ML20207K205

Special Rept 99-01:on 990128,DG Tripped After 2 H of Operation During Loaded Operation for Monthly Test.Caused by Several Components That Were Degraded or Had Intermittent Problems.Parts Were Replaced & Initial Run Was Performed

ML20207K205
Person / Time
Site:	McGuire
Issue date:	03/05/1999
From:	Barron H DUKE POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
99-01, 99-1, NUDOCS 9903170081
Download: ML20207K205 (5)

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McGuire Nudear Station 12700 Hagers Ferry Road Huntersville, NC 28078-9340

' (704) 875-4800 OFFICE

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, (704) 875-4809 FAX j

March 5, 1999 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555

Subject:

McGuire Nuclear Station, Unit 1 and 2 Docket No. 50-369 and 50-370 Special Report Number 99-01 i Problem Investigation Process No. 0-M99-0366 l Attached is Revision 0 of Special Report Number 99-01. This l report is being submitted in accordance with the McGuire Nuclear l Station, Updated Final Safety Analysis Report, Selected Licenses Commitment 16.9-7, Standby Shutdown System (SSS). Special Report 99-01 complies with remedial action (c), which provides a commitment to submit a report to the Commission for an occurrence of a period with the SSS inoperable in excess of 7 days. ]

The SSS diesel generator was inoperable for a period 8 days, 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> and 29 minutes, beginning on January 28, 1999 and ending on February 5, 1999. The SSS was successfully restored to an operable status and remedial actions were performed in compliance with the commitment.

The attached report provides details of the root cause investigation and the corrective actions that were implemented to restore the SSS diesel generator to operable status and planned corrective actions to improve reliability.

The planned corrective actions identified in this report are regulatory commitments.

Very truly yours,

.[gy H. B. Barron }

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,aV Attachment , l' ' ^

9903170081 990305 PDR ADOCK 05090369 8 PDR2

', *U.S. Nuclecr R gulatory Commiccion Document Control Desk March 5, 1999 Page 2 cca Mr. L. A. Reyes INPO Records Center U.S. Nuclear Regulatory Commission 700 Galleria Parkway Region II Atlanta, GA 30339 Atlanta Federal Center 61 Forsyth St., SW, suite 23T85 Atlanta, GA 30323 F. Rinaldi Mr. Scott Shaeffer U.S. Nuclear Regulatory Commission NRC Resident Inspector Office of Nuclear Reactor Regulation McGuire Nuclear Station Washington, D.C. 20555 l

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Duke Energy Corporation McGuire-Nuclear Station Special Report 99-01 Revision 0 Backaround McGuire Nuclear Station, Updated Final Safety' Analysis Report, Selected Licensee Commitment 16.9-7 provides the commitment to

-which the Standby Shutdown System (SSS) is to be maintained operable and remedial actions for allowed outage time.

Specifically remedial action (c)-provides a commitment to submit a report to theLCommission for an occurrence of a period with the SSS inoperable in excess of 7 days.

The SSS diesel generator was inoperable for a period of 8 days, 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> and 29 minutes; beginning on January 28, 1999 at 15:49 and ending on February 5, 1999 at 23:18. During the period of inoperability, all compensatory actions as required by the q remedial actions were performed. These remedial actions include i verification of operable fire detection and suppression systems, i operability of off-site power and emergency diesel generator i operability. i i

1The Updated Final Safety Analysis Report, Selected Licensee Commitment 16.9-7 includes testing requirements to demonstrate SSS operability. Included in the testing requirements, is a 31-day test requirement to verify that the diesel generator starts from ambient conditions and operates at least 30 minutes at greater than or equal-to 700 kW. This requirement is performed per a periodic test procedure (PT/0/A/4200/002). This periodic test of the diesel generator identified the condition of inoperability.

The following outlines the cause of the inoperability, and corrective actions taken to restore the SSS to operable status.

Description of Occurrence The SSS diesel generator was determined to be inoperable upon experiencing a trip during loaded operation for the monthly test (PT/0/A/4200/002) on January 28, 1999 at 15:49. The diesel generator tripped after two hours and twenty minutes of operation.

i Initial troubleshooting and a subsequent operational run of the j diesel generator did not immediately identify an apparent cause.

