ML18139C091

Technical Evaluation Rept on Adequacy of Station Electric Distribution Voltages for Surry Power Station,Units 1 & 2.

ML18139C091
Person / Time
Site:	Surry
Issue date:	09/01/1982
From:	Selan J LAWRENCE LIVERMORE NATIONAL LABORATORY
To:	NRC
Shared Package
ML18139C089	List: ML18139C091
References
CON-FIN-A-0250, CON-FIN-A-250 UCID-19454, NUDOCS 8211010584
Download: ML18139C091 (16)

Text

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e e UCID- 19454 I.

TECHNICAL EVALUATION REPORT ON THE

~ ADEQUACY OF STATION ELECTRIC DIS.TRIBUTION SYSTEM VOLTAGES FOR TRE SURRY POWER STATION, UNITS 1 AND 2 (Docket Nos. 50-280, 50-281)

James C. Selan This is.an informal report intended primarily for internal or limited external distribution.

The opinions and conclusions stated are those of the author and may or may not be those

- of the Laboratory.

This work .,,;.as supported by the United States Nuclear Regulatory Commission under a Memorandum of Understanding with the Unite*d States Department of Energy.

NRC FIN No. A-0250 .

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• ABSTRACT e

l This report documents the technical evaluation of the adequacy of the station electric distribution system voltages for the Surry Power Station, Units 1 and 2. The evaluation is to determine if the onsite distribution system in conjunction ';olith the offsite power sourc,es".:"h~ sufficient capacity to autowatically start and operate all Class lB loads within the equipment voltage ratings under certain conditions established by the Nuclear Regulatory

• Commission. The analyses submitted demonstrate that the station's* electric distribution system will~supply adequate voltage to the Class lE equipment under the worst case conditions analyzed.

FOREWORD This report is supplied as part of the Selected Electrical, Instr-iraentation, and Control Systems Issues Program being conducted f_or the U. s. Nuclear Regulatory Comi:tlssion, Office of Nuclear Reactor Regulation, Division of Licensing, by Lawrence Livermore National Laboratory.

The U. s. Nuclear Regulatory Commissipn funded the_ work under the authorization entitled "Electrical, Instrumentation and Control System Support,"

B&R 20 19 04 031, FIN A-0250.

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e e TABLE OF CONTENTS Page

1. INTRODUCTION . . . 1 2

2. DESIGN BASIS CRITERIA *-

2

3. SYS':'E~ .DESCRIPTION 4 4 *. ANALYSIS . *

• 4 4

4.1 Analysis Conditions 4.2 Analysis Results . -:

6 7

4.2.1 Overvoltage

• Undervoltage. 7 4.2.2 Analysis Verification 7 4.3 9

5. EVALUATION 11

6. CONCLUSION REFERENCES .. . 13 ILLUSTRATIONS FIGURE 1 Surry Power Station, Units 1 and 2 3

Electrical One-Line Diagram.

TABLE 1 ,.Surry Power Station, Units 1 and 2 Class lE Equipment Voltage Ratings and 8

Analyzed Worst Case Terminal Voltages

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l TECHNICAL EVALUATION REPORT ON THE i' ADEQUACY OF STATION ELECTRIC

-l DISTRIBUTION SYSTEM VOLTAGES l~ FOR THE SURRY POWER STATION, UNITS 1 AND'2 l

l (Docket Nos. 50-280, 50-281)

James c. Selan l Lawrence Livermore National Laboratory, Nevada i .(

I 1. INTRODUCTION The Nuclear Regulatory Commission (NRC) by a letter dated August 8, 1979 [Ref, l] expanded its generic review of the adequacy of the station electric distribution systems for all operating nuclear power fa~ilities. This review is to deterreine if the onsite distribution system in conjunction with the offsite 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 suggested guidelines a.nd to meet certain requirements in* the analysis.* These requir_ements are deta~led in Section 5 of this report.

• By letters dated May 26, 1981 [Ref. 2], December 31, 1981 [Ref. 3],

March 31, 1982 [Ref. 4], June 11, 1982 [Ref. 5], and June 30, 1982 [Ref. 6),

Virginia Electric and Power Company (VEPCO), the licensee, submitted their analysis and conclusion regarding the adequacy of the electrical distribution system's voltages ~t the Surry Power Station, Units 1 and 2.

