ML19344F189

Degraded Grid Protection for Class IE Power Sys for Monticello Nuclear Generating Plant, Technical Evaluation Rept

ML19344F189
Person / Time
Site:	Monticello
Issue date:	08/31/1980
From:	Cleveland C EG&G IDAHO, INC., EG&G, INC.
To:	Shemanski P Office of Nuclear Reactor Regulation
References
CON-FIN-A-6256 EGG-EA-5186, NUDOCS 8009120579
Download: ML19344F189 (14)

Text

Q S ldsho.tnc f OLM EG4G 398 (R@v (179)

INTERIM REPORT

, Accession No.

Report No. EGG-EA-5186. Rev. 1 C:ntract Program or Project

Title:

Electrical, Instrumentation, and Control System Support Sublect of this Document:

Technical Evaluation Report on the Degraded Grid Protection for Class lE Power Systems for the Monticello Nuclear Generating Plant, Docket No. 50-263 Type of Document:

Technical Evaluation Report ,

Author (s):

C. J. Cleveland

- Date of Document:

August 1980 R:sponsible NRC Individual and NRC Office or Division:

Paul C. Shemanski, NRC-NRR This document was prepared primarily for preliminar/ or internal use. It has not received full review and approval. Since there may be subs' < , ave changes, this document should not be considered final.

EG&G Idaho, Inc.

Idaho Falls, Idaho 83415 l

l Prepared for the U.S. Nuclear Regulatory Commission l

Washington, D.C.

Under DOE Contract No. DE-AC07-761D01570 NRC FIN No. A6256 l INTERIM REPORT Soo7GoF77 iN, $C ROSC'jfCh nf Techgjeaf A'tSSISI8DC0 R.ep0d

1585F u

. TECHNICAL EVALUATION REPORT DEGRfsED GRID PROTECTION-FOR CLASS 1E POWER SYSTEMS -

MONTIGLLO NUCLEAR GENERATING PLANT Docket No. 50-263 C. 'J. Cleveland Augus t 1980 I

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1. Revision 1

AESTRACT In June 1977, the.NRC sent all licensees a letter outlining three positions the staf f had taken in regards to the _onsite ecergency paser system. Northern States Power Co=pany (NSP) was to assess the suscepti-bility of the safety-related electrical equiptent at the Ebuticello Gener-ating Plant (NSP-1) to a sustained voltage legcadation of the offsite source and interaction of the of fsite and onsite energency power systems. This report contains an evaulation of NSP's analyses, modifications, and Techni-. .

cal Specification changes to comply with these NRC positions.

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s CONTENTS 1.0 I NTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.0 DESIGN BASE CRITERIA . . . . . . . . . . . . . . . . . . . . . . .  !

3.0 EVALUATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3.1 Existing Chdervoltage Protection . . . . . . . . . . . . . . . 2 3.2 Modifications . . . . . . . . . . . . . . . . . . . . . . . 3 3.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4.0 CONCLUSION

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5.0 REFERENCES

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TECHNICAL EVALUATION REPORT DEGRADED GRID PROTECTION FOR CLASS 1E POWER SYSTEMS MONTI LLO NUCLEAR GENERATING PLANT Q

1.0 INTRODU CTION on June 3,1977, the NRC requested the Northern States Power Company (NSP) to assess the susceptibility of the safety-related electrical equip-ment at the Monticello Nuclear Generating Plant (NSP-1) to a sustained voltage degradation of the offsite source and interaction of the offsite and onsite emergency power systems.1 The letter contained three posi-tions with which the current design of the plant was to be compared. After comparing the current design to the staf f positions, NSP was required to either propose modifications to satis fy the positions and criteria or fur-nish an analyis to subs tantiate that the existing facility design has equivalent capabilitie s.

By letter, dated April 21, 1978, NSP proposed certain design modifica-tions to satis fy the criteria and staf f positions. A ' request for additional information, to clarify some points in NSP's proposal, was sent to NSP by the NRC on June 18, 1979.3 NSP responded by letters dated Spetember 14, 19794 and October 31, 1979 5

. The modifications consist of the instal-lation of a'second-level undervoltage protection system for the class 1E equipment, and blocking of the load-shedding feature when the diesel gener-ator is supplying power to the emergency buses. The NRC required tha, th e setpoint, surveillance requirements, test requirements, and allowable limits were to be included by NSP in the plant Technical Specificaticas.

2.0 DESIGN BASE CRITERIA The oesign base criteria that were applied in determining the accept-

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ability of the system modifications to protect the safety-related equipment

, from a sustained degradation of the offsite grid are:

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1. General Design Criterion 17 (GDC 17), " Electrical Power Sys tems," of Appendix A, " General Design Criteria for Nuclear Power Plants," of 10 CFR 50 6

2. IEEE Standard 27 9-1971, "Clas s IE Power Systems for .

Nuclear Power Generating Stations'd

3. IEEE Standard 30 8-1974, "Clas s 1E Power Systems for Nuclear Power Generating Stations"

4. Staf f positions as detailed in a letter sent to the l ic en see, dated June 3,1977 1

5. ANSI Standard C84.1-1977, " Voltage Ratings for Electri-cal Power Systems and Equipment (60 HZ)."9 3.0 EVALUATION ,

This section provides, in Subsection 3.1, a brief description of the '

existing undevoltage protection available to the NSP-1; in Subsection 3.2, a description of the licensee's proposed modifications for the second-level undervoltage protection; and, in Subsection 3.3, a discussion of how the proposed modifications meet the design base criteria.

