05000395/LER-2017-003

LER-2017-003, FAILED LIGHTNING ARRESTER ON MAIN TRANSFORMER CAUSES REACTOR TRIP

Vc Summer - Unit 1
Event date:	08-28-2017
Report date:	12-12-2017
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3952017003R01 - NRC Website

LER 17-003-00 For Virgil C. Summer Nuclear Station, Unit 1, Regarding Failed Lightning Arrester On Main Transformer Causes Reactor Trip

ML17299A951
Person / Time
Site:	Summer, Vc Summer - Unit 1
Issue date:	10/26/2017
From:	Lippard G A SCANA Corp, South Carolina Electric & Gas Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 17-003-00
Download: ML17299A951 (4)

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

05000- VC SUMMER - UNIT 1 395 1.0 EVENT DESCRIPTION On August 28, 2017, at 0837, VCSNS Unit 1 automatically tripped due to a turbine trip. The turbine trip was caused by the Main Generator Differential Lockout due to a fault on the center phase lightning arrester on XTF-1.

Plant trip responses were normal. All control rods fully inserted, BOP buses automatically transferred to their alternate power source XTF-31/32, and all EF pumps started as required.

2.0 EVENT ANALYSIS In the normal alignment, power produced at VCSNS is transmitted offsite through XTF-1 via the 230 kV system and also feeds normal onsite loads through the Auxiliary Transformer (XTF-2). An alternate source of power for the service power system is available immediately, upon loss of XTF-2, via the 230 kV system and XTF-31 and XTF-32.

The engineered safety features power system originates offsite from three independent sources: two credited, 230kV source via XTF-31 and 115kV source via the Engineered Safety Feature Transformers (XTF-4 or XTF-5), and one alternate 13.8kV source via Engineered Safety Feature Transformer (XTF-5052). In the event of a loss of all offsite power, emergency power for the engineered safety features is available from two onsite diesel generators. The AC electrical power sources provide sufficient capacity, capability, redundancy, and reliability to ensure the availability of necessary power to engineered safety systems so that the fuel cladding, reactor coolant system (RCS), and containment design limits are not exceeded. This event had no impact on the Class lE electric system.

Each phase of XTF-1 has an associated lightning arrester (LA) connected to the 230 kV line between the transformer high voltage bushing and the first transmission line tower out from the transformer yard. The LAs are designed to protect the transformer and its components against impulse and lightning surges on the transmission line.

In this event, examination results indicate that the center phase LA failed as a result of moisture ingress past the end seal of its upper housing. With moisture being present, the upper housing block structure flashed over immediately placing the lower housing in an overvoltage condition, which resulted in a flashover of its metal oxide varistors. This LA failure grounded the center phase of XTF-1, which caused the XTF-1 Overall Differential Backup Lockout Relays to actuate and initiated the trip of the Main Generator Field Supply Breaker, resulting in a turbine trip and ultimately an automatic reactor trip.

3.0 SAFETY SIGNIFICANCE A PRA sensitivity study was performed as a result of this reactor trip. PRA model 8b was used with baseline testing and maintenance values. This event was modeled with "reactor trip" set to 1/yr and the results were compared with the baseline values.

The resulting change in Core Damage Frequency (CDF) is 1.08E-07/yr and the resulting change in Large Early Release Frequency (LERF) is 5.47E-09/yr. The baseline CDF and LERF for the model (version 8b) used for this study are 3.3208E-06/yr and 1.0439E-07/yr, respectively. These changes constitute a 3.24% increase in CDF and a 5.24% increase in LERF.

The changes in CDF and LERF described herein are not considered significant.

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05000- VC SUMMER - UNIT 1 395 4.0 PREVIOUS OCCURRENCE No previous occurrence within the last three years.

5.0 CORRECTIVE ACTIONS All three LAs on XTF-1 were replaced, under Work Order 1713128, in an effort to avoid the possibility of a common cause or mode failure occurring on either of the two remaining LAs.

During repair efforts, collateral damage to the Center Phase High Voltage Bushing was identified. To ensure proper operation of XTF-1, the Center Phase High Voltage Bushing was also replaced under Work Order 1713128.

The failed LA, along with the other two LAs that were in service on XTF-1 during the reactor trip, were sent to an independent lab, NEETRAC — Georgia Tech, for testing and evaluation to determine the cause of the failure. The results of this examination were discussed in section 2.0 Event Analysis of this report.