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1. AN OVERVIEW ABOUT THE COMPANY

NTPC, the largest power Company in India, was setup in 1975 to accelerate power development

in the country. . It is emerging as an ‘Integrated Power Major’, with a significant presence in the

entire value chain of power generation business. NTPC’s core business is engineering,

construction and operation of power generating plants. It also provides consultancy in the area of

power plant constructions and power generation to companies in India and abroad. NTPC has set

new benchmarks for the power industry both in the area of power plant construction and

operations. Its providing power at the cheapest average tariff in the country.

Fig.1

NTPC ranked 341st in the ‘2010, Forbes Global 2000’ ranking of the World’s biggest companies.

With a current generating capacity of 34,854 MW, NTPC has embarked on plans to become a

75,000 MW company by 2017.

NTPC has gone beyond the thermal power generation. It has diversified into hydro power, coal

mining, power equipment manufacturing, oil &gas exploration, power trading & distribution.

NTPC is now in the entire power value chain and is poised to become an Integrated Power Major.

NTPC’s share on 31 Mar 2008 in the total installed capacity of the country was 19.1% and it

contributed 28.50% of the total power generation of the country during 2007-08. NTPC has set

new benchmarks for the power industry both in the area of power plant construction and

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operations. With its experience and expertise in the power sector, NTPC is extending consultancy

services to various organizations in the power business.

2. POWER GENERATION IN INDIA

Presently, NTPC generates power from Coal and Gas. With an installed capacity of 34,854 MW,

NTPC is the largest power generating major in the country. It has also diversified into hydro

power, coal mining, power equipment manufacturing, oil & gas exploration, power trading &

distribution. With an increasing presence in the power value chain, NTPC is well on its way to

becoming an “Integrated Power Major.”

2.1 OVERALL POWER GENERATION

Be it the generating capacity or plant performance or operational efficiency, NTPC’s Installed

Capacity and performance depicts the company’s outstanding performance across a number of

parametres.

Regional Spread of Generating Facilities

NO. OF PLANTS CAPACITY (MW)

NTPC Owned

Coal 15 27,535

Gas/Liquid Fuel 7 3,955

Total 22 31,490

Owned By JVs

Coal & Gas 6 3,364

Total 28 34,854

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REGION COAL GAS TOTAL

Northern 8,015 2,312 10,327

Western 7,520 1,293 8,813

Southern 4,100 350 4,450

Eastern 7,900 - 7,900

JVs 1,424 1,940 3,364

Total 28,959 5,895 34,854

2.2 OPERATIONS

In terms of operations, NTPC has always been considerably above the national average. The

availability factor for coal based power stations has increased from 89.32% in 1998-99 to 91.62%

in 2010-11, which compares favourably with international standards. The PLF has increased from

76.6% in 1998-99 to 88.29% during the year 2010-11.

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The table below shows that while the installed capacity has increased by 73.33% in the last

twelve years the generation has increased by 101.39%.

•Excluding JVs and Subsidiaries

The table below shows the detailed operational performance of coal based stations over the years

DESCRIPTION UNIT 1998-99 2010-11 % OF INCREASE

Installed Capacity MW 17,786 30,830 73.33

Generation MUs 1,09,505 2,20,540 101.39

OPERATIONAL PERFORMANCE OF COAL BASED NTPC STATIONS

Generation(BU) PLF(%) Availability

Factor(%)

2012-13 220.54 88.29 91.62

2010-11 218.84 90.81 91.76

2009-10 206.94 91.14 92.47

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Fig.2

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2.3 ENVIRONMENT MANAGEMENT

While leading the nation’s power generation league, NTPC has remained committed to the

environment. It continues to take various pro-active measures for protection of the environment

and ecology around its projects.

NTPC was the first among power utilities in India to start Environment Impact Assessment (EIA)

studies and reinforced it with Periodic Environmental Audits and Reviews.

All stations of NTPC are ISO 14001 certified

Various groups to care of environmental issues

The Environment Management Group

Ash Utilization Division

A forestation Group

Centre for Power Efficiency & Environment Protection

Group on Clean Development Mechanism

TPC is the second largest owner of trees in the country after the Forest department.

Partnering government in various initiatives

Consultant role to modernize and improvise several plants across the country

Disseminate technologies to other players in the sector

Consultant role “Partnership in Excellence” Programme for improvement of PLF of 15

Power Stations of SEBs.

Rural Electrification work under Rajiv Gandhi Grameen Vidyutikaran Yojana

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3. INTRODUCTION TO THEMAL POWER PLANT

Introduction

Classification

Functioning

3.1 INTRODUCTION

Power Station (also referred to as generating station or power plant) is an industrial facility for the

generation of electric power. Power plant is also used to refer to the engine in ships, aircraft and

other large vehicles. Some prefer to use the term energy center because it more accurately

describes what the plants do, which is the conversion of other forms of energy, like chemical

energy, gravitational potential energy or heat energy into electrical energy.

At the center of nearly all power stations is a generator, a rotating machine that converts

mechanical energy into electrical energy by creating relative motion between a magnetic field and

a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly

on what fuels are easily available and the types of technology that the power company has access

to. In thermal power stations, mechanical power is produced by a heat engine, which transforms

thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power

stations produce steam, and these are sometimes called steam power stations. About 80% of all

electric power is generated by use of steam turbines. Not all thermal energy can be transformed to

mechanical power, according to the second law of thermodynamics. Therefore, there is always

heat lost to the environment.

If this loss is employed as useful heat, for industrial processes or district heating, the power plant

is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In

countries where district heating is common, there are dedicated heat plants called heat-only boiler

stations. An important class of power stations in the Middle East uses byproduct heat for

desalination of water.

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A

Fig.3 coal-fired Thermal Power Plan

3.2 CLASSIFICATION

3.2.1 BY FUEL

Nuclear power plants use a nuclear reactor’s heat to operate a steam turbine generator.

Fossil fuelled power plants may also use a steam turbine generator or in the case of natural

gas fired plants may use a combustion turbine.

Geothermal power plants use steam extracted from hot underground rocks.

Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste,

landfill methane, or other forms of biomass.

