LECTURE 27 Controlled Rectifiers

Dr. Rostamkolai

ECE 452Power Electronics

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Single-Phase Series Converters

For high voltage applications, two or more converters can be connected in series to share the voltage and also to improve the power factor

The following figure shows two semiconverters that are connected in series

Each transformer secondary has the same number of turns, and the turns ratio is 2

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Twelve-Pulse Converter

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 configurations are shown in the following figure

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A 30 degrees phase shift between secondary windings can be accomplished by connecting one secondary in wye and the other in delta

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Design of Converter Circuits

The design of converter circuits requires determining the ratings of switching devices and diodes

The switches and diodes are specified by: Average current RMS current Peak current Peak inverse voltage 8

In case of the controlled rectifiers, the current rating of devices depend on the delay angle

The ratings of power devices must be designed under the worst case condition

This occurs when the converter delivers the maximum average output voltage 9

The output of converters contain harmonics that depend on the delay angle

The worst-case condition is generally when the minimum output voltage occurs

Input and output filters must be designed under the minimum output voltage conditions 10

Effect of Load and Source Inductances

The load current harmonics depend on load inductances

Also, the input power factor depends on the load power factor

So far, in derivations of output voltages and the performance criteria of converters, we have assumed that the source has no inductances and resistances

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The source resistance is generally very small

The amount of voltage drop due to source inductance is independent of the delay angle, and:

The voltage drop is not dependent on the delay angle α

dccx ILfV 66

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However, the commutation (overlap) angle μ varies with the delay angle

As the delay angle increases, the overlap angle decreases

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If Vx is the average voltage drop per commutation due to overlap and Vy is the average voltage reduction due to phase angle control that is zero, then average output voltage is (when ignoring commutation overlap):

ydmdc

ydcdc

VVV

VVV

)0(

max

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The average output voltage with overlap due to two commutations and phase angle control is:

xdcdc

xydmdc

yxdcdc

VVV

VVVV

VVVV

2)()(

2)(

2)0()(

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Then:

The overlap angle μ can be determined from the above equation for a single-phase full converter

)()(22

2)()(

dcdccdcx

xdcdc

VVLIfV

VVV

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Work on Example 10.10

α = 10o → µ = 4.66o

α = 30o → µ = 1.94o

α = 60o → µ = 1.14o

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