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PROFILE OF B.H.E.L.

Bharat Heavy Electrical Limited (BHEL) is today the largest engineering enterprise of India

with an excellent track record of performance. Its first plant was set up at Bhopal in 1956 under

technical collaboration with M/s. AEI, UK followed by three more major plants at Haridwar,

Hyderabad and Tiruchirapalli with Russian and Czechoslovak assistance.

These plants have been at the core of BHEL’s efforts to grow and diversify and become

India’s leading engineering company. The company now has 14 manufacturing divisions, 8 service

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.

BHEL has installed equipment for over 62,000 MW of power generation-for Utilities, Captive

and Industrial users. Supplied 2,00,000 MVA transformer capacity and sustained equipment

operating in Transmission & Distribution network up to 400kV – AC & DC, Supplied over 25,000

Motors with Drive Control System Power projects. Petrochemicals, Refineries, Steel, Aluminium,

Fertiliser, Cement plants etc., supplied Traction electric and AC/DC Locos to power over 12,000 Km

Railway network.

Supplied over one million Valves to Power Plants and other Industries. This is due to the

emphasis placed all along on designing, engineering and manufacturing to international standards by

acquiring and assimilating some of the best technologies in the world from leading companies in

USA, Europe and Japan, together with technologies from its-own R & D 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 up to 560 MW and is in the

process of going up to 660 MW. It has also the capability to take up the manufacture of ratings unto

1000 MW suitable for thermal power generation, gas based and combined cycle power generation

as-well-as for diverse industrial applications like Paper, Sugar, Cement, Petrochemical, Fertilisers,

Rayon Industries, etc. Based on proven designs and know-how backed by over three decades of

experience and accreditation of ISO 9001. The Turbo-generator is a product of high-class

workmanship and quality. Adherence to stringent quality-checks at each stage has helped BHEL to

secure prestigious global orders in the recent past from Malaysia, Malta, Cyprus, Oman, Iraq,

Bangladesh, Sri Lanka and Saudi Arabia. The successful completion of the various export projects in

a record time is a testimony of BHEL’s performance.

Established in the late 50’s, Bharat Heavy Electrical Limited (BHEL) is, today, a name to

reckon with in the industrial world. It is the largest engineering and manufacturing enterprises of its

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kind in India and is one of the leading international companies in the power field. BHEL offers over

180 products and provides systems and services to meet the needs of core sections like: power,

transmission, industry, transportation, oil & gas, non-conventional energy sources and

telecommunication. A wide-spread network of 14 manufacturing divisions, 8 service 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 specialised needs with total solutions-efficiently

and economically. An ISO 9000 certification has given the company international recognition for its

commitment towards quality. With an export presence in more than 50 countries BHEL is truely

India’s industrial ambassador to the world.

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Preface

Power is the basic necessity for economic development of a country. The production

of electrical energy and its per capital consumption is deemed as an index of standard of living in a

nation in the present day civilization. Development of heavy or large-scale industries, as well as

medium scale industries, agriculture, transportation etc, totally depend on electrical power resources

of engineers and scientists to find out ways and means to supply required power at cheapest rate. The

per capital consumption on average in the world is around 1200KWH, the figure is very low for our

country and we have to still go ahead in power generation to provide a decent standard of living for

people.

An AC generator is a device, which converts mechanical energy to electrical

energy. The alternator as it is commonly called works on the principle of ‘Electro Magnetic

Induction’. Turbo generators are machines which can generate high voltages and capable of

delivering KA of currents .so the designer should be cautious in designing the winding insulation. So

insulation design plays a major role on the life of the Turbo Generator. In our project we deal with the

“Manufacture process of turbo generator and its insulation design by VPI process.”

The first half of project is concerned with the aspects of generator manufacturing

comprising of stator manufacturing, in a step by step procedure involving different stages, and the

latter stage includes the insulation design of the generator by VPI process in a detailed manner, which

completes the generator design.

We more over stress mainly on VPI insulation process. Before going deep into the

topic, we will start with a brief introduction.

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Introduction

Electrical insulating materials are defined as materials that offer a large resistance to

the flow of current and for that reason they are used to keep the current in its proper path i.e. along

the conductor. Insulation is the heart of the generator. Since generator principle is based on the

induction of e.m.f in a conductor when placed in a varying magnetic field. There should be proper

insulation between the magnetic field and the conductors. For smaller capacities of few KW, the

insulation may not affect more on the performance of the generator but for larger capacities of few

MW (>100MW) the optimisation of insulation is an inevitable task, moreover the thickness of

insulation should be on par with the level of the voltage, also non homogenic insulation provisions

may lead to deterioration where it is thin and prone to hazardous short circuits, also the insulating

materials applied to the conductors are required to be flexible and have high specific (dielectric)

strength and ability to withstand unlimited cycles of heating and cooling.

Keeping this in view among other insulating materials like solids gases etc liquid

dielectrics are playing a major role in heavy electrical equipment where they can embedded deep into

the micro pores and provide better insulating properties. Where as solid di-electrics provide better

insulation with lower thickness and with greater mechanical strength. So the process of insulation

design which has the added advantage of both solid and liquid dielectrics would be a superior process

of insulation design. One such process which has all the above qualities is the VPI (vacuum

pressurised impregnation) process and has proven to be the best process till date.

Drawbacks of Early VPI Process:

DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the

year 1956. It has been used for many years as a basic process for thorough filling of all interstices in

insulated components, especially high voltage stator coils and bars. Prior to development of

thermosetting resins, the widely used insulation system for 6.6kv and higher voltages was a VPI

system in which, Bitumen Bonded Mica Flake Tape is used as main ground insulation. The

bitumen is heated up to about 180C to obtain low viscosity which aids thorough impregnation.

To assist penetration, the pressure in the autoclave was raised to 5 or 6 atmospheres. After

appropriate curing and calibration, the coils or bars were wound and connected up in the normal

manner. These systems performed satisfactorily in service provided they were used in their thermal

limitations.

In the late 1930’s and early 1940’s, however, many large units, principally turbine generators,

failed due to inherently weak thermoplastic nature of bitumen compound.

Failures were due to two types of problems:

1. Tape separation

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2. Excessive relaxation of the main ground insulation.

Much development work was carried out to try to produce new insulation systems, which

didn’t exhibit these weaknesses.

The first major new system to overcome these difficulties was basically a fundamental

improvement to the classic Vacuum Pressure Impregnation process: Coils and bars were

insulated with dry mica flake tapes, lightly bonded with synthetic resin and backed by a thin layer of

fibrous material. After taping, the bars or coils were vacuum dried and pressure impregnated in

polyester resin. Subsequently, the resin was converted by chemical action from a liquid to a solid

compound by curing at an appropriate temperature, e.g. 150C. this so called thermosetting process

enable coils and bars to be made which didn’t relax subsequently when operating at full service

temperature. By building in some permanently flexible tapings at the evolutes of diamond shaped

coils, it was practicable to wind them without difficulty. Thereafter, normal slot packing, wedging,

connecting up and bracing procedures were carried out. Many manufacturers for producing their large

coils and bars have used various versions of this Vacuum Pressure Impregnation procedure for almost

30 years.

The main differences between systems have been used is in the type of micaceous tapes used

for main ground insulation and the composition of the impregnated resins. Although the first system

available was styrenated polyester, many developments have taken place during the last two decades.

Today, there are several different types of epoxy, epoxy-polyester and polyester resin in common use.

Choice of resin system and associated micaceous tape is a complex problem for the machine

manufacturer.

Although the classic Vacuum Pressure Impregnation technique has improved to a significant

extent, it is a modification to the basic process, which has brought about the greatest change in the

design and manufacture of medium-sized a.c. industrial machines. This is the global impregnation

process. Using this system, significant increases in reliability, reduction in manufacturing costs and

improved output can be achieved.

Advantage of present resin poor VPI process:

VPI is a process, which is a step above the conventional vacuum system. VPI includes

pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is

improved mechanical strength and electrical properties. With the improved penetration, a void free

coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution,

the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the

average rise.

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In order to minimise the overall cost of the machine & to reduce the time cycle of the

insulation system vacuum pressure Impregnated System is used. The stator coils are taped with

porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator

is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin

bath under pressure of Nitrogen gas.

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Abstract

In developing countries like India, power generation is a major break through to meet

the present demands of the nation. Power generation of several types are on forefront, the dominant

component of power generation is TURBO-GENERATOR which produces large capacity, the word

“TURBO” stands for turbine drive. Generally the turbines used to drive these turbo-generators are of

reaction type.

In large-scale industries manufacturing generators, insulation design plays a vital role.

Insulation is known to be the heart of the generator. If insulation fails, generator fails which leads to

the loss of crores of rupees. The latest technology for insulation in the world and adopted by BHEL,

(Hyderabad) unit is “VACUUM PRESSURE IMPREGNATION “which is of resin poor

thermosetting type. This type is preferred as it is highly reliable and possesses good mechanical,

thermal properties and di-electric strength. As the quantity of resin used is less, hence the over all cost

of insulation is reduced.

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

system is employed by BHEL first in the country and second in the world next to Germany.

Project Associates:

G.Venkatesh Babu (04A21A0258)

M.K.Chaitanya Sarma (04A21A0216)

M.V.Satya Teja (04A21A0254)

L.Praneeth Chaitanya (03A21A0226)

Project Guide External: Project Guide Internal:

R.K.Manohar., Sr.D.G.M, T.Ravi. M.E..,

Quality Control (E.M) , Asst prof.

