Electric power system 1

Electric power system

A steam turbine used to provide electric power.

An electric power system is a network of electrical componentsused to supply, transmit and use electric power. An example of anelectric power system is the network that supplies a region's homesand industry with power - for sizable regions, this power system isknown as the grid and can be broadly divided into the generatorsthat supply the power, the transmission system that carries thepower from the generating centres to the load centres and thedistribution system that feeds the power to nearby homes andindustries. Smaller power systems are also found in industry,hospitals, commercial buildings and homes. The majority of thesesystems rely upon three-phase AC power - the standard forlarge-scale power transmission and distribution across the modernworld. Specialised power systems that do not always rely uponthree-phase AC power are found in aircraft, electric rail systems,ocean liners and automobiles.

History

A sketch of the Pearl Street Station

In 1881 two electricians built the world's first power system atGodalming in England. It was powered by a power stationconsisting of two waterwheels that produced an alternating currentthat in turn supplied seven Siemans arc lamps at 250 volts and 34incandescent lamps at 40 volts.[1] However supply to the lampswas intermittent and in 1882 Thomas Edison and his company,The Edison Electric Light Company, developed the first steampowered electric power station on Pearl Street in New York City.The Pearl Street Station initially powered around 3,000 lamps for59 customers.[2][3] The power station used direct current andoperated at a single voltage. Direct current power could not beeasily transformed to the higher voltages necessary to minimisepower loss during long-distance transmission, so the maximumeconomic distance between the generators and load was limited toaround half-a-mile (800 m).[4]

That same year in London Lucien Gaulard and John Dixon Gibbsdemonstrated the first transformer suitable for use in a real powersystem. The practical value of Gaulard and Gibbs' transformer wasdemonstrated in 1884 at Turin where the transformer was used tolight up forty kilometres (25 miles) of railway from a singlealternating current generator.[5] Despite the success of the system, the pair made some fundamental mistakes.Perhaps the most serious was connecting the primaries of the transformers in series so that active lamps would affect

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the brightness of other lamps further down the line. Following the demonstration George Westinghouse, anAmerican entrepreneur, imported a number of the transformers along with a Siemens generator and set his engineersto experimenting with them in the hopes of improving them for use in a commercial power system. In July 1888,Westinghouse also licensed Nikola Tesla's US patents for a polyphase AC induction motor and transformer designsand hired Tesla for one year to be a consultant at the Westinghouse Electric & Manufacturing Company's Pittsburghlabs.[6]

One of Westinghouse's engineers, William Stanley, recognised the problem with connecting transformers in series asopposed to parallel and also realised that making the iron core of a transformer a fully enclosed loop would improvethe voltage regulation of the secondary winding. Using this knowledge he built a much improved alternating currentpower system at Great Barrington, Massachusetts in 1886.[7]

By 1890 the electric power industry was flourishing, and power companies had built thousands of power systems(both direct and alternating current) in the United States and Europe. These networks were effectively dedicated toproviding electric lighting. During this time a fierce rivalry known as the "War of Currents" emerged betweenThomas Edison and George Westinghouse over which form of transmission (direct or alternating current) wassuperior.[8] In 1891, Westinghouse installed the first major power system that was designed to drive a 100horsepower (75 kW) synchronous electric motor, not just provide electric lighting, at Telluride, Colorado.[9] On theother side of the Atlantic, Oskar von Miller built a 20 kV 176 km three-phase transmission line from Lauffen amNeckar to Frankfurt am Main for the Electrical Engineering Exhibition in Frankfurt.[10] In 1895, after a protracteddecision-making process, the Adams No. 1 generating station at Niagara Falls began transferring three-phasealternating current power to Buffalo at 11 kV. Following completion of the Niagara Falls project, new powersystems increasingly chose alternating current as opposed to direct current for electrical transmission.[11]

Developments in power systems continued beyond the nineteenth century. In 1936 the first experimental HVDC(high voltage direct current) line using mercury arc valves was built between Schenectady and Mechanicville, NewYork. HVDC had previously been achieved by series-connected direct current generators and motors (the Thurysystem) although this suffered from serious reliability issues.[12] In 1957 Siemens demonstrated the first solid-staterectifier, but it was not until the early 1970s that solid-state devices became the standard in HVDC.[13] In recenttimes, many important developments have come from extending innovations in the information technology andtelecommunications field to the power engineering field. For example, the development of computers meant loadflow studies could be run more efficiently allowing for much better planning of power systems. Advances ininformation technology and telecommunication also allowed for remote control of a power system's switchgear andgenerators.

