generator rotor desing

GE Power Systems

GE Generator Rotor Design,Operational Issues,and Refurbishment Options

Ronald J. ZawoyskyKarl C. TornroosGE Power SystemsSchenectady, NY

GER-4212

g

GE Power Systems ■ GER-4212 ■ (08/01) i

GE Generator Rotor Design, Operational Issues, and Refurbishment Options

Contents

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1Function of a Generator Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1Types of Generator Rotors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Conventional Windings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4Generator Rotors with Aluminum Alloy Windings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Direct-Cooled Windings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Radial Flow Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Radial-Axial-Radial Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Diagonal Flow Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Laminated Rotors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Current 4-Pole Salient Pole Rotors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Problems Encountered with Generator Rotors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Shorted Turns and Field Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Thermal Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11Collector, Bore Copper and Connection Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Copper Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Forging Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Retaining Ring Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Misoperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Generator Rotor Reliability and Life Expectancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Generator Rotor Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Generator Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Generator Rotor Refurbishment and Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Generator Rotor Rewind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Reasons for Rewinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Types of Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Generator Rotor Modifications, Upgrades and Uprates . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18Impact on Other Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19Generator Rotor Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Exchange Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20New Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

GE Power Systems ■ GER-4212 ■ (08/01) ii


Rewind, Refurbishment and Replacement Recommendations Versus Risk . . . . . . . . . . . . . . .20New Replacement Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Exchange Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Rewind with New Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21Rewind Reusing Old Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

High Speed Balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21High Speed Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21Flux Probe Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21Thermal Sensitivity Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Frequently Asked Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

OverviewWith the average age of the GE generator fleetrapidly approaching the limit of the originalintended life, utilities and industrial users areseeking alternatives to replace this aging equip-ment with new generators. One component ofthe generator that is typically refurbished,upgraded or uprated is the generator rotor(field). Degradation of the generator field canbe caused by a number of factors, including abreakdown in insulation due to time and tem-perature and mechanical wear. This degrada-tion can lead to shorted turns, a field ground,or an in-service operational incident that canrequire premature maintenance work. The typeof work needed to repair and upgrade dependsupon the generator rotor design, length of timein service and the manner in which the rotorwas operated.

This paper covers various types of generatorfields, including both conventionally-cooled(indirect copper cooling) windings and direct-cooled copper windings as well as those withspindle and body mounted retaining rings. Theoptions for rewinding, modifying, upgrading oruprating are provided for each field type. Alsoaddressed in this text are the problems typicallyencountered when dealing with generatorrotors, including:

■ Shorted turns

■ Field grounds

■ Thermal sensitivity

■ Negative sequence heating

■ Contamination

■ Misoperation

■ Forging damage

The issue of balancing generator rotors afterrework or modifications is also discussed. Thispaper concludes with a discussion on generator

rotor reliability and its life expectancy—whichvaries considerably based on the type and con-figuration of the generator rotor and the man-ner in which it is operated.

Function of a Generator Rotor

This section covers the generator field’s func-tion in two main areas: a brief description of themechanical configurations, and a brief descrip-tion of the electrical theory.

The generator rotor represents an excellentcombination of electrical, mechanical and man-ufacturing skills in which the field coils are wellinsulated, supported and ventilated in a com-pound structure rotating at very high speed(typically 1800 or 3600 rpm). Furthermore,though the rotor experiences great mechanicalstress and high temperatures (in some cases upto 266˚F–311˚F/130˚C–155˚C) while subjectedto electrical voltage and current, it is expectedto function in this manner for years without fail-ure. The three design constraints that limitthe size and life of generator rotors are temper-ature, mechanical force and electrical insula-tion.

Figure 1 shows a basic mechanical outline for atypical generator field. Note the major compo-nents:

■ Turbine coupling

■ Main cooling fans

■ Retaining rings

■ Coil slot

■ Balance plug

■ Collector rings

■ Collector fans

There are, of course, variations on this configu-ration. For example, while the illustrated designuses radial fans, other designs use axial fans.


GE Power Systems ■ GER-4212 ■ (08/01) 1



COUPLING

COIL SLOT

RETAININGRING

BALANCEPLUG

FANCOLLECTORRING

COLLECTORFAN

Figure 2. Collector end of generator field

RETAININGRING

BORE COPPER

MAINTERMINAL

FIELD COIL ENDWINDINGS

COLLECTORS

COLLECTOR TERMINAL

MAIN LEADS

AXIAL FAN

A typical collecter end configuration is shown inFigure 2, which also shows a cutaway view of vitalelectrical components such as:

■ Collectors

■ Collector terminals

■ Bore copper

■ Main terminal

■ Main lead

■ Retaining ring

■ Coil endwindings (shown from the side)

■ Axial fan

Figure 3 shows a typical cross-section of a radial

cooled slot section. Other configurations will be

described later. Note the main components of

the slot:

■ Coil wedge

■ Creepage block

■ Slot armor

■ Turn insulation (groundwall insulation)

■ Copper turns

■ Subslot cover

Figure 1. Generator field

To understand the intricacies of the field wind-ing design, it must be remembered that thebasic function of the rotor is to produce a mag-netic field of the size and shape necessary toinduce the desired output voltage in the stator.The rotor can be visualized as a large rotatingelectromagnet with north and south poles. Asillustrated in Figure 4, the magnetic flux thatradiates from the rotor follows the magnetic cir-

cuit across the air gap, through the stator coreand then back across the air gap into the rotorto complete the loop.

Simply stated, the primary function of the fieldwinding is to provide the path for the DC currentneeded to magnetize the field. However, reach-ing this goal is not so simple. It involves manytradeoffs in trying to satisfy all the mechanical,electrical, thermal, and manufacturing con-straints. Consider just this basic list of require-ments for the field winding and its components:

■ The winding and associated compo-nents must withstand centrifugal load-ing at speed and possible overspeed.

■ The winding and its insulation systemmust fit within the space available forthe rotor slot. The amount of spaceavailable for the rotor slot is depend-ent on the stresses in the rotor teeth—the more area used for the slot, thehigher the tooth stresses.

■ The insulation system must be suffi-cient to protect the winding fromground faults and turn-to-turn shortsthroughout the operating envelope.



COIL WEDGE

CREEPAGEBLOCK SLOT ARMOR

(RIGID)

TURNINSULATION

COPPERTURN

SLOTCLEARANCE

(WIDTH)

SUB SLOTCOVER

SLOTCLEARANCE

(RADIAL)SUB SLOT

ROTOR

Figure 3. Radial cooled slot

Figure 4. Rotor magnetic flux linking rotor and stator

STATOR

AIR GAP

GENERATORFIELD


■ The insulation system must be strongenough to withstand the physical wearand tear of assembly and winding, andduring operation (particularly when inthe turning gear mode).

