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FACILITIES INSTRUCTIONS, STANDARDS,AND TECHNIQUES

Volume 1-4

PERMISSIBLE LOADING OFGENERATORS AND LARGE MOTORS

Internet Version of This Manual CreatedJuly 2000

The appearance of the Internet Version of This ManualMay Differ From the Original, but the Contents Do Not

FACILITIES ENGINEERING BRANCH

DENVER OFFICE

DENVER, COLORADO

UNITED STATES DEPARTMENT OF THE INTERIORBUREAU OF RECLAMATION

REVISED MARCH 1991

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Purpose and ScopeThis volume is intended to fill the need for practical information concerning the temperature and mechanicaland electrical overload limits of rotating electric equipment such as generators and large motors. Rotating

electrical equipment cannot be overloaded on the same basis as transformers and is not as able to standshort-time overloads. This is largely because equipment cooled by air does not have the heat transfer abilitywhich the higher thermal conductivity of insulating oil gives to transformers; also, the windings in rotatingequipment are confined in slots in the steel and are subject to temperature expansion problems. For anyabnormal loading of generators, the electrical, mechanical, and thermal characteristics must be evaluated todetermine the machines limitations. Machines should not be loaded beyond the manufacturer's definedoverload capability unless a complete study of the unit has been made to define safe limits. Additional testsmight have to be conducted to substantiate the possibilities for higher-than-rated loadings.

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1. TYPES OF INSULATION USED ONROTATING MACHINES, THEIR

1.1 Thermoplastic (asphalt-mica).-Theuseful lifeof a thermoplastic (asphalt-mica) insulation systemis practically ended when the insulation has become

brittle enough to develop cracks under themechanical stresses to which it is subjected. A directcause of embrittlement is operation at hightemperatures; cracking results from mechanicalstresses imposed upon conductors having brittleinsulation. The mechanical stresses are caused by(a) short-circuit currents, (b) thermal expansion andcontraction of the conductors, and (c) vibration. Thedielectric strength of insulation is not significantlyreduced by brittleness alone; however, electricalbreakdown may quickly follow the development ofensuing cracks, especially when moisture andcontaminants invade the system through the cracks.

Another cause of failure in asphalt-mica windings isthe migration of the asphalt compound when the coiloperating temperature reaches the flow point of theasphalt. As the compound migrates, the spaceformerly occupied by the compound becomes avoid, subject to localized interior coronadeterioration and resultant failure. This type ofdeterioration is more predominant in the phaseterminal end of the winding where the voltages toground are high enough to produce coronadischarges. Evidence of asphalt compoundmigration would be bulges in coil tape, usually at thelowest point in the coil. It has been known for coils toremain in service, without failure, for years even ifall the asphalt compound had migrated. This is nota healthy condition, but the only remedies arereduced load or rewinding the generator.

1.2 Thermosetting (polyester-mica or epoxy-mica).-Theuseful life of thermosetting (polyestermica or epoxy-mica) insulation systems has not yetbeen determined as they have not been in servicelong enough to determine the eventual effects ofaging. To date, the main cause of failure ofthermosetting insulation systems has been

vibration due to looseness in slots. Thermosettinginsulation systems have proven to be especiallyprone to developing loose wedges and slotdischarge because the "hard" nature of the insulationsystem does not mold itself to fit the slot as the oldasphalt-mica system did.

New installation methods, such as the use of springtype wedge fillers, are being used to control thetendency of thermosetting systems to become loosein the slots. It appears that proper installationtechniques may solve this problem.

2. MECHANICAL LIMITATIONS

Large and high-voltage hydrogenerators built inaccordance with ANSI C50.12- 1965 may be operatedup to 115 percent load at rated power factor,frequency, and voltage, with the stator and rotor

temperatures in excess of normal for these machines.Although this load level does not define the actualtemperature rises, it does define the mechanical limitas the value on the machine at rated power factor and115 percent kVA. For example, a generator with arated power factor of 0.9 would actually be operatingat 128 percent of the mechanical rating if it wasloaded to 115 percent kVA and unity power factor. Ifit is intended to operate units with rated power factorsless than unity at 115 percent kVA and unity powerfactor, the Denver Office should obtain themanufacturer's concurrence that the machine has thismechanical capability.

