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Electric power distribution

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A 50 kVA pole-mounted distribution transformer

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 used by lighting, industrial equipment or household appliances. Often several customers are supplied from one transformer through secondary distribution lines. 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.

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Generation and transmission

Power stationTransformerElectric power transmissionTransformer
Simplified diagram of AC electricity delivery from generation stations to consumers' service drop.

Electric power begins at a generating station, where the potential difference can be as high as 13,800 volts.[1] AC is usually used. Users of large amounts of DC power such as some railway electrification systems, telephone exchanges and industrial processes such as aluminium smelting use rectifiers to derive DC from the public AC supply, or may have their own generation systems. High-voltage DC can be advantageous for isolating alternating-current systems or controlling the quantity of electricity transmitted. For example, Hydro-Québec has a direct-current line which goes from the James Bay region to Boston.[2]

From the generating station it goes to the generating station’s switchyard where a step-up transformer increases the voltage to a level suitable for transmission, from 44kV to 765kV. Once in the transmission system, electricity from each generating station is combined with electricity produced elsewhere. Electricity is consumed as soon as it is produced. It is transmitted at a very high speed, close to the speed of light.

Distribution overview

General layout of electricity networks. The voltages and loadings are typical of a European network.

The transition from transmission to distribution happens in a power substation, which has the following functions:[3]

  • Circuit breakers and switches enable the substation to be disconnected from the transmission grid or for distribution lines to be disconnected.
  • Transformers step down transmission voltages, 35kV or more, down to primary distribution voltages. These are medium voltage circuits, usually 600-35,000 V.[4]
  • From the transformer, power goes to the busbar that can split the distribution power off in multiple directions. The bus distributes power to distribution lines, which fan out to customers.

Urban distribution is mainly underground, sometimes in common utility ducts. Rural distribution is mostly above ground with utility poles, and suburban distribution is a mix.[4] Closer to the customer, a distribution transformer steps the primary distribution power down to a low-voltage secondary circuit, usually 120/240 volts in the US for residential customers. The power comes to the customer via a service drop and an electricity meter. The final circuit in an urban system may be less than 50 feet, but may be over 300 feet for a rural customer.[4]

Primary distribution

Primary distribution voltages range from 4 kV to 35 kV phase-to-phase (2.4 kV to 20 kV phase-to-neutral)[5] Only large consumers are fed directly from distribution voltages; most utility customers are connected to a transformer, which reduces the distribution voltage to the low voltage "utilization voltage", "supply voltage" or "mains voltage" used by lighting and interior wiring systems.

Network configurations

Substation near Yellowknife, in the Northwest Territories of Canada

Distribution networks are divided into two types, radial or network.[6] A radial system is arranged like a tree where each customer has one source of supply. A network system has multiple sources of supply operating in parallel. Spot networks are used for concentrated loads. Radial systems are commonly used in rural or suburban areas.

Radial systems usually include emergency connections where the system can be reconfigured in case of problems, such as a fault or required replacement. This can be done by opening and closing switches. It may be acceptable to close a loop[ambiguous] for a short time.

Long feeders experience voltage drop (power factor distortion) requiring capacitors to be installed.

Reconfiguration, by exchanging the functional links between the elements of the system, represents one of the most important measures which can improve the operational performance of a distribution system. The problem of optimization through the reconfiguration of a power distribution system, in terms of its definition, is a historical single objective problem with constraints. Since 1975, when Merlin and Back[7] introduced the idea of distribution system reconfiguration for active power loss reduction, until nowadays, a lot of researchers have proposed diverse methods and algorithms to solve the reconfiguration problem as a single objective problem. Some authors have proposed Pareto optimality based approaches (including active power losses and reliability indices as objectives). For this purpose, different artificial intelligence based methods have been used: microgenetic,[8] branch exchange,[9] particle swarm optimization[10] and non-dominated sorting genetic algorithm.[11]

Rural services

Rural electrification systems tend to use higher distribution voltages because of the longer distances covered by distribution lines (see Rural Electrification Administration). 7.2, 12.47, 25, and 34.5 kV distribution is common in the United States; 11 kV and 33 kV are common in the UK, Australia and New Zealand; 11 kV and 22 kV are common in South Africa and China.[12] Other voltages are occasionally used. Distribution in rural areas may be only single-phase if it is not economical to install three-phase power for relatively few and small customers.

