Information Notice 1991-81, Switchyard Problems That Contribute to Loss of Offsite Power
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.
20555
December 16, 1991
NRC INFORMATION NOTICE 91-81: SWITCHYARD PROBLEMS THAT CONTRIBUTE TO
LOSS OF OFFSITE POWER
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 several problems associated with plant switchyards.
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
On April 23, 1991, a complete loss of offsite power occurred at the Vermont
Yankee Nuclear Power Station as a result of maintenance activities in the
Maintenance workers were installing a new battery for one of the
two non-lE 125 VDC buses.
The two normally independent buses were cross tied
through the swing battery charger 4A-5A after defeating a mechanical interlock
(Figure 1).
Furthermore, the licensee was paralleling the battery chargers
on bus DC-4A without a battery on this bus.
When the 4A-5A battery charger
output breaker feeding bus DC-5A was opened prior to connection of the new
battery 4A to bus DC-4A, a voltage transient propagated through the switchyard
DC control system that caused all but one of the 345kV and 115kV circuit
breakers to trip and lock open.
The loss of offsite power which occurred
caused a main turbine and automatic reactor trip.
Restoration of full offsite
power took thirteen hours.
The NRC dispatched an augmented inspection team (AIT) to investigate this
event, and the following generic concerns were identified by the team in its
report (50-217/91-13):
(1) a lack of preventive maintenance on battery
chargers, (2) presence of sensitive components in logic cards making them
susceptible to failure, (3) switchyard protective relays that were susceptible
to a single failure, and (4) a lack of clear authority over the switchyard
during emergency conditions.
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Anntpriance
Lack of Preventive ,--
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char
Three of the four non-lE battery chargers (4A, B, and 4A-5A) in the switchyard
DC control system were in a degraded condition at the time of the event. fn
each of the degraded battery chargers, manufactured by Exidf, the output filter
circuit was not functional. The problems included blown fuses in one
iiter
circuit, blown and wrong-sized fuses in another, and a failed capacitor *in
the
third filter circuit. If the 4A-5A battery charger output filter circuit r.ad
been functional, the effect of the voltage transient may have been m(itigate
and loss of offsite power might have been prevented.
The problems resultCC
directly from infrequent preventive maintenance on the battery chargers. IlTe
licensee had no regular maintenance program for the chargers and it appeared
that the last maintenance was performed in 1985. These battery chargers aigre
not safety related and no surveillance testing or preventive maintenance is
required. However, at this facility, the switchyard battery system, includit.u
the
quitchyard batteries, their associated chargers, and associated distribu- tion panels, is part of the auxiliary electrical power system and is required
to be operable by the plant's Technical Specifications.
Sensitive
Components
in Logic
Cards
The voltage spike in the switchyard DC control 'y"u,, .baI_
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in the stuck breaker failure units (SBFUS) of two switchyard circuit breakers
which led to a chain reaction that caused most of the breakers in the
-4zks
ard
115kV switchyard to trip and lock open.
These SBFUs are manufactured by AREA
Brown Boveri, and previous models were made by the Westinghouse Electric
Corporation. The manufacturer has developed new units that are not as suscep- tible to voltage spikes because the new units do not contain zener diodes.
Protective Relays Subject to a Single Failure
All of the SBFU relays are powered from a single bus (DC-4A) even though t%.,
DC buses are available in the switchyard DC control system.
This desig. a
subject to a single failure.
Other facilities may have similar proteccti;-
relaying designs that could be vulnerable to a common cause failure.
c-..4+,4iiizrnrd Under Emergenc Coditions
The restoration of offsite power was delayed because of a lack of coIrmunications
between plant staff and the transmission and distribution (T&D) personnel.
Under normal conditions, T&D is responsible for operating the switchyard.
The
licensee's procedure specifies that, under emergency conditions, the responsi- bility is transferred to the plant staff.
However, the procedure does not
define "emergency."
