Project 8

1

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

T.Ravi. M.E.., Asst prof. Swarnandhra College Narsapuram

R.K.MANOHAR Sr DGM Quality Control(E.M) BHEL, Ramachandra puram

2

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 by G.VENKATESH BABU (04A21A0258) M.KRISHNA CHAITANYA SARMA (04A21A0216) M.V.SATYA TEJA (04A21A0254) L.PRANEETH CHAITANYA (03A21A0226) In partial fulfillment of “BACHELORS DEGREE IN ELECTRICAL AND ELECTRONICS ENGINEERING” for the academic Year 2007-2008 of IV-Year from SWARNANDHRA COLLEGE OF ENGINEERING AND TECHNOLOGY, affiliated to JNT UNIVERSITY, WEST GODAVARI DIST., A.P, INDIA. A record of bonafide work carried by them under my guidance in “BHEL, RAMACHANDRAPURAM, HYDERABAD-32”. SIGNATURE OF PROJECT GUIDE SHRI R.K.MANOHAR DGM,B.Tech(Elect),(SQC&OR) Electrical Machines,(Quality Control), BHEL,Ramachandrapuram.

3

1. ABSTRACT In developing countries like India, power generation is a major break through to meet the present demands of the nation. Power generation of several types are on forefront, the dominant component of power generation is TURBO-GENERATOR which produces large capacity, the word “TURBO” stands for turbine drive. Generally the turbines used to drive these turbo-generators are of reaction type. In large-scale industries manufacturing generators, insulation design plays a vital role. Insulation is known to be the heart of the generator. If insulation fails, generator fails which leads to the loss of crores of rupees. The latest technology for insulation in the world and adopted by BHEL, (Hyderabad) unit is “VACUUM PRESSURE IMPREGNATION “which is of resin poor thermosetting type. This type is preferred as it is highly reliable and possesses good mechanical, thermal properties and di-electric strength. As the quantity of resin used is less, hence the over all cost of insulation is reduced. In our project we have made a detailed study of the VPI system of insulation. This system is employed by BHEL first in the country and second in the world next to Germany. Project Associates: G.Venkatesh Babu

(04A21A0258)

M.K.Chaitanya Sarma

(04A21A0216)

M.V.Satya Teja

(04A21A0254)

L.Praneeth Chaitanya

(03A21A0226)

Project Guide External:

Project Guide Internal:

Mr. R.K.Manohar.,

Mr. T.Ravi. M.E..,

Sr.D.G.M,

Assistant prof.- EEE dept.

Quality Control (E.M),

Swarnandhra College of

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

Engg and technology

APPROVED BY HOD OF EEE

4

TABLE OF CONTENTS 1. ABSTRACT

3

1.1 ACKNOWLEDGEMENTS

10

1.2. PROFILE OF BHEL

11

1.3. PREFACE

13

2. INTRODUCTION

14

2.1 DRAWBACKS OF EARLY VPI PROCESS

14

2.2 ADVANTAGE OF PRESENT RESIN POOR VPI PROCESS

15

3. INTRODUCTION TO VARIOUS PARTS OF A GENERATOR

17

3.1 STATOR

17

3.2 ROTOR

18

3.3 FIELD CONNECTIONS AND MULTI CONTACTS

19

3.4 EXCITATION SYSTEM

19

3.5 PERMANENT MAGNET GENERATOR AND AVR

20

3.6 VARIOUS LOSSES IN A GENERATOR

23

4. MANUFACTURE OF GENERATOR

26

VARIOUS STAGES IN MANUFACTURE OF GENERATOR

26

4.1 STATOR MANUFACTURING PROCESS

26

4.1.1 STATOR CORE CONSTRUCTION

27

4.1.2 PREPARATION OF STATOR LAMINATIONS

27

4.1.3 RECEPTION OF SILICON STEEL ROLLS

27

4.1.4 SHEARING

27

4.1.5 BLANKING AND NOTCHING

27

4.1.6 COMPOUND NOTCHING

27

4.1.7 INDIVIDUAL NOTCHING

28

4.1.8 DEBURRING

28

4.1.9 VARNISHING

28

5. STATOR CORE ASSEMBLY

29

5.1 TRAIL PACKET ASSEMBLY

29

5.2 NORMAL CORE ASSEMBLY

29 5.2.1 STEPPED PACKET

ASSEMBLY

29 5.2.2 NORMAL PACKET

ASSEMBLY

29 5.2.2.1 IN PROCESS PRESSING

30

5 5.2.2.2 FITTING OF CLAMPING BOLTS

30

6. STATOR WINDING

31

6.1 CONDUCTOR MATERIAL USED IN COIL MANUFACTURING

31

6.2 TYPES OF CONDUCTOR COILS

31

7. ELECTRICAL INSULATION

33

7.1 STATOR WINDING INSULATION SYSTEM FEATURES

35

7.1.1 STRAND INSULATION

35

7.1.2 TURN INSULATION

39

7.1.3 GROUND WALL INSULATION

40

7.1.4 SLOT DISCHARGES

41

7.2 INSULATING MATERIALS

41

7.2.1 CLASSIFICATION OF INSULATING MATERIALS

42

7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES

42

7.3 ELECTRICAL PROPERTIES OF INSULATION AND FEW DEFINITIONS

44

7.3.1 INSULATION RESISTANCE

44

7.3.2 DIELECTRIC STRENGTH

44

7.3.3 POWER FACTOR

44

7.3.4 DIELECTRIC CONSTANT

44

7.3.5 DIELECTRIC LOSS

44

8 RESIN IMPREGNATION

45

8.1 INSULATION MATERIALS FOR LAMINATIONS

46

8.2 VARNISH

47

8.3 RESIN POOR SYSTEM 8.4 RESIN RICH SYSTEM

48

9. MANUFACTURE OF STATOR COILS

48

9.1 FOR RESIN POOR PROCESS

48

9.1.1 RECEPTION OF COPPER CONDUCTORS

50

9.1.2 TRANSPOSITION

50

9.1.3 PUTTY OPERATION

50 9.1.4 STACK

CONSOLIDATION

51

9.1.5 BENDING

51

9.1.6 FINAL TAPING

51

9.2 FOR RESIN RICH PROCESS

52

6 9.2.1 PUTTY WORK 52 9.2.2 FINAL TAPING

52

9.2.3 FINAL BAKING 53 10. AN OVERVIEW

54

10.1 ADVANTAGES OF RESIN POOR SYSTEM

54

10.2 DISADVANTAGES OF RESIN POOR SYSTEM

54

10.3 ADVANTAGES OF RESIN RICH SYSTEM

54

10.4 DISADVANTAGES OF RESIN RICH SYSTEM

54

11. ASSEMBLY OF STATOR

54

11.1 RECEPTION OF STATOR CORE

55

11.2 WINDING HOLDERS ASSEMBLY

55

11.3 STIFFENER ASSEMBLY

55

11.4 EYE FORMATION

55

11.5 CONNECTING RINGS ASSEMBLY

56

11.6 PHASE CONNECTORS

56

12. THE VPI PROCESS

56

12.1 INTRODUCTION TO VPI PROCESS

56

12.2 HISTORY

57

12.3 VPI PROCESS FOR RESIN POOR INSULATED JOBS

59

12.3.1 GENERAL

59 12.3.2 PREHEATING

59 12.3.3 VACUUM CYCLE 60 12.3.4 IMPREGNATION 61 12.3.5 POST CURING 62 12.3.6 ELECTRICAL TESTING

63

12.4 GLOBAL PROCESSING

63

12.5 RESIN MANAGEMENT

63

12.6 SPECIFIC INSTRUCTIONS

63

12.7 PRECAUTIONS

64

12.8 FEATURES AND BENEFITS

64

7 13. FACILITIES AVAILABLE IN VPI PLANT BHEL

66

13.1 DATA COLLECTION OF SAMPLES

68

13.1.1 INDO-BHARAT –II ROTOR

68

13.1.2 INDO BHARAT –II STATOR

70

13.1.3 HIGH VOLTAGE LEVELS OF STATOR/ROTOR WINDINGS FOR MULTI74

TURN MACHINES

13.1.4 TESTING RESULTS OF INDO BHARAT-II ROTOR

75

13.1.5 TESTING RESULTS OF INDO BHARAT-II STATOR

75

14. COMPARISON BETWEEN RESIN POOR AND RESIN RICH SYSTEMS

77

14.1 DRAWBACKS

78

14.2 SUGGESTIONS

78

14.3 JUSTIFICATION

78

15. PRESENT INSULATION SYSTEMS USED IN THE WORLD 15.1 WESTINGHOUSE ELECTRIC CO: THERMALASTIC™

83 84

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

85

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

RESIVAC™ AND DURITENAX™

15.4 SIEMENS AG, KWU: MICALASTIC

85 86

15.5 ABB INDUSTRIE AG: MICADUR™, MICADUR COMPACT™, MICAPACT™ AND MICAREX™

86

15.6 TOSHIBA CORPORATION: TOSRICH™ AND TOSTIGHT-I™

87

15.7 MISTUBISHI ELECTRIC CORPORATION

87

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

87

15.9 SUMMARY OF PRESENT DAY INSULATION

87

16. A NEW TREND IN INSULATION SYSTEM

88

16.1 MICALASTIC

88

16.2 MICALASTIC INSULATION IN ITAIPU’

89

17. CONCLUSION

91

18. BIBLIOGRAPHY

92 A. LIST OF TABLES

A.1

CLASSIFICATION OF INSULATIONS ACCORDING TO TEMPERATURE

A.2

INSULATING MATERIALS FOR ELECTRICAL MACHINES

A.3

PROPERTIES OF AN ELECTRICAL INSULATION

A.4

MATERIALS USED IN RESIN POOR PROCESS

A.5

MATERIALS USED IN RESIN RICH PROCESS

8 A.6

TABLE SHOWING TEMPERATURE AND TIME TO BE MAINTAINED FOR DIFFERENT TYPE OF JOBS IN

VPI

A.7

PREHEATING OF INDO BHARAT II ROTOR

A.8

IMPREGNATION OF INDO BHARAT II ROTOR

A.9

VACUUM CYCLE OF INDO BHARAT II ROTOR

A.10 PREHEATING OF INDO BHARAT II STATOR A.11 VACUUM CYCLE OF INDO BHARAT II STATOR A.12 POST CURING OF INDO BHARAT II STATOR A.13 HIGH VOLTAGE LEVELS OF STATOR/ROTOR WINDINGS FOR MULTI TURN MACHINES A.14 COMPARISON BETWEEN RESIN RICH AND RESIN POOR PROCESS

76

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

79

A.16 TABULAR FORM FOR SHORT CIRCUIT CURRENTS AND VOLTAGES

79

A.17 TABULAR FORM SHOWING OPEN CIRCUIT VOLTAGES AND CURRENTS 80

OBTAINED IN THE TESTING

B. LIST OF FIGURES B1

PHOTOGRAPH OF A SMALL ROUND ROTOR

19

B2

FIGURE SHOWING THE FLOW OF EDDY CURRENTS IN ROTOR BODY WITH AND WITHOUT LAMINATIONS 25

B3

FLOW DIAGRAM SHOWING VARIOUS STAGES IN GENERATOR MANUFACTURE

B4

FIG SHOWING THE SHAPE OF LAMINATIONS AFTER COMPLETION OF NOTCHING AND

26 28

DEBURRING OPERATION

B5

SCHEMATIC DIAGRAM FOR A 3-Ф Y CONNECTED STATOR WINDING WITH 2 PARALLEL CONDUCTORS PER PHASE 32

B6

PHOTOGRAPHS OF END WINDINGS AND SLOTS OF RANDOM WOUND STATOR (COURTESY TECO WESTING HOUSE) 33

B7

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

34

B8

A SINGLE FORM WOUND COIL BEING INSERTED INTO TWO SLOTS

34

B9

C.S OF A RANDOM STATOR WINDING SLOT

36

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

37

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

38

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

40

ARE SAME

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

BAR.

