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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 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.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 fulfilment 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.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.

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

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APPROVED BY HOD OF EEE

TABLE OF CONTENTS

1. ABSTRACT 3

1.1 ACKNOWLEDGEMENTS 9

1.2. PROFILE OF BHEL 10

1.3. PREFACE 12

2. INTRODUCTION 13

2.1DRAWBACKS OF EARLY VPI PROCESS 13

2.2 ADVANTAGE OF PRESENT RESIN POOR VPI PROCESS 14

3. INTRODUCTION TO VARIOUS PARTS OF A GENERATOR 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. MANUFACTURE OF GENERATOR

4.1VARIOUS STAGES IN MANUFACTURE OF GENERATOR 25

4.1.1 STATOR MANUFACTURING PROCESS 26

4.1.2 STATOR CORE CONSTRUCTION 26

4.1.3 PREPARATION OF STATOR LAMINATIONS 26

4.1.4 RECEPTION OF SILICON STEEL ROLLS 26

4.1.5 SHEARING 26

4.1.6 BLANKING AND NOTCHING 26

4.1.7 COMPOUND NOTCHING 26

4.1.8 INDIVIDUAL NOTCHING 27

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4.1.9 DEBURRING 27

4.1.10 VARNISHING 27

5 STATOR CORE ASSEMBLY 28

5.1 TRAIL PACKET ASSEMBLY 28

5.2 NORMAL CORE ASSEMBLY 28

5.2.1 STEPPED PACKET ASSEMBLY 28

5.2.2 NORMAL PACKET ASSEMBLY 28

5.2.3 IN PROCESS PRESSING 29

5.2.4 FITTING OF CLAMPING BOLTS 29

6. STATOR WINDING 29

6.1 CONDUCTOR MATERIAL USED IN COIL MANUFACTURING 29

6.2 TYPES OF CONDUCTOR COILS 29

7. ELECTRICAL INSULATION 31

7.1 STATOR WINDING INSULATION SYSTEM FEATURES 34

7.1.1 STRAND INSULATION 34

7.1.2 TURN INSULATION 38

7.1.3 GROUND WALL INSULATION 39

7.1.4 SLOT DISCHARGES 40

7.2 INSULATING MATERIALS 40

7.2.1 CLASSIFICATION OF INSULATING MATERIALS 41

7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES 42

7.3 ELECTRICAL PROPERTIES OF INSULATION AND FEW DEFINITIONS 43

7.3.1 INSULATION RESISTANCE

7.3.2 DIELECTRIC STRENGTH

7.3.3 POWER FACTOR

7.3.4 DIELECTRIC CONSTANT

7.3.5 DIELECTRIC LOSS

8 RESIN IMPREGNATION 44

8.1 INSULATION MATERIALS FOR LAMINATIONS 45

8.2 VARNISH 46

8.3 MATERIAL FOR RESIN POOR BARS 46

8.4 RESIN RICH SYSTEM

MATERIALS FOR RESIN POOR HALF BARS

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9. MANUFACTURE OF STATOR COILS 48

9.1 FOR RESIN POOR PROCESS 48

9.1.1 RECEPTION OF COPPER CONDUCTORS

9.1.2 TRANSPOSITION

9.1.3 PUTTY OPERATION 49

9.1.4 STACK CONSOLIDATION

9.1.5 BENDING

9.1.6 FINAL TAPING

9.2 FOR RESIN RICH PROCESS 50

9.2.1 PUTTY WORK

9.2.2 FINAL TAPING

9.2.3 FINAL BAKING 51

10. AN OVERVIEW

10.1 ADVANTAGES OF RESIN POOR SYSTEM 52

10.2 DISADVANTAGES OF RESIN POOR SYSTEM 52

10.3 ADVANTAGES OF RESIN RICH SYSTEM

10.4 DISADVANTAGES OF RESIN RICH SYSTEM

11. ASSEMBLY OF STATOR 52

11.1 RECEPTION OF STATOR CORE 53

11.2 WINDING HOLDERS ASSEMBLY

11.3 STIFFENER ASSEMBLY

11.4 EYE FORMATION

11.5 CONNECTING RINGS ASSEMBLY

11.6 PHASE CONNECTORS

12. THE VPI PROCESS

12.1 INTRODUCTION TO VPI PROCESS 54

12.2 HISTORY 54

12.3 VPI PROCESS FOR RESIN POOR INSULATED JOBS 57

12.3.1 GENERAL 57

12.3.2 PREHEATING

12.3.3 VACUUM CYCLE

12.3.4 IMPREGNATION

12.3.5 POST CURING

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12.3 .6 ELECTRICAL TESTING 61

