ger3809- generator rotor thermal sensitivity

GE Power Systems

Generator Rotor ThermalSensitivity — Theory andExperience

Ronald J. ZawoyskyWilliam M. GenoveseGE Power SystemsSchenectady, NY

GER-3809

g

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1General Generator Rotor Vibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Generator Rotor Thermal Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Testing for Thermal Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Causes of Thermal Sensitivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Shorted Turns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Blocked Ventilation or Unsymmetrical Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Insulation Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Wedge Fit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Distance Block Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Retaining Ring/Centering Ring Assembly Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Tight Slots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Heat Sensitive Rotor Forging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

GE Thermal Sensitivity Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Examples of Thermally Sensitive Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Case A: Hydrogen-Cooled Generator — Shorted Turns. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Case B: Air-Cooled Generator Insulation Degradation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Case C: Hydrogen-Cooled Generator — Variable Friction Influence . . . . . . . . . . . . . . . . . . 11

Proven Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Commonly Asked Questions Regarding Generator Rotor Thermal Sensitivity . . . . . . . . . . . 14List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Generator Rotor Thermal Sensitivity — Theory and Experience

GE Power Systems ■ GER-3809 ■ (04/01) i


GE Power Systems ■ GER-3809 ■ (04/01) ii

IntroductionGenerator rotor thermal sensitivity is a phe-nomenon which may occur on the generatorrotor causing the rotor vibration to change asthe field current is increased. This has occurredon generator fields of all manufacturers at onetime or another. The thermal sensitivity can becaused by an uneven temperature distributioncircumferentially around the rotor, or by axialforces which are not distributed uniformly inthe circumferential direction. The primary driv-er of this second cause is the large difference incoefficient of thermal expansion between thecopper coils and the steel alloy rotor forgingand components. If the rotor winding is notbalanced both electrically and mechanically inthe circumferential direction, the generatorrotor will be unevenly loaded which can causethe rotor to bow and cause the vibration tochange. In most cases, a thermally sensitiverotor will not prevent a generator from run-ning, but may limit the operation at high fieldcurrents or VAR loads due to excessive rotorvibration.

This paper will discuss the types of thermal sen-sitivity, thermal sensitivity mechanisms and caus-es, and testing to diagnose for a thermally sen-sitive field. The paper will conclude with exam-ples of thermally sensitive generator fields, cor-rective actions taken to eliminate the thermalsensitivity and recommendations for fields thatare thermally sensitive. An appendix is includedwhich answers commonly asked questionsregarding thermally sensitive fields. (Note: theterms generator rotor and generator field areused interchangeably throughout this paper.)

General Generator Rotor VibrationThe vibration objective for a generator rotor isfor the vibration to remain within acceptablelimits and remain smooth at all operating

speeds and under all operating conditions atrated speed within the capability curve.Inspection of the capability curve in Figure 1shows three distinct regions. Region A-B is lim-ited by field heating, region B-C is limited byarmature heating and region C-D is limited byarmature core end heating. In general, a ther-mally sensitive field is not affected when operat-ed in regions B-C or C-D since the field currentis not high and, therefore, the rotor does notreach rated temperature.

However, as the field winding approaches thegenerator rating point at B, a thermally sensitivefield will respond to the increase in field cur-rent by changing its vibration. This changemight be in the form of a vibration increase ordecrease, or a change in phase angle. All fieldshave some degree of thermal sensitivity; howev-er, if the vibration levels remain within accept-able limits (2-3 mils Pk-Pk at the journals), ther-mal sensitivity is not normally an issue. Itbecomes an issue when the vibration exceeds


GE Power Systems ■ GER-3809 ■ (04/01) 1

0.85

0.95

P.F.

P.F.

.9

.8

.7

.6

.5

.4

.3

.2

.1

0

.1

.2

.3

.4

.5

Lead

Per

Un

itK

ilo

vars

Lag

Per Unit Kilowatts

Curve as Limited by Field HeatingCurve BC Limited by Armature HeatingCurve CD Limited by Armature Core and Heating

A

B

CD

GE

R3809-1

Figure 1. Typical reactive capability curve

acceptable levels during operation within thecapability curve.

There are many causes of high vibration on agenerator field. The most common aremechanical unbalance, thermal sensitivity, mis-alignment and bearing degradation. Othercauses include rubbing, bent overhangs, rotorstiffness dissymmetry, out-of-round journals andother design deviations caused by abnormal in-service operation. Each of these causes has apredominate frequency and a characteristicresponse. The cause of the vibration can bediagnosed by a thorough analysis of the vibra-tion data. For example, the most frequentlyobserved cause of vibration is mechanicalunbalance. This type of vibration is synchro-nous; that is, the vibration frequency equals therotor rotational speed frequency. It does notrespond to changes in operating conditions,such as generator load or field current. In mostcases, unless the unbalance is excessive,mechanical vibration can be corrected by bal-ancing. The remainder of this paper will onlydiscuss generator field thermal sensitivitybecause it is generally the least understood andis relatively common.

