[isidor kerszenbaum] inspection of large synchrono

8/19/2019 [Isidor Kerszenbaum] Inspection of Large Synchrono

1/190

Inspection ofLarge

Synchronous

Machines


2/190

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3/190

Inspection of Large

Synchronous Machines

Checklists, Failure Identification,

and Troubleshooting

Isidor Kerszenbaum

Department

of

Electrical Engineering

California State University, Long Beach

IEEE Power Engineering Society,

Sponsor

IEEE Press Power Systems Engineering Series

Dr. Paul M.Anderson,

Series Editor

•

IEEE

U PRESS

The Institute of Electrical and Electronics Engineers, Inc., New York


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Library of Congress Cataloging-In-Publication Data

Kerszenbaum, Isidor.

Inspection of largesynchronous machines: checklists, failure-

identification, and troubleshooting I IsidorKerszenbaum.

p. em.

Includes bibliographical references and

index.

ISBN0-7803-1148-5

(cloth)

1.

Electricmachinery, Synchronous-Inspection. I. Title.

TK2731.K47 1996

621.31 33-dc20

95-51730

CIP


5/190

To Jackie,

Livnat, and Yigal


6/190

Contents

List of Illustrations

Preface

Acknowledgments

Part

1 Preparation

Chapter 1: Site Preparation

1.1

Foreign Material Exclusion

1.2 Safety Procedures-Electrical Clearances

1.3 Inspection Frequency

1.3.1

Condition BasedMaintenance (CBM)

References

Chapter 2: Inspection Tools

Chapter 3: Inspection Forms

Form 1: Basic Information

Form2: Nameplate Information

Form

3:

Inspection

Accessibility

Form4: StatorInspection

Form

5:

Rotor Inspection

Form

6:

Excitation Inspection

Form

7:

SalientPolesCondition Report

Form8: Comments

Form9: Wedge Survey

Form 10: Electric TestData

xi

xv

xix

1

3

3

9

12

13

13

15

19

25

26

26

27

30

32

33

34

35

37

vii


7/190

viii

Contents

Part 2 Inspection 39

Chapter 4: Descriptionof Stator Items (Form 4) 41

SO1: Cleanliness of Bore 41

S02: Air/Gas DuctsCloggedlUnclogged 42

S03: IronOxideDeposits 43

S04: Hardware Condition 47

S05: High-Voltage Bushings 49

S06: Stand-OffInsulators 50

S07: Bushing Vents 51

S08: GreasinglRed OxideDeposits on CoreBolts andSpringWashers 52

S09: SpaceHeaters 55

S10: Fan-Baffle Support Studs 56

SII

and S12:Heat Exchanger Cleanliness and Leaks 56

S13: Hydrogen Desiccant/Dryer 57

514: Core-Compression ( Belly )Belts 58

S15: Bearing Insulation (at Pedestal or Babbitt) 58

S16: CoilCleanliness (including watercooledwindings) 60

S17: Blocking Condition 61

S18

and S19:TiesBetween Coils 63

S20

and S21:Ties to Surge-Rings 64

522: Surge-Ring Insulation Condition 64

S23: Surge-Rings Support

Assembly

65

S24: Additional End-Winding SupportHardware 67

S25: RIDs and TCs Wiring Hardware 70

526: AsphaltBleeding/Soft Spots 70

S27:

Tape

Separation/Girth Cracking 72

528: Insulation GallinglNecking Beyond Slot 75

829: Insulation BulgingintoAir Ducts 75

S30: Insulation Condition 75

S31: Circumferential Bus Insulation 76

532: CoronaActivity 78

S33:

Wedges Condition 84

534: Wedges Slipping Out 87

S35: FillersSlipping Out 87

536: BarsBottomed in Slot 88

S37: Laminations Bent/Broken in Bore 88

538: Laminations Bulging intoAirDucts 90

539: Terminal Box CurrentTransformer (CT)Condition 92

S40: Bushing

Well

Insulators and 2 SealantCondition 93

841:

End-Winding

Expansion-Bearing BoltsCondition 93

References 95


8/190

Contents

Ix

R21:

R22:

R23:

R24:

R25:

R26:

R27:

Chapter 5: Description of Rotor Items (Form 5)

RO I: Rotor Cleanliness

R02: RetainingRings' Visual Appearance

R03: Centering Rings' VisualAppearance

R04: Fan Rings' VisualAppearance

R05: FrettingIMovement at Rings' Interference-FitSurfaces

R06: Fan Blades Condition

R07: Bearing Journals Condition

R08: BalanceWeightslBolts Condition

R09: End-Wedges Condition

RIO: Other wedges

Rll: End-Windings Condition

R12: Top Series Connections

R13: BottomSeries Connections

R14

and R15: Field-PoleKeys in Dovetail and Inter-PoleBlocking

RI6

and R17:Insulation BetweenTurns

RI8: Starting-Bars (DamperWinding)Condition

RI9

and R20: Bull-Ring Segments and Bracing to

Starting-BarsCondition

CollectorRingsCondition

Collector InsulationCondition

Brush-SpringPressure and General Condition

Brush-RigCondition

Shaft-Voltage Discharge-Brush Condition

Inner/OuterHydrogenSeals

Circumferential Pole Slots Condition

References

97

97

98

103

103

105

105

106

108

108

110

111

114

114

114

115

117

119

119

122

123

123

123

123

124

125

130

131

131

128

129

130

127

127

128

128

Chapter 6: Description of Excitation Items (Form 7)

EO1: Cleanliness

E02: Shaft-MountedDiodes Condition

E03: Diodes Connectionsand Support Hardware

E04

to E07: Commutator, Brushes, Springs and

Brush-RigCondition

DC GeneratorStatorCondition

DC GeneratorArmatureCondition

E08:

E09:

EI0

and Ell: Exciter-Drive Motor Cleanliness and StatorCondition

E12: Exciter-MotorRotor

E13: Field DischargeResistor Condition


9/190

x

Chapter 7: Description of Generator Auxiliaries

7.1 LubricationSystem

7.2

HydrogenSeal Oil System

7.3 StatorWaterCooling System

7.4 Hydrogen System

Chapter 8: Standard Electrical and MechanicalTests

8.1 ElectricTests

8.1.1 WindingResistance(DC)

8.1.2 InsulationResistance (DC)

8.1.3 PolarizationIndex (PI)

8.1.4 Dielectric(Over-Potentia) or Hi-Pot) test

8.1.5 Tum-to-Tum Insulation(SurgeComparison) Test

8.1.6 ShortedTurns in ExcitationFieldWindings

8.1.7 OpenCircuit Test (for Detectionof Rotor ShortedTurns)

8.1.8 Power Factor Test

8.1.9 Tip-UpTest (for Stator Winding)

8.1.10 DielectricAbsorption

8.1.11

Partial Discharge(PO) Test (for StatorWindings)

8.1.12 Slot-Discharge (SO)Test

8.1.13 Stator InterlaminarInsulationTests

8.1.14 Core-Compression Bolts Insulation Test

8.1.15 Bearing Insulation

Test

8.2

Mechanical

Tests

References

Appendix: Principles of Operation of Synchronous Machines

A.I GeneralDiscussion

A.2

Construction

A.3 Operation

A.3.t No-LoadOperation

A.3.2 Motor Operation

A.3.3 GeneratorOperation

A.3.4 EquivalentCircuit

A.3.5 Performance Characteristics: V curves and RatingCurves

A.4 OperatingConstraints

A.4.1

Volts per Hertz (VlHz)

A.4.2 NegativeSequenceCurrentsand {I

2

)2t

A.4.3 Maximum

Speed

References

Index

Contents

133

133

135

135

136

137

138

138

138

139

141

141

141

144

144

145

146

147

148

148

148

149

149

149

153

153

154

156

157

159

159

160

161

165

165

167

170

170

171


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List of Illustrations

Chapter 1

Fig. I-I.

