VPI for turbo generator

Automation of Vacuum Pressure Impregnation process of Insulation for an Air cooled Turbo generator
DEPT. OF EEE 1 AITS-HYD
1. INTRODUCTION
Electrical insulating materials are defined as materials that offer alarge resistance to
the flow of current and for that reason they are used to keepthe current in its proper path i.e.
along the conductor. Insulation is the heart ofthe generator.
Since generator principle is based on the induction of e.m.f in aconductor when
placed in a varying magnetic field. There should be properinsulation between the magnetic
field and the conductors.
For smaller capacitiesof few KW, the insulation may not affect more on the
performance of thegenerator but for larger capacities of few MW (>100MW) the
optimization ofinsulation is an inevitable task.
Moreover the thickness of insulation should beon par with the level of the voltage,
also non homogenic insulation provisionsmay lead to deterioration where it is thin and
prone to hazardous short circuits.
Also the insulating materials applied to the conductors are required to be flexibleand
have high specific (dielectric) strength and ability to withstand unlimitedcycles of heating
and cooling.
Keeping this in view among other insulating materials like solidsgases etc liquid
dielectrics are playing a major role in heavy electrical equipmentwhere the can embedded
deep into the micro pores and provide betterinsulating properties.
Whereas solid di-electrics provide better insulation withlower thickness and with
greater mechanical strength.
So the process ofinsulation design which has the added advantage of both solid and
liquiddielectrics would be a superior process of insulation design.
One such processwhich has all the above qualities is the VPI (vacuum pressurised
impregnation)process and has proven to be the best process till date.
Vacuum impregnation as an industrial process has been in commercial use for more
than 60 years.
For the world’s largest manufacturers, it continues to be the preferred process through
which to guarantee the pressure-proof, leak-proof, and corrosion-proof requirements of
parts and components in critical operations.

1.1NECESSITY
During normal operation,Electrical machines like Generators, Transformers etc are
subjectedto the failure of the insulation system. How long an insulation system will be
serviceable depends on the materials chosen and the service environment.Thermal,
mechanical, voltage and environmental stresses all combine to reduce the service life of the
Electrical machines.
a) To reduce this losses, We have to manufacture all Electrical machines like
Generators, Transformers using the Vacuum Pressure Impregnation (VPI) process.
b) This system strengthens the insulation system and extends the service life of the
transformer. The VPI process is the most advanced system in use today.
c) VPI includes pressure in addition to vacuum, thus assuring goodpenetration of the
varnish in the coil.
d) The result is improved mechanical strengthand electrical properties.
e) With the improved penetration, a void free coil isachieved as well as giving greater
mechanical strength.
1. 2OBJECTIVE
The ultimate goal of vacuum impregnation is to seal leak / migration paths without
impacting the functional,assembly or appearance characteristics of a part. Functional
characteristics include the ability for fluids or gasses to flow only where needed in order to
enhance in-service performance of the components’ design. Assembly characteristics, which
must be maintained, include performance of tapped holes; the integrity of mating and
sealing surfaces; the elimination of residual internal contamination in water jackets; sockets;
surfaces; and dimensional areas. Appearance characteristics include oxidation and
discoloration.
The processing methods may be used to variety of impregnate parts. The method
selected depends on the sealant and the requirements of the parts. Fundamentally, vacuum
impregnation sealing of porosity addresses a pair of fluid mechanics problems. The laws of
fluid mechanics govern the flow problem of removing the air from the pores and the flow
problem of filling the pores with liquid sealant.

1.3 THEME
To insulate winding elements in rotating high voltage electrical machines the
impregnation technique based on vacuum pressure impregnation has become very popular in
recent years, In doing so, the winding elements for construction engineering reasons are made
either as preformed coils or conduct and scolds preferably performed bars. The winding
elements are provided with mica-containing mail solution and are further tired in a vacuum
pressure impregnation process.
The main theme/characteristics of this insulation system are:
 a. Better heat transfer resulting from penetration into minute air
gaps in between laminations and bar insulation.
b. Low dielectric loss resulting in increased life of insulation and so
the machine.
c. High resistance against the effect of moisture.
d. Reduction of time cycle of insulation.
1.4 ORGANISATION
In this project documentation we have initially put the definition and objective of the
project as well as the design of the project which is followed by theimplementation and
testing phases
Chapter 1: It explains about the introduction to the project, necessity of the project.
Chapter 2: This chapter explains the Literature survey about VPI.
Chapter 3: This chapter explains about development of the VPI.
Chapter 4: This chapter explains the analysis of the VPI process for the stator of Turbo-
generator
Finally the project has been concludedsuccessfully and also the future enhancements
of the project were given in this documentation.

2. LITERATURE SURVEY
2.1 INTRODUCTION
The VPI process is the most effective way known to eliminate the dead air spaces that
cause hot spots within the Electric Machines like large generators and transformer coils.
These hot spots can be 20* higher than the average coil temperature. The VPI process, along
with a good resin, provides a low thermal resistance path that lowers the average operating
temperature of the Machine.
During the VPI process, the resin seals the machine against environmental conditions
and bonds all components of the insulation system together for good mechanical strength.
This is very effective in reducing mechanical vibrations. This greatly reduces the audible
noise level of the generator. The VPI process and resin also enhances the dielectric capability
between windings and between the windings and ground. This allows the transformer to
survive higher voltage stress levels without failure.
2.2 EXISTING SYSTEM
Electrical machines quality is highly dependent on the vacuum pressure impregnation
insulation system. All the high voltage machines, pole coils irrespective of size and shape are
being impregnated under vacuum and pressure of self-developed resin systems. The stringent
quality tests on the resin mixtures and strictly following the vacuum pressure impregnation
and systematic cooling and heating cycle of resin mixture and sophisticated automatic control
systems made the insulation systems for better and better quality for more than 30 years. The
insulating materials used for wedges are resin poor and accelerator treated. For example Main
insulation tapes, mica paper tapes, overhang protective tapes, shrink tape and glass mates
HM693 are treated with accelerator.
After impregnation, they become hard and experiments were conducted for voltage
endurance at room temperature and at evaluated temperature continuously for more than 3
years. Though the mica tape can withstand 20kv per mm, the extrapolation has been done at
4kv/mm and life expectancy is around 100yrs. With an operation stress level of less than
4kv/mm, factor of safety is considerable. The Vacuum pressure impregnation system was
brought by Dr.Meyer with the collaboration of wasting house in the year 1956. The resins
used were of polyester. The mica tapes used for Vacuum pressure impregnation systems are
ROGS 275, ROGS 275.1 and ROV 292. ROGS 275 tapes are with glass cloth baking up to

13.8 kV voltage levels ROV 292 mica paper tapes are with polyester fleece above and more
penetration of resin. ROGS 275.1 tape is special glue varnish for tropical countries like India
and Brazil to resist higher humidity. The glue being used for main insulation tape is X2026
and for conductor insulation is X2027.
The resin used for Vacuum pressure impregnation is ET 884, a mixture of epoxy resin
E1023 (lekuther m x 18) and hardener H1006 in 1:1.2 ratio by weight. In kwu, the
components are mixed in 1:1 ratio.
E1023: The resin is in drums of 220 kgs weight. It is in crystal form at temperature of 14 or
20deg.C the container is resin is available in drum the reason is faster heating in furnacethe
resin in liquid state shall not come out of the container. The drums are kept in oven and
heated up to 1000
C for about 18hrs. If the resin is not fully in liquid condition, it can be
heated up to 1250
C. The storage tank is filled with resin first depending on the volume and
ratio of mixture at a temperature of 600
C through hose pipes. Resin filling is being done by
creating 0.2 bar vacuum in the tank.
2.2.1 RESIN MIXTURE
The mixing ratio of resin to harden is 46:54 parts. The resin mixture required for the
Impregnation tank is 27000lts. A job of 1.9m height and 4.5m diameter can be impregnated.
a )Size of the tanks:
Main impregnation tank = 4.5m (pie) x 3.0m ht.
There are 3-inch vessels for different sizes jobs impregnation.
I. Vessel (1) – 3.8m pie x 2.25m ht.
II. Vessel (2) – 3.0m pie x 2.3m ht.
III. Vessel (3) – 2.0m pie x 2.3m ht.
Three Storage tanks of each resin capacity9000 litres are in the operation for storing.
The resin mixture cooling and heating cycle is by circulating the resin through the heat
exchangers. Oil heated by water is being used for heat exchangers.
The Vacuum pressure impregnation cycle is as per WIV 114.1 standard. The job is
kept in an oven for a period of 12hrs at a temperature of 70O
C. Six no. of thermocouples are
inserted on the back of the core and measured the temperature. Job insertion in the
impregnation tank is at 70o
c. The lid of the impregnation tank will be in open condition. The
vessels are kept clean. Resin available is wiped out by Methylene. Traces of resin shall not be

