ML17352A951

LER 94-006-00:on 941130,Unit 4 Tripped Automatically.Caused by Failure of Flexible Link Connection Between Main Generator B Phase Bus & Associated Isolated Phase Bus Bar. All Bolts on Flexible Link checked.W/941213 Ltr

ML17352A951
Person / Time
Site:	Turkey Point
Issue date:	12/13/1994
From:	Knorr J, Plunkett T FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
L-94-311, LER-94-006-02, LER-94-6-2, NUDOCS 9412210168
Download: ML17352A951 (12)

Text

P R.ICOR.I RIDS PROC!'.SSI EY'ACCLLERATED REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

ACCESSION NBR:9412210168 DOC.DATE: 94/12/13 NOTARIZED: NO DOCKET FACIL:50-251 Turkey Point Plant, Unit 4, Florida Power and Light C 05000251 AUTH. NAME AUTHOR AFFILIATION KNORR,J.E. Florida Power & Light Co.

PLUNKETT,T.F. Florida Power & Light Co.

RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 94-006-00:on '941130,Unit -4 tripped automatically. Caused by failure of flexible link connection between main generator B phase bus & associated isolated phase bus bar.

All bolts on flexible link checked.W/941213 ltr.

DISTRIBUTION CODE: IE22T COPIES RECEIVED:LTR ENCL ) SIZE:

TITLE: 50.73/50.9 Licensee Event Report (LER), Inciden&Rpt, etc.

NOTES RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL PD2-2 PD 1 1 CROTEAU,R 1 1 INTERNAL: AEOJ~3-S -D JRA'B 2 2 AEOD/S PD/RRAB 1 MI-LE=GENT 02 1 1 NRR/DE/ECGB 1 NRR/DE/EELB 1 1 NRR/DE/EMEB 1 NRR/DI S P/PI PB 1 1 NRR/DOPS/OECB 1 NRR/DRCH/HHFB 1 1 NRR/DRCH/HICB 1 NRR/DRCH/HOLB 1 1 NRR/DRSS/PRPB 2 NRR/DSSA/SPLB '1 1 'RR/DSSA/SRXB 1 RES/DSIR/EIB 1 1 RGN2 FILE 01 1 EXTERNAL: L ST LOBBY WARD 1 1 LITCO BRYCE,J H 2 2 NOAC MURPHY,G.A 1 1 NOAC POOREiW 1 1 NRC PDR 1 1 NUDOCS FULL TXT 1 1 DOCUMENT'I'S YOTE TO ALL"Rl DS" RECIPIENTS:

PLEASE HELP US TO REDUCE EVKSTE! CON'TAP'I"I'IIE DOCI.'ifEiTCONTROL DESh. ROOM PI -37 I EAT.'504-~OS3 ) TO I'.LIXflNATE 5'Ol R X'AhfL I ROif DISTRI8 O'I'IOi LIS'I'S I'OR YOI: DOi "I'EI'.D!

FULL TEXT CONVERSION REQUIRED TOTAL NUMBER OF COPIES REQUIRED: LTTR 27 ENCL 27

Cl I L-94-311 10 CFR 50 '3 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, 'D. C. 20555 Gentlemen:

Re: Turkey Point Unit 4 Docket No. 50-251 Reportable Event: 94-006-00 Automatic Reactor Trip Due to Main Generator Ground The attached =-

Licensee Event Report, 251/94-006-00, is being provided in accordance with 10 CFR 50.73(a) (2) (iv) .

Xf there are any questions, please contact us.

Very truly yours,

/7~'I T. F. Plunkett Vice President Turkey Point Plant jek enclosure cc Stewart USNRC D. Ebneter, Regional Administrator, Region II, Thomas P. Johnson, Senior Resident Inspector, USNRC, Turkey Point Plant 943.223.03.68 5'43.21 i PDR ADOCVi 050r'r0>53 8 PDR an FPL Group company

Il LICENSEE EVENT REPORT (LER)

'I FACILZTY NAME (I ) DOCKET NUMBER (2) PAGE (3)

TURKEY POINT UNIT 4 05000251 oF 1 3 TITLE (4)

AUTOMATIC REACTOR TRIP DUE TO MAIN GENERATOR GROUND EVENT DATE (5) LER Nl&DER(6) RPT DATE (?) OTHER PACILITIES INV. (8)

MON DAY SEQ S RS 'ON DAY PACILITY NAMES DOCKET S (S) 30 94 '4 006 00 12 13 94 OPERATING MODE (9) 1 POWER LEVEL (10) 100%

10 CFR 50.73 a 2 iv LICENSEE CONTACT FOR THIS LER (12)

TELEPHONE N(B(BER J. -E. Knorr, Regulation and Compliance Engineer 305-246-6757 COMPLETE ONE LINE FOR EACH COMPONENT PAZLURE DESCRIBED IN THIS REPORT (13)

CAUSE SYSTEM COMPONENT NPRDS? CAUSE SYSTEM NPRDS'?

