05000250/LER-2023-003, Grid Disturbance in Switchyard Causes Automatic Reactor Trip
| ML23324A275 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 11/20/2023 |
| From: | Strand D Florida Power & Light Co |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-2023-166 LER 2023-03-00 | |
| Download: ML23324A275 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2502023003R00 - NRC Website | |
text
FloridaPower&LightCompany
9760SW344thStreet,Homestead,FL33035
November 20, 2023 L-2023-166 10 CFR 50.73 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 Re:
Turkey Point Unit 3 Docket No. 50-250 Reportable Event: 2023-03-00 Date of Event: September 22, 2023 Grid Disturbance in Switchyard Causes Automatic Reactor Trip The attached Licensee Event Report 2023-03 is being submitted pursuant to the requirements of 10 CFR 50.73 to provide notification of the subject event.
Respectfully, Dianne Strand General Manager, Regulatory Affairs Attachment cc:
Turkey Point NRC Senior Resident Inspector Turkey Point Station NRC Program Manage
Abstract
On September 22, 2023, with Unit 3 at 100% power, a grid disturbance caused by a lightning strike in the switchyard resulted in a generator lock out and subsequent turbine trip. The Reactor Protection System (RPS) automatically actuated as a result of the turbine trip. The automatic trip was uncomplicated with systems responding normally post-trip, including an expected Auxiliary Feedwater (AFW) actuation. Subsequent analysis identified that due to the magnitude of the lightning strike, an induced voltage was created through the switchyard ground grid that arced across the relay contact that drives the string bus differential relay, resulting in a generator lockout. Corrective actions include replacement of the affected relay as well as a review of the ground grid and lightning protection scheme. This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) for any event or condition that resulted in the manual or automatic actuation of the RPS and AFW.
Description of Event
On September 22, 2023, at 1819 EDT, with Unit 3 at 100% power, a grid disturbance in the switchyard [EIIS: FK] caused a contact [EIIS: 93] to be bypassed in the string bus differential relay [EIIS: 87]. This actuated a generator [EIIS: GEN]
lockout, which then tripped the turbine {EIIS: TRB]. Upon the turbine trip, the RPS [EIIS: JE] automatically actuated. The trip was uncomplicated with systems responding normally, including the actuation of the AFW system [EIIS: BA] to remove decay heat. This is an expected condition after reactor trips. This licensee event report is being reported pursuant of 10 CFR 50.73(a)(2)(iv)(A) for any event or condition that resulted in the manual or automatic actuation of the RPS and AFW. In addition, there were no inoperable Structures, Systems, or Components that contributed to this event.
Cause of Event
The magnitude of the lightning strike caused a larger than normal voltage difference between the switchyard and plant ground grids. Because the string bus differential relay is tied to the switchyard ground system and the generator lockout is tied to the plant ground system, an induced voltage was created that arced across the differential relay contact. This initiated the generator lockout and ultimately the actuation of RPS. The was relay was manufactured by Westinghouse.
Safety Significance
The plant trip was uncomplicated with systems responding as expected post-trip. In response to the expected low steam generator level, emergency feedwater actuated as expected. This event did not result in a Safety System Functional Failure.
Corrective Actions
- 1. The string bus differential relay has been replaced.
- 2. Ground grid and lightning protection scheme will be reviewed as well as additional planned actions in accordance with the corrective action program.
Similar Events
A review of events over the past 5 years did not identify any previous events that involved the same underlying cause of this event.