ML20082T212

Special Rept:On 910820,EDG Secured During Surveillance Test Due to Power Dropping Resistor Failure.Power Dropping Resistor Assembly Replaced.Health & Safety of Public & Plant Personnel Not Affected by This Event

ML20082T212
Person / Time
Site:	Waterford
Issue date:	09/13/1991
From:	Packer D ENTERGY OPERATIONS, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
W3B5-91-0240, W3B5-91-240, NUDOCS 9109180151
Download: ML20082T212 (5)

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W3BS-91-0240 A4.05 QA September 13, 1991 U.S. Nuclear Regulatory Commission -

ATTENTION: Document Control Lesk Washington, D.C. 20555

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SUBJECT:

Waterford 3 SES .

P. . Docket No. 50 '82 g' / . -

- License No Nr 38 4 Reporting of Special Report Gentlemen:

Attached is Special Report . umber SR-91 006-00 for Waterford Steam Electric Station Unit 3. This Special Report is submitted per 10CfR50.36(c)(2) and Technical Specifications 4.8.1.1.3 and 6.9.2.

Very truly yours,

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0;F. Packce j General Manager Plant Operations N DFP/WEF/rk Attachment

-cc: Messr: R.D. Martin G.L. Florreich J.T. When'.ock - INP0 Records Center E.L. P'.ake N.C. Reynolds NRC Resident Inspectors Office 1

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EEECIAL RERQRI SR-91-006 '

EMERGENGLDIESEL CENERATOR SECERED DURING SURVEILLANCE TEST DUE TO POWER DROPPING RESISTOR FAILURE ,

REPORTABLE OCCURRENCE On August 20, 1991, at 1J38 hours, Waterford 3 Emergency Diesel Generator (EDG) "A" was started to perform an EDG operability verification in accordance with Operations Procedure (OP) 903-068, Surveillance Procedure-Emergency Diesel Generator Operability Verification. Within seconds following a successful start, the diesel generator speed started to slowly decrease.

Shortly after frey ,cy decreased to fifty-five hertz, voltage began to decrease. The nuclear auxiliary operator conducting the test reported that i the generator speed started to fluctuate during the test, dropping as low as 280 revolutions per minute (rpm). Seconds after the voltage decrease, the EDG

- surged, and speed, voltage and frequency returned c- normal operating parameters. After a few reconds, the EDG repeated a similar sequence of fluctuations. The fluctuations were the result of a failure in one of the two

• voltage dropping resistors in the power supply to the generator's electric governor speed control unit. In the power supply circuit for the electrical governor is a resistor assembly that functions to reduce the 125 volt direct current (DC) system voltage to 24 volts DC. The resistor assembly aias two parallel, three hundred ohm, seventy watt resistors. When a resistor fails open, the higher voltage drop on the remaining resistor results in a lower  ;

reference voltage at the amplifier. Therefore, as the generator approaches f

operating speed, the lower reference voltage produces a high amplifier output voltage, which causas the EDG to lose speed. After observing unother similar ,

cycle of fluctuations, the nuclear auxiliary operator secured t.se EDG, Technical Specification (TS) 3.8.1.1, limiting condition for speration (LCO),

was entered at 1339 hours0.0155 days <br />0.372 hours <br />0.00221 weeks <br />5.094895e-4 months <br />.

The EDG "A" problem qual;fies as a valid failure as defined in Regelatory  ;

Caf.de 1.106, the second in the last twenty tests on the "A" train EDC. In accordance with (TS) 3/4.8.1, A.G. Sources-Operating, ar.d Table 4.8-1, Diesel Generator Test Schedule, test frequency is to be increased from once every l thirty-one days to once every seven days.

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i After troubleshooting the EDG, replacing the power dropping resistor assembly, and testing EDG "A", TS 3.8.1.1. LCO was exited at 2126 hours0.0246 days <br />0.591 hours <br />0.00352 weeks <br />8.08943e-4 months <br />.

