insulation of turbogenerators by vpi process.pdf

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1 CHAPTER -1 INTRODUCTION TO TURBO GENERATORS 1. INTRODUCTION Electricity does not occur naturally in usable form and it cannot be stored usefully in large quantities. Therefore it must be generated continuously to meet the demand at all times. It also improves the economy of a country. A generator means an efficient & convenient way to generate electrical power by conversion of mechanical energy to electrical energy in a rotating device. Turbo generator means a generator directly coupled to a turbine which can be either steam or gas. These are used for power production on large-scale basis. 1.1 PRINCIPLE A generator works on the basic principle of Faraday’s Law of Electromagnetic Induction. According to the law, “When a conductor is mover in a stationary magnetic field (or) when the magnetic fields is moved across the stationary conductor, an E.M.F is induced in the conductor. When the conductor cuts the magnetic flux produced by the magnetic field, current flows through the load when the circuit is closed.’ ESSENTIAL PARTS OF A GENERATOR The basic essential parts of a generator are A magnetic field A conductor (or) conductors which can move so as to cut the flux.

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  • 1

    CHAPTER -1

    INTRODUCTION TO TURBO GENERATORS

    1. INTRODUCTION

    Electricity does not occur naturally in usable form and it cannot be stored usefully

    in large quantities. Therefore it must be generated continuously to meet the demand at all

    times. It also improves the economy of a country. A generator means an efficient &

    convenient way to generate electrical power by conversion of mechanical energy to

    electrical energy in a rotating device. Turbo generator means a generator directly

    coupled to a turbine which can be either steam or gas. These are used for power

    production on large-scale basis.

    1.1 PRINCIPLE

    A generator works on the basic principle of Faradays Law of Electromagnetic

    Induction. According to the law, When a conductor is mover in a stationary magnetic

    field (or) when the magnetic fields is moved across the stationary conductor, an E.M.F is

    induced in the conductor. When the conductor cuts the magnetic flux produced by the

    magnetic field, current flows through the load when the circuit is closed.

    ESSENTIAL PARTS OF A GENERATOR

    The basic essential parts of a generator are

    A magnetic field

    A conductor (or) conductors which can move so as to cut the flux.

  • 2

    1.2 CLASSIFICATION OF GENERATORS

    Generators can be broadly classified into two types as:

    1) D C G e n e r a t o r

    2) A C G e ne r a t o r

    DC GENERATORS

    In DC Generators, the armature rotates and the field system is stationary.

    AC GENERATORS

    AC Generators are also known as Alternators. Here the field system rotates and

    the armature is stationary.

    CLASSIFICATION OF DC GENERATORS

    DC Generators can be broadly classified into three types.

    1. Shut Generators.

    2. Series Generators.

    3. Compound Generators.

    CLASSIFICATION OF AC GENENRATORS

    AC Generators can be broadly classified into two types.

    1. Asynchronous generators.

    2. Synchronous generators.

  • 3

    ASYCHRONOUS GENERATORS

    Asynchronous generators are those in which the speed of rotor and flux are not in

    synchronism. E.g.: Induction motor.

    SYNCHRONOUS GENERATORS

    Synchronous generators (or) alternators are those in which the speed of the rotor

    and flux are in synchronism. The 3-phase synchronous generators are widely used

    machines for power production on large scale basis. These when connected to turbines

    are called turbo generators. Gas turbine generators and steam turbine generators are

    widely used for power generation. Synchronous generators can be classified into various

    types based on the medium used for generation.

    They are:

    1. Turbo- alternators:

    Steam

    Gas

    2. Hydro Generators.

    3. Engine Driven Generators.

    Turbo generator mainly consists of 3 parts

    1. Stator

    2. Rotor

    3. Excitation system.

    1.3 STATOR

    Armature windings are mounted on a stationary element called the stator. The

    main parts of a stator are

    1. Stator Frame.

    2. Stator Core

    3. Stator Winding.

  • 4

    STATOR FRAME

    The stator frame is of horizontally split type and welded construction and supports

    the laminated core and the winding. Ventilation holes are provided in the frame itself and

    helps in cooling the machine.

    STATOR CORE

    The stator core is made up of stacked insulated silicon steel laminations. The core

    is laminated to minimize loss due to eddy currents. Spaces are provided between the

    laminations to allow the cooling air to pass through. The slots for housing the armature

    conductors lie along the inner periphery of the core.

    STATOR WINDING

    The stator winding is a fractional pitch double layer lap winding. The bars are

    located in slots which are uniformly distributed on the circumference of the stator core.

    1.4 ROTOR

    Field windings are mounted on a rotating element called rotor.

    The main parts of the rotor are

    1. Rotor Shaft.

    2. Rotor Winding.

    3. Retaining Rings.

    4. Field Connections.

    5. Bearings.

  • 5

    ROTOR SHAFT

    Rotor shaft is a solid forging into which slots for insertion of field winding are

    milled using the Heller machine. The longitudinal slots are distributed over the

    circumference so that solid poles are obtained. It is then sent for red gel painting.

    ROTOR WINDING

    Rotor windings are made up of copper strips. Each individual conductor is placed

    over a template and passed under the ventilation punching machine. On both sides of the

    conductor 900 bending is carried out. The conductors are then subjected for Annealing,

    Hydraulic pressing on both the bends and checking with gauges carried out. On both the

    ends of the conductor dovetailed punching is done. Air dry varnish is applied and relief

    filing is done on both sides of the conductor. All the bars are assembled on a dummy

    rotor and brazed to make one full coil. After red gel painting, the rotor slots are checked

    for foreign matter presence and windings are assembled. Footings assembly is carried

    out on both sides and diameter is checked. Input lead is assembled into the rotor shaft on

    the exciter side. It is enclosed with H G L (Hardened Glass Lamination) insulation and

    two D-leads are separated and surrounded with insulation and is checked for H.V. The

    output studs are assembled onto the rotor for connections. Wedging is carried out using

    high electrical conductivity material which act as damper winding Overhang braces are

    assembled in between the conductors to protect from electrical short circuits. HV AC

    and Impedance tests are conducted.

    RETAINING RINGS

    Assembly of retaining rings, the contact surface of which is sprayed with silver, is

    carried out on both turbine and exciter side on the overhang part of the rotor body by

    heating it to 2500C. Then the snap ring is released into the groove of the retaining ring.

    Before assembling the retaining rings ensure the snap ring movement into the rotor

  • 6

    groove and lock it. After cooling the retaining rings, the rotor is subjected for HV and

    impedance tests.

    FIELD CONNECTIONS

    The two output leads which are brought out towards the exciter side are connected

    tote excitation system and the field current is supplied to the rotor.

    BEARINGS

    The rotor is supported in two sleeve bearings. The temperature of the bearings is

    maintained with two RTDs (Resistance Temperature Detector) embedded in the lower

    bearing sleeve so that the ensuring point is located directly below the Babbitt. All

    bearings have provisions to monitor the shaft vibrations. The oil supply to the bearing is

    obtained from the turbine oil system.

    1.5 COOLING SYSTEM

    There are various losses occurring in a generator due to which heat is generated.

    Hence cooling system is a basic requirement for any generator. The insulation used and

    cooling system employed are inter-related. The various losses in a generator are:

    1. Iron Losses

    Hysteresis Losses

    Eddy Current Losses

    2. Copper Losses

    3. Mechanical Losses

    Friction losses

    Winding losses

    These losses dissipate as heat and raise the temperature of the generator which effects the

    insulation. Therefore it should be cooled to avoid excessive temperature rise. So the

  • 7

    class of insulation used depends mainly on the cooling system installed. There are

    various methods of cooling.

    They are:

    1). Air cooling - 60MW

    2). Hydrogen Cooling - 100MW

    3). Water Cooling - 500MW

    4). H2 & Water Cooling - 1000MW

    Hydrogen cooling has the following advantages:

    1. H2 has seven times more heat dissipation capacity.

    2. Higher specific heat.

    3. Since H2 is 1/14 thof air weight, it has higher compressibility.

    4. It does not support combustion.

    Hydrogen cooling has the following disadvantages:

    1. It is an explosive when mixed with oxygen

    2. Cost of running is higher

  • 8

    CHAPTER -2

    MANUFACTURING OF STATOR

    MANUFACTURING OF STATOR

    The different stages involved in the manufacturing of stator are:

    1. Lamination Preparation.

    2. Stator Core Assembly.

    3. Stator Winding.

    4. Stator Assembly.

    2.1 LAMINATION PREPARATION

    The building up of the core using laminations plays a vital role to minimize the

    magnetic losses which are of two types,

    Hysteresis Losses occur due to residual magnetism in the material.

