05000251/LER-2020-001, Automatic Reactor Trip Caused by Main Generator Lockout Due to Exciter Fault
| ML20242A001 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 08/28/2020 |
| From: | Stamp B Florida Power & Light Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-2020-132 LER 2020-001-00 | |
| Download: ML20242A001 (3) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2512020001R00 - NRC Website | |
text
Florida Power & Light Company 9760 SW 344th St., Homestead, FL 33035 10 CFR 50.73 L-2020-132 August 28, 2020 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555-0001 RE: Turkey Point Unit 4 Docket No. 50-251 Reportable Event: 2020-001-00 Date of Event: July 5, 2020 Title: Automatic Reactor Trip caused by Main Generator Lockout due to Exciter Fault The attached Licensee Event Report 05000251/2020-001-00 is submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A), due to automatic actuation of the Unit 4 Reactor Protection System and Auxiliary Feedwater System.
If there are any questions, please call Mr. Robert Hess at 305-246-4112 or e-mail Robert.Hess@fpl.com.
Sincerely, Brian Stamp Site Vice President - Turkey Point Nuclear Plant Florida Power & Light Company Attachments: USNRC Forms 366 and 366A, current revision cc: USNRC Senior Resident Inspector, Turkey Point Plant USNRC Regional Administrator, Region II
Abstract
Turkey Point Unit 4 251 2
Automatic Reactor Trip caused by Main Generator Lockout due to Exciter Fault 07 05 2020 2020 001 00 08 28 2020 1
100%
David Stoia - Licensing Engineer (305) 246-6538 B
EL EXC W120 Yes
On 7/5/2020 at 21:09 hours, while Unit 4 was operating in Mode 1 at 100% power, a Main Generator Lockout caused the Unit 4 Turbine to trip, automatically tripping the Reactor. The Main Generator Lockout was caused by a loss of two Voltage Regulator power supplies. All safety systems operated as required in response to the equipment failure. The Auxiliary Feedwater System automatically initiated as designed. This event is reportable pursuant to 10 CFR 50.73 (a)(2)(iv)(A), due to automatic actuation of the Reactor Protection System and Auxiliary Feedwater System.
Corrective actions included replacement of the Exciter PMG and rotating assembly and re-sealing the Exciter housing.Page of 05000-
- 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
EVENT DESCRIPTION
On 7/5/2020 at 21:09 hours, during a heavy rainstorm while Unit 4 was operating in Mode 1 at 100% power, a lockout of the Main Generator [TB, GEN] occurred, causing a trip of the Unit 4 Turbine [TA, TRB]. This initiated a Reactor Protection System (RPS) [JC] actuation signal that caused the Reactor [AC, RCT] to automatically trip. The Main Generator lockout was caused by the loss of two power supplies to the Voltage Regulator. All safety systems functioned as designed during the event. The Auxiliary Feedwater (AFW) System [BA] automatically started to maintain Steam Generator [SG] levels within normal bands. Unit 4 was stabilized in Mode 3 following the reactor trip.
The RPS actuation was reported in accordance with 10 CFR 50.72 in Event Notification 54762. Automatic AFW actuation was inadvertently omitted from the initial event notification. The automatic RPS and AFW actuations are included in this Licensee Event Report pursuant to 10 CFR 50.73(a)(2)(iv)(A).
CAUSE
The event was initiated by an electrical fault of the Exciter Permanent Magnet Generator (PMG) [PMG], which caused both Voltage Regulator power supplies to fail. The resulting actuation of the Voltage Regulator lockout relay caused a lockout of the Unit 4 Main Generator, tripping the Turbine as designed. As a design input to RPS actuation logic, the turbine trip caused an automatic Reactor trip. Investigation found that the PMG fault was caused by a combination of the aged condition of the stator windings and exposure to moisture that was present due to a degraded weather seal on the exterior housing.
SAFETY SIGNIFICANCE
The safety significance of this event is very low because the unit responded as designed to the trip. There were no failures of safety-related equipment.
CORRECTIVE ACTIONS
The Exciter PMG and rotating assembly were replaced. The Exciter housing was re-sealed following reassembly.
Additional actions will be prescribed by AR 2361794.
ADDITIONAL INFORMATION
EIIS Codes are shown in the format [IEEE system identifier, component function identifier, second component function identifier (if appropriate)].
SIMILAR EVENTS
A review of reportable events over the previous 15 years was performed. No similar events were identified, and no pattern of such events is evident.
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Turkey Point Unit 4 251 2020 001 00