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PRO CEED IN G S OF THE IEEE. VOL. 56. NO. 10. OCTOBER 1968 1691

Air Cooled Two-PoleGenerators for GasTurbine Peaking Service

Abstrac t -Comideratkm m the applicat ion nd matching of two-polecylindrical rotor generatorso gas turbines forpeakingseni e are dkussed.V a h riteria re reviewed i cl ling generator thermal rating ad W a -coasiderptiom generator v e n t i l a t i o o ad air Htration mise steadydatea d ransient performance nd special generator features.ti omgas tnrbiae r tingsaltitude fact- cyclic Operptingduty rapid oading

INTRODUCTION caR COOLED two-pole steam turbine driven generators

have long been built to IEEEand National ElectricManufacturers Association standardso meet a cL

narrow range of operating conditions. These generators xIhave normally been of totally enclosed construction withintegral air to water heat exchangers. Design was based onstator and rotor temperatureises giving accepted tandardtotal temperatures based on an ambient air temperatureair leaving heat exchangers)of 40C. Cooling air heat

transfer wasbased on air density corresponding toanaltitude of300 feet. The standard generator rating was AMBIENT TEMPERATURE O Fmatched to the steam turbine rating at design steam condi- Fig. 1 Typical gasurbine erformance power. Outp uttions. Modifications of generator rating were rare, and due versus ambientemperature.primarily to operations at altitudes greater than 3300 feetor cooling water temperatures greater than 95F 35C).

The gas turbine uses compressed and heated air as thepower medium. Power output is a function of air flow,pressure, and temperature. The maximum temperature towhich the air can be heated before entering the turbine islimited by metallurgical considerations. Hence, for a givenlimiting turbine speed and inlet air temperature, power out-put will depend upon air density and pressure, and greateroutput can normally be obtained withreduced ambientinlet air temperature and/or altitude. For a given altitudeand turbine inlet temperature, a curve can be plotted ofpower output versus ambient inlet air temperature. Fig. 1shows a family of typical curves for various turbine inletair temperatures. Note the considerable increase in poweroutput at lower ambient air temperatures.

GENERATORHERMALATI NGFor most operating conditions, the two-pole generator is

a thermally limited device. Increased electrical output willresult in a proportionate increase in losses and windingtemperature rise. Thus, the total temperature limitations of

Manuscript received September 18, 1967. This was Paper 68 C P126-PWR presented at the 1968 IEEE Winter Power Meeting. It wasreferred to the PROCEEDINGSby the Power Generation Committee of theIEEE Power Group.The authors are with the Electric Machinery M fg. Company, Minne-apolis, Minn. 55413

the windings can be maintained and higher temperaturerises allowedf the temperature of the cooling air is reduced.Present standards or air cooled wo-pole generatorslimit the stator winding temperature rise by resistancetemperature detector to 60Cover a 40C ambient. Therotor winding temperature rise s limited to 85C y resis-tance over a 40C ambient. These give total temperaturelimits of 100C for the stator and125Cfor the rotor.

The use of room air cooled or open generator con-struction allows us to make use of the lower ambient tem-perature toachieve greater power output without exceedingthe established total temperature limits. Knowledge of thechange in generator stator and rotor winding temperaturerise with change in load allows us to calculate a theoreticalload at each ambient temperature that will giveus the samerated winding total temperature. Such a theoretical ratingcurve is shown in Fig. 2.

Fig. 2 was plotted on the basis of constant power factoroperation. It is based on equivalent thermal loading of thestator and rotorwith a 40C mbient temperature. It is ap-parent that for such a design, the output is limited at re-duced ambient temperatures by the rotor heating and atambient temperatures above 40C by the stator heating.

Comparative ratings of peaking gas turbines are usuallybased on an ambient temperature of 80F 27C). A genera-tor design based on a 40C ambient becomes somewhat

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1692 PRO CEED IN G S O F TH E IEEE OCTOBER 1968

1 1 0w 1002

9W

80

2 0 4 8 0 100 1 2 0AMBIENT TEMPERATURE f

Fig. 2. Gen erator load for constant total winding temperature.unbalanced at an ambient temperature of 80F with therotor windings more heavily thermally loaded than thestator windings. From the theoretical standpoint, it wouldappear that if the designer were to utilize a lower ambienttemperature as the basis for his design, optimization woulddictate a somewhat different approach to obtain the mosteconomical machine configuration and size. In actuality,the design must be balanced to best achieve match with thegas turbine output rating curve.So far our discussion has considered only the total wind-ing temperature as a basis for generator rating. However,the temperature rise itself becomes a limiting factor eventhough the total winding temperature has not beenex-ceeded.

