For the Partial Fulfilment for the

Degree of Bachelor of Technology in

Electrical Engineering from

Rajasthan Technical University, Kota

DEPARTMENT OF ELECTRICAL ENGINEERINGSri Balaji College of Engineering and Technology,

1

Submitted By-

Akshaya Varchswa PandeyAmit VatsSaju S. Nair

B.Tech. VII SemElectrical Engg.

Submitted To-

Mrs. Parul MathuriaH.O.D. [E.E.]

A

PRACTICAL TRAINING REPORT

ON

ELECTRICAL MACHINES MANUFACTURING

Benad Road, Macheda, Jaipur(Raj.)-302013ACKNOWLEDGEMENT

Training is the medium of systemic development of personality and attitude. Training gives

up the awareness of the atmosphere prevailing in the Industry, the workmanship and the

challenges faced.

With pleasure I would like to express my deep sense of gratitude to the Senior Training

Manager Mr. Sudhanshu Pathak, Senior Manager Mr. B.C. Sharma under whose valuable and

precise guidance I have done my vocational training.

I would also grateful to all employees of B.H.E.L, HARIDWAR Division working in various

departments who guided us time to time. Naming a few will be unjust on their other

counterparts.

I would also like to thank our HOD who gave us opportunity to do our vocational training.

One thing that we have learnt from the training that one should always to eager to grasp the

information, without proper querying and logical questioning one cannot get in depth

knowledge, one should always be curious and open to new thought.

The employees of B.H.E.L, HARIDWAR Division were very cooperative and were always

there to explain even our smallest queries.

Date: - 14/June/2010

2

AKSHAYA V. PANDEYAMIT VATSSAJU S. NAIR

B.TECH VII SEM.Electrical Engg.

INDEX OF CONTENTS

1. Introduction2. BHEL – A brief profile

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4

5

CHAPTER-1INTRODUCTION

BHEL is the largest engineering and manufacturing enterprise in India in the energy-related

sector today. BHEL was established more than 40 years ago, ushering in the indigenous

Heavy Electrical Equipment industry in India - a dream that has been more than realized with

a well-recognized track record of performance. The company has been earning profits

continuously since 1971-72 and paying dividends since 1976-77.

B.H.E.L appeared on the power map of India in 1969 when the first unit supplied

by it was commissioned at the Basin Bridge Thermal Power Station in Tamil Nadu.

Within a decade, BHEL had commissioned the 100th unit at Santaldih. West Bengal.

BHEL had taken India from a near total dependence on imports to complete self-reliance

in this vital area of power plant equipment BHEL has supplied 97% of the power

generating equipment. BHEL has already supplied generating equipment to various

utilities capable of generating over 18000 MW power. Today BHEL can produce

annually; equipment capable of generating 6000MW. This will grow further to enable

BHEL to meet all of India projected power equipment requirements as well as sizeable

portion of export targets.

Probably the most significant aspect of BHEL.s growth has been its

diversification. The constant reorientation of the organization to meet the varied needs in

time with time a philosophy that has led to the development of a total capability

fromconcepts to commissioning not only in the field of energy but also in industry and

transportation.

BHEL manufactures over 180 products under 30 major product groups and caters to core

sectors of the Indian Economy viz., Power Generation & Transmission, Industry,

Transportation, Telecommunication, Renewable Energy, etc. The wide network of BHEL's

14 manufacturing divisions, four Power Sector regional centres, over 100 project sites, eight

service centres and 18 regional offices, enables the Company to promptly serve its customers

6

and provide them with suitable products, systems and services -- efficiently and at

competitive prices. The high level of quality & reliability of its products is due to the

emphasis on design, engineering and manufacturing to international standards by acquiring

and adapting some of the best technologies from leading companies in the world, together

with technologies developed in its own R&D centres. BHEL caters to core sectors of the

Indian Economy viz., Power Generation's & Transmission, Industry, Transportation,

Telecommunication, Renewable Energy, Defence, etc. BHEL has already attained ISO 9000

certification for quality management, and ISO 14001 certification for environment

management. The Company is engaged in engineering, development and manufacturing of a

wide variety of electrical and mechanical equipment for generation, transmission and

utilization of energy and electrical power. The company today enjoys national and

international presence featuring in the "Fortune International 500" and is ranked among the

top 10 companies in the world manufacturing power generation equipment.

BHEL has now thirteen manufacturing divisions, eight service centres and four power sector

regional centres, besides a large number of projects sites spread all over India and abroad.

BHEL's operations are organised around three business sectors, namely Power, Industry -

including Transmission, Transportation, Telecommunication & Renewable Energy - and

Overseas Business. This enables BHEL to have a strong customer orientation, to be sensitive

to his needs and respond quickly to the changes in the market.

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CHAPTER 2

BHEL-A BRIEF PROFILE

Over the years, Bharat Heavy Electricals Limited has emerged as world class Engineering and Industrial giant, the best of its kind in entire South East Asia. Its business profile cuts across various sectors of Engineering/Power utilities and Industry. The Company today enjoys national and international presence featuring in the "Fortune International-500" and is ranked among the top 12 companies in the world, manufacturing power generation equipment.

The Company is embarking upon an ambitions growth path through clear vision, mission and committed values to sustain and augment its image as a world class enterprise.

2.1 VISION

World-class, innovative, competitive and profitable engineering enterprise providing total business solutions.

2.2 MISSION

The leading Indian engineering enterprise providing quality products systems and services in the fields of energy, transportation, infrastructure and other potential areas.

2.3 VALUES

Meeting commitments made to external and internal customers. Foster learning creativity and speed of response. Respect for dignity and potential of individuals. Loyality and pride in the company. Team playing. Zeal to excel. Integrity and fairness in all matters.

The fourteen manufacturing Divisions are located at

Bhopal(Madhya Pradesh)

Bharat Heavy Electrical Limited, Ranipur, Haridwar (Uttarakhand) [4]

Hyderabad (Andhra Pradesh)

Jhansi (Uttar Pradesh)

Tiruchirapalli(Tamil Nadu)

8

Ranipet (Tamil Nadu)

Bangalore (Karnataka)

Jagdishpur (Uttar Pradesh)

Rudrapur (Uttrakhand)

Goindwal (Punjab)

Bharat Heavy Plates and Vessels Limited (Vizag)

Besides these manufacturing units there are four power sectors which undertake EPC

contract from various customers. The Research and Development arm of BHEL is situated in

Hyderabad and two repair shops are at HERP(Heavy Equipment Repair Plant),Varanasi and

EMRP(Electric machines repair plant) Mumbai. In 1976, BHEL entered into a collaboration

agreement with M/s Kraftwerk Union, AG of Germany for design, manufacturing, erection

and commissioning of large size steam turbines. More than 40 percent of the country's

electrical energy is generated from the power equipment supplied by BHEL, Haridwar.

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CHAPTER 3

BHEL-AN OVERVIEW

BHEL business operations cater to core sectors of the Indian Economy like:-

a) Power Generation

b) Transmission

c) Transportation

d) Industry

e) Defenses etc..

3.1 POWER GENERATION

Power generation sector comprises thermal, gas, hydro and nuclear power plant business as

of 31.03.2001, BHEL supplied sets account for nearly 64737 MW or 65% of the total

installed capacity of 99,146 MW in the country, as against nil till 1969-70. BHEL has proven

turnkey capabilities for executing power projects from concept to commissioning, it

possesses the technology and capability to produce thermal sets with super critical parameters

up to 1000 MW unit rating and gas turbine generator sets of up to 240 MW unit rating. Co-

generation and combined-cycle plants have been introduced to achieve higher plant

efficiencies. to make efficient use of the high-ash-content coal available in India, BHEL

supplies circulating fluidized bed combustion boilers to both thermal and combined cycle

power plants.

The company manufactures 235 MW nuclear turbine generator sets and has commenced

production of 500 MW nuclear turbine generator sets. Custom made hydro sets of Francis,

Pelton and Kapian types for different head discharge combination are also engineering and

manufactured by BHEL. In all, orders for more than 700 utility sets of thermal, hydro, gas

and nuclear have been placed on the Company as on date. The power plant equipment

manufactured by BHEL is based on contemporary technology comparable to the best in the

10

world and is also internationally competitive.

The Company has proven expertise in Plant Performance Improvement through renovation

modernisation and uprating of a variety of power plant equipment besides specialised know

how of residual life assessment, health diagnostics and life extension of plants.

3.2 POWER TRANSMISSION & DISTRIBUTION

BHEL offer wide ranging products and systems for T & D applications. Products

manufactured include power transformers, instrument transformers, dry type transformers,

series – and stunt reactor, capacitor tanks, vacuum – and SF circuit breakers gas insulated

switch gears and insulators.

