Project

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MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR

AIR COOLED TURBO GENERATOR BY V.P.I PROCESS

A PROJECT REPORT SUBMITTED IN PARTIALFULFILLMENT OF THE REQUIREMENTS

FOR THE AWARD OF

BACHELOR DEGREEIN

ELECTRICAL ENGINEERING

SUBMITTED BY

G.VENKATESH BABU(04A21A0258)

M.K.CHAITANYA SARMA(04A21A0216)

M.V.SATYA TEJA(04A21A0254)

L.PRANEETH CHAITANYA(03A21A0226)

UNDER THE ESTEEMED GUIDANCE OF

R.K.MANOHARSr DGM

Quality Control(E.M)BHEL, Ramachandra puram

T.Ravi. M.E..,Asst prof.

Swarnandhra CollegeNarsapuram

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CERTIFICATE

This is to certify that the project entitled “MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR AIR COOLED TURBO GENERATOR BY V.P.I PROCESS” Submitted byG.VENKATESH BABU (04A21A0258)M.KRISHNA CHAITANYA SARMA (04A21A0216) M.V.SATYA TEJA (04A21A0254) L.PRANEETH CHAITANYA (03A21A0226) In partial fulfillment of “BACHELORS DEGREE IN ELECTRICAL AND ELECTRONICS ENGINEERING” for the academic Year 2007-2008 of IV-Year from SWARNANDHRA COLLEGE OF ENGINEERING AND TECHNOLOGY, affiliated to JNT UNIVERSITY, WEST GODAVARI DIST., A.P, INDIA.A record of bonafide work carried by them under my guidance in “BHEL, RAMACHANDRAPURAM, HYDERABAD-32”.

SIGNATURE OF PROJECT GUIDE SHRI R.K.MANOHARDGM,B.Tech(Elect),(SQC&OR)Electrical Machines,(Quality Control), BHEL,Ramachandrapuram.

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ABSTRACT

In developing countries like India, power generation is a major break through to meet

the present demands of the nation. Power generation of several types are on forefront, the dominant

component of power generation is TURBO-GENERATOR which produces large capacity, the word

“TURBO” stands for turbine drive. Generally the turbines used to drive these turbo-generators are of

reaction type.

In large-scale industries manufacturing generators, insulation design plays a vital role.

Insulation is known to be the heart of the generator. If insulation fails, generator fails which leads to

the loss of crores of rupees. The latest technology for insulation in the world and adopted by BHEL,

(Hyderabad) unit is “VACUUM PRESSURE IMPREGNATION “which is of resin poor

thermosetting type. This type is preferred as it is highly reliable and possesses good mechanical,

thermal properties and di-electric strength. As the quantity of resin used is less, hence the over all cost

of insulation is reduced.

In our project we have made a detailed study of the VPI system of insulation. This

system is employed by BHEL first in the country and second in the world next to Germany.

Project Associates:

G.Venkatesh Babu (04A21A0258)

M.K.Chaitanya Sarma (04A21A0216)

M.V.Satya Teja (04A21A0254)

L.Praneeth Chaitanya (03A21A0226)

Project Guide External: Project Guide Internal:

Mr. R.K.Manohar., Sr.D.G.M, Mr. T.Ravi. M.E..,

Quality Control (E.M), Assistant prof.- EEE dept.

B.H.E.L. R.C.Puram. Swarnandhra College of

Engg and technology

APPROVED BY HOD OF EEE

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TABLE OF CONTENTS

1. ABSTRACT 3

1.1 ACKNOWLEDGEMENTS 10

1.2. PROFILE OF BHEL 11

1.3. PREFACE 13

2. INTRODUCTION 14

2.1 DRAWBACKS OF EARLY VPI PROCESS 14

2.2 ADVANTAGE OF PRESENT RESIN POOR VPI PROCESS 15

3. INTRODUCTION TO VARIOUS PARTS OF A GENERATOR 17

3.1 STATOR 17

3.2 ROTOR 18

3.3 FIELD CONNECTIONS AND MULTI CONTACTS 19

3.4 EXCITATION SYSTEM 19

3.5 PERMANENT MAGNET GENERATOR AND AVR 20

3.6 VARIOUS LOSSES IN A GENERATOR 23

4. MANUFACTURE OF GENERATOR 26

VARIOUS STAGES IN MANUFACTURE OF GENERATOR 26

4.1 STATOR MANUFACTURING PROCESS 27

4.1.1 STATOR CORE CONSTRUCTION 27

4.1.2 PREPARATION OF STATOR LAMINATIONS 27

4.1.3 RECEPTION OF SILICON STEEL ROLLS 27

4.1.4 SHEARING 27

4.1.5 BLANKING AND NOTCHING 27

4.1.6 COMPOUND NOTCHING 27

4.1.7 INDIVIDUAL NOTCHING 28

4.1.8 DEBURRING 28

4.1.9 VARNISHING 28

5. STATOR CORE ASSEMBLY 29

5.1 TRAIL PACKET ASSEMBLY 29

5.2 NORMAL CORE ASSEMBLY 29

5.2.1 STEPPED PACKET

ASSEMBLY 29

5.2.2 NORMAL PACKET

ASSEMBLY 29

5.2.2.1 IN PROCESS

PRESSING 30

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5.2.2.2 FITTING OF

CLAMPING BOLTS 30

6. STATOR WINDING 30

6.1 CONDUCTOR MATERIAL USED IN COIL MANUFACTURING 31

6.2 TYPES OF CONDUCTOR COILS 31

7. ELECTRICAL INSULATION 32

7.1 STATOR WINDING INSULATION SYSTEM FEATURES 35

7.1.1 STRAND INSULATION 35

7.1.2 TURN INSULATION 39

7.1.3 GROUND WALL INSULATION 40

7.1.4 SLOT DISCHARGES 41

7.2 INSULATING MATERIALS 41

7.2.1 CLASSIFICATION OF INSULATING MATERIALS 42

7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES 42

7.3 ELECTRICAL PROPERTIES OF INSULATION AND FEW DEFINITIONS 44

7.3.1 INSULATION RESISTANCE 44

7.3.2 DIELECTRIC STRENGTH 44

7.3.3 POWER FACTOR 44

7.3.4 DIELECTRIC CONSTANT 44

7.3.5 DIELECTRIC LOSS 44

8 RESIN IMPREGNATION 45

8.1 INSULATION MATERIALS FOR LAMINATIONS 46

8.2 VARNISH 47

8.3 RESIN POOR SYSTEM

8.4 RESIN RICH SYSTEM 48

9. MANUFACTURE OF STATOR COILS 48

9.1 FOR RESIN POOR PROCESS 48

9.1.1 RECEPTION OF COPPER CONDUCTORS 50

9.1.2 TRANSPOSITION 50

9.1.3 PUTTY OPERATION 50

9.1.4 STACK

CONSOLIDATION 51

9.1.5 BENDING 51

9.1.6 FINAL TAPING 51

9.2 FOR RESIN RICH PROCESS 52

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9.2.1 PUTTY WORK

52

9.2.2 FINAL TAPING 52

9.2.3 FINAL BAKING

53

10. AN OVERVIEW 54

10.1 ADVANTAGES OF RESIN POOR SYSTEM 54

10.2 DISADVANTAGES OF RESIN POOR SYSTEM 54

10.3 ADVANTAGES OF RESIN RICH SYSTEM 54

10.4 DISADVANTAGES OF RESIN RICH SYSTEM 54

11. ASSEMBLY OF STATOR 54

11.1 RECEPTION OF STATOR CORE 55

11.2 WINDING HOLDERS ASSEMBLY 55

11.3 STIFFENER ASSEMBLY 55

11.4 EYE FORMATION 55

11.5 CONNECTING RINGS ASSEMBLY 56

11.6 PHASE CONNECTORS 56

12. THE VPI PROCESS 56

12.1 INTRODUCTION TO VPI PROCESS 56

12.2 HISTORY 57

12.3 VPI PROCESS FOR RESIN POOR INSULATED JOBS 59

12.3.1 GENERAL 59

12.3.2 PREHEATING

59

12.3.3 VACUUM CYCLE

60

12.3.4 IMPREGNATION

61

12.3.5 POST CURING

62

12.3.6 ELECTRICAL TESTING 63

12.4 GLOBAL PROCESSING 63

12.5 RESIN MANAGEMENT 63

12.6 SPECIFIC INSTRUCTIONS 63

12.7 PRECAUTIONS 64

12.8 FEATURES AND BENEFITS 64

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13. FACILITIES AVAILABLE IN VPI PLANT BHEL 66

13.1 DATA COLLECTION OF SAMPLES 68

13.1.1 INDO-BHARAT –II ROTOR 68

13.1.2 INDO BHARAT –II STATOR 70

13.1.3 HIGH VOLTAGE LEVELS OF STATOR/ROTOR WINDINGS FOR MULTI-

TURN MACHINES 74

13.1.4 TESTING RESULTS OF INDO BHARAT-II ROTOR 75

13.1.5 TESTING RESULTS OF INDO BHARAT-II STATOR 75

14. COMPARISON BETWEEN RESIN POOR AND RESIN RICH SYSTEMS 77

14.1 DRAWBACKS 78

14.2 SUGGESTIONS 78

14.3 JUSTIFICATION 78

15. PRESENT INSULATION SYSTEMS USED IN THE WORLD 83

15.1 WESTINGHOUSE ELECTRIC CO: THERMALASTIC™ 84

15.2 GENERAL ELECTRIC CO: MICAPALS I AND II™, EPOXY MICA MAT™,

MICAPAL HT™ AND HYDROMAT™ 85

15.3 ALSTHOM, GEC ALSTHOM, ALSTOM POWER: ISOTENAX™, RESITHERM™,

RESIFLEX™, RESIVAC™ AND DURITENAX™ 85

15.4 SIEMENS AG, KWU: MICALASTIC 86

15.5 ABB INDUSTRIE AG: MICADUR™, MICADUR COMPACT™,

MICAPACT™ AND MICAREX™ 86

15.6 TOSHIBA CORPORATION: TOSRICH™ AND TOSTIGHT-I™ 87

15.7 MISTUBISHI ELECTRIC CORPORATION 87

15.8 HITACHI LTD: HI-RESIN™ AND SUPER HIGH-RESIN 87

15.9 SUMMARY OF PRESENT DAY INSULATION 87

16. A NEW TREND IN INSULATION SYSTEM 88

16.1 MICALASTIC 88

16.2 MICALASTIC INSULATION IN ITAIPU’ 89

17. CONCLUSION 91

18. BIBLIOGRAPHY 92

A. LIST OF TABLES

A.1 CLASSIFICATION OF INSULATIONS ACCORDING TO TEMPERATURE

A.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES

A.3 PROPERTIES OF AN ELECTRICAL INSULATION

A.4 MATERIALS USED IN RESIN POOR PROCESS

A.5 MATERIALS USED IN RESIN RICH PROCESS

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A.6 TABLE SHOWING TEMPERATURE AND TIME TO BE MAINTAINED FOR

DIFFERENT TYPE OF JOBS IN VPI

A.7 PREHEATING OF INDO BHARAT II ROTOR

A.8 IMPREGNATION OF INDO BHARAT II ROTOR

A.9 VACUUM CYCLE OF INDO BHARAT II ROTOR

A.10 PREHEATING OF INDO BHARAT II STATOR

A.11 VACUUM CYCLE OF INDO BHARAT II STATOR

A.12 POST CURING OF INDO BHARAT II STATOR

A.13 HIGH VOLTAGE LEVELS OF STATOR/ROTOR WINDINGS FOR MULTI TURN MACHINES

A.14 COMPARISON BETWEEN RESIN RICH AND RESIN POOR PROCESS

A.15 TAN Δ VALUES OF THE DIFFERENT PHASES OF A GENERATOR

A.16 TABULAR FORM FOR SHORT CIRCUIT CURRENTS AND VOLTAGES

A.17 TABULAR FORM SHOWING OPEN CIRCUIT VOLTAGES AND CURRENTS

OBTAINED IN THE TESTING

LIST OF FIGURES

B1 PHOTOGRAPH OF A SMALL ROUND ROTOR

B2 FIGURE SHOWING THE FLOW OF EDDY CURRENTS IN ROTOR BODY WITH AND WITHOUT

LAMINATIONS

B3 FLOW DIAGRAM SHOWING VARIOUS STAGES IN GENERATOR MANUFACTURE

B4 FIG SHOWING THE SHAPE OF LAMINATIONS AFTER COMPLETION OF NOTCHING AND DEBURRING

OPERATION

ROEBEL AND DIAMOND PULLED COILS

B5 SCHEMATIC DIAGRAM FOR A 3-Ф Y CONNECTED STATOR WINDING WITH 2 PARALLEL

CONDUCTORS PER PHASE

B6 PHOTOGRAPHS OF END WINDINGS AND SLOTS OF RANDOM WOUND STATOR (COURTESY TECO

WESTING HOUSE)

B7 PHOTOGRAPH OF A FORM WOUND STATOR WINDING (COURTESY TECO WESTING HOUSE)

B8 A SINGLE FORM WOUND COIL BEING INSERTED INTO TWO SLOTS

B9 C.S OF A RANDOM STATOR WINDING SLOT

B10 C.S OF A FORM WOUND MULTI-TURN SLOTS CONTAINING FORM WOUND MULTI-

TURN COILS.

B11 C.S OF A FORM WOUND MULTI-TURN SLOTS DIRECTLY COOLED ROEBEL BARS

B12 C.S OF MULTI-TURN COIL, WHERE THE TURN INSULATION AND STRAND INSULATION

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ARE SAME

B13 SIDE VIEW SHOWING ONE WAY OF TRANSPOSING INSULATED STRANDS IN STATOR

BAR.

B14 C.S OF MULTI-TURN COIL WITH 3 TURNS AND 3 STRANDS PER TURN

B15 LAYOUT OF MOULD USED IN BAKING OF STATOR BY RESIN RICH PROCESS

B16 VERTICAL VPI TANK FOR SMALLER JOBS

B17 RESIN TANK IN WHICH RESIN IS STORED

B18 MODERN STATOR BAR TAPING MACHINE THAT APPLIES THE TAPE ON BOTH IN THE

BOTH SIDES

B19 ROTOR OF THE WORLDS LARGEST HYDRO GENERATOR ITAIPU AT THE ASSEMBLY

LIST OF SYMBOLS ABBREVATIONS AND NOMENCLATURE

C.S. CROSS SECTION

AVR AUTOMATIC VOLTAGE REGULATOR

PMG PERMANENT MAGNETIC GENERATOR

VPI VACUUM PRESSURIZED IMPREGNATION

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1.1 ACKNOWLEDGEMENTS

The successful completion of any task would be incomplete without greeting those who made it

possible and whose guidance and encouragement made our effort success.

With profound gratitude, respect and pride, we express our sincere thanks to Sri.

Ramesh Babu, Secretary and correspondent of our college for providing necessary facilities for

doing the project.

We wish to express our gratitude to Dr. M. Muralidhara rao, our principal, for

having given us permission to carry out the project.

We express our deep sense of gratitude to Sri. P.V.V. Satya Narayana, Head of Dept.

of Electrical Engineering for his learned suggestions and encouragement which made this project a

success. We express sincere thanks to our internal guide Mr. T. Ravi, Asst prof, Department of

Electrical and Electronics Engineering for his encouragement, which made this project a success.

We express our earnest thanks to Mr. R. K. Manohar our external project guide who

had given valuable suggestions throughout our project. Finally we thank every one who directly or

indirectly helped for our project.

-Project Associates.

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1.2 PROFILE OF B.H.E.L.

Bharat Heavy Electrical Limited (BHEL) is today the largest engineering enterprise of India

with an excellent track record of performance. Its first plant was set up at Bhopal in 1956 under

technical collaboration with M/s. AEI, UK followed by three more major plants at Haridwar,

Hyderabad and Tiruchirapalli with Russian and Czechoslovak assistance.

These plants have been at the core of BHEL’s efforts to grow and diversify and become

India’s leading engineering company. The company now has 14 manufacturing divisions, 8 service

centers and 4 power sector regional centers, besides project sites spread all over India and abroad and

also regional operations divisions in various state capitals in India for providing quick service to

customers.

BHEL manufactures over 180 products and meets the needs of core-sectors like power,

industry, transmission, transportation (including railways), defense, telecommunications, oil business,

etc. Products of BHEL make have established an enviable reputation for high quality and reliability.

BHEL has installed equipment for over 62,000 MW of power generation-for Utilities, Captive

and Industrial users. Supplied 2,00,000 MVA transformer capacity and sustained equipment

operating in Transmission & Distribution network up to 400kV – AC & DC, Supplied over 25,000

Motors with Drive Control System Power projects. Petrochemicals, Refineries, Steel, Aluminium,

Fertiliser, Cement plants etc., supplied Traction electric and AC/DC Locos to power over 12,000 Km

Railway network.

