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:

www.mnhs.org/library/findaids/sv000083.xml

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.

collections.mnhs.org/cms/web5/media.php?irn=10204803&width=640&height=640