49

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

50

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

51

B16 VERTICAL VPI TANK FOR SMALLER JOBS

61

B17 RESIN TANK IN WHICH RESIN IS STORED

63

B18 MODERN STATOR BAR TAPING MACHINE THAT APPLIES THE TAPE ON BOTH IN THE BOTH SIDES B19 ROTOR OF THE WORLDS LARGEST HYDRO GENERATOR ITAIPU AT THE ASSEMBLY

83

10 C. LIST OF SYMBOLS ABBREVATIONS AND NOMENCLATURE C.S.

CROSS SECTION

AVR

AUTOMATIC VOLTAGE REGULATOR

PMG

PERMANENT MAGNETIC GENERATOR

VPI

VACUUM PRESSURIZED IMPREGNATION

Up

Final test voltage

Un

Rated Voltage of Generator

RTD

Resistance temperature Detectors

HGL

High Glass Laminations

IR

Insulation Resistance

11

1.1 ACKNOWLEDGEMENTS The successful completion of any task would be incomplete without greeting those who made it possible and whose guidance and encouragement made our effort success. With profound gratitude, respect and pride, we express our sincere thanks to Sri. Ramesh Babu, Secretary and correspondent of our college for providing necessary facilities for doing the project. We wish to express our gratitude to Dr. M. Muralidhara rao, our principal, for having given us permission to carry out the project. We express our deep sense of gratitude to Sri. P.V.V. Satya Narayana, Head of Dept. of Electrical Engineering for his learned suggestions and encouragement which made this project a success. We express sincere thanks to our internal guide Mr. T. Ravi, Asst prof, Department of Electrical and Electronics Engineering for his encouragement, which made this project a success. We express our earnest thanks to Mr. R. K. Manohar our external project guide who had given valuable suggestions throughout our project. Finally we thank every one who directly or indirectly helped for our project.

-Project Associates.

12 1.2 PROFILE OF B.H.E.L. Bharat Heavy Electrical Limited (BHEL) is today the largest engineering enterprise of India with an excellent track record of performance. Its first plant was set up at Bhopal in 1956 under technical collaboration with M/s. AEI, UK followed by three more major plants at Haridwar, Hyderabad and Tiruchirapalli with Russian and Czechoslovak assistance. These plants have been at the core of BHEL’s efforts to grow and diversify and become India’s leading engineering company. The company now has 14 manufacturing divisions, 8 service centers and 4 power sector regional centers, besides project sites spread all over India and abroad and also regional operations divisions in various state capitals in India for providing quick service to customers. BHEL manufactures over 180 products and meets the needs of core-sectors like power, industry, transmission, transportation (including railways), defense, telecommunications, oil business, etc. Products of BHEL make have established an enviable reputation for high quality and reliability. BHEL has installed equipment for over 62,000 MW of power generation-for Utilities, Captive and Industrial users. Supplied 2,00,000 MVA transformer capacity and sustained equipment operating in Transmission & Distribution network up to 400kV – AC & DC, Supplied over 25,000 Motors with Drive Control System Power projects. Petrochemicals, Refineries, Steel, Aluminium, Fertiliser, Cement plants etc., supplied Traction electric and AC/DC Locos to power over 12,000 Km Railway network. Supplied over one million Valves to Power Plants and other Industries. This is due to the emphasis placed all along on designing, engineering and manufacturing to international standards by acquiring and assimilating some of the best technologies in the world from leading companies in USA, Europe and Japan, together with technologies from its-own R & D centers BHEL has acquired ISO 9000 certification for its operations and has also adopted the concepts of Total Quality Management (TQM). BHEL presently has manufactured Turbo-Generators of ratings up to 560 MW and is in the process of going up to 660 MW. It has also the capability to take up the manufacture of ratings unto 1000 MW suitable for thermal power generation, gas based and combined cycle power generation aswell-as for diverse industrial applications like Paper, Sugar, Cement, Petrochemical, Fertilizers, Rayon Industries, etc. Based on proven designs and know-how backed by over three decades of experience and accreditation of ISO 9001. The Turbo-generator is a product of high-class workmanship and quality. Adherence to stringent quality-checks at each stage has helped BHEL to secure prestigious global orders in the recent past from Malaysia, Malta, Cyprus, Oman, Iraq, Bangladesh, Sri Lanka and Saudi Arabia. The successful completion of the various export projects in a record time is a testimony of BHEL’s performance.

13 Established in the late 50’s, Bharat Heavy Electrical Limited (BHEL) is, today, a name to reckon with in the industrial world. It is the largest engineering and manufacturing enterprises of its kind in India and is one of the leading international companies in the power field. BHEL offers over 180 products and provides systems and services to meet the needs of core sections like: power, transmission, industry, transportation, oil & gas, non-conventional energy sources and telecommunication. A wide-spread network of 14 manufacturing divisions, 8 service centers and 4 regional offices besides a large number of project sites spread all over India and abroad, enables BHEL to be close to its customers and cater to their specialized needs with total solutions-efficiently and economically. An ISO 9000 certification has given the company international recognition for its commitment towards quality. With an export presence in more than 50 countries BHEL is truely India’s industrial ambassador to the world.

14 1.3 PREFACE Power is the basic necessity for economic development of a country. The production of electrical energy and its per capital consumption is deemed as an index of standard of living in a nation in the present day civilization. Development of heavy or large-scale industries, as well as medium scale industries, agriculture, transportation etc, totally depend on electrical power resources of engineers and scientists to find out ways and means to supply required power at cheapest rate. The per capital consumption on average in the world is around 1200KWH, the figure is very low for our country and we have to still go ahead in power generation to provide a decent standard of living for people. An AC generator is a device, which converts mechanical energy to electrical energy. The alternator as it is commonly called works on the principle of ‘Electro Magnetic Induction’. Turbo generators are machines which can generate high voltages and capable of delivering KA of currents .so the designer should be cautious in designing the winding insulation. So insulation design plays a major role on the life of the Turbo Generator. In our project we deal with the “Manufacture process of turbo generator and its insulation design by VPI process.” The first half of project is concerned with the aspects of generator manufacturing comprising of stator manufacturing, in a step by step procedure involving different stages, and the latter stage includes the insulation design of the generator by VPI process in a detailed manner, which completes the generator design. We more over stress mainly on VPI insulation process. Before going deep into the topic we will start with a brief introduction and we conclude with the recent trends in the insulation systems used all over the world.

15

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 optimization of insulation is an inevitable task, moreover the thickness of insulation should be on par with the level of the voltage, also non homogenic insulation provisions may lead to deterioration where it is thin and prone to hazardous short circuits, also the insulating materials applied to the conductors are required to be flexible and have high specific (dielectric) strength and ability to withstand unlimited cycles of heating and cooling. Keeping this in view among other insulating materials like solids gases etc liquid dielectrics are playing a major role in heavy electrical equipment where they can embedded deep into the micro pores and provide better insulating properties. Where as solid di-electrics provide better insulation with lower thickness and with greater mechanical strength. So the process of insulation design which has the added advantage of both solid and liquid dielectrics would be a superior process of insulation design. One such process which has all the above qualities is the VPI (vacuum pressurised impregnation) process and has proven to be the best process till date. 2.1 DRAWBACKS OF EARLY VPI PROCESS: DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the year 1956. It has been used for many years as a basic process for thorough filling of all interstices in insulated components, especially high voltage stator coils and bars. Prior to development of thermosetting resins, the widely used insulation system for 6.6kv and higher voltages was a VPI system in which, Bitumen Bonded Mica Flake Tape is used as main ground insulation. The bitumen is heated up to about 180°C to obtain low viscosity which aids thorough impregnation. To assist penetration, the pressure in the autoclave was raised to 5 or 6 atmospheres. After appropriate curing and calibration, the coils or bars were wound and connected up in the normal manner. These systems performed satisfactorily in service provided they were used in their thermal limitations. In the late 1930’s and early 1940’s, however, many large units, principally turbine generators, failed due to inherently weak thermoplastic nature of bitumen compound. Failures were due to two types of problems: a. Tape separation

16 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 used is in the type of micaceous tapes used for main ground insulation and the composition of the impregnated resins. Although the first system available was styrenated polyester, many developments have taken place during the last two decades. Today, there are several different types of epoxy, epoxy-polyester and polyester resin in common use. Choice of resin system and associated micaceous tape is a complex problem for the machine manufacturer. Although the classic Vacuum Pressure Impregnation technique has improved to a significant extent, it is a modification to the basic process, which has brought about the greatest change in the design and manufacture of medium-sized A.C industrial machines. This is the Global Impregnation Process. Using this system, significant increases in reliability, reduction in manufacturing costs and improved output can be achieved.

2.2 ADVANTAGE OF PRESENT RESIN POOR VPI PROCESS: VPI is a process, which is a step above the conventional vacuum system. VPI includes pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is improved mechanical strength and electrical properties. With the improved penetration, a void free coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution, the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the average rise.

17

In order to minimize the overall cost of the machine & to reduce the time cycle of the insulation system vacuum pressure Impregnated System is used. The stator coils are taped with porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin bath under pressure of Nitrogen gas.

18

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 3.1.1

STATOR: 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 minimize eddy current losses of rotating magnetic flux which interacts with the core, the entire core is built of thin laminations. Each lamination layer is made of individual segments. The segments are punched in one operation from electrical sheet steel lamination having high silicon content and are carefully deburred. The stator laminations are assembled as separate cage core without the stator frame. The segments are staggered from layer to layer so that a core of high mechanical strength and uniform permeability to magnetic flux is obtained.

On the outer

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

19

To obtain optimum compression and eliminate looseness during operation the laminations are hydraulically compressed and heated during the stacking procedure. To remove the heat, spaced segments are placed at intervals along the core length, which divide the core into sections to provide wide radial passages for cooling air to flow. The purpose of stator core is 1.

To support the stator winding.

2.

To carry the electromagnetic flux generated by rotor winding.

So selection of material for building up of core plays a vital role. 3.1.3 STATOR WINDING: The stator winding is a fractional pitch two layer type, it consisting of individual bars. The bars are located in slots of rectangular cross section which are uniformly distributed on the circumference of the stator core. In order to minimize losses, the bars are compared of separately insulated strands which are exposed to 360.degrees transposing To minimize the stator losses in the winding, the strands of the top and bottom bars are separately brazed and insulated from each other.

3.2 ROTOR: 3.2.1 ROTOR SHAFT: Rotor shaft is a single piece solid forging manufactured from a vacuum casting. Slots for insertion of field winding are milled into the rotor body. The longitudinal slots are distributed over the circumference. So that solids poles are obtained. To ensure that only high quality forgings are used, strengthen test, material analysis and ultrasonic tests are performed during manufacture of the rotor. After completion, the rotor is based in various planes at different speeds and then subjected to an over speed test at 120% of rated speed for two minutes. 3.2.2. ROTOR WINDING AND RETAINING RINGS: The rotor winding consisting of several coils, which are inserted into the slots and series connected such that two coils groups from one pole. Each coil consists of several connected turns, each of which consists of two half turns which are connected by brazing in the end section. The individual turns of the coils are insulated against each other, the layer insulation L-shaped strips of lamination epoxy glass fibre with nomax filler are used for slot insulation. The slot wedges are made of high electrical conductivity material and thus act as damper winding. At their ends the slots wedges are short circuited through the rotor body.

20 The centrifugal forces of the rotor end winding are contained by single piece of non magnetic high strengthen steel in order to reduce stray losses, each retaining rings with its shrinks fitted insert ring is shrunk into the rotor body in an overhang position. The retaining rings are secured in the axial position by a snap ring. F e 1:

igur

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

3.3 FIELD CONNECTION AND MULTICONTACTS: The field current is supplied to the rotor through multi contact system arranged at the exciter side shaft end. 3.3.1 BEARINGS: The generator rotor is supported in two sleeve bearings. To eliminate shaft current the exciter and bearing is insulated from foundation plate and oil piping. The temperature of each bearing is maintained with two RTD’s (Resistance Temperature Detector) embedded in the lower bearing sleeve so that the ensuring point is located directly below the Babbitt. All bearings have provisions for fitting vibration pick up to monitor shaft vibrations. The oil supply of bearings is obtained from the turbine oil system. 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.

21 When the first a.c generators were introducing a natural choice for the supply of field systems was the DC exciter. DC exciter has the capability for equal voltage output of either polarity, which helps in improving the generator transient performance. DC exciters, how ever, could not be adopted for large ratings because of the problems in the design commutator and brush gear, which is economically unattractive. Of –course, the problems are not uncommon in power stations but Of the environment with sulphur vapours, acidic fumes as in the cases of petrochemical and fertilizer industries, exposure of DC exciter. This adds to the problem of design. Types of a.c exciters are: (1) High frequency excitation (2) Brush less excitation (3) Static excitation The high frequency D.C exciter is a specially designed “inductor type alternator” with no winding on its rotor. It is designed to operate at high frequency to reduce the size of the rotor; the a.c exciter was very reliable in operation. Though this system eliminates all problems associated with commutator, it is not free from problems attributable to slip rings and its brush gear. Thus brushless excitation system was introduced. The BL exciter consists of field winding on the stator. This system proved to be highly reliable and required less maintenance. Absence of power cables and external ac power supplies males the system extremely reliable. The problem associated with brushes like fast wear out of brush, sparkling etc, are eliminated. This suffers from the disadvantage of lack of facility for field suppression in the case of an internal fault in generator. The system comprises shaft driven AC exciter with rotating diodes.

