ML18139C090
| ML18139C090 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 10/06/1982 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML18139C089 | List: |
| References | |
| NUDOCS 8211010582 | |
| Download: ML18139C090 (9) | |
Text
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. ENCLOSURE 1 e
SAFETY EVALUATION SURRY POWER STATION UNITS l AND 2*
DOCKET NOS. 50-280/281 ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES INTRODUCTION AND
SUMMARY
Virginia Electric and Power Company (VEPCO) was requested by NRC letter dated August 8, 1979 to reyiew the electric power system at Surry Power Station Units l and 2.
The review was to consist of:
a)
Determining analytically the capacity and capability of the offsite
~ power system and onsite distribution system to automatically start as well as operate all required loads within their required voltage ratings in the event of 1) an anticipated transient, or
- 2) an accident (such as LOCA) without manual shedding of any electric loads.
b)
Determining if there are any events or conditions which could result in the simultaneous or, consequential loss of both required circuits from the offsite network to the onsite electric distribution _system and thus violating the requirement~ of GDC 17.
The August 8, *1979 letter included staff guidelines for performing the required voltage analysis and the licensee was further required to perform a test in order to verify the validity of the analytical results.
VEPCO_ res ponded. by J e.tters _date_d May 26, 1981, December 31, 1981, March.31, 1982, June 11, 1982 ~nd June 30, 1982. A detailed review and technical evaluation of the submittals was performed by LLL under contract to the 8211010582 821006 - ~~ -
PDR ADOCK 05000280 P
. ' e NRC, with general supervision by NRC staff. Th1s work is feported by LLL in Technical Evaluation Report (TER),
11Adequacy of Station Electric Distribution System Voltages, Surry Power Station Units 1 and 2, 11 dated September 1, 1982 (attached).
We have revie'11ed fhis report and concur in the conclusions that the offsite power system and the onsite 4
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di.stri but ion system areicapabl e of providing acceptable.voltages for worst case station electric load and grid voltages.
EVALUATION CRITERIA The criteria used by LLL in this technical evaluation of the analysis focJudes GDC 5 (
11Sharing of Structures, Systems, and Components"),
of Appendix A to 10 CFR 50; IEEE Standard 308-1974 ("Class 1E Power Systems for Nuclear Power Generati.ng Stations 11-), ANSI C84.l-1977
("Voltage Ratings for Electric Power Systems and Equipment Hz 11 ),
and the staff positions and guidelines in* N~C letter to VEPCO dated August 8, 1979.
ANALYSIS AND TEST FEATURES VEPCO analyzed each offsite po.wer ?ource to the onsite power system under extremes of load with the 500 kV grid at its maximum (535 kV) and minimum (505 kV) voltage.
To insure that the worst case conditions were analyzed the followi.ng assumptions were*u*sed.
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- 1. The maximum switchyard vo1tage drop caused by either one or both units tripping is 15 kV.
The automatic load tap changer (LTC) position was determined by the loading on the reserve station service transformers (RSSTs) prior to th~ 1oading ~ondit~ori ana1yzed and with the switchyard vo1tage at 520 kV.
At the instant the condition 4..
being analyzed occurs, the switchyard voltage drops jnstantaneously to 505 kV and the loading for the analyzed condition occurs.. Voltages calculated at this time are based on LTC tap position prior to the occurrence.
Fina1 vo1tages are based on the LTC correcting for the loading with the grid voltage sti11 at 505 kV.
- 2.
The transfer of the station service buses (non-C1ass lE) to the reserve station service system occurs immediate1y upon occurrence of the*
conditio*n being ana1yzed.
- 3. All motors that start upon receipt of a safety injection signal (SI) or consequence limiting signal (CLS) were assumed not to be running prior to the condition analyzed.
All starting motors were assumed to have a 0.0 power factor.
- 4.
Worst case loading ~f the station service buses was assumed for the transfers.
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- 5.
No manual load shedding or reduction in motor current due to decreased pum~ loads was assumed to occur for 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> after the event analyzed.
- 6.
Sequential loading occurs as designed.. f~e-:-sequenced loads are the auxiliary sfeam generator feedwater pump (50 seconds af~er SI), inside 4
recirculation~spray pump ( 120 seconds after SI), and the outside recirculation spray pump (300 seconds after SI).
- 7.
The existing automatic load-shedding scheme on each individual RSST.
occurs as designed.
- 8.
Ampere values used were based on measured plant values.
In cases where measured values were not known they were estimated.
- 9.
All anticipated modifications and operat~ng restrictions were included in the analysis except for replacement of the 4 kV underground cables from the RSST to the transfer buses.
Usi_ng the above assumptions the analysis determined that the worst case Class lE equipment volt.ages occur under the following conditions:
- l. Maximum voltage occurs when the grid is at its maximum anticipated
_ volt.age of 535. kV, Unit.1 is at. l 00% power and Unit 2_ is in _t~e.
refueling mode.
The Unit l 1oadi_ng on the RSST 1s consi§t of Class 1E
-bus loads only and Unit 2 1oad~ng on the RSSTs consist of normal
operating Class lE bus loads and the 480 volt loads normally
-supplied from the station service buses.
- 2.
Mjnimum voltage occurs v1hen the offsite grid is at its minimum anticipated voltage of 505 kV, loss of one 500/230/36.5 kV 4
autotransformer and a:utomatic transfer of the affected 34.5 kV bus to the 230/36.5 kV transformer.
