TRANSMISSION SYSTEM

system that includes both the transmission and distribution systems. To explain the two systems, we will begin with the transmission system. TRANSMISSION SYSTEM Refering to figure 2-1, you will see that the starting point for electrical power is its place of generation, or generating station, which uses fossil fuels, water pressure, or, in some locations, nuclear energy to drive turbine generators. The energy generated in these stations is generally in the range of 13,200 to 24,000 volts. That voltage is insufficient for economical transmission over long distances. Therefore, the voltage is raised to transmission levels of 138,000 to 765,000 volts at a transmission substation located at the generating station. A substation is a facility that contains transformers, switches, and other equipment that is used to raise or lower voltages to transmission or distribution levels and to protect the substation and the transmission lines or distribution feeders against faults. Sets of conductors that are energized with high voltage and transmit large bulks of power over relatively long distances are known as transmission lines or transmission circuits. Usually, these circuits are run overhead with structures supporting the conductors, which are attached to insulators. In some locations where it is not practical or permissible to have overhead high-voltage lines, the transmission lines may be run underground. The transmission lines shown in figure 2-1 are overhead and supported by towers. As shown in the figure, the transmission lines, or circuits, deliver power from the transmission sub- station located at the generating plant to customers located along the route. Where required throughout its length, transmission circuits are equipped with additional transmission substations that lower the voltage to reduced transmission (or subtransmission) levels. The transmission circuits are also equipped with distribution substations that reduce the voltage to required distribution levels. It is at the distribution substations that the distribution system begins. DISTRIBUTION SYSTEM The distribution system is that portion of the electrical power system that connects the transmission system to the user’s equipment. It includes distribution substations, feeder circuits, distribution centers, primary mains, distribution transformers, protective devices, secondary circuits, and services. Figure 2-2 shows the principal elements of a distribution system. A power distribution system may be either an overhead distribution line or an underground cable Figure 2-2.—Elements of a power distribution system. Figure 2-3.—Radial distribution system. system. Since it is less costly to construct, the overhead system is more common. However, in some instances, such as near an airfield, an underground system may be required. This chapter will discuss mainly the overhead distribution system. Substations The distribution substation transforms the trans- mission voltage to the proper distribution voltage levels and protects the substation and transmission lines against faults occurring in the feeder circuits. At advanced bases, the source of power may be generators 2-2