allowed on the inner side of the tank. It reacts with humidity and scale formation will takes
place. These components obstruct the filters also. The resin at the time of cleaning is
carefully removed by wiping with rubber sheets. Keeping the vessel in slant position on the
ground also cleans the inner vessels. After ensuring the perfect cleaning, the tank should be
allowed for further operation. The job is inserted in the tank the temperature monitoring
thermocouples are placed on the back of the core. The lid is allowed to come down by
hydraulic motor. Silicon grease is applied on the surface of tank where the lid is touching.
A rubber gasket is also provided on the rim not to allow any leakage. Air pipes are closed and
vacuum pumps will be started.
b)Vacuum creation – 0.35 torr for 2 hrs:
The job temperature is to be maintained always above 65O
C, if found less, tank can be
heated up. In practice, the vacuum can be created in 2 hrs. Siemens adept before starts of 2nd
shift (3.0 pm), they create 0.35 torr vacuum and it will be continued till next day morning 1st
shift (6.00 am) min. requirement is 2hrs.During this time the resin cooling is being carried
out to reach 10deg.C and heated up automatically to 70deg.C.
2.2.2 IMPREGNATION
The resin mixture is to be heated to 70deg.C. Every day morning a 20ml sample will
be taken to laboratory tests. Viscosity will be measured at 70deg.C. It should not be more
than 45 CD +10%. Anew resin will beat 15 CP. New resin and hardener mixture is to be
added if the viscosity is more. The resin filling is being completed in 25 minute. At this time,
the vacuum reduces to 0.5 Torr – 1 Torr level. The resin is to be allowed to settle for 15 min.
The level of resin is above 100mm over the job.
a) Pressuring – 3 bar:
With the hydrostatic pressure of the resin, only surface of the insulation can be filled with
resin. To have an effective penetration up to the end of a barrier, pressure is to be created to 3
bar (2 bar over atm. Pressure of 1 bar).
b)Gelling time:
The polymerization of resin and accelerator take place at this time. At 65O
C, the time
required is 170 min. The insulation gets hardened.
c) Curing – 14 hour at 140 O
C:
The resin is to be pumped back to the storage tank. The job is to be removed from the tank
and allowed for dripping. It is kept in oven at 140deg.C for min of 14 hrs. The accelerator

B1057.1 is to be placed at 4 corners of the oven. In curing process the accelerator vapors will
react with surface resin and cures.
2.3DISADVANTAGES OF EXISTING SYSTEM
a. If any short circuit is noticed, the repairing process is difficult and need of excess
resin from outside.
b. Dependability for basic insulating material on foreign supply
2.4 CONCLUSION
Hence Vacuum-Pressure Impregnation technology can be used in a wide range of
applications from insulating electrical coil windings to sealing porous metal castings. It
normally produces better work in less time and at a lower cost than other available
procedures.
These systems can be large or small, simple or highly sophisticated and equipped with
manual, semi-automatic or automatic controls. Vacuum Pressure Impregnation (VPI) yields
superior results with better insulating properties, combined with “flexible” rigidity, resulting
in greater overall reliability and longer life. VPI reduces coil vibration by serving as an
adhesive between coil wires, coil insulation, and by bonding coils to their slots.

3. SYSTEM DEVELOPMENT
3.1. INTRODUCTION (Vacuum Pressure Impregnation)
VPI produces a better insulation system than can be obtained by conventional
methods, better environmental protection and superior chemical and moisture resistance
(salt water immersion tests).The removal of air voids from the windings assures longer
electrical life and less opportunity for corona. In addition, more solid fill means heat will be
conducted to the outside more efficiently, better thermal endurance, lower hot-spot
temperatures, and lower temperature rise. Further, VPI with a solvent less product provides
greater mechanical and structural strength and may eliminate the need for a surge ring.
Blocking or tying may be replaced with Dacron felt pads that will form the necessary
blocking when impregnated with resin.
Global vacuum pressure impregnation insulation system exhibits various merits in
insulation performance and reliability for operation including maintenance. Therefore, the
system is suitable for ordinary turbine generators, especially, the generators for geothermal
power plant. Developed a global vacuum pressure impregnation insulation system (F-resin/G
insulation system) for large size turbine generators and had put into practical use in 1993.
In the process of evaluation of insulation system, evaluated impregnating ability,
electrical, and mechanical characteristics. From evaluation able to achieve the improvement
of heat cycle, heat resistance and V-t characteristics against usual insulation system, because
of the utilization of internal electrical field relaxation layer, thermal stress relaxation layer,
insulation tapes with excellent impregnation ability and epoxy resin with high heat resistance.
As the insulation characteristics are affected not only utilized materials and their composition
but also manufacturing process, it is reasonable to suppose that we can obtain the
improvement of insulation performances and stabilization of quality by using the taping
simulation technique.
In the global vacuum impregnation insulation system, stator winding inserted into
stator is immersed in epoxy resin, and is impregnated under vacuum and pressure. Through
this VPI process not only the coil insulation is formed but also the resin penetrates into stator
slot clearance, wedges and coils. Additionally, all stator parts are coated by the resin.
Furthermore, the stator coil insulation is impregnated by the resin continuously andrigidly
from slot portion to coil end conductor connecting portion. Therefore, there are a lot of merits

such as high reliability of winding, no looseness of stator core, corrosion proof, and
prevention of coil vibration as well as excellent heat conductivity from the conductor to the
core owing to filling up of stator slot clearance by the resin. Furthermore, replacement of
wedges is not required because no wedge looseness is induced. These features are suitable for
ordinary turbine generators, especially, the generators for geothermal power plant,
surrounded by atmosphere containing hydrogen sulphide Though this insulation system is
mainly applied to air cooled turbine generators with output range of from 20 to 260MVA, it
is also possible to manufacture hydrogen cooled turbine generators with output range of from
50 to340MVA, utilizing the merits of global vacuum pressure impregnation insulation
system.
3.1.1. FEATURES OF F-RESIN/G INSULATUION SYSTEM
Fig. 3.1 Appearance of 126MVA air cooled turbine generator stator.
Global vacuum pressure impregnation insulation system retains many merits such as
improvement of insulation performances, stabilization of quality, shortening of
manufacturing period and mitigation of maintenance load. In particular, since a thermal stress

relaxation layer is provided in the F-resin/G insulation system, a stable heat cycle resistance
characteristic is realized and enables to serve for the DSS (daily start and stop) operation
mode which corresponds to current power demand.
The main features of the F-resin/G insulation system are listed below, and Figure 2 shows
cross section of winding.
a) Utilization of epoxy impregnation resin with long enduring life and high heat
resistance.
b) Utilization of main insulation tapes with excellent impregnation ability.
c) Utilization of internal electrical field relaxation layer
d) Utilization of thermal stress relaxation layer
3.2. Glossary of VPI terms:
1)Bond Strength:The measure of force required to break the bond of varnished helical coils
of enamelled magnet wire.
2)Bump: Briefly revert from vacuum to atmospheric pressure and again draw the vacuum.
Applied in the wet vacuum cycle to help dislodge trapped air and improve penetration.
3)Centipoise:Unit of viscosity. Usually measured by the drag on a turning spindle immersed
in the liquid, Brookfield viscosity. A force of 0.01 dyne per centimetre.
4)Film Build: Average build-up of cured resin on one side of a metal panel.
5)Copolymer: A polymer formed by the of the resin.
6)Deaerate:Remove air and other gasses by vacuum. Note that initial deaeration after a tank
fill can take from several hours to as much as 3 or 4 days depending on the amount, type and
condition of the resin.
7)Dielectric Constant:The property of a material that determines how much charge is stored
per unit volume when unit voltage is applied. The capacitance of a material compared with
the capacitance of an equal volume of air or vacuum.
8)Dielectric Strength: The voltage a material can withstand before breakdown occurs.
Usually expressed in “Volts Per Mil”. Interestingly, a thicker section of material has a higher

total breakdown but a lower dielectric strength, i.e. dielectric strength for one mil Mylar tape
may be 3000 VPM but for 2 mils, breakdown would be only 5000 Volts (2500 VPM).
9)Dissipation Factor:An indication of energy loss in the circuit, as in the production of
unused heat. A multiplier used to obtain useful energy compared to supplied energy.
10Electrical Varnish:A resinous material used to protect and insulate electrical apparatus,
which is applied as a liquid and converted by chemical action, with heat or without, to form a
solid film or mass.
11) Flash Point: The temperature at which enough vapour is generated to flash if a spark or
flame is introduced.
12) Foaming: An accumulation of frothy bubbles caused under vacuum by the expansion of
air and other gasses trapped within the resin.
13) Form Wound: Describes a coil that is formed or shaped over a fixture. Often made with
rectangular conductors laid precisely together, interleaved with flexible insulation. Also
usually covered with one or several wraps of half lapped tape. Also a motor incorporating
such coils.
14)Green: Describes coils or devices that have not been treated, coated or sealed.
15)Half Lap: Spiral tape wrap in which each turn overlaps the previous one by a half tape
width. Provides a double thickness of tape.
16)Hertz: A term indicating the frequency of one cycle per second.
Hg: Chemical symbol for the element, mercury.
18)Holding Tank: A reservoir for keeping the varnish when it is not in use. Should be
equipped with heavy duty mixer and vacuum capability. Refrigeration may be needed in
warmer climates and/or where hot dipping or continuous use is anticipated. Also consider
cooling when infrequent use (low tank turnover) is anticipated. Storage @ <75°F is
suggested. Vacuum cycles can be short- ended by storing the resin under vacuum to prevent
build up of air and other gasses in the resin.
19)Millibar: A unit of atmospheric pressure: 0.75 mm Hg (75 microns). One mm equals 1.33
mbar.