    Eddy Current Losses occur due to the emf produced in the core.

    In order to minimize the Hysteresis losses, silicon alloyed steel sheets are used for

    building up of the core. These sheets are 4% Silicon Alloyed COLD ROLLED NON

    GRAIN ORIENTED (CRNGO). The sheets have the following composition,

    Steel : 95.8%

    Sillicon : 4%

    Impurities : 0.2%

    In order to minimize the Eddy Current losses, the core is built up of 0.5mm thick

    laminations which are insulated from each other using class B type oil varnish. The

    preparation of the lamination involves the following process.

    1. RECEPTION OF SILICON SHEETS

    The silicon roll sheets are received in the form of bundles.

  • 9

    2. EXAMINATION OF SILICON SHEETS

    The received silicon sheets are examined for the specified electrical, magnetic and

    mechanical properties.

    3. BLANKING

    It is the process where the required shape of the lamination is obtained by passing

    on the rollers and cutting into required size. The specified dimension sheet obtained from

    cutting process is called Blanking and the remaining waste material is called

    Perforation.

    4. NOTCHING

    This is the process where slots are punched into the blanked sheet. There are two

    types of notching.

    INDIVIDUAL NOTCHING: Each operation is carried out independently & the

    probability of error is high.

    COMPOUND NOTCHING: Processing the laminations at single stroke & the

    probability of error is less.

    5. DUBURRING

    Each lamination is processed for deburring operation i.e. removing the bur level

    which prevents from short circuit. The acceptable limit of the bur is 5 microns

    6. VARNISHING

    This is done to insulate the lamination using ALKYD PHENOL VARNISH.

    The laminated sheets are passed through a conveyor which has an arrangement to

  • 10

    Sprinkle a coat of varnish. The coating thickness should be 7 to 10 microns/side. The

    Varnish used should be of correct viscosity which is measured using a din-four-cup.

    After varnishing, laminations are passed through furnace where temperature is

    Maintained at 3000C 4000C.

    TESTS PERFORMED AFTER VARNISHING

    Checking hardness, the hardness of the varnish coating is checked using a 7H pencil.

    Bonding adhesive test: Pour Xylol on the lamination and wait for one minute. The

    varnish coating should not dissolve the xylol.

    IR Value test: This test is performed using Megger. When twenty laminations are

    stacked under a pressure of 26kg/cm2 the IR value should be greater than 1 Mohm.

    Uniformity test: It is measured using a mini tester after giving two coats of varnish.

    2.2 STATOR CORE ASSEMBLY

    The purpose of stator core is:

    a) To support the winding.

    b) To carry the flux. The assembly of the stator core involves the following processes:

    1. ASSEMBLY OF TRIAL PACKETS

    A clamping plates is placed on the assembly bed which is already aligned

    horizontally with spirit level. Laminations are assembled on this clamping plate one after

    the other to form 3600 or inside diameter and up to a width of 50 to 100 mm. All the slots

    are checked with inspection drift. The inside diameter of the core is checked with inside

    micrometer. After fulfilling all the above requirements, the trial packed assembly is

    dismantled.

  • 11

    2. ASSEMBLY OF NORMAL PACKETS

    The stepped packets are assembled on the clamping plate by inserting assembly

    drifts into the slots and mandrels in all the respective holes. The stepped arrangement of

    the laminations at the core ends provides an efficient support to the tooth portion and

    reduction of eddy current losses and heating. It is carried out by laying individual

    laminations to obtain the required width of the packet. Over it one layer of HGL sheet

    and one layer of ventilation lamination are assembled. Once again the normal packet

    assembly is carried out up to required width. After completion of two packets, the inside

    diameter of the core is checked and also inspection drifts is passed in all slots, The above

    process is repeated up to 800mm and first pressing is done. Similarly the above process

    is again repeated up to 800mm and second pressing is done.

    3. ASSEMBLY OF GUIDE BARS

    All the guide bars assembly is carried out by placing required number of holding

    half rings. One guide bar is earthed called Earth Bar. Required hydraulic pressure is

    given to the rings until the guide bars are seated into the dovetailed slots. All the guide

    bars & holding rings are welded in a systematic manner. Winding brackets are welded

    and checked for 900 on both sides.

    4. TESTS PERFORMED ON THE CORE

    Dipenetrant test: To check for any cracks during welding.

    Core Flux test : To detect the presence of hot spots.

    5. PROVISION OF CORE RTD AND TOOTH RTD

    RTDS are placed to detect temperature in between the winding and on the core.

  • 12

    6. FIRE DETECTORS

    On the overhang portion, fire detectors are placed to detect occurrence of fire due

    to short circuit.

    2.3 STATOR WINDING

    The winding used in the stator is of Roebel type. The manufacturing of the

    winding involves the following processes.

    1. RECEPTION OF THE METERIAL

    The material used for the winding consists of 99% copper and1% silver which has

    class-F type of insulation.

    2. CHECKING OF THE RAW MATERIAL

    The raw material is cross checked for electrical mechanical an chemical

    properties.

    3. CUTTING

    The copper strips are cut according to the given design.

    4. TRANSPOSITION

    The copper strips are 1800 transposed by applying a pressure of 150kg/cm2.

    Transposition is done to equalize the induced emf in all the strands, to minimize I2R

    losses and skin effect. Also the heat distribution is equal.

  • 13

    5. BUNDLING

    The 1800 transposed coils are placed one above the other to form a bundle. Hlaf

    insulation is placed between the two transposed coils and the bundle is tied with cotton

    tape.

    6. PUTTY WORK

    The uneven surface formed during transposition are filled with nomex sheets to

    prevent inter-half and inter-strip short Mica fleece is placed on the width of the bar and

    PTFE (Poly Tetra Fluoro Ethylene) is wrapped on the straight portion.

    7. STRAIGHT PART CONSOLIDATION

    The bar is subjected to a pressure of 150kg/cm2 horizontally and vertically and

    temperature of 1600C for 2-3 hours. The bar is consolidated such that there are no air

    gaps.

    8. DIMENSION CHECK:

    The dimension i.e. both width and height are checked using a guage.

    9. TESTING

    The tests performed on the bar are interstrip and inter half. These tests are

    performed using a lamp which is connected between a phase and neutral. The two

    terminals are connected for:

    Inter strip -> between strips.

    Inter half -> between two coils of a bundle.

    If the lamp glows, it indicates that a short circuit has occurred.

  • 14

    10. BENDING

    Bending process is carried out on the bending fixture. After bending, the bar is in

    the shape of half diamond and is hence called as half diamond coil.

    11. OVERHANG CONSOLIDATION

    Manufacturing &Insulation System By VPI Process For Air Cooled Turbo

    Generators Nomex pieces are inserted by applying rotopax and hardner from first bend to

    third bend between the two coils a bundle. Both the overhang portion and consolidated

    using clamps and heating to a temperature of 600C to 700C for a duration of 30 minturs.

    12.COPPER FOIL SOLDERING

    A copper foil is soldered on the width of the bar to prevent internal corona discharges.

    13. FINISHING, TESTING & DIMENSIONAL CHECK

    Finishing, testing and dimensional check is carried out before taking the bar for

    final taping.

    14. FINAL TAPING

    It is carried out with machine or manually to obtain the designed insulation wall

    thickness around the periphery of the stator bar in the straight portion, overhang portion

    and third bend portion

    Resin poor tape is wrapped throughout the bar with 1*1/2 overlap.

    Resin poor tape is wrapped on the overhang portion of the bar with 6*1/2 over lap.

    Copper foil is placed along the width of the bar and ICP (internal corona protection tape

    is wound.

  • 15

    Resin poor tape is again wrapped through out the bar with 8*1/2 overlap.

