Page 123 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 121

Figure 4.1 Two waveforms with the same harmonic content. (a) A quasi-square wave
with harmonics up to the thirty-first. (b) The fifth, eleventh, seventeenth, twenty-third,
and twenty-ninth harmonics are reversed in phase.

TABLE 4.2 Data Relating to Fig. 4.1

Quasi square wave (4.1a) Peaky wave (4.1b)
Fundamental rms a 1 0.780 0.780
Total rms a 0.813 0.813
Distortion factor 0.959 0.959
Total harmonic distortion 0.294 0.294
Peak or crest factor 1.244 2.404

A three-phase bridge rectifier operates as two complementary star
groups, each arm conducting for 120° in each cycle and the two groups
being displaced by 180°. The operating sequence of the arms and the
idealized current waveforms are indicated in Fig. 4.2.
It can be shown from a Fourier analysis that the harmonic content of
such quasi square waveforms includes only the harmonic numbers
derived from the formula:

n px 1 (4.7)

where n the harmonic number
p the number of current pulses per cycle (6 for a three-phase
bridge)
x any integer

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