Text

Nonproprietary Version of Transformer Rept on Events at North Anna Power Station
	ML20080Q732
Person / Time
Site:	North Anna
Issue date:	06/16/1983
From:	Cossaart J, Petrie E, Skooglund J; WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:	;
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ML19268E158	List: ML20080Q720; ML20080Q727; ML20080Q732;
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ATTACIDIENT 2 8310170107 831005 PDR ADOCK 05000338 S PDR

4 f

TRANSFORMER REPORT i

ON EVENTS AT NORTH ANNA POWER STATION s

4 June 16, 1983

' - POWER TRANSFORMER DIVISION l

WESTINGHOUSE ELECTRIC CORPORATION GREENTREE, PA 15220 t

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PRINCIPAL REPORTERS J. Cossaart E. Petrie J. Skooglund A. Sletten J. Templeton L. Wagner D. Yannucci I

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. l TABLE OF CONTENTS Page No._

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1 Scope . .

..........., .. ..... 2 1.

Executive Suraary . . . . . . . . . . . . . 10 II.

III. Introduction Conclusions . . . .

17 IV. .... *****

Correlation Analysis .- *** **

V. ...

33 VI. Summary of Areas of Investigation ............

...... 39 System Investigation . .. . . . . . .

VII.

VIII.

IX. Gas Evolution Causes Transformer Processing Record Review 42 .

..... 58

< X. Oil Electrification Effect Configuration .. . .

Dielectric Analysis of Line Lead XI. 70 Impulse Distribution and Resonant.................

XII. Frequency Analysis ......73 System Sensitivity Tests of P.D. Waveguide XIII.

APPENDICES 75 88 Trip Reports Appendix A: .

Transformer Report on Events .

169 Appendix B:

Processing Records .

214 Appendix C: . ... .

Appendix D:

Calculation of Tank Vacuum 231 i

Appendix E:

011 Characteristics and Gas Analys s7001965 and C-0-645 of Serial .- ********

Gas-In-011 Analysis Results . . .

Appendix F: .

266 Trip Report on Hydraulic.............. 269 Appendix G: Experiments in HAM 707 . ........

Blue Ribbon Committee Organization nization . . . .

271 Appendix H:

Transformer Evaluation Committee Orga Appendix I:

1. SCOPE This report covers the investigation of the transformer events at VEPCO's North Anna power station from November of 1980 through March of 1983. The report is applicable to shor order HAM 707 on which 7 units were manuf actured.

Included are dielectric, fluid flow, gas evolution and processing considerations relating to the transformer. Also included is a system analysis relating to the event occurring on December 5, 1982.

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II. EXECUTIVE

SUMMARY

Seven transformer failures occurred at VEPCO's North Anna power station from November of 1980 to December of 1982. This report covers the investigation of these failures.

None of the failures could be reproduced in the laboratory, thus no definitive conclusions can be reached as to the exact cause of any of the failures.

The following statements can be made on each failure:

O 1491E 2

FAILURE #1: NORTH ANNA #2 - HV LEAD TO LV COIL (2099)

Rebuilt ship: 1/77 Processed last: 2/79 Failed at N.A.: 11/80 Phase 'A' Additional transportation involved prior to rebuild.

. Unit involved in failure at Bowen and rebuilt at Muncie.

. No f actory test difficulties on original or rebuilt unit.

. Unit did have overexcitation.

. Processing of main unit was done for a considerable length of time before the failure.

. Mis operation of inertnire system did occur.

. Gas bubbles do occur in the oil under this condition.

Dielectric strength degradation of lead configuration due to gas bubbles in the oil does occur.

Simulated lead configuration withstood 400 kV for 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months with and without bubbles present -- corona was observed for large bubbles.

. No other source of bubbles found. .

1491E 3

FAILURE #2: NORTH ANNA #2 - HV BUSHING (2100)

Original ship: 2/74 Processed last: 2/79 Failed at N.A.: 6/81 Phase 'C'

. No f actory test problems.

. Additional transportation was involved.

In bank at Georgia Power when 2099 failure occurred.

. Unit overexcited.

. In bank when 2099 failure occurred at N.A.

. Failure involved bushing only. .

.. Processing was done a considerable length of time before failure.

. HV bushing was not stored properly.

1491E 4

FAILURE #3: NORTH ANNA #2 - HV BUSHING, LV COIL AND TANK

.(2098)

Original ship: 2/74 Processed last: 6/81 (Energized)

Failed at N.A.: 7/81 Phase 'B'

. No f actory test difficulties

. In bank at Georgia Power when 2099 failure occurred.

Additional transportation was involved.

. Unit processed one month prior to failure.

. In bank when 2100 and 2099 failures occurred. .

. Lead configuration changed 6/81.

. HV bushing was not stored properly.

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FAILURE #4: NORTH ANNA #2 - G.P. UNIT - HV LINE COIL, LV COIL, TANK (1527)

Original ship: 5/70 Processed last: 6/81 Energized: 7/81 Failed at N.A.: 7/81 Phase 'C'

. No f actory test difficulties.

. In bank when two other transformer failures occurred at G.P.

Dielectrically tested above rated design level at Muncie plant.

. In bank when 2098 difficulty occurred.

Failed immediately af ter energizing subsequent to 2098 f ailure at N.A. .

. Large quantities of carbon observed during failure observations.

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FAILURE #5: NORTH ANNA #2 - HV BUSHING, CORONA SHIELD AND LV WINDING AND TANK (2100 Repaired at Muncie)

Rebuilt ship: 10/81 Processed last: 3/82 Energized: 3/82 Failed at N.A.: 8/82 Phase 'B'

. Bushing transportation.

At Muncie, lead configuration changed to present practice during rebuild.

. No f actory test difficulties.

. Processed five months prior to failure -- factory tested bushing installed 1/82 - unit was final processed 3/82 -- energized at intervals for five months. -

Mechanically fatigued bolt in bushing assembly found -- failure location in bushing correlates with physical evidence of bolt and washers.

. Possible physical evidence of oil electrification in coil assembly.

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FAILURE #6: NORTH ANNA #1 - HV BUSHING SHIELD AND LV COIL (1995) original ship: 8/73 Processed last: 8/82 Energized: 11/82 Failed at N.A.: 11/82 Phase 'C' No f actory test difficulties.

In service since 1978 without difficulty.

. System configuration changed with addition of generator breaker prior to problem.

. Pumps removed and refurbished.

. Processed prior to difficulty. ,

.- Ambient temperature change of 30*C noted two days prior to energizing.

Cooler initiation and operation in accordance with task force report (coolers not operated prior to energization.)

To date, gas bubbles have not been produced from the lead or oil in laboratory experiments except under vacuum.

Bushing / lead configuration simulation withstcod 400 kV for 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months with and without bubbles -- corona observed for large bubbles.

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FAILURE #7: NORTH ANNA #1 - HV LEAD, LV COIL, TURN-TO-TURN FAILURE (1994)

Original ship: 7/73 Processed last: 12/82 Energized: 12/82 Failed at N.A.: 12/82 Phase 'B'

. No f actory test difficulties.

In service since 1978 without difficulty.

. System configuration changed with addition of generator breaker prior to problem.

. Pumps removed and refurbished due to metallic contamination, unit flushed.

. In bank during 1995 failure.

. Retrofitted with COPS.

. Removed barrel barrier,

, Partial discharge field test with waveguides installed.

. Pumps run prior to energization per instructions.

. To date, gas bubbles have not been produced from lead experiment except under vacuum.

. Bushing / lead configuration simulation withstood 400 kV for 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months with and without bubbles - corona observed for large l

bubbles.

l j . To date, lab tests have not produced high turn-to-turn stresses in area of turn-turn failure observed for a high-low failure I mode.

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III. INTRODUCTION Seven single phase generator step up transformers were manufactured and delivered to Virginia Electric Power Company in 1973 and 1974 for operation at the North Anna station. These are identified as S.O.11AH707. The units are rated 330 MVA with a 55"C rise and 369.6 MVA with a 65'C rise. The high voltage winding is 1300 kV BIL on the line end and 150 kV BIL at the neutral.

The low voltage is 150 kV BIL. The nominal voltage is 500000 grd. Y/288675 to 22000 volts. There are de-energized taps at 303110, 295890, 281460 and 274240 volts. The cooling system is "FOA" and the oil preservation system is -

"inertaire" designed to operate between 0.5 and 8.0 psi pressure.

In addition there was involved a spare transformer that Virginia Electric Power Company borrowed f rom the Georgia Power Company. It is identified as S.O. HAM 390. It is a single phase 277 MVA, 500000 grd. Y/288675 to 23800 volts.

The history of each of these units is detailed as follows:

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Serial 7001965 S.O. HAM 707

1. Tested originally 7/20/73.

2. Operated commercially.

3. Installed waveguide and " COPS" system - 11/82. .

4 Removed barrel barriers from H.V. bushing and made general internal inspection -- 11/22/82. See J. B. Templeton's trip report of 12/15/82. (1)

5. Field corona tests by VEPCO -- 11/30/82 to 12/3/82. See W. J. Carter's trip report of 12/16/82. (2)

(1) Appendix A - Item 1 (2) Appendix A - Item 2 Serial 7001993 S.O. HM4707

1. Tested originally 7/24/73. .

2. Originally the spare for the North Anna Station.

3. Put in place of serial 7002099 which failed on 11/29/80.

4 500 kV lead changed from wire of coil and brush copper section to cable --

June, 1981. (Muncie Personnel)

5. 500 kV lead rechecked for quality -- July, 1981. (Muncie Personnel).

6. 500 kV bushing removed and replaced with bushing from serial 7002099.

Bushing from serial 7002099 tested 7/11/81 in Muncie and returned to VEPCO.

Pumps replaced -- July, 1981. New pumps were sent from Muncie.- hhe

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replaced pumps returned to Muncie were dismantled and two indicated signs of bearing wear.

8. Unit flushed to remove bearing particles -- July,1981.

l 9. VEPCO accident -- vater introduced inside unit.

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' 10. Unit put on dry out process 7/19/81.

11. Waveguide installed 8/26/82.

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Serial 7001994 S.O. HAM 707

1. Tested originally 7/26/73,

2. Operated commercially.

3. Installed " COPS" system - 11/82.

4. Removed barrel barriers from H.V. bushing and made general internal inspection -- 11/22/82. See J. B. Templeton's trip report of 12/15/82. (3)

5. Field corona tests by VEP00 -- 11/30/82 to 12/3/82. See W. J. Carter's trip report of 12/16/82. (4)

6. Failed 12/5/82. See W. J. Carter's and D. White's teardown report of 1/10/83. (5)

Serial 7001995 S.O. HAM 707

1. Originally tested 8/3/79.

2. Operated commercially starting in 1974 in North Anna #1.

3. De energized May 1982 4 Oil drained and refilled with new oil August 1982.

5. Failed 11/16/82 after three hours and six minute after energizing. See H. R. Moore's report of 12/3/82. (6)

(3) Appendix A - Item 1 (4) Appendix A - Item 2 (5) Appendix B - Item 1, .

(6) . Appendix'B - Item 2 1491E 12 l

Serial 7002098 S.O. HAM 707

1. Tested originally 2/20/74 4 2. Stored at VEPCO.

3. Loaned to Georgia Power Company for operation at Plant Bowen -- May, 1976.

4 Returned to Muncie on S.O. XDM1324 for retest -- tested 9/2/76.

5. Reshipped to VEPCO.

6. Overexcitation reported (140.9% for 42 seconds) -- 2/6/79.

7. 500 kV lead changed from wire of coil and brush copper section to cable --

June, 1982. (Muncie Personnel)

8. Failed while in service at North Anna Station -- 7/3/81.

9. Remains of 500 kV bushing returned to Muncie. See bushing report, serial 6, of 8/3/81 by L. B. Wagenaar. (7)

10. Pumps returned to Muncie.

11. Scrapped in the field - 9/81.

(7) Appendix B - Item 8 D

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Serial 7002099 S.O. HAH 707

1. Tested originally 2/22/74

2. Stored at VEPCO.

3. Loaned to Georgia Power for operation at Plant Bowen -- May,1976.

4 Failed while in service at Plant Bowen -- May, 1976. See Engineering Lab Report #41-5F by Puri/Hansen - 7/30/76. (8)

5. Returned to Muncie for repair on S.O. XDM1323.

6. XDM1323 tested on 1/31/77.

7. Reshipped to VEPCO.

8. Missing hardware problem. Conclusion it was never shipped. See I. L. ,

Hansen's trip report of 3/24/77. (9)

9. Overexcitation reported (140.9% for 42 seconds) -- 2/6/79.

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10. Failed while in service at North Anna Station -- 11/29/80. See trip report by B. W. Hugon of 12/17/80 (10) and disassembly report by L. E Luke of 6/5/81 (11).

11. Repaired on S.O. XDM1742.

12. Retested on 7/24/81 and reshipped to VEPCO.

13. Waveguide installed - 8/26/82.

(8) Appendix B - Item 3 (9) Appendix A - Item 3

-(10) Appendix B - Item 4 (11) Appendix B - Item 5 i

1491E 14

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Serial 7002100 S.O. HAM 707 I

J 1. . Tested originally on 3/4/74 i i

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2. Stored at VEPCO.

3. Loaned to Georgia Power Company for operation at Plant Bowen -- May,1976.

4. Returned to Muncie on S.O. KDM1324 for retest -- tested 9/23/76.

5. Reshipped to VEPCO.

6. Overexcitation reported (140.9% for 42 seconds) -- 2/6/79.

See disassembly

7. Failed while in service at North Anna Station -- 6/19/81.

report of 10/13/81 by B. W. Hugon. (12)

8. Remains of 500 kV bushing returned to Muncie. See bushing report, serial 7, of 8/3/81 by L. B. Wagenaar. (13)

9. Pumps returned to Muncie.

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10. Repair order -- KDM1779. Rebuilt with new phase. Tested 10/21/81.

11. Failed while in service at North Anna Station - 8/22/82. See R&D report 82-7D7-MUNCI-R1, 10/6/82. (14)

12. Repair order - KDM1867. See teardown report of 2/23/83 by Dale White. (15)

(12) Appendix B - Item 6 (13) Appendix B - Item 9 (14) Appendix B - Itra 10 (15) Appendix B - Item 7 Serial 7001527 S.O. HAM 390

3. Tested originally 5/21/70.

2. Operated at Georgia Power Company, Bowen Plant.

3. Removed from service after failure of serials 7001526 and 7001528 --

March, 1976.

4 Returned to Muncie for tests on S.O. KDK1316.

5. Dielectric tests made at 1175, 1300, 1425, 1550 kV BIL.

6. Returned to Georgia Power Company, Bowen Plant, as a spare.

7. Shipped to VEPCO to replace Serial 7002100 -- June, 1981.

8. Failed at VEPCO North Anna Station -- 7/25/81.

9. Unit disassembled in the field and scrapped - 8/81.

1491E 15

i A summary of the failure points for each of the incidents is as follows:

FAILURE POINTS HV HV HV To HV Corona HV Line Series LV LV Serial Shipped Failed Bushing Shield Lead Coils Conn._ % % Tank X X 7002099 1/77 11/80 7002100 2/74 6/81 X X X X 7002098 2/74 7/81 X X X 7001527 5/70 7/81 X X X X 7002100 10/81 8/82 X X 7001995 8/73 11/82 X X X 7001994 7/73 12/82 .

7001993 7/73 ---

7001965 7/73 ---

1491E 16

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IV. CONCLUSIONS The investigation has not been able to simulate or duplicate the failure mechanism observed at the North Anna power station.

In general, the dielectric studies have indicated the following:

1. For large gas bubbles at high velocity oil flow rates, partial

! discharge was experienced but ao flashovers occurred.

2. The high voltage line lead configuration was stressed at 400 kV to ground for a time equivalent to 5.7 years service at nominal operating voltage. This time equivalent is based on a volt-time probability function.

3. The effect of an improper taper joint in the taping of the high voltage line lead did not signficantly affect the partial discharge levels.

