ML20027D219

Adequacy of Station Electric Distribution Sys Voltages, North Anna Power Station,Units 1 & 2.

ML20027D219
Person / Time
Site:	North Anna
Issue date:	07/31/1982
From:	Udy A EG&G, INC.
To:	Prevatte R Office of Nuclear Reactor Regulation
References
CON-FIN-A-6429 EGG-EA-5927, NUDOCS 8211030206
Download: ML20027D219 (13)

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U.S. Department of Energy

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6 This is an informal report intended for use as a preliminary or working document.

Prepared for the U.S. Nuclear Regulatory Comission Under DOE Contract No. DE-AC07-76ID01570 FIN No. A6429 [ggg ,

8211030206 820731 PDR RES 8211030206 PDR

h EGcG,.. . -

FOHM EG4G 39tt (Rev 03 a/)

INTERIM REPORT Accession No.

Report No. EGG-EA-5927

• C:ntract Program or Project

Title:

Selected Operating Reactors Issues Program (III)

Subject of this Document: -

Adequacy of Station Electric Distribution System Voltages, North Anna Power Station, Unit Nos. I and 2 Type of Document:

Informal Report Author (s):

A. C. Udy Date of Document:

July 1982 R .sponsible NRC Individual and NRC Office or Division:

R. L. Prevatte, Division of Systems Integration This document was prepared primarily for preliminary or internal use. it has not received full review and approval. Since there may be substantive changes, this document should j not be considered final.

i EG&G Idaho, Inc.

Idaho Falls, Idaho 83415 i

e .

l Prepared for the U.S. Nuclear Regulatory Commission Washington, D.C.

Under DOE Contract No. DE-AC07-761D01570 NRC FIN No. A6429 INTERIM REPORT

0114j i

i ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES NORTH ANNA POWER STATION, UNIT NOS. 1 AND 2 July 1982 A. C. Udy

. Reliability and Statistics Branch Engineering Analysis Division EG&G Idaho, Inc.

TAC No. 13109 Docket Nos. 50-338 and 50-339

I y ABSTRACT This EG&G Idaho, Inc., report reviews the capacity and the capability of the onsite power distribution system at the North Anna Power Station, in conjunction with the offsite power sources, to automatically start and

, continuously operate all required safety-related loads.

FOREWORD This report is supplied as part of the " Selected Operating Reactors Issues Program (III)" being conducted for the U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Licensing, by EG&G Idaho, Inc., Reliability and Statistics Branch.

The U.S. Nuclear Regulatory Commission funded the work under the o authorization, B&R 20 19 10 11, FIN No. A5429.

i O

l Selected Operating Reactors Issues, Programs (III)

FIN No. A6429 ii

CONTENTS

1.0 INTRODUCTION

.................................................... 1 2.0 DESIGN BASIS CRITERIA ....... ................................... 1 3.0 S Y STE M DE S CR I PT I ON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

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4.0 ANALYSIS DESCRIPTION ............................................ 4 4.1 Design / Operation Changes .................................. 4 4.2 Analysis Conditions ....................................... 6 4.3 Analysis Results .......................................... 6 4.4 Analysis Verification ..................................... 7 5.0 E VAL U AT I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

6.0 CONCLUSION

S ..................................................... 9

7.0 REFERENCES

...................................................... 9 FIGURE

1. North Anna Statio n one-li.ie di agram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 TABLES

1. Class lE Equipment Voltage Ratings and Analyzed Worst Case Load Terminal Voltages ...................... 5

2. Comparison of Analyzed Voltages and Undervoltage Relay Setpoints .................................... 8 3

l iii

_ ADEQUACY OF STATION ELECTRIC DISTRI'BUTION SYSTEM VOLTAGES

! NORTH ANNA POWER STATION, UNIT NOS. 1 AND 2

1.0 INTRODUCTION

i An event at the Arkansas Nuclear One station on September 16, 1978, is described in NRC IE Information Notice No. 79-04. As a result of this event, station conformance to General Design Criteria (GDC) 17 is being questioned at all nuclear power stations. The NRC, in the generic letter ofAugustj,requiredeachlicensee-toconfirm1979, Vol tages ," " Adequacy of Station Elec

, by analysis, the adequacy

of the voltage at the Class IE loads. This letter included 13 specific guidelines to be followed in determining if the load terminal voltage is adequate to start and continuously operate the Class lE loads.