Generator Rotor Thermal SensitivityA thermally sensitive rotor is characterized by aonce-per-revolution frequency response signa-ture due to a change in the rotor balance aris-ing from the rotor bow. If the total vibration ofthe field stays within acceptable limits, the fieldis not considered “thermally sensitive.”Vibration performance is frequently plotted ona polar chart, such as that shown on Figure 2,because vibration is characterized by amplitudeand phase angle. If the vibration vector stayswithin the 2 or 3 mil circle, or whatever is cho-sen as an acceptable vibration level, the vibra-tion is not considered to be a problem. This is

true even if the phase angle changes and thevibration moves around the interior of this cir-cle. The change in vibration and phase anglewithin the polar plot from the starting operat-ing point to the end operating point is calledthe thermal vector.

There are two types of thermal sensitivity:repeatable (or reversible) and irreversible. Bothtypes vary with field current; however, thereversible type follows the field current as it isincreased and decreased. For example, if thevibration on a field increases from 1 mil to 3mils as field current is increased and thendecreases in the same manner as the field cur-rent decreases, then the thermal sensitivity isconsidered to be reversible. If this is the case, inmany instances the field can be compromisedbalanced so that the thermal vector passesthrough zero and the maximum vibrationremains within acceptable limits.

However, if the vibration increases as the fieldcurrent is increased but does not respond to adecrease in field current, then this type of ther-



0 1 2 3 4 5 6 7

Unacceptable Vibration Excursion

Acceptable Vibration Excursion

GE

R3809-2

Figure 2. Typical polar plot showing athermal vibration vector

mal sensitivity is referred to as irreversible orslip-stick. If this situation occurs, the generatorfrequently must be taken off-line and broughtdown to turning gear speed to unlock the forcesthat induced the rotor bow. This type of ther-mal sensitivity is particularly troublesome and,in a few cases, there were no effective remediesto relieve this condition without disassembly ofthe winding. As a result, a field winding with thiscondition will limit load options for the ownersince the generator will not be able to operateover its full electrical capability. Figure 3 shows aplot of an irreversible field where the vibrationincreased with field current but locked in at thehigh vibration level when the field current wasremoved.

Testing for Thermal SensitivityIf a rotor is suspected to be sensitive to field cur-rent, there are tests that can be performed toconfirm this and to ensure that the condition isnot due to megawatt loading of the turbine gen-erator set. One of the tests that is important is aflux probe test. This will give a diagnosis of thecondition of the field winding turn insulationand indicate which coils have shorts in the fieldwinding. In most cases, the test will detect thenumber of shorts present in each coil and in

which pole the shorts are located. (It should benoted that the presence of magnetic wedgesmay prevent accurate detection). This is anextremely useful diagnostic test since windingturn shorts are the most common cause of ther-mal sensitivity.

The other test that should be performed is oneto isolate the effects of megawatt loading fromVAR loading on the field. Vibration changing asa function of megawatt loading is not a thermalsensitivity mechanism. Megawatt loading mayresult in rotor vibration excursions as a result ofbearing alignment shifts. The three-part ther-mal sensitivity diagnostic test is shown graphi-cally in Figure 4.

The first part of the test is to apply constantfield current to the field and then to vary themegawatt loading on the generator from15–60%. Detailed vibration readings as well asother key generator parameters, such as gener-ator voltage, currents and temperatures, shouldbe monitored throughout all stages of the test-ing. Any significant changes in the generatorvibration during any part of the testing should



300 350

Field Current – Amps

Vib

rati

on

Am

plitu

de

–M

ils

400 450 500 550 600 650 700

7

6

5

4

3

2

1

0

GE

R3809-3

30.0 PSIG H2

15.0 PSIG H

0.5 PSIG H

2

2

50

40

30

20

10

0

10

10 20 30 40 50 60 70

20

30

40

Lead

Meg

avars

Lag

3

2

8

7

5

4

12

9

6

11

1

0.85 PF

0.95 PF

TEST POINTNO.

10

GE

R3

80

9-4

Figure 4. Testing procedure for thermallysensitive field

Figure 3. Vibration data showing irreversiblethermal sensitivity

be noted. This first part of the testing wouldcorrespond to moving from point 1 to point 4on Figure 4.

The second part of the test is to apply a constantmegawatt load to the generator (approximately60–80%) and then raise field current to maxi-mum rated field current. Each test point shouldbe held until steady state is reached. If the unitis unable to reach maximum field currentattainable without a vibration excursion, theseries should be repeated but be limited to themaximum field current attainable withoutexceeding acceptable vibration limits. Again,detailed test data should be taken. A significantchange in vibration or phase angle with anincrease in field current at constant megawattload would indicate that the field is thermallysensitive.