20)Preheat:To bake the device before processing.
21)Preheated Oven:Oven heated until the skins(inside walls) are at temperature and
temperaturehas stabilized. May take several hours.
22)psi :Abbreviation for “Pounds per Square Inch”.Random Wound: Describes a coil in
which thewires do not lie in an even pattern. Not shapedbefore insertion in the devise. Also a
motor containing such coils. Sometimes called “Mush Wound”.
23)Resins:A class of organic, liquid, fusible materials of synthetic or natural origin that are
polymeric in structure. Storage Life: The time during which a liquid resin can be stored @
70°F and remain suitable for use. Also called “Shelf Life”. See Tank Life.
24)Stress Crack: A fissure in the cured resin caused by unequal expansion and contraction
of the core, flexible insulation, resin, etc.
25)Tank Life:The time the product remains usable in service. Tank life is affected by the
frequency of use, processing temperature, turnover of material, storage temperature, and
occasionally by contaminants. Also called “Pot Life”. Thermal Conductivity: The ability of a
material to conduct heat. Usually expressed as: Calories/sec/cm2/°F/cm thickness.
26)Thixotropic (Thixotropy):Describes materials that liquefy or flow when agitated
(mixed) and return to a thick consistency when allowed to rest, e.g. ketchup. A thixotropic
material can therefore, be used at both high and low viscosities.
27)Torr: Unit of pressure (vacuum): 1 mm Hg
28) Vacuum Chamber:Vessel where devices are processed. May be equipped for both
vacuum and pressure. Usually also includes 2 portholes, the sight port and the light port, one
for illumination, the other for viewing the process .
29)VapourPressure: An indication of the evaporation rate. The pressure in an enclosed
container when the vapour and liquid are in equilibrium.
30)Viscosity:The resistance of a material to flow. Higher viscosity liquid flows more slowly,
lower more quickly. May be measured in centipoise, or in minutes and seconds.
31)Volume Resistivity: The ability of a material to resist the passage of electricity through
its bulk. The value is expressed in “Ohm-Cm”.

3.3. Typical VPI cycle...
This process should be varied according to the VPI equipment, resin and apparatus to
be treated. Equipment to be processed must be green (untreated), and tapes untreated, open,
or permeable so as not to block the resin. Resin filled or B-Staged tapes that cure with heat
should not be used.
A)Preheat:
The part is placed in an oven and heated to 250°-325°F. The preheat serves to
evaporate moisture and any volatile oils, which may be present. It also improves penetration
and fill by lowering resin viscosity surrounding the part, and creates suction when the part is
cooled by immersion in the resin. Before proceeding to the next step, cool to 150°F or cooler.
B)Dry Vacuum:
After placing the part in the vacuum chamber, apply vacuum, typically 1 - 4 mm Hg,
for 30 minutes. During this phase, air and any remaining moisture, oil, etc. is removed.
NOTE: During the dry vacuum, the resin in the holding tank should be deaerated and
thixotropic products should be agitated (mixed) in the holding tank for at least 15 minutes.
Agitation will reduce viscosity for effective penetration and fill.
C)Wet Vacuum:
Immediately after mixing, introduce the resin into the vacuum chamber allowing it to
flow up from the bottom so as not to block further penetration. The resin should cover the
part by a depth of at least 1 inch. If excessive foaming occurs during the vacuum process,
slow down the introduction of resin to allow time for air and gasses to escape. Maintain
recommended vacuum for 20-60 minutes. Larger units and those with more layers of tape
will require a longer time under vacuum. For fine wire coils and constricted parts, bumping
the vacuum may increase penetration.
3.4. Pressure Cycle:
When the wet vacuum portion of the cycle is complete and the parts are still totally
immersed, pressurize to 90 – 100 psi with air for an hour or longer. Note: Depending on resin
characteristics, an inert gas may be required to bring vacuum up to atmospheric pressure.
Form wound devices will require about 15 minutes per half lap of tape. Release pressure.

Thixotropic products will have a higher build if allowed an atmospheric soak for 30-60
minutes.
a)Removal and Drain:
Vent pressure and remove the part or drain the resin. A removal rate of 4 inches per
minute or slower should be used so that the resin forms a uniform coating.
Drain may take place over the tank so that runoff can be captured and returned to the
reservoir. While draining, the part should hang at an angle so that flat surfaces can drain
readily.
This will tend to eliminate thick sections, which might promote stress cracks. Drain
until major runoff stops. Follow specific recommendations on the product data sheet.
If using a thixotropic product, allow a period of 1-2 hours or more after drain to
promote resin retention during cure. Thixotropic products should show minimal drain in the
oven.
b) Bake: Place the treated part in a fully preheated oven. Cure using DOLPH’
recommendations for time and temperature according to the product data sheet.
Applications for VPI...
1) High Voltage Machines
2) High Temperature Apparatus
3) Transformers
4) HID Ballasts
5) Random Wound Stators
6) Chemical Duty Motors
7) Rugged Duty Motors
8) Inverters
9) Form Wound Coils
10) Armatures With Coils Installed
11) Precision Wound Transformers
12) Ferro-Resonant Units

3.5 EPOXY RESINS:
Epoxy resins are poly ethers derived from epi-chlorohydrin and Bis-phenol monomers
through condensation polymerization process. These resins are product of alkaline condensed
of epi-chlorohydrin and product of alkaline condensed of epi-chlorohydrin and poly-hydric
compounds.
In epoxy resins cross-linking is produced by cure reactions. The liquid polymer has
reactive functional group like oil etc, otherwise vacuum as pre polymer. The pre polymer of
epoxy resins allowed to react curing agents of low inductor weights such as poly-amines,
poly-amides, poly-sulphides, phenol, urea, formaldehyde, acids anhydrides etc, to produce
the three dimensional cross linked structures.Hence epoxy resins exhibit outstanding
toughness, chemical inertness and excellent mechanical and thermal shock resistance. They
also possess good adhesion property. Epoxy resins can be used continuously up to 300F, but
with special additions, the capability can be increased up to a temperature of 500F.
Epoxy resins are made use as an efficient coating material. This includes coating of
tanks containing chemicals, coating for corrosion and abrasion resistant containers. Epoxy
resins are made up of as attractive corrosion and wear resistant floor ware finishes.
These are also used as industrial flooring material. They are also used as highways
Surfacing and patching material. Moulding compounds of epoxy resins such as pipe fitting
electrical components bobbins for coil winding and components of tooling industrial finds
greater application in industries.The epoxy resins similar to polyester resins can be laminated
and Fibre Reinforced (FPR) and used in glass fibre boats, lightweight helicopters and
aeroplanes parts.
In the modern electronic industry, the application of epoxy resins is great. Potting and
encapsulation (coating with plastic resin) is used for electronic parts. Most of the printed
circuits bodies are made of laminated epoxy resin which is light but strong and tough.
3.5.1 PROPERTIES:
1. Epoxy resins have good mechanical strength less shrinkage and excellent
dimensional stable after casting.
2. Chemical resistance is high.
3. Good adhesion to metals.
4. To impact hardness certain organic acid anhydrides and alphabetic amines are mixed.

3.5.2 APPLICATIONS:
1. They are used in the manufacture of laminated insulating boards.
2. Dimensional stability prevents crack formation in castings. They are also used as
insulating varnishes.
Epoxy resins are polyether resins containing more than one epoxy group capable of
being converted into thethermoset form. These resins, on curing, do not create volatile
products in spite of the presence of a volatile solvent. The epoxies may be named as oxides,
such as ethylene oxides (epoxy ethane), or epoxide. The epoxy group also known as oxirane
contains an oxygen atom bonded with two carbon atoms, which in their turn are bound by
separate bonds as in Scheme I:
Fig.3.2 Seheme I
The simplest epoxy resin is prepared by the reaction of bisphenol A (BPA) (80-05-7)
with epichlorohydrine(ECH) (106-89-8) (Scheme II). The value of n varies from 0 to 25.
This determines the end-use applications of the resin.
Fig.3.3 Scheme II

Applications for epoxy resins are extensive: adhesives, bonding, construction
materials (flooring, paving, and aggregates), composites, laminates, coatings, molding, and
textile finishing. They have recently found uses in the air- and spacecraft industries.
A)EPOXIDATION :
There are three important methods of producing epoxides. First is catalytic
epoxidation. Here the oxidation of olefins is carried out by directly oxidizing them in the
vapor phase in the presence of a catalyst such as silver Second is epoxidation by organic
peroxides and their esters. Unsaturated compounds such as hydrocarbon fatty acids and their
esters are epoxidized by peroxyacetic acid.Third is epoxidation by inorganic peroxides and
inorganic peroxy-acids. Sodium peroxide or tungstic acid deposited on a inert surface is used
for the epoxidation of olefins by hydrogen peroxide.
B)CHEMISTRY:
Epoxy resins are prepared by the reaction of active hydrogen-containing compounds
with epichlorohydrinfollowed by dehydro-halogenation. BisphenolA (BPA) (80-05-7), on
reaction with epichlorohydrin (ECH) (106-89-8) in the presence of caustic soda, produces
diglycidyl ether of bisphenol A (DGEBPA) (1675-54-3). Here nis nearly zero (0.2). The resin
is liquid when n < 1 and solid when n > 2.
C)Curing:
The curing of the epoxy group takes place either between the epoxide molecules
themselves or by the reaction between the epoxy group and other reactive molecules with or
without the help of the catalyst.The former is known as homopolymerization, or corrective
curing; and the latter is an addition or catalytic curing reaction. Both reactions result in
coupling as well as crosslinking (Scheme III).