    OCP (outer corona protection) tape is wound on the straight part along with split mica

    tape on the width of the bar simultaneously so that mica is not over lapped.

    15. TESTING

    Inter-half testing is carried out before sending the bar to stator assembly.

    2.4 STATOR ASSEMBLY

    1. RECEPTION OF STATOR BARS

    All the bars are checked physically for dimensions and quality. Each bar is

    pressed at a pressure of 60kg/cm2 in the pressing fixture for duration of 30-45 minutes to

    obtain the desired width such that the bar is easily placed into the slot. during pressing

    the bar is tested for inter half shorts.

    2. RECEPTION OF STATOR CORE

    The stator core received from the core assembly is checked for foreign matter.

    The insulation drift is passed in all the slots to check for the laminations projections .It is

    rotated continuously so that all the foreign matter comes out.

    3. WINDING HOLDER ASSEMBLY

    The winding holder s are assembled onto the winding brackets on the turbine side

    as well as the exciter side.

    4. HGL RINGS CENTERED TO THE CORE

  • 16

    HGL rings are assembled and centered to the core on both the turbine and exciter

    sides. All the slots and the RTD (resistance temperature detector) slots are identified with

    numbers.

    5. LAYING OF THE BOTTOM BAR

    All the bars are inserted into the respective slots and checked for pitch matching.

    Before laying the bottom bars, a conductive fleece is laid into the slot to discharge the

    charges. A 5 mm glass mat is placed underneath the winding holders. Two bars are laid

    in the consecutive slots and tied to the winding holders with Neoprene glass sleeve by

    inserting spaces in between them. All the bars are laid in the slots by following the above

    procedure. The seating of the bottom bar is checked with a guage and weding is carried

    out for the bottom bars. All the bars are subjected to H.V.DC test i.e. 16.8KV or

    17.2KV.

    6. LAYING OF THE TOP BAR

    Before laying the top bars, stiffners are adapted on the winding holders on which

    5mm glass mat is laid and interlayer inserts are inserted on the both turbine and exciter

    side. All the bars are laid into the respective slots and checked for pitch matching.

    Subject the top bars for H.V DC i.e. 16.8KV or 17.2KV. Wedging is carried out in all the

    slots. Both the bottom and top bars are subjected for H.V test.

    7. EYES JOINING ON BOTH THE SIDES

    Manufacturing & insulation System by VPI Process For Air cooled Turbo

    GeneratorsStrip to strip bracing of the conductors in the overhang portion is done using a

    silver foil which contains 14% silver and remaining is tin.

  • 17

    8. ASSEMBLY OF CONNECTING RINGS

    Inter half test is carried out for the three phases. Assemble all the connectors and

    join or brace all the twelve eyes to the connectors. Terminate the three phases and three

    nautrals.

    9. INSULATION OF EYES

    Insert nomex sheets between the two halves of the eye insulate each eye with

    3X1/2 layers of semica folium glass plate. Ultimately wrap hyper seal tape.

    ROTOR WINDING

  • 18

    STATOR WINDING

  • 19

    ROTOR

    Solid rotors are manufactured from forged alloy steel with suitable alloying

    elements to achieve very high mechanical and superior magnetic properties.

    Rectangular or trapezoidal rotor slots are accurately machined to close tolerances

    on slot milling machine. For indirectly cooled generator rotors, ventilation slots are

    machined in the teeth. For directly cooled rotors, sub slots are provided for cooling

    Generator rotors of 1500 RPM are of round laminated construction. Punched and

    varnished laminations of high tensile steel are mounted over machined shaft and are

    firmly clamped by end clamping plates.

    2.1 ROTOR SHAFT

    Rotor shaft is a single piece solid forming manufactured from a vacuum casting.

    It is forged from a vacuum cast steel ingot. Slots for insertion or the field winding are

    milled into rotor body. The longitudinal slots are distributed over the circumference such

    that two solid poles are obtained.

    To ensure that only a high quality product is obtained, strength tests, material

    analysis and ultrasonic tests are performed during the manufacture of rotor. The high

    mechanical stresses resulting from the centrifugal forces and short circuit torques call for

    a high quality heat treated steel. Comprehensive tests ensure adherence to the specified

    mechanical and magnetic properties as well as homogenous forging. After completion,

    the rotor is balanced in various planes at different speeds and then subjected to an over

    speed test at 120% of the rated speed for two minutes.

    The rotor consists of electrically active portion and two shaft ends.

    Approximately 60% of rotor body circumference has longitudinal slots which hold the

    field winding. Slot pitch is selected so that the two solid poles are displaced by 180

    degrees. The rotor wedges act as damper winding within the range of winding slots. The

    rotor teeth at the ends of rotor body are provided with axial and radial holes enabling the

    cooling air to be discharged into the air gap after intensive cooling of end windings.

  • 20

    2.2 ROTOR WINDINGS

    The rotor windings consist of several coils inserted into the slots and series

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

    connected turns, each of which consists of two half turns connected by brazing in the end

    section. The rotor bearing is made of silver bearing copper ensuring an increased thermal

    stability.

    The individual turns of coils are insulated against each other by interlayer

    insulation. L-shaped strips of laminated epoxy glass fibre fabric with nomex filter are

    used for slot insulation.

    The slot wedges are made of high electrical conductivity material and thus act as

    damper windings. At their ends the slot wedges are short circuited through the rotor

    body.

    CONSTRUCTION

    The field winding consists of several series connected coils inserted into the

    longitudinal slots of rotor body. The coils are wound so that two poles are obtained. The

    solid conductors have a rectangular cross section and are provided with axial slots for

    radial discharge or cooling air. All conductors have identical copper and cooling duct

    cross section. The individual bars are bent to obtain half turns. After insertion into the

    rotor slots, these turns are brazed to obtain full turns. The series connected turns of one

    slot constitute one coil. The individual coils of rotor are connected in a way that one

    north and one south pole is obtained.

    CONDUCTOR MATERIAL

    The conductors are made of copper with a silver content of approximately 0.1%.

    As compared to electrolytic copper, silver alloyed copper features high strength

    properties at high temperatures so that coil deformations due to thermal stresses are

    eliminated.

  • 21

    INSULATION

    The insulation between the individual turns is made of layer of glass fiber

    laminate. The coils are insulated from the rotor body with L-shaped strips of glass fiber

    laminate with nomex interlines.

    To obtain the required leakage paths between the coil and the rotor body thick top

    strips of glass fiber laminate are inserted below top wedges. The top strips are provided

    with axial slots of the same cross section and spacing as used on the rotor winding.

    ROTOR SLOT WEDGES

    To protect the winding against the effects of centrifugal forces. The winding is

    secured in the slots with wedges. The slot wedges are made of copper alloy featuring

    high strength and good electrical conductivity. They are also used as damper winding

    bars. The slot wedges extend beyond the shrink seats of retaining rings act on the damper

    winding in the event of abnormal operations. The rings act as short circuit rings in the

    damper windings.

    END WINDING BRACING

    The spaces between the individual coils in the end winding are filled with

    insulated members that prevent coil movement. Two insulation plates held by HGL high

    glass laminate plates separate the different cooling zones in the overhangs on either sides.

    2.3 ROTOR RETAINING RINGS

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

    retaining rings. Retaining rings are made of non-magnetic high strength steel in order to

    reduce stray losses. Each retaining ring with its shrink fitted. Insert ring is shrunk on to

    the rotor body in an overhang position. The retaining ring is secured in the axial position

    by snap rings.

  • 22

    The rotor retaining rings withstand the centrifugal forces due to end windings.

    One end of each ring is shrunk fitted on the rotor body while the other end overhangs the

    end windings without contact on the rotor shaft. This ensures and unobstructed shaft

    deflection at the end winding.

    The shrunk on hub on the end of the retaining ring serves to reinforce the

    retaining ring and secures the end winding in the axial direction at the same time.

    A snap ring is provided against axial displacement of retaining ring. The shrunk

    seat of the retaining ring is silver plated, ensuring a low contact resistance for induced

    currents. To reduce the stray losses and have high strength, the rings are made of non

    magnetic, cold worked materials.