The Virginia Electric and Power Company (VEPCO), at the time of this generic letter, was submitting independently, for NRC review, analysis of the projected voltage conditions at the North These analyses, d and August 16, 1979, for Unitgo.1,gtedAugust7,1979,forUnitNo.'2gnnaPowerStatio were tied to this review by the VEPC0 response of' October 9, 1979. At the request of the NRC, for this review on August 20,1980,gEPC0provided.additionalingormation and on September 24, 1980. Other

' correspondence and telephone gonversations resulted in a schedule, submitted on Augus t ' 21, 1981, for the submittal of the remaining information germane to this review.

NRC in letters of December 1, 1981,8 Thisinformationwasgubmittedtothe and January 20, 1982, with the bulk of the inform Februgy 26,1982.gionprovidedandcondensedintheVEPCOsubmittalof Ad 1982, on June 4,1982,gtional information

'and on July 1,1982. waj3 Fo d ed on May 20, d

Based on the information supplied by VEPC0, this report addresses the capacity and capability of the onsite distribution system of the North Anna Power Station, in conjunction with the offsite power system, to maintain the voltage for the required Class 1E equipment within acceptable limits for the worst-case starting and load conditions.

2.0 DESIGN BASIS CRITERIA The positions applied in determining the acceptability of the offsite voltage conditions in supplying power to the Class lE equipment are derived from the following:

l.

General Design Criterion 17 (GDC 17), " Electrical Power Systems,"

of Appendix A, " General Design Criteria for Nuclear Power Plants," of 10 CFR 50.

2. General Design Criterion 5 (GDC 5), " Sharing of Structures, Systems, and Components," of Appendix A, " General Design Criteria for Nuclear Power Plants," of 10 CFR 50.

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3. General Design Criterion 13 (GDC.13), " Instrumentation and <

Control," of Appendix A, " General Design Criteria for Nuclear Power Plants," of 10 CFR 50.

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

5. Staff positions as de datedAugust8,1979.{ailedinalettersenttothelicensee, -

6. ANSI C84.1-1977, " Voltage Ratings for Electric Power Systems and Equipment (60 Hz)." ,

SixrgviewpositionshavebeenestablishedfromtheNRCanalysis guidelines and the above-listed documents. These positions are stated in Section 5.0.

3.0 SYSTEM DESCRIPTION Figure 1 shows the electrical distribution system at the North %na Power Station. Each unit has two 4160V Class lE buses. These are designated lJ,1H, 2J and 2H. Each of these buses provides 4160V power to high horsepower safety-related equipment and to two 4160V/480V transformers for safety-related equipment that requires less power. 480V Motor Control Center (MCC). loads have individual 460V/ll5V control power' transformers.

The breaker control circuits for the 4160V and the 480V buses are supplied power by the DC power distribution system, and thus are independent of this review. 115 VAC vital loads and instrumentation are supplied power from uninterruptable power supplies (DC backed). Should these power supplies fail, self-regulating transformers provide a stable-(to within 2%) voltage.

