ML20052C569

Degraded Grid Protection for Class IE Power Sys,Dresden Station,Units 1 & 2, Preliminary Informal Rept

ML20052C569
Person / Time
Site:	Dresden
Issue date:	03/31/1982
From:	Udy A EG&G, INC.
To:	Prevatte R Office of Nuclear Reactor Regulation
References
CON-FIN-A-6429 EGG-EA-5811, NUDOCS 8205050169
Download: ML20052C569 (11)

Text

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EGG-EA-5811 March 1982 l DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS, ORESDEN STATION, UNIT NOS. 2 AND 3 A. C. Udy PRELIMQNARY U.S. Department of Energy Idaho Operations Office

• Idaho National Engineering Laboratory

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s This is an informal report Intended for use as a preliminary or working document l

Prepared for the U.S. Nuclear Regulatory Comission Under DOE Contract No. DE-AC07-76ID01570 I FIN No. A6429 [l EGnG ,s.n.

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• INTERIM REPORT Accession No.

Report No. EGG-EA-5811 C:ntract Program or Project

Title:

Selected Operating Reactors Issues Program (III)

Subject of this Document Degraded Grid Protection for Class lE Power Systems, Dresden Station, Unit Nos. 2 and 3 Type of Document:

Informal Report Auth:r(s):

A. C. Udy D;t] of Document:

March 1982 Responsible NRC Individual and NRC Office or Division:

R. L. Prevatte, Division of Systems Integration 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 I not be considered final.

l EG&G Idaho. Inc.

Idaho Falls. Idaho 83415 Prepared for the U.S. Nuclear Regulatory Commission Washington, D.C.

Under DOE Contract No. DE-AC07 761D01570 NRC FIN No. A6429 INTERIM REPORT l

0025j DEGRADED GRID PROTECTION FOR CLASS 1E POWER SYSTEMS DRESDEN STATION, UNIT NOS. 2 AND 3 Docket No. 50-237 and 50-249 March 1982 A. C. Udy Reliability and Statistics Branch Engineering Analysis-Division EG&G Idaho, Inc.

C TAC Nos. 10019 and 10021

ABSTRACT This EGM Idaho, Inc. report reviews the susceptibility of the safety-related electrical equipment, at the Dresden 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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CONTENTS 1.0 I N TR O DU CT I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.0 D ES I GN B AS E CR I TOR I A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3.0 E VA L U AT I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3.1 Ex is ting Un dervol tage Pro tection . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3.2 Mo d i f i c a t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3.3 Discussion ................................................ 2 i

4.0 CO NCL U S IO NS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.0 R E F ER EN C E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1

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DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS DRESDEN STATION, UNIT NOS. 2 AND 3

1.0 INTRODUCTION

On June 3,1977, the IRC requested Commonwealth Edison (CE) to assess the susceptibility of the safety-related electrical equipment at the Dresden Station to a sustained voltage degradation of the offsite action of the offsite and onsite emergency power systems.) Thesource letterand inter-contained three positions with which the current design of the plant was to be compared. Af ter comparing the current design to the staff positions, CE

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.

CE responded to the NRC lgtter on July 27, 1977.2 CE propcsed design modifications on June 26, 1980 a modifications on October 1,1980.gd Draft provided additional technical details on specifications the were submitted on Octgber 28, 1981.b These were modified by a suomittill of January 6, 1982,0 and replaced with a submittal of March 4,1982.'

i 2.0 DESIGN BASE CRITERIA i

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 Ag " General Design Criteria for Nuclear Power Plants,"

of 10 CFR 50

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

3. IEEE Standard 308-197 Generating Stations."g " Class IE Power Systems for Nuclear Power

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

5. ANSI Standard C84.1-1977, "Volty Ratings for Electrical Power Systems and Equipment (60 HZ)."

3.0 EVALUATION l This section provides, in Subsection 3.1, a brief description of the existing undervoltage protection at the Dresden Station; in Subsection 3.2, a descr.ption of the licensee's proposed scheme for the second-level under-voltage protection; and, in Subsection 3.3, a discussion of how the system meets the design base criteria.

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3.1 Existing Undervoltage Protection. The present design utilizes two undervoltage relays on eacn 4:euv t, tass lE bus. They are arranged in a two-out-of-two logic scheme that senses complete loss of voltage. The relays and their logic circuitry start the diesel generator, initiate load-shedding, and trip the incoming line breaker.

The existing logic circuitry of the undervoltage protection system does not disable the load-shed feature once the diesel generators are sup-plying power to the Class lE buses.

3.2 Modifications. To protect the Class lE safety-related equipment from the effects of a degraded grid condition, tha licensee has proposed adding another set of undervoltage relays to each of the 4160V Class 1E buses. Each set will be comprised of two solid-state undervoltage relays that have an inherent time delay of seven seconds, arranged in a two-out-of-two logic scheme, with associated auxiliary relays and a timer added to the undervoltage logic circuitry. The setpoint of the second-level protec-tion relays will be between 3708V and 3784V, with a time delay of 5 minutes.

Should the two undervoltage relays remain tripped for 5 minutes, or if a LOCA signal occurs during that 5 minute period, the diesel is started, and the undervoltage condition is annunciated in the control room, the incoming line breakers are tripped, load-shedding is initiated, and finally the diesel generator breaker is closed when the voltage and the frequency from the diesel generator are satisfactory. The loss-of-voltage relays function as before, with a setpoint between the limits of 3092V and 3255V.

