ML20212P842

Auxiliary Power Sys Voltage Study for Yankee Nuclear Power Station

ML20212P842
Person / Time
Site:	Yankee Rowe
Issue date:	08/22/1986
From:	William Jones, Riela J, Rosenberg S YANKEE ATOMIC ELECTRIC CO.
To:
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ML20212P836	List: ML20212P834 ML20212P842
References
NUDOCS 8609030308
Download: ML20212P842 (27)

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AUXILIARY POWER SYSTEM VOLTAGE STUDY FOR THE YANKEE NUCLEAR POWER STATION By J. A. Piela Prepared By: . S &-

J. Q ela, Engineer (Date)

Reviewed By: * '

~

S. A. Rosenberg, k (Date)

Acting Lead Electrical Engineer Approved By: - W EL b ineering Manager (Date)

W.G.Jon(

Yankee Atomic Electric Company Nuclear Services Division 1671 Worcester Road Framingham, Massachusetts 01701 8609030308 DR 860829 ADOCK 05000029 PDR

DISCLAIMER OF RESPOSIBILITY a

This document was prepared by Yankee Atomic Electric Company

(" Yankee"). The use of information contained in this document by anyone other than Yankee, or the Organization for which this document was prepared under contract, is not authorized and, with respect to any unauthorized use, neither Yankee nor its officers, directors, agents, or employees assume any obligation, responsibility, or liability or make any warranty or representation as to the accuracy or completeness of the material contained in this document.

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ABSTRACT YAEC Report 1561 presents the results of the review of the Auxiliary Power System at the Yankee Nuclear Power Station. The adequacy of the Auxiliary Power System is determined by analyzing a maximum load case with a minimum grid voltage and a light load case with a maximum grid voltage.

This report demonstrates that the off-site and Auxiliary Power Systems at the Yankee Nuclear Power Station are of sufficient capacity to automatically start and operate all safety loads during a maximum load condition, assuming that all on-site power systems are not available. This report also concludes that all loads will not receive voltage of more than the

, umximum allowable level during light load conditions.

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TABLE OF CONTENTS Pane DISCLAIMER OF RESPONSIBILITY..................................... 11 ABSTRACT......................................................... ill TABLE OF CONTENTS................................................ iv ,

LIST OF FIGURES.................................................. Vi LIST OF TABLES................................................... Vli

1.0 INTRODUCTION

..................................................... 1 2.0 AUKILIARY POWER SYSTEM........................................... 2 2.1 Description................................................ 2 2.1.1 Normal Plant Configuration......................... 2 2.1.2 Bus Tie configurations............................. 2 2.2 First and Second Level Undervoltage Protection Relay Setpoint........................................... 3 2.3 Off-Site Power System Voltage Operating Range.............. 3 2.4 Auxiliary Power System Voltage Requirements................ 4 2.4.1 Motors and Contactors.............................. 4 2.5 2400 Volt Circuit Breaker Modification..................... 4 2.5.1 Problem Description................................ 4 2.5.2 Corrective Actions................................. 4

! 3.0 ANALYSIS......................................................... 8 3.1 Problem Description........................................ 8 3.1.1 Maximum Load Case.................................. 8 3.1.2 Minimum Load Case.................................. 8 3.2 Methods.................................................... 8 3.2.1 Computer Solution.................................. 8 3.2.2 Maximum Load Case.................................. 9 3.2.3 Minimum Load Case.................................. 9 3.3 Assumptions...... ........................................ 10 3.3.1 Impedance Model.................................... 10 3.3.2 Maximum Load Model................................. 10 3.3.3 Minimum Load Model................................. 11 3.3.4 Plant Electrical Configurations.................... 11

-iv-

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TABLE OF CONTENTS (Continued)

Page 3.3.4.1 Maximum Load Case........................ 11 3.3.4.2 Minimum Load Case........................ 11 3.3.4.3 Complimentary Bus Configurations......... 12

4.0 CONCLUSION

S...................................................... 15 5.0 VERIFICATION..................................................... 18

