Lectures Multi-level converters

11

EE8407Power Converter Systems

Topic 6Multilevel Cascaded H-Bridge (CHB) Inverters

Textbook: Bin Wu, High-Power Converters and AC Drives, Wiley-IEEE Press, 2006, ISBN: 0-471-73171-4

CHB Inverter Fed Drive Source: Toshiba - General Electric

H-bride power cells

2

Lecture Topics H-Bridge Inverter CHB Inverter Topologies Phase Shifted PWM Voltage Shifted PWM PWM Scheme Comparison

Topic 6Multilevel CHB Inverters


EE8407

23

To increase inverter operating voltage without devices in series To minimize THD with low switching frequencies fsw To reduce EMI due to lower voltage steps

Why Use Multilevel Inverters?

Switching frequency for high power converters:fsw = 60Hz ~ 1000Hz



EE8407

4

Topic 6 Multilevel CHB Inverters Multilevel Converter Topologies

EE8407


dCdC dC

dC

dC

Per-Phase Diagram

35Textbook: Bin Wu, High-Power Converters and AC Drives, Wiley-IEEE Press, 2006, ISBN: 0-471-73171-4

Topic 6 Multilevel CHB Inverters H-Bridge Inverters

EE8407

H-bridge Power Cell

dCdV

1S

2S

3S

4S

1D

2D

3D

4D

1g 3g

2g4g

A

B

P

N

ABv

6

H-bridge Power Cell

Typical Industrial Applications


= 0

= 0

= 0

M

= 30

= 30

= 30

A

B

C

Neutral

H-bridge

Phase ShiftingTransformer

UtilityGrid

Five-level CHB inverter

dCdV

1S

2S

3S

4S

1D

2D

3D

4D

1g 3g

2g4g

A

B

P

N

ABv


EE8407

47

Bipolar Modulation


Bipolar PWM:vAB from -Vd to +Vd

or from +Vd to -Vd

dCdV

1S

2S

3S

4S

1D

2D

3D

4D

1g 3g

2g4g

A

B

P

N

ABv1gv

3gv

dV

dV

0

-1.0

1.0

0

BNv

ANv

0

ABv

dV

1ABv

2mv crv

0

0.2

0.4

0.6

12 fm32 fm 32 +fm

2fm 2+fm 23 fm 14 fm1=n

dn VV AB /

0

(a) W aveform s

(b) H arm onic spectrum

dV

n1 10 20 30 40 50 60


EE8407

8

Bipolar Modulation (FFT)

d

n

V

V AB

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8

2fm

00

32 fm

12 fm

0.707

fm

am

1=n



EE8407

59

Unipolar Modulation (1)


0

-1.0

1.0

mv

2

1gv

3gv

0

BNv

ANvdV

dV

0

ABvdV

0

0.2

0.4

dn VV AB /

32 fm 32 +fm14 fm

0.6

12 fm

mv crv

1=n

(b) Harmonic spectrum

(a) W aveform s

n1 10 20 30 40 50 60

dCdV

1S

2S

3S

4S

1D

2D

3D

4D

1g 3g

2g4g

A

B

P

N

ABv

Two modulation wavesvm and vm-

One carrier wave vcr

Unipolar PWM:vAB from 0 to +Vd

or from 0 to -Vd


EE8407


One modulating wave vm Two carrier waves

vcr and vcr-

dCdV

1S

2S

3S

4S

1D

2D

3D

4D

1g 3g

2g4g

A

B

P

N

ABv0

-1.0

1.0

0

ABvdV

mv crv

crv

1gv

3gv

Unipolar Modulation (2)


EE8407

611

Five-Level CHB Inverter


Topic 6 Multilevel CHB Inverters CHB Inverter Topologies

EE8407

Converters in cascade, but no switching devices in series.

ComplementarySwitch pairs:S11 and S41;S31 and S21;

S12 and S42;S32 and S22;

1Hv

2Hv

N

21S

11S

41S

31S

22S42S

32S12S

E

E

E

E

E

E

A B C

O

LOAD


Output Voltage and Switching Status (five-level)

Waveform of vAN is composed of five voltage levels: 2E, E, 0, -E, and -2E

1Hv

2Hv

21S

11S

41S

31S

22S42S

32S12S

E

E

A

N

Switching State Output Voltage

ANv 11S 31S 12S 32S 1Hv 2Hv

2E 1 0 1 0 E E 1 0 1 1 E 0 1 0 0 0 E 0 1 1 1 0 0 E

E

0 0 1 0 0 E 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0 0 0 0 1 1 1 1 0 0 1 0 0 1 E -E

0

0 1 1 0 -E E 0 1 1 1 -E 0 0 1 0 0 -E 0 1 1 0 1 0 -E -E

0 0 0 1 0 -E -2E 0 1 0 1 -E -E


EE8407


Seven- and Nine-Level Inverters (Per phase diagram)

