Project Done by Rastogi

8/9/2019 Project Done by Rastogi

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CERTIFICTE

This is to certify that MR. RAJAT RASTOGI student of

2ndyear ELECTRICAL & ELECTRONICS ENGG. Bearing roll

no. B050288ee of NATIONAL INSTITUTE OF

TECHNOLOGY, CALICUT has undergone project training

in the areas of MANUFACTURING OF TURBO

GENERATOR at BHEL, R.C.Puram, Hyd-32 under my

guidance from 4.5.07 to 3.6.07

His conduct is satisfactory.

DEBABRATA BALADy. MANAGEREM PRODUCTIONBHEL R C PURAM

HYDERABAD-32


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ABSTRACT

Today energy is the basic necessicity of our life like water and food.

Energy is needed in core sectors like industry, transportation, defense,

and telecommunication. Energy is found in many forms like electrical,

mechanical, thermal, nuclear and solar. Among these forms electrical

energy has its pivotal role because of its flexibility to be converted into

other forms.

For generating the electrical energy alternators are needed. The alternators

convert the mechanical energy to electrical energy. Alternators have mainly

two parts (1) Rotor (2) Stator. It works on the principle of Faradays law. D.C.

current is given to the rotor winding due to which it produces magnetic field.

There is a relative motion between the magnetic field and stator conductors,

due to which an e.m.f. is induced in stator winding. If a load is connected to

the terminals of stator winding, there will be current flow.

It is quite possible for an individual to operate an alternator that is adequate

for his loads but in the most populated areas, it is more economical and

convenient to have a utility company that generates and distributes electrical

power to all or most of the users in specific areas. Such a utility company can

make use of very large alternators, which are inherently more efficient and for

this reason alternators with as much as 500 1000 MW are now in service. In


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recent years of growing concern about efficient use of energy has made users

reconsider the economics of generating all or large of their own electrical

requirements. Hospitals, large educational institutes, industrial plants like oil

refineries and even large office buildings sometimes find it economical to

generate their own power. It seems likely that the number of smaller user

owned alternators will increase considerably in forthcoming years. This time

the total electrical power generated by various means in India is 130000 MW

and required amount of energy is 200000 MW. Every year there is increment

of 20000 25000 MW energy in production. For this big alternators are

needed.

Here we will start by examining the construction of alternators. Then we

will briefly discuss the basic theory of its operation and typical characteristics,

different types of excitation systems and their principle of operation , cooling

methods and also different types of insulation methods and after all the

various testing required.


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PROFILE OF B.H.E.L.

Bharat Heavy Electricals Limited (BHEL) is today the largest

engineering enterprise of India with an excellent track record of

performance. Its first plant was set up at Bhopal in 1956 under

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

plants at Haridwar, Hyderabad and Tiruchirapalli with Russian and

Czechoslovak assistance.

These plants have been at the core of BHELs efforts to grow and

diversify and become Indias 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.


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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 substained

equipment operating in Transmission & Distribution net work upto 400

KV AC & DC Supplied over 25,000 Motors with Drive Control System

Power projects. Petrochemicals, Refineries, Steel, Aluminium,

Fertilizer, Cement plants etc., supplied Traction electrics and AC/DC

locos to power over 12,000 Kms Railway network.

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

This is due to the emphasis placed all along on designing, engineering

and manufacturing to international standards by acquiring and

assimilating some of the best technologies in the world from leading

companies in USA, Europe and Japan, together with technologies from

its-own R & D centres BHEL has acquired ISO 9000 certification for its

operations and has also adopted the concepts of Total Quality

Management (TQM).


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BHEL presently has manufactured Turbo-Generators of ratings upto

560 MW and is in the process of going upto 660 MW. It has also the

capability to take up the manufacture of ratings upto 1000 MW suitable

for thermal power generation, gas based and combined cycle power

generation as-well-as for diverse industrial applications like Paper,

Sugar, Cement, Petrochemical, Fertilizers, Rayon Industries, etc.

Based on proven designs and know-how backed by over three

decades of experience and accredition of ISO 9001. The Turbo-

generator is a product of high-class workmanship and quality.

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

to secure prestigious global orders in the recent past from Malaysia,

Malta, Cyprus, Oman, Iraq, Bangladesh, Sri Lanka and Saudi Arabia.

The successful completion of the various export projects in a record

time is a testimony of BHELs performance.

Established in the late 50s, Bharat Heavy Electricals Limited (BHEL)

is, today, a name to reckon with in the industrial world. It is the largest

engineering and manufacturing enterprises of its kind in India and one

of the leading international companies in the power field. BHEL offers

over 180 products and provides systems and services to meet the


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needs of core sections like : power, transmission, industry,

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

telecommunication. A wide-spread network of 14 manufacturing

divisions, 8 service centres and 4 regional offices besides a large

number of project sites spread all over India and abroad, enables

BHEL to be close to its customers and cater to their specialized needs

with total solutions-efficiently and economically. An ISO 9000

certification has given the company international recognition for its

commitment towards quality. With an export presence in more than 50

countries BHEL is truly Indias industrial ambassador to the world.


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COMPONENTS OF TURBO - GENERATOR

The general components of a turbo generator are

# Stator

- Stator Frame

- Stator Core

- Stator Windings

- End Covers

# Rotor

- Rotor Shaft

- Rotor Windings

- Rotor Retaining Rings

# Bearings

# Cooling Systems

The following auxiliaries are required for operation:

# Oil Supply system

# Excitation System


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In order to minimize 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. Lamination plates are

of two types, depending upon the thickness

.65 mm thickness plates These plates are used on both sides

of a stack. Eye bars are welded on it for providing ventilation.

5 mm thickness plates These are general lamination sheets

which are placed within end plates for making the stack.

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 segments


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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. Hysterysis Loss: Due to the residual magnetism in the

Core-material. Hysterysis loss is given by

Wh max1.6 ft

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

Core of the stator. Eddy current loss is given by

We max f t


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In order to reduce the hysterysis loss, silicon alloyed steel, which has

low hysterysis 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.

PREPATION OF LAMINATIONS


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Initially the material comes in the form of rolled sheets and then it is cut

in trapezoidal form for reducing the copper losses since the material is

very costly.

For high rating machines each lamination is build of 6 sectors

(stampings), each of 60 cut according to the specifications. Press

tools are used in the manufacture of laminations. Press tools are

mainly of two types.

1. Compounding tools.

2. Blanking and slot notching tools.

Laminations are manufactured in two different ways.