Supplied over one million Valves to Power Plants and other Industries. This is due to the

emphasis placed all along on designing, engineering and manufacturing to international standards by

acquiring and assimilating some of the best technologies in the world from leading companies in

USA, Europe and Japan, together with technologies from its-own R & D centers BHEL has acquired

ISO 9000 certification for its operations and has also adopted the concepts of Total Quality

Management (TQM).

BHEL presently has manufactured Turbo-Generators of ratings up to 560 MW and is in the

process of going up to 660 MW. It has also the capability to take up the manufacture of ratings unto

1000 MW suitable for thermal power generation, gas based and combined cycle power generation as-

well-as for diverse industrial applications like Paper, Sugar, Cement, Petrochemical, Fertilizers,

Rayon Industries, etc. Based on proven designs and know-how backed by over three decades of

experience and accreditation of ISO 9001. The Turbo-generator is a product of high-class

workmanship and quality. Adherence to stringent quality-checks at each stage has helped BHEL to

secure prestigious global orders in the recent past from Malaysia, Malta, Cyprus, Oman, Iraq,

Bangladesh, Sri Lanka and Saudi Arabia. The successful completion of the various export projects in

a record time is a testimony of BHEL’s performance.

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Established in the late 50’s, Bharat Heavy Electrical Limited (BHEL) is, today, a name to

reckon with in the industrial world. It is the largest engineering and manufacturing enterprises of its

kind in India and is one of the leading international companies in the power field. BHEL offers over

180 products and provides systems and services to meet the needs of core sections like: power,

transmission, industry, transportation, oil & gas, non-conventional energy sources and

telecommunication. A wide-spread network of 14 manufacturing divisions, 8 service centers and 4

regional offices besides a large number of project sites spread all over India and abroad, enables

BHEL to be close to its customers and cater to their specialized needs with total solutions-efficiently

and economically. An ISO 9000 certification has given the company international recognition for its

commitment towards quality. With an export presence in more than 50 countries BHEL is truely

India’s industrial ambassador to the world.

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1.3 PREFACE

Power is the basic necessity for economic development of a country. The production

of electrical energy and its per capital consumption is deemed as an index of standard of living in a

nation in the present day civilization. Development of heavy or large-scale industries, as well as

medium scale industries, agriculture, transportation etc, totally depend on electrical power resources

of engineers and scientists to find out ways and means to supply required power at cheapest rate. The

per capital consumption on average in the world is around 1200KWH, the figure is very low for our

country and we have to still go ahead in power generation to provide a decent standard of living for

people.

An AC generator is a device, which converts mechanical energy to electrical

energy. The alternator as it is commonly called works on the principle of ‘Electro Magnetic

Induction’. Turbo generators are machines which can generate high voltages and capable of

delivering KA of currents .so the designer should be cautious in designing the winding insulation. So

insulation design plays a major role on the life of the Turbo Generator. In our project we deal with the

“Manufacture process of turbo generator and its insulation design by VPI process.”

The first half of project is concerned with the aspects of generator manufacturing

comprising of stator manufacturing, in a step by step procedure involving different stages, and the

latter stage includes the insulation design of the generator by VPI process in a detailed manner, which

completes the generator design.

We more over stress mainly on VPI insulation process. Before going deep into the

topic we will start with a brief introduction and we conclude with the recent trends in the insulation

systems used all over the world.

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2. INTRDUCTION

Electrical insulating materials are defined as materials that offer a large resistance to

the flow of current and for that reason they are used to keep the current in its proper path i.e. along

the conductor. Insulation is the heart of the generator. Since generator principle is based on the

induction of e.m.f in a conductor when placed in a varying magnetic field. There should be proper

insulation between the magnetic field and the conductors. For smaller capacities of few KW, the

insulation may not affect more on the performance of the generator but for larger capacities of few

MW (>100MW) the optimization of insulation is an inevitable task, moreover the thickness of

insulation should be on par with the level of the voltage, also non homogenic insulation provisions

may lead to deterioration where it is thin and prone to hazardous short circuits, also the insulating

materials applied to the conductors are required to be flexible and have high specific (dielectric)

strength and ability to withstand unlimited cycles of heating and cooling.

Keeping this in view among other insulating materials like solids gases etc liquid

dielectrics are playing a major role in heavy electrical equipment where they can embedded deep into

the micro pores and provide better insulating properties. Where as solid di-electrics provide better

insulation with lower thickness and with greater mechanical strength. So the process of insulation

design which has the added advantage of both solid and liquid dielectrics would be a superior process

of insulation design. One such process which has all the above qualities is the VPI (vacuum

pressurised impregnation) process and has proven to be the best process till date.

2.1 Drawbacks of Early VPI Process:

DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the

year 1956. It has been used for many years as a basic process for thorough filling of all interstices in

insulated components, especially high voltage stator coils and bars. Prior to development of

thermosetting resins, the widely used insulation system for 6.6kv and higher voltages was a VPI

system in which, Bitumen Bonded Mica Flake Tape is used as main ground insulation. The

bitumen is heated up to about 180C to obtain low viscosity which aids thorough impregnation.

To assist penetration, the pressure in the autoclave was raised to 5 or 6 atmospheres. After

appropriate curing and calibration, the coils or bars were wound and connected up in the normal

manner. These systems performed satisfactorily in service provided they were used in their thermal

limitations.

In the late 1930’s and early 1940’s, however, many large units, principally turbine generators,

failed due to inherently weak thermoplastic nature of bitumen compound.

Failures were due to two types of problems:

a. Tape separation

b. Excessive relaxation of the main ground insulation.

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Much development work was carried out to try to produce new insulation systems, which

didn’t exhibit these weaknesses.

The first major new system to overcome these difficulties was basically a fundamental

Improvement to the classic Vacuum Pressure Impregnation process:

Coils and bars were insulated with dry mica flake tapes, lightly bonded with synthetic resin

and backed by a thin layer of fibrous material. After taping, the bars or coils were vacuum dried and

pressure impregnated in polyester resin. Subsequently, the resin was converted by chemical action

from a liquid to a solid compound by curing at an appropriate temperature, e.g. 150C. this so called

thermosetting process enable coils and bars to be made which didn’t relax subsequently when

operating at full service temperature. By building in some permanently flexible tapings at the evolutes

of diamond shaped coils, it was practicable to wind them without difficulty. Thereafter, normal slot

packing, wedging, connecting up and bracing procedures were carried out. Many manufacturers for

producing their large coils and bars have used various versions of this Vacuum Pressure Impregnation

procedure for almost 30 years.

The main differences between systems have been used is in the type of micaceous tapes used

for main ground insulation and the composition of the impregnated resins. Although the first system

available was styrenated polyester, many developments have taken place during the last two decades.

Today, there are several different types of epoxy, epoxy-polyester and polyester resin in common use.

Choice of resin system and associated micaceous tape is a complex problem for the machine

manufacturer.

Although the classic Vacuum Pressure Impregnation technique has improved to a significant

extent, it is a modification to the basic process, which has brought about the greatest change in the

design and manufacture of medium-sized A.C industrial machines. This is the Global Impregnation

Process. Using this system, significant increases in reliability, reduction in manufacturing costs and

improved output can be achieved.

2.2 Advantage of present resin poor VPI process: VPI is a process, which is a step above the conventional vacuum system. VPI includes

pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is

improved mechanical strength and electrical properties. With the improved penetration, a void free

coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution,

the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the

average rise.

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In order to minimize the overall cost of the machine & to reduce the time cycle of the

insulation system vacuum pressure Impregnated System is used. The stator coils are taped with

porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator

is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin

bath under pressure of Nitrogen gas.

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3 Introduction to various parts of a Generator:The manufacturing of a generator involves in manufacturing of all the parts of the

generator separately as per the design requirements and assembling them for the operation. It is worth

knowing the parts of the Turbo Generator. Usually for larger generators the assembling is done at the

generator installation area in order to avoid the damage due to mechanical stresses during

transportation, also this facilitates easy transportation. Let us have a view about various parts of a

turbo generator. Parts of a turbo generator:

1. Stator

2. Rotor

3. Excitation system

4. Cooling system

5. Insulation system

6. Bearings

3.1 STATOR:

3.1.1 STATOR FRAMEThe stator frame is of welded steel single piece construction. It supports the laminated core

and winding. It has radial and axial ribs having adequate strength and rigidity to minimise core

vibrations and suitably designed to ensure efficient cooling. Guide bards are welded or bolted inside

the stator frame over which the core is assembled. Footings are provided to support the stator

foundation.

3.1.2 STATOR CORE The stator core is made of silicon steel sheets with high permeability and low hysteresis and

eddy current losses. The sheets are suspended in the stator frame from insulated guide bars.

Stator laminations are coated with synthetic varnish; are stacked and held between sturdy steel

clamping plates with non-magnetic pressing fingers, which are fastened or welded to the stator frame.

In order to minimize eddy current losses of rotating magnetic flux which interacts with the

core, the entire core is built of thin laminations. Each lamination layer is made of individual

segments.

The segments are punched in one operation from electrical sheet steel lamination having high

silicon content and are carefully deburred. The stator laminations are assembled as separate cage

core without the stator frame. The segments are staggered from layer to layer so that a core of high

mechanical strength and uniform permeability to magnetic flux is obtained. On the outer

circumference the segments are stacked on insulated rectangular bars, which hold them in position.

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To obtain optimum compression and eliminate looseness during operation the laminations are

hydraulically compressed and heated during the stacking procedure. To remove the heat, spaced

segments are placed at intervals along the core length, which divide the core into sections to provide

wide radial passages for cooling air to flow.

The purpose of stator core is

1. To support the stator winding.

2. To carry the electromagnetic flux generated by rotor winding.

So selection of material for building up of core plays a vital role.

3.1.3 STATOR WINDING: The stator winding is a fractional pitch two layer type, it consisting of individual bars.

The bars are located in slots of rectangular cross section which are uniformly distributed on the

circumference of the stator core.

In order to minimize losses, the bars are compared of separately insulated strands which

are exposed to 360.degrees transposing

To minimize the stator losses in the winding, the strands of the top and bottom bars are

separately brazed and insulated from each other.

3.2 ROTOR:

3.2.1 ROTOR SHAFT:Rotor shaft is a single piece solid forging manufactured from a vacuum casting. Slots

for insertion of field winding are milled into the rotor body. The longitudinal slots are distributed

over the circumference. So that solids poles are obtained. To ensure that only high quality forgings

are used, strengthen test, material analysis and ultrasonic tests are performed during manufacture of

the rotor. After completion, the rotor is based in various planes at different speeds and then subjected

to an over speed test at 120% of rated speed for two minutes.

3.2.2. ROTOR WINDING AND RETAINING RINGS: The rotor winding consisting of several coils, which are inserted into the slots and

series connected such that two coils groups from one pole. Each coil consists of several connected

turns, each of which consists of two half turns which are connected by brazing in the end section. The

individual turns of the coils are insulated against each other, the layer insulation L-shaped strips of

lamination epoxy glass fibre with nomax filler are used for slot insulation. The slot wedges are made

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of high electrical conductivity material and thus act as damper winding. At their ends the slots

wedges are short circuited through the rotor body.

The centrifugal forces of the rotor end winding are contained by single piece of non

magnetic high strengthen steel in order to reduce stray losses, each retaining rings with its shrinks

fitted insert ring is shrunk into the rotor body in an overhang position. The retaining rings are secured

in the axial position by a snap ring.

Figure 1: Photograph of a small round rotor. The retaining rings are at the each end of the rotor.

3.3 FIELD CONNECTION AND MULTICONTACTS: The field current is supplied to the rotor through multi contact system arranged at the

exciter side shaft end.

3.3.1 BEARINGS: The generator rotor is supported in two sleeve bearings. To eliminate shaft current the

exciter and bearing is insulated from foundation plate and oil piping.

The temperature of each bearing is maintained with two RTD’s (Resistance Temperature

Detector) embedded in the lower bearing sleeve so that the ensuring point is located directly below

the Babbitt. All bearings have provisions for fitting vibration pick up to monitor shaft vibrations.

The oil supply of bearings is obtained from the turbine oil system.

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3.4 EXCITATION SYSTEM:In all industrial applications, the electrical power demand is ever increasing. This

automatically demands for the design, development and construction of increasingly large capacity

Synchronous generators. These generators should be highly reliable in operation to meet the demand.

This calls for a reliable and sophisticated mode of excitation system.

When the first a.c generators were introducing a natural choice for the supply of field systems

was the DC exciter. DC exciter has the capability for equal voltage output of either polarity, which

helps in improving the generator transient performance. DC exciters, how ever, could not be adopted

for large ratings because of the problems in the design commutator and brush gear, which is

economically unattractive. Of –course, the problems are not uncommon in power stations but Of the

environment with sulphur vapours, acidic fumes as in the cases of petrochemical and fertilizer

industries, exposure of DC exciter. This adds to the problem of design.

Types of a.c exciters are:

(1) High frequency excitation

(2) Brush less excitation

(3) Static excitation

The high frequency D.C exciter is a specially designed “inductor type alternator” with no

winding on its rotor. It is designed to operate at high frequency to reduce the size of the rotor; the a.c

exciter was very reliable in operation. Though this system eliminates all problems associated with

commutator, it is not free from problems attributable to slip rings and its brush gear. Thus brushless

excitation system was introduced.

The BL exciter consists of field winding on the stator. This system proved to be highly reliable

and required less maintenance. Absence of power cables and external ac power supplies males the

system extremely reliable. The problem associated with brushes like fast wear out of brush, sparkling

etc, are eliminated.

This suffers from the disadvantage of lack of facility for field suppression in the case of an

internal fault in generator.

The system comprises shaft driven AC exciter with rotating diodes.

3.5 PERMANENT MAGNET GENERATOR AND AVR:

This system is highly reliable with least maintenance and is ideally suitable for gas driven

generators.

The static excitation system was developed contemporarily as an alternative to brush less

excitation system. This system was successfully adapted to medium and large capacity Turbo

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generators. Though the system offers very good transient performance, the problems associated with

slip rings and brush gear system are still present.

This system consists of rectifier transformer, thyristor converts, field breaker and AVR. This

system is ideally suitable where fast response is called for. The system is flexible in operation and

needs very little maintenance.

Thus, each excitation system has its own advantages and disadvantages. The selection of

system is influenced by the transient response required, nature of pollution and pollution level in the

power plant and cost of equipment.

Exciters are those components, which are used for giving high voltage to the generator during

the start up conditions. The main parts that are included in the exciter assembly are:

(1) Rectifier wheels

(2) Three phase main exciter

(3) Three phase pilot exciter

(4) Metering and supervisory equipment

3.5.1 RECTIFIER WHEELS:The main components of the rectifier wheels are Silicon Diodes, which are arranged in the

rectifier wheels in a three-phase bridge circuit. The internal arrangement of diode is such that the

contact pressure is increased by centrifugal force during rotation.

There are some additional components contained in the rectified wheels. One diode each is

mounted in each light metal heat sink and then connected in parallel. For the suppression of

momentary voltage peaks arising from commutation, RC blocks are provided in each bridge in

parallel with one set of diodes. The rings from the positive shrunk on to the shaft. This makes the

circuit connections minimum and ensures accessibility of all the elements.

3.5.2 THREE PHASE PILOT EXCITER:The three phase pilot exciter is a six-pole revolving field unit; the frame accommodates the

laminated core with the three-phase winding. The rotor consists of a hub with poles mounted on it.

Each pole consists of separate permanent magnets, which are housed, in non-metallic enclosures. The

magnets are placed between the hub and the external pole shoe with bolts. The rotor hub is shrunk on

to the free shaft end.

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3.5.3 THREE PHASE MAIN EXCITER:Three phases main exciter is a six-pole armature unit; the poles are arranged in the frame with

the field and damper winding. The field winding is arranged on laminated magnetic poles. At the pole

shoe, bars are provided which are connected to form a damper winding.

The rotor consists of stacked laminations, which are compressed through bolts over

compression rings. The three- phase winding is inserted in the slots of the laminated rotor. The

winding conductors are transposed with in the core length and end turns of the rotor windings are

secure with the steel bands. The connections are made on the side facing of the rectifier wheels. After

full impregnation with the synthetic resin and curing, the complete rotor is shrunk on to the shaft.

3.5.4 AUTOMATIC VOLTAGE REGULATOR:The general automatic voltage regulator is fast working solid thyristor controlled equipment.

It has two channels, one is auto channel and the other is manual. The auto channel is used for the

voltage regulation and manual channel is used for the current regulation. Each channel will have its

own firing for reliable operation.

The main features of AVR are:

(1) It has an automatic circuit to control outputs of auto channel and manual channel and

reduces disturbances at the generator terminals during transfer from auto regulation to

manual regulation.

(2) It is also having limiters for the stator current for the optimum utilization of lagging

and leading reactive capabilities of turbo generator.