A strong engineering base enables the Company to undertake turnkey delivery of electric

substances up to 400 kV level series compensation systems (for increasing power transfer

capacity of transmission lines and improving system stability and voltage regulation), shunt

compensation systems (for power factor and voltage improvement) and HVDC systems (for

economic transfer of bulk power). BHEL has indigenously developed the state-of-the-art

controlled shunt reactor (for reactive power management on long transmission lines).

Presently a 400 kV Facts (Flexible AC Transmission System) project under execution.

3.3 INDUSTRIES

BHEL is a major contributor of equipment and systems to industries, cement, sugar, fertilizer,

refinances, petrochemicals, paper, oil and gas, metallurgical and other process industries. The

range of system & equipment supplied includes: captive power plants, co-generation plants

DG power plants, industrial steam turbines, industrial boilers and auxiliaries. Wate heat

recovery boilers, gas turbines, heat exchangers and pressure vessels, centrifugal compressors,

electrical machines, pumps, valves, seamless steel tubes, electrostatic precipitators, fabric

filters, reactors, fluidized bed combustion boilers, chemical recovery boilers and process

control.

11

The Company is a major producer of large-size thruster devices. It also supplies digital

distributed control systems for process industries, and control & instrumentation systems for

power plant and industrial applications. BHEL is the only company in India with the

capability to make simulators for power plants, defense and other applications. The Company

has commenced manufacture of large desalination plants to help augment the supply of

drinking water to people.

3.4 TRANSPORTATION

BHEL is involved in the development design, engineering, marketing, production,

installation, maintenance and after-sales service of Rolling Stock and traction propulsion

systems. In the area of rolling stock, BHEL manufactures electric locomotives up to 5000

HP, diesel-electric locomotives from 350 HP to 3100 HP, both for mainline and shunting

duly applications. BHEL is also producing rolling stock for special applications viz.,

overhead equipment cars, Special well wagons, Rail-cum-road vehicle etc., Besides traction

propulsion systems for in-house use, BHEL manufactures traction propulsion systems for

other rolling stock producers of electric locomotives, diesel-electric locomotives, electrical

multiple units and metro cars. The electric and diesel traction equipment on India Railways

are largely powered by electrical propulsion systems produced by BHEL. The company also

undertakes retooling and overhauling of rolling stock in the area of urban transportation

systems. BHEL is geared up to turnkey execution of electric trolley bus systems, light rail

systems etc. BHEL is also diversifying in the area of port handing equipment and pipelines

transportation system.

3.5 TELECOMMUNICATION

BHEL also caters to Telecommunication sector by way of small, medium and large switching

systems.

12

3.6 RENEWABLE ENERGY

Technologies that can be offered by BHEL for exploiting non-conventional and renewable

sources of energy include: wind electric generators, solar photovoltaic systems, solar lanterns

and battery-powered road vehicles. The Company has taken up R&D efforts for development

of multi-junction amorphous silicon solar cells and fuel based systems.

3.7 INTERNATIONAL OPERATIONS

BHEL has, over the years, established its references in around 60 countries of the world,

ranging for the United States in the West to New Zealand in the Far East. These references

encompass almost the entire product range of BHEL, covering turnkey power projects of

thermal, hydro and gas-based types, substation projects, rehabilitation projects, besides a

wide variety of products, like transformers, insulators, switchgears, heat exchangers, castings

and forgings, valves, well-head equipment, centrifugal compressors, photo-voltaic equipment

etc. Apart from over 1110MW of boiler capacity contributed in Malaysia, and execution of

four prestigious power projects in Oman, Some of the other major successes achieved by the

Company have been in Australia, Saudi Arabia, Libya, Greece, Cyprus, Malta, Egypt,

Bangladesh, Azerbaijan, Sri Lanka, Iraq etc. The Company has been successful in meeting

demanding customer's requirements in terms of complexity of the works as well as

technological, quality and other requirements viz extended warrantees, associated O&M,

financing packages etc.

BHEL has proved its capability to undertake projects on fast-track basis. The company has

been successful in meeting varying needs of the industry, be it captive power plants, utility

power generation or for the oil sector requirements. Executing of Overseas projects has also

provided BHEL the experience of working with world renowned Consulting Organisations

and inspection Agencies.

13

In addition to demonstrated capability to undertake turnkey projects on its own, BHEL

possesses the requisite flexibility to interface and complement with International companies

for large projects by supplying complementary equipment and meeting their production needs

for intermediate as well as finished products.

The success in the area of rehabilitation and life extension of power projects has established

BHEL as a comparable alternative to the original equipment manufacturers (OEMs) for such

plants.

3.8 TECHNOLOGY UPGRADATION AND RESEARCH &

DEVELOPMENT

To remain competitive and meet customers' expectations, BHEL lays great emphasis on the

continuous upgradation of products and related technologies, and development of new

products. The Company has upgraded its products to contemporary levels through continuous

in house efforts as well as through acquisition of new technologies from leading engineering

organizations of the world.

The Corporate R&D Division at Hyderabad, spread over a 140 acre complex, leads BHEL's

research efforts in a number of areas of importance to BHEL's product range. Research and

product development centers at each of the manufacturing divisions play a complementary

role. BHEL's Investment in R&D is amongst the largest in the corporate sector in India.

Products developed in-house during the last five years contributed about 8.6% to the revenues

in 2000-2001.

BHEL has introduced, in the recent past, several state-of-the-art products developed in-house:

low-NQx oil / gas burners, circulating fluidized bed combustion boilers, high-efficiency

Pelton hydro turbines, petroleum depot automation systems, 36 kV gas-insulated sub-stations,

etc. The Company has also transferred a few technologies developed in-house to other Indian

companies for commercialisation.

14

Some of the on-going development & demonstration projects include: Smant wall blowing

system for cleaning boiler soot deposits, and micro-controller based governor for diesel-

electric locomotives. The company is also engaged in research in futuristic areas, such as

application of super conducting materials in power generations and industry, and fuel cells

for distributed, environment-friendly power generation.

3.9 HUMAN RESOURCE DEVLOPMENT INSTITUTE

The most prized asset of BHEL is its employees. The Human Resource Development

Institute and other HRD centers of the Company help in not only keeping their skills updated

and finely honed but also in adding new skills, whenever required. Continuous training and

retraining, positive, a positive work culture and participative style of management, have

engendered development of a committed and motivated work force leading to enhanced

productivity and higher levels of quality.

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CHAPTER 4

BHEL-HEEP

[HEAVY ELECTRICAL EQUIPMENT PLANT]

At Hardwar, against the picturesque background of Shivalik Hills, 2 important manufacturing units of BHEL are located viz. Heavy Electrical Equipment Plant (HEEP) & Central Foundry Forge Plant (CFFP). The hum of the construction machinery woke up Shivalik Hills during early 60s and sowed the seeds of one of the greatest symbol of Indo Soviet Collaboration – Heavy Electrical Equipment Plant of BHEL. Following is the brief profile of Heavy Electrical Equipment Plant:-

4.1 ESTABLISHMENT AND DEVELOPMENT STAGES

Established in 1960s under the Indo-Soviet Agreements of 1959 and 1960 in the area of Scientific, Technical and Industrial Cooperation.

DPR – prepared in 1963-64, construction started from October '63. Initial production of Electric started from January, 1967. Major construction / erection / commissioning completed by 1971-72 as per original

DPR scope. Stamping Unit added later during 1968 to 1972. Annual Manufacturing capacity for Thermal sets was expanded from 1500 MW to

3500 MW under LSTG. Project during 1979-85 (Sets upto 500 MW, extensible to 1000/1300 MW unit sizes with marginal addition in facilities with the collaboration of M/s KWU-Siemens, Germany.

Motor manufacturing technology updated with Siemens collaboration during 1984-87. Facilities being modernized continually through Replacements / Reconditioning-

Retrofitting, Technological / operational balancing.

4.2 INVESTMENTS

Gross Block as on 31.3.95 is Rs. 355.63 Crores (Plant and Machinery – Rs. 285.32 Crores).

Net Block as on 31.3.95 is Rs. 113.81 Crores (Plant & Machinery – Rs. 76.21 Crores).

4.3 CLIMATIC AND GEOGRAPHICAL

Hardwar is in extreme weather zone of the Western Uttar Pradesh of India and temperature varies from 2oC in Winter (December to January) to 45oC in Summer (April-June); Relative humidity 20% during dry season to 95-96% during rainy season.

16

Longitude 78o3' East, Latitude 29 o55'5" North. Height above Mean Sea Level = 275 metres.

4.5 POWER & WATER SUPPLY SYSTEM:

40 MVA sanctioned Electric Power connection from UP Grid (132 KV / 11KV / 6.6 KV) (Connected load – around 185 MVA)

26 deep submersible Tube Wells with O.H. Tanks for water supply. A 12 MW captive thermal power station is located in the factory premises.

