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Field-Excitation-in-Relation-to-Machine-and-System-Operation_Farnham.pdf

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  continuous-currentrating increases the specific loading of the main current-carrying elementsdoes not decrease as one would expect, thinking in terms of the skin effect at higher current capacities. I am curious regarding this. Regarding the seconditem I mentioned, theuse of a stored-energy closing mechanism certainly answersthe problem of closing abreaker when the controlsource is taken from the primary circuit. The question then naturally arises: is this advantageworth the increased complexity and loss ofaccessibility wheich is thereby encoun- tered?Inplace 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 undoubtedly critical in their adjustmentsand in their relations to each other. Very little is said in this paper regardingthe overcurrent trip devices, other than that the direct-actingprinciple has been ex- tended to the full line up to and including the 4,000-ampere frame. By this, I assume that their present standard device hasbeen used. I hadhoped to read that they had provided a better means of adjustment for the long-time delay, and at least some adjustment for the short-time delay. My company has found that both theseadjust- ment features are most desirable from the user s standpoint, especially in selective tripping applications. The introduction of a completely new line of circuit breakers should certainly notcarry with it certain definite limitations of the superseded line.I find no reference in this paper to 5,000- and 6,000-ampere frame breakers, and I should like to ask what the General Electric Company intends to do in these sizes. Even though these sizes may not remain standard in the future, there will always be considerable application for them. There- fore, is it the intention ofthis company to extend their developments higher, or will they continue to offer their present equip- mentwhichwould now appear ratherobso- lete by comparisonwith their new line?L. H.Romzick(The Detroit Edison Com- pany, Detroit, Mich.): This new line of low-voltage aircircuit breakers appears to be an improvement in design of this manu- facturer s circuit breakers. However, the stored-energy closing mechanism seems to be more complex than a solenoid mecha- nism. I believe thata hazard exists to main- tenance personnel, inasmuch as the closing mechanism is energized when in theopen position. Although provision is made for blockingtheloaded springs, I believe that a means of automaticallyunloading the spring mechanism when the breaker is withdrawn from its cubicle or blocking so thatthe breakercannotbe withdrawn beyond the test position unless the springs have been released would provide sufficient safety. The overcurrent trip devices are satis- factory for general applications; however, for essential auxiliary motor feeds in power plants it would be desirable to have high- current instantaneous trip on three poles forfault protection, long inverse time delay on at least two poles for overload protection, and a long-time delay with a making con-tact for a remotealarm onone pole. This is a common practice forutility companies. (Author s closureappears on page 1354.) S.B. FARNHAM MEMBER AIEE U NDER normal conditions, anopera- tor can do only two things to a syn- chronousmachine to influence its be- haviorwith respect to the system to which itis connected, that is, he can ad- just the throttle valve or change the driven load, thereby changing the shaft torque; and he can turn the rheostat, and hence change the field current. Even in these operations, hedoes not have unlimited freedom of choice, for usually the torque can be in one direction only, depending on whether the machine happens to be a generator or a motor. If itis a synchronous condenser, he may not be able to doanything at all about shaft torque, since the shaft may be sealed up inside the housing where itis inaccessible. Happily,however, theoperator does have somewhat more freedom of action in what he does to the field current. It is the purpose ofthis discussion to indicate, qualitatively and quantitatively, the effects of the operator s manipulation of the field rheostat, bothon the individ- ual machineandon the system of which that machine is a part. As referred to here, field rheostat is used in its broadest sense. The discussion will beconcerned R. W. SWARTHOUT ASSOCIATE MEMBER AIEE not only with the conventional resistance box with handwheel or motor mechanism but equally with the automatic volt- age regulator, with which most important machines are today equipped andwhich can be adjusted by theoperator to hold a desired voltage level. The current interest in field excitation