(3) There will be automatic transfer from auto regulation to manual regulation in case do

measuring PT fuse failure or some internal faults in the auto channel.

(4) The generator voltage in both channels that is in the auto channel and the manual

channel can be controlled automatically.

3.5.5 COOLING SYSTEM: Cooling is one of the basic requirements of any generator. The effective working of generator

considerably depends on the cooling system. The insulation used and cooling employed is inter-

related.

The losses in the generator dissipates as the heat, it raises the temperature of the generator. Due

to high temperature, the insulation will be affected greatly. So the heat developed should be cooled to

avoid excessive temperature raise. So the class of insulation used depends mainly on cooling system

installed.

There are various methods of cooling, they are:

a. Air cooling- 60MW

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b. Hydrogen cooling-100MW

c. Water cooling –500MW

d. H 2 & Water cooling – 1000MW

Hydrogen cooling has the following advantages over Air-cooling:

1. Hydrogen has 7 times more heat dissipating capacity.

2. Higher specific heat

3. Since Hydrogen is 1/14th of air weight. It has higher compressibility

4. It does not support combustion.

DISADVANTAGES:

1. It is an explosive when mixes with oxygen.

2. Cost of running is higher.

Higher capacity generators need better cooling system.

3.6 VARIOUS LOSSES IN A GENERATOR In generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the generator, are considered as losses and may be defined as follows:

3.6.1 Copper loss in the winding.

3.6.2 Magnetic Losses.

3.6.3 Mechanical Losses

3.6.1 Copper loss:

The power lost in the form of heat in the armature winding of a generator is known as Copper loss. Heat is generated any time current flows in a conductor.

I2R loss is the Copper loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire. These includes rotor copper losses and Stator copper losses

3.6.2 Magnetic Losses (also known as iron or core losses)(i) Hysteresis loss (Wh) Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated Silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.

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(ii) Eddy Current Loss (We):The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss. Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Figure, view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to cross-sectional area.) The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core.

B 1: Circuit showing flow of eddy currents in a rotor with and without laminations

As you can see, eddy current losses are kept low when the core material is made up of many thin

sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The

laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by

the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are

being annealed. The insulation value need not be high because the voltages induced are very small.

Most generators use armatures with laminated cores to reduce eddy current losses.

These magnetic losses are practically constant for shunt and compound-wound generators, because in

their case, field current is constant.

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3.6.3 Mechanical or Rotational Losses:

These consist of

(i) friction loss at bearings.

(ii) Air-friction or windage loss of rotating rotor armature.

These are about 10 to 20% of F.L losses.

Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean

bearings and proper lubrication are essential to the reduction of bearing friction. Brush friction is

reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush

tension.

Usually, magnetic and mechanical losses are collectively known as Stray Losses. These are also

known as rotational losses for obvious reasons.

As mentioned above, these losses are responsible for the rise in temperature of the generator

body hence an appropriate insulation should be used. Also the insulation should withstand the

generator voltage and currents. So an insulation whose breakdown voltage is of 5 to 6 times the

normal voltage is taken as Safety factor.

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4. MANUFACTURE OF GENERATOR

Various stages in generator manufacturing:In our project we have a detail study of only stator, rotor and the insulation system

used for it. The parts excitation system, cooling system and bearings are external to the generator and

are treated as a completed one and are out of scope of our record. Now, generator manufacturing can

be broadly divided into three main parts:

1. Stator manufacture

2. Rotor manufacture

3. Exciter manufacture

The various stages involved in the generator manufacture and their sub processes are shown in

the flow diagram given below. This facilitates manufacture erection and transport of the stator.

B.3: flow diagram showing various stages involved in generator manufacture.

Now these sub processes are explained in detail below. Let us start with Stator.

4.1 STATOR MANUFACTURE PROCESS:This stator manufacturing is a combination of two individual sub processes, namely

Stator core construction and

Coil construction and their assembly.

4.1.1 STATOR CORE CONSTRUCTION:

The stator core isn’t a solid iron type and is the assembly of strips of laminations. As the

reasons are explained in the Section 3.1 in the introduction to stator.4.1.2 PREPARATION OF STATOR LAMINATIONS

As explained above, stator laminations are the important parts of the stator core and they

should be manufactured as per the design requirements and involves the following sub processes.

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4.1.3 RECEPTION OF SILICON STEEL ROLLS:

The silicon steel rolls received are checked for their physical, chemical, mechanical and

magnetic properties as per the specifications mentioned above.

In order to reduce the Hysterisis loss, silicon alloyed steel, which has low Hysterisis

constant is used for the manufacture of core. The composition of silicon steel is

Steel - 95.8 %

Silicon - 4.0 %

Impurities- 0.2 %

From the formula for eddy current loss it is seen that eddy current loss depends on the

thickness of the laminations. Hence to reduce the eddy current loss core is made up of thin

laminations which are insulated from each other. The thickness of the laminations is about 0.5

mm. The silicon steel sheets used are of COLD ROLLED NON-GRAIN ORIENTED

(CRANGO) type as it provides the distribution of flux throughout the laminated sheet.

4.1.4 SHEARING: The cold rolled non grained oriented (CRNGO) steel sheets are cut to their outer

periphery to the required shapes by feeding the sheet into shearing press. For high rating

machines each lamination is build of 6 sectors (stampings), each of 60 cut according to the

specifications.

4.1.5 BLANKING AND NOTCHING:

Press tools are used in making the core bolt holes and other notches for the laminations.

Press tools are mainly of two types.

i. Compound notching tools.

ii. Individual notching tools.

4.1.6 COMPOUND OPERATION:

In this method the stamping with all the core bolt holes, guiding slots and winding

slots is manufactured in single operation known as Compound operation and the press tool

used is known as Compounding tool. Compounding tools are used for the machines rated

above 40 MW. Nearly 500 tons crank press is used for this purpose.

4.1.7 INDIVIDUAL OPERATIONS:

In case of smaller machines the stampings are manufactured in two operations. In the

first operation the core bolt holes and guiding slots are only made. This operation is known as

Blanking and the tools used are known as Blanking tools. In the second operation the

winding slots are punched using another tool known as Notching tool and the operation is

called Notching.

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4.1.8 DEBURRING OPERATION:

In this operation the burrs in the sheet due to punching are deburred. There are chances

of short circuit within the laminations if the burrs are not removed. The permissible is about 5

micrometer. For deburring punched sheets are passed under rollers to remove the sharp burs

of edges.

B4: Figure showing the shape of laminations after the completion of notching and

deburring operations.

4.1.9 VARNISHING:

Depending on the temperature withstand ability of the machine the laminations are

coated by varnish which acts as insulation. The lamination sheets are passed through

conveyor, which has an arrangement to sprinkle the varnish, and a coat of varnish is obtained.

The sheets are dried by a series of heaters at a temperature of around 260 – 350 oC. Two

coatings of varnish are provided in the above manner till 12-18 micrometer thickness of coat

is obtained. Here instead of pure varnish a mixture of Tin and Varnish is used such that the

mixture takes 44sec to empty a DIN4 CUP.

The prepared laminations are subjected to following tests.

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i) Xylol test - To measure the chemical resistance.

ii) Mandrel test - When wound around mandrel there should not be any cracks.

iii) Hardness test - Minimum 7H pencil hardness.

iv) IR value test - For 20 layers of laminations insulation resistance should not be less than

1M.

5. STATOR CORE ASSEMBLY:

5.1 TRAIL PACKET ASSEMBLY:Clamping plate is placed over the assembly pit; stumbling blocks are placed between the

clamping plates and the assembly pit. Clamping plate is made parallel to the ground by checking with

the spirit level. One packet comprising of 0.5 mm thickness silicon steel laminations is assembled

over the clamping plates by using mandrels and assembly pit .after assembling one packet thickness

of silicon laminations, inner diameter of the core is checked as per the drawing also the slot freeness

is checked with inspection drift .There should not be any projections inside or outside the slot. If all

the conditions are satisfied the normal core assembly is carried out by dismantling the trial packets.

5.2 NORMAL CORE ASSEMBLY

5.2.1 Stepped packed assembly: Stepped packets are assembled from the clamping plate isolating each packet with ventilation

laminations up to 4 to 5 packets of thickness 10cms for an air cooled turbo generator of 120MW.

5.2.2 Normal packet assembly: Normal packet assembly is carried out using 0.5 mm silicon steel laminations up to

required thickness of 30mm by using mandrills and inspection drift after normal packet assembly

completion, one layer of HGL laminations are placed and one layer of ventilation lamination are

placed over them. Then again normal packet assembly is carried as above. The thickness of each

lamination is 0.5 mm and the thickness of lamination separating the packets is about 1 mm. The

lamination separating each packet has strips of nonmagnetic material that are welded to provide radial

ducts. The segments are staggered from layer to layer so that a core of high mechanical strength and

uniform permeability to magnetic flux is obtained. Stacking mandrels and bolts are inserted into the

windings slot bores during stacking provide smooth slot walls.

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5.2.2.1 In process pressings To obtain the maximum compression and eliminate under setting during operation, the

laminations are hydraulically compressed and heated during the stacking procedure when certain

heights of stacks are reached.

The packets are assembled as above up to 800mm as above and 1 st pressing is carried

using hydraulic jacks up to 150kg/cm2 and the pressing is carried out for every 800mm and a pre final

pressing is done before the core length almost reach the actual core. Now the core is tested for the

design specifications and the compensation is done by adding or removing the packets.

5.2.2.2 Fitting of clamping bolts: The complete stack is kept under pressure and locked in the frame by means of clamping

bolts and pressure plates. The clamping bolts running through the core are made of nonmagnetic steel

and are insulated from the core and the pressure plates to prevent them from short circuiting the

laminations and allowing the flow of eddy currents.

The pressure is transmitted from the clamping plates to the core by clamping fingers. The

clamping fingers extend up to the ends of the teeth thus, ensuring a firm compression in the area of

the teeth. The stepped arrangement of the laminations at the core ends provides an efficient support

to tooth portion and in addition contributes to the reduction of stray load losses and local heating in

that area due to end leakage flux.

The clamping fingers are also made of non-magnetic steel to avoid eddy-current losses. After

compression and clamping of core the rectangular core key bars are inserted into the slots provided in

the back of the core and welded to the pressure plates. All key bars, except one, are insulated from

the core to provide the grounding of the core.

WINDINGThe next important consideration is winding. The stator winding and rotor winding consist of several

components, each with their own function. Furthermore, different types of machines have different

components. Stator windings are discussed separately below.

6. Stator WindingThere are three main components in a stator, they are

a. copper conductors (although aluminum is sometimes used).

b. The stator core.

c. Insulation.

6.1 Conducting material used in coil manufacturing:

Copper material is used to make the coils. This is because

i) Copper has high electrical conductivity with excellent mechanical properties

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ii) Immunity from oxidation and corrosion

iii) It is highly malleable and ductile metal.

6.2 TYPES OF CONDUCTOR COILS:

Basically there are three types of stator winding structures employed over the range from 1 KW to

1000 MW.

1. Random wound stators.

2. Form-wound stators using multi turn coils.

3. Form-wound stators using Roebel bars.

Out of these, two types of coils are manufactured and used in BHEL, Hyderabad.

1) Diamond pulled multi-turn coil (full coiled).

2) Roebel bar (half-coil).

In general, random-wound stators are typically used for machines less than several hundred

KW. Form-wound coil windings are used in most large motors and many generators rated up to 50 to

100 MVA. Roebel bar windings are used for large generators. Although each type of construction is

described below, some machine manufacturers have made hybrids that do not fit easily into any of the

above categories; these are not discussed in the project.

Generally in large capacity machines ROEBEL bars are used. These coils were constructed

after considering the skin effect losses. In the straight slot portion, the conductors or strips are

transposed by 360 degrees.

The transposition is done to ensure that all the strips occupy equal length under similar

conditions of the flux. The transposition provides for a mutual neutralization of the voltages induced

in the individual strips due to the slot cross field and ensures that no or only small circulating currents

exists in the bar interior. Transposition also reduced eddy current losses and helps in obtaining

uniform e.m.f. More about transposition is discussed later in the section with diagrammatic quote.

The copper is a conduit for the stator winding current. In a generator, the stator output current

is induced to flow in the copper conductors as a reaction to the rotating magnetic field from the rotor.

In a motor, a current is introduced into the stator, creating a rotating magnetic field that forces the

rotor to move. The copper conductors must have a cross section large enough to carry all the current

required without overheating.

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B5 schematic diagram for a three –phase Y-connected stator winding with two parallel

conductors per phase

The diagram shows that each phase has one or more parallel paths for current flow. Multiple

parallels are often necessary since a copper cross section large enough to carry the entire phase

current may result in an uneconomic stator slot size. Each parallel consists of a number of coils

connected in series. For most motors and small generators, each coil consists of a number of turns of

copper conductors formed into a loop. The rationale for selecting the number of parallels, the number

of coils in series, and the number of turns per coil in any particular machine is beyond the scope of

our project.

The stator core in a generator concentrates the magnetic field from the rotor on the copper

conductors in the coils. The stator core consists of thin sheets of magnetic steel (referred to as

laminations). The magnetic steel acts as a low-reluctance (low magnetic impedance) path for the

magnetic fields from the rotor to the stator, or vice versa for a motor. The steel core also prevents

most of the stator winding magnetic field from escaping the ends of the stator core, which would

cause currents to flow in adjacent conductive material.

7. Electrical Insulation:The final major component of a stator winding is the electrical insulation. Unlike copper

conductors and magnetic steel, which are active components in making a motor or generator function

the insulation is passive. That is, it does not help to produce a magnetic field or guide its path.

Generator and motor designers would like nothing better than to eliminate the electrical insulation,

since the insulation increases machine size and cost, and reduces efficiency, without helping to create

any torque or current. Insulation is “overhead,” with a primary purpose of preventing short circuits

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between the conductors or to ground. However, without the insulation, copper conductors would

come in contact with one another or with the grounded stator core, causing the current to flow in

undesired paths and preventing the proper operation of the machine. In addition, indirectly cooled

machines require the insulation to be a thermal conductor, so that the copper conductors do not

overheat.

The insulation system must also hold the copper conductors tightly in place to prevent

movement. The stator winding insulation system contains organic materials as a primary constituent.

In general, organic materials soften at a much lower temperature and have a much lower mechanical

strength than copper or steel. Thus, the life of a stator winding is limited most often by the electrical

insulation rather than by the conductors or the steel core. Furthermore, stator winding maintenance

and testing almost always refers to testing and maintenance of the electrical insulation.

B6 Photographs of end windings and slots of random wound stator (Courtesy TECO Westing

house)

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B7 Photograph of a form wound stator winding (courtesy TECO Westing house)

High purity (99%) copper conductors/strips are used to make the coils. This results in high

strength properties at higher temperatures so that deformations due to the thermal stresses are

eliminated.

B8 A single form wound coil being inserted into two slots

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7.1 STATOR WINDING INSULATION SYSTEM FEATURESThe stator winding insulation system contains several different components and features which together ensure that electrical shorts do not occur, that the heat from the conductor I2R losses are transmitted to a heat sink, and that the conductors do not vibrate in spite of the magnetic forces.The basic stator insulation system components are the:1. Strand (or sub conductor) insulation2. Turn insulation3. Ground wall (or ground or earth) insulation

Figures 1.8 and 1.9 show cross sections of random-wound and form-wound coils in a stator slot, and identify the above components. Note that the form-wound stator has two coils per slot; this is typical.

Figure 1.10 is a photograph of the cross section of a multi-turn coil. In addition to the main insulation components, the insulation system sometimes has high-voltage stress-relief coatings and end-winding support components.The following sections describe the purpose of each of these components. The mechanical, thermal, electrical, and environmental stresses that the components are subjected to are also described.

7.1.1 Strand InsulationIn random-wound stators, the strand insulation can function as the turn insulation, although extra sleeving is sometimes applied to boost the turn insulation strength in key areas. Many form-wound machines employ separate strand and turn insulation. The following mainly addresses the strand insulation in form-wound coils and bars. Strand insulation in random wound machines will be discussed as turn insulation. Section 1.4.8 discusses strand insulation in its role as transposition insulation. There are both electrical and mechanical reasons for stranding a conductor in a form wound coil or bar. From a mechanical point of view, a conductor that is big enough to carry the current needed in the coil or bar for a large machine will have a relatively large cross-sectional area. That is, a large conductor cross section is needed to achieve the desired ampacity. Such a large conductor is difficult to bend and form into the required coil/bar shape. A conductor formed from smaller strands (also called sub-conductors) is easier to bend into the required shape than one large conductor.

B9 C.S of a random stator winding slot

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From an electrical point of view, there are reasons to make strands and insulate them from one

another. It is well known from electromagnetic theory that if a copper conductor has a large enough

cross-sectional area, the current will tend to flow on the periphery of the conductor. This is known as

the skin effect. The skin effect gives rise to a skin depth through which most of the current flows.