4.6 HEEP PRODUCT PROFILE

THERMAL AND NUCLEAR SETS

(Turbines, Generators, Condensers and Auxiliaries of unit capacity upto 1000 MW)

HYDRO SETS INCLUDING SPHERICAL AND DISC VALVES

(Kaplan, Francis, Pelton and reversible Turbines of all sizes and matching generators

and auxiliaries maximum runner dia – 6600 mm)

ELECTRICAL MACHINES:

(For various industrial applications, pump drives & power station auxiliaries, Unit

capacity upto 20000 KW AC / DC)

CONTROL PANELS

(For Thermal / Hydro sets and Industrial Drives)

LARGE SIZE GAS TURBINES

(Unit Rating : 60-200 MW)

LIGHT AIRCRAFT

DEFENSE PRODUCTS

17

CHAPTER 5

ELECTRICAL MACHINES

5.1 INTERODUCTION

1. Block-1 is designed to manufacture hydro-generators and large and

medium size AC and DC Electrical machines. Equipment layout plan is as

per drawing No.003 appended in section lll.

2. The Block consists of 3 Bays: Bay-1(36*482 meters), Bay- 2, (36*360 meters) and Bay-3

of size (24*360 meters) each. For handling and transporting the various components over-

head crane facilities are available, depending upon the products manufactured in each bay.

There

are also a number of self propelled electrically driven transfer trolleys for

the inter-bay movements of components/assemblies

3. Conventional Bay-wise broad distribution of products is as

Follows:

LSTG Area Large Size Turbo Generators

BAY-1 hydro generators, their exciters;PMG and Large size turbo

generators

BAY-2 Turbo-generators, their exciters and Heavy Electrical Motors

BAY-3 Medium size Electrical Motors

4. Testing facilities for turbo generators and heavy motors are available in bat-2 and for

medium size motors in bay-3

5. There is a special test bed for testing of T.G. of capacity of 500 MW units sizes and above.

There are three BAY in BLOCK 1

BAY-I [Hydro Generator]

Machine Section

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Assembly Section

Stator Winding

BAY-II [Turbo Generator]

Machine Section

Iron Section

Heavy Rotor Assembly Section

Strator Winding Section

Armature/Rotor Section

Armature/Rotor Impregnation Section

General Assembly Section

Test Stands

BAY-III [Medium Size Motors]

Machine Section

Iron Assembly Section

Commutator Section

Pole Core Section

Painting Section

Bus bar & filling Section

Winding Section

General Assembly Section

Test Stand

5.2 Bay-1

(A) LARGE SIZE TURBO GENERATORS (LSTG AREA)

Following facilities are available in different sections of this area:-

a. Stators core assembly section

Two number core pits with core building and pressing facilities are available in this section.

The section is also equipped with optical centering device, core heating installation and core

loss testing facilities.

b. Stator winding section

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The section is located in dust-proof enclosure and equipped with one number-winding

platform with two number rotating installations for assembly of winding. Resistance brazing

machines and high voltage transformer are also available in this section.

c. Rotor assembly section

This section is also in a dust-proof enclosure with number of rotators, rotors bar laying

facilities and MI heating and mounting of retaining rings, rotor winding assembly and rotor

assembly like retaining ring fitting, four assemblies are carried out in this section.

d. Bar preparation section

This section consist of milling machine for long preparation, installation for insulation of

tension bolts for stator and preparation of stator winding before assembly.

e. Test bed section

This section this section is equipped with bedplates, height blocks, supporting blocks and

12MW drive motors. Large size turbo generators and exciters are assembled and tested in this

section.

5.3 Bay-2

(B)Heavy Electrical Motors

The following are the main sections:-

(a.)Machine section

This section is equipped with large size CNC and

conventional machine tools such as lathes and vertical boring, horizontal

boring machine, rotor slot milling and radial drilling machines for machining

stator body, rotor shaft end shields, bearing etc. for turbo-generators.

(b.) Iron assembly section

This section has facilities for stator core assembly of

turbo-generators and heavy electrical motors. 1000T and 250T umbrella type

presses for pressing the cores and transformers for induction heating of the

armature core of large size electric motor are available in this section.

(c.) Armature section

This section is equipped with installations like

bandaging m//c, tensioning devices, and magnetic putty application machine

and 45KW MF brazing m/c for laying windows in large size DC armatures.

(d.) General assembly section: -

20

General assembly of large size AC and DC motors is carried out in this section

CHAPTER 6

TURBO-GENERATOR

The turbo-generator is common-shaft excitation AC synchronous generator with 3 phases, 2

poles or with 3 phases, 4 poles.

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. These turbo generators are supplied together

with the turbines and matching excitation systems.

6.1 500 MW Turbo generators at a glance

2-Pole machine with the following features:-

Direct cooling of stator winding with water.

Direct hydrogen cooling for rotor.

Mica insulation system

Spring mounted core housing for effective transmission of vibrations.

Brushless Excitation system.

Vertical hydrogen coolers

Salient technical data

Rated output : 588 MVA , 500 MW

Terminal voltage : 21 KV

21

Rated stator current : 16 KA

Rated frequency : 50 Hz

Rated power factor : 0.85 Lag

Efficiency : 98.55%

Important dimensions & weights

Heaviest lift of generator stator : 255 Tons

Rotor weight : 68 Tons

Overall stator dimensions [LxBxH] : 8.83Mx4.lMx4.02M

Rotor dimensions : 1.15M dia x 12.11 M length

Total weight of turbo generator : 428 Tons

Unique installations

Heavy Electrical Equipment Plant, Haridwar is one of the best equipped and most modern

plants of its kind in the world today.

6.2 General components

The general components of a turbo generator are :

Stator

1. Stator Frame

2. Stator Core

3. Stator Windings

4. End Covers

Rotor

1. Rotor Shaft

2. Rotor Windings

3. Rotor Retaining Rings

Bearings

22

Cooling System

Excitation

Fig 6.1- Turbo Generator

23

6.3CONSTRUCTIONAL FEATURES OF STATOR BODY

6.3.1 Stator Frame

Stator body is a totally enclosed gas tight fabricated structure made up of high quality mild

steel and austenitic steel. It is suitably ribbed with annular rings in inner walls to ensure high

rigidity and strength .The arrangement, location and shape of inner walls is determined by the

cooling circuit for the flow of the gas and required mechanical strength and stiffness.

The natural frequency of the stator body is well away from any of exiting frequencies. Inner

and sidewalls are suitably blanked to house for longitudinal hydrogen gas coolers inside the

stator body.

6.3.2 Pipe Connection

To attain a good aesthetic look, the water connection to gas cooler is done by routing

stainless steel pipes; inside the stator body; which emanates from bottom and emerges out of

the sidewalls.

These stainless steel pipes serve as inlet and outlet for gas coolers. From sidewall these are

connected to gas coolers by the means of eight U-tubes outside the stator body. For filling the

generator with hydrogen, a perforated manifold is provided at the top inside the stator body.

6.3.3 Terminal Box

The bearings and end of three phases of stator winding are brought out to the slip-ring end of

the stator body through 9 terminal brushing in the terminal box. The terminal box is a welded

construction of (non magnetic) austenitic steel plates. This material eliminates stray losses

due to eddy currents, which may results in excessive heating.

6.3.4 Testing Of Stator Body –

On completion of manufacture of stator body, it is subjected to a hydraulic pressure of 8

kg/cm for 30 minutes for ensuring that it will be capable of withstanding all expansion

24

pressure, which might arise on account of hydrogen air mixture explosion. Complete stator

body is then subjected to gas tightness test by filling in compressed air.

6.4. CONSTRUCTIONAL FEATURES STATOR CORE

6.4.1. Core

It consists of thin laminations. Each lamination made of number of individual segments.

Segments are stamped out with accurately finished die from the sheets of cold rolled high

quality silicon steel. Before insulation on with varnish each segment is carefully debarred.

Core is stacked with lamination segments. Segments are assembled in an interleaved manner

from layer to layer for uniform permeability.

Stampings are held in a position by 20 core bars having dovetail section. Insulating paper

pressboards are also put between the layer of stamping to provide additional insulation and to

localize short circuit. Stampings are hydraulically compressed during the stacking procedure

at different stages. Between two packets one layer of ventilating segments is provided. Steel

spacers are spot welded on stamping.

These spacers from ventilating ducts where the cold hydrogen from gas coolers enter the core

radialy inwards there by taking away the heat generated due to eddy current losses. The

pressed core is held in pressed condition by means of two massive non-magnetic steel

castings of press ring. The press ring is bolted to the ends of core bars.

