ML19261B973
| ML19261B973 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 03/02/1979 |
| From: | Moody D Maine Yankee |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| WMY-79-14, NUDOCS 7903090225 | |
| Download: ML19261B973 (6) | |
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ENGINEERING OFFICE W ESTBORO. MASS ACHUSETTS 01581 617 4 6-9011
- 1. "W- =. O y37 79_14 March 2, 1979 United States Nuclear Regulatory Cocnission Washinbton, D. C.
20555 Attention:
Office of Nuclear Reactor Regulation
Reference:
License No. DPR-36 (Docket No. 50-309)
Dear Sir:
As requested in your letter of February 5,1979 we are providing you with information in connection with the failure of one of Maine Yankee's half size main transforcers.
1.
Failure Description The insulation failed on a jumper from an inner high voltage winding to an outer high voltage winding. The cause of the insulation failure is unknown and because of the damage caused by the arc it may never be known. An arc was established among the outer high voltage winding, the jumper, and the core clamp.
Readings taken on the transfor=er af ter the fault showed the high voltage winding H1 open and the low voltage winding grounded. Oil samples revealed that the oil had turned black and conr ~ 1ned cocbustible gas. The gas space above the oil contained combustible gas in concentrations above the lower cosbustible limit.
The transfor=er has been untanked and subsequent in pections have revealed no apparent cause for the failure. Copper beads from the burned of f ju=per were observed throughout the transforcer core and windings.
2.
Sequence of Events The following sequence of events is based on the su==ary of events by the Plant Shif t Superintendent, and on the Plaat Shift Superintendent's log book. The computer was not fully operational during the transforcer failure as it was restarting; therefore, only a few computer generated sequence of events were available.
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U.S. Regulatory Commission March 2, 1979 Attn: Of fice of Nuclear Reactor Regulation Page 2 Date:
8-31-78, Time: 0257 hours0.00297 days <br />0.0714 hours <br />4.249339e-4 weeks <br />9.77885e-5 months <br /> Transformer fault Transformer dif ferential lock out; Relay 86L/P, annunciated at window HV l-3 Primary lockout relay 86P; Computer data log Generator excitation breaker open and locked out; KG1 and KGl/375 Turbine trip; 20AST, 20ET Reactor Trip Unit Breakers open and locked out; 1U, 2U, 3U, & 4U Reserve breakers fail to close; 1R, 2R, 3R, & 42 Bus Undervoltage; 27-5, 27-6 Emergency Diesel generator fast start and load; DGlA, DGlB Generator output motor operated disconnect open and locked out; 89/TlH Time: 0258 hours0.00299 days <br />0.0717 hours <br />4.265873e-4 weeks <br />9.8169e-5 months <br /> Operator Action; close breakers 1R, 2R, 3R, & 4R Shutdown E=ergency Diesel Generator; DGlA & DGlB Perform Emergency Procedure EP-2-1 3.
Evaluation (a) Power Systems Interconnected Power Systems are not rigid systems; they are elastic systems which can bend and flex to accommodate disturbances without disintegrating. Disturb;
.es can strike suddenly - or build slowly; they can be major-or minc. The ability of a system to maintain steady-state operation following a disturbance is dependant on the nature and magnitude of the disturbance and the structure of the system; however, all systems will distort to some extent when subjected i.o disturbances. Major disturbances will usually produce momentary local stresses.
(b) 115 kV System The 345 kV and 115 kV Systems that are interconnected with Maine Yankee are shown in rigure 1.
The 115 kV System provides Station startup and standby power through two transformers, X14 and X16. These transformers provide reserve power for the 4.16 kV and the 6.9 kV buses.
A 4.16 kV tertiary winding on the X16 transformer is connected to one of the emergency 4.16 kV buses, thus ensuring that either of the reserve transformers will be able to supply power for a safe shutdewn of the pla nt.
U.S. Regulatory Cornission March 2, 19 79 Atta: Of fice of Nuclear Reactor Regulation Page 3 The 115 kV source for the reserve transforcers is the 115 kV switchyard.
Two 115 kV transsission lines enter the switchyard, one from the Mason Station and one from the Suroviec Substation. Both stations are ettensively interconnected with 115 kV and 345 kV trans=ission network in the New England area. The Mason Station, only 3 =iles from Maine Yankee is in a northeasterly direction, while the Suroviec Substation is located 25 =iles froc Maine Yankee in a southwesterly direction.
