Power from the generating station may be
carried to the various points of consumption by
overhead transmission and distribution lines, by
underground cable, or by a combination of both.
At most advanced bases, OVERHEAD feeder
lines are commonly used because such lines are
cheaper to build, simpler to inspect, and easier
to maintain than UNDERGROUND cables.
Obviously, the use of underground cables is
preferred at airports and runways to prevent
hazardous flight conditions.
Overhead Power Distribution
Figure 9-2 shows a three-phase, three-wire
OVERHEAD power distribution system. Assume
that the system has an alternator generating 220
V (fig. 9-3). From the generating station, three-
phase, three-wire feeders carry the power
overhead to the distribution points (or centers),
from which two primary mains branch off. One
of these mains carries power to a lighting system
and a single-phase motor in a motor pool, each
of which is designed to operate on 110 V, and to
a three-phase motor designed to operate on 220
V. The 220-V, three-phase motor is connected
directly to the 220-V, three-phase primary main.
However, for the lighting system and 110-V
motor, two wires in the primary main are tapped
off to a transformer, which reduces the 220-V
primary main voltage to 110 V. The use of two
wires creates a single-phase voltage in the
secondary main to the motor pool. Similarly,
power to secondary mains running to the
operational headquarters, living quarters, and the
mess hall is reduced to 110 V and converted to
single phase.
A system may be a THREE-WIRE or a
FOUR-WIRE system, depending upon whether
the alternators are connected DELTA (A) or
WYE (Y). Figure 9-4 is a schematic diagram
showing a delta connection. The coil marked
Figure 9-2.-A typical overhead power distribution system.
9-2