3.1. Existing Undervoltage Protection. The present design uses two undervoltage relays on each of the two station 4160 V class 1E safety buses t o de tec t a los s o f o f f sit e powe r. These relays have a setpoint of 2625 V (63%). When the offsite voltage drops to this value and persists for at least ten seconds, the of fsite source breakers are tripped, load shedding is initiated, and the emergency diesel generator breaker is allowed to close automatically as soon as the unit has attained rated speed and vol-tage. Both emergency diesel generator units are started immediately upon a loss of voltage. .

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. The existing system, as ' des igned, does ' disable the load-shedding cir-cuits once the diesel-generator breaker is closed. .If.the' diesel generator braaker is_ tripped, load -sh'edding is automatically reinstate'd, when an

' emergen y power source becomes available.

3.2 Modifications. - %e licensee has proposed adding eight new under-voltage relays to the two 4160 V class lE buses. There will be four relays-

-per bus, arranged in a one-out-of-two taken twice coincidence logic. These.

relays will have a nominal setpoint of. 3885 V (93.4% of bus voltage) with a time delay of ten seconds. Een an undervoltage condition persists below .

the setpoint 'for at least' ten seconds, the incoming line breakers to-the emergency 4160 V. buses are tripped. Een these breakers trip, the loss- '

of-voltage relays. start the diesel generators. The licensee has also added one new undervoltage relay to make the los s-of-voltage relay -logic; a three- l out-o f-three coincidence scheme. E en the second-level undervoltage relays activate and the ten-second time delay is satisfied, the automatic fast transfer to the start-up transformer is' inhibited. After an additional five-second delay, initiated by the actuacion of the loss-of-voltage relays, load shedding is acconplished and the die'sel generators energize the. safety-buses as rated-speed and voltage are reached. Wenever an undervoltage relay is removed for testing or maintenance a trip signal for-that relay l

will be initiated.

Ioad-shed blocking, once the diesel generator is supplying power to the emergency buses, is a part of the plants current design. This is accomplished by interlocks which sense the closed position of the onsite-

.: surce supply breaker s. If the onsite source breakers are tripped, load shedding. is automatically reinstated.

Proposed changes to the plant's Technical Specifications, adding the surveillance requirements, allowable limits for the setpoint and time delay, and limiting conditions for operation for the second-level ~undervoltage

. - monitor s, were also fuknished- by the' licensee. An analysis to substantiate the limiting conditions and minimum and maximum setpoint limits was also part.of the proposal.

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3.3 Discussion. 'Mie first position of the NRC staff letterl required that a second level of undervoltage protection for the onsite power system be provided. The letter stipulates other criteria that the under voltage protection must mee t. Each criterion is restated below fol-lowed by a discussion regsrding the licensee's compliance with that .

criterion.

1. "The selection of voltaf s and time setpoints shall be determined from an analysis of the voltage requirements of the safety-related loads at all onsite system distri-bution ' le vel s."

The licensee's proposed setpoing of 3885 V at the 4160 V bus is 97% of the motor-related voltage of 4000 v. This setpoint reflected down to the 480 V buses will be

, ater than 90% of the motor-rated voltage. As the mctors are the most limiting equipment in the system, .

this setpoint is acceptable. The licensee's analysis considered these factors. ' '

2. "The voltage protection shall include coincidence logic to preclude spurious trips of the of fsite power sources."

The proposed modification incorporates a one-out-of-two taker. twice logic scheme, thereby satisfying this I

criterion.

3. "The time delay selected shall be based on the following condition s:

a. "The allowable time delay, including margin, shall i not exceed the maximum time delay that is assumed in the FSAR accident analysis." -

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2 The proposed time delay ofiten ' seconds' does not exceed this maximum time delay. - This is ' subs tanti-ated _by 'the licensee in his proposal.

_, The proposed time delay will not be the cause of any

, thermal damage to the safety-related equipment. The setpoint is within voltage ranges recommended by-f ANSI.C84.1-1977 for sustair.ed: operation.

b. "The time delay shall minimize the effect of .short-duration disturbances from reducing the unavailabil-

, ity of the' of fsite power source (s) ."

l Ihr. License e 's proposed t Lee delay. of tet, seconds is l'

.1reng enough to override _any short incor. quential q y; rid disturbances.- Further, we have rev wed the licensee's analysis and agree with the lic

, Tee's finding that any voltage dips caused from the start-

'ing.of large motors will not trip the .offsite source. .

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c. "The ' allowable- time duration of a degraded voltage j- condition at all distribution ~ system levels shall f not result in failure of safety systems or j' components."

