ML20197F819

Proposed Design Mods & Tech Specs Changes on Grid Voltage Degradation for Vermont Yankee Nuclear Power Station, Technical Evaluation Rept

ML20197F819
Person / Time
Site:	Vermont Yankee
Issue date:	08/25/1982
From:	Selan J LAWRENCE LIVERMORE NATIONAL LABORATORY
To:	NRC
Shared Package
ML20197F824	List: ML20197F819 ML20202J415 ML20202J421
References
CON-FIN-A-0250, CON-FIN-A-250 UCID-19453, NUDOCS 8209230493
Download: ML20197F819 (11)

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I TECHNICAL EVALUATION REPORT ON THE PROPOSED DESIGN MODIFICATIONS AND TECFNICAL SPECIFICATION CHANGES .

ON GRID VOLTAGE DEGRADATION FOR THE VERMONT YANKEE NUCLEAR POWER STATION l

(Docket No. 50-271) .

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1 James C. Selan E

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This is an informal report intended primarDy for internal or limited external distribution. ?hig F , ~^ The opinions and conclusions <tated are those of the author and may or may not be those qn

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This work was supported by the United States Nuclear Regulatory Commission under ' ' 3;0f. a Memorandum of Understanding with the United States Department of Energy. E3 * - . - W. NRC FIN No. A-0250 ,ip . aWC . Y g ,'* a . ,e e- = 4 - -s e ABSTRACT \ . This report documents the technical evaluation of the proposed design modifications and Technical Specification changes for protection of Class 1E equipment from grid voltage degradation for the Vermont Yankee Nuclear . Power Station. The review criteria are based on several IEEE standards-and the Code of Federal Regulations. The evaluation findc that the proposed design modifications and Technical Specification' changes will-ensure that the Class lE equipment will be automatically protected from sustained voltage degradation concurrent with an accident. For non-accident conditions, plant procedures and the availability of redundant systems (pending NRC acceptance)_ ensure that if required, a safe shutdown of the plant could be obtained and . =aintained should the norcally operating Class lE equipment be lost under degraded grid conditions. i FORIWORD E This report is supplied as part of the Selected Electrical, Instrunentation, and Control Syste=s issues Program being conducted for the U. S. Nuclear Regulatory Cc==1ssion, Office of Nuclear Reactor Regulation, Division of Licensing, by Lawrence Livermore ljational Laboratory. The U. S. Nuclear Regulatory Commission funded the work under the authorization entitled " Electrical, Instrumentation and Control System Support," B&R 20 19 04 C31, FIN A-0250. , -we. 1 TABLE OF CONTENTS \ Page t~

1. INTRODUCTION. . . . . . . . . . . . . . 1

! - 2. DESIGN BASIS CRITERIA . . . . . . . . . . . 2

3. EVALUATION . . . . . . . . . . . . . . 2 3.1 Existing Undervoltage Protection . . . . . . . 2

'3. 2 Modifications . . . . . . . . . . . . 3 3.3 Discussions. . . . . . . . . .. . . . 3

3.3.1 NRC Staff Position 1

Second Level of Under- .

l Voltage or Overvoltage Protection with a , j Time Delay . . . . . . . . . . . 3 l 3.3.2 NRC Staff Position 2: Interaction of Onsite Power Sources with Load Shed Feature. . . . . 6 { . ,3.3.3 NRC Staff Position 3: Onsite Power Source Testing . 6  ; 3.4 Technical Specifications ~ , , . . . . . . . . . 6 , i I

4. CONCLUSION . . . . . . . . . . . . . . 7  ;

REFERENCES . . . . . . . . . . . . . . . 8 r* i J I e -iii- q TECHNICAL EVALUATION RrPORT ON THE PROPOSED DESIGN MOD 7?ICATIONS AND: TECHNICAL SPECIFICATION CHANGES a0N GRID VOLTAGE DEGRADATION - FOR THE. l VERMONT YANKEE NUCLCAR POWER STATION l (Docket No.'50-271)- , . 1 James C. Selan Lawrence Livermore-National Laboratory l

