Information Notice 2005-15, Three-Unit Trip and Loss of Offsite Power at Palo Verde Nuclear Generating Station: Difference between revisions

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{{#Wiki_filter:UNITED STATES
{{#Wiki_filter:UNITED STATES


NUCLEAR REGULATORY COMMISSION
===NUCLEAR REGULATORY COMMISSION===
 
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NUCLEAR REACTOR REGULATION


WASHINGTON, D.C. 20555-0001 June 1, 2005 NRC INFORMATION NOTICE 2005-15:                THREE-UNIT TRIP AND LOSS OF OFFSITE
WASHINGTON, D.C. 20555-0001


POWER AT PALO VERDE NUCLEAR
===June 1, 2005===
NRC INFORMATION NOTICE 2005-15:
THREE-UNIT TRIP AND LOSS OF OFFSITE


===POWER AT PALO VERDE NUCLEAR===
GENERATING STATION
GENERATING STATION


Line 39: Line 41:
addressees to electrical equipment failures and design deficiencies identified following recent
addressees to electrical equipment failures and design deficiencies identified following recent


transients at Palo Verde Nuclear Generating Station (PVNGS), Units 1, 2, and 3. As a result, the units lost offsite power, tripped, and experienced other problems, including the loss of an
transients at Palo Verde Nuclear Generating Station (PVNGS), Units 1, 2, and 3. As a result, the units lost offsite power, tripped, and experienced other problems, including the loss of an


emergency diesel generator (EDG). It is expected that recipients will review the information for
emergency diesel generator (EDG). It is expected that recipients will review the information for


applicability to their facilities and consider actions, as appropriate, to avoid similar problems.
applicability to their facilities and consider actions, as appropriate, to avoid similar problems.
Line 52: Line 54:
the PVNGS switchyard originated with a fault across a degraded insulator on a 230 kV
the PVNGS switchyard originated with a fault across a degraded insulator on a 230 kV


transmission line. Protective relaying detected the fault and isolated the line from the remote
transmission line. Protective relaying detected the fault and isolated the line from the remote


substation. The protective relaying scheme at the other substation received a transfer trip
substation. The protective relaying scheme at the other substation received a transfer trip


signal actuating an auxiliary relay (Westinghouse Type AR) in the tripping scheme for two
signal actuating an auxiliary relay (Westinghouse Type AR) in the tripping scheme for two


breakers connected to the faulted line. The AR relay had four output contacts, all of which were
breakers connected to the faulted line. The AR relay had four output contacts, all of which were


actuated by a single lever arm. The tripping scheme used two contacts in redundant trip coils
actuated by a single lever arm. The tripping scheme used two contacts in redundant trip coils


for each breaker.
for each breaker.
Line 66: Line 68:
One breaker tripped, demonstrating that the AR relay coil picked up, and at least one of the AR
One breaker tripped, demonstrating that the AR relay coil picked up, and at least one of the AR


relay contacts closed. The other breaker did not trip. Bench testing of the AR relay
relay contacts closed. The other breaker did not trip. Bench testing of the AR relay


showed that, even with normal voltage applied to the coil, neither of the tripping contacts for the
showed that, even with normal voltage applied to the coil, neither of the tripping contacts for the


failed breaker closed. The breaker failure scheme for the failed breaker featured a design
failed breaker closed. The breaker failure scheme for the failed breaker featured a design


where the tripping contacts for the respective redundant trip coils also energized redundant
where the tripping contacts for the respective redundant trip coils also energized redundant


breaker failure relays. Since the tripping contacts for the failed breaker apparently did not
breaker failure relays. Since the tripping contacts for the failed breaker apparently did not


close, the breaker failure scheme was not activated, resulting in a persistent uncleared fault on
close, the breaker failure scheme was not activated, resulting in a persistent uncleared fault on
Line 83: Line 85:


