bharat heavy electrical limited training report

7/30/2019 Bharat Heavy Electrical Limited training report

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BHARAT HEAVY ELECTRICALS LIMITEDRAMACHANDRAPURAM

HYDERABAD - 502 032

REPORT ON

Design features of Turbo Generator

A DISSERTATION WORK SUBMITTED TO THE

FACULTY OF ENGINEERING OF

NATIONAL INSTITUTE OF TECHNOLOGY ,RAIPUR

Submitted in partial fulfillment of the requirement for the award of the degree of

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL ENGINEERING.

UNDER THE GUIDANCE OF: PREPARED BY

M.N.V.SURYA PRASAD

(Sr.DGMEME)

BHEL, Hyderabad.


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CERTIFICATE

This is to certify that the dissertation work entitled Design &

Manufacture of Turbo Generators is a bonafide work carried out by

KATAKAM VENKATA RAMESH, Roll No.10117035 in partial

fulfillment of the requirements for the award of the Degree of

BACHELOR OF TECHNOLOGYIN ELECTRICAL ENGINEERINGfrom

NATIONAL INSTITUTE OF TECHNOLOGY, RAIPURduring the

year 2010-2014.


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ACKNOWLEDGEMENT

I wish to express my gratitude to my beloved guide M.N.V.SURYA PRASAD(SR.DGM)

BHEL, Hyderabad for her valuable guidance in the successful completion of this dissertation

work. I am very much indebted to him for suggesting this topic and helping me at every stage for

its successful completion.

I express my profound thanks to Sri. NARENDRA D.LONDHE, Assoc. Professor of Electrical

Department, NATIONAL INSTITUTE OF TECHNOLOGY, RAIPUR for their cooperation

throughout this work and efforts for arranging this dissertation work.

Also, I wish to thank all those who have involved directly or indirectly, for the successfulcompletion of my project work.

KATAKAM VENKATA RAMESH

10117035


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Bharat Heavy Electricals Limited (BHEL)

Bharat Heavy Electricals Limited (BHEL) is an Indian state-owned integratedpower

plant equipment manufacturer and operates as anengineering and

manufacturing company based in New Delhi, India. BHEL was established in 1964, ushering in the indigenous Heavy Electrical

Equipment industry in India. The company has been earning profits

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

It is one of the only 7 mega Public Sector Undertakings (PSUs) of India clubbed

under the esteemed 'Maharatna' status. On 1 February 2013, theGovernment of

India granted Maharatna status to Bharat Heavy Electricals Limited.

In the early sixties 3 more major plants were set up at Hyderabad, Haridwar and

Tiruchirapalli, which all together form the core of the diversified product range,

systems and services that BHEL offer today.

The company has 14 manufacturing units, 4 power sector regional centers, 8 servicecenters and 18 regional offices besides project sites spread all over India and abroad.

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

core sectors of the Indian economy like Power generation and Transmission,

Industry, Transportation, Telecommunication, Renewable energy etc.

BHEL has acquired ISO 9000 & 14000 certification for its operation and has also

adopted the concepts of Total Quality Management (TQM) and Environmental

Management

BHEL's manufacturing units and other entities have been accredited to QualityManagement Systems (ISO 9001:2008), Environmental Management Systems (ISO

14001:2004) and Occupational Health & Safety Management Systems (OHSAS

18001:2007).

BHEL is exporting power and industry segment products and services for over 40

years. BHELs global references are spread across 75 countries.

The cumulative overseas installed capacity of BHEL manufactured power plants

exceeds 9,000 MW across 21 countries including Malaysia, Oman, Iraq, the UAE,

Bhutan, Egypt and New Zealand. BHEL's physical exports range from turnkey projects

to after sales services.
http://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/New_Delhi,_Indiahttp://en.wikipedia.org/wiki/New_Delhi,_Indiahttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Maharatnahttp://en.wikipedia.org/wiki/Maharatnahttp://en.wikipedia.org/wiki/Maharatnahttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/New_Delhi,_Indiahttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Power_plant


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BHEL,HYDERABAD, RAMACHANDRA PURAM

It manufactures variety of products such as Turbo

Generators, Steam & Gas Turbines, Switchgear equipment,

Compressors, Heat exchangers, Pumps, Pulverizes, Oilrigs

and so on.

ELECTRICAL MACHINES AT BHEL, RAMACHANDRA PURAM BHEL, Hyderabad unit manufactures Turbo Generators of rating up to 125

MW for industrial applications and for power generation in Steam Power

Plants. The Turbo Generators manufactured here range from 4 MW to 125

MW.

The Turbo Generators are supplied with the turbines and matching excitation

systems and are used mostly in paper, sugar, cement, petrochemical, fertilizer

industries etc., and thermal power stations

BHEL has borrowed technology for manufacturing generators from SKODA

Exports Czechoslovakia in 1960's. Also from M/s. Siemens, Germany and its

sister company KWU (Kraft Werk Union) in Germany.

They borrowed less than 12 modules, from semen's Germany. Till now the

BHEL had developed more than 70 in-house modules.

The turbo generators are based on proven designs and know how backed by

over three decades of experiences gained by BHEL engineers in this field

keeping pace with the latest development in insulation systems to optimize

the design.

BHEL Hyderabad is the only one in Asia that has the latest type of insulation

system called the Vacuum Pressure Impregnation (VPI) system.


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SPECIAL FEATURES OF THE TURBO GENERATORS DESIGNED BY BHEL:

High output to weight ratio.

Thermo setting class F epoxy insulation, both resin rich and Vacuum Pressure

Impregnation (VPI).

Low loss high-grade silicon steel for laminations.

Optimally designed fans on the rotors.

Better voltage waveform with less harmonic content.

Low wind age loosed and low noise.

Static/blushless excitation.

Split casing design for low manufacturing cycle for VPI design.

Generator Nomenclature:


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PRODUCT PROFILE OF ELECTRICAL MACHINES

ELECTRICAL MACHINES


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Introduction to Electrical Machines:

Energy can neither be created nor be destroyed. We can only change its forms, using

appropriate energy- conversion processes.

Machine acts as a generator converts the mechanical energy into electrical energy. The

machine, which acts as a motor, converts electrical energy into mechanical energy. The

basic principle of rotating machine remains the same i.e. Faradays Laws of Electro

Magnetic Induction.

Faradays first law states that whenever conductor cuts magnetic flux, dynamically

induced Emf is produced. This Emf causes a current flow if the circuit is closed.

Faradays second law states that Emf induced in it is proportional to rate of change of

flux. Mathematically,

e = -Nd/dt

e- Induced emf

N- Number of turns of coil

d/dt rate of change of flux

Emf induced will oppose both the flux and the rate of change of flux.

Efficiency of a machine is equal to the ratio f output to input.

= Output / input = Output / output + losses

To increase the efficiency of any machine we must decrease the losses, but losses are

inevitable. There are different types of losses that occur in a generator.They are

broadly divided into two types.

Constant losses Variable losses

a. iron losses. a. copper losses

b. Friction and windage losses


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Electrical machines are of two types AC machines & DC machines. AC machines are divided

into single-phase AC machines and poly phase AC machines.

1.1. SYNCHRONOUS MACHINES:

Synchronous Generators (or) Alternators are those in which the speed of the rotor and

flux are in synchronism.

1.2. ASYNCHRONOUS MACHINES:

These are the machines in which the flux speed and rotor speed and rotor speed will not

be the same.

Inherently all the machines are AC machines. AC or DC depends upon the flow of current

in the external circuit.

Synchronous generators can be classified into various types based on the medium used

for generation.

1. Turbo-Alternators Steam(or) Gas

2. Hydro generators

3. Engine driven generators

In every machine they are two carrying parts.

1. Flux carrying parts

2. Load carrying parts

In large synchronous machines the stators have the load carrying parts, i.e. armature and

the rotor has the flux carrying parts i.e.; field winding.

Iron losses are also called as magnetic losses and core losses. They are broadly divided

into.

1. Hysteresis losses

2. Eddy current losses

These losses occur in the stator core.

Copper losses occur in both stator and rotor winding.

The general efficiency of a synchronous generator is 95-98%.


