continuous-current rating increases thespecific loading of the main current-carryingelements does not decrease as one wouldexpect, thinking in terms of the skin effectat higher current capacities. I am curiousregarding this.

Regarding the second item I mentioned,the use of a stored-energy closing mechanismcertainly answers the problem of closing abreaker when the control source is takenfrom the primary circuit. The questionthen naturally arises: is this advantageworth the increased complexity and loss ofaccessibility wheich is thereby encoun-tered? In place of two time-proven devices,namely, the solenoid and control relay, wenow find a motor and its control scheme,plus a 1,000-to-1 reduction unit, springs,mechanisms, cams, etc., all undoubtedlycritical in their adjustments and in theirrelations to each other.Very little is said in this paper regarding

the overcurrent trip devices, other than thatthe direct-acting principle has been ex-tended to the full line up to and includingthe 4,000-ampere frame. By this, I assumethat their present standard device has been

used. I had hoped to read that they hadprovided a better means of adjustment forthe long-time delay, and at least someadjustment for the short-time delay. Mycompany has found that both these adjust-ment features are most desirable from theuser's standpoint, especially in selectivetripping applications. The introduction ofa completely new line of circuit breakersshould certainly not carry with it certaindefinite limitations of the superseded line.

I find no reference in this paper to 5,000-and 6,000-ampere frame breakers, and Ishould like to ask what the General ElectricCompany intends to do in these sizes.Even though these sizes may not remainstandard in the future, there will always beconsiderable application for them. There-fore, is it the intention of this company toextend their developments higher, or willthey continue to offer their present equip-ment which would now appear rather obso-lete by comparison with their new line?

L. H. Romzick (The Detroit Edison Com-pany, Detroit, Mich.): This new line of

low-voltage air circuit breakers appears tobe an improvement in design of this manu-facturer's circuit breakers. However, thestored-energy closing mechanism seems tobe more complex than a solenoid mecha-nism. I believe that a hazard exists to main-tenance personnel, inasmuch as the closingmechanism is energized when in the openposition. Although provision is made forblocking the loaded springs, I believe that ameans of automatically unloading the springmechanism when the breaker is withdrawnfrom its cubicle or blocking so that thebreaker cannot be withdrawn beyond thetest position unless the springs have beenreleased would provide sufficient safety.The overcurrent trip devices are satis-

factory for general applications; however,for essential auxiliary motor feeds in powerplants it would be desirable to have high-current instantaneous trip on three poles forfault protection, long inverse time delay onat least two poles for overload protection,and a long-time delay with a making con-tact for a remote alarm on one pole. This isa common practice for utility companies.

(Author's closure appears on page 1354.)

S. B. FARNHAMMEMBER AIEE

U NDER normal conditions, an opera-tor can do only two things to a syn-

chronous machine to influence its be-havior with respect to the system towhich it is connected, that is, he can ad-just the throttle valve or change thedriven load, thereby changing the shafttorque; and he can turn the rheostat, andhence change the field current.Even in these operations, he does not

have unlimited freedom of choice, forusually the torque can be in one directiononly, depending on whether the machinehappens to be a generator or a motor. Ifit is a synchronous condenser, he may notbe able to do anything at all about shafttorque, since the shaft may be sealed upinside the housing where it is inaccessible.Happily, however, the operator does havesomewhat more freedom of action inwhat he does to the field current.

It is the purpose of this discussion toindicate, qualitatively and quantitatively,the effects of the operator's manipulationof the field rheostat, both on the individ-ual machine and on the system of whichthat machine is a part. As referred tohere, field rheostat is used in its broadestsense. The discussion will be concerned

R. W. SWARTHOUTASSOCIATE MEMBER AIEE

not only with the conventional resistancebox with handwheel or motor mechanismbut equally with the automatic volt-age regulator, with which most importantmachines are today equipped, and whichcan be adjusted by the operator to hold adesired voltage level.The current interest in field excitation

stems from the fact that many systemsare now operating at power factors higherthan have previously been experienced.This is true for several reasons, amongthem being the application of capacitorsin substantial quantities, the increasinguse of underground cable, and the inter-connection of system to system, resultingin substantial new mileage of high-volt-age lines.

Since all of these things help supply excitation to every machine on the system,the general voltage level tends to rise.As a result, it becomes necessary to re-duce the d-c field strengths of synchronousmachines so as to hold an acceptable sys-tem voltage level. The operator, there-fore, logically wonders: what are the ef-fects of operation with weak field, andwhat are the limits to which this fieldweakening can reasonably be carried?

Also, there have been within the past fewyears several prominent cases of systemshutdowns attributable to complete ac-cidental loss of field excitation, or to lackof means to increase the field strengthquickly after a system disturbance.With this background of interest, a

fundamental understanding of machinecapabilities will be helpful in answeringthe questions which arise.

Kilowatts and Kilovars

Just as there is a direct and well-under-stood relationship between shaft torqueand kilowatts, so also is there an equallydistinct relationship between field currentand that other well-defined commodity,kilovars. Broadly speaking, all synchro-nous machines are capable of both produc-ing and consuming each of these twoseparate and distinct kinds of commodity.By convention, kilowatts are considered

positive when they flow from the machineout into the system. Hence, generatorkilowatts are plus kilowatts, while motorkilowatts are minus kilowatts. Similarly,when the machine is overexcited, it gen-erates kilovars and delivers them to thesystem. By analogy to the positivedirection of power flow, this is accepted

Paper 53-387, recommended by the AIEE SystemEngineering Committee and approved by theAIEE Committee on Technical Operations forpresentation at the AIEE Fall General Meeting,Kansas City, Mo., November 2-6, 1953. Manu-script submitted June 8, 1953; made available forprinting August 6, 1953.

S. B. FARNHAM and R. W. SWARTHOUT are with theGeneral Electric Company, Schenectady, N. Y.

Farnham, Swarthout-Field Excitation in Machine Operation

Field Excitation in Relation to Maclineand System Operation

DECEMBER 1953 1215

POSITIVEKILO VARS

AREA OFMOTOR OPERATION,OVER- EXCITED

NEGATIVE _ _

KILOWATTS

AREA OFMOTOR OPERATION,UNDER - EXCITED

v

NEGATIVEKILOVAR S

as also being the positive direction ofkilovar flow. Negative kilovars, then,are those which flow from the system intothe machine to maintain its magnetiza-tion when its own field is underexcited.

