Chapter 5 DC to AC Converters

Outline

5.1 Commutation

5.2 Voltage source inverters

5.3 Current source inverters

5.4 Multiple- inverter connections and multi- level inverters

5.1 Commutation types

Basic operation principle of inverters

A classification of inverters

–Square- wave inverters (are discussed in this chapter)

–PWM inverters ( will be discussed in Chapter 6)

The concept of commutation

l oad

t

S 1

S 2

S 3

S 4

i o

u o

U d

u o

i o

t 1 t 2

4 types of commutation

1)Device commutation:

Fully- controlled devices: GTO, IGBT, MOSFET

2)Line commutation:

Phase- controlled rectifier,Phase- controlled AC controller,

Thyristor cycloconverter

3)Load commutation

4)Forced commutation

(1) Load commutation

t

t

t

t R L

C

O

O

O

O

i

i

t 1

b) a)

E d

L d

VT 1

VT 2

VT 3

VT 4 u o

i o

i d

u o u o i o

i o

u VT

i VT 1 i VT 4

i VT 2 i VT 3

u VT 1 u VT 4

(2) Forced commutation (capacitance commutation)

S

VT

+

-

Direct- Coupled

C

L

+

VD

S

C

VT +

L

S

VT VD

b) a)

With Coupling-Inductor

Another classification of commutations

Self-commutation

External

commutation

4 types of Commutations Device commutation

Forced commutation

Line commutation Load commutation

For fully-controlled devices

For thyristors

2 classes of inverters

Voltage Source Inverter

(VSI)

Current Source Inverter

(CSI)

R L

C

E d

L d

VT 1

VT 2

VT 3

VT 4 u o

i o

i d VD 1

VD 2

VD 3

VD 4

u o

L R C

i o

V1

V2

V3

V4

Ud

+

-

5.2 Voltage source inverter (VSI)

FeaturesDC side is constant voltage, low impedance (voltage source, or bulk cap)

AC side voltage is square wave or quasi- square wave.

AC side current is determined by the load.

Anti- parallel diodes are necessary to provide energy feedback path.

(freewheeling diodes , feedback diodes)

VD 1

VD 2

VD 3

VD 4

u o

L R C

i o V1

V2

V3

V4

Ud

+

-

Single-phase half bridge VSI

The current conducting path is determined by the polarity of load voltage and load current. (This is true for analysis of many power electronics circuits.)

The magnitude of output square- wave voltage is Ud/2.

VD 1

VD 2 u o

L R i o

V1

V2

Ud 2

Ud

Ud 2

Single-phase full bridge VSI

Operation principle

VD 1

VD 2

VD 3

VD 4

u o

L R C

i o V1

V2

V3

V4

Ud

+

-

Quantitative analysis

Fourier series extension of output voltage

Magnitude of output voltage fundamental component

Effective value of output voltage fundamental component

ttt

Uu

5sin

5

13sin

3

1sin

4 do （5-1）

（5-2） dd

o1m 27.14

UU

U

（5-3） dd

1o 9.022

UU

U

Output voltage control by phase-shift

VD 1

VD 2

VD 3

VD 4

u o

L R C

i o V1

V2

V3

V4

Ud

+

-

t

t

t

t

t

u o u o i o i o

o

o

o

o

o u G1

u G2

u G3

u G4

θ

t1 t2

t3

Inverter with center- tapped transformer—push-pull inverter

l oad

+

-

i o u o

U d V 1 V 2

VD 1

VD 2

Three-phase VSI

W

V1

V4

V

VD4

Ud

V6 V2

V3 V5

N

VD6 VD2

VD1 VD3 VD5 2

N´

U

+

-

Ud

2

180° conduction

Dead time (blanking time) to

avoid “shoot through”