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1. INTRODUCTION TO TURBOGENERATOR:

A synchronous generator is the core of any generating power plant. A synchronous

generator is a rotating electromagnetic device that converts mechanical energy into

electrical energy by taking the mechanical input from a prime mover (Gas turbine or Steam

turbine) and magnetic energy from excitation.

The different electromagnetic or active parts of a generator are as follows:

1. Stator core

2. Stator coils/ bars

3. Stator winding

4. Output leads, brushings and conductors

5. Rotor excitation leads

6. Rotor coils and rotor winding

7. Exciters

1. Stator core

The stator core serves the two fold function of providing the mechanical support for

the stator coils and carrying effectively the electromagnetic flux generated by the rotor

winding. In order to minimize hysteresis and eddy current losses the entire core is built of

thin laminations. Each lamination layer is made up from a number of individual segments.

2.Stator coils/ bars

The stator coils are the individual entities, which are placed in the slots of the stator

core and finally connected to each other as per a pre-designed scheme to form a three phase

winding. The prime purpose of the stator bars is to carry the load current at minimal

winding losses. These coils are provided with high voltage stresses. Depending upon the size

and rated voltage of the machine different types of stator bars are designed.

3. Stator windingThe stator winding is a short pitch; two-layered type made of individual bars. The

bars are located in slots of rectangular cross section, which are uniformly distributed on the

circumference of the stator core. In order to minimize the losses, the bars are composed of

separately insulated stands transposed by 360 degrees. To minimize the stray losses in the


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end winding, the standards of the top and bottom bars are separately brazed and insulated

from each other.

4. Output lead brushings and phase connectors

Output leads are taken out from the exciter end of stator from the top and supported

on an insulated glass fabric plate. Six terminals are brought out, three for phase and three

for neutral connections. The phase connectors are connections between the stator winding

phase bars /coils to the output lead bushings.

5. Rotor excitation leads

The excitation leads provide electrical connection between rotor winding and output

from brush less exciter.

6. Rotor Coils and winding:

The construction of the rotor winding consists of placement of pre-formed rotor coils

as per the winding scheme in the slotted rotor body, providing necessary insulation both in

the straight position and overhang, making connections of the excitation leads, wedging of

the straight part and mounting of the retaining rings.


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2. CONSTRUCTIONAL FEATURES OF TURBOGENERATOR

The turbo generator is designed for continuous operation with voltage variations +/-

5% of the rated voltage and a frequency variation of +/- 1%. In general, the machine is

designed for the altitudes of 1000 meters and above sea level and below an ambient

temperature of 600 maximum, with cooling water temperature of 380 maximum at the inlet.

The generator consists of the following components:

2.1 Stator:

2.2 Rotor

2.3 Bearings

2.4 Ventilation and Protection System

2.5 Cooling System

2.6 Insulation

2.7 Vaccum pressure impregnation system

2.8 Exciter

2.9 Base frame


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2.1.STATOR

Stator part of the Generator after VPI process

2.1.1.Stator Frame:

The stator frame is of welded construction, supports the core and the windings. In

consists of air duct pipes and radial ribs, which provide rigidly to the frame. Footings are

provided to support the stator on the skid. The stator frame should be rigid due to the

various forces and torque during operation. The welded stator frame consists of the two

end plates, axial and radial ribs. The arrangement and dimensioning of the ribs are

determined by the cooling air passages, the required mechanical strength and stiffness.

The end covers are Aluminum alloy castings. The stator frame is fixed to the skid with the


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help of hexagonal bolts. The skid is temporarily fixed to the concrete foundation through

bolts.

2.1.2.Stator Core:

The stator core is made up of stacked insulation electrical sheet steel lamination with a low

loss index and suspended in the stator frame from insulated rectangular guide bars. Axial

compression of the stator core is obtained by clamping fingers, pressure plates and non

magnetic clamping bolts, which are regulated from the core. The clamping finger ensures a

uniform clamping pressure, especially within the range of the teeth and provided for

uniform intensive cooling of stator core ends.

2.1.3. Lamination Preparation:

For high rating machines each lamination is built up of six sectors (stampings), each of six

cut according to specifications. Press tools are used in the manufacture of laminations. Press

tools are mainly two types:

1. Compounding Tools

2. Blanking and Slot Notching Tools

Lamination Sheets after preparation process

2.1.3.1. COMPOUNDING OPERATIONS:

In this method, the stampings with all core bolts holes guiding slots and winding slots are

manufactured in a single operation known as COMPOUNDING OPERATION. Press tools used

as known as COMPOUNDING TOOLS. They are used for the machines rated above 40MW.

2.1.3.2. BLANKING AND NOTCHING OPERATION:


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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 press tools used known as NOTCHING.

In the second operation the winding slots are punched using another tool known as

NOTCHING TOOL and the operation performed by the tool is known as NOTCHING.

The different operation taking places in the manufacture of laminations are:

1. Shearing

2. Notching

3. Deburring

4. Varnishing

2.1.3.3. ASSEMBLY OF CORE:

The stator laminations are assembled as separate cage without stator frame. The entirecore length is made in the form of packets separate by radial ducts to provide ventilatingpassage for the cooling of core. The thickness of lamination is about 0.5mm and thethickness of lamination separating the packets is about1mm. The segments arestaggered from layer to layer so that a core of high mechanical strength and uniformpermeability of magnetic flux is obtained.

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 stack is reached. The complete stack is kept under pressure

and located in stator frame by means of clamping bolts and pressure plates.

Assembly of Stator core with help of Guide bars.


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2.1.4. End Covers:

The end covers are the castings of the aluminum alloy and are bolted to the side plates of

the stator frame. The inlet passage is specially designed with built in guide vanes, which

ensure uniform distribution of the air to the fan. Air ceiling is provided around the shaft

and at the parting plane of the top and bottom parts of the end covers so that suction of

oil vapor from the bearings does not take place.

2.1.5.Stator Winding:

The stator winding is a fractional pitch two layer type, it consisting of individual bars. The

bars are located in slots of rectangular cross section which are uniformly distributed on the

circumference of the stator core.

In order to minimize losses, the bars are of separately insulated strands which are

exposed to 360.degrees transposing.

To minimize the stator losses in the winding, the strands of the top and bottom bars are

separately brazed and insulated from each other.

Fig - Wound Stator

2.1.6. Location of Bars:


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A semi-conducting wrapper of graphite paper in the slot protects the bar. The stator

winding is protected against the effects of current forces in the slot section. To ensure

tight seating of the bar at the slot bottom, a slot bottom-equalizing strip of stress path is

inserted. A top ripple spring is arranged between two compression strips to exert a

continuous pressure on the bars. The bars are shaped so that, cone shaped end windings

are obtained. In order to reduce the stray losses a small cone taper of (13-20deg) is used.

On the wide sides of the bars spacers of insulating material are inserted at regular

intervals.

2.1.7. Enclosure:

The enclosure consists of the inner and outer components. The inner components

comprises of the winding covers, which from an angular enclosure of top and bottom

parts and is designed as required for particular degree of protection, as indicated in the

dimension drawing or in the Technical data. The ventilating circuit is of the double-

ended symmetrical arrangement.

2.1.8. Electrical Connections of Bars and Phase Connection:

Brazing makes electrical connection of Bars: Electrical connection between the top and

bottom bars, one top bar being brazed to the associated bottom bar. The coil connections

are wrapper depends on the machine voltage. After tapping, an insulating varnish is

applied.

2.1.9. Phase Connectors:

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

a low specific current loading. The connections to the stator winding are of riveted and

soldered type. The phase connectors are wrapped with resin rich mica type, which

contain synthetic resin having very good penetration properties. The phase connectors


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are then cured at a certain temperature, with the shrinking tapes contracting so that a

void free insulation is obtained.


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2.2. ROTOR:The rotor is forged from a homogeneous steel ingot of specially alloy steel properly

heat treated to meet the required mechanical, metallurgical and magnetic properties. Axial

slots are milled through out the active length of the rotor body to accommodate the

conductors. The slots are dovetailed at the top of housing the wedges.

2.2.1. Rotor Shaft:

Solid rotors are manufactured from forged alloy steel with suitable alloying elements to

achieve very high mechanical and superior magnetic properties. Rectangular or trapezoidal

rotors slots are accurately machined to close tolerances on slot milling machine.

