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Thyristor Converters
Chapter 6
• In some applications (battery charger, some ac/dc drives),
the dc voltage has to be controllable
• Thyristor converters provide controlled conversion of ac
into dc
• Primarily used in three-phase, high power application
• Being replaced by better controllable switches
6-1
Thyristors (Review Class)
• Semi-controlled device
• Latches ON by a gate-current pulse
if forward biased
• Turns-off if current tries to reverse
6-2
Thyristor in a Simple Circuit (Review Class)
•
For successful turn-off, reverse voltage required
6-3
Thyristor Converters
• Fully controlled converter shown in Fig. 6-1a
• Average dc voltage Vd can be controlled from a positive maximum to a
negative minimum on a continuous basis
• The converter dc current Id can not change direction
• Two-quadrant operation
• Rectification mode (power flow is from the ac to the dc side): +Vd & +Id
• Inverter mode (power flow is from the dc to the ac side): : -Vd & +Id
• Inverter mode of operation on a sustained basis is only possible if a source of
6-4
power, such as batteries, is present on the dc side.
• Basic thyristor circuits: Line-frequency voltage source connected to a load
resistance
• In the positive half cycle of vs, the current is zero until wt=a, at which a
gate pulse of a short duration is applied
• With the thyristor conducting, vd = vs
• vd becomes zero at wt = p
• By adjusting the firing angle a, the average dc voltage Vd and current Id
can be controlled
6-5
o Basic thyristor circuits: Line-frequency voltage source connected to a RL
load
o Initially, the current is zero until wt=a, at which the thyristor is fired
during the positive half cycle of vs
o With the thyristor conducting, current begins to flow, vd = vs
o Voltage across the inductor: vL=vs-vR
o During a to q1, vL is positive, and the current increases
o Beyond q1, vL is negative, and the current begins to decline
 q2 is the instant at which current becomes zero and stays at zero until 2p+a
at which the thyristor is fired again
6-6
o Basic thyristor circuits: The load consists of L and a dc voltage Ed
o The thyristor is reverse biased until q1
o The thyristor conduction is further delayed until q2 at which the thyristor is
fired
o With the thyristor conducting, vd = vs
o Between q2 to q3, vL is positive, and the current increases
o Beyond q3, vL is negative, and the current begins to decline
o When A1 is equal to A2, current goes to zero at q4
6-7
Thyristor Gate Triggering
• Generation of the firing signal
• The sawtooth waveform
(synchronized to the ac input) is
compared with the control signal
vcontrol, and the delay angle a with
respect to the positive zero crossing
of the ac line voltage is obtained in
terms of vcontrol and the peak of the
sawtooth waveform Vst.
o  v control
a  180 
o
 V st




6-8
Full-Bridge (Single- and Three-Phase) Thyristor Converters
6-9
Single-Phase Thyristor Converters
• One thyristor of the top group and one of the bottom group will conduct
• If a continuous gate pulse is applied then this circuit will act like a full
bridge diode rectifier and the web forms are as shown below
 a=0 for 1 and 2 and a=p for thyristors 3 and 4
6-10
1-Phase Thyristor Converter Waveforms
• Assumptions: Ls=0 and purely dc current
Id
 a: delay angle or firing angle
• Prior to wt=0, current is flowing through 3
and 4, and vd = -vs
• Beyond wt=0, thyristors 1 and 2 become
forward biased, but cannot conduct until
a.
• vd becomes negative between 0 and a as a
consequence of the delay angle
• At wt=a, gate pulse applied and current
commutation from thyristors 3 and 4 to 1
and 2 is instantaneous (Ls = 0), and vd = vs
• Thyristors 1 and 2 will keep conducting
until 3 and 4 are fired
6-11
Average dc Voltage as a Function of the Delay Angle
The expression for the average voltage Vd:
Vda 
1
a p

a
p
2Vs sin wt d wt   0.9Vs cos a
Let Vd0 be the average dc voltage with a=0,
Vd 0 
1
p
p
2Vs sin wt d wt   0.9Vs
0
Then, drop in average voltage due to a,
Vda  Vd 0  Vda  0.9Vs 1  cos a 
The average power through the converter,
1T
1T
P   pt dt   v d i d dt
T 0
T 0
With a constant dc current (id=Id),
1 T