Fig.3.4 Scheme III
Curing of DGEBPA with a diamine occurs in three stages: propagation of the linear chain,
formation of a branched structure, and crosslinking. Primary and secondary amines are
widely used to cure epoxy resins. The reaction between the oxiranegroup of the epoxy resin
with primary amines is shown in Scheme IV.
Fig.3.5 Scheme IV
Tertiary amines also are used to bring about catalytic polymerization of epoxy resin
and the mechanism given inScheme V. To suit the requirements of the end products, other
nitrogen compounds used for curing are triamines (DETA, TETA), polyamides (two)
obtained from vegetable oils, polyureas (two), polyisocyanates, dicyanamide, polyurethane,
and imidazole. Polymercaptans, polyhydric alcohols, polyphenols, novalacs, and silanes also
are usedfor epoxidations. Magnetic fields and photoinitiation also are used for
polymerization. Hydantoin-based epoxyresin (15336-81-9) is used to form DGEBPA.
Glycidyl esters of dimerfatty acids can also be produced fromvegetable oils. Curing agents
such as cyclic acid anhydrides are used. The reaction is shown in Scheme VI.

Fig.3.6 Scheme V
Fig.3.7 Scheme VI
DGEBPA also is produced from aliphatic diols such as butane-1,4-diol (2425-79-8),
propylene glycol(16096-30-3), hydrogenated BPA (13410-58-7), triglycidyl adduct of p-
aminophenol, hetrocyclicglycidyl amidesand imides, and triglycidylisocyanurate (2451-62-
9). Lewis acids such as boron trifluoride complexes are also usedas curing agents. Cationic

catalysts such as metal halides, coordination catalysts such as metal chelates
andphotoinitiation are used to bring about polymerization.
3.6. THE MANUFACTURING PROCESS
The epoxy resins can be obtained in either liquid or solid states.
a)Liquid Epoxy Resins :
In this process ECH and BPA are charged into a reactor in the ratio of 10:1. A
solution of 20-40% caustic sodais added slowly to the reaction vessel as the solution is
brought to the boiling point. The solution is kept boilinguntil 2 mol of caustic soda per mole
of BPA have been added. The solution breaks up into two layers. UnreactedECH is removed
by vacuum distillation. An inert solvent is then added to the resin and the reaction is
completedwith excess of caustic soda solution. The resin separates into brine solution, which
is thoroughly washed with waterto obtain a clear resin. The solvent is removed by vacuum
distillation.
b)Solid Epoxy Resin :
Here ECH and BPA are added to the reactor in theoretical molar ratio with a little
excess of ECH. Aqueouscaustic soda is well mixed into the system. After one hour the
reaction is complete and a taffylike mass is obtained.Phase separation is brought about by
adding an inert solvent. Brine is withdrawn and the resin solution isthoroughly washed with
water to remove traces of salts. The solid resin is obtained by removing the solvent byvacuum
distillation.
c)Modified Epoxy Resins :
Epoxy resins form adducts with vinyl, acrylic, polyester resins, phenol novolac (9003-
35-7), cresol novolac(37382-79-9), bis-[4(2,3-epoxy propyoxy) phenyl] methane (2467-02-
9), and phenol hydrocarbon novalac(13446-85-0).
3.7. STRUCTURE
Ethylene oxide is a cyclic ether.The carbon atoms in oxirane are trigonallysp2
hybridized. One orbital fromeach carbon atom overlaps with the atomic orbitals of the
oxygen atom to form molecular orbital in the center ofthe ring. Atomic p orbitals, in the plane
of the ring, overlap sideways.
This explains the conjugative ability of epoxyring, and results in a bent bond structure. Since
the H-C-H bond is 116° 15à, the carbon will be sp2 hybridized. But italso shares the

triangular ring, so it is possible to give the correct hybridization structure.Very often the
epoxy groups deform to keep the area of the ring constant. The ring atoms do not lie along
thelines of greatest electron density of the atomic orbitals from the neighboring atoms. The
smaller amount ofoverlapping is more than offset by a decrease in the strain energy.
3.8. CHARACTERIZATION AND PROPERTIES
The end groupsÌepoxy, hydroxide, glycol, chlorine, and bisphenolAÌare estimated by
the usual chemical methods.Researchers have found aqueous concentrated hydrochloric acid
with dioxane to be a suitablereagent for estimating epoxide content. Curing agents such as
amines are characterized by refractive index andspecific gravity. Thin-layer chromatography
has been used for identification.
The reactive dye labeling techniquehas also been used for studying the curing
reaction.The epoxy resins do not soften at a specific temperature but appear to undergo a
gradual and imperceptiblechange. The softening of epoxy resins can be empirically graded by
the ring and ball method.
The curing of epoxy resins is an exothermic process, resulting in the production of
limited-size molecules,having molecular weights of a few thousands. Their weight is
determined by the usual physical methods.
Thenumber average molecular weight (Mn) is determined by gas density, and
cryoscopic, ebulioscopic, and osmoticpressure methods. The weight average molecular
weight (Mw) is estimated by viscosity and light scattering methods.Solubility parameters and
critical surface tension (surface tension below which a liquid drop makes zero contactangle)
have been used to find the molecular weights between the cross-links (Mc).
Epoxy resins have a very wide molecular weight distribution. This can be estimated
by comparing the Mw and Mnvalues. The greater the difference, the wider the distribution.
Gel permeation chromatography has been used forfinding this distribution.Liquid epoxy
resins or their solids in 40% diethylene glycol solution do not behave as Newtonian liquids.
Thekinematic or intrinsic viscosities of epoxy resins are measured by viscometers, which can
measure the dependenceof viscosity on shear rate.
Viscosity studies also provide information regarding the formation of networks and
theiraging. The linear viscoelastic response of epoxies displays the internal characteristics of
tensile relaxation andfracture response.
Chemorheology studies display the changes in deformation and flow properties of the
resin.Continuous and intermittent stress relaxation measures the appearance of network
formation.

Researchers haveused a variety of techniques to study the thermal and environmental
stability of epoxy polymers.The effect of moisture on epoxy resins has been studied from the
point of view of both absorption that bringsabout degradation and of sorption behavior itself.
Water absorption has been found to decrease the Tg because ofstrong hydrogen bonds.
It has been shown that in amine-cured resins, water is homogeneously distributed as
aplasticizer and that water clusters are present at microcracks.The strength and toughness of
epoxy resins below the glass transition temperature Tg depend on the mechanismof the
movement of short segments in the solid state, and above Tg mechanical properties are
influenced bycross-linking density and Mc.
Epoxy resins shrink on curing. Thus both the density and refractive index increase. It
is necessary to distinguishbetween shrinkage in the liquid stage and shrinkage in the gel or
solid state. Shrinkage in the gel or solid stateintroduces stress.
Thus gelation temperature has a direct effect on the degree of shrinkage. Refractometry
ordilatometric measurements are used to measure shrinkage, which determines the cure rate.
Epoxy resins are noncrystalline, and cured resin finds its structural applications below
the heat distortion orglass transition temperature (Tg). Continuous and intermittent stress
relaxation measures the appearance of networkformation.
A variety of techniques have been used to study the thermal and environmental
stability of epoxypolymers. Several mechanical properties of epoxy resins are closely related
with chemical composition and internalcohesive energy of the resin. This has been found to
be true well below the transition temperature, whererotational motion related to
configurational entropy and stress relaxation processes do not occur.
Epoxy resins undergo alpha, beta, and gamma thermal transition through proper
selection of epoxy monomersand curing agents. The temperature, location, and magnitude of
these transitions directly influence thethermo-mechanical properties of the resin.
These transitions also are influenced by the mechanism of crosslinkingproduced by
amines, BF3-amine complex, and anhydride curing agents. These transitions can be studied
bydynamic mechanical analysis (DMA) and nuclear magnetic resonance (NMR).
Epoxy resins have good electrical insulation properties. They have a 3-6 dielectric constant, a
low dissipationand loss factor, and good arc, surface, and volume resistance.
These properties are affected by moisture andincrease in temperature. The resins can
be made conductive and semiconductive by use of suitable fillers andcuring agents.
3.9. APPLICATIONS

