05000498/LER-2020-001, Automatic Actuation of Emergency Diesel Generators Due to Lockout of Switchyard Electrical Bus
| ML20143A188 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 05/22/2020 |
| From: | Schaefer M South Texas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NOC-AE-20003737, STI:35022728 LER 2020-001-00 | |
| Download: ML20143A188 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 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)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 4982020001R00 - NRC Website | |
text
Nuclear Operating Company South Texas Protect Electric Generating Station P.O. Box 289 Wadavorth, Texas 77483 May 22, 2020 NOC-AE-20003737 10CFR 50.73 Attention: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Units 1 & 2 Docket Nos. STN 50-498, STN 50-499 Licensee Event Report 2020-001-00 Automatic Actuation of Emergency Diesel Generators due to Lockout of Switchvard Electrical Bus Pursuant to reporting requirements of 10 CFR 50.73(a)(2)(iv)(A), STP Nuclear Operating Company hereby submits the attached South Texas Project Licensee Event Report (LER) 2020-001-00.
The event did not have an adverse effect on the health and safety of the public.
There are no commitments in this submittal.
If there are any questions, please contact Wendy Brost at 361-972-8516 or me at 361-972-7888.
lichael A. Schaefer Site Vice President Attachment: LER 2020-001-00, Automatic Actuation of Emergency Diesel Generators due to Lookout of Switchyard Electrical Bus ec:
Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511 STI:35022728
NOC-AE-20003737 Attachment Attachment LER 2020-001-00 Automatic Actuation of Emergency Diesel Generators due to Lockout of Switchyard Electrical Bus
NRC FORM 366 (04-2020)
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION (04-2020)
LICENSEE EVENT REPORT (LER)
(See Page 2 for required number of digits/characters for each block)
(See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2020
- 1. Facility Name South Texas Unit 1
- 2. Docket Number 05000498
- 3. Page 1
OF 5
- 4. Title Automatic Actuation of Emergency Diesel Generators due to Lockout of Switchyard Electrical Bus
- 5. Event Date
- 6. LER Number
- 7. Report Date
- 8. Other Facilities Involved Month Day Year Year Sequential Number Rev No.
Month Day Year Facility Name South Texas Unit 2 Docket Number 05000499 03 24 2020 2020 001
- - 00 05 22 2020 Facility Name N/A Docket Number 05000
- 9. Operating Mode
- 11. This Report is Submitted Pursuant to the Requirements of 10 CFR §: (Check all that apply)
Defueled 20.2201(b) 20.2203(a)(3)(i) 50.73(a)(2)(ii)(A) 50.73(a)(2)(viii)(A) 20.2201(d) 20.2203(a)(3)(ii) 50.73(a)(2)(ii)(B) 50.73(a)(2)(viii)(B) 20.2203(a)(1) 20.2203(a)(4) 50.73(a)(2)(iii) 50.73(a)(2)(ix)(A) 20.2203(a)(2)(i) 50.36(c)(1)(i)(A) 50.73(a)(2)(iv)(A) 50.73(a)(2)(x)
- 10. Power Level 20.2203(a)(2)(ii) 50.36(c)(1)(ii)(A) 50.73(a)(2)(v)(A) 73.71(a)(4) 000 20.2203(a)(2)(iii) 50.36(c)(2) 50.73(a)(2)(v)(B) 73.71(a)(5) 20.2203(a)(2)(iv) 50.46(a)(3)(ii) 50.73(a)(2)(v)(C) 73.77(a)(1) 20.2203(a)(2)(v) 50.73(a)(2)(i)(A) 50.73(a)(2)(v)(D) 73.77(a)(2)(ii) 20.2203(a)(2)(vi) 50.73(a)(2)(i)(B) 50.73(a)(2)(vii) 73.77(a)(2)(iii) 50.73(a)(2)(i)(C)
Other (Specify in Abstract below or in summary of the event On March 24, 2020 at 1046 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.98003e-4 months <br />, the 345 kV south bus was unexpectedly de-energized when a switchyard breaker failure trip signal was initiated in the south bus differential relay. The failure trip signal was initiated when a test signal was injected into an incorrect non-isolated transmission line relay instead of the intended electrically-isolated transmission line relay during a maintenance activity that is routinely performed during refueling outages. The maintenance activities in the switchyard were being performed by the Transmission and Distribution Service Provider (TDSP) personnel.
The following is a summary timeline of relevant events (note: all times are Central Daylight Time):
March 13, 2020 [2300]:
Unit 1 Main Output Breaker opened and refueling outage 1RE22 began March 22, 2020 [2315]:
Unit 1 defueled and core alterations suspended March 23, 2020 [0717]:
TDSP personnel (TDSP) began performing work in the switchyard
(Continued)
March 24, 2020 [1046]:
345 kV south bus unexpectedly de-energized Unit 1 experienced a loss of power to Engineered Safety Features (ESF) busses E1A and E1C. Emergency Diesel Generators (EDGs) 11 and 13 automatically started and restored power to the de-energized ESF busses. Spent Fuel Pool (SFP) Cooling Pump 1B lost power, but SFP Cooling Pump 1A remained running. A-and C-Train safety equipment started automatically including the Reactor Containment Fan Coolers (RCFCs).
Unit 2 experienced a loss of power to ESF bus E2B. EDG 22 automatically started and restored power to lost bus. SFP Cooling Pump 2A lost power and forced circulation was lost in the Unit 2 SFP. B-Train safety equipment started automatically including Auxiliary Feedwater Pump (AFW) Pump 22 and RCFCs.