The skin depth of copper is 8.5 mm at 60 Hz. If the conductor has a cross section such that the

thickness is greater than 8.5 mm, there is a tendency for the current not to flow through the center of

the conductor, which implies that the current is not making use of all the available cross section. This

is reflected as an effective AC resistance that is higher than the DC resistance. The higher AC

resistance gives rise to a larger I2R loss than if the same cross section had been made from strands

that are insulated from one another to prevent the skin effect from occurring. That is, by making the

required cross section from strands that are insulated from one another, all the copper cross section is

used for current flow, the skin effect is negated, and the losses are reduced.

In addition, Eddy current losses occur in solid conductors of too large a cross section. In the slots, the

main magnetic field is primarily radial, that is, perpendicular to the axial direction. There is also a

small circumferential (slot leakage) flux that can induce eddy currents to flow. In the end-winding, an

axial magnetic field is caused by the abrupt end of the rotor and stator core. This axial magnetic field

can be substantial in synchronous machines that are under-excited.

By Ampere’s Law, or the ‘right hand rule’, this axial magnetic field will tend to cause a current to

circulate within the cross section of the conductor (Figure 1.11). The larger the cross sectional area,

the greater the magnetic flux that can be encircled by a path on the periphery of the conductor, and

the larger the induced current. The result is a greater I2R loss from this circulating current. By

reducing the size of the conductors, there is a reduction in stray magnetic field losses, improving

efficiency.

The electrical reasons for stranding require the strands to be insulated from one another. The voltage

across the strands is less than a few tens of volts; therefore, the strand insulation can be very thin. The

strand insulation is subject to damage during the coil manufacturing process, so it must have good

mechanical properties. Since the strand insulation is immediately adjacent to the copper conductors

that are carrying the main stator current, which produces the I2R loss, the strand insulation is exposed

to the highest temperatures in the stator. Therefore, the strand insulation must have good thermal

properties. Section 7.1.1 describes in detail the strand insulation materials that are in use. Although

manufacturers ensure that strand shorts are not present in a new coil, they may occur during service

due to thermal or mechanical aging. A few strand shorts in form-wound coils/bars will not cause

winding failure, but will increase the stator winding losses and cause local temperature increases due

to circulating currents.

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B10 C.S of a form wound multi-turn slots containing form wound multi-turn coils.

7.1.2 Turn Insulation

The purpose of the turn insulation in both random- and form-wound stators is to prevent shorts

between the turns in a coil. If a turn short occurs, the shorted turn will appear as the secondary

winding of an autotransformer. If, for example, the winding has 100 turns between the phase terminal

and neutral (the “primary winding”), and if a dead short appears across one turn (the “secondary”),

then 100 times normal current will flow in the shorted turn. This follows from the transformer law:

npIp = nsIs (1.1)

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Where n refers to the number of turns in the primary or secondary, and I is the current in the

primary or secondary. Consequently, a huge circulating current will flow in the faulted turn, rapidly

overheating it. Usually, this high current will be followed quickly by a ground fault due to melted

copper burning through any ground-wall insulation. Clearly, effective turn insulation is needed for

long stator winding life.

B11 C.S of a form wound multi-turn slots directly cooled roebel bars

The power frequency voltage across the turn insulation in a random-wound machine can range

up to the rated phase-to-phase voltage of the stator because, by definition,

the turns are randomly placed in the slot and thus may be adjacent to a

phase-end turn in another phase, although many motor manufacturers may

insert extra insulating barriers between coils in the same slot but in

different phases and between coils in different phases in the end-windings.

Since random winding is rarely used on machines rated more than 600 V

(phase-to-phase), the turn insulation can be fairly thin. However, if a

motor is subject to high-voltage pulses, especially from modern inverter-

fed drives (IFDs), inter-turn voltage stresses that far exceed the normal

maximum of 600 Vac can result. These high-voltage pulses give rise to

failure mechanisms, as discussed in Section 8.7.

The power frequency voltage across adjacent turns in a form-wound multi-turn coil is well defined.

Essentially, one can take the number of turns between the phase terminal and the neutral and divide it

into the phase–ground voltage to get the voltage across each turn. For example, if a motor is rated

4160 Vrms (phase–phase), the phase–ground voltage is 2400V.

This will result in about 24 Vrms across each turn, if there are 100 turns between the phase end and

neutral. This occurs because coil manufacturers take considerable trouble to ensure that the

inductance of each coil is the same, and that the inductance of each turn within a coil is the same.

Since the inductive impedance (XL) in ohms is:

XL = 2_fL (1.2)

Where f is the frequency of the AC voltage and L is the coil or turn inductance, the turns appear as

impedances in a voltage divider, where the coil series impedances are equal. In general, the voltage

across each turn will be between about 10 Vac (small form-wound motors) to 250 Vac (for large

generator multi turn coils).

The turn insulation in form-wound coils can be exposed to very high transient voltages associated

with motor starts, IFD operation, or lightning strikes. Such transient voltages may age or puncture the

turn insulation. This will be discussed in later sections. As described below, the turn insulation

around the periphery of the copper conductors is also exposed to the rated AC phase–ground stress, as

well as the turn–turn AC voltage and the phase coil-to-coil voltage.

39

Before about 1970, the strand and the turn insulation were separate components in multi turn coils.

Since that time, many stator manufacturers have combined the strand and turn insulation. Figure 1.12

shows the strand insulation is upgraded (usually with more thickness) to serve as both the strand and

the turn insulation. This eliminates a manufacturing step (i.e., the turn taping process) and increases

the fraction of the slot cross section that can be filled with copper. However, some machine owners

have found that in-service failures occur sooner in stators without a separate turn insulation

component.

Both form-wound coils and random-wound stators are also exposed to mechanical and thermal

stresses. The highest mechanical stresses tend to occur in the coil forming process, which requires the

insulation-covered turns to be bent through large angles, which can stretch and crack the insulation.

Steady-state, magnetically induced mechanical vibration forces (at twice the power frequency) act on

the turns during normal machine operation. In addition, very large transient magnetic forces act on

the turns during motor starting or out-of-phase synchronization in generators. These are discussed in

detail in Chapter 8. The result is the turn insulation requires good mechanical strength.

The thermal stresses on the turn insulation are essentially the same as those described above for the

strand insulation. The turn insulation is adjacent to the copper conductors, which are hot from the I2R

losses in the winding. The higher the melting or decomposition temperature of the turn insulation, the

greater the design current that can flow through the stator.

In a Roebel bar winding, no turn insulation is used and there is only strand insulation. Thus, as will be

discussed in later sections, some failure mechanisms that can occur with multi turn coils will not

occur with Roebel bar stators.

7.1.3 Ground wall Insulation

Ground wall insulation is the component that separates the copper conductors from the

grounded stator core. Ground wall

insulation failure usually triggers a

ground fault relay, taking the motor or

generator off-line.* Thus the stator

ground wall insulation is critical to the

proper operation of a motor or

generator. For a long service life, the

ground wall must meet the rigors of the

electrical, thermal, and mechanical

stresses that it is subject to.

B12 C.S of multi-turn coil, where the turn insulation and strand insulation are same

40

7.1.4 Slot Discharges: Slot discharges occur if there are gaps within the slot between the surface of the insulation and

that of the core. This may cause ionization of the air in the gap, due to breakdown of the air at the

instances of voltage distribution between the copper conductor and the iron.

Within the slots, the outer surface of the conductor insulation is at earth potential, in the

overhanging it will approach more nearly to the potential of the enclosed copper. Surface discharge

will take place if the potential gradient at the transition from slot to overhang is excessive, and it is

usually necessary to introduce voltage grading by means of a semi-conducting (graphite) surface

layer, extending a short distance outward from the slot ends.

So insulation of these stator bars is an inevitable task. It is worth now to know about

insulation. As we told before insulation is the heart of the generator now let us move to the most

interesting and important topic insulation.

7.2 INSULATING MATERIALS:Insulating materials or insulators are extremely diverse in origin and properties. They are

essentially non-metallic, are organic or inorganic, uniform or heterogeneous in composition, natural

or synthetic. Many of them are of natural origin as, for example, paper, cloth, paraffin wax and

natural resins. Wide use is made of many inorganic insulating materials such as glass, ceramics and

mica. Many of the insulating materials are man-made products and manufactured in the form of

resins, insulating films etc., in recent years wide use is made of new materials whose composition and

organic substances. These are the synthetic Organo-silicon compounds, generally termed as silicones.

Properties of a good Insulating Material: The basic function of insulation is to provide insulation live wire to live wire or to the earth. A

good insulating material needs the following physical and electrical properties.

1. It should be good conductor to heat and bad conductor to electricity.

2. It should withstand the designed mechanical stress.

3. It should have good chemical and thermal resistively and environmental resistively.

4. High dielectric strength sustained at elevated temperatures.

5. High resistivity or specific resistance

6. Low dielectric Hysterisis.

7. Good thermal conductivity.

8. High degree of thermal stability i.e. it should not deteriorate at high temperatures.

9. Low dissipation factor.

10. Should be resistant to oils and liquid, gas flames, acids and alkalis.

11. Should be resistant to thermal and chemical deterioration.

41

7.2.2 CLASSIFICATION OF INSULATING MATERIAL:The insulating material can be classified in the following two ways.

I. Classification according to substance and materials.II. Classification according to temperature.

a. Classification according to substance and materials:1. Solids (Inorganic and organic)

EX: Mica, wood slate, glass, porcelain, rubber, cotton, silks, rayon, ethylene, paper and cellulose

materials etc.

1. Liquids (oils and varnishes)

EX: linseed oil, refined hydrocarbon minerals oils sprits and synthetic varnishes etc.

2. Gases

EX: Dry air, carbon dioxide, nitrogen etc.

A.1 CLASSIFICATION ACCORDING TO TEMPERATURE:

Class Permissible temperature

Materials

Y 90 Cotton, silk, paper, cellulose, wood etc neither impregnated nor immersed in oil. These are unsuitable for electrical machine and apparatus as they deteriorate rapidly and are extremely hygroscopic.

A 105 Cotton, silk & paper, natural resins, cellulose esters, laminated wool, varnished paper.

E 120 Synthetic material of cellulose base B 130 Mica, asbestos, glass fibre with suitable bonding substance F 155 Material of class B with binding material of higher thermal

stability. H 180 Glass fibre and asbestos material and built up mica with

silicon resins. C Above

180Mica, porcelain, quartz, glass (without any bonding agent) with silicon resins of higher thermal stability.

42

7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES:

Name of Material

InsulationClass

Shelf life(In months) Application

At20 oC

At 5oc

1. Samicatherm calmica glass-n, mimica, domica, folium, filamic novobond-s, epoxy therm laxman isola calmicaflex

F 6 12 Main insulation of stator bars

2. Samica flex H 4 8 Overhang insulation of motor coils, at 3rd bends of multi turn coil

3. Vectro asbestos (365.02/365.32)

4. (used in resin rich)

B/F 2 8 Main pole coils of synchronous machines

Epoxide impregnated glass cloth

F6 12 Winding holders and

inter-half insulation5. Polyester resin mat &

rope6 Bar to winding holder

& stiffener groove of support segment of clamping plate

6. GlassoflexTurbo laminate

F 6 12 Inter-turn insulation of rotor winding

7. Hyper seal tape F 6 12 As finishing layer in overhangs of motor coils

8. SIB775 or 4302 varnish F 6 12 Stack Consolidation of stator bars

9. SIB475 or 4301 varnish F 6 12 Base coat varnish before taping of stator bars

10. SIB 643 or8003 Varnish or K8886 varnish

4 8 Conductive coat in straight portion of stator bars

11. SIB 642 or 8001 varnish 4 8 At slot emerge portion on stator bars

A.2 Insulating materials for electrical machines

7.3 FEW DEFINITIONS OF ELECTRIAL PROPERTIES OF INSULATION:

7.3.1 INSULATON RESISTANCE:

It may be defined as the resistance between two conductors usually separated by insulating

materials. It is the total resistance in respect of two parallel paths, one through the body and

other over the surface of the body.

Insulation Resistance is influenced by the following factors.

1) It falls with every increase in temperature.

43

2) The sensitivity of the insulation is considerable in the presence of moisture.

3) Insulation resistance decrease with increase in applied voltage.

7.3.2 DIELECTRIC STRENGTH:

The voltage across the insulating material is increased slowly the way in which the leakage

current increases depend upon the nature and condition material.

7.3.3 POWER FACTOR:

Power factor is a measure of the power losses in the insulation and should be low. It varies

with the temperature of the insulation. A rapid increase indicates danger.

7.3.4 DIELECTRIC CONSTANT:

This property is defined as the ratio of the electric flux density in the material .To that

produced in free space by the same electric force.

7.3.5 DIELECTRIC LOSS:

The dielectric losses occur in all solid and liquid dielectric due to

(b) Conduction current

(c) Hysterisis.

Additional to the Electrical properties there are many factors such as thermal, chemical etc.., they are

tabulated as below.

S.No Thermal Properties Chemical Properties Mechanical Properties

1.

2.

3.

4.

5.

6.

7.

Specific heat

Thermal conductivity.

Thermal plasticity

Ignitability

Softening point

Heat Aging

Thermal expansion.

Resistance to external chemical

effects.

Resistance to chemical in soils.

Effect of moisture and water.

Density

Viscosity

Moisture absorption

Hardness of surface

Surface tension

Uniformity.

A.3 properties of electrical insulation

44

8. Resin Impregnation:

Resin impregnation fills the porosity of a part with a resin to create a pressure-tight part for

hydraulic applications which can withstand several thousand psi, to improve machine ability, or to

allow electroplating. The parts are placed in a mesh basket and loaded into a vacuum tank. This is

then submerged in a bath of Anaerobic resin. A vacuum is pulled to remove all air from the porosity

of the parts. This vacuum is released to and the tank is pressurised, causing the resin to be drawn into

the porosity of the parts. Parts that typically undergo resin impregnation include hydraulic fittings for

pressure tightness and plating, covers and plated for pressure tightness, as well as machined

components.

The previous method of sealing parts was a furnace treatment, which formed a hard oxide layer on

the internal and external surfaces of a part, filling the porosity. Most machining operations were

performed prior to sealing the part because the hard oxide layer adversely affected mach inability.

Residue left by traditional cutting fluids tended to inhibit the formation of an oxide layer. With resin

impregnation, conventional cutting fluids can be used because the furnace treatment is eliminated

resulting in improved mach inability. These fluids efficiently remove heat from the cutting tool,

extending the tool life. Machining a porous part effectively creates a continuous interrupted cut.

Each time the tool impacts metal after passing through a pore, it may chip and become dull. Resin

impregnation reduces that effect and may also provide added lubrication to the cutting tool. Before

resin impregnation, many parts were mechanically plated. Resin impregnation allows the use of

electroplating.

EPOXY RESINS:Epoxy resins are poly ethers derived from Epi-Chloro Hydrin and Bis-Phenol

monomers through condensation polymerization process. These resins are product of alkaline

condensed of Epi-Chloro Hydrin and product of alkaline condensed of Epi-Chloro Hydrin and Poly-

Hydric compounds.

In Epoxy Resins cross-linking is produced by cure reactions. The liquid polymer has reactive

functional group like oil etc, otherwise vacuum as pre polymer. The pre polymer of epoxy resins

allowed to react curing agents of low inductor weights such as poly-amines, poly-amides, poly-

sulphides, phenol, urea, formaldehyde, acids anhydrides etc, to produce the three dimensional cross

linked structures.

Hence epoxy resins exhibit outstanding toughness, chemical inertness and excellent

mechanical and thermal shock resistance. They also possess good adhesion property. Epoxy resins

can be used continuously up to 300F, but with special additions, the capability can be increased up to

a temperature of 500F.

45

Epoxy resins are made use as an efficient coating material. This includes coating of tanks

containing chemicals, coating for corrosion and abrasion resistant containers. Epoxy resins are made

up of as attractive corrosion and wear resistant floor ware finishes.

These are also used as industrial flooring material. They are also used as highways Surfacing

and patching material. Moulding compounds of epoxy resins such as pipe fitting electrical

components bobbins for coil winding and components of tooling industrial finds greater application

in industries.

The epoxy resins similar to polyester resins can be laminated and Fiber Reinforced (FPR) and

used in glass fiber boats, lightweight helicopters and aero planes parts.

In the modern electronic industry, the application of epoxy resins is great. Potting and

encapsulation (coating with plastic resin) is used for electronic parts. Most of the printed circuits

bodies are made of laminated epoxy resin which is light but strong and tough.

PROPERTIES:

1) Epoxy resins have good mechanical strength less shrinkage and excellent dimensional stable after casting.

2) Chemical resistance is high.

3) Good adhesion to metals.

4) To impact hardness certain organic acid anhydrides and alphabetic amines are mixed.

APPLICATIONS:

1) They are used in the manufacture of laminated insulating boards.