The pressure of the pressure ring is transmitted to stator core stamping through press fringes

of non-magnetic steel and duralumin placed adjacent to press ring. To avoid-heating of press

ring due to end leakage flow two rings made of copper sheet are used on flux shield. The ring

screens the flux by short-circuiting. To monitor the formation of hot spots resistance

transducer are placed along the bottom of slots. To ensure that core losses are with in limits

and there are no hot spots present in the core. The core loss test is done after completion of

core assembly.

6.4.2. Core Suspension

25

The elastic suspension of core consist of longitudinal bar type spring called core bars. Twenty

core bars are welded to inner walls of stator body with help of brackets. These are made up of

spring steel having a rectangular cross section and dove-tail cut at tap, similar type of dove-

tail is also stamped on to stamping and fit into that of core bar dove-tail.

Thus offering a hold point for stamping core bars have longitudinal slits which acts as inertial

slots and help in damping the vibrations. The core bars are designed to maintain the

movement of stator core with in satisfactory limits.

26

Fig 6.2 Stator core

6.4CONSTRUCTIONAL FEATURES OF STATOR WINDING

6.5.1. General

27

The stator has a three phase, double layer, short pitched and bar type of windings having two

parallel paths. Each slots accommodated two bars. The slot lower bars and slot upper are

displaced from each other by one winding pitch and connected together by bus bars inside the

stator frame in conformity with the connection diagram.

6.5.1. Conductor Construction

Each bar consist of solid as well as hollow conductor with cooling water passing through the

latter. Alternate arrangement hollow and solid conductors ensure an optimum solution for

increasing current and to reduce losses. The conductors of small rectangular cross section are

provided with glass lapped strand insulation.

A separator insulates the individual layers from each other. The transposition provides for

mutual neutralization of voltage induced in the individual strands due to the slots cross field

and end winding field. The current flowing through the conductor is uniformly distributed

over the entire bar cross section reduced.

To ensure that strands are firmly bonded together and give dimensionally stability in slot

portion, a layer of glass tape is wrapped over the complete stack. Bar insulation is done with

epoxy mica thermosetting insulation. This insulation is void free and posses better

mechanical properties. This type of insulation is more reliable for high voltage.This

insulation shows only a small increase in dielectric dissipation factor with increasing test

voltage. The bar insulation is cured in an electrically heated process and thus epoxy resin fill

all voids and eliminate air inclusions.

6.5.3. Method Of Insulation

Bar is tapped with several layers of thermosetting epoxy tape. This is applied continuously

and half overlapped to the slot portion. The voltage of machine determines the thickness of

insulation. The tapped bar is then pressed and cured in electrical heated press mould for

certain fixed temperature and time.

6.5.4. Corona Prevention

28

To prevent corona discharges between insulation and wall of slots, the insulation in

slot portion is coated with semiconductor varnish. The various test for manufacture

the bar are performed which are as follows-

Inter turn insulation test on stuck after consolidation to ensure absence of inter short.

Each bar is subjected to hydraulic test to ensure the strength of all joints.

Flow test is performed on each bar to ensure that there is no reduction in cross section

area of the ducts of the hollow conductor.

Leakage test by means of air pressure is performed to ensure gas tightness of all

joints.

High voltage to prove soundness of insulation.

Dielectric loss factor measurement to establish void free insulation.

6.5.5. Laying Of Stator Winding –

The stator winding is placed in open rectangular slots of the stator core, which are uniformly

distributed on the circumference. A semi conducting spacer is placed in bottom of slots to

avoid any damage to bar due to any projection. Driving in semi conducting filler strips

compensates any manufacturing tolerances.

After laying top bar, slot wedges are inserted. Below slots wedges, high strength glass

texolite spacers are put to have proper tightness. In between top and bottom bars, spacers are

also put.

6.5.6. Ending Winding –

In the end winding, the bars are arranged close to each other. Any gaps due to design or

manufacturing considerations are fitted with curable prepag with spacer in between. The

prepag material is also placed between the brackets and binding rings. Lower and upper

layers are fixed with epoxy glass ring made in segment and flexible spacer put in between

two layers.

Bus bars are connected to bring out the three phases and six neutrals. Bus bars are also

hollow from inside. These bus bars are connected with terminal bushing. Both are water-

cooled. Brazing the two lugs properly makes connection.

6.6 CONSTRUCTIONAL FEATURES OF ROTOR

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The rotor comprises of following component:

Rotor shaft

Rotor winding

Rotor wedges and other locating parts for winding

Retaining ring

Fans

Field lead connections

6.6.1 Rotor Shaft

The rotor shaft is a single piece solid forging manufactured from a vacuum casting.

Approximately 60 % of the rotor body circumference is with longitudinal slots, which hold

the field winding. The rotor shaft is a long forging measuring more than 9m in length and

slightly more than one meter in diameter. The main constituents of the steel are chromium,

molybdenum, nickel and vanadium. The shaft and body are forged integral to each other by

drop forging process.

Following tests are done: -

Mechanical test

Chemical analysis

Magnetic permeability test

Micro structure analysis

Ultrasonic examination

Boroscope examination

On 2/3 of its circumference approximately the rotor body is provided with longitudinal slot to

accommodate field winding. The slot pitch is selected in such a way that two solid poles

displaced by 180o C are obtained. For high accuracy the rotor is subjected to 20% over

speeding for two minutes. The solid poles are provided with additional slots in short lengths

of two different configurations.

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One type of slots served as an outlet for hydrogen which has cooled the overhang winding

and other type used to accommodate finger of damper segments acting as damper winding.

Fig 6.3 Rotor Shaft

6.6.2 Rotor Winding

After preliminary turning, longitudinal slots are milled on sophisticated horizontal slot

milling machine. The slot house the field winding consists of several coils inserted into the

longitudinal slots of rotor body

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Fig 6.4 Rotor Winding

6.6.2.1 Copper Conductor

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The conductors are made of hard drawn silver bearing copper. The rectangular cross section

copper conductors have ventilating ducts on the two sides thus providing a channel for

hydrogen flow. Two individual conductors placed-one over the other are bent to obtain half

turns. Further these half turns are brazed in series to form coil on the rotor model.

6.6.2.2 Insulation

The individual turns are insulated from each other by layer of glass prepag strips on turn of

copper and baked under pressure and temperature to give a monolithic inter turn insulation.

The coils are insulated from rotor body by U-shaped glass laminate module slot through

made from glass cloth impregnated with epoxy varnish.

At the bottom of slot D-shaped liners are put to provide a plane seating surfaces for

conductors and to facilitate easy flow of gas from one side to another. These liners are made

from molding material. The overhang winding is separated by glass laminated blocks called

liners. The overhang winding are insulated from retaining rings segments having L-shape and

made of glass cloth impregnated by epoxy resin.

6.6.3 Cooling Of Winding

The rotor winding are cooled by means of direct cooling method of gap pick-up method. In

this type of cooling the hydrogen in the gap is sucked through the elliptical holes serving as

scoop on the rotor wedges and is directed to flow along lateral vent ducts on rotor cooper

coils to bottom of the coils. The gas then passes into the corresponding ducts on the other side

and flows outwards and thrown into the gap in outlet zones.

In this cooling method the temperature rise becomes independent of length of rotor. The

overhang portion of the winding is cooled by axial two systems and sectionalized into small

parallel paths to minimize temperature rise. Cold gas enters the overhang from under the

retaining rings through special chamber in the end shields and ducts under the fan hub and

gets released into the air gap at rotor barrel ends.

6.6.3.1 Rotor Wedges

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For protection against the effect of centrifugal force the winding is secured in the slots by slot

wedge. The wedges are made from duralumin, an alloy of copper, magnesium and aluminum

having high good electrical conductivity and high mechanical strength.

The wedges at the ends of slot are made from an alloy of chromium and copper. These are

connected with damper segments under the retaining ring for short circuit induced shaft

current. Ventilation slot wedges are used to cover the ventilation canals in the rotor so that

hydrogen for overhang portion flows in a closed channel.

6.6.3.2 Retaining Ring

The overhang portion of field winding is held by non-magnetic steel forging of retaining ring

against centrifugal forces. They are shrink fitted to end of the rotor body barrel at one end;

while at the other side of the retaining ring does not make contact with the shaft.

The centering rings are shrink fitted at the free end of retaining ring that serves to reinforce

the retaining ring, securing, end winding in axial direction at the same time. To reduce stray

losses, the retaining rings are made of non-magnetic, austenitic steel and cold worked,

resulting in high mechanical strength.

6.6.3.3 Fans

Two single stage axial flow propeller type fans circulate the generator cooling gas. The fans

are shrink fitted on either sides of rotor body. Fans hubs are made of alloy steel forging with

three peripheral grooves milled on it. Fan blades, which are precision casting with special

alloy, are machined in the tail portion so that they fit into the groove of the fan hub.

