

References

9 Distribution System Reliability

9.1 Motivation

9.2 Basic Definitions

9.3 Reliability Indices

9.4 Major Event Day Classification

9.5 Causes of Outages

9.6 Outage Recording

9.7 Predictive Reliability Assessment

9.8 Regulation of Reliability

Problems

References

10 Distribution System Grounding

10.1 Basics of Grounding

10.2 Neutral Grounding

10.3 Substation Safety

10.4 National Electric Safety Code (NESC)

10.5 National Electric Code (NEC)

References

11 Distribution System Protection

11.1 Overview and Philosophy

11.2 Role of Protection Studies

11.3 Protection of Power‐carrying Devices

11.4 Classification of Protective and Switching Devices

11.5 New Generation of Devices

11.6 Basic Rules of Classical Distribution Protection

11.7 Coordination of Protective Devices

11.8 New Digital Sensing and Measuring Devices

11.9 Emerging Protection System Design and Coordination

Problems

References

12 Power Quality for Distribution System

12.1 Definition of Power Quality

12.2 Impacts of Power Quality

12.3 Harmonics and PQ Indices

12.4 Momentary Interruptions

12.5 Voltage Sag and Swell

12.6 Flicker

Problems

References

13 Distributed Energy Resources and Microgrids

13.1 Introduction

13.2 DER Resources and Models

13.3 Interconnection Issues

13.4 Variable Solar Power

13.5 Microgrids

13.6 Off‐Grid Electrification

References

Appendix A: Per‐unit Representation

A.1. Single‐phase Systems

A.2. Three‐phase Systems

A.3. Base Values for Transformers

A.4. Change of Base

A.5. Advantages of Per‐unit Representation

Appendix B: Symmetrical Components

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Common terminologies [1].

Table 1.2 Characteristics of distribution and transmission systems [1].

Chapter 2

Table 2.1 OA and FA ratings of transformers.

Chapter 4

Table 4.1 Suggested values of sensitivities of real power and reactive power...

Table 4.2 Voltage, reactive power, and real power control function requireme...

Table 4.3 Power flow models.

Chapter 5

Table 5.1 Energy usage in the US homes in 2015 [2].

Table 5.2 Data for load duration curve obtained from Figure 5.12.

Table 5.3 Data for Problem 5.1.

Table 5.4 Data for Problem 5.2.

Table 5.5 Energy consumption data of houses for Problem 5.3.

Table 5.6 Data for Problem 5.4.

Table 5.7 Data for Problem 5.5.

Chapter 6

Table 6.1 Percentage of conductors with different loading ranges in an examp...

Table 6.2 Best conductor sets.

Table 6.3 Data for a 100‐kVA pad mount transformer.

Table 6.4 Example of rate schedule for residential customers.

Table 6.5 Conductor data for Problem 6.7.

Table 6.6 Loading ranges and weights for conductors for Problem 6.7.

Table 6.7 Transformer data for Problem 6.8.

Chapter 7

Table 7.1 Mapping the benefits of distribution automation functions.

Table 7.2 Data for the two example systems.

Table 7.3 Summary of all costs and benefits for the urban system.

Table 7.4 Summary of all costs and benefits for the rural system.

Chapter 8

Table 8.1 Calls log of 18 June 2010.

Table 8.2 Escalation of calls and status of device.

Table 8.3 Calls log of 13 August 2010.

Table 8.4 Load duration data.

Table 8.5 System data for the test system.

Table 8.6 Results of 10 runs of the algorithm with variable number of locati...

Table 8.7 Minimum solutions for each objective for the system.

Table 8.8 Diversified and undiversified loads of sections of the example sys...

Table 8.9 Input data for the voltage drop limited distribution system.

Table 8.10 Results for the voltage drop limited distribution system.

Chapter 9

Table 9.1 Log of interruptions in the system of Figure 9.1.

Table 9.2 Log of momentary interruptions in the system of Figure 9.1.

Table 9.3 Probability of exceeding

T

MED

as a function of

k

.

Table 9.4 Causes and number of outages in a service territory in Kansas in 2...

Table 9.5 Data for the distribution system shown in Figure 9.1.

Table 9.6 Computation of CMI.

Table 9.7 Log of outages for Problem 9.1.

Table 9.8 Failure data for Problem 9.5.

Chapter 11

Table 11.1 Constants for different time‐inverse overcurrent relays.

Table 11.2 Recloser K‐factors for coordination with source‐side fuse links....

Table 11.3 Recloser K‐factors for coordination with load‐side fuse links.

Table 11.4 Data for Problem 11.1.

Chapter 12

Table 12.1 Current distortion limits for systems rated 120V through 69 kV fo...

Chapter 13

Table 13.1 Typical criteria used for a transition process.

Appendix A

Table A.1. Base value selection and relations for wye (Y)‐ and delta (Δ)‐con...

List of Illustrations

Chapter 1

Figure 1.1 Typical distribution system layout.

Figure 1.2 A 115‐kV to 12.47‐kV step‐down distribution substation. The right...

Figure 1.3 The 12.47‐kV side of the substation showing the transformer, circ...

Figure 1.4 A 12.47‐kV primary feeder.

Chapter 2

Figure 2.1 Two‐winding diagram (a) and schematic (b) of a single‐phase trans...

Figure 2.2 Model of a practical single‐phase transformer.

Figure 2.3 Simple model of a single‐phase transformer.

Figure 2.4 Phasor diagram of a simplified transformer model.

Figure 2.5 Three‐wire secondary distribution transformer.

Figure 2.6 Three‐wire secondary distribution transformer with 120 V loads.

Figure 2.7 Two three‐wire secondary distribution transformers connected in p...

Figure 2.8 A two‐winding transformer and its connections to create an autotr...

Figure 2.9 A three‐phase wye–delta transformer with three‐phase balanced and...

Figure 2.10 Phasor diagram due to three‐phase load.

