ML13317A242
| ML13317A242 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/15/1982 |
| From: | Baskin K Southern California Edison Co |
| To: | Crutchfield D Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8208020214 | |
| Download: ML13317A242 (15) | |
Text
Southern California Edison Company P. 0. BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD. CALIFORNIA 91770 K. P. BASKIN TELEPHONE MANAGER OF NUCLEAR ENGINEERING, (213) 572-1401 SAFETY, AND LICENSING July 15, 1982 Director, Office of Nuclear Reactor Regulation Attention:
D. M. Crutchfield, Chief Operating Reactors Branch No. 5 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 Adequacy of Station Electric Distribution System San Onofre Nuclear Generating Station Unit 1 By letter dated February 24, 1982 you forwarded the results of your review of the adequacy of station electric distribution system voltages at San Onofre Unit 1.
Your letter indicated that the San Onofre Unit 1 design was acceptable subject to the results of verification testing.
The required verification testing was conducted on February 27, 1982. In addition, the calculated minimum and maximum voltages on the distribution buses have been revised to reflect new conditions at the plant.
The results of the tests and the revised calculations are provided as an enclosure to this letter.
As indicated in the enclosure, the tests demonstrate the adequacy of the electric distribution system. Furthermore, the computer simulations of the tests are accurate to within 1.2%, thereby verifying the calculational procedures. The extrapolation of these test results to the minimum voltage conditions demonstrates that the minimum voltages meet the acceptance criteria of 80% for transient voltages and 90% for steady state voltages.
With respect to maximum voltages, the revised calculations indicate an overvoltage condition of 12% on the 4.16 kV buses and 22% on the 480 V buses. Although this potential for overvoltage is not considered significant, as discussed in the enclosure, a voltage monitoring program is planned to be initiated with the next plant startup.
Based on the results of this monitoring, an optimization of the transformer tap settings may be 8208020214 820715 PDR ADOCK 05000206 P
Mr. D.
July 15, 1982 implemented. Any revised settings will be based on the same criteria and calculational procedures as utilized in the enclosure. We will advise the NRC staff 90 days following startup from the current outage of the results of this monitoring program and of any changes in the transformer tap settings.
If you have any questions on the enclosed information, please let us know.
Very truly yours,
ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES SAN ONOFRE NUCLEAR GENERATING STATION, UNIT 1
- 1. INTRODUCTION By letter dated February 24,
- 1982, the NRC provided the results of their review of the adequacy of the electric distribution system voltages at San Onofre Unit 1. The NRC indicated that the minimum and maximum voltages, indicated by SCE's analysis in a letter to the NRC dated August 19, 1981, were acceptable, provided the minimum voltages could be verified by tests within 3%.
The minimum voltages were calculated assuming the worst possible condi tion; including a minimum voltage of 217.8 kV, and initiation of a Safety Injection Signal during the hot shutdown mode. Since this calculation was
- made, the sequence of the start of the safety injection loads was modified. Under the new sequence, the feedwater pumps are first tripped, at the initiation of a Safety Injection Signal, and then restarted with an 11-second time delay. Therefore, the calculations of the minimum voltages have been revised to reflect these new conditions.
On February 27, 1982, during the plant shutdown, tests were conducted to verify the minimum voltages, using the new safety injection sequence.
The calculations of the maximum voltages have also been revised because of other new conditions.
This report presents the results of the minimum voltage tests conducted on February 27, 1982 and the results of the revised calculations of the minimum and maximum voltages.
- 2. MINIMUM VOLTAGE CONDITIONS Minimum voltages would occur following a Safety Injection Signal (SIS) during the hot shutdown mode with the minimum grid voltage of 217.8 kV as described in our August 19, 1981 letter. The resulting simultaneous start of safety injection loads, initiated by the sequencer, causes a large voltage dip.
Calculations and tests have been performed to verify that under these extreme conditions:
(a) Safety injection loads start and operate normally, (b) The voltage does not dip below 80% of the motor nameplate
- voltage, (c) The steady state voltage is not below 90% of the motor nameplate voltage.
The calculations submitted in SCE's letter to the NRC dated August 19, 1981, have been revised to consider the new safety injection sequence, the initial operating loads and the actual sequencing of loads.
These items are discussed in the following paragraphs.
-2 2.1 New Safety Injection Sequence As a result of a failure of the safety injection valves, the sequence of the startup of safety injection loads was changed.
Under the new
- sequence, the feedwater pumps are tripped at the beginning of the sequence and then started after an 11-second time delay, as shown by Table 1.