12.4 GLOBAL PROCESSING 61

12.5 RESIN MANAGEMENT 61

12.6 SPECIFIC INSTRUCTIONS 62

12.7 PRECAUTIONS

12.8 FEATURES AND BENEFITS

13. FACILITIES AVAILABLE IN VPI PLANT BHEL 63

13.1 DATA COLLECTION OF SAMPLES 65

13.1.1INDO-BHARAT –II ROTOR 65

13.1.2 INDO BHARAT –II STATOR 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 systems 74

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

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17. Conclusion 83

18. BIBLIOGRAPHY

LIST OF TABLES

Classification of insulations according to temperature

Insulating materials for electrical machines

Properties of an electrical insulation

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-Ф 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 BHEL’s efforts to grow and diversify and become

India’s 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

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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, 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 BHEL’s performance.

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 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

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.

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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 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 optimisation 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.

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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. 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.

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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.

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 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 minimise 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

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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.

3.1.3 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.

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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.

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:

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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.

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

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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

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

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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.

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.

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(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

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.

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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.

(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.

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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 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.

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

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.

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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.

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.

SES LAB, 03/26/08,

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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.

4.1.8 Deburring operation :

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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.

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 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:

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 inspection drift 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 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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5.2.3 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 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 WindingThere are three main components in a stator, they are

6.2 copper conductors (although aluminum is sometimes used)

6.3 The stator core

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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 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).

Add here diag of roebel and diamond pulled coilsIn 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

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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.

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 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

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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 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 I 2R

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 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.

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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

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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

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

41

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 turn–turn 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 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 InsulationGround 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

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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.

7.1.4 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 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 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.

Till now we have discussed the manufacturing process, but the

manufacture is incomplete without insulation design.

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

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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.

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.

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.

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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.

7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES:

Name of Material

InsulationClass

Shelf life(In months)

7.2.2.1.1.1.1.1 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

5. Epoxide impregnated glass cloth 7.2.2.1.1.1.2

F

6 12 Winding holders and inter-half insulation

6. Polyester resin mat & rope

6 Bar to winding holder & stiffener groove of support segment of

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clamping plate7. Glassoflex

Turbo laminateF 6 12 Inter-turn insulation of

rotor winding8. Hyper seal tape F 6 12 As finishing layer in

overhangs of motor coils

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

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

11. SIB 643 or8003 Varnish or K8886 varnish

4 8 Conductive coat in straight portion of stator bars

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

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.

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.

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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.

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

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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.

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.

48

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.

8.1INSULATING 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

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:

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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.

8.3 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)

Materials used in resin rich process:

MATERIAL FOR RESIN RICH BARS:

Preprag Nomex Epoxy resin rich mica tape Glass tape PTFE tape

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VARNISH

Mica powder Graphite powder Conductive varnish Semiconductor varnish

8.4 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%

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.

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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

13.2.1.1.1.1MANUFACTURE 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.

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.

Figure 4: Side View showing one way of transposing insulated strands in stator bar.

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

52

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 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 3 rd 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.

53

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 is done. 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 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.

54

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

9.2.3 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.

55

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 later.

11. Assembly of stator:

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

assembly.

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11.1 Reception of stator core:

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.

57

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 minimising three circulating currents.

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.

12. The vpi process

12.1 INTRODUCTION TO VACUUM PRESSURE IMPREGNATION SYSTEM (VPI)

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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 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

59

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 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.

60

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 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.

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

61

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

Insertion of tub with job into the impregnation tank:

62

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

12.3.3 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.

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

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.

12.4 resin management

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

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

63

is 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.

Resin settling:

The resin is allowed to settle for about 4mins in such a way that bubble formation ceases

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. Fig: vertical

VPI tanks for smaller jobs

64

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.

Taking out the tub with job from impregnation tank

The 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.

12.3.5 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.

65

Cleaning : entire wound stator is cleaned for resin drips, after which its subjected to HV and tan

delta tests

12.3.6 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.

