ML20237G859

Forwards Justification to Sustain Continued Use of Existing Plant Switchgear in Support of Unit 2 Restart.Util Will Update FSAR to Address Most Recently Calculated Medium Voltage Sys Fault Levels.List of Commitments in Ltr Encl

ML20237G859
Person / Time
Site:	Sequoyah
Issue date:	08/10/1987
From:	Gridley R TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
NUDOCS 8708140223
Download: ML20237G859 (14)

Text

{

TENNESSEE VALLEY AUTHORITY CH ATTANOOG A. TENNESSEE 37401 SN 157B Lookout place 3

AUG 101987 l

U.S. Nuclear Regulatory Commission l

ATTN: Document Control Desk Washingi m, D.C.

20555 j

Gentlemen:

In the Matter of

)

Docket Nos. 50-327 Tennessee Valley Authority

)

50-328 SEQUOYAH NUCLEAR PLANT (SQN) - COMMENTS ON SAFETY EVALUATION REPORT (SER) -

l SHORT-CIRCUIT STUDY, MEDIUM VOLTAGE (6.9-kV) SYSTEM

)

References:

1.

Memorandum from Charles E. Rossi to John A. Zwolinski dated April 2, 1987, " Evaluation of Sequoyah Units 1 and 2 Short-Circuit Study-Medium Voltage (6.9KV) System" 2.

Letter from R. L. Gridley to B. J. Youngblood dated I

June 12, 1987, "Sequoyah Nuclear Plant, Electrical calculations - Revised Final Status Report" During a recent site inspection, an NRC inspector provided TVA with a copy of the referenced memorandum (reference 1).

This memorandum transmits the " Technical Evaluation Report of TVA Sequoyah Electrical l

Medium Voltage Short Circuit Analysis Calculations" (SAIC-87/3030) and the draft SER on the short-circuit study, medium voltage system.

Both documents substantiate that "the calculational methods and assumptions used by TVA for determining the available short circuit currents in the 6.9KV electrical supply system are consistent with industry standards and practices." However, both documents express the concern that ".

. the installed electrical protective devices (6.9KV circuit breakers) are undersized for the available short circuit currents.

As a result i

of a failure.

. to interrupt the maximum available fault current, the

'd 6.9KV Class 1E or non-Class 1E switchgear may fail catastrophically and could possibly involve cascading failure of the normal and alternate supply sources." NRC has identified potential concerns in three areas:

acceptability of testing used to establish breaker interrupting capability, actual breaker rating, and potential for fault propagation.

This information was prepared as a result of an NRC audit conducted on March 2-4, 1987, in Knoxville. The negative conclusions are in conflict with the information presented during the exit meeting.

TVA would also like to indicate that these calculations were also reviewed by NRC during audits in January 1986 and February 1987. These calculations and a discussion of the identified problems with the unit board breakers were submitted to NRC in reference 2.

TVA would like to take this opportunity to restate its position to ensure that all concerns are properly f

addressed.

g 8700140223 h h 27 PDR ADOCK PDR P

An Equal Opportunity Employer a

,, U.S.' Nuclear Regulatory Commission p(}Q{Q}hh]

.TVA is.providing sufficient justification in enclosures 1, 2, and 3 to sustain the continued.use of the existing plant switchgear in support-of the SQN unit 2 restart.

This information' demonstrates that the unit board feeder breakers and all the shutdown board breakers will interrupt the calculated fault currents.

In addition, the design of the electrical system precludes the propagation of faults that would disable both sources of offsite power or affect other trains of onsite safety-related power..This remains in line with original SQN design to meet and maintain General Design Criteria 17 for the safe shutdown of.the plant.

