05000306/LER-2011-002, For Prairie Island Nuclear Generating Plant Unit 2, Regarding Unit 2 Trip as a Result of a Weather Related Turbine Trip
| ML111810621 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  (DPR-060) |
| Issue date: | 06/30/2011 |
| From: | Schimmel M Northern States Power Co, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-PI-11-052 LER 11-002-00 | |
| Download: ML111810621 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3062011002R00 - NRC Website | |
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@ Xcel Energye June 30,201 I L-PI-11-052 10 CFR 50.73 U S Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Prairie lsland Nuclear Generating Plant Unit 2 Docket: 50-306 Renewed License No. DPR-60 LER 50-3061201 1-002-00, Unit 2 Trip As A Result Of A Weather Related Turbine Trip Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, herewith encloses Licensee Event Report (LER) 50-3061201 1-002-00.
On May 9, 201 1, Prairie lsland Nuclear Generating Plant (PINGP) Unit 2 tripped from approximately 100% steady-state power as a result of a turbine trip from a generator lockout signal. At the time of the trip, thunderstorms were in the vicinity of the plant.
Post event diagnostics determined that the relay ground potential circuit had a missing link (wire) between substation panels which resulted in the loss of the reference ground potential to the associated protective relaying in the Switchyard panels. This resulted in the improper operation of the breaker protection relays.
Summarv of Commitments This letter contains no new commitments and no changes to existing commitments.
Mark A. Schimmel Site Vice President, Prairie lsland Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:
Administrator, Region Ill, USNRC Project Manager, Prairie lsland Nuclear Generating Plant (PINGP), USNRC Resident Inspector, PINGP, USNRC Department of Commerce, State of Minnesota 171 7 Wakonade Drive East Welch, Minnesota 55089-9642 Telephone: 651.388.1 121
ENCLOSURE LICENSEE EVENT REPORT 50-3061201 1-002-00 4 Pages Follow
imated burden per response to comply vnth thls mandatow wllectio uest 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br /> Reported lessons learned are incorporated tnto the licensin cess and fed back to industry Send wmments regard~ng burden estimate t LICENSEE EVENT REPORT (LER) 20.2203(a)(3)(i) 50.73(a)(Z)(i)(C)
C]
20.2203(a)(3)(ii)
C]
50.73(a)(2)(ii)(A) 50.73(a)(2)(viii)(A) 1 20.2203(a)(I) 1 20.2203(a)(4)
C]
50.73(a)(2)(ii)(B)
C]
50.73(a)(2)(viii)(B) 20.2203(a)(2)(i) 1 50,36(c)(l)(i)(A)
C]
50.73(a)(2)(iii) 1 50,73(a)(2)(ix)(A) 1 20.2203(a)(2)(ii) 1 50.36(c)(l)(ii)(A) 50.73(a)(2)(iv)(A) 50.73(a)(2)(x) 1 20.2203(a)(2)(iii) 1 50.36(~)(2)
C]
50,73(a)(2)(v)(A) 73.71(a)(4) 1 20.2203(a)(2)(iv)
C]
50.46(a)(3)(ii)
C]
50.73(a)(2)(v)(B) 73.71 (a)@)
C]
20.2203(a)(2)(v)
C]
50.73(a)(2)(i)(A) 1 50.73(a)(2)(v)(C) 1 OTHER 20.2203(a)(2)(vi) 5OS73(a)(2)(i)(B)
C]
50,73(a)(2)(v)(D)
Spectfy In Abstract below or In 12. LICENSEE CONTACT FOR THlS LER NAME Sam J. DiPasquale, P.E.
TELEPHONE NUMBER (Include Area Code) 651.388.1 121 x7350 CAUSE ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines)
On May 9, 201 1, Prairie Island Nuclear Generating Plant (PINGP) Unit 2 tripped from approximately 100%
steady-state power as a result of a turbine trip from a generator lockout signal. At the time of the trip Unit 1 was in a planned refueling outage in Mode 6 Operation and thunderstorms were in the vicinity of the plant.
Post event diagnostics determined that the relay ground potential circuit had a missing link (wire) between substation panels which resulted in the loss of the reference ground potential to the associated protective relaying in the Switchyard panels. This resulted in the improper operation of the breaker protection relays.
The causal evaluation determined that the proximate cause of the relaying scheme failure was inadequate ground wire installation during a modification to the substation equipment. The evaluation also determined that NSPM (Transmission and Nuclear) did not establish the appropriate standard for procedure quality with respect to work under System Control Center jurisdiction in the PINGP substation such that a means of effectively utilizing human performance tools to minimize errors during work, which directly impacts the safe and reliable operation of nuclear plants, was not in place.
NRC FORM 366 (10-2010)
SYSTEM COMPONENT MANU-FACTURER REPORTABLE TO EPlX
CAUSE
SYSTEM COMPONENT MANU-FA CTURER REPORTABLE TO EPlX
EVENT DESCRIPTION L~CENS EE EVENT RE PORT (LER)
U.S. NUCLEAR REGULATORY COMMlSSlON (10-201 0)
CONTINUATION SHEET The Prairie Island Nuclear Generating Plant (PINGP) switchyard breakers' use a selective isolation design. The relays2 are designed to isolate the breakers closest to a detected fault first with the intent of maximizing reliability while protecting the grid from a localized fault. Some PINGP substation relays utilize distance and directional-based sensing to determine the location of a fault. These relays enable selective tripping of the breakers closest to the fault.
