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Chapter 6
Thyristor Converters
• Thyristor circuits and their control
• Line-frequency phase-controlled rectifiers and inverters
• line-frequency ac <-> controlled dc
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-1
Thyristor Converters
• Two-quadrant conversion
• positive Vd implies rectification
• negative Vd implies inverter mode
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-2
Primitive circuits with thyristors (1)
R load
• The current is zero until t=
• The current and voltage vd are in phase
• Average voltage of load R controlled by adjusting 

vd
Copyright © 2008
by Jose Bastos
1
V

V sin x dx 
(1  cos )

2 
2
Chapter 6 Thyristor Converters
6-3
Primitive circuits with thyristors (2)
R L load
v
R
• The current is zero until t=
• The current lags voltage vd
• At Instant 2 current becomes zero
di
vL  L
 vd  v R
dt
t
1
v ()  vR ()d 
i (t ) 
L  d
1
i ( 2 ) 
L
Copyright © 2008
by Jose Bastos
1
1


v
(

)

v
(

)
d


R
 d
L
2
 v
d
()  vR ()d  0
1
Chapter 6 Thyristor Converters
6-4
Primitive circuits with thyristors (3)
L Load with dc voltage
di
v L  L  vd  E d
dt
i (t ) 
t
 v
d
( )  Ed d
2
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-5
Thyristor Gate Triggering
Delay angle  is
vcontrol
  180
Vˆst
o
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-6
Full-Bridge Thyristor Converters (1)
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-7
Full-Bridge Thyristor Converters (2)
• constant dc current load
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-8
Thyristor Converter Waveforms (1)
 0
• Assuming zero ac-side inductance
• at t=0 thyristors 1,2 are triggered
•At t=  thyristors 3,4 are triggered
vd 
Copyright © 2008
by Jose Bastos
1


 V sin( t )d (t ) 
0
2V


2 2

VRMS  0.9VRMS
Chapter 6 Thyristor Converters
6-9
Thyristor Converter Waveforms (2)
 0
vd 
1

 

Copyright © 2008
by Jose Bastos
V sin( t )d (t ) 
2V

cos  
2 2

VRMS cos   0.9VRMS cos 
Chapter 6 Thyristor Converters
6-10
Thyristor Converter Waveforms (3)
• The average power in the converter is
T
1
P   vd id dt
T 0
T
1
 I d  vd dt  I d vd  0.9 I d VRMS cos 
T 0
• Average power becomes negative when  > 90º ; this is inverter mode
of operation
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-11
Inverter mode
  90
º
• 90º <  < 180º
• average vd is a negative value
• on dc side average power <vd>Id is negative
• on ac side average power VS RMS Is1 RMS cos 1 is negative
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-12
Average DC Output Voltage
• Assuming zero ac-side inductance
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-13
Input Line-Current Waveform
• input line current is harmonics
is (t )  I s1 (sin t   )  I s 3 (sin 3t   )  I s 5 (sin 5t   )  
with
I s1 
2

Copyright © 2008
by Jose Bastos
2 I d  0.9 I d
I sh 
I s1
h
Chapter 6 Thyristor Converters
6-14
Power, Power Factor
• Power
P  Vs _ RMS I s1_ RMS cos   0.9 Vs _ RMS I d cos 
• Power Factor PF
I s1_ RMS
I
P
PF 

DPF  s1 cos   0.9 cos 
Vs _ RMS  I s _ RMS I s _ RMS
Is
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-15
Effect of Ls
• Finite ac-side inductance; constant dc output
current
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-16
Finite ac-side inductance (1)
• finite comutation interval u
• during u all thyristors conduct (vd=0)
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-17
Finite ac-side inductance (2)
dis
dt
to find u, integrate over the comutation interval :
vd  0  v s  v L  L
Id
 u
 V sin( t ) d (t )   L  di
s
 2 L Id
Id
 u
Au 
 V sin( t ) d (t )  2  L I
d
Au  V cos( )  cos(  u )  2  L I d
2  L Id
V
vd can be obtained by
cos(  u )  cos( ) 
 
 u

1
vd    V sin (t ) d (t )   V sin( t ) d (t )



2  L Id
2V

cos  


2 2 VRMS cos  2  L I d
2


 0.9 VRMS cos    L I d



Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-18
Input line current is
• Is has a trapezoidal waveform
• DPF is
DPF  cos 1  cos(  u / 2)
• The RMS value of fundamental frequency is
Vs RMS I s1 RMS DPF  vd I d
I s1 RMS 
Copyright © 2008
by Jose Bastos
0.9 Vs RMS I d cos   2 /  L I d2
Vs RMS cos(  u / 2)
Chapter 6 Thyristor Converters
6-19
Practical Thyristor Converter
• waveform for =45º
• average value of vd
Copyright © 2008
by Jose Bastos
vd  0.9 Vs RMS cos  
Chapter 6 Thyristor Converters
2

 L I d ,min
6-20
Inverter mode with finite AC inductance
• 90º <  < 180º
Copyright © 2008
by Jose Bastos
Chapter 6 Thyristor Converters
6-21