For other unusual conditions such as short-timeoverloads, unbalanced load operation, harmoniccurrent loadings, etc., industry standards do not makeany specific provisions. Additionally if persistentvibrations at some fixed frequency during operationshould occur due to hydraulic forces, the resonantfrequencies of rotating parts and associatedcomponents should be investigated because usuallyno allowance has been made for this condition in theoriginal design. Each such case should again bediscussed with the manufacturer to establish safelimits.

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3. DAMAGE CAUSED BY EXPANSION OFCOPPER

Temperature rise places stricter limits on loadsthan consideration of total temperature alone,Copper windings have a greater coefficient ofexpansion than stator steel, and in addition, are

usually at a higher temperature. Under widelyfluctuating temperatures, portions of the coilsmove in the steel slots with which they are incontact and abrasion and cracking of insulationmay result. This movement is greater for machineswith long slots than short slots, and increases withtemperature rise. Since temperature rise variesapproximately as the square of the load, it can beseen that a machine which must be overloadedshould have its load maintained as constant aspossible. This may be accomplished by holdingconstant load on the overloaded machine as muchas possible, and carrying the fluctuating portion of

the system load on other machines which are notoverIoaded. The rate of load increase on coldmachines should be limited to not more than 10percent of rated load per minute except inemergencies when it is essential to pick up loadpromptly. This restriction does not apply todecreasing load, nor to varying load on a

warmed-up generator. Hydroelectric generatorsused for peaking power generation are sometimesloaded at the gate opening speed when peakingpower is required on short notice. An example ofthe damage (tape separation) done to asphalt-micainsulated coils by excessive coil expansion andcontraction is shown infigure 1.

4. LIMITING TEMPERATURES FORINSULATION

The safe operating temperature of rotatingmachine windings is limited by the heat whichinsulating material will stand without abnormaldeterioration. This varies with different classes ofinsulating materials as shown in table 1. Thehottest spot temperatures shown in the table havevery little, if any, margin of safety for continuousoperation. The IEEE Standard No. 492 indicatesthat life of class B insulation is halved with each 10

C that the hottest spot temperature is above thelimiting temperatures given in table 1 and anumber of manufacturers believe that 8C is amore realistic value. A curve ofinsulation life isshown infigure 2 for class B insulation. This curveis based on an assumed life of 30 years whenoperating continuously at the temperature shown.

Figure 1. - Example of stator end turn insulation cracking due to coil expansion and contraction.Photo 3-5828

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Insulation life is affected by many other factors, suchas cyclical loading duty, cleanliness, and rate ofloading. so general conclusions on effect oftemperature on insulation life are difficult to make.

5. HOTTEST SPOT ALLOWANCE

Insulation deterioration at a given temperature isapproximately proportional to the length of time thatthe temperature is above the limiting value. Hottestspot temperatures shown intable 1 are not directlymeasurable values in actual machines and are,therefore, not used in machine ratings. Usualmethods for measuring winding temperatures areembedded RTD's (resistance temperature detectors)or by measuring the winding resistance. The lattermethod is dated and is not used with newermachines; new machines have RTD's to measurewinding temperatures and RTD's are being phased

in to replace old temperature measuring methods asolder machines are rewound or uprated. Themeasurable or observable" temperature indicatedby these devices will be less than the hottest spottemperature by an amount known as "temperaturegradient" or "hottest spot allowance.' The actualhottest spot allowance varies with machine design,and, therefore, the limiting temperature rise is notthe same for all machines. Values of limitingobservable temperature are shown in table 1. Itshould be noted that data in table 1 apply only toinsulating materials themselves and not toequipment in which they are used. To provide areasonable service life for insulation in equipment,rated temperature rise should be used as themachine base allowable temperature.

6. LIMITING TEMPERATURE RISE

Observable temperature rise of each of the variousparts of a rotating machine above the temperature ofthe cooling air should not exceed values given intable 2when the machine is operated at rated powerfactor, voltage, and kVA (hereafter called rated

3

load). For open machines and for parts of enclosedmachines which are cooled by open ventilation, suchas collector rings, cooling air temperature is theaverage temperature of the external air at entrancesto the ventilation opening of the machine. For totallyenclosed machines, cooling air temperature is theaverage temperature of air leaving the coolers. In no

event should cooling air temperature exceed 40 C.Machines whose observable temperature rise atrated load exceeds values given intable 2 should beconsidered to have a reduced operational capacityto correspond to a loading value which does notexceed temperature rise values given in the table.

