Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. Direct current may be obtained from an alternating current supply by use of a current-switching arrangement called a rectifier, which contains electronic elements or electromechanical elements that allow current to flow only in one direction.
Direct current is used to charge batteries, and in nearly all electronic systems as the power supply. Very large quantities of direct-current power are used in production of aluminum and other electrochemical processes. The first and simpler type of electricity is called direct current. A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems.
For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted. The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. High-voltage direct current transmission allows efficient use of energy sources remote from load centers. The disadvantages of HVDC are in conversion, switching and control. The required static inverters are expensive and have limited overload capacity.