1. COMPOUNDING OPERATION :

In this method the stamping with all the core bolt holes, guiding slots

and winding slots is manufactured in single operation known as

Compounding operation and the press tool used is known as

Compounding tool. Compounding tools are used for the machines

rated above 40 MW.

2. BLANKING AND NOTCHING OPERATIONS :


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This method is used for those generators which are rated less than 40

MW. 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.

The different operations taking place in the manufacture of laminations

are

a) The cold rolled non grained oriented (CRNGO) steel sheets in

the required shapes according to the size of the laminations

are cut by feeding the sheet into shearing press.

b) Compounding operation or Blanking & Notching operation is

done. Nearly 500 tons crank press is used for this purpose.

c) Deburring operation :

In this operation the burrs in the sheet due to punching are

deburred. There are chances of short circuit within the


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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.

d) Varnishing :

Then depending on the temperature withstandability of the

machine the laminations are coated by varnish which acts as

insulation. Varnish is mixed with thinner in such a manner that

one IS 9 cup filled with this mixture will be empty in 50 sec.

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 C. Two coatings of varnish are

provided in the above manner till 12-18 micrometer thickness of

coat is obtained. Thickness of the obtained coat should be 7 cm

and its hardness should be 7H.

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


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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 1 mega ohm.

ASSEMBLY OF CORE

The stator laminations are assembled as separate cage core without

stator frame. The entire core length is made in the form of packets

separated by radial ducts to provide ventilating passages for the

uniform cooling of the core. The thickness of each lamination is 0.5

mm and the thickness of lamination separating the packets is about .65

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.


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Stacking mandrels and bolts are inserted into the windings slot bores

during stacking provide smooth slot walls.

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 complete stack is kept under pressure and located

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


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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.

The core building or 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.

3.


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STATOR WINDING

Stator winding is the one which induces emf and supplies the load.

Stator winding is placed in the slots of stator core. Due to the

advantages of generation and utilization of 3 phase power we use

three phase windings 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, chorded to about 5/6 pitch which

practically eliminates 5th and 7th harmonics from the flux wage or open

circuit induced emf wave is used. The stator coil is made up of number

of strips instead of single solid piece to reduce the skin effect.

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..

There are two types of coils manufactured in BHEL, Hyderabad.


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1) Diamond pulled multiturn coil (full coiled):

2) Roebel bar (half coiled).

Generally diamond pulled multiturn coils are used for low capacity

machine. In this coils are pulled in a particular shape similar as

diamond thats why they are called so.

In large capacity machines we use ROEBEL bars. 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 neutralization 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.

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. The high

voltage insulation is provided according to the resin poor mica base of


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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.

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

ionization of he 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.


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MANUFACTURE OF STATOR COILS

Various operations carried out during manufacture of stator coil

are

1. Set the straightening and cutting machine using guide pilot.

2. Cut the conductor strips as per the requirement.

3. Set the press for Roebel Transposition.

4. Assemble strips with respect to template and transpose.

5. Assemble both halves of coil sides to from

i) One Roebel half bar

ii) Insert insulation of halves between quarter bars matching the

straight part zone as per drawing.

6. Cure half coil on hydraulic press. This process is known as Baking.

7. (a) Remove insulation at the ends of the strips.

(b) Test for inter-strip and inter-halves shorts.

8. Set the universal former as per standards. Check the setting of

universal former for

(i) Length of straight part also mark diagonals/former walls inside

for cross check.

(ii) Check for marking made by template.

9. (a) Place the bar on former.


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(b) Form the overhang bends as per standards.

Remove clamps and inserts overhand insulation to both roebel

halves with an application of araldite mixture.

(d) The bar is allowed to cure by giving supply to heating clamps.

10) (I) Remove heating clamps and take out the bar halves

from former.

(ii) Round off sharp edges of straight part and dress up

overhang halves insulation of both halves with out damage

to copper strip insulation and to copper stacks.

11) Process of taping:

a) Tape the bar with Resin poor fine mica paper tape on

straight part of bar taking copper foil outside the tape.

b) Tape with one layer of conductive polyester fleece tape.

c) Provide main insulation

d) OCP protection tape

(i) Tape the straight part of bar with conductive

polyester fleece tape with starting and ending shall

be on straight part of bar.


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(ii) Tape with mica splitting tape with accelerator taking

Ocp layer into and leaving.

(iii) Tape the straight part of bar with polyester

Conductive fleece tape.

e) Provide End Corona protection taping.

f) Provide overhang with protective tape (Polyester glass

tape)

g) Test for inter-strip shorts.

After the manufacture of stator half coils (Roebel bars), they are

sent to stator winding shop: In winding shop stator bars are

arranged in the core slots as per the design. First bottom layer of

bars is placed and then top one. Between them stiffner made up

of HGL are placed for insulation. At the ends according to pitch

factor top and bottom bars are brazed. Due to that at the ends a

particular type of design is obtained which is called stator eye. In

BHEL Hyd only lap connected winding is done.Stator winding

has a transposed coil in each phase such that the flux distribution

is equal and hence the induced e.m.f.


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STATOR END COVERS:

The stator end covers are attached to end flanges of stator

frame and also rest on the foundation plate. The end covers are

made up of non-magnetic material (Aluminium castings) to

reduce stray load and eddy current losses.

PHASE CONNECTORS:

The phase connectors consist of flat copper sections, which results in

low specific current loading. The phase connectors are wrapped with

resin rich mica tape. After curing the connectors are attached to the

pressure plate with clamps and bolts.

RESISTANCE TEMPERATURE DETECTORS :

The temperature measurements on the generator are made with

RTDs. They are placed at various sections of the core and winding.

When making measurements with RTDs the resistance element is

exposed to the temperature to be measured. The RTD works on the

principle of the change in electrical resistance of a conductor due to

temperature.

R= Ro (1+ T)

Where Ro = reference resistance at room temperature


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= temperature coefficient of resistance

T = temperature difference in C.

INSULATION SYSTEMS

1. Bitumen Mica System :

The system consists of flakes of mica in the form of tape and with the

use of natural Asphalt (Bituman) as binder and is class B. The bitumen

mica folium tape is continuously wrapped in the slot position and in the

overhang and the winding is impregnated in bitumen compound under

pressure. Thermoplastic class B system with increased flexibility &

Thermo plasticity was generally satisfactory. But is has the problems of

tape migration, poor dielectric strength, insulation swelling, thermal

deterioration & moisture susceptibility.

