ML19319B462
| ML19319B462 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/18/1977 |
| From: | Roe L TOLEDO EDISON CO. |
| To: | Stolz J Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8001220870 | |
| Download: ML19319B462 (12) | |
Text
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b oOCKET%UMS3SV G U.S. NUCLEAD CE!ULATLOY COM' ilON N.RC P RM 196
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,NRC DISTRIBUTION PoM PART 50 DOCKET MATERIAL TrJ:
FROM:
oATE op ooCuMENT Toledo Edison Company 7/18/77 Toledo, Ohio oATE REcEivEo Mr John F. Stolz Lowell E. Roe 7/19/77 a
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stability with attached drawings to V. l A' i complete their submittal of an evaluation
&proposedmodificationsinaccordancewith the requirements of Facility Operating License No. NPF 3 condit on 2.C.(3)(q).....
.M.CIG70WLEDGED m do"'T PLANT NAME: Davis-Besse (1-P)
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0 RJL 7/21/77 4AF?W FOR ACTION /INFORMATION ENVIPSNMENTAL A99TCNED ADe ASSIGNED AD; V. MOORE (LTR) nWANcM CMTT7?
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TOLEDO EDISON LOWELL E. ROE Docket No. 50-346 v,.......
FacskLes Osvelopment 141H 259-5242 Serial No. 293
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July 18, 197 Regulaterf Dockef Fiis.r
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d Director of Nuclear Reactor Regulation Attention:
Mr. John F. Stolz, Chief
\\'k LightWaterReactorsBranchNo.1%,h Division of Project Management idd./
U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Mr. Stolz:
Attached with this letter is the response regarding Davis-Besse Unit No. I grid stability to which we agreed in meeting with you on June 6, 1977. The response is organized per the summary of that meeting as transmitted by Enclosure A to your letter of June 23, 1977 requesting response by July 18, 1977.
This letter completes our submittal of an evaluation and proposed modi-fications 11 accordance with the requirements of Facility Operating License No. NPF-3 condition 2.C. (3)(q). We await your approval of the proposed modifications before incorporating them into our designs.
Yours very truly, Attachment db a/1 W AA n y n., -
THE TCLEDO EDISCN COMPANY EDISON PLAZA 30C MACISCN AVENUE TOLEDO. CHIO 43652
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Davi -Be co Nuclear Power N,titn Unit No. 1
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Attachment' to Lett:r Scriar do. 293 1..
The f 11owing drawings were submitted by the applicant to facilitate review of the modifications made to detect and preclude sustained degraded voltage conditions from offsite power sources.
E3 Rev. 8 E 34B Sheet 9 Rev. 7 E 22 Sheet 1 Rev. 5 E 34B Sheet 11
- Rev. 5 E 22 Sheet 2 Rev. 5 E 34B Sheet 14 Rev. 5 E 34B Sheet 1 of 1
- Rev. 11 E 34B Sheet 14A Rev. 0 E 34B Sheet 3 Rev. 4 E 64B Sheet 1A Rev. 4 E 34 Sheet 5
- Rev. 6 E 64B Sheet 2A Rev. 5 The applicant was requested to submit these drawings formally on the docket.
Response
These drawings are attached with this lette The drawings with asterisked revision numbers are actually a later revision than what was submitted June 6.
The technical content is identical however.
2.
The staff identified their concerns regarding the applicants incomplete responses to various questions identified in our letter of January 25, 1977, regarding their of fsite powa:- system. The following specific 4
questions of the January 25, 1977, letter were discussed.
A. Item Ic.
The applicant was requested to provide their basis and justification of acceptability to permit exceeding allowable voltage levels on some of the equipment before administrative procedures are taken to lower the voltage levels to within acceptable limits. The applicant indicated that the design may be c',anged to maintain the voltage level within allowable limits at all times, and thus ' eliminate this concern. The applicant, however, did commit to modify their response and describe the adequacy of their present design or describe the f
modifications that will be made.
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Response
In order to assure that the bus voltages do not exceed the allowable voltage of equipment an additional overvoltage alam will be added to the safety-related 4.16KV buses with a setpoint of 102% (4.16KV base).