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

APPROVED BY HOD OF EEE

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Introduction to various parts of a Generator:

Manufacturing of Generator:

The manufacturing of a generator involves in manufacturing of all the parts of the

generator separately as per the design requirements and assembling them for the operation. Usually

the assembling is done at the generator installation area in order to avoid the damage due to

mechanical stresses during transportation, also this facilitates easy transportation. So, now it is worth

knowing the parts of the Turbo Generator.

Parts of a turbo generator

1. Stator

2. Rotor

3. Excitation system

4. Cooling system

5. Insulation system

6. Bearings

In our project we have a detail study of only stator, rotor and the insulation system used for it.

And the parts excitation system cooling system and bearings are external to the generator and is

treated as a completed one and is out of scope of our record. Generator manufacturing can be broadly

divided into three main parts:

1. Stator manufacture.

2. Rotor manufacture and

3. Insulation system.

The various stages involved in the generator manufacture and their sub processes are shown in the

flow diagram given below.

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Figure 1: flow diagram showing various stages involved in generator manufacture.

Now these sub processes are explained in detail below. Let us start with Stator.

1. Stator Manufacture:

STATOR description:

To facilitate manufacture erection and transport the stator consists of following parts.

1.1STATOR FRAME

The stator frame is of welded steel single piece construction. It supports the laminated core and

winding. It has radial and axial ribs having adequate strength and rigidity to minimise core vibrations

and suitably designed to ensure efficient cooling. Guide bards are welded or bolted inside the stator

frame over which the core is assembled. Footings are provided to support the stator foundation.

1.2 STATOR CORE

The stator core is made of silicon steel sheets with high permeability and low hysteresis and

eddy current losses. The sheets are suspended in the stator frame from insulated guide bars.

Stator laminations are coated with synthetic varnish; are stacked and held between sturdy steel

clamping plates with non-magnetic pressing fingers, which are fastened or welded to the stator frame.

In order to minimise eddy current losses of rotating magnetic flux which interacts with the

core, the entire core is built of thin laminations. Each lamination layer is made of individual

segments.

The segments are punched in one operation from electrical sheet steel lamination having high

silicon content and are carefully deburred. The stator laminations are assembled as separate cage

core without the stator frame. The segments are staggered from layer to layer so that a core of high

mechanical strength and uniform permeability to magnetic flux is obtained. On the outer

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

To obtain optimum compression and eliminate looseness during operation the laminations are

hydraulically compressed and heated during the stacking procedure. To remove the heat, spaced

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segments are placed at intervals along the core length, which divide the core into sections to provide

wide radial passages for cooling air to flow.

The purpose of stator core is

1. To support the stator winding.

2. To carry the electromagnetic flux generated by rotor winding.

So selection of material for building up of core plays a vital role. The losses in the core are of two

types

1. Hysterisis Loss: Due to the residual magnetism in the core material. Hysterisis

loss is given by

Wh Bmax1.6

* f * t

2. Eddy Current Loss: Due to the e.m.f. induced in the core of the stator. Eddy

current loss is given by

We Bmax ² * f ² * t²

In order to reduce the Hysterisis loss, silicon alloyed steel, which has low Hysterisis constant is used

for the manufacture of core. The composition of silicon steel is

Steel - 95.8 %

Silicon - 4.0 %

Impurities- 0.2 %

From the formula it is seen that eddy current loss depends on the thickness of the laminations. Hence

to reduce the eddy current loss core is made up of thin laminations which are insulated from each

other. The thickness of the laminations is about 0.5 mm. The silicon steel sheets used are of COLD

ROLLED NON-GRAIN ORIENTED (CRANGO) type as it provides the distribution of flux

throughout the laminated sheet.

Now let us see the detailed study of stator manufacturing process.

STATOR MANUFACTURE PROCESS:

This stator manufacturing is a combination of two individual sub processes, namely

Stator core construction and

Coil construction and their assembly

STATOR CORE CONSTRUCTION:

PREPARATION OF STATOR LAMINATIONS

a. Reception of silicon steel rolls: The silicon steel rolls received are checked for their

physical, chemical, mechanical and magnetic properties as per the specifications

mentioned above.

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b. Shearing: The cold rolled non grained oriented (CRNGO) steel sheets are cut to their

outer periphery to the required shapes by feeding the sheet into shearing press. For high

rating machines each lamination is build of 6 sectors (stampings), each of 60 cut according

to the specifications.

c. Blanking and notching: Press tools are used in making the core bolt holes and other

notches for the laminations. Press tools are mainly of two types.

i. Compound notching tools.

ii. Individual notching tools.

i. COMPOUND OPERATION: In this method the stamping with all the core bolt holes,

guiding slots and winding slots is manufactured in single operation known as Compound

operation and the press tool used is known as Compounding tool. Compounding tools are

used for the machines rated above 40 MW. Nearly 500 tons crank press is used for this

purpose.

ii. INDIVIDUAL OPERATIONS:

In case of smaller machines the stampings are manufactured in two operations. In the first

operation the core bolt holes and guiding slots are only made. This operation is known as Blanking

and the tools used are known as Blanking tools. In the second operation the winding slots are

punched using another tool known as Notching tool and the operation is called Notching.

d. Deburring operation :

In this operation the burrs in the sheet due to punching are deburred. There are chances of

short circuit within the laminations if the burrs are not removed. The permissible is about 5

micrometer. For deburring punched sheets are passed under rollers to remove the sharp burs

of edges.

e. Varnishing :

Depending on the temperature withstand ability of the machine the laminations are coated by

varnish which acts as insulation. The lamination sheets are passed through conveyor, which

has an arrangement to sprinkle the varnish, and a coat of varnish is obtained. The sheets are

dried by a series of heaters at a temperature of around 260 – 350 oC. Two coatings of varnish

are provided in the above manner till 12-18 micrometer thickness of coat is obtained. Here

instead of pure varnish a mixture of Tin and Varnish is used such that the mixture takes

44sec to empty a DIN4 CUP.

The shape of these laminations are as shown in the fig 1 below, the figure is for when

the core is made up of 4 sectors as shown in fig 2

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The prepared laminations are subjected to following tests.

i) Xylol test - To measure the chemical resistance.

ii) Mandrel test - When wound around mandrel there should not be any cracks.

iii) Hardness test - Minimum 7H pencil hardness.

iv) IR value test - For 20 layers of laminations insulation resistance should not be less than

1M

STATOR CORE ASSEMBLY:

TRAIL PACKET ASSEMBLY:

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Clamping plate is placed over the assembly pit; stumbling blocks are placed between the

clamping plates and the assembly pit. Clamping plate is made parallel to the ground by checking

with the spirit level. One packet comprising of 0.5 mm thickness silicon steel laminations is

assembled over the clamping plates by using mandrels and assembly pit .after assembling one

packet thickness of silicon laminations, inner diameter of the core is checked as per the drawing

also the slot freeness is checked with inspection drift .There should not be any projections inside

or outside the slot. If all the conditions are satisfied the normal core assembly is carried out by

dismantling the trial packets.

NORMAL CORE ASSEMBLY

A. Stepped packed assembly:

Steeped packets are assembled from the clamping plate isolating each packet with ventilation

laminations up to 4 to 5 packets of thickness 10cms for an air cooled turbo generator of 120MW.

B. Normal packet assembly: Normal packet assembly is carried out using 0.5 mm silicon steel laminations up to

required thickness of 30mm by using mandrills and inspection drift after normal packet assembly

completion 1 layer of HGL laminations are placed and one layer of ventilation lamination are placed

and again normal packet assembly is carried as above. The thickness of each lamination is 0.5 mm

and the thickness of lamination separating the packets is about 1 mm. The lamination separating each

packet has strips of nonmagnetic material that are welded to provide radial ducts. The segments are

staggered from layer to layer so that a core of high mechanical strength and uniform permeability to

magnetic flux is obtained. Stacking mandrels and bolts are inserted into the windings slot bores

during stacking provide smooth slot walls.

In process pressings

To obtain the maximum compression and eliminate under setting during operation, the

laminations are hydraulically compressed and heated during the stacking procedure when certain

heights of stacks are reached.

The packets are assembled as above up to 800mm as above and 1 st pressing is carried

using hydraulic jacks up to 150kg/cm2 and the pressing is carried out for every 800mm and a pre final

pressing is done before the core length almost reach the actual core. Now the core is tested for the

design specifications and the compensation is done by adding or removing the packets.

Fitting of clamping bolts:

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The complete stack is kept under pressure and locked in the frame by means of clamping

bolts and pressure plates. The clamping bolts running through the core are made of nonmagnetic steel

and are insulated from the core and the pressure plates to prevent them from short circuiting the

laminations and allowing the flow of eddy currents.

The pressure is transmitted from the clamping plates to the core by clamping fingers. The

clamping fingers extend up to the ends of the teeth thus, ensuring a firm compression in the area of

the teeth. The stepped arrangement of the laminations at the core ends provides an efficient support

to tooth portion and in addition contributes to the reduction of stray load losses and local heating in

that area due to end leakage flux.

The clamping fingers are also made of non-magnetic steel to avoid eddy-current losses. After

compression and clamping of core the rectangular core key bars are inserted into the slots provided in

the back of the core and welded to the pressure plates. All key bars, except one, are insulated from

the core to provide the grounding of the core.

STATOR WINDING: (W.C.No 3216, STATOR WINDING SHOP)

Now we have a completed stator core. The next comes the winding. Winding is an important

consideration. Stator winding is the one, which induces e.m.f and supplies the load. Stator winding is

placed in the slots of stator core. Due to the advantages of generation and utilisation of 3-phase

power, three-phase winding is designed for generation. So number of slots must be a multiple of 3 (or

6 if two parallel circuits are required).

