I came across a batch of old photos taken in Minnneapolis at Minneapolis General Electric Company (Predessesor to NSP) and later NSP's facilities including power plants and substations in the mid 1920s:


The Garfield and Hiawatha Stations shown still exist as a buiding.

I even found a photo of the NSP St. Clair 13.8/4 kV substation when it was being built in 1937.