Fig. 1 shows these concepts translatedinto diagramatic form, in which kilowattsand kilovars are plotted along co-ordinateaxes, with the positive direction upwardand toward the right, as is customary.As previously indicated, a synchronous

machine, in the broadest sense, can beoperated in any quadrant of Fig. 1. Ifdriven by a prime mover, it operates as agenerator somewhere in the positive kilo-watt area (right). If the rotation of itsshaft is restrained by a driven load, wecall it a motor, and it operates in thenegative kilowatt area (left). If itsfield strength is more than enough tosupply its own excitation requirements,the excess appears as a component ofarmature current representing kilovarsexported to the system, and the machineoperates in the positive kilovar area(above). If its field is underexcited, thedeficiency must be made up by armaturecurrent, representing kilovars drawn fromthe system, and the machine operates inthe negative kilovar area (below).A machine operating at unity power

factor is just self-sufficient in its excita-tion. It neither produces nor consumessystem kilovars. Hence its operation isdepicted along the horizontal axis ofFig. 1. When used as a synchronouscondenser, on the other hand, the ma-chine operates very nearly along thevertical axis (except for losses) since use-

POSITIVEKILOVARS A

(PER UNIT)0.53

AREA OFGENERATOR OPERATION,

OVER- EXCITED

POSITIVEKILOWATTS

AREA OFGENERATOR OPERATION,

UNDER-EXCITED

NEGATIVE vKILOVARS

Fig. i (left). Defini-tion areas of syn-chronous machine

operation

ful power is neither deliver to, nor re-moved from, its shaft.As a practical matter, most machines

are designed for one specific duty only,hence there are definite limitations to thearea within which any given machinemay be operated successfully. Sincethis discussion is primarily about genera-tors, and if it is agreed to omit referenceto those abnormal circumstances underwhich they may motor temporarily, weimmediately cut the area in half. Fig. 2,then, is merely the right-hand or positivekilowatt side of Fig. 1.

Generator Capabilities

In Fig. 2, there is plotted at point Athe name-plate rated conditions for anassumed typical 0.83 power factor genera-tor. Rated kilovolt-amperes of the ma-chine are taken as 1.0 per unit on its ownrated kva base. Hence, the rated condi-tions for machine operation, upon whichits performance guarantees are based, are0.85 per unit kilowatts and 0.33 per unitkilovars (cos 0=0.85)), (sin 6=0.53).Point A, however, is just one point in arather extensive area; and few, if any, ma-chines are operated for any length of timeat exactly the conditions stated on thename plate. The question is then: whatare the boundaries of the area on thekilowatt-kilovar diagram within whichthe machine may be operated?

Armature Current Limit

In addition to point A, it is usual thatgenerators be suitable for delivering kilo-watts equal to rated kilovolt-amperes atunity power factor, corresponding topoint B in Fig. 2. Thus, by drawing the

X MACHINE RATING.A = 1.0 P. U. KVA

/(i 0.85 PRF.

2° I I B POSITIVEBo* KILOWATTS0.85 1.0 (PER UNIT)

Fig. 2. Typical generator capabilities as de-termined from name-plate rating

arc AB, having its center at 0, and radiusequal to rated armature amperes, wehave begun to outline an area of per-missible operation. The operator of theassumed typical machine, of course, doesnot hesitate to operate at reduced loadanywhere within the sector OAB. Hemay also on occasion operate at overloadin the region to the right of the arc AB,but in so doing he encroaches upon themargins which the designer provided tocover the variables that mav occur inmaterials, workmanship, maintenance, orthe demands of emergency loading, andover which he has no control. These mar-gins are essential both to the designer'sown peace of mind and to the preserva-tion of the good name of the company herepresents. It is outside the scope of thisdiscussion to explore overload operation.Rather, it is our intent to define the entirearea within which operation within ratingis possible, so that full advantage of thisflexibility may be taken in securing opti-mum over-all system operation. Hencethe immediate problem is to close off theopenings at the top and bottom of thenow partially bounded area of Fig. 2.This can be begun from information ob-tainable from the simple phasor diagramof the assumed typical machine, as shownin Fig. 3.

Field Current Limit

Starting with rated terminal voltage Etand rated armature current Ia, each equalto 1.0 per unit, and at an angle 0 with re-spect to one another (cos 0=0.85 in the

0F E,=.ORUXd

0 4 EE, 1.0 P. U.

Fig. 3. Typical generator phasor diagram

Farnham, Swarthout-Field Excitation in Machine Operation

l

-- I

DIECEMBER 19531216

assumed typical case), we can lay offIaXd at right angles to Ia, where Xd isthe synchronous reactance of our ma-chine. E. then is internal or generatedvoltage corresponding to rated terminalconditions. It would also be equal tothe terminal voltage if full load were re-moved without making any change infield current.

Keeping the phasor triangle in mind,refer now to Fig. 4. Here Et, IaXd, andE0 are each divided by Xd. Also, the tri-angle is inverted and reoriented so that itfalls on the kilowatt-kilovar co-ordinates,with the side representing IaXdlXd =Ia so placed as to form the radius OA ofthe previously determiined constant arma-ture current arc AB.The side of the triangle representing

Et/Xd= 1.0 Xd falls along the kilovaraxis. Note however that, neglectingsaturation, the quantity 1.(Xd is equalto the short-circuit ratio of the assumedtypical machine. Hence, point C isestablished on the negative kilovar axis, ata point corresponding to the short-circuitratio. This value might typically be0.80, corresponding to Xd = 1.25 for theassumed machine (1/1.25=0.80).

This leaves only the third side, EglXd,of the phasor diagram triangle to be ac-counted for. Dimensionally, since it is aratio of voltage to reactance, it must repre-

sent some kind of current; and since weare dealing in per unit quantities, there isno constant of proportionality with whichto be concerned. E9 is generated voltageand is proportional to air gap flux, whichis in turn proportional to field current,neglecting saturation. Thus E, Xd rep-resents per unit field current. Hence,just as OA represents rated armaturecurrent, so CA represents rated full-loadfield current. Then with C as a center,and CA as a radius, the arc AD may bedrawn representing the locus of rated fieldcurrent, thereby closing off the top of thearea within which the machine may beoperated.

