ML20217D425

Updates Schedule for Implementation of inter-unit 4.16 Kv Bus DG Cross-Tie.Ltr Also Provides Addl Info Requested by NRC in Support of Review of Request to Implement DG Cross Tie

ML20217D425
Person / Time
Site:	San Onofre
Issue date:	03/26/1998
From:	Rainsberry J SOUTHERN CALIFORNIA EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20217D430	List: ML20217D425 ML20217D470
References
NUDOCS 9803300017
Download: ML20217D425 (19)

Text

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March 26, 1998 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:

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Subject:

Docket Nos. 50-361 and 50-362 Additional Information Regarding the Inter-Unit 4.16 kV Diesel Generator Bus Cross-Tie San Onofre Nuclear Generating Station, Units 2 and 3

References:

1) Letter dated December 31, 1996, from J. L. Rainsberry (SCE) to Document Control Desk (NRC),

Subject:

Docket Nos. 50-361 and 50-362, Inter-Unit 4.16 kV Bus Cross-Tie, San Onofre Nuclear Generating Station Units 2 and 3

2) Letter dated July 15, 1997, from J. L. Rainsberry (SCE) to Document Control Desk (NRC),

Subject:

Docket Nos. 50-361 and 50-362 Individual Plant Examination of External Events Commitments, San Onofre Nuclear Generating Station Units 2 and 3

3) Letter dated October 24, 1997, from J. L. Rainsberry (SCE) to Document Control Desk (NRC),

Subject:

Docket Nos. 50-361 and 50-362, Inter-Unit 4.16 kV Bus Cross-Tie, San Onofre Nuclear Generating Station f Units 2 and 3

4) Letter dated November 20, 1997, from J. L. Rainsberry (SCE) to Document Control Desk (NRC),

Subject:

Docket Nos. 50-361 and 50-362, Inter-Unit 4.16 kV Bus "o U ,'V V :"

Cross-Tie, San Onofre. Nuclear Generating Station, Units 2 and 3 /~h/

This letter updates the schedule for implementation of an Inter-Unit 4.16 kV Bus Diesel Generator Cross-Tie. This letter also provides additional information requested by the NRC in support of NRC review of San Onofre Nuclear Generating Station J. .P. O.Ika 128 L- . San Clemente, CA 926744128 714 368-7420'-

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Document. Control Desk . .

Southern California Edison's (SCE's) request to implement the Diesel Generator Cross-Tie.

By letter dated December 31, 1996 (Reference 1), SCE requested NRC approval to install an Inter-Unit Diesel Generator Cross-Tie in order to meet commitments in support of the Individual Plant Examination of External Events (IPEEE). By letter dated July 15, 1997 (Reference 2),

SCE committed to complete implementation of the Diesel Generator Cross-Tie by the end of the Unit 3 Cycle 9 Mid-Cycle outage, pending NRC approval. The end of the Unit 3 Cycle 9 Mid-Cycle outage is currently scheduled for March 26, 1998, however, NRC approval to implement the Diesel Generator Cross-Tie has not yet been received. Therefore, SCE

-intends'to implement the Diesel Generator Cross-Tie following NRC approval.

During telephone conversations dated January 28, 1998 and February 12, 1998, the NRC requested additional information in support of review of the design of the Diesel Generator Cross-Tie. The NRC requests and SCE responses are provided below.

Question 1: Provide voltage plots to demonstrate that a fully loaded Diesel Generator (DG), in a non-Station Blackout (SBO) unit is capable of starting the largest shutdown load (i.e., the Auxiliary Feedwater Pump) in the SB0 Unit.

Response

Dynamic simulation was performed to verify that the DG has sufficient capacity to support starting of the largest load, which in this case is the Auxiliary Feedwater (AFW) pump of the SB0 unit, while feeding hot standby loads of both units as listed in Table 4.1, CASE 1 in Calculation E4C-088, CCN-18 (transmitted to the NRC by Reference 3). In addition to the CASE 1 loads, an additional phantom load of 300 KW and i 160 KVAR was connected to load the DG up to or beyond its normal rating when the Unit 3 AFW pump is connected to the bus (total DG load was4795KW).

Enclosure -1,- Figure 1 shows the alignment simulated in the analysis.

Figure 2 and Figure 3 show the DG 2G002 terminal voltage and frequency j respectively. The voltage remained above 75% of nominal during the l acceleration of the V3-AFW pump. The DG frequency did not dip below 58.8 hertz. These results meet the voltage and frequency criteria of l Regulatory, Guide (RG) 1.9, " Selection, Design, and Qualification of

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I Document Control Desk . .

Diesel Generator Units Used as Standby (Onsite) Electric Power Systems at Nuclear Power Plants."

