ML20140C592

Forwards Response to 970117 RAI Re Details on Analysis That Evaluates Adequacy of Offsite Power Sys.Analysis Supports SQN Operation & Accident Mitigation & Methods Used to Keep SQN Personnel Aware of Offsite Power Sys Condition

ML20140C592
Person / Time
Site:	Sequoyah
Issue date:	06/02/1997
From:	Shell R TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
TAC-M93319, TAC-M93320, NUDOCS 9706090335
Download: ML20140C592 (5)

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Tennessee Valley Authority. Post Office Box 2000, Soddy Daisy, Tennessee 37379-2000 June 2,1997 U.S. Nuclear Regulatory Commission  !

ATTN: Document Control Desk 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 REGARDING RELIABILITY OF OFFSITE POWER Si STEM (TAC NOS. M93319 AND M93320)

Reference:

NRC letter to TVA dated January 17,1997, " Request for Additional information - Reliability of Offsite Power System - Sequoyah Nuclear Plant Units 1 and 2 (TAC Nos. M93319 and M93320)

The enclosure to this letter provides the additionalinformation requested in the referenced letter. This information provides details on the analysis that evaluates the adequacy of the offsite power system to support SON operation and accident rn*:gation and the methods used to keep the SON operation personnel aware of the offsite power system condition.

Please direct questions concerning this issue to Keith Weller at (423) 843-7527.

Sincerely, f b, k }

R. H. Shell Site Licensing and Industry Affairs Manager h

Enclosure cc: See page 2 COO b-lilll{lilllll}ll!EE!E@I 9706090335 970602 PDR ADOCK 05000327 '

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l U.S. Nuclear Regulatory Commission Page 2 June 2,1997 cc (Enclosure):

Mr. R. W. Hernan, Project Manager l Nuclear Regulatory Commission I One White Flint, North j 11555 Rockville Pike Rockville, Maryland 20852-2739 l l

l NRC Resident Inspector j Sequoyah Nuclear Plant '

2600 Igou Ferry Road Soddy-Daisy, Tennessee 37379-3624 Regional Administrator U.S. Nuclear Regulatory Commission Region Il  !

61 Forsythe St., SW, Suite 23T85 l

Atlanta, Georgia 30303-3415 l

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t ENCLOSURE RESPONSE TO NRC REQUEST FOR ADDITIONAL INFORMATION REGARDING SEQUOYAH NUCLEAR PLANT OFFSITE POWER

1. Analysis of immediate Access Offsite Circuit.

1 This question requested information on the analyses performed for the design basis event of l one unit LOCA with a simultaneous trip of both units and with simultaneous start of all l required design loads on both units in accordance with design and licensing basis requirements under various assumptions / contingencies. Transmission System Studies (TSS) or plant auxiliary power system analyses as described below have evaluated the offsite 1 power system adequacy under the above design basis event concurrent with the worst case assumption / contingency. Each of the assumptions / contingencies listed in the question have been considered in those studies.

Background:

The eight 161-kv transmission lines connected to the 161-kv switchyard, the 500/1G1-kv intertie transformer bank, two 84 MVAR capacitor banks for the 161-kv switchyard, and the five 500-kv transmission lines have sufficient capacity to supply the total required power to the plant's electrical auxiliary power system under normal, shutdown, and loss of coolant (LOCA) conditions concurrent with any single transinission system contingency. The 500-kv intertie transformer bank and the 161-kv system, including the 161-kv capacitor banks, are the immediately available power sources to the troferred power system, and are capable, at all times, of providing adequate voltage and power. In the event the 161-kv capacitor banks become unavailable, the 500/161-kv intertie transformer bank becomes the immediately available power source and the eight 161-kv transmission lines are the delayed power source in the event the 500/161-kv intertie transformer bank also becomes

! unavailable, the system dispatcher will adjust the transmission network so that the delayed source will be fully capable of providing adequate voltage and power within 10 minutes.

For this event, loss of both the 161-kv capacitor banks and the 500/161-kv intertie transformer bank, applicable Sequoyah technical specification LCO actions will be entered as described in Sequoyah Switchyard Operations Procedure SWYD-18.

l Transmission System Studies:

Transmission System Studies (TSS) have been performed at Sequoyah in accordance with IEEE 765-1995, Preferred Power Supply for Nuclear Power Plants, and NRC Letter to All Power Reactor Licensees, August 8,1979, " Adequacy of Station Electric Distribution System Voltages". There are two types of TSS's performed. A planning TSS evaluates steady-state and transient stability conditions at Sequoyah and is performed on a three year cycle. This TSS evaluates the current year when issued and a five year look-ahead. Each year an Operational TSS is performed which for the current year defines the 161-kv and a 500-kv operating voltage schedule, and Sequoyah operating parameters for the main generators, 500/161-kv intertie transformer bank, and 161-kv capacitor banks to ensure adequate offsite power to Sequoyah. The operating TSS's are a continuation of the J

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l t planning TSS's and evaluate for the current year, the worst case contingencies identified in the planning TSS's.

l The current planning TSS, SON-GRID-STUDY-004 dated February 5,1996, has been l performed for the normal steady-state power flow around the Sequoyah 500-kv and 161-kv