Figure 2.11 Y/Y transformer with unbalanced loading.

Figure 2.12 A three‐winding transformer with single‐phase load.

Figure 2.13 Distribution transformer serving single‐phase loads.

Chapter 3

Figure 3.1 556‐kcmil ACSR overhead conductor.

Figure 3.2 #2 triplex cable used for overhead service drops.

Figure 3.3 Cross section of a 15‐kV class insulated copper cable.

Figure 3.4 Overhead lines and their images below earth.

Figure 3.5 Typical three‐phase overhead line configuration.

Figure 3.6 Three‐phase overhead line configuration.

Figure 3.7 Three‐phase underground cable with nonconcentric neutral. Distanc...

Figure 3.8 Schematic of a single‐phase cable with concentric neutral conduct...

Figure 3.9 Schematic of a three‐phase cable with concentric neutral conducto...

Figure 3.10 A single‐phase 7.2‐kV overhead line.

Figure 3.11 A three‐phase 12.47‐kV overhead line.

Figure 3.12 Cross section of a 600‐V all aluminum conductor (AAC) quadraplex...

Figure 3.13 Cross section of a 600‐V all aluminum conductor (AAC) triplex ca...

Chapter 4

Figure 4.1 Minimum reactive power capability of Category A and B DER.

Figure 4.2 Volt‐var characteristic for DER control.

Figure 4.3 Watt‐var characteristic for DER control.

Figure 4.4 Volt–watt characteristic for DER control.

Figure 4.5 Schematic of a three‐phase line connected between buses

i

and

j

....

Figure 4.6 Representation of a three‐phase Y‐connected load on bus

i

.

Figure 4.7 Representation of a three‐phase Δ‐connected load on bus

i

.

Figure 4.8 A two‐phase load connected between phases

b

and

c

.

Figure 4.9 Schematic showing line sections and connected loads.

Figure 4.10 Feeders splitting at bus

k

.

Figure 4.11 A feeder of resistance

R

and reactance

X

.

Figure 4.12 Phasor diagram for Eq. (4.58).

Figure 4.13 A single‐phase feeder supplying three point loads.

Figure 4.14 Feeder with load distributed uniformly in a rectangular service ...

Figure 4.15 Current on the feeder.

Figure 4.16 Feeder serving a triangular area with fixed load density.

Figure 4.17 An example distribution system.

Figure 4.18 Three‐phase fault at bus

i

.

Figure 4.19 DLG fault between phases

b

and

c

at bus

i

.

Figure 4.20 SLG fault on phase

a

at bus

i

.

Figure 4.21 LL fault between phases

b

and

c

at bus

i

.

Figure 4.22 Prefault positive‐, negative‐, and zero‐sequence equivalent circ...

Figure 4.23 Three‐phase fault in sequence domain.

Figure 4.24 DLG fault in sequence domain.

Figure 4.25 SLG fault in sequence domain.

Figure 4.26 LL fault in sequence domain.

Figure 4.27 The 12.47‐kV system for Problem 4.3.

Figure 4.28 Service area for Problem 4.5.

Chapter 5

Figure 5.1 S‐curve showing load growth in a small area.

Figure 5.2 Load growth of a service area over the years.

Figure 5.3 Typical aggregate load demands of residential, commercial, and in...

Figure 5.4 Demand of a house on 15‐minute basis.

Figure 5.5 Demand of a house on 30‐minute basis.

Figure 5.6 Demand of a house on one‐hour basis.

Figure 5.7 Fifteen‐minute average load of two houses.

Figure 5.8 Fifteen‐minute average load of five houses.

Figure 5.9 Fifteen‐minute average load of 10 houses.

Figure 5.10 Fifteen‐minute average load of 20 houses.

Figure 5.11 Coincidence factor as a function of number of houses.

Figure 5.12 Hourly load at a substation. The load shown at a given time is t...

Figure 5.13 Load duration curve corresponding to the load characteristics of...

Figure 5.14 Discrete load duration curve.

Figure 5.15 Discrete loss duration curve.

Figure 5.16 Typical layout of a substation for a semiurban area (NC, normall...

Figure 5.17 Single‐feeder layout of distribution feeders.

Figure 5.18 Multifeeder layouts of distribution feeders.

Figure 5.19 Service territory division for four primary feeders.

Figure 5.20 Service territory division for six primary feeders.

Figure 5.21 Feeders along with the roads in a typical distribution system in...

Figure 5.22 Illustration of a secondary system for service to eight customer...

Figure 5.23 Load upon restoration following a long outage during winter reco...

Figure 5.24 Delayed exponential model for cold load pickup.

S

U

is the undive...

Chapter 6

Figure 6.1 Economic characteristics of a set of four conductors.

Figure 6.2 Reach at different peak loads for selected conductors.

Figure 6.3 Economic cost characteristics of a set of four selected conductor...

Chapter 7

Figure 7.1 An architecture for transactive energy in distribution systems.

Chapter 8

Figure 8.1 An automated distribution feeder with outage location, fault isol...

Figure 8.2 One‐line diagram of part of a typical distribution system showing...

Figure 8.3 Circuit diagram and call scenario for 13 August 2010.

Figure 8.4 Thirty‐bus three‐phase radial test distribution system.

Figure 8.5 Plot showing variation in the best cost with number of locations ...

Figure 8.6 One‐line diagram of a distribution system with 12 sections.

Figure 8.7 Load of section

i 

upon restoration following an extended ou...

Figure 8.8 An example showing load on the substation transformer as a functi...

Chapter 9

Figure 9.1 An example distribution feeder.

Figure 9.2 Monthly outages in a service territory in Kansas.

Figure 9.3 Two components connected in series.

Figure 9.4 Two components connected in parallel.

Figure 9.5 A network of six components.

Figure 9.6 Steps for network reduction.

Figure 9.7 State transition diagram for the system of Figure 9.1 for outages...

Figure 9.8 Distribution system for Problems 9.1 and 9.5.