7. OPERATION OF MACHINES UNDERWARRANTY

New, newly uprated, or newly rewound machinesoperating under the manufacturer's warranty shouldnot be loaded above design values. If the results of

acceptance tests or the observation of stationinstruments should show that a machine will exceedtemperature rise limitations of specifications, themachine should not be operated above theselimitations until the contractor has had theopportunity to make the necessary corrections to theequipment. This is necessary to avoid nullifyingcontractual warranties on equipment as well as topermit a period of close observation of performanceof new equipment to evaluate the desirability ofallowing increased loading. After acceptance of themachine and analysis of heat runs, the DenverOffice will prepare and issue capability curves forsubsequent operational use (par.20).

8. OPERATION UP TO RATED CAPACITY

Hydroelectric generators fail into the following ofthe five categories:

a. Old machines that have not been rewound oruprated.

b. Old machines that have been rewound.

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except as described inparagraph 9, as upratedand new machines may not have overloadcapabilities.

The loading limitations given above are primarily

set to guarantee a mechanical safety factoradequately below the endurance limit for the

machines and, therefore, the loading limit of a unitshould not be raised on the basis of thermal andelectrical capabilities alone.

Specific recommendations on a machines loading

capabilities can be requested from the Code D8440, Denver Office.

Figure 2. - Life of generator insulation.

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Table 2A. - Limiting observable temperature rise of indirectly cooled and directly water-cooled salient polesynchronous generators and synchronous generator/motors for hydraulic turbine applications - indirectlycooled machines manufactured before 1982

Method of Temperature rise(C)Item Machine part Temperature Class B Class F

determination

(1) Armature windings of machines 1500 kV A RTD 60 80and above.

(2) Field windings of machines 1500 kV A and Resistance. 80 100above.

(3) Collector rings Thermometer. 65 85(4) Cores and mechanical parts in contact with Thermometer. 55 75

or adjacent to insulation (except (except

motors) motors)70 90(motors) (motors)

(5) Miscellaneous parts such as brushholders, May attain such temperature as will notbrushes, etc. injure the machine in any respect.

Table 2B. - Limiting observable temperature rise of indirectly cooled and directly water-cooled salient polesynchronous generators and synchronous generator/motors for hydraulic turbine applications - indirectlycooled machines manufactured after 1982

Method of Temperature rise (C)Item Machine part Temperature Class B Class F

determination(1) Armature winding1+

(a) 7000 V and less Embedded detector.* 80 100(b) over 7000 V to 15 000 V Embedded detector.* 75 90

inclusive.(c) over 15 000 V Embedded detector.* 70 85

(2) Field winding. Resistance. 80 100(3) Collector rings. Thermometer. 85 85(4) Cores and mechanical parts in contact Thermometer Not to exceed the value

with or adjacent to insulation or detector, for the associatedadjacent insulation.**

(5) Miscellaneous parts (such as amortisseur windings, brush holders, brushes, etc.) may attainsuch temperatures as will not injure the machine in any respect.

* Embedded detectors are located within the slot of the machine and can be either resistance elements orthermocouples.

+ The temperature rise limits listed are for insulation system with thermosetting materials. For thermoplasticmaterials, thetemperature rise limit shell be 70 C.

** When core temperatures are measured at the outside diameter of the core, the limiting temperature rise shallbe 5 C less than the associated armature winding insulation limiting temperature rise.

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Table 2. - Limiting observable temperature rise of indirectly cooled and directly water-cooled salient polesynchronous generators and synchronous generator/motors for hydraulic turbine applications - directlycooled machines manufactured after 1982

Method of Water-cooled windingsItem Machine part temperature Class B insulation

determination temperatures (C)

(1) Temperature of cold coolant Detector or thermometer. 45 - 50(Note 1) (Note 2)

(2) Temperature rise of directly cooled Coolant 55 -50armature windings (Note 3) (Note 2)

(3) Temperature rise of directly cooled Resistance. 55 - 50field windings. (Note 2)

(4) Temperature rise of cores and Detector or thermometer. 85 -80mechanical parts in contact (Notes 4with or adjacent to insulation and 5)

(5) Temperature rise of collector rings. Thermometer. 85

(6) Temperature of miscellaneous parts such as amortisseur windings, rotor surface, brush holders,brushes, etc., may attain such levels as will not injure the machine in any respect.