In integrated steel mills, blast furnace exhaust gas is a low-cost, although low energy density,

fuel.

Waste heat from industrial processes is occasionally concentrated enough to use for power

generation, usually in a steam boiler and turbine.

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3.2.2 BY PRIME MOVER

Steam turbine plants use the dynamic pressure generated by expanding steam to turn the

blades of a turbine. Almost all large non-hydro plants use this system.

Gas turbine plants use the dynamic pressure from flowing gases to directly operate the

turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply “peak”

energy during periods of high demand, though at higher cost than base-loaded plants. These

may be comparatively small units, and sometimes completely unmanned, being remotely

operated. This type was pioneered by the UK, Prince town being the world’s first,

commissioned in 1959.

Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler

and steam turbine which use the exhaust gas from the gas turbine to produce electricity.

This greatly increases the overall efficiency of the plant, and many new base load power

plants are combined cycle plants fired by natural gas. Internal combustion Reciprocating

engines are used to provide power for isolated communities and are frequently used for small

cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities

also use them to provide backup power in case of a power outage. These are usually fuelled

by diesel oil, heavy oil, natural gas and landfill gas.

Micro turbines, Sterling engine and internal combustion reciprocating engines are low cost

solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment

plants and waste gas from oil production.

3.3 FUNCTIONING

In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert the

water into steam. The steam is used to turn a turbine, which is connected to a generator. When the

turbine turns, electricity is generated and given as output by the generator, which is then supplied

to the consumers through high-voltage power lines.

3.3.1 Detailed Process Of Power Generation In A Thermal Power Plant

1) Water intake: Firstly, water is taken into the boiler through a water source. If water is

available in a plenty in the region, then the source is an open pond or river. If water is scarce, then

it is recycled and the same water is used over and over again.

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Fig.4: Process of a Thermal Power Plant

2) Boiler heating: The boiler is heated with the help of oil, coal or natural gas. A furnace is used

to heat the fuel and supply the heat produced to the boiler. The increase in temperature helps in

the transformation of water into steam.

3) Steam Turbine: The steam generated in the boiler is sent through a steam turbine. Theturbine

has blades that rotate when high velocity steam flows across them. This rotation of turbine blades

is used to generate electricity.

4) Generator: A generator is connected to the steam turbine. When the turbine rotates, the

Generator produces electricity which is then passed on to the power distribution systems.

5) Special mountings: There is some other equipment like the economizer and air pre-heater. An

economizer uses the heat from the exhaust gases to heat the feed water. An air pre-heater heats

the air sent into the combustion chamber to improve the efficiency of the combustion process.

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6) Ash collection system: There is a separate residue and ash collection system in place to collect

all the waste materials from the combustion process and to prevent them from Escaping into the

atmosphere.

4. OPERATION

Introduction

Steam Generator or Boiler

Steam Turbine

Electric Generator

4.1 INTRODUCTION

The operating performance of NTPC has been considerably above the national average. The

availability factor for coal stations has increased from 85.03 % in 1997-98 to 90.09 % in 2006- 7,

which compares favourably with international standards. The PLF has increased from 75.2% in

1997-98 to 89.4% during the year 2006-07 which is the highest since the inception of NTPC.

In a Thermal Power Station, steam is produced and used to spin a turbine that operates a

generator. 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; this is

known as a Rankine cycle.

Shown here is a diagram of a conventional thermal power plant, which uses coal, oil, or natural

gas as fuel to boil water to produce the steam. The electricity generated at the plant is sent to

consumers through high-voltage power lines.

The Thermal Power Plant has Steam Turbine-Driven Generators which has a collective capacity

of 705MW. The fuel being used is Coal which is supplied from the Jharia Coal Field in

Jharkhand.

There are basically three main units of a thermal power plant:

1. Steam Generator or Boiler

2. Steam Turbine

3. Electric Generator

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Fig.5 Operation Room of Power Plant

We have discussed about the processes of electrical generation further. A complete detailed

description of the three units is given further.

4.2 TYPICAL LAYOUT OF A POWER PLANT

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Fig.6 :Typical Diagram of a Coal based Thermal Power Plant

5. ELECTRICITY FROM COAL

Coal from the coal wagons is unloaded with the help of wagon tipplers in the C.H.P. this coal is

taken to the raw coal bunkers with the help of conveyor belts. Coal is then transported to bowl

mills by coal feeders where it is pulverized and ground in the powdered form.

This crushed coal is taken away to the furnace through coal pipes with the help of hot and cold

mixture P.A fan. This fan takes atmospheric air, a part of which is sent to pre heaters while a part

goes to the mill for temperature control. Atmospheric air from F.D fan in the air heaters and sent

to the furnace as combustion air.

Water from boiler feed pump passes through economizer and reaches the boiler drum . Water

from the drum passes through the down comers and goes to the bottom ring header. Water from

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the bottom ring header is divided to all the four sides of the furnace. Due to heat density

difference the water rises up in the water wall tubes. This steam and water mixture is again taken

to the boiler drum where the steam is sent to super heaters for super heating. The super heaters

are located inside the furnace and the steam is super heated (540 degree Celsius) and finally it

goes to the turbine.

Fuel gases from the furnace are extracted from the induced draft fan, which maintains balance

draft in the furnace with F.D fan. These fuel gases heat energy to the various super heaters and

finally through air pre heaters and goes to electrostatic precipitators where the ash particles are

extracted. This ash is mixed with the water to from slurry is pumped to ash period.

The steam from boiler is conveyed to turbine through the steam pipes and through stop valve and

control valve that automatically regulate the supply of steam to the turbine. Stop valves and

controls valves are located in steam chest and governor driven from main turbine shaft operates

the control valves the amount used.

A Thermal Power Station comprises all of the equipment and a subsystem required to produce

electricity by using a steam generating boiler fired with fossil fuels to drive an electrical

generator. Some prefer to use the term ENERGY CENTER because such facilities convert forms

of energy, like nuclear energy, gravitational potential energy or heat energy (derived from the

combustion of fuel) into electrical energy. However, POWER PLANT is the most common term

in the united state; While POWER STATION prevails in many Commonwealth countries and

especially in the United Kingdom.

