ML19318B134

Technical Evaluation of Proposed Design Mods & Tech Spec Changes on Grid Voltage Degradation for Prairie Island Nuclear Generating Plants Units 1 & 2.

ML19318B134
Person / Time
Site:	Prairie Island
Issue date:	03/31/1980
From:	White R LAWRENCE LIVERMORE NATIONAL LABORATORY
To:
Shared Package
ML19318B124	List: ML19318B134
References
CON-FIN-A-0231, CON-FIN-A-231 UCID-18654, NUDOCS 8006250052
Download: ML19318B134 (15)

Text

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TECHNICAL EVALUATION OF THE PROPOSED DESIGN M00!FICATIONS AND TECHNICAL SPECIFICATION "HANGES ON GRIO VOLTAGE DEGRADATION FOR THE PRAIRIE ISt>ND NUCLEAR GENERATING

. PLANT UNITS 1 AND 2

, Robert L. White

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Neither de United States Government nor any agency thereof, nor any of their employees.makes any warranty, expressed or implied, or assurnes any legalliability or respensibility fer the accura..y, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spec:Ee cornmer:dal product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessanly cor.stitute or imply its .

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ABSTRACT This report doctments the technical evaluation of the proposed design modifications and technical specification changes on grid voltage degradation 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 Reculations, Title 10, part 50, Appendix A requirements for determining the acceptability of the proposed system to protect the safety-related equipment frem degradation of grid vol tages .

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FOREWCR0 This report is supplied as part of the Sel ected El ectrical ,

Instrumentation, and Control Systems Issues (SEICSI) Program being conducted for the U. S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Operating Reactors, by Lawrence Livemore Laboratory, Field Test Systems Division of the Electronics Engineering Depart: rent.

The U. S. Nuclear Regulatory Commission funded the work under the

, authori zation entitled "El ectric al , Instrunentation and Control System Support," B&R 20 19 04 031, FIN A-0231.

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9 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 o

4. CONCLUSIONS . . . . . . . . ,. . . . . 15 REFERENCES . . . . . . . . . . . . . . . 17 e

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ILLUSTRATIONS Figure Page

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1 Existing bus 15 potential transformer circuit at Prairie Island Nuclear Generating Plant . . . . . 6 2 Existing loss of voltage logic at Prairie Island Nuclear Generating Plant . . . . . . . . . 7 3 Modified bus 15 potential transformer circuit for Prairie Island Nuclear Generating Plant . . . . . 9 4 Modified degraded voltage and loss of voltage detection circuitry for Prairie Island Nuclear Generating Plant . . 10 0

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TECHNICAL EVALUATION OF THE PROPOSED DESIGN MCOIFICATIONS AND TECHNICAL SPECIFICATION CHANGES ON GRIO VOLTAGE DEGRADATION FOR THE PRA4RIE ISLAND NUCLEAR GENERATING PLANT UNITS 1 AND 2 (Docket Nos. 50-282,50-306)

Robert L. White Lawrence Livermore Laboratory, Nevada

1. INTRODUCTION By letter dated June 3,1977 [Ref.1], the U.S. Nuclear Regula-tory Comission (NRC) requested the Northern States Power Canpany 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 add' tion, the NRC requested that the licensee compare the current design of the emergency power systems at the plant facilities with the NRC staff positions, as statea in the June 3,1977 letter [Ref. 13, 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-tions ce incorporated into all facility operating licenses.

By letters dated May 4,1978 [Ref. 2] and October 12, 1979 [Ref.

3], Northern States Power Ccmpany proposed certain design mcdifications and additions to the licensee's Technical Specifications. The design modifica-tiens include the installation of a degraded voltage protection system for the Class IE 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 proposec' design modifications and Technical Specification changes and to determine that they meet the criteria established by the NRC for the protection of Class 1E equipment from grid voltage degradation.

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2. DESIGN BASIS CRITERIA The design basis criteria that were applied in determining the acceptability of the system modification to protect the Class 1E equipment from degradation 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-308-1974, " Class 1E Power Systems for Nuclear Power Generating Stations" [Ref. 6].

(4) NRC positions as stated in a letter dated June 3,1977

[Ref.1].

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3. EVALUATION 3.1 EXISTING UNDERVOLTAGE PROTECTION The present design uses undervoltage relays to sense the loes 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. in undervoltage condition will result in isolating the Class 1E buses from all offsite sources, initiati.ng energency diesel generator start and load shedding on

, the Class 1E buses, and lastly, permi~. ting closure of the emergency diesel ganerator supply breakers. Figures 1 and 2 indicate the existing design.

The existing system does not bypass the load-shedding feature once the emergency diesels are energizing the Class 1E buses.

,' 3.2 MODIFICATIONS

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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 on each 4160-volt Class 1E bus. Also, the licensee will make circuit design changes to prevent load shedding when the Class 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 operation and sur-veillance requirements for the proposed design changes as presented in this evaluation are documentec in the licensees's proposed Technical Specifications.

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A 27R 68% 27A 68%

I BIS Inst. BIS 1 Sec B

C

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Logic test I-tin

. 27B 68% 27S 68%

BIS 1 Sec BIS Inst.

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Logic test Note: Only bus 15 logic'is shown.

Figure 1. Existing bus 15 potential transformer circuit at Prairie Island Nuclear Generating Plant. .

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- I Sec - 1 Sec --Ins t. --I ns t .

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t 27AX 27SX 3 N N s z s.

Initiate:

automatic voltage 27BX 27RX restoring N 'N scheme g

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Figure 2. Existing loss of voltage logic at Prai.-ie Island Nuclear Generating Plant.

