Information Notice 1991-57, Operational Experience on Bus Transfers
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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
reactors.
-
permits for
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p
'nuclear power
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 199D, 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 require- ments; 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 ClasslE loads of both trains, and
repeated bus transfers could damage the safety-related motors'.' This informa- tion prompted the NRC to conduct an-in-depth study of the bus transfer prac- tices 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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IN 91-57 September 19, 1991 difference will cause transient current flows in the system that candamage the
transformers, the buses, and theWconnected loads.
There is.a differen~ce of opinion within the industry about criteria that-assure
safe bus transfer. In 1977, 'the Americtrr National Stridards 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 Associa- tion (NEMA) accepted this as a safe criteriQnin 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 plaIts that, can be
directly attributed to eXce'ssive' difference' in'
voltage between an auxiliary
load"'bus and the incomihg.power, source, although some equipment could be
stressed U this pr6cess.' Such stresses experienced by connected equipment
would be. cumula'ti've,: and',' uinless specifki
action is taken.to limit' these
stresses', they can remainj'undetectedi.ntit failure'occurs.
The second safety'concern cov'ers cases in whi ch 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 offsitepower to plant auxiliary
loads-,-which-n-some-cases,-include-the-Class--IE loads.
Hence, with-this-.type- of bu's-transfer'failUre,';th'e Clas's'lE lo'ads-will be powered by. the.emergency.
diese1 gerntor's, and 7reactor sstem coblin will be
.accomplished
by-natural
circul'ation. Althobgh nu-clear plaants ati d'signed to safely shut down.on a
loss of'offslteepower, it is desirable' th4't such failures be kept to a minimum.
to m'finiiize;c'hllen'gesto the' emerg ency dese6lgenerators.:
Analy`sis
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z
i
,
The data fror AtODIE190-65, ind'i'cttes'that the' irot causes for.the most signifi
caiet'events (those that% in'ved'ln'aotual fa'il.re to'. transfer or a significant
design deficienky) canb-
6eb'roken down as follows:'
c'
42 percent were caused by design deficiencies, i
25'-percent were' caused by equi pment failures
18 percent were c
persnneel ererors and
15 percent were caused by protective relays thqt blockedthe bus transfer, The major design deficiencies.includIed slot bus~transfer,(35 percent of all
designudef-ciency:events), 1ow bsv61tagef(.4percent f all design deficiency
n speed'of operation of sync-check relays
(17 percent of all design deficiency events).
IN 91-57 September 19, 1991 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 trainsfer 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 person- nel 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 Dfes&i
Generator Degradation,"86-87, "Loss of Offsite Power Upon an AutomaticjBus
Transfer,"86-100, "Loss of Offstte Power to Vital .Buses at Salem:2," and
88-50, "Effect of Circuit.Breaker Capacitance on.Availab.ility of Emergency
Power."
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NRC Engineering Evaluation Report-AEOD/E90-05 is available n.-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 cohtacts listed below'or the appropriate Office of Nuclear
Reactor Regulation project manager.
.
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Division of Operational E
Office of Nuclear Reactor Regulation
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r ' ;.2 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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91-55,
05-18,
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91-41 Attachment
Septaber 19. 1991
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.4 LIST Of MCENTLY ISSUED'
IRC IUKMfOaTIQIU NOTICES
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Subject
Potential Radiotive
Lekage to TniK Vented
to Atmosphee
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Failures Ceusatb An:
Improperly Adjustkd Test
L-ink In 4.16'V General
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Failures of Undervoltae
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the Westinghoee-Oesigned
Splid State Protect4oh
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Foreign Experience Regard- ng Baoron Ollution
Pressurizer Saf ty Valve
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Fellure of Remote Shutd&wn
System Instramentation
Because of Incorrectly
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In.
Overtemperature Delte- Temperature (0Tot) Set- point Caused by Improper
Goin Settings
Overspead Trips of AFW.
NIPCI end RCIC Turbines'
Inadequate Fuse Centrol
Progre
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Bate of
Issuance
Issued to
09/1S/91 ill ,holders of A0S or CPs
'for puclear power reactors.
09/16/1 All holdre' of OLt or CPs
fori nuclear tower reactors.
09/10/91 All holders'of OLS or CPS for
Wdest~inghouse (40-designed
nucear. power reactors.
09/06/91 All holders. o-OLs or CPs
for pressurized water
reactors (?irr.
09/05/91 All hIolersot OLs- or CPs
ore nclaar poweIrsrectorS.
