THE DESIGN OF LARGE VERTICAL-SHAFT WATER-TURBINE-DRIVENA.C. GENERATORS

By E. M. JOHNSON, M.Sc.Tech., Member, and C. P. HOLDER, B.A., Associate Member.(The paper was first received 10th July, and in revised form 9th December, 1947. It was read before THE INSTITUTION \9th February, the SOUTHMIDLAND CENTRE 5th April, the NORTH-WESTERN CENTRE 6th April, the NORTH-EASTERN CENTRE 8th November, and the RUGBY SUB-CENTRE

30th November, 1948.)

SUMMARYThe paper deals with vertical-shaft a.c. generators of large diameter'

in the speed range 50-500 r.p.m. The influence of turbine andhydraulic characteristics is first considered. This is followed by adiscussion of factors which determine the generator dimensions. Abrief summary of the considerations affecting rotor design leads to adetailed review of different types of rotor and pole construction andtheir range of application.

An important feature of this type of machine is the thrust bearingand its support, which carry very heavy loads. Alternative types ofbearing and bearing arrangement are therefore examined and theirmerits compared. Methods of bearing cooling are briefly con-sidered.

In a Section on the special arrangement of air coolers required forthis class of generator, reference is made to the use of generatorventilating air for station heating. This is followed by a Section onthe arrangement and operation of brakes and jacks.

The stator frame, core and windings; the rotor windings; theexciters and governor generator are briefly considered, and the paperconcludes with a short review of special electrical characteristics.

output and low head, therefore, operate at very low speeds. Inconsequence, generators driven by such turbines are of largedimensions, and many problems in the design and manufactureof these generators arise solely from their physical size. Table 1gives the approximate overall diameters of some recent examplesof the type of machine with which the paper is concerned.

Table 1

TYPICAL LARGE VERTICAL WATER-TURBINE-DRIVEN GENERATORS

(1) INTRODUCTIONThe development of inland waterways and of large-scale land

irrigation and reclamation schemes in combination with thesupply of electrical energy has increased greatly in recent years.The power plants are usually of the low-head type with low-speed vertical-shaft generating sets, many of them of very largedimensions. The generators of such sets differ profoundly fromhorizontal-shaft generators of similar output, and their designinvolves many special problems. Some of these problems occuralso in the design of high-speed vertical-shaft generators.

Descriptions of individual machines of this general type haveappeared in the technical Press, but, so far as the authors areaware, there does not exist anywhere a critical examination ofthe basic problems that arise. The paper discusses these prob-lems and indicates various alternative solutions. These maydiffer widely according to circumstances. The problems aremainly mechanical; electrical features will be dealt with onlywhere they are peculiar to this type of generator.

Limitations of space make it impossible to deal with the cal-culation of mechanical stresses in the discussion of alternativetypes of construction. In consequence of this, no attempt hasbeen made to cite permissible limits of stress, which often dependas much on the method of calculation as on the materialsemployed.

(2) CHARACTERISTICS OF WATER-TURBINE DRIVES ANDTHEIR EFFECT ON GENERATOR DESIGN

(2.1) Effect of Output and Head on Permissible SpeedThe principal factors that determine the choice of speed for a

water turbine are the available head and the output required.For a given output the speed falls with the head, and for a givenhead the speed falls with increasing output. Turbines of large

Output, kVA

100 00090 00075 00082 50070 00052 50048 00033 33314 000

Speed, r.p.m.

15083-3

128-618015042875

166-746-9

Approximateoverall

diameter, ft

324138313016402937

Country ofmanufacture

GermanyU.S.A. 'CanadaU.S.A.FranceSwitzerlandU.S.A.EnglandSweden

(U.D.C.621.313.322-82)Messrs. Johnson and Holder are with the Metropolitan-Vickers Electrical Co., Ltd.

The overall height of such machines from coupling face to thetopmost point, including exciter, will be 25-35 ft, with a further10 ft in the case of generators driven by Kaplan turbines.

The two factors of output and head determine not only thespeed, but also the type of turbine. Since, however, the typeof turbine has only a secondary effect on the generator design, itwill be mentioned only where its effect on a particular part isimportant.

(2.2) Variation of Speed with LoadThe governing of water turbines presents a serious problem

because of the great inertia of the moving mass of water; a toorapid change in the rate of water flow into a turbine would causesurges and possibly dangerous variations in pressure in thepenstock. The adjustment of the water flow following a suddenchange in the load must, therefore, be relatively gradual, and,in the interval of time during which this adjustment is takingplace, the turbine speed may rise or fail considerably beforebeing finally restored to normal. This so-called "momentary"speed variation will depend on the moment of inertia of therotating masses, and can by the provision of sufficient inertiabe limited to any desired degree. In turbines driving a.c.generators the momentary rise in speed on the loss of full loadmay be required to be between 10 and 25%, although a rise ashigh as 32% may sometimes be permitted. The flywheel effectneeded for this purpose is roughly proportional to the h.p.output, and inversely proportional to the square of the speed,so that in large low-speed units the total flywheel effect requiredbecomes very great, and it is usually more economical to embodyit in the generator rotor than in a separate flywheel (see Section3.1). As a result, generators driven by water turbines have oftento be built with very high flywheel effects.

VOL. 95, PART II. [757] 48

758 JOHNSON AND HOLDER: THE DESIGN OF LARGE

There is a further consideration, almost more important in itseffect, and arising from the same causes. If a sudden changefrom full load to no-load were, by misadventure, accompaniedby a governor failure, the speed would rise to approximatelytwice normal. It is not practicable to check this by means of anemergency governor, and the machines must therefore be capableof running safely, for a short time, at this overspeed. WithKaplan turbines the requirement is still more severe, because,theoretically at least, a speed approaching three times normalmay be attained.

(23) Use of Vertical ShaftsPerhaps the most important advantage of the vertical-shaft

design is that it permits the generator to be placed above, andthe turbine to be placed below, the maximum tail-water level.Flooding of the generator is thereby avoided without sacrificeof head.

Other factors that influence the choice between horizontal-and vertical-shaft generating units are relative cost of plant,relative cost of foundations and station building, available space,and requirements of station layout. Such questions can besettled only with reference to particular circumstances, and areoutside the scope of the paper. In practice, large electricalunits of medium and slow speed are now most frequently of thevertical-shaft type.

In the vertical-shaft design the weight of the rotating parts ofthe generator and turbine act in the same direction as the axialhydraulic thrust. This necessitates a thrust bearing capable ofcarrying a relatively very heavy load and of operating at themaximum runaway speed of the turbine. The preferred positionfor this bearing is immediately above or below the generatorrotor, and the usual modern practice is to make it part of thegenerator. In consequence, the design of the thrust bearing andits support is an important part of vertical-shaft generator design.The type of problem is indicated by the loads and dimensions inTable 2.

Table 2TYPICAL THRUST BEARINGS FOR LARGE VERTICAL-SHAFT

GENERATORS

Load,tons

13401 130

750140

Outsidediameter, in

1129682-548

Speed,r.p.m.

75120167428

Normalperipheral

speed, ft/sec

36-750-2600900

Country ofmanufacture

U.S.A.U.S.A.EnglandSwitzerland

In the vertical-shaft design the various parts of the machineare placed one above the other, and the handling of these partsfor inspection and repair needs special attention. Indeed, thearrangement of the whole machine may be decided by thisconsideration.

