ENS 47858

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v t e ENS Event
23:15 Apr 21, 2012
Title	Apparent Phase Imbalance Between Normal and Alternate Power Circuits
Event Description	On April 21, 2012, at 1615 PDT, Diablo Canyon Power Plant (DCPP) operators identified that an attempted manual transfer of the Unit 1 AC power sources from the normal offsite circuit (500 kv) to the alternate offsite circuit (230 kv) failed. The attempted manual transfer of the offsite power source from the normal 500 kv to the alternate 230 kv was a planned activity to shut down Unit 1 for its 17th refueling outage. The failure to manually transfer was concluded to be caused by an indicated out of phase condition between the 500 kv and 230 kv system. DCPP control transfer systems prevent paralleling and transferring the power supplies from the normal to alternate offsite circuit with an out of phase condition. The offsite AC power sources remain capable of being manually transferred from the normal to the alternate source by way of transferring to diesels as an intermediate step. The 230 kv system ability to perform its accident mitigation safety function following a reactor trip or safety injection was not affected by the inability to perform a direct manual transfer from the 500 kv offsite power system. DCPP staff are currently troubleshooting the out of phase indication and will provide an update when the cause is concluded. The licensee notified the NRC Resident Inspector. * * UPDATE FROM MIKE QUITTER TO HOWIE CROUCH AT 2013 EDT ON 4/25/12 * * * This is an update to EN#47858 reported on April 22, 2012, where Pacific Gas & Electric Company (PG&E) reported that an attempt to manually transfer the DCPP Unit 1 AC power sources from the normal offsite circuit (500 kV) to the alternate required offsite circuit (230 kV) failed. PG&E determined that the out-of-phase condition between the 500kV system and the 230 kV system resulted from the offsite power system configurations and conditions existing at the time of the attempted transfer. PG&E established alternate operating guidance to allow operators to perform bus transfers between the 500kV source and the 230 kV source in the event that an out-of-phase condition exists when bus transfer is desired. PG&E personnel informed the NRC Resident Inspector. Notified R4DO (Proulx).

ENS 47858 +/-
Where
Diablo Canyon California (NRC Region 4)
Reporting

Information Only
Time - Person (Reporting Time:+3.58 h0.149 days <br />0.0213 weeks <br />0.0049 months <br />)
Opened:	Dan Stermer 02:50 Apr 22, 2012
NRC Officer:	Pete Snyder
Last Updated:	Apr 25, 2012
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v • d • e

v t e Emergency Notification System
Diablo Canyon with Information Only
WEEKMONTHYEAR2012-04-21T23:15:00 [Table view]
[ENS list for Diablo Canyon]

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