ML19210E394

Submits Final General Design Criteria-17 Compliance Review Results.Minor Mods to Switchyard to Meet Intent of NRC 790808 Requirements.Operator Procedures Revised to Eliminate Loss of Both Offsite Sources Due to Tie Breaker Failure

ML19210E394
Person / Time
Site:	North Anna
Issue date:	11/28/1979
From:	Stallings C VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	Harold Denton, Gammill W Office of Nuclear Reactor Regulation
References
673A-080879, 673A-80879, NUDOCS 7912040496
Download: ML19210E394 (3)

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' 7h VxnoxNIA ELECTRIC AND POWER COMPANY Brcuxown.VrnorwrA n 3 n 61 November 28, 1979

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Mr. Harold R. Denton, Director Serial No: 673A/080879 Office of Nuclear Reactor Regulation P0/FHT:baw Attn: Mr. William Gammill, Acting Docket Nos: 50-338 Assistant Director for Operating 50-339 Reactor Projects License No: NPF-4 Division of Operating Reactors Permit No: CPPR-78 U. S. Nuclear Regulatory Cocmission Washington, D. C. 20555

Subject:

North Anna Power Station Units 1 and 2 GDC-17 Compliance Review Results

Dear hr. Denton:

In our letter of October 9,1979, Serial 673/080879, we referenced the results of our review of the adequacy of the station electric distribution systems for North Anna Units 1 and 2, and committed to provide additional information ec-the compliance with GDC-17 of the offsite power supply for North Anna. This letter provides the final results of our review.

The design of our offsite power supply for North Anna Units 1 and 2 is in full compliance with GDC 17. This statement is based on the sentence in GDC-17 which states "a switchyard common to both circuits is acceptable". However, the concern expressed in your letter of August 8,1979, was "to determine if there are any events or conditions which could result in the simultaneous or consequential loss of both required circuits to the offsite network". We have determined that some minor modifications will be required in our switchyard to meet the intent of your August 8,1979, requirements.

The first area of concern we mentioned in our October 9 1979, letter involved a 34.5KV tie breaker between the two 34.5KV buses which supply the redundant offsite power supplies. This tie breaker would norr. ally be open and would be automatically closed upon the loss of supply power to either 34.5KV bus.

However, to eliminate any possibility of simultaneous loss of both offsite sources due to tie breaker failure, we will incorporate operating procedures to leave one. or both 34.5KV disconnects open (one disconnect is installed on each side of the breaker). The autocatic close feature will be removed and manual closing will be required. ,

The second item of concern we expressed in our October 9,1979, letter involv-ed the switchyard service transformers. Each of these 1500KVA transformers can supply the entire switchyard load. The primaries of these transformers are connected to each 34.5KV bus and the secondaries are routed in a common cable trough to manual throwover switches. The purpose of the throwover switches is to allow the loads to be transferred to either transformer. It Ik , ., ,

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4 v3MOINIA ELECTEIC AND Powsn COMPANY TO Mr. Barold R. Denton 2 order to eliminate the possibility of simultaneous loss of both offsite sources, switches will be installed in the secondary leads of both transformers and all the secondary leads of one of the transformers will be removed from the com.n trough and installed in conduit. The rew switches will be operated such that only one transformer source will be available at the throwover switch at any time.

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Enclosed is a sketch which shows the danges described above. The remainder of the offsite power source is in f'211 compliance with GDC-17 and these changes eliminate the concern of a single event causing simultaneous loss of both required circuits to the offsite source.

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Very truly yours,

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.h.hdfirNff C. M. Stallings Vice President-Power Supply and Production Operations cc: Mr. James P. O'Reilly 4

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