Electricity generation and transmission concept
The purpose of the electric transmission system is the interconnection of the electric energy producing power plants or generating stations with the loads. A three-phase AC system is used for most transmission lines.
The operating frequency is 60 Hz in the U.S. and 50 Hz in Europe, Australia, and part of Asia.
The three-phase system has three phase conductors. The system voltage is defined as the RMS voltage between the conductors, also called line-to-line voltage. The voltage between the phase conductor and ground, called line-to-ground voltage, is equal to the line-to-line voltage divided by the square root of three.
The generating station produces the electric energy. The generator voltage is usually 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 Figure 1 the voltage is increased to 500 kV and an extra-high voltage (EHV) line transmits the generator-produced energy to a distant substation.
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.
Electricity distribution concept
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.
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 Figure 1 above.
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