■ The ventilation scheme must be suffi-cient to keep the temperature rises ofthe winding within the acceptablerange. In the case of ventilated endwindings the vent paths must be opti-mized to provide sufficient coolingwhile not exceeding the stress limits ofthe copper.

■ The field MMF (magnetomotive force)must be sinusoidal in shape and of thedesired magnitude.

■ The winding must be symmetrical andbalanced to prevent unacceptablevibrations.

The winding and components should bedesigned to require little maintenance duringthe 30 or more years of expected operation,which is the typical life for a base loaded controlpower station. Rewinds may be more frequentunder extreme conditions such as an open ven-

tilated gas turbine generator in a dirty environ-ment, or frequent start/stops or load cycling.

Types of Generator RotorsThere are two basic types of generator rotors:conventional windings (indirect-cooled) anddirect windings (conductor-cooled). Both typesand their variations are discussed.

Conventional Windings Smaller generators, which are not providedwith conductor cooling, have ventilating ductsthrough which the cooling air passes. (SeeFigure 5.) With this arrangement, the heat gen-erated in the coil is conducted through the slotinsulation to the field forging, then to the cool-ing gas in the ventilating duct. The dielectricbarrier forming the slot insulation is also theprimary thermal barrier in the circuit; as cur-rent levels increase, additional rotor heat dissi-pation is required. The solution is to use a con-ductor-cooling arrangement, in which coolinggas flows directly through the conductors. Thiseliminates the thermal barrier of the slot insu-lation, allowing a continued increase in the cur-rent-carrying capability of a given size rotor.


VENTILATINGDUCT

WEDGEWEDGE

CHAFINGSTRIP

COPPERWINDING

SLOTINSULATION

SLOTINSULATION

CAP

Figure 5. Indirect cooled coil slot

Generator Rotors with Aluminum AlloyWindings As conventionally cooled generators wereincreased to larger sizes, the generator rotorstresses increased to unacceptable levels in therotor and retaining rings. Aluminum alloy (con-dal) windings were incorporated on some gen-erator rotors, enabling the rotor size and rat-ings to increase and still allow conventionalindirect cooling to be used in the design ofthese units. These units have provided manyyears of reliable operation. However, to ensurefuture long-term reliability when they arerebuilt, the design of these units requires spe-cial design and process considerations.

Direct-Cooled Windings Several different arrangements of direct-cooledwindings have been used by domestic and for-eign manufacturers to accomplish the conduc-tor-cooling principle. The two primary methodscurrently used by GE for two-pole generatorsare radial flow cooling and diagonal flowcooling, as shown respectively in Figure 6 andFigure 7.

Radial Flow Cooling Radial flow cooling is used for small and medi-um sized two-pole units and for large four-poleunits. The ventilation arrangement shown inthe slot cross-section of Figure 6 permits gas toenter the subslot in an axial direction. The gasthen discharges radially through holes in thecopper winding and through the wedges shownin the figure. This conductor cooling arrange-ment brings the cooling gas into direct contactwith the copper conductors, eliminating thethermal drop through the insulation. Thewedges are constructed with pre-drilled holes toallow for the passage of gas to the rotor surface.



SUB SLOT

COPPERWINDING

SLOTINSULATION

TURNINSULATION

CREEPAGEBLOCK

WEDGE

Figure 6. Radial cooled coil slot

EXTRUDEDCOPPERCHANNEL

SUB-DIVIDEDFIELD

CONDUCTOR

TURNINSULATION

FLUSHINLET

SCOOPCREEPAGEBLOCK

INLETREGION

SLOTARMOR

OUTLETREGION

INSULATIONSTRIP

ROTATION

Figure 7. Diagonal cooled coil slot

The rotor slot in Figure 6 may be tapered to pro-vide an optimum balance between total copperarea and rotor forging stresses. The size andcontour of the subslot, along with the size andnumber of radial holes in the copper andwedges, are parameters designed specifically tokeep the copper and insulation temperaturesand rotor forging stresses within standard andmaterial limits.

Four-pole rotor windings are subject to muchless duty than those with two-pole designs. Thefour-pole rotor turns at half the speed, so thecentrifugal loading of the copper windingagainst the wedges and retaining rings is con-siderably reduced. Much larger diameters ofthe large four-pole generators permit use ofdeeper and wider slots to accommodate a larg-er cross-section of copper winding. Increasedrotor diameter also increases the availablepumping head for forced convection cooling.All these factors ease the problem of designingthe rotor cooling circuit

The radial flow direct-cooling arrangement wasoriginally developed for large hydrogen-cooledfour-pole generators. Once perfected, it wasadapted to two-pole hydrogen cooled genera-

tors, and then to air-cooled generators. This isjust one example of the development of tech-nology for large generators that is then utilizedto enhance smaller units.

Radial-Axial-Radial Cooling This type of cooling system was GE's first provendesign at gap pickup cooling. (See Figure 8.) Thegenerator rotor and stator incorporated inletand outlet sections along their axial lengthsto achieve uniform cooling along the length ofthe generator field. This uniform cooling elim-inated axial hotspots and allowed the ratings ofthe generators to be increased. The design wasnamed radial-axial-radial because of the coolingflow which enters the rotor radially, goes in aradial direction into the winding, then proceedsin an axial direction and finally in a radial direc-tion out of the winding. (See Figure 9.) This cool-ing scheme is accomplished by using extrudedcopper conductors that have been intricatelymachined to achieve the desired cooling pat-tern.

This design had been used very successfully andreliably for over 40 years. However, the designwas replaced by diagonal flow design, whichalso incorporated a gap pickup. Diagonal flow



OUTLET IN IN IN INOUT OUT OUT OUT

CORE

Figure 8. Rotor and stator cooling zones


offered the additional benefit of a simplerdesign that allowed it to be manufactured moreeasily, while increasing long term reliabilitywithout sacrificing performance. If a radial-axial-radial field requires a rewind, it is typicallyconverted to a diagonal flow design or replacedwith a new diagonal cooled rotor.

Diagonal Flow Cooling The arrangement used in large two-pole gener-ators is referred to as diagonal flow cooling.Figure 7 shows a detailed cross-section of a typi-cal diagonal flow field coil. Gas flows downthrough a series of slotted holes, offset in eachlayer from those in the previous layer. The bot-tom turn is a channel that redirects the gas toanother series of slotted holes which force thegas upward in a diagonal progression to the topof the coil. The pumping action to provide gasflow is obtained in the configuration of the slotwedges, requiring little fan pressure to circulategas through the rotor winding. The holes forgas inlet are inclined in such a manner thatrotation of the field forces the gas through thewedge and down into successive turns of thecoil. Discharge holes in the wedges have araised section preceding the hole in the direc-tion of rotation. This creates a pressure reduc-tion at the hole with the lower pressure induc-ing gas flow by suction from the discharge end.There are several alternate inlet and discharge

sections throughout the length of the rotor,providing multiple parallel paths through thewinding (as shown in Figure 8). The stator corehas corresponding inlet and outlet sections,matching those in the generator field.

