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NORTH CHENNAI

THERMAL POWER STATION

INPLANT TRAINING REPORT

Submitted by

P.Thondaiman

R.Harikumar

D.D.Arokiaraj

K.Balamurugan


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INTRODUCTION

North Chennai Thermal Power Station is situated about25 KMs from Chennai on Northern side.

NCTPS has a total installed capacity of 630 M.Wcomprising 3 units of 210 M.W each.All the three units are coal based. Coal for NCTPS is received from Mahanadhi coal fields Limited (Talchar & IB

Valley), Orissa, Eastern coal fields Limited, Ranikanj, West Bengal.

Generation and Plant load factor(PLF) for the year 2010-11 is 4510.863 MUand 81.74 % respectively.

COAL USED:

Bituminous coal is NCTPS.Bituminous coal or black coal is a relatively soft coal

containing a tarlike substance called bitumen.

Cabon content - 60 -80%

Moisture - 05%

Ash content - 45%

calorific value - 38003850

ALLOTED DIVISIONS

DAYS FN AN

1 TRG Chemical lab

2 Boiler Maintenance Technical service

3 Meter Relay Testing Electrical Maintenance 1

4 Control &

instrumentation

Common Auxiliaries

5 Turbine and generator

maintenance

Coal monitoring
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THERMAL POWER STATION

A thermal power station is a power plant in which the prime mover is steam driven. Water

is heated, turns into steam and spins a steam turbine which drives an electrical generator.

After it passes through the turbine, the steam is condensed in a condenser and recycled to

where it was heated; this is known as a Rankine cycle.

MAIN PARTS OF THERMAL POWER STATION

1. BOILER2. TURBINE3. GENERATOR

THE BLOCK DIAGRAM OF A TYPICAL ARRANGEMENT OF EQUIPEMENTS USED

IN POWER SYSTEM
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Typical diagram of a coal-fired thermal power station

1. Condensor 10. Steam Control valve 19. Superheater

2. Cooling water pump11. High pressure steam

turbine20. Forced draught (draft) fan

3. transmission line (3-phase) 12. Deaerator 21. Reheater

4. Step-up transformer (3-

phase)13. Feedwater heater 22. Combustion air intake

5. Electrical generator (3-

phase)14. Coal conveyor 23. Economiser

6. Low pressure steam turbine 15. Coal hopper 24. Air preheater

7. Condensate pump 16. Coal pulverizer 25. Precipitator

8. Surface condenser 17. Boiler steam drum 26. Induced draught (draft) fan

9. Intermediate pressure steam

turbine18. Bottom ash hopper 27. Flue gas stack

BOILER

The boiler is a rectangular furnace about 60 feet on a side and 140 feet tall. Its walls are made

of a web of high pressure steel tubes about 2.3 inches (58 mm) in diameter.

In NCTPS water tube boilers are used, where water flowing in the water tubes areconverted into High pressure High Temperature steam which is used to run the turbine

Here Tangential firing is done in order to improve the heat transfer from boiler core to

the water tubes.

Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it

rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats

the water that circulates through the boiler tubes near the boiler perimeter. The water

circulation rate in the boiler is three to four times the throughput and is typically driven by

pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700

F (370 C)and 3,200 psi (22,000 kPa). It is separated from the water inside a drum at the top

of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the

hottest part of the combustion gases as they exit the furnace. Here the steam is superheated

to 1,000 F (540 C) to prepare it for the turbine.
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BOILER FURNACE AND STEAM DRUM

The boiler transfers energy to the water by the chemical reaction of burning coal.The water

enters the boiler through a section in the convection pass called the economizer. From the

economizer it passes to the steam drum. Once the water enters the steam drum it goes down

to the lower inlet water wall headers. From the inlet headers the water rises through the water

walls and is eventually turned into steam due to the heat being generated by the burners

located on the front and rear water walls. As the water is turned into steam in the water walls,

the steam once again enters the steam drum. The steam is passed through a series of steam

and water separators and then dryers inside the steam drum. The steam separators and dryers

remove water droplets from the steam and the cycle through the water walls is repeated. This

process is known asnatural circulation.