Basics of electric power

An external AC to DC power adapter used forhousehold appliances

Electric power is the mathematical product of two quantities: currentand voltage. These two quantities can vary with respect to time (ACpower) or can be kept at constant levels (DC power).

Most refrigerators, air conditioners, pumps and industrial machineryuse AC power whereas most computers and digital equipment use DCpower (the digital devices you plug into the mains typically have aninternal or external power adapter to convert from AC to DC power).AC power has the advantage of being easy to transform betweenvoltages and is able to be generated and utilised by brushlessmachinery. DC power remains the only practical choice in digitalsystems and can be more economical to transmit over long distances atvery high voltages (see HVDC).[14][15]

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The ability to easily transform the voltage of AC power is important for two reasons: Firstly, power can betransmitted over long distances with less loss at higher voltages. So in power systems where generation is distantfrom the load, it is desirable to step-up (increase) the voltage of power at the generation point and then step-down(decrease) the voltage near the load. Secondly, it is often more economical to install turbines that produce highervoltages than would be used by most appliances, so the ability to easily transform voltages means this mismatchbetween voltages can be easily managed.[14]

Solid state devices, which are products of the semiconductor revolution, make it possible to transform DC power todifferent voltages, build brushless DC machines and convert between AC and DC power. Nevertheless devicesutilising solid state technology are often more expensive than their traditional counterparts, so AC power remains inwidespread use.[16]

Components of power systems

Supplies

The majority of the world's power still comes from coal-fired powerstations like this.

All power systems have one or more sources of power.For some power systems, the source of power isexternal to the system but for others it is part of thesystem itself - it is these internal power sources that arediscussed in the remainder of this section. Directcurrent power can be supplied by batteries, fuel cells orphotovoltaic cells. Alternating current power istypically supplied by a rotor that spins in a magneticfield in a device known as a turbo generator. Therehave been a wide range of techniques used to spin aturbine's rotor, from steam heated using fossil fuel(including coal, gas and oil) or nuclear energy, fallingwater (hydroelectric power) and wind (wind power).

The speed at which the rotor spins in combination withthe number of generator poles determines the frequency of the alternating current produced by the generator. Allgenerators on a single synchronous system, for example the national grid, rotate at sub-multiples of the same speedand so generate electrical current at the same frequency. If the load on the system increases, the generators willrequire more torque to spin at that speed and, in a typical power station, more steam must be supplied to the turbinesdriving them. Thus the steam used and the fuel expended are directly dependent on the quantity of electrical energysupplied. An exception exists for generators incorporating power electronics such as gearless wind turbines or linkedto a grid through an asynchronous tie such as a HVDC link — these can operate at frequencies independent of thepower system frequency.

Depending on how the poles are fed, alternating current generators can produce a variable number of phases ofpower. A higher number of phases leads to more efficient power system operation but also increases theinfrastructure requirements of the system.[17]

Electricity grid systems connect multiple generators and loads operating at the same frequency and number of phases, the commonest being three-phase at 50 or 60 Hz. However there are other considerations. These range from the obvious: How much power should the generator be able to supply? What is an acceptable length of time for starting the generator (some generators can take hours to start)? Is the availability of the power source acceptable (some renewables are only available when the sun is shining or the wind is blowing)? To the more technical: How should the generator start (some turbines act like a motor to bring themselves up to speed in which case they need an

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appropriate starting circuit)? What is the mechanical speed of operation for the turbine and consequently what are thenumber of poles required? What type of generator is suitable (synchronous or asynchronous) and what type of rotor(squirrel-cage rotor, wound rotor, salient pole rotor or cylindrical rotor)?[18]

Loads

A toaster is great example of a single-phase load thatmight appear in a residence. Toasters typically draw 2to 10 amps at 110 to 260 volts consuming around 600

to 1200 watts of power

Power systems deliver energy to loads that perform a function.These loads range from household appliances to industrialmachinery. Most loads expect a certain voltage and, for alternatingcurrent devices, a certain frequency and number of phases. Theappliances found in your home, for example, will typically besingle-phase operating at 50 or 60 Hz with a voltage between 110and 260 volts (depending on national standards). An exceptionexists for centralized air conditioning systems as these are nowtypically three-phase because this allows them to operate moreefficiently. All devices in your house will also have a wattage, thisspecifies the amount of power the device consumes. At any onetime, the net amount of power consumed by the loads on a powersystem must equal the net amount of power produced by thesupplies less the power lost in transmission.[19][20]