The purpose of this report is to evaluate the licensee's submittal with respect to the NRC criteria and present the reviewer's conclusion on the adequacy of the station electric distribution systems to maintain the voltage within the design limits of the required Class lE equipment for the worst case starting and load conditions.

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2. DESIGN BASIS CRITERIA The design basis criteria that were applied in determining the adequacy of station electric distribution system voltages to start and operate all required safety loads within their required voltage ratings are as follows:*

(1) General Design Criterion 17 (Gbc 17), "Electri~ Power Systems," of Appendix A, '.'General Design Criteria for Nuclear Power Plants," in the Code of Federal Regulations, Title 10,~ Part 50 (10 CFR 50) [Ref. 7].

(2) General Design Criterion 13 (GDC 13), "Instruraentation 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. 7).

(3) General Design Criterion 5 (GDC 5), ~Sharing of Structures, Systems and Components," of Appendix A, "General Design Criteria for Nuclear Power Plants," in the Code of Federal Regulations, Title 10, Part 50 (10 CFR SO) [Ref. 7].

(4) A:."l'SI C84.l-1977, "Voltage Ratings for Electric Power Syste!:!s and Equipment" [Ref. 8).

(5). IEEE Std 308-1974, "Class lE Power Systems for Nuclear Power Generating Stationsu [Ref. 9].

(6) "Guidelines for Voltage Drop Calculations," Enclo~ure 2, to NRC letter dated August 8, 1979 [Ref. 1].

3. SYSTEM DESCRIPTION An electrical one-line diagram for Surry Power Station, Units 1 and 2 is shown in Figure 1. Each unit's generate~ is connected to the transoission system through it's own main transformer. The output of the the generator is stepped up from 22 kV to 230 kV and 22 kV to 500 kV for Units l and 2, re'spectively. The 230 kV system and the 500 kV system is tied

• together by two 500/230/36.5 kV autotransformers (preferred offsite source).

The autot.ransformer.s. supply two 34. 5 kV buses from which the th.ree 34 *. 4l 4.16 kV reserve station service transformers (RSST's), two 34.4/4.16 kV intake structure transformers and four 34. 5 kV reacto.r banks a*re fed. The RSST' s A, B, and C supply the 4 kV transfer buses D, E, and F resp~ctively. Transfer.bus D supplies Class lE bus lJ and station service buses 1A and 2A. Transfer bus E supplies

• I 230KV SWYD 500KV SWYD

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DIESEL . DIESEL GEN. 1 GEN. 2

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1 480V CLASS lE BUSES (TYPICAL BOTH 1:JN~TS)

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I j) 2H FIGURE 1 - SURRY UNITS 1 AND 2 - ELECTRICAL ONE-LINE DIAGRAM

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e e Class lE bus 2H and station service buses lB and 2B. Transfer bus F supplies Class lE buses lH and 2J and station service buses lC and 2C. The Class lE buses (two .4160-volt and four 480-volt Class lE buses at ea.ch unit) are fed from the RSST's at all times. The RSST's have a load tap changer (LTC) on their secondary *winding which is set to maintain a 4300-volt output and which will provide a+ 10% voltage adjustment over the full range of operation. The adjustment capability is provided by 32 taps each of 0.625% voltage adjustment.

The station service buses are normally f.ea::fr:o-m the_ 22/4.16 kV station service transformers which are supplied by the.main generator. During plant startup, the station service buses are then supplied from the RSST's. A unit trip will initiate an automatic transfer of the station service buses to the RSST' s. }

The 230/36.S kV transformer which is supplied from the 230 kV switch-yard serves as an alternate supply to the reserve station service system. On loss of either 500/230/36.5 kV autotransformer, the alternate supply is automati-cally switched to supply the affected 34.S kV bus. The 230/36.5 kV transformer is *equipped with a LTC on the secondary windi:ig ;.;hich is set to maintai.n 3*6. 4 kV and will provide a+ 10% voltage adjustr:ient over the full range of operation.

The adjustment capability is provided by 32 taps each of a 0.625% voltage adjust-ment.

The Class iE equipment will be protected from undervoltage conditions by two protection schemes. The first schene (loss of voltage) consists of three undervoltage relays (2-out-of-3 logic) on each 4160-volt Class lE bus. The loss-of-voltage relays are set to actuate at a voltage setpoint of 75% + 0.1% of 4160 volts (3120 volts) with a time delay ot' 2 seconds,+ 5 seconds, .,.. 0.1 seconds.