Either of the two 115 kV incoming lines are independently capabic of supplying the plant auxiliary power require:ents. The tie breaker shown in between the two lines is r.ornally closed.
(c) Transfer of Scheres - Aute:atic and Manual Autocatic transfer of equip =ent from a nor=al source to an alternate source =ust be accc=plished very fast, or, af ter a specific ti=e delay.
Transfers can be extrecely dangerous in the inter =ediate period.
In order to enhance safety and to opticize equip ent availability, Maine Yankee has been designed to automatically transfer power fro =
the unit station service transforcers to the reserve station service transfor=ers usirg the fast dead bus transfer cethod. Transfer ti=es with this sche:e are in the order of 6 cycles (0.1 second) and result in no transients or interruptions in service.
For a fast dead bus transfer to be successful the voltage, frequency, and phase angle between the bus and the reserve station service transfor:er cust be within tolerable litits. If a transfer takes place when these para:eters are outside the desirable litits; serious transforcer, switchgear, and equip:ent da= age can result.
For this reason high speed instantaneous voltage relays cone.tantly onitor the reserve station service transforcer voltage and on detecting no voltage, or a voltage below the li=its established for successful transfer, the auto =atic fast transfer is blocked.
Backing up the auto =atic iast transfer are two canual transfer sche es.
The first cethod consists of =anually synchronizing the two sources and then closing the incocing scurce breaker; the nor=al breaker trips auto =atically af ter a shart tire delay. The second rethod assu=es the nor=al source is lost and permits canual closure of the alternate source breaker. Blocking of the auto atic f ast transf er does not inhibit the =anual tracsfe rs.
(d) Occurence At 0257 hours0.00297 days <br />0.0714 hours <br />4.249339e-4 weeks <br />9.77885e-5 months <br /> on S/31/78 Maine Yankee was opecating with a net output of 385MW (47%).
Auxiliary power for the s tation was being supplied I
1
U.S. Regulatory Commission March 2,1979 Attn: Office of Nuclear Reactor Regulation Page 4 by the unit stacion service transformers. The transmission system was under relatively light load conditions, and no other generators were operating in the immediate vicinity of Maine Yankee.
It is postulated that when the transformer fault occured, the voltage got depressed on both the 345 kV and the 115 kV Systems. The voltage relay in the fast transfer circuit, being instantaneous, sensed the low voltage cc adiri in and blocked the fast transfer. Had the transfer not been block '.
te two reserve station service transformers, the 6.9 kV switchgs
'he 4.16 kV switchgear, and the connected loads on these buses ma, *. ave all been seriously damaged. On isolating the fault the sys tem voltage recovered to its normal pre-fault condition almost immediately.
The loss of offsite power was sensed by the two 4.16 kV emergency buses.
Both emergency diesel generators started and energized the buses.
At 0258 the operator observed that of fsite power was still available and proceeded to connect this source to the buses.
(e) Conclusion 1.
100% load rejection has been experienced in the past under similar aystem light load conditions with no loss of fast transfer capability; therefore, the conclusion is that the massive transformer fault resulted in the voltage transient which blocked the fast transfer.
2.
Offsite power was degraded momentarily for the duration of the fault. Voltage recovery was almost instanteneous on isolation of the f; ult.
3.
Of fsite power was not lost. Power from the 115 kV switchyard was always available to assure that specified acceptable fuel design limits and design condition of the reactor coolant pressure boundary were not exceeded.
4.
The automatic fast transfer scheme performed as designed in that it prevented a potentially dangerous situation from occuring. Once the danger had passed, of fsite power was restored within minutes to all station buses.
U.S. Regulatory Commission March 2, 1979 Atta: Office of Nuclear Reactor Regulation Page 5 5.
The occurrence demonstrated the ledependence and lack of interaction between the preferred of fsite power supply and the onsite standby power supply under extremely adverse conditions.
We trust that you will find this information satisfactory; however, should you desire additional information, please contact us.
Very truly yours, MAINE YANKEE ATOMIC POWER COMFAhi
,, CCC " \\
.r,
-r D. E. Moody Manager of Operations
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