ML20039G465

Degraded Grid Protection for Class IE Power Sys,Crystal River Nuclear Generating Plant,Unit 3.

ML20039G465
Person / Time
Site:	Crystal River
Issue date:	11/30/1981
From:	Udy A EG&G, INC.
To:	Donohew J Office of Nuclear Reactor Regulation
References
CON-FIN-A-6429 EGG-EA-5672, NUDOCS 8201180275
Download: ML20039G465 (14)

Text

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This is an informal report intended for use as a preliminary or working document Prepared for the U.S. Nuclear Regulatory Cornission Under DOE Contract No. DE-AC07-76ID01570 FIN No. A6429 8201180275 811130 E Idaho PDR RES 8201180275 PDR

[ EGnG,.. . ,-

FORM EG&G 398 (Rev 1979)

INTERIM HriPORT Accession No.

Report No. EGG-EA-5672 Contract Program or Project

Title:

Selected Operating Reai. ors Issues Program (III)

Subject of this Document:

- Degraded Grid Protection for Class lE Power Systems, Crystal River Nuclear Generating Plant, Unit No. 3, Docket No. 50-302, TAC No. 10017 Type of Document:

Informal Report Author (s):

A. C. Udy D;te of Document:

November 1981 l

Responsible NRC Individual and NRC Office or Division:

J. N. Donohew, Division of Licensing This document was prepared primarily for preliminary or internal use. it has not received full review and approval. Since there may be substantive changes, this document should not be considered final.

EG&G Idaho, !nc.

Idaho Falls, Idaho 83415 s

Prepared for the U.S. Nuclear Regulatory Commission Washington, D.C.

Under DOE Contract No. D -A 07 761D01570 NRC FIN No. A 2 INTERIM REPORT l

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0624J DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS CRYSTAL RIVER NUCLEAR GENERATING PLANT, UNIT N0. 3 Docket No. 50-302 November 1981 A. C. Udy Reliability and Statistics Branch Engineering Analysis Division EG&G Idaho, Inc.

11/18/81 TAC No. 10017 J

ABSTRACT This EG&G Idaho, Inc. report reviews the susceptibility of the safety-related electrical equipment, at Unit 3 of the Crystal River station, to a -

sustained degradation of the offsite power sources.

FOREWORD This report is supplied as part of the " Selected Operating Reactor Issues Program (III)" being conducted for the U.S. Nuclear Regulatory Com-mission, Office of Nuclear Reactor Regulation, Division of Licensing, by EG&G Idaho, Inc., Reliability and Statistics Branch.

The U.S. Nuclear Regulatory Commission funded the work under Authoriza-tion B&R 20-19-01-06.

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I CONTENTS 1.0 I NTR O D UC T I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 2.0 DESIGN BASE CRITERIA ............................................ 2 3.0 EVALUATION ...................................................... 2 3.1 Existing Undervoltage Protection .......................... 3 3.2 Modifications ............................................. 3 3.3 Discussion ................................................ 4 4.0 C O NC L U S I O N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5.0 REFERENCES

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DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS CRYSTAL RIVER NUCLEAR GENERATING PLANT, UNIT NO. 3

1.0 INTRODUCTION

On June 6,1977, the NRC requested the Florida Power Corporation (FPC) to assess the susceptibility of the safety-related electrical equipment at Unit 3 of the Crystal River station, to a sustained voltage degradation of the offsite source ar.d interaction of the offsite and onsite emergency power systems.I The letter contained three positions with which the current design of the plant was to be compared. After comparing the current design to the staff positions, FPC was required to either propose modifications to satisfy the positions and criteria or furnish an analysis to substantiate that the existing facility design has equivalent capabilities.

By letters dated July 21, 1977,2 and July 25, 1977,3 FPC responded to the NRC letter proposing design modifications and changes to the plant's Technical Specifications to satisfy the criteria and staff positions.