2. Thermo-reactive Resin rich System :

The system is class I epoxy mica paper thermo reactive employing B

stage impregnated epoxy mica resin rich tape, which consolidated

under heat & pressure. In this system B stage epoxy mica tape


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material has limited shelf life and requires controlled condition of lower

temperature storage.

3. Micalastic VPI Resin poor System :

Micalastic class F insulation system is based on resin poor technology.

This consists of high strength mica and Thermo-setting solvent less

epoxy resin with vacuum impregnation. The system employing

elaborate manufacturing facility, gives higher volume of production and

more consistent quality because of lesser manual operation & more

automation.

Till early fifties bitumen mica insulation system was in vogue with most

of the manufacturers for medium as well as large utility sets. Epoxy-

mica system, resin rich or resin poor, is used in manufacture of Turbine

Generators.


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BHEL INSULATION SYSTEM FOR TURBO GENERATORS

BHEL had Bitumen insulation system for low & medium rating TGS

and switched over to resin rich Thermo setting type as a step towards

increasing reliability and upgrading technology. Micalastic system has

been adopted for high rating machinery.

BITUMEN SYSTEM & LIFE EXTENSION

The experience with Bitumen system has been generally satisfactory &

practically negligible service failure has been reported on these sets.

Mechanical damage most commonly associated with this system ie.,

tape separation, due to thermal expansion of the winding during

normal or abnormal temperature eyeing is not met any of sets. Though

outage due to insulation failures has been considerably low, yet these

machinery would need to be attended to have life extension above

their estimated life of 25 years. Major inspection of the machine

condition is by checking the healthiness of windings & life of bar

insulation. Rehabilitation, if needed, requires restoration of varnish,

removal of bitumen & cleaning, tightening of fasteners/supports,

modification of busbars, use of new wedges & other winding


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components. The replacements are required because of vibration /

external damage etc.

VARIOUS INSULATION SYSTEMS & PRACTICES

Large & medium range motors are provided with following insulation

system.

(a) Resiflex Insulation System :

This system is used on earlier designs & where duplicate or spare

motors to suit the customer requirements are required. In the coming

years this system may become absolute.

(b) Resin Rich micalastic Insulation System :

The system provides use of Resin rich polyester backed epoxy

micafolium on straight portion & resin rich polyester backed epoxy

mica paper tape on overhang with a final layer of polyester shrink tape.

The system is highly productive during coil manufacture and housing.

The wound stators are totally impregnated in unsaturated polyester

resin through rolling technique. For bracing of coils use of woven


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polyester felt around moulded casting in common which provides bond

between coils after impregnation. Insulation thicknesses in mm

adopted for main insulation are much less in comparison to resiflex

insulation system.

Resin poor Micalastic Insulation System:

Resin poor micalastic system is adopted for large range Ac Induction

and synchronous machines. Theses are designated to meet specific

customer requirement hence for unique in nature to each other. The

main insulation consists of resin poor epoxy mica paper tape all over

the oil periphery with varying number of layers on straight and

overhang portions.

The wound stators are impregnated under vacuum and pressure to

provide avoid free monolithic insulation all over the winding. The

insulation thicknesses are slightly higher than those used for resin rich

micalastic insulation system.


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This system satisfies class F requirements but are being thermally

utilized to class B temperatures only. Therefore there is a thermal

reserve which results in - prolonged life

increased reliability

capacity for occasional overloads.

The brief comparison of Resin poor over Resin rich is as follows:

Resin Poor Resin Rich

1.Epoxy Resin content is about 8%. 1.Epoxy Resin content is about 40%.

2. This method follows Thermo- 2. Same as in Resin poor system.

Setting process.

3. There is a need for addition of 3. Further addition of resin is not

resin from outside. Required.

4. Reduction in time cycle for this 4. It is very long process and time

process. Consuming.

5. Repairing is very difficult. 5. Repairing work is easy.

6. Overall cost is less compared 6. Overall cost is more.

to resin rich system.


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1.

INTRODUCTION TO VACUUM PRESSURE IMPREGNATION

SYSTEM (VPI)

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 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 provide it was used in its thermal limitations. In the late

1930s and early 1940s, however, many large units, principally turbine generators, failed


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due to inherently weak thermoplastic nature of bitumen compound. Failures were due to

two types of problems:

1. Tape separation

2. Excessive relaxation of the main ground insulation.

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

didnt 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 didnt 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 with out 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 thisVacuum Pressure Impregnation 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.


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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 interturn and high-

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

unimpregnated 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 interturn 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 thermoset 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 upto 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 improved mechanical strength

and electrical properties. With the improved penetration, a void free coil is


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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.

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.

Vacuum Pressure Impregnation of resin poor insulated jobs:

Variant Description

01 Brushless exciter armature, PMG stators and

Laminated rotors

02 Stator wound with diamond pulled coils.

3 Stator with half coils


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Variant-01 Variant-02 Variant-03 Any otherinformation

Preheating 60 5C for3hrs

60 5C for 12hrs 60 3C for12hrs

Vacuum to be

maintained

0.4mbar 0.2mbar/0.4mbar


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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. Preheating:

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

monitored by the RTD elements placed on the job and the readings are logged by

production. The time of entry into 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.

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 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 insertedcarefully into impregnation tank without damage toinsulation.

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.


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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 upto 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 raising from

job temperature to this temperature as per relevant annexure. The time


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at which the heating is started, achieved and maintained is to be

logged.

5. Electrical testing:

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 oper


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TESTING


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TESTING OF STATOR

Two types of tests are carried out during production.

1) Process tests

2) Performance tests

PROCESS TESTS

STATOR CORE FLUX TEST :

Immediately after the core is built up and before it is wound, a test is

made to detect the presence of local hot spots. Whenever there are

shorts between adjacent core laminations, due to break of inter laminar

insulation or burns on the edges, high eddy current flow giving rise to

temperature rise in that zone. Any hot spots found are rectified by

carrying out Electrolysis using phosphoric acid as electrolyte.

INSULATION RESISTANCE MEASUREMENT TEST :

The resistance of insulation is measured by placing two copper plates

above and below the laminations. The maximum acceptable value

should be 1 Mohms per K V of the rated voltage. These tests ensures

the quality of the insulation varnish of the laminations.


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MEASUREMENT OF WINDING RESISTANCE :

DC resistance of the stator is measured separately using micro

ohmmeter. The instrument terminals are connected to the machine

terminals and proper range in meter is selected. The stabilized reading

is recorded.

