ML20071N830

Adequacy of Station Electric Distribution Sys Voltages, Technical Evaluation Rept

ML20071N830
Person / Time
Site:	Vermont Yankee
Issue date:	09/03/1982
From:	Selan J LAWRENCE LIVERMORE NATIONAL LABORATORY
To:	NRC
Shared Package
ML20071N832	List: ML20071N830
References
CON-FIN-A-0250, CON-FIN-A-250 UCID-19459, NUDOCS 8210120191
Download: ML20071N830 (12)

Text

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TECHNICAL EVALUATION REPORT ~

ON THE ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES FOR THE VERMONT YANKEE NUCLEAR POWER STATION (Docket No. 50-271)

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James C. Selan .

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ABST?aCT 4

8 This report docu=ents the technicar evaluation of the adequacy

, of the station electric distribution syste= voltages for the Vermont Yankee Nuclear Power Station. The evaluation is to deter..ine if the onsite distri-bution system, in conjunction with the uffsite power sources, has sufficient capacity to automatically start and operate all Class lE loads. Vithin the equipment voltage ratings under certain conditions established by the Nuclear Regul'atory Commission. The evalu'ation finds that the voltage analyses sub-j mitted demonstrates that adequate voltage will be supplied to the Class lE

equipment under worst case cenditions.

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, FOREWORD .

This report is supplied as part of the Selected Electrical, Instru '

,, , mentation, and Control Systems Issues ~(SEICSI) Program being conducted for* , , ,

the, U. S. Nuclear Regulatory Co= mission , Office of Nuclear Reactor Regulation, Division of Licensing, by Lawrence Livermore National Laboratory.

i The U. S. Nuclear Regulatory Con =ission funded the work und' er the authorization entitled " Electrical, ' Instrumentation and Control System Support,"

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1. INTRODUCTION . . . . . . .

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2. DESIGN BASIS CRITERIA . . . . _. . .

. . . 2

3. SYSTEM DESCRIPTION . . . . . . . . . . . . . 2

4. ANALYSIS . . . . .' . . .

. . . . . . . . 4 4.1 Analysis Conditions . . . . . . . . . . . 4 4.2 Analysis Results . . .

. .. . . . . . 5

• 4. 2.1 Undervoltage. . . . . . . . . . . . 5 4.2.2 Overvoltage . ,.' . . . .

. . . . . . 5

4. 3

• Analysis Verification . . . . . . . . . 5

5. EVALUATION . . . . . . . . . . . . . . . 5

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6. CONCLUSIONS . . . . . . . . . . . . . . . 8 FIFERENCES . . . . . . . . . . . . . . ,

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ILLUSTRAIIONS FIGURE 1 Vermont Yankee' Nuclear Power Station Electrical One-Line Diagram . . . . . . . . . .. 3-TABLE 1 Vermont Yankee Nuclear Power Station Class'lE Equipnent Voltage Patings and Analyzed .

Worst Case Terminal Voltages. . . . . . . . . . 6 b -- .

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TECHNICAL EVAL.UATION FIFORT ON THE ADEQUACY OF STATION ELECTRIC ,

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DISTRI'BUTION SYSTEM VOLTAGES -

'FOR THE VERMONT YANKEE NUCLEAR P0rr R STATION (Docket No. 50-271)

, James C. Selan Lawrence Livermore National Laboratory, Nevada

., ,- 1. INTRODUCTION -

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The Nuclear Regulato'ry Commission (NRC) by a letter dated .

Augus t 8,1979 [Ref.1), expanded its generic review of the adequacy of the station electric distributicn systems for all operating nuclear power

. . facilities.

This review is to determine if the oRsite distribution system, - - .

in . conjunction with the of fsite power sources, has sufficient capacity and' capability to automatically start and operate all required safety loads within the equipment voltage ratings. In addition, the NRC requested each .

licensee to follow su'ggested guidelines and to meet certain requirements - -

in the analysis. These requirements' are detailed in Section 5 of this '

report. .

By letters dated March 17, 1980 [Ref, 2) and December 29, 1980

[Ref. 3], Vermont Yankee Nuclear Power Corporation, the licensee, submitted- .. ,

' their analysis of the adequacy of the electrical- distribution syste.m's voltages at Vermont Yankee Nuclear Power Station.

The purpose of this report is to evaluate the licensee's sub=ittal .

with respect to the NRC criteria and present the reviewer's conclu'sion on the adequacy of the station electric distribution system to maintain the voltage for the required Class lE equipment within acceptable licits for the worst-case starting and load conditions. .

