05000247/LER-2001-007

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LER-2001-007,
Docket Number
Event date: 12-26-2001
Report date: 02-25-2002
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
2472001007R00 - NRC Website

FACILITY NAME (1)

DOCKET

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SEQUENT R REVISI

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PLANT AND SYSTEM IDENTIFICATION

Westinghouse 4-Loop Pressurized Water Reactor

EVENT IDENTIFICATION

Automatic Reactor trip initiated by a main turbine trip on auto oil.

stop

EVENT DATE

December 26, 2001

REFERENCE

Condition Reporting System Number: 200112878

PAST SIMILAR EVENTS

Licensee Event Report Number: 1997-018-00

EVENT DESCRIPTION

On December 26, 2001 at

0720 hours
0.00833 days
0.2 hours
0.00119 weeks
2.7396e-4 months

Indian Point Unit 2 experienced an automatic reactor trip with all control rods fully inserting. The trip was initiated by a main turbine trip on auto stop oil.

The auto stop oil turbine trip was caused by a trip of over frequency relays actuated by a disturbance associated with the 345 Kv Bus W93. This disturbance caused generator output breaker 9 to open. The cause of the over frequency relays actuation was failure of the blocking relay [EIIS:FK:68] on Consolidated Edison's 345 Kv feeder Y94. This failure caused the breakers on Y94 to open causing loss of load to the main generator and an over frequency trip. The over frequency trip caused the turbine trip. The turbine trip resulted in the generator trip. The generator trip caused the Indian Point 2 345 Kv generator output breaker 7 to open.

The resultant trip placed the plant in natural circulation with 480-volt buses 2A and 3A de-energized as per design. All three Emergency Diesel Generators (EDGs) started and buses 2A and 3A were manually energized by 22 EDG, this was an expected response. 480-volt buses 5A and 6A remained energized from off-site sources during this event. No steam generator or pressurizer safety valves lifted and actuation of the Safety Injection System was not required. No radioactive release to the environment occurred as a result of this transient.

DOCKET

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EVENT ANALYSIS

This event is reportable in accordance with 10CFR50.73(a)(2)(iv)(A) which requires a Licensee Event Report (LER) for any event that resulted in manual or automatic actuation of the Reactor Protection System (RPS) including: reactor scram or reactor trip.

EVENT SAFETY SIGNIFICANCE

This event was initiated as a result of a grid disturbance on the North 345 Kv ring bus at the Buchanan switchyard. This is an expected plant response due to the actuation of the over-frequency protection circuit.

These relays were added as part of a plant modification after a similar event in July 1997 resulted in a 100% load reject. Since this event is bounded by section 14.1.12 (Loss of all power to the Station Auxiliaries) of the Updated Final Safety Analysis Report (UFSAR) the safety significance was determined to be minimal.

FACILITY NAME (1)

DOCKET

(1 SEQUENT REVISI IAL � ON LER NUMBER6) PAGE (3 )

CORRECTIVE ACTION

The root cause of this event was the failure of a blocking relay on Consolidated Edison's 345 Kv line Y94. Under normal current carrying conditions the blocking relay operated as designed, however under full transient voltage conditions the circuit internal to the relay developed a ground. This caused the breakers on Y94 to open resulting in the over frequency trip of the turbine. The failed blocking relay was replaced prior to plant restart. Consolidated Edison is continuing its investigation into cause of the blocking relay failure.

PREVIOUS OCCURRENCES

A similar event occurred July 26, 1997 and is documented in LER 1997- 018-00. A 100% lost of external load occurred due to a Buchanan switchyard fault in coincidence with a line outage. The root cause of the July 26, 1997 event was a mis-operation of a directional relay device associated with transformer TA5. As a result of that event, over-frequency relays were added to the overall unit protection scheme.

The over-frequency relays actuated as per design for the December 26, 2001 event.