ML20028B900
| ML20028B900 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 12/01/1982 |
| From: | Mills L TENNESSEE VALLEY AUTHORITY |
| To: | Adensam E Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8212070176 | |
| Download: ML20028B900 (11) | |
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a TENNESSEE VALLEY AUTHORITY CHATTANOCGA TENNESSEE 374o1 400 Chestnut Street Tower II December 1, 1982 Director of Nuclear Reactor Regulation Attention:
Ms. E. Adensam, Cnief Licensing Branch No. 4 Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Ms. Adensam:
In the Matter of
)
Docket Nos. 50-327 Tennessee Valley Authority
)
50-328 As requested by Melanie Miller of your staff in a telephone conversation on November 29, 1982, Enclosure 1 provides TVA's response to NRC verbal questions concerning our proposed technical specification change related to degraded voltage relaying. Our proposed technical specification change was submitted by letter from me to H. R. Denton on September 17, 1982. Also as requested, Enclosure 2 provides revised pages to the attachment to the justification provided in our September 17, 1982 submittal.
If you have any questions concerning this matter, please get in touch with Jerry Wills at FTS 858-2683 Very truly yours, TENNESSEE VALLEY AUTHORITY h
L. M. Mills, $anager Nuclear Licensing Sworn to gnd subscr4)ed before me thig /8L day of /LL' 6'982 l
AltL,1f lla, LLLLg,fy My C ission Expires //)
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Notar Public Enclosure cc:
U.S. Nuclear Regulatory Commission (Enclosure)
Region II f
Attn:
Mr. James P. O'Reilly Administrator RO}l 101 Marietta Street, Suite 3100 v
Atlanta, Georgia 30303 8212070176 821201 PDR ADOCK 05000327 P
PDR An Eaual Oppor turuty Employer
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ENCLOSURE 1 SEQUOYNI ?UCLEAR PLA!TP - RESMNSE 'IO IRCs VERBAL CUESTIONS (DNCERNI?U DEGRADED VOLTAGE REIAYI!G 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 the following information:
(1) Give a synopsis of our analysis to show that adequate voltages are provided under all plant conditions and how our voltage setpoints were selected, (2) clarification that no spurious trip could occur as a result of relay drift on the degraded voltage relaying, and (3) clarification of considerations in calculating worst case overvoltage conditions and the proposed steps to be taken to handle this situatics We have the following comments:
The de' graded voltage relay setpoints were initially selected in 1.
accordance with the range B minimuin and maximum service voltages (6560V and 7260V, respectively) described in ANSI C84-1-1970 and ANSI C84.la - 1973. After selection of these tentative setpoints, we used an in-house developed computer program to calculate the allowable minimum 161-kV grid voltages for each of the CSST tap positions that would allow at least 6560V plus relay tolerance at the 6.9-kV shutdown boards during both a design basis accident with the other unit in full load rejection and for a two unit full load rejection. The upper limit is based on the excitation limit of the transformer. The results of that analysis is listed below:
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-kV RATED 163-kV 177-kV
-2.5%
159-kV 173-kV l
-5%
155-kV 168-kV l
-7.5%
151-kV 164-kV l
1 B.
If one CSST is out-of-service The minimum 161-kV grid voltage for each CSST voltage tap must be incrcased by 2 kV.
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Due to the addition of CSSP-C at Sequoyah, the start board and transformer configuration will be different when the degraded voltage relaying scheme is installed than was shown in Figures D and E of the supplementary technical information package. See Figures D1 and D2 for the new configuration and calculated voltages. The present long-term 161-kV grid schedule at Sequoyah Nuclear Plant call for operation at 168 kV i 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 at least 6560V at the 6.9-kV shutdown boards during normal operation with the main generator operated at its minimum voltage of 22.8-kV. Also, a design basis accident was postulated with this connection because the generator is not tripped in the switchyard nor is the 6.9-kV unit boards transferred for 30 seconds following a turbine trip and reactor scram if not caused by a switchgear or generator fault. During the 30 seconds, all of the safety injection actuated loads are started and the generator behaves as a synchronous motor holding the voltage constant (approximately the same as before the trip).
After it was shown that 6560V is compatible with the 161-kV grid operating range and main generator operating ranges with available station service transformer voltage taps, the 480V safety-related APS was examined. In this analysis, the voltage at the terminals of each 460V safety motor was calculated under the design basis accident condition (safety injection with containment isolation). This presents the largest load demand on the 480V Class lE APS. In this analysis the 6900 to 480V shutdown board transformer voltage taps were chosen as to ensure, adequate voltages at the motors during the times they would be required to operate. The allowable range of operation for the 480V boards are 440-504V. The 6900V shutdown boards were assumed to be at a transient voltage of 6160V for up to 4 seconds follcwing a DBA and 6560V steady-state afterwards. The transient voltage of 6160V was determined from preop test W6.1F " Integrated emergency safeguards activation test."
i Item 2.
