ML20195B504

Safety Evaluation Supporting Interim Degraded Grid Voltage Procedures

ML20195B504
Person / Time
Site:	Millstone
Issue date:	05/20/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20195B494	List: ML20195B491 ML20195B504
References
NUDOCS 8605290374
Download: ML20195B504 (6)

Text

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D NORTHEAST NUCLEAR EhERGY COMPANY HILLSTONE NUCLEAn. POWER STATION, UfeIT NO _ . '

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1.0 INTRODUCTIO,N ,

During nurmalp' lant operation, the safety class IE pawer distribution busses are supplied electrical power by the plant's main turbine generator through 1

the normal station service trar.sformftr. Power can also be supplied to the safety class IE di$tribution busses from the off-site transmission grid Vis the r.eserve station service transfomer. When off-site power is not -

-availabie, redundant emergency generatcrs are available to supply power to the safety clast IE power di.stribution busses, 7

P A degraded gr'id voltage condition occurred at Millst ne Nuclear Power istation on July 5, 1976 that caused coimonent failures in the class.1E electric system af Unit 2. The staff requestad (see refere.nced) that the license'a analyze the class IE systems to determine the grid voltage at which damage could of. cur. As a recilt, the li;ensee made plans to install vsltage sensing devices on the class lE busses with coincident logic and with the low voltage set points abova values where equipment

' damage could occbr. It wu planned that the vo?tage ' sensing devices would cause the off-site grid power supp'y ta be disconnected automatically from the class IE System chove voltages where the Asage could occur.

The class IE busies upon deing discennected crould then be supplied power from their respective einergency generator.

However, the licensee was cor.cerned because of the large number of nuclear plants 'in the New England area that significant degradation of the grid would result if the Millstone Unit No. I disconnection was automatic. ,

8605290374 860520 PDR ADOCK 05000245-P PDR

It was the licensco's view that automatic disconnection from the grid should only be required if a low grid voltage occurred at the seme tiine asalossofCoo?antAccideet(LOCA). Tne staff. agreed that a LOCA coincident with in degraded grid voltige candition, should cause automatic disconnect frun the of f-site grid. Iso, for a degraded grid voltage condit1on Without a LOCA, t.he operator could be relied on to take the

necessary manual cctica to prot (ct the class 1E system.

This proposal was reviewed and acceoted by a NRC letter, dated June 23, 1982, (Reference 6) that required the licensee to develop an operatir.g procedure to provide the nececsary operator gyid3nce to protect the class iE system under degraded grid voltage conditions without a LOCA conditicn. By letter dateo Octcbar 9, 1985, the ilcentae submitted ,

for staff review a revised plant procedure to cDer aperator actions, The staff has compluted tne evaluation of this pro'edure c and the latest revision (Revision 23) made available by the licensee.

2.0 DISCUSSION The operating procedure, designed to assure protection of class IE systems under degraded grid voltage conditions without a concurrent LOCA, was reviewed during a site visit discussed in the staff's Inspection Report No. 50-245/85-25.

The staff noted that, when the low voltage alarm signals 345KV on the reserve station service transformer primary, the operator:

Requests Connecticut Valley Electric Exchango (CONVEX) to raise the system voltage above 345KV. If unit 1 is supplying power via the 4.16KV system to Unit 2, the voltage request would be for 347KV. If the 345KV or 347KV requirement cannot be met, the operator notifies the Duty Officer.

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If the voltage cannot be raised above the alarm point or the main generator current is greater than 17.5 Kiloamps, the operator:

- Notifies CONVEX that reactor power is being reduced until the generator current is normal. The power reduction, about 10%, is accomplished by reducing the speed of the recirculation pumps.

- Starts the emergency gas turbine and diesel generator and lets them idle if after the above reduction the 345KV system voltage does not return to normal. (Theemergency generator start provisio.n has since been changed; and is discussed below in the evaluation.)

