05000362/LER-1993-001, :on 930116,main Generator Tripped on Stator Ground Protection Signal Causing Turbine/Reactor Trip.Caused by Water Intrusion.Addl Sealing Provided for Main Generator Terminal Encl to Minimize Water Intrusion

:on 930116,main Generator Tripped on Stator Ground Protection Signal Causing Turbine/Reactor Trip.Caused by Water Intrusion.Addl Sealing Provided for Main Generator Terminal Encl to Minimize Water Intrusion

ML20134B297
Person / Time
Site:	San Onofre
Issue date:	02/16/1993
From:	Krieger R SOUTHERN CALIFORNIA EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-93-001, LER-93-1, NUDOCS 9302230133
Download: ML20134B297 (5)

LER-1993-001, on 930116,main Generator Tripped on Stator Ground Protection Signal Causing Turbine/Reactor Trip.Caused by Water Intrusion.Addl Sealing Provided for Main Generator Terminal Encl to Minimize Water Intrusion

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(1) 10 CFR 50.73(a)(2)(ii) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(i)
3621993001R00 - NRC Website

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e cu Southam Califomia Edison Company SAN ONOF HC NUCLE AM OCNt RATING STAYlON P. O. BOX t es LAN CLEMENT E. CALWORNIA92074-0128 N. W. KfVL GE Ft TEttesowy February 16, 1993 U. S. Nuclear Regulatory Commission Document Control Desk Washington, D.C.

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Subject:

Docket !Jo. 50-362 30-Day Poport Licensee Event Report No.93-001 San Onofre Nuclear Generating Station, Unit 3 Pursuant to 10 CFR 50.73 (d), this submittal provides the t' squired 30 day written Licensee Event Report (LER) for an occurrence involving an automatic reactor trip. Neither the health nor the safety of plant personnel or the public was affected by this occurrence.

If you require any additional information, please no advise Sincerely,

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Enclosure:

LER Ho.93-001 cc:

C.

W.

Caldwell (USNRC Senior Resident Inspector, Units 1, 2 and 3)

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B.

Kartin (Regional Administrator, USNRC Region V)

Institute of Nuclear Power Operations (INPO) 22009 4 fp maamat2%2 r

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LICENSEE EVENT RtTORT (LER) f5Tility Name (1) lDocketkwber(2)

_Page v3) _

SAN ONOFRE NUCLEAR GINfDATING STAftDN UNIT 3

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AUTOMATIC TURBlNE/ REACTOR TRIP DUE TO MAIN GENERATOR STATOR GROUND PROTECTION ACTUATION CAUSED BY WATER INTRUSION EVENT DAff M T LER NUMi>ER (6)

REFoki DATE (7) 01TTR71TiflilES INVOLVED (8)

Month ay Year Veer l///5equen'ial

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Other (Specify in 73.71(c)

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20.405(a)(1)(v) 50.73(a)(2)(iii) __ 50.73(a)(2)(i)

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Ye g lKitTsic. (Lys. cospete 9PL:TED SUBMISSION DATE) timit to 141i0 spaces, i.e.,

upproximately fifteen single-space types-itten lines) (16)

On January l' 1993, Unit 3 was operating at 75% power with a 'neavy rain storm occurring.

at 1641, the hiain Generator tripped on a stator ground protection signal, causi' turbine / reactor trip.

The plant was stabilized in Mode 3 at normal tempel. are and pressure at approximately 1715, Rain water leaked down from the turbine deck into the main generator terminal enclosure, which containu the transition from the main generator conductors'to the isophase bus.

The water accumulated on some insulating material and provided a low resistance path from the exposed B phase conductor ta ground, actuating a Main Generator / Main Turbine trip signal.

Additional sealing wes provided for the Main Generator terminal enclosure to minimize the possibility of rain water intrusion, and the insulating material was replaced.

SCE will evaluate whether additional measures can be taken to minimize the possibility of l

rain water intrusion into the Main Generator components.

The e in no safety significance to this event since all Reactor Protection System compenents operated ao designed.

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.O LICENSEE EVENT REPORT (LER) TEXT CONTINUATION SAN ONOFRE NUCLEAR GENERATION STATION DOCKET NUMBER LER NUMBER PAGE UNIT 3 05000362 93-001-00 2 of 4 Plant: San Onofre Nuclear Generating Station Unit: Three Reactor Vendor: Combustion Engineering Event Date: 01-16-93 Time: 1641 A.

CONDITIONS AT TIME OF THE EVEITT:

Mode:

1, Power Operation (75% Power)

B.

BACKGROUND INFORMATION

1.

Main Generator:

The Main Generator (TB, GEN] at San Onofre is located on the turbine deck which is not an enclosed structure.

The Main Generator is protected by-relaying devices

[. LY] designed to detect abnormal conditions, including R

stator grounds.