The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot

blowers, water lancing and observation ports (in the furnace walls) for observation of the

furnace interior. Furnace explosions due to any accumulation of combustible gases after a

trip-out are avoided by flushing out such gases from the combustion zone before igniting the

coal.

The steam drum have air vents and drains needed for initial start up. The steam drum has

internal devices that removes moisture from the wet steam entering the drum from the steam

generating tubes. The dry steam then flows into the super heater coils.

SUPERHEATER

Fossil fuel power plants can have a superheater and re-heater section in the steam generating

furnace. In a fossil fuel plant, after the steam is conditioned by the drying equipment inside

the steam drum, it is piped from the upper drum area into tubes inside an area of the furnaceknown as the superheater, which has an elaborate set up of tubing where the steam vapor

picks up more energy from hot flue gases outside the tubing and its temperature is now

superheated above the saturation temperature. The superheated steam is then piped through

the main steam lines to the valves before the high pressure turbine.
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FEED WATER SYSTEM

The feed water used in the steam boiler is a means of transferring heat energy from the

burning fuel to the mechanical energy of the spinning steam turbine. The total feed water

consists of recirculate condensate water. Because the metallic materials it contacts are

subject to corrosion at high temperatures and pressures, the makeup water is highly purified

before use. A system ofwater softeners and ion exchange demineralizers produces water so

pure that it coincidentally becomes an electrical insulator, with conductivity in the range of

0.31.0 microsiemens per centimetre

The feed water cycle begins with condensate water being pumped out of the condenser after

traveling through the steam turbines. The water flows through a series of intermediate feed

water heaters, heated up at each point with steam extracted from an appropriate duct on the

turbines and gaining temperature at each stage. Typically, the condensate plus the makeup

water then flows through a deaerator that removes dissolved air from the water, further

purifying and reducing its corrosiveness.

ECONOMISER

It is located below the LPSH in the boiler and above pre heater. It is there to improve theefficiency of boiler by extracting heat from flue gases to heat water and send it to boiler

drum.

Advantages of Economiser include

1) Fuel economy:used to save fuel and increase overall efficiency of boiler plant.

2) Reducing size of boiler:as the feed water is preheated in the economiser and enter boiler

tube at elevated temperature. The heat transfer area required for evaporation reduced

considerably.

AIR PREHEATER

The heat carried out with the flue gases coming out of economiser are further utilized for

preheating the air before supplying to the combustion chamber. It is a necessary equipment
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for supply of hot air for drying the coal in pulverized fuel systems to facilitate grinding and

satisfactory combustion of fuel in the furnace

REHEATER

Power plant furnaces may have a reheater section containing tubes heated by hot flue gases

outside the tubes. Exhaust steam from the high pressure turbine is rerouted to go inside the

reheater tubes to pickup more energy to go drive intermediate or lower pressure turbines.

BOILER FEED PUMP

Boiler feed pump is a multi stage pump provided for pumping feed water to economiser. BFP

is the biggest auxiliary equipment after Boiler and Turbine. It consumes about 4 to 5 % of

total electricity generation.

STEAM CONDENSING

The condenser condenses the steam from the exhaust of the turbine into liquid to allow it tobe pumped. The surface condenser is a shell and tube heat exchanger in which cooling water

is circulated through the tubes. The exhaust steam from the low pressure turbine enters the

shell where it is cooled and converted to condensate by flowing over the tubes as shown in

the adjacent diagram. Such condensers use steam ejectors or rotary motor-

driven exhausters for continuous removal of air and gases from the steam side to

maintain vacuum.
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For best efficiency, the temperature in the condenser must be kept as low as practical in order

to achieve the lowest possible pressure in the condensing steam. Since the condenser

temperature can almost always be kept significantly below 100 C where the vapor

pressure of water is much less than atmospheric pressure, the condenser generally works

under vacuum. Thus leaks of non-condensible air into the closed loop must be prevented.

Typically the cooling water causes the steam to condense at a temperature of about 35

C (95 F) and that creates an absolute pressure in the condenser of about 27 kPa (0.592.1

inHg), i.e. a vacuum of about 95kPa (28.1 inHg) relative to atmospheric pressure. The

large decrease in volume that occurs when water vapor condenses to liquid creates the low

vacuum that helps pull steam through and increase the efficiency of the turbines.