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2. DESIGN LASIS CRITERIA

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The design basis criteria that were, applied,in determining the adequacy of station electric distribution syste= voltages to c art and operate all required safety loads within their required voltage ratings.

are as follows:

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(1) General Design Criterion 17.(CDC 17), # Electric Power

. Syscens," of Appendix A, " General Design Criteria for Nuclear Power Plants," in the Code of Federal Regulations, .

. Title 10, Part 50 (10 CFR 50) [Ref. 4]. ,

(2) General Design Criterion 13 (GDC 13), " Instrumentation and Control," of Appendix A, " General Design Criteria for

. Nuclear Power Plants," in the Code of Federal Regulations, Title 10, Part 50 (10 CFR 50) [Ref. 4].

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(3) ' ANSI C84.1-1977, " Voltage Ratings for Electric Power Systems

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and Equipoent" [Ref. 5).

(4) IEEE Std. 308-1974, " Class 1E Power Systems for Nuclear Power

.. . Generating Stations" [Ref. 6). . .

(5)- " Guidelines for Voltage Drop Calculations," Enclosure 2, to NRC letter dated August 8, 1979 [Ref. 1).

3. SYSTEM DESCRIPTION

, A one-line diagram of Vermont Yankee Nuclear Power Station's e.lec- .

trical distribution system is shown in Figure 1. This figure was , adapted . .

from Figures 2.1 through 2.5 of Reference 2. As shown in Figure 1, there i are two 4160-volt Class 1E buses and two 480-volt Class 1E buses. During normal plant operation, 4160-volt Class 1E buses 3 and 4 are connected to station auxiliary buses 1 and 2, respectively. Auxiliary' buses 1 and 2 are energized by unit auxiliary transformer T-2. The 480-volt Class 1E buses 8 and 9 are en'ergized through station service transformer T-8 and station '

service transformer T-9, respectively. Upon loss of the main generator, buses 1 and 2 are auto =atically transferred to start-up transformers (SUT's)

T-3A and T-33, respectively.

- The cain generator is connected to the 345 kV switchyard throug'h -

the =ain transfor=er T1. There are three lines which are connected to the 345,kV switchyard and one line connected to the 115 kV switchyard from which the startup transformers T-3A and T-3B are supplied. A 345/115 kV autotrans-former connect; the two switchyards together. . . -

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FIGURE 1 VERHONT YANKEE NUCLEAR POWER STATION - ONE-LINE DIAGRAH

Protecting the Class 1E equip =ent frc= undervoltage conditions is accc plished by two levels of protection. The firs: level (loss of-voltage)  ;

scheme which utiliz,es two rel-ays on each 4160-volt Class iE bus. ThE relays ,

l (2-out of-2 logic) will operate in 1.25 seconds at zero voltage, 6.0 seconds at 41% of 4160 volts, and will not operate above 46% voltage. The second level scheme (proposed) also utilices two relays per 4160-volt Class 1E bus in a 2-sut-of-2 logic. These relays will have a setpoint of 3700 volt's + 40 vo,lts (88.9% + 1% of 4160 volts) with a time delay of 10 seconds + 1 second.

4. ANALYSIS - -

4.1 ,

ANALYSIS CONDITIONS ~

Vermont Yankee Nuclea'r Power Corporation (VYNPC) analyzed the dis-tribution systec voltages using computer load flow programs. Several cases ,

were analyzed to determine worst case transient voltage drops and steady state voltages. These cases evaluated voltages for maximum load / minimum

' ' offsite grid voltage, mininum load / maximum offsite grid voltage, and for , ,

the effects of starting a large non-Class lE Icad for all possible source connections. The minimum and maximum offsite grid voltages used were 340 kV

, and 362 kV for the 345 kV system cnd 110 kV and 121 kV for the 115 kV system, respectively. The analyses evaluated conditions of load and no-lo.ad transfersi from one source to another with safeguard loads starting and steady state .

voltages following safeguard load starting. In addition to these conditions, several other assumptions were made and are as follows:

(a) The plant was considered to be' at normal operation when an acci-dent signal initiated a turbine trip, a transfer of loads, and the start of the e=ergency roads. .

(b) Cable impedence to the terminals of each- safety load was calcu- '

lated. ' Cable capacitance was neglected.

(c) Transformer nameplate impedences were used.

(d) Motor nameplate values were used.