Laminated rotors are made of punched and varnished laminations of high tensile steel are

mounted over machined shaft are firmly clamped by end clamping plates.

2 pole machine rotors are directly cooled for which sub slots are provided for cooling

Generator rotors. 4 pole machine rotors are indirectly cooled for which ventilation ducts

will be provided.

The complete part of the Rotor with winding and Rotor fans.

Rotor Body:


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This is the RAW Product of the Rotor shaft which used in Generator.

2.2.2. Rotor Winding:

The coils used for the windings are rectangle in cross-section made up of copper with silver

content of approximately 0.1%. This features high conductivity and more of the strength i.e.,

more of the fatigue and creep resistance. Basically they arise due to frequent changes in

temperature. This may lead to deformation of coils. Thus they must be resistant to thermal

stresses.

The rotor winding consists of several coils which are inserted into the longitudinal slots and

series connected such that the coil groups form one pole. Each coil consist of several series

connected turns each, which consists of two half turns, which are connected by the brazing

in the end section. The rectangular cross-section is provided with axial slots for radial

discharges of cooling gases.

2.2.2.1. Placement of Rotor coils in the machine:

Coils are made into two halves.


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Rotor half coils after annealing process.

The joint of two halves of the rotor coil is called full coil formation.

Fig: The rotor full coil formation after joining the halves coils.

2.2.2.2. Process of Rotor Winding construction:Ventilation Punching:

First the conductors are checked for their quality and ventilation holes are punched and

they are checked for burr. Then edge wise bending is made. The holes are punching for air

gap to enter the air for the cooling of the rotor coils and rotor wedges.

The gap between holes to hole is 10mm or 15mm.

Fig: The copper conductors after the ventilation &champering process.

2.2.2.3. Champering process:

In this process the holes which are punched on the copper conductors are making

smoothing without sharp edges on conductor. Due to punch on conductor the sharp edges

are formed in between the holes. By this the conductor may be cuts and short circuited in the

rotor part and gets damaged. So, Champering process is used.


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2.2.2.4. Edgewise Bending:

The conductors are bent more than 90o so that it will sustain spring back effect.

Debuggingventilationslotsbytherelevant tools.

2.2.2.5. Annealing &Pressing:

Then the conductors are heated and pressed at the bending so that the cross section of the

conductors will be maintained equal through out. This process is called annealing.

The smoothening at the edges of half copper conductors. By the pressing the coil the

thickness is decreases.

2.2.2.6. 90 Rectification and Marking:

Here the marking the copper conductors making 90 formation at end of the copper

conductor. To form the equal size at the end of the copper coils. At the end of the coils the

two edges is a male and female edge are champered to join the two halves copper conductors

at end.

2.2.2.7. Dovetail Punching & Window dimension:A small portion near the bend is removed so that it does not cause any damage to the

insulation trough while lying in the slots. This process is called relief filing. Then dovetail

punching is made which provides good brazing process when two conductors are joined.

Window dimensions for the conductors are checked. The dimension of the window

decreases from top to bottom conductors.

Bending at the end of the copper coils in the curve shape for the increasing the efficiency.

And also join the both ends of coils.

The copper conductor after the dove tail punching.

2.2.2.8. Reliefing the coils:To remove the roughness on the copper coils become smooth on the surface for the

supporting the windings of the machine.


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2.2.2.9. Cleaning& Radial Bending:

Then the conductors are cleanedwith thinner (acetone) and then air-dry varnish is applied.

Then keeping the conductors on a dummy rotor makes radial bending. For the conductors

away from the poles prebrazing is done. The cleaning is done copper bearings by the sand

paper, Termer oil. The copper bearings are radially bending for the joint of the other half to

join at the end of another half of the copper bearings.

2.2.2.10. Coil formation:

The edges of coils are joined together at the end of the coils by adding the steel alloy in

between the two edges of copper conductors. By using the Brazing process both are joined

together.

2.2.2.11. Insulation Punching:

By adding the insulation punching in between the copper conductors the silicon sheets are

used in the rotor winding. Then it increases the efficiency and output power of the

machine. This is the protective layer of the winding and coils.

2.2.3. Rotor Retaining Rings:

Just as the rotor wedges secure the rotor coils in the slots along the lift, the overhang

winding is secured by the retaining rings. The retaining ring is non-magnetic alloy steel

forging with very high mechanical properties. This material is stress corrosion resistant.

One end of the retaining ring is shrink fitted on the rotor body and the other end overhangs

the end winding without making contact with the rotor. This arrangement ensures that the

shaft deflection during operation is not obstructed by the retaining ring. The free end of the

retaining ring is reinforced by shrinking a hub an inner diameter of the retaining ring. The

amount of shrink fit is so chosen that the releasing speeds of retaining ring on rotor body

and on hub are beyond the over speed of rotor (i.e. 120% of rated speed).

2.2.4.RotorSlot Wedges:

To protect the winding against the effects of the centrifugal force, the winding is secured

with wedges. The slot wedges are made from an alloy high strength and good electrical

conductivity, and are also used as damper wedged bars. The retaining rings act as short

circuit rings to induced current in the damper windings.


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2.2.5. Rotor Fans:

The cooling air to the generator is circulated by two axial flow fans located on the shaft at

both ends. Each of fans comprises of a number of individual blades, which are directly

screwed onto the rotor. The number and profile of the blades are so chosen that threaded

root fastening facilitates the change of blade angle depending on operating requirement.

Every fan blade is secured at its root with a threaded pin.The fans circulate required

quantity of cooling air at required head to over come the pressure drop inside the generator.

Fan blades are all minimum alloy die forgings. Threading is mad on the fan blade root such

that they can be fixed into the threaded holes provided on the rotor.

Rotor fans which is used on the rotor part after winding.

2.2.6. Slip Rings:

These are made of forged steel and shrunk on either side of the rotor between the end

cover and the bearing. The mica splitting is used to insulate the slip rings from the rotor

body. The excitation to the rotor winding is taken from these slip rings. The connection

leads are suitably insulated and taken through slots milled on the surface of the rotor.

Wedges are provided to keep the leads in position. A helical groove is machined on the


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outer surface of the slip rings to have better dissipation of heat, thus minimizing the

brush wear.

2.2.7. Rotor Balancing:

The rotor is balanced with the help of sophisticated balancing machine. The balancing

weights are provided in the hubs under retaining rings and in the fans. The rotor is

dynamically balanced and subjected to an over speed of 20% for 2min.

2.2.8.Field Connections:

The field connections provide the electrical connection between the rotor winding and

the exciter.

2.2.9. Terminal Lugs:

Consists of a copper conductor of rectangular cross-section. One end of the terminal lug is

brazed to the rotor winding, while the other end is screwed to the radial bolt.

2.2.10.Radial Bolt:

The field current lead located in the shaft bore is connected to the terminal lug through a

radial bolt. The radial bolt is made from steel and screwed into the field current lead into

the shaft bore.


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2.3 BEARINGS

2.3.1. PRINCIPLE OF OPERATION:

In steam turbines, the rotating shaft is supported on two journal bearings, and is axially

located by a thrust bearing. Both these types of bearings in steam turbines work with a

hydrodynamic oil film formed between the two mating surfaces. The hydrodynamic action

can be explained as follows. The rotor pulls the oil into the space between the two mating

surfaces. Because of the viscosity of oil, the layer of oil closest to the shaft drags in the next

layer, which drags in the next layer, and so on. This action forms an oil film between the

rotating shaft and the stationary bearing surface.

Now if the stationary surface is inclined such that the gap between the moving part

and the stationary part gradually reduces along the direction of movement, then the

dragged-in oil gets compressed, and a pressure film is built up. This pressure film takes the

normal load on the moving part. The friction is only because of the shear in the oil, and the

heat generated is also only because of this shear. The heat is carried away by the oil flow. It

may be noted that the pressure built up depends upon the viscosity of the fluid. If, for a given

bearing configuration, a fluid of less viscosity is used, pressure built up will be less, and the

load carrying capacity will be less.

The generator rotor is supported at two-journal bearing. The bearings consist of a

bearing pedestal and bearing shell is split into two halves to facilitate assembly. The bearing

pedestals are iron castings and the bearing shells are the steel castings. The bearing

pedestals are providing with a spherical seating surface and bearing shell rests into with its


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outer spherical surface. The inner surface of the bearing shell is provided with spherical

grooves and cast with Babbitt metal.