In any balanced design, point A, thepoint representing name-plate rating, isalso the point at which the designer ar-ranged for the thermal limits of both fieldand armature to be reached together.Thus the output of the typical machine islimited by field heating from D to A, andby armature heating from A to B.As an example of the usefulness of Fig.

4, it gives a ready answer to the fre-quently encountered question: whatmaximum kilovars can this machinegenerate at zero kilowatt load? Inother words, what is its capability as asynchronous condenser? To answer this,it is only necessary to measure the ver-tical intercept OD, which represents themaximum permissible per unit kilovars toscale. Or, mathematically

KILOVARSOD= CD= CO= CA =CO

where, for the constants it is assumed

NEGATIVEKI LOVARS

Et IXd Xd

0.8SCR

CA = V/(O.80+0.53)2+(0.85)2= 1.58

CO = 0.80

Hence

OD= 1.58 = 0.80 = 0.78

This machine, then, can generate maxi-mum kilovars equal to 78 per cent ofname-plate rated kilovolt-amperes.

End Region Heating Limit

There remains now only the lower partof the area to be bounded. Since this isa region of low field current, being wellbelow the limiting field current arc DA.it might be said offhand that the limitmust therefore be armature current, andso the constant armature current arc ABcould be extended all the way around to E.This would, however, be wrong for severalreasons. For one, system stability wouldhave been completely overlooked. Foranother, localized heating in the machinewould very likely become a problem, inthe case of steam turbine generators.Now, as the boundary line is extended

into the region below the horizontal axis,we come to the one place where general-ized reasoning fails, and where specificknowledge of the individual machine isrequired, namely, in establishing thatpart of the operating limit resulting fromlocalized heating in the machine iron.The reason for the problem is that all

synchronous machines have an armaturereaction end leakage flux at both endsof the stator. This flux is produced by

Fig. 4. Composite capability limits of typical generator at rated termi- Fig. 5. Sectional view of end-region construction of a modern turbinenal voltage generator

RFarnham, Swarthout-Field Excitation in Machine Operation

Pn4,, riv;r

1217DECEMBER 1953

load current flowing in the stator conduc-tors. It revolves at synchronous speedwith respect to the stator, and hence isstationarv with respect to the rotor. Itcrosses from one side of the stator toanother point on the stator 180 electricaldegrees away. In so doing, it takes thelow-reluctance path, which as shown inFig. 5, representing this portion of atypical steam turbine generator, carriesit through the stator core flange and endfingers, across the air gap into the rotorretaining ring, circumferentially aroundthe retaining ring, and so on back acrossthe air gap, fingers, and flange, to thestator core. While the main flux in thebody of the stator is parallel to the lam-inations, it is to be noted that this endleakage flux enters and leaves the ends ofthe stator in a direction essentially per-pendicular to the laminations. Hencethe effect of the laminations in reducingeddy currents caused by the end leakageflux is minimized. To understand thesignificance of this change in flux direc-tion with respect to the laminations, itmust be appreciated that the core lossesare typicaly something in the neighbor-hood of 100 times greater for perpendicu-lar flux than for flux parallel to the lam-inations. Hence, considerable additionalheat is generated; and since it is appliedto only a relatively small volume of ma-terial, dangerously high temperatures maybe produced within only a matter ofminutes.Now, how is field current related to this

end leakage flux and its resultant heating?Simply in this way: Normal values offield current keep the retaining ring sat-urated, so that only a relatively smallamount of armature end leakage fluxtraverses the path described. However,when the field excitation is reduced,corresponding to operation of the machinein the region of unity and leading powerfactor, then the retaining ring is no longersaturated, and permits an increase inarmature end leakage flux. As we haveseen, this increased leakage flux producesheating in those areas of restricted ma-terial cross section, and where the fluxdirection is at right angles to the plane ofthe laminations.

This plhenomenon has been recog-nized since the middle 1920's. Severaldifferent approaches have been used,either singly or in combination, to reducethe armature end leakage flux and theresultant heating. Among these are theuse of nonmagnetic materials for the re-taining rings and parts of the stator endstructure; changing the end structure sur-face configuration so that leakage flux isreduced and so that the remaining flux

paths are not at right angles to the planeof stator laminations; and also the useof magnetic shields to control the fluxpaths.The success of these methods is at-

tested by the fact that modern generatorsmay be operated successfully in the under-excited region down to a line such as HJin Fig. 4, where point H is at 60-per-centrated kva at zero power factor leading,and point J is at rated kva, 0.95 powerfactor leading.These limits, however, may not apply

to older machines; and it becomes neces-sary to investigate the capabilities of eachsuch machine in question. Sometimesthe manufacturer mav have test data onthat particular machine or on one of verysimilar design, from which he will bewilling to give a reasonably close esti-mate of the expected capabilities. Inother instances the users may have thechoice either of accepting some ratherconservative estimate based on generalknowledge and experience, or of making anactual test onthe particular machine itself.If a test is to be undertaken, the manufac-turer will generally be willing to suggestthe locations at which thermocouples aremost likely to reveal the limiting tempera-tures.

Often, however, the operator alreadyhas a fairly clear conception of the capa-bilities of his older machines, based onhis operating temperature records, visualinspections, and maintenance experienceover the years. Evidences of havingreached the limits of underexcited opera-tion may occasionally be found in theblueing of iron parts of the end structure,or the charring of insulation on the arma-ture bars where they emerge from thecore stacking.Having determined, by whatever means

are appropriate to the particular ma-chine, the limits imposed bv end heating,these limits may be plotted, Fig. 4, tocomplete the boundary of the permissibleoperating area. For illustration, in Fig. 4this is the line HJ which as has been indi-cated, is typical of a modern machine.For some older machines, this line willbe displaced upward toward the hori-zontal axis, and in a few cases it may befound that point J will actuallv be abovethe axis, since the end heating limitationsof some machines may be such that theycan not be operated near full load evenat unity power factor. Fortunately, thisrestriction will apply to only a few of theoldest steam turbine generators still re-maining in service. It is not a limitationto the operation of water-wheel genera-tors, because of their generally differentconstruction.