Generally Two layer lap winding, corded to about 5/6 pitch which practically eliminates 5th

and 7th harmonics from the flux wage or open circuit induced e.m.f wave is used.

The stator coil is made up of number of strips instead of single solid piece to reduce the skin

effect. The bundle of copper strips consolidated is called as stator bar. Hence stator winding involves

two stages

1. Construction of stator coils.

2. Stator coils assembly

1. Conducting material used in coil manufacturing:

Copper material is used to make the coils. This is because

i) Copper has high electrical conductivity with excellent mechanical properties

ii) Immunity from oxidation and corrosion

iii) It is highly malleable and ductile metal.

Basically there are three types of stator winding structures employed over the range from 1 KW to

1000 MW.

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1. Random wound stators.

2. Form-wound stators using multi turn coils.

3. Form-wound stators using Roebel bars.

Out of these, two types of coils are manufactured and used in BHEL, Hyderabad.

1) Diamond pulled multi-turn coil (full coiled).

2) Roebel bar (half-coil).

Generally in large capacity machines ROEBEL bars are used. These coils were constructed after

considering the skin effect losses. In the straight slot portion, the conductors or strips are transposed

by 360 degrees. The transposition is done to ensure that all the strips occupy equal length under

similar conditions of the flux. The transposition provides for a mutual neutralisation of the voltages

induced in the individual strips due to the slot cross field and ensures that no or only small circulating

currents exists in the bar interior. Transposition also reduced eddy current losses and helps in

obtaining uniform e.m.f.

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High purity (99%) copper conductors/strips are used to make the coils. This results in high

strength properties at higher temperatures so that deformations due to the thermal stresses are

eliminated.

Considerations in coil manufacturing and assembly:

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Slot Discharges:

Slot discharges occur if there are gaps within the slot between the surface of the insulation

and that of the core. This may cause ionisation of the air in the gap, due to breakdown of the air at the

instances of voltage distribution between the copper conductor and the iron.

Within the slots, the outer surface of the conductor insulation is at earth potential, in the

overhanging it will approach more nearly to the potential of the enclosed copper. Surface discharge

will take place if the potential gradient at the transition from slot to overhang is excessive, and it is

usually necessary to introduce voltage grading by means of a semi-conducting (graphite) surface

layer, extending a short distance outward from the slot ends.

So insulation of these stator bars is an inevitable task. It is worth now to know about

insulation.

Till now we have discussed the manufacturing process, but the manufacture is incomplete without

insulation design.

Need for insulation?

In Electrical Machines insulation is most important requirement to sustain high voltages. They

are quite a few reasons for the insulation, one of which has already been discussed earlier, the other

being that in a generator there are inevitable losses, due to which there would be high amounts of

temperature ,which in turn hampers life of the generator , because generator once installed in a power

system should serve for decades together without any considerable damage .So hence the study of

various types of losses in a generator is the foremost consideration for an insulation designer.

Various losses in a Generator:

In generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the generator, are considered as losses and may be defined as follows:

1. Copper loss in the winding.

2. Magnetic Losses.

3. Mechanical Losses

Copper loss:

The power lost in the form of heat in the armature winding of a generator is known as Copper loss. Heat is generated any time current flows in a conductor.

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

Magnetic Losses (also known as iron or core losses)(i) Hysteresis loss (Wh)

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

(ii) Eddy Current Loss (We):The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss.

Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Figure, view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to cross-sectional area.) The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core.

As you can see, eddy current losses are kept low when the core material is made up of many thin

sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The

laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by

the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are

being annealed. The insulation value need not be high because the voltages induced are very small.

Most generators use armatures with laminated cores to reduce eddy current losses.

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These magnetic losses are practically constant for shunt and compound-wound generators, because in

their case, field current is constant.

Mechanical or Rotational Losses:

These consist of

(i) friction loss at bearings.

(ii) Air-friction or windage loss of rotating rotor armature.

These are about 10 to 20% of F.L losses.

Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean

bearings and proper lubrication are essential to the reduction of bearing friction. Brush friction is

reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush

tension.

Usually, magnetic and mechanical losses are collectively known as Stray Losses. These are also

known as rotational losses for obvious reasons.

As mentioned above, these losses are responsible for the rise in temperature of the generator

body hence an appropriate insulation should be used. Also the insulation should withstand the

generator voltage and currents. So an insulation whose breakdown voltage is of 5 to 6 times the

normal voltage is taken as Safety factor.

INSULATING MATERIALS:Insulating materials or insulators are extremely diverse in origin and properties. They are

essentially non-metallic, are organic or inorganic, uniform or heterogeneous in composition, natural

or synthetic. Many of them are of natural origin as, for example, paper, cloth, paraffin wax and

natural resins. Wide use is made of many inorganic insulating materials such as glass, ceramics and

mica. Many of the insulating materials are man-made products and manufactured in the form of

resins, insulating films etc., in recent years wide use is made of new materials whose composition and

organic substances. These are the synthetic Organo-silicon compounds, generally termed as silicones.

A good insulating material needs the following properties.

1. The basic function of insulation is to provide insulation live wire to live wire or to the earth.

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

3. It should withstand the designed mechanical stress.

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

5. An insulator should satisfy the following properties for an electrical system are

An ideal insulating material should have:

(1) High dielectric strength sustained at elevated temperatures.(2) High receptivity or specific resistance(3) Low dielectric hysterics(4) Good thermal conductivity(5) High degree of thermal stability i.e. it should not determine at high temperatures. (6) Low dissipation factor

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(7) Should be resistant to oils and liquid, gas flames, acids and alkalis.(8) Should be resistant to thermal and chemical deterioration.

CLASSIFICATION OF INSULATING MATERIAL:

The insulating material can be classified in the following two ways.

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

Classification according to substance and materials:

1. Solids (Inorganic and organic)

EX: Mica, wood slate, glass, porcelain, rubber, cotton, silks, rayon, ethylene, paper and cellulose

materials etc.

1. Liquids (oils and varnishes)

EX: linseed oil, refined hydrocarbon minerals oils sprits and synthetic varnishes etc.

2. Gases

EX: Dry air, carbon dioxide, nitrogen etc.

CLASSIFICATION ACCORDING TO TEMPERATURE:

Class Permissible temperature

Materials

Y 90 Cotton, silk, paper, cellulose, wood etc neither impregnated nor immersed in oil. These are unsuitable for electrical machine and apparatus as they deteriorate rapidly and are extremely hygroscopic.

A 105 Cotton, silk & paper, natural resins, cellulose esters, laminated wool, varnished paper.

E 120 Synthetic material of cellulose base B 130 Mica, asbestos, glass fiber with suitable bonding substance F 155 Material of class B with binding material of higher thermal

stability. H 180 Glass fiber and asbestos material and built up mica with

silicon resins. C Above

180Mica, porcelain, quartz, glass (without any bonding agent) with silicon resins of higher thermal stability.

INSUL ATING MATERIAL FOR MACHINES:

Name of Material

InsulationClass

Shelf life(In months) ApplicationAt

20 oCAt 5oc

1. Samicatherm calmica glass-n, mimica, domica, folium, filamic novobond-s, epoxy therm laxman isola

F 6 12 Main insulation of stator bars

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calmicaflex2. Samica flex H 4 8 Overhang insulation of

motor coils, at 3rd bends of multi turn coil

3. Vectro asbestos (365.02/365.32)

4. (used in resin rich)

B/F 2 8 Main pole coils of synchronous machines

5. Epoxide pepreg glasscloth

F 6 12 Winding holders and interhalf insulation

6. Polyester resin mat&rope

6 Bar to winding holder&stiffner groove of support segment of clamping plate

7. GlassoflexTurbo laminate

F 6 12 Interturn insulation of rotor winding

8. Hyper seal tape F 6 12 As finishing layer in overhangs of motor coils

9. SIB775 or 4302 varnish F 6 12 Stack Consolidation of stator bars

10. SIB475 or 4301 varnish F 6 12 Base coat varnish before taping of stator bars

11. SIB 643 or8003 Varnish or K8886 varnish

4 8 Conductive coat in straight portion of stator bars

12. SIB 642 or 8001 varnish 4 8 At slot emerge portion on stator bars

I. ELECTRIAL PROPERTIES:

1. INSULATON RESISTANCE:

It may be defined as the resistance between two conductors usually separated by insulating

materials. It is the total resistance in respect of two parallel paths, one through the body and

other over the surface of the body.

2. DIELECTRIC STRENGTH:

The voltage across the insulating material is increased slowly the way in which the leakage

current increases depend upon the nature and condition material.

3. POWER FACTOR:

Power factor is a measure of the power losses in the insulation and should be low. It varies

with the temperature of the insulation. A rapid increase indicates danger.

4. DIELECTRIC CONSTANT:

This property is defined as the ratio of the electric flux density in the material .To that

produced in free space by the same electric force.

5. DIELECTRIC LOSS:

The dielectric losses occur in all solid and liquid dielectric due to

(a) Conduction current

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(b) Hysterisis.

II. THERMAL PROPERTIES:

1. Specific heat

2. Thermal conductivity.

3. Thermal plasticity4. Ignitability5. Softening point6. Heat Aging7. Thermal expansion.

III. CHEMICAL PROPERTIES:

1. Resistance to external chemical effects2. Resistance to chemical in soils3. Effect of water.

IV. MECHANICAL PROPERTIES:

1. Density 2. Viscosity3. Moisture absorption4. Hardness of surface5. Surface tension6. Uniformity.

EFFCT OF MOISTURE ON INSULATION:

Thermal propertyChemical propertyElectrical propertyPhysical and mechanical property.

INSULATION RESISTANCE IS EFFECTED BY THE FOLLOWING FACTOR (Resistance between two conductor):

1) It falls with every increase in temperature.