6.6.4 Field Lead Connections

6.6.4.1 Slip Rings

The slip ring consists of helical grooved alloy steel rings shrunk on the body shaft and

insulated from it. The slip rings are provided with inclined holes for self-ventilation. The

helical grooves cut on the outer surfaces of the slip rings improve brush performance by

breaking the pressurized air pockets that would otherwise get formed between the brush and

slip rings.

6.6.4.2 Field Lead

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The slip rings are connected to the field winding through semi flexible copper leads and

current-carrying bolts placed in the shaft. The radial holes with current carrying bolts in the

rotor shafts are effectively sealed to prevent the escape of hydrogen.

A field lead bar, which has similar construction as, does the connection between current

carrying bolt and field winding that of semi flexible copper leads (they are insulated by glass

cloth impregnated with epoxy resin for low resistance and ease of assembly).

6.7 COOLING SYSTEM

Heat losses arising in generator interior are dissipated to secondary coolant (raw water,

condensate etc.) through hydrogen and Primary water. Direct cooling essentially eliminates

hot spots and differential temperature between adjacent components, which could result in

mechanical stresses, particularly to the copper conductors, insulation, rotor body and stator

core.

6.7.1 Hydrogen Cooling Circuit

The hydrogen is circulated in the generator interior in a closed circuit by one multistage axial

flow fan arranged on the rotor at the turbine end. Hot gases is drawn by the fan from the air

gap and delivered to the coolers where it is recooled and then divided into three flow paths

after each cooler:

Flow path I

Flow path I is directed into the rotor at the turbine end below the fan hub for cooling of the

turbine end half of the rotor.

Flow path II

Flow path II is directed from the cooler to the individual frame compartments for cooling of

the stator core.

Flow path III

Flow path III is directed to the stator end winding space at the exciter end through guide

ducts in the frame of cooling of the exciter end half of the rotor and of the core end portion.

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The three flow paths miss the air gaps. The gas is then returned to the coolers via the axial

flow fan.The cooling water flow through the hydrogen coolers should automatically control

to maintain a uniform generator temperature level for various loads and cold-water

temperature.

6.7.2 Cooling Of Rotors

For direct cooling of rotor winding cold gas is directed to the rotor end wedges at the turbine

and exciter ends. The rotor winding is symmetrical relative to generator centerline and pole

axis. Each coil quarter is divided into two cooling zones consists of the rotor end winding and

the second one of the winding portion between the rotor body end and the midpoint of the

rotor. Cold gas is directed to each cooling zone through separate openings directly before the

rotor body end. The hydrogen flows through each individual conductor is closed cooling

ducts. The heat removing capacity is selected such that approximately identical temperature

is obtained for all conductors. The gas of the first cooling zone is discharged from the coils at

the pole center into a collecting compartment within the pole area below the end winding

from the hot gases passes into air gap through the pole face slots at the end of the rotor body.

The hot gas of the second cooling zone is discharged into the air gap at the mid length of the

rotor body through radial openings in the hollow conductors and wedges.

6.7.3 Cooling of stator core

For cooling of the stator core, cold gas is passes to the individual frame compartment via

separate cooling gas ducts.

From these frames compartment the gas then flow into the air gap through slots and the core

where it absorbs the heat from the core. To dissipate the higher losses in core ends the

cooling gas section. To ensure effective cooling. These ventilating ducts are supplied from

end winding space. Another flow path is directed from the stator end winding space paste the

clamping fingers between the pressure plate and core section into the air gap along either side

of flux shield.

All the flows mix in the air gap and cool the rotor body and stator bore surfaces. The air gap

is then returned to the coolers via the axial flow fan. To ensure that the cold gas directed to

the exciter end cannot be directly discharged into the air gap. An air gap choke is arranged

with in the stator end winding cover and the rotor retaining rings at the exciter end.

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6.7.3.1Primary Cooling Water Circuit In The Generators

The treated water used for cooling of the stator winding, phase connectors and bushings is

designated as primary water in order to distinguish it from the secondary coolant (raw water,

condensator etc.). The primary water is circulated in a closed circuit and dissipates the

absorbed heat to the secondary cooling in the primary water cooler. The pump is supplied

with in primary water cooler. The pump is supplied with in the primary water tank and

delivers the water to the generator via the following flow paths:

Flow path I

Flow path I cools the stator winding. This flow path passes through water manifold on the

exciter end of the generator and from there to the stator bars via insulated bar is connected to

the manifold by a separate hose. Inside the bars the cooling water flows through hollow

strands. At the turbine end, the water is passed through the similar hoses to another water

manifold and then return to the primary water tank. Since a single pass water flow through

the stator is used, only a minimum temperature rise is obtained for both the coolant and the

bars. Relatively movements due to the different thermal expansions between the top and the

bottom bars are thus minimized.

Flow Path II

Flow path II cools the phase connectors and the bushings. The bushing and the phase

connectors consist of the thick walled copper tubes through which the cooling water is

circulated. The six bushings and phase connectors arranged in a circle around the stator

winding are hydraulically interconnected so that three parallel flow paths are obtained. The

primary water enters three bushings and exits from the three remaining bushings. The

secondary water flow through the primary water cooler should be controlled automatically to

maintain a uniform generator temperature level for various loads and cold-water

temperatures.

6.8 EXCITATION SYSTEM:

In large synchronous machines, the field winding is always provided on the rotor, because it

has certain advantages they are:

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It is economical to have armature winding on the stator and field winding on the rotor.

Stationary armature windings can be insulated satisfactorily for higher voltages, allowing

the construction of high voltage synchronous machines.

Stationary armature winding can be cooled more efficiently.

Low power field winding on the rotor gives a lighter rotor and therefore low centrifugal

forces. In view of this, higher rotor speeds are permissible, thus increasing the

synchronous machine output for given dimensions.

6.8.1 Design features

The excitation system has a revolving field with permanent magnet poles. The three-phase ac

output of this exciter is fed to the field of the main exciter via a stationary regulator &

rectifier unit. Three-phase ac induced in the rotor of the main exciter is rectified by the

rotating Rectifier Bridge & supplied to the field winding of the generator rotor through the dc

lead in the rotor shaft.

A common shaft carries the rectifier wheels, the rotor of the main exciter & PMG rotor. The

shaft is rigidly coupled to the generator rotor. The generator & exciter rotors are supported

on total three bearings. .

6.8.1.1Three Phase Pilot Exciter

It is a six-pole revolving field unit. The frame accommodates the laminated core with the

three-phase winding. Each pole consists of separate permanent magnets that are housed in a

non-magnetic metallic enclosure.

6.8.1.2Three phase main exciter

The three phase main exciter is a six-pole armature-revolving unit. The field winding is

arranged on the laminated magnetic poles. At the pole shoe, bars are provided which are

connected to form a damper winding. Between the two poles, a quadrature-axis coil is

provided for inductive measurement of the field current. After completing the winding &

insulation etc., the complete rotor is shrunk on the shaft.

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6.8.1.3 Rectifier wheels

The silicon diode is the main component of the rectifier wheels, which are arranged in a

three-phase bridge circuit. With each diode, a fuse is provided which serves to cut off the

diode from the circuit if it fails. For suppression of the momentary voltage peaks arising from

commutation, R-C blocks are provided in each bridge in parallel with each set of diodes. The

rings, which form the positive & negative side of the bridge, are insulated from the rectifier

wheel which in turn is shrunk on the shaft. The three phase connections between armature &

diodes are obtained via copper conductors arranged on the shaft circumference between the

rectifier wheels & the main exciter armature.

6.8.2 Voltage regulator

The voltage regulator is intended for the excitation and control of generators equipped with

alternator exciters employing rotating uncontrolled rectifiers. The main parts of the regulator

equipment are two closed-loop control systems including a separate gate control set and

thyistor set each, field discharge circuit, an open loop control system for exchanging signal

between the regulator equipment and the control room, and the power supply circuits.

6.8.2.1 Voltage regulation

The active and reactive power ranges of the generator ve require a wide excitation setting

range. The voltage regulator in the restricted sense, i.e. the control amplifiers for the

generator voltage controls via the gate control set the thyristors so as they provide quick

correction of the generator voltage on changing generator load. For this purpose the gate

control set changes the firing angle of the thyristors as a function of the output voltage of the

voltage regulator. The main quantities acting on the input of the voltage regulator are the

setpoint and the actual value of the generator voltage. The setpoint is divided into a basic

setpoint (e.g. 90% rated voltage) and an additional value (e.g. 0 to 20%), which can be

adjusted from the control room. In this case the setting range is 90 to 110%. With operation

at the respective limits of the capability curve, further, influencing variable are supplied by

the under and over excitation limiters. To partly compensate the voltage drop at the unit

transformer, a signal proportional to the reactive current can be added to the input, the

controlled voltage level then rising together with the reactive current (overexcited) thereby

increasing the generator degree of activity in compensating system voltage functions.