2.3.2. Bearing Oil Supply

The oil required for the bearing lubrication and cooling is obtained from the turbineoil supply system supplied to the lubricating groove in the bottom-bearing sleeve. The upper

bearing sleeve consists of a wide overflow groove through which oil is distributed over the

shaft journal and fed to the lubricating pump.

2.3.3. Bearing Temperatures

One double-element resistance temperature detectors monitor the temperatures of

each bearing. The resistance temperature detector is screwed in the position on side of thelow bearing sleeve from outside with the detector extending to the Babbitt liner.


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2.4. VENTILATION AND PROTECTION EQUIPMENT:

2.4.1. Ventilation Arrangement:

The turbo generator is cooled by air circulated by means of two axial fans. Air coolers

cool the air after circulation. The air is drawn through suction ducts by axial fans

mounted on either side of the rotor. The warm air flows out through the exhaust at the

bottom of the stator frame.

2.4.2. Space heaters:

These heaters are used to circulate warm air inside the turbo generator and during

outages to prevent condensation of the moisture inside the machine. They are of strip

type and robust design. The heating elements are enclosed in a steel sheet with specific

rating of 15W per sq. inch of the surface. They are so designed that they may be fixed in

the suction ducts of the turbo generator. The heaters are completely covered in order to

prevent the accidental contact with the heat units.

2.4.3. Resistance Temperature Detectors:

The resistance temperature detectors are made up of Platinum resistance elements. The

detectors are placed in a groove cut in a rectangular glass laminate and embedded in

different positions like stator teeth, stator core, and slots. There are 12 active and three

spare elements distributed in different locations in 3 different planes, 5active plus 3

spare elements are placed in stator slots, 4 active are placed in stator core, 3 are placed in

teeth to measure the hot and the cold air temperatures. The resistance thermometers are

fixed in the exhaust hood of the stator frame and the end covers. The leads from these

resistance thermometers are brought out and connected to the terminal board. The leads

coming from the spare elements are brought up to the terminal board and left inside the

machine. These resistance temperature detectors operate on the principle that the


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resistance of the elements will change depending on the temperature coefficient of the

element. The change in resistance can be accurately measured in a bridge circuit. A graph

is drawn showing the variation of resistance with temperature, which is used to know the

temperature rise under different operating conditions of the turbo generator.

2.4.4. Fire Detectors:

For the protection of turbo generator against any possible fire hazards 12 fire detectors

relays are provided on either side of the stator winding. These relays have a set of

normally open contacts. The set of contacts will close when the temperature surrounding

the first relay exceeds 80deg Celsius. The other relay set of contacts close when the

temperature exceeds 1000. These contacts are wired up to the terminal board provide on

the stator frame for the resistance temperature detectors. Both the sets of contacts are

used for automatic fire alarm shutting down of the turbo generator system and for the

release of CO2 gas from the Carbon dioxide system

2.4.5. Finish Machined Rotor:

The finish-machined rotor will be dynamically balanced at rated speed in a vacuum-

balancing tunnel to an accuracy . The rotor is also subjected to over speed at 120% rated

speed for 2 minutes. Balance condition is also checked over the entire speed range from 0

rpm to 3000 rpm

2.5. Cooling System:


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2.5.1. Indirect Cooling:Indirectly cooled machines dissipate their losses to a cooling medium, which is entirely

outside the coil insulation. All air-cooled machines, with rare exceptions, are cooled in the

manner, as well as most hydrogen-cooled machines under 100mm VA. Turbo-generators

rated above 100mVA usually employ direct cooling.

2.5.2. Direct Cooling:Direct cooling is the process of dissipating the armature and field coil losses to a cooling

medium within the main conductor insulation wall. Machines cooled in this manner are also

called as super charges or inner cooled by various manufacturers. The cooling medium is

in either direct contact with the conductor copper or is separated only by thin materials

having little thermal resistance. Direct cooling eliminates the temperature differential

resulting from heat flow through the coil insulation, providing greater current-carrying

capability for the same hot-spot temperature rise.

2.5.3. Cooling methods of Turbo Generator:a) Air cooled Turbo Generator (TARI):In Air Cooled Turbo generator stator winding is indirectly air cooled whereas therotorwinding and stator core is directly air cooled. This type of cooling isapplicable for rating of

30 MW- 60 MW generators. In this type of turbo generatorthere are vertically side mountedcooler in a separate housing.

b) Hydrogen cooled Turbo Generator (THRI):When the problem of increasing generator rating was talked in it became clearthat the aircooled machine did not provide the necessary scope for progress. Notonly in circulating therequisite of air through the machine but also because highfan power required to circulate.Evidently to push up generator ratings hydrogenis used as cooling medium.

Advantages of Hydrogen as Cooling Medium:1. Increased efficiency2. Increase in rating

3. Elimination of fire hazard4. Smaller size of coolers

c) Hydrogen/water cooled T.G. (THDF):In large rating machines, hydrogen cooling is not sufficient to remove the entireheatgenerated. For additional cooling, a Primary Water (PW) cooling system withdemineralisedwater flowing through the hollow stator conductors is used. Therotor conductors arehydrogen cooled.


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2.6. Insulating systems are of two types:

1. Resin rich system of insulation

2. Resin poor system of insulation

2.6.1. Resin rich system:

- Stacking of coils is done. In this case high resin glass cloth is used for preventing

inter half shorts.

- Putty work.

- Nomex is used as transposition pieces. Putty mixture is a composition if mica

powder, china clay and SIB 775 Varnish.

- Straight part baking is done for 1hour at a temperature of 160OC and a pressure

of 150kg/ sq.cm

- Then bending and forming is done.

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

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

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

layers.

- Final baling is done for 3hrs at a temperature of 160OC in cone furnace.

- Gauge suiting is done.

High voltage testing is done at four times that of rated voltage and tan testing,

inter strip, inter half testing are done.

Finally glass taping followed by epoxy gel coating is carried out.

Advantages of resin rich system of insulation:

Better quality and reliability is obtained.


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

very easy.

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

2.6.2. Resin poor system:

Resin poor micalastic system is adopted for large range Ac Induction and synchronousmachines. These are designated to meet specific customer requirement hence for uniquein nature to each other. The main insulation consists of resin poor epoxy mica paper tapeall over the oil periphery with varying number of layers on straight and overhangportions.

RESIN POOR RESIN RICH

1. Epoxy resin content is about 8%.2. This method follows Thermo SettingProcess.3. There is a need for addition of resinfrom outside.4. Time required for this cycle is less.5. Repairing is very difficult.6. Overall cost is less compared to resinrich.

1. Epoxy resin content is about 40%.2. This method also follows ThermoSetting Process.3. Further addition of resin is notrequired from outside.4. its a very long process and time

consuming.5. Repairing is easy.6. Overall cost is more.


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2.7. VACCUM PRESSURE IMPREGNATION:

Preheating of the stator 60C, Duration : 1 Hour

Vacuum Drying at 60C, 0.2 mbar Duration: ~16-18 Hours

Entire stator assemblies are immersed into liquid thermosetting epoxy

resin insulation and vacuum-pressure impregnated.

Resin Temp: 60DEG. C +/- 2DEG. C, Resin Fill ~ 20 min

Resin Level: 100 mm Above job, Settling Time ~ 10 min

Pressurization:

N2 Pressure ~ 4 bar, Raising Time ~ 80 min

Holding Time: 2 Hours.

Resin is withdrawn back

2.7.1. Characteristics of VPI insulation system:

1) Higher mechanical bond

2) Void free resin with high mica content ensures better heat transfer.

3) High dielectric strength, low dissipation factor, hence longer electrical life.

4) Higher thermal stability, ensures class-F under running conditions.

5) Less maintenance

6) Cost effective

7) Low inflammability, hence limited damage during abnormal operations.

8) High resistance to oil, acid, alkali and moisture.

9) Manufacturing cycle is less

10)Frame size is small, machine cheaper.

11)Elastic response to thermo-mechanical stress, machine suitable peak load

operation.

12)Tan Delta temperature tie up is small,Voltage grading at ends will be effective.