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3.3 DISCUSSION This section 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.

3.3.1 Position 1: 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 safety-related loads at all onsite system

- distribution levels."

The licensee's degraded undervoltage setpoint is 89 percent (+1 percent) of 4160 volts (3702 volts). The undervoltage time delay setting is 6 seconos (+2 seconds). The licensee's analysis shows that this setpoint and time delay are satisfactory for 4160-volt and 480-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 preclude spurious trips of the offsite power sources."

The proposed modification incorporates a " modified 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 (+l seconds), '

time-delay relay 27Al (2731) 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 (278) would drop out 27AX (278X), which along with 27SX (27RX) would initiate the autcmatic

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Bus 15 (4160 Volts) 27R 89% 27A 55% 27Al 89%

A BIS 1 Sec BIS 1 Sec BIS 6 Sec g (new)

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Logic test

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40 27B 55% 2781 89% 27S 89%

BIS 1 Sec BIS 6 Sec BIS 1 Sec (new) _.

l Logic test Note: Only bus 15 logic is shown.

A similar modification is _-

required on buses 16, 25, 7 and 26.

Figure 3. Modifled bus 15 potential transformer circuit for Prairie Island Nuclear Generating Plant.

1 55% 55% 89% 89%

27A 27B 27R 27S 1 Sec 1 Sec 1 Sec 1 Sec 89% 89%

27Al 2781 6 Sec 6 Sec 27AX 27BX 279X 27SX 1

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Figure 4 Modified degraded voltage and loss of voltage circuitry for Prairie Islanc Nuclear Generating Plant.

voltage restoring scheme. The time for this relay dropout is inversely related to voltage taking 1 second at the 55 percent (+5 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-ing conditions."

(a) "The allowable time delay, including margin, shall not exceed the maximum time delay that is assumed in the FSAR 3ccident analysis."

The proposed time delay of 6 seconds (12 seconds) does not exceed this maximum time delay. This is based on the ECCS analysis assuning 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 voltage restora-tion. Following this action, 4 seconds are required to trip the source breakers. At this time the gene?ator output breakers close and the safety f r.jection pumps 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 time delay shall minimize the effect of i short-duration disturbances from reducing the  !

, unavailability of the offsite power sources." l The licensee's proposed time delay of 6 seconds (12 seconds) is long enough to override any short l grid disturbances. This will be confirmed by testing once the circuitry is installed.

(c) "The allowable time duration of a 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 cause any failure of any equipment connected to and associated with the Class lE emergency power system.

(4) "The undervoltage monitors shall automatically ini-tiate the disconnection of offsite power sources whenever the voltag'e setpoint and time delay 1 trr :ts have been exceeded.

The " modified 2 out of 4" logic automatically dis-connects offsite power from the Class 1E emergency buses experiencing degraded voltage and initiates the voltage-restoring scheme.

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

The licensee states that the relays and the relaying scheme will be in cenpliance with the requirements of s IEEE Std 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 pro-tection monitors."

Limiting conditions for operatien and surveillance requirements, as well as trip setpoints for allowable values for degraded voltage protection, are included in the licensee's proposed Technic 11 Specifications.

3.3.2 Position 2: Interaction of Onsite Power Sources with the Load-sned Feature The second position requires that the system be designed to .

prevent load shedding of the emergency buses automatically once the onsite

,' sources are supplying power to all sequenced loads. If' an adequate 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 block the undervoltage trip signal to the diesel generator source breaker with the safety injection signal, and thus prevent load shedding of the emergency buses when they are being 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 bruker is trip-ped.

3.3.3 Position 3: Onsite Power Source 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 conjunction with a safety injection actuation signal and to simulate interruption and subsequent reconnection of onsite power sources. These 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 verify the requirements of the NRC by testing the system by initiating loss of offsite power in conjunction with a simulated safety injection v 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.

1 The licensee affirms that interruption of the diesel generators to test l load shedding and load sequencing is not necessary, as this logic is tested l during the simulated loss of offsite power in conjunction with a safety injection actuation signal.

3.4 TECHNICAL SPECIFICATIONS 1

1 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 Q1 percent] and 6 seconds [+2 -

seconds]).

(2) Provide the required coincidence logic (" modified two-out-of-four").

(3) Incorporate action statements regarding limiting conditions for operations when the numoer of operable channels for degraded voltage protection is reduced.

. (4 ) Provide the surveillance requirements for channel calibration end channel functional tests. 1 (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 1E bus de-energization, load shedding, voltage restoration, and load _ sequencing.

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4. CONCLUSIONS Based on the infornation 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 the time delays will protect the Class 1E equipment from a sustained degraded voltage condition of the offsite power source.

The modifications to the logic of the load-shed circuitry comply with Position 2, and will prevent adverse interaction of the offsite and onsite emergency power systems.

The proposed additions to the Technical Specifications and the method of testing the logic circuitry have been reviewed and found to meet the intent of Position 3.

Accordingly, we recommend that the NRC approve the proposed

,' design modifications and Technical Specifications.

15 -

REFERENCES

1. NRC letter to Northern States Power Co. letter (L. O. Mayer), dated June 3, 1977.

2. Northern States Power Co. letter (L. O. Mayer) to NRC, dated May 4, 1978.

3. Nonthern States Power Co. letter (L. O. Mayer) to NRC, dated October 12, 1979.

4. Code of Federal Regulations, Title 10, Part 50 (10 CFR 50), General Oesign Criterion il (GDC 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. IEEE Std. 308-1974, " Criteria for Class 1E Power Systems for Nuclear Power Generating Stations."

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