05000499/LER-2023-002, Automatic Actuation of Emergency Diesel Generator Due to Lockout of Switchyard Electrical Bus
| ML24030A968 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 01/30/2024 |
| From: | Tomlinson J South Texas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NOC-AE-24004009 LER 2023-002-00 | |
| Download: ML24030A968 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(iv)(B)(8) |
| 4992023002R00 - NRC Website | |
text
Nuclecr Cornpcny U:r South Tens Pmiect Electric Generating Station EO. &ox 289 Wadsworth, Texas 77483 January 30,2024 NOC-AE-24004009 10 cFR 50.73 STI: 35552671 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Unit 2 Docket No. STN 50499 Licensee Event Report 2023-002-00 Automatic Actuaiion of Emerqencv Diesel Generator due to Lockout of Switchvard Electrical Bus Pursuant to reporting requirements of 10 CFR 50.73(a)(2)(iv)(A), STP Nuclear Operating Company (STPNOC) hereby submits the attached South Texas Project Licensee Event Report (LER) 2023-002-00 for valid actuation of an emergency AC electdcal power system included under 10 CFR 50.73(a)(2)(ivXBXS).
The event did not have an adverse effect on the health and safety of the public.
There are no commitments in this letter lf there are any questions, please contact Stephanie Rodgers at361-972-4527 or me at361-972-8945.
Jason Tomlinson Site Vice President LER 2023-002-00, Automatic Actuation of Emergency Diesel Generator due to Lockout of Switchyard Electrical Bus 7
ST cc:
Attachment:
Regional Administrator, Region lV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 7601 1451 I
NOC-AE-24004009 Attachment Attachment LER 2023-002-00 Automatic Actuation of Emergency Diesel Generator due to Lockout of Switchyard Electrical Bus
Abstract
On December 2, 2023, with both units operating at 100% power, the South Texas Project (STP) 345 kV south switchyard electrical bus unexpectedly de-energized. The de-energization of the south bus resulted in a loss of power to the Unit 2 standby transformer which was supplying power to the Engineered Safety Features (ESF) 4160V busses for the Unit 2 B Train. The associated Emergency Diesel Generator 22 automatically started in response to the under-voltage condition, as designed.
The cause of this event was the failure of a stand-off insulator occurring on Phase A in 13.8kV Terminal Cabinet. A root cause could not be determined after Engineering evaluations. Corrective actions included replacing the insulator and performing visual inspections of both the transformer and bus for signs of arcing and/or water intrusion. There were no previous indications of issues with the insulator.
This event is reportable as an event which resulted in the automatic actuation of the Unit 2 emergency AC electrical power systems. Additionally, all ESF equipment operated as designed.
I. Description of Reportable Event
A. Reportable event classification
This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in the valid actuation of an emergency AC electrical power system (50.73(a)(2)(iv)(B)(8)).
B. Plant operating conditions before the event
Prior to the event on December 2, 2023, STP Unit 2 was at 100% power in Mode 1.
C. Status of structures, systems, and components that were inoperable at the start of the event and that contributed to the event No other structures, systems or components were inoperable at the start of this event that contributed to this event.
D. Background information
The Offsite Electrical System consists of two standby transformers, the 138kV emergency transformer, two main power transformers, 345kV lines connecting the main power transformers and the standby transformers to the switch yard, the 345kV switch yard, nine 345kV transmission circuits from the South Texas Project Electric Generating Station (STPEGS) 345kV switchyard to the interconnecting grids, and the 138kV radial line connected to the 138kV emergency transformer. The transmission system provides reliable sources of offsite power for supplying plant auxiliary power systems for plant startup, shutdown or at any time that power is unavailable from the unit's main generator.
The standby transformers are individually supplied by separate and independent overhead 345kV ties from the 345kV switchyard. These 345kV ties to the standby transformers are connected by two separate 345kV busses (north and south) in the switchyard. Each standby transformer has the capacity to supply all Engineered Safety Features (ESF) busses in both Units and two 13.8kV auxiliary busses. These transformers can be shared between Units 1 and 2 and can supply the two preferred power sources (the north and south 345kV busses).