(2.4) Effect of Turbine Efficiency on Generator designAt a constant speed and constant head, which is the normal

operating condition of hydro-electric units, the efficiency ofreaction turbines falls rapidly as the load is reduced. With suchtypes of turbine it is therefore an advantage to operate as nearfull load as possible. This is usually achieved by dividing thetotal output of a station between a number of generating units inparallel, so that variations in load can be met by varying thenumber of units in service, while keeping those that are in serviceat or near their full-load output. Seasonal variations in head

can be provided for in the same way. This solution of the prob-lem of low efficiency at low loads has little effect on generatordesign. An alternative solution, which has a direct effect, isthe use, where the effective head permits, of Kaplan turbines inwhich the angle of the runner blades is adjusted automatically bythe governor, so as to maintain a high efficiency over a wide rangeof load and head. This characteristic makes it economical touse fewer and larger generating units, and has led to an increasein the size of large vertical-shaft water-turbine-driven generatorsin recent years.

(2.5) Summary of Special Requirements resulting from theTurbine Characteristics

It may be convenient at this point, before proceeding toconsider the generator design in detail, to summarize briefly theprincipal water-turbine characteristics that affect it. Thesecharacteristics may be expressed in terms of their consequentialeffects on the mechanical design of the vertical-shaft generator,thus:

(a) The generator will almost invariably be of the salient-poletype, and may be of low speed and very large diameter.

(b) The rotating parts will usually be required to embody aflywheel effect greater than would otherwise be normal in orderto ensure satisfactory governing.

(c) The rotor must be capable of withstanding the stressesproduced by running at the maximum overspeed of the turbine.

(d) The lowest critical speed of the rotating system should beabove the maximum turbine overspeed.

(e) A thrust bearing must be provided to take the weight ofthe generator rotor and turbine runner and the hydraulic thrust.

(3) DESIGN OF GENERATORS TO MEET THE REQUIREMENTSOF THE DRIVE AND OF THE LOAD

(3.1) Considerations that affect the Physical Sizeof Generators

The two factors that principally determine the cylindricalvolume of a large vertical-shaft a.c. generator are the powerrating and the speed. Electrical characteristics such as reactance,short-circuit ratio and permissible temperature rise, affect thedimensions to a very limited extent, because in a machine of largediameter and slow speed the amount of electromagnetic materialmay be varied within wide limits with relatively little change inthe diameter.

The problem that faces the generator designer is to proportionthe cylindrical volume by choosing the diameter and axiallength most suited to the particular case. Maximum efficiencyis obtained by making the diameter as small as possible and theaxial length correspondingly great. The axial length that canbe permitted is limited, however, by two factors: the criticalspeed of the rotor, and the effect of increasing the core length onthe ventilation of the machine.

The question of critical speed arises only in very high speedmachines. The factor that limits the core length in the largemajority of medium- and low-speed generators is ventilation.General practice sets the limit at about three times the pole pitch,although in high-speed or forced-ventilated machines ratios ashigh as 5:1 are successfully used. In low-speed machines,particularly if more than normal flywheel effect is necessary andthe machine diameter is increased for that purpose, a ratio ofcore length to pole pitch as low as unity may result.

As already explained, the flywheel effect of the rotating parts isdetermined by the requirements of satisfactory governing. Inmany high-speed machines, where the diameter is limited bystress, a rotor of the minimum diameter is used and any extra

VERTICAL-SHAFT WATER-TURBINE-DRIVEN A.C. GENERATORS 759

flywheel effect is provided by a separate flywheel. In the majorityof large low-speed generators, however, it is more economicalto embody the whole of the flywheel effect in the generator rotor,even if this means an increase in the diameter.

The foregoing considerations permit of a simple and con-venient generalization relating to the flywheel effect and diameterof machines of the type under review. It has been shown thatthe dimensions are largely independent of the required electricalcharacteristics. We may therefore assume a mean value forthe output coefficient. Similarly an approximation can be madeto the minimum flywheel effect for satisfactory governing. Wewill assume as a basis 50-c/s generators rated at 0-8 p.f., anddesigned to be stable under line-charging conditions whendeveloping 50 % of full output at normal rated voltage. Theoutput coefficient of such machines may be taken as

= 15 000

and minimum flywheel effect = 11 x 106 x — lb-ft2

n2

where d — Stator bore, in./ = Stator core length, in.n — Speed, r.p.m.P = Rated output, kW.

A chart (Fig. 1) can then be prepared which permits theapproximate diameter of a generator to be found for any given

The frame diameter given on the chart is that taken over thestator yoke, but not including coolers or air casing.

An example will illustrate the use of the chart and will alsoindicate the way in which the proportions of a machine may varywith the required flywheel effect.

A 50-c/s generator rated at 37 5OOkVA at 0-8 p.f. and167 r.p.m. is to have a flywheel effect of 22 000 000 lb-ft*. Sincethe number of poles is 36, the value of (poles)* x kVA X 10~«= 49. The ordinate at this value gives the desirable minimumflywheel effect as 12-5 x 1061b-ft2, and the frame diameternecessary for the output as 382 in for a machine with a ratiolength/pole-pitch = 1 0 . A horizontal line drawn from thepoint of intersection of the chosen ordinate with the curve offrame diameter across to the 36-pole curve and projected up-wards to the scale of flywheel effect, gives a value of34-5 x 106Ib-ft2, showing that:

(a) The stipulated flywheel effect (22 0 x 106) is considerablymore than the desirable minimum (12-5 X 106).

(ff The flywheel effect (34-5 x 106) o f a machine of ratiolength/pole-pitch = 1 is greater than that stipulated (22 • 0 x 106).

An intermediate value of length/pole-pitch is therefore re-quired. Let us assume 1-35; a correction factor of 1 0 9obtained from the inset curve gives a new frame diameter of382/1 09 = 350 in. For this frame diameter and 36 poles thechart gives a flywheel effect of 22-5 x 106, which is practicallythe stipulated figure. The generator will thus have an overalldiameter of 350 in, or say 29 ft.

,80Wk\ lb-ft2x 106 (upper curves)60 50 40 30 20

Curve of overall dia.plotted for length/pole-pitch-1. for othervalues of length/pole-pitch, divideD by /Treadfrom curve W ?above

Figures adjacent to thecurves are numbers of

poles

Curves drawn for length/pole-pitch -1For other values of lergth/pole-pitch

Wk2 varies in proportion

150, 80 120 A 160(Poles)2xkVAxKf6Wk\ lb-ft2x 10° (lower curves)

Fig. 1.—Chart for estimating approximate overall diameter of generator for given output, speed, and moment of inertia.

operating conditions. It will be noted that the curves are drawnfor generators having a ratio length/pole-pitch = 1. A correc-tion curve is given for other ratios, but experience alone canpredict the ratio to be used in particular cases. As a guide,however, it may be taken that where the flywheel effect requiredto meet the governing conditions is far above the minimum, theratio length/pole-pitch will be approximately unity, whilst formachines with small flywheel effect, the ratio will approach 3 0.

(3.2) Rotor Design(3.2.1) General.

