Columbia Generating Station is shutdown for a refueling outage and is currently in Mode 4. At 1222 PDT June 15, 2013, the
115 kV offsite power to the backup transformer relayed off and then came back on. Both Division 1 and Division 2
4.16 kV critical switchgear buses were powered from the backup transformer at the time of the loss of backup power.
Upon detection of under voltage conditions, Emergency Diesel Generators 1 and 2 started and powered the Division 1 and Division 2 4.16 kV critical switchgear buses after approximately 5 seconds. The temporary loss of power to the Division 1 and Division 2 4.16 kV critical switchgear buses also resulted in the closure of containment isolation valves in the Reactor Water Cleanup System, Equipment Drain System and the Floor Drain System. Shutdown cooling had previously been secured in preparation of performing the reactor pressure vessel hydro test. Investigation into the cause of the temporary loss of the 115 kV offsite power is on-going.
The Division 1 and Division 2 4.16 kV critical switchgear buses are currently being powered from the 230 kV offsite power through the startup transformer.
There were no radiological releases as a result of this event."
The licensee notified the NRC Resident Inspector.
