05000461/LER-1992-001, :on 920104,failure of B Phase Main Power Transformer Led to Turbine Generator Trip & Reactor Scram. Caused by Internal Fault Leading to Transformer Replacement

:on 920104,failure of B Phase Main Power Transformer Led to Turbine Generator Trip & Reactor Scram. Caused by Internal Fault Leading to Transformer Replacement

ML20092A371
Person / Time
Site:	Clinton
Issue date:	01/31/1992
From:	Rasor S, Spangenberg F ILLINOIS POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-92-001, LER-92-1, U-601929, NUDOCS 9202100084
Download: ML20092A371 (8)

LER-1992-001, on 920104,failure of B Phase Main Power Transformer Led to Turbine Generator Trip & Reactor Scram. Caused by Internal Fault Leading to Transformer Replacement

Event date:
Report date:
4611992001R00 - NRC Website

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' January 31~, 1992 10CFR50.73 Docket No $0 461' Document Control Desk

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- Clinton' Power Station --Unit 1

- kicensee Event Report No. 92 001-00

Dear Sir:

'Please find enclosed Licensee Event Report No. 92 001 00:

Jnternal Fault In The- "B" Phase Main Power Transformer Resulted in A

j Turbine Generator Trio And An Automatic Retietor SCRAM. This report is being;subtnitted.in accordance with the requiressents of 10CFR50,73.

Sincerely yours, F C

F. A. Span enbe

, IIIl Manager, Licensing and afety-

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NRC Resident Office, V-690 NRC' Region III. Re61onal Administrator Illinois Department of Nuclear Safety

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On January 4,1992, with the plant in POWER OPERATION at 99 percent reactor power, the "B" phase main power transformer (MPT 1B) failed'due

=to an internal fault, The transformer failure resulted in a turbine generator trip-and an automatic reactor SCRAM.

The automatic reactor scram occurred due to the turbine control valve fast closure. Vishin seconds of the SCRAM, the turbine driven reactor feed pump (TDRFP) IB tripped.

Additionally, after the reactor SCRAM was reset, a SCRAM-discharge volume (SDV) drain valve failed to reopen and a SDV vent valve L

only opened to an intermediate position, The cause of the SCRAM was an l-internal fault in MPT 1B, The cause of the TDRFP 1B trip was attributed to a worn thrust bearing, and the cause of the SDV vent and drain valves j'

failing to reopen was. attributed to air-leakage past the seat of a three-way solenoid. valve, Corrective actions for this event include replacing I

the failed MPT with a spare MPT, restoring the TDRFP 1B thrust. bearing clearance to original manufacturer specifications, and rebuilding the l

three-way solenoid valve,

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we i., aw., nn DESCRIPTION-0F-EVENT On January _4, 1992, at 0217 hours0.00251 days <br />0.0603 hours <br />3.587963e-4 weeks <br />8.25685e-5 months <br />, with the plant at 99 percent reactor power-in Mode 1 (POWER OPERATION), an-internal fault in the "B" phase main power transformer (MPT IB) [XFMR) occurred result'.pg in a turbine generator [TO) trip'and a fast closure of the turbine control valves which caused an automatic reactor [RCT) SCRAM. Within seconds of the reactor SCFAM,. turbine driven reactor feed pump (TDRFP) [P) IB tripped.

At :218 hours, control room operators reset the SCRAM logic.

I By 0220 hours0.00255 days <br />0.0611 hours <br />3.637566e-4 weeks <br />8.371e-5 months <br />, main control room personnel noted on the sequence of events [IQ)-printout that the MPT 18 sudden pressure trip relay had actuated.

Control room operators also verified that the SUDDEN PRESSURE MPT 1B annunciator was alarming.

- By 0225 hours0.0026 days <br />0.0625 hours <br />3.720238e-4 weeks <br />8.56125e-5 months <br />, the opera.cors had determined that the automatic deluS6 system for.MPT 1B and MPT 10 had initiated. Operators were dispatched dnd did not= observe any_ fire on MPT 1B or on MPT IC.

At 0310 hours0.00359 days <br />0.0861 hours <br />5.125661e-4 weeks <br />1.17955e-4 months <br />; non-licensed operators isolated the deluge system for MPT IB and MPT IC to facilitate access to the lockout relays [86] and to allow the fire pumps to be restored to standby.

lBy0323 hours,controlroomoperatorsnotedthatSCRAMDischarSeVolume (SDV)--drain valve [V) IC11-F011 had not reopened and that SDV vont valve

- IC11-F010 indicated an intermediate position.

