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PROGRESS IN REVISION OF IEEE/ANSI C50 SERIES OF STANDARDS FOR LARGE STEAMND COMBUSTION TURBINE GENERATORS AND HARMONIZATION WITH THE IEC

60034 SERIES

B.E.B.GottC.A. KaminskiGeneral Electric C ompanySchenectady, NY U.S.A.

W.R. McCown, J.R. MichalecL.W. MontgomerySiem ens Westinghouse AEP Pro Serv, IncOrlando, FL U.S.A. Columbus, OH U.S.A.

INTRODUCTIONRECOMMENDATIONSA working group of the IEEE PES Electrical Machinery

Committee has completed the task of comparing theAmerican National Standards Institute (ANSI) standardsfor electrical machinery with the correspondingInternational Electrotechnical Commission (IEC) standards.For the synchronous generator standards, a summary ofthis work was presented at a panel session in Berlin,Germany in July, 1997. In association with this initiative,a second working group was commissioned by theSynchronous Machinery Sub-committee of the IEEE PESElectrical Machinery Committee to revise the IEEE/ANSIC50 series of standards. This SMSC Working Group No. 4(Revision of C50.1X Series) reported initial progress at apanel session in New York in February, 1999. Theworking group is now refining its current draft of a revisedC50.13 standard for steam and combustion turbinegenerators before submitting a final version for balloting bythe IEEE PES Electric Machinery Committee and the ANSIC50 Committee. As part of those final steps the workinggroup has prepared this paper to publicize some highlightsof refinements added since 1999 to modernize the C50standards for large synchronous electrical machines.SCOPEIEC 600341~ covers general requirements for all rotatingelectrical machinery and 1EC 60034-34 covers the specificsof synchronous machinery to be applied with steam andcombustion turbines. IEEE/ANSI C50.105 coverssynchronous machines in general, while C50.12(6,C50.13,C50.14@and C50.1 5(9cover specific types or applications.C50.12 covers large salient pole hydraulic generators andmotor/generators while C50.13 covers large cylindrical rotorsynchronous generators. C50.14 and C50.15 essentiallytake the basic requirements of (30.13 and extend them foropen-air-cooled and hydrogen-cooled generatorapplications to combustion turbines respectively.

ORGANIZATION OF SCOPE ND CONTENTThe current approach is to consolidate C50.10, C50.12,(30.13, C50.14 and C50.15 into two standards. C50.12 willbe a consolidated standard for large salient pole generatorsand generator/motors for hydraulic turbine applications.The applicable parts of C50.10 will be incorporated intoC50.12. C50.13 will be a consolidated standard for largecylindrical rotor synchronous generators. It willconsolidate (250.13, C50. 14 and C50.15 in their entire scopeand incorporate applicable parts of C5O. 10.The logic for consolidating C50. 13, (250.14 and C50.15 intoone standard is primarily that (30.14 and C50.15 contain asignificant amount ofduplicate material from C50.13 just tohandle different applications of the same configuration ofgenerators. Further, with additional applications havingsprung up over the past 10 years, the working group facedeither creating additional highly duplicated standards orfinding a way to consolidate them.The decision to drop the general standard C50.10 andincorporate appropriate parts of its content into C50.12 andC50.13 was made after an initial attempt to retain andupdate (250.10. It was recognized that the content directlyapplicable to C50.12 and C50.13 respectively was not thatgreat, and that the advantage to the ultimate users of thestandardsof having a single standard for each of these twomajor types of machines would be significant. Thisapproach would minimize the risk of conflict or ambiguitybetween the general and type specific standards, and mayreduce the risk of missed requirements or ineffectivelycommunicated requirements. It should also easemaintenance of the standards with a better focus andalignment of the standards to users and manufacturersknowledgeable in each product and affected by thestandard.

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GENERATOR CONFIGURATIONSBecause of the following developments during the past 10years, the current IEEE/ANSI cylindncal rotor standardsdo not cover certain generator configurations:

2 0 80 70 6 -.

introduction of alternative configurations by somemanufacturerssignificantly increased outputs of combustionturbines requiring generator configurations notused previously in this applicationincreased use of combined cycle plants requiringambient following generators for steam turbineapplicationsdesire to include coverage for someconfigurations applied in limited areas of the world(because of efforts to reconcile with IEC standardswhere possible).

