Document: NUREG-0800
Document ID: 61198d64-419c-45ea-aebb-cf34c3b171fe
Document Type: srp
Title: Revision 0 – July 2015
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1505/ML15057A085.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8
CFR Part: 
CFR Title: 

Content:
ention: Reproduction and Distribution Services Section, by fax to (301) 415-2289; or by email to DISTRIBUTION@nrc.gov. Electronic copies of this section are available through the NRC's public Web site at http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/ , or in the NRC's Agencywide Documents Access and Management System (ADAMS), at http://www.nrc.gov/reading-rm/adams.html, under ADAMS Accession No. ML15057A085. NUREG-0800 U.S. NUCLEAR REGULATORY COMMISSION STANDARD REVIEW PLAN 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 Byron Station is a two-unit pressurized water reactor plant. The electrical distribution system for each unit consists of four nonsafety 6.9-kilo Volt (kV) buses, two nonsafety 4-kV buses, and two engineered safety features (ESF) 4-kV station buses. Both the ESF 4-kV and the two nonsafety 6.9-kV station buses that power the two reactor coolant pumps (RCPs) are normally supplied by station auxiliary transformers (SATs) connected to the 345-kV offsite power switchyard. On January 30, 2012, 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 the RCPs. The undervoltage condition was caused by a broken inverted porcelain insulator stack of the Phase C conductor for the 345-kV power circuit that supplies both 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 BTP 8-9-2 Revision 0 – July 2015 an unbalanced voltage condition on the two 6.9-kV nonsafety-related RCP buses and the two 4.16-kV ESF buses. Some ESF loads that were energized relied on equipment protective devices to prevent damage from an unbalanced overcurrent