- By _0325 hours, a non-11cer :sd operator had restored the horizontal fire pump (0FP03P) to standby status.

By 0335 hours0.00388 days <br />0.0931 hours <br />5.539021e-4 weeks <br />1.274675e-4 months <br />, a non licensed operator had restored the "B" G rc pump (0FP01PB)'to standby status.

At 0436 hours0.00505 days <br />0.121 hours <br />7.208995e-4 weeks <br />1.65898e-4 months <br />,-'a non-licensed operator manually opened valve 1C11-F001 to drain the SDV.

- At 0441 hours0.0051 days <br />0.123 hours <br />7.291667e-4 weeks <br />1.678005e-4 months <br />,-a non-licensed operator returned the handwheel for valve' -

11011-F011 to:the neutral position,-and valve ICll-F01_1 immediately returned to the closed position.

Since the SCRAM logic had been_ reset, i

valve 1011-F011-should have remained open.

-.By-0535 hours; theLfire protection deluge system for MPT 1B and MPT IC had been restored to standby condition.

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0l0 l1 0l0 0l 3 0F ;0l7 nxTw=.a w...m weci wv4m No other automatic or manually initiated safety system responses were necessary to place the plant in a safe, stable condition, No other-equipment or components were inoperable at the start of this event such.

that their inoprable condition contributed to this event.

CAUSE OF THE EVENT

The cause of the evene is due to an internal fault in MPT 1B.

Illinois Power (IP) is evaluating if the failed transformer:should be shipped to a repair shop t. have a failure analysis performed to determine, if possible,. the cause of the internal fault.

IP will submit a supplemental report if the cause of the failure is identified and if the cause of the failure significantly changes the information provided in this report.

The.following paragraphs discuss the equipment problems that occurred subsequent to the reactor SCRAM:

TDRFP 1B Trin Troubleshooting conducted on:the TDRFP 1B indicated that the most probable cause af the TDRFP trip was due: to -a worn thrust bearing on the feed pump. A check of the feed pump thrust bearing clearance indicated that-the clearance was larger-than the-r

. clearance specified by the manufacturer.

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. increased feedwater demand resulted in an increased thrust on the TDRFP thrust bearing. This increased thrust, coupled with a larger-than specified thrust beatin5 clearance,.is suspected to have caused.the. thrust bearing wear trip to actuate, The increased feedvater demand resulted from-the set point set down-logic initiating..The set-point set down logic initiated due to the reactor water low level 3 condition, which normally occurs-after a reactor SCRAM. The set-point set-drwn. logic causes:a

larger demand for feedwater for about-ten seconds, followed ty-a

' demand decrease. This-logic is designed to minimize the reactor level decrease that occurs after a reactor SCRAM due to the collapse of voids,

Egilure-of the SDV Vent Valve and Drain Valve To Open Subsequent to the; event, troubleshooting of the SDV drain and vent valves identified the following deficiencies (refer to Figure 1):

Air leaked out of the vent ports of the three way solenoid

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Valve IC11-F011 immediately c;. ied and remained open when the instrument air time delay valve (1C11-F159A) was manually opened about one-eighth of a turn.

A~small piece (less than 1/16 inch) of foreign material (belisved to be solder) was found between the backseat anc

- disc of valve IC11 F009.

The bench settings for valve IC11-F010 were found to be 6 and 30 pounds per square inch gauge (psig).

Bench settings of 6 and 30 psig means that the valve starts to open at 6 psig and is fully open or backseated at 30 psig.

The correct opening bench settings for IC11-F010 are 18.5 and 60 psig.

The bench settings for valve IC11-F011 were found to be 20 and 62 psig. The correct. bench settings for IC11 F011 are 18.5 and 60 psig.

- Maintena'nce noted that the SDV vent valve (IC11 F180) operated erratically when stroked repeatedly.

The valve-stem was found to be bent and galling was noted on the disc and stem. Additionally, the bench settings for IC11-F180 were found to be 8 and 25 psig whereas the correct bench settinga are 3 and 15 psig.

l Based on the deficiencies identified above, the cause of valve L

IC11-F011 failing to open and valve IC11-F010.only opening to.an-intermediate position 'is attributed to the air leakage past the seat of valve 1011-F009.