- - \\I

Some specific additions being incorporated are:

direct air coolingof stator and rotor windingsapplication of totally enclosed air to water cooledgenerators to combustion turbinesapplication of configurations used for largerratings to ambient following operationwater cooled rotor windingsconfiguration with air and water cooling

TEMPERATURE BASIS OF RATINGIEEEJANSI standards have required winding hot spot limitsas well as observable temperature limits whereas the IEC60034 standards are silent on hot spot limits. One couldsurmise that IEC and IEEE/ANSI set the observables at thattime with the intent that they would keep the hot spottemperatures within their limits of 130C for Class B and155C for Class F as noted in IEC 85 and in theIEEE/ANSI C50 series However, there are many designconfiguration parameters than can change the relationshipof hot spot to observable. IEC appears to have tried toaccommodate some of these by including such things asthe number of rotor zones in setting the rotor observabletemperature, but there are many variations beyond those.Unfortunately, this can work in two ways. One designcould meet observables while exceeding the hot spottemperatures of the insulation class while another could beunduly restricted by the observable temperatures while thehot spot is well below that allowed by the insulation class.The working group intent is to retain the currentIEEEIANSI hot spot requirements and is currently workingtoward establishing observable temperature limits that willnot unduly restrict capability based upon hot spotrequirements.Care will also be taken to specify the rated cold coolanttemperature at which the rise and resulting observable total

temperature limits apply as they should only apply at thatpoint. Maintaining a constant hot spot temperature over arange of cold coolants as needed for ambient followingmachines will result in significantly non-constantobservable temperatures. The corollary is that maintaininga constant observable total temperature over a cold coolantrange will result in hot spot total temperatures that may besignificantly under limits in part of that range, andor overlimits in another part of that range, depending upon thecold coolant temperature where the observable temperatureand hot spot temperature limits are matched. Figure Iillustrates this for a real case as reflected in machinecapability holding one or the other temperatures constant

old Gas Degree

Figure 1 Dependence of Generator Output Power uponHot Spot Observable Temperatures

Thus, as noted above, the working group plans to establishthe observable limits at only one cold coolant temperatureand allow the capability to be calculated by themanufacturer for different cold coolants based uponconstant hot spot temperatures. This should avoid asignificant mismatch of generator real (hot spot based)capability with driver capability for ambient followingmachines. It should be noted that this is contrary to theIEC standards that require observable total temperatures tobe held constant to determine capability for all coldcoolants down to 10C.TURBINE OUTPUT CAPABILlITlES WHICH VARYWITHAMBIENT TEMPERATURERequirements have been clarified for power outputcapabilities for ambient following generators that mustfunction over an ambient temperature range from -20C to+50C. The intent is to require a manufacturer to providecurves like those in Figure 2 to define permissiblerelationships between generator output power and primarycold coolant temperatures for an ambient temperature rangethat is within that -20C to +50C range. The guidingprinciple for these curves is that the permissible hottestspot temperatures for the specified insulation class shallnot be exceeded for base capability.

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1 5 T

Steady state stability limits with fixed manGI voltagecontrol usually affect how far the unit can operate into the

I-31.2a

Transient Reactance Subtransient Reactanceo?? +30

PEAK CWABIUTV ATRATEDPailVERFXTOR

'tFigure 2 Typical Output Power Capability Curve for

Ambient Following GeneratorPOWER FACTOR(30.13 was silent on this topic. It is being revised tospecify that power factor at output rating should be fixedby agreement, and it provides the following guidance forthis agreement.Rated power factor is the power factor specified foroperation at rated megawatt load and rated megavars over-excited. Specifying a higher rated power factor results in aturbine application. For air-cooled generators, the ratedpower factor is usually set at 0.90,0.85 or 0.80 over-excited.For hydrogen-cooled generators, the rated power factor isusually set at 0.90 or 0.85 over-excited.