The condenser generally uses either circulating cooling water from a cooling tower to reject

waste heat to the atmosphere, or once-through water from a river, lake or ocean.

.The cooling water used to condense the steam in the condenser returns to its source without

having been changed other than having been warmed. If the water returns to a local water

body it is tempered with cool 'raw' water to prevent thermal shock when discharged into that

body of water.

In NTPC as sea water is present very near, readily available in large volumes and its

temperature is much lower than the steam temperature from L.P Turbine, its used for

condensing the stream.

From the bottom of the condenser, powerful condensate pumps recycle the condensed

steam (water) back to the water/steam cycle.

COMMON AUXILIARIES

Boiler make-up water treatment plant and storage

Since there is continuous withdrawal of steam and continuous return ofcondensate to the

boiler, losses due to blowdown and leakages have to be made up to maintain a desired water

level in the boiler steam drum. For this, continuous make-up water is added to the boiler

water system. Impurities in the raw water input to the plant generally consist

ofcalcium and magnesium salts which impart hardness to the water. Hardness in the make-up

water to the boiler will form deposits on the tube water surfaces which will lead to
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overheating and failure of the tubes. Thus, the salts have to be removed from the water, and

that is done by a water demineralising treatment plant (DM). A DM plant generally consists

of cation, anion, and mixed bed exchangers. Any ions in the final water from this process

consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water.

Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere

because of its very high affinity for oxygen.

The capacity of the DM plant is dictated by the type and quantity of salts in the raw water

input. However, some storage is essential as the DM plant may be down for maintenance. For

this purpose, a storage tank is installed from which DM water is continuously withdrawn for

boiler make-up. The storage tank for DM water is made from materials not affected by

corrosive water, such as PVC. The piping and valves are generally of stainless steel.Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on

top of the water in the tank to avoid contact with air. DM water make-up is generally added at

the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only

sprays the water but also DM water gets deaerated, with the dissolved gases being removed

by a de-aerator through an ejector attached to the condenser.

FUEL PREPARATION SYSTEM

In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into

small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is

next pulverizedinto a very fine powder. The pulverizers may be ball mills, rotating

drum grinders, or other types of grinders.

Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour

point) in the fuel oil storage tanks to prevent the oil from congealing and becoming

unpumpable. The oil is usually heated to about 100 C before being pumped through the

furnace fuel oil spray nozzles.

Boilers in some power stations use processed natural gas as their main fuel. Other power

stations may use processed natural gas as auxiliary fuel in the event that their main fuel

supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the

boiler furnaces.
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OIL SYSTEM

An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine

generator. It supplies the hydraulic oil system required for steam turbine's main inlet steam

stop valve, the governing control valves, the bearing and seal oil systems, the relevanthydraulic relays and other mechanisms.

At a preset speed of the turbine during start-ups, a pump driven by the turbine main shaft

takes over the functions of the auxiliary system.

COAL PREPARATION

Fuel preparation system: In coal-fired power stations, the raw feed coal from the coalstorage area is first crushed into small pieces and then conveyed to the coal feed hoppers at

the boilers. The coal is next pulverized into a very fine powder, so that coal will undergo

complete combustion during combustion process.pulverizer is a mechanical device for the

grinding of many different types of materials. For example, they are used to pulverize coal

for combustion in the steam-generating furnaces of fossil fuel power plants.

Bowl mill:

Bowl mill is used to powder the coal into fine particles


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Dryers: they are used in order to remove the excess moisture from coal mainly wetted

during transport. As the presence of moisture will result in fall in efficiency due to

incomplete combustion and also result in CO emission.

Magnetic separators: coal which is brought may contain iron particles. These iron particles

may result in wear and tear. The iron particles may include bolts, nuts wire fish plates etc. so

these are unwanted and so are removed with the help of magnetic

separators.

The coal we finally get after these above process are transferred to the storage site.

Purpose of fuel storage is two

Fuel storage is insurance from failure of normal operating supplies to arrive. Storage permits some choice of the date of purchase, allowing the purchaser to take

advantage of seasonal market conditions. Storage of coal is primarily a matter of

protection against the coal strikes, failure of the transportation system & general coal

shortages.

