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REPORTON

“500 MW TURBOGENERATORSTATOR WINDING ASSEMBLY”

BHARAT HEAVY ELECTRICALSLIMITED, HARDWAR

Submitted by,Akshita Gupta

DEPARTMENT OF ELECTRICAL ENGINEERINGCOLLEGE OF TECHNOLOGY

G.B. PANT UNIVERSITY OF AGRI. AND TECH.

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PREFACE

It is a matter of great pleasure for me to present the following report on myINDUSTRIAL TRAINING at B.H.E.L (Haridwar) for 30 days. The objects of training inengineering couse is to correlate the theory with practical aspects and to make studentsfamiliar with the difficulties arises during practical application so that they can facechallenges boldly while working in the field.As I am a student of electrical engineering so training at “BHEL” had been particularlybeneficial for me. I observe various electrical machineries that are used in different largescale or a small scale industries and different types of power plants.BHEL is very large industry for making the different types of equipments as well asmechanical equipments.

The project report consists of manufacturing of turbo generators i.e. constructionalfeatures of main parts of turbo generator (500MW), general aspects of large turbogenerator, generator series, generator modules, classification of turbo generators etc. withthe main stress given on the detailed description of Stator Winding Assembly Design.

The basic aim of this report is to study THE CHARACTERISTICS OF WINDINGSIN CASE OF 500MW TURBO GENERATOR as well as the latest manufacturingtechniques employed to produce a quality product. It presents a detailed analysis on themanufacture of Turbo generator bars and various insulating materials used in BLOCK-4& manufacture of various turbo generator parts in BLOCK-1.

All in all I have tried my best to present this project report on the summer industrialtraining done in BHEL, in a very precise and profitable manner. Any suggestions in thisdirection will be gratefully accepted.

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ACKNOWLEDGEMENTWe cannot achieve anything worthwhile in the field of technical education unless or untilthe theoretical education acquired in the classroom is effectively wedded to its practicalapproach that is taking place in the modern industries and research institutes. It gives mea great pleasure to have an opportunity to acknowledge and to express gratitude to thosewho were associated with my training at BHEL.

I express my gratitude to BHEL authorities for allowing me to undergo my training inthis prestigious organization. My sincere thanks goes to Sh. P.S. Jangpangi (Sr. DGM)for his prodigious guidance, painstaking, attitude reformative and suggestion throughoutmy summer training schedule.

Last but not the least, my sincere thanks to all the staff members of “BLOCK-1andBLOCK-4, BHEL, RANIPUR, HARIDWAR”

Akshita Gupta

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CONTENTS

Study of Block-1 500 MW Turbo generator Stator Winding Assembly

General Aspects of Turbo generatorDesign Aspects of Turbo generator

Generator Series & Cooling Systems Generator Modules Specifications of TG sets Type of Turbo generator

Design Data of 500 MW TG Stator winding Stator core Rotor Design features

Design and Constructional features of main parts of TG(500 MW) and Ventilation SystemMain components of stator winding assembly design500 MW Turbo generator Stator BarManufacturing DetailsPlacing the bars in slot partBar Support System including support ringTerminal BushingsElectrical Connection of barsImproved version of 500 MW TG with new designfeaturesComparison between 500 MW and 660 MW TG

Conclusion

STUDY OF BLOCK-1

Block-1 is known as ELECTRICAL MACHINE BLOCK. It has got 4 bays as already

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mentioned i.e. Bay-1, Bay-2, Bay-3 & Bay-4 of sizes 36*482, 36*360, 24*360 &24*360 m respectively. The Electrical Machine Block is designed for manufacturingTurbo generators, Hydro generators, Heavy and medium sized A.C. or D.C. electricalmachines.

The production programme of this block is as under:

ITEM TYPE CAPACITYTurbo generators Air-cooled bar type upto 200 MW

Air-cooled V.P.I type upto 160MWHydrogen-cooled THRI 130-270MWHydrogen & water cooled 200-800MWTHDF type

Exciter Brushless upto 800MW

This block comprises of the following departments, test stations, electrical test parts andother auxiliary facilities:

DESCRIPTION PURPOSEMechanical Department Machining of parts of TG, heavy &

medium sized electrical machineAssembly Department Assembly & finishing of TG, heavy &

medium sized electrical machinesTest station Testing of finished products manufactured by

the block.Electrical Test of parts Electrical test of parts for the products

manufactured by the blockAuxiliary services and (a) Routine repair tools of equipmentHandling (b) Centralized operation of handling

facilities the block

TURBOGENERATORS:

Facilities available in this bay are: -(a) Machine Section:This section is equipped with large size machine tools such as lathes, vertical boring and

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drilling machines for machining stator body, rotor shaft, end-shields, bearing etc. forturbo generators. Also it has a special rotor slot-milling machine for cutting slots inrotor shafts.

(b) Iron Assembly:This section has facilities for stator core assembly of turbo generators and heavyelectrical motors including 1000 T umbrella type press for pressing the cores andtransformer for induction heating of retaining rings and armature cores of large sizeelectric motors.

(c) Heavy Rotor Assembly Section:It has a 250-ton horizontal press and other necessary facilities for assembly of large sizerotors.

(d) Stator Winding Section:It has two dust proof stands for stator winding of turbo generators and aninstallation for heating of stator bars.

(e) Armature Rotor Section:It is equipped with installations for laying windings in turbo generators and large size o.k.Armatures. Also it has a dynamic balancing machining for rotors up to 16-ton weight.

(f) Armature Rotor Impregnation Section:In this general section assembly of turbo generators, large size arc and d.c. Motors arecarried out.

(g) Test Stands: Turbo generators Test Stand:It is equipped with 6 MW drive motor and a test pit open circuit, sudden short circuittemperature rise, hydraulic and hydrogen leakage tests are carried out her for turbogenerators.

(h) Over speed Balancing Tunnel:The dynamic balancing of turbo generator rotors is done on this installation. In thattunnel rotor is rotated at 3000 i.e. if rotor is unbalanced then wt. are put in the ELLENBOLTS.

LSTG (Large size turbo generators) sectionLSTG section is divided into four sections: -

1. Main assembly2. Rotor winding section3. Stator winding section4. Core assembly

CORE ASSEMBLY:In this section stator core is assembled i.e. assembly of core bar, dovetail, stampings etc.

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ROTOR WINDING SECTION:In this section rotor winding is assembled as explained earlier.

STATOR WINDING SECTION:In this section stator winding is assembled as explained earlier.

MAIN ASSEMBLY:After assembling rotor and stator windings the whole assembly of turbo generator is donein that section.

GENERAL ASPECTS OFTURBO GENERATOR

With the increasing pace of industrial & technological developments, powerful andhigh-speed machines have become a common feature of many industries. Electric motors,turbo generators, hydro generators, high-speed turbines & compressors have becomeintegral and indispensable components of modern industry.Turbo generator is an A.C. Synchronous Machine. These are the Synchronous

Generators driven by steam turbines at high speed. They have ratings as high as 1000MW. The basic operation principle involves the conversion of mechanical energy intoelectrical energy. It is necessary to remove the generated losses (heat) in the machinefrom a point as close as possible to the heat source. So the designer aims at the removalof the heat losses in the most efficient way.

Here in BHEL Turbo generators are manufactured having capacity up to 660 MW. Theirlatest projects going on are the manufacturing of 500 MW and 660 MW Turbogenerators. BHEL is the leading manufacturer of high capacity Turbo generators up to660 MW in India. All modern Turbo alternators are 2-pole machines and their speed is3000 rpm corresponding to a frequency of 50 Hz. Turbo generators are characterized bylong lengths & short diameters. This is because it is not possible to increase the rotordiameter beyond a certain value (1.2 m) owing to the limitations imposed by mechanical

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considerations like centrifugal force, deflection of shaft and the critical speed. Thediameter being limited, the only way to raise the rating is to increase the length; theactive core length must be of the order of 10 mm per MVA.The generator is provided with a fast acting fully static / brush less excitation system &

dependable services to give prolonged trouble –free operation over the years.

“500MW Turbo generator is a three phase, horizontally mounted 2 polecylindrical rotor type machine driven by a directly coupled steam turbineat 3000 rpm.”

