ML19324C161

LER 89-020-00:on 891011,inverter Which Feeds Channel IV Class IE Vital Ac Distribution Panel Failed Causing ESF Actuations in Control Room.Caused by Failure of Bridge Rectifier Circuit.Inverter repaired.W/891110 Ltr

ML19324C161
Person / Time
Site:	South Texas
Issue date:	11/10/1989
From:	Ayala C, Chewning R HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-020-01, LER-89-20-1, ST-HL-AE-3290, NUDOCS 8911150093
Download: ML19324C161 (6)

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November 10, 1989  ;

ST-HL-AE-3290 File No.: G26 l 10CFR50.73 l i

U. S. Nuclear Regulatory Commission Attention: Document Control Desk  ;

Washington, DC 20555 >

South Texas Project Electric Generating Station Unit 1 Docket No. STN E0-498  ;

Licensee Event Report 89-020 Regarding Unplanned Engineered Safety Features Actuations Due to an Inverter Failure Pursuant to 10CFR50.73 Houston Lir,hting & Power (HL&P) submits the i attached Licensee Event Report 89-020 regarding an unplanned Engineered Safety

• Features actuations due to an inverter failure. This event did not have any adverse f.rnpact on the health and safety of the public. 3 If you should have any questions on this matter, please contact Hr. C. A. Ayala at (512) 972-8628.

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R.' W. Chevning [

Vice President l Nuclear Operations ,

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Attachments LER 89-020, South Texas, Unit 1 ,

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( cet Regional Admissistrator, Region IV Rufus S. Scott Nuclear Regulatory Commission Associate General Counsel 61,1 Ryan Plaza Drive Suite 1000 Houston Lighting & Power Company Arlington, TX 76011 P. O. Box 1700 Houston, TX 77001 l George Dick, Project Manager U. S. Nuclear Regulatory Commission INPO Washington, DC 20555 Records Center 1100 Circle 75 Parkway J. 2. Tapia Atlanta, GA 30339-3064 i Senior Resident Inspector

c/o U. S. Nuclear Regulatory Commission Dr. Joseph H. Hendrie P. O. Box 910 50 Be11 port Lane Bay City, TX 77414 Be11 port, NY 11713 R. J. Evans D. X. Lacker Resident Inspector Bureau of Radiation Control c/o U. S. Nuclear Regulatory Commission Texas Department of Health P. O. Box 910 1100 West 49th Street Bay City Texas 77414 Austin, TX 78756-3189 J. R. Newman, Esquire Newman & Holtzinger, P.C.

1615 L Street, N.W.

Washington, DC 20036 R. L. Range /R. P. Verret Central Power & Light Company P. O. Box 2121 Corpus Christi, TX 78403 R. John Miner (2 copies)

Chief Operating Officer City of Austin Electric Utility 721 Barton Springs Road 1

Austin, TX 78704 R. J. Costello/M. T. Hardt City Public Service Board P. O. Box 1771 San Antonio, TX 78296 Revised 11/08/89 NL.DIST

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.mi n ac, ,o ,. .m .. . .., . 4. n . . . n ei On October 11, 1989, Unit I was in Mode 3 prior to restart froio a refueling outage. At approximately 1533 hours0.0177 days <br />0.426 hours <br />0.00253 weeks <br />5.833065e-4 months <br />, the inverter which feeds the Channel IV Class IE Vital AC distribution panel failed. This caused Engir etired Safety Features actuations of the Control Room, Reactor Containment Building and Fuel Handling Building HVAC eystems due to lose of power to their respective radiation monitors. The cause of this event was f ailure of a bridge rectifier circuit on the inverter DC to DC converter board. The circuit appeared to have failed due to excessive output voltage over an extended period of time which overheated the components. The inverter has been repaired and the DC to DC converter voltage adjusted. Preventive maintenance procedures will be revised to require periodic DC to DC converter board output voltage adjustments. Inverters will be added to the existing plant thermography program which will assist in identifying equipment that could be approaching a similar failure.

