The substations do not form a whole that is worth mapping. Instead, if for any reason you want to have access to all substations with (say) a certain voltage or operator in a certain region, you would formulate an Overpass query that gives you these. In OSM we don't make relations like "all buildings designed by architect X" or "all schools operated by provider Y" - we simply add the architect or operator to the individual things and then rely on database queries to generate the lists. Everything else would be too much maintenance effort.