The degraded voltage scheme consisis of ~hree u~dervoltage relays (2-out-of-3 logic) on each 4160-volt Class lE bus. These relays are set to actuate at 90% + -

1% of 4160 volts (3744 volts). The time delay associated with the voltage set--

point is 7 + 0.35 seconds for a safety injection (SI) or a consequence limiting safeguard (CLS) and 60 + 3 seconds for non-CLS or non-SI conditions.

4. &~ALYS IS 4.1 ANALYSIS CONDITIONS VEPCO has analyzed each of the offsite circuits .to the reserve station service system from which the Class lE buses are fed .through the RSST's.

Various 1.oading condit*ions were analyzed.which included combinations of the units at 100% power, startup, tripping, tripping with a CLS, and refueling.* "From 'these i-*

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~~rious loading combinations under minimum and ~3ximum offsite grid voltages of 505 kV and 535 kV respectively, the RSST system was analyzed to ensure the system

! can supply adequate voltage to the Class lE equipment. The analysis included several assumptions to ensure the "worst case" was analyzed* and are as follows:

(1) The maximum switchyard voltage drop caused by either one or both of the units tripping off-line is 15 kV. The load tap changer tap position is determined by the loading on the RSST's prior to the condition analyzed and with the_ s~it:i:*hyard_ voltage at 5-20 kV.

At the instant the condition being analyzed occurs, the switchyard voltage drops instantaneously to 505 kV and.the loading caused by the condition is assumed to occur. Voltages calculated at this time are hlised on the LTC tap position prior to the condition occurrence/. Final voltages ar-e based on t.he LTC correcting for the loading with the switchyard still at 505 kV.

(2) Transferring of the station service buses (non-Class lE) to the 1 reserve station service system occurs immediately upon occur~ence of the condition being analyzed.

(3) All motors receiving an SI or CLS signal were assumed not to be running prior to the accident. Upon receiving the signal, all motors ~ere assumed to start.

(4) Worst case loading of the station service buses was as~umed for the transfers.

(5) No manual load shedding or reduction in moto-r current due to decreased pump loads is assumed to occur for 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> after the occurrence analyzed.

(6) Sequence loading occurs as designed. The sequenced loads are the auxiliary steam generator feedwater pumps (50 seconds after an SI signal), inside recirculation spray pump (120 seconds after an SI signal), and the outside recirculation spray pump (300 seconds after an SI signal).

(7) The existing load-shedding scheme on each individual RSST occurs as designed. The affected loads are tripped and locked out

[Ref. 4].

(8) Ampere values used were measured values except for those which had to be estimated.

(9) Anticipated modifications and operating restrictions.were included in the analysis except for the replacement of the 4 kV und_erground cab3..e-s *f:i:.:om the RSST 's. 4-lso analyzed was the effect of s~arting large non-Class lE loads during steady state condition*s *.following an SI or CLS occurrence.

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ANALYSIS RESULTS e

I As a result of the initial analyses of the various loading combi-nations, several modifications and operating restrictions *were determined necessary to eliminate equipment overloading and to assure that adequate voltage is supplied by the RSST system. These modifications and restrictions I) a.re as follows:

(1) The existing load shedding scheme which...occurs automatically I when t-wo uni ts load onto the RSST 1*s ~il1 be enabled when one unit is on line and the other unit is in startup, both units on I line, and both units in startup.

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(2) -

Follow t~e transmission system voltage r~gulation action plan.

(3) Disconnects on both sides of the 34.5 kV bus tie breaker will be normally open to prevent the loss of both offsite source circuits should the tie breaker fail.

(4) Rerate the MOV's starting capability to 80% of 460 volts.* If the MOV's cannot be rerated, replace those with MOV's rated at 80% of 460 volts.

(5) Autooatically trip the four 34.5 kV reactor banks following an SI or CLS on either unit when the station service buses (non-Class lE) are transferred to the RSST's.

(6) Modify the LTC control to provide:

(a) Instantaneous voltage correction for approximately 3 minutes -

upon an SI or CLS at either unit.

(b) Instantaneous voltage correction for approximately 1 minute when a unit transfers to.the RSST's during an SI or CLS.