As shown on Figure B of the supplementary technical information package, the 6.9-kV shutdown board lowest calculated voltage is 6580V 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 relay drifts to its upper limits it is possible to have a spurious operation of these relays. TVA finds this slight overlap acceptable during normal 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 operator 5 minutes in which to raise the voltage, such as slightly l
i increasing the generator voltage. Also, the diesel generators will not start until the lapse of the 5 minutes.
2
b.
Our calculations are on the conservative side. For example:
(1) for voltage calculations, the cable impedances used are assumed to be at the. industries maximum tolerance, and (2) 480V board leading assumed is generally 15 to 25 percent higher than actual operating data at the plant indicates. -
Item 3.
To determine the amount of overvoltage the 6.9-kV shutdown boards will have, two cases have been examined. First, for normal operation with the main generator operated at its maximum of 25.2-kV the voltages on the 6.9-kV shutdown boards were calculated to be approximately 7350V.
Secondly, we examined the 6.9-kV shutdown voltage of one unit' when it is in cold shutdown with the 161-kV grid at its upper limit of 173 kV when the CSST voltage tap is at the -2.5 percent position. The voltages on the 6.9-kV shutdown boards would be approximately 7500 volts. Corrective action will be taken to correct the voltage before equipment damage occurs.
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+2.5%
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6666/6702 6688/6714 6594/6598 6619/6622 182
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7303/7335 7323/7346 7233/7236 7257/7259 168
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7277/7309 7296/7320 7206/7210 7230/7233 164
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5 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) the I
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 allow 4
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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-6
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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.
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INCLOSURE 2 REVISED PAGES 1D 1EE ATIACHMENT ID THE JUSTIFICATION FOR INE PROPOSED IECHNICAL SPECIFICATION CHANGE
to initiate three time delay sequences (see Figure A-I). The first sequence of 30 seconds will ride through normal system voltage transients (motor ~ starts - both safety and nonsafety related) before annunciating the undervoltage in the main control room. The second sequence is short enough to allow safety-related equipment to be i
powered within the time required by the safety analysis. At the end of 10 seconds if an SIS has been initiated, or is subsequently initiated, the shutdown board degraded voltage relays will initiate load shedding and subsequently transfer the shutdown board to its diesel generator. The return of bus voltage initiates load sequencing of 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 shutdown board to its diesel generator if degraded voltage has not been corrected. The error associated with these voltage sensors is 10.5 percent. The error associated with the timers is 110 percent f or timers with settings greater than or equal to 200 seconds and i 5 percent for timers with settings less than 200 seconds.
To protect the Class IE buses from a sustained over-voltage, each of the two 6.9-kV Class IE buses per unit will be provided with a set of three instantaneous solid-state overvol tage relays (ITE-type $911).
These relays will be arranged in a one-out-of-three coincidence logic which will annunciate in the control room. The relays will have a nominal voltage setpoint of 7260 volts i 1 percent (105 percent of nominal). The operator will take the action necessary to reduce the vol ta ge.
Load shedding for a los's of bus voltage
(< 70 percent) is being maintained once the diesel generators are supplying their respective buses. Degraded voltage relaying will not open the standby supply breaker and will not initiate load shedding and resequencing if a 6900-volt shutdown board is supplied by its diesel generator. The output of these relays is blocked when the standby breaker is closed. TVA' s bases for this is discussed in section 3.3.2.
Proposed changes to the plant's technical specifications,. adding the surveillance requirements, allowable limits for the setpoint and time delay, and limiting conditions f or operation f or the second Icvel undervoltage monitors are furnished in appendix B.
An analysis to substantiate the limiting conditions and minimum and maximum setpoint limits 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 undervoltage protection must meet.
Each criterion is restated below followed by a discussion regarding TVA's compliance with that criterion.
t
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 are set at 80 percent of nominal for four seconds, the band of voltages that nonaccident degraded voltage condition can exist is f rom 80 to 95 percent of nominal for five minutes. At 80 percent of nominal the voltage at the tenninals of running motors will not drop below 71 percent of motor rated voltage. NEMA Class B motors will not stall out or be damaged above this point for the time delay of five minutes.
Also, during the five-minute time delay the 125V de vital battery boards could be powered by the batteries instead of the battery chargers. H ow ev e r, the vital batteries have sufficient capacity to meet 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 the loss-of-voltage relays to initiate transf er to the alternate supply, if it is greater than 95 percent of nominal, before tripping and transf erring to the diesels.
An error of f 0.5 percent f or the voltage sensors in the degraded voltage protection circuits has been considered in the design. The error associated with the timers used in the degraded voltage design is 110 percent for timers with settings greater than or equal to 200 seconds and f 5 percent for timers with settings less than 200 se c on d s.
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