Recent trends in turbogenerators

Recent trends in turbogenerators

V.J. Vickers, B.Sc.(Eng.), C.Eng., F.I.E.E.

Indexing terms: A.C. generators, Turbogenerators

Abstract

The paper begins with an historical account of the evolution of the turbogenerator, emphasing the land-marks in its development. Against the background of probable future requirements, a critical assessmentis made of current design philosophies and their limitations are examined. The barriers to advancementin technology, in materials, and in physical size and weight are identified and the resulting trends indesign investigated. The interdependence of generator and transmission-system characteristics is consideredin relation to likely advances in generator design. The problem areas of stator core and winding vibrationand core-end heating are reviewed. Developments in structural materials, cooling systems, insulation andhandling and transport are covered. Recent developments in excitation systems, notably in the 'brushless'concept, that have obviated the need for sliprings are also examined. The final section includes a reviewof the potential advantages and practical problems associated with the possible use of design principlesinvolving increasingly radical departures from current established practice. These progress from the com-pletely liquid-cooled machine without hydrogen, prototypes of which have already been manufactured,through the more or less conventional generator but with stator and rotor windings accommodated inthe airgap, to designs using evaporative and cryogenic cooling principles. An extensive bibliography isincluded.

1 Introduction

Less than a century ago, in 1888, C.A. Parsons,who foresaw the steam turbine as the prime mover for theelectrical power stations of the future, made the first high-speed a.c. generator by fitting sliprings to the armature of adynamo. The success of this experimental machine was suchthat the first 'alternators' were put into production thefollowing year. These had a rating of 75 kW, single phaseat 1000 V, and were driven at 4800 rev/min, giving afrequency of 80 Hz.

The first 3-phase a.c. generator of this type was installedin 1900, but already the limitations of the revolving arma-ture were very apparent, and attention had been turned tothe development of the rotating field machine. Again pro-gress was rapid and by 1903 units of 3500 kW were beinginstalled. These initial designs were basically of the salient-pole type, which is very difficult and uneconomical toadapt for the 2-pole fields that are necessary to match theinherently high speed of the steam turbine. Over the nextfew years, many experimental rotors of more appropriatemechanical construction were made, until by 1910 a cross-section essentially of the form used today had been evolved.

In 1912, only 23 years after he had made the firstcommercial high-speed a.c. generators of 75 kW, Parsonsundertook the manufacture of a 25 MW a.c. turbogeneratorfor the Commonwealth Edison Company of Chicago. Thisrepresented an increase in output of well over 300 times, ata period in history when the technologies of design andmanufacture were in their infancy.

By this time the advantages of a.c. over d.c. for publicpower supplies had gained wide acceptance, but no seriousattempt was made in Britain to adopt a standard frequency.It was not until the Central Electricity Authority wasformed in 1925, with powers to create a high-voltage inter-connecting network, that 50 Hz became the national stand-ard. In the intervening years many different frequencies had

Paper 7220P. Commissioned IEE Review

Mr Vickers is retired, and lives at Lane's End, Bromstead, Newport,Salop., England. He was formerly with GEC Turbine GeneratorsLtd., Generator Division, Stafford, England

been employed, of which 25,40 and 50 Hz were the mostcommon. Inevitably, therefore, a number of generator formspersisted.

Once it had become accepted that the steam turbinewould be the power source for the bulk electricity suppliesof the future, development work became centred on thehigh-speed generator, both 4-pole and 2-pole. In Europegenerally, a preference for 50 Hz gradually emerged, whilein North America 60 Hz was becoming widely adopted.Because of its lower stress levels and easier construction,the 4-pole machine made the most rapid headway. However,designers of both turbines and generators soon found them-selves blocked by limitations of size and weight, both forcomponent parts and for handling and transport of thecomplete generator. Attention was therefore directed increas-ingly to 2-pole designs. After encouraging early achievements(20 MW at 3000 rev/min in 1924) progress in uprating wasrelatively slow because of lack of adequate forgings forrotors and retaining rings in particular, and because of thetrade recession in the early 1930s.

At the outbreak of war in 1939, the output of the largestgenerator running at 3000 rev/min built in this country was50 MW. The next decade saw stagnation, in that, to con-serve effort, large numbers of 30 MW sets built to existingdesigns were installed and no significant development wasundertaken. Although the first commercial machines usinghydrogen as the sole coolant had been installed in the USAprior to the Second World War (synchronous compensatorsin 1928 and generators in 1937), it was not until 1949 thatgenerators of a high enough rating to justify its use werebuilt in this country. These were of 60 MW output, andhave been conspicuously successful and troublefree.

Thereafter, as the demand for electric power was increas-ing at rates up to, or even over, 10% per annum, progress inunit ratings was rapid. In western Europe, it has been thealmost universal practice to install single-line tandem unitsrunning at 3000 rev/min. Fig. 1, which illustrates the increasein rating, has been drawn to a base of year of order, takenas being the most significant date, since it represents theearliest time at which there was sufficient confidence as theresult of service experience and further laboratory work tojustify a major uprating.

PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEE REVIEWS 1273

In Britain and in France, the early integration of theelectricity supply industry into national undertakings result-ed in greater concentrations of generating plant, of more orless standardised designs, than was possible in countriesserved by a number of individual power companies withrelatively weak interties. It therefore followed that, in the50 Hz areas, the most rapid development has been engen-dered by the policies of the generating boards in Britainand of Electricite de France. The former ordered 200 MWsets in 1955, 350 MW sets in 1958, 500 MW sets in 1960 and660 MW sets in 1967. No less than 49 500 MW units havebeen supplied by British manufacturers to the CEGB alone,and increasing numbers of 660 MW sets are in process ofinstallation or construction, see Fig. 2.

In other 50 Hz territories, progress has been slower, butin Australia 500 MW units are now in commission andothers on order.

Because of the different structure of the power-supplyindustry in the German Federal Republic, there has not,until recently, been a call for particularly large units, but 2-pole generators of around 800 MW and 4-pole types up to1200MW are under construction.

In the USSR the 500 MW set has been a standard forsome years, and this is being followed by 800 MW, the firstof which was commissioned at Slavyansk. The next stepappears to be to 1200 MW, the prototype of which is tobe installed at Kostroma.

1400r

1200

(U1000

•§ 800Eo 600

f 40°200

1950 1954 1958 1962year of order

1966 1970 1973

Fig. 13000 rev/min generators: pattern of ordering in Britaina Predictedb Actual

Fig. 2Stator core and winding of '500 MW, 3000 rev/min generator

[C.A. Parsons & Co. Ltd.)

The main 60 Hz areas are the USA, South America,Canada and parts of Japan. The population concentrationsand economic conditions generally in the USA havenecessitated a more rapid expansion there than elsewhere.Today, 2-pole generators of around 800 MW are in com-mission, and it is predicted that by 1978 1200 MW units at3600 rev/min will be in service.

Comparatively few 4-pole generators have been manufac-tured because the overall economic considerations of genera-tion using fossil fuels are so strongly in favour of the 3000 or3600 rev/min turbine. Where 4-pole generators have beenused, they have formed part of the low-pressure line ofcrosscompound sets, notably in the USA. However, withthe advent of nuclear power the 4-pole machine has comeinto its own, particularly with wet-steam systems prevalentin the USA, Canada and to some extent on the continent ofEurope. With these systems, the volume of steam to behandled per kilowatt generated is very large, and turbine-design considerations strongly favour the slower-speed setparticularly at the larger ratings. Consequently, 1800 rev/mingenerators of around 1200 MW are being installed in fairnumbers in the USA, and orders for sets of this rating havebeen placed in Britain by US utilities, see Fig. 3.

In Britain, nuclear development has so far taken placearound the gas-cooled reactor with its inherently highersteam conditions, so that it has not been necessary to departfrom the high-speed unit. In Europe, however, the wet-steam-reactor philosophy is being increasingly followed and1200 MW 4-pole generators are now being installed.

The incentive for development has been almost entirelythe substantial overall economic advantage accruing from

1274 PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEE REVIEWS

Fig. 3Stator of 1215MW, 1800 rev/min generator

[GEC Turbine Generators Ltd.]

stepping up the unit output. Consequently the effort hasgone into refining design techniques within what may bebroadly termed conventional approaches to the electric,magnetic and thermal problems, and the steady increase insize and weight has been accepted. The stage is now beingreached when the economic gains are being seriously erodedas the result of the need to finance the enormous capitalinvestment necessary for manufacture, testing and transport.This, coupled with the growing realisation that the world'sresources of essential raw materials are being depleted at analarming rate, will undoubtedly result in much greatereffort being put into the development of methods of ex-ploiting the potential savings inherent in design changes ofa much more radical nature than have yet been seen.

2 Evolution

The course of development of the turbogeneratorset has been well reported,1""26 particularly over the lasttwo or three decades, and it will be assumed that the readerrequiring a more detailed study of particular topics thancan be covered in an overall review of this nature will referto appropriate references in the bibliography.

It is at once evident that the basic form has not alteredsince it was evolved in the early 1900s. A study of thepatent literature, for example, shows that virtually all thefundamental principles that are being exploited today wererecognised (by a remarkably small number of people) eitherjust before, or shortly after, the turn of the century. Thepersistent characteristic of this continuing advance in sizeand performance is the absence of any significant techno-logical breakthrough. Virtually all the progress has beenachieved by the steady development and refinement ofexisting design processes, constructional methods andmaterials. It is, however, one thing to have the vision to seethe possibility of harnessing the use of a basic principle tothe solving of a particular problem, but it is quite anotherto develop economical and reliable methods of doing so.The real evolution has therefore been a relatively slow pro-cess covering a multiplicity of fine technical problems. Manyof these admit of different solutions, and one of the pointsto be appreciated is that so many different detailedapproaches have been successfully adopted in practice.

Although there may have been no epoch-making innova-tions as yet in the history of the turbogenerator comparablewith those in other fields of engineering, there have beensignificant landmarks. The most important of these are

(a) the use of hydrogen in place of air as the overall coolingmedium, first employed in the USA in 1928 for

synchronous compensators,.and in 1937 for generators(b) the use of coolant ducts within the main conductor

insulation on stators and rotors. Although the principlehad been used on small low-voltage generators inBritain and elsewhere as early as the 1910s, it was notuntil the early 1950s that its full potential began to beexploited

(c) the use of liquids, notably transformer oil or syntheticfluids, and water, circulating in ducts within the high-voltage stator insulation. The first recorded use of water(at first sight an unpromising liquid for the purpose) wasin a 30 MW generator installed in 1956

(d) the introduction, over a number of years, of high-voltage stator-insulation systems based on the use ofthermosetting synthetic resins in place of the naturalthermoplastic materials, shellac and bitumen

(e) the development of alternative sources of excitation toreplace the d.c. generator.

The impact of these steps can be seen by reference toFig. 4, which shows the reductions achieved in overallgenerator weight and in the utilisation of copper to a baseof output in megawatts.

Apart from the successive introduction of these basicsteps in methods of heat removal, no other dramaticincreases in rating can be attributed to any one area ofdesign. Nevertheless, there have been many other second-ary advances without which full exploitation would nothave been possible. One of these areas is that of computer-aided design, both in the form of programs for comprehen-sive optimisation of designs and also for routine calculations.

The writing of these programs has necessitated a re-appraisal of a number of basic design techniques, which, initself, has yielded a bonus in terms of a better understandingof the principles and the elimination of rule-of-thumbmethods. With this new ability to achieve much more refinedoptimisation and to employ more sophisticated calculationprocesses, significant steps have been made in recent yearstowards obtaining the maximum possible output from agiven volume of material. The impact has been greatest inthe field of rotor design. On the electromechanical andthermal sides, it is now possible to achieve rapidly a far finerdegree of optimisation than has been possible before,between the three conflicting requirements of mechanicalstresses in teeth and body, of conductor and coolant-ductcross-section, and of magnetic saturation.

3 Future requirements

Historically, unit outputs have roughly doubledevery ten years, and there seems to be no basic reason whythis pace should not continue at least for the next twodecades. However, the recent fairly general slackening ofthe rate of increase in electrical energy consumption hasslowed up the ordering of new generating plant. The CEGBhas not so far proceeded above 660 MW, although a step toaround 1300 MW in a single unit was envisaged for orderingin 1972.27

A period of slower progress, at least in the 2-polemachine, will undoubtedly be beneficial to manufacturerand user alike, in that it will permit consolidation andfurther refinement of design and manufacturing philosophiesin the light of extended running experience. The importanceof consolidation will be obvious when it is remembered thatindividual design features are basically developed in labora-tory rigs. While many can subsequently be confirmed insmall-scale, and often even in full-scale models, this is afar from perfect substitute for proving on an integratedfull-size unit in commercial service. The basic economicadvantages of large units are very quickly eroded if un-reliability causes extended outage time. Hitherto, financeand time have, in general, only been made available for full-

PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEEREVIEWS 1275

scale testing when some basically untried principle, e.g.water cooling of stator or rotor windings, has beeninvolved. Extrapolation to larger ratings using alreadyestablished design philosophies has almost always beenaccepted. Experience in recent years has, however, shownthat the stage has now been reached when this approachneeds reconsideration if undue outage times to correctbasic weaknesses, that only manifest themselves after aperiod of running, are to be avoided. The present slackerpace would therefore seem to afford an ideal opportunityto advance the development of a high-speed set of around1300MW.

of the largest set on such a system to be related, not to thetotal connected capacity as is commonly done, but ratherto the load-pickup ability of the tie lines, which in turn isrelated to trie transmission voltage. Voltages are rising andthe figures in Table 1 are given to indicate the sort ofpressures on unit size that could develop if this principlewere accepted.

• The many factors influencing the direction that futuredevelopment is likely to take will be considered in somedetail in the following Sections against the background ofcurrent limitations and changing economic climate.

y

|V)

3230

25

20

10

05

0-10

0

V total generator weight

copper weight

0 200 400 600 800output, MW

1000 1200

Fig. 4Increase in power output per unit weight for 2-pole 3000 rev/min generatorsa Hydrogen cooledb Direct hydrogen-cooled rotorc Direct-cooled one-piece statord Direct-cooled, two piece for transport

As has been said earlier, the future of the slow-speedmachine is very dependent on reactor technology. Thelight-water reactors will undoubtedly be used for some yearsyet, and as their ratings increase so does the slow-speedmachine become more attractive. The generator design is,however, considerably easier than for the same rating at2-pole speeds, so that only passing references will be madeto the 4-pole generator in this review.

Many predictions have been made of the dates by whichlarger unit sizes will be required, and a number of these areshown plotted in Fig. 5. As might be expected, there iswide divergence of opinion, and it is not a purpose of thisreview to attempt to reconcile the forecasts. Suffice it tosay that there is every indication that 2-pole generators ofaround 1300 MW will be economically justified in theimmediate future, and 2000 MW in the 1980s. Towards theend of the century, even larger outputs could well be apossible requirement. It is even more difficult to predictthe future growth rate for the 4-pole machine because ofthe uncertainties in the field of reactor development. How-ever, the present ceiling of around 1200MW at 50 and 60 Hzis likely to be lifted considerably, and 2500 MW has beenmooted. In this context it is interesting to observe a pro-posal in Reference 28 to relate the optimum generatingunit size to the transmission-network voltage in the case ofvery large power pools. An example is cited of the easternregion of the USA, where the interconnected system totalsover 200000MVA, with the result that the largest unit isonly a fraction of one per cent of the total. A case is made,from the performance standpoint, for the appropriate size


TABLE 1

GENERATOR SIZE RELATED TOTRANSMISSION-SYSTEM VOLTAGE28

System voltage

kV230345500800

1300

Generator unit size

MW200500

100025007000

1276 PROC.

4 Electrical design features

The basic form of the turbogenerator was estab-lished in the first decade or so of this century and surpris-ingly little change has taken place during its subsequentevolution. As experience and knowledge have been accumu-lated, it has been possible to stepup the unit output,initially by extrapolation using established design principles,and latterly by adopting more sophisticated methods ofcooling, loss reduction, mechanical construction etc. Onlynow is really serious attention being given to design con-cepts that would radically alter the construction, such aslocating the stator, and even the rotor, windings in the air-gap space, and adapting evaporative and cryogenic coolingmethods, see Section 13.

The biggest advances have been, and for the present arelikely to continue to be, made in the fields of electrical and

IEE, Vol 121, No. 11R, NOVEMBER 1974, IEEREVIEWS

thermal design, as distinct from magnetic design, becausepractical flux-density limits have already been reached foravailable steels. The specific electric loading, convenientlyexpressed as the ratio of the product of the number ofstator conductors and the phase current to the circum-ference at the diameter of the rotor, has risen from valuesof a maximum of 65 A/mm for the largest air-cooleddesigns, to 250 for present day machines having water-cooled windings. These higher loadings have been madepossible by the development of increasingly efficient andprecise systems of heat removal in conjunction with moreexact methods of loss determination, both in respect oflocation and magnitude. Uprating in this way inevitablyadversely affects a number of the operating characteristics,notably the short-circuit ratio and the transient reactance.Furthermore, because only marginal increases in generatingvoltage have been possible with established techniques, theresulting very heavy currents and the associated leakagefields give rise to serious problems of stator-winding vibra-tion and intense heating in local areas.

reduction possible compared with designs for the lowerpower factors that were common when machines wererequired to feed directly into local load systems.

On the other hand, short-circuit ratios (s.c.r.) haveremained relatively stable at around 0-5. In the USA,appreciably higher values have been normal, but as intertiesbetween systems are strengthened lower figures approachingthose common elsewhere are becoming accepted. No furtherappreciable reduction seems likely because of the need tomaintain capability at leading power factors to assist in theproblem of system voltage control, particularly at times oflight load on the e.h.v. lines, and because a stage of diminish-ing returns has been reached at the lowest values already inuse. In very large networks, such as the British Grid system,it is generally neither economical, nor even feasible, todesign generators with sufficient leading capacity to meetthe system requirements under all optimum-loadingconditions, so that some separate form of line compensationis often necessary.

3000r

2000

8

en

"o1000

1965 1970 1975year of order

1980 1985

Fig. 5Forecasts of maximum unit ratings

Because methods of cooling individual components areto a large degree interdependent and influence the overalldesign concept of the generator as a whole, they are coveredseparately in Section 8. Aspects of insulation are discussedin Section 9.

4.1 Power factor and short-circuit ratio

Generating units in the range with which thisreview is concerned are almost invariably used in largepower systems having substantial interconnections at highvoltage. Consequently generator power factors tend to berelatively high because charging currents are large. Commonpractice today is to rate machines at power factors of 0-85or 0-90 so that advantage may be taken of the weight

Even so, it has been possible to hold down the s.c.r. onlybecause the development of rapid-response excitation andcontrol systems has enabled the necessary stability marginsunder both transient and steady-state conditions to bemaintained. Much fundamental work, both theoretical andpractical, has been done in the field of the integration ofturbines, generators and control methods into powersystems.29"37

4.2 Reactances

In general terms, the subtransient reactance x"d

should be high to reduce fault levels and to minimise wind-ing and shafting stresses. The transient reactance x'd shouldbe low to achieve maximum transient stability. These two


machine characteristics are largely interdependent, so thatan increase in subtransient is accompanied by an increase intransient. Both are closely related to the specific electricloading, so that as machine ratings are raised so both react-ances tend to rise also. Some control over their magnitudeis possible by attention to the stator slot size and configura-tion. The economically achievable variations are, however,relatively small, and in the largest machines are generallyinsignificant because of the need to give preference to otherconflicting requirements.

The largest present designs of 2-pole generators havevalues of subtransient reactance in the range 0-20—0-25 p.u..These are adequate to afford protection to the stator wind-ing and shafting in the event of faults close up to themachine, and, with the inclusion of the reactance of theusual step-up transformer, are acceptable from the pointof view of the system.

The transient reactance is normally in the band 0-30—0-35 p.u. and does not pose any particularly serious problemsin respect of transient stability. However, projected designsof generator for larger ratings are giving values up to0-45 p.u., so that much greater attention will have to bepaid to the integration of such machines into transmissionsystems and to the achievement of improved performanceof other associated equipment, e.g. excitation response,circuit-breaker clearing times and steam-valve control.

