ML20079C330
| ML20079C330 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 06/12/1991 |
| From: | Cottle W ENTERGY OPERATIONS, INC. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GNRO-91-00102, GNRO-91-102, NUDOCS 9106190230 | |
| Download: ML20079C330 (3) | |
Text
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Entbrgy g r y = "a"* 'ac-Operations Syg =
W. T. Cottle 7
J-June 12, 1991 1..
U.S. Nuclear Regulatory Commission Mail Station P1-137 Washington, D.C.
20555 Attention: Document Control Desk Gentlemen:
SUBJECT:
Grand Gulf Nuclear Station Unit 1 l.
Docket No. 50-416 License No. NPF-29 l
Special Report 91-004 Emergency Diesel Generator 11 Failure l-GNRO-91/00102 On May 13, 1991 at approximately 2036 hours0.0236 days <br />0.566 hours <br />0.00337 weeks <br />7.74698e-4 months <br />, Diesel Generator 11 (Division I) was started for the performance of the Monthly Standby Diesel Generator
-(SDG 11) Functional Test.
The SDG was successfully started and an attempt to load the SDG was commenced.. Prior to reaching maximum load, oscillations were observed in the DC Field 'lolt, DC Field Amp and AC Amps.
At an approximate. load of 4.5 megawatts, more-intense oscillations were-observed.
Shortly afterwards, the SDG' output breaker (152-1508) tripped i
open due to An overcurrent condition. The diesel continued to run until l
I it was manually shutdown by operations personnel.
A work order was initiated to' investigate the SDG failure.
The results of
-the investigation indicated that the cause of the overcurrent condition was a change in the resistance across two contacts associated with the vol_tage regulator rectifier bridge selector switch in the generator field circuit.
The selector switch is a manual switching device employed ~ to place a redundant (i.e., Bridge 2) rectifier bridge in service in the event the inservice rectifier bridge (i.e., Bridge 1) failed. The selector switch contacts which connect the negative poles of bridge 1 and bridge 2 to the generator field circuit indicated excessive resistances during the invest'igation.
An inspection of the contacts revealed a thin layer of j
contact-lubricant smeared over the surfaces of the two contacts. 'The E
lubricant appeared to have hardened due to age.
The hardened lubricant, along with inadequate tension between the contact surfaces, did not allow adequate-continuity within the circuit at all times.
-SPRP9104/SCMPFLR - 1
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9106190230 910612 PDR ADOCK 05000416 1 :
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June 12, 1991 GNRO-91/00102 Page 2 of 4-
- The current within the gens ator field circuit would vary with a change in resistance resulting in erroneous signals being sent to the voltage regulator. The erroneous signals created an overcurrent condition at the output breaker which resulted in an output breaker trip.
During the investigation, other SDG 11 components were examined and no abnormalities were identified. An inspection of the SDG 12 (Division II) bridge selector switch revealed similar conditions but not to the extent to cause a SDG 12 failure.
A similar event occurred on November 27, 1990 during a test of SDG 11, in that the output breaker tripped under similar conditions. During the investigation, attempts to repeat the failure were unsuccessful.
Therefore, the cause of the failure could not be positively identified.
The actions taken to correct the SDG 11 failure were as follows:
o Inspection frequency of the generator carbon brushes and slip rings was increased from an annual to a quarterly frequency.
o Periodic cycling of the voltage regulator bridge relay (selector switch) will be performed.
The cause of the November 27th and May 13th SDG 11 failures are attributed to the selector. switch contacts not maintaining continuity in the circuit due to inadequate tension between contact surfaces and the degradation of the contact lubricant due to aging.
The lubricant was used for the initial _ lubrication by the vendcr. Neither the switch or the lubricant had been changed since being placed inservice at Grand Gulf Nuclear Station.
The surfaces of the contacts were cleaned and relubricated. Additionally, the contacts were tightened to maintain circuit continuity.
A repetitive task will be generated to inspect-the switch contacts during refueling outages. Monitoring of the diesel generator will be continued to ensure the effectiveness of the corrective actions to prevent recurrence.
SPRP9104/SCMPFLR - 2
June 12, 1991 GNRO-91/00102 Page 3 of 4 Entergy Operations, Inc. considered the test to be a valid failure pursuant to Position C.2.e(5) of the Regulatory Guide 1.108 since the ability of the diesel generator in this condition to power ESF loads in response to a bonafide signal was indeterminate.
This was the second valid failure in the last 20 tests and the fifth in the last 100 valid tests.
Therefore, the testing frequency was increased to once per 7 days in accordance with the test schedule of Technical Specification Table 4.8.1.1.2-1.
Yours truly, w r c----
WTC/RR/cg cc:
Mr. D. C. Hintz Mr. J. L. Mathis Mr. R. B. McGehee Mr. N. S. Reynolds Mr. H. L. Thomas Mr. F. W. Titus Mr. Stewart D. Ebneter Regional Administrator U.S. Nuclear Regulatory Commission Region II l
101 Marietta St., N.W., Suite 2900 Atlanta, Georgia 30323 1
Mr. L. L. Kintner, Project Manager Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Mail Stop 11021 l
Washington, D.C.
20555 l
I 1
l SPRP9104/SCMPFLR - 1