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On October !!, 1989. Unit I was in Mode 3 prior to restart from a refueling outage. At approximately 1533 hours0.0177 days <br />0.426 hours <br />0.00253 weeks <br />5.833065e-4 months <br />, power was lost to the Class 1E 120 Volt AC vital distribution panel DP002. This caused Engineered Safety Features (ESF) actuations of Control Rocm, Reactor Containment Building and Fuel Handling Building HVAC systema due to loss of power to their respective i radiavtan monitors. Once it waa determined that the actuations occurred as a  !

result of the power failure, oporations personnel took action to restore power to panni DP002 at 1552 hours0.018 days <br />0.431 hours <br />0.00257 weeks <br />5.90536e-4 months <br /> and restore the HVAC systems to their normal mode.  ;

Inspection of the inverter which supplies panel DP002 revealed a failed output bridge rectifier on the DC to DC converter board. This board supplies power to various inverter circuit cards. As a result of a previous failure on a similar DC to DC converter card on another inverter, a corrective action was.

identified which required periodic measurement of output voltages and adj ustment if necessary. Heasurements were taken of the DC to DC converter board output voltace in January of 1989 per the instrue tions of a preventive ,

maintenance procedure. At that time, the voltage was oetermined to be high and a Work kequest was written to adjust the voltage to its correct value. i This adjustment was performed on October 11, 1989, approximately four hours prior to the board failure. The maintenance and engineering personnel {

performing the adjustment noted evidence of overheating of some of the board's i components. This finding was being evaluated and board replacement was being considered at the time of the failure.

Cg1SE OF EVENT:

i The most likely cause of this event has been identified as excessive output voltage of the inverter DC to DC converter board for an extended period of time which resulted in heating and preinature f ailure of the output bridge  ;

rectifier. This problem may have been aggravated when the board was disturbed to perform the output voltage adjustment earlier that day, i

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ANALYSIS OF EVENT:

This event resulted in the unplanned Engineered Safety Features actuations of the Control Room, Fuel Handling Building and Reactor Containment Building HVAC systems. Had a radiological release occurred during this event, these systems would have already been in their safest mode. While any unnecessary challenge to an ESF system is undesirsble, actuation of these systems represunts a minimal hazard since it could not cause, exacerbate or prevent mitigation of an accident.

Unplanned actuation of an ESF system is reportable pursuant to 10CFR50.73(a)(2)(iv).

CORRECTIVE ACTION:

The following corrective actions are being taken as a result of this event:

1. The inverter DC to DC converter board has been replaced, its voltage adjusted and the inverter has been returned to service.

2. The other similar inverters have been checked for proper DC to DC converter board output voltage. They have also been subjected to thermographic examination which will assist in detecting excessive component temperatures.

3. The preventiva maintenance procedures for similar inverters will be revised to include instructions to adjust the DC to DC converter board output when it is found out of tolerance in lieu uf only measuring this output. This will assure that DC to PC converter board output voltage is l promptly adjusted when needed. This action will be completed by December 18, 1989.

4. periodic inspection of inverters will be added to the plant thermography program by December 31, 1989. This will assist in identifying emponents which are experiencing e;teessive heating prior to f ailure.

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ADDITIONAL INFORMATION: i i

The failed inverter is a 25 KVA, single phase, 120 volt AC unit manufactured l

by Elgar Corporation. The model number is INV 253-1-105.

Two previous events have been reported regarding the failure of components on -

I the DC to DC converter board of this same model of inverter:

L LER 88-021 (Unit 1) reported an inverter failure which caused numerous  !

EST actuations.

LER 89 023 (Unit 2) reported a non-safety related inverter which failed 6 and caused a turbine trip and reactor trip. ,

In each case the specific component which f ailed on the DC to DC converter i board has been different.  ;

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