less expensive and more efficient generator for a given

generator capability curve. Underexcitation limiters areoften set based upon this. With modem automatic voltageregulators this limit can be significantly extended,particularly if sophisticated features such as a powersystem stabilizer are included to avoid any possible systemdynamic stability issues that might result because of fastregulator action. Redundant regulators can be consideredto assure automatic control availability.As a final guide, C50.13 is being revised to specify thatunless otherwise agreed upon, a minimum short circuit ratioof 0.35 should be applied, regardless of rating or method ofcooling.REACTANCES(250.13 did not address this topic. It is being revised totreat reactances in ways similar to IEC 60034-3. While nottypically specified by a user, direct axis transient and sub-transient reactances may be specified or agreed upon. Thisshould be based upon operating conditions both at outputrating and across the ranges of base and peak capability. Itmay be appropriate to specify or agree to a minimum valueof the direct axis subtransient reactances at the saturationlevel of rated voltage as this can affect short circuitcurrents and circuit breaker sizing. It may also be desirableto specify or agree to a maximum value for direct axistransient reactance at the unsaturated conditions of ratedcurrent as this may have an impact on transient stability.Since the two reactances depend to a great extent oncommon fluxes, care must be taken to ensure that thevalues specified or agreed are compatible, i.e. that the lowerlimit of the sub-transient reactance is not set too close tospecified, the manufacturer should provide estimatedvalues of these reactances and many other parameters thatare useful in the design o f power systems.For definitions of saturated and unsaturated transient and

the upper limit of the transient reactance. I f no limits are

subtransient reactances, see IEEE 100' '. If it is agreed thatvalues are to be determined by test, the test shall be inaccordance with IEEE/ANSI 115 .

For both air-cooled and hydrogen-cooled generators, theminimum under-excited power factor at rated megawattoutput power is usually set at 0.95 or higher.SHORT CIRCUITRATIO(30.13 was silent on this topic also. It is being revised tospecify that the short circuit ratio at output rating be fixedby agreement. This approach is being chosen because alower short circuit ratio results in a less expensive and moreefficient generator. The revision suggests that a minimumshort circuit ratio should be fixed after consideration of

Where limit values of transient or subtransient reactancehave been specified or agreed to, these limits shall besubject to tolerances. As outlined in the following table,these tolerances shall be set so that there is no toleranceon the value provided by the manufacturer in thesignificant direction, i.e. no negative tolerance on minimumvalues and no positive tolerance on maximum values. Inthe other direction, a tolerance of 30 shall apply.

steady state stability in the power system where thegenerator is to be installed. Tolerances on Limits for Transient and SubtransientReactances, When Limits Are Specified or Agreed to:

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If no values have been specified or agreed to, themanufacturer shall provide estimated values. These valuesare to be subject to a tolerance of +IS .NUMBER OF STARTBTOP CYCLES

the standards of performance established for operation atrated voltage and rated frequency. This is defined by theshaded area ofFigure 3.

Currently, both IEC60034-3 and IEEE/ANSI C50.13/14/15requirements for stadstop cycles are somewhatambiguous. IEC60034-3 does specify a total number ofcycles of 3000 as a general rule. However IEEE/ANSI(30.14 and (250.15 specify that the number of starts shallnot exceed 500 per year, but these standards do notspecify what number of years should be considered. In thesection of IEC 60034-3 for application to combustionturbines, the same language as C50.14 is used. The totalnumber of starts is important to specify because of theneed to design to prevent cyclic fatigue degradation.Therefore, 250.3 is being revised to include requirementsdefined in the following paragraph:A generator shall be designed for a minimum number ofstarts over the operating life of the machine. The minimumnumber of starts from zero speed or tuming gear speed tooperating speed and then up to rated load shall be:

3,000 starts for base load units10,000 starts for peaking units or other frequentlycycled units

GENERATOR SSTARTING MOTORNeither the C50 nor the IEC 60034 standards currentlyaddress the topic of starting a combustion turbine usingthe generator as a starting motor. C50.13 is being revisedto specify that when a generator and variable frequencystatic starting power electronics system are to be used as astarting motor drive system for a combustion turbine, co-ordination of the generator design with the static startingpower electronics system is required. As part of thatcoordination several attributes shall be considered toensure that temperatures of the generator windings andcomponents do not exceed limits that are appropriate forthe insulation class specified. These attributes are toinclude the following: characteristics of the powerelectronics equipment, drive train loss and inertiacharacteristics, starting speed versus time, and re-startinterval requirements.FREQUENCYDEVIATIONThis topic is not addressed in IEEEIANSI C50.10 and(30.13. C50.13 is being revised to specify off-frequencyrequirements that are closely aligned with those inIEC60034-3. Generators shall be thermally capable ofcontinuous operation within the confines of their reactivecapability curves over the ranges o f f 5 in voltage and2 in frequency, but not necessarily in accordance with

