ML20207G386

Summary of 880803 Meeting W/Util in Rockville,Md Re Facility Auxiliary Transformer Fire & Reactor Trip Event Initiated by Combination of Cracked Insulating Matl & Dirt Accumulation.Briefing Slides Encl

ML20207G386
Person / Time
Site:	Palo Verde
Issue date:	08/12/1988
From:	Marlone Davis Office of Nuclear Reactor Regulation
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 8808240107
Download: ML20207G386 (41)

Text

{{#Wiki_filter:f~ . in. .e [ UNITED STATES !/* s@( 'k-NUCLEAR REGULATORY COMMISSION i 4

. i WASHINGTON,0 C. 20555 j

/ August 12, '138 DOCKET N0.: 50-528 LICENSEE: AR120!1A PUBLIC SERVICE C0ftPANY FACILITY: PALO VERDE NUCLEAR GENERATING STATION

SUMMARY

OF MEETING ON PALE VERDE UNIT 1 AUXILIARY TRANSFORMER FIRE AND REACTOR TRIP EVENT OF JULY 6, 1988. A meeting was held in Rockville, Maryland on August 3,1988, with represent-atives of the licensee to discuss the evaluation and planned corrective actions from the Unit 1 auxiliary transformer failure / fire and reactor trip event that occurred on July 6, 1988. is a copy of the briefing slides used by the licensee. is a list of meeting attendees.

SUMMARY

The licensee provided a detailed discussion of the event using the briefing slides attached as Enclosure 1. The discussion included a chronology of events, a description of the electrical distribution system and protective relaying design, bus design and failure mechanism for the ground fault on the 13.8 KV bus IE-NAN-S02, and transformer design and reasons for the failure of the unit auxiliary transformer. The initiating event was determined to be a combination of cracked insulating material and dirt accumulation in the 13.3 KV bus IE-NAN-502. A single phase fault on B phase propagated into a three phase fault of approximately 27,500 amps. This fault current was within the three fault rating of the unit auxiliary transformer. The thru fault eggravated a local material or winding turn to turn defect in the transformar winding coil. The turn to turn short propagated into a coil to coil short which caused coil ejection and major fault'.9 inside the transformer. The resulting internal gas pressure ruptured the transformer casing, which resulted in a major fire as arcing ignited the oil vapor exposed to air. The licensee stated that a similar transformer failure on other units would be unlikely since the probability of defect repetition is low, increased frequency of transformer oil analysis for dissolved gas in the oil (dga) to detect development of similar defect is planned. The licensee also plans to revise the preventive maintenance program for cleaning switchgear. Cleaning of switchgear on all three units has been conducted or is

planned, l

8808240107 880912 ADOCK0500g8 PDR S

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• Investigation of the cause of insulation cracking continues. The staff will monitor the results as they become available.

The staff requested copies of any supplements to the licensee's post trip review report which document the results of various investigations that'are still in progress. The staff complemented the licensee on the thoroughness of the event analysis. Other staff comments cautioned the licensee that a number of concerns generated as a result of this event show that improvement is still needed in areas such as compliance with preventive maintenance requirement, personnel performance, and procedural adequacy which have been discussed in previous management meetings. 7 AA/I 4K ^' u Michael,. Davis, Et ject Manager Project Directorate V Division of Reactor Projects - 111, IV, V and Special Projects

Enclosure:

As stated cc: See next page 4-, y ..e.-- r.---..