Such power stations are most usually constructed on a very large scale and designed for

continuous operation.

Typical diagram of a coal fired thermal power station

1. Cooling water pump

2. Three-phase transmission line

3. Step up transformer

4. Electrical Generator

5. Low pressure steam

6. Boiler feed water pump

7. Surface condenser

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8. Intermediate pressure steam turbine

9. Steam control valve

10. High pressure steam turbine

11. Deaerator Feed water heater

12. Coal conveyor

13. Coal hopper

14. Coal pulverizer

15. boiler steam drum

16. Bottom ash hoper

17. Super heater

18. Forced draught(draft) fan

19. Reheater

20. Combustion air intake

21. Economizer

22. Air preheater

23. Precipitator

24. Induced draught(draft) fan

25. Fuel gas stack

The description of some of the components written above is described as follows:

1. Cooling Towers

Cooling Towers are evaporative coolers used for cooling water or other working medium to near

the ambivalent web-bulb air temperature. Cooling tower use evaporation of water to reject heat

from processes such as cooling the circulating water used in oil refineries, Chemical plants,

power plants and building cooling, for example. The tower vary in size from small roof-top units

to very large hyperboloid structures that can be up to 200 meters tall and 100 meters in diameter,

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or rectangular structure that can be over 40 meters tall and 80 meters long. Smaller towers are

normally factory built, while larger ones are constructed on site.

The primary use of large , industrial cooling tower system is to remove the heat absorbed in the

circulating cooling water systems used in power plants , petroleum refineries, petrochemical and

chemical plants, natural gas processing plants and other industrial facilities . The absorbed heat is

rejected to the atmosphere by the evaporation of some of the cooling water in mechanical forced-

draft or induced draft towers or in natural draft hyperbolic shaped cooling towers as seen at most

nuclear power plants.

2. Three Phase Transmission Line

Three phase electric power is a common method of electric power transmission. It is a type of

polyphase system mainly used to power motors and many other devices. A Three phase system

uses less conductor material to transmit electric power than equivalent single phase, two phase, or

direct current system at the same voltage. In a three phase system, three circuits reach their

instantaneous peak values at different times. Taking one conductor as the reference, the other two

current are delayed in time by one-third and two-third of one cycle of the electrical current. This

delay between “phases” has the effect of giving constant power transfer over each cycle of the

current and also makes it possible to produce a rotating magnetic field in an electric motor.

At the power station, an electric generator converts mechanical power into a set of electric

currents, one from each electromagnetic coil or winding of the generator. The current are

sinusoidal functions of time, all at the same frequency but offset in time to give different phases.

In a three phase system the phases are spaced equally, giving a phase separation of one-third one

cycle. Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the

power station, transformers: step-up” this voltage to one, more suitable for transmission.

After numerous further conversions in the transmission and distribution network the power is

finally transformed to the standard mains voltage (i.e. the “household” voltage).

The power may already have been split into single phase at this point or it may still be three

phase. Where the step-down is 3 phase, the output of this transformer is usually star connected

with the standard mains voltage being the phase-neutral voltage. Another system commonly seen

in North America is to have a delta connected secondary with a center tap on one of the windings

supplying the ground and neutral. This allows for 240 V three phase as well as three different

single phase voltages( 120 V between two of the phases and neutral , 208 V between the third

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phase ( known as a wild leg) and neutral and 240 V between any two phase) to be available from

the same supply.

3. Electrical Generator

An Electrical generator is a device that converts kinetic energy to electrical energy, generally

using electromagnetic induction. The task of converting the electrical energy into mechanical

energy is accomplished by using a motor. The source of mechanical energy may be a

reciprocating or turbine steam engine, The turbines are made in a variety of sizes ranging from

small 1 hp (0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other

shaft driven equipment , to 2,000,000 hp(1,500,000 kW) turbines used to generate electricity.

There are several classifications for modern steam turbines.

Steam turbines are used in all of our major coal fired power stations to drive the generators or

alternators, which produce electricity. The turbines themselves are driven by steam generated in

‘Boilers’ or ‘steam generators’ as they are sometimes called.

Electrical power station use large steam turbines driving electric generators to produce most

(about 86%) of the world’s electricity. The turbines used for electric power generation are most

often directly coupled to their-generators. As the generators must rotate at constant synchronous

speeds according to the frequency of the electric power system, the most common speeds are

3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems.

Energy in the steam after it leaves the boiler is converted into rotational energy as it passes

through the turbine. The turbine normally consists of several stage with each stages consisting of

a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy

of the steam into kinetic energy into forces, caused by pressure drop, which results in the rotation

of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical

energy.

4. Boiler Feed Water Pump

A Boiler feed water pump is a specific type of pump used to pump water into a steam boiler. The

water may be freshly supplied or retuning condensation of the steam produced by the boiler.

These pumps are normally high pressure units that use suction from a condensate return system

and can be of the centrifugal pump type or positive displacement type.

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5. Control Valve

Control valves are valves used within industrial plants and elsewhere to control operating

conditions such as temperature, pressure, flow, and liquid Level by fully partially opening or

closing in response to signals received from controllers that compares a “set point” to a “process

variable” whose value is provided by sensors that monitor changes in such conditions. The

opening or closing of control valves is done by means of electrical, hydraulic or pneumatic

systems.

6. Deaerator

A Deaerator is a device for air removal and used to remove dissolved gases (an alternate would

be the use of water treatment chemicals) from boiler feed water to make it non-corrosive. A

deaerator typically includes a vertical domed deaeration section as the deaeration boiler feed

water tank. A Steam generating boiler requires that the circulating steam, condensate, and feed

water should be devoid of dissolved gases, particularly corrosive ones and dissolved or suspended

solids. The gases will give rise to corrosion of the metal. The solids will deposit on the heating

surfaces giving rise to localized heating and tube ruptures due to overheating. Under some

conditions it may give to stress corrosion cracking.

Deaerator level and pressure must be controlled by adjusting control valves- the level by

regulating condensate flow and the pressure by regulating steam flow. If operated properly, most

deaerator vendors will guarantee that oxygen in the deaerated water will not exceed 7 ppb by

weight (0.005 cm3/L).

