Rusko A., Thompson M. Power Quality in Electrical Systems

Подождите немного. Документ загружается.

Summary 107

References 108

Chapter 8. Methods for Correction of Power-Quality

Problems 109

Introduction 109

Correction Methods 110

Voltage disturbances versus correction methods 111

Reliability 113

Design of load equipment 115

The design of electric-power supply systems 117

Power harmonic ﬁlters 119

Utilization-dynamic voltage compensators 119

Uninterruptible power supplies 119

Transformers 120

Standby power systems 122

Summary 126

References 126

Chapter 9. Uninterruptible Power Supplies 129

Introduction 129

History 131

Types of UPS Equipment 133

Commercial equipment 134

Energy storage 137

Batteries 138

Flywheels 139

Fuel cells 141

Ultracapacitors 144

Summary 145

References 145

Chapter 10. Dynamic Voltage Compensators 147

Introduction 147

Principle of Operation 148

Operation on ITIC curve 151

Detection of disturbance and control 152

Commercial equipment 153

Summary 154

References 154

Chapter 11. Power Quality Events 155

Introduction 155

Method 1 155

Method 2 156

Personal Computers 156

Power-quality characteristics 157

Modes of malfunction 160

Sensitivity to voltage sags and interruptions 160

Correction measures 162

Contents ix

Correction measures 164

AC Contactors and relays 165

Operation 165

The Impact of Voltage Disturbance 168

Correction methods 169

Summary 170

References 170

Chapter 12. Electric Motor Drive Equipment 173

Electric Motors 173

Induction Motors 173

Operation 174

Hazards 174

Phenomena 175

Protection 176

Adjustable Speed Drives 177

Application 178

Voltage disturbances 180

Voltage unbalance 181

Protective measures 183

Summary 188

References 188

Chapter 13. Standby Power Systems 189

Principles: Standby Power System Design 189

Components to Assemble Standby

Power Systems 190

Sample Standby Power Systems 191

Engine-Generator Sets 194

Standards 195

Component parts of an E/G set installation 196

Transfer switches 198

Summary 200

References 200

Chapter 14. Power Quality Measurements 201

Multimeters 201

Oscilloscopes 202

Current Probes 203

Search Coils 204

Power-Quality Meters and Analyzers 205

Current Transformer Analysis in Detail 205

Summary 213

References 213

Index 215

x Contents

Preface

This book is intended for use by practicing power engineers and man-

agers interested in the emerging field of power quality in electrical sys-

tems. We take a real-world point of view throughout with numerous

examples compiled from the literature and the authors’ engineering

experiences.

Acknowledgments

PSPICE simulations were done using the Microsim Evaluation

version 8.0.

The authors gratefully acknowledge the cooperation of the IEEE with

regard to figures reprinted from IEEE standards, and with permission

from the IEEE. The IEEE disclaims any responsibility or liability result-

ing from the placement and use in the described manner.

ALEXANDER KUSKO, SC.D., P.E.

MARC T. THOMPSON, PH.D.

This page intentionally left blank

Power Quality

in Electrical

Systems

This page intentionally left blank

Chapter

Introduction

In this introductory chapter, we shall attempt to define the

term “power quality,” and then discuss several power-quality

“events.” Power-quality “events” happen during fault

conditions, lightning strikes, and other occurrences that

adversely affect the line-voltage and/or current waveforms. We

shall define these events and their causes, and the possible

ramifications of poor power quality.

Background

In recent years, there has been an increased emphasis on, and concern

for, the quality of power delivered to factories, commercial establish-

ments, and residences [1.1–1.15]. This is due in part to the preponder-

ance of harmonic-creating systems in use. Adjustable-speed drives,

switching power supplies, arc furnaces, electronic fluorescent lamp bal-

lasts, and other harmonic-generating equipment all contribute to the

harmonic burden the system must accommodate [1.15–1.17]. In addi-

tion, utility switching and fault clearing produce disturbances that

affect the quality of delivered power. In addressing this problem, the

Institute of Electrical and Electronics Engineers (IEEE) has done sig-

nificant work on the definition, detection, and mitigation of power-

quality events [1.18–1.27].

Much of the equipment in use today is susceptible to damage or serv-

ice interruption during poor power-quality events [1.28]. Everyone with

a computer has experienced a computer shutdown and reboot, with a loss

of work resulting. Often, this is caused by poor power quality on the 120-V

line. As we’ll see later, poor power quality also affects the efficiency and oper-

ation of electric devices and other equipment in factories and offices

[1.29–1.30].

