ML20029D630

LER 94-003-00:on 940405,reactor Tripped Due to Main Power Transformer Failure.Corrective Actions:Detailed Investigation Performed for Collateral Damage & Isophase Bus Cleaned & inspected.W/940505 Ltr

ML20029D630
Person / Time
Site:	Braidwood
Issue date:	05/05/1994
From:	Archterberg J, Kofron K COMMONWEALTH EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
BW-94-0072, BW-94-72, LER-94-003-08, LER-94-3-8, NUDOCS 9405090099
Download: ML20029D630 (5)

Text

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Ccmmsnw:alth Edis:n s Braidwood Nuclear Power Station C Route #1, Box 84 k L/ Braceville, Illinois 60407 Telephone 815/458-2801 May 5, 1994 BW/94-0072 4

U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555

Dear Sir:

The enclosed Licensee Event Report from Braidwood Generating Station is being transmitted in accordance with the requirement of 10CFR50.73 (a) (2) (iv) , which requires a 30-day written report.

This report is number 94-003-00, Docket No. 50-457.

'[l 'ron K. L. K Station Manager Braidwood Station Enc: Licensee Event Report No. 457/94-003-00 cc: NRC Region III Administrator NRC Resident Inspector INPO Record Center CECO Distribution Center 090014 i 9405090099 940505

{DR ADOCK 0500o457 v PDR

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NitC FC3M 366 U.S. NUCLEAR REeJLAT G V COMMIS$1 C APPROVED B7 OMB No. 3150-0104 (5-92) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH

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L'ICENSEE EVENT REPORT (LER) uS REGARDING BUR EN S IMATE b THE INFORMATION AND RECORDS MANAGEMENT BRANCH i (MNBB 7714), U.S. NUCLEAR REGULATORY COMMISSION, l (See reverse for required number of digits / characters for each block) ON, O ECT ( 1 0 0104), OF 0 MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.

FACILITY NAME (1) DOCKET NUMBER (2) PAGE (3)

Braidwood 2 05000457 1 OF 5 TITLE (4)

Reactor Trip Due to Main Power Transformer Failure EVENT DATE (5) LER NLMBER (6) REPORT DATE (7) OTHER FACILITIES INVOLVED (8) i SE AL MONTH DAY YEAR YEAR MONTH DAY YEAR 0$0$

ME E 04 05 94 94 -- 003 -- 00 05 05 94 hA$UNAME O OPERATING THIS RE N T IS M Mi m D M M NT TO M R M IREMENTS OF 10 CFR H (Check one or more) 01) 1 MODE (9) 20.402(b) 20.405(c) X 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)

Jg LEVEL (10) 20.405(a)(1)(li) 50.36(c)(2) 50.73(a)(2)(vil) OTHER 20.405(a)(1)(lii) 50.73(a)(2)(i) 50.73(a)(2)(vi i i )( A) (SPecify in 20.405(a)(1)(tv) 50.73(a)(2)( t i ) 50.73(a)(2)(viii)(B) nd e 20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x) NRC Form 366A)

LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBER (include Area Code)

J. Achterberg, Work Planning (815)458-2801 x2221 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13) 0 E R CAUSE SYSTEM COMPONENT MANUFACTURER CAUSE SYSTEM COMPONENT MANUFACTURER 3

X EBF TRANSF W120 YES SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MONTH DAY YEAR YES SUBMISSION x NO (If yes, complete EXPECTED SUBMIS$!ON DATE). DATE (15)

ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single spaced typewritten lines) (16)

At 1539 on April 5,1994 a Reactor Trip was received. The 2E Main Power Transformer (MPT) had a sudden pressure and a differential current trip which caused a turbine trip and a subsequent reactor trip. The cause of this reactor trip was Equipment Failure. A review of the oscillograph traces indicated a high side B phase fault on the 2E Main Power Transformer. The fault was initiated at the transformer and damage was extensive to the 2E MPT. A detailed investigation for collateral damage was performed which covered the following items: A generator crawlthru inspection; the 2W MPT and both UAT'S were electrically tested; and oil samples on the 2W MPT and both UAT'S were taken. The results of all of these activities indicated that there was no additional damage to any equipment beyond the 2E MPT. The 2E MPT will be replaced with a spare transformer. Additionally, instrumentation associated with the transformer will be calibrated prior to operation. Finally, a Company task force has been developed to investigate this recent failure. There have been previous similar occurrences where electrical system problems have resulted in a reactor trip. The root cause(s) and corrective actions of the previous similar occurrences do not apply to this event. There is no adverse trend.

NRC FORM 366 (5 92)

1. .

It2C FORM 366A U.S. NUCLEAR RECJLATC3Y COMMISSIC] APPROVED BY OMB NO. 3150-0104 (5-92) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST: 50.0 HRS.