This test should then be reversed; that is,decrease field current from its maximum valueback down to the starting point. Again monitorall test data. If the vibration and phase anglereturn to their initial values, then the type ofthermal sensitivity can be considered reversibleand, in many cases, can be overcome with acompromised balance that moves the thermalvector through zero so that vibration limits arenot violated. However, if the vibration levels donot return to the original values and remainhigh, then this field vibration is considered tobe irreversible and corrective actions mayinvolve modification to the field. The fluxprobe and thermal sensitivity tests are veryimportant in monitoring and diagnosing a ther-mally sensitive field. GE offers specializedequipment and trained personnel to work withcustomers in performing this testing.

Causes of Thermal SensitivityIt was mentioned previously that one of theprime reasons that generator fields are ther-

mally sensitive is because of the large differencein coefficients of expansion between the copperconductors that make up the winding and thesteel field forging. Whenever field current isapplied, the copper tends to expand more thanthe steel forging. As field current is increased tolarge values that approach the rating of theunit, the difference in expansion between thecopper and the steel can become significantand the forces generated large. If these forcesare not distributed uniformly around the fieldcircumferentially, they can cause the generatorrotor to bow. This bowing is what causes thethermal sensitivity and it varies as field currentis increased or decreased. This principle is sim-ple, but because of the complex configurationof a generator field, there are many things thatcan influence and affect the susceptibility of agiven field to thermal vibration. The followingare some items which by themselves or in com-bination can cause a generator field to be ther-mally sensitive.

Shorted TurnsShorted turns occur when there is a breakdownin the insulation between turns. They are themost common cause of thermal sensitivity.Depending on the distribution and number ofshorted turns, there may or may not be an oper-ating problem. Shorted turns in the coils adja-cent to the poles are most significant. Whenthere are shorted turns in a field, the pole of thefield that has the higher number of shortedturns, there may or may not be an operatingproblem. Shorted turns in the coils adjacent tothe poles are most significant. When there areshorted turns in a field, the pole of the field thathas the higher number of shorts has a lowerelectrical resistance and, as a result, will be at aslightly lower temperature than the oppositepole. Therefore, the higher temperature polewill elongate in the axial direction more than



the other pole and, as a result, the field will bowin that direction. (See Figure 5.) As field currentis increased, the amount of bowing will increaseand the field vibration and phase angle will besimilarly affected. Shorted turns result in areversible thermal sensitivity.

Blocked Ventilation or UnsymmetricalCoolingBlocked ventilation, like shorted turns, can sig-nificantly affect the circumferential thermal bal-ance of a generator field. This could occur if aforeign object were introduced into the fieldand disrupted the normal ventilation and cool-ing of the field. Direct-cooled windings arecooled by the cooling medium passing directlythrough holes that are designed and manufac-tured into the copper. A shifting of the insula-tion or plugging of these cooling passages couldcause these fields to become thermally sensitive.The uneven temperature distribution wouldaffect a field in the same manner as shortedturns. This type is reversible.

Insulation VariationIf a field is not wound uniformly from pole topole in regards to insulation thickness and

buildup, binding and uneven friction forces inthe coil slots and under the retaining ringscould result. Should this occur, the field coilsmight not be free to expand uniformly in theaxial direction as field current is applied and, asa result, the field forging may be loadedunevenly and cause the field to bow. In thiscase, the coils with the highest friction or theones that are binding will apply more load tothe forging in the axial direction and cause thefield to bow in that direction. Increasing thefield current will cause the bow in the field toincrease further. In some cases, the conductorsin some slots may slip and cause a step changein the vibration. In other cases, the binding ofthe coils will persist and the rotor will remainbowed even after the field current has beenremoved. This condition has occurred on fieldsthat have been in service for many years as theinsulation has broken down or migrated andshifted in the slots. Care must be taken in theassembly of new fields and during field rewindsto ensure that the insulation is installed uni-formly and according to proper design proce-dures and clearances. (See Figure 6.) In manycases, this type of thermal sensitivity is consid-ered irreversible or slip-stick.



Figure 5. Temperature induced thermal sensitivity

Wedge FitGenerator rotors can become thermally sensi-tive if wedges are modified or replaced. This isespecially true when only a portion of thewedges is replaced, such as a slot or two-in-onepole. If the tightness of the wedges does notremain uniform, then it can cause binding inthe axial direction which could lead to bowingof the rotor. If wedges are replaced or modified,it is very important that this be performed care-

fully so that all wedges in the field have thesame clearances and fit. (See Figure 7.) This con-dition usually produces irreversible thermalsensitivity.

Distance Block FittingThe distance blocks that provide the spacing inthe generator field end windings must bespaced and fit properly. Uneven spacing and/orfitting can cause non-uniform forces to be trans-mitted into the field forging through the retain-ing rings or centering rings and, like all theother possible causes of thermal sensitivity,make the rotor bow and change dynamic char-acteristics. (See Figure 8 and Figure 9.) Unevendistance block fitting will cause reversible vibra-tion.