Once the diesel generator is supplying its associated Class lE bus, load-shedding is blocked by the "b" contact of the diesel generator breaker.

Proposed changes to the station'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 monitors, were furnished by the licensee.7 3.3 Discussion. The first position of the NRC staff letterI required that a second level of undervoltage protection for the onsite power system t be provided. The letter stipulates other criteria that the undervoltage I

protection nust 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 distribution levels."

The licensee's proposed nominal setpoint of 3746V at the 4160V bus is 93.6% of the motor nominal voltage rating of 4kV. This -

voltage segoint was chosen after a system voltage analysis was completed. The time delay allows operator action to improve the voltage levels. The time delay is defeated immediately should a LOCA signal occur.

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

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The relay logic is arranged in a two-out-of-two logic scheme, thereby satisfying this criterion.

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

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

There is no induced time delay for undervoltage protection should an accident signal be present. The Class lE bus will be ready for transfer to diesel-generator power before the diesel is up to speed.

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

The licensee's proposed minimum time delay of 5.6 seconds is long enough to override any short inconsequential grid dis turb ances.

c. "The allowable time duration of a degraded voltage condition at all distribution system levels shall not result in fail-ure of safety systems or components."

A review of the licensee's voltage analysis 12 indicates that the time delay will not cause any failures of the safety-related equipment since the nominal voltage setpoint is within the allowable tolerance of the equipment voltage rating.

4. "The voltage monitort shall automatically initiate the disconnec-tion 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 cri-terion is met.

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

The licensee has stated that the circuits associated with the widervoltage relays meet the applicable requirements of IEEE Standard 279-1971. -

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

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The licensee's proposal for technical specification changes includes the required items. The voltage setpoint, with the max-imum allowable limit included, is 3784V. The lowest bus voltage available to the undervoltage rela as determined by CE, is 3784V (Unit 2) and 3856V (Unit 3).g, Therefore, spurious trips of the offsite source due to operation of the undervoltage relays is not possible with the most limiting tolerance. The limiting conditions for operation, the surveillance requirements, the chan-nel test frequency and the calibration frequency are included in the technical specifications, and while not in conformance with the model technical specifications, do meet the criteria of the staff's pos itions. ,

The second NRC staff position requires 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.

The licensee has modified the Dresden Station to incorporate this fea-ture in the circuit design at Units 2 and 3. The load-shed is blocked by an auxiliary contact of the diesel generator circuit breaker.

The third NRC staff 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. The tests are to simulate loss of offsite power in conjunction with a simulated safety injection actuation signal and to simulate interruption and subse-r;uent reconnection of onsite power sources. These tests verify the proper operation of the load-shed system, 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 l sources.

The testing procedures proposed by the licensee comply with most of this position. Load-shedding on the trip of offsite power is tested. Load-sequencing, once the diesel generator is supplying the safety buses, is tes ted. A simulated loss of the diesel generator and subsequent load-shedding and load-sequencing once the diesel generator is back on-line is not tested. A loss of the diesel generator prohibits automatic restart-ing. The load sequencer is controlled by a contact of the diesel-generator breaker, and therefore, by design, tripping of the diesel generator breaker will reset the load shedding and the load sequencing circuitry. The time duration of the test will verify that there is no interaction between the offsite and the onsite power sources and that the load-shed bypass circuit is functioning properly. ,

4.0 CONCLUSION

S Based on the information provided by CE, 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 4

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and time delay will protect the Class 1E equipment from a sustained degraded voltage condition of the offsite power source.

The load-shed circuitry has been modified to comply with staff posi-tion 2 and it will prevent adverse interaction of the offsite and onsite emergency power systems.

The changes to the technical specifications adequately test the system modifications and canply with staff position 3. The surveillance require-ments, limiting conditions for operation, minimum and maximum limits for the trip point, and allowable values satisfy staff position 1.

Therefore, CE's proposed changes and technical specification changes are acceptable. It is recommended that the proposed technical specification changes be approved and implemented to coincide with completion of the modifications.

5.0 REFERENCES

1. NRC letter to R. L. Bolger, CE, June 3,1977.

2. CE letter, R. L. Bolger to K. R. Goller, NRC, "Onsite Emergency Power Systems," July 22, 1977.

3. CE letter, R. F. Janecek to D. G. Eisenhut, NRC, "Second Level of Undervoltage Protection for 4KV Onsite Emergency Power Systems,"

June 26,1980.

4 CE letter, R. F. Janecek to T. M. Novak, NRC, " Response to Request for Information Concerning Undervoltage Protection," October 1,1980.

5. CE letter, T. J. Rausch to D. G. Eisenhut, NRC, " Draft Technical Specifications Concerning Undervoltage Protection," October 28, 1981.

6. CE letter, T. J. Rausch to P. O' Conner, NRC, " Draft Technical Specification Concerning Undervoltage Protection," January 6,1982.

7. CE letter, T. J. Rausch to H. R. Denton, NRC, " Proposed Technical Specification Changes to Implement Second-level Undervoltage Relays,"

March 4, 1982.

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

" General Design Criteria for 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 Systens for Nuclear Power Generating Stations."

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

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12. " Adequacy of Station Electric Distribution System Voltages, Dresden Station, Unit Nos. 2 and 3," EG&G Idaho, Inc. informal report i EGG-EA-5322, A. Udy, August 1981.

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