Factors in Excitation Systems of Large Central Station Steam Plants

Presented at the 36th Annual Conventionof the American Institute of Electrical Engi-neers, White Sulphur Springs, W. Va.,July 2, I920.

Copyright 1920. By A. I. E. E.

FACTORS IN EXCITATION SYSTEMS OFLARGE CENTRAL STATION STEAM PLANTS

BY J. W. PARKER AND A. A. MEYERBoth of .The Detroit Edison Co., Detroit, Mich.

THE purpose of this paper is: To point out the mostessential requirements of excitation schemes; tooutline two general methods followed in the de-

sign of such systems and from which a variety ofschemes are built up; and to discuss briefly the meritsas well as demerits of factors determining the successof various schemes.

A. GENERAL REQUIREMENTSIn the installation of a generating unit, there are

many component parts all of which must functionproperly for continuity of service. Of these, theexcitation system is a very vital one deserving carefulattention. Its rank of importance compared to othercomponent parts is not to be argued, because success-ful operation of a generator unit depends on the properfunctioning of all component parts. There seems tobe a wide diversity of opinion, however, regarding theequipment and its assembly as required for furnishingexcitation to the main generator units. Many of theschemes of course, are determined largely by local con-ditions and no attempt is made in this paper to arguefor standardization, nor for a most ideal scheme. Theintention is to point out some of the underlying prin-ciples affecting an excitation system and determiningcontinuity as well as quality of service of the maingenerating units.

Simplicity is the keynote to successful operation.Some designers in laying out an exciter system, and intheir anxiety to approach perfection, will considermany possibilities rather than the comparatively few

1563

1564 J. W. PARKER AND A. A. MEYER [July 2

probabilities which might interfere with continuousoperation of a generator unit. As a consequence, acomplication of safeguards are incorporated, some-times at great expense, and all for good purpose, butnot always with positive assurance that each willfunction as intended. Other engineers will care-fully weigh the probabilities against the possibilitiesand risk the chance of some possibilities not occurring,rather than introduce complications .equally apt tocause interference. It must not be forgotten thata multiplicity of safeguards multiplies the chance fortrouble.

Reliability of excitation is of course most desirable,but its relation to the reliability of other componentparts of the main generator unit must be carefully con-sidered. Too much effort is sometimes expended toobtain a very high degree of reliability in the excita-tion system alone without much consideration of thereliability of other essential component parts of the mainunit. The turbine and generator proper, as wellas the accessories such as the excitation system, thegovernor, the oil pump, and various other auxiliaries,are all subject to failures. Each is a link in a -chainwhose overall strength depends upon the weakest link.Reliability should be provided for proportionatelyamong the various links, rather than indiscriminatelyby biased attention to one component part or other,as e. g. the excitation system.

Of course, a high degree of reliability must be reason-ably provided for. This can be attained by selectingsimple equipment composed of few parts, each liberallydesigned mechanically as well as electrically. Furtherinsurance of reliability can be obtained by makingexcitation an independent system, free from ties toother service auxiliaries. Going a step further andsegregating the exciter equipment into separate inde-pendent units, each having equipment assembled whichserves only its respective main generator unit, wouldafford not only immunity from external disturbances,but would also isolate excitation troubles within oneunit.Working in opposition to simplicity and reliability

1920] J. W. PARKER AND A. A. MEYER 1565

are commonly found provisions for flexibility in anexcitation system. It is felt by some engineers thatgreat flexibility is required and as a result, all sorts ofswitching arrangements and reserve apparatus of extralarge capacity are provided for. It would be muchmore practicable to curtail flexibility in an excitationsystem to an absolute minimum in favor of simplicity.Adaptability of the exciter units to take energy fromseveral sources of power depends upon the generalscheme of excitation and will be referred to again below.