2.7.2. VPI SYSTEM OF INSULATION:

For vacuum pressure impregnation (VPI)of cage core stators up to 120 MW capacity which,

is the largest of its kind is India. This system conforms to the latest insulation system

adopted by KWU-SIEMENS technology. The stator coils are taped the porous resin poor mica

tapes before inserting them in the slots in cage core. Subsequently, the wound stator is


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subjected to a special VPI process in which first the stator is vacuum dried and the

impregnated in a resin bath under pressure of nitrogen gas. Then the stator is cured in an

oven.

The main characteristic if this system of insulation is also follows:

1. Better heat transfer resulting from resin penetration into minute air gaps in between

laminations and bar insulations.

2. Low dielectric loss resulting in increased life of the machine.

3. High resistance against the effect of moisture.

4. Reduction of time cycle of manufacture.

2.7.3. Advantages of VPI:

Longer electrical life.

Higher thermal conductivity

Lesser thickness of insulation higher voltage stress.

Small dissipation factor tip-up (void free insulation).

Better elastic response to thermo mechanical stresses

Insensitivity to high temperature and temperature changes(suitable for gas

turbine application).

Low inflammability

Higher resistance to effect moisture

Higher chemical resistance to corrosive gaseslubes oils,acids&alkalis.

Lesser manufacturing cycle of machine.

Higher p/w ratio,lower cost of machines


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Wound Stator at VPI Plant

Impregnation Plant


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2.8. Excitation system:

2.8.1. Brushless Excitation system:The main parts of the brushless excitation system are:a) Pilot Exciterb) Main Exciter

c) Rectifier Wheelsd) Automatic voltage regulator

a) Pilot Exciter:Three phase pilot exciter is 16 pole revolving field units. The stator accommodatesthree

phase armature winding and magnetic poles are placed on the rotor. Thusrotating flux is

produced which cuts the stationary armature conductors and threephase a.c. is generated.


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PMG Rotor and Fan

b)Main Exciter:

The three phase main exciter is a 6 pole armature type unit. The stator frameaccommodates

the field winding. The field winding is placed on the magneticpoles. The armature consists of

stacked lamination and the three phase winding isinserted into the slots of the laminated

armature.

c) Rectifier wheel:


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Components in the rectifier wheel are as follows1. Silicon diodes2. Aluminium heat sink3. Fuses4. RC circuit

The main component in the rectifier wheel is silicon diodes which are arranged inrectifier

wheel in three phase bridge circuit. The direct current from rectifierwheel is fed to DC leads

and then to the field winding of the rotor.

Rectifier wheel

d) Automatic voltage regulator:

The main features of AVR are:

It has an automatic circuit to control outputs of auto channel and manual channel and

reduces disturbances at the generator terminals during transfer from auto regulation to

manual regulation.

It is also having limiters for the stator current for the optimum utilization of lagging and

leading reactive capabilities of turbo generator.

There will be automatic transfer from auto regulation to manual regulation in case do

measuring PT fuse failure or some internal faults in the auto channel.

The generator voltage in both channels that is in the auto channel and the manual channelcan be controlled automatically.

2.8.2. Advantages of Brushless Excitation:

Eliminates slip rings and brushes


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Eliminates all problems associated with transfer of current via sliding contacts

Eliminates the hazard of changing brushes on load

Brush losses are eliminated

Minimum operating and maintenance cost

High response excitation with fast acting AVR

Rotor Earth Fault Measurement through provision of Instrument Slip Rings

Brush less Exciters Nomenclature:

E A R 50 / 15 - 30 / 8 - 3

Exciter

Air-cooled

Rotating Rectifier Diode Wheel

Diameter

Length

rpm

No. of poles

Number of diodes in a bridge arm

2.8.3. PERMANENT MAGNET GENERATOR (PMG):

This is a very small 3 phase synchronous generator , in which armature windings are located

in the stator and the rotor is provided with permanent magnet poles .Since permanent

magnet poles are provided , no field winding(or excitation) is required. With the help of

PMG, the generator can build up voltage in the absence of any DC excitation.


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The 3 phase AC produced in the PMG stator is rectified and fed to the main exciter field

through an AVR.

The PMG is also a high frequency generator, generally at 150 Hz or 300Hz.

Fig :Permanent Magnetic Regulator


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3. TESTING OF TURBOGENERATOR

Testing is an activity, which basically evaluates a component, and or a product (built

up of component assemblies) as to whether it has the technical capability that has been built

into it by way of design, materials, and technological processes employed while

manufacturing and workmanship.

As such, testing activities can broadly be classified in to a number of categories as follows:

- Type tests.

- Routine tests.

- Process tests.

a) Advantages of testing:

Provides quality assurance.

Meets the requirements of legal & contract requirements.

Ensures process capability & develops checklist.

Have an approved procedure.

Check the equipment before use.

Calibrate the test equipment & instruments.

Ensure interlocks of the equipment

b) Performance tests on turbo generator:

With the increasing trend on standardization every country has its National

Standards covering a wide range of subjects, In India; the Indian Standards are valid and

applicable. The machines produced at Hyderabad fully conform to Indian and also

International Standards and many machines are being exported to various countries. The

performance tests on turbo generators include the following

Measurement of Insulation resistance.

Measurement of ohmic resistance of windings in cold state.

Applied H.V. tests.

Measurement of vibrations and mechanical losses.

Measurement of no load characteristics.

Measurement of short circuit characteristics.

Determination of excitation on load and checks of voltage rise (regulation).

Measurement of leakage and potier reactance.


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Measurement of losses and determination of efficiency.

Heat run tests.

Retardation test.

c) Test Procedure:

During manufacturing of Turbo-generators the following stage tests are to be carried out on

individual components to ensure quality of the product and to reduce last minute delay

during acceptances tests. The standards for these tests will differ from plant to plant.

1. Stator

Checking up of Resistance temperature detectors while core building.

Ring flux test.

Tan delta measurement on stator winding after impregnation.

H.V. test on coils during manufacturers and assembly.

Capacitance measurement.

2. Rotor

H.V. test on excitation connecting leads.

H.V. & inter- turn insulation tests on field coils during the winding process.

Impedance measurement field coils.

3. Over-speed test.

Over-speed test involves mechanical running of the rotor at the prescribed % over-speed for the stipulated period of time. It is later subjected to a very close mechanical

inspection to investigating into the effects of over speed if any.

The rotor isbalancedto the required levels as per standards. This brings us to measurement

of characteristics and losses of the generator.

Measurement or determination of efficiency of the machine is an important step.

For determination of efficiency, losses measurement on the drive system is to be done and

derive machine losses by subtracting drive motor losses.

4. Determination of efficiency

Having measured the losses, the efficiency can be estimated from the formula.

Efficiency =(inputlosses / input = output / (Output + losses)

Tolerance on guaranteed efficiency is 0.1 (1-efficiency) when measured by summation of

losses method.


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5. Measurement for leakage &potier reactance

Leakage reactance is measured on the stator alone when winding is completed. The

procedure involves supplying the winding from a 3-phase variable voltage source and

measuring currents voltage and power at the stator terminals. Depending on the source

capacity upto 0.25In may be passed. Potier reactance as per accepted standard practices is

taken as 0.6 to 0.65 of total leakage reactance. It can alternatively be calculated from the

zero power factor test measurements.

6. Line-to-Line sustained short circuit test

The negative phase sequence reactance is can be determined from the line-to-line

sustained short circuit test.

Negative phase sequence reactance (X2) = P / 3. (Ik2)2

Where P = Power measured and Ik2 is negative phase sequence current measured during

line-to-line sustained short circuit test.

7. Line to line and to neutral sustained short circuit test

From this test zero phase sequence reactance can be determined.

Zero phase sequence reactance (Xo) = Vo / Io.

Where Vo = Voltage from measured in open phase voltage and Io is current measured in line

to neutral during line to line sustained short circuit test.

8. Retardation test for determination of Moment of Inertia

The machine speed and time are noted during free coasting down of the machine.

Moment of inertia = (4 x 365000 x P x T)/ N2 Kg-m2

Where: P = Power input in kW to keep machine at rated speed. N = Rated Speed in rpm.