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~ _a 9:- i Mr. Donald B. Karner . Arizona Nuclear Power Project Palo Verde Executive Vice President Post Office. Box 52^34 Phoenix, Arizona 85072-2034 CC: Arthur C. Gehr, Esq. Ms. Lynn Bernabei .Snell & Wilmer Government Accountability Project 3100 Valley Center of the Institute for Policy Studies Phoenix, Arizona 85073 1901 Oue Street, NW Washington,.DC 20009 Mrs James M. Flenner, Chief Counsel F,izona Corporation Commission Mr. Charles B. Brinkman, Manager 1200 West Washington Washington Nuclear Operations Phoenix, Arizona 85007 Combustion Engineering, Inc. 7910 Woodmont Avenue Suite 1310 Charles R.~ Kocher, Esq. Assistant Bethesda, Maryland 20814 Council ' James A. Bneletto, Esq. Southern California Eoison Company P. O. Box 800 Rosemead, California 91770 Mr. Mark Git:sberg Energy Director Office of Economic ?lanning and Development 1700 West Washington - 5th Floor Phoenix, Arizona 85007-Mr. Wayne Shirley Assistant Attorney General Bataan Memorial Building Santa Fe, New Mexico 87503 Mr. Tim Polich U.S. Nuclear Regulatory Commission P. O. Box 97.- Palo Verde Tonopah, Arizona 85354-0097 Regional Administrator, Region V .U. S. Nuclear Regulatory Commission 1450 Maria Lane Suite 210 Walnut Creek, California 94596 i l l l i

Arizona ?!uclear Power Project Palo Verde cc: 6 Chairman Arizona Corporation Commission Post Office Box 6019 Phoenix, Arizona 85003 Arizona Radiation Regulatory Agency ATTil: Ms. Clara Palovic, Librarian 4814 South 40 Street Phoenix, Arizona 85040 Mr. Charles Tedford, Director Arizona Radiation Regulatory Agency 4814 South 40 Street Phoenix, Arizona 85040 Chairman Maricopa County Board of Supervisors 111 South Third Avenue Phoenix, Arizona 85003 v

r .n August 12, 1988 l Investigation of the cause of insulation cracking continues. The staff will monitor the results as they become available. The staff requested copies of any supplements to the litansee's post trip review report which document the results of various bx".stigations that are still in progress. The-staff complemented the licensee on the thoroughness of the event analysis. Other staff comments cautioned the licensee that a number of concerns generated as a result of this event show that improvement is still needed in areas such as compliance with preventive maintenance requirement, personnel performance, and procedural adequacy which have been discussed in previous management meetings. original signed by G. Knighton Michael J. Davis, Project Manager Project Directorate V Division of Reactor Projects - III, IV, V and Special Projects

Enclosure:

As stated cc: See next page DISTRIBUTION E""' Docket file ? NRCTLocal PDRs PD5 Reading i MJDavis 0GC-Rockville. EJordan BGrimes NRC Participants ACRS(10) MJohnson (Region V) MMJ ydr DRS DR D5 s. GW s ton 8/19488 8/pf88

s e BRIEFING OF NRC STAFF ON UNIT 1 AUXILIARY TRANSFORMER FIRE ROCKVILLE, MD AUGUST 3, 1988

AGENDA UNIT 1 AUXILIARY TRANSFORMER FIRE PRESENTER I. INTRODUCTION A.C. R0GERS II. DESCRIPTION OF THE EVENT J.M. ALLEN III. ELECTRICAL DISTRIBUTION AND RELAYING L.L. HENSON IV. BUS DESIGN AND FAILURE L.L. HENSON l V. TRANSFORMER DESIGN AND TRANSFORMER J.T. BARROW FAILURE l l VI. CONCLUSIONS AND QUESTIONS A.C. ROGERS

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i a EVENTS AND CAUSAL FACTORS CHART SUDDEN PRESSURE RELAY ACTION INDICATES UAT CRACKED AND FAULT. BRITTLE SUDDEN PRESSURE INSUL ATIO N. FAULT CURRENT OF RELAY ABOU T 27,000 AMPS CAUSES GENERATOR-DIRT TO S02 AND INITIATED TRIP TO BUILDUP 127,000 AMPS THE MAIN TURBINE TO FAULTED UAT TURBINE GENERATOR GROUND FAULT IN OCCURS ON 13.8KV 4 AUXILIARY TRIP INITIATION BUS 1 E-N AN-SO2 TRANSFORMER 7/6/88 12:07:55 PM 12:07:55.122 PM 12:07:55 PM (14.5 CYCLES)

f TRIP INITIATION TO THE TURBINE.