With the completion of planned modifications (see Section 4.1), the' <

4160V Class lE buses will be supplied power from the following sources:

Bus lJ will normally be powered-by reserve transformer A via transfer bus D. Altarnately, a manually established connection to Unit 2 bus 2B is possible, with either the Unit 2 generator'or reserve transformer B as its-source. Bus 1H will normally be powered by reserve transformer C via transfer bus F. Alternately, a manually established connection to bus 1B is possible, with the Unit 1 main generator or main transformer being its preferred source. Bus 2J will also be normally powered by reserve transformer C via transfer bus F. Alternately, a manually established connection to bus lA is possible, with the Unit 1 main generator or main transformer being its preferred source, or reserve transformer A as an alternate source. Bus 2H will be normally powered by reserve transformer B .

via transfer bus E. Alternately, a manually established connection to bus 2C is possible. This bus derives power from the Unit 2 generator, or reserve transformer C. Thus, it is possible, for buses 1H, 2J and 2H to be ,

powered by reserve transformer C. VEPC0 has proposed operating restric-tions that prevent more than two of these buses being connected to this transformer simultaneously. There are no automatic transfers of these buses between offsite power sources.

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I SW HY RD I GEN. GEN.

UNIT Lv 500-36.5 KV 500-36.5 KV y g UNIT TRANS. TRANS. #1 TRANS. #2 TRANS.

1. 1 r3 D T rn #2 22 KV BUS #3 y 34.5 KV BUS #4 U 34.5 KV 22 KV UNIT STATION (._A.) W W RESERVE W SERVICE lA IB IC TRANS.

g g ( A.) LA_/ RESERVED' W WUNIT STATION 28 2A SERVICE TRANS. 2C TRANSFORMERS O G P 9 C @ . RESERVE TRANS. A DO MB P 9 F G TRANSFORMERS h h 1A h h 1B h IC h 2C h h 2B h h 2A

-4160V w

6 4160V 4160V 6 TRANSFER BUS D TRANSFER

[^)4160V BUS F h 6 4160VTRANSFER BUS E 4160V 6 4160V 6 6 4160V 64160V DG DG DG DG lJ lH 2J 2H h h CLASS lE 9 h CLASS hh CLASS E h h._ CLASS 4160V g 4160V g 4160V 4160V

\_A.) \ 1.) LA) LA) \.A.) W L1_/ N 09 en lJ lJ1 lH 1H1 2J 2J1 h 2H h 2H1 480V 480V 480V 480V 480V 480V 480V 480V lE lE lE .lE lE 1E 1E lE NORTH ANNA STATION ONE LINE DIAGRAM .

FIGURE 1

4.0 ANALYSIS DESCRIPTION 4.1 Design /0peration Changes. The voltages shown on Table 1 are based on the following licensee proposed changes:

1. VEPCO, by analysis and by test, will attempt to rerate the motor operated valves (MOV) to assure operation at less than design voltage limits. Where this is not possible, VEPC0 will replace the motors of the MOVs. This will be completed by the second ,

refueling outage of each unit after September 1,1982.

2. Load shedding when Unit 2 transfers to the reserve transformers ,

simultaneously with a safety injection (SI) or Containment Depressurization Actuation (CDA)a in Unit 1.

3. Trip the 34.5kV switchyard reactors when either unit has a SI or CDA.

4 Install overvoltage alarms on all Class 1E buses.

5. Modify the Load Tap Changer (LTC) controls on the reserve transformers to eliminate response delays in the LTC for the first three minutes following either an SI or a CDA in either unit or on transfer of the unit loads to the reserve transformers.

6. Block the auto-start of large non-Class lE motors when their power source is the same as an emergency bus of a unit with either an SI or CDA.

7. Elimination of the automatic transfer of the Class lE buses from the reserve transformers to the Normal Station Service Buses.

8. Trip all circulating water pumps when a unit has an SI or CDA if Buses 16 and 26 (not shown in Figure 1) are supplied power from the same reserve transformer.