09/04/91 All
oofdsrs
eOLs or CPs
for ulr
power reactors.
0829/0/91 AI
for Uestiflghousa (W)-designed
nuclear
poear reaetors.
0826/91 All holders of'OLe or CPa
for nuclear power feectors.
08/20/91
All holders eof 0L or CPs
for nuclear power reactors.
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- Operatin License
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CP u Construction Permit
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IN 91-57 September 19, 1991 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 person- nel 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.
Original Signed by
Charles F. Rossi
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
- SEE PREVIOUS CONCURRENCS
D/
D
/OGCB:DOEA:NRR*RPB:ADM
- C/SELB:DST:NRR
CeD L/( CHBerlinger
TechEd
FRosa
9//'91 i
8/26/91
06/28/91
09/11/91
- OGCB:DOEA:NRR*ROA l3SP:AEOD
- I3AB:DSP:AEOD *C/ROAB:DSP:AEOD
- D/DSP:NRR
AJKugler
SMazumdar
EJBrown
JERosenthal
TMNovak
08/08/91
08/09/91
08/13/91
08/13/91
08/19/91 DOCUMENT NAME: IN 91-57
IN 91-XX
September xx, 1991 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 person- nel 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."
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 References:
Ie NRC Engineering Evaluation Report, AEOD/E90-05,
"Operational Experience On Bus Transfers," June 1990.
Available in the Public Document Room, Washington, D.C.,
Accession Number 9007310012
Attachment:
List of Recently Issued NRC Information Notices
Document Name: BUS TRANSFER IN
- SEE PREVIOUS CONCURRENCES
D/DOEA:NRO
- C/OGCB:DOEA:NRR*RPB:ADM
CERossi
CHBerlinger
TechEd
09/
/91V
08/26/91
06/28/91
- OGCB:DOIPA:NRR*ROAB:DSP:AEOD
- ROAB:DSP:AEOD
- C/ROAB:DSP:AEOD
AJKugler
S)IM ndar
EJBrown
JERosenthal
08/08/91
08/69/9^1
08/13/91
08/13/91 ti/SEL?.DS'T:NRR
04~osa
09/:{ /91
- D/DSP:NRR
TMNovak
08/19/91
IN 91-XX
August xx, 1991 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 person- nel vigilance.
Related Generic Communications
The staff has issued the following information notices on this and related
topics: Notices 85-28, "Partial Loss of AC Power and Diesel Generator Degrada- tion," 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."
This information notice requires no specific
you have any questions about the information
of the technical contacts listed below or the
action or written response. If
in this notice, please contact one
! appropriate NRR 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
References:
1. NRC Engineering Evaluation Report, AEOD/E90-05,
"Operational Experience On Bus Transfers," June 1990.
Document Name:
BUS TRANSFER IN
D/DOEA:N1t
CERossS I
08/ /911
)SP:A OD ROAB:DSPAEOD
nda ;
EJBrown !?
/91
08//3/91 C/OG
CHBerlinger
08/Z&91 C/ROAB:DSP:AI);
JERosenthal
08//3'91
- RPB:ADM
TechEd
06/28/91 D/Di
08/l ¶191 OGCB:DOEA:NRR
AJKuglerclK
08/ ,/91"
ROAB: C
SMazur
08/ 9i
.
also indicates that many of the transfer failures resulting from equipment
malfunction and personnel errors can be avoided by improving equipment
maintenance and personnel vigilance.
Related Generic Communications
Previous information notices have been issued on this and related topics.
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" should be consulted
for further information.
No specific action is required by the notice. However, if you have any
questions on this issue, please contact the technical staff 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:
Subinoy Mazumdar, AEOD
(301) 492-4308
Andrew Kugler, NRR
(301) 492-0834 Attachments:
List of Recently Issued NRC Information Notices
References:
1.
NRC Engineering Evaluation Report, AEOD/E90-05, "Operational
Experience On Bus Transfers, June 1990.
D/DOEA:NRR
CERossi
07/
91 D/DSP:AEOD
TMNovak
06/
/91 C/OGCB:DOEA:NRR
CHBerlinger
07/
/91 RPB:ADM
TechEd
06/21 /91 OGCB:DOEA:NRR
AJKugler v
06/12./91 ROAB:DSP:AEOD
SMazumdar
06/
/91 C/ROABtDSP:AEOD
JERosenthal
06/
/91
)1Nin 91h