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TENNESSEE VALLEY AUTHORITYCHATTANOCGA TENNESSEE 374o1400 Chestnut Street Tower II

December 1, 1982

Director of Nuclear Reactor RegulationAttention: Ms. E. Adensam, Cnief

Licensing Branch No. 4Division of Licensing

U.S. Nuclear Regulatory Commission'

Washington, D.C. 20555

Dear Ms. Adensam:

In the Matter of ) Docket Nos. 50-327Tennessee Valley Authority ) 50-328

As requested by Melanie Miller of your staff in a telephone conversation onNovember 29, 1982, Enclosure 1 provides TVA's response to NRC verbalquestions concerning our proposed technical specification change related todegraded voltage relaying. Our proposed technical specification change wassubmitted by letter from me to H. R. Denton on September 17, 1982. Also asrequested, Enclosure 2 provides revised pages to the attachment to thejustification provided in our September 17, 1982 submittal.

If you have any questions concerning this matter, please get in touch withJerry Wills at FTS 858-2683

Very truly yours,

TENNESSEE VALLEY AUTHORITY

h,L. M. Mills, $anagerNuclear Licensing

Sworn to gnd subscr4)''ed before methig /8L day of /LL' 6'982

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My C ission Expires //)'//Notar Public

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Enclosurecc: U.S. Nuclear Regulatory Commission (Enclosure)

Region II fAttn: Mr. James P. O'Reilly Administrator RO}l101 Marietta Street, Suite 3100 vAtlanta, Georgia 30303

8212070176 821201PDR ADOCK 05000327P PDR

An Eaual Oppor turuty Employer._ _ , _c_

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ENCLOSURE 1' * ,. .

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SEQUOYNI ?UCLEAR PLA!TP - RESMNSE 'IO IRCs VERBAL CUESTIONS (DNCERNI?UDEGRADED VOLTAGE REIAYI!G

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This is in response to ?RCs verbal questions in a telecon on October 22,and November 29, 1932, concerning TVA's submittal on degraded voltage relaying -Supplementary Technical Information. Mr. Dick Privatt,?RC, asked for thefollowing information: (1) Give a synopsis of our analysis to show thatadequate voltages are provided under all plant conditions and how ourvoltage setpoints were selected, (2) clarification that no spurious tripcould occur as a result of relay drift on the degraded voltage relaying,and (3) clarification of considerations in calculating worst caseovervoltage conditions and the proposed steps to be taken to handle thissituatics

We have the following comments:

The de' graded voltage relay setpoints were initially selected in1.accordance with the range B minimuin and maximum service voltages(6560V and 7260V, respectively) described in ANSI C84-1-1970 andANSI C84.la - 1973. After selection of these tentative setpoints,we used an in-house developed computer program to calculate theallowable minimum 161-kV grid voltages for each of the CSST tappositions that would allow at least 6560V plus relay tolerance atthe 6.9-kV shutdown boards during both a design basis accident withthe other unit in full load rejection and for a two unit full loadrejection. The upper limit is based on the excitation limit of thetransformer. The results of that analysis is listed below:

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TARTP 1

ArrtWARTR 161-KV GRID RANr:M FOR EACH CSST VOLTAGE TAP IOSITION *,

A. If all three CSSTs are available

CSST Voltage Tap Minime Grid Voltage Maxin e Grid Voltace+2.5% 167-kV 382-kVRATED 163-kV 177-kV

. -2.5% 159-kV 173-kV| -5% 155-kV 168-kV| -7.5% 151-kV 164-kVl1 B. If one CSST is out-of-service

The minimum 161-kV grid voltage for each CSST voltage tap must beincrcased by 2 kV.

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Due to the addition of CSSP-C at Sequoyah, the start board andtransformer configuration will be different when the degraded voltagerelaying scheme is installed than was shown in Figures D and E of thesupplementary technical information package. See Figures D1 and D2for the new configuration and calculated voltages. The present long-term161-kV grid schedule at Sequoyah Nuclear Plant call for operation at 168 kVi 1 kV. The proposed CSST voltage tap is the -2.5 percent position.

For the unit connected case, the USST voltage tap was selected to ensure atleast 6560V at the 6.9-kV shutdown boards during normal operation with themain generator operated at its minimum voltage of 22.8-kV. Also, a designbasis accident was postulated with this connection because the generator isnot tripped in the switchyard nor is the 6.9-kV unit boards transferred for30 seconds following a turbine trip and reactor scram if not caused by aswitchgear or generator fault. During the 30 seconds, all of the safetyinjection actuated loads are started and the generator behaves as asynchronous motor holding the voltage constant (approximately the same asbefore the trip).

