1. 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

2. Thyristors (Review Class)
Semi-controlled device
Latches ON by a gate-current pulse if forward biased
Turns-off if current tries to reverse

3. Thyristor in a Simple Circuit (Review Class)
For successful turn-off, reverse voltage required

4. 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 power, such as batteries, is present on the dc side.

5. 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. Basic thyristor circuits: Line-frequency voltage source connected to a RL load
Initially, the current is zero until wt=a, at which the thyristor is fired during the positive half cycle of vs
With the thyristor conducting, current begins to flow, vd = vs
Voltage across the inductor: vL=vs-vR
During a to q1, vL is positive, and the current increases
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

7. Basic thyristor circuits: The load consists of L and a dc voltage Ed
The thyristor is reverse biased until q1
The thyristor conduction is further delayed until q2 at which the thyristor is fired
With the thyristor conducting, vd = vs
Between q2 to q3, vL is positive, and the current increases
Beyond q3, vL is negative, and the current begins to decline
When A1 is equal to A2, current goes to zero at q4

8. 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.

9. Full-Bridge (Single- and Three-Phase) Thyristor Converters

10. 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

11. 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

12. The expression for the average voltage Vd:
Average dc Voltage as a Function of the Delay Angle
The expression for the average voltage Vd:
Let Vd0 be the average dc voltage with a=0,
Then, drop in average voltage due to a,
The average power through the converter,
With a constant dc current (id=Id),

13. Average dc Output Voltage
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

14. 1-Phase Thyristor Converter
AC side inductance is included, which generally cannot be ignored in practical thyristor converters.
For a given delay angle, there will be a finite commutation interval
Commutation process is similar to that in diode bridge rectifiers
During the commutation interval, all four thyristors conduct, and therefore, vd=0, and the voltage vLs=vs.

15. 1-Phase Thyristor Converter
During the commutation interval, all four thyristors conduct, and therefore, vd=0, and the voltage vLs=vs.

16. 1-Phase Thyristor Converter: with and without Ls
without Ls with Ls
Voltage drop due to the inclusion of Ls.

17. 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 m and Vd/Vd0 with the rated input voltage, power of 3 kW, and a=30o.

18. Solution

19. 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

20. 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 three-phase full bridge diode rectifier and, as a result,

21. 3-Phase Thyristor Converter Waveforms

22. Average Output DC Voltage

23. dc-side voltage waveforms as a function of a
Vd repeats at six times the line frequency

24. 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