4. This combination of gas bubbles in the oil system and induced static electrification of the oil did increase the partial discharge levels but no flashovers occurred. ,

5. No evidence of problems with winding resonance effects could be determined.

6. No evidence of high transient voltage effects could be determined.

7. Gas trapped in taped insulated leads did not evolve into the oil as bubbles unless a negative pressure occurred above the oil.

8. No apparent system related problems have been determined.

A summary of each individual transformer is as follows.:

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FAILURE #1: NORTH ANNA #2 - MV LEAD TO LV COIL (2099)

Rebuilt ship: 1/77 Processed last: 2/79 Failed at N.A.: 11/80 Phase 'A' Additional transportation involved prior to rebuild.

. Unit involved in failure at Bowen and rebuilt at Muncie.

No f actory test difficulties on original or rebuilt unit.

. Unit did have overexcitation.

Processing of main unit was done for a considerable length of time before the failure.

. Mis operation of inertaire system did occur.

4

. Gas bubbles do occur in the oil under this condition.

Dielectric strength degradation of lead configuration due to gas bubbles in the oil does occur.

Simulated lead configuration withstood 400 kV for 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months with and without bubbles present - corona was observed for large bubbles.

. No other rource of bubbles found. ,

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L FAILURE #2: NORTH ANNA #2 - HV BUSHING (2100)

Original ship: 2/74 Processed last: 2/79 Failed at N.A.: 6/81 Phase 'C'

. No factory test problems.

. Additional transportation was involved.

In bank at Georgia Power when 2099 f ailure occurred.

. Unit overexcited.

. In bank when 2099 failure occurred at N.A.

. Failure involved bushing only. ,

. Processing was done a considerable length of time before failure.

. HV bushing was not stored properly.

1491E 19

FAILURE #3: NORTH ANNA #2 - HV BUSHING, LV COIL AND IANK (2098)

Original ship: 2/74 Processed last: 6/81 ,

(Energized)

Failed at N.A.: 7/81 Phase 'B'

. No factory test difficulties In bank at Georgia Power when 2099 f ailure occurred.

Additional transportation was involved.

. Unit processed one month prior to failure.

. In bank when 2100 and 2099 f ailures occurred.

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. Lead configuration changed 6/81.

. HV bushing was not stored properly.

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FAILURE #4: NORTH ANNA #2 - G.P. UNIT - HV LINE COIL, LV COIL. TANK (1527)

Original ship: 5/70 Processed last: 6/81 Energized: 7/81 Failed at N.A.: 7/81 Phase 'C' No f actory test difficulties.

. In bank when two other transformer failures occurred at G.P.

Dielectrically tested above rated design level at Muncie plant.

In bank when 2098 difficulty occurred.

Failed immediately af ter energizing subsequent to 2098 f ailure at N.A. .

Large quantities of carbon observed during failure observations.

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FAILURE #5: NORTH ANNA #2 - HV BUSHING, CORONA SHIELD AND LV WINDING AND TANK 4

(2100 Repaired at Muncie)

Rebuilt ship: 10/81 Processed last: 3/82 Energized: 3/82 Failed at N.A.: 8/82 Phase 'B'

. Bushing transportation.

. At Muncie, lead configuration changed to present practice during rebuild.

. No f actory test difficulties.

. Processed five months prior to failure -- factory tested bushing installed 1/82 - unit was final processed 3/82 - energized at intervals for five months.

. Mechanically fatigued bolt in bushing assembly found -- failure location in bushing correlates with physical evidence of bolt and washers.

. Possible physical evidence of oil electrification in coil '

assembly. ,

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i FAILURE #6: NORTH ANNA #1 - HV BUSHING SHIELD AND LV COIL (1995)

Original ship: - 8/73 Processed last: 8/82 Energized: 11/82 Failed at N.A.: 11/82 Phase 'C'

. . No f actory test difficulties.

In service since 1978 without difficulty.

. System configuration changed with addition of generator breaker prior to problem.

. Pumps removed and refurbished.

. Processed prior to difficulty. .

. Ambient temperature change of 30*C noted two days prior to energizing.

f Cooler initiation and operation in accordance with task force report (coolers not operated prior to energization.)

.- To date, gas bubbles have not been produced from the lead or oil in laboratory experiments except under vacuum.

. Bushing / lead configuration simulation withstood 400 kV for 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months with and without bubbles -- corona observed for large

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bubbles. .

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FAILURE #7: NORTH ANNA #1 - HV LEAD, LV COIL, TURN-TO-TURN FAILURE (1994)

Original ship: 7/73 Processed last: 12/82 Energized: 12/82 Failed at N.A.: 12/82 Phase 'B'

. No f actory test difficulties.

In service since 1978 without difficulty.

System configuration changed with addition of generator breaker prior to problem.

. Pumps removed and refurbished due to metallic contamination, unit flushed.

. In bank during 1995 failure.

. Retrofitted with COPS.

Removed barrel barrier, Partial diacharge field test with waveguides installed.

. Pumps run prior to energization per instructions.

To date, gas bubbles have not been produced from lead experiment except under vacuum.

. Bushing / lead configuration simulation withstood 400 kV for 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months with and without bubbles -- corona observed for large bubbles.

. To date, lab tests have not produced high turn-to-turn stresses in area of turn-turn failure observed for a high-low failure mode.

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-V. CORRELATION STUDIES A multidimensional correlation study has been performed on all 500 kV generator step up transformers (GSU) manuf actured by Westinghouse and subsequently put into service. The categories selected for the cross correlation are as follows: ,

1) Customer (VEPCO and Others)

2) Winding BIL (1300 kV, 1450 kV+)

3) Internal work (performed on site) (Yes, No)

4) Previous involvement (in a bank during failure) (Yes, No) ,

5) Bushing BIL (1300 kV, 1450 kV+)

6) High voltage lead (horizontal, vertical)

Each of the above six categories are subdivided into two classes as indicated, thus creating 26 = 64 cells. One of the two classes in each category is perceived to have possible adverse effects on the f ailure rate.

(Notes: Three phase transformers are counted as three transformers since each phase has had a chance to f ail. Failure of a three phase transformer is counted as one failure. A rebuilt transformer is counted as a new transf o rme r. Internal work is defined as the situation when a transformer is ,

drained and actual physical work is performed inside before reprocessing and reene rgization. Initial installation and filling of the transformer on site is not counted as Internal work. The remaining counting rules are self explanatory).

l 160 transformers (500 kV GSU) have been manufactured and rut into service and of these 16 have failed. The failure rate is therefore 0.1 or 10% of the population. Each of these 160 transformers was classified into one of the 64 cells. Table I shows the number of transformers in the upper lef t corner and the number' of failed transformers in the lower right hand corner. As had been

anticipated only a small portion of the cells, actually 11, contain any transformers. These 11 cells are listed.in Table II in descending order of

the number of t'ransformers in ea'ch cell.

Data analysis was carried out on these 11 cells as shown in Table III. Each cell is divided into success and failure. Expected success (90%) and failure (10%) is calculated and the contribution to the X2 value for each cell ;

calculated. .

y 2 ,-J{j{

1913 - Eij)2 Aji Eij where Oij are' observed and Eij are expected values. (i designates the column and j the row in Table III).

Eij Oij 1491E 25 l

By definition the degrees of freedom is:

D = (1-1)(j-1) i which in this case is 11. X2 is used as a measure of the deviation from a random distribution. If the failures 2were truly randomly distributed there would be only a 1% chance to obtain X greater than 24.7. The actual calculated vaine is 83.3. Therefore the probability of the failures coming Cell 7 cont ftorom the X a pgrely randos distribution is extremely small.value, but is even if cell 7 is di still higher than expected from a random distribution.

If next the elimination of any one of the chosen six categories is considered, cell 7 stays intact in all cases. This can easily be seen by noting that cell 7 in Table I has no non-empty cell in the vertical or horizontal direction.

Cell 7 has the combination of all the perceived adverse effects on the failure rate from all the six categories.

' Cells 8 to 11 contain only two or one transformers and are therefore 4

statistically not very meaningful. They were therefore next combined into two cells based on whether the transformer has a horizontal or vertical high voltage lead. The first seven cells were kept intact. Table IV shows the result of a similar analysis to that in Table III. Cell 8x in Tr.ble IV is the combination of the original cells 8 and 11, while es119x is the combination of the original cells 8 and 11, while cell 9X is the combination of the original cells 9 and 10 which also results in losing the category of ;

C Cell 9x now contributes 18 to the Xgstomer(VEPCO,Others) value while cell 8x is relatively incell 9X. benign.

L Similar results are obtained with two other approaches dealing with cells 8 to l

11, if cells 8 to 11 are lumped into one new cell or lumped into two new cells

, based on whether the customer was VEPCO or not. In both these cases one new cell has a substantial contribution to X2 indicating that cell 7 is not the only contributor ,to the lack of random behavio'r of th.e f ailure rate.,

Eliminating any two of the original categories, similar results are obtained.

That is, there remains at least two cells contributing significantly to X2, Eliminating three of the categories, cell 7 stays intact with its 100% failure rate in most of the possible combinations of the remaining three perceived adverse effects on the failure rate. One particular case gives an interesting result and is described below:

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i of the five non-empty cells, cells A and D contain only VEPCO transformers ,

except on ein cell D. Only cell D has a failure rate which deviates I

significantly from the average failure rate. Cell A and D are comparable in size and both have a low winding BIL. Cell D contains nearly half the f ailed transformers. However, cell 7 stays intact with its 100% f ailure rate if the

categories of Previous involvement, Customer and Internal work are the three categories retained. Those are the categories eliminated in cell D. Another example resulting in a cell with 100% failure rate, actually six failures out of six transformers, occurs when the categories of Customer, Bushing BIL and Winding BIL are retained, in which cells 7 and 10 combine. Conversely 3 if the categories Internal work, Previous involvement and H.V. lead are retained, cell 7 stays intact.

i The sliainations of categories described above demonstrates the lack of data for comparison in the available data base. No particular combination of the perceived adverse effects appears to be significantly worse or better than any other combination because cell 7 has such a small amount of neighboring data *

(Table 1).

a i Discussion

~

The results of this analysis indicate that there is a significant deviation ,

f rom random behavior of the failure rate with a strong correlation to a combination of perceived adverse effects on the failure rate. Yet no single one of the perceived adverse effects is needed to show the strong deviation from a truly random failure rate.

It is speculated that a physical reason or reasons for many of the failures have occurred in correlation with many of the perceived adverse effects, but j

these physical reasons are not known or associated with any particular

)

combination of the initially perceived adverse effects. The extreme deviation l

.from random behavior of the combination of all the perceived adverse ef fects (cell 7) suggests that one or more unknown adverse physical conditions might have been associated with that group. However, such physical conditions may or may not be associated with any or all the' perceived adverse effects on the i failure rate.

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A 1491E 27

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VI. AREAS OF INVESTIGATION Subsequent to the seventh failure of the Westinghouse transformers at the VEPCO North Anna power generating station a comprehensive program was initiated to study various aspects of the situation. The Westinghouse study included investigations into the transformers as well as the power system into which it was integrated.

The power system study was conducted by reviewing the data accumulated from the recording devices on the system and through analytical calculations.

Calculations of voltages and currents for various scenarios of operation were completed and concentrated on the generator side of the transformer.

Along with the power system study, several investigations were made concurrently concentrating on the internal operation of the transformer. A study of the transformer hydraulic circuit consisted of a review of field

! processing records, a hydraulic flow analysis, analysis of gas and moisture content in oil samples, and a gas in oil equilibrium analysis. In support of the previous tests, various electrical tests were performed with a high The voltage lead configuration as existed in the North Anna transformers.

purpose of the electrical tests was to determine if the insulation integrity ,

degraded as the temperature and pressure changes of the oil produced on equilibrium change in the dissolved gases in the oil thus producing bubbles.

Additionally, experiments were performed to determine any electrical effects from static electrification of the oil on the integrity of the high voltage lead insulation.

One of the failures identified in the seventh incident was a turn-to-turn fault in coil 1 of the low voltage winding. In order to determine the sequence as to when the fault occurred, a low voltage impulse distribution test was done to simulate a high voltage to low voltage are with measurerants to quantify the subsequent turn-to-turn stresses. Also, a review was maee of the resonant frequency response of the transformer to determine if a low voltage bus ' duct flashover c' o u'1d. result' in transient voltages in the high vo,ltage winding.

The transformer involved in the seventh incident had been equipped with an acoustical waveguide inside the transformer to detect incipient electrical problems during the operation of the transformer. A test was performed to determine if the waveguide would have had suf ficient sensitivity to detect the turn-to-turn fault in the low voltage if the fault existed prior to the failure of the transformer.

In reviewing the seven incidents at North Anna relative to the field experience of other 500 kV units, the need was identified to conduct a statistical analysis of 500 kV transformers. Thus, a statistical correlation analysis involving multiple variablen was completed.

The analysis of the North Anna incidents has entailed a comprehensive investigation. The details of the various studies are included in this report.

1491E 32

VII. SYSTEM INVESTIGATIONS FOR VEPCO NORTH ANNA TRANSFORMER - GENERATOR FAILURE OF DECEMBER 5, 1982 This report summarises the results of the system investigations made by T&DSE to determine the possible causes of the VEPCO North Anna transformer and generator f ailures on December 5,1983.

Analysis of Oscillograms and Events Records i

The oscillograms for the December 5 failure as well as those from the previous August and November f ailures were examined. The only quantities measured were those from the 500 kV side of the step up transformer with no generator or

' low-side quantities monitored. From this limited data nothing unusual could be determined. No external system disturbance was apparent that could have caused the failure.

The handwritten and typed notes on the sequence of events prior to and following the failure prepared by the VEPCO personnel was examined. Ce rtain items from these noted led to the following investigations.

3E nBus Relay Operation On Friday, December 3 it was noted that the 3Eo bus relay operated when the transformer was being backfed from the 500 kV system with the generator breaker open. This relay is connected across the broken delta of a set of potential transformers connected to the low voltage bus between the generator breaker and the low voltage winding of the transformer. A 48 ohn resistor is connected across the relay to stabilize the neutral of the low voltage system. The relay was set to trip at 19 volts which represents an Eo on the system of 19/208 or approximately 10 percent.

It was originally thought that the relay trippi g was due to the unbalanced reactances of the one McGraw single phase unit and the two Westinghouse units which make up the three phase bank. The setting of the relay was raised and the' bank reenergized.

l The unbalanced reactances could not have caused the unbalance since there was essentially no load current flowing through the bank. This indicated that something else caused the unbalance, possibly indicative of a turn-to-turn

' fault existing in the low voltage winding of the transformer. Since these windings are connected in delta, it is impossible to calculate the resulting 3Eo without knowing the exact loading impedances on the system. Other possible causes were investigated.

A difference in taps on the McGraw unit compared to the Westinghouse units

could be significant. It was determined, however, that the difference in tap settings were only .0007 percent, so this should not cause the relay trip.

Since the only ground source on the low voltage bus is this wye-broken delta potential transformer with only a 48 ohm resistor and relay across the broken delta, it is probable that unequal capacitances on the bus could cause the 1

1491E 33 f

- n- - ----------v,-.w- , ~ ~ , - - ,,---m,e,mm,-.-,, -s--- -r----_ m -- - - ,

I unbalance. While most of the capacitances (such as that of the iso phase bus) are relatively equal for each phase, the capacitance of the McGraw unit is lower than the Westinghouse unit. The McGraw unit has a capacitance of 0.02 microfarade while the capacitance of the Westinghouse units is 0.03 microfarade. With this difference it was found that a 3Eo of greater than 10% would be obtsined.

Figures 1 and 2 show the calculations for this condition. Figure 1 shows that the LV windings of the three single phase step up units result in phase-to ground capacitances of 0.025, 0.025 and 0.03 microfarads on phase "a", "b" and "c" respectively. Assuming 100 feet of isolated phase bus adds another 0.002 microfarads to each phase, and the three station service auxiliaries each have 0.005 microfarads per phase, the total would be 0.015 microfarads for each phase. Thus, the total capacitance on two of the phases is 0.042 microfarads with 0.047 aicrofarads on the third. the capacitances represent .