The rotors' of water-turbine-driven a.c. generators are almostinvariably of the salient-pole type. Superficially therefore, theyresemble the rotors of engine-driven a.c. generators. Butwhereas the latter mostly have bolted-on poles, this method ofconstruction is usually impracticable in the case of generatorsdriven by water turbines, because of the large centrifugal forces

760 JOHNSON AND HOLDER: THE DESIGN OF LARGE

that would occur at the runaway speed of the turbine. Withlaminated poles some form of dovetailed or equivalent construc-tion is generally necessary, but an alternative for high-speedgenerators of small and medium diameter is to use solid poles,cast or forged integral with the rotor body.

The question of pole design is linked closely with the designof the rotor body, which in its turn is governed by the flywheeleffect to be achieved. Rotors will therefore be divided into twobroad classes, namely those having laminated poles and thosehaving solid poles, and each class will be dealt with separately.The design of field windings and damper windings will be con-sidered in later Sections of the paper.

(3.2.2) Laminated-Pole Rotor.Separate considerations govern the design of the poles, and of

the rim or wheel which carries them. Poles will therefore bedealt with in one Section, and wheels and rims, under the generaltitle "Rotor Body," in another.

(3.2.2.1) Laminated Poles.Laminated poles are used on all medium- and low-speed

generators of the type under consideration. In machines with aperipheral speed not exceeding about 5 000 ft/min the poles maybe bolted to the wheel rim. This construction is, however,limited to machines of small flywheel effect or to those withseparate flywheel, since otherwise the depth of rim would involvebolts inconveniently long. For speeds higher than the above,the pole is usually secured in the rotor by dovetail or equivalentprojections on the pole. Typical constructions in common useare shown in Fig. 2. In Fig. 2{a) the pole is pressed into the

Fig. 2.—Typical dovetailed and T-headed poles.

recess in the rotor with a slight interference fit. This is thestrongest form of dovetailed pole construction and is particularlyvaluable for machines of high speed and long core length. Analternative is shown in Fig. 2(b); the necessary tightness betweenthe pole and wheel is secured by means of taper keys driven infrom opposite ends. The construction most favoured to-day forall but the most highly stressed rotors is illustrated in Fig. 2(c).It is simpler to produce than its equivalent in Figs. 2(a) and 2(b)and it is simpler to erect and dismantle. One further advantage,which the construction of Fig. 2(c) has in common with that ofFig. 2(b), is that the slot in the rotor need not have a smoothfinish. The importance of this point is indicated in Section 3.2.2.2.

A limitation of the construction shown in Fig. 2{c) becomesapparent from a comparison with Fig. 2(ti). The centrifugal forceof the poles is carried on the neck between adjacent pole dove-tails, and the stress in this neck is greater with the T-shapeddovetail. However, by the use of multiple T-heads, as shown inFig. 2{d), the stress in the neck can be reduced, and the range of

application of the keyed-in pole, as compared with the morecostly and less convenient pressed-in pole, can thereby beextended. As many as three T-heads are sometimes used.

An advantage possessed by the construction shown in Figs.2(c)and 2(d) is that the air gap of the machine can be adjusted bymeans of liners under the poles, thus allowing greater latitude inthe electromagnetic design.

(3.2.2.2) Rotor Body.The rotor body may vary from a solid cylinder to a com-

paratively thin rim carried on a spider. Each of the forms oflaminated pole referred to above may be used with various typesof rotor body. The construction will, in- most o£ the .machinesin the category considered in the paper, be governed by thefundamental necessity of taking the machine to pieces for transportbetween the maker's works and the site.

The simplest type of all is the cast-steel magnet wheel such asis commonly used for engine-driven generators. Only in thesmaller sizes is a single casting practicable because of difficultiesof manufacture and of transport. Such wheels may, however, besplit diametrically and the sections joined after assembly bymeans of shrink rings. This construction is generally confinedto rotors up to about 15 ft diameter. Within this limit, longermachines may be constructed by placing two or more separatewheels side by side on the shaft. For higher-speed machines,however, castings are incapable of supporting the stresses in-volved, and one alternative construction employs a rotor bodybuilt up of rolled-steel discs. The individual discs may be3-9 in thick, machined all over, and spigoted together, or theymay be made from i-in boiler plate and held in correct alignmentwith one another by fitted bolts. Where stresses permit, theshaft is usually pressed into the built-up rotor. For higher speeds,where a hole through the centre of the rotor body is not per-missible, bolted-on stub shafts may be employed. The con-struction last mentioned is appropriate to large high-speedmachines, e.g. 50 000 kVA at 428 r.p.m. In the rare cases inwhich this construction is not possible because of high stresses,it is necessary to employ a non-salient pole design, but this is notpeculiar to vertical-shaft machines and will not be dealt with inthe present paper.

The disc rotor is not ideal for obtaining flywheel effect sincethe centres of the discs contribute little to the flywheel effect andmuch to the weight. The ideal construction from this point ofview would be a heavy rim on a light spider, but such a rim, if itwere cut out of rolled-steel plate, or cast, would be uneconomical,and in regard to size would be subject to the same limitations assolid discs. These disadvantages are overcome by building it ofrelatively thin steel plates, suitably shaped, overlapped andbolted together. This is now the generally accepted practice forrotors of water-turbine-driven generators exceeding about 15 ftdiameter. The segmental plates are usually ^ to £ in thick, andare punched or guillotined to the desired shape. Each segmentcovers one or more pole pitches, and successive layers are"staggered" so that each segment overlaps the one below.Fig. 3 shows part of a rotor of this type in which each segmentcovers three pole pitches.

It will be observed that, in the example illustrated in Fig. 3,the effective cross-sectional area of the rim carrying the hoopstress is two-thirds of the total. By making a segment cover morepole pitches and by suitably staggering the segments in one layerwith those in adjacent layers, it is possible to increase the effectivecross-sectional area to as much as five-sixths of the total. Beyondthis the cost is prohibitive. Exceedingly accurate workmanshipis necessary in the production of the rim segments to ensure thatthey register accurately on one another in consecutive layers inthe rim. The difficulty of producing them increases with the

VERTICAL-SHAFT WATER-TURBINE-DRIVEN A.C. GENERATORS 761

Fig. 3.—Portion of typical laminated rim and poles..

number of pole pitches per segment, because of the increase inthe number of ways in which the segments must register.

The bolts through the rim may either be fitted, the holes beingreamed after the rim has been completely assembled, or they mayhave a slight clearance in their holes. If the segments have to betaken apart for shipment, the preference is for clearance holesas this avoids the necessity of having to ream the holes again onsite. In this case the usual practice is to'clamp the rim so tightlythat no slipping of the segments relative to one another can takeplace at any speed up to the maximum runaway speed of theturbine. Otherwise the laminated rim, expanding under thecentrifugal forces to the extent of the clearance in the bolt holes,may be permanently and undesirably distorted, and the air gapmay thus be reduced. The thinner the laminations of which therim is composed, the greater the frictional force to resist slipping,for a given tension in the bolts. It is for this reason that thinpunchings are often preferred to plates in the construction ofbuilt-up rims. By a suitable choice of the segment thickness,and a proper disposition and proportioning of the bolts, rimscan be made non-slipping at the highest working stresses.