Rural services normally try to minimize the number of poles and wires. Single-wire earth return (SWER) is the least expensive, with one wire. It uses higher voltages (than urban distribution), which in turn permits use of galvanized steel wire. The strong steel wire allows for less expensive wide pole spacing. In rural areas a pole-mount transformer may serve only one customer. In New Zealand, Australia, Saskatchewan, Canada, and South Africa, single-wire earth return systems (SWER) are used to electrify remote rural areas.

Higher voltage or three phase service, at a higher infrastructure and a higher cost, provide increased equipment efficiency and lower energy cost for large agricultural facilities, petroleum pumping facilities, or water plants.

In other rural areas, a "4-wire" system is used but, to reduce the number of conductors, the 4th neutral wire is connected to the ground to use that as a return (an unground wye system).

Secondary distribution

Electricity is delivered at a frequency of either 50 or 60 Hz, depending on the region. It is delivered to domestic customers as single-phase electric power In some countries as in Europe a three phase supply may be made available for larger properties. Seen in an oscilloscope, the domestic power supply in North America would look like a sine wave, oscillating between -170 volts and 170 volts, giving an effective voltage of 120 volts.[13] Three-phase power is more efficient in terms of power delivered per cable used, and is more suited to running large electric motors. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

A ground connection is normally provided for the customer's system as well as for the equipment owned by the utility. The purpose of connecting the customer's system to ground is to limit the voltage that may develop if high voltage conductors fall down onto lower-voltage conductors which are usually mounted lower to the ground, or if a failure occurs within a distribution transformer. Earthing systems can be TT, TN-S, TN-C-S or TN-C.

Regional variations

Worldwide electrical voltage and frequency

220-240 volt systems

Most of the world uses 50 Hz 220 or 230 V single phase 400V 3 phase for residential and light industrial services. In this system, the primary distribution network supplies a few substations per area, and the 230 V power from each substation is directly distributed. A live (hot) wire and neutral are connected to the building for each phase of three phase service. Single-phase distribution is used where motor loads are light. In Europe, electricity is normally distributed for industry and domestic use by the three-phase, four wire system. This gives a three-phase voltage of 400 volts wye service and a single-phase voltage of 230 volts. In the UK a typical urban or suburban low-voltage substation would normally be rated between 150 kVA and 1 MVA and supply a whole neighborhood of a few hundred houses. For industrial customers, 3-phase 690 / 400 volt is also available, or may be generated locally.[14] Large industrial customers have their own transformer(s) with an input from 11 kV to 220 kV.

110-120 volt systems

Most of the Americas use 60 Hz AC, the 120/240 volt split phase system domestically and three phase for larger installations. North American transforms usually power homes at 240 volts, similar to Europe's 230 volts. It is the split-phase that allows use of 120 volts in the home.

Japan's utility frequencies are 50 Hz and 60 Hz.

In the electricity sector in Japan, the standard frequencies for AC are 50 and 60 Hz. Parts of the country use 50 Hz, while other parts use 60 Hz.[15] This is a relic of the 1800s. Some local providers in Tokyo imported 50 Hz German equipment, while the local power providers in Osaka brought in 60 Hz generators from the United States. The grids grew until eventually the entire country was wired. Today the frequency is 50 Hz in Eastern Japan (including Tokyo, Yokohama, Tohoku, and Hokkaido) and 60 Hertz in Western Japan (including Nagoya, Osaka, Kyoto, Hiroshima, Shikoku, and Kyushu).[16]

Most household appliances are made to work on either frequency. The problem of incompatibility came into the public eye when the 2011 Tōhoku earthquake and tsunami knocked out about a third of the east’s capacity, and power in the west couldn’t be fully shared with the east, since the country does not have a common frequency.[15]