2.2 Initial Operating Loads The initial operating loads are as shown in Table 2.
As previously
- assumed, they are the hot shutdown loads including the additional loads of feedwater pumps A and B and condensate pumps A, B, C and D.
2.3 Starting Loads The assumptions made for the starting loads in the previous calcula tions were conservative.
As shown by Table 1, under the previous sequence, all of the loads listed in Table 1 were started simultan eously at the initiation of SIS (time t = 0).
Under the new sequence, in addition to the 11-second delay for the start of the 4.16 kV feedwater pumps, the starting loads on the 480 V buses were revised to be more realistic.
During the hot shutdown condition, prior to SIS, at least one component cooling water pump and one salt water cooling pump have to be operating, and one com ponent cooling water pump is kept in standby.
In addition, the refueling water pumps are not started at the initiation of SIS.
The refueling water pumps are started when the pressure within the containment, reaches 2 psi.
For calculation of the minimum voltage these pumps are assumed to be started coincident with the feedwater pumps at 11 seconds, which results in the lowest possible voltages.
The "New Sequence" shown in Table 1 reflects these changes with component cooling pump A and salt water cooling pump A starting at time t = 0, and refueling water pump A starting at time t
= 11 seconds.
These pumps were chosen on 480 Volt Bus 1 to obtain the worst voltage dip on this bus.
Refueling water pump B on 480 Volt Bus 2 is also started at 11 seconds.
- 3. MINIMUM VOLTAGE TESTS -
CONDITIONS Voltage tests were conducted on February 27,
- 1981, during the shutdown operations of San Onofre Unit 1.
Voltmeters and ammeters with a strip chart recorder were installed as shown on Figure 1, to measure voltages on the 230 kV bus, the 4.16 kV buses 1C and 2C, and the 480 V buses 1, 2 and 3 as well as currents on the cables feeding these 4.16 kV and 480 V buses.
The test conditions were as follows:
3.1 System Configuration:
As shown by Figure 1.
3.2 Initial Conditions:
Hot shutdown mode as described by Table 2.
-3 3.3 Grid Voltage The grid voltage which was available at the time of the test was 221 kV.
3.4 Safety Injection Sequence The "New Sequence", shown in Table 1, was attempted to be followed as described in the test procedures by manually depressing the SIS push button simultaneously on both sequencers 1 and 2.
Refueling water pump A was to be started manually 11 seconds later.
3.5 Auxiliary Transformer Tap Settings The present transformer tap settings were used:
218.5/4.36 kV and 4.16/0.48 kV.
- 4. MINIMUM VOLTAGE TESTS -
RESULTS Analysis of the results indicates that the tests were not conducted exact ly as planned in the test procedures, as shown by the "Actual Sequence" in Table 1. These results are illustrated by the voltage curves in Figure 2 obtained for the 4.16 kV buses and the 480 V buses which show the voltage dips caused by pump starts.
4.1 At the Initiation of SIS (t = 0)
Because of the manual operation, the initiation of the sequencers was not simultaneous; Sequencer 1 (4.16 kV Bus 1C and 480 V Buses 1 and
On 480 Volt Bus 2, where no pump start was required, component cooling pump B was started.
4.2 At Time t = 8 Seconds After Initiation of SIS Refueling water pump A was not started at 11 seconds at the same time as the feedwater pumps but at time t = 8 seconds.
4.3 At Time t = 11 Seconds After Initiation of SIS Sequencer 2 was lagging Sequencer 1 by 0.2 seconds.
The Safety Injection loads were started successfully.
As shown by Figure 2, voltage dips were experienced at time t = 0, 8 and 11 seconds.
The largest dip occurred at 11 seconds and the voltages remained well above the 80% limit.
(Minimum of 3,765 volts on the 4.16 kV buses and 412 Volts on the 480 V buses.)
These bus voltages returned to at least 4,281 volts and 466 volts respectively, which are above the 90% limit.
- 5. MINIMUM VOLTAGE TEST - SIMULATION The actual test conditions were simulated with a
computer load flow program for the various periods of the test, as shown by the Actual Sequence in Table 1:
-'4 (a) Prior to test, t < 0 (Steady State)
(b) Initiation of test, t = 0 (Transient)
(c) 0 < t < 8 sec.
(Steady State)
(d) At t = 8 sec.
(e) 8 sec.<t < 11 sec.
(Steady State)
(f) At t = 11 sec.
(g) Completion of test, t > 11 sec.
