system that includes both the transmission anddistribution systems. To explain the two systems, wewill begin with the transmission system.TRANSMISSION SYSTEMRefering to figure 2-1, you will see that the startingpoint for electrical power is its place of generation, orgenerating station, which uses fossil fuels, waterpressure, or, in some locations, nuclear energy to driveturbine generators. The energy generated in thesestations is generally in the range of 13,200 to 24,000volts. That voltage is insufficient for economicaltransmission over long distances. Therefore, the voltageis raised to transmission levels of 138,000 to 765,000volts at a transmission substation located at thegenerating station. A substation is a facility that containstransformers, switches, and other equipment that is usedto raise or lower voltages to transmission or distributionlevels and to protect the substation and the transmissionlines or distribution feeders against faults.Sets of conductors that are energized with highvoltage and transmit large bulks of power over relativelylong distances are known as transmission linesortransmission circuits. Usually, these circuits are runoverhead with structures supporting the conductors,which are attached to insulators. In some locationswhere it is not practical or permissible to have overheadhigh-voltage lines, the transmission lines may be rununderground. The transmission lines shown in figure2-1 are overhead and supported by towers.As shown in the figure, the transmission lines, orcircuits, deliver power from the transmission sub-station located at the generating plant to customerslocated along the route. Where required throughout itslength, transmission circuits are equipped withadditional transmission substations that lower thevoltage to reduced transmission (or subtransmission)levels. The transmission circuits are also equipped withdistribution substations that reduce the voltage torequired distribution levels. It is at the distributionsubstations that the distribution system begins.DISTRIBUTION SYSTEMThe distribution system is that portion of theelectrical power system that connects the transmissionsystem to the user’s equipment. It includes distributionsubstations, feeder circuits, distribution centers,primary mains, distribution transformers, protectivedevices, secondary circuits, and services. Figure 2-2shows the principal elements of a distribution system.A power distribution system may be either anoverhead distribution line or an underground cableFigure 2-2.—Elements of a power distribution system.Figure 2-3.—Radial distribution system.system. Since it is less costly to construct, the overheadsystem is more common. However, in some instances,such as near an airfield, an underground system may berequired. This chapter will discuss mainly the overheaddistribution system.SubstationsThe distribution substation transforms the trans-mission voltage to the proper distribution voltage levelsand protects the substation and transmission linesagainst faults occurring in the feeder circuits. Atadvanced bases, the source of power may be generators2-2
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