2) The sensitivity of the insulation is considerable in the presence of moisture.

3) Insulation resistance decrease with increase in applied voltage.

EPOXY RESINS:

These resins are product of alkaline condensed of epichlorohydrin and product of alkaline

condensed of epichlorohydrin and polyhydric compounds.

PROPERTIES:

17

1) Epoxy resins have good mechanical strength less shrinkage and excellent dimensional stable after casting.

2) Chemical resistance is high.

3) Good adhesion to metals.

4) To impact hardness certain organic acid anhydrides and alphabetic amines are mixed.

18

APPLICATION:

1) They are used in the manufacture of laminated insulating boards.2) Dimensional stability prevents crack formation in castings.3) They are also used as insulating varnishes.

EPOXY RESINS:

Epoxy resins are poly ethers derived from epi-chlorohydrin and Bis-phenol monomers through condensation polymerization process.

In epoxy resins cross-linking is produced by cure reactions. The liquid polymer having reactive functional group like oil etc, otherwise vacuum as pre polymer. The pre polymer of epoxy resins allowed to react curing agents of low inductor weights such as polyamines, polyamides, polysulphides, phenol, urea formaldehyde, acids anhydrides etc, to produce the three dimensional cross linked structures.

Hence epoxy resins exhibit outstanding toughness, chemical inertness and excellent mechanical and thermal shock resistance. They also posses good adhesion property. Epoxy resins can be used continuously up to 300F, but withy special addition can withstand a temperature of up to 500F.

Epoxy resins are made use as an efficient coating material. This includes coating of tanks containing chemicals, coating for corrosion and abrasion resistant containers. Epoxy resins are made up of as attractive corrosion and wear resistant floor ware finishes.

These are also used as industrial flooring material. They are also used as highways Surfacing and patching material. Molding compounds of epoxy resins such as pipe fitting electrical components bobbins for coil winding and components of tooling industrial finds greater application in industries.

The epoxy resins similar to polyester resins can be laminated and fiber reinforced (FPR) and used in glass fiber boats, lightweight helicopters and aeroplanes parts.

In the modern electronic industry, the application of epoxy resins is great. Potting and encapsulation (coating with plastic resin) is used for electronic parts. Most of the printed circuits bodies are made of lamination epoxy resin which light but strong and tough.

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INSULATING MATERIAL FOR LAMINATIONS: -

The core stacks in modem machines are subjected to high pressers during assembly and

subjected to high pressures during assembly and there fore to avoid metal-to-metal contact,

laminations must be well insulated. The main requirements of good lamination insulation are

homogeneously in thin layers toughness and high receptivity.

We use varnish as insulating material for laminations.

VARNISH

This is most effective type of insulation now available. It makes the laminations nest proofs

and is not effected by the temperature produced in electrical machines varnish is usually applied to

both sides of lamination to a thickness of about 0.006mm. On plates of 0.35mm thickness varnish

gives a stacking factor about 0.95.In order to achieve good insulation properties the following

processes are in BHEL.

THERMOPLASTIC PROCESS OF INSULATION THERMOSETTING PROCESS OF INSULATION

BHEL is practicing only thermosetting process of insulation so

Thermosetting types of insulation are of two types:

RESIN RICH SYSTEM OF INSULATION RESIN POOR SYSTEM OF INSULATION

MATERIAL FOR RESIN RICH BARS:

Preprag Nomex Epoxy resin rich mica tape Glass tape PTFE tape

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VARNISH

Mica powder Graphite powder Conductive varnish Semiconductor varnish

MATERIAL FOR RESIN POOR HALF BARS

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

VARNISH

Polyester glass tape Rutapox Hardener (H-90)

MATERIAL FOR RESIN POOR DIAMOND COILS

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

RESIN RICH SYSTEM:

In olden days, Resin Rich system of insulation is used for all Electrical

Machines. In insulator contains nearly 40% of EPOXY RESIN, so it gives good thermal stability

Resin Rich Insulation consists of the following materials in percentage

1. MICA PAPER TAPE -40-50%

2. GLASS PAPER TAPE-20%

3. EPOXY RESIN-40%

The bars are insulated (or) taped with RESIN RICH TAPE and place in the Pre-assembled

stator core including stator frame.

In resin rich system of insulation Mica paper will give a good dielectric strength and Glass fiber tape

will give a good mechanical strength and Epoxy resin can withstand up to 155 degree Centigrade so it

gives a good thermal properties. Resin rich and Resin poor insulating materials are

21

characterized by the contact of the Epoxy Resin. In Resin rich system the content of Epoxy Resin

tape is 40% so it is named as RESIN RICH SYSTEM, and in Resin poor system the content of Resin

tape is 8%. By VIP impregnation process, the required amount is added to then conductor bars after

assembling the core and placing the winding in the core. In resin rich system before placing of coils

in the stator slots the rich tape will be wrapped over the bars. Nevertheless, this system has the

following disadvantages:

1. This system is very time consuming and very long procedure.

2. Total cost of the system is more.

In order to minimize the over all cost of the machine and to reduce the time cycle of the system, the

VACUUM PRESSURE IMPREGNATION SYSTEM is being widely used. This process is very

simple, less time consuming and lower cost.

BHEL, HYDERABAD is equipped with the state of the art technology of VACUUM

PRESSURE IMPREGNATION.

Resin Impregnation:

Resin impregnation fills the porosity of a part with a resin to create a pressure-tight part for

hydraulic applications which can withstand several thousand psi, to improve machine ability, or to

allow electroplating. The parts are placed in a mesh basket and loaded into a vacuum tank. This is

then submerged in a bath of Anaerobic resin. A vacuum is pulled to remove all air from the porosity

of the parts. This vacuum is released to and the tank is pressurised, causing the resin to be drawn into

the porosity of the parts. Parts that typically undergo resin impregnation include hydraulic fittings for

pressure tightness and plating, covers and plated for pressure tightness, as well as machined

components.

The previous method of sealing parts was a furnace treatment, which formed a hard oxide layer on

the internal and external surfaces of a part, filling the porosity. Most machining operations were

performed prior to sealing the part because the hard oxide layer adversely affected mach inability.

Residue left by traditional cutting fluids tended to inhibit the formation of an oxide layer. With resin

impregnation, conventional cutting fluids can be used because the furnace treatment is eliminated

resulting in improved mach inability. These fluids efficiently remove heat from the cutting tool,

extending the tool life. Machining a porous part effectively creates a continuous interrupted cut.

Each time the tool impacts metal after passing through a pore, it may chip and become dull. Resin

impregnation reduces that effect and may also provide added lubrication to the cutting tool. Before

resin impregnation, many parts were mechanically plated. Resin impregnation allows the use of

electroplating.

See here resin management

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The core or coil building and assembling method depends on the insulation system used.

1. For Resin rich insulation system the laminations are stacked in the frame itself.

2. For Resin poor insulation system (VPI) cage core of open core design is employed.

Thus the manufacturing of coils also is of two types as explained above for core.

a. For resin poor process

b. For resin rich process

MANUFACTURE OF STATOR COILS: (W.C.No 3215, COIL SHOP):

Manufacturing of stator coils depends on the type of the insulation process used for the stator. I.e. the process is different for resin rich and resin poor process although few of the sub processes are same for both.

A.) For resin poor process:

In this process the high voltage insulation is provided according to the resin poor mica base of

thermosetting epoxy system. Several half overlapped continuous layers of resin poor mica tape

are applied over the bars. The thickness of the tape depends on the machine voltage.

i. Reception of copper conductors:The copper conductors rolls are received is checked for physical and mechanical properties. First

piece is checked for specifications such as length and if found satisfactory, mass cutting to desired length is carried out by feeding into the cutting mills.

ii. Transposition:

Conductors are adjusted one over another for a given template and the bundles are

transposed by 360 degrees by setting the press for “Roebel Transposition”. Now they are bundled and

consolidated by tying with cutter tape at various places. Similarly all the bundles are processed. Thus

each stator bundle has a transposed coils in each phase such that the flux distribution is equal and

hence the induced e.m.f.

iii. Putty operation:

All the transposed bars are shifted to putty operation. Here a single bar is taken for

putty operation by filling up the uneven surfaces on the width face by filling with

NOMAX. I.e., NOMAX sheets are inserted in the crossovers on the width face to the both

ends. Form mica net is placed over the width face of the bar on both sides & wrapped with

PTFE (poly tetra flamo ethane) tape.

iv. Stack consolidation:

Now 2 to 3 bars are inserted into hydraulic presser and they are pressed horizontally

and vertically to a pressure up to 150kg/cm2. At the same time the bars are subjected o

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heating from 140 to 160 degrees for duration of 2-3 hrs. Then the bars are unloaded and

clamped perfectly. Now inter half and inter strip testing is carried out and the dimensions

are checked using a gauge.

v. Bending:

Each of the samples is placed over the universal former & the universal former is

aligned to the specifications. The bar is bent on both the sides i.e. on turbine side (TS) and

exciter side (ES).the 1st bend and the 2nd bend is carried out and continued by over hang

formation. Now the 3rd bend is carried by inserting nomax sheet from the end of straight

part to the end of 3rd bend and the bars are clamped tightly. Now the clamps are heated to

60 degrees for 30mins. Inter half and inter strip tests follows.

vi. Final taping:

The taping may be machine or manual taping and the taping is done according to the

type of insulation used. In case of resin poor system, resin poor tape is wrapped by 9*1/2 over lap

in the straight portion up to overhang and 6*1/2 over lap layers in the intermittent layers. The

intermittent layers are follows….