TVA will update its Final Safety Analysis Report' (FSAR) to address the most recently calculated medium voltage system fault levels. This revision to the FSAR will be completed in the next annual update. A list of commitments made in this letter is included in enclosure 4 Very truly yours, TENNESSEE VALLEY AUTHORITY I

)

R. Gridley, D rector Nuclear Safe and Licensing Enclosures cc (Enclosures):

Mr. G. G. Zech, Assistant Director for Inspection Programs Office of Special Projects U.S.. Nuclear Regulatory Commission 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Mr. J. A. Zwolinski, Assistant Director L

for Projects Division of TVA Projects Office of Special Projects U.S. Nuclear Regulatory Commission 4350 East West Highway EWW 322 Bethesda, Maryland 20814 Sequoyah Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379

ENCLOSURE 1 BREAKER CAPABILITIES JUSTIFICATION, GOULD-BROWN BOVERI CORRESPONDENCE, AND 6.9-kV BREAKER TEST DATA NRC expressed a concern in reference 1 that the vendor tests were performed on a new circuit breaker that was not representative of SQN's existing 14-year-old equipment. Furthermore, NRC stated that the test voltage used was too low.

Thus, i

NRC has concluded that "The staff cannot accept a one time test which is not accepted as a certified breaker rating apon which rests the qualification of a nuclear power plant's power supply system which is necessary for the safe shutdown j

of the plant under normal, transient, and accident conditions."

1 Although the vendor test was performed on a new breaker, this breaker had been conditioned through a battery of required testing before successfully interrupting a 550-MVA fault.

This pretesting is tabulated below and accounts for breaker

)

aging:

i 1.

Close/Open - 15 seconds

- Close/Open (8.47 kV/35.5 kA/521 MVA)

- (8.43 kV/35.5 kA/518 MVA) 2.

Close/Open - 15 seconds

- Close/Open (6.84 kV/44.8 kA/531 MVA)

- (6.95 kV/45.7 kA/550 MVA) i Based upon vendor test and documentation TVA is recognizing a demonstrated one-time interrupting capebility for this breaker.

Design tests show four l

successful successive interruptions abcVe 500 MVA.

Note that breaker maintenance was ' performed after each of the two test series. At voltages as high as 8.47 kV, 521 MVA was interrupted. This voltage is well above the 7.26-kV marimum voltage at SQN.

These breaker tests adequately bound all SQN 6.9-kV shutdown board conditions for both the feeder and load breakers with respect to the voltage levels and breaker interrupting requirements. Only the 6.9-kV unit board load breakers are not within these breaker test boundaries.

TVA will commit to develop a schedule to correct this problem of the 6.9-kV unit board load breakers after unit 2 startup.

Refer to the manufacturer's letter, included within this enclosure, for confirmation that the tests performed will substantiate that these breakers are adequate for SQN application with the stipulation that maintenance be performed after any 550-MVA interruption.

Such maintenance would most likely include the l

replacement of the breaker contacts and are chutes.

In addition, SQN Maintenance Instruction (MI)-10.4 calls for complete disassembly and inspection every five years or immediately following a multiphase fault interrupted by a 6.9-kV breaker.

Furthermore, fault currents in excess of the breakers demonstrated capacity of I

550-MVA can only occur during normal operation, while the unit station service transformers (USSTs) and the main generator are connected. When shutdown of the plant occurs through the common station service transformers (CSSTs), the maximum available fault at the 6.9-kV unit boards is 541 MVA and at the 6.9-kV shutdown boards is 416 MVA.

This is within the breaker's demonstrated interrupting capacity, i

j

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4:52 EEE-CENTRA l STAFF

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• C t 5 J. r gw.c.ne mitM 001 til E N 2.DS l92R F-M.V EtB 801107 O ll s

November 6,1980

. APS fault AN ALYS1S J

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• Tennessee Valley Authority

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WBC126 Connerciel Realty Management Bldg.

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400 Comerce Avenue Knoxville. Tennessee 37902

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ttention:

F.iW. Chandler, Chief Electrical Engineering Branch l

3

Subject:

Sequoyah Nuclear Plant, Units 1 & 2 l

-6900 Volt Switchgear TVA Contract.

71C2-54499

'i

~

, Gould-Brown Boveri S.O.

37-47035,

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Referent,e:

TVA Letter dated December 14, 1979

[

g 8

Gentlemen:

i You have requested a written response concerning the application of our 7.5HK500 circuit breaker at 7.17KV and 44.18KA.

