Manufacturing and Design of Insulation System For

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MANUFACTURING AND DESIGN OFINSULATION SYSTEM FOR

AIR COOLED TURBO GENERATORBY V.P.I PROCESS

A PROJECT REPORT SUBMITTED IN PARTIAL

FULFILLMENT OF THE REQUIREMENTS

FOR THE AWARD OF

BACHELOR DEGREE

IN

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

REGD.OFFICE: BHEL, SIRIFORT, NEWDELHI-110 049

R.K.MANOHAR

Sr DGM

Quality Control(E.M)

BHEL, Ramachandra puram

T.Ravi. M.E..,

Asst prof.

Swarnandhra College

Narsapuram


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CERTIFICATE

This is to certify that the project entitledMANUFACTURINGAND DESIGN OF INSULATION SYSTEM FOR AIR COOLEDTURBO GENERATOR BY V.P.I PROCESS

Submitted by

G.VENKATESH BABU (04A21A0258)

M.KRISHNA CHAITANYA SARMA (04A21A0216)

M.V.SATYA TEJA (04A21A0254)

L.PRANEETH CHAITANYA (03A21A0226)

In partial fulfilment ofBACHELORS DEGREE IN ELECTRICAL AND

ELECTRONICS ENGINEERING for the academicYear 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 GUIDESHRI R.K.MANOHAR

DGM,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:

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

Quality Control (E.M), Asst prof.


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

College

APPROVED BY HOD OF EEE

TABLE OF CONTENTS

1. ABSTRACT 3

1.1 ACKNOWLEDGEMENTS 9

1.2. PROFILEOF BHEL 10

1.3. PREFACE 12

2. INTRODUCTION 13

2.1DRAWBACKSOFEARLYVPIPROCESS 13

2.2 ADVANTAGEOFPRESENTRESINPOORVPIPROCESS 14

3. INTRODUCTIONTOVARIOUSPARTSOFAGENERATOR 16

3.1 STATOR 16

3.2 ROTOR 17

3.3 FIELD CONNECTIONS AND MULTI CONTACTS 19

3.4 EXCITATON SYSTEM 20

3.5 PERMANENT MAGNET GENERATOR AND AVR 21

3.6 VARIOUS LOSSES IN A GENERATOR 23

4. MANUFACTUREOFGENERATOR

4.1VARIOUS STAGES IN MANUFACTUREOFGENERATOR 25

4.1.1 STATORMANUFACTURINGPROCESS 26

4.1.2 STATORCORECONSTRUCTION 26

4.1.3 PREPARATIONOFSTATORLAMINATIONS 26

4.1.4 RECEPTIONOFSILICONSTEELROLLS 26

4.1.5 SHEARING 26


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4.1.6 BLANKINGANDNOTCHING 26

4.1.7 COMPOUNDNOTCHING 26

4.1.8 INDIVIDUALNOTCHING 27

4.1.9 DEBURRING 27

4.1.10 VARNISHING 27

5 STATORCOREASSEMBLY 28

5.1 TRAILPACKETASSEMBLY 28

5.2 NORMALCOREASSEMBLY 28

5.2.1 STEPPEDPACKETASSEMBLY 28

5.2.2 NORMALPACKETASSEMBLY 28

5.2.3 INPROCESSPRESSING 29

5.2.4 FITTINGOFCLAMPINGBOLTS 29

6. STATORWINDING 29

6.1 CONDUCTORMATERIALUSEDINCOILMANUFACTURING 29

6.2 TYPESOFCONDUCTORCOILS 29

7. ELECTRICALINSULATION 31

7.1 STATORWINDINGINSULATIONSYSTEMFEATURES 34

7.1.1 STRANDINSULATION 34

7.1.2 TURNINSULATION 38

7.1.3 GROUNDWALLINSULATION 39

7.1.4 SLOTDISCHARGES 40

7.2 INSULATINGMATERIALS 40

7.2.1 CLASSIFICATIONOFINSULATINGMATERIALS 41

7.2.2 INSULATINGMATERIALSFORELECTRICALMACHINES 42

7.3 ELECTRICALPROPERTIESOFINSULATIONANDFEWDEFINITIONS 43

7.3.1 INSULATIONRESISTANCE

7.3.2 DIELECTRICSTRENGTH

7.3.3 POWERFACTOR

7.3.4 DIELECTRICCONSTANT

7.3.5 DIELECTRICLOSS

8 RESINIMPREGNATION 44

8.1 INSULATIONMATERIALSFORLAMINATIONS 45

8.2 VARNISH 46

8.3 MATERIALFORRESINPOOR BARS 46

8.4 RESINRICHSYSTEM

MATERIALSFORRESINPOORHALFBARS


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9. MANUFACTUREOFSTATORCOILS 48

