Page 139 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
Harmonic Distortion of the Supply 137

monics that surround it. The basic power supply comprises a series
arrangement of a single-phase rectifier, a shunt-connected capacitor, a
chopper with associated control circuit, and an output filter. The recti-
fier and capacitor produce a peaky current flow in the middle of each
voltage half cycle—a waveform rich in harmonics particularly the third.
A typical current waveform is illustrated in Fig. 4.10.
The waveform of Fig. 4.11 is a useful approximation of Fig. 4.10, it is
made up of the following harmonic components:

Harmonic order (n) 1 3 5 7 9 11 13
Magnitude (%) 100 85 60 30 18 12 8

Triplen harmonic currents in a three-phase system have zero phase
sequence and become additive in the neutral (if there is no neutral con-
ductor there can be no triplen harmonic currents). In an installation
that includes a large proportion of IT equipment there are likely to be
large triplen harmonic currents and these must be taken into consid-
eration at the planning stage. These currents may flow in the UPS neu-
tral, the UPS bypass neutral conductor, the neutral conductors in the
distribution system, and the standby generator neutral.
Triplen harmonic currents may be removed from a distribution sys-
tem by providing a low-impedance shunt path in the form of a trans-
former with a delta winding which can be either the primary or a

Figure 4.10 Typical waveform of current taken by a switched mode power supply.

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.