ML20056H403

Submits Addl Info in Support of Plant Compliance W/Station Blackout Rule

ML20056H403
Person / Time
Site:	Vermont Yankee
Issue date:	09/02/1993
From:	Tremblay L VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
BVY-93-94, NUDOCS 9309090273
Download: ML20056H403 (9)

Text

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.g VERMONT YANKEE l NUCLEAR POWER CORPORATION Ferry Road, Brattleboro VT 05301-7002 ENGINE tlNG OFFICE

$50 VAIN STREET BOL TON. M A 01740 (50b> 779 f,711 September 2,1993 BVY 93-94 United States Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555

References:

a) License No. DPR-28 (Docket No. 50-271) b) Letter USNRC to VYNPC, NVY-91-98, dated June 5,1991 c) Letter VYNPC to USNRC, BVY 91-69, dated July 17,1991 d) Letter VYNPC to USNRC, BVY 90-038, dated March 30,1990 e) Letter USNRC to VYNPC, NVY 92-163, dated September 1,1992 f) Letter VYNPC to USNRC, BVY 93-12, dated February 1,1993 g) Letter USNRC to VYNPC, NVY 93-091, dated June 18,1993 h) Letter VYNPC to USNRC, BVY 92-94, dated July 31,1992 i) Letter VYNPC to USNRC, BVY 91-21, dated February 28,1991 j) Letter USNRC to VYNPC, NVY 93-109, dated July 27,1993 k) Letter VYNPC to USNRC, BVY 89-36, dated April 12,1989 Attachments: a) Site Plan: Vernon Station b) Vernon Station Interim Configuration c) Vernon Station Final Configuration

Subject:

AdditionalInformation in Support of Vermont Yankee Compliance with the Station Blackout Rule

Dear Sir:

On July 22,1993, VYNPC representatives met with USNRC staff to discuss the status of Vermont Yankee's Station Blackout plan and schedule. In particular, the issues raised in References f) and g) regarding periodic testing of the Alternate AC (AAC) source were discussed, Reference j). During the meeting, NRC staff members requested Vermont Yankee to submit additional information regarding: 1) the status of the design change for the AAC source upgrade and plans for periodic testing,2) the design of the AAC source and how it meets the requirements for weather protection for AAC sources,3) the time required to manually load the safety buses from the tie line, and 4) the agreement between VYNPC and New England Power Company regarding use of Vernon Station as an AAC source. The purpose of this letter is to f provide this requested information.

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1. Desian Chanae Status and Plans for Testina As of this time, Vermont Yankee has installed a new 3750 kVA 13.2 kV / 4.16 l kV transformer on its property, installed an underground 4.16 kV cable from the l transformer to connect to the existing underground Vernon Station tie line -4.16 kV cable, and installed an underground 13.2 kV cable from the transformer to the 13.2 kV section of the Vernon Station switchyard. Initial testing of the cable and transformer has been completed.

An electrical connection at Vernon Station was completed on August 13,1993.

This connection is an interim one using a sectionalizer with an existing vacuum recloser. The permanent connection will use an existing circuit breaker within I the switchyard. This circuit breaker is currently connected to the hydro statio 1 via underground cable and is being used as one of the generator breakers for the station. This breaker will become available to VY as modifications on the station progress. Du' - the interim phase, the 13.2 kV section of the switchyard is fed directly om one hydro station generator via the underground cable as well as from the 69 kV section of the switchyard.

Vermont Yankee completed its connection to the existing underground 4.16 kV l cabie on August 21,1993. Vermont Yankee will no longer rely on the overhead portion of the Vernon Tie line.

A load test on the new line is scheduled during the 1993 refueling outage. The outage is scheduled to begin on August 28th. Because of difficulty in scheduling available load due to outage constraints, Vermont Yankee shall perform the test using the maximum available 4160 V system load and perform the test to a load as close as practical to the maximum SBO load of 2300 kW.

