ML20115H471

Forwards Response to RAI in NRC Ltr to Util Dtd 960327, RAI - Reliability of Offsite Power Study - Sequoyah Nuclear Plant,Units 1 & 2. Justifications That Voltage at Safety Buses Will Not Go Below 5220 Volts,Included

ML20115H471
Person / Time
Site:	Sequoyah
Issue date:	07/17/1996
From:	Shell R TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
TAC-M93319, TAC-M93320, NUDOCS 9607230142
Download: ML20115H471 (7)

Text

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IUA Tennessee Valley Authonty Pos! OMce Bon 2000 Sod @Da>sy. Tennessee 37379 July 17,1996 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk l Washington, D.C. 20555 Gentlemen:

In the Matter of ) Docket Nos. 50-327 Tennessee Valley Authority ) 50-328 SEQUOYAH NUCLEAR PLANT (SON)- RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION - RELIABILITY OF OFFSITE POWER STUDY (TIA 94-021)(TAC i NOS. M93319 AND M93320) 1

Reference:

NRC Letter to TVA dated March 27,1996," Request for Additional Information - Reliability of Offsite Power Study (TIA 94-021)- Sequoyah j Nuclear Plant Units 1 and 2 (TAC Nos. M93319 and M93320)"

The purpose of this letter is to provide NRC with the response to the request for additional information in the referenced letter. By the reference, NRC requested TVA to provide analysis (or other justification) to demonstrate that the immediate access offsite circuits from the transmission network to the Sequoyah facility will remain connected to safety system loads as their source of power for all anticipated transmission network contingencies, including the worst-case contingency (intertie transformer out of service with a postulated loss-of-coolant accident [LOCA) in Unit 2). The enclosure provides TVA's response and includes justifications that voltage at the safety buses will not go below 5520 Volts for greater than the minimum technical specification (TS) limit of one second and this voltage will not go below 6456 Volts without recovery to a voltage above 6595.5 Volts within the (

minimum TS limit of 7.5 seconds. These justifications show that the SON under I voltage protection design will not trip the offsite circuits and sequence the safety loads onto the diesel generators due to transient voltages on the transmission network gp j immediately after trip of Unit 2 due to a LOCA condition. <-

l The calculations referenced in the enclosure are available for NRC review at the SON site.

l 9607230142 960717 PDR ADOCK 05000327 P PDR L

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L U.S. Nuclear Regulatory Commission

- Pa0e 2 July 17,1996 Please direct questions concerning this issue to Keith Weller at (423) 843-7527.

Sincerely, 4M.RW '

R. H. Shell Manager SON Site Licensing Enclosure l

cc (Enclosure): l Mr. R. W. Hernan, Project Manager Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike l

Rockville, Maryland 20852-2739-NRC Resident inspector

, Sequoyah Nuclear Plant -

! 2600 Igou Ferry Road Soddy-Daisy, Tennessee 37379-3624  !

Regional Administrator l U.S. Nuclear Regulatory Commission Region ll' 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323-2711 i

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1 ENCLOSURE i

RELIABILITY OF OFFSITE POWER TO SEQUOYAH NUCLEAR PLANT UNITS 1 AND 2.

I Calculation SON-EEB-MS-T106-0002 evaluates the minimum voltage levels at the shutdown and unit boards utilizing the 153 kV from the offsite power supply. That is, with the intertie transformer out of service, the limiting case is for the 161-kV switchyard going from 166 kV, prior to the event, to 153 kV, after the event I (SON EEB-MS-Tl06-0002,Section 4.15). The load tap changers for the common  !

station service transformers (CSSTs) will be in a boosting position since the CSST l secondary voltage will be dropping below 7,004 volts. Load tap changers are set to maintain 7052 Volts f_48 Volts at the start buses ahead of the unit boards which I supply power to the shutdown boards (SON-EEB-MS-T106-0002,Section 6.1).

Prior to the loss-of-coolant accident (LOCA) event for Unit 2 on the 161-kV system, the steady state voltages for the unit and shutdown boards will be above 100% of

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6,900 rated Volts (SON-EEB-MS-T106-0002, running voltage summary). At T= 0 sec., the board voltages will drop to as low as 5861.6 Volts (SON-EEB-MS-Tl06-0002, l Motor Start Voltage Summary, Source 2, Condition 4; S1 PHASE B at T= 0 sec.). This voltage level is above 5,520 Volts, the 80% loss-of-voltage (LOV) TS trip value. Six seconds later, the board voltages will rise above 96%, lowest value 6641.3 Volts (SON-EEB-MS-Tl06-0002, Motor Start Voltage Summary, Source 2, Condition 4; St PHASE B at T= 6 sec.). The voltage rise value, 6641.3 Volts, is above 6595.5 Volts, the TS maximum voltage reset value.