T Time in seconds from curve. (Tangent drawn at rated rpm)

3.1. OBJECTIVES OF TESTING:

Testing is the most important process to be conducted on a machine after it is designed. The

testing of machine is necessary primarily to establish that the machine performance

complies with the customer specifications. Tests ensure that the piece of equipment

concerned is suitable for and capable for performing duty for which it is intended.


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Testing is done under condition simulating closely as possible to those, which will

apply when the set is finally installed with a view to demonstrate to purchasers

representative its satisfactory operation. Test provides the experimental data like

efficiency, losses, characteristics, temperature limits, etc. for the use of design office, both as

confirmation of design forecast and also as basic information for the production of future

designs.

a) Introduction

With ever increasing rating of the modern turbo generators and reliability of service

expected, testing at manufacturers works has become of paramount importance. The

machine performance is evaluated from the results of the equivalent tests.

b) Advantages of testing

- Provides data for optimization of design

- Provides quality assurance

- Meets the requirement of legal and contract requirements.

- Reduction in rework cost.

- Ensures process capability and develops checklist.

- Increases confidence levels in manufacture.

- Establishes control over raw materials.

- Helps in building of safety and general operation and manual.

3.2. TYPES OF TESTS

Tests on turbo generators are classified under the following headings, which is also

the order in which these are performed during the course of manufacture.

Tests on the materials and components during the manufacture so as to control the

quality of the materials in process also known as Process tests.

Performance tests on the machine to prove the performance of the generator in

accordance with the required standard.

3.3. TESTS DURING MANUFACTURE/ PROCESS TESTS:

Tests on rotor winding

Tests on stator coils.


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Balancing and over speeding of rotor.

3.3.1. Rotor winding

The rotor coils and their insulation are subjected to a high stress when in normal

operation. Thus a thorough inspection of these is necessary before putting them in the rotor

slots as otherwise any replacement of the faulty insulation or cell would necessitate

removing the coil binding rings and the wedges, which is a cumber job. A series of graded

voltage tests are conducted on the rotor cell and coils dusting assembly and also when

finished to test its electrical strength against likely creep age to ground or inter turn

breakdown.

3.3.2. Tests on the Rotor Coils

1. Between turns:

The rotor coils turns are made up of a number of turns which are formed in halves and

then assembled with their inter turn insulation and boned with an adhesive in the steam

heat press. Although the normal working voltage per turn is very small, a turn to test of 240

volts is done to exercise quality control.

2. Top turn trough:

The insulation the top and bottom of the rotor slot provides adequate and safe creep

age distance between copper and rotor steel, in case of slotted copper 500 volts for one

minute to test the top trough.

3. Collecting leads:

A high voltage equivalent to the shipping test plus 1500v is applied to the collector

leads when fitted in bore with studs of seals fitted but before connecting to the windings.

3.3.3. Tests on stator coils

As per the standards the stator winding has to be subjected to a shipping pressure test of

(2 x line voltage +1kv). This power frequency voltage applied for one minute.

In order that machine when found with stands this final voltage test and any faulty coil is

eliminated during various stages of coil manufacture and winding. Because of the

continued application of HV tests overstress the winding insulation, the voltage is

reduced in various stages.

1. Tests between parallel strips


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This is a test of strip insulation provided for eddy emf, on low voltage at the order of 350V

for 3 seconds is applied.

2. Coils in manufacturing and Winding Sections:

When the coils are first tested, a voltage of 8kV in excess of shipping pressure test is applied,

and this voltage tests are repeated.

3. Tests on Thermocouples:

1000V Megger test is applied to the thermocouples.

3.4 BALANCING AND OVER SPEEDING OF ROTORS

1. Balancing:

One of the most important preliminaries to testing is that of balancing the rotors.

Before over speeding, the rotor is dynamically balanced, in cold as well as hot conditions. A

set of run up and run down is taken sure those critical speeds are well away from the

running speed.

2. Over Speeding:

In order to check the soundness of all parts and fitting on rotor assembly, the rotor is

run at an over speed of 14% for five minutes or 30% for one minute.

3.5. PERFORMANCE TESTS/ TESTS ON COMPLETED MACHINE:

The machine is assembled and erected at the heavy rotating plant test bay for test.

a. Dry out insulation resistance of rotor & stator windings

Before starting with running tests, the stator windings are dried out by circulating

current in the winding from an external dc source Input of power is so controlled as to limit

the temperature of the end windings to a maximum of 800C by thermometer.

Progress of dry out is observed by one minute insulation resistance reading with

1000v Megger. With the application of heat, the insulation resistance will initially drop and

then will rise again over a period of time and finally becomes approximately constant

temperature. Ration of ten-minute reading, i.e. polarization index, when more than 2 gives

an indication of good dry out. Insulation resistance readings of rotor winding to ground are

taken.


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b. Resistance of machine windings

Measurement of cold winding resistance, both for stator and rotor must be very

accurate since it forms the basis of calculating rise in temperature of rotor winding under

cold and hot conditions by resistance method.

All precautions are taken to ascertain correct temperature of the winding white

measuring cold resistance. Since the winding resistance of turbo generator is quite low: a

modified form of wheat stone bridge i.e. Kelvins double bridge does away with the necessity

of accounting for the resistance of loads. Resistance between phases for stator and between

slip rings for is recorded along with the cold winding temperature at the time of

measurement.

c. Phase sequence test:

The phase sequence test is to check the agreement of the terminal markings that

have been specified using the Phase Sequence Indicator.

3.5.1 Performance tests

The performance tests on the turbo generator are classified as:

- Type tests

- Routine tests

- Heat run tests

3.5.2. Type tests

These are specially requested tests form the customer. They are not performed on all

machines i.e., they are specific to machine. They include

Mechanical measurement of leakage reactance of stator winding

Measurement of residual voltage of stator winding at rated speed

Line to line sustain short test and determination of negative sequence reactance [X2]

Line-to-line and neutral sustain short circuit test and determination of negative

sequence reactance [XO]

Retardation test for determination of GD2

3.5.3. Measurement of leakage reactance of stator winding


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This test is done on the stator winding with out rotor, generally before assembly on

test bed for running test.

Purpose

To determine total leakage reactance

To determine potier reactance

To determine armature leakage reactance

3.5.4. Before running the machines ensure:

Lubricating oil is flowing through bearings and the gear box.

All the instruments are working

Roll the machine and check all the parameters. Slowly raise the speed to one-sixth

rated speed. Observe slow roll vibrations, temperature and oil flows. Raise the speed

to one-third rpm slowly and record the vibrations, temperature and oil flows.

The vibrations are measured at rated speed on both the bearing housings (pedestals)

in horizontal, vertical and axial directions with the help of vibration meters, which

are internally connected to the monitor and the vibrations, are noted in the form of

graphs.

The temperature of stator is monitored by monitoring resistance temperature

detectors embedded in core, tooth and winding. Now raise the speed to two third the

rpm by observing all the parameters, critical speed and record them. The machine is

rolled and run at rated speed after ensuring the bearing oil and left at rated for

stabilization of bearing temperatures.


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INSULATION

History of Insulation

Since the beginning of civilization, humans have recognized the need for insulation. Man

clothed himself with wool and skins from animals. He built homes of wood, stone, earth, and

other materials for protection from the cold winter and the heat of summer.

Ancient Greeks and Romans discovered asbestos and found many uses because of its

resistance to heat and fire. The Romans used cork for insulation. One use was in shoes, to

keep their feet warm. As industrialization expanded, cork was used as insulation for ice

houses. Blocks of ice were cut from frozen lakes in winter and stored in cork-lined ice

houses for use in summer. When mechanical refrigeration came into use, cork was used to

insulate pipes and equipment.

Mineral fiber - another important insulating material - was first used by the natives of the

Hawaiian Islands to blanket their huts. The fibers came from volcanic deposits where

escaping steam had broken the molten lava into fluffy fibers.

Man-Made mineral fibers were developed in the early industrialization period. Steam was

injected into molten slag, a waste product from iron furnaces. It has been widely used for

both building and industrial insulation. As more and more uses were found for it, mineral

fibers were modified and molded into different shapes such as pipe covering.

A pipe covering made from corrugated layers of asbestos paper was developed for hot

applications. This type of insulation obtained its efficiency from air pockets in the

corrugation. The development of the more efficient fiberglass made this material obsolete.

