The Principles of Electronic and Electromechanic Power Conversion E-Book

Braham Ferreira

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

Seitenzahl: 377

Veröffentlichungsjahr: 2013

Beschreibung

A top-down approach that enables readers to master and apply core principles Using an innovative top-down approach, this text makes it possible for readers to master and apply the principles of contemporary power electronics and electromechanic power conversion, exploring both systems and individual components. First, the text introduces the role and system context of power conversion functions. Then the authors examine the building blocks of power conversion systems, describing how the components exchange power. Lastly, readers learn the principles of static and electromechanic power conversion. The Principles of Electronic and Electromechanic Power Conversion opens with a chapter that introduces core concepts in electrical systems and power conversion, followed by a chapter dedicated to electrical power sources and energy storage. Next, the book covers: * Power, reactive power, and power factor * Magnetically coupled networks * Dynamics of rotational systems * Power electronic converters * DC machines * AC machines The text offers readers a concise treatise on the basic concepts of magnetic circuits. Its simple approach to machines makes the principles of field-oriented control and space vector theory highly accessible. In order to help readers fully grasp power electronics, the authors focus on topologies that use a series transistor and diode combination connected to a DC source, a standard building block of today's power conversion systems. Problem sets at the end of each chapter enable readers to fully master each topic as they progress through the text. In summary, The Principles of Electronic and Electromechanic Power Conversion provides the most up-to-date, relevant tools needed by today's power engineers, making it an ideal undergraduate textbook as well as a self-study guide for practicing engineers.

Leseprobe

Table of Contents

Title Page

Copyright

Preface

Chapter 1: Introduction to Electrical Systems and Power Conversion

1.1 Electricity as an Energy Carrier

1.2 Development of Electrical Energy Conversion Systems

1.3 System Building Blocks

1.4 Guide to the Book

Problems

Chapter 2: Electrical Power Sources and Energy Storage

2.1 Introduction

2.2 Primary Sources

2.3 Secondary Sources

2.4 Highlights

Problems

Chapter 3: Power, Reactive Power and Power Factor

3.1 Introduction

3.2 Power in DC Circuits

3.3 Power in Resistive AC Circuits

3.4 Effective or rms Values

3.5 Phasor Representation

3.6 Power in AC Circuits

3.7 Apparent Power, Real Power and Power Factor

3.8 Complex Power

3.9 Electrical Energy Cost and Power Factor Correction

3.10 Fourier Series

3.11 Harmonics in Power Systems

3.12 Power and Non-Sinusoidal Waveforms

3.13 Effective or rms Value of Non-Sinusoidal Waveforms

3.14 Power Factor of Non-Sinusoidal Waveforms

3.15 Harmonics in Power Systems

3.16 Three-Phase Systems

3.17 Harmonics in Balanced Three-Phase Systems

3.18 Highlights

Problems

Further Reading

Chapter 4: Magnetically Coupled Networks

4.1 Introduction

4.2 Basic Concepts

4.3 Mutual Inductance

4.4 Ideal Transformer

4.5 Highlights

Problems

Further Reading

Chapter 5: Dynamics of Rotational Systems

5.1 Introduction

5.2 Preliminaries

5.3 Rotational Dynamics

5.4 Coupling Mechanisms

5.5 Highlights

Problems

Further Reading

Chapter 6: Power Electronic Converters

6.1 Introduction

6.2 Linear Voltage Regulator

6.3 Switched Approach

6.4 Basic Assumptions

6.5 Buck Converter

6.6 Discontinious Conduction Mode

6.7 Other Basic Converter Structures

6.8 DC–DC CONVERTERS WITH ISOLATION

6.9 Highlights

Problems

Further Reading

Chapter 7: Simple Electrical Machines

7.1 Introduction

7.2 Motional Voltage and Electromagnetic Force

7.3 Simple Linear dc Machine

7.4 Basic Operation of the dc Machine

7.5 Practical DC Machine Construction

7.6 Practical DC Machine Configurations

7.7 DC Machine as A Component in A System

7.8 Highlights

Problems

Further Reading

Chapter 8: AC Machines

8.1 Introduction

8.2 Three-Phase AC Electrical Port

8.3 Ac Machine Stator

8.4 Synchronous Machine

8.5 Induction Machine

8.6 Highlights

Problems

Further Reading

Index

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, [phone], fax [phone], or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, [phone], fax [phone], or online at http://www.wiley.com/go/permission.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at [phone], outside the United States at [phone] or fax [phone].