This value,6641.3 Volts,is above the 6.9 kV shutdown board degraded voltage set point,6456 Volts (or 93.5% of 6900 Volt at timer set point = 9.5 seconds). Also, it l Is above the minimum voltage recovery at six seconds following block start of accident loads, 6599 Volts (SON-EEB-MS-Tl06-0008, Degraded Voltage Protection  !

Calculation).

I l The Transmission Group Study, SON-GRID-STUDY-004, has provided verification for the adequacy of the offsite power supply. These studies include the new 161-kV capacitors at SON. The load flow studies show that the voltage does not drop below the 153-kV required voltage post-event (steady state voltage one-second post-vent).

For a LOCA in Unit 2, and a simultaneous fault in the intertie bank, the 161-kV voltage will drop to 155.7-kV.

Transient stability studies were also done in SON-GRID-STUDY-004 to examine l voltage recovery times. Two types of studies were performed. The first type was a l

LOCA plus another transmission contingency. These were performed to ensure safe shutdown of the units following an accident. The second type was two transmission i contingencies. These were performed to determine whether transmission events

! could accidentally trip the units. In both studies, voltage recovery times were compared to the following relay settings. The reactor coolant pump (RCP) voltage  ;

must recover to 5076 Volts within 38.5 cycles. The shutdown board LOV relays must recover to 5686 Volts within 75 cycles.

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4 For a LOCA and a simultaneous fault in the intertie bank such that the bank is tripped, ;

the 161-kV voltage recovered to the 153-kV levelin 6.25 cycles which is well within any of the relay settings.

For a phase to phase to ground fault with a stuck breaker on the 161-kV system the voltage recovers to the RCP reset level in 34.6 cycles and to the LOV reset level in 35.6 cycles. This does not violate any of the relay settings and would therefore not result in an accidental trip of a generator unit.

SON-GRID-STUDY-006is a further Transmission Group Study in evaluating the adequacy of the SON offsite power supply. The approach is to provide two offsite sources that maintain the SON 161-kV bus voltage during normal system conditions, that is to exceed 153 kV during a simulated LOCA of one unit and a simultaneous transmission contingency.

SON-GRID-STUDY-006 concluded that the SON offsite power supply is adequate with two immediate sources, whenever the two 84 MVAR capacitors connected to the SON 161-kV bus are available. Two immediate sources are possible only if specific system indicators are met. The indicators are SON Unit 2 reactive output. SON 161-kV bus voltage, and the SON 500/161-kVintertie transformer bank reactive flow.

The SON capacitors must be in automatic control mode continuously except when the dispatchers deem it necessary to manually switch the capacitors in order to maintain these system indicators. Furthermore, another load flow study, SON-GRID-STUDY-008, augments SON-GRID-STUDY-006 and provides voltage schedules and operation instructions. This study includes a coincident extreme load forecast for the net system load of 28,797 MW. This study also takes into account the outage of 161-kV switchyard breaker 974 which will be out until the summer of 1997.

Conclusion:

Transmission system studies have been performed, which demonstrate that the 161-kV bus will maintain a minimum voltage of 153 kV, both steady state and during transients. SON-EEB-MS-Tl06-002 demonstrates that the 153-kV bus voltage is adequate to support the required safety features. As documented and illustrated in this response, the offsite power under transient conditions will not cause nuisance tripping / alarms as a result of undervoltage at the 6.9-kV unit boards or 6.9-kV shutdown boards.

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1 500kv switchyard (offsite supply) l  ;

wm w m 500-161kv intertie l

161kv switchyard (offsite' supply)  ;

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wmum CSST A  !

um- um 161-6.9kv l x-winding j l

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'6.9kv Start bus IA -l l

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6.9kV Unit BD 1C l) l)(EMFdecay+earlytimefor undervoltage & timer relays i

I 38.5 cycles)=

I 27 62 UV Relay & Timer .