B. Two GENERAL SCHEMESOne very common practise has been to supply excita-

tion energy to the fields of all main generators from acommon exciter bus, the latter in turn served by severalseparately driven exciter generators operating inparallel. In some cases the exciters are motor-driven,in others turbine or engine-driven. In any case con-tinuity of supply and voltage regulation of greataccuracy are essential. If only motor-driven excitersare employed, taking their energy from the main sta-tion bus,--often the case in small stations-very badregulation may result from the cumulative effects ofsystem disturbances which might otherwise be quiteunimportant. This would be especially characteristicof a system employing exciters driven by inductionmotors. If the exciters are steam-driven the govern-ing characteristics of small turbines or engines mustbe contended with. A great many turbine-drivenexciter sets of approximately 200-kw. size, built in thelast six years, have performed verypoorly in this respect.The important part to be considered in regard to acommon exciter bus is, that the latter entails the main-tenance of an additional energy system, secondary tothe main energy system, but equally important andrequiring safeguards and careful attention to insurecontinuity of service and the required degree of accuracyof voltage regulation.Another method of supplying excitation, which is

growing in popularity and avoids some of the undesir-able features of the common bus system is the individ-ual exciter scheme. In this, individual exciters aredevoted solely to serving their respective main genera-


tor units. By this individual arrangement, simplifi-cation in many ways is easily attained. Moreover,disturbances of the system are limited to a singlegenerator unit instead of endangering operation of theentire plant, e.g. a ground coming on the field circuitof one unit would permit continued operation until theopportunity arrived for shutting down that particularunit. Whereas, if the fields of all generatorswereserved from a common exciter bus, it would not begood practise to continue operation of a unit with sucha fault, because the appearance of a second groundwould not only affect the one unit but endanger theoperation of every exciter and main generator in thestation. Another favorable feature of the individualscheme is seen inits adaptation torapidlyiuccreasingstation capacity. With increase in number of maingenerator units additional exciter equipment can beinstalled in like ratio and without disturbing existingsystems. In the common excitation bus scheme, theexciter capacity is sometimes necessarily out of pro-portion to the main generator capacity.Means of driving individual exciters is another

phase of the problem. Recourse to any driver com-monly employed with exciter units is permissible, butif motor drive is used, the same care must be taken tosecure independence of plant electrical disturbances.If steam drive is used, performance is still dependentupon the speed characteristics of some small primemover. Latterly, it has become rather commonpractise to supply the auxiliary power requirements oflarge power plants from several auxiliary turbines.Such turbines ensure continuous running of essentialmotor-driven plant auxiliaries, even in the event of amain a-c. system shut-down. A throwover connectionfrom this auxiliary power bus to the main station busgives additional reliability. It would seem goodpractise to connect motor-driven exciter sets to thisauxiliary power bus, but taking advantage of thesecurity thus afforded as far as continuity of drive isconcerned has two distinct disadvantages. A verymuch more accurate standard of speed and voltagecontrol would be required for the auxiliary energy


system and even more important, the exciters would besubject to serious disturbances incident to the operationof numerous pieces of apparatus throughout the plantwhich are connected to the auxiliary power bus.

Direct connection of exciter generators to the mainshafts of main turbines seems an excellently simplesolution. Attention of the turbine room operator toan additional machine is not required. The exciterbenefits by the good speed regulation of the mostaccurately governed prime mover in the plant, themain unit. The question of reliability of the primemover driving the exciter is automatically eliminated,since shutting down the main turbine simultaneouslyremoves the need for excitation of that unit.

There seems to be only one main objection bysome engineers to the direct-connected exciter, that theloss of the exciter entails losing from commissionthe corresponding main unit. However, with theexciter very liberally designed both mechanicallyand electrically, the chance for trouble with this unitmay be so minimized that sole dependence upon it isno more hazardous than a dozen other vital accessoriesof a main unit, to say nothing of the turbine andgenerator proper.

C. STANDBY EXCITATIONStandby equipment for every component part of a

main turbo-generator unit might appear desirable onfirst sight, but every engineer realizes its impractica-bility. Within practicable limitations standby can beprovided only for the more extensive components, suchas e.g. the excitation system. By standby, is meantthe substitution of excitation from another sourcefor the normal excitation of a generator unit.The kind of reserve and method of applying samedepend upon the general scheme of normal excitation.Assuming that the common excitation bus has enough

exciter capacity connected at all times to permit asudden shut-down of one unit, additional standby isfrequently provided by means of a storage battery.This is usually floated on the common bus and is in-tended to supply the bus automatically with sufficientcurrent for complete excitation of all main generator


units, at least for a reasonable period, in the eventof a partial or complete failure of the exciter system.Incidentally, it might be mentioned that a battery soconnected acts very efficaciously as a stabilizer forthe load fluctuations as well as the vagaries of the sev-eral exciters paralleled upon the bus, and thus affordsbetter voltage regulation of the exciter bus.