Inherently, 4-pole machines have higher reactances, butas far as transient stability is concerned the effect is largelycompensated by the naturally higher inertia constant H ofthe set as a whole.

The reactances of possible future generators using theslotless and superconductivity concepts (Section 13) areinherently considerably lower. While a degree of optimisationis possible between low reactances and high fault currents,it may not be feasible to avoid incorporating reactors inseries to reduce fault currents to acceptable levels.

4.3 Inertia constant H

In condensing turbines, the major contribution tothe inertia of the unit comes from the l.p. rotors. Conse-quently, unlike generators for water-turbine drive, it isfortunately not necessary, in designing a generator, toattempt to attain any specific value. Since rotor diametersare determined by considerations of allowable stress levels,very little variation in inertia is in fact economically feasible.While the value of H for the set as a whole is a very signifi-cant factor in calculating the transient stability, the com-ponents for the individual rotors determine the stress levelsin the shafting under the oscillating conditions arising fromsystem faults. Here, the relatively low value for the genera-tor, compared with that for the turbine, has already posedproblems in the mechanical design of the l.p.-to-generatorcoupling and associated shafts. On occasions, differentialheat treatment of the shaft ends has had to be undertakento obtain higher strength. This problem will become moredifficult as the disparity in inertia between turbine andgenerator increases. A number of manufacturers alreadyuse coupling flanges forged integral with the shafts toachieve greater strength and minimise stress concentrationeffects.

4.4 Stator winding

Theso-called basic basket or lap winding is nowuniversally used rather than the concentric or hair-pin type.It has many advantages, arising principally from its physicalsymmetry, e.g. it has lower stray load losses, only two half-coil shapes, it is less difficult to insert into the slots and itis more easily insulated. Furthermore, it can be readilychorded and subdivided into two or more parallel circuits.

1278 PROC.

Copper is, and is likely for the foreseeable future tocontinue to be, employed, largely because of the absoluteneed to keep the coil cross-section to a minimum. Currentdensities vary with the form of cooling. For the largestwater-cooled windings values up to 11 A/mm2 are common.

Voltage levels have risen only marginally to a maximumof around 30 kV, which is about the practical limit for con-ventional insulating and cooling systems. It has been repor-ted that a generator operating at 110 kV is in service andthe design of one at 220 kV is being studied in the USSR.13

Such windings would require the application of techniquessuch as immersion in oil or perhaps encapsulation - familiarin the transformer field. In this respect, the concept of the'airgap' winding referred to later, has potential.

The limitation in voltage has necessitated very largeincreases in current as ratings have grown. The resultingforces, both between coils and between coils and theirsupporting structures, are proportional to the square of thecurrent, and are at twice the operating frequency. Theirmagnitude, even in present ratings, is such as to requirereappraisal of the methods of wedging in the slots andclamping in the end turns. In ratings up to 450 MVA, wind-ings having supports adequate to withstand without damagethe forces arising under transient fault conditions have beenshown to have sufficient margin to sustain full load in-definitely. At higher ratings, the continuous vibratory stresslevels at full load have been in some cases high enough tocause fatigue fracture of conductor strands or tubes, usuallynear the point where they are brazed into the waterboxes.26

Severe fretting of the insulation against support membersor packing blocks has also led to electrical failure.17 Evenfretting of the steel of the slot surfaces has been noted.38

Much theoretical and practical work has been done in recentyears, both to obtain a better understanding of the problemsand also to evolve superior winding-support systems.38~43

In a typical large 50 Hz generator, the pulsating forceexperienced by a pair of conductors in a core slot can be ofthe order of 101 at a frequency of 8-5 X 106 cycles perday. The problem is to ensure that the coils are, and remain,tight in the slots of a core that is of laminated construction,and is itself subjected to high vibratory and distorting forcesdue to the main and leakage fluxes. The analysis of corevibration and distortion is extremely complex.38'44"48 Aparticularly significant aspect of direct concern in relationto the suppression of coil vibration is that the slot profile isdistorted by the elongation of the teeth by the main flux.45

Hence, in addition to a slot-wedging system that exerts asubstantial radial force by, for example, incorporatingtapered slides under the wedges, corrugated glass-fibremouldings are sometimes used to exert side or radial press-ure on the coils.43 Manufacturers employ elaborate full-scale test rigs to obtain long-term confirmation of theeffectiveness of wedging systems, see Fig. 6.

It is also increasingly common for the core laminationsto be bonded together to ensure maximum mechanicalstability without relying on the interplate friction, whichis very dependent on the degree of flatness and surfacefinish on the steel and on the effect of temperature changeson the axial compression applied by the core-clampingmeans.

The support and anchoring of the end turns pose evengreater problems, partly because of the complex 3-dimensional force pattern and partly because manufacturingtolerances on the shape of insulated coils are of necessityrelatively large. A supporting structure of brackets and ringsof insulating material is common to all designs, but thereare many variations in detail. Banding with glass fibre, orpolyester cord, which shrinks and therefore tightens on risein temperature, is widely used. Extensive use is also made ofconformable pads of epoxy-impregnated matt both betweencoils and supports, and between coil sides. In some designs

IEE, Vol 121, No. 11R, NOVEMBER 1974, IEEREVIEWS

Fig. 6Rig to investigate stator-bar vibration and wedging systems


the 'basket' is clamped back against its supports by non-magnetic steel or nonmetallic studs passing through thewinding layers. The size of stud that can be used is verylimited and the presence of metal in this region representssome hazard, so that the trend is to confine such boltingtechniques to the area around the coil ends where there isample space. Again, full-scale test rigs are used to enable theleakage-field calculations and force direction and" magnitudeto be checked, see Fig. 7. Some manufacturers make pro-vision for the support structure to move axially as a unit toavoid subjecting the coils to any stress that might arise owingto differential thermal expansion.43 Sometimes the com-pleted end windings are impregnated with a synthetic resinto fill any voids and to ensure as rigid a structure as possible.

Very little further development of this conventionalapproach is possible, and other means of either reducing thevibratory forces or containing them have been under investi-gation for some years, particularly for 2-pole machines,which are conventionally limited to two parallel currentpaths. This restriction, together with the limitation ongenerating voltage, has necessitated the use of a smallnumber of stator slots with conductors carrying very heavycurrents. If the winding could be subdivided further, thecurrent per circuit would be reduced in proportion and thevibratory force acting in the slots, being dependent on thesquare of the current, would be very significantly reducedfor a given rating. Alternatively, much higher outputswould be possible without exceeding a force level that hasbeen proved to be acceptable.

Three basic methods are available. The first is the use ofa higher number of slots, and consequently a lower currentper slot for a given rating and voltage, by dividing eachpole-phase-winding group into two parallel circuits, giving awinding having four parallel paths per phase. The distribu-tion of the coils between two such groups requires verydetailed analysis to ensure that the voltages generated inthe two sections are virtually balanced, and also that thereactances are nearly equal to avoid harmful circulatingcurrents and unbalanced loading between groups. Withcareful design, remarkably close equality can be attained.

The second method is to wind the stator with twoseparate 3-phase windings displaced by 30° electrically. Thecorrect phase relationship for paralleling to give a balanced3-phase output can then be obtained by choosing an appro-priate transformer system.

The third is to connect the generator internally in deltainstead of star configuration. The application of thisprinciple is, however, rather limited because of the con-straints imposed on the winding design by the need to

Electromagnetic model used in the studies of stator endwinding vibration and support systems

[General Electric Co., USA)

minimise harmonic circulating currents to avoid excessiveparasitic losses, particularly in the rotor surface, and theproblems of protection.

All three approaches have been used, and wider adoption,at least of the first two, can be expected. The choice be-tween them in a particular case depends on a full technicaland economic analysis of the many factors involved.41'49'S0

Complete encapsulation of the end windings has beenproposed, but coil replacement in the event of insulationfailure would be very difficult, and the already vulnerablearea of mechanical discontinuity between the slot portionand the end windings would almost certainly be subjected toadditional stress. It is therefore unlikely that such a systemwill be used. However, now that airgap lengths of 150 mmor more are necessary to maintain an acceptable s.c.r. thereis a strong revival of interest in the concept of accommodat-ing the stator coils in this otherwise wasted space and elim-inating the stator teeth. Freed from the constraints imposedby core slots, the winding as an entity can in theory betotally encapsulated in a cylinder of insulating material. Thiswould not only afford uniform mechanical support, butalso give scope for raising the generating voltage. There are,however, formidable practical problems in this approachand these are discussed in conjunction with other aspects ofthe 'slotless' concept in Section 13.

In any heavy-current winding, the end leakage fluxesinduce correspondingly large voltages between conductorstrands or tubes, which can in turn give rise to high circula-ting currents, unless very thorough balancing by suitabletransposition is ensured. Further, in order to minimise thenumber of water-connection points it is the usual practiceto braze together all the tubes, forming the composite con-ductor at each end of each half coil, so that all transpositionhas to be complete within every half coil. Increasing resortis therefore being made to transposition, not only in theslot length but in the end turns also.sl~~ss

All transpositions are still based on the classical Roebelmethod.

4.5 Core-end heating

From experience with the earliest designs it wasapparent that the leakage field entering the core and teethaxially at the ends of the stator would impose limitationson the maximum specific electric loading that would bepossible for a given construction and cooling system. As

PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974,1EEREVIEWS 1279

loadings have risen, control of the problem has been main-tained by the fitting of conductive screens of copper oraluminium over the core clamping plates, or, less commonly,by incorporating magnetic-flux traps, either between theend-winding support structure and the clamping plates oreven as part of the structure itself. In the higher-rated designsthe fields are high, and it is difficult to ensure adequatecooling of the screens, or to build-in sufficient cross-sectionof iron in the flux traps. Some 500 MW generators have thescreens water cooled, and an extension of this practiceseems certain.

Effective shielding of the teeth is not feasible, andrecourse has to be had to improved cooling and to lossreduction by dividing each tooth into one or more sectionsin its width by narrow slits. The design of these slits and thedistance they are taken axially into the core requires verycareful calculation backed up by extensive flux-plottinganalysis and measurements using pickups on actual machines,particularly under conditions of high load and leading powerfactor.

The axial component of leakage flux increases rapidlyas the operating power factor approaches unity and goes overto leading, so that steep rises in temperature occur in theseareas under conditions that the operator may well imagineto be less onerous.29' 56~65 Consequently modern generatorsare provided with permanent temperature detectors in theseareas, connected, along with all others in the machine, to acontinuously scanning instrument.

Core-end heating is also one of the limitations on outputwhen running asynchronously on low loads without excita-tion to absorb excess leading reactive power from the trans-mission system. Although this mode of operation is notwidely used, largely because of the practical complications,it can be a relatively cheap method. Tests carried out by theCEGB on a standard 588 MVA unit showed that an outputof 0*37 p.u. was possible before the core-end temperatures,notably in the teeth, reached allowable limits. The advancesin knowledge, gained in recent years from the large amountof investigational work carried out as the result of collabora-tion between manufacturers and users, should also permitthis limit to be raised.

4.6 Stator terminals

The very heavy stator currents necessitate the useof some form of direct cooling of the terminal bushings.Earlier designs employed hydrogen with the terminalsarranged so that the blower head was applied either directlyacross them, or in series with the appropriate leads from thewindings, which were made of hollow section copper. Thesesystems have the disadvantage that openings have to beprovided through the insulation for entry and exit of thegas, and therefore involve creepage paths to earth.

With the introduction of water cooling it becamerelatively simple to include the terminals in the stator watercircuit and most designs now follow this practice. Provisionmust be made to take up differential thermal expansionbetween the copper and the insulation of the bushing toavoid leakage of hydrogen.

Intense heating can result from eddy currents induced inadjacent metal parts if proper precautions are not observed.Nonmagnetic steel must be used in appropriate areas of theterminal support pockets, and current-carrying joints mustbe made with nonmagnetic clamping bolts having a co-efficient of thermal expansion as nearly as possible the sameas copper.

The connections between the generator and the system,whatever form they take, are bulky, and their accommoda-tion in and through the foundation block is becomingincreasingly difficult. Some of the largest units are nowbeing designed with the terminals at the top or at the side

of the stator frame. These arrangements too have theirdisadvantages, in that the run of the connections aboveoperating floor level results in severe restriction of craneaccess.

4.7 Rotor winding

For easier correlation with the stator, cooling andinsulation systems are covered separately in Sections 8 and9, and only more general matters are discussed here.

The basic concentric coil rotor winding is still universallyemployed, and only changes in detail have been introducedas required to exploit more efficient systems of heat removal.The diamond - or basket - shaped coil has practicaladvantages in divided-winding and water-cooled designs, andexperimental rotors using it have been built.24'32'66 It is,however, much more difficult to insulate and supportagainst thermal expansion and centrifugal forces.

Copper remains the only economical conductor material.Careful specification and control of its mechanical proper-ties is even more critical than it was with indirectly-cooledstrip windings. In direct-cooled designs using axial ductswithin the copper, very high compressive stresses have to becarried by the radial side walls of the ducts of those con-ductors at the mouth of the slots arising from the centri-fugal loading imposed by the conductors beneath them. Inaddition, there can still be high axial compressive forcesfrom inhibited differential thermal expansion between thewinding and the steel of the rotor body. These axial forceswere the root cause of the phenomenon that became knownas 'copper shortening', in the 1930s and early 1940s inrotors that were wound with annealed copper that wasunable to sustain the stress without either yielding orcreeping.67 Consequently silver-bearing copper, which hasbetter creep resistance and a higher annealing temperature,is still used. The conductors are produced either by rollingor drawing, depending on the cross-section required. Inorder to avoid problems of mechanical or thermal unbalancein the completed rotor, very stringent dimensional toler-ances are essential. These, coupled with the simultaneousrequirement of close control of hardness, and hence mechan-ical strength, impose considerable difficulties on the coppersupplier.

It is not practicable to preform this type of conductorinto complete coils, therefore they are normally assembledinto the slots as half turns, having previously been veryaccurately bent and shaped. Joints have not only to beelectrically sound but also of high mechanical strength,particularly if they are located in the circumferential partof the end turns where they are subjected to high tensilestress resulting from the expansion of the retaining ring atspeed. For this reason joints are brazed rather than soldered,and their quality is often checked by comparison with acalibrated sample, using ultrasonic means. The design of thejoints is such that they retain adequate strength after sub-jection to the temperature required for brazing. Weakeningof the conductors by heat travelling away from the areaduring the brazing process has also to be prevented.

The same basic principles are used for water-cooled coils,which require modifications in detail because of the needto ensure they withstand the very high internal pressuregenerated due to rotation.

For many years, rectangular-section slots and uniformlytapered teeth have been used because machining is straight-forward and only a single copper section is required. Withthe increased need to avoid underworking materials as far aspossible, there is a general trend towards tapered slots andparallel teeth. The practice has been followed for a con-siderable time in the more difficult field of generation at60 Hz. The advantage is that the steel is more uniformlystressed throughout the length of the tooth, and extra space

1280 PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEEREVIEWS

(up to 30%) is made available for copper. Not all designersgo the whole way and there is a wide variety of formsbetween the two extremes. Some use a tapered section forthe lower part of the slot and a length that is parallel forthe upper part, others employ one or more steps to givea number of shorter parallel sections. None of these designspresents any particularly difficult machining problems formodern machine tools, see Fig. 8. However, they ali intro-duce complications from the point of view of winding and toa lesser extent of insulating, because such conductor has tobe of a shape to correspond to its exact position in the slot,and the support packings in the circumferential lengths ofthe endturns have to be in a form that can be assembled ina trapezoidal space having the wide section innermost. Never-theless, as the extra copper space can be so high, the overalladvantages in reduction of machine size and cost heavilyoutweigh the disadvantages.

Fig. 8Machining slots in rotor for 660 MW generator

[C.A. Parsons & Co. Ltd.]

Considerable interest is being taken in Britain in theadaptation of the so-called divided-winding rotor (d.w.r.)to large generators, particularly because it offers a sub-stantial increase in the steady-state stability limit. A studyfor a 500 MW turbogenerator has shown that the reactive-power absorption in the leading power-factor regime for ad.w.r. rotor is 2-4 times that of the conventional rotor. Thesystem also improves the transient stability and postfaultvoltage recovery.32'33'3St x

In principle the rotor winding is divided into two separatesections, each connected to sliprings, in such a way that theaxes are displaced, ideally by 90°, so that independentexcitation control in the direct and quadrature axes can beeffected. In practice, it is not feasible on economic groundsto slot more than approximately two-thirds of the rotorperiphery, because of magnetic saturation in the poles,consequently the maximum angle possible between the twowinding sections has to be a compromise, being about 60°The benefits accrue from the fact that the direction of therotor m.m.f. is no longer rigidly related to the angularposition of the rotor, but can be varied by adjusting thecurrents in the two sections of the winding, thereby elimina-ting the time delay otherwise involved in accelerating ordecelerating the high-inertia rotor system of the set.

If the physical position of the rotor is controlled through-out the load range by varying the current in one winding sothat the m.m.f. axis of the second winding always coincideswith that of the airgap flux, a change in the current in thesecond winding will directly vary the reactive absorptionor generation. The current in the second winding may evenbe reversed without inducing pole slipping.

The practical realisation of this design concept virtuallyprecludes the use of the conventional concentric coilwinding, because of the difficulty of finding space for,insulating, and supporting the additional connections. The

wave or lap winding is more suitable but also involveschanges in packing and insulating techniques. In the con-ventional rotor all coils carry the same excitation currentso that thermal differentials are minimal. In the d.w.r. thewinding sections not only have different currents but theratio between them changes with load and power factor.The design of a support system for the end turns is corres-pondingly more difficult, and the particularly vulnerableinsulation at the ends of the slots may be subjected to bend-ing stresses that are not normally present with a concentriccoil winding.

In spite of the potential advantages of the d.w.r., thesepractical difficulties have so far limited its application to atrial on a 5 MW generator to prove the principles, and themanufacture of a prototype 500 MW rotor.

4.8 Unbalanced electrical loading

Any unbalance in loading between phases gives riseto negative sequence currents that have to be mirrored bycompensating currents flowing in the rotor. Being at twicethe generating frequency, these currents flow near theperiphery of the rotor, axially in the active region andcircumferentially between poles in the retaining-ring area.They therefore have to flow across interfaces between theslot wedges and the rotor teeth, and between the rotor andthe retaining rings. The resistivities of these areas can behigh in relation to those of the solid metal, and in additioncan be variable because of differing surface conditions.Unless particiilarxare is taken, notably at the point wherethe rings are attached to the rotor body, the local loss andtemperature rise can be high enough to cause, at best, pittingand further deterioration of the contact surfaces, or, atworst, softening and extrusion of the wedges or reduction inmechanical properties of the ring material to the point atwhich it fails.

The magnitude of the negative-sequence airgap field fora given degree of unbalance is directly related to the specificelectric loading for which the generator is designed. As hasbeen said in Section 4, these loadings have risen very rapidlysince the introduction of improved cooling techniques.Since the losses associated with the flow of negative sequencecurrents are a function of the square of the current,modern large generators have significantly lower capabilityin respect of the unbalance they can safely accept, bothtransiently as in the case of a high-level line fault, or perm-anently should the type of load demand it.3'68"70

The normal method of specifying this capability ontransient faults is with reference to the integrated (current)2

X time function expressed as I22t = C, where 72 = per-unit

negative sequence component, t = time in seconds and C =a constant.

The value of the constant for any particular design isdetermined principally from consideration of the specificelectric loading that is closely allied with the cooling systememployed.

National standards71"73 specify minimum values for thisconstant. In the past the figures have been chosen on a veryarbitrary basis because of the hitherto largely intractablenature of the problem. Recently considerable attention hasbeen devoted to it both analytically and practically in thelaboratory and by tests on commercial machines. A rotorequipped with a large number of temperature gaugespositioned in the vulnerable areas is shown in Fig. 9.

Whereas I22t values in the range of 10-20 apply to

indirectly-cooled designs, the figure drops sharply to around5 for present-day highly rated direct-cooled generators.While some concern has been felt at these low levels, theyare within the capability of modern protection systems.

The ability to withstand sustained negative-sequenceloading has also declined from figures of around


Fig. 9Rotor fitted with temperature sensors to assess heating dueto unbalanced loading

(GEC Turbine Generators Ltd.]