With large lengths of core, it is very difficult to cool the machine, especially itscentral portions. Infect the cooling of turbo alternators is the most complexengineering problems. All the materials that go into the manufacture of thismachine are subjected to rigorous tests and each sub-assembly or componentundergoes a series of stage wise tests. Every Turbo generator is fully tested atthe plant’s test bed as per National Electro technical Commission Standards.

Synchronous machine construction. Schematic cross section of a salient-polesynchronous machine. In a large generator, the rotor is magnetized by a coil wrappedaround it. The figure shows a two-pole rotor. Salient-pole rotors normally have manymore than two poles. When designed as a generator, large salient-pole machines aredriven by water turbines. The bottom part of the figure shows the three-phase voltagesobtained at the terminals of the generator, and the equation relates the speed of themachine, its number of poles, and the frequency of the resulting voltage.

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DESIGN ASPECTS OFLARGE TURBOGENERATOR

The continuous development in the design of Turbo generators is possible due toimprovements in:

Materials Insulation Systems Cooling Systems Design features

Turbo generators are classified according to cooling adopted in windings of rotor & statorand medium filled inside the machine. Air, hydrogen and water are the coolants used forcooling. Also whether the medium comes in contact directly with the conductor orthrough the insulation is another aspect. With the present day technology it istheoretically possible to design Turbo generators up to 2000 MVA. Annexure-1 showsthe various cooling schemes of Turbo generators. In addition BHEL has acquired gappick up cooling technology from Russia for 210 MW generators.Annexure -2 presents the various ratings possible with various combinations. TLII with

indirect cooling is not discussed as it is used in small generators. TLRI generators arepossible from 32 MVA to 300 MVA. THRI generators are generally offered from 180MVA to 450 MVA. THDD generators (not popular in India) are possible from 400 MVAto 1000 MVA. THDF generators can be offered from 500 MVA to 1300 MVA. THFFgenerators in which stator and rotor windings are cooled by water is more popular inlarge ratings of 1500 MVA, 4-pole versions for nuclear applications.

Various issues in selecting the modulea) Ratingb) Application whether GTG or STG and direction of rotationc) Site conditions like cold-water temperatured) Standard IEC, ANSI

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e) Operational experience requirementsf) Special requirements like SCR, Voltage and Frequency variation, Overload Capabilityg) Efficiencyh) Cost & deliveryI) Transportj) Availability of toolingk) Critical speeds and temperature including hot spots

TECHNICAL ISSUES

Selecting a module starts from the knowledge of some or all of the above. Diameters arestandardized e.g. 750, 800, 860, 930, 1000, 1075, 1150, and 1230 mm for rotor. Voltagesare generally selected from 10.5/11, 15.75/16.5, 21/22, 27 KV although the manufacturercan select the voltage. The permissible stator slots can be in the range of 48 to 84 slotsand can be 36 - 48 also. For a given output after deciding the cooling, the rotor diameteris selected from past experience. Active length is selected to obtain the output as well asacceptable from vibration behavior and cooling point of view. The design has to beverified from the above listed issues and customer requirements.

Core diameter is constrained from vibrations, type of core suspension, saturation levels,and effect on temperature rises and transport. The vibrations are permitted from type ofcore suspension, which could be rigid, elastic & spring.Rigid suspension is used in TLRI and THRI machines with built in core in stator frameand elastic suspension is in which core is supported in electric beam for separately builtcore and is used in air cooled machines unto 190 MVA TLRI (108/41). The vibrationsare restricted to 15 vibrations in both.

Spring suspension in which the vibrations can be permitted more than 16 microns (up to20 microns) is used in bigger air-cooled machines , separately built up cores of THRI andall THDD and THDF machines. Additional care has to be taken in restricting core fluxdensity to 1.5 Tesla as it may lead to overheating of steel parts beyond core and also thesetting of over fluxing protection is to be checked. Additional temperature rise in statorwinding due to extra losses in core is also to be considered. The permissible diameter forrail transport is about 4060 mm in Indian railways and for road transport 4200 mm ispossible. However care has to be taken regarding suitability of springs for road transport.With 4200 mm casing diameter and spring suspension core outer diameter is possible upto 2900 mm. Magnetic shunt is used in high capacity machines. Like so the stepping andslitting are to be decided based on requirements.

Length of the machine is decided from output, type of cooling, no .of slots, voltage, airgap flux density and permissible behavior from vibrations and critical speedconsiderations. If experience on previous length - diameter combination is not possible,the design may have to be reworked after detailed calculations in many cycles. Ironlength to total length ratio depends upon the type of machines, acceptable combinationsof ventilation ducts and lamination packets. 5/8/10 mm is used in indirectly cooledmachines and 3/5 mm ducts are used in directly cooled machines. The width of packet isselected, ventilation and temperature raises considerations and also the temperature

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distribution along the length.

Slot width is decided from permissible tooth density and gives good temperature ofwinding. The conductor dimensions and no. is decided based on the feasibility ofmanufacture, current density, slot dissipation factor, efficiency, reactance andtemperature including hot spots. It is mandatory to work out slot balance upon practicesand standards. The knowledge of the following in deciding this as well as slot dimensionsis essential.a) Transposition width to pitch ratiob) 360º / 540º transposition and 1 or 2 plan bending is requiredb) Effective elementary insulationc) Intercolumn insertd) Ripple spring requirementse) Main insulation requirementsg) Inner and outer corona protectionh) Maximum / minimum permissible conductor width and width to thickens ratioi) Clearances in bar-bar and winding-retaining ringj) Permissible lip and wedge heightk) Overhang involutes anglesl) Mid-phase transposition, bundling, strip to strip or sleeve connection

Rotor slotting is generally standardized for various diameters and type of machine.These are done from the considerations of stresses at various parts, retaining–ringdimensions, type of machine, flux densities in rotor core and teeth and availability oftooling. The conductor width, insulation parts and wedge dimensions are also optimized.However for a new variant through investigations are required before implementation.Length is decided after fixation of stator length. The rotor length is more by 30 - 40 mmof stator. The requirements of damper slots are to be foreseen.

GENERATOR SERIES & COOLING SYSTEMS:

1. T A R I2. T H R I3. T H D D4. T H D I5. T H D F

(STATOR COOLING)I-For indirect coolingD-Direct cooling with gasF-Direct cooling with fluid (water)

(ROTOR COOLING)

R-Radial cooling with gasD-Direct axial cooling with gas

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(COOLING GAS IN CASING)

A-Air coolH-Hydrogen cooled

(TYPE of GENERATOR OR PRODUCT)

T- Turbo generator A.C.

GENERATOR MODULES

According to Russian Design:

TURBOGENERATOR MODULE: THW TYPECAPACITY: 210 MW - TPS

235 MW - NPS

According to Siemen’s Design (Germany)

THRI: 108/44 A, 108/44 B, 108/39 & 108/55TARI: 93/38, 108/36, 108/41 & 108/46THDF: 115/59 & 11 /67

In case of THRI type module 108/44 A, 108/44 B, 108 /39 & 108/55 have thefollowing meanings:u

Numerator i.e. 108 means rotor diameter is 1075 mmDenominator i.e. 44, 39 & 55 means the stator length is 4400 mm/3900 mm/5500 mm44 A means the module is suitable for Gas turbine application (bar impregnation).

44 B means total impregnation including stator core

In case of TARI Type Module 93/38, 108/36, 108/41 & 108/46 have the followingmeaning:

Numerator 93 means that the rotor diameter is 930 mmNumerator 108 means that the rotor outer diameter is 1075 mm.Denominator 38, 36, 41 & 46 means that the stator length is 3800 mm, 3600 mm,4100 mm & 4600 mm respectively.

In case of THDF type module the dimensions have the following meaning:

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Numerator 115 means that the rotor diameter is 1150 mmDenominator 59 & 67 means that the stator length is 5900 and 6700 mm respectively.

It is to be noted that in Russian design there are no dimensions.

