ML18139B334

Forwards Info & Nine Drawings Re Status of Reserve Station Svc Sys & Degraded Grid Mods,In Response to NRC 790808 Ltr Concerning Util Analysis of Electrical Distribution Sys. Aperture Cards Are Available in PDR

ML18139B334
Person / Time
Site:	Surry
Issue date:	05/26/1981
From:	Sylvia B Virginia Power (Virginia Electric & Power Co)
To:	Harold Denton, Varga S Office of Nuclear Reactor Regulation
Shared Package
ML18139B335	List: ML18139B334 ML20004B462 ML20004B463 ML20004B465 ML20004B469 ML20004B472 ML20004B473 ML20004B474 ML20004B475
References
233, PSE&C-KSB-DAC, NUDOCS 8105280297
Download: ML18139B334 (4)

Text

• '

Vepco VIRGINIA ELECTRIC AND POWER COMPANY, RICHMOND, VIRGINIA 23261 May 26, 1981 Mr. Harold R. Denton, Director Serial No. 233 Office of Nuclear Reactor Regulation PSE&C/KSB/dac

• • Attn: Mr. Steven A. Varga, Chief Operating Reactors Branch No. 1 Docket Nos. 50-280 Division of Licensing 50-281 U. S. Nuclear Regulatory Commission Washington, DC 20555 License Nos. DPR-32 DPR-37

Dear Mr. Denton:

RESERVE STATION SERVICE SYSTEM AND DEGRADED GRID MODIFICATIONS SURRY POWER STATION UNIT NOS. 1 AND 2 In your letter titled "Adequacy of Station Electric Distribution System Voltages" dated August 8, 1979, you requested a Vepco analysis of the station electrical distribution system to the gufdeJines of GDC-17.

The purpose of this letter is to inform you of our progness to date and to advise you of our planned course of action.

The RSS System at Surry supplies the emergency buses and the intake structure buses (supplied at the 34.5 KV level) at all times (see attached one line diagram). It also supplies all unit auxiliaries required during a start up, until these loads are manually transferred to the Normal Station Service System. In the case of a unit trip, all loads on the Normal Station Service System are automatically transferred to the RSS System.

The original design basis for the RSS System was for it to be capable of handling the loads required for one unit to start up, or to handle the load automatically transferred in the case of a unit trip. The analyses that we have performed recently are for worst case conditions of two unit simultaneous loading on the RSS System. Results of our review have shown that for the very conservative worst case loading (full load nameplate ratings) on each individual reserve transformer, there exists the potential for overloading of the underground feeder cables from the reserve transformers to the transfer buses, as well as overloading of the transfer bus switchgear. For this scenerio, there is a potential for voltage levels on the emergency buses to be below acceptable values.

However, under actual operating conditions under this scenerio, the loads on the RSS System would be significantly less than the limits of the equipment in the system.

~ 4.Mr. Harold R. Denton~~ector In an effort to upgrade the RSS System, we have installed a load

• shedding scheme which will reduce the postulated overloading and improve the postulated undervoltage. This load shedding scheme is automatic and will be initiated whenever both units are being fed

• , simultaneously from the RSS System. The loads to be shed are all non-safety related. The following operating preconditions will normally be in effect as required to implement the automatic load shed scheme:

Unit l

1. Operating procedures will have 11 A11 FW pump running during unit start up and when on the line.

2. Operating procedures will have 11 8 11 or "C" condensate pump running during unit start up and when on the line.

Unit 2

1. Operating procedures will have 11 8 11 FW pump running during unit start up and when on the line.

2. Operating procedures will have "A" or*"B" condensate pump running during unit start up and when on the line.

A manual override switch is provided in the control room to allow manual restarting of the shed loads under a controlled condition.

Preliminary studies have been performed for the RSS System loading conditions of one unit in start up combined with one unit tripping, and one unit in start-up combined with other unit tripping with a consequence limiting safeguard (CLS) actuation. Results of these studies indicate that with the automatic load shed feature, the very conservative worst case loading on the transfer bus switchgear will be less than 15% overload. The switchgear vendor has indicated to us that calculations, completed in accordance with ANSI C37.010-1979 Section 4.4.3.3.1 using worst case continuous loading prior to the overload situation, indicates that a 15%

overload is allowable for 22 minutes on the transfer bus switchgear. An operating procedure will be implemented to insure that within 20 minutes a sufficient amount of non-safety related load will be manually shed to reduce the loading of the switchgear to within its continuous rating. The underground cables will be loaded beyond their 90 degrees Celcius maximum continuous rating, but will not approach the emergency 130 degrees Celcius rating for at least four hours, prior to which further unloading will have been effected on the unit in startup by administrative controls. The voltage conditions on the emergency buses will be acceptable; that is, the voltage available during the worst motor starting transient will be above the minimum starting voltage, and the worst case steady state voltage will be above the motor's minimum continuous running capability. Further details of these analyses will be forwarded to you at a later date.

brc/0069/2

-* Mr. Harold R. Denton, Director As indicated in our October 15, 1979 letter, Serial no. 583A,

• -to you entitled "Surry Power Station Units 1 and 2, Degraded Grid Voltage", the undervoltage protection presently used on the 4KV emergency buses for both units will be revised. The new protective scheme will protect for both loss of voltage (below 74% voltage) and degraded voltage (90%-74% voltage). As shown in the attached logic diagrams for Unit 1 (Unit 2 is similar), the protective scheme for each bus will consist of voltage monitoring on each phase with separate loss of voltage and degraded voltage relays. Each separate set of relays will provide the input to a coincident 2-out-of-3 logic. After a time delay, the output of these separate logic schemes will initiate an emergency diesel generator start and loading of the emergency bus to the emergency diesel generator. The degraded grid circuitry will be tested for operability upon installation and covered by normal station Periodic Tests which are conducted at each refueling outage. These modifications will be installed on each unit at its next refueling outages.

Although we consider continued operation of both units justified with the installation of the load shed scheme, we are investigating various additional options for a permanent upgrade. We expect to develop a final decision on the permanent solution by late 1981. At that time, we will forward this decision to you, along with supporting analyses.

If you have any further questions on this subject, please advise.

Very truly yours,

~

Manager - Nuclear Operations and Maintenance Attachments cc: J. P. O'Reilly, Director Office of Inspection and Enforcement Region II Washington, D. C. 20555 brc/0069/3

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