Fig.

1-2.

Fig. 1-3.

Fig.

1-4.

Fig.

1-5.

Fig.

1-6.

Fig.

1-7.

Fig. 1-8.

Fig.

1-9.

Fig.

1-10.

Fig.

I-II.

Fig.

1-12.

Wooden

coverwithdoorat the entranceto the borearea.

Temporary barriererectedto preventcontamination.

Typical hot-airblower.

Lowering a protective tent on the rotorof a largeturbogenerator.

Applying masking tape to vent holesof a 4-pole turbogenerator

rotor.

Same rotoras previous figure, with the ventscovered.

Rotorbodycoveredto avoidcontamination.

Flashlight strappedto the

wrist.

Site of a 1350-MVA unit undergoing overhaul.

Provisions allowing safe accessto theboreof a largegenerator.

Groundleadsappliedto a generatorunit beingoverhauled.

The knuckle area of directlygas-cooled statorcoils.

Chapter 2

Fig. 2-1. Easy-to-carry tool case with themostessential tools.

Fig.

2-2. Commercially available wedgetightness electromechanical

tester.

Fig. 2-3. Commutation performance chartsfrom various manufacturers.

Fig. 2-4.

Boroscope

usedfor

visual inspection

of otherwise inaccessible

areas.

Chapter 3

Fig. 3-1.

An air-cooled, 35-MVA synchronous condenser.

Fig. 3-2. Ahydrogen-cooled, 60-MVA synchronous condenser.

xi


11/190

xii

Listof Illustrations

Fig. 3-3. A

vertical, air-cooled, synchronous hydrogenerator.

Fig. 3-4.

A

1182-MVA, 3000-rpm

2-pole

generator.

Fig. 3-5.

A

1620-MVA,

1500-rpm 4-pole

generator.

Fig.3-6. Schematic representation of a typical large turbogenerator.

Chapter 4

Fig. 4-1.

Fig. 4-2.

Fig. 4-3.

Fig.

4-4.

Fig. 4-5.

Fig. 4-6.

Fig.4-7.

Fig. 4-8.

Fig. 4-9.

Fig.4-10.

Fig. 4-11.

Fig. 4-12.

Fig. 4-13.

Fig. 4-14.

Fig.

4·15.

Fig. 4-16.

Fig. 4-17.

Fig. 4-18.

Fig. 4-19.

Fig. 4-20.

Fig.

4-21.

Fig. 4-22.

Fig. 4-23.

Fig. 4-24.

Fig. 4-25.

Fig. 4-26.

Fig. 4-27.

Fig. 4-28.

Fig. 4-29.

Fig.

4-31.

Fig. 4-32.

[SOl] Broken piecesof laminations found in borearea.

[S03] Section of a turbine-generator boreshowing ironoxide

deposits.

[S03] Closeviewof Fig.4-2.

[S03]

Worn

wedges in the statorof a synchronous condenser.

[S03]Sectionof statorlaminations showing insulation partially

lost.

[S03] Close-up of section of lamination shown

in

Fig. 3-5.

[S03]

Twopackets of

broken

laminations.

[504]Section of a large4-poleturbine generator showing gas-

guides.

[S04] Close-up viewof gas deflectors showing damage.

[S06] Stand-offinsulators damaged by surface contamination.

[S07] Crosssection of a typical bushing well.

[S08]

Core-compression hardware of a many-pole machine.

[S08] Springwashers coveredwithresin-soaked glasstape.

[S10]Fan-baffle supportthreaded stud shown.

[S17]Theend-winding of a turbogenerator.

[S17]Close-up of a blocking arrangement.

[522]

Insulated surge-ring supporting hydrogenerator end-winding.

[523]End-winding withtwosurge-rings and support assembly.

[S23]

Schematic of a typical end-winding supportassembly.

[S24] Portion of an end-winding of a water-cooled stator.

[S24]

Manifolds and tubing for4-pole

1150-MVA

turbine

generator.

[S24] Schematics of end-windings, watercooledstators.

[S27]

Tapeseparation shownincoilof a steam-turbine

generator.

[S27] Tapeseparation andgirthcracks.

[527]Coilwithgirthcrackin wall insulation nearcooling

vent.

[S27]

A fissure on the insulation of a coilnear the surge-ring.

[S31] Circumferential busses at connection endof salient-pole

machine.

[531] Portion of circumferential

busses

in a steam-turbine

generator.

[S31]

Greasing at the

region

of the support due tomovement.

[S32] Closeviewof oneof affected areasshownin Fig.4-30.

[532] Installation of embedded statorslot coupler(SSe).


12/190

Listof

Illustrations

xiii

Fig.

4-33.

Fig. 4-34.

Fig.

4-35.

Fig. 4-36.

Fig. 4-37.

Fig.

4-38.

Fig.

4-39.

Fig.

4-40.

Fig. 4-41.

Fig. 4-42.

Chapter 5

Fig. 5-1.

Fig. 5-2.

Fig. 5-3.

Fig. 5-4.

Fig.

5-5.

Fig. 5-6.

Fig. 5-7.

Fig. 5-8.

Fig. 5-9.

Fig.

5-10.

Fig. 5-11.

Fig. 5-12.

Fig. 5-13.

Fig. 5-14.

Fig. 5-15.

Fig.

5-16.

Fig. 5-17.

Fig. 5-18.

Fig.

5-19.

Fig. 5-20.

Fig. 5-21.

[833] Movementof loosewedges in oil createsgreaselike

deposits.

[833]Close-upof a looseend-wedgeand greasing.

[S33]Tapping wedges insideboreof a

475-MVA

4-polegenerator.

[S35] Side-coil fillersof a salient-pole synchronous machine.

[S37]Bent laminations from the first packet (closest to edge of

bore).

[837] Samemachineas in Fig. 4-37,differentpackets.

[S38]Lamination bulginginto ventduct in steam-turbine

generator.

[S38]Cross-section of slot showingmigration of duct-spacer

assembly.

[841]A windingsupportbearing-boltwith greasedeposits.

[841] Grease deposits at theheadof thebearing-bolt of Fig.4-31.

[R02lPortionof a retainingring showingrust deposits.

[R02]Retainingringbeingpreparedfor removalby electrical

heating.

[R02]Retainingring of a 2-poleturbinegeneratorwith vent

holes.

[R02]Close viewof venthole from the retainingring in Fig. 5-3.

[R02l Schematicsof spindle-mounted andbody-mounted retain-

ing rings.

[R04]Fan ring and fan bladesof a 2-polecylindrical

rotor.