06315 and .0562 megohns capacitive reactance respectively. This compares to the resistance of 0.2 megohns when the 48 ohn broken delta resistor is expressed as Ro on the primary side of the potential transformer.

' Figure 2 shows the symmetrical component solution to this unbalanced capacitance condition.

Solving the circuit of Figure 2(c) gives an E, value of 0.0366 per unit or a 3E, across the relay of 10.9 percent. This voltage is sufficient to cause the relay to operate.

It can be concluded from this study that it is possible that the 3E, relay operation on December 3 could have indicated a turn-to-turn fault or some i other abnormality in the transformer. If such were the case, however, one

]' would have expected that the ultimate failure would have occurred before December 5. It is therefore more probable that the relay operation was caused by the unbalanced capacitance of the low voltage bus system.

3E, Potential Transformer Resonance

The above calculations clearly indicated that the 48 ohn resistor in the broken delta of the bus potential transformers was. not sufficient' to .st'abilize the neutral of the low' voltage s'ystem with the generator breaker open. A logical question therefore arises, "could ferroresonance have occurred to cause the transformer failure?" A detailed ANA00M study of the North Anna installation would have to be run to determine the actual voltages involved, and whether ferroresonance could occur. As a substitute, the paper "Ferroresonance of Grounded Potential Transformers on Ungrounded Power Systems" by R. F. Karlicek and E. R. Taylor, Jr. (AIEE Power Apparatus &

Systems, August 1959) was reviewed for general observations pertinent to the December 5 failure.

From this paper it could be concluded that the ohmic value of the resistor

' should be considerably less than 48 ohns to absolutely prevent ferroresonance, but the line-to ground voltage resulting from any ferroresonance (should it occur) would probably not produce high enough voltages to breakdown the transformer insulation.

1491E 34

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Distribution Transformer Grounding Bank The lack of' adequate grounding on the low voltage bus suggests another '

possible failure mode. Assume a line-to ground fault in the bus between the generator breaker and the transformer. The system is well' grounded through the generator neutral resistor and af ter a certain time delay the relay across this resistor will trip the generator. The ground fault now exists on a '

system only grounded through the 48 ohn resistor in the potential circuit.

This results in a fault current on the order of 0.1 ampere which creates a very unstable arc. This arcing ground fault can cause high transient overvoltages and could cause a f ailure of the transformer.

This probably did not occur on December 5, since it requires the generator being isolated from the bus which would have protected the generator from damage. It is a real possibility for happening at some time, however, and most people using generator breakers provide a full scale grounding bank on the low voltage bus to protect against this and any other condition where an arcing fault could occur with the breaker open. In fact, the new ANSI standard C37.101-1983 " Generator Ground Protection Guide" contains a section on the relaying for such an installation.

The generally accepted rules for high resistance or distribution transformer ~

grounding states that the grounding resistor size should be such that the resistor kilowatts should be equal to or greater than the capacitive charging kVA oj[ that the ground fault current should be equalSince to or with greater than 5 the generator amperes; whichever gives the lowest resistor ohms.

breaker open, the capacitive charging kVA of the bus is quite low, in this case the 5 ampere requirement would hold. Figure 3 shows that for the North Anna application a 1.08 ohm resistor should be used.

Note that when the generator breaker is closed, the generator neutral grounding system will be in parallel with the bus grounding system. Both ground relays should see the bus fault. C37.101 covers this condition with two suggested tripping modes: .

1. Set the tripping time of the bus relay at a longer time than the generator relay. The generator will always be tripped and if the fault is in the generator, the integrity of the low voltage bus and thus the station auxiliary bus will be maintained. If the fault is on the bus side of the generator, then the high side transformer breakers will be tripped by the bus relay at some time af ter the r generator breaker. Since in this case, the fault current is only 5 amperes (and no high transient overvoltages are produced) this additional time delay should not be objectionable.

2. An alternative tripping scheme suggested by C37.101 is to supervise the tripping of the bus relay with the auxiliary contacts of the generator breaker. Thus tripping by the bus relay cannot occur unless the generator breaker is open. Note that the operation of this scheme is the same as the one described above.

Generator Breaker VEP00 has retained Power Technologies, Inc. to perform a detailed analysis of generator breaker performance. The results of that study are not included as part of this report.

1491E 35

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k VIII. TRANSFORMERS PROCESSING RECORDS REVIEW Processing Procedures The t ield processing procedures on Westinghouse t ransf ormer uni t s at Nort h Anna in general and on the last failure, 7001994, in pa rticula r were reviewed to determine any possible processing related contributant to failure. The f ollowing information was utilized: (1) VEPCO records and verbal informat ton from VEPCO personnel as to the reprocessing of 7001994 in November, 1982, (2) test results of oil samples taken from 7001965, an unfailed unit processed similarly and at the same tin.2 as 7001994, and (3) VEPC0 records as to the reprocessing of other Westinghouse units at North Anna. Available information does not indicate that the failure of 7001994, or the West'inghouse f ailures in general, are related to processing conditions or procedures.

Unit 7001994 was retrofitted with a COPS tank, reprocessed with new oil in late Novembe r, 1982, and the transformer f ailed in December. Detailed "EHV Equipment Fill Record" data for all Westinghouse North Anna units were supplied by VEPCO and can be found in Appendix C. Units 7001994 and 7001965 were evacuated for vacuum filling with oil on November 26, 1982 During this process two pressure readings were monitored: (1) at the vacuum pump via a Stokes gauge and (2) by a McLeod gauge separated from the main tank cover by approximately 25 feet of 3/8" i.d. Tygon tubing. The following schematic illustrates the relative position cf these gauges and the average pressure readings on both units.

Calculation of Tank Pressure - HAM 707 T

er sco, 2 1D I n 4 Z D.

6 A9MM McdEOD : SAWE GAu6f \

.dMy O. MMM '

W9c A%p Because of the configuration of the reprocessing system and the fact that a McLeod gauge measures only partial pressure of the non-condensables, the pressure within the tank is different than measured by either gauge. Tank pressure can be calculated based on the Stokes gauge reading and conductance of the 100 feet of 4" vacuum hose.

1491E 39

l s i This calculation (Appendix D) shows that the internal tank pressure was 1.4 mm Hg which is greater than the specification limit of 1.0 mm maximum (per Westinghouse Instruction Leaflet 48-668) prior to oil filling; although the vacuna meets the specification for pressure during oil filling, 2 mm maximum.

The 20 hours2.314815e-4 days 0.00556 hours 3.306878e-5 weeks 7.61e-6 months during which the vacuum pump was on prior to oil filling is sufficient time to evacuate the tank completely and reach equilibrium.

Although the 1.4 mm absolute pressure is greater than the specification requirement prior to oil filling, the insulation should have been effectively impregnated and dried because of circulating hot, de gassed oil through the system prior to energizing. Moisture in the oil during processing and prior to energization was within specification, 10 ppa maximum. There was very little difference in processing characteristics between 7001994 and 7001965.

Unit 7001994 failed in December,1982, and unit 7001965 did not show any irregular tendency during operation.

After failure of 7001994, samples of oil were taken from 7001965 and a COPS-type McGraw-Edison transformer C-0-6459-5-1 which was processed similarly These results, and run on the same bank (North Anna #1) as the f ailed unit.

shown in Appendix E, indicate that both units have very similar oil characteristics. All tests were performed by Westinghouse in Muncie. The results agreed well with those measured by VEPCO after the 7001994 failure. .

These oil characteristics are indicative of proper processing and do not show high nitrogen levels which would be expected if the Westinghouse COPS retrofit procedure were not followed accurately or if the units were not processed completely. All tested oil results are what would be expected from normally operating power transformers of this type.

4 1491E 40

Gas-In-011 Analysis Gas-in oil analysis of the Westinghouse North Anna units were completed by VEPCO on approximately a monthly basis. Oil samples were analyzed by both Doble and VEPCO laboratories, but primarily by VEPCO. These data are included in Appendix F.

It can generally be stated that abnormal transformer conditions would be represented by: (1) total combustible gas content greater than 500 ppa, (2) rate of combustible gas generation greater than 100 ppa for a 24 hour2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months period under relatively constant load or (3) acetylene in excess of 20 ppa. In all cases the North Anna units appear normal. Total combustible gas levels seldom exceeded 100 ppa and were never measured greater than 200 ppe. The following data represent maximum combustible gas levels measured for each transformer regardless of testing laboratory.

Cas-In-Oil Analysis North Anna Maximus Ultimate TCG, ppa Test Data Failure Date Serial 152 10/16/80 06/19/81 7002100 -

28 06/11/82 08/22/82 7002100*

49 11/08/79 07/03/61 7002098 7001965 172 09/18/80 t

147 06/12/81 11/16/82 7001995 133 11/20/80 11/29/80 .

7002099 --

7002099* 185 10/07/82 7001993 103, 04/16/81, ,

180 06/12/81 12/05/82 7001994

Unit rebuilt af ter initial failure From the recorded data there is no indication of abnormal gas pattern or gas

! generation rates. In all respects the data are typical of normally operating units.

i l

I 1491E 41

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IX. GAS EVOLUTION CAUSES Gas Bubble Evolution L

The possibility of gas bubble evolution within the transformers at North Anna and within gas-space power transformers in general were investigated as to reliability considerations. Past research has shown that under certain circumstances gas bubbles will lead to reduced electric strength of the Insulation system. The extent of strength loss is dependent on the type l

insulation structure, the condition of oil itself, including its degree of contamination and other factors which may be unpredictable. The presence of gas bubbles is hazardous where the bubbles can collect beneath collars, inverted channels, and other insulation pieces. Gas bubbles in large power transformers can result from the following sources: r (1) Leak in a low pressure area allowing outside air into the unit (2) Gaseous cavitation of supersaturated oil j (3) Improper processing (4) Thermal degradation of oil or insulation resulting f rom incipient .

faults.

An analysis of calculated local oil pressures at various locations in the initial RAM 707 design (Figure 1) show that gas space pressures less than 0.7 Under psig would result in negative pressures in the top cooler assembly.

these conditions should there be a leak in the upper valve assembly or packing gland, air would be sucked into the unit rather than oil being forced out.

The resulting air bubble would be circulated with the oil until it dissolved j or entered the gas space. If the oil was saturated with nitrogen such as would be expected of a gas-space transformer, the introduced air bubble would not dissolve. However, the probability of air entering the unit is very small since ,the transformer is pressure tested at the factory and indirectly Furthermore, with the COPS '

pressure te'sted during shipa'ent and pro' cessing.

retrofit design (such as was on the December 1982 failure of 7001994), the. oil head caused by the height of the COPS tank generates sufficient static preesure so that all locations within the transformer are under positive pressure, preventing air from leaking into the unit.

The possibility of reduced reliability in large power transformers due to j

supersaturated transformer oil and the release of nitrogen bubbles because of Laboratory test gaseous cavitation has been speculated on for many years.

have shown that bubbles can form in gas-space transformers under special circumstances such as af ter an extraordinary rapid temperature drop, f ailure of periphery gas regulating equipment and improper maintenance procedures.

I 42

! 1491E i ~'~ - - - - . - . . _ . . . , _ , _ _ , __,_

J Gesas dissolve in all transformer oils, but the amount of gas which is dissolved at equilibrium will depend on the gas, the chemical nature of the oil (both its source and condition) and on the temperature and applied gas pressure. Figure 2 illustrates the effect of temperature on the solubility of nitrogen in a new transformer oil prior to aging or conditioning. At higher i

temperatures the amount of nitrogen that the oil absorbs will increase. If the gas pressure on the space over the oil is increased above atmosphere, the equilibrium gas content rises in direct proportion to the absolute pressure ,

(Henry's Law) as shown in Figure 3. Thus, to determine the gas concentration in oil at any temperature and pressure it is necessary to apply the nitrogen solubility curve and Henry's Law.

For example, if a unit is at stable load at 6.5 psig gas-space pressure and 80*C, the amount of gas dissolved in the oil is (9.7%) (14.7 +

6.5)/14.7 = 14.0%. This condition is the maximum theoretical gas content at an oil temperature of 80*C because the relief device will bleed out at ;

pressures over 6.5 psig. .

As long as equilibrium conditions are maintained, the gas content of the oil is not a problem since only free bubbles will af fect the dielectric strength of the oil. However, when the oil temperature is reduced (e.g. decrease in load, loss of excitation, reduced ambient), the gas pressure is also reduced.

If the temperature drop is substantial and it occurs over a fast rate (several*

hours or less), the gas dissolved in the oil cannot escape through the oil gas i

space interface fast enough to maintain equilibrium. The result is a condition of supersaturation in the oil.

Using the previous example, suppose the unit is deenergized and that there is no oil circulation (FOA equipment) and rapid cooling to 25'C. The pressure in the gas space would likely be in the range of 0.5 to 3 psig (we will assume 2 psig). The maximum gas content that the oil can hold is (8.9%) (14.7 + 2.0)/14.7 = 10.1%. However, the oil has 14.0% gas because sufficient time was not allowed for orderly dif fusion. The saturation leve1~then at the new equilibrium is 14.0/10.1 = 139%, and oil can be said to be supersaturated by a factor of 39%. .

Supersaturated oil is not a problem in itself. However, when the local oil pressure is reduced below the gas saturation pressure, gas bubbles will be found. Local pressures within a transformer can be calculated knowing the '

static and dynamic head of oil and pressure drop incurred 'oy circulating the oil through the cooling equipment. Figure 1 illustrates local pressures, ,

PL , that may exist in the HAM 707 design. The data are based on a gas-space pressure of one atmosphere, and the localized pressures can be adjusted for various gas spa e pressures by moving up or down the X-ordinate. Low pressure regions could also occur by transients such as the mechanical shock of short circuit, electrical shock of transient voltages, and vibration.

Gas-saturation pressure (pressure of the gas-in-oil) can be calculated using a derivation of Henry's Law:

Gas Saturation Pressure = Pg = (C/Cs)(14.7) - 14.7 Where C = measured gas content in oil !

Cs = gas content in oil at equilibrium (100% saturation) and C/Cs X 100% = percent saturation 1491E 43

a To determine the Percent gas saturation necessary for the first theoretical indication of gas bubbling:

Pg = PL (C/Cs)(14.7) - 14.7 = PL (C/Cs) = (PL + 14.7)/14.7 Table I shows the value of C/Cs to just begin forming bubbles at various gas-space pressures and locations within a typical transformer tank. Values 4

I of C/Cs greater than those presented in Table I will lead to spontaneous generation of gas bubbles.

Table I For the first bubble to form Pc = PL Pressures and saturation levels where first bubble will format locations F,1, m, n are indicated on Figure 1.

Cas Space I C C 'C "C Pressure C -

P C P C Pg psig Pg C 3 S S 3 0.0 -1.2 92 9.0 161 2.7 118 0.6 104 0.5 -0.7 95 9.5. 165 3.2 122 1.1 107 1.4 0.2 101 10.4 171 4.1 128 2.0 114 6.5 5.3 136 15.5 204 9.2 162 7.1 148 l

l l Note that supersaturation is not needed to generate bubbles. Only the criteria Pg greater than PL need be satisfied. However, bubbles forming i

in undersaturated oil will quickly redissolve in the oil. Bubbles forming in

' supersaturated oil cannot dissolve but wil,1 expand and rise. Should these bubbles migrate to a h'igh-stre'st region 'of the~ transformer, the dielectric-characteristics may be seriously impaired. Prior investigations.have shown as much as a 40% reduction in dielectric strength of oil (measured with VDE electrodes, 80 mil gap) due to the presence of gas bubbles evolving from supersaturated oil.

Gas bubbles could be found anywhere in the transformer depending on the level .

of saturation and the local pressure. However, they are not likely to evolve in a large power transformer unless:

1. The pumps are en causing low pressure regions.

j 2. The gas-saturation level is such that the gas pressure in the oil exceeds the local pressure, i

3. Temperature and pressure changes are rapid (i.e. allowing no or very little dif fusion through the interf ace).

I 1491E 44

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J A

i

Such conditions would most likely be possible if: (1) a gas-space, FOA transformer is deenergized af ter establishing equilibrium, and ambient conditions are such that temperature drop of the oil is rapid, and the unit is reenergized before a new equilibrium condition could be established or (2) the gas space regulation system malfunctions or the nitrogen supply is exhausted in which case a negative pressure could develop in the gas space. The only method found to prevent the possibility of bubble formation due to '

I supersaturated oil is by using a constant pressure oil preservation systen (COPS) which essentially removes the ;1s space (insuring continued low gas content in the oil) and does not allow pressure variation.

