PROFILE OF B.H.E.L

Bharat Heavy Electricals Limited (BHEL) is today the largest engineering enterprise of India with an excellent track record of performance. Its first plant was set up at Bhopal in 1956 under technical collaboration with M/s. AEI, UK followed by three more major plants at Hardwar, Hyderabad and Tiruchirapalli with Russian and Czechoslovak assistance.

These plants have been at the core of BHEL’s efforts to grow an diversify and become India’s leading engineering company. The company now has 14 manufacturing divisions, 8 service centres and 4 power sector regional centres, besides project sites spread all over India and abroad and also regional operations divisions in various state capitals in India for providing quick service to customers.

BHEL manufactures over 180 products and meets the needs of core-sectors like power, industry, transmission, transportation (including railways), defence, telecommunications. Oil business, etc. Products of BHEL make have established an enviable reputation for high quality and reliability.

Supplied 2,00,000 M V S transformer capacity and substained equipment operating in Transmission & Distribution net work upto 400 KV – AC & DC Supplied over 25,000 Motors with Drive Control System Power projects. Petrochemicals, Refineries, Steel, Aluminium, Fertilizer, Cement, plant etc., supplied Traction electrics and AC/DC locos to power over 12,000 Kms Railway network.

Supplied over one million Valves to Power Plants and other Industries.

This is due to the emphasis placed all along on designing, engineering and manufacturing to international standards by acquiring and assimilating some of the best technologies in the world from leading companies in USA, Europe and Japan, together with technologies from its-own R & D centres BHEL has acquired ISO 9000 certification for its operations and has also adopted the concepts of Total Quality Management (TQM).

BHEL presently has manufactured Turbo-Generators of ratings upto 560 M W and is in the process of going upto 660 M W. It has also the capability to take up the manufacture of ratings upto 1000 MW suitable for thermal power generation, gas based and combined cycle power generation as-well-as for diverse industrial applications like Paper, Sugar, Cement, Petrochemical, Fertilizers, Rayon Industries, etc. Based on proven designs and know-how backed by over three decades of experience and accredition of ISO 9001. The Turbo-generator is a product of high-class workmanship and quality. Adherence to stringent quality-checks at each stage has helped BHEL to secure prestigious global orders in the recent past from Malaysia, Malta, Cyprus, Oman, Iraq, Bangladesh, Sri Lanka and Saudi Arabia. The successful completion of the various export projects in a record time is a testimony of BHEL’s performance.

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

Contents:

Profile of b.h.e.l

Abstract

Chapter 1:introduction

Chapter 2:manufacturing of stator and rotor

Chapter 3:insulating materials

Chapter 4:introduction to insulating system

Chapter 5:vaccum pressure impregnation process

Chapter 6:perfrormance testing

‘ABSTRACT

Electrical power has become a basic necessity of our daily life. Maximum percentage of total power generation is obtained by conventional power plants. Of these, steam, diesel and gas turbine power plants are high-speed systems where always 3-phase alternators are used. Generators play a major role in the production of electricity. In large scale industries manufacturing generators, insulation design plays a vital role. Insulation is known to be the heart of the generator. If the insulation fails, generator fails which leads to loss of crores of rupees. The latest technology for the insulation in the world adapted is “VACUUM PRESSURE IMPREGNATION” which is of resin poor thermosetting type. This type is preferred as it is highly reliable and possesses good mechanical, thermal properties and dielectric strength. As the quantity resin used is less, the overall cost of insulation is reduced.

In our project we have made a detailed study of the VPI System of insulation.

E

T R

BFP

WATER SOURCE

CONDENSORDM PLANT

PULVERISER

COAL

BOILER

CHAPTER -1

INTRODUCTION TO TURBO GENERATORS

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.

1.1PRINCIPLE

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.

1.2CLASSIFICATION 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.

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.

1.3STATOR

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

1. Stator Frame.2. Stator Core3. Stator Winding.

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.

1.4ROTOR:

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.

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 250 0C. 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 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 RTD’s (Resistance Temperature Detector) embedded in the lower bearing sleeve so that the ensuring point is located directly below the Babbitt. All bearings have provisions to monitor the shaft vibrations. The oil supply to the bearing is obtained from the turbine oil system.

1.5COOLING 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 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 oxygen2. Cost of running is higher

`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.

2.1LAMINATION 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.

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 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.

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:

RTD’S are placed to detect temperature in between the winding and on the core.

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.

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 inter–strip 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.

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.

