Power Converter Systems Graduate Course EE8407

EE8407 Topic 7

1 Textbook: Bin Wu, ‘High-Power Converters and AC Drives’, Wiley - IEEE Press, 2006

Power Converter Systems

Graduate Course EE8407

Bin Wu PhD, PEng

ProfessorELCE DepartmentRyerson University

Contact Info Office: ENG328Tel: (416) 979-5000 ext: 6484Email: [email protected] http://www.ee.ryerson.ca/~bwu/

Ryerson Campus

EE8407 Topic 7


Three-Level NPC Inverter Based MV Drive

Topic 7

Multilevel Neutral Point Clamped (NPC) Inverters

Courtesy of ABB (ACS1000)

EE8407 Topic 7


Lecture Topics

• Three-level NPC Inverter

• Space Vector Modulation• Neutral Point Voltage Control

• High-level NPC Inverters

Multilevel NPC Inverters

EE8407 Topic 7


• Inverter Configuration

Clamping diodes: DZ1 and DZ2 (Phase A)

1dC

1S

2S

3S

4S

A

B

CZ

2dC

1D

2D

3D

4D

1ZD

2ZD

E

E

ZidV

LOAD

Ai

OBi

Ci

Three-Level NPC Inverters

EE8407 Topic 7


• Switching State

Complementary Switch pairs:

S1 and S3; S2 and S4;

Device Switching Status (Phase A) Switching

State 1S 2S 3S 4S

Inverter Terminal Voltage

ZAV

P On On Off Off E

O Off On On Off 0

N Off Off On On E


EE8407 Topic 7


• Gate Signal Arrangements

Inverter phase voltage vAZ has three levels: E, 0 and –E

ONONONOPOPOPO

0

1gv

2gv

3gv

4gv

AZv

E

E


EE8407 Topic 7


• Inverter Output Waveforms

2

EAZv

BZv

CZv

ABv

0

0

0

0

t

t

t

tBZAZAB vvv

E

E2

E2

E


EE8407 Topic 7


• Space Vectors

• Three-phase voltages

0)()()( tvtvtv COBOAO

• Two-phase voltages

)(

)(

)(

3

4sin

3

2sin0sin

3

4cos

3

2cos0cos

3

2)(

)(

tv

tv

tv

tv

tv

CO

BO

AO

• Space vector representation

)()()( tvjtvtV

(2) (3)

3/43/20 )()()(3

2)( j

CO

j

BO

j

AO etvetvetvtV

where xjxe jx sincos

(3)

(1)

(2)

(4)

Space Vector Modulation

EE8407 Topic 7


Switching state [POO] on-state switches:Phase A: upper two switches [P]Phase B: middle two switches [O]Phase C: middle two switches [O]

dBOdAO VtvVtv6

1)(,

3

1)( dCO Vtv

6

1)( and

Substituting (5) to (4) gives a space vector

(5)

(6)01 3

1 jd eVV

• Space Vectors (Example)

from which

Total switching states: 27Total space vectors: 19


EE8407 Topic 7


• Space Vectors Diagram

19 space vectors:

Zero vector: V0

Small vectors: V1 – V6

Medium vectors: V7 – V12

Large vectors: V13 – V18

dVV

3

213

6/

3

37

jedVV

3/

3

214

jedVV

0V

3V

2V

4V

5V

8V

6V

15V

18V

12V

11V

10V

9V

17V

16V

3/1 dVV


EE8407 Topic 7


Space Vector

Switching State Vector Classification Vector

Magnitude

0V [PPP][OOO] [NNN] Zero Vector (ZV) 0

P-type N-type

PV1

[POO]

1V

NV1

[ONN]

PV2

[PPO]

2V

NV2

[OON]

PV3

[OPO]

3V

NV3

[NON]

PV4

[OPP]

4V

NV4

[NOO]

PV5

[OOP]

5V

NV5

[NNO]

PV6

[POP]

6V

NV6

[ONO]

Small Vector (SV)

P-type Small Vector (PSV) N-type Small Vector (NSV)

dV3

1

• Switching States and Space Vectors

Redundancy: Zero vector – three switching statesSmall vectors – two states per vector


EE8407 Topic 7


Space Vector

Switching State Vector Classification Vector

Magnitude

7V [PON]

8V [OPN]

9V [NPO]

10V [NOP]

11V [ONP]

12V [PNO]

Medium Vector (MV) dV3

3

13V [PNN]

14V [PPN]

15V [NPN]

16V [NPP]

17V [NNP]

18V [PNP]

Large Vector (LV) dV3

2

No redundant switching states for medium or large vectors

• Switching States and Space Vectors


EE8407 Topic 7


• Sector Definition

1V

13V

7V

14V

0V

3V

2V

4V

5V

8V

6V

15V

18V

12V

11V

10V

9V

17V

16V

refV

Sector I

Sector II

Sector III

Sector IV

Sector V

Sector VI

Vref : Reference vector, rotating in space at a certain speed;

All other vectors are stationary.


