The figure below shows the concept of typical energy transmission and distribution systems. The generating station produces the electric energy. The generator voltage is around 15 to 25 kV. This relatively low voltage is not appropriate for the transmission of energy over long distances.
At the generating station a transformer is used to increase the voltage and reduce the current. In Fig. 4.3 the voltage is increased to 500 kV and an extra-high-voltage (EHV) line transmits the generator-produced energy to a distant substation.
Such substations are located on the outskirts of large cities or in the center of several large loads. As an example, in Arizona, a 500-kV transmission line connects the Palo Verde Nuclear Station to the Kyrene and Westwing substations, which supply a large part of the city of Phoenix.
The voltage is reduced at the 500 kV/220 kV EHV substation to the high-voltage level and high voltage
lines transmit the energy to high-voltage substations located within cities.
At the high-voltage substation the voltage is reduced to 69 kV. Sub-transmission lines connect the high-voltage substation to many local distribution stations located within cities. Sub-transmission lines are frequently located along major streets.
The voltage is reduced to 12 kV at the distribution substation. Several distribution lines emanate from each distribution substation as overhead or underground lines.
Distribution lines distribute the energy along streets and alleys. Each line supplies several step-down transformers distributed along the line.
The distribution transformer reduces the voltage to 230/115 V, which supplies houses, shopping centers, and other local loads. The large industrial plants and factories are supplied directly by a subtransmission line or a dedicated distribution line as shown in the figure.
The overhead transmission lines are used in open areas such as interconnections between cities or along wide roads within the city. In congested areas within cities, underground cables are used for electric energy transmission.
The underground transmission system is environmentally preferable but has a significantly higher cost. In the figure the 12-kV line is connected to a 12-kV cable which supplies commercial or industrial customers.
The figure also shows 12-kV cable networks supplying downtown areas in a large city. Most newly developed residential areas are supplied by 12-kV cables through pad-mounted step-down transformers as shown in the figure.