1 Waterford 3 issued a letter to the U.S. Nuclear Regulatory Commission on August 27, 1991, requesting that the power dropping resistor assembly failure ,

on EDG "A" not bo classified as a valid failure. This request was based on the determination that the problem with the power dropping resistor assembly i was known to the nuclear industry and that the assembly was being redesigned. l In January of 1991, Waterford 3 placed an order for the new design assembly, before it was available to the industry, in order to ensure receiving the new designed assembly as soon as it was available. Classifying the last failure l -on the EDG as a valid failure would increase the frequency of testing and add ,

1 unnecessary wear to the EDCs.

t IHIIIAL 00NDITIOPE Plant Power - 100%

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l Procedure being performed - OP-903 068, Surveillance Procedure-Emergency  ;

Diesel Generator Operabil: :y Verification  ;

\ t Major Equipment Out of Service - No major equipment out of service that

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impacted, or was impacted by the EDG problem.

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i CVENT SEQUERGE dator Occurrence Chronology All events occurred on August 20, 1991.

-1338 Started EDG "A" per OP-903 068, EDG Opetability Verification

• 1339 Secured EDG "A" due to e normal fluctuations Entered ~5 3.8.1.1 i,CO 1412 Completed OP 903 066, Electric Breaker Alignment Check, per TS 3.8.1.1 LCO, action b 1505 Started EDG "A" for troubleshooting  ;

-1507. Secured EDC "A" j 1537 Started EDG-"A" for trcubleshooting  !

1547 Secured EDO "A"

-1554 EIX) "A" was determir,ed to be available since 1537 hours0.0178 days <br />0.427 hours <br />0.00254 weeks <br />5.848285e-4 months <br /> af ter prope:- ,

operation of the EDG using the mechanical governor

-1840 Started EDG "A* for troubleshooting 1845 Glosed-EDG "A" output breaker, loaded to approximately 2 HW . -

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- 1853 Opened.EDG "A" output breaker

-1859 Secured-EDC "A"

-1907 Started EDG "A" per OP-903 068 l

-1910 Parallel EDG "A" with 3A3 S i -- 1916 EDG "A" loaded to 4600 KW

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-2025 Unicaded EDG "A" and opened t'e breaker to 3A3-S l

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-2035 Secured EDG "A"

-2044 Placed EDG "A" in stand by ,

-2125 Completed OP 903-068 for EDG "A", test satisfactory 2126 Declared EDC ".$" operable, Exit TS 3.8.1.1 LCO l- ,

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EQQLGidlSE The root cause of this event was failure ne power dropping resistor in the electric governor circuitry. When a resistor fails open, the higher voltage drop on the remaining resistor results in a lower reference voltage at the amplifier. Therefore, as the generator approaches operating speed, the lower referenen voltage nroduces a high amplifier output voltage, which causes the EDG to lose speed.

CORRECTIVE ACTIONS The power dropping resistor assembly was replaced on EDG "A" with a redesigned unit during troubleshooting. The new unit is designed to be ' fail safe';

therefore, if the resistor fails open, the speed of the EDG will increase and the mechanical governor will maintain EDG speed control, The EDG "B" power dropping resistor assembly was replaced on September 10, 1991, and was tested satisfactorily. Previous to the EDG "B" resistor assembly replacement, Electrical Maintenance monitored the old assembly on a daily basis, SAFETY SIGNIFICANCE The Waterford 3 Final Safety Analysis Report concerning the onsite pawer system is based on a single failure of two EDGs, Since there was one EDG operable at all times, the health and safety of the public and plant personnel was not af fected t / this event.

PREVIOQS_Ey::"IS Special Repu_ 002, reported a valid failure of the EDG due to a crankcase overpressurization. The root cause was not positively identified but all possible root causes identified were investigated and corrective actions performed.

Special Report 86-004, reported a valid failure of the EDG due to the power dropping resistor assembly. The assembly was replaced and was determineet to be an isolated failure.

Plant Contact W.R. Brian, Plant Engineering Superintendent, (504) 464 3127

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