12 seconds after fault inception, several transmission lines on the interconnected 69 kV, 230
12 seconds after fault inception, several transmission lines on the interconnected 69 kV, 230
kV, 345 kV, and 500 kV systems tripped on overcurrent. Also during the first 12 seconds, three
kV, 345 kV, and 500 kV systems tripped on overcurrent. Also during the first 12 seconds, three


cogeneration plants tripped, two with combustion turbines and one with a steam turbine, and
cogeneration plants tripped, two with combustion turbines and one with a steam turbine, and


the fault alternated between a single-phase-to-ground fault and a two-phase-to-ground fault, apparently as a result of a failed shield wire bouncing on the faulted line. After 12 seconds, the
the fault alternated between a single-phase-to-ground fault and a two-phase-to-ground fault, apparently as a result of a failed shield wire bouncing on the faulted line. After 12 seconds, the


fault became a three-phase-to-ground fault and additional 500 kV lines tripped.
fault became a three-phase-to-ground fault and additional 500 kV lines tripped.
Line 97: Line 99:
of their negative sequence relaying, thereby isolating the fault from the several cogeneration
of their negative sequence relaying, thereby isolating the fault from the several cogeneration


plants connected to that substation. Approximately 24 seconds after fault inception, the last two
plants connected to that substation. Approximately 24 seconds after fault inception, the last two


500 kV lines connected to the PVNGS switchyard tripped, isolating the PVNGS switchyard from
500 kV lines connected to the PVNGS switchyard tripped, isolating the PVNGS switchyard from


the transmission system. At approximately 28 seconds after fault inception, the three PVNGS
the transmission system. At approximately 28 seconds after fault inception, the three PVNGS


generators were isolated from the switchyard and, by approximately 38 seconds, all remaining
generators were isolated from the switchyard and, by approximately 38 seconds, all remaining
Line 111: Line 113:
Because of the loss of offsite power (LOOP), a Notice of Unusual Event was declared for all
Because of the loss of offsite power (LOOP), a Notice of Unusual Event was declared for all


three Palo Verde units at approximately 7:50 a.m. MST. The Unit 2 train A emergency diesel
three Palo Verde units at approximately 7:50 a.m. MST. The Unit 2 train A emergency diesel


generator started but failed early in the load sequence process due to a diode which short- circuited. The subject diode had less than 70 hours of run time in the exciter rectifier circuit. As
generator started but failed early in the load sequence process due to a diode which short- circuited. The subject diode had less than 70 hours of run time in the exciter rectifier circuit. As


a result, the train A engineered safeguards features busses deenergized, limiting the availability
a result, the train A engineered safeguards features busses deenergized, limiting the availability


of certain safety equipment for operators. Because of this failure, the emergency declaration
of certain safety equipment for operators. Because of this failure, the emergency declaration


for Unit 2 was elevated to an Alert at 7:54 a.m. MST. All three units were safely shut down and
for Unit 2 was elevated to an Alert at 7:54 a.m. MST. All three units were safely shut down and


stabilized under hot shutdown conditions. Units 1, 2, and 3 were without offsite power for
stabilized under hot shutdown conditions. Units 1, 2, and 3 were without offsite power for


approximately 4 hours and 9 minutes, 1 hour and 46 minutes, and 2 hours 15 minutes, respectively.
approximately 4 hours and 9 minutes, 1 hour and 46 minutes, and 2 hours 15 minutes, respectively.


==DISCUSSION==
==DISCUSSION==
External fouling on a 230 kV insulator resulted in the deenergizing of a 500 kV switchyard, removing all sources of power to three nuclear units. The single-failure susceptibility of a
External fouling on a 230 kV insulator resulted in the deenergizing of a 500 kV switchyard, removing all sources of power to three nuclear units. The single-failure susceptibility of a


transmission line protective system was the primary cause of the cascading blackout. The insulator degradation was caused by external fouling and did not, by itself, represent a
transmission line protective system was the primary cause of the cascading blackout. The insulator degradation was caused by external fouling and did not, by itself, represent a


concern about the reliability of the insulators on the 230 kV transmission system. Nevertheless, the failed AR relay and the lack of a robust tripping scheme raised concerns about the
concern about the reliability of the insulators on the 230 kV transmission system. Nevertheless, the failed AR relay and the lack of a robust tripping scheme raised concerns about the


maintenance, testing, and design of 230 kV system protective relaying. The 230 kV substation
maintenance, testing, and design of 230 kV system protective relaying. The 230 kV substation