3.5 PERMANENT MAGNET GENERATOR AND AVR: This system is highly reliable with least maintenance and is ideally suitable for gas driven generators. The static excitation system was developed contemporarily as an alternative to brush less excitation system. This system was successfully adapted to medium and large capacity Turbo 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.

22 Thus, each excitation system has its own advantages and disadvantages. The selection of system is influenced by the transient response required, nature of pollution and pollution level in the power plant and cost of equipment. Exciters are those components, which are used for giving high voltage to the generator during the start up conditions. The main parts that are included in the exciter assembly are: (1) Rectifier wheels (2) Three phase main exciter (3) Three phase pilot exciter (4) Metering and supervisory equipment

3.5.1 RECTIFIER WHEELS: The main components of the rectifier wheels are Silicon Diodes, which are arranged in the rectifier wheels in a three-phase bridge circuit. The internal arrangement of diode is such that the contact pressure is increased by centrifugal force during rotation. There are some additional components contained in the rectified wheels. One diode each is mounted in each light metal heat sink and then connected in parallel. For the suppression of momentary voltage peaks arising from commutation, RC blocks are provided in each bridge in parallel with one set of diodes. The rings from the positive shrunk on to the shaft. This makes the circuit connections minimum and ensures accessibility of all the elements.

3.5.2 THREE PHASE PILOT EXCITER: The three phase pilot exciter is a six-pole revolving field unit; the frame accommodates the laminated core with the three-phase winding. The rotor consists of a hub with poles mounted on it. Each pole consists of separate permanent magnets, which are housed, in non-metallic enclosures. The magnets are placed between the hub and the external pole shoe with bolts. The rotor hub is shrunk on to the free shaft end.

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

23 secure with the steel bands. The connections are made on the side facing of the rectifier wheels. After full impregnation with the synthetic resin and curing, the complete rotor is shrunk on to the shaft. 3.5.4 AUTOMATIC VOLTAGE REGULATOR: The general automatic voltage regulator is fast working solid thyristor controlled equipment. It has two channels, one is auto channel and the other is manual. The auto channel is used for the voltage regulation and manual channel is used for the current regulation. Each channel will have its own firing for reliable operation. The main features of AVR are: (1) It has an automatic circuit to control outputs of auto channel and manual channel and reduces disturbances at the generator terminals during transfer from auto regulation to manual regulation. (2) It is also having limiters for the stator current for the optimum utilization of lagging and leading reactive capabilities of turbo generator. (3)There will be automatic transfer from auto regulation to manual regulation in case do measuring PT fuse failure or some internal faults in the auto channel. (4)The generator voltage in both channels that is in the auto channel and the manual channel can be controlled automatically.

3.5.5 COOLING SYSTEM: Cooling is one of the basic requirements of any generator. The effective working of generator considerably depends on the cooling system. The insulation used and cooling employed is interrelated. 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.

24

Disadvantages: 1. It is an explosive when mixes with oxygen. 2. Cost of running is higher. Higher capacity generators need better cooling system.

3.6 VARIOUS LOSSES IN A GENERATOR In generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the generator, are considered as losses and may be defined as follows: 3.6.1 Copper loss in the winding. 3.6.2 Magnetic Losses. 3.6.3 Mechanical Losses 3.6.1

Copper loss: The power lost in the form of heat in the armature winding of a generator is known as Copper

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

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

25

(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. 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.

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

26

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.

27

4. MANUFACTURE OF GENERATOR

Various stages in generator manufacturing: In our project we have a detail study of only stator, rotor and the insulation system used for it. The parts excitation system, cooling system and bearings are external to the generator and are treated as a completed one and are out of scope of our record. Now, generator manufacturing can be broadly divided into three main parts: 1. Stator manufacture 2. Rotor manufacture 3. Exciter manufacture The various stages involved in the generator manufacture and their sub processes are shown in the flow diagram given below. This facilitates manufacture erection and transport of the stator.

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

Now these sub processes are explained in detail below. Let us start with Stator. 4.1 STATOR MANUFACTURE PROCESS: This stator manufacturing is a combination of two individual sub processes, namely 

Stator core construction and



Coil construction and their assembly.

4.1.1 STATOR CORE CONSTRUCTION: The stator core isn’t a solid iron type and is the assembly of strips of laminations. As the reasons are explained in the Section 3.1 in the introduction to stator. 4.1.2 PREPARATION OF STATOR LAMINATIONS As explained above, stator laminations are the important parts of the stator core and they should be manufactured as per the design requirements and involves the following sub processes.

28 4.1.3 RECEPTION OF SILICON STEEL ROLLS: The silicon steel rolls received are checked for their physical, chemical, mechanical and magnetic properties as per the specifications mentioned above. In order to reduce the Hysterisis loss, silicon alloyed steel, which has low Hysterisis constant is used for the manufacture of core. The composition of silicon steel is Steel

- 95.8 %

Silicon - 4.0 % Impurities- 0.2 % From the formula for eddy current loss it is seen that eddy current loss depends on the thickness of the laminations. Hence to reduce the eddy current loss core is made up of thin laminations which are insulated from each other. The thickness of the laminations is about 0.5 mm. The silicon steel sheets used are of COLD ROLLED NON-GRAIN ORIENTED (CRANGO) type as it provides the distribution of flux throughout the laminated sheet. 4.1.4 SHEARING: The cold rolled non grained oriented (CRNGO) steel sheets are cut to their outer periphery to the required shapes by feeding the sheet into shearing press. For high rating machines each lamination is build of 6 sectors (stampings), each of 60 cut according to the specifications. 4.1.5 BLANKING AND NOTCHING: Press tools are used in making the core bolt holes and other notches for the laminations. Press tools are mainly of two types. i.

Compound notching tools.

ii.

Individual notching tools.

4.1.6 COMPOUND OPERATION: In this method the stamping with all the core bolt holes, guiding slots and winding slots is manufactured in single operation known as Compound operation and the press tool used is known as Compounding tool. Compounding tools are used for the machines rated above 40 MW. Nearly 500 tons crank press is used for this purpose. 4.1.7 INDIVIDUAL OPERATIONS: In case of smaller machines the stampings are manufactured in two operations. In the first operation the core bolt holes and guiding slots are only made. This operation is known as Blanking and the tools used are known as Blanking tools. In the second operation the winding slots are punched using another tool known as Notching tool and the operation is called Notching.

29 4.1.8 DEBURRING OPERATION: In this operation the burrs in the sheet due to punching are deburred. There are chances of short circuit within the laminations if the burrs are not removed. The permissible is about 5 micrometer. For deburring punched sheets are passed under rollers to remove the sharp burs of edges.

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

4.1.9 VARNISHING: Depending on the temperature withstand ability of the machine the laminations are coated by varnish which acts as insulation.

The lamination sheets are passed through

conveyor, which has an arrangement to sprinkle the varnish, and a coat of varnish is obtained. The sheets are dried by a series of heaters at a temperature of around 260 – 350 oC. Two coatings of varnish are provided in the above manner till 12-18 micrometer thickness of coat is obtained. Here instead of pure varnish a mixture of Tin and Varnish is used such that the mixture takes 44sec to empty a DIN4 CUP. The prepared laminations are subjected to following tests.

30 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

1MΩ.

5. STATOR CORE ASSEMBLY: 5.1

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

clamping plates and the assembly pit. Clamping plate is made parallel to the ground by checking with the spirit level. One packet comprising of 0.5 mm thickness silicon steel laminations is assembled over the clamping plates by using mandrels and assembly pit .after assembling one packet thickness of silicon laminations, inner diameter of the core is checked as per the drawing also the slot freeness is checked with inspection drift .There should not be any projections inside or outside the slot. If all the conditions are satisfied the normal core assembly is carried out by dismantling the trial packets.

5.2 NORMAL CORE ASSEMBLY 5.2.1 Stepped packed assembly: Stepped packets are assembled from the clamping plate isolating each packet with ventilation laminations up to 4 to 5 packets of thickness 10cms for an air cooled turbo generator of 120MW. 5.2.2 Normal packet assembly: Normal packet assembly is carried out using 0.5 mm silicon steel laminations up to required thickness of 30mm by using mandrills and inspection drift after normal packet assembly completion, one layer of HGL laminations are placed and one layer of ventilation lamination are placed over them. Then again normal packet assembly is carried as above. The thickness of each lamination is 0.5 mm and the thickness of lamination separating the packets is about 1 mm. The lamination separating each packet has strips of nonmagnetic material that are welded to provide radial ducts. The segments are staggered from layer to layer so that a core of high mechanical strength and uniform permeability to magnetic flux is obtained. Stacking mandrels and bolts are inserted into the windings slot bores during stacking provide smooth slot walls.

5.2.2.1 In process pressings To obtain the maximum compression and eliminate under setting during operation, the laminations are hydraulically compressed and heated during the stacking procedure when certain heights of stacks are reached.

31 The packets are assembled as above up to 800mm as above and 1 st pressing is carried using hydraulic jacks up to 150kg/cm2 and the pressing is carried out for every 800mm and a pre final pressing is done before the core length almost reach the actual core. Now the core is tested for the design specifications and the compensation is done by adding or removing the packets.

5.2.2.2 Fitting of clamping bolts: The complete stack is kept under pressure and locked in the frame by means of clamping bolts and pressure plates. The clamping bolts running through the core are made of nonmagnetic steel and are insulated from the core and the pressure plates to prevent them from short circuiting the laminations and allowing the flow of eddy currents. The pressure is transmitted from the clamping plates to the core by clamping fingers. The clamping fingers extend up to the ends of the teeth thus, ensuring a firm compression in the area of the teeth. The stepped arrangement of the laminations at the core ends provides an efficient support to tooth portion and in addition contributes to the reduction of stray load losses and local heating in that area due to end leakage flux. The clamping fingers are also made of non-magnetic steel to avoid eddy-current losses. After compression and clamping of core the rectangular core key bars are inserted into the slots provided in the back of the core and welded to the pressure plates. All key bars, except one, are insulated from the core to provide the grounding of the core.

32

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. STATOR WINDING There are three main components in a stator, they are a. copper conductors (although aluminum is sometimes used). b. The stator core. c. Insulation. 6.1 CONDUCTING MATERIAL USED IN COIL MANUFACTURING: Copper material is used to make the coils. This is because i)

Copper has high electrical conductivity with excellent mechanical properties

ii)

Immunity from oxidation and corrosion

iii)

It is highly malleable and ductile metal.

6.2 TYPES OF CONDUCTOR COILS: Basically there are three types of stator winding structures employed over the range from 1 KW to 1000 MW. 1.

Random wound stators.

2.

Form-wound stators using multi turn coils.

3.

Form-wound stators using Roebel bars.

Out of these, two types of coils are manufactured and used in BHEL, Hyderabad. 1) Diamond pulled multi-turn coil (full coiled). 2) Roebel bar (half-coil). In general, random-wound stators are typically used for machines less than several hundred KW. Form-wound coil windings are used in most large motors and many generators rated up to 50 to 100 MVA. Roebel bar windings are used for large generators. Although each type of construction is described below, some machine manufacturers have made hybrids that do not fit easily into any of the above categories; these are not discussed in the project. Generally in large capacity machines ROEBEL bars are used. These coils were constructed after considering the skin effect losses. In the straight slot portion, the conductors or strips are transposed by 360 degrees. The transposition is done to ensure that all the strips occupy equal length under similar conditions of the flux. The transposition provides for a mutual neutralization of the voltages induced in the individual strips due to the slot cross field and ensures that no or only small circulating currents

33 exists in the bar interior. Transposition also reduced eddy current losses and helps in obtaining uniform e.m.f. More about transposition is discussed later in the section with diagrammatic quote. The copper is a conduit for the stator winding current. In a generator, the stator output current is induced to flow in the copper conductors as a reaction to the rotating magnetic field from the rotor. In a motor, a current is introduced into the stator, creating a rotating magnetic field that forces the rotor to move. The copper conductors must have a cross section large enough to carry all the current required without overheating.

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

The diagram shows that each phase has one or more parallel paths for current flow. Multiple parallels are often necessary since a copper cross section large enough to carry the entire phase current may result in an uneconomic stator slot size. Each parallel consists of a number of coils connected in series. For most motors and small generators, each coil consists of a number of turns of copper conductors formed into a loop. The rationale for selecting the number of parallels, the number of coils in series, and the number of turns per coil in any particular machine is beyond the scope of our project. The stator core in a generator concentrates the magnetic field from the rotor on the copper conductors in the coils. The stator core consists of thin sheets of magnetic steel (referred to as laminations). The magnetic steel acts as a low-reluctance (low magnetic impedance) path for the magnetic fields from the rotor to the stator, or vice versa for a motor. The steel core also prevents most of the stator winding magnetic field from escaping the ends of the stator core, which would cause currents to flow in adjacent conductive material.