After it was shown that 6560V is compatible with the 161-kV grid operatingrange and main generator operating ranges with available station servicetransformer voltage taps, the 480V safety-related APS was examined. Inthis analysis, the voltage at the terminals of each 460V safety motor wascalculated under the design basis accident condition (safety injection withcontainment isolation). This presents the largest load demand on the 480VClass lE APS. In this analysis the 6900 to 480V shutdown boardtransformer voltage taps were chosen as to ensure, adequate voltages at themotors during the times they would be required to operate. The allowablerange of operation for the 480V boards are 440-504V. The 6900V shutdownboards were assumed to be at a transient voltage of 6160V for up to 4seconds follcwing a DBA and 6560V steady-state afterwards. The transientvoltage of 6160V was determined from preop test W6.1F " Integratedemergency safeguards activation test."

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Item 2. As shown on Figure B of the supplementary technical informationpackage, the 6.9-kV shutdown board lowest calculated voltage is 6580V,

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when the main generator is operated at its minimum voltage of 22.8-kV.The degraded voltage relays are set at 6560V i 33 volts. If the relaydrifts to its upper limits it is possible to have a spurious operationof these relays. TVA finds this slight overlap acceptable duringnormal unit operation because of the following:

Should a spurious operation occur during this mode of operation,a.there will be a main control room annunciation giving the operator5 minutes in which to raise the voltage, such as slightly|

i increasing the generator voltage. Also, the diesel generators willnot start until the lapse of the 5 minutes.

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b. Our calculations are on the conservative side. For example:(1) for voltage calculations, the cable impedances used are assumedto be at the. industries maximum tolerance, and (2) 480V boardleading assumed is generally 15 to 25 percent higher than actualoperating data at the plant indicates. -

Item 3. To determine the amount of overvoltage the 6.9-kV shutdown boards willhave, two cases have been examined. First, for normal operation withthe main generator operated at its maximum of 25.2-kV the voltages onthe 6.9-kV shutdown boards were calculated to be approximately 7350V.Secondly, we examined the 6.9-kV shutdown voltageof one unit' when it is in cold shutdown with the 161-kV grid at itsupper limit of 173 kV when the CSST voltage tap is at the -2.5 percentposition. The voltages on the 6.9-kV shutdown boards would beapproximately 7500 volts. Corrective action will be taken to correct

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the voltage before equipment damage occurs.

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6.9-XV SHUTDOWN BOARDS (KV)

161-kV CSST voltage 1A-A IB-B 2A-A 28-8GA.ID-kV tao FLR/DBA FLR/DBA FLR/DBA FLR/DBA

167 +2.5% 6651/6688 6674/6700 6580/6584 6605/6608163 RATED 6655/6691 6677/6703 6583/6587 6608/6611159 -2.5% 6658/6694 6681/6707 6587/6531 6612/6615155 -5% 6662/6698 6684/6710 6590/6594 6615/6618

i 151 -7.5% 6666/6702 6688/6714 6594/6598 6619/6622

182 +2.5% 7309/7341 7329/7352 7238/7242 7262/7265177 RATED 7284/7316 7304/7327 7213/7217 7237/7240173 -2.5% 7303/7335 7323/7346 7233/7236 7257/7259168 -5% 7277/7309 7296/7320 7206/7210 7230/7233164 -7.5% 7297/7329 7317/7340 7227/7230 7251/7253

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In response to NRC's verbal questions concerning (1) actions to be taken,

to correct high voltage on the 6,900-volt shutdown boards and (2) theI

manner in which the expected system grid voltage levels are coordinated

with transformer tap settings, we have the following comments. .

1. The 6,900-volt shutdown boards are fed from the 6,900-volt unit

boards in all instances except emergency power. If an overvoltage

condition exists on the 6,900-volt shutdown boards during normal

operation, the generator outgoing reactive will be reduced to lower

the voltage to within acceptabic limits. If an overvoltage condition

exists on the 6,900-volt shutdown. boards while being supplied from

the offsite power system with the unit in a shutdown mode, the

electrical operator must inform the chief load dispatcher of an

overvoltage condition on the 161-kV system. The power system load

dispatcher will then take corrective acti'on to lower the 161-kV .

system voltage to within the upper limits as set forth in the ,

system grid voltage schedule. If system conditions do not allow4

( correction of the overvoltage condition, the operator can start'

auxiliary equipment to reduce the voltage or transfer the safety-

related equipment to the onsite power system until the offsite

power system overvoltage condition is corrected.

