Electric power distribution

Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage of household appliances and typically feed several customers through secondary distribution lines at this voltage. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.

Electric power distribution only became necessary in the 1880s when electricity started being generated at power stations. Before that electricity was usually generated where it was used. The first power distribution systems installed in European and US cites were used to supply lighting: arc lighting running on very high voltage (usually higher than 3000 volt) alternating current (AC) or direct current (DC), and incandescent lighting running on low voltage (100 volt) direct current. Both were supplanting gas lighting systems, with arc lighting taking over large area/street lighting, and incandescent lighting replacing gas for business and residential lighting.

Due to the high voltages used in arc lighting, a single generating station could supply a long string of lights, up to 7-mile (11 km) long circuits, since the capacity of a wire is proportional to the square of the current traveling on it, each doubling of the voltage would allow the same size cable to transmit the same amount of power four times the distance. Direct current indoor incandescent lighting systems (for example the first Edison Pearl Street Station installed in 1882), had difficulty supplying customers more than a mile away due to the low 110 volt system being used throughout the system, from the generators to the final use. The Edison DC system needed thick copper conductor cables, and the generating plants needed to be within about 1.5 miles (2.4 km) of the farthest customer to avoid excessively large and expensive conductors.

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Wikipedia