IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 11 November 1981AN AUXILIARY POWER SYSTEM

FOR A500 TO 600MW COAL POWER PLANT

R. M. Damar, member, IEEE Bechtel Power CorporationGaithersburg, Maryland

Coal-fired generating stations will constitute an increasing portionof the electric power generation for the United States in the yearsahead. Many of the new units will be rated in the 500 to 600 MWrange and will incorporate complex pollution control equipment. Thepollution control equipment represents a significant increase instation load requiring a larger auxiliary power system. This paperdescribes suitable design methods and results of a reliable andeconomical auxiliary power system design suitable for a typical single-unit generating station.

INTRODUCTION

Coal is again assuming an important role in the new powergeneration facilities being planned by the electric utility industry.Unlike the general trend for building giant coal-burning units in the1950's and early 1960's, the present trend is for smaller units in therange of 500 to 600 MW.

It is expected that the boiler and turbine-generator manufacturerswill be able to standardize their major auxiliary equipment andcomponents to serve these units. This trend, if it continues, will helpto streamline the manufacture and installation of future units andreduce their cost considerably.

The reasons for the decrease in unit size will not be explored in thispaper. The discussion will concern the general features of an auxiliarypower distribution system. This system is very flexible and thereforeadaptable to units in the range of 500 to 600 MW. It is designed tohandle all unfavorable power system and plant operating conditions.It requires minimum operator action under these variable conditions.

The system reliability is enhanced by redundant feeders andequipment. The system economy, first cost and operating cost, isevaluated.

The auxiliary system described is not site sensitive. The plantcould be located in any part of the country. The assumed powersystem operating conditions are conservative and therefore applicableto any power system.

The plant auxiliary loads would be different for 500 MW and600 MW plants. However, the difference in loads is not significantenough to change the auxiliary system configuration. The boundaryof given capability curves envelopes all equipment starting andoperating conditions for 500MW as well as 600MW plants.

These new plants, although smaller in net generation, require alarger percentage of gross unit generation to supply the conventionaland new flue gas desulfurization (FGD) auxiliary load requirements.The new FGD loads are not only comparatively large, but are often

81 SM 359-9 A paper recommended and approved by theIEEE Power Generation Committee of the IEEE PowerEngineering Society for presentation at the IEEE PESSummer Meeting, Portland, Oregon, July 26-31, 1981.Manuscript submitted February 2, 1981; made availablefor printing April 21, 1981.

remote from the turbine generator/power block area. The methodused for the auxiliary power system design consists of determinationof the electrical equipment ratings that optimize the balance betweenshort circuit ratings and voltage regulation requirements at the lowestachievable cost under variable system and unit operating conditions.

It is not the intention of the authors to claim that the auxiliarysystem shown in Figure 9 is the only system that satisfies the assumedsystem conditions. Other system configurations may also satisfy thegiven conditions. The preference should be based on operatingpriorities as well as economic advantages.

IDEAL AUXILIARY SYSTEM CHARACTERISTICS

The auxiliary system design engineer is faced with various systemrequirements which need to be satisfied for optimum plant per-formance.

Two sets of variables must be considered. One set of variablesoccurs during the design stage. The magnitude of auxiliary load atevery voltage level changes frequently during the design period andsometimes even after commercial operation. In addition to changes innumber of the individual large loads, such as pumps and fan drives,ratings also change during the early phases of the conceptual design.The second set of variables to be considered is the power system andthe plant operating conditions. When the plant auxiliary loads areenergized from the offsite system through the service or startuptransformer, the transmission system voltage and frequency fluc-tuations affect the performance of the plant auxiliary system. Offsitesystem maximum and minimum voltage conditions must be taken intoconsideration so that the plant auxiliary system performance is withinacceptable operating limits. In addition to variable system conditions,the plant itself is subject to different operating requirements whichmay be the result of normal daily load variations or due to occasionalequipment malfunctions. The plant may be subject to operatingconditions which change from the minimum load to maximum load.In summary, the plant and its auxiliary systems are dynamic innature, changing at almost any moment during the normal operatingcycle.