Stability Limitations

It has been hinted that system stabilitymight constitute a limitation. Withrespect to steady-state stability, it canbe shown that if the machine in questionis connected through negligibly smallimpedance into an infinitely large system,then the stability limit may be repre-sented by a horizontal straight line passingthrough the point on the negative verticalaxis representing short-circuitratio. Thisline is CF in Fig. 4. If the machine isoperated at any kilowatt and kilovar load-ing above this line, it will be stable. Onthe other hand, operation along the arcEK, even if it did not exceed any heatinglimitation, would be impossible, as themachine would not remain in synchro-nism with the system. Actually few, ifany, machines operate through negligiblysmall impedance into a system so "stiff"that it approaches the infinite. In mostpractical cases, the machine operatesthrough impedance representing trans-formers, lines, and the paralleled value ofthe impedances of all the other machineson the system. This resultant impedanceis typically about 0.20 to 0.40 per unit,based on the individual machine rating,although it is of course determined in anyspecific case by the system configurationand constants. At any rate, the effect ofthis external impedance Xe is to bend up-ward the straight line CF to some positionsuch as CG. With terminal voltage heldconstant at 1.0 per unit, it can be shownthat CG is the arc of a circle whose centerlies on the vertical axis at a point SCR/2+1/2Xe above point C. Hence, with theaid of numbers already known or readilydeterminable, the part of the boundaryestablished by steady-state stability canbe established. This limit is slightlyconservative, in keeping with its deter-mination by the commonly accepted prac-tices of approximating saturation andneglecting saliency.

Transient stability also is of course af-fected to some degree by the machine ex-citation as dictated by the kilowatt andkilovar load which it carries, as well asby its operating voltage. However,many other factors outside the scope ofthis discussion such as type and locationof the fault, operating times of relays andclearing times of circuit breakers, sys-tem grounding, machine inertias, andautomatic reclosing play a so much moredominant part in transient stability con-siderations that the effect of field excita-tion in thisregard is greatly overshadowed.It would appear to be a very extreme casewhere system transient stability werecritically dependent on field excitation.

Farnham, Swarthout-Field Excitation in illachine Operation1218 DE-CEMBER 195-3

It is reasonable to conclude, therefore,that in most practical cases transientstability will not become a barrier to suc-cessful underexcited operation if it hasnot previously been an obstacle whileoperating in the overexcited region.To summarize, the limits of generator

capability in each of the several potentialareas of operation, as illustrated in Fig. 4are

DA-field heatingA J-armature heatingJH-armature end region heating

In addition, there may also be a steady-state stability limit CG but as has beenseen, this would not constitute an operat-ing limit unless the machine under con-sideration were connected to the systemthrough a tie so weak that the curve CGbent upward to intersect HJ.

Automatic Voltage Regulators

As a basis for appreciating the role ofthe automatic voltage regulator in permit-ting the widest possible flexibility of ma-chine operation within the boundariesestablished, it may be in order first toconsider the case of a machine operatingwithout a voltage regulator. Assume thatthis machine is carrying a kilowatt andkilovar load as represented by point Pin Fig. 6. On this figure will immediatelybe recognized the composite boundary ofpermissible machine operation DAJH as

previously determined, for rated ter-minal voltage. Also shown is the steady-state stability limit CG, for this machineand connected system.

It will be apparent from our earlierderivation that the field current of thismachine is represented by CP. (Remem-ber that CA is proportional to rated fieldcurrent. Hence CP/CA is simply theproportion of name-plate rated field cur-rent; and this is just the amount neces-sary to permit the machine to handle thekilowatts and kilovars represented bypoint P.)Now, suppose this machine is called

upon to pick up load. This might well bein consequence of the system normal dailyload pattern, or perhaps because someother machine on the system has had todrop load. At any rate, let us assumethat the load increase on the machineoccurs before the operator notices theresulting drop in terminal voltage, andhence that he makes no change in the fieldrheostat setting. On the co-ordinatediagram of Fig 6, this situation, namelyincreasing kilowatts with constant fieldexcitation, is depicted as a movement ofthe operating point from P along thecircular arc PK. This path runs almostdirectly into the end region heating limitHJ and the system steady-state stabilitylimit CG. Whether or not one or both ofthese limits will be exceeded depends ofcourse on the amount of the kilowatt loadincrease. The final operating point lies

at the intersection of the circular arcwith a vertical line, such as XY, repre-senting the new kilowatt load, and ofcourse the terminal voltage is substan-tially below normal.

Several conditions immediately be-come apparent from inspection of Fig. 6:

1. Load increase without correspondingadjustment of field current pushes the ma-chine operating power factor toward theleading (underexcited) region.2. The more lagging (overexcited) theinitial load power factor, the greater thetotal load the machine can carry beforerunning into one of the limits.3. If adequate field strength could alwaysbe assured, steady-state stability would, forall practical purposes, never be a problem,because the limit is greater than the capa-bility of the typical prime mover.

A somewhat broader picture of thisvariation of steady-state stability limitwith initial operating power factor isshown in Fig. 7, based on typical con-stants. It will be apparent from thisfigure why some operators (who perhapsexperienced a shutdown resulting frominstability while operating in the leadingor near unity power factor region) havea phobia against approaching the unitypower factor line. Actually, however, asFig. 7 shows, there is no precipitousbreaking point at unity power factor.The criterion, therefore, should not bewhat the power factor is, but rather whatthe maximum available synchronizingpower is, relative to the maximum load

zN

Xz

zr

c Z

3M

ow

_ INITIAL LOAD:1.0 RPU. KW

INITIAL LOAD=0.5 P U. KW

0.8F

0.6 i _ Xe = 0.4Xd = 1.25SCR =0.8

0.4 F

0.2 _

l 01 I I0.80 0.85 0.90 0.95 1.0 0.95 0.90 0.85 0.80

LEADING LAGGING(UNDER-EXCITED) (OVER-EXCITED)

POWER FACTOR OF INITIAL LOAD

Fig. 7. Ultimate maximum synchronizing power versus power fac-tor of initial load

Farnham, Swarthout-Field Excitation in Machine Operation

POSITIVEKILOVARS

ARMATURE-CURRENTLIMIT

Xe=0.4-(WITHOUT REGULATOR)

POSITIVE-1 KILOWATTS

(PER UNIT)

,G - Xe=0.4(WITH REGULATOR)

Fig. 6. Generator capability limits asaffected by automatic voltage regulatorKILOVARS

I1.2-

DECEMBER 1953 1219

POWER FAMACHINE

z

w 1.0a.