2. The Division of Power System Operations (PS0) establishes system

grid voltage schedules and coordinates the required common station-

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service tran_sfprmer, unit station-service transformer, and generator

step-up transformer tap positions as required. The voltage schedules

and required tap positions are transmitted to all affected divisions.

The required tap positions are incorporated into plant operating

procedures while the voltage schedules are incorporated into the

power system load dispatcher procedures. Any variance from the

established system grid voltage schedules and/or transformer tap

positions must be coordinated with PSO and all affected divisions.A

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INCLOSURE 2

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REVISED PAGES 1D 1EE ATIACHMENT ID THEJUSTIFICATION FOR INE PROPOSED IECHNICAL SPECIFICATION CHANGE

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to initiate three time delay sequences (see Figure A-I) . The firstsequence of 30 seconds will ride through normal system voltagetransients (motor ~ starts - both safety and nonsafety related) beforeannunciating the undervoltage in the main control room. The second

; sequence is short enough to allow safety-related equipment to bei powered within the time required by the safety analysis. At the end

of 10 seconds if an SIS has been initiated, or is subsequentlyinitiated, the shutdown board degraded voltage relays will initiateload shedding and subsequently transfer the shutdown board to itsdiesel generator. The return of bus voltage initiates load sequencingof sat ~ety-related equipment. The third time delay is long enough to ,allow operator action but not result in damage to connected safety- *related equipment. At the end of five minutes, the shutdown board

will initiate load shodding and subsequently transfer the shutdownboard to its diesel generator if degraded voltage has not beencorrected. The error associated with these voltage sensors is 10.5percent. The error associated with the timers is 110 percent f ortimers with settings greater than or equal to 200 seconds and i 5percent for timers with settings less than 200 seconds.

To protect the Class IE buses from a sustained over-voltage, each ofthe two 6.9-kV Class IE buses per unit will be provided with a set ofthree instantaneous solid-state overvol tage relays (ITE-type $911) .These relays will be arranged in a one-out-of-three coincidence logicwhich will annunciate in the control room. The relays will have a

nominal voltage setpoint of 7260 volts i 1 percent (105 percent ofnominal). The operator will take the action necessary to reduce thevol ta ge .

Load shedding for a los's of bus voltage (< 70 percent) is beingmaintained once the diesel generators are supplying their respectivebuses. Degraded voltage relaying will not open the standby supplybreaker and will not initiate load shedding and resequencing if a 6900-volt shutdown board is supplied by its diesel generator. The outputof these relays is blocked when the standby breaker is closed. TVA' sbases for this is discussed in section 3.3.2.

Proposed changes to the plant's technical specifications,. adding thesurveillance requirements, allowable limits for the setpoint and timedelay, and limiting conditions f or operation f or the second Icvelundervoltage monitors are furnished in appendix B. An analysis tosubstantiate the limiting conditions and minimum and maximum setpointlimits is furnished in appendix A.

3.3 Discussion

3.3.1 NRC staff position 1 requires that a second level of under-voltage protection f or the onsite power system be provided.The position stipulates other criteria that the undervoltageprotection must meet. Each criterion is restated belowfollowed by a discussion regarding TVA's compliance with thatcriterion.

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Aeoendly C to the_ Attachment

shutdown board will transf er to its diesel if voltage has not been

corrected. Since.the loss of voltage relays on nor=n1 feeder only areset at 80 percent of nominal for four seconds, the band of voltagesthat nonaccident degraded voltage condition can exist is f rom 80 to 95percent of nominal for five minutes. At 80 percent of nominal thevoltage at the tenninals of running motors will not drop below 71percent of motor rated voltage. NEMA Class B motors will not stallout or be damaged above this point for the time delay of five minutes.Also, during the five-minute time delay the 125V de vital batteryboards could be powered by the batteries instead of the batterychargers. H ow ev e r, the vital batteries have sufficient capacity tomeet this requirement, as well as meet the original design require-ments as identified in section 8.3.2 of the Sequoyah FSAR.

For a loss of voltage, both the selected time delays allow for theloss-of-voltage relays to initiate transf er to the alternate supply,if it is greater than 95 percent of nominal, before tripping andtransf erring to the diesels.

An error of f 0.5 percent f or the voltage sensors in the degradedvoltage protection circuits has been considered in the design. Theerror associated with the timers used in the degraded voltage design

is 110 percent for timers with settings greater than or equal to200 seconds and f 5 percent for timers with settings less than 200se c on d s .

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