ML19322B869
| ML19322B869 | |
| Person / Time | |
|---|---|
| Site: | Oconee, Millstone  |
| Issue date: | 08/12/1976 |
| From: | Schwencer A Office of Nuclear Reactor Regulation |
| To: | Parker W DUKE POWER CO. |
| References | |
| NUDOCS 7912060700 | |
| Download: ML19322B869 (5) | |
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Occket.'os. 50-269 50-270 AUG 1 g MS and 1:0-237 Ouke Pcwr Co :oany ATT?!: fir. Willian 0. Parker, Jr.
Yice President Stean Production Post Office cox 2173 422 Scuth Church Street Charlotte, P.crth Carolina 20242 Gentle'e':
RE:
CC@EE WCLEAR STATIO.1, QlITS HOS.1, 2, ANO 3 Providec herein as Ent.Msure 1 is a cescrintion of events which cccurre.1 at :iillstana Unit Fo. 2 durinc July 1976 relating to plant creration and ecuip.cnt failures durir? 9 degraded gri1 vol tage condi tion.
Cn July 27, 1976, all utilities.,ith ocerating reactor facilities received tele @one notification frem t.'1e GC of t.he events ai tre "illstene Unit No. 2 facility. At that tire aer.bers of ycur st#f were asked to investicata the vulnerability of your facility te sinflar de:.raded v31tage conditions and te provide a res onse ay telschone vithin 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
's e resul t of our initial investiption o! evaluatic, of :.me entential neeeric inplications of tha events et '!illstene ir.d our cre114fniry escussions with several licensaes, w consiaer it necessary to recai e all oneratirn reactor licensees to ccitet:c t*arnuch 9-/alta tion of the tre.alui and to subi t for al re ce:s.
Therefore, c req mst that you c nduct an invectiartion of tre issua as 1: affects ycur facility usin>T tne 7equest for Infor a:icn Jcutle in Fnclosure 2 as a guide, and pr0 Vide the analyses and rssul ts wi thia 3e cays of your recaint of thi; letter.
Tqe ci1ned origi7sl and 39 copies of your response sill u rcccmry, OFFICE P ou==*=*
... _ i O ATE De Forts AEC 318 (Rev. 9 53) AICM 0240
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=h Duke Power Co:npany AUG 121976 This recuest for generic infomaticn was approved by GA0 under a blanht cicarance nu:ser 8-lP0225 ('10072); this clearance expires July 31, 1377.
Sincerely, grigiest signed kY A. Schwencer, Chief Operstina Reactors Branch #1 Division of Operating Reacters Enclosuras:
DISTRIBUTION 1.
Description of Events NRCPDR(3)
Pillstone Unit No. 2 Local PDR 2.
Recuest for Infomation Docket (3)
ORB #1 Reading ec: tir. '.41111an L. Porter KRGoller Duke Powar Carpany TJCarter P. O. Box 217f>
OELD J22 South Cnurch Street OI&E(3)
Charlotte, North Carolina 29242 GZech TVerdery Mr. Troy 8. Conner ASchwencer Conner 4 Xnotts SMSheppard 1747 Pennsylvania Avenue, H. W.
DEisenhut Washington, D. C.
20006 TBAbernathy JRBuchanan Oconee Pelic Library ACRS(16) 201 Souca Spring Street VStello
'dalhall a, South Carolina 29991 Dross A
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Form AEC 318 (Rev. 9 53) AECd 0240
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s ENCLOSURE NO. 1 j
DESCRIPTION'0F EVENTS MILLSTONE UNIT NO. 2 On July 20, 1976, Northeast Nuclear Energy Cwpa'ny (NNECO) reported that, following a trip of Millstone Unit No. 2 on July 5,1976, several motors powered from 480 volt (v) motor control centers failed to start as required. The failure of the 480 y motors to start was traced to blown control power fuses on the individual motor controllers.
These controllers receive control power through 480 v/120 y transformers within i
the controller.
NNECO's investigation disclosed that, as a result of the plant trip, the grid voltage dropped from352 kv to 333 kv.
This voltage drop, in conjunction with additional voltage drops asscciated with the transformers involved, reduced the control power and voltage within individual 480 y controllers to a voltage which was insufficient to actuate the main line controller contactors.