34

7. ELECTRICAL INSULATION The final major component of a stator winding is the electrical insulation. Unlike copper conductors and magnetic steel, which are active components in making a motor or generator function the insulation is passive. That is, it does not help to produce a magnetic field or guide its path. Generator and motor designers would like nothing better than to eliminate the electrical insulation, since the insulation increases machine size and cost, and reduces efficiency, without helping to create any torque or current. Insulation is “overhead,” with a primary purpose of preventing short circuits 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

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

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.

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

High purity (99%) copper conductors/strips are used to make the coils. This results in high strength properties at higher temperatures so that deformations due to the thermal stresses are eliminated.

36

B8 A single form wound coil being inserted into two slots

7.1 STATOR WINDING INSULATION SYSTEM FEATURES The stator winding insulation system contains several different components and features which together ensure that electrical shorts do not occur, that the heat from the conductor I2R losses are transmitted to a heat sink, and that the conductors do not vibrate in spite of the magnetic forces. The basic stator insulation system components are the: 1. Strand (or sub conductor) insulation 2. Turn insulation 3. Ground wall (or ground or earth) insulation Figures B9 , B10 and B11 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 B14 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 Insulation In random-wound stators, the strand insulation can function as the turn insulation, although extra sleeving is sometimes applied to boost the turn insulation strength in key areas. Many form-wound machines employ separate strand and turn insulation. The following mainly addresses the strand

37 insulation in form-wound coils and bars. Strand insulation in random wound machines will be discussed as turn insulation. Section 1.4.8 discusses strand insulation

in

its

role

as

transposition insulation. There are both electrical and mechanical

reasons

for

stranding a conductor in a form wound coil or bar. From a mechanical point of view, a conductor that is big enough to carry the current needed in the coil or bar for a large machine will have a relatively large cross-sectional area. That is, a large conductor cross section is needed to achieve the desired ampacity. Such a large conductor is difficult to bend and form into the required coil/bar shape. A conductor formed from smaller strands (also called sub-conductors) is easier to bend into the required shape than one large conductor.

B9 C.S of a random stator winding slot

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.

38 By Ampere’s Law, or the ‘right hand rule’, this axial magnetic field will tend to cause a current to circulate within the cross section of the conductor (Figure 1.11). The larger the cross sectional area, the greater the magnetic flux that can be encircled by a path on the periphery of the conductor, and the larger the induced current. The result is a greater I2R loss from this circulating current. By reducing the size of the conductors, there is a reduction in stray magnetic field losses, improving efficiency. The electrical reasons for stranding require the strands to be insulated from one another. The voltage across the strands is less than a few tens of volts; therefore, the strand insulation can be very thin. The strand insulation is subject to damage during the coil manufacturing process, so it must have good mechanical properties. Since the strand insulation is immediately adjacent to the copper conductors that are carrying the main stator current, which produces the I2R loss, the strand insulation is exposed to the highest temperatures in the stator. Therefore, the strand insulation must have good thermal properties. Section 7.1.1 describes in detail the strand insulation materials that are in use. Although manufacturers ensure that strand shorts are not present in a new coil, they may occur during service due to thermal or mechanical aging. A few strand shorts in form-wound coils/bars will not cause winding failure, but will increase the stator winding losses and cause local temperature increases due to circulating currents.

39

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

7.1.2 Turn Insulation The purpose of the turn insulation in both random- and form-wound stators is to prevent shorts between the turns in a coil. If a turn short occurs, the shorted turn will appear as the secondary winding of an autotransformer. If, for example, the winding has 100 turns between the phase terminal and neutral (the “primary winding”), and if a dead short appears across one turn (the “secondary”), then 100 times normal current will flow in the shorted turn. This follows from the transformer law: npIp = nsIs (1.1) 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

40 overheating it. Usually, this high current will be followed quickly by a ground fault due to melted copper burning through any ground-wall insulation. Clearly, effective turn insulation is needed for long stator winding life. B11 C.S of a form wound multi-turn slots directly cooled roebel bars

The power frequency voltage across the turn insulation in a random-wound machine can range up to the rated phase-to-phase voltage of the stator because, by definition, the turns are randomly placed in the slot and thus may be adjacent to a phase-end turn in another phase, although many motor manufacturers may insert extra insulating barriers between coils in the same slot but in different phases and between coils in different phases in the end-windings. Since random winding is rarely used on machines rated more than 600 V (phase-to-phase), the turn insulation can be fairly thin. However, if a motor is subject to high-voltage pulses, especially from modern inverter-fed drives (IFDs), inter-turn voltage stresses that far exceed the normal maximum of 600 Vac can result. These high-voltage pulses give rise to failure mechanisms, as discussed in Section 8.7. The power frequency voltage across adjacent turns in a form-wound multi-turn coil is well defined. Essentially, one can take the number of turns between the phase terminal and the neutral and divide it into the phase–ground voltage to get the voltage across each turn. For example, if a motor is rated 4160 Vrms (phase–phase), the phase–ground voltage is 2400V. This will result in about 24 Vrms across each turn, if there are 100 turns between the phase end and neutral. This occurs because coil manufacturers take considerable trouble to ensure that the inductance of each coil is the same, and that the inductance of each turn within a coil is the same. Since the inductive impedance (XL) in ohms is: XL = 2_fL (1.2) Where f is the frequency of the AC voltage and L is the coil or turn inductance, the turns appear as impedances in a voltage divider, where the coil series impedances are equal. In general, the voltage across each turn will be between about 10 Vac (small form-wound motors) to 250 Vac (for large generator multi turn coils). The turn insulation in form-wound coils can be exposed to very high transient voltages associated with motor starts, IFD operation, or lightning strikes. Such transient voltages may age or puncture the turn insulation. This will be discussed in later sections. As described below, the turn insulation around the periphery of the copper conductors is also exposed to the rated AC phase–ground stress, as well as the turn–turn AC voltage and the phase coil-to-coil voltage. 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

41 the

turn

insulation.

This

eliminates

a

manufacturing step (i.e., the turn taping process) and increases the fraction of the slot cross section that can be filled with copper. However, some machine owners have found that in-service failures occur sooner in stators without a separate turn insulation component. Both form-wound coils and random-wound stators are also exposed to mechanical and thermal stresses. The highest mechanical stresses tend to occur in the coil forming process, which requires the insulation-covered turns to be bent through large angles, which can stretch and crack the insulation. Steady-state, magnetically induced mechanical vibration forces (at twice the power frequency) act on the turns during normal machine operation. In addition, very large transient magnetic forces act on the turns during motor starting or out-of-phase synchronization in generators. These are discussed in detail in Chapter 8. The result is the turn insulation requires good mechanical strength. The thermal stresses on the turn insulation are essentially the same as those described above for the strand insulation. The turn insulation is adjacent to the copper conductors, which are hot from the I2R losses in the winding. The higher the melting or decomposition temperature of the turn insulation, the greater the design current that can flow through the stator. In a Roebel bar winding, no turn insulation is used and there is only strand insulation. Thus, as will be discussed in later sections, some failure mechanisms that can occur with multi turn coils will not occur with Roebel bar stators. 7.1.3 Ground wall Insulation Ground wall insulation is the component that separates the copper conductors from the grounded stator core. Ground wall insulation failure usually triggers a ground fault relay, taking the motor or generator off-line.* Thus the stator ground wall insulation is critical to the proper operation of a motor or generator. For a long service life, the ground wall must meet the rigors of the electrical, thermal, and mechanical stresses that it is subject to. B12 C.S of multi-turn coil, where the turn insulation and strand insulation are same

42 7.1.4 Slot Discharges: Slot discharges occur if there are gaps within the slot between the surface of the insulation and that of the core. This may cause ionization of the air in the gap, due to breakdown of the air at the instances of voltage distribution between the copper conductor and the iron. Within the slots, the outer surface of the conductor insulation is at earth potential, in the overhanging it will approach more nearly to the potential of the enclosed copper. Surface discharge will take place if the potential gradient at the transition from slot to overhang is excessive, and it is usually necessary to introduce voltage grading by means of a semi-conducting (graphite) surface layer, extending a short distance outward from the slot ends. So insulation of these stator bars is an inevitable task. It is worth now to know about insulation. As we told before insulation is the heart of the generator now let us move to the most interesting and important topic insulation.

7.2 INSULATING MATERIALS: Insulating materials or insulators are extremely diverse in origin and properties. They are essentially non-metallic, are organic or inorganic, uniform or heterogeneous in composition, natural or synthetic. Many of them are of natural origin as, for example, paper, cloth, paraffin wax and natural resins. Wide use is made of many inorganic insulating materials such as glass, ceramics and mica. Many of the insulating materials are man-made products and manufactured in the form of resins, insulating films etc., in recent years wide use is made of new materials whose composition and organic substances. These are the synthetic Organo-silicon compounds, generally termed as silicones. Properties of a good Insulating Material: The basic function of insulation is to provide insulation live wire to live wire or to the earth. A good insulating material needs the following physical and electrical properties. 1.

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

2.

It should withstand the designed mechanical stress.

3.

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

4.

High dielectric strength sustained at elevated temperatures.

5.

High resistivity or specific resistance

6.

Low dielectric Hysterisis.

7.

Good thermal conductivity.

8.

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

9.

Low dissipation factor.

10.

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

11.

Should be resistant to thermal and chemical deterioration.

43 7.2.2 CLASSIFICATION OF INSULATING MATERIAL: The insulating material can be classified in the following two ways. I. II.

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

a. Classification according to substance and materials: 1. Solids (Inorganic and organic) EX: Mica, wood slate, glass, porcelain, rubber, cotton, silks, rayon, ethylene, paper and cellulose materials etc. 2. Liquids (oils and varnishes) EX: linseed oil, refined hydrocarbon minerals oils sprits and synthetic varnishes etc. 3. Gases EX: Dry air, carbon dioxide, nitrogen etc.

A.1 CLASSIFICATION ACCORDING TO TEMPERATURE: Class Y

Permissible temperature 90°

A

105°

E B F

120° 130° 155°

H

180°

C

Above 180°

Materials 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. Cotton, silk & paper, natural resins, cellulose esters, laminated wool, varnished paper. Synthetic material of cellulose base Mica, asbestos, glass fibre with suitable bonding substance Material of class B with binding material of higher thermal stability. Glass fibre and asbestos material and built up mica with silicon resins. Mica, porcelain, quartz, glass (without any bonding agent) with silicon resins of higher thermal stability.

A.1 Classification according to temperature

44 7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES:

Name of Material 1. Samicatherm calmica glass-n, mimica, domica, folium, filamic novobond-s, epoxy therm laxman isola calmicaflex 2. Samica flex 3. Vectro asbestos (365.02/365.32) 4. (used in resin rich) Epoxide impregnated glass cloth 5. Polyester resin mat & rope

Shelf life Insulation (In months) Class At At 20 oC 5oc F 6 12

H

4

8

B/F

2

8

F

6

12 6

6. Glassoflex Turbo laminate 7. Hyper seal tape

F

6

12

F

6

12

8. SIB775 or 4302 varnish

F

6

12

9. SIB475 or 4301 varnish

F

6

12

10. SIB 643 or8003 Varnish or K8886 varnish

4

8

11. SIB 642 or 8001 varnish

4

8

Application Main insulation stator bars

of

Overhang insulation of motor coils, at 3rd bends of multi turn coil Main pole coils of synchronous machines Winding holders and inter-half insulation Bar to winding holder & stiffener groove of support segment of clamping plate Inter-turn insulation of rotor winding As finishing layer in overhangs of motor coils Stack Consolidation of stator bars Base coat varnish before taping of stator bars Conductive coat in straight portion of stator bars At slot emerge portion on stator bars

A.2 Insulating materials for electrical machines

7.3 FEW DEFINITIONS OF ELECTRIAL PROPERTIES OF INSULATION: 7.3.1 INSULATON RESISTANCE: It may be defined as the resistance between two conductors usually separated by insulating materials. It is the total resistance in respect of two parallel paths, one through the body and other over the surface of the body. Insulation Resistance is influenced by the following factors. 1) It falls with every increase in temperature.

45 2) The sensitivity of the insulation is considerable in the presence of moisture. 3) Insulation resistance decrease with increase in applied voltage.

7.3.2 DIELECTRIC STRENGTH: The voltage across the insulating material is increased slowly the way in which the leakage current increases depend upon the nature and condition material.

7.3.3 POWER FACTOR: Power factor is a measure of the power losses in the insulation and should be low. It varies with the temperature of the insulation. A rapid increase indicates danger.

7.3.4 DIELECTRIC CONSTANT: This property is defined as the ratio of the electric flux density in the material .To that produced in free space by the same electric force. 7.3.5 DIELECTRIC LOSS: The dielectric losses occur in all solid and liquid dielectric due to (b) Conduction current (c) Hysterisis. Additional to the Electrical properties there are many factors such as thermal, chemical etc.., they are tabulated as below. S.No 1.