Another obsolete material widely used for many years was 85 percent magnesia. This

material was similar to the calcium silicate used today but it contained asbestos fibers as a

binder. This material was used extensively until the mid-fifties when calcium silicate made it

obsolete.


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Materials made from layers of high-fiber-content felt or paper, with layers of asphalt

saturated felt formerly were used on moderate-to-cold temperature applications.

Today materials manufactured from fiberglass, ceramic, mineral wool, calcium silicate,

foamed plastic, glass, and other substances are used in many shapes and forms. The most

widely used material is fiberglass which is available in loose fill, blanket, board, and molded

pipe shapes.

Insulation uses range from conventional building and pipe insulation to insulation for

equipment and systems operating at extremely low or high temperatures. Insulation is a

vital part of industry. It is used in many ways and forms to improve our environment. Its

importance will continue to increase as technology advances and energy resources and

conservation are high priority.

Electrical Insulation

An electrical insulation system (EIS) is defined as an insulating structure containing one or

more electrical insulating materials (EIM) together with associated conducting parts

employed in an electrotechnical device. This is a rather simple definition for what can be a

very complex combination of materials. An EIS is composed of two sets of components

major ground insulation components and minor components.

Major ground insulation components are EIM. That is, these materials are the electrically

stressed components used to separate conducting parts at different electrical potentials.

Typical examples of major components include magnet wires, varnishes, and flexible sheet

materials used for core insulation, as high-low barrier insulation, or slot liners in motors.

Minor components are those materials used in combination with the major ground

insulation for mechanical, heat transfer, decoration, or other non-electrically stressed

applications. Typical examples of minor components include pressure-sensitive tapes,

sleeving and tubing, lead wires, phase insulation, and potting compounds.INSULATING

MATERIALS

Insulating materials

This report lists a number of commonly used insulating materials found in electronic and

electric equipment. Although this listing is somewhat long, it is not comprehensive- the


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choices available canbe overwhelming. Fortunately many superb and inexpensive materials

are available and the finalchoice may be somewhat arbitrary and the designer is advised to

obtain samples of several candidates before making a final decision. Trade names do not

represent a particular brand preference but areincluded for cla:rity.

Table for classes of insulation:

Insulation

class

Maximum Permissible

operating Temperature (C)

Y 90

A 105

E 120

B 130

F 155

H 180

C Over 180

The following are brief explanations of those insulation techniques.

i) Class-Y insulation: Withstands atemperature of up to 90C; typically made of cotton, silk,

or paper

ii) Class-A insulation: Withstands a temperature of up to 105C; reinforced

Class-Y materials with impregnated varnish or insulation oil

iii) Class-E insulation: Withstands a temperature of up to 120C

iv) Class-B insulation: Withstands a temperature of up to 130C. This has a form that

inorganic material is hardened with adhesives. This is the first insulator using this structure.

v) Class-F insulation: Withstands a temperature of up to 155C; for example, made of Class-B

materials that are upgraded with adhesives, silicone, and alkyd-resin varnish ofhigherthermal endurance

vi) Class H insulation: Withstands atemperature of up to 180C; for example, made of

inorganic material glued with silicone resin or adhesives of equivalent performance

vii) Class-C insulation: Withstands a temperature of up to 180C or higher; made of 100%

inorganic material


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As explained above, electrical insulation is classified with its maximum allowable

temperature. By adopting an insulation technique of higher thermal endurance, electric

instruments can be downsized.

INSULATING MATERIALS

A.B.S.: Acrylonitrile, butadiene, and styrene combine to form this common plastic often

usedto make housings or other mechanical parts.ACETATE: Acetates have good electrical

insulating properties and is the material used to make movie and microfilm.

ACRYLIC: Lucite and Plexiglass are trade names for acrylic which has widespread use

wheretoughness and transparency are required. Solvent cement is quite effective for

welding pieces together.

BERYLLIUM OXIDE: A hard white ceramic-like material used as an electrical insulatorwhere

high thermal conductivity is required. Beryllium oxide is highly toxic in powder form

andshould never be filed or sanded and consequently has fallen out of common use. Power

semiconductorheat sinks can still be found with beryllium oxide spacers for electrical

insulation.

CERAMIC: Ceramics are used to fabricate insulators, components, and circuit boards.

Thegood electrical insulating properties are complemented by the high thermal

conductivity.

DELRIN: This Dupontacetal resin is made from polymerized formaldehyde and finds

usessimilar to nylon. The material is rigid and has excellent mechanical and electrical

properties making itsuse common in appliances and electronics.

EPOXY/FIBERGLASS: This laminate is quite common due to its superior strength

andexcellent electrical properties even in humid environment. Most modern circuit boards

are made from a

grade of epoxy/fiberglass. (Grades include G10/FR4 and G11/FR5 extended temperature

grade.)

GLASS: Glass insulation comes in a wide variety of forms including solid glass, fiber

tapes,fiberglass sheets and mats, woven tubing and cloth, and various composites. High

temperatureoperation is a key feature.

KAPTON: Polyimide film has exceptionally good heat resistance and superb mechanical

andelectrical properties. Kapton tapes are fairly expensive but often indispensable.

KYNAR: As is Teflon, Kynar is a floropolymer with excellent chemical and

abrasionresistance. It is readily machined and welded.


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LEXAN and MERLON: These polycarbonates have excellent electrical insulating

properties.Optical grades are available and the material is so tough that it meets U.L.

requirements for burglaryresistance. Non-transparent grades are machined to make strong

insulators, rollers, and othermechanical parts.

MELAMINE: Melamine laminated with woven glass makes a very hard laminate with good

dimensional stability and arc resistance. (Grades G5 is the mechanical grade and G9 is the

electrical

grade.)

MICA: Mica sheets or "stove mica" is used for electrical insulation where high

temperaturesare encountered. Thermal conductivity is high so mica insulators are useful for

heatsinking transistorsor other components with electrically conductive cases. Puncture

resistance is good but the edges of themica should be flush against a flat surface to prevent

flaking. Mica finds uses in composite tapes andsheets which are useful to 600 degrees

centigrade with excellent corona resistance. Sheets and rods ofmica bonded with glass can

tolerate extreme temperatures, radiation, high voltage, and moisture. Thisrather expensive

laminate may be machined and it will not burn or outgas.

NEOPRENE: Neoprene rubber is the material used for most wet suits. This black rubber

iscommonly used for gaskets, shock absorbers, grommets, and foams.NOMEX: Nomex is a

Dupont aromatic polyamide with an operating temperature range over220 degrees

centigrade and with superb high voltage breakdown. It is an excellent choice

forstandardization since it outperforms many other materials.

NYLON: Nylon has good resistance to abrasion, chemicals, and high voltages and is oftenused

to fashion electro-mechanical components. Nylon is extruded and cast and is filled with a

varietyof other materials to improve weathering, impact resistance, coefficient of friction,

and stiffness.

P.E.T.: Polyethylene terephthalate is a highly dimensionally stable thermoplastic with

goodimmunity to moisture. This excellent insulator has a low coefficient of friction and is

excellent forguides and other moving parts.

P.E.T.G.: A clear, tough copolyester commonly used for durable "bubble-packs" or

foodcontainers.

PHENOLICS: Phenolic laminated sheets are usually brown or black and have

excellentmechanical properties. Phenolics are commonly used in the manufacture of

switches and similarcomponents because it is easily machined and provides excellent


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insulation. Phenolic laminates arewidely used for terminal boards, connectors, boxes, and

components. (Grades x, xx, xxx arepaper/phenolic and grades c, ce, l, le are cotton/phenolic

which is not the best choice for insulation.

Grade N-1 is nylon/phenolic and has good electrical properties even in high humidity but

exhibitssome cold flow.)

POLYESTER (MYLAR): A strong material often used in film sheets and tapes for graphic

artsand electronics. Those shiny balloons and "space blankets" are usually made from

metalized Mylar.

Mylar is also used as a dielectric in capacitors.

POLYOLEFINS: Polyethylene is the white Teflon-like material used for food cutting board.

Different densities are available with the ultra-high molecular weight grade at the top

offeringtoughness outlasting steel in some applications. Polypropylene is another widely

used polyolefin.

