Page 125 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 123

Whatever the pulse number the magnitude of the nth harmonic is
inversely proportional to the harmonic number, thus the fifth harmonic
will be 20 percent of the fundamental and the thirteenth harmonic will
be 7.7 percent. The total harmonic content of the current for a three-
phase bridge (six-pulse) is 30 percent of the fundamental and for a dou-
ble bridge (12-pulse) 13.9 percent.

Commutation
The rectangular waveforms of Fig. 4.2 would require an instantaneous
transfer of current from the conducting phase to the next in sequence.
Such instantaneous transfer cannot occur owing to the presence of induc-
tance in the supply; in practical circuits commutation between diodes
requires a time measured in milliseconds. Commutation has the effect of
slightly reducing the higher harmonic content of the line current.
During commutation the direct current is supplied from two diodes
connected to different transformer phases. As both diodes are conduct-
ing and connected to a common point the transformer phases may be
considered as being connected together. The common point assumes a
voltage that is the mean of the two phases until commutation is com-
plete. Figure 4.3 illustrates the effect of diode commutation on the sup-
ply voltage and on the current output.
With a diode rectifier commutation occurs at practically zero volt-
age difference, and rates of change of currents are not high. If the
rectifier is of the controlled type it will incorporate thyristors instead
of diodes and reduction of the dc output voltage will be achieved by
delaying the commutation. During delayed commutation the voltage
difference between the phases can be high, which leads to a high rate

Figure 4.3 Effect of diode commutation on voltage and current.

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