SPECIFICATIONS OF TURBOGENERATOR SETS:

THRI: 210 MW, 235 MW & 250 MW TG Sets (GERMAN Design)

T refers to TURBOGENERATORH refers to HYDROGEN GAS COOLEDR refers to RADIAL COOLING OF ROTORI refers to INDIRECT COOLING

THDF: 500 MW (Already manufactured): 660 MW TG Sets (Designing being done)

T refers to TURBOGENERATORH refers to HYDROGEN GAS COOLEDD refers to DIRECT COOLING OF ROTORF refers to FLUID COOLING OF WINDING BARS

THW: 200 MW Sets (Russian Design )

T refers to TURBOGENERATORH refers to HYDROGEN GAS COOLEDW refers to WATER COOLING

Therefore the Turbo generators manufactured at Haridwar so far covers the wide range from200MW to 500 MW.

TYPE OF TURBOGENERATORS:

The coupling turbine-wise classification is as follows:

Type of Turbine Type of Generator Capacity

Steam Turbines THDF type 500 MWTHRI type 130 MW, 200 - 250 MW

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THW type 200 / 210 MWTARI type Up to 170 MW

Nuclear Turbines THDF type 500 MWTHW type 235 MW

Gas turbines TARI type Up to 170 MW

The following is the type of stator winding cooling for these generators:

Type of Generator Type of Stator winding cooling Type of stator bar conductors

THDF Type Direct Water cooled Hollow & Solid Cu conductorsTHRI Type Indirect Hydrogen cooled Solid Cu conductorsTARI Type Indirect Air cooled Solid Cu conductorTHW Type Direct Water cooled Hollow & Solid Cu conductor

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DESIGN DATA OF 500 MWTURBO GENERATOR

Generator Type : THDF -115 / 59

Capacity : 500 MW

Load : Base

MVA : 588

Power Factor : 0.85 lagging

Terminal Voltage/ : 21 KVStator Voltage

Speed : 3000 rpm

Rated Current/ : 16.16 KAStator Current

Frequency : 50 Hz

Hydrogen pressure : 3.5 bars (g)

Excitation (V & A) : 317 V & 4040 A

Phase : 3

Short ckt. Ratio : 0.48

Insulation Class : F

Generator Efficiency : 98.64 %

Coolant : Water & Hydrogen

Voltage Var. : 5 %

Frequency Var. : 5 % to 3 %

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STATOR WINDING (RATINGS):

1. Conductor Size : 8 x 4.6 mm (hollow)

2. Conductor Size (uninsulated) : 8 x 1.3 mm (solid)

3. Bar Insulation : 5.85 mm

4. No. of Conductors : 5 (hollow)

5. No. of Conductors per bar per column : 10 (solid)

6. Mean turn Length : 2 x 9845 mm

7. No. of slots : 48

8. Short Chording : 20 / 24

9. Type of Winding : Lap, Double layer

10. Connections : Double Star

11. No. of parallel paths : 2

12. No. of coils : 48

13. Cooling Duct : 5 x 1.6 mm

14. Net straight winding copper weight : 8180 Kg

STATOR CORE (RATINGS):

Outer diameter : 2630 mm

Bore diameter : 1330 mm

Core length net : 5592 mm

Core length gross : 5850 mm

ET sheet steel (thickness) : 0.50 mm

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Grade of ET steel : 1.5 W / Kg

Slot size : 48 x 160 mm

No. of slots : 48

Net Core weight : 141784 Kg

ROTOR (RATINGS):

Air gap : 90 mm

Rotor diameter : 1150 mm

Barrel length : 5800 mm

Total length end to end : 12220 mm

DESIGN FEATURES:

Brushless Excitation

Micalastic Insulation

End-shield mounted bearing

Individual Bar impregnated design

Hydrogen Pressure : 3.5 bar

Direction of rotation : Anticlockwise at turbine-end towards generator/ clockwiseat exciter end.

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DESIGN AND CONSTRUCTIONAL FEATURES OF MAIN PARTSOF TURBOGENERATOR (500 MW) & VENTILATIONSYSTEM:

The general cross-section of the Turbo generator consists of the followingcomponents:

STATORStator frame Spring basket Testing

* Alignment of spring basket* Hydraulic testing* Pneumatic testing

Stator core Core bar Dovetail assembly Stampings Studs End rings Magnetic shunt Support rings Bus bar assembly Header assembly

Stator WindingRipple springs

Hydrogen coolers and end-shields

ROTORRotor ShaftRotor WindingRotor Retaining ringsField connections

BEARINGS AND SHAFT SEALS

Various additional auxiliaries for generator operation are:

(I) Seal Oil System(ii) Gas System

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(iii) Primary Water System(iv) Excitation System

STATOR:

The stator comprises of a stator frame as well as stator core and winding assembly.Manufacture of stator frame is performed independently of the stator core and windingassembly production and prior to installation of the winding; the core is linked to thestator frame by means of flat springs.

STATOR FRAME:

The stator frame consists of cylindrical center -section and two end-shields, which aregas-tight and can, withstand explosion pressure in the event of likely hydrogen explosion.The stator end-shields are jointed and sealed to the stator frame with O-rings and boltedflange connections. The stator frame accommodates the stator core and windings.The generator cooler is subdivided into four single sections arranged vertically in theend-shield on the turbine side. In addition, the stator end-shields contain the shaft sealand bearing components. The stator is firmly connected to the foundation with anchorbolts through the stator feet, which are welded on to the frame.

SPRING BASKET:

Spring basket is used to control the vibrations in the core of stator. These are made ofMild steel. About 7 baskets are used in the 500 MW T.G. It consists of springs at its backwhich controls the vibrations in the coreUsing Telescope, Collimeter etc. by finding the center of the frame, does alignment ofspring baskets.

TESTING:

Using two methods does stator frame testing: -1. HYRAULIC TESTING2. PNEUMATIC SYSTEM

In hydraulic testing frame with end shields on both sides is filled with water at10 Kg/cm^2 to check the elasticity and plasticity of the material.

Maximum plasticity should be 2 mmMaximum elasticity should be 3 mm

Pneumatic testing is performed on stator frame to check any leakage in the frame and inthis air is filled in the stator body at 6 Kg/cm^2 for 6 to7 hours and after this pressure ischecked again. If pressure decreases it means there is leakage in the body. This test iscarried out because we have to fill hydrogen in it and if there is any leakage that results infire.

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STATOR CORE:

It is one of the most significant parts. Apart from providing support to the stator windingit provides a closed path for flux produced by the winding.

The stator core is stacked from insulated electrical sheet steel laminations &mounted in supporting rings over insulated dovetailed guide bars. It is to be notedthat the stacking of core laminations is done in the stator frame in the verticalposition with the exciter side downward and is normally done in the stacking pit.Also to obtain smooth slot walls, mandrels / stacking guides are inserted in thestator slots as well as in the holes meant for tension bolts. Axial compression ofstator core is obtained by clamping fingers, pressure plates and non-magneticthrough type clamping bolts, which are insulated from the core. The supportingrings form part of an inner frame cage. This cage is suspended in the outer frameby a number of separate flat springs (fig. Shown). The flat springs aretangentially arranged on the circumference in sets with three springs each, i.e. twovertical supporting springs on both sides of core and one horizontal springbelow the core. The suspension of core is such that the transmission offorced vibration of core to the frame and foundation is restrictedeffectively. The end-portions of the core are guarded from the end-leakagefluxes by magnetic shunt type shielding provided at both the ends of thegenerators

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STATOR WINDING:

The high voltage insulation is provided according to the proven MICALASTICSYSTEM. This system, which adopts Vacuum Pressure Impregnation (VPI) technique

with resin poor tape, ensures a nearly void-free insulation and is characterized by itsexcellent electrical and mechanical properties. The winding bars are made with 540ºtransposition in the slot-portion. The individual bars consist of hollow and solid strandsevenly distributed in the cross-section. At the bar-ends all solid strands are jointly brazedinto a connecting sleeve and the hollow strands into a water box from which the coolingwater enters and leaves via Teflon insulating hoses connected to the annular manifolds.The electrical connection between the top and bottom bars is made by a boltedconnection at the connecting sleeve.