12.4 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.

12.5 Resin management:

After impregnation the VPI bath is pumped into storage tanks and cooled down to 5-10 C

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 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 fig: 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

66

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.

12.6 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

12.7 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.

12.8 Features and Benefits:

• 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

67

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

Preheating 605C for 3hrs

605C 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 pressure

3bar 4bar 4bar

Pressure holding

3hrs 3hrs 3hrs

Post curing At1405C for 14hrs

At1405C for 32hrs

At1405C for 32hrs

68

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

69

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


(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.


70

b.2 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.

b.2.8 INDO-BHARAT-II ROTOR

Preheating:

Indo Bharat II rotor is loaded for preheating in steam furnace on 30-5-2003 at 18:00hrs.

Date and timeRTD-I(C) RTD-II(C) Furnace air

temperatureRemarks

30.5.2003 19:00 32.0 30.0 45.6

Rotor temperature is reached to 603C at 2:00hrs on 31.5.2003 and it is maintained for 4 hrs i.e., up to 6:00 on 31.5.2003

30.5.2003 20:00 45.4 48.6 57.930.5.2003 21:00 49.9 50.9 63.430.5.2003 22:00 52.5 54.3 70.530.5.2003 23:00 53.3 55.1 73.4

30.5.2003 24:00 56.6 57.3 75.6

Rotor is switched to vac 140 tank at 7:00 hrs on 31.5.2003

31.5.2003 1:00 59.9 60.2 75.131.5.2003 2:00 62.4 63.9 77.031.5.2003 3:00 62.3 64.7 77.031.5.2003 4:00 63.3 64.1 75.031.5.2003 5:00 63.3 64.0 75.631.5.2003 6:00 63.1 63.7 75.6

71

12.9

12.10 Date and

time

12.11

12.12R

12.13

12.14R

12.15Room

temperature

12.16(C)

12.17

12.18 Remarks

12.19 31.5.2003 19:30

12.206

12.216

12.2236.0

12.23 Resin tank 025 is heated for impregnation

12.24 31.5.2003 21:30

12.257

12.266

12.2736.7

12.28

12.29 31.5.2003 23:30

12.308

12.318

12.3236.7

12.33 1.6.2003 1:30

12.341

12.359

12.3635.6

12.37 Resin admission started at 14:10 hrs on

72

31.5.2003

73

12.38 1.6.2003 3:30

12.391

12.401

12.4134.8

12.42 Resin admission completed at 14:25 hrs on 31.5.2003

12.43 1.6.2003 5:30

12.441

12.451

12.4633.8

12.47 Pressurisation started at 14:45 hrs on 31.5.2003

12.48 1.6.2003 7:30

12.491

12.501

12.5133.2

12.52 Pressurisation completed at 15:30hrs on 31.5.2003

12.53 1.6.2003 9:30

12.541

12.551

12.5636.5

12.57 Pressurisation hod up completed at 18:30hrs on 31.5.2003

12.58 1.6.2003 11:30

12.591

12.601

12.6138

12.62 Resin withdr

74

awal to storage tanks is from 18:30 to 18:45hrs on 31.5.2003

75

12.63 1.6.2003 13:30

12.641

12.651

12.6642.8

12.67 Rotor loaded in gas furnace at 19:15hrs on 31.5.2003

12.68 1.6.2003 15:30

12.691

12.701

12.7143.8

12.72 Rotor temperature is reached to 131.6 to 145.7C at 8:30hrs on 1.6.2003 and it is maintained for 14hrs i.e., up to 22:30hrs on 1.6.2003

12.73 1.6.2003 17:30

12.741

12.751

12.7642.8

12.77

12.78 1.6.2003

12.791

12.801

12.8141.0

12.82 Furnace

76

19:30 is switched at 22:30hrs on 1.6.2003 and circulation fans are kept running till the job temperature is reached to 70C to 75C.

12.83 1.6.2003 21:30

12.841

12.851

12.8640.8

12.87

12.87.5.1.1 Vacuum cycle:

Date and time Vacuum in

graph (mbar)Vacuum in

meter (mbar)

Job temperature

(C)Remarks

31.5.2003 7:00 -- -- 62.2

Vacuum pump started at 7:30 hrs on 31.5.2003.

