a branch of a main post office.
an auxiliary power station where electrical current is converted, as from AC to DC, voltage is stepped up or down, etc.

1885–90; sub- + station Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
substation (ˈsʌbˌsteɪʃən)
1.  a subsidiary station
2.  an installation at which electricity is received from one or more power stations for conversion from alternating to direct current, reducing the voltage, or switching before distribution by a low-tension network

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1881, in the policing sense, from sub- + station. Power grid sense is attested from 1901.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The culprit: a large rodent, identified as a rat in some reports, burned out
  switches in a campus substation.
The cars have to be designed so they can be recharged safely and easily and
  without taking down the neighborhood power substation.
Another possibility: roping together thousands of batteries, planting them at a
  substation, and storing wind or solar power.
He was handcuffed, taken to the airport's police substation, and interrogated.
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