9.1 FORRESINPOORPROCESS 48

9.1.1 RECEPTIONOFCOPPERCONDUCTORS

9.1.2 TRANSPOSITION

9.1.3 PUTTYOPERATION 49

9.1.4 STACKCONSOLIDATION

9.1.5 BENDING

9.1.6 FINALTAPING

9.2 FORRESINRICHPROCESS 50

9.2.1 PUTTYWORK

9.2.2 FINALTAPING

9.2.3 FINALBAKING 51

10. ANOVERVIEW

10.1 ADVANTAGESOFRESINPOORSYSTEM 52

10.2 DISADVANTAGESOFRESINPOORSYSTEM 52

10.3 ADVANTAGESOFRESINRICHSYSTEM

10.4 DISADVANTAGESOFRESINRICHSYSTEM

11. ASSEMBLYOFSTATOR 52

11.1 RECEPTIONOFSTATORCORE 53

11.2 WINDINGHOLDERSASSEMBLY

11.3 STIFFENERASSEMBLY

11.4 EYEFORMATION

11.5 CONNECTINGRINGSASSEMBLY

11.6 PHASECONNECTORS

12. THEVPIPROCESS

12.1 INTRODUCTIONTO VPI PROCESS 54

12.2 HISTORY 54

12.3 VPI PROCESSFORRESINPOORINSULATEDJOBS 57

12.3.1 GENERAL 57

12.3.2 PREHEATING

12.3.3 VACUUMCYCLE

12.3.4 IMPREGNATION

12.3.5 POSTCURING

12.3 .6 ELECTRICALTESTING 61

12.4 GLOBALPROCESSING 61

12.5 RESINMANAGEMENT 61


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12.6 SPECIFICINSTRUCTIONS 62

12.7 PRECAUTIONS

12.8 FEATURESANDBENEFITS

13. FACILITIESAVAILABLEIN VPI PLANT BHEL 63

13.1 DATACOLLECTIONOFSAMPLES 65

13.1.1INDO-BHARAT IIROTOR 65

13.1.2 INDOBHARAT IISTATOR 68

13.1.3 High voltage levels of stator/rotor windings for multi turn machines 71

13.1.4 Testing results of indo bharat-ii rotor 72

13.1.5 Testing results of indo bharat-ii stator 73

14. Comparision between resin poor and resin rich systems74

14.1 Drawbacks 74

14.2 Suggestions

14.3 justification 75

15. Present insulation systems used in the world 75

15.1 Westinghouse electric co: Thermalastic 76

15.2 General electric co:

Micapals i and ii, epoxy mica mat, micapal ht and hydromat 77

15.3 Alsthom, gec alsthom, alstom power:

Isotenax, resitherm, resiflex, resivac and duritenax 77

15.4 Siemens ag, kwu: micalastic 78

15.5 Abb industrie ag:

micadur, micadur compact, micapact and micarex 79

15.6 Toshiba corporation: tosrich and tostight-i 79

15.7 Mistubishi electric corporation 80

15.8 Hitachi ltd: hi-resin and super high-resin 80

15.9 Summary of present day insulation 80

16. a new trend in insulation system

16.1 Micalastic 81

16.2 Micalastic insulation in itaipu 82

17. Conclusion 83

18. BIBLIOGRAPHY

LIST OF TABLES

Classification of insulations according to temperature

Insulating materials for electrical machines

Properties of an electrical insulation


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Materials used in resin rich and resin poor process

Table showing temperature and time to be maintained for different

type of jobs in VPI

LIST OF FIGURES

Photograph of a small round rotor

Figure showing the flow of eddy currents in rotor body with and

without laminations

Flow diagram showing various stages in generator manufacture

Fig showing the shape of laminations after completion of notching and

deburring operation

Roebel and diamond pulled coils

Schematic diagram for a 3- Y connected stator winding with 2 parallel

conductors per phase

Photographs of end windings and slots of random wound stator

Photograph of a form wound stator winding

A single form wound coil being inserted into two slots

C.S of a random stator winding slot

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

a.) form wound multi-turn coils.

b.) directly cooled roebel bars

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

are same

Side view showing one way of transposing insulated strands in stator

bar

C.S of multi-turn coil with 3 turns and 3 strands per turn

Layout of mould used in baking of stator by resin rich process

Vertical VPI tank for smaller jobs

Resin tank in which resin is stored


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LIST OF SYMBOLS ABBREVATIONS AND NOMENCLATURE

C.S. Cross Section

AVR Automatic Voltage Regulator

PMG Permanent Magnetic Generator

VPI Vacuum Pressurised Impregnation


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1.1. ACKNOWLEDGEMENTS


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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 BHELs efforts to grow and

diversify and become Indias leading engineering company. The company now

has 14 manufacturing divisions, 8 service centres and 4 power sector regional

centres, 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),

defence, 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 centres 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


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diverse industrial applications like Paper, Sugar, Cement, Petrochemical,

Fertilisers, 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 BHELs performance.

