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On December 2, 2011, with the plant at 100 percent power, Vermont Yankee (VY) was modifying the tagging lineup on the "B" Emergency Diesel Generator (EDG) that was out of service for scheduled maintenance.

During the tagging evolution, an operator mistakenly entered the "A" EDG room and tripped the "A" EDG fuel rack making the "A" EDG inoperable. This resulted in both EDGs being inoperable requiring entry into a 24 hour limiting condition for operation. This event is reported in accordance with 10CFR50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of a safety function since both EDGs were inoperable.

The investigation determined that this event was caused by a lack of adherence to procedures that provide administrative controls over tagging evolutions and direct the use of human performance tools to prevent occurrence of this type of an event. The condition was immediately identified by operations personnel due to alarms received in the main control room and the "A" EDG was returned to operable status in two minutes.

There were other sources of AC power available and therefore, this event did not pose a threat to public health and safety.

On May 26, 2010, at approximately 1526 hours, a generator lockout and automatic reactor trip occurred on differential current between a current transformer (CT) installed in the newly commissioned switchyard and a CT installed in the plant. During startup, when the plant reached 72% power, both channels of the reactor protection system (RPS) actuated and all control rods inserted. Following the reactor trip reactor vessel level decreased causing primary containment isolation system (PCIS) actuation for groups 2, 3, 4, and 5.. All associated valves functioned correctly. Additionally, both trains of standby gas treatment system actuated.

Immediate plant actions included entering appropriate trip response procedures. The operators stabilized the plant and reset both RPS and PCIS. During RFO 28, Vermont Yankee (VY) and the Vermont Electric Power Company (VELCO) commissioned a new 345kV switchyard. The direct cause of the trip was that VELCO changed the CT ratio settings within the 345kV Switchyard and failed to communicate the new ratio setting to VY. As power was increased, the differential current caused by the difference in CT ratio settings resulted in the generator lockout. Corrective actions include adjustment of the VY CT to the correct settings and establishment of the necessary programmatic controls to preclude recurrence. This event is reportable as a licensee event report (LER) per 10CFR50.73(a)(2)(iv)(A) as an event or condition that resulted in actuation of the any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B), which includes RPS and PCIS.