1. INTRODUCTION l

By,. letter dated June 3,1977 [Ref.1), the U. S. Nuclear Regulatory Commission (FRC) requested the Vermont Yankee Nuclear Power Corporation (VYNPC), the licensee, to assess the susceptibility of the Class IE electrical equipmeqt . to sustained degraded voltage conditions at the offsite power sources and to the interaction between the offsiteLand onsite emergency power systems at the Vermont YaI5Eee Nuclear Power Station. In addition, the NRC requested that the licensee compare the current design of the emergency power syctems at the plant facilities with the NRC staff positions 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 protective features. Further, the NRC required certain Technical Specifications be incorporated into the facility's operating license. By letters dated July 18, 1977 [Ref. 2], December 6, 1979 [Ref. 3], July 24,1980 [Ref. 4], March 17,1980 [Ref. 5], February. 6,1981- [Ref. 6], June 8, 1982 [Ref. 7] and July 22, 1982 [Ref.8], the licensee submitted certain design modification details, additions to the Technical Specifications, and limiting conditions for. operation (LCO's). The design modification details include a degraded voltage protection system for the Class 1E equipment.. The additions to the Technical Specifications and LCO's are in regard to calibra- { , tions, surveillance. requirements, test requirements, and " action" statements associated with the existing undervoltage protection system. -The purpose of this report is to evaluate the licensee's p'roposed l design modifications, Technical Specification changes, and proposed LCO's to i determine that they meet the criteria established by the NRC for the protec- ' tion of Class 1E equipment from grid voltage degradation. -1_ -i .o .

2. DESIGN BASIS CRITERIA

( The design basis criteria that were applied _in determining the acceptability of the system modification to protect the Class 1E (quipment 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," Code of Federal Regulations, Title 10, Part 50 (10 CFR 50) [Ref. 9]. (2) IEEE Standard 279-1971, " Criteria for Protection Syste=s for Nuclear Power Generating Stations" [Ref. 10]. (3) IEEE Standard 308-1974, " Class IE Power Syste=s for Nucle'ar Power Generating Stations" [Ref. 11}. - (4) NRC staff positions as stated in a letter dated June 3, 1977 [Ref. 1}. E

3. EVALUATION 3.1 EXISTING UNDERVOLTAGE PROTECTION The existing undervoltage protection scheme uses two undervoltage

, relays (loss-of-voltage) on each of the three 4160-volt Class lE buses. The relays (induction disc type) are set to actuate in 1.25 seconds upon complete loss of power and in 6.0 seconds at 1698 volts (41% of 4160 volts) with the tap setting of 55 volts. These relays will not operate if the voltage on the buses re=ain above 46% of 4160 volts (1914 volts). This-d'tpoint e corresponds to 217' volts (45% of 480 volts) on the 480-volt buses. Actuation of these relays will automatically disconnect the offsite source, shed the loads, and start the diesel generators with subsequent load sequencing. In addition to the less-of-voltage relays (first level), two voltage transducers are installed on each 4160-volt and 480-volt Class 1E bus. These sensors (degraded voltage protection) will actuate an alarm above the minicum required voltage to ensure continuous operation of the Class 1E equipment. The associated time delay is 8 seconds. Upon actuation of these sensors an alars is annunciated in the control room to alert the operator of the degraded voltage condition. The operator will then follow plant procedures for voltage restoration. The load shedding feature is automatically bypassed when the diesel generator breaker closes. This feature is not. automatically reinstated should the breaker trip. l, ~ ~ ^~ T . . '. . :. . .. . - ~: .. . .: .. . .': Q.:: . ,. . L. ~. . . __ ~ - 3. 2' MODIFICATIONS

• The licensee is proposing a design change to the existing undervoltage protection system. - This design change adds-two additional relays on each 4160- ,

4. volt Class lE bus. The relays ~are instantaneous type with a trip Jetpoint of 3700 volts 140 volts (88.9% i-l% of 4160 volts) with a time delaf of 10 seconds i 1 second. The relays'will be arranged in a 2-out-of-2 coincident logie scheme.

The design change interlocks the degraded voltage relays with an accident signal. i Operation of this design change is as follows: , (1) 'Should the voltage degrade below that required for continuous operation for the Class 1E equipment (exceeding'the relay set-points) .the relays will initiate an alarm to alert the operator of a degraded voltage condition. Upon receiving this alarm,~the operator (through established plant procedures) will contact the , Rhode Island, Eastern Massachusetts and Vermont Energy Control

(REMVEC) system operator to, request an assessment of the degraded

, voltage condition. Following the assessment, appropriate actions 'will then'be taken to restore voltage. Should the restoration' fail, the operator will initiate.the offsite. source disconnection l 2 . and the onsite source re-connection. (2) Should the voltage degrade below the relay setpoints concurrent' i with an accident signal, the disconnection of the degraded off-i site source, load . shedding, diesel generator starting, and 'subse- .__ quent load sequencing will occur automatically. The licensee is proposing a design modification to automatically rein-j state the load shed feature and subsequent load sequencing onto the diesel: gene-rators following diesel generator breaker tripping. Details of this circuitry modification is not available. 1 3.3 DISCUSSION This section presents a statemeno on the NRC staff position from l their June 3,1977 ' letter [Ref. l} followed by an evaluation of the licensee's , design. ] 3.3.1 NRC Staff Position 1: Second Level of Undervoltage