(e) Load factors were assigned for inter =ittent loads. ' '

(f) For motor starting, all loads were converted to constant real current and constant icaginary reactance equivalents .(all resis-

• tive loads are treated as motors). -

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. (g) No load shedding was considered. -

In evaluating the starting of a large non-Class lE load (reactor feed ~ '

pump), the anglysis assumed that two condensate pumps, two circulating water

! pumps, three circulating water booster pumps, and two recirculating M-G ' sets

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had been shed prior to the =otor start. The reactor feed pump will accelerate i

in 6 seconds at 80% of 4000 volts.

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1 4.2 ANALYSIS RESULTS - -

The results of the first round analysis re' quired-changing taps on startup transformers T-3A and T-3B and the 4160/480-volt station service

t rans f o rmers. This change is-necessary when the plant is in a closed cooling cycle mo,de. The analysis vas performed again with the tap changes and resulted i

in the worst case Class lE equipment terminal voltages occuring un. der the foll'owing conditions and are presented in Table 1:

4.2.1 Overvoltage The unit is in a cold shutdown with minimum loads of three station -

service water pumps, one residual heat remeval pump, one RER service water pdmp, lighting loads, and various other 480-volt system loads.

The grid voltage was 3,62 kV and 121 kV for the two transmission systems and. only one startup.: transformer in service.

4.2.2 Undervoltage Unit is in an accident mode with only one startup transformer in -

. - - service, the startup transformer is carrying the plant 's auxiliary, ,

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loads (no transfer) with simultaneous starting of the Class lE loads. ,

The grid is at 340 kV and 110'kV for the two switchyard voltage systems A' tap change is necessary for single startup transformer operation (112 kV tap for the SUT and 4060 volt tap for the 4160/480 volt-station service transformers). .This tap change is a manual operation. .

e 4.3 ANALYSIS VERIFICATION -

a VYNPC verified their computerized voltage analyses calculations by using the. computer load flow program and a model, of the auxiliary power system 1

to predict bus voltages for actual plant conditions. Two verification tests

) were =ade with the distribution system at 80% of maximum load during steady state '

conditions. The combined test data resulted in percentage errors of + 1.211 to i

-C1' 2.95% on the 4160-volt Class lE buses and of + 1.09% to - 1.73% on the 480-volt ass 1E buses. A negative error per.centage indicates tha't the measured voltages were higher than the calculated.

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5. EVALUATION The NRC generic letter [Ref.1] stated several requirements that',each I plant must meet in its voltage analyses. These requirements and an evaluation i of the licensee's submittals are as follows: I

.. j i (1) With the minimum expected grid voltage and maximum load condi- '

tion, each offsite source and distribution system connection ~

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TAILE 1 .

, VEFJiOliT YAITr2E ICOLI.AR PC'4IR STATION _ ,, ,

, CLASS 1E EQUIP 121C VOLIAGE FATINGS AND WORST CASE TI??.INAL. VOLTAGES.

(in % of Equip =ent No inal Voltage Rcting)

Maximum Voltage Minimum Voltage

.. Analyzed Rated Analyzed (a)

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Rated Hominal .

. Voltage Steady Steady ,

• State Equipment Rating State Transient Motors 4000 Start

• 90 (a) 85.3(b)

Operate 11.0 110.8 90 90.7 ,

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460 .

• Start 90 83.9(b)

Operate 110 112.2(c) ,

90 89.6

~.i' Starters 460 Pickup 80 84.8 -

Dropout 70 .

83.3(d)

Operate 110 ll2.2(c) . 90 90.2 .

Other(e) 120/240 110 -

85 86.7 83.3 Equipment

. (a) Two 4160-volt Class 1E motors (RHR and core spray pumps) are rated for' 80% .

. start voltage. ,

(b) This voltage is the lowest experienced during the bulk starting'of the large, 4000-volt RHR and core spray motors. The voltage transient caused from these motors starting lasts on the order of 2-3 seconds. Upon motor acceleration, adequate starting voltage will be supplied to the re=aining Clu s lE equipment.

(c) Percent voltage is at bus and motor control center (HCC) level.

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(d) Lowest transient voltage experienced during start of the largest non-Class lE motor (rear. tor feed pump) following steady state conditions of the fully loaded Class lE buses.

(e) Low voltage AC buses normally supplied from 120/240 volt UPS. Worst c .se

. occurs when buses are supplied fron the.ir =aintenance tie from a MCC.

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=ust ba capable of starting and continuously operating all Class lE equipment within the equiptent 's voltage ratings.

>VYNPC analyzed their onsite distribution system being encr-gized by offsite sources throof,h either two s'tartup trans-for ers, the unit auxiliary transformer, or one startup, transformer under worst case conditions. For the above source connections, transformar tap changes are necessary for two conditions. With the plant in' a closed c'ooling cycle mode, tap changes on SUT's T-3A and T-3B and the 4160/480-

." volt station service transformers is required. For.the condi-

' tion with only one SUT in service, the taps on the remaining SUT will be changed to the 112 kV tap and the 4160/480 volt service station transformers to the 4060 volt tap.

