Internally generated transients depend on the
configuration of power distribution inside the AIS
facility. The effects of internal transients can be
minimized by isolating the AIS hardware from other
facility loads. Ideally, the computer area power
distribution panels should be connected directly to the
primary feeders and should not share step-down
transformers with other high-load equipment.
The risk analysis should include a complete power
transient and failure study. It should also carefully
consider the projected growth in particularly sensitive
applications (such as real-time or teleprocessing) in
projecting future loss potential.
In some cases it may be economically feasible to
connect the AIS facility to more than one utility feeder
via a transfer switch. If one feeder fails, the facility’s
load may be transferred to the alternate feeder. This
technique is of greater value if the two feeders connect
to different power substations.
If the AIS facility is in a remote area, an
uninterrupted power supply (UPS) is usually required
as a backup power source. The UPS system can be
manually or automatically controlled from prime power
sources or from the AIS computer site. The typical UPS
consists of a solid-state rectifier that keeps batteries
charged and drives a solid-state inverter. The inverter
synthesizes alternating current for the computer. A
simplified block diagram is shown in figure 4-8.
Depending on the ampere-hour capacity of the
battery (or batteries), the UPS can support its load for a
maximum of 45 minutes without the prime power
source. At the same time, it will filter out transients. To
provide extra capacity to protect against a failure of the
UPS, a static transfer switch can be inserted between
the UPS and the computer, as shown in figure 4-9. The
control circuitry for the static switch can sense an
overcurrent condition and switch the load to the prime
power source without causing noticeable transients.
Figure 4-8.—Simplified block diagram of an uninterrupted
power supply (UPS).
Figure 4-9.—UPS with transfer switch.
4-22
If the facility’s current needs exceed its UPS
capacity, it may be economically feasible to use
multiple, independent UPS units, as shown in figure
4-10. Since each unit has its own disconnect switch, it
can be switched offline if it fails.
Finally, if the risk analysis shows a major loss from
power outages lasting 30 to 45 minutes or beyond, an
onsite generator can be installed, as shown in figure
4-11. The prime mover may be a diesel motor or a
turbine. When the external power fails, UPS takes over
and the control unit starts the prime mover
automatically. The prime mover brings the generator
up to speed. At this point, the UPS switches over to the
generator. Barring hardware failures, the system
supports the connected load as long as there is fuel for
the prime mover. Note that the generator must be large
enough to support other essential loads, such as air
conditioning or minimum lighting, as well as the UPS
load.
Figure 4-10.—Multiple, independent UPS units.