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Figure 3 Operation Ranges of Voltage Frequency

The same variation in voltage and frequency is to apply tothe operation of hydrogen-cooled generators at reducedgas pressure and also to base and peak output reactivecapability curves for ambient following generators.As the operating point moves away from rated values ofvoltage and frequency, temperature rises or temperaturesofcomponents may progressively increase. Continuousoperation near certain parts of the boundary of the shadedarea in Figure 3at outputs near the limitsof the generatorsreactive capability curve may cause insulation to agethermally at approximately2 to 6 times its normal rate.Generators shall also be capable of operation within theconfines of their reactive capability curves within theranges of 5 in voltage and +3 / -5 in frequency, asdefined by the outer boundary of Figure 3, with furtherreduction of insulation life. However, operation at peakcapability on ambient following machines is restricted tothe shaded area of Figure 3.To minimize reductions of a generators lifetime caused byeffects of temperature and temperature differences,operation outside the shaded area should be limited inextent, duration, and frequency of occurrence. The outputshould be reduced or other corrective measures taken assoon as practicable.The sloping boundaries at the upper left and lower right ofFigure 3result in a generator (and its transformer) beingover- or under-fluxed by no more than 5 .

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Under some operating conditions represented in Figure 3,stability margins and excitation margins will be reduced,and a generator may even be unstable.As the operating frequency departs from rated frequency,effects outside the generator may become important toconsider. For example, the turbine manufacturer mayspecify ranges of frequency and corresponding periodsduring which the turbine can operate. The ability of theauxiliary equipment to operate over a range of voltage andfrequency should also be considered.Operation over a still wider range of voltage and frequency,if required, should be subject to agreement between thepurchaser and the manufacturer.SUDDENSHORT CIRCUITSAlthough both the C50 and IEC60034 standards nowaddress this topic, IEC60034-3 does so with greater clarity.With the following text (250.13 is being revised toincorporate much of the sudden short circuit specificationsfrom IEC60034-3 and to clarify what is meant by the termsfit for service:A generator shall be designed so that it can be fit forservice after experiencing a sudden short circuit of any kindat its terminals while operating at rated load and 1.05 perunit rated voltage, provided that the fault is limited by thefollowing conditions:

The maximum phase current does not exceed thatobtained from a three phase sudden short circuit.The armature winding short time thermalrequirements are not exceeded.

A generator shall be judged fit for service after the incidentif it requires no more than the following minor repairs:

For the stator winding the term minor repairsimplies that some attention to the end turn bracingsystem and to coil ground insulation may benecessary to ensure that the winding willwithstand a maintenance high potential test afterthe repairs. The term minor repairs does notimply replacement of stator coils.For the rotor shaft the term minor repairs impliesthat some attention to coupling bolts, couplingsand rotor shaft balancing may be necessary toensure that shaft dynamic motion and bearingvibration will be within acceptable limits after therepairs. The term minor repairs does not implymodification of the joumals or bearings,adjustment of the shrink fits of components otherthan couplings, or replacement of the rotor.

FAULTY S Y I z A T l 0 N SC50 standards are silent on this topic. IEC60034-3 brieflymentions faulty synchronizations in its section on suddenshort circuits. To clarify this topic, C50.13 is being revisedto incorporate the following requirements:Generators shall be designed to be tit for service withoutinspection or repair after synchronizing that is within IO.Faulty synchronizing is that which is outside these limits.Under some power system conditions, faulty synchronizingcan cause intense, short duration currents and torques thatexceed those experienced during sudden short circuits.These intense currents and torques may cause internaldamage to the generator.Generators shall be designed so that they are capable ofcoasting down from synchronous speed to a stop or toturning gear speed after being immediately tripped off linefollowing a faulty synchronization. Any generator that hasbeen subject to a faulty synchronization shall be inspectedfor damage and repaired as necessary before being judgedfit for service after the incident. Any loosening of statorwinding bracing and blocking and any deformation ofcoupling bolts, couplings and rotor shafts should becorrected before returning the generator to service. Even ifrepairs are made after a severe out of phasesynchronization, it should also be expected that repetitionof less severe faulty synchronizations might furtherdeteriorate the components.It should be expected that the most severe faultysynchronizations, such as 180 or 120 out of phasesynchronizing to a system with low system reactance to theinfinite bus, might require partial or total rewind of thestator, or extensive repair or replacement of the rotor, orboth.M TRIcmIn line with current IEEE Standards Board rules all units areexpressed in metric terms only. No dual dimensioning isused, and some attributes previously described in Englishunits are being expressed only in metric units. All metricunits in (30.13 are to be the units used in IEC60034standards. For example, hydrogen pressures will be inbars rather than in psig as in prior C50 standards.

