Page 133 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 131

The Effect of Rectifier Commutation
It has already been stated that the period of commutation of a rectifier
depends on the commutating reactance of its supply. If there is no rec-
tifier transformer or series inductor the commutation period is deter-
mined solely by the supply system reactance. A normal supply system
will have a low reactance leading to a short commutation time, and a
small standby generator will have a comparatively high reactance
leading to a longer commutation time. The longer commutation time
leads to a reduction of dc output voltage and more pronounced notch-
ing of the supply voltage waveform.
When a three-phase generator supplies a steady linear load the sta-
tor currents produce a steady magnetic flux rotating, with the rotor, at
synchronous speed. When the linear load is exchanged for a rectifier an
entirely different set of conditions arises. The current will not be the
idealized square waves previously considered (Fig. 4.2) but is likely to
approach a trapezoidal shape due to the extended commutation peri-
ods. Between the commutation periods the stator flux remains station-
ary and moves forward in discrete steps as each commutation transfers
current from one phase to the next.
With a diode rectifier this results in the type of distortion illustrated
in Fig. 4.7. The magnitude of the distortion will depend on the reac-
tance of the supply, as already discussed.
With a thyristor rectifier the operation is similar to that of diodes but
the depth of the notching will be deeper depending on the commutation
delay angle and, in idealized form is illustrated in Fig. 4.8.
When running from a local generator, owing to the irregular rotation
of the stator flux the notching in the succeeding phase can be consider-
ably deeper (and the rise in the preceding phase correspondingly less)

Figure 4.7 Typical distortion caused by rectifier loads on a local generator. (a) 50 percent
load. (b) 100 percent load.

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