    2.4 ROTOR FANS

    The cooling air in generator is circulated by two axial flow fans located on the

    rotor shaft one at each end. To augment the cooling of the rotor winding, the pressure

    established by the fan works in conjunction with the air expelled from the discharge parts

    along the rotor.

    The blades of the fan have threaded roots for being screwed into the rotor shaft.

    The blades are drop forged from an aluminium alloy. Threaded root fastenings permit

    angle to be changed. Each blade is secured as its root with a threaded pin.

    BEARINGS

    The turbo generators are provided with pressure lubricated self-aligning elliptical

    type bearings to ensure higher mechanical stability and reduced vibration in operation.

    The bearings are provided with suitable temperature element devices to monitor bearing

    metal temperature n operation.

    The temperature of each bearing is monitored with two RTDs (Resistance Thermo

    Detectors) embedded in the lower bearing sleeve such that the measuring point is located

    directly below the babitt. These RTDs are monitored a temperature scanner in the

    control panel and annunciated if the temperature exceeds the prescribed limits. All

    bearings have provisions for fitting vibration pickups to monitor shaft vibrations.

  • 23

    To prevent damage to the journals due to shaft currents, bearings and oil piping

    on either side of the non drive end bearings are insulated from the foundation frame.

    For facilitating and monitoring the healthiness of bearing insulation, split insulation is

    provided.

    VENTILATION AND COOLING

    Turbo generators are designed with the following ventilation systems:

    Closed circuit air cooling with water or air coolers mounted in the pit.

    Closed circuit hydrogen cooling with water or hydrogen mounted axially on the stator

    frame.The fan design usually consists of two axial fans on either made of cast aluminium

    with integral fan blades or forged and machined aluminium alloy blades screwed to the

    rotor. In case of 1500 RPM generators, fabricated radial fans are provided.

    TESTING OF TURBO GENERATOR

    To ensure that all functional requirements are fulfilled, and to estimate the

    performance of generator, the TURBO GENERATORS are required to undergo some

    tests. For testing, the TURBO GENERATOR was mechanically coupled to a drive

    motor-motor generator set with gearbox. The rotor was excited by thyristor converter

    system located in an independent test room and the operation was controlled from the test

    gallery.

    The following first two tests will be conducted on the stator and rotor before

    assembling and the third and final routines tests will be conducted after after assembling

    the turbo generator.

    a) Tests conducted on rotor

    b) Tests conducted on stator

    c) Routine tests on turbo generators

  • 24

    Rotor of Brushless

    Exciter

    Rotatin

    g

    Diodes

    &

    Fuses

    Perma

    nent

    Magne

    ts

    E/f

    Monito

    ring

    Slip

    Rings

    Armatu

    re

    Diode

    Wheel

  • 25

    Brushless Exciter Complete

    Assembly

    Cooling

    Fan

    Stator

    Armatur

    e

    Diode

    Wheel

  • 26

    TESTING OF TURBO GENERAROR ROTOR WINDING

    Details of Process tests to be performed at various stages

    HIGH VOLTAGE TEST

    1. After mounting the excitation lead and slip rings and before actually commencing the

    winding , the slip rings are to be tested.

    First, measure the insulation resistance with 1000v Megger, if the insulation condition is

    found satisfactory, then perform High Voltage test for one minute, the test of which is to

    be determined according to the following equation.

    U2 =Ut + 1 K V

    Where U2 is test voltage

    Ut is 10* rated rotor voltage

    However the resulting test voltage U2 should be neither lower than 2.5 K V nor

    above 4.5 KV.

    After the high voltage test, measure the insulating condition again with 1000 V

    Megger.

    2. The next test is to be carried out after placing all the coils in the respective rotor slots

    and before clamping the pressing equipment. Measure the insulating condition with a

    1000V megger. It must not be lower than I MO for each K V of the tested voltage. Then

    measure the ohmic resistance of the winding.

    3. After tightening the winding with the pressing and tightening equipment and before

    actually baking the winding, measure the ohmic resistance of the winding. Then check

    polarity of the winding.

    While clamping care should be taken to see that the pressing rings and other

    equipment are insulated from the winding and rotor body, by inserting insulation in every

    slot under the shims of the equipment.

    4. After baking and forming of the winding and removing of the clamping equipment and

    after the rotor cools down to ambient temperature, measure the insulation resistance with

    1000V Megger..

    If the insulation condition is satisfactory, perform High Voltage test for one

    minute with a value of 1.15 Ut.

  • 27

    When Ut is 10 times the rated rotor voltage.

    After performing the High Voltage test, measure again the insulation condition.

    5. After driving the central wedges only in position, measure the insulation resistance and

    if found satisfactory, perform High Voltage test with a value of 1.10 Ut for 10 sec, i.e.,

    just reaching the value and then bringing down to zero.

    After driving all the wedges in position, measure the insulation resistance and if

    found satisfactory, perform High Voltage test with a value of 1.10 Ut for one minute.

    6. After putting all the bracings, mounting of the end retaining ring and just before

    Dispatch of the rotor for further machining.

    7. After machining of the rotor, and before is dispatch to the centrifugal tunnel, measure

    the insulation resistance.

    8. After setting the rotor in the centrifugal tunnel, check the insulation resistance and the

    ohmic resistance, while the rotor is at rest. Check again the insulation condition at

    3000rpm.

    Measure again the insulation resistance after the rotor is balanced and just before

    its dispatch to the winding shop.

    9. Finally, just before the dispatch of the finished rotor measure the insulation resistance

    and perform High Voltage test with a value of 1.0 Ut for one minute.

    MEASUREMENT OF D.C.RESISTANCE

    The D.C. resistance value of rotor winding is measured by using a Micro

    Ohmmeter. First connect the micro ohmmeter to 230V AC supply. And measure the

    resistance and the temperature using RTD. This resistance at T temperature has to be

    converted to resistance at 20 Degrees C by using the formula:

    R20 = Rt * (235+20)/(235+T) milli ohms.

    Where R20 = Resistance at 20 Degrees C in mO

    T = temp in degree Celsius

    Rt = measured resistance of winding in mO

    A deviation of 10% from design values is acceptable.

  • 28

    MEASUREMENT OF IMPEDANCE

    By applying 50-200 V in steps of 50V, Impedance value is measured at standstill

    and at the rated speed.

    Impedance is measured by using the formula :

    Z = V/I

    Where Z impedance in ohms;

    V = voltage in volts;

    I = current in amps;

    In the measurement of Impedance there will be a graph plotted between voltage

    v/s current. In this, there is no perfect value for the impedance but the only condition is

    that the impedance should increase with increase in voltage.

    TESTING OF TURBO GENERATOR STATOR BARS

    For resin rich systems, stator bars will be tested in the following order

    1) After bars manufacturing bars are tested at four times the rated voltage.

    Ut = 4* Urated

    2) Individual bars will be tested for tan d d is the angle between actual current and line

    current. When the insulation is perfect and dielectric strength is optimal d is zero. But

    due to the presence of impurities there will be a phase angle difference between the two

    currents.

    This tan d measurement is known as loss angle measurement or dielectric loss

    measurement. Tan d values should be within 2%.

    3) Outer corona protection resistance is measured and this value should be within the range

    of 75-300 0/Sq. cm

    4) Inter strip and Inter half shorts are checked. Inter strip means between the

    conductor strips and inter half means between the halves. This shorts are checked by a

    series bulb test.

  • 29

    Manufacturing of Turbo - Generators

    EXCITERS

    BRUSHLESS EXCITER

    Suitable for mounting on synchronous generator.

    CONSTRUCTION

    The exciter is brush-less and takes the form of a stationary field generator. Its

    rotor is mounted on the overhang of main machine shaft end. The stator may be fixed

    either to be base frame of the main machine or to a separate steel or concrete foundation.

    A permanent magnet three phase pilot exciter driven directly by the common shafting or

    a static auxiliary excitation unit is used for exciting the field of the stationery field

    generator via a voltage regulator. The auxiliary excitation equipment is described

    elsewhere. The three phase current flowing in the rotor winding is rectified by Silicon

    diodes in the rotating rectifier and fed into the field winding of main machine via the

    excitation leads which pass through the hallow shaft of the main machine.

