Information Notice No. 91-57: Operational Experience on Bus Transfers

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                             September 19, 1991



All holders of operating licenses or construction permits for nuclear power 


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.


The problems associated with the bus transfer process raise two basic safety 

(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 


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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.  


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 

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 

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

List of Recently Issued NRC Information Notices


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