GENERATOR CONTINUED TO FEED THE FAULTED UAT. JMA 08/03/88

u EVENTS AND CAUSAL FACTORS CHART LOSS OF 13.8 KV FAULT CURRENT POWER FROM CAUSES PROPAG ATION NAN S01 AND OF FAILURE NAN S02 CAUSED RCPs l IN H2Y WINDING TO LOSE POWER. IN BOTTOM DISK NUE DECLARED AUXILIARY REACTOR TRIP DUE TO FIRE IN TRANSFORMER PROTECTED AREA 4 FIRE ON LO DNBR 4 FOR >10 MINUTES 17.5 TO 20.5 CYCLES 12 07 55 DM 12:15 PM LOSS OF ALL NON CLASS lE AC POWER TO PLANT AUXILIARIES. l MA 08/03/88 l

EVENTS AND CAUSAL FACTORS CHART S02 RE ENERGIZATION WAS PRECEDED BY A BUS WALKDOWN AND CHECK OF FLAGS AND TARGETS BY PR&C AND OPERATIONS ATTEMPTED TO AUXILIARY BOTH DIESEL FROM NAN-SO4 AND TRANSFORMER GENERATORS WERE STARTED AND SU XFMR. BREAKER FIRE 4 LOADED TO SUPPLY 4 TRIPPED AND FIRE 4 EXTINGUISHED CLASS IE 4.16 Kv STARTED IN S02 NUE TERMINATED BUSES 12:21 PM 12:50 PM NUE REDECLARED 1:03 PM DELUGE SYSTEM AD BEEN ACTUATED LOSS OF S01 FOR ESF SERVICE AND S02 CAUSED TRANSFORMERS SO A LOSS OF PLANT NON. THERE HAD BEEN SPRAY ON ENERGlZED CLASS POWER. NO TRANSFORMERS. POWER FOR INSTRUMENT DECISION WAS MADE AIR, NUCLEAR COOLING TO DE ENERGlZE WATER, AND TURBINE COCLING WATER AND POWER CLASS BUS WITH DIESELS 08/03/83

EVENTS AND CAUSAL FACTORS CHART EXTENSIVE OPS SUPPORT INSPECTION CENTER ACTIVATED AND TESTING PER PLANT MANAGER WAS CONDUCTED DIRECTION AND DUE TO FAILURE OF TECHNICAL LIAISON N AN.S02. GROUP ASSEMBLED TO ENSURE BUS WAS MEGGERED SUFFICIENT AS WERE THE SUPPORT WAS IN DIVIDU AL AVAILABLE FOR BREAKERS RECOVERY RESTORED NORMAL POWER SUPPLY TO 1 E-N AN-S02 RESTORED POWER CLASS lE 4.16 KV BUS TO 13.8 KV BUS 1 E-P B A-S03 VIA ESF FIRE 1 E-N AN-S01 SERVICE XFMR AND EXTINGUISHED STOPPED DG A 1:22 PM 5:49 PM 10:01 PM AUXILIARIES WERE RESTORED AFTER RE.EN ERGlZ ATION OF NAN.S01 l JMA 08/03/88

EVENTS AND CAUSAL FACTORS CHART 1 VOLTAGE ON F B ATTERY BUS WAS LOW DUE TO EXTENDED LOSS OF NON CLASS AC. INCREASED VOLTAGE NECESSARY TO START RCPs. CONTROL POWER FOR S01 RCPs IS SUPPLIED FROM F i PROVIDED STARTED 1 A RCP TERMIN A~?ED TEMPORARY POWER TO ESTABLISH THE NUE y TO F BATTERY FORCED + CHARGER CIRCULATION 10:45 PM 7/7/88 12:33 AM 7/7/83 1:02 AM JMA 08/03/88