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com.

Library of Congress Cataloging-in-Publication Data:

Ferreira, Braham, 1958-

The principles of electronic and electromechanic power conversion : a systems approach / Braham

Ferreira, Wim van der Merwe.

1 online resource.

Includes bibliographical references and index.

Description based on print version record and CIP data provided by publisher; resource not viewed.

ISBN 978-1-118-79884-3 – ISBN 978-1-118-79885-0 (ePub) – ISBN 978-1-118-65609-9 (cloth) 1. Power electronics. 2. Electric generators. I. Van der Merwe, Wim, 1977- II. Title.

TK7881.15

621.31′7–dc23

2013029833

Preface

During the past forty years, electrical power conversion systems have evolved considerably. Some technologies have matured, while others have undergone substantial new development. Conversion systems have become more diverse and sophisticated, and feature complex architectures that have improved performance. Central and embedded controllers are now playing a prominent role in managing power flow. Power is applied to loads with high precision, and energy conversion efficiency is optimized at the same time. The volume of power conversion knowledge has increased, and the average electrical engineer deals increasingly with system aspects and less with component details.

The typical undergraduate student needs to study fundamentals as well as the various disciplines of electrical and electronics engineering during a three or four year study program. It is a full program, and it is not possible to introduce more course material in a power conversion course on system-related issues without condensing the content on components. In this book, we have strived to strike a new balance between systems and components in a one semester course that is intended for a bachelors undergraduate program. Alternatively, the book may be used as a self-study book on the principles of power conversion by a professional in science and engineering, who has sufficient mathematical background.

This textbook is a result of many discussions with colleagues. Our observation was that available undergraduate textbooks at this level follow a bottom-up approach that is forty years old. Issues such as magnetic circuits and dc machine field winding configurations receive a substantial amount of attention, while few modern engineers need this knowledge. Power electronics is frequently treated in a 1970s style, describing circuit topologies that are now being phased out and using old-fashioned power devices. In this book, a compact treatise on magnetic circuits is presented where the principles are introduced to gain a good qualitative understanding, leaving practical magnetic circuit analysis and design for follow-up courses. A modern simple approach to machines that makes the principles of field-oriented control and space vector theory approachable to undergraduate students is introduced. In power electronics, the focus is on topologies that use a series transistor and diode combination that is connected to a dc source because this has become a standard building block. The trend is that semiconductor switches are becoming ever better and, therefore, this treatise uses ideal switches.

A top-down approach is followed in this textbook, where the role and system context of power conversion functions are first introduced. The building blocks of the system are defined, and the theory of how they exchange power with each other is described. Then the building blocks are opened, and principles of static and electromechanical power conversion are discussed. Chapter 1 introduces the system architectures used in modern power conversion systems and defines the functions of the building blocks. The first components in a power conversion chain are the sources. As the theory of most energy sources falls outside the scope of the book, the treatise in Chapter 2 is limited and focuses only on a few key concepts. Chapters 3–5 cover electric, magnetic and mechanical power transfer. In Chapter 3, electrical power theory is expanded to include periodic nonsinusoidal voltage and current waveforms because shapes of waveforms other than dc and sinusoidal ac now exist in systems; this phenomena is increasingly due to the prevalence of power electronics in the modern grid. Magnetic coupling is addressed in Chapter 4, and it is not only relevant for coupling different electrical circuits but also plays a role in electromechanical conversion. Chapter 5 is a short chapter on mechanical components and was included because of the important role that mechanical inertias play in the dynamics of systems and because mechanical coupling and electrical coupling are often interchangeable in system architectures. Chapter 6 introduces power electronics at the hands of switchmode dc–dc converters. The main focus is on the quasi-steady-state circuit analysis and the basic conversion functions where step-up, step-down, and conversion involving magnetic coupling is introduced. Chapter 7 introduces electromechanics at the hand of the Lorenz force and evolves the principle from linear motion through rotational motion to a practical dc machine. The concept of orthogonality between torque and speed control in separately excited machines receives attention because of its important role in high-performance electrical drive systems. In Chapter 8, the source of variable frequency, variable amplitude ac, namely a power electronics inverter, is first introduced followed by a treatise on the ac machines stator as a device that can be connected to a three-phase ac source. Two types of rotors then yield either a synchronous or an asynchronous machine. The loss mechanisms are superficially discussed and the calculation of power conversion efficiency is also treated in the two chapters on electrical machines.