010 to 0-05 p.u. I2. Even so, system requirements are easilymet.

There are considerable differences of approach to designin these important areas. It is essential to achieve the lowest-possible contact resistance between the retaining ring andthe ends of the slot wedges and rotor-body steel. Thesesurfaces are often silver plated, and the rings are shrunkonto them with an interference that ensures that some pres-sure remains at speed and operating temperature. Somemanufacturers also fit a damper winding in the form ofcontinuous loops lying on top of the insulated coils and inelectrical contact with the underside of the wedges andwith the retaining rings. Others use what might be describedas a circular comb of copper contained on the inner surfaceof the rings with the 'teeth' projecting a short distance intothe slots to form a shunt across the body—ring interfaces.Others, again, prefer to rely entirely on the silver-platedsurfaces to carry the current. This approach avoids theproblems of insulating the coil ends from the rings intro-duced by any form of damper to the detriment of reliabilityin normal operation.

Along the active length of the rotor, the distribution ofnegative-sequence current and resulting losses not onlydepend on the relative resistivities of the body and wedgematerials but also on the depths of penetration at twice thefundamental frequency. These factors are therefore impor-tant in the selection of wedge material.

4.9 Sliprings and brushgear

Over the years the possibility of using a liquid metal,such as mercury, sodium or sodium-potassium alloy, hasbeen investigated and rigs have been built, but the obviousserious difficulties and objections have prevented their usein practice.

In recent years much more emphasis has been placed onthe development of 'brushless' excitation systems usingsemiconductor rectifiers mounted on the shaft as describedin Section 11, and such systems are being increasinglyemployed throughout the whole range of generator sizes.

5 Magnetic design features

Very little further development in the magneticcircuit of the conventional machine is likely, becausepractical limits in the permeability of the available steelshave been reached.

In order to keep the short-circuit ratio as high as possible,the degree of saturation of the magnetic circuit has beenmarginally raised as improvements in metallurgy haveensured more consistent and repeatable permeabilitycharacteristics.

5.1 Materials

Some manufacturers use oriented-grain steel forthe stator laminations, to take advantage of its higherpermeability in the direction of rolling rather than of anysaving in loss, because the core loss in turbine-type genera-tors is a very small proportion of the total loss. The greatestpotential is seen in the 2-pole generator with its very deepback-of-slot dimension, which can be significantly reducedif the laminations are punched with the direction of rollingrunning tangentially. Some further small savings could bemade in the total weight of the stator core if oriented steelcould be used in the teeth with the direction of rollingrunning radially. Various methods of making such compositecoreplates have been covered by patents in the USA, but asfar as is known none has ever been used.

Against these attractions must be weighed the seriousdisadvantage that the Young's modulus is only about two-thirds of that of nonoriented steels, which implies that fora given stiffness the core diameter would have to be increased.In practice, those manufacturers who use the material haveadopted a compromise and have designed for a higher vibra-tion amplitude to enable them to take some advantage ofthe weight-saving possibility.

5.2 Vibration

The design, assembly and maintenance of high-speed sliprings for 5000 A and more are all highly critical ifbrush and ring wear are to be held to acceptable rates. Ifcurrent sharing between brushes is not achieved and con-sistently maintained, catastrophic failure can easily follow.Rubbing speed, voltage drop, temperature, brush materialand composition of the environmental atmosphere allhave a big influence on wear rates, and the scope forfurther development is small.

Work on current collection, particularly for the heavycurrents at low voltages associated with homopolar machines,suggests that carbon- and perhaps metal- fibre brushes havepotential for turbogenerators.

Shutting down for adjustment of brushgear cannot betolerated in the case of high-merit sets, so that the design isfurther complicated by the need to incorporate features toensure the operator's safety when changing brushes on load.Constant-force coiled clock-type springs are used to followup brush wear.1282 PROC.

The natural radial resonant frequency in the funda-mental ring mode of any large 2-pole core is as low as140—170 Hz. The rotation of the magnetic poles of therotor subjects the core to a very large radial distorting forceat an effective frequency of twice the fundamental. Themargin is clearly uncomfortably small, particularly in 60 Hzmachines. The construction of a core from thin laminatedmaterial, in which the tolerances of flatness and surfacefinish are necessarily large in relation to the thickness,renders it subject to appreciable variation in mechanicalcharacteristics. Further variation is introduced as the resultof the effect of temperature changes in operation. Whilethere is a growing tendency to bond the laminations together,either in packets or throughout the core, to eliminate thesevariables and to facilitate core building, it is, nevertheless,essential for theoretical analysis to be backed up withextensive tests on models and full-sized generators, so thatthe overall characteristics can be adequately determined atthe design stage. The complexity of the whole field of corevibration, of which the above is only one facet, is wellbrought out in the literature.38'44""48

IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEEREVIEWS

5.3 Core clamping

Many different means are used to apply a pre-determined degree of axial compression. It is necessary tominimise variations in pressure over the operating tempera-ture range of the generator in order to ensure that the stiff-ness of the core structure does not relax, thereby loweringthe resonant frequency too close to the exciting frequency.Conversely, the pressure should not increase unduly, toavoid imposing too high a specific pressure that could leadto breakdown of the interlamination insulation. This aspectis particularly significant in the case of cores having radialventilation ducts where the local pressures under the spacersare inevitably high.

In the majority of designs, heavy section plates, either incomplete rings or in segments, of ferritic steel or non-magnetic cast iron are used. The inner face is often taperedto a degree such that the required pressure on the teeth isapplied when the plate has 'dished' until the surface is flat.A few manufacturers employ clamping bolts passing throughthe body of the core just behind the slots. While controlover the pressure exerted on the core is facilitated, it isnecessary to insulate these bolts and their nuts very care-fully, because a voltage approaching full turn voltage isgenerated in them and any insulation failure could be cata-strophic in its effect.

The need to screen the core ends from the penetrationof axial and peripheral leakage fields has been referred toin Section 4.5. Developments in this area are consequentlydirected to simplifying and improving the screening. Themost interesting design trends are based on the conceptionof using modern high-strength adhesives to bond corelaminations to form composite clamping plates and screens.66

6 Mechanical design features

Where particularly interesting and challengingmechanical features are very closely allied to aspects ofelectrical or magnetic design, they have already been dis-cussed. Mention has also been made of the severe constraintsimposed on the overall design of generators by the inherenthigh speed of the steam turbine and by the problems ofsheer size and weight. In the present Section those featuresnot so covered will be examined.

and windings, with protective and noise reducing coverssimilar to normal air-cooled generators.

6.1 Stator frame

The frame of any generator that has hydrogen asa cooling medium is necessarily a complex fabrication,demanding the highest welding skills. Because of the fireand explosion risks, the principle of containing the com-plete cooling system within the frame is virtually universallyfollowed so as to restrict both the volume of gas and thearea of potential leakage. Furthermore, the structure has tobe able to withstand without rupture an explosion of ahydrogen—air mixture. Such explosions as have been record-ed appear to have been the result of omitting the flushingoperation using an inert gas, usually CO2, during the fillingor empyting process. In order to meet these requirements,as well as to sustain the normal operating gas pressure,which for modern large machines is in the range 5—6 X 10s Pa,the frame has to be very robust and is consequently wellable to carry the weight of the core and windings. Themanner in which these are supported and the dispositionof the hydrogen/water coolers are very largely determinedby the limitations of weight and size imposed by handlingand transport facilities, see Section 10.

The concept of the completely liquid-cooled machine, inwhich hydrogen is no longer required, is receiving increasingdevelopment effort, and one such trial machine is alreadyin operation, see Section 13.2. In this, the frame reverts tobeing no more than a ribbed structure to support the core

6.2 Stator core

Reference has been made in Sections 4.4 and 5.2to the complex electromagnetic forces to which the coreand windings are subjected and how these impose majorproblems in the design of these components. Considerationmust also be given to the prevention of unacceptably highlevels of vibration being transmitted to the foundationblock, and thence to neighbouring items of equipment. Ifadequate steps are not taken, fatigue failures of structuralparts as well as high noise levels can be expected, particularlywhen natural frequencies are excited. At the high airgapflux densities used in modern designs, the magnetic pullexerted on the stator core is sufficient to induce vibrationamplitude in 2-pole generators as high as 25 /urn. Withinthe restrictions of size and weight, it is not practicable toachieve figures significantly lower than this. It is thereforenecessary to support the core within the frame in such amanner that this radial vibration is attenuated, while at thesame time maintaining adequate rigidity in the tangentialdirection to sustain the high oscillating torques arising underfault conditions.

Many methods are employed to achieve the necessaryattenuation. Perhaps the simplest is that in which the coreis built into a skeleton frame supported within the outergas-tight housing at discrete points along the length out ofline with the ribs, in order to take advantage of the flexibil-ity of the support members. In another system, the bars onwhich the coreplates are assembled are slotted in such a wayas to reduce the transmission of vibration to the frame ribs.In a third, commonly used, method the skeleton-frameholding the core is supported along its length on thin plate'springs' to absorb radial vibration. Additional membersat the bottom, and usually also at the top, are fitted toprovide tangential stiffness. These systems, which can pro-vide attenuations up to 10:1, have been described previously,and no significant developments have been introduced thatwarrant inclusion in this review.

In 4-pole machines, the magnetically induced vibration is8-node compared with 4-node for 2-pole designs. Theresultant vibration levels are very much lower, so that evenat the highest flux densities possible from magnetic con-siderations there is no need to resort to special attenuationmeans.

6.3 Rotor

The limitations on the rate of application of theadvancements in other fields of technology imposed bymaterials are reviewed in Section 7. The most critical areas inthe generator are those of the most highly stressed com-ponents, the rotor body and shaft and the end windingretaining rings. In the present Section, the mechanical designof these is reviewed in more detail, and other features ofthe rotor that are primarily governed by mechanical con-siderations are covered.

6.3.1 Body and shaft

As the regions of highest stress are those at theroots of the teeth, the roots of the wedge-retaining slotsand the bore of the central inspection hole, the form of thewinding slots is critical. The increasing trend towards theuse of tapered, or at least nonuniform-width, slots has beendiscussed in Section 4.7. To minimise the excitation lossand consequently to reduce the space and power needed tocirculate the coolant, the winding area must be a maximum.In conflict with this is the need to provide the greatestpossible iron section to carry the flux, both in the teeth, and,


particularly in 2-pole machines, across the section beneaththe slots that has to carry not only the useful but also theinevitably heavy leakage fluxes. Optimising between theseand other significant factors requires the most thoroughknowledge of flux distributions, stress patterns, stressconcentration effects etc. before a sufficiently comprehen-sive computer program can be devised to achieve the bestoverall compromise.

Because of the very high running stress at the surface ofany inspection hole that is bored along the axis of the forg-ing, it is becoming generally accepted to rely on non-destructive-testing methods at various stages in the manu-facture of forgings as evidence of freedom from harmfuldefects. Boring is consequently confined to the shaft at theoutboard end, where stresses are low, to accommodate theconnections between winding and sliprings or rectifiers.

6.3.2 End-winding retaining rings

The retaining ring is the most highly stressed com-ponent in the generator, and it has been, and at presentremains, the major limitation on adopting larger rotordiameters. Again, as shown in Table 2 of Reference 12,there has been some slow raising of strength levels over theyears, but, as is seen in Section 7.2, the limits of develop-ment of conventional materials have virtually now beenreached. The almost universal practice is to employ non-magnetic steel, because of the magnetic short-circuitingeffect across the poles of the rotor, and the increased lossand heating resulting from the-use of magnetic material inan area of intense leakage flux. However, the very long air-gaps in modern large generators somewhat mitigate theseobjections, and a number of 500 MW units are in servicewith magnetic rings. Nevertheless, with any of the steels,up to 75% of the permissible stress is due to the rotationof the mass of the ring itself, leaving little strength to supportthe applied load of the winding and its packings.

Fortunately the configuration of the ring is such thatareas of high stress concentration can be avoided. Thecalculation of the stresses is, nevertheless, an involved onebecause of the bending introduced by the nonuniform load-ing imposed by the windings, and by the shrinking of theinner end onto the rotor body for retention purposes. Thesubject has been well covered in a number of papers.74'7S

Practical confirmation of theoretical and model investiga-tions has also been obtained from strain gauges fixed to ringson production rotors.

The urgent need to develop alternative lightweightmaterials is exemplified in Fig. 10. This illustrates the situa-tion that would apply to a rotor of 1330 mm diameter, suchas might be suitable for a 2000 MW generator, running at3000 rev/min. The curves show the ratio of copper cross-section in the endwindings to that copper that can beaccommodated in the rotor-body slots plotted against theoutside diameter of the ring. They demonstrate that, foreach diameter and available forging strength, there is alimiting value of ring thickness beyond which no increasein load-carrying capacity is obtained. Some increase incapacity can be achieved by decreasing the bore diameter,which increases the thickness without increasing the self-stress (it does however make the attainment of the sameproof-stress level more difficult in austenitic steel becauseof the greater force required for cold-working the thickersection). This solution necessitates a reversion to the oldpractice of stepping down the end windings to a smallerdiameter as they leave the slots. Experience has shown thatsuch designs can be prone to variation in balance becauseof the ununiform restraint offered to thermal expansion ofthe copper and to the restricted access for end ventilation.A variation, aimed at minimising these objections, in whichthe stepping is carried out within the slot length has beenproposed.76

Another approach that is quite practicable with a gas-cooled winding is to reduce the mass of copper in the endturns by increasing the area of the gas ducts. The increasedresistance losses can be offset by the improved coolingwithout the overall capability being impaired. In practice,the amount of copper that can be removed is limited bythe allowable compressive stress set up in the outermostturns by those beneath them, but in the example of Fig. 10this method would permit the construction of a 1330 mmrotor using available materials.

When cooling is by a relatively dense medium, e.g. water,these solutions offer little help, and the development ofnew low-density materials (see Section 7.2) and theiradaptation to the particular environment is becomingincreasingly necessary. One of the main problems to beresolved is the method of attachment to the rotor. To avoidthe fatigue cracking, which can occur as the result of deflec-tion of the shaft if the inner and outer ends of the ring areboth supported, it is universal practice with steel rings tocentre them by shrinking them onto the end of the rotorbody and to incorporate at that point some form of bayonet,screw or split-ring retention means against the axial forcesto which they are subjected from the differential thermalexpansion of the winding. Because of the different physicalproperties of the proposed new materials, these methodscould not be used in their existing forms.

6.3.3 Inertia equalisation

To avoid an undesirably high-level of vibration attwice the running frequency, it is necessary .on all 2-polegenerators above about 60 MW to make some mechanicalcompensation for the nonuniform distribution of thewinding slots around the circumference of the rotor, so assubstantially to equalise the deflection at all angularpositions. Full compensation is possible if the pole centresare also slotted, but such extra slots must be filled withsteel bars to avoid unacceptable magnetic saturation. Barsare not only difficult and time-consuming to fit but arealso subject to fretting in service, so that a compromise isoften resorted to in which shallower unfilled slots areused.

An alternative method of weakening the pole centres, bycutting narrow but deep gashes transversely to the axis atintervals along the length, is frequently employed. Again, thecompensation obtained cannot be perfect, but is adequate.Under conditions of unbalanced electrical loading, highcurrents are induced in the surface of the rotor. These flowaxially, and in highly rated machines it is necessary to pro-vide bridges across these gashes to avoid the very highconcentration of current that would otherwise occur at thenarrow path between the ends of the gashes and the firstwinding slots. These currents can, if not adequately con-trolled, cause excessive local temperature that may leadto cracking.

The much greater stiffness and relatively narrower polesinherent in 4-pole generators render inertia equalisationunnecessary.

6.3.4 Critical speeds and balancing

The length/diameter ratio of 2-pole generators isalways such that the first critical speed is below runningspeed. As outputs have risen, this ratio has increased withthe result that many of the largest machines running todayhave their second criticals also below running speed. Extremeaccuracy of calculation and of balancing are vital if vibration-free running is to be ensured, both as an isolated rotorduring testing and as a coupled unit with the turbine andexciter on their respective foundations in service. For smallsets, it is normally sufficient to assume that the bearings andtheir supports are infinitely rigid, but as the bearing spans


and weights increase it becomes essential to allow for theeffects of flexibility of the bearings, their support structureand of the oil film. While the calculation presents nodifficulty in this age of computers, there has been inade-quate knowledge in respect of the values of many of thesenow all important variables. Much investigational work hasbeen carried out both in the laboratory, particularly on thecharacteristics of oil films, and in the field so that theseeffects can now be taken account of realistically in thedesign.77'78 Even so, provision is sometimes made in thebearing supports for varying the flexibility.

is sometimes incorporated in the form of a convolutedsleeve between the two sections of the coupling, extremeaccuracy in alignment of all the shafting is neverthelessessential if acceptable vibration levels are to be obtainedand unpredicted criticals avoided. Furthermore, there isserious risk of overloading one or other of two adjacentbearings and of subjecting the shafting to abnormal bendingstresses if variation in vertical alignment with temperaturechanges in casings and bearing supports is excessive. Forthis reason, the trend is away from the bearing-in-endshielddesign, particularly at the turbine end where it is becoming

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1375 1400 1425 1450 1475 1500 1525

outside diameter of ring, mmFig. 10Limitations imposed by stresses in rotor-coil retaining rings for conventional and possible future materialsa Titanium alloy, 0-2% proof stress = 94 kgf/mm2

b Carbon-fibre reinforced epoxy of ultimate tensile strength 74 kgf/mm2

c Austenitic steel, 0-2% proof stress = 110 kgf/mm2

d Austenitic steel, 0-2% proof stress =107 kgf/mm2

e Austenitic steel, 0-2% proof stress = 102 kgf/mm2

[GEC Turbine Generators Ltd.

It is relevant to note here that there is an increasing bodyof opinion that under-tuned steel foundations are superiorto the traditional stiff concrete block. The great intrinsicstiffness of the turbine l.p. and generator stator casingsmeans that they can be used as self-supporting structuresmuch stiffer than any steel foundation that could be builtaround them.12

Because of the tendency to lock and of the difficulty ofensuring adequate lubrication, flexible couplings are nowseldom used even on the smallest sets. Normal practice is toemploy rigid flanges, either forged solid with the shaft orshrunk and keyed onto it. Increasingly, the bolts in theturbine—generator coupling are designed to shear undersevere electrical-fault conditions to protect the shafts againstcatastrophic damage, but any broken or deformed boltsthemselves represent a hazard, and the design of such com-ponents is not easy. While a small degree of angular flexibility

more common to mount the bearing in the turbine casingto minimise temperature differentials.

The length of turbine—generator rotors necessitatesdynamic balancing not only in three or more planes alongthe body but also in the endwinding retaining rings,coupling flange and sliprings. Balancing is carried out inspecial-purpose tunnels or pits provided with pickup equip-ment that measures vibration amplitude and phase angle.The balancing procedure is integrated with those of settlingand heat seasoning the winding at normal operating tempera-ture and of overspeed proving, which is normally at 20—25%over normal speed. Finally, the necessary number of runs aremade to establish that mechanical/thermal stability has beenreached to avoid any risk of outage time for trimming thebalance in service.

As will be apparent from Fig. 11, these balancing andoverspeeding facilities involve a very high level of capital

PROC IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEEREVIEWS 1285

Fig. 11Overspeeding and balancing pit with rotor of660MWgenerator being assembled

[C.A. Parsons & Co. Ltd.]

expenditure, not only because of the necessary heavy-dutydriving, lubricating, heating and cooling equipment but alsobecause of the need to ensure that in the event of a burstduring overspeed testing the enormous energy released canbe absorbed within the tunnel.79 To reduce the drivingpower there is a tendency towards surrounding the rotorwith a box that can be partially evacuated, or, particularlyfor turbine rotors, to design the tunnel itself for evacuation.

6.4 Bearings

In order to maintain acceptable critical speeds andstress levels, bearing diameters, even on 660 MW units, aresuch as to result in operation in the turbulent range, produc-ing very high oil-film losses and temperature. Full-scale testrigs are essential for full understanding of the problemsinvolved and for obtaining adequate design data. Workalready done shows that bearings based on conventionalpractice, but incorporating refinements in design detail,will be available for the largest units projected at present.However, their losses will be very high because of the rapidincrease with peripheral speed in the turbulent region. Theorder of escalation is indicated by figures that have beenquoted12 for 660 MW and 1300 MW sets, having the samenumber of bearings, of 2-4 MW and 6-6 MW, representing0-36% and 0-51% of the respective net outputs.