Breakers 8H13 and 8H14 serve as Unit 2 en era tor^ output isolations. Unit 1 Generator output isolations are 8H16 and 8H17. Breakers 8H15 and 8H14 isolate the Blue Lake transmission line4 from 345kV Buses 2 and 1 (respectively). Breakers 8H9 and 8H8 isolate the Byron transmission line from 345kV Buses 2 and 1 (respectively). In November 201 0, NSPM implemented a modification to the breakers in the switchyard (under System Control Center (SCC) jurisdiction) to upgrade protective relaying on those breakers to a digital system. The modification was performed under Transmission System Operator (TSO) Work Order (WO) 11209851.
- 3. PAGE 2OF4
- 1. FACILITY NAME Prairie Island Nuclear Generating Plant Unit 2 At approximately 07:22 CDT on May 9, 201 1, thunderstorms in the vicinity of PINGP and other portions of Southeast Minnesota produced a lightning strike which contacted ground on the Byron transmission line. The strike occurred approximately 25 miles from the Byron station and 15 miles from PINGP. The strike created a grid disturbance and actuated PINGP protective relaying on breakers for the Unit 2 Generator, Blue Lake, and Byron Lines.
As a result, PINGP Unit 2 tripped from approximately 100% steady-state power as a result of a turbine trip from a generator lockout signal. At the time of the trip, Unit 1 was in a planned refueling outage in Mode 6. The Unit 2 steam plant and reactor protection system5 responded properly to the turbine trip.
- 2. DOCKET NUMBER 05000 306
EVENT ANALYSIS
- 6. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO 201 1 -
002
- - 00 Based upon post event diagnostics, Xcel Energy System Relay Specialists determined that the relay ground potential circuit was missing a link (wire) between substation panels6 (from a latent wiring error) resulting in the loss of the reference ground potential to associated protective relaying in the Switchyard panels associated with the Unit 1 Generator, Unit 2 Generator, and the Blue Lake transmission line.
The loss of the reference ground potential was sufficient to "disorient" the distance and directional-sensing of the relays for the affected breakers; ultimately resulting in the relays detecting a "fault1' on Ells Component Identifier: BKR 2
3 Ells Component Identifier: RLY Ells System Code:
EL Ells System Code:
5 FK 6 Ells System Code:
JC Ells Component Identifier: PL
the Blue Lake and/or Generator sides of the breaker when no fault existed. In essence, breakers 8H13, 8H14, and 8H15 tripped when a fault requiring relay actuation was not present, with the net effect of causing a generator, turbine, and reactor trip. This event is reportable under 10 CFR 50.73(a)(2)(iv)(A) since there was an automatic actuation of the Reactor Protection system (RPS) and the auxiliary feedwater system (AFW). LICENSEE EVENT REPORT (LER)
U.S. NUCLEAR REGULATORY COMMlSSlON (10-201 0)
CONTINUATION SHEET
SAFETY SIGNIFICANCE
This event did not challenge nuclear safety as all plant systems responded as designed. Therefore, this event does not represent a safety system functional failure for Unit 2. However, the lightning strike and latent system installation flaws combined to create an initiating event which did challenge reliable operation. Additionally, the root cause investigation determined that the Unit 1 Generator output breakers would also have been affected if Unit I were online at the time (creating the potential for a dual-unit trip). Any unplanned reactor trip is a significant event.
- 3. PAGE 3 0 F 4
- 1. FACILITY NAME Prairie Island Nuclear Generating Plant Unit 2 The plant systems responded as designed so there were no radiological, environmental, or industrial impacts associated with this event. Therefore, this event did not affect the health and safety of the public.
CAUSE
2, DOCKET NUMBER 05000 306 The causal evaluation determined that the proximate cause of the relaying scheme failure was inadequate ground wire installation during the November 201 0, modification to the substation equipment.
- 6. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO 2011 -
002
- - 00 The root cause of the Unit 2 trip was that NSPM (Transmission and Nuclear) did not establish the appropriate standard for procedure quality with respect to work under SCC jurisdiction in the PlNGP substation such that a means of effectively utilizing human performance tools to minimize errors during work which directly impacts the safe and reliable operation of nuclear plants was not in place.
A contributing cause was that the design of the protective relaying circuit includes a single point vulnerability whereby a single floating ground can result in misoperation of multiple breakers, thereby reducing transmission reliability.
Another contributing cause was that PlNGP has, in the past, not rigorously implemented corrective actions from external assessments and recommendations, thereby resulting in vulnerabilities in oversight of substation work not being adequately addressed.
CORRECTIVE ACTION LICENSEE EVENT REPORT (LER)
U.S. NUCLEAR REGULATORY COMMlSSlON (1 0-201 0)
CONTINUATION SHEET WO 430439 replaced the missing section of the reference ground circuit on May 9, 201 1. Unit 2 was placed online on May 10, 201 1.
In accordance with the Corrective Action Program (CAP), NSPM will implement a program to identify new installation and modification work under SCC jurisdiction that may impact nuclear plant reliability and implement procedural controls to enable human performance tool usage. This program will ensure that, prior to conduct of work, procedures of sufficient quality to enable human performance tool usage are in place.
- 3. PAGE 4 0 F 4
- 1. FACILITY NAME Prairie Island Nuclear Generating Plant Unit 2 In accordance with the CAP, NSPM will implement a modification to the ground circuit for protective relaying to remove the single point vulnerability.
In accordance with the CAP, NSPM will evaluate recommendations of industry documents against current procedures and practices. The evaluation will, at a minimum, address recommendations and describe how current practices and procedures support implementation or identify actions needed to address identified gaps.
- 2. DOCKET NUMBER 05000 306
PREVIOUS SIMILAR EVENTS
- 6. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO 2011 -
002
- - 00 A LER search was conducted and no similar events at PlNGP involving grid disturbances were identified in the last three years.