ML20212G867
| ML20212G867 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 01/09/1987 |
| From: | Warembourg D PUBLIC SERVICE CO. OF COLORADO |
| To: | Berkow H NRC OFFICE OF ADMINISTRATION (ADM), Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20212G872 | List: |
| References | |
| P-87008, TAC-59801, NUDOCS 8701210217 | |
| Download: ML20212G867 (10) | |
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2420 W. 26th Avenue, Suite 100D, Denver, Colorado 80211 January 9, 1987 Fort St. Vrain Unit No. 1 P-87008 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C.
20555 Attention: Mr. H. N. Berkow, Director Standardization and Special
{
Projects Directorate Docket No.
50-267
SUBJECT:
Emergency Diesel Generator Eler.trical Independence and DC Control Power Modifications
REFERENCE:
- 1) NRC Letter, Heitner to Williams, dated November 12, 1986 (G-86599)
- 2) NRC Letter, Heitner to Berkow, dated December 24, 1986
Dear Mr. Berkow:
Public Service Company of Colorado (PSC) met with the NRC staff in a meeting on December 9, 1986 in Bethesda. The purpose of this meeting was to further discuss the emergency electrical power system independence and the DC control power systen single failure issues.
These issues were addressed in an NRC letter (with an enclosed draft Technical Evaluation Report) dated November 12, 1986 (Ref. 1).
A summary of the December 9,
1986 meeting discussions and PSC commitments are addressed in a NRC letter dated December 24, 1986 (Ref 2).
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P-87008 Pag 2 January 9, 1987 The purpose of this letter is to provide the NRC staff with a formal submittal and schedule for the resolution of the above referenced issues for NRC review and approval. The following information and/or analyses are incit.ded herein: :
Contains PSC's evaluation of the recommendations and findings addressed in the Engineering Evaluation (Attachment 2) performed on the EDG's independence. :
Contains the Engineering Evaluation en the emergency diesel generator control circuitry which demonstrates, upon implementation of certain recommended modifications, that:
Manual capabilities exist to energize the associated 480 VAC essential bus (es) from at least one EDG set for postulated single failures in the permissive control logic circuitry.
Postulated single failures in the permissive control logic circuitry will not result in paralleling the two redundant EDG sets.
Certain postulated single failures in the permissive control logic circuitry will not preclude the automatic energization of the associated 480 VAC essential bus (es) and subsequent automatic load sequencing by at least one EDG set. :
Contains an itemized list of PSC's commitments, both short term and long term, associated with the resolution of the referenced issues. A schedule for the completion of each commitment item is also addressed.
Public Service Company of Colorado is confident that the above referenced attachments contain the necessary information and analyses to adequately demonstrate the reliability and operability of the onsite AC and DC electrical systems as committed to in the Final Safety Analysis Report for the Fort St. Vrain Nuclear Generating Station.
Should you have any questions concerning this matter, please contact Mr. M. H. Holmes at (303) 480-6960 Very truly yours, 0 W Wun~$
D. W. Warembourg, ger Nuclear Engineering Division DWW/JS:jmt Attachments
ATTACHMENT 1 TO P-87008 PSC'S EVALUATION OF THE RESULTS OF THE ENGINEERING EVALUATION ON THE EDG INDEPENDENCE I
to P-87008 Page 2 The results of the Engineering Evaluation (Attachment 2) performed on the EDG power system permissive control circuitry determined that there is a single component failure which results in the paralleling of the two redundant EDG sets unsynchronized and therefore compromising the EDG power system independence.
Figure 1 shows a simplified one-line of the EDG power system.
For the specific failure addressed above, the following conditions must occur:
An imposed single ground already exists on the floating DC power system.
A single failure to ground of contact 7-8 of CR-9203 results in a blown fuse in the trip circuit of the 252BT12 breaker. The same final result would be if contact 7-8 of CR-9203 failed in the open position.
In both cases the trip circuit of Breaker 252BT12 is imparied.
NOTE:
In the case of the short to ground of contact 7-8 and the blown fuse, the control room operator would be alerted because of the loss of breaker position lights on the control board.
Voltage degradation continues until the Degraded Voltage logic or Loss of Voltage logic of the undervoltage protection system is satisfied to initiate EDG start.
Generator 1B is selected to power two buses (first to achieve voltage) which would result in closure of 252BT32.
Bus tie breaker 252BT12 is closed prior to losing control power to the trip circuit fo the tie break 252BT12.
NOTE:
Breaker 252BT12 was placed in the closed position manually for maintenance or it was closed automatically via the undervoltage protection system.
With the above conditions met, the single failure results in paralleling the two redundant EDG sets unsynchronized.
This scenario is also applicable to component CR-9204 Recommended modifications are provided to eliminate this single failure scenario.
Re-analyses of the modified design indicates that
Attachment I to P-87008 Page 3 upon completion of the modifications, the EDG system will be free from all known single failure points which could compromise EDG power system independence.
PSC has reviewed the system design and operational requirements and concurs that the single failure scenario presents an undesirable configuration.
PSC concurs with the recommended modifications and is pursuing immediately to initiate a Change Notice to eliminate the single failure point.
The modifications will be completed prior to restart.
PSC acknowledges that the NRC has requested in their letter dated November 12, 1986 that design changes be reviewed and approved by the staff prior to actual changes being made.