P  I d   v d dt   I d V d  0.9V s I d cos a
T 0

6-12
Average dc Output Voltage
Vda 0.9V s cos a

 cos a
Vd 0
0.9V s
The variation of Vd as a function of a:
Average dc voltage is positive until a=90o: this region is called
the rectifier mode of operation
Average dc voltage becomes negative beyond a=90o: this
region is called the inverter mode of operation
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1-Phase Thyristor Converter
o AC side inductance is included, which generally cannot be ignored
in practical thyristor converters.
o For a given delay angle, there will be a finite commutation interval
o Commutation process is similar to that in diode bridge rectifiers
o During the commutation interval, all four thyristors conduct, and
therefore, vd=0, and the voltage vLs=vs.
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1-Phase Thyristor Converter
o During the commutation interval, all four thyristors conduct, and
therefore, vd=0, and the voltage vLs=vs.
di s
v s  v Ls  L s
dt

a 
A  
a
a 
Id
2V s sin wt dt  wL s  di s  2wL s I d
Id
2V s sin wt dt  2V s cos a  cosa     2wL s I d

A  


2wL s I d
  cos cos a 
2V s

a
1

 a

6-15
1-Phase Thyristor Converter: with and without Ls
without Ls
with Ls
o Voltage drop due to the inclusion of Ls.
V d 
A
p

2wL s I d
p
V d  0   0.9V s cos a
V d   0   0.9V s cos a 
2wL s I d
p
6-16
Example
In the converter circuit, Ls is 5% with the rated voltage of
230 V at 60 Hz and the rated volt-ampere of 5 kVA.
Calculate the commutation angle  and Vd/Vd0 with the
rated input voltage, power of 3 kW, and a=30o.
6-17
Solution
5000
 21 .74 A
230
V
 rated  10 .58 
I rated
I rated 
Z base
0.05 Z base
 1.4 mH
377
a  30 0
Ls 
2


Pd  V d I d  0.9V s cos a  wL s I d  I d  3000
p


I d  17 .3 A

2wL s I d 
0
  cos cos a 
  a  5.9
2V s 

2
V d  0.9V s cos a  wL s I d  173 .5 V
1
p
6-18
Thyristor Converters: Inverter Mode (Vd is negative)
• Average value of vd is negative for
90o<a<180o. Average power Pd is
negative (Pd=VdId) and thus power
flows from the dc to the ac side
• On the ac side, Pac=VsIs1cosf1 is
also negative because f1>90o
• Inverter mode of operation is
possible because there is a source
of energy on the dc side
• ac side voltage source provides
commutation of current from one
pair of thyristors to the others
6-19
3-Phase Thyristor Converters
• Current Id flows through the one thyristor of the top group and one of the
bottom group
• If a continuous gate pulse is applied then this circuit will act like a threephase full bridge diode rectifier and, as a result,
Vd 0  1.35 VLL
6-20
3-Phase Thyristor Converter Waveforms
6-21
Average Output DC Voltage
V da  V d 0 
A
p 3
V ac  2V LL sin wt 
The reduction in the average dc voltage due to the delay anglea
a
A   2V LL sin wt d wt   2V LL 1  cos a 
0
 V da
2V LL 1  cos a 
A
 Vd 0 
 1.35V LL 
p 3
p 3
 1.35 V LL cos a  1.35 V d 0
Average Power
Pda  V da I d  1.35 V LL I d cos a
6-22
dc-side voltage waveforms
as a function of a
Vd repeats at six times the
line frequency
6-23
Conclusions
• Thyristor converters provides controlled transfer of power
between the line frequency ac and adjustable-magnitude dc
• By controlling a, transition from rectifier to inverter mode
of operation can be made and vice versa
• Thyristor converters are mostly used at high-power levels
• Thyristor converters inject large harmonics into the utility
system
6-24