During the development of the text, we have placed considerable emphasis on keeping the descriptions and notations mathematically correct and consistent. Although for simplification it might be possible to rewrite the text using different notation methods and conventions for different parts of the book and to shy away from the more complicated mathematical derivations, it is our view that such an approach would be to the eventual detriment of the student. It is our sincere hope that this text strikes the balance of both making the contents accessible to a wide range of undergraduate students, not least of all the students who will eventually specialize in a different engineering field, and, at the same time, to lay a solid and mathematically sound foundation for those students who wish to specialize in power conversion systems either as a career or through postgraduatestudies.

We are grateful to colleagues and students for constructive discussions and advice to improve the content of the book. In particular, we want to acknowledge the effort and support of Sjoerd de Haan and Fatih Çalayan.

Braham Ferreira

Wim van der Merwe

Delft, the Netherlands

Baden, Switzerland

August 2013

Chapter 1

Introduction to Electrical Systems and Power Conversion

1.1 Electricity as an Energy Carrier

All through human existence, some of the greatest advancements in the standard of living came about by learning how to convert energy from one form to another. The first controlled and intentional fire (converting chemical energy to heat energy) was the first step in this process. Man employed the energy from biomass for heating and cooking by burning dried leaves, wood and animal waste. The chemical energy trapped in the organic material was released and converted into heat and light energy.

Later, humans discovered that wind could be used for transportation on water. By using sails, the energy present in moving air can be used to propel a ship in water. Wind energy is a form of kinetic energy that is used to overcome the resistance of water and make the ship move.

The generation of mechanical energy to replace human or animal power came later in human history with the development of simple devices to harness the energy of flowing water and wind. The earliest machines built were waterwheels used initially for grinding grain but later adopted for various other functions such as for driving saw mills and pumps. The oldest reference to a water mill dates to about 85 BC, appearing in a poem by an early Greek writer. The source of energy harnessed here is the potential energy of water, which flows from high areas along rivers to the sea.

Windmills, like waterwheels, were among the original machines that replaced animal muscle as a source of power. They were used for centuries in various parts of the world, converting the energy of the wind into mechanical energy for grinding grain, pumping water and draining lowland areas.

The rapid growth of industry from the mid-18th century created a need for new sources of motive power, particularly solutions that are independent of geographic location and weather conditions. This situation, together with certain other factors, set the stage for the development and widespread use of the steam engine, the first practical device for converting thermal energy into mechanical energy. In 1765, James Watt, a Scottish instrument maker and inventor, made important modifications to the steam engine which resulted in a fuel cost reduction of about 75%. This was the first modern breakthrough in improving the efficiency of a machine that converts energy. The unit for electrical power, the rate of energy flow or energy conversion, was named after JamesWatt.

Electrical energy conversion emerged during the 19th century when the English physicist and chemist Michael Faraday discovered a means by which to convert mechanical energy into electricity. This set the scene for the use of electricity to provide light. Lighting was an important driver for the large scale use of electrical energy because incandescent lamps were much easier and safer to use than oil lamps.

The conversion of mechanical energy into electrical energy can also be reversed; the same generator can be operated as an electrical motor, which converts electrical energy into mechanical energy. The advantage of electrical motors in factories was soon realised. Instead of using a complex system of belts and pulleys to distribute power to the various work stations in the factory, copper cables and electrical motors could be used. In this way, the first small electrical grid came into being with a steam engine driving a generator that supplied the electrical energy for lamps and electrical motors.

An advantage of electrical energy is that it can be easily controlled, by using, for example, switches. An oil lamp can be ignited with a match, while it is much simpler to push the button of a switch to turn on a light. Today, you do not even have to get out of the chair to turn on the light or television; you can use a remote control that is carried in your pocket.

An alternative source of electrical energy is batteries in which chemical energy is directly converted to electrical current. Electrical automobiles using batteries succeeded steam-powered self-propelled vehicles but were later displaced by combustion engine-powered automobiles because the energy density of gasoline is much higher than that of a battery.

