#### LECTURE 26 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics Principles of Three-Phase HalfWave Converters Three-phase converters provide higher average output voltage, and the frequency of.

download report#### Transcript LECTURE 26 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics Principles of Three-Phase HalfWave Converters Three-phase converters provide higher average output voltage, and the frequency of.

LECTURE 26 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics 1 Principles of Three-Phase HalfWave Converters Three-phase converters provide higher average output voltage, and the frequency of the ripples on the output voltage is higher compared to that of single-phase converters Therefore, the filtering requirements for smoothing out the load current and load voltage are simpler 2 3 If the phase voltage is van Vm sin t , then the average output voltage for a continuous current will be: 3 Vdc 2 5 / 6 /6 Vm sin t d (t ) 3 3 Vm cos 2 The rms value of the output voltage will be: Vrms 3 2 Vrms 1 3 3 Vm ( cos 2 )1/ 2 6 8 5 / 6 /6 2 2 Vm sin t d ( t ) 1/ 2 4 If α ≥ 30o, then the average output voltage for a resistive load will be: 3 Vdc 2 /6 Vm sin t d (t ) 3Vm 2 1 cos( ) 6 The rms value of the output voltage will be: 1/ 2 Vrms 3 2 V sin t d ( t ) / 6 Vrms 1 5 3 Vm sin( 2 ) 3 24 4 8 2 m 2 1/ 2 5 Three-Phase Full Converters Three-phase converters are extensively used in industrial applications up to 120 kW level, where two-quadrant operation is required This circuit is known as a three-phase bridge converter The thyristors are fired at an interval of 60 degrees 6 The frequency of the output ripple voltage is 6fs The following figure shows a full converter circuit with a highly inductive load 7 8 The line-to-neutral voltages are: van Vm sin t 2 vbn Vm sin ( t ) 3 2 vcn Vm sin ( t ) 3 Then the line-to-line voltages are: vab van vbn 3 Vm sin ( t ) 6 vbc vbn vcn 3 Vm sin ( t ) 2 5 vca vcn van 3 Vm sin ( t ) 6 9 The average output voltage is found from: Vdc 3 / 2 / 6 vab d ( t ) 3 3 Vm cos The rms value of the output voltage is: 1/ 2 Vrms 3 2 vab d ( t ) / 6 Vrms 1 3 3 3 Vm ( cos 2 )1/ 2 2 4 / 2 10 A three-phase bridge gives a six-pulse output voltage For high-power applications such as highvoltage dc transmission, a 12 pulse output is generally required to reduce the output ripples and to increase the ripple frequencies Two six-pulse bridges can be combined either in series or in parallel to produce a 12-pulse output 11 Three-Phase Dual Converters In variable-speed drives application, the four quadrant operation is generally required Three-phase dual converters are extensively used in applications up to 2 MW The following figure shows three-phase dual converters where two three-phase converters are connected back-to-back 12 13 Due to instantaneous voltage differences between the output voltages of converters, a circulating current flows through the converters This circulating current is limited by a reactor The two converters are controlled in such a way that if α1 is the delay angle of converter 1, the delay angle of converter 2 is α2 = π- α1 14 The following figure shows the waveforms for input voltages, output voltages, and the voltage across the inductor 15 16 Pulse-Width-Modulation Control The power factor of phase-controlled converters depends on delay angle , and it is generally low at low output voltage range These converters inject harmonics into the supply Forced commutation can improve the input power factor and reduce the harmonics levels 17 18 With advancement of devices (GTO or IGBT), the forced commutation can be implemented in practical systems Now we will discuss the basic techniques of forced commutation which are: Extinction Angle Control Symmetrical Angle Control Pulse-Width Modulation Single-Phase Sinusoidal Pulse-Width Modulation Three-Phase PWM Control 19 Extinction Angle Control 20 21 Symmetrical Angle Control 22 Pulse-Width Modulation Control 23 Single-Phase Sinusoidal PulseWidth Modulation 24