TECHNICAL SERVICES

Fan or draught systemIn a boiler it is essential to supply a controlled amount of air to the furnace for effective

combustion of fuel and to evacuate hot gases formed in the furnace through the various heat

transfer area of the boiler. This can be done by using a chimney or mechanical device such as

fans which acts as pump.

i) Natural draught

When the required flow of air and flue gas through a boiler can be obtained by the stack(chimney) alone, the system is called natural draught. When the gas within the stack is hot, its

specific weight will be less than the cool air outside; therefore the unit pressure at the base of

stack resulting from weight of the column of hot gas within the stack will be less than the

column of extreme cool air. The difference in the pressure will cause a flow of gas through

opening in base of stack. Also the chimney is form of nozzle, so the pressure at top is very

small and gases flow from high pressure to low pressure at the top.


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ii) Mechanized draught

There are 3 types of mechanized draught systems

1) Forced draught system

2) Induced draught system

3) Balanced draught system

Forced draught: In this system a fan called Forced draught fan is installed at the inlet of

the boiler. This fan forces the atmospheric air through the boiler furnace and pushes out the

hot gases from the furnace through superheater, reheater, economiser and air heater to stacks.

Induced draught:Here a fan called ID fan is provided at the outlet of boiler, that is, just

before the chimney. This fan sucks hot gases from the furnace through the superheaters,

economiser, reheater and discharges gas into the chimney. This results in the furnace pressure

lower than atmosphere and affects the flow of air from outside to the furnace.

Balanced draught:-In this system both FD fan and ID fan are provided. The FD fan is

utilized to draw control quantity of air from atmosphere and force the same into furnace. The

ID fan sucks the product of combustion from furnace and discharges into chimney. The point

where draught is zero is called balancing point.

Control and Instrumentation

New technologies to maximise power plant efficiency

In order to operate power stations at an optimum level, power technology has, over the years,

become more and more dependent on automation and control. Increasing computerisation has

enabled the use of some very sophisticated techniques for controlling and monitoring power

systems. Earlier, control and monitoring systems were usually made up of simple circuits.

Operators supervised processes continuously and used their judgement to make necessary

changes to set values. Now, monitoring and control in power plants are based almost entirely

on computerised equipment, which has considerably simplified the control of processes

involving multiple variables. In fact, control and instrumentation or automation at power

plants has become critical to


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maximising efficiency and availability. It has allowed faster collection and processing of all

data from various parts of the plant and has even provided for remote control of all devices. It

has also helped optimise fuel utilisation and lower operational costs. In a typical power

station, the major systems for power plant monitoring and control are:

Process control system

This is a closed loop control system that takes its direction from the energy management

system (EMS) and automatically collects plant data by reading instruments. Physical and

electrical parameters associated with the boiler, turbine and generator are monitored on a

continuous-cycle basis. Alarms and events are logged. Control of pumps, valves and switches

for routine functions and for start-ups or shutdowns are provided.

Plant monitoring system

This is strictly a data collection system for fuel monitoring and performance calculations. No

control actions are performed. Data is stored and retrieved as required to prepare reports and

performance analysis.

Operational monitoring system

This is used by plant operators to enter manually collected operational data for record

keeping, report writing and analysis. In addition to these systems, the power plant may also

use computers for security, environmental, controlled access and chemical analysis systems.

Power plant maintenance

The power plant maintenance systems stores pertinent information for analysis of

maintenance costs and evaluation of equipment performance. The interactive portion of the

system provides plant personnel with the capability to enter problem data, planning data and

work execution data. Interactive functions are also provided for entry and maintenance of an

equipment database and for access to equipment history.


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Automatic generation control

The automatic generation control (AGC) system performs a very critical taskit adjusts

generation against load. The objective is maintenance of quality and minimisation of costs

of energy production and transmission. This is done primarily through load frequency control

and economic dispatch. Both these functions are done in real time.

