ML20003C092

Adequacy of Station Electric Distribution Sys Voltages, Beaver Valley Power Station,Unit 1, Interim Preliminary Rept

ML20003C092
Person / Time
Site:	Beaver Valley
Issue date:	01/31/1981
From:	Udy A EG&G IDAHO, INC., EG&G, INC.
To:	Shemanski P Office of Nuclear Reactor Regulation
References
CON-FIN-A-6256 EGG-EA-5326, NUDOCS 8102260613
Download: ML20003C092 (13)

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O EGG-EA-5326 ww o-o January 1981 ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES. BEAVER VALLEY POWER STATION, UNIT NO. 1, DOCKET No. 50-344, TAC NO. 13110 q,,

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'm This is an informal report intended for use as a preliminary or working document Prepared for the U.S. Nuclear Regulatory Connission Under DOE Contract No. DE-AC07-761D01570 FIN No. A6256

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FORu EcsG 398 e.. u rw INTERIM REPORT Accession No.

Report No. EGG-EA-5326 Contract Program or Project

Title:

Electrical, Instrumentation and Control System Support Subject of this Document:

Adequacy of Station Electric Distribution System Voltages, Beaver Valley Power Station, Unit No. 1, Docket No. 50-334, TAC No. 13110 Type of Document:

Informal Report Author (s):

N E Researca anc

,ecanical A. C. Udy Date of Document:

January 1981 Responsible NRC Individual and NRC Office or Division:

Paul C. Shemanski, Division of Licensing 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 not be considered final.

EG&G Idaho, Inc.

Idaho Falls, Idaho 83415 Prepared for the U.S. Nuclear RegJiatory CommlSSlon Washington, D.C.

Under DOE Centract No. DE-AC07 761D01570 NRC FIN No.

A6256 INTERIM REPORT

4839N ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTAGES BEAVER VALLEY POWER STATION - UNIT NO. 1 Docket No. 50-334 January 1981 A. C. Udy Reliability and Statistics Branch Engineering Analysis Division EC&G Idaho, Inc.

TAC No. 13110

ABSTRACT The Nuclear Regulatory Commission has required all licensees to analyze the electric power system at each nuclear station. This review is to deter-mine 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. This Technical Evaluation Report reviews the submittals for the l

Beaver Valley Power Station.

The offsite power sources, in conjunction with the onsite distribution system, have been shown to have sufficient capacity and capability to auto-matica11y start, as well as continuously operate, all required class 1E equipment within rated voltage limits in the event of either an anticipated transient or an accident condition.

FOREWORD This report is supplied as part of the selected Electrical, Instrumen-tation, and Control Systems (EICS) issues program being conducted for the U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Operating Reactors, by EG&G Idaho, Inc., Reliability and Sta-tistics Branch.

The U.S.-Nuclear Regulatory Commission funded the work under the authorization entitled " Electrical, Instrumentation, and Control System Support," B&R 20 19 01 03, FIN A6256.

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CONTENTS

1.0 INTRODUCTION

1 2.0 DESIGN BASIS CRITERIA.

1 3.0 SYSTEM DESCRIPTION 2

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4.0 ANALYSIS DESCRIPTION 2

4.1 Design Changes.

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4.2 Analysis Conditions.

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14. 3 Analysis Results 5

4.4 Analysis Verification 5

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5.0 EVALUATION 6

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6.0 CONCLUSION

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7.0 REFERENCES

8 FIGURE l

1.

Beaver Valley Station Unit One Line Diagram.

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1 TABLES 1.

Class 1E Equipment Voltage Ratings and

. Worst Case Available-Load Terminal Voltage 4

2.

Comparison of Analyzed Voltages and

'Undervoltage Relay Setroints 6

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ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTACES BEAVER VALLEY POWER STATION - UNIT NO. I 1.0 IRTRODUCTION 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 of August 8, 1979, " Adequacy of Station Electric Distribution Systems Volt-ages," I required each licensee to confirm, by analysis, the adequacy of the voltage to the class 1E loads. This letter included 13 specific guide-lines to be followed in determining if the voltage is adequate to start and continuously operate the class 1E loads.

l with letters of 15,1979ght(DL)respondedtotheNRCletter Duquesne Li and February 22, 1980.3 The Final Safety Analysis October Report (FSAR), additional analyses submitted on June 17, 19804 and a telephone call on July 11, 1980,5 provided information for this report.

DL documented this telephone call on July 14, 1980.6 A letter of November 11, 1976,7 was also reviewed in preparing this report.

Based on the informatioa supplied by DL, this report addresses the capacity and capability of the onsite distribution system of the Beaver Valley Power Station, in conjunction with the offsite power system, to maintain the voltage within acceptable limits for required class 1E equip-ment for the worst-case starting and load conditions. Unit 2 is not an operating reactor, therefore this report covers only the Unit 1 distribution system.

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

1.

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 Struc-tures, Systems, and Components," of Appendix A,

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

3.

General Design Criterion 13 (GDC 13), " Instrumentation and Control," of Appendix A, " General Design Criteria for Nuclear Power Plants," of 10 CFR 50.

l 4.

.IEEE Standard 308-1974, " Class 1E Power Systems for Nuclear Power Generating Stations."

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Staff positions as detailed in a letter sent to the licensee, dated August 8, 1979.1 l

6.

ANSI C84.1-1977, " Voltage Ratings for Electric Power l

Systems and Equipment (60 Hz).

Six review positions have been established from the NRC analysis guide-linesl and the above-listed documents. These positions are stated in Section 5.

3.0 SYSTEM DESCRIPTION Figure 1 of this report is a unit one-line diagram of the power sources and the class IE distribution system taken from Figure 8.1-1 of the FSAR.

Class 1E 4160V buses 1AE and IDF are supplied power from auxiliary buses IA and ID, respectively. When the unit generator is operating, these buses are powered by independent unit transformers. On a unit generator trip, these buses are automatically and independently connected via separate system auxiliary transformers 1A and IB to the 138 kV switchyard. The unit generator cannot be isolated from the unit transformers; therefore, the main transformer cannot supply offsite power to the class 1E buses from the 345kV switchyard.

Eacn 4160V class IE bus supplies power to two 480V class IE buses via separate 4160V/480V transformers.

120V vital buses are normally supplied power from uninterruptable power supplies (UPS), however, when a UPS is undergoing maintenance, separate 480V/120V transformers supply power to the 120V vital buses. Other 120V buses that supply instruments and control systems as required by GDC 13 are powered by 480/240/120V transformers.

There are two class 1E undervoltage relays on each 4160V class IE bus.2,6 The relays separate the class 1E bus from offsite power should there be degradation of offsite power. There are also class IE undervoltage relays on 480V buses IN and IP that operate the same as the 4160V relays.

Loss of voltage relays are on these same buses.

The FSAR indicates that control power for the class 1E 4160V and 480V switchgear is 125V DC.

480V MCCs use AC control power.7 DL supplied the equipment operating ranges identified in Table 1.

4.0 ANALYSIS DESCRIPTION 4.1 Design Changes. DL submitted analyses 3,4 based on the following proposed changes:

1.

DL will install automatically controlled 6.3 MVAR capacitor banks on 4160V normal buses IA and 1D.3 (These buses supply the class IE buses.) The capaci-tors will be automatically placed on their respective bus when the bus voltage is less than 101% (of-4160V),

and will be automatically removed from their bus when the bus voltage rises above 108%. Manual operation will also be possible.

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CLASS 1E EQUIPMENT VOLTAGE RATINGS AND l

ANALYZED WORST CASE TERMINAL VOLTAGES

(% of nominal voltage)

Maximum Minimum ae Ana yze ae Analyzed Nominal Voltage Steady Equipment (100%)

state Transient Motors 4160V 84.0b 80a Start i

Operate 110 108.5 90 99.6b 460V 80a Start 78.5 Operate 110 110 90 91.8 MCC Starters 480V Pickup 80 75.2 Dropout 55 75.2 Continuous 110 105.4 80 90.0 Other 120V Equipmente Instruments 3

and Controls a.

DL states 6 that they, "do not have documentation that all motors will start at 80% voltage, but by experience, we believe they will." 80% is conservative for the ratings of similar equipment in other nuclear stations.

b.

Bus voltage has adequate margin to allow for feeder cable voltage drop.

c.

The 120V~ vital buses are normally supplied power by inverters. Should an inverter by out of service, self-regulating transformers are used to maintain the vital bus voltage within +2% for a +10% change on the input

-voltage.6 Until these capacitors are installed and operational, DL has administrative procedures to manually change the taps on the system station service transformers fol-lowing reactor startup to ensure that no undervoltage condition will occur following a reactor trip and auto-matic transfer to the system station service trans-formers.3. Taps will be reset prior to normal shut- '

down to ensure that no.overvoltage-condition will occur.

2.-

The taps on the 480/240/120V transformers will be changed to give a 2-1/2% voltage boost.4 4

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Ine transformers that supply backup power to the vital buses are being replaced with self-regulating trans-formers which will maintain the bus voltage within the required limits.4 The discussion in the text and the values in Table 1 of this report reflect that these modifications have been completed.

4.2 Analysis Ponditions. DL has determined by load-flow studies that the maxiraum 138kV seitchyard voltage is 143.4kV and the minimum is 134.9kV.

DL has analyzed each offsite source to the onsite distribution system under extremes of load and offsite voltage conditions to determine the terminal voltages to the class IE equipment. The worst case class IE equip-ment terminsi voltages occur under the following conditions:

1.

The maximum analyzed load terminal voltages occur when the grid is maximum, each system auxiliary transformer is supplying power for its class IE buses, and no unit loads exist.

2.

The minimum analyzed continuous load terminal voltages occur when the grid is minimum and each system auxiliary transformer is supplying the maximum connected auxiliary and class lE loads.

3.

The minimum analyzed transient load terminal voltages occur under the same conditions as 2, concurrent with the start of a 6000 hp reactor coolant pump.

4.3 Analysis Results. Table 1 shows the projected worst case class lE equipment terminal voltage.

A review of Table 1 shows that, with the exception of the transient condition on the 460V motors and 480V contactors, all the class 1E loads are within'the equipment rated voltages. This worst case transient condi-tion will last less than 22.secords3 before normal operating voltages (i.e., greater than 90%) are restored.4 Contactor pickup for the 480V MCC loads is prevented 'until the voltage recovers.

It will not cause con-tactor dropout or spurious shedding of any loads.

Table'2 compares the undervoltage relay setpoints and time delays with bus voltages derived from Table 1.

4.4 Analysis Verification. The computer analysis is to be verified 4 by measuring the grid and bus voltages, and the actual load of the buses and selected equipment while the unit is being restarted from the current shutdown.5 An analysis to determine bus voltages will then be completed using the measured offsite source voltage. The accuracy of the DL analysis will be verified by comparing the results with the measured bus voltages.

DL must establish that the load conditions and instrument accuracies are adequate to account for impedence and t.ransformer voltage drops in the distribution system.

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TABLE 2 COMPARISON OF ANALYZED VOLTAGES AND UNDERVOLTAGE RELAY SETPOINTS

(% of nominal voltage)

Minimum Analyzed Relay Setpoint Location a

Action Voltage Time Voltage (Tolerance)

Time 4160V bus Degraded 99.6 continuous 90 (+3) 90 sec Loss 84.0 22 sec 75 I sec 480V bus Degraded 90.0 continuous 90 (+3) 90 see Loss 75.2 22 sec 75 41 sec a.

DL submitted voltages.

5.0 EVALUATION Six review positions have been established from the NRC analysis guide-linesl and the documents listed in Section 2.0.

Each review position is stated below, followed by the evaluation of the licensee submittals.

Position 1--With the minimum expected offsite grid voltage and maximum load condition, each offsite source and distribution system connection combiniation must be capable of starting and of continuously operating all class IE equipment within the rated equipment voltages.

DL has shown that, with the modifications of Section 4.1 completed, the Beaver Valley Power Station, Unit 1, hus sufficient capacity and capa-bility for starting and continuously operating the class IE equipment.

While the class IE 460V motors and 480V contactors can have transient volt-ages below the motor starting rating and the contactor pickup rating, this should not be a problem, as the class 1E motors are running when this transient voltage occurs and the contactor dropout voltage rating is not exceeded.

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 all class lE equip-ment without exceeding the rated equipment voltage.

DL has stated that, with the grid voltage at the maximum expected value and no unit loads, the voltage ratings of the class IE equipment are not exceeded.

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Positioji 3--Loss of offsite power to either the redundant class lE distribution systems or the individual class lE loads, due to operation of voltage protection relays, must not occur when the offsite power source is within analyzed voltage limits.

The voltage protection relays will not separate the class IE buses f rom the offsite power source during the analyzed starting transients.

However, as can be seen in Table 2, the setpoint (including +3% tolerance) of the 480V degraded grid undervoltage relays (93%) is above the minimum expected continuous voltage (90%). This presents a potential for unneces-sary tr$pping of the class IE distribution system fram the offsite power source.

Position 4--Test results should verify the accuracy of the voltage analyses supplied.

DL has not yet shown the calculations to be an accurate representation of actual conditions of the class IE buses and loads; however, they have an acceptable test method and have scheduled the testing 4 and the comparison of test results to an analysis of identical conditions.

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 (CDC 17).

DL has analyzed the Beaver Valley, Unit I connections to the offsite power grid and determined that no potential exists for either a simultaneous or consequential loss of both circuits to the offsite grid.4 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 lE loads with an accident in one unit and an orderly shutdown and cooldown in the remaining units.

l The Beaver Valley Power Station is presently a single unit station, therefore, this position does not apply.

6.0 CONCLUSION

S The voltage analyses submitted by DL for Unit 1 of the Beaver Valley Power Station were evaluaced in Section 5.0 of this report.

It was found that, upon the completion of the changes described in Section 4.1:

1.

Voltages within the operating limits of the class lE equipment are supplied for all projected combinations of plant load and offsite power grid conditions.

Two of the three proposed changes (Section 4.1) have been made. DL has provided suitable analysis for the use of capacitors oa the 4160V auxiliary buses, but has not installed these capacitor banks. DL is investiga-ting the use of capacitor banks on the class IE 480V buses instead.5 This would be acceptable provided DL 7

shows, by analysis, that the revised capacitor instal-lation provides voltage to the class IE buses that is equal to or better than the originally proposed capaci-tor installation.

2.

The proposed test will verify the analysis accuracy.

3.

DL has determined that no potential for either a simul-tanous or consequential loss of both offsite power sources exists.

4.

The class 1E 480V bus undervoltage relay setpoints are above the analyzed minimum bus voltage. Unnecessary separation of offsite power to the class 1E distribution system could result. This does not appear to comply with the requirements of GDC 17.

DL should be required to make the changes necessary to keep the class 1E buses powered by the offsite power source as long as acceptable voltage is available with that source.

7.0 REFERENCES

1.

NRC letter, William Gammill, to All Power Reactor Licensees (Except Humboldt Bay), " Adequacy of Station Electric Distribution Systams Voltages," August 8, 1979.

2.

DL letter, C. N. Dunn to Director of Nuclear Reactor Regulation, U. S.

NRC, " Response to Requests for Information on Station Service Bus Voltages", October 15, 1979.

3.

-DL letter, C. N. Dunn to' Director of Nuclear Reactor Regulation, U. S.

NRC, " Station Service Bus Voltage Study", February 22, 1980.

4.

DL letter, C.'N. Dunn to-Director of Nuclear Reactor Regulation,'U. S.

NRC, " Adequacy of Station Electric Distribution System Voltages,"

June 17, 1980.

5.

Telecon, Ray Burski, DL, Tom Mayers, DL and Alan Udy, EG&G Idaho, i

Inc., July _ 11, 1980 at 4 p.m. EDT.

6.

'DL letter,'C. N. Dnnn, to Director of Nuclear Reactor Regulation, U.S. NRC, " Station Service Busses Undervoltage Relays," July 24, 1980.

7.

DL letter, C.'N. Dunn,-to R. W.-Reid, U.S. NRC, " Operation Under Degraded Voltage," November 11, 1976.

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