    ROTOR

    The rotor is fitted on the shaft extension of the main machine and locked to it in

    the circumferential direction by parallel keys which are capable of accepting shock loads

    caused by short circuit in the main machine without being over stressed.

    The rotor hub is of welded construction and called the laminated core which is

    compressed axially by means of a clamping ring welded to the hub. Specially shaped

    supporting elements for the rotating rectifier modules are welded between the arms of the

    rotor spider within the ring formed by laminated core.

  • 30

    ROTOR WINDING

    The 3-phase rotor winding inserted in the slots of the laminated core is connected

    in star. It is a two layer winding to insulation of class F. The end leads of the individual

    windings are on the A end and connected to the u,v,w and neutral bus rings arranged at

    the same end. Both winding overhangs are bound with heat setting glass fiber tapes to

    afford protection against centrifugal forces. The rotor winding is impregnated with

    epoxy resin.

    RECTIFIER

    The rectifier accommodated inside the rotor core and rotor winding comprises six

    diode assemblies and the protection circuit. The diode assemblies each consist of a light

    metal heat sink with integrally formed cooling fans containing one disc type diode

    secured by means of a clamping plate. As the heat sinks are electrically live, they are

    insulated from the rotor hub to which they are fixed. A contact face provided on the

    inside of each heat sink is connected by meanks of links to the appropriate bus ring on the

    3-phase side. The connections to the dc bus rings are established by longitudinally

    arranged bus connector, which is connected to the contact bolts protruding from the

    clamping plates.

    Diode assemblies situated on opposite sides of the rotor spider have opposite

    polarities. The sign of polarity, which appears on the front face of the heat sink, should

    be observed. The dc bus rings carry the protective varistors are screwed to the B end of

    the rotor spider by means of insulating mounts. The two bus rings, each have a terminal

    lug for the copper bars which are connected to the excitation cable of the main machine.

    The excitation cables are led through the insulated hollow shaft of the main

    machine and are provided with special cable lugs at the shaft openings.

    VARISTOR

    To protect the rectifier bridge against over voltages occurring during starting or

    during fault conditions, a non linear resistor is provided. This protective varistor

  • 31

    consists of 12 varistor discs in parallel, connected between the positive and negative bus

    rings.

    The varistor discs are clamped between the bus rings by means of insulated

    screws. Electrical contact between the varistor discs and the bus rings is ensured by discs

    of annealed copper inserted between them.

    MAIN EXCITER

    The 3 phase pilot exciter is a 6 pole revolving armature unit. Arranged in the

    frame are the poles with the field and damper windings. The field winding is arranged on

    the laminated magnetic poles. Each coil is made from individually insulated tube. To

    reduce eddy current in the coil, copper strips in each coil is transposed. At the pole shoe,

    hair is provided which are connected to form a damper winding. Between the 2 poles of

    quadrature axis, a coil is fixed for inductive measurement of field current.

    The rotor consists of stacked silicon steel laminations forming the rotor core. The

    3 phase winding is inserted in the slots of laminated rotor. The winding conductors are

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

    steel bands.

    The stator slots form indentations in the air gap boundary. Therefore as the rotor

    flux moves across the stator teeth the change in performance due to the slot opening

    introduces median frequency pulsations. These pulsations induce harmonic voltages in

    the surface of the stator teeth. But due to the laminated construction, the resultant leaves

    are kept to minimum. The winding ends are connected to a burring system to which the 3

    phase leads loading to the rectifier wheel are also connected. A journal bearing is

    arranged between main the pilot exciters and has forced oil lubrication from the turbine

    oil supply; rotor windings and core are air-cooled.

    ROTATING RECTIFIER WHEEL

    As power from the main exciter is fed to RR wheel it is converted to dc. The

    main components of the rectifier wheel are the silicon diodes, which are arranged inside

  • 32

    the retaining ring in a 3-phase bridge circuit. The internal arrangement of the diode is

    shown in fig. The arrangement of the diode is such that the contact pressure produced by

    plate spring assembly is increased by the centrifugal force during rotation. The rotating

    rectifier includes 20% standby capacity ensuring continued and restricted operation in the

    unlikely event of the diode failure. Anode based diodes are used in positive arms and

    cathode based diodes in negative arm of the bridge. Additional components contained in

    rectifier wheel are heat sinks, RC networks, fuses, Each diode is mounted in each light

    metal heat sink and thus connected in parallel associated with each diode with HRC fuse,

    which serves to switch off the diode if it fails.

    Rotating rectifier wheel is provided with 6 RC networks each consisting of one

    capacitor and one damping resistor, which are connected, in single resin encapsulated

    unit.

    When high voltage surges occur, the capacitor gets charged until normal

    conditions occur When a low voltage surge occurs, the charge through the capacitor is

    dissipated through the damping resistor.

    Three-phase alternating current is obtained via copper conductors arranged on the

    shaft circumference between rectifier wheel and 3 phase main exciter. One 3 phase

    conductor originating as a abus ring system of the main exciter is provided for each

    diode.

    The dc current from the rectifier wheels is fed to the DC leads arranged in the central

    bore of the shaft via radial bolts.

    PILOT EXCITER

    Some of different types of pilot exciters are salient pole, inductor type, and

    homopolar and heteropolar designs. Salient pole PMG is a 3-phase medium frequency

    machine providing a constant voltage supply to the thyristor converter and AVR circuits.

    PMG poles are manufactured from high-energy material such as Alcomax. The

    permanent magnet pieces are bolted to a steel hub and held in place in place by pole shoe.

    The bolts are made from non-magnetic steel to prevent formation of magnetic shunt. To

    improve the waveform of the output voltage and reduce electrical noise, the pole shoes

  • 33

    are skewed one pole pitch over the stator length, Stator core is constructed from a stack

    of low loss sheet steel laminations assembled within the fabricated steel frame. Radial

    and axial cooling ducts are provided at intervals along the core length to allow cooling of

    core and windings. The stator windings is a two layered, each conductor consisting of a

    number of small diameter copper wires insulated with polyster enamel. The coils are

    connected to give rated 3 phase voltage output and insulated with class F epoxy glass

    material.

    A steel frame is fitted over PMG stator provides mechanical protections and

    reduces medium frequency noise emitted from the PMG to an acceptable level. Cooling

    of PMG is achieved by drawing air through mesh-convered apertures in the frame.

    AUTOMATIC VOLTAGE REGULATORS

    The AVR is solid-state thyristor controlled equipment with very fat response. It

    has two channels. Auto channel for voltage regulation and manual channel for feld

    current regulation. Each channel has its own firing circuit and thyristor converter for

    reliability.

    Normally automatic regulation system is operative, including the startup and shut

    down of the machine. The set point adjuster of the excitation current controller

    automatically follows up, so that change over to excitation current control is possible at

    any time. Under certain emergency and fault conditions, change over is initiated

    automatically.

    The two self-ventilated thyristor sets for voltage control (AUTO) and excitation

    current control (MANUAL) are designed to meet the normal safety requirements

    regarding the current and voltage. In case of higher capacity thyristor bridge a separate

    fuse protects each thyristor. The individual thyristor fuses of both AUTO and MANUAL

    control systems are being monitored using miniature circuit breakers.

    The voltage generated by the generator has to be maintained constant. This

    constant voltage, taken as reference voltage, is fed to the error detector. The terminal

    voltage of the generator is also fed to the error detector. Error signal is amplified in the

    error amplifier. The output of the amplifier is fed to the gate pulse generator where the

  • 34

    pulsed are generated. These gate pulses are given to the gate terminals of the thyristors in

    the bridge circult of either AUTO channel or MANUAL channel, thus triggering the

    thyristors at required regular intervals.

    The three-phase output of the permanent magnet generator is fed to the thyristor

    bridge. The rectified signal from the thyristor bridge is fed to the main exciter field, so

    that the pole are excited.