AC resistance of the stator winding is taken as 1.6 times that of DC

resistance.

TESTING OF STATOR BARS :

After laying of bottom bars in the stator core they are tested at for

2Un+7KV.

After laying of top bars they are tested for a voltage of 2 Un + 5 KV.

After laying of top and bottom bars, their balancing is done and then

they are tested for a voltage of 2 Un + 3 KV.

In front of the customer test is carried out at 2 Un + 1 KV

Where Un = Rated voltage of the machine.

PERFORMANCE TESTS

MECHANICAL RUN TEST :

In this test the machine is run at rated speed with the help of prime

mover. The vibrations of the rotor and bearings are measured in three


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directions- horizontal, vertical and axial. The vibrations must be within

the limitations. In order to achieve it the rotor must be properly

balanced before the assembly. For balancing the rotor weights are

added to rotor.

OPEN CIRCUIT TEST :

In this test the machine is run at rated speed with the help of prime

mover. The excitation to rotor is increased in steps and the

corresponding terminal voltages of the stator winding are noted. In this

test the input to machine is the indication of core or iron losses.

Core loss = no load input input of drive motor.

SHORT CIRCUIT TEST :

In this test the machine is run at rated speed with the help of

prime mover. The output terminals of the stator are short circuited and

the excitation is slowly increased such that rated current flows through

stator winding. The input to machine is the indication of copper losses.

HIGH VOLTAGE TEST :

When high voltage is done on the phase winding all other phase

windings, rotor winding, instrumentation cables and stator body are

earthed.


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High voltage is applied to winding by increasing gradually to

required value and maintained for one minute and reduced gradually to

minimum. The transformer is switched off and winding is discharged to

earth by shorting the terminal to earth using earthing rod connected to

the earthed cable. The test is conducted to all phases and rotor

windings separately.


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COOLING

NECESSITY OF COOLING SYSTEM :

Cooling is one of the basic requirement of any generator. The effective

working of generator considerably depends on the cooling system. The

insulation used and cooling employed are inter-related.

The various losses in the generator are broadly classified as below:

1. Iron losses/Core losses/Magnetic losses

i) Hysterisis loss

ii) Eddy current loss

2. Copper losses/Winding losses

3. Mechanical losses

i) Frictional loss

ii) Windage loss

These losses in the generator dissipates as heat which raises the

temperature of the generator. Due to high temperature the insulation

will be affected greatly. So the heat generated should be cooled to

avoid excessive temperature raise.


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There are various methods of cooling. They are

i) Air cooling 60 MW

ii) Hydrogen cooling 100 MW

iii) Water cooling 500 MW

iii) Hydrogen and water cooling 100 MW

Advantages of Hydrogen cooling over Air cooling :

a) Hydrogen has 7 times more heat dissipating capacity.

b) Hydrogen has higher specific heat.

c) Since Hydrogen weight is 1/14th of air it has higher

compressibility.

d) Hydrogen does not support combustion.

Disadvantages :

a) Hydrogen is explosive when it combines with oxygen.

b) Greater precautions are to be taken to avoid leakage of

Hydrogen.

c) Cost of cooling system is high compared to air cooling system.


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The two-pole generator uses direct cooling for rotor winding and

indirect cooling for the stator winding. Director cooling of the rotor

essentially eliminates hot spots and differential temperature between

adjacent components which could result in mechanical stresses,

particularly to the copper conductors, insulation and rotor body.

AIR COOLING :

The cooling air is circulated in generator interior in a open circuit by

two axial flow fans arranged on the rotor shaft. Cold air is drawn by the

fans from the atmosphere through air filter the cooling air flow is

divided into three flow paths after each fan.

Flow path 1 is directed into the rotor end winding space and cools the

rotor winding, part of the cooling air flows past the individual coils for

cooling the rotor end winding and then leaves the end winding space

via bores in the rotor teeth at the end of the rotor body. The other

portion of the cooling air flow is directed from the rotor end winding

space into the slot bottom ducts axially from where it is discharged into

the air gap radially via a large number of radial ventilating slots in the


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coils and bores in the rotor wedges. Along these paths the heat of rotor

winding is directly transferred to the cooling air.

Flow path 2 is directed over the stator end winding to the cold air

ducts and into the cold air compartments in the stator frame space

between the generator housing and the rotor core. The air then flows

into the air gap through slot in the stator core were it absorbs the heat

from the stator core and stator winding.

Flow path 3 is directed into the air gap via the rotor retaining ring. The

air then flows past the clamping fingers via ventilating slot in the stator

core into the hot air compartments in the stator frame being discharged

to the air cooler. The flow path mainly cools the rotor retaining rings,

the ends of the rotor body and the end portions of the stator core.

Flows 2 & 3 mix in the air gap with flow 1 leaving the rotor. The cooling

air flows radially outward through ventilating slots in the core within the

range of the hot air compartments for cooling of winding and core. The

hot air is then discharged to air cooler.


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ROTOR

Rotor is the rotating part of alternator. It is used to support field winding

placed in slots on the rotor.

FOR 2-POLE GENERATOR:

Solid rotors are manufactured from forged alloy steel with suitable

alloying elements to achieve very high mechanical and superior

magnetic properties. This type of rotor can withstand even upto speed

of 3000 rpm.

Rectangular or trapezoidal rotor slots are accurately machined to close

tolerances on slot milling machine.

For indirectly cooled generator rotors, ventilation slots are machined in

the teeth.

FOR 4-POLE GENERATOR:

For directly cooled rotors, sub slots are provided for cooling Generator

rotors of 1500 RPM are of round laminated construction. In this case

rotor is made up of two parts (1) core, (2) lamination. The outer

diameter of core and the inner diameter of laminations are equal. So

for inserting the core inside the laminations the laminations are first red


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heated at medium temperature for 15 hours in BELL FURNACE. After

that the core is shrunk fitted inside the laminations. Thus punched and

varnished laminations of high tensile steel are mounted over machined

shaft and are firmly clamped by end clamping plates.

2.1 ROTOR SHAFT

Rotor shaft is a single piece solid forming manufactured from a

vacuum casting. It is forged from a vacuum cast steel ingot. Slots for

insertion or the field winding are milled into rotor body. The longitudinal

slots are distributed over the circumference such that two solid poles

are obtained.