2) Dimensional stability prevents crack formation in castings.

3) They are also used as insulating varnishes.

8.1 INSULATING MATERIAL FOR LAMINATIONS: -

The core stacks in modem machines are subjected to high pressers during assembly and there

fore to avoid metal-to-metal contact, laminations must be well insulated. The main requirements of

good lamination insulation are homogeneously in thin layers toughness and high receptivity.

We use varnish as insulating material for laminations.

8.2 VARNISH

This is most effective type of insulation now available. It makes the laminations nest proofs

and is not affected by the temperature produced in electrical machines varnish is usually applied to

both sides of lamination to a thickness of about 0.006mm. On plates of 0.35mm thickness varnish

46

gives a stacking factor about 0.95.In order to achieve good insulation properties the following

processes are in BHEL.

THERMOPLASTIC PROCESS OF INSULATION THERMOSETTING PROCESS OF INSULATION

BHEL is practicing only thermosetting process of insulation so

Thermosetting types of insulation are of two types:

RESIN RICH SYSTEM OF INSULATION

RESIN POOR SYSTEM OF INSULATION

The various types of materials used in the resin rich and resin poor process are given below.

Let us have an overview.

Materials used in resin poor system:

MATERIAL FOR RESIN POOR

DIAMOND COILS HALF BARS

Treated trivoltherm Impregnated polyester fleece Glass mat with accelerator Hostofon folium Synthetic fibre tape Resin poor mica tape Polyester fleece tape with graphite Semiconductor asbestos tape Polyester glass tape Polyester fleece tape Nomex polyamide adhesive tape

EPOXY glass cloth Nomex glass fleece Fine mica polyester glass cloth Nomex Form micanite Form mica tape Copper foil Polyester fleece tape with graphite for ICP Polyester fleece for OCP Polyester fleece tape with silicon carbide Mica splitting tape

VARNISH

Polyester glass tape Rutapox Hardener (H-90)

A.4 materials used in resin poor process

8.3 RESIN RICH SYSTEM:

In olden days, Resin Rich system of insulation is used for all Electrical Machines. In

insulator contains nearly 40% of EPOXY RESIN, so it gives good thermal stability Resin Rich

Insulation consists of the following materials in percentage

1. MICA PAPER TAPE -40-50%

2. GLASS PAPER TAPE-20%

3. EPOXY RESIN-40%

47

The bars are insulated (or) taped with RESIN RICH TAPE and place in the Pre-assembled

stator core including stator frame.

MATERIAL FOR RESIN RICH BARS

Preprag Nomex Epoxy resin rich mica tape Glass tape PTFE tape

VARNISH

Mica powder Graphite powder Conductive varnish Semiconductor varnish

A.5 Materials used in resin rich process:

In resin rich system of insulation Mica paper will give a good dielectric strength and Glass fiber tape

will give a good mechanical strength and Epoxy resin can withstand up to 155 degree Centigrade so it

gives a good thermal properties. Resin rich and Resin poor insulating materials are characterized by

the contact of the Epoxy Resin. In Resin rich system the content of Epoxy Resin tape is 40% so it is

named as RESIN RICH SYSTEM, and in Resin poor system the content of Resin tape is 8%. By VIP

impregnation process, the required amount is added to then conductor bars after assembling the core

and placing the winding in the core. In resin rich system before placing of coils in the stator slots the

rich tape will be wrapped over the bars. Nevertheless, this system has the following disadvantages:

1. This system is very time consuming and very long procedure.

2. Total cost of the system is more.

In order to minimize the over all cost of the machine and to reduce the time cycle of the system, the

VACUUM PRESSURE IMPREGNATION SYSTEM is being widely used. This process is very

simple, less time consuming and lower cost.

BHEL, HYDERABAD is equipped with the state of the art technology of VACUUM

PRESSURE IMPREGNATION.

The core or coil building and assembling method depends on the insulation system used. The

difference in core building is

1. For Resin rich insulation system the laminations are stacked in the frame itself.

2. For Resin poor insulation system (VPI) cage core of open core design is employed.

The manufacturing of coils also differs for both as explained above for core.

1. For resin poor process

2. For resin rich process

48

MANUFACTURE OF STATOR COILS:Manufacturing of stator coils depends on the type of the insulation process used for the stator.

I.e. the process is different for resin rich and resin poor process although few of the sub processes are

same for both.

9.1 For resin poor process:In this process the high voltage insulation is provided according to the resin poor mica base of

thermosetting epoxy system. Several half overlapped continuous layers of resin poor mica tape

are applied over the bars. The thickness of the tape depends on the machine voltage.

9.1.1 Reception of copper conductors:

The copper conductors rolls are

received is checked for physical and

mechanical properties. First piece is

checked for specifications such as

length and if found satisfactory, mass

cutting to desired length is carried out

by feeding into the cutting mills. B13 Side View showing one way of transposing insulated strands in

stator bar.

9.1.2 Transposition: Conductors are adjusted one over another for a given template and the bundles are

transposed by 360 degrees by setting the press for “Roebel Transposition”. Now they are bundled and

consolidated by tying with cutter tape at various places.

Similarly all the bundles are processed. Thus each stator bundle has a transposed coils in each phase

such that the flux distribution is equal and hence the induced e.m.f.

9.1.3 Putty operation:

All the transposed bars are shifted to putty operation. Here a single bar is taken for putty

operation by filling up the uneven surfaces on the width face by filling with NOMAX. I.e., NOMAX

sheets are inserted in the crossovers on the width face to the both ends. Form mica net is placed over

the width face of the bar on both sides & wrapped with PTFE (poly tetra flamo ethane) tape.

9.1.4 Stack consolidation:

Now 2 to 3 bars are inserted into hydraulic presser and they are pressed horizontally and

vertically to a pressure up to 150kg/cm2. At the same time the bars are subjected o heating from 140

49

to 160 degrees for duration of 2-3 hrs. Then the bars are unloaded and clamped perfectly. Now inter

half and inter strip testing is carried out and the dimensions are checked using a gauge.

9.1.5 Bending:

Each of the samples is placed over the universal former & the universal former is aligned to

the specifications. The bar is bent on both the sides i.e. on turbine side (TS) and exciter side

(ES).the 1st bend and the 2nd bend is carried out and continued by over hang formation. Now the

3rd bend is carried by inserting nomax sheet from the end of straight part to the end of 3 rd bend and

the bars are clamped tightly. Now the clamps are heated to 60 degrees for 30mins. Inter half and

inter strip tests follows.

9.1.6 Final taping:

The taping may be machine or manual taping and the taping is done according to the

type of insulation used. In case of resin poor system, resin poor tape is wrapped by 9*1/2 over lap

in the straight portion up to overhang and 6*1/2 over lap layers in the intermittent layers. The

intermittent layers are follows….

1st intermittent layer is ICP (internal corona protection) tape. This is wrapped by

butting only in straight portion.

2nd is split mica tape. One layer of split mica is wrapped by butting & using conductive

tape at the bottom so that split mica is not overlapped.

Next layer is O.C.P (outer corona protection). OCP tape is wrapped final in straight

portion by but joint up to end of straight portion on both the sides.

Next intermittent layer is ECP (end corona protection). ECP tape is wrapped from the

end of straight portion up to over hang over a length of 90-110mm. Now the bars are wrapped

finally with hyper seal tape from straight portion to the end of 3rd bend in overlapping layers for

protecting the layers from anti fingering. The IH & IS tests follows and the bars are discharged to

the stator winding.

9.2 For Resin rich system:

The coil manufacture is same as

in case of resin poor but differ in a few

stages. The Conductor cutting and

material used is same as resin poor

system. Transposition is done same as

50

that of resin poor system. Stacking of coils is done. In this case high resin glass cloth is used for

preventing inter half shorts. There is a difference in putty work. B.14 Cross-section of a

multi turn coil, where three turns and three strands per turn.

9.2.3 Putty work:

Nomex is used in between transposition pieces. 775 varnish is applied over the straight

portion of bar and mica putty is applied on the width faces of the bars.

Mica Putty mixture is a composition of SIB 775 Varnish, mica powder and china clay in the ratio of

100:50:25.

Straight part baking is done for 1hour at a temperature of 160C and a pressure of 150kg/sq.cm.Then

bending and forming is done. Half taping with resin rich tape is done for over hangs and reshaping is

done. To ensure no short circuits half testing of coils is done.

9.2.4 Final taping:

Initial taping and final tapings is done with resin rich tape (Semica Therm Tape) to about 13-

14 layers. The main insulation layers are 12*1/2 overlap in the straight portion and 9 layers in the

overhang.

51

B15 Layout of a mould used in baking of stator by Resin rich process

9.2.5 Final baking:

Final baking is done for 3hrs at a temperature of 160C in cone furnace. The bar is fed into the

baking mould.

The bar is heated for 1 hr at 90 degree to get gelling state.

The temperature of the mould is increased to 110 degrees in 30 mins and simultaneously the

moulds are tightened. Now in this process 155 of the resin is oozed out only 25% will be

remain. Now the bar is unloaded and checked for final dimensions, sharp corners, depressions,

charring, hollow sounds etc.,

Gauge suiting is done. I.e. the dimensions are made to compromising with the design.

Conductive/graphite coating (643) is applied on the straight portion and semi-conductive

coating 642 from end of straight portion to 3rd bend to pre transition coating on both sides.1st

coating for 90mm, 2nd coating for 100mm and 3rd coating for 120mm on both sides.

The bar is allowed for drying and epoxy red gel is applied from the end of straight portion to

the 3rd bend on both sides and allow for drying.

52

High voltage testing is done at 4 times that of rated voltage and tan testing, inter strip, inter

half testing are done. Tan values must be less than 2%.

10. An Overview

10.1 ADVANTAGES OF RESIN POOR SYSTEM OF INSULATION:

It has better dielectric strength

Heat transfer coefficient is much better

Maintenance free and core and frame are independent

It gives better capacitance resulting in less dielectric losses due to which the insulation life will be

more

The cost will be less and it is latest technology

Reduction in time cycle and consumption for MW also less and it gives high quality

10.2 DISADVANTAGES OF RESIN POOR SYSTEM OF INSULATION:

If any short circuit is noticed, the repairing process is difficult and need of excess resin from

outside.

Dependability for basic insulating material on foreign supply

10.3 ADVANTAGES OF RESIN RICH SYSTEM OF INSULATION:

Better quality and reliability is obtained

In case of any fault (phase - ground/ phase – phase short) carrying the repair process is very easy.

Addition of excess resin will be avoided because of using resin rich mica tape

10.4 DISADVANTAGES OF RESIN RICH SYSTEM OF INSULATION:

It is a very long procedure

Due to fully manual oriented process, the cost is more

It is possible to process stator bars only.

Even though the advantages and disadvantages of both the process are explained above, resin

poor process is the best of all, as the resin content used is almost only 35% compared to resin poor

process and also show good insulation properties justified in the later sections.

11. Assembly of stator:

The completed core and the copper bars are brought to the assembly shop for

assembly.

11.1 Reception of stator core:

53

Stator core after the core assembly is checked for the availability of foreign matter, so coil

projections are checked in each slot. HGL gauge is passed in each and every slot to detect bottom

core projections.

11.2 Winding holder’s assembly:

Assemble all the winding holders on both sides by adapting to the required design size.

Check all the wedge holders by a template and they are assembled as per the design requirement.

Tighten all the bolts relevant to winding holders and lock them by tag welding. Assemble HGL rings

on both the sides by centring with respect to core. Subject each individual for pressing in pressing

fixture at a pressure of 60 kg/cm2 for 30 minutes. Inter half test is conducted for each individual bar

before assembling into the stator.

Now stator bar assembling is carried out by centring to the core and check for proper

seating of bottom bars with T-gauge and checked for third bend matching, over hang seating etc..,

rein force the overhang portion of stator bars by inserting glass mat in between the bars and tying

them with neoprene glass sleeve. This process is carried out for all respective bottom bars .now the

pitch matching is checked on both sides both the generator and the exciter side.

Now high voltage testing is carried out on the stator.

11.3 Stiffeners assembly: Stiffeners are assembled on both sides and then checked for physical feasibility of top bar

by laying into the respective slot. Check for uniform gap in the over hang and top bar matching to the

bottom bar pitch on both sides. Assemble all the top bars by inserting inner layer inserts and also

assemble relevant RTD’s (Resistance Temperature detectors) where ever they are required as per the

design.

After completion of top bars, reinforce overhangs by inserting Glass-mat and tying with

Neoprene glass sleeve and also check for the third bend matching on both the sides. Then the core is

subjected to high voltage DC test and inter half short circuit tests.

11.4 Eye formation: Join bottom conductors and top conductors forming an eye, by brazing the conductors with

silver foil. Segregate eyes into two halves on both sides and test for inter half shorts. Insert Nomax

into two halves and close them.

Brazing makes the electrical connection between the top and bottom bars. One top bars strand

each is brazed to one strand of associated bottom bar so that beginning of the strand is connected with

out any electrical contact with the remaining strand. This connection offers the advantage of

minimizing three circulating currents.

54

11.5 Connecting rings assembly: The connecting rings are assembled on exciter side as per the drawing and connect all the

connectors to the phase groovers by joining and brazing with silver foil. Clean each individual phase

groove, insert nomax sheet and tape with semica folium. Subject the whole stator for HVDC test.

Terminate the three RTD’s in the straight portion and the 3-RTD’s in the over hang portion on both

turbine and exciter side except one for earthing requirement.

11.6 Phase connectors: The phase connectors consist of flat copper sections, the cross section of which results in a low

specific current loading. The connections to the stator winding are of riveted and soldered tape and

like wise wrapped with dry mica/glass fabric tapes. The phase connectors are firmly mounted on the

winding support using clamping pieces and glass fabric tapes.

Thus we have a completed stator here. Now this stator is sent for VPI process because in there is a

chance of damage to the insulation due to the following reasons

During the stator assembly, the bars are beaten with rubber hammers to fit into the slots

Also there is a chance of void spaces in between the stator conductors and the core due to the

use of solid insulating materials, which lead to slot discharges.

So in order to fill these voids and to gain good insulating properties the stator is VPI processed. Let

us start with an introduction to the process and the early materials used for this process and the

advancement of this process to our present resin poor VPI process.

55

12. THE VPI PROCESS

12.1 INTRODUCTION TO VACUUM PRESSURE IMPREGNATION SYSTEM (VPI)

12.2 HISTORY

DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the

year 1956. Vacuum Pressure Impregnation has been used for many years as a basic process for

thorough filling of all interstices in insulated components, especially high voltage stator coils and

bars. Prior to development of Thermosetting resins, a widely used insulation system for 6.6kv and

higher voltages was a Vacuum Pressure Impregnation system based on Bitumen Bonded Mica

Flake Tape is used as main ground insulation. After applying the insulation coils or bars were placed

in an autoclave, vacuum dried and then impregnated with a high melting point bitumen compound.

To allow thorough impregnation, a low viscosity was essential. This was achieved by heating the

bitumen to about 180C at which temperature it was sufficiently liquid to pass through the layers of

tape and fill the interstices around the conductor stack. To assist penetration, the pressure in the

autoclave was raised to 5 or 6 atmospheres. After appropriate curing and calibration, the coils or bars

were wound and connected up in the normal manner. These systems performed satisfactorily in

service provided they were used in their thermal limitations. In the late 1930’s and early 1940’s,

however, many large units, principally turbine generators, failed due to inherently weak thermoplastic

nature of bitumen compound.

Failures were due to two types of problems:

a. Tape separation

b. Excessive relaxation of the main ground insulation.

Much development work was carried out to try to produce new insulation systems, which

didn’t exhibit these weaknesses. The first major new system to overcome these difficulties was

basically a fundamental improvement to the classic Vacuum Pressure Impregnation process. Coils

and bars were insulated with dry mica flake tapes, lightly bonded with synthetic resin and backed by

a thin layer of fibrous material. After taping, the bars or coils were vacuum dried and pressure

impregnated in polyester resin. Subsequently, the resin was converted by chemical action from a

liquid to a solid compound by curing at an appropriate temperature, e.g. 150 C. this so called

thermosetting process enable coils and bars to be made which didn’t relax subsequently when

operating at full service temperature. By building in some permanently flexible tapings at the evolutes

of diamond shaped coils, it was practicable to wind them without difficulty. Thereafter, normal slot

56

packing, wedging, connecting up and bracing procedures were carried out. Many manufacturers for

producing their large coils and bars have used various versions of this Vacuum Pressure Impregnation

procedure for almost 30 years. The main differences between systems have been in the types of

micaceous tapes used for main ground insulation and the composition of the impregnated resins.

Although the first system available was styrenated polyester, many developments have taken place

during the last two decades.

Today, there are several different types of epoxy, epoxy-polyester and polyester resin in

common use. Choice of resin system and associated micaceous tape is a complex problem for the

machine manufacturer.