stems from the fact that many systems are now operating at power factors higherthan have previously been experienced. This is true for several reasons, among them being the application of capacitors in substantial quantities, the increasing use of underground cable, and the inter- connection of system to system, resulting in substantial new mileage of high-volt- age lines. Since all of these thingshelp supply ex citation to every machine on thesystem,thegeneralvoltage level tends to rise. As a result, it becomes necessary to re- duce the d-c field strengths of synchronous machines so as to holdan acceptable sys- tem voltage level. The operator, there-fore, logically wonders: what are the ef- fectsof operationwith weak field, andwhat are the limits to which this field weakening canreasonablybe carried? Also, there have been within the past few years several prominent cases of system shutdowns attributable to complete ac- cidental loss of field excitation, orto lack of means to increase the field strengthquickly after a system disturbance. With this background of interest, a fundamental understanding of machine capabilities will be helpful in answeringthe questions which arise. Kilowatts and Kilovars Just as there is a direct and well-under- stood relationship between shaft torque and kilowatts, so also is there an equally distinct relationship between field current and that other well-defined commodity, kilovars. Broadly speaking, all synchro- nous machines are capable of both produc- ing andconsuming each of these two separate and distinct kinds of commodity. By convention, kilowatts are considered positive when they flow from the machine out into the system. Hence, generator kilowatts are plus kilowatts, while motor kilowatts are minus kilowatts. Similarly, when the machine is overexcited, it gen- erates kilovars and delivers them to thesystem. By analogy to the positive direction of power flow,this is accepted Paper 53-387, recommendedby the  IEE System Engineering Committee and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Fall General Meeting, Kansas City, Mo., November 2-6, 1953. Manu- script submitted June 8, 1953; made availablefor printing August 6, 1953. S. B. FARNHAM and R. W.SWARTHOUT are with the General Electric Company, Schenectady, N. Y. Farnham, Swarthout-Field Excitation in Machine Operation Field Excitation in Relation to Macline and SystemOperation DECEMBER 1 953 1215  POSITIVE KILO VARS AREA OF MOTOR OPERATION, OVER- EXCITEDNEGATIVE __ KILOWATTS AREA OF MOTOR OPERATION, UNDER - EXCITED v NEGATIVE KILO VAR Sas also being the positive direction of kilovar flow. Negative kilovars, then, are those which flow from the system into the machine to maintain its magnetiza- tion when its own field is underexcited. Fig. 1 shows these concepts translatedinto diagramaticform, in which kilowatts and kilovars are plotted along co-ordinate axes, with the positive direction upward and toward the right, as is customary. As previouslyindicated, a synchronous machine, in the broadest sense, can be operated in any quadrant of Fig. 1. If driven by a prime mover, it operates as a generator somewhere in the positive kilo- watt area (right). If the rotation of its shaft is restrained by a driven load, we call it a motor, and it operates in the negative kilowatt area  left . Ifits field strength is more than enough to supply its own excitation requirements, the excess appears asa component of armature current representing kilovars exported to the system, and the machine operates in the positive kilovar area (above). If its field is underexcited, the deficiency must be made up by armature current, representing kilovars drawn from thesystem, and the machine operates in the negative kilovar area (below).   machine operating at unitypower factor is just self-sufficientin its excita- tion. It neither produces nor consumes system kilovars. Hence its operation is depicted along the horizontal axis of Fig. 1. When used as a synchronous condenser, on theother hand, the ma- chineoperates very nearly along the vertical axis (except for losses) since use- POSITIVE KILOVARS A (PER UNIT) 0.53 AREA OF GENERATOR OPERATION, OVER- EXCITED POSITIVE KI LOWATTS AREA OF GENERATOR OPERATION, UNDER-EXCITED NEGATIVE v KILOVARS 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, mostmachines are designed for one specific duty only, hence there are definite limitations to the area within which any given machine may beoperated successfully. Since this discussion is primarily about genera- tors, and if it is agreed to omit reference to those abnormal circumstancesunder which they may motor temporarily, we immediately cut the area inhalf. Fig. 2, then, is merely the right-hand or positive kilowatt side ofFig. 