Page 138 - Uninterruptible Power Supplies

Harmonic Distortion of the Supply
136 Chapter Four

discussed in connection with power factor correction capacitors in the
section titled “The Effect of a Bridge Rectifier on a Supply System.”
Shunt-connected harmonic filters and capacitors will take a leading
fundamental frequency current which may result in the UPS operating
at a leading power factor, particularly at light loads, and almost cer-
tainly if a diode rectifier is involved. At the expense of some complica-
tion it may be possible to install a capacitor bank with automatic
switching, but there may well be conflict between the requirements to
maintain harmonic attenuation and to control power factor.

Active Power Filters
This is a system of “cleaning up” a distorted current wave by continu-
ously monitoring the wave shape of the current taken by the load, com-
paring it to a sine wave, and injecting into the upstream system a
current equal to the distortion. It follows that the filter equipment sup-
plies the distorting harmonic currents which do not have to be taken
from the supply. Active power filters became widely available in the
1990s and may well become more widespread in the next decade. They
use inverters to generate the complex wave shapes, for a particular
installation the rating of the inverter will be related to the degree of
correction required. It follows that there will be additional energy con-
sumption, related to the degree of correction, the cost of which must be
taken into consideration.

Pulse Width Modulation of the Input Current
Instead of taking the current in blocks at the rate of one block per half
cycle the current is taken in short pulses occurring at a high frequency
which may be up to tens of thousands of hertz. The width of the pulses
is continuously controlled so that they are widest at the center of each
half cycle and narrow towards the zero crossover points. The high fre-
quency components are removed by a shunt-connected filter and only
the fundamental component of current is drawn from the supply. The
power factor (cos ) is determined by the pattern of pulses and the
equipment is normally arranged to operate at unity power factor. UPS
equipment incorporating input rectifiers of this type became available
in the late 1990s.

Switched Mode Power Supplies
This is a rectifier configuration which is universally used in electronic
equipment and is infamous for the peaky current which it takes, it is
said of the current that the fundamental cannot be seen for the har-

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.