Electricity distribution

Electricity distribution

Overview

File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers. Transmission system elements are shown in blue, distribution system elements are in green.
rect 2 243 235 438 Power station
Power station
A power station is an industrial facility for the generation of electric energy....


rect 276 317 412 556 Transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...


rect 412 121 781 400 Electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...


rect 800 0 980 165 Transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...


desc bottom-left


Electricity distribution is the final stage in the delivery
Power delivery
Electricity delivery is the process that goes from generation of electricity in the power station to the use by the consumer.The main processes in power delivery are, by order:* Central generation* Transmission* Distribution* And retailing...

 of electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 to end users.
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File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers. Transmission system elements are shown in blue, distribution system elements are in green.
rect 2 243 235 438 Power station
Power station
A power station is an industrial facility for the generation of electric energy....


rect 276 317 412 556 Transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...


rect 412 121 781 400 Electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...


rect 800 0 980 165 Transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...


desc bottom-left


Electricity distribution is the final stage in the delivery
Power delivery
Electricity delivery is the process that goes from generation of electricity in the power station to the use by the consumer.The main processes in power delivery are, by order:* Central generation* Transmission* Distribution* And retailing...

 of electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 to end users. A distribution system's network
Grid (electricity)
An electrical grid is a vast, interconnected network for delivering electricity from suppliers to consumers. It consists of three main components: 1) generating plants that produce electricity from combustible fuels or non-combustible fuels ; 2) transmission lines that carry electricity from power...

 carries electricity from the transmission system
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...

 and delivers it to consumers. Typically, the network would include medium-voltage (less than 50 kV) power lines, substations
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

 and pole-mounted transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, low-voltage (less than 1 kV) distribution wiring and sometimes meters
Electricity meter
An electricity meter or energy meter is a device that measures the amount of electric energy consumed by a residence, business, or an electrically powered device....

.

Modern distribution systems


The modern distribution system begins as the primary circuit leaves the sub-station and ends as the secondary service enters the customer's meter socket. Distribution circuits serve many customers. The voltage used is appropriate for the shorter distance and varies from 2,300 to about 35,000 volts depending on utility standard practice, distance, and load to be served. Distribution circuits are fed from a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 located in an electrical substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

, where the voltage is reduced from the high values used for power transmission.

Conductors for distribution may be carried on overhead pole lines, or in densely-populated areas where they are buried underground. Urban and suburban distribution is done with three-phase
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

 systems to serve both residential, commercial, and industrial loads. Distribution in rural areas may be only single-phase if it is not economic to install three-phase power for relatively few and small customers.

Only large consumers are fed directly from distribution voltages; most utility customers are connected to a transformer, which reduces the distribution voltage to the relatively low voltage used by lighting and interior wiring systems. The transformer may be pole-mounted or set on the ground in a protective enclosure. In rural areas a pole-mount transformer may serve only one customer, but in more built-up areas multiple customers may be connected. In very dense city areas, a secondary network may be formed with many transformers feeding into a common bus at the utilization voltage. Each customer has an "electrical service" or "service drop" connection and a meter for billing. (Some very small loads, such as yard lights, may be too small to meter and so are charged only a monthly rate.)

A ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 connection to local earth is normally provided for the customer's system as well as for the equiment 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 on the lower-voltage conductors, or if a failure occurs within a distribution transformer. If all conductive objects are bonded to the same earth grounding system, the risk of electric shock is minimized. However, multiple connections between the utility ground and customer ground can lead to stray voltage
Stray voltage
Stray voltage describes the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages are often measured between two grounded objects in distant locations, due to normal current flow in the power system...

 problems; customer piping, swimming pools or other equimpent may develop objectionable voltages. These problems may be difficult to resolve since they often originate from places other than the customer's premises.

International differences


In many areas, "delta" three phase service is common. Delta service has no distributed neutral wire and is therefore less expensive. In North America and Latin America, three phase service is often a Y (wye) in which the neutral is directly connected to the center of the generator rotor. The neutral provides a low-resistance metallic return to the distribution transformer. Wye service is recognizable when a line has four conductors, one of which is lightly insulated. Three-phase wye service is excellent for motors and heavy power use.

Many areas in the world use single-phase
Single-phase electric power
In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors...

  or residential and light industrial service. In this system, the high voltage distribution network supplies a few substations per area, and the power from each substation is directly distributed. A hot wire and neutral are connected to the building from one phase of three phase service. Single-phase distribution is used where motor loads are small.

Americas
In the U.S. and parts of Canada and Latin America, split phase
Split phase
A split-phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial applications. It is the AC equivalent of the original Edison 3-wire direct current system...

 service is the most common. Split phase provides both and service with only three wires. The house voltages are provided by local transformers. The neutral is directly connected to the three-phase neutral. Socket voltages are only , but is available for heavy appliances because the two halves of a phase oppose each other.