[R04]2-polecylindricalrotor.

[R06]Salient-polemachinewith radial fansbolted to rotor's

pole-support.

[R06]Arrangement similar to Fig. 5-8, but in a smallermachine.

[R06]Closeviewof a fan blade shown in Fig. 5-8.

[R06]Closeviewof elaborate attachment of fanbladesin Fig. 5-6.

[R09]Narrowstrips are rotorwedgesof a cylindrical rotor,

[R09]Rotorwedgesthat havemigrated.

[R09]A turbinegenerator's rotor witha singlewedgeper slot.

[RIO] Cross-section of aluminum rotorwedgeshowing stressarea.

[R11]Front viewof end-winding of a 2-polecylindricalrotor.

[RII] Topviewof the sameend-winding shown in Fig. 5-16.

[RII] Loose insulatingblockingof end-winding of gas-turbine

generator.

[RI5] A

V-block

wedgingtwo adjacentpoles in a salient-pole

generator.

[RI6] A salient pole with a strip-on-edge winding.

[RI6] A wire-wound salient-pole.


13/190

xiv Listof Illustrations

Fig. 5-22. [R18]Short-circuiting ringof starting winding of a 60-MVA

condenser.

Fig. 5-23. [RI8] Closeviewof Fig.5-22,pole-face withstarting bars

sheared off.

Fig. 5-24. [R20] Short-circuiting segments.

Fig. 5-25. [R21] Thecollector ring (slip-ring) of a large turbine generator.

Fig. 5-26.

[R27]

Schematic representation of thecircumferential pole

slots.

Chapter 6

Fig. 6-1. [E04] DCgenerator commutator withthe

brushes

removed.

Fig. 6-2. [E09]

Armature

of shaft-mounted exciter.

Chapter 7

Fig. '-I. Typical arrangement of a lubrication system.

Chapter

8

Fig. 8-1.

Fig.8-2.

Fig. 8-3.

Fig. 8-4.

Fig. 8-5.

Fig. 8-6.

Fig. 8-7.

Appendix

Fig. A-I.

Fig.

A-2.

Fig.

A-3.

Fig. A-4.

Fig. A-5.

Fig.A-6.

Fig.A-'.

Fig.

A-8.

Fig.A-9.

Fig.A-IO.

Insulation resistance as a function of timeand dryness.

Typical curve

showing

variation of insulation resistance

with time.

Typical waveform

recorded

from flux probein a 2-pole

turbine

generator.

Typical

arrangement for C-core test to detectshorted

turns.

Typical

traces for theC-coretestof Fig.8-4.

Dependence of insulation powerfactoron number of

voids

in

insulation.

Absorption currentas a function of time.

Hydroelectric generator

from

Lauffen, nowin theDeutsches

Museum.

Schematic cross-section of a synchronous

machine.

Phasordiagrams fora synchronous cylindrical-rotor idealma-

chine.

Steady-state equivalent circuitof a synchronous machine.

Steady-state equivalent circuitandvectordiagram.

Typical V-curves for generator operation.

Typical

capability curves fora synchronous generator.

Typical

saturation curvefor transformers and

machines.

Hysteresis

losses

undernormal and abnormal conditions.

Temperature risemeasured at endof rotorbody.


14/190

Preface

Many synchronous machines that were designed and manufactured in the last

decadeof the nineteenth centuryare still ineverydayoperation. Theseare mainly

hydrogenerators belonging to long-established utilities. The theory of the syn-

chronousmachinewas alreadywell advancedwhen thesemachinesweremanu-

factured. Since then, a profuse amountof theoretical literaturehas been added,

especially with the adventof the computerto facilitate the implementation of nu-

merical

analysis techniques. Over the years, the continuous push for higher and

higher ratings and operating voltages resulted in the development of more com-

plex mechanical structures, cooling arrangements, and insulation materials. As

the complexity of the machine increased, design margins became less forgiving.

In addition, the growingdependence of societyon electricpower and the prohib-

itive cost of failuresput the issue of reliability foremost. Reliability has been the

drivingforce for the continuous improvement of techniques and instrumentation

designed to monitor the conditionof the machineand for the creationof a wealth

of industrystandards.

Hand in hand with the development of hardware and written literature, a

wealthof expertisehasbeenaccumulated on the practicalaspectsof theoperation

of these machines. This expertise becomes evident during troubleshooting and

inspection activities. In spite of all the instruments andwrittenliterature, there is

no effective substitute for the expert on the spot to troubleshoot or evaluate the

conditionof the machineduringa visual inspection. The recognition of this ex-

pertise is demonstrated by the recent implementation of so-called expert sys-

tems to diagnoseproblemsin large synchronous machines.

xv


15/190

xvi

Preface

In spite of the universal reliance on visual inspection of large synchronous

machines as part of their operation andmaintenance, there is no written compre-

hensive compendium to be found on the subject. Succinct guidelines can be

found

in numerous standards, technical papers, manufacturers' bulletins, and

otherpublications. This bookfills this gap by providing a comprehensive

refer-

enceon the visual inspection of large synchronous

machines.

It is basedon the

largebodyof accumulated experience that can be

found

in a

myriad

of publica-

tions, thepersonal experience of the

author,

and foremost, on the contribution of

manyassociates.

Having the in-house capability to performreliable inspection of its genera-

tors and rotary condensers

allows

electric utilities and independent power pro-

ducers to make informed choices regarding repairs/refurbishment of these large

machines.

This independent capacity for evaluating the condition of the ma-

chinescan resultin substantial savings.

This book was written with the machine's operatorand inspector in

mind.

Although

not designed to providea step-by-step guidefor the troubleshooting of

largesynchronous machines, it servesas a valuable sourceof information thatcan

proveto

be

useful

duringtroubleshooting operations. The topics covered arealso

cross-referenced toothersources. Manysuch references are included to facilitate

the

search.

Equations describing the operation of the

machine

were intentionally

leftout frommostof the discussions. There is a vastamount of theoretical litera-

ture available for this

purpose.

The only theoryincluded in thisworkconsists of

descriptions of phenomena affecting the reliability of the

machine

components.

In addition, an appendix is included to review thebasicconcepts of synchronous

machine

operation. This appendix provides a

measure

of understanding of how

to utilize machine performance characteristics and theirsources, and shows how

to perform simplecircuitcalculations.

Aftera description of sitepreparation andinspection

tools,

thebookpresents

a numberof forms adequate for entering

findings

duringthe inspection of a large

synchronous machine. They are written in such a way that most of the items

found in major typesof machines can be accommodated. Items found only in

salient-pole machines are marked and items found only in round-rotor ma-

chinesare marked RR. Subsequently, eachof the itemsis described inChapters 4

through

7 regarding visualappearance andtheessenceof theprocesses involved.

Figures

are introduced

when

available. Theitemscovered appearunderthemain

members of the machine; that is, stator, rotor, and excitation. A list of themost

common

electric and mechanical tests performed in the field is presented in

Chapter8. Eachtestis referenced to thecorresponding ANSI/lEEE standards and

other publications.