Although the classic Vacuum Pressure Impregnation technique has improved to a significant

extent, it is a modification to the basic process, which has brought about the greatest change in the

design and manufacture of medium-sized a.c. industrial machines. This is the global impregnation

process. Using this system, significant increases in reliability, reduction in manufacturing costs and

improved output can be achieved. Manufacture of coils follows the normal process except that the

ground insulation consists of low-bond micaceous tape. High-voltage coils have corona shields and

stress grading applied in the same way as for resin-rich coils, except that the materials must be

compatible with the Vacuum Pressure Impregnation process. Individual coils are inter turn and high-

potential-tested at voltages below those normally used for resin-rich coils because, at the un-

impregnated stage, the intrinsic electric strength is less than that which will be attained after

processing. Coils are wound into slots lined with firm but flexible sheet material. Care has to be taken

to ensure that the main ground insulation, which is relatively fragile, is not damaged. After inter-turn

testing of individual coils, the series joints are made and coils connected up into phase groups. All

insulation used in low-bond material, which will soak up resin during the impregnation process. End-

winding bracing is carried out with dry, or lightly treated, glass-and/or polyester-based tapes, cords

and ropes. On completion, the wound stator is placed in the Vacuum Pressure Impregnation tank,

vacuum-dried and pressure-impregnated with solvent less synthetic resin. Finally, the completed unit

is stoved to thermo set all the resin in the coils and the associated bracing system.

After curing, stator windings are high-potential-tested to the same standard. Loss-tangent

measurements at voltage intervals up to line voltage are normally made on all stators for over 1kv. A

major difference between resin-rich and vacuum pressure impregnation lies in the importance of this

final loss-tangent test; it is an essential quality-control check to conform how well the impregnation

has been carried out. To interpret the results, the manufacturer needs to have a precise understanding

of the effect of the stress-grading system applied to the coils. Stress grading causes an increase in the

loss-tangent values. To calculate the real values of the ground insulation loss-tangent, it is necessary

to supply from the readings the effect of the stress grading. For grading materials based on the

57

materials such as silicon carbide loaded tape or varnish, this additional loss depends, to a large extent

upon the stator core length and machine voltage.

VPI is a process, which is a step above the conventional vacuum system. VPI includes

pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is

improved mechanical strength and electrical properties. With the improved penetration, a void free

coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution,

the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the

average rise.

In order to minimize the overall cost of the machine & to reduce the time cycle of the

insulation system vacuum pressure Impregnated System is used. The stator coils are taped with

porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator

is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin

bath under pressure of Nitrogen gas.

The chemical composition of our resin type and its advantages are explained in the later

sections. Now let us discuss the various stages involved in VPI process for resin poor insulated jobs.

VPI process is done in the VPI camber. For higher capacity stators of steam turbine or gas

turbine generator stators, horizontal chamber is used where as vertical chamber is used for smaller

capacity systems such as Permanent Magnet Generator (PMG), coil insulation of small pumps and

armature of motors etc..,

12.3 Vacuum Pressure Impregnation of resin poor insulated jobs:

VPI process for a stator involves the following stages.

2. Preheating

3. Lifting and shifting

4. Vacuum cycle

5. Vacuum drop test

6. Heating the resin

7. Resin admission.

8. Resin settling

9. Pressure cycle

10. Aeration.

11. Post curing cycle

12. Cleaning

58

12.3.1 General instructions before VPI process:The jobs that are entering tank for Vacuum Pressurised Impregnation shall not have any oil

based coatings. Any such, rust preventive/ corrosion preventive viz., red oxide etc., shall be

eliminated into the tank. Jobs shall be protected with polyethylene sheet for preventing dust or dirt on

jobs, till it is taken up for impregnation. Resin in the storage tank shall be stored at 10 to 12 C and

measured for its viscosity, viscosity rise. Proper functioning of the impregnation plant and curing

oven are to be checked by production and cleared for taking up of job for impregnation.

12.3.2 Pre heating: The foremost stage of VPI, the completed stator is placed in the impregnation vessel and kept

in an oven for a period of 12 hours at a temperature of 60 deg. Six thermocouples are inserted at the

back of the core to measure the temperature. The temperature should not exceed to 85 deg .Smaller

stator can be inserted directly into the impregnation chamber. The job is to be loaded in the curing

oven and heated. The temperature is to be monitored by the RTD elements placed on the job and the

readings are logged by production. The time of entry into the oven, time of taking out and the

temperature maintained are to be noted. Depending on convenience of production the jobs can be

preheated in impregnation tank by placing them in tubs.

The impregnation tubs used for impregnation of jobs are to be heated in the impregnated tank itself,

when the jobs are preheated in the curing oven

59

12.3.3 Insertion of tub with job into the impregnation tank:The wound stator is lifted and shifted into the tub. By the time, the preheating of job is completed, it

is to be planned in such a way that the heating of tub and tank heating matches with the job. This is

applicable when the job is heated in the curing oven separately. The preheated job is to be transferred

into the tub by crane handling the job safely and carefully with out damage to the green hot insulation

the tub is then pushed in the 140 tank furnace or also called as vacuum tank, after which the lid is

closed and the tank furnace was heated to 60 +/- 3 deg The warm tub with job is inserted into

impregnation tank by sliding on railing, in case of horizontal tank. The thermometer elements are to

be placed at different places on the job. The connection for inlet resin is to be made for collection of

resin into tub. After ensuring all these lid of the impregnation tank is closed. In case of vertical tank

the job along with tub is slinged and inserted carefully into impregnation tank without damage to

insulation

12.3.4 Vacuum cycle:

The pre heated job will be placed in the impregnation chamber by a hydraulic mechanism.

The vessels are kept clean and the resin available in the vessel is wiped out. Methylene and traces of

resin should not be allowed on the inner side of the tank. Now the vacuum pumps are all switched on

and a vacuum pressure of about 0.2 mb is maintained for about 17 HRS, after which the wound stator

is subject to vacuum drop test.

12.3.5 Vacuum drop test: This drop test is important phase, all the vacuum pumps are switched off for about 10 mins,

and the vacuum drop is measured and it is checked whether it exceeds 0.06mb, if it exceeds 0.06mb

then it is subject to repetition of vacuum cycle for another 6 to 8 hrs, else it is sent to the next cycle

12.3.6 Drying the job in vacuum The job is to be dried under vacuum. Drain out the condensed moisture/ water at the exhausts

of vacuum pumps for efficient and fast vacuum creation. Also check for oil replacement at pumps in

case of delay in achieving desired vacuum.

12.3.7 Heating the resin in the storage tank The completion of operations of drying and the heating of the resin in the storage tank are to

be synchronised. The heating of resin in the tank and pipeline is to be maintained as at preheating

temperature .i.e. the temperature is maintained at 60+/- 3 deg ,including pipeline

12.3.8 Admission of resin into impregnation tank The resin is allowed into the impregnation tank tub if required from various storage tanks

one after the other, such that the difference in pressure fills the tank, up to a level of 100mm above

the job generally, after which the resin admission is stopped. After 10mins of resin settling the tank is

60

to be pressurised by nitrogen. While admitting resin from storage tanks pressurise to minimum so that

nitrogen will not affect resin to spill over in tank.

12.3.9 Resin settling:The resin is allowed to settle for about 4mins in such a way that bubble formation ceases

12.3.10 Impregnation Pressurising/gelling After the resin has settled the job

is subject to pressure cycle of 4 kg/ cm2

of dry nitrogen into the vacuum tank

after obtaining 4 kg/cm2, this is subject

for 2 hrs. in this stage the resin is

impregnated into the micro pores of the

stator and is very firmly embedded into

the crevices of the stator ,so thus acting

as a tough layer of insulation for the

stator ,being indestructible in the long run. B16 vertical VPI

tanks for smaller jobs

12.3.11 Withdrawal of resin from impregnation tank to storage tank The resin that is pressurised as

per pressure cycle is drawn into the

tank by the opening of relevant valves

will allow the resin to come back to

the storage tank. The job also shall be

allowed for dripping of residue of

resin for about 10min. After dripping, withdrawal of resin in various storage tanks is to be carried out.

This is necessary because resin is a very costly material.

61

12.3.12 Taking out the tub with job from impregnation tankThe lid is then opened after taking precautions of wearing mask and gloves for the operating

personnel as a protection from

fumes. The job is withdrawn from

impregnation tank by

sliding on railing for horizontal

and slinging on to crane for

vertical impregnation tanks.

12.3.13 Post curing:The job is post heated. The

time and temperature to which

the job has to be scheduled is varies

according to the type of job and is

given in the table. The time at which the heating is started, achieved and maintained is to be logged.

The wound stator is subject to 140 +/- 5 deg. After obtaining 140 deg the stator is subject for 32 hrs.

The stator is then made to rotate at 1 rpm up to 120 deg. It is then allowed for cooling without

opening the doors till the temperature reaches 80 deg, after attaining the temperature of 80 deg, the

doors are opened and wound stator is sprayed with epoxy red gel on the overhangs and is allowed for

drying.

12.3.14 Cleaning: Entire wound stator is cleaned for resin drips, after which its subjected to HV and tan delta tests

12.4 Electrical testing:

All jobs that are impregnated till above process are to be tested for electrical tests. After

ensuring that all the temperature/vacuum conditions stipulated for drying, impregnation and curing

operations have been properly followed, the job is to be released for this operation.

12.5 Global processing:

Processing details depends very much on the machine type, on customer’s defined parameters and

type of mica tapes.

Generally the VPI system is used in impregnation vessels up to 30T where the rotor/stator is

impregnated at elevated temperatures. Machine parts usually are preheated (also under vacuum) in

order to remove moisture and to reduce viscosity during impregnation.

12.6 Resin management:

After impregnation the VPI bath is pumped into storage tanks and cooled down to 5-10C and

should be stored in dry conditions in order to obtain a long bath life. Actual bath life depends on

additional parameters, e.g., impregnation temperature and duration of impregnation, impurities in the

bath, wash-out of catalyst from mica tapes into the un- accelerated resin system (B), replenishment

62

rate, moisture exposure etc,. The viscosity of the bath should be checked periodically in order to

maintain a suitable viscosity for impregnation.

Impregnated, yet uncured machine parts in unconditioned atmosphere may pickup moisture.

Therefore curing directly after

impregnation or storage in moisture

controlled area is recommended.

Generally machine parts are rotated

when removed

B17 Showing

the resin tank in which resin is

stored.

from the bath and during the first part of

curing in order to avoid drip off.

Evaporation of hardener during the

vacuum cycle leads to a change in the resin/hardener ratio in the bath and has to be compensated.

Therefore replenishment is mixing ratios of 100-120pbw of hardener HY 1102 per 100pbw MY 790-

1 are generally used. Replenishment mixing ratios depend on actual processing parameters and

conditions and have to be evaluated at the customer site.

Due to excellent latency of the system (A) MY 790-1/HY 1102/DY 9577/DY073 the replenishment

volume to maintain a constant viscosity is comparatively small, even if impregnation is performed at

40-50C.

On single coils and roebel-bars the mica insulation is normally covered with a tight glass tape to

prevent drainage of the impregnation resin.

12.7 Specific Instructions:

Depending on the insulation materials and the accelerating agent in use, a ramped curing

schedule is recommended.

In systems with high reactivity, where the accelerator can be include in the mica-tape, a fast

gelation can be obtain with a temperature-shock, and draining can so be reduced or avoided.

Standard curing with the standard accelerated mixture (system A) is:

11 at 90C plus 18 h at 140C

12.8 Precaution:

To determine whether cross linking has been carried to completion and the final properties are

optimal, it is necessary to carry out relevant measurements on the actual object or to measure

the glass transition temperature. Different gelling and cure cycles in the manufacturing

process could lead to a different cross linking and glass transition temperature respectively.

12.9 Features and Benefits:

63

• State-of-the-art process for completely penetrating air pockets in winding insulation.

• Increases voltage breakdown level. (Even under water!)

• Proven submergence duty system

• Improved heat transfer- windings are cooler, efficiency is improved.

• Improves resistance to moisture and chemicals.

• Increases mechanical resistance to winding surges.

An overview of entire VPI process with the time taken for each process according to the type of the

job used is given below in a tabular form

Vacuum Pressure Impregnation of resin poor insulated jobs:

Variant Description

01 Brushless exciter armature, PMG stators and Laminated rotors 02 Stator wound with diamond pulled coils.

03 Stator with half coils

Variant-01 Variant-02 Variant-03Any other

information

Preheating605C for

3hrs605C for 12hrs

603C for 12hrs

Vacuum to be maintained

0.4mbar 0.2mbar/0.4mbar

<0.2mbar(both together

shall not exceed 50hrs

including rising time)

Vacuum heating time 3hrs

0.2mbar for 9hrs 0.4mbar for 17hrs

Stopping vacuum

pumps for 10min shall check 17hrs

vacuum drop. The vacuum

drop shall not exceed by

0.06mbar for 10min

Increase in pressure

40min 80min 80min

Maximum 3bar 4bar 4bar

64

pressure

Pressure holding

3hrs 3hrs 3hrs

Post curingAt1405C for

14hrsAt1405C for

32hrsAt1405C

for 32hrsA.6 Table showing temperature and time to be maintained for different type of jobs in VPI

13. FACILITIES AVAILABLE IN VPI PLANT IN BHEL:

The major facilities available in VPI plant are:

Steam furnace for preheating

Size of chamber: 2 * 2 * 6.5 M

Maximum temperature: 160C

Electrical power consumption: 75KW

Work place: 1425

Work centre: 3215

Stream inlet: 200-250C

Impregnated tubs for keeping jobs

For vertical impregnation: As per respective tech. Document.

For horizontal impregnation: As per respective tech. Document.

Specifications of plant:

Impregnation medium

(a) Epoxy resin (class F solvent free) and hardener mix in 1:1 ratio as per TG34967

(b) Epoxy resin (class F solvent free) and hardener mix in 1:1 ratio as per TG34931

Horizontal impregnation chamber

Diameter: 4000mm

Cylindrical length: 9000 mm

Operating over pressure: 6 bar

Operating vacuum: 0.15 mbar

Operating temperature: 90C

Loading weight of impregnation object: maximum of 120 tonnes

Maximum leakage rate: less than 1mbar/lit/sec.

Moving load: 140 tonnes.

Static load: 170 tonnes

Pressure medium for impregnation

Pressure medium: dry nitrogen

Operating pressure: 6 bar.

Nitrogen storage capacity: 52cubic meter at 25 bar.

Resin storage capacity

65

Total storage: 5*9000L+1*3000L

Operating parameters of each tank

Operating vacuum: 0.5mbar

Operating over pressure: 0.5bar

Operating temperature: 80C

Resin filters(stainless steel washable)

Filter fineness: 150microns

Output (maximum): 1000lits/min

Vacuum system

Root pumps: 2No.s, 5.5KW each

Suction capacity: 2000cubic meter/hr

Vacuum pumps(4No.s, 7.5KW each)

Suction capacity: 250 cubic meter/hr

This system is provided with separator filter with activated carbon filters, to protect the vacuum

pumps from resin and hardener vapours.

Refrigeration system

The resin inside the tanks has to be stored at 102C. this can be stored for indefinite period with

a brine chilling/refrigeration system.

The brine storage capacity: 1*25000L+1*26000L

Composition of brine: 40%Mono Ethylene glycol and 60%water

Heating and cooling system

The heating of resin in the storage tanks and the impregnation chamber is by circulating the

heated brine through the heat exchangers, to heat by saturated steam. The hot brine is cooled to

about 40C by circulating water through coolers and then the brine is chilled to -10C and stored

in the tanks.

Post heating of job

(a) Explosion proof steam drier and electrical heating superposed.

Size: 7*4.5*4.5M

Maximum weight of job: 80 tonnes

Maximum temperature: 150C

(b) Indirectly heated hot air circulating oven (gas fired)

Size: 9*4.5*4.5M

Maximum weight of job: 170 T/120T with facility for rotation.

Maximum temperature: 150C

66

13.1 DATA COLLECTION OF SAMPLES

During the project two jobs have been impregnated in VPI Plant, the data has been collected and

recorded in the project report.

13.1.1 INDO-BHARAT-II ROTORPreheating:

Indo Bharat II rotor is loaded for preheating in steam furnace on 30-5-2007 at 18:00hrs.