The diesel generator performed acceptably with only minor power fluctuations (+-20kW). Additional run time and troubleshooting led to discovery of several components that were either potentially degraded or had intermittent problems. Specifically, a diode and surge. capacitor were discovered to have failed which resulted in overloading one phase of the armature windings. These parts were replaced and an initial run wa.? performed on February 4,1999 at

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Page 2 of 3 J 04:12. Additional runs identified a problem that made closing the generator breaker difficult. The problem was due to instantar.eous over current relay (50 DGT) trips immediately after closing the

. generator breaker to the energized bus.

Jul operability evaluation concluded and documented that trips experienced from the over current relay during synchronization attempts are a condition that is acceptable to the diesel generator performing its intended function during an SSS event. The over current relay trips are a result of difficulty in synchronizing the generator to an energized bus for testing purposes. The intended I function of the SSS diesel generator for SSS events is to provide power to a de-energized bus which would not require synchronization. For events requiring operation of the SSS diesel, the v er current relay is bypassed in the circuit. Therefore, after repairing the diesel generator, any remaining difficulty in synchronizing the generator was a condition acceptable to declaring ,

the SSS diesel generator operable.  !

l On February 5, 1999 at 16:50 the diesel generator was started and i loaded per the monthly periodic test procedure (PT/0/A/4200/002). 1 The test procedure was performed successfully with no problems. I The SSS diesel generator was declared operable on February 5, 1999 )

at 23:18, resulting in a period of inoperability of 8 days, 7 )

hours, and 29 minutes. '

A root cause investigation continued after restoration of the diesel generator to' identify causes of these difficulties and possible related reliability issues. This investigation identified a degraded power factor controller during a periodic test on February 25, 1999. The power factor controller is bypassed in the circuit for the diesel generator to perform its intended function, but is utilized for synchronization during testing.

Safety Sionificance This occurrence is not considered to be significant. At no time was the safety or health of the public or plant personnel affected as a result of the conditions that existed.

The SSS is intended to respond to those low-probability fire and/or sabotage events which render both the control room and automatic safety systems inoperable. Because of the low probability of occurrence of these events, the remedial actions rely-on compensatory action, and timely repair or return to operability. The Updated Final Safety Analysis Report requires a justification for continued operation upon exceeding an inoperable

-period of 60 days for the SSS diesel generator.

There is no safety significance associated with the occurrence described by this report as all remedial actions were performed and operability was restored without challenging the 60 day period

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of inoperability that has been evaluated to require justification for costinued operation.

Causes The Root Cause of the initial trip is attributed to a degraded subcomponent of the voltage regulation / excitation system due to aging. The degraded subcomponent was a generato* diode, which resulted in failure of one phase of the exciter armature windings.

Synchronization is not a function. required during an SSS event.

However, the root cause investigation also focused on resolving '

difficulties with synchronization. The difficulties with synchronizing the generator to the bus have been concluded to -

result from two separate factors. First, the 50DGT over current l relay has an overly conservative trip setting of 110% of rated j full load current for generator protection during testing with no I time delay feature. Second, the synch-check relay minimum setting allows breaker closure further out of phase than desired.

Corrective Actions Immediate

1. Technical representatives from the diesel, generator and voltage regulation system manufacturers were involved as part of the investigation and recovery team.

2. Replaced voltage regulator, voltage regulator motor operated '

potentiometer, and frequency motor operated potentiometer.

3. Rewound the exciter armature, and verified the stator was not 1 damaged.

Subsequent

1. Replaced the p ser factor controller.

Planned (The below planned actions represent NRC commitments):

1. Implement modification to replace the instantaneous over current relay (50 DGT) with an instantaneous over current relay with a time delay.

2. Evaluate design enhancements to improve Operator control of the SSS diesel generator.

3. Evaluate the need for additional monitoring and trending of the operating parameters of the SSS diesel generator.

4. Evaluate the need for enhanced preventative maintenance of the l SSS diesel generator.

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