March 24, 2020 [1054]:
Secured SFP Cooling Pump 1A in preparation for restarting both SFP Cooling Pumps 1A and 1B March 24, 2020 [1056]:
Started SFP Cooling Pumps 1A and 1B March 24, 2020 [1208]:
Paralleled EDG 11 with offsite power and re-energized ESF Bus E1A from offsite power March 24, 2020 [1215]:
Paralleled EDG 22 with offsite power and re-energized ESF Bus E2B from offsite power March 24, 2020 [1229]:
Secured AFW Pump 22 March 24, 2020 [1231]:
Started SFP Cooling Pump 2A, forced circulation restored in Unit 2 SFP March 24, 2020 [1257]:
Paralleled EDG 13 with offsite power and re-energized ESF Bus E1C from offsite power March 24, 2020 [1615]:
NRC notification due to automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B)
March 24, 2020 [2142]:
Required offsite circuits declared operable following restoration of switchyard south bus and Standby Transformer 2 F.
Method of discovery
This event was self-revealed when the switchyard south bus de-energized, resulting in a loss of power to Standby Transformer 2 which was supplying power to the ESF busses for the Unit 1 A-and C-Trains and the Unit 2 B-Train.
EDGs 11, 13, and 23 automatically started in response to the undervoltage condition.
II. Component failures
A.
Failure Mode, mechanism, and effects of failed component
The failed component in this event was the 345 kV Switchyard South Breaker Y550. A Y550 breaker failure trip signal was initiated to the south bus differential relay when a test signal was injected into an incorrect non-isolated transmission line relay (Y540/Y550) instead of the intended electrically-isolated transmission line relay (Y500/Y510) during a planned maintenance activity.
(Continued)
B.
Cause of component failure
The cause of this event was a human performance error by the TDSP personnel conducting relay testing in the switchyard. Specifically, a test signal was mistakenly injected into an energized transmission line relay instead of into the intended de-energized relay.
C.
Systems or secondary functions that were affected by failure of components with multiple functions During this partial Loss of Offsite Power event, Standby Transformer 2 was supplying power to Trains A and C on Unit 1 and Train B on Unit 2. All three of these trains lost normal power when the south bus was inadvertently isolated. No other electrical busses in the switchyard were affected by this event - the North Bus remained energized.
In response to the undervoltage condition, EDGs 11, 12, and 22 automatically started and provided power to their respective busses as designed.
The loss of busses E1A and E1C in Unit 1 resulted in a loss of power to SFP Cooling Pump 1B. All fuel had been offloaded to the Unit 1 SFP and the Residual Heat Removal (RHR) system was not in service. The loss of bus E2B in Unit 2 resulted in a loss of power to SFP Cooling Pump 2A and a loss of forced circulation to the Unit 2 SFP.
As a result of the transient, Integrated Computer System (ICS) displays, RM-11 radiation monitoring, and RT-8010B indications were lost in the Unit 1 Control Room. Compensatory measures for these indications were available and in place during the time-period this equipment was non-functional.
D.
Failed component information
Switchyard System { FK }
Breaker { BKR }
Manufacturer: Mitsubishi { Mitsubishi }
Model: {300SFMT63B}
III. Analysis of the event
A.
Safety system responses that occurred EDGs 11, 13, and 22 automatically started in response to the undervoltage condition. Subsequently, the Unit 1 A-and C-Train RCFCs and Unit 2 B-Train RCFCs and AFW Pump actuated automatically.
B.
Duration of safety system inoperability
When the south bus was de-energized, Unit 2 entered TS Action 3.8.1.1.e on March 24, 2020 at 1046 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.98003e-4 months <br /> due to two independent offsite circuits being inoperable - the switchyard south bus and ESF bus E2B were not connected to their offsite source. Upon re-energization of ESF Bus E2B approximately 89 minutes later at 1215 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.623075e-4 months <br />, Unit 2 exited TS Action 3.8.1.1.e and entered TS Action 3.8.1.1.a due to one independent offsite circuit being inoperable. Unit 2 exited TS Action 3.8.1.1.a approximately 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> and 27 minutes later at 2142 hours0.0248 days <br />0.595 hours <br />0.00354 weeks <br />8.15031e-4 months <br /> when the offsite circuits were declared operable following the restoration of the switchyard south bus and Standby Transformer 2.
TS 3.8.1.1 was not applicable to Unit 1 at the time of the event because Unit 1 was in the defueled Mode.
C.
Safety consequences and implications
This event resulted in the actuation of the emergency AC electrical power system. The initiating event was caused by human error and not by external events. This event did not result in any offsite release of radioactivity or increase of offsite dose rates, and there were no personnel injuries or damage to any other safety-related equipment associated with this event. Therefore, there was no adverse effect on the health and safety of the public.
(Continued)
IV. Cause of the event
The root cause for this event was determined to be:
TDSP personnel had less than adequate work practices during the performance of transmission line relay testing for Jones Creek Circuit 18 (Y500/Y510).
The contributing cause for this event was determined to be:
STP had less than adequate oversight of switchyard field activities when work was in progress.
V. Corrective actions
Planned corrective actions are as follows:
1.
Assign a dedicated STP Switchyard Coordinator with primary duties for oversight and observation of switchyard work activities.
2.
Revise site procedures to incorporate current industry best practices and to expand the scope of switchyard work activities that require oversight by the STP Switchyard Coordinator.
VI. Previous similar events
A review of internal operating experience identified a similar event in 2010 involving a TDSP human performance error and insufficient STP oversight that resulted in the loss of the north switchyard electrical bus. STP submitted Licensee Event Report 2010-004-00 in response to this event.
STP reviewed the corrective actions associated with the 2010 event and determined that STP needs to provide increased direct oversight and ownership of work performed in the switchyard. In addition, an effectiveness review will be performed on the corrective actions to be implemented for the 2020 event.