6.5 Gas circulators

Axial-flow and radial-flow hydrogen blowers areused. The choice is determined partly by the duty required,which in turn is decided by the cooling concepts used andpartly by the predilections of individual engineers, since,in the middle range of pressure and volume requirements,either basic type is suitable.

Generators having hitherto conventional cooling ofthe stator core by radial gas ducts and multipath gas-cooledrotor conductors, need a relatively high volume at lowdifferential pressure. This can be most simply provided by

single-stage axial 'fans' mounted at each end of the rotor,either on a separate hub, or, when the pressure requires, onthe larger diameter afforded by the endring centering disc.Sometimes the pressure for the rotor is boosted by asupplementary ring of small blades located in the annulusat the entry to the feed space between the shaft and theend windings. Such overall ventilation systems tend to beinefficient in terms of ventilation power and cooler cross-section, which has to be larger because of the low pressuredrop that can be allowed. Consequently many designersprefer axial cooling of the core with its much lower volumebut higher-pressure requirements. For current 2-pole ratings,two or three stage axial or single-stage radial blowers,usually at one end of the generator, suffice. Both are used.The axial design offers a wide range of pressure/volumecharacteristics while using standard parts. The optimumspeed for the duty required is however very much higherthan the available 3000 or 3600 rev/min, with the resultthat the blades are short. Consequently the tip clearancehas to be determined not by a desirable stage—stage leakagebut by the need to ensure mechanical safety, having regardto the relatively large clearance in the bearings. Particularcare is therefore necessary when assembling a generatorhaving this type of blower.

While the radial blower is rather less adaptable to a rangeof duties, it is cheaper, simpler and much less sensitive toinlet velocity distribution. Accordingly, when the pressure/volume characteristic of the generator can be met by a singlestage, the radial design is often used in preference to theaxial.

For the largest 4-pole designs having direct-gas-cooledstator coils, axial blowers having up to eight stages are used.

6.6 Shaft seals

The two basic forms of seal originally employed atthe points where the shaft emerges from the hydrogenenclosure have been further developed as higher gas pressuresand peripheral speeds have been introduced. In both, normallubricating oil is used in the triple role of sealant, lubricantand coolant.

The axial, or ring, seal is inherently robust, permits com-pletely free movement of the shaft through it, acceptsnormal vibration levels without distress and is reasonablytolerant of residual particles of dirt in the oil. It consists inprinciple of a ring surrounding the shaft just inboard of theadjacent bearing and has a circumferential groove intowhich the sealing oil is fed at a pressure automaticallycontrolled to be slightly above the prevailing hydrogenpressure. The running clearances are scaled to give a largeflow towards the air side to remove the considerable lossesinherent in the high peripheral speed, but only a small flowto the gas side. This restriction is necessary, to minimise theremoval of hydrogen from the frame by solution in the oil.Seals of this type require the oil to be vacuum treated toremove the dissolved oxygen and nitrogen that wouldotherwise be carried into the frame and pollute the hydro-gen. The treatment plant is simple and is incorporated in aself-contained seal-oil-supply unit for ease of operation. Withmodern seal designs, the flow of oil to the gas side is lowenough to give an acceptably small hydrogen-removal rate.However, an interesting variation that eliminates the needfor vacuum treatment and avoids extraction of hydrogen atthe expense of introducing a further pumping circuit andmore sophisticated pressure controls has been developed. Inthis, the ring is basically divided axially into three sectionsby two circumferential oil-feed grooves. By fine oil-pressurecontrol it is possible to prevent any interchange of oilbetween the air-side and hydrogen-side flows. These twoflows may consequently be supplied from separate pumpswith their circuits running saturated with air or hydrogen,


respectively, thereby eliminating the need for vacuumtreatment.

The alternative form is the radial, or face, seal. This isan adaptation of the fixed-pad white-metalled thrust bear-ing, and operates against a collar on the shaft. Again, theoil is fed to a groove in the face where it divides, the bulkflowing radially outwards to provide cooling and lift genera-ted by the ramped 'pads', and only a very small quantitypassing inwards across a narrow plain land to form the sealagainst egress of hydrogen. Seals of this type are verysensitive to the effects of the centrifugal action on the oilresulting from the very high peripheral speeds, but withthe necessary data from full-scale test rigs it is possible toreduce flow rates to the gas side to as low as 150—250 cm3/sor about a one-tenth of the value for equivalent ring seals.At this level, vacuum treatment is no longer necessary, andthe oil supply may be derived directly from the main tur-bine oil supply via suitable differential-pressure regulators.Unfortunately, the ramp on the thrust 'pads' has to be verysmall, and as seals of this type are very susceptible to wearcaused by fine particles in the oil or overloading resultingfrom binding in the carrier when attempting to follow theaxial movement of the shaft, the trend is away from theiruse because of the significantly better service record of theaxial seal.

7 Structural materials

The rate of growth of unit output has been in-fluenced as much by the availability of suitable materials,particularly steel forgings, as by progress in design technology.It is fortuitous that advances in metallurgy have enabledturbine and generator development to be kept in step.80'81

Again, these advances have been largely achieved by therefinement of existing production techniques and by modi-fications of the composition of the basic alloys rather thanby any major technological breakthrough. One of thebiggest factors contributing to the attainment of higher-quality, as well as larger, forgings has been the introductionin the last 15 years of vacuum degassing, which, althoughcostly in terms of additional plant, has very significantlylowered the rejection rate.

The picture is much the same in.the case of otherlimiting materials.

7.1 Rotor forgings

Great credit is due to the forgemasters for themanner in which they have kept pace with the demands forever larger and more uniform rotor forgings having higherphysical properties. For 2-pole machines, forging weightsof 85 t and diameters of around 1150 mm are common-place. The recent upsurge in demand for large 4-polegenerators has imposed a severe strain on the forge capacities,particularly in respect of melting, ingot size and weight,forging and heat treatment. Nevertheless, a number ofentirely satisfactory single-piece forgings weighing up to2201 have been produced in the USA and Japan.

A point has now been reached, however, beyond whichany further increase in size will be difficult to achieve byconventional methods, even by the installation of equip-ment requiring a capital outlay entirely out of proportionto its potential return. Alternative approaches are thereforereceiving increasing attention. Vacuum-arc remelting andelectroslag remelting or refining offer prospects of produc-ing large ingots of high quality with cheaper equipment andlower costs accompanied by smaller top and bottom dis-cards. However, neither method would seem to offer theprospect of any major size increase.

More promising are developments in the technique ofbuilding up composite rotors from a number of smallerforgings. For many years mechanical methods of

assembly have been used successfully, and the rotor for a1150 MW (Fig. 12) 1800 rev/min generator has been madein this manner.23 However, as sizes increase the problemsinherent in the mechanical design of such composite rotorsbecome more severe, not only because of the longer spansbetween bearings but also on account of the higher trans-ient torques under system fault conditions. There is, there-fore, considerable interest in the electroslag welding techniquefor joining two or more billets. Although the resulting rotorwould have to be subsequently forged and heat treated, thisapproach would eliminate the severe technical difficultiesand heavy costs associated with very large melts and ingots.Laboratory work in the USA and Europe suggests that theprocess could be developed to give entirely acceptablerotors. Some of the experimental work has already beenreported.82*83

There has been a steady improvement in physical proper-ties over the years, as can be seen from Table 2 of Reference12, which lists the proof-stress values generally available atthe time the first units in their respective output rangeswere designed. Some further small advance can be expectedto be achievable by the time, say, a 1300 MW 2-pole genera-tor is required. This would enable a body diameter of1270 mm or even slightly more to be used at 3000 rev/min.Any further raising of the strength level without undulysacrificing other essential properties is becoming more andmore difficult, and only marginal gains can reasonably behoped for.

In 4-pole designs, the stressing is much easier. For ratingsaround 1200MW in use today, and for those projected inthe immediate future, the main metallurgical problems arethose associated with the mass of metal to be produced andprocessed;

The improvements in steel-making have resulted in sub-stantial reductions in impurities and in nonmetallic inclus-ions, which, coupled with advances in ultrasonic and othernondestructive testing methods, have not only made growthin size possible, but also have resulted in much improvedsoundness and quality generally.

Increasing attention is being given to fracture toughness,which becomes of greater significance as yield strength andstress levels increase. While it is not technically or economic-ally feasible to perform direct tests on production rotors,much laboratory effort has been devoted to establishing anadequate means of estimation from correlation with Charpyimpact fracture appearance transition temperature f.a.t.t.tests,84 which are performed as routine on material takenfrom the rotor surface and central core. Transition tempera-tures are sensitive to composition and rate of cooling. Fromwork done in the USA83 consistently low values have beenobtained using Ni—Cr—Mo—V and water quenching.

Another major limitation in the overall design of genera-tors, particularly 2-pole, lies in the permeability of the steelsforming the magnetic circuit. In the past only marginal im-provements have been obtained in rotor forgings, and thereseems little likelihood of any further significant advance.

7.2 End-winding retaining rings

These rings, which are very highly stressed, havebeen and remain the major limitation on adopting largerrotor diameters. For the reasons given in Section 6.3.2, it isalmost universal practice to use nonmagnetic austenitic steelwhich is of the manganese-chrome type. The production ofsuch rings is a formidable task, because the physical proper-ties have to be developed by 'cold' working. In order toobtain the greatest possible uniformity in metal structureand in physical properties, this is normally carried out at300°C by an expansion process using tapered mandrelsunder a very high-capacity press, which is liable to seriousdamage should the ring fracture. Because of this risk and the

PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974,1EEREVIEWS 1287

high capital cost of the press, considerable work, notably inFrance, has been done towards adapting the explosive form-ing technique for this purpose.

As rotor diameters are raised, the compressive stresses inthe sidewalls of hollow rotor conductors, resulting fromthe centrifugal loading, are already high. There is, therefore,

Fig. 12Building up 4-pole rotor for 1333MVA 1800 rev/min generator

Ferritic steel offers marginally better properties with theadded advantage that these are obtained by heat treatmentonly. However, because the material is magnetic, it isrelatively little used for the reasons stated in Section 6.3.2.

Very little more development can be expected in thesecurrent materials, and alternatives are being sought. High-strength low-density materials offer the best prospects; inparticular, titanium, steel strip wound in gun-barrel fashion,and epoxy resins reinforced with glass or carbon fibresappear possible. Because of their low Young's moduli andsusceptibility to loss of strength with rise in temperature,much development work will be necessary on methods offitting and avoidance of contact with any possible areas oflocally high temperature before they can be safely used.

7.3 Coreplate steel

The most significant recent innovation has beenthe development of grain-oriented steel, which exhibitslower loss and higher permeability when magnetised in thedirection of rolling. These properties can be fully exploitedin the transformer industry, but only to a lesser extent inthe machines field. The topic has been discussed in moredetail in Section 5.1.

More recently, the steel producers have concentrated onadapting cold-reducing techniques to the conventionalsilicon steels, thereby obtaining more consistent mechanicalproperties, a more uniform thickness and a better surfacefinish.

7.4 Copper

No great changes in conductor materials haveappeared except in higher-strength, but lower-conductivity,alloys, which have found application on large rotors in theconnections between sliprings and winding. The need forhollow rectangular sections for stator and rotor conductorshas required refinement of production techniques to ensurefine control of dimensional tolerances, and, in the rotor,hardness.

[ Brown Boveri AG ]

an increasing need for a higher-strength high-conductivityalloy than is available at present.

8 Systems of cooling

8.1 General

The overall physical arrangement of any generatoris largely determined by the manner in which its active partsare cooled. It is therefore helpful to review cooling systemsin a separate Section. The present Section accordingly coversestablished practices. Methods still under development arereviewed in Section 13.

Economic considerations normally determine the change-over points from one system to another. Hydrogen is usedinstead of air at ratings over about 50 MW for 2-pole designsand at rather higher ratings for 4-pole designs. The point inthe output range at which direct conductor cooling is intro-duced is less clearly defined, but the bottom limit has beenprogressively lowered as techniques have been refined andcosts reduced. Direct-cooled rotors have been used for ratingsas low as 30 MW at 3000 rev/min.

The methods of using hydrogen were first established atpressures just above atmospheric, and as experience wasgained the pressure was progressively stepped up to takeadvantage of the improved cooling obtained by virtue ofthe higher mass flow, without any undue penalty on windageand circulation losses resulting from the higher density beingsuffered. However, with the diminishing net returns, thereis little economic advantage in exceeding about 6—7 X 10s Paabsolute, and in practice most modern machines are in therange to 5-6 X 10s Pa.

In any winding, the thermal drop across the insulation isby far the largest, up to 60% or more, of the componentsgoing to make up the conductor temperature rise. Thepotential gain in passing the coolant through, or in near con-tact with, the conductor within the insulation is thereforevery great. Attention was first paid to the 2-pole rotorbecause of the constraints imposed on maximum size bylimitations of available materials. There is more latitude on

1288 PROC IEE, Vol 121, No. 11R, NOVEMBER 1974, IEEREVIEWS

the stator because its size can be increased until consider-ations of cost and handling set a ceiling beyond which it isuneconomical to go. Accordingly, generators having direct-conductor-cooled rotor windings in conjunction withindirectly-cooled stator windings are widely used up toabout 150MW.

Direct cooling by air of rotor and stator conductors hadbeen tried on a small scale immediately after the FirstWorld War but it did not find wide application, largelybecause there was not the same need to reduce size andperhaps because no serious attempt appeared to have beenmade to prevent the ingress of dirt and oil that led tofouling of the ducts. With the introduction of the com-pletely sealed hydrogen-cooled machine, the latter problemwas overcome and the first direct-cooled rotor winding inBritain went into service in 1952.

Early direct-cooled machines relied entirely on hydrogen,but as liquids offer great advantages in terms of relative heatabsorption (see Table 2) development work to adapt thedesign to permit their use followed quickly.

Most of the major manufacturers are expending con-siderable effort in arriving at practical solutions to thedifficult problems involved. A number have produced testrotors, and in some cases, notably in the USSR (where thefirst such rotor to be used commercially was installed in1959), Germany, Sweden and Switzerland, are now offeringand supplying integrated designs using water-cooled rotorsfor ratings as low as 300—400 MW.

In Britain, experimental rotors suitable for the standard500 MW generators have been tested, but current thinking isthat there is still ample potential in the gas-cooled windingwithout the added complication of introducing water intothe rotating parts.

Design studies12 of 2-pole generators of 1300 MW and2000 MW running at 3000 rev/min indicate quite clearly thatthe former is perfectly possible using materials that are avail-able now and employing either hydrogen or water coolingfor the rotor. Because of the very high capitalised value thathas to be placed on the losses, it is always essential to strivefor the highest generator efficiency consistent with

TABLE 2

APPROXIMATE COOLING PROPERTIES OF HYDROGEN AND LIQUIDS RELATIVE TO AIR

Fluid

AirHeliumHydrogen at 1 X 10s Pa absoluteHydrogen at 3 X 10s Pa absoluteHydrogen at 4 X 10s Pa absoluteTransil oilPyranolWater

Specificheat

1-005-25

14-3514-3514-352091094-16

Density

1000-1380070-210-2884815101000

Volumeflow

101-010101000120-0120012

Heatabsorption

1-00-751-0304 0212050

For stators, development of systems using hydrogen inducts lightly insulated from the conductors and alternativelyliquids flowing directly in tubular strands went on inparallel. The first direct-cooled stator winding in Britain wasinstalled in 1956 on an experimental 30 MW machine,which pioneered the use of water as the coolant in a high-voltage winding.8s Apart from its overall thermal advantage,water avoids the risks from fire associated with oil andfrom the toxic products that can be formed by the decompo-sition of the synthetic fluids by electric arc.

In order to reduce the differential pressure necessary topass adequate hydrogen through the length of a stator coilto an acceptable level, the gas ducts have to be relativelylarge so that the space factor is poor compared with aliquid-cooled coil. Consequently gas-cooled stators tend tobe larger and have higher reactances; nevertheless ratingsup to 815 MW 2-pole and 1170 MW 4-pole have beenachieved.16 The method would however seem to have littlefurther potential.

Experience with the use of water in stator windingshas been so universally good that, not only in Britain butaround the world generally, it has superseded the otherliquids.

Direct-hydrogen-cooled rotor windings have givenexcellent service and are competitive for the largest sizesat present contemplated, but again there are advantages, atleast in the largest sizes, in using liquids. There is now aconsiderable revival of interest in the concept. A numberof small turbogenerators and locomotive phase changershaving water-cooled rotors were built in the 1910s and1920s, but in these the water flowed in ducts in the steelrather than in the conductors so that the experience,although useful, is not very relevant.

maximum reliability and lowest first cost. It is thereforereasonable to compare the two rotor cooling systems onthe basis of the same efficiency. In the case of the 1300 MWrating, it would be possible to reduce the weight of thewater-cooled-rotor body by about 15% compared with thegas-cooled alternative. However, as the flux and currentloadings would be much the same for the stators for bothalternatives, there would be comparatively little or noreduction in transport weight of the stator. Furthermore,the use of a smaller rotor diameter with the more compactwinding slots would result in a somewhat higher transientreactance and lower inertia constant, which are adversefactors. It is therefore considered that there is relativelylittle advantage in adopting water cooling for the rotor atthis rating.

The picture is similar for the 2000 MW rating, but herethe potential reduction in rotor diameter and weight, ofthe order of 5% and 15%, possible with water cooling,could make it attractive by minimising the strength require-ments and physical size of forgings, which would be at theextreme upper limit of present experience.

In the USA the position is much the same as in Britain.While the manufacturers have carried out extensive analyti-cal and laboratory work in connection with water coolingof rotor windings, they too prefer to continue to exploitthe further potential in gas cooling.1S'16%86 It is claimed thatthe proved reliability of hydrogen-cooled rotors resultingfrom simplicity of construction, installation and operationmakes for the best opportunity for achieving the lowest-costpower.

In sharp contrast are the opinions and policies adoptedby a number of the manufacturers in Europe, who see out-puts of around 1000 MW as being the upper economical


limit of gas cooling, particularly for 2-pole designs. Spacedoes not permit a detailed analysis of the argumentsadvanced in support of the use of water at this and evenlower ratings, but the subject is well covered in a numberof papers,23'66t 87 which in turn make reference to otherpublished articles that deal with the topic in much moredetail. Considerable importance is placed on the possibilityof substantial uprating if a water-cooled rotor is used. Whileit may be essential to take advantage of this in the verylargest sizes, it will probably not be desirable to do so ingeneral because the greater part of any uprating can only beachieved by increasing the current loading of the generator.The inherently higher current-induced losses in the machineas a whole can more than offset the reductions in specificexcitation loss resulting from the superior rotor-slot spacefactor and in the component of pumping power forcirculating the rotor coolant, so that there can be an adverseoverall effect on the generator efficiency.

8.2 Stator-coil cooling

A summary of stator-coil-cooling methods is bestprovided by reference to the representative cross-sectionsin Fig. 13. For direct gas cooling, the high pressure dropprecludes the use of hollow conductor strands; consequentlylightly-insulated thin-walled ducts, in phosphor bronze orstainless steel to minimise eddy-current losses, have to bebuilt in between the Roebel stacks. At the ends of the coil,these tubes are left protruding to allow for entry and exit ofthe gas. Even with these relatively large-section ducts, thedifferential pressure needed in the case of a long core is high.Design studies have been made and patents obtained for anumber of schemes to permit gas to be exhausted at pointsalong the slot in order to reduce the flow length. All theseinvolve introducing inaccessible creepage paths to earth,and as far as is known none have ever been used. Anotherapproach has been to increase the mass flow by sub-stantially raising the hydrogen pressure in the coil circuitof the generator.

the losses in the two coil sides in a slot is obtained byadjusting the conductor makeup to take account of thehigher loss intensity in the coil at the top of the slotposition.

