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Electric Power Transmission
Electric power transmission is one process in the delivery of electricity to consumers. It refers to the 'bulk' transfer of electrical power from place to place. Typically power transmission is between the power plant and a substation in the vicinity of a populated area. This is distinct from electricity distribution which is concerned with the delivery from the substation to the consumers. Due to the large amount of power involved, transmission normally takes place at high voltage (110 kV or above). Electricity is usually sent over long distance through overhead power transmission lines. Power is transmitted underground in densely populated areas (such as large cities) but is typically avoided due to the high capacitive and resistive losses incurred.
A power transmission system is sometimes referred to colloquially as a "grid". However, for reasons of economy, the network is rarely a grid (a folly connected network) in the mathematical sense. Redundant paths and lines are provided so that power can be routed from any power plant to any load center, through a variety of routes, based on the economics of the transmission path and the cost of power. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical limit of the line. Deregulation of electricity companies in many countries has lead to renewed interest in reliable economic design of transmission networks. The separation of transmission and generation functions is one of the factors that contributed to the 2003 North America blackout.
AC power transmission
AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. In electric railways, sometimes single phase AC current is used as traction current for railway traction.
Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Bulk power transmission
A transmission grid is a network of power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with 3-phase alternating current (AC).
The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the variable load than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the United States might cause many people there to turn on their air conditioners), imported electricity must often come from far away. Because of the irresistible economics of load balancing, transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow even if one link is disabled.