Taking into account the above-mentioned variables, the designengineer must create a system with acceptable response to all of theuncontrolled variables.

EQUIPMENT LIMITATIONS

All power plant equipment is designed to operate within certaindesign limits. The maximum performance is obtained at a definedoperating condition, such as nominal applied voltage at rated load.The equipment will operate below its maximum efficiency when theoperating parameters differ from the nominal design parameters suchas overload, underload, and undervoltage.

The major apparatus that determine the main characteristics ofthe auxiliary system performance are the transformers, switchgear,and large motors. The power transformers used for the plant auxiliarysystems are either connected to the generator bus (unit auxiliary trans-formers) or to the utility transmission system (startup or servicetransformers).

The unit auxiliary transformers generally have a high voltagerating of 22 to 26 kV. The startup transformer's primary windings are

1981 IEEE

Abstract

457'

J. P. Henschel

4572

usually high voltage or EHV type (138 to 765 kV). The number andratings of these transformers have always been a subject for con-siderable discussion. This design incorporates a redundant powersource for the unit auxiliary loads. This requirement is best fulfilledby having one full-size startup and one full-size unit auxiliary trans-former. Other studies have shown that under certain conditions thestartup transformer can be eliminated when a generator breaker isinstalled in the isolated phase bus. This is a viable alternative to thestartup transformer. However, an evaluation of reliability should bemade when this alternative is selected. Although the addition of agenerator breaker permits the elimination of the startup transformer,it also eliminates a second route of auxiliary power through thestartup transformer. To achieve a similar degree of reliability, thenumber of unit auxiliary transformers is often increased. If under thiscondition the cost evaluation favors the generator breaker, it shouldbe given serious consideration. The economic analysis will be dif-ferent for a single unit plant versus a multiple unit plant utilizing ashared startup transformer.

The voltage rating of the secondary windings generally determinesthe maximum rating of the transformer. Secondary voltage affects thesystem design in two ways. First, it determines the secondary current,and second, it determines the switchgear voltage and load currentrating. When the decision is made for the transformer secondaryvoltage, the switchgear voltage class is also determined.

Switchgear ratings play a very important role in the auxiliarysystem design. The switchgear and the transformer secondaries mustbe compatible. This requirement usually determines the minimumlimit of the transformer impedance.

SYSTEM STUDY

The design engineer must consider all the above-mentioned in-teractions of the major equipment characteristics. These interactionsare first quantified and then solved analytically and representedgraphically to determine the auxiliary system design.

A detailed, computer-aided study was performed to determine theauxiliary power system for units rated from 500 MW to 600 MW.The following parameters were the guiding factors of the study:

* Short circuit capability* Voltage rating and regulation* Maximum bus loading capability and optimum transformer

impedance* Lowest system cost and plant availability.

Short Circuit Capability

The short circuit calculation was performed in accordance withANSI/IEEE 37.010-1979 [1]. The fault current at the medium voltagebus consists of contributions from both the transmission systemthrough the startup transformer, and the connected motors. Thesystem contribution is determined by the system equivalent faultimpedance and the startup transformer impedance. The motorcontribution is a function of the motor subtransient reactance ad-justed by the appropriate multiplying factors to account for ac and dcdecrements of the actual fault current.

The transformer impedance can be determined such that the faultcurrent contribution from the system (through the transformer) plusthe motor contribution are within the switchgear short circuit rating[2]. As the motor contribution increases, the system fault currentcontribution must decrease so that the net fault current remainswithin the switchgear short circuit rating. One method of reducing thesystem fault current contribution is to increase the transformer im-pedance. Thus, the minimum allowable transformer impedancebecomes a function of the maximum connected motor load. Thisrelationship is shown in Figure 1. The curves are nonlinear for boththe interrupting and closing and latching current limits of the switch-

INTERRUPTING

:R55>s 55- \ / / CLOSE & LATCHE 50 LIMIT

450j 40-40u 35w 30zZ 250c 20DX 15

1050o3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TRANSFORMER Z (% @ 22.5 MVA BASE)