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

material analysis and ultrasonic tests are performed during the

manufacture of rotor. The high mechanical stresses resulting from the

centrifugal forces and short circuit torques call for a high quality heat

treated steel. Comprehensive tests ensure adherence to the specified

mechanical and magnetic properties as well as homogenous forging.

After completion, the rotor is balanced in various planes at different


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speeds and then subjected to an over speed test at 120% of the rated

speed for two minutes.

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

Approximately 60% of rotor body circumference has longitudinal slots

which hold the field winding. Slot pitch is selected so that the two solid

poles are displaced by 180 degrees. The rotor wedges act as damper

winding within the range of winding slots. The rotor teeth at the ends of

rotor body are provided with axial and radial holes enabling the cooling

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

windings.

2.2 ROTOR WINDINGS

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

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

consists of several series connected turns, each of which consists of

two half turns connected by brazing in the end section. Thickness of

each strip can be made upto 10.5 mm but here in BHEL we make only

upto 5.3 mm. The rotor bearing is made of silver bearing copper


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ensuring an increased thermal stability. For ventilation purpose the

slots are provided on the coil and on inter strip insulation layer both.

The individual turns of coils are insulated against each other by

interlayer insulation. L-shaped strips of laminated epoxy glass fibre

fabric with nomex filter are used for slot insulation.

The slot wedges are made of high electrical conductivity material and

thus act as damper windings. At their ends the slot wedges are short

circuited through the rotor body. The inter space between the overhang

is called slot through.

CONSTRUCTION

The field winding consists of several series connected coils inserted

into the longitudinal slots of rotor body. The coils are wound so that two

poles are obtained. The solid conductors have a rectangular cross

section and are provided with axial slots for radial discharge or cooling

air. All conductors have identical copper and cooling duct cross

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


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into the rotor slots, these turns are brazed to obtain full turns. The

series connected turns of one slot constitute one coil. The individual

coils of rotor are connected in a way that north and south poles are

obtained.

CONDUCTOR MATERIAL

The conductors are made of copper with a silver content of

approximately 0.1%. As compared to electrolytic copper, silver alloyed

copper features high strength properties at high temperatures so that

coil deformations due to thermal stresses are eliminated.

INSULATION

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

fiber laminate.

The coils are insulated from the rotor body with L-shaped strips of

glass fiber laminate with nomex interlines.

To obtain the required leakage paths between the coil and the rotor

body thick top strips of glass fiber laminate are inserted below top


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wedges. The top strips are provided with axial slots of the same cross

section and spacing as used on the rotor winding. Insulation b/w

overhang is done by blocks made of HGL.

ROTOR SLOT WEDGES

To protect the winding against the effects of centrifugal forces, the

winding is secured in the slots with wedges. The slot wedges are made

of copper alloy featuring high strength and good electrical conductivity.

They are also used as damper winding bars. The slot wedges extend

beyond the shrink seats of retaining rings. The wedge and retaining

rings act on the damper winding in the event of abnormal operations.

The rings act as short circuit rings in the damper windings.

END WINDING BRACING

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

with insulated members that prevent coil movement. Two insulation

plates held by HGL high glass laminate plates separate the different

cooling zones in the overhangs on either sides.


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2.3 ROTOR RETAINING RINGS

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

piece rotor retaining rings. Retaining rings are made of non-magnetic

high strength steel in order to reduce stray losses. Each retaining ring

with its shrink fitted. Insert ring is shrunk on to the rotor body in an

overhang position. The retaining ring is secured in the axial position by

snap rings.

The rotor retaining rings withstand the centrifugal forces due to end

windings. One end of each ring is shrunk fitted on the rotor body while

the other end overhangs the end windings without contact on the rotor

shaft. This ensures an unobstructed shaft deflection at the end

winding.

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

the retaining ring and secures the end winding in the axial direction at

the same time.

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

The shrunk seat of the retaining ring is silver plated, ensuring a low


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contact resistance for induced currents. To reduce the stray losses and

have high strength, the rings are made of non magnetic, cold worked

materials.

2.4 ROTOR FANS

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

on the rotor shaft one at each end. To augment the cooling of the rotor

winding, the pressure established by the fan works in conjunction with

the air expelled from the discharge parts along the rotor.

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

rotor shaft. The blades are drop forged from an aluminium alloy.

Threaded root fastenings permit angle to be changed. Each blade is

secured at its root with a threaded pin.

BEARINGS

The turbo generators are provided with pressure lubricated self-

aligning elliptical type bearings to ensure higher mechanical stability

and reduced vibration in operation. The bearings are provided with

suitable temperature element devices to monitor bearing metal


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temperature in operation. From inside the bearings are made of very

soft metal called Babbitt so that rotor doesnt get harmed even if it

comes in contact with Babbitt. Inside this Babbitt there is a very thin

film of pressurized lubrication oil on which the shaft rotates.

The temperature of each bearing is monitored with two RTDs

(Resistance Thermo Detectors) embedded in the lower bearing sleeve

such that the measuring point is located directly below the babitt.

These RTDs are monitored a temperature scanner in the control panel

and annunciated if the temperature exceeds the prescribed limits. All

bearings have provisions for fitting vibration pickups to monitor shaft

vibrations.

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

oil piping on either side of the non-drive end bearings are insulated

from the foundation frame. For facilitating and monitoring the

healthiness of bearing insulation, split insulation is provided.

VENTILATION AND COOLING


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Turbo generators are designed with the following ventilation systems:

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

Closed circuit hydrogen cooling with water or hydrogen coolers

mounted axially on the stator frame.

The fan design usually consists of two axial fans on either made of

cast aluminium with integral fan blades or forged and machined

aluminium alloy blades screwed to the rotor.

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

TESTING OF TURBO GENERATOR

To ensure that all functional requirements are fulfilled, and to

estimate the performance of generator, the TURBO GENERATORS

are required to undergo some tests. For testing, the TURBO

GENERATOR was mechanically coupled to a drive motor-motor

generator set with gearbox. The rotor was excited by thyristor

converter system located in an independent test room and the

operation was controlled from the test gallery.


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

before assembling and the third and final routine tests will be

conducted after assembling the turbo generator.

TESTS CONDUCTED ON ROTOR

TESTS CONDUCTED ON STATOR

ROUTINE TESTS ON TURBO GENERATORS

TESTING OF TURBO GENERATOR ROTOR WINDING

Details of Process tests to be performed at various stages :

HIGH VOLTAGE TEST :


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

commencing the winding, the slip rings are to be tested.

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

insulation condition is found satisfactory, then perform High Voltage

test for one minute, the test of which is to be determined according

to the following equation.