OO- l l O RCP  !

l 6.9kv Shutdown Bd 1B-B 27D DS SI Degraded voltage & timer O-O n- Re1ay.. na Safety 93.5% 9.5s Injection Contact 80% 1.25s

, -O-O  ;

27T LV Loss of Voltage & Timer Rlys TYPICAL SINGLE LINE SUPPLY TO 6.9KV UNIT & SHDN BOARDS

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OFFSITE VOLTAGE RECOVERY CURVE (IN TERNS OF 6900V SYSTEN):

PHASE-PHASE-GROUND FAULT WITH STUCK BKR SITE VOLTAGE RECOVERY CURVE: LOCA + INTERTIE OUT Unit a G.n.cator will b. g.n.roting T-30 sec

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T-0 T-1 T2 T-3 T-4 T-5 T-6 T-7.5 (166KV) 7114V - -

M1-------------1-----l 7004V - - +- -p - - - - - - - - -t - - - - l l l Y l I

'IC l'a9 6.9KV Shutdown Bd (160KV1 6857V ry i

i Recov.ry low.st volu.l l gg4}y ,_ []f,f elt . Vo l t og. Recovey _ A

_ l fil n Voltage following i I I A' block stortI 6599V I I I I 6595.5V t - - - - - - - - - - - - -t - - - p (153KV) 6557V - - Li 10.g r d.d v t t /

og.I TSpec mox l l y I reset volu. l Sit. Voltog. g Recov.ry 6456V - TT--- - - - - ----

t---

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7 I I l0. eod.d Vol1o9.l 6.9KV Shutdown Bd 9 .5% of 6900V

/ Iowest vatu. l dropout with SIl 5861.6V -- on =orty on TT (TSpec LOV I 7.5 e.c < Tsp.c l timer trip e.1 1l j* _ Tal.25 see ti..r Lood eh.a e.tting) l point . g.s ,,c 3l 80% LOV r...t volu.

5686V l l p- 'l 34-(Dropout at 5520V os

.orly as T=1 e.c) l (73.6% of 6900V) 5076V -a, 4*~ RCP UV res.1 rDeopout valueos at 5022v .or t y os 38.5 cyi LOV recov.ry in 35.6 cy Phos.-Phoe.-Ground foult RCP voltog. recovery i .n 34.6 cy ,

[85KV) 3644V - -

VOLTAGE RECOVERY D I A G R A l1 l

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• OFFSITE VOLTAGE RECOVERY CURVE IIN TERNS OF 6900V SYSTEMn LOCA + 500/161KV INTERTIE FAULT SITE VOLTAGE RECOVERY CURVE: LOCA + INTERTIE DUT Uni 1 2 Generator wiiI b. 9.n roting T 30 see

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T-0 T-1 T-2 T3 T-4 T-5 T-6 T-7.5 (166KV) 7114V -

E l- - - - - - - - - - - - - - L - - - _ _ l 7004V , - - t- + - - -t - - - - -l I I

• I I I LTC Boosting 6.9KV Shutdown Bd l I I a.cov.cy to...t voio.1 1 6641V - -.L_L _ _ _ _ _ _ _ _ _ _ _ _'_^ fl i n Voltog. rol l

l l Voltog. recovery at 6.25 cy l / b lock . tor t l low i ng 6599V g l  ;

6595.5V -

t---------- -

-t - - - - p I155.7KV) 6673V r  :

(l53KV1 6557V I Iy\0 rr.it. Voitog.

tsp., ". ' /** I I I R cov.ry l r...t valu.

y\

g g Sit. Voltog. g Recovery l 6456V - TT--- i t-l l 10.groded VoItog.l

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6.9KV Shutdown Bd 93.5% or 6900V

/ tow..t votu. Ideopout with 51l 5861.6V a' =ar l y aa TT T=1.25 .e (TSpee LOV 1 7. t ... ITSp.c 1 t i .r trip ..t ll l4 Load sh.d 16..r ..tting) lpo i n t = 9.5 ..c Il 80X LOV r...t volu.

5686V -- - b 14 10r opout at 5520V l l

o. .orty o. T=1 ..ol l

RCP UV re..t volu. (73.6% or 6900V) i 5076V - -4 *~ l Dr op ou t ,t 5022V a. .orty o. 38.5 eyl l (109KV) 4671V - ---

VOLTAGE RECOVERY DIAGRAM