t

t

t

t

t

t

t

t

o

o

o

o

o

o

o

o

a）

b）

c）

d）

e）

f）

g）

h）

uUN´

uVN´

uWN´

uUV

uNN´

uUN

iU

id

Ud 2

Ud

Ud

2Ud

6

3 3 Ud

Basic equations to obtain voltage

UN'WN'WU

WN'VN'VW

VN'UN'UV

uuu

uuu

uuuF o r l i n e v o l t a g e

F o r p h a s e v o l t a g e o f t h e l o a d

'NN WN'WN

NN' VN'VN

NN' UN'UN

uuu

uuu

uuu

)(3

1)(

3

1WNVNUN WN' VN' UN' NN' uuuuuuu

)(3

1 WN' VN' UN'NN' uuuu

u U N u V N u W N + + = 0

Quantitative analysis

Fourier series extension of output line- to- line voltage

Magnitude of output voltage (line- to- line) fundamental component

Effective value of output voltage (line- to- line) fundamental component

16 kn

n

k tnn

tU

tttttU

u

sin)1(1

sin32

13sin13

111sin

11

17sin

7

15sin

5

1sin

32

d

dUV

（5-8）

（5-10） dd

UV1m 1.132

UU

U

（5-11） ddUV1m

UV1 78.06

2UU

UU

5.3 Current source inverter (CSI) FeaturesDC side is constant current , high i

mpedance (current source, or large inductor)

AC side current is quasis-quare wave. AC side voltage is determined by the load.

No anti-parallel diodes are needed. sometimes series diodes are needed to block reverse voltage for other power semiconductor devices.

VT 1

VT 2

VT 3

VT 4 u o

L R i o

Ed

Ld

i d C

Single-phase bridge CSI

Parallel Resonant Inverter

VT 1

VT 2

VT 3

VT 4

u o L R i o

Ld

I d C

LT1

LT2 LT3 LT4

Switching frequency is a little higher

than the resonant frequency so that the

load becomes capacitive and load

current is leading voltage to realize

load commutation.

O

t O

O

O

O

O

O

O

u G 1 , 4

u G 2 , 3

i T

i o

I d

t 1 t 2

t 3 t 4

t 5

t 6

t 7 t

u o

t

u AB

t

t

I d

i VT 1 , 4 i VT 2 , 3

u VT 2 , 3

u VT 1 , 4

t

t

t

t

t

t

t

Three- phase self-commutated CSI

VT 2

U Ud

id

V W

VT 1 VT 3

VT 4

VT 5

VT 6

iU

iV

iW

uUV

t

t

t

t o

o

o

o

Three- phase force- commutated CSI

VD 1

L

C1

3

VT1

Ud

+

-

Id

C5

C3

C4

C2

C6

VT3 VT5

VT4 VT6 VT2

VD 3 VD 5

VD 4 VD 6 VD 2

U V W

M

VT1 VT3

Id

+ - C13

VD1 VD3

VD2

VT2

U V

W

Three- phase load-commutated CSI

O

O

O

O

O

O t

u u U u V u W

i V

i W

i U

u dM

VT2 u VT 1

t

t

t

t

t

VT6 VT5

VT1 VT3

VT4

Pulse Distributor

Ud UdM

VT1

VT3

VT5

VT4

VT6

VT2

U V W

BQ MS 3~

5.4 Multiple- inverter connections and multi-level inverters Series connection of 2 single- phase VSIs

120°

60°

180° t O

t O

t O

3rd Harmonics u 1

u 2

u o

3rd Harmonics

Ud

T1

T2

u 1

u 2

· ·

· ·

Series connection of 2 3- phase VSIs

U V W

N

T 1 A 1

T 2

B 1

A 21

C 1

B 21

A 22

C 21

B 22 C 22

u UN

u U2

u U1

U d

I

II

tO

tO

tO

tO

tO

31

31 )

(1+ )

5-24图

UU1

UA21

-UB22

UU2

UUN

(UA1) Ud

Ud

32 Ud

31 Ud

32 Ud

(1+ Ud

31 Ud

Multi-level Inverters 3- level inverter

+

-

+

U

V

+ W

U d

VD 1 V 11

O'

V 12

VD 11

VD 12

VD 4 V 41

V 42

VD 41

VD 42

i U L s

R s