ML20044D133

LER 91-007-01:on 910629,two Neutral Overcurrent Relays on Unit 1B Auxiliary Transformer Actuated,Resulting in Generator Trip & Reactor Trip.Caused by Electrical Fault. Loose Strand Removed from conductor.W/930511 Ltr

ML20044D133
Person / Time
Site:	Farley
Issue date:	05/11/1993
From:	Hill R, Woodard J SOUTHERN NUCLEAR OPERATING CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-007, LER-91-7, NUDOCS 9305170262
Download: ML20044D133 (4)

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A Southem Nuclear Operating Cornpany Post O*fce Ben 1295 Birmingham. Alabama 3s201

• Telephone 205 868 5086 m

3. o. ,, ,,,, Southern Nudear Operating Company et the southem electnc system

)'pPrep 10 CFR 50.73 dby 11,1993 Docket No. 50-348 U. S. Nuclear Regulatory Commission ATIN: Document Control Desk Washington, DC 20555 Joseph M. Farley Nuclear Plant - Unit 1 Licensee Event Report No. LER 91-007-01 Gentlemen:

Joseph M. Farley Nuclear Plant Unit 1 Licensee Event Report Number LER 91-007-01 is being submitted in accordance with 10 CFR 50.73. If you have any questions, please advise.

Respectfully submitted,

(

J. D. Woodard vn EFB:cht-licevent.nrc Enclosure cc: Mr. S. D. Ebneter i Mr. G. F. Maxwell 170024

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, LICENSEE EVENT REPORT (LER)

FACIL111 hAmE (1) LOCAEl hum 6Ek (2) 3113 f3i Joseph M. Farley Nuclear Plant - Unit 1 05000348 1 g,l3 111LE (4)

Reactor Trip Following Actuation Of Protective Relays On The IB Unit Auxiliary Transformer Evihi DATE (5) LER hum 6ER (6) REPChi DAIE (7) 01hER FAC16111ES INVOLbED (8)

MOh1H DAY YEAR YEAR SEQ hum EEV MON 1H DAY YEAR FACILITY hAMES DOCKET hum 6Ek(S) 05000 06 29 91 91 007 01 05 11 93 05000 OPERATlhG HI & 1 ubO T1ED WW MThE Wi&NSM MW (11)

MODE (9) 1 20.402(b) 20.405(c) ^ 50.73(a)(2)(iv) 73.71 (b)

POWER -

20.405(a)(1)(i) ~

50.36(c)(1) -

50.73(a)(2)(v) -

73.71(c)

LEVEL 100 20.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(vii) OTHER (Specify in 20.405(a)(1)(iii) 50.73(a)(2)(i) 50.73(a)(2)(viii)(A) Abstract below)

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20.405(a)(1)(iv) -

50.73(a)(2)(ii) -

50.73(a)(2)(viii)(B) 20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x)

LICENSEE CONTACI FCR THIS LER (12) 4AME TELEPHONE huMBER LEEA CODE R. D. Hill, General Manager - Nuclear Plant 205 899-5156 COMPLETE CAE LihE FOR EACM FAILURE DESCR16ED IN 1HIS EEPOR1 (13)

CAUSE SYSTEM COMP 0hENT MAhuFAC- R PCRT CAUSE SYSTEM COMPONENy MAhUFAC- R PORT g pp3 B EA XFMR W120 Y

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SUPPLEMENIAL REPORT EXPECTED (14) MOh1M DAY YEAR EXPECTED SUEMISSION

] YES(If yes, complete EXPECTED SUBMISSION DATE) DATE (15)

% NO

, A551RACI (16)

At 2045 on 6-29-91, a Unit one reactor trip occurred. Two of the neutral overcurrent relays on the IB unit auxiliary transformer (UAT) actuated resulting in a generator trip. By design, the generator trip caused the turbine to trip ,

and the turbine trip caused the reactor to trip.

The low and high setpoint neutral overcurrent relays on the 1B UAT actuated as a result of an electrical fault. The fault was caused by a broken strand of a multiple stranded conductor coming into contact with the grounded core frame structure of the transformer. The multiple stranded conductor is located in the transformer oil box and connects the transformer winding to the transformer output bushing.

i The conductor was repaired by removing the loose strand and reinsulating the conductor. The integrity of the conductor and crimp connection was verified prior to reinstallation of the insulation.

The IB UAT was returned to service on 3-16-93 at 0227.

