Most electrical power lines carry alternating current. Very little direct current is used for electroplating, electro-deposition, electric lighting etc.
There are many good reasons for this choice of Alternating Current over Direct Current for electric power transmission. AC voltage can be stepped up or stepped down easily and without appreciable power loss, through the use of a transformer, but on the other hand direct current voltages cannot be changed without a considerable power loss. This is a very important factor in the transmission of electric power, since large amounts of power must be transmitted at very high voltages. At generating station the voltage is stepped up with the help of the transformers to very high voltages and sent over transmission line and at receiving end of the line the step down transformer step down the voltage to values which can be used for lighting and power.
Various kinds of electrical equipment require different voltages for proper operation, and these voltages can easily be obtained by using a transformer and AC power transmission line.
Since the power transmitted equals the voltage multiplied by the current (P = VI), and the size of the wire limits the maximum current which can be used, the voltage must be increased if more power is to be transmitted over the same size wires. Also, excessive current flow causes overheating of the wires, resulting in larger power loss, so that maximum current is kept as low as possible. The voltage, however, is limited only by the insulation of transmission line. Since the insulation can be easily strengthened, the voltage can be increased considerably, permitting the transfer of larger amounts of power with smaller wires and much less power load.
When current flows through a wire to reach the electrical device using power, there is a power loss in the wire loss in the wire proportional to the square of the current. Any reduction in the amount of current flow required to transmit power results in a reduction in the amount of power lost in transmission line.