Page 284 - Introduction to Marine Engineering

258 Electrical equipment

Direct current supply
The supply to a distribution system will usually come from two or more
generators operating in parallel. Each generator must be provided with
certain protective devices to ensure against reverse currents, low voltage
or an overcurrent. There must also be ammeters and voltmeters in the
circuits to enable paralleling to take place.
The circuit for two generators operating in parallel is shown in Figure
14.4. A triple-pole circuit breaker connects the supply to the bus-bars and
also the equaliser bus-bar. The arrangement of the various protective
trips can be seen, with excess current protection being provided in each
pole. The reverse current trip prevents the generator operating as a
motor if, for instance, the prime mover stopped.
The voltmeters and ammeters are provided in the generator supply
circuits for paralleling purposes. A voltmeter is positioned across the
bus-bars to indicate their voltage. Consider the situation where one

Bus ban

Equaliser
Ammeter
Circuit
breaker

Series field
1
—CA-A-T-
Shunt field ,
Resistance
Figure 14.4 Protective trips for the parallel operation of two d,c. generators

generator is supplying the bus-bar system and a second generator is to
be paralleled with it. The second machine is run up to speed and its field
current adjusted until the two machines are at the same voltage. The
circuit breaker connecting the second machine to the bus-bar can now be
closed and the Field current adjusted to enable the generator to take its
share of the load. When the load is evenly shared the two machines can
then be left to operate in parallel. The equalising connection will cater
for any slight changes in load sharing that occur.