ML20135H559

Forwards Truth-Analysis/Action-Reaction Review of 480-Volt Essential Bus Tie Breaker Control Scheme, Final Rept & Provides Evaluation of Control Logic Design

ML20135H559
Person / Time
Site:	Fort Saint Vrain
Issue date:	09/13/1985
From:	Warembourg D PUBLIC SERVICE CO. OF COLORADO
To:	Hunter D NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML20135H560	List: ML20135H559 ML20135H563
References
P-85318, TAC-59801, NUDOCS 8509240012
Download: ML20135H559 (4)

Text

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Company of Colorado 2420 W. 26th Avenue, Suite 1000, Denver, Colorado 80211 September 13, 1985 Fort St. Vrain Unit No. 1 P-85318 Regional Administrator Region IV ,.

p p r' q ' gjjpq U. S. Nuclear Regulatory Commission 'j H k a gl}

611 Ryan Plaza Drive, Suite 1000 ?l jj Arlington, Texas 76011 lSEPl81985 l!I Attn: Mr. Dorwin R. Hunter . U[- ,i Docket No. 50-267

SUBJECT:

Emergency Electrical Power System Independence

REFERENCES:

(1) PSC Letter dated June 14, 1985 Warembourg to Johnson (P-85208)

(2) NRC Letter dated July 10, 1985 Johnson to Lee (G-85268)

Dear Mr. Hunter:

In a letter dated June 14, 1985 (Reference 1), Public Service Company (PSC) provided additional information in response to questions raised in the NRC's review of the Emergency Electrical Power System design as described in the FSAR. In addition to the information provided, PSC comitted to perform an evaluation of the Emergency Diesel Generator (EDG) control logic circuitry and to provide the results to the NRC staff. In a letter dated July 10, 1985 (Reference 2), the NRC issued a safety evaluation for continued operation in the interim until any necessary design changes are proposed and to reaffinn PSC's comitment to evaluate the control logic design. This letter is providing PSC's response to this comitment.

Fort St. Vrain (FSV) was designed using a combination of principles, i.e. redundancy, independence, diversity and single failure. Each of , () [) l these principles was used and/or combined to achieve a highly '

y reliable onsite power system. The FSAR comitments concerning independence, redundancy and single failure criteria as they relate t to FSV's. onsite power system cannot be taken to be solely the single b4 t

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t sentence containe'd in FSAR Section 8.2.5.1, "The onsite power supplies are completely independent and meet the single failure criterion." It was not PSC's intention, by the use of that sentence, to imply.that " complete independence" is maintained at every level, i.e. from sensor to actuation device input terminal, but rather at a system level.

The principle of independence was incorporated with the other principles and good design practices to ensure a high quality' and reliable power system. Other portions of FSAR Section 8.2.5.1 specifically allude to this, e.g. "A number of separate and independent sources of power'are provided. The design maintains the independence of alternate sources to the greatest possible degree."

Appendix C, specifically Criterions 20 and 21, and Section 7.1.1.2 of the FSAR address in more detail the single failure criteria considered in the design for independence at FSV.

Total and exclusive independence was not. utilized in the design of the plant. PSC's design commitment is to assure (where total independence is not achieved) that no single failure in the system logic will cause loss.of the protective function. Therefore, to comply with this coninitment to meet independence and-redundancy requirements such that no single failure will interfere with the proper operation of a redundant counterpart or system, PSC contracted with Proto-Power Corporation to conduct an overall evaluation. The scope of the analysis included the 480 VAC essential bus undervoltage relays and sensors, the 480 VAC switchgear bus supply breakers, the 480 VAC essential tie breakers, the EDG output circuit breakers, the load sequencer, the timers T1 and T2 and the EDG start /run circuitry.

PSC has reviewed the results of the analysis conducted by Proto-Power Corporation in the attached report dated August 1985 and concurs with the recommended corrective actions. It should be emphasized that items 1,2,.and 3 below are highly unlikely failure modes. PSC will implement the design changes required to the EDG permissive logic control circuitry as-follows:

.1) Control Relays CR-9203 or CR-9204 Problem: Adjacent' contacts subject to contact block failure or concurrent physical damage.

Correction: No detrimental effect on system, no action required. (See Section 5.0, Item 1 of the attached Proto-Power Report) 2)- Control Relays 286G1A or 286G1B Problem: Adjacent contacts subject to contact block failure or concurrent physical damage.

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=- Correction: Rewire contact 11-11C to contact 7-7C to ,

provide separation and alleviate the consequences of a contact block failure or concurrent physical damage.

3) Control Relays CR-9205 or CR-9206 Problem: Adjacent contacts subject to contact block failure or concurrent physical damage.

Correction: CR-9205, rewire contacts 7-8 to contact location 13-14 and vice versa which will provide separation and alleviate the consequences of a contact block failure or concurrent physical'dama5e-CR-9206, rewire contacts 3-4 to contact location 11-12 and vice versa which will provide separation and alleviate the consequences of a contact block failure or concurrent physical damage.

4. 480V AC Supply Breakers 252TR1, 252TR2, 252TR3 Problem: Failure of operator to return Sync Switch to "off", or failure of Sync Switch, will allow a degraded bus to be paralleled with an operating bus without synchronizing the power.

Correction: Wire a spare normally closed contact of the supply breaker in series with the contact that closes the bus tie breaker, which will not allow the bus tie breaker to close if the supply breaker has not opened.

The NRC concern and PSC response that the present system design did not assure automatic operation of the redundant EDGs was addressed in Concern No. 2 of Reference 1. Additional analyses have indicated that automatic closure of each EDG output breaker is provided for through two diverse paths, energizing CR9203 and TR9201 for output breaker 252DGIA or CR9204 and. TR9202 for output breaker 252DG1B.

Timing relays TR9201/TR9202 are provided for the automatic closure of the redundant EDG depending upon which control relay (CR9203 or CR9204) was energized first. Since these diverse paths exist, automatic operation of a redundant- EDG is assured and no longer-presents a concern.

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The above design changes will be implemented by November 30, 1985 and will assure that no single failure will interfere with the proper operation of a redundant EDG .- PSC will, in the next revision (Revision 4) of the FSAR, clarify the electrical system design commitments in applying .the principles of redundancy, independence and single failure criterion.

'Should 'you Mr. M. H. haveatany(questions Holmes 303) 571-8409.concerning this matter, please contact Very truly yours,

$YY & hL D. W. Warembourg, Manager Nuclear Engineering Division i

DW/JCS:pa - ,

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