The annular manifolds are insulated from the stator frame, permitting the measurementsof insulation resistance of the water filled winding. During operation, the annularmanifolds are grounded. The MICALASTIC system is fully waterproof and oil-resistant.

To protect the stator winding against the damaging effects of magnetic bar bouncingforces under normal load and short circuit in the slots, bottom filler, side ripple springand ripple spring located beneath the slot wedges ensures permanent firm sitting of thebars in the slot during operation. The gaps between the bars in the stator end windings arecompletely filled with insulating conformable material and cured after installation.

For radial support the end-windings are clamped to a rigid ring of insulating material,which in turn is fully supported by the core pressure plate. The bars are clamped to thesupport ring with insulating segments held by clamping bolts made from a high strengthinsulating material (fig. shown). The support ring is free to move axially within the statorframe so that movements of the winding due to thermal expansions are not restricted andis held rigidly to prevent movement in circumferential direction. The stator windingconnections are brought out to six bushing located in a compartment of welded non–magnetic steel below the generator at the exciter-end. CTs (Current Transformers) formetering and relaying purposes can be mounted on the bushings.

HYDROGEN COOLERS:

The hydrogen cooler is a shell and tube type heat exchanger, which cools the hydrogengas in the generator.

END-SHIELDS:

The ends of the stator frame are closed by pressure containing end-shields. The end-shields feature a high stiffness and accommodate the generator bearings, shaft seals andhydrogen coolers. The end-shields are horizontally split to allow for assembly.

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ROTOR:

A rotating body with journals supported in bearings is a rotor. Generator rotor housesfield windings and provides essential excitation energy needed for the induction ofelectrical power in stator windings. The rotor of the turbo generator is the most loadedpart as far as the mechanical stresses, heating and magnetic saturation are concerned. Thedesign and size of rotor depends mainly on the output of the generator. It also dependson the speed of operation, extent of optimization in mechanical, electromagnetic design,type of cooling employed, design of materials and the insulation system.

ROTOR SHAFT:

The rotor shaft is a solid single piece forging manufactured from a vacuum casing. Slotsare milled on a rotor body to accommodate the field winding. The longitudinal slot polesare obtained. Transverse slots are machined on rotor body to reduce double systemfrequency rotor vibrations caused by deflections in the direction of pole and neutral axis.To ensure quality standard material analysis, only high quality forgings are used, strengthtests, mechanical tests and ultrasonic tests are performed on the forging during rotormanufacture. After assembly, the rotor is balanced & subjected to an over speed test at20 % over speed for 2 minutes.

ROTOR RETAINING RINGS:

Retaining rings are used in the electric generator rotor at both the ends to hold coppercoil ends firmly in position against the action of centrifugal forces arising out of the rotormovement and they are some of the most highly stressed and critical components of thegenerator rotor. Rotor and Retaining rings are very expensive and long lead items.

ROTOR WINDING:

The rotor winding consists of several coils, which are inserted into the slots, and seriesconnected such that two coil groups form one pole. Each coil consists of several seriesconnected turns, each of which consists of two longitudinal and transverse turns, whichare connected by brazing in the end–section.

The rotor winding consists of several coils, which are inserted into the slots, and seriesconnected such that two coil groups form one pole. Each coil consists of several seriesconnected turns, each of which consists of two half turns, which are connected by brazingin the end-section. The rotor winding consists of silver-bearing de-oxidized copperhollow conductor with two lateral cooling ducts. L-shaped strips of laminated epoxyglass fiber fabric with Nomex filler are used for slot insulation. The slot wedges are madeof high conducting material and extended below the seating surface of the retaining rings.

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The retaining rings are made of non-magnetic high strength steel in order to reduce straylosses. On one side these rings are shrink-fitted onto the rotor body and are of floatingtype design with the other end free. The rings are further secured in axial position by snaprings.

FIELD CONNECTIONS:

The field current is supplied to the rotor winding through radial terminal bolts and twosemi-circular conductors to the exciter leads at the exciter coupling with multi contactplug in contact which allow for unobstructed thermal expansion of the field currentleads.

BEARINGS:

The generator rotor is supported on the end-shield mounted on the journal bearings onboth ends. A third bearing is located between the exciters. Provision is made for thehydraulic jacking of the rotor shaft operation. To eliminate shaft currents all bearings areinsulated from the stator frame and foundation plate resides inside to eliminate shaftcurrents. A temperature sensing thermocouple is embedded in the lower bearing sleeve sothat the measuring points are located directly below the babbit. Vibrations of the bearingsare measured by vibration pick-ups. The two generator bearings and the exciter bearingare connected to the turbine oil assembly.

SHAFT SEALS:

Shaft seals are provided at the points where the rotor shaft passes through the statorcasing. These radial seal rings are guided in the seal carrier rings which in turn, are boltedto the end-shields. These are insulated to prevent the flow of shaft currents. The seal ringsare lined with babbit in the shaft journal side and the gap between the seal ring and shaftis kept optimum to provide effective sealing by forming a continuous stable oil film. Thehydrogen side seal oil is supplied to the seal ring via an annular groove in the seal guidethis oil emerges out through several circumferentially situated holes. The airside seal oilis supplied to the sealing gap from the seal ring chamber via radial bores and the airsideannular groove of the seal ring. To ensure effective sealing, the seal oil pressure in theannular gap is maintained at a higher level than the gas pressure within the generatorcasing. The airside seal oil pressure is set such that a small quantity of the air - side sealoil only flows to the hydrogen side and vice-versa.

SEAL-OIL SYSTEM:

The shaft seals are supplied with seal oil from two seal oil circuits, which consists ofHydrogen side seal oil circuit, and airside seal oil circuit.

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GAS SYSTEM:

The gas system contains all equipment necessary for filling the generator with CO2,hydrogen or air and removal of these media, and for operation of the generator filled withhydrogen. In addition the gas system includes a Nitrogen supply.

PRIMARY WATER SYSTEM:

The primary water required for cooling the stator winding is circulated in a closedsystem. In order to prevent corrosion, only copper, stainless steels or similar corrosionresistant materials are used throughout the entire cooling circuit.

EXCITATION SYSTEM:

An important characteristic of large turbo generators is that the excitation requirementsincrease sharply with the rating of the machine. Brush less excitation system comprises adirect driven revolving armature arc. Exciter connected through shaft-mounted rectifiersto the rotating field of the turbo generator with no tapping of excitation power betweenthe source of generation and point of supply to the generator field. Today, leadingmanufacturers offer brush less excitation with rotating diodes as the preferred excitationsystem.

Brush less Excitation System employed in 500 MW TG has the following merits:

a) Completely eliminates brush gear, slip rings, field-breaker and exciter bus or cable.

b) Eliminates the hazard of changing brushes on load or the need to shut down the set tochange brushes.

c) Carbon dust is no longer produced and hence the operation is fully dust-free.

c) Brush losses are eliminated.

d) Operating costs are reduced

e) The system is best suited for atmospheres contaminated with oil, salt, chemicaletc. and were sparking may be a fire hazard.

e) The system is simple and requires practically no maintenance except for anoccasional inspection. Maintenance costs are thus reduced. Ideally suited for locationswhere maintenance is likely to be rare due to continuous demand on the machine.

g) Brush less system with shaft mounted pilot exciter is of self generating type andThe excitation is unaffected by system faults and disturbances.

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h) Reliability is better

I) Ideally suited for large sets

j) Increasingly popular system the world over.

Thus the brush less excitation system is so well proven in service with practically nofailure due to rotating diodes as to offer great promise for the excitation of the highestrating turbo generators as that of 500 MW envisaged at present.

(500 MW brushless exciter)

VENTILATION SYSTEM AND COOLING CIRCUIT:

The generator is designed for direct water cooling for the stator winding including phaseconnectors, main bushings and direct hydrogen cooling for the stator core and rotorwinding. The losses in the remaining generator components such as friction, windageand stray losses are also dissipated through hydrogen. The type of cooling beingexclusively direct, largely eliminates any hot spots and higher differential temperaturebetween adjacent components. As a result the thermal displacement leading tomechanical stresses is eliminated. This applies particularly to the copper conductors,insulation, and rotor body and stator core.

