ML20010B735
| ML20010B735 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 03/31/1981 |
| From: | White R LAWRENCE LIVERMORE NATIONAL LABORATORY |
| To: | |
| Shared Package | |
| ML20010B729 | List: |
| References | |
| UCID-18654, NUDOCS 8108170483 | |
| Download: ML20010B735 (22) | |
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TECHNICAL EVALUATION OF THE PROPOSE 0 CESIGN MGDIFICATIONS AND TECHNICAL SPECIFICATION CHANGES ON GRIO '.'OLTAGE DEGRADATION FOR THE PRAIRIE ISLAND NUCLEAR GENERATING
. PLANT UNITS 1 AND 2
- Robert L. White
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e-i ABSTRACT This report docunents the technical evaluation of the proposed design modifications and technical specification changes on grid voltage d2 gradation for the Prairie Island Nuclear Generating Plant.
The review criteria are based on IEEE Std-279-1971, IEEE' Std-308-1974, and General Design Criterion 17 of the Code of Federal Regulations, Title 10, part 50, Appendix A requirements for detennining the acceptability of the proposed system to protect the safety-related equipment from degradation of grid vol tages.
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e FOREWORD This report is supplied as part of the Selected Electrical,
Instrumentation, and Control Systems !ssues (SEICSI) Program being conducted for the U. S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Operating Reactors, by Lawrence Livennore Laboratory, Field Test Systems Division of the Electronics Engineering Department.
The U. S. Nuclear Regulatory Commission funded the work under the
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huthorization entitled " Electrical, Instrutentation and Control System
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Support," B&R 20 19 04 031, FIN A-0231.
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TABLE OF CONTENTS Page 1.
INTRODUCTION.
1 2.
DESIGN BASIS CRITERIA.
3 3.
EVALUATION 5
3.1 Existing Undervoltage Protection 5
3.2 Modifications.
5 3.3 Discussion.
8 3.3.1 Position 1: Second Level of Undervoltage or Overvoltage Protection with a Time Delay 8
3.3.2 Position 2:
Interaction of Onsite Power Sources with the Load-shed Feature 12 3.3.3,. Position 3: Onsite Power Source Testing.
13 3.4 Technical Specifications 13 4
CONCLUSIONS 15 REFERENCES.
17
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1
ILLUSTRATIONS Figure Pay 1
Existing bus 15 potential transformer circuit at Prairie Island Nuclear Generating Plant 6
2 Existing less of voltage logic at Prairie Island Nuclear Generating Plant 7
3 Mcdified bus 15 potential transformer circuit for Prairie Island Nuclear Generating Plant 9
4 Modified degraded voltage and loss of voltage detection circuitry 'or Prairie Island Nuclear Generating Plant.
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TECHNICAL EVALUATION OF THE PROF 0SEO DESIGN MODIFICATIONS ANG TECHNICAL SPECIFICATION CHANGES ON GRIO VOLTAGE DEGRADATION FOR THE PRAIRIE ISLAND NUCLEAR GENERATING PLANT UNITS 1 AND 2 (Docket Nos. 50-282,50-306)
Robert L. White Cawrence Livermore Laboratory, Nevada 1.
INTRODUCTION By letter dated June 3,1977 [Ref.1], the U.S. Nuclear Regula-tory Ccenission (NRC) requested the Northern States Powar Company to assess the susceptibility of the Prairie Island Nuclear Generating Plant Class 1E electrical equipment to sustained degraded voltage conditions at offsite power sources and to the interaction between the offsite and onsite emer-gency power system.
In addition, the NRC requested that the licensee ccmpare the current design of the emergency power systems at the plant facilities with the NRC staff posicions, as stated in the June 3,1977 letter [Ref. 1), and that the licensee propose plant modifications, as necessary, to meet the NRC staff positions or provide a detailed analysis which shows that the facility design has equivalent capabilities and pro-tection features.
Further, the NRC required certain Technical Specifica-tiens be incorporated into all facility operating licenses. _
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By letters dated May 4,1978 [Ref. 23 and October 12, 1979 [Ref.
3], Northern States Power Company proposed certain design modifications and additions to the licensee's Tecnnical Specifications.
The design modifica-tions include the installation of a degraded voltage protection system for the Class 1E equipment. The proposed additions to the Technical Specifica-tions are in regard to the setpoints, calibrations, and surveillance re-quirements associated with the proposed voltage protection system.
The purpose of this report is to evaluate the licensee's proposed design mod;fications and Technical Specification changes and to detennine that they meet the criteria established by the NRC for the protection of-Class IE equipnent from grid voltage degradation.
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DESIGN BASIS CRITERIA The design basis criteria that were applied in detemining the acceptability of the system modification to protect the Class 1E equipment fran degrad,ation of grid voltages are as follows:
(1)
General Design Criterion 17 (GDC 17), " Electric Power Systems," of Appendix A, " General Design Criteria for Nuclear Power Plants," in the Code 3 of Federal Reg-ulations, Title 10, part 50 (10 CFR 50) [Ref. 4].
(2)
IEEE Std-279-1971, " Criteria for Protection Systems for Nuclear Power Generating Stations" [Ref. 5].