6.0 REFERENCES

....................................................... 20 I

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4 5

d

-V-i l

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LIST OF FIGURES Number Title Page 2.1-1 Main One-Line Diagram, Yankee Nuclear Power Station 6 2.5-1 2400 Volt circuit Breaker Modification 7 3.3.2-1 Bus Loading - Maximum Load Case 13 3.3.3-1 Bus Loading - Minimum Load Case 14 i

1 T

4 d

i L

I 1 1

-vi-i

-~, ----y-.---.--. . _ . . _ _ _ _ . , . . . . . . _ , . . _ . , , , _ _ _ _

LIST OF TABLES Number Title Page 4.1 Voltage Sununary - Maximum Load Case 16 4.2 Voltage Summary.- Minimum Load Case 17 5.1 DAPPER Verification - DAPPER Volta 5e Versus SWEC Voltage 19

-vil-

1.0- INTRODUCTION An incident at Arkansas Nuclear one brought into question the ability of that station's Electrical Distribution System to perform its safety function. Consequently, in 1979, the NMC required all power reactors to review the adequacy of their electric power systems. The off-site power system was to be of sufficient capacity to automatically start and operate all l safety loads during maximum loading and minimum grid voltage conditions assuming that all on-site power systems are not available. In addition, all loads were not to receive more than the maximum allowable steady-state operating voltage during light load and maximum grid voltage conditions.

YAEC Report 1206, Reference (a), presented the results of the review of the adequacy of the Station Electrical Distribution System at the Yankee Nuclear power Stations. l l

To incorporate the latest assumptions and changes made to the Station Electrical Service System since YAEC Report 1206 was issued in 1980, we have i updated our analyses. This report presents the results.

2.0 AUXILIARY POWER SYSTEM 2.1 Description 2.1.1 Normal Plant Configuration The Auxiliary Power System of the Yankee Nuclear Power Station is comprised of three systems, each normally supplied from a different source.

Two are normally supplied by the 115 kV transmission lines, and the third is supplied from the station generator. The main one-line diagram of the Auxiliary Power System at Yankee is shown on Figure 2.1-1.

Electric power is supplied from the transmission network to the on-site Electric Distribution System by two independent circuits. One is the 115 kV Cabot line (Y-177) and the other is the 115 kV Harriman line (Z-126). The 115 kV 1r.?.oming Cabot and Harriman lines are tapped on the line sides of the oil circuit breakers in the switchyard and are connected via overhead lines to the 115/2.4 kV Station Service Transformers Nos. 2 and 3.

Voltage regulators at the secondary windings of Station Service Transformers Nos. 2 and 3 maintain a regulated 2400 volt load side voltage with a variation in source voltage of plus or minus 15%.

The two station service transformers are connected to the 2400 volt indoor metal clad Switchgear Buses No. 2 and 3, and also to the 480 volt Buses 5-2 and 6-3, via 2400/480 volt Station Service Transformers Nos. 5 and 6.

The generator supplies Station Service Transformer No. 1, which normally feeds 2400 volt Bus 1 and 480 volts Bus 4-1 via the 2400/480 volt Station Service Transformer No. 4.

2.1.2 Bus Tie Configurations When the generator is not connected to the grid, such as during startup or shutdown, the center 2400 voit and 480 volt buses are connected to off-site power by closing the normally open bus tie breakers to the appropriate outer buses. Thus, 2400 volt Bus 1 may be tied to 2400 volt Bus 2 and 480 volt Bus 4-1 may be tied to 480 volt Bus 6-3; or alternatively, 2400 volt Bus 1 may be tied to 2400 volt Bus 3 and 480 volt Bus 4-1 may be tied to 480 volt Bus 5-2.

After the generator is synchronized to the grid, the center buses are powered from the generator and the tie breakers described above are manually opened.

2.2 First and Second Level Undervoltage Protection Relay Setpoints A loss of voltage on an emergency bus will result in complete isolation of that bus from its normal supply and starting of the associated emergency diesel generator. The loss of voltage is sensed by the first level protection undervoltage relays which are set to operate in 1.8 seconds upon complete loss of power; 3.0 seconds with 58% of rated emergency bus voltage; and in 7.0 seconds with 77% of rated emergency bus voltage.

The second level undervoltage protection relays actuate an alarm if a degraded voltage of 421 or 429 volts exists on an emergency or nonemergency bus, respectively, for 10 seconds. The ten-second time delay has been provided to prevent actuation of the alarm during transients caused by motor starting or short-term grid disturbances. The operator has been provided with instruction on the actions required should the alarm be received.