The stator ground trip circuit protects the generator against a ground en the stator or on equipment connected to any of the stator phases (e.g.,

generator terminals, which are located in a terminal enclosure below the Main Generator),

Upon sensing a ground, a stator ground protection signal is initiated to trip the Main Generator and the Main Turbine [TRB).

The Main Generator terminal enclosure is fabricated from aluminum and f.iber board and is bolted between the underside of the Main Generator and the top of the Isophase Bus (EA, IPBU),

The enclosure contains six terminal bushings (one for each phase and three connected to the neutral ground).

The bushings connect each phase of the Main Generator stator windings to the respective output conductors of the Isophase Bus.

The Isophase Bus conducts the power produced by the Main Generator to the main and unit auxiliary transformers. [XFMR]

The transition from the Main Generator output conductors to the isophase bus _is contained-within the Main Generator terminal enclosure.

2.

Reactor Protection System (RPS) (JC]-

When above 55% reactor power, a Main Turbine trip causes the RPS to initiste a Loss of Load signal to trip the reactor [AC).

3.

Main Feedwater Pump (MFWP) (SJ, P] Speed Control (JK]

Following a reactor trip, MFWP speed automatically slows to a minimum speed setting. This setting is intended to maintain MFWP discharge pressure higher than expected steam generator (SG) [SG.1 pressure, thereby maintaining flow to the SG's.

o LICENSEE EVENT REPORT (LER) TEXT CON'I'INUATION SAN ONOFRE UUCLEAR GENERATION STATION DOCKET NUMBER LER NUMBER PAGE p))IT 3 Q1222]E2 9}-001-00 3 of 4 C.

DESCRIPTION OF THE EVENT:

1.

Event:

On January 16, 1993, Unit 3 wan operating at 75% power with a heavy rain storm occurring. At 1643, the Main Gonai ntor tripped on a stator ground protection signal, causing a turbine / reactor trip.

The plant was stabilized in Mode 3 at normal temperature and pressure at approximately 1715.

2.

Inoperable Structures, Systemn o:' Components that Contributed to the Event:

None 3.

Sequence of Evento:

TIME S Q QN 1641 Main Generator atator ground protection nignal caused a turbine / reactor trip.

1715 Plant conditionn were atabilized in Mode 3, 4.

Method of Discovery

Control room alarma and indications alerted the control room operators to the turbine / reactor trip.

5.

Personnel Actions and Analyais of Actionn:

Control room operatorn renponded properly to the reactor trip, implementing normal post-trip procedures to stabilize the plant in Modo 3.

6.

Safety System Responnen:

The RPS componento actuated as required by design.

D.

CAUSE OF THE EVENT

1.

Immediate Caune:

A Main Generator stator ground protection signal caused a Main Generator / Main Turbine trip.

The turbine t: 1p caused a reactor trip on a loca of load signal.

2.

Root Cause:

An inspection of the Unit 3 main generator terminal enclonure was performed.

Thin enclosura contains the tranaition from the main generator. conductor to the.iuophase bus.

Rain water (the site received an unusually high amount r rainfall combined with high winda prior to the_ trip) leaked down from the turbine deck through openingo into this enclocure. The water accumulated on

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4 LICENSEE EVENT-REPORT (1.ER)- TEXT CONTINUATION SAN ONOFRE NUCLEAR GENERATION STATION DOCKET NUMBER LER NUMBER PAGE UNIT 3 05030362 931001-00' 4 of 4 some insulating material and provided a low resistance' path;from the exposed B phase conductor to ground. This resulted in phase B;to ground current being detected by the stator ground relay, actuating a Main Generator / Main Turbine trip signal.

E.

CORRECTIVE ACTIONS

1.

Corrective Actions Taken:

Since the turbine building is open to the elements and due to'the configuration of the main generator to isophase bus housing, it is not feasible to provide absolute protection against wind driven water intrusion.

However, additional sealing was provided for the Main Generator terminal enclosure to minimize the possibility of rain wat3r intrusion, and the insulating material was replaced. During the subsequent plant recovery, the Main Generator was monitored to ensure acceptable performance.

2.

Planned Corrective Actions

SCE will evaluate whether additional measures can be taken to minimize the possibility of rain water intrusion into Main Generator components.

F.

SAFETY SIGNIFICANCE OF THE EVENT:

There is no safety significance to this event since all RPS components operated as designed.

G.

ADDITIONAL INFORMATION

1.

Component Failure Information

Not applicable 2.

Previous LERs for Similar Events:

None 3.

MFWP Response:

During the post trip response, MFWP minimum speed setting.was such that MFWP=

discharge pressure was below SG pressure, causing SG levels to decrease.

Control room operators (utility, licensed) properly took manual control of MFWP speed to maintain SG levels. The Main Feed Water Pump minimum speed settings were adjusted. During the subsequent plaat recovery, the Main Feed Water Pump speed was monitored to ensure acceptable cerformance. SCE will evaluate whether additional measures should be taken to minimize the possibility of low MFWP specd during post trip conditions.