Page 78 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
76 Chapter Two

Standby Essential load Supply Authority's
generator circuit breakers circuit breaker
Standby supply Normal supply
CB1 CB2
R

Y Normal
supply
B
N & E
Neutral
contactor
NC

Local
earth Essential Non-essential
electrode load load

Device CB1 CB2 NC
Essential load from O I O
normal supply
Essential load from
standby supply I O I
The two supplies
in parallel I I O

Synchronizing facilities are provided across each of the essential load circuit breakers.
Figure 2.17 Typical connections and switching status table for low-voltage parallel run-
ning when the normal supply is multiple earthed (PME, TN-C-S, or TN-C).

Overcurrent Protection of the Standby
Supply
Behavior of Generators under Fault
Conditions
When a generator experiences a fault, a large current, dependent upon
the subtransient reactance (X″), flows for a short time, this will
rapidly decay to a value dependent upon the transient reactance (X′)
and at the same time the voltage regulator will start to increase the
excitation. After a short time the current will have stabilized to a

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.