In addition Unit 2 then experiences a CLS with a simultaneous Unit 1 trip and transfer of its station loads to the RSSTs.
All Class lE motors required for a CLS are assumed to start at time= 0.
The voltage analysis shows that under worst case, minimum grid voltage, maximum load condition, the voltage to some 460 volt Class lE motor operated val.ves (MOVs) did not meet the minimum ~ated starting voltage*
at time= 0.
The analysis assumed that all MOVs have an 80% of 460 volt starting rati_ng.
The existi_ng 460 volt MOVs will start at 90% and 440
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volt MOVs at 85% volt~ge.
The licensee is currently analyzing manufacturers data with the inte~t of rerating MOVs that would not have the required voltage under the worst ~ase conditions.
VEPCO has committed to replace any MOV *that cannot be rerated to meet the minimum voltage analyzed.
Under the worst case analyzed condition the MOVs could be exposed to a voltage of approxi~ately 2.5% below the 80% rated starting voltage.
This
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lower volt.age will *only be pre"se~t' duri_ng*the.block load starting of...
4000 volt motors.
As the motors accelerate the voltage to the 460 vo1t MOVs will recover to an acceptable level within a f~w seconds.
Based upoQ the fact that a highly conservative computer model was used in the analysis; with verification fest results that show an average of
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approximately 5% conservatism for the 480 volt buses; and the extremely low probability of all events required to produce the worst case conditions analyzed occurring at the same time there is reasonable assurance that acceptable voltage will be provided to insure ~atisfactory operation of the 460 volt MOVs.
The voltage analysis was verified by taking voltage measurements in.the switchyard and voltage and bu~ loading measurements on the Class lE buses during a Unit 1 startup.
The bus loading on the 4160 volt and 480 volt Class lE buses ranged from 11% to 65% of the maximum bus load.
A comp.a ri son of the measured to ca lcu1 at ed.. voJ tages show the computer model to be highly conservative.
The percentage errors ranged from 3.35% to 7.18% on the 4160 volt buses and from 4.29% to 6.26% on the 480 volt buses with a11 measured values befog higher than thos-e used in the computer model for the analysis.
DES I GN CHANGES As a result of the voltage analysis VEPCO has proposed the following design changes and operati_ng restrictions:
- 1. The existing load shedding scheme which occurs automatically when two units are placed on the reserve station service transformers (RSSTs) wil-1 be enabled" w*hen one *unit is on *1 ine and second unit i_s in startup, both u~its are on line, and when both units are in startup.
e 2.
Transmission system (grid) will. be operated within limits prescribed by established voltage schedules.
- 3.
Disconnects on both sides of the 34. 5 kV bus t ie*.:-br.ea ker between the two 34.5 kV reserve station service buses will *be normally open to prevent the loss of ~oth offsite power sources which could occur if the above tie breaker should fail.
- 4. ~erate those MOVs which the analysis determined to have less than design rated starting voltage. If manufacturers data shows that motor operators or certain MOVs cannot be rerated they will be repla~ed with MOVs rated for 80% of 460 volts for starting.
- 5.
Automatic disconnection of the.four 34.5 kV reactor banks following an SI or CLS on either unit when the station service buses are transferred to the RSSTs.
- 6.
Modify the automatic load tap cha_ngers on the RSSTs to provide:
a.-
Instantaneous voltage correction for the first 3 minutes upon receipt o.-f an SI or CLS on either unit.
- b.
Instantaneoui voltage correction for the ~irst minute when a
-unit transfers to the.RSSTs duri.~g an SI or CLS.
- 7.
Block automatic starti_ng of the condensate, ~igh pressure.heater ~rain,
. bearing cooli_ng and component cooli~g water pumps for approximately 315 seconds after an SI or CLS occurs.
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- 8.
Replace and reroute the underground 4 kV cables fr*om the RSSTs to the transfer buses.
- 9.
Add two radiators with fans to each RSST to ~ncrease their 55°C temperature rise r?ting from 24 MVA to 10 MVA.
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-10.
Replace MOV 480/120 volt control transformers with larger size units.
- 11.
Change the 4160/480 volt transformer taps to 4056/480 volt tap.
1~.
Reroute the RSST control cables to meet separation requirements for fire protection.
13, Install overload alarms of BS MVA on th~ 500/230/36.5 kV transforme~
and 95 MVA on the 230/36/5 kV transformer.
CONCLUSIONS We have reviewed the LLL technical evaluation report and concur in its findings that:
- 1.
VEPCO has provided verified voltage analysis to d;monstrate that upon completion of the proposed design changes and with adherence to system voltage schedules, there is reasonable assurance that acceptable voltage will be provided to all Class lE equipment under the worst case conditions analyzed.
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The tests used to verify the analysis were valid and showed the analysis to be conservative.
- 3.
The offsite power sources shared between Units l and 2 has the capacity to supply adequite voltage for an accident on one unit 4
and a safe shutdown of the second unit.
- 4.
The proposed modifications will insure Surry's compliance with GO-C 17 requirements.
- 5.
The licensee is currently collecting manufacturers data and completing studies which will result in either the rerating or replacement of some 460 volt.MOV's that are not provided with satisfactory starting capabil itie$ under the worst *case conditions analyzed.
- We request that the licensee provide the results of the completed studies to NRC for review.
In addition, if these studies show that replacement is required for any MOV's, we request that a schedule for these replacements be also provided.