Thisbookcan

be

useful to themachine-designing engineerand systems op-

erations engineer. It provides a wealthof information obtained in the field about

the behavior of thesemachines, including typical problems andconditions of op-

eration. Byserving as a sourcefor descriptions of different typesof synchronous


16/190

Preface

xvii

machines

and machine components, it canalsobe useful to the studentof electri-

cal rotating machinery.

The author's intention is to keep updating the contents of thisbookfromhis

own and others' experience. Therefore, he wouldappreciate it if readers would

please submit their comments or additions to the publisher for incorporation in

future editions.


17/190

Acknowledgments

Thecontents of thisbookare

almost

impossible to learn in a class. They are the

resultof personal experience accumulated over

years

of workwith largeelectric

machinery.

Mostof all, theyare the resultof the invaluable long-term contribu-

tionof co-workers and associates. The following two individuals, in particular,

havebeen a continuous sourceof knowledge and inspiration over the

years,

as

wellas

having

beenkindenoughto

review

themanuscript and

provide

numerous

suggestions: Mr. Jack Cohon, large

machines

expert, retired fromSouthern Cali-

fornia

Edison, and

Mr.

TomBaker, SteamDivision Electrical Engineer, Southern

California Edison.

The author is highly indebted to Professor Alan Wallace of Oregon State

University

at Corvallis, ProfessorRadhe DasofCalifornia State

University,

Long

Beach,

James

Edmonds

of MCMEnterprises, Ltd., and GeoffKlempner of On-

tario Hydro for painstakingly reviewing the manuscript and making important

suggestions thatwereincorporated intothe finalmanuscript.

Theauthor

would

alsoliketo thankMr.

Dudley

Kay, the

members

of theed-

itorial department of IEEEPress,reviewers, and all othermembers of the IEEE

Press involved in thepublication of thisbook, for theirsupport inmaking its pub-

lication possible.

IsidorKerszenbaum

Irvine,

California

xix


18/190

PART 1

Preparation

Chapter

1

Site

Preparation

Chapter 1 describes how to minimize the risk of machine contamination and to

ensure a safe environment in which to perform the inspection.

Chapter 2 Inspection Tools

Chapter 2 discusses tools needed to perform the inspection.

Chapter

3

Inspection Forms

Chapter 3 includes The Synchronous Machine Inspection and Test Report, com-

prising ten forms applicable to the inspection of most large synchronous ma-

chines.


19/190

1

Site Preparation

Site preparation is the first significant action to perform immediately before an

inspectioncarried out on, or in the vicinityof, a large electricalmachine.Every

inspectionof a large machine-scheduled or not, longor short-requires a sensi-

ble effort toward site preparation.

The

goal is to minimize the risks of contami-

natingthe machinewithany foreignmaterialor object, aswell as to ensure a safe

environment in which to perform the inspection. Site preparation should be

plannedaheadof time, and it shouldbe maintainedfrom the momentthe machine

is opened for inspectionuntil the time it is sealed and readied for operation.Ne-

glecting to prepare and maintaina properworkingenvironmentin and around the

machine can result in undue risks to personnel safety and machine integrity (see

Figs.

1-1

and 1-2).

1.1

FOREIGN

MATERIAL EXCLUSION

Foreignmaterialexclusion(FME),a termoriginated in thenuclear industry, is the

set of proceduresgeared tominimizethe possibilityof intrusion into the machine

of foreignmaterialsbefore, during, and after the inspection.

In principle, the definitionof foreignmaterial is anythingnot normallypres-

ent during the operationof the machine that might adverselyaffect its constituent

components if left there. For instance, although ambient air is not necessarily

considered a foreign material, the water content of the air is. Water definitively

is an extraneous element that should be kept from condensing on the machine

3


20/190

4

Preparation PartI

Fig.

I-I

Wooden

coverwithdoorat theentrance to theborearea

(both sides). These

allow

control

of access to themachine of personnel and tools.

Fig. 1·2 Temporary barrier erected

to

keep vital components from being contami-

nated with foreign objects or

materials.


21/190

Chap. J Site

Preparation

5

windings, retaining rings, and other parts susceptible to mechanical failure from

corrosion,or fromelectric breakdownof the insulation.

Keepingwater from condensingonto the machinecomponentscan be read-

ily accomplishedby containingboth statorand rotorunderprotectivecovers (i.e.,

tents), and maintaininga flowof hot air.The hot air and the positivepressuredif-

ferential inside the tent eliminate the condensationof any significant amount of

water(Figs. 1-3and 1-4).Althoughthe flowof hot air is normallysuspendeddur-

Flg.1-3

Typical

hot-air

blower.

Fig.

1-4

Loweringa protectivetent on the rotor of a largeturbogenerator.


22/190

6

Preparation

Part

1

ing the actual inspection for personnel comfort reasons,thus allowing somecon-

densation to occur, the subsequentflow of hot air will most probablyevaporate

themoisture and remove it from the containmentarea [1].

It is important to perform any scheduled electric tests with dry windings.

Otherwise, results obtained will not be representative of the winding condition

under normal operatingconditions.

It is also importantnot to contaminate themachineinadvertently withcorro-

siveliquidssuchas solvents,certainoils, andso forth.Sometimes extraneous flu-

ids can be introduced by walkingover them and thenwalking into the machine.

Therefore, in situationswherestringentFME rulesare applied, paperbootiesare

wornover the shoes.Some inspectors prefer the use of rubberbooties over their

shoesfor better and safergrip.

Paper, rubber, or cloth booties will go a long way in eliminating the intro-

ductionof small pebbles thatmaybe stuck to the soleof the shoe.Whenpressure

is appliedto the end-winding by walkingover it, a small pebblecan puncturethe

insulation, thus creating a region of electric-field concentration. This is worth

avoiding. It is good practicenot to step on the bare coils.A cloth will suffice to

protectthe windingfrom the shoe.

The worst enemies of the windings are any foreign metallicobjects.They

can becomeairbornedue to the highspeedof the coolinggas, and break the insu-

lationwhen striking it. Magnetic particles have been known to cause failures in

water-cooled coils by penetrating the insulation over long periodsof time,due to

the electromagnetic forces acting on the particle. Magnetic as well as nonmag-

netic metallic objects may be subject to eddy-current heating, detrimentally af-

fecting the insulation with which they are in contact. Foreign metallic objects

such as nails, welding beads, or pens inadvertently left in the bore can short-

circuit the laminations of the core.Continuedoperationunderthis conditionmay

result in a winding failure due to localized temperature rise of the core. Precau-

tionsshouldbe takento eliminatethepossibility of metallicpartsor other foreign

objectsenteringthe machine. One step in thatdirectionis maskingthe vent holes

of the rotorwheretheseare locatedoutside the statorbore,andcoveringthe rotor

when not under inspection or refurbishment (see Figs. 1-5 to 1-7).Metallicob-

jects not requiredfor the inspection shouldbe left outside.This entails removing

any coins and otherobjects (suchas medallions, beepers, unnecessary pens,pen-

cils, etc.) frompocketsprior to entering the machine. Inspectiontools shouldbe .

carried into the machineon an as needed basis.When using mirrors or flash-

lights in otherwise inaccessible areas, theseshouldbe securedby strappingthem

toone's wrist(Fig. 1-8).In particularly compromising situations, suchas withnu-

clear-powered generators, takingan inventory of tools is recommended both be-

foreenteringthe machineand afterexiting it. This is a time-consuming practice,

but recommended for all largegeneratorinspections.