In the individual exciter scheme, provision for stand-by for normal excitation is not so clearly established.The reason for this is of course quite evident in that theexcitation system belonging to each main generatorunit is less extensive, more independent and less sub-ject to troubles. This is especially so with direct-connected exciters.An individual standby exciter for each main unit

would certainly be unnecessary. A common standbybus however, to which the field of any main generatorunit can be connected is more practicable and is verycommonly employed. The importance of the commonstandby bus is more or less a matter of opinion. Insome cases a special bus used exclusively for excitationis installed and connected to a floating battery. Thishardly seems warranted for an occasional duty. If acontinuous-current auxiliary-service bus is employedit is usually made reliable for various other reasonsand would answer all requirements for standby excita-tion. In some installations of direct-connected ex-citers no standby whatever is provided, acting on thetheory that although a chain is no stronger than itsweakest link, on the other hand it is no weaker by theaddition of another link built as strong or strongerthan the other links. Here the generator with itsauxiliaries is analogous to the chain and the separateexciter to the additional link. In at least one largeplant employing direct-connected exciters, five years'sucessful operation has proved that no undue riskshave been taken with direct-connected exciters.During this period, the plant increased from 40,000to 100,000 kw. and only one emergency shut-down ison record, chargeable to the exciter a preventableaccident caused by a foreign agent short-circuiting anunprotected brush holder.


Throwover to the standby bus is accomplishedusually manually, but sometimes automatically. Auto-matic throwover is somewhat questioned as to whetheror not it is practicable or even possible with some typesof turbo generators. The sensitiveness of the governorand steam valves and the synchronizing power of thegenerator when the load is suddenly dropped, as wellas the speed of the relay and of the closing field breakeronto the throwover bus are very big factors apt to fallshort sometimes. With a well and liberally designedexcitation system the occasions -for throwoverwould beso rare that the apparent gain by the automatic featuremight be offset by the added complications givingrise to trouble.

D. CONTROL OF EXCITATIONThe method of varying the exciting current to the

main generating field for voltage regulation dependsof course, upon the general scheme of exciter con-nections. With a common excitation bus, rheostatsmust be provided for both the exciter field and themain generating field circuit. The rheostat in theexciter field is usually small in range and used to main-tain a predetermined and constant exciter bus voltagefor good parallel operation of exciters. The rheostatin the main generating field has a wide range of smallsteps, and is employed for controlling the voltageregulation of the main generating unit. The exciterfield rheostat is adjusted sometimes to assist the mainfield rheostat in meeting the excitation requirements ofheavy load or other demands on the main generator,but demands within reasonable limitations are usuallymet by means of the main field rheostat alone, thussimplifying the routine of switchboard regulation.With individual excitation the exciter field rheostat

alone is usually employed to vary the main field current.The exciter field should be designed liberally and therheostat made capable of adjusting the-exciter terminalvoltage through a wide range in order to meet occasionslike the dropping of the exciter voltage due to abnormalslowing down of the main unit on account of heavyoverload or low steam pressure. With exciter fieldcontrol a comparatively small rheostat performs the


duty which otherwise would be required of a large andinefficient main field rheostat. But where standbyexcitation from a common bus is provided for it is quitecommon practise to install also a main field rheostatfor each unit, but which normally is inoperative.It is only in emergencies when normal excitation isshut off and excitation taken from a new source thatthis rheostat is used to regulate the main field current.It is evident from the foregoing that the main fieldrheostat is quite essential in a common bus excitationscheme, but may be eliminated in the individualexcitation scheme, except for its need as a reserve inconnection with some standby schemes. In theindividual exciter scheme, the exciter field rheostatonly is essential for normal operation.

In large central stations where the load fluctuations oc-cur at a comparatively slow rate, these rheostats areregulated very satisfactorily by the station operators.In some generating stations supplying energy to asystem or part of a system which fluctuates rapidly orto long transmission lines, automatic operation of fieldrheostats would perhaps be more desirable. Provi-sions are sometimes made to automatically com-pensate for line drop on long transmission lines throughexcitation of the main generators. Such cases, how-ever, are in the minority and are impracticable exceptwhere the load on the generating plant is of such pro-portions and of such nature as to permit operation ofthe main generator buses in separate and distinct units.Such splitting up of the main generator buses exceptin special cases is very undesirable from several otherconsiderations in plant operation. In most largecentral stations carrying a mixed load which is trans-mitted to substations from which it is distributed, theload fluctuations are comparatively slow and can bemet by manual control. The latter method also per-mits better flexibility in dividing the energy itself aswell as the reactive component among the variousmachines according to the best overall plant economyof the main units in service at any one time.High quality of regulation and good control in

