ML20029B143

LER 91-003-00:on 910129,Diesel Generator 2A & 2B Experienced Valid Failures During Testing.Cause of Failures Under investigation.K4 Transfer Relays Replaced & Relay Contacts on Diesel Generator 1A tested.W/910228 Ltr

ML20029B143
Person / Time
Site:	Vogtle
Issue date:	02/28/1991
From:	Hairston W, Idom R GEORGIA POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
ELV-02564, ELV-2564, LER-91-003, LER-91-3, NUDOCS 9103050513
Download: ML20029B143 (5)

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0849 Docket No. 50-425 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk i Washington, D. C. 20555 i

, Gentlemen:  ;

V0GTLE ELECTRIC GENERATING PLANT LICENSEE EVENT REPORT DIESEL GENERATOR FAILURES MAY HAVE RESULTED IN LOSS i

OF ABILITY TO MITIGATE ACCIDENT CONSL@ELS In accordance with 10 CFR 50.73, Georgia Power Company hereby submits the i enclosed report related to an event which occurred on January 29. 1991. ,

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. ft7d W. G. Hairston IJI WGH,111/NJS/gm, ,

Enclosure:

LER 50-425/1991-003 .f xc: Georaia-Power Company Mr. C. K. McCoy Mr. W. B. Shi) man Mr. P. D. Rusiton Mr. R. M. Odom NORMS U. S. Nucicar Reaulatory Commission Mr. S. D. Ebneter, Regional Administrator Mr. D. S. Hood, Licensing Project Manager, NRR _

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Ab61tACI (16) on 1 29 91, Diesel Generators (DCs) 2A and 2B experienced valid failures during testing, due to indicated reactive power reaching a value of negative 6000 kVARs as the DCs were loaded to the grid. Investigation has been unable to determine a definitive cause for these failures, and further testing was unable to repeat the failuro for eitbor DG. Initially, high resistance across the contacts of the voltage regulating bridge circuit transfer relay was suspected as a cause of the events and the relays were replaced. However, it has since been determined that this condition could not have caused a constant leading value for reactive power as was observed during these events.

These DC failures may represent a cor.dition that alone could have prevented the fulfillment of the safety function of a system needed to mitigate the consequences of an accident, as described in 10 CFR 50.73 (a)(2)(v). Although a telephone notification to the NRC Operations Center was made on 1 30 91, the causes of the failures are not known with certainty to be the same, and these events are conservatively being reported per 10 CFR 50.73. In addition, this report is required per Technical Specification 4.8.1.1.3 because DG failures occurred.

An investigation to deteritine a definitive root cause is continuing.

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YLAE EfD hum ktv VCCTLI 122CTRIC CINIRATIlp piAYr . tNIT 2 05000425 91 003 00 2 0F 4 ItkT A. REQUIREMENT FOR REPORT This report in conservatively being submitted per 10 CFR 50.73 (a)(2)(v) because a condition may have existed that alone could have prevented the fulfillment of the safety function of a systee needed to mitigate the consequences of an accident, although an investigation has been unable to determlne that a single condition was the cause of both events. It also reprrsents a follow up report to a four hour telephone notification made per 10 CPR 50.72 on 1 30 91 at 0258 CST. This report is also required per '

fechnical Specification (Tr) 4,8.1.1.3 because diesel generator (DG) failures occurred.

B. UNIT STATUS AT TIME OF L* VENT At the time of-this event. Unit 2 was operating in Mode 1 (power Operation) at 90% of rated thermal power. Other than that described herein, there was no inoperable equipment which contributed to the occurrence of this event.

C. DESCRIPTION OF EVENT I On 1 29 91 at 1506 CST, DG 2A was started for surveillance testing per procedure 14980 2,

• Diesel Generator Operability Test." At 1521 CST, operators paralleled DC 2A to .he grid and the indicated reactive power value immediately reached negative 6000 kVARs. An attempt was made to raise the reactive power value, but was unsuccessful. The output breaker was opened, the system engineer was notified and DG 2A was declared inoperable. .

The DG remained running while the redundant rectifier bridge circuit was I selected and, at 1556 CST, DG 2A was again loaded to the grid. Reactive power remained at-normal levels and no other problems occurred. At 1721 CST, the DG surveillance was completed, and at 1953 CST, DC 2A was declared operable. The DG had been out of service for 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 32 minutes.

Due to TS 3.8,1.1 requirements for demonstrating the operability of the redundant diesel, DG 2B was started at 2349 CST. When operators paralleled DC 2B to the grid at 2359 CST, the reactive power value immediately reached negative 6000 kVARs. An attempt was made to raise the reactive power value, and it was raised to negative 4500 kVARs but this was still unacceptable.

