ET3380

Principles and Methods of Electric Power Conversion

David Morrisson MS,MBA

Week 1

2

CONTENTS

1. Principles and Methods of Electric Power Conversion

2. Semiconductor Power Switches

3. Supplementary Components and Systems

4. AC-to-DC Converters

5. AC-to-AC Converters

6. DC-to-DC Converters

7. DC-to-AC Converters

8. Switching Power Supplies

9. Power Electronics and Clean Energy

3

Principles and Methods of Electric Power Conversion

The Power Grid

From Generation to the Home

The Basics

• Every power grid in the U.S. has a few essential components.

• These components include the following:• A source: the power plant• A transmission system• A hub: the substation• A distribution system• A user: the home or business

5

6

The Source: The Power Plant

• Essentially, there are only a few ways to generate AC electricity.

• For the vast majority of electricity in the U.S. a fuel (coal, natural gas, a nuclear reaction) is used to create electricity.

• In addition, solar, wind and hydroelectric methods are used to generate electricity.

7

The Heart: The Steam Turbine

• Once a fuel has created sufficient heat, steam is created.

• Pressure from the steam is used to rotate the steam turbine.

• The turbine has magnets attached to the end.

• These magnets rotate within coils, thus generating an AC signal.

8

Electrical Generation:Coal, Natural Gas, & Diesel

9

Relevant Links

• http://www.earthlyissues.com/nuclearplants.htm

• http://www.southerncompany.com/learningpower/powerinfo_5.aspx

10

Nuclear Generation

11

Transmission and Distribution

• Once produced, electricity must be distributed.

• The main device used to achieve this is the transformer.

• Transformers convert AC voltages.• Step-up transformers convert

from low to high voltages.• Step-down transformers convert

from high to low voltages.

12

Transmission and Distribution

• Power plant transformers step up voltages to reach substations and are sent at approximately 550kV.

• Once at the substation, transformers are used to step down voltages to approximately 13kV.

• These 13kV voltages are sent via distribution lines to your neighborhood home or business.

• Once in the neighborhood, transformers are used (on poles or set on the ground) to step down the electrical voltage to 120/240.

13

Transmission and Distribution

• From the power plant, via transmission lines to the substation.

• From the substation, via distribution lines to the home.

• All through the use of the transformer.

14

The Entire System

Power Electronics

Power electronics is the application of electronic apparatus for the control and conversion of electric power.

Also to design, control, computation and integration of nonlinear, time varying energy processing electronic systemsThe first high power electronic devices were mercury-arc tubes. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors

Mercury-arc Tube

17

Types of electric power conversion

DCCHOP P ERS

INVERTERS

RECTIF

IERS

CYCLOCO NVE RTERS

A C V OLTA GE CO NTRO LLE RS

DC

AC AC

INP

UT

CO

NS

TA

NT

MA

GN

ITU

DE

AN

D F

RE

QU

EN

CY

OU

TPU

T

AD

JUS

TA

BLE

MA

GN

ITU

DE

AN

D/O

R F

RE

QU

EN

CY

18

Generic power converter

vov i

I1 O 1

LO AD SO U R CE

i i io

S5

I2 O 2

S1

S3

S4

S2

19

AC input voltage waveform

2

Vi,p

-Vi,p

vi

40 t

20

Output voltage and current waveforms in the generic rectifier

io

vi

2

vo

40 t

21

Output voltage and current waveforms in the generic inverter

io

vo

2

vi

40 t

22

Configurations of power electronic converters: (a) current-source(b) voltage-source

S O UR C E

LOA

D

(a)

C O N VER T ER

LOA

D

S O UR C E

(b)

C O N VER T ER

Total Harmonic Distortion, or THD

• Measurement of the harmonic distortion

• Defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

• Measurement is most commonly defined as the ratio of the RMS amplitude of a set of higher harmonic frequencies to the RMS amplitude of the first harmonic, or fundamental

• Harmonic distortion adds overtones that are whole number multiples of a wave's frequencies

24

Decomposition of the output voltage waveform in the generic rectifier

vo,ac

vo,dc

2

vo

40 t

25

Decomposition of the output voltage waveform in the generic inverter

vo,1

vo,h

2

vo

40 t

26

Decomposition of the output current waveform in the generic inverter

io,h

io,1

2

io

40 t

27

Input current waveform and Its fundamental component in the generic inverter

ii,1

2

ii

40 t

28

Resistive control schemes: (a) rheostatic control, (b) potentiometric control

o

R

R rh

(a)