Electrical energy can, in general, be converted into other forms of energy with very high efficiency. It is also possible to convert many other forms of energy into electrical energy. The introduction of electrical machines set the first step towards making electrical energy the best universal energy carrier. Semiconductor-based energy converters made it possible to accurately control the flow of electrical energy. The era of electronics began in the 1950s with rapid advances in the design and construction of semiconductor diodes and transistors. When thyristors appeared a few years later in 1957, it became possible to convert energy using electronic circuits.

Figure 1.1 illustrates the interaction between different forms of energy. All energy comes from nuclear reaction, taking place either in the sun or on the earth. Energy from water, wind and solar radiation is called renewable because it cannot be depleted. Fossil fuels, on the other hand, while a cheap and easily available source of energy, can only be consumed once and, as a result, these sources of energy are being depleted.

Figure 1.1 The interaction between different forms of energy.

Most energy is first converted into mechanical energy as an intermediate step before it is converted into electrical energy. By using power electronics, electrical energy is converted into heat, light, mechanical energy and chemical energy for homes, offices and factories. Note that all of these energy flows can be bidirectional, with the exception of heating.

One of the major problems with electrical energy is the willing buyer/willing seller principle. It is impossible to generate electrical energy if there is no consumer of that form of energy. Electrical energy can only be used as a carrier to transport energy from one place (of generation) to another (of consumption). Although it is possible, as we will discuss later in this book, to store limited amounts of electrical energy, we can only generate electrical energy if we have a consumer, even if the storage device is the consumer for that period in time. One of the big challenges to be solved in future energy systems is the fact that the electrical grid will be supplied by a growing amount of renewable energy, which fluctuates between day and night and when wind conditions vary. This will imply that energy may not be available when it is needed by the consumer or the energy could be in abundance it is not needed. Large energy storage systems can solve this problem but it is easier said than done. This puzzle will need to be solved with the Smart Grid in the future.

1.2 Development of Electrical Energy Conversion Systems

The process of energy conversion needs to be controlled. The early humans quickly learned how much wood was required to prepare food for an evening meal. Then they had to feed fuel to the fire at a specific rate to ensure that the flame was large enough, but not too big, to cook food or to roast meat. The same process of controlling the application of energy was used when coal was shovelled into the furnace chamber built into the boiler of a steam locomotive, see Fig. 1.2. Generating the right amount of steam for propulsion and controlling the pressure in the boiler was complicated, and driving a steam engine requires years of practice and apprenticeship training.

Figure 1.2 A steam locomotive.

In a steam locomotive, a complicated system of regulators and throttle valves is employed to control the steam pressure and the flow of energy to the pistons that are directly mounted on the driving wheels of the locomotive. The steam engine was improved by feeding the steam back through boiler tubes to superheat (to heat something to beyond its boiling point) the steam to increase the engine efficiency and power. The modification of the steam engine set the trend to apply advanced technology to improve the efficiency of energy conversion systems. As a result, modern energy conversion systems are complex and have many conversion stages.

Continuing with the example of the train locomotive, we observe in Fig. 1.3 how the distance between the furnace or chamber where the combustion takes place and mechanical energy conversion has increased. In a diesel-electrical locomotive, the combustion takes place in a diesel engine that is connected to an electrical generator. The energy required by the train is then supplied, in electrical form, to an electronic power converter that controls both the speed and traction power delivered to the wheels by controlling the flow of electrical energy delivered to the electrical motors. This power electronic converter fulfils the same function as that of the gears and throttle in an automobile. In the final stage of the conversion chain, the electrical energy is converted to mechanical energy by an electrical motor that is directly connected to the axle.

Figure 1.3 A diesel-electrical locomotive.

In the case of an electrical locomotive, Fig. 1.4, the combustion of fuel takes place at a power station that feeds the electrical grid. The electrical energy flows hundreds of kilometres and goes through many conversion stages before it reaches the locomotive. The electrical locomotive has to share its source of energy with homes, offices, factories and other systems that are connected to the grid. Once again a power electronic converter in the locomotive controls the flow of electrical energy from the grid to the driving electrical motors to control the speed and the delivered traction power.

Figure 1.4 An electrical locomotive.

