Information Notice 2005-15, Three-Unit Trip and Loss of Offsite Power at Palo Verde Nuclear Generating Station
| ML050490364 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| 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) | |
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
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