(Steady State)
For each case, the operating loads were estimated from the list of Table 2.
The totals on each bus were adjusted to the recorded actual loads.
The starting loads at time t = 0 and t = 11 seconds were repre sented as shunt loads derived from motor characteristics.
For the largest dip at time t = 11 seconds, the following results were obtained:
Minimum Voltage (Volts at Time t =11 Seconds)
Auxiliary Bus Actual Simulated Difference 4.16 kV Bus 1C 3,765 3,794 0.8%
4.16 kV Bus 2C 3,765 3,807 1.1%
480 V Bus 1 416 417 0.2%
480 V Bus 2 412 417 1.2%
480 V Bus 3 428 426 0.5%
It can be seen that the accuracy of the computer simulation is satisfac tory with a maximum discrepancy between the actual value and the simulated value of 1.2%.
Similar accuracy was obtained for the other periods of the test as shown by Figure 2.
- 6. MINIMUM VOLTAGE TEST -
EXTRAPOLATION OF RESULTS Using the results of the test for the operating loads, the results of the tests were extrapolated from the actual test conditions
("Actual Sequence")
to the more severe conditions
("New Sequence")
as shown by Table 1, with the absolute minimum grid voltage of 217.8 kV.
Because of the accuracy of the simulations demonstrated above, it is concluded that this extrapolation is reliable.
The following minimum voltages (in volts and percent of motor nameplate rating) were determined from these calculations.
It should be noted that the nameplate motor voltages are 4,160, 460 and 440 Volt.
For the minimum voltages on the 480 Volt buses the voltage limit was based on the higher nameplate voltage of 460 Volts.
-5 Minimum Voltage Tests Results (Extrapolated to New Sequence)
Time (Seconds) 4.16 kV Buses 480 V Buses 1C 2C 1
2 3
Prior to Test Volt 4,180 4,197 466 470 472 (Steady State) 100 101 101 102 103 0
Volt 4,019 4,054 419 453 453 (Transient) 96 97 91 98 98 11 Volt 3,619 3,538 376 390 405 (Transient) 87 85 82 85 88 After Test Volt 4,167 4,188 462 468 470 (Steady State) 100 101 100 102 102 It can be seen that the acceptance criteria of the minimum dip of 80% for transient voltages and the minimum value of 90% for steady voltages were met.
- 7. MAXIMUM VOLTAGES -
CALCULATION RESULTS Since the analysis of maximum voltages was submitted in SCE's letter to the NRC dated August 19,
- 1981, the calculations and assumptions of the analysis were reviewed.
It was found that the maximum 230 kV voltage at San Onofre should be increased from 234.5 kV to 240.0 kV (with all three generating units operating at the maximum generator terminal voltage of 105% of rated voltage).
This increase results in higher maximum auxiliary bus voltages, from 109.7% to 112% of the 4.16 kV rated motor voltage on the 4.16 kV
- Bus, and from 118.3% to 122% of the 440 Volt rated motor voltage on the 480 Volt Bus.
Although these voltages exceed the 110% continuous limit, lower voltages can be expected with the normal operating generator terminal voltage of 100% of the rated voltage, as shown by Figures.3 and 4 and as indicated below:
-6 Generating Units Maximum Bus Voltages 4.16 kV Bus 480 V Bus Terminal Voltage 230 kV Bus
% of 4.16 kV % of 440 V Operating
(% of Rated)
(kV)
Rating Rating 1, 2 and 3 105 (Maximum) 240 112 122 1, 2 and 3 100 (Normal) 231 107 118 1 and 2 100 (Normal) 231 107 118 1 only 100 (Normal) 225 106 115 Because of this overvoltage situation, a voltage monitoring program is planned to be initiated with the next unit startup, to verify the above results and help in the optimization of the auxiliary transformer tap settings (presently 218.5/4.36 kV and 4.16/0.48 kV).
In the interim, this potential for overvoltage is not considered signifi cant, considering the fact that Units 2&3 are not yet operational and that high voltages on the 230 kV system have a low probability.
In addition, overvoltage limits on electric motors are thermal limits.
The 110% limit can be exceeded by 5% to 10% for several hours without resulting in signi ficant loss of life of the motor insulation.
- 8. CONCLUSION The test of the minimum voltages was conducted on February 27, 1982, with a grid voltage of 221 kV. A computer simulation of the tests was made and provided values within 1.2% or less of the recorded actual values.
Because of the accuracy of this simulation, it is concluded that the extrapolation of the test results to the somewhat more severe minimum voltage conditions, is reliable.