HYDROGEN COOLING CIRCUIT:

The hydrogen is circulated in the generator in a closed circuit by a multistage axialcompressor located on the turbine-end. The compressor sucks hot gas from the air-gapand delivers it to the cooler, where it is cooled and re-circulated. The hydrogen at theoutlet of the coolers is divided into three flows: the first portion is admitted into therotor at the turbine-end below the fan-hub for cooling of the turbine-end half of therotor. The second portion is passed from the cooler sections to the individual frame

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compartments for cooling of the stator core. The third portion is passed to the stator end -winding space at the exciter-end through guide ducts in the frame for cooling of theexciter-end half of the rotor-end of the core-end portions. After carrying away the heatgenerated at various parts in their paths, these part flows are discharged into the air-gapwhere they are mixed and then return to the compressor and further to the cooler.

Hydrogen-cooled turbo generator

COOLING OF ROTOR:

The cold gas is admitted at both the ends of the rotor for direct cooling of the rotorwinding. The rotor winding is symmetrical relative to the generator centerline and poleaxis. Each coil quarter obtained by this symmetry is divided into two cooling zones.The first cooling zone consists of the rotor end winding and the second of the windingportion between the rotor body-end and the mid-point of the rotor. The cold gas isdirected to each cooling zone through separate openings directly to the rotor body-end.The hydrogen flows through each individual conductor in closed cooling ducts. The heatremoval capacity is selected such that approximately identical temperatures are obtainedfor all conductors. The gas of the first cooling zone is discharged from the pole centercoils into a collecting compartment within the pole area below the end winding. Fromthere the hot gas passes into the air-gap through pole face slots at the end of rotor body.The hot gas of the second cooling zone is discharged into the air gap at mid-length ofrotor body through radial openings in the hollow conductors and wedges.

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COOLING OF STATOR CORE:

The cooling of the stator core is achieved through the cold gas admitted to the individualframe compartments, the gas flows through ventilating ducts in the core where itabsorbs the heat from the core. To dissipate the higher losses in the core end, theventilating ring ducts are provided in the core-end portion. These ventilating ducts areprovided with the cooling gas directly from the end-winding space. Another part flow isdirected from the stator end winding space past the clamping fingers between thepressure plate and core end portion into the air-gap. All the flows mix in the air-gap andcool the rotor body and stator bore surfaces. The gas is then returned to the coolers viathe axial-flow fan. An air gap seal fixed onto the stator-winding overhang on exciter-endflows through rotor winding.

PRIMARY WATER COOLING CIRCUIT:

The losses occurring in the generator stator winding, main bushings and phaseconnectors are dissipated through direct water-cooling. This water, known as primarywater is circulated in a closed circuit by means of centrifugal pumps. At the entry to thegenerator the cooling water flow is divided into two parts. The first part cools the statorwinding. This part of the flow enters the generator, the annular manifold on the terminalside and from there to the stator winding bars via insulating flow hoses.

Each individual bar is connected to the annular manifold by a separate hose. At the otherend, the water passes through similar hoses to another annular manifold and then returnsto the primary water tank. This single flow system ensures the elimination of relativemovements due to different thermal expansion between the top and bottom bars.

The second part cools the phase connectors and the bushings. The bushings and phaseconnectors consist of thick walled copper tubes through which the water is circulated.The six bushings and the phase connectors arranged in a circle around the stator endwinding are hydraulically interconnected so that three parallel flow-paths are obtained.The primary water enters three bushings and exits from the remaining bushings.

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MAIN COMPONENTS OF STATORWINDING ASSEMBLY DESIGN:

1. Stator Winding bar

2. Placing of bars in slot part

3. Bar support system including support ring

4. Terminal Bushings

5. Electrical Connection of bars

6. Components for water-cooling of Stator winding

1. STATOR WINDING BAR:

Manufacturing of stator Winding bar includes the following:

(A) Manufacture of transposed bars

(B) Overhang Forming

(C) Brazing of bar-ends

(D) Insulation of bar (MHV Insulation)

(E) Corona Protection

(F) Testing of bar

GENERAL CONSTRUCTIONAL FEATURES:

The three-phase stator winding is a fractional pitch two layer consisting of individualbars. Each stator slot accommodates two bars. The slots are uniformly distributed on thecircumference of the stator core. The slot bottom bars and top bars are displaced fromeach other by one winding pitch and connected at their ends to form coil groups. The coilgroups are connected together with connecting bus bars inside stator frame. Theconnecting bus bars are connected to terminal bushings.

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500 MW TG STATOR BARMANUFACTURING DETAILS

The bar consists of a large number of strands. These may be only solid Cu-conductors ora combination of hollow and solid Cu-conductors as in case of 500 MW TG dependingupon the type of Stator Winding Cooling as described earlier. To minimize the straylosses, the individual conductor strands are separately insulated and transposed in theslot portion. The transposition is 540º for 500 MW and 360º for all other generatorsmanufactured so far. The bar consists of four columns side by side in case of 500 MWand two columns for other generators. A vertical separator insulates these columnsagainst each other.

(A) MANUFACTURE OF TRANSPOSED BARS:

1. Cutting & strengthening of strands:

The copper strand i.e. double glass covered rectangular Copper conductor anddouble glass covered rectangular hollow conductor is straightened and cut to length.It must be ensured that the strand insulation is not damaged. The markedand visually defective portions are to be removed.

2. Removal of strand insulation:

FOR SOFT BRAZED CONNECTION: The insulation at both the ends ofstrand is removed.

FOR HARD BRAZED CONNECTION: The strands for water-cooled windingbars are stripped off insulation at both ends according to dimensions & at 50 to 75mm for all other type of winding bars.

BENDING OF STRANDS: Then bending of strands is done.

TRANSPOSITION OF STRANDS & ASSEMBLING OF BAR HALVES:The strands belonging to a bar half are transposed for number of transpositions

and bound together with cotton tape. Both the bars are so assembled andtransposed that they form one bar. The cotton tape is then removed. The bar is

secured at the untransposed ends in a distance of about 500 mm against filling ofstrands in one another.

CHECKING & IDENTIFICATION OF BARS:The first manufactured bar for a winding is to be checked by the SupervisorExecutive. In the course of further manufacture random checks are conducted.

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It is to be noted that in the course of manufacture, the bars are subjected tonumerous electrical tests for quality control, these include, Short test, the DielectricDissipation Loss Factor (tan delta measurement) and High Voltage Test at various

Stages of manufacture and assembly.

Parts of Stator winding bar are:

1. Separators2. Crossover insulation

Separators are further classified as Vertical Separator, Separator in overhang(end-part) and Separator extension at bend.

3. Vertical Separator:

For all types of bars, a vertical separator is inserted between the two halves of theBar.

CROSS-OVER INSULATION:The crossover positions in slot area are provided with crossover insulation i.e.Epoxy Macanese Fleece. The material is cut to bar width. If the strands on theoverhang are also transposed than also crossover insulation material 0.25 thicki.e. Polyamide paper Nomex type is to be put. The pieces shall be pushed on oneanother at respective half spacing. During lifting of the bends, it is ensured that

the strand insulation is not damaged.

SEPARATOR EXTENSION AT BEND: The separator extension i.e. Nomexglass fleece is inserted in two layers between the bar halves.

TRANSPOSITION OF BARS (540º ELEC.):

The bar consists of a large number of separately insulated strands, which are transposedto reduce the skin effect losses. The strands of small rectangular cross-section areprovided with braided glass insulation & arranged side by side over the slot width. Avertical separator insulates the individual layers from each other. In the straight slotportion, the strands are transposed by 540º. The transposition provides for a mutualneutralization of the voltages induced in the individual strands due to the slot cross-field & end-winding flux leakage and ensures that minimum circulation currents exist.The current flowing through the conductor is thus uniformly distributed over the entirebar-cross-section so that the current dependent losses will be reduced.

Thus transposition of bars is done:(1) To reduce Eddy current losses.(2) To equalize the voltage generation

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(3) To minimize the skin effect of A.C. supply so small cross-section of conductor isused & also hollow conductors are used to effect cooling by water.