Further, signals can be added if necessary via free inputs.

39

40

The various schemes, for supplying D.C. excitation to the field winding to large turbo generators are given below:

The Pilot Exciter and the main exciter are driven by the turbo generators main shaft.

The pilot Exciter, which is a small D.C. shunt generator, feeds the field winding of

main exciter is given to the field winding of the main alternator, through slip-rings

and brushes. The function of the regulator is to keep the alternator terminal voltage

constant at a particular value.

In this second scheme it consists of main A.C. exciter and stationary solid-state

rectifier. The A.C. main exciter, which is coupled to shaft of generator, has rotating

field and stationary armature. The armature output from the A.C. exciter has a

frequency of about 400 Hz. This output is given to the stationary solid-state controlled

rectifier. After rectification, the power is fed to the main generator field, through slip

rings and brushes.

In third scheme the A.C exciter, coupled to the shaft that drives the main generator,

has stationary field and rotating 3-phase armature. The 3-phase power from the A.C

exciter is fed, along the main shaft, to the rotating silicon-diode rectifiers mounted on

the same shaft. The output from these rectifiers is also given, along the main shaft, to

the man generator field, without any slip rings and brushes. In the other words, the

power flows along the wires mounted on the main shaft, from the A.C. exciter to the

silicon diode rectifiers and then to the main generator field. Since the scheme does not

require any sliding contacts and brushes, this arrangement of exciting the turbo

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Fig. 6.6 BRUSHLESS EXCITOR STATOR

generators has come to be called as Brush less Excitation system.

For large turbo generators of 500 MW excitation systems, the direct cooling required by the

rotating field winding increases considerably (up to 10 kA or so). In such cases, the brush

gear design becomes more complicated and reliability of turbo generator operation decreases.

The only promising solution of feeding the field winding of large turbo generator is the brush

less excitation system. In view of its many advantages, the brush less excitation system is

employed in almost all large turbo generators being designed and manufactured now days.

Here are some merits of Brush less Exciters.

Eliminates slip rings, brush gear, field breaker and excitation bus/cables.

Eliminates all the problems associated with transfer of current via sliding contacts.

Simple, reliable and ideally sited for large sets.

Minimum operation and maintenance cost.

Self-generating excitation unaffected by system faults or disturbances of shaft

mounted pilot exciter.

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CHAPTER 7

BAR WINDING

Bar winding is carried out in separate shop in BHEL factory, Haridwar .This shop is meant

for manufacturing of stator winding coils of generator that may be Turbo-generator

or Hydro-generator. Bars manufactured are of standard capacity such as 100MW, 130MW,

210/235MW, 500MW.

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Fig 7.1 Bar Windings

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Fig 7.2 Stator and bar winding

It is quite difficult (rather impossible) to manufacture, handle and wind the coil in slot of

generator of higher generation capacity because of its bigger size and heavy weight.

That is why we make coil in two parts. They are:

Bottom part (called as lower bar)

Top part (called as upper part)

7.1 Manufacturing process of Bars

The manufacturing procedure for bars can be classified as follows-

7.1.1 Conductor cutting

This process is done by automatic CNC machine In this process the pre

insulated copper conductor is cut into number of pieces of required length

(as per given design).Insulation is removed from the both ends of copper

conductor cut.

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7.1.2 Transposition

Transposition means changing/shifting of position of each conductor in active

core (slot) part. After cutting the required number of conductor ,the

conductors are arranged on the comb in staggered manner and then bends

are given to the conductor with the help of bending die at required distance

.Then the conductors are taken out from the comb and die and placed with

their ends in a line and transposition is carried out. This process is repeated

for making another half of the bar which would be mirror image of the first

half .The two halves of the bar are overlapped over each other and a spacer

is placed between the two halves.

7.1.3 Crossover insulation

The pre insulation of the copper conductor may get damaged due to

mechanical bending in die during transposition, hence the insulation spacers

are provided at the crossover portion of the conductors .A filler material

( insulating putty of moulding micanite ) is provided along the height of the

bar to maintain the rectangular shape and cover the difference of level of

conductors.

7.1.4 Stack Consolidation

The core part of the bar stack is pressed in press (closed box) under pressure (varies from

product to product) and temperature of 1600C for a given period .The consolidated stack is

withdrawn from the press and the dimensions are checked.

7.1.5 Inter strand short test

The consolidated bar stack is tested for the short between any two conductors in the bar. If

found then it has to be rectified.

7.1.6 Forming

The straight bar stack is formed as per overhang profile (as per design). The overhanging

portion is consolidated after forming.

7.1.7 Brazing of coil lugs

For water cooled generator bars, the electrical connection contact and water box for inlet and

outlet of water are brazed.

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7.1.8 Nitrogen leak test

The bar is rested for water flow test , nitrogen leak test and pressure test for given duration.

7.1.9 Thermal Sock Test

The cycles of hot (800C) and cold (300C) water are flew through the bar to ensure the

thermal expansion and contraction of the joints.

7.1.10 Helium Leakage Test

After thermal shock test bar is tested for any leakage with the help of Helium gas.

7.1.11 Insulation

The bar is insulated with the given number of layers to build the wall thickness of insulation

subjected to the generating voltage of the machine.

Fig 8.4 Insulated Bar

7.2Impregnation and Baking:

7.2.1 Thermoelectric system

In case of rich resin insulation the bar is pressed in closed box in heated condition and baked

under pressure and temperature for a given period.

7.2.2 Micalastic Sytem

In case of poor resin system the insulated bars are heated under vaccum and the impregnated

(dipped) in heated resin so that all the air gaps are filled layer by layer with resin. Then extra

resin is draied and bars are heated and baked under pressure condition in closed box fixture.

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7.2.3 VPI Micalastic System

The bars already laid in closed fixture and full fixture is impregnated (dipped) in resin and

then fixture with box is baked under given temperature for given duration.

7.2.4 VIP Micalastic System

The individual (separate) bar is heated in vaccum and impregnated in resin. Then bar is taken

out and pressed in closed bx fixture and then baked at given temperature for given duration.

7.2.5 Finishing

The baked and dimensionally correct bars are sanded-off to smoothen the edges and the

surfaces is calibrated, if required, for the dimension.

7.2.6 Conducting Varnish Coating

OCP (Outer Corona Protection) Coating: The black semi conducting varnish coating is

applied on the bar surface on the core length ECP (End Corona Protection) Coating: The gray

semi conducting varnish is applied at the bent outside core end of bars in gradient to prevent

from discharge and minimize the end corona.

7.3 Testing

7.3.1 Tan Test: This test is carried to ensure the healthiness dielectric (insulation) i.e. then or

rare and measure the capacitance loss.

7.3.2 HV Test: The each bar is tested momentarily at high voltage increased gradually to

three times higher than rated voltage.

7.3.3 Dispatched for Winding

The bar preserved with polythene sleeves to protect from dust, dirt, oil, rain, etc are send to

block 1 for winding.

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CHAPTER 8

Failure of generator stator conductor

8.1 INTRODUCTION

During the 1960s and early 1970s many large hydrogen-cooled generators of 500 MW and upwards with water-cooled stator windings were brought into service. A number of these machines - irrespective of manufacturer - exhibited core end heating problems. [Tavner & Anderson Nov 2005] On installation of the first 500 MW generators on the CEGB system in 1968 there were some difficulties in operating at high leading power factor, there were a number of relatively serious core failures in which parts of generator cores were  damaged by melting, and a few generators were found to have  rather unstable end-heating problems which gave rise to long-term operational restrictions. 

A major core failure at Turkey Point No 3 (Florida Power and Light) was reported in 1968 in which molten metal had been ejected from one of the stator slots over  a fault length  of 17 feet and had spilled into the generator housing. 

Core failures can be expensive. Firstly, there is the cost of the repair. Secondly, there is the cost of the loss of revenue while the generator is out of commission. The core failure that occurred at PacifiCorp Hunter Unit I in Utah on 24th November 2000 and which required a complete rebuild of the stator is estimated to have incurred US$ 270.1 million in net purchased power costs by the time the unit went back into service on May 8th 2001.  

8.2. Eddy currents in the machine magnetic circuit

Eddy currents in electrical machines restrict the rate at which flux can change in the magnetic circuit and may cause appreciable heating losses. Eddy currents are normally minimised by constructing magnetic circuits from thin laminations punched from steel strip. These laminations are de-burred, cleaned and then insulated with varnish before being assembled. The principles of core construction for transformers and rotating machines are fairly similar. 