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. Voltage analyses and further information were provided on April 3, 1980 and on December 22, 1980.5 FPC responded to NRC questions on the subject on October 30, 1981.0 This information was clarified by telephone on November 12, 1981.7 l

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2.0 DESIGN BASE CRITERIA The design base criteria that were applied in determining the accept-ablility of the system modifications to protect the safety-related equipment from a sustained degradation of the offsite grid voltage are:

1. General Design Criterion 17 (GDC 17), " Electrical Power .

Systems," of Appendix A, " General Design Criteria for Nuclear Power Plants," of 10 CFR 50.8 ,

2. IEEE Standard 279-1971, " Criteria for Protection Sys-tems for Nuclear Power Generating Stations."9

3. IEEE Standard 308-1974, " Class lE Power Systems for Nuclear Power Generating Stations."10

4. Staff positions as detailed in a letter sent to the licensee, dated June 3, 1977.I

5. ANSI Standard C84.1-1977, " Voltage Ratings for Electri-cal Power Systems and Equipment (60 Hz)."Il 3.0 EVALUATION This section provides, in Subsection 3.1, a brief description of existing undervoltage protection at Unit 3 of the Crystal River Station; in -

Subsection 3.2, a description of the licensee's proposed scheme for the ,

second-level undervoltage protection; and, in Subsection 3.3, a discussion of how the existing system meets the design base criteria.

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3.1 Existing Undervoltage Protection. The present design utilizes three inverse tinie-delay undervoltage relays on each 4160V Class lE bus.

They are arranged in a two-out-of-three logic scheme with a setpoint of 2375V (59.4% of the equipment rated voltage). The lower the bus voltage the faster these relays operate. The relays and their logic circuitry also start the diesel generator, initiate load-shedding, and trip the incoming line breaker. The undervoltage relays also activate an annunciator in the control room, e

The existing logic circuitry of the undervoltage protection system does disable the load-shed feature once the diesel generators are supplying power to the Class lE buses. The load-shed feature is re-instated should the diesel gw erator output circuit breaker trip.

3.2 Modifications. To protect the Class lE safety-related equipment from the effects of a degraded grid condition, the licensee has proposed adding another set of undervoltage relays to each of the 4160V Class lE buses. Each set will be comprised of three relays, arranged in a three-out-of-three logic scheme, with associated auxiliary relays and timers added to the undervoltage logic circuitry. The setpoint of the second-level pro-tection relays will nominally be 90% of 4160V (3744V) with a tolerance of

+2% (thus the setpoint will always be between 3660V and 3827V). When a

. degraded grid voltage operates any one relay the undervoltage condition is annunciated in the control room. The diesel generator is started should the undervoltage condition persist for seven seconds. If the voltage recovers before T = 20 seconds, the relays reset and offsite power is 3

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maintained. If the voltage does not recover, the offsite source breakers are tripped, the bus loads are stripped from the bus and then the diesel generator breaker is closed at T = 23 seconds. Then the first block of loads.is sequenced onto the bus. A safety injection signal, in addition to starting the diesel generators, bypasses 13 seconds of the time delay.

Once the diesel generator is supplying its associated Class lE bus, load-shedding is blocked by interlocks and auxiliary relays in the under-voltage protection logic circuitry. As stated above, this is already .

incorporated in the existing logic circuits.

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 I

monitors, have not been furnished by the licensee. The limiting conditions of operation and surveillance requirements pertaining to these relays have j

not been supplied.

l 3.3 Discussion. The first position of the NRC staff letter required that a second level of undervoltage protection for the onsite power system be provided. The letter stipulates other criteria that the undervoltage protectioti must meet. Each criterion is restated below followed by a dis- .

cussion regarding the licensee's compliance with that criterion.

1. "The selection of voltage and time setpoints shall be determined from an analysis of the voltage requirements of the safety-related loads at all onsite system dis-tribution levels."

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The licensee's proposed setpoint of 3744V at the 4160V bus is 93.6% of the motor nominal voltage rating of 4000V. This setpoint reflected down to the 480V buses will be greater than the minimum allowable motor voltage (90% of nominal voltage). As the motors are the most limiting equipment in the system, this set-point is acceptable. The licensee's analysis con-sidered other factors, such as MCC contactor pick-up voltage and drop-out voltage.

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

The relay logic is arranged in a three-out-of-th:ee logic scheme, thereby satisfying this criterion.

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

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

The proposed time delay of 23 seconds (10 seconds with a safety injection) until diesel generator supplied power is applied to the loads does not exceed this maximum time delay. This is sub-

. stantiated by the licensee in his proposal, and in the Final Safety Analysis Report.

b. "The time delay shall minimize the effect of short- l duration disturbances from reducing the unavail-ability of the offsite power source (s)."