Retaining Ring/Centering Ring Assembly

MovementSignificant forces from the field coils are trans-mitted into the retaining ring and centeringring as field current is increased. If these ringsare not installed properly, the field can be non-uniformly loaded and cause the rotor to bow.Also, if the shrink fit is insufficient, these ringscan move on their shrink fits and cause achange in center of mass of the retaining rings.In this case, the field vibration signature will be



Figure 6. Typical slot configuration of agenerator field

Wedge

Insulation Configuration and ClearancesMust Be Uniform in All Slots

Conductor

Turn Insulation

Slot Armor

GE

R3

80

9-6

Figure 7. Coil wedge fitting

Interference HereCan Causea Thermal Bow

Wedge Fit Must be Uniform

WEDGE FIT

Wedge

GE

R3809-7

variable and the problem cannot be resolveduntil the light shrink is corrected.

It should be noted that rotors that have spindle-mounted retaining rings as compared to thosethat are body-mounted are much more suscep-tible to thermal sensitivity since the retainingrings are mounted on the more flexible spindlesection of the shaft. Because of this, for thesame amount of axial force, a spindle-mountedrotor will bow to a much larger extent. (See

Figure 10 and Figure 11.) Movement of theretaining ring and/or centering ring can causereversible and irreversible vibration.

Tight SlotsThis rare condition will usually occur if, duringa field rewind, the insulation system is changedand/or the copper is reused and is no longerflat due to distortion caused by handling andoperation. It is important that the requireddesign clearance is incorporated in a field



Figure 8. Generator end blocking design

Figure 9. Bowing caused by uneven blocking

rewind. Tight slots will cause the copper tomove unevenly in the axial direction as fieldcurrent is applied and result in rotor bowing.This condition typically causes the irreversibletype of vibration. (See Figure 12.)

Heat Sensitive Rotor ForgingGE has no generator rotors that have exhibitedheat sensitive rotor forgings. However, othermanufacturers are reported to have experi-enced this phenomenon. This rare conditionoccurs due to non-uniform material character-

istics throughout the generator rotor forgingand has no relation to the configuration of thefield or the copper. Because of the non-uniformproperties, as field current is applied the rotorforging expands unevenly in the axial directionand causes the rotor to bow. This condition iscaused by problems in the manufacture andheat treatment of the forging at the materialvendor.

The preceding causes for generator rotor ther-mal sensitivity are those that are most common-



Figure 10. Typical retaining ring mounting configurations

FieldBodyLock

LockFloatingCenteringRing

(Body Mounted)

Centering Ring

Magnetic orNon-Magnetic Retaining RIng

Magnetic orNon-Magnetic Retaining RIng

(Spindle Mounted)

Shaft

Body

GER-3809-10

Shaft

Figure 11. End winding blocking force transfer

Spindle Mounted Retaining Rings

Body Mounted Retaining Rings

GE

R3

80

9-1

1



ly encountered, but are by no means a completelisting. Other things which can cause dissymme-try, such as misuse of adhesives, use of incorrectmaterials and some types of misoperation, canalso cause fields to be thermally sensitive.Anything that creates non-uniform heating,expansion forces, friction, etc., as field currentis changed can result in thermal sensitivity. Insome cases, the problem is not due to just oneof the above or other causes, but a combinationof them. As mentioned previously, all rotorshave some degree of thermal sensitivity. The keyis to control the level of sensitivity through gooddesign and manufacture.

GE Thermal Sensitivity Findings

In a fleet of more than 7,000 generators, lessthan 0.5% of all active generator fields havereported operational problems associated withthermal sensitivity. Of these, only one field wasnot able to operate as a result of the condition.The others could operate but were limited byhigh vibration which exceeded acceptable lim-its at high VAR loading conditions. In general,thermal sensitivity does not cause a forced out-age but may limit operational flexibility of thegenerator.

GE has done a great deal of testing andresearch to better understand the phenomenonof generator field thermal sensitivity. This workhas led to the changing of design parametersfor new fields and field rewinds to minimize therisk and effects of thermal sensitivity.

GE has also done an extensive amount of workto define solutions for fields that have beenfound to be thermally sensitive. One of the ini-tial areas of work was to determine which of thelarge number of types of generator fields in theGE fleet were most susceptible to thermal sensi-tivity. There are a large number of rotor config-urations in the GE fleet that operate at a widerange of temperatures. A summary of the dif-ferent configurations in the GE fleet is shown inFigure 13. These fields range from the smallspindle-mounted retaining ring designs withconventional cooling to the large body-mount-ed retaining ring design with direct-conductor-cooling.