23/190

Chap. J Site Preparation

7

Fig. 1-5 Applying maskingtape to ventholesof a large4-poleturbogenerator rotor,

withthe purposeof eliminatingcontamination of rotorwinding.

Fig. 1-6 Samerotoras previousfigure,with the ventscovered.


24/190

8

Preparation

Part

1

Fig. 1-7 Rotorbodycoveredto avoidcontamination whileworkingonotherareasof

the rotor (end-windings).

Fig. 1·8 Flashlight strapped to the wrist, to

eliminate the possibility of dropping

it

in inac-

cessibleplaces.


25/190


1.2 SAFETYPROCEDURES-

ELECTRICAL CLEARANCES

9

Whencarryingout work in an industrial environment, nothingis more important

than adhering to all required safety precautions. Large machines opened for in-

spection often present obstacles in the formof big openings in the floor surface,

crevicesto crawl through,rodsandmachinemembers stickingout, and soon (see

Fig. 1-9).They all demandevaluationof requiredtemporary additionsto the site,

such as beams over the open floor spaces, handrails (Fig. 1-10), secure ladders,

and so on.

The obstaclesjust mentioned are all visible to the peopleengaged in the in-

spection. However, an invisibleand verypowerfulelementto contendwith is the

voltage potential (or range of voltages) that may be present in a machine. Al-

thoughrare, electrical accidents can occur whenwork is performed in large ma-

chines.

A comprehensive inspection of a large machine requires direct physical

contact with all windingsand other elements that are normallyenergizedduring

the operationof the machine. Walkingthe clearance is jargon usedby some to

describetheprocess of inspectingall breakers, cables, and connections that may

besourcesof electric power to any part of the machine,and makingsure theyare

deenergized and secure.This means that none of these will be accidentallyener-

Fig.I-9 Site of a 1350-MVAunit undergoing overhaul.


26/190

10

Preparation Part 1

Fig. 1·10 Provisions allowingsafe accessto the boreof a largegenerator.

gized during the inspection. The following is a typical (but by no means all-

inclusive) list of safety procedures:

• Personalgrounds (groundingcables) atbothendsof the windingof each

phase will minimize the possibility of receiving an unexpected electric

discharge(Fig. 1-11).

• Phase leads must be open.

• Neutral

transformer (if

present) must be disconnected, or have its leads

opened.

•

Voltage

regulatorsand other excitationequipment must be disconnected.

• Potential transformers are an additional source of voltage to the main

windings, and therefore they must be disconnected and secured. Space

heaters are often overlooked.

To

keep

themoisture out,

space heaters are

normally left on after disassemblingthe machine; thus, it is imperative

to makesure they are disconnectedduring the inspection.

• All switches that may energize any part of the machine must

be

clearly

tagged. A tag can only be removed by the person who installed the tag

originally.

• When inspectingmachineswith direct gas cooling of the stator windings,

discharge resistors are often found on the coil knuckles (see Fig. 1-12).


27/190

Chap.

1 Site Preparation

11

Fig.

I-II

Groundleadsappliedto a generator unitbeingoverhauled.

Flg.t-ll Theknuckleareaof directlygas-cooled statorcoils.Thedischarge resistors

are insidethe knucklearea.


28/190

12

Preparation Part 1

When faulty,

theymay

remain

charged for substantial lengths of

time.

Pre-

caution should be taken when inspecting such

windings,

in particular if

high-voltage testswereperformed beforedisassembling the

machine.

You

canneverbetoo safety-conscious

when

dealing withhigh-voltage

appara-

tus.

• Turning gear must be disconnected (fuses removed) and clearly

tagged

wheninspecting withthe rotor in place.

• Gasmonitors for confined areasmustbe used.

• When inspecting hydrogenerators, in addition to the electrical clearance

walk, a mechanical clearance walkmustbecarriedout.This shouldverify

thewaterturbineand valves are lockedandsecurefrominadvertent move-

ment.If possible, the turbine/penstock should

be

de-watered.

• Additional itemsas each specific casewarrants.

• Followall relevant safetyrulesandregulations.

1.3 INSPECTION FREQUENCY

Certain components in large synchronous machines require routine inspections

(and sometimes maintenance) between scheduled major outages. Other more

comprehensive inspections, requiring various degrees of machine disassembly,

areperformed duringthemorelengthy outages.

However,

experience hasshownthata majorinspection afteroneyearof op-

eration is

highly

recommended for new

machines. During

the initialperiod,

wind-

ingsupport hardware andsomeothercomponents experience harsherthannormal

wear.

Retightening of core-compression bolts may also be required during this

first outage.

Subsequent outages and inspections can be performed at longerplannedin-

tervals. How long an interval? Minor outages/inspections every 30 months, to

majoroutages/inspections every60

months

are typical periodsrecommended by

machine

manufacturers. Thesemajoroutages include

removal

of the rotor, com-

prehensive electrical and

mechanical (nondestructive) tests, and visual inspec-

tions.

Obviously,

these intervals tend to be longerfor machines spending long

periods without operation. Most stations havelogscontaining the actualnumber

of hours theunitwas running andthe number of starts/stops. This information, to-

getherwiththemanufacturer's recommendations, canbe used to schedule the in-

spections and overhauls.

Largeutilities thathavemanygenerators in their systems andmanyyearsof

experience

running

these

machines

have formed their own maintenance and in-

spection criteria and schedules. Although

working

closelywith therespective ma-

chines' manufacturers, theseutilities tend to extend the periods between outages

for thosemachines thatexperience has shownto havegood records of operation,


29/190


13

and shorten the periods between outages/inspections for

machines

thathavebeen

characterized by morefrequent failures (asmaybe thecasewithold hydrogener-

atorsand rotating condensers).

Frequently, themajoroutages during

which

theopportunity presents itself to

carry out a major inspection

follow

the need to maintain the prime movermore

thanthegeneratoritself.

1.3.1

Condition-Based Maintenance (CBM)

Highequipment reliability, high outagecosts, and the newcompetitive out-

look permeating the electricpower industry have resulted in a newapproach to

machine maintenance. Condition-based maintenance (CBM) reliesheavily on so-

phisticated on-line instrumentation and evaluation techniques to assess the con-

dition of the machine. In this manner, the periods between major outages/

inspections can be increased

beyond

the fixed, scheduled intervals of the past.

The mainconcept is to basemaintenance on the actual condition of the machine

ratherthanon a fixedschedule.

One suggested method of obtaining significant information on thecondition

of themachine is to retrieve temperature, vibration, PD (partial discharge) activ-

ity, air/gas-gap flux, andother readings on themachine underload(priorto shut-

down), andcompare themwith thesame testdataobtained on previous occasions

undersimilaroperating conditions.

REFERENCES

[1] ANSIIIEEE Std43-1974, Recommended Practice forTesting Insulation Re-

sistance of Rotating Machinery, ItemA2, p. 15.


30/190


31/190

16

Preparation Part 1

Fig.2-)

Easy-to-carry

toolcase with the

most essential tools

for visual inspection of

themachine.