EVENTS,AND CAUSAL FACTORS CHART COMMENCED COOL ENTERED ENTERED DOWN FROM MODE MODE m MODE 3 4 5 TO MODE 4 7/7/88 1:12 AM 7/7/88 1:23 PM 7/12/88 5:46 AM l TEMPORARY POWER WAS REQUIRED FOR THE GAS STRIPPER TO ALLOW DEGASSING OF THE RCS PRIOR TO CONTINUING COOLDOWN TO MODE 5 JMA 08/03/88 l

ELECTRICAL DISTRIBUTION AND RELAYING

• SWITCHYARD AND UNIT ELECTRICAL DISTRIBUTION SYSTEM

- LOCATION OF INITIAL FAULT ON S02 - LOCATION OF SUBSEQUENT TRANSFORMER FAULT

• RELAY PROTECTION SCHEME

- STARTUP TRANSFORMER - AUXILIARY TRANSFORMER - 13.8 KV BUS LLH 08/03/88

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BUS DESIGN AND FAILURE

• SEQUENCE OF EVENTS
• BUS DESIGN
• ROOT CAUSE
• CORRECTIVE ACTIONS
• REC 0VERY ACTIONS i

l l LLH 08/03/88 c

DIGITAL FAULT RECORDER SEQUENCE OF EVENTS TIME (CYCLES) EVENT 0 CYCLES B PHASE-TO-GROUND FAULT ON 1E-NAN-S02. 1 CYCLE SINGLE PHASE FAULT PROPAGATES INTO THREE PHASE FAULT (27,500 AMPS), l 12 CYCLES UAT STARTS INTERNAL FAULT. PRESSURE AND GASSES BUILD. SUDDEN PRESSURE RELAY LOCK-0UT 286T2 INITIATES. 14.5 CYCLES SUDDEN PRESSURE RELAY LOCK-0UT PICKS UP WHICH RESULTS IN: - GENERATOR EXCITATION TRIPS. - TRIP SIGNALS INITIATED FOR BREAKERS S01A AND S02A AND MAIN GENERATOR BREAKERS 915 AND 918. - FAST-BUS TRANSFER SIGNAL INITIATED. - TURBINE TRIP SIGNAL INITIATED. 17.5 CYCLES UAT GOES INTO THREE PHASE FAULT. MAIN GENERATOR BREAKERS OPEN. UAT CASE RUPTURES. I 20.5 CYCLES BREAKERS S01A AND S02A OPEN. FAST BUS TRANSFER DOES NOT TAKE PLACE. l LLH l l 08/03/88 l \\ 1 L

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ROOT CAUSE INVESTIGATION VISUAL INSPECTION RESULTS

• CRACKED INSULATING MATERIAL (NORYL)

- unit 1:

1) outdoor switchgear S03, S05 and S06 experienced the greatest amount of cracking, SO4 had no cracks.

2) indoor switchgear S02 had very little cracking but had indications of foreign material.

3) indoor switchgear S01 had only several small cracks.

4) indoor switchgear NBN-S02, PBA-S03 and PBB-SO4 had no cracks.

- unit 2:

1) outdoor switchgear SO4 had no cracks.

• DIRT ON BUS AND INSULATING SI'PPORTS:

- unit 1:

1) outdoor switchgear S03, S05 and S06 were dirty.

2) outdoor switchgear SO4 and Indoor switchgear S01 and S02 were not as dirty.

3) NBN-S02 was fairly clean.

4) PBA-S03 and PBB-S04 were clean.

LLH 08/03/88

VISUAL INSPECTION RESULTS (CONTINUED)

• EVIDENCE OF ARC TRACKING

-indications were found:

1) under insulators on S03, S05 and S06.

2) on noryl adjacent to insulators.