Figure 9.9 A network of components for Problem 9.2.

Figure 9.10 System for Problem 9.3.

Chapter 10

Figure 10.1 Four‐wire multigrounded distribution system.

Figure 10.2 Three‐wire unigrounded distribution system.

Figure 10.3 Current through human body due to step potential.

Figure 10.4 Current through human body due to touch potential.

Figure 10.5 Safety ground for homes and buildings.

Figure 10.6 Standard 120‐V household receptacle used in the United States.

Figure 10.7 Hazard due to reversal of ground and neutral wires at load.

Chapter 11

Figure 11.1 Fuse‐link construction.

Figure 11.2 Comparison of various fuse links.

Figure 11.3 Example of a distribution system fuse cutout.

Figure 11.4 Examples of modern reclosers: (a) Nova NXT and (b) IntelliRupter

Figure 11.5 Example of recloser characteristics with one fast (A) and two sl...

Figure 11.6 Typical recloser operating sequence to lockout.

Figure 11.7 Control box for a recloser.

Figure 11.8 Operational sequence of a sectionalizer.

Figure 11.9 Examples of 15‐kV and 38‐kV circuit breaker.

Figure 11.10 A vintage electromechanical overcurrent relay.

Figure 11.11 Time–current characteristics of CO‐8 time‐inverse overcurrent r...

Figure 11.12 Time–current characteristics of different time‐inverse overcurr...

Figure 11.13 A modern digital relay for protection of distribution system fe...

Figure 11.14 A typical distribution system depicting various components incl...

Figure 11.15 Operational convention for protective devices.

Figure 11.16 Time–current model of 10K fuse link.

Figure 11.17 Fuse–fuse coordination of a simple radial system.

Figure 11.18 Part of a distribution system protected by fuse links.

Figure 11.19 MMC and TCC curves of 30T and 50T fuse links and their coordina...

Figure 11.20 Time–current characteristic curve for a recloser and a downstre...

Figure 11.21 Recloser–fuse locations in a simple distribution feeder system....

Figure 11.22 Example distribution system.

Figure 11.23 Operating time of L‐type recloser and E‐type slow upstream fuse...

Figure 11.24 Adjusted fast curve (Curve A) of recloser and MMC of selected f...

Figure 11.25 Slow curve (Curve C) of recloser and TCC of selected fuse links...

Figure 11.26 Basic sectionalizer–recloser coordination.

Figure 11.27 Main feeder of a distribution system.

Figure 11.28 Recloser and CO‐8 relay‐operating curves.

Figure 11.29 Phasor measurement unit.

Figure 11.30 A microphasor measurement unit.

Figure 11.31 Overcurrent relay with current transformer (CT) for Problem 11....

Figure 11.32 Part of a distribution system for Problem 11.2.

Figure 11.33 Distribution system for Problem 11.3.

Figure 11.34 Distribution system for Problem 11.4.

Chapter 12

Figure 12.1 Voltage and current waveforms recorded at a veneer plant.

Figure 13.2 Example of voltage sag.

Figure 12.3 Voltage sag and swell limits for information technology equipmen...

Figure 12.4 Example of periodic voltage change that causes flicker.

Chapter 13

Figure 13.1 Mechanical power output of wind turbines as a function of wind s...

Figure 13.2 Solar PV cell characteristics.

Figure 13.3 Examples of solar irradiation recorded with 30‐second resolution...

Figure 13.4 Real power variation at a bus due to changing solar irradiation ...

Figure 13.5 Voltage at a bus with unity power factor operation and dynamic c...

Figure 13.6 An 18‐bus radial/looped example system with four microgrids (MGs...

Figure 13.7 CERTS microgrid test bed.

Appendix A

Figure A.1 An example system.

Appendix B

Figure B.1 Sequence components and the resultant phase voltages.

Guide

Cover Page

Title Page

Copyright

Dedication

Biography

Preface

Organization of the Book

Acknowledgments

About the Companion Website

Table of Contents

Begin Reading

Appendix A: Per‐unit Representation

Appendix B: Symmetrical Components

Index

Wiley End User License Agreement

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IEEE Press445 Hoes LanePiscataway, NJ 08854

IEEE Press Editorial Board

Sarah Spurgeon,

Editor in Chief

Jón Atli Benediktsson

Andreas Molisch

Diomidis Spinellis

Anjan Bose

Saeid Nahavandi

Ahmet Murat Tekalp

Adam DrobotPeter (Yong) Lian

Jeffrey ReedThomas Robertazzi

Electric Power and Energy Distribution Systems

Models, Methods, and Applications

Subrahmanyam S. VenkataVenkata Consulting Solutions, LLCTuscon, Arizona, US

Anil PahwaKansas State UniversityManhattan, Kansas, US

Copyright © 2023 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.

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Dedicated to our spousesPadma and Mukta

Biography

Subrahmanyam S. Venkata is Affiliate Professor of Electrical and Computer Engineering with the University of Washington (UW), Seattle, Washington, where he has taught since 1979. He was Dean and Distinguished Professor of Wallace H. Coulter School of Engineering at Clarkson University, Potsdam, New York, during 2004–2005. In 2003, he was Palmer Chair Professor of Electrical and Computer Engineering Department at Iowa State University, Ames, Iowa. From 1996 to 2002, he was Professor and Chairman of the department at ISU. Before joining ISU, he taught at the University of Washington, Seattle, West Virginia University, and the University of Massachusetts, Lowell, for 25 years. He received his B.S.E.E and M.S.E.E. degrees from India, and his Ph.D. degree from the University of South Carolina, Columbia, in 1971. In 2016, he received the Robert M. Janowiak Outstanding Leadership and Service Award from ECEDHA. In 2015, he received the IEEE PES Douglas M. Staszesky Distribution Automation Award. He received the IEEE Millennium Award in 2000. In 1996, he received the Outstanding Power Engineering Educator Award from the IEEE Power Engineering Society.

