chapter 5 dc to ac converters outline 5.1 commutation 5.2 voltage source inverters 5.3 current...
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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
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