The liquid-cooled coil has an inherent advantage interms of reliability compared with all other forms, becauseof the very low (~ 1°C) temperature difference betweenthe copper and the coolant, which virtually eliminates anyinternal stress. Also, as liquids are operated at lower tem-peratures, the thermal differentials between the windingand the core and support structure are also low and arereadily adjustable by control of the temperature of thefluid.

Depending on design philosophy and on thermal andhydraulic requirements, water flow may be simply fromone end of each halfcoil to the other, or through a top andbottom halfcoil in series, confining all external waterconnections to one end of the machine. As ratings and corelengths are increased, it is becoming general practice to limiteach water path to a single halfcoil.

Because of the large number of conductor strands,practical considerations necessitate their being brazedtogether at each end of each halfcoil. In some designs, thecurrent and water-flow paths are separated at these joints,Fig. 14; in others, a single joint box serves to carry currentand water. The water distribution and collection manifoldsin the stator frame are at earth potential, so that insulatingconnection pipes of sufficient length to withstand machinevoltage are required. The usual practice is to use p.t.f.e.,which has suitable electrical and mechanical properties. Amachine having this arrangement is shown in Fig. 3. In analternative design (Fig. 2), all the coil ends in each phasegroup are brought into a waterbox of insulating material.This has the advantages of needing only one supply ordischarge pipe for a group, and of giving complete waterimmersion of the coil—coil joints, although access toindividual coil ends is more restricted.

Fig. 13Some cross-sections ofstator coils


Fig. 14Arrangement ofstator coil ends having separate current andwater connections

[ Kraftwerk Union AG ]

With liquid coolants, hollow-section strands may be usedwith acceptable levels of pressure drop and eddy-currentloss. There are opportunities for variations in detail designdepending on the requirements in particular cases. At thehighest current densities, all strands may consist of ductedcopper. At lower densities, a mixed arrangement of tubesand strands (which may be made thinner to reduce eddy-current loss) is practicable and widely used. In one suchmixed design,54'88 the hollow strands are in stainless steel,and do not carry current. Often a close balance between

8.3 Rotor cooling by gas

Systems of direct cooling of rotors can be broadlyclassified in five basic categories within which there aremany variations:

(a) Radial flow: Gas is fed along axial channels cut in thebase of the slots, and, at intervals throughout the length,is fed through the insulation to radial ducts punched inthe conductors, whence it is discharged to the airgap.The gains with this method are relatively small, being

1290 PROC. IEE, Vol. 121, No. UR, NOVEMBER 1974, IEEREVIEWS

L

Fig. 15Stator having rings dividing airgap into high and low pressure gas zones [General Electric Company, USAJ

limited by the maximum size of subslot and the needfor heat to travel axially along the conductors to reachthe ducts.

(b) Axial flow: The basis is a copper section, or a combina-tion of two sections, incorporating one or two ductseither within the width or at the edges fed from each endand discharging at the centre of the rotor. This shows abig improvement on the simple radial system (a), butthe differential-pressure requirements become excessiveat the larger core lengths.

(c) Axial/radial flow: The combination of (a) and (b) greatlyimproves the heat transfer rate of (a) and reduces thedifferential pressure needed by (b). All modern designsexcept (e), below, use the principle in some form.

(d) Gap pickup:89 This term is derived from a method ofusing scoops or special shaped wedges protruding abovethe rotor surface, to develop sufficient differentialpressure due to rotation to pickup hydrogen from theairgap, pass it through an axial/radial configuration ofducts in the coil stack, and discharge it back to the gap.By arranging for the airgap to be divided axially intoinlet and discharge zones, it is possible to break downthe winding into any desired number of cooling sections,thereby removing any limit on core length, see Fig. 15.

(e) Gap pickup with cross flow: A recently announceddesign90'91 uses the gap pickup principle with conductorshaving many shallow and narrow grooves running trans-versely across the width of the slot. Very high heattransfer is claimed for the arrangement.

Cooling of the end turns in shorter rotors can be integratedwith that of the slot length, but usually the end-turn spaceunder the retaining ring is divided into supply and exhaust-pressure zones in various manners to give separate ventila-tion.

On large heavy-current rotors, cooling of the connectionsbetween windings and slip rings may require supplementingby direct cooling.

In a general review, it is not feasible to go into anydetail of the many different and ingenious methods in which

these basic principles have been applied, but there is muchof interest in References 86—93, many of which in turnrefer to other more detailed papers.

8.4 Rotor cooling by water1 2 ' 2*2 5 ' 8 6 -8 7 -9 4"9 7

The additional m.m.f. ratings that can be achievedby water-cooled conductors are considerable. An exampleis given in Reference 87, in which the ratio compared withhydrogen at 5 X 10s Pa is 1*7, but it is pointed out that inpractice it could not be economically justified to go so farbecause the associated much higher I2R loss significantlylowers the overall machine efficiency. An optimum figureof 1-3 is suggested.

Before the concept can be used, solutions to manytechnical and practical problems peculiar to the applicationmust be found. In addition to the normal corrosion anderosion aspects, electrolytic corrosion has to be guardedagainst because of the direct potential difference across theinsulating lengths incorporated in the feed- and drain-piperuns between the shaft and the winding. The length of thiswater column, the material of the fittings and the oxygencontent of the water are all critical.

The usual concentric coil winding can be used with waterconnections at the top and bottom of each coil and also atintermediate positions when it is desired to cut down thelength of individual flow paths. Further subdivision ispossible by arranging the inlet connections at one end andthe outlets at the other, either discharging the heated waterfrom the shaft at the turbine end, or transferring it back tothe inlet end via pipes running through the unslotted polecentres to rejoin the ducting system incorporated in thebore of the shaft end. The possible number of water con-nections is however limited by the restricted access betweenthe inner surface of the winding and the shaft. Two arrange-ments are shown in Figs. 16 and 17, the former for a589 MVA and the latter a 400 MW generator, respectively.

To avoid subjecting the pipes to movement due to flexingof the shaft, which, although extremely small, could cause


to avoid fatigue. In Fig. 16, the manifolds are arrangedaxially with the connections taken off in the space under

Fig. 16Arrangement of water connections to rotor winding usingaxial distribution manifolds

(GEC Turbine-Generators Ltd.]

fatigue failure, the pipes themselves are either made inreinforced p.t.f.e. (which also incorporates an insulatingsection) as in Fig. 16, or are taken in grooves in the surfaceof the shaft up to the shoulder at the rotor body where thedeflection is a minimum, as in Fig. 17. In the latter, theinsulating lengths are arranged outside the area of the wind-ing and held in position with a retaining ring.

Fig. 17Arrangement of water connections to rotor winding usingcircumferential distribution manifolds

[Kraftwerk Union AG]

In an alternative arrangement (Fig. 18) the end of thewinding to which water connections are made is of lap ordiamond form, which, although more difficult to support,brings the connection points into a more accessible area.

The water is supplied either by a motor-driven pump to aseal system at the free end of the shaft, or by a directly-coupled pump forming part of the seal assembly. The sealingarrangement is a critical part of the design requiring closerunning clearances, yet being insensitive to shaft vibrationand substantial axial movement due to thermal expansioneffects. To avoid corrosion, all piping and ducting up to thewinding is in stainless steel. These ducts are subjected to thefatiguing effects of shaft flexing during rotation; the designand manufacture of the joints between the axial and radialruns, particularly those in the inaccessible region at theshaft bore, are therefore critical.

Distribution manifolds, to which the pipes to individualcoils are connected, are fitted at the surface of the shaft.The design of these also is very much influenced by the need

Fig. 18Water-cooled rotor using diamond form of winding


the winding. In Fig. 17, a single annular manifold dividedinto feed and return chambers is used.

In the top conductors of large 2-pole rotors, the waterpressure developed in the ducts is very high, of the order of "18 Pa. Since it is necessary to minimise the number ofparallel flow paths, deep-section copper is used so thatthe only problem in containing such pressures occurs at thejoints, both copper—copper between turns and copper—stainless-steel when the latter is employed for the pipes intothe winding. Again, the design of these joints is critical.Although the latter can be made in a controlled atmospherebefore assembly on the rotor, the former have to be brazedin position in such a manner as to ensure not only that apredetermined mechanical strength is achieved (checked byultrasonic means against a standard), but also that each turnlies in exactly its correct radial position.

Although long term operating experience is still verylimited, it appears to have been good. In particular, it hasbeen established that sudden changes in water flowrate orthe injection of air into the stream do not result in anysignificant change in vibration level. It has further beenproved that, in the event of a leak, the water is instantlyatomised on leaving the rotor and does not present a hazardprovided that provision is made to prevent it accumulatingunder the retaining ring to avoid increasing the stress in thering. It is also essential to protect the surfaces of the ringagainst free water because of the susceptibility of austeniticsteels to stress-corrosion cracking.

8.5 Overall machine cooling

In all current designs, the stator core and structuralparts subject to heating by stray fields are cooled by hydro-gen, which in turn gives up its heat to water-cooled finned-tube heat exchangers housed in the frame. The physicalarrangement of these coolers is decided by considerations ofthe internal gas-flow paths, handling and transport of theframe, and head room in the power station for assembly.They are usually disposed either vertically at one or bothends, or horizontally, running axially in the upper part ofthe frame.

The arrangement of gas ducts in the core is considerablyinfluenced by the pressure differentials required by othercomponents. For maximum economy of power for circulat-ing the gas, the pressure drops across the main circuits ofcore, rotor and, where applicable, stator windings should bematched. Hence, when gas-cooled stator coils are used,requiring a high differential, the core will have axial ducts.The stator winding limitation is removed when water coolingis employed, so that the core duct arrangement is thenentirely related to the method of rotor cooling. For rotorsystems using long axial paths, the core is normally axially

1292 PROC. IEE, Vol. 121, No. UR, NOVEMBER 1974, IEEREVIEWS

cooled either from end to end or, m.long cores, from endto middle. For radial and gap-pickup systems involvingrelatively low heads, the core is radially cooled. Becauseof the higher loss density at the core ends, cooling in thisarea is supplemented, often by a combination of radial andaxial ducts. Two typical designs are shown in cross-section inFigs. 19 and 20.

silk or rayon using shellac or bitumen were still in regularuse for the major stator and rotor insulation. As ratings andphysical size increased, the insulation became subjected toelectrical and mechanical stresses that began to show up basicweaknesses. In particular, the thermoplastic nature ofshellac and bitumen, while helpful in the manufacturingstages, can result in loss of bond and the consequent

Fig. 19Section of generator having axially-cooled stator core and rotor winding [C.A. Parsons & Co. Ltd.]

Fig. 20Section of generator having radially-cooled stator core

In order to divide the airgap into defined zones ofalternate high and low pressure, to optimise the potentialof rotor cooling systems in which hydrogen is picked upfrom and returned to the gap, rings are inserted in the boreof the stator at the required intervals to restrict axial flowof gas along the gap, see Fig. 15. Co-operating rings may befitted to the rotor also, as shown in Fig. 21.

Circulation of gas is by axial- or radial-flow blowers; theseare reviewed in Section 6.5.

9 Electrical insulation

Up to the end of the Second World War, the originalmaterials based on mica splittings built up on cotton, paper,

(Brown Boveri AG]

development of voids between the stator conductor stackand the insulation wall, in which corona discharges takeplace. The effect is cumulative in that the organic materialspresent are attacked by the acids produced, causing furtherdegradation and permitting conductor vibration thatultimately leads to mechanical erosion of the insulation walland final electrical breakdown.

On long stators having indirectly-cooled coils insulatedwith thermoplastic materials, particularly when the bondcontent is relatively high, the phenomenon known as tapeseparation, or girth cracking, has been experienced. Owingto the high temperature difference between conductor andcore in this type of generator, there is considerable differ-ential movement. On axial expansion, the coil carries the


Fig. 21Section of generator having airgap divided into high and low gas-pressure zones [Westinghouse Electric Corporation]

insulation with it, but at the slot ends the side restraint islost and the insulation is then free to swell under theinfluence of temperature and vibration. On cooling, thecopper contracts, but the insulation being trapped rucklesup at the slot edges. The effect is repeated for each thermalcycle until, in extreme cases, the wall thickness at the slotend becomes so reduced that breakdown occurs.

On the bigger-diameter rotors, the centrifugal forcesacting on the slot-insulation troughs are high enough tocause plastic flow in the old materials resulting in seriousthinning or in cracking of the cell near the root. At theslot ends this effect is compounded with that arising fromaxial expansion and contraction, as encountered on stators,so that very careful reinforcement and support has had tobe provided in these areas. Even so, the incidence of outagesdue to rotor-earth faults became unduly high on the largergenerators.

However, the rapid developments in the fields of man-made fibres and synthetic resins have made available analmost embarrassing choice of new materials, and theadaptation of these for use in specific areas of the turbo-generator is outlined in the following Sections.

9.1 Stator windings

Apart from some experimental machines in Japan,in which glass fibre is used, mica is still universally employedas the main dielectric in all high-voltage insulation systems,and as far as can be foreseen is likely to remain so. Never-theless the methods of application have changed radicallyin recent years on both economic and technical grounds.Large splittings are now both very expensive and lower inquality. They do not lend themselves easily to automatedmachine methods of sheet building or of coil wrapping,and they are subject also to further delamination, makingthe attainment and maintenance of low-loss-tangent coilsdifficult.

Against this background, the adaptation of mica paperfor machine insulation was undertaken. In mica paper themica is split up into flakelets a few square millimetres inarea, and these are then formed into sheets, usually in aFourdrinier paper-making machine. In this state, thematerial has virtually no mechanical strength, being heldtogether only by electrostatic forces. A strong backing isessential for handling purposes, and a thin glass cloth isnormally used. The resulting sheet can be made with greatuniformity in thicknesses, in the range equivalent to50—250 g/m2, suitable for a wide range of applications. Forcoil insulation, it can be wrapped on, either in sheet formusing the basic Haefely process or, after slitting into tape,by conventional taping machines.98

A very wide range of synthetic resins having broadlysuitable electrical and mechanical properties are now avail-able and are being constantly added to. The insulationengineer has necessarily to confine himself at any one timeto selecting a limited number, which he must submit toexhaustive laboratory testing, alone and in conjunctionwith the dielectrics he intends to use, to ensure that theconstituents are compatible and that the combination hasthe necessary characteristics for long electrical, mechanicaland thermal life.98""106

The polyesters were the first suitable synthetic resins,and the earliest (1951) of the new insulation systems wasbased on these.99 The silicones were also considered, but asthese are more difficult to process and are expensive theyare not used in turbogenerators. The latest group to bedeveloped, the epoxides, have proved to be the mostversatile, and resins in one or other of the subgroups,particularly epoxy novolak and cycloaliphatic, are nowwidely used either alone or sometimes in conjunction witha polyester.

With the choices available, it is not suprising that virtu-ally all the major manufacturers have developed their ownsystems, many of them covered by registered trade names.


Some retain flake mica, some rely entirely on mica paper,and others use flake and paper in various combinations,both in the slots and on the endturns. The most generaltrend is towards all mica paper in the slots because of itsbetter inherent stability, and flake-based material on theends because of its rather superior mechanical properties,but there is no really clear-cut uniformity at present.

Two basic methods of application of the resin are avail-able, and both have their adherents. In the earliest developedmethod, the dielectric is applied dry with only sufficientresin incorporated in it to hold the constituents together.The completed coils are then subjected to a vacuum/pressure cycle during which the insulation is impregnatedwith resin. The coils are finally clamped in sizing fixturesand heated to cure the resin. This system requires extensivecapital plant and imposes restraints on the resins that maybe used, because of the need to have a long pot life toendure repeated cycling as batches of coils are processed.Nevertheless it is very widely used, see Fig. 22.

Fig. 22Vacuum/pressure impregnation and curing plant forsynthetic-resin-bonded stator-coil insulation

[Westinghouse Electric Corporation!

The more recently evolved system has been made possibleby the introduction of tapes in which the full quantity ofresin is built in, in the same manner as with the old shellacand bitumen materials. After application the insulation isconsolidated and cured in the usual heated presses. Thisprocess is becoming more frequently used because it avoidsthe many problems inherent in an impregnation processusing thermosetting resins. The end result is virtually thesame in both cases.

These modern insulation systems have high thermalstability, and very superior mechanical strengths comparableeven with those of structural materials. They have dieability to withstand the vibration levels and shock loads ofcurrent and future large generators, provided that properattention is given to integrating them into the mechanicaldesign as a whole.

9.2 Rotor windings

The three critical areas in the application of insula-tion are between turns, to earth in the slot length and underthe retaining rings. Because of the very high premium onspace, it is essential that all insulation thicknesses should be

at the minimum consistent with long electrical and mechani-cal life. For many years flake mica, usually bonded withshellac, was used in. all positions because of its high electricalstrength. As rotors became larger in diameter and longer,the inherent mechanical weakness of these old materialsoften resulted in disintegration and ultimate electricalfailure from the effects of high compression loading anddifferential thermal expansion. In modern designs, syntheticresin-bonded glass fibre is widely used because it offers agood compromise between high mechanical rigidity andreasonable electrical strength. The material is howeverrelatively brittle, and consequently difficult to handle,particularly in the thin sections necessary for slot insulation,and present trends are towards composite laminates incor-porating plastics-film materials to give some small degree offlexibility and toughness.

While the operating voltage between turns is only a fewvolts, the choice of interturn insulation is surprisinglycritical because of complex chemical and electrolytic actionsthat can occur between resin and copper in the confinedenvironment of the slot length, either during processing inmanufacture or in normal service. Some of the breakdownproducts are electrically conducting, which can result insevere short circuiting between turns. For this reason andbecause of its excellent resistance to abrasion, asbestos-based material is still used in this location, but precuredresin/glass laminates are becoming more common.

The insulation to earth in the end turn region not onlyhas to withstand the high compressive loading imposed bythe coils and their packings, but also has to be resistant toabrasion from relative movement arising from differentialthermal effects. It also has to have sufficient elasticity toaccept, without cracking, the strains imposed on it fromthe expansion of the retaining ring at speed. Phenolic resin-treated asbestos sheet has proved very suitable except forits low and erratic electrical strength. There has thereforebeen a movement towards the use of epoxy-glass materialscoated on the inner surface with some antifriction material,such as p.t.f.e., to reduce the risk of abrasion and to mini-mise the stresses imposed on the coils and their joints.

National standards differ in respect of the level of testvoltage that should be applied to the winding, as the finalproof that the insulation system has been properly designedin terms of thicknesses and creepage distances and has notbeen damaged during manufacture. For example, in Britainand in Europe, the current British71 and the InternationalElectrotechnical Commission (1EC)72 standards correspondin specifying an a.c. test level of 10 times the rated excita-tion voltage with a minimum of 1500 V and a maximum of3500 V. Others, notably the US standards,73 althoughgenerally the same, do not set any upper limits. This meansthat, as generator ratings are increased and the excitationvoltages are raised to prevent the rotor current becomingexcessive, the test voltage for the largest machines currentlyin manufacture to this standard is of the order of 7500 V.Clearly this wide disparity has a very significant effect onrotor winding and insulation design and, because of the verylimited space available, particularly in 2-pole machines, mustresult in a larger rotor than would be necessary to meet theless onerous specifications.

All test specifications must be related to conditions metwith, both in normal and in credible abnormal operation.In assessing the electrical tests for rotor windings, accountmust therefore be taken not only of the slipring voltage atsteady full load, but also of the maximum voltages that canbe experienced under field forcing, field breaker openingand pole slipping, particularly when the excitation is froman a.c. source through rectifiers that block the flow ofcurrent in the reverse direction.

Furthermore, current practice is towards the use ofthyristors as the rectifying means, see Section 11. With


these, voltage regulation is obtained by control of thethyristor firing angle; the rotor winding is therefore subjectedto unsmoothed direct current, which, at large firing delayangles, is composed of steep wavefront spikes. It is conse-quently necessary to consider this in the design of theinsulation system both to earth and between turns.

The whole subject is complex and open to considerablediscussion. It is reviewed in detail in References 106 and107, which also make proposals on the basis of experimentalwork for revisions of the standards to take proper accountof these new factors.

10 Transport

The handling of the component parts of steamturbines and of generators within the factory and the powerstation need never impose a limit on the size of unit thatcan be manufactured. Nevertheless, the cost of civil worksand mechanical equipment in the form of cranes, machinetools etc. mounts rapidly as weight and physical sizeincrease, thereby offsetting some of the overall economicadvantage in adopting larger units.