1Hv

2Hv

21S

11S

41S

31S

22S42S

32S12S

E

E

A

3Hv

23S43S

33S13S

E

N

1Hv

2Hv

E

E

A

3HvE

4HvE

N


EE8407

14

Unequal dc Bus Voltages


1Hv

2Hv

21S

11S

41S

31S

22S42S

32S12S

E

E2

A

N

1Hv

2Hv

21S

11S

41S

31S

22S42S

32S12S

E

E3

A

N


EE8407

815

Switching State Output Voltage

ANV 11S 31S 12S 32S 1HV 2HV

3E 1 0 1 0 E 2E 1 1 1 0 0 2E

2E 0 0 1 0 0 2E 1 0 1 1 E 0 1 0 0 0 E 0 E 0 1 1 0 -E 2E 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0 0 0 0

0

1 1 1 1 0 0 1 0 0 1 E -2E 0 1 1 1 -E 0 -E 0 1 0 0 -E 0 1 1 0 1 0 -2E

-2E 0 0 0 1 0 -2E

-3E 0 1 0 1 -E -2E

Unequal dc Bus Voltages (Two-cell seven-level topology)


1Hv

2Hv

21S

11S

41S

31S

22S42S

32S12S

E

E2

A

N


EE8407


Carrier Based PWM Phase Shifted

Amv1crv 2crv 3crv

11gv

31gv

1Hv

12gv

32gv

2Hv

13gv

33gv

3Hv

0ANv

E

3E

0

1.0

-1.0

E

1crv 2crv 3crv

E

321 HHH vvvv AN ++=

# of voltage levels: m = 7 # of carriers: mc = m - 1 = 6

Phase shift: 360 / mc = 60

Carriers for H1 bridge:vcr1 and vcr1-



m = 7

Topic 6 Multilevel CHB Inverters Phase Shifted PWM

EE8407

917

Inverter Waveforms (7-level, phase shifted)


1Hv

2Hv

3Hv

0

ANv

0

ABv

3E

6E

Switching occurs at different times

fsw(device) = 60 mf = 600Hz

Inverter phase voltage levels: 7

Low EMI

Line-to-line voltage levels: 13

Close to a sinusoid Low THD


EE8407

18

FFT (7-level, phase shifted)



EE8407

Lowest harmonics: around 2mf

Lowest harmonics: around 6mf

Containing triplen harmonics

No triplen harmonics Equivalent fsw(inverter)

= 60(6mf ) = 3600Hz

12 fm14 fm 16 fm

36 fm96 fm 96 +fm

16 fm76 fm 76 +fm

dn VV /H1

dANn VV /

dABn VV /

10



EE8407

Harmonic Content (7-level, phase shifted)

00.2 0.4 0.6 0.80 am

0.1

0.2

0.3

d

n

V

VAB

16 fm56 fm 76 fm

116 fm

x

y 2241.xy =

1=n


Topic 6 Multilevel CHB Inverters Level Shifted PWM

EE8407

Carrier Based PWM Level Shifted

# of voltage levels: m = 5

# of carriers: mc = m - 1 = 4

IPD provides the best harmonic profile.

mv

2

3

1crv

2crv

3crv

4crv

2 3

mv

2

mv

1crv

2crv

3crv

4crv

1crv

2crv

3crv

4crv

IPD

APOD

POD

11



EE8407

cr1v

cr2vcr3v

g11vg31v

1Hv

12gv32gv

2Hv

13gv33gv

3Hv

0

ANv3E

mAv

-cr1v

-cr3v-cr2v

3H2H1HAN vvvv ++=

E

E

E

# of voltage levels: m = 7

# of carriers: mc = m - 1 = 6

fsw(device): - not equal to fcr, and - not the same for all switches.

Device conduction angle:- not equal.

Necessary to swap switching pattern.

Gating Arrangement (7-level)



EE8407

Inverter Output Voltages (seven-level)

m = 7

vAB close to a sinusoid Low THD, low EMI

Equivalent fsw(inv) = fc = 3600Hz

Switching occurs at different times

fsw(device) = fcr /mc =600Hz (avg)

1Hv2Hv

3Hv

ANv

ABv

2fm8+fm8fm

16+fm16fm

2 4

2 4

6fm 6+fm

dABn VV /

dANn VV /

12

23


EE8407

Prototype at Ryerson(Seven-level CHB Inverter)


24

Measured Waveforms (IPD, 7-level)

Inverter phase voltage vAZ Line-to-line voltage vAB


EE8407


13


Topic 6 Multilevel CHB Inverters PWM Scheme Comparison

EE8407

PWM at Low ma

- Phase shifted PWM: THD = 96.7%

- Level shifted PWM: THD = 48.8%

At ma = 0.2:

1Hv2Hv3Hv

0

ANv

0

ABv

E

2E

1Hv

2Hv3Hv

0

ANv

0

ABv2E

E

(a) Phase-shifted modulation

(b) Level-shifted modulation (IPD)

THD = 96.7%

THD = 48.8%

26

Total Harmonic Distortion (THD)



EE8407

am

Hz600, =devswf

14

27

Comparison Phase-shifted Modulation Level-shifted

Modulation (IPD)

Device Switching Frequency Same for all devices Different Device Conduction

Period Same for all devices Different

Rotating of switching patterns No required Required THD of inverter output

line-to-line voltage Good Better

Low Order Harmonics No Yes (Very low amplitude)


Summary


EE8407

28


EE8407


Documents

Lectures Multi-level converters