He is President of Venkata Consulting Solutions LLC. He was with GE Power/Alstom Grid Inc. from January 2011 to September 2017. Dr. Venkata is a Life Fellow of the IEEE. At the IEEE level, he served as a member of the IEEE Fellows Committee for six years. He also served on the PES Board as Vice‐President, Publications PES, during 2004‐07.

Anil Pahwa is University Distinguished Professor at Kansas State University and holds the Logan‐Fetterhoof Chair in Electrical and Computer Engineering. He received the B.E. (honors) degree in electrical engineering from Birla Institute of Technology and Science, Pilani, India, in 1975, the M.S. degree in electrical engineering from University of Maine, Orono, in 1979, and the Ph.D. degree in electrical engineering from Texas A&M University, College Station, in 1983. He has been a faculty member in Electrical and Computer Engineering Department at Kansas State University since 1983. The National Academies selected him for the Jefferson Science Fellowship in 2014. He served as a Senior Scientific Advisor in Economic Policy Office of East Asian and Pacific Affairs Bureau of the U.S. Department of State as a part of the fellowship. He served as Program Director for power and energy at the National Science Foundation from 2018 to 2020. He has served in several officer positions in IEEE Power and Energy Society (PES) including chair of Power and Energy Education Committee from 2012 to 2013, and an editor of IEEE Transactions on Power Systems from 2010 to 2015. He has received several awards over his professional career including the IEEE PES Douglas M. Staszesky Distribution Automation Award in 2012, IEEE PES Prize Paper Award in 2013; Distinguished Researcher Award in 2020, Snell Excellence in Undergraduate Teaching Award in 2017, Frankenhoff Outstanding Research Award in 2012, and Erickson Public Service Award in 2011 from Kansas State University College of Engineering; and Outstanding Alumni Award from Birla Institute of Technology and Science in 2014. He is a Life Fellow of IEEE. He is presently serving as a member of the IEEE Fellows Committee. His research focuses on reliability, automation, and optimization of power distribution systems. His research has provided innovative and practical solutions for application of advanced communication and cyber technologies for automation of electricity distribution to customers and large‐scale integration of renewable energy resources in the system.

Preface

Electric power systems have seen enormous changes over the past 15 years due to the growth of renewable resources and the integration of information technology. Power and energy distribution systems have had the most changes. They are at the lowest end with less than 69 kV and close to customers. In the past, they were completely passive with very little information available beyond the substation. Now they are becoming active with consumer‐owned rooftop solar generation, electric vehicles, energy storage, and advanced metering infrastructure. It is imperative to integrate cyber technologies to automate operation of the emerging distribution systems for higher efficiency, reliability, resiliency, and flexibility. We can expect higher deployment of distributed energy resources (DERs), more electric vehicle ownership, and consumers becoming more engaged in the future. As a result, the planning and operation of power distribution systems becomes more challenging.

Power distribution systems did not receive much attention until about 15 years ago. With the introduction of the Smart Grid concept and related investment by industry in distribution systems for integration of advanced technologies, interest in distribution systems grew. Increased deployment of distributed resources and microgrids further fueled interest in the subject. Many universities now offer a course in distribution systems. We expect that this book will encourage more faculty to offer courses, as the lack of a suitable book is often a deterrent to offering a new course. We also expect that parts of this book may be used as a part of other courses in power systems. As we have expertise and combined experience of about 90 man‐years in distribution systems, several colleagues asked for our help and urged us to write a textbook. This book will be a valuable resource for professors teaching a course on distribution systems and for students to prepare for their future careers. In addition, it will be a comprehensive reference for professionals who want to expand their knowledge about distribution systems.

The book provides a comprehensive treatment of various aspects associated with distribution systems including modeling, methodologies, analysis, planning, economics, distribution automation, reliability, grounding, protection, power quality, DERs, and microgrids. Modeling and analysis provide rigorous modeling of distribution system components including distribution transformers, feeders, and load followed by system analysis. Planning and economics include topics such as load characteristics, feeder layouts, and optimal selection of devices. Distribution automation focuses on operation functions and their analysis. Case studies of distribution automation are provided. Reliability, grounding, protection, and power quality topics deal with legacy issues as well as address issues relevant to emerging distribution systems. The chapter on DERs and microgrids discusses various types of distributed resources and challenges associated with operation and planning of distribution systems with increased proliferation of these resources, and design and operation of microgrids.

The book is designed to provide comprehensive treatment of various topics relevant to classical and emerging electric power and energy distribution systems. The topics address legacy as well as contemporary issues supported by rigorous analysis and practical insight. The book provides a good blend of theory and practice. It is ideally suited to teach both senior and first‐year graduate courses at any university. We are assuming that the students have taken a basic course in power systems. The material can be covered in two semesters, but the instructors can select topics for a one‐semester course based on the emphasis of the course. Some topics are more relevant for a graduate‐level course.

Organization of the Book

The book consists of 13 chapters.

Chapter 1 introduces power distribution systems with a historical perspective and looks into the future, guided by the new developments. Differences in terminologies as relevant to distribution systems in the United States and the rest of the world are included. Descriptions of various devices in distribution substations and feeders are provided.

Chapter 2 presents different types of transformers used in distribution systems and associated standards. Models for steady‐state analysis and performance measures, such as efficiency and regulation, are discussed. Various schemes and associated analyses for transformer connections both for single‐ and three‐phase transformers for distribution of electricity to customers are included.

Chapter 3 presents a rigorous analysis for modeling overhead lines and underground cables. Effects of unbalances including mutual coupling between phases and effects of ground are included in models for series impedance.

Chapter 4 includes models for sources and loads. DERs are introduced, and their integration with the system as per IEEE Standard 1547 is discussed. This is followed by a source-load iteration method for power flow analysis of unbalanced three‐phase power distribution systems. Voltage regulation is explained with simple examples. The chapter concludes with methods for computing different unbalanced faults both in the phase domain and the sequence domain.