Gases may also be trapped in the insulation structures af ter draining and l

refilling of the transformer with oil. Figure 4 represents a cross section of trapped gas in an insulation structure and between layers of insulation.

Attention thus far has focused on spontaneous formation of free gas bubbles in

' the bulk oil. But potential also exists for reduction of dielectric strength of major insulation structures from bubbles which are trapped in a gas pocket (Figure 4a) or between paper wraps and conductors (Figure 4b). In these places they may be effectively prevented from escaping during draining and filling operations. To eliminate these pockets one must rely on good vacuum treatment, good vacuur-filling procedures, and the circulating of oil with j

very low gas content through the transformer to dissolve the bubble prior to

energizing. It must also be realized that the rate of dissolution of the gas -

in the bubble to the oil is very temperature dependent.

I A high-voltage transformer is generally vacuum filled with oil after draining. Then the oil is circulated through a degassing plant which removes

' gases and moisture from the oil. Manufacturer's suggested gas content has been 1% or less, and moisture content maximum has been 10 ppa, for EHV units.

However, these parameters are measured from the average bulk oil. Insu12 tion l

! areas could have high gas or moisture values yet not be large enough to affect I

the bulk oil properties. Since the solution of gas and moisture into oil is I time and temperature dependent, it is best to adjust processing (circulation) l

time for oil temperature. Af ter a unit has been sitting deenergized for a

' long period of time, the temperature'of'the' core and coils are affected by the ambient. Processing in winter months should as a result continue longer, or the temperature of the circulating oil should be increased. Hot oil is usually obtainable f rom standard degassing plants, and the processing time should take into consideration the temperature of the oil being circulated.

Figure 5 describes suggested temperature-dependent processing conditions for filling transformers. These new procedures have been developed primarily as a guide for draining and filling in cold weather months, but they are generally useful independent of the season.

VEP00 claims to have followed these procedures during reprocessing of the COPS retrofit units in November,1982. To determine whether the processing procedures were adequate to impregnate the insulation around the HV lead in 7001994, a full size model lead was taped, processed and visually observed in a 350 gallon test tank equipped with an observation window. The results of various processing condition with respect to bubble evolution are shown in the following table.

1491E 45 l

l

u

. Lead processed using processing . Lead processed Poorly -- 20 information supplied -- 2.4 MM Ho MM Mc vacuus Internal heater was implanted . Heated to 70*C adjacent to copper Geometric configuration same . No bubbles observed as HAH 707

. System brought to equilibrium to . Gas pressure over the oil systen simulate inertaire was reduced, but no bubbles until i pressure went regative

. Internal temperature raised in 10' . Number of bubbles was profuse steps to 136*C when generated, and continued ,

for a long period of time while i

under vacuum

. No bubbles were observed Drained oil and processed with poor vacuus -- 20 MM Ho ,

. Internal temperature raised in 10'C increments to 136*C - no bubbles observed

. Gas pressure over the oil system was reduced, but no bubbles observed unti1' pressure went negative The only cases in which bubbles evolved from the lead insulation was when the

! gas space was under vacuum. Under positive or atmospheric pressure, bubbles were not noticed at the HV lead even when.the insulation was grossly underprocessed.

Another possible source of gas bubbles is due to an incipient fault. In 7001994 a turn-turn fault was discovered in coil #1. An analysis was made to determine if gases generated f rom this f ault could find there way to the HV lead to precipitate a breakdown to coil #21. Gas bubbles will dissolve in oil depending on their size, temperature and degree of oil saturation. It is likely that if gas bubbles were generated at coil #1 they would not dissolve for at least one period of circulation through the unit. Figure 5 shows the relative position of coil #1 and the HV lead. The pumps that were operating during the period immediately prior to failure are also noted. It would appear that due to the flow of oil a bubble found at the fault in coil #1 would move directly up through the windings and then transverse across the top of the coils toward the HV lead. However as shown in Figures 6 and 7, a bridge structure compartmentalizes the top of the unit so that the bubbles would have to creep under the bridge section supports and then come up inside the HV lead section of the bridge structure.

1491E 46

To determine the actual flow characteristics in the KAM707 design, field The experiments were made on 7001965 which was not energized at the time.

Trip Report attached as Appendix G describes these tests and their results.

The following was observed:

'. On turning on the pumps no bubbles were noticed in the circulating oil.

When bubbles were purposely injected through one of the pumps, they were found to progress straight up through the windings and then straight-up thrcugh the bridge structura.

The quantity of injected bubbles were at least twice as great around the core and coils as opposed to through the coils.

Calculations indicate that a given volume of gas, if present, will take seven minutes to complete one cycle through the main tank and cooling system.

Reference data indicates that if the bubble size is greater than 1/4" in diameter, it would not dissolve in the oil before one cycle of circulation.

it was not likely that a bubble generated at coil #1 would approach the HV lead unle,ss it was circulated through the cooling system and then come up through the HV lead bridge section.

4 1491E 47

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X. OIL ELECTRIFICATION EFFECTS Static Electrification

' Static electrification phenomena in large power transformers has been studied (notably by Japanese researchers) with regard to reliability of high voltage, larger capacity units. The distribution of electrostatic charges in a transformer is fundamentally similar to that produced by oil flowing through a plain pipe. In a large power transformer, positive and negative chargea in oil are separated by the flow of oil on the surface of paper insulation. The charges, positive or negative, are accumulated when the charges generated by oil flow exceed leakage charge to ground. The charges accumulated in the oil (generally positive) or on the solid insulation (generally negative) cause an electric field, and if it proceeds beyond a certain level may cause

. electrostatic discharge or creepage discharge in oil. Charging tendency in a power transformer is a complex interaction of a number of factors some of which follow.

Primary Factors Which Influence Flow Electrification

. Oil flow rate .

. Bulk oil properties (temperature, conductivity, etc.)

. Surface conditions and structure of solid insulation

. Oil flow paths Moisture content of oil and paper

. Nature of ionic content of oil

.We' have examined the charging ~ t'endency' of various commercial sources of-l transformer oil during various stages 'cf oil processing and aging. The

charging tendency is closely associated with the amount of substances such as

! ions or polar material that are in the oil. These substances can come from a number of sources such as: (1) products of thermal decomposition, (2) outside sources of contamination, or (3) the refining operation relative to a specific supplier. We had found that the concentration of these ions or polar substances need not be very great to significantly increase charging tendency. In fact the presence of such substances cannot be detected by standard analytical chemical methods, standard transformer oil specification tests, or any other known test other then by directly measuring charging tendency via a laboratory device specifically designed for the purpose (Exxon mini-static tester, Figure 8).

1491E 53

In experiments conducted at Muncie in 1982, .various commercial sources of transformer oil where analyzed for charging tendency with the Exxon mini static tester. The investigation was initially to determine if hydro-refined oil had greater charging tendency than acid refined oil.

(Texaco the last and only domestic supplier of acid refined oil is a chief source of supply for the Muncie plant.) The results of the investigation did not show correlation between acid or hydro-refined oil and charging tendency, but it did show that one source of supply (Gulf Trancrest H) exhibited significantly greater charging tendency than any other source, Figure 9.

Further investigation has shown that the higher charging tendency of the Gulf oil is likely to be due to organic sulfur compounds in the oil. These contaminates can be removed f rom the oil and the charging tendency reduced to normal levels by clay filtering.

Figure 10 illustrates the trend of Gulf Trancrest H IFT (interf acial tension values) as measured by the semi-annual Doble oil survey. Although the degree of contamination required to provide significant charging tendency may not lower the IFT below the specification limit (40 dynes /cm), it would lower IFT somewhat. Figure 10 shows that the IFT values of Trancrest H dropped significantly after changing to the hydro-refining process, and an indication of a drop in IFT also occurs in mid-1980 through 1981. Oil produced in this .

time period was likely to have been used during factory testing or field filling some of the North Anna units.

The above data only indicate that Trancrest H has a relatively higher static charging tendency then most other oils. To determine whether this f actor would have a significant bearing on the reliability of power transformers would require a great expenditure in resources. The complex interaction of parameters which effect charge generation is not fully understood and possibly could only be reliably reproduced in an actual transformer.

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1491E 54

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Eo TEMPERQTUCE 3 C Figure. 9 -- Charging Tendency of Cu2f and Texaco Transformer Oils 56

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Figure 10 - n't Values for Gulf Oil, 1971-81 taken from Doble Semi.-Annwt1 011 Survey 57

i XI. DIELECTRIC ANALYSIS OF LINE LEAD CONFIGURATION Purpose The project was undertaken to provide experimental data for the investigation of dielectric phenomenon around the HV line lead and bushing of the North Anna transformers. The effects of gas bubbles, lead taping, and electrification of oil were studied individually and in combination (gas bubbles / poor lead taping, gas bubbles / electrification of oil).

Experimental Assembly To simulate the HV line lead assembly in the laboratory, a 500 kV bushing was mounted in a test tank. Attached to this bushing was a simulated lead consisting of two parts -- 1) the vertical section is composed of thin copper sheets that are soldered together at each end; this lead section is bolted to the bottom of the bushing; 2) the horizontal portion is made of solid copper bar; both items 1) and 2) were the same as existed in the North Anna units.

Refer to Figure 1 for side and front elevation views of the experimental ,

assembly. To simulate the LV coils and insulation, taped copper strap was placed inside pressboard angles; eight coils were simulated by this assembly.

All dimensions were the same as the actual transformer. Gas bubbles were introduced into the experimental assembly using a tygon line positioned directly underneath the lead simulation.

Experimental Results (Standard Transformer 011)

--- C.

8 1491E 58

d,C.

For correlation purposes, tests were performed before doping at 400 kV for the duration listed in Table 6. Three separate tests were performed -- 1) no oil After flow /no bubbles; 2) oil flow /no bubbles; and 3) oil flow with bubbles.

doping, tests 2 and 3 were repeated. As shown in Table 7, the partial discharge increased when the oil was flowing with bubbles flowing past the lead. Although the extent of partial discharge increase with the combination of bubbles and electrification was not large, a qualitative trend does exist indicating an increase in partial discharge with the combination. No flashovers occurred during any of these tests.

59 1491E

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=O HV LINE CONFICURATION WITHOUT TAPE (BARE)_

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1491E 62 e

HV LINE CONFIGURATION (TAPED)

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TABLE 5 1491E 65

STATIC CHARGE TEST RESULTS NO OIL FLOW /NO BUBBLES /NO AEROSOL OT

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TABLE 6 1491E 66

bl STATIC CHARGE TEST RESULTS TESTS WITH AEROSOL OT ADDITIVE IN THE OIL OIL FLOW WITHOUT BUBBLES

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XII. IMPULSE DISTRIBUTION AND RESONANT FREQUENCY ANALYSIS The seventh incident at the VEP00 North Anna station involving a Westinghouse generator step up transformer exhibited a flashover from the high voltage line lead to low voltage coil 21. In addition, there were multiple flashovers in the isolated phase bus duct connecting the generator to the transformer.

Similar flashovers in the isolated phase bua duct were also evident in the sixth incident. The seventh incident involved transformer serial number 7001994, which was also in the same three phase bank with transformer serial 7001995 which was involved in the sixth incident.

Serial 7001994 was completely disassembled and inspected at the Muncie Plant.

The evidence from the disassembly included failure points on the HV line lead and low voltage coil 21 which were presumably the two endpoints to a flashover and a turn-to-turn fault in LV coil 1. Figure 1 shows a winding schematic of the coil arrangement with the arrow signifying a flashover to coil 21 and the "X" indicating the turn-to-turn failure in coil 1. The HV winding consists of the series combination of coils 5-14 and 23-34 All other coils are in the LV winding and are connected in four parallel paths. Coils 21 and 1 are in different parallel paths, however, both coils are approximately at the electrical midpoint of their respective parallel paths. .

Upon finding the turn-to-turn f ault in coil 1, the objective was to investigate as to whether the fault was a result of the high-low flashover or if the fault was incipient to the high-low flashover. Therefore, a test was devised to simulate the application of a transient voltage on the last turn of coil 21 and measure the subsequent turn-to-turn stresses in other parts of the low voltage winding. To accomplish this, transformer serial 7002100 was used. The transformer core and coil assembly was still intact and thus modelling of the windings was not required. A low voltage recurrent surge generator was used to inject a transient voltage into the LV winding with the last turn of coil 21 being the injection point. Measurements of the resulting turn-to-turn voltage were taken on coil 2 since the edge of coil 1 was

inaccessible. However, the voltages in coil 2 should be approximately , '

equivalent to the v'oltages in coil 1.'

The waveshape of the voltage applied to coil 21 during the seventh incident is unknown, however, the recurrent surge generator was used to apply a full wave, a chopped wave, and a steep front wave in three separate tests. The results of the turn-to-turn voltages on coil 2 are as follows:

Full Wave 3.5% of applied voltage Chopped Wave 7.5% of applied voltage Steep Front Wave 5.5% of applied voltage With these results and assuming the most pessimistic situation in which the chopped wave voltage is used and that no voltage drop exists in the high-low flashover crc, one would calculate the crest value of the voltage on coil 21 as 424 kV ( 2 X 300 kV line to ground for the 500 kV system). The resulting turn-to-turn voltage would be (.075)(424) = 32 kV.

I 1491E 70

The turn-to-turn insulation for the LV consists of paper tape on the conductor with an additional strip of pressboard located in the turn-to-turn space. One of the observations from the disassembly was that the pressboard between turns was shif ted out of place thus leaving only the paper tape on the conductors for insulation. The design value for the electrical strength of the specified paper insulation on the conductors was 45 kV. Therefore, the 32 kV turn-to-turn was well below the design level of the paper alone which leads to the low probability of the turn-to-turn f ault occurring as a result of the high-low flashover.

From this analysis the probability of the turn-to-turn fault being in existence prior to the high-low flashover is much greater than existing af ter the high-low flashover. A possible mechanism for producing the turn-to-turn fault involved the sixth incident with serial 7001995. As stated previously, the isolated phase bus ducts flashed over in the sixth incident. It is possible that when the C phase bus duct flashed over a negative impulse wave was applied to the terminal to which the parallel path containing coil 1 was connected. The magnitude of the wave would have been approximately equal to the crest flashover value of the bus duct. This situation was not modelled because it involves a complex interaction between two transformers and the associated isolated phase bus ducts. However, the point to be made is that serial 7001994 (seventh incident) was involved in another incident by being .

connected to another transformer which failed in a manner that transient voltages could have been applied to serial 7001994 In summary, it is most probable that the turn-to-turn fault in coil 1 of-serial 7001994 was the result of an event prior to the seventh incident.

Another aspect to the transfer of transient voltages involves the concept of producing voltage transients as the result of a resonant frequency condition.

This may occur if a dynamic forcing function with a frequency component theIn same as the resonant frequency of the winding was applied to the winding.

the specific cases analyzed at North Anna in which the isolated phase bus ducts flashed over, the . objective was to determine if a low side are could 4 produce. potential' difference's between'the HV and LV of sufficient magnitude to cause the high-low flashover, l

l One of the North Anna units was tested to determine the resonant frequency at which the HV line voltage is maximum when excited by a sinusoidal voltage on the low voltage terminals. This test produced results for determining as to whether a low side are in the isolated phase bus duct could produce voltages in the HV in excess of the withstand high-low. Two resonant frequencies were identified for the HV line terminal; 9 kHz and 40 kHz. A flashover in the bus duct could produce a voltage approximated by a square wave with a period t

dependent on the reflection time from the f ault location to the transformer i

terminal. Assuming an approximate 100 feet of bus duct length with a capacitance of 0.002 microfarads per phase, the voltage wave would be a square wave with a frequencv Of 2.5 MHz. The 2.5 MHz square wave would not contain any frequency components at c near the 9 kHz and 40 kHz resonant frequencies. Therefore, the conclusion is that the low side isolated phase bus duct flashovers did not produce the high-low flashovers in the transformer l but rather the opposite occurred.