2.3 Off-site Power System Voltage Operatina Range The Off-Site Power System and On-Site Power Distribution System at the Yankee Nuclear Power Station are designed to provide adequate voltage to support the operation of required loads under any mode of operation.

The 115 kV transmission lines, which provide Yankee with off-site power, operate between a maximum expected voltage of 121 kV and a minimum expected voltage of 109 kV. However, the voltage regulators at the secondary windings of Station Service Transformers Nos. 2 and 3 maintain a regulated load side voltage of 2400 volt with a 115% variation of the 115 kV source.

r 2.4 Auxiliary Power System Voltate Requirements 2.4.1 Motors and Contactors Motors used in the Auxiliary Power System at Yankee require a continuous operating voltage 'etween o 90% and 110% of their rated voltage. The bases of the 110% operating voltage range are NEMA standards.

The 460 volt contactors have a 370 volt pickup and 322 volt dropout.

The 480 volt High Pressure Safety Injection (HPSI) pump motors and the 460 volt Low Pressure Safety Injection (LPSI) pump motors will start within acceptable time limits at 80% and 60% of their rated voltages, 384 volts and 276 volts.

2.5 2400 Volt Circuit Breaker Modification 2.5.1 Problem Description While performing this study, a potential problem was discovered and required resolving prior to completion of the study. When the generator is off line, and the center buses are tied to the outer buses, the Auxiliary Power System might not have the capacity to start and operate all safeguard loads if a safety injection actuation signal was received when the buses are in a heavily loaded condition.

The study also indicated that the 2400/480 volt station service transformer which feeds the two tied 480 volt buses could exceed its rated capability if a safety injection actuation signal occurs. We reported this potential problem as Licensee Event Report 50-29/86-07 (Reference (b)).

2.5.2 Corrective Actions In order to resolve the above concerns, Yankee intends to install 2400 volt Circuit Breaker 424 to feed Station Service Transformer No. 4 from 2400 volt Bus No. 1 as shown in Figure 2.5-1. This modification will result in 480 volt Bus 4-1 being fed directly from 2400 volt Bus 1. The new 2400 volt breaker will be installed during the 1987 refueling outage.

e-During normal plant operation, Breaker 424 will be closed and the 2400 volt and 480 volt bus tie breakers will be open. When the generator is not connected to the grid, Breaker 424 will also be closed and 2400 volt Bus 1 will be tied to 2400 volt Bus 2 or 3. With 480 volt Bus 4-1 fed directly from 2400 volt Bus 1 and a 2400 bus Lie breaker c19 sed, the need to tie 480 volt buses together - an arrangement which causes the voltage and loading problems - is eliminated.

Reference (c) provided additional details of the problem and proposed interim operating procedures which would assure that emergency buses are supplied with adequate voltage at all times. In Reference (d) the Nuclear Regulatory Commission approved Yankee's actions to resolving the transformer overloading and safeguard load starting and operating concerns.

The conclusions and results presented in this report are based on a new 2400 volt breaker installed in the Auxiliary Power System.

Y177 Z126 CABOT 115 KV n n 115 KV HARRIMAN 3 C 3 C

/ \

d MAIN mm XFMR

/ \ 3S115

/\2S115 3SST 2SST 1SST 1.d l V.R.I GEN 1324 1224 r ")324 9 Q ")124 9 r Q r 9)224

\ \

2400V BUS 3 2400V BUS Y 2400V BUS 2

$ $ 6SST $ M 4SST "m M 5SST 4648 4548 g 480V BUS 4-1 ") 448 480V

?)648BUS 6-3 9 g 9) S48 480V BUS 5-2

)BT1A )BT2A )BT3A EBUS 1 EBUS 2 9 EBUS 3 5 5 8 5 5 8 6 8 ET n/ n n a o o n a o PSI PSI DG PS PS DG PS 'PS DG 1 1 1 2 2 2 3 3 3 FIGURE 2.1-1 Main One-Line Diagram, Yankee Nuclear Power Station