COMPONENT SUPPLEMENTAL REPORT EXPECTED (14) NO gl YES 'C3 EXPECTED SUBM I5 5 ION MONTH DAY DATE (15)

(if yee, coaylete EXPECTED SUBMISSION DATE)

ABSTRACT (16)

On November 30,. 1994, Unit 4 tripped automatically because of the failure of a flexible link connection between the main generator B phase bus and its associated isolated phase bus bar. The flexible link came loose and grounded against the isolated phase bus duct work causing a generator lockout, turbine trip and automatic reactor trip. At the time of the Unit 4 was in Mode 1 operating at approximately 100% power. All systemstrip responded as expected to the automatic reactor trip.

LICENSEEVENT REPORT (LER) TEEONTINUATION FACILITY NAME DOCKET NOMBER LER NUMBER PAGE NO.

TURKEY POINT UNIT. 4 05000251 94-006-00 02 OF 04 I. DESCRIPTION OF THE EVENT On November 30, 1994, at 1542, Unit 4 tripped automatically because of the failure of a flexible link connection between the main generator B phase bus and its associated isolated phase bus bar. Unit 4 was in Mode 1 operating at approximately 100% power steady state operation. When the connection failed, the flexible link fell against the adjacent isolated phase bus duct thereby grounding that phase, causing a main generator ground, generator lockout, turbine trip and an automatic reactor trip. All safety systems responded as expected to the reactor trip. Reactor trip recovery actions were completed in accordance with. procedure 4-EOP-E-O, Reactor Trip or Safety Injection.

The ground current was limited due to the design value for the ground circuit for the main generator. Therefore, no damage occurred to the bus work or the isolated phase bus.

The NRCOC was notified at 1648 on November 30, 1994. This event is being reported in accordance with 10 CFR 50.73(a)(iv), as an automatic actuation of the reactor protection system.

II. CAUSE OF THE EVENT The cause of the failure of the flexible link connection was a personnel error resulting in an inadequate torque of the bolted connection. The required torque for the connection was 40 foot-pounds. These connections were disconnected, cleaned and reinstalled as part of the maintenance scheduled for this equipment during the recent refueling outage. After the trip, one flexible link was found fallen away from the bus and touching the isolated phase bus ductwork. Five bolts on the north face of the B phase of the generator bus were found loose.

Procedure O-PME-090.1 Power Generator Grounding For 'Safety and Testing Preparation, was used as work control for the flexible link removal and reinstallation'his work control was not adequate to ensure all flexible link connections were retorqued to 40 foot pounds. A contributor to the event was more than one worker completing torquing of the bolts on the B phase flexible links without verification of each link during .the process.

The A and C phases were each completed by one worker.

III. ANALYSIS OF THE EVENT S stem Descri tion The main generator output voltage at Turkey Point Unit 4 is a nominal 22 kV. The grounding circuit of the main generator and isolated phase bus is designed to limit current during a ground fault. A primary main generator ground neutral relay monitors the generator ground neutral which is connected through a transformer and resistor between ground and the neutral of the wye connected generator. A secondary generator ground voltage phase imbalance on the generator output to indicate a relay'onitors ground by sensing the change in the phase voltages when a ground is present on any of the phases of the generator. Both relays actuated as expected for an actual generator ground. These relays initiate a generator lockout. As discussed above, this relay operation ultimately results in an automatic reactor trip.

LICENSE~NT REPORT (LER). TEX9QNTINUATION FACILITY NAME DOCKET NUMBER LER NUMBER PAGE No.

TURKEY POINT UNIT 4 05000251 94-006-00 03 OF 04 Each output phase of the main generator is connected to its associated isolated phase bus by a series of 20 pairs of flexible links. Each pair of links is installed back to back and requires two bolts at each end to connect it,to the main generator bus at one end and the isolated phase bus on the other. Each bolt must be-torqued to 40 foot-pounds. A total of 240 bolts are required to complete the installation of all flexible links between the each of three phases of the main generator buses and their associated isolated phase bus.

ISO-PHASE DUCT AREA I SO-PHASE L INKS (olde vIew)

I SO-PHASE OVCT (bock side of ores)

LINK T CAME LOOSE 80LTS LINKS 8OLTS I SO-PHASE BVS SPACER IQT SHAN

~ I SO-PHASE BVS L INKS (front sIde of work cree)

Safet Anal sis analysis for a transient of this type assumes a total loss of load The UFSAR without a. direct or immediate reactor trip. 'n this event, a reactor -trip occurred upon receipt of the turbine trip. The UFSAR conclusion was that this event poses no hazard to the integrity of the reactor coolant system and steam system.

No other engineered safety systems or reactor protection systems actuated or were required to actuate during or after the trip. Therefore, the health and safety of the public were not affected.

IV.. CORRECTIVE ACTIONS All 240 connecting bolts on the main generator flexible links were checked for proper torque.

2. The main generator was megger tested with satisfactory results.

3. The procedure used to install the mai:n generator flexible links will be reviewed to determine necessary work process improvements. These improvements, if necessary, will be completed piior to the next outage requiring work on these links.

4l ~,

Ct'

LZCENS1EVENT REPORT (LER) TEXONTZNUATZON FACILITY NAME DOCKET NUMBER LER NOMBER PAGE NO.

TURKEY POINT UNIT 4 0500025l 94-006-00 04 OF 04 V. ADDZT ZONAL INFORMATION 4

EIIS Codes are shown in the format [EIIS SYSTEM: IEEE component function identifier, second component function identifier (if appropriate)].

No other LERs were issued during the past two y'ears for a personnel error caused reactor trip.

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