Feasibility of Using a Single Auxiliary Transformer as Shutdown Power for Dual M310+ Nuclear Power Units

To mitigate unit shutdown risks in M310+ nuclear power plants caused by auxiliary transformer (Aux-TF) unavailability, this study evaluates the feasibility of relying on a single remaining Aux-TF to supply shutdown power for two operational units. Through a case analysis of a representative plant, actual shutdown loads were determined by analyzing station service power switching logic, system configurations, and empirical equipment operating data. Key parameters including busbar voltage during motor group auto-starting, high-power motor startup transients, and cable/equipment capacity limits were systematically assessed. Results demonstrate that upgrading specific switchgear components enables a single Aux-TF to meet shutdown power demands, provided both units utilize auxiliary feedwater pumps for steam generator water level maintenance and avoid concurrent reactor coolant pump startups. Implementation of these findings can prevent forced shutdowns during Aux-TF maintenance, with selected recommendations already deployed in new plants.