Y177 Z126

. CABOT 115 KV 115 KV HARRIMAN TI/k d MAIN

/\3S115 / \ 2S115 O 3SST 1S18 & 2SST P

• mm P

I V.a.1 IV.a.1 GEN 1324 1224 r ")324

? O ")124

? Q r  ?)224

\ '

2400V BUS 3 2400V BUS 1 2400V BUS 2 424 4 4 6SST 4 4 ASST 4 4 5SST 8 8 548 480V BUS 6-3 480V BUS 4-1 480V BUS 5-2 ac "'

6

,)BT1A 6)BT2A n n)ET3A EBUS 1 EBUS 2  ? EBUS 3 Ag 63 63 63 Ag 63 63 63 63 c) of of n) of n) of of of PS IPSI DG PS PSI DG PSI PSI DG 1 1 1 2 2 2 3 3 3 FIGURE 2 5-1 2400 Volt Circuit Breaker Modification 3.0 ANALYSIS 3.1 Problem Description 3.1.1 Maximum Load case The purpose of this analysis is to assure that all safeguard loads will start and operate within their required voltage limits.

The study was performed assuming the need for power is initiated by a safety injection actuation signal without a loss of off-site power and with the generator off-line. These conditions present the largest load demand.

The analysis also assumes that the grid voltage is at the minimum expected value, however, voltage regelators will maintain the desired load voltage of 2400 volts.

3.1.2 Minimum Load case This analysis assures that all loads will not receive more than the maximum steady-state operating voltage during light load conditions.

This case _ assumes that the plant is at cold shutdown with minimum operating load. The analysis is based on an off-site voltage of 121 kV, the maximum expected value with the voltage regulated to 2400 volts.

3.2 Methods 3.2.1 Computer Solution A computer solution of a load flow calculation is needed to determine each bus voltage in the power system because an interactive algorithm is required to solve numerous simultaneous equations. Load flow computer programs require as input such parameters as the load at each bus and the impedance between each bus in the system.

3.2.2 Maximum Load case This voltage analysis was performed using the Distribution Analyses for Power Planning, Evaluation, and Reporting Program (DAPPER). Since DAPPER is a steady-state program and this analysis involves transient conditions, such as motor starting and voltage regulation, the following series of DAPPER calculations is required.

Time (seconds) Condition t=0~ Initial condition; prior to voltage regulation.

t=0 Initial condition; voltage regulated, t=0 Safety injection actuation signal is initiated; start LPSI pumps.

t=3 LPSI pumps at speed.

t = 10 Start HPSI pumps.

t = 13 HPSI pumps at speed, t = 33 Fitial running condition; voltage regulated.

The voltage at each bus is calculated by the DAPPER Program and is compared to the required levels.

3.2.3 Mipimum Load Case This analysis was performed using the Stone and Webster Engineering Corporation Power Flow Program.

The steady-state voltage at each bus is calculated for the final running condition, with the voltage regulators in service. The voltage at each bus is compared to the maximum allowable voltage level.

_g.

3.3 Assumptions 3.3.1 Impedance Model

a. Cable lengths and sizes were taken from the Yankee plant cable schedules. Cable impedances were taken from ICEA standards.

b. Impedances of breakers are assumed to be insignificantly small.

c. Transformer impedances were obtained from nameplates.

3.3.2 Maximum Load Model The bus loadings used in this analysis are provided in Figure 3.3.2-1.

The following assumptions were used to develop these loadings:

a. The maximum load was determined to exist when an accident occurs without loss of off-site power and with the generator off-line.

When the generator is off-line, the center 2400 volt bus is connected to off-site power by closing the normally open bus tie breaker to the appropriate outer bus.

b. Motor kVAs were conservatively calculated by using nameplate horsepower instead of the actual horsepower required.

c. The loading for the Electric Distribution System was determined from discussions with plant personnel, and a review of station logs,

d. For motor starting studies, all loads are treated as constant impedance loads at a .15 power factor.

o. No load shedding occurs.

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3.3.3 Minimum Load Model The loading assumptions for this case are presented in Figure 3.3.3-1.

In developing the minimum load model, the plant was assumed to be at cold shutdown with minimum operating load.

3.3.4 plant Electrical Configurations 3.3.4.1 Maximum Load case This analysis was performed with the plant's Electrical System configured as follows:

Air Breaks: 2S115, 3S115 closed; IS18 open.