If the insulation between adjacent laminations should break down, for instance because of mechanical damage to to a stator tooth, this will result in an increase in the local eddy currents and perhaps result in a local hotspot. Surface hotspots can usually be detected by a static ring flux test. Internal hotspots, as for example where there has been an interlaminar breakdown between lamination surfaces, are much more difficult to detect.

In large a.c. machines it is customary to construct stator cores from a number of lamination segments arranged to form a ring and located in position within a stator frame by means of key bars. The joints in successive layers of these laminations are offset by typically half or one third of the pitch of a lamination segment, so that the flux at the butt joints can find an easy path via adjacent laminations and so keep the overall reluctance of the iron path down.

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8.3. Interlaminar breakdowns caused by foreign bodies

It is not unknown for a metallic foreign body - a nut, a bolt, a washer or a piece of swarf etc. - to find its way into the rotor/stator gap or into an axial cooling duct in the stator core. Over time this foreign body may vibrate against the lamination edges and damage the interlaminar insulation, causing adjacent laminations to come in contact with one another. This may lead to a significant number of laminations becoming connected together and therefore to a local hotspot. 

The interlaminar voltage in the main part of the core is modest - typical figures for 0.35 mm laminations being of the order of 50 mV for a large two pole machine. Therefore only if the damage were to occur over sufficient laminations and if the core-to-frame contact were to be consistently good, could a runaway fault condition occur. Usually the damage - visible as a polishing of tooth surfaces - is detected during a routine inspection and, in most cases; the lamination insulation can be restored. 

One method of restoration used is to etch the lamination edges electrolyticaly using phosphoric acid, clean surfaces and then apply a penetrating epoxy resin that  is drawn between the laminations by capilliary action.

8.4. Interlaminar breakdowns caused by vibrating stator teeth

One of the most difficult aspects of core building is to ensure that the core is tightly and uniformly packed and that no part of an individual lamination - a stator tooth for example - is free to vibrate axially. 

The main reason for poor packing is that laminations are not of uniform thickness. The steel strip from which they are punched is always thinnest at the edges and thickest in the middle. Since large lamination segments are nearly always cut so that the yoke portion is aligned with the direction of rolling, the teeth and the back of core are always thinner than the portion just below the bottom of slot. Therefore unless sufficient measures are taken to compensate for the cumulative difference  of thickness during core building, or unless the core is bonded, it is likely that somewhere in the core there will be loose tooth laminations.  If these loose tooth laminations happen to be near a radial vent duct, they will be subject to axial forces tending to separate them from the mass of the core. Given time, a portion of lamination may fatigue and break off, leaving space for the adjacent laminations to vibrate and in due course fatigue and break off in the same way. Eventually a whole portion of tooth may disappear. The broken portions may cause further damage in the same way as any other foreign body. 

8.5. End Region Heating due to eddy currents

Within the main active section of the magnetic circuit of a conventional type of a.c. machine the flux has radial and circumferential components. Towards each end of the core, as a result of the circumferential components of excitation current from the end windings, there will be

50

axial flux components in the stator core which will tend to set up circulating eddy current in the plane of the laminations. 

In large a.c. rotating electrical machines, the axial fluxes  caused by circumferential currents flowing in the rotor and stator end windings are sufficiently great to induce significant eddy currents in the laminations at each end of the stator core and in the core clamping plates. These circumferential/radial eddy currents give high losses, particularly  in the bottom of slot regions and especially  in large turbine generators. because of their very high specific power outputs.  The end region losses are highest when operating far in the leading power factor region. 

Various methods are used to minimise eddy current losses in the core end regions. (1) conducting screens on the core end plates to act as flux divertors (2) profiling the end of the core, i.e. locally increasing the reluctance of the rotor/stator gap (3) segmentation of the laminations (4) the use of narrow slits - "pistoye slots" - in the rotor teeth to lengthen the path taken by the eddy currents, thereby increasing the path resistance and decrease the current/losses (5)use of extra coatings of insulating varnish on the laminations. . Core end region design is therefore a compomise between keeping the eddy current losses small yet maintaining adequate magnetic, thermal and mechanical properties. 

Attention to overall design would be of little effect however without equal attention to the detail: use of low-loss steel for the laminations ; maintenance of punch and die quality ; adequate deburring after punching ; cleaning after deburring ; cleanliness of the  varnish insulation process ; the appropriate segmentation and lapping of laminations; careful core-building ;  maintenance of adequate and uniform core pressure, both during core build and later in operation.  Even with careful attention to design,  interlaminar voltage levels at the ends of cores may be as much as 40 or 50 times as high as in the main body of the core under steady  state conditions and higher still under transient conditions. As a consequence, the core ends are extremely vulnerable to the slightest imperfection in interlaminar insulation. 

An indication of significant core end heating having occurred at some time is often provided by  discolouration of varnish on the outermost laminations behind the slot. Typically, heat discolouration contours will be visible behind most slots and at both ends of the machine. In addition there may be signs of more localised overheating, for example at the butt joint positions and sometimes between

surfaces away from lamination edges. See also the possible  role of  transients in damaging the interlaminar insulation, see note below. 

8.6 Problems caused by back-of-core leakage flux

An additional source of troublesome eddy currents in the stator core end regions of  large machines is caused by the "back-of-core leakage flux", which  is  the small component of armature flux that is not contained by the core and which permeates the space behind the core and tends to be drawn into circumferential members of the core frame. The axial members of the core frame are exposed to this leakage flux and act as a squirrel cage, with the circumferential members of the core frame at the ends of the machine providing return paths. At least, this would be the situation were it not for the fact that the core is often in

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direct contact with the frame via the axial keybars which communicate the torque reaction to the core frame. Because the laminated core represents a lower impedance path than the frame, there is a tendency for the back of core currents to complete the circumferential path at the core ends via the core and not the frame.

Conduction of back-of-core currents circumferentially via the back of core would not be a problem if the core was not subject to torque reaction and ovalising forces. The relative movement that results can cause intermittent core to keybar contacts and consequential arcing at the back of core. Some manufacturers have attempted to overcome the problem by placing a copper strip between each keybar and the frame and joining these to circumferential copper strap, one at each end of the core, to form a true squirrel cage. This is on the basis that the circumferential currents will prefer the copper strip to either the core frame or the core. 

Other manufacturers have chosen to weld the keybars to the back of core in order to guarantee that the currents will always take the back of core path provided for them. Yet others go for a semi-insulated core with one keybar earthed to provide earth leakage protection in the event of a bar to core insulation breakdown. 

No clearcut answer can be given as to which of the many possible solutions is best.  In most cases, arcing at the back of core merely results in local core to frame welding and, in consequence, and in time, the whole of the core becomes well grounded and arcing will cease. However when  back of core arcing takes place damage may not be confined to the back of core. The interlaminar insulation may be stressed and an interlaminar breakdown may occur as a point of weakness well away from the back of core - for instance,  on a tooth, at the bottom of slot or in an axial vent duct. Given sufficient energy fed into interlaminar breakdown, this may cause significant local damage. Over time a number of these interlaminar breakdowns may occur and cause a significant rise in the core end losses.

8.7. Exacerbating factors 1 : The role of transient disturbances

Under transient conditions - sudden short circuit, line clashing, pole slipping etc. -  the stator currents may well rise to several times their steady state value. This will not produce significant changes in the main flux because the moment that the stator flux starts to rise, eddy currents will spring up in the rotor damper winding tending to keep the flux linkages constant. However in the end regions  the end winding ampere turns and the damping currents in the rotor damper winding tend to augment each other rather than cancel each other out. The result is a considerable increase both  in axial flux and in the back of core leakage flux at the ends of the core. As a result the interlaminar insulation will be more highly stressed and therefore more likely to break down at points of insulation weakness. 

The situation is complicated by relative vibration between core and frame which will clearly be greatest under transient conditions and will cause a greater degree of  intermittent contact between the core at its ends and  the core frame keybars than under steady loads. 

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8.8.  Exacerbating factors 2 : interlaminar capacitance.

A feature of many generator cores that have been in service is the existence of small spit marks - sometimes so small that they may be missed altogether - that may be found at lamination edges in the end regions. These may be found at the back of core, on the teeth, in the slots in vent ducts and between surfaces, indeed anywhere where there might be points of insulation weakness. These turn out usually to be microscopic spot welds. Sometimes the welding extends over several laminations. 

A possible explanation lies in considering the core end laminations in their pristine state to be a large parallel plate capacitor whose plates are short-circuited by the core to keybar contacts. Whenever the core to frame contacts are broken the inductive currents continue to flow into the open-circuited capacitor, giving rise to a rapid rise in voltage across the laminations. This process of capacitive charging will continue either until there is an interlaminar breakdown, when all the capacitive energy is discharged into the fault, or until the stored capacitive energy equals the initial inductive energy and the conditions for an oscillatory discharge are established. 