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. TEXT CONTINUATION

, FACILITY NAME (1) DOCKET WUMBER (2) LER NUMEER (5) PAGE (3)

TEAR 5EO hum REW Joseph M. Farley Nuclear Plant - Unit 1 05000348 91 007 01 2 N 3 IEu Plant and System Identification Westinghouse - Pressurized Water Reactor Energy Industry Identification System codes are identified in the text as [XX).

Summary of Event At 2045 on 6-29-91, a reactor trip occurred. Two of the neutral overcurrent relays on the IB UAT [EA) actuated resulting in a generator trip. By design, the generator trip caused the turbine to trip and the turbine trip caused the reactor to trip, e

Description of Event on 6-29-91, Unit 1 was operating at approximately 100 percent power. At 2045, the low and high setpoint neutral overcurrent relays on one of the low side windings (Y vinding) of the 1B UAT actuated. By design, this protective relay scheme caused the main generator to trip. The main turbine and reactor tripped per design in response to the generator trip.

Following the trip the operators implemented FNP-1-EEP-0 (Reactor Trip or Safety  !

Injection) and FNP-1-ESP-0.1 (Reactor Trip Response), ensuring that the unit was safely in Mode 3 (Hot Standby). The unit was maintained in a stable condition.

After the trip occurred, an investigation was conducted to determine the cause.

Testing performed on the transformer and its associated 4160 volt cables and protective relaying was indeterminate. The transformer was then energized and partially loaded and instrumented with test equipment during the Unit 1 eleventh refueling outage (September - December, 1992) with no problems indicated.

Upon Unit 1 startup on December 2, 1992, while at approximately 13 percent reactor power, a turbine trip and exciter breaker trip occurred shortly after generator field voltage was applied. Generator output voltage had reached approximately 10 kv or 45 percent rated terminal voltage when the trip occurred.  ;

Initial investigation revealed that a ground fault had occurred on the IB UAT Y vinding circuit. The test equipment which had been set up after the initial  ;

trip indicated that a fault had occurred inside the transformer. When the transformer oil box was partially disassembled and inspected, the broken conductor strand was located. ,

i Cause of Event  !

The fault was the result of a broken strand of a multiple stranded conductor internal to the transformer coming into contact with the grounded core frame ,

structure of the transformer. A contributing factor was the proximity of the l loose strand to the transformer oil pump discharge. Another factor was the i unraveled insulation on the conductor which allowed the oil pump discharge to ,

force the strand from the crimp towards the grounded core frame support structure.

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. TEXT CONTINUATION -

PAGE (3)

- TACILITY NAME (1) DOCKET NUMEER (2) LER NUMBER (5) vtAk sto hum Riv  !

Joseph M. Farley Nuclear Plant - Unit 1 05000348 91 007 01 3 0F 3 un l Reportability Analysis and Safety Assessment This event (on 6-29-91) is reportable because of the actuation of the reactor l protection system. After the trip, the following safety systems operated as j designed; j t

- main feedwater was isolated with flow control valves and bypass valves  :

closed,

- auxiliary feedwater pumps started automatically and provided flow to  !

the steam generators,  !

- source range nuclear instrumentation automatically energized,

- pressurizer heaters and spray valves operated automatically as required to maintain system pressure. t l

There was no effect on the health and safety of the public.  !

t Corrective Action [

i The conductor was repaired by removing the loose strand and reinsulating the .  !

conductor. The integrity of the conductor and crimp connection was verified [

prior to reinstallation of the insulation.  !

, )

The IB UAT was returned to service on 3-16-93 at 0227.  ;

Inspection of the other UATs will be performed during the next refueling outage  ;

on each unit. The next refueling outage on Unit 1 is scheduled to begin in

• March of 1994 and the next refueling outage on Unit 2 is scheduled to begin in j-September, 1993. A study was conducted to determine if the design of the Farley l l Nuclear Plant start-up transformers (SUls) could present similar problems. The  !

E study indicated that the UATs and the SUTs are similar in design, thus the j start-up transformers will also be scheduled for inspection during the next  ;

refueling outages on each unit. f Additional Information [

I

] No similar 1.ER's have been submitted by Farley Nuclear Plant. [

I This event would not have been more severe if it had occurred under different i operating conditions. i s

I The unit returned to power operation at 0642 on 7-1-91 after the 6-29-91 event. l l

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