The Poseidon power substation PX-02 is a location in the Forest region of Appalachia in 2102, and can be found to the north of Summersville.
The Poseidon substation is similar to other ones, with the exception being that this one is inhabited by mole rats. There is a power box here that requires activating the Poseidon Energy Plant WV-06.
Potential workshop plan - On a rolling table in the metal shack.
The Poseidon power substation PX-02 appears only in Fallout 76.