Laminated RotorsUntil 1940 some older, smaller generator rotorswere constructed with laminated steel (e.g., 5MVA in 1935) in limited number. GE occasion-ally will get a request for spare parts, which wecan generally support. Laminated rotors wereconstructed of a shaft forging and laminatedfull circle punchings shrunk onto the shaft withthe collector end secured by a large nut to keepthe punchings tight. A few of these units expe-rienced vibration problems because the punch-ings shifted, causing a kink. Finding the kinkedlocation and straightening out the punchingpackage would usually fix the vibration. Theselaminated rotors were built with a stator bore ofup to 23.75 inches. Beyond that, the rotorshould be a solid design. The fields had a one-or two-piece coil slot wedge made of brass, andon some early units the wedges were actuallyinsulated from the rotor laminations. The lami-nated rotors had radiating plates and some-times holes in the retaining rings for end turncooling. Generally such a rotor cannot beuprated, mainly due to flux density limitationsof the original design.


Figure 9. Radial-axial-radial cooled coil slot

Current 4-Pole Salient Pole Rotors GE continues to produce state-of-the-art salientlaminated rotors to support the GE 4-PoleAlterrex Excitation System (typical ratings areup to 4375 KVA). Of course vast improvementshave been made in the design over the past 50years compared to what was discussed in theprevious paragraph. However, the rotors arelimited in diameter and are most appropriatefor machines in the smaller MVA ratings.

Problems Encountered With GeneratorRotorsAs a generator rotor ages, its insulation can beaffected by temperature, mechanical wear andoperating incidents. Rotor forging and otherrotor components are also at risk. The mostcommon problems occurring with generatorrotors are shorted turns and breakdown ingroundwall insulation. These two concerns willbe discussed in detail.

Shorted Turns and Field GroundsA short or ground occurs when the insulation inthe field is damaged. A short results when theinsulation between the copper turns is alteredlocally, allowing the adjacent turns to make con-tact. Although this is not desirable, generatorfields have operated—and will operate—with alimited number of shorted turns without signif-icant effect to the operation of the generator.Shorts can occur anywhere in the winding ofthe generator; however, they are most commonin the endwinding area under the retainingrings.

Grounds occur because of a breakdown in thegroundwall insulation. This can be in the slotportion of the field, under the retaining ring, atthe main lead and terminals or collector andcollector terminals, or in the bore copperregion. However, unlike having a short, it is rec-

ommended not to operate a field with aground. As the source of excitation isungrounded, a single ground will not cause cur-rent to circulate in the field forging. Yet if a sec-ond ground should occur, current will circulatein the forging, and in a very short period oftime melting and serious damage to the forgingcan result.

Figure 10 shows an example of both a short anda ground in the slot section of a generator field.It should be noted that the short only affects theinsulation between the turns of copper wind-ings, while the ground wall insulation is notaffected. Figure 11 is an example of a ground inthe endwinding area between the top turn ofthe winding and the retaining ring.

The following conditions can lead to field insu-lation breakdown:

■ Time. The longer a generator has beenin service, the higher the probabilitythat the soundness of the insulationhas been compromised by mechanicalstress or heat related damage.



COIL SLOT

WEDGE

CREEPAGEBLOCK

SLOTARMOR

COPPERWINDING

WINDINGGROUND

LOCATION

WINDINGSHORT

TURNINSULATION

Figure 10. Coil slot insulation breakdown

■ Type of operation. A generator fieldthat is subject to many start-stopsand/or VAR cycling is more prone to adegrading of the insulation systemthan a field that is baseloaded.

■ Contamination. The integrity of theinsulation can be jeopardized if con-tamination has been introduced intothe machine or if there has been anyburning in the generator (which pro-duces conductive material that can cir-culate inside the generator). This phe-nomenon is discussed in detail later inthe paper.

■ Operating incidents. Any operatingincidents that induce heating, burning,arcing, high stresses, etc., can be detri-mental to the insulation. This abnor-mal operation includes motoring, anegative sequence event (closing gen-erator breaker at standstill or turninggear or single phase operation), burn-ing within the generator or an over-speed incident.

As mentioned previously, in many cases the gen-erator can operate satisfactorily with shortedturns in the field, whereas it is not recommend-ed that a field be operated with a ground. While

shorts are undesirable, they do not present anysignificant risk to the machine. Consequencesof operating a field with shorts include theinability to reach the nameplate rating on themachine and the possibility of developing athermally sensitive generator field. In the worstcase, this would require a full field rewind inwhich all winding insulation would be replaced.

In the case of a single field ground, current willnot flow in the field forging. However, if a sec-ond ground occurs, current will immediatelystart to flow between the two ground points.Within a matter of seconds this current couldgenerate enough heat to melt the field forging,wedges or retaining ring. This could causeirreparable damage to the affected compo-nents, and in the worst case, result in the rotorbursting.

If a field ground occurs, the cause should beimmediately determined and corrective actionshould be taken. Operating with a ground canlead to serious field damage and even to cata-strophic failure should a second ground occur.Unfortunately, there is no way to determine ifand when a second ground will occur.

As units age and experience many start-stopcycles, the turn insulation will degrade. In suchconditions it is not uncommon for shorted



GROUNDLOCATION

END WINDINGS

RETAINING RINGINSULATION RETAINING RING

Figure 11. Field endwinding insulation breakdown



turns to develop. Many units have operated foryears with shorted turns without affecting thefunction of the field or the generator. It is onlynecessary to repair the shorted turns when theoperation of the generator is unacceptablyaffected (i.e, the unit rating cannot beobtained, or unacceptable thermal sensitivitydevelops).

Regular inspection and testing of the generatorfield is the best way to ensure the integrity of theinsulation system. If an insulation fault devel-ops, a number of diagnostic tests can pinpointthe location and severity of the fault. This abili-ty to quickly and accurately diagnose the prob-lem minimizes the time required to implementcorrective action, allowing the field to bereturned to service in the shortest possibletime.

There are a number of other concerns that alsoaffect generator rotors.

Thermal Sensitivity Thermal sensitivity is the term used to describean excessive vibration of the generator rotor,induced by the heating effect of the field cur-rent. As field current flows in the winding, thecopper heats up. Two things happen as a con-sequence:

1. The copper, having a greater coefficient ofthermal expansion, expands more than thesteel forging. This disparity in expansionresults in the transmission of forces to theforging through the rotor slots, wedges,retaining ring and centering ring.