Per STP Technical Specification (TS) 3.8.1.1.e, with two of the above required offsite AC circuits inoperable, within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> restore at least one of the inoperable offsite sources to OPERABLE status or apply the requirements of the Configuration Risk Management Program (CRMP) or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Unit 2 entered Technical Specification Actions 3.8.1.1.e. in response to this event.
E. Narrative summary of the event
On 12/2/2023 at 06:10 Unit 2 Control Room received annunciators 10M2-B-3 "345kV South Bus Diff" and 10M2-A-4 "345kV BKR Y610 Trip". The Control Room observed indication that Switchyard Breaker Y610 had tripped open, and the 345kV Switchyard South Bus was de-energized. The following Standby Transformer 2 relays tripped: 51N-ST2Y, both 85N relays, 86-ST2B, and 86-ST2A. Standby Transformer 2 experienced a lockout resulting in Emergency Diesel Generator 22 (EDG22) starting to supply power to the standby bus. This resulted in loss of two offsite sources due to one ESF bus not being connected to a load tap changer in automatic and standby bus not being available. On 12/2/23 at 08:30 the 4160V Bus E2B was powered from U1 standby transformer. On 12/2/23 at 12:35 the South Bus was re-energized after CenterPoint (Transmission Distribution Service Provider) opened disconnect S024 to remove Standby Transformer 2 from the South Bus.
Note: Dates and approximate times of occurrences are listed in Central Daylight Time.
December 2, 2023 [06:10]
Per alarm page, at time 06:10:58, received U2 standby transformer 2A lockout. At time 06:11:02 received 345kV S bus voltage low. Bus 4160v E2B is being supplied by ESF DG 22.
Declared loss of two offsite sources due to one ESF bus not connected to a load tap changer in auto and U2 standby transformer/south bus not available.
Entered TS 3.8.1.1.e. with two of the required offsite A.C. circuits inoperable, within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> restore at least one of the inoperable offsite sources to operable status or apply the requirements of the CRMP or be in at least hot standby within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
December 2, 2023 [06:11]
Received annunciators 10M2-B-3 "345kV South Bus Diff" and 10M2-A-4 "345kV BKR Y610 Trip". Observed Control Room indication that Switchyard Breaker Y610 has tripped open and Switchyard South Bus is de-energized.
Standby Diesel Generator 22 started in response to "B" Train loss of power and is running SAT. Contacted CenterPoint energy. Need to investigate and determine the cause of Y610 trip.
December 2, 2023 [08:30]
4160V Bus E2B is being powered from U1 Standby Transformer. Exited TS 3.8.1.1. Action E.
Entered TS 3.8.1.1.A for one offsite source inoperable, perform 0PSP03EA0002, ESF Power Availability, within one hour and at least once per eight hours thereafter. Within 72hrs (from 06:10), restore the offsite circuit to operable status or apply CRMP, or be in at least hot standby within the next 12hrs and in cold standby within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
December 2, 2023 [12:35]
Re-energized the South Bus after CenterPoint opened disconnect S024 to remove Standby Transformer 2 from the South Bus.
EXITED TS 3.8.1.1.A.
December 2, 2023 [12:47]
Reported to NRC HOO (EN 56877) under 10 CFR 50.72(b)(3)(iv)(A) as an event that resulted in the valid actuation of an emergency AC electrical power system (50.72(b)(3)(iv)(B)(8)).
Notification Time: 12/02/23 12:47 Time of Discovery: 12/02/23 06:10 Documents reviewed: Reporting Manual, Technical Specifications Firm evidence: Loss of electrical power
F. Method of discovery of each component or system failure or procedural error
This event was self-revealed when a bus lockout in the switchyard caused the loss of a credited offsite electrical power source. This resulted in the automatic actuation of on-site emergency power sources (EDG).
