Page 126 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
124 Chapter Four

of change of current and a short commutation period. The idealized
current shapes are not affected, but in practice, since the load induc-
tance is finite, delaying the commutation leads to a change of current
shape.
As in the case of diodes, during commutation the voltage of the com-
mon point assumes a voltage that is the mean of the two phases, lead-
ing to a voltage rise in the preceding and a dip in the succeeding phase.
When commutation is complete the previously conducting thyristor
abruptly assumes a reversed bias state. If the supply inductance is
small it may be necessary to add series inductance to limit the com-
mutating di/dt. Figure 4.4 illustrates the effect of delayed commutation
on the supply voltage and on the current output.

The Effect of a Bridge Rectifier on a
Supply System
Distortion of the Supply Voltage
A rectifier may be regarded as a load which takes power from the sup-
ply system at fundamental frequency and generates harmonic currents
which are fed back into the system. In flowing through the system the
harmonic currents cause voltage drops along the various current paths,
and these voltages distort the system voltage waveform. The distortion
will be greatest at the rectifier terminals, and the degree of penetration
into the system will depend on the system impedance. If a number of
rectifiers feed harmonic currents into different points of a system the
effect is cumulative, the penetration can be deep, and a large number
of consumers may be affected.

Figure 4.4 Effect of delayed commutation on voltage and current.

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