As machine loading and generator winding temperaturerise increases, the difference between he hottest point in thewinding and its average temperature also increases. In con-ventionally cooled generators, the stator temperature riseusually is based on detectors embedded between the upperand lower coil sides in the slots. Jerrard [2] has shown thatthe temperature indicated by such detectors is somewhatless than the actual copper temperature, due to thermalgradient in the main winding insulation. This differencewill increase with generator loading, so we must also in-crease the temperature margin to keep the hot spot tem-perature within limits.

As we increase the temperature rise on a given generator,the differential temperature between the windings andadjacent parts increases. Thus, the effects of thermal ex-pansion must be considered in the development of a finalgenerator rating curve [11. Winding and insulation stressesincrease rapidly with increasing temperature rise. Theeffects of rotor winding deformation due to heating afterthe application of centrifugal force must be seriously ex-amined [3]. In addition,he effects of these increased stressesare aggravated by the cyclic nature of gas turbine peakingapplications. It is expected that the generator will be loadedand unloaded hundreds of t imes during its operating life.

At lower altitudes the increased density of air results in aslight improvement in heat transfer rate in an electric ma-chine. However, since the thermal gradient at heat transfersurfaces in a conventionally cooled turbogenerator isusually only 20 to 30 percent of the total temperature rise,it isevident that any change in rating due to site elevation is

Fig. 3. Suggested generator loading versus ambient temperature.

quite small. One rule of thumb is that the rating of a genera-tor can be increased approximately one percent per 200meters below the standard altitudeof lo00 meters. This in-crease in rating is about one half of the increase in outputobtained when a gas turbine is operated at a lower altitude.

Giving due consideration to the preceding factors, a sug-gested generator output versus ambient temperature curveis shown in Fig. 3. For a given design, he base rating wouldbe adjusted to suit the site altitude. It is understood thatthis curve is based on intuitive judgement and experience,and not on factors that can be calculated or predicted ac-curately.MATCHINGURBINEND GENERATORATINGS

Fig. 1 showed typical gas turbine output curves as a func-tion of ambient temperature for different limiting turbineinlet temperatures. In effect, each curve corresponds to adifferent operating life for the turbine. For practical pur-poses these curves can be replaced by two basic ones or anoperating guide. The base load or normal rating curve cor-responds to relatively long time periods between turbineinspections or overhaul. This might be in the neighborhoodof SO00 operating hours. The other operating curve mightbe called the peaking or maximum output curve, based onan operating lifebetween inspections of ess than 1000hours.

Since the economics of gas turbine peaking power ap-plications are based primarily on providing kilowatt out-put at minimum installed cost, it is common practice tomatch the generator to the gas turbine rating on its normalor base load curve at 80F ambient and 1OOO-foot elevation.This is shown in Fig. 4. To utilize the maximum outputcurve of the turbine, the generator excitation should belimited so that he operating power factor approachesunity. Note that it is necessary to use a larger generator toutilize the turbine capability at lower ambient temperatures.

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SMITH A N D DOESCHE R: TWO-POLE GENERATORS FOR G AS TURBINE PEAK ING 1693

0 20 4 0 60 8 0 1 12AMBIENT TEMPERATURE OF

Fig. 4. Power output versus ambient temperatureor gas turbine and generator matched at 80F ambient and 1000-foot altitude.MECHANICALONSIDERATIONS

One of the prime advantages of the gas turbine for peak-ing applications is its ability to be started and stoppedquickly. The gas turbine generator set is loaded rapidly,often achieving full load in less than two minutes after re-ceiving the signal tostart. Naturally, this severecyclicloading must be considered in the design of the air cooledtwo-pole generator. The stress levelof mechanical partsmust be considered in the light of wide temperature ex-tremes and very rapid thermal expansion. It is expectedthat the generator will start and operate to its capabilitywithout adverse effects on its operating life regardless f theambient temperature at the time of startup, the speed ofsuch a startup, or he number of times hat this condition isrepeated. Rotor forging transition temperatures, fatiguelimits, stress levels, and mechanical configuration must becarefully considered. The fits between parts must be madewith consideration for the wide range of temperatures ex-pected. Bearings andhe lubrication systemmust bedesigned to operate regardless of ambient temperature.