It is the movement of equipment between the manu-facturers' works and the power station that presents thedesigner with the real challenges and threatens to imposean ultimate limit. For the turbine, there are never likely tobe any insuperable problems because the largest items,such as the l.p. casing, can be subdivided in such a way thatindividual pieces can be moved by normal methods. Eventhough it may be necessary for the large l.p. rotors of1500 and 1800 rev/min machines to have the last row ofblades removed for transport, the objections are basicallyonly the economic ones of cost and time for reassembly.For the generator, however, the limitations, whetherthey be of loading gauge or of weight, are serious becausethere is not the same possibility of breaking down the majorcomponents into easily handled pieces.

The problem is most serious for the stator because onlyin special cases, such as single-phase generators supplyingtraction systems, in which the winding can be suitablyarranged, can the factory-built core and windings beeconomically split for shipment. The solutions that have tobe evolved vary in detail, depending upon national trans-port investment policies of the countries in which movementhas to take place. Hence, manufacturers in countries inwhich, for example, transport has to be by road are facedwith the need to develop designs capable also of beingmoved by rail when exporting to countries where this modeof transport is necessary.

Much ingenuity has gone into design both of generatorsand of vehicles. In Britain, movement by rail is out of thequestion because of the very restricted loading gauge. There-fore inland transport has to be by road, and the first limitmet with is weight. By careful route planning, it has so farbeen possible to avoid any serious restriction on dimensions.The universal method of minimising the weight for a givenframe size is to build the core in a skeleton framework,which is finally assembled as a unit in an outer gas-tightand explosion-resisting frame. The outer frame, althoughof large size, is comparatively light and is unlikely to poseinsuperable transport problems. Fig. 23 shows one form ofassembly in which the caged core is threaded axially into asingle-piece outer frame. An example of a generator thathad to be designed for transport by road in Britain and byrail in Australia is shown in Fig. 24. Owing to rail loadinggauge restrictions, the outer frame had to be split on thehorizontal centre line.

Much development is taking place in road vehicles, andslow but definite steps in weight-carrying capability havejust kept abreast of requirements.108 The most significantadvance is that of harnessing the air-lift or hovercraft

Fig. 23Axial insertion of660MW caged stator core and windinginto OUter frame [C.A. Parsons & Co. Ltd.]

Fig 24.Assembly ofSOOMW caged core and winding in lower halfof outer frame, split horizontally for ease of transport

[GEC Turbine Generators Ltd.)

principle to the conventional swan-neck vehicle. By thismeans, uplifts of up to 125 t are possible.

When factories and power stations are located on orclose to the coast, shallow-draught roll-on/roll-off ships areused, so that heavy-duty dock-side cranes or ships' derricksare no longer needed, and the designer's task is correspond-ingly eased.

In other countries, notably many parts of Europe andthe USA where weight considerations require the heaviestpieces to be moved by rail, the need to keep within themore generous but nevertheless still very restrictive(4200mm-diameter) loading gauges generally rules out theuse of anything but the so-called Schnabel principle. Atypical car of this type in use in the USA is shown in Fig. 25having a payload capacity of 5501. Similar vehicles capableof carrying 4201 are in service on a number of railways inEurope. An ultimate capacity of up to 6001 is consideredpossible. The stator frame itself forms an integral part ofthe car assembly, and must be designed to accept the result-ing tensile and compressive stresses.

An interesting exploitation of the environmentalpossibilities is that adopted by one of the West Germanmanufacturers, see Fig. 26. In this instance, a cantileveredgantry enables a machine to be lifted off the factory testberth and loaded directly onto a barge on the canal.

Clearly, transport difficulties give added stimulus to theinvestigations into new forms of generator constructionoutlined in Section 13 because of the potential for sub-stantial weight and size reduction.


Fig. 25Transport by rail of heavy stator using Schnabel car [Westinghouse Electric Corporation]

Fig. 26Extension of crane gantry to permit direct loading onto canalbarge [ Kraftwerk Union AG ]

11 Excitation systems

The design and maintenance problems of direct-coupled d.c. generators impose a limit on this form ofexcitation at 60— 100MW unit rating. Similarly, theeconomic and practical limit of application of low-speedgear-driven exciters occurs generally at about 250 MW. Thedevelopment of alternative systems based on the solid-staterectifier, notably the silicon diode and, more recently, thethyristor, has meant that the d.c. generator is being largelysuperseded, and that the excitation requirements of thelargest sets that can be envisaged will present no significantexcitation problems. These new systems are already provingto be extremely reliable.22' 109~116

Two main lines of development have taken place:

(a) 'static' systems in which the rectifiers are cubicle moun-ted and the current is fed to the rotor through sliprings

(b) 'brushless' systems, so called because the rectifiers andtheir associated equipment are shaft-mounted to rotatewith the main rotor, avoiding the need for sliprings andbrushes altogether.

11.1 Static rectifier systems

These can be subdivided into two groups accord-ing to the type of rectifier:

(a) diodes supplied from a shaft-driven a.c. exciter

(b) thyristors (controlled rectifiers) supplied either from ashaft-driven a.c. generator as for diodes, or from themain alternator terminals via a transformer.

PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, fEEREVIEWS

In both systems, a rotating-field a.c. generator with a con-stant-voltage pilot exciter is normally used. The fieldcan be either solid or laminated and with or without adamper winding. With the trend towards higher-responsesystems, laminated-rotor machines, usually 100, 150 or200 Hz, now predominate.

The rectifier cells along with their associated fuses,capacitors etc. are cubicle mounted and can be cooled bynatural, or by forced, air circulation, or, as is now becomingusual, by water. For large generators, the excitation poweris such as to require a number of cubicles operating inparallel if air cooling is used, but with water a single unitis normally sufficient. Spare capacity up to 20% is usuallybuilt in, and, as the cells are assembled in racks, maintenancecan be carried out on individual racks without affecting theperformance of the system, even under the most severeoperating conditions. In the early installations, a completespare cubicle was often provided, but the proven reliabilityof cells is so high that present practice is to rely entirely onthe 20% built-in overcapacity.116

With diode systems, excitation-voltage control is obtainedby action on the field of the main exciter in the conven-tional manner, so that overall response is very dependent onthe exciter time constants. Where high response rates havebeen required in the past, mercury-arc rectifiers have beenemployed to avoid the exciter delay time. These equip-ments are bulky and expensive, but. with the developmentof the thyristor a relatively cheap and compact substituteis now available. Since voltage control is by variation of thefiring angle, a thyristor bridge can be supplied at a constantalternating input voltage to eliminate exciter delay.22'116"118

The supply to the thyristors may therefore be taken fromany suitable constant-voltage source. A shaft-driven exciteris still used when maximum security against disturbance tothe excitation is required under system fault conditions.This is relatively expensive, and in practice the cheaperalternative of drawing the supply from the main generatorterminals through a transformer is often employed. It suffersthe disadvantage that under system fault conditions thegenerator voltage is reduced just at the time an excitationboost is required. This effect can, however, be to a largedegree offset by designing for a higher supply voltage to therectifiers than would be necessary under steady conditionsand also by various compounding means derived from thefault current itself using a saturating airgap transformer ora power current transformer.112

Because sliprings are required with all static excitationsystems, conventional field-suppression methods using abreaker and discharge resistor to limit the voltage rise canbe applied. However, d.c. circuit breakers for the very heavycurrents of modern large generators present severe designproblems and are bulky and expensive. With thyristorsystems, field suppression is achieved directly and simply

1297

by inverting the thyristor bridge, and therefore the fieldswitch and resistor are no longer necessary,

11.2 Brushless systems: general

Heavy-current sliprings and brushgear representdifficult and potentially-hazardous maintenance problems.The small size of solid-state rectifiers, together with theirinherent robustness and reliability, make possible thebuilding of compact assemblies that can be mounted on,or coupled to, the main generator rotor. When this is fedfrom a rotating-armature exciter, also direct coupled, theneed for sliprings is eliminated.

In order to monitor conditions of current and voltage inthe rotor circuit, and to provide earth-leakage protection,either small measurement sliprings, with retractable brush-gear, or telemetry must be provided.

The design of rotating-armature exciters for brushlesssystems presents mechanical and electrical problems in thelargest sizes. There is a low limit on diameter arising fromthe need for a laminated rotor construction, and the means ofretaining the end windings are much more restricted forwindings carrying alternating current than they are withthose carrying direct current. Any form of rectifier givesrise to additional losses in the generator supplying it, byvirtue of the harmonics in the current wave form. Thyristors,particularly when operating at large delay angles such aswill occur at low loads, impose very high harmonic loadings.

11.3 Brushless systems using diodes

Systems using rotating diodes have proved soreliable that they are now almost exclusively used ongenerators up to 100 MW and are rapidly superseding otherforms on sets of the largest sizes, see Fig. 27. The 3-phasebridge connection is normally used, but polygon arrange-ments using multiphase exciters may also beemployed.22'112-116'119'120-123

For the larger machines, it is general practice to connecta fuse in series with each diode, or diode string, so that inthe event of diode failure the faulty cell can be isolatedand any risk of disintegration on heavy current tliroughflowavoided.

been quoted. The resulting fuses are considerably largerand less reliable than the cells, and there is an increasingbody of opinion that considers overall safety might even beincreased if fuses were omitted on all sizes, or confined tothe main leads from the exciter.

Various means have been devised to indicate when afuse has blown, such as subsidiary fuses incorporating amechanical telltale viewed by stroboscopic light,119 orphotoelectric cells and magnetic monitoring.120 Other lesssophisticated but non-fail-safe methods are also used.

It is obviously impracticable to incorporate any form ofcircuit breaker in a rotating system, and de-energising ofthe main generator field is achieved by suppressing theexciter field, which can be done very rapidly by invertingthe thyristor bridge that supplies it. The time constants ofthis type of exciter are small, and the de-energisation timeis comparable with that obtained using field breakers.112'116

11.4 Brushless systems using thyristors

There is considerable current interest in adaptingthyristors for rotating systems to take advantage of thehigh overall response rates.116'121'122 However, not only arethe cells themselves inherently less robust than diodes, butalso many additional components and their interconnectingleads for the control circuits have to be provided. There isalso the problem of transmitting the control signals to theshaft reliably and without the use of sliprings.

Because of these new problems, the application has notyet progressed beyond the experimental stage. Fig. 28illustrates the assembly of the rotating parts of an exciterfor a 660 MW generator.

Fig. 27Brushless excitation unit for 660 MW 3000 rev/min genera-tor using Silicon diodes [GEC Turbine Generators Ltd.]

All components have to withstand centrifugal accelera-tions of up to 6000 g. The rectifier cells present no greatproblem, but the design of fuses and capacitors for thiscondition is much more difficult. In addition, the fuses haveto be capable of clearing should a rectifier fail under pole-slipping conditions, when high induced voltages are super-imposed; values as high as 5 or 6 times31' 12° normal have1298 PROC.

Fig. 28Rotating thyristor assembly with main and permanent-magnet control exciter armatures

[C.A. Parsons &. Co. Ltd.l

Field suppression can be achieved rapidly by invertingthe main thyristor bridge.

11.5 Pilot exciters

Where shaft-driven main exciters are employed, itis usual for pilot exciters to be fitted to provide the suppliesfor the main exciter field. For many years d.c. machineswere used, but as currents were low, commutation problemscaused by glazing were common and a.c. generators are nowuniversal. Various forms are used, either permanent-magnetsalient-pole air-stabilised, or inductor-type homopolar orheteropolar, all of which are brushless, having no windingson the rotor. These have a small voltage-regulation droop,and if a truly constant voltage is essential it is necessary to

IEE, Vol. 121, No. 11R, NOVEMBER 1974,1EEREVIEWS

use a wound-rotor machine with the attendant disadvantagesof sliprings.

The trend on these machines is towards quite large out- •puts; lOOkW is now common and requirements for 150—200 kW are becoming evident. More attention is being paidto mechanical integration of the pilot exciter with the mainexciter to minimise the number of bearings and couplings.

11.6 High-response systems

For some years there has been a growing demandfor fast-response high-performance excitation systems toimprove the overall transient stability of synchronousgenerators, both to assist transmission-system design andalso to enable lower short-circuit ratio generators to beused. In addition to the normal design methods of reducingtime constants, higher frequencies in the region of 250—300 Hz are sometimes employed.

With the wide variety of excitation systems now in use orproposed, the old definitions covering exciter response areno longer adequate to give a proper comparison. Efforts arebeing made to establish more realistic means of definingand comparing the performances of different 'high-response'systems, and then to devise practical tests to verifythem.113'115'124'125

11.7 Automatic voltage regulators

Automatic voltage regulators, once regarded asdevices intended largely to relieve operators of a tedioustask, are now an essential link in the operation of electricalpower systems. For diode-excitation schemes the regulatorcontrols the main exciter field, and for thyristor equipmentsit operates on the firing circuits.

The principal functions demanded are:

(a) to maintain constant system voltage(b) to control the division of reactive load between

machines running in parallel(c) to raise the transient stability limit of the system by

•increasing excitation power subsequent to a fault(d) to modify artificially the generator characteristics under

steady-state conditions, so that under-excited operationin the dynamic region becomes possible.

Requirements a and b are readily satisfied by electro-mechanical regulators of all types, but c and d necessitatecontinuously acting devices with no dead band, combinedwith amplification properties and high response. Hence thepurely electrical type was introduced, and this has almostentirely superseded the electro-mechanical versions onlarger systems.126

Earlier types of continuously acting regulators usemagnetic amplifiers, but these have given way to thyristorsand solid-state circuitry.

Great emphasis has now to be placed on reliabilitybecause of the trend towards lower short-circuit ratiogenerators, which results in greater dependence on theexcitation system as a whole to maintain power-systemstability.114 On larger units, dual-channel regulators arecommonly employed. In these, components are duplicatedin whole or in part, and, should one channel become faulty,control is automatically switched to the other. Many regula-tors are also provided with a manual-control system that isused mainly during commissioning and maintenance but isalways available as an emergency standby.

Other features include devices for limiting excitation sothat it does not either fall below or rise above preset limits.Stability circuits, the design of which is based on systemparameters, are sometimes provided to limit the effects ofsudden changes on the system, e.g. full-load rejection.

Fault conditions and load-rejection studies to determinethe transient behaviour of the main regulating circuitelements are now usual. Simplified block diagrams areemployed, and attempts are being made to standardisecomputer representations of the various regulator para-meters.22' ni

12 Protection

The upratings made possible by the introductionof increasingly effective methods of heat removal areaccompanied by considerable decreases in thermal timeconstants. Consequently modern highly-rated generatorsare significantly more susceptible to damage not only byinterruption of coolant flow but also by overcurrent instator and rotor windings and by unbalanced loading.

Whereas in the past it was often sufficient to give analarm of abnormal conditions, this is no longer so. Properco-ordination of automatic control and tripping functionsis essential to afford adequate plant protection and yetavoid unnecessary disconnection. Protection circuitsincreasingly incorporate means of automatically initiatingcorrective action as a first step. A more detailed summaryof modern requirements and of the principles used in meet-ing them is given in Reference 128.

13 Future developments

13.1 General

Incidental mention has been made in earlierSections of a number of possible lines of development thatcould push back, or in some cases even eliminate, the barriersagainst the attainment of outputs very significantly inexcess of the ceiling of around 2000 MW that is widely seenas being the limit, at least for 2-pole designs, for currenttechnology. Outputs at least as high as 6000 MW have beenpredicted as being achievable.

While current interest is still largely in ways of raising out-put levels, it must be remembered that the steam turbineand the steam-raising plant are also facing severe problemsin development. Consequently the incentive to evolve newforms of generator construction is as likely to receiveimpetus from their potential in terms of lower capitalisedcost, smaller size and lower reactances as from ability toachieve higher ratings.

The more significant of these future possibilities are out-lined in the following Sections. One of them has alreadybeen developed to the stage when a prototype machine of300 MVA has been manufactured and tested. In another,a full-size rotor has been run under test conditions simulat-ing those encountered in commercial service. Very activeinvestigation on others is being carried out in universitiesand industrial research establishments. However it is tooearly to try and predict to what extent and when any ofthem may be accepted as offering commercially viablealternatives to current practice, particularly as the mostadvanced concepts pose formidable technical and construc-tional problems.

13.2 Completely liquid-cooled generator

Following the introduction commercially of watercooling of the rotor windings, the realisation of the principleof the completely water-cooled generator, without hydro-gen, discussed in the literature over the years,94 becomespossible. The advantages are not so much in any ability toincrease utilisation of active materials over and above whatis possible using water-cooled windings only, but rather tosimplify construction and operation by avoiding the needfor hydrogen. In the ultimate, no flow of gaseous mediumis required at all, so that blowers, coolers and ducting in the


frame, all of them requiring considerable space and extraweight, are eliminated.

The original concepts for liquid cooling of the stator,which were the subject of patents in the 1920s, envisagedthe stator and rotor spaces being separated by a nonmetalliccylinder in the airgap, so that the stator core and windingscould be immersed in oil just as transformer windings are.A few such machines were made in the years following theSecond World War, but the small gains hardly justified theadditional complication of another potentially hazardouscoolant, and these designs have not been taken up.

Interest has been revived now that the technology ofusing water in both windings has been established. Anexperimental generator66 of 300 MVA, shown in section inFig. 29, has been manufactured in which the designprinciples in this approach have been taken to their prob-able limits. Immersion of the stator in oil has been aband-oned in favour of water cooling of all active parts and alsoof those structural members subjected to leakage fields.

In the absence of a gas flow in the airgap, removal of therotor windage and negative-sequence losses has to be

. effected by other means. In the design referred to, an airgapcylinder with cooling pipes on its inner surface is used toremove the heat and to reduce the windage loss. As thesepipes are in the main field, their design and method of con-nection have to be such as to avoid undue eddy-currentlosses. In 4-pole generators, the windage loss is a lowerproportion of the total, and a design is proposed in whichthe cylinder is omitted and a small flow of nitrogen iscirculated along the airgap by a simple axial-flow fan andthe heat is removed by coolers at one end of the frame.

Cooling of the rotor surface is by conduction to thedamping loops immediately under the slot wedges. Theseloops are much heavier than normal and are connected intothe water circuit of the excitation winding.

The overall design permits a separate choice in respectof the filling of the stator and rotor free spaces. In the testmachine, nitrogen is used in the stator and air in the rotor

Fig. 29Simplified longitudinal section of a 300 MVA generator having complete water cooling [ Brown Boveri AG ]

While this system has the additional advantage of givingan excellent opportunity of obtaining calorimetric verifica-tion of loss intensities, it requires a reliable solution to thedifficult problems of water cooling the core. Three basicmethods are available:

spaces, respectively. If shaft seals are fitted, the systemallows partial evacuation of the rotor space further toreduce windage losses, provided that the lower heat transferrate and inferior electric creepage capability at the lowerpressure can be tolerated.

(a) provision of lightly insulated tubes passing axiallythrough the core behind the slots as in the experimentalmachine. The same risks of core damage from failureby abrasion of the insulation exists as for through-coreclamping bolts.

(b) insertion radially of water-cooled pads of stainless steel.These have been used in the past, but, as heat flow hasto be axially across the interlamination insulation, alarge number are required and replacement is notnormally possible.

(c) reliance on conduction of heat to the back of the corewhere it is transferred to cooling pads attached to thesurface, with some additional conduction to the statorwinding when it is at a lower temperature. This approachis more suited to multipole hydroelectric generatorshaving small depths of core, but could prove possible for4-pole turbogenerators.

13.3 Slotless generators

The lengths of airgap, 150 mm or more, nownecessary with the largest outputs to maintain an acceptableshort-circuit ratio make more attractive the concept of theslotless machine in which the stator, and even the rotor,windings are located in the otherwise wasted space and slots aredispensed with. The outside diameter of the core can thenbe reduced by twice the slot depth, or, if the gap is notadequate to take the full winding thickness, by some ratherlesser but still valuable amount.129"132

The principle illustrated in Fig. 30 affords the bestapproach to obtaining either significantly higher outputs orreduced size and weight with the minimum of departure fromproven constructional features. Other advantages havealready been mentioned, e.g. in Section 4.4. Quite fullanalytical treatment and design studies are set out in anumber of papers.