! buses and assumes a design basis event (LOCA) on one unit, an orderly shutdown of the i other unit, and one of the following: 1) a normal transmission network,2) the loss of the l 500/161-kv intertie transformer bank,3) loss of the 161-kv capacitor bank,4) loss of either l the 161-kv bus 1 or 2, or 500-kv bus 1 or 2,5) the loss of the largest generating unit (which includes loss of the other SON unit), or 6) the loss of the most critical transmission line. None of the simulations showed that a LOCA in one unit and a simultaneous full load rejection of the other unit was the worst case contingency. A LOCA in one unit with the simultaneous loss of a transmission element (i.e. intertie transformer bank, capacitor bank, etc.) have been shown to be the worst case. This TSS also performed transient stability analyses that show that the offsite power sources remain intact as reliable sources for the following postulated transmission disturbances: 1) a 3-phase fault and a stuck breaker on either the 500-kv bus 1 or 2 or the 161-kv bus 1 or 2, or 2) a phase-phase-ground fault on the 161-kv side of the 500/161-kv intertie transformer bank and a stuck breaker in the 161-kv switchyard. Voltage recovery times for the postulated transmission disturbances were within the time limits required to ensure that the Reactor Coolant Pumps do not trip l and the 6.9-kv Shutdown Board loss of voltage relays do not trip the offsite power circuits.

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) The current operational TSS, SON-GRID-STUDY-003, dated January 30,1997, evaluated l the offsite power supply at Sequoyah for an extreme load forecast of 28,797 MW net

system load. in determining adequate offsite power for SON, the TVA transmission system t

must provide at least two offsite sources that maintain the SON 161-kv bus voltage at a specified range during normal system conditions in order to supply at least 153-kv during a l postulated LOCA on one unit and a simultaneous transmission system contingency.

Eighteen pre-existing transmission configurations were studied for pre-event outages of the l 500/161-kv intertie transformer bank and/or the 161-kv capacitor banks. The TSS l concludes that the SON offsite power supply is adequate for a net system load up to 28,797 MW with two immediately available power sources whenever the 161-kv capacitor

! banks are available and are maintained in automatic control on the wide band mode continuously, and if specific system indicators are monitored and controlled. These

indicators are SON Unit 2 reactive output, SON 161-kv bus voltage, and the SON l 500/161-kv intertie transformer bank reactive f;ow. The study also concludes that the offsite power sources can support a LOCA in one unit with the 500/161-kv intertie transformer bank out of service and an outage of the 161-kv capacitor banks, without taking another transmission system contingency.

Plant Auxiliary Power System Analyses:

SON Electrical calculation SON-EEB-MS-Tl06-0002 evaluates the Plant AC Auxiliary Power

Distribution System under the conditions: Normal Operations, Full Load Rejection, Safety injection Phase A(SI-A), and Safety injection Phase B(SI-B). In the scenarios for all cases for j loading conservatism, the other unit is considered to be in a fullload rejection. This j calculation analyzes 161-kv switchyard voltage and determines 153-kv as the minimum acceptable voltage used in the analysis of steady state and transient cases. The 153-kv is i

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the limit as the bases for acceptability in the Transmission / Power Supply Group (TPS) studies. The site voltage recovery curves provided in TVA's letter to NRC, dated

July 17,1996, depict the worst- case transient voltage response (LOCA + Full Load Rejection for SLB from the calculation results). Also, as shown in TVA's letter to NRC, dated July 17,1996, the offsite and site voltage recovery curves demonstrate that the voltages at the safety buses do not go below 5520 volts (80% loss of voltage dropout) for greater than 1 second or that the voltage goes below 6456 volts (degraded voltage dropout) without recovery to a voltage above 6595.5 volts (degraded voltage T/S maximum reset value) within 7.5 seconds. Thus, the offsite circuits will not be tripped by the uridervoltage l protective relays due to transient voltages, if the auto load tap changer is out of service,it is required to be placed in the manual position 2R ( 2.5% boost ) in modes 1-4. In Modes 5 and 6 the auto load tap changer is placed in manual and adjusted to maintain the 6.9-kv shutdown boards associated with it between 6900 and 7100 olts. If one of the Common Station Service Transformers (CSST) A or C is out of service, CSST B provides the alternate source.

2. Describe the process which assures the Sequoyah control room operational staff that the transmission network is operating within analyzed parameters and that the immediate offsite power circuits to the Sequoyah plant are operable and will remain available following a LOCA. l The Sequoyah Nuclear Plant 161- and 500-kv Grid Voltage Schedules and Operating Instructions, dated October 31,1996, list the three indicators which are used to determine the adequacy of the offsite power system to provide an unlimited condition of operation with the Sequoyah 161-kV capacitor banks available. These indicators are Sequoyah Unit 2 reactive output,161-kV bus voltage, and intertie transformer bank reactive flow. They are monitored continually by transmission dispatchers located at TVA's Chickamauga Area l Dispatcher & Control Center. This operating instruction requires the transmission dispatcher to communicate changes that affect the capability of the offsite power supply to the Sequoyah Control Room. SON Switchyard Operations Procedure SWYD-18 implements the operating gu;delines issued by TPS. In additPan to the notifications required by the operating l

instructions described above, frequent communication between the dispatchers and the SON operation personnel is conducted. These informal communications are initiated to provide information regarding planned activities that could affect the 161- and 500-kv lines that are connected to the SON switchyard, severe weather notices in the Chattanooga and SON area that could impact the transmission system, and planned activities that may result i

in a change of approximately 1-kv or more in the grid voltage. Discussions are froquently j held between the dispatchers and SON operation personnel following unplanned grid transients to evaluate the impact of the disturbance. These informal communications provide an additional level of grid condition information to the operation personnel that enhances their awareness of the capability and availability of the offsite power circuits.

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