05000440/LER-2025-001, Automatic Actuation of Division 3 Diesel Generator Resulting from Startup Transformer Testing
| ML25141A212 | |
| Person / Time | |
|---|---|
| Site: | Perry  |
| Issue date: | 05/21/2025 |
| From: | Penfield R Vistra Operations Company |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-25-087 LER 2025-001-00 | |
| Download: ML25141A212 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(8) 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 4402025001R00 - NRC Website | |
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May 21, 2025 L-25-087 Attention: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Perry Nuclear Power Plant Rod Penfield Site Vice President 10 Center Road Perry, Ohio 44081 440-280-5382 724-462-0816 (cell) 10 CFR 50.73(a)(2)(iv)(A)
Subject:
Perry Nuclear Power Plant, Docket No. 50-440, License No. NPF-58:
Licensee Event Report Submittal Enclosed is Licensee Event Report (LER) 2025-001, "Automatic Actuation of Division 3 Diesel Generator Resulting from Startup Transformer Testing." No regulatory commitments are contained in this letter.
If there are any questions, or if additional information is required, please contact Mr. Tony Kledzik, Manager-Regulatory Compliance, at (440) 280-6188.
Enclosure:
LER 2025-001 Cc:
NRC Project Manager NRC Resident Inspector NRC Region III Regional Administrator 6555 SIERRA DRIVE IRVI G. TEXAS 75039 o 214-812-4600 VISTRACORPCOM
Enclosure L-25-087 Licensee Event Report 2025-001
Abstract
At 0915 on March 29, 2025, with Unit 1 in Mode 5 at 0% power, an automatic start of the Division 3 (high pressure core spray) emergency diesel generator (DG) occurred. This DG automatically started when it received a low voltage signal on a 4.16 kV safety bus for which this DG is designed as an emergency power source. During relay testing associated with a startup transformer, an interlock switch tripped the breaker that had been energizing the 13.8 kV bus, which in turn was powering the 4.16 kV bus.
The apparent cause was insufficient impact and work order reviews of transformer relay testing. The direct cause was engineering provided erroneous technical information regarding breaker control schematics. Contributing causes were omission of engineering reviews of a work order revision with the tests steps, and lack of stakeholder participation in meetings and communications.
Corrective actions include training on electrical interlocks and relay schemes, installing a caution placard on a breaker door, issuing of a night order to operators regarding lessons learned, and incorporating this operating experience into a work order for similar work in the next refueling outage.
This event was reported at 1616 on March 29, 2025, as a 10 CFR 50.72(b)(3)(iv)(A) event resulting in a valid actuation of the Emergency Diesel Generator System.
EIIS Energy Industry Identification System and Component Function codes are identified in the text as [XX].
BACKGROUND:
This event involved work upon the Unit 2 startup transformer [XFMR] and the loss of power to Perry 13.8 kV bus [BU] L20. Although construction of Perry Unit 2 was halted and not completed, some electrical equipment on Unit 2 is required to support operation of Unit 1.
Bus L20 can receive power from power sources [JX] characterized as preferred from Unit 2 startup transformer via the buss main supply breaker [BKR] L2003, or as alternate-preferred from Unit 1 startup transformer via breaker L2004.
Bus L20 is the normal power source to loads including Unit 2 non-safety air compressors [CMP] and the C non-safety Service Water system [KG] pump [P]. Bus L20 can also energize Perrys Class 1E Unit 1 4.16 kV bus EH13, as can Unit 1s Bus L10. Electric power systems and components essential for the plants safety are designated Class 1E. Engineered safety features loads are assigned to three independent load groups designated as Division 1, Division 2, and Division 3.
Power is normally supplied to each of the 4.16 kV Class 1E buses from an interbus transformer associated with that unit. The preferred power supply to a units interbus transformer is the same units startup transformer.
One unit interbus transformer secondary winding feeds the 4.16 kV Class 1E load of the associated unit.
The other secondary winding of the interbus transformer feeds the 4.16 kV Class 1E load of the other unit.
The Unit 2 interbus transformer is capable via cross-tie to supply power to the 4.16 kV Class 1E buses for Unit 1.
The Unit 1 emergency diesel generators [DG] can also supply power to the Class 1E 4.16 kV buses for Unit 1.
Perry Unit 1s 4.16 kV Division 1, 2, and 3 DGs provide the sites Emergency Onsite Power Supply [EK]. Division 3 DG is the onsite emergency power source to the Division 3 class 1E 4.16 kV bus EH13, which has as its main load the motor [MO] for the High Pressure Core Spray (HPCS) system [BG], to drive the HPCS pump. This pump provides and maintains an adequate coolant inventory inside the reactor vessel to limit fuel cladding temperatures in the event of breaks in the reactor coolant pressure boundary. Bus EH13 also supplies power to Division 3 battery chargers.