Making sure that the voltage, frequency and amount of powersupplied to the loads is in line with expectations is one of the great challenges of power system engineering.However it is not the only challenge, in addition to the power used by a load to do useful work (termed real power)many alternating current devices also use an additional amount of power because they cause the alternating voltageand alternating current to become slightly out-of-sync (termed reactive power). The reactive power like the realpower must balance (that is the reactive power produced on a system must equal the reactive power consumed) andcan be supplied from the generators, however it is often more economical to supply such power from capacitors (see"Capacitors and reactors" below for more details).[21]

A final consideration with loads is to do with power quality. In addition to sustained overvoltages and undervoltages(voltage regulation issues) as well as sustained deviations from the system frequency (frequency regulation issues),power system loads can be adversely affected by a range temporal issues. These include voltage sags, dips andswells, transient overvoltages, flicker, high frequency noise, phase imbalance and poor power factor.[22] Powerquality issues occur when the power supply to a load deviates from the ideal: For an AC supply, the ideal is thecurrent and voltage in-sync fluctuating as a perfect sine wave at a prescribed frequency with the voltage at aprescribed amplitude. For DC supply, the ideal is the voltage not varying from a prescribed level. Power qualityissues can be especially important when it comes to specialist industrial machinary or hospital equipment.

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ConductorsConductors carry power from the generators to the load. In a grid, conductors may be classified as belonging to thetransmission system, which carries large amounts of power at high voltages (typically more than 50 kV) from thegenerating centres to the load centres, or the distribution system, which feeds smaller amounts of power at lowervoltages (typically less than 50 kV) from the load centres to nearby homes and industry.[23]

Choice of conductors is based upon considerations such as cost, transmission losses and other desirablecharacteristics of the metal like tensile strength. Copper, with lower resistivity than aluminium, was the conductor ofchoice for most power systems. However, aluminum has lower cost for the same current carrying capacity and is theprimary metal used for transmission line conductors. Overhead line conductors may be reinforced with steel oraluminum alloys.[24]

Conductors in exterior power systems may be placed overhead or underground. Overhead conductors are usually airinsulated and supported on porcelain, glass or polymer insulators. Cables used for underground transmission orbuilding wiring are insulated with cross-linked polyethylene or other flexible insulation. Large conductors arestranded for ease of handling; small conductors used for building wiring are often solid, especially in lightcommercial or residential construction.[25]

Conductors are typically rated for the maximum current that they can carry at a given temperature rise over ambientconditions. As current flow increases through a conductor it heats up. For insulated conductors, the rating isdetermined by the insulation.[26] For overhead conductors, the rating is determined by the point at which the sag ofthe conductors would become unacceptable.[27]

Capacitors and reactorsThe majority of the load in a typical AC power system, is inductive; the current lags behind the voltage. Since thevoltage and current are out-of-sync, this leads to the emergence of a "useless" form of power known as reactivepower. Reactive power does no measurable work but is transmitted back and forth between the reactive power sourceand load every cycle. This reactive power can be provided by the generators themselves but it is often cheaper toprovide it through capacitors, hence capacitors are often placed near inductive loads to reduce current demand on thepower system. Power factor correction may be applied at a central substation or adjacent to large loads.Reactors consume reactive power and are used to regulate voltage on long transmission lines. In light loadconditions, where the loading on transmission lines is well below the surge impedance loading, the efficiency of thepower system may actually be improved by switching in reactors. Reactors installed in series in a power system alsolimit rushes of current flow, small reactors are therefore almost always installed in series with capacitors to limit thecurrent rush associated with switching in a capacitor. Series reactors can also be used to limit fault currents.Capacitors and reactors are switched by circuit breakers, which results in moderately large steps in reactive power. Asolution comes in the form of static VAR compensators and static synchronous compensators. Briefly, static VARcompensators work by switching in capacitors using thyristors as opposed to circuit breakers allowing capacitors tobe switched-in and switched-out within a single cycle. This provides a far more refined response than circuit breakerswitched capacitors. Static synchronous compensators take it a step further by achieving reactive power adjustmentsusing only power electronics.