EE8407 Topic 7


• SVM Principle

• For a given length and position in space, Vref can be approximated by three nearby stationary vectors; • Based on the chosen stationary vectors, switching states are

selected and gate signals are generated; • When Vref passes through sectors one by one, different sets of

switches are turned on or off; • When Vref rotates one revolution in space, the inverter output

voltage varies one cycle over time; • The inverter output frequency corresponds to the rotating speed

of Vref ; • The inverter output voltage can be adjusted by the magnitude of Vref.


EE8407 Topic 7


• Dwell Time Calculation

4

bT PNNcT

bT

3

PON

PPN

PPO

OON

1

aT

ONNPOO

2

cT

39

13V

7V

1V

14V

0V

2V

refV

aT

• Use volt-second balancing principle

• Select three nearest stationary vectors

721 and, VVV

Sector I

scba

srefcba

TTTT

TVTVTVTV

271

Four Regions

Dwell time is the duty-cycle time of selected switches during the sampling period Ts .

(a)


EE8407 Topic 7


• Dwell Time Calculation

])3

(sin21[

]1)3

(sin2[

]sin21[

asc

asb

asa

mTT

mTT

mTT

Ta , Tb and Tc – dwell times for V1 , V7 and V2

d

refa V

Vm 3 – modulation index

From equation (a)


EE8407 Topic 7


• Switching Sequence (Seven-segment)

General Design Requirements

a) The transition from one switching state to the next involves only

two switches in the same inverter leg, one being turned on and

the other turned off; and

b) The transition for Vref moving from one sector (or one region)

to the next requires no or minimum number of switchings.

Note:

The switching sequence design is not unique, but the above

requirements should be satisfied for switching frequency

minimization.


EE8407 Topic 7



Voltage Vector NV2

7V

14V

PV2

14V

7V

NV2

Dwell Time 4cT

2bT

2aT

2cT

2aT

2bT

4cT

Phase A O P P P P P O

Phase B O O P P P O O

Switching

State Phase C N N N O N N N

,0]O[,]P[ E E]N[ .

Assuming Vref is in Region 4 of Sector I,

three vectors are selected: V2 , V7 and V14


EE8407 Topic 7



2aT

BZv

AZv

CZv

0E

4cT

2bT

sT

0

0

ABv0

E

E

E

2cT

2bT

4cT

NV2

OON7V

PON14V

PPN PPOPV2

14V

PPN7V

PONNV2

OON

2aT

Switching sequence requirement a) is satisfied.


EE8407 Topic 7




EE8407 Topic 7



14V

15V

17V

18V

13V

16V 2a

1a

2b1b

3

4SECTOR I

SECTOR VI

SECTOR II

SECTOR III

SECTOR IV

SECTOR V

refV

2a2b

1a1b

4 3

refV

ExtraSwitching

3

4

2b

2a1a

1b

Switching sequence requirement b) is satisfied.

2/2/ 1, fff spdevsw

ssp Tf /1

Device switching frequency:

Sampling frequency:

Fundamental frequency:

1f


EE8407 Topic 7


AZv

ABv

0

1gv

4gv

0+E

-E

+2E

-2E

2 3

• Simulated Waveforms (Seven-segment)

f1 = 60Hz, Ts = 1/1080 sec, ma = 0.8, fsw = 570Hz

vAB is not half wave symmetrical; and

contains both even- and odd-order harmonics.


EE8407 Topic 7


• Simulated Waveforms (Seven-segment)

f1 = 60Hz, Ts = 1/1080 sec, ma = 0.8, fsw = 570Hz

0

0.05

0.1

dAZ VV n /

0.15THD = 69.8%

0

0.05

0.1

dAB VV n /

0.15THD = 41.0%

n1 5 10 15 20 25 30 35 40 45 50 55 60

n1 5 10 15 20 25 30 35 40 45 50 55 60

14 16 20 22 26 28


EE8407 Topic 7


dnAB VV /

(a) Even order harmonics

(b) Odd order harmonics

0

0.02

0.04

0.06

0.08

20n = 2 4 8 10 1416

0 0.2 0.4 0.6 0.8

n =1

am

0

0.02

0.04

0.06

0.08

n = 19 17 13 11 7 5

0 0.2 0.4 0.6 0.8 am

dnAB VV /

n =1

0

50

100

150

200

THD(%)

THD

201610

20

16

14

10

19

17

• Harmonic Content (Seven-segment)


EE8407 Topic 7


• Laboratory Prototype at Ryerson


EE8407 Topic 7


• Measured Waveforms

AZv

ABv

Waveforms (2ms/div)

Spectrums (500Hz/div)

18

3

9

35 54

3537

17 47

77

7761 65

Waveforms (2ms/div)


18

3

14 16 222029

35

4771

61 65

35 5471

8.0)a( am 9.0)b( am

9

3729

d

n

V

VAB

d

n

V

VAZ

0.1

0.1


EE8407 Topic 7


• Measured waveforms (with even-order harmonic elimination)

Waveforms (2ms/div)