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1491E 71 l

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I 1491E 73

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APPENDIX A - ITEM i TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION 9

e 75

APPENDIX A - ITEM 1 Ef:G!!iEER'S TRIP REPORT Hoiti i . The Envia.or esperting is ressensibt, fe, sh, ,.e. , dslivsry of his esco,,, ,e . Ine,.e itsfinenoust FGate 3611 G p.,. . ,,g (,, . ;n,3 ,het all peines requiring action are cleered up without deley.

2 For lens reports give e su.amery of impervent peines and rect,amendeeiene.

For Weevi.vp.euse Rea,eeenterives saly. OATc 12/15/82 CATE QF ARRiv&L AT PACPERTY CATC QF LCAtflNG P40p 021GINAL TO ISSUING DEPARTMEtiT'S FILE NO.

COPY 70s ( FOR DEPORT DISTRiduTION GUIDE

AEFER TO REVERSE SICE OF THl! SHEET )

! G Muncie Plant MR. D.A. Yannicci G Muncie Plant f/R. H . R . Mne.ra G M'incia Pl a.' t ua. 7.3. Coc9aart G Muncie Pla.it MR. M. Wood

.G Muncie Plant vR. .T. Burcess

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.=uaFoSE OF TRI P 2PPov? barral barriets fren H.V. bushiae and r.ake 7eneral teternal ins.ectice. _

j CONFERENCE HELD A7 North Anna Nbelear Plant DATE 11/22/0"

- .yo:g pegsENT !!r. .T. Markins and Mr. J. Te .nleton, We:.; tin?houze: Mr. J. !!accre :or. VETCO.

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TRIP REPORT TO VEPCO NORTH ANNA POWER STATION ON 11/22/82 Mr. Joe Markins and Mr. Jim Templeton of the Muncie Westinghouse Plant and Mr.

Joe Macgregor of VEPCO removed the barrel barriers from the HV bushing in serials 7001994 and 7001965 on 11/22/82. The two transformers were part of the transformer bank connected to the North Anna #1 generator. The third transformer in the bank was a McGraw Edison unit.

VEPCO had already prepared both transformers for inspection by draining all of the oil but they had not broken the seal of the manhole covers. With each unit, VEPCO placed a tent over the manhole which maintained a humidity level of about 20-30% above the open machole.

The phase B unit, serial 7001994, was entered first. The only personnel to enter the transformer were J. Markins and J. Templeton of Westinghouse and J.

Macgregor of VEPCO. The first operation completed was to remove the barrel barrier from the internal end of the HV hushing. This was accomplished rather simply by removing the permali nuts and bolts connecting the barrel barrier to '

the bridge walls. The pressboard barrier uas then slipped from around the bushing .ithout any cutting or tearing of the material required. However, as a precautionary measure the phase was packed with cloth material in the area where uork was being performed.

Af ter the barrel barriers vera removed a general tuspection of the transformer was este. The HV line. lead and bushing shield were closely examined. The tape on the lead uas firm with no soft spots. The bushingThe shield was in the top end of all proper crientation with no evidence of any abnormalities.

coils, both HV and LV, appeared acceptable.

In the HV line group f.our insulation items were poun,ded back into placc. The items were the ' outermost engles for' coils 23-28. The items were only reis~cd about 1/2'* and loccted approximately at the centerline of the phase. This condition was not identified as being critical.

In summary, the phase B transf ormer was in satisf actory condition. The total open time for the transformer was approximately two hours.

The phase A transformer, serial 7001965, was then worked upon in order to i

remove the barrel barrier and conduct a general inspection. The same procedures were followed with the second unit as used with the first. That is, the humidity was e.ontrolled, the phase was packed, the barrier removed without cutting any material, and the windings inspected.~ Here again, the barrier was removed with no difficulty. The HV line lead and bushing shield were examined and were satisfactory. One point to note about the configuration of the line lead is that the copper bar comprising the horizontal section of the lead was positioned flat in the serial 7001994 whereas it was on edge for the serial 7001965. Again, there was no evidence of any abnormalities with the HV line lead in either unit. The top end of all coils was satisf actory and minor corrections were made with the LV bus har 1302E 77

connections in that about 70% of the bolted connections were tightened. The connections were not excessively icose but were tightened .in order to full ^y

~

compress the lockwashers. '

As with the phase B unit, the phase A transf,omer wqs open for approximately two hours. Nothing was found out of the ordinary and the VEPCO representative was satisfied with both units and subsequently instrucced the rew on-site to pull vacuum on both units. At the time of leaving the power plant, both units were being processed under vacuum with the plan to refill the units per the instruction book vacuum filling procedure.

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O APPENDIX A - ITEM 2 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION I

i 1

79

, APPENDIX A - ITEM 2' .

E.'4G'.:#EER"3 TRIP RE?OftT Nots 1 he en,ineer e:p.,,tz, i. 4b:2 f., th3 e,e d:li..,y ei ha. , ,.,,. ,, <it ir,s..

ec Timenewst 7023 3418 G por:en. . sad for esc 4xg thet all points regvicing ceti.e e<o steered up withove del:y.

2 . free loa, rep.et. give e vn nory .i I ,.,, e peine. end rece endeeien .

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, For Westfacheese Repeseesweefves entr. DATE 11

K+- 8 b samt , - = _ . .w cats or annivat af peoptary j satt or Leavens poept CAICINAL To ISSQlNG oEPARTMENT'S FILE No. l 2 007Y 70s ( FCA REPORT DISTRIBUT'ON GUIDE - REFER TO REVERSE SIDE OF THIS SHEETl_

. O Muncie PTD *a 9. a. cns<,,rt C MR.S. A. Aarrott ~

O 9= . H . R. Moore wa.T. D. Poyser O _. va.J. n. Te,nteton O

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7001994

i. .c ATi es Richmond, VA c.o. s.o. HAM 707 s:n. o. 3,3 raros: OF TRIP To WStch ccronP test on two tran9 4 P* art.

sFEREscc HELc AT VEPCO - North Anna Station cATE 11/30 19 /32

. 7.-osE P9EstnT Jce McGrecaor, Ron Barker and Haden Keafauver of VEDCO: John H ubert of IWi Richmond: W. J. Carter of (W) Muncie -

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f'Je4ARY:

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, i Trip Report Sammary The purpose of the trip was to observe the corona tests made on two units of shop order HAM 707.

Vepco was having problems obtaining the required oil breakdown voltage and was not ready to make tests when I arrived. After reviewing their procedures, it was observed that:

1. Their test set was being operated on a low line voltage.

2. The oil samples were being tested at about 10-15'C temp. where a 20*C minimum temperature is required by the ASTM procedure.

3. The oil sanples may have been taken using dirty fittings. -

These problems were corrected and acceptable breakdown voltages were obtained. The coisture contents measure in serial 7001994 were 5.01 ppn in the bottom sample, 2.44 ppm in the top sanple and 10.95 ppn in the COPS tank. -

The gas in oil was 1.05% in the top and .4% in the bottom. The measured moisture contents in serial 7001965 were 6 ppm top and bottom and 17 ppm in the COPS tank. The gas in oil results were .48% top and .6% bottom. Since the measurements in both COPS tanks were above the limit of 10 ppm, severcl gallops cf oil was drained from the bottom of the COPS and new samples were were tested. These tests gave similar results. No free water was observed in the samples. Clearance was obtained from Muncic to accept these levels.

During the tests en unit 7001994 difficulties were encountered uhich resulted in hiah corons leveln st about 50% voltage 4nd higher. The corona levels were The apparent on the RIV meters and on both acoustic detection systems.

acoustic signals vete characteristic of conducted interference and not

. accustically detected corona internal to thef transformer. The corona would disappect with tine and would'then reappe'ar at high'er volta'ges. I't was th'enThe determined that the source was corors observed and heard on the test set. I accustic meter on the waveguides measured approximately 45 during these events.

The weather during the entire test period was rainy and foggy. The moisture caused the problems encountered during the test. The final test results are tabulated on the attached page. These results were accepted by all parties.

The corona levels were stable during these tects and no corona was detected by either accustic system.

An attempt was made during the evening of 12/3 to back feed the unit. Before the unit was energized, the waveguide detected several switching evcuts in the 500 kV yard. These events caused a response of about 2,000 to 20,000 on the meter. When the transformer was energiced the meter jumped momentarily from 40 to about 45. After about five seconds, relaying on the low voltage bus caused the unit to trip off. The low voltage bus was meggered and a problem at the generator breaker was found and corrected. The transformer was re-energized from the high voltage' side at 4:08 a.m. on 12/4/82. As the unit l

81 1281E E

, ') >

- w?s energized the meter indicated a icvol of about 200 sinits on both waveguides, This signal remained steady for seven minutes and then suddently decreased to 40 on each waveguide. The 40 reading represented an ambient level present without voltage. The waveguide meters remained steady for over one hour after enargization.

Interviews with people in the area of the trannformer during the period of energization disclosed that in the interval corresponding with the 200 unit reading on the waveguide meter, severe external corona existed on the 500 kV bus. This external corona probably caused the 200 reading.

At this point, probleas with the generator breakers were discovered, and it was decided that it would be several hours before the unit would start carrvine load.

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B. 7001994 Hi H2 Acoustic

% Time V V Waveguide CP 0 0 12-14 25-30 40 -

100 - 12-14 30-35 125 - 12-14 35-40 135 0 20-25 35-40 146% max. voltage 135 5 20-25 35-40 120 5 20-25 30-35 10 20-25 30-35 40

20. 20-25 30-35 40 30 14-18 30-35 39 1 40 18-22 30-35 39 50 15-18 40-45 39 55 14-16 30-35 39 135 55 14-16 40-45 39-42 4

56 18-20 35-40 57 16-18 30-35 38 58 18 30-35 38

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59 16-18 30-35 38 135 60 16-18 30-35 38 120 60 20-25 30-35 1

ALL CORONA READINGS ARE STAELE.

i Weather conditions: Fog with light rain at end of test.

9 0

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83 1281E

- . _ _ - _ . _ , _ . - _ _ . _ . , . . _ . . -- . _ _,-__. ._ _.._ ,-_.-._,. ~. - - _ _ - . - ,

j A. 7001965 Hi H2 Acoustic Time V V Waveguido GP O 18-20 7-8 40 OK O

30-32 8-10 40 100 25-30 12-14 40 135 0 20-25 8-10 40 2

20-25 7-8 40 5

25-30 6-8 40 120 5 20-25 6-8 40 10 20-25 12-14 40 15 22-27 14-16 47 20 .

30-35 14-16 40 25 20-25 16-18 40 30 20-25 14-16 40 35 20-25 20-25 40 40 20-25 16-18 40 45 14-16 40 OK 50 20-25 25-33 20-25 40 55

^

20-25 25-30 40 OK 13 5 55 20-25 25-30 4b ~0K 56 20-25 30-35 40 57 20-25 25-30 40 OK 58 25-30 40 OK 59 20-25 25-40 40 OK 60 20-25 20-25 16-18 40 OK 120 -

10-12 40 OK 100 - 20-25 STEADY Weather Conditions: Heavy Fog .

84 1281E

APPENDIX A - ITEM 3 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION 6

85

APPENDIX A - ITEM 3

L't',i?CER"; Tff!;' REP 0l:I -)T E 1 e TM Engineet reporting is essponsib. .i., pr:mpt d2 livery cf his espees to all intereste.f etstentnou,t f ars 3611 G .

p.,,,n, c d f:r ascing tint all p>ints esquering action ets e ssersd up wit!.out Jalny.

2 e for long r:perts giro o sun miry el impartent points end escommIndstisas.

For Westinshouse Repoosenroelves only. DATE 3l2NlII a . . , . . _ _ _ _- - - - . -

r ...... .. .

n87t 0F ANNIVAL af PROPfRTY Davr op Lts v e %e. Pierff s u ty csecitaat to :: SUING OEPARTMENT'S FILE No. _

HAM 707-08 3/22/77 3/23/77 COPr To: ( FOR PEPORT DISTRIOUTION GUtDF = REFER To REVPRF' SIDE oF THit SHEET )

p Mu!!CIE PIAliT Md. R. J.~Cossaart _

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O Mu::ctu PLA:Ir uu. c. B. Metzter O Mu:. cit rLAsT uR. L. n. curtis O MascIt etANT uR. R. L. Amos __

Q MuMCIS_PL.CT un. N. M. Yovetich O Muncic PLANT uR. c. F. Millis O MuncIe PLANT un. V. A. Holford O "a-a_ uR.

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(Original S.O. HAM 707)

L :ATios Fredericksburg, Virginia c.o. S.o. X,DM13? 3 SER. o. 7002097 P.% POSE CF ThlP To search for missing hardeare used in the shipping braces.

c:.'4rE'ENCE HELD AT North Anna _P_ower Station DATr. 3/23/77 T-3SE PRESENT Alfred Moore and "Smiley" Francis of VEPC0 and C. F. Millis , V. A. Holford, and __

I. L. Hansen of Muncia. '

i.* NARY:

A bolt in the shipping braces was minus a nur and washer upon arrival at the site. Several investigations by VEPCO and one by Muncio personnel failed to locate the missing hardware. It was the conclusion of the personnel from Mancic tnat the hardware was never on the bolt.

86 f,, A PP Ao v t.

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1 TRIP REPORT Virginia Electric & Power Company S.O. XDM1323 (Orir,inally IUdf/07)

Scrial #7002099 When this unit was received at the North Anna Cencrating Station an internal inspection was perforr.ed with no discrepancies noted. After the unit was placed in an upright position and during the removal of the laydown shipping braces, a bolt was found with no nut or washer on ic. Using the attached portion of Drawing HAM 707-75 as reference, the location of the missing nut and washur is indicated. There are three bolts through this particular post and uall, and the one without a nut or washer was located near the bottom (or nearest the side member). The missing nut and washer were not discovered in the earlier inspection because of its inaccessibility.

The crew that discovered the missing hardware searched for several hours, but failed to locate it. Mr. Alfred Moore of VEPCO also spent several hours on cach of three different cccasions searching for the hardware with no success, ik.' Moore 3nd the crew also searched the tank bottom by inserting a r.agnet through the cooler valves. Since it is impossible to see into the tank bottom, there are undoubtedly large areas that were not covered.

On 3/22/77 V. A. Holford, C. F. lEllis, cnd I. L. Hansen from Mancic inspected the shipping braces and hardware that were remcved fren the transformer. Thcre was no indication cf this bolt ever being tightened baccuse neither the post nor wall h:d any indentations. All bolts were in one large bag so there was no uay of locating and inspecting the bolt of interest to determine uhother it had evar had a nut en it. Sin:e it was raining the transformer was not c::amined interna 11v untti 3/?3/.77. This inspection revealed no loose hardware. Wedges between tha

~

shiciding and phase were not rceoved, but gaps in the cc: ness ware examindd 'ulth a borascope.

,In the final discussion with tk. Moore, I. L. Hansen stated that he was satisfied that the nut and washer were never placed on the bolt at Fbncie.

Mr. Moore asked about the possibility of cutting into the tank bottcm to determine whether anything had fallen into it. Mr. Hanson acknowledged the postibility of doing this, but attempted to discourage it. Mr. Maare said they would not cut into the tank without discussing it first with Mur. ic.

t/) W $

I. L. Hansen Design Ensincor 3/24/77 Page 2 87

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I APPENDIX B - ITEM 1 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION t

88

. - = _ _ _ - _ _ _ _ - . - _ . . -

Teardown Report VEPCO North Anna Unit 500/22 GSU Si 7001994 4 S.O. XDM1888 (RAM 707) 1/10/83 Summary

' This transformer failed on December 5,1982 and was returned to Muncie for teardown and inspection. Visual examination of the phase indicated that the greatest damage occurred in the area of coil 21 located approximately below the HV bushing flexible connector. The edge of coil 21 was burned and the outermost layers of the coil displaced. The corners of washers 66, 67 and 68 were broken and missing. Arc burning was also found on the edge ot the HV bushing flexible connector. Detailed inspection of the coils confirmed the assumption that an are had occurred between the HV bushing flexible lead and the edge of coil 21.

