United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-50: Effect of Circuit Breaker Capacitance on Availability of Emergency Power

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C. 20555

                                  July 18, 1988


NRC INFORMATION NOTICE: NO. 88-50:  EFFECT OF CIRCUIT BREAKER CAPACITANCE ON 
                                    AVAILABILITY OF EMERGENCY POWER 


Addressees: 

All holders of operating licenses or construction permits for nuclear power 
reactors. 

Purpose: 

This information notice is being provided to alert addressees to the potential 
for circuit breaker capacitance having an adverse effect on the operation of 
emergency power sources.  It is expected that recipients will review the 
information for applicability to their facilities and consider actions, as 
appropriate, to preclude similar problems from occurring at their facilities. 
However, suggestions contained in this information notice do not constitute 
NRC requirements; therefore, no specific action or written response is 
required. 

Description of Circumstances: 

As a result of projected increases in generation capacity, Duke Power Company 
was replacing the power circuit breakers (PCBs) in the 230-kilovolt 
transmission switchyard at the Oconee station with breakers having a higher 
interrupting capacity.  With an increase in the interrupting capability of the
PCBs, a higher capacitance is needed to prevent restrike across the breaker 
contacts following fault interruption.  

The increased capacitance value used across the breaker contacts, when coupled
with the electrical impedance characteristics of the Unit 2 startup 
transformer and its associated switchyard circuits, established a stable 
induced voltage in the startup transformer circuit.  The circuit provided 
4000 volts on the secondary of a 230-kilovolt to 4160-volt transformer when 
the primary and secondary breakers were open.  Under these circumstances the 
unpowered 4160-volt bus would appear to be near rated voltage to the circuitry 
used to make actual emergency bus transfers during loss-of-power events.  The 
possibility of such a resonance effect was not considered in the modification, 
and the post modification testing did not exactly duplicate all possible 
operating conditions of the switchyard. 

Discussion: 

The Oconee station has three units.  Unit 3 was shutdown for steam generator 
repairs at the time this potential problem was identified.  The PCBs for the 
Unit 2 startup transformer had been replaced with new breakers as part of a 


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                                                            July 18, 1988 
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modification to upgrade all the breakers in the switchyard to the larger 
capacity breakers. 

Oconee is unique in that its onsite emergency power generation is provided to 
the emergency buses from the Keowee hydro plant via an above-ground 
transmission system that connects to each startup transformer via the 
230-kilovolt switchyard and a 13.8-kilovolt underground transmission system 
with a 13.8/4.16-kilovolt transformer.  Therefore, each emergency bus has 
three power sources; i.e., the main generator via the auxiliary transformer, 
the switchyard via the startup transformer, and the Keowee station via the 
underground transmission system.  These sources are automatically controlled, 
primarily by the emergency power switching logic (EPSL), to ensure that 
emergency power is provided for various accident scenarios.  Under normal 
circumstances a loss of the unit generator will cause a shift of the emergency 
bus feed to the startup transformer.  If there is a loss of voltage sensed on 
the secondary of the startup transformer the system will shift to the 
underground source of emergency power.  

The licensee was testing these new PCBs as they were installed.  This 
consisted primarily of testing the breaker function and the various relays 
associated with the control circuits.  The licensee did not test to determine 
if an induced voltage was occurring in the circuit because this was not 
considered a problem after discussions were held with the manufacturers.  

On March 29, the utility identified a leakage voltage and current developing 
across the capacitors of the newly installed PCBs.  The licensee concluded at 
this time that the resonance condition would not cause an operating problem.  
On April 26, 1988 after further review of this problem while trying to provide 
corrective action for this condition, the licensee determined that the induced 
voltage would be of sufficient magnitude to exceed the pickup setting of the 
EPSL.  If the pickup setting is exceeded, the EPSL system will not 
automatically select the appropriate power supply for emergency power.  The 
electrical system was immediately placed in a lineup to preclude this problem 
from affecting the operating units.  The capacitors have subsequently been 
changed to ones with a different capacitance thus eliminating this problem.  

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the 
technical contacts listed below or the Regional Administrator of the 
appropriate regional office. 




                                Charles E. Rossi, Director 
                                Division of Operational Events Assessment 
                                Office of Nuclear Reactor Regulation 

Technical Contacts:  Raymond F. Scholl, NRR 
                     (301) 492-1171 

                     Thomas Peebles, Region II 
                     (404) 331-4196 

Attachment: List of Recently Issued NRC Information Notices 
.                                                            Attachment
                                                            IN 88-50 
                                                            July 18, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-49          Marking, Handling,            7/18/88        All holders of OLs
               Control, Storage and                         or CPs for nuclear
               Destruction of Safe-                         power reactors and
               guards Information                           all other licensed
                                                            activities 
                                                            involving a 
                                                            formula quantity 
                                                            of special nuclear
                                                            material. 

88-48          Licensee Report of            7/12/88        All holders of OLs
               Defective Refurbished                        or CPs for nuclear
               Valves                                       power reactors. 

88-47          Slower-Than-Expected          7/14/88        All holders of OLs
               Rod-Drop Times                               or CPs for PWRs. 

88-46          Licensee Report of            7/8/88         All holders of OLs
               Defective Refurbished                        or CPs for nuclear
               Circuit Breakers                             power reactors. 

88-45          Problems In Protective        7/7/88         All holders of OLs
               Relay and Circuit                            or CPs for nuclear
               Breaker Coordination                         power reactors. 

88-44          Mechanical Binding of         6/24/88        All holders of OLs
               Spring Release Device                        or CPs for nuclear
               in Westinghouse Type                         power reactors. 
               DS-416 Circuit Breakers 

88-43          Solenoid Valve Problems       6/23/88        All holders of OLs
                                                            or CPs for nuclear
                                                            power reactors. 

88-42          Circuit Breaker Failures      6/23/88        All holders of OLs
               Due to Loose Charging                        or CPs for nuclear
               Spring Motor Mounting Bolts                  power reactors. 

88-41          Physical Protection           6/22/88        All holders of OLs
               Weaknesses Identified                        or CPs for nuclear
               Through Regulatory Ef-                       power reactors. 
               fectiveness Reviews (RERs) 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013