ML19319E182

Requests Info Re Facility Vulnerability to Plant Operation & Equipment Failures During Degraded Grid Voltage Conditions Similar to Millstone 2 Jul 1976 Occurrence.Description of Millstone 2 Event & Request for Addl Info Encl

ML19319E182
Person / Time
Site:	Rancho Seco, Millstone
Issue date:	08/12/1976
From:	Reid R Office of Nuclear Reactor Regulation
To:	Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT
References
TAC-10968, NUDOCS 8003310698
Download: ML19319E182 (5)

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Docket No'. 50-312.

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Sacramento Municipal Utility District

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ATTN: Mr. 'J.lJ.,Matticoe

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- Assistant General Manager

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.and~ Chief' Engineer 6201 S Street 1

P. O. Box,15830 - ~

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Sacramento, California ' 95813

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. Gentl emen:

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- Provided 'hereis a's Ynclosur}e11'.is a de',cription of evdnts w

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foccurred at Hillstone" Uni.t No.- ~ 2 'during" July 1976L relating to.4 :n MM.9%.a

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plant operation and, equipment failures during-aldegraded grid?E a$$bOr vol tage condition.. '

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f 10n:" July 27',l19766 all; utilities with'operatinga reactor facilities- '~'

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' received telenhone notification from the tRC of the events at the

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Tiillstone Unit %. 2 facility.

At that time members of your staff

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. were asked to investigate ;the ' vulnerability of.your facility to:

~ similar: deyaded voltagefconditionsand :to provide a 'respon'se byi telephone witnin 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

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As ai result of our'.iditial.. investigation and' evaluation of the' %

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ipotential.igeneric :inolicatins 'of.. the events at Millstone andiouri.gD.f:Og

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rpreliminiry discussions wit.c severalclicensees, we co. n.si. der sit. :. ~

-M Hnecessary ;to require;all: onfating;reactorrlicensees to conductfe.Qa g@g.m x

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~ thoroughLevaluation of the ;prohlta and toj submit?fornal renorts.;-

< ~" T Therefore, we request tha t you conduct 'an-investinatier' of the

~ issue as.f t affects your facility-using the. Tequet:for Infornation

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T fdetailtd-17 Encinshre J2 ts a cuide, and provide the analyses ~and resul ts vi tnirO3'i days of your receipt of this. letter..

Tse-sf goed original-ami 39 conies of vcur response will ne nec'essary.

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Sacramento ibnicipal Utility District [

a This reouest for generic infomation wa:: anproved by rM under a blanket clearance number B-180225 (R0072); this clearance expires July 31,-1977.

1 Sincerely, Robert W. Rela, Chief Operating Reactors Branch 44 1-Division of Coerating Reactors I

Enclosures:

DISTRIBUTION:

1.

Description of. Events Docket Millstone Unit No. 2 '

NRC PDR 2.-

Request for Information Local POR I

.l ORB-4 Reading cc w/encls:

VStello David S. Kaplan,' Secretary and

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General ' Counsel..

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M 6201 S-Street ;... 1 M N.J

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~Sacramerito, California 95813 RIngram Post 'Of fice Box ;15830.' ~

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Attorney, OELD

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Bissiness and Hunicipal Departnent OI&E (5)

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l Sacramento City-County L1brary DEisenhut 828.I Street -

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' Sacramento, California ; 95314-EVerdery

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ENCLOSURE NO. 1

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' DESCRIPTION ~0F' EVENTS MILLSTONE UNIT NO. 2 On July 20, 1976, NortheastNuclearEnergyCompa'ny(NNECO) reported that, follo' wing a trip of Millstone Unit No. 2 on July 5,1976, several motorspoweredfrom480 volt (v)motorcontrolcentersfailedtostart 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 the controller.

NNECO's investigation disclosed that, as a result of the plant trip, the grid voltage dropped from352 kv to 333 ky.

This voltage drop, in conjunction with additional voltage drops associated with the trar,sformers involved, reduced the control power and voltage within individual 480 y controllers to a voltage which was insufficient to l

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.As a resu t, w en t e actuate the main line controller contactors.

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.

NNECO concluded that under s'imilar low voltage conditions, the opera-bility of 480 v Engineered Safety Feature equipment could not be assured.

NNECO's immedia'te 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 er,irgency 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 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 was no longer considered appropriate because during starting of a circulating water pump, the voltage dropped below the new ESAS under-voltage relay setting.

This de-energized the emergency buses, caused

.L load shedding to occur, started the diesel generators and began sequencing

_ loads onto the emergency buses in accordance with the design.

Howev~er, 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 not1,oads supplied.

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REQUEST FOR INFORMATION 1.. Evaluate the design of your facility's Class IE electrical distribution system to detemine if the operability of' safety related equipnent, including associated control circuitry or instrumentation, can be adversely affected by short tenn or long tem 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 liniited to the following:

a.

Describe the plant conditions under which the plant auxiliary systems (safety related and non-safety related) will be suppliud by offsite power.

Include an estimate of the fraction of normal plant operating time in which this is the case, b.

The voltage used to describe the grid distribu' fon system is usually t

a " nominal" value.

Define the nomal operating range of your grid system voltage and the corresponding voltage values at the safety related beses.

c.

The transfomers utilized in power systems for providing the required voltage at the various system distributina levels are nomally provided with taps to allow voltage adjunment.

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 generatcr, provide the voltage profiles at the safety buses for grid voltage at the nomal maximum value, the normal minimum value, and at the degraded conditions (high or 1ow voltage, current, etc.) which would require generator trip.

e.

Identify the sensor location and provide the trip 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.

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g.. Utilizing the safety related bus voltage values identified in (f),

evaluate the capability of all _ safety related loads, including related

' control circui_try 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 performance of their de-ign function.

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Describe the bus voltage monitoring and abnonnal 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]

systens) and the capability of the onsite systems to perfonn 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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