Note 1. - The method of coolant temperature measurement shell be optional with the manufacturerunless otherwise agreed upon. Only one method of temperature measurement shall be required in anyparticular case.

Note 2. - Cold coolant temperatures shall be provided within the range of 45 to 50 C, at themanufacturer's option, so long as compensating adjustments are made in the rise of the respective partsso that the sum of the cold coolant temperature and respective part rise does not exceed 100 C for watercoolant.

Note 3. - Temperature rise of coolant at the outlet of the hottest coil shall be considered the observabletemperature rise of the directly cooled armature winding.

Note 4. - Temperature of the core and mechanical parts in contact with or adjacent to insulating material including that of the winding and of core laminations shall not exceed the values in the table.Temperature of other metal parts, including structural members and shielding devices in the end region, isnot required to be within the limiting temperature, provided that these parts do not appreciably influencethe temperature of insulating material either by contact or radiation. These parts may be operated attemperatures which are considered safe for the particular metals used.

Note 5. - The values shown for item 4 are limiting regardless of the operating power factor.

9. SHORT-TIME AND EMERGENCYOVERLOADS In extreme emergencies where lack of generation

might cause a system breakup, it may be

During short-time emergencies, loads of 115 necessary to overload machines briefly in excesspercent of the normal loading limit are permissible of 115 percent of the loading limit and/or

at the discretion of the plant superintendent.

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maximum temperature permitted intable 2, but ifand when this is done, a sacrifice in insulation lifemust be expected and the risk of mechanicaldamage to the machine must be considered.If it is anticipated that the emergency overloadmust be continued for 1 or more days, temporarysupplemental cooling, as discussed in paragraph

13should be installed.

10. LOAD LIMITATIONS OF ASSOCIATEDEQUIPMENT

It is important that overloads should not be carriedon rotating machines without an investigation ofthe limitations of associated equipment. Equipmentsuch as cables, buses, reactors, circuit breakers,disconnecting switches, current transformers, andpower transformers should be checked. Any one ofthese may constitute the practical limit in loadcarrying ability of the unit. On the machine itself,

auxiliary equipment such as exciters or rheostatsshould be checked. The exciter should havesufficient margin while carrying the overload totake care of small fluctuations in load and voltagethat may occur with minor system disturbances. Insome cases it may be possible to ease the burdenon the exciters of the machine being overloaded bytransferring reactive kVA to other units of the samesystem.

11. COOLING WATER

If the mechanical limits of a machine are notexceeded, overload capacity of air-cooledmachines with water cooled air coolers can beincreased in some cases by increasing flow ofcooling water so as to not exceed temperaturelimits intable 2. This is particularly true where coldcooling water is available. It should beemphasized that in addition to limiting the totaltemperature, temperature rise should be heldwithin indicated limits. In a non-overload conditionfor class B insulation, if the limiting statortemperature rise is 60 C, and the limiting totaltemperature is 100 C, the difference between thetemperature of the stator and the air leaving thecooler should not be more than 60 C even though

it may be possible to hold the total statortemperature to less than 100 C by increasing flowor reducing the temperature of cooling water.

12. COOLING WATER REGULATION

As stated earlier, winding insulation life (a) isshortened by high temperature, and (b) is subjectto mechanical damage by temperature cycling.Unfortunately, these facts place conflictingdemands on any cooling system which is

designed to lengthen the insulation service life,since, if maximum cooling of insulation, thepossibility of mechanical damage due totemperature cycling under varying load isincreased; likewise, if temperature cycling is to beminimized, the insulation temperature must beheld constant at a relatively high value.

Therefore, there are presently in use two methodsfor controlling cooling water, each of whichoperates to lengthen insulation service life bycontrolling one (but not both) of the aboveconflicting requirements a. and b. as follows:

a. In cooling systems not provided withautomatic regulation of water flow, thecooling water should be adjusted to produceminimum cooling air temperature withoutexceeding temperature rise limits intable 2when the machine is carrying the maximumexpected load. This flow should be constantfor all other loads in order to maintainminimum insulation temperature at all times.However, winding temperatures shouldalways be held above10 C.

Where cold cooling water is used, it isimportant to watch for condensation onpiping and cooler surfaces within thegenerator and see that water is not beingcarried into the windings, causing rust orcorrosion of metal parts. The amount ofcooling water may have to be reduced toprevent condensation, or mixed with thewarmer discharge water through a bypassconnection to raise its temperature.

b. On some units, an automatic coolingwater flow control system has been installed

to reduce the range of temperature

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from no load to full load. On these units,the cooling water temperature is varied bymixing warm discharge water with freshcool water as required to minimizetemperature variations.