Thermal Properties Specific heat

Chemical Properties Resistance to external chemical

Mechanical Properties Density

2.

Thermal conductivity.

effects.

Viscosity

3.

Thermal plasticity

Resistance to chemical in soils.

Moisture absorption

4.

Ignitability

5.

Softening point

6.

Heat Aging

7.

Thermal expansion.

Hardness of surface Effect of moisture and water.

A.3 properties of electrical insulation

Surface tension Uniformity.

46

8. RESIN IMPREGNATION Resin impregnation fills the porosity of a part with a resin to create a pressure-tight part for hydraulic applications which can withstand several thousand psi, to improve machine ability, or to allow electroplating. The parts are placed in a mesh basket and loaded into a vacuum tank. This is then submerged in a bath of Anaerobic resin. A vacuum is pulled to remove all air from the porosity of the parts. This vacuum is released to and the tank is pressurised, causing the resin to be drawn into the porosity of the parts. Parts that typically undergo resin impregnation include hydraulic fittings for pressure tightness and plating, covers and plated for pressure tightness, as well as machined components. The previous method of sealing parts was a furnace treatment, which formed a hard oxide layer on the internal and external surfaces of a part, filling the porosity. Most machining operations were performed prior to sealing the part because the hard oxide layer adversely affected mach inability. Residue left by traditional cutting fluids tended to inhibit the formation of an oxide layer. With resin impregnation, conventional cutting fluids can be used because the furnace treatment is eliminated resulting in improved mach inability. These fluids efficiently remove heat from the cutting tool, extending the tool life. Machining a porous part effectively creates a continuous interrupted cut. Each time the tool impacts metal after passing through a pore, it may chip and become dull. Resin impregnation reduces that effect and may also provide added lubrication to the cutting tool. Before resin impregnation, many parts were mechanically plated. Resin impregnation allows the use of electroplating. EPOXY RESINS: Epoxy resins are poly ethers derived from Epi-Chloro Hydrin and Bis-Phenol monomers through condensation polymerization process. These resins are product of alkaline condensed of Epi-Chloro Hydrin and product of alkaline condensed of Epi-Chloro Hydrin and PolyHydric 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, polysulphides, 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 300°F, but with special additions, the capability can be increased up to a temperature of 500°F.

47 Epoxy resins are made use as an efficient coating material. This includes coating of tanks containing chemicals, coating for corrosion and abrasion resistant containers. Epoxy resins are made up of as attractive corrosion and wear resistant floor ware finishes. These are also used as industrial flooring material. They are also used as highways Surfacing and patching material. Moulding compounds of epoxy resins such as pipe fitting electrical components bobbins for coil winding and components of tooling industrial finds greater application in industries. The epoxy resins similar to polyester resins can be laminated and Fiber Reinforced (FPR) and used in glass fiber boats, lightweight helicopters and aero planes parts. In the modern electronic industry, the application of epoxy resins is great. Potting and encapsulation (coating with plastic resin) is used for electronic parts. Most of the printed circuits bodies are made of laminated epoxy resin which is light but strong and tough. Properties: 1) Epoxy resins have good mechanical strength less shrinkage and excellent dimensional stable after casting. 2) Chemical resistance is high. 3) Good adhesion to metals. 4) To impact hardness certain organic acid anhydrides and alphabetic amines are mixed. Applications: 1) They are used in the manufacture of laminated insulating boards. 2) Dimensional stability prevents crack formation in castings. 3) They are also used as insulating varnishes. 8.1

INSULATING MATERIAL FOR LAMINATIONS: The core stacks in modem machines are subjected to high pressers during assembly and there

fore to avoid metal-to-metal contact, laminations must be well insulated. The main requirements of good lamination insulation are homogeneously in thin layers toughness and high receptivity. We use varnish as insulating material for laminations. 8.2 VARNISH This is most effective type of insulation now available. It makes the laminations nest proofs and is not affected by the temperature produced in electrical machines varnish is usually applied to both sides of lamination to a thickness of about 0.006mm. On plates of 0.35mm thickness varnish

48 gives a stacking factor about 0.95.In order to achieve good insulation properties the following processes are in BHEL. • •

THERMOPLASTIC PROCESS OF INSULATION THERMOSETTING PROCESS OF INSULATION

BHEL is practicing only thermosetting process of insulation so Thermosetting types of insulation are of two types: •

RESIN RICH SYSTEM OF INSULATION

•

RESIN POOR SYSTEM OF INSULATION

The various types of materials used in the resin rich and resin poor process are given below. Let us have an overview. Materials used in resin poor system: MATERIAL FOR RESIN POOR DIAMOND COILS • • • • • • • • • • •

HALF BARS

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

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

VARNISH • • •

Polyester glass tape Rutapox Hardener (H-90)

A.4 materials used in resin poor process

8.3RESIN 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%

49 The bars are insulated (or) taped with RESIN RICH TAPE and place in the Pre-assembled stator core including stator frame. MATERIAL FOR RESIN RICH BARS • • • • •

Preprag Nomex Epoxy resin rich mica tape Glass tape PTFE tape

• • • •

Mica powder Graphite powder Conductive varnish Semiconductor varnish

VARNISH

A.5 Materials used in resin rich process

In resin rich system of insulation Mica paper will give a good dielectric strength and Glass fiber tape will give a good mechanical strength and Epoxy resin can withstand up to 155 degree Centigrade so it gives a good thermal properties. Resin rich and Resin poor insulating materials are characterized by the contact of the Epoxy Resin. In Resin rich system the content of Epoxy Resin tape is 40% so it is named as RESIN RICH SYSTEM, and in Resin poor system the content of Resin tape is 8%. By VIP impregnation process, the required amount is added to then conductor bars after assembling the core and placing the winding in the core. In resin rich system before placing of coils in the stator slots the rich tape will be wrapped over the bars. Nevertheless, this system has the following disadvantages: 1. This system is very time consuming and very long procedure. 2. Total cost of the system is more. In order to minimize the over all cost of the machine and to reduce the time cycle of the system, the VACUUM PRESSURE IMPREGNATION SYSTEM is being widely used. This process is very simple, less time consuming and lower cost. BHEL, HYDERABAD is equipped with the state of the art technology of VACUUM PRESSURE IMPREGNATION. The core or coil building and assembling method depends on the insulation system used. The difference in core building is 1. For Resin rich insulation system the laminations are stacked in the frame itself. 2. For Resin poor insulation system (VPI) cage core of open core design is employed. The manufacturing of coils also differs for both as explained above for core. 1. For resin poor process 2. For resin rich process

50

9. MANUFACTURE OF STATOR COILS Manufacturing of stator coils depends on the type of the insulation process used for the stator. I.e. the process is different for resin rich and resin poor process although few of the sub processes are same for both. 9.1 FOR RESIN POOR PROCESS: In this process the high voltage insulation is provided according to the resin poor mica base of thermosetting epoxy system. Several half overlapped continuous layers of resin poor mica tape are applied over the bars. The thickness of the tape depends on the machine voltage. 9.1.1 Reception of copper conductors: The copper conductors rolls are received is checked for physical and mechanical properties. First piece is checked for specifications such as length and if found satisfactory, mass cutting to desired length is carried out by feeding into the cutting mills. B13 Side View showing one way of transposing insulated strands in stator bar.

9.1.2 Transposition: Conductors are adjusted one over another for a given template and the bundles are transposed by 360 degrees by setting the press for “Roebel Transposition”. Now they are bundled and consolidated by tying with cutter tape at various places. Similarly all the bundles are processed. Thus each stator bundle has a transposed coils in each phase such that the flux distribution is equal and hence the induced e.m.f. 9.1.3 Putty operation: All the transposed bars are shifted to putty operation. Here a single bar is taken for putty operation by filling up the uneven surfaces on the width face by filling with NOMAX. I.e., NOMAX sheets are inserted in the crossovers on the width face to the both ends. Form mica net is placed over the width face of the bar on both sides & wrapped with PTFE (poly tetra flamo ethane) tape. 9.1.4 Stack consolidation: Now 2 to 3 bars are inserted into hydraulic presser and they are pressed horizontally and vertically to a pressure up to 150kg/cm2. At the same time the bars are subjected o heating from 140 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.

51

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 B.14 Cross-section of a multi turn coil, where three turns and three strands per turn

finally with hyper seal tape from straight portion to the end of 3rd bend in overlapping layers for protecting the layers from anti fingering. The IH & IS tests follows and the bars are discharged to the stator winding.

52 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. 9.2.3 Putty work: Nomex is used in between transposition pieces. 775 varnish is applied over the straight portion of bar and mica putty is applied on the width faces of the bars. Mica Putty mixture is a composition of SIB 775 Varnish, mica powder and china clay in the ratio of 100:50:25. Straight part baking is done for 1hour at a temperature of 160°C and a pressure of 150kg/sq.cm.Then bending and forming is done. Half taping with resin rich tape is done for over hangs and reshaping is done. To ensure no short circuits half testing of coils is done. 9.2.4 Final taping: Initial taping and final tapings is done with resin rich tape (Semica Therm Tape) to about 1314 layers. The main insulation layers are 12*1/2 overlap in the straight portion and 9 layers in the overhang.

53

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

9.2.5 Final baking: Final baking is done for 3hrs at a temperature of 160°C 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.

54 •

Conductive/graphite coating (643) is applied on the straight portion and semi-conductive coating 642 from end of straight portion to 3rd bend to pre transition coating on both sides.1st coating for 90mm, 2nd coating for 100mm and 3rd coating for 120mm on both sides.

•

The bar is allowed for drying and epoxy red gel is applied from the end of straight portion to the 3rd bend on both sides and allow for drying.

•

High voltage testing is done at 4 times that of rated voltage and tanδ testing, inter strip, inter half testing are done. Tanδ values must be less than 2%.

55 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 10.2DISADVANTAGES 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

10.3ADVANTAGES 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 10.4DISADVANTAGES OF RESIN RICH SYSTEM OF INSULATION: •

It is a very long procedure

•

Due to fully manual oriented process, the cost is more

•

It is possible to process stator bars only. Even though the advantages and disadvantages of both the process are explained above, resin

poor process is the best of all, as the resin content used is almost only 35% compared to resin poor process and also show good insulation properties justified in the later sections.

56

11. ASSEMBLY OF STATOR The completed core and the copper bars are brought to the assembly shop for assembly.

11.1 RECEPTION OF STATOR CORE: Stator core after the core assembly is checked for the availability of foreign matter, so coil projections are checked in each slot. HGL gauge is passed in each and every slot to detect bottom core projections.

11.2 WINDING HOLDER’S ASSEMBLY: Assemble all the winding holders on both sides by adapting to the required design size. Check all the wedge holders by a template and they are assembled as per the design requirement. Tighten all the bolts relevant to winding holders and lock them by tag welding. Assemble HGL rings on both the sides by centring with respect to core. Subject each individual for pressing in pressing fixture at a pressure of 60 kg/cm2 for 30 minutes. Inter half test is conducted for each individual bar before assembling into the stator. Now stator bar assembling is carried out by centring to the core and check for proper seating of bottom bars with T-gauge and checked for third bend matching, over hang seating etc.., rein force the overhang portion of stator bars by inserting glass mat in between the bars and tying them with neoprene glass sleeve. This process is carried out for all respective bottom bars .now the pitch matching is checked on both sides both the generator and the exciter side. Now high voltage testing is carried out on the stator.

11.3 STIFFENERS ASSEMBLY: Stiffeners are assembled on both sides and then checked for physical feasibility of top bar by laying into the respective slot. Check for uniform gap in the over hang and top bar matching to the bottom bar pitch on both sides. Assemble all the top bars by inserting inner layer inserts and also assemble relevant RTD’s (Resistance Temperature detectors) where ever they are required as per the design. After completion of top bars, reinforce overhangs by inserting Glass-mat and tying with Neoprene glass sleeve and also check for the third bend matching on both the sides. Then the core is subjected to high voltage DC test and inter half short circuit tests.

11.4 EYE FORMATION: Join bottom conductors and top conductors forming an eye, by brazing the conductors with silver foil. Segregate eyes into two halves on both sides and test for inter half shorts. Insert Nomax into two halves and close them.

57 Brazing makes the electrical connection between the top and bottom bars. One top bars strand each is brazed to one strand of associated bottom bar so that beginning of the strand is connected with out any electrical contact with the remaining strand. This connection offers the advantage of minimizing three circulating currents.

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.