Load frequency control

The load frequency control (LFC) system monitors generation load, constantly looking for

imbalances. The LFC system needs to maintain frequency at the scheduled value, net power

interchanges with neighboring control areas at the scheduled values and power allocation

among generating units at economically desired values.

Economic despatch

Different generating units that are online have different costs of generation. It is therefore

necessary to optimise the contribution of each of these units so that the load is met at

minimum cost. This system takes into account not only the per unit generating costs of power

plants but also their geographical location. This enables the despatch manager to minimise

transmission losses and thus achieve the true minimum cost. Other factors such as reserve

margins and transmission limits are also considered.

Deionised/Demineralised Water

Deionisation: Process utilizing special-manufactured ion exchange resins which remove

ionised salts from water. Can theoretically remove 100 % of salts. Deionisation typically does

not remove organics, virus or bacteria except through accidental trapping in the resin and

specially made strong base anion resins which will remove gram-negative bacteria.

Demineralisation: Any process used to remove minerals from water, however, commonly

the term is restricted to ion exchange processes.

Ultra pure water:Highly-treated water of high resistivity and no organics; usually used in

the semiconductor and pharmaceutical industries

Deionisation entails removal of electrically charged (ionised) dissolved substances by binding

them to positively or negatively charged sites on a resin as the water passes through a column
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packed with this resin. This process is called ion exchange and can be used in different ways

to produce deionised water of various qualities.

Strong acid cation + Strong base anion resin systems

These systems consist of two vessels - one containing a cation-exchange resin in the

hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form (see

picture below). Water flows through the cation column, whereupon all the cations are

exchanged for hydrogen ions. The decationised water then flows through the anion column.

This time, all the negatively charged ions are exchanged for hydroxide ions which then

combine with the hydrogen ions to form water (H2O).

These systems remove all ions, including silica. In the majority of cases it is advisable to

reduce the flux of ions passed to the anion exchanger by installing a CO2 removal unit

between the ion exchange vessels. This reduces the CO2 content to a few mg/l and brings

about a reduction of the following strong base anion resin volume and in the regeneration

reagent requirements.

In general the strong acid cation and strong base anion resin system is the simplest

arrangement and a deionised water that may be used in a wide variety of applications can be

obtained with it.
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Strong acid cation + weak base anion + Strong base anion resin systems

This combination is a variation of the previous one. It provides the same quality of deionised

water, while offering economic advantages when treating water which contains high loads of

strong anions (chlorides and sulphates). The subtitle shows that the system is equipped withan extra weak base anion exchanger before the final strong base anion exchanger. The

optional CO2 removal unit may be installed either after the cation exchanger, or between the

two anion exchangers (see picture below). The regeneration of the anion exchangers takes

place with caustic soda (NaOH) solution first passing through the strong base resin and then

through the weak base resin. This method requires less caustic soda than the method

described before because the remaining regeneration solution after the strong base anion

exchanger is usually sufficient to regenerate the weak base resin completely. Moreover, when

raw water contains a high proportion of organic matter, the weak base resin protects the

strong base resin.

Mixed-bed DeionisationIn mixed-bed deionisers the cation-exchange and anion-exchange resins are intimately mixed

and contained in a single pressure vessel. The two resins are mixed by agitation with

compressed air, so that the hole bed can be regard as an infinite number of anion and cation

exchangers in series (mixed bed resin).To carry out regeneration, the two resins are separated

hydraulically during the loosening phase. As the anion resin is lighter than the cation resin it

rises to the top, while the cation resin falls to the bottom. After the separation step the

regeneration is carried out with caustic soda and a strong acid. Any excess regenerant is

removed by rinsing each bed separately.
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Measuring of the purity

Water purity can be quickly estimated on the basis of electrical conductivity or resistance

very pure water conducts electricity poorly, so its resistance is high.

STEAM TURBINE:

1) Steam flow:-

210 MW steam turbine is a tandem compound machine with HP, IP & LP parts. TheHP part

is single flow cylinder and IP & LP parts are double flow cylinders. The individual turbine

rotors and generator rotor are rigidly coupled. The HP cylinder has a throttle control.Main

steam is admitted before blending by two combined main stop and control valves. TheHP

turbine exhaust (CRH) leading to reheater have two swing check valves that prevent

back flow of hot steam from reheater, into HP turbine. The steam coming from

reheater ca ll edHRH is passed to turbine via two combined stop and control valves. The IP

turbine exhaustsdirectly goes to LP turbine by cross ground pipes.

