ML20091A848
| ML20091A848 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 03/04/1977 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Ziemann D Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9105210512 | |
| Download: ML20091A848 (4) | |
Text
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NS58 NOMTHEMN STATES POWEM COMPANY M I N N S A f* O W S, M I N N E S O T A 95400 ReguldOW OM b D
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'O9 67TIo Dear Mr Zierunni MONTICELLO NUCLEAR GENERATING PIA!U Docket No. 50'263 License No. DPR-22 Supplemental Infomation Concerning Equipment Perfonnance Under Degraded Voltage Conditioni.
In your August 13, 1976 letter, you asked us to conduct an investigation of our facility to detemine if ve are susceptible to a Millstone type low grid voltage event. A fonnal report of this investigation was sutsuitted on Septwber 17, 1976. 'this reported stated that further investigation was con-tinuing in the following areas:
a.
Precise determination of the voltage range over which safety related components and non-safety related componente can operate continuously h the performnce of their function, b.
Determining if the voltage variations on the safeguards busses due to worst case grid or generator voltage fluctuations and plant loading lie within the acceptable band defined in (a)
- above, c.
Proposed changes or plant modifications to assure that accept-able voltages will exist on safeguards busses under all conditions of voltage fluctuation and plant loading. This includes procedural changes or modifications to protect the plant in the unlikely event safeguard bus voltages fall to just above the under voltaga trip setting.
'the purpose of this letter is to report the progress of continued investi-gations in this area and to describe our proposed actions and modifications to assure that adequate voltages will be cresent under all conditions on the safety related busses.
9105210512 770304 P
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NOR7 ERN OTATEO POWER Cok ANY D b Zietann page 2 Perch 4, 1977 Item (a). Determination oJ Range of Acceptable Vol_tages Motor Cont-ol Centers (MCC's)
Testing is nov in progress to measure the minimum pickup voltage of safety related MCC contactors.
This testing vill be substantially completed by April 1, 1977. Smne testing, however, must be scheduled for the 1977 autumn outage. Preliminary test results indicate that minimum pickup voltages are less than 400 volts.
Earlier calculations based on conserva.
tive methods indicated that pickup voltages were significantly higher.
Motors safety related motors on the 480 volt busses are rated at 440 and 460 volts.
Safety related motors on the 4160 volt busses are rated at 4000 volts. The arceptable + 107. operating ranges are therefore:
gus Minimum voltage M.ximum voltage 3
480 V 414 V 4 84 V 4160 V 3600 V 4400 Y i
Final station auxiliary power voltage limits will be established when the MCC pickup voltage measurements are completed.
If, as expected MCC pickup voltages I
are not the limiting factor in detcomining these limits, limits dictated by motor requirements will be adopted.
ltem (b). comnarinn worst case voltane variations to Range of Acceptable Voltage This comparison will be made af ter the MCC pickup voltage measurements are completed.
If the MCC pickup voltages are not limiting, the range of acceptable voltages becomes 414 to 484 volts on the 480 volt busses and 3600 to 4400 volts on the 4160 volt busses as noted above. Assuming this is the case, referring to Items (Ic) and (Id) in our September 17, 1976 report shows that acceptable minimum voltages are provided under worst case conditions with the existing transformer taps.
Under conditions of maximum expected grid voltage and zero load (a condition which cannot be physically achieved), the maximum voltage limits are exceeded somewhat.
When significant load is applied, however, these voltages will fall to acceptable values.
Transformer tap setting changes are being con-sidered to optimize the range of voltages on safety related busses due to worst case grid voltage variations.
As noted in our September 17, 1976 report, no motors or contactors will operate correctly if grid voltage is assumed to deteriorate to the point where the under-voltage trip setting of the safeguard busses is reached.
Item (c). Proposed Changes or Plant Hodifications To provide assurance that the voltages supplied to safety related loads remain within allowable ranges we will improve our bus voltage monitoring capability
NORT,.tRN GTATED POWER Coh
\\NY D 1. Z i eru nn l' age 3 Ita rch 4,1977 and provide high-low alarms on of f atte supply bunnes, the generator outputs, and on the safety related 4160 volt busses.
As nce envisioned, the proposed alarm syste2n will include:
1.
345 Kv bus voltage monitored by coinputer or undervoltage relays with high low set points to alann when bus voltage leaves the nonnal operating range.
2.
115 Kv bus voltage monitored by computer or under voltage relays with high lov set points to alann when bus voltage Icaves the nonul operating range.
This voltage is regulated by an automatic load tap changer.
3.
Generator tenninal voltage monitored by computer with high-lov set points set at rated generator voltage plus and minus five percent.
This corresponds to the allowable voltage operating range of the genera tor.
4.
4.16 Ky station safeguarti bus voltages monitored by computer or undervoltage relays with high-low set points to alann when voltage on the bus leavec che allowable operating range.
With proper station auxiliary transfonner tap settings, maintaining proper substation bus voltages and generator tenninal voltages viu ensure that the r,tation auxiliary voltages are acceptabic.
Should a disturbance create a low voltage condition which cannot be 12nediately corrected by plant operators or the system dispatcher, the station auxiliary bus voltage alanns will alert the plant operator that action is required to protect station auxiliary equip.
tnent from excessively high or low voltages.
In the event other corrective actions fail to restore voltages to the nonnal operating range, the unit will be tripped and safety related loads will be carried by the diesel generators.
Corrective actions will be outlined in written procedures.
'Itc proposed upgraded monitoring and alarm system will be completed during the 1977 autumn outage, liecessary changes to operating procedures will be completed by the end of the outage.
Contact us if you have any additional questions concerning the potential for ittproper voltages or the actions we have taken to provide additional assurance that the problem will not be encountered at our facility.
Yours very truly, W
A L 0 Mayer PE Manager of liuclear Support Services LOM/D!tt/deh cci J G Keppler G Cha rnof f MPCA Attn:
J W Ferman
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Northern States Power Company 3/4/77 MI"""*P I I ' ' M I"""
l' D^' t "' C E 'V E D Mr. D. L. Zier. ann L. O. Mayer 3/10/77 Ott tit n ONotomitiD P R oP INPUT F oRM NUutt M of CoPit 5 Nt et av( 0 D omioiN AL p(vNe t assie it o One si ned copy De or y.
E DE SC hip T ION ENCLo6Umt Ltr. re our 8/13/76 Itr... furnishing supplemental infomation concerning equipment performance under degraded voltage conditions......
ACKNOWLEDGED (3-N runt nAME DO NOT REMOVE Monticello
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