58 12.THE VPI PROCE SS

12.1 INTRODUCTION TO VACUUM PRESSURE IMPREGNATION SYSTEM (VPI) 12.2 HISTORY DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the year 1956. Vacuum Pressure Impregnation has been used for many years as a basic process for thorough filling of all interstices in insulated components, especially high voltage stator coils and bars. Prior to development of Thermosetting resins, a widely used insulation system for 6.6kv and higher voltages was a Vacuum Pressure Impregnation system based on Bitumen Bonded Mica Flake Tape is used as main ground insulation. After applying the insulation coils or bars were placed in an autoclave, vacuum dried and then impregnated with a high melting point bitumen compound. To allow thorough impregnation, a low viscosity was essential. This was achieved by heating the bitumen to about 180°C 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: c. Tape separation d. 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,

59 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 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 highpotential-tested at voltages below those normally used for resin-rich coils because, at the unimpregnated 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. Endwinding 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

60 materials such as silicon carbide loaded tape or varnish, this additional loss depends, to a large extent upon the stator core length and machine voltage. VPI is a process, which is a step above the conventional vacuum system. VPI includes pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is improved mechanical strength and electrical properties. With the improved penetration, a void free coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution, the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the average rise. In order to minimize the overall cost of the machine & to reduce the time cycle of the insulation system vacuum pressure Impregnated System is used. The stator coils are taped with porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin bath under pressure of Nitrogen gas. The chemical composition of our resin type and its advantages are explained in the later sections. Now let us discuss the various stages involved in VPI process for resin poor insulated jobs. VPI process is done in the VPI camber. For higher capacity stators of steam turbine or gas turbine generator stators, horizontal chamber is used where as vertical chamber is used for smaller capacity systems such as Permanent Magnet Generator (PMG), coil insulation of small pumps and armature of motors etc..,

12.3 VACUUM PRESSURE IMPREGNATION OF RESIN POOR INSULATED JOBS: VPI process for a stator involves the following stages.

2. Preheating 3. Lifting and shifting 4. Vacuum cycle 5. Vacuum drop test 6. Heating the resin 7. Resin admission. 8. Resin settling 9. Pressure cycle 10. Aeration.

61

11.Post curing cycle 12.Cleaning 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

12.3.3 Insertion of tub with job into the impregnation tank: The wound stator is lifted and shifted into the tub. By the time, the preheating of job is completed, it is to be planned in such a way that the heating of tub and tank heating matches with the job. This is applicable when the job is heated in the curing oven separately. The preheated job is to be transferred into the tub by crane handling the job safely and carefully with out damage to the green hot insulation the tub is then pushed in the 140 tank furnace or also called as vacuum tank, after which the lid is

62 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 12.3.4Vacuum cycle: The pre heated job will be placed in the impregnation chamber by a hydraulic mechanism. The vessels are kept clean and the resin available in the vessel is wiped out. Methylene and traces of resin should not be allowed on the inner side of the tank. Now the vacuum pumps are all switched on and a vacuum pressure of about 0.2 mb is maintained for about 17 HRS, after which the wound stator is subject to vacuum drop test. 12.3.5 Vacuum drop test: This drop test is important phase, all the vacuum pumps are switched off for about 10 mins, and the vacuum drop is measured and it is checked whether it exceeds 0.06mb, if it exceeds 0.06mb then it is subject to repetition of vacuum cycle for another 6 to 8 hrs, else it is sent to the next cycle 12.3.6 Drying the job in vacuum The job is to be dried under vacuum. Drain out the condensed moisture/ water at the exhausts of vacuum pumps for efficient and fast vacuum creation. Also check for oil replacement at pumps in case of delay in achieving desired vacuum. 12.3.7 Heating the resin in the storage tank The completion of operations of drying and the heating of the resin in the storage tank are to be synchronised. The heating of resin in the tank and pipeline is to be maintained as at preheating temperature .i.e. the temperature is maintained at 60+/- 3 deg ,including pipeline 12.3.8 Admission of resin into impregnation tank The resin is allowed into the impregnation tank tub if required from various storage tanks one after the other, such that the difference in pressure fills the tank, up to a level of 100mm above the job generally, after which the resin admission is stopped. After 10mins of resin settling the tank is to be pressurised by nitrogen. While admitting resin from storage tanks pressurise to minimum so that nitrogen will not affect resin to spill over in tank. 12.3.9 Resin settling: The resin is allowed to settle for about 4mins in such a way that bubble formation ceases

63 12.3.10 Impregnation Pressurising/gelling After the resin has settled the job is subject to pressure cycle of 4 kg/ cm2 of dry nitrogen into the vacuum tank after obtaining 4 kg/cm2, this is subject for 2 hrs. in this stage the resin is impregnated into the micro pores of the stator and is very firmly embedded into the crevices of the stator ,so thus acting as a tough layer of insulation for the stator ,being indestructible in the long run.

B16 vertical VPI tanks for smaller

jobs

12.3.11 Withdrawal of resin from impregnation tank to storage tank The resin that is pressurised as per pressure cycle is drawn into the tank by the opening of relevant valves will allow the resin to come back to the storage tank. The job also shall be allowed for dripping of residue of resin for about 10min. After dripping, withdrawal of resin in various storage tanks is to be carried out. This is necessary because resin is a very costly material. 12.3.12 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.

64 12.3.13 Post curing: The job is post heated. The time and temperature to which the job has to be scheduled is varies according to the type of job and is given in the table. The time at which the heating is started, achieved and maintained is to be logged. The wound stator is subject to 140 +/- 5 deg. After obtaining 140 deg the stator is subject for 32 hrs. The stator is then made to rotate at 1 rpm up to 120 deg. It is then allowed for cooling without opening the doors till the temperature reaches 80

deg,

after

attaining

the

temperature of 80 deg, the doors are opened and wound stator is sprayed with epoxy red gel on the overhangs and is allowed for drying. 12.3.14 Cleaning: Entire wound stator is cleaned for resin drips, after which its subjected to HV and tan delta tests 12.4ELECTRICAL 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. 12.5GLOBAL PROCESSING: Processing details depends very much on the machine type, on customer’s defined parameters and type of mica tapes. Generally the VPI system is used in impregnation vessels up to 30T where the rotor/stator is impregnated at elevated temperatures. Machine parts usually are preheated (also under vacuum) in order to remove moisture and to reduce viscosity during impregnation. 12.6 RESIN MANAGEMENT: After impregnation the VPI bath is pumped into storage tanks and cooled down to 5-10°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.

65 Impregnated, yet uncured machine parts in unconditioned atmosphere may pickup moisture. Therefore curing directly after impregnation or storage in moisture controlled area is recommended. Generally machine parts are rotated when removed

B17 Showing the resin

tank in which resin is stored.

from the bath and during the first part

of

curing in order to avoid drip off. Evaporation of hardener during the vacuum cycle leads to a change in the resin/hardener ratio in the bath and has to be compensated. Therefore replenishment is mixing ratios of 100120pbw of hardener HY 1102 per 100pbw MY 790-1 are generally used. Replenishment mixing ratios depend on actual processing parameters and conditions and have to be evaluated at the customer site. Due to excellent latency of the system (A) MY 790-1/HY 1102/DY 9577/DY073 the replenishment volume to maintain a constant viscosity is comparatively small, even if impregnation is performed at 40-50°C. On single coils and roebel-bars the mica insulation is normally covered with a tight glass tape to prevent drainage of the impregnation resin. 12.7 SPECIFIC INSTRUCTIONS: Depending on the insulation materials and the accelerating agent in use, a ramped curing schedule is recommended. In systems with high reactivity, where the accelerator can be include in the mica-tape, a fast gelation can be obtain with a temperature-shock, and draining can so be reduced or avoided. Standard curing with the standard accelerated mixture (system A) is:11 at 90°C plus 18 h at 140 °C 12.8 PRECAUTION: To determine whether cross linking has been carried to completion and the final properties are optimal, it is necessary to carry out relevant measurements on the actual object or to measure the glass transition temperature. Different gelling and cure cycles in the manufacturing process could lead to a different cross linking and glass transition temperature respectively. 12.9 FEATURES AND BENEFITS: • State-of-the-art process for completely penetrating air pockets in winding insulation. • Increases voltage breakdown level. (Even under water!)

66 • Proven submergence duty system • Improved heat transfer- windings are cooler, efficiency is improved. • Improves resistance to moisture and chemicals. • Increases mechanical resistance to winding surges.

An overview of entire VPI process with the time taken for each process according to the type of the job used is given below in a tabular form VACUUM PRESSURE IMPREGNATION OF RESIN POOR INSULATED JOBS: Variant

Description

01

Brushless exciter armature, PMG stators and Laminated rotors

02

Stator wound with diamond pulled coils.

3

Stator with half coils Variant-01

Preheating

Vacuum to be maintained

Vacuum heating time

Increase in pressure Maximum pressure Pressure holding

60±5°C for 3hrs

Variant-02 60±5°C for 12hrs

Variant-03

Any other information

60±3°C for 12hrs <0.2mbar (both together shall not exceed 50hrs including rising time) Stopping vacuum pumps for 10min shall check 17hrs vacuum drop. The vacuum drop shall not exceed by 0.06mbar for 10min

0.4mbar

0.2mbar/0.4mbar

3hrs

0.2mbar for 9hrs 0.4mbar for 17hrs

40min

80min

80min

3bar

4bar

4bar

3hrs

3hrs

3hrs

At140±5°C for At140±5°C for At140±5°C for 32hrs 14hrs 32hrs A.6 Table showing temperature and time to be maintained for different type of jobs in Post curing

VPI

67

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:

160°C

Electrical power consumption:

75KW

Work place:

1425

Work centre:

3215

Stream inlet:

200-250°C

 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: Cylindrical length:

4000mm 9000 mm

Operating over pressure: Operating vacuum:

6 bar

0.15 mbar

Operating temperature:

90°C

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 Total storage: 5*9000L+1*3000L

•

Operating parameters of each tank Operating vacuum:

0.5mbar

68

•

Operating over pressure:

0.5bar

Operating temperature:

80°C

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 10±2°C. 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 40°C by circulating water through coolers and then the brine is chilled to -10°C and stored in the tanks.

 Post heating of job (a) Explosion proof steam drier and electrical heating superposed. Size:

7*4.5*4.5M

Maximum weight of job: 80 tonnes Maximum temperature:

150°C

(b) Indirectly heated hot air circulating oven (gas fired) Size:

9*4.5*4.5M

Maximum weight of job: 170 T/120T with facility for rotation. Maximum temperature:

150°C

69 13.1DATA COLLECTION OF SAMPLES During the project two jobs have been impregnated in VPI Plant, the data has been collected and recorded in the project report. 13.1.1 INDO-BHARAT-II ROTOR PREHEATING: Indo Bharat II rotor is loaded for preheating in steam furnace on 30-5-2007 at 18:00hrs. RTD-I(°C)

Date and time

RTD-II(°C)

Furnace air temperature

30.5.2007

19:00

32.0

30.0

45.6

30.5.2007 30.5.2007 30.5.2007 30.5.2007

20:00 21:00 22:00 23:00

45.4 49.9 52.5 53.3

48.6 50.9 54.3 55.1

57.9 63.4 70.5 73.4

30.5.2007

24:00

56.6

57.3

75.6

31.5.2007 31.5.2007 31.5.2007 31.5.2007 31.5.2007 31.5.2007

1:00 2:00 3:00 4:00 5:00 6:00

59.9 62.4 62.3 63.3 63.3 63.1

60.2 63.9 64.7 64.1 64.0 63.7

75.1 77.0 77.0 75.0 75.6 75.6

A 7 PREHEATING OF INDO BHARATH II ROTOR

Remarks Rotor temperature is reached to 60±3 °C at 2:00hrs on 31.5.2007 and it is maintained for 4 hrs i.e., up to 6:00 on 31.5.2007

Rotor is switched to vac 140 tank at 7:00 hrs on 31.5.2007

70 VACUUM CY CLE: RTDII(°C)

Date and time 31.5.2007 19:30 62.0

Room temperature (°C)

62.0

36.0

31.5.2007 31.5.2007 1.6.2007

21:30 70.1 23:30 86.2 1:30 101.5

69.2 81.6 97.6

36.7 36.7 35.6

1.6.2007

3:30 116.2

113.1

34.8

1.6.2007

5:30 129.6

125.7

33.8

1.6.2007

7:30 137.6

133.2

33.2

1.6.2007

9:30 145.7

140.2

36.5

1.6.2007

11:30 145.7

141.6

38

1.6.2007

13:30 144.7

143.4

42.8

1.6.2007

15:30 144.1

143.0

43.8

1.6.2007 1.6.2007

17:30 144.0 19:30 143.0

144.5 143.0

42.8 41.0

1.6.2007

21:30 142.8

142.6

40.8

RTDI(°C)

Remarks Resin tank 025 is heated for impregnation Resin admission started at 14:10 hrs on 31.5.2007 Resin admission completed at 14:25 hrs on 31.5.2007 Pressurization started at 14:45 hrs on 31.5.2007 Pressurization completed at 15:30hrs on 31.5.2007 Pressurization hod up completed at 18:30hrs on 31.5.2007 Resin withdrawal to storage tanks is from 18:30 to 18:45hrs on 31.5.2007 Rotor loaded in gas furnace at 19:15hrs on 31.5.2007 Rotor temperature is reached to 131.6 to 145.7°C at 8:30hrs on 1.6.2007 and it is maintained for 14hrs i.e., up to 22:30hrs on 1.6.2007 Furnace is switched at 22:30hrs on 1.6.2007 and circulation fans are kept running till the job temperature is reached to 70°C to 75°C.