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- r The licensee's proposed time delay of 20 seconds (7 seconds with a safety injection) is long enough to override any short inconsequential grid distur-I bances. Further, review of the FPC analysis shows that any voltage dip caused by starting large motors will not trip the offsite source.

c. "The allowable time duration of a degraded voltage condition at all distribution system levels shall .

not result in failure of safety systems or com-ponents." .

A review of the licensee's voltage analysis 5 indicates that the time delay will not cause any failures of the safety-related equipment since the voltage setpcint is within the allowable tolerance of the equipment voltage rating.

4. "The voltage monitors shall automatically initiate the disconnection of offsite power sources whenever the voltage setpoint and time-delay limits have been exceeded."

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

5. "The voltage monitors shall be designed to satisfy the-requirements of IEEE Standard 279-1971."

The licensee has stated in his submittal that all- cir- .

cuits associated with the undervoltage relays meet IEEE -

Standard 279-1971.6 ,

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6. "The Technical Specifications shall include limiting s

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.Tt.e licensee has not proposed. technical specification changes. - Instead, FPC will request the technical specification changes after-tbey'hdve received a favor-Ja ble safety, evaluation report from the NRC.6 The second NRC staff positio'n.reqdires that the system design auto-

matically 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.

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The licensee states in his submittal that this feature is already 1-incorporated in the circuit design.

'The third NRC staff position requires that certain test requirements

'be added to the Technical Specifications. These tests were to demonstrate

he full-functional cperability and independence of the onsite power sources and are to be performed at least once per 18 months during shutdown. The tests are to simulate loss of offsite power in conjunction with a simulated safety ipjection actuation signal and to simulate interruption and subse-5,uent reconnection of onsite power sources. These tests verify tne proper operation of the load-shed system, the load-shed bypass when the emergency n.- , ,,

< ., < diesel generators dre supplying power to their respective buses, and that i there ls no ' adverse interaction between the onsite and offsite power

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These positions are also yet to be addressed by FPC in a request for technical specification changes.

4.0 CONCLUSION

S Based on the information provided by FPC, it has been determined that the proposed changes do comply with NRC staff position 1. All of the staff's requirements and design base criteria have been met. The setpoint dnd time delay will protect the Class lE equipment from a sustained degraded voltage condition of the offsite power source. However, technical specifi-cation charges have not been supplied.

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~ Theexistjng~ load-shedcircuitrydoescomplywithstaffposition2and will prevent ' adverse interaction of the offsite and onsite emergency power I. '

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.Tbgefore,-FPbtsproposedchangesareacceptable. As the second-level undervoltage protection modifications are to be installed in the first m .s ^

u nutage of_suff;icient; duration, it is recommended that FPC propose Technical

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, Specificarion7 changes, to be approved as a supplement to this report and v

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ImplementId e-it the earliest opportunity.

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'S .0 REFERENCES

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fRC letter (J. Stolz) to FPC, June 6, 1977.

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2. FPC letter, J. T. Rodgers to Director, Office of Nuclear Reactor Regulation, NRC, " Operating License No. DPR-72," July 21, 1977, 3-0-4e, 3-0-17.

3. FPC letter, J. T. Rodgers to R. W. Reid, NRC, " Operating License No. DPR-72," July 25, 1977, 3-0-3-a-3.

4. FPC letter, R. M. Bright to W. Gammill, NRC, " Adequacy of Station Electric Distribution Systems Voltages," April 3, 1980, 3-0-3-a-3.

5. FPC letter, P. Y. Baynard to Division of Licensing, NRC, " Adequacy of Station Electric Distribution Systems Voltages," December 22, 1980, 3-0-3-a-3.

6. FPC letter, W. A. Cross to J. F. Stolz, NRC, " Degraded Grid Voltage,"

October 30, 1981, #3F-1081-27, File 3-0-3-a-3.

7. Telecon, A. Udy, EG&G Idaho, Inc., Gary Castleberry FPC, and Ken Baker, FPC, November 12, 1981.

8. 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."

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

10. IEEE Standard 308-1974, " Standard Criteria for Class lE Power Systems for Nuclear Power Generating Stations."

11. ANSI C84.1-1977, " Voltage Ratings for Electric Power Systems and Equip-ment." (60 Hz).

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