in an electric utility system. the beginning of the distribution network; a facility that steps-down voltages between transmission lines and primary lines or distribution feeders.

Merriam-Webster Online Dictionary
substation (noun)
a subordinate or station as - subsidiary
a) a branch post office
b) a subsidiary station in which electric current is transformed
c) a police station serving a particular area
substation (Wikipedia)

Substation can refer to:

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