Document: NUREG-0800
Document ID: b75d082b-2493-4543-981a-2ee136d737cf
Document Type: srp
Title: BTP 8-9-1
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1405/ML14057A433.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8
CFR Part: 
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Content:
BTP 8-9-1 Draft Revision 0 – May 2014 NUREG-0800 U.S. NUCLEAR REGULATORY COMMISSION STANDARD REVIEW PLAN DRAFT BRANCH TECHNICAL POSITION (BTP) 8-9 OPEN PHASE CONDITIONS IN ELECTRIC POWER SYSTEM REVIEW RESPONSIBILITIES Primary - Organization responsible for electrical engineering Secondary - None A. BACKGROUND On January 30, 2012, Byron Station, Unit 2 experienced an automatic reactor trip from full power because the reactor protection scheme detected an undervoltage condition on the 6.9-kV buses that power reactor coolant pumps (RCPs). The undervoltage condition was caused by a broken insulator stack of the Phase C conductor for the 345 kV power circuit that supplies both station auxiliary transformers (SATs). The insulator failure resulted in a high impedance fault through the fallen Phase C conductor and a sustained open phase condition on the high voltage side of the SAT. The open circuit created an unbalanced voltage condition on the two 6.9-kV nonsafety-related RCP buses and the two 4.16-kV engineered safety features (ESF) buses. ESF loads remained energized momentarily, relying on equipment protective devices to prevent damage from an unbalanced overcurrent condition. The overload condition caused several ESF loads to trip. Approximately 8 minutes after the reactor trip, the control room operators diagnosed the loss of Phase C condition and manually tripped breakers to separate the unit buses from the offsite power source. When the operators opened the SAT feeder breakers to the two 4.16-kV ESF buses, the loss of ESF bus voltage resulted in the emergency diesel generators (EDGs) automatically starting and restoring power to the ESF buses. In the event that the operators failed to diagnose the condition in a timely manner, a potential damage to the RCP seals due to loss of seal cooling water may have resulted in a loss-of-coolant event within a few more minutes. A second event also occurred at Byron Station Unit 1 on February 28, 2012. This event was also