(3)
IEEE Std-308-1914, " Class lE Power Systems for Nuclear Power Generating Stations" [Ref. 6].
(4)
HRC ' positions as stated in a letter dated June 3, 1977
[Ref. 1].
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EVALUATION 3.1 EXISTING UNDERVOLTAGE PROTECTION The present design uses undervoltage relays to sense the loss of offsite power.
This design consists of four relays:
two instantaneous acting and two time delay.
The time relays are one second operate - one second release relays.
The concept of this design is to reduce spurious
' operation both on voltage loss and voltage restoration.
An undervoltage condition will result
. isolating the Class 1E buses from all offsite sources, initiating emergency diesel generator start and load shedding on the Class 1E buses,'and lastly, permitting closure of the emergency diesel generator supply breakers. Figures 1 and 2* indicate tid existing design.
The existing system does not bypass the load-shedding feature ence the emergency diesels are energizing the Class 1E buses.
3.2 MODIFICATIONS The licensee has proposed a design change which includes auto-matic degraded voltage. protection.
This modification consists of the addition of two time-delayed, undervoltage relays en each 4160-volt Class 1E bus.
Also, the licensee will make circuit design changes to prevent load shedding when the C1 ass 1E buses are being. powered by the diesel generators, and will reinstate this load-shedding feature if the diesel generator breaker trips.
The limiting conditions for operaticn and sur-veillance requirements for the proposed design changes as presented in this evaluation are documented in the licensees's proposed Technical Specifications.
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Existing loss of voltage logic at Prairie Island fluclear Generating Plant..
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3.3 OISCUSSION This s'ction provides a statement on the NRC staff position taken from their June 3,1977 letter [Ref.13, followed by an evaluation of the licensee's design.
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Second Level of Undervoltage or Overvoltage Protection witn a Time Delay This position is to be met by the licensee meeting certain criteria.
Each criterion has been evaluated against the licensee's pro-
,posal and is addressed below.
(1)
"The selection of voltage and time setpoints shall be determined from an analysis of the voltage require-ments of the safet elated loads at all onsite system distribution level The licensee's degraded undervoltage setpoint is 89 percent (11 percent) of 4160 volts (3702 volts).
The undervoltage time delay setting is 6 seconds (+2 seconds).
The licensee's analysis shows that this set;oint and time delay are satisfactory for 4160-volt ant a30-volt Class 1E loads as well as for relays, contactors, and other components whose functional performance would be inadequate because of under-voltage.
(2)
"The voltage protection shall include coincidence logic to creclude spurious trips of the effsite power sources."
The proposed modification incorporates a " modi fied two-out-of-four" coincidence logic scheme (see Figures 3 and 4).
If the offset voltage remains below 89 percent (+1 percent) longer than one second, 27S (27R) drops out, causing 275X (27RX) to drop out also.
If the voltage remains below 89 percent (+1 percent) continuously for a total of 6 seconds (+7 secoads),
time-delay relay 27Al (2781) will drop out, causing 27AX (27BX) to drop out, and thus will initiate the automatic voltage restoring scheme.
For loss of voltage, 55 percent (+5 percent) of 4160 or less, 27A (273) would drop out 27AX (27BX), which along with 27SX (27RX) would initiate the automatic _
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A similar modification is required on buses 16, 25 and 26.
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Modified bus 15 potential transformer circuit for Prairie Island Nuclear Generating Plant.
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Mccified degraded voltage anu loss of voltage circuitry for Prairie Island Nuclear Generating Plant.
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voltage restoring scheme.
The tinie for this relay dropout is inversely related to voltage taking 1 second at the 55 percent (15 percent) voltage level down to nearly instantaneous operation at zero volts.
This same logic will be used on all four of the Class 1E emergency buses.
(3)
"The time delay selected shall be based on the follow-i ing conditions."
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(a)
"The allowable time delay, including margin, shall not exceed the maximum time delay that is 4
assumed in the FSAR accident analysis."
The proposed time delay of 6 seconds (12 seconds) does not ext.aed this maximum time delay. This is based on the ECCS analysis assuming a 25-second time delay in safety injection system delivery.
Upon initiation of the voltage restoration scheme by a degraded voltage condition, 7 seconds are required for diesel startup and voitage restora-
- tion, Following this action, 4 seconds are required to trip the source breakers.
At this t.ime the generator output breakers.close and the i
s'afety 'njection punps are powered.
Ten seconds are then required for the motor-operated valves to completely actuate to their emergency posi-tions. Total time is 21 seconds.
(b)
"The tima delay shall minimize the effect of short-duration disturbances from reducing the unavailability of the offsite power sources."
The licensee's proposed time delay of 6 seconds (12 seconds) is long enough to override any short grid disturbances.
This will be confirmed by testing once the circuitry is installed.
(c)
"The allowable time duration of o degraded volt-age condition at all distribution system levels shall not result in failure of safety systems or components."
A review of the licensee's voltage analysis indicates that the time delay will not caure any failure of any equipment connected to and associated with the Class IE emergency power system.
