Electricity distribution
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Electricity distribution is the penultimate process in the delivery of electric power, i.e. the part between transmission and user purchase from an electricity retailer. It is generally considered to include medium-voltage (less than 50kV) power lines, low-voltage electrical substations and pole-mounted transformers, low-voltage (less than 1000V) distribution wiring and sometimes electricity meters.
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Description
In the early days of electricity generation, direct current (DC) generators were connected to loads at the same voltage. The generation, transmission and loads had to be of the same voltage because at the time there was no known way of doing DC voltage conversion (other than inefficient motor-generator sets). The voltages had to be fairly low with such systems due to the fact that it is difficult and dangerous to distribute high voltages to small loads. The losses in a cable are proportional to the square of the current, the length of the cable, and the resistivity of the material, and are inversely proportional to cross-sectional area. Early transmission networks were already using copper, which is one of the best economically feasible conductors for this application. To reduce the current while keeping power transmission constant requires increasing the voltage which, as previously mentioned, was problematic. This meant in order to keep losses to a reasonable level the Edison system needed thick cables and local generators.
The adoption of alternating current (AC) for electricity generation following the War of Currents dramatically changed the situation. Power transformers, installed at substations, could be used to raise the voltage from the generators and reduce it to supply loads. Increasing the voltage reduced the current in the transmission and distribution lines and hence the size of conductors required and distribution losses incurred. This made 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 with transmission systems to interconnect generating stations and distribution networks.
In North America, early distribution systems used a voltage of 2200 volts corner-grounded delta. Over time, this was gradually increased to 2400 volts. As cities grew, most 2400 volt systems were upgraded to 2400/4160 Y three-phase systems, which also benefited 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 7200/12470Y.
European systems used higher voltages, generally 3300 volts to ground, in support of the 220/380Y volt power systems used in those countries. In the UK, urban systems progressed to 6.6 kV and then 11 kV (phase to phase), the most common distribution voltage.
North American and European power distribution systems also differ in that North American systems tend to have a greater number of low-voltage step-down transformers located closer to customers' premises. For example, in the US a pole-mounted transformer in a suburban setting may supply only a single or a very few houses, whereas in the UK a typical urban or suburban low-voltage substation might be rated at 2 MW and supply a whole neighbourhood. This is because the higher voltage used in Europe (230 V vs 120 V) may be carried over a greater distance without unacceptable power loss. An advantage of the North American setup is that failure or maintenance on a single transformer will only affect a few customers. Advantages of the UK setup are that fewer transformers are required; larger and more efficient transformers are used, and due to diversity there need be less spare capacity in the transformers, reducing power wastage.
Rural Electrification systems, in contrast to urban systems, tend to use higher voltages because of the longer distances covered by those distribution lines (see Rural Electrification Administration). 7200 volts is commonly used in the United States; 11 kV and 33 kV are common in the UK, New Zealand and Australia; 11 kV and 22 kV are common in South Africa. Other voltages are occasionally used in unusual situations or where a local utility simply has engineering practices that differ from the norm.
In New Zealand, Australia,Saskatchewan,Canada and South Africa, single wire earth return systems (SWER) are used to electrify remote rural areas.
Characteristics of the supply given to customers are generally mandated by law and by contract between the supplier and customer. Variables include:
- AC or DC - Virtually all public electricity supplies are AC today. Users of large amounts of DC power such as some electric railways, telephone exchanges and industrial processes such as aluminium smelting either operate their own generating equipment or have equipment to derive DC from the public AC supply)
- Voltage, including tolerance (usually + or - a certain percentage)
- Frequency (50 Hz and 60 Hz are most common)
- Maximum current that may be drawn
- Phase configuration (single phase, split phase, polyphase including two phase and three phase)
- Restrictions on power factor of connected load
- earthing arrangements - TT, TN-S, TN-C-S or TN-C
- Maximum prospective short circuit current
- Maximum level and frequency of occurrence of transients
See List of countries with mains power plugs, voltages & frequencies.
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.
U.S. and U.K. terminology
| U.S.A. | U.K. |
| Grounded | Earthed |
| Wye or Y | Star |
See also
Other processes in power delivery are generation, transmission and retailing.
Reference
- Saskpower single-wire ground return electrification in 1949 (http://www.saskpower.com/pm/rural_elect/rural_elect2.shtml)
Further reading
- Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system". (Transmission of power; polyphase system; Tesla patents)