These operational problems suggest the need to clarify
switchyard authority during a loss of offsite power or other emergency condi- tions when timely switchyard activities are important.
Additional
Example of Switchyard Communication Problems
Another example of communication problems in the switchyard can be seen in Ar
event that took place at the McGuire Nuclear Generating Station, Unit i, on
February 11, 1991. The unit tripped from 100% power when T&D personnel caused
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IN 91-81 December 16, 1991 a loss of all offsite power while testing a modification to the protective
relay circuitry in the 230 kV switchyard.
Since the majority of the switchyard
is outside of the plant equipment boundary, the T&D personnel did not notify
the station operations personnel that work was in progress even though switch- yard activities can impact the station.
Prior to this event there was no
agreement between station operations and T&D personnel on how to handle switch- yard activities outside the plant equipment boundary.
Subsequent to this
event, a work control policy was established to provide improved communication
between the station operations personnel and T&D personnel.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the technical
contacts listed below or the appropriate Office of Nuclear Reactor Regulation
(NRR) project manager.
arles E. Rossi Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts:
Cliff Anderson, Region I
(215) 337-5376
Peter Kang, NRR
(301) 504-0779
David Skeen, NRR
(301) 504-1174 Attachments:
1. Figure 1:
Switchyard House DC Distribution
One Line Diagram
2. List of Recently Issued NRC Information Notices
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Attachment 2
December 16, 1991 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
I tformation
Date of
Notice to.
Subject
Issuance
Issued to
91-80
88-92, Supp.
1
9j-57, S..pp. 1
01 -77
91-76
91-75
Failure of Anchor
Head Threads on Post-
Tensioning System During
Surveillance Inspection
Deficiencies in the
Procedures for Instal- ling Thermo-Lag Fire
Barrier Materials
Potential for Spent
Fuel Pool Draindown
Status Indication of
Control Power for
Circuit Breakers Used
in Safety-Related Appli- cations
Substandard, Refur- bished Potter & Brum- field Relays Repre- sented as New
Shift Staffing at
Nuclear Power Plants
10 CFR Parts 21 and
50.55(e) Final Rules
Static Head Corrections
Mistakenly not Included
in Pressure Transmitter
Calibration Procedures
Changes in Pressurizer
Safety Valve Setpoints
Before Installation
12/11/91
All holders of OLs or CPs
for nuclear power reactors.
12/06/91
All holders of OLs or CPs
for nuclear power reactors.
11/29/91
All holders of OLs or CPs
for nuclear power reactors.
11/28/91
All holders of OLs or CPs
for nuclear power reactors.
11/27/91
All holders of OLs or CPs
for nuclear power reactors.
11/26/91
All holders of OLs or CPs
for nuclear power reactors.
11/26/91
All holders of OLs or CPs
and vendors for nuclear
power reactors.
11/25/91
All holders of OLs or CPs
for nuclear power reactors.
11/25/91
All holders of OLs or CPs
for nuclear power reactors.
91-74 OL = Operating License
CP = Construction Permit
IN 91-81 December 16, 1991 a loss of all offsite power while testing a modification to the protective
relay circuitry in the 230 kV switchyard.
Since the majority of the switchyard
is outside of the plant equipment boundary, the T&D personnel did not notify
the station operations personnel that work was in progress even though switch- yard activities can impact the station.
Prior to this event there was no
agreement between station operations and T&D personnel on how to handle switch- yard activities outside the plant equipment boundary.
Subsequent to this
event, a work control policy was established to provide improved communication
between the station operations personnel and T&D personnel.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the technical
contacts listed below or the appropriate Office of Nuclear Reactor Regulation
(NRR) project manager.
Orgnal Signid by
Charles E. Rossi, Director
Charles E.