or Overvoltage Protection with a Time Delay i

j This position is to be met by the licensee =eeting certain criteria. [ Each criterien has been evaluated against the licensee's proposal and is

addressed below. -

1 i i I I 9 i h e-v, . ,,---,,,--,=----..i, --.+--.p , ,,...,v.,-,,ry-.- ,,y%,,y y - - - ,,.-ey,,-p ,.,,-,-~y g.-, , , , - - -.-go,. -.- --,- (1) "Th2 salection of voltogn end tima satpoints shsll ba determined from an analysis of.the. voltage requirements of the safety-related loads at all onsite system distribution , i levels." The proposed setpoint of 88.9% + 1% of 4160 volts (3700 volts) is above the -10% continuous voltage rating for the Class IE equipment. The 10 second + 1 second time delay wasyselected to prevent spurious actuation caused by grid fluctuhtions or voltage transients on the auxiliary power system. . (2) "The voltage protection shall include coincidence logic to preclude spurious trips of the offsite power sources." The proposed second-level of undervoltage protection scheme ~ is designed using a 2-out of-2 coincident. logic to preclude spurious tr'ips. (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 licensee states that the time delay of 10 seconds + 1 second does not exceed the time delay assumed in the FSAR accident analysis {Ref. 8). (b) "The time delay shall minimize the effect of short-duration disturbances from reducing the availability of the offsite ' power sources. The licensee's proposed time delay of 10 seconds + 1 second was selected to override short duration voltage transients on the transmission and auxiliary power systems. (c) "The allowable time duration of a degraded voltage condi- ~ 3 tion at all distribution system levels shall not result in failure of safety systems or components." The licensee's analysis has shown that the 10 seconds + 1 ' second time delay will not cause any failure of any equip-ment connected to and associated with the Class 1E power system. (4) "The undervoltage monitors shall automatically initiate the disconnection of offsite power sources whenever the voltage setpoint and time delay limits have been exceeded." + w w g e w- , - i *

• The undervoltage. protection system design changes will only allow for automatic disconnection fr'om the degraded offsite sources'whenever an accident condition occurs concurrently. ~

with.a degraded grid voltage...Without an' accident' condition- l present, actuation of the second-level'undervoltage protection ~ scheme will initiate an alarm which signals the operator of a degraded voltage condition. Plant procedures will.then direct the operator for corrective' actions. r The licensee has provided justification for not providing auto 2 matic disconnection from the offsite source unless an accident. signal is present.. Detailed information on the following points of justification can be fo'und in Refs. 2, 3, 4 and 7: , (a) Operating experience -i.e. the number of incidents of sus- ], tained degraded grid voltages experienced. (b) Auto-tripping of plant could 'eause a " cascading effect" in the transmission distribution system (effect on other nuclear power plants). ' (c) Availability of redundant systems not. exposed to degraded i  ; voltages for obtaining and maintaining the plant in a safe shutdown. . g ~ The licensee has not provided the following information to . _ _ . support this methed of providing undervoltage protection for the Class IE equipment from sustained voltage degradation: ) ) (a) Detailed plant procedures outlining operator actions for

. voltage restoration during non-accident. conditions.

I I (5) "The voltage monitors sh'all be designed to satisfy the require-ments of IEEE Standard 279-1971." .l The licensee states that the proposed design changes will satisfy the. requirements of IEEE. 279-1971 [Ref. 8]. (6) "The Technical Specifications shall inclu' e limiting conditions for operation, surveillance requirements, trip setpoints with =inimum and maxicum li=its, and allowable values for the second-l le' vel voltage protection monitors." j The'11censee has submitted draft Technical Specification changes to include the design changes of the undervoltage protection system. The changes include setpoints with tolerances, surveil- - lance require =ents,'and LCO's. The licensee is required to submit formal Techn,ical Specification changes for'the undervoltage pro-tection system. l l J s i I '. 3.3.2 KRC Staff Position 2: Interaction of Onsite Power Sources with-Load Shed Feature i The second position requires the system be designed to prevent auto =atic load shedding of the emergency buses onceLthe onsite sources are supplying power to all sequenced loads. If an adequate basis can.be _ provided for retaining the load shed feature, the licensee cust assign maximum and l =ini=um values to the setpoint of the load-shed feature. These setpoints must !_ be documented in the Technical Specifications. The load-shedding feature must l also be reinstated if the onsite source supply breakers are tripped. , l I ,' The licensee is bypassing the load-shed feature when the diesel gene- l rators are supplying the Class 1E buses and is proposing to auto-reinstate the  ! load-shed feature following breaker tripping. . The licensee is required to sub- I cit details on the circuitry modifications for accomplishing this feature. 3.3.3 NRC Staff Position 3: Onsite Power Source Testing

The third position ~ requires that certain test requirements'be q

included in the Technical Specifications. These tests are7to '... demons rate

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 o~ffsite power in conjunction with a safety-injection actuation I 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 circuitry, and that there is no adverse inter- l action between the onsite and offsite power sources.

