ML20101H725

Special Rept 92-003-00:on 920525,Div 1 Standby DG 11 Experienced Nonvalid Failure.Caused by Excessive Contact Resistance.Comprehensive Loading Increased on One of Six Sets of Contacts in Svc & Selector Switch Bypassed

ML20101H725
Person / Time
Site:	Grand Gulf
Issue date:	06/26/1992
From:	Cottle W ENTERGY OPERATIONS, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
92-003-00, 92-3, GNRO-92-00075, GNRO-92-75, NUDOCS 9206300156
Download: ML20101H725 (3)

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W. T. Cottle June 26, 1992 ,

U.S. Nuclear Regulatory Commission Mail Station F1-137 Washington, D.C. 20555 Attention: Document Control Desk

SUBJECT:

Grand Gulf Nuclear Station Unit 1 Docket No. 50-416 Special Report 92-003-00 GNRO-92/00075 Gentlemen:

This special report is being submitted pursuant to Grand Gulf Nuclear Station Technical Specification 4.8.1.1.3.

On May 25,1992 at approximatfy 1713 hours0.0198 days <br />0.476 hours <br />0.00283 weeks <br />6.517965e-4 months <br /> while perfonning post-maintenance and post-modification testing, Division 1 standby diesel generator (DG 11) experienced a non-valid failure. DG 'll was manually started at 1559 hours0.018 days <br />0.433 hours <br />0.00258 weeks <br />5.931995e-4 months <br /> and incrementally loaded from 1 megawatt (MW) to 5.45 MW. DG 11 had been running at 5.45 MW load for 2.28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br /> with no anomalies indicated in the control room or at the local control panel. The non-licensed operator (NOB) noticed the field voltage dropped to zero then immediately pegged high prior to DG 11 output breaker 152-1508 tripping open. DG 11 continued to run until it was manually shut dmvn by operators. DG 11 was then secured for corrective maintenance.

A_ work order was initiated to investigate the output breaker trip. Investigation determined i that 152-1508 tripped open due to large oscillations in DG 11's field voltage. The l oscillations were evaluated to have been caused by excessive resistance due to erratic

l. continuity at DG 11's rectifier bridge selector switch.

u The DG 11 exciter-regulator system includes redundant rectifier bridge circuits. The circuits l

are controlled via a bridge selector switch which features blade / clip style xntacts.

. Contact resistance of the bridge selector switch was measured as part of troubleshooting for a non-valid failure which occurred May 13, 1991. Troubleshooting for that event determined the contact resistance was excessive. Compressive loading and alignment of switch contacts were inspected; compressive loading was increased on one of six sets of 9206300156 920626

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,. GNRO 92/00075 Page 2 of_4 contacts in service until satisfactory resistance values were obtained. Measurements taken after cleaning and relubrication ( ' the other five sets of contacts in service indicated that resistance values had diminished. Cleaning and relubrication of the other six sets of contacts for the redundant bridge was also performed and resulted in improvements in contact resistance values. The failure mechanism was believed to have been degradation of contact lubricant. Mainte .ance activities were developed to address pe-iodic cleaning,

, relubrication, and resistance measurements of the bridge selector switch contacts.

This recent failure indicates th it the maintenance program has not been successful in eliminating the failure mechmu3m as exr ^ted. Investigation has revealed that contact resistance can vary without bridge selector switch manipulation.

4 The bridge selector switch has been bypassed from DG 11's field circuit as a result of this recent failure. The redundant rectifier bridge circuit and selector switch are unnecessary for performance of DG 11's safety function. Division 2 standby diesel generator (I)G 12) has a identical exciter-reguiator system. DG 12 has not experienced this ;aihre mechanism, i Modification to DG 12's circuit will be considered.

The bridb selector switch apparently caused the previous failures of DG 11 which occurred on November 27,1990 and May 13, 1991. The December 18,1989 failure of DG 11 had similar symptoms as the other failures referenced, but the cause was attributed to loose potential transformer fuse connections. The bridge selector switch may have caused the December 18,1989 failure. Those failures were previously discussed in correspondences from Entergy Operations, Inc. (EOI) dated January 17,1990, December 27,1990, and June 12,1991 respectively. Correspondence from EOI dated August 20,1991 discussed revision of commitments contained in the December 27,1990 correspondence.

Entergy Operations, Inc regards the event as being a valid test and non-valid failure _ _ .

pursuant to Position C.2.e(7) ef Regulatory Guide 1.108 since DG 11 was unable to power ESF loads in response to a bona fide signal in this condition. There have been no valid failures of DG 11 in the last 20 test., and four valid failures in the last 100 tests.

Yours truly, 1

w r~w WTC/BAB/cg cc: (See following page)

June 26, 1992

., GNRO 92/00075 Page,3 of 4 cc: Mr. D. C. Ilintz Mr. J. L Mathis Mr. R. B. McGehee Mr. N. S. Reynolds Mr.11. L Thomas Mr. Stewart D. Ebneter Regional Administrator U.S. Nuclear Regulatory Commission Region II 101 Marietta St., N.W., Suite 2900 Atlanta, Georgia 30323 Mr. P. W. O'Connor, Project Manager Office of Nuclear Reactor Regulation l U.S. Nuclear Reguiatory Commission Mail Stop 13H3 Washington, D.C. 20555 t

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