71 RESIN CYCLE AND POST CURING CYCLE: Date and time 31.5.2007 31.5.2007 31.5.2007 31.5.2007 31.5.2007 31.5.2007 31.5.2007 31.5.2007

7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00

Vacuum in graph (mbar) --0.85 0.54 0.39 0.38 0.37 0.36

Vacuum in meter (mbar) -3.0 0.86 0.55 0.4 0.4 0.4 0.39

A.9 vacuum cycle of Indo Bharath II rotor

Job temperature (° C) 62.2 61.5 61.3 61.1 61.1 61.1 61.0 61.0

Remarks

Vacuum pump started at 7:30 hrs on 31.5.2007.

72 13.1.2 INDO-BHARAT-II STATOR: PREHEATING: Indo-Bharat-II stator is loaded for preheating in steam furnace on 7-5-2007 at 23:30hrs. Date and Time

RTD-I(°C)

RTD-II(°C)

Furnace air temperature(°C)

7.5.2007

23:30

36.3

36.1

Remarks

Stator temperature is reached to 60.5°C to 62.9°C(60±3°C) at 7:30hrs on 8.5.2007 and it is maintained for 12hrs i.e., up to 19:30hrs on 8.5.2007

8.5.2007 8.5.2007 8.5.2007 8.5.2007

1:30 3:30 5:30 7:30

43.6 52.0 55.9 60.5

42.9 51.74 56.0 62.9

Stator is loaded in vac(140) tank at 21:00hrs on 8.5.2007

8.5.2007 8.5.2007 8.5.2007

9:30 11:30 13:30

61.3 60.3 60.3

62.9 62.4 62.6

Vac. Pump is started at 2:30hrs on 9.5.2007

8.5.2007 8.5.2007 8.5.2007

15:30 17:30 19:30

62.5 62.9 62.4

A.10 Preheating of Indo Bharat II stator

62.9 62.66 62.1

73 VACUUM CYCLE: Date and Time

Vacuum in

Vacuum in

Job

graph (mbar)

meter (mbar)

temperature (°

Resin cycle

C) 8.5.2007

22:00

--

--

54.37

Resin tanks 025,102 are

8.5.2007

0:00

--

--

54.89

heated for impregnation Viscosity of resin at 60°C

9.5.2007

2:00

--

--

59.02

is 33CP Viscosity after aging is

9.5.2007 9.5.2007

3:30 5:30

0.65 0.41

0.65 0.40

61.6 63.59

36.10CP 9.5.2007 and 10.5.2007 Resin admission started at

9.5.2007

7:30

0.28

0.29

64.2

19:45hrs Resin admission

63.2

completed at 19:55hrs Pressurisation started at

62.3

20:00hrs Pressurisation of

9.5.2007 9.5.2007

9:30 11:30

0.22 0.19

0.22 0.19

4kg/sq.cm reached at 9.5.2007

13:30

0.18

0.18

62.1

21:20hrs Pressurisation hold up for

9.5.2007

15:30

0.17

0.17

62.0

3hrs is at 0:20hrs Resin withdrawn to storage tanks is from

9.5.2007

17:30

0.14

0.14

61.8

0:30hrs –1:00hrs Stator loaded in hot air furnace from 1:00hrs – 1:30hrs on 10.5.2007

9.5.2007

19:30

0.14

0.14

A.11 Vacuum cycle of Indo Bharat II stator

61.3

74 POST CURING: Date and

ESOH

TSOH

ESW

TSW

Time 10.5.2007

15T

06B

02

13

70.0

76.4

62.4

62.5

63.4

33.1

126.7

131.4

94.7

102.3

98.8

31.7

144.3

154.1

125.4

134.5

126.1

31.6

147.7

154.9

139.9

145.1

140.6

34.8

137.6

144.4

139.3

141.6

140.7

38.0

136.9

144.2

140.0

140.9

140.6

38.4

140.2

143.6

140.1

140.7

140.2

37.2

1:30hrs 10.5.2007 4:30hrs 10.5.2007 7:30hrs 10.5.2007 10:30hrs 10.5.2007 13:30hrs 10.5.2007 16:30hrs 10.5.2007 19:30hrs

Core

Room temperature

Remarks

Job temp. is reached to 140±5° C i.e., from 136.2°C to 145.6° C at 9:30hrs on 10.5.2007 and 10.5.2007 22:30hrs 11.5.2007 1:30hrs 11.5.2007 4:30hrs 11.5.2007 7:30hrs 11.5.2007 10:30hrs 11.5.2007 13:30hrs

it is maintained for 32hrs i.e. 144.4

151.3

143.7

145.1

144.1

35.9

143.1

146.7

145.2

145.1

145.2

33.8

144.3

151.0

143.6

144.0

144.7

31.1

135.7

142.1

144.3

145.1

145.0

31.3

135.0

135.7

135.1

135.0

135.8

34.8

135.6

141.4

135.4

135.6

135.9

38.3

up to 17:30hrs on 11.5.2007.

Furnace is switched off at 17:30hrs on 11.5.2007 and circulation fans kept running till the job temperature is

11.5.2007

148.0 149.2 142.8 142.2 17:30hrs A.12 Post curing of Indo Bharat II stator

142.1

39.8

reached from 70°C- 75°C

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

S.No.

Description

HV level Stator winding

HV in kv

remarks

75 1. 2. 3. 4.

5.

After laying and wedging of coils After OH spacers and forming eyes Before impregnation After impregnation

18.9/1’ 18.03/3’

RTD,IT test RTD,IT test

17.5/1’

R, RTD test

26.0/1’

25.0/1’

R, RTD, Tanδ, leakage reactance test Rotor winding

UT+1400

2.9

Pole drops

UT+1250

2.75

Pole drops

UT+1100

2.6

Pole drops

UT+950

2.45

Pole drops

UT+800

2.3

Pole drops

UT+650

--

Pole drops

--

--

R, Pole drops

--

2.15

---

2.0 1.9

R, Pole drops R, Pole drops

--

1.8

R, Pole drops

UT+200

1.7

R,Z with 50Hz

Customer acceptance Rotor winding

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

After laying first coil After laying second coil After laying third coil After laying fourth coil After laying fifth coil After laying sixth coil After all connections After tech. rings assembly After bandage After impregnation After excitation cable assembly After balancing

A.13 High voltage levels of stator/rotor windings for multi turn machines

13.1.4TESTING RESULTS OF INDO-BHARAT-II ROTOR

Customer Name: INDO-BHARAT-II ROTOR M/c rating:

10.8MW, 12kv, 1500rpm.

76 Test:

Z, R and H.V test.

Stage:

After impregnation.

Ambient temperature: 35°C Ohmic resistance:

0.264Ω (rotor temperature was more) Voltage( volts) 215.0

Current(amps) 0.5

367.5 523.0

1.0 1.5

High voltage test: IR value before H.V. test at 15”/60” -- 200/300 MΩ H.V. applied at 1.9kv /1’ – withstood IR value after H.V. test at 15”/60” -- 200/300 MΩ 13.1.5TESTING RESULTS OF INDO-BHARAT-II STATOR: Customer Name: INDO-BHARAT-II STATOR M/c rating:10.0MW, 12kv, 0.8pf, 650A, 1500rpm. Test: H.V test. Stage: after impregnation. Ambient temperature: 36°C A PHASE:

IR value at 2.5kv

IR value before H.V. test -- 1000/2000 MΩ H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000MΩ B PHASE:

IR value at 2.5kv

IR value before H.V. test -- 1000/2000 MΩ H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000MΩ C PHASE:

IR value at 2.5kv

77 IR value before H.V. test -- 1000/2000 MΩ H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000MΩ

INDO-BHARAT-II STATOR: Customer Name: INDO-BHARAT-II STATOR M/c Rating: 10.0MW, 12kv, 0.8pf, 650A, 1500rpm. Test: H.V test-RTD measurement, resistance measurement. Stage: After impregnation. Ambient Temperature: 36°C Excitation Side: 26 10 62 50 20 38 14

113.8Ω 114.0Ω 113.8Ω 113.8Ω 113.8Ω 113.9Ω 113.8Ω

49 13 25 21 61 37 1

125.0Ω 113.8Ω 113.9Ω 113.8Ω 113.8Ω 113.9Ω 113.8Ω

Turbine Side:

A-Aο --29.4mΩ B-Bο -- 29.3mΩ C-Cο -- 29.4mΩ

78

14.COMPARISION BETWEEN RESIN POOR AND RESIN RICH SYSTEMS RESIN POOR SYSTEM 1.

RESIN RICH SYSTEM

The insulation tape used in this system

1.

has 40% resin. 2.

This

method

The insulation tape used in this is 7% of 40% resin.

follows

thermosetting

2.

Same as in resin poor.

There is a need for addition of resin from

3.

Further addition of resin is not required

process. 3.

outside.

from outside.

4. Reduction in time cycle for this process

4.

It

is

very

long

process

and

time consuming while at processing stage. 5. No tests are carried out while at

5.

Tests are being carried out Stage.

6.

Processing

processing 6. Processing of bars along with stator and with conductors and processing of exciter

only

Coils along with exciter is possible.

systems.

7. The cost of repair is more 8. The

overall

cost

is

less

of

possible

stator in

resin

7.

Repairing work is easy.

8.

The total cost in this process is more.

compared to resin rich system. A.14 Comparison between resin rich and resin poor process

Applications: • All critical machines • Equipment exposed to frequent surges/starting • Harsh or moist environments • Motors that run at service factor

14.1 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 8-12°C is complicated. 14.2 SUGGESTIONS:

bars

is rich

79

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: 14.3 JUSTIFICATION We can say that VPI by resin poor system is more superior to other types of insulation by conducting HV and tan δ tests, and the results of which are clearly indicated in the graphs below: After impregnation of the stator core by VPI process the following tests are conducted: 1. TAN δ TEST. 2. HIGH VOLTAGE Tests.

15.3.1 High Voltage test: AC High voltage test is conducted on VPI system after impregnation to verify proper impregnation and dielectric strength of insulation. This test was conducted at 105% of winding test voltage i.e. Up=2Un+1KV Where Up-Winding test voltage Un-rated voltage of machine. Equipment  50 Hz A.C High voltage transformers and its induction regulator/input autotransformer.  Potential transformer (35 or100KV/100V)  Voltmeter  Binding wire  Earthing Rod and Earthing wire/cable When H.V test is done on one-phase winding, all other phase windings, rotor winding, instrumentation cables and stator body are earthed. The high voltage is applied to winding by increasing gradually to required value and maintance for one minute & reduced gradually to minutes. The transformer is switched off & winding discharged to earth by shorting the terminal to earth using earthing rod connected to earth wire/cable. The test is conducted on all the phases & rotor winding separately.

80 HV Test Levels: Stator winding: (2Ut+1) KV =23 for 11 KV machine Rotor winding: (10 Up) volts (with min of 1500v & max of 3500v), Where, Ut= Rated of machine under test Up= Excitation voltage.

15.3.2 TAN δ TEST: Equipment: Schering Bridge After impregnation and curing of the winding a dissipation factor Vs voltage measurement as stipulated in the application national and international standard specification is performed for each bar between all-individual phase winding to ground. Guiding values for the deception factor and its rice with the voltage merely. Given in the KEMA specification the maximum value should not exceed 0.001 at 20% of rated voltage and rise shall not be greater than 0.006 per 20% of rated voltage up to 60% of rated voltage and 0.08 per 20% of rated voltage up to a rated voltage. Winding manufacture by the Vacuum Pressure Impregnation Process comply with these limits. The above test results are specified in the following graphs. First graph shows that Voltage Vs Tan δ curve, it shows different Tan values at different percentage of rated voltage 20%, 40%, 60%, 80%, 100% of rated voltage respectively. The second graph is Stages of materials Vs life of insulation material, it shows that resin poor system of insulation has very long life compared to resin rich system of insulation. At 10 KV the resin poor system insulation as a lifetime of 540 years. Any good machine as life span of 25-30 years by using this insulation we will get a very long life with standard machine. This test is conducted to check the presence of impurities in the insulation & tanδ value for each phase & also for combined phases is noted down. Tan δ value should be generally less than or equal to 2%.

