Power Transmission And Distribution

The major advantage of AC three-phase over DC power systems isthat the electrical power is generated economically in large power stationsrelatively far from the end users, transported at high voltage over longdistances with very little power loss and finally made available to theconsumers providing them with two different levels of voltage depending on theapplication needs. Today, the public electric power is supplied almostexclusively using three-phase systems with frequency of 50 or 60 Hz, dependingon the country.

The major componentsof electric power transmission and distribution systems are:
Transmission lines:overhead power lines are mainly used to transmit electrical energy from thepower stations to the consumers. However, in densely populated areas the powercan only be supplied via cables. 
Transformers:step up transformers increase the generated voltage to values suitable for highvoltage transmission systems, isolation transformers are used to exchange powerbetween networks, and step down transformers decrease the voltages to mediumvoltage level and further down to low voltage to be distributed to theconsumer.
Busbars, disconnectors and power circuit breakers: they arethe main components found in a switching station used for power distribution.
Various voltage levels are used fortransmitting power; the levels are determined by the amount of power and thedistance; the higher the transmission voltages, the lower the currents as wellas the transmission losses. However, it must also be considered that networkinvestment costs increase with the voltage.