While all generator rotors are thermally sensi-tive to some degree, these studies have shownthat rotors with spindle-mounted retainingrings and conventional cooling are more likelyto experience high levels of vibration ampli-tudes, particularly at high reactive loads. If therotor operates near unity power factor and does

Figure 12. Mechanically induced thermal sensitivity

A

B

– Binding in Slot #1 in Pole A

– Copper is Restricted When Current Increased

– Pole B is Unrestricted

CL

GE

R3

80

9-1

2



not push the field current limits, even rotorswhich are prone to thermal sensitivity areunlikely to experience vibration excursions.This is assuming that other factors are notinvolved, such as misassembly of the rotor or anoperational incident.

GE has incorporated the findings of these stud-ies in both its new rotor designs and rewinds offields in its existing fleet. There are a number ofactions that have been taken by GE to minimizethe susceptibility of its new and existing fields tothermal sensitivity. All configurations of rotorswere monitored to assess their susceptibility tothermal sensitivity. This data was summarizedand rotors were categorized regarding their sus-ceptibility to thermal sensitivity. All fieldrewinds were reviewed and the most appropri-ate features used to minimize the risk of thefield being thermally sensitive. The fields havebeen categorized by design so that for newfields and rewinds, the optimum field winding,blocking and insulation system is used. Thesechanges have been permanently made in theGE generator design procedure so that they areused in all future generator developmentdesign and rebuild work. As a result of thisextensive work, the number of thermally sensi-tive fields in the GE fleet has decreased signifi-cantly. The fleet will continue to be monitoredto assure that thermal sensitivity is not a majorissue regarding the reliability and availability ofthe GE generator fleet.

Examples of Thermally Sensitive FieldsThere have been a number of items used toimprove or eliminate the effects of thermal sen-sitivity on a generator field. For rewinds, thisincludes new insulation systems, modificationsto the field forging and wedges, or convertingfrom conventional to conductor cooling. Mostnew fields have been designed to incorporatebody-mounted retaining rings and conductorcooling which have virtually eliminated thermalsensitivity on these fields.

GE has taken action to determine the rootcause(s) of problem sensitivity and to devisemeans to correct it. Some of the more interest-ing cases which GE has analyzed (and continuesto monitor) in its work are discussed next, andmost have been improved to where rotor vibra-tion is no longer an operational issue.

Case A: Hydrogen-Cooled Generator —Shorted TurnsA large steam turbine generator had been inoperation for many years. With time, this fielddeveloped a vibration pattern directly related tofield current. This vibration problem graduallyincreased with time. A flux probe test showed alarge number of shorted turns in the field wind-ing. The distribution of the shorted turns wassuch that one pole had a significantly largernumber than the other. The behavior of thisvibration was reversible in nature.

Figure 13. Typical GE generator rotor configurations

LARGE GENERATOR MEDIUM GENERATOR

DiagonalFlowBody-

Mounted

RadialFlowBody-

Mounted

Conv.CooledSpindle-Mounted

Conv.Cooled

Spindle-Mounted

Conv.CooledBody-

Mounted

DirectCooled

Spindle-Mounted

DirectCooledBody-

Mounted

GER3809-13



The field was disassembled and evidence ofdeterioration of turn insulation and shortedturns was present. The field was reassembledwith a new insulation system and returned toservice. The vibration levels of the rotor werewithin normal values and there was no evidenceof any significant thermal sensitivity.

Case B: Air-Cooled Generator Insulation

Degradation

An air-cooled gas turbine-generator wasbrought down for routine maintenance. Duringthe outage, the generator’s spindle-mountedretaining rings were inspected for moisturedamage. The process involved removal of therotor from the stator, removal and inspection ofthe retaining rings and slot wedges for pittingand rust damage, some minor machining of theretaining rings to remove surface indications,reassembly of the components and restoringthe unit to service. Upon startup, the rotordemonstrated a significant increase in field cur-rent sensitive vibration — a condition that hadnot been present before the inspection. A com-promise balance provided marginal relief, anda flux probe test failed to show sufficient shortsto account for the problem. Six months later, asister unit at the site exhibited an almost identi-cal problem. This vibration was irreversible innature.

Attempts to resolve the problem by recheckingturbine-generator alignment, tightening gener-ator end shield bolts and repairing turbineexhaust leaks into the coupling compartmentproved unsuccessful. Further investigationfound that there was a degradation in the insu-lation at the top of the slots and in the end-winding blocking. A plan was then developed torepair the rotors using hard copper in the topturns, new coil insulation caps, extra Teflon™ atthe top coil turns, modified and repositioned

blocking, and modified wedges. Subsequentimplementation of these modifications provedsuccessful in restoring normal rotor vibrationlevels to the generator. Similar proceduresdeveloped by GE have proven very effective inreducing thermal sensitivities on other highpower-density generators with spindle-mountedrotor retaining rings, and ultimately resolvedthis situation when implemented.