1.3 System Building Blocks

Look around you and notice all the advanced power conversion systems that have become part of modern life. We briefly discussed the evolution of the train, but similar developments have taken place in automobiles, ships and aircraft. The trend is to have more power conversion stages and to increase the levels of electrification in systems. Growing complexity, improved performance and energy efficiency can also be observed if we study the evolution of telecommunication and computer systems. For example, the original personal computer of 1980 had one power supply and a single microprocessor, while any modern cell phone, game console, laptop or desktop computer today has multiple processors and power supplies that are all specialised for their specific functions.

A systems engineer puts a system together from various building blocks. He needs to consider many issues and has to meet certain performance criteria. For example, if it is a power conversion system, then energy efficiency is an important parameter. The system building blocks are often physical building blocks. Consider, as an example, the PVS300 converter shown in Fig. 1.5. This converter can connect up to 8 kW of solar panels to the electrical grid. Although this sounds simple, this process implies that firstly the flow of dc power from the solar panels must be controlled to make it possible to extract maximum power from the panels. Then, for the second step, the low dc voltage delivered by the panels must be transformed into a higher dc voltage from where an ac voltage waveform can be created which can be connected to the electrical grid. A power converter such as this can easily consist of more than ten different smaller power supply units, several micro controllers and a multitude of other component systems such as sensors.

Figure 1.5 ABB PVS300 solar power converter.

Another example is the induction machine that is shipped with all the drive circuitry included in one simple housing, as shown in Fig. 1.6. We still have one set of electrical terminals to connect wires that provide the electrical power. However, instead of an electrical output, we now have a rotating shaft that is connected to a mechanical system.

Figure 1.6 An induction machine with integrated drive electronics.

Although it is possible to design and build many systems by using various building blocks and by reading the instructions carefully, a successful engineer will always need a fair amount of knowledge of the inner working of each of these blocks. In this book, we will ‘open’ each of these system blocks to discuss their individual operation.

1.4 Guide to the Book

The purpose of the book is to provide a hands-on understanding of the fundamentals of electrical power processing. The examples and exercises are designed to help students apply their knowledge to practical situations. The power conversion systems that were discussed earlier can be broken down into system building blocks that convert energy from one form to another, convert electrical energy into mechanical energy, change mechanical energy from one form into another or convert electrical energy into a form where the energy can be stored as depicted in Fig. 1.7. Using these blocks it is, in principle, possible to build a variety of modern power conversion systems ranging from microgrids to vehicle drivetrains.

Figure 1.7 The energy conversion system blocks of interest: (a) electrical machines, (b) mechanical transmission systems, (c) electrical energy converters, and (d) electrical energy storage systems.

1.4.1 Generation, Storage and Consumption of Electricity

Chapter 2 serves as an introduction to power generation and energy storage components. Because the theory of most sources falls outside the scope of the book, the focus is on just a few key concepts including power balancing, energy housekeeping and efficiency calculations. Chapters 7 and 8 discuss conversion of electrical power to and from mechanical power, which is important from a system viewpoint because it deals with energy that enters or leaves the electrical power system. Most of the energy handled by the electrical grid comes from electromechanical power conversion.

1.4.2 Power Transfer and Matching of Loads and Sources

In addition to balancing power and energy, the interaction between the different building blocks in the system must also be optimised from a performance point of view. Drawing up a specification of how the building blocks must behave is usually the task of the engineer in charge of integration of the system. Some parameters play an important role in determining the performance of the system and the effectiveness of operation.

Chapter 3 discusses the theory of how electrical power is transferred between the building blocks by the voltage and current waveforms on the electrical conductors and introduces figures of merit that describe their effectiveness.

Since power is defined as the product of two terms, for example voltage times current or torque times speed, converter building blocks can be used to better match the characteristics of a source and a load to get a good power transfer. Techniques that are used to adjust the ratio of the two terms in the power equation are

1. changing the ratio of windings (Chapter 4);

2. the diameter of gears and pulleys (Chapter 5);

3. the duty cycle of power electronic converters (Chapter 6) and

4. the pole pairs in electrical machines (Chapters 7 and 8).