The new line, as governed by the rating of the transformer and cable, has a significantly higher capacity than the anticipated SBO requirement. The intent of the NUMARC 87-00 recommendation to perform a rated load capacity test is met by testing to a load level approximately equal to the 530 requirement. A capacity test on cables and transformers is performed at the factory. Field tests, as specified in ANSI C57.12 are designed to show the condition of a i transformer without requiring that it be loaded to its full rating. Additionally, we currently plan on performing load testing on the Vernon tio line during each scheduled refueling outage.

l 2A. Description of the AAC Source One line and layout diagrams describing the upgraded Vernon Station, the interim, and the final configuration of the Vernon Tie are provided as Attachments A, B and C. Vernon Station generation will be connected to two

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~ United States Nuclear Regulatory Commission September 2,1993 l Page 3 l indoor 13.8 kV generMon buses separated by a normally open tie breaker. i Each bus will be connected by underground cable to its associated generator step up transformer located in the 69 kV switchyard. Each step up transformer is sized to carry the full station output. The 69 kV switchyard is arranged in a dual bus configuration. The output of the station will feed four 69 kV transrnission lines and a 13.2 kV feeder to Green Mountain Power Company.

The 69 kV lines can be supplied by either bus. At this time the 13.2 kV feeder is supplied by 69 kV Bus No.1.

An underground cable runs from the structure in the 13.2 kV section of the hydro station yard to a 13.2 kV / 4.16 kV transformer located on Vermont Yankee property. From the transformer a 4.16 kV cable runs underground to the Vermont Yankee switchgear room.

The only portions of the AAC that may be potentially affected by weather related events are the portions within the outdoor switchyard.

The 69 kV switchyard is located approximate ly 100' west of the Vernon Station turbine hall. The switchyard bus structure is approximately 30' tall. Provision for mounting disconnect switches and line takeoffs are from the towers. Circuit breakers for the transmission lines are located within the switchyard.

Connections from the circuit breakers to the structure mounted equipment are by short lengths (less than 15') of uninsulated conductors. Conductors connecting the station to the switchyard will be run underground. The only exposed cable in the switchyard will be run vertically attached to a steel support I approximately 15 feet from the bus work down to the ground. At no time will l this short section of cable be subject to the forces of wind, ice, snow or trees  ;

in a manner which could cause it to fail.  ;

l 2B. Compliance with SBO Rule i

We conclude that the proposed design meets the requirements for an AAC i source.  ;

Alternate AC source is defined in 10CFR50.2 as: " Alternate ac source means an alternating current (ac) power source that is available to and located at or l nearby a nuclear power plant and meets the following requirements:

(1) is connectable to but not normally connected to the offsite or onsite emergency power systems; (2) Has minimum potential for common mode failure with offsite power or the onsite emergency ac power sources;

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United States Nuclear Regulatory Commission r September 2,1993 I Page 4 1

(3) is available in a timely manner after the onset of station blackout; and (4) Has sufficient capacity and reliability for operation of all systems required for coping with station blackout and for the time required to bring and maintain the plant in safe shutdown (non design basis accident)."

The proposed design using Vernon Station meets the requirements of an alternate ac source in that:

(1) It is connectable to Vermont Yankee's emergency busses but it is not  ;

normally connected to these busses. The 4 kV breakers in the VY switchgear room which connect the tie line to either of the buses are l open during normal operation, but can be closed readily from the VY Control Room. The Vernon Station is also not normally connected to the  ;

offsite or onsite emergency power systems at Vermont Yankee.

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l (2) It has minimum potential for common mode failure with Vermont Yankee's offsite power or onsite power. '

Vernon Station generators are not normally connected to the VY emergency busses, the equipment is of different manufacture than the VY onsite power sources, the equipment is maintained and operated by a separate organization; therefore, it has minimum potential for common mMe failure with VY's onsite power system.

l Vernon Station is connected to a 69 kV transmission system which is not directly electrically connected to VY's offsite power sources thus providing electrical independence and minimizing the potential for l common causi. . ilure due to electrical faults, switching problems or l other grid related losses of power. Vernon Station is connected to its l own switchyard which is physically separated from VY's switchyard (by

, about a mile) and the transmission lines emanating from the station are I

routed on separate rights of way. The majority of the lines emanating l from the station are routed in completely different directions from the lines supplying offsite power to VY.

