ML19322C617

Forwards Description of Events on 760720 Re Equipment Failures During Degraded Grid Voltage Condition at Millstone Unit 2

ML19322C617
Person / Time
Site:	Crane, Millstone
Issue date:	10/15/1976
From:	Stolz J Office of Nuclear Reactor Regulation
To:	Roe L TOLEDO EDISON CO.
References
TASK-TF, TASK-TMR NUDOCS 8001180120
Download: ML19322C617 (5)

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t. L D R. C. DeYcung IE (3)

ACRs (16)

F. J. Willi'ams

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tiiller J. Stolz

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E. Hylton Docket !!o. 50-346 L. Engle

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'ss caes e toledo Edison Company J. Knight, SS ATTit: Mr. Lowell E. Roe edesco Vice President, Facilities YA" "'

Development H. Denton Edison Plaza

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300 l'.adison Avenue bcc:

J. R. Buchanan, flSIC Toledo, Ohio 43652 T. B. Abernathy, TIC

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Gentieten:

EQUIPMEtiT FAILURES DURI!iG A DEGRADED GRID VOLTAGE CGiiDITIO!! AT MILLST0 tie, UtlIT 2 Provided herein.as Enclosure 1 is a 'de griptiddfjevents.uich4 occurred atfiillstone Unit iio. 2 during< July 1976 relating to plant

, operation and; equipment failuresidur:ing.ajdegraded grid voltage!

. condition..

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27, 1976, all utilities with operating reactor facilities On July received telephone notification from the i!RC of the events at the Millstone Unit tio. 2 facility At that tima members of your staff were asked to investigate the vulnerability of your facility to similar degraded voltage conditions and to provide a response by telephone within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

As a result of our initial investigation and evaluation of the potential generic implications of the events at !!illstone and our preliminary discussions with several licensees, we consider it necessary to require all plants presently in review for an operating license to conduct a thorough evaluation of the problem and to submit fonnal reports. Therefore, we request that you conduct an investi-gation of the issue as it affects your facility using the Request for Infurcation detailed in Enclosure 2 as a guide, and provide the analyses rnd results within 30 days of receipt of this letter or explain why you cannot meet this schedule and provide the schedule you will meet.

EXHIBIT 6 page 1 of 5 The signed original and 39 copies of your response will be necessary.

CCI 15 i976 P00R~0RIGINAL go01188 $ 8

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Toledo Edfson company

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r This request for generic inforretion was approved by GAO under a bla6ket clearance number B-130225 (20072); this clearance expires

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July 31,1977.

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Sincerely, I

prip siped br.

]Q n T* M0h. l.

h John F. Stolz, Chief Light Water Reactors Branch I'o.1 Division of Project ;1anagement

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Enclosures:

1.

Description of Events Millstone Unit tio. 2 2.

Request for Infomation

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Mr. Canald H. Hauser Esq.

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The Cleveland Electric Illuminating Ccapany P. O. Box 5000 Cleveland, Ohio 44101 Gerald Charnoff, Esq.

Shaw, Pitt:.an, Potts and Trowbridge

"" 17th Street, II. W. -

Washington. D. C.

20036 Leslie Henry, Esq.

Fuller, Seney, Henry and Hodge 300 Madison Avenue Toledo, Ohio, 43604 :

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EXHIBIT 6 page 2 cf 5

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DESCRIPTIO[OF' EVENTS

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'HILLSTONE UNIT NO. 2 On July 2b,.1976, Northeast Nuclear Energy CcmpaIiy (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 The failure of the 480 y motors to start was traced to as required.

These blown control pcwer fuses on the individual motor controllers.

controllers receive control power through 480 v/120 y transformers within the controller.

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

in conjunction with additional voltage drops associated with the transfomers involved, reduced the control power and voltage within 1

indiYidual 480 y controllers to a voltage which was insufficient to As a result, when the actuate the main line controller contactors.

notors were signalled to start, the control power fuses were blown.

Subsequent testing by NNECO showed that the contactorst required at least 410 y to function properly.

HNECO concluded that under similar low voltage conditions, the opera-bility of 480 v Engineered Safety Feature equipment could~not be assured.

NNECO's immediate corrective action was to raise the setpoint of the I

Engineered Safeguards Actuation System (ESAS) " loss of power" under-Voltage relays to assure that the plant would be separated from thed before

, grid and emergency power system (dual) operation would be A trip of the undervoltage relays causes the emergency buses to be

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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 bu;es.

NNECO reported that the earlier corrective action taken On July 21, 1976, was no longer considered appropriate because during starting of a circulating water pump, the voltage dropped below the new ESAS under-This de-energized the emergency buses, caused Yoltage relay setting.

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

However, loads onto the emergency buses in accordance with the design.

during sequencing of the loads onto the buses, the voltage again dropped below the undervoltage relay setting which caused the load shed signal The result was energi.ed e.dergency buses with to strip the buses.

no' loads supplied. -

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EXHIBIT 6 page 3 of 5

.e P00R ORGINAL

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IfcUCST T C:s ! N F 0.a P.A T ! C m 1.

Evaluate the design of your facility's Class IE electrical distribition system to detemine if the operability of safety related equipment, including assdciated control circuitry or instrumentation, can be adversely affected by short tem 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 limited to the following:

Describe the plant conditions under which the pla'nt auxiliary systems a.

(safety related and non-safety related) will be supplied by offsite Include an estinate of the fraction of nomal p1 ant power.

operating time in which this is the case.

b.

The voltage used to describe the grid distribution system is usually a " nominal" value. Define the nomal operating range of your grid system voltage and the corresponding voltage values. at the safety related buses.

j c.

The transfomers utilized in power systems for providing the required voltage at the various system distritution levels are nomally 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.

4 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 n'omal value, the nomal minimum value, and at

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the degraded conditis snigh or low voltage, current, etc.) which would require generator trip. ;

y Identify the sensor location and provide tne trip setpoint for your e.

facility's Loss of Offsite. Power (undervoltage trip) instrumentation.

Include the basis for your trip setpoint selection.

f.

Assuning operation on ofYsite 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 rel$ted bus voltage values identified in (f),

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 ccuponents, can operate continuously in the performance of their design function.

EXHIBIT 6 p$ge 4 of 5 G

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Describe the bus vol tage monitoring and abnonnat 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) arid the capability of the onsite systems to perform their function if the lead 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 b& sed on the results of the analyses performed in response to-items 1-3 above.

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P00R~0RIGINAL

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EXHIBIT 6 page 5 of 5 e

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