Although this event was initially reported in Event Report 47799 on April 2, 2012, this subsequent report provides further details and some related safety issues. To date it is unknown why Event Report 47875 has been delayed over a month. It is also unknown whether there will be a corresponding Licensee Event Report (60 Day).
In this event (report 47875, dated 4/30/12), the plant workers having completed the maintenance, were attempting to RESTORE that portion of the Switchyard Bus. They closed Bus Tie 9-10 Bus 10 without removing the protective grounding device. This caused an electrical transient which was responsible for the requiring that the Unit be taken off line to comply with Technical Specifications with respect to Offsite Power Supplies.
It appears too that that the Switchyard mis-operation also caused the loss of two trains of Core Spray (due to their associated Keep-Fill Pumps being de-energized) and several other Key Safety Systems.
In addition, Unit 1 received Panel 901-8 A7 U1 Emergency Diesel Generator (EDG) Trouble alarm. Equipment Operators were dispatched and the Unit 1 EDG was found running unloaded, without a generator field flash, and no auto start signal received.
Troubleshooting identified that a 125 VDC ground had caused the Unit 1 EDG to start. As a result, the Unit 1 EDG was declared inoperable.
This was potentially serious because the Emergency Diesels are required for times when there is a Switchyard malfunction that would prevent Offsite Power from being fed to the Unit. EDG 1 was restored to operability 7 hours later. Without the Licensee Event Report the condition of the other emergency and standby power sources is unknown.
Event Report 47875 also describes that the Main Turbine Generator Bearing #10 was undergoing trouble-shooting activities due to higher than allowable vibrations. The Switchyard mis-operation may have been related to the Bearing problem, since an electrical disturbance can affect the Alternator.
These grounding devices are usually mechanical devices, which are cranked into place and locked, one has to remember to crank them down, prior to energizing the portion of the bus.
In the video above shows opening an energized portion, those three “poles” are the disconnect switches which were Closed and energized the grounded bus
This was an unnecessary transient (Reactor Shutdown) preventable by good Operating Practices.
Event Reports Listed on Page 2…