U2 = Ut + 1 KV

Where U2 is test voltage

Ut is 10* rated rotor voltage

However the resulting test voltage U2 should be neither lower

than 2.5 KV nor above 4.5 KV.

After the high voltage test, measure the insulating condition

again with 1000V Megger.

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

respective rotor slots and before clamping the pressing equipment.

Measure the insulating condition with a 1000V megger. It must not

be lower than 1 MO for each KV of the tested voltage. Then

measure the ohmic resistance of the winding.


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3. After tightening the winding with the pressing and tightening

equipment and before actually baking the winding, measure the

ohmic resistance of the winding. Then check polarity of the winding.

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

and other equipment are insulated from the winding and rotor body,

by inserting insulation in every slot under the shims of the

equipment.

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

clamping equipment and after the rotor cools down to ambient

temperature, measure the insulation resistance with 1000V

Megger.

If the insulation condition is satisfactory, perform High Voltage test

for one minute with a value of 1.15 Ut.

Where Ut is 10 times the rated rotor voltage.

After performing the High Voltage test, measure again the

insulation condition.


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

insulation resistance and if found satisfactory, perform High Voltage

test with a value of 1.10 Ut for 10 sec, i.e., just reaching the value

and then bringing down to zero.

After driving all the wedges in position, measure the insulation

resistance and if found satisfactory, perform High Voltage test with

a value of 1.10 Ut for one minute.

6. After putting all the bracings, mounting of the end-retaining ring

and just before dispatch of the rotor for further machining.

Measure the insulation resistance.

Measure the ohmic resistance of the winding and perform High

Voltage test with a value of 1.05 Ut for one minute.

7. After machining of the rotor, and before its dispatch to the

centrifugal tunnel, measure the insulation resistance.


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

resistance and the ohmic resistance, while the rotor is at rest.

Check again the insulation condition at 3000 rpm.

Measure again the insulation resistance after the rotor is balanced

and just before its dispatch to the winding shop.

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

insulation resistance and perform High Voltage test with a value of

1.0 Ut for one minute.

MEASUREMENT OF D.C.RESISTANCE :

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

a Micro Ohmmeter. First connect the micro ohmmeter to 230V AC

supply. And measure the resistance and the temperature using RTD.

This resistance at T temperature has to be converted to resistance at

20 Degrees C by using the formula:

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

Where R20 = Resistance at 20 Degrees C in mO


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T = temp in degree Celsius

Rt = measured resistance of winding in mO

A deviation of + 10% from design values is acceptable.

MEASUREMENT OF IMPEDANCE :

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

measured at standstill and at the rated speed.

Impedance is measured by using the formula :

Z = V/I

Where Z = impedance in ohms;

V = voltage in volts;

I = current in amps;

In the measurement of Impedance there will be a graph plotted

between voltage v/s current. In this, there is no perfect value for the

impedance but the only condition is that the impedance should

increase with the increase in voltage.

TESTING OF TURBO GENERATOR STATOR BARS


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FOR RESIN RICH SYSTEMS, STATOR BARS WILL BE TESTED IN

THE FOLLOWING ORDER :

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

voltage.

Ut = 4* Urated

2) Individual bars will be tested for tan . is the angle between

actual current and line current. When the insulation is perfect and

dielectric strength is optimal is zero. But due to the presence

of impurities in the insulation there will be a phase angle

difference between the two currents.

This tan measurement is known as loss angle

measurement or dielectric loss measurement. Tan

values should be within 2%.

3) Outer corona protection resistance is measured and this value

should be within the range of 75-300 0 /Sq. cm

4) Inter-strip and Inter-half shorts are checked. Inter-strip means

between the conductor strips and inter-half means between the

halves. This shorts are checked by a series bulb test.

TEST OF TURBO GENERATOR STATOR WINDING


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HIGH VOLTAGE TEST :

FOR RESIN RICH SYSTEMS THE STATOR WINDING WILL BE

TESTED IN THE FOLLOWING ORDER :

1) After lying bottom bars, bars are subjected to (2Un+7) KV Where

Un is the rated voltage.

2) After lying top bars, bars are subjected to (2Un+5) KV Where Un

is the rated voltage.

3) After lying bottom, top and eyes joining, High voltage test is

conducted for (2 Un+3) KV

Where Un is the rated voltage.

4) After final assembling and connections, customer witness test is

carried at (2Un+1) KV Where Un is the rated voltage.

5) Inter-half shorts are also checked.

FOR RESIGN POOR SYSTEMS THE FOLLOWING TEST IS

CARRIED OUT :

Bars are subjected for Inter strip and Inter half shorts tests.

INTER TURN INSULATION TEST :

The insulation between the windings of the rotor is tested by

applying a high frequency current of about 500 HZ. The insulation

should be able to withstand this test.


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RING FLUX TEST ON STATOR CORE :

Ring flux test is carried out on the stator core before winding is

put in the slots. The rated flux density is generated in the stator core by

passing current in it. This results in the temperature rise and

generation of heat. The stator core is observed for the temperature rise

through its surface by using RTDs. If there is any hot spot found in the

core, it is detected. Then it is rectified by carrying out electrolysis using

phosphoric acid as electrolyte.

MEASUREMENT OF D.C.RESISTANCE :

The D.C. Resistance of stator winding is measured by using

Micro Ohmmeter. Connect the micro ohmmeter to 230V AC supply.

Connect the measuring leads of micro ohmmeter across R phase of

stator terminals. Measure the resistance and repeat the step for Y and

B phases. Record the stator RTDs value.

R20 = Rt * (235+20) / (235+T) m

Where R20 = Resistance at 20 C in m

T = temp in degree Celsius

Rt = measured resistance of winding in m

The variation of maximum and minimum value of stator DC resistance

upto 5% is acceptable.


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MEASUREMENT OF LEAKAGE REACTANCE :

This test is done without rotor inside the stator.

Base Impedance Xn = En/((3) * In) Where En = rated line to line voltage

In = rated line current

Total Armature leakage reactance (XL) :

XL = (Z - R)

Where Z = U/ ((3) * I)

R = P/(3* I)

U = voltage measured during the test

I = current measured during the test

P = Power measured during the test

Resistance per phase is negligible compared to Z. Therefore

Measurement of P is not required.