where the relay failure occurred was subject to annual maintenance and testing. Following the
where the relay failure occurred was subject to annual maintenance and testing. Following the


event, the failed AR relay was visually inspected. No apparent signs of contamination or
event, the failed AR relay was visually inspected. No apparent signs of contamination or


deterioration were found.
deterioration were found.
Line 142: Line 144:
As noted earlier, the tripping scheme lacked redundancy that could have prevented the failure
As noted earlier, the tripping scheme lacked redundancy that could have prevented the failure


of the protective scheme to clear the fault. The review of the design of the substations
of the protective scheme to clear the fault. The review of the design of the substations


connected to the PVNGS switchyard indicated that two transmission lines at the subject
connected to the PVNGS switchyard indicated that two transmission lines at the subject


substation featured a tripping scheme with only one AR relay. The newer lines had two AR
substation featured a tripping scheme with only one AR relay. The newer lines had two AR


relays. However, the review found that the bus-sectioning breakers at the subject substation
relays. However, the review found that the bus-sectioning breakers at the subject substation


contained only one trip coil instead of two trip coils.
contained only one trip coil instead of two trip coils.
Line 154: Line 156:
To improve reliability, the tripping schemes for the two identified lines were modified to have
To improve reliability, the tripping schemes for the two identified lines were modified to have


two AR relays energizing separate trip coils for each breaker. The utility is considering
two AR relays energizing separate trip coils for each breaker. The utility is considering


installation of two trip coils in all single-trip-coil breakers. The tielines that connected 500 kV
installation of two trip coils in all single-trip-coil breakers. The tielines that connected 500 kV


and 230 kV switchyards did not have overcurrent or ground fault protection. The installation of
and 230 kV switchyards did not have overcurrent or ground fault protection. The installation of


overcurrent protection for these tielines were completed in a later modification.
overcurrent protection for these tielines were completed in a later modification.
Line 164: Line 166:
The apparent failure of the Unit 2 train A EDG was a failed diode in phase B of the voltage
The apparent failure of the Unit 2 train A EDG was a failed diode in phase B of the voltage


regulator exciter circuit. The diode failure resulted in a reduced excitation current and the
regulator exciter circuit. The diode failure resulted in a reduced excitation current and the


current was unable to maintain the voltage output with the applied loads. The failed EDG did
current was unable to maintain the voltage output with the applied loads. The failed EDG did


not have a significant impact on plant stabilization and recovery, but it did result in limited
not have a significant impact on plant stabilization and recovery, but it did result in limited
Line 177: Line 179:
S
S


This information notice requires no specific action or written response. Please direct any
This information notice requires no specific action or written response. Please direct any


questions about this matter to the technical contact(s) listed below or the appropriate Office of
questions about this matter to the technical contact(s) listed below or the appropriate Office of
Line 184: Line 186:


/RA/
/RA/
                                          Patrick L. Hiland, Chief


===Patrick L. Hiland, Chief===
Reactor Operations Branch
Reactor Operations Branch


Division of Inspection Program Management
===Division of Inspection Program Management===
Office of Nuclear Reactor Regulation
 
Technical Contacts:


Office of Nuclear Reactor Regulation
===Amar N. Pal, NRR===
Thomas Koshy, NRR


Technical Contacts: Amar N. Pal, NRR                Thomas Koshy, NRR
301-415-2760
301-415-1176 E-mail: anp@nrc.gov


301-415-2760                  301-415-1176 E-mail: anp@nrc.gov            E-mail: txk@nrc.gov
E-mail: txk@nrc.gov


Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
Line 200: Line 207:
Attachment (exempt from public disclosure in accordance with 10 CFR 2.390)
Attachment (exempt from public disclosure in accordance with 10 CFR 2.390)


PACKAGE: ML051520154, IN (PUBLIC): ML050490364 ATTACHMENT (NON-PUBLIC) ML051520164 OFFICE OES:IROB:DIPM Tech Editor           EEIB:DE               EEIB:DE       LPD4:DLPM
PACKAGE: ML051520154, IN (PUBLIC): ML050490364 ATTACHMENT (NON-PUBLIC) ML051520164 OFFICE OES:IROB:DIPM
 