Chapter 5 focuses on distribution system planning while comparing traditional vs. modern approaches for planning to achieve optimal designs. Effects of load forecasting, load demand characteristics, and standards on system design are discussed. Load characteristics include coincidence and effects of temporal and spatial aggregation of loads. Topological designs for the substation, primary feeders, and secondary feeders are presented. An introduction to cold load pickup and its effects on planning are discussed.

Chapter 6 introduces fundamental economics concepts, such as present worth, and annuity, relevant to power distribution systems. Applications of these concepts for optimal selection of conductors for feeders and transformers are presented. Tariffs for selling electricity to consumers of different types are discussed.

Chapter 7 focuses on distribution automation for the operation of distribution systems. Basic ideas on communication infrastructure needed for automation are provided. Details of various common operation functions are discussed along with benefits of their automation. Cost–benefit evaluation of distribution automation with examples of cost and benefits is presented. Case studies of automating various functions in urban and rural systems are provided.

Chapter 8 provides an in‐depth analysis of various operation functions presented in the previous chapter. Details on novel approaches with mathematical formulations for outage management, voltage and var control, system reconfiguration, and systems restoration including cold load pickup are presented. This chapter relies heavily on integrating the results of the authors' research on these topics.

Chapter 9 introduces the concept of distribution system reliability supported by various indices that are used by industry. Mathematical approaches for component modeling are discussed. Analytical methods as well as Monte Carlo methods for reliability evaluation of distribution systems are presented. This is followed by discussion on regulations for reliability.

Chapter 10 provides the definition and need for system grounding for distribution systems. The effects of soil resistivity and frequency on neutral grounding are discussed. Applicability of National Electric Safety Code for primary system grounding and the National Electric Code for secondary system grounding are presented.

Chapter 11 focuses on philosophy and architecture of protection with relevance to distribution systems. Selection of fuses, reclosers, and overcurrent relays and their coordination for overcurrent protection are discussed. Equations for relay characteristics are included. Students can use these and also standard characteristics of other devices provided by the manufacturers for computer‐aided design of distribution system protection.

Chapter 12 introduces various indices used in power quality evaluation of distribution systems. This is followed by approaches for harmonic analysis. Effects of motor starting and its effects on flicker in the system are discussed. Voltage sag and swell and behavior of sensitive loads, such as computers, which are vulnerable to transients in the system, are presented.

Chapter 13 defines DERs with a focus on wind, solar, and battery storage. This is followed by interconnection issues with a focus on the role of all applicable standards including IEEE 1547 and other standards addressing the interconnection issue. Architecture, control, and protection of microgrids along with the methods for their performance evaluation are presented.

While there are various other topics that are relevant and several issues still evolving, we feel that the book provides a comprehensive coverage of the most relevant topics. We will address additional topics as necessary in the next edition of the book.

Acknowledgments

First and foremost, we thank our parents for nurturing us and providing us the right opportunities for education and inspiration for professional success. We thank our respective spouses, Padma Venkata and Mukta Pahwa, for their unrelenting support throughout our professional careers. Without their support and encouragement, writing this book would not have been possible. We thank our respective children Sri and Harish (Venkata) and Samir and Mrinal (Pahwa) and their families for their love and respect and guidance during preparation of the book. We thank Padma Venkata for her proof reading and editing. Several of our professional colleagues reviewed our book proposal and provided valuable comments, which allowed us to improve the quality of the book. Thanks to H. Lee Willis, whose books on power distribution planning inspired the authors to write some of the chapters of this book. Thanks to Dr. Chanan Singh (Texas A&M University) for his help in preparation of the book proposal. Special thanks to Dr. Sukumar Brahma (Clemson University), who suggested several enhancements to the book and served as a sounding board during the writing of the book. Thanks to Dr. Ned Mohan (University of Minnesota) for his general advice on book writing. Thanks to our graduate students from whom we learned a lot. Although we have not listed individual names, the work of several graduate students has contributed to portions of the book. We want to thank Eaton Corporation, S&C Electric, ABB, Schweitzer Engineering Laboratories, Power Standard Laboratories, and the Information Technology Industry Council (ITI) for providing diagrams and product images for inclusion in the book. Finally, we thank the staff of Wiley and IEEE Press for their help throughout the course of writing this book.

Subrahmanyam S. (Mani) VenkataAnil Pahwa

About the Companion Website

The book is accompanied by a companion website which has data sheets, supplementary files, additional problems, and problem solutions. Problem solutions are accessible only by instructors, but the rest of the material is accessible to both instructors and students.

www.wiley.com/go/Pahwa/ElectricPowerDistributionSystems

1Introduction

1.1 Prologue

The development of distribution systems poses new challenges in the changing world, where levels of electrification need to be increased and electricity served reliably for sustainable, economic, and social development. Technological development and adequate regulations are required at the distribution level to respond to new energy challenges and the restructured environment. The need for a change in the way distribution systems are designed, planned, operated, and managed is a must for both developed and developing countries. All changes should ultimately ensure optimal and economic service to the consumers of electricity. While the basic parameters remain the same, the challenges to be met are substantially different. The past, present, and the future of the distribution systems are reviewed.

1.2 The Past

Towards the end of the nineteenth century, direct current (dc) distribution systems came into existence. Recognizing the value of electric energy and the need for development of economic sources of electricity, our forefathers wisely replaced dc with alternating current (ac) during the early part of the twentieth century. Subsequently, ac systems grew enormously, making the development of the electric power system the greatest achievement of the century. Unfortunately, several persistent problems with distribution systems have lingered on for many decades. They did not receive the attention they deserved under the regulated environment, when compared to generation and transmission. Very little attention was paid to the planning, design, operation, and management of these nonbulk systems. Performance optimization for efficiency, regulation, and other measures were not adequately addressed.