2. The heat generated in the copper dissipatesinto the forging and is drawn away by thecooling medium (air or hydrogen).

As long as both of these conditions occur sym-metrically about the rotor centerline, there willbe no forces that tend to "bow" the rotor.

However, when the coil forces act unevenly, orwhen a temperature differential exists acrossthe rotor, the rotor will tend to bow, causing animbalance and subsequent vibration.

A thermally sensitive field will exhibit a changein vibration magnitude and/or phase anglewith a change in field current. The circum-stances that might cause a non-uniform distri-bution of forces are varied. Some of the morecommon factors that may cause thermal sensi-tivity (either singularly or in combination)include:

■ Shorted turns. When a significantnumber of adjacent field turns areshorted, the pole with the greaternumber of turn shorts will have a lowerresistance than the other pole. Withthe same current flowing, the higherresistance pole (the one with lessshorts) will heat up and expand morethan the other, causing a bow in itsdirection.

■ Blocked ventilation. A blockedventilation path may cause an unevenheat distribution in the rotor in muchthe same way that shorted turns can.

■ Insulation variation. Non-uniformityin field insulation can result in bindingof the field coils and an unevendistribution of friction forces, both inthe slots and under the retaining ring.In this case, the coils with the greaterbinding or friction will transmit agreater axial load to the rotor, againcausing the rotor to bow.

■ Wedge fit. Uneven tightness of thefield wedges can also result in non-uniform distribution of axial forcesaround the rotor. This situation is mostprevalent when a portion of a field's



wedges are replaced and the fit of thenew wedges is inconsistent with thoseof the existing wedges.

■ Endwinding blocking fit. Similar touneven wedge fits, unevenly spacedand fitted distance blocks can cause anon-uniformity of forces to betransmitted to the rotor, againresulting in bowing of the rotor and achange in its dynamic characteristics.This situation is most common in fieldshaving spindle-mounted retainingrings.

While these are the most common causes ofthermal sensitivity, there are other less preva-lent causes such as misuse of adhesives, incor-rect materials and certain types of misopera-tion.

There are two general types of thermal sensitiv-ity: reversible and irreversible. As the nameimplies, reversible thermal sensitivity is charac-terized by its reversible and repeatable behavior.If a field's vibration increases and decreases as adirect function of field current, the thermalsensitivity can be considered reversible. How-ever, if the field vibration increases with fieldcurrent, but does not respond directly to a sub-sequent decrease in field current, the thermalsensitivity is considered irreversible, or slip-stick. The reversibility of a field's thermal sensi-tivity can be determined through a series oftests, the results of which will generally givesome clues as to the most effective remedy.GER-3809 (Generator Rotor Thermal Sensi-tivity—Theory and Experience) covers genera-tor field thermal sensitivity in great detail.

ContaminationThe type and extent of contamination to beexpected in a generator primarily depends

upon its cooling configuration. A hydrogen-cooled generator is well sealed and should seevery little contamination. A TEWAC (totallyenclosed water to air cooled) unit will requiresmall amounts of make-up air that can intro-duce particulates into the generator. An OV(open ventilated) generator is most likely to seelarge amounts of contamination introducedinto the field.

Contamination of generator rotors can comefrom many sources. Carbon, which representsone of the more common contaminates, cancome from collector brush wear or gas turbineexhaust. Other particulates likely to be found ina generator (such as silicon or petroleum by-products) can come from nearby operations orprocesses. While the inlet filters eliminate mostof the contaminates from the air, the flowthrough the generator is so great that even asmall percentage in the air stream equates tosignificant deposits over time. Other types ofcontamination can come from the generatoritself. Worn insulation, blocking and wedgescan introduce particulates into the ventilationstream that can accumulate in the field.

Liquid contamination may also be present andcan compound problems by combining withthe particulate contamination and sticking onall areas of the rotor. Generally, liquid contami-nation is limited to oil from the hydrogen sealsand bearings. The oil can get drawn into the sta-tor and coat both the stator and rotor ventila-tion paths. As a result, particulates in the venti-lation stream that otherwise would proceedunimpeded will stick to the oily film and even-tually create a significant build-up.

Problems that can arise from contaminationbuild-up in a generator rotor include low meg-gar (resistance to ground) readings, overheat-ing, creepage failures, and turn shorts.



Collector, Bore Copper and ConnectionProblems The collectors, bore copper and main leads rep-resent a vital link in the excitation path. Oneproblem encountered in this area is "collectorflashover," in which the positive collector flash-es to ground due to contamination. This createsa creepage path (a degradation of the collectorshell insulation) that ultimately results in aforced outage.

As a rotor ages, loosening of components maydevelop in the rotor bore, bore copper, termi-nal studs, copper coils, retaining rings, terminalstud gooseneck, or mainlead 90° bend. Theloosening may lead to increased relative motionbetween the stud/main lead and the #1 coil.This can result in low and high cycle fatigue,leading to breakage of the connection and aforced outage due to "loss of field current."

A large number of mechanical connectionsbetween the collectors and the #1 coils are inclose proximity to ground potential. The insu-lation in these areas is designed with generouscreepage paths, but over time the insulation candegrade, leading to a field ground and a forcedoutage. (See Figure 12.)

Copper Distortion Distortion in the copper winding is sometimesfound after a generator field has been in servicefor a period of time and maintenance work isperformed on the rotor. (See Figure 13.) Thiscondition most frequently occurs in the end-winding area. Distortion can occur due to thefollowing reasons:

■ The presence of soft or annealed cop-per

■ Friction between the top turns and theretaining ring insulation

■ Frequent cycling of the winding

■ Overheating of the winding

■ Coil foreshortening

Any damaged copper must be repaired orreplaced prior to returning the generator rotorto service. Failure to do so can result in shortedturns or field grounds.

Forging ConcernsGenerator rotor forgings should be inspectedprior to a rewind to determine the long-termstructural integrity. This is especially true if therotor has been exposed to negative sequence

Figure 12. Collector and brush holder neglect

currents or a motoring incident, or if the forg-ing was manufactured prior to 1959. Negativesequence currents can cause burning, hardspots and cracking on the surface of the forg-ing. Rotors manufactured prior to 1959 tend tohave significantly lower toughness than modernday forgings and the increased levels of impuri-ties found in the forgings make many of themmarginal for continued use.

During inspections in the period from 1995 to2001, three out of seventeen of our large steamturbine generators were found to have crackedgenerator rotor teeth. In two of these cases therotors also had experienced in-service negativesequence events, which resulted in arc strikeson the rotor teeth, dovetail load surfaces, andon the mating surfaces of the slot wedges. Thethird generator rotor that experienced crackinghad an operational history that included exten-sive turning gear operations (25,600 hours inan eight-year period). The cracks were detectedduring rewind inspections and were in the radi-al circumferential direction.