There are four high-voltage direct current (HVDC) converter stations that move power across Japan’s AC frequency border. Shin Shinano is a back-to-back HVDC facility in Japan which forms one of four frequency changer stations that link Japan's western and eastern power grids. The other three are at Higashi-Shimizu, Minami-Fukumitsu and Sakuma Dam. Together they can move up to 1.2 GW of power east or west.[17]

240 volt systems and 120 volt outlets

Most modern North American homes are wired to receive 240 volts from the transformer, and through the use of split-phase electrical power, can have both 120 volt receptacles and 240 volt receptacles. The 120 volts is typically used for lighting and most wall outlets. The 240 volt outlets are usually placed where the oven and stovetop, water heater, and clothes dryer would go (if they are electric, rather than using natural gas). Sometimes a 240 volt outlet is mounted in the garage for machinery or for charging an electric car.

See also

References

  1. ^ "Power Transmission and Distribution | Hydro-Québec". www.hydroquebec.com. Retrieved 2016-03-08.
  2. ^ "Extra-High-Voltage Transmission | 735 kV | Hydro-Québec". www.hydroquebec.com. Retrieved 2016-03-08.
  3. ^ "How Power Grids Work". HowStuffWorks. Retrieved 2016-03-18.
  4. ^ a b c Short, T.A. (2014). Electric Power Distribution Handbook. Boca Raton, Florida, USA: CRC Press. pp. 1–33. ISBN 978-1-4665-9865-2.
  5. ^ Csanyi, Edvard (10 August 2012). "Primary Distribution Voltage Levels". http://electrical-engineering-portal.com. EEP - Electrical Engineering Portal. Retrieved 9 March 2017. {{cite web}}: External link in |website= (help)
  6. ^ Abdelhay A. Sallam and Om P. Malik (May 2011). Electric Distribution Systems. IEEE Computer Society Press. p. 21. ISBN 9780470276822.{{cite book}}: CS1 maint: year (link)
  7. ^ Merlin, A.; Back, H. Search for a Minimal-Loss Operating Spanning Tree Configuration in an Urban Power Distribution System. In Proceedings of the 1975 Fifth Power Systems Computer Conference (PSCC), Cambridge, UK, 1–5 September 1975; pp. 1–18.
  8. ^ Mendoza, J.E.; Lopez, M.E.; Coello, C.A.; Lopez, E.A. Microgenetic multiobjective reconfiguration algorithm considering power losses and reliability indices for medium voltage distribution network. IET Gener. Transm. Distrib. 2009, 3, 825–840.
  9. ^ Bernardon, D.P.; Garcia, V.J.; Ferreira, A.S.Q.; Canha, L.N. Multicriteria distribution network reconfiguration considering subtransmission analysis. IEEE Trans. Power Deliv. 2010, 25, 2684–2691.
  10. ^ Amanulla, B.; Chakrabarti, S.; Singh, S.N. Reconfiguration of power distribution systems considering reliability and power loss. IEEE Trans. Power Deliv. 2012, 27, 918–926.
  11. ^ Tomoiagă, B.; Chindriş, M.; Sumper, A.; Sudria-Andreu, A.; Villafafila-Robles, R. Pareto Optimal Reconfiguration of Power Distribution Systems Using a Genetic Algorithm Based on NSGA-II. Energies 2013, 6, 1439-1455.
  12. ^ Chan, F. "Electric Power Distribution Systems". Electrical Engineering (PDF). Retrieved 12 March 2016.
  13. ^ "How Power Grids Work". HowStuffWorks. Retrieved 2016-03-18.
  14. ^ "The Bumpy Road to Energy Deregulation". EnPowered. 2016-03-28.
  15. ^ a b Gordenker, Alice (2011-07-19). "Japan's incompatible power grids". The Japan Times Online. ISSN 0447-5763. Retrieved 2016-03-12.
  16. ^ "Electricity in Japan". Japan-Guide.com. Retrieved 2016-03-12.
  17. ^ "Why Japan's Fragmented Grid Can't Cope". Spectrum.IEEE.org. Retrieved 2016-03-12.
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