I

w3 0.90a-0

zRi 0.8z0cr

0z(I)

x

1.0 P.F

INITIAL LOAD - 0.8Xd = 1.25

0.7 _-X -0 4Xe =0.4, ' SCR = 0.8

0.1_

0.90 0.95 1.00 1.05 1.10GENERATOR TERMINAL VOLTAGE - PE

demand that may be made on the par-ticular machine in question. On thisbasis, any machine under hand controlcan have its loading so scheduled as toinsure stable operation under whateveremergency load condition may be im-posed upon it.

WVith an automatic voltage regulator,however, any practical concern oversteady-state stability can be dismissed,for the regulator will hold the machineexcitation at a level corresponding tosynchronizing power in excess of the maxi-mum capability of the prime mover.

Lower Excitation Limit

Notwithstanding the practical assur-ance against loss of steady-state stabilityprovided by the automatic voltage regula-tor holding normal terminal voltage, theremay be instances where an operator, inhis efforts to reduce system voltage dur-ing light load, may lower the setting of theregulator voltage adjusting rheostat sothat the machine excitation and terminalvoltage are reduced substantially belownormal. This action on the part of theoperator may reintroduce a steady-statestability problem for, as shown in Fig. 8,the limits, which may be ample at ratedterminal voltage, become materially re-duced at lower values of terminal voltage.

Recognizing this possibility, some mod-ern voltage regulators include a lowerexcitation limit which automaticallycomes into play below a predeterminedlevel of excitation to prevent loss of svn-chronism. It may also be used advan-tageously to prevent operation where endregion heating might become a problem.The region within which this lower limitis typically set to come into operation isshown by the shaded area of Fig. 6.

NCTOR a Fig. 8. Ultimate maximum syn-TERM. chronizing power versus generator

terminal voltage for assumed initialF. LAG toad of 0.8 per unitEXC.)

F. LEADt-EXC.)

PU KW

R UNIT

Actually the characteristic of this deviceis a straight line, such as JUN, and maybe located anywhere within the area byindependent adjustments of its slope andpoint of intersection with the verticalaxis. Hence this characteristic can beset so that the lower excitation limit willcome into operation just short of eitherthe end-heating limit or the steady-statestability limit, as determined for theparticular machine and associated system.

This obviously is not a fixed minimumfield current limit, but rather it varies theminimum allowable excitation automat-ically with the load on the machine, asmeasured through instrument transform-ers connected to the machine terminals.Thereby it permits the maximum ofoperating flexibility, right up to the limitsimposed by the machine and system.This lower excitation limit is a standardfeature of all amplidyne-type automaticvoltage regulators, but is not adaptable torheostatic-type regulators.

Loss of Excitation Relay

The extreme case of underexcitation isof course, complete loss of excitation, asmay happen, for example, when a fieldlead is broken or when someone inadvert-ently trips the field breaker. Loss ofsynchronism with the system is, under thiscondition, a foregone conclusion, althoughthe machine may continue to producekilowatts as an induction generator. Theamount of kilowatts thus generated willdepend both on the initial setting anddroop characteristics of the speed gover-nor, and on whether the excitation fail-ure left the field circuit completely open,or closed (perhaps through a field dis-charge resistor). In anv event, however,it becomes desirable to remove the ma-

chine immediately from the system, fortwo reasons: 1. The machine may bedamaged through heating caused by largeeddy currents flowing in the surface of therotor. 2. The system may not be able towithstand successfully the large kilovarload suddently imposed on it. Systemvoltage will drop, and other machinesmay lose synchronism.With respect to the first possibility,

namely, rotor damage, it is true that ma-chines have been operated for extendedperiods without apparent damage; andsome power companies feel that an opera-tor can correctly diagnose the symptomsin time either to restore the excitation(perhaps by switching over to a spareexciter) or to trip the machine off thesystem manually. On the other hand,generators have had to be repaired be-cause of damage resulting from this sortof procedure. Regardless, however, ofwhether the machine suffers obviousdamage or not, there is the very realpossibility of reduction of field-windinginsulation life or other incipient damagewhich may be impossible to evaluate eventhrough careful examination, and the ma-chine will certainly be out of service whilethe examination is being made. In con-trast, if an automatic protective relay isemployed to trip the main breaker beforethe machine begins to slip poles, thereneed be no delay in returning the ma-chine to service immediately after thecause of lost excitation is corrected, andwithout the necessity for examining andtesting the rotor insulation.The amount of the increased kilovar

load imposed on the system as a conse-quence of generator excitation failurewill vary, depending on the machineconstants and the rate at which it is slip-ping poles, from something about equalto its name-plate kilovolt-ampere ratingto as much as four times this rating.Especially if, previous to the disturbance,the machine was overexcited, that is,delivering kilovars to the system, the netchange so far as the system is concernedmay be very serious indeed. The newestand largest generator on a well-plannedsystem usually represents the largestblock of power that can be lost to thesystem without causing undue hardship.Often overlooked, however, is the factthat the larger kilovar load resulting fromloss of excitation on such a machine maybe more of a shock to the system thanthe loss of its kilowatt output. If othermachines on the system are equipped withautomatic voltage regulators, and if theresulting demand on them is not exces-sive, they may be able to supply the neces-sary additional kilovars and so help main-

Farnham, Swarthout-Field Excitation in Machine Operation

I I

1220 DECEMBER 1 953

tain system voltage. If, however, theseother machines are inadequate to thedemand, or if their excitation controls donot provide immediate response to theneed, system voltage levels will fall, syn-chronizing power among them will bereduced, and instability may occur. Ifthis happens, a major system shutdownis probable.