Established in the late 50s, 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 centres 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 specialised 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 Indias 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 isdeemed 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 livingfor 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 ourproject 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.


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2. Introduction

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 keepthe 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 optimisation of

insulation is an inevitable task, moreover the thickness of insulation should beon 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 ofthermosetting 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.


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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 1930s and early 1940s, 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 didnt 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 didnt 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.


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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 isachieved 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 minimise 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 FRAME

The stator frame is of welded steel single piece construction. It supportsthe 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 permeabilityand 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 minimise eddy current losses of rotating magnetic flux which

interacts with the core, the entire core is built of thin laminations. Eachlamination 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


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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.

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.

3.1.3STATOR 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.


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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 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.

F

igur e 1:

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

rotor.


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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 RTDs

(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.

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


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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

b. Hydrogen cooling-100MW


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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 todecrease the efficiency. These forces, as they affect the generator, areconsidered 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 generatoris 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 withits cross- sectional area. Copper loss is minimized in armature windings by usinglarge 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 thearmature. When an armature core is in a magnetic field the magnetic particles ofthe core tend to line up with the magnetic field. When the armature core isrotating, its magnetic field keeps changing direction. The continuous movementof the magnetic particles, as they try to align themselves with the magneticfield, produces molecular friction. This, in turn, produces heat. This heat is


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transmitted to the armature windings. The heat causes armature resistances toincrease. To compensate for hysteresis losses, heat-treated Silicon steellaminations are used in most dc generator armatures. After the steel has beenformed to the proper shape, the laminations are heated and allowed to cool. Thisannealing process reduces the hysteresis loss to a low value.

(ii) Eddy Current Loss (We):The core of a generator armature is made from soft iron, which is a conductingmaterial with desirable magnetic characteristics. Any conductor will havecurrents induced in it when it is rotated in a magnetic field. These currents thatare induced in the generator armature core are called EDDY CURRENTS. Thepower dissipated in the form of heat, as a result of the eddy currents, isconsidered a loss.

Eddy currents, just like any other electrical currents, are affected by theresistance of the material in which the currents flow. The resistance of anymaterial 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 softiron. Figure, view B, shows a soft iron core of the same size, but made up ofseveral small pieces insulated from each other. This process is called lamination.The currents in each piece of the laminated core are considerably less than inthe solid core because the resistance of the pieces is much higher. (Resistance isinversely proportional to cross-sectional area.) The currents in the individualpieces 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.

fig 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 maybe 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
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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.

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 beused. 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.

4. MANUFACTUREOFGENERATOR
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4 . 1 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:

4.1.1 Stator manufacture.

The various stages involved in the generator manufacture and their sub

processes are shown in the flow diagram given below.

Figure 2: flow diagram showing various stages involved in generator manufacture.

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

To facilitate manufacture erection and transport the stator consists of

following parts.

Now let us see the detailed study of stator manufacturing process.


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4.1.1 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 .2 STATOR CORE CONSTRUCTION:

4.1.3 PREPARATION OF STATOR LAMINATIONS

4.1.4 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.


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4.1.6COMPOUND 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.

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.


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Figure 2: Figure showing the shape of laminations after the

completion of notching and deburring operations.

4.1.10 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 ofTin

and Varnish is used such that the mixture takes 44sec to empty a DIN4

CUP.

The prepared laminations are subjected to following tests.

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.


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5.2 NORMAL CORE ASSEMBLY

5.2.1 Stepped packed assembly:

Steeped 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 inspectiondrift after normal packet assembly completion 1 layer of HGL laminations are

placed and one layer of ventilation lamination are placed and 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 magneticflux is obtained. Stacking mandrels and bolts are inserted into the windings slot

bores during stacking provide smooth slot walls.

5.2.3 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 1st

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.

5.2.4 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


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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.

6 WINDING:

The 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.

6.1 Stator Winding

There are three main components in a stator, they are

6.2 copper conductors (although aluminum is sometimes used)

6.3 The stator core

6.4 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

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 therange from 1 KW to 1000 MW.

1. Random wound stators.

2. Form-wound stators using multi turn coils.


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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).

Add here diag of roebel and diamond

pulled coils

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

neutralisation 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.

Figure 1.4 is the circuit diagram of a typical three-phase motor or

generator stator winding.


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Figure 1.4 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 asa 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


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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 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.


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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.


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7.1 STATOR WINDING INSULATION SYSTEM FEATURES

The 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) insulation

2. Turn insulation

3. 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 componentsare subjected to are also described.