The detailed inspection also uncovered an electrical failure between turns 7 and 8 of coil 1.

Detailed Inspection l Tank: The top section of the tank,was distorted and ruptured in several places. ,

4 Bridge: Nearly all fiber studs in the bridge structure were broken and the bridge fell off the phase when the tank top section was removed.

Core Steel: The core steel stack was straight and undamaged.

Coil Support: The coil supports were undamaged.

~

Phase: Coil 1 - Had a turn-to-turn failure between turns 7 and 8 on the HV side 36 inches above the centerline. The turn-to-turn pressboard fill was displaced at the point of f ailure.

l Coil 15 - A "S" shaped piece of copper wire was found near the start-start connection. No damage to this coil was found.

Coil 19 - The coil insulation directly adjacent to the damaged area l

l of coil 21 was charred.

Coil 21 - The outer edge of the coil was burned and bare copper pitted on ths A end, HV side. The outer layer of the coil l was displaced.

! Coil 22 - The insulation of the "A" and coil edge was skinned in 3 places.

Coil 38 - The insulation on the "A" end coil edge was skinned in 1 place.

l 1338E 89

Washers 66, 67, 68 - The "A" end HV side corners of the washers were

, broken off and missing.

HV Lead - There was electrical burning on the edge of this HV bushing flexible connection at about mid-length.

Conclusion It is concluded that an electrical arc occurred between the HV bushing flexible lead and the edge of coil 21.

W. J. Carter D. White Engineering Operations d

S 4

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i 1338E 90 i

D APPENDIX B - ITEM 2 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION 91

l t

APPENDIX B - ITEM 2 VEPCO TRANSFORMEP HAM 707, SERIAL 7001995, VEPCO PHASC C FAILED NOVEMBER 16, 1982 p,Afr.CROUND DATA Shipped 1973

. Went into service 1974 (limited, full service 1975).

Taken of f line, May 1982.

Drained, refilled with new oil, August 1982.

Re-energized for first time on November 16, 1982.

Failed 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months , 6 minutes after re-energizing.

Weather reported to have been mild through the fall season until two nights before the failure. .

Failure occurred while unit was being back fed from the 500 kV systen.

Pump bearing wear had occurred in pumps on other two serials (7001965 and 7001994). These units cleaned, flushed and refilled with new oil. Harley Pump Co. report states that there was no bearing wear in pumps from serial 7001995.

s .

DATh FROM VEPCO RECORDS AND OBSERVATIONS i

. Fault Currents Phase C - 16200 amperes Phase A - 5250 amperes Normal 120' Apart

" Phase B - 5250 anperes Ereakers cleared fault in three cycles.

No unusual occurrences or observations prior to failure.

One stage of coolers initiated when unit was energized.

Pipes to disconnected gas relay broken and distroted on Phase A and B tanks. Condition prior to Phase C failure thought to be normal.

. No. failure on HV bushing. .

FAILURE DAMACE AND FAILURE PATH See Figure 1-1/2" diameter hole in lower edge of HV bushing shield.

l .

? 1.

Flashover from this hole across the surface of the cyltndrical '

barrier down to the edge of L.V. coil #21 about 18" from the centerline of the phase. The damar, a coil 21 was erosion of the 1/32". See Figure 1.

strand surfaces for a depth of about l . Smoked area on insulation around failure on coil 21 was small.

Insulation in good condition with exception of mechanical demage caused by broken porcelain and bridge walls. No evidence d Both upper and lower porcelains on H.V. bushing broken.

ll of a bushing failure!.

.) .

One small are mark on tank wall oppostte the corona shield which'uas lgl felt to be a low energy secondary failure. d f

'j Tank cover had some minor distortion and the welds at the en s o some cover braces were partially ruptured.

l 92

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No ruptures in tank so that there was no loss of oil.

. Turn ratto correct af ter failure.

Meggar values in air 1000 megohms minimum indicating no failures to ground within the transformer int:ulation system.

Flashovers reported in bus duct for phases A and C. See Figure 2.

Did not Found flashover in Phase A hus duct 8 feet f rom transformer.

find where Phase C flashed over as of 11/18/82.

HYPOTHESIS OF FATLUPE CAUSE The oil became saturated with nitrogen during the period between August and November. The saturation level would he the equilibrium values for' ambient temperatures that existed. It appears that a maximum ambient during of 30*C occurred two nights during the period. Ambient temperatures dropped to -3*

precceding the re-energization. The tenperature of the oil in coolers, piping and pumps vould decrease rather rapidly resulting in super saturation of this

" oil and possihty formation of some small gas pockets.

f When the pumps were started, nitrogen bubbles were evolved in the oil. Cas bubbles col.1.ected around the lower edge of the corona shield on the H.V.

j-bushing. Corona was initiated by these gas huhbles which ultimately resulted in a flashover from the shield to the edge of L.V. coil #21. .

The potential of the L.V. uinding was elevated when the flashover from the I H.V. bushing occurred. The potential became high enough to cause flashovers to ground in phases A and C bus duct. The bus duct was the weak link since it was insulated for 110 HV BIL while the transformer LV winding and bushings were insulated for 150 kV BIL. See Figures 1 and 2 for sketches of the failure paths.

This failure sequence agrees with the magnitudes and phase angle relationships of the fault currents.

H. R. Moore 12/3/82 .

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4 e APPENDIX B - ITDi 3 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION f I i 1 ) f i t f ( 96

APPEIGIX B - ITEM 3

                                        )                                   )

ENGINEERING IABORATORY P2 PORT #41-5F DEVELOPMENT ENGINEERING HUNCIE PIRiT TITLE: FIELD FAILURE DI S.O. HAM 707 ABSTRACT: One of the of ngle phase units (Ser. #7002099), built on S.O. HAM 707 failed in the field as a result of flashovers across the ducts between H.V. coils #31, #32 and #33. Flashovers also occurred to the L.V. series connections. This report describes teardown of this unit and outlines a probable sequence of flashovers. PREPARED BY:

                                                                          .                      Jo J. L. Pu?/.                         (Date)

Dpvelopment Engineer

                                                                   %        MW                       /50     O (Date)
                                          -                   I. L. Hansen

Design Enginchr

! APPROVED BY: l 49s. ALLmau L. S. licCormick (Date) l Supervising Engineer Electrical Development , i

                         >        4     .Y'          Y II %h (Dste)/ '

H.'R'.*Woore,' Manager f Electrical Davelopment i l i Page 1 97 l l

1 i j DESCRIPTION OF UNIT S.O.: Ham 707 Customer: VEPCO Type: Shell Fora, Single Phase, 60 Hertz, 4 H-L Generator Rating: 330 MVA (FOA) H.V. - 288675 V (1300 kV BIL) L.V. - 22000 V (150 kV BIL) FAILURE EVENTS The tranasformer was brought up to full voltage level and it carried . full load only for a few Hertz and failed. Heavy flow of ground currents was also recorded. Various cases and events that resulted in the railure is still under investigation. 9 9 98 - ., . - _ . - . = . _ - _ _ _ _ - _ _ - -- - . - . . - - - - . - -

e -

                                                                   )                                                  -)

4 TEARDOWil OBSERVATIO!!S , The phase was removed from the tank bottom July 12, 1976. No cvidence was found of a failure to the tank bottom, core, T-beam, or corona shicids.

                 .            The bridge was remove'd; nothing unusual was not'ed except for the
           , ebvious failure which will be described later.

Customer representatives arrived at the Muncie Plant July 13, 1976. Dismantling of the phase began with coil 38. All items were inspected prior to being scrapped -- no evidence of corona,was found anywhere..

./

Initial observations prior to dismantling vere that the phase was very dirty with carbon deposits. everywhere and copper shot in most of he A end. Channels were not intact in the A end of the phase over coils 30 through 33. A fault with apparently heavy currents had destroyed all but 1 strand of the outside turn on coil 33, burned through the edge strip on coil 32, and completely burned open the out-side turn in 3.1. plus about 807,of the second turn. Burn marks were detected on the series connections between coils 35 - 1 and coils 38 - 4. Coil 3 . - .

                                'Ihis ' coil had no burn mark on it; however, the series connection betueen coils 38 - 4 did. The crcpe paper over the series connection had been burned through just above the washer line. The resulting cavity in the copper was approximately 1/4" in diameter and approximately 1/8" deep.                                                                     -

Coils 37 and 36 . These coils were removed with no evidence of failure except for heavy carbon deposits resulting from the . fault in the ll.V.

                                                                                     , 99                               ;
               '                                  -                                a

py ,

il 35 ,

This coil had burned spots all along tho outside turn at the A cnd (see sketch). There were 14 cavitics approximately 1/4" in di1 meter & 1/8" - 1/4" deep on Coil 36 side and 1 cavity at tho'45 cn Coil 34 side. In addition one of the outside strands were burned through just beyond the 45o. Most of the cavitics were in the out-side 4 layers of the turn, but two cavitics were detected in the 5th cnd 6th layers. The series connection was burned in two places just cb:,vc the washer level. The series connection split in three parts in tha turn height direction (Fig. 22 5 3 layers cach split'); only tuo of the three parts had cavitics in them corresponding to the layers that h d cavities on the coil. The washers above & below Coil 35 were badly discolored but not " burned" through. The severe discoloration occurred wherever a cavity was formed. The high-low insulation was dismantled with no evidence of any , crcing or corona. . Coil 34 . This HV line coil and it's associated stat,ic plate were free from unusual mcrkings. The washers between Coils 34 & 33 were badly enarred, but did not indicate any coil to coil punctures. Coil 33 .

          .This coil had all but 1 strand of. the outside turn burned out.

This' was located about 2 feet from the coil center line on the HV side cf'the coil. The failure also burned open seve al strands of the s:cond turn on the Coil 32 side. 'This coil also had two cavitics on C:ll 32 side much like the cavitics on Coil 35; they were located near the center line. Coil 3_2 = . This coil had it's edge strip burned away in the area of the fault.

In a,ddition there were two cavitics on the first layer & one cavity on th] second layer; they were located at the 45 line at the A cud on the LV side of the center line. Another cavity was noted on the 2nd layer ct the tie. Washers betwcon Coils 32 & 33 were burned open at the area of the fault.

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L 31 . This coil had it's outside turn burned open plus 3 strands in layers 1 and 2 and 1 strand in layers 3 of the second turn. The burned out area was in the A end on the HV side of the coil. There w2ro four cavitics on the outside turn also. One was under the tic cn Coil 32 side; two were on Coil 30 side on the LV side 900 line; cnd one was on the coil center line on Coil 32 side. The finish-finish connection between Coils 30 - 31 was burned n:arly open. The burn was located approximately 1 foot toward the nv side of the braze; the burned out area was between and on the outside of the two figure 15 parts. The paper covering this area was not

blackened as one might expect from a heating probicm; rather, it cpp ared to have happened in a very short time. - The uashers betwcon Coils were burned through at the fault . 1: cation.-

Coil 'f D .

This coil did not have portions of the gturn burned open, but there a two ccvities on the outside two layers;about center line on the Coil 31 side. There was also one cavity on the third layer about center line on Coil 29 side. . Coil 29

                                                                                                    .'        ~
              . , This coil ~had one cavity on the third layer on Coil. 30 side at the cdge of the tie. Another cavity was located on the Coil 30 side 1-1/2" toward tie from center line. On Goll 30 side,             A   cavity third s trand,10" from was noted between cdge of tie another cavity was detected.

the first and second layer of the third turn at about the center line; ' this was the decpese penetration of cavities noted. There were no cavities noted on Coil 28 side; however, a cavity was located on Coil 28 side of the finish-finish connection. .

               -  .The ' remaining coils did not have any uin6sual marks or distortions cvident.
                             -        .              . 101   .'                              .

_ ~ . . _ . . . . Q e. usnelusion

                       'In the area of the failure.across the second ductConsequently                                                                         from t.hc 11V line, Coils 31 & 33 are " closed" and 32 is "open".

the failure path from Coil 31 to 33 and over the top of'32 is

      . ossentially a straight line. The heavy currents that flowed during the coil-to-coil fault apparently generated a great deal of gas.

The gas uns distributed over most of the A cud The dueionized to channels gas then over the faulted coils and general gas expansion. provided a low impedance path for multiple secondary failurcs from the 11V, over the high-low washers, to the LV (Coil 35). There wcre a total of 15 cavitics on the LV Coil 35 plus one burned open strand. There were also three cavitics in the LV series connections. An estimate of the current ficwing betueen high and low would be marginal at best since it is difficult to oscimate the , current required to form the cavitics and impossible to know how many paths were faulting simul _.incously. f Wy} , I. L. Hansen . Design. Engineer J. L. Furi ' Development Engineer e 0 -

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APPENDIX B - ITEM 4 TRANSFORMER REPORT ON EVENTS AT N. /3'NA POWER STATION 6 8 9 107

l 6 0 9 e I 1 l i .J l l t I l l 108 , t i I t i r h

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APPENDIX B - ITEM 4 ') H;7 g 1. m s.,i , ,.,.,,ing i ,....n.ibi. te, en, ,,4m et deli..,y of hi. ee ,,. .. sie in,...u.4 EftGINEE.r5 TRIP RE?CRT 6,hsvt do!sy. restinensyst seIN 3811 G p.,: . g (3, ,ein.,thg,ggl pein,. ,g g;,; , sc,; ce ,:(,c,,q .3 2 . Fe, ten, reports sive e, summary of impe,rene peines er.d recommtadsvisas. Fe, Westin,heve. A$ne.. ,1, enty, cart UCCCSDUT 17e Ib0

                                                                                                                                                                         =att se aa.svat af eccetarv                          : art :n .tave=a e.cagiry C2fGINAL 70 ISSUING O!?AR W E*47*$ FILI NO.

COPY 70: ( FCR *E?CDT Ol571130T!CN OUICE . REFER TO 4 E*/EPSE S I CE99. C F T9i

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O um. Q File L - hAla 10'/-05 va-C va-O "a-O -- un. a_ a vR. v.. O VR. t] un. G u=. a va. _ O vR. Q Vi.rrinia Electric Power Compar'v ngs.

                          ..Mmt                                                                                                                                                              (Original S.O. FAM707 )

Fredericksburn, Virginia s.o. XDM13?3 sza. c. 7002090 L:cf.T1cN 7'APQfE CF TMlp to make internal insDection cart 17/15/80 CCNFERENCE FELD 17 North Anna !!uelear Generatine Station most PRgsINT Messrs. W. Birckhead and R. Barker of VEPCO: R. L Bewrn in of Wes tinchoose Richmond - and B. W. iTunon of Westinehouse Muncia TPTD

             $'.30dARY :

There was a flashover from H.V. line lead to L'.V. coil #16. We recommend the l installation of complete new phase (coils, insulation and bridge). 109 Tru.. i s .s.. ..z. >,

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                                                                                                                                                                                                           )

TRIP REPORT

                                                                                                                                               '                                                      Virginia Electric and Power Company S.O. XDM 1323 (originally HAM 707)

Serisi 7002099 VEPC0 reported that subject unit tripped of f the line on November 29, 1980 during normal weatrer conditions. The SPR, mechanical relief device and cif ferential releys had operated and the fault was cicated within 3-1/2 cycles. There was also a flashouer to ground in the L.V. isolated phase tus duct and H.V. arrester damage VEPCO which VEPCO that also reported feelsthe was rechanical unit will rather than electrical failure. ratio correctly but the D-hic readings are slightly Icw. An interna 1' inspection was made by Messrs. W. Birckhead and R. Barker of VEPCO and B. W. Hugon of Muncie LPTD ca 12/15/80. The H.V. line coil wire of coil comes up vertically through a clamp and

then turns 900 and brazes to a horizontal run of copper bar. Brush copper bolts to the end of' this bar and goes vertically upward to the H.V.*Line bushing which is located on the tank centerline.