Side view showing one way of transposing insulated strands in stator bar

CROSSOVER INSULATION:

To eliminate inter-turn short at bends during edge-wise bending and leveling of bars inslot portion for proper stack pressing.

PRESSING OF BARS:

Firstly the bars are prepared for pressing. The bars with wrapper insulation are coatedwith Silicon compound (silicon rubber compound and silicon rubber hardener) in about100 mm length at wrapping ends on all strands towards bar-ends. The treated position istapped with a tight layer of protecting tape i.e. Cotton tape (supplied in rolls).The barswith micalastic insulation are not treated with silicon rubber application; TechnicalSeparator & technical tapes shall only be removed when the pressing process is stopped.

The bars after preparation for pressing, are pressed in temperature controlled heatedpresses. The bars are wrapped in releasing foil (polyester foil) and so placed in the pressthat the pressed part of bar lies between the pressing planks/strips. Then the press isclosed. Curing is done for minimum 30 minutes. At 160 ± 10°C, the temperature rise isto be recorded.

Note: During rapid heating, the press is checked many times till the pressure plate areremoved & the pressing strips are placed.

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COOLING OF PRESS & REMOVAL OF BARS:

The press is then cooled down to 50°C and then opened. The bars are removed &released of releasing foil. The edges of the bar are smoothened by sand paper.

Pressing of stack bar is done to achieve proper size of bar & consolidation of stack sothat it becomes a monolithic.

ELECTRICAL TESTING:

In case of water-cooled transposed bars, the electrical testing is done after pressing. Toensure that the copper conductors are firmly bonded together & to give dimensionalstability in the slot portion, the bars are cured in an electrically heated press.

(B) OVERHANG - FORMING:

Prior to applying the bar insulation, the bar–ends are bent with a special device whichshapes the involutes over a cone shell. This involutes shape ensures uniform spacing ofthe bars over the entire length of overhang after assembly.

(C) BRAZING OF BAR ENDS:

Before applying the insulation on bar stack and curing, contact sleeves for electricalconnection of the bars and water boxes for the cooling water connections are brazed tothe bar ends.After manufacturing of the bars, the bars are subjected to numerous electrical and leakagetests for quality control.

(D) INSULATION OF BARS:

The stator winding bars are insulated with Micalastic (trade name) insulation. Highquality mica, selected epoxy resins and a matching vacuum impregnation process arethe characteristic features of the micalastic insulation for large turbo generators. Aconsistent development has led to a high quality insulation system, the reliability ofwhich is ensured by continuous quality control. The stator winding of the world’s largestsingle-shaft generators with an output rating of 1640 MVA and a rated voltage of 27 KVwere provided with micalastic insulation.The surface of the bar must be clean and grease-free and to be cleaned by a solvent.

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TAPING OF BARS/MICALASTIC INSULATION TAPING:

The bars are first tapped. In the slot part there are a total no. of 17 layers and in overhangpart i.e. front-end there are a total no. of 13 layers. Main insulation comprises of finemica fleece tape (0.15 x 40) & Mica Splitting Tape (0.18 x 40). The taping is done layerby layer in the same direction with a uniform tape tension (i.e. proper force as possible).Firstly both the base tape layers of fine mica fleece tape are to be tapped 45 to 49%overlapped over the total taping length. For machine taping every layer should be tapped45 to 49% overlapped. The overlapping position of individual layer is to be adjustedagainst each other. It must be ensured in the bends that the taping is done 45 to 49%overlap at the outer ends.

During machine taping in the slot portion the transition from machine to hand taping isadjusted at the start of the bend for a tape width for every layer from bar-center. Afterabout half the number of layers, the taping is to be started further at the bend.

If necessary the taping must be reinforced with a layer of polyester fleece tape afterevery 5th to 8th layers of main taping at the bends of bar. On the main taping two layersof fine mica fleece tape are applied 45 to 49% overlapped over the entire insulationlength of the bar. The bar is tapped slightly overlapped with a separating foil. Every rollof separating foil on its periphery is drilled through the core with 8 holes ofapproximately 5 mm diameter at the tape center.In the slot part 1st and 2nd tape layers of fine mica fleece tape are taped continuouslyfrom one end to other end. Next 13 layers are of mica splitting tape. Last two layers i.e.16th and 17th are of fine mica fleece tape.

Whereas on the overhang part, reduced insulation thickness on the overhang is to beconsidered. Two layers of basic taping and surface taping respectively should reach upto the end of insulation. First the continuous layers are taped, and then the reducedlayers and thereafter-minimum continuous layers are applied.

In the overhang part, layers 1 and 2 are of fine mica fleece tape, layers 3-11 are of micasplitting tape and remaining two layers i.e. 12-13 are of fine mica fleece tape.

METHOD OF INSULATION & IMPREGNATION:

For insulation with micalastic, the conductor strands (with internal gas cooling, theventilation ducts as well) are arranged together to form a compact assembly and set to therequired shape. This assembly is then baked with epoxy resin to give it the mechanicalstrength required for further processing. Following this, several layers of mica tape areapplied continuously and half-overlapped to the end-turn and slot portions of the bar.The mica tape consists of a thin high strength baking material to which the mica isbonded by synthetic resin. The number of layers i.e. the thickness of insulation isdetermined by the voltage of the machine. The taped bars are then dried under vacuumand impregnated with synthetic resin, which by reason of its low viscosity penetrates the

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insulation thoroughly and eliminates all voids. After impregnation under vacuum, thebars are subjected to pressure, with nitrogen being used as Pressurizing Medium (VPIprocess). The impregnated bars with direct conductor cooling are then brought to therequired dimensions in moulds and cured in an oven at a high temperature. Withindirectly cooled windings, up to 20 stator bars are placed in moulds with by insulationand curing.

PROPERTIES OF MICALASTIC:

Class-F, highly reliable micalastic insulation has the following advantages:

Micalastic is an extremely dependable winding insulation system developed for highvoltage Turbo-generators. The insulation is applied end to end on the stator barsproviding effective protection against over voltages arising during normal and againstthe high stresses that may occur at the slot ends when HIGH TEST VOLTAGES are

applied.

Micalastic has a long electrical life.

Micalastic is a good conductor of heat by reason of the high mica content and void-free synthetic resin. Efficient heat transfer is particularly important in machines havingthick insulation and having indirect cooling.

Micalastic owes insensitivity to high temperatures.

Micalastic does not burn. The flammability is so low that it does not continue to burnafter the arc is extinguished & therefore CO2 fire extinguisher systems are not necessary.

Micalastic has elasticity and accommodates thermo mechanical stresses.

Micalastic provides high resistance to moisture and chemical action. Corrosive gases,vapors, lubricating oil and weak acids, to which the winding may be subjected duringoperation, do not attack the insulation.

Micalastic retains outstanding properties even after years of operation.

DRYING, IMPREGNATION & CURING OF WINDING WITHINSULATION FOR MICALASTIC SYSTEM:

DRYING: The stator winding is to be dried under vacuum 0.1 m bar at (60 ± 5)°Cfor 15 hours, minimum. The drying temperature is to be increased to

(65 ± 2)°C if the initial viscosity of the impregnating resin mixture is high &for generators with a rated voltage > 16.5 KV.The temperature of the overhang must not be more than 80° C. The

temperature distribution should be as uniform as possible. The drying undervacuum can be stopped if the pressure rises, 10 minutes after closing of

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vacuum valve is less than 0.06 m bar.

IMPREGNATION: The impregnating resin mixture is to be heated in the workingtank, to (60 ± 3)°C or in case of higher initial viscosity and also forgenerators with a rated voltage > 16.5 KV, it is to be heated to

(65 ± 2)° C. At a temperature of 50°C, the impregnating resin mixture isto be degassed with 1–5 m bar vacuum. Subsequently the stator windings

(core) are to be dipped continuously in resin hardener mix such that thehighest locations of the winding are at least 100 mm below the resin level.After 10 minutes of resin stabilization, pressure is increased by applicationof nitrogen. Pressure is to be raised gradually in uniform stages within 80minutes to 4 bars and to be maintained for 120 minutes in the impregnation

tank. The impregnation of the stator winding is to be monitoredcontinuously. Further it is to be decided whether to increase the pressureor to stop the impregnation process, however the total period of nitrogenpressure cycle shall in any case not exceed 4 hours. The impregnation tank

during shut down is to be closed and kept either filled with nitrogen(1.1 bar) or low vacuum.

