Power Electronics
Chapter 5
DC to AC Converters
( Inverters )
Power Electronics
Applications of Inverters
Conversion of electric power from DC type energy
sources to AC type load
– Battery
– Photovoltaic cell (Solar cell)
– Fuel cell
As a part of composite converter
– AC-DC-AC frequency converter (for AC motor drive)
– AC-DC-AC constant-voltage constant-frequency converter (for
uninterruptable power supplies)
– AC-DC-AC Converters for induction heating
– AC-DC-AC-DC switching power supplies
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Power Electronics
Outline
5.1 Commutation
5.2 Voltage source inverters
5.3 Current source inverters
5.4 Multiple-inverter connections and multi-level inverters
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Power Electronics
5.1 Commutation types
Basic operation principle of inverters
uo
S1
Ud
S2
io
Load
uo
S3
S
4
io
t1 t2
t
A classification of inverters
– Square-wave inverters (are discussed in this chapter)
– PWM inverters ( will be discussed in Chapter 6)
The concept of commutation
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Power Electronics
4 types of commutation
Device commutation:
Fully-controlled devices: GTO, IGBT, MOSFET
Line commutation
Phase-controlled rectifier
Phase-controlled AC controller
Thyristor cycloconverter
Load commutation
Forced commutation
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Power Electronics
Load commutation
Condition: Load current is leading load voltage
Application: capacitive load, synchronous motor
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Power Electronics
Forced commutation
(capacitance commutation)
Direct-Coupled
With Coupling-Inductor
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Power Electronics
Another classification of commutations
4 types of Commutations
Device commutation
Self-commutation
For fully-controlled
devices
Forced commutation
Line commutation
External
commutation
For thyristors
Load commutation
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Power Electronics
2 classes of inverters
Voltage Source Inverter
(VSI)
Current Source Inverter
(CSI)
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Power Electronics
5.2 Voltage source inverter (VSI)
+
V3
VD 1
C
V1
R io
Ud
VD3
L
uo
V2
VD 2
VD4
V4
-
Features
DC 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)
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Power Electronics
Single-phase half bridge VSI
V1
Ud
2
VD
io R
Ud
Ud
2
1
L
uo
VD
V2
2
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.
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Power Electronics
Single-phase full bridge VSI
Operation principle
+
V3
VD 3
VD1
C
V1
R io
Ud
L
uo
V2
VD2
VD 4
V4
-
The magnitude of output square-wave voltage is Ud.
The effective value of output voltage (or fundamental
output voltage) can be changed by changing Ud.
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Power Electronics
Single-phase full bridge VSI
Quantitative analysis
Fourier series extension of output voltage
4U d ⎛
1
1
⎞
t
+
t
+
t
+
"
ω
ω
sin
5
ω
sin
3
sin
(5-1)
⎜
⎟
π ⎝
5
3
⎠
Magnitude of output voltage fundamental component
uo =
U o1m =
4U d
π
= 1.27U d
(5-2)
Effective value of output voltage fundamental component
U o1 =
2 2U d
π
= 0.9U d
(5-3)
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Power Electronics
Single-phase full bridge VSI
Output voltage control by phase-shift
uG1
+
O
V3
VD1
C
V1
R io
U
d
L
uo
V2
VD2
-
VD 3
VD 4
V4
t
uG2
O
uG3
O
uG4
t
θ
t
O
uo
io
O
t
io
t1 t2
uo
t3
t
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Power Electronics
Inverter with center-tapped transformer
—push-pull inverter
Load
io
uo
+
Ud
-
V1
V2
VD1
VD2
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Power Electronics
Three-phase VSI
180o conduction
Dead time (blanking time) to
avoid “shoot through”
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Power Electronics
Three-phase VSI
Basic equations to obtain voltage waveforms
For line voltage
For phase voltage of the load
U UN + U VN + U WN = 0
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Power Electronics
Three-phase VSI
Quantitative analysis
Fourier series extension of output line-to-line voltage
u UV =
=
2 3U d ⎛
1
1
1
1
⎞
sin 11ω t +
sin 13 ω t − " ⎟
⎜ sin ω t − sin 5ω t − sin 7ω t +
π
13
5
7
11
⎝
⎠
2 3U d ⎡
⎢ sin ω t +
π
⎣
∑
n
⎤
1
( − 1) k sin n ω t ⎥
n
⎦
(5-8)
Magnitude of output voltage (line-to-line) fundamental component
2 3U d
U UV1m =
= 1.1U d
(5-10)
π
Effective value of output voltage (line-to-line) fundamental
component
U UV1 =
U UV1m
2
=
6
π
U d = 0.78U d
(5-11)
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Power Electronics
5.3 Current source inverter (CSI)
Features
DC side is constant
current, high impedance
(current source, or large
inductor)
AC side current is quasisquare 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.
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Power Electronics
Single-phase bridge CSI
Parallel Resonant Inverter
Ld
Id
A
VT1
VT3
C
LT1
io
LT2
R
VT2
LT3
uo
LT4
L
VT4
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.
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Power Electronics
Three-phase self-commutated CSI
120o conduction
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Power Electronics
Three-phase force-commutated CSI
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Power Electronics
Three-phase load-commutated CSI
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Power Electronics
5.4 Multiple-inverter connections
and multi-level inverters
Series connection of 2 single-phase VSIs
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Power Electronics
Series connection of 2 3-phase VSIs
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Power Electronics
Multi-level Inverters
3-level inverter
26