These values compute to 549MVA, which exceeds the MVA rating specified for this breaker, as we all,,know, and consequently we cannot certify this value as a ' rating.

We do, however, have two (2) interrupting tests at 4.4_JNA and 45.7KA which we can fall back on to support that the breaker can handG3s-

~

amount of current.

~-

l A review of the application test data indicates that the breaker contacts ~

were heavily eroded and that the are chutes were at the ultimate limit of l

'their capacity and life expectancy at the conclusion of the tests.

Interrupting the 550*1VA in test report R-S253D, Sheet 6 of 6, demonstrated -

a capability of this circuit breaker to interrupt 550MVA satisfactory'il v.a,. am sm'.

The interrupting performance, during this test, de7mo.strates capability of L

interrupting the 5492-!VA fault level at TVA.

There is no probler uf et&

switchgear cepability hou:ing these breakers to withstand these shock s 'g1 e gircui_t__fah.es.

4 A,.,, a %

w c:

7.he cirgrit bmket must_be $nsm.t.ed i::ine.diAte.ly After interrupting 0 TS~see - close/open operation at _tinis f3 ult ley.e_1, He'aTse tWe'~i5rcui d, '

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breeteg fgj_J=._s.l.te.r.p_CsuffieTEh51Ty t:r remiiie maintenance.-"ThC_53ch =. w.;_x,'. -

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Ti-~the :::aximum fault capsM1ity ever Yc~mb7is'tPa5d liy IMs~ circuit'bitM aW 57

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-/ UUULU BHUWN 130VEF:

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M *V C,!g 4 g

. Tennessee Valley Authority F. W. Chcndler, Chief APs FAULT ANALYSIS 71C2-54499 l

37-47035

[...,

November 6, 1980

)

j y,.

1 s s In the course of our investigations, we have discovered a misnomer,ip Appendix C on this shop order.

l.f. rated short circuit current, KA-rms shows 40 KA.

This is the nominal rating for the nominal 5000/A class listed.

We find however that a close investigation of ANSI C37.06 tabic shows that the rated short circuit current corresponds to the maximum voltage. XV-r:qs -

i which ANSI lists as 33KA and which we list as 35KA.

In view of this.

I the 40KA should read 35KA. par.1.f

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k'

Atta' ched pleese find copy of Bulletin 8.2-1E.

Pages 42, 43 and 44 cover a thorough discussion on " Class" symmetrical and "true" symmetrical rating and shows the comparative table.

l Very truiy yours, GOU1.D-BP.0MN B0VEP.I

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R &.T.w /

se r, y

Chester E. Wood s.

Senior Electrical Sale.s Engineer CEW:mf J

En. closure n' /7/80 - Fuc:Isu cc: IEDS, Eh337 C-K I L Cer.t:cu, 204 G3-K (2)

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ENCLOSURE 2

' BREAKER RATING JUSTIFICATION (1) Asymmetrical, " Class"' Symmetrical, and.

"True" Symmetrical Interrupting' Ratings 1

(2) Guaranteed Performance Data l

i NRC also stated that the circuit breakers at SQN are not rated 500 MVA but k

rather 471 MVA in accordance with American National Standards Institute J

(ANSI) C37.06-1964.

This ANSI standard of breaker ratings lists nominal MVA class ratings that exceed the hVA' calculated from rated voltage and current.

Therefore, a breaker may be designated a 500-MVA class, even though it can only achieve'a 471-MVA symmetrical rating. Please refer to the enclosed information for a discussion of the differences between asymmetrical " class" symmetrical, and "true" symmetrical interrupting ratings. This information clearly demonstrates the superiority of Gould-Brown Boveri (GBB) ratings as compared with the present ANSI standards since CBB rates its breakers on a true symmetrical basis. Further, in accordance with.the guaranteed performance contract data for SQN's 7.5HK500 breakers also included within this enclosure, GBB certified that the breakers could interrupt 44 kA at 6.9 kV (i.e., 525 MVA).

