Page 234 - Uninterruptible Power Supplies

Notes on Systems Installation
232 Chapter Nine

Consideration should also be taken of the well-known fact that pre-
dicted loads have a habit of increasing, when it is evident that a high
security power supply is envisaged for a stated location.
Clearly consideration should be given as to the type of UPS system
to be employed, static or rotary and which circuit is most suitable. As a
guide, rotary sets these days are of higher reliability but at a signifi-
cantly higher cost. Static systems certainly are almost universally in
use up to about 50 kVA, and gradually beyond this rating rotary sets
should be given consideration. Price and reliability have to be balanced
for the project in hand.
Assuming that the UPS power rating has been ascertained, attention
may now be given to the rating of the generator (usually a diesel gen-
erator set). Thus, consideration should be given to the following:
Computer rating and possible waveform problems
Air conditioning and variations in load due to operational requirements
Emergency lighting: there may be two sections, one portion directly
from the UPS and (with a delay in operation) from the generator
Security systems

There may be other loads to consider depending on site conditions.
The generator will usually assume loads in the following way: ini-
tially air conditioning, lighting, and security. Note that it may be advis-
able to switch these loads on with small delays using a delay timer
system to avoid heavy surge loads onto the prime mover. In most
instances the UPS is the last load to be presented to the prime mover
as it will be working from the battery system. The UPS also has a load
ramp-up feature which allows a relatively slow rise in power demand
from the UPS.
It should be remembered that the power demand from the UPS will
consist of three sections: UPS load, losses in the UPS system, and
recharging the battery. This extra load, present when recharging the
battery, may be deferred until the system is restored to mains opera-
tion, but this deferment of battery recharge does have the penalty that
a second outage of mains power soon after mains restoration may
result in a complete loss of UPS power due to a battery incapable of
supplying sufficient power.
Care should be taken to ensure that any harmonics induced onto the
prime mover are taken into consideration. As stated in previous chap-
ters harmonics induced onto the mains or the prime mover from the
UPS will vary with UPS design. If, for example, a fifth harmonic filter
is employed on the input to the UPS, low load conditions and variations
in frequency from the prime mover may have a detrimental effect.

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