Power Factor Improvement

Power Factor Improvement

Normally, two types of practice are in common for the improvement of the power factor. They are:

1)Extinction Angle Control

2)Symmetrical Angle Control

Extinction Angle Control

The circuit diagram of a single phase full wave half-controlled (semi) force-commutated bridge converter is shown in Fig_1. The thyristors, T1 & T2, are replaced by the switches, self-commutated devices, such as power transistor or equivalent. The power transistor is turned on by applying a signal at the base and turned off by withdrawing the signal at the base. A gate turn-off thyristor (GTO) also may be used, in which case, it may be turned off by applying a short negative pulse to its gate, but is turned on by a short positive pulse, like a thyristor. In extinction angle control, switch, S1 is turned on at ωt 0 = , and then turned off by forced commutation at ω = π −β t ( ) . The switch, S2 is turned on at ωt = π, and then turned off at
ω = π −β t 2 ( ) . The output voltage is controlled by varying the extinction angle, β. Fig_2 shows the waveforms of input voltage, output voltage, input current, and the current through thyristor switches. The fundamental component of input current leads the input voltage and power factor are leading. This feature may be desirable to simulate a capacitive load, thus compensating the line voltage drops.

We can see from the figure that Fundamental component of input current leads the input voltage byβ/2

Symmetrical Angle Control

This control can be applied for the same half-controlled force commutated bridge converter with two switches, S1, and S2 as shown in Fig. 16.1(a). The switch, S1 is turned on at ω = (π −β)/ 2 and then turned off at ω = (π + β)/ 2 . The other switch, S2 is turned on atω = (3 π − β)/2 and then turned off at ω = (3π + β )/2 . The output voltage is varied by varying conduction angle, β. The gate signals are generated by comparing triangular waves with a dc signal as shown in Fig_3 . In this case, the conduction angle varies linearly with the dc signals, but in inverse ratio, i.e., when the dc signal is zero, full conduction (β = π) takes place, and the dc signal being same as the peak of the triangular reference signal, no conduction (β = 0) takes place. Fig_4 shows the waveforms of input voltage, output voltage, input current and the current through the switches. The fundamental component of the input current is in phase with the input voltage, and the displacement factor is unity (1.0). Therefore, the power factor is improved.

REFERENCES

IIT,Kharagpur. (2009, 12 31). Power Electronics. Retrieved from NPTEL: http://www.nptel.ac.in/courses/108105066/