XL = Z = U/(( 3) * I)

% XL = (XL/ Xn) * 100

MEASUREMENT OF INSULATION RESISTANCE AND CONTINUITY

TEST OF RESISTANCE TEMPERATURE DETECTOR (rtd):


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Short all the RTD leads together and connect one lead of megger

to it. Run the megger and note down the Insulation resistance value

after 60 seconds. This insulation resistance value should not be less

than 1 M

Remove the RTD terminals i.e. open the RTD terminals and

connect to the multimeter. Note down the resistance value of RTD. For

three wire RTD check the continuity between shorted terminals.

CAPACITANCE AND TAN MEASUREMENT OF STATORWINDING :

Stator winding has two values of capacitances.

1) Capacitance with respect to ground called ground capacitance

(Cg).

2) Capacitance with respect to other windings called mutual

capacitance.

Measurement of capacitance is done using Schering Bridge and

a standard capacitor.


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1) High Voltage applied to one of the phases and remaining

phases are connected to body of stator Cg + 2 Cm.

2) High Voltage to all the phases . 3 Cg.

Cx = capacitance to be measured.

Cn = standard capacitor

G = galvanometer

R3+S = variable resistance

C4 = variable capacitance

N = Parallel step fixed resistance.

R4 = standard resistor

Raise the transformer voltage to 0.2 Un

Where Un = rated voltage of machine.

Balance the Schering Bridge with proper selection (R3+s) and

C4.

And note down the values of C4 and R3+S

Take reading at 0.4 Un, 0.6 Un, 0.8Un and Un.

Cx = (Cn * R4*(R3+100)/(N*(R3+s) uf

% tan = (( * R4* C4)/10000) * 100

C4 and Cn are in uf

R3, R4, N and S are in


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ROUTINE TETSTS ON TURBO GENERATOR

MECHANICAL RUN AND MEASUREMENT OF VIBRATIONS AT

RATED SPEED:

The machine is rolled and run at rated speed after ensuring the

bearing oil and kept at rated speed for stabilization of bearing

temperatures.

The vibrations are measured at rated speed on both the bearing

housings in Horizontal, Vertical and Axial directions.

The temperature of stator is monitored by monitoring RTDs

embedded in core, tooth and winding.

The vibrations should be less than 5 microns and noise level

should be in between 75-90 db.

SHORT CIRCUIT TEST :


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The machine is prepared for short circuit characteristic using

current transformers and shorting the terminals as shown in fig.

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

and current are noted and machine is excited gradually in steps, at

20%, 40%,60%,80%,100% rated current of machine (In).

The excitation is reduced and cut off. The speed is reduced and

the machine is cooled at lower speed. The temperature are checked

from machine RTDs. The machine is stopped when it is sufficiently

cooled down. The stator winding temperature should be less than 60

C) From the Short Circuit test, we will get copper losses.

The short circuit characteristics is plotted from SCC results by

selecting X-axis as field current and Y-axis as % rated current.

OPEN CIRCUIT TEST :

The machine is prepared for Open Circuit Characteristic as

shown in the fig.

The machine is run at rated speed and the motor input voltage

and current are noted and machine is excited gradually in steps, cat


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20%,40%,60%,80%,90%,95%,100%,105%,110% and 120% of rated

voltage of machine (En).

At 100% rated voltage the following parameters are noted :

Shaft voltage

Checking of phase sequence

Bearing vibration

RTDs readings

The excitation is reduced, cut off, the speed is reduced, and the

machine is cooled at lower speed. The temperatures are checked from

machine RTDs. The machine is stopped when it is sufficiently cooled

down. The stator core temperatures to be less than 60C.

From the Open Circuit test, we will get Iron losses.

The Open Circuit Characteristics is plotted on a graph paper from

OCC results by selecting X-axis as field current and Y-axis as % rated

voltage.

MEASUREMENT OF SHAFT VOLTAGE :

When the rotor shaft rotates inside the stator there will be some

induced EMF will be developed inside the rotor. In addition, this

voltage will go to the bearings and pedestal and to the earth as it is

grounded and it will again come back to the pedestal, to the bearings


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through the earth. It will become a cyclic process. This voltage has to

be reduced otherwise, the rotor will get heated. For this bearing

pedestal is placed on, insulation called HGL.

When the machine is under Open Circuit Characteristic testing

shaft voltage is measured with multimeter and high input impedance

AC voltmeter across the two ends of the rotor at 100% rated voltage.

The shaft voltage should be as minimum as possible.

CHECKING OF PHASE SEQUENCE :

When the machine is under OCC condition at 100% rated

voltage, phase sequence of generator is checked using a phase

sequence indicator across PT.

MEASUREMENT OF ROTOR IMPEDANCE (rotor inside stator) :

A variable 50 HZ A.C voltage of single phase is applied across

the input leads and readings of voltage and current are noted down

from 50v-200 v in steps of 50V.

Impedance is measured by using the formula :

Z = V/I

Where Z = impedance in ohms;

V = voltage in volts;


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I = current in amps;

Rotor Impedance is measured at standstill and at rated speed of

the machine.

The impedance of rotor at standstill and at rated speed is plotted

as applied voltage v/s Impedance.

MEASUREMENT OF INSULATION RESISTANCE OF STATOR AND

ROTOR WINDINGS BEFORE AND AFTER HIGH VOLTAGE TEST

(Machine at rest):

Insulation Resistance of the stator and rotor windings is

measured separately before and after high voltage test using Megger

of 2500 V for stator & 1000 V for rotor windings.

The Insulation Resistance values are taken at 15 sec and at 60 sec .

The ratio of insulation resistance at 15 sec and 60 sec is known

as Absorption Coefficient.

Absorption Coefficient = IR at 60/IR at 15

This Absorption Coefficient for High Voltage test should be > =1.3

HIGH VOLTAGE TEST ON STATOR AND ROTOR WINDINGS

(MACHINE AT REST):


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The High Voltage is applied to windings by increasing gradually

to required value and maintained for one minute and reduced gradually

to minimum. The transformer is switched off and winding is discharged

to earth by shorting the terminal to earth using earthing rod connected

to earthen wire. The test is conducted on all the phases and rotor

winding separately.

When High Voltage test is done on one phase winding, all other

phase windings, rotor winding, instrumentation cables and stator body

is earthed.

High Voltage test levels :

Stator winding = (2 Ut +1) KV

Rotor winding = (10 * Up) V

Where Ut = Rated voltage of the machine under test

Up = Excitation voltage

MEASUREMENT OF POLARISATION INDEX OF STATOR WINDING


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In the measurement of the polarization index of stator winding,

stator output terminals are subjected to 2.5KV Megger for a duration of

1 minute and 10 minutes. And the respective insulation values are

noted down.

