Judgment of the General Court (Third Chamber, Extended Composition) of 22 September 2021.#Dimosia Epicheirisi Ilektrismou AE (DEI) v European Commission.#State aid – Electricity supply tariff – Decision of an arbitration tribunal setting the tariff charged to Alouminion – Decision to take no further action on the complaint – Decision finding no State aid – Challengeable act – Standing as party concerned – Interest in bringing proceedings – Locus standi – Admissibility – Whether attributable to the State – Advantage – Private operator principle – Serious difficulties.#Joined Cases T-639/14 RENV, T-352/15 and T-740/17.