1st intermittent layer is ICP (internal corona protection) tape. This is wrapped by

butting only in straight portion.

2nd is split mica tape. One layer of split mica is wrapped by butting & using conductive

tape at the bottom so that split mica is not overlapped.

Next layer is O.C.P (outer corona protection). OCP tape is wrapped final in straight

portion by but joint up to end of straight portion on both the sides.

Next intermittent layer is ECP (end corona protection). ECP tape is wrapped from the

end of straight portion up to over hang over a length of 90-110mm.

Now the bars are wrapped finally with hyper seal tape from straight portion to the end

of 3rd bend in overlapping layers for protecting the layers from anti fingering. The IH & IS tests

follows and the bars are discharged to the stator winding.

2. For Resin rich system:

The coil manufacture is same as in case of resin poor but differ in a few stages. The

Conductor cutting and material used is same as resin poor system. Transposition is done same as that

of resin poor system. Stacking of coils is done. In this case high resin glass cloth is used for

preventing inter half shorts. There is a difference in putty work.

Putty work:

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Nomex is used in between transposition pieces. 775 varnish is applied over the straight portion of bar

and mica putty is applied on the width faces of the bars.

Mica Putty mixture is a composition of SIB 775 Varnish, mica powder and china clay in the ratio of

100:50:25.

Straight part baking is done for 1hour at a temperature of 160C and a pressure of 150kg/sq.cm.Then

bending and forming is done. Half taping with resin rich tape is done for over hangs and reshaping is

done. To ensure no short circuits half testing of coils is done.

Final taping:

Initial taping and final tapings is done with resin rich tape (semica therm tape) to about 13-14

layers. The main insulation layers are 12*1/2 overlap in the straight portion and 9 layers in the

overhang.

Figure 2: Layout of a mould used in baking of stator by Resin rich process

Final baking:

Final baking is done for 3hrs at a temperature of 160C in cone furnace. The bar is fed into the

baking mould.

The bar is heated for 1 hr at 90 degree to get gelling state.

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The temperature of the mould is increased to 110 degrees in 30 mins and simultaneously the

moulds are tightened. Now in this process 155 of the resin is oozed out only 25% will be

remain. Now the bar is unloaded and checked for final dimensions, sharp corners, depressions,

charring, hollow sounds etc.,

Gauge suiting is done. I.e. the dimensions are made to compromising with the design.

Conductive/graphite coating (643) is applied on the straight portion and semi-conductive

coating 642 from end of straight portion to 3rd bend to pre transition coating on both sides.1st

coating for 90mm, 2nd coating for 100mm and 3rd coating for 120mm on both sides.

The bar is allowed for drying and epoxy red gel is applied from the end of straight portion to

the 3rd bend on both sides and allow for drying.

High voltage testing is done at 4 times that of rated voltage and tan testing, inter strip, inter

half testing are done. Tan values must be less than 2%.

Let us have a overview about the various materials used in resin rich and resin poor processes.

Materials used in resin poor system:

S.NO. Material Size Composition supplier

1. Epoxy glass cloth Isola

2. Nomex fleece Isola

3. Fine mica polyester

glass cloth

Isola

4. Nomex Isola

5. Fom micanite Isola

6. Fine mica tape

(KUN 561.44)40*0.15mm

Isola

7. Copper foil Isola

8. Polyester fleece tape

with graphite for ICP

Isola

9. Polyester fleece tape

with graphite for OCP

Isola

10. Polyester fleece tape

with Silicon Carbide

Isola

11. Mica splitting tape 0.18mm*40mm*

50m

Isola

12. Polyester glass tape Isola

ADVANTAGES OF RESIN POOR SYSTEM OF INSULATION:

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It has better dielectric strength

Heat transfer coefficient is much better

Maintenance free and core and frame are independent

It gives better capacitance resulting in less dielectric losses due to which the insulation life will be

more

The cost will be less and it is latest technology

Reduction in time cycle and consumption for MW also less and it gives high quality

DISADVANTAGES OF RESIN POOR SYSTEM OF INSULATION:

If any short circuit is noticed, the repairing process is difficult and need of excess resin from

outside.

Dependability for basic insulating material on foreign supply

ADVANTAGES OF RESIN RICH SYSTEM OF INSULATION:

Better quality and reliability is obtained

In case of any fault (phase - ground/ phase – phase short) carrying the repair process is very easy.

Addition of excess resin will be avoided because of using resin rich mica tape

DISADVANTAGES OF RESIN RICH SYSTEM OF INSULATION:

It is a very long procedure

Due to fully manual oriented process, the cost is more

It is possible to process stator bars only.

Even though the advantages and disadvantages of both the process are explained above, resin

poor process is the best of all, as the resin content used is almost only 35% compared to resin poor

process and also show good insulation properties justified later.

2. Assembly of stator:

The completed core and the copper bars are brought to the assembly shop for

assembly.

Reception of stator core:

Stator core after the core assembly is checked for the availability of foreign matter, so coil

projections are checked in each slot. HGL gauge is passed in each and every slot to detect bottom

core projections.

Winding holder’s assembly:

Assemble all the winding holders on both sides by adapting to the required design size.

Check all the wedge holders by a template and they are assembled as per the design requirement.

27

Tighten all the bolts relevant to winding holders and lock them by tag welding. Assemble HGL rings

on both the sides by centring with respect to core. Subject each individual for pressing in pressing

fixture at a pressure of 60 kg/cm2 for 30 minutes. Inter half test is conducted for each individual bar

before assembling into the stator.

Now stator bar assembling is carried out by centring to the core and check for proper

seating of bottom bars with T-gauge and checked for third bend matching, over hang seating etc..,

rein force the overhang portion of stator bars by inserting glass mat in between the bars and tying

them with neoprene glass sleeve. This process is carried out for all respective bottom bars .now the

pitch matching is checked on both sides both the generator and the exciter side.

Now high voltage testing is carried out on the stator.

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

Eye formation: Join bottom conductors and top conductors forming an eye, by brazing the conductors with silver foil. Segregate eyes into two halves on both sides and test for inter half shorts. Insert Nomax into two halves and close them. Brazing makes the electrical connection between the top and bottom bars. One top bars strand each is brazed to one strand of associated bottom bar so that beginning of the strand is connected with out any electrical contact with the remaining strand. This connection offers the advantage of minimising three circulating currents.

Connecting rings assembly: The connecting rings are assembled on exciter side as per the drawing and connect all the connectors to the phase groovers by joining and brazing with silver foil. Clean each individual phase groove, insert nomax sheet and tape with semica folium. Subject the whole stator for HVDC test. Terminate the three RTD’s in the straight portion and the 3-RTD’s in the over hang portion on both turbine and exciter side except one for earthing requirement.

Phase connectors: The phase connectors consist of flat copper sections, the cross section of which results in a low specific current loading. The connections to the stator winding are of riveted and soldered tape and like wise wrapped with dry mica/glass fabric tapes. The phase connectors are firmly mounted on the winding support using clamping pieces and glass fabric tapes.

Thus the complete stator completed is sent for vpi process. The vpi process is a new technology

introduced in ……….. and the best for ever

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INTRODUCTION TO VACUUM PRESSURE IMPREGNATION SYSTEM (VPI)

HISTORY DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the year

1956. Vacuum Pressure Impregnation has been used for many years as a basic process for thorough

filling of all interstices in insulated components, especially high voltage stator coils and bars. Prior to

development of thermosetting resins, a widely used insulation system for 6.6kv and higher voltages

was a Vacuum Pressure Impregnation system based on bitumen bonded Mica flake tape is used as

main ground insulation. After applying the insulation coils or bars were placed in an autoclave,

vacuum dried and then impregnated with a high melting point bitumen compound. To allow thorough

impregnation, a low viscosity was essential. This was achieved by heating the bitumen to about 180C

at which temperature it was sufficiently liquid to pass through the layers of tape and fill the interstices

around the conductor stack. To assist penetration, the pressure in the autoclave was raised to 5 or 6

atmospheres. After appropriate curing and calibration, the coils or bars were wound and connected up

in the normal manner. These systems performed satisfactorily in service provided they were used in

their thermal limitations. In the late 1930’s and early 1940’s, however, many large units, principally

turbine generators, failed due to inherently weak thermoplastic nature of bitumen compound.

Failures were due to two types of problems:

3. Tape separation

4. Excessive relaxation of the main ground insulation.

Much development work was carried out to try to produce new insulation systems, which

didn’t exhibit these weaknesses. The first major new system to overcome these difficulties was

basically a fundamental improvement to the classic Vacuum Pressure Impregnation process. Coils

and bars were insulated with dry mica flake tapes, lightly bonded with synthetic resin and backed by

a thin layer of fibrous material. After taping, the bars or coils were vacuum dried and pressure

impregnated in polyester resin. Subsequently, the resin was converted by chemical action from a

liquid to a solid compound by curing at an appropriate temperature, e.g. 150 C. this so called

thermosetting process enable coils and bars to be made which didn’t relax subsequently when

operating at full service temperature. By building in some permanently flexible tapings at the evolutes

of diamond shaped coils, it was practicable to wind them without difficulty. Thereafter, normal slot

packing, wedging, connecting up and bracing procedures were carried out. Many manufacturers for

producing their large coils and bars have used various versions of this Vacuum Pressure Impregnation

29

procedure for almost 30 years. The main differences between systems have been in the types of

micaceous tapes used for main ground insulation and the composition of the impregnated resins.

Although the first system available was styrenated polyester, many developments have taken place

during the last two decades. Today, there are several different types of epoxy, epoxy-polyester and

polyester resin in common use. Choice of resin system and associated micaceous tape is a complex

problem for the machine manufacturer.

