05000271/LER-2010-001
| Vermont Yankee Nuclear Power Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2712010001R00 - NRC Website | |
Description of Event
On May 26, 2010, at approximately 1526 hours0.0177 days <br />0.424 hours <br />0.00252 weeks <br />5.80643e-4 months <br />, 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 RFO 28, Vermont Yankee (VY) and the Vermont Electric Power Company (VELCO) commissioned a new 345kV switchyard [EIIS = FK].
Just prior to the trip VY was raising power following the 2010 refueling outage (RFO 28) from 72% at 1% every three minutes using the recirculation flow system [EIIS = SK]. At the time of the trip, two of three reactor feed pumps [EIIS = SK] were inservice and there were no significant equipment challenges for the control room operators. Both channels of the reactor protection system (RPS) [EIIS = JD] actuated and all control rods inserted. Following the reactor trip reactor vessel level decreased as expected causing an expected primary containment isolation system (PCIS) [EIIS = BD] groups 2, 3, 4, and 5 actuation signals. 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.
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. On May 26, 2010, at approximately 1725 hours0.02 days <br />0.479 hours <br />0.00285 weeks <br />6.563625e-4 months <br /> VY completed the required NRC notification in accordance with 10CFR50.72(b)(2)(iv)(B) - RPS Actuation — Critical, and 10CFR50.72(b)(3)(iv)(A) - Valid Specific System Actuation.
Cause of Event
The direct cause of the trip was VELCO's changing of CT ratio settings within the interface between the 345kV Switchyard and not communicating the new ratio settings to VY and not receiving concurrence for the new settings from VY. This change in settings caused a differential current reading between the CT in the new switchyard and the existing CT in the plant to cause a generator lockout on differential current which caused the reactor trip.
Analysis of Event
Although the incident resulted in a trip, operators responded in accordance with established procedures and all plant systems responded as designed. This event did not impact or reduce the margin of nuclear safety and did not result in an industrial safety concern where injuries could have resulted. Further, there were no radiological safety issues associated with the trip. The safety function of the differential relay is to actuate on demand to prevent damage to large electrical components, such as the main generator or main transformer, and to isolate an electrical fault to stabilize the transmission grid. These protective functions operated as designed with the mismatch in settings.
Corrective Actions
Immediate Corrective Actions
- Identified and corrected the CT ratio mismatch that caused the automatic reactor shutdown and performed an extent of condition review.
Ongoing Corrective Actions
- Revise procedures to include verification of CT ratio settings for interface relays.
- Develop or enhance the process which engineering personnel control, review and approve protective relay setting calculations and application.
- Establish written documentation delineating acceptance criteria for verification that the off-site power system is viable prior to re-establishing the connection to the grid following a loss of off-site power, reactor trip, or scheduled shut down.
- Develop a process to control switchyard configuration management to support safe plant operation.
Previous Similar Events
No previous similar events.