Based on the design of these fields and the lowstress and original fatigue life calculations, it isnot expected that generator rotor tooth crack-

ing will occur through normal operation.However, if the generator rotor is subjected tosignificant negative sequence events, GE EnergyServices should be consulted prior to the com-mencement of repairs or return to service. GEEnergy Services can then determine the severi-ty of such an incident and advise in the inspec-tions that should be performed. Units with sig-nificant hours of turning gear operations(greater than 10,000 hours) should contact GEEnergy Services regarding inspection recom-mendations.

Testing of the bore of the forgings is recom-mended for all of those manufactured before1959 or if the generator is subject to frequentstart-stops. Also, for those forgings manufac-tured after 1959 that have been in service over25 years and 5000 start-stop cycles, it is recom-mended that the rotor bores of these forgingsbe inspected. Prior to 5000 start-stop cycles, therotor teeth, dovetails and field wedges shouldbe inspected using magnetic particle and fluo-rescent techniques.

Retaining Ring Concerns

Figure 14 shows the retaining rings used torestrain the centrifugal force of the rotor wind-ing end turns. They require very careful atten-tion during design and manufacture since theyare the highest stressed components of the gen-erator. The centrifugal load of the end windingcontributes 5000 to 8000 pounds for eachpound of copper under the ring. This producesa hoop stress, which attempts to stretch the ringduring operation into a slightly elliptical shapefor two-pole rotors due to the inherent non-uni-form weight distribution of the end turns andthe associated blocking. In modern generatorsthe retaining ring is shrunken on a machinedfit at the end of the body. This forces the retain-ing ring to remain cylindrical at full speed and



Figure 13. Moderate copper distortion

prevents differential movement of the ring withrespect to the body.

There are two types of retaining ring mountingschemes: spindle-mounted and body-mounted.

■ Body-mounted. A retaining ring mount-ed on the field body is subject to highcirculating currents during certainunbalanced loading conditions thatproduce negative-sequence currents inthe rotor body. The current circulatesin a closed loop pattern: first alongthe length of the field body, thenentering the retaining ring and flowingcircumferentially around the ring for ashort distance before returning to thefield body. Unless good electrical con-tact exists at the junction between theretaining ring and the rotor body,

excessive heat may occur and possiblydamage the ring.

■ Spindle-mounted. Spindle-mountedretaining rings allow flexure betweenthe rotor body and retaining ring,which can lead to insulation and coilfailure at this location. This is particu-larly true for units that operate with fre-quent start-stops and/or load cycling.

Two classes of materials are used for retainingrings: magnetic and nonmagnetic. Nonmag-netic rings are used for higher-rated generatorsto minimize leakage flux and to reduce losses.

The older generator rotors in the GE fleet useretaining rings that were manufactured frommagnetic forging materials. These rings haveprovided reliable service and have not been aproblem when well maintained. However, some



RETAININGRING

LOCKINGRING

CENTERINGRING

A. BODY-MOUNTED RETAINING RING ASSEMBLY

B. SPINDLE-MOUNTED RETAINING RING SHOWINGPHENOMENA OF RING AND COIL FLEXURE

= INDICATES SHRINK FIT

= INDICATES SHRINK FIT

Figure 14. Retaining ring mounts

magnetic rings that were exposed to extensivemoisture have developed severe corrosion andrusting and have required rework or replace-ment. Magnetic retaining rings were replacedwith those made from nonmagnetic materials.Two of the most common materials wereGannalloy and 18 Manganese-5 Chromium(18Mn-5Cr). Both of these materials have beenrecognized to have problems that arise fromtheir operating environment. Gannalloy hasbeen found to be subject to embrittlementwhen operated in a hydrogen environment. Asa result, it is recommended that those type ringsbe replaced.

Across the industry it has been found thatretaining rings made from 18Mn-5Cr are sub-ject to stress corrosion cracking (SCC).Because of the high incidence of SCC on theserings, it is recommended that all utility andindustrial 18Mn-5Cr retaining rings bereplaced with an 18Mn-18Cr material which ishighly resistant to stress corrosion cracking.Since the generator rotor must be removedfrom the stator to replace the retaining rings,this is most often done when the generator ison a major outage and when the rotor isbeing rewound. Failure to properly maintainretaining rings could result in a catastrophicfailure. (See Figure 15.)

Misoperation There are various modes of generator rotormisoperation. While some are rather benign tothe rotor, some are catastrophic to the rotorand can cause secondary damage to the gener-ator stator as well as to the prime driver.Misoperation of the rotor can occur due to anumber of reasons including: internal genera-tor failure, auxiliary equipment failure, abnor-mal system conditions, and operator error.

The most common modes of misoperation thatcan affect a generator rotor are shown in Table 1.

Generator Rotor Reliability and LifeExpectancy

Generator Rotor Life The life expectancy of a generator rotordepends upon mode of operation, rotor designand operating incidents. Generators that areoperated as peaking units with many start-stopsand also with high var loads (high field cur-rents) generally will have a lower life expectan-cy—much shorter than a base load unit with fewstart-stops and operating near unity power fac-tor with lower field current. More frequentstart-stops tend to induce more mechanicalwear on the insulation and will lead to morelong term distortion on the copper conductor.This is because the insulation and copper aresubject to extremely high "g-forces" every timethe generator is accelerated to speed. When theunit is at speed and high field current is appliedto the copper winding, high forces are devel-



� �

� �

� �

� �

� �

� �

� �

� �

Field overheating Abnormal frequency and voltage

Motoring Electrical faults

Reduced seal oil pressure High speed reclosing

Loss of excitation Breaker failure

Rotor or stator vibration Voltage surges

Synchronizing errors Transmission line switching

Unbalanced armature currents Subsynchronous resonance

Loss of synchronism Accidental energization

Figure 15. Catastrophic retaining ring failure

Table 1. Common modes of misoperation

oped in the axial direction. This can cause cop-per deformation and distortion and also cancause abrasion on the insulation, which canlead to premature failure.

The older conventionally-cooled rotors havehigher "hot-spot" temperatures. If operated athigh field currents they would tend to haveshorter life expectancies than a direct-cooledfield, which tends to distribute heat removalmore evenly. Generator rotors with spindle-mounted retaining rings tend to require main-tenance and repair work more frequently thanthose with body-mounted retaining rings. Thisis due to the relative motion between the spin-dle-mounted retaining rings and the field bodycaused by start-stops and once-per-revolutionbending. This may lead to top turn breaks andother operational problems such as thermalsensitivity.

Operating incidents such as motoring or nega-tive sequence operation can lead to rotor forg-ing and retaining ring damage. Other incidentsof high voltage spikes have caused insulationfailures that have led to shorted turns and fieldgrounds. Minimizing operational incidents canprevent premature maintenance work on a gen-erator rotor and can prolong its useful life.