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07/02/1987 14t56 EEE-CENTAL STA:F

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LET'S EXAMINE THE DIFFERENCE BETWEEN ASYMMETRICAL'," CLASS" SYMMETRICAL AND "TRUE" SYMMETRICAL INTERRUPTING RATINGS.

Until mld 1964, ANSI standards called for asymmetncel in 1969 ANSI also converted the 15 kV 1000 MVA breaker ratings on all circuit breakers from 5 kV.75 MVA tnrough to a symmetencal current retmg basis.

1$ kV.1000 MVA. To achieve this rating, the circuit breener MVA was calculated by use of the foHowmg formula; To futher comp!lcate matters. ANSI symmettlcal caicula.

Applied Voltage x Highest Asym. Phase Current x tions still ce not fully meet the "true" symmetrical test.

V3 m MVA A compromise method of rating was reached whereby an a

ey m nos been en>d.

This approach led to some misappilcation since it did not

(

duplicate the ccnditions that occur on en actual system.

EXAMPLE: en a 15 kV 500 MVA Breaker:

The recovery voltage on a test circuit might drop consider.

ANSI Stancares Requirement-18.000 Sym.

ably below the applied voltage.'This is contrary to a real Ampere interruptf rig f

system wnere the recovery voltage would be almost equiv.

l.T.E "TRUE" Symmetrical Reauiremont-alent to the applied voltage.1T.E pioneered tne use of 19.300 Sym. Ampere Interrupting recovery voltage (instead of applied voltage) and sym.

When you calculate the MVA rating of the breaker, you see metrical currents (m place of asymmetrical) to calculate that; breaker ratings, and in 1959 began to test all cf its 15 kV x 19,300 x V3 = 500 MVA -This is a full breakers on this basis, in all tests, l.T E used methods symmetrical rating which meets the "TRUE" symmet.

that produced recovery Voltages equal to applied voltages, rical test Following this aporoach, l.T.E used the following formula EUT.

to calculate its rating.

15 kV x 18.000 x V3 = 468 MVA-.-This is allthat Recovery Vettege x Average Sym. Current x la recuired by ANSI to call this a 500 MVA breaker.

Y3 = MVA Because the ANS1 requirement does not meet a full sym-

• tt is now easy to see that when tnese two methods are metrical MVA rating. ANSI has intreouced a new term to compared, the 8-T.E rating and the ANSI rating were con.

tne industry called t"C4ess" MVA.

siderably different, lT.E circuit broskers had a greart plus Therefore, a breaker rnay be mesignated a 500 MVA "claas"

\\

value and tne appilcation to the system to be protected

,ven trrostgnit termot artsnrve a **trus** tull $CD MVA sym.

was constdorably simplified.

metrical ratmg.

Let's compare with the following example:

17.E is tne only manufacturer to rate its breaners on the Test generator voltage (apphed)

..... 17.2 kV

..true" symmerneal basis.

Recovery voltage (normal frecue icy)

. 25.0 kV Short.circutt current: Pnase A.

... 26.5 kA he Mowmg table demonstrates the superiority of I.T.E Pne s e C,.,..... 21.2 kA. ratings compared with,the present ANSI stand Pne se B........... 25.5 kA ANSI " CLASS" $YMMETRICAL STANDARD VS.

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Average a.c component (symmetrical)....21.2 kA 1.T.E "TRUE" SYMMETRICAL STANDARD Total current (asymmetr eal).....

. '. 26.6 k A Assi cuesu,, uv$,,

From the following calculation, we will see that a circud n'tYrb iM.

".M l

breaker could be rated 750 MVA on an asymmetnest basre:

Typ.

"U"

^M." ~[,7 lC*g"'8 s ym".

wnersas, l.T.E would only rate the breaker 500 MVA sym.

L,.75 gg g ag 3

g 3

metncal.

sH5250' 2s.000 ac.sco 250 -

• se.t 2ao

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FT-E "trus" MYA = 21.2 k'A X 15.D W x 3.73 3: 550 SHKaso 4 000 42.400 asc ass.c aso

?.SH54co 31.000 35.000 i soo 47:.s sco I

tSHKaco 1a.000 ta tan 500 487.7 aco ANSI sNowebee MVA = 26.6 kA x 17.2 W X 3 73 = 793 N 790

• • nna ts. goo 750 72n 7so

• • 4 coo 27.coe suco seo seu sono MVusymmetncel l'urther, the "true" esb. u ; rating of this broeher (using recovery voltage) should be onsy 690 MVA. La.

