Electricity distribution is the penultimate process in the delivery of electric power, i.e. the part between generation and user consumption. Other processes in power delivery are transmission and retailing.
In the early days of electricity generation, direct current (DC) generators were connected to loads at the same voltage. This imposed limitations on how far the distribution system could extend because of the voltage drop. It also meant that cables and lines had to be made from relatively large diameter copper in order to carry the high currentss required to meet the demand of distributed load. (Power lost in generating heat in a conductor is proportional to the square of the current ie Losses = I2R. These losses can be reduced by reducing the resistance (R) of the conductor, hence increasing the diameter; or, more effectively, by reducing the current (I).)
The adoption of alternating current (AC) for electricity generation dramatically changed the situation. Power transformers could be used to raise the voltage from the generators and reduce it to supply loads. Increasing the voltage reduced the current and hence the size of conductors and distribution losses, making it more economic to distribute power over long distances. The ability to transform to extra-high voltages enabled generators to be located far from loads and transmission systems to interconnect generating stations and distribution networkss.
In North America, early distribution systems were single phase and used a voltage of 2200 volts corner-grounded delta. Over time, this was gradually increased to 2400 volts. As cities grew and the use of three-phase power became more widespread, most 2400 volt systems were upgraded to 2400/4160Y three-phase systems, which also benefitted from better surge suppression due to the grounded neutral. Some city and suburban distribution systems continue to use this range of voltages, but most have been converted to 7200/12480Y.
European systems used higher voltages, generally 3300 volts to ground, in support of the 220Y/380 volt power systems used in those countries. In the UK, urban sysytems progressed to 6.6 kV and then 11 kV, the most common distribution voltage.
Rural Electrification systems, in contrast to urban systems, tend to use higher voltages because of the longer distances covered by those distribution lines. 7200 volts is commonly used in the United States. Other voltages are occasionally used in unusual situations or where a local utility simply has engineering practices that differ from the norm.
Electricity industry reform has led to the creation of electricity markets through the separation of contestable retailing from distribution, a natural monopoly and the separation of the monopoly transmission from generation. It also led to the development of new terminology to describe the distributor such as Line company, Wires Business and Network Company.
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