In May of 1991, personnel discovered that

- valve 1C11LF011 took approximately 17 minutes to reopen, and Maintenaner Work Request MVR D17319 was written to address the slow -

reopening time. However,-MVR=D17319 was not scheduled to be worked until'the next refueling outage, currently scheduled to begin March c

1992.

Based on the. findings noted above, the slow opening time of L.

. valve IC11 F011 noted in May 1991 is attributed to-the small-amount i

of air leaking past the seat of valve IC11-F009 and out the vent port.

In May of 1991, valve 1C11 F1594 provided sufficient air volume-to eventually pressurite the air header enough to open valve s

1C11-F011', but the leakage past valve 1C11-F009 continued to p

increase. -Valve 1C11-F159A is a throttle valve which creates a L

time delay for the closing'of valver IC11eF011-and 1011-F010.

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January, due to the. increased leakage past the seat'of valve IC11--

F009,-valve IC11 F159A at the throttled position-was not capable of supplying-enough air volume to pressurize the air header

completely.. Therefore, due to the variance between the bench settings of valves 1C11-F010 and 1C11-F011 and the reduced pressure in the air header, valve 1C11-F010 opened to an intermediate g.onu m..

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CORRECTIVE ACTION

MPT IB was replaced with a spare MPT undar MVR D10238.

- The-following corrective actions address the equipment problems which occurred subsequent to the reactor SCRAM:

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TDRFPlB The TDRFP 1B thrust bearing was machined and shims were added to restore the thrust clearance to be within original manufacturer specifications per MVR D28271.

SDV Vent and Drain Valves

- Valves 1C11-F009 and 1C11-F182 have been rebuilt per preventive maintenance tasks-(PM) PMMRDM003 and FMMRDM004 In addition, the bench settings for valves 1C11-F010, IC11 F180, and 1C11 F011 were reset to their correct values, and valve IC11-F180 was rebuilt per MWR D17319.

The Nuclear Station Engineering Department (NSED) has performed a review to determine. if similar applications exist where an ietstrument air throttle valve is used to perform a time delay for the opening or closing of air operated valves. There were no other-applications ~found, _ Additionally, the five year PMs.to rebuild valves 1C11-F009 and IC11-F182 are being evaluated to determine if they are adequate and if an increased frequency is needed. -NSED will also evaluate the need for a routine PM to check the. bench-settings on valves 1C11-F010, IC11-F011, IC11-F180,-and 1011-F181.

Maintenance vill blow down the air lines which supply _ air to the SDV vent aid drain valves. These actions are scheduled to be-completed by May 30, 1992.

Finally, to determine if system enhancements are needed, !"'ED will evaluate alternative means of providing the opening an.. Losure delay for the SDV vent and drain valves. - This action is % - ed to_ be completed by July 30, 1992.

Station procedure 9012.01 " SCRAM Discharge Volume Vent and Drain

- Valve Operability Test" will be revised to ensure that the opening and closure times for valves 1C11-F011, IC11-F010, IC11-F180 and' IC11 F181 are recorded and verified against the design criteria.

This action is scheduled to be completed by May 30, 1992.

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h ANAINSIS OP EVENT This event is reportabic under dia provisions of 10CTRSO.73(a)(2)(iv) due to the automatic initiation of t waactor Protection System lJC).

Assessment of the safety consequer.cca and inoplications of this event indicates that this event was not nuclear safety significant. This, i

transient van ecmpared to a cin.dar transient, " Generator Load Rejection with Ilypass", described in Chapter 1$ of the Updated Safety Asalysis l

Report and the Transicut Garety Analysis Design Report (CEZ 7355).

The event was found (- be within the design brais of the plant.

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ADDITIONAL INFORMATION

i The failed MPT in a Ceneral Electric suodel 390E648, single phase, 310/347 MVA, forced oil and air, 60 liertz, with a low voltage side of 22,000 V and a high voltage side of 34$,000 v.

LEE 88 028-00 discussed.4ruiti transfortner fault which resulted in a turbine generator trip and a reactor serarn.

LER 89 028 00 discussed the failure of a sudden press'tre relay on HPT which resulted in a turbine generator trip and a reactor SCRAM, For further infortsation r.'gardin6 this event, contact. S. E. Rasor, Director Plant Maintenance, extension 3204.

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