Additionally, VEPC0 is implementing operating restrictions to:

1. Maintain the generator bus voltage above 21.7kV when a "J" bus is fed from a station service bus.

2. Limit the load on a station service bus when a "J" bus is fed from it.

3. Prevent powering bus 2H from reserve transformer C, when bus 2J is also powered by this source.

VEPC0 has implemented a voltage schedule and pro '

thegridvoltageismaintainedbetween505and535kV.ggdurestoensurethat

a. A CDA includes all motors started for an SI plus additional motors i required for the CDA. '

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b TABLE 1. CLASS 1E EQUIPMENT V0LTAGE RATINGS AND ANALYZED WORST CASE BUS VOLTAGES (% of nominal voltage)

Maximum Minimum

. Analyzed a Equipment Condition Rated Analyzed Rated Steady State Transient 4000V Motors Start -- -- 70 --

82.5 Operate 110 114.3 90 96.3 --

460V Motors Start -- --

70 --

67.9 Operate 110 117.6 90 91.4 --

460V MOV Start -- --

c --

68.1 Operate d 110 117.6 90 --

MCC Starters Pickup -- --

75 --

65.1 Dropout -- --

60 --

65.1 Operate 110 127 90- 91.5 --

a. VEPC0 supplied bus voltage minus the average feeder voltage drop. The average feeder voltage drops were: 4160V steady state-0.4%; transient-1.2%;

460V steady state-3.3%; transient-5.0%.

b. VEPC0 is installing an overvoltage alarm on each Class lE bus. Each will be set at 110% of the nominal load voltage. This will enable station personnel to take action to reduce the voltage available to the equipnent.

c. VEPC0 is presently rerating MOVs replacing motor operators as needed so that all MOVs will start at 80, 84 or 86%. VEPCO indicates that with these ratings, all MOVs will operate within 4 seconds of an initial accident, and that this ensures proper actuation of the valves.

d. Considered as a transient case only. The MOV operation will be complete in less than 3.0 seconds. In that case, the steady state voltage would be approximately 91.4%.

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4.2 Analysis Conditions. VEPC0 has determined by contingency planning that the maximum expected offsite grid voltage is 535kV and the minimum expected offsite grid voltage is 505kV. VEPC0 is implementing procedures to ensure the grid voltage will not deviate from these limits.

VEPC0 has analyzed each offsite source to the onsite distribution system under extremes of load and offsite voltage coaditions to determine the voltages available to the Class 1E equipment. With the corrective actions of Section 4.1 taken, the following conditions resulted in the '

worst Class lE equipment terminal voltages:

l. The worst case steady state 4160V volf. ages occur to buses powered by a unit auxiliary transformer when the offsite grid voltage is '

minimum and with a CDA in Unit 2 and Unit 1 generating at 21.5kV.

The lowest 480V steady state voltages occur when power is supplied by unit auxiliary transformers of Unit 1, the offsite grid voltage is minimum, Unit 2 has a CDA and Unit 1 is tripped.

2. The worst transient voltages occur with the minimum offsite grid voltage and with a CDA in Unit I concurrently with Unit 2 transferring to the reserve transformers for a power source.

3. The maximum steady state voltage occurs with the maximum offsite grid voltage and with no station loads or transformer or feeder cable losses, while backfeeding through the normal buses via the unit station service transformers.

4.3 Analysis Result. Table 1 shows the projected worst case voltages available to the Class IE equipment.

It shows that steady state voltages will be maintained above the minimum required for any operating condition, and that the minimum transient voltage at the 4000V loads will not cause these motors to stall.

It also shows the potential to stall 460V loads. VEPC0 indicates that j all M0Vs will be actuated and operating within an acceptable 4 second l period, and that other 460V loads, in particular the quench spray pump, will start accelerating within 2 seconds as the voltage recovers (the 4kV motors are completely accelerated and the voltage is stablized within 4 seconds). This is acceptable. Some of the motor control center l contactors may not be able to pickup to operate additional loads during this transient period. The loads will pickup within a few (less than 4) seconds as the vcitage recovers. No contactors will drop out during this period.

• Table 1 also shows that for the no station load condition, the equip-l ment voltage limits can be exceeded. Minimum station loads (lights, e battery chargers, pumps and so forth) will reduce this potential. VEPC0 is l installing an overvoltage alarm on each Class lE AC bus to allow operator action to correct the condition.

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6 4.4 Analysis Verification. The computer analysis was verified 10 by recording the grid voltage, the position of the load tap changers and the voltages and currents of the 4160V and the 480V buses. This was done while the Unit 2 loads were transferred from the unit station service transformers to the reserve transformers and the subsequent start of a charging pump. An analysis was then done using the recorded grid and load and load tap changer data, and the results compared with the measured bus voltages.

The comparison shows that the Class lE bus calculated voltages are within 2.1% of the measured voltage for the steady state condition and -

within 3.1% of the measured voltage for the transient condition. These i errors were generally in the conservative direction. This close correla-tion shows that the assumptions used in the voltage analysis give an

, accurate representation of actual station voltages.

5.0 EVALUATION SixryviewpositionshavebeenestablishedfromtheNRCanalysis guidelines and the documents listed in Section 2.0 of this report. Each review position is stated below followed by an evaluation of the licensee submittals. The evaluations are based on completion of changes described in Section 4.1.

Position 1--With the minimum expected offsite grid voltage and maximum load condition, each offsite source and distribution system connection combination must be capable of starting and of continuously operating all Class 1E equipment within the equipment voltage ratings.

VEPC0 has shown, by analysis, that the North Anna Power Station has sufficient capability and capacity for starting and continuously operating the Class lE loads within the equipment voltage ratings (Table 1 and Section 4.3).

Position 2--With the maximum expected offsite grid voltage and minimum load condition, each offsite source and distribution system connection combination must be capable of continuously operating the required Class lE equipment without exceeding the equipment voltage ratings.

VEPC0 has shown, by analysis, that the voltage ratings of the Class lE equipment could be exceeded'if no station load exists. VEPC0 is installing an overvoltage alarm on each Class lE AC bus (4g0V & 480V) to allow opera-tor action to reduce the potential overvoltage

. Position 3 -Loss of offsite power to either of the redundant Class lE distribution systems due to operation of voltage protection relays must not occur when the offsite power source is within expected voltage limits.

As shown in Table 2, voltage relays will not cause loss of Class lE distribution systems when the offsite grid voltage is within expected voltage limits.

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TABLE 2. COMPARIS0N OF ANALYZED VOLTAGES AND UNDERVOLTAGE RELAY SETPOINTS

(% of nominal voltage)

Minimum' Analyzed a Relay Setpoint Location / Relays Voltage Time Voltage (Tolerance) Time Unit 1 4160V bus Degraded grid 94.0 continuous 90% (+1.4V) 5(56)b sec.

Loss of grid 80.6 4.2 sec. 71% 2 sec. ,

Unit 2 4160V bus Degraded grid 93.0 continuous 90% (+1.4V) 7(60)b sec.

Loss of grid 80.5 6.5 sec. 71% 2 sec.

a. Licensee has determined by analysis the minimum bus voltages with the offsite grid at the minimum expected voltage and the worst case plant and Class lE loads.

b. The first time is for an SI or CDA condition. The second time is for non-accident conditions.

Position 4--The NRC letterl requires that test results verify the accuracy of the voltage analyses supplied. '

VEPC0 has supplied the required information which shows the analysis to be an accurate representation of the worst ~ case voltage conditions for the Class lE buses and loads.

Position 5--No event or condition should result in the simultaneous or consequential loss of both required circuits from the offsite power network to the onsite distribution system (GDC 17).

VEPC0 has analyzed the connections to the offsite power grid, and has determined that no potential exists for either the simultaneous or the consequential loss of both circuits from the offsite grid.