1)Foams :
Epoxy resins are used to form rigid, lightweight, foamy structures with good
insulation properties. They areparticularly used for foam-in-place applications in the
“potting” process, as well as in casting. They are producedeither by chemical reaction or by
incorporating aprefoamed filler in the liquid system.
2)Adhesives :
The versatile properties of epoxy resins make them valuable as adhesives in civilian
and military applications.About five percent of total epoxy resin production is consumed as
adhesive in a wide range of structuralapplications. Epoxy resin adhesives form strong bonds
with almost all surfaces, with the exception of somenonpolar substrates. Very often special
modifiers and curing agents must be used to produce specific properties.
The formulation of epoxy adhesives into a serviceable adhesive binding system is a
highly specialized technology.Adhesives based on epoxide resins are available as room-
temperature-curing two-component liquids, heat-curingliquids, powders, hot-melt adhesives,
films, and tapes.Adhesive formulation based on epoxy resins requires a wide variety of
curing and modifying agents. GenerallyDGEBA and oligomers are used, but to produce some
specific effects alicyclic or heterocyclic epoxides are alsoincluded.
Polyvinyl modified resins are used to increase flexibility and as toughening agents. Epoxy
polyurethane resinsmake high-strength structural adhesives. Acrylates are also used to
modify epoxy adhesives. Rubber- andelastomer-modified resins have been used to produce
adhesive that cures under water.
3)Construction :
Epoxy resins are now used as binders in materials for construction. Generally a two-
component systemcontaining liquid epoxy resin, diluents, fillers, thickening agents, and
curing agents is used. They are used to bondconcrete, and to produce industrial seamless thin-
set tarrazzo floors. This use has been extended to the laying ofroads, construction of
buildings, and filling cracks in concrete structures.
3.10. COATINGS:
The coatings industry is the biggest consumer of epoxy resins. These resins are used
mostly as chemical andspecial purpose coatings. Epoxy resins provide thin-layer durable
coatings having mechanical strength and goodadhesion to a variety of substrates. They are
resistant to chemicals, corrosion, and solutions. They find applicationsin washing machines
and appliances, ships and bridges, pipelines and chemical plants, automobiles,
farmimplements, containers, and floor coatings. Epoxy coating formulations are available as

liquid resins, solid resins,high molecular weight thermoplastic resins, multifunctional resins,
radiation curable resins, and special purpose resins.
Aliphatic amines, aromatic amines, and ketamines are used as curing agents for
package epoxy systems.Epoxy baking finishes are obtained by high molecular weight epoxy
resins crosslinked by phenolic or aminoresins. These resins are used as lining for tanks, cars,
drums, pails, pipes, downhole oilfield tubing, and food cans.Epoxy acrylic systems provide
excellent coatings for appliances, kitchen cabinets, outdoor furniture, aluminumsiding, and
other metal products.
High-solid coating solution formulations attain maximum film properties (adhesion,
appearance, and freedomfrom defects). These are based on liquid epoxy resin acrylic adducts
with epoxy resins. These adducts have proveduseful in automotive primers. Epoxy resins
cured with aliphatic amines, polyamides, or aliphatic liquid amineadducts are used in
seamless floors. Industrial floors require extra epoxy resin.Waterborne coatings are made by
dispersing or emulsifying the resins with surfactants. Such coatings also havebeen based on
emulsified liquid epoxy resins cured with emulsified polyamide resins. These formulations
are usedin anionic electrodeposited coatings. They provide exterior and interior coatings for
underground pipes, andelectrical equipment appliances reinforcement. High-solid coatings
have an additional advantage, as they are usefulon steel, brass, metal furniture, buildings, and
miscellaneous products. Application of powders is accomplished byelectrostatic spray
fluidized-bed coating and electrostatic fluidized-bed coating. For marine use, epoxy resins
thatcure under water or are resistant to seawater have been developed.
3.11. OTHER APPLICATIONS
Epoxy resins improve the crease resistance or breaking of fibers. They are also used
as intermediates for stabilizers and plasticizers.
This is the over view of Resins. This gives information about the development and
usage of Resins in manufacturing process of Turbo generators.
3.12. DEVELOPMENT OF TURBO GENERATOR TECHNOLOGIES:
Since the 1901 invention of the cylindrical rotor of Charles Brown for a high-speed
generator, the turbogenerator has been the unique solution for converting steam turbine
power into electrical power. The continuously transposed stator bar, invented by Ludwig
Roebel in 1912, opened the door for large scale winding application. Up to the 1930ies the
generators were designed in 2-, 4- and even 6- pole, in accordance with the speed optimums

of the steam turbines in those days. The 1920ies ended with impressive power generation
plants, having generator units in the 100 MVA range. The stator winding insulation consisted
in the beginning of plied-on mica-paper, compounded by Shellac varnish, later substituted by
asphalt. Voltages were up to 12 kV.
In the early 1930ies two European manufacturers were introducing 36 kV stator
windings, thus eliminating the machine transformer. All such designs were suffering of
continuous heavy electrical discharges, and were soon discontinued. After a 60-year time-out,
a manufacturer surprised the world in 1998 with a cable-based high-voltage generator up to
400 kV. However again, the cable technology was not ready for turbo generator
requirements, and a breakthrough for commercial application was not achieved. In the 1930
US manufacturers were introducing hydrogen as coolant. When combined with direct
conductor hydrogen cooling in the rotor, and later in the stator, this allowed a considerable
increase in specific utilization and efficiency.
By early 1960 the unit ratings were achieving 500 MVA. At that time deionized water
cooling in the stator winding was introduced. Around 1960 all major manufacturers changed
their insulation system to mica tape with synthetic resin impregnation, a technology for
thermal qualification at 155°C, and which has been lasting into these days. By end of the
1960, with the power semiconductors becoming mature, the dc machine excitation was
superseded by the static excitation, and by an ac exciter machine with rotating diodes.
The 1970ies brought again a tremendous growth in unit ratings, going along with the
introduction of nuclear power. Units of 1200 MVA at 3000 rpm and 1600 MVA at 1500 rpm
at up to 27 kV were designed and put in operation. The rotor diameters were arriving at their-
physical limits. Water-cooling of the rotor winding was introduced. Along with plans for
2000 MVA and beyond, superconducting rotor windings and stator air-gap windings were
studied. However, in early 1980 the market focus was shifting to gas turbine technology, with
some 100 MW beginning to grow into the area of large power plants, and initiating a new
round of uprating the simple and robust air-cooling technology in the 300 MVA range by
1996. The generator has for a long time been developed by repeating the cycle: design – test
– adjust design tools – extrapolate design. A tremendous breakthrough came with the large
computers in the 1960ies, immediately being used for the key competences, such as magnetic
field calculations, nonlinear coolant flow networks and mechanical turbinegenerator shaft
calculations. Some programs of that area are even in use in the today’s PC environment. As
an example, magnetic equivalent circuits were established to determine excitation currents.

Once these programs were calibrated on measured data, they have been proven very accurate
and still today, for most applications make obsolete any FEM method.
TODAY’S TURBOGENERATOR TECHNOLOGIES
1.Small units up 150 MVA :
The size of these small air cooled units has evolved quite quickly. These machines are
mainly devoted for gas turbines and steam turbines accepting cycling expansion. The gas
turbines market has led to a very standardized range of machine based on the evolution of the
turbine technologies and on the market requests. The models developed in 1980 for 40 MW
50 Hz/60 Hz; same generator for 50 Hz and 60 Hz with a gear box wheel and pinion
adaptation; are nowadays joined by models in the 130-150 MW range. These generators are
always designed using the simplest solution in order to reach low costs using modular
solutions. For example the stator is cooled using one chamber and the excitation system does
not need a third bearing and no pilot exciter. By this way, the models used for gas turbines
are easily adapted for steam turbine or double drive solutions.
All these machines are easy to transport and to mount on site and are very often
mounted and coupled to the turbine by the turbine manufacturer. They are delivered in a short
time and a lot of engineering is done to improve the through put time of these models. The
maintenance of these groups is quite simple requiring a small storage of spare parts.
A recent trend is the increase of the power of the electrical drives used in the oil and
gas industry, mainly for liquid natural gas pumps. Such drive motors require options similar
to those developed for the generators, however having a variable speed drives controlled by
static frequency converters. The performance is evolving quite strongly: a world record for
this kind of motor at 21 MW 5900 rpm in 1985, seems modest in view of today’s 100 MW.
The speed values are close to generation with values between 3600 and 4200 rpm.
2.Medium range up to 500 MVA :
Since the introduction of the 300 MVA class ten years ago, subsequent development
has extended the rating up to the 400 MVA range. One of the main technology drivers has
been the improvement of the rotor axial cooling and winding indirect cooling using a modular
stator multi-chamber airflow. These generators are characterized by their simplicity and ease
of operation and maintenance. They have also proven their maturity in GT24/GT26 gas
turbine applications as well as on numerous steam turbines and turbines of other
manufacturers. The new ratings of the air-cooled generator series allow for the application of

air-cooled technology in power ranges where hydrogen cooled generators were used
previously. As a result of electrical and cooling optimization the present air-cooled
turbogenerators achieve efficiency up to 98.8 % and are used with a maximum voltage of 21
kV. Aircooled turbogenerators technology with highest ratings has now accumulated more
than 1.8 million of successful operating hours with more than 100 units in operation. In two
decades the power output of air-cooled generators has been increased from 200 MVA to 400
MVA. Fig.4 shows this exceptional increase in generator power as a function of the time. It is
clear that this strong increase in power that has occurred in the last decade was a direct
response to the market demands.
Recently, the increase of air-pressure inside the generator was realized. This measure
allows a better cooling and consequently enhances the capability of the air-cooled turbo
generators. The hydrogen-cooled types have hydrogen filling up to 5.5 bar. They are
designed for single-shaft and combined-cycle applications and are increasingly used with
steam turbines. The main features of the gas-cooled design are the same as the air-cooled.
The cooling principle, end winding support system, the retightening system and the
aluminum press plate are excellent examples of the design similarities. The hydrogen-cooled
types are setting the benchmark for efficiency, large units commonly achieving 99.0 %. Since
1996, ALSTOM has supplied more than 50 units hydrogen-cooled turbogenerators of the 500
MVA range. However, the achievable power is much higher and will be soon at 600 MVA.
Fig.3.8Evolution of the air-cooled turbogenerators in the last decades.
3.Large units up to 2000 MVA :

