ML20053F032
| ML20053F032 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 05/31/1982 |
| From: | Udy A EG&G, INC. |
| To: | Prevatte R Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20053F033 | List: |
| References | |
| CON-FIN-A-6429 EEG-EA-5677, EEG-EA-5766, EGG-EA-5677, NUDOCS 8206100327 | |
| Download: ML20053F032 (7) | |
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EGG-EA-5766 May 1982 DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS,
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ZION STATION, UNIT NOS. l AND 2 A. C. Udy U.S. Department of Energy
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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 i
Precared for the U.S. Nuclear Regulatory Comission Under DOE Contract No. DE-AC07-761001570 FIN No A6429
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% uni INTERIM REPORT Accession No.
EGG-EA-5766 Report No.
Csntr:ct Program or Project
Title:
Selected -0perating Reactors Issues Program (III)
Subject of this Document:
Degraded Grid Protection for Class 1E Power Systems, Zion Station, Unit Nos. I and 2 Type cf Document:
Informal Report Auth:r(s):
A. C. Udy Date cf Document:
May 1982 R:sponsible 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 not be considered final.
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 76tD01570 NRC FIN No.
A6429 INTERIM REPORT l
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i DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS ZION STATION, UNIT NOS.1 AND 2 May 1982 A. C. Udy Reliability and Statistics Branch Engineering Analysis Division EG&G Idaho, Inc.
8 Draft 2/5/82 s
TAC Nos. 10061 and 10062 Docket No. 50-295 and 50-304
d ABSTRACT This EG&G Idaho, Inc. Report reviews the susceptibility of the safety-related electrical equipment at the Zion Station to a sustained degradation of the offsite power sources.
FOREWORD This report is supplied as part of the " Selected Operating Reactors 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 20-19-10-11.
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I CONTENTS
1.0 INTRODUCTION
1 2.0 DESIGN BASE CRIT 8RIA............................................
1 3.0 EVALUATION......................................................
1 3.1 Ex i sti ng Undervolt age P rotection..........................
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3.2 Modifications.............................................
2 3.3 Discussion................................................
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4.0 CONCLUSION
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5.0 REFERENCES
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DEGRADED GRID PROTECTION FOR CLASS lE POWER SYSTEMS ZION STATION, UNIT NOS. 1 AND 2
1.0 INTRODUCTION
On June 3, 1977, the NRC requested Commonwealth Edison (CE) to assess the susceptibility of the safety-related electrical equipment at the Zion Station to a sustained voltage degradation of the offsite) source and inter-action of the offsite and onsite emergency power systems.
The letter 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 f acility design has equivalent capabilities.
27, 1977.2 CE proposed design 26,1980,jter on July CE responded to the NRC le modifications on June modifications on October g,1980.gnd provided additional details on the Draft technical specifications were submitted on May 5,1982.
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
" General Design Criteria for Nuclear Power Plants,"
of 10 CFR 50 Nuclear Power Generating Stations."gr Protection Systems for IEEE Standard 279-1971, " Criteria f 2.
Generating Stations."g, Class lE Power Systems for Nuclear Power IEEE Standard 308-197 3.
dated June 3,1977.jetailed in a letter sent to the licensee, 4
Staff positions as 5.
ANSI Standard C84.1-1977, "Voltgge Ratings for Electrical Power Systems and Equipment (60 HZ)."
3.0 EVALUATION This section provides, in Subsection 3.1, a brief description of the existing undervoltage protection at the Zion Station; in Subsection 3.2, a description 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 each 4160v class lE bus. They are arranged in a too-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 IE buses.
3.2 Modifications. To protect the Class 1E 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 is comprised of two solid-state undervoltage relays, each Cith a built in 8 + 2 second time delay and arranged in a two-out-of-two logic scheme, with associated auxiliary relays and timers added to the undervoltage logic circuitry. The setpoint of the second-level protection relays is 3846 volts + 2% and the time delay is 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, 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.
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, monitors, have been furnished by the licensee.gecond-level undervoltage and limiting conditions for operation for the A basis for limiting conditions of operation, as well as a bases for surveillance requirements pertaining to these relays, was also furnished.
I 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 protection 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 distribution levels."
The licensee's proposed setpoint of 3846V at the 4160V bus is 96.2% of the motor nominal voltage rating of 4kV. This point 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 inniediately should a LOCA signal occur.
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"The voltage protection shall include coincidence logic to pre-clude spurious trips of the offsite power sources."
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:
a.
"The allowable time delay, including margin, shall not exceed the maximum time delay that is asstaned in the FSAR accident analysis."
There is no greater than a 10 second time delay for under-voltage protection should an accident signal be present.
The diesels start on the accident signal and transfer would then be possible. This is within the time analyzed in the FSAR accident analysis.
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 6 seconds is long enough to override any short inconsequential grid disturbances.
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 l0 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 monitors 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 undervoltage relays meet the applicable requirements of IEEE Standard 279-1971.
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6.
"The Technical Specificatio'1ssshall include limiting conditions for operations, surveillance requirements, trip setpoints with 3
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minimum and maximum limits, and allowable values for the second-level voltage protection monitors."
The licensee's proposal for technical specification changes includes all the required items. The voltage setpoint, with the maximum allowable limit included, is 3923V. A high setpoint is needed to assure adequate voltages to all Class lE equipment.
The lowest bus voltage avaijgble to the undervoltage relays, as determined by CE, is 3916V.
Therefore, if both undervoltage relay setpoints drifted to the upper limit, spurious trips of the offsite source due to operation of the undervoltage relays could occur.
It is unlikely that both setpoints would drift to the extreme upper limit concurrently. Therefore, the proposed set-point and tolerences are acceptable. The limiting conditions for operation, the surveillance requirements, the channel test fre-quency and the calibration frequency are included in the technical specifications, and while not in conformance with the model tech-nical specifications, do meet the criteria of the staff's positions.
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 Zion Station to incorporate this feature in the circuit design. 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 are 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 saf ety injection actuation signal and to simulate interruption and subse-quent 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 sources.
The testing procedures proposed by the licensee canply 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 tested. 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 restarting of that diesel generator. 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 thh onsite power sources and that the load-shed bypass circuit is functioning properly.
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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 and time delay will protect the Class lE 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 proposed changes to the technical specifications adequately test the system modifications and comply with staff position 3.
The surveil-lance requirements, limiting conditions for operation, minimum and maximum limits for the trip point, and allowable values satisfy staff position 1.
Based on the above conclusions, CE's f acility modifications and pro-posed technical specification changes are acceptable.
It is recommended that the proposed technical specification changes be approved and implemented to coincide with the 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 27, 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, F. G. Lentine to H. R. Denton, NRC, "Draf t Technical Specifications on Undervoltage Protection," May 5, 1982.
6.
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."
7.
IEEE Standard 279-1971, " Criteria for Protection Systems for Nuclear Power Generating Stations."
8.
IEEE Standard 308-1974, " Standard Criteria for Class lE Power Systems for Nuclear Power Generating Stations."
9.
ANSI C84.1-1977, " Voltage Ratings for Electric Power Systems 'and Equip-ment (60 HZ)."
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" Adequacy of Station Electric Distribution System Voltages, Zion Sta-tion, Unit 1 (Unit 2)," EG&G Idaho, Inc. Infomal Report EGG-EA-5305 (5306), A. Udy, December 1980.
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