Although the classic Vacuum Pressure Impregnation technique has improved to a significant

extent, it is a modification to the basic process, which has brought about the greatest change in the

design and manufacture of medium-sized a.c. industrial machines. This is the global impregnation

process. Using this system, significant increases in reliability, reduction in manufacturing costs and

improved output can be achieved. Manufacture of coils follows the normal process except that the

ground insulation consists of low-bond micaceous tape. High-voltage coils have corona shields and

stress grading applied in the same way as for resin-rich coils, except that the materials must be

compatible with the Vacuum Pressure Impregnation process. Individual coils are inter turn and high-

potential-tested at voltages below those normally used for resin-rich coils because, at the un-

impregnated stage, the intrinsic electric strength is less than that which will be attained after

processing. Coils are wound into slots lined with firm but flexible sheet material. Care has to be taken

to ensure that the main ground insulation, which is relatively fragile, is not damaged. After inter-turn

testing of individual coils, the series joints are made and coils connected up into phase groups. All

insulation used in low-bond material, which will soak up resin during the impregnation process. End-

winding bracing is carried out with dry, or lightly treated, glass-and/or polyester-based tapes, cords

and ropes. On completion, the wound stator is placed in the Vacuum Pressure Impregnation tank,

vacuum-dried and pressure-impregnated with solventless synthetic resin. Finally, the completed unit

is stoved to thermo set all the resin in the coils and the associated bracing system.

After curing, stator windings are high-potential-tested to the same standard. Loss-tangent

measurements at voltage intervals up to line voltage are normally made on all stators for over 1kv. A

major difference between resin-rich and vacuum pressure impregnation lies in the importance of this

final loss-tangent test; it is an essential quality-control check to conform how well the impregnation

has been carried out. To interpret the results, the manufacturer needs to have a precise understanding

of the effect of the stress-grading system applied to the coils. Stress grading causes an increase in the

loss-tangent values. To calculate the real values of the ground insulation loss-tangent, it is necessary

to supply from the readings the effect of the stress grading. For grading materials based on the

materials such as silicon carbide loaded tape or varnish, this additional loss depends, to a large extent

upon the stator core length and machine voltage.

VPI is a process, which is a step above the conventional vacuum system. VPI includes

pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is

30

improved mechanical strength and electrical properties. With the improved penetration, a void free

coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution,

the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the

average rise.

In order to minimise the overall cost of the machine & to reduce the time cycle of the

insulation system vacuum pressure Impregnated System is used. The stator coils are taped with

porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator

is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin

bath under pressure of Nitrogen gas.

Vacuum Pressure Impregnation of resin poor insulated jobs:In the process of manufacturing of the stator the stator core had under go several in-process

modifications there fore there may be chances of damage due to insulation due to plastic hammer

used in the wedging process. the insulation layer used also may not be uniform and the presence of

voids due to solid insulation may adversely affect the insulation of the generator. So the stator after

completion of resin poor process

the completed stator is sent for VPI process. VPI process is done in the vpi chamber. For

higher capacity stators of steam turbine or gas turbine generator stators, horizontal chamber is used

where as vertical chamber is used for smaller capacity systems such as permanent magnet generator,

coil insulation of small pumps and armature of motors etc..,

Variant Description

01 Brushless exciter armature, PMG stators and Laminated rotors

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02 Stator wound with diamond pulled coils.

03 Stator with half coils

Vpi process for a stator involves the following stages.

1. preheating

2. vacuum cycle

3. vacuum drop test

4. heating the resin

5. Resin admission.

6. pressure cycle

7. post curing cycle

8. performance tests

Variant-01 Variant-02 Variant-03 Any other information

Preheating 605C for 3hrs

605C for 12hrs 603C for 12hrs

Vacuum to be maintained

0.4mbar 0.2mbar/0.4mbar <0.2mbar(both together shall not exceed 50hrs including rising time)

Vacuum heating time 3hrs

0.2mbar for 9hrs 0.4mbar for 17hrs

Stopping vacuum pumps for 10min shall check 17hrs vacuum drop. The vacuum drop shall not exceed by 0.06mbar for 10min

Increase in pressure

40min 80min 80min

Maximum pressure

3bar 4bar 4bar

Pressure holding

3hrs 3hrs 3hrs

32

Post curing At1405C for 14hrs

At1405C for 32hrs

At1405C for 32hrs

Process: 1. General:

The jobs that are entering tank for Vacuum Pressurised Impregnation shall not have any oil

based coatings. Any such, rust preventive/ corrosion preventive viz., red oxide etc., shall be

eliminated into the tank.

The jobs shall be protected with polyethylene sheet for preventing dust or dirt on jobs, till it is

taken up for impregnation.

Resin in the storage tank shall be stored at 10 to 12C and measured for its viscosity, viscosity

rise.

Proper functioning of the impregnation plant and curing oven are to be checked by production

and cleared for taking up of job for impregnation

2. Pre heating: the completed stator is placed in the impregnation vessel and kept in an oven for a

period of 12 hors at a temperature of 60 deg. Six thermocouples are inserted at the back of the

core to measure the temperature. The temperature should not exceed to 85 deg. smaller stator can

be inserted directly into the impregnation chamber.

RTD elements placed on the job and the readings are logged by production. The time of entry into

The job is to be loaded in the curing oven and heated. The temperature is to be monitored by the

the oven, time of taking out and the temperature maintained are to be noted. Depending on

convenience of production the jobs can be preheated in impregnation tank by placing them in

tubs.

The impregnation tubs used for impregnation of jobs are to be heated in the impregnated tank

itself, when the jobs are preheated in the curing oven.

1) Vacuum cycle: the pre heated job will be placed in the impregnation chamber by a hydraulic

mechanism. The vessels are kept clean, resin available in the vessel is wiped out. Methylene and

traces of resin are not be allowed on the inner side of the tank

3. Impregnation:

Job insertion into preheated tub and insertion into tank

By the time, the preheating of job is completed, it is to be planned in such a way that the

heating of tub and tank heating matches with the job. This is applicable when the job is heated in the

33

curing oven separately. The preheated job is to be transferred into the tub by crane handling the job

safely and carefully with out damage to the green hot insulation.

Insertion of tub with job into the impregnation tank

The warm tub with job is inserted into impregnation tank by sliding on railing, in case of

horizontal tank. The thermometer elements are to be placed at different places on the job. The

connection for inlet resin is to be made for collection of resin into tub. After ensuring all these the lid

of the impregnation tank is closed. In case of vertical tank the job along with tub is slinged and

inserted carefully into impregnation tank without damage to insulation.

Drying the job in vacuum

The job is to be dried under vacuum. Drain out the condensed moisture/ water at the exhausts

of vacuum pumps for efficient and fast vacuum creation. Also check for oil replacement at pumps in

case of delay in achieving desired vacuum.

Heating the resin in the storage tank

The completion of operations of drying and the heating of the resin in the storage tank

are to be synchronised. The heating of resin in the tank and pipeline is to be maintained as at

preheating temperature.

Admission of resin into impregnation tank

The resin is allowed into the impregnation tank tub if required from various storage tanks

one after the other up to a level of 100mm above the job generally, after which the resin admission is

stopped. After 10mins of resin settling the tank is to be pressurised by nitrogen. While admitting resin

from storage tanks pressurise to minimum so that nitrogen will not affect resin to spill over in tank.

Pressurising/gelling

The pressure cycle is to be maintained.

Withdrawal of resin from impregnation tank to storage tank

The resin that is pressurised as per pressure cycle by which the opening of relevant valves will

allow the resin to come back to the storage tank. The job also shall be allowed for dripping of residue

of resin for about 10min. After dripping, withdrawal of resin in various storage tanks is to be carried

out.

Taking out the tub with job from impregnation tank

The lid is then opened after taking precautions of wearing mask and gloves for the operating

personnel as a protection from fumes. The job is withdrawn from impregnation tank by sliding on

railing for horizontal and slinging on to crane for vertical impregnation tanks.

4. Post curing: The job is post heated. The time for rising from job temperature to this

temperature as per relevant annexure. The time at which the heating is started, achieved and

maintained is to be logged.

5. Electrical testing:

34

All jobs that are impregnated till above process are to be tested for electrical tests. After

ensuring that all the temperature/vacuum conditions stipulated for drying, impregnation and curing

operations have been properly followed, the job is to be released for this operation.

Global processing:

Processing details depends very much on the machine type, on customer’s defined parameters and

type of mica tapes.

Generally the VPI system is used in impregnation vessels up to 30T where the rotor/stator are

impregnated at elevated temperatures. Machine parts usually are preheated (also under vacuum) in

order to remove moisture and to reduce viscosity during impregnation.

Resin management:

After impregnation the VPI bath is pumped into storage tanks and cooled down to 5-10C and

should be stored in dry conditions in order to obtain a long bath life. Actual bath life depends on

additional parameters, e.g., impregnation temperature and duration of impregnation, impurities in the

bath, wash-out of catalyst from mica tapes into the un- accelerated resin system (B), replenishment

rate, moisture exposure etc,. The viscosity of the bath should be checked periodically in order to

maintain a suitable viscosity for impregnation.

Impregnated, yet uncured machine parts in unconditioned atmosphere may pickup moisture.

Therefore curing directly after impregnation or storage in moisture controlled area is recommended.

Generally machine parts are rotated when removed from the bath and during the first part of curing in

order to avoid drip off.

Evaporation of hardener during the vacuum cycle leads to a change in the resin/hardener ratio in the

bath and has to be compensated. Therefore replenishment is mixing ratios of 100-120pbw of hardener

HY 1102 per 100pbw MY 790-1 are generally used. Replenishment mixing ratios depend on actual

processing parameters and conditions and have to be evaluated at the customer site.