These generators are driven by steam turbines in large coalfired power plants and
nuclear power plants. They are all equipped with hydrogen-cooling with up to 6 bar
overpressure, and with direct water cooling in the stator winding bars. The two-pole
generator series begins at 500 MVA, and units up to 1300 MVA are in commercial operation.
They are of highest specific utilization and therefore need complete direct cooling.
Depending on the size the rotor, cooling is performed by axial flow of hydrogen through all
conductors of a slot, either in one path over half-length of the rotor, or in two paths,
supported by a sub slot. The stator core is axially flown by hydrogen, symmetrically fed from
both ends driven by a radial fan, arranged on the non-driving end of the rotor shaft. The stator
winding is cooled by water-flown stainless steel tubes embedded in the Roebel bars. Thanks
to the watercooling the stator winding has ever been open factor for upratings. The rotor
winding has revealed to be the limiting part for upratings. At 1.25 m for 50 Hz, the rotor
diameter is at the limits of mechanical stress. Any extension in active length beyond 8m
needs careful consideration of the shaftline dynamics. Potential lies in multi-zone cooling
concepts for the rotor winding, in an increase of hydrogen absolute pressure and fan pressure.
All the described measures will lead to a consolidation at 1400 MVA unit rating.
Any higher unit rating must go along with a break in rotor winding cooling, and the
parasitic effects due to stray flux will remain a challenge as such. The four poles machines
are running at 1500 rpm up to 1700 MVA. This is a key advantage for nuclear units, where
the temperature of the steam is relatively low and its flow in the low pressure parts of the
turbines huge. This allows the turbine to have very large diameter by using very long blades.
The hydrogen/water-cooled generators coupled to these turbines are the largest electric turbo
machines both in term of size and performance. This type of machine is ensuring 80% of the
electrical production in France, which is a country with a very high electrical nuclear
production. Some 50 machines in operation of this type have shown a very good reliability in
operation and have a potential of improvement in performance.
Based on this situation, the solution preferred in the nuclear market are not based on
new technologies, but, more safely, the tend to still improve the existing validated well-
running units. The 2000 MVA limit for turbogenerators for the 3rd
generation of reactors is
now close to be reached with improved life time and reliability. In order to reach this level of
power, following choices have been done:
I. Use the basic solutions validated by years of operation on running nuclear
units.

II. Analyze those parts which have led to the faults on existing machines.
III. Implement improvements validated on full-speed hydrogen and water-cooled
machines in the last decades.
IV. Adapt the cantilever type of excitation technology and adapt it to be even less
sensitive to diode aging.
V. Implement an improved type of cooling in the rotor copper ducts.
The maintenance of such a machine has to be done very carefully in order to reach the
guaranteed lifetime. The periodic stops to refuel the reactor are to be used for optimum
maintenance. The trend on the modern reactors is also to reduce the time between refueling
and the maintenance has to be adapted accordingly. A wide experience has been accumulated
on the existing machines.
1. Market trends :
As a part of the energy chain, the turbogenerator requires present and future
developments that have to comply with the market requirements as following:
i. Higher efficiency
ii. Higher reliability
iii. Low cost energy production
iv. Grid stability enhancement
To fulfill continuously these requirements huge developments are in progress as presented in
the following sections.
Substitution of hydrogen-cooled units by air-cooled units for higher reliability and
low cost energy productionThe substitution of hydrogen-cooled units by air-cooled andof
hydrogen/water-cooled by hydrogen-cooled will becontinuing to shift the ratings upwards.
The limits are given by transport dimensions, by the established temperatureclasses, and by
the degree of complexity of design. The engineering will further exploit these limits
involving mainlycooling and insulation materials developments. Air-cooled turbogenerators
offer many benefits to the operator. Some of which are listed below:
i. Excellent reliability
ii. Less civil work, simpler foundation
iii. No hydrogen treatment system
iv. No seal oil system and less sealing
v. Less piping
vi. Simple engineering work due to its advanced technology

These advantages are the consequences of not using hydrogen gas as a cooling
medium. This results in much simpler and shorter maintenance periods as well as a shorter
delivery time and an increased reliability. The good experience with large air-cooled
turbogenerators demonstrates the high potential of these generators. The largest air-cooled
generator was designed for 500 MVA. This design has been proven by tests and represents
the maximum achievable capability of air-cooled generators.
2. Efficiency enhancement :
The improvement of the efficiency is of first importance for the turbogenerator of all
kind in particular in air-cooled 60 Hz units for closing the gap to the benchmark values of
hydrogen-cooled units. Actually, it is one of the first issues considered in any new
turbogenerator development. In this section, some examples of new design solutions and new
technologies implementation to increase the efficiency will be described.
D. Stability improvement: Excitation booster :
Generally speaking, the excitation current of large generators is managed thanks to an
avr-controlled thyristorrectifier, directly supplied by the three phase voltages generated by the
generator itself. In the case this voltage drops down, the avr will counteract. This can become
impossible when the thyristor rectifier reaches limits given by its AC feeding. A possible
solution is to oversize the transformer connected between the output of the generator and the
thyristor rectifier, to keep the control of the excitation current for obtaining the over-
excitation of the generator, even if its output voltages are at a low level.
A solution is to add an additional energy source in the form of a pre-charged
supercapacitor bank that allows the control of the excitation current, even in case of heavy
drop of the main supply. Supercapacitors seem to be well adapted in view power density. A
study in collaboration with EPFL and Technical University of St-Petersburg is in progress to
demonstrate the positive impact of such a system on the stability of an electrical grid. To
assess the real benefit, the system is incorporated to every turbogenerator of the modeled
electrical grid. The simulation is going to be performed with SIMSEN and Matlab Simulink
E. Engineering calculation tools :
Concerning the future of engineering tools we expect the further linking of the design
geometry with the modeling tools, and this in both ways. Furthermore we expect more use of
CFD, eventually replacing large test arrangements. FEM will be interactively combined with
machine parameter tools, a precursor is the virtual short circuit testing program of EPFL.
FEM integrated in machine models which themselves are part of simulation tools could be
used for investigations of harmonics of currents or voltages on the generator components.

Two main categories of programs are used at the present. The first is employed to simulate
events in the electrical grid and to demonstrate the consequences on grid stability. These
programs integrate transient and subs transient models of generators. For the turbogenerator,
a complete result including stator and rotor current, stator voltage, powers, torques in
function of time can be obtained. Some of the more used are:
1) SIMSEN
2) Matlab Simulink
The second category of programs is based on FEM calculation and gives detailed
information of what happens in the generator. The magnetic field, eddy currents, losses and
forces are calculated. Ventilation models are introduced in order to know the resulting
temperature distribution in the generator. For this kind of calculation the detailed design of
the generator is required. Some of the most used programsare :
1. Magnet (Infolytica)
2. Electra (Vector Fields)
3. Maxwell (AusSoft)
4. ANSYS/ Emag
5. UNIFELD (in-house development)
An approach, which mixes both categories of programs and gives combined results
for the grid as well as the generator, is the most interesting for turbogenerator applications.
The application of such a program provides a better understanding of the turbogenerator
coupled with the electrical grid.
3.13 CONCLUSION
Since more than 100 years turbogenerators have been in use for steam turbine and gas
turbine applications of any size. The technical evolution has not stopped; new market
requirements and new material technologies ask for adaptations in design. The future market
will be characterized by a revitalized need for very large turbogenerators, both two-pole and
4-pole. The future will also be characterized by an exciting competition between well-
established conventional solutions and new “high tech” solutions. In any case highly skilled
engineers paired with the best available design tools will be required.

4. ANALYSIS
4.1 INTRODUCTION
Vacuum Pressure Impregnation has been used for many years as a basic process for
thorough filling of all interstices in insulated components, especially high voltage stator coils
and bars. Prior to development of Thermosetting resins, a widely used insulation system for
6.6kv and higher voltages was a Vacuum Pressure Impregnation system based on Bitumen
Bonded Mica Flake Tape is used as main ground insulation. After applying the insulation
coils or bars were placed in an autoclave, vacuum dried and then impregnated with a high
melting point bitumen compound. To allow thorough impregnation, a low viscosity was
essential. This was achieved by heating the bitumen to about 180C at which temperature it
was sufficiently liquid to pass through the layers of tape and fill the interstices around the
conductor stack. To assist penetration, the pressure in the autoclave was raised to 5 or 6
atmospheres. After appropriate curing and calibration, the coils or bars were wound and
connected up in the normal manner. These systems performed satisfactorily in service
provided they were used in their thermal limitations. In the late 1930’s and early 1940’s,
however, many large units, principally turbine generators, failed due to inherently weak
thermoplastic nature of bitumen compound.
4.2 CONTENT OF PROJECT
This Vacuum Pressure Impregnation (VPI) system is mainly used for insulation
of Electrical machines like Turbo generators, Large motors and transformers etc . So as I
want to explain about VPI, I have to explain about Turbo Generator, Insulation systems of
Turbo Generator. This is the basic and required introduction about Turbo Generator and
others.
4.2.1 Introduction to Turbo Generator:
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.

