49627168 Manufacturing Amp Insulation System for Air Cooled Turbo Generators

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generators

Manufacturing & Insulation System For Air Cooled Turbo Generators

Document BySANTOSH BHARADWAJ REDDYEmail: [email protected]

Engineeringpapers.blogspot.comMore Papers and Presentations available on above site

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 of 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.

mailto:[email protected]


CHAPTER1:

INTRODUCTION

TO

TURBOGENERATORS


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 moved in a stationary magnetic

field (or) when the magnetic field 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.


1.2 CLASSIFICATION OF GENERATORS :

Generators can be broadly classified into two types as:

1) DC Generators.

2) AC Generators.

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. Shunt Generators.

2. Series Generators.

3. Compound Generators.

CLASSIFICATION OF AC GENERATORS:

AC Generators can be broadly classified into two types.

1. Asynchronous generators.

2. Synchronous generators.

ASYNCHRONOUS GENERATORS:

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

in synchronism.

E.g.: Induction motor.

SYNCHRONOUS GENERATORS:

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo 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:

Turbo-alternators.

• Steam

• Gas

Hydro Generators.

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.

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:

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generators 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.

ROTOR SHAFT:

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

using the Hellar 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 90° bending is carried out. The conductors are then

subjected for “Annealing”, Hydraulic pressing on both the bends and checking with gauge

is 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

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsHGL (Hardened Glass Lamination) insulation and the 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° C. 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 to

the 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.

1.5 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

• Windage losses.

These losses dissipate as heat and raise the temperature of the generator which affects 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 1/14th of 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


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%

Silicon : 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 processes:

1. RECEPTION OF SILICON SHEETS:

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsThe 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. DEBURRING :

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 300°C - 400°C.

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 1Mohm.

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 plate 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 360º 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 drift is passed in all the slots, The above process is repeated up to 800 mm and

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generatorsfirst 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 90º 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 and

1% 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.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generators4 . TRANSPOSITION:

The copper strips are 180º 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 180° transposed coils are placed one above the other to form a bundle. Half

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 160°C 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 of a bundle. Both the overhang portion and consolidated

using clamps and heating to a temperature of 60°C to 70°C for a duration of 30 minutes.

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

overlap.

Copper foil is placed along the width of the bar and ICP (Internal Corona

Protection) tape is wound.

Resin poor tape is again wrapped throughout 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

overlapped.

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

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generatorsobtain the desired width such that the bar is placed easily 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 lamination projections. It is

rotated continuously so that all the foreign matter comes out.

3. WINDING HOLDER ASSEMBLY:

The winding holders 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 wedging

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 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 neutrals.

8. INSULATION OF EYES:

Insert nomex sheets between the two halves of the eye insulate each eye with 3

× ½ layers of semica folium glass plate. Ultimately wrap hyper seal tape.

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.

3.1 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:

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsThe insulating materials can be classified according to:

1. Substances and materials

2. Temperature.

3.3 PROPERTIES OF INSUIATING 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 insulating 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 liquid dielectrics due to:

a. Conduction Current

b. Hysteresis.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsTHERMAL PROPERTIES:

• Specific heat thermal conductivity.

• Thermal plasticity.

• Ignitability.

• Softening point.

• Heat Ageing

CHEMICAL PROPERTIES:

• Resistant to external chemical effects.

• Resistant 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:

INTRODUCTION

TO

INSULATION SYSTEM


INTRODUCTION TO INSULATION SYSTEM:

In Electrical Machines insulation is the basic requirement to sustain high voltages.

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

conductivity i.e., the forbidden gap (or Fermi level) between the valency 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:

1. Thermoplastic 2. Thermosetting.

4.2 THERMOPLASTIC:

Thermoplastic process is that where the resin softens on heating and hardens on

cooling.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsTHERMOSETTING:

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 content is 8%

• Zinc Napthenate.

• Glass cloth

• Fine Mica content.

• Volatiles.

4.3 EPOXY RESINS:

These resins are product of alkaline condensed of epichlorohydrin and polyhydric

compounds. Epoxy resins are polyethers derived from epichlorohydrin and bisphenol

monomers 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 pre

polymer low inductor weights such as polyamines, polymides, polysulphides,phenol, urea

formaldehyde, acids anhydrides etc, to produce the three dimensional cross linkage

structures.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generators Epoxy resins can be used continuously up to 300º F, but with special additions can

withstand a temperature of up to 500º 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.