The, analysis results for worst case conditions indicate that during bulk Class 1E motor starting, adequate starting voltage is available to the RER and core spray punp motors. The spend and torque curvei for these motors show that the voltage tran-sient will last'only for 2-3 seconds before reaching operating speed. Following this short 2-3 second voltage dip, the voltage will recover to acceptable levels which ensure adequate star. ting

,. . and operating voltage'to the remaining Class lE equipment. .. .

(2) Vith the naximum expected' offsite grid voltage and minimum locd condition, each offsite source and distribution system connection

=ust be capable of continuously operating the required Class lE equiptant without exc'eeding the equip =ent's voltage ratings,.

The analysis results show that the voltage could exce,ed the upper

" voltage design rating for the 4,000-volt motors and the 460-volt motors by 0.8% and 2.2%, respectively. The, magnitude of these ..

overvoltages is considered negligible as the effect on the motors will be inconsequential to the life or performance o'f the motors.

(3) The analysis must show that there will be no spurious separation.

from the offsite power source to the Class lE buses'by the t

! voltage protection relays when the grid is within the normal expected limits and the loading conditions established by the NRC are being met. .

The analyses submitted demonstrate that the worst case voltage transients are not of sufficient time duration to cause spurious separations from the offsite sources. However, for worst case j

steady state conditions, the upper tolerance of the degraded .

voltage relays is such that spurious separations could occur.

' Since these retpoints are only. proposed, final. installation and testing r.ay result in lowering t'he tolerance band to preclude the possibility of spurious separations. Also, due to the close j l

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control of ~the grid voltage by the plant's operators, experiencing  ;

grid voltages near the mininum expected in conjunction with the

' worst case loading and having both relays drif t in the same direction has a low probability'of occurring. . l

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. (4) Test results are required to verify the voltage analyses calcu-laticas submitted. The licensee performed tests which ' verified -

that the analysis ~results submitted are acceptable.

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(5) Review the plant's electrical p'ower systems to deter =ine if l any events or conditions could result.in the si=ultaneous loss of both offsite circuits to the onsite distribution system

, (compliance with CDC 17).

The licensee states that they have reviewed the electric power system at Vermont Yankee Nuclear Power Station and have found total compliance with GDC-17.  ;

They state also that to the extent practical there are no events or conditions which could result in the sieultaneous or. consequential loss of both required circuits to the offsite network [Ref. 2).

6.' CONCLUSIONS .

.' Based on information submitted. by VYNPC for the Vermont Yankee Nuclear Power Station, it is concluded that:

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(1) Theoffsitesources(withcertainrequiredtapchanges)incon-junction with the onsite distribution system have the capability and capacity to auto =atically start' and continuously operate the.

Class 1E equipment within their design voltage ratings under worst case conditions. .

(2) The potential overvoltages are of negligible magnitude to have .

any adverse effect' on che life or operability of the Class lE - -

motors. -

(3) No event or condition will result in the simultaneo'us or conse-quential loss of b'oth required circuits to the onsite distribution system.

(4) The voltage analysis was verified by test with the cf ror percen-tages and judged acceptable.

(5) There will. be no spurious separations from the offsite sources due to the low probability. of experiencing minimum expected grid voltages with worst case loading in addition to both undervoltage

, relays drif ting to the upper tolerance setpoints.- - .

Accordingly, I reco= mend that the NRC approve the voltage analy' sis sub-hitted which shows that the station electric distribution system is adequate to supply acceptable voltages to the Class 1E equipment for the worst case conditions..

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." REFERENCES i

l. NRC letter (W. Ga= mill) to all Power Reactor Licensees,' dated August 8, 1979. '

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2 Vermont Yankee Nuclear Power Corporation letter (-W. F. Conway) to NRC (W.,Gammill), dated March 17, 1980. ~

3. Ver=ont Yankee Nuclear Power Corporation letter (R. 'L. Smith) to NRC i

(T. A. Ippolito), dated December 29, 1980.

4. Code'of Federal Regulations, Title 10, Part 50 (10 CFR 50), General Design Criterion 13 and 17 of Appen. dix A for Nuclear Power Plants. -

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5. ANSI C84.1-1977, " Voltage Ratings for Electric Power Systems and Equipment."

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6. IEEE Standard 308-1974, " Class lE Po.wer Systems for Nuclear Power Generating

• Stations."

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