ST-HL-AE-2709, Final Special Rept:On 880316,nonvalid Failure Occurred While Performing Operability Surveillance Testing of Standby Diesel Generator 13.Caused by Spurious Actuation of Overcurrent Relay.Relays Replaced W/Relays W/Time Delays

Final Special Rept:On 880316,nonvalid Failure Occurred While Performing Operability Surveillance Testing of Standby Diesel Generator 13.Caused by Spurious Actuation of Overcurrent Relay.Relays Replaced W/Relays W/Time Delays

ML20150F864
Person / Time
Site:	South Texas
Issue date:	07/01/1988
From:	Vaughn G HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
ST-HL-AE-2709, NUDOCS 8807190097
Download: ML20150F864 (4)

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The Light company " " " ' "

llouston Lighting 8: Power .

Jisly 1, 1988 ST-HL-AE-2709 File No.: G02 10CFR50.36(c)(5)

U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 South Texas Project Electric Generating Station Unit 1 Docket No. STN 50-498 Final Special Report Regarding a Nonvalid Failure of #13 Standby Diesel Generator Reference A. Letter ST-HL-AE-2623 dated April 15, 1988 from G. E. Vaughn to the NRC.

B. Letter ST-HL-AE-2489 dated January 28, 1988 from G. E. Vaughn to the NRC.

Pursuant to the South Texas Project Electric Generating Station Technical Specifications 4.8.1.1.3 and o.9.2 Houston Lighting & Power submits the attached Final Special Report regarding a nonvalid failure of the #13 Standby Diesel Generator. The first report of this event was sent to the NRC on April 15, 1988 (reference A). A similar event occurred and is documented in l reference B.

1 If you should have any questions on this matter, please contact Mr. C.A. Ayala at (512) 972-8628.

G. E. Vaughn n~

l Vice President Nuclear Plant Operations l

l GEV/CAA/n1 l

Attachment:

Final Special Report Regarding a l Nonvalid Failure of #13 Standby l Diesel Generator l

8807190097 ACOCK  % hPDC 9e i

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NL.88.104.01 A Subsidiary of liouston Industries Incorporated f [I k

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-liouston Lighting & Power Company July 1, 1988 ST-HL-AE-2709 File No.:G02 Page 2 cc Regional Administrator, Region IV Rufus S. Scott Nuclear Regulatory Commission Associated General Counsel 611 Ryan Plaza Drive, Suite 1000 Houston Lighting & Power Company Arlington, TX 76011 P. O. Box 1700 Houston, TX 77001 George Dick U._S. Nuclear Regulatory Commission INPO Washington, DC 20555 Records Center 1100 Circle 75 Parkway Jack E. Bess Atlanta, Ga. 30339-3064 Resident Inspector / Operations c/o U. S. Nuclear Regulatory Commission Dr. Joseph M. Hendrie P. O. Box 910 50 Be11 port Lane Bay City, TX 77414 Bellport, NY 11713 Don L. Garrison Resident Inspector / Construction

.c/o U. S. Nuclear Regulatory Commission P. O. Box 910 Bay City, TX 77414 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 l R. John Miner (2 copies) l Chief Operating Officer City of Austin Electric Utility 721 Barton Springs Road l Austin, TX 78704 l

R. J. Costello/M. T. Hardt l City Public Service Board P. O. Box 1771 San Antonio, TX 78296 Revised 06/16/88 NL.DIST

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f I' Attachment ST-HL-AE-2709 Page'1.of 2-South' Texas Project Electric ' Generating Station Unit-1 Docke't No. STN-50-498 Final Special Report Regarding-a Nonvalid Failure of #13 Standby Diesel Generator s

DESCRIPTION OF EVENTS:

A nonvalid failure occurred while performing operability surveillance testing on Standby Diesel Generator (SDG) #13 on March 16, 1988.