As a result, when the motors were signalled to start, the control power fuses were blown.
Subsequent testing by NNEC0 showed that the contactors required at least 410 y to function properly.
f NNECO concluded that under similar low voltage conditions, the opera-bility of 480 y Engineered Safety Feature equipment could not be assured.
NNEC0's immediate corrective action was to raise the setpoint of the Engineered Safeguards Actuation System (ESAS) " loss of power" under-voltage relays to assure that the plant would be separated from the grid and emergency power system (dual) operation would be initiated before the control voltage fell below that required for contactor operation.
A trip of the undervoltage relays causes the emergency buses to be i
de-energized and a load shed signal to strip the emergency buses, the diesel generators to start and power the emergency buses, and required safety related loads to sequence start on the buses.
On July 21, 1976, NNECO reported that the earlier corrective action taken g
was no longer considered appropriate because during starting of a circulating water pump, the voltage dropped below the new ESAS under-A voltage relay setting.
This de-energized the emergency buses, caussd load shedding to occur, started the diesel generators and began sequencing loads onto the emergency buses in accordance with the design.
- However, during sequencing of the loads onto the buses, the voltage again dropped below the undervoltage relay setting which caused the load shed signal to strip the. buses.
The result was energized emergency buses with not loads supplied.
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ENCLOSURE NO. 2
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g REQUEST FOR INFORMATION 1.
Evaluate the design of your facility's Cfass IE electrical distribution system to determine if the operability of safety related equipment, including associated control circuitry or instrumentation, can be adversely affected by short term or long term degradation in the grid system voltage within the range where the offsite power is counted on to supply important equipment.
Your response should address all but not be limited to the following:
a.
Describe the plant conditions under which the plant auxiliary systems (safety related and non-safety related) will be supplied by offsite power.
Include an estimate of the fraction of nomal plant operating time in which this is the case, b.
The voltage used to describe the grid distribution system is usually a " nominal" val ue.
Define the nomal operating range of your grid '
system voltage and the corresponding voltage values at the safety related buses.
p c.
The transformers utilized in power systems for providing the required voltage at the various system distribution levels are normally provided with taps to allow voltage adjustment.
Provide the results of an analysis of your design to determine if the voltage profiles at the safety related buses are satisfactory for the full load and no load conditions on the system and the range of grid voltage.
d.
Assuming the facility auxiliary loads are being carried by the station generator, provide the voltage profiles at the safety buses for grid voltage at the normal maximum value, the nomal minimum value, and at the degraded conditions (high or low voltage, current, etc.) which would require generator trip.
e.
Identify the sensor lo<
ion and provide the tr'ip setpoint for your
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facility's Loss of Offsite Power (undervoltage trip) instrumentation.
Include the basis for your trip setpoint selection.
f.
Assuming operation on offsite power and degradation of the grid system voltage, provide the voltage values at the safety related buses corresponding to the maximum value of grid voltage and the degraded grid voltage corresponding to the undervoltage trip setpoint.
g.
Utilizing the safety related bus voltag' values identified in (f),
e evaluate the capability of all safety related loads, including related control circuitry and instrumentation, to perfom their safety functions.
Include a definition of the voltage range over which the safety related components, and non-safety components, can operate continuously in the perfomance of their design function.
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Describe the bus voltage monitoring and abnormal voltage alarms available in the control room.
2.
The functional safety requirement of the undervoltage trip is to detect the loss of offsite (preferred) power system voltage and initiate the necessary actions required to transfer safety related buses to the onsite power system.
Describe the load shedding feature of your design (required prior to transfering to the onsite [ diesel generator]
systems) and the capability of the onsite systems to perform their function if the load shedding feature is maintained after the diesel generators are connected to their respective safety buses.
Describe the bases (if any) for retention or reinstatement of the load shedding function after the diesel generators are connected to their respective buses.
3.
Define the facility operating limits (real' and reactive power, voltage, frequency and other) established by the grid stability analyses cited in the FSAR.
Describe the operating procedures or other provisions presently in effect for assuring that your facility is being operated within these limits.
4.
Provide a description of any proposed actions or modifications to your facility based on the results of the analyses performed in response to items 1-3 above.
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