Date and timeRTD-I(C) RTD-II(C) Furnace air

temperatureRemarks

30.5.2007 19:00 32.0 30.0 45.6

Rotor temperature is reached to 603C at 2:00hrs on 31.5.2007 and it is maintained for 4 hrs i.e., up to 6:00 on 31.5.2007

30.5.2007 20:00 45.4 48.6 57.930.5.2007 21:00 49.9 50.9 63.430.5.2007 22:00 52.5 54.3 70.530.5.2007 23:00 53.3 55.1 73.4

30.5.2007 24:00 56.6 57.3 75.6

Rotor is switched to vac 140 tank at 7:00 hrs on 31.5.2007

31.5.2007 1:00 59.9 60.2 75.131.5.2007 2:00 62.4 63.9 77.031.5.2007 3:00 62.3 64.7 77.031.5.2007 4:00 63.3 64.1 75.031.5.2007 5:00 63.3 64.0 75.631.5.2007 6:00 63.1 63.7 75.6A 7 PREHEATING OF INDO BHARATH II ROTOR

Date and time

RTDI(C) RTDII(C)Room temperature(C)

Remarks

31.5.2007 19:30 62.0 62.0 36.0 Resin tank 025 is heated for impregnation

31.5.2007 21:30 70.1 69.2 36.731.5.2007 23:30 86.2 81.6 36.71.6.2007 1:30 101.5 97.6 35.6 Resin admission started

at 14:10 hrs on 31.5.2007

1.6.2007 3:30 116.2 113.1 34.8 Resin admission

67

completed at 14:25 hrs on 31.5.2007

1.6.2007 5:30 129.6 125.7 33.8 Pressurization started at 14:45 hrs on 31.5.2007

1.6.2007 7:30 137.6 133.2 33.2 Pressurization completed at 15:30hrs on 31.5.2007

1.6.2007 9:30 145.7 140.2 36.5 Pressurization hod up completed at 18:30hrs on 31.5.2007

1.6.2007 11:30 145.7 141.6 38 Resin withdrawal to storage tanks is from 18:30 to 18:45hrs on 31.5.2007

1.6.2007 13:30 144.7 143.4 42.8 Rotor loaded in gas furnace at 19:15hrs on 31.5.2007

1.6.2007 15:30 144.1 143.0 43.8 Rotor temperature is reached to 131.6 to 145.7C at 8:30hrs on 1.6.2007 and it is maintained for 14hrs i.e., up to 22:30hrs on 1.6.2007

1.6.2007 17:30 144.0 144.5 42.81.6.2007 19:30 143.0 143.0 41.0 Furnace is switched at

22:30hrs on 1.6.2007 and circulation fans are kept running till the job temperature is reached to 70C to 75C.

1.6.2007 21:30 142.8 142.6 40.8

A.8 impregnation of Indo Bharath II rotor

Vacuum cycle:

Date and time Vacuum in

graph (mbar)Vacuum in

meter (mbar)

Job temperature

(C)Remarks

31.5.2007 7:00 -- -- 62.2

Vacuum pump started at 7:30 hrs on 31.5.2007.

31.5.2007 8:00 -- 3.0 61.531.5.2007 9:00 0.85 0.86 61.331.5.2007 10:00 0.54 0.55 61.131.5.2007 11:00 0.39 0.4 61.131.5.2007 12:00 0.38 0.4 61.131.5.2007 13:00 0.37 0.4 61.031.5.2007 14:00 0.36 0.39 61.0A.9 vacuum cycle of Indo Bharath II rotor

RESIN CYCLE AND POST CURING CYCLE:

68

13.1.2 INDO-BHARAT-II STATOR:

Preheating:

Indo-Bharat-II stator is loaded for preheating in steam furnace on 7-5-2007 at 23:30hrs.

Date and Time RTD-I(C) RTD-II(C) Furnace air

temperature(C)Remarks

7.5.2007 23:30 36.3 36.1 Stator temperature is

reached to 60.5C to

62.9C(603C) at

7:30hrs on 8.5.2007

and it is maintained

for 12hrs i.e., up to

19:30hrs on

8.5.2007

8.5.2007 1:30 43.6 42.9

8.5.2007 3:30 52.0 51.74

8.5.2007 5:30 55.9 56.0

8.5.2007 7:30 60.5 62.9 Stator is loaded in

vac(140) tank at

21:00hrs on

8.5.2007

8.5.2007 9:30 61.3 62.9

8.5.2007 11:30 60.3 62.4

8.5.2007 13:30 60.3 62.6 Vac. Pump is started

at 2:30hrs on

9.5.2007

8.5.2007 15:30 62.5 62.9

8.5.2007 17:30 62.9 62.66

8.5.2007 19:30 62.4 62.1

A.10 Preheating of Indo Bharat II stator

Vacuum cycle:

Date and TimeVacuum in

graph (mbar)

Vacuum in

meter (mbar)

Job

temperature

(C)

Resin cycle

8.5.2007 22:00 -- -- 54.37 Resin tanks 025,102 are

69

heated for impregnation

8.5.2007 0:00 -- -- 54.89 Viscosity of resin at 60C

is 33CP

9.5.2007 2:00 -- -- 59.02 Viscosity after aging is

36.10CP

9.5.2007 3:30 0.65 0.65 61.6 9.5.2007 and 10.5.2007

9.5.2007 5:30 0.41 0.40 63.59 Resin admission started at

19:45hrs

9.5.2007 7:30 0.28 0.29 64.2 Resin admission

completed at 19:55hrs

9.5.2007 9:30 0.22 0.22 63.2 Pressurisation started at

20:00hrs

9.5.2007 11:30 0.19 0.19 62.3 Pressurisation of

4kg/sq.cm reached at

21:20hrs

9.5.2007 13:30 0.18 0.18 62.1 Pressurisation hold up for

3hrs is at 0:20hrs

9.5.2007 15:30 0.17 0.17 62.0 Resin withdrawn to

storage tanks is from

0:30hrs –1:00hrs

9.5.2007 17:30 0.14 0.14 61.8 Stator loaded in hot air

furnace from 1:00hrs –

1:30hrs on 10.5.2007

9.5.2007 19:30 0.14 0.14 61.3

A.11 Vacuum cycle of Indo Bharat II stator

Post curing:

Date and

Time

ESOH

15T

TSOH

06B

ESW

02

TSW

13Core

Room

temperatureRemarks

10.5.2007

1:30hrs70.0 76.4 62.4 62.5 63.4 33.1

10.5.2007

4:30hrs

126.7 131.4 94.7 102.3 98.8 31.7

70

10.5.2007

7:30hrs144.3 154.1 125.4 134.5 126.1 31.6

10.5.2007

10:30hrs147.7 154.9 139.9 145.1 140.6 34.8

10.5.2007

13:30hrs137.6 144.4 139.3 141.6 140.7 38.0

10.5.2007

16:30hrs136.9 144.2 140.0 140.9 140.6 38.4

10.5.2007

19:30hrs140.2 143.6 140.1 140.7 140.2 37.2

10.5.2007

22:30hrs144.4 151.3 143.7 145.1 144.1 35.9

Job temp. is reached to

1405C i.e., from

136.2C to 145.6C at

9:30hrs on 10.5.2007 and

it is maintained for 32hrs

i.e. up to 17:30hrs on

11.5.2007.

11.5.2007

1:30hrs143.1 146.7 145.2 145.1 145.2 33.8

11.5.2007

4:30hrs144.3 151.0 143.6 144.0 144.7 31.1

11.5.2007

7:30hrs135.7 142.1 144.3 145.1 145.0 31.3

11.5.2007

10:30hrs135.0 135.7 135.1 135.0 135.8 34.8

11.5.2007

13:30hrs135.6 141.4 135.4 135.6 135.9 38.3

11.5.2007

17:30hrs148.0 149.2 142.8 142.2 142.1 39.8

Furnace is switched

off at 17:30hrs on

11.5.2007 and

circulation fans kept

running till the job

temperature is

reached from 70C-

75C

A.12 Post curing of Indo Bharat II stator

71

13.1.3 High voltage levels of stator/rotor windings for multi turn machines:

S.No. Description HV level HV in kv remarks

Stator winding

1. After laying and wedging of

coils

18.9/1’ RTD,IT test

2. After OH spacers and

forming eyes

18.03/3’ RTD,IT test

3. Before impregnation

17.5/1’ R, RTD test

4. After impregnation

26.0/1’ R, RTD, Tan, leakage

reactance test5. Customer

acceptance25.0/1’ Rotor winding

Rotor winding

1. After laying first coil

UT+1400 2.9 Pole drops

2. After laying second coil

UT+1250 2.75 Pole drops

3. After laying third coil

UT+1100 2.6 Pole drops

4. After laying fourth coil

UT+950 2.45 Pole drops

5. After laying fifth coil

UT+800 2.3 Pole drops

6. After laying sixth coil

UT+650 -- Pole drops

7. After all connections

-- -- R, Pole drops

8. After tech. rings assembly

-- 2.15

9. After bandage -- 2.0 R, Pole drops10. After

impregnation-- 1.9 R, Pole drops

11. After excitation cable assembly

-- 1.8 R, Pole drops

12. After balancing UT+200 1.7 R,Z with 50Hz

A.13 High voltage levels of stator/rotor windings for multi turn machines

72

13.1.4 TESTING RESULTS OF INDO-BHARAT-II ROTOR

Customer name: INDO-BHARAT-II ROTOR

M/c rating: 10.8MW, 12kv, 1500rpm.Test: Z, R and H.V test.

Stage: after impregnation.

Ambient temperature: 35C

Ohmic resistance: 0.264 (rotor temperature was more)

Voltage( volts) Current(amps)215.0 0.5

367.5 1.0

523.0 1.5

High voltage test: IR value before H.V. test at 15”/60” -- 200/300 M H.V. applied at 1.9kv /1’ – withstood IR value after H.V. test at 15”/60” -- 200/300 M

13.1.5TESTING RESULTS OF INDO-BHARAT-II STATOR:

Customer name: INDO-BHARAT-II STATOR

M/c rating: 10.0MW, 12kv, 0.8pf, 650A, 1500rpm.

Test: H.V test.

Stage: after impregnation.

Ambient temperature: 36C

A PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M

B PHASE: IR value at 2.5kv

73

IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M

C PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M

INDO-BHARAT-II STATOR:

Customer name: INDO-BHARAT-II STATOR

M/c rating: 10.0MW, 12kv, 0.8pf, 650A, 1500rpm.

Test: H.V test-RTD measurement, resistance measurement.

Stage: After impregnation.

Ambient temperature: 36C

Excitation Side:

26 113.8 10 114.062 113.850 113.820 113.838 113.914 113.8

Turbine Side:

49 125.013 113.825 113.921 113.861 113.837 113.91 113.8

A-A --29.4mB-B -- 29.3mC-C -- 29.4m

74

14. COMPARISION BETWEEN RESIN POOR AND RESIN RICH SYSTEMS:

RESIN POOR SYSTEM RESIN RICH SYSTEM

1. The insulation tape used in this system

has 40% resin.

2. This method follows thermosetting

process.

3. There is a need for addition of resin from

outside.

4. Reduction in time cycle for this process

5. No tests are carried out while at

processing

6. Processing of bars along with stator and

with conductors and processing of exciter

Coils along with exciter is possible.

7. The cost of repair is more

8. The overall cost is less

compared to resin rich system.

1. The insulation tape used in this is 7% of

40% resin.

2. Same as in resin poor.

3. Further addition of resin is not required

from outside.

4. It is very long process and

time consuming while at processing stage.

5. Tests are being carried out Stage.

6. Processing of stator bars is

only possible in resin rich

systems.

7. Repairing work is easy.

8. The total cost in this process is more.

A.14 Comparison between resin rich and resin poor process

Applications:• All critical machines

• Equipment exposed to frequent surges/starting

• Harsh or moist environments

• Motors that run at service factor

DRAWBACKS OF VPI SYSTEM:

Number of RTD’s required are more

The whole operation is time consuming

It depends largely on moisture and season of operation

Maintenance of resin below room temperature about 8-12C is complicated.

75

SUGGESTIONS:

Processed in a Clean Room Environment To ensure optimum rewind integrity, all rewinds should be conducted in a clean,

temperature- and humidity-controlled environment. It ensures optimum material performance and

prevents dirt or moisture contamination during the process.

VPI Process Control Throughout the VPI process, each stator is continuously monitored by computer to ensure

homogenous fill.

How can we say that the present VPI BY RESIN POOR process currently used in BHEL is a superior

process as compared to VPI by resin rich process ? inorder to find an answer to this question the

justification follows:

15. JUSTIFICATIONWe can say that VPI by resin poor system is more superior to other types of insulation by conducting

HV and tan δ tests, and the results of which are clearly indicated in the graphs below:

After impregnation of the stator core by VPI process the following tests are conducted:

1. TAN TEST.2. HIGH VOLTAGE Tests.

15.3.1 High Voltage test: AC High voltage test is conducted on VPI system after impregnation to verify proper

impregnation and dielectric strength of insulation. This test was conducted at 105% of winding

test voltage i.e. Up=2Un+1KV Where Up-Winding test voltage Un-rated voltage of machine.

Equipment

50 Hz A.C High voltage transformers and its induction regulator/input autotransformer.

Potential transformer (35 or100KV/100V)

Voltmeter

Binding wire

Earthing Rod and Earthing wire/cable

When H.V test is done on one-phase winding, all other phase windings, rotor winding,

instrumentation cables and stator body are earthed.

76

The high voltage is applied to winding by increasing gradually to required value and maintance for

one minute & reduced gradually to minutes. The transformer is switched off & winding discharged to

earth by shorting the terminal to earth using earthing rod connected to earth wire/cable.

The test is conducted on all the phases & rotor winding separately.

HV Test Levels:

Stator winding: (2Ut+1) KV =23 for 11 KV machine

Rotor winding: (10 Up) volts (with min of 1500v & max of 3500v),

Where, Ut= Rated of machine under test Up= Excitation voltage.

15.3.2 TAN TEST:Equipment: Schering Bridge

After impregnation and curing of the winding a dissipation factor Vs voltage measurement as

stipulated in the application national and international standard specification is performed for each

bar between all-individual phase winding to ground.

Guiding values for the deception factor and its rice with the voltage merely. Given in the

KEMA specification the maximum value should not exceed 0.001 at 20% of rated voltage and rise

shall not be greater than 0.006 per 20% of rated voltage up to 60% of rated voltage and 0.08 per 20%

of rated voltage up to a rated voltage.

Winding manufacture by the Vacuum Pressure Impregnation Process comply with these limits.

The above test results are specified in the following graphs. First graph shows that Voltage Vs Tan δ

curve, it shows different Tan values at different percentage of rated voltage 20%, 40%, 60%, 80%,

100% of rated voltage respectively.

The second graph is Stages of materials Vs life of insulation material, it shows that resin poor

system of insulation has very long life compared to resin rich system of insulation. At 10 KV the

resin poor system insulation as a lifetime of 540 years. Any good machine as life span of 25-30 years

by using this insulation we will get a very long life with standard machine.

This test is conducted to check the presence of impurities in the insulation & tan value for each

phase & also for combined phases is noted down. Tan value should be generally less than or equal

to 2%.

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TESTING RESULTS:Rating: Vph (0.2Un ) Rated KV= 10.5KV , 3000RPM.

A.15 tan δ values of the different phases of a generator

Rating: 31.25, 250 MW, 11KV, 1640A, 0.8pf, 3000rpm.

SCC Testing:Ia Ib %In If Vd Id Dm O/P

0 0 0 0.01 559.0 404.58 226.16

337.2 337.6 20.57 94.7 515.95 443.1 228.60

664.8 666.2 40.5 187.1 516.04 481.4 248.44

1005.8 1006.1 61.34 281.51 516.52 533.6 275.6

1324.9 1326.5 80.84 369.16 516.54 651.25 336.35

1495.2 1496.9 91.2 415.2 516.18 690.3 356.32

A.16 tabular form for short circuit currents and voltages

Wph Tan

Wph

Tan

uph

Tan

vph

Tan

Wph uph

vph

2.1 0.806 0.815 0.811 1.18

4.2 0.820 0.832 0.830 1.209

6.3 0.857 0.869 0.868 1.230

8.4 0.899 0.903 0.905 1.254

10.5 0.941 0.938 1.268

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OCC Testing:Vab Vbc Vca %En If Vd Id Dm O/P

35.3 35.3 35.3 35.32 0.01 512.4 419.85 215.16

2281.2 2282.5 2281.8 20.74 40.44 512.93 423.15 216.62

4447.42 4441.4 4448.2 40.44 77.05 512.4 442.45 226.71

6662.0 6665.0 6663.0 66.58 116.65 512.3 466.43 288.95

8845.0 8849.0 8846.0 80.4 160.3 512.19 501.08 256.64

10015.0 10019.0 10017.0 91.06 186.56 512.25 517.25 264.93

A.17 tabular form showing open circuit voltages and currents obtained in the testing

79

14.3.4. Graphs

KV

TA

N δ

80

Fig. 2Comparative functional testing using the stator slot model confirms the progress achieved in extending the service life of 13.8-kV insulation of varying compositions.