Case C: Hydrogen-Cooled Generator —Variable Friction InfluenceGE’s analysis of rotor vibration changes on ahydrogen-cooled generator with conventional-ly-cooled coil windings and spindle-mountedretaining rings revealed both gradual and slip-stick types of thermal sensitivity – depending onthe severity of field current changes.Calculations and testing by GE indicated thesensitivity was driven by forces developed fromthe axial expansion of the coils and asymmetri-cally transmitted to the rotor, causing it to bow.A “pre-warming” test demonstrated that theforce dissymmetry was principally due to signif-icant variations in the binding friction along thecoil/insulation interface. By pre-warming therotor coils at low rotor speeds (approximately300 rpm) prior to synchronizing the generator,the coils were allowed to expand without bind-ing from the centrifugal load, resulting in a sig-nificant lower vibration. This helped pinpointthe cause of the problem as non-uniform axialforces and friction. The rotor was later rewoundwith an enhanced treatment of the retainingring insulation and coil cover insulation to min-imize frictional restraint of the coils, as well asan improved copper ventilation scheme toreduce both the coil expansion and its friction-al path to the rotor. The thermal sensitivity waseliminated and the unit is operating at accept-able vibration levels over the full generator rat-ing. This case exhibited both reversible and



irreversible vibration which was directly relatedto the amount of field current applied.

Proven SolutionsGE has addressed and studied the thermal sen-sitivity issue for many years and has developedseveral solutions to this problem. Some of thedifferent ways GE has found to eliminate a ther-mally-sensitive field are:

GE Patented “Slip-Plane”. One of the commoncauses of thermal sensitivity is non-uniformmechanical forces in the field winding. This canbe due to uneven friction or binding in theslots. GE has developed a “slip-plane” whichmakes the axial winding forces that are trans-mitted to the field forging very uniform so thatlittle if any field bowing occurs as field currentis applied. The “slip-plane” equalizes the fric-tion and forces at the top of the coil stack, thuseliminating the non-uniform forces that causerotor bowing. This fix can be applied to a com-plete rewind as well as integrated into an exist-ing winding. This solution would be particular-ly effective for indirectly cooled fields that oper-ate at high field currents and field tempera-tures. It would also be appropriate for a fieldthat demonstrates an irreversible lock-in type ofvibration.

Generator Field Redesign. For indirectly-cooledfields that operate at high field current andVAR levels, an option would be to modify thefield to direct-cooled. Many times, these types offields only have a thermal sensitivity problem athigh VAR loads which results from the high dif-ferential temperatures between the copperwindings and the field forging. Modifying thefield to direct cooling will significantly lowerthis differential temperature and, in some cases,even allow the generator to be uprated. In caseswhere the field cannot be modified or outagetime is critical, a new direct-cooled winding can

be manufactured to accomplish the sameresults. Both new and modified direct-cooledfields have been installed in GE generators thathave been thermally sensitive, and all sensitivityhas been eliminated after the modification.This is appropriate for fields with bothreversible and irreversible thermal sensitivity.

Generator Field Thermal Sensitivity Modifi-cations. Since most of the differential motionbetween the copper winding and the field forg-ing and retaining ring is at the top of the coilstack, the insulation, blocking and top turns ofcopper are subject to wear and distortion at thislocation. This is particularly true for fields thatsee many start-stops or operate at high field cur-rent with frequent load changes. A set of modi-fications has been developed to apply to thesetypes of fields which modifies such componentsas insulation, creepage blocks, distance blocksand wedges. This can be done at a completefield rewind or by only modifying required com-ponents on an existing winding. These modifi-cations, designed to renew the winding/insula-tion contact surfaces at the top of the slot, havebeen performed on many in-service generatorfields as well as new field rewinds and, in allcases, have resulted in a significant decrease inthermal sensitivity. These modifications areappropriate for both reversible and irreversiblethermal sensitivity.

Hydrogen Pressure Increase. For those genera-tors that are hydrogen-cooled, increasing thehydrogen pressure will result in a reduced dif-ferential in temperature between the copperand the forging. If the thermal sensitivity is onlyseen at high field currents and VAR levels, theincrease in pressure could be very useful ineliminating this restriction. The increasedhydrogen pressure allows more heat to beremoved from the copper winding. Increasedhydrogen pressures have been used for many

years on new generator designs to providehigher ratings.

Conclusion

GE has applied significant effort towards achiev-ing improved understanding of thermal sensi-tivity and incorporating design techniques tominimize the effects of this phenomenon in thefuture. Whether it is a new field or a rewind ofan existing field, the following items as a mini-mum need to be addressed during design andassembly to avoid thermal sensitivity problems:

■ Retaining ring assembly

■ End winding blocking

■ Clearances in coil slots

■ Improper application of adhesives

■ Condition of copper

■ Contamination in the field

■ Wedge assembly

■ Type of insulation

■ Field rewind procedure

If careful attention is paid to these items, therisk of having a thermally sensitive field will begreatly minimized. GE recognizes that achiev-ing good thermal sensitivity performanceinvolves close attention to design details andmanufacturing process quality. The followingactions are being taken to gain improvedunderstanding of the mechanisms involved andto improve performance.