2) HP Turbine:-

The HP casing is a barrel type casing without axial joint. Because of its rotationsymmetry the

barrel type casing remain constant in shape and leak proof during quick changein

temperature. The inner casing too is cylinder in shape as horizontal joint flange arerelieved

by higher pressure arising outside and this can kept small. Due to this reason barreltype

casing are especially suitable for quick start up and loading. The HP turbine consists of 25

reaction stages. The moving and stationary blades are inserted into appropriately shapesinto

inner casing and the shaft to reduce leakage losses at blade tips.

3) IP Turbine:-

T h e I P p a r t o f t u r b i n e i s o f d o u b l e f l o w c o n s t r u c t i o n . T h e c a s i n g o f

IP tu rb in e is sp li t horizontally and is of double shell construction. The double

flow inner casing is suppor ted kinematically in the outer casing. The steam from HP


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turbine after reheating enters the inner cas ing from above and below th rough two

inlet nozzles. The centre flows compensates theax i al th ru st an d pr ev en t

s t e a m i n l e t t e m p e r a t u r e a f f e c t i n g b r a c k e t s , b e a r i n g e t c .

Thearrangements of inner casing confines high steam inlet condition to

admission branch of casing, while the joints of outer casing is subjected only to lower

pressure and temperature atthe exhaust of inner casing. The pressure in outer casing relieves

the joint of inner casing sothat this joint is to be sealed only against resulting differential

pressure.33

The IP turbine consists of 20 reaction stages per flow. The moving and

st at ionary blades are inserted in appropriately shaped grooves in shaft and inner casing.

4) LP Turbine:-

The casing of double flow type LP turbine i s of three shell design. The shells

areaxially split and have rigidly welded construction. The outer casing consist

of the front andrear walls, the lateral longitudinal support bearing and upper part.The outer

casing is supported by the ends of longitudinal beams on the base plates of foundation. The

double flow inner casings consist of outer shell and inner shell. The inner shell is attached to

outer shell with provision of free thermal movement.Steam admitted to LP turbine from

IP turbine flows into the inner casing from bothsides through steam inlet nozzles.

ELECTRICITY GENERATOR

To the end of the turbine rotor generator is coupled, so that when turbine rotatesthe rotor

turns with it. The rotor is housed inside the stator having coil o f copper bars

inwhich electricity is produced through the movement of magnetic fi eld created

by rotor Theelectricity passes from the stator winding to th e transformer which

steps up the voltage sothat it can be transmitted effectively over the power line of grid34

The steam which has given up its heat energy in changed back into a condenser so that it is

ready for reuse. The cold water continuously pumped in condenser. The

steam passing around the tubes loose heat and rapidly change into water. But

these two types of water (boiler feed water and cooling water ) must never mix

together . The cool ing wate r is drawn from the river but the Boiler Feed Water must be

pure than potable water (DM Water).


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TURBO HYDROGEN WATER GENERATOR

Stator volts24,700 V Rotor volts310 V

Amphere9050 A Amphere2600 A

Stator coolantwater Rotor coolant - hydrogen

ConnectionY Y

TURBO GENERATOR is manufactured by B.H.E.L and incorporated withmost modern

design concepts and constructional features, which ensures reliability,

withconstructional & operational economy. The generator stator is a tight construction,

supporting& enclosing the stator windings, core and hydrogen coolers. Cooling

medium hydro gen iscontained within frame & circulated by fans mounted at either ends of

rotor. The generator isdriven by directly coupled steam turbine at a speed of 3000 r.p.m. the

Generator is designedfor continuous operation at the rated output. Temperature detectors and

other devicesinstalled or connected within then machine, permit the windings, teeth core &

hydrogentemperature, pressure & purity in machine under the conditions. The source of

excitation ofrotor windings is thyristor controlled D.C. supply. The auxiliary equipments

supplied withthe machine suppresses and enables the control of hydrogen pressure and purity,

shaft sealinglubricating oils. There is a provision for cooling water in order to maintain a

constanttemperature of coolant (hydrogen) which controls the temperature of windings

GENERATOR TRANSFORMER:

The output from thegenerator is fed to the generator transformer, which step up the voltage

from 15.75 KV to 230KV and supplies power to grid. Threephase supply from the

generator is connected to the low voltage side bushings and the output is taken from the

opposite side. Neutral point on the H.V. side is provided at the side of the tank. Neutral is

grounded. In case neutral is solidly connected to the earth a very small current flowing

through the neutral causes the tripling of the transformer. So in this case more care is to be

taken.