GENERATORENTILATIONPreventing the buildup of dirt and other foreign sub-

stances within the generator is important to insure longtrouble-free life. The volume of cooling air required isquite high and the air is distributed internally for maximumcooling efficiency. Because of mechanical limitations, thecooling ducts within the generator rotor are small and partof the rotor windings are subjected directly to the flow ofcooling air. Buildup of dirt or other foreign particles can

plug these cooling ducts, reducing the cooling efficiency. Inaddition, severe dirt buildup could minimize the electricalcreepage distances maintained within the rotor.

Hence, filters are used to removeasmuch dirt as possiblefrom the outside air drawn into theenerator. Filter area ismade as large as possible to give maximum filtering effi-ciency. The filters may be either of the viscous cleanabletype or of the fiber or bag replaceable type. Filter main-tenance must be strictly followed to allow full air flow andgenerator cooling and o prevent premature buildup ofdirt within the generator. Obviously, dirt buildup will bemore rapid in terms of hours of operation thanwith a totallyenclosed water cooled machine, but because of the reducedoperating time, generator cleanliness consistent with yearsof installation will be comparable.

In some cases, particularly with larger generators, it maybe advantageous to recirculate some or all of the generatorcooling air o minimize the wide temperature changespossible. Thermostatically controlled louvers can be usedto prevent natural circulation of air through thegeneratorduring shutdown and assist the space heaters in maintainingthe generator shutdown temperature well above the out-side ambient air temperature. These louvers can then beused to recirculate cooling air discharged from the generatorto prevent the air entering the generator from droppingbelow a specified temperature, such as 60 to 70F. Notonly does th s air recirculation reduce the possible tempera-ture differential seen by the mechanical parts of the genera-tor, but it can also in some cases raise the minimum tem-perature to which the generator will be subjected and allow

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1694 PROC EEDIN GS OF THE IEEE OCTOBER 1968a moreeconomic choice in materials such as the rotorforging with regard to transition emperatureand hepossibility of brittle fracture.

GENERATOROISEBecause of the high rotor peripheral velocity and large

number of cooling ducts in a two-pole generator, its in-herent broad-band noise level will be quite high. In addi-tion, a very high noise component at twice line frequencyi.e., 120 Hz for a 60-Hz generator) is inherent. To achievetwo-pole magneticconfiguration on the rotor, he windingsare inserted in slots milled between the center line of thepole faces. This gives a stiff bending moment with the poleaxis vertical and a somewhat essstiffbendingmomentwith the pole axis horizontal. This results in a change inrotor deflection twice per revolution with the attendantdouble frequency vibration which s transmitted directlyto the cooling air in the air gap and mechanically to thegenerator bearings and bearing supports. In addition, themagnetic attraction and repulsion on the stator punchingsashe fieldevolves introducesnotherouble frequency OCTAVEAN0IDFREOUENCY Hzvibration which s transmitted to the generator frame and Fig. 5 Typical open generator noise level ndecibelsat 7 feetto the air passing through the stator ventilation passages.Isolation of this mechanical core vibration from the gen-

referenced to 0.0002 microbar.

eratormounting feet by means of lexible mounting isstandard on larger generators. However, this flexiblemountingcannot eliminate the noise generated by thevibrating core itself and transmitted directly to the coolingair.

Thus the air cooled two-pole generator, without soundattenuating equipment, will exhibit noise source levelssimilar to those shown in Fig. 5 with the peak sound powerlevel occurring at twice line frequency in the range of120 to130 dB. Considerable deviation from Fig. 5 can occur withdifferent generator sizes and mechanical configurations.

To meet even minimal sound riteria, the generator cool-ing air inlet and discharge must be acoustically treated.Approximately 20- to 3 0 d B higher noise levels are trans-mitted from the generator air discharge so acoustic treat-ment there is considerably more extensive than at the airinlet. In addition, where strict residential sound levels mustbe met, it s often necessary to enclose the generator wi thnan acoustic treated house.

GENERATORERFORMANCEMany gas turbine generators are installed not only forpeaking service, but also to provide startup power for an

adjacent steam power lant, or emergency power or nuclearplant auxiliary motors. Two-pole generators are inherentlylow-reactance machines and exhibit good transient voltageperformance upon theudden application of load. However,when used o start theuxiliaries of a steam power lant, thestarting sequence and motor inrush evels may be such thatadequate voltages cannot be achieved with a standard gasturbine generator ombination even though the gas turbinehas sufficientpower output o match he combinedfullload rating of the auxiliaries. In this case it may be necessary

l o9

w 8022 7 0I5 60? 5 0U

430

0 0 5 1 0 1 5 2 05 3 0

Fig. 6. Minimum and restored voltages with sudden applicationof low PF load . Based on no initial load.INRUSH LVWCENERATOR kVA RATING

to utilize an oversize, or low-reactance, generator toachieve adequate voltage performance.