7. Feed Water Heater

A Feed water heater is a power plant component used to pre-heat water delivered to a steam

generating boiler. Preheating the feed water reduces the irreversible involved in steam generation

and therefore improves the thermodynamic efficiency of the system.[4] This reduces plant

operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is

introduces back into the steam cycle.

8. Pulverizer

A pulverizer is a device for grinding coal for combustion in a furnace in a fossil fuel power plant

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9. Super Heater

A Super heater is a device in a steam engine that heats the steam generated by the boiler again

increasing its thermal energy and decreasing the likelihood that it will condense inside the engine.

Super heaters increase the efficiency of the steam engine, and were widely adopted. Steam which

has been superheated is logically known as superheated steam.Non-superheated steam is

called saturated steam or wet steam.

10. Economizers

Economizer is a mechanical device intended to reduce energy consumption, or to perform

another useful function like preheating a fluid. The term economizer is used for other purposes as

well. Boiler, power plant, and heating, ventilating and air conditioning. In boilers, economizer are

heat exchange devices that heat fluids , usually water, up to but not normally beyond the boiling

point of the fluid. Economizers are so named because they can make use of the enthalpy and

improving the boiler’s efficiency. They are a device fitted to a boiler which saves energy by using

the exhaust gases from the boiler to preheat the cold water used the fill it (the feed water).

Modern day boilers, such as those in cold fired power stations, are still fitted with economizer

which is decedents of Green’s original design. In this context they are turbines before it is

pumped to the boilers. A common application of economizer is steam power plants is to capture

the waste hit from boiler stack gases (flue gas) and transfer thus it to the boiler feed water thus

lowering the needed energy input , in turn reducing the firing rates to accomplish the rated boiler

output .

11. Air Preheater

Air preheater is a general term to describe any device designed to heat air before another process

(for example, combustion in a boiler). The purpose of the air preheater is to recover the heat from

the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat

lost in the fuel gas. As a consequence, the flue gases are also sent to the flue gas stack (or

chimney) at a lower temperature allowing simplified design of the ducting and the flue gas stack.

It also allows control over the temperature of gases leaving the stack.

12. Precipitator

An Electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate device that removes

particles from a flowing gas (such As air) using the force of an induced electrostatic charge.

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Electrostatic precipitators are highly efficient filtration devices, and can easily remove fine

particulate matter such as dust and smoke from the air steam.

13. Fuel gas stack

A Fuel gas stack is a type of chimney, a vertical pipe, channel or similar structure through which

combustion product gases called fuel gases are exhausted to the outside air. Fuel gas is usually

composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen

remaining from the intake combustion air. It also contains a small percentage of pollutants such

as particulates matter, carbon mono oxide, nitrogen oxides and sulfur oxides. The flue gas stacks

are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants

over a greater area and thereby reduce the concentration of the pollutants to the levels required by

governmental environmental policies and regulations.

6. SWITCH GEAR

It makes or breaks an electrical circuit.

1. Isolation: - A device which breaks an electrical circuit or when circuit is switched on to no

load. Isolation is normally used in various ways for purpose of isolating a certain portion when

required for maintenance.

2. Switching Isolation: - It is capable of doing things like interrupting transformer

magnetized current, interrupting line charging current and even perform load transfer switching.

The main application of switching isolation is in connection with transformer feeders as unit

makes it possible to switch out one transformer while other is still on load.

3. Circuit Breakers: - One which can make or break the circuit on load and even on faults is

referred to as circuit breakers. This equipment is the most important and is heavy duty equipment

mainly utilized for protection of various circuits and operations on load. Normally circuit

breakers installed are accompanied by isolators

4. Load Break Switches: - These are those interrupting devices which can make or break

circuits. These are normally on same circuit, which are backed by circuit breakers.

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5. Earth Switches: - Devices which are used normally to earth a particular system, to avoid any

accident happening due to induction on account of live adjoining circuits. These equipment do

not handle any appreciable current at all. Apart from this equipment there are a number of relays

etc. which are used in switchgear.

6.1 LT SWITCH GEAR

It is classified in following ways:-

1. Main Switch:- Main switch is control equipment which controls or disconnects the main

supply. The main switch for 3 phase supply is available for the range 32A, 63A, 100A, 200A,

300A at 500V grade.

2. Fuses: - With a very high generating capacity of the modern power stations extremely heavy

currents would flow in the case of fault and the fuse clearing the fault would be required to

withstand extremely heavy stress in process.It is used for supplying power to auxiliaries with

backup fuse protection. Rotary switch up to 25A. With fuses, quick break, quick make and double

break switch fuses for 63A and 100A, switch fuses for 200A, 400A, 600A, 800A and 1000A are

used.

3. Contractors: - AC Contractors are 3 poles suitable for D.O.L Starting of motors and protecting

the connected motors.

4. Overload Relay: - For overload protection, thermal over relay are best suited for this purpose.

They operate due to the action of heat generated by passage of current through relay element.

5. Air Circuit Breakers: - It is seen that use of oil in circuit breaker may cause a fire. So in all

circuits breakers at large capacity air at high pressure is used. This reduces the possibility of

sparking. The pressure may vary from 50-60 kg/cm^2 for high and medium capacity circuit

breakers.

6.2 HT SWITCH GEAR

1. Minimum oil Circuit Breaker: - These use oil as quenching medium. It comprises of simple

dead tank row pursuing projection from it. The moving contracts are carried on an iron arm lifted

by a long insulating tension rod and are closed simultaneously pneumatic operating mechanism

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by means of tensions but throw off spring to be provided at mouth of the control the main current

within the controlled device.

2. Air Circuit Breaker: - In this the compressed air pressure around 15 kg per cm^2 is used for

extinction of arc caused by flow of air around the moving circuit . The breaker is closed by

applying pressure at lower opening and opened by applying pressure at upper opening. When

contacts operate, the cold air rushes around the movable contacts and blown the arc.

It has the following advantages over OCB:-

Fire hazard due to oil are eliminated.

Operation takes place quickly.

There is less burning of contacts since the duration is short and consistent.