2400 V Breakers: 224, 324, 424, 1324 closed; 124, 1224 open.

480 V Breakers: 448, 548, 648 closed; 4548, 4648 open.

480 V Emergency Bus Tie Breakers: BT1A, BT1B, BT2A, BT2B, BT3A, BT3B closed.

3.3.4.2 Minimum Load Case The minimum load analysis was performed with the plant configured as follows:

Air Breaks: 2S115, 3S115 closed; IS18 open.

2400 V Breakers: 224, 324, 424, 1224 closed; 124, 1324 open.

480 V Breakers: 448, 548, 648 closed; 4548, 4648 open.

480 V Emergency Bus Tie Breakers: BTIA, BT1B, BT2A, BT2B, BT3A, BT3B cic1 sed.

3.3.4.3 Complimentary Bus configurations As previously stated there can be two bus configurations when the plant is off-line; 2400 volt Bus 1 tied to 2400 volt Bus 3 or 2400 volt Bus 1 tied to 2400 volt Bus 2. The study of the complimentary bus configurations is applicable to the analyzed cases since:

a. The loading on the buses normally connects

• o the Harriman line is equivalent to the loading on the buses normally connected to the Cabot line,

b. The impedance of 115 kV/2400 volt Station Service Transformer No. 2 is equal to the impedance of 115 kV Station Service Transformer No.

3.

3 Y177 Z126 n 115 KV HARRIMAN CABOT 115 KV n oe ac TI/\

MT mm

/\3S115 2S115 d 3SST I8I8

-2SST g.R.I 1SST " " tr.aJ GEN 2 OoV 124 BUS 3 )324 1324 BUS 1g ac

")224 g2 y ae g g)424 3214 3213 3215 6SST w g KVA 4SST p y KVA 5SSTWW ym KVA 480V 4648 480V o 4548 480Vo BUS 6-3 u)648

? g BUS 4-1?)448 QBUS5-2?)548 BT1A

) BT2A) BT3A )

494 624 525 KVA KVA KVA EBUS 2 BT3B EBUS 1 EBUS 3 8 0 8 KVA KVA KVA FIGURE 3 3.2-1 Bus Loading - Maximum Load Case

Y177 Z126 CABOT 115 KV n n 115 KU HARRIMAN ae ac o

TI /\

MT mm 2S115

/(.3S115

' 1S18 M 3SST '

2SST

. I V.R.I L 1SST " " [LIL}

CEN

- 2400V. o 00V o 124 -1224 2400V 324 1324 BUS 1 '")224 BUS 2 BUS 3 g 9 9 f  ?

~

424 214 0 0 W

6SST p w 4SST p w KVA KVA KVA 5SST y *m 480V 4648 480V 0 4548 480V BUS 6-3 u)648

? g BUS 4-1?)448 QBUS5-2?)548 BT1A

) BT2A) BT3A )

247 360 294 KVA KVA KVA

~

BT1D T3B, EBUS 1 EBUS 2 EBUS 3 4 1 0 8 KVA KVA KVA FIGURE 3.3.3-1 F

Bus Loading -~ Minimum Load Case

4.0 CONCLUSION

S ,

The results of the voltage study for the maximum and minimum load case are presented in Tables 4.1 and 4.2. These tables contain operating and motor starting voltages and the allowable voltage range. The results are dependent upon the addition of a new 2400 volt breaker to the Electrical Distribution System which was found necessary as a result of this study.

The maximum load study of the Auxiliary Power System at 'lankee was performed using the worst case loading, minimum expected grid voltages, and other conservative assumptions. The results show that adequate voltage exists for the start of safeguard loads. The voltages in the 480 volt system momentarily dip to slightly lower than acceptable continuous running values when the LPSI and HPSI pump motor start. This voltage dip is of no concern because it will not cause any contactors to drop out and the 480 volt system voltage will recover to within allowable operating limits after the safeguard motors are at speed. The results also demonstrate that the voltages are:

(a) adequate for the continuous operation of all loads under the worat case loading, and (b) will not cause activation of the undervoltage protection relays. We concludo that the Auxiliary Power System at the Yankoo Nuclear Power Station is of sufficient capacity to automatically start and operate all safety loads assuming that all On-Site Power Systems are not available.

