Information Notice No. 91-29, Supplement 2:Potential Deficiencies Found During Electrical Distribution System Functional Inspections

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               December 22, 1993

                                             DURING ELECTRICAL DISTRIBUTION
                                             SYSTEM FUNCTIONAL INSPECTIONS


All holders of operating licenses or construction permits for nuclear power


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. 


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  


                                                            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

List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Wednesday, March 24, 2021