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Electrical power travels along transmission wires from power plants to homes and businesses. Along the way, transformers raise or "step up" and lower or "step down" the voltage.
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Electric Power Delivery System

Electric power supplied to homes and businesses is typically AC (alternating current). The electrons do not travel along the power lines overhead but vibrate back and forth at 60 times per second within these lines. The AC outlet in your home delivers energy not electrons. When you plug in an appliance, the outlet supplies the power to move electrons that are already in the wiring of the appliance, around a closed circuit to produce a current. The energy is supplied to your home as a voltage through a large and very complex power distribution network. After electric power is generated within a power station, it is sent through a network of power lines to consumers. This is done in several steps.

  • Electric power leaves the power plant at very high current levels and with voltages of several thousand volts.
  • The voltage is "stepped up" (increased) to several hundred thousand volts because less energy is lost during the trip along the overhead power lines at these high voltages.
  • Before the power is distributed to industrial users, the voltage is "stepped down" (decreased) to several thousand volts again.
  • For home use, the voltage is stepped down even further to 110 volts.

Each time the voltage is stepped down, a transformer is used. Transformers are devices that work only with alternating current flows to either step up or step down voltages. They begin to behave badly when direct currents (DC) are superimposed on the normal AC currents that are handled routinely in the power grid. This is the main reason that power grids can be compromised by space weather events.

Last modified February 17, 2009 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA