ML20049J618

Technical Evaluation of Adequacy of Station Electric Distribution Sys Voltages for AR Nuclear One,Unit 1.

ML20049J618
Person / Time
Site:	Arkansas Nuclear
Issue date:	12/16/1981
From:	Selan J LAWRENCE LIVERMORE NATIONAL LABORATORY
To:	NRC
Shared Package
ML20049J614	List: ML20049J618
References
CON-FIN-A-0250, CON-FIN-A-250 UCID-19118, NUDOCS 8203180499
Download: ML20049J618 (13)

Text

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i I i TECHNICAL EVALUATION OF THE ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEf1 VOLTAGES FOR THE ARKANSAS NUCLEAR ONE, UNIT 1 (Docket No. 50-313) ,

James C. Selan >

-l SELECTED ISSUES PROGRAM i

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This is an informal report intended pnmardy fcr internal or limited external dhtribucon. '^\ ,' ~

The opuuona and conclusaons stated are those of the author and may or may not be those g.g, ]'

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This work was supported by the United States Nuclear Regulatory Commu.saon under - M r. "

, a Memorandum of Understandag with the Unsted States Department of Energy. .

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ABSTRACT ,

This report documents the technical evaluation of the adequacy

• of the station electric distribution system voltages for the Arkansas Nuclear One, Unit 1. The evaluation is to determine if the onsite distri-bution system in conjunction with the offsite power so'urces has sufficient

. capacity to automatically start and operate all Class lE loads within the equipment voltage ratings under certain conditions established by the -

Nuclear Regulatory Commission. The analyses submitted de-onstrate that the station's electric distribution system supplies adequate voltage to the Class lE equipment under the worst case conditions analyzed.

FOPEWORD This report is supplied as part of the Selected Electrical, Instrumentation, and Control Systems Issues Program baiag conducted for the U. S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Operating Reactors, by Lawrence Livermore National Laboratory.

The U. S. Nuclear Regulatory Commission funded the work under the atthorization entitled " Electrical, Instrumentation and Control System Support,"

B&R 2019 04 031, FIN A-0250.

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TABLE OF CONTENTS Page

. . . . . . . . 1 .

1. INTRODUCTION . . . . . .

. . . . . . . 2 ,

l 2. DESIGN BASIS CRITERIA . . . . .

. . . . . . . 2

3. SYSTEM DESCRIPTICN . . .

. . . . . . . . 4

4. ANALYSIS . . . . . . . .

4 4.1 Analysis Conditions . . . . . . . . . .

5 .

4.2 Analysis Results . . . . . . . . . .

. 5 4.2.1 overvoltage . . . . . . . . .

. . . . 5 4.2.2 Undervoltage . . . . . . .

7 4.3 Analysis Verification . . . . . . . . .

. . . . . . . . 7

5. EVALUATION . . . . . . .

. . . . . . . . . 8

6. CONCLUSION . . . . . .

. . . . . . . . . 9 REFERENCES . . .

ILLUSTRATIONS P9ge FIGURE 1 Arkansas Nuclear One, Unit 1 3 c Electrical one-Line Diagram . , . .

TABLE 1 Arkansas Nuclear One, Unit 1 Class lE Equipment Voltages Ratings 6 and Analyze 0 Worst Case Bus Voltages . . . . . .

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TECHNICAL EVALUATION OF THE ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEM VOLTACES FOR THE ARKANSAS NUCLEAR ONE, UNIT 1 o

(Docket No. 50-313) e James C. Selan Lawrence Livermore National Laboratory, Nevada

1. INTRODUCTION The Nuclear Regulatory Commission (NRC) by a letter dated August 8, 1979 (Ref. 1] expanded its generic review of the adequacy of the station electric distribution systems for all operating nuclear power facilities. This review is to determine if the onsite distribution system in conjunction with the of fsite power sources has suf ficient 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 and to meet certain requirements in the analysis. These requirements are detailed in Section 5 of this report.

By letters dated August 23, 1978 [Ref. 2], October 25, 1978

[Ref. 3], March 13, 1979 [Ref. 4], May 21, 1979 (Ref. 51, July 12, 1979

[Ref. 6], August 24, 1979 [Ref. 7], October 19, 1981 [Ref . 8 ] , and October 30, 1981 (Ref. 9], Arkansas Power and Light Company (AP&L), the licensee, submitted their analysis and conclusion regarding the adequacy of the electrical distribution system's voltages at Arkansas Nuclear One, Unit 1.

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 I starting and load conditions.

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2. DESIGN BASIS CRITERIA 6

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 (GDC 17), " Electric 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. 10].

(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. 10].

(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 50) (Ref. 10].

(4) ANSI C84.1-1977, " Voltage Ratings for Electric Power Systems and Equipment" (Ref. 11].

(5) IEEE STD. 308-1974, " Class 1E Power Syctems for Nuclear Power Generating Stations" (Ref. 12].

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

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l I 3. SYSTEM DESCRIPTION An electrical one-line diagram for Arkansas Nuclear One, Unit 1, is shown in Figure 1. The main generator is connected to the station switchyard through the main power transformer. The station switchyard is ,

supplied by a 500 kV system and a 161 kV system which are connected together by a bus tie autotransformer. The tertiary side of the autotransformer supplies the onsite distribution system for both units 1 and 2 through their i

l respective startup transformers No. 1 and No. 3 (SUT-1 and SUT-3) which are ,

l the units preferred offsite sources. The second offsite source for both l

units 1 and 2 is startup transformer No. 2 (SUT-2) which is fed directly from l

the 161 kV ring bus. Both units 1 and 2 also supply the station distribution system l

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The onsite distribution system (unit 1) consists of two each 4160-volt and 480-volt load center Class 1E buses. The Class lE equipment is protected from voltage degradation by two 'evels of undervoltage protection.

The first level located on the 4160-volt buses has a setpoint of 3115 volts (75% of 4160 volts). This scheme will isolate the Class lE buses in approxi-mately 3 seconds upon complete loss of power. The second scheme is located on the 480-volt load center buses and has a setpoint greater than 423 volts (92% of 460 volts) but less than 431 volts (93.7% of 460 volts) with a time delay of 8 seconds + 1 second.

4. ANALYSIS 4.1 ANAL'tSIS CONDITIONS AP&L has analyzed each of f site source to the onsite distribution system under minimum and maximum load demand conditions in conjunction with the minimum and maximum expected of fsite grid voltages to determine the voltage at the terminals of the Class lE equipment. The minimum and maximum grid voltages are 1.0 and 1.05 per unit (of the 161 kV and 500 kV systems),

respectively. An analysis was also performed with both units (1 and 2) transferring their station load to the alternate source (SUT-2). Transient analyses were analyzed for both the starting of the Class lE equipment as i well as the starting of large non-Class lE loads. The above analyses were '

conducted with the use of computer power flow prograus. The input to the programs were conservatively based and with actual f actory test data on the transformers instead of nameplate data.

4.2 ANALSIS RESULTS The analysis results indicated that to ensure acceptable onsite distribution voltages, several circuit and hardware changes would have to be made and are as follows:

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(1) During the automatic fast transfer f rom the UAT to SUT-1, all auxiliary loads except the Class lE loads are trans-ferred in bulk with subsequent load sequencing of the Class lE loads.

(2) During the automatic fast transfer f rom the UAT to SUT-2 (SUT-1 not available), certain selected non-Class lE loads will be shed before the transfer. The loads to be shed

- are; 2 reactor coolant pumps, 3 circulating water pumps, 2 condensate pumps, and 2 chiller pumps (Ref. 6].

(3) Interlocks to be provided to prevent the simultaneous, automatic transfer of unit 1 and unit 2 loads to SUT-2.

(4) Delete automatic slow transfer from the UAT to SUT-1 which will prevent simultaneous starting of station auxiliary loads. ,

(5) Manually block the operation of the second-level under-voltage protection scheme during the starting cf a reactor coolant pump or a condensate pump.

(6) Install a second-level undervoltage protection scheme on each 480-volt Class 1E bus. The scheme will include a 2-out-of-2 coincident logic with a voltage setpoint between 92% - 93.7% of 460 volts with a time delay of 8 seconds +

1 second.

(7) Increase size of control transformers as required to insure adequate voltage for contactor pickup.

The above modifications were reviewed by the NRC and found acceptable [Ref. 3].

Based on the above modifications, the worst case Class 1E equipment voltages occur under the following conditions:

4.2.1 Overvoltage Minimum station load, maximum grid voltage at 1.05 per unit at either the 500 kV or 161.kV system, either SUT-1 or SUT-2 in service.

1 4.2.2 Undervoltage

- Reactor trip with fast transfer to the SUT-2 with a safety signal, grid voltage at 1.0 per unit of the 161 kV system, all of the 500 kV lines are lost and both of the 161 kV lines are intact.

These worst case Class lE equipment voltages are shown in Table 1.

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_ . a .m TABLE 1 ARKANSAS NUCLEAR ONE-UNIT 1 CLASS lE EQUIPMENT VOLTAGE RATINGS AND ANALYZED WORST CASE BUS VOLTAGES '

(in % of Equipment Nominal Voltage Rating)

Maximum Minimum ,

Rated Analyzed (a) Rated Analyzed (a) -

Nominal Voltage Rating Steady Steady Equipment (100 %) State State Transient Motors 4000 Start 80 96.5

, Operate 110 111 90 98.6 ,

460

Start 80 86.5 i

Operate 110 110 90 93.0 Starters (b) 115 Pickup 80 --

Dropout 60 Operate 110 - 90' -

Other(c)

Equipment (a) The maximum allowable voltage drop % f rom bus to load terminal is as follows: 2% for running 460-volt loads; 6% for starting 460-volt loads; 1% for running 4000-volt loads; and 2% for starting 4000-volt c

loads. Class lE motors are designed to provide full load torque during I momentary dip to 75% of rated voltage when other motors accelerate on the system.

(b) Voltage drop calculations were performed to assure that with 90% of l

j 460 volts at the motor terminals, adequate voltage (including control l cable lengths) for contactor operation is present. Existing power control transformers were replaced with larger size transformers [Ref. 8]

where calculations showed that they did not meet the operating criteria.

(c) The 115 volt relays (HFA and HGA) have a pickup rating of 80% and dropout ratings of 40% and 55%, respectively. The 120-volt timers j ( AGASTAT) have a pickup rating of 80% and a dropout rating of 50%.

I Class lE instrumentation circuits are supplied from inverters.

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hew - = - %.- a 4.3 ANALYSIS VERIFICATION By letter dated October 30, 1981 [Ref. 9], the licensee has stated that AP&L will not perform a verification test for the voltage analyses. Their main position for not performing the test is based upon cost / plant safety benefit. They reiterated that the computer model used incorporates conservatively based inputs. Also that the addition of the second-level of undervoltage protection scheme is added assurance that the Class lE equipment will be protected from voltage degradations.

5. EVALUATION The NRC generic letter (Ref.1] stated several requirements that the plant must meet in the voltage analysis. These requirements and an evaluation of the licensee's submittals are as follows:

(1) With the minimum expected grid voltage and maximum load condition, each of fsite source and distribution system connection must be capable of starting and continuously operating all Class lE equipment within the equipment's voltage ratings.

The voltage analysis submitted by AP&L shows that the offsite sources in conjunction with the onsite distribution system, have the capacity and capability to automatically start and continue to operate the Class lE equipment within their design voltage ratings.

(2) With the maximum expected of fsite grid voltage and minimum 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 Class lE equipment's voltage rating is not exceeded for minimum plant load and maximum expected offsite grid voltages. .

(3) The analysis must show that there will be no spurious sepa-ration from the offsite power source to the Class lE buses by the voltage protection relays when the grid is within the g normal expected limits and the loading conditions established by the NRC are being met.

The licensee has proposed to install a Class lE blocking circuit to the second-level of undervoltage protection scheme to prevent spurious tripping caused by the starting of the reactor coolant pumps (RCP). The blocking circuit uses a

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-v - e ~ 4-buffer relay for each RCP and a slave relay to block the undervoltage trip for 20 seconds. A 5-second delayed timer is used to annunciate an alarm if the blocking circuit does not auto-reset to reinstate the undervoltage protection scheme in 25 seconds [Ref. 13].

(4) Test results are required to verify the voltage analyses calculations submitted. .

The licensee has not submitted an analysis verification. .

(5) Review the plant's electrical power systems to determine tf any events or conditions could result in the simultaneous loss of both offsite circuits to the onsite distribution system (compliance with GDC 17).

The NRC has reviewed the electric power system at ANO and has found that with the required circuitry and hardware changes detailed in Reference 13, conformance with GDC 17 is me t.

(6) As required by GDC 5, each offsite source shared betwece 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 remaining unit (s).

The licensee provided the required analysis and found that the the shared offsite source (SUT 2) would be capable of supplying adequate voltage to the Class 1E equipment provided that an interlock be installed. The interlock would prevent the simultaneous and automatic transfer of Unit 1 and Unit 2 loads to SUT 2 (Ref. 14]. Adequate voltage can be supplied by SUT 2 for one unit's complete electrical load and sequentially started ESF loads or the combined sequentially started ESF loads of both units.

6. CONCLUSIONS I

Based on the information submitted by AP&L for Arkansas Nuclear One, Unit 1, which is evaluated in Secton 5 of this report, it is concluded that: d' (1) By deleting the automatic slow transfer from the UAT to SUT-2, providing Class lE load sequencing during the automatic fast transfer from the UAT to SUT-1, load shedding of selected non-Class lE loads when fast transferring from the UAT to SUT-2 and increasing control transformer sizes as required, the

' offsite sources in conjunction with the onsite distribution

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start and continuously operate the Class lE equipment within their design voltage ratings.

(2) Spurious separation from the offsite sources will not occur as the second-level of undervoltage protection system is manually blocked during the starting of the RCP's.

(3) The installed hardware and circuit changes ensure the electric power system is in conformance with GDC 17 requirements.

(4) The requirement of GDC 5 for multi-unit stations was met by installing an interlock to prevent the simultaneous automatic transfer of both station loads to SUT-2.

(5) A test verification of the voltage analysis results be made and submitted in a timely manner to demonstrate that the station electric distribution computer program inputs represent a valid model of the actual plant configuration.

Accordingly, I recommend the NRC approve the analyses submitted j by Arkansas Power and Light Company for Arkansas Nuclear One, Unit 1, which demonstrates that the station electric distribution system is adequate to supply acceptable voltages to the Class lE equipment for the worst case conditions analyzed.

REFERENCES

1. NRC letter (W. Gammill) to all power reactor licensees, dated August 8, 1979.

2. AP&L letter (D. H. Williams) to NRC (R. W. Reid), dated August 23, 1978.

3. AP&L letter (D. H. Williams) to NRC (R. W. Reid), dated October 25, 1979.

4. AP&L letter (D. A. Rueter) to NRC (R. W. Reid), dated March 13, 1979.

5. AP&L letter (D. C. Trimble) to NRC (R. W. Reid), dated May 21, 1979. .

6. AP&L letter (D. C. Trimble) to NRC (R. W. Reid), dated July 12, 1979.

7. AP&L letter (D. C. Trimble) to NRC (R. W. Reid), dated August 24, 1979.

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8. AP&L letter (D. C. Trimble) to NRC (J. F. Stolz), dated October 19, 1981.

9. AP&L letter (D. C. Trimble) to NRC (J. F. Stolz), dated October 30, 1981.

10. Code of Federal Regulations, Title 10, Part 50 (10 CFR 50), Ceneral Design ,

Criterion 5, 13 and 17 of Appendix A for Nuclear Power Plants.

11. ANSI C84.1-1977. " Voltage Ratings for Electric Power Systems and Equipsnt." .

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

13. NRC letter (R. W. Reid) 'to AP&L (W. Cavanaugh, III), dated October 27, 1978.

14. NRC letter (G. Lainas) to NRC (R. Reid), dated November 9, 1979.

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