The grid is a network that connects electric power generation plants to high voltage lines which carry electricity over some distance to substations – “transmission”. When a destination is reached, the substations decrease the voltage for “distribution” to medium voltage lines and then further to low voltage lines. Finally, a transformer on a telephone pole reduces it to a household voltage of 120 volts. See the diagram below.

The overall grid can be thought of as composed of three major sections: generation (plants and step up transformers), transmission (lines and transformers operating above 100,000 volts – 100kv) and distribution (lines and transformers under 100kv). Transmission lines operate at extremely high voltages 138,000 volts (138kv) to 765,000 volts (765kv). Transmission lines can be very long – across state lines and even country lines.

For the longer lines, more efficient high voltages are used. For example, if the voltage is doubled, the current is cut in half for the same amount of power being transmitted. Line transmission losses are proportional to the square of the current, so long line “losses” are cut by a factor of four if the voltage is doubled. “Distribution” lines are localized across cities and surrounding areas and fan out in a radial tree-like fashion. This tree-like structure grows outward from a substation, but for reliability purposes, it usually contains at least one unused backup connection to a nearby substation. This connection can be quickly enabled in case of an emergency so that a substation’s territory can be fed by an alternative substation.