• Disposablepaper or cloth overall.

• Workshoeswithsoft sole (rubber).

• Floodlightwith an extensioncord.

• Flashlights and a set of spare batteries. If a flashlight is at risk of fallingto

inaccessible places, it shouldbe attached to the wrist with string and tape.

• Clean rags.

• A small sealed container with white rags to be used as swabs; useful for

takingsamplesof contamination when required.

• A

set of mirrorswith articulatedjoints andexpandablehandles. If

the

mir-

rors are at risk of falling to inaccessible places, they shouldbe attachedto

the wristwith a stringand tape.

• A hammerwith both a soft (rubber) and hard (plastic) heads, for probing

wedges, insulation blocking, etc. For a wedge survey of the entire ma-

chine, hand-held electromechanical probes are commercially available

(see Fig. 2-2). After an initial setting, the probe identifieseach wedge as

tight

or

loose/hollow.

• A set of magnifying glasses or hand-heldmicroscopes to probe for corro-

sion or electrically originatedpitting on retaining rings and in other criti-

cal locations.

• Charts from manufacturers of commutator brushes depicting observable

signs of bad commutation (Fig. 2-3).


32/190

Chap.

2 Inspection Tools

17

Fig.2-2 Commercially available wedge tightness electromechanical tester.

Fig. 2-3 Commutation performance charts from various manufacturers of brushes,

and other useful information.


33/190

18

Preparation

Part 1

• Boroscope, especiallysuitableto inspectunderthe retainingrings,air/gas

ducts, air/gas-gap of machineswiththe rotor in place, and.other inaccessi-

ble spots (Fig. 2-4).

• Agoodcameracapableof takingclose shots of small areas.

• A smallmagnetfor the extractionof loose ironparticles.

Figure 2-4 Boroscope used for visualinspection of otherwise inaccessible areas of

the

machine.

Lightsourceandoptionalvideoequipmentnot shown.


34/190

3

Inspection Forms

Chapter 3 includes the Synchronous

Machine

Inspection and Test Report,

which comprises ten inspection forms for largesynchronous machines. The forms

accommodate both salient-pole and round-rotor machines, as well as motors,

generators, and rotarycondensers. Some individual itemson the forms apply to

only one of the above; some apply to all of them. Items that are found only in

salient-pole machines are

marked SP

on the forms. Items found only in round-

rotor (cylindrical) machines aremarked RR on the forms.

Theformsare not intended to cater toevery typeof machine currently being

used in the industry. Their purpose is to serve as an example of howsuch forms

mightbe organized. However, withminoradditions or changes, theycan suffice

to aid in the inspection of mostmachines.

Figures 3-1 to 3-3 depict typical synchronous machines encountered across

the

industry.

Figure 3-4 is a schematic representation of a large 2-pole turbine

generator.

This hybrid-cooled generator has a water-cooled statorand hydrogen-

cooled rotor. The bearings are mounted on the machine'sbrackets (end-shields).

Figure 3-5 shows a large 4-pole turbine generator. The machine has stator and

rotorwindings, bothwater-cooled. Its bearings aremounted on pedestals. In both

Figures 3-4and3-5,certaindetailsof thewaterfeed

system

to the statorwindings

can be seen.

Figure3-6 depictsa typical generatorand itscomponents. Thefigureshows

the

frame

construction and the end-winding supports attached to the

frame.

The

figurealso showssomeof theexternal detailsof thecylindrical

rotor.

19


35/190

20

Preparation Part J

Fig.3-1 An

air-cooled,

35-MVA

synchronous

condenser.

Fig. 3-2 A hydrogen-cooled, 60-

MVA

synchronous condenser.


36/190

Chap. 3 Inspection Forms

21

Fig.3-3 Avertical, air-cooled, synchronous hydrogenerator.

One of the important steps to be taken at the onset of an overhaul is the pri-

oritization of those items to be inspected immediately after disassembly of the

machine. For instance, if a wedge survey is considered necessary,

it

should

be carried out as soon as possible after removal of the rotor. The reason for this is

the relatively long lead time required to purchase a new set of wedges. Pressure-

testing of the hydrogen seals of the rotor of a turbine generator should also be

done as early into the overhaul as possible. The same can be said about non-

destructive examinations (NDEs) of retaining rings and other critical items that

may require replacement.


37/190

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40/190

Chap.

3 Inspection

Forms

SYNCHRONOUS MACHINE INSPECTION ANDTEST REPORT

Form 1: Basic Information

mark withX

GENERATOR

CONDENSER

MOTOR

SALIENT·POLE

ROUND·ROTOR

Station/Company where installed

Unit no.

25

Primemover type:

Serialno. of generator

Steam

Gas

Hydro

__

Diesel

__

Manufacturer _

Date of manufacture _

Frame _

Year

installed _

Dateof last

rewind:

Stator _

Dateof last major inspection

Rotor _

Total

operating hours

__

Operating hourssincelast overhaul

__

Total

numberof starts/stops

__

Numberof hours in turning gear__

Presentinspection performed by

Assisted by _

Dateof inspection


41/190

26

Form 2: Nameplate Information

Preparation Part J

Rated

MVAo_

Powerfactor_ Shortcircuit ratio _ Field 1

2

1_

Stator:

Field:

Line voltage kV Rated current amps

DC voltage volts DC current amps

Nominal speed

___ rpm No. of poles_ Frequency__Hz

Stator cooling: Open air _ AirlWater _ Direct

water_

Hydrogen _

Rotorcooling: Air _ Hydrogen_

Water

_

Stator insulation: Asphalt_ EpoxylResin-Mica _ VPI _ Other__

Max.

H

2

pressure[psi]

__

Form

3: Inspection Accessibility

Yes No

Rotor out of bore

Inboardtop-bracket removed

Inboardbottom-bracket removed

Outboard top-bracket removed

Outboardbottom-bracket removed

Bushingwell open

Inboardretaining ring off

Outboard retaining ring off

Exciter's rotor out of bore

Electrical and mechanical clearance

Additional details:


42/190

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45/190

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47/190

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48/190

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49/190

34

Form8:

omments

Preparation

Part J

Comments:

- - - - - - - - - - - - - - - - - - - - - - - - -


50/190

Chap. 3 Inspection Forms

Form

9: Wedge Survey

35

This is a typical tablefor performing a

wedge

survey. A larger number of

columns

for wedges and/orrows for slotsmaybe required for largermachines.

