Page 83 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 81

Z = High impedance relay Z Z Z

Figure 2.22 Typical connections to provide differential cur-
rent protection.

D D = Definite time relay Figure 2.23 Typical connections
to provide standby earth fault
protection.

The generator should be deexcited and the engine shut down if the
fault persists after the expiry of the time delay. This protection is a
useful supplement to the restricted earth fault or differential current
protection, it also protects the earthing resistor against continuous
loading which would have disastrous consequences. Typical connec-
tions are indicated by Fig. 2.23.
■ Reverse Power Protection. This system is needed for any generator
which is required to run in parallel with another supply or another
generator. A reverse power situation indicates that the generator is
acting as a motor and is driving the engine, an undesirable situation.
Under reverse power conditions the generator circuit breaker should
be opened, it is not important to shut down the engine immediately
but it must not be left running unloaded for longer than say 15 min,
it depends on whether the generator is attended or not. It is usual for
the relay to include a short time delay to prevent any transient
effects, particularly during synchronizing operations, from tripping
the circuit breaker.
■ Unbalanced Load Protection. This system is applicable where there
is concern that the generator may be running for long periods on a

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