There war a flashover from the bottom of the end of the This coilhorizontal is 6" run of the H.V. line lead to the radius of L.V. coil #16. beyond rather than directly below the end of the horizontal run of the H.V. line lead. Bare copper was exposed for approximately ona square 1he four conductors of the outside layer of

                                                   -                                                                              inch on th.e ll.V. line lead.                                                                                               to coil 916 were severely pitted for about 12" and the strand adjacent coil #15 was burned open. The free end of the open strand toward                                                    The the other coil centerline remained essentially in its nornal position.

j free end projected upwards 4 or 'S inchesand 'then turned downwards for' about 2 inches. No are pits could be seen on L.V. coil #15, however, it appeared that the washer between coil #16 and #15 may have been ruptured, and there was In this area considerable carbon on the coil #15 side of this washer. the outside edge of coil #16 is approximately 5" higher than that of coil

                                                                                                                                       #15 sir.ce #16 is a " closed" coil while #15 is an "open" coil.

The round barriers around the H.V. line lead and the bridge walls supporting these barriers were broken. corona shield The hanger streps for the lower half of the double do nut on the H.V. bushing were distorted so that the side of the lower shield toward the L.V. side of the tank was lower than its normal position. Many of the fiber studs in the bridge walls supporting the L.V. series connectio,ns were broken. 110

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No are to ground was observed during this inspection. It is, of course, possible that one or more L.V. coils may have flashed to the core in a location not visible at this time. The amount of copper particles and carbon was fairly small for a field failure but still of sufficient magnitude that we would recommend the installation of complete new phase (coils, insulation and bridge). The tank, bushings, coolers and most core steel should be reusable.

              .j% M r-  p        avalu B. W.-Hugon Design Engineer BWH:sjt:0154E O

i 1 ep C e 6 9 I 0 111

t i 9 4 L 112

D APPENDIX B - ITEM 5 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION O l r e 113

U e l P I ( O t i 114

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                                                                                                        'l     's
              *

I APPENDIX B - ITEH 5 DISASSEMBLY REPORT SUBJECT 7002099, built originally on shop Disassembly of transformer serial numberThe transformer is a single phase, 330MVA, order HAM 707. the North Anna Station of Virginia step-up transforir.cr and was located at Electric and Power Company.

HISTORY 29, 1980. An internal inspection was

                'This unit failed in service on Novemberby Mr. W. Birckhead and R. Barker          This of Virginia performed on December 15, 1980 Electric and Power Company and by Mr. B. Hugon of Westinghouse LPTD.

inspection indicated there had Itbeen was an are between determined thetime, at that RV line dueIcad and the to the center low voltage coil group. contamination present, a new phase would be manufactured for re-assembly of this transformer. REPORT OF DISAS9EMBLY ,

On May 17, 1981 the The transformer was untanked and the core was removed. phase was dismantled by LPTD personnel in the presence of Mr. W. Birchhead Mr. R. Barker of Virginia Electric and Power Company, Mr. J. MechlerThe of the phase Hartford Boiler Insorance Company, and Mr. E. Luke of Westinghouse. The entire assembly was split at the high-low space between coils 14 and 15 Low voltage coil grou group of coils, I through 14, was removed intact.The only coil in this group that through 22 was then dismantled coil by coil.The other coils and their associated showed evidence of failure was coil 16. insulation were dar!:ened as a result of the carbon and smoke f rom the The HV line Icad was de, signed to come vertically up from coil number 34 and bend. . Wire of. coil was then brazed to a rectangular copper then make a 900 plate which nad four holes for connection ~ to'a' flexible lead w to the'HV bushing. The arc burned one strand of the cuter to the edge of low voltage coil 16.The other three strands of this outside layer of coil 16 conductor apart. There were also are marks on the layer were pitted but had not burned open. it is believed this occurred when second layer of this turn on coil 16, butThe are had melted a snali crater in the corner the outer strand burned open. Particles of copper frou this arcing were of the HV copper place connector. Coil washers in coil 15 to 22 group and some of the observed on the phase. high-low washers between coils 22 and 23 verc broken along the top edge by bridge items that collapsed as a rcoult of the are energy. e 115 G

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                . Attached is a portion of drawings llAM707-48 and ilAM707-50 and Page 6 of L-ilAM707-08. Drawing HAM 707-50 shows that relative position of the llV lead with respect to the coil groupings.

This' unit is being re-assembled utilizing a new phase which contains all rew coils and insulation. The H.V. line lead has been redesigned such that the rectangular copper plate has been eliminated and the connection, from the coil wire to the li.V. ' bushing, will be made with insulated cable. . @L. E. Luke, ~Manager. Design Applications 04065 8/P/ s =

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O APPENDIX B - ITEM 6 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION e e 1 21

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i APPENDIX B - ITDi 6

~,

hI r L. ,Lif.:s ~'t,).r f' DISASSElfBLY l(i: PORT SUl: JECT Dinassembly of transformer, serial nunter 7002100, built origtnally on shop This order HAM 707 nnd retierned to !!uncic f or repair on shop order XD:11779. transforcer in a singic phase, 330 !!VA, 500kV generator sicp-up trannforner and was located at the 1Mrth Anna Station of Virginia ticc.tric and Power Company. HISTORY ThJs unit f ailed in service on June 19, 1981. An investigation at that time indicated it was a high voltage line bushing f ailure and did not involve the windings. REPO'?T OF DISASSD!BLY The conplete phase was disasseabled coil by coil on October 7th and 8th,1981-There was a with "r. Joe McGregor and Mr. Robby Bridges of VEPCO present. very large.anount of carbon deposited on both the coils at:d the insulation,

       - but no f ailure uns fcund. The termination of the tape, on t:e high voltage lina lead from the coil to a poirt just below it's borizontal                                   There     run, had was, however,
       -essentially no taper rather than the specified 7" taper.

na evidence of tracking or any other electrical difficulty at this point. Na other irregularitics were found. Prepared by: 6310 X"W "h3/u B. W. Hugon (Date)

       ' Design Engineer t

Approved by:

               <3      6 1.. E. Luke '

7(Dtte)Iy[f2 Mant:ger lh'ulnn Applications i 123 L 02m. 1 L

) 1 i i e 0 e l $ 4 124 x.___,_,. . . .-. . ----. -.-- - - -- - - ~ --- - - - - - - - - - - - - - - ^ - ~ ~ ~ ~ - '

O APPENDIX B - ITEM 7 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION l I 9 l . i l 1 i 125

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V O 4 1 o e e p 126

m AFFEND1X B - ITEM 7 i

Tearduun Report VEpCO North Anna Unit 500/22 GSU Si 7002300 S.O. XDM1867 (IIAM707) 2/22/33 s Summary iciled August 2?,1982 and returced to Muncie for teardown phanc This transforect Visual inspectien of the phase indicated greatest hin:: and inspection. damage occurred in the hV coil 22 located approximately belou and next to a II-h spacc. Many nre spots were 22 and the out er layern of the coil were displaced. located on obrerved on the HV bur.hing shield and alno on 22.- the llV grcund the tank covcr.

 '       cceurred        between the llV bushing shield and the edge of the LV coil Detailed Inspection Tanh:      The top bection of the tank was distorted and ruptured in loccrions (at brace cadc).

ground planc. The botton _section uas dirty but otherwise undcuaged. Nearly all Bridge: The bridge structure war brokca into several pieces. liber bolts connecting the bridge structure were bJoken. The core uas straight and undemaged. Core Stec1: The coil supports were in place and undanaged. Coil Supports: Phace: Coil 22' - The coil outer edge vac 'bstred and the copper pitted linc and th2 at, seven locatic.as en the A cnd and spaced be.tt.een the A end centerThe largest bu a h A cnd. tangent betre n the HV leg and the corner radius. occurred a

            'A large charred area (18" x 36") was located on the face of the coil et t                           1 tangent between the HV Icg and the A cnd radiun.No turn-to-turn f lue                                   ailuren va the outer      three  turns.

Three very small spots shoufna pitting of the copper ucre foun.1 Coc on area. One between layers 3 and 4, and tuo betweca layerc 8 and 9 outer turn: additional small arca shoved bare copper free of pitting. f the

         -    Layers of the outer turn were distorted perpandicular                                 to the Spacer        plaue blocks     en o coil. These distortions were located in the charredd area.                      h diutorted washer 69_(under cell 22) were found to be overlapping un er t e area.

6, 27, 29, 30, 31, 33 and 34 - Carbon ddeponito channels. Co t i s 5, ,6, 7, 9, 12, 23, 24,in the forra of branching ntreamers were on the B 1407E 127 . i 6 ,

                                                                    - - - - - . . , . _ _ ,                        9 d1rw--e ,eg.   ,_

                            ;                                  }

s : 1 Coiln 5, 9, 23 and 26 - rarbon depostts in the form of branching streamero f were found on tight precsboard channels on the liv leg. Coils 5, 23 and 26 - Carbon deposits in the form of branching streamers were ; on the tight, pressboard channels on the LV Icg._ in the form of branching streamers HV and LV side chcets - Carbon deposit were found originating f rom the string tics and along the vertical pressboard strips. Streamerc were also found along a crack in the HV side chect. Coil Washers - Several of the coil washers had sarton deposits in the form of

 ' branching streamers. They were especially noticeabic on the following:

Wacher 6 had streameru on the A cnd originating at the coil spacer blocks on the side facing-coil 36. The oppocite side of washer 6 (facing coil

35) was unifornly dark outside the coil perincter.

Uasher 7 had streanern along the A end edge originating at the edge. Also, a very long streamer at the B cnd llV corner. Washer 56 hed streamers betwoon spacer bloche on coil 23 side on both llV and LV legs. : Washar 97 located between tap coils 9 and 8 had streancrn on the bottom side (facing coil 8) instead of the top side. Washer 99, betreen coils 7 a'nd 8, had strecracrc on both sides. Washer 109 had strenners between spacer blocks on both legs on the coil 4 side. I l

  - II-L Space Washers - Streaners betecen the spacer blocks and top and botton             l cdges ucre on gll washers in the H-L space between coils 14 and 15, coils 22 and 23 and c'i3:o   34 and 35.. ' The streancr's occurred on one side uith the other side uniformly dark.

HV Jusbing - Chips of poreclain found on top of the phase shouThese signschips of arcing are ) on the inside curface and inside the porcelain cross-section. from the upper half of the oil side porcelain. hbh Dale Uhl.to Engineering Operations , t e e 128 1407c

I

APPENDIX B - ITEM b TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION 9 t 129 e

y . . I 4 2 9 0 s i 130

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APPENDIX B - ITEM 8 , REPORT ON FAILED BtlSilING ' - . August 3, 1981

Euching 255D500, Croup 2

         ' 500 kV system voltage, 1300 kV BIL kilt as llAM 707, Serial 6.in 1974 The remains of the bushing uere- received in Muncie on July 16, in a VEPCO shipping crate. ' Parts received were the condenser, flange, expansion cap assembly, lower porcelain assembly, lower shields, ficxible cable to the transformer vinding and broken pieces of the upper porcelain. Broken pieces of the lower porcelain werc. received in a separate shipping crate.

The bur.hing was discssembled on July 22 and the condenser was tnvound on July 23. The lower 24-in'chec of the straight portion of the condenser;.i.e., the paper area over approximately the Ic.'er half the ground foil layerBoth (No. 55), vas' burned and torn for cbout one half the circumference. tapers had mechanien1 gouges due to broken procclain and a few singled areas. The support sleeve had shif ted upward about 2.88 inches relacive to the - condenser.. The voltcge top insulating board was firmly in its correct position. parts of After about five inyerc of paper. the burned and torn paper ended. the circunference of the condenser paper cppeared to have. spats Thuae whichucro region, werefron ir.pregnete.1 but vold of oil between layers of paper. about foil laye; 52 'to foil layer 53. No other damage or The entire. condenser oss unsound to the lead. irrcgul.crLies were found.

             'the 2 inch shielt ws still l'artially attached co the 1cuer -pocccle.in Appport.
                          'Jhe lower chield was couplete disloegt;d frcn the suppcrt,Moth   bs.th of
          . the support bolt hands having pulled through the coppdr material.
           -ohields had several are marks on their cides, some of the holcs burned completely through the material. The upper 2 inch shield also had five 0.5 to 1.5 inch seni-circles burned out on its top curface chere it occts up vith the The lower support clso hcd pock matku top surface of the louer support.

adjccent to the areas. . i The spanner r.uf. . The bottom cod of' the exp.rnsion cap was den.ed inward. Socc securing the spring asset.ibly had j.ome stripped threada an? had cracked. of the matchir.g thicads on the 1e'ad tube had also stripped., L. B. Wagenant 0603E 131 B 9

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e t e 3 a 4 e 9 e 1 132 r .,,-e-,- ,- - - . , - - - - , ..,,,._-.-.-,,,.,,,--,,,m,n,.e_,enwn-w.m.w.,.m,,,,,nm,,,,.n_ nww,,wm_,,m,-a,-,,,vw.m,, --w<

APPENDIX B - ITEM 9 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION ( e 133 w - r-----, -- _ _ - , _

as e,- - h e t O 6 9 [ ) I i i b 134 Y

APPENDIX B - ITEM 9 t REPORT ON FAIT.ED BUS!IltiG

                     -                    August 3, 1981 Bushing 255D500, Croup 2 500 kV System Voltage,1300 kV BIL Built as I!A!!707, Serial 7 in 1974                            .

The rer.ains of the bushing were received in Muncie on July 16 in a VEPCO shipping crate. parts received were the condenser, flange, expeccion cap assccbly, lower support assenbly, louer chicid, have terminal and flexible cable to the transfocuer uinding; neither of the porcelains was received.

                                                                                         *he
   'Ihc bushing ucr disassembled and the coedenser was unwound on July 22.

exterior of the condenser was burned, .erinkled and fluffed up on the entire circumf erence on about the bottom 41 inches of the 56 inch long straight artion. About 16 inches of the grcund layer foil (No. 55) between the top of

   'he t       support sleere and the ground layer groove uns exposed. About 14 inches of the bottcn of the foil was also expcsed. About 0.144 and 0.308 inches
                                                      ~

The of paper originally covered the ground and tap foil layer, respectively. vc.ltnge tap i. nulatien board uas fituly in its correct position. . The lower taper of the condenser had a surface burn 4 to 24 inches vide running the full length of the taper, the burn getting vider as it progressed up the raper. The remainder of the taper surfcce was singed, but not badly tur:.ed. 1hc top tapas had a feu singed creas, all of u'aich were only on the surfacc. The suppert sleeve has been shif ted uputrd abcur 1.75 inches relative to the condenscr. Dic outer 5.74 incl.es of the coader ser ht 3 shif ted about 0.31 inches opvard relative to the lead. The lead ruhe showed several barned narl.a f rem electrical arcing just belou

Some ci. the' bor. tom of the condenscr' for about one half of its circumfer2nce. Sor.te of the marks the burned marks formed a continuous 2 inch long crater. were just below the burned area on the condenser taper, but most were within c 90 de' gree are to one side of the darkest area en the taper. There was also evidcace of arcing on the lower procelain nepport and the lock not located on top of the former. Thenc umrks get.arally lined up with the burnc3 area on the lower taper of the coi. denser. There t.cre also arcing macLa in both lowcr shicids. Both shicids had been disloc.ted frem the lower support, the heads of the screws attaching Lucm to t.hc support havi.ig pullcd cither off or through the copper raaterial.