1. Generator Capabilities In Fig. 2, there is plotted at point A the name-plate ratedconditions for an assumed typical 0.83 power factor genera- tor. Rated kilovolt-amperes of the ma- chine are taken as 1.0 per unit on its own rated kva base. Hence, the rated condi- tions for machine operation, uponwhich its performance guarantees are based, are 0.85 per unit kilowatts and 0.33 perunit kilovars (cos 0=0.85)), (sin 6=0.53). Point A, however, is just one point in a rather extensive area; and few, if any, ma- chines are operated for any length of time at exactlytheconditionsstated onthe name plate. The question is then: what are the boundaries of the area on the kilowatt-kilovar diagram within which the machine may be operated? Armature Current Limit In addition to point A, itis usualthatgeneratorsbe suitable for delivering kilo- watts equal to rated kilovolt-amperes at unity power factor, corresponding to point B in Fig. 2. Thus,by drawing the X MACHINE RATING .A = 1.0 P. U. KVA /(i 0.85 PRF. 2°   I B POSITIVE o KILOWATTS 0.85 1.0 (PER UNIT) Fig. 2. Typicalgenerator capabilities as de- terminedfrom name-plate rating arc AB, having its center at 0, and radius equal to rated armature amperes, we have begun to outline an area of per- missible operation. The operator of the assumed typical machine, of course, does not hesitate to operate at reduced load anywhere within thesector OAB. He may also on occasionoperate at overload in theregion to the right of the arc AB, but inso doing he encroaches upon the margins which the designer provided to cover thevariablesthat mav occur in materials, workmanship, maintenance, or the demands of emergency loading, and over which he has no control. These mar- gins are essential both to the designer s own peace of mind and to the preserva- tion of the good name of the company he represents. It is outside the scope of this discussion to explore overload operation. Rather, it is our intent to define the entire area withinwhich operation withinrating is possible, so that full advantage of this flexibility may be taken in securing opti- mum over-all system operation. Hence the immediateproblem is to close off the openings at thetop and bottom of the now partially bounded area of Fig. 2. This canbe begunfrom information ob- tainable from the simple phasor diagram of the assumed typical machine, as shown in Fig. 3. Field Current Limit Starting with ratedterminal voltage Et and ratedarmature current Ia each equal to 1.0 per unit, and at an angle 0 with re- spect to one another (cos 0=0.85 in the 0F E,=.ORUXd   E   1.0 P. U. Fig. 3. Typical generator phasor diagram Farnham, Swarthout-Field Excitationin Machine Operation l --   DIECEMBER 1 953216  assumed typical case), we can lay off IaXd at right angles to Ia, where Xd is the synchronous reactance of our ma- chine. E. then is internalor generated voltagecorresponding to ratedterminal conditions. It would also be equal to the terminal voltage if full loadwere re- moved without making anychange infield current. Keeping the phasor triangle in mind, refer now to Fig. 4. Here Et, IaXd, and E0 are each divided by Xd. Also, the tri- angle is inverted and reoriented so that it falls on thekilowatt-kilovar co-ordinates, with the side representing IaXdlXd = Ia so placed as to form the radius OA of thepreviously determiined constant arma- ture current arc AB. The sideof the triangle representing Et/Xd= 1.0 Xd falls along thekilovar axis. Notehowever that, neglecting saturation, the quantity 1.  Xd is equal to the short-circuit ratio of the assumed typical machine.Hence,point C is established on thenegativekilovar axis, at a point corresponding to the short-circuit ratio. This value might typically be 0.80, corresponding to Xd = 1.25 for the assumedmachine (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 a ratio of voltage to reactance, it must repre-sent some kind of current; and since we are dealing in perunit quantities, there is no constant of proportionality with which to be concerned. E9 is generated voltage and is proportional to air gap flux, which is in turn proportional to field current, neglecting saturation. Thus E, Xd rep- resents per unit field current. Hence just as OA represents rated armature current, so CA represents rated full-load field current. Then with C as a center, and CA as a radius, the arc AD may be drawn representingthelocus of rated field current, thereby closing off the top of the area within which the machine may be operated.In any balanced design, point A, the point representing name-plate rating, is also thepoint at which the designer ar- ranged for the thermal limits of both field and armature to bereached together. Thus the output of the typical machine is limited by field heating from D to A,and by armatureheating from A to B. As an example