ML19317F516

RO 270/76-11:on 760924,230 Kv Switchyard Red Bus & Startup Transformer CT2 Were Isolated.Caused by Error in Implementation of Wiring Changes.Relay Contact Modified & Mod Implementation Procedure Reviewed

ML19317F516
Person / Time
Site:	Oconee
Issue date:	10/08/1976
From:	DUKE POWER CO.
To:
Shared Package
ML19317F512	List: ML19317F511 ML19317F516
References
RO-270-76-11, NUDOCS 8001140744
Download: ML19317F516 (2)

Text

c DUKE POWER COMPANY OCONEE UNIT 2 e: '

Report No.: R0-270/76-11 Report Date: October 8, 1976 Occurrenge Date: September 24, 1976 ,

Faci?ity: Oconee Unit 2, Seneca, South Carolina Identification of Occurrence: 230 kV Switchyard Red Bus and Startup Transformer CT2 isolated Conditions Prior to Occurrence: Unit at 100 percent full power Description of Occurrence:

On September 24, 1976, during the testing of interlocks between switchyard circuits PCB-28 and PCB-30, the 230 kV switchyard red bus and startup trans-former CT2 were isolated. PCB-28 and PCB-30 were being tested as the final step in Nuclear Station Modification ON-0140-D. A wiring revision nraessary to perform this modification had not been identified. Due to this unper-formed wiring change, contacts of relay 86TX/CT3 were wired in parallel with contacts of relay 86TX/CT2. Testing of the 86TX/CT3 contacts energized the 86TX/CT2 contacts resulting in a breaker failure indication in PCB-26.

Since PCB-26 was closed, the logic circuit sensed a closed failure mode and l

isolated the red bus by tripping all the PCB breakers associated with the red bus. The PCB-26 breaker failure relay also tripped PCB-27 and, therefore, isolated startup transformer CT2.

PCB breakers necessary to restore the 230 kV red bus were reset within 8 minutes of the occurrence. Startup transformer CT2 was returned to service within an additional 10 minutes.

Apparent Cause of Occurrence:

'This occurrence was apparently caused by an error in the implementation of a Nuc1 car Station Modification. The wiring changes necessary to perform the modification were specified by revised wiring and schematic diagrams.

Personnel reviewed these diagrams, identified the wiring revisions and executed these changes. The review, however, failed to identify and implement one of the wiring changes necessary to have precluded this incident.

Analysis of Occurrence:

The Oconee Nuclear Station 230 kV switching station includes two full capacity main buses, the 230 kV red bus and the 230 kV yellow bus. Due to the redundant full. capacity bus, the loss of the 230 kV red bus did not interrupt service to any component receiving power via the 230 kV switching station.

For power operation two startup transformers, in this case CT2 and CT1, are required to be operable and available by Oconee Technical Specification 3.7.1(b). The loss of one startup transformer is permitted by Oconee 80011407py

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the other startup transformer

' itechnical' Specification 3.7.2(a) provided that Startup is available for. automatic connection to the unit's main' feeder bus.

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transformer CT1 was not automatically available to the main feeder bus, how-Startup transformer CT3 was also manually ever,-it.was available.

manually available. Alternate sources of power automatically available to Un included the 4160V standby bus powered via the Keowee 13.8 kV underground feeder ~and the 4160V standby bus powered via the 100 kV transmission line from the Central Switchyard Station.

During normal power operation startup transformer CT2 serves as a standby source to the Unit 2 ' auxiliary transformer 2T. At'the time of the occurrence It is, the Unit 2 generator was on line and transformer 2T was operable.

therefore, concluded that in the event of an ES actuation, a suf ficient number of-power sources were available to handle any emergency loads.

Additionally it is felt that due to the brief interval over which power was lost and in. light of the above conclusion, that the health and safety of the public was not affected by this incident.

Corrective Action:

The 86TX/CT2 relay contact was modified as required by Duclear Station Modifi-cation ON-0140-D specifications and the associated circuits were successfully tested. Additionally, modification implementation procedures involving a large number of circuit changes are being reviewed to determine if the pro-bability of implementation errors due to inadvertent personnel errors can be reduced.

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