Electricity distribution is the final stage in the delivery of electricity. It is generally considered to include medium-voltage (less than 50 kV) power lines, electrical substations and pole-mounted transformers, low-voltage (less than 1000 V) distribution wiring and sometimes electricity meters.
Description
History
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 impossible to use more than 2500 volts within generators, and 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 (DC) system needed thick cables and local generators. Early DC generating plants needed to be within about 1.5 miles of the farthest customer to avoid the need for excessively large and expensive conductors.
Introduction of alternating current
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 economical 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 more stable voltages 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, 7620/13200Y, 14400/24940Y, and 19920/34500Y.
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 close to customers' premises. For example, in the US a pole-mounted transformer in a suburban setting may supply 1- 3 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 (380 V vs 230 V) may be carried over a greater distance with acceptable 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.
While power electronics do now allow for DC voltage conversion AC has retained is ubiquity in normal transmission and distribution due to the low cost, reliability and efficiancy of transformers. DC has only been reintroduced to power grids in some very large trasmission lines and other specialist applications.
Distribution network configurations
Distribution networks are typically of two types, radial or interconnected (see spot network substations [1]). A radial network leaves the station and passes through the network area with no normal connection to any other supply. This is typical of long rural lines with isolated load areas. An interconnected network is generally found in more urban areas and will have multiple connections to other points of supply.
These points of connection are normally open but allow various configurations by the operating utility linemen carefully closing and opening switches. The benefit of the interconnected model is that in the event of a fault or required maintenance a small area of network can be isolated and the remainder kept on supply.
Within these networks there may be a mix of overhead line construction utilizing traditional poles and wires and increasingly underground construction with cables and indoor or cabinet substations. However, such costs 11 times more than overhead lines -- air is a good insulator. Distribution feeders emanating from a substation are generally controlled by a circuit breaker which will open when a fault is detected. Automatic Circuit Reclosers may be installed to further segregate the feeder thus minimising the impact of faults.
Long feeders experience voltage drop requiring capacitors or voltage regulators to be installed, and the phase physical relationship to be interchanged.
Characteristics of the supply given to customers are generally mandated by contract between the supplier and customer. Deviations from the normal usage pattern usually invokes monthly surcharges. 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 or have adjacent dedicated generating equipment or have equipment to derive DC from the public AC supply
- Voltage, including tolerance (usually +10 or -15 percentage)
- Frequency, commonly 50 & 60 [[Hertz|Hz], exactly, over 24 hours, (16-2/3 for railways)
- * Phase configuration (single phase, split phase, polyphase including two phase and three phase)
- Maximum demand | amount of power delivered within a 15 minute period
- Power factor of connected load
- Earthing arrangements - TT, TN-S, TN-C-S or TN-C
- Maximum prospective short circuit current {not in USA)
- Maximum level and frequency of occurrence of transients
See List of countries with mains power plugs, voltages and frequencies.
Electricity industry "deregulation" reform started in the mid1990s has led to the creation of electricity markets through the separation of generation from transmission and distribution, and having electricity as a commodity, thru the breakup of the natural monopoly granted power companies prior to then. It also led to the development of new terminology to describe the distributor such as line company, wires business and network company.
See also
References
External links
Further reading
- Brown, R. E., Electric Power Distribution Reliability, Marcel Dekker, Inc., 2002.
- Burke, J., Power Distribution Engineering, Marcel Dekker, Inc., 1994.
- Hoffman, P., Scheer, R., Marchionini, B., Distributed Energy Resources: A Key Element of Grid Modernization DE - March/April 2004 [3]
- Short, T. A. Electric Power Distribution Handbook, CRC Press, 2004.
- Westinghouse Electric Corporation, Distribution Systems, vol. 3, 1965.
- Westinghouse Electric Corporation, Electric power transmission patents; Tesla polyphase system. (Transmission of power; polyphase system; Tesla patents)
- Willis, H. L., Power Distribution Planning Reference Book, Marcel Dekker, Inc., 2nd ed., 2004.