At approximately 0448 CST on March 16, 1988 a nonvalid failure occurred during

loading of ;SDG #13 with the SDG unit operating in parallel with offsite power.

Following a successful start the operator paralleled the unit with offsite

- power and began adding load. At approximately 90%.fu11 load, the instantaneous directional overcurrent r61ay (67/50H) operated tripping the'SDG-output breaker. The SDG successfully handled the near full load rejection and continued operating at rated voltage. The surveillance test was terminated

. pending investigation'of the relay actuation.

The SDG output current at the time of the trip was approximately 700 Amps with a portion of'this current supplying the loads of 4~16KV switchgear Elc and the remainder flowing back into the system through the current transformer of the 67/50H Relay. The operating setpoint of the 67/50H Relay is 1008 Amps.

Based on.the SDG output current being well below the operating setpoint of the relay and the investigation of a recent similar event on SDG #11 (Refer to letter ST-HL-AE-2489 dated January 28, 1988), instrumentation was connected to the 67/50H Relay to monitor its operating parameters and the operability-surveillance test was repeated.

The'SDG #13 operability surveillance was completed satisfactorily at 0025 on March 17, 1988. The unit was started, paralleled with offsite power, and

' operated under full load conditions for a minimum of one hour in accordance with' surveillance requirements.

It is noted that actuation of the 67/50H Relay.would not have prevented the SDG from performing its safety function. The SDG cannot be paralleled while a a Safety Injection (SI) signal is present. The SDG's are paralleled with offsite power during performance of the monthly operability surveillance in order to satisfy the full load test requirement. The 67/50H relay requires current flow back into the system to be operational, which requires parallel operation of the SDG with offsite power.

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NL.88.104.01

Attachment ST-HL-AE-2709 Page 2 of 2 The SDG remained operable and was available to perform its safety function throughout the event.

ROOT CAUSE:

The nonvalid failure has been attributed to a spurious actuation of the instantaneous _ directional overcurrent relay. The function of the relay is to trip the SDG output breaker in case a Loss of Offsite Power (LOOP) condition occurs while the SOG is operating in parallel with offsite power. Pe rmanent plant instrumentation has verified that the current as seen by the relay was well below its operating setpoint. Discussions with the relay manufacturer following a similar event on SDG #11 revealed that a transient on the D.C.

supply voltage to the relay might cause an inadvertent actuation. A review of the relay application and setpoint performed prior to the event confirmed that the relay was properly applied and set.

CORRECTIVE ACTIONS:

The following corrective actions were established in the special report written on April 15, 1988. (Reference letter ST-HL-AE-2623.)

1. An Engineering evaluation has been initiated to consider modifications to the 67/50H relay circuitry to avoid spurious actuations.

'2. Instrumentation will be connected to the 67/50H relay to monitor A.C.

current and D.C. voltage parameters during performance of the monthly operability surveillance tests on each SDG.

THE RESULTS OF THESE CORRECTIVE ACTIONS ARE AS FOLLOWS:

1. The Engineering evaluation has been completed. Based upon this evaluation, an Engineering Change Notice was initiated to replace the existing 67/50H relays with 67/50D relays which have time delay provisions. This modification will prevent future spurious actuations of the instantaneous directional overcurrent relay during parallel operation of the SDG with offsite power. This design thus enhances system reliability during SDG testing, and maintains the system's designed safety function.

2. Instrumentation was connected to the instantaneous directional overcurrent relay (67/50H) on each SDG during the performance of each monthly operability surveillance test through June 28, 1988. No further spurious actuations were observed.

ADDITIONAL INFORMATION:

Per the criteria of Regulatory Guide 1.108 the failure described in this report has been classified as a nonvalid failure. As such, the current test interval for SDG #13 remains 31 days. There have been twenty-one (21) valid l tests performed on SDG #13 since completion of the 100-hour run test on August 6, 1987 and, to date, no valid f ailures br<e occurred.

NL.88.104.01