Enclosure 1, Figure 4, depicts the terminal voltage and acceleration of the AFW pump 3P141, which shows that the pump successfully accelerated in approximately three seconds.

Question 2: Demonstrate coordination of the Diesel Generator Cross-Tie breakers with the Diesel Generctor (DG) output breaker such that a fault in the SB0 unit will not trip the DG breaker in the non-SB0 Unit.

Response

The Cross-Tie breakers do not coordinate with the DG output breaker.

This is justified as follows:

The 4.16 kV Class 1E electrical power systems are designed such that an electrical fault in the system will be isolated in a manner to have the minimum impact on overall system performance. This is achieved through coordination of the trip setpoints of the load breakers, the DG breaker, and the incoming preferred power breaker. For instance, if a fault I

occurred in the cabling for a shutdown load in either unit, this load would be isolated by the trip of its breaker before the trip setpoint of the DG breaker was reached. Therefore, the DG would remain available to support all other required loads.

Enclosure 2 shows the coordination among load breakers, the DG breaker, the incoming preferred power breaker, and the tie breaker of ESF bus 2A04. The load breakers in both units are well coordinated with all source breakers, including the DG breaker.

The coordination plot shows that selective coordination does not exist between the Cross-Tie breakers and DG breakers. During design basis operation, coordination between the DG breaker and the associated Cross-Tie breaker is not required because the DG breaker and the tie breaker cannot be closed concurrently. The control circuit of the DG breaker is such that when the associated tie breaker is closed its closed position switch will initiate a DG breaker trip. Each DG is provided with protective devices (critical or noncritical) which function to shutdown the DG should any malfunction occur.

Document Control Desk . .

The 10 CFR 50.54(x) DG Cross-Tie operation is a beyond-design-basis event. During DG Cross-Tie operation the Cross-Tie breaker-initiated trip of the associated DG breaker discussed above is defeated by the 5054X Hand Switch contacts. All DG protective devices (critical and noncritical) are kept in service. During the DG Cross-Tie operation all design basis operation protective devices are in service and provide selective coordination among the 4.16 kV Emergency Safety Feature (ESF) bus loads, incoming preferred source breakers, and Cross-Tie breakers.

The DG breaker and load breakers are also coordinated. However, the selective coordination between the DG breaker and the associated Cross-Tie breakers does not exist.

Coordinating the Cross-Tie breakers with the existing DG breakers presented design difficulties. A General Electric model-IJCV time over-current relay with voltage restraint is used for the DG breaker and DG trip (this is considered as a noncritical trip device). The IJCV relay protects the DG from a bus fault. With the existing relays it is not possible to provide coordination between the breakers such that the Cross-Tie breaker will trip before the DG breaker. The Cross-Tie breaker over-current trip setting may be lowered only to the point where the largest load can accelerate without tripping the Cross-Tie breaker.

The DG over-current setting could be increased without sacrificing the DG. However, this approach still did not result in the desired coordination.

In the absence of coordination of the breakers, an alternate method of preventing the DG from separating from the 4.16 kV bus during Cross-Tie operation would be to disable the over-current (IJCV) initiated trip of the DG breaker. However, this was not implemented due to the potential of damage to the DG, which at the time would be the only available source of AC power to the site.

Despite_the fact that coordination of the Cross-Tie breakers and the

.Dg breakers could not be achieved, the design of the modification is considered to be acceptable for the following two reasons:

1) For a pre-existing fault in the zone consisting of a Cross-Tie cable bus and the companion unit's 4.16 kV ESF bus, the tie breakers will not close because the over-current lockout relay will prevent the breakers from closing. Therefore, only a fault that develops during Diesel Generator Cross-Tie operation could affect the DG of the non-SB0 unit.

Per NUREG/CR-2815 "Probabilistic Safety Analysis Procedures

. Document Control' Desk 'l Guide," the probability of. failure of a 4.16 kV bus for a

. period of.24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> is 7.2E-7. The probability of failure.of

. associated cable is' similarly unlikely.- By comparison, the-probability of a DG failing to run for the same 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> is_8.5E-2 (based ~on San Onofre-specific data). Therefore, an electrical fault provides a negligible contribution to the-overall probability of DG failure during Diesel Generator Cross-Tie operation.

2) If the DG breaker trips prior to the tie breaker trip for a ,

' fault either in the Cross-Tie cables or in the SB0 unit bus I during the Cross-Tie operation, operator action would be to isolate the facit and reconnect the DG to the healthy bus.

Therefore, the shutdown loads of the non-SB0 unit can be supported following a fault'in the SB0 unit during Diesel Generator Cross-Tie operation. ,

Question 3: Surveillance Requirements for Cross-Tie Hand Switches will 4 be incorporated into the Licensee Controlled -

Specifications (LCS). Provide a description of the types and frequency of those surveillances.