    BL EXCITER

    Machined Rotor

  • 35

    STATOR CORE ASSMBLY

    PMG

  • 36

    END SHIELD ASSEMBLY BEARING

    FUSE

  • 37

    BEARING SHELL ASSEMBLY

  • 38

  • 39

    AIR FLOW DIAGRAM OF GENERATOR

    ARMATURE CORE ASSEMBLY

  • 40

    CHAPTER 3

    INSULATING MATERIALS

    3.0 INSULATING MATERIALS

    Electrical insulating materials are defined as those which offer high resistance to

    the flow of current. In the electrical machines and transformers, the insulating materials

    applied to the conductors are required to be flexible and have high dielectric strength and

    ability to withstand unlimited cycles of heating and cooling.

    3.1 CHARACTERSTICS OF A GOOD INSULATING MATERIAL

    Large insulation resistance.

    High dielectric strength.

    Uniform viscosity.

    Should be uniform throughout least thermal expansion.

    When exposed to arcing they should be non ignitable.

    Resistant to oils, liquids, as flames, acids and alkalies.

    No deteriorating effect on the material in contact with it.

    Low dissipation factor.

    High mechanical strength

    High thermal conductivity.

    3.2 CLASSIFICATION OF INSULATING MATERIALS

    The insulating materials can be classified according to:

    1. Substances and materials

    2. Temperature.

  • 41

    3.3 PROPERTIES OF INSULATING MATERIALS

    I. Electrical Properties.

    II. Thermal Properties.

    III. Chemical Properties.

    IV. Mechanical Properties.

    ELECTRICAL PROPERTIES

    INSULATION RESISTANCE

    It is defined as the resistance between two conductors usually separated by

    materials i.e., one through the body and other over the surface of the body.

    DIELECTRIC STRENGTH

    The voltage across the insulating materials is increased slowly, the way in which

    the leakage current increase depends upon the nature and condition of material.

    POWER FACTOR

    Power factor is a measure of the power losses in the insulation. It Should be low.

    It increases with the rise in temperature of the insulation. A rapid increase indicates

    danger.

    DIELECTRIC CONSTANT

    The property is defined as the ration of the electric flux density in the material to that

    produced in free space by the same electric force.

  • 42

    DIELECTRIC LOSS:

    The dielectric losses occur in all solid and lidquid dielectrics due to:

    a. Conduction Current

    b. Hysteresis.

    THERMAL PROPERTIES

    Specific heat thermal conductivity.

    Thermal plasticity.

    Ignitability.

    Softening point.

    Heat Ageing

    CHEMICAL PROPERTIES

    Resistant to external chemical effects.

    Restistant to chemicals in soils

    Effect of water.

    MECHANICAL PROPERTIES

    Density.

    Viscosity.

    Moisture absorption.

    Hardness of surface.

    Surface tension.

    Uniformity.

  • 43

    EFFECT OF MOISTURE ON INSULATION

    Thermal property

    Chemical property

    Electrical property

    Physical property

    Mechanical property

    FACTORS AFFECTING INSULATION RESISTANCE

    The factors which affect the insulation resistance(i.e., resistance between two

    conductors) are:

    It falls with every increase in temperature.

    The sensitivity of the insulator is considerable in presence of moisture.

    It decreases with increase in applied voltage

  • 44

    CHAPTER-4

    INTRODUCTION TO INSULATION SYSTEM

    4.0 INTRODUCTION TO INSULATION SYSTEM

    In Electrical Machines insulation is the basic requirement to sustain high

    voltates.Insulation is the heart of the electrical machines and has enormous resistance to

    conductivity i.e., the forbidden gap (or Fermi level) between the valence and the

    conduction bands is very large.

    4.1 PROPERTIES OF A GOOD INSULATION MATERIAL

    1. Non-conductive to electricity & good conductor of heat.

    2. Provides isolation between live wires or live wire & earth.

    3. Should withstand the designed mechanical stress.

    4. Good thermal and chemical resistivity.

    METHODS FOR INSULATION

    There are two methods of insulation.

    They are:

    Thermoplastic

    Thermosetting.

    4.2 THERMOPLASTIC

    Thermoplastic process in that where the resin softens o heating and hardens on

    cooling.

  • 45

    THERMOSETTING

    Thermosetting process is that where the resin once hardened cannot be softened

    even on heating. Thermosetting is again divided into two types.

    They are

    1. Resin Rich System.

    2. Resin Poor System

    RESIN RICH SYSTEM

    CONTENTS

    Resin content is 40%

    Binder content.

    Glass cloth.

    Mica content

    Volatiles.

    RESIN POOR SYSTEM

    CONTENTS

    Resin contnt is 8%

    Zine Napthenate

    Glass cloth

    Fine Mica content.

    Volatiles.

  • 46

    4.3EPOXY RESINS

    These resins are product of alkaline condensed of epichlorohydrin and polyhydric

    compounds. Epoxy resins are polyethers derived from epichlorohydrin and

    bisphenolmonomers through condensation polymerization process

    In epoxy resins, cross linking is produced by cure reaction. The liquid polymer

    has reactive functional group like oil etc. Otherwise vacuum as pre polymer. The

    prepolymer low inductor weights such as polyamines, polyamides, polyamides,

    polysulphides, phenol, ureaformaldehyde, acids anhydrides etc, to produce the three

    dimensional cross linkage structures.

    Epoxy resins can be used continuously to 3000 F, but with special additions can

    withstand a temperature of up to 5000 F.

    PROPERTIES OF EPOXY RESINS

    Good mechanical strength, less shrinkage and excellent dimensional stable after casting.

    Exhibit Chemical Inertness.

    High resistance.

    Good adhesion to metals.

    APPLICATION OF EPOXY RESINS

    1. Epoxy resins are used in the middle of laminated insulating boards.

    2. Dimensional stability prevents crack formation in castings.

    3. They are also used as insulating varnishes.

  • 47

    CHAPTER-5

    VACCUM PRESSURE IMPREGNATION PROCESS

    BLOCK DIAGRAM OF VPI PROCESS

    VACCUM PRESSURE IMPREGNATION PROCESS

    The Vacuum Pressure Impregnation system was introduced by Dr. Meyer in

    collaboration with Westing House in the year 1956. The resins used were of polyester

    SIEMENS developed VPI system with EPOXY RESIN and treated accelerator VPI

    system can be useful for manufacture of insulation and also windings are guaranteed to

    expected quality. The stator coils are taped with porous resin poor tapes before inserting

    into the slots of the cage stator. Subsequently wound stator is subjected to a special

    VACUUM

    PUMP

    VACUUM

    TRAP VACUUM

    PUMP

    VALVE

    NITROGEN

    SUPPLY

    TANK

    NITROGEN

    LINE

    VALVE

    VACUUM

    PRESSURE

    IMPREGENT

    ION TANK

    RESIN

    SUPPLY

    TANK

    RESIN

    SUPPLY

    VALUE

  • 48

    process called VPI Process in which first the stator is vacuum dried and then impregnated

    in a resin bath under a pressure of Nitrogen gas. Then the stator is curried in an oven. In

    olden days Resin Rich System of insulation was used where the stator coils are wound

    with Resin Rich tape which contains 40% of resin. But for good dielectric strength 25%

    is required. The extra 15% of resin is to be oozed out which is a tedious process and is

    carried out in medium. Hence it is not an ideal process is not employed

    Now-a-days Resin Poor System is employed where the stator coils are wound

    with Resin Poor tape which contains 8% of resin. For good dielectric strength, the extra

    17% of resin is to be injected into the pores of the resin poor tape by impregnation and

    is done by VPI Process.

    RESIN MIXTURE

    The resin used in VPI process is ET884, a mixture of Epoxy Resin E1023 and

    Hardener H1006 in 1:12 rations by weight and the two components are mixed in 1:1

    ratio. The resin tank contains Resin Mixture (Epoxy Resin + Hardener) and catalyst for

    good insulation system.

    RESIN

    Resin is a polymer. The process of polymerization under condensation gives

    Resin. The chemical name of resin is DIPHENOL PROPANE and its commercial

    name is BISPHNOLA-A. The chemical structure of Diphenol Propane is

    (C6H50H)2C3H8.

    HARDNER

    Hardener is used to solidify the resin. Hardener means Anhydride which means

    removal of water (i.e. H2O) molecule.

