ML18026B220

Forwards Response to 840619 Request for Addl Info Re Adequacy of Station Electric Distribution Sys Voltages. Analysis Performed Showed Expected Voltages Will Not Result in Shortening of Svc Life for 480-volt Equipment

ML18026B220
Person / Time
Site:	Browns Ferry
Issue date:	09/18/1984
From:	Mills L TENNESSEE VALLEY AUTHORITY
To:	Harold Denton Office of Nuclear Reactor Regulation
References
NUDOCS 8409240210
Download: ML18026B220 (9)

Text

'REGULATORY NFORMATION DISTRIBUTION S EM (RIDS)

'ACC'ESSI ON NBR: 8409240210 DOO ~ DATEJ 84/09/1P NOTARIZED: YES, DOCKET< 0 FACIL!50 259 Browns Fei ry Nucle'ar Power Stations 'Unit 1< Tennessee'5000259

'50 260 Browns Ferry Nuclear.Power Stations Unit 2< Tennessee 05000260

'50~296 Browns Ferry Nuclear Power Stations Unit-3~

Tennessee 05000296 AUTH,NAME AUTHOR AFF II.IATION MILLS'pL', M ~

Tennessee'alley.

Authority RECIP ~ NAME RECIPIENT AFFII;IATION, DENTONzHeRi Office'f Nuclear Reactor'Regulationi Director>>,

SUBJECT For, wards response "to 840619 request for addi-infa re>>

adequately, of station electric distribution sys= voltages Analysis.,performed showed expected >>voltages will,not result.

in shortening of svc life for 480 volt equipment.

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'P TENNESSEE VALLEYAUTHORITY CHATTANOOGA. TENNESSEE 3740'I 400 Chestnut Street Tower II September

'l8, 1984 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C. 20555

Dear Mr. Denton:

In the Matter of the Tennessee Valley Authority Docket Nos. 50-259 50-260 50-296 By letter from D. B. Vassallo to H. G.

Par ris dated June 19,

1984, we were requested to provide additional. information regar ding adequacy of station electric distribution system voltages for the Browns Ferry Nuclear Plant.

Specifically requested was that we review a potential over-voltage condition and provide an assessment of in-plant voltages, discussion of proposed resolution, and a schedule for submitting design information on any-needed modifications.

Our response to the request is enclosed.

As explained in more detail in 'the'enclosure, the analyses we performed showed that the expected voltages will not result in a shortening of the service life for the 480-Volt equipment.

Consequently, no modifications are needed.

However, we plan to perform additional evaluations on methods to reduce the voltage level when power is supplied from the 161-kV system.

If you need additional information, please get in touch with us through the Br owns Ferry Project Manager.

Very truly yours, j

Subscribed Mtd sworn to

-'me this da f

efore TENNESSEE VALLEY AUTHORITY L.

. Mills, Ma ager Nuclear Licensing 1984.

Notary Public My Commission Expires Enclosur e 84092402i0 8409i8 PDR ADOCK 05000259 P

PDR An Equal Opportunity Employer U

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Mr. Har old R. Denton September 18, 1984 cc (Enclosure);

U.S. Nuclear Regulatory Commission Region II ATTN:

James P. O'Reilly, Regional Administrator 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Mr. R. J. Clark Browns Ferry Project Manager U.S. Nuclear Regulatory Commission 7920 Norfolk Avenue

Bethesda, Maryland 20814

0

ENCLOSURE

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RESPONSE

TO REQUEST FOR ADDITIONAL INFORMATION REGARDING ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM UOLTAGES BROWNS FERRY NUCLEAR PLANT

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"Show that the 480U equipment (by locking at the most limiting equipment:)

will not have its service unjustifiably shortened by the described overvoltage conditicns so as to cause failure during a later accident situatiat Resocnse There is no problem with any of the 480V equipment because none of the equipment will be overheated.

The equipment involved includes 480V switchgear, 480V motor control centers (MCCs) with ac control, and motors.

The 4160 480U transformers will also be discussed.

480-V Switchgear Switchgear applied at 480V is rated 600V, thus voltage stress is not a

concern.

The controls are dc, which is not affected by the ac voltage.

480U MCCs MCCs applied at 480U are rated 600U.

The controls are supplied cn an individual circuit basis by 480-120V control pmer transformers (CPTs).

Electric industrial type control equipment is designed to operate at 10 percent above and bel mr rated voltage (per NEMA Publicaticn ICS 2-Part ICS 2-110.41.).

The control transformer is rated for 480V primary voltage

+ 10 percent.

Thus the controls will perform within rating when the voltage is 528V (480

+ 10 percent).

With any increase in voltage the control devices will draw an increased current.

Hmrever, the CPTs have continuous current ratings considerably higher than the continuous load current.

For example GE typically furnishes for a size 1 starter a 60 VA (0.5 A for 120V control) control transformer where the coil requires 24 VA (0.2 A).

The actual coil voltage of 133.25V (533 turn ratio of CPT) is cnly 1 percent above the rated

+10 percent voltage) this would cnly increase the I2 R loss coil by 2 percent.

This is inconsequential, particularly since rio allowance was made in the study for voltage drop between the 4160-480V transformer and the MCC.

Mat sts Refer to the attached table.

For a voltage variation of +20 percent above rated voltage, the general effect cn inducticn-moor characteristics is to increase torque by 44

percent, decrease full Road current by 11 percent, and decrease the full load temperature rise by 5 "6 C.

Since the temperature is decreased, there would be no damage to the motor insulaticn.

While the values stated are

general, they do indi.cate there should be no over heating problem due to

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overvoltage.

These figures would be scmewhat lower for 15 percent overvoltage (above naninal rating) which is cnly 5 percent above the maximum voltage for which motors are designed to operate per NEM standard NG-1.

'I In the unlikely event that a 480V motor-operated valve operates while the voltage is high, its locked rotor current will be correspondingly high.

The valve load inertia reflected through the gears will appear high to the motor.

To be conservative i.t will be assumed that the current during the short

"-travel time is locked rotor current (i.e.,'"the current is proporticnal to voltage rather than essentially inversely proportional as in the above case of ccntinuous cperaticn motors where the load torque varies as a power of speed).

The rate of I R heating in the motor would increase as the square of voltage.

But the torque should also increase as the voltage squared resulting in a corresponding decrease in operating time.

Thus, the I t heating effect would not be increased above that for rated voltage during the short operaticn time.

4160-480V Transformers The analysis requested by NRC only covered 480V equipment.

However, the source of the overvoltage cn the 480V system, (i.e., high voltage in the 161-kV system) could cause sane saturaticn effects in the transformer which could cause more heating than nonsaturation if all other conditions were the same.

However, the other condi;ticns are not the same.

The subject 480V system overvoltage questicn is predicated upon the condition of light load.

Thus the reducticn in transformer heating frcm load loss, which is the major heat source and is proportimal to the square of the load current, offsets any

-effect of heating due to saturaticn.

Conclusions The above analyses shows that the expected voltages will not result in a shortening of theervice life for the 480-Volt equipment.

Consequently, no modifications are needed.

However, TUA plans to perform additional evaluations on methods to reduce the voltage level when power is supplied from the 161-. kV system.

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