ML20010C314

LER 81-009/03L-0:on 810708,while Attempting Start of Cooling Tower Pump 122,undervoltage Relay Operation Opened Breaker CT 11-1,closed Bus Tie CTBT-112 & Caused Trip of Bus CT-12. Caused by Improperly Set Undervoltage Relay Setpoint

ML20010C314
Person / Time
Site:	Prairie Island
Issue date:	08/07/1981
From:	Hunstad A NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML20010C310	List: ML20010C309 ML20010C314
References
LER-81-009-03L-01, LER-81-9-3L-1, NUDOCS 8108190343
Download: ML20010C314 (3)

Text

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tio 61 DOCI(ET NUf48En 68 69 EVENT DATE 14 75 REPORT OATE 80 EVENT DESCRIPTION AND PROBAflLE CONSEQUENCES h O a l During normal operation, an attempted start ot No. 122 Cooling Tower l 0 3 l Pump caused undervoltage relay ogc-ration which opened breaker CT 11-1 and l (g l closed bus tic CTBT-ll2. This transferred the Cooling Tower Bus CT 11 loads l

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August 7, 1981 Attachment NORTilERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT LER 81-009 Supplemental Report pait One Operation With One Of fsite Source of Power Detailed Description of Event

1. Cooling tower pump 122 tripped. Later found to be due to high vibration reading on malfunctioning instrument channel.

2. Operators attempted to restart 122 cooling tcwer pump. -

3. Cooling tower bus CT11 un<lervultage due to motor starting load.

Relay 27/CT11 drops out tripping open breaker CT11-1 and closing bustic CTBT-112.

4. Cooling tower bus CT12 undervoltage occurs due to transfer of bus CT11 loads and continued starting of 122 cooling tower pump bus. Bus doss not trip due to undervoltage on both buses. (27/CT11 and 27/CT12 both droped out).

5. Cooling tower buses (still tied together through bustie) begin to recover from undervoltage. Although volage is the same on both buses, relay 27/CT11 resets (picks up) first. Since relay 27/CT12 is still dropped out, breaker CT12-7 trips open leaving the cooling tower buses (CT11 and CT12) tied together and without a source.

6. Safeguards buses 16 and 26 sense the loss of source and initiate their voltcge restoration schemes. Both diesel generators start and bus tie breakers 16-8 and 26-8 cloc' providing sources for the safeguards buses 16 and 26 from the siternate offsite source.

7. Operators restore cocling tower buses to normal configuration and restart 123 and 124 cooling tower pummps.

8. 121 and 12 cooling tower pumps are restarted. Bus CT11 transfer is prevented by opening trip switch for undervcitage relay.

Designation of Appareat Cause of the Occurrence The undervoltage relay, a Westingl ouse type SV relay, was found with an incorrect set point. The SV type alay is an overvoltage relay on which we use the dropout contact. The setpoint card stated a pickup voltage of 110 V and a dropout voltage of 90% of pickup (99v) . When the relay was calibrated (2/81) the pickup was set at 110V. The dropout is not adjustable separately. Due to bus trans fer problems this relay was recbecked (5/81) and the pickup was found to be correct. In both cases the actual dropout voltage was not checked.

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On 7/9/81 the relay was rechecked and the dropout voltage was measured.

The actual setting turned out to be: pickup 110V, dropout 10SV or 98%. The pickup voltage was set at 99V.

This relay should not have tripped the second source b.caker. This was dite to a slight difference in the pickup voltage for the two coils. J 3 is difference is unavoidable in calibration of the relay and is unitportant as long as this relay is prever.ted from tripping the second source breaker, Corrective Action The cooling sources were restored to the safeguards buses and the undervoltage relay was recalibrated. Following this recalibration 122 Cooling Tower Pump was stopped and restarted to verify that the relay was properly calibrated and would function as designed.

If a fault condition occurred which caused an undervoltage on CT11 bus and was not cleared quickly the potential exists for this logic to trip both cooling tower source breakers. The cooling tower transfer scheme will be revised to prevent this.

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