Generator Experience Generator rotors that operate primarily at baseload duty and have minimal operating incidentscan expect to have an average useful life ofapproximately 30 years. This approximate lifes-pan, of course, applies to the forging. The insu-lation and the copper may need to be repairedor replaced during this time. On the otherhand, rotors that see frequent start-stops andload cycling can be expected to have a muchshorter useful lifespan. Older rotors with forg-ing issues and/or spindle (flush) mountedretaining rings are more prone to acceleratedlife degradation. Although there is no exact for-

mula, stop/start application does significantlyreduce service life. A unit operated in a fre-quent start/stop mode can expect the insula-tion life of roughly 30% to 50% of that of a baseload unit. If longer life expectancy is required,a modern-designed field with body-mountedretaining rings and direct conductor coolingmay yield a significant improvement.

Knowing the life expectancy of a generatorrotor is very helpful in planning future mainte-nance and minimizing forced outages.Frequent inspections, and electrical and fluxprobe testing can help diagnose insulation dete-rioration and assist in decision making forfuture repairs and rotor rewinds.

Generator Rotor Refurbishment andReplacement

Generator Rotor Rewind

Reasons For Rewinding

Experience has shown the rotor to be the gen-erator component requiring the most mainte-nance. This is not surprising considering that itoperates under very high centrifugal load, andthat typical operating incidents have the great-est impact on the field (motoring, contamina-tion, etc.). Rewind of the field normally focuseson re-insulation of the field winding. However,the owner should not lose sight of other con-siderations. It is common for older units to beoperated at lower power factors to carry morereactive power. This places greater duty on thefield leading to accelerated wear and, at times,field current sensitive vibration (thermal sensi-tivity). GE has developed several componentmodifications to the designs, including apatented "slip plane modifications" to improvethe field vibration stability at high thermalloads. These modifications are available forretrofit or as part of a rewind.



The first step in considering a generator rotorrewind is to define its intended use. For exam-ple, what are the service life, reliability require-ments, outage interval requirements, loadrequirements (MW and MVAR) load and per-formance requirements (such as change inpower factor, terminal voltage, etc.).Realistically defining these parameters willestablish the extent of the rotor rewind neces-sary. GE can produce several options with vari-ous levels of risk for future operation.

Types Of Insulation

When a new winding is being provided, thethree basic decisions the designer must makeare number of turns per slot, turn cross-section,and method of cooling. Normally, the methodof cooling the new winding will be the same asthe original winding. However, there are caseswhere an improved cooling scheme should beevaluated to facilitate an uprate or for reliabili-ty considerations. The number of winding turnsand turn cross-sections is determined throughanalysis, in conjunction with the selected turnand ground insulation systems.

There are three types of turn insulation systems.The first is a system of taped turns where everyother turn is taped, including the end turns, witha mica mat tape. This system is the least costly but

most labor intensive (hand taped) since the turninsulation is applied prior to winding and pro-vides the most protection against contamination.The disadvantage of this system is that it requiresnarrower copper in the slot section because tapeadds to the coil width. (See Figure 16.)

The second system consists of strip turn insula-tion in the slots and taped ends. This systemallows for wider copper in the slots, yet still pro-vides for contamination protection in the endregion since every other turn is taped with micamat tape. The third system is an "all strip turninsulation" system and it utilized whenimproved endwinding cooling is required. Thestrips can be either Nomex or a glass laminateas shown in Figure 17.



TURN INSULATIONCONSISTING OFHALF LAPPEDGLASS BACKEDMICA TAPE

SLOT SECTION INSULATIONWITH TWO .005" NOMEX SLOT STRIPSWITH .001" OF STRIPPED PRESSURESENSITIVE ADHESIVE SIDE TOWARDCOPPER WITH TAPED END STRAP

NOMEX CORNER PIECEUNDER GLASS BEGINNINGAT THIS CORNER

ONE LAYER HALF LAPPED.003" GLASS TAPE ONTOP TURN OF EACH COILAND THE THIRD TURNDOWN ON THE LONGESTCOIL

END SECTION INSULATIONCONSISTING OF .005"NOMEX STRIPS TOP ANDBOTTOM OF TURN WITHHALF LAPPED .003" GLASSTAPE AND VARNISH AS ASTICKER

TOPTURN

Figure 16. Taped insulation system

Figure 17. Taped and strip insulation system

There are two types of ground insulation (orslot armor) material that are currently used inthe slot section. The first type is a rigid armorthat has high mechanical strength. This materi-al has a glass base and may contain a film layer.In addition, in higher-voltage fields this materi-al has a grading applied to prevent creepagefailures at the top and bottom of the slot. Theother type of material is Nomex, which is bothflexible and tough.

Ground insulation in the end region is provid-ed by the retaining ring insulation, shown inFigure 18, and by the designed creepage paths atthe slot exit and centering ring. The retainingring insulation must be mechanically strong towithstand the centrifugal loading, yet flexibleenough to absorb the discontinuities of theendwinding's outer surface. It also has to allowfor movement of the end turns due to thermalexpansion. An outer layer of glass and an innerlayer of Nomex typically provide these features.The inner surface is also treated with a low fric-tion coating to allow more uniform movementof the winding.

The endwinding blocking must support thewinding to prevent permanent distortion, yetalso allow for thermal expansion. (See Figure19.) The blocking materials that are currentlyutilized are epoxy glass laminates. It is impor-

tant to employ a service-proven blocking pat-tern that is compatible with the specific end-winding geometry being used, since it is theblocking pattern which allows for thermalexpansion movement and ventilation. Also, spe-cial consideration must be given to the blockingand support of pole-to-pole, coil-to-coil and ter-minal connectors. It should be noted thatasbestos was used extensively in older genera-tion distance blocks and rotor insulation; main-tenance/repair processes must take this intoaccount.

Some design features are added when highcyclic duty is anticipated. These include reliefsin either the copper or body at the end of thecoil slots to prevent armor damage, reliefsbetween the slot armor ends, and the blocks justoutside the body when rigid armors are utilized.

Generator Rotor Modifications, Upgradesand Uprates

An uprate often can be realized while perform-ing a rewind, modification or replacement to a



COILWEDGE

RETAININGRING

TOP ACTIVE TURN

SECOND ACTIVE TURN

RETAININGRING

INSULATION

Figure 18. Retaining ring insulation

COPPER

COILS

ENDWINDING

BLOCKING

Figure 19. Endwinding blocking

generator rotor. For example, a rewind couldbe an opportunity to install new Class F tem-perature-insulating materials. Running at ahigher temperature from higher field amps canproduce more flux and more MVAR and/orMW. The same applies to a direct-cooled con-version or a replacement rotor with perhapsmore uprate capability.