An undervoltage signal or a degraded voltage signal at an emergency DGs associated division bus will automatically start the DG.
DESCRIPTION OF EVENT
Plant conditions immediately prior to the event were as follows.
The plant was in Mode 5, with Division 1 and 2 DGs and Residual Heat Removal B and C Emergency Core Cooling Systems [BO] operable to satisfy Emergency Core Cooling Systems (ECCS) Technical Specification (TS) 3.5.2 requirements for Modes 4 and 5.
Division 3 DG status was inoperable and available. The HPCS system was inoperable and not needed to meet TS requirements.
Bus EH13 was energized via the alternate preferred source: from the Unit 1 startup transformer, through breaker L2004, through bus L20. The Unit 2 startup transformer was in a deenergized state to allow performance of switchyard differential relay [87] testing associated with newly installed current transformers [XCT] on the Unit 2 startup transformer. Breaker L2003 was in the test position and thus not transmitting electricity to bus L20.
At 0915, operators closed breaker L2003 in the test position to perform relay testing. An electrical system interlock [IEL] exists between L2004 and L2003 to ensure the Unit 1 and Unit 2 startup transformers do not feed bus L20 simultaneously. Breaker L2004 opened, as designed, due to interlock switch [33] action after L2003s closing in the test position. As bus L20 had been receiving power via breaker L2004, bus L20 subsequently lost power.
All loads energized from bus L20 then lost power. Alarms [ALM], [JA] notified the Control Room immediately of the loss of power to bus L20 and to bus L20s downstream buses, non-safety air compressors, and non-safety service water pump. The Control Room was also notified immediately of the automatic start of the Division 3 DG.
Bus EH13 had been receiving power from a circuit going back to bus L20: bus L20 supplied power to Unit 2 interbus transformer LH-2-A, and transformer LH-2-A (via bus TH21) supplied bus EH13. When bus L20 lost power, so did transformer LH-2-A, bus TH21, and bus EH13. The Division 3 DG automatically started upon receipt of the bus EH13 undervoltage signal.
This LER is being written due to Division 3 DGs actuation: 10 CFR 50.73(a)(2)(iv)(A) requires reporting of actuation of 50.73(a)(2)(iv)(B)(8), emergency ac electrical power systems including diesel generators.
Operators restored power to bus EH13 from the available preferred source (via Unit 1 bus L10 and the Unit 1 interbus transformer), shut down Division 3 DG, and restored power to bus L20 and its associated loads.
At 1616, Event Notification 57638, Automatic Actuation of Division 3 Diesel Generator, was completed.
CAUSES OF EVENT:
The apparent cause was insufficient impact review and work order review by operations of testing upon Unit 2 startup transformer relay scheme changes. These reviews did not identify that manipulation of an auxiliary transfer switch position from AUTO to OFF, before the closing of breaker L2003 in the test position, would prevent the trip of breaker L2004. The direct cause was that engineering, in addressing a verbal request lacking formality, provided erroneous technical information regarding the relay testing; a failure to thoroughly interpret breaker control schematics was not corrected when providing the response to the informal request. Contributing causes were omission of engineering reviews of a work order revision with the tests steps, and lack of stakeholder participation in meetings and communications.
CORRECTIVE ACTIONS
To address the apparent cause, conduct refresher training to operations on the pertinent electrical system operating procedure. This training will review the breaker interlocks, breaker racking positions, relay schemes, and lessons learned from this event.
As a bridging strategy before completion of the refresher training, a night order has been issued to all operators (licensed and nonlicensed) with the lessons learned from this event.
A CAUTION placard referring to the automatic closure feature has been installed on the L2004 breaker door, similar to the placard on Unit 1s counterpart L1004 breaker door.
To address the contributing causes, ensure all changes, lessons learned, and operating experience are incorporated into the Work Order for the next refueling outages similar Unit 1 transformer work to install slip-on current transformers and to perform remote test/trip with relays.
ANALYSIS OF EVENT
This event is reportable due to the actuation of the low voltage automatic start of Division 3 DG: 10 CFR 50.73(a)(2)(iv)(A) requires reporting of actuation of 50.73(a)(2)(iv)(B)(8), emergency ac electrical power systems including diesel generators. No other 50.73(a)(2)(iv)(B) systems were actuated, and there was no impact to the in-service offsite power or decay heat removal systems.
PREVIOUS SIMILAR EVENTS
No similar events have occurred at Perry within the last three years.
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