13. SUPPLEMENTAL COOLING

For air/water-cooled units, when not limited by themechanical capability, the capacity may beincreased by using colder water, or by increasingthe quantity of water circulated through the coolers.Open-type units depend on the surrounding air forthe removal of heat. By increasing circulation ofthe air, or by cooling the air in some manner,capacity of open-type machines can be increased,but temperature rise limitations of table 2must bekept in mind. Ventilating fans may be used to directair toward the machine. Advantage should betaken of windows and doors to admit outside air. In

several instances, cooling has been improved bybuilding a duct to bring outdoor air directly into theturbine pit and eliminating the possibility of warmair exhausted from the generator being recirculatedinto the machine, if artificial cooling of the air isattempted, care should be taken to preventmoisture or dusty air from being drawn into thegenerator.

14. KEEPING MACHINES CLEAN

Another factor which should be watched inmachines, especially if they are to be overloaded,is accumulation of dust and dirt. Dust and dirthinder heat dissipation and can adversely affectthe voltage grading system; therefore, internalcleanliness of the machine is important to assureproper cooling and retention of the voltage gradingshould be cleaned regularly to assure maximumheat transfer. When generator temperaturesappear to be getting higher, temperature datashould be compared with log sheet data of pastyears, or cleaned to determine the amount oftemperature when the machine was new, or justafter being rise due system. Particular attentionshould be given to field winding because dirt ismore apt to deposit on irregular surfaces.

Inspection should be made of ventilating passagesof the stator core for dirt accumulations. Formachines having recirculating systems, the coolershould be cleaned regularly to assure maximumheat transfer. When generator temperaturesappear to be getting higher, temperature data

should be compared with log sheet data of pastyears, or when the machine was new, or just afterbeing cleaned to determine the amount oftemperature rise due to accumulation of dirt andestimate the benefit obtainable by cleaning.Reduction of approximately 10 C in the statortemperature of of an open-type Bureau generator

resulted from a thorough cleaning and enable theplant to carry more load during a critical period.Where possible, suction should be used to cleanmachines, since with this method the grit anddust is not merely moved from one resting placeto another. Clean, dry compressed air at apressure of not over 276 kPa (40 lb/in2 ) may beused to blow dust out of the wound section orother places not accessible to the suctionattachment. If dust is allowed to accumulate onthe windings, it not only prevents properdissipation of heat and circulation of cooling airthrough ducts, but also tends to hold moisture and

oil against insulation. In the cleaning process, sliprings and commutators should be cleaned of dustto prevent abrasion. Oil should never be allowedanywhere on machines except where it is requiredfor lubrication. Oil harms commutators bycarbonizing mica insulating segments betweenbars. Oil on windings catches dirt and dust andthe resulting gummy compound hastens ultimatefailure of insulation. Flammable or highly toxiccleaning mixtures such as carbon tetrachlorideshould not be used. Several satisfactorycommercial solvents are now available for thispurpose. Windings should not be allowed to soakin any solvent, but just enough solvent should beused to loosen grease so that it can be wiped off.Ample ventilation should be supplied forworkmen. Any oil leaks should, of course, bepromptly repaired.

15. TEMPERATURE OF OLD MACHINES

For older machines, special care should be takenin determining the temperature of all parts ifoperation at overload is contemplated. Many oldermachines have stator core laminations of poorquality steel and inadequate ventilating systemsfor taking care of losses in the area close to the

air gap. Increased load or operation above normalvoltage means more leakage flux and even highertemperatures for these machines. Duringunderexcited operation, some machines aresubject to above normal temperatures on endand finger plates or laminations at each end of

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the stator core because of excessive leakage fluxnear the winding end turn area. Most machines arenot subject to this trouble, but the condition shouldnot be overlooked in the few cases where it exists.Table 2 shows allowable temperature rises ofvarious components of the machine.

16. CHANGING VOLTAGE TO REDUCETEMPERATURE

Some reduction in operating temperature issometimes possible, particularly on synchronousmotors, without reducing load, by changing theoperating voltage. Stator iron losses andtemperatures increase with increased appliedvoltage, and vice versa. Copper losses andtemperature are proportional to the square of thestator current. If stator laminations run cool andcoils comparatively hot, an increase in statorvoltage by changing transformer taps will decrease

the copper temperature and increase the irontemperature, without a change in output. Theminimum stator winding temperature for any kWload will be obtained at 100 percent power factor.

