Page 59 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 57

possible to close the normal supply circuit breaker and supply the
nonessential load. Indeed, it is not necessary to trip the normal supply
circuit breaker because the two supplies are separated by bus section
breaker CB4, on restoration of the normal supply the nonessential load
is then energized.
The Fig. 2.3 interconnections provide another operating advantage.
The generator section of the busbars does not have any other circuits
connected to it; this allows the two generating sets to be run up to
speed, paralleled and connected to the busbars for test purposes with-
out interfering with normal running.
A key-operated manual override facility can be added to electrically
interlocked circuit breakers and some uses for such a system are
described in the following paragraphs. They require the presence of
skilled personnel. Each key must be unique, not interchangeable with
others used locally and must be clearly and permanently identifiable.
Where the override involves a “two out of three” selection a separately
mounted key exchange mechanism is provided; this holds the three
keys but allows only two to be extracted.

Alternative Operating Regimes
Many variations of the interconnections with the normal supply are
possible. In Fig. 2.2 changing the electrical interlock from CB1 and
CB3 to CB1 and CB5 makes it possible, during standby running, to use
surplus standby power to feed selected items of the nonessential load.
This is achieved at the expense of losing the ability to restore the nor-
mal supply to the nonessential load before shutting down the standby
supply. Similar reasoning can be applied to Fig. 2.3.
A more complex system can be devised which retains the ability to
use surplus standby power to feed selected items of the nonessential
load or to restore the normal power to the nonessential load before
shutting down the standby supply. Referring to Fig. 2.2, if the electri-
cal interlock between CB1 and CB3 is retained for automatic operation,
but a manual override is provided which replaces it with a key inter-
lock which allows two, and only two, circuit breakers of the set CB1,
CB3, and CB5 to be closed at any time, versatile operation is possible.
Considering only these three circuit breakers the following rules apply:

In normal operation CB3 and CB5 are closed
In standby operation only CB1 is closed
To supply some nonessential load from the standby supply CB1 and
CB3 are closed
To use the normal and standby supplies at the same time CB1 and
CB5 are closed.

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.