1.3 The Present

The way the business was regulated contributed to the current situation in many distribution systems worldwide. While analyzing just efficiency we find that many electric companies in several countries, both public and private, are still experiencing extremely high system losses, in the range of 30–50%. In addition, voltage regulation at the customers' premises sometimes is very poor, placing undue stress on the loads at this end. In many developing countries this has an added complex socioeconomic dimension: the need to have access to affordable electricity supply as a basic human need. The cumulative benign neglect of the past is now coming home to roost in the form of aging distribution infrastructure, that is still operational though it has far exceeded its intended life span. Its ability to survive natural disasters is more because of chance than design. For example, there are underground cables and overhead poles installed in the 1930s that are still in service. Aging conductors of inadequate capacity (from the current demand point of view) are still supplying power, but with poor performance. It is amazing that these major components have survived this long. The conservativeness of the design and operation may have prolonged their life expectancy. The question that naturally arises is: How long will they survive and at what cost to the utilities, or for that matter, to the world citizenry?

1.4 The Future

The outlook for the future is not all doom and gloom. Many positive changes have been rapidly occurring during the past decade, perhaps due to the deregulation (or reregulation) of the industry. We have become increasingly dependent on electricity being a necessity for our existence. It is also the backbone for future economic development if we are committed to improving the quality of life for all mankind. We have witnessed electric power systems becoming larger and more complex in the past 60 years due to the unprecedented growth in the demand for electricity coupled with the population growth and with the higher standards demanded by society. Distribution systems are no exception. Globally, these lower voltage power systems are facing intense competition, with tremendous challenges to cover the ground of past neglect and to deliver cost‐effective electric supply, while meeting ever‐increasing customer expectations. Globalization is yet another factor to keep in mind when designing, planning, and operating distribution systems of the future.

1.5 New Developments

On the technology front, the penetration of new technologies and materials for efficient distribution systems, including distributed generation, and the availability of efficient computation and analysis tools provide the encouragement and the impetus to make the distribution systems of the future more efficient and effective. As a result, monitoring, control, protection, and automation of these systems in real time are becoming a reality. Demand management at the consumer level to match the availability of supplies to lower costs is a distinct possibility in such an environment. The distribution community should work now to make these lower voltage systems safer, more secure, and more reliable while meeting the ever‐increasing demand with the highest possible performance. The asset management intended to prolong the life of the existing equipment while integrating the new technologies, is receiving increased attention. The overall risk management of resources, including finances, will assist the utilities to utilize them wisely and effectively. Many optimistic trends are emerging as we started our journey into the twenty‐first century.

1.6 Epilogue

Distribution systems require regular upgrading and modernization to continue providing quality service to consumers. In countries where the demand for electricity has reached a plateau, systems suffer from aging infrastructure and reliability issues. In countries where the power demand is high, the extension of an upgraded electricity infrastructure in urban areas is becoming a necessity and this will require investment. Reliability of supply in many cases is compounded by the shortage in power supply and by inadequate power delivery systems. The need to stimulate efficiency in investment and operation is a must. The tools to evolve the solutions for the problems and the technology for implementation are available. The cost of implementation will, however, be substantial. Therefore, solutions must be structured for phased implementation to ensure acceptability. Challenging, but interesting, times are faced by engineers contributing to these developments. For further insight into any of the topics, readers are encouraged to go through the recent literature and the reference cited at the end of the chapter. While the basic principles for distribution of electricity are the same throughout the world, there are some differences between North America and the rest of the world with respect to topology and terminology. Some aspects of these differences are highlighted in Table 1.1.

Table 1.1 Common terminologies [1].

North America

Rest of the world

Distribution systems

Distribution networks

Primary distribution

Medium‐voltage, high tension

Secondary distribution

Low‐voltage, low tension

Consumption: kWh

Units

Topology: radial tree structure

Radial with primary and/or secondary selective

Primary feeder protection: reclosers, fuses

Circuit breakers

1.7 The Electric Power System

Electric power systems have three main building blocks: generation, transmission, and distribution. In terms of capital expended, generation systems have approximately 40%, transmission systems have 20%, and distribution systems have 40% of the total. It is the last block and subsystem, which is closest to the consumers, that is the main focus of this textbook. Table 1.2 shows comparison of transmission and distribution systems based on various characteristics. Figure 1.1 shows the layout of a typical distribution system. The starting point for a distribution system is a distribution substation that steps down the power flowing through it from a transmission (or a subtransmission) level, say 115 kV, to a primary distribution level between 4.16 and 34.5 kV level. The power‐handling capability of a distribution substation usually varies from 5 to 25 MVA. A substation may feed two to eight three‐phase primary feeders. Several three‐ or single‐phase laterals branch off from the primary feeders. Depending on the nature of the load, the power is further stepped down to a secondary distribution level of 600 V and less via distribution transformers. The secondary lines feed residential loads, whereas the commercial and industrial loads are predominantly three phase in structure and may be fed directly from the primary level, depending on the connected load and power. Most of the distribution systems around the world have overhead feeders due to cost considerations, but underground feeders have become popular due to aesthetics and higher reliability.

Table 1.2 Characteristics of distribution and transmission systems [1].

Characteristics

Distribution

Transmission

Topology

Radial

Network or loop

Power

100 MVA or below

Bulk (100–1000 MVA)

Voltage

<69 kV class

>120 kV class

No. of phases

Both 1 and 3

Only 3

Load

Distributed

Concentrated

Unbalance

20–30%

5%

No. of components

10 times more

10 times less

Capital outlay

40%

20%

Figure 1.1 Typical distribution system layout.

1.8 Distribution System Devices

Distribution systems have a substantial number of devices, all the way from the substation to the service entrance at customer locations. The devices include transformers, switchgear, compensating devices, protection equipment, and control and monitoring devices. In this chapter, we explore these devices.

