Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 19, 1991
NRC INFORMATION NOTICE 91-57: OPERATIONAL EXPERIENCE ON BUS TRANSFERS
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 to alert addressees to findings contained in Engineering Evaluation
Report AEOD/E90-05, "Operational Experience on Bus Transfer," June 1990, on
medium-voltage (2 kV to 15 kV) bus transfer failures at nuclear power
reactors. 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
In 1988 and 1989, Northeast Utilities issued licensee event reports (LERs)
50-422/88-026 and 50-422/89-030 indicating that at the Millstone Nuclear
Power Station, Unit 3, under certain scenarios, the existing bus transfer
scheme could result in common-mode failure of Class 1E loads of both trains,
and repeated bus transfers could damage the safety-related motors. This
information prompted the NRC to conduct an in-depth study of the bus
transfer practices and operational experience at U.S. nuclear plants. The
results of this study are delineated in NRC Engineering Evaluation Report
AEOD/E90-05, "Operational Experience on Bus Transfer," June 1990. The
report indicates that between 1985 and 1989 bus transfers either failed to
take place on demand or were accompanied by some equipment malfunction on
more than 50 occasions.
Discussion
The problems associated with the bus transfer process raise two basic safety
concerns:
(1) Damage to the equipment connected to the auxiliary buses.
(2) Failures of bus transfers to take place on demand.
The first safety concern relates to an excessive difference in voltage
between the auxiliary load bus and the incoming power source. An excessive
voltage
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difference will cause transient current flows in the system that can damage
the transformers, the buses, and the connected loads.
There is a difference of opinion within the industry about criteria that
assure safe bus transfer. In 1977, the American National Standards
Institute (ANSI) introduced the recommendation for limiting resultant
voltage to 1.33 per unit volts per hertz by issuing ANSI standard C50.41,
"Polyphase Induction Motors for Power Generating Stations." The National
Electrical Manufacturers Association (NEMA) accepted this as a safe
criterion in NEMA MG-1-1978, "Motors and Generators." However, further
research showed that limiting the resultant voltage to 1.33 per unit volts
per hertz does not ensure that motor shaft torques will be within acceptable
limits. In 1987, NEMA withdrew the criterion from MG-1.
The NRC staff is not aware of equipment failures at nuclear plants that can
be directly attributed to excessive difference in voltage between an
auxiliary load bus and the incoming power source, although some equipment
could be stressed in this process. Such stresses experienced by connected
equipment would be cumulative, and, unless specific action is taken to limit
these stresses, they can remain undetected until failure occurs.
The second safety concern covers cases in which the transfer does not take
place on demand, resulting in a loss of power to buses. In most nuclear
plants, such a failure leads to the loss of offsite power to plant auxiliary
loads, which in some cases, include the Class 1E loads. Hence, with this
type of bus transfer failure, the Class 1E loads will be powered by the
emergency diesel generators, and reactor system cooling will be accomplished
by natural circulation. Although nuclear plants are designed to safely shut
down on a loss of offsite power, it is desirable that such failures be kept
to a minimum to minimize challenges to the emergency diesel generators.
Analysis
The data from AEOD/E90-05 indicates that the root causes for the most
significant events (those that involved an actual failure to transfer or a
significant design deficiency) can be broken down as follows:
42 percent were caused by design deficiencies,
25 percent were caused by equipment failures,
18 percent were caused by personnel errors, and
15 percent were caused by protective relays that blocked the bus
transfer.
The major design deficiencies included slow bus transfer (35 percent of all
design deficiency events), low bus voltage (24 percent of all design
deficiency events), and inadequate setting and speed of operation of
sync-check relays (17 percent of all design deficiency events).
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In addition, the report includes a discussion of various design
modifications that can reduce the probability of bus transfer failures,
including modifying the power distribution scheme, using static sync-check
relays and devices, and incorporating faster bus transfer practices. The
report also indicates that many of the bus transfer failures resulting from
equipment malfunction and personnel errors can be avoided by improving
equipment maintenance and personnel vigilance.
Related Generic Communications
The staff has issued the following information notices on this and related
topics: Information Notices 85-28, "Partial Loss of AC Power and Diesel
Generator Degradation," 86-87, "Loss of Offsite Power Upon an Automatic Bus
Transfer," 86-100, "Loss of Offsite Power to Vital Buses at Salem 2," and
88-50, "Effect of Circuit Breaker Capacitance on Availability of Emergency
Power."
NRC Engineering Evaluation Report AEOD/E90-05 is available in the Public
Document Room, Washington, D.C. (Accession Number 9007310012).
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 project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Subinoy Mazumdar, AEOD
(301) 492-4308
Andrew Kugler, NRR
(301) 492-0834
Attachment:
List of Recently Issued NRC Information Notices
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