Thyristor Converters
EE 442-642
6-1
Thyristor Converters
• Two-quadrant conversion
6-2
Simple half-wave circuits with thyristors
6-3
Thyristor Triggering
vcontrol
  180
Vˆst
o
o
• ICs available
6-4
Case of Pure Resistive Load
6-5
Full-Bridge Thyristor Converters – Constant DC Current
6-6
DC-Side Voltage
Average DC voltage:
Vd  Vdo cos 
where
Vdo  0.9Vs
6-7
AC-Side Current
P  Vd I d  0.9Vs I d cos 
RSM value of source current
Is  Id
RMS value of fundamental current
I s1  (2 2 /  ) I d  0.9 I d
RMS value of harmonic current
I sh  I s1 / h, h  3,5,7,...
Current THD
THD  100 ( 2 / 8)  1  48.43%
Displacement Power Factor
DPF  cos 
PF  0.9 cos 
Power Factor
6-8
Effect of Source Inductance
2Ls I d
2Vs
Commutation angle:
cos(   )  cos  
Average of DC-side voltage:
Vd  0.9Vs cos  
Displacement Power Factor
DPF  cos(  0.5 )
RMS fundamental current
Vd I d
0.9Vs I d cos   (2 /  )Ls I d2
I s1 

Vs DPF
Vs cos(  0.5 )
2Ls I d

6-9
Thyristor Converter with DC Source
Continuous current conduction mode
Discontinuous current conduction mode
6-10
AC-Side Current Waveform
(continuous conduction mode)
PSpice-based simulation example: Vs = 240 V, f = 60 Hz,
Ls = 1.4 mH, α = 45 deg., Ld = 9 mH, Ed = 145 V.
Solution: Is = 60.1 A, Is1 = 59.7 A, DPF = 0.576, PF =
0.572, THD = 12.3%
6-11
DC Voltage versus Load Current
6-12
Inverter Mode (α > 90o)
6-13
Inverter Mode with DC Voltage Source
• For a large value of Ld, id can be assumed constant (= Id), then
Ed  Vd  0.9Vs cos  
2

LS I d
6-14
Inverter Mode: Extinction Angle
  180o  (   )
Importance of extinction angle in inverter mode: The extinction time
interval should be greater than the thyristor turn-off time:

t   t q

6-15
3-Phase Thyristor Converters: Simplified Case
6-16
DC-side voltage waveforms
assuming zero ac-side inductance
Vd  Vdo cos 

3

2VLL cos 
 1.35VLL cos 
6-17
Input Line-Current Waveform
6-18
Input line-current waveforms assuming zero ac-side inductance
I s  2 / 3I d  0.816 I d
I s1  ( 6 /  ) I d  0.78 I d
I sh  I s1 / h, h  3,5,7,...
THD  100[ ( 2 / 9)  1]  31%
DPF  cos 
3
PF  cos   0.955 cos 

6-19
3-Phase Thyristor Converter with AC-side Inductance
2Ls I d
cos(   )  cos  
2VLL
Vd  1.35VLL cos  
3Ls I d
DPF  cos(  0.5 )

6-20
Input Line-Current Harmonics
6-21
Input Line-Current Harmonics
Typical Passive Filter Block (for each phase)
6-22
12-Pluse Phase Controlled Rectifier
Harmonic Order: 1, 11, 13, 23, 25, …
6-23
3-Phase Thyristor Converter with Realistic Load
Continuous conduction
Mode
Discontinuous conduction
mode
6-24
3-Phase Thyristor Inverter – Constant Current
6-25
Thyristor Inverter – Constant Voltage & Current
6-26
Thyristor Inverter Operation: Extinction Angle
6-27
Thyristor Converters: Voltage Notching
Ls1  Ls
Ls 2  0
Depth:
Vn  2VLL sin 
Area:
An  2Ls I d
Width:
2Ls I d

2VLL sin 
6-28
Limits on Notching and Distortion
In practice, the notch depth at PCC depends on Ls1 relative to Ls2. Let depth
factor be defined by

Ls1
Ls1  Ls 2
Given Ls1 , a higher value of Ls2 results in a smaller notch.
6-29