While automatic voltage regulatorsshould always be regarded as the firstline of defense, backup protection againstthis difficulty can be provided by a suita-ble loss-of-excitation relay on each largeor otherwise important machine on thesystem. Like the lower excitation limiton the regulator, this relay responds tocurrent and voltage at the a-c terminalsof the machine, rather than to some arbi-trary minimum value of field current orvoltage alone. Hence it is immune tofalse operation when the field strength ispurposely reduced, as during times oflight load. Its application is clean-cut,

DiscussionW. R. Brownlee (Southern Services, Inc.,Birmingham, Ala.): This comprehensivesummary of various factors influencing theoperating limitations on the excitation ofgenerators is most timely because of twoimportant trends. One is the continuingupsurge of unswitched capacitor installa-tions, and the other is the use of largerconductors on and the more economicalloading of transmission lines. These fac-tors require greater control of kilovar sup-

plies and generator voltages.One caution is in order in the use of the

curves and equations with respect to theeffective external reactance. For example,consider a generating plant with three 60-megawatt units, connected to each otherand to the transmission system through a

high-voltage bus. If the short-circuit contri-bution of the transmission system, togetherwith that of generators no. 2 and 3, istaken in determining the excitation limit ofgenerator no. 1, then the only criterion de-rived might be the ability of this generatorto remain at parallel with the other two.For stability with the power system, theexternal reactance is a function of the short-circuit contribution from the transmissionlines only, and is applied on a base of allthree generators rather than just one.

While the general tone of the paper is-clear and simple, a serious defection appearsin the section entitled "Kilowatts andKilovars," namely the expression "negativekilovars." Usage of this type encouragessuch dangerous equivocations as "leadingreactive," "leading or lagging current,"and "impressed voltage reference or inducedvoltage reference." In view of the actionof the AIEE Standards Committee in1946,1 it is urged that such expressions beavoided. Actually, there should be no more

temptation to say "negative kilovars" thanto say "negative kilowatts." The latter

and it is fail-safe in the event of opencircuit in its supply leads.

Conclusions

Modern system operation is increasinglyin the region of unity and leading powerfactor. Generators may safely be oper-ated in this region, up to limits whichusually are readily determinable. Suchoperation generally is not a determiningfactor in system transient stability, al-though steady-state stability may be-come a problem if machine kilowatt load-ings are increased from light-load opera-tion without corresponding increases inexcitation. Automatic voltage regula-tors are very desirable, since they effec-tively eliminate this problem; and if of atype incorporating a lower excitationlimit, they provide a means for auto-matically avoiding the regions where endheating and stability become limitations.A protective relay, arranged to isolate the

would lead to a description of a steam-elec-tric generating station as a "disposal" fornegative kilowatts and a means for supply-ing negative coal to the mines.

REFERENCE1. THE SIGN OF REACTIVE POWER, AIEE Commit-tee Report. Electrical Engineering, vol. 65, Nov.1946, pp. 512-16.

John F. Watson (Union Electric Companyof Missouri, St. Louis, Mo.): The authorshave done a commendable job, first intheir presentation of a method of determin-ing the capabilities of a synchronousgenerator for a complete range of powerfactors, and second, in viewing these limitsfrom a standpoint of stability in order toreach some important conclusions regardingmachine and system operation.

In the development of the capabilitycurves, the assumption is made that ar-mature and field thermal limitations arereached at the point of name-plate rating.Despite the fact that this assumption isquite commonly made, there may be somequestion as to whether this actually occurs.Perhaps some elaboration on this pointwould be helpful.

In regard to the method of rating a gener-ator in the underexcited region of operation,the authors suggest specific limitations formodern machines but go on to say that testdata, operating experience, etc., furnish theonly definite means of rating a certainmachine. While this is true, it does imposea limitation on the usefulness of the ratingmethod because very often it is the absenceof specific data and experience that makesome method of rating imperative. It maybe that no better way of establishing limitsis possible, but if a method existed wherebylimits could be determined from the morereadily available machine data and curves,the usefulness of the whole scheme would begreatly enhanced.

machine from the system in the event offailure of its excitation, is desirable, bothas primary protection against damage tothe machine itself and as backup protec-tion against serious disturbance to theinterconnected system.

References

1. UNDEREXCITED OPERATION OF TURBOGENERA-TORS, C. G. Adams, J. B. McClure. AIEE Trants-actions, vol. 67, pt. I, 1948, pp. 521-28.

2. UNDEREXCITED OPERATION OF LARGE TURBINEGENERATORS ON PACIFIC GAS AND ELECTRICCOMPANY'S SYSTEM, V. F. Estcourt, C. H. Holley,W. R. Johnson, P. H. Light. AIEE Transactions,vol. 72, pt. III, Feb. 1953, pp. 16-22.

3. POWER SYSTEM STABILITY, VOLUMES I AND II(book), S. B. Crary. John Wiley and Sons, Inc.,New York, N. Y., 1947.

4. EFFECT OF A MODERN AMPLIDYNE VOLTAGEREGULATOR ON UNDEREXCITED OPERATION OFLARGE TURBINE GENERATORS, W. G. Heffron,R. A. Phillips. AIEE Transactions, vol. 71, pt. III,Aug. 1952, pp. 692-97; discussion pp. 1134-35.

5. SYSTEM STABI ITY LIMITATIONS AND GEN-ERATOR LOADING, H. C. Anderson, H. 0. Simmons,Jr., C. A. Woodrow. AIEE Transactions, vol. 72,pt. III, June 1953, pp. 406-23.

In the initial months of plant operation,operators at the Meramec Plant of the UnionElectric System noticed certain oscillatoryphenomena which instigated a thoroughstudy of machine stability at high and un-derexcited power factors. The results ofcalculations and a network analyzer studyagree very well with conclusions presentedby the authors regarding transient stability,steady-state stability, and the advantagesof an automatic voltage regulator for main-taining stability under conditions of rapidload change. However, the results of thisstudy show an important disadvantage ofthe automatic voltage regulator which is notmentioned by the authors.Machine steady-state stability depends on

external impedance as well as loading andload changes. If the impedance betweenthe generator and the "infinite" system isincreased suddenly as a result, for example,of losing a transmission line, the machinewould be forced to deliver the same amountof power to the constant voltage systemthrough an increased external impedance.Under certain conditions this can result in areduction of excitation and a consequent re-duction in stability margin.

This is not as remote a possibility as mightfirst be assumed, for it is rather common fora generator or a plant to be connected intoa large system by two or three lines withmachine and external impedances, as in theMeramec case, to the typical values used bythe authors in their work. It is not sug-gested that this particular detrimental ef-fect of the regulator is of sufficient impor-tance to be comparable to the many advan-tages of automatic voltage regulation, but itis believed to be of general interest andpossibly worthy of mention and perhapsfurther investigation.