“FARADAY’S LAWS OF ELECTRO MAGNETIC INDUCTION”.
Faraday’s 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.
Faraday’s second law states that EMF induced in it, is proportional to rate of change
of flux.
𝑒 =
−𝑁d
dt
Eq.4.1
EMF induced will oppose both the flux and the rate of change of flux.
In the case of AC generators the armature winding is acts as stator and the field
winding acts as rotor.
Efficiency of a machine is equal to the ratio of output to input
 =
Output
input
=
Output
output +losses
Eq. 4.2
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 2 types
(1) Constant losses
(a) Iron losses
(b) Friction and windage losses (air friction losses).
(2) Variable losses
(a) Copper losses
3 Phase all machines are of two types AC machines & DC machines. AC machines
are divided into single-phase AC machines and poly phase AC machines. poly phase AC
machines divided into
1 Synchronous Machines:
Synchronous Generators (or) Alternators are those in which the speed of the
rotor and flux are in synchronism (or) The machine which rotates with its
Synchronous speed.
Synchronous speed (Ns) =
120𝑓
𝑝
rpm. Eq. 4.3

2 Asynchronous Machines:
These are the machines in which the flux speed and rotor speed will not be the
same. (or) The machine which rotates less than its Synchronous speed.
Ex: Induction motors.
a) Inherently all the machines are AC machines. AC or DC depends upon the flow
of current in the external circuit.
b) 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 parts
(1) Flux carrying parts
(2) Load carrying parts
In large synchronous machines the stator 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) Hystersis 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.
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.
Generators driven by steam or gas turbines have cylindrical/ round rotors with slots
into which distributed field windings are placed. These round rotor generators are usually

referred to as turbo generators and they usually have 2 or 4 poles. Generators driven by
hydraulic turbines have laminated salient pole rotors with concentrated field winding and a
large number of poles.
Turbo Generator:
Fig.4.1Turbo Generator.
A turbo generator is the combination of a turbine directly connected to an electric
generator for the generation of electric power. Large steam powered turbo generators provide

the majority of the world's electricity and are also used by steam powered turbo-electric
ships.
Smaller turbo-generators with gas turbines are often used as auxiliary power units.
For base loads diesel generators are usually preferred, since they offer better fuel efficiency,
but on the other hand diesel generators have a lower power density and hence, require more
space. The efficiency of larger gas turbine plants can be enhanced by using a combined cycle,
where the hot exhaust gases are used to generate steam which drives another turbo generator
4.2.2 History of turbo generator :
The Turbo generator was invented by a Hungarian engineer OttóBláthy.A turbo
generator is a turbine directly connected to electrical generator for the generation of electric
power. An electrical generator is a machine which converts mechanical energy to electrical
energy.
Generators are based on the theory of electromagnetic induction, which was
discovered by Michael Faraday in 1831, a British Scientist. Faraday discovered that if an
electric conductor, like a copper wire, is moved through a magnetic field, electrical current
will flow(be induced) in the conductor. So the mechanical energy of the moving wire is
converted into the electric energy of the current that flows in the wire.
4.2.3 Components of turbo generator:
1) The generator consists of the following components
2) Stator
3) Rotor
4) Ventilation and Protection System
5) Cooling System
6) Insulation
7) Vacuum pressure impregnation system
8) Exciter
9) Base frame
4.2.4 STATOR
a)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 help of hexagonal bolts. The skid is temporarily fixed to the concrete foundation
through bolts.
b) Stator Core:
Stator core is stacked from the insulated electrical sheet laminations and in the stator
frame from insulated dovetailed guide bars. Axial compression is from clamping fingers,
clamping plates and non-magnetic clamping bolts which are insulated from the core. In order
to minimize the hysteresis and eddy current losses of the rotating magnetic flux, which
interacts with the core, the entire core is built up of lamination, each layer of which is made
from a no. of individual segments. The segments are punched from the silicon steel. In the
outer circumference the segments are stacked in insulated trapezoidal guide bars, which hold
them in position. The guide bar is not insulated to provide for grounding the core. The
laminations are hydraulically compressed and heated during the stacking procedure. The
complete stack is kept under pressure and fixed in the frame by means of cells.
The core packed into the stacking frame is pressed firmly together between the end
plates of the machine frame and fixed in this position by welding the axial ribs of the core
and end of the plates of frame. End fingers on the inside diameter of the end plates transmit
the pressure to the teeth of the core. The compressive force produced prevents the
laminations and teeth from vibrating. An eye is welded to each end plate for attaching
suitable lifting gear with adequate lifting capacity for transporting the complete machine. All
the forces that occur during normal operation or on short circuits are transmitted from the
stator yoke to the frame via the seating plates and into the foundation.
c) Stator Winding:
The winding is a double layer multi turn lap winding. The half coils are made up of
electrolytic copper strips insulated with mica based epoxy insulation of suitable thickness to
give a long and uninterrupted service. Each strip is staggered to 360degrees and it passes
through all the sides of the coil. This process is called transposition. The purpose of
transposition is to avoid the circulation currents due to eddy current and also to avoid corona

losses. The straight parts of the half bar are coated with conductive varnish to prevent corona
discharges in the slot. The end winding is specially shaped to form a basket with an inviolate
shaped over hang of the bars. The straight portion of the winding is secured by means of
wedges driven into the slot position. The resistance thermometer elements are placed in the
core teeth at carefully selected points to measure the temperature rise of the machine. Epoxy
glass laminated brackets support the end winding. Epoxy glass laminated spacers to give a
rigid structure to withstand the short circuit forces of the three-phase winding are connected
to the connecting strips, which are also insulated and secured in position. Six output terminals
are brought out from the rings of the insulated covers.

Fig.4.2 Wound Stator

Fig.4.3Stator Winding

d) 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.
e) Location of Bars:
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.
f)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.
g) 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.
h) 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 are then cured
at a certain temperature, with the shrinking tapes contracting so that a void free insulation is
obtained.

i) Output leads:
The beginning and ends of three phase windings are solidly bolted to the output leads
with flexible. The output leads consist of flat copper sections with mica insulation. To
prevent eddy-current losses and inadmissible temperature rises: the output leads are brought
out.
Fig.4.4 Phase connectors and rings

Table 4.1: Insulating materials used in the stator winding:
Usage Material description Specifications
A foam insulation for slot bottom
layer of stator coil
Semi conductive foam fleece HL657
Slot bottom insulation Semi conductive fleece HL656
Inter layer insert Glass mat (compressible) HM693
Top insulation Glass mat (compressible) HM693
Spacers in overhang Glass mat (compressible) HM693
Bandage ring spacers Glass mat (compressible) HM693
Slot wedge Glass mat (Hard) HM694.52
Fillers for Slot Merge spacer Glass mat (Hard) HM693
Slot Merge spacer Glass mat (Hard) HM694.5
Stiffeners between top & bottom
layers
Glass mat (repressed in a
hydraulic fixture)
SN763353
Interlacing for ring and stiffeners Glass Sleeve SCHL559
Typing of spacers b/w collector rings Glass tape GSBD552
Insulation of rings Fine mica paper glass tape GSBD552
Shrink and protection layer Polyester shrink tape L611
Adhesive varnish for tape ends Adhesive varnish KIL 875.2
Bandage rings Polyester resin + glass
ravings
PG822 + GSSTR588
Terminal boards Glass mat (hard) HM 694.5

4.2.5 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 throughout the active length of the rotor body to accommodate the
conductors. The slots are dovetailed at the top of housing the wedges.
1. Rotor shaft:
The rotor shaft is forged from a vacuum cast steel ingot. The high mechanical stresses
resulting from the centrifugal forces and short circuit torque call for high quality heat-treated
steel. The rotor consists of an electrically active portion and two shafts end. Approximately
60% of the rotor body circumference has longitudinal slots, which hold the field winding.
Slot pitch is selected so that 1800
displace the two solid poles. The rotor wedges act as
damper winding within the range of winding slots. The rotor teeth at the ends are provided
with the axial and radial holes, enabling the cooling gas to be discharged into the air gap
after, intensive cooling of the end windings.
2. Rotor Winding:
The field winding consists of several series connected coils inserted into the
longitudinal slots of the rotor body. The coils are wound so those two poles are obtained. The
solid conductors have a rectangular cross-section and are provided with axial slots for radial
discharge of the cooling gas. The individual conductors are bent to obtain half turns. After
insertion into the rotor slots, these turns are combined to form full turns of the series
connected turns of one slot constituting one coil. The individual coils of the rotor winding are
electrically series connected so that one north and one south magnetic pole are obtained.
Fig.4.5 Rotor shaft

Fig.4.6Laminated Rotor before winding
3. Rotor Slot 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.
4. Cooling of Rotor Windings:
Each turn is subdivided into four parallel cooling zones. One cooling zone includes
the slot from the center to the end of the rotor body, while another covers half the end
winding to the center of the rotor body. The cooling air for the slot portion is a limited into
the slot bottom ducts below the rotor winding.
The hot gas at the end of the rotor body is then discharged into the air gap between the
rotor body and stator core through the radial openings in the conductors and in the rotor slot
wedges. The cooling air for the end windings is drawn from below the rotor-retaining ring. It
rises radically along the individual coils and is then discharged into the air gap.