17. LOAD TEMPERATURE TESTS

For all types of machines, an indication of theamount of overload which can be carried can bedetermined from the mechanical limits and a seriesof temperature tests. A prediction can be madefrom results of a single temperature run bycomputing the temperature rise as outlined inparagraph 19. More accurate indications areobtained by making temperature rise test runs at50, 75, and 100 percent load. However, the bestplan is to make a temperature test at the actualoverload which the machine is desired to carry.Voltage and power factor should be held constantfor all runs so that the internal voltage and coreloss will be about the same. Switchboardinstruments should be supplemented withaccurately calibrated instruments duringoverloading of the machine. Each loadtemperature test should be run with constant load,excitation, cooling water, etc., until the temperature

rise above ambient air temperature reaches aconstant value, and readings should be taken at30-minute intervals for several hours alterconditions become stabilized. Specific testprocedures may be obtained from the DenverOffice. A curve of generator amperes versusstator temperature rise plotted from the test data

may be of value for future reference.

18. MEASURING STATOR TEMPERATURE

Temperature of the stator coils should be taken asthe highest reading obtained. Where embeddedRTDs are provided for measuring the statortemperature and a temperature check should bemade by placing a few thermocouples on thestator iron (core). Where no embedded RTDs areinstalled, it is desirable to use a number ofthermocouples for measuring stator temperature;thermocouples should be placed on the hottest

parts of the stator steel, well protected from theairstream, and the highest thermocouple readingsshould be used. Thermocouples may be heldagainst the surface with duct seal. Precautionsshould be taken if thermocouples are used tomeasure end turn temperatures, end turn coilscould be charged to nearly the turn-to-groundvoltage during machine operation. Specific testprocedures may be obtained from the DenverOffice.

19. MEASURING FIELD TEMPERATURE

Field temperature for both static or rotatingexcitation systems shall be determined by theresistance method from field voltage and currentreadings while the machine is carrying the load forwhich data are desired. Specific test proceduresmay be obtained from the Denver Office. Anaccurate voltmeter and ammeter should be used.Field voltage should be measured at collectorrings by pilot brushes. If no pilot brushes areprovided, one of the main brushes on each ringcan be disconnected and insulated from thebrush holder by a layer of paper and used as

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temporary pilot brushes. Field winding temperaturecan be found from the following formula:

R2T2 = (234.5 + T1) - 234.5

R1

where:

T2 = temperature (OC) corresponding to final

resistanceT1 = temperature (

OC) corresponding toInitial resistance R1

R2 =field volts

field amperes

Field resistance R1 at temperature T1 is usually

given on the generator manufacturer's test report.If these data are not available, resistance shouldbe measured after the machine has been shutdown for at least 12 hours with constant ambienttemperature and using ambient temperature for T1.

Data obtained from field temperature tests can beused for making up curves as shown infigure 3bfor future use in determining field temperaturefrom readings of field volts and amperes. Thesecurves will be particularly valuable if the fieldtemperature is the limiting factor in overloadingthe generator. While making the field temperature

test, readings of the accurate voltmeter andammeter used should be compared with theswitchboard voltmeter and ammeter and the lattershould be corrected if necessary. It is importantthat the field voltages be measured at collectorrings. If pilot brushes are not provided, anallowance for brush drop can be made as foundfrom comparison of voltage measured on the mainbrushes and temporary pilot brushes, Thetemperature-resistance curve, figure 3a, is astraight line and can be plotted through any twopoints of resistance and correspondingtemperature, such as R1, T1, and R2, T2 used in theabove formula. Curves in figure 3b should beplotted in the range of about 80 to 130 percent offull load field amperes and corresponding voltage,so as to obtain a broad spread in the temperaturescale. Field temperature readings are not

considered important unless they are near thelimiting temperature. Each curve infigure 3b is astraight line plotted between two values of voltageand its corresponding current giving the samevalue of resistance. That is,

E1 E2R = =

I1 I2

The value of R is first found fromfigure 3aforeach value of temperature for which a curve isto be made.

