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On 7/24/17, with Oconee Nuclear Station (ONS) Unit 3 operating at 100 percent power, Transmission Department Relay personnel were in the ONS 525kV Switchyard Relay House performing preventive maintenance on a Breaker Failure Relaying device for Power Circuit Breaker PCB-57. This is a non- safety PCB that isolates a commercial transmission line from the commercial bus in the 525kV switchyard.

The maintenance was intended to actuate the protective relaying for PCB-57. The crew inadvertently connected test equipment to the adjacent relaying for PCB-58. The activation of the PCB-58 relay resulted in a Unit 3 separation from the electrical grid and a generator "Lockout." The lockout generates a turbine trip which in turn trips the reactor via the Reactor Protection System (RPS). This actuation of the RPS is reportable per 10 CFR 50.73(a)(2)(iv)(A).

Post trip plant response was normal and plant conditions were controlled and maintained within the allowances of Technical Specifications with no personnel injuries or safety system actuations.

A cause analysis attributed the cause of this event to human error in that test equipment was inadvertently connected to relaying for the incorrect PCB. The cause analysis corrective actions will address the likelihood of comparable human errors from occurring.

At 1512 on March 6, 2016, Oconee Nuclear Station (ONS) Unit 1 experienced a Reactor Trip, initiated by an electrical bushing failure on the Unit 1 Main Step-up Transformer that resulted in a transformer fire. The plant response to the trip was evaluated, and was found to be acceptable. The reactor trip is a RPS actuation and requires an LER per 10 CFR 50.73(a)(2)(iv)(A).

Due to the transformer fire a "Notification Of Unusual Event" was declared at 1520. The fire eventually led to a transformer overhead line failure that caused a switchyard bus lockout and the loss of one emergency power path for the site. The second emergency power path and multiple offsite sources remained operable during the event. The emergency classification was upgraded to an "Alert" condition at 1658, upon determining that the fire had an impact on safety related equipment.

The 4-hour notification related to this reactor trip was bounded by the notifications made for the Emergency Classifications described above (See Event Notification 51770).

Post trip conditions and Emergency event conditions were controlled and maintained within the allowances of Technical Specifications with no personnel injuries or challenges to other safety system actuations. The transformer deluge system activation along with the response of onsite and offsite fire teams promptly contained the fire.

For the 13.8kV Keowee Hydroelectric Station (KHS) power cables contained in the underground trench to both transformer CT-4 and to the Protected Service Water (PSW) system, single failure analysis considered a three-phase bolted fault at the end connections and a phase-to-ground fault as bounding conditions. This single failure analysis was questioned in a recent NRC inspection as not being bounding. Duke Energy evaluated the issue and concludes the cables to be operable but nonconforming with the current licensing basis due to a lack of documentation. Additional cable analysis and/or testing is needed. The results of planned power cable analysis and/or testing will provide the necessary information needed to update the licensing basis documentation. This condition is conservatively being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.

The preliminary apparent cause of this event is a missed opportunity to update the licensing basis in 2002 when the underground trench and cables were modified, and again in 2013, when the PSW system 13.8kV power cables and control cables were placed in service. Duke will complete the causal evaluation and as necessary, supplemented the report following the conclusion of that evaluation. Duke Energy used a risk-informed approach to determine the risk significance associated with the condition. The analysis results concluded that the postulated single failure of the Keowee emergency power supply represents an insignificant impact to plant risk.

On 2/6/2013, with all three Oconee Units at 100 percent power, Oconee Nuclear Station (ONS) personnel concluded that emergency power equipment could be adversely impacted by a licensee identified original design issue involving inadequate analysis of electrical equipment heat loads and weaknesses in the Heating Ventilation & Air Conditioning (HVAC) system design.

The investigation determined that the principle causes were associated with inadequate and incomplete inputs and methods in development of HVAC systems during original plant design and Blockhouse, and the 230kV Switchyard Relay House susceptible to single failures.

Compensatory measures were put into place. Corrective actions include modifications to resolve the inadequacies with the original plant design issues.

This event is reportable under the following criteria: Operations Prohibited by Technical Specifications, Unanalyzed Condition, Event that could have prevented Fulfillment of a Safety Function, and Common Cause.