ELECTRICAL DISTRIBUTION
INTRODUCTION
As aConstruction Electrician second class, youmay have to supervise the installation, maintenance,and repair of overhead primary and secondary powerdistribution systems. This chapter will provide thenecessary informationto enable you to calculateelectrical loads and perform fundamental tasks in theselection, by size and type, of distribution equipment.When you perform the above-mentioned tasks,remember, your primary goal should be the safety ofyour troops.
A power distribution system includes all parts ofan electrical system between the power source and thecustomer’s service entrance. The power source may beeither alocal generating plant or a high-voltagetransmission line feeding a substation that reduces thehigh voltage to a voltage suitable for local distribution.At most advance bases, the source of power will begenerators connected directly to the load.
DISTRIBUTION SYSTEMSCONFIGURATION
The configurations of four distribution systems aredefined inthe following paragraphs. These fourdistribution systems — radial, loop (ring), network,and primary selective — are briefly described. Foradditional information, review the Electric PowerDistribution Systems Operations, NAVFAC MO-201.
RADIAL DISTRIBUTIONSYSTEM
A representative schematic of a radial distributionsystem is shown in figure 4-1. You should note that theindependent feeders branch out to several distributioncenters without intermediate connections betweenfeeders.
The most frequently used system is the radialdistribution system because it is the simplest and leastexpensive system to build. Operation and expansionare simple. It is not as reliable as most systems unlessquality components are used. The fault or loss of acable, primary supply, or transformer will result in anoutage on all loads served by the feeder. Furthermore,electrical service is interrupted when any piece ofservice equipment mustbe de-energized to performroutine maintenance and service.
Service on this type of feeder can be improved byinstalling automatic circuit breakers that will reclosethe service at predetermined intervals. If the faultcontinues after a predetermined number of closures,the breaker will lock out until the fault is cleared andservice is restored by hand reset.
Figure 4-1.—Radial distribution system.
LOOP/RING DISTRIBUTIONSYSTEM
The loop, or ring, system of distribution starts atthe substation and is connected to or encircles an areaserving one or more distribution transformers or loadcenters. The conductor of the system returns to thesame substation.
The loop system (fig. 4-2) is more expensive tobuild than the radial type, but it is more reliable. It maybe justified in an area where continuity of service is ofconsiderable importance, for example, a medicalcenter.
In theloop system, circuit breakers sectionalizethe loop on both sides of each distribution transformerconnected to the loop. The twoprimary feeder breakersand the sectionalizing breakers associated with theloop feeder are ordinarily controlled by pilot wirerelaying or directional overcurrent relays. Pilot wirerelaying is used when there are too many secondarysubstations to obtain selective timing with directionalovercurrent relays
Figure 4-2.—Loop, or ring, distribution system.
A fault in the primary loop is cleared by thebreakers in the loop nearest the fault, and power issupplied the other way around the loop withoutinterruption to most of the connected loads. Becausethe load points can be supplied from two or moredirections, it is possible to remove any section of theloop from service for maintenance without causing anoutage at other load points. If a fault occurs in a sectionadjacent to the distribution substation, the entire loadmay have to be fed from one side of the loop untilrepairs are made. Sufficient conductor capacity mustbe provided in the loop to permit operation withoutexcessive voltage drop or overheating of the feederwhen either side of the loop is out of service. If a fault occurs in the distribution transformer, it is cleared bythe breaker in the primary leads; and the loop remainsintact.
NETWORK DISTRIBUTIONSYSTEM
The network and radial systems differ with respectto the transformer secondaries. In a network system (fig.4-3) transformer secondaries are paralleled; in a radialsystem, they are not.
The network is the most flexible type of primarysystem; it provides the best service reliability to thedistribution transformers or load center, particularlywhen the system is supplied from two or moredistribution substations. Power can flow from anysubstation to any distribution transformer or loadcenter in the network system. The network system ismore flexible with regard to load growth than the radialor loop system and is adaptable to any rate of loadgrowth. Service readily can be extended to additionalpoints of usage with relatively small amounts of newconstruction. The network system, however, requireslarge quantities of equipment and extensive relaying;therefore, it is more expensive than the radial system.From the standpoint of economy, the network systemis suitable only in heavy-load-density areas where theload centerunits range from 1,000 to 4,000kilovoltamperes(kVA)
Figure 4-3.—Network distribution system