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] The results of the minimum load case demonstrate that under light load conditions voltages at all buses are well within the maximum voltage limits for the operation of electrical equipment. We conclude that all loads will not receive voltsges of more than the maximum allowable levels.

TABLE 4.1 Voltane Sununary Maximum Load Case Eaulpaent .

Voltane (Volts)

LPSI and Initial LPSI LPSI HPSI HPSI l Condition Start Run Start Run "ous/ Motor T=0 sec. T=0+ sec. T=3 sec. T=10 sec. T=33 sec. 2400 V Bus 1 2393 2247 2370 2302 2392 2400 V Bus 2 2399 2329 2389 2357 2396 2400 V Bus 3 2395 2249 2371 2303 2394 480 V Bus 4-1 462 376 447 407 448 l l

480 V Bus 5-2 467 398 456 424 454 480 V Bus 6-3 467 384 453 414 454 l

480 V E Bus 1 467 368 448 401 447 l

480 V E Bus 2 + 361 442 394 441 480 V E Bus 3 467 382 451 412 447 LPSI 1 + 365 447 400 446 LPSI 2 + 358 441 393 440 LPSI 3 + 378 450 410 446 HPSI 1 + + + 398 446 HPSI 2 + + + 391 439 HPSI 3 + + + 408 445 NOTES:

(1) 276 volts required.

(2) 384 volts required.

(3) 414 volts required.

(4) 432 volts raquired.

+ no load.

TABLE 4.2 Voltage Summary Minimum Load Case Maximum Allowable Bus Voltage (Volts) Voltage (Volts) 2400 V Bus 1 2400 2530 2400 V Bus 2 2400 2530 2400 V Bus 3 2400 2530 480 V Bus 4-1 471 506 480 V Bus 5-2 473 506 480 V Bus 6-3 475 506 480 V E Bus 1 474 506 480 V E Bus 2 471 506 480 V E Bus 3 473 506

I 5.0 VERIFICATION The Stone and Webster Engineering Corporation computer program, used to perform the minimum load case, has been verified to manual calculations and to field tests per Stone and Webster's Engineering Assurance Procedure 5.25.

To assure that the DAPPER results for the maximum load case are accurate, the DAPPER program has been verified to the Stone and Webster program. The results are provided in Table 5.1. Comparison of the voltages calculated by DAPPER to the Stone'and Webster program calculated values demonstrate that the DAPPER computer nodel is valid.

TABLE 5.1 DAPPER Verification DAPPER Voltage Versus SWEC Voltage Case 1 Case 2 DAPPER SWEC  % DAPPER SWEC  %

Equipment / Bus (Volts) (Volts) Deviation (Volts) (Volts) Deviation Source 109000 109000 0.00 109000 109000 0.00 2400 V Bus 3 2350 2336 0.60 2350 2336 0.60 2400 V Bus 2 2279 2256 1.02 2290 2269 0.93 480 V Bus 6-3 408 406 0.49 408 406 0.49 480 V Bus 4-1 395 391 1.02 347 343 1.17 480 V Bus 5-2 395 391 1.02 347 343 1.17 E Bus 1 392 389 0.77 392 389 0.77 E Bus 2 381 376 1.33 334 330 1.21 E Bus 3 380 375 1.33 333 329 1.22 LPSI 1 389 387 0.51 389 387 0.51 LPSI 2 378 374 1.07 331 328 0.91 LPSI 3 376 372 1.08 332 326 1.84 i

6.0 REFERENCES

a. P. R. Johnson, S. F. Urbanowski, Auxiliary Power System Voltage Study for the Yankee Nuclear Power Station, YAEC 1206.

b. License Event Report 50-29/86-07, 480 V AC Buses Cross-Tie Electrical Loading Problem, dated July 18, 1986.

c. YAEC Letter to USNRC, Electrical Loading at Yankee Nuclear lower Station During Bus Cross-Tie Configurations FYR 86-059, dated June 19, 1986,

d. USNRC Letter to YAEC, Summary of Meeting Held on June 23, 1986 to Discuss Electrical System Voltanes, dated June 27, 1986.

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