Power details come from a variety of sources. 
- Sand Hill Substation: Category "Educated Guess". Substations are always named after nearby places - preferably after the next populated place. Exceptions are when there multiple substations at one populated place. This is my experience from a lot of other substations where I have explicit data about the name available.
H o w e v e r, because Sand Hill is only an unincorporated community and not very far away from Ellisville, there might be a very low chance that it is actually named after something else & in hindsight I wouldn't have named it like that (even through I'm still relatively certain about it)
- Cooperative Energy Lines: An excellent reference is the EIA Form 860 data, government collected data about every US power plant. One entry is the operator of the interconnecting line (and their voltage). 
https://www.eia.gov/electricity/data/eia860/
Just a few miles south of Sand Hill Substation is the Cooperative Energy's Moselle Power Plant  - the Grid Operator is listed as "South MS Electic Association", which is Coop. Energy (renamed themselfes a few years ago). Now using a combination of other power plant data which also feed cooperative energy lines, the Coop.Energy system area map https://cooperativeenergy.com/energy-resources/transmission-system-map/, rarely some parcel ownership data, annual reports (according to which they operate 1800 miles of transmission line), construction/expansion project related documents/websites and more. Each source must be viewed somewhat critical, but all in all, it draws a good picture. 
Still, I'm sometimes unsure and then I don't tag an operator (or add it later when I have more clarity).
And by the way, sometimes I'm closer than it seems ;)
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Most power lines were not exactly untouched for 15 years, but most are still basically unimproved, even after being touched multiple times. But well, a few were actually improved good enouth & shouldn't have been deleted whatsoever, I understand.
I don't think I offended anyone, certainly not everyone. But ok, I'll get back to editing now. Mass edits will be avoided, turns out deleting the lines directly before retracing would've now been faster anyway...