FIGURE 1. SHORT CIRCUIT LIMITS FOR6900 V, 500 MVA SWITCHGEAR

gear. The area to the right of the curve represents acceptable designsfor the selection of the transformer impedance and connected loadwith respect to the switchgear short circuit rating. The minimumtransformer impedance boundary changes from the switchgear in-terrupting rating to the close and latch capability as the connectedload increases. This occurs due to our conservative assumption thatthe utility system momentary and interrupting short circuit im-pedances are equal. Since the motor momentary fault contribution ishigher than the interrupting contribution and as the portion of themotor contribution of total fault current increases, the close and latchcapability of the switchgear becomes the limiting factor.

The data used to determine the curves of Figure 1 are given inAppendix A. The switchyard voltage and system short circuitcapability are assumed to be maximum (worst case) at the time of thefault. A -10 percent and -7.5 percent manufacturing impedancetolerance was allowed for three-winding and two-winding (loadcenter) transformers, respectively.Voltage Regulation

The voltage drop calculations were performed by an iterativemethod to solve for the transformer impedance in terms of the switch-yard voltage, and impedance voltage drop due to the running load.

These calculations were verified by a computer program utilizingthe accelerated Gauss-Seidel iterative algorithm for solution of theload flow [3].

The running motors are represented as a constant MVA load. Thestarting motors are represented as a constant impedance. The system(or generator) is representated by an equivalent impedance and aninfinite bus behind the equivalent impedance. The system equivalentimpedance is based on 1000 MVA minimum system short circuitcapability. The voltage of the infinite bus is determined from thesteady-state load flow when the lowest switchyard voltage is 0.95 perunit and the auxiliary system load is at the nominal level. The voltageat the infinite bus (swing bus) is then held constant for the calculationof the voltage dip during motor starting. This procedure allows for avoltage dip at the switchyard bus as well as the medium voltage buswhen starting the large motors. This is based on the conservativeassumption that voltage regulating devices do not respond during themotor starting transient.

The transformer turn ratios are selected to be equal to the nominalsystem voltage ratios to prevent overvoltage at the motor terminals atlight load and high switchyard voltage (1.05 per unit). A + 10 percent

4573

manufacturing tolerance was allowed for the transformer impedancefor the voltage drop calculations.

Figure 2 shows the relationship of the medium voltage bus runningload with respect to the startup transformer impedance for both thelarge motor starting voltage limits (80 percent of the rated terminalvoltage) and the low voltage (480 V) bus regulation limits (90 percentof the rated terminal voltage). All coordinates to the left and belowthe 480 V bus regulation curve represent acceptable designs for theselection of the transformer impedance and bus loading with respectto low voltage regulation requirements.

6055

7000 HP50- MOTOR

2 45E 45- / 6000 HP3 40 MOTOR0-1

z

z

25D,

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20TRANSFORMER Z (% @ 22.5 MVA BASE)

FIGURE 2. VOLTAGE REGULATION LIMITS FOR6900 V, 500 MVA SWITCHGEAR

Maximum Bus Loading Capability, and Optimum TransformerImpedance

Optimization of the power plant auxiliary system requires thateach medium voltage bus be loaded to its maximum capability andthat the quantity of buses, unit auxiliary- transformers, and startuptransformers be minimized. Figures 1 and 2 indicate that there areopposing constraints placed on the selection of the transformerimpedance to maximize the bus load capability. The short circuit con-straints require an increasing impedance as the load increases and thevoltage regulation constraints require a decreasing impedance as theload increases. The determination of an acceptable design bus load ismade by selecting a transformer impedance that simultaneouslysatisfies the requirements of both the short circuit and voltage regula-tion constraints. The unique transformer impedance that satisfiesthese requirements at the largest load is the optimum impedance. Thisvalue can be determined by combining Figures I and 2 on the sameplot.