12.FINISHING, TESTING & DIMENSIONAL CHECK:

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

13.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. 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.

2.4STATOR ASSEMBLY1. 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:

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 Generators

Strip to strip bracing of the conductors in the overhang portion is done using a silver foil which contains 14% silver and remaining is tin.

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

STATOR WINDING

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 torque’s 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.

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.

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.

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 RTD’s 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.

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.

Rotor of Brushless Exciter

Rotating Diodes & Fuses

Permanent Magnets

E/f Monitoring Slip Rings

Armature

Diode Wheel

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 rotorb) Tests conducted on statorc) Routine tests on turbo generators

Brushless Exciter – Complete Assembly

Cooling Fan

Stator

ArmatureDiode Wheel

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.

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 bracing’s, 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.

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.

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.

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 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 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 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 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.

Machined Rotor

BL EXCITER

STATOR CORE ASSMBLY

PMG

END SHIELD ASSEMBLY BEARING

FUSE

BEARING SHELL ASSEMBLY

AIR FLOW DIAGRAM OF GENERATOR

ARMATURE CORE ASSEMBLY

CHAPTER 3:

INSULATING MATERIALS

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.

3.1CHARACTERSTICS 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.

3.2CLASSIFICATION OF INSULATING MATERIALS:

The insulating materials can be classified according to:

1. Substances and materials

2. Temperature.

3.3PROPERTIES 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.

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.

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

CHAPTER 4:

INTRODUCTIONTOINSULATION SYSTEM

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.

4.1PROPERTIES 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:ThermoplasticThermosetting.

4.2THERMOPLASTIC:

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

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 contentVolatiles.

RESIN POOR SYSTEM: CONTENTS:

Resin contnt is 8% Zine Napthenate Glass cloth Fine Mica content. Volatiles.

4.3EPOXY RESINS:These resins are product of alkaline condensed of epichlorohydrin and polyhydric compounds. Epoxy resins are polyether’s 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.

CHAPTER 5:

VACCUM PRESSUREIMPREGNATION PROCESS

5.1 STEPS INVOLVED IN VPI PROCESS5.2 QUALITY CHECKS ON RESIN MIXTURE5.3 TESTING PERFORMANCE OF RESIN POOR SYSTEMBEFORE IMPREGNATION PROCESS.

Block Diagram of VPI Process

VACUUM PUMP

VACUUM TRAP

VACUUM PUMP VALVE

NITROGEN SUPPLY TANK

NITROGEN LINE VALVE

VACUUM PRESSURE IMPREGENT ION TANK

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 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:

RESIN SUPPLY TANK

RESIN

SUPPLY VALUE

Hardener is used to solidify the resin. Hardener means Anhydride which means removal of water (i.e. H2O) molecule.

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”. 5.1STEPS INVOLVED IN VPI PROCESS:The different steps involved in the Vacuum pressure Impregnation process for awound stator are:

HV Test.

Termination of the RTD’s.

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.

TERMINATION OF THE RTD’s:All the salient RTD’s 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.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.

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=5 0C 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.

5.2QUALITY 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.

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.

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.

5.3TESTING PERFORMANCE OF RESIN POOR SYSTEM BEFOREEIMPREGNATION 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 TESTAfter 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.

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.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.

COMPREISON BETWEEN RESIN RICH AND RESIN POOR:

RESIN POOR SYSTEM RESIN RICH SYSTEM1) 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

Finished Rotor with Retaining Ring

Complete finished rotorCHAPTER 6:

PERFORMANCE TESTING

TESTING OF TURBOGENERATORS:6.1OBJECTIVES 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.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.

6.2STATIC TEST 6.2.1 MEASUREMENT OF INSULATION RESISTANCE OF STATOR AND ROTOR WINDINGS BEFORE AND AFTER HV TEST.Equipment:

i. Megger (1000v/2500v)

ii. Ear thing rod and ear thing 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 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.

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

6.3RUNNING 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 MECHANCIALLOSSES, 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 RTD’s. 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:

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 transformeriii. 100-1000 PF Standard capacitor.iv. Isolation shunt box.v. High tension cable.vi. Earth cable.vii. Voltmeterviii. Megger (2 KV)

ix. Null indicator 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 methodii. MMF methodiii. ZPF methodiv. ASA method

Overhang vibration test:This test is conducted to check the rigidity and life of the overhang portions of the stator windings.

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 today’s 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.SCOPE OF THE FUTURE:In view of the above, in the coming decades, the Indian grids will use more of generators usingVPI 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.