Alternating-current (ac) transmission is the movement of ac electrical energy from one location to another, such as from a terminal at a generating station to a substation near a load center.

Direct-current (dc) transmission is the movement of dc electrical energy from one location to another. One terminal may be at a generating station. Direct current is economically used to transmit large blocks of power long distances.

Transmission Circuit History. The first electric generators produced direct current. These generators operated at a voltage matched to the load to be served. A Swiss engineer named Thury developed a high-voltage dc transmission system in 1899.

The invention of the ac transformer made it possible to step voltages up and down. Alternating-current generators used with transformers made it practical to locate the generators remote from the utilization area. Polyphase systems were developed because of improved efficiency over single-phase systems and reduced costs for a given quantity of power required to meet the customers' needs. The early systems generated, transmitted, and utilized two-phase ac power. The two-phase system is now obsolete. Three-phase ac systems with a frequency of 60 Hz (hertz) exist throughout the United States and Canada. The development of waterpower sites made it necessary to transmit electrical energy long distances from the generating stations to the factories located in the cities. High-voltage equipment was developed to permit the economic transmission of electrical energy from the hydroelectric generating stations to the load.

The development of transmission circuits across the continent has increased the...