1300 PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974,1EEREVIEWS

active length {cooling-water

connection

laminated stator core

stator,winding and insulation

solid rotor core

central hole

Fig. 30Slot less alternator

binding driveshaft

[Proc. IEE]

It has been estimated131 that a 6000 MW generator couldbe designed having a power/weight ratio of up to twice thatof a 2000 MW machine built on conventional lines andhaving approximately half the transient reactance. Thepotential in this approach is also brought out by a directcomparison at 660 MW with a standard conventionaldesign. This shows a saving of 4-5 m in length, 0 4 m inoutside diameter, 1501 in weight and up to 23% in losses.A cost estimate indicates that, although the equivalentrotor would be slightly more expensive, the completemachine should be significantly cheaper with some furtheradvantage from lower-cost civil works. At present-dayvalues, it is claimed that the loss reduction would becapitalised at over £200000.

The magnitude of these estimated gains is such that itis not surprising that the principle is now receiving con-siderable development effort.

13.3.1 Slotless stators

The elimination of slots affects design philosophyin other ways than those mentioned above. Since toothsaturation is no longer a factor, higher airgap flux densities,up to 2 T instead of 1 -0—1-2 T for conventional designs,may be employed. While the simpler geometry at the endsof the core may facilitate the estimation of the axialpenetration of leakage flux, it may prove to be more diffi-cult to combat it than it is when teeth, which can be split,are present. This is an important consideration, particularlywhen much higher electrical loadings, up to 400 A/mm, arebeing contemplated.

There is much greater freedom of choice of conductorform with a uniformly distributed winding, but as theconductors are located in the main field they need to befinely stranded and twisted to reduce eddy-current lossesto an acceptable level. In theory, the whole windingincluding the end turns can be fully encapsulated; inpractice, there would be severe difficulties in the sizeinvolved and the accuracy necessary. It therefore seemslikely that development will be along more conventionallines.

Reactances are inherently lower so that transient-stability margins are higher, but fault currents are larger.

Against the advantages must be set a number of challeng-ing practical problems. The method of securing the insulatedwinding to the core has not only to be able to acceptindefinitely full-load torque and the high tangential forcesunder system-fault conditions, but also to sustain, withoutfretting and loosening, ovalling of the core at twice theoperating frequency. Various proposals have been made,from glueing to wedging using tapered wedges in conjunc-

tion with shallow axial grooves in the bore of the statorcore.

If complete encapsulation of the winding does not provefeasible, adequate support of the end turns against vibrationon normal load and distortion due to the heavier faultcurrents becomes more difficult because of the higher spec-ific loading proposed and of the relatively finely strandedconductors having lower inherent rigidity. While water cool-ing will present no great difficulty, the small size of the con-ductor strands will preclude the use of hollow-sectioncopper, so that separate cooling tubes will be necessary.

13.3.2 Slotless rotors

If means could be devised to hold the rotor windingsagainst centrifugal action and to transmit the load and faulttorques without the use of slots in the rotor surface, thebenefits would be farreaching. The areas of high stressand magnetic saturation at the roots of the teeth andwedge grooves would be eliminated, and as a result themaximum rotor diameter could be increased by 10-15%,and the surface flux density by 50% or more. Furthermore,the maximum-length limit would be raised because thelarger diameter rotor would be inherently stiffer and wouldnot be weakened by slotting. The additional potential gainsover and above merely using a slotless stator are thereforeconsiderable.

The technical and manufacturing problems are, however,very formidable. They are analysed and solutions are pro-posed in References .130—132. They vary from theshrinking over the winding of a series of overlapping non-magnetic steel tubes, to banding in the form of a continu-ous cylinder or of discontinuous hoops, using steel ortitanium strip, or, more advantageously, carbon fibre.

In addition to the centrifugal stress, the design of theretaining cylinder will have to take account of additionalloading due to thermal-expansion effects in the windingconductors and in the steel. It will also be subjected tocyclic bending stresses from the flexing of the rotorbetween the bearings and to differential thermal axialstrain, both of which can damage a filament-woundmaterial.

The transmission of torque from the shaft to the windingnecessitates some means of positive drive between them,and this transfer has to occur through the insulation.

Cooling by gas or by liquid is possible on broadly con-ventional lines.

13.4 Evaporative cooling

From time to time there has been discussion onthe possibility of using the latent heat of evaporation of a


liquid in the cooling of electrical machines. For a turbo-generator it is not enough merely to inject a volatile liquidinto the enclosure, as has been done on small motors forexample. Schemes have, however, been proposed forcooling rotor windings either using one of the Freons in aclosed circuit,86 or water that, after evaporation in itspassage through the conductors, is ejected as vapour intothe airgap whence it is collected, condensed and recirculated.

Since only 1/13 of the mass flow of water is requiredfor a given heat-removal capacity if it is vaporised ratherthan used entirely in the liquid state, the potential advantageis considerable. For example, it is claimed that satisfactorycooling can be ensured at current densities of 16 A/mm2 ormore.

Before the principle can be realised in practice, manyfundamental problems associated with corrosion and erosion,hydrodynamics and flow distribution, scaling, insulation,starting and stopping procedures etc. require detailedinvestigation for the particular conditions of operation inan environment of very high centrifugal loading.

As far as is known, work on the use of Freon has beenconfined to the laboratory, but a recent paper133 summarisesthe development and rig running in vapour of a 200 MVA,3600 rev/min rotor having evaporative cooling of the wind-ings by water. This test rotor is reported to have confirmedthe design and not to have given rise to any unforeseenproblems, at least in the short term. In previous discussions,fears have been expressed that possible lack of symmetry ofvaporisation would result in vibration problems. The testrotor did not exhibit any symptoms of this kind on startingup, under steady running conditions with varying current oron emptying before shutting down.

The basic cooling circuit is shown in Fig. 31. The pressure-control valve enables the boiling temperature to be con-trolled by adjustment of the pressure in the space around therotor. Corrosion is minimised by ensuring that the steam isheated slightly above the saturation temperature, afterleaving the winding, by the friction of rotation.

insulating cylinder

device for catchingoverflow water

water-distributing ring

nn

de ioniser pump

pressure-control valve

condenser

Fig. 31Principle of the evaporative-cooled rotor winding

[IEEE Trans.)

13.5 Superconducting generators

In recent years, increasing interest has been shownin adapting the principles of superconductivity to electricalequipment generally.134"144 The early work in the field ofmachines has been concentrated on the homopolar typebecause it offers the simplest physical arrangement of thesuperconducting coils even though it involves difficultcurrent-collection problems. In Britain, trial machines havebeen built for the Ministry of Defence (Navy) for ship pro-pulsion and for the CEGB for circulating water-pump drivesat Fawley with a rating of 2450 kW at 200 rev/min. Areview of the operating principles and running experienceof the latter are given in Reference 134.

More recently, attention has been turned to generators.A considerable amount of investigational and model workhas already been reported in the USA,13S'136' 142~144

Britain,138"140 France and the USSR. Superconductors cancarry immense direct-current densities without loss, and socan in principle be used for the field winding. Because theresistivity in the'superconducting state is zero, all changesin current and field are propagated from the surface of thewire inwards — an effect having the same origins as the skineffect in copper, but which is hysteretic. This hysteresiscauses a loss with alternating currents and fields, which,with present technology, renders it impracticable to makethe armature superconducting.

Superconductors must be held down to temperaturesaround 5°K, which necessitates the use of liquid helium asthe cooling medium. From this point of view, design andoperation of the cryogenic coils would be greatly facilitatedif the machine could be turned 'inside out' with the fieldstationary and the armature rotating. However, this approachintroduces the even more difficult problems of armaturemechanical construction, and transference from it of veryheavy currents at high voltage. While recently reported •work on liquid-metal-slipring technology suggests that thelatter is not necessarily insoluble,14S present investigationsappear to be concentrated on obtaining solutions on thebasis of retaining the philosophy of a rotating supercon-ducting field with a static armature at ambient temperature.

A solid-steel rotor would conduct heat to the cryogeniccoils, and so current designs envisage a hollow cylindricalconstruction. One form proposes stainless steel and anothera fibre-composite material, with stiffening discs at intervals,to reduce the heat gain still further. The basic concept isshown in Fig. 32.

While the superconducting coils themselves are light, theelectromagnetic forces acting on them are very large andadd to the centrifugal forces. The design of the retainingbinding will therefore not be easy.

As superconductors must be protected from the con-tinuous a.c. fields set up by load unbalance and harmonics,and the much greater fields experienced during transmission-system faults, it is necessary to include a thin copper oraluminium cylinder or damper shield. This has to beattached to the rotor and rotate with it, at least understeady-state conditions. An interesting application of the3-element-machine concept to this shield has been suggestedin the discussion on Reference 141 as a means of sub-stantially reducing the fault torques transmitted to thewinding and rotor shell and shafting. The principle requiresthe shield to be mounted in bearings on the rotor and to beconstrained to rotate synchronously under normal con-ditions but to be free to be displaced when high transienttorques during faults are induced in it. If sufficient inertiacan be incorporated in the shield and the mechanical prob-lems overcome, only a small fraction of the fault torquewill be felt by the winding.

Although the magnetic path in the rotor is comparativelylong because of the absence of steel, very high magneticfields can be achieved without incurring excitation losses.Values of gap flux density measured at the rotor as high as5 T, compared with 1 -0—1-2 T for conventional machinesor 20 T for slotless designs, are considered to be theoretic-ally possible. Other constraints would, however, probablyimpose a significantly lower limit in practice.

Furthermore, as there is no penalty in increased excita-tion power through adding reluctance in the magneticcircuit, there is no need for a stator core in the normalflux-carrying role. It is, however, obviously necessary toconfine the flux to the immediate area of the machine.This can be done either by providing a laminated yoke, i.e.in effect a core, or by surrounding the stator winding andits supports at a suitable distance with a transposed copper


or aluminium eddy-current screen. There are arguments forand against both approaches, and no clear preference is yetapparent.

The transient and subtransient reactances are relativelylow. Some adjustment is economically possible by selectingan appropriate radial depth of stator winding. Because ofthe very large air path, the synchronous reactance is very

.low, and so the machine has a high steady-state stabilitymargin and the need for rapid field changes is reduced.

For a design having a superconducting rotor, the excita-tion loss is almost negligible. For example, the 4-5 MW orso losses for a typical 660 MW 2-pole generator would bereduced to little more than lOOkW, virtually all of whichis the power for driving the helium-liquifier compressor.Because the rotor is so light, the bearing losses, too, are muchlower.141

laminated magnetic shield a r m a t u r e . windingstationary nonconductingvacuum jacket

section A-Asuperconductorand structure(rotating)

'•A electrothermalshield(rotating)

Fig. 32Elements of large superconducting synchronous generator

[IEEE Trans. ]

Estimates that have been made show the order of savingin generator length and weight to be large, see Table 3.

TABLE 3

COMPARATIVE LENGTHS AND WEIGHTS OF CONVENTIONALAND SUPERCONDUCTING GENERATORS139

Conventional Superconducting

Machine output, MW 500 1300 500 1300Length, m 13 18 8-6 10Weight, t 400 700 210 280

Clearly there is immense opportunity for reducing sizeand weight, or increasing output while at the same timeimproving efficiency (0-4% has been quoted140) and steady-state stability. If the very high investment in developmentis ignored, the economics, as far as can be assessed at thisvery early stage, appear to be favourable in the larger sizes.When the capitalised value of the much reduced overalllosses is taken into account, the incentive is furtherenhanced.

Equally clearly, there are many formidable theoreticaland constructional problems to be solved in depth beforeeven the basic form such a machine might take can befinally decided. It is beyond the scope of a review of thisnature to attempt to identify them all. The major problemsare set out and some solutions are profferred in a numberof the References.

Reports of practical progress indicate that a number ofgenerators having rotating superconducting field windingsranging in output up to 5 MVA have been tested.143 Designstudies have also been carried out for machines up to10 000 MVA.

While a start has been made, there is a very great way togo before the feasibility of extending the use of the principle

to the sizes at which the economics appear attractive can beestablished. It has recently been suggested141 that a proto-type 60 MW experimental machine that could be installedin a power station would be the next logical step in theprogression of development.

13.6 Reciprocating generators

The requirement of avoiding subjecting super-conducting armature conductors to flux reversal is met ifthe winding can be given a reciprocating motion in aconstant field.140'146'147

Arrangements that achieve this aim are shown in Figs. 33and 34. In the first, the field B is produced by a cryogenictoroidal winding. This has the advantages of high uniformityof field and no leakage, but the toroid is difficult andexpensive to manufacture. The second proposal uses asolenoid form of field winding that requires only 20% ofthe superconductor necessary for the toroid and is muchsimpler to produce but has the disadvantage that the fieldis no longer totally enclosed and homogeneous.

Such systems suffer from all the problems inherent inthe conversion of rotary to reciprocating motion, with theadded difficulties of support of the armature, and the needfor reliable high-voltage heavy-current connections havinga flexibility range equal to the stroke. It is therefore difficultto imagine that the principle will see practical application,at least in large sizes, although studies show that outputswell into the 100—1000 MW range are theoretically feasiblewith, at 500 MW, a weight reduction of more than 50% andand efficiency of over 99%.

Fig. 33Principle of reciprocating a.c. generator with toroidal fieldwinding131

14 Conclusion

In the 70 or so years since the cylindrical-rotorturbogenerator was first conceived, its basic form hashardly changed. The advances have been made possible notso much by any revolutionary technological developments,but rather by steady refinement of design techniquessupported by parallel advances in materials, manufacturingand handling. The stage is now being reached when furtherdevelopment along established lines, particularly at 2-polespeeds, is impracticable with available materials. Conse-quently research establishments and industry are nowdevoting considerable effort to evaluating and finding waysof adapting hitherto unexploited principles that offer pros-pects of either substantially raising the output limit or, asis becoming increasingly important as the world's mineraland energy sources are depleted, reducing weight and lossesfor any given rating. Theoretical studies backed up by modelwork are encouraging and indicate that the ceiling of 2000 MW,


widely seen as about the limit for a 2-pole design usingpresent technology, could be raised by a factor as high as 3.

Fig. 34Principle of reciprocating a. c. generator with solenoidalfield winding™

However, while the incentives to breakaway from theconventional machine form may be great, optimism mustbe very severely restrained because an immense amount ofprogressive development needs to be done before theviability of the more advanced concepts can be trulyestablished.

15 Acknowledgments

The author gratefully acknowledges the help ofmany former colleagues in the industry in providinginformation and illustrations. He also thanks the variouscompanies concerned for permission to publish illustrationsand information, and GEC Turbine Generators Ltd. forsecretarial assistance.

16 References

1 HORSLEY, W.D.: 'Turbo-type generators: review ofprogress', Proc. IEE, 1963, 110, (4), pp. 695-702

2 HORSLEY, W.D.: 'The high-speed generator - eighty yearsof progress', 29th Parsons Memorial Lecture, 1964

3 HAWLEY, R., and THOMAS, D.L.: 'Definition, investigationand resolution of design limitations for large turbo-typegenerators'. CIGRE, Paper 11-01, 1974

4 LAMBRECHT, D.: 'Some problems in the construction oflarge turbogenerators', AEG Progress, 1963, 2, pp. 178-191

5 BOOTH, E.S.: 'Power supply for 1970', Proc. IEE, 1967,114,(1), pp. 89-101

6 KUSKO, A.: 'A prediction of power system development1968-2030', IEEE Spectrum, 1968, 5, pp. 75-80

7 BENNETT, R.R.: 'Planning for power - a look at tomorrow'sstation sizes', ibid., 1968, 5, pp. 67-72

8 HOUSER, H.G., and REKER, J.A.: 'Forecasting unit sizepatterns for the seventies', Westinghouse Eng., 1969, pp. 76-79

9 GRGIC, A.: 'Output limits for two and four-pole turbo-generators imposed by mechanical stresses', Brown BoveriRev., 1969, 56, pp. 394-415

10 HAWLEY, R.: 'Turbo-type generators of the future', Electr.Times, 12th June 1969, p. 6

11 KRICK, N.: 'Turbogenerators today', Brown Boveri Rev.,1969, 56, pp. 368-379

12 FRANKEL, A.: 'Large turbine generators: survey of progress',Proc. IEE, 1970, 117, (4), pp. 799-810

13 ARNOLD, J.J.: 'Mechanical problems in large rotating electricalmachines', Chart. Mech. Eng., 1970, 17, pp. 478-485

14 KRICK, N., and HIEBLER, H.: 'Generators for nuclear powerstations;, Brown Boveri Rev., 1970, 57, pp. 208-222

15 HARRINGTON, D.B., and JENKINS, S.C.: 'Trends andadvancements in the design of large generators'. Proceedingsof the American Power Conference, 1970, pp. 963-973

16 AKERS, H.T.: 'Present trends in design of large turbinegenerators'. Proceedings of the American Power Conference,1970, pp. 951-962

17 WOLFF, G.M., KUZANWINSKI, J.G., and JENKINS, S.C.:'Large generators - a look at reliability'. Presented at theSoutheastern Electric Exchange, Florida, 1970

1304

18 CUNNINGHAM, J.C., and LOKAY, H.E.: 'Future turbine-generator characteristics related to system requirements'.Proceedings of the American Power Conference, 1971

19 HAAS, H., HEINRICHS, F., SCHMOCH, O., HAPPOLDT, H.,KOCH, E., SIMON, M., and SPIRK, F.: 'Development trendsof large power generating machinery'. Presented at the 8thWorld Energy Conference, 1971, Paper 2.1-95

20 ABEGG, K., and RAUHUT, P.: 'Les grandes machineselectriques d'hier, l'aujourd'hui et de demain', Assoc. SuisseElectr. Bull., 1971, 62, pp. 865-874

21 CENTRAL ELECTRICITY GENERATING BOARD:'Modern power station, practice' (Pergamon, 1971)

22 ABOLINS, A., ACHENBACH, H., and LAMBRECHT, D.:'Design and performance of large 4-pole turbogenerators withsemi-conductor excitation for nuclear power stations'.CIGRE, Paper 11-04, 1972

23 KRICK, N., WAELCHLI, H., and HIEBLER, H.: 'Advanceddesign of large 4-pole turbogenerators'. CIGRE, Paper 11-06,

24 HAWLEY, R., TREECE, D.R., and RALPH, M.C.: 'Largegenerators of the future - a 2000 MW experimental machine',Reyrolle Parsons Rev., 1973, l,pp. 1-8

25 WEGHAUPT, E.: 'Die Entwicklung von Grossturbogeneratoren'(Technische Rundschau, Bern, 1972)

26 JAMES, L.W.: 'Large turbine generators - the British story',Electron. & Pwr, 1973, 19, pp. 304-306

27 'Test order for 1300 MW sets', Electr. Rev., 1970, 186, p. 31428 CONCORDIA, C, and BROWN, P.G.: 'Effects of trends in

large steam turbine driven generator parameters on powersystem stability', IEEE Trans., 1971, PA-S90, pp. 2211-2218

29 MASON, T.H., AYLETT, P.D., and BIRCH, F.H.: 'Turbo-generator performance under exceptional operating con-ditions', Proc. IEE, 1959, 106A,pp. 357-373

30 SCOTT, E.C., CASSON, W., CHORLTON, A., and BANKS,J.H.: 'Multigenerator transient-stability performance underfault conditions', ibid., 1963, 110, (6), pp. 1051-1064

31 HUMPAGE, W.D., and SAHA, T.N.: 'Digital-computer methodsin dynamics-response analysis of turbogenerator units', ibid.,1967, 114, (8), pp. 1115-1130

32 SOPER, J.A., and FAGG, A.R.: 'Divided-winding-rotorsynchronous generator', ibid., 1969, 116, (1), pp. 113-126.Discussion contributions, ibid., 1969, 116, (10), pp. 1720-1721

33 HARLEY, R.G., and ADKINS, B.: 'Stability of synchronousmachine with divided-winding rotor', ibid., 1970, 117, (5),pp. 933-947