Polarization Index is the ratio of insulation Resistance value at 10

min and Insulation Resistance value at 1 Min.

Polarisation Index = Insulation resistance at 10 /Insulation resistance

at 1

The polarization index value should be greater than 2.

MEASUREMENT OF D.C.RESISTANCE OF STATOR AND ROTOR

WINDINGS IN COLD CONDITION :

In cold condition means that measuring the D.C. resistance of

the stator and rotor windings before testing.

D.C. Resistances of stator and rotor windings are measured

separately using micro ohmmeter. The instrument terminals are

connected to the machine terminals and proper range in the meter is

selected.

Variation in the values of D.C. Resistance of 3 phases of stator

windings up to 5 % is acceptable.


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MEASUREMENT OF D.C.RESISTANCES AND INSULATION

RESISTANCE OF RTDs:

The D.C. Resistances and insulation resistances of RTDs are

measured using multimeter and Megger respectively.

EVALUATION OF SHORT CIRCUIT RATIO :

From the test data Short Circuit Ratio is calculated using the

formula.

S.C.R= Field current at 100% Rated voltage from OCC/Field

current at 100% rated current from SCC.


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BURSHLESS EXCITATION SYSTEM:

BASIC ARRANGEMENT OF BRUSHLESS EXCITATION SYSTEM

WITH ROTAITNG DIODES :

The Excitation system consists of :

(i) Rectifier wheels

(ii) 3 phase main exciter

(iii) 3 phase pilot exciter

(iv) Cooler

(v) Meter and supervising equipment

The 3 phase pilot exciter has a revolving field with permanent magnet

poles. The 3-phase ac is fed to the field of revolving armature main

exciter via a stationary regulator and rectifier unit. The 3 phase ac

induced in the rotor of main exciter is rectified by the rotating rectifier

bridge and fed to the field winding of generator rotor through dc lead in


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the rotor shaft. A common shaft carried the rectifier wheels, the rotor of

main exciter and permanent rotor of the pilot exciter. The shaft is rigidly

coupled to the generator rotor and supported on bearings between

main and pilot exciters. The generator and exciter rotors are thus

supported on a total of 3 bearings. Mechanical coupling of the 2 shaft

assemblies results in simultaneous coupling of dc leads in the central

shaft bore. This also compensates the length variations of leads due to

thermal expansion.

RECTIFIER WHEELS:

The main components are silicon diodes, which are arranged in

rectifier wheels in a 3-phase bridge circuit. A plate spring assembly

produces the contact pressure for silicon wafer. The arrangement is

such that the pressure is increased by centrifugal force during rotation.

For suppression of the momentary volt peaks arising form

commutaion, each wheel is provided with 6 RC networks consisting of

1 capacitor and 1 damping resistor each. The wheels are identical in

their mechanical design and differ only in the forward direction of the

diodes. The dc from rectifier wheels id fed to the dc leads via radial

bolts. The 3-phase ac is obtained via copper conductors arranged on

the shaft circumference between the rectifier wheels and 3-phase main


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exciter. One 3 phase conductor is provided for each diode. The

conductors originate at a bus ring system of the main exciter.

1 PHASE PILOT EXCITER :

The 3 phase pilot exciter is a 6-pole revolving field unit. The frame

accommodates the laminated core with 3 phase winding. The rotor

consists of a hub with mounted poles. Each pole consists of a separate

permanent magnet, which is housed in non-magnetic metallic

enclosure. The magnets are braced between the hub and external pole

shoe with bolts. The rotor hub is shrunk onto free shaft end.

3. PHASE MAIN EXCITER :

3-phase main exciter is a 6-pole revolving armature unit. Arranged in

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

is arranged on laminated magnetic poles. At pole shoe, bars are

provided which are connected to form a damper winding. The rotor

consists of stacked laminations, which are compressed by through

bolts over compression rings. The 3 phase winding is inserted into the


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slots of the laminated rotor. The winding conductors are transposed

within the core length and end turns of the rotor winding are secured

with steel bands. The connections are made on the side, facing

rectifier wheels. The winding ends are run to a bus ring system to

which the 3 phase leads leading to the rectifier wheels are also

connected. After full impregnation with synthetic resin and cooling, the

complete rotor is shrunk onto the shaft.

AVR :

It consists of a generator voltage regulator with subsidiary current

controller and field forcing limiter for the main exciter field current, a set

point adjuster, over and under excitation limiters. At the maximum

control setting, the field forcing limiter limits the output current of

thyristor set assigned to control system, to the value allowed for field

forcing.

CONCLUSION :

1) The choice of excitation system largely depends upon the

complexity of grid and the loading pattern. In cases where power


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BRUSHLESS EXCITER :

Suitable for mounting on synchronous generator

CONSTRUCTION :

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

generator. Its rotor is mounted on the overhang of main machine shaft

end. The stator may be fixed either to be base frame of the main

machine or to a separate steel or concrete foundation. A permanent

magnet three phase pilot exciter driven directly by the common

shafting or a static auxiliary excitation unit is used for exciting the field

of the stationery field generator via a voltage regulator. The auxiliary

excitation equipment is described elsewhere. The three phase current

flowing in the rotor winding is rectified by Silicon diodes in the rotating

rectifier and fed into the field winding of main machine via the

excitation leads which pass through the hallow shaft of the main

machine.

ROTOR :


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The rotor is fitted on the shaft extension of the main machine and

locked to it in the circumferential direction by parallel keys which are

capable of accepting shock loads caused by short circuit in the main

machine without being over streessed.

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

which is compressed axially by means of a clamping ring welded to the

hub. Specially shaped supporting elements for the rotating rectifier

modules are welded between the arms of the rotor spider within the

ring formed by laminated core.

ROTOR WINDING :

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

connected in star. It is a two layer winding to insulation of class F. The

end leads of the individual windings are on the A end and connected to

the u,v,w and neutral bus rings arranged at the same end. Both

winding overhangs are bound with heat setting glass fiber tapes to

afford protection against centrifugal forces. The rotor winding is

impregnated with epoxy resin.


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RECTIFIER :

The rectifier accommodated inside the rotor core and rotor winding

comprises six diode assemblies and the protection circuit. The diode

assemblies each consist of a light metal heat sink with integrally

formed cooling fans containing one disc type diode secured by means

of a clamping plate. As the heat

Project Done by Rastogi

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