The keyed-in pole is admirably suited to a laminated rim.As shown above, one of its advantages is that it is not dependentupon smooth machined surfaces, as is the pressed-in pole, so thatthe slightly irregular surfaces of the built-up rim present nodifficulties.

The laminated rim merely rests on the spider arms, i.e. it isnot attached to them, but is driven by floating keys. By thismeans the arms are relieved of all centrifugal stresses exceptthose due to their own weight. The spider arms may either becast integral with a central hub, pressed on and keyed to theshaft, or they may be fabricated from steel plate and bolted to aseparate cast hub or to flanges on the shaft. A typical methodof attaching the spider arms to a separate hub is shown in Fig. 4.The arms are secured by means of bolts (preferably tapered) to

Fig. 4.—Section through typical laminated-rim rotor, showing methodof attaching spider to shaft.

two steel rings which are themselves attached in the same wayto flanges on the central cast-steel hub. The taper on the boltshas a twofold purpose. It ensures that the stresses due to torqueshall be reasonably equal in all the bolts, and it enables the boltsto be removed easily. A principal object of the constructionillustrated is to allow the rotor to be detached from the hub.This is done by the removal of the two inner circles of bolts.The rotor can then be lifted without its shaft, the arms beingunited by the two rings. This construction not only makes itpossible to use a smaller station crane than otherwise, but maylead in certain circumstances to a lower height of building. Theway in which this comes about will be described later when thedifferent types of bearing arrangement are considered (Section3.3). The construction described above has the further advantagein certain applications of allowing all the bearings to be inposition before the generator rotor is erected.

Factory tests are of great value in determining accurately theelectrical characteristics and efficiency of a generator in a waythat cannot so readily be done on site. Moreover, the assemblytime on site can frequently be reduced if a certain amount ofpre-assembly is done in the factory. The suitability of a designfor dismantling for shipment is therefore of importance. Rotorbodies of the laminated-rim type can be shipped in several waysaccording to the transport facilities available. For very largemachines it is usually necessary to dismantle the rim into itscomponent segments, and to separate the arms of the rotorspider. In smaller machines the rim, without poles, may beseparated from the spider, and arrangements have been made tohandle complete rims up to 22 ft diameter on special trucks andbarges. The rim is sometimes made divisible axially, into twoor more separate rings, to reduce weight and dimensions fortransport.

In addition to the types of laminated-pole rotor constructiondescribed above there are several others that are peculiar toindividual manufacturers or special circumstances and are notin general use. Space does not permit of their being dealtwith here.

(3.2.3) Solid-Pole Rotor.In this type the rotor body is solid (usually cast) and the pole

body is integral with it. The pole shoes are bolted on. The useof solid poles leads to a very simple and robust rotor constructionwhich is particularly suited to high-speed generators of smalldiameter and long core length. In such machines the absence ofslots for pole dovetails, which are necessary with laminatedpoles, contributes to the strength of the rotor body and permitsthe use of smaller rotor diameters than would otherwise bepossible. Alternatively, for a given diameter, the solid-polerotor will have a higher critical speed. Such rotors are, however,not peculiar to vertical-shaft generators, and their design willnot be considered further.

(3-3) Bearing Arrangement(3.3.1) General Considerations.

Some vertical-shaft a.c. generators have a thrust bearing andtwo guide bearings, the latter being placed one above and onebelow the rotor. The water turbine usually has one guide beari ng,making a total of three for the whole unit. There is, nevertheless,an increasing preference for one guide bearing only in each of thetwo machines. However, the principal factor in the bearingarrangement is usually the position of the thrust bearing. Themost common practice at one time was to place this bearingabove the rotor. But an increasing proportion of the largersizes of generator are now built with the thrust bearing belowthe rotor. Particularly has this been the case with large-diameterlow-speed machines, including a variant in which the upper

762 JOHNSON AND HOLDER: THE DESIGN OF LARGE

guide bearing is omitted, giving what is known as the "umbrella"type of construction.

Each of these three types of bearing arrangement has itsadvantages and disadvantages, and will be considered in turn.

(3.3.4) Umbrella Type.The advantages, listed above, of placing the thrust bearing

below the rotor, apply also to the umbrella type of construction,illustrated diagrammatically in Fig. 5(c). In addition, the

(c)

Fig. 5.—Alternative arrangements of thrust and guide bearing.Bearing surfaces are indicated in heavy black.

(3.3.2) Thrust Bearing above the Rotor.Fig. 5(a) shows diagrammatically the arrangement with the

thrust bearing above the rotor. The axial load due to the weightof the generator rotor and turbine runner, and to the hydraulicthrust, is carried by transverse or radial arms spanning the statorframe. The arms also carry the exciters, if these are direct-coupled. The upper guide bearing is shown above the thrustbearing, but it is sometimes placed immediately below it. Therelative merits of these two arrangements will be discussed laterwhen bearing design is considered in detail.

The principal consequences of placing the thrust bearing abovethe rotor are as follows:—

(a) The thrust bearing is generally easier of access forinspection.

(b) The shaft diameter being smaller above the rotor thanbelow, the diameter of the thrust bearing, and hence the lossesin it, will be a minimum.

(c) The stator frame has to carry the load transmitted to it bythe thrust bearing arms.

(3.3.3) Thrust Bearing below the Rotor.As already indicated, a thrust bearing placed below the rotor,

as shown in Fig. 5(6), is larger, on account of the greater shaftdiameter, and less accessible than if it were placed above. Ina particular example where the two alternative positions wereconsidered, the bearings were 84 in and 79 in outside diameterrespectively, and the losses were 180 and 160 kW. Against thesedisadvantages, however, must be set the following importantadvantages of this type of bearing arrangement:—

(a) The vertical load on the thrust bearing is transmitteddirectly to the concrete of the foundations, and to the turbinecasing, without having to be carried by the stator frame.

(b) The arms supporting the thrust bearing have only to spanthe turbine pit, which is necessarily smaller in diameter than thestator frame. Consequently, the depth and weight of the armcan be less than if the thrust bearing were above the rotor.

(c) If the rotor is easily removable from its shaft, as describedin Section 3.2.2.2, it can be lifted out of the stator, when neces-sary, without uncoupling the turbine shaft, the weight of thetwo shafts and the turbine runner continuing to be carried by thethrust bearing. When the thrust bearing is above the rotor ithas to be dismantled before the rotor can be removed.

(d) On account of the shallower arms needed to support thethrust bearing, the overall height of the generator is less whenthe thrust bearing is below the rotor than when it is above.This may permit of a lower height of station.

following further advantages result from the omission of theupper guide bearing, which characterizes the umbrella type ofconstruction:—

(a) The first cost is less.(b) The bearing friction is less.(c) The arrangements for bearing lubrication are simplified.(d) The rotor is more easily accessible.(e) When a generator has separately-driven exciters, further

simplification results from there being no need for a heavy upperbracket, except with a Kaplan turbine, when a bracket is usuallyneeded to carry the blade operating mechanism.

(/) The station headroom and crane height can be somewhatless than for a machine with two guide bearings.

