ML20041D810

Responds to O Chopra Power Sys Questions.Method of Calculation of Min Sys Voltage Valid,Therefore,Calculated Min Offsite Voltage Was Accurate.Procedure Followed in Determining Min Offsite Voltage Encl

ML20041D810
Person / Time
Site:	Summer
Issue date:	03/01/1982
From:	Nichols T SOUTH CAROLINA ELECTRIC & GAS CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
NUDOCS 8203090252
Download: ML20041D810 (6)

Text

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e A south CAROLINA ELECTRIC a gas COMPANY g cost o,nce som re. E CotuMeA, SOUTH CARollNA 29218

%D T C.NicwoLs.Jn. b "

v.a ema 4c=g,nc~ March 1, 1982 2 m 081982" !

Mr. Harold R. Denton, Director #

Office of Nuclear Reactor Regulation aW h susar U. S. Nuclear Regulatory Conmission 'b 9 /

Washington, D.C. 20555 1G

Subject:

Virgil C. Sunmer Nuclear Station Docket No. 50/395 Power System Questions

Dear Mr. Denton:

This response from South Carolina Electric and Gas Capany is in regard to conments from the Nuclear Regulatory Conmission Power Systems Reviewer, Om Chopra.

The following method was used to determine the minimum permissible offsite voltage at which the system will operate satisfactorily. First, it was determined that maximum loads existed under accident load conditions. Motor terminal voltages were calculated assuming 100% offsite voltage. This required the calculation of the voltage drops of all parts of the system including bus ducts, transformers, feeder cables, etc. The notor which experienced the lowest terminal voltage was the Controlled Access Cooling Supply Fan #A, fed from ICC #XMC1DA2X. The terminal voltage of this motor was set equal to the mininum permissible notor terminal voltage, 90% of rated voltage (460). To this voltage, voltage drops of the various parts of the systen from the notor terminals to the l

primary side of the offsite power transformers were added to determine the mininum permissible offsite voltage.

To ensure that this method of calculation was valid, a test was performed under which the engineered safety feature (ESP) buses were carrying various loads. Cmparisons between the calculated and measured voltages were very close. This proved that the method of calculation of the minimum system voltage was valid, and therfore, the calculated minimum offsite voltage was an accurate value.

An undervoltage scheme for the 120 volt class lE vital AC system is not rquira3 The system consists of four redundant channels.

Channel I and III consist of two buses each and Channel II and IV consist of one bus each. Each bus is normally powered from a dual input (AC and DC) inverter. The preferred source to the inverter is a 480 VAC Class lE Motor Control Center. The backup source is two 120 volt Class lE batteries, one for Channel I.and III inverters and one for Channel II and IV inverters. When the tc input to the inverters is lost, a solid state auctioneering circuit automatically switches to the DC source without any output voltage degradation or interruption. Since the batteries are kept fully charged during plant operation, the inverter output voltage can be maintained for a

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8203090252 820301 PDR ADOCK 05000395 Sk A PDR

Mr. Ilarold R. Derton March 1, 1982 Page 2 minimum period of two hours following a total loss of AC power. In addition to the inverter, each bus can be powered from one of two 120 volt vital AC transformers for mintenance purposes. Ioss of any one vital AC bus does not initiate a reactor trip, but an alarm in the control room warns the operator so that corrective action can be taken.

. A concern was expressed that we may have unnecessary diesel generator starts due to voltage drops caused by starting large motors on the ESF buses. During hot functional testing, various loads including essential motors were placed on the ESF buses, and no degraded voltage actuations were initiated.

If you require any additional information, please let us know.

Very truly yours,

/ A , j T. C. Nichols, Jr.

DII:'ICN:lkb cc: V. C. Sumner (w/o attach.)

G. II. Fischer (w/o attach.)

11. N. Cyrus T. C. Nichols, Jr. (w/o attach.)

M. B. Whitaker, Jr.

J. P. O'Reilly

11. T. Babb D. A. Natman C. L. Ligon (NSRC)

W. A. Willians, Jr.

R. B. Clary O. S. Bradham A. R. Koon M. N. Browne G. J. Braddick J. C. Ruoff

! J. L. Skolds J. B. Knotts B. A. Bursey NECF File

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4 rnoczuunn roLLowna In DETrnaIuluc 1;

.. THE M7t!IMUM PEFF.ISSIBI.E OFFBITE VOLTAGE

.s In determining the minimum permissibie .of fcite voltage the .hM following steps were taken: , -Q A. A calculation was pcrformed using. the following . method: 'j

1. Conditions under which synt'em carries maximum load- . Q'1 wore determincd. Accident----2oad

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2. Ausuming 100% offsite voltage, all motor terminal 1sE"

. .. . ..... volt 49c s woro calculated. - This required the calculation of the voltage . drops of all parte of [

the system including the bus duct, transformers,

. . . . . . . . . . . . _ _ . - feeder cables, etc. The ' motor which experienced 2 the lowcet terminal voltage was determined. This motor was the Contro11od . Access cooling Supply .

Fan "A", fed from Motor Control Center XMC1DA2X'.

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3. The terrninal voltage of :the worst case motor -

(determined above) was set equal to the minimum permicnible motor terminal voltage, 90% of rated (90% of 460) . To this voltage, all voltage drops of the various parts: of the system from the motor terminals to the primary side of the offsite powcr transformer.i were added, to actor-mine the minimum permissible off aite voltage.

N'ote that these' voltage drops were ad-lusted for reduced voltage conditions. At reduced voltage

,--- the current was higher and consequently the voltage drop was higher.

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B. To ensure that the method of; calculation was veliv the following action was taken:

1. A test was performed at tiie v. c. Summer Sito

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under which all ESP buses worge carrying various amounts of load. . In the , test all buc loads and-voltages, and the offsite. voltage wire measurco and recorded.

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throughottt the 87 stem hpplying the sme stethod i of analysis as that of part.A above. The cniculated

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