Response: ,

Two new Surveillance Requirements (3.8.102.1 and 3.8.102.2) will be ,

incorporated into the Licensee Controlled Specifications (LCS) as part of the implementation of the DG Cross-Tie. LCS 3.8.102.1 will require performance of a visual inspection to verify that the 5054X Hand Switches are'in the " NORMAL" position at an interval of 31 days.

-LCS 3.8.102.2 will require performance of a functional test of the 5054X Hand Switches by placing the Hand Switches in the "50.54X" position and verifying proper contact operation at an interval of

24 months.

Question 4: Provide an explanation that inadvertent operation of the Diesel' Generator Cross-Tie hand-switches will not cause the DG to go.into droop mode.

Response

In'the pre-DG Cross-Tie configuration (Enclosure 3), wh11e the DG is supplying power to the 4.16 kV ESF bus, the DG will enter droop mode

'when.either the Reserve. Auxiliary Transformer (RAT) feb er breaker, the

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. Document Control Desk . .

Unit' Auxiliary Transformer (UAT) feeder breaker, or the associated bus Cross-Tie breaker is closed.onto the bus. 1 DCP U2&3-7048.00SE reconfigured the DG droop circuit (Enclosure 3).

The DG~ droop mode control circuit is modified for 10CFR 50.54(x) operation.to add new logic-that will force the DG supplying the -

4.16 kV ESF bus to operate-in the droop mode when either~the RAT-feeder breaker, the UAT feeder breaker, or.the DG breaker of the S80 unit is closed and both bus Cross-Tie. breakers are closed. This is required to recover the 580 unit'4.16 kV bus when any of the onsite or offsite sources are.available without dropping' loads from the bus.-

During ' design' basis operation, droop circuit contacts from HS-5054XA and HS-5054XB are open. This allows-for continued maintenance on the

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.4.16 kV ESF bus breakers of either unit without affecting the opposite unit. An inadvertent operation or failure of one or both Hand Switch contacts does not alter the DG operation mode by itself. For example,

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with bus 2A04 receiving power from DG 2G002, and bus 3A04 energized, Unit 3 tie-breaker 3A0416 and both 5054X Hand Switches must be inadvertently closed, followed'by inadvertent closure of Unit 2 tie breaker 2A0417 before the DG would be forced into droop mode.

Therefore, it requires multiple operator actions and/or Hand Switch contact' failures to cause the DG to enter droop mode. Therefore the new droop permissive circuit exceeds single failure criteria.

If the DG is already operating in droop mode during testing (parallel i operation with any one of its own preferred sources), and the operator actions or contact failures necessary to connect to the opposite unit should occur, this would have no effect on DG operation.

1 Question 5: The Failure Modes and Effects Analysis provided to the NRC by SCE letter dated 11/20/97 is missing page 4a. Forward this page to the NRC.

Response

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' A' revised version of the Failure. Modes'& Effects Analysis is included as Enclosure 4. : This version includes the missing information and clarifies the functions of the individual 5054X switch contacts.

Question 6: SCE's let'ter of 11/20/97 withdrew the request to perform an IntegratedFunctional-Test ~(IFT)ontheCross-Tie. In lieu of the IFT, _ overlap testing was performed to. demonstrate the  ;

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~ Document Control Desk- j functional'~ capability to implement the Cross-Tie. Provide to'the:NRC a description of the circuits tested, and the types of tests performed.

Response: -

'The tests.were performed by use of Construction Work Orders (CW0s)-to ensure,that the DG Cross-Tie will function as designed.. Existing components affected while performing this-test and modification were retested and restored to their original designed form and function.

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Tests"were performed to demonstrate the following: l

• The 5054X hand switches located in the Exposure Fire Isolation Panels were tested for proper contact form and function. It was verified that the contact configuration changes according to the intent of the design when the hand switches are operated from the " Normal" to the "50.54X" 4 position and vice versa.

• Demonstrate by static test that DG breaker control and alarm circuits, and modified breaker auxiliary position switches function as designed.

e Verify that Standby Power System Inoperable Annunciators, Bus Paralleled Annunciators, and ESF System Bypassed / Inoperable Status Indications affected by this DCP performed as designed, e Demonstrate by static test that the UAT Breaker modified breaker auxiliary position switches functioned as designed.

• Demonstrate by static test that the RAT Breaker modified breaker auxiliary position switches functioned as designed.

• Demonstrate by static test.that the Cross-Tie Breaker Control and modified breaker auxiliary position switches functioned as. designed, s,

e . Demonstrate by static test that the Diesel Generator Governor Control functioned asfdesigned.

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Document Control Desk . .

