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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. IN 88-50
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, March 09, 2021