1.8.1 Substation Devices

Distribution substations are the link between the transmission system and the distribution system. Figures 1.2 and 1.3 show a distribution substation. The right‐hand side of Figure 1.2 shows the 115‐kV side including the incoming feeder, switching equipment, and busbars. The left‐hand side of this figure shows the 12.47‐kV side. A power transformer to step down the voltage is in the middle of the figure. Figure 1.3 shows an enlarged view of the 12.47‐kV side. Substations are typically located on the periphery of cities, but they can also be inside the cities. Sometimes the distribution substation can be part of a large transmission substation. Distribution substations can have air‐ or gas‐insulated equipment. Modern substations are typically gas insulated because gas reduces the size of equipment and provides additional advantages. Sulfur hexafluoride (SF6) has been used for substation equipment for many years. However, there are some environmental concerns associated with SF6, which is prompting scientists to look for alternatives.

1.8.1.1 Power Transformers

Power transformers are large transformers that receive power from the transmission system and reduce the voltage for distribution of power to consumers. These transformers have multiple power ratings, such as 15/20/25 MVA OA/FA/FOA. The rating implies that the transformer will handle up to 15 MVA with cooling provided by convective flow of oil through fins (OA), up to 20 MVA with additional cooling provided by fans circulating air through fins (FA), and up to 25 MVA with cooling aided by forced air as well as forced oil circulation (FOA). These transformers typically have a load tap changer (LTC) on the secondary side to change the low‐voltage side voltage up or down, depending on the load on the system.

Figure 1.2 A 115‐kV to 12.47‐kV step‐down distribution substation. The right side of the picture shows the 115‐kV equipment, and the left side of the picture shows the 12.47‐kV equipment.

Figure 1.3 The 12.47‐kV side of the substation showing the transformer, circuit breakers, and the rest of the equipment.

1.8.1.2 Switchgear

Switchgear includes all the devices that are used for opening or closing an electrical path. The most important of them is the circuit breaker, which is designed to open under fault conditions. A circuit breaker can have air, oil, vacuum, or gas as the media. Modern circuit breakers at high voltages are SF6 based, but most of the circuit breakers in the 15‐kV class use vacuum‐based interruption mechanism. Load break switches are designed to interrupt a circuit, but their capacity is limited to the maximum expected load on the circuit. They cannot interrupt a circuit under fault conditions. Disconnect switches are purely manual devices that are used to isolate a circuit or component that has already been deenergized.

1.8.1.3 Compensating Devices

Compensating devices are used to adjust the voltage or reactive power flow. The Regulator is like an autotransformer, which can change the output voltage by moving the tap up or down. Capacitors provide reactive power. They can be fixed or switched. Switch capacitors are switched on to provide reactive power in response to voltage, reactive power flow, or temperature, or they can have a fixed time‐based switching schedule. Similar to capacitors, reactors can be installed in substations to absorb reactive power. Usually, reactors are not used in distribution substations, but they are sometimes used in transmission substations to compensate for the reactive power of high‐voltage transmission lines under light load conditions.

1.8.1.4 Protection Equipment

These are the equipment that respond to abnormal conditions, such as high current due to faults and high voltage caused by circuit switching or lightning. Inverse time overcurrent and instantaneous relays are the most common types of relays deployed in distribution systems. They monitor the current flowing on the distribution feeders emanating from the substation and send trip signals to associated circuit breakers to trip if the current exceeds the threshold values. High‐voltage fuses are sometimes used on the high‐voltage side of power transformers to protect them from faults. Other protective equipment include surge arresters for limiting voltage on equipment by discharging or bypassing surge current created by switching or lightning. In addition, the substations have static wires, which are at the top of the poles bringing an overhead transmission line into the substation or an overhead distribution line going out of the substation. These wires protect the substation equipment during lightning storms by diverting the lightning surges to the ground.

1.8.1.5 Control and Monitoring Devices

These devices include current transformers (CTs) that reduce the current flowing on the lines to lower values for meters and relays. Similarly, voltage transformers (VTs) or potential transformers (PTs) reduce the high voltage to a lower value for metering and protection. Traditional voltage transformers use the inductive principle to reduce the voltage, but capacitive principle can also be used for reductive voltage. The devices that use this principle are called capacitive voltage transformers (CVTs). Substations also have various transducers to measure different quantities such as ambient temperature, oil temperatures of transformers, and dissolved gases in transformer oil. Since modern substations substantially integrate cyber technology, they have computers and communication links. Microwave, fiber optics, and radio are some of the options for communications. In addition, the substations have remote terminal units (RTUs), which collect information and convey this information to the supervisory control and data acquisition (SCADA) system in the control room through the communication link.

1.8.2 Primary System Components

The primary system consists of feeders that emanate from the substation and go all the way to the distribution transformers. Figure 1.4 shows an example of an overhead primary distribution feeder along a city street. These feeders can have different configurations and conductor types. They also have different associated devices. In this section, we present a brief overview of these devices.

Figure 1.4 A 12.47‐kV primary feeder.

1.8.2.1 Feeders and Laterals

Feeders and laterals can be overhead or underground. Overhead feeders have bare conductors mounted on poles. These conductors can be copper, aluminum, or aluminum conductor steel reinforced (ACSR). ACSR conductors are the most used bare wires for overhead distribution feeders. Underground feeders use insulated cables with copper or aluminum conductors and are insulated with ethylene propylene rubber (EPR) or cross‐linked polyethylene (XLPE) polymeric insulation. A third option for feeders are the tree wires, which are copper or aluminum conductors coated with insulation for overhead feeders. They are used in areas with dense vegetation with high likelihood of contacts with trees.

