Page 79 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 77

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

Y Normal
supply
B
N

E
(if provided)
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
I I O
in parallel

Synchronizing facilities are provided across each of the essential load circuit breakers.
Figure 2.18 Typical connections and switching status table for low-voltage parallel run-
ning when the normal supply is single point earthed (TN-S or TT).

steady-state value determined by the synchronous reactance; this is
the steady-state short circuit current which is available for relay
operation (the prospective fault current). The time constants of these
decrements depend on the generator size and design, the subtran-
sient time constant is measured in milliseconds, the transient time
constant in tens of milliseconds and the synchronous time constant in
tenths of a second.
For satisfactory fault clearance it is important that each genera-
tor is provided with an effective excitation system. The simplest
excitations systems use a shaft-mounted rectifier, an ac exciter, and

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