An Air Canada Airbus A330-300, registration C-GEGI performing flight AC-7231 from Madrid,SP (Spain) to Montreal,QC (Canada) with 5 crew, was enroute at FL400 over the Atlantic Ocean about 350nm west of Madrid prior to ETOPS entry when the crew received an ECAM warning about the ESS TR (Essential Bus Transformer Rectifier) and completed the related checklist actions. The crew consulted with maintenance and subsequently returned to Madrid for a safe landing about one hour later.
The Canadian TSB reported the crew did not declare Mayday or PAN PAN.