LECTURE 26 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics 1

Principles of Three-Phase Half- Wave 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 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 Therefore, the filtering requirements for smoothing out the load current and load voltage are simpler 2

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If the phase voltage is, then the average output voltage for a continuous current will be: If the phase voltage is, then the average output voltage for a continuous current will be: The rms value of the output voltage will be: The rms value of the output voltage will be: 4

If α ≥ 30 o, then the average output voltage for a resistive load will be: If α ≥ 30 o, then the average output voltage for a resistive load will be: The rms value of the output voltage will be: The rms value of the output voltage will be: 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 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 This circuit is known as a three-phase bridge converter The thyristors are fired at an interval of 60 degrees The thyristors are fired at an interval of 60 degrees 6

The frequency of the output ripple voltage is 6f s The frequency of the output ripple voltage is 6f s The following figure shows a full converter circuit with a highly inductive load The following figure shows a full converter circuit with a highly inductive load 7

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The line-to-neutral voltages are: The line-to-neutral voltages are: Then the line-to-line voltages are: Then the line-to-line voltages are: 9

The average output voltage is found from: The average output voltage is found from: The rms value of the output voltage is: The rms value of the output voltage is: 10

A three-phase bridge gives a six-pulse output voltage A three-phase bridge gives a six-pulse output voltage For high-power applications such as high- voltage dc transmission, a 12 pulse output is generally required to reduce the output ripples and to increase the ripple frequencies For high-power applications such as high- voltage 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 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 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 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 The following figure shows three-phase dual converters where two three-phase converters are connected back-to-back 12

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Due to instantaneous voltage differences between the output voltages of converters, a circulating current flows through the converters 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 This circulating current is limited by a reactor The two converters are controlled in such a way that if α 1 is the delay angle of converter 1, the delay angle of converter 2 is α 2 = π- α 1 The two converters are controlled in such a way that if α 1 is the delay angle of converter 1, the delay angle of converter 2 is α 2 = π- α 1 14

The following figure shows the waveforms for input voltages, output voltages, and the voltage across the inductor The following figure shows the waveforms for input voltages, output voltages, and the voltage across the inductor 15

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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 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 These converters inject harmonics into the supply Forced commutation can improve the input power factor and reduce the harmonics levels Forced commutation can improve the input power factor and reduce the harmonics levels 17

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With advancement of devices (GTO or IGBT), the forced commutation can be implemented in practical systems 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: Now we will discuss the basic techniques of forced commutation which are: Extinction Angle Control Extinction Angle Control Symmetrical Angle Control Symmetrical Angle Control Pulse-Width Modulation Pulse-Width Modulation Single-Phase Sinusoidal Pulse-Width Modulation Single-Phase Sinusoidal Pulse-Width Modulation Three-Phase PWM Control Three-Phase PWM Control 19

Extinction Angle Control 20

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Symmetrical Angle Control 22

Pulse-Width Modulation Control 23

Single-Phase Sinusoidal Pulse- Width Modulation 24