This design change is to initiate administrative procedures prior to the bus voltages exceeding the allowable voltage of equipment.
B. Item ig. The applicant comitted to modify their response of this Item (1g) to identify all safety related equipment (i.e. valves, j
l fans, pumps, etc.) and redefine the voltage ranges over which safety related equipment c.an operate continuously and perform their function.
The applicant stated that safety related motors (valves, fans and pumps) are designed and tested to function at 70% voltage and will address this in the anended response of this item.
Response
The voltage range over which safety related equipment (i.e. valves, fans, pumps, etc.) can operate continuously in the performance of their design function is tabulated below:
Equipment Range Base Value Source 110%
120V, 240V, 460V, 4KV NEMA MG-1 Safsty reli-4 motors
- 90%
Stfsty related 4.16KV Switchgear Maximum of 114%
4.16KV ANSI C37.06 Safsty related 480V Switchgear Maximum of 132%
480V ANSI C37.13 120V Contactors 85%
110%
120V NEMA ICS 2-110 120V Relays 85%
110%
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tSaf;ty. related motsro (vcivec, fcn3 cud pump 3) era den 3and cad type-tz2tsd to start at 70% voltage and to provida full load terqua during a momenttry dip to 65% of rated notor nameplate voltage at the motor terminals for 15 seconds. The only exceptions to this requirement are the following electrical equipment which have starting capacity of 80% voltage:
- * (1)
Containment recirculation fans 1 and 2.
- * (2)
Battery room vent fans 1-1 and 1-2.
- The-loss of these fans for 9 seconds is inconsequential to the safe shutdown of the plant.
C. Item 4 The applicant agreed to amend the response to item 4 to state that the modifications made to the setpoints in the relay system would not effect the required starting times assumed in the accident analysis, and that adequate margins are provided in their system.
In addition the setpoint Table listed, will be modified to reflect the setpoint setting discussed in the meeting.
Response
The additions and modifications made to the setpoints in the relay system as enumerated in the TECo letter of January 4,1977, Serial No. 179, do not affect the starting times assumed in 'he accident analysis except for the very improbable case of a SA signal occurring simultaneously with a voltage condition that was less than 90% and greater than 59% of 4.16KV safety-related bus voltage. This exception, which was previously indi-cated in the aforementioned letter, would result in a slight reduction of existing margins for starting times. In order to eliminate the possi-bility of a slight reduction in margins for this very improbable case, the following annendment is made to the setpoints of the relay system:
(1) The 10 second time delay setting of the 90% undervoltage relays on the 4.16KV safety-related buses will be changed to 9 seconds. These undervoltage relays automatically initiate the disconnection of the offsite power source whenever the voltage setpoint of 90% (4.16KV base) and time delay limit of 9 seconds have been erceeded.
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m (2) Tha cdditi:nal 1.0 cicond time d21ry rdd:d to tha di al g:nzrztor breaker closure will be changed to.5 seconds. This relay is actuated by the 59% undervoltage relays with a.5 second time delay.
These undervoltage relays automatically initiate the disconnection of the offsite power source, load shed the bus and start the diesel generator whenever the voltage setpoint of 59% (4.16KV base) and time delay of.5 seconds have been exceeded.
"ach 4.16KV safety-related bus is provided with four undervoltage relays for each level of voltage protection (i.e. 90% and 59%). These relays function in a coincidental logic to preclude spurious trips of the off-site power source. The time delays (i.e. 9 seconds and.5 seconds) are chosen so as to minimize the effect of short duration disturbances from reducing the availability of the offsite power source and to assure that the allowable time duration of a degraded voltage condition does not re-suit in failure of safety systems or components.
As discussed in the meeting,'the following alarm setpoints were revised to further enhance the coordination between the alarm system and the protective system:
Previous Revised Alarms Setpoint Setpoint 4.16KV Safety Related Bus Voltage Low 90.8%
95%
480V Safety Related Bus Voltage Low 90.0%
92%
13.8KV Bus Voltage Low 93.9%
95%
4.16KV Bus Voltage Low 90.8%
95%
480V Bus Voltage Low 90.0%
92%
- 4.16KV Safety Related Bus Voltage High 103.3%
103.3% & 102%
- The additional alarm was not presented to NRC on June 6, but is referenced in response 2A above. --
e D.' Item 5.