31.5.2003 8:00 -- 3.0 61.531.5.2003 9:00 0.85 0.86 61.331.5.2003 10:00 0.54 0.55 61.131.5.2003 11:00 0.39 0.4 61.131.5.2003 12:00 0.38 0.4 61.131.5.2003 13:00 0.37 0.4 61.031.5.2003 14:00 0.36 0.39 61.0

RESIN CYCLE AND POST CURING CYCLE:

77

b.2.9 INDO-BHARAT-II STATOR:

Preheating:

Indo-Bharat-II stator is loaded for preheating in steam furnace on 7-5-2003 at 23:30hrs.

Date and Time RTD-I(C) RTD-II(C) Furnace air

temperature(C)12.87.5.1.1.1.1.1.1

7.5.2003 23:30 36.3 36.1 Stator temperature

is reached to 60.5C

to 62.9C(603C) at

7:30hrs on 8.5.2003

and it is maintained

for 12hrs i.e., up to

19:30hrs on

8.5.2003

8.5.2003 1:30 43.6 42.9

8.5.2003 3:30 52.0 51.74

8.5.2003 5:30 55.9 56.0

8.5.2003 7:30 60.5 62.9 Stator is loaded in

vac(140) tank at

21:00hrs on

8.5.2003

8.5.2003 9:30 61.3 62.9

8.5.2003 11:30 60.3 62.4

8.5.2003 13:30 60.3 62.6 Vac. Pump is started

at 2:30hrs on

9.5.2003

8.5.2003 15:30 62.5 62.9

8.5.2003 17:30 62.9 62.66

8.5.2003 19:30 62.4 62.1

78

Vacuum cycle:

12.87.5.1.1.1.1.2Vacuum in

graph (mbar)

Vacuum in

meter (mbar)

Job

temperature

(C)

8.5.2003 22:00 -- -- 54.37 Resin tanks 025,102 are

heated for impregnation

8.5.2003 0:00 -- -- 54.89 Viscosity of resin at 60C

is 33CP

9.5.2003 2:00 -- -- 59.02 Viscosity after aging is

36.10CP

9.5.2003 3:30 0.65 0.65 61.6 9.5.2003 and 10.5.2003

9.5.2003 5:30 0.41 0.40 63.59 Resin admission started at

19:45hrs

9.5.2003 7:30 0.28 0.29 64.2 Resin admission

completed at 19:55hrs

9.5.2003 9:30 0.22 0.22 63.2 Pressurisation started at

20:00hrs

9.5.2003 11:30 0.19 0.19 62.3 Pressurisation of

4kg/sq.cm reached at

21:20hrs

9.5.2003 13:30 0.18 0.18 62.1 Pressurisation hold up for

3hrs is at 0:20hrs

9.5.2003 15:30 0.17 0.17 62.0 Resin withdrawn to

storage tanks is from

0:30hrs –1:00hrs

9.5.2003 17:30 0.14 0.14 61.8 Stator loaded in hot air

furnace from 1:00hrs –

1:30hrs on 10.5.2003

9.5.2003 19:30 0.14 0.14 61.3

79

Post curing:

Date and

Time

ESOH

15T

TSOH

06B

ESW

02

TSW

13Core

Room

temperatureRemarks

10.5.2003

1:30hrs70.0 76.4 62.4 62.5 63.4 33.1

10.5.2003

4:30hrs126.7 131.4 94.7 102.3 98.8 31.7

10.5.2003

7:30hrs144.3 154.1 125.4 134.5 126.1 31.6

10.5.2003

10:30hrs147.7 154.9 139.9 145.1 140.6 34.8

10.5.2003

13:30hrs137.6 144.4 139.3 141.6 140.7 38.0

10.5.2003

16:30hrs136.9 144.2 140.0 140.9 140.6 38.4

10.5.2003

19:30hrs140.2 143.6 140.1 140.7 140.2 37.2

10.5.2003

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 10.5.2003 and

it is maintained for 32hrs

i.e. up to 17:30hrs on

11.5.2003.

