Information Notice No. 91-34: Potential Problems in Identifying Causes of Emergency Diesel Generator Malfunctions
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
June 3, 1991
Information Notice No. 91-34: POTENTIAL PROBLEMS IN IDENTIFYING CAUSES
OF EMERGENCY DIESEL GENERATOR
All holders of operating licenses or construction permits for nuclear power
This information notice is intended to alert addressees of problems that
could occur in identifying the cause(s) of malfunctions in emergency diesel
generators (EDGs). 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 March 20, 1990, a loss of offsite power to the vital buses occurred at
the Alvin W. Vogtle Nuclear Plant, Unit 1. EDG 1A, the only available EDG,
automatically started and energized its associated 4.16 kilovolt (kV) safety
bus. The EDG tripped after operating for approximately 1 minute, leaving
the plant with no Class 1E ac electrical power. The operators could not
determine the cause of the EDG trip. The operators did not record the EDG
trip conditions that were annunciated before resetting the annunciator
panels. Annunciated conditions for the EDGs are not automatically recorded.
Operators attempted a second start of the EDG approximately 18 minutes
later. The EDG started, ran for approximately 1 minute, and tripped again.
The operators observed a number of alarms on EDG annunciator panels but
could not determine which signal had actually tripped the EDG.
Approximately 36 minutes into the event, operators again restarted the EDG,
this time using a manual emergency start, which bypasses many of the EDG
protective trips. The EDG started, loaded, and continued to run without
further incident, thereby restoring power to one Class 1E bus. The licensee
later identified a spurious high jacket water temperature trip as the most
probable cause of the EDG trip, although this protective trip was not
bypassed by the manual emergency start. The licensee has taken action to
prevent spurious high jacket water temperature trips in the future by
installing a bypass for this trip signal.
June 3, 1991
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Annunciators and control panels for EDGs are located in the EDG room and in
the control room. In most cases, the annunciators and EDG parameters are
not automatically recorded during operation and testing. Operators usually
record EDG parameters manually during operation, but do not generally record
annunciators. Following an EDG trip, a number of annunciators are
illuminated. These annunciators include both the alarms associated with the
cause of the trip and the alarms that result from the trip. Operators can
often determine the cause of a trip only if an effective "first out"
indication is installed. An effective "first out" feature indicates the
first cause of an abnormal condition. Regulatory Guide 1.9, "Selection,
Design, and Qualification of Diesel Generator Units Used as Standby (Onsite)
Electric Power Systems at Nuclear Power Plants," recommends the installation
of such a feature, but there is no regulatory requirement associated with
it. Although a "first out" feature was available locally at Vogtle, it was
not effective because its existence was not generally recognized by the
operators, and its operation was confusing.
Analyzing EDG incidents on the basis of alarm indications, operator memory
and observations, and the manual recording of data may not provide an
accurate assessment of an event, especially if the alarm system does not
have an effective "first out" capability. Monitoring systems have been
installed or are being considered by some licensees to better analyze EDG
parameters. These are computer-driven on-line monitoring and diagnostic
systems that have been developed to support and supplement the EDG
instrumentation and control systems that are generally supplied by the EDG
manufacturer. This type of system can significantly enhance the licensee's
capability to monitor the operation of an EDG, to troubleshoot EDG problems,
and to provide the information necessary to restore an EDG to operation.
The licensee for North Anna Power Station has installed such a
computer-driven monitoring and diagnostic system to support the EDG
preventive maintenance program. The system processes signal inputs from
approximately 50 sensors and provides a digital output for viewing and
recording in several formats. While the EDG is operating, data is
continuously stored in memory and may be transferred to a hard disk to be
retrieved later for trend analysis. At any time, the on line data may be
viewed from a local terminal. The system allows the user to perform
plotting and trending analyses to determine the limits and guidelines for
establishing or predicting inspection or maintenance requirements. The
licensees for the Prairie Island Nuclear Generating Plant and the Crystal
River Nuclear Plant, Unit 3, are also considering the acquisition of
computer-driven monitoring systems primarily for the EDG preventive
maintenance program to enhance EDG performance by the early identification
of areas that could become problems.
Available systems have various capabilities including some or all of the
1. Performing general surveillance, recording alarms, and reporting
(local and remote) EDG parameters.
June 3, 1991
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2. Automatically determining the trends of recorded data points and
analyzing EDG parameters in three different operating modes:
* Standby (keep-warm conditions)
* The first 30 or 60 seconds after a start
* Engine running (collecting data at preset intervals)
3. Determining the trends of EDG operating parameters to monitor
performance and predict required maintenance.
These systems may be installed to supplement the existing EDG annunciator
system rather than as a redundant system.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: S. N. Saba, NRR
Attachment: List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Wednesday, March 24, 2021