Facility for frequent operation since the cooling medium is replaced constantly.

Rated Voltage-6.6 KV

Current-630 A

Auxiliary current-220 V DC

3. SF6 Circuit Breaker: - This type of circuit breaker is of construction to dead tank bulk oil to

circuit breaker but the principle of current interruption is similar to that of air blast circuit

breaker. It simply employs the arc extinguishing medium namely SF6. the performance of gas .

When it is broken down under an electrical stress. It will quickly reconstitute itself.

4. Vacuum Circuit Breaker: - It works on the principle that vacuum is used to save the purpose

of insulation and it implies that pressure Of gas at which breakdown voltage independent of

pressure. In regards of insulation and strength, vacuum is superior dielectric medium and is

better than all other medium except air and sulphur which are generally used at high pressure.

7. STEAM GENERATOR OR BOILER

The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls are

made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.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

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steam at 700 °F (370 °C) and 3,200 psi (22.1MPa). 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.

The steam generating boiler has to produce steam at the high purity, pressure

And temperature required for the steam turbine that drives the electrical generator. The

generator includes the economizer, the steam drum, the chemical dosing equipment, and

the furnace with its steam generating tubes and the Superheater coils. Necessary safety

valves are located at suitable points to avoid excessive boiler pressure. The air and flue

gas path equipment include: forced draft (FD) fan, air Preheater (APH), boiler furnace,

induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse) and the

flue gas stack.

Fig.7

7.1 BOILER FURNACE AND STEAM DRUM

Once water inside the boiler or steam generator, the process of adding the latent heat of

vaporization or enthalpy is underway. The boiler transfers energy to the water by the chemical

reaction of burning some type of fuel. The water enters the boiler through a section in the

convection pass called the economizer. From the economizer it passes to the steam drum.

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Once the water enters the steam drum it goes down the down comers 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 (typically). As the water is turned into

steam/vapor in the water walls, the steam/vapor once again enters the steam drum. The

steam/vapor is passed through a series of steam and water separators and then dryers inside the

steam drum. The steam separators and dryers remove the water droplets from the steam and the

cycle through the water walls is repeated. This process is known as natural 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 tripout are avoided by

flushing out such gases from the combustion zone before igniting the coal.

The steam drum (as well as the superheater coils and headers) have air vents and drains needed

for initial startup. The steam drum has an internal device that removes moisture from the wet

steam entering the drum from the steam generating tubes. The dry steam then flows into the

superheater coils.

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Geothermal plants need no boiler since they use naturally occurring steam sources. Heat

exchangers may be used where the geothermal steam is very corrosive or contains excessive

suspended solids. Nuclear plants also boil water to raise steam, either directly passing the

working steam through the reactor or else using an intermediate heat exchanger.

7.2 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 pulverized

into 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.

7.3 STEAM TURBINE

Steam turbines are used in all of our major coal fired power stations to drive the generators or

alternators, which produce electricity. The turbines themselves are driven by steam generated in

‘Boilers’ or ‘Steam Generators’ as they are sometimes called Energy in the steam after it leaves

the boiler is converted into rotational energy as it passes through the turbine. The turbine

26

normally consists of several stages with each stage consisting of a stationary blade (or nozzle)

and a rotating blade. Stationary blades convert the potential energy of the steam (temperature and

pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating

blades convert the kinetic energy into forces, caused by pressure drop, which results in the

rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the

electrical energy. The rotational speed is 3000 rpm for Indian System (50 Hz).

7.4 NOZZLES AND BLADES

Steam enthalpy is converted into rotational energy as it passes through a turbine stage. A turbine

stage consists of a stationary blade (or nozzle) and a rotating blade (or bucket). Stationary blades

convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity)

and direct the flow onto the rotating blades. The rotating blades convert the kinetic energy into

impulse and reaction forces caused by pressure drop, which results in the rotation of the turbine

shaft or rotor.

Steam turbines are machines which must be designed, manufactured and maintained to

high tolerances so that the design power output and availability is obtained. They are

subject to a number of damage mechanisms, with two of the most important being:

Erosion due to Moisture: - The presence of water droplets in the last stages of a turbine

causes erosion to the blades. This has led to the imposition of an allowable limit of about 12%

wetness in the exhaust steam;

7.5 BEARINGS AND LUBRICATION

Two types of bearings are used to support and locate the rotors of steam turbines:

Journal bearings are used to support the weight of the turbine rotors. A bearing consists

of two half-cylinders that enclose the shaft and are internally lined with Babbitt, a metal

alloy usually consisting of tin, copper and antimony; and

Thrust bearings axially locate the turbine rotors. A thrust bearing is made up of a series of

Babbitt lined pads that run against a locating disk attached to the turbine rotor. High-

pressure oil is injected into the bearings to provide lubrication. The oil is carefully

filtered to remove solid particles. Specially designed centrifuges remove any water from

the oil.

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8. ELECTRIC GENERATOR

The steam turbine-driven generators have auxiliary systems enabling them to work satisfactorily

and safely. The steam turbine generator being rotating equipment generally has a heavy, large

diameter shaft. The shaft therefore requires not only supports but also has to be kept in position

while running. To minimize the frictional resistance to the rotation, the shaft has a number of

bearings. The bearing shells, in which the shaft rotates, are lined with a low friction material

likeBabbitt metal. Oil lubrication is provided to further reduce the friction between shaft and

bearing surface and to limit the heat generated.

Fig.8 :A 95MW Generator at Thermal power station

8.1 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 relevant hydraulic relays and other

mechanisms.

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Auxiliary Systems

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. Generator Heat Dissipation.The

electricity generator requires cooling to dissipate the heat that it generates. While small

units may be cooled by air drawn through filters at the inlet, larger units generally require

special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing, is used because it

has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces

windage losses. This system requires special handling during start-up, with air in the chamber

first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly

flammable hydrogen does not mix with oxygen in the air. The hydrogen pressure inside the

casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress. The

hydrogen must be sealed against outward leakage where the shaft emerges from the casing.

Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing

between the shaft and the seals. Seal oil is used to prevent the hydrogen gas leakage to

atmosphere.