(7) Block the automatic starting of the condensate, high pressure h~ater drain, bearing cooling and component cooling pumps for approximately 315. seconds after an SI or CLS occurs.

• The blocking featuFe is accomplished* by the use of auxiliary relays initiated from the SI contacts and will occur in the automatic close path of the*pump's 4* kV breakers. The automat-ic starting is aga~n enabled after the 315 seconds.

(8) Replace and reroute the underground 4 kV cable from the RSST's to the transfer buses.

.( 9) Add two radiators with fans to each RSST to increase their 55° C ris~ *r~dng to* 30 KVA from 24 MVA.

1 r (10) e e Replace control transformers (480/120 volt) to larger sizes for adequate voltage to the MOV contactor coils.

I (11) Change all the 4160/480-volt transformer tap_s* to 4056/480-volts.

I l (12), Reroute the RSST control cables to meet the separation requirement for fire protection.

I (13)

Install overload alarms of 85 MVA and 95 }fVA on the 500/230/

I 36.5 kV and 230/36.5 kV transformers.

Based on the ab~~e assumptions and including the modifications and operating restrictions:: into the _final analysis, the worst case Class lE equipment terminal voltages occur under the following conditions and are summarized in Table 1:

4.2.1 Overvoltage The offsite grid voltage at 535 kV, Unit 1 at 100% power, Unit 2 in refueling, Unit 1 loading on the RSST's consists of Class lE bus loads only, Unit 2 loading on the RSST's consists of Class lE bus loads and some 480-volt loads fed from the station service buses.

4. 2. 2 , Undervoltage .

Tne offsite grid voltage at 505 kV, loss of one of the 500/230/36.5 kV autotransformer with auco* transfe~ of the affected 34~5 kV bus to the 230/36.5 kV transformer. Unit 2 experiences a CLS with a Unit 1 trip transferring its station ietvice 16adg to the RSST's at the same instant the CLS occurs, All Class lE ~otors receive an instantaneous start signal with LTC movement.

4.3 k~ALYSIS VERIFICATION VEPCO verified their computerized voltage analyses calculations by performing a voltage profile test, The profile test was conducted during the startup of Unit 1 in which voltage and current measurements were taken on the reserve system. System parameters. were monitored from the 500 kV and 230 kV buses down to the 480-volt buses. The measuring of system parameters was accomplished by using strip-chart recorders and the readings from*permanent mounted meters. The bus loading conditions on the 4160-volt buses ranged from 11% to 65% of maximum bus load. Inputting the measured parameters into the computer codel, a voltage profile was then computed. Comparing the actual measured voltage conditions to the calculated values resulted in a computer model which is highly conservative, The percentage errors ranged from 3.35%

to 7.18% 6n the 4160-volt*buse&, and from A.29% to 6.26% on the ~80-volt buses with the measured values being higher than the calculated.

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TABLE 1-I SURRY POWER STATION, UNITS 1 AND 2 CLASS lE EQUIP!-IBNT VOLTAGE RATINGS AND ANALYZED WORST CASE BUS VOLTAGES (in% of Eq;ipment Nominal Voltage Rating)

Maximum Minimum Rated Analyze~ (-a} ~ted Analyzed Ca)

Nominal Voltage Rating Steady Stea-dy Equipment (JOO %) State State Transient Mot*ors 4000 Start 70 83. 93 Operate 110 107.4 90 98~28 460 Start 70 69.70 Operate 110 108.5 90 92. 66 XOV's 46o(b)

St~rt 80 82.40 Operate llO 108.5 90 92. 66 Starters Cc) 120 Pickup 83 (d)

Dropout 50 (d)

Operate llO (d) 90 (d)

Other<e)

Equipment (a) Minimum required bus voltages were calculated for the Class lE loads incl~ding all voltage drops in cables to meet the starting and running design voltages (Appendix D, Ref. 4).

(b) Existing Class lE motor ope~ating vaives (MOV's) are rated; 1) 460 volt+ 10%

continuous and starting, 2) 440 volt+ 10% continuous, -* 15% starting analysis assumes all MOV's will be rerated or replaced with a .460-volt + 10% continuous,

-20% starting rating. The MOV's are modeled as starting loads-:- drawing locked

• rotor amps, through the first 5 seconds.

• (c) Fusing is not used for primary protection of a Class lE load or for control transformer pi'."otection in Glass lE control circuits. *

(d) The licensee has committed to upgrading the control transformers to ensure adequate operation of the MCC contactors under worst case conditions.