PSC feels that this issue is of prime importance and is proceeding expeditiously to its resolution.
The modified schematics in Attachment 2 are determined to provide adequate design information for the staff's review and approval.
PSC concludes from this evaluation upon implementation of certain recommended modifications that no single component failure precludes the manual capabilities (as a minimum) to energize the associated 480 VAC essential bus (es) from at least one EDG set. As a part of the single component failure analyses, PSC has field verified that there are no physical component installations / configurations that compromise the separation and segregation requircments nor which could impose any additional credible failure modes.
In addition to the above, the Engineering Evaluation has also recomended several modifications to enhance or improve reliability by eliminating the undesirable effects of certain component mode failures.
It should be noted that these recommended modifications are not required to achieve desired manual operating capabilities (as a minimum) for a least one EDG set.
These recommended modifications primarily provide for automatic functions in lieu of the need for manual operator actions.
PSC concurs with these recommended modifications and since they are not required for the EDG power system to perform its design function, these modifications will be implemented at the 4th refueling outage.
A final item addressed in the Engineering Evaluation is the potential for certain single component failures affecting the load shedding and load sequencing portions of the EDG control circuitry to allow additional loading on an EDG set to occur.
PSC is in process of performing an evaluation to analyze the loading requirenint on the EDG sets.
PSC will submit, in the June 1, 1987 submittal (addressed in Attachment 3 of this letter), the results of this evaluation.
to P-87008 OFFSITE OFFSITE OFFSITE POWER POWER POWER RJJ R JJ RJJ (TT)
WT WT 252TRI 252TR2 252TR3 252BTl2 252BT32 480VAC (SWING BUS) 252DGIA 2520G18 Diesel Diesel Diesel Diesel Engine Engine Engine Engine Generator Generator IA IB FIGURE I FORT ST. VRAIN ONSITE AC P0yER SYSTEM u - -
ATTACHMENT 2 TO P-87008 ENGINEERING EVALUATION EE-92-0008, REY. A L.-
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to P-87008 Page 2 The following is a list of FSC commitments, both short term and long term, to further ensure the resolution of the independence and single failure issues related to the electrical systems:
PSC commits that the two circuit breakers in each DC control power feed from DC buses 1A and IB (preferred and alternate) to the ATO scheme in each 480 VAC essential switchgear will be load tested to demonstrate operability.
In addition, the ATO scheme will be tested to demonstrate operability for loss of the power source for both directions Both of these tests will be performed prior to restart.
PSC commits to perform an tvaluation of the utilization of the third battery system (battery 1C) to supply DC control power to at least the undervoltage protection system on 480 VAC essential bus No. 2.
The scope of the evaluation will also include the capability to supply the DC requirements for the bus No. 2 switchgear. Utilization of a third DC bus will eliminate the single failure issue for the undervoltage protection system for the long term without utilizing the ATO scheme.
A conceptual design of this approach is shown in Figure 1.
PSC will submit the results of the evaluation and preliminary design concepts by June 1,
1987.
Following NRC approval by January 1, 1988, implementation will be made at the 4th refueling outage. As can be seen from the figure, the third DC bus could also be used as the alternate source of DC control power to one of the other 480 VAC essential switchgear.
PSC cornits that administrative controls will be implemented to ensure that the two circuit breakers in the alternate DC power feed to the ATO scheme in the 480 VAC essential switchgear will be in the open position and will require manual closing upon loss of the preferred DC source (while opening its associated two circuit breakers).
These controls, in conjunction with the previous commitment will, without requiring modifications to the ATO scheme, ensure that a postulated fault cannot be transferred automatically from a DC bus to another DC bus thus compromising the two DC buses.
These controls will be inplemented in conjunction with the design modifications for the third DC bus at the 4th refueling outage.
PSC commits to update the FSAR in Revision 5 to more precisely describe the FSV electrical system design and operation.
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. Attachment 3 to P-87008 Page 3 Specifically the following:
1)
Section 8.1 to state that "... no single failure will interfere with the proper operation (manually as a minimum) of the redundant counterpart nor result in paralleling of the EDG sets.
2)
Section 8.2.2.1 and 8.2.3.3 to remove statements that the EDGs "will be started automatically as an additional precautionary measure" and "The generators are started automatically on loss of the unit auxiliary transformer power supply," respectively.
This is justified by the removal of the non-safety related contacts from DEV 86 GT1 in the EDG start circuitry.
3)
Section 8.2.3.3.1 to describe the design function of the system when only one engine on each EDG set successfully starts.
4)
Section 8.2.3.5 to clarify that there are no loads associated with 480 VAC essential bus no. 2 that are sequenced on. The loads are either permanently connected or manually loaded as required.
5)
Section 8.2.5.1 to clarify that no single failure will affect the proper operation of the redundant EDG set in the manual mode (as a minimum) nor result in paralleling of both EDG sets.
6)
Section 8.2.5.2 to clarify the existence of the ATO feature and limitations of the redundant DC control power supplies to the 480 VAC essential switchgear.
7)
Section 8.2.5.3.2 to clarify the system design, the levels of undervoltage protection, and the EDG starting requirements.
8)
Various FSAR figures to reflect design changes and as built conditions.
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