The generator also uses water cooling. Since the generator coils are at a potential of about

15.75kV and water is conductive, an insulating barrier such as Teflon is used to interconnect the

water line and the generator high voltage windings. Demineralized water of low conductivity is

used.

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8.2 GENERATOR HIGH VOLTAGE SYSTEM

The generator voltage ranges from 10.5 kV in smaller units to 15.75 kV in larger units. The

generator high voltage leads are normally large aluminum channels because of their high current

as compared to the cables used in smaller machines. They are enclosed in well-grounded

aluminum bus ducts and are supported on suitable insulators.

The generator high voltage channels are connected to step-up transformers for connecting to a

high voltage electrical substation (of the order of 220 kV) for further transmission by the local

power grid. The necessary protection and metering devices are included for the high voltage

leads. Thus, the steam turbine generator and the transformer form one unit. In smaller units,

generating at 10.5 kV, a breaker is provided to connect it to a common 10.5 kV bus system.

8.3 OTHER SYSTEMS

Monitoring and Alarm system Most of the power plant’s operational controls are automatic.

However, at times, manual intervention may be required. Thus, the plant is provided with

monitors and alarm systems that alert the plant operators when certain operating parameters are

seriously deviating from their normal range.

An Engineer monitoring the various parameters Battery Supplied Emergency Lighting &

Communication. A central battery system consisting of lead acid cell units is provided to supply

emergency electric power.

When needed, to essential items such as the power plant’s control systems, communication

systems, turbine lube oil pumps, and emergency lighting. This is essential for a safe, damage-free

shutdown of the units in an emergency situation.

9. COAL HANDLING PLANT

Coal is delivered by highway truck, rail, barge or collier ship. Some plants are even built near

coal mines and coal is delivered by conveyors. A large coal train called a “unit train” may be a

kilometers (over a mile) long, containing 60 cars with 100 tons of coal in each one, for a total

load of 6,000 tons.

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A large plant under full load requires at least one coal delivery this size every day. Plants may get

as many as three to five trains a day, especially in “peak season”, during the summer months

when power consumption is high. A large thermal power plant stores several million tons of coal

for use when there is no wagon supply.

Fig.9 :Coal Handling Plant Layout

9.1 RUN-OF-MINE (ROM) COAL

The coal delivered from the mine that reports to the Coal Handling Plant is called Run-of-mine,or

ROM, coal. This is the raw material for the CHP, and consists of coal, rocks, middlings, minerals

and contamination. Contamination is usually introduced by the mining process and may include

machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large

variability of moisture and maximum particle size.

9.2 COAL HANDLING

Coal needs to be stored at various stages of the preparation process, and conveyed around the

CHP facilities. Coal handling is part of the larger field of bulk material handling, and is a

complex and vital part of the CHP.

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9.3 COAL STACKING

Traveling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal

stockpiles. Stackers are nominally rated in tph (tonnes per hour) for capacity and normally travel

on a rail between stockpiles in the stockyard. A stacker can usually move in at least two

directions typically: horizontally along the rail and vertically by luffing its boom. Luffing of the

boom minimizes dust by reducing the height that the coal needs to fall to the top of the stockpile.

The boom is luffed upwards as the stockpile height grows.

9.4 STOCKPILES

Stockpiles provide surge capacity to various parts of the CHP. ROM coal is delivered with large

variations in production rate of tonnes per hour (tph). A ROM stockpile is used to allow the wash

plant to be fed coal at lower, constant rate.

Fig.10

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9.5 RECLAIMING

Tunnel conveyors can be fed by a continuous slot hopper or bunker beneath the stockpile to

reclaim material. Front-end loaders and bulldozers can be used to push the coal into feeders.

Sometimes front-end loaders are the only means of reclaiming coal from the stockpile. This has a

low up-front capital cost, but much higher operating costs, measured in dollars per tonne handled.

9.6 COAL SAMPLING

Sampling of coal is an important part of the process control in the CHP. A grab sample is a one

off sample of the coal at a point in the process stream, and tends not to be very representative. A

routine sample is taken at a set frequency, either over a period of time or per shipment.

9.7 SCREENING

Fig.11

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Screens are used to group process particles into ranges by size. These size ranges are also called

grades. Dewatering screens are used to remove water from the product. Screens can be static, or

mechanically vibrated. Screen decks can be made from different materials such as high tensile

steel, stainless steel, or polyethylene.

9.8 MAGNETIC SEPARATION

Magnetic separators shall be used in coal conveying systems to separate tramp iron (including

steel) from the coal. Basically, two types are available. One type incorporates permanent or

electromagnets into the head pulley of a belt conveyor. The tramp iron clings to the belt as it goes

around the pulley drum and falls off into a collection hopper or trough after the point at which

coal is charged from the belt. The other type consists of permanent or electromagnets

incorporated into a belt conveyor that is suspended above a belt conveyor carrying coal. The

tramp iron is pulled from the moving coal to the face of the separating conveyor, which in turn

holds and carries the tramp iron to a collection hopper or trough. Magnetic separators shall be

used just ahead of the coal crusher, if any, and/or just prior to coal discharge to the in-plant

bunker or silo fill system.

9.9 COAL CRUSHER

Before the coal is sent to the plant it has to be ensured that the coal is of uniform size, and so it is

passed through coal crushers. Also power plants using pulverized coal specify a maximum coal

size that can be fed into the pulverizer and so the coal has to be crushed to the specified size using

the coal crusher. Rotary crushers are very commonly used for this purpose as they can provide a

continuous flow of coal to the pulverizer.

9.10 ASH HANDLING

The ever increasing capacities of boiler units together with their ability to use low grade high ash

content coal have been responsible for the development of modern day ash handling systems. The

widely used ash handling systems are

1. Mechanical Handling System

2. Hydraulic System

3. Pneumatic System

4. Steam Jet System

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10. ELECTRIC MOTORS

An electric motor uses electrical energy to produce mechanical energy. The reverse process that

of using mechanical energy to produce electrical energy is accomplished by a generator or

dynamo. Traction motors used on locomotives and some electric and hybrid automobiles often

performs both tasks if the vehicle is equipped with dynamic brakes.

