page_title: Electric power system
section_title: Supplies
hierarchical_section_title: Electric power system / Components of power systems / Supplies
caption_attribution_description: English: Big Bend Power Station near Apollo Beach, Florida. Warm water from the plant is a favorite habitat for Florida manatees during the winter so a manatee viewing station is nearby (where this photo was taken). This &#32;image&#32;was created with Photoshop Photomerge (stitched images may differ from reality).
context_page_description: An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industry within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centres to the load centres, and the distribution system that feeds the power to nearby homes and industries. Smaller power systems are also found in industry, hospitals, commercial buildings and homes. The majority of these systems rely upon three-phase AC power—the standard for large-scale power transmission and distribution across the modern world. Specialised power systems that do not always rely upon three-phase AC power are found in aircraft, electric rail systems, ocean liners, submarines and automobiles.
context_section_description: All power systems have one or more sources of power. For some power systems, the source of power is external to the system but for others, it is part of the system itself—it is these internal power sources that are discussed in the remainder of this section. Direct current power can be supplied by batteries, fuel cells or photovoltaic cells. Alternating current power is typically supplied by a rotor that spins in a magnetic field in a device known as a turbo generator.  There have been a wide range of techniques used to spin a turbine's rotor, from steam heated using fossil fuel (including coal, gas and oil) or nuclear energy to falling water (hydroelectric power) and wind (wind power).
  The speed at which the rotor spins in combination with the number of generator poles determines the frequency of the alternating current produced by the generator. All generators on a single synchronous system, for example, the national grid, rotate at sub-multiples of the same speed and so generate electric current at the same frequency. If the load on the system increases, the generators will require more torque to spin at that speed and, in a steam power station, more steam must be supplied to the turbines driving them.  Thus the steam used and the fuel expended directly relate to the quantity of electrical energy supplied. An exception exists for generators incorporating power electronics such as gearless wind turbines or linked to a grid through an asynchronous tie such as a HVDC link — these can operate at frequencies independent of the power system frequency.
  Depending on how the poles are fed, alternating current generators can produce a variable number of phases of power. A higher number of phases leads to more efficient power system operation but also increases the infrastructure requirements of the system. Electricity grid systems connect multiple generators operating at the same frequency: the most common being three-phase at 50 or 60 Hz.
  There are a range of design considerations for power supplies. 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 appropriate starting circuit)? What is the mechanical speed of operation for the turbine and consequently what are the number 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)?
caption: The majority of the world's power still comes from coal-fired power stations like this
exif: {"Image Make": "Canon", "Image Model": "Canon EOS DIGITAL REBEL XSi", "Image Orientation": "Horizontal (normal)", "Image XResolution": "240", "Image YResolution": "240", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop Elements 8.0 Windows", "Image DateTime": "2011:05:30 10:53:26", "Image ExifOffset": "232", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "770", "Thumbnail JPEGInterchangeFormatLength": "4927", "EXIF ExposureTime": "1/250", "EXIF FNumber": "10", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "100", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2011:03:12 15:26:21", "EXIF DateTimeDigitized": "2011:03:12 15:26:21", "EXIF ShutterSpeedValue": "995723/125000", "EXIF ApertureValue": "415241/62500", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "29/8", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "18", "EXIF SubSecTime": "46", "EXIF SubSecTimeOriginal": "46", "EXIF SubSecTimeDigitized": "46", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "4256", "EXIF ExifImageLength": "3232", "EXIF FocalPlaneXResolution": "2136000/439", "EXIF FocalPlaneYResolution": "356000/73", "EXIF FocalPlaneResolutionUnit": "2", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard"}