05000348/LER-1995-001, :on 950113,reactor Trip Occurred Due to Loss of Turbine DEHC Overspeed Protection.Root Cause Analysis Investigation Was Performed.Mods Performed to Reduce Vulnerability of OPC Processor

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:on 950113,reactor Trip Occurred Due to Loss of Turbine DEHC Overspeed Protection.Root Cause Analysis Investigation Was Performed.Mods Performed to Reduce Vulnerability of OPC Processor
ML20080G876
Person / Time
Site: Farley 
Issue date: 02/06/1995
From: Hill R, Dennis Morey
SOUTHERN NUCLEAR OPERATING CO.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-95-001, LER-95-1, NUDOCS 9502140136
Download: ML20080G876 (4)
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LER-1995-001, on 950113,reactor Trip Occurred Due to Loss of Turbine DEHC Overspeed Protection.Root Cause Analysis Investigation Was Performed.Mods Performed to Reduce Vulnerability of OPC Processor
Event date:
Report date:
3481995001R00 - NRC Website
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Southern Nuclear Operating Company Post Offic3 Box 1295 B;rrmngh:m, Alibzma 35201 Telephone (205) 868-5131 4

Southern Nudear Operating Company o.ve morey Vice President Farfey Project the Southem electnc System February 6, 1995 Docket No.: 50-348 10 CFR 50.73 t

t U. S. Nuclear Regulatory Commission ATrN: Document Control Desk Washington, DC 20555 Joseph M. Farley Nuclear Plant-Unit 1 Licensee Event Report No. 95-001-00 i

Reactor Trip Due To Loss of Turbine Digital Electro-Hydraulic Control Overspeed Protection j

Gentlemen:

i Joseph M. Farley Nuclear Plant Licensee Event Report No. 95-001-00 is being submitted in accordance with 10 CFR 50.73. If you have any questions, please adsisc.

Respectfully submitted, j

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$l} }71ru Dave Morey, REM / cit:lttbdchl. doc Enclosure cc: Mr. S. D. Ebneter Mr. B. L Siegel i

Mr. T. M. Ross i

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DOCKET NUMBER (2)

PAqFft Joseph M. Farley Nuclear Plant - Unit 1 0l5l0l0l0l3l4l8 1 OF 3

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Reactor Trip Due To A Loss ofTurbine DEHC Overspeed Protection EVENT DATE (5)

LER NUMBER (6)

REPORT DATE (7)

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OPERATING MODE (S)

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LICENSEE CONTACT FOR THIS LER (12)

N1.ME TELEPHONE NUMBER

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R. D. Hill, General Manager - Nuclear Plant 3l314 819191-15l11516 COMPLETE ONE LINE FOR EACH COMPONENT F ALLURE DESCRIDED IN THIS REPORT (13)

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At 0758, on January 13, 1995 with Unit 1 in mode 1 operating at 100 percent power, the reactor tripped due to a turbine trip in response to a loss of digital electro-hydraulic control (DEHC) overspeed protection. The primary overspeed protection (OPC) controller (DROP 2) failed in a manner which prevented the redundant OPC controller (DROP 52) from assuming control of the turbine DEHC overspeed protection function within the designed time frame. This resulted in a turbine trip due to a loss of both OPC controllers.

An investigation determined this event was most likely caused by the response of the DEHC OPC processors and their associated power supplies to minor voltage transients. An evaluation of the DEllC system design indicated a vulnerability of the DEHC system to minor voltage transients.

Modifications have been performed to the DEHC system in order to reduce the vulnerability to minor voltage transients.

The unit was returned to power operation at 2203 on January 17,1995.

NAC Form 366 ($49)

e IIR POftm 30eA U.S.NUCLSAR REOULATORY COGNIISeloN APPRO No 50 0104

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Joseph M. Farley Nuclear Plant - Unit 1 l

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Plant and System Identification

t Westinghouse -- Pressurized Water Reactor Energy Industry Identification System codes are identified in the text as [XX).

Description of Event

At 0758, on January 13, 1995 with Unit 1 in mode 1 operating at 100 percent power, the reactor tripped due to a turbine trip in response to a loss of digital electro-hydraulic control (DEHC) [JJ) overspeed protection.

Cause of Event

A root cause investigation indicated that within the DEHC system, the primary overspeed protection (OPC) controller (DROP 2) failed in a manner which prevented the redundant OPC controller (DROP 52) from assuming control of the DEHC overspeed protection function within the designed time frame. This resulted in a turbine trip due to a loss of both OPC controllers.

On-site testing identified a vulnerability of the DEHC power supplies and associated OPC processors to minor voltage transients. A DEHC system configuration specific to Farley Nuclear Plant was assembled in a test system at the vendor's facility. An evaluation of the test results conducted at the vendor's facility confirmed the on-site test results.

These tests concluded that under certain conditions the DEHC system's response to minor voltage transients would result in an inability to transfer OPC control from DROP 2 to DROP 52 within the designed time frame. This would result in a turbine trip due to a loss of both OPC controllers.

Safety Assessmet11 This event is reportable because of the actuation of the reactor protection system.

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All systems operated as designed.

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This event would not have been more severe ifit had occurred under different operating conditions.

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.Cprrective Action

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A root cause analysis investigation was performed.

I Modifications have been performed to the Unit I and Unit 2 DEHC systems which have reduced the l

vulnerability of the DEHC OPC processors and their associated power supplies to minor voltage transients.

l AdslitionalInformation The turbine's mechanical overspeed trip device, which is the turbine's primary overspeed trip protection, was operable during this event.

Additional DEHC system modifications are planned to further enhance the reliability of the DEHC system.

A temporary recorder was installed to monitor DEHC system voltage. To date, no unexpected l

voltage transients have been observed.

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The unit returned to power operation at 2203 on January 17,1995.

Further review of data associated with the December 25,1994 Unit 2 reactor trip (LER 94-004, l

Unit 2) indicates that both the 12/25/94 Unit 2 trip and the 1/13/95 Unit I trip could have been caused by the response of the DEHC OPC processors and their associated power supplies to minor voltage transients. Modifications to Unit 2 DEHC system were completed on January 30,1995, to reduce the vulnerability to the minor voltage transients.

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The following LER's involved reactor trips associated with DEHC system failures.

Reactor trip due to a loss of turbine DEHC overspeed protection: LER 94-004, Unit 2.

l Reactor trip due to turbine control system intermittent failure: LER 94-003, Unit 2.

j Manual reactor trip due to governor valve closure caused by a degraded DC voltage output i

from the primary Operator Auto Controller power supply iri the main turbine DEHC system l

and a failure of the circuitry which should have transferred the power supply: LER 91-010, Unit 1.

Reactor trip caused by a voltage transient on the DEHC inverter: LER 89-015, Unit 2.

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