      *1he condenscr was cotr.nletely unwound. Burning war found through foil layer 53 (about 0.39 inches of paper thickness) and the amount of nuch decreased as this foil was approached. About 70% of the approximately 49 inch circumferenec of the lower ends of the last L.wo foil layeta (Nos. 54 and 55)

The arcan burned away from had been, burned away for 8 to 9 inchen in length. the foils were generally aligned and extcuded abot.L 6.5 inchen beneath the flande. , '1he surface burn on the lowc taper had progressed to within an ineb of the bottom end of foil layer 53, but uns only on t he tapered surface below this point. No other damage or irregularitieu vere found beyond foil layer SL 135 i T.. E. L'ar.cean t

I APPENDIX B - ITEM 10 TRANSFORMER REPORT ON EVENTS AT N. ANNA POWER STATION 9 e 136

t 0 9 e 9 1 a f 137

   - ,_.,m,   -
                  . . . _ . _ . - . , _ , , - _ , _ , . , , _ _ _ _ . _ _ _ . _ , , - _ . , . _   -__._____._,..--___.,..,,y.     - . -

APPENDIX B - ITEM 10 R&D REPORT 82-7D7-MUNCI-R1 INVESTIGATION OF FRACTURE OF A POWER TRANSFORMER CAP SCREW FRCH VEPCO'S NORTH ANNA NUCLEAR STATION A. Madeyski Materials Engineering Department October 6, 1982 APPROVED

                  .b R. C. Bates, Manager Materials Engineering Department l, e           *
         /k.l sw G. W. Wiener, Manager Materials Science Division us O Westinghouse R&D Center 1310 Beulah Road Pittsburgh. Pennsylvania 15235

- , - , - - - ~ _ . , , , - - , , , , - - . . . - - - - , , . - , . - . . . . , . _ . , - , , - , , - - . - , - _ . - . -

                                                                                                                                                  . , . - , - . , . ~ . - , . - , . . - , _ - -

CONTENTS 1-1

1. I NT RO D U CT IO N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-1

2. EXPERIMENTAL PROCEDURES AND RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . .

2-1 2.1 Optical Examination..................................... 2-1 2.2 Fraccography............................................ . 2.3 Energy-dispersive X-ray Spectrometry (EDS) . . . . . . . . . . . . . . 2-1 2.4 Hardness Tests.......................................... 2-1

3. DISCUSSION...................................................

3-1 4-1

4.

SUMMARY

OF THE OBSERVATIONS AND CONCLUSIONS.................. 5-1

5. ACKN0WLEDGEMENTS.............................................

9 i i V I l I l r h. f i i l 139 l l

October 6, 1982 R&D REPORT 82-7D7-MUNCI-R1 INVESTIGATION OF FRACTUP.E OF A POWER TRANSFORMER CAP SCREW FROM VEPCO'S NORTE ANNA NUCLEAR STATION A. Madayski Materials Engineering Department ABSTRACT A forced outage of the main generator stepup transformer at VEPCO's North Anna nuclear power station occurred on August 22, 1982. One hypothesis is that this was caused by the fracture of a cap screw supporting the insulator bushing. An investigation revealed that the final overload f racture of the screw started from a very deep crack produced by a low stress, high. cycle bending fatigue. The fatigue cracking may have occurred earlier when the bushing was transported by truck several times. In transport to and from the factory, the bushing laid in a horizontal position in a crate, exerting a bending moment on the screw. Vibrations of the truck made the stress variable, resulting in the fatigue cracking of the screw. 140

1. INTRODUCTION A recent forced outage of the main generator step-up transformer at the Virginia Electric and Power Company's North Anna nuclear station was found to have been caused by the fracture of a cap screw. The cap screw was part of the support structure of an insulator bushing, as shown schematically in Figures 1 and 2.* The 500 kV bushing assembly (Drawing No. 255D500) was manufactured in 1965 by the Westinghouse

transformer plant in Sharon, Pennsylvania. The bushing was shipped to the customer when originally built. In 1976 the bushing was sent to the Westinghouse Power Transformer Division (PTD) plant in Muncie, Indiana,

          ,    to correct an electrical connection, after which it was returned to the customer. In 1941 the bushing made another round trip to Muncie for electrical testing.                                             .

The broken cap scraw was submitted by PTD to the Materials Engineering Department of the Westinghouse R&D Center for investi-gation. It was stipulated.that the method used in the investigation should be non-destructive, if possible. Fortunately, we have succeeded in determining the causes of the cracking in the screw by non-destruc-tive procedures, and the screw will be returned to PTD essentially as received. The investigation consisted of macro-examination, optic;l and scanning electron microscope (SEM) fraccography, energy-dispersive X-ray spectrometric (EDS) analysis of the materiel, and hardness testing. Following is a report on the methods of investigation, the results obtained, aad the conclusions reachi.d. This design is not used anfmore in presently built transformers. 141 y-_ e - , _ . , . - , . . - . , -,m.-%. , ,-,-. - r ---. 7,. . - - . _ . - _ _ _ - - . _ - , . . - . - . - , ,m,., ,-._.y.e. - , , . , . . . _, _ _ . , - - - - ~ , - - - - -

l 2. EXPERIMENTAL PROCEDURES AND RESULTS 2.1 Optical Examination The broken cap screw was carefully examined with unaided eye , and using a low power optical microscope. Figures 3 through 6 show the general appearance and details of the scrtw. 2.2 Fraccography The fracture of the screw was examined using an optical microscope and an SEM. and 13 through 21. The results are shown in Figures 7 through 10, 2.3 Energy-dispersive X-ray Spectrometry (EDS1 While inside the SEM chamber, the fracture face on the shorter piece of the cap screw was analyzed using the. EDS attachment.The .

                      .results.are shown in Figures 11 and 12.                                                                                                      .

In interpreting these results it must be remembered that the EDS technique cannot detect any elements with the atomic number l

11. ower than This includes hydrogen, helium, nitrogen, oxygen, fluorine, and neon. lithium, beryllium, boron, carbon, However, all the other elements, including important alloying additions such as chromium nickel, ,

molybdenum, vanadium, etc., can be detected and measured. 2.4 Hardness Tests The side of the shank was used for hardness testing (Figure 4) . Both Rockwell C and B scales were used. agreement with each other: The results shown below were in 142

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                          ,   .       -,.  , . , . -        -  . - , , , . , . - - . . , - - , , - - - - . -           - . , - , , - - , - - -       ,  . , - - - ,     - - - - - - - - , - - ~

i 9 RC 21-23; RB 98-101; The corresponding ultimate tensile strength is 112-120 k41 approximately. I O 143

3. DISCUSSION Fractography has shown that the screw cracked very deeply in high cycle fatigue (Figure 9, areas 1 through 11), before it finally broke completely in shear overload (Figure 9, area 14). The fatigue crack started from multiple origins on the side of the screw marked 0 0 in Figure 9, and gradually moved across the screw to area 14. At this point the remaining cross-section carrying the load was very small, and .

eventually some relatively minor load produced the final catastrophic fracture. The overload fracture of this remaining ligament was of the

                 " dimpled rupture" type (Figure 19), indicating that the screw material was fully ductile.

The " primary" origins of the fatigue fracture were all located r at the same side of the screw (Figure 8), but it is interesting to note i that there was also a very shallow " secondary" fatigue crack which started from the 1800location (Figure 9).. Thi's crack propagated only t to the depth of about 0.01' in (Figure 20) before the screw finally broke completely. The type of fatigue in which the fracture traverses the cross-section from one side to the other almost in a straight line is characteristic of bending in a single plane. In a normal operation of the transformer each cap screw acts only as a cuice between the clamp l ring and the cylindrical flange (Figures 1 and 2). Therefore, the cap i l screws are not expected to be subjected to any major bending moments. Furthe rmore , there is no obvious source of cyclic loading on the bushing support to produce fatigue cracking of the screws in normal service. However, when the bushing is shipped to and from the , factory, it_ is , crated in a horizontal position, and part of its weight rests on the center portion of a cap screw. Vibrations of the truck provide the 144 l l . l

                                                                                                                                               +
                                                                                                                                           'gr variability of the load necessary for fatigue. A close examinscion of                                                                                                                                           i the broken S..w provided additional evidence substantiating the                                                                                                                                          .

hypothesis that the cracking occurred in transport. Figure 6 ~ illustrates the wear areas in the middle of the screw. The wear area at the O' location shows heavy peening and burnishing of the steel, indicating that a high transverse load acted at this point. This load j provided the bending moment at the screw thread. A variable bending stress was thus superimposed on the constant tensile stress due to the tightening of the screw in the threaded hole. The resultant stress range apparently exceeded the fatigue threshold level and initiated the j crack. The fact that this particular bushing was shipped 5 times prior to its forced outage, makes this explanation even more plausible.

A smaller amount of peening and burnishing in the middle of the
screw at the 180 location (Figures 4 and 6) was presumably caused by an upward load due to botacing of the truck on uneven roads. This load

',i apparently started the secondary f atigue crack. The hardness test indicated that the screw had a tensile , strength in the vicinity of 115'ksi which is quite acceptable for a grade 2 screw (74 kai ain.). The EDS analysis has shown that the steel was of plain carbon grade, presumably middle carbon. There was also no fractographic evidence of any material or manufacturing defects which would have started fatigue cracking at an unusually low stress level. Thus, the quality of the screw does not seem to have been a contributive factor in the cracking. The second cap screw did not break, but was severely bent, presumably after the first screw broke. The second screw was not subritted to the R&D Center for examination. l 145 i

4

SUMMARY

OF THE OBSERVATIONS AND CONCI.USIGNS

1. The screw cracked deeply in high cycle, low stress fatigue before it eventually broke in ehear overload at some minor load.

2. In normal operation of the transformer the bushing support cap screws are not expected to be subjected to any significant bending, or to cyclic stresses.

3. The evidence suggested that the screw cracked when the bushing was

 .                   being shipped in the horizontal position, with its weight bending                                                                                     -

the screw. Bouncing of the truck on uneven roads provided the variable load necessary for fatigue cracking. The bushing was in transport 5 times before its forced outage.

4. The bushing was made in 1965. This design of the bushing suspension system is not used in transformers currently produced by Westinghouse.

G 146 _ _ _ - - _ _ _ _ _ _ . _ _ . , , - -m--_____,___,.. - . _ _ ~ . . , , , . . _ _ , . , _ - - - . _ _ - _ - , , _ _ , _ _ . , _ _ , _ _ _ _

O

5. ACKNOWLEDGEMENTS The background information and the excellent cooperation

' provided by J. Templeton of the Power Transformer Division in Muncie, Indiana, is gracefully acknowledged. Thanks are also due to the following members of the Research and Davelopment Center staff: R. C. Bates and J. W. Cunningham - general guidance and techical discussions; J. P. Yex - macrophotography; T. J. Mullen - SEM . fractography and EDS analysis. J. Selchan - typing and assembling of the report before printing. l l l l l l l l I 147 t .. _

r ) l owg.777 f A75 Bushing

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Cap Screw ac- - -

yThrough Holes ac :- Threaded Hole s J -Cylindrical Flange , Hanger g l Straps ( Clamp Ring f i

l l Fig.1 - Schematic sketch showing the location of the two cap screws in the bushing support assembly. Not to scale. l l l 1 148 l i

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                    -                     Cap Screw g ock Washer L

3 3 E C GE 3D c Hanger Straps r i Section of Cylindrical" Y " Flange -t - i 5 Clamp Ring Fig. 2- Schematic sketch showing the central detail from Fig.1. Not to scale 149

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! i l 1 l Mag. 3.5X . . Figure 4. Details from Fig. 3. Note the wear marks under the head and i in the middle of the screw. This side cf the screw is located 180* from the main fracture origins (see Fig. 9). The inden- ! tations visible in the bottom photograph were produced in the hardness tests. i ' l I 151 RM-97391

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l SPECTRun TRANSF0 AMER SCREW SEP. 20. 1982 FNACTURE NEAR ORIGIN STAWBARDLESS EDS ANALfSIS (Z4F CORRECTIONS NIA RAGIC V) ELEMENT UEIGHT ATOMIC PRECISION 8 LINE PERCENT PERCENT 2 SIGMA K-RAlIO ITER CR K4 0.24 0.26 0.06 0.0032 MN KA 1.04 1.06 0.09 0.0107 FE KA 98.38 98.37 0.39 0.9841 NI KA 0.34 0.32 0.12 0.0031 2 TOTAL 100.00 NORMALIZATIDH FAC10R: 1.001 Figure 12. Semi-quantitative EDS analysis of the fracture from Fig. 11. The analysis indicates that the cap screw was made of plain carbon steel. i 159

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;                                                                                                                                                                                                             Mag. 2100X                                          .         .

I Figure 14. Bottca of the thread groove at the primary crack origins. Note the spalling of the oxide due to metal strain, and

                                                                               .           a number of small cracks, marked with arrows. The lower photograph (No. 7) shows one of the cracks at a higher                                                                                                                                       i magnification.

161 1 i EM-97400 e-m-,,-ww -w--- e-ww--.w -w+w-- ----.~-- - ,ww. ._ yWwp r ,W.

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i Mag. 1050X Figure 15. The first major beach mark of the fracture (marked No. 8 in Fig. 10). The crack probably stopped here for some l' time, and then resumed the propagation.

l l t l 162 < l RM-97401

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Mag. 2100X Figura 16. Fracture areas 9 and 10 from Figs. 9 and 10. Note the t topography characteristic of fatigye at a moderately high strass intensity range. 163

Di 97402

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2OOPM 20KV O2 012 S N Mag. 105X Figure 17. Fracture area 11 from Figs. 9 and 10. Note the side cracking characteristic of a high stress intensity fatigue fracture. 164 RM-97403 1 i

i i

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m '- :. < r , Mag. 1050X Figure.18. Fracture area 13 from Fig. 17. j 165 RM-97404

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l Figuta 19. Fracture area 14 from Figs. 9 and 17. Note the typical l

                                                                                                                   " dimpled rupture" topography, characteristic of an                                                                                                                      l overload fracture in a ductile material. Since the                                                                                                                      ;

dimples are all ' oriented in the same direction, there i must have been a strong shear component in the final ' l load. I i l l l l I 166 EM-97405

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l ... -.,.s , . Mag. 2100I Figure 20. Fracture area 16 from Fig. 9. When the cap screw broke in the final overload, the area marked "A" was rubbed and smeared. , However, small portions of this area, such as "B" in the photograph 16, which were left undamaged, show that this was originally a fatigue crack that started from the "180*" side (Fig. 9), but did not propagc.te as far as the crack which started from the "O*" side. The rubbing of area "A" must hava ; produced enough haat to create oxide "C". 167 au-974o6 _ _ _ _ . _ _ . - _ _ _ - - _ _ - - _ . _ - . - _ _ _ _ _ _ - . _ . _ _ - . . - _ ~ . _ - . . . _ -

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l Mag. 2100X Fip,ure 21. Appearance of the bottom of the thread when viewed in the direction of the arrow 18 in Fig. 20, Note the spalling of the l oxide, indicating strain, and some sn.all original fatigue cracks marked with arrows. 168 RM-97407 l r F ws---w_. ,e--<-.e~ __-r,m_.

O APPENDIX C TRANSFORMER REPORT ON EVENTS AT N. ANNA POWEh STATION 8 I 169

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EHVEQUlbNTFILLR5 CORD .

l , ' Substation _ N . I.,, [I A , Readings to be taken every ha!f hour. . Equip. ment d 7 _-; 7- tm. () 6 - -

                                           ..       Manufacturer Nf5 r/#r.///s us /?                                        . .                                                         ..

Probe # Oil in

!                                                   Serial Nunber 7s o / S'2. 7

) 6-J a.- 8l Probe # Oil, Out 2 '1 2- / H i

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i Date Time ma hg am hg . Probe . Dew : - . OC KV .Readirig Remarks C C Reading Point 4 P Ps Cs P?M g -> n I W .1,ew.timr'T"

.,.io IN

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! C = r.o'Isture foncentration in sample in PPM. ' I C=C Cs =,100% saturation of moisture in samole at temperature of measurement. s (Pg, )- i P p== . Vater vapor pressure measured by prob laturated test.c sapor pressure == a- ...-*"-:---- * -

s ., EHV EQUI- NT FILL RICORD - -

                                       ,                                         ' Substation                              f! d . . . [.                                                 Readings to be taken every half hour.

Equipment .- T x, ,

Hanufacturer y f ,. . ! : , , . , , , , , . , , Probe # Oil in - * - Serial Hunber 7 0 0 / S'y. 7 Probe # Oil Out .2 -7.7/ N T er t. e F .

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                                                                                , ,) I         .                                                 ~/.I OUT                          -                    l            1 .                1                I           -

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ML20080Q732

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