Resistance Measurement:Instrument: Micro ohm meter

Resistance at 25c (m) Resistance at 20c (m)

Rotor 264 258.92

R75 = (( 235+75)/(235+20)) x R20= (310/255) x (0.2587)=0.3147

Rotor current = 562 A

Efficiency = (output)/(output+losses)

Losses = 99.532 + 9.9532 + 39.39 + 385.15 + 286. 38 = 820.40

Efficiency= (25000/25000 + 820 .40 ) = 96.82%

18.PRESENT INSULATION SYSTEMS IN THE WORLD Four major manufacturing processes have been and are still widely used to form and consolidate

insulation systems for form-wound stators. They are:

1. Vacuum pressure impregnation (VPI) of individual coils and bars

2. Global VPI of complete stators

3. Hydraulic molding of individual coils and bars using resin-rich tapes

4. Press curing of individual coils and bars, also using resin-rich tapes

There are some combinations of these methods also in use. The binder resins can be

categorized as high- or low-solvent-containing and solvent less, as well as by their chemical nature.

Although no longer manufactured for coils in new stators, there are many machines still in service,

and expected to remain in use for several more decades, that are insulated with asphaltic mica

splitting.

There are four principal drivers that govern the selection of the insulation systems currently being

manufactured. They are:

1. Good service experience with earlier versions of the same basic system

2. Commercial availability of the materials to be used

3. Relative costs of the raw materials and processes in the competitive machine-sales environment

4. Design advantages or limitations each insulation system and process brings to the final generator or

motor for its expected service life and economy of operation

81

New insulating materials may require the development of new or significantly modified

manufacturing processes to obtain the insulation system improvements inherent in the materials. By

the 1990s, the major insulation suppliers were offering full insulation systems, including the basic

processing know-how, to their customers. For smaller OEMs and most repair shops, the insulation

supplier’s materials, other acceptable materials, and processing specifications are all that is needed to

support work. The final insulation system may use the materials supplier’s trade name, e.g., VonRoll

ISOLA’s Samicatherm™ and Samicabond™.

Larger OEMs still work with insulation suppliers to optimize both the materials and processes for

new or changed insulation systems.

The first consideration in using modern insulation systems is the method of applying the

ground-wall materials to form-

wound stator coils and bars

discussed in previous sections, the

Haefley process was widely used

decades ago to apply wide sheets of

insulation material to coils.

Presently, however,

virtually all ground-walls are

fabricated by the application of

relatively narrow (2–3 cm wide)

tapes. When tapes were first

introduced, and for many decades thereafter, they were applied with hand by skilled tradesmen. There

are many companies into insulation manufacture, mainly all the companies have these insulation

systems as a trade secrets. So one cannot point out which is the best insulation system as there are

many factors such as availability in a particular country, so insulation systems are given different

names ,though the composition just differs a wee bit, so let us have a brief overview

16. Westinghouse Electric Co.: Thermalastic™

Westinghouse Thermalastic™, the first modern synthetic insulation system. The first

Thermalastic insulated generator went into service in 1950, in the 1960’s minor changes that were

made included introduction of glass cloth as a backing material for the mica, resin modifications to

help VPI resin tank stability, and improvements in the partial discharge suppression treatments on

generator coil surfaces. Although large turbine generators continue to use the individual bar

impregnation and cure method, motors and smaller generators shifted to the global VPI method in the

early 1970s. The hybrid epoxy VPI resin used for turbine generators was optimized for the previously

B18 A modern stator bar taping machine that applies tape both in the slot and end winding portions of the bar.

82

developed processing equipment and insulation requirements. It is comprised of a modified epoxy

resin, prepared in a resin cooker to create polyester linkages, and is compatible with styrene for

viscosity control. The final resin cure was achieved by cross-linking through the epoxy or oxirane

group.

After Siemens acquired Westinghouse in the late 1990s, the Thermalastic system underwent

many refinements in materials and processing while maintaining the same resin system. Now a days

there are not much changes though.

17. General Electric Co. :Micapals I and II™, Epoxy Mica Mat™, Micapal HT™, and hydromat™

It was introduced to the industry in an IEEE Technical paper in 1958, after several years of

limited production. Micapal 1 contained approximately 50% GE Micamat™ (paper), made with

calcined muscovite, and 50% muscovite splitting.

winding operation.

After a 12-year development program, General Electric announced the MICAPAL II™ insulation for

large turbine generator stator windings in 1978. This solvent less, resin-rich, second-generation epoxy

mica paper insulation system has been used on most large steam turbine generators since that time.

In 1999, GE began to offer a reduced-build strand-and-turn insulation, using similar metal

oxide fillers in the large-motor business. These machines use the global epoxy VPI process to make

the glass-fabric-supported Mica-mat insulation systems for machines at least up to 13.8 kV ratings.

Several generations of VPI resins have been used by GE for motor manufacture. Two of these epoxy

resin systems have been based on controlled reactivity chemistry. The most recent improvement

creates polyether linkages in cured Di-Glycidyl ether Bis-Phenol, an epoxy resin provides high

reactivity at curing temperatures with excellent shelf life at room temperature.

18. Alsthom, GEC Alsthom, Alstom Power: Isotenax™, Resitherm™, Resiflex™,

Resivac™, and Duritenax™

During the 1950s, Alsthom licensed the resin technology used in the GE Micapal I system to create

the first Isotenax™ system. There were several differences in materials and processes between the

two systems. Isotenex used only mica paper, not mica splittings. The resin-rich impregnating epoxy

contained significant amounts of a solvent mixture that had to be removed after the glass-backed mica

paper tape was wrapped around the stator bars. Since the 1980s the UK operations of Alsthom have

also worked with global VPI processing and an insulation system called Resivac™. Recent advances

in the VPI system have used bisphenol epoxy resins with a latent Lewis acid catalyst system

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19. Siemens AG, KWU: Micalastic™

Siemens began using the individual-bar VPI process with polyester resins and mica splittings as early

as 1957 for hydro and steam turbine generators, with initial help from Westinghouse. This system

was trade named Micalastic. Production continued with this combination of resins and processes for

at least 10 years. Except for indirect cooled generators and direct-cooled generators rated at more

than about 300 MVA or so, which still use the individual bar epoxy VPI methods, the global VPI

process has been standard for all motor and turbo generator stators since 1986. For its large global

VPI stators, this manufacturer avoids difficulties due to shear stress at the interface of the bar to the

stator core by employing a slip plane. The slip plane consists of mica splitting sandwiched between

two semi conductive tapes.

20. ABB Industrie AG: Micadur™, Micadur Compact™, Micapact™

and Micarex™

Brown Boveri AG started changing from resin-rich asphalt mica flake ground-wall insulation about

1953, first using modified polyester resins and then switching to epoxy resins to make resin-rich

tapes. The ABB Group name for bars and coils manufactured from the epoxy resin-rich system is

Micarex™. Initially, these tapes were applied by hand and later by machine taping, followed by hot-

press consolidation and curing. New machine production with this system will stop with the end of

turbo generator production in Sweden, although some repair licensees will continue using Micarex

for some time.

The Micadur™ insulation system was introduced in 1955 by Brown Boveri as an individual

bar VPI method before the merger of ASEA with Brown Boveri to form ABB in the mid 1980s,

ASEA also developed the technology for individual bar VPI production, using similar materials. The

result was Micapact™, introduced in 1962 for the stator insulation of large rotating machines. It was

made with glass-backed mica paper, impregnated with a special mixture of an epoxy resin, curing

agent, and additives. Unlike most other VPI tapes, the glass backing and mica paper lack any

impregnant or bonding resin. The adhesion between mica paper and glass was accomplished by an

extremely thin layer of material, which was melted at a high temperature during formation of the

tape. The tape did not contain any volatile matter, which means that the completed machine taped bar

insulation was more easily evacuated and impregnated.

15.6 Toshiba Corporation: Tosrich™ and Tostight- I™

The Toshiba Tosrich™ insulation system for low-voltage, small-capacity generators with a relatively

small number of insulation layers was based on a resin-rich mica paper tape. The solvent containing

84

synthetic resin was impregnated into the mica tape, wound onto a coil and cured in a mold. Although

used successfully for many years for smaller machines, its replacement with a solvent less epoxy,

resin-rich mica paper tape during the 1990s allowed the improved Tosrich to be applied to medium-

capacity generators; it is still gaining manufacturing and service experience.

For larger machines, the Tostight-I™ insulation system was developed.A new generation of the

Tostight-I VPI insulation system was introduced in 1998. It has been optimized to improve heat

resistance and to be environmentally friendly in materials, equipment, production methods, and

disposal of waste. The mica paper has been changed to replace the aramid fibrids with short glass

fibers. The new impregnating resin is principally a high-purity, heat-resistant epoxy resin, employing

a complex molecular capsule, latent hardening catalyst that is activated by heat to quickly cure and

produce a high-heat-resistant, mechanically and electrically strong filling material for the mica. The

revised system is manufactured using new production equipment, including a fully automatic taping

machine and a new vacuum pressure impregnation facility and curing oven. The VPI tank is equipped

to control vacuum and impregnation as a parameter of the coil capacitance. The new Tostight-I is

intended to be usable for all types of medium and large generators.

21. Mitsubishi Electric Corporation

The ground-wall insulation systems employed by Mitsubishi until the 1990s were largely

based on licenses obtained from Westinghouse. During the late 1990s, Mitsubishi introduced a new

global VPI insulation system for air-cooled generators up to 250 MVA. The new system

supplemented an older global VPI system, used for air-cooled generators of up to 50 MVA rating.

The new system uses a glass-fabric-backed mica paper tape, bonded with a very small amount of

hardener-free epoxy resin as an adhesive. The global VPI resin is an epoxy anhydride.

22. Hitachi, Ltd.: Hi-Resin™, Hi-Mold™, and Super Hi-Resin™

Hitachi also introduced a pre impregnated or resin-rich mica paper insulation, called the Hi-

Mold™ coil in 1971 This press-cured system uses an epoxy resin to impregnate glass-cloth-backed

mica paper, which is partially cured to the B stage. The high-performance resin was selected to obtain

superior electric and thermal characteristics for use in machines rated for up to Class F insulation

performance. The Hi-Mold system is used for hydro and gas turbine peaking generators and for heavy

duty or other unfavorable environments in synchronous and induction motors.

23. Summary of Present-Day Insulation Systems:

A review of subsections 4.2.1 through 4.2.8 shows that all of the world’s larger OEMs are

currently using various mixtures and types of epoxy resins and mica paper to make their stator coil

ground-wall insulation systems. The compositions are adjusted or tailored to accommodate the exact

process used in their manufacture. The end results are comparable in terms of inherent insulation

85

quality as related to the machine and insulation design parameters, provided that consistent quality

control practices are routinely carried out. This fact is recognized by some large suppliers of rotating

machines, who will, in times of extraordinary demand, out-source or purchase generators to their own

design from competitors, while allowing the supplier to use their own insulation systems. We

presented here the most efficient and reliable system of insulation, the Micalastic insulation

A NEW FLUSH IN INSULATION SYSTEM

1. MICALASTIC

Of all these

insulation processes

the insulation which

was preferred by

ITAIPU power plant

was the

MICALASTIC the

features of which

will be briefed

below:

“As central

components of

hydroelectric power

plants, generators are

subjected to

operating stresses

which influence the

long-term

performance of the

winding insulation.

B19 Rotor of the worlds largest hydro generator ITAIPU at the assembly

Failure of the insulation can lead to lengthy downtimes. The un surpassed reliability of products such

as MICALASTIC® insulation is therefore of great economic significance.”

The capacity of a hydroelectric power plant is determined by the available water flow and

head. Both of these parameters vary widely, and generators can be dimensioned for any rating

between 10 kW and 800 MW. The head determines the turbine type as well as the speed, which can

86

lie between 50 and 1500 rpm. Additional parameters include the generator voltage, the rotor’ s

moment of inertia, the runaway speed of the turbine, the physical design of the generator (horizontal

or vertical) and various requirements imposed by the grid. Hydroelectric generators are therefore

always custom designed. Dimensions and weights can assume enormous proportions External

diameters of up to nearly 23 meters are possible, and total weight can amount to as much as 3500

metric tons. Generators of this size cannot be assembled and tested at the factory. Nevertheless, the

generators can be expected to operate well right after their initial installation at the power plant. It

was once correctly stated that” the construction of a hydroelectric generator can be compared to

making a tailor made suit without trying it on”.

To date, Siemens has manufactured more than 1200 large hydroelectric generators with a combined

capacity in excess of 80,000 MVA. Of these, 360 generators (over 50,000 MVA) have

MICALASTIC windings. These machines are characterized by their outstanding reliability, which

can be attributed in large measure to their high quality MICALASTIC insulation system.

2. The MICALASTIC Insulation in ITAIPU™

MICALASTIC is the registered trademark for Siemens insulation systems for high-voltage windings

of rotating electrical machines. These systems use mica, a material capable of withstanding high

electrical and thermal loads, together with curable, elastic epoxy resins as bonding material. Since the

early days of electrical machine construction, the naturally occurring, inorganic mineral mica has

been an indispensable constituent of high voltage insulation systems. The most important criterion for

the use of mica is its ability to durably withstand the partial electrical discharges which can occur

inside the insulation due to high electrical stresses.

16.2.1 Manufacturing and Design

As early as 1957, Siemens-Dynamo werk in Berlin manufactured the first stator windings that made

use of mica tape and a vacuum-pressure impregnation process. With this method, single coils and

Roebel bars for hydroelectric generators are continuously wrapped with mica tape in the slot and end

sections. The taped winding elements are then dried out and degassed in a vacuum impregnation tank,

and flooded with low-viscosity, curable synthetic resin. High nitrogen pressure applied to the

impregnating bath completely impregnates the mica tape. After being placed in accurately sized,

portable pressing molds, the insulation is cured at high temperatures in large chamber ovens.

Continued development of this insulation technology ultimately led to the use of a film of ground

mica on mechanically strong glass fabric as the carrier material with epoxy resin as the impregnant,

which produced a very durable electrically, thermally and mechanically),modern insulation system.

87

Long duration tests in a slot model were unnecessary, since the desired voltage endurance had already

been achieved in the previous development stages (Fig. 2) using lower-quality carrier materials.

Short-duration tests were performed, however, for verification.

16.2.2 Fitting of Roebel Bars into Slots:

Winding elements with cured MICALASTIC insulation are secured in the slots by filling up the

tolerances between the slot wall and the conductive surface (coil side corona shielding) of the bar

insulation. Initially, Siemens used graphite-treated paper as filler material. Since about 1969, however

a special bar fitting procedure has been used for hydroelectric generators. The main features of this

procedure are U-shaped slot liners made of polyester fleece impregnated with a conductive material,

and a conductive, curable synthetic resin paste between the surface of the bar insulation and the slot

liner (Fig. 3). Therefore, the insulation does not stick to the stator core, and the option of removing

the bars, even though seldom required, is retained. In the radial direction, the slot portion of the

winding elements is secured by means of various packing strips or ripple springs, and slot wedges.

Bracing the end windings and jumpers by using glass fiber reinforced spacers and epoxy-resin

impregnated cording makes the winding resistant to electro dynamic forces during operation and to

possible short-circuit faults. This resistance is also aided considerably by the mechanical stiffness of

the MICALASTIC insulation, which is also cured within the end winding.

16.2.3 Thermal Stability

The MICALASTIC insulation system was developed strictly for a continuous load in accordance

with temperature class F (155°C). Nevertheless, generator design engineers generally guarantee

compliance with class B (130°C) temperature limits for nominal operating conditions, as is also

required in most invitations to tender. In practice, the stator windings of hydroelectric generators are

frequently dimensioned for even lower operating temperatures, because the stators will usually be

optimized for good efficiency by adding electrically active material (winding copper and core

lamination). Particularly low operating temperatures can be expected in the case of stator windings

with direct water cooling. With an appropriately dimensioned de mineralized-water cooling system,

the maximum winding temperature can be reduced to 70°C and lower. Thermal aging of the

insulation is therefore essentially eliminated, and thermo mechanical stresses are also substantially

reduced. The resulting increase in operational reliability makes a real difference in the case of

hydroelectric generators which are essential to safe grid operation

88

17. CONCLUSION:

Hence Vacuum-Pressure Impregnation technology can be used in a wide range of applications from

insulating electrical coil windings to sealing porous metal castings. It normally produces better work

in less time and at a lower cost than other available procedures.

Our VPI systems can be configured in a variety of ways, depending on the size and form of the

product to be impregnated, the type of impregnant used and other production factors. System

packages include all necessary valves, gauges, instruments and piping. These systems can be large or

small, simple or highly sophisticated and equipped with manual, semi-automatic or automatic

controls.

Vacuum Pressure Impregnation (VPI) yields superior results with better insulating

properties, combined with “flexible” rigidity, resulting in greater overall reliability and longer life.

VPI reduces coil vibration by serving as an adhesive between coil wires, coil insulation, and by

bonding coils to their slots.

18 Bibliography

##############################Abbey eeeeedddde see pictures of ABB n bhel pics ,also tabular forms to venky show, complete graphs ,roebel shit of venky!!