25 August 2017

Power Factor Improvement in the Converter

  • The operation of Phase controlled converter is very simple, reliable and less costly. 
  • The commutation circuit is not necessary in this type of converter. 
  • The power factor becomes low when the output voltage less than the supply voltage ( particularly when the firing angle is high ).
  • The displacement angle ( The angle between fundamental component of alternating line current and phase to neutral voltage is known as displacement angle. ) between the supply voltage and supply current increases as the firing angle increases. 
  • This will result in power factor decreases and lagging reactive power flows from load to supply side and power factor decreases. 
  • There are following methods to improve power factor in the phase controlled converters.

Power factor of phase controlled converters

(a) Phase angle control

  • The output voltage decreases as the firing angle of the SCR increases. 
  • The input displacement factor and input power factor decreases as the output voltage decreases in the semi converter and full converter. 
  • The effect of the firing angle on the output voltage is shown in the figure A.
effect of firing angle on the output voltage

(b) Semiconductor operation of full converter

  • The full converter is used when regeneration is required. 
  • The operation of full converter in the semi converter mode is explained below. 
  • The change in controlled circuit is necessary in order to operate semi converter in to full converter.
power circuit of the full converter
(1) Rectifier mode
  • The SCR T4 and SCR T2 are turned on during positive and negative half cycle of alternating supply respectively. 
  • The SCR T4 and SCR T2 acts as switch in this mode.  
  • The output voltage is adjusted by controlling firing angle of SCR T1 and SCR T3.
  •  The firing of SCR T4 and SCR T2 is kept at zero in this mode of operation therefore the output voltage is adjusted from maximum to minimum by controlling the firing angle of SCR T1 and SCR T3. 
  • When the firing angle of positive group SCR T1 and SCR T3 is kept zero, the output voltage becomes maximum positive and the output voltage becomes zero when the firing angle becomes 180o.
waveform of the rectification for the power factor improvement
(2) Inverting mode
  • The SCR T1 and SCR T3 are turned on during positive half cycle and negative half cycle of alternating supply respectively. 
  • The SCR T1 and SCR T3 works as switch in this mode and the output voltage is controlled by controlling firing angle of SCR T2 and SCR T4. 
  • The firing angle of positive group of SCR T1 and SCR T3 is kept 180o
  • When the firing angle of negative group of SCR T2 and SCR T4 is kept at 180o, the output voltage becomes maximum negative. 
  • The output voltage becomes zero when the firing angle is kept 0o
  • Actually the firing angle is kept less than 180o in order to keep commutation margin. 
  • The system firing angle improves when the full converter operates in the semi converter mode.
waveform of the inversion for the power factor improvement

(c) Asymmetrical firing
  • The firing angle of SCRs is kept same in the symmetrical firing angle control method whereas the firing angle of SCRs is kept different in the asymmetrical firing angle control scheme. 
basic asymmetrical semiconverter circuit
  • Let us consider that the firing angle is kept 90o for output voltage of 0.5 pu in the symmetrical firing angle control. 
  • Let us consider that the firing of the SCR T1 is kept at 60o and SCR T2 is kept at 120o in the asymmetrical firing angle control. 
  • The power factor improves too some extent when the firing angle of SCR T1 is kept small. 
  • There are following disadvantages of asymmetrical firing angle control scheme.(1) It generates DC and 2nd, 4th and 6th harmonics. (2) The motor current becomes discontinuous.
  • The asymmetrical firing angle control scheme is only for theory point of view due to above mentioned disadvantages.
waveform of the asymmetrical triggering

Power factor of forced commutated converters

  • The commutation circuit of the SCR is given in the forced commutated converter.
  •  The commutation of SCR is done at any time. The power factor of the forced commutated converter is done by following methods.

(a) Extinction angle control (EAC)

  • The single phase semi converter is shown in the figure G. The SCR or GTO is used as switch S1 and S2. The square shows commutation circuit of the SCR or GTO
single phase semiconverter circuit
  • The waveform of the extinction angle control is shown in the figure H.The switch S1 is turned on at ωt = α angle and turned off at angle ωt = β. The switch S2 is turned on at ωt = π + α and turned off at angle ωt = π + β. 
  • The output voltage is controlled by controlling extinction angle of switch S1 and S2.
  • The waveform for input voltage, output voltage, input current and output current is shown in the figure H.
  • The fundamental component of supply current I1 leads the supply voltage therefore the displacement angle becomes leading. 
  • The displacement factor becomes leading for extinction angle control and lagging for firing angle control.
waveform of the single phase semi converter for power factor improvement

(b) Symmetrical angle control (SAC)

  • The symmetrical configuration of the single phase semi converter is shown in the figure. 
single phase semiconverter in the symmetrical configuration
  • The cathode potential of SCR T1 and SCR T2 is kept same and gate signal is given from common gate signal. 
  • The fundamental component of supply current is in phase with supply voltage as shown in the figure H. Therefore the displacement angle becomes unity. This will improve the power factor.
waveforms of the symmetrical angle control


(c) Pulse width modulation (PWM) method

  • The output voltage of the single phase converter is done by phase angle control, extinction angle control or symmetrical angle control. 
  • There is only one pulse generated for a given half cycle in all the above method and third harmonic is generated. 
  • The lower order harmonics is not easily filter out. If there is more than one pulse per half cycle, the lower order harmonics is not generated. 
  • The semi conductor switch is on and off several times during half cycle in order to change the width of pulse. 
  • The output voltage changes according to width of pulse. 
comparing dc signal with triangular waveform
  • The gate signal is generated by comparing DC signal and triangular waveform. 
  • The output voltage and other performance parameters are determined as follows.
  • One pulse start at ωt = α angle and complete at ωt = α + δ angle. Similarly the second pulse starts at ωt = π + α angle and complete at ωt = π + α + δ.
                 Where α = Firing angle and δ = Pulse width
  • When all the pulses are considered, the nth pulse start at ωt = αn angle and its width is equal to δn. The average output voltage for p number of pulses can find out by using Fourier series.

Sinusoidal pulse width modulation

  • The sinusoidal pulse width modulation control is shown in the figure K. 
  • The gate pulses are generated by comparing sinusoidal waveform and triangular waveform. 
  • Let us consider that the voltage of triangular waveform is Vt whereas the rectified sinusoidal waveform is Vc
  • The output voltage is adjusted by changing either modulation index M or amplitude of rectified sinusoidal voltage Vc.
                 Modulation index M = Amplitude Vc / Amplitude Vt
  • The gate signal is generated by comparing intersection point of Vc and Vt
  • As the number of pulses per half cycle increases, the higher order harmonics increases which can be filtered out easily. 
  • The displacement factor becomes unity in this method.
sinusoidal pulse width modualation
  • When all the pulses are considered, the nth pulse start at ωt = αn angle and its width is equal to δn
  • The average output voltage for p number of pulses can find out by using Fourier series.