Power electronicsPower electronics are semi-conductor based devices that are able to switch quantities of power ranging from a few hundred watts to several hundred megawatts. Despite their relatively simple function, their speed of operation (typically in the order of nanoseconds[28]) means they are capable of a wide range of tasks that would be difficult or impossible with conventional technology. The classic function of power electronics is rectification, or the conversion of AC-to-DC power, power electronics are therefore found in almost every digital device that is supplied from an AC

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source either as an adapter that plugs into the wall (see photo in Basics of Electric Power section) or as componentinternal to the device. High-powered power electronics can also be used to convert AC power to DC power for longdistance transmission in a system known as HVDC. HVDC is used because it proves to be more economical thansimilar high voltage AC systems for very long distances (hundreds to thousands of kilometres). HVDC is alsodesirable for interconnects because it allows frequency independence thus improving system stability. Powerelectronics are also essential for any power source that is required to produce an AC output but that by its natureproduces a DC output. They are therefore used by many photovoltaic installations both industrial and residential.Power electronics also feature in a wide range of more exotic uses. They are at the heart of all modern electric andhybrid vehicles - where they are used for both motor control and as part of the brushless DC motor. Powerelectronics are also found in practically all modern petrol-powered vehicles, this is because the power provided bythe car's batteries alone is insufficient to provide ignition, air-conditioning, internal lighting, radio and dashboarddisplays for the life of the car. So the batteries must be recharged while driving using DC power from the engine - afeat that is typically accomplished using power electronics. Whereas conventional technology would be unsuitablefor a modern electric car, commutators can and have been used in petrol-powered cars, the switch to alternators incombination with power electronics has occurred because of the improved durability of brushless machinery.[29]

Some electric railway systems also use DC power and thus make use of power electronics to feed grid power to thelocomotives and often for speed control of the locomotive's motor. In the middle twentieth century, rectifierlocomotives were popular, these used power electronics to convert AC power from the railway network for use by aDC motor.[30] Today most electric locomotives are supplied with AC power and run using AC motors, but still usepower electronics to provide suitable motor control. The use of power electronics to assist with motor control andwith starter circuits cannot be underestimated and, in addition to rectification, is responsible for power electronicsappearing in a wide range of industrial machinery. Power electronics even appear in modern residential airconditioners.Power electronics are also at the heart of the variable-speed wind turbine. Put simply, conventional wind turbinesrequire significant engineering to ensure they operate at some ratio of the system frequency (the ratio beingaccounted for using gears), however by using power electronics this requirement can be eliminated as can the gearsleading to quieter, more flexible and (at the moment) more costly wind turbines. A final example of one of the moreexotic uses of power electronics comes from the previous section where the fast-switching times of powerelectronics were used to provide more refined reactive compensation to the power system.

Protective devicesPower systems contain protective devices to prevent injury or damage during failures. The quintessential protectivedevice is the fuse. When the current through a fuse exceeds a certain threshold, the fuse element melts, producing anarc across the resulting gap that is then extinguished, interrupting the circuit. Given that fuses can be built as theweak point of a system, fuses are ideal for protecting circuitry from damage. Fuses however have two problems:First, after they have functioned, fuses must be replaced as they cannot be reset. This can prove inconvenient if thefuse is at a remote site or a spare fuse is not on hand. And second, fuses are typically inadequate as the sole safetydevice in most power systems as they allow current flows well in excess of that that would prove lethal to a humanor animal.The first problem is resolved by the use of circuit breakers - devices that can be reset after they have broken current flow. In modern systems that use less than about 10 kW, miniature circuit breakers are typically used. These devices combine the mechanism that initiates the trip (by sensing excess current) as well as the mechanism that breaks the current flow in a single unit. Some miniature circuit breakers operate solely on the basis of electromagnetism. In these miniature circuit breakers, the current is run through a solenoid, and, in the event of excess current flow, the magnetic pull of the solenoid is sufficient to force open the circuit breaker's contacts (often indirectly through a tripping mechanism). A better design however arises by inserting a bimetallic strip before the solenoid - this means