(e) 120-volt vital bus loads -and instrumentation are fed from either uninterruptable power supplies or from regulating transformers.

5. EVALUATION e

The ~C generic letter [Ref. l] stated several requirements that the plant must meet in their voltage analysis. These requirements and an evaluation of the licensee's submittals are as follows:

-(1) With the minimum expected grid volt_~ge_~nd maximum load condition, each off site source. and d~stribution system connection must be capable of starting and continuously operating all Class lE equipnent within the equipment's voltage ~atings.

The volt~ge analyses submi_tted for minimum grid voltage and maximum load demand conditions resulted in various voltage , {

profiles where the voltage to the 460-volt Class lE MOV' s t fl"\0-+t )

did not meet the minimum required starting voltage at T = 0 seconds for the conditions analyzed. The analyses , -

assumed that all the MOV'~ have a 80% of 460-volt starting.

rating. The worst case analyzed voltage was 4.1% of 480: __ _

volts below the required mininum starting voltage;::**-A;highly..

conservative computer model was used to analyze these scenarios. The following conservatisms were used in -the model:

(a) Both units tripping with load transferring (with one unit experiencing a CLS) at the same instant that the electrical abnor~alities- occur is a very improbable event.

(b) All motors upon receiving a CLS or SI were assumed not to be running and to start at T = 0 seconds. Some Class lE motors are normally running.

(c) Starting loads were modeled at a power factor of 0.0.

(d)

• A 15 kV drop in the 500 kV voltage to 505 kV was assumed to occur at the ~ame inst~nt of the condition analyzed.

This 15 kV drop is 3 kV greater than the maximum. expected.

(e) The measured test verification results ranged from 4.29%

to 6.26% higher than those calculated on the 480-volt buses~

Therefore, due to these conservatisms, modifications, and plant operating restrictions, adequate voltage within the design voltage ratings will be ensured down to the 480-volt level under the worst case conditions. The licensee has committed to submit the results oi th~ MOV re--rating and"/or replacement and the results of the _MCC control transformer upgrading with the worst case undervoitage and overvoltage analyses.

Ir e e (2) With the maximum expected offsite grid voltage and m1n1muo.

load condition, each offsite source and distribution system connection must be capable of continuously operating the required Class lE equipment without exceeding the equipment's voltage ratings.

The voltage analysis shows that the Clabs lE equipment's voltage design rating is not exceeded for minimum plant load and r:1axi.nuo. expected offsite_ gricf-_;,oltage conditions. *

(3) The analysis must show that *there will be no spurious sepa-ration from the offsite power source to the Clas~ lE buses by the!voltage protection relays when the grid is within the normal iexpect:E!d limits and the loading* conditions established by the NRC are being met.

The voltage analyses profiles with instantaneous LTC movement resulted in several scenarios where at T = 7 seconds the-Voltage on the Class lE buses :TiaY be below the degraded grid protection scheme setpoint. The profiles show that a voltage of 1%-2% below the setpoint could be experienced. For CLS or SI conditions, the setooint is 90% + 1% of 4160 volts with a time delay of 7 ~ 0.35 sec~nds. Evaluation of these scenarios -;-1i th the degree of-conser-vatism in the co~puter model and the test verification results finds that spurious trips from the offsite source will not occur for these conditions analyzed.

}1odifications wql be made to block t_he automatic starting of the condensate, high pressure.heater drain, bearing cooling, and component cooling pumps for 315 seconds after -the occurence of a CLS or SI. This bloc~ing will prevent spurious separations from occuring should one of these motors start before steady state conditions are reached. Manually starting (non-automatic capability) the reactor or steam generator feedwater pumps could cause spurious separation during a CLS or SI. Caution statements have been incorporated into the plant's emergency procedures to ensure that the operators are aware of the consequences of starting these motors.-

(4) Test results are required to verify the voltage analyses calcula-tions submitted.

VEPCO verified their voltage analyses by performing a voltage pro-file test. The test results proved the computer model to be highly conservative with the percentage errors judged acceptable.

(5) Review the plant's eleotrical power systems to dete+m:!-ne* if any events or conditions could result in the simultaneous lo*ss of both offsite circuits to the onsite distribution system (compliance of GDC 17).