Current Technical Specifications include tests which demonstrate the i operability and independence of the onsite power sources (i.e. once per'18 months - ( the simulating of loss-of power in conjunction with a safety injection). The j licensee submitted a draft Technical Specification change which included a test to simulate the interruption of the onsite sources with subsequent load shedding, reconnection, and load sequencing. The licensee is required to submit a formal Technical Specification change to include this test requirement. - .a 3.4 TECHNICAL SPECIFICATION ' - I. l The licensee has provided draft Technical Specification changes to . P reflect the proposed design modifications to the undervoltage protection system. - ' Specifically, the proposed changes: . l t (1) Include the trip setpoints of voltage and time with tolerances for the undervoltage protection relays-(3700 volts + 40 volts, time delay of 10 seconds + 1 second). l (2) Provide the required coincident logic (2-out-of-2). (3) Incorporate action statements regarding limiting conditions for 2 operation when the number of operable channels for undervoltage !, protection is reduced. i 1 I .g[^& b (4)'.Provida th2 surveil 1snee requireennts for channel calibration during the refueling shutdown'and the-monthly channel functional test. . (5) Include test requirecents to demonstrate the operability and independence of the onsite-sources and the operation of the-undervoltage relaying modifications. Y l

4. CONCLUSIONS Based on the information provided,by Vermont Yankee Nuclear Power i

Corporation, it has been determined that protection of.the Class IE equipment fron sustained degraded grid voltages concurrent with an accident condition meet the requirements of NRC Staff Position 1. For non-accident conditions, j the automatic disconnection. requirement is not met. Instead of providing this auto =atic disconnection feature, the licensee maintains that by utilizing cor-3 rective meas,ures by the plant operator and the REMVEC system control operator ! continued plant operation reduces the possibility of total collapse of the

trans=issiod syste= (i.e. cascading ef fect to other nuclear plants). . The ,

licensee provided a list of the available systems (not exposed to the degr~adea ~ voltage condition) that can, if required, obtain and maintain the plant in a safe shutd~own. Upon evaluation and acceptance of the list of available syste=s (not exposed to degraded voltages) by the Reactor Systems Branch of the NRC, I reco= mend that the' alternative method be accepted. i the licensee is bypassing the load-shed feature'to prevent adverse , interaction when the onsite sources are supplying the Class IE buses. The l licensee is proposing to auto-reinstate the load-shed feature following diesel ! generator breaker tripping. Thus, NRC Staff Position 2 is met. l l i The proposed additions to the Technical Specifications and the testing of the logic circuitry meet the requirements of NRC Staff' Position 3. Accordingly, the licensee is required to subm t the following informa-tion for NRC review: l 1 (1) Detailed plant procedures defining operator actions being taken i j to r.estore adequate voltage to the Class lE buses during non- l accident conditions. (2) Details of the circuitry modifications which accomplish the disabling of the load-shed feature when on the diesel generators and the auto reinstate =ent of the load-shed feature following generator breaker tripping. (3) For=al Technical Specification changes for the modifications to the undervoltage protection system and associated test require-ments. o *** . I REFERENCES l

1. NRC letter (A. Schwencer) to'YAECO, dated June 3, 1977. ,

2. VYNPC letter (E. W. Jackson) to the NRC, dat ed July 18, 1977._ .

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3. VYNPC letter (D. E. Moody) to the NRC (T. A. Ippolito), da'ted December 6, 1979.

4 YAECO letter (D. E. Vandenburgh) to the NRC, dated July 24, 1980.

5. VYNPC letter (W. F. Conway) to the NRC (W. Cammill), dated March 17, 1980.

l l 6. VYNPC letter (L. H. Heider) to the NRC (T. A. Ippolito), dated February 6,1981. i

7. VYNPC letter (J. B. Sinclair) to the NRC (D'. B. Vassallo), dated June 8, 1982.

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8. VYNPC le'tter (L. H. Heider) to the NRC (D. B. Vassallo), dated July 22, 1982.

1 9. Code of Federal Regulations, Ticle 10, Part 50 (10 CFR 50), Ceneral Design Criterion 17 (GDC 17), " Electric Power Systems" of Appendix A " General  : Design Criteria for Nuclear Power Plants." *

10. IEEE Standard 279-1971, " Criteria for Protection Systems for' Nuclear Power

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, Cenerating Stations."  ! I

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

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