Impregnation tank

CURING: To prevent heating in the overhang portion, the curing of the impregnatedstator winding is to be done with a maximum 160°C hot air. The curing

period is extended for such a long time till the measurement positions in the

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core (slot resistance thermometer or adjacent thermocouple) indicate(140 ± 5)°C for minimum 8 hours. During curing, the course of temperatureis to be monitored with the help of RTDs placed in overhang and stator slot

RTDs.

(E) CORONA PROTECTION:

Typical buildup of corona protection:

1. INNER CORONA PROTECTION

2. OUTER CORONA PROTECTION

3. END CORONA PROTECTION

Including Transition coating, High Voltage Insulation, Stator bar (slot end), Stator bar(end winding) and glass tape epoxy protective layer.To prevent potential differences and possible corona discharges between the insulationand the slot wall, the slot sections of the bar are provided with an outer corona protection.This protection consists of a wear–resistant, highly flexible coating of conductive alkydvarnish containing graphite.

At the transition from the slot to the end-winding portion of the stator bars, a semi-conductive coating is applied. On top of this, several layers of semi-conductive andcorona protection coating are applied in varying length. This ensures uniform control ofthe electric field and prevents the formation of corona discharge during operation andduring the performance of high voltage tests.A final wrapping of glass fabric tapes impregnated with epoxy resin serves as surfaceprotection.

1. INNER CORONA PROTECTION:

One layer of fine mica fleece tape is tightly taped approximately 45 to 49% overlapped.Over the whole length of packing strip, a single length copper strip is laid, soldered onthe tinned strand with a soldered iron. A layer of conducting fleece tape overlapped is tobe taped on the bar as inner corona protection or inner potential control.

Note: It is to be noted that no copper strips or residues of solder must be left between thestrands.

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2. OUTER CORONA PROTECTION: It includes:

(I) APPLICATION OF CONDUCTIVE COATING ON BARS(II) PROTECTIVE SLEEVE (FOR WINDING BARS)

(I) APPLICATION OF CONDUCTIVE COATING (WINDING BARS):

The surfaces of the bars must be roughened that these do not show any shining spot.These must be flat, smooth and without any raised portion. The thickness of conductivecoating is neglected while calculating slot build-up. Conductive varnish is first coated inthe reduced length. Brush can also be used for re-application of small area. The usedvarnish roller and brush must be clean, solvent-free and dry before dipping into varnish.The varnish is applied uniformly thick with a roller.

For bars with MI length = 2 x 30 mm (insulation is continuous in slot and overhang)For bars with wrapper insulation length is reduced so that 1-minute HV test at 1.5 test

voltage with the remaining wrapping ends can be performed.

Drying time before electrical testing is minimum of 4 hours. The surface resistance of thecoating is tested. The length of conducting coating is checked by quality control. Theconducting coating is applied further before carrying end corona protection.

(II) PROTECTIVE SLEEVE (FOR WINDING BARS):

Protective sleeves are to be made from conductive polyester fleece material.

Length of wrapper (cutting): Core length-20 mmWidth of wrapper (cutting): Bar periphery-10 to 20 mm

The protective sleeves (wrapper) for the upper layer bar in the region of corecompartment are cut in trapezoidal form according to the dimensions of the bar so thatthese do not lie loose. All the bars are to be checked visually for mechanical damages andpainted in the slot region with a conductive varnish. Each time on fresh coats, theprotective sleeve (wrapper) is so glued over the bar (made trapezoidal) that the roughfleece side at the bar and overlapping laid on the narrow side of the bar.

3. END CORONA PROTECTION:

The end corona protection is divided in the following application stages:

STAGES VOLTAGE LEVEL

Up to 6.6 KV, no end corona protection

Stage-I Over 6.6 KV up to 16.5 KV

Stage-II Over 16.5 KV up to 22 KV

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Stage-III Over 22 KV up to 33 KV

In case of 500 MW TG, Voltage is 21 KV, so we are concerned with stage-II only.

Surface treatment:

For wrapper insulation, the ends of slot insulation outside outer surface coronaapplication are to be cleaned and smoothened with sand paper. For micalastic insulation,the upper surface of insulation is cleaned for the end of outer corona coating up to 20 mmover ends of specified end corona protection and smoothened with sand paper.

A template and colored writing pencil (e.g. brush pen) without electrically conductingmedia are used for marking of end corona protection. Carbon or greased pencils must notbe used for this purpose.

Stage –II for micalastic

COATINGS DRYING TIME1st 4 hrs.2nd 4 hrs.3rd 10 hrs.

1. On straight part of bar:

If the first and second coatings are not accommodated on the straight part, theouter surface corona protection is to be extended over the bends and the end corona

protection is to be finished on the front sides (overhang).

2. On Overhangs:

The conducting varnish is applied overlapping whole of transition coating and indifferent lengths for the various stages of rated voltage. The conducting varnish is

usually so applied that no deposits are formed.

(F) TESTING OF BAR:The tests are for checking the healthiness of bar.

1. TESTS AFTER STACK PRESSING:

(I) Inter-Turn Short Test / Inter Strand Short Test i.e. with 300 V: This test isapplicable after manufacturing of the bar but before insulation.

PURPOSE: This test is to check the self-insulation of strands and to ensure thatthere is no inter-turn short after forming of bars.

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Different tapes before test

(II)H.V.Test: HV test is conducted at 1.5 up i.e. test voltage.

SCOPE: The HV test is conducted after manufacturing of the stator winding bars.

PURPOSE: To exercise QC(Quality Control).

TEST EQUIPMENTS: ---- Varian transformer (Variable Voltage Control)---- Instrument transformer---- Voltmeter---- Supply source: Sinusoidal ac voltage, 50 Hz

TEST PROCEDURE: While conducting the tests general safety precautions shallbe observed. Voltage shall be raised continuously from 0 to the rated test voltage,

held at this value for 1 minute and then reduced to the initial value.

2. TESTS ON STATOR WINDING BARS:

To obtain a good (conductive) connection of all insulation surfaces for the entire OCPlength is covered with an applied conductive strip e.g. Al foil having a width of atleast1/3 of the bar height in such a manner so that any OCP length is not more than 200 mmfrom this test electrode which is directly connected with the grounded test unit. The barsare wrapped tightly with temporary ECP after finish of outer corona paint by way ofsemi-conducting polyester tape half-overlapped.

The bar is then joined at one-end to the HV connection of the test unit.Tests on stator winding bar are given below:

(I)Tangent Delta Test: Tan delta test (0.2 Un-1.4 Un) at each 0.2 Un (limit value) i.e. at0.2 Un (rated Voltage)

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SCOPE: This test covers the details of dissipation factor measurement on the statorwinding bar of TG prior to lying.

PURPOSE: This test is aimed at determining the insulation condition & for qualitycontrol of generator bars.

TEST REQUIREMENTS: Stator winding bars are to be insulated, cured & providedwith an outer corona protection.

TEST EQUIPMENTS:

---- HV transformer with continuously variable voltage control element.---- Sinusoidal alternating test voltages (50/60 Hz)---- Voltage Transformer

---- High Voltage – loss factor measuring bridge, measuring accuracy ≤ 10 – 4---- Compensation Condenser approximately 100 pf

TEST PROCEDURE: For dissipation factor measurement, the bar is prepared asfollows:

All conductor strands and cooling pipes are to be connected to each other at bothends. The bar is provided on its outer corona coating with a conductive strip of Cuor Al foil, having a width of atleast 1/3 of the bar height. There should be a goodcontact of all insulation surface area with outer corona protection. Protecting ringelectrode is to be put on both the OCP ends at a distance of approximately 1mm up tothe maximum half insulation thickness. There should be no contact between outercorona protection and protecting ring. Semi-conducting polyester tape is applied withhalf-overlap for a length of 100 mm, starting from the protecting ring electrodes. The

bar is connected to Cox. The protection rings are earthed.

