Page 122 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
120 Chapter Four

The power factor, cos , of a diode rectifier is close to unity whereas
that of a thyristor rectifier is determined by the commutation delay
angle such that

output dc voltage with phase control
cos (4.6)
maximum dc voltage with no delay

which can result in unexpectedly low power factors for lightly loaded
thyristor rectifiers. Typical values for and cos are:

cos

Diode rectifier 0.96 0.98
Thyristor rectifier 0.94 0.5–0.9

Wave Shape or Profile
The profile of a nonsinusoidal quantity is determined by the magni-
tudes and phase relationships of its harmonic components. It cannot be
determined from a knowledge of the harmonic magnitudes alone,
phase relationships are involved; hence the use of the peak or crest fac-
tor. The currents indicated by Figs. 4.1a and b both have the harmonic
components (from fifth to thirty-first) characteristic of a six-pulse rec-
tifier. For Fig. 4.1b the signs or “polarities” of the fifth, eleventh, seven-
teenth, twenty-third, and twenty-ninth harmonics have been reversed to
achieve the high current peak. For Fig. 4.1a the addition of the infinite
number of higher characteristic harmonics would result in a waveform
having vertical edges and a flat top.
The rms magnitudes of the harmonics of both waveforms are:

n 1 5 7 11 13 17 19 23 25 29 31
rms 0.78 0.156 0.11 0.07 0.06 0.046 0.04 0.034 0.031 0.027 0.025

Data relating to the two waveforms may be calculated:

Harmonics Generated by Bridge Rectifiers
Rectifier Operation
Many UPS assemblies take power from the mains to feed a rectifier
input module. A common rectifier configuration is the three-phase
bridge, which theoretically demands a current with a harmonic content
30 percent of the fundamental. These harmonic currents, in flowing
through the impedances of the supply system, cause distortion of the
supply voltage for local consumers.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.