Effect of transformer connection and construction on single phasing detection

On January 30, 2012, Unit 2 at Exelon Corporation´s Byron Generating Station experienced an event that resulted in a unit trip and a loss of offsite power to the unit. The root cause of the event was determined to be the failure of an under hung insulator on the phase C feed to the Unit 2 Station Auxiliary Transformers (SAT). An insulator on a bus support broke and grounded the high voltage bushing of the SAT, causing a short section of 345 kV bus to become disconnected. The section on the SAT side also fell to ground. See Figure 1. This bus section was associated with the phase C offsite source to the SATs. This event caused an open circuit on phase C; however, neither the protective relay system in the switchyard nor the plant relaying detected the loss of phase C. This paper will provide an outline of the event, failure analysis and follow-up actions as a result of the event, including a microprocessor based relay solution, as well as transformer theory relevant to single phasing issue.