- Monitors buss voltage on 4.16KV class 1E busses. If the voltage drops to 3646 volts on any two busses, the operator notifies CONVEX that the station wants to open the Unit 1 main generator breaker at the 345KV switchyard.

Opens the main steam bypass valves about 10% using the speed load changer prior to opening the main generator breaker.

Opens the breakers and adjusts the generator speed and excitation after CONVEX permission.

When the station electrical service is being supplied power from the isolated Unit 1 main generator it is not subject to the degraded grid voltage condition.

3.0 EVALUATION The operating procedure 24KV Electrical System, OP 340, Revision 21, for degraded grid ve age was revised (effective February 1, 1985) to include operatcr attention to the fact that alarms also may be received from new sensors on 4.16KV busses 14A,148,14C and 140. These alarias are identified in opert. ting procedure 4160 volt Electrical System,

OP 341, Revision 13 (effective February 1,1985). The operator monitors the voltage of the above busses. The operator action of starting the emergency gas turbine and the diesel generator has been changed in Revision 23, OP 340, effective December 10, 1985. The gas turbine only will be started. The diesel generator will not be started until needed because unloaded operation is restricted to less than 5 minutes dura tion.

The class IE motors' design voltage is 4000 volts. These motors can operate continuously with a ten percent voltage reduction without insulation degradation or life reduction. This reduced voltage, accounting for cable 4.16 voltage drop, measured at the 4.16KV busses is 3646 volts. Should a degraded grid voltage occur, the operator would take corrective action when any two of the four 4.16KV busses voltmeters indicate 3646 volts. The corrective action, opening the main generator breaker, separates the generator from the degraded grid voltage condition. The main generator, then isolated from the grid, supplies power to the motors rated at 4000 volts. Thus, during a degraded grid voltage condition the class 1E motors would not be operated beyond their rated voltage limits and will meet their intended safety function. The procedure provides the operator with guidance to meet this safety objective.

4.0 CONCLUSION

-The staff concluded, based on the consideration discussed above, that:

1. The operating procedure, OP 340, Revision 23, provides an acceptable means for operator action to protect the Class 1E electrical system during a degraded grid voltage condition without a coincident LOCA condition.

2. There is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner.

5-References

1. Reportable Occurrence Report 76-42/IP, July 5, 1976, Millstone Unit 2 Degraded Grid Voltage and Related Event On July 21, 1976.

2. Licensee Event Report 76-42/IT, August 3, 1976, same subject as 1.

3. G. Lear letter to D. C. Switzer, dated June 2,1977.

4. D. M. Crutchfield letter to W. G. Counsil, dated November 24, 1981.

5. W. G. Counsil letter to D. M. Crutchfield, dated April '21,1982.

6. D. M. Crutchfield letter to W. G. Counsil, dated June 23, 1982.

7. J. Shea letter to W. G. Counsil, dated June 30, 1982.

8. Integrated Plant Safety Assessment, NUREG-0824, Millstone Nuclear Power Station, Unit No.1, Final Report, February 1983.

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9. W. G. Counsil letter to D. M. Crutchfield, dated January 17, 1984.

10. W. G. Counsil letter to D. M. Crutchfield, dated April 26, 1984.

11. J. R. Miller letter to W. G. Counsil, dated April 27, 1984,

12. W. G. Counsil letter to D. M. Crutchfield, dated May 11, 1984.

13. W. G. Counsil letter to J. R. Miller, dated June 11, 1984.

14. J. F. Opeka letter to C. I. Grimes, dated May 17, 1985.

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15. H. Le Tho:npson letter to J. F. Opek:, dated July 31, 1985.

16. J. F. Opeka letter to C. I. Grimes, dated October 9, l'385.

17. S. D. Ebneter letter to J F. Opeka, dated November 26, 1985, ,

Dated: K1y 20,1986 Principal Contributor: Frederick 9. Paulitz Division of Reect.or Safety, Region i L

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