.The IFT was intended to demonstrate the following:

1. Closure of the tie breaker on a Class 1E 4.16kV bus will not trip its DG breaker with the 5054X hand switches in the "50.54X" position.

2.- A Class 1E 4.16kV bus tie breaker can be closed onto a dead bus with the 5054X hand switches in the "50.54X" position.

3. Alarm and indications function as per design.

4. The DG of the non-SB0 unit can be cross connected to the same train dead bus of the opposite unit with DG governor control not in droop mode and that recovery is possible using offsite power supplies and the opposite unit's DG without dropping loads.

Item 4, above, of this test requires connecting loads from the same trains of both units to the Diesel Generator of the non-SB0 unit.

During this test, a 4.16 kV ESF bus in the operating unit would be disconnected from its normal source of offsite power and auto-sequencing of the loads would be disabled. Because of this, SCE informed the NRC (Reference 4), that an IFT is not required and withdrew the request for NRC approval of the IFT.

In Enclosure 5, the Diesel Generator Cross-Tie modification is shown within the dotted bubbles and was tested by CW0s as described earlier.

SR 3.8.1.8 functionally tests the portion of the circuit shown within the solid bubbles by a transfer test (Surveillance Operating Instructions 023-3-3.19) and crediting the overlap test. SR 3.8.1.8 states that every 24 months, " Verify capability of automatic and manual transfer of AC power sources from the normal offsite circuit to each alternate required offsite circuit."

'SR 3.8.1.8 is performed to demonstrate the OPERABILITY of the alternate preferred power _ distribution source to power the post accident and 1 shutdown loads via the train aligned 4.16 kV Cross-Tie between Unit 2 and Unit 3. Surveillance Operating Instruction S023-3-3.19 is used to

' demonstrate the ability to manually and automatically transfer each Class 1E distribution bus from the normal preferred power source to the

Document-Control Desk . .

alternate preferred power source. The following steps are performed to manually transfer bus 2A04 to the Alternate preferred Power Source (RAT of U3):

- Verify that bus 2A04 is fed from RAT (breaker 2A0418) and bus 3A04 is fed from the RAT (breaker 3A0418).

Verify 2A0417 open and transfer 3A0416 control to manual and then close 3A0416, observing that there is no bus parallel alarm.

Transfer 2A0417 control to manual and then Close 2A0417'and observe that the bus parallel alarm annunciated.

- Open the breaker 2A0418. Observe that the bus parallel alarm cleared.

- Restore normal offsite power to bus 2A04.

- Open Cross-Tie breakers 2A0417 and 3A0416 and return the controls to automatic.

Similar tests are performed on the opposite train.

The transfer test is not generally performed to verify transfer from the UAT or the RAT of one unit to the UAT of the other unit. To complete this surveillance, an overlapping circuit test is performed on that portion of the circuit which permits the companion unit's Cross-Tie breaker to close with its ESF bus aligned to the VAT. In this example breaker 3A0419 is tested to verify that all the interface elements from this breaker to the other control circuit are actuated and function as designed. By doing these tests and taking credit for the overlapping tests, the surveillance requirements of SR 3.8.1.8 are met.

Similarly, the functional transfer test of SR 3.8.1.8 and the circuit tests on the modified control circuits of the Diesel Generator Cross-Tie are considered sufficient to verify intended design function without performing the IFT. The only exception is the design feature to recover the SB0 unit from its own Diesel Generator, because this would require momentary paralleling of the two Diesel Generators. The provision for paralleling two Diesel' Generators during 10CFR 50.54(x) operation to pickup loads is not considered essential (see SCE letter to NRC dated November 20, 1997, Reference 4).

Document Control Desk , .

Conclusion The DG Cross-Tie was designed and installed as a result of the San Onofre IPEEE to respond to beyond-design-basis. events. As such, it will only be used.in a beyond-design-basis event at the site;in accordance with the provisions of 10CFR 50.54(x). This letter provides the information requested by the NRC request in support of review of the DG Cross-Tie. Following NRC approval, SCE intends to proceed with

- implementation of the Cross-Tie.

If you have'any further questions on this subject, please call me.

Sincerely,

},- N Ni Q l

. Enclosures cc: E. W. Merschoff, Regional Administrator, NRC Region IV K. E. Perkins, Jr., Director, Walnut Creek Field Office, NRC Region IV J. A.-Sloan, NRC Senior Resident Inspector, San Onofre Units 2 & 3 M. B. Fields, NRC Project Manager, San Onofre Units 2 and 3 i

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2. NORMALLY ENERGlZED. DE ENERGlZED TO PLACE THE DG IN THE DROOP MODE AND PROVIDE DROOP INDICATION.

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• ENCLOSURE 4 REVISED FAILURE MODES AND EFFECTS ANALYSIS i