The absence of the upper guide bearing in the umbrella typeof construction might seem to endanger the stability of therotating system. This, however, is not the case so long as thedistance between the lower guide bearing and the centre ofgravity of the rotor is not too great. With a view to making thisdistance as-small as possible, the rotors of umbrella-type genera-tors are usually "dished" as shown in Fig. 5(c). Such generatorshave been factory tested at overspeed without the assistance ofthe turbine guide bearing, i.e. running on the thrust bearing andcentred by the single guide bearing of the generator.

Mention has already been made of the advantage of a lowheight of station which results if the thrust bearing is placedbelow the generator rotor. The advantage is increased if therotor is easily removable from its shaft (see Section 3.2.2.2 above).Since this can be accomplished most readily when there is noguide bearing above the rotor, the advantage may be regarded,in effect, as being peculiar to the umbrella-type machine.

The minimum height of the crane hook above the stationfloor, and hence the station height, is usually determined, wherethere are several units in a station, by the necessity of being ableto lift a rotor over or past an adjacent generator. Unless thestation is exceptionally wide, a rotor lifted together with its shafthas to be lifted higher for this purpose than has a rotor withoutits shaft.

(3.4) Bearing Design(3.4.1) Thrust Bearing.

The thrust bearing consists essentially of a collar or runner,as it is usually called, which is rigidly mounted on the generatorshaft and bears upon a specially formed stationary member to bedescribed below. Such bearings run submerged in oil (seeFig. 6), circulation of oil through the bearing itself being main-tained by the centrifugal pumping action of the collar. Coolingof the oil is dealt with in Section 3.5.

VERTICAL-SHAFT WATER-TURBINE-DRIVEN A.C. GENERATORS 763

fie Ground and lapped bearingface of runner

Oil potWhite metal-faced pad

Fig. 6.—Arrangement of typical thrust bearing and bracket.

Correct alignment of the runner face in relation t o the shaftaxis is of great importance, otherwise eccentric running of therotor , journals and couplings would result. Accurate machiningof the large par ts involved is therefore necessary, and the runnermust be rigidly seated on the shaft. This is assured by makingthe seat sufficiently long and providing an adequate interferencefit. The axial thrust is transmitted from the shaft to the runnerthrough a r ing key embracing the shaft and accurately fitted intoa slot in its circumference. The key is divided into two or moresegments (preferably two) and these are prevented from workingout radially by locking plates on the thrust runner .

The bearing surface of the runner is of mild steel or of specialclose-grained cast iron, ground and lapped to optical accuracy.The surface of the stationary par t is usually of white metal . Thestationary pa r t is divided radially, and consists of individualpads so mounted that they are free to tilt slightly in operationand thus to form a wedge-shaped film of oil between the rubbingsurfaces.

Except when the machine is stationary, there is no contactbetween the metal surfaces, for as soon as the runner begins tomove an oil film begins to be formed. Wear in the bearing canoccur, therefore, only during the first fraction of a revolution atstarting, and the last fraction of a revolution at stopping. Thecoefficient of friction is between 0 001 and 0 005 when running,whereas at standstill it may be 0-20 or more . Some operators ,before starting a machine of this kind, lift the ro tor t o allow thebearing surfaces to become wetted with oil. This practice hasmuch to recommend it.

Correct alignment of the bearing surfaces is essential to ensuretha t the load is equally divided between the several pads . Mos tof the variety in thrust bearing design arises from the differentmethods of providing for (a) the tilting of the pads , and (b) thealignment of the stationary and rotat ing members . In the well-known Michell and Kingsbury types of thrust bearing, thetilting of the pads is provided for by support ing each one on apivot about which it may rock in one or more directions, thecorrect distribution of the load being ensured by screw adjust-ment of the pivot height. In a somewhat similar tilting-pad typeof bearing the pad is integral with a small neck or column soplaced that the pad is able to tilt under the pressure of the oilwedge. These types of bearing are too well known in otherapplications to require further description here.

When thrust bearings of the above types are used the deflectionof the bracket carrying the bearing must be small, otherwise thealignment of the pads will suffer. A n alternative, free from thislimitation, is the spring-supported thrust bearing, which meritsmore detailed consideration, since its use is confined to vertical-

shaft thrust bearings. In this type of bearing the tilting of thepads and the equal distribution of the load are accomplished bymount ing each pad on a number of short spiral springs s tandingon a rigid baseplate. In order to prevent too much flexibility inthe springs and the possibility that vertical oscillations may beset up , the springs are pre-compressed by means of a screw andwasher, as shown in Fig. 7. The amount of pre-compression is

Fig. 7.—Typical thrust-bearing spring.

approximately equal to the load that the spring will have to carryin service, and the overall height of each spring is adjustedaccurately, to a gauge, to ensure tha t it shall carry its fair shareof the load. The segmental pads , of which there may be six t otwelve according to the size of the bearing, rest on t o p of thesprings and are prevented from turning with the runner bymeans of keys at tached to the baseplate. A large bearing of thistype may contain over 1 000 springs.

A n important consequence of the complete immersion of thebearing in oil (Fig. 6) is that rota t ion of the runner impar tsrotary movement to the whole of the oil which, unless prevented,would form a vortex. The oil level a t the inner diameter wouldfall, whilst that a t the outer would rise. There would thereforebe a risk of the pads being deprived of oil, with consequentdestruction of the bearing and, indeed, risk of damage to thewhole machine. T o eliminate this risk an adequate head of oilin the oil po t is essential. If this means alone were adopted,however, the overall height of the machine would be undesirablyincreased. I t is therefore usual practice to fit a stationary baffleround the runner , as shown in Fig . 6. The propor t ions of thebaffle are critical and must be well chosen. The object is to allowthe swirling oil that leaves the runner to dissipate its rotat ionalenergy within the baffle and then to escape in to the oil po t withpurely radial velocity through suitable passages at the base of thebaffle. If the baffle is incorrectly proport ioned, i .e. in the relationof its diameter to the radial outlets and to the speed, or inac-curately positioned, the oil may be forced up inside the baffle byvirtue of its rota t ional energy, and thrown clear of the bearing.Much experimental work lies behind the successful designs of thedifferent makers , which, whilst differing in detail, follow the samegeneral principle. I t is perhaps interesting to record that in athrust bearing of this type the peripheral speed of the runner atoverspeed may reach 145 ft/sec (approximately lOOm.p.h.) .Tests on full-scale models have proved the successful functioningof the design at speeds in excess of this.

(3.4.2) Guide Bearings.

Guide bearings for large vertical-shaft generators are com-monly either of the plain-sleeve or of the pivoted-pad type. Theymay be 5 ft or more in diameter, but the loads to be carried areusually low, and the axial length is small compared with thediameter. The length of a bearing of this size would be abou t12 in. The radial clearance in the guide bearings of vertical-shaftgenerators should be kept as small as is compatible with freedomfrom trouble in service due to differential expansion of the journa l

764 JOHNSON AND HOLDER: THE DESIGN OF LARGE

and housing. The clearances are commonly no more than one-third of those used in horizontal sleeve bearings.

The vertical arrangement of the bearing introduces problemsof lubrication not met in horizontal machines. Fig. 6 shows away of lubricating a guide bearing placed immediately above thethrust bearing; oil is fed to it by centrifugal action throughpassages in the runner. Guide bearings that are separate fromthe thrust bearings are sometimes lubricated in a similar mannerby making the journal on a collar on the shaft and partiallysubmerging it in oil.