10.1 AC MOTOR

An AC motor is an electric motor that is driven by an alternating current. It consists of two basic

parts, an outside stationary stator having coils supplied with AC current to produce a rotating

magnetic field, and an inside rotor attached to the output shaft that is given a torque by the

rotating field. There are two types of AC motors, depending on the type of rotor used. The first is

the synchronous motor, which rotates exactly at the supply frequency or a sub multiple of the

supply frequency. The magnetic field on the rotor is either generated by current delivered through

slip rings or a by a permanent magnet. The second type is the induction motor, which turns

slightly slower than the supply frequency.

The magnetic field on the rotor of this motor is created by an induced current. Synchronous

Motor. A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils

passing magnets at the same rate as the alternating current and resulting magnetic field which

drives it. Another way of saying this is that it has zero slip under usual operating conditions.

Contrast this with an induction motor, which must slip in order to produce torque. Sometimes a

synchronous motor is used, not to drive a load, but to improve the power factor on the local grid

it’s connected to. It does this by providing reactive power to or consuming reactive power from

the grid. In this case the synchronous motor is called a Synchronous condenser. Electrical power

plants almost always use synchronous generators because it’s very important to keep the

frequency constant at which the generator is connected.

10.2 GENERATOR FUNDAMENTALS

The transformation of mechanical energy into electrical energy is carried out by the Generator.

Working Principle

The A.C. Generator or alternator is based upon the principle of electromagnetic induction and

consists generally of a stationary part called stator and a rotating part called rotor. The stator

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houses the armature windings. The rotor houses the field windings. D.C. voltage is applied to the

field windings through slip rings. When the rotor is rotated, the lines of magnetic flux (viz

magnetic field) cut through the stator windings. This induces an e.m.f in the stator windings.

The magnitude of this e.m.f is given by the following expression.

E = 4.44 /ø FN volts

ø = Strength of magnetic field in Weber’s.

F = Frequency in cycles per second or Hertz.

N = Number of turns in a coil of stator winding

F = Frequency = Pn/120

Where P = Number of poles

n = revolutions per second of rotor.

From the expression it is clear that for the same frequency, number of poles increases with

decrease in speed and vice versa. Therefore, low speed hydro turbine drives generators have 14 to

20 poles where as high speed steam turbine driven generators have generally 2 poles.

10.3 GENERATOR COMPONENT

10.3.1 ROTOR

The electrical rotor is the most difficult part of the generator to design. It revolves in most modern

generators at a speed of 3,000 revolutions per minute. The problem of guaranteeing the dynamic

strength and operating stability of such a rotor is complicated by the fact that a massive non-

uniform shaft subjected to a multiplicity of differential stresses must operate in oil lubricated

sleeve bearings supported by a structure mounted on foundations all of which possess complex

dynamic behavior peculiar to themselves. It is also an electromagnet and to give it the necessary

magnetic strength the windings must carry a fairly high current.

The passage of the current through the windings generates heat but the temperature must not be

allowed to become so high, otherwise difficulties will be experienced with insulation. To keep the

temperature down, the cross section of the conductor could not be increased but this would

introduce another problems. In order to make room for the large conductors, body and this

would cause mechanical weakness. The problem is really to get the maximum amount of copper

into the windings without reducing the mechanical strength. With good design and great care in

construction this can be achieved. The rotor is a cast steel ingot, and it is further forged and

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machined. Very often a hole is bored through the centre of the rotor axially from one end of the

other for inspection. Slots are then machined for windings and ventilation.

Rotor winding

Silver bearing copper is used for the winding with mica as the insulation between conductors. A

mechanically strong insulator such as micanite is used for lining the slots. Later designs of

windings for large rotor incorporate combination of hollow conductors with slots or holes

arranged to provide for circulation of the cooling gas through the actual conductors.

When rotating at high speed. Centrifugal force tries to lift the windings out of the slots and they

are contained by wedges. The end rings are secured to a turned recess in the rotor body, by

shrinking or screwing and supported at the other end by fittings carried by the rotor body. The

two ends of windings are connected to slip rings, usually made of forged steel, and mounted on

insulated sleeves.

Rotor balancing

When completed the rotor must be tested for mechanical balance, which means that a check is

made to see if it will run up to normal speed without vibration. To do this it would have to be

uniform about its central axis and it is most unlikely that this will be so to the degree necessary

for perfect balance. Arrangements are therefore made in all designs to fix adjustable balance

weights around the circumference at each end.

10.3.2 STATOR

Stator frame: The stator is the heaviest load to be transported. The major part of this load is the

stator core. This comprises an inner frame and outer frame. The outer frame is a rigid fabricated

structure of welded steel plates, within this shell is a fixed cage of girder built circular and axial

ribs. The ribs divide the yoke in the compartments through which hydrogen flows into radial

ducts in the stator core and circulate through the gas coolers housed in the frame. The inner cage

is usually fixed in to the yoke by an arrangement of springs to dampen the double frequency

vibrations inherent in 2 pole generators. The end shields of hydrogen cooled generators must be

strong enough to carry shaft seals. In large generators the frame is constructed as two separate

parts. The fabricated inner cage is inserted in the outer frame after the stator core has been

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constructed and the winding completed. Stator core: The stator core is built up from a large

number of ‘punching” or sections of thin steel plates. The use of cold rolled grain-oriented steel

can contribute to reduction in the weight of stator core for two main reasons:

a) There is an increase in core stacking factor with improvement in lamination cold rolling and in

cold buildings techniques.

b) The advantage can be taken of the high magnetic permeance of grain-oriented steels of work

the stator core at comparatively high magnetic saturation without fear or excessive iron loss of

two heavy a demand for excitation ampere turns from the generator rotor.

Stator Windings

Each stator conductor must be capable of carrying the rated current without overheating. The

insulation must be sufficient to prevent leakage currents flowing between the phases to earth.

Windings for the stator are made up from copper strips wound with insulated tape which is

impregnated with varnish, dried under vacuum and hot pressed to form a solid insulation bar. The

fabricated inner cage is inserted in the outer frame after the stator core has been constructed and

the winding completed. Stator core: The stator core is built up from a large number of ‘punching”

or sections of thin steel plates. The use of cold rolled grain-oriented steel can contribute to

reduction in the weight of stator core for two main reasons.