  • 49

    CATALYST:

    Catalyst is used to accelerate or decelerate the rate of a reaction. The catalyst

    used in resin in the VPI process is Zine Napthenate.

    5.1 STEPS INVOLVED IN VPI PROCESS

    The different steps involved in the Vacuum pressure Impregnation process for

    awound stator are:

    HV Test.

    Termination of the RTDs.

    Preheating the job.

    Shifting the job into the impregnation chamber.

    Vacuum cycle.

    Vacuum Drop test.

    Heating of Resin.

    Admission of Resin.

    Resin Settling time.

    Pressure Cycle.

    Refilling of Resin.

    Aeration.

    Post Curing.

    Cooling.

    HV TEST:

    The total wound stator which is brought from the stator assembly is subjected for

    HV test before impregnation.

  • 50

    TERMINATION OF THE RTDs

    All the salient RTDs in the straight portion and the body of the core are

    terminated towards one side to monitor the temperature of the total winding.

    PREHEATING THE JOB

    The total wound stator is subjected for preheating to 60+30C in am oven for

    duration of 12 hours.

    SHIFTING THE JOB INTO THE HORIZONTAL CHAMBER

    The impregnation chamber is to be kept clean without any traces of resin on the

    resin inner side of the horizontal tank. If present, it reacts with moisture and scale

    formation takes place. The resin traces present in the tank is wiped with methylene.

    The total wound stator is lifted and shifted into a tub. The tub is shifted into the

    impregnation chamber and the lid of the tank is closed by a hydraulic mechanism.

    VACUUM CYCLE

    The total wound stator is lifted and shifted into a tub. The tub is 60+30 C by

    circulating hot brine solution through the surface of the impregnation chamber which is

    heated up by heat exchangers. The vacuum pumps are switched on and vacuum is

    created in the chamber up to 0.2 mbar. Then the total wound stator is subjected for

    duration of 17 hours. Vacuum is created in the chamber to remove any moisture present

    in the stator core and chamber as it greatly affects the dielectric strength of the insulation.

    This is the most important factor considered during the manufacture and operation of the

    generator.

  • 51

    VACUUM DROP TEST

    This test is carried out at the end of vacuum cycle and before the admission of

    resin. In this test all the vacuum pumps are switched off for 10 minutes and the vacuum

    drop is measured. The vacuum drop should not be greater than 0.06mbar. If the drop is

    greater than 0.06mbar, it suggests that there are some impurities present in the pores of

    the insulation and the stator is again subjected for vacuum cycle for duration of 8 hours.

    HEATING OF RESIN

    All the resin tanks including the input pipelines of the resin are heated to 60+30 C.

    ADMISSION OF RESIN

    The valves of resin tanks are opened one after the other and the resin is filled

    within 25-30 minutes into the tub due to difference in pressure i.e., resin in the resin tank

    is at atmospheric pressure and the impregnation chamber is at 0.2mbar. During this time

    there is a change in pressure inside the chamber and should not be more than 0.06mbar,

    the vacuum will be created inside the chamber up to 0.2mbar. The level of resin should

    be 100mmmore than the job height.

    RESIN SETTLING TIME

    Resin is allowed to settle for duration of 15 minutes such that all the air bubbles

    are vanished.

    PRESSURE CYCLE

    The impregnation chamber is pressurized to 4kg/cm2 by dry. Nitrogen gas and

    then the total wound stator is subjected for 3 hours.

  • 52

    REFILING OF RESIN

    The resin remaining in the tub is filtered an sent back to the resin tanks.

    AERATION

    Here the pressure inside the impregnation chamber is made equal to that of

    atmospheric pressure and the job is brought out.

    POST CURING

    The job is placed in the gas furnace. All the RTD terminals which are brought out

    are connected to the temperature monitor for monitoring the post curing cycle. The total

    wound stator is roated a t1rpm up to 1020 C, then the rotation is stopped. The

    temperature is now increased to 140=50C and the stator is subjected for 32 hours.

    COOLING

    The job is allowed to cool down to 800 C naturally. Now the furnace is opened

    and epoxy red gel is sprayed on the overhangs to serve as anti fungus.

    5.2 QUALITY CHECKS ON RESIN MIXTURE

    The resin mixture is a combination of epoxy resin and hardener, the container of

    which is stored in a cool and dry place and should be protected against humidity and

    hence stored under vacuum below 200C., but chilled not below 80C. The impregnating

    resin mixture is in the ratio of 100 parts of epoxy resin and hardener in a resin tank. The

    epoxy resin and hardner are heated in an oven at 1750 C and sample is taken from every

    drum to test before release. After thorough mixing the resin mixture is tested.

  • 53

    TESTS ON RESIN MIXTURE

    Before beginning impregnation and after standstill period of more than 15 days,

    the resin mixture is tested in the following manner:

    The resin mixture is tested for viscosity at 600C and the limiting value is 50m

    Poise above which the resin is rejected.

    The resin is again tested for the increase in its viscosity at 600 C after 20 hours

    heating at 1000 C. The maximum value at this point is 9m poise.

    o After this resin is tested for the ester number which is the difference

    between saponification number and total acid number. Its maximum

    limiting value is 10.

    5.3 TESTING PERFORMANCE OF RESIN POOR SYSTEM

    BEFORE IMPREGNATION PROCESS

    The different tests that are carried out after laying the bars in the stator slots are:

    Complete bottom layer high voltage test

    Complete top layer high voltage test

    Winding resistance measurement.

    Mechanical run test.

    BOTTOM LAYER TEST

    After laying the bottom bars high voltage test is conducted with 1.5Up for One

    minute, where Up=2Un=1. Up=Final test voltage. Un=Rated voltage of generator.

    TOP LAYER TEST

    After laying the top bars high voltage test is conducted with 1.1Up for one

    minute, where Up=2Un=1.

  • 54

    INTER CONNECTION CHECKING

    After completion of connection, winding and baking, high voltage test is

    conducted with 1.05 Up for one minute. When one phase is under testing, the other

    phases are earthed. The measurement of resistance of individual phases gives the

    checking of interconnection.

    AC HIGH VOLTAGE TEST

    After laying the top and bottom bars AC high voltage test carried out by

    connecting all other phases to ground.

    MECHANICAL AND ELECTRICAL RUN TEST

    Dynamic test is carried out to find various losses. They are;

    Mechanical losses

    Iron losses

    Copper losses

    IMPREGNATION PLANT

    Horizontal Impregnation Chamber for higher capacity stators of steam turbine or

    gas turbine generators and Vertical Impregnation Chamber for small capacity systems

    such as Permanent Magnet Generator stators for brushless excitation systems, coil

    insulation of small pumps and armature of motors etc. are used.

    RESIN RICH SYSTEM OF INSULATION

    ADVANTAGES

    1. Better quality and reliability is obtained.

  • 55

    2. In case of any fault the repair process is very easy.

    3. Addition of excess resinis avoided.

    DISADVANTAGES

    It is very long procedure.1. Due to fully manual oriented process, the cost is more.

    RESIN POOR SYSTEM OF INSULATION

    ADVANTAGES

    1. It has better dielectric strength and heat transfer coefficient.

    2. Cost is very less and maintenance free.

    3. Insulation life will be more and lifetime is about 540 years.

    4. Reduction in time cycle and it gives high quality.

    DISADVANTAGES

    1. If any short circuit occurs, the repair process in difficult.2. There is need of excess resin

    from outside.

  • 56

    COMPARISION BETWEEN RESIN RICH AND RESIN POOR

    RESIN POOR SYSTEM RESIN RICH SYSTEM

    1) The insulating tape used in this system has

    only 8% of resin.

    2) This method follows thermosetting process.

    3) There is a need for addition OF resin from

    outside

    4) Reduction in time cycle for this process.

    5) No tests are carried out here at processing

    stage.

    6) The cost of repairing is more.

    7) Processing of bars along with stator and

    processing of exciter coils (along with exciter

    coil) are possible in resin poor.

    8) The overall cost is less compared to resin rich

    system.

    9) Windings are easy

    10) Insulation strength remains almost same due to

    more layers of insulation materials (tapes)

    11) Cycle time is less

    1) The insulating tape material used in this

    system has 40% resin.

    2) This method follows thermosetting process.

