| |
Back
Power Plant Maintenance | Up
Construction Electrician Intermediate - Electricity handbook for electricians | Next
Network Distribution System |
CHAPTER 4
ELECTRICAL DISTRIBUTION
INTRODUCTION
As a Construction Electrician second class, you
may have to supervise the installation, maintenance,
and repair of overhead primary and secondary power
distribution systems. This chapter will provide the
necessary information to enable you to calculate
electrical loads and perform fundamental tasks in the
selection, by size and type, of distribution equipment.
When you perform the above-mentioned tasks,
remember, your primary goal should be the safety of
your troops.
A power distribution system includes all parts of
an electrical system between the power source and the
customer’s service entrance. The power source may be
either a local generating plant or a high-voltage
transmission line feeding a substation that reduces the
high voltage to a voltage suitable for local distribution.
At most advance bases, the source of power will be
generators connected directly to the load.
DISTRIBUTION SYSTEMS
CONFIGURATION
The configurations of four distribution systems are
defined in the following paragraphs. These four
distribution systems — radial, loop (ring), network,
and primary selective — are briefly described. For
additional information, review the Electric Power
Distribution Systems Operations, NAVFAC MO-201.
RADIAL DISTRIBUTION SYSTEM
A representative schematic of a radial distribution
system is shown in figure 4-1. You should note that the
independent feeders branch out to several distribution
centers without intermediate connections between
feeders.
The most frequently used system is the radial
distribution system because it is the simplest and least
expensive system to build. Operation and expansion
are simple. It is not as reliable as most systems unless
quality components are used. The fault or loss of a
cable, primary supply, or transformer will result in an
outage on all loads served by the feeder. Furthermore,
electrical service is interrupted when any piece of
Figure 4-1.—Radial distribution system.
service equipment must be de-energized to perform
routine maintenance and service.
Service on this type of feeder can be improved by
installing automatic circuit breakers that will reclose
the service at predetermined intervals. If the fault
continues after a predetermined number of closures,
the breaker will lock out until the fault is cleared and
service is restored by hand reset.
LOOP/RING DISTRIBUTION SYSTEM
The loop, or ring, system of distribution starts at
the substation and is connected to or encircles an area
serving one or more distribution transformers or load
centers. The conductor of the system returns to the
same substation.
The loop system (fig. 4-2) is more expensive to
build than the radial type, but it is more reliable. It may
be justified in an area where continuity of service is of
considerable importance, for example, a medical
center.
In the loop system, circuit breakers sectionalize
the loop on both sides of each distribution transformer
connected to the loop. The two primary feeder breakers
and the sectionalizing breakers associated with the
loop feeder are ordinarily controlled by pilot wire
relaying or directional overcurrent relays. Pilot wire
relaying is used when there are too many secondary
substations to obtain selective timing with directional
overcurrent relays.
4-1
|
