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Power distribution in a large combatant ship |
regulator (fig. 12-6) controls the voltage output
of the generator portion of the MG. This voltage
may be either 450 volts or 120 volts, depending
on the application. The frequency regulator
controls the speed of the MG motor, and therefore
the output frequency of the MG. Voltage- and
frequency-sensing circuits continuously sample the
output of the MG and provide feedback to their
respective regulators. By doing this, they maintain
the output voltage and frequency at the required
level (usually to plus or minus one-half percent
of the rated value). A magnetic controller is
provided to start and stop the unit. It also
protects the MG from operating at continuous
overload and removes power to the MG if there
is an undervoltage condition.
STATIC FREQUENCY CHANGERS
Frequency changers step up and refine the
frequency of the ship’s 60-Hz electrical power to
400 Hz. Most of the frequency changers installed
on board combat ships are static frequency
changers. Static frequency changers have no
rotating parts—they are all solid state. Static
frequency changers are reliable and efficient; they
are the only ones that provide the high-quality
power demanded by modern weapon systems.
A static frequency changer usually consists of
a three-phase rectifier and a three-phase inverter.
The rectifier changes the 60-Hz ac incoming
power. The inverter converts the dc power
delivered by the rectifier into 400-Hz output power
through the use of many input filters and
transformers.
SHIPBOARD POWER
DISTRIBUTION
Most ac power distribution systems in naval
ships are 450-volt, three-phase, 60-Hz, three-wire
systems. The ship’s service generator and
distribution switchboards are interconnected by
bus ties. This arrangement makes it possible to
connect any switchboard to feed power from its
generators to one or more of the other switch-
boards. The bus ties also connect two or more
switchboards so that the generator plants can be
operated in parallel. In large installations (fig.
12-7), distribution to loads is from the generator
and distribution switchboards or switchgear
groups to load centers, distribution panels, and
the loads, or directly from the load centers to
some loads.
On some ships, such as large aircraft carriers,
zone control of the ship’s service and emergency
distribution is provided. A load center switch-
board supplies power to the electrical loads within
the electrical zone in which it is located. Thus,
zone control is provided for all power within the
electrical zone. Emergency switchboards may
supply more than one zone.
GENERATOR AND
DISTRIBUTION SWITCHBOARDS
Ship’s service 450-volt, ac switchboards are
generally of the dead-front type (no live con-
nections exposed). These switchboards are built
to provide efficient and safe operation of the
electrical system. A typical power distribution
system in a destroyer consists of four generators
(two forward and two aft) and two distribution
switchboard. The distribution switchboards are
set up so that each one controls two generators.
All the necessary apparatus for generator control
and power distribution is incorporated in its
associated switchboard (fig. 12-8).
The ship’s forward distribution switchboard
is also used as the control switchboard. This
switchboard has instruments and controls for the
aft generators. These instruments and controls are
necessary to parallel the generators to equalize the
load. An automatic voltage regulator is mounted
on each switchboard to control the generator field
excitation and to maintain a constant ac generator
voltage during normal changes in load.
Two emergency diesel generator sets provide
electric power for limited lighting and for
vital auxiliaries if the ship’s service power
should fail. These units are located in the forward
and aft emergency generator rooms. The forward
emergency switchboard is normally energized
from the forward ship’s service switchboard. The
aft emergency switchboard is normally energized
from the aft ship’s service switchboard.
Dc power distribution systems are in use on
some older ships that have large deck machinery
loads. These systems, which consist of the ship’s
service generator and distribution switchboards,
are similar to the ac systems. On newer ships, dc
power is provided at the load with rectifiers that
change the ac power to dc power, when required.
COMPONENTS OF A SWITCHBOARD
Each switchboard includes one or more units,
such as a bus tie unit, a power distribution unit,
lighting distribution units or transformers, and
12-7
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