ML19259C222

Submits Rept Re Potential Overloads of Electrical Sys Supplying Offsite Power for Startup Calculations. Review Shows Automatic Shedding of Certain Non Essential Normal Loads Will Provide Adequate Voltage Profile

ML19259C222
Person / Time
Site:	North Anna
Issue date:	05/23/1979
From:	Brown S VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
References
403, NUDOCS 7906140421
Download: ML19259C222 (3)

Text

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     'Mr. James P. O'Reilly, Director                                           Serial No. 403 Office of Inspection & Enforcement                                         PSELC/WLT:mac: wang U. S. Muclear Regulatory Comission Region II                                                                  Docket No. 50-339 101 Marietta Street, Suite 3100 Atlanta, Georgia 30303

Dear Mr. O'Reilly:

On April 27, 1979, a report was made under the provisions of 10CP50.55(e) concerning potential overloads of electrical systems which supply offsite power for unit start-up as well as emergency loads Nr botn Units 1 and 2 at Ncrth Anna. The potential overloid involves the underground cibles from the reserve station service transformers to the 4160 volt transfer huses, the 4160 voit circuit breakers which act as feeder breakers for the transfer buses, and the 4160 volt transfer buses D, E, and F. A review of various station ccnditions showed that when both Units 1 and 2 were using power from the reserve transformer source, the systems T.entioned aoave would not be capable of continuously carrying the potential full load current. It was also determined that with unrestricted start-up Icading of one unit the loading on the reserve source underground cables could exceed their continuous current carrying capaoility as calculated for a 40 year cable life. In order to alleviate the potential overload situation, we are in the process of upgrading the reserve power supply for Units 1 and 2. Specifically, an overhead bus is being built from the reserve transformers to the area of the north wall of the turbine building. Frcm this point, cables routed in cable trays will carry the pcwer to the 4160 volt normal station buses for both units. The emergency buses (1H & J and 2H & J) will be fed from the transfer buses, (D, E, & F) which will be fed by the existing underground cables frcm the reserve station transformers. Enclosed is a revised one-line diagram showing the changes which we are undertaking. These changes will be reflected in the next amendment to the FSAR. Detailed loading and voltage profile studies have been performed in an effort to fully assess the capabilities of the upgraded system. Various station contingencies were examined including one unit in start-up and the oth.er unit tripped with loads transferred automatically to the reserve source, and one unit tripped under a loss of coolant acticent condition witn transfer of loads to reserve source while the other unit is in start-uo. These studies \ have shown all components involved in carrying power from offsite to the station loads are capable of carrying the worst-case postulated load for cqe D5 hour without any degradation of pcwer feeds. The concerns of voltage profile i and breaker f ault interrupting capability have required automatic sheddicq cf certain non-essential normal loads during the conditions postulated above. With this loao shedding in effect, calculations have shown the voltage profi e

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mom suct..c aso powin ccupany to Mr. James P. O'Reilly, Director . '**E7* - 2 This letter is our interim report of this situation. A final report will be submitted when the work is. completed. However, we feel that this interim report provides all information required for your review of our application for an Operating License. If additional information is required fcr your review, please notify us. Very tr ly yours,

                                                            /

Sam C. , Jr. Senior Vice President - Power Station Engineering and Construction ec: Mr. John G. Davis. Acting Director Office of Inspection & Enforcement Mr. Harold R. Denton, Director Office of ?!uclear Reactor Regulation 2M4 160 Enclosure

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