Aamember. 8-T-E's are the ceny encuit breekers that fulfit "True" av....a.

MVA = 2dL6 kA x 15.0 W x 1.73 =

the "true".7c..

rating throusnesst the enure fins.

-v 490 MVA Fork-more comoesti~Mr=aEof synaneancal satings in 1964 ANSI converted to a syrrervetncal system with the see bT E Bul6 etic No. A.5-1A.

use et recovery soltage, but oaty on part at the tidi twesear fine. 3 kY75 MVA,5 W.250 MVA and'.5 W.500 MVA-

--. Den in 1968 ttvey added the 5 W.390 MYA. 7.5 kV.SDD

~ ' ' '

MVA and the 15 kV.750 MVA.

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Interrupting time, from energitation of the trip circuit to extinguishment of the crc, et 25 to 100 percent.

interrupting rating with 250-volt d-c i*

control voltage applied, cycles 5

5

<5 1.

Interrupting time, from energization of the trip circuit to extinguishment of the arc, at less then 25 percent interrupting rating with 125-volt d-c 1

control voltage applied, cycles

_5 l

Interrupting tinc, from energization

\\

n.

of the trip circuit to extinguishment-I of the arc, at icsc than 25 percent interrupting rating with 250-volt d-c

)

control voltage applied, cycles

,5 5

5-i n.

Closing time, tahcn frcz instant of f

energizing the closing circuit,"at l

g 123-volt d-e control voltages, cyc1'es 10

o.. Closingtime,takenfromins2nntof cnercir.ing the clocing circuit, at

/

250-volt d-c control voltaces, cycles 10 10 10 i

• p.' Iow-frequency 1-ninute withstand dic1cetric test, kr.-

36 36 36

• q.

Impulse vithstand' level, hv 95 95 95 r.

Interrupting current ce.pacity based on ANSI duty cycle CO-15 soc-CO,

^

s nt6yovolts,apercs

../ mg gp

%K~l'44K-50K j

G-c 1

5. Mcment$.ry ratin;;, c: peres (per total current rating, basis asI c37.6)

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NRC's position that a maximum fault could propagate back to the normal and alternate offsite sources and could potentially disable both power sources is not correct. The following analysis demonstrates this fact.

contains a simplified electrical single-line diagram that can be used to follow the TVA analysis.

1.

For a bolted three-phase fault at a 6.9-kV shutdown board, the maximum available fault level is within the breaker's tested 550-HVA rating.

Thus, the fault would be cleared and not propagated.

2.

For a bolted three-phase fault at a 6.9-kV unit board, the maximum available fault level exceeds the load breaker's interrupting rating of 550 MVA because of motor contribution.

The main board breaker, however, experiences only the source fault contribution that is below the 550-MVA rating. Thus, the main board breaker will clear the fault. This scenario results in loss of power to one 6.9-kV shutdown board. The affected l

shutdown board will subsequently transfer to the associated diesel generator.

The shutdown board alternate source would still be available pending manual operator action to transfer from the diesel generator to the shutdown board's alternate offsite connection.

This scenario would affect no more than one power train and is thus bounded by the single failure criterion and also meets the intent of GDC-17.

In addition, a fault would not be transferred across trains that are separately fed from the other unit's unit station service transformers.

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9 ENCLOSURE 3 ELECTRICAL SINGLE-LINE DIAGRAM l

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ENCLOSURE 4 COMMITME.NTS 1.

TVA will revise the FSAR, in the next annual update, to reflect the most recently calculated medium voltage system fault levels. This will include sections 8.2.1.3 and 8.3.1.1, table 8.2.1-2, and figure 8.3.1-33.

2.

TVA will develop a schedule to correct the fault current problem for the 6.9-kV unit board load breakers after unit 2 rectart.

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