II. Component failures
A. Failure Mode, mechanism, and effects of failed component
The failure mode was a phase-to-ground short. The protective relay detected the fault and cycled the breaker which momentarily removed all voltage from the bus to isolate the south bus from the fault.
B. Cause of each component or system failure or personnel error
Failure of a stand-off insulator occurred on Phase A in 13.8kV Terminal Cabinet. A specific cause could not be determined.
C. Systems or secondary functions that were affected by the failure of components with multiple functions
The de-energization of the south bus disconnected Unit 2 standby transformer from the grid. Since this transformer is the normal supply to Train B ESF bus, the resulting undervoltage condition initiated a partial loss of offsite power (LOSP), which sent a valid actuation signal to the Unit 2 Train B Standby Diesel Generator 22.
D. Failed component information
System: { XFMR }
Component: { INS }
Manufacturer: Crown Electric { Crown Electric }
Model: {CE 10-0002}
III. Analysis of the event
A. Safety system responses that occurred The loss of the Unit 2 standby transformer caused an undervoltage condition on the Unit 2 Train B 4.16kV bus, the automatic start of the Unit 2 Train B Standby Emergency Diesel Generator 22 and the automatic sequenced loading of ESF equipment.
B. Duration of safety system inoperability
When the south bus was de-energized, Unit 2 entered TS Action 3.8.1.1.e on December 2, 2023, at 0610 hours0.00706 days <br />0.169 hours <br />0.00101 weeks <br />2.32105e-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 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 20 minutes later at 0830 hours0.00961 days <br />0.231 hours <br />0.00137 weeks <br />3.15815e-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 at approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 5 minutes later at 1235 hours0.0143 days <br />0.343 hours <br />0.00204 weeks <br />4.699175e-4 months <br /> when the offsite circuits were declared operable following the restoration of the switchyard south bus and Standby Transformer 2. Therefore, the total safety system inoperability duration was 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and 25 minutes.
C. Safety consequences and implications
A quantitative analysis was used for Unit 2, which remained at full power for the duration of the event. The loss of the south bus did cause a partial loss of offsite power leading to an ESF actuation in Unit 2. All mitigating systems remained functional and there were no mitigating system failures as a result of the event. No fission product barriers were challenged in Unit 2. The loss of the South Bus has no increase in Core Damage Frequency (CDF) and Large Early Release Frequency (LERF) in Unit 2.
This event did not result in any offsite release of radioactivity or increase of offsite does 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.
IV. Cause of the event
The cause of the event was the failure of a stand-off insulator occurring on Phase A in 13.8kV Terminal Cabinet, which resulted in a phase-to-ground fault and subsequent bus lock out. There were no previous indications of issues with the insulator. Based on vendor recommendation, Electric Power Research Institute, Inc. (EPRI) guidelines (TR-112784), Industry practices, and Institute of Electrical and Electronics Engineers (IEEE) standards, these standoff insulators do not have a proposed replacement schedule and are expected to not fail. Visual inspections and meggering the bus are the industry best preventative maintenance that ensure the insulators remain reliable. STP's Preventative Maintenance (PM) program currently aligns with these recommendations.
Furthermore, EPRI guidance preventative maintenance is to perform visual inspections for any water intrusion signs or damage to the bus, perform thermography bi-annually, and performing a partial discharge test following maintenance. A review of the Unit 2 Standby transformer was performed for the prior 10 years and as found condition codes and inspection values were all within tolerance with no adverse trend. No changes to STPs PM program are required and the consequence is aligned with the maintenance strategy.
V. Corrective actions
- 1. Replaced faulty insulator.
- 2. Performed electrical testing of the non-segregated phase bus.
- 3. Performed Doble Power factor testing of the transformer.
- 4. Performed visual inspections of both the transformer and bus for signs of arcing and/or water intrusion.
VI. Previous similar events
A review identified a similar event in 2022 involving a failed insulator owned by the Transmission Distribution Service Provider, which resulted in a loss of the south switchyard electrical bus. STP submitted Licensee Event Report 2022-001-00 in response to this event. Page 5 of 5 002 00