- no indications were found on indoor switchgear S01 and S02, except where the fault occurred in the B cubicle. l l l l I LLH 08/03/88

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(3) 1E-NAN-S02B REAR PANEL COVER i NOTE: ARROW SHOWS BURN SPOT CAUSED BY .\\ INITIAL FAULT ~T ) NOTE: Il0LES CAUSED BY SECONDARY FAULT ~ l t "'r (4) IE-NAN-S02B ABOVE FAULT GE TO CALVERT BUS TIE NOTE: SHOKE DAMAGE CAUSED BY FIRE 1 I ?#'l"Z Z Z_7,W, t t ~~ 1 i. l l l t l T. 's . o r t

ROOT CAUSE

• ROOT CAUSE CONCLUSION

- noryl cracking alone can not cause fault. - two conditions are necessary for the fault to occur:

1) cracking of the insulating material, and

2) dirt on the horizontal insulated supports.

- fault conditions are most likely to occur on the vertical indoor bus sections.

• CONTINUING INVESTIGATIONS

- revise the preventive maintenance program for cleaning switchgear. - noryl cracking is still under investigation. possibly due to the use of an inappropriate cleaning material or exposure to heat. I LLH 08/03/88

CORRECTIVE ACTIONS

• VERIFICATION GF PROTECTIVE RELAYING CALCULATIONS AND REl,AY SETTINGS IN UNIT 1.
• UNIT 1

- inspect all 13.8 kv switchgeer busses. - clean all dirt from the busses and supports. - tape cracked noryl insulation in areas susceptible to accumulation of dirt.

• UNIT 2

- preventive maintenance tasks were performed for the 13.8 kv busses in 1987-88 except for S02 (S02 scheduled for next refueling). - visual inspection to be performed on S02.

• UNIT 3

- visual inspections to be performed on S03 and S05. LLH 08/03/88

RECOVERY ACTIONS

• SPARE AUXILIARY TRANSFORMER INSTALLED

- TESTING PERFORMED:

1) DOBLE TEST

2) TRANSFORMER TURNS RATIO TEST

3) OIL ANALYSIS

4) DEVICE VERIFICATION

• MAIN TRANSFORMER CONTROL CABINET DAMAGE

- RETEST OF MAIN TRANSFORMER

• MAIN GENERATOR INSPECTION

- VISUAL - MEGGER - TEST OF CURRENT TRANSFORMERS

• 525 KV LINE REPAIR AND INSULATOR REPLACEMENT
• FINAL TESTING

- BACK-FEED ENERGIZATION TEST l LLH 08/03/88

UNIT AUXILIARY TRANSFORMER DESIGN AND FAULT

• DESIGN

- RATINGS - CONFIGURATION - QUALITY

• FAULT

- INDICATIONS - EVALUATION - SCENARIO - CONCLUSIONS JTB i 08/03/88

UNIT AUXILIARY TRANSFORMER DESIGN

• RATINGS

- 3 PHASE, 24 Kv / 13.8 KV Y - 105-52.5-52.5 / 140-70-70 MVA OA/FA - HIGH VOLTAGE TAPCHANGER:

1) ONE 2.5% TAP ABOVE 24 KV

2) THREE 2.5% TAPS BELOW 24 KV

• CONFIGURATION

- MANUFACTURED BY WAGNER ELECTRIC CORP. CORE FORM - SPLIT HIGH VOLTAGE (H1 AND H2) WINDING TO BALANCE LOW VOLTAGE WINDIMGS (X AND Y) - CONTINUOUS DISC HIGH VOLTAGE WINDING 4 - HELICAL LOW VOLTAGE WINDING - COPPER CONDUCTORS 4

• QUALITY

- ANPP/APS CONDUCTED CORE AND COIL INSPECTIONS I PRIOR TO TANKING - UNITS WERE VERY CONSERVATIVELY DESIGNED. l THRU FAULT RATING LIMITED TO 12.5 TIMES RATED CURRENT FOR 5 SECONDS (13,75 AT 110% OVER-VOLTAGE) - HIGH TO LOW VOLTAGE WINDINGS WELL BALANCED JTB 08/03/88

