power electronics engineering UNIT IV.ppt

What is an inverter?
• Fixed DC power can be converted into AC
power at desired output voltage and
frequency by using a power electronics circuit,
called an inverter.
• DC input can be a rectifier, battery or fuel cell

Applications of inverter
• Adjustable speed drive
• UPS
• Induction heating
• HVDC transmission
• Solar and wind energy conversion systems

Types of inverter
There are two common types of inverters,
• i) Voltage Source Inverters and
•ii) Current Source Inverters.
Single Phase Inverter
Half bridge inverter
Full Bridge Inverter
Three phase inverter
Series and parallel Inverter

Why thyristors are not preferred in
inverter circuit?
The reverse withstanding capability of thyristor
is very less.
Thyristors in inverter circuit requires forced
commutation.

Difference between voltage and
current source inverter
S.No Voltage Source Inverter Current Source Inverter
1. DC source with small resistance
forms fixed dc voltage
DC Source with high resistance forms
fixed dc current
2. Large value of capacitor connected
across the source to maintain
constant voltage
Large value of inductor connected in
series with the source to maintain
constant current
3. Any change in load voltage does
not affect the source voltage
Any change in load current does not
affect the source current

ECE 442 Power Electronics 7
Single-phase half-bridge inverter

Operational Details
• Consists of 2 choppers, 3-wire DC source
• Transistors switched on and off alternately
• Need to isolate the gate signal for Q1 (upper device)
• Each provides opposite polarity of Vs/2 across the load
3-wire DC source

Q1 on, Q2 off, vo = Vs/2
Peak Reverse Voltage of Q2 = Vs

Q1 off, Q2 on, vo = -Vs/2

Waveforms with resistive load

Look at the output voltage
1
2
2
2
0
2
4 2
o
T
s s
o
o
V V
V dt
T
 
 
 
 
 
 

rms value of the output voltage, Vo

Fourier Series of the instantaneous output
voltage
 
1
0
0
1,3,5,..
cos( ) sin( )
2
, 0
1
sin( ) ( ) sin( ) ( )
2 2
2
1,3,5,...
2
sin( )
o
o n n
n
o n
s s
n
s
n
s
o
n
a
v a n t b n t
a a
V V
b n t d t n t d t
V
b n
n
V
v n t
n


 
   









  

 

 
 
 
  


 


rms value of the fundamental component
1,3,5,..
1
1
2
sin
2
1
2
0.45
s
o
n
s
o
o s
V
v n t
n
V
V
V V










When the load is highly inductive

Turn off Q1 at t = To/2
Current falls to 0 via D2, L, Vs/2 lower
+
Vs/2
-
+
Vs/2
-

Turn off Q2 at t = To
Current falls to 0 via D1, L, Vs/2 upper
+
Vs/2
-
+
Vs/2
-

Load Current for a highly inductive load
Transistors are only switched on for a quarter-cycle, or 90

Fourier Series of the output current for an RL
load
2 2
1,3,5,...
1
2
sin( )
( )
tan ( )
o o s
o n
n
n
v v V
i n t
Z R jn L n R n L
n L
R
 
  





   
 



Fundamental Output Power
In most cases, the useful power
2
1 1 1 1 1
2
1 2 2
cos
2
2 ( )
o o o o
s
o
P V I I R
V
P R
R L

 
 
 
 
  
 

DC Supply Current
• If the inverter is lossless, average power absorbed
by the load equals the average power supplied by
the dc source.
• For an inductive load, the current is approximately
sinusoidal and the fundamental component of the
output voltage supplies the power to the load.
Also, the dc supply voltage remains essentially at Vs.
0 0
( ) ( ) ( ) ( )
T T
s s o o
v t i t dt v t i t dt

 

1 1
0 0
1
1
1
( ) 2 sin( ) 2 sin( )
cos( )
T T
s o o s
s
o
s o
s
i t dt V t I t dt I
V
V
I I
V
  

  

 
DC Supply Current (continued)

Performance Parameters
• Harmonic factor of the nth harmonic (HFn)
1
on
n
o
V
HF
V
 for n>1
Von = rms value of the nth harmonic component
V01 = rms value of the fundamental component

Performance Parameters (continued)
• Total Harmonic Distortion (THD)
• Measures the “closeness” in shape between a
waveform and its fundamental component
1
2 2
2,3,...
1
1
( )
on
n
o
THD V
V


 

• Distortion Factor (DF)
• Indicates the amount of HD that remains in a
particular waveform after the harmonics have been
subjected to second-order attenuation.
1
2 2
2
2,3,...
1
2
1
1 on
n
o
on
n
o
V
DF
V n
V
DF
V n


 
 
  
 
 
 
 


for n>1

• Lowest order harmonic (LOH)
• The harmonic component whose frequency is
closest to the fundamental, and its amplitude
is greater than or equal to 3% of the
amplitude of the fundamental component.

Single-phase full-bridge inverter

Operational Details
• Consists of 4 choppers and a 3-wire DC source
• Q1-Q2 and Q3-Q4 switched on and off alternately
• Need to isolate the gate signal for Q1 and Q3 (upper)
• Each pair provide opposite polarity of Vsacross the load

Q1-Q2 on, Q3-Q4 off, vo = Vs
+ Vs -

Q3-Q4 on, Q1-Q2 off, vo = -Vs
- Vs +

When the load is highly inductive
Turn Q1-Q2 off – Q3-Q4 off

Turn Q3-Q4 off – Q1-Q2 off

Load current for a highly inductive load

180 degree mode of operation
• In 1800
conduction scheme,each device
conducts for 1800
. They are turned ON at
regular interval of 600
in the sequence
Q1,Q2,Q3,Q4,Q5,Q6.The output terminals A B and
C of this bridge are connected to the terminals
of a 3-phase star or delta connected load.

Hence, load voltages are
VAN=V/3,
VBN=-2V/3,
VCN=V/3
Line volages are
VAB=VAN-VBN=V,
VBC=VBN-VCN=-V ,
VCA=VCN-VAN=0

power electronics engineering UNIT IV.ppt

1200
CONDUCTION
• In 1200
conduction scheme each device conducts for 1200
.It is
preferable for a delta connected load because it provides a six step
waveform across any phase.As each device conducts for 1200 , only
two devices are in conduction state at any instant.
• During the period 00
to 600
, Q1 to Q6 are conducting. Load terminals
A is connected to positive terminal and load terminal B is connected
to negative terminal of the source. Load terminal C is in floating
state.
• Phase Voltages=Line Voltages
• VAB= V
• VBC= -V/2
• Vca= -V/2

Concept of pulse width modulation
In this method, a fixed AC voltage is given
to the converter and controlled DC output
voltage is obtained by adjusting the ON and OFF
periods of the pulses. This is the most popular
method of controlling the output voltage and
this method is termed as pulse width
modulation control.

M modulation index and output voltage
expression is

Multiple PWM
• The harmonic content can be reduced using
several pulses in each half cycle of output
voltage.

Current Source Inverter
• DC side is constant current, high impedance
(current source, or large inductor)
• AC side current is quasi square 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.

Single phase current source inverter
( Auto Sequential current source
Inverter ASCI
0

Mode II EQUIVALENT CIRCUIT OF
Mode 2

Three phase self commutated CSI

Three phase current source inverter

Thyristor Th1 and Th2 conducting

power electronics engineering UNIT IV.ppt

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