It has been common to support a gas turbine orsteam turbine uprate by taking advantage of theexisting generator margin (i.e., just operate thegenerator at a higher power factor than origi-nally designed). An example of this would be tooperate at 0.95 lag rather than the original 0.9lag, while recognizing a reduction in MVARcapability. Assuming the generator is in "as-new"condition, this requires no changes to the gen-erator hardware or performance curves, as longas it operates within the existing reactive capa-bility curve.

With recent energy shortages causingbrownouts and blackouts, this practice of uprat-ing generators at the expense of existing MVARcapability—along with projected reinforce-ments of NERC Planning Standard Section G30(.9 pf lag minimum) uprates to rotors—shouldbe strongly reconsidered.

Impact on Other Components

When any change in the rating of a generatoris contemplated, a coordinated examination ofall the generator components is necessary. Thiscan best be understood by referring toFigure 20.

As shown in Figure 20, Area 1 indicates theincreased capability that is potentially availableif the stator is rewound. Area 2 shows addition-al capability that is potentially available if thefield is also rewound. Notice that if the statoralone is rewound, the power factor in the lag-ging region at the new maximum rating is

increased unless the rotor has a margin that wasunused at the previous rating. Similarly, unlessthere is unused temperature capability in thecore end region, the leading power factor capa-bility becomes more restrictive. To control coreend heating, some cases may require changingfrom magnetic to non-magnetic retaining rings

These changes also require a design study andpossible upgrade/replacement of the coolersand excitation system. For this reason, rewindscan best be addressed by the original equip-ment manufacturer who has the knowledge ofthe unit's electromagnetic design and is in aposition to make the necessary design studiesand capability assessments.

The magnitude of the performance improve-ment that can be achieved varies widely frommachine to machine. In a conventional hydro-gen-cooled generator, uprate potential canrange from 10% (with a new armature winding)to as high as 35% (with a new armature wind-ing, new direct-cooled field and new exciter).



KVARs

KWATTS

AREA 2

AREA 1

ORIGINAL RATEDPOWER FACTOR

NEW LEADING POWERFACTOR CAPABILITY

ORIGINAL LEADING POWERFACTOR CAPABILITY

NEW RATED POWERFACTOR (STATORREWIND ONLY)

INCREASED CAPABILITY WITH FIELD REWIND

INCREASED CAPABILITY WITH STATOR REWIND

Figure 20. Uprating capability curve

Generator Rotor Replacement

It is often possible to repair/refurbish an exist-ing rotor to satisfactory working condition.However, in certain circumstances, it maybecome necessary to replace the existing rotor.The following situations are instances whenreplacement is preferable to repair:

■ When outage duration is critical, thedifferential cost between a rewind anda replacement may be offset by poten-tial loss of revenue.

■ A significant uprate, or other benefit suchas increased efficiency, may be achievablewith a replacement field, but not with amodification to the existing field.

■ An operational, or other, incident mayhave damaged the rotor forging suffi-ciently to make further operationimpossible or unsafe.

There are generally two choices to obtaining areplacement field: a newly manufactured fieldor an exchange field. These are discussed in thefollowing sections.

Exchange Field

GE has implemented an exchange field pro-gram for some of the more numerous genera-tor models, where the customer will receive acompletely refurbished field as a replacement.The existing field is removed and returned toGE for refurbishment for another customer.The benefits include a shorter outage and lowercost than a new replacement field. The originaldesign of the field remains the same, thoughthe retaining rings are upgraded to 18Mn-18Cr.

New Field

In addition to the benefit of a short outageduration, a new field would incorporate the lat-est design features such as:

■ Direct-cooled coils, either radially ordiagonally cooled

■ Body-mounted retaining rings (for larg-er units)

■ Creepage blocks

■ "State-of-the art" insulation system

In some cases, a new field will allow for moreefficient operation and will also permit the gen-erator to be uprated.

Rewind, Refurbishment and ReplacementRecommendations Versus Risk Below are listed recommendations to repairrotors, in order of risk.

New Replacement Rotor

The best technical option is to uprate to a newClass F rotor with a body-mounted, 18Mn-18Crretaining ring and a direct-cooled winding witha state-of-the-art high quality forging). Itextends life the most, offers the most addition-al uprate capability, allows for the quickest out-age and will address TILs. Though it is initiallythe most expensive option, it may ultimately bethe most cost effective when considering poten-tial back-end costs such as replacement rotorsand downtime.

Exchange Field

Currently there are exchange fields for themore numerous gas turbine generator models.Additional exchange fields are being added asthe fleet ages. Other than a new replacementfield, an exchange field is the best technicaloption since it replaces most hardware whilereusing another NDT rotor forging for econo-my. Cost is less than a replacement field buthigher than a rewind with new copper. Theexchange fields are balanced.



Rewind With New Copper

This is the next best repair. It consists of a newcopper field winding, field winding turn andground insulation, slot filler, blocking, andretaining ring insulation. New copper shouldstrongly be considered if the rotor is more aged,as the copper can soften and distort over a peri-od of time (approximately 15-30 years). Thedecision to replace or reuse existing copper issomewhat subjective. It is best to inspect thecopper at an outage and make the decision toorder new copper for rewinding at the next out-age. Unlike the previous two repair options, thisrepair does not renew the collector ring insula-tion, the bore copper insulation, collector studinsulation or main lead and terminal insulation.These options should be ordered if the field isto be completely renewed. A high speed bal-ance is strongly recommended with new cop-per. The risk of this option is not replacing thecollector ring insulation, the bore copper insu-lation, collector stud insulation or main leadand terminal insulation and not addressing anypre-existing conditions with the forging orretaining rings, etc.

Rewind Reusing Original Copper

This is perhaps the most economical repair butcarries commensurate longer-term risk and isnot recommended for older aged rotors, as itsrepair scope is limited. The scope of material isbasically field winding turn and ground insula-tion, slot filler, blocking and retaining ring insu-lation. The original copper is cleaned butreused. This repair does not normally renewthe collector ring insulation, the bore copperinsulation, collector stud insulation or mainlead and terminal insulation. A high speed bal-ance is not normally recommended as there isnot the significant change in mass introducedthat would require a balance. This repair hasgreater risk in the long run because it does not

normally replace the original copper, collectorring insulation, the bore copper insulation, col-lector stud insulation or main lead and terminalinsulation. Nor does it address any pre-existingconditions with the forging or retaining rings,etc., although these components may needreplacing at any time after the basic rewindrepair. Any of these repairs can be added to theworkscope if higher reliability is desired.