To combine Figures 1 and 2 a correlation of the ordinate axes isrequired. The short circuit constraints are related to the bus connectedload, since the motor subtransient reactance and consequently themotor fault current feedback are functions of the motor design ratingand are independent of the actual motor running load. On the otherhand, the system voltage drop is a function of the load current asdetermined by the actual running load. What is required, then, is ascaling factor or load factor that relates the actual running load to themaximum connected (nameplate) load. This is determined by calcu-lating the expected driven equipment brake horsepower requirementsat the unit maximum continuous rating and dividing by the connectedload. Figures 1 and 2 can now be combined to form Figure 3. Thebroken line indicates the optimum transformer impedance and themaximum bus loads for the system parameters given in Appendix A.

The voltage regulation requirements are determined by theallowable motor terminal voltage. The applicable NEMA standards[4] specify a standard steady-state terminal voltage of from 90 percentto 110 percent of rated motor voltage, resulting in a 20 percent( 10 percent) range. One-half of the range is used by the generatorand/or the utility transmission system, leaving the remaining 10percent range for the auxiliary system regulation [5]. Satisfying thelow voltage loads (less than 600 V) that are two or more tran-sformations from the generator or transmission system is a difficultdesign requirement.

70- 67000 HP INTERRUPTING

65- 5 MOTOR LIMIT60-.g50 6000 HP COSE & LATCH55- 4 MOTOR

50-caO45- Ln3 30-O Zj 5000 HP: 30-CD 25 MOTORz /zZ 25-z 2O z0

20- ) 480 V BUS REGULATIONcc15 -/

co 15--)

6 7 8 9 10 11 12 13 14 15 16 17 18TRANSFORMER Z (% @ 22.5 MVA BASE)

FIGURE 3. BUS CAPABILITY FOR6900 V, 500 MVA SWITCHGEAR

Increasing the allowable voltage range of the auxiliary system hasthe effect of shifting the regulation curves of Figures 2 and 3 to theright. This results in a different Optimum transformner impedance anda higher maximum bus load& This is easily accomplished by incorpor-ating a load tap changer (LTC) on the startup transformer. Since thegenerator voltage is usually kept at its nominal or higher level, theauxiliary transformers do not require LTCs. The tap changer can beautomatically or manually operated to control the secondary voltageat the nominal bus rating (1.0 p.u.). Figure 4 shows the effect of theLTC on the system design. For the same conditions given in Ap-pendix A, the maximum bus load capability of Figure 4 is approx-imately twice that of Figure 3. Figures 5 and 6 are the analogoussystem capability curves for a 4160 V system.

It should be borne in mind that if the utility system is well-regulated,the switchyard voltage will not drop below its nominal level.Therefore, LTCs on the startup transformer tnay not be necessary.

Lowest System Cost

The choice of medium voltage levels, number of transformers,and number of switchgear lineups can be reduced to an economicanalysis of the technically acceptable alternatives.

The technically acceptable designs must meet the followingcriteria:

1. The transformer and switchgear application should be inaccordance with the short circuit and voltage regulation re-quirements shown in Figures 3 through 6.

2. A single failure of a transformer or bus should allow the unitto operate at 50 to approximately 100 percent nominalrating.

/1 7000 HP 6000 HPMOTOR MOTOR

I III5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TRANSFORMER Z (% @ 22.5 MVA BASE)FIGURE 4. BUS CAPABILITY WITH LTC FOR

6900 V, 500 MVA SWITCHGEAR

6 7 8 9 10 11 12 13 14 15 16 17 18 19TRANSFORMER Z (% @ 22.5 MVA BASE)FIGURE 5. BUS CAPABILITY FOR4160 V, 350 MVA SWITCHGEAR

X/A00 Hfi/ MOTOR @N 00 HP

MOTRI MOTORI7 ' 'b 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

TRANSFORMER Z (% @ 22.5 MVA BASE)

3. The quantity of auxiliary and startup transformers should beminimized to avoid congestion and to simplify automatic bustransfer schemes.

4. The number of separately operated medium voltage busesshould be consistent with the redundancy and back-upschemes designed into the mechanical systems. This is oftennecessary to meet the criterion of item 2 above.