H. C. Anderson (General Electric Company,Schenectady, N. Y.): The authors are to becongratulated on a paper that explains, in

Farnham, Swarthout Field Excitation in Machine Operation1 1221DECEMBER 1953

3 PHASE,6-CYCLE FAULTet

INFINITE BUSXeI OR Xe2 e2=SYSTEM VOLTAGE

U)

0

I

±4-

0-J

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4KW

1.6

Fig. 9. Transient stability limits for (A)0.20/0.40 system and (B) 0.40/0.60 system

simple terms, the factors which affect thelower limit on field current.

In their paper a statement concerning thetransient stability limit is made. "It isreasonable to conclude, therefore, that inmost practical cases transient stability willnot become a barrier to successful under-excited operation, if it has not previouslybeen an obstacle while operating in the over-

excited region." This statement is correctif the day-to-day generator voltage is notallowed to vary appreciably. In otherwords, if the reactive oultput of a given plantis decreased by raising system voltage, as

opposed to lowering generator terminal volt-age, the transient stability margin is notreduced.

However, in the day-to-day operation of a

given plant, generator reactive output isusually varied considerably to meet systemrequirements. If system voltage is heldnearly constant, and generator voltage islowered to produce a reduction in reactiveoutput, there will be an appreciable decreasein transient stability limit. It is the pur-

pose of this discussion to show how reactiveoutput, and hence field excitation (withsystem voltage constant), affects the tran-sient stability limit.

In a so-called "low-reactance" system,such as the examples Mr. Farnham and Mr.Swarthout have used, where reactance ex-

ternal to the generator does not exceed 40per cent (with maximum turbine capa-

bility as a kilovolt-ampere base), the gen-

erator may be operated over its entire capa-

bility range without fear of its becomingunstable. There are other factors, as

pointed out in the paper, which will limitits range of operation. However, if thesame generator is operated on a higher re-

actance system, say 50 per cent to 60 per

cent, it is safe to assume that transient sta-bility will be a factor determining part of itsoperating range.

Fig. 9 shows operating limits of a gen-erator when it is connected to either of twodifferent systems. The equivalent system

voltage in both cases is that correspondingto initial operation of the generator at maxi-mum turbine capability with unit terminalvoltage at 0.95 power factor overexcited.In both systems (0.20 initial reactance and0.40 initial reactance) the equivalent systemvoltage is 0.95.

Curve A in Fig. 9 is the limit of opera-tion, based on transient stability con-siderations, of a generator when it is con-nected to a system whose reactance is 20per cent initially and 40 per cent after aline or other facility is switched out on theoccurrence of a 3-phase fault, 6 cycles induration. Note that the limit of stable oper-ation for this case is well outside the limitsoutlined in the paper.

Curve B is the limit of stable operationfor the same generator operating on a systemwhose initial reactance is 40 per cent andwhose reactance after switching out a lineor other facility is 60 per cent. Notethat in both cases the transient power limitdecreases quite rapidly as the kilovar outputis decreased by lowering the field excita-tion, when system voltage is held constant.However, in the low-reactance system, thedecrease in power limit is of no conse-quence since maximum turbine capabilitylimits the amount of power which can betransmitted.

In the higher reactance system, curve B,operation of the generator is somewhat re-stricted, as long as system voltage is notallowed to increase. The generator maybe operated at maximum turbine capa-bility in the overexcited region at power fac-tors up to about 98 per cent (correspondingto about 20 per cent kilovar output) withoutfear of transient instability. If it becomesnecessary to operate at still lower values ofkilovar output, the power must be reducedas the kilovars are reduced until at unitypower factor the machine may be operatedat only about 90 per cent of its maximumturbine capability. If underexcited opera-tion is necessary, the power must be reducedstill further. The foregoing explanationapplies when the equivalent system voltageis held constant.

If the system is called on to supply more ofits own kilovar requirements, either bychanging transformer taps or by actually

1.2

Fig. 10. Gener-ator terminal vol-tage as systemreactance is in-creased tosteady-state stabilitylimit, maintainingconstant field

excitation

CL

H

zwa-w(9

I0

-4z

wi-0H

ErwzwLI)

1.0

Q8

raising system voltage, and generator volt-age is maintained nearly constant, thestability margins are increased. As amatter of fact, if equivalent system react-ance is tmly high, on the order of 70 percent to 80 per cent, the generator can supplyvery little kilovar to the system, if stableoperation is to be maintained at maximumturbine capability.

S. B. Farnham and R. W. Swarthout: Theauthors are appreciative of the interestshown by all those who presented discus-sions, both written and oral.Mr. Anderson has pointed out the reduc-

tion in system transient stability that re-sults, at lower than normal voltage, whenthe external system reactance is abruptlyincreased. He has thus extended the scopeof the paper, which is predicated on the con-dition of rated machine terminal voltage,except for Fig. 8 wherein is shown the typicalvariation in steady-state stability limit withchanges in terminal voltage. To an oper-ator faced with the practical necessity formaintaining lower than normal voltagelevels, transient stability may indeed be-come a limiting factor, particularly if thereactance between the machine and the sys-tem can be substantially increased, as, forexample, by the faulting and subsequentisolation of the shorter of two lines whichnormally connect the machine to the system.Mr. Watson describes another manifesta-

tion of this 2-tie-line problem as it pertainsto steady-state stability. Fig. 10 illustrateshis statement: "An automatic voltageregulator would, by attempting to maintaina constant generator terminal voltage, de-crease the stability limit of the machine."Here, generator terminal voltage is plottedas a function of the increase in system re-actance when one of two parallel lines isopened. It is to be noted that, dependingon the degree to which the machine is ini-tially operating overexcited (generatingkilovars), there is actually a rise in the volt-age seen by a regulator at the machineterminals. Hence, the regulator neces-sarily acts in the direction to reduce thestability limit. However, each curve ofFig. 10 is terminated at the value of react-

Et-1=.0 1.0 1.0 1.

PF= 1.0 0.95 0.90 0.wut-I ,, OVER EXCITED

0.6 _

0.4 V

1.0 1.0 1.