■ All thermally sensitive fields will bemonitored and their operation andvibration characteristics will befollowed closely.

■ Features are incorporated into thedesign of new generator fields tominimize the effects of thermalsensitivity.

■ All existing generator fields arereviewed prior to rewind to determinewhich insulation, blocking and coolingsystem to use to minimize thepotential for thermal sensitivity. Theoptions would include one of thefollowing:

— Rewind the field in kind

— Modify the existing field to includenew insulation and blocking systems

— Rewind with conductor cooling and anew insulation system, including mod-ified blocking

— Replace the existing field with a newconductor-cooled field with body-mounted retaining rings

Appendix

Commonly Asked Questions RegardingGenerator Rotor Thermal Sensitivity1. Question: What is thermal sensitivity of a

generator field?

Answer: Because of the difference in coeffi-cients of thermal expansion between cop-per and the field forging, the copper willexpand axially much more than the steelalloy forging as field current is increased. Iffriction and mechanical forces are equal inthe circumferential direction, no bendingforces are induced which cause the rotor tobow and cause the vibration to change. Thischange in vibration magnitude and phaseangle is termed the thermal vector. If thisvibration change exceeds acceptable limits,the field is said to be thermally sensitive.

2. Question: What causes thermal sensitivity?

Answer: There are a number of causes forthermal sensitivity of a generator field and,in some cases, the condition results from acombination of the causes. The most com-



monly thought cause is shorted turns whichdevelop as the field insulation system dete-riorates with age. However, there are manyother things that can cause thermal sensitiv-ity, including blocked ventilation or unsym-metrical cooling, insulation variation,wedge fit, distance block fitting, retainingring/centering ring assembly movement,tight slots and heat sensitive forgings.

3. Question: What generator fields are affect-ed?

Answer: All generator fields have somedegree of thermal sensitivity; however, it isnot considered a problem unless the vibra-tion levels exceed acceptable limits undernormal operation. The most vulnerablefields are those that are indirectly-cooledand operate at high field currents. Typically,direct-cooled fields are much less suscepti-ble to thermal sensitivity unless they devel-op blocked ventilation or a large number ofshorted turns.

4. Question: How can it be determined if afield is thermally sensitive?

Answer: Normally, a field is suspected to bethermally sensitive if the vibration magni-tude and phase angle changes as the loadon the generator is changed. However, it isnot always clear if it is the megawatt or VARload change that is causing the vibrationchange. To determine the mechanism thatis causing the vibration change, a controlledthermal sensitivity test as described earlierin this paper should be performed.

5. Question: A field operated for 20 yearswithout any significant thermal sensitivity.After a recent rewind of the field, the fieldis thermally sensitive. Why?

Answer: The steps that went into the fieldrewind should be reviewed and any changes

from the original field should be investigat-ed. Typically, anything that was done to thefield to make it non-uniform in the circum-ferential direction can cause thermal sensi-tivity. This can be from mechanical and/orthermally induced force. Examples ofthings that can cause the problem are tightslots, non-uniform insulation thickness andsize, distorted copper causing binding,uneven use of glues or resins in the rewind,non-uniform wedge fit, poor distance blockfitting or positioning, and blocked ventila-tion.

6. Question: What restrictions should beplaced on a field that is thermally sensitive?

Answer: Unless acceptable vibration limitsare exceeded, there should be no restric-tions placed on the generator field as longas the generator is operated within its capa-bility curve. For those cases where the vibra-tion magnitude exceeds limits, it is some-times possible to perform a compromisebalance on the field such that the thermalvector passes through zero and, as a result,does not exceed vibration limits throughoutits entire operating range.

7. Question: Why does some thermal sensitiv-ity follow field current and is repeatable,while other types lock in and do not followfield current?

Answer: Typically, thermal sensitivity that iscaused by shorted turns, blocked ventilationor mechanical variations not related to non-uniform friction or binding are repeatable.If certain coils in a winding tend to stick orslip as field current is applied, and movenonlinearly in steps, vibration changes canbe locked in and move very rapidly. Theseproblems usually occur when the friction atthe top of the coil slots is non-uniform or if



there is binding in one or more slots whichcauses uneven bending forces in the fieldforging. This problem sometimes occurs asthe field insulation deteriorates with age atthe top of the coil and non-uniform frictionand mechanical forces develop.

8. Question: What would cause a field that hasnot been thermally sensitive to developthermal sensitivity while in operation.

Answer: As a field ages, both thermal andmechanical loading on the insulation andblocking can cause changes in friction anddistribution of mechanical forces. This cancause a change in the bending moment thatis transmitted into the forging. Also, an in-service incident, such as negative sequenceoperation or motoring, operating out of thecapability curve or overspeed, can causechanges which could induce thermal sensi-tivity into a field that was previouslyimmune.