(4)
"The undervoltage monitors shall automatically ini-tiate the disconnection of cff site power sources whenever the voltage setpoint and time delay limits have ::een exceeded.
4 The " modified 2 out. of 4" logic automatically dis-connects offsite power from the Class 1E emergency buses experier.cing degraded voltage and initiates the voltage-restoring scheme.
1 (5)
"The voltage monitors shall be designed to satisfy the requirements of IEEE Standard 279-1971."
j The licensee states that the relays and the relaying scheme will be in compliance with the requirements of s
(6)
"The Technical Specifications shall include limiting conditions for operations, surveillance requirements, trip setpoints witn minimum and maximum limit 3, and i
allowable values fcr the second-level voltage pro-tection monitors."
Limiting conditions for operation and surveillance requiremmts, as well as tri.9 setpoints for allowable values fpr degraded voltage protection, are included in the' 11censee's proposed Technical Spec'fications.
3.3.2 Position 2:
Interaction of Onsite power Sources with the Load-sned Feature Tne second positial requires that the system be designed to prevent load shedding of the emergency buses autoenatically once the onsite sources are supplying power to all sequenced loads.
If an adaquate basis can be provided for retaining the load-shed feature, the licensee must assign maximum and minimum values to the setpoint of the load-shed feature.
These setpoints must be documented in the Technical Specifications.
The load-shedding feature must also be reinstated if the onsite source supply breakers are tripped.
The licensee's proposal states that a design has been completed to blo1k the undervoltage trip sisaal to the diesel generator source breaker with the safety injection signal, and thus prevent load shedding of the emergency buses when they are heing energized by the diesels. Further, the licensee states that the load-shedding logic circuitry will automatic-
' ally be reinstated once the diesel generator load source breaker is trip-ped.
e 3.3.3 Position 3: Onsite power Scurce Testing The third position requires that certain test requirements be included in the Technical Specifications.
These tests are to "... demon-strate the full functional operability and independence of the onsite power sources at least once per 18 months during shutdown."
The tests are to simulate loss of offsite power in conjunctio' with a safety 14.j ection actuation signal and to simulate interruption and subsequent reconnection of onsite power sources.
Thest tests will verify the proper operation of the load-shed system, the load-shed bypass when the emergency diesel generators are supplying their respective buses, and that there is no adverse interaction between the onsite and offsite power sources.
The licensee will ve'rify the requirements of the NRC by testing the system by initiating loss of offsite-power in -conjunction with a simulated safety injection si gnal.
The test sequence will be bus de-
, energization, load shedding, voltage restoration, and load sequencing. The operating time on emergency onsite power will be at least five minutes.
The licensee affirms that interruption of the diesel generators to test load sheddirig and load sequencing is not necessary, as this logic is tested i
during the simulated loss of offsite power in conjunction with a safety injection actuation signal.
3.4 TECHNICAL SPECIFICATIONS The changes proposed by Northern States Power Company to the l
Prairie Island Nuclear Generating Plant Technical Specifications reflect the proposed design modifications.
Specifically, the proposed changes are as follows:
(1)
Include the trip setpoints for the degraded voltage protection sensors and the associated time delays with tolerances (39 percent (11 percent) and 6 seconds [12 seconds]).
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9 (2)
Provide the required coincidence logic (" modified two-out-of-four").
(3)
Incorporate action statements regarding limiting conditions for operations when the number of operable channels for degraded voltage protection is reduced.
(4)
Provide the surveillance requirements for channel calibration and channel functional tests.
(5)
Provide the surveillance requirements to demonstrate at least once per 18 months that.the loss of offsite power in conjunction with a safety injection actuation signal will provide the sequence of Cl ass IE bus de-energization, load shedding, voltage restoration, and load sequencing.
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CONCLUSIONS Based on the information provided by Northern States power Company, it has been determined that the proposed modifications comply with Position 1.
All of the staff's requirements and design base criteria have been met.
The voltage setting and t'te time delays will protect the Class IE equipment from a sustained degraded voltage cordition of the offsite power source.
The modificatiors to the logic of the load-shed circuitry comply with Position 2, and will prevent adverse interaction of the cffsite and onsite emergency power systems.
The proposed additions to the Technical Specifications and the method c' testing the logic circuitry have been reviewed and found to meet the inte.1t of Position 3.
Accordingly, we recommend that the NRC approve the proposed design modifications and Technical Specifications.
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a REFERENCES 1.
NRC letter to Northern States Power Co. letter (L. O. Mayer), dated
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June 3,1977.
2.
Northern States Power Co. letter (L. O. Mayer) to NRC, dated May 4, 1978.
3.
Northern States Power Co. letter (L. O. Mayer) to NRC, dated October 12, 1979.
4.
Code of Federal Reaulations, Title 10, Part 50 (10 CFR 50), General Design Criterion 17 (G0C 17), " Electric Power Systems" of Appendix A,
" General Design Criteria for Nuclear Power Plants."
5.
IEEE Std. 279-1971, " Criteria for Protection Systems for Nuclear Power Generating Stations."
6.
.1EEE Stv. 3G8-1974, " Criteria for Class 1E Power Systems for Nuclear Power Generating Stations."
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