On January 6, 2014 at
1659 hours0.0192 days <br />0.461 hours <br />0.00274 weeks <br />6.312495e-4 months <br />, Beaver Valley Power Station (
BVPS) Unit 1 was operating at 100 percent power. An
automatic reactor trip occurred due to a
main transformer differential protection main unit generator trip. All three
Auxiliary Feedwater (
AFW) pumps automatically started, as expected, due to lowering
steam generator levels. The
reactor trip response was as expected with no immediate complications. The
main transformer differential protection actuation was determined to be caused by an internal winding fault in the 'B' phase windings due to a dielectric breakdown. The transformer has been replaced.
This event is being reported under 10 CFR 50.73(a)(2)(iv)(A) as a condition that resulted in the valid automatic actuation of systems listed in (a)(2)(iv)(B) - (1) automatic Reactor trip, (6) automatic Auxiliary Feedwater pump start. A 10 CFR 50.72 notification was made at 1909 hours0.0221 days <br />0.53 hours <br />0.00316 weeks <br />7.263745e-4 months <br /> on January 6, 2014 to report an automatic reactor trip and a Specified System, Auxiliary Feedwater actuation (EN 49697).
' - NRC FORM 366 (01-2014)
- NRC'FORM 366A (01-2014) U.S. NUCLEAR REGULATORY COMMISSION Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
CONDITIONS PRIOR TO OCCURRENCE
Unit 1: Mode 1, 100% Power There were no systems, structures, or components (SSCs) that were inoperable at the start of the event that contributed to the event.
DESCRIPTION OF EVENT
On January 6, 2014 at 1659 hours0.0192 days <br />0.461 hours <br />0.00274 weeks <br />6.312495e-4 months <br />, Beaver Valley Power Station (BVPS) Unit 1 was operating at 100 percent power. An automatic reactor trip occurred due to a main transformer [XFMR] differential protection [87] main unit generator [GEN] trip. All three Auxiliary Feedwater (AFW) [BA] pumps [P] automatically started as expected due to lowering steam generator [SG] levels. The normal and emergency AC busses [BU] were powered from the offsite supply during this event. Since power to the Emergency Busses was not lost, the Emergency Diesel Generators [DG] were not required to start. The reactor trip response was as expected with no immediate complications. At 1757 hours0.0203 days <br />0.488 hours <br />0.00291 weeks <br />6.685385e-4 months <br /> the plant was stabilized in Mode 3.
At 1848 hours0.0214 days <br />0.513 hours <br />0.00306 weeks <br />7.03164e-4 months <br /> the Control Room received an alarm indicating that the Turbine Driven AFW (TDAFW) pump had stopped. The responding operator reported that the pump had tripped and the two Motor Driven AFW (MDAFW) pumps were operating properly. At 1902 hours0.022 days <br />0.528 hours <br />0.00314 weeks <br />7.23711e-4 months <br /> the MDAFW pumps were secured. Subsequent investigation revealed that the TDAFW pump had tripped on over-speed due to governor [65] oscillations.
(Reference BVPS Unit 1 LER 2014-002-00, Beaver Valley Unit 1 Turbine Driven Auxiliary Feedwater Pump Governor Oscillations Result in Pump Trip, for TDAFW pump details)
CAUSE OF EVENT
The cause of the main transformer differential trip was determined to be an internal winding fault in the 'B' phase windings due to a dielectric breakdown. This is supported by the results of post fault oil analysis, transformer turns ratio testing, fault recorder data, internal inspections and the "B" Phase coil package inspection. A fault was found on one of the High Voltage coils of the transformer. Due to the extent of damage, the exact cause of the dielectric breakdown cannot be identified. The following conditions may have caused the event to occur: migration of contamination due to overheated oil, a latent manufacturing defect, and/or static electrification.
ANALYSIS OF EVENT
On January 6, 2014 at 1659 hours0.0192 days <br />0.461 hours <br />0.00274 weeks <br />6.312495e-4 months <br />, Beaver Valley Power Station (BVPS) Unit 1 was operating at 100 percent power. An automatic reactor trip occurred due to a main transformer differential protection main unit generator trip. All three Auxiliary Feedwater (AFW) pumps automatically started, as expected, due to lowering steam generator levels. The reactor trip response was as expected with no immediate complications.
The plant risk associated with the BVPS Unit 1 automatic reactor trip and an Auxiliary Feedwater actuation, on January 6, 2014, due to a main unit transformer differential trip is considered to be very low.
This is based on the conditional core damage probability and conditional large early release probability for the event when considering the actual plant conditions that were present at the time of the event.
Based on the above, the safety significance of the automatic reactor trip and Auxiliary Feedwater actuation event on January 6, 2014, was very low.
CORRECTIVE ACTIONS
1. Replacement of the Main Unit Transformer. (Complete) 2. Establish a sequence of operation, for both Units' transformers, that minimizes the potential for static electrification.
3. Evaluate training for the appropriate departments on lessons learned from this event.
Completion of the above and other corrective actions is being tracked through the BVPS Corrective Action Program.
PREVIOUS SIMILAR EVENTS
A review identified the following historical BVPS Unit 1 events involving a failure of the main unit transformer.
BVPS LER 94-005-00 - BVPS LER 78-043-00 - Main Transformer Bushing Failure Results In Electrical Grid Disturbance and Dual Unit Reactor Trip.
Main Transformer Failure and Inadvertent
SI CR 2014-00175, 12779