1.8.2.2 Switches

Primary feeders, which are mainly three phase, have different types of switches installed on them to provide operating flexibility. Manual switches are installed to isolate a part of the feeder for maintenance. They can only operate on a feeder that is deenergized. Sectionalizing switches or sectionalizers are devices that isolate parts of a feeder under fault conditions. They do not have fault‐clearing capabilities but operate in conjunction with reclosers, which have fault‐clearing capabilities. Sectionalizers have a counting mechanism to count the number of times the recloser has operated, and after a predetermined number of operations they open when the recloser is open to disconnect the downstream part of the feeder. In automated systems, advanced sectionalizing switches with communication capabilities are being deployed. Communication capabilities allow the automated switches to precisely locate the fault to isolate the faulted part from the rest of the system. Three‐phase main feeders also have a switch at the end of the feeder, which is called the tie switch, which is normally open. This switch can be manual or automated. In legacy systems, these switches were typically manual; but in modern systems, they are automated to provide higher flexibility for system reconfiguration.

1.8.2.3 Compensating Devices

Similar to substations, feeders also have compensating devices. The most common are capacitors and voltage regulators. Capacitors are installed at strategic locations to inject reactive power to maintain proper voltage on the feeders under changing load conditions. Capacitors can be switched based on local control using time, temperature, voltage, reactive power, or a combination of them, or they can be switched in coordination with other capacitors and devices in the system. Regulators are like autotransformers and are used on exceedingly long feeders to boost voltage under heavy load conditions.

1.8.2.4 Protection Equipment

Fuses are the most common protection equipment used on distribution feeders. Every lateral branching off the main feeders has a fuse to protect it. Also, the main feeder can have fuses in certain situations. They are also used to protect distribution transformers. Fuses are very inexpensive and have provided reliable protection for over a century. A disadvantage with them is that they must be manually replaced. One can argue that they should be replaced by automated protective devices. However, the cost advantage they offer outweighs any benefits an automated protective device would provide. Hence, they will continue to be used for protection of downstream portions of distribution systems. Reclosers are like circuit breakers and have fault‐clearing capabilities. Unlike circuit breakers, which depend on separate relays to initiate operation during faults, reclosers have their own fault‐detection mechanism. However, reclosers are smaller in size and are mounted on top of poles in overhead feeders. They also have a reclosing feature, which allows them to reclose a selected number of times before locking out. Since many faults in distribution systems are temporary, this feature permits fuse saving for such faults. The feeders also have surge arresters and static lines, which have the same functions as those for substations.

1.8.2.5 Control and Monitoring Devices

Legacy distribution systems had very little control and monitoring devices on the feeders. But emerging distribution systems with automation have larger proliferation of such equipment. Monitoring devices include current transformers (CTs), voltage transformers (VTs), transducers, and RTUs. Automated distribution systems also have an overlay of communication network for communication between different devices and the control center to make optimal operating decisions in real time.

1.8.2.6 Distribution Transformers

Distribution transformers are at the end of the primary distribution system. They reduce the voltage to utilization level for distribution to the customers. They are pole mounted for overhead systems and pad mounted for underground systems. They are single phase for residential customers but can be three phase for commercial and industrial customers, depending on the size of the load. A single‐phase transformer typically feeds one to eight residential customers through the secondary part of the system.

1.8.2.7 Types of Primary Systems

Primary systems can be public or private. Typically, large consumers have their own primary distribution system, which is connected to the local utility. For example, the process industry, such as Boeing at Everett, WA, has a load higher than 10 MVA, and University of Washington in Seattle, has load higher than 40 MVA. While the focus on this book is not on terrestrial power systems, ships, aircrafts, and space use power systems that are different. Ships use three‐phase distribution at 60 Hz, but aircraft and space systems use 400 Hz.

1.8.3 Secondary System Components

Secondary systems connect the distribution transformers to service entrance in homes and businesses. For an overhead system, triplex cable provides this connection, and for an underground system, aluminum cable is used for this connection. Service entrance has a meter to record energy consumption. Smart meters, which are prevalent now, allow remote metering capabilities with the ability to meter energy consumption over a 15‐minute period. They can also have capability to report loss of power to the utilities. This feature is useful for locating outages when enough meters report loss of power. In addition, the customers are installing their own devices for generation and storage of energy. For residential customers, rooftop solar photovoltaic (PV) and battery are a viable option. Industrial and commercial customers can typically have co‐generation (1–25 MW): solar sources (100 kW to 25 MW), wind parks (100 kW to 25 MW), batteries (1–25 MW), and fuel cells (1–25 MW).

1.9 Frequently Asked Questions on Distribution Systems [1]

While we explore many issues related to distribution systems in this book, it is worth considering the following questions:

(1) Should secondary distribution systems be single or three phase?

(2) What voltage should be used for primary distribution?

(3) Should a unit or modular substation or a gas‐insulated substation (GIS) be used instead of a conventional substation?

(4) How do we judge the economics of installing voltage regulators, capacitors, and automating the system?

(5) Can distribution systems be designed optimally?

(6) Can distribution systems be designed, planned, and operated automatically with the help of computers?

(7) How can distribution systems be protected effectively?

(8) How can distribution systems be planned and operated with dispersed storage and distributed energy resources (DER)?

(9) How are energy losses evaluated in a distribution system?

(10) What are the effective methods for load forecasting?

(11) How can the highest level of power quality be delivered to the customers?

(12) What will be the impact of electric vehicles (EV) on distribution systems?

(13) Can real-time pricing and electricity markets be implemented in distribution systems?

Reference

1

. Venkata, S. S., Pahwa, A., Brown, R. E. and Christie, R. D. “What Future Distribution Engineers Need to Learn,”

IEEE Transactions on Power Systems

, Vol. 19, No. 1, February 2004, pp. 17–23.

2Distribution System Transformers

2.1 Definition

Transformers with a rating of 1000 kVA or less are classified as distribution transformers. Those with larger than 1000 kVA are grouped as power transformers. In a distribution system, distribution transformers are those that are located close to the loads and are used to reduce voltage to the utilization level, such as 120/240 V. Power transformers are those that are located in substations. In some situations, for large industrial or commercial loads, power transformers can be deployed close to the loads.

2.2 Types of Distribution Transformers