Where pumps are used for circulating the thrust-bearing oilthrough external coolers, the same pumps are frequently arrangedto give a continuous supply of oil to the guide bearings. Oilpumps, gear-driven from the main shaft, are not often usednowadays for the guide bearings of vertical generators.

(3.5) Cooling of BearingsIn most vertical-shaft generators artificial cooling is necessary,

at least for the thrust bearing. Water is the usual cooling medium.Water-cooled bearings can be divided into two broad classes,those in which the coolers are inside the bearing housing, andthose in which the coolers are external to the machine. Thesimplest way of water-cooling the oil in a thrust or guide bearingis by means of a cooler placed in the bearing reservoir. Thecooler may be a single nest of tubes surrounding the shaft, or itmay consist of a number of separate banks of tubes. For largemachines the latter type is generally preferred.

The question of accessibility is again of some importance. Notonly must it be possible to remove a cooler readily for cleaningor repair, but there must be convenient access to the thrustbearing for inspection. When the thrust bearing is above therotor, a cooling coil of the nest type can sometimes be removedvertically without much dismantling of the machine, but this israrely possible when the thrust bearing is below the rotor.A cooler made up of several units has an advantage in suchcases, as the units can be arranged for removal, one at a time,through openings in the side of the thrust-bearing housing. Thesame openings then serve for the removal of the thrust pads.

Internal coolers are ordinarily associated with a self-containedsystem of bearing lubrication, and are preferred by someoperators because of the absence of pumps and motors. If,however, it is desired to circulate the bearing oil outside themachine for continuous filtration, external coolers may beemployed. The bearings and the coolers then become moreaccessible. The authors incline to the view that the latter systemis the better for large generators, provided adequate steps aretaken to safeguard the circulation. Such precautions are wellknown. It will be appreciated that submerged bearings willoperate safely for some time, say thirty minutes, without artificialcooling. Thus, time is available to deal with an emergency withboth internal and external coolers. It may be remarked thatthere is no material difference in cost between the two systems.

In a recent development, motor-driven pumps are dispensedwith and oil is circulated through external coolers by the cen-trifugal pumping action of the thrust-bearing runner itself. Thisarrangement is not, however, in general use.

(3.6) VentilationUntil comparatively recently, large a.c. generators in hydro-

electric power stations had open-type ventilation. In hotclimates the heated ventilating air was sometimes dischargedoutside the generator room through ducts. To-day suchmachines are usually provided with closed-circuit ventilation,the fans being integral with the machine.

The arrangement of coolers and ducts differs, in general, fromthat employed for other large electrical machines, in which one

or two coolers are disposed in a duct system between the machineoutlets and inlets. Whilst such an arrangement is possible alsowith large vertical generators, a better plan is to divide the coolingplant into a number of units (four to twelve) mounted on flats onthe outer periphery of the stator frame. Sometimes the generatorstands on the station floor and is surrounded by a steel casing.Alternatively, the station floor may be level with the top of thegenerator stator, the stator standing in a pit which serves as thecollecting chamber for the cooled air. As far as the generator isconcerned neither arrangement has an advantage over the other;the choice is determined by structural considerations andoperating convenience. In either case the cooled air in the statorenclosure is returned through ducts in the foundations and inthe enclosure to the fans mounted above and below the rotor.

In many hydro-electric power stations artificial heating isnecessary in the cold season. It is customary and convenient touse the generator ventilating air for this. In a machine withclosed-circuit ventilation, a part of the air may be bled off forstation heating, make-up air being taken in through a filter.One of the great advantages of closed-circuit ventilation, how-ever, is the protection it gives against major fire damage in themachine, both by restricting the supply of oxygen and by per-mitting the injection of carbon dioxide. This advantage is tosome extent lost if ventilating air is bled for station heating,because it is desirable to tap the system at two or more pointsin order not to upset the uniform cooling of the generator, andat each point there must be regulating dampers which have tobe closed in the event of fire. This system is not withoutobjectionable features. The full advantages of the closed-circuit system are retained in an alternative system whichemploys secondary heat-exchangers. In this the heat in theventilating air is given up to an entirely separate stream of airused for station heating. The closed-circuit system then remainsclosed at all times, and the emergency operation of dampers isavoided.

(3.7) Braking and Jacking(3.7.1) Brakes.

The rotor of a water-turbine-driven a.c. generator, revolvingfreely, may take half-an-hour or more to come to rest on accountof its high moment of inertia. This may have serious conse-quences in the event of a fire or a mechanical failure; it can alsobe a great inconvenience to the operating staff in routine shuttingdown. Chiefly for this reason, verticaj-shaft generators arealmost invariably provided with mechanical brakes. An excep-tion is sometimes made in the case of generators driven byimpulse-type turbines, because the latter can be braked hy-draulically, but such cases are comparatively rare.

The use of brakes for bringing vertical-shaft generators rapidlyto rest has a further advantage in that it reduces wear in the thrustbearing. So long as an oil film is maintained between the rubbingsurfaces, the wear in a thrust bearing is infinitesimal, but whenthe speed falls below about 1 r.p.m. the oil film tends to fail, andmetal-to-metal contact can occur. The more quickly the rotoris brought to rest from this point, the less is the wear in thebearing.

The number of brakes varies according to requirements, asmany as 32 having been used. They are located on the bracketbelow the rotor, and bear against a circular track on its under-side. Each brake has its own cylinder, and may be air- or oil-operated at about 100 lb/in2 pressure. When oil is the operatingmedium it may be applied directly, or, preferably, by being forcedinto the brake cylinders from a closed vessel by the admission ofcompressed air above the oil. The air has a cushioning effectwhich prevents a too violent application of the brakes.

Brakes are not usually applied with the rotor running at full

VERTICALrSHAFT WATER-TURBINE-DRIVEN A.C. GENERATORS 765

speed, because wear on the brake linings would be excessive.The usual practice in shutting down is to wait until the speed hasfallen to about half normal. This involves no great loss of timesince the windage and friction torque, down to about half speed,is usually sufficient to maintain an adequate rate of retardation,and it is only between half speed and standstill that extra brakingtorque is needed. However, in case of fire or an internal fault,the brakes may with advantage be applied at full speed so as tobring the rotor to rest as quickly as possible. In an automaticstation, therefore, the brake-operating relays should distinguishbetween emergencies of the kind mentioned and normal routine.

(3.7.2) Jacking.The brakes provide a convenient means of lifting the rotor of

a vertical-shaft generator for flooding the thrust-bearing face atstarting, and for the periodical inspection of thrust-bearing pads.The upward thrust required to lift the generator rotor and turbinerunner greatly exceeds the thrust required for braking. Apressure of about 1 000 lb/in2 is usually employed, and is obtainedfrom a small hand- or motor-operated oil pump. This pump isfrequently made portable so that it can serve any one of a numberof generators in a station without the need for an extensive high-pressure pipe system.

The vertical lift in jacking is usually about $ in. If the rotoris to remain in the raised position for a long time, the weight istransferred from the brake pistons to solid mechanical supports.Some makers provide remote operation for these supports, butthe authors prefer simple packing placed manually under thebrake shoes.

(3.8) Stator(3.8.1) Frame.