As a result of this analysis, VEPC0 will maintain the 34.5kV tie breaker open, with two manual disconnects also open between the two 34.5kV '

buses, and the automatic close feature of the breaker removed. The power cable routing from the switchyard service transformers was also changed to eliminate a potential for a comon cause failure of both offsite sources. ,

Position 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 for all required Class IE loads with an accident in one unit and an orderly shutdown and cooldown in the remaining units.

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The North Anna Power Station is the site of two nuclear units. The units are not independently connected to the offsite power sources and have electrical power interconnections between units that are restricted by technical specification and operating procedures. VEPC0 has shown that the shared offsite sources will be capable of supplying adequate starting and operating voltages far all required Class lE loads with an accident in one .

unit and an orderly shutdown and couldown of the other unit. ~

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6.0 CONCLUSION

S The voltage analys s submi tad by VEPC0 f o'r the Eorth Anna Power Station were evaluated in Section 5.0 of'this report. Upon the completion of changes described in Section 4.1, it was found that:'

1. Voltages within the operating limits of the Class lE equipment ,

are supplied for all proje'cted combinations of plant load and normal offsite power grid conditions, including an accident in one unit and the safe shutdown of the ot_her unit. However, operator action is required should an c4ervoltage condition occur.

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2. The test used to verify the analysis shows the analyses to be an accurate representation 6f the worst case conditions analyzed; ,

3. VEPC0 has determinAds,that no potential:for' either a simultanous or consequential loss \ of both offsite power sources exists.

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4. Loss of offsite power to .ClassflE buses, due to spurious operation of voltage protection relays, will not occur with the offsite grid voltage withiri its expected limits.

7.0 REFERENCES

s _

l. NRC letter, William Gansnill, to All Power Reactor Licensees (Except Humboldt Bay), " Adequacy of Station Electric Distribution Systems Voltage," August 8,1979. i

2. VEPC0 letter, S. C. ~ Brown to J. P. O'Reilly, NRC Region II, August 7, 1979, Serial No. 403A.

3. VEPC0 letter, C. M. Stallings to H. R. Denton, NRC, " North Anna Unit No.1, Station Electrical Distribution System Voltages," August 16, 1979, Serial No. 352A.

, 4. VEPC0 letter, C. M. Stallings to H. R. Denton, NRC, " Adequacy of Station Electric Distribution System Voltages," October 9, 1979, Serial No. 673/080879. ,

5. VEPCO letter, B. R. Sylvia t to H. R. Denton, NRC, " Request for Additional Information, Electrical Distribution System Voltages,"

Augus t 20, 1980, Serial No. 725.

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6. VEPC0 letter, B. R. Sylvia to H. R. Denton, NRC, " Request for Additional Information, Electrical Distribution System Voltages,"

September 24, 1980, Serial No. 725A.

7. VEPC0 letter, R. H. Leasburg to H. R. Denton, NRC, " Completion Schedule, General Design Criteria 17 Analysis," August 21, 1981, Serial No. 908A.

8. VEPC0 letter, R. H. Leasburg to H. R. Denton, NRC, " General Design '

Criteria 17 Analysis," December 1,1981, Serial No. 9088.

9. VEPC0 letter, R. H. 'Leasburg to H. R. Denton, NRC, " General Design 4 Criteria 17 Analysis," January 20, 1982, Serial No. 017.

10. VEPC0 letter, R. H. Leasburg to H. R. Denton, NRC, " General Design Criteria 17 Analysis," February 26, 1982, Serial No. 076.

11. VEPC0 letter, R. H. Leasburg to H. R. Denton, NRC, " General Design Criteria 17 Analysis," May 20, 1982, Serial No. 233.

12. VEPC0 letter, R. H. Leasburg to H. R. Denton, NRC, " General Design Criteria 17 Analysis," June 4,1982, Serial No. 316.

13. VEPC0 letter, R. H. Leasburg to H. R. Denton, NRC, " General Design Criteria 17 Analysis," July 1,1982, Serial No. 374.

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