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UNIT AUXILIARY TRANSFORMER FAULT

• INDICAT10NS:

- HV AND LV WINDINGS WERE NOT AXIALLY DISPLACED. - PRESSURE RINGS AND CLAMPS WERE NOT DISTORTED. COILS WERE NOT SKEWED (RADIAL SPACERS STRAIGHT). - ALL 15 TURNS OF THE 40TH DISC WERE EXPELLED. - EXPELLED TURNS ARCED TO TAPCHANGER LEADS AND TO TANK WALL. - TANK WALL BULGED AND SPLIT AT SEAMS. - EXTENSIVE FIRE DAMAGE.

• EVALUATION

- THRU FAULT DID NOT EXCEED RATING. - EXPULSION OF ALL 40TH DISC TURNS INDICATES FAULT INITIATION POINT AND IMPLIES THE l CAUSE TO BE A LOCAL DEFECT. i - ARCING INDICATES ORDER OF FAULT PROGRESSION: TURN TO TURN / PHASE TO PHASE / THREE PHASE TO GROUND. - CASE DAMAGE DUE TO GAS PRESSURE DURING INTERNAL FAULT. FIRE DAMAGE DUE TO ARC AND OIL VAPOR HITTING AIR DUE TO RAPID OIL EXPULSION. JTB l 08/03/88

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O O 6 1. Probable Initiation ,m ,m md "fmm . t i.u - - - . iM u,, m_. _ T ' ' ^ ,m m ~ P,, a ~ h M Turn to Turn Short ( 22 volts) (Very low impact during operation) 2. Second Stane ,v ,m&b ~ymn ~_A ^ ^ ' ll

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,7 y V /)Jvm >L _u ,,)r tT t (CoiltoCoilShort( 330 volts) (Significant impact during operation-Failure Initiation During thru fault) Coil l 1 1 I f _ W l ~ 1 i This was probably initiated by the thru fault. It causes massive heating in the winding. This heating results in conductor melt, turn and coil movement, and subsequent increase in local electromagnetic forces. l t l l l [ l

/ t 3. Third Stage A To A Tapchanger cc C C F( ') ) - r~~ d e a ~ j d{

m.,

'~ -y- ^N Coil Eicction -*A hf i Maior Faultina, Helting A L From #A 4 Fourth Stage A To Tapchanger CP 4 h Coil ' f'z) 13( l tw_ A7 JM 3 d! n 7/,/, +o Fcu t \\ s j 1 i. FromQA /toGroundFault Against Tank Wall l L

s Tcnk Top s ^ j 6 6 n x ::::::

_:: ::- -^^^

1. Fault Initiation 2. Cas Bubble Core Form Tapchanger 3. Normal Oil level 4 Oil Piston to Tank Top 5. Oil pressure to w seams (Top) 6. Ruptures top 2 A p & bottom 7. Exploston 1,g Fire when top t 3 leaks expose I fault to air N ~ ~ '5 6- , n, 1 4, 6 2 s //CM J%:. L / W Core Coils (Bottom) 4 4 + 4 des d ve 1. Same 2. Same 3. Same Tapchanger Shell 4. Same - but tank Form top lifts up 5. Same f 3 6. Same - but Coils bottom only generally no fire 4 4 because tank top p f 1 gg p settles down after pressure V VL_y_u =7 subsides a Core '~'N'- y' - "2 ~6 6 p5 5 6 t ./ 6

.o, UNIT AUXILIARY TRANSFORMER FAULT (CONTINUED)

• CONCLUSIONS

- transformer fault was caused by the thru fault which exacerbated an existing singular material or winding defect. - failure mode is not transportable to other units since likelihood of defect repetition is low. - defects can be detected by periodic dissolved gas in oil analysis (dga). - currently evaluating additional measures to detect dissolved gasses. I I JTB 08/03/88

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