The special problems that arise in designing rotors of largevertical-shaft water-turbine-driven generators have no counter-part in the design of stators. In their essentials the stators ofsuch machines follow conventional lines except for their hori-zontal position and the provision that has to be made in theframe for carrying the load transmitted by the upper bracket.•The stator frame of a large machine is usually made in four partsfor ease of transport, with bolted and dowelled joints betweenadjacent parts. The construction of the stator core followsestablished practice for a.c. generators.(3.8.2) Thrust-Bearing Bracket.

The particulars given in Tables 1 and 2 indicate that the thrust-bearing bracket may have to support a load approaching 1 500tons, and may have to span 30-40 ft. In generators driven byKaplan turbines the hydraulic thrust may account for 30-70%of the total load on the thrust bearing. The load on the brackettherefore increases considerably when the water thrust is applied.Clearances in the turbine are small, and in consequence thedeflection permitted in the thrust bearing bracket is also small.A total of £ in is considered reasonable for large brackets; greaterrigidity is, however, sometimes required for the less flexible typesof thrust bearing. It is always necessary to ensure that criticalvibration frequencies cannot occur in the structure.

Two main arrangements of thrust-bearing bracket are recog-nized :—

(a) A self-contained bracket carrying the thrust bearing upon it.(b) A bracket of which the thrust-bearing housing is an

integral part.The first arrangement is generally preferred where the thrust

bearing is not made by the maker of the bracket, i.e. by thegenerator maker. In that arrangement, the centre of the bracketis not interrupted by the large hollow of the thrust-bearingreservoir; consequently the requisite rigidity is obtained with alighter and shallower bracket than in the second arrangement.On the other hand, since the thrust bearing is placed on top of

the bracket the overall height of the machine may be greater withthe first arrangement than with the second. The difference is,however, not as great as might be expected, because the guidebearing can be disposed within the bracket, i.e. below the thrustcollar, instead of above the thrust collar as is necessary forminimum height when the thrust bearing itself is within thebracket.

Two principal forms of bracket are common:—(i) A simple bridge comprising two parallel girders.

(ii) Radial arms, six or eight in number, supporting a centralhub.

The former construction is usually confined to machines notexceeding about 25 ft diameter, and having thrust-bearing loadsof not more than about 300 tons. Beyond such limits, the simplebridge becomes uneconomical in cost and height. The radial-armbracket is used in all the larger machines. The arms are boltedto the central member, keys or spigots being employed to take theheavy shear load at the joint (see Fig. 6).

The bridge type of bracket is used mainly for thrust bearingsmounted above the rotor, i.e. the bridge is carried on the statorframe. Radial arm brackets are used for thrust bearings bothabove and below the rotor. With the bearing above the rotor,a multi-arm bracket gives a more uniform distribution of thethrust load on the stator frame and foundations, and, for thisreason also, is preferred to a simple bridge for heavy loads.When the thrust bearing is below the rotor the span of the bracketis reduced. A saving in cost and height therefore results. In thisposition, also, the bracket forms a convenient support for thebrakes and jacks.

(3.8.3) Exciter Bracket.In umbrella-type generators not driven by Kaplan turbines,

a bracket carried on the stator frame must be provided to supportthe exciters, if these are direct coupled. This bracket, althoughcarrying only a relatively light load, is nevertheless of considerablesize, and its elimination is one of the advantages claimed forseparately-driven exciters. However, an air casing enclosing thetop of the machine is required, and its supporting structureamounts almost to a bracket. In generators driven by Kaplanturbines, a bracket to support the blade-operating mechanism isnecessary and the same bracket will also carry the exciters.

(3.9) Windings(3.9.1) Stator Windings.

The stator windings of large vertical-shaft water-turbine-drivengenerators do not differ in any essential respect from those ofother types of a.c. generator except in the number of coils, whichmay reach several hundred. They are, in general, of the two-layerdiamond-coil type, and lie in open slots in the stator core. Onaccount of the heavy currents which are met the windings areoften arranged in a number of parallel paths, and, in addition,it is frequently necessary to divide each conductor into severalparallel sections which are then insulated from one anotherthroughout the winding and transposed at intervals. Joints mayeither be soldered or welded.

As the coil throw is relatively small and the transient reactancerelatively high, the bracing of the stator end-windings does notpresent the same problems as in large steam-turbine-drivengenerators. A single bracing ring, and packing blocks betweenadjacent coils, are usually sufficient.

(3.9.2) Field Windings.Like the stator windings, the field windings of large vertical-

shaft water-turbine-driven generators are essentially similar tothose of other large salient-pole machines. The coils are almostinvariably of copper strip, and the insulation may consist of micaor asbestos according to the practice of individual manufacturers.

766 JOHNSON AND HOLDER: THE DESIGN OF LARGE

Whilst some makers support the field coil on springs, fitted atits base, to take up any relative movement between pole and coilin service, others submit it during manufacture to a process thatremoves all risk of subsequent shrinkage. It can then be clampedtightly between pole tip and rotor body without fear of move-ment. The authors consider the latter to be the better method.

On account of the high overspeed, special attention is paid tothe clamping of the coil connections and the coil ends. Thelatter purpose is usually achieved by means of an overhanginglip on the pole endplate. V-clamps between adjacent poles areoften necessary on machines of long core-length.

(3.9.3) Damper Windings.The reasons for fitting damper windings to large water-turbine-

driven generators are:—(a) To increase stability under system disturbances.(b) To reduce the risk of over-voltages in certain conditions

of unbalanced short-circuit on long open-ended h.v. transmissionlines.

Attention has been directed to the latter characteristic ofdamper windings as a result of investigations by Wagner,*Clarket and others in recent years.

The principal disadvantages of a damper winding, apart fromits cost and the additional complication involved, are the slightlyincreased losses on load, and the increased fault currents thatresult from the lowering of the subtransient, negative-sequenceand zero-sequence reactances. On account of the increase in thelength and capacitance of transmission systems at the presenttime, the advantages of damper windings are now generally con-sidered to outweigh their disadvantages. Large numbers ofwater-turbine-driven generators are, however, working satis-factorily without them.

(4) AUXILIARY MACHINES(4.1) Exciters

Large vertical-shaft water-turbine-driven generators are, ingeneral, connected to transmission lines; they have therefore tobe capable of working at low leading power-factors for linecharging. This usually necessitates an overall range of excitationgreater than can be obtained from a shunt exciter. Most largegenerators of this type are therefore provided with a pilot- orsub-exciter to energize the field of the main exciter.

In addition to a wide range of excitation, large water-turbine-driven generators usually require also a rapid rate of exciterresponse to ensure adequate stability under fault conditions.To meet this requirement the main exciter is designed to have ahigh maximum voltage and small time-constant. A maximumvoltage equal to twice the normal excitation voltage and a rate ofexciter response (averaged over the first half second) equal to themaximum voltage per second, are found to cover all but the mostexceptional cases.

Exciters are usually direct-coupled, and their armatures arethen overhung from the main shaft. At the low speeds ofrotation for which so many machines of this type are built,direct-coupled exciters tend to have very large dimensions, hightime-constants and high cost. For this reason separate high-speed exciters, motor- or water-turbine-driven, are sometimesemployed. The design of such machines, and of the protectivemeasures required to ensure continuity of excitation in anemergency, are outside the scope of the present paper.