LICENSEE EVENT REPORT (LER) YE 1NF0EN AERECORDS NAGE BRAN TEXT CONTINUATION " ^" RE JLAT R Q Q oj.S'055I'0001 2 9 TO THE REDUCTION PROJECT (3180-0104), OFFICE OF

_j MANAGEMENT AND BUDGET. WASHINGTON, DC 20503.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

Braidwood 2 YEAR SEQUENTIAL REVISION 05000457 94 -- 003 -- 00 l TEXT (if more space is required, use additional copies of NRC Form 366A) (17)

A. PLANT CONDITIONS PRIOR TO THE EVENT:

i Braidwood Unit 2 Event Date: 04/05/94 Event Time: 1539 CDST Mode 1- Power Operation; Reactor Power 99.6%

l RCS Temperature / Pressure: NOT/NOP l

B. DESCRIPTION OF EVENT:

l At 1539 on April 5,1994 with Unit 2 at 99.6% power operation, a Reactor Trip was received . The first out annunciator on the generator panel was

" Main Transformer Sudden Pressure Generator Trip." This caused a turbine trip which resulted in a reactor trip. The 2E Main Power Transformer (MPT) had a sudden pressure and a differential current trip indication up. The differential current trip also actuated fire protection deluge as required.

Also per procedure the plant fire brigade was dispatched, although no fire l occurred. During the reactor scram all systems performed as expected with l the exception of control rod K2 which did not insert beyond 210 steps. A separate internal root cause investigation team and NRC Augmented Inspection Team (AIT) is addressing the failure of control rod K2 to insert below 210 steps.

Offsite power continued to be supplied to the plant via the normal System  !

Auxiliary Transformer (SAT) throughout the event.

l l

The appropriate Emergency Notification System (ENS) notification was made at 1739 CDST pursuant to 10CFR50.72 (b) (2) (ii) .

1 This report is being submitted pursuant to 10CFR50.73 (a) (2) (iv) - any event l

or condition that resulted in manual or automatic actuation of any

[ Engineered Safety Feature (ESF) , including the Reactor Protection System (RPS).

C. CAUSE OF EVENT:

Summary of Equipment Failure Modes The cause of this reactor trip was Equipment Failure. A review of the oscillograph traces by Commonwealth Edison Operational Analysis Department (OAD) indicated a high side B phase fault on the 2E Main Power Transformer.

The fault was initiated at the transformer and damage was extensive to the 2E MPT. Detailed inspections were performed on the 2E MPT and associated bus work and generator. Commonwealth Edison Company (CECO) Technical Center transformer specialists performed a detailed internal inspection of the 2E Main Power Transformer, with significant damage found on the B phase 1

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NRC FCM 366A U.S. CELEAR REOJLATOY C0milSSIO APPROVED BY OMB No. 3550-0104 (5-92) EXPIRES 5/31/95 l EST! MATED BURDEN PER RESPONSE TO COMPLY WITH l THIS INFORMATION COLLECTION REQUEST: 50.0 HRS.  !

LICENSEE EVENT REPORT (LER) $"#"rNF0ES AI^ REC 0RDS NAGE BRAN TEXT CONTINUATION g a g ,p .S*

"$'0001^"A " YD TO$H 2o55 REDUCTION PROJECT (31$00104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON. DC 20503.

FACILITY NAME (1) DOCKET NUMBER (2) LER NLDeBER (6) PAGE (3)

Braidwood 2 SEQUENTIAL REVISION YEAR 05000457 94 -- 0 0 3 -- 00 TEXT (If more space is required, use additional copies of NRC form 366A) (17) winding package. Additionally, a detailed crawlthru of the main generator was performed and no indications of damage was observed. Iso phase bus duct inspections were also completed and no indications of damage were observed. All inspections to date indicate the fault damage was isolated i to the 2E MPT.

In addition to visual inspections, OAD performed several other inspections and tests. A relay target analysis was performed which indicated the fault occurred on B phase. Three transformer electrical tests were performed: a low voltage excitation test, a megger, and a transformer turns ratio test all indicated a severe fault to the 2E MPT.

An oil sample analysis was performed and the results indicated severe arcing occurred internal to the 2E MPT.

Additionally, weather at the time.of the trip was partly sunny with temperatures at 37 degrees F. This, combined with the inspections performed of the lightning arrestors, effectively rule out lightning as j being a potential cause.

Analysis of 2E MPT Prior To The Event An analysis of the latest Commonwealth Edison System Material Analysis '

Department (SMAD) oil sample from the 2E MPT prior to the fault indicated that the transformer had normal combustible gas levels present. Based on this latest analysis the transformer was on an oil sampling frequency of six months. This is the normal sampling frequency and indicates the transformer is operating normally. Additionally, temperature trends prior to the transformer failure were examined, and indicated that the 2E MPT was running with normal temperatures and thus no indication of any abnormalities were present. Thermography data obtained prior to the fault on 03/25/94 indicated that temperature profiles were as expected with no signs of localized heating. All parameters for the 2E MPT indicated normal operation, and thus the unexpected fault appears to be immediate and

! catastrophic in nature. The decision to do additional investigation on the faulted transformer via a complete tear down will be made by the appropriate levels of company management at a later date.

CRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB Wo. 3150-0104 (5 02) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH

. THIS INFORMATION COLLECTION REQUEST: 50.0 HRS.

LICENSEE EVENT REPORT (LER) $"#"1NF0Ef07AE'#"$RDS REC MANAGEYNBRAN TEXT CONTINUATION u QQ oc.S'0555-O001""t^"

2 RE,GlfLA,T0R w9 9 H  !

REDUCTION PROJECT (31$0-0104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON. DC 20503.

FACILITY NAME (1) l DOCKET NUMBER (2) LER WUMBER (6) PAGE (3)

Braidwood 2 ygg, SEQUENTIAL REVISION 05000457 94 00 j

-- 0 0 3 --

TEXT (If more space is required, use additional copies of NRC Form 366A) (17)

D. SAFETY ANALYSIS: i The safety of the plant and public were not affected. The MPT sudden pressure and a differential current trip isolated the MPT from the system grid. Redundant trains of reactor protection (RP) {JG) and engineered 1 safety features (EF) (JE) were operable, available and effective in l I

performing their design functions.

Under the worst case condition, if the MPT fault caused a total loss of offsite power, emergency diesel generators (DG) {EK) would supply the EF electrical power requirements. This event is analyzed in section 8 of the i Updated Final Safety Analysis Report.

During the event control rod K2 failed to insert beyond 210 steps. The Updated Final Safety Analysis Report Section 15 assures protection for the l most reactive control rod stuck in the full out position. Control rod K2 l

is not the most reactive control rod and therefore this anomaly is within the bounds of our Updated Final Safety Analysis Report. ,

! l E. CORRECTIVE ACTIONS:

LTunediate Actions Af ter The Event A detailed investigation for collateral damage was performed which 2 l

covered the following items: A generator crawlthru inspection; the 2W MPT l and both UAT'S were electrically tested; and oil samples on the 2W MPT and both UAT'S were taken. The results of all of these activities indicated that there was no additional damage to any equipment beyond the 2E MPT. I l

Additional actions taken were as follows:

1. Isophase bus duct clean and inspection

2. Relaying and metering potential transformer test

3. Bus duct lightning arresters meggering

4. Potential transformer current limiting resistor resistance measurement

5. Generator neutral transformer megger and turns ratio test

6. Generator neutral transformer resistor resistance measurement

7. A review of 2E/W MPT oil temperatures

8. As-found calibrations of the instrumentation on the transformer was performed and the instruments were found within tolerance l

4 .

NRC FORM 366A U.S. NUCLEAR REGULATORY C00041SS10N APPROVED BY OMB NO. 3150-0104 (5 02) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST: 50.0 HRS.

LICENSEE EVENT REPORT (LER) EE INF0 E $ *AE #" REC'"0RDS MANAGE BRAN TEXT CONTINUATION g g . ,j.S20555-0001 REDUCTION PROJECT

" ' YD T0 THNNEr (31$0-0104),

A OFFICE OF MANAGEMENT AND BUDGET. WASHINGTON. DC 20503.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

ScouENTIAL REVISION Braidwood 2 YEAR 05000457 94 -- 003 -- 00 VEXT (If more space is required, use additional copies of NRC Form 366A) (17)

Further Actions to be completed The 2E MPT will be replaced with a spare transformer. The spare transformer is a like-for-like replacement and will be inspected and tested prior to operation. Additionally, the temperature gages and other instruments physically located on the transformer will be calibrated prior to operation and a calibration frequency will be established. The 2E MPT relays will be calibrated prior to operation and a surveillance will be performed on the switchgear fed from the UAT's. Finally, a Company task ,

force has been developed to investigate this recent failure.

Further actions will be evaluated based on this task force recommendations  ;

at the conclusion of their investigation.

F. PREVIOUS OCCURRENCES:

There have been previous similar occurrences where electrical system problems have resulted in a reactor trip. The root cause(s) and corrective actions of the previous similar occurrences do not apply to this event.

There is no adverse trend. i LER NO: TITLE 50-456/87-052 REACTOR TRIP DUE TO MAIN POWER TRANSFORMER OVEREXCITATION RELAY ACTUATION FOR UNKNOWN REASON 50-456/88-012 REACTOR TRIP DUE TO PHASE B OVERCURRENT PROTECTIVE RELAY CO-7 DEFECTIVE CURRENT SWITCH 50-456/89-002 REACTOR TRIP DUE TO A 345 KV SWITCHYARD BREAKER DEFECTIVE TRIP COIL 50-457/92-007 REACTOR TRIP DUE TO GENERATOR TRIP AS A RESULT OF l SENSED NEUTRAL GROUND OVERCURRENT FROM FUSE FAILURE G. COMPONENT FAILURE DATA:

MANUFACTURER NOMENCLATURE MODEL #/ MFG PART #

Westinghouse 2MP01E - Main N/A Power Transformer