One way to enter the information is:

• 0 fora tight wedge

• H for a hollow wedge

• L for a loose wedge

s

w

ww

ww

ww

ww ww ww

ww w

ww ww ww

ww

ww ww

W

L

E

E E E E E E

E E E E E E

E E

E E

E E

E E E E E E

E E E E

o D

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D D

D D

DD DD

D D

D D D D D D

D D

D D

DD D D D D

T G G G

G G

G G

GO

o

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G G

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0

o 0

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0

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E E

E E

E E E E E E E E E E E E E E E E E E E E E

E

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1

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1 1

1

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1 2

2 2

2 2 2 2 222

I 2

3 4 5 6

7 8

9 0

1 2

3 4 5

6

7 8

9 0

1 2

3 4

5 6

789

1

2

3

4

5

6

7

8

9

10

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12

13

14

15

16

17

18

19

20

21

22

23

24

2S

26

27

28

29

30

31

32


51/190

36

Wedge

Survey continued

Preparation

Pari J

s

w

WW

WW

WW

WW WW WW

WW WW WW WW

WW WW

WW

WW

L E E E E E E E

E E E E E E E E

E E E E

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E E E E E E

E E

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D D D D

D D

DD D 0

D D

T G

G G

G G

G G

o G G 0

G G G G

G G

G 0

G G G G

0 0 G G

o G

E E E E

E E

E E

E E

E E

E E E

E E

E E

E E

E E

E E E E E E

#

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2 3

4 5

6 7

8 9

o 1

2 3

4

S

6 7 8 9

33

34

S

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

S9

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75


52/190

Chap. 3

Inspection

Forms

Form

10: Electric Test Data

37

The following data is a sample of data from tests performed after machine shut-

down (some of these tests to be performed under hydrogen for hydrogen-cooled

machines)

refer to the pertinent standards for

the correct test procedures

- see references for chapter 8 -

MACHINE STATOR

• Measured conductivity for liquid-cooled machines:

__

micro-mhos/em

• Measured meggerreadings of windings to ground(with2500V

megger)

_

3D

s 1 min 10min _

Polarization Index(10 mini min) _

Ambienttemperature Hours after shutdown

_

StatorRTDs temperature I) 2) 3) 4) _

5) 6) (twoof each phase)

Note: Forwater-cooled statorsonly 1 min megger

reading

required

MACHINE

ROTOR

• Measured meggerreadingsof windings to ground(with500V megger) _

30 s 1min 10min _

Polarization Index(10 min/I min) _

ALTERNATOR EXCITER

Stator

• Measured statormeggerreadingsof windings to ground(with 500 V

megger)

_

3D

s 1 min 10min _

Polarization Index(10 minll min) _

Winding temperature _

Rotor

•

Measured

meggerreadings of windings to ground(with 500V

megger)

_

3D

s 1 min 10min _



DC

EXCITER

• Measured meggerreadingsof windings to ground(with 2500V megger) _

3D s 1 min 10 min _




53/190

38

Electric Test Data (continued)

RTDs

•

Megger

test to groundwith500V

Megger

• Measure eachRTD'sresistance with a bridge

•

Compare reading

with

measured

temperature of thewinding

STATOR WATER OUTLET THERMOCOUPLES

Preparation

Part

1

•

Measure

millivolts andcompare

readings

with

measured

temperature of wateror

ambient

air (when

empty)

ADDITIONAL TESTS

• Corona probe(when required-large salient-poles)

• PDActivity readings (beforeshutdown if instrumented)

• Rotor-flux waveforms (if fluxprobeinstalled)

ALARM CHECKS

The following is a sampleof the alarmcircuitsand activators that

require

check

(differ-

ent machines willhavea different set of alarms):

• Air

filters

clogging

alarms

• Statorcoolingwaterpressure low

• Waterpressure at

machine

•

Water

flow

• Statorwaterfilter

• Stator

water-cooling

pump

• Water andoil leakage detectors

• Hydrogen seal-oil enlargement detector


54/190

PART 2

Inspection

Chapter

4

escription

of

Stator

IteDls

Chapter 4 describes each stator item on Form 4, Stator Inspection.

Chapter 5 escription ofRotor ItelIls

Chapter 5 describes each rotor item on Form 5, Rotor Inspection.

Chapter

6 escription ofExcitation IteDls

Chapter 6 describes each excitation item on Form 7, Excitation Inspection.

Chapter

7 Generator uxiliaries

Chapter 7 describes auxiliary systems that should not be overlooked in an inspec-

tion, such as lubrication systems, hydrogen seal oil systems, stator-cooling water

systems, and hydrogen systems.

Chapter

8

Standard Electrical

andMechanical Tests

Chapter 8 giyes an overview of the electrical and mechanical tests, which should

only

be

performed

by

trained personnel.


55/190

4

Description of Stator Items

The Stator Inspection form refers to items comprising the actual stator, as well as

the frame, bearings, and other machine-related components. Each item on the

form is described below with reference to its item number. Figure captions also

include the reference number for the item to which they correspond.

501: Cleanliness of Bore

Important information on the condition of the machine may be obtained

from a general view of the bore area and frame. For instance:

• Excessive discoloration (and perhaps flaking) of paint on the casing,

frame, and bore indicates a probable case of overheating. This could be a

result of overloading, or improper flow of air, gas, or water.

• The presence of large amounts of oil or a dust-oil mixture attests to possi-

ble hydrogen-seal problems.

• In certain types of air-cooled machines, large amounts of carbon dust are

evidence of deficient sealing between the collectors' enclosure and main

bore areas.

• In cylindrical machines (turbine generators), loose copper dust, or dust

mixed with oil andlor other dusts, indicates excessive pounding of the

rotor-field conductors.

41


56/190

42

Inspection

Part2

•

Water

found in the bottom of the

machine

may indicate a leak in the heat

exchanger.

• Excessive amounts of ironpowdermixed with oil anddustor found alone

in theborearea tendto indicate a loosecore.

Nuts, bolts,

small pieces of lamination iron,or otherlooseobjects

found

in-

sidethemachine, oftentimes at the

bottom

ofthecasing,

should be

investigated as

to their origin. They may point to loose space heaters, broken laminations or

cooler

fins, and/orotherabnormalities in needof

attention.

Figure 4-1 shows metallic objects found in theboreand

bottom

of thecasing

of an

air-cooled,

gas-turbine generator. In order to retrieve these objects, side

plates

were

opened

and long instruments

with

a grip device at their end were

used.

Subsequent examination of theboreareaidentified thesemetallic objects as

remnants

of

broken

pieces of laminations (seeFig.4-7).

:.

•

.

oil·

.:

....::

•.

A

••

•

,

Fig. 4-1

[SO

1]Brokenpiecesof laminations foundin theboreareaandbottomof

the

casingof an air-cooled gas-turbine

generator.

802:

Air/Gas-Ducts Clogged/Unclogged

Clogged

vents

effectively derate a machine byrestricting theflowof cooling

gas

through

the laminations andcoils. Thisphenomenon is particularly evidentin

open-air machines, which tend to be older and slower: mainly hydrogenerators,

condensers, and industrial motors. Clogged

vents

are particularly

common

in

open-air machines contaminated withoil.


57/190

Chap. 4 Description ofStator Items

43

The restrictions can show up during operationof the machine as hot spots;

i.e., temperature readings from one or several temperature detectors will

be

higher than those obtained from the rest. (This can also indicate core-insulation

problems,as discussedbelow.)

The clogging need not only be dust or show itself only in old or open ma-

chines. In one case, massive cloggingof the ducts by red iron oxide powderwas

found in a new large hydrogeneratorhavinga very loose core.

Normally, a visual inspection with the aid of a frontal source of light may

suffice. Where possible, a light placed in the back of the core while observing

from the inside can result in good observations of the ducts. In cases where the

rotor has been left in place and clogging of the ducts is suspected, insertion of a

side-view boroscope through the airgap can provide an excellent view of the air

ducts in the inspectionarea.

