ML19329A177

Safety Evaluation Supporting Mods to 230 Kv Switchyard

ML19329A177
Person / Time
Site:	Oconee
Issue date:	05/03/1974
From:	DUKE POWER CO.
To:
Shared Package
ML19329A176	List: ML19329A175 ML19329A177
References
NUDOCS 7912300204
Download: ML19329A177 (4)

Text

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' DUKE POWER COMPANY OCONEE NUCLEAR STATION I SAFETY EVALUATION MODIFICATIONS TO THE 230 kV SWITCHYARD 4-Introduction

On_ January 4, 1974, a spurious signal in a multi-conductor cable between the Oconee Nuclear Station and the 230 kV switchyard actuated multiple solid state breaker relays located in the switchyard relay house. A report describing this incident was transmitted to the AEC by A. C. Thies's letter of January 14, 1974. Mr. V. A. Moore's letter of January 21, 1974, requested a safety evaluation be provided prior to modification of the breaker failure relay circuits; this evaluation responds to that letter.

Corrective Action The following revisions to the.affected circuit breaker failure relay circuits

,l will be made to prevent actuation by spurious induced signals: '

(1) The five transformer lockout relay contact circuits from station to i l

switchyard will be run in r,eparate cables.

(2) Additional ground cables will be installed in trenches between the station

and the switchyard for increased electrostatic shielding.

4 (3) Interposing relays with 125V DC coils will be installed in these circuits l in the switchyard relay house.

(4) Connecting circuitry from interposing relays to breaker failure relays will be by shielded twisted pair cable.

(5) The pickup setpoint of the breaker failure relays will be increased from the present value of 15 volts to over 70 volts.

1 (6) Overcurrent monitor relays will be installed to monitor the startup transformers for low side faults, and to supervise the interposing relay circuit.

In addition to these six corrective measures, two modifications will be made to assure greater operational reliability. The red and yellow bus differential relaying will be placed on separate panels and supplied by separat DC power sources. The 24V DC trip interposing relays utilized in the individual circuit breaker control circuits will be replaced with relays having 125 V coils.

Safety Evaluation J

The separation of the five transformer lockout contact circuits into five individual cables will prevent any induced currents in one cable frow affecting more than one breaker failure relay initiating circuit. The additional ground cables installed in the trenches between the station and the switchyard will 7912300MOf

.- t L provide increased electrostatic shielding and will protect the cables from external influences. The installation of the interposing relays with 125V DC coils in the circuit requires that any induced currents produce a voltage of such magnitude that.its probability of occurrence is negligible. In the event that such an induced current did occur, it could not operate the interposing relay in the startup transformer lockout circuits unless the transformer overcurrent monitor relay, which supervises the circuit, had operated.

In addition, contacts, from the interposing relay will be used to trip the associated circuit breakers. Since this circuit is supervised by PCB breaker auxiliary contacts,-this precludes misoperation of the interposing relay which could, heretofore, initiate a false breaker failura signal. To preclude operation of the breaker failure relay by induced current in the circuit from 1 the interposing relay, shielded twisted pair cable is utilized, and the breaker failure relay pick-up setpoint will be increased to a value of approxi-mately 70 volts.

These modifications will preclude the breaker failure relays from inadvertently tripping and isolating the switchyard from induced currents. The schedule for completion for these modifications calls for completion of the required engineering by May 15, 1974, and completion of the modification as soon as practicable thereafter. These modifications will not require any additional surveillance or testing not presently required by the Oconee Technical Specifi-cations.

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