Power generated from power stations are distributed to cities over a large distance through metal cables. Energy or power is lost during the transmission wire or line due to heating of the wires. The energy lost or dissipated in the wires is given by
Energy loss = I2Rt
Power can be transmitted from stations to other parts of the country on either low voltage or high current or low current and high voltage.
Low current leads to low energy loss during transmission, hence electrical power which is transmitted at a high voltage and low current is called high tension transmission
A step-up transformer is usually used to step up the voltage from the power station to about 132KVA or more before transmission to cities, and a step down transformer is used to step down the voltage before distributing it to consumers.