3

15 21

2735

45 57

17 19

3537

29 47 61 65 77

AZv

ABv

315 21

3527 45

57

1719

353729

47 61 65

71

13

71

Waveforms (2ms/div)


8.0)a( am 9.0)b( am

77

d

n

V

VAB

d

n

V

VAZ

0.1

0.1


EE8407 Topic 7


• Neutral Point Voltage Deviation

The neutral point voltage vz can be controlled byP- and N-types of small vectors

ZLOAD

dVLOAD

dV

LOAD

dVLOAD

dV

(a) Zero voltage vector (b) P-type small vector

(c) N-type small vector (d) Medium vector

1dC

2dC

1dC

2dC

1dC

2dC

1dC

2dC

Zv

Zv

Zv

Zv

Zi

Zi

Z

ZZ

A

B

C

A

B

C

A

B

C

A

B

C

Zv]POO[affectednot]PPP[ Zv

Zv]ONN[ definednot]PON[ Zv

LOAD

dV

(e) Large vector

Zv

1dC

2dC

Z

A

B

C

affectednot]PNN[ Zv

Neutral Point Voltage Control

EE8407 Topic 7


• Neutral Point Voltage Control

aNaPa TTT

tT

T

tT

T

aaN

aaP

12

12 11 T

Neutral Point Deviation Level

Motoring Mode 0di

Regenerating Mode 0di

ddd Vvv )( 21 0t 0t

ddd Vvv )( 12 0t 0t

ddd Vvv 21 0t 0t

V maximum allowed voltage deviation ( 0 dV ).


EE8407 Topic 7


• Neutral Point Voltage Control

R is used on purpose to make the dc voltage unbalance.

0 0.01 0.02 0.03 0.04 0.052600

2700

2800

29001dv

(sec)t

)(V21, dd vv

2dZ vv

Without Neutral PointVoltage Control

With Neutral PointVoltage Control

A

Z

2dv

ZidV

1dv

R

(a) (b)

(5600V)


EE8407 Topic 7


• Inverter Topologies

1S

2S

3S

1S

A

1D

22 D

2D

E

E

dV E

E

E

dV

E

E

A

(a) Four-level (m = 4) (b) Five-level (m = 5)

2S

3S

21D

1S

2S

3S

4S

1S

2S

3S

4S

1D

X

22 D

33D

31D

22 D

3D

X

Y

Y

Z

dC

dC

P

N

ANv ANv

N

P

Ai Ai

High-Level NPC Inverters

EE8407 Topic 7


• Switching State

Switch Status

Four-level Inverter

1S 2S 3S 1S 2S 3S

ANv

1 1 1 0 0 0 3E

0 1 1 1 0 0 2E

0 0 1 1 1 0 E

0 0 0 1 1 1 0

Five-level Inverter

1S 2S 3S 4S 1S 2S 3S 4S ANv

1 1 1 1 0 0 0 0 4E

0 1 1 1 1 0 0 0 3E

0 0 1 1 1 1 0 0 2E

0 0 0 1 1 1 1 0 E

0 0 0 0 1 1 1 1 0


EE8407 Topic 7


• Component Count

Voltage Level

Switches Clamping Diodes*

dc capacitors

m 6(m-1) 3(m-1)(m-2) (m-1)

3 12 6 2

4 18 18 3

5 24 36 4

6 30 60 5

* The clamping diodes have the same voltage rating as other switches.

Note:

The number of clamping diodes increases substantially with the voltage level.


EE8407 Topic 7


• IPD Modulation (four-level)1.0

-1.0

3gv2gv

1gv

0

ANv

0

ABv

1crv

2crv

3crv

3E

3E

n1 5 10 15 20 25 30 35 40 45 50 55 60

0.05

0.10

dAB VV n /

0

0

THD = 25.2%

THD = 25.2%


EE8407 Topic 7


• Harmonic Content (four-level, IPD Modulation)

0.2 0.4 0.6 0.8 am

0.1

0.2

00

n=1 n=13,17 n=11,19n=7

dAB VV n /

n=5

vAB = 3-level vAB = 5-level 7-level


EE8407 Topic 7


• APOD Modulation (four-level)

1crv

2crv

3crv

1.0

-1.0

0.05

0.10

0 1000 2000 3000 f (Hz)

0.15

ABv

0

0

Ai

13

23rd29th

47th

n = 11 19th

37th

17th

53rd 61st

3E

2 3

dn VVAB /


EE8407 Topic 7


• Harmonic Content (four-level, APOD Modulation)

0.2 0.4 0.6aM

0.1

0.2

0.3

00

x

y 612.0

x

y

n=1

n=13,17n=11

n=19

0.8

d

n

V

VAB


EE8407 Topic 7


• The 3-level NPC inverter widely used in MV drives

Main features

- Low device count

- No switches in series

- Suitable for medium voltage operation

• The practical use of 4- or 5-level NPC inverters not reported

Main reasons

- Difficulties in dc capacitor voltage control

- Large number of clamping diodes

Summary

EE8407 Topic 7


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