.0 INITIAL

.85 OPERATINGCCONDITIONS

1.0

/ Xd \ INFINITE) --Fs7 SYSTEM

Et= 1.0 Xe= 0.20.2 _

0.l 0.3 0.4 0.5 0.6 0.7PER UNIT Xe WITH ONE LINE OUT

0.8

Farnham, Swarthout -Field Excitation in Machine Operation

K.W.=I.V

2

1222 DE-CEMBE-R 1953

ance representing the stability limit for itsassumed initial operating conditions; andit is to be noted that in all cases the terminalvoltage has fallen below normal before thispoint is reached. Hence, it can be con-cluded that, while the regulator may act inthe direction to impair stability in thoseregions where stability is not a problem,nevertheless it does come into play in thedesirable direction before a critical stabilitysituation is reached.

In response to the questions raised byMr. Watson, it is the usual objective of abalanced machine design to have all partsof that machine meet their thermal require-ments at name-plate rating. In this way,the most economical design is achieved.In the case of modern machines, it is a rea-sonable assumption that the designer hassufficient knowledge to accomplish thisobjective. AWTith an extremely old machine,

however, particularly if it has been rebuiltand rerated, it is possible that the fieldand armature windings may not both attainthe same temperature rise at name-platerating, and specific knowledge of the par-ticular machine may therefore be desirableif precise operating limits are to be drawn.The published percentage increases in rat-ings of modern hydrogen-cooled machinespermit the same relative increases in loadingof both field and armature, again on thepremise that the designer has the knowledgeto make equally effective application of thecooling medium to both the field and arma-ture windings.The statement in the paper to the effect

that test data, operating experience, etc.,provide the only means for determining theoperating capabilities in the underexcitedregion applies only to machines built priorto the mid-1920's. For more modern ma-

chines, the limits are definitely set forth inthe paper. Hence, it appears that thealleged limitation to the usefulness of thegeneralized capability data as outlined inthe paper would pertain to only a relativelyfew of the less important machines remain-ing in service today.Mr. Brownlee's dissent with the expres-

sion "negative kilovars" is well worthy ofnote. There is only one kind of kilovar.While, for purpose of recording data orindicating direction on co-ordinate diagrams(as in Figs. 1, 2, 4, and 6) the terms "posi-tive" and "negative" or their counterpartplus and minus signs are very convenient,and are widely used, such use must be under-stood to pertain to direction of flow only.It must not be permitted to lead to a con-cept that there are two kinds of kilovars,any more than there are two kinds of kilo-watts.

THE USE of power-line carrier as apilot channel for high-speed relaying

is an accepted practice for transmission-line protection. However, there has beena lack of concrete information on the per-formance and reliability of carrier-relay-ing channels. In addition, there is con-siderable variation among power com-panies as to the testing and maintenanceof the various components of a carrierinstallation. Recognizing these condi-tions, the AIEE Carrier Committeeseveral years ago established a projectsubcommittee to obtain data on the ex-perience and reliability of carrier-relayingchannels.A detailed questionnaire was drawn up

to obtain information which would servetwo purposes: first, to help users of car-rier-relaying equipment in determiningreasonable maintenance intervals andprocedure; and second, to guide manu-

Paper 53-377, recommended by the AIEE CarrierCurrent Committee and approved by the AIEECommittee on Technical Operations for presenta-tion at the AIEE Fall General Meeting, KansasCity, Mo., November 2-6, 1953. Manuscriptsubmitted March 13, 1953; made available forprinting July 29, 1953.

The personnel of the Project Subcommittee are:H. W. Lensner, Chairman; T. A. Cramer, G. M.Babcock, J. R. Curtin, W. J. Googe, V. J. Hayes,L. E. Ludekens, M. Warren, J. Youngblood, 0. A.Starcke, and B. W. Storer.

The committee wishes to express their appreciationof the co-operation given by the contributingpower companies for the data in this report.

facturers in producing more trouble-freeequipment. The tabulated results of thequestionnaires will also be of benefit tothe user of carrier relaying in indicatingwhat degree of reliability he can expectfrom his equipment.With these objectives in mind, the

following questionnaire on carrier relay-ing was sent out. It is made up of twoparts. Part I covers circuit informationand performance data, and each ques-tion requires one answer per line sectionprotected by carrier relaying. Part IIcovers maintenance on the carrier com-ponents, and the questions in this partrequire only one answer from each powercompany.

Summary of Questionnaire

PART I CIRCUIT INFORMATION ANDPERFORMANCE DATA

1. Length of circuit in miles?2. Nominal line voltage rating in kilovolts?

3. Number of carrier-relaying terminals onthis line?4. Carrier frequency or frequencies?5. A. Transmitter output watts for re-laying?

B. Transmitter output watts for voiceor telemetering?6. A. Nominal operating range of relay-ing transmitter-receiver in decibels?

B. Nominal operating range of voicetransmitter-receiver in decibels?7. Type of relaying system (directional orphase comparison, transfer trip)?8. Is this relay carrier channel used forother services?9. What other carrier channels are on thiscircuit?10. Years of service of the carrier-relayingchannel?11. Total outage time of carrier relaychannel for routine maintenance?

12. Total outage time of carrier relaychannel from terminal equipment defects?

13. Number of outages of carrier relaychannel caused by carrier equipment de-fects?14. Number of outages of carrier relaychannel caused by relaying equipment de-fects?15. Number of correct operations?16. Number of incorrect operations?

PART II-MAINTENANCE17. How often is the carrier transmissiontested? A. Manual. B. Automatic.18. Is a reserve signed test (or sleet test)made, and if so, at what intervals?

19. What is the inspection schedule onrelays? Describe routine very briefly.A. Inspection. B. Calibration and opera-tion check.20. What is the inspection schedule on thecarrier equipment? Describe briefly. A.Transmitter-receiver. B. Coupling capaci-tor and potential device. C. Line trap.

21. How often are the vacuum tubeschecked?22. What tests are made to determine thata tube has reached the end of its useful life?23. What are the reasons for removingtubes from service? State results in per-centage.24. WVhat is the average tube life? List bytypes.

Reliability of Carrier-Relaying Channels

Experience and Reliability of Carrier-Relaying ChannelsAIEE COMMITTEE REPORT

1223DECE-MBIER 1953