Europe
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 and a single-phase voltage of . For industrial customers, 3-phase is also available.

Japan
Japan has a large number of small industrial manufacturers, and therefore supplies standard low-voltage three phase-service in many suburbs. Also, Japan normally supplies residential service as two phases of a three phase service, with a neutral. These work well for both lighting and motors.

Rural services


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 high voltages, which in turn permit use of galvanized steel wire. The strong steel wire permits inexpensive wide pole spacings. Other areas use high voltage split-phase or three phase service at higher cost.

Metering


Electricity meters use different metering equations depending on the form of electrical service. Since the math differs from service to service, the number of conductors and sensors in the meters also vary.

Terms


Besides referring to the physical wiring, the term electrical service also refers in an abstract sense to the provision of electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 to a building.

History


In the early days of electricity distribution, 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 there was no way of changing DC voltage levels, other than inefficient motor-generator sets. Low DC voltages were used (on the order of 100 volts) since that was a practical voltage for incandescent lamps, which were the primary electrical load. Low voltage also required less insulation for safe distribution within buildings.

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 used copper cable
Copper wire and cable
Copper has been used in electric wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 proved to be another early boon for copper wire....

, which is one of the best economically feasible conductors for this application. To reduce the current and copper required for a given quantity of power transmitted would require a higher transmission voltage, but no efficient method existed to change the voltage of DC power circuits. To keep losses to an economically practical level the Edison DC system needed thick cables and local generators. Early DC generating plants needed to be within about 1.5 miles (2.4 km) of the farthest customer to avoid excessively large and expensive conductors.

Introduction of alternating current


The competition between the direct current (DC) of Thomas Edison
Thomas Edison
Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

 and the alternating current (AC) of Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

 and George Westinghouse
George Westinghouse
George Westinghouse, Jr was an American entrepreneur and engineer who invented the railway air brake and was a pioneer of the electrical industry. Westinghouse was one of Thomas Edison's main rivals in the early implementation of the American electricity system...

 was known as the War of Currents
War of Currents
In the "War of Currents" era in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison's promotion of direct current for electric power distribution over alternating current advocated by several European companies and Westinghouse Electric based out of Pittsburgh,...

. At the conclusion of their campaigning, AC became the dominant form of transmission of power. Power transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, installed at power stations, could be used to raise the voltage from the generators, and transformers at local substations
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

 could reduce voltage to supply loads. Increasing the voltage reduced the current in the transmission and distribution lines and hence the size of conductors and distribution losses. This made it more economical to distribute power over long distances. Generators (such as hydroelectric sites) could be located far from the loads.

In North America, early distribution systems used a voltage of 2.2 kV corner-grounded delta. Over time, this was gradually increased to 2.4 kV. As cities grew, most 2.4 kV systems were upgraded to 2.4/4.16 kV, three-phase systems. In three phase networks that permit connections between phase and neutral, both the phase-to-phase voltage (4160, in this example) and the phase-to-neutral
Ground and neutral
Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding of equipment and structures...

 voltage are given; if only one value is shown, the network does not serve single-phase loads connected phase-to-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 3.3 kV 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 7-8 houses, whereas in the UK a typical urban or suburban low-voltage substation would normally be rated between 315 kVA and 1 MVA and supply a whole neighbourhood. This is because the higher voltage used in Europe (415 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 the transformers may be fewer, larger and more efficient, and due to diversity there need be less spare capacity in the transformers, reducing power wastage. In North American city areas with many customers per unit area, network distribution will be used, with multiple transformers and low-voltage buses interconnected over several city blocks.

Rural Electrification
Rural electrification
Rural electrification is the process of bringing electrical power to rural and remote areas. Electricity is used not only for lighting and household purposes, but it also allows for mechanization of many farming operations, such as threshing, milking, and hoisting grain for storage; in areas...

 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). 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, New Zealand and Australia; 11 kV and 22 kV are common in South Africa. Other voltages are occasionally used.

In New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, Saskatchewan, Canada
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....

, and South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

, single wire earth return
Single wire earth return
Single wire earth return or single wire ground return is a single-wire transmission line for supplying single-phase electrical power from an electrical grid to remote areas at low cost...

 systems (SWER) are used to electrify remote rural areas.