Three schemes that meet these criteria are presented in Figures 7,8, and 9. Each of these schemes utilizes a load tap changer on thestartup tranformer's secondary windings. A listing of the major loadsis given in Appendix B.

Figure 7 is representative of a 4160 V system consisting of one22.5/30 MVA and one 30/40 MVA transformer (with optional LTC)with split secondary windings for the transformers. This system iscapable of starting a 5500 hp induction motor. Four 4160V switch-gear buses are utilized.

Figure 8 is representative of a 6900 V system (Scheme 1) consistingof one 22.5/30 MVA and one 30/40 MVA transformer (with optionalLTC) with split secondary windings for the transformers. This systemis capable of starting a 7500 hp induction motor. Four 6900 Vswitchgear buses are utilized.

Figure 9 is representative of a different 6900 V system utilizing one70 MVA transformer with optional LTC and split secondary windings(Scheme 2). Each 6900 V winding is rated at 35 MVA and is capableof starting a 5000 hp induction motor. Four 6900 V switchgear busesare utilized.

CONCLUSION

An auxiliary system design for a 500 to 600 MW coal-firedgenerating station has been presented. The design tools and analyticalmethods are straightforward and applicable to power plants of otherdesign and ratings.

We have determined that the 6900 V system utilizing one auxiliaryand one startup transformer is the most economical choice. Thecapital cost of the equipment and the construction cost make the6.9 kV system of Figure 9 the obvious choice. The simplicity of thedesign and ease of operation are additional benefits.

REFERENCES

[1] American National Standard Application Guide for AC High-Voltage Circuit Breaker Rates on a Symmetrical Current Basis,ANSI/IEEE C37.010-1979.

[2] American National Standard Schedules of Preferred Ratingsand Related Required Capabilities for AC High-VoltageCircuit Breakers Rated on a Symmetrical Current Basis,ANSI/IEEE C37.06-1979.

[3] G. W. Stagg and A. H. El-Abad, Computer Methods in PowerSystem Analysis. New York; McGraw Hill Book Company,1968.

[41 National Electrical Manufacturer's Association StandardPublication "Motors and Generators," MG 1-1978.

[5] American National Standard Voltage Ratings for ElectricPower Systems and Equipment (60 Hz), C84.1-1977.

FIGURE 6. BUS CAPABILITY WITH LTC FOR4160 V, 350 MVA SWITCHGEAR

4574

70-65-

4575

UTILITY'S SYSTEM (138 kV)

7T MAIN -

UTILITYS SYSTEM 4138 kV _

T MAINTRANSFORMER

UNIT AUXILIARY STARTUPTRANSFORMER TRANSFORMER

uJ U 52.6/70 MVA, OA/FA AJ 52.6n70 MVA, OA/FA23/6.9 69 kV 138/16 -T6.9kV

WitlO LTC(OPTIONAL)

I'53 .69L(TYPICAL)N0 _0

L i 6.9 kV BUS NB02 6.9 kV BUS NBO3 6.9 kV BUS NB04

.TOTAL 5TRANSFORMERS

1 6901100011333

APPENDIX A

SYSTEM DATA

The calculations and curves were developed from the following data:

a. Maximum switchyard voltage: 1.05 per unit

b. Minimum switchyard voltage: 0.95 per unit

c. Maximum three-phase short circuit capability of the system:15,000 MVA

d. Minimum three-phase short circuit capability of thesystem: 1,000 MVA

e. Maximum allowable motor voltage: 10 percent of motorrating

f. Minimum allowable motor voltage:

Steady state - 90 percent of motor ratingStarting dip - 80 percent of motor rating

g. Motor locked rotor current: 6.5 x full load amps

h. Load center transformer impedance: 5.75 percent at 1000 kVA

i. X/R multiplier of combined fault network: 1.