R

po t

R po t

(b)

I

V V

Ii

i o

Vi Vo

I i oI

I

29

Harmonic spectra of output voltage with the firing angle of in: (a) phase-controlled generic rectifier(b) phase-controlled generic AC voltage controller

(b)

(a)

HARMONIC NUMBER

0 10 20 30 40 50

AM

PLIT

UD

E (

pu)

0.0001

0.001

0.01

0.1

1

HARMONIC NUMBER

0 10 20 30 40 50

AM

PLIT

UD

E (

pu)

0.0001

0.001

0.01

0.1

1

30

Output voltage and current waveforms in the generic chopper

tOFF tON

io

vi

t

vo

0

31

Output voltage and current waveforms in the generic chopper: switching frequency twice as high as in the previous figure

io

vi

t

vo

0

32

RL load circuit

v o

oi

R

L

33

Fragments of output voltage and current waveforms in a generic PWM ac voltage controller

34

Single-pulse diode rectifier

oV

oiii

V i

35

Output voltage and current waveforms in the single-pulse diode rectifier with an R load

3

io

4

vo

20 t

36

Output voltage and current waveforms in the single-pulse diode rectifier with an RL load

e 3

io

4

vo

20 t

37

Single-pulse diode rectifier with a free-wheeling diode

ii

V i DF oV

oi

What are the possible types of loads and what are the effects of each?

38

Output voltage and current waveforms in the single-pulse diode rectifier with a freewheeling diode and an RL load

3

io

4

vo

20 t

39

Single-pulse diode rectifier with an output capacitor

ii

V i oV

oiiC

40

Output voltage and current waveforms in the single-pulse diode rectifier with an output capacitor and an RL load

iC

64

vo

2

vi

0 t

41

Two-pulse diode rectifier

oi

D 1

D 3

D 4

D 2

vo

i i

Vi

42

Cycloconverter

43

Timing diagram of switches in the generic cycloconverter

44

Output voltage waveform in the generic cycloconverter

0 20 40 60 80 100 120 140 160

-200

-150

-100

-50

0

50

100

150

200

v o (

V)

t (ms)

45

Cycloconverter for three-phase alternating current

Block diagram of a dc power supply.

Rectifier Circuits

Rectifier CircuitsHalf-wave Rectifier

vo 0 vs VDO

voR

R rDvs VDO

R

R rD vs VDO

rD R vo vs VDO

PIV Vs

Input and output waveforms.

PIV 2 V s VDO

Rectifier CircuitsFull-wave rectifier utilizing a transformer with a center-tapped secondary winding

Input and output waveforms.

Rectifier CircuitsBridge Rectifier

PIV = Vs VDO-

Voltage and current waveforms in the peak rectifier circuit with CR T. The diode is assumed ideal.

Rectifier CircuitsWith A Filter Capacitor

Rectifier CircuitsDiode – Applications

52

Pulse-width Modulation

53

• Pulse-width modulation (PWM) is a modulation technique that controls the width of the pulse based on modulator signal.

• Main use is to allow the control of the power supplied to electrical devices.

• The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load.

• The PWM switching frequency has to be much higher than what would affect the load (the device that uses the power. Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.

54

• The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.

• The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on and power is being transferred to the load, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

Chapter 1 55

Pulse Width Modulated Step-Down Converter Circuit Schematic

56

Output voltage and current waveforms in the generic PWM rectifier

io

vo

0 2 4 6 8 100

40

80

120

160

200

i o (

A)

v o (

V)

0

20

40

60

80

100

t (ms)

57

Output voltage and current waveforms in a (a) generic PWM rectifier (b) generic PWM ac voltage controller

58

Control characteristics of (a) generic PWM rectifier(b) generic PWM ac voltage controller

59

Harmonic spectra of output voltage in(a) generic PWM rectifier(b) generic PWM ac voltage controller (N = 24)

60

Output voltage and current waveforms in the generic PWM inverter(a) M = 1(b) M = 0.5

io

vi

vo

vi

vo

io

t0

(a)

t0

(b)