In the arrangement of direct-coupled exciters the armaturesare usually disposed with their commutators upwards. In arecent improvement the armature of the pilot exciter is inverted

• WAGNER, C. F.: "Unsymmetrical Short Circuits on Waterwheel Generatorsunder Capacitive Loading," Transactions of the American I.E.E., 1937, 56, p. 1385.

f CLARKE, E., WBYOANDT, C. N., and CONCORDIA, C : "Overvoltages caused byUnbalanced Short Circuits," ibid., 1938, 57, p. 453.

Slip rings"of maingenerator

Mamexciter

/

Fig. 8.—Sectional view of main and pilot exciters.

so that the two commutators are more nearly adjacent to oneanother. In this arrangement (Fig. 8) the generator slip-ringsare placed between the two commutators, so that the three setsof brushgear, the commutators, and the slip-rings, are all in onelarge compartment (instead of three small ones), and are allwithin easy reach of an attendant standing on a platform at thebase of the main exciter. An alternative construction, aimedmore at reducing height than increasing accessibility, employsa pilot exciter built partly within the main exciter.

Exciters are usually of the open or screen-protected type.Successful installations have, however, been carried out withexciters with closed-circuit ventilation. A filter should pre-ferably be included in the circuit to reduce the deposit of carbondust on the windings and commutator connections. Propersupervision and maintenance are liable to be impeded whenclosed-circuit exciter ventilation is employed.

(4.2) Governor GeneratorThe use of an electric drive for governor pendulums instead of

belts or gears has become almost universal in recent years forlarge vertical-shaft units. The governor is driven by a self-starting synchronous motor supplied from a small three-phasegenerator, of 1-5 kVA, mounted on the main generator shaft-system. For simplicity and security, the revolving field of thegovernor generator has permanent-magnet poles of an aluminium-nickel-cobalt alloy. Thus slip-rings, brushes and excitation con-nections are avoided. It is convenient to provide a winding onthe poles for the initial magnetization and any subsequentre-magnetization.

The governor generator, being a machine of small air gap, isusually mounted near a main guide bearing. In machines ofcomparatively high speed, the air gap of the governor generatorwill be larger and it may then safely be mounted at the summit ofthe set. This arrangement is particularly to be recommended ifthe main generator has a guide bearing above the rotor.

(5) ELECTRICAL CHARACTERISTICS(5.1) Voltage Regulation

The exciters operate under the control of a high-speed auto-matic voltage regulator. The problem of voltage control is made

VERTICAL-SHAFT WATER-TURBINE-DRIVEN A.C. GENERATORS 767

difficult by the governing characteristics of the turbine to whichreference has been made in Section 2.2. If large blocks of loadare switched off, the momentary rise of speed of the turbine,influencing not only the generator but also the exciters, may leadto a rise of voltage dangerous to other consumer apparatus stillconnected to the system. The course of events without auto-matic voltage control is shown in Fig. 9. To cope with this

100

| l 2 5

1

full-

load

sp

eed

5 tn

c

X of

nor

mal 7

/

/

/

2

/

i 0-2 0-4 0-6 0-8 KTime, sec

Fig. 9.—Typical curves showing rise of speed and generator voltageon loss of full load, without automatic voltage control.

through the exciter fields introduces an undesirable time lag,and it would be desirable, therefore, to regulate the field currentof the generator directly. This, however, is almost impossiblewith conventional means, because of the large amount of energystored in the magnetic field. A system recently put into com-mission is based on the assumption that the sudden loss of alarge block of load is an isolated event and will not immediatelyrecur. Thus a single non-repeating step of control in the mainfield circuit will suffice to check the voltage rise, and, being asingle non-repeating step, cannot lead to dangerous powerswinging such as might occur if fully regulated control of themain field were attempted. The system of control consists inswitching into the generator field circuit a resistance sufficient tolimit the voltage rise to, say, 10%, with the generator on opencircuit at, say, 30% above normal speed. The automatic voltageregulator then introduces its resistance into the exciter fieldsystem, and after a sufficient lapse of time for the exciter voltageto have fallen to a value corresponding roughly to an open-circuit generator voltage 10% above normal (say two seconds),the resistance in the main field circuit is short-circuited. Theemergency voltage-control system then becomes inoperative untilreset by hand or automatically.

(5.2) Reactance

Large vertical-shaft water-turbine-driven a.c. generators have,in general, a relatively large number of poles and relatively fewarmature turns per pole. They tend in consequence to bemachines of high transient and subtransient reactance. Typical

figures of reactance ("per unit" values) for such machines areas follows:—

With damper Without damperwinding winding

Subtransient reactance .. 0-19-0 -30 0-23-0-37Transient reactance .. 0-25-0-40 0-25-0-40

The synchronous reactance of large water-turbine-driven a.c.generators is usually in the region of 1 0-1 -3 (per unit). Con-siderations of system stability rarely permit 1 -3 to be exceeded,but values as low as 0- 5 are sometimes necessary when generatorshave to work in parallel at opposite ends of a long transmissionline.

(5.3) Line ChargingProvision for line charging is necessary when water-turbine-

driven generators are connected, as frequently happens, to longtransmission lines. In such cases a generator has to be capableof stable operation at normal voltage (or lower), and zeroleading power-factor. The output required for this duty dependson the characteristics of the transmission line.

The effect of this requirement on the generator design is two-fold. In order to provide adequate voltage control at the fullleading-current output an adequate margin of positive excitationis necessary, and for this reason the short-circuit ratio of thegenerator must not be less than a certain value, generally asindicated in Fig. 10. Secondly, the exciter must not be unstable

20

1-6.2

I0-4

P

0 K 47) 60 8 0 1 0 0LeaduTg-current output, % of rated current

Fig. 10.—Relation between leading-current output and short-circuitratio.

at the low values of excitation required for line charging. This is-one of the reasons why, as stated above, a.c. generators of thekind under review usually have pilot exciters.

(6) ACKNOWLEDGMENTSAcknowledgment is made to the directors of Metropolitan-

Vickers Electrical Co., Ltd., for permission to publish the paper.The authors wish to acknowledge also their indebtedness toMr. O. Thott, Managing Director of Boving and Co., Ltd., forvaluable help with the section on water-turbine characteristics;to Mr. F . T. M. Kissel, General Manager, State Hydro-ElectricDepartment, New Zealand, for permission to include details ofthe overvoltage protection used on part of the New ZealandGovernment system; and to their colleagues of the Metropolitan-Vickers Electrical Co., whose work has made the paper possible.

DISCUSSION BEFORE THE INSTITUTION, 19TH FEBRUARY, 1948Mr. J. W. Howard: The developments in size and output of

waterwheel alternators which have been achieved during thepresent century are very remarkable and are comparable withthose which have taken place in turbo-alternators.

In 1900, a power output of 1 000 kW was considered excep-tional, whereas at present impulse turbines of 40 000 kW and

reaction turbines of 100 000 kW are possible. The necessityfor these large units is due mainly to the progress in hydraulicengineering. In the early days, the power from a particularwaterfall was utilized either for some local industry or for alocal network, and the possibility of development upstream ordownstream was largely ignored, whereas now every effort is