34 HAMMONS, T.J., and WINNING, D.J.: 'Comparisons of syn-chronous-machine models in the study of the transientbehaviour of electrical power systems', ibid., 1971, 118, (10),pp. 1442-1458

35 SOPER, J.A., JAMES, L.W., CONWAY, A.C., and MILLER,T.: 'Dual axis excitation and control of synchronous turbo-generators'. CIGRE, Paper 11-01, 1970

36 SHACKSHAFT, G.: 'Generator stability improved by an inter-connected winding rotor', Electr. Times, 29th Jan. 1971,pp. 33-37

37 SHACKSHAFT, G., and NEILSON, R.: 'Results of stabilitytests on an underexcited 120 MW generator', Proc. IEE, 1972,119, (2), pp. 175-188. Discussion contributions, ibid., 1972,119,(10), pp. 1487-1494

38 RICHARDSON, P., and HAWLEY, R.: 'Generator statorvibration'. IEEE Winter Power Meeting, 1970, PaperCP70-186, pp. 76-81

39 LAWRENSON, P.J.: 'Forces on turbogenerator endwindings',Proc. IEE, 1965, 112, (6), pp. 1144-1158

40 TEGOPOULOS, J.A.: 'Forces on the endwinding of turbine-generators', IEEE Trans., 1966, PA-S85, pp. 114-121

41 HOLLEY, C.H., and WILLYOUNG, D.M.: 'Stator windingsystems with reduced vibratory forces for large turbine-generators'. IEEE Winter Power Meeting, 1970, Paper 70TP 187-PWR

42 SCHULER, R., and HEYMANNS, H.: 'Full-scale investigationinto the wedging of stator windings in high-output generators',Brown Boveri Rev., 1971, 58, pp. 25-29

43 BUCCI, C.A., COGGESHALL, A.D., DREXLER, K.F., andGIBBS, E.E.: 'Control of electromagnetic forces on statorwindings of large turbine-generators', IEEE Trans., 1971,PAS-90, pp. 2548-2560

44 KUCERA, J.: 'Stator core vibration in high-power 2-poleturbo-alternators', Rev. Gen. Elec, April 1964, p. 227

45 RICHARDSON, P.: 'Stator vibration in large two-polegenerators'. IEEE Winter Power Meeting, 1966, PaperCP66-50, pp. 5-11

46 HAWLEY, R., HINDMARSH, R., and CRAWFORD, A.J.:'Generator load vibration: some preliminary observations'.IEEE Winter Power Meeting, 1974, Paper CP74 217-6

47 EASTON, V.: 'Magnetic vibration in turbo-generator stators'.CIGRE, Paper 11-07, 1970

PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEE REVIEWS

48 RICHARDSON, P., and HAWLEY, R.: 'Generator stator coreand end winding vibration'. IEEE Winter Power Meeting, 1972,Paper C72 241-3

49 ROBERT, P., DISPAUX, J., and DAC^ER, J.: 'Improvementof turbo-alternator efficiency'. CIGRE, Paris, Paper 124, 1966

50 KHUTORETSKII, G.M., and VORONOV, G.G.: 'Six-phasewindings for turbo-generators', Elektrotehnika, 1968, 39, p.l

51 RINGLAND, W.L., and ROSENBERG, L.T.: 'A newtransposition for large machines', Trans. Am. Inst. Elec. Eng.,1959, 78, pp. 743-747

52 MACDONALD, D.C.: 'Losses in Roebel bars: effect of slotportion on circulating currents', Proc. IEE, 1970, 117, (1),pp. 111-118

53 NEIDHOFER, G.: 'Roebel bar windings for large synchronousmachines', Brown BoveriRev., 1970, 57, pp. 4-14

54 MARTI, P., and SCHULER, R.: 'Manufacture and testing ofRoebel bars', ibid., 1970, 57, pp. 25-31

55 VOGELE, H.: 'Measures for reducing circulating-currentlosses in the armature windings of turbo-generators', ibid.,1970, 57, pp. 32-40

56 CARPENTER, C.J.: 'The application of the method ofimages to machine end-winding fields', Proc. IEE, 1960, 107A,pp. 487-500

57 ASHWORTH, D.S., and HAMMOND, P.: 'The calculationof the magnetic field of rotating machines - Pt. 2', ibid.,1961, 108A, pp. 527-538

58 LAWRENSON, P.J.: 'The magnetic field of the end-windingsof turbo-generators', ibid., 1961, 108A, pp. 538-553

59 REECE, A.B.J., and PRAMANIK, A.: 'Calculation of theend-region field of a.c. machines', ibid., 1965, 112, (7),pp. 1355-1368

60 STOLL, R.L., and HAMMOND, P.: 'Calculation of themagnetic fields of rotating machines', ibid., 1966, 113, (11),pp. 1793-1804

61 HAWLEY, R., EDWARDS, I.M., HEATON, J.M., and STOLL,R.L.: 'Turbogenerator end-region magnetic fields: Qualitativeprediction by flux plotting', ibid., 1967, 114, (8), pp. 1107 —1114

62 LAWRENSON, P.J.: 'Calculation of machine end-windinginductances with special reference to turbogenerators', ibid.,1970, 117,(6), pp. 1129-1134

63 GLEBOV, I.A., DANILEVITCH, Ya.B., KHOUTORETSKY,G.M., POSTNIKOV, I.M., STANISLAVSKY, L.Ya., andTCHISTIKOV, A.P.: 'Investigation of the losses and tempera-ture rises in the end parts of direct-cooled turbo-generators'.CIGRE, Paper 11-05, 1970

64 MASON, T.H., FAIRNEY, W., ARNOLD, J.J., andTHELWELL, M.J.:('Asynchronous operation of turbo-generators'. CIGRE, Paper 11-02, 1972

65 KAPLUNOV, V.B., KIL'DISHEV, V.S., LINDORF, L.S.,and CHISTIKOV, A.P.: 'Heating in the end packets of thestator core of a high-power turbo-generator under slip con-ditions', Elektr. Stantsii, 1972, 43, pp. 55-58

66 NOSER, R.R., and POHL, H.: 'Cooling large turbo-generatorswithout hydrogen', IEEE Trans. ,1971, PAS-90, pp .2101-2107

67 JUHLIN, G.A.: 'Deformation of turbo-alternator rotorwindings due to temperature rise',/. IEE, 1939, 85, pp. 544-552

68 LINKINHOKER, C.L., SCHMITT, N., and WINCHESTER,R.L.: 'Influence of unbalanced currents on the design andoperation of large turbine-generators', IEEE Trans., 1973,PAS-92,pp. 1597-1604

69 BARRET, Ph., COUSTERE, A., and HEUILLARD, J.F.:'Stresses on turbo-alternators under unbalanced conditions'.CIGRE, Paper 11-11, 1970

70 'IEEE Committee Report: A standard for generator continuousunbalanced current capacity', IEEE Trans., 1973, PAS-92,pp. 1547-1549

71 BS2613: 1957. 'The electrical performance of rotatingelectrical macines'

72 IEC34: 1969.'Rotating electrical machines'73 ANSI, C 50 13: 'American standard requirements for cylindrical

rotor synchronous generators'74 LAMBRECHT, D.: 'Some problems in the construction of large

turbogenerators', AEG Progress, 1963,2, pp. 178-19175 LAMBRECHT, D., and WEGHAUPT, E.: 'Rotor endbells with

threaded locking rings', ibid., 1967, 6, p. 2176 US Patent 3 395 29977 LINDLEY, A.L.G., and BISHOP, R.E.D.; 'Some recent

research on the balancing of large flexible rotors', Proc. Inst.Mech. Eng., 1963, 177, pp. 811-841

78 MORTON, P.G.: 'On the dynamics of large turbo-generatorrotors', ibid., 1966, 180, Pt. 1, pp. 295-329

79 '£ 11 m development programme completed', Reyrolle ParsonsRev., 1973, l ,pp. 15-18

80 SULLY, A.H.: 'Progress in the manufacture of large forgings',Proc. Inst. Mech. Eng., 1967, 181, Pt. 1, pp. 877-899

81 CHITTY, A., and GRAHAM, M.R.: 'New steels - theirproperties and applications in large turbo-generators', Electr.Rev., 1971, 189, pp. 886-888

82 SICHA, F.: 'Extra heavy forgings built-up from electroslag-welded blanks', Tech. Dig., 1968, pp. 793-797

83 NEWHOUSE, D.L., and DEFOREST, D.R.: 'Meeting require-ments for larger generator rotors - a metallurgical challenge'.Presented at the International Forgemasters' Meeting, Terni,Italy, 1970

84. GREENBERG, H.D., WESSEL, E.T., CLARK, W.G., andPRYLE, W.H.: 'Application of fracture mechanics technologyto turbine-generator rotors'. Presented at the ASME AnnualMeeting, Los Angeles, Calif., 1969

85 TUDGE, J.: 'A new water-cooled turbo-generator',Metrop.Vickers Gaz., 1957, 28, pp. 91-96

86 HOLLEY,C.H., and WILLYOUNG, D.M.: 'Conductor-cooledrotors for large turbine generators: experience prospects'.CIGRE, Paper 11-06, 1970

87 HEINRICHS, F.: 'Large turbine-generators with water-cooledrotors', Proceedings of the American Power Conference, 1970,pp. 940-950

88 SCHULER, R.: 'Insulation systems for high-voltage rotatingmachines', Brown Boveri Rev., 1970, 57, pp. 15-24

89 SCHMITT, N., WILLYOUNG, D.M., and WINCHESTER,R.L.: 'Diagonal-flow cooling of gap-pickup rotors for largeturbine-generators', AIEE Trans., 1963, PAS-81, Pt. Ill

90 WALLENSTEIN, M., and TUSCHAK, R.: 'Some problem?concerning big hydrogen-cooled turbo-alternators', CIGRE,Paper 11-02, 1970

91 ASZTALOS, P.A.: 'Direct cooling systems for turbo-alternator rotors in view of the maximum rating of hydrogencooling', IEEE Trans., 1970, PAS-89, pp. 1935-1945

92 HLAVAC, J., and PIVRNEC, M.: 'Development of the pool-ing system for a 500 MW turbo-generator rotor', CIGRE,Paper 11-04, 1970

93 HIEBLER, H.: 'The 1333 MVA generator for 'Donald C. Cook'nuclear power plant of AEP', Brown Boveri Rev., 1972, 59,pp. 20-29

94 WIEDEMANN, E.: 'Fully water-cooled turbo-generators',ibid., 1966, 53, pp. 501-511

95 CSILLAG, I.K.: 'Experimental study of water flow in full-size test rig for water-cooled turbo-generator rotor', Proc. Inst.Mech. Eng., 1967, 181, Pt. 1, pp. 53-73

96 BENNETT, R.B.: 'Water cooling of turbine generator rotorwindings', English Elec. J., 1968^23, pp. 18-26

97 LAMBRECHT, D.: 'Problems of flow and heating with water-cooled rotor winding', Proc. Inst. Mech. Eng., 1969-70, 184,Pt. 3E, pp. 41-54

98 NEAL, J.E.: 'Development of high-voltage insulation systemsfor rotating machines',/. Sci. & TechnoL, 1972, 39, pp. 2 1 -27

99 LAFFOON, CM. et al.: 'A new high-voltage insulation forturbine-generator stator-winding insulation', Trans. Am. Inst.Elec. Eng., 1951, 70, pp. 721-726

100 FLYNN, E.J., et al.: 'An advanced concept for turbine-generatorstator-winding insulation', ibid., 1958, 77, Pt. Ill, pp. 358-365

101 KUZAWINSKI, J.G., and WOLFF, G.M.: 'A new look atreliability of asphalt-mica insulation in large, conventionallycooled turbine-generators'. IEEE Summer Meeting, 1969,Paper69TP711-PWR

102 MAUGHAN, C.V., GIBBS, E.E., and GIAQUINTO, E.V.:'Mechanical testing of high voltage stator insulation systems',IEEE Trans., 1970, PAS-89, pp. 1946-1954

103 FARMER, E., 'Insulating large electrical machines'. Presentedat the BE AM A Electrical Insulation Conference, 1970

104 DOUGLAS, J.L., LACOTTA, M.J., and SMITH, J.W.R.:'Assessment of stator insulation condition of large turbo-generators'. Presented at the BEAMA Electrical InsulationConference, 1970

105 WICHMANN, A.; 'Reliability and testing of high-voltage statorinsulation for large rotating machines'. IEEE Winter Meeting,1972, Paper T72 056-5

106 RICHARDSON, P., HAWLEY, R., and WOOD, J.W.: 'Insula-tion levels for turbo-generator rotors'. IEEE Winter Meeting,1972, Paper T72 063-1

107 IEEE Committee Report: 'High potential test voltages of highvoltage synchronous machines — proposed changes to ANSIC50 Standard', IEEE Trans., 1973, PAS-92, pp. 1594-1596

108 FARRALL, R., WARDROP, R., BUNTING, R.H., andFLURSCHEIM, C.H.: 'Land and sea transportation of veryheavy power-station equipment', Proc. IEE, 1968, 115, (5),pp. 671-684. Discussion contributions, ibid., 1968, 115,(11),pp. 1671-1676


109 EASTON, V.: 'Excitation of large turbogenerators', ibid., 1291964, 111, (5), pp. 1040-1048

110 ABOLINS, A., and HEINRICHS, F.: 'Brushless exciters with 130rotating rectifiers for large turbogenerators', Elektrotech. Z,1966, 87, pp. 1-8

111 PANIS, L.M., and PROLL, E.: 'Rectifier excitation of large 131turbo-generators', Brown Boveri Rev., 1967, 54, pp. 69-75

112 DEBERNARDI, J., FRITSCH, Th., BOULET, R., THERBY, 132M., BARRAL, A., and DUREAULT, M.: 'Excitation systemsfor large a.c. generators. Arrangement, tests, performance 133criteria'. CIGRE, Paper 11-09, 1970

113 BARRET, P.: 'New development and main problems of excita-tion systems in large synchronous machines', CIGRE, Paper 13411-08,1970

114 BOYD, J.G., COOPER, B., DOMERATZKY, L.M., andWOOD, R.D.: 'Reliability of electronic generator excitation 135equipment'. Proceedings of the American Power Conference,1970, pp. 982-991

115 DILLMAN, T.L., SKOOGLUND, J.W., KEAY, F.W., SOUTH,W.H., and RACZKOWSKI, C: 'A high initial response brush- 136less excitation system'. IEEE Winter Power Meeting, 1971,Paper 71 TP29-PWR 137

116 HUMPHRIES, Rev. H.J., and FAIRNEY, W.: 'Excitationrectifier schemes for large generators', Proc. IEE, 1972, 119,(6), pp. 661-671 138

117 WARTO, P.: 'Thyristorised excitation for large turbo-generators', Brown Boveri Rev., 1971, 58, pp. 41-48

118 FARMER, R.G., CRENSHAW, M.L., SCHULZ, R.P., and 139TEMOSHOK, M.: 'System design considerations andoperating experience with high performance thyristor excita-tion systems'. Presented at the American Power Conference,1971 140

119 HOLBURN, W.W.: 'Brushless excitation of 660 MW generators',J. Sci. & Technol, 1970, 37, pp. 85-90

120 MEYERHOF, W.:'Brushless excitation of synchronous 141machines by rotating semiconductors', Brown Boveri Rev.,1967, 54, pp. 539-553

121 FAIRNEY, W., LODGE, I., and WATERS, R.: 'Developmentsin thyristor excitation of large alternators', Electr. Times, 1421971, 160, pp. 37-40

122 WRIGHT, W.F., HAWLEY, R., and DINELY, J.L.: 'Brushlessthyristor excitation systems', IEEE Trans., 1972, PAS-91,pp. 1848-1854 143

123 ROBERT, P., BAWIN, P., DISPAUX, J., and LE PRINCE, J.:'Development of rotating rectifier exciters'. CIGRE, Paper11-05,1968

124 IEEE Committee report: 'Proposed excitation system definitionsfor synchronous machines', IEEE Trans., 1969, PAS-88, 144pp. 1248-1258

125 DANDENO, P.L., KORAS, A.N., McCLYMONT, K.R., andWATSON, W.: 'Effects of high speed excitation rectifiersystems on generator stability limits', ibid., 1968, PAS-87, 145pp. 190-201

126 ROBERT, P., and FORTPEID, G.: 'Performance of alternator 146excitation systems', CIGRE, Paper 11-03,1970

127 IEEE Committee report: 'Computer representation of excita- 147tion systems', IEEE Trans., 1968, PAS-87, pp. 1460-1464

128 BERDY, J., CRENSHAW, M.L., and TEMOSHOK, M.: 'Pro-tection of large steam turbine generators during abnormaloperating conditions'. CIGRE, Paper 11-05, 1972

DA VIES, E.J.: 'Airgap windings for large turbogenerators',Proc. IEE, 1971,118,(3/4), pp. 529-536AICHHOLZER, G.: 'New ways of building turbo-generatorsup to 2 GVA, 60 kV, Elektrotech. & Maschinenbau, 1972,89,pp. 1-11SPOONER, E.: 'Fully slotless turbogenerators', Proc. IEE,1973, 120,(12), pp. 1507-1518REECE, A.B.J., and PRESTON, T.: 'Technical and economicfeasibility of slotless stator turbine-generators' (to be published)KOIZUMI, H., and OHSHIMA, T.: 'Evaporative cooling ofturbine-generator rotor windings', IEEE Trans., 1971, PAS-90,pp. 2749-2758APPLETON, A.D.: 'Status of superconducting machines,Spring 1972'. Presented at the Applied SuperconductivityConference, Annapolis, USA, 1972, Paper A2THULLEN, P., DUDLEY, J.C, GREENE, D.L., SMITH, J.L.,and WOODSON, H.H.: 'An experimental alternator with asuperconducting rotating field winding', IEEE Trans., 1971,PAS-90, pp. 611-627MALANDAIN, A.: 'Superconductivity - present situationand outlook', Brown Boveri Rev., 1971, 58, pp. 34-40MOLE, C.J., HALLER, H.E., and LITZ, D.C.: 'Superconduc-tor synchronous generators'. Presented at the Applied Super-conductivity Conference, Annapolis, USA, 1972, Paper M-4WARNE, D.F., and HADLOW, M.E.: 'Superconducting a.c.machines: an approach to development'. Proceedings of the4th International Cryogenic Conference, Eindhoven, 1972APPLETON, A.D., and ANDERSON, A.F.: 'A review of thecritical aspects of superconducting a.c. generators'. Presentedat the Applied Superconductivity Conference, Annapolis,USA, 1972, Paper M-2HADLOW, M.E.G., BAYLIS, J.A., and LINDLEY, B.C.:'Superconductivity and its applications to power engineering',Proc. IEE, 1972, 119, (8R), pp. 1003-1032LORCH, H.O.: 'Feasibility of turbogenerator with supercon-ducting rotor and conventional stator', ibid., 1973, 120, (2),pp. 221-227. Discussion contributions, ibid., 1973, 120, (10),pp. 1256-1259KIRTLEY, J.L., SMITH, J.L., THULLEN, P., and WOODSON,H.H.: 'The MIT-EEI program on large superconductingmachines'. Presented at the IEEE Power Engineering SocietyWinter Power Meeting, 1973PATERSON, A., JONES, C.K., WALKER, M.S., CHANG,Y.W., LITZ, D.C, and KARPATHY, S.: 'The developmentof a 5 MVA superconducting generator. Pt. 4 - Testing andevaluation', Presented at the IEEE Power EngineeringSociety .Winter Power Meeting, 1973JEFFERIES, M.J., GIBBS, E.E., FOX, G.R., HOLLEY, C.H.,and WlLLYOUNG, D.M.: 'Prospects for superconductivegenerators in the electric utility industry', IEEE Trans., 1973,PAS-92, pp. 1659-1669THOMAS, L.A.A.: 'Physics in the service of the engineer',Proc. IEE, 1972, 119, (1), pp. 99-107HARROWELL, R.V.: 'Preliminary studies of superconductingalternators', Cryogenics, 1972, 12, pp. 109-115HARROWELL, R.V.: 'Superconducting reciprocatingmachines', Phil. Trans. A (to be published)

1306 PROC. IEE, Vol. 121, No. 11R, NOVEMBER 1974, IEE REVIEWS

Recent trends in turbogenerators

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