The proposed design subjects the AAC source to a limited exposure to l weather related events. The only portion of the electrical configuration which could be considered exposed to the weather is the rigid bus within the switchyard, and a short (15 feet) length of armored cable in transition j from the bus to underground. Because of its construction, this bus is not  ;

prone to the same weather-induced failure (ice, snow, wind, trees) as overhead cable. The small section of armored cable will be attached to i

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' United States Nuclear Regulatory Commission September 2,1993 Page5 a steel structure, and will also not be prone to weather-induced failure.

From the switchyard, all cable is run underground to a pad mounted i transformer. The connections at the transformer are totally enclosed and protected from the weather. Cable from the transformer secondaryis run .

underground to the VY 4 kV switchgear. t Reference h) details our previous response regarding common mode failure of VY and Vernon Station power, includino a regional blackout.

Recent discussions with the organization responsible for transmission system operations for the area, REMVEC, confirm that system ,

improvements have been implemented since 1965 to prevent the occurrence of a regional blackout. These improvements include the installation of underfrequency load shedding relays and establishing organizations such as REMVEC to enable early detection and correction of potential grid failures. A review of REMVEC records indicates that Vernon Station tripped off during the 1965 blackout (an event which predates Vermont Yankee); however, the station was reenergized in 5 minutes.

(3) Power from Vernon Station is available within ten minutes after the onset i of station blackout. All that is required is the closure of two breakers l from the VY Control Room; and j l

(4) It has sufficient capacity and reliability for operation of all systems I required for coping with station blackout and for the time required to bring and maintain the plant in safe shutdown (non design basis accident). The capacity of the AAC source is sufficient to power the l maximum loss of offsite power loads for one emergency bus for an I indefinite time. The reliability and availability of Vernon Station has historically been well above 99% which exceeds the required AAC l source availability of 95%. l

3. Time Recuired to Manually Load the Safety Buses The time required to manually load a safety bus is not governed by the source of power to that bus, but rather by system requirements. There is no time restriction on the application of load after the closure of the Vernon Tie breaker.

Energization of a safety bus from the Vernon Tie line requires the closare of two circuit breakers (Breaker 3V4, and either Breaker 3V or 4V) from the Vermont Yankee Control Room. Breaker closure would be accomplished as soon as the operator has diagnosed that a loss of offsite power has occurred, a diesel I generator has failed to start, and one attempt to restart a diesel generator is l.

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made. The SBO scenario is practiced in simulator training. Typical time for power restoration is considerably less than 5 minutes, well within the required l time of 10 minutes from the onset of a SBO. A timed test of this capability will j be conducted this outage on Bus 4.

4. Aareement between VYNPC and NEPCo l As previously stated in Reference (i), Vermont Yankee has confirmed with Vernon Station management that, to the best of the hydro station operators' i ability, they will keep the Vernon tie line energized at all times and that both the  !

hydro station and NEPOOL consider this line to be a high priority line. A contract is in place obligating Green Mountain Power /NEPCo to supply i emergency power to Vermont Yankee from the Vernon Hydro Station. This I contract is aval!able for NRC inspection through the on-site NRC Resident inspectors if needed. In addition, Vermont Yankee is in the process of formalizing, via a memo of understanding, our relationship with NEPCo, regarding the Vernon tie line. This also will be available for inspection, through the onsite NRC Resident inspector, when finalized.

We trust that this letter is responsive to your request for additional information and resolves all remaining concerns relative to Vermont Yankee's compliance with the Station Blackout Rule,10CFR50.63. However, should you have any further questions, please contact this office.

Sincerely,  ;

Vermont Yankee Nuclear Power Corp.

Q, 4Sc W Leonard A6mmblay Senior Licensing Engineer cc: USNRC Region i Administrator USNRC Resident inspector - VYNPS USNRC Project Manager - VYNPS 1

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