Various health organizations have also shown an increased interest

in stray magnetic and electric fields, resulting in guidelines on the levels

of these fields [1.31]. Since currents create magnetic fields, it is possi-

ble to lessen AC magnetic fields by reducing harmonic currents present

in the line-voltage conductors.

Harmonic pollution on a power line can be quantified by a measure

known as total harmonic distortion or THD.

High harmonic distortion can

negatively impact a facility’s electric distribution system, and can gener-

ate excessive heat in motors, causing early failures. Heat also builds up

in wire insulation causing breakdown and failure. Increased operating tem-

peratures can affect other equipment as well, resulting in malfunctions and

early failure. In addition, harmonics on the power line can prompt com-

puters to restart and adversely affect other sensitive analog circuits.

The reasons for the increased interest in power quality can be sum-

marized as follows [1.32]:

■

Metering: Poor power quality can affect the accuracy of utility

metering.

■

Protective relays: Poor power quality can cause protective relays

to malfunction.

■

Downtime: Poor power quality can result in equipment downtime

and/or damage, resulting in a loss of productivity.

■

Cost: Poor power quality can result in increased costs due to the pre-

ceding effects.

■

Electromagnetic compatibility: Poor power quality can result in

problems with electromagnetic compatibility and noise [1.33–1.39].

Ideal Voltage Waveform

Ideal power quality for the source of energy to an electrical load is rep-

resented by the single-phase waveform of voltage shown in Figure 1.1

and the three-phase waveforms of voltage shown in Figure 1.2. The

amplitude, frequency, and any distortion of the waveforms would remain

within prescribed limits.

When the voltages shown in Figure 1.1 and Figure 1.2 are applied to

electrical loads, the load currents will have frequency and amplitudes

dependent on the impedance or other characteristics of the load. If the

waveform of the load current is also sinusoidal, the load is termed

“linear.” If the waveform of the load current is distorted, the load is

termed “nonlinear.” The load current with distorted waveform can produce

2 Chapter One

THD and other metrics are discussed in Chapter 4.

distortion of the voltage in the supply system, which is an indication of

poor power quality.

Nonlinear Load: The Rectiﬁer

The rectifier, for converting alternating current to direct current, is the

most common nonlinear load found in electrical systems. It is used in

equipment that ranges from 100-W personal computers to 10,000-kW

Introduction 3

0 0.005 0.01 0.015 0.02 0.025 0.03

−200

−150

−100

−50

100

150

200

Time [sec]

Voltage [V]

Ideal 60 Hz sinewave

Figure 1.1 Ideal single-phase voltage waveform. The peak value is 170 V, the rms

value is 120 V, and the frequency is 60 Hz.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

−400

−300

−200

−100

100

200

300

400

VLL = 480

Time, [sec]

Voltage, [V]

Phase a Phase b Phase c

Figure 1.2 An ideal three-phase voltage waveform at 60 Hz with a line-line-voltage of

480 V rms. Shown are the line-neutral voltages of each phase.

The line-line-voltage is 480 volts rms; the line-neutral voltage for each phase is 480/

277 V. Therefore, the peak value for each line-neutral voltage is 277 V 392 V.

23 

adjustable speed drives. The electrical diagram of a three-phase bridge

rectifier is shown in Figure 1.3a. Each of the six diodes ideally conducts

current for 120 degrees of the 360-degree cycle. The load is shown as a

current source that maintains the load current, I

, at a constant level—

for example, by an ideal inductor. The three-phase voltage source has

the waveform of Figure 1.2. The resultant current in one source phase

is shown in Figure 1.3b. The current is highly distorted, as compared

to a sine wave, and can distort the voltages of the supply system.

As will be discussed in Chapter 4, the square-wave rectifier load cur-

rent is described by the Fourier series as a set of harmonic currents. In

the case of a three-phase rectifier,

the components are the fundamen-

tal, and the 5th, 7th, 11th, 13th (and so on) harmonics. The triplens

are

eliminated. Each of the harmonic currents is treated independently in

power-quality analysis.

4 Chapter One

D1 D3 D5

D2 D4 D6

(a)

load

Three-Phase

service

Phase

current

−I

60 120 180 240 300

(b)

Electrical

degrees

360

Figure 1.3 A three-phase bridge rectifier. (a) The circuit. (b) The ideal

phase current drawn by a three-phase bridge rectifier.

Often called a “six pulse” rectifier.

Triplen (or “triple-n”) are harmonics with numbers 3, 9, and so on.