    3) Further addition of resin is not required from

    outside

    4) It is very long process and time-consuming.

    5) Tests are carried out while processing stage.

    6) Repairing work is easy.

    7) Processing of stator bars is only possible in

    Resin rich system.

    8) The total cost of this process is more.

    9) Windings is difficult

    10) Insulation strength decreases

    11) Cycle time is more

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    Complete finished rotor

    Finished Rotor with Retaining Ring

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    CHAPTER-6

    PERFORMANCE TESTING

    TESTING OF TURBOGENERATORS

    6.1 OBJECTIVES OF TESTING

    Testing is the most important process conducted on a machine after it is designed,

    to ensure that equipment concerned is suitable and capable for performing duty for which

    it is intended & complies with the customer specifications. Testing is done under

    conditions as closely as possible to those which apply when the set is finally installed

    with a view to demonstrate the customer its satisfactory operation. The tests provide the

    experimental data like efficiency, losses, characteristics, temperature, limits etc, both for

    confirmation of design forecast and as basic information for the production of future

    designs. The machine performance is evaluated from the results of the equivalent tests.

    ADVANTAGES OF TESTING

    1. Provides data for optimization of design & quality assurance.

    2. Meets the requirements of legal and contract requirements.

    3. Reduction in rework cost.

    4. Ensures process capability and develops check list.

    5. Establishes control lover raw materials.

    PERFORMANCE TESTS

    The performance tests carried out on the turbo generator are classified as:

    1.Mechanical run test.

    2.Routine tests.

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    3.Type tests.

    MECHANICAL RUN TEST

    The generator should be run for 24 hours. This test is done to ensure that there

    are no losses (friction and windage and the heat generated should be less. Vibrations

    occurring in the generator are also detected.

    ROUTINE TESTS

    These tests are carried out on a generator to ascertain that it is electrically and

    mechanically sound. The routine tests are classified as:

    a. Static test.

    b. Running test.

    6.2 STATIC TEST

    6.2.1 MEASUREMENT OF INSULATION RESISTANCE OF STATOR AND

    ROTOR WINDINGS BEFORE AND AFTER HV TEST

    Equipment:

    i. Megger (1000v/2500v)

    ii. Earthing rod and earthing wire or cable

    IR of the stator and winding are measured separately & value are taken at 15 seconds

    and60 seconds before and after HV test using megger of 2500V for stator and 1000V for

    rotor windings.

    Absorption coefficient of insulation is found out suing. Absorption coefficient=IR at 60

    sec IR at 15 sec RMS value should be greater than or equal to 2. IF IR values are high,

    the absorption coefficient is not considered because of early saturation. With dry

    windings its value will be somewhere in the vicinity of 2 or more. With damp windings

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    it decreases to one. Absorption coefficient of 1.8 & 1.7 may be satisfactory while a value

    below 1.5 indicates a damp machine.

    The minimum value if insulation Resistance (Rm) At 60 minutes is recommended as;

    Rm= (KV=1) ohms where KV is voltage in kilovolts to be applied for test. In practice a

    fairly high value is obtained .

    6.2.2 MEASUREMENT OF POLARIZATION INDEX OF STATOR

    WINDING

    The polarization index of stator winding, all the three phases together, is measure

    during 2500v megger after HV test. The IR values are noted at one minute and ten

    minutes from starting of the measurement. The minimum allowable value of PI is 2.0

    The value of Polarization Index is valuated as< P.I = IR value at 10 min IR value at 1

    min

    High Voltage Test

    Equipment

    :i. Voltmeter.

    ii.Binding 50 HZ AC High voltage transformer and its induction regulator (or) input

    autotransformer.

    iii.Potential Transformer (35 or 100KV/100V).

    Iv. Wire.

    v. Ear thing rod and Ear thing Wire (or) cable.

    When HV test is done on one phase winding, all other phase windings, rotor

    winding , instrumentation cables and stator body are earthed. High voltage is applied to

    the winding by gradually increasing to the required values and maintained for 1 min and

    reduced gradually. The transformer is switched off and winding is earthed by connecting

    into ear thing rod connected to earth wire. The test is conducted on all the phase & rotor

    winding separately.

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    HV test levels

    Stator winding: (2Ut= 1) KV=23 for 11 KV machines Rotor winding: 10Up volts

    (with min of 1500V and 3500V) where Ut=Rated Voltage of machine under test Up=

    Excitation voltage

    6.3 RUNNING TESTS

    6.3.1MEASUREMENT OF SHORT CIRCUIT CHARACTERISTICS

    The machine is prepared for short circuit characteristic using current transformers

    and shorting links. The machine is run at rated speed and dive motor input voltage and

    current are noted and is excited gradually in steps of 20%,40%,60%,80%,90%,100% In

    (rated current) At each sep the following parameters are noted:

    Stator current (Ia & Ib)

    Rotor current (If) corresponding to stator current

    Drive motor voltage (Vd) and current (Id) corresponding to stator current.

    6.3.2 MEASUREMENT OF MECHANCIAL LOSSES, OPEN CIRCUIT

    CHARACTERISITICS

    The machine is run at rated speed and drive motor input voltage and current are

    noted and machine is excited gradually in steps of 20% En (En=rated voltage of

    machine) At each step the following parameters are noted.

    Stator Voltage (Vab, Vbc, Vca).

    Rotor current (If) corresponding to stator voltage.

    Drive motor voltage (Vd) and current corresponding to stator Voltage. The excitation is

    reduced and cutoff, the speed is reduced and the machine is cooled at lower speed. The

    temperatures are checked using RTDs. The machine is stopped when it is sufficiently

    cooled down (stator core temperature should be less than 600C. From the above data,

    characteristic curves are plotted as follows:

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    En (vs) if

    En (vs) machine losses in KW

    6.4 TYPE TESTS

    The tests are conducted for customer satisfaction. The different type tests done

    are:

    BDV (Brad Down Voltage)

    Tand Test

    Voltage regulation.

    Over hang vibration.

    BDV (Break down voltage) test

    Break down voltage at which the insulation breaks down. This test is conducted

    to check the reliability and life of the insulation.

    Tand test

    Equipment:

    i. Schering bridge.

    ii. 50 HZ HV transformer

    iii. 100-1000 PF Standard capacitor.

    iv. Isolation shunt box.

    v. High tension cable.

    vi. Earth cable.

    vii. Voltmeter

    viii. Megger (2 KV)

    ix. Null indicator

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    The test is conducted to check the presence of impurities in the insulation. Tand value is

    significant factor for testing dielectric strength of the insulation. D is loss angle. The

    stator body, is isolated from ground by placing insulation package between body and

    phase connections to the Schering Bridge. HV Supply is switched on and the bridge 1.0

    Un in steps of 0.2 Un. By varying the voltage, Tand value for each phase and also for

    combined phases is noted down. Tand value should be generally less than or equal to 2%

    Voltage regulation

    Voltage regulation is defined as the change in voltage from no load to full load

    expressed as the percentage of full load. For generator to be ideal and efficient, voltage

    regulation should be less. There are four methods to find voltage regulation.

    They are:

    i. EMF method

    ii. MMF method

    iii. ZPF method

    iv. ASA method

    Overhang vibration test

    This test is conducted to check the rigidity and life of the overhang portions of the

    stator windings.

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    CONCLUSION

    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 lower cost than other available

    procedures. VPI yields superior results with better insulation properties, 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. As todays world is

    concentrating on reliability, maintainability, cost reduction and time cycle reduction, the

    leading manufacturers in the world are adapting VPI system of insulation for generators

    up to 400 MW with hydrogen cooling VPI system of insulation for electrical, mechanical

    and chemical properties and is highly reliable.

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    SCOPE OF THE FUTURE

    In view of the above, in the coming decades, the Indian grids will use more of

    generators using VPI system of insulation. In the scenario of world market which

    demands generators with less cost at the best possible time with better reliability VPI

    system of insulation will provide viable solution compared to Resin Rich type of

    insulation system.

    15. TESTING2.4 STATOR ASSEMBLY1. RECEPTION OF STATOR BARSSTATOR CORE ASSMBLYEND SHIELD ASSEMBLY BEARING