81 TESTING RESULTS: Rating: Vph (0.2Un ) Rated KV= 10.5KV , 3000RPM. Wph

2.1 4.2 6.3 8.4 10.5

Tan δ

Tan δ

Tanδ

Tan δ

Wph

uph

vph

Wph uph

0.806 0.820 0.857 0.899 0.941

0.815 0.832 0.869 0.903 0.938

0.811 0.830 0.868 0.905

vph 1.18 1.209 1.230 1.254 1.268

A.15 tan δ values of the different phases of a generator

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

SCC TESTING: Ia 0 337.2 664.8

Ib 0 337.6 666.2

%In 0 20.57 40.5

If 0.01 94.7 187.1

Vd 559.0 515.95 516.04

Id 404.58 443.1 481.4

Dm O/P 226.16 228.60 248.44

1005.8 1324.9 1495.2

1006.1 1326.5 1496.9

61.34 80.84 91.2

281.51 369.16 415.2

516.52 516.54 516.18

533.6 651.25 690.3

275.6 336.35 356.32

A.16 tabular form for short circuit currents and voltages

82 OCC TESTING: Vab 35.3 2281.2 4447.42 6662.0 8845.0 10015.0

Vbc 35.3 2282.5 4441.4 6665.0 8849.0 10019.0

Vca 35.3 2281.8 4448.2 6663.0 8846.0 10017.0

%En 35.32 20.74 40.44 66.58 80.4 91.06

If 0.01 40.44 77.05 116.65 160.3 186.56

Vd 512.4 512.93 512.4 512.3 512.19 512.25

Id 419.85 423.15 442.45 466.43 501.08 517.25

Dm O/P 215.16 216.62 226.71 288.95 256.64 264.93

A.17 tabular form showing open circuit voltages and currents obtained in the testing

83

TAN δ

14.3.4. Graphs

KV

Fig. 2 Comparative functional testing using the stator slot model confirms the progress achieved in extending the service life of 13.8-kV insulation of varying compositions.

84 RESISTANCE MEASUREMENT: Instrument: Micro ohm meter Rotor

Resistance at 25°c (mΩ) 264

Resistance at 20°c (mΩ) 258.92

R*75 = (( 235+75)/(235+20)) x R20= (310/255) x (0.2587)=0.3147 Ω 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%

85

15. 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 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.

86 The first consideration in using modern insulation systems is the method of applying the ground-wall materials to form-wound stator coils and bars discussed in previous sections, the Haefley process was widely used decades ago to apply wide sheets of insulation material to coils. Presently, virtually

all

however,

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

B18 A modern stator bar taping machine that applies tape both in the slot and end winding portions of the bar.

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

15.1 WESTINGHOUSE ELECTRIC CO.: THERMALASTIC™ Westinghouse Thermalastic™, the first modern synthetic insulation system. The first Thermalastic insulated generator went into service in 1950, in the 1960’s minor changes that were made included introduction of glass cloth as a backing material for the mica, resin modifications to help VPI resin tank stability, and improvements in the partial discharge suppression treatments on generator coil surfaces. Although large turbine generators continue to use the individual bar impregnation and cure method, motors and smaller generators shifted to the global VPI method in the early 1970s. The hybrid epoxy VPI resin used for turbine generators was optimized for the previously 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.

87 15.2 GENERAL ELECTRIC CO. :MICAPALS I AND II™, EPOXY MICA MAT™, MICAPAL HT™, AND HYDROMAT™ It was introduced to the industry in an IEEE Technical paper in 1958, after several years of limited production. Micapal 1 contained approximately 50% GE Micamat™ (paper), made with calcined muscovite, and 50% muscovite splitting. winding operation. After a 12-year development program, General Electric announced the MICAPAL II™ insulation for large turbine generator stator windings in 1978. This solvent less, resin-rich, second-generation epoxy mica paper insulation system has been used on most large steam turbine generators since that time. In 1999, GE began to offer a reduced-build strand-and-turn insulation, using similar metal oxide fillers in the large-motor business. These machines use the global epoxy VPI process to make the glass-fabric-supported Mica-mat insulation systems for machines at least up to 13.8 kV ratings. Several generations of VPI resins have been used by GE for motor manufacture. Two of these epoxy resin systems have been based on controlled reactivity chemistry. The most recent improvement creates polyether linkages in cured Di-Glycidyl ether Bis-Phenol, an epoxy resin provides high reactivity at curing temperatures with excellent shelf life at room temperature.

15.3 ALSTHOM, GEC ALSTHOM, ALSTOM POWER: ISOTENAX™, RESITHERM™, RESIFLEX™, RESIVAC™, AND DURITENAX™ During the 1950s, Alsthom licensed the resin technology used in the GE Micapal I system to create the first Isotenax™ system. There were several differences in materials and processes between the two systems. Isotenex used only mica paper, not mica splittings. The resin-rich impregnating epoxy contained significant amounts of a solvent mixture that had to be removed after the glass-backed mica paper tape was wrapped around the stator bars. Since the 1980s the UK operations of Alsthom have also worked with global VPI processing and an insulation system called Resivac™. Recent advances in the VPI system have used bisphenol epoxy resins with a latent Lewis acid catalyst system 15.4 SIEMENS AG, KWU: MICALASTIC™ Siemens began using the individual-bar VPI process with polyester resins and mica splittings as early as 1957 for hydro and steam turbine generators, with initial help from Westinghouse. This system was trade named Micalastic. Production continued with this combination of resins and processes for at least 10 years. Except for indirect cooled generators and direct-cooled generators rated at more than about 300 MVA or so, which still use the individual bar epoxy VPI methods, the global VPI process has been standard for all motor and turbo generator stators since 1986. For its large global VPI

88 stators, this manufacturer avoids difficulties due to shear stress at the interface of the bar to the stator core by employing a slip plane. The slip plane consists of mica splitting sandwiched between two semi conductive tapes.

15.5 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 hotpress consolidation and curing. New machine production with this system will stop with the end of turbo generator production in Sweden, although some repair licensees will continue using Micarex for some time. The Micadur™ insulation system was introduced in 1955 by Brown Boveri as an individual bar VPI method before the merger of ASEA with Brown Boveri to form ABB in the mid 1980s, ASEA also developed the technology for individual bar VPI production, using similar materials. The result was Micapact™, introduced in 1962 for the stator insulation of large rotating machines. It was made with glass-backed mica paper, impregnated with a special mixture of an epoxy resin, curing agent, and additives. Unlike most other VPI tapes, the glass backing and mica paper lack any impregnant or bonding resin. The adhesion between mica paper and glass was accomplished by an extremely thin layer of material, which was melted at a high temperature during formation of the tape. The tape did not contain any volatile matter, which means that the completed machine taped bar insulation was more easily evacuated and impregnated. 15.6 TOSHIBA CORPORATION: TOSRICH™ AND TOSTIGHT- I™ The Toshiba Tosrich™ insulation system for low-voltage, small-capacity generators with a relatively small number of insulation layers was based on a resin-rich mica paper tape. The solvent containing 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 mediumcapacity 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

89 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. 15.7 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. 15.8 HITACHI, LTD.: HI-RESIN™, HI-MOLD™, AND SUPER HI-RESIN™ Hitachi also introduced a pre impregnated or resin-rich mica paper insulation, called the HiMold™ 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.

15.9 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

90

16 A NEW FLUSH IN INSULATION SYSTEM 16.1 MICALASTIC Of

all

these

insulation processes the insulation which was

preferred

by

ITAIPU power plant was

the

MICALASTIC

the

features will

of

be

which briefed

below: “As

central

components

of

hydroelectric power plants, generators are subjected

to

operating

stresses

which influence the long-term performance of the winding insulation. B19 Rotor of the worlds largest hydro generator ITAIPU at the assembly

Failure of the insulation can lead to lengthy downtimes. The un surpassed reliability of products such as MICALASTIC® insulation is therefore of great economic significance.” The capacity of a hydroelectric power plant is determined by the available water flow and head. Both of these parameters vary widely, and generators can be dimensioned for any rating between 10 kW and 800 MW. The head determines the turbine type as well as the speed, which can 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

91 generators can be expected to operate well right after their initial installation at the power plant. It was once correctly stated that” the construction of a hydroelectric generator can be compared to making a tailor made suit without trying it on”. To date, Siemens has manufactured more than 1200 large hydroelectric generators with a combined capacity in excess of 80,000 MVA. Of these, 360 generators (over 50,000 MVA) have MICALASTIC windings. These machines are characterized by their outstanding reliability, which can be attributed in large measure to their high quality MICALASTIC insulation system. 16.2 THE MICALASTIC INSULATION IN ITAIPU™ MICALASTIC is the registered trademark for Siemens insulation systems for high-voltage windings of rotating electrical machines. These systems use mica, a material capable of withstanding high electrical and thermal loads, together with curable, elastic epoxy resins as bonding material. Since the early days of electrical machine construction, the naturally occurring, inorganic mineral mica has been an indispensable constituent of high voltage insulation systems. The most important criterion for the use of mica is its ability to durably withstand the partial electrical discharges which can occur inside the insulation due to high electrical stresses. 16.2.1 Manufacturing and Design As early as 1957, Siemens-Dynamo werk in Berlin manufactured the first stator windings that made use of mica tape and a vacuum-pressure impregnation process. With this method, single coils and Roebel bars for hydroelectric generators are continuously wrapped with mica tape in the slot and end sections. The taped winding elements are then dried out and degassed in a vacuum impregnation tank, and flooded with low-viscosity, curable synthetic resin. High nitrogen pressure applied to the impregnating bath completely impregnates the mica tape. After being placed in accurately sized, portable pressing molds, the insulation is cured at high temperatures in large chamber ovens. Continued development of this insulation technology ultimately led to the use of a film of ground mica on mechanically strong glass fabric as the carrier material with epoxy resin as the impregnant, which produced a very durable electrically, thermally and mechanically),modern insulation system. Long duration tests in a slot model were unnecessary, since the desired voltage endurance had already been achieved in the previous development stages (Fig. 2) using lower-quality carrier materials. Short-duration tests were performed, however, for verification. 16.2.2 Fitting of Roebel Bars into Slots: Winding elements with cured MICALASTIC insulation are secured in the slots by filling up the tolerances between the slot wall and the conductive surface (coil side corona shielding) of the bar

92 insulation. Initially, Siemens used graphite-treated paper as filler material. Since about 1969, however a special bar fitting procedure has been used for hydroelectric generators. The main features of this procedure are U-shaped slot liners made of polyester fleece impregnated with a conductive material, and a conductive, curable synthetic resin paste between the surface of the bar insulation and the slot liner (Fig. 3). Therefore, the insulation does not stick to the stator core, and the option of removing the bars, even though seldom required, is retained. In the radial direction, the slot portion of the winding elements is secured by means of various packing strips or ripple springs, and slot wedges. Bracing the end windings and jumpers by using glass fiber reinforced spacers and epoxy-resin impregnated cording makes the winding resistant to electro dynamic forces during operation and to possible short-circuit faults. This resistance is also aided considerably by the mechanical stiffness of the MICALASTIC insulation, which is also cured within the end winding. 16.2.3 Thermal Stability The MICALASTIC insulation system was developed strictly for a continuous load in accordance with temperature class F (155°C). Nevertheless, generator design engineers generally guarantee compliance with class B (130°C) temperature limits for nominal operating conditions, as is also required in most invitations to tender. In practice, the stator windings of hydroelectric generators are frequently dimensioned for even lower operating temperatures, because the stators will usually be optimized for good

efficiency by adding electrically active material (winding copper and core

lamination). Particularly low operating temperatures can be expected in the case of stator windings with direct water cooling. With an appropriately dimensioned de mineralized-water cooling system, the maximum winding temperature can be reduced to 70°C and lower. Thermal aging of the insulation is therefore essentially eliminated, and thermo mechanical stresses are also substantially reduced. The resulting increase in operational reliability makes a real difference in the case of hydroelectric generators which are essential to safe grid operation

93 17. CONCLUSION: Hence Vacuum-Pressure Impregnation technology can be used in a wide range of applications from insulating electrical coil windings to sealing porous metal castings. It normally produces better work in less time and at a lower cost than other available procedures. Our VPI systems can be configured in a variety of ways, depending on the size and form of the product to be impregnated, the type of impregnant used and other production factors. System packages include all necessary valves, gauges, instruments and piping. These systems can be large or small, simple or highly sophisticated and equipped with manual, semi-automatic or automatic controls. Vacuum Pressure Impregnation (VPI) yields superior results with better insulating properties, combined with “flexible” rigidity, resulting in greater overall reliability and longer life. VPI reduces coil vibration by serving as an adhesive between coil wires, coil insulation, and by bonding coils to their slots. 18 BIBLIOGRAPHY