Due to excellent latency of the system (A) MY 790-1/HY 1102/DY 9577/DY073 the replenishment

volume to maintain a constant viscosity is comparatively small, even if impregnation is performed at

40-50C.

On single coils and roebel-bars the mica insulation is normally covered with a tight glass tape to

prevent drainage of the impregnation resin.

Specific Instructions:

Depending on the insulation materials and the accelerating agent in use, a ramped curing

schedule is recommended.

35

In systems with high reactivity, where the accelerator can be include in the mica-tape, a fast

gelation can be obtain with a temperature-shock, and draining can so be reduced or avoided.

Standard curing with the standard accelerated mixture (system A) is:

12 h at 90C plus 18 h at 140C

Precaution:

To determine whether cross linking has been carried to completion and the final properties are

optimal, it is necessary to carry out relevant measurements on the actual object or to measure

the glass transition temperature. Different gelling and cure cycles in the manufacturing

process could lead to a different cross linking and glass transition temperature respectively.

Features and Benefits:

• State-of-the-art process for completely penetrating air pockets in winding insulation.

• Increases voltage breakdown level. (Even under water!)

• Proven submergence duty system

• Improved heat transfer- windings are cooler, efficiency is improved.

• Improves resistance to moisture and chemicals.

• Increases mechanical resistance to winding surges.

FACILITIES AVAILABLE IN VPI PLANT IN BHEL:

The major facilities available in VPI plant are:

Steam furnace for preheating

Size of chamber: 2 * 2 * 6.5 M

Maximum temperature: 160C

Electrical power consumption: 75KW

Work place: 1425

36

Work centre: 3215

Stream inlet: 200-250C

Impregnated tubs for keeping jobs

For vertical impregnation: As per respective tech. Document.

For horizontal impregnation: As per respective tech. Document.

Specifications of plant:

Impregnation medium

(a) Epoxy resin (class F solvent free) and hardener mix in 1:1 ratio as per TG34967

(b) Epoxy resin (class F solvent free) and hardener mix in 1:1 ratio as per TG34931

Horizontal impregnation chamber

Diameter: 4000mm

Cylindrical length: 9000 mm

Operating over pressure: 6 bar

Operating vacuum: 0.15 mbar

Operating temperature: 90C

Loading weight of impregnation object: maximum of 120 tonnes

Maximum leakage rate: less than 1mbar/lit/sec.

Moving load: 140 tonnes.

Static load: 170 tonnes

Pressure medium for impregnation

Pressure medium: dry nitrogen

Operating pressure: 6 bar.

Nitrogen storage capacity: 52cubic meter at 25 bar.

Resin storage capacity

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

Operating parameters of each tank

Operating vacuum: 0.5mbar

Operating over pressure: 0.5bar

Operating temperature: 80C

Resin filters(stainless steel washable)

Filter fineness: 150microns

Output (maximum): 1000lits/min

Vacuum system

Root pumps: 2No.s, 5.5KW each

Suction capacity: 2000cubic meter/hr

Vacuum pumps(4No.s, 7.5KW each)

37

Suction capacity: 250 cubic meter/hr

This system is provided with separator filter with activated carbon filters, to protect the vacuum

pumps from resin and hardener vapours.

Refrigeration system

The resin inside the tanks has to be stored at 102C. this can be stored for indefinite period with

a brine chilling/refrigeration system.

The brine storage capacity: 1*25000L+1*26000L

Composition of brine: 40%Mono Ethylene glycol and 60%water

Heating and cooling system

The heating of resin in the storage tanks and the impregnation chamber is by circulating the

heated brine through the heat exchangers, to heat by saturated steam. The hot brine is cooled to

about 40C by circulating water through coolers and then the brine is chilled to -10C and stored

in the tanks.

Post heating of job

(a) Explosion proof steam drier and electrical heating superposed.

Size: 7*4.5*4.5M

Maximum weight of job: 80 tonnes

Maximum temperature: 150C

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

Size: 9*4.5*4.5M

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

Maximum temperature: 150C

ABOUT RESIN TYPE, MAINTANENCE AND STORAGE FACILITIES

Araldite Impregnating Resin System :

System: A B

Araldite MY 790-1 100 100pbw

Hardener HY 1102 90 90pbw

Accelerator DY 9577 0.16 --pbw

Accelerator DY 073 0.04 --pbw

38

Liquid, hot-curing impregnating system based on distilled Bis phenol-A resin and Anhydride

hardener with long pot life, high class transition temperature and excellent electrical and mechanical

end properties.

Applications:

Full-bath impregnation of motors and generators with medium and high power ratings and

impregnation of single conductors (Roebel bars) in combination with porous mica-tapes.

Processing method:

Vacuum pressure impregnation (VPI-process)

Properties:

Low viscosity and good impregnation capability

Solvent free system

Long pot-life due to high latency (in absence of tape catalyst)

Excellent mechanical and electrical end properties

Liquid, solvent-free, distilled Bis-A epoxy resin

Araldite MY 790-1:

Viscosity at 25C: 4000-6400mpa s

Epoxy content: 5.60-5.90equiv/kg

Chlorine content, hydrolysable: 0.05%

Density at 25C: 1.15-1.20g/cu.cm

Flash point: 200C

Vapour pressure at 20C: 1mpa

At 180C: 0.133pa

Liquid, unmodified anhydride-curing agent

Hardener HY 1102 :

Viscosity at 20C: 0.70mpa s

Density at 20C: 1.13-1.17g/cu.cm

Flash point: 145C

Vapour pressure at 20C: 0.3pa

Latent accelerator based on BCl3 (solid or liquid state)

Accelerator DY 9577:

Melting range: 26-35C

Density at 20C: 1.12-1.15g/cu.cm

Flash point: 118C

Vapour pressure at 20C: 0.01 pa

Latent accelerator based on amine complex

Accelerator DY 073:

Viscosity at 25C: 10-30mpa s

39

Density at 20C: 0.93-0.97g/cu.cm

Flash point: 82C

Vapour pressure at 20C: 6pa

Storage:

Store the components at 18-25C, in tightly sealed and dry, if possible, in original containers.

Under these conditions, the shelf life will correspond to the expiration date stated on the label. After

this date, the product may be processed only following reanalyses. Partly emptied containers should

be closed tightly immediately after use. For information on waste disposal and hazardous products of

decomposition in the event of fire refer to material data sheet.

DATA COLLECTION OF SAMPLES

During the project two jobs have been impregnated in VPI Plant, the data has been collected and

recorded in the project report.

INDO-BHARAT-II ROTOR

Preheating:

Indo Bharat II rotor is loaded for preheating in steam furnace on 30-5-2003 at 18:00hrs.

Date and Time RTD-I(C) RTD-II(C) Furnace air temperature

Remarks

Rotor temperature is

40

30.5.2003 19:00 32.0 30.0 45.6

reached to 603C at 2:00hrs on 31.5.2003 and it is maintained for 4 hrs i.e., up to 6:00 on 31.5.2003

30.5.2003 20:00 45.4 48.6 57.930.5.2003 21:00 49.9 50.9 63.430.5.2003 22:00 52.5 54.3 70.530.5.2003 23:00 53.3 55.1 73.4

30.5.2003 24:00 56.6 57.3 75.6

Rotor is switched to vac 140 tank at 7:00 hrs on 31.5.2003

31.5.2003 1:00 59.9 60.2 75.131.5.2003 2:00 62.4 63.9 77.031.5.2003 3:00 62.3 64.7 77.031.5.2003 4:00 63.3 64.1 75.031.5.2003 5:00 63.3 64.0 75.631.5.2003 6:00 63.1 63.7 75.6

Vacuum cycle:

Date and time Vacuum in

graph (mbar)Vacuum in

meter (mbar)

Job temperature

(C)Remarks

31.5.2003 7:00 -- -- 62.2

Vacuum pump started at 7:30 hrs on 31.5.2003.

31.5.2003 8:00 -- 3.0 61.531.5.2003 9:00 0.85 0.86 61.331.5.2003 10:00 0.54 0.55 61.131.5.2003 11:00 0.39 0.4 61.131.5.2003 12:00 0.38 0.4 61.131.5.2003 13:00 0.37 0.4 61.031.5.2003 14:00 0.36 0.39 61.0

RESIN CYCLE AND POST CURING CYCLE:

41

Date and time RTDI(C) RTDII(C)

Room

temperature

(C)

Remarks

31.5.2003 19:30 62.0 62.0 36.0 Resin tank 025 is heated for impregnation

31.5.2003 21:30 70.1 69.2 36.7

31.5.2003 23:30 86.2 81.6 36.7

1.6.2003 1:30 101.5 97.6 35.6 Resin admission started at 14:10 hrs on 31.5.2003

1.6.2003 3:30 116.2 113.1 34.8 Resin admission completed at 14:25 hrs on 31.5.2003

1.6.2003 5:30 129.6 125.7 33.8 Pressurisation started at 14:45 hrs on 31.5.2003

1.6.2003 7:30 137.6 133.2 33.2 Pressurisation completed at 15:30hrs on 31.5.2003

1.6.2003 9:30 145.7 140.2 36.5 Pressurisation hod up completed at 18:30hrs on 31.5.2003

1.6.2003 11:30 145.7 141.6 38 Resin withdrawal to storage tanks is from 18:30 to 18:45hrs on 31.5.2003

1.6.2003 13:30 144.7 143.4 42.8 Rotor loaded in gas furnace at 19:15hrs on 31.5.2003

1.6.2003 15:30 144.1 143.0 43.8 Rotor temperature is reached to 131.6 to 145.7C at 8:30hrs on 1.6.2003 and it is maintained for 14hrs i.e., up to 22:30hrs on 1.6.2003

1.6.2003 17:30 144.0 144.5 42.8

1.6.2003 19:30 143.0 143.0 41.0 Furnace is switched at 22:30hrs on 1.6.2003 and circulation fans are kept running till the job temperature is reached to 70C to 75C.