On the basis of this argument, the better the interlaminar insulation, the bigger the build up of stored charge between the laminations before breakdown takes place and the greater the capacity for causing significant interlaminar damage.  The inductive/capacitive phenomenon can be simply demonstrated in a controlled manner in the laboratory with two laminations connected as the plates of a capacitor across a switch that is connected in series with a battery and an inductance. Opening the switch will produce a breakdown at the weakest point of insulation. By inserting insulation at the point of breakdown, and repeating the switching, a breakdown  will occur at the next  weakest point : this time the flash will be slightly brighter, and so on. Eventually the energy input will be sufficient to cause welding at the point of breakdown. The experiment may be varied to show, for example, the modifying effect of oil, which increases the dielectric strength at the point where breakdown formerly occurred and shifts the breakdown elsewhere.

Fig 8.1 Circuit used to artificially induce interlaminar voltage

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8.9. Exacerbating factors 3 : The possible effect of hydrogen disassociation

The coolant in large  generators is usually hydrogen gas, which  when subject to the high temperatures of an electric arc disassociates into atomic hydrogen H, which is extremely reactive. This disassociation  may provide a possible link between the localised core burning  at the ends of  machines - including the spitting mentioned above - and a full scale core fault. 

Hydrogen arcs are localised (see Langmuir, G.E. Review XXIX No 3 152 March 1926) and are characterised by a very high rate of heat transfer which cannot be accounted for by conduction, convection or radiation. A process of disassociation takes place in which the molecule H2 is converted into atomic hydrogen H, with the absorption of heat from the arc. Subsequently the hydrogen atoms recombine in the presence of a catalyst, giving up their heat to the catalyst in the process. Metals such as iron are good catalysts and may be heated to incandescence or be melted at some distance from the arc (3-5 cm). Heat transfer rates are about 11 times greater than might be expected from Hydrogen in its molecular form. Large quantities of hydrogen gas are not required to transfer large quantities of heat. The efficiency of the heat transfer process is high (82% 3mm from a metal surface to 55% at 35mm). The energy is transferred preferentially to metal in the vicinity of the arc causing greater damage than might be expected in air.  Whilst the disassociation of hydrogen is well understood and indeed is exploited in the hydrogen arc welding process, it has not been proven definitively to be a factor in core fault propagation. Nevertheless the core failures in large machines have involved arcing in a hydrogen atmosphere and therefore it is entirely logical to believe that disassociation of hydrogen does play a part.

8.9 Discussion and Conclusions

The specific power output in  a large turbogenerator is some 70 MW per metre of active length of core and the heat content of 1 gram of silicon steel at its melting point is approx 1330 J/gm. Therefore if  the energy output from 1m length of core were to be diverted into a single core hotspot the core could, in theory, melt at a rate of 54 kg/second, ignoring  radiation, convection and conduction losses. Core faults are known  in which several hundred kg of core have been melted. It is salutory to realise that even such large scale melting may have occurred in just a few seconds. 

We have seen that there are mechanisms capable of producing transiently very high interlaminar voltages and causing localised breakdowns, both in the form of spot welds or meandering breakdowns.  Furthermore it is clear that hydrogen gas, the stator coolant  on large machines, can act as an extremely efficient heat transfer medium if it disassociates in the presence of an arc. 

Bearing in mind the high energy densities present in the stator core and the possibility of the presence of inter-laminar breakdowns causing increased eddy currents, localised heating and further breakdown, it is remarkable how relatively infrequent major core failures are.

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Generally speaking, design and manufacturing techniques are fit for purpose and only exceptionally does some particular set of circumstances lead to a truly dramatic fault of the kind described in this working paper. One moment's relaxation in preparation of the laminations, a little lack of attention in core building, careless insertion and wedging of the conductor bars and it might be a different story. If you haven't yet experienced a core fault, then it may be your turn tomorrow. And by the way, like troubles  and London buses, core faults seldom come singly, they  like to keep one another company.

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CHAPTER 9

ELECTRICAL GENERATOR PROTECTION

Generator may be endangered by short circuit, ground fault, over voltage, under excitation and excessive thermal stresses. The following protective equipment is recommended

Differential protection Stator ground fault protection Rotor ground fault protection Under excitation protection Over current protection Load unbalance protection Rise in voltage protection Under-frequency protection Reverse power protection Over voltage protection

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CHAPTER 10

FEATURES AND SPECIFICATION

10.1 TURBO GENERETOR

10.1.1 Air Cooled Turbo Generators Up To 200 MW Range (Type.- TARI)

Stator core and rotor winding direct air cooled

Indirect cooling of stator winding

Horizontally split casing design of stator

Vertically side mounted coolers in a separate housing

Micalastic bar type insulation system

Separately assembled stator core and winding for reducing the manufacturing cycle

Brush less/static excitation system

10.1.2 Hydrogen & Water-Cooled Turbo Generators Of 200-235 MW range (Type:

THW)

Stator winding directly water cooled

Rotor winding directly hydrogen cooled by gap pick up method

Resiliently mounted stator core on flexible core bars

Thermo reactive resin rich insulation for stator winding

Top ripple springs in stator slots

Enclosed type slip rings with forced ventilation

Ring/thrust type shaft seal

Two axial fans for systematic ventilation and four hydrogen coolers

Static excitation

10.1.3 Hydrogen Cooled Turbo Generators Of 140-260 MW range (Type: THRI)

Stator core and winding directly hydrogen cooled

Indirect cooling of stator windingRigid core bar mounting

Micalastic insulation system

End shield mounted bearings

Top ripple springs in stator slots

Ring type shaft seals

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Symmetrical ventilation

Brush less/ static excitation

Integral coupling of rotor

10.1.4 Hydrogen & Water-Cooled Turbo Generators Of 500 MW range (Type: THW)

Stator winding directly water cooled

Rotor winding direct hydrogen cooled (axial)

Leaf spring suspension of stator core

Micalastic insulation system

End shield mounted bearings

Support ring for stator over hang

Magnetic shunt to trap end leakage flux

Ring type shaft seals with double flow

Multistage compressor and vertical coolers on turbine end

Brush less/static excitation

Integral coupling of rotor

10.2 TECHNICAL DATA OF TURBO GENERATORS

TECHNICAL PARAMETER

130MW

210MW

235MW

210MW

250MW

500MW

560MW

1. Generator Type

THRI93/98

THW-210-2

THW-235-2

THRI108/44

THDF108/44

THDF115/59

THDF115/59

2. Rated MVA 162.5 247.1 264.1 247.1 294.1 588.2 659.03. Rated MW 130.0 210.0 237.7 210.0 250.0 500.0 560.04. Rated Volt,

KV10.5 15.75 16.5 15.75 16.5 21.0 21.0

5. Power Factor 0.80 0.85 0.90 0.85 0.85 0.85 0.856. Stator Current 8935 9054 9240 9054 10291 16166 181187. Speed, RPM 3000 3000 3000 3000 3000 3000 30008. Frequency, Hz 50 50 50 50 50 50 509. Hydrogen

Pressure,Kg/Cm2(g)

3.0 3.5 3.5 2.0 3.0 3.5 4.0

10.

Type ofExcitation

STATIC STATIC/HFG

STATIC/HFG

B’LESS/STATIC

B'LESS B'LESS B'LESS

10.2.1 THW, THRI AND THDF TYPE10.2.2 TARI TYPE

TECHNICAL PARAMETER

TROMBAYSTG

KAYAMKULAM

KAYAMKULAM

FARIDABADGTG

FARIDABADSTG

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GTG GTG1. Generator

TypeTARI 93/98

TARI108/36

TARI108/36

TARI 108/41

TARI108/46

2. Base LoadMVAMW

10080

137.5116.8

154.3131.1

170.1144.6

188.2160.0

3. Rated Volt,KV

10.5 10.5 10.5 10.5 15.75

4. Power Factor 0.80 0.85 0.85 0.85 0.855. Stator Current 5499 7561 8484 9354 69006. Speed, RPM 3000 3000 3000 3000 30007. Frequency, HZ 50 50 50 50 508. Type of

ExcitationB'LESS B'LESS B'LESS STATIC B'LESS

CHAPTER 11

CONCLUSION

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I fell highly overwhelmed as I got such opportunity of being a part of BHEL as a trainee. It

was a great experience and I learnt a lot from it. I find no suitable words to express my

profound, indebtness and heartful thanks to BHEL for their prestigious guidance, support and

supervision. There occasional words of advice would surely act as a beacon of light. It was

due to their cheerful and sincere co-operation which made my training a fruitful, pleasant and

life time experience.

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