The final results from these extrapola tions show that the safety injection loads can be successfully started and that the minimum voltages will meet the acceptance criteria of 80% for transient voltages and 90% for steady state voltages.
For the maximum voltages, because of an increase of the expected maximum grid voltage from 234.5 kV to 240 kV, the revised calculations show an increase in the maximum voltages from 109.7% to 112% of the 4.16 kV rating on the 4.16 kV bus, and from 118.3% to 122% of the 440 V rating on the 480 V buses.
These values, however, are considered to be extreme condi tions, corresponding to Units 1, 2 and 3 operating at 105% of their rated terminal voltage and light load.
Under the normal operating terminal voltage of 100%,
with only Unit 1 operating, the above voltages would be reduced to 106% on the 4.16 kV buses, and 115% on the 480 V buses.
A
-7 monitoring program is planned to be conducted, following the next unit startup to verify these high voltages.
Based on the results of this monitoring, an optimization of the transformer tap settings may be imple mented. Meanwhile, it is concluded that exceeding the 110% limit by 5% to 10% under maximum grid voltage conditions will not result in a significant loss of life of the equipment.
SAN ONOFRE UNIT 1 AUXILIARY SYSTEM - VOLTAGE TEST SEQUENCES PUMPS TRIPPED OR STARTED DURING SAFETY INJECTION Time Previous Sequence New Sequence Actual Sequence (As submitted on 8-19-81)
(Actual Tests 2-27-82) 0 4.16 kV Bus 1C 4.16 kV Bus 1C 4.16 kV Bus 1C (Actual 0.2 sec.).
Trip:
Condensate P. C and D Trip:
Condensate P.
C and D Trip:
Feedwater P.
B Feedwater P.
B Start:
Safety Injection P. B Start:
Safety Injection P.
B Start:
Safety Injection P. B 4.16 kV Bus 2C 4.16 kV Bus 2C 4.16 kV Bus 2C Trip:
Condensate P. A and B Trip:
Condensate P. A and B Trip:
Condensate P. A and B Feedwater P.
A Feedwater P.
A Start:
Safety Injection P. A Start:
Safety Injection P. A Start:
Safety Injection P. B Feedwater P.
A 480 V Bus 1 480 V Bus 1 480 V Bus 1 (Actual 0.2 Sec.)
Start: Component Cooling P. A Start:
Component Cooling P. A Start:
Salt Wtr. Cool. P.
A Salt Wtr. Cooling P. A Salt Wtr. Cooling P. A Refueling Water P.
A 480 V Bus 2 1480 V Bus 2 Start:
Component Cooling P. B Start:
Component Cooling P. B Salt Wtr. Cooling P. B Refueling Water P.
B 480 V Bus 3 Start:
Component Cooling P. C 8.0 480 V Bus 1 Start:, Refueling Water Pump 11.0 4.16 kV Bus 1C 4.16 kV Bus 1C Start:
Feedwater P. B Start:
Feedwater P.
4.16 kV Bus 2C 4.16 kV Bus 2C (Actual 11.2 Sec.)
Start:
Feedwater P. A Start:
Feedwater P.
480 V Bus 1 Start:
Refueling Water P.
A 480 V Bus 2 Start:
Refueling Water P.
B (not part of test)
'ABLE 2 SAN ONFRE UNT 1 AUXELIARY S'STEM MNIu TOLTACE TES FEBRRY 2, 19B2 E1ETRC LDAIE OPERATI IRIC TO INITIATRDN CF TISTS Loads Qeatinr an each BAs leads 4.16 kV Bs 1C 4I.16 kV Bus X Feedwater Rmps B
A circulatirg Pup B
A Cordensate Rmps C and D A and B (harzirg PuI B
A off LigItirs Itansfbrmer (Normal)
Stadby Total %coded load (MPS) 82 916 40D-V BRes 1
2 3
Pressurizer haters A
B Bttery Orger A
D Instrunent Qxopressr A
B Sphere Ehlosure Biildirg Fas A-40 ard A-41 A-42 ard A-43 Motor (bntrol Cbnters 1, 1A, and 1B 2, 2, and 2B 3
Ah.
Feedvater Pmp A
Component (boling Pump A
B off (Aito)
C Sandby Salt wter Coolirg ap A Orf (Auto)
B Reftelirg Wter Rnp A CIT (Aito)
B Sandby Ibric Acid atch ank Hater
-on
'Arbine Ailiary lube (1 Rap 01 Total hcorded Loai (R4FS) 1123 1010 757
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