APPLICATIONS 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 PRESSURE

IMPREGNATION

PROCESS


5.1 STEPS INVOLVED IN VPI PROCESS.

5.2 QUALITY CHECKS ON RESIN MIXTURE.

5.3 TESTING PERFORMANCE OF RESIN POOR SYSTEM

BEFORE IMPREGNATION PROCESS.

VACUUM 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 and 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 1:1.2 ratios 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

“BISPHENOL-A”. The chemical structure of Diphenol Propane is (C6H5OH)2C3H8.

HARDNER:

Hardner is used to solidify the resin. Hardner 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 “Zinc Napthenate”.


BLOCK DIAGRAM OF VPI PLANT


5.1 STEPS INVOLVED IN VPI PROCESS:

VACUUM TRAP

VACUUM PRESSURE IMPREGNATION TANK

RESIN SUPPLY TANK

NITROGEN LINE VALVE

RESIN SUPPLY VALVE

NITROGEN SUPPLY TANK

VACUUM PUMP

VACUUM PUMP VALVE

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generators The different steps involved in the Vacuum pressure Impregnation process for a

wound 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 overhang 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+3ºC in an 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 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.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo Generators 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 heated to 60+3º 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

0.06 mbar, 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+3º 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 100mm

more 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.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsREFILLING OF RESIN:

The resin remaining in the tub is filtered and 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 rotated at 1 rpm up to 120º C , then the rotation is stopped. The temperature is now

increased to 140+5º C and the stator is subjected for 32 hours.

COOLING:

The job is allowed to cool down to 80º 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 hardner, the container of which is

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

under vacuum below 20ºC., but chilled not below 8ºC. The impregnating resin mixture is in

the ratio of 100 parts of epoxy resin and hardner in a resin tank. The epoxy resin and

hardner are heated in an oven at 175º 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 60º C and the limiting value is

50mPoise above which the resin is rejected.

The resin is again tested for the increase in its viscosity at 60º C after 20 hours

heating at 100º C. the maximum value at this point is 9 mPoise.


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 1.1 Up 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 resin is avoided.

DISADVANTAGES:

It is a 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 is difficult.

2. There is need of excess resin from outside.


COMPARISON 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.

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.


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 conformation 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 over raw materials.

PERFORMANCE TESTS:

The performance tests carried out on the turbogenerator 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.


6.2 STATIC TEST:

6.2.1 MEASUREMENTOF 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 the rotor windings are measured

separately & values are taken at 15 seconds and 60

seconds before and after HV test using megger of 2500V

for stator and 1000V for rotor windings.

• Absorption coefficient of insulation is found out using.

Absorption coefficient=IR at 60sec

IR at 15sec

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 of 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 measured

using 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 Polarisation Index is evaluated 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. Earthing rod and Earthing 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 1min and

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

to earthing 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 11KV 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:

6.3.1 MEASUREMENT 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 step 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 MECHANICAL 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 Voltages (Vab,Vbc,Vca).

• Rotor current (If) corresponding to stator voltage.

• Drive motor voltage (Vd) and current (Id) 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

60ºC).

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 (Break Down Voltage)

• Tanδ Test

• Voltage regulation.

• Over hang vibration.

BDV (Break down voltage ) test:

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

is conducted to check the reliability and life of the insulation.

Tanδ Test:

Equipment:

i. Schering bridge.

ii. 50HZ HV transformer

iii. 100-1000PF Standard capacitor.

iv. Isolation shunt box.

v. High tension cable.

vi. Earth cable.

vii. Voltmeter.

viii. Megger (2KV)

ix. Null indicator

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

value is significant factor for testing dielectric strength of the insulation. δ 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, Tanδ value for each phase and

also for combined phases is noted down.

Manufacturing & Insulation System By VPI Process For Air Cooled Turbo GeneratorsTanδ 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 a 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.


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 insulating 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 has

better thermal, 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 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.


BIBLOGRAPHY:

Electrical Machines: Dr.P.S.BIMBHRA

Electrical Machines: M.G.SAY

Electrical Insulating Materials: R.K.RAJPUT

Document BySANTOSH BHARADWAJ REDDYEmail: [email protected]

Engineeringpapers.blogspot.comMore Papers and Presentations available on above site

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49627168 Manufacturing Amp Insulation System for Air Cooled Turbo Generators

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Transcript of 49627168 Manufacturing Amp Insulation System for Air Cooled Turbo Generators