5. Rotor Retaining Rings:
The rotor retaining rings with stand the centrifugal forces due to the end windings one
end of each ring is shrunk on the rotor body, while the other end of the ring overhangs the
end winding without contact on the shaft. The shrunk on the hub at the free end of the
retaining serves to reinforce the retaining ring and secures the end winding in the axial
director at the same time. The shrink seat of the retaining ring is silver plated, ensuring a low
contact resistance for the induced current. To reduce the stray losses and have high strength,
the rings are made of non-magnetic core worked materials.
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 outer surface of
the slip rings to have better dissipation of heat, thus minimizing the brush wear.
7. Rotor Fan:
The generator cooling air circulated by the two axial flow fans located on the rotor
shaft at either end. To augment the cooling of the rotor winding the pressure established by
the fan works in conjunction with the air expelled from the discharge ports along the rotor
shaft. The blades are screwed into the rotor shaft. The blades are forged from an aluminum
alloy. Threaded root fastening permits the blade permits the blade angle to be changed.
8. 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.
9. 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.

Fig.4.7 Rotor over hang Portion
4.2.6 Ventilation and protection equipment:
A) 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.
B) 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.
C) 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 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.
d) 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.
4.2.7 COOLING SYSTEM
Cooling is one of the basic requirements of any generator. The effective working of
generator considerably depends on the cooling system. The insulation used and cooling
employed is inter-related.
The losses in the generator dissipates as the heat, it raises the temperature of the
generator. Due to high temperature, the insulation will be affected greatly. So the heat
developed should be cooled to avoid excessive temperature raise. So the class of insulation
used depends mainly on cooling system installed.
There are various methods of cooling, they are:
a. Air cooling- 60MW
b. Hydrogen cooling-100MW
c. Water cooling –500MW
d. H2& Water cooling – 1000MW
Hydrogen cooling has the following advantages over Air-cooling:

1. Hydrogen has 7 times more heat dissipating capacity.
2. Higher specific heat
3. Since Hydrogen is 1/14th of air weight. It has higher compressibility
4. It does not support combustion.
A. DISADVANTAGES:
1. It is an explosive when mixes with oxygen.
2. Cost of running is higher.
Higher capacity generators need better cooling system.
The two-pole generator uses direct cooling for the rotor winding and indirect air-
cooling for the stator winding. The losses in the remaining generator components, such as
iron losses, wind age losses, and stray losses are also dissipated through air.
The heat losses arising in the generator interior are dissipated through air. Direct
cooling of the rotor essential eliminates hot spots and differential temperatures between
adjacent components, which could result in mechanical stresses, particularly to the copper
conductors, insulation and rotor body. Indirect air-cooling is used for stator winding.
Axial-flow fans arranged on the rotor via draw the cooling air for axial-flow
ventilated generator via. Lateral openings in the stator housing. Hot air is discharged via.
Three flow paths after each fan.
FLOW PATH 1:
It is directed into the rotor end windings space and cools the rotor windings, part of
the cooling air flows past the individual coils for cooling the rotor end windings space via
bores in the rotor teeth at the end of the rotor body.
The other portion of the cooling airflow is directed from the rotor end winding space
into the slot-bottom ducts from where it is discharged into the air gap via.
A large number of radial ventilating slots in the coils and bores in the rotor wedges
along these paths the heat of rotor winding is directly transferred to the cooling air.

FLOW PATH 2:
It is directed over the stator end windings to the cold air ducts and into the cold air
compartments in the stator frame between the generator housing and rotor core. The air then
flows into the air gap through slot in the stator core where it absorbs the heat from the stator
core and stator winding.
FLOW PATH 3:
It is directed into the air gap via, the rotor retaining-ring. The air then flows past the
clamping fingers via. Ventilating slot in the stator core into the hot air compartments in the
stator frame being discharged to the air cooler. The flow path mainly cools the rotor retaining
rings, the ends of the rotor body and the ends of the stator core.
Flow 2&3 mix in the air gap with 1 leaving the rotor. The cooling air then flows
radially outward through ventilating slots in the core within the range of the hot air
compartments for cooling of the core and winding. The hot air is discharged to air cooler.
B. AIR AND HYDROGEN COOLING
Many of the internal generator components do not have the capability in their design
to have direct liquid cooling and yet they incur substantial losses during operation. In
addition there is the problem of rotation of the rotor and the windage and friction that goes
with it. Therefore large generator designs need a cooling medium that has good heat transfer
properties and low windage and friction characteristics.
Turbo generators employ either air or hydrogen as the internal cooling medium of the
generator. Air is used in the smaller machines (now a days up to about 300 MVA and
growing), but hydrogen is the most effective gas for ventilating a rotating machine and is
used in the larger machines to achieve higher ratings. Generally, hydrogen is used in all large
turbine generators and most of the medium-size machines, but it has been also used in some
smaller generators.
When hydrogen is for used as a coolant in generators, it is supplied at a purity of
approximately 98% or better. It is usually maintained from a continuous supply of
commercial grade hydrogen of high purity. It is necessary to maintain a large supply for
filling the generator after overhauls and to replace gas lost during operation. Hydrogen

consumption occurs in the generator by absorption into the seal oil and through small leaks in
the hydrogen coolers, stator winding, rotor terminal stud seals, or out of the casing. A
pressure regulator holds the hydrogen pressure at the rated level specified by the generator
design. Hydrogen’s density at 98% purity is of the order of one-tenth that of air at a
comparable pressure. This reduces the fan and windage losses to an extremely low value.
Because of this, it is possible to increase hydrogen pressure in machines to as high as 75 psi
relative to atmospheric pressure. Because of low windage and friction, the higher pressure
does not compromise efficiency.
The main benefit of increasing the hydrogen pressure is that it greatly increases the
heat removal capability of the hydrogen. Hydrogen’s properties are such that its heat transfer
coefficient is 50% more effective than that of air at the same pressure. Therefore hydrogen is
much more effective in removing heat from a surface.
The heat capacity per unit volume (the product of specific heat at constant pressure
and density) of hydrogen is approximately equal to that of air at the same pressure. Therefore
the temperature rise of hydrogen would be approximately the same as that of air if the same
volume flow rate of the two gasses were used to remove the same amount of heat. The
temperature rise is substantially reduced because the fan and windage loss is reduced in
hydrogen. The hydrogen is circulated throughout the generator by shaft mounted fans or
blowers. The hot rotor gas is discharged to the air gap, after having absorbed the heat from
the field winding losses. The hydrogen is also circulated through the core and stator terminals
and then back to the coolers for cooling and re-circulation. To remove or introduce hydrogen
in the generator, an external system is connected that employs CO2 for hydrogen purging on
removal and air purging when admitting hydrogen into the machine. This ensures that an
explosive mixture of hydrogen and oxygen cannot happen, as would be the case if purging
were done with air.Instrumentation is also generally provided for monitoring of hydrogen
gaspurity, dewpoint and temperatures. Air-cooled turbine generators are commonlyopen
ventilated taking air from outside the machine and discharging the warmair back to the
outside in another location.
C. HYDROGEN COOLERS
As the hydrogen cooling gas picks up heat from the various generator components
within the machine, its temperature rises significantly. This can be as much as 46◦C, and
therefore the hydrogen must be cooled down prior to being recirculatedthrough the machine
for continuous cooling. Hydrogen coolers or heat exchangers are employed for this purpose.

Hydrogen coolers are basically heat exchangers mounted inside the generator in the
enclosed atmosphere. Cooling tubes with “fins” are used to enlarge the surface area for
cooling, as the hydrogen gas passes over the outside of the finned tubes. “Raw water”
(filtered and treated) from the local river or lake is pumped through the tubes to take the heat
away from the hydrogen gas and outside the generator. The tubes must be extremely leak-
tight to ensure that hydrogen gas does not enter into the tubes, since the gas is at a higher
pressure than the raw water.
Fig.4.8Relative Cooling Properties of theVarious Cooling Mediums Used in Turbogenerators
4.2.8. INSULATING SYSTEMS
1.Resin rich system of insulation:
1) Conductor cutting and material used is same as in resin poor system.
2) Transposition is done same as that of resin poor system.
3) Stacking of coils is done. In this case high resin glass cloth is used for
preventing inter half shorts.
4) Putty work.
5) Nomex is used as transposition pieces. Putty mixture is a composition if mica
powder, china clay and SIB 775 Varnish.
6) Straight part baking is done for 1hour at a temperature of 160O
C and a
pressure of 150kg/ sq.cm
7) Then bending and forming is done.
8) Half taping with resin rich tape is done for over hangs and reshaping is done
9) To ensure no short circuits half testing of coils is done.
10) Initial taping and final tapings is done with resin rich tape to about 13-14
layers.

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