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that instead of always producing a magnetic force, the solenoid only produces a magnetic force when the current isstrong enough to deform the bimetallic strip and complete the solenoid's circuit.In higher powered applications, the protective relays that detect a fault and initiate a trip are separate from the circuitbreaker. Early relays worked based upon electromagnetic principles similar to those mentioned in the previousparagraph, modern relays are application-specific computers that determine whether to trip based upon readings fromthe power system. Different relays will initiate trips depending upon different protection schemes. For example, anovercurrent relay might initiate a trip if the current on any phase exceeds a certain threshold whereas a set ofdifferential relays might initiate a trip if the sum of currents between them indicates there may be current leaking toearth. The circuit breakers in higher powered applications are different too. Air is typically no longer sufficient toquell the arc that forms when the contacts are forced open so a variety of techniques are used. One of the mostpopular techniques is to keep the chamber enclosing the contacts flooded with sulfur hexafluoride (SF6) - a non-toxicgas that has sound arc-quelling properties. Other techniques are discussed in the reference.[31]

The second problem, the inadequacy of fuses to act as the sole safety device in most power systems, is probably bestresolved by the use of residual current devices (RCDs). In any properly functioning electrical appliance the currentflowing into the appliance on the active line should equal the current flowing out of the appliance on the neutral line.A residual current device works by monitoring the active and neutral lines and tripping the active line if it notices adifference.[32] Residual current devices require a separate neutral line for each phase and to be able to trip within atime frame before harm occurs. This is typically not a problem in most residential applications where standardwiring provides an active and neutral line for each appliance (that's why your power plugs always have at least twotongs) and the voltages are relatively low however these issues do limit the effectiveness of RCDs in otherapplications such as industry. Even with the installation of an RCD, exposure to electricity can still prove lethal.

SCADA systemsIn large electric power systems, Supervisory Control And Data Acquisition (SCADA) is used for tasks such asswitching on generators, controlling generator output and switching in or out system elements for maintenance. Thefirst supervisory control systems implemented consisted of a panel of lamps and switches at a central console nearthe controlled plant. The lamps provided feedback on the state of plant (the data acquisition function) and theswitches allowed adjustments to the plant to be made (the supervisory control function). Today, SCADA systems aremuch more sophisticated and, due to advances in communication systems, the consoles controlling the plant nolonger need to be near the plant itself. Instead it is now common for plant to be controlled from a with equipmentsimilar to (if not identical to) a desktop computer. The ability to control such plant through computers has increasedthe need for security and already there have been reports of cyber-attacks on such systems causing significantdisruptions to power systems.[33]

Power systems in practiceDespite their common components, power systems vary widely both with respect to their design and how theyoperate. This section introduces some common power system types and briefly explains their operation.

Residential power systemsResidential dwellings almost always take supply from the low voltage distribution lines or cables that run past the dwelling. These operate at voltages of between 110 and 260 volts (phase-to-earth) depending upon national standards. A few decades ago small dwellings would be fed a single phase using a dedicated two-core service cable (one core for the active phase and one core for the neutral return). The active line would then be run through a main isolating switch in the fuse box and then split into one or more circuits to feed lighting and appliances inside the house. By convention, the lighting and appliance circuits are kept separate so the failure of an appliance does not leave the dwelling's occupants in the dark. All circuits would be fused with an appropriate fuse based upon the wire

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size used for that circuit. Circuits would have both an active and neutral wire with both the lighting and powersockets being connected in parallel. Sockets would also be provided with a protective earth. This would be madeavailable to appliances to connect to any metallic casing. If this casing were to become live, the theory is theconnection to earth would cause an RCD or fuse to trip - thus preventing the future electrocution of an occupanthandling the appliance. Earthing systems vary between regions, but in countries such as the United Kingdom andAustralia both the protective earth and neutral line would be earthed together near the fuse box before the mainisolating switch and the neutral earthed once again back at the distribution transformer.[34]

There have been a number of minor changes over the year to practice of residential wiring. Some of the mostsignificant ways modern residential power systems tend to vary from older ones include:•• For convenience, miniature circuit breakers are now almost always used in the fuse box instead of fuses as these

can easily be reset by occupants.•• For safety reasons, RCDs are now installed on appliance circuits and, increasingly, even on lighting circuits.•• Dwellings are typically connected to all three-phases of the distribution system with the phases being arbitrarily

allocated to the house's single-phase circuits.•• Whereas air conditioners of the past might have been fed from a dedicated circuit attached to a single phase,

centralised air conditioners that require three-phase power are now becoming common.•• Protective earths are now run with lighting circuits to allow for metallic lamp holders to be earthed.• Increasingly residential power systems are incorporating microgenerators, most notably, photovoltaic cells.