Rossi
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Cliff Anderson, Region I
(215) 337-5376
Peter Kang, NRR
(301) 504-0779
David Skeen, NRR
(301) 504-1174 Attachments:
1. Figure 1:
Switchyard House DC Distribution
One Line Diagram
2. List of Recently Issued NRC Information Notices
- See previous concurrence
OFC
- OEAB:NRR
- *RI
- SC:OEAB:NRR:*ADM:RPB
- C:SELB:NRR :*C:OEAB:NRR
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NAME :DSkeen
- CAnderson :RDennig
- JMain
- FRosa
- AChaffee
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DATE :10/01/91
- 10/01/91
- 11/05/91
- 10/01/91
- 11/05/91
- 11/14/91 OFC
- C:OGCB:NRR :D:DOEA
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NAME :CBerlinger :
DATE :11/25/91
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OFFICIAL RECORD COPY
Document Name: IN 91-81
IN 91-XX
November xx, 1991 there was no agreement between station operations and T&D personnel on how to
handle switchyard activities outside the plant owned equipment boundaries.
Subsequent to this event, a work control policy was established to provide
improved communication between the station operations personnel and T&D
personnel.
The staff is providing this information as an early notification of a matter
that could become significant and that the NRC staff is continuing to consider.
Upon completing its evaluation, the staff may require licensees to take
specific actions. No specific action or written reponse is required by this
information notice. If you have any questions about this matter, please contact
the technical contacts 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: Cliff Anderson, Region I
(215) 337-5376
Peter Kang, NRR
(301) 492-0779
David Skeen, NRR
(301) 492-1174 Attachment:
1. Figure 1:
Switchyard House DC Distribution
One Line Diagram
2. List of Recently Issued NRC Information Notices
- See previous concurrence
OFC :*OEAB:NRR
- *RI
- SC:OEAB:NRR:*ADM:RPB
- C:SELB:NRR :*C:OEAB:NRR
_____
____________
-
____________
____________
____________
-
NAME :DSkeen
- CAnderson :RDennig
- JMain
- FRosa
- AChaffee
DATE :10/01/91
- 10/01/91
- 11/05/91
- 10/01/91
- 11/05/91
- 11/14/91
- 11 9)
OFC
- C: UP,
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NAME :CBerlinger :CRossi I::
DATE : 1;/I91
- / /91
OFFICIAL RECORD COPY
Document Name:
IN/SWITCHYARD/SKEEN
IN 91-XX
October xx, 1991 The staff is providing this information as an early notification of a matter
that could become significant and that the NRC staff is continuing to consider.
Upon completing its evaluation, the staff may require licensees to take
specific actions. No specific action or written reponse is required by this
information notice. If you have any questions about this matter, please contact
the technical contacts 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:
Cliff Anderson, Region I
(215) 337-5376
Peter Kang, NRR
(301) 492-0779
David Skeen, NRR
(301) 492-1174 Attachment:
List of Recently Issued NRC Information Notices
- See previous concurrence
OFC
- OEAB:NRR
- *RI
NAME :DSkeen
- CAnderson
DATE :10/01/91
- 10/01/91
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OFFICIAL RECORD COPY
Document Name:
IN/SWITCHYARD/SKEEN
No specific action or written reponse is required by this information notice.
If you have any questions about this matter, please contact the technical
contacts 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: Cliff Anderson, Region I
(215) 337-5376
Peter Kang, NRR
(301) 492-0779
David Skeen, NRR
(301) 492-1174 Attachment: List of Recently Issued NRC Information Notices
pfr TGLECoA
OFC
- OEAB:NRR
- RI
- SC:OEAB:NRR :ADM:RPB
- C:SELB:NRR :C:OEAB:NRR
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NAME :DSkeen j
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DATE :/O/ 1/91
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- 10/1 /91
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- / /91 OFC
- C:OGCB:NRR :D:DOEA
NAME :CBerlinger
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DATE: / /91
- / /91
OFFICIAL RECORD COPY
Document Name:
IN/SWITCHYARD/SKEEN