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STATION TRANSFORMER :-

When the unit is to be started, power supplied to the auxiliaries is taken from thestation

transformer. The rating of the station transformer is 50 MVA. It takes power from the grid at

230 kV and steps it down to 6.6 kV. At the time of starting all the auxiliaries are supplied

from the station transformer. When the generator is synchronized and starts producing power,

about 80% of the load is shifted on to the unit auxiliary transformer. The load that requires

uninterrupted supply is left connected on the station transformer.

UNIT AUXILIARY TRANSFORMER:-

Each unit has two unit auxiliary transformers. When the unit starts generating electricity these

transformers are energized and then supplies power to the auxiliaries. Before starting of the

unit, UAT bus is connected to the station bus. Auxiliaries of one unit take about 20MW

of power. UAT is connected between the generator and the GT. A tapping is taken from

the power coming from the generator to the GT. It is a step down transformer, which steps

down the voltage from 15.75 kV to 6.6kV. The ratingof UAT is 20 MVA. UAT bus supplies

only those auxiliaries, which are necessary to be energized in case of sudden tripping ofgenerator.

UNIT SERVICE TRANSFORMER:-

It is also a 6.6 kv / 415 v transformers which is used to supply the auxiliaries connected to the

unit secondary switchgear bus.

SWITCH YARD 220 KV SYSTEM:-

Two 220 KV bus bars have been provided in switch yard and are inter-connected through a

bus coupler. Each of the two 110 MW generator is connected to this system through a step up

of 125 MVA 240/ 11 KV yard generator transformer. Thereare two step down transformer

each feeding 6.6 KV system (Station Switch yard ) viz. BS-IS& SB-IB. Each of four feeders

are provided with bypass isolators which is connected across line breaker and breaker
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isolator. By closing bus coupler between 220 KV buses and putting line feeders whose

breaker required maintenance of anyone bus through by pass isolators and all other line

feeders whose breaker is by passed is then transformed to bus coupler breaker..

ASH HANDLING

Fly ash collection

Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag

filters (or sometimes both) located at the outlet of the furnace and before the induced draft

fan. The fly ash is periodically removed from the collection hoppers below the precipitators

or bag filters. Generally, the fly ash is pneumatically transported to storage silos for

subsequent transport by trucks or railroad cars

Bottom ash collection and disposal

At the bottom of the furnace, there is a hopper for collection ofbottom ash. This hopper is

always filled with water to quench the ash and clinkers falling down from the furnace. Some

arrangement is included to crush the clinkers and for conveying the crushed clinkers and

bottom ash to a storage site. Ash extractor is used to discharge ash from Municipal solid

waste fired boilers.

ASH DISPOSAL

The disposal of ash from a large capacity power station is of same importance as ash is

produced in large quantities. Ash handling is a major problem.

i) Manual handling: While barrows are used for this. The ash is collected directly through

the ash outlet door from the boiler into the container from manually.

ii) Mechanical handling: Mechanical equipment is used for ash disposal, mainly bucket

elevator, belt conveyer. Ash generated is 20% in the form of bottom ash and next 80%

through flue gases, so called Fly ash and collected in ESP.
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iii) Electrostatic precipitator: From air preheater this flue gases (mixed with ash) goes to

ESP. The precipitator has plate banks (A-F) which are insulated from each other between

which the flue gases are made to pass. The dust particles are ionized and attracted by charged

electrodes. The electrodes are maintained at 60KV.Hammering is done to the plates so that

fly ash comes down and collect at the bottom. The fly ash is dry form is used in cement

manufacture.

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