9. Question: What can be done to a generatorfield design to minimize/eliminate thermalsensitivity?

Answer: Most new GE generator fields aredesigned to be direct-conductor-cooled withbody-mounted retainer rings. These designshave been virtually free of thermal sensitivi-ty due to the maximized cooling schemeand provisions made at the top of the coil toprovide for uniform friction and mechani-cal forces at the copper-to-insulation inter-face. For existing field rewinds, many modi-fications have been made to individual gen-erator field components and procedures toassure uniform loading on the copper andfield forging. One of the potential designchanges that can be incorporated is the GEpatented “Slip-Plane” modification that willminimize the possibility of non-uniform

loading developing at the top of the coilslots.

10. Question: A field has run many years as abase load unit at high megawatt loading andnear unity power factor. It was recentlychanged to a peaking unit with operationnear rated VAR loading. The field now has asignificant thermal vector. Why?

Answer: Operating near unity power factordoes not challenge the capability of thefield in terms of temperature and heating.However, as field current is increased andthe field is operated near its VAR rating, thefield will experience much higher tempera-tures and the differential copper-field forg-ing axial expansion will increase. If the fieldwinding has shorted turns or other windingproblems which can induce non-uniformforces into the forging, these effects will bemagnified as the field current is pushed toits limit.

11. Question: What advances in technologyhave been developed to eliminate thermalsensitivity?

Answer: GE recently developed and patent-ed a “Slip-Plane” system to be incorporatedat the top of the coil stack to provide uni-form friction between the top copper turnand the adjacent insulation. This results inuniform forces being transmitted into thefield forging and little, if any, field bowing asfield current is applied. New fields that aredirect-cooled with body-mounted retainingrings tend to be very tolerant to thermalsensitivity. Modifications have been incorpo-rated into the design of rewinds for existingfields to minimize the effect of thermal sen-sitivity.

12. Question: If a field has a minimal amountof thermal sensitivity, what can be done to



minimize its effect on the overall operationof the generator?

Answer: If the change in vibration does notresult in vibration levels that exceed accept-able limits, nothing needs to be done to thefield. However, if limits are exceeded veryfrequently, a compromise balance shot,which will move the “no-load” starting vibra-tion point, can be tried such that the vibra-tion will never exceed limits throughout theentire operating range.

13. Question: For a new field or one that hasbeen rebuilt, how can the risk of the fieldbeing thermally sensitive while in operationbe minimized?

Answer: A high speed balance whichincludes an overspeed run, a thermal sensi-tivity test and a flux probe test will providethe most confidence that the field will notbe thermally sensitive when returned toservice. The flux probe test checks for short-ed turns while the thermal sensitivity test isan attempt to apply field current to thewinding which will model actual operationconditions. Experience has shown that if afield passes the thermal sensitivity test crite-ria in the high speed balance facility, theprobability of having any significant thermalsensitivity during operation is very remote.

14. Question: Two identical generators areoperated by two different users. One userhas a field that is thermally sensitive and, asa result, limits operation. Why?

Answer: When this occurs it is usually foundthat the units are operated very differently.If one user operated at base loads with min-imal field current, normally, thermal sensi-tivity would never be an issue. However, if aunit is run with many start-stops and highfield current, thermal sensitivity would bemore likely — especially as the unit agesand the insulation deteriorates due to high-er temperatures and mechanical wear dueto differential motion between the copperand the insulation.

15. Question: What factors can affect a genera-tor field when it is being rewound (bothmaterial and assembly methods)?

Answer: It is of utmost importance that thecorrect insulation, blocking, resins, gluesand other materials are used to maintainthe insulating, mechanical and thermalcharacteristics of the field. Incidents haveoccurred where non-compatible materialshave been used which resulted in highertemperatures, slot binding and insulationfailure; all have led to thermal sensitivity.The same can be said about assembly methods. Thermal sensitivity can and hasoccurred if faulty assembly techniques, such as uneven use of glue, poor blockingfit and mixed insulation systems, are used. This would result in non-uniform cir-cumferential loading, leading to thermalsensitivity.



List of FiguresFigure 1. Typical reactive capability curve

Figure 2. Typical polar plot showing a thermal vibration vector

Figure 3. Vibration data showing irreversible thermal sensitivity

Figure 4. Testing procedure for thermally sensitive field

Figure 5. Temperature induced thermal sensitivity

Figure 6. Typical slot configuration of a generator field

Figure 7. Coil wedge fitting

Figure 8. Generator end blocking design

Figure 9. Bowing caused by uneven blocking

Figure 10. Typical retaining ring mounting configurations

Figure 11. End winding blocking force transfer

Figure 12. Mechanically induced thermal sensitivity

Figure 13. Typical GE generator rotor configurations



ger3809- generator rotor thermal sensitivity

Documents