7.1.1 Strand Insulation

In 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


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(also called sub-conductors) is easier to bend into the required shape than one

large conductor.

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 notto 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 aneffective 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 crosssection. 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


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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 Amperes 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 3.8 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 (see Chapter

8). 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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7.1.1.1.1.1


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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)

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.

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 motormanufacturers 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 inSection 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 phaseground voltage to

get the voltage across each turn. For example, if a motor is rated 4160 V rms

(phasephase), the phaseground voltage is 2400V.

This will result in about 24 Vrms across each turn, if there are 100 turns betweenthe phase end and neutral. This occurs because coil manufacturers take

considerable trouble to ensure that the inductance of each coil is the same, and


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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 Section 8.7. 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 turnturn AC voltage and the phase coil-to-coil

voltage.

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 [1.11].

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


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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 Chapter 8, 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.

7.1.4Slot 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 ionisation 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


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7.2.1 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.

i. 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.

2. Liquids (oils and varnishes)

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

etc.

3. Gases

EX: Dry air, carbon dioxide, nitrogen etc.

CLASSIFICATION ACCORDING TO TEMPERATURE:

Class Permissible

temperature

Materials

Y 90 Cotton, silk, paper, cellulose, wood etc neitherimpregnated nor immersed in oil. These areunsuitable for electrical machine andapparatus as they deteriorate rapidly and areextremely hygroscopic.

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

E 120 Synthetic material of cellulose base

B 130 Mica, asbestos, glass fibre with suitablebonding substance

F 155 Material of class B with binding material ofhigher thermal stability.

H 180 Glass fibre and asbestos material and built upmica with silicon resins.

C Above180

Mica, porcelain, quartz, glass (without anybonding agent) with silicon resins of higherthermal stability.

7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES:


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Name ofMaterial

Insulation

Class

Shelf life(In

months)At20oC

At5oc

7.2.2.1.1.1.1.1Application

1. Samicathermcalmica glass-n,mimica, domica,folium, filamicnovobond-s, epoxytherm laxman isolacalmicaflex

F 6 12 Main insulation of stator bars

2. Samica flex H 4 8 Overhanginsulation ofmotor coils, at 3rd

bends of multiturn coil

3. Vectro asbestos(365.02/365.32)

4. (used in resin rich)

B/F 2 8 Main pole coils of synchronousmachines

5. Epoxideimpregnated glasscloth

7.2.2.1.1.1.2 F

6 12 Winding holdersand inter-half insulation

6. Polyester resin mat& rope

6 Bar to windingholder & stiffener

groove of supportsegment of clamping plate

7. GlassoflexTurbo laminate

F 6 12 Inter-turninsulation of rotorwinding

8. Hyper seal tape F 6 12 As finishing layerin overhangs ofmotor coils

9. SIB775 or 4302varnish

F 6 12 StackConsolidation of

stator bars10.SIB475 or 4301

varnishF 6 12 Base coat varnish

before taping ofstator bars

11.SIB 643 or8003Varnish or K8886varnish

4 8 Conductive coat instraight portion ofstator bars

12.SIB 642 or 8001varnish

4 8 At slot emergeportion on statorbars


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7.3 FEW DEFINITIONS OF ELECTRIAL PROPERTIES OFINSULATION:

7.3.1 INSULATONRESISTANCE:

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.

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

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


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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.

8. Resin Impregnation:

Resin impregnation fills the porosity of a part with a resin to create apressure-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 ofAnaerobic 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 pressuretightness 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. Withresin 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 partswere mechanically plated. Resin impregnation allows the use of electroplating.


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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.

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 Fibre

Reinforced (FPR) and used in glass fibre boats, lightweight helicopters and

aeroplanes 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:


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

2) Chemical resistance is high.

3) Good adhesion to metals.

4) To impact hardness certain organic acid anhydrides and alphabetic aminesare 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.

8.2VARNISH

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 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

given below.

Let us have an overview.


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8.4RESIN 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%

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

Pre-assembled stator core including stator frame.

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.


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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 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 3rd bend

and the bars are clamped tightly. Now the clamps are heated to 60 degrees

for 30mins. Inter half and inter strip tests follows.

9.1.6Final 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


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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.

Fig 2. Cross-section of a multi turn coil, where three turns and three strands per turn.

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 that of resin poor system. Stacking of coils isdone. In this case high resin glass cloth is used for preventing inter half shorts.

There is a difference in putty work.

9.2.1 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


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tape is done for over hangs and reshaping is done. To ensure no short circuits

half testing of coils is done.

9.2.2 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.

Figure 3: Layout of a mould used in baking of stator by Resin rich process

9.2.3Final 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


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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.

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 m

Manufacturing and Design of Insulation System For

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