AC-to-DC Rectifier LAB3 - Operating Principle

Figure 19 shows the diagram of a three-phase thyristor bridge rectifier.
Observe that the circuit topology is the same as that of a power diode three-phase full-wave rectifier (see figure 14), except that all diodes are replaced with thyristors.
Using thyristors instead of diodes in a three-phase full-wave rectifier allows the beginning of the conduction interval of each thyristor to be delayed, and thereby, the values of the average (DC) voltage and current at the rectifier output to be varied.
The operation of the thyristor three-phase rectifier is studied in this laboratory.

Figure 19: Three-phase Thyristor-based Rectifier

The operation of a power diode three-phase full-wave rectifier can be reproduced in a thyristor three-phase bridge by firing each thyristor at the same instant as the corresponding diode in a three-phase full-wave rectifier naturally enters into conduction.
This is achieved by using a firing angle (α) of 0° (i.e., without delaying the conduction of the thyristors).

To generate firing signals that are properly synchronized with the ac power source voltages, the thyristor firing circuit samples one of the line-to-line voltages (e.g., line-to-line voltage v_AB in figure 20 below).
Since the firing angle is set to 0°:

• Thyristor T₁ is fired at phase angle 60^o of line-to-line voltage v_AB (or at phase angle 30^o of phase voltage v_AN). This turns thyristor T₅
• 120^o later, thyristor T₃ is fired while thyristor T₁ turns off.
• 120^o later, thyristor T₅ is fired while thyristor T₃ turns off.

Also, the complementary thyristors T₂, T₄, and T₆ are fired 180^o later than thyristors T1, T3, and T5. Therefore,

• Thyristor T₂ is fired at phase angle 240^o of line-to-line voltage v_AB (or at phase angle 210^o of phase voltage v_AN). This turns thyristor T₆
• 120^o later, thyristor T₄ is fired while thyristor T₂ turns off.
• 120^o later, thyristor T₆ is fired while thyristor T₄ turns off.

The firing sequence previously described repeats over and over.
The pulses in each firing signal have a duration of 120°, which corresponds to the conduction interval of each diode in a three-phase full-wave diode bridge rectifier.
Consequently, the thyristors conduct current by pairs, one after the other, during equal intervals of 60, as presented in table 15 below.

Figure 20: Waveforms of voltages and thyristor firing signals for a firing α angle set to 0°

Table 15: Order of Conducting Thyristors for a Firing Angle of 0° when the Load is Resistive

The average output voltage in the controlled three-phase bridge rectifier is:

From (18), one can see that the amount of power supplied to the load can be varied by changing the firing angle of the thyristors.
The values of the DC current and voltage, and thus the power supplied to the load, are maximum when the firing angle is 0°.
When the firing angle is increased, the firing pulses for each thyristor are delayed, which reduces the average values of the DC current and voltage at the DC side of the bridge, and thus, the amount of power supplied to the load.

The capability to change the amount of power to the load, by changing the firing angle of the thyristors, represents a major advantage of the thyristor three-phase bridge over the three-phase diode bridge rectifier.