1.4.3 Electromechanics

The conversion of electrical power into mechanical power has its roots in physics. For this reason, Ampere's circuital law and Faraday's law are introduced in Chapter 4, followed by the Lorentz force equation in Chapter 7. Starting with very simple electromechanical devices, the theory for practical machines is derived in Chapters 7 and 8. Taking the viewpoint that power electronics-generated ac will dominate in future, thanks to variable speed drives replacing fixed speed motors, synchronous and asynchronous ac machines are introduced by assuming that they are fed by variable frequency and variable amplitude ac voltage.

1.4.4 Power Electronics

Power electronics is introduced in Chapter 6 where the application of the switching approach to state space theory is discussed without going into too much detail. As the next step, a simplified analysis method is introduced to analyse the buck and boost converter. It is then shown that the buck and boost converter can be combined to create a phase arm, which is the power electronics building block that is needed to construct converters that can drive electrical machines. In Chapters 7 and 8, this basic power electronics converter then becomes the power source for electrical machines.

For a study program that would include dc power supplies, isolated converters are dealt with in the second part of Chapter 6. Using knowledge on transformers and inductors that was covered in Chapter 4, the flyback and forward converter are introduced and analysed.

Problems

This introductory chapter has not discussed much theory. An important observation has been that modern energy systems contain many power conversion steps. The efficiency can easily be calculated by tracing the power flow through the various conversion stages. The overall system power efficiency is obtained by simply multiplying the conversion efficiencies of the system building blocks with each other.

1.1 Energy conversion chains have become longer despite the fact that the system has become more complex. Explain the improvement in terms of energy efficiency and effective application of power for the following two applications.

1. A steam train of 1900 compared to an electrical train of 2010.

2. The transmission system automobile of 1980 (using gearboxes with three or four gear ratios compared to a 2012 automobile (using gearboxes with six to nine gear ratios or a hybrid electrical drive train).

1.2 Two options of providing power to several subsystems from a single 18 V supply are shown in Fig. 1.8. Each of the blocks represents a linear regulator. Remembering that the losses of a linear regulator is where is the voltage difference between the input and output and I is the current through the regulator.

1. Compare the power efficiency of centralised and decentralised systems.

2. What will the power efficiency be if switchmode converters with an efficiency of 96% are used in stead of linear regulators?

Figure 1.8 Problem 1.2: A centralised and a decentralised power delivery system.

1.3 The power required by an electrical boat, in kilowatts, at a specific speed (in kilometres per hour) can be found by the relationship

The boat uses a 12 V lead-acid battery with a rating of 100 Ah. The electrical motor is 95% efficient. The electrical motor is coupled to the battery with a converter rated 5 kW maximum. If is the proportion of maximum power delivered by the converter, then the efficiency of the converter is determined as

Calculate the maximum range at

1. 5 km/h,

2. the maximum speed.

Chapter 2

Electrical Power Sources and Energy Storage

2.1 Introduction

In this chapter, we focus on the different energy sources that are normally associated with our modern society. As we are predominantly interested in the conversion of electrical energy, this chapter focuses on the sources normally associated with electrical systems.

In general, we can identify two types of energy sources: primary and secondary sources. The distinction is that primary sources are one-directional sources where energy is transferred from one state to another without a viable option of reversing the process. On the other hand, secondary sources are, in essence, storage sources where we first store the energy we want to use and then extract it again at a later (and hopefully more convenient) time.

In modern systems, energy sources are connected to the grid using power electronic converters as intermediate coupling interfaces. These converters are required since the grid frequency is 50 or 60 Hz, and when the energy source is dc or works at a different ac frequency, a method of converting the power to the form required by the grid is needed. Because primary sources are unidirectional, the converter needs to handle power in one direction. In contrast, a power electronic converter that exchanges electrical energy between a system and the grid in both directions is needed for a secondary electrical energy source. One example of such a converter is the drive unit of an electrical locomotive. When the train is accelerating or running at a constant speed, the converter is taking energy from the grid and providing the train with propulsion. However, when the train is decelerating, the converter effectively converts the excess unwanted mechanical energy into electrical energy and therefore the power flow through the converter reverses, and the converter can now feed the excess energy back in the grid. It is, however, true that these converters are often found operating in tandem with other energy storage devices, for example, in a hybrid electrical vehicle (EV), or even in a full EV, in which the excess kinetic energy of the vehicle during braking is stored, using the converter, in the vehicle's battery pack.