Do you mean the "voltage=115000;13860" tags on substations? They are from FERC Form 1 annual reports -elibrary.ferc.gov-, which include listings of operated substations+associated data, such as primary and secondary voltage, see https://data.catalyst.coop/ferc1_xbrl/substations_426_instant/158598 https://data.catalyst.coop/ferc1_xbrl/substations_426_instant/1903 https://data.catalyst.coop/ferc1_xbrl/substations_426_instant/1779
three substations which I added here with 13.86kV as the listed "secondary voltage".

Disclaimer I have no electrical background.