Tech Editor
 
EEIB:DE
 
EEIB:DE
 
LPD4:DLPM
 
NAME
 
CVHodge
 
PKleene
 
ANPal
 
TKoshy
 
MBFields
 
DATE
 
02/24/2005
02/16/2005
02/24/2005
02/24/2005
02/28/2005 OFFICE PDIV-1:DLPM
 
EEIB:DE
 
A:SC:OES:IROB:DIPM
 
C:IROB:DIPM
 
NAME
 
WDReckley
 
JACalvo


NAME CVHodge              PKleene          ANPal                TKoshy        MBFields
EJBenner


DATE    02/24/2005      02/16/2005        02/24/2005            02/24/2005    02/28/2005 OFFICE PDIV-1:DLPM        EEIB:DE          A:SC:OES:IROB:DIPM C:IROB:DIPM
PLHiland


NAME WDReckley            JACalvo          EJBenner                PLHiland
DATE


DATE    03/01/2005       03/01/2005       05/16/2005             06/01/2005}}
03/01/2005
03/01/2005
05/16/2005
06/01/2005}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Latest revision as of 22:16, 15 January 2025

Three-Unit Trip and Loss of Offsite Power at Palo Verde Nuclear Generating Station
ML050490364
Person / Time
Site: Palo Verde  Arizona Public Service icon.png
Issue date: 06/01/2005
From: Hiland P
NRC/NRR/DIPM/IROB
To:
Hodge, CV, NRR/DIPM/IROB, 415-1861
Shared Package
ML051520154 List:
References
IN-05-015
Download: ML050490364 (5)
v • d • e


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

June 1, 2005

NRC INFORMATION NOTICE 2005-15:

THREE-UNIT TRIP AND LOSS OF OFFSITE

POWER AT PALO VERDE NUCLEAR

GENERATING STATION

ADDRESSEES

All holders of operating licensees for nuclear power reactors, except those who have

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert

addressees to electrical equipment failures and design deficiencies identified following recent

transients at Palo Verde Nuclear Generating Station (PVNGS), Units 1, 2, and 3. As a result, the units lost offsite power, tripped, and experienced other problems, including the loss of an

emergency diesel generator (EDG). 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

On June 14, 2004, at 7:41 a.m. Mountain Standard Time (MST), the 500 kV system upset at

the PVNGS switchyard originated with a fault across a degraded insulator on a 230 kV

transmission line. Protective relaying detected the fault and isolated the line from the remote

substation. The protective relaying scheme at the other substation received a transfer trip

signal actuating an auxiliary relay (Westinghouse Type AR) in the tripping scheme for two

breakers connected to the faulted line. The AR relay had four output contacts, all of which were

actuated by a single lever arm. The tripping scheme used two contacts in redundant trip coils

for each breaker.

One breaker tripped, demonstrating that the AR relay coil picked up, and at least one of the AR

relay contacts closed. The other breaker did not trip. Bench testing of the AR relay

showed that, even with normal voltage applied to the coil, neither of the tripping contacts for the

failed breaker closed. The breaker failure scheme for the failed breaker featured a design

where the tripping contacts for the respective redundant trip coils also energized redundant

breaker failure relays. Since the tripping contacts for the failed breaker apparently did not

close, the breaker failure scheme was not activated, resulting in a persistent uncleared fault on

the 230 kV line.

Various transmission system event recorders show that, during approximately the first

12 seconds after fault inception, several transmission lines on the interconnected 69 kV, 230

kV, 345 kV, and 500 kV systems tripped on overcurrent. Also during the first 12 seconds, three

cogeneration plants tripped, two with combustion turbines and one with a steam turbine, and

the fault alternated between a single-phase-to-ground fault and a two-phase-to-ground fault, apparently as a result of a failed shield wire bouncing on the faulted line. After 12 seconds, the

fault became a three-phase-to-ground fault and additional 500 kV lines tripped.

Approximately 17 seconds after fault inception, the three transmission lines between the

PVNGS switchyard and the nearby 500 kV substation tripped simultaneously due to the action

of their negative sequence relaying, thereby isolating the fault from the several cogeneration

plants connected to that substation. Approximately 24 seconds after fault inception, the last two

500 kV lines connected to the PVNGS switchyard tripped, isolating the PVNGS switchyard from

the transmission system. At approximately 28 seconds after fault inception, the three PVNGS

generators were isolated from the switchyard and, by approximately 38 seconds, all remaining

lines feeding the fault had tripped and the fault was isolated.

