ML20086H193
| ML20086H193 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 01/03/1973 |
| From: | Vandenburgh D VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | Boyd R US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML20086H198 | List: |
| References | |
| TWX-120, NUDOCS 8401160054 | |
| Download: ML20086H193 (2) | |
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- 12. ROGER E0YD
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'RF-IN ANSUER To QUESTIONS POSED IN J. OUSLEY\\S TELEPHONE CALL OF JANUARY 2, 1973, THE FOLLOUING ANSUERS ARE SUPPLIED \\
REFERENCEN LICENSE NO. DPR-28 \\ DOCKET NO. 50-271\\
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NO PLANT TRANSIENT OCCURRED PRIOR TO THE TRANSFORMER FAILURI.
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sTESTING TO DATE INDICATES THAT THE FAILURE WAS DUE TO AN INTERN FAULT EITHER IN THE "Y" SECONDARY WINDING OR BETWEEN THE ".Y"
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AND "2" WINDINGS.
THE FAILURE IS APPARENTLY UNRELATED TO THE l-I FAILURE OF THE AUXILIARY TRANSFORMER ON NOVEMBER 8.SINCE THE
X" SECONDARY UINDING WAS INVOLVED IN THAT FAILURE.
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All ELECTRICAL BALANCE l!AS DEEN PERFOREED TO CHECK THE ADEQUACY OF THE 25 MVA REPLACEMENT TRANSFORMER WHICH.WILL SUPPLY POWER FOR STATION SERVICE AND THE EMERGENCY BUSES.
THE PLANT AUXILIARY LOAD IS 20 MW AND Tl!E N0!!-RUNNING SAFETY LOAD IS 2.1 MW ON EACH EMERGENCY BUS \\FSAR FIGURE 8.5.1\\.
WITH THE S AFETY LOAD STARTItlG INRUSit OF FIVE TIMES RbMNING LOAD \\23.3 MVA\\ SUPERIMPOSED Oil Tite !!ORMAL PLANT AUXILI ARY LOAD, CALCULATIONS INDICATE THAT THE VOLTAGE DIP DURING THE STARTING PERIOD WILL BE APPROXIMATELY 6.5\\ AS COMPARED TO T}iE CALCULATED VOLTAGE DIP OF S.2\\ USING THE ORIGINAL STARTUP TRANSFORMER.
CALCULATIONS ALSO INDICATE THE TRANSFORMER WILL BE CAPABLE OF CARRYING THIS INRUSH AND i
t STEADY STATE CURRENT.
FOR THE SHORT PERIOD OF TIME UNTIL RUN-NING LOAD UILL BE 26.7 MVA WHICH IS WITHIN TNI CAPABILITY OF THE TRAliSFORMER.
THAT PORTION OF THE A16 PREOPERATI0t!AL r
TEST WHICH CALLS FOR ECCS STARTING WILL DE PERFORMED TO PRODUCE f THE INRUSH CONDITION AND TEST THE ADECUACY OF THE REPLACEMENT 1
TRANSFORMER TO START THE ECCS LO/DS, PRIOR'TO RETURNING THE PLANT TO POUER OPERATI0tl.
4 D. E. VANDENDURGH, VICE PRESIDENT VERT!OtiT YANKEE ' NUCLEAR POWER CORP.
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- 3) VERMONT YANKEE NUCLEAR POWER CORPORATION ScVcNTY ScVcN ORoVC STRCCT RU'rt AND, VERMORT 03701 REPLY TO:
VYV-2364 p.o. sox :57 vcn sas4 December 13, 1972 ga
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u.s. AT0',t C ENERGY COMMISSION REFEREUCE:
Operating License DPR-28 y$T$a Docket No. 50-271
Dear Sir:
This letter is to provide you with detailed information concerning the incident reported in my initial report dated 9 November 1972 (VYV-2327).
On November 8,1972 the Unit Auxiliary Transformer was extensively damaged by fire. The incident occurred approximately three minutes after j
a reactor scram, executed as the initial step of a shutdown from outside the Control Room which was being performed in conjunction with the Startup Test Program. Reconstruction of the incident utilizing computer printouts, recorder charts, and reports of participating personnel (refer to enclosure 1) fixed the origia of the problem as a mechanical failure of a limit switch gg; arm on No. 2 Turbine Stop Valve.
This prevented generator lock-out when the turbine was tripped and also prevented automatic transfer of in-plant (7
loads to the Startup Transformer. The generator was manually isolated from y;.N the system by opening the tie breakers in the 345 KV Switchyard. The generator began to coast down and the decreasing frequency caused over-kib(
excitation of the Unit Auxiliary Transformer. The exciter field breaker
! failed to_ trip during the under-frequency conditien because two leads to the~ volts / hertz relay were not securely connected to the terminal board y sur 1
(at_the back of the relay. At a point when generator frequency was n
UN' significantly less than system frequency, the in-plant loads were manual'y transferred from the Unit Auxiliary Transformer to the Startup -
Transformer. This caused a monentary application of system frequency to the Unit Auxiliary Transformer. Apparently, this momentary frequency mismatch, while the transformer was under the stress of overexcitation, caused the failure of the transformer.
On-site assessment of damage to the transformer on the following day revealed that it was essentially a total loss. The unit was returned to f
the manufacturer for examination and salvage,.to the extent possible. A ew transformer has been ordered and is expected in April, 1973.
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m VERMONT YAILJ.E NUCLEAR POWER CORPORATION Directorate of Licensing December 13, 1972 Page 2 Based upon initial investigation, a progran was developed to: a) evaluate the extent of possible equipment damage, b) implement corrective action as necessary, c) preclude recurrence of the incident and d) provide adequate assurance that plant operations could be safely continued.
