Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 16, 1988
Information Notice No. 88-75: DISABLING OF DIESEL GENERATOR OUTPUT
CIRCUIT BREAKERS BY ANTI-PUMP CIRCUITRY
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems where the capability to either automatically or manually close diesel
generator circuit breakers from the control room may be lost. 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 do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
On June 5, 1988, operators at Browns Ferry Unit 3 were unable to reclose the
diesel generator output breakers to the 4-kV shutdown boards from the control
room during a loss-of-power/loss-of-coolant-accident (LOP/LOCA) test. During
the test, the output breakers connected the diesel generators to their respec-
tive 4-kV shutdown boards after the LOP signal was generated; however, the
breakers tripped and remained open after receiving the LOCA signal. Operators
diagnosed the output breaker lockout problem using system electrical configu-
ration drawings. An operator was sent to the 4-kV shutdown board to manually
transfer output breaker control power to its alternate source. This
momentarily removed power, enabling the breaker to close when the power was
restored.
On October 14, 1987, an operator at Wolf Creek Generating Station manually
tripped the output circuit breaker of the emergency diesel generator (EDG)
from the control room. At the time, the EDG was supplying a vital bus. When
the operators tried to reenergize the vital bus from the still-operating EDG,
they found that they could not close the EDG output breaker from the control
room. The vital bus was finally reenergized from the offsite power supply.
Through examination of the breaker control schematics the licensee later found
that the EDG circuit breaker could be closed by cycling the EDG mode switch at
the EDG local control station.
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Discussion:
The anti-pump circuit configuration will protect large breakers from rapid
cycling and, under certain circumstances, will prevent breaker closure. At
Browns Ferry Unit 3, a unique sequence of events, a LOP signal followed within
6 seconds by a LOCA signal, led to the discovery of a design deficiency of
this circuit configuration. Contacts from the undervoltage relay will seal in
the breaker anti-pump relay until the undervoltage condition on the 4-kV
boards clears.
The circuit is designed such that following a LOP, the undervoltage condition
must exist for at least 5 seconds and the diesel must reach rated speed before
the diesel generator output breaker will close on the bus. Once the breaker
has closed, the closure spring recharge motor and the breaker anti-pump relay
will be energized. A fully discharged closure spring requires 2 seconds for
the spring to be fully recharged. During this 2 second window the anti-pump
coil will remain energized via contacts sensing spring position, and if an
undervoltage condition exists on the 4-kV bus it will seal in and lock open
the breaker.
During the Browns Ferry event, the EDG output breaker closed 5 seconds after
the LOP signal, the undervoltage condition was eliminated, and the
undervoltage relay began its 5 second cycle to reset from the undervoltage
condition. About 1.5 seconds later, the LOCA signal retripped the breaker and
created another undervoltage condition on the 4-kV bus. At this point,
because the undervoltage relay had not completed its reset cycle, the
undervoltage relay remained in its undervoltage state. Therefore, the
undervoltage relay sealed in the still-energized anti-pump relay because the
undervoltage condition occurred before the breaker charging spring was fully
charged. Thus, the breaker could not be closed from the control room either
manually or automatically until control power was removed, which deenergized
the anti-pump relay.
The Browns Ferry licensee modified the breaker control logic to prevent the
anti-pump relay from sealing in during a LOCA condition by adding a time-delay
relay in the breaker trip coil circuitry. This relay will be energized by a
LOCA signal and its contact in the anti-pump coil seal-in path will open after
a 2-to 5-second delay to prevent anti-pump coil seal-in and breaker lockout.
The Wolf Creek EDG output circuit breaker has automatic closing logic to close
the circuit breaker when the following five permissives are satisfied:
(1) Both offsite circuit breakers are open.
(2) The EDG mode switch is in the automatic mode.
(3) Lockout relays are deenergized.
(4) A 3-second time delay has elapsed.
(5) The EDG has reached operating speed and voltage.
This logic sends a constant close signal to the circuit breaker that keeps the
breaker's internal anti-pump relay energized as long as the logic permissives
are satisfied. The anti-pump relay prevents the circuit breaker from cycling
if attempts are made to hold the breaker closed against a valid trip signal.
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September 16, 1988
Page 3 of 3
When the Wolf Creek operator manually tripped the EDG output breaker, the
auto-matic closing logic permissives remained satisfied. Therefore, the
anti-pump relay remained energized, preventing reclosure of the circuit
breaker. Cycling the EDG mode control switch at the local control station
allowed the circuit breaker to reclose by momentarily interrupting the
automatic close signal, there-by resetting the anti-pump logic. When the mode
switch contact was reclosed by returning the switch to the "auto" position,
the circuit breaker's automatic closing logic closed the breaker.
The Wolf Creek licensee modified the EDG breaker control switch located in the
control room to enable the operator to reclose the EDG circuit breaker from
the control room. This switch was originally intended only as a means of
paralleling the EDG with the offsite power supply. With the current
modification, the switch can be used to reset the anti-pump logic and allow
the automatic circuit to re-close the breaker. The modification added a
contact that is closed in the "normal" position and open in the "trip" and
"pull to lock" positions of the control switch. When operators manually trip
the EDG circuit breaker from this control switch, the contacts open to
interrupt the close circuit and reset the circuit breaker anti-pump relay. If
the operator wishes to keep the breaker open, he must put the switch in the
"pull to lock" position. Returning the switch to the "normal" position
completes the automatic close circuit and the breaker recloses.
It should be noted that although the above discussion has dealt only with EDG
output circuit breakers, anti-pump circuit problems could also apply to other
breakers that use automatic closing logic, such as load-sequencing breakers
and offsite supply breakers to the emergency buses.
The information herein is being provided as an early notification of a
potentially significant matter that is still under consideration by the NRC
staff. If NRC evaluation so indicates, specific licensee actions may be
requested.
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: James Lazevnick, NRR
(301) 492-0814
Carl Schulten, NRR
(301) 492-1192
Fred Burrows, NRR
(301) 492-0783
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 88-75
September 16, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-74 Potentially Inadequate 9/14/88 All holders of OLs
Performance of ECCS in or CPs for W and
PWRs During Recirculation B&W-designed
Operation Following a LOCA nuclear power
reactors.
88-73 Direction-Dependent Leak 9/8/88 All holders of OLs
Characteristics of or CPs for nuclear
Containment Purge Valves power reactors.
88-72 Inadequacies in the Design 9/2/88 All holders of OLs
of dc Motor-Operated Valves or CPs for nuclear
power reactors.
88-71 Possible Environmental 9/1/88 All holders of OLs
Effect of the Reentry or CPs for nuclear
of COSMOS 1900 and power reactors,
Request for Collection fuel cycle
of Licensee Radioactivity licensees, and
Measurements Attributed Priority 1
to That Event material
licensees.
88-70 Check Valve Inservice 8/29/88 All holders of OLs
Testing Program or CPs for nuclear
Deficiencies power reactors.
88-69 Movable Contact Finger 8/19/88 All holders of OLs
Binding in HFA Relays or CPs for nuclear
Manufactured by General power reactors.
Electric (GE)
88-48, Licensee Report of Defective 8/24/88 All holders of OLs
Supplement 1 Refurbished Valves or CPs for nuclear
power reactors.
88-68 Setpoint Testing of Pres- 8/22/88 All holders of OLs
surizer Safety Valves with or CPs for nuclear
Filled Loop Seals Using power reactors.
Hydraulic Assist Devices
88-67 PWR Auxiliary Feedwater Pump 8/22/88 All holders of OLs
Turbine Overspeed Trip or CPs for nuclear
Failure power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
