LECTURE 26 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics Principles of Three-Phase HalfWave Converters  Three-phase converters provide higher average output voltage, and the frequency of.

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Transcript LECTURE 26 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics Principles of Three-Phase HalfWave Converters  Three-phase converters provide higher average output voltage, and the frequency of.

LECTURE 26
Controlled Rectifiers
Dr. Rostamkolai
ECE 452
Power Electronics
1
Principles of Three-Phase HalfWave Converters

Three-phase converters provide higher average
output voltage, and the frequency of the ripples
on the output voltage is higher compared to that
of single-phase converters

Therefore, the filtering requirements for
smoothing out the load current and load voltage
are simpler
2
3

If the phase voltage is van  Vm sin  t , then the
average output voltage for a continuous current
will be:
3
Vdc 
2

5 / 6  

/6 
Vm sin t d (t ) 
3 3 Vm
cos
2
The rms value of the output voltage will be:
Vrms
 3

 2
Vrms
1
3
 3 Vm ( 
cos 2 )1/ 2
6 8
5 / 6  

/6 

2
2
Vm sin  t d ( t )

1/ 2
4

If α ≥ 30o, then the average output voltage for a
resistive load will be:
3
Vdc 
2



/6 
Vm sin t d (t ) 
3Vm
2



1

cos(


)


6
The rms value of the output voltage will be:
1/ 2
Vrms
 3

 2

V
sin

t
d
(

t
)
 / 6  

Vrms

1

5

 3 Vm  

sin(  2 )
3
 24 4 8


2
m
2
1/ 2
5
Three-Phase Full Converters

Three-phase converters are extensively used in
industrial applications up to 120 kW level, where
two-quadrant operation is required

This circuit is known as a three-phase bridge
converter

The thyristors are fired at an interval of 60
degrees
6

The frequency of the output ripple voltage is 6fs

The following figure shows a full converter
circuit with a highly inductive load
7
8

The line-to-neutral voltages are:
van  Vm sin  t
2
vbn  Vm sin ( t  )
3
2
vcn  Vm sin ( t 
)
3

Then the line-to-line voltages are:

vab  van  vbn  3 Vm sin ( t  )
6

vbc  vbn  vcn  3 Vm sin ( t  )
2
5
vca  vcn  van  3 Vm sin ( t  )
6
9

The average output voltage is found from:
Vdc 

3
 / 2 
 
/ 6 
vab d ( t ) 
3 3 Vm

cos
The rms value of the output voltage is:
1/ 2
Vrms
3

2
 
vab
d ( t )
   / 6 

Vrms
1 3 3
 3 Vm ( 
cos 2 )1/ 2
2 4
 / 2 
10

A three-phase bridge gives a six-pulse output
voltage

For high-power applications such as highvoltage dc transmission, a 12 pulse output is
generally required to reduce the output ripples
and to increase the ripple frequencies

Two six-pulse bridges can be combined either in
series or in parallel to produce a 12-pulse output
11
Three-Phase Dual Converters

In variable-speed drives application, the four
quadrant operation is generally required

Three-phase dual converters are extensively used
in applications up to 2 MW

The following figure shows three-phase dual
converters where two three-phase converters are
connected back-to-back
12
13

Due to instantaneous voltage differences
between the output voltages of converters, a
circulating current flows through the converters

This circulating current is limited by a reactor

The two converters are controlled in such a way
that if α1 is the delay angle of converter 1, the
delay angle of converter 2 is α2 = π- α1
14

The following figure shows the waveforms for
input voltages, output voltages, and the voltage
across the inductor
15
16
Pulse-Width-Modulation Control

The power factor of phase-controlled converters
depends on delay angle , and it is generally low
at low output voltage range

These converters inject harmonics into the
supply

Forced commutation can improve the input
power factor and reduce the harmonics levels
17
18

With advancement of devices (GTO or IGBT),
the forced commutation can be implemented in
practical systems

Now we will discuss the basic techniques of
forced commutation which are:
Extinction Angle Control
 Symmetrical Angle Control
 Pulse-Width Modulation
 Single-Phase Sinusoidal Pulse-Width Modulation
 Three-Phase PWM Control

19
Extinction Angle Control
20
21
Symmetrical Angle Control
22
Pulse-Width Modulation Control
23
Single-Phase Sinusoidal PulseWidth Modulation
24