Use of tapes in tan delta test

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PLACING OF BARS IN SLOT PART:

* The stator windings i.e. the manufactured bars are placed in rectangular slots, whichare uniformly distributed around the circumference of the stator core.

* A cemented graphitized paper wrapper protects the bars over the slot portion of thebar. The bars fit tightly in the slots.

* Side-gaps are filled with side ripple springs, which ensure tight fitting of bars in theslot.

Radial positioning of the bar is done with slot wedges. A top ripple spring of highstrength fiberglass fabric is placed between the filler and slide strip below the slotwedges. Ripple spring presses the bar against the slot bottom with a specific pre-loading.An equalizing strip is inserted at the slot bottom to compensate any unevenness in thebar shape and the slot bottom surface during bar insertion. The strip is cured afterinsertion of the bars. These measures prevent vibrations. The specified pre-loading ischecked at each slot-wedge. Supporting arrangement of bars in overhang with thewindings placed in the slots, the bar ends form a cone-shaped end winding. A small conetaper is used to keep the stray losses at a minimum. (Any gaps in the end winding due tothe design or manufacturing are filled with curable plastic fillers, ensuring solid supportof the cone-shaped top and bottom layers). The two bar layers are braced with theclamping bolts of high strength fiberglass fabric against a rigid tapered supporting ringof insulation material. Tight seating is ensured by plastic fillers on both sides of thebars, which are cured on completion of winding assembly.

Each end winding thus forms a compact, self supporting arch of high rigidity whichprevents bar vibrations during operation and can withstand short-circuit forces.In addition, the end-turn covering provides good protection against external damage.Thesupporting rings rest on supporting brackets which are capable of moving in the axialdirection. This allows for a differential movement between the end windings and the coreas a result of different thermal expansions.

BAR SUPPORT SYSTEM INCLUDING SUPPORT RING:

To protect the stator winding against the effects of magnetic forces due to load and toensure permanent firm seating of the bars in the slots during operation, the bars areinserted with a top ripple spring located beneath the slot wedge. The gaps between thebars in the stator end winding are completely filled with insulating material which in turnis fully supported by the frame. The stator end windings rest on the supporting rings incase of 500 MW TG. The support rings rests on the support brackets.These are capable ofmoving in the axial direction within the stator frame so that there is a differentialmovement between the end windings and the core as a result of thermal expansions. Hotcuring conforming fillers arranged below the stator bars and the support ring ensures a

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firm support of each individual bar against the support ring. The bars are clamped to thesupport ring with pressure plates held by clamping bolts, made from a high strengthinsulating material.The stator winding connections are brought out to six bushings located in a compartmentof welded non-magnetic steel below the generator at the exciter end. Current transformersfor metering and relaying purposes can be mounted on the bushings.

TERMINAL BUSHINGS:

Arrangement of terminal bushings:

The terminal bushings are water/hydrogen/air cooled depending upon the type of statorwinding cooling.The beginnings and ends of the three phase windings are brought out from the statorframe through terminal bushings which provide for HV insulation and seal againsthydrogen leakage.The bushings are bolted to the bottom plate of the generator terminal box by themounting flanges.The generator terminal box located beneath the stator frame at the exciter end is madefrom non-magnetic steel to avoid eddy current losses and resulting temperature rises.Bushing – type generator for relaying and metering purposes are mounted on thebushings outside the generator terminal box. The bus is further connected to the air-side connection flange via terminal connectors.Phase-connector and terminal bushings are connected with flexible terminalconnections.The cylindrical bushing conductor consists of high conducting copper with a centralbore for direct primary water cooling.

Construction of bushings:

The shrunk on mounting sleeve consists of a gas-tight casting of non-magnetic steelwith a mounting flange and a sleeve type extension extruding over entire height of thecurrent transformers. The cylindrical connection ends of the terminal bushingconductors are silver-plated. Connection to the beginning, end, each phase inside theterminal box and to the external bus is by means of flexible connectors. To maintain auniform and constant contact pressure Belleville washers or tension disc or Bellevillelocking are used for all bolted connections. Covers with brazed sockets for connectionto the water supply are flanged to the ends of the terminal bushing conductors.

ELECTRICAL CONNECTION OF BARS:

The electrical connection between the top and bottom bars is by bolted contact sleeves.At their ends, the strands are brazed into a connecting sleeve, the strand rows beingseparated from each other by spacers. Non-magnetic clamping bolts press the contact

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surfaces of the connecting sleeves for the top and bottom bars against each other. Specialcare is taken to obtain flat and parallel contact surfaces. In order to prevent any reductionin contact pressure or any plastic deformations due to excessive contact pressure,Belleville washers are arranged on the clamping bolts, which ensure a uniform andconstant contact pressure.

Water supply:

The water connection at the stator bar is separate from the electrical connection. As aresult no electrical forces can act on the water connection.

While the solid strands of the stator bars terminate at the connecting sleeve, the hollowstrands are brazed into water boxes, with solid spacers inserted to compensate for thesolid strands. Each water box consists of the two parts i.e. the sleeve shaped lower partenclosing the hollow strands and the cover-type upper part. Spacers separate the strandrows from each other.

Each water-box is provided with a pipe connection of non-magnetic stainless steel forconnection of the hose.

The exciter-end water boxes serve for water admission and distribute the cooling wateruniformly to the hollow strands of the bar. The hot water is collected on leaving thehollow strands in turbine-end water boxes. The cooling water is then discharged fromthe generator via the hoses & the ring header.

During manufacturing of the stator bars, various checks are performed to ensure watertightness and unobstructed water passages.

The flow check ensures that no reduction in the cross-sectional area of the strand ductshas occurred, and that all strands are passed by identical water flows. After brazing of theupper part of water box, all brazed joints are subjected to a Helium Leakage Testfollowed by a “Thermal Shock treatment.”

The air clearance between the water boxes and bar connections and the clearance relativeto the end shield, which is at ground potential, is so kept that additional insulation is notrequired.

Phase Connectors (Connecting Bus bars):

The phase connector interconnects the coil groups and links the beginning and ends of thewinding to the bushings. They consist of thick walled copper tubes. The stator bar endscoupled to the phase connectors are provided with connecting fittings which are joinedto the cylindrical contact surface with Belleville washers on the bolts to maintain auniform and constant contact pressure.

The phase connectors are provided with a MI. In addition a grounded outer coronaprotection consisting of a semi conducting coating is applied over the entire length. At

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the beginnings and ends of the phase connectors several layers of semi-conducting andcorona protection is applied in varying lengths.

The phase connectors are mounted on end winding supporting ring over supportingbrackets. Neighboring phase connectors are separated with spacer and tied securely inposition. This ensures a high short–circuit strength and differential movements betweenphase connectors and end windings are thus precluded.

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COMPARISON BETWEEN 500 MWAND 660 MW TG:

660 MW 500 MW

# GEN. TYPE THDF 115 / 67 THDF 115 / 59

# RATING 660 MW / 776 MVA 500 MW / 588MVA(0.85 P.F.) (0.85P.F.)

# TERMINAL VOLTAGE 21 KV 21 KV

# RATED CURRENT 21.33 KAMPS. 16.16 KAMPS.

# HYDROGEN PRESSURE 5 BAR (g) 3.5 BAR (g)

# EXCITATION 445 V 340 V5587 A 4040 A

# SHORT CIRCUIT RATIO 0.52 0.48

# GEN. EFFICIENCY 98.72 % 98.64 %

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CONCLUSION

Under the kind guidance of the helpful staff of BHEL, Haridwar, I have successfully

completed the training. This practical training has proved to be very useful. It provided

opportunity to encounter with such huge machine like Turbo generators. Training was

started on 7th June, 2012 and ended on 6th July, 2012.

The whole training was good learning experience. Not only the knowledge of huge

machine was gained but I also got the feel of “Professional World”.

The way of working in discipline, makes the trainee realize that engineering is not just

studying the structured description but greater is of planning and proper management.

Working in the BHEL for a period of a month and keenly observing all the preleadings in

the company has enabled me to understand a lot about Electrical Machines and

Professional World.

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