POLYSTYRENE: A clear insulator with superb dielectric properties. Polystyrene

capacitorsexhibit little dielectric adsorption and virtually no leakage. Liquid polystyrene or

Q-dope is a low-losscoil dope used to secure windings and other components in RF circuits.

POLYURETHANE: Polyurethane is another common polymer which features abrasion

andtear resistance along with a host of desirable characteristics. Degrading little over time

or temperature,polyurethane is popular in both commercial and consumer applications.

PVC: Polyvinylcloride or PVC is perhaps the most common insulating material. Most wiringis

insulated with PVC including house wiring. Irradiated PVC has superior strength and

resistance toheat. PVC tapes and tubing are also quite common.

Electrical and electronic housings are commonly molded from PVC.

SILICONE/FIBERGLASS: Glass cloth impregnated with a silicone resin binder makes

anexcellent laminate with good dielectric loss when dry. (Grades include G7.)

SILICONE RUBBER: A variety of silicone foam rubbers are available for insulating

andcushioning electronic assemblies. Silicone rubbers exhibit a wish list of characteristics

includingsuperb chemical resistance, high temperature performance, good thermal and

electrical resistance, longterm resiliency, and easy fabrication. Liquid silicone rubbers are

available in electrical grades forconformal coating, potting, and gluing. Silicone rubbers

found in the hardware store should be avoided.

in electronic assemblies because they produce acetic acid. Silicone rubbers filled with

aluminum oxideare available for applications requiring thermal conductivity.


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TFE (TEFLON): Teflon is an excellent high temperature insulation with superb

electricalproperties. Teflon tubing and wire insulation comes in a variety of colors and

typically feels slippery.

The insulation is impervious to the heat and chemicals normally encountered in

electronicsmanufacturing but the material will "cold flow" so Teflon insulation is avoided

where sharp corners or

points are encountered. Laminated TFE circuit boards take advantage of Teflon's excellent

microwavecharacteristics.

Teflon emits a dangerous gas when exposed to extreme heat. White Teflon terminals are

commonlyused where extremely good insulation is required. The slick surface repels water

so the insulationproperties are fantastic even in high humidity. High quality I.C. sockets are

made from Teflon toreduce leakage currents. Teflon and Teflon composite tapes with

adhesive are available. FEP is a lower

temperatureTeflon.THERMOPLASTICS: Other thermoplastics include Polysulfone,

Polyetherimide, Polyamideimide, and polyphenylene with trade names like Noryl, Ultem,

Udel, Vespel, and Torlon. Thesematerials are grouped here for completeness and are not

particularly similar. For example Vespel is SPpolyimide with amazing properties but

commanding an equally amazing price- a 10 inch sheet couldcost thousands of dollars,

whereas Polysulfone (Udel) is a rather good engineering material with a costfor the same 10

inch sheet near thirty dollars.

ELECTRICAL INSULATING PAPERS

A variety of insulating papers are available specifically designed for insulating electrical

circuits.

Rag and craft paper often called Transformer Paper is often used to separate windings

intransformers or in applications where no sharp edges might poke through the relatively

weak paper.

Grey and tan are common colors.

Fishpaper is a curious name referring to a grey cotton rag paper usually vulcanized and

oftenlaminated with Mylar. The Mylar may have paper on one or both sides and many

thickness grades areavailable. Tear and puncture resistance are excellent and the thinner

grades are easily cut with scissors.

Other "sandwich materials" are available including 100% polyester laminates and are

usually a distinctcolor. The paper/Mylar laminates resist soldering heat better since the


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paper doesn't melt and theDacron/Mylar laminates resist moisture best. Laminates with

thicker polyester centers are fashionedinto insulating plates in many electro-mechanical

devices. A typical application may be observedinside most older electrical timers where a

printed and folded piece of laminated paper keeps the user'sfingers away from the high

voltage when adjusting the position of the on and off trippers. Papers madewith

temperature resistant nylon and/or glass weaves have excellent electrical properties and

goodtemperature resistance.

Thin sheets of epoxy-fiberglass usually green in color are commonly used for insulating

PCB's andelectronic assemblies with potentially sharp projections. Puncture resistance is

superb even for sheet thin enough to be quite flexible.

A simple clear polyester sheet is sometimes used for insulation but is offers far less puncture

andtemperature resistance than the laminates. The ordinary appearance may prove to be a

liability also:one computer maker uses such a sheet to insulate the motherboard from the

chassis and many noviceshave left this critical insulator out when reassembling their

computer with disastrous results. Die-cutlaminates look important and are easily printed.

TAPES

Tapes are made from many of the above materials. Vinyl tapes are commonly used for wire

insulationand are available in all the colors necessary for color coding. Mylar tapes are

common in electronics:

film capacitors often have a final wrap of yellow Mylar tape. Acetate tapes are used where

goodconformability is desired as when covering coils as is white cotton cloth tape. Glass

cloth electricaltape with thermosetting adhesive (adhesive that permanently sets with

temperature) is used to secureand protect heater windings or insulate components exposed

to heat. Kapton, Teflon, and otherinsulators from the above list are used to make high

performance specialty tapes for harsh temperatureor chemical environment.

FOAMS

Foams are available for both thermal insulation and mechanical / acoustical insulation.

Choosing afoam for vibration damping can prove difficult. Many foams become stiff at cold

temperatures and will"take a set" at elevated temperatures. Some foams may have excellent

temperature characteristics butexhibit too much "spring" giving the assembly an

unacceptable resonance. Evaluate several materialsbefore choosing- foams are made from

many of the insulating materials mentioned above. Some of themore common foams are

listed below.


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NEOPRENE: Neoprene foam (black) is often used for shock absorbing and vibrationdamping.

POLYSTYRENE: Styrofoam is the white foam used in inexpensive ice chests and

packingpeanuts. It is an excellent insulator but cannot tolerate elevated temperatures.

POLYURETHANE: Urethane foams are available in both rigid and flexible forms.

Theinsulating properties are excellent and elevated temperature tolerance is good.

Machined pieces of rigidpolyurethane are often used as thermal insulators in electronic

equipment. The soft foams are good for

vibration and sound attenuation and are available with a wide variety of properties.

SILICONE: Silicone foams provide excellent vibration damping characteristics and

excellenthigh temperature performance and chemical resistance.

VINYL: Vinyl foam has very little "spring" and is useful for vibration damping.

LAMINATES: Various foams are often laminated with a heavy center layer to create a sound

and vibration barrier. Lead has been used as the massive layer but the obvious concerns

have led todifferent materials such as metal oxide filled plastics.

The complete list of foam rubbers, plastics, and other foam materials could fill a bookshelf so

thispartial list should not confine the imagination. The yellow pages of any large city will

yield the namesof plastic companies which usually carry the solid insulating materials

mentioned. Gasket supplierswill have a surprising assortment of sheets and foams including

specialty electronic materials. Themanufacturers can often supply the name of distributors

but if they don't it doesn't mean that localsuppliers aren't there. Check thoroughly before

buying some huge minimum quantity from the factoryalmost all of the materials mentioned

are available from distributors in small quantities. Industry directories will supply the name

of material suppliers if the local distributors cannot. Used bookstoresoften have old copies

of "E.E.M.", "Goldbook" or "Thomas Register" which can give you a list ofmanufacturers. Ask

for the name of the local factory representative since he will probably know thenames of

local suppliers in his territory since he probably visits them on sales calls.

VPI insulation system

INTRODUCTION:

BHEL, Hyderabad manufactures 2 pole and 4 pole turbo-generators in the range of 1.5 MW to

200 MW with voltage levels varying from 440 volts to 13.8 KV. It meets the demand from


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state electricity boards, process industries like sugar, paper and pulp, cement, petro-

chemicals, oil, steel, aluminum etc. The first ever generator manufactured was of 11 KV, 60

MW capacity in the year 1966 and the stator bar with epoxy resin rich insulation system was

ejected out of the mould. After two decades of rich experience in this resin rich insulation

system, for a better and more reliable insulation system of stator windings, Total Vacuum

Pressure Impregnation (VPI) Insulation System with epoxy resins had been introduced in

the year 1984 for the generators at BHEL, Hyderabad.

The total VPI insulation system presents the following advantages:

Void free insulation

B

bharat heavy electrical limited training report

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