Thyristor Three-Phase, Six-Pulse Converter

OBJECTIVE

1. To study the operation of the three-phase, six-pulse rectifier/inverter.
2. To plot the static transfer function of the three-phase, six-pulse rectifier/inverter, and compare this with the theoretical curve.

DISCUSSION

The three-phase, six-pulse thyristor converter, or rectifier inverter, shown in Figure 1, is used in power electronics. This type of circuit gives the highest and most regular output voltage with the least amount of ripple. It can function as a rectifier or, when connected to a correctly polarized dc source, as an inverter.

This is the static transfer function of a three-phase, six-pulse converter, and is valid only when the on-time of the thyristors is equal to 120°. That is, when the series inductor is large enough to ensure continuous conduction.

Figure 1: Three-phase, six-pulse converter with resistive load.

Figure 2: Waveforms for the three-phase, six-pulse converter.

With respect to the three-phase restifier/inverter, the six-pulse circuit has the following differences:

1. the average value of E_D is twice as great
2. the transfer of active power is two times greater with the same value of current
3. the ripple frequency of E₀ is 360 Hz instead of 180 Hz
4. the average value of current I_A , I_B , I_C is zero.

This last difference is important because it prevents saturation of the transformers supplying the converter. According to Kirchhoff’s current law, I_A = I₁ - I₄. We can therefore draw the curve of I_A as shown in Figure 2. I_A changes polarity each half cycle. being equal to I₁ than -I₄ , and then repeating again. I_A flows for 240° or two-third of the cycle.

Three-phase bridge using three thyristors and three diodes

A three-phase bridge can be made using three thyristors and three diodes. Figure 3 shows an example of such circuit.

Figure 3: Three-phase bridge using three common-cathode thyristors.

The free-wheeling diode D₄ is necessary to ensure that the circuit be able to turn off an inductive load. Without this diode, when the gate pulses are stopped, the current may never drop to zero and one thyristor may continue to conduct.The freewheeling diode also relives the thyristors from freewheeling duty, allowing the use of lower power thyristors.

One advantage of the circuit in Figure 3 is that the firing control circuit can be simplified since the cathode of the three thyristors are at a common potential. This circuit is of lower cost than a three-phase six-thyristor bridge of comparable power, and both allow control of power from 0 to 100%. Unlike the six-thyristor bridge, however, this bridge cannot be used to make a line-commutated inverter.

Procedure summary

In the first part of this exercise, you will set up the equipment.

In the second part of this exercise, you will operate the three-phase, six pulse converter in both the rectifier and the inverter modes. You will plot the static transfer function of the converter, and compare it to the theoretical curve.

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