Transcript LECTURE 27 - Rose

LECTURE 27
Controlled Rectifiers
Dr. Rostamkolai
ECE 452
Power 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 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

Two configurations are shown in the following
figure
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6

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

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
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
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
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
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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:
V6 x  6 f Lc I dc

The voltage drop is not dependent on the delay
angle α
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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):
Vdc  Vdc  max  V y
Vdc (  0)  Vdm  V y
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
The average output voltage with overlap due to
two commutations and phase angle control is:
Vdc (   )  Vdc (  0)  2Vx  V y
Vdc (   )  Vdm  V y  2Vx
Vdc (   )  Vdc ( )  2Vx
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
Then:
Vdc (   )  Vdc ( )  2Vx
2Vx  2 f I dc Lc  Vdc ( )  Vdc (   )

The overlap angle μ can be determined from the
above equation for a single-phase full converter
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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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