The trips resulted in a total loss of nearly 5,500 megawatts electric of local electric generation.

Because of the loss of offsite power (LOOP), a Notice of Unusual Event was declared for all

three Palo Verde units at approximately 7:50 a.m. MST. The Unit 2 train A emergency diesel

generator started but failed early in the load sequence process due to a diode which short- circuited. The subject diode had less than 70 hours8.101852e-4 days <br />0.0194 hours <br />1.157407e-4 weeks <br />2.6635e-5 months <br /> of run time in the exciter rectifier circuit. As

a result, the train A engineered safeguards features busses deenergized, limiting the availability

of certain safety equipment for operators. Because of this failure, the emergency declaration

for Unit 2 was elevated to an Alert at 7:54 a.m. MST. All three units were safely shut down and

stabilized under hot shutdown conditions. Units 1, 2, and 3 were without offsite power for

approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 9 minutes, 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 46 minutes, and 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 15 minutes, respectively.

DISCUSSION

External fouling on a 230 kV insulator resulted in the deenergizing of a 500 kV switchyard, removing all sources of power to three nuclear units. The single-failure susceptibility of a

transmission line protective system was the primary cause of the cascading blackout. The insulator degradation was caused by external fouling and did not, by itself, represent a

concern about the reliability of the insulators on the 230 kV transmission system. Nevertheless, the failed AR relay and the lack of a robust tripping scheme raised concerns about the

maintenance, testing, and design of 230 kV system protective relaying. The 230 kV substation

where the relay failure occurred was subject to annual maintenance and testing. Following the

event, the failed AR relay was visually inspected. No apparent signs of contamination or

deterioration were found.

As noted earlier, the tripping scheme lacked redundancy that could have prevented the failure

of the protective scheme to clear the fault. The review of the design of the substations

connected to the PVNGS switchyard indicated that two transmission lines at the subject

substation featured a tripping scheme with only one AR relay. The newer lines had two AR

relays. However, the review found that the bus-sectioning breakers at the subject substation

contained only one trip coil instead of two trip coils.

To improve reliability, the tripping schemes for the two identified lines were modified to have

two AR relays energizing separate trip coils for each breaker. The utility is considering

installation of two trip coils in all single-trip-coil breakers. The tielines that connected 500 kV

and 230 kV switchyards did not have overcurrent or ground fault protection. The installation of

overcurrent protection for these tielines were completed in a later modification.

The apparent failure of the Unit 2 train A EDG was a failed diode in phase B of the voltage

regulator exciter circuit. The diode failure resulted in a reduced excitation current and the

current was unable to maintain the voltage output with the applied loads. The failed EDG did

not have a significant impact on plant stabilization and recovery, but it did result in limited

availability of certain safety equipment during a design basis event.

Refer to Attachment 1 for additional discussion.

CONTACT

S

This information notice requires no specific action or written response. Please direct any

questions about this matter to the technical contact(s) listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

/RA/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Technical Contacts:

Amar N. Pal, NRR

Thomas Koshy, NRR

301-415-2760

301-415-1176 E-mail: anp@nrc.gov

E-mail: txk@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

Attachment (exempt from public disclosure in accordance with 10 CFR 2.390)

PACKAGE: ML051520154, IN (PUBLIC): ML050490364 ATTACHMENT (NON-PUBLIC) ML051520164 OFFICE OES:IROB:DIPM

Tech Editor

EEIB:DE

EEIB:DE

LPD4:DLPM

NAME

CVHodge

PKleene

ANPal

TKoshy

MBFields

DATE

02/24/2005

02/16/2005

02/24/2005

02/24/2005

02/28/2005 OFFICE PDIV-1:DLPM

EEIB:DE

A:SC:OES:IROB:DIPM

C:IROB:DIPM

NAME

WDReckley

JACalvo

EJBenner

PLHiland

DATE

03/01/2005

03/01/2005

05/16/2005

06/01/2005


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