of the usefulness of Fig. 4, it gives a ready answer to the fre- quently encountered question: what maximum kilovars can this machine generate at zero kilowatt load?In other words, what is its capability asa synchronous condenser? To answer this, it is onlynecessary to measure the ver- tical intercept OD, which represents the maximum permissible per unit kilovars to scale. Or, mathematically KILOVARS OD CD CO= CA  CO where, for the constants it is assumed NEGATIVE KI LOVARS Et I Xd Xd 0.8 SCR CA = V/(O.80   0.53)2+(0.85)2= 1.58 CO = 0.80 Hence OD= 1.58 = 0.80 = 0.78 Thismachine, then, can generate maxi- mum kilovars equal to 78 per cent of name-plateratedkilovolt-amperes. End RegionHeating Limit There remains now only thelowerpart of the area to bebounded.Since this is a region of low field current, being well below the limiting field current arc DA. it might be said offhand that the limit must therefore be armature current, and so the constant armature current arc AB couldbe extended all the way around to E. This would, however, be wrong for several reasons. For one, system stability wouldhave beencompletelyoverlooked.For another, localized heating in the machine would very likely become a problem, in the case of steamturbine generators. Now, as the boundary line is extended into the region below the horizontal axis, wecome to the one place where general- ized reasoning fails, and where specific knowledge of the individual machine is required, namely, in establishing that part of the operating limit resulting from localized heating in the machine iron. The reason for the problem is that all synchronous machineshave an armature reaction end leakage flux at both ends of 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 turbine nal voltage generator RFarnham, Swarthout-Field Excitation in Machine Operation Pn4,, riv;r 1217 ECEMBER 1 953  load current flowing in the stator conduc- tors. It revolves at synchronousspeed with respect to the stator, and hence is stationarv with respect to the rotor. It crosses from one side of the stator to anotherpoint on the stator 180 electrical degrees away. In so doing, it takes the low-reluctance path, which as shown in Fig. 5, representing this portion of a typical steam turbine generator, carries it through the stator core flange andend fingers, across the air gap into the rotor retaining ring, circumferentially around the retaining ring, and so on back across the air gap, fingers, and flange, to the stator core. While the main flux in the body of the stator is parallel to the lam- inations, it is to be noted that this end leakage flux enters and leaves the ends of the statorin a direction essentially per- pendicular to the laminations. Hence the effect of thelaminations in reducing eddy currents caused by the end leakage flux is minimized. To understand the significanceof this change influxdirec- tion with respect to the laminations, it must be appreciated that the core losses are typicaly something in the neighbor- hood of 100 times greater for perpendicu- lar flux than for flux parallel to the lam- inations. Hence, considerableadditional heat is generated; and since it is applied to only a relatively small volume of ma- terial, dangerouslyhigh temperatures may be produced within only a matter of minutes. Now, how is field current related tothis end leakage flux and its resultant heating? Simply in this way: Normal values offield current keep the retaining ring sat- urated, so thatonly a relatively small amount of armature end leakage fluxtraverses the path described. However, when the field excitation is reduced, corresponding to operation of the machine in the region of unity and leading power factor, then the retainingring is no longer saturated, and permits an increase in armature end leakage flux. As we have seen,this increased leakage flux produces heating in those areas of restricted ma- terial cross section, andwhere the flux direction is at right angles to theplane of the laminations. This plhenomenon hasbeen recog- nized since the middle 1920 s. Several different approaches have been used, either singly or in combination, to reduce the armature end leakage flux and the resultantheating. Among these are the use of nonmagnetic materials for the re- taining rings and parts of thestator end structure; changing the end structure sur- face configuration so that leakage flux is reduced and so thatthe remaining flux paths are not at right angles to the plane ofstator laminations; and also the use of magnetic shields to control the flux paths. The success of these methods is at- tested by the fact that modern generators may be operated successfully in the under- excited region down to a line such as HJ inFig. 