Information Notice No. 90-51: Supplement 1:Failures of Voltage-Dropping Resistors in the Power Supply Circuitry of Electric Governor Systems
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 24, 1991
NRC INFORMATION NOTICE 90-51, SUPPLEMENT 1: FAILURES OF VOLTAGE-DROPPING
RESISTORS IN THE POWER SUPPLY
CIRCUITRY OF ELECTRIC GOVERNOR
SYSTEMS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice supplement to alert addressees to a recent failure of a
voltage-dropping resistor in the power supply for the electronic control
module of an emergency diesel generator (EDG) governor system at the
Waterford Steam Electric Station. The resistor had been in service for
approximately 15 months. The resistor failure resulted in the inoperability
of the EDG. 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
On August 20, 1991, at the Waterford Steam Electric Station (Waterford), EDG
"A" failed to maintain rated speed following a successful start during a
surveillance test. After reaching rated speed of 600 rpm, engine speed
began to fluctuate, dropping to a low of 280 rpm. This prompted the
licensee to shut down the EDG and declare it inoperable. The licensee
performed an investigation and determined that the speed anomaly resulted
from the failure of one of the two voltage-dropping resistors in the power
supply for the electronic control module for the governor system on the EDG.
The NRC staff described this failure mode in NRC Information Notice (IN)
90-51, issued August 8, 1990.
In IN 90-51, the staff discussed failures of voltage-dropping resistors in
the power supplies for the electronic control modules for EDGs at the
Commonwealth Edison Company's (CEC's) Braidwood and Byron Stations. CEC
conducted a root cause analysis and concluded that normal "aging" was the
most likely cause of the resistor degradation or failure. CEC subsequently
began a preventive maintenance program that included periodic replacement of
the voltage-dropping resistors. After reviewing the original information
notice, the Waterford licensee instituted a preventive maintenance program
to replace the resistors every 18 months. However, the resistor that
recently failed at the
9110210298
.
IN 90-51, Supplement 1
October 24, 1991
Page 2 of 2
Waterford Station had been in service for only about 15 months after having
been replaced as part of the newly adopted preventive maintenance program.
Although this is the only early failure of these voltage-dropping resistors
of which the NRC staff is aware, it appears that these resistors can
prematurely fail even before 18 months and that periodic replacement may not
ensure adequate availability of the EDGs to perform their intended function.
Following the publication of IN 90-51, Woodward Governor Company (Woodward)
stated, in a letter to the NRC, that they did not concur with using periodic
replacement of the two resistors, of the original design, as an acceptable
preventive maintenance procedure. Woodward stated that whenever the battery
charging system is in operation, system voltage approached the design
capacity of the dual resistor assembly. The higher voltage developed by the
charging system caused more current to flow through the resistors, resulting
in more heat generation. The increased heat generated at the higher voltage
level would tend to reduce the life expectancy of the assembly.
The letter from Woodward described the design of the governor control system
and stated that several product improvements had been made that offer
certain advantages over the original design. Specifically, the letter
stated that a replacement assembly for the dual voltage-dropping resistor
configuration had been developed. The replacement assembly substitutes a
single resistor for the dual resistor design. Besides the fact that the
single resistor assembly is of greater thermal capacity than the dual
resistor assembly, the letter states that, because of the design of the
governor system, use of the single resistor has distinct advantages over the
dual resistor configuration.
With the single voltage-dropping resistor of the replacement assembly, a
resistor failure would result in a backup mechanical governor taking control
of speed. The EDG would then continue to operate at rated speed. The
licensee for Waterford has concluded that the new design has advantages over
the dual resistor design and has installed the new resistor assembly in each
of its EDG governor systems.
This 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.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: N. Fields, NRR
(301) 492-1173
O. Chopra, NRR
(301) 492-3265
Attachment: List of Recently Issued NRC Information Notices
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