Page 80 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
78 Chapter Two

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

Y Normal
supply
B
E
Earthing (if provided)
contactor
EC

Earthing
resistor
Essential Non-essential
load load
Local
earth
electrode

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

Synchronizing facilities are provided across each of the essential load circuit breakers.
Figure 2.19 Typical connections and switching status table for high voltage parallel running.

a pilot exciter with permanent magnet excitation (see Fig. 1.4).
Alternatively, instead of a pilot exciter, power may be derived from
the generator output terminals (see Figure 1.5); with this arrange-
ment, under short circuit conditions, there will be a loss of exciter
field unless a system to maintain it is included. One system uses
current transformers to maintain excitation during a fault. Another
system derives power from a dedicated third harmonic winding in
the stator.
The steady-state short circuit current available from an ac generator
is normally of the order of three times the rated current.

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