4, where point H is at 60-per-cent rated kva at zero power factorleading, and point J is at rated kva, 0.95 power factor leading. These limits, however, may not apply to older machines; and it becomes neces- sary to investigate the capabilities of each such machine in question. Sometimes the manufacturer mav have test data onthat particular machine or onone of very similar design, from which he will be willing to give a reasonably close esti- mate of the expected capabilities. In other instances the users may have the choice either of accepting some rather conservativeestimate based on general knowledgeand experience, or of making an actual test onthe particular machine itself. If a test is to beundertaken, the manufac- turer will generally be willing to suggest the locations at whichthermocouples are most likely to reveal the limiting tempera- tures. Often, however, the operator already has a fairlyclear conception of the capa- bilities of his older machines, based on his operating temperature records,visual inspections, and maintenance experience over the years. Evidences of having reached the limitsof underexcited opera- tion may occasionally be found in the blueing of iron parts of the end structure, or the charring of insulation on thearma- ture bars where they emergefrom the core stacking. Having determined, by whatever means are appropriate to the particular ma- chine, the limits imposedbv end heating, these limits may be plotted, Fig. 4, to complete the boundary of the permissible operating area. For illustration,in Fig. 4 this is the line HJ which as has been indi- cated, is typical of a modern machine.For some older machines, thislinewill be displaced upward toward the hori- zontal axis, and in a few cases it may be found that point J will actuallv be above the axis, since the end heating limitations of somemachines may be such that they cannot beoperated near full load even at unity power factor. Fortunately, this restriction will apply to onlya few of the oldest steam turbinegenerators still re- maining in service. It is not a limitation to the operation of water-wheel genera- tors, because oftheir generally different construction. Stability Limitations It hasbeen hintedthat system stability might constitute a limitation. With respect to steady-state stability, it can be shown that if the machine in question is connected through negligibly small impedance into an infinitely large system, then the stability limit may be repre- sented by a horizontal straight line passing through the pointonthe negative vertical axis representing short-circuitratio. This line is CF in Fig. 4. If the machine is operated at any kilowatt and kilovar load- ing above this line, it will be stable. On theother hand, operationalongthe arc EK, even if it did notexceed any heating limitation, would be impossible, as the machine would not remain in synchro-nism with thesystem. Actually few, if any, machines operate through negligibly small impedance into a system so  stiff that it approaches the infinite. In most practical cases, the machine operates through impedance representing trans- formers, lines, and the paralleled value of the impedances of all theother machines on thesystem. This resultant impedance is typically about 0.20 to 0.40 per unit, based on the individual machine rating, although it is of course determined in any specific case by the system configuration and constants. At any rate, the effect ofthisexternal impedanceXe is to bend up- ward the straight line CF to some position such as CG. With terminal voltageheld constant at 1.0 per unit, it can be shown that CG is the arc of a circle whose center lies on the verticalaxis at a point SCR/2+ 1/2Xe above point C. Hence,with the aid of numbers already known or readilydeterminable, thepart of the boundary established by steady-state stability can be established. This limit is slightly conservative, in keepingwith its deter- mination by the commonly accepted prac- ticesof approximating saturation and neglecting saliency. Transient stability also is of course af- fected to some degree by the machine ex- citation as dictated by thekilowatt and kilovarload which it carries, as well as by its operating voltage. However, many other factors outside the scope of this discussion such as type and location of the fault, operating times of relays and clearing times of circuit breakers, sys- tem grounding, machine inertias, and automatic reclosing play a so much more dominant part in transient stability con- siderations that the effectof field excita- tion in thisregard is greatly overshadowed. It would appear to be a veryextreme case where system transient stability were critically dependent on field excitation. Farnham,Swarthout-Field Excitation in illachine Operation218 DE-CEMBER 195-3
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