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STATUS OF WORKING GROUPThe membership of the working group Revision of C50.1XSeries was initially established at the IEEE PES WinterMeeting in January 1998. Since that time the membershipof the working group has grown to include the followingpeople:

Name AffiliationAEP Pro Serv, Inc.. Michalec, chair

R.C. BartheldW. Bartley

. K.E. BlanchardM. BusalaR.E. FentonB.E.B. GottG. GriffithR. JohoC.A. KaminskiW.R. McCownW.M. McDermidL.W. MontgomeryG.A. MottersheadN.E. NilssonP.I. NippesJ.A. OliverT.R. WaitL.A. Wall

IECHartford Steam & BoilerGeneral Electric CanadaNippes Bell AssociatesGeneration Technology Cons..General Electric (Retired)Florida Power & LightAlstom PowerGeneral Electric Power SystemsSiemens Westinghouse PowerManitoba HydroSiemens Westinghouse PowerVoith Siemens Hydro PowerFirst EnergyNippes Bell AssociatesJARSCO EngineeringPG&E National Energy GroupSouthern Company

This working group is now resolving a few final issuesbefore issuing the final (250.13 or balloting by the IEEE PESElectric Machinery Committee.SUMMARY NDCONCLUSIONSSeveral years of examining the similar but differentIEC 60034 and IEEE/ANSI C50.1X standards forlarge synchronous electric machines by an lEEE PESworking group of users, manufacturers andconsultants has led to the draft of a revisedIEEE/ANSI C50.13 standard that is discussed in thispaper. Where it has been possible for this group toagree to the appropriateness of requirements fiom theIEC standards, these requirements have beenincorporated into the revised (30.13. This revisedIEEE/ANSI C50.13 standard must be subjected toconsensus amongst a broad range of manufacturers,users, consultants, and other interested groups. Nextsteps for the C50.1X Working Group are to includeh a l refinements to the proposed C50.13, issuing thisproposed final version for balloting by the ElectricMachinery Committee of IEEE, and then for ballotingby the ANSI C50Committee.

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Gott, B E. B. ,McCown, W. R. ,Montgomery, L. W.,and Michalec, J. R., Implications of DifferencesBetween the ANSI C50 Series and the IEC 60034 SeriesStandards for Large Cylindrical Rotor SynchronousMachines, Panel Discussion, IEEE-PES SummerMeeting, Berlin, Germany, July, 1997Gott, B. E. B., McCown, W. R., Montgomery, L. W.,and Michalec, J. R., Update of Revision of ANSI C50Series and the IEC 60034 Series Standards for LargeCylindrical Rotor Synchronous Machines, PanelDiscussion, IEEE-PES Winter Meeting, New York, NewYork, February, 1999IEC 60034-1, Rotating Electrical Machines Part 1:Rating and Performance, 10Edition, 1996IEC 60034-3, Rotating Electrical Machines Part 3:Specific Requirements for Turbine-Type SynchronousMachines, 4 Edition, 1988IEEEIANSI (30.10, Rotating Electrical MachinerySynchronous Machines, 1990IEEE/ANSI C50.12, Requirements for Salient-PoleSynchronous Generators and Generator/Motors forHydraulic Turbine Applications, 1989IEEE/ANSI C50. 13, Requirements for Cylindrical RotorSynchronous Generators, 1989IEEEIANSI C50.14, Requirements for Combustion GasTurbine Driven Cylindrical Rotor Synchronous Gen-erators, 1977IEEE/ANSI C50.15 Hydrogen cooled, CombustionGas Turbine Driven Cylindrical Rotor SynchronousGenerators Requirements, 1989IEC 85, Thermal Evaluation and Classification ofElectrical Insulation, 2dEdition, 1984IEEE 100-1996, Standard Dictionary of Electric andElectronic TermsIEEE 115, Test Procedures for SynchronousMachines, 1995

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