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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 December 22, 1993 NRC INFORMATION NOTICE 91-29, SUPPLEMENT 2: POTENTIAL DEFICIENCIES FOUND DURING ELECTRICAL DISTRIBUTION SYSTEM FUNCTIONAL INSPECTIONS Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to provide additional information on potential deficiencies identified by the NRC during electrical distribution system functional inspections at nuclear plants. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances Licensees have reported degraded grid voltage conditions at several plants in the United States. During the electrical distribution system functional inspections at selected plants, the staff found deficiencies in the protection against degraded grid voltage conditions as noted in Information Notice 91-29 dated April 15, 1991, and its Supplement 1 dated September 14, 1992. This supplement describes a potential design weakness related to the degraded grid undervoltage relay protection at the Hope Creek plant. Under postulated accident and degraded grid voltage conditions, the eletrical bus transfer logic scheme allows repeated transfer between the two offsite sources supplying power to the safety-related buses. The transfer of power can be stopped by manual operator action; however, the repeated transfer of electrical power between the offsite sources could cause delayed responses or failures of equipment needed to mitigate an accident. Discussion At the Hope Creek plant, the normal power source for the safety-related buses is one of two offsite power sources. If the grid voltage level is just above the actuation setpoints of the degraded grid voltage relays during postulated accident conditions, starting accident-mitigating equipment (emergency loads) will lower the voltage for the emergency buses below the degraded voltage 9312150075. IN 91-29, Supp. 2 December 22, 1993 Page 2 of 2 setpoint. However, instead of transferring the buses to the emergency diesel generators, the bus transfer logic scheme transfers the buses to the second offsite source. The shedding of these emergency loads from the normal power source will restore this source to its initial voltage level. Since the grid is still slightly above the degraded voltage setpoint, the second offsite source also will reject the emergency loads as the emergency bus voltage again dips below the degraded voltage relay setpoint. Since the bus transfer logic scheme was designed to select offsite power in preference to the diesel generators, it will transfer the safety buses back to the "restored" normal offsite power source. The degraded grid undervoltage relays will again trip and the emergency power will again be transferred between the two offsite sources. This cycle will continue to repeat unless the operator intervenes or equipment fails. Repeated transfer of power between the offsite power sources also may delay the safe shutdown of the plant. As a solution to this potential problem, the licensee required plant operations staff to depress the "Auto Close Block" pushbutton switches after the feeder breakers for the normal power supply to the Class 1E 4160-V buses have been placed in the closed position. The "Auto Close Block" pushbutton feature is a part of the original plant design that overrides the bus transfer logic scheme and provides "one way" transfers (normal power supply- alternate power supply- emergency diesel generators), when selected. Thus, this provision will prevent the automatic transfer of the buses from the alternate power supply source back to the normal power supply source and allow the emergency diesel generators to power the buses, when the alternate power supply is lost. This supplemental information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate office of Nuclear Reactor Regulation (NRR) project manager. /S/'D BY BKGRIMES Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: Gene Lazarowitz, RI Frederick H. Burrows, NRR (215) 337-5392 (301) 504-2901 Neil Della Greca, RI (215) 337-5046 Attachment: List of Recently Issued NRC Information Notices .
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