The applicant agreed to modify their presently documented response and describe in detail the methodology used to verify the adequacy of their design. The applicant agreed to describe the analysis used to verify that the equipment will maintain their operability for the required length of time.
(i.e., until isolation of degraded bus voltages is accomplished) and describe the tests, and or standards used to assure that the equipment will sustain, without degradation, these faulted conditions.
Response: In order to verify the adequacy of the plant electric power syste?
design an analysis of the design has been performed. This analysis was conducted to complement the comprehensive initial testing and startup testing program which is being conducted at the Davis-Besse Nuclear Pouar Station. The purpose of the testing program is to assure that the electrical equipment and systems perform in accordance with the design.
For the analysis, three modes of operation were developed taking into account the plant's electrical system configurations and protective systems including the protective system described in the TECo letter of January 4,1977 Serial Nc.179. Utilizing the safety related 4.16KV bus voltage as a reference value, the three modes were:
Mode No. 1 Overvoltage (103.3%)
Mode No. 2 90% to 103.3%
Mode No. 3 59% to 90% for 10 seconds y
3 The basis of the analysis was a review of the voltage requirements of safety-related loads and their performance for each appropriate voltage and time setpoints corresponding to the above modes of operation. The equipment rating was determined by industry standards to which the equipment had been specified, manufactured and purchased
'(Ref: Modified response to Item Ig; Item B of attachment A above).
These industry standards are the approved minimum requirements that govern the manufacture of electrical equipment.
Mode No. 1 Operation The analysis for Mode No. 1 indicated at 103.3% (4.16KV base) the 460V motors may be operating at their maximum permissible voltage according to NEMA MG-1.
Therefore, overvoltage alarms are provided.
The overvoltage condition occurs at light unit auxiliaries load, maximum system grid voltage and all transformers one tap below nominal. Should this operating condition occur, administrative procedures will be initiated to change the taps. Therefore, the anal sis concluded that the plant's electrical equipment would not be adversely affected by Mode No. 1.
Mode No. 2 Operation The analysis for Mode No. 2 indicated that, with one exception, in-dustry standards assure satisfactory performance of components with-in the stated voltage range. The one exception, 120V contactors, requires an applied voltage of 85% (120V base) according to NEMA
.ICS 2-110.
For the worst case, the analysis has indicated that 90%
voltage (480V base) is required at the primary of the 480V-120V control power transformer to assure 85% voltage at the contactor.
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Tha 90% valt go (,.16KV bn.00) crrre: ponds to 88%..' tags on 480V base. The Toledo Edison Company therefore performed tests at the Davis-Besse jobsite which descustrated that the contactors will pickup and hold with 80% voltage en the primary of the control power trans-former. These tests wcre simulations of worst case configurations at Davis-Besse with respect to heavy loading on the control power.
transformer due te auxiliary coils and lights and heaters, low voltage at the contactor coil due to voltage drop on long runs of external control cable, greatest pickup voltage due to largest contactor size used with a given size control power transformer.
No failures of control power fuses were experienced, 80% primary voltage was maintained for a minimum of *10 seconds after contactor pickup, all auxiliary relays performed as required.
- Actual time delay is to be set at 9 seconds. See C. item 4.
Mode No. 3 Operation The analysis, for Mode No. 3, was to demonstrate that the electrical equipment is not degraded during Mode No. 3 and would operate sa-tisfactorily after this condition is over. The grid system voltages corresponding to the 59% to 90% range (4.16KV base) for Mode No. 3 are approximately 67% to 93%, respectively (345KV base) from computer analysis. For this range of degraded system grid voltage, it must be emphasized that the performance of the grid system would be ex-tremely difficult to predict due to the likelihood of transients associated with system load shedding, loss of generation, tripping of interconnections, etc. These events would occur in an effort s
to correct the degraded grid voltage and to protect electrical equip-ment throughout the grid system. Likewise, the plant's auxiliary system would be subjected to these grid system transients for a maximum of 9 seconds. The 9 second time interval is the maximum time which the pr:tscriva cystem will allow th2 p unt'o auxilicry system to be exposed to the grid system transients.