High Speed BalancingIn addition to the obvious benefit of maximiz-ing dynamic stability, a high speed balanceallows for a comprehensive evaluation of thegenerator rotor's overall suitability for service.The quality of work performed on the rotor canbe fully assessed, which minimizes the potentialfor a costly outage extension. A fully equippedhigh speed balance facility can perform each ofthe following:

High Speed Balance

In most cases a high speed balance, under strin-gent acceptance criteria, will eliminate the needfor subsequent "trim" balancing on site—aprocess that can be extremely time consumingdue to the extent of disassembly and reassembly ofthe generator necessary to gain access to the rotor.

GE recommends a high speed balance whenev-er the rotor is rewound with new coils. However,experience has shown that a rewind with exist-ing copper does not require a high speed bal-ance to successfully return to service withacceptable vibration levels.

Flux Probe Test

A flux probe test can be performed if the facili-ty is equipped with a static excitation system toenergize the rotor coils. A flux probe test canthen be conducted to determine whether anyshorted turns exist. The field is spun up to oper-ating speed and excitation applied to the col-



lectors. The flux probe then measures the rela-tive magnetic flux from each coil. If there areone or more shorts in a coil, the relative fluxwill be lower in that coil compared to its corre-sponding coil. The number of shorts and num-ber of turns per coil determines acceptability.

Thermal Sensitivity Test

In some GE balance facilities sufficient power isavailable from the excitation system to performa thermal sensitivity test and a related test—aprewarming test. The thermal sensitivity testinvolves first running the rotor up to operatingspeed and establishing a stable state prior toapplying excitation. Excitation is then applied.With current through the coils, the tempera-ture increases gradually during which timevibration magnitudes and phase angles arerecorded at regular intervals. Once a targettemperature is reached and maintained for 20minutes, a final set of data is recorded. Fromthe hot and cold vibration data a thermal vectorcan be calculated. The field passes thermal sen-sitivity if the thermal vector is below a thresholdvalue, generally 3 mils.

If the field fails the thermal sensitivity test, aprewarming test can be performed to aid indiagnosing the cause of the thermal sensitivity.To perform a prewarming test the rotor isbrought to operating temperature by spinningat low speed with excitation applied. After oper-ating temperature is reached, the rotor is spunup to operating speed and vibration data taken.A comparison of the vibration levels from theprewarming test with those of the thermal sen-sitivity test can help in pinpointing the cause ofthe thermal sensitivity.

ConclusionThe average age of the GE generator rotor isapproaching the limit of the original intended

life. The integrity of the insulation systems canbe monitored using flux probe and other elec-trical testing. As the generator rotors age, itshould be expected that rewinds, modificationsor replacements will be necessary, especially fol-lowing an in-service operating incident. Manyoptions are available to the user in which therotor can be restored to the original condition,modified to present day design condition orreplaced with a new, upgraded design.Modifying or replacing the generator rotorsalso gives the user the possibility of uprating thegenerator.

Frequently Asked Questions

1) Q. What is the typical lifespan of a genera-tor rotor?

A. The life is dependent upon mode ofoperation, in-service operating inci-dents and misoperation. Generatorrotors are typically rewound, upgradedor replaced in the 10-30 year timeframe. Those used in stop/start modecan expect a shorter lifespan.

2) Q. What are the most common causes of agenerator rotor insulation breakdown?

A. The degradation in insulation is causedby heating and/or mechanical wearand/or operating incidents. A break-down in the insulation will cause short-ed turns between conductors or aground between the conductor, andthe field forging or retaining ring.

3) Q. When can a flux probe test be per-formed on a generator rotor?

A. The test can be performed under no-load with the stator short-circuited orduring operation at load. When the testis performed at load, it must be doneat various load points to test all coils inthe rotor.



4) Q. If a generator rotor is a conventional(indirect-cooled) design, can the rotorbe converted to a direct-cooled wind-ing?

A. Depending on the design of the rotor,in some cases it is possible to convert toa direct-cooled winding. Convertinginvolves machining subslots in the rotorforging below the coil slots. Because ofrotor geometry and size, this modifica-tion is not possible on all rotors.

5) Q. Is there asbestos in generator rotorinsulation and blocking materials?

A. On older units, each GE generatorrotor that is being rewound or modi-fied will have all components withasbestos identified, while new non-asbestos materials will be included inthe rewind materials package.

6) Q. Why should a generator rotor not beoperated with a field ground?

A. While a single breakdown in ground-wall insulation will not damage therotor or its components, should a sec-ond ground occur, high current willpass through the rotor forging whichcan cause melting, wedge and ringdamage and in the worst case, a forgingfailure.

7) Q. What uprate potential does my genera-tor have if it is rewound?

A. It depends on the particular design. GEGenerator Engineering can do a quickproposal design and offer severaloptions with various uprates and possi-bly even increases in efficiency.

8) Q. Can just changing out a magneticretaining ring to a non-magnetic retain-ing ring uprate my rotor?

A. Yes. For example, for a small air cooledgenerator (say 20 MW), 2–5% uprate infield capability is possible.

9) Q. Should a field be high speed balancedfollowing a rewind?

A. GE recommends a high speed balancefollowing a rewind with new copper. Ifcopper is re-used, the field will general-ly not require a balance.

10) Q. Can thermal sensitivity result in aforced outage?

A. Generally, thermally sensitive fields willexhibit high vibrations that may limitoutput, but it is very rare for thermalsensitivity to force an outage. GE fieldshave had only one such incident

11) Q.When should a replacement field beconsidered?

A. A replacement field is worth consider-ing when the customer is looking for: ■ A very short outage duration■ Uprate potential■ Replacement of a bad forging■ Improved efficiency■ Extended life

12) Q. Is there a GE Technical InformationLetter on rotor cracking ?

A. Yes. It is "TIL 1292", issued Decemberof 2000 and is titled "Large SteamTurbine Generator Dovetail InspectionRecommendation." It briefly cites sev-eral recent dovetail cracks in older gen-erator forgings and recommendsinspection for those cracks.



List of FiguresFigure 1. Generator fieldFigure 2. Collector end of generator fieldFigure 3. Radial cooled slotFigure 4. Rotor magnetic flux linking rotor and statorFigure 5. Indirect cooled coil slotFigure 6. Radial cooled coil slotFigure 7. Diagonal cooled coil slotFigure 8. Rotor and stator cooling zonesFigure 9. Radial-axial-radial cooled coil slotFigure 10. Coil slot insulation breakdownFigure 11. Field endwinding insulation breakdownFigure 12. Collector and brush holder neglectFigure 13. Moderate copper distortionFigure 14. Retaining ring mountsFigure 15. Catastrophic retaining ring failureFigure 16. Taped insulation systemFigure 17. Taped and strip insulation systemFigure 18. Retaining ring insulationFigure 19. Endwinding blockingFigure 20. Uprating capability curve

List of TablesTable 1. Common modes of misoperation



generator rotor desing

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