11.5.2003

1:30hrs143.1 146.7 145.2 145.1 145.2 33.8

11.5.2003

4:30hrs144.3 151.0 143.6 144.0 144.7 31.1

11.5.2003

7:30hrs135.7 142.1 144.3 145.1 145.0 31.3

11.5.2003

10:30hrs135.0 135.7 135.1 135.0 135.8 34.8

11.5.2003

13:30hrs135.6 141.4 135.4 135.6 135.9 38.3

80

11.5.2003

17:30hrs148.0 149.2 142.8 142.2 142.1 39.8

Furnace is switched

off at 17:30hrs on

11.5.2003 and

circulation fans kept

running till the job

temperature is

reached from 70C-

75C

12.87.5.1.2

b.2.10 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


3. After laying third coil


4. After laying fourth coil


5. After laying fifth coil


6. After laying sixth coil

UT+650 -- Pole drops

7. After all -- -- R, Pole drops

81

connections8. 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

b.2.11 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 1.0523.0 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

b.2.12 TESTING RESULTS OF INDO-BHARAT-II STATOR:

Customer name: INDO-BHARAT-II STATOR

82

M/c rating: 10.0MW, 12kv, 0.8pf, 650A, 1500rpm.

Test: H.V test.

Stage: after impregnation.


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 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

34.5 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.


Excitation Side:

26 113.8 10 114.062 113.8

83

50 113.820 113.838 113.914 113.8

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

c. COMPARISION BETWEEN RESIN POOR AND RESIN RICH SYSTEMS:

RESIN POOR SYSTEM RESIN RICH SYSTEM

84

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.

Applications:• All critical machines

• Equipment exposed to frequent surges/starting

• Harsh or moist environments

• Motors that run at service factor

c.2 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.

c.3 SUGGESTIONS:

85

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:

c.4 JUSTIFICATION

We can say that VPI by resin poor system is more superior to other types of insulation by conducting

h v and tan delta tests, and the results of which are clearly indicated in the graphs below:

3. High Voltage test:

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.

The high voltage is applied to winding by increasing gradually to required value and

maintance for 1 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.

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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.

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TAN TEST:

Equipment: Schering Bridge

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%.

TESTING RESULTS:

Vph (0.2Un ) Rated KV= 10.5KV , 3000RPM.

Uph Tan

2.1 0.815

4.2 0.832

6.3 0.869

8.4 0.903

10.5 0.938

Wph Tan

2.1 0.806

4.2 0.820

6.3 0.857

8.4 0.899

10.5 0.941

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Vph Tan

2.1 0.811

4.2 0.830

6.3 0.868

8.4 0.905

Uph Vph Wph Tan

2.1 1.18

4.2 1.209

6.3 1.230

8.4 1.254

10.5 1.268

Rating: 31.25, 250 MW, 11KV, 1640A, 0.8pf, 3000rpm.

SCC:

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

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OCC:

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

4447.4

2

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

Graphs

90

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%

d. 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

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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. As discussed in Section 3.10,

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

d.2 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

Fig: A modern stator bar taping machine that applies tape both

in the slot and end winding portions of the bar.

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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 [4.4]. The hybrid epoxy VPI resin used for turbine generators was optimized for

the previously 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.

d.3 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 [4.5] 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 [4.8]. 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.

d.4 Alsthom, GEC Alsthom, Alstom Power: Isotenax™, Resitherm™, Resiflex™, Resivac™, and

Duritenax™

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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

d.5 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 [4.18]. For its

large global VPI stators, this manufacturer avoids difficulties due to shear stress at the interface of

the bar to the stator core (Section 8.2) by employing a slip plane. The slip plane consists of mica

splitting sandwiched between two semi conductive tapes.

d.6 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

94

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.

d.7 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

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.

d.8 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.

d.9 Hitachi, Ltd.: Hi-Resin™, Hi-Mold™, and Super Hi-Resin™

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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.

d.10 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 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

e. A NEW FLUSH IN INSULATION SYSTEM

e.2 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:

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As central components of hydroelectric power plants, generators are subjected to operating

stresses which influence the long-term performance of the winding insulation. 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 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

97

windings. These machines are characterized by their outstanding reliability, which can be attributed

in large measure to their high quality MICALASTIC insulation system.

e.3 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.

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. 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.

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

98

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.

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

f. 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.

99

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.

g. Bibliography

Book

Documents

insulation design

vpi system of insulation

design of insulation

bhel abstract

cost of insulation

narsapuram bhel

project guide t

air cooled turbo generator