20. COMPUTING STATOR WINDINGTEMPERATURE RISE

At times it is difficult or inconvenient to measurestator winding temperature rise by tests at all

desired loadings; therefore, a method ofcomputing temperature rise, TR1 from availabletemperature test data is outlined below:

The basic equation to be used is:

TR = C1 C2 (kVA)2

This equation has two unknowns, C1 and C2, andtest data are required from two heat runs to findthese unknown values. By substituting TR1 andkVA1 in equation for heat run No. TR2 and kVA2 forheat run No. 2 and subtracting one equation fromthe other, C2 can be found. Substituting C2 backinto either equation gives a value for C1. Theconstant C1 represents stator winding temperaturerise for no-load losses. If it is more convenient, orif data are already available, stator windingtemperature rise attained during a no-load, ratedvoltage heat run may be used for C1.

After C1 and C2 are known, the stator windingtemperature rise, TR, for other desired loadingconditions may be calculated.

21. GENERATOR CAPABILITY CURVES

Capability curves for large rotating machines arerequired to establish safe operational limits. Thesecurves are required for inclusion involved.

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Figure 3. - Typical field resistance temperature curves.

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21.1. Overexcited (boosting voltage).-Acapability curve is defined as a curve which showsboundaries of the area on the kilowatt-kilo-vardiagram within which a machine may be operatedcontinuously. Capability curves are furnished foreach installation which show kVA capabilitythroughout the expected range of operation and

which include the range of permissible operatingvoltages. One such curve sheet for a generatorrated 65 789 kVA, 13 800 volts, and 95 percentpower factor is shown in figure 4. The highestoperating voltage is limited to 105 percent of ratedterminal voltage in order to meet voltagelimitation requirements of the Standards forSynchronous Generators. Capability curves formachines in categories a and b in paragraph 7 notoperating under manufacturer's warranty willnormally show limitations for 115 percent ofnameplate ratings. Limitations for new machines(post 1982) will normally be the nameplate rating,

For uprated generators, the kilowatt rating ratherthan the kVA rating is set at a maximum fixedvalue which should not be exceeded. TheCapability curve will be based on a constantkilowatt load.

21.2. Underexcited (bucking voltage).-Generator capability when operating underexcitedis limited by the permissible armature current andsteady-state stability limit of the machine. Acapability curve sheet for underexcited operationis shown infigure 5. Steady-state stability limit isa function of system at this point are above

terminal voltage. As the kilovar load is increasedat zero-kilowatt load, the generator will eventuallybecome unstable and start to slip poles. The kVA

reactance, and generator and armature currentrated values so that continuous operation nearthis point is out of the generator reactances,

external question because of armature heating.

Most modern generators may be operatedcontinuously at zero field current and rated

voltage except for a few machines with specialelectrical characteristics such that stator heatingbecomes a problem. Modern, continuously actingvoltage regulators are equipped withunderexcited reactive ampere limiters whichpermit machine operation at underexcitedcapability determined by thermal limitationswithout danger of the machine becomingunstable. These limiters respond to underexcitedreactive stator current and to terminal voltage.As a part of the regulating function, limitersautomatically hold the underexcited reactiveamperes at lower values as kilowatt load on the

machine is increased. Limiter settings areadjustable so that their operating characteristicsmay be coordinated with the thermal capabilitycurve and the steady state stability curve. TheDenver Office, should be consulted when anyoperating problems arise concerningunderexcited operating capability either underload or strictly as a synchronous condenser.Additional underexcited synchronous condensercapacity is sometimes made available byoperating with negative field current underautomatic control of voltage regulators.However, any changes in limiter settings toaccommodate this operation must becoordinated with machine thermal limitations andwith the settings of the loss of excitation relays.

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22. REFERENCES

General Principles upon which TemperatureLimits are Based in the Rating of Electrical Machines and Other Equipment, IEEE Stan-dard No. 1, 1969

Guide for Functional Evaluation of Insulation Systems for Large High Voltage MachinesIEEE Standard No. 434-1973

Test Procedures for Synchronous Machines, IEEE publication No. 115, 1965

General Principles of Temperature measurements as Applied to Electrical ApparatusIEEE Publication No. 119, 1974

American Standard for Rotating Electrical Machinery, American National Standard C50.10,1977; C50.11, 1965; C50.12, 1982; C50.13,1977

Guide for Operation and Maintenance ofHydrogenerators, IEEE Standard No. 492

1974.

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