Lesson 2 Power Factor Improvement, Harmonic Reduction, Filter

Transcription

1 Lesson 2 Powe Facto Impovement, Hamonic Reduction, Filte 1. Powe Facto Impovement Fo phase-contolled opeation in both single phase full wave half and full contolled bidge convetes, the displacement facto (o powe facto, which is lagging) deceases, as the aveage value of output voltage (V dc ) deceases, with the incease in fiing angle delay, α. This is also applicable fo both thee phase half wave and full wave (bidge) convetes. The thee schemes used fo powe facto (pf) impovement ae: Extinction angle contol Symmetical angle contol Pulse width modulation (PWM) contol 2. Extinction Angle Contol The cicuit diagam of a single phase full wave half-contolled (semi) foce-commutated bidge convete is shown in Fig. 16.1(a). The thyistos, T 1 & T 2, ae eplaced by the switches, self-commutated devices, such as powe tansisto o equivalent. The powe tansisto is tuned on by applying a signal at the base, and tuned off by withdawing the signal at the base. A gate tun-off thyisto (GTO) also may be used, in which case, it may be tuned off by applying a shot negative pulse to its gate, but is tuned on by a shot positive pulse, like a thyisto. In extinction angle contol, switch, S 1 is tuned on at, and then tuned off by foced commutation at. The switch, S 2 is tuned on at, and then tuned off at. The output voltage is contolled by vaying the extinction angle, β. Fig.6(b) shows the wavefoms fo input voltage, output voltage, input cuent, and the cuent though thyisto switches. The fundamental component of input cuent leads the input voltage, and the displacement facto (and powe facto) is leading. This featue may be desiable to simulate a capacitive load, thus compensating the line voltage dops.

2 Fig. 6: Single phase foced commutated semiconvete The aveage output voltage is

3 The value of V dc is vaied fom to 0, as vaies fom 0 to. The ms value of output voltage is Hee also, V o vaies fom V to 0. This scheme of extinction angle contol can also be used fo single phase full wave full contolled bidge convete with fou switches, instead of two needed in the ealie case. The students ae equested-to study this matte fom text books, but details ae not included hee. 3. Symmetical Angle Contol This contol can be applied fo the same half-contolled foce commutated bidge convete with two switches, S 1 and S 2 as shown in Fig. 6(a). The switch, S 1 is tuned on at and then tuned off at. The othe switch, S 2 is tuned on at and then tuned off at. The output voltage is vaied by vaying conduction angle, β. The gate signals ae geneated by compaing half-sine waves with a dc signal as shown in Fig. 7(b). The half-sine waves can be obtained using a full wave diode (uncontolled) bidge convete. The gate signals can also be geneated by compaing tiangula waves with a dc signal as shown in Fig. 7(c). In the second case, the conduction angle vaies linealy with the dc signals, but in invese atio, i.e., when the dc signal is zeo, full conduction takes place, and the dc signal being same as the peak of the tiangula efeence signal, no conduction takes place. Fig. 7(a) shows the wavefoms fo input voltage, output voltage, input cuent and the cuent though the switches. The fundamental component of input cuent is in phase with input voltage, and the displacement facto is unity (1.0). Theefoe, the powe facto is impoved.

4 Fig. 7: Symmetical angle contol

5 The aveage output voltage is The value of V dc vaies fom to 0 as β vaies fom π to 0. The ms value of output voltage is 4. Pulse Width Modulation (PWM) Contol If the output voltage of single phase half-contolled convete is contolled by delay angle, extinction angle o symmetical, thee is only one pulse pe half cycle in the input cuent of the convete, and as a esult, the lowest ode hamonic is thid. It is difficult to filte out the lowe ode hamonic cuent. In Pulse Width Modulation (PWM) contol, the convete switches ae tuned on and off seveal times duing a half cycle, and the output voltage is contolled by vaying the width of pulses. The gate signals ae geneated by compaing a tiangula wave with a dc signal as shown in Fig. 8(c). In this case, all the pulse widths obtained ae equal. Fig. 8(a) shows the input voltage, output voltage, and input cuent. The lowest ode hamonic can be eliminated o educed by selecting the numbe of pulses pe half cycle. Howeve, inceasing the numbe of pulses would also incease the magnitude of highe ode hamonics, which could easily be filteed out. The ealie case of symmetical angle contol can be consideed as single pulse PWM.

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7 Fig. 8: Pulse width modulation contol The details of output voltage and cuent wavefoms of the convete ae given. The output voltage (i.e., pefomance paametes) can be obtained in two steps: (i) by consideing only one pai of pulses such that, if one pulse stats at, and ends at, the othe pulse stats at, and ends at If m th pulse stats at and its width is p numbe of pulses is found as, and (2) then by combining the effects of all pais of pulse., the aveage output voltage due to If the load cuent with an aveage value of I a is continuous and has negligible ipple, the instantaneous input cuent is expessed in a Fouie seies as Due to symmety of the input cuent wavefom, even hamonics ae absent, and I dc is zeo. The Fouie coefficients ae obtained as

8 =α So, the equation fo i s (t) is witten as whee, and 5. Sinusoidal Pulse Width Modulation (SPWM) Contol Vaious types of modifications in PWM techniques have been poposed. One impotant method is sinusoidal pulse width modulation (SPWM) contol, the pulse widths ae geneated by compaing a tiangula efeence voltage v of amplitude A and fequency f, with a caie half sinusoidal voltage v c of vaiable amplitude A c and fequency 2f s. The sinusoidal voltage v c is in phase with the input phase voltage v s and has twice the supply fequency f s. The widths of the pulses (and the output voltage) ae vaied by changing the amplitude A o the modulation index M fom 0 to 1. The modulation index, M is A c /A. It may be noted that the width of the pulses obtained ae vaiable. The width is smalle at the cente of the caie signal (sinusoidal), and inceases as one goes to the stat and end of the above signal. Fig. 9 shows the vaious wavefoms, including the cuents though thyistos and the input cuent and load cuent (assumed to be continuous). It may be noted that, in the ealie case (multiple PWM contol), the pulse widths ae unifom (equal). In this type of contol, the displacement facto is unity, and the powe facto is impoved. The lowe ode hamonics one eliminated o educed. Fo example, width fou pulses pe half cycle, the lowest ode hamonic is the fifth, and so on.

9 Fig. 9: Sinusiodal pulse width modulation contol