S03: Iron Oxide Deposits

Iron oxide appears as red powder deposited mainly on the bore and in the

air/gas-ducts of the machine. When mixed with oil, these deposits may be con-

cealed in a mixture of dirt and oil. This mixture should be chemically analyzed

when iron dust is present (or suspected), for content in proportion to weight.A

quick identificationduring the inspectioncan be made by subjecting small por-

tions of the mixture to the field of a magnet (one of the desirable inspection

tools). If the dirt responds to the magneticattraction,then irondust is mostcer-

tainlypresent in significantproportions.

Iron oxide can result both from loose laminationsand loose wedges [1]. In

the case of loose wedges, the iron dust is mainly seen in the contact region be-

tweenwedgeand iron. In the caseof loose laminations,the ironoxide powderde-

posits are distributed on larger sections of the machine, on the iron itself. The

deposits will tend to concentrate in places adjacent to the air/gas-ducts, having

been left there by the flowing gas (see Figs. 4-2 and 4-3). In severe cases, the

large amounts of iron oxide powdergeneratedmight clog air/gas-ducts.

When the amountsof ironoxide powderpresent in the machine are substan-

tial, its origins should be thoroughly investigated. If loose wedges are the cause,

then they should be tightenedby re-wedgingor another effective method. Loose

wedgesmay abrade themselves to the extent they come out of the slot (see Fig.

4-4). They may also indicate a loose winding condition,with detrimental conse-

quences to coil insulation; in particular, the lossof semiconducting paint [2, 3].

If the iron oxide originatesfrom the movementof metallicparts, it probably

indicatesa loose core or loose portionsof the core [4].

Cores are pressure-loadedto given valuesduring the manufactureof the ma-

chine. Duringoperation, the core is subjected to continuouselongationand con-

traction of the laminations (magnetostriction) at twice supply frequency, and


58/190

44

Inspection Part 2

Fig.4-2

[S03] Section of a largeturbine-generator boreshowing deposits of red iron

oxide

powder.

Fig.4-3

[503] Close view of Figure

4-2.

The accumulation of the red powder de-

pends on the origin of the powder and the pattern of the flow of cooling gas.


59/190

Chap. 4 Description

of

Stator Items

45

Fig.4-4 [S03] Wedges in the stator of a synchronous condenserworndue to vibra-

tions to theextentthatseveralcameout of theslot, andotherswerefoundto

beon theirwayout.

elongation and contraction of supporting structures due to thermal cyclesandvi-

brations. Machines having properly designed and stacked cores are supposed to

withstand theseonerous conditions. In many cases, however, after yearsof oper-

ation, this constant movement of the core components and the accompanying

metalfatigue, abrasion, anddeformation resultin a reduction of thecore's loaded

pressure. Theend result, if notcorrected, is abrasion of theinterlaminar insulation

(Figs. 4-5 and 4-6). The consequences of such abrasion are spot-heating of the

core, broken laminations leading to machine contamination with iron particles,

and serious failures of core-compression bolts, bolt insulation, andcore-compres-

sion fingers [4]. Otherdetrimental effectsaredeterioration of the coil insulation

due to hot spotsin thecore,additional core losses, andaugmented vibrations and

increased audible sound levels. An additional test to confirm the presence of a

loosecore is the insertion of a knifebetween the laminations at several locations.

If a lo-mil blade penetrates morethan a quarterof an inch, the core may not be

sufficiently tight.Extreme care shouldbe taken not to break the blade, leaving a

piecein the laminations. This technique shouldbe usedverycarefully, especially

in anymachine withits

windings

in place. (Fora description of the properproce-

dure,seeReference [3].)

Some utilities check the torqueof a sampleof the machine's compression

boltsat every secondor thirdoverhaul, or someotherchosen interval. The mea-

suredtorquevalueis compared withthoserecommended by themachine'smanu-


60/190


61/190

Chap.

4

Description of StatorItems

47

facturer. If thesebolts are found to be

loose,

theentirecore is retorqued in

accor-

danceto procedures laid downby themanufacturer.

It is important to note that not all lamination

problems

are the result of a

loose

core. For instance, Figure4-7 showsentirepackets of broken laminations

because of lackof sufficient support by the I-shaped duct separators at the top of

the tooth. Thecore of this particular machine (an air-cooled gas-turbine genera-

tor)wasotherwise found tobe tight. In thiscase,repairs included the introduction

of epoxy glass laminates between laminations, and application of penetrating

(low-viscosity) epoxyto the damaged area.

Thesolution to looselaminations is

varied,

depending on location, severity,

and typeof

problem.

It

ranges

from retightening the core-compression bolts to

changing compression platesor parts thereof, introducing nonmetallic shims, and

so

forth.

Fig.4-7 [S03]Two packets of broken laminations belonging to an air-cooled gas-

turbine generator.

504: Hardware Condition

All parts in a generator are exposed to continual vibration, temperature

changes, and other mechanical stresses. They may

become

loose, fractured, or

broken. It is therefore important to

search

for these abnormalities during the in-

spection

before

theydevelop intomajortroubles.


62/190

48

Inspection

Part2

In particular, all

components

of the winding

support

assembly are subjected

to

mechanical

stresses due to sudden and

large

loadchanges, suchas are present

during lossof load, shortcircuit, pole slipping, and closing out of synchronism.

Machines

subjected to the above-mentioned conditions, as

well

as

machines

op-

erated with

many

starts, aremoresusceptible than

others

to

hardware

failure.

Some of themostsensitive components are:

•

Compression

bolts-Observe if any

greasing

(oilanddustmixed together

bythefriction of twocomponents vibrating within themachine) is present

indicating

looseboltsor

core,

and

integrity

of

nut-locking

device.

•

Surge-ring

supports-Look for cracking and looseness.

• Finger-plates-Look for cracked orbent

fingers.

Observe space heaters for looseness of bolts and nuts, and integrity of con-

nections. Figures ~ and 4-9

present

an example of hardware in the bore of the

machine damaged

during removal

of the

rotor.

Fig.4-8 [S04]Section of a large4-poleturbine generator showing gas-guides made

of insulating material.


63/190

Chap. 4 Description of-Stator

Items

49

Fig.4-9 [504] Close-upview of the gas deflectorsshowingdamageduring the re-

movalof therotor (seeFig.4-8).

805: High-Voltage Bushings

There are too many different arrangements of terminal boxes and bushing

types in large synchronous machines to describethemall in this book.However,

the following general inspection guidelines andcomments apply to any arrange-

ment.

Lead-bushings are susceptible to damagearisingfrom suddenloadchanges,

excessive vibration, overheating of the leads,and normal vibration over long pe-

riods of time. Stator high-voltage bushings should be inspected for evidenceof

cracks, oil leakage (when oil-filled), and looseness of components. All dirt and

tracking residues should

be thoroughly cleaned.

In large turbogenerators, the high-voltage bushings are partly contained in

sealedbushingwells.Someof the lead-bushings haveducts allowingthe flowof

hydrogen. The ducts shouldbe free of oil, grease,or any foreignelements. Many

othersarewater-cooled. In these,connections to thebushings shouldbe inspected

for cracksand leaks.

[isidor kerszenbaum] inspection of large synchrono

Documents