The licensee reviewed the plant's electrical power systems and determined that three modifications were necessary to eliminate l

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the possibility of a simultaneous loss of both offsite

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l circuits. The three modifications are as follows [Ref. 4]:

(a) Both disconnects (one on each side) of the 34.5 kV tie breaker which is normally open between the two 34.5 kV buses (5 and 6) will be left open. This will eliminate any possibility due to the tie breaker failure. Oper-ating procedureg will be incorporated frir this alignment

. when both the 34. 5 kV f.eeder':""oreake.rs from the *230/36. 5 kV transformer are in the manual operation only because the loss of power to eit~er 34.5 kV bus will cause the auto-matic closing of the tia breaker.

A modification, not yet determined, will be incorporated to eliminate the possibility of a fire causing the loss of all three RSST control cables from the turbine building to the switchyard.

(6) As required by GDC 5, each offsite source shared between units in a multi-unit station must be capable of supplying adequate starting and operating voltage to all required Class lE loads with an accident in one unit and a safe shutdown in the reraainine unit(s).

Based on the coruputer model conservatisms, plant modifications and operating capabilities and restrictions, and the test veri-fication resluts, the analyses demonstrate that the shared o~f-site sources have the -capability and capaci.ty to supply adequate voltage to the Class lE eq~ipment for an SI or CLS in orte-uni~

and a safe shutdown of the remaining unit.

6. CONCLUSIONS Based on the information submitted by VEPGO for the Surry Nuclear Power Station, Units 1 and 2, it is_concluded that:

(1) With the re-rating and/or replacement of the MOV's, implementa-tion of an automatic load shedding scheme, adherence to strict plant operating capabilities and restrictions, control circuit modifications and hardware upgrading, the offsite sources in conjunction with.the *onsite distribution system have the capacity

.and .capabili.ty to automatically s*tart and continu_ou.sly operate the Class lE equipment (to the 480-volt level) within.theii design ratings under worst case.conditions.

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? - (2) Spurious separations from the offsite sources will not occur as the result of voltage.transients caused from the automatic starting of equipment or the manual starting following steady state conditions of the condensate, high pressure heater drain, bearing cooling or component cooling pumps. Caution statements have been incorporated into plant proced~res on the starting of the reactor coolant or steam generator feedwater pumps which may cause spurious trips during a CLS Qr s.r..

(3) The computer analyzed voltage profiles were verified by test with the result confirming the model's conservatism.

(4) The sharing of the offsite sources has the capacity and capability to supply adequate voltage to the units' Class IE equipment for an accident condition in one and a safe shutdown of the remaining unit.

(5) With the incorporation of operating procedures to ensure proper breaker alignnent and disconnect positioning (tu the open posi-tion) on both sides of the 34.5 kV tie breaker and 34.5 kV feeder breakers and with the prevention modification to the RSST control cables will e~sure that the simultaneous loss of both required offsite circuits (GDC 17) will not occur.

The licensee has committed to replacing any HOV which cannot be rerated to 80% voltage starting with 80% rated MOV's and to upgrading the MCC control transformers to ad.equate sizing to ensure adequate contactor operation under worst case conditions. Therefore, .I recomraend -the NRC accept the voltage ana-lyses of the station's electrical distribution system to supply adequate voltage under worst case conditions.

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e REFERENCES

1. NRC letter (.W. Gammill) to all Power *Reactor Licensees, dated August 8, 1979.

2. VEPCO letter (B. R. Sylvia) to NRG (S. A.* Varga), dated May 26, 1981.

3. VEPCO letter (R. H. ~easburg) to NRC (S. A_. Varga), dated Deceober 31, 1981.

4. VEPCO letter (R. H. L~asburg) to NRG (S. A. Varga), dated March 31, 1982.

5. VEPCO letter (R.H. Leasburg) to NRG (S. A. Varga), dated June 11, 1982.

6. V~?CO letter (R.H. Leas~urg) to NRG (S. A. Varga), dated June 30, 1982.

i. Code of Federal Regulations, Title 10, Part 50 (10 CFR 50), General Design Crjterion 5, 13 and 17 of Appendix A for Nu~lear Power Plants.

8. ANSI C84. l-197i, "Voltage Ratings for Electric Power Systems and Equipment."

9. IEEE STD. 308-1971, "Class lE Power Systems for Nuclear Power Generating Stations."