1.6.2003 21:30 142.8 142.6 40.8

42

INDO-BHARAT-II STATOR:

Preheating:

Indo-Bharat-II stator is loaded for preheating in steam furnace on 7-5-2003 at 23:30hrs.

Date and Time RTD-I(C) RTD-II(C) Furnace air

temperature(C)

Remarks

7.5.2003 23:30 36.3 36.1 Stator temperature is

reached to 60.5C to

62.9C(603C) at

7:30hrs on 8.5.2003

and it is maintained

for 12hrs i.e., up to

19:30hrs on

8.5.2003

8.5.2003 1:30 43.6 42.9

8.5.2003 3:30 52.0 51.74

8.5.2003 5:30 55.9 56.0

8.5.2003 7:30 60.5 62.9 Stator is loaded in

vac(140) tank at

21:00hrs on

8.5.2003

8.5.2003 9:30 61.3 62.9

8.5.2003 11:30 60.3 62.4

8.5.2003 13:30 60.3 62.6 Vac. Pump is started

at 2:30hrs on

9.5.2003

8.5.2003 15:30 62.5 62.9

8.5.2003 17:30 62.9 62.66

8.5.2003 19:30 62.4 62.1

43

Vacuum cycle:

Date and Time Vacuum in

graph (mbar)

Vacuum in

meter (mbar)

Job

temperature

(C)

Resin cycle

8.5.2003 22:00 -- -- 54.37 Resin tanks 025,102 are

heated for impregnation

8.5.2003 0:00 -- -- 54.89 Viscosity of resin at 60C

is 33CP

9.5.2003 2:00 -- -- 59.02 Viscosity after aging is

36.10CP

9.5.2003 3:30 0.65 0.65 61.6 9.5.2003 and 10.5.2003

9.5.2003 5:30 0.41 0.40 63.59 Resin admission started at

19:45hrs

9.5.2003 7:30 0.28 0.29 64.2 Resin admission

completed at 19:55hrs

9.5.2003 9:30 0.22 0.22 63.2 Pressurisation started at

20:00hrs

9.5.2003 11:30 0.19 0.19 62.3 Pressurisation of

4kg/sq.cm reached at

21:20hrs

9.5.2003 13:30 0.18 0.18 62.1 Pressurisation hold up for

3hrs is at 0:20hrs

9.5.2003 15:30 0.17 0.17 62.0 Resin withdrawn to

storage tanks is from

0:30hrs –1:00hrs

9.5.2003 17:30 0.14 0.14 61.8 Stator loaded in hot air

furnace from 1:00hrs –

1:30hrs on 10.5.2003

9.5.2003 19:30 0.14 0.14 61.3

44

Post curing:

Date and TimeESOH

15T

TSOH

06B

ESW

02

TSW

13Core

Room

temperatureRemarks

10.5.2003

1:30hrs70.0 76.4 62.4 62.5 63.4 33.1

10.5.2003

4:30hrs126.7 131.4 94.7 102.3 98.8 31.7

10.5.2003

7:30hrs144.3 154.1 125.4 134.5 126.1 31.6

10.5.2003

10:30hrs147.7 154.9 139.9 145.1 140.6 34.8

10.5.2003

13:30hrs137.6 144.4 139.3 141.6 140.7 38.0

10.5.2003

16:30hrs136.9 144.2 140.0 140.9 140.6 38.4

10.5.2003

19:30hrs140.2 143.6 140.1 140.7 140.2 37.2

10.5.2003

22:30hrs144.4 151.3 143.7 145.1 144.1 35.9

Job temp. is reached to

1405C i.e., from

136.2C to 145.6C at

9:30hrs on 10.5.2003 and

it is maintained for 32hrs

i.e. up to 17:30hrs on

11.5.2003.

11.5.2003

1:30hrs143.1 146.7 145.2 145.1 145.2 33.8

11.5.2003

4:30hrs144.3 151.0 143.6 144.0 144.7 31.1

11.5.2003

7:30hrs135.7 142.1 144.3 145.1 145.0 31.3

11.5.2003

10:30hrs135.0 135.7 135.1 135.0 135.8 34.8

11.5.2003

13:30hrs135.6 141.4 135.4 135.6 135.9 38.3

11.5.2003

17:30hrs148.0 149.2 142.8 142.2 142.1 39.8

Furnace is switched off at

17:30hrs on 11.5.2003

and circulation fans kept

running till the job

temperature is reached

from 70C- 75C

45

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

S.No. Description HV level HV in kv remarks

Stator winding

1. After laying and wedging of

coils

18.9/1’ RTD,IT test

2. After OH spacers and

forming eyes

18.03/3’ RTD,IT test

3. Before impregnation

17.5/1’ R, RTD test

4. After impregnation

26.0/1’ R, RTD, Tan, leakage

reactance test5. Customer

acceptance25.0/1’ Rotor winding

Rotor winding

1. After laying first coil

UT+1400 2.9 Pole drops

2. After laying second coil

UT+1250 2.75 Pole drops

3. After laying third coil

UT+1100 2.6 Pole drops

4. After laying fourth coil

UT+950 2.45 Pole drops

5. After laying fifth coil

UT+800 2.3 Pole drops

6. After laying sixth coil

UT+650 -- Pole drops

7. After all connections

-- -- R, Pole drops

8. After tech. rings assembly

-- 2.15

9. After bandage -- 2.0 R, Pole drops10. After

impregnation-- 1.9 R, Pole drops

11. After excitation cable assembly

-- 1.8 R, Pole drops

12. After balancing UT+200 1.7 R,Z with 50Hz

46

TESTING RESULTS OF INDO-BHARAT-II ROTOR

Customer name: INDO-BHARAT-II ROTOR

M/c rating: 10.8MW, 12kv, 1500rpm.Test: Z, R and H.V test.

Stage: after impregnation.

Ambient temperature: 35C

Ohmic resistance: 0.264 (rotor temperature was more)

Voltage( volts) Current(amps)215.0 0.5

367.5 1.0

523.0 1.5

High voltage test: IR value before H.V. test at 15”/60” -- 200/300 M H.V. applied at 1.9kv /1’ – withstood IR value after H.V. test at 15”/60” -- 200/300 M

INDO-BHARAT-II STATOR:

Customer name: INDO-BHARAT-II STATOR

M/c rating: 10.0MW, 12kv, 0.8pf, 650A, 1500rpm.

Test: H.V test.

Stage: after impregnation.

Ambient temperature: 36C

A PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M

B PHASE: IR value at 2.5kv

47

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

C PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M

INDO-BHARAT-II STATOR:

Customer name: INDO-BHARAT-II STATOR

M/c rating: 10.0MW, 12kv, 0.8pf, 650A, 1500rpm.

Test: H.V test-RTD measurement, resistance measurement.

Stage: After impregnation.

Ambient temperature: 36C

Excitation Side:

26 113.8 10 114.062 113.850 113.820 113.838 113.914 113.8

Turbine Side:

49 125.013 113.825 113.921 113.861 113.837 113.91 113.8

A-A --29.4mB-B -- 29.3mC-C -- 29.4m

48

COMPARISION BETWEEN RESIN POOR AND RESIN RICH SYSTEMS:

RESIN POOR SYSTEM RESIN RICH SYSTEM

1. The insulation tape used in this system

has 40% 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 while at

processing

6. Processing of bars along with stator and

with conductors and processing of exciter

Coils along with exciter is possible.

7. The cost of repair is more

8. The overall cost is less

compared to resin rich system.

1. The insulation tape used in this is 7% of

40% resin.

2. Same as in resin poor.

3. Further addition of resin is not required

from outside.

4. It is very long process and

time consuming while at processing stage.

5. Tests are being carried out Stage.

6. Processing of stator bars is

only possible in resin rich

systems.

7. Repairing work is easy.

8. The total cost in this process is more.

Applications:• All critical machines

• Equipment exposed to frequent surges/starting

• Harsh or moist environments

• Motors that run at service factor

DRAWBACKS OF VPI SYSTEM:

Number of RTD’s required are more

The whole operation is time consuming

It depends largely on moisture and season of operation

Maintenance of resin below room temperature about 8-12C is complicated.

49

SUGGESTIONS:

Processed in a Clean Room Environment To ensure optimum rewind integrity, all rewinds should be conducted in a clean,

temperature- and humidity-controlled environment. It ensures optimum material performance and

prevents dirt or moisture contamination during the process.

VPI Process Control Throughout the VPI process, each stator is continuously monitored by computer to ensure

homogenous fill.

CONCLUSION:

Hence Vacuum-Pressure Impregnation technology can be used in a wide range of applications from

insulating electrical coil windings to sealing porous metal castings. It normally produces better work

in less time and at a lower cost than other available procedures.

Our VPI systems can be configured in a variety of ways, depending on the size and form of the

product to be impregnated, the type of impregnant used and other production factors. System

packages include all necessary valves, gauges, instruments and piping. These systems can be large or

small, simple or highly sophisticated and equipped with manual, semi-automatic or automatic

controls.

Vacuum Pressure Impregnation (VPI) yields superior results with better insulating

properties, combined with “flexible” rigidity, resulting in greater overall reliability and longer life.

VPI reduces coil vibration by serving as an adhesive between coil wires, coil insulation, and by

bonding coils to their slots.

50