AC to DC converters (Controlled) –

Thyristor Rectifiers

1 Introduction to Power Electronics Spring 2020

 Single phase controlled rectifier

Vs Vlo ad
Is

T
R=0

20.0
FP1

0.3
• Firing angle (triggering angle, delay angle – a is the common symbol) is adjusted using a
control circuit to delay or advance the turn on instance of the SCR to obtain variable output
voltage.
• Angle a is measured from the zero crossing point of the AC waveform that will lead to
conduction through SCR.
• SCR will turn off when its current goes to zero. This angle (g) is called the extinction angle. It is
also measured as for the a.

2 Introduction to Power Electronics Spring 2020

 Single phase controlled rectifier
Fig9
Vs Vlo ad
Is Vs
400

T
R=0

200

Vsource (V)
2
0

20.0
FP1 -200
-400

0.3
Is
6.0
4.0

Isource (A)
2.0
0.0
CPanel -2.0
Alpha Vloa d Vload
400
175
200

Vload (V)
Is 0
deg

-200
-400
2 Vs
Time ... 0.140 0.150 0.160 0.170 0.180 0.190 0.200 ...
107.5 ...
...

• Firing angle (triggering angle, delay angle – a is the common symbol) is adjusted using a
control circuit to delay or advance the turn on instance of the SCR to obtain variable output
voltage.
• Angle a is measured from the zero crossing point of the AC waveform that will lead to
conduction through SCR.
• SCR will turn off when its current goes to zero. This angle (g) is called the extinction angle. It is
also measured as for the a.
3 Introduction to Power Electronics Spring 2020
 Single phase controlled rectifier

4 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

• This circuit is the same as the three phase diode rectifier circuit but all diodes are
replaced with SCRs
• 6 gate pulse generation circuits are now required to trigger the SCRs (In PSCAD all
this can be simulated using the provided library. As the gate pulses have to be
synchronised to incoming mains using a phase locked loop)

ComBus
AM

Van 1 3 5
A Ia A A GM
1.0e-005
Vbn

20.0
B B B
Vload
Vcn
C C C AO

0.8
Alpha
4 6 2
KB 1.0

Alpha (firing angle) input is AO. Input KB=1 (ensures normal firing, this input
can be used to inhibit devices)

5 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier
ComBus
AM

Van 1 3 5
A Ia A A GM
1.0e-005
Vbn

20.0
B B B
Vload
Vcn
C C C AO

0.8
Alpha
4 6 2
KB 1.0

6 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

• The natural firing begins at wt = 30 degree of all incoming line-neutral voltage. So

care is needed in measuring the firing angle.
Fig10

400
Van Vbn Vcn
a = 0 degrees
200
gives the maximum
Vsource (V)

0
output voltage
-200
-400
Ia
30
20
10
Isource (A)

0
-10
-20
-30
Vload
600
500
400
Vload (V)

300
200
100
0
Time ... 0.940 0.950 0.960 0.970 0.980 ...
...
...

7 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

a = 30 degrees
gives a reduced
output voltage
compared to a = 0
degrees

8 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

Fig10
a = 30 degrees
Van Vbn Vcn
400
gives a reduced
200
output voltage
Vsource (V)

0
compared to a = 0
-200 degrees
-400
Ia
30
20
10
Isource (A)

0
-10
-20
-30
Vload
600
500
400
Vload (V)

300
200
100
0
Time ... 0.940 0.950 0.960 0.970 0.980 ...
...
...

9 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

a = 60 degrees
gives a reduced
output voltage
compared to a = 30
degrees

10 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

Fig10
Van Vbn Vcn
a = 60 degrees
400
200
gives a reduced
output voltage
Vsource (V)

0
compared to a = 30
-200
degrees
-400
Ia
30
20
10
Isource (A)

0
-10
-20
-30
Vload
600
500
400
Vload (V)

300
200
100
0
Time ... 0.940 0.950 0.960 0.970 0.980 ...
...
...

11 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

 2 a
1
  6 a
v load( avg )  3 Vm sin( w t  30 0
)d(wt )
( )
3 1.35VLL
3 3
v load( avg )  Vm cos a

3 2 a=0 a=90 a=180
 VLL cos a

 1.35 VLL cos a

-1.35VLL
An important observation:
The derivation implies negative output voltage for a > 90 deg. This should correspond to
power flow from the dc load to ac source indicating that there should be an appropriately
connected dc power source on the load side as the direction of dc current cannot
change through the converter.

12 Introduction to Power Electronics Spring 2020

 Three phase controlled rectifier

• This mode of operation is called inversion as dc power is fed back to the ac mains.
• This is commonly used in regenerative braking of DC motors where energy stored in
the armature is fed back to ac mains. Also this mode is used in HVDC transmission
systems where DC power is converted to AC power
Iload(avg)

+ Iload(avg) -
Vload(avg)

Vload(avg)

- +
90 > a > 0 180 > a > 90
Power transfer from ac to dc Power transfer from dc to ac
(rectification) (inversion)
13 Introduction to Power Electronics Spring 2020
 Three phase controlled rectifier

a = 120 degrees
gives a negative
output voltage

14 Introduction to Power Electronics Spring 2020