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. A review of the licensee's voltage analysis 10 indicates that the time delay will not cause any l failures of the s.tfety-related equipment since the voltage setpoiat is within the allowable tolerance of the equipment-rated voltage.

4. _ "The voltage monitors shall automatically initiate the

. disconnection iaf offsite power sources whenever the

. voltage septoint and time delay limits have been j e xceeded."

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A review of the licencee's proposal substantiates that this criterion is met.

5. "The voltage monitors shall be designed to satisfy the '

requirements ci IEEE Standard 279-1971." ,

The licensee has stated in his proposal that the modifi-cations are designed to meet or exceed IEEE Standard 279.

6. "The Technical Specifications shall include limiting conditions for operations, surveillance requirements, trip sctpoints with minimum and maximum limits, and allowable values for the second-level voltage protection monitors."

The licensee's draf t proposal for Technical Specification changes does include all the required items. An analyses ,

had been performed which assures that the range between the minimum and maximum trip point settings, as well as -

the allowable limits, will not be the cause of spurious trips of the offsite source nor will they allow the voltage to be so low as to allow damage to the safety equipmen t.

The second NRC staf f position requires that the system design automat-ically prevent load shedding of the emergency buses once the onsite sources are supplying power to all sequenced loads. The load shedding must also be reinstated if the onsite breakers are tripped.

The licensee s tated in his proposal that the current design meets this posi tion fully. A review < f his submittal and logic diagrams confirms this contention.

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r 1he third NRC stcf f position requires that certain test requirements be added to the Technical Specifications. These tests were to demonstrate the full-functional operability and independence of the onsite power sources and are to be performed at least once per 18 months during shutdown. Th e

, tests are to simulate ' loss of offsite power in conjunction with a safety injection actuation signal and to simula te interruption and subsequent reconnection of onsite power sources. These tests verify the proper opera-tion of the load-shed sys tem, the load-shed bypass when the emergency diesel generators are supplying power to their respective buses, and that there is no adverse interaction between the onsite and offsite power sources.

Die licensee has stated tha t, with the exception of the 5 minute time dura' tion of the diesel generator tests, this position is currently being met.2 They have further committed to add this time duration to the diesel tests when they submit their Technical Specification changes. Load shedding on offsite power trio is tested. Load sequencing, once the . diesel generator is supplying the safety buses, is tested. Tha time durations of the tests (five minutes with full safety loads) will verify that the time delay is suf ficien t to avoid spurious trips and that the load-shed bypass circuit is functioning properly.

4.0 CONCLUSION

S Ba sed on the information provided by NSP, it has been determined that the proposed modifications comply with NRC staf f positioa 1. All o f th e s' af f's requi rements and design base criteria have been met. The modifica-tions will protec t the clas s 1E equipment from a sustaired degraded voltage condition of the of fsite power source.

l The existing load-shed circuitry does comply with staf f position 2 and will present adverse interaction of the offsite and onsite emergency power systems.

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l The proposed draf t changes to the Technical Specifications and change commitments do adequately test the system modifications and do comply with staf f . position 3. The surveillance requirements, limiting conditions for operation, minimum and-maximum limits for the trip point, and allowable values meet the intent of staf f' position 1. .

It is therefore concluded that NSP's proposed modifications are accept-able. Fbrther, it is recommended that the changes to the Technical Speci-fications, if similar to the supplied draf t and if they meet the licensee's commitment, be incorporated in the NSP-1 Technical Specifications when the modifications are c *mplete.

5.0 REFEREN CE5

1. NRC (D. K. Davis) letter _ to NSP (L. O. Mayer), dated June 3,1977.

2. NSP (L. O. M a yer) letter to NRC, dated April 21, 1978. ,

3. NRC (T. Ippolito) letter to NSP (L. O. Mayer), dated June 18, 1979. -

4. NSP (L. O. Phyer) letter to NRC, dated September 14, 1979.

5. NSP (L. O. Mayer) letter to NRC, dated Oc tober 31, 1979.

6. General Design Criterion 17, " Electric Power Systems," of Appendix A,

" General Design Criteria of Nuclear. Power Plants," to 10 CFR Part 50,

" Domestic Licensing of Production and Utilization Facilities."

7. IEEE' Standard 279-1971, " Criteria for Protection Systems for Nuclear Power Generating Stations."

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8.- IEEE Standard 308-1974, " Standard Criteria for Class IE Power Sys tems for Nuclear Power Generating Stations." .

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9. A11SI C84.1-1977, " Voltage Ratings for Electric Power Systems and

. Equipment" (60'HZ).

10. NSP (L. O. ' Hayer) letter; to NRC (D. L. Zeinann), dated September 17,

, 1976.

11. USP (L. O. Mayer) letter to NRC (D. L. Zeimann), dated HMrch 4,1977.

12. NSP- (L. O. Mayer) letter to NRC (D. K. Davis), dated July 25, 1977.

13. HSP (L. O, hayer) letter to NRC (D. K. Davis), dated October 14, 1977.

14. Final Safety haalysis . Report (FSAR) for the Monticello !bclear Gener-a ting Plant (NSP-1).

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