While power electronics now allow for conversion between DC voltage levels, AC is still used in distribution due to the economy, efficiency and reliability of transformers. High-voltage DC is used for transmission of large blocks of power over long distances, or for interconnecting adjacent AC networks, but not for distribution to customers.
Electric power is normally generated at 11-25kV in a power station. To transmit over long distances, it is then stepped-up to 400kV, 220kV or 132kV as necessary. Power is carried through a transmission network of high voltage lines. Usually, these lines run into hundreds of kilometres and deliver the power into a common power pool called the grid. The grid is connected to load centres (cities) through a sub-transmission network of normally 33kV (or sometimes 66kV) lines. These lines terminate into a 33kV (or 66kV) substation, where the voltage is stepped-down to 11kV for power distribution to load points through a distribution network of lines at 11kV and lowe

Distribution network configurations



Distribution networks are typically of two types, radial or interconnected (see spot network
Spot Network Substation
In electricity distribution networks, spot network substations are used in interconnected distribution networks. They have the secondary network with all supply transformers bussed together on the secondary side at one location. Spot networks are considered the most reliable and most flexible...

). 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 by closing and opening switches. Operation of these switches may be by remote control from a control center or by a lineman. 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 utility pole
Utility pole
A utility pole is a pole used to support overhead power lines and various other public utilities, such as cable, fibre optic cable, and related equipment such as transformers and street lights. It can be referred to as a telephone pole, power pole, hydro pole, telegraph pole, or telegraph post,...

s and wires and, increasingly, underground construction with cables and indoor or cabinet substations. However, underground distribution is significantly more expensive than overhead construction. In part to reduce this cost, underground power lines are sometimes co-located with other utility lines in what are called common utility duct
Common utility duct
A common utility duct, sometimes called a common utility conduit, is any structure – above, on, or below ground – that carries more than two types of public utility lines...

s. Distribution feeders emanating from a substation are generally controlled by a circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

 which will open when a fault is detected. Automatic circuit reclosers may be installed to further segregate the feeder thus minimizing the impact of faults.

Long feeders experience voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...

 requiring capacitors or voltage regulators to be installed.

Characteristics of the supply given to customers are generally mandated by contract
Contract
A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...

 between the supplier and customer. Variables of the supply include:
  • AC or DC
    Direct current
    Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

     - Virtually all public electricity supplies are AC today. Users of large amounts of DC power such as some electric railway
    Railway electrification system
    A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...

    s, telephone exchange
    Telephone exchange
    In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

    s and industrial processes such as aluminium
    Aluminium
    Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

     smelting usually either operate their own or have adjacent dedicated generating equipment, or use rectifiers to derive DC from the public AC supply
  • Voltage
    Voltage
    Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

    , including tolerance (usually +10 or -15 percent
    Percentage
    In mathematics, a percentage is a way of expressing a number as a fraction of 100 . It is often denoted using the percent sign, “%”, or the abbreviation “pct”. For example, 45% is equal to 45/100, or 0.45.Percentages are used to express how large/small one quantity is, relative to another quantity...

    )
  • Frequency, commonly 50 or 60 Hz, 16. Hz for some railways and, in a few older industrial and mining locations, 25 Hz.
  • Phase configuration (single phase, polyphase
    Polyphase system
    A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

     including two-phase and three phase)
  • Maximum demand (usually measured as the largest amount of power delivered within a 15 or 30 minute period during a billing period)
  • Load factor, expressed as a ratio of average load to peak load over a period of time. Load factor indicates the degree of effective utilization of equipment (and capital investment) of distribution line or system.
  • Power factor
    Power factor
    The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

     of connected load
  • Earthing arrangements
    Earthing system
    In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply, and regulations can vary considerably among...

     - TT, TN-S, TN-C-S or TN-C
  • Prospective short circuit current
  • Maximum level and frequency of occurrence of transients

Distribution industry


Traditionally the electricity industry has been a publicly owned institution but starting in the 1970s nations began the process of deregulation
Deregulation
Deregulation is the removal or simplification of government rules and regulations that constrain the operation of market forces.Deregulation is the removal or simplification of government rules and regulations that constrain the operation of market forces.Deregulation is the removal or...

 and privatisation, leading to electricity market
Electricity market
In economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...

s. A major focus of these was the elimination of the former so called natural monopoly
Natural monopoly
A monopoly describes a situation where all sales in a market are undertaken by a single firm. A natural monopoly by contrast is a condition on the cost-technology of an industry whereby it is most efficient for production to be concentrated in a single form...

of generation, transmission, and distribution. As a consequence, electricity has become more of a commodity. The separation has also led to the development of new terminology to describe the business units (e.g., line company, wires business and network company).

See also


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 http://www.distributedenergy.com/march-april-2004/distributed-energy-resources.aspx
  • SE Group Planning & Design for Vermont Dept of Public Service, Utility Line Location Issues Paper, Summary Report, January 2003 http://publicservice.vermont.gov/dockets/5496/5496fullreport.pdf
  • Short, T. A. Electric Power Distribution Handbook, CRC Press, 2004.
  • von Meier, A. Electric Power Systems: A Conceptual Introduction, John Wiley/IEEE Press, 2006.
  • 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.