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However, for purposes of analysis for Mode No. 3, the effects of extremely degraded grid system voltages of a sustained nature for 10 seconds were evaluated. The analysis was divided into two subanalyses:
-Subanalysis No. 1.
Plant electrical equipment starting during Mode,No. 3 Subanalysis No. 2.
Plant electrical equipment operating during j
i Mode No. 3 For subanalysis No. 1, the motors have a starting capability of 70% of rated voltage (Ref: Modified response to Item Ig; Item B of attachment A above).
Toledo Edison has performed tests and found some contactors pick up at below 70% voltage. In the case where bus voltage is less than 70% on a 480 volt base, some contactors may pick up, but the motor may not start. The motor will experience reduced locked rotor current due to lower voltage for a maximum 9 second duration. Motor suppliers have stated that the motors will not be damaged. The molded case circuit breaker thermal trip in the case of motor control centers or the air circuit breaker power sensor in the case of unit substations will not trip in this 9 second period. This maximum delay time of 9 seconds for motor starting is consistent with the required starting times assumed in the accident analysis.
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i in those cases where bus voltage falls below 80% on the 480 volt i
base where the contactor fails to pick up, there is the remote l
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~y prccibility that.the control circuit futs in 1stcu circuito may blow due to inrush current for 9 seconds to the contactor and auxiliary relays. In these cases, which include only size 1 and size 2 contactors, the one amp fuse will be incre. sed to a 2.5 amp slow blow fuse which will not blow in the assumed 9 second period.
A For voltage in excess of 80% on the 480 volt base, the contactor will pick up and the motor will start.
Motors supplied frcm switchgear and unit substations will attempt to start because the breaker control circuits are D.C.
controlled. For voltages of 70% of rated voltage or above, all the motors will start. For voltages below 70% of rated voltage and depending upon load characteristics, some motors may successfully start. For those motors which do not successfully start, the maximum delay time is 9 seconds; as indicated above, no motor damage will occur.
To assure that the safety related motors will not trip and lock-out during this 9 second period, the motor protective relay setting will be suitably modified to preclude this from happening.
For subanalysis No. 2 the motors have the capability of operating at 65% of rated voltage for 15 seconds (Ref: Modified response to Item ig; Item B of attachment A above). For voltages below 65%,
some motors may stall depending upon their particular load characteristics. As previously indicated, 9 seconds is the maxi-mum time that the motors would be subjected to these extremely degraded voltages.
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Mottra cupplied tros motor c:ntrol ccnt rs would. i ctcp opzrzting due to contractor drop out. The 120V contactors are either latching type or non-latining type with a drop out voltage lever than the minimum voltage for Mode No. 3.
Motors supplied from switchgear and load centers will continue operating for 9 seconds because the breaker control circuits
' C. controlled.
Subanalysis No. 2 was considered the worst case for degradation of equipment since the time duration could be a maximum of 9
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seconds. The complete analysis of Mode No. 3 demonstrated that the electrical equipment would not be degraded for sustained voltages of that range.
Thus, the analysis of the design for the three modes of operation (including proposed modifications) concluded that the plant's electric power system will be adequate for reactor operations under various grid system operating conditions.
In summary, then, the following are proposed changes to Davis-Besse Unit #1 which were previously identified in the text but are included again here for convenience.
1.
The time delay on the 90% 4.16KV voltage relays will change from 10 seconds to 9 seconds, and the time delay to the diesel breaker closure will change from 1 second to.5 second.
2.
An additional computer alarm point on the essential 4.16KV buses will be added.
3.~
Overentrent relay setpoints for essential 4.16KV motors will be revised.
4.
One amp fuses for motor control canter control power will change to 2.5A slow blow fuses..-
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