excitation are obtained by means of exciters with


shunt fields and interpole windings, a type which hascome to be quite universally used. There is a tendencyto increase the exciter voltage from 125 volts forthe relatively smaller generator units, to 250 voltsfor the large units on account of the reduction in thesize of the exciter as well as in the leads from the.exciter to the main generator field. In a large unitthe current values, even at 250 volts are not to bedisregarded. The old objection to automatic trippingof the main generator field circuit seems to have beenwithdrawn in the present tendency to provide gener-ators with protective relay schemes which automat-ically cut off the field excitation on occasion of internaltrouble in the main generator circuit.

E. ECONOMYEnergy for excitation, though relatively of small

amount when compared with the main generator out-put, is nevertheless of considerable importance whenmeasured in absolute terms, and the economy of itsgeneration is accordingly deserving of some attention.Its importance, however, is only secondary to con-siderations of reliability and good regulation. Anarrangement devised to obtain the maximum of re-liability may, nevertheless, be likewise the mosteconomical, as will be pointed out.The steam-driven prime movers of the plant fall

into two classes those exhausting to condensersfrom which the latent heat of the exhaust steam iscarried away by the circulating water and wasted-and those units which exhaust to condensers utilizingthe exhaust heat. Obviously the energy that can bedeveloped advantageously by the latter class of steamunits is limited in plants not complicated by centralheating or process work, limited practically by theheat absorbing capacity of the feed water in being raisedfrom the temperature of main unit condensate to thetemperature desired for boiler feed. Every kilowatt-hour generated by the main unit, wasting its exhaustheat to the circulating water, costs the plant upwards of19,000 B.t.u. chargeable to such generation, whereasevery kilowatt-hour generated by prime movers of thesecond class, costs the system very little more than the


heat equivalent of the electrical energy generated.This is all very common knowledge, and very manyplants have availed themselves of this source of cheapenergy by employing small turbines and engines todrive plant auxiliary machinery. What seems not tohave been so clearly realized, however, is the factthat economy dictates the employment of the mostefficient auxiliary turbine available, in order to skim themaximum of cheap electrical energy from the live steamdevoted to feed water heating. Hence the employmentof the extraction type of main turbine from whichsteam is bled from an intermediate stage in quantitiessufficient for feed water heating. Here the earlystages of the main unit perform the part of an auxiliaryturbine. An alternative is the employment of severalauxiliary turbine generators supplying energy to anauxiliary power bus, as described above. The amountof energy developed should be controlled at all timesby the demand for exhaust steain to heat the feedwater. The load thus carried will bear little or norelationship, however, to the momentary power demandof the motor-driven auxiliary machinery. Arrange-ment should be made to deliver all excess energy notdemanded by plant auxiliaries to the main station bus.Similarly, the auxiliary bus should be able to deriveenergy from the system at certain periods of the dayor in case of shut-down of an auxiliary turbine. Theseauxiliary turbines can be made of sufficient size toattain very respectable economy, approaching closely,if not equalling that of the early stages of the mainunit.

Practically all station auxiliary machinery is thenmade motor-driven with consequent simplicity andflexibility of control. Every such piece of apparatusshares the reliability afforded by a twofold energysupply and, as already explained, excellent heateconomy has been attained. Further economy canbe gained by direct connection of auxiliary apparatusto either the main or auxiliary prime mover, and theconsequent elimination of intervening generator andmotor losses. This arrangement is hardly feasiblefor any machinery except exciter generators. The

1920] J. W. PARKER AND A. A. MEYER, 1573

exciter direct-connected to the shaft of the main tur-bine may, therefore, be employed under certain con-ditions, with maximum economy, besides affording themajor advantages of reliability and simplicity. Inplants which do not employ auxiliary turbine gener-ators supplying an auxiliary bus, on the other hand, itmay be necessary to make exciters turbine-driven inorder to utilize all of the available energy of the steamdevoted to feed water heating. The heat economyof such small exciter sets is ordinarily rather low,however, and some of the advantages of the othermethod are forfeited.

Documents

Factors in Excitation Systems of Large Central Station Steam Plants