Commercial power systemsCommercial power systems such as shopping centers or high-rise buildings are larger in scale than residentialsystems. Electrical designs for larger commercial systems are usually studied for load flow, short-circuit fault levels,and voltage drop for steady-state loads and during starting of large motors. The objectives of the studies are to assureproper equipment and conductor sizing, and to coordinate protective devices so that minimal disruption is causewhen a fault is cleared. Large commercial installations will have an orderly system of sub-panels, separate from themain distribution board to allow for better system protection and more efficient electrical installation.Typically one of the largest appliances connected to a commercial power system is the HVAC unit, and ensuring thisunit is adequately supplied is an important consideration in commercial power systems. Regulations for commercialestablishments place other requirements on commercial systems that are not placed on residential systems. Forexample, in Australia, commercial systems must comply with AS 2293, the standard for emergency lighting, whichrequires emergency lighting be maintained for at least 90 minutes in the event of loss of mains supply.[35] In theUnited States, the National Electrical Code requires commercial systems to be built with at least one 20A sign outletin order to light outdoor signage.[36] Building code regulations may place special requirements on the electricalsystem for emergency lighting, evacuation, emergency power, smoke control and fire protection.

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xpl/ freeabs_all. jsp?arnumber=5242078b). Proceedings of the IEEE - Part A: Power Engineering: 341–354. .[31] http:/ / ocw. kfupm. edu. sa/ user/ EE46603/ Circuit%20Breakers. pdf[32] How does an RCD work? (http:/ / www. greenbrook. co. uk/ pdf/ pbcatpg21-22. pdf), PowerBreaker, accessed on 14-Mar-10.[33] Report: Cyber Attacks Caused Power Outages in Brazil (http:/ / www. wired. com/ threatlevel/ 2009/ 11/ brazil/ ), Kevin Poulsen, WIRED

Threat Level blog, November 7, 2009.[34] "The MEN System of Earthing" (http:/ / www. docep. wa. gov. au/ energysafety/ PDF/ Newsletters/ electricians_newslet. pdf). Electricians

Newsletter No. 1 (Office of Energy (WA)): p2. May 2001. . Retrieved 30 Dec 2010.[35] http:/ / www. electricalsolutions. net. au/ articles/ 1546-Emergency-lighting-an-essential-service[36] http:/ / ecmweb. com/ nec/ code-basics/ nec-commercial-loads-2-20100201/

Electric power system 10

External links• IEEE Power Engineering Society (http:/ / www. ieee. org/ portal/ site/ pes)• Power Engineering International Magazine Articles (http:/ / pepei. pennnet. com/ articles/ print_toc.

cfm?Section=ARTCL& p=17)• Power Engineering Magazine Articles (http:/ / pepei. pennnet. com/ articles/ print_toc. cfm?Section=ARTCL&

p=6)• American Society of Power Engineers, Inc. (http:/ / www. asope. org/ )• National Institute for the Uniform Licensing of Power Engineer Inc. (http:/ / www. niulpe. org/ )

Article Sources and Contributors 11

Article Sources and ContributorsElectric power system  Source: http://en.wikipedia.org/w/index.php?oldid=533127874  Contributors: Ahmedmustafamahmoud, Arch dude, Arnavchaudhary, Beeblebrox, Cedars, Chris thespeller, CommonsDelinker, DynamoDegsy, Edison, Fountains of Bryn Mawr, FrankDev, GLPeterson, Gaius Cornelius, Inwind, John of Reading, Kylemaring, OnlyInAmerica, PleaseStand,R'n'B, Randolph02, Rjwilmsi, ShelfSkewed, Spinningspark, Wknight94, Wtshymanski, 29 anonymous edits

Image Sources, Licenses and ContributorsFile:Dampfturbine Montage01.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dampfturbine_Montage01.jpg  License: GNU Free Documentation License  Contributors: D-Kuru,Markus Schweiss, MichaelDiederich, StraSSenBahn, Tetris LFile:PearlStreetStation.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PearlStreetStation.jpg  License: Public Domain  Contributors: Earl Morter. Original uploader was Cedars aten.wikipediaFile:Switched mode power adapter.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Switched_mode_power_adapter.jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: Silverxxx (talk)File:Big Bend Power Station.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Big_Bend_Power_Station.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:Wknight94 talkFile:Toaster.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Toaster.jpg  License: GNU Free Documentation License  Contributors: Donovan Govan

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