Salient features of thin program included the following:
1.
Cleaned up debris and removed danaged transformer 2.
Doble Tested the Main Transformer and collected and analyzed oil sampics on the Main and Startup Transformers 3.
Inspected all control cabic to Main Transformer 4.
Inspected 345 KV dead end insulators and inspected bussing 5.
Inspected generator stator end-turns and slots 6.
Checked all relaying associated with the generator lockout relay 7.
Doble Tested the Main Generator 8.
Inspected bus ducts and bussing 9.
Inspected breakers 12 and 22 10.
Meggered all motors on the 4 KV busses 11.
Modified the Stop Valve Collats and replaced damaged limit switch 12.
Inspected and verified satisfactory operation of all Stop Valve limit switches 13.
Repaired and tested the volts / hertz relay 14.
Modified applicable Plant Operating Procedures.
Execution of the individual steps involved in satisfactory conpletion of the program outlined above began on 9 November 1972 and extended to 22 November. On 20 November 1972 all significant test results had been obtained and there was clear evidence that damage had been limited to the Unit Auxiliary Transformer only and all other associated plant equipnent was in satisfactory operating condition. At a meeting of appropriate company and vendor personnel it was concluded that plant operations could be conducted with reasonable assurance that an incident.of this nature would not reoccur.
It is gernane to point out ' that a 4160-volt power line from the adjacent Vernon llydroelectric Station may also be connected to the station switchgear-and is a source of off site electrical power in addition to that provided by the Startup Transformer.
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VERMONT YANew.aE NUCLEAR POWER CORPORATION i
Directorate of Licensing December 13, 1972 Page 3
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Preparation for startup.was begun 21 November and Vermont Yaakee returned to power operation-22 November.
j Very truly yours, j
VE ONT YANKEE NUCLEAR POWER CORPORATION Robert F. Jones Plant Superintendent DUE/rph I
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Enclosura 1 o
O SEQUENCE OF EVENTS LEADING TO LOSS OF THE AU C IARY TRANSFORMER 11/8/72 S
Time Event 1440:31 Manual scram from Control Room 1441:11 Stop valves closed as a result of High Moisture Separator Level Trip. Bypass valves opened to maintain pressure.
1441:20 Turbine tripped manually in the Control Room and at the front s tandt.rd.
Although the turbine was tripped it indicated that it was still on the line as #2 stop valve ligh*
indicated " intermediate" and RPM was holding at 1800. A furthet look showed the load coming off the machine indicating that it had to be disconnected.
1441:27 Breakers 379T and 381T vere opened manually --RPM and frequency
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began to decrease immediately.
1442:05 RPS MG sets tripped on under frequency (54Hz) at sl620 rpm.
Recirc MG sets tripped. Voltage / Hertz relay failed to operate.
1442:07 MSIV's closed as did all Group I through V isolation valves.
1442:19 FW pump tripped on high reactor water level, 49".
1443:22 Breaker 13 was closed paralleling the start-up transformer with the Auxiliary Transformer, the latter being at N36Hz 1443:22 Auxiliary Transformer High Pressure Fault.
Generator Backup Relay tripped.
1443 (From
" Word" passed on public address system " Fire in Transformer" Security Log) 1443:22 Breakers 12 & 22 opened auto. Generator field breaker opened.
1443:26 Breaker 4T2 auto opened 1443:26 Breaker 23 closed manually f;
1443:31 Diesel generator "A" started " auto" and picked up bus 4.
(All times from this point are approximate, based on log entries, recorder charts and computer print-outs.)
1444 Diesel generator "B" started manually and paralled to bus 3.
y 3T1 breaker opened manually.
1444 Brattleboro and Vernon Fire Departments called.
Dry powder extinguishers were used to er.tinguish fire prinr to j
arrival of fire. departments.
Hoses were rigged and the turbine building inside wall, adjacent to the transformer, was sprayed t
for cooling.
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Time Event N1449 All vent fans secured, and turbine roof vents shut.
1453 First town fire units arrived 1455 Five fire units on site 1500 control Rod Drive System secured to prevent reactor. water level from increasing further (%65" at this time).
1501 Reactor Cleanup System which had isolated was restarted and draining of reactor water started.
1507 Seven fire units on site.
1522 Normal station service was restored and diesels A and B were secured.
I 1527 Turbine turning gear was engaged.
1530 The fire was declared officially out by fire department personnel.
I 1532 Manual initiacion of relief valves to control reactor pressure
(%1050 psi) and level (>50")
1534 RCIC on line pumping CST to CST to reduce vessel level.
1555 MSIV's were opened after equalizing upstream and downstream headers. These had isolated initially as a result of the loss of the protection MG sets and remained shut due to high vessel level.
1600 SJAE's restarted to continue a normal reactor cooldown.
Reactor Recifculation pumps were not restarted to prevent excessive thermal stress due to differential temperatures.
i 1639 MSIV's were reopened after equalizing upstream and downstream headers. RCIC and SJAE's were restarted and a normal cool-down continued.
1648 MSIV's again isolated due to a transient high vessel level caused by operator partially opening one bypass valve to continue the vessel cooldown. At this time further attempts to continue a normal cooldown using the' main condenser were terminated.
1657 Torus cooling was established to prepare for a normal cooldown without the main condenser.
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HPCI was manually started and operated on minimum flow.
A normal reactor cooldown without the main condenser continued i
at %70*F/hr.
1 2320 Shutdown cooling was placed into normal service.
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