Fig. 6 shows typical two-pole generator transient voltageperformance for both adirect connected exciterand a staticor brushless excitation system with minimum and restoredvoltages shown as a function of the ratio of applied loadinrush to generator rating. In the power range of 3 MWthrough 40 MW, inherent reactances for air cooled gen-erators are very similar and little deviation from Fig.would normally be expected.

Economic studies of most gas turbine peaking installa-tions have indicated that the standard 0 5 excitation systemspeed of response is adequate. Typical peaking units arenot large compared with total powersystem generatingcapacity, and their possible contribution to system stabilitycan not be sufficient to justify the cost of a high-responseexcitation system.

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S M I T HN DH ER: TWO -PO LE G EN ERA T O RSO RA SU R B I N EEAKING 1695

0 10 20 30 40 50 6 7 80 90 100 11 120PERCENT KILOWATTS

Fig. 7. Sync hrono us genera tor 0.85-PF reactive capability.100 perce nt=rate d kVA in 104F 4OC) ambient.GENERATORROTECTION

The gas turbine generator is usually installed as part of aremote control, semiautomatic or completely unattendedoperation. Since the capability of the turbine at the siteelevation and operating ambient temperature is frequentlygreater than the generator capability, and since emergencyand peaking operation often requires substantial reactivepower to sustain system voltage, it is important that auto-matic thermal protection of the generator be installed.This may operate from direct measurement of generatortemperature. Usually it is simpler and adequate to bias thevoltage regulator circuits to keep generator operation withinits reactive capability curve.

Fig. 7 shows a typical reactive capability curve for laggingpower factor operation. Special circuits can be used in thevoltage regulator to permit operation at leadingpowerfactor. Reactive capability curves for such operation areusually based on stability considerations for the particularapplication.

INSTALLATIONND MAINTENANCEOne of the attractive features of any gas turbine generator

installation is its packaged design. These units are in-stalled in a wide variety of locations, many of them veryremote. The generator design must be simple and allow formaximum aseof installation and maintenance. Two-

bracket bearing design allows shipment as a completeassembled unit. This allows the lifting or skidding of thecomplete generator to the foundationwhere only lignmentand grouting is required. Threading of the rotor into thestator can be eliminated.

Many gas turbine sets are built as power plants completewith turbine and generator control and switchgear devicesinstalled and connected. The various components of theplant can be installed and interconnected quickly.Thiscon-struction permits rapid installation and economical move-ment of the plant to a different location in the future. Infact, some plants have been built on highway trailers orrail cars.

CONCLUSIONSThe application of a two-pole air cooled cylindrical rotor

generator to a gas turbine driver is considerably differentthan hat historically used for application with steamturbines. Considerable confusion exists regarding the differ-ences in ratings of two-pole generators and gas turbineswith regard to ambient temperature and altitude. Limita-tions in extension of generator rating have not been estab-lished. While onsiderable study has been made of the effectofambient temperature extremes, rapid loading, continuouscyclic operation, noise, and other factors, no standardshave been set by either the user or the industry. The genera-tor manufacturer must tillbase his design and ratingmethods upon historic steam turbine generator standardsand optimum economic choice often cannot be given tothe ultimate user. Specifications for gas turbine sets oftenreflect confusion and can impose an economic penalty onboth the manufacturer and the user. It is highly recom-mended, in light of the rapidly expanding application ofgas turbine generator sets, that well thought out and mean-ingful standards be established by both industry and theuser for the application of two-pole generators to gasturbine peaking sets.

REFERENCE[ l ] J . S Johnson and J C. Botts, Physical effects of thermal cycling onstator coil insulation of turbinegenerators, Trans. AZEE PowerApparatus nd Systems), vol. 75, pp. 249-253, J un e 1956.[2] R. P. Jerrar d, Temperature dro p to resistance temperature detectorsin stator windings of turbine generators, Trans. AIEE Power A pparatus nd Systems), vol. 73, pp. 66 56 70 , Ju ne 954.[3] J. G. Noest, Prevention of rotor-winding deformation on turbo-generators, Trans. AZEE, vol. 63, pp. 5 145 17, July 1944.