TOTAL 17MOTORS

j. Contact parting time of switchgear breaker: 3 cyclesk. Startup or auxiliary transformer impedance tolerance: 10 per-

cent

1. Load characteristics:

30 percent - 480 V loads70 percent - 4000 V or 6600 V motor loads(47 percent - above 100 hp at 1800 rpm, or above 250 hp at3600 rpm; 23 percent - all others)

m. Motor starting impedance: 4000 V or 6600 V motor =I /(locked rotor current)

n. Feeder cable voltage drop when starting large motors: 2.5 per-cent

o. Feeder cable voltage drop of 480 V running motors: 2.0 percent

p. Running load power factor: 0.85

q. Starting motor power factor: 0.15

r. 480 V load center running load: 1000 kVA (self cooled ratingof the transformer)

s. Nominal voltage levels: 138 kV, 6.9 kV, 4.16 kV, 480 V

t. Motor rated voltages: 6600 V, 4000 V, 460 V

4576

GENERATOR0-

I

69 kV BUS NBO1

TOTAL 11MOTORS

A A -1200 A50 MVA(TYPICAL)

TTI TT TTTOTAL5 TOTAL 12 TOTAL 16 TOTAL 6 TOTAL6

TRANSFORMERS MOTORS MOTORS TRANSFORMERS TRANSFORMERS01480 V J L 69OO,480 V-1AVA, OA/FA 5-1000/1333 AVA, OA/FA

1-1500/2000 /VA, OA/FA

FIGURE 9. 6900 V AUXILIARY SYSTEM - SCHEME 2

4577

u. Switchgear ratings (ANSI/IEEE C37.06-1979):

Nominal voltage class

Nominal three-phase MVA

Rated maximum voltage

Rated short circuit currentat rated maximum voltage

Close and latch capability

Rated interrupting time

4.16kV

350

4.75 kV

41,000A

78,000 A

5 cycles

7.2 kV

500

8.25 kV

33,000 A

66,000 A

5 cycles

v. Load factor: 85 percent (running load/connected load)

w. Load tap changer does not operate during motor starting.

APPENDIX B

600 MW PLANT AUXILIARY LOADS

Ragip M. Damnar (M'56) was born inTurkey on November 1, 1921. He receivedthe B.S. degree in Electrical Engineeringfrom Robert College, Istanbul, Turkey in1951, and the M.S. degree in ElectricalEngineering from Cornell University,Ithaca, New York in 1953.From 1954 to 1962 he worked in Etibank,Turkey, which is a state-owned electricpower administration. As the systemsplanning chief engineer, his responsibilitiesincluded the system planning of the North-

West and West Anatolian Interconnected Power System.

F.rom 1963 to 1964 he worked in the Tennessee Valley Authority(TVA) Power Operations Department, Computer Research staff.Since 1964 he has been employed by Bechtel Power Corporation,Gaithersburg Power Division. His experience with Bechtel in dif-ferent capacities, including Senior Engineer, Engineerinig Supervisor,and Staff Assistant, encompasses broad aspects of electrical design ofnuclear and fossil plants.

He is a member of IEEE and a Registered Professional Engineer inMaryland, Florida and Georgia.

Name Quantity

Induced Draft FanBooster FanForced Oxidation CompressorForced Draft FanCirculating Water PumpCondensate Water PumpPrimary Air FanStartup Boiler Feed PumpBoiler Circulation PumpPulverizerCoal Reclaim ConveyorAir CompressorVacuum PumpScrubber PumpLoad Center

3 (2-50 percent, I spare)3 (2-50 percent, I spare)4 (3 running, I spare)3 (2-50 percent, spare)222I363332014

Hp/Each

40003500200020002500125020003500450500350450250750

(1000 kVA)

James P. Henschel was born in NiagaraFalls, New York on November 7, 195N1. Hereceived the BSEE and MSEE degreesfrom Clarkson College of Technology,

~;Potsdam, New York, in 1973 and 1976,respectively.

< He joined Bechtel Power Corporation,Gaithersburg Power Division in 1974 andhas worked in the Engineering Departmentprimarily in coal-fired power plant design.He is presently an engineering supervisorworking on fossil-fueled generating stationand coal conversion design.

He is a member of Eta Kappa Nu, Tau Beta Pi, and Phi Kappa Phi.