DG 2B van shut down and declared inoperable at 0003 CST on 1 30 91. The DG 2B bridge circuit was then instrumented to ass,ist in pinpointing any abnormal conditions and at 1412 CST on 1 30 91, DG 2B was started. The DC

-was paralleled to the grid at 1423 CST, with no apparent problems. At 1512 CST, the output breaker was opened and at 1515 CST DG 2B was stopped. The DG 2B bridge circuit was transferred to the redundant rectifier bridge, which was instrumented, and DG 2B was started at 1730 CST. DG 2B was tied to the grid, without incident, ther, removed from the grid and stopped at 1751 CST. DG 2B was started at 1915 CST to prove operability. This-surveillance was successfully completed and the DG was declared operabh at 2154 CST. DG 2B had beer. out of service for 21 hours2.430556e-4 days <br />0.00583 hours <br />3.472222e-5 weeks <br />7.9905e-6 months <br /> and 55 minutes.

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YIAR EIQ huu R[v V07T12 ELICTRIC CINIRATING PiNir UNIT 2 0500042$ 91 003 00l3 or 4 IL71 To demonstrate continued operability of the redundant diesel, DC 2A was started at 2229 CST on 1 30 9) and successfully completed TS testing requirements. On 2 1 91 at 1621 CST, DG 2A was removed from service to make l resistance measurements on transfer relays K3, K4, and K$. These relays transfer control of voltage regulation from automatic to manual and vice ,

versa. Contact resistance for contacts 2 and 8 of the K4 relay were found to be abnormally high and the K4 relay was replaced. The rectifier bridge was switched so that the bridge that hae failed on 1 29 91 could be instrumented and tested. After instrumentation was attached, DG 2A was started and loaded to the grid with no anomalies observed. The DG was

. stopped, a surveillance test was performed, and DG 2A was declared operable and returned to service. The DC had been out of service for 19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> and 48 minutes.

D. CAUSE OF EVENT Testing of transfer relay contacts in the DG 2A control panel found a higher than normal resistance condition that was initially thought to be the cause of these events. However, after discussing it with the vendor we have determined that this condition could cause reactive power to fluctuate, but it would not cause a constant leadin6 value for reactive power. Futhermore, the transfer relay contacts on the DG 2B rectifier bridge that had been in service on 129 91 when the DG 2B failure occurred did not exhibit the same high resistance condition when tested. Therefore, a definitive root cause for these events is not known at this time. Our investigation is continuing in an effort to identify a cause or causes.

4 E. ANALYSIS OF EVENT Operating procedures provide instructions for switching of bridges in case of a failure during emergency conditions. The actual transfer could have been performed in a matter of 510 minutes, had an emergency DC start been made and kVARs exhibited leading values. Additionally, the DGs were not called on to perform their safety rslated function during the period of time involved. Based on these considerations, there was no adverse effect on plant safety or the health and safety of the public as a result of this event. _

t These events are classified as valid failures per Regulatory Guide 1.108 Section C.2.e. -Diesel Generator 2A has experienced one valid failure in the last 20 valid tests and six valid failures in 76 valid tests. The test frequency for DG 2A remains at once per 7 days in accordance with Table 4.8 1 of1the TS. Diesel Generator 2B has experienced one valid failure in the last 20 valid tests and one valid failure in 47 valid tests. The test frequency for DG 2B remains at once per 31 days in accordance with Tabic 4.8 1 of the TS.

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• LICENBEE EVENT REPORT (LER)

, TEXT CONTINUATION (ACIL117 hAN[ (1) 00CLil hvMitR (f) LIA huMBik (6) PALL (3) h AR HU HUM HV VOCTTLE E1II*IRIC CDiERATING PIR(T itiIT 2 05000425 91 003 00 4 0F 4 TEAT F. CORRECTIVE ACTIONS

1. The K4 transfer relays for both DGs 2A and 2B have been replaced and each DG has been demonstrated to be operabic by completion of ,

surveillance procedure 14980 2,

2. The transfer relay contacts on Unit 1 DG 1A have been tested and no problems were found. The transfer relay contacts on DG 1B will be tested during the next monthly surveillance in an effort to gather additional information on the relays.

3. The voltage regulating bridge circuits on each DC (Units 16 2) will be instrumented, prior to the nortral surveillance testing, to record any abnormalities during the upcoming test (s). The number of tests is to be determined by management review. Furthermore, we are continuing to review these events with the vendor. If investigation deterinines a definitive cause of the 129 91 failures, a suppleinental LER will be submitted.

G. AL ITICMAL INFORMATION

1. Failed Components:

Transfer relay manufactured by Deltrol Controla Model #B600

2. Previous Similar Events:

None

3. Energy Industry Identification System Code:

Diesel Generator System EK l

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