These bars are then place in the stator slots and held in with wedges to form the complete winding

which is connected together at each end of the core forming the end turns. The generator

terminals are usually arranged below the stator. On recent generators (210 MW) the windings are

made up from copper tubes instead of strips through which water is circulated for cooling

purposes. The water is fed to the windings through plastic tubes.

11. GENERATOR COOLING SYSTEM

The 200/210 MW Generator is provided with an efficient cooling system to avoid excessive

heating and consequent wear and tear of its main components during operation. This Chapter

deals with the rotor-hydrogen cooling system and stator water cooling system along with the shaft

sealing and bearing cooling systems.

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11.1 ROTOR COOLING SYSTEM

The rotor is cooled by means of gap pick-up cooling, wherein the hydrogen gas in the air gap is

sucked through the scoops on the rotor wedges and is directed to flow along the ventilating canals

milled on the sides of the rotor coil, to the bottom of the slot where it takes a turn and comes out

on the similar canal milled on the other side of the rotor coil to the hot zone of the rotor. Due to

the rotation of the rotor, a positive suction as well as discharge is created due to which a certain

quantity of gas flows and cools the rotor. This method of cooling gives uniform distribution of

temperature. Also, this method has an inherent advantage of eliminating the deformation of

copper due to varying temperatures.

11.2 HYDROGEN COOLING SYSTEM

Hydrogen is used as a cooling medium in large capacity generator in view of its high heat

carrying capacity and low density. But in view of its forming an explosive mixture with oxygen,

proper arrangement for filling, purging and maintaining its purity inside the generator have to be

made. Also, in order to prevent escape of hydrogen from the generator casing, shaft sealing

system is used to provide oil sealing.

The hydrogen cooling system mainly comprises of a gas control stand, a drier, an liquid level

indicator, hydrogen control panel, gas purity measuring and indicating instruments, The system is

capable of performing the following functions : Filling in and purging of hydrogen safely

without bringing in contact with air. Maintaining the gas pressure inside the machine at the

desired value at all the times. Provide indication to the operator about the condition of the gas

inside the machine i.e. its pressure, temperature and purity. Continuous circulation of gas inside

the machine through a drier in order to remove any water vapour that may be present in it.

Indication of liquid level in the generator and alarm in case of high level.

11.3 STATOR COOLING SYSTEM

The stator winding is cooled by distillate. Which is fed from one end of the machine by Teflon

tube and flows through the upper bar and returns back through the lower bar of another slot.

Turbo generators require water cooling arrangement over and above the usual hydrogen cooling

arrangement. The stator winding is cooled in this system by circulating demineralised water

39

(DM water) through hollow conductors. The cooling water used for cooling stator winding calls

for the use of very high quality of cooling water. For this purpose DM water of proper specific

resistance is selected. Generator is to be loaded within a very short period if the specific

resistance of the cooling DM water goes beyond certain preset values. The system is designed to

maintain a constant rate of cooling water flow to the stator winding at a nominal inlet water

temperature of 40 deg.C.

Rating of 95 MW Generator

Manufactured by Bharat heavy electrical Limited (BHEL)

Capacity - 117500 KVA

Voltage - 10500V

Speed - 3000 rpm

Hydrogen - 2.5 Kg/cm2

Power factor - 0.85 (lagging)

Stator current - 6475 A

Frequency - 50 Hz

Stator wdg connection - 3 phase

12 .TRANSFORMER

A transformer is a device that transfers electrical energy from one circuit to another by magnetic

coupling with out requiring relative motion between its parts. It usually comprises two or more

coupled windings, and in most cases, a core to concentrate magnetic flux.

An alternating voltage applied to one winding creates a time-varying magnetic flux in the core,

which includes a voltage in the other windings.

Varying the relative number of turns between primary and secondary windings determines the

ratio of the input and output voltages, thus transforming the voltage by stepping it up or down

between circuits.By transforming electrical power to a high-voltage, low-current form and back

again, the transformer greatly reduces energy losses and so enables the economic transmission of

power over long distances.

40

Fig.12

It has thus shape the electricity supply industry, permitting generation to be located remotely

from point of demand. All but a fraction of the world’s electrical power has passed through a

series of transformer by the time it reaches the consumer.

12.1 BASIC PRINCIPLES

The principles of the transformer are illustrated by consideration of a hypothetical ideal

transformer consisting of two windings of zero resistance around a core of negligible reluctance.

A voltage applied to the primary winding causes a current, which develops a magneto motive

force (MMF) in the core. The current required to create the MMF is termed the magnetizing

current; in the ideal transformer it is considered to be negligible, although its presence is still

required to drive flux around the magnetic circuit of the core. An electromotive force (MMF) is

induced across each winding, an effect known as mutual inductance. In accordance with faraday’s

law of induction, the EMFs are proportional to the rate of change of flux.

The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the

back EMF”. Energy losses An ideal transformer would have no energy losses and would have no

energy losses, and would therefore be 100% efficient. Despite the transformer being amongst the

most efficient of electrical machines with ex the most efficient of electrical machines with

experimental models using superconducting windings achieving efficiency of 99.85%, energy is

dissipated in the windings, core, and surrounding structures

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A small transformer such as plug-in “power brick” used for low-power consumer electronics may

be less than 85% efficient. Transformer losses are attributable to several causes and may be

differentiated between those originated in the windings, some times termed copper loss, and those

arising from the magnetic circuit, sometimes termed iron loss.

At higher frequencies, skin effect and proximity effect create additional winding resistance and

losses. Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost

due to hysteresis within the core

The eddy current loss is a complex function of the square of supply frequency and inverse square

of the material thickness. Magnetostriction Magnetic flux in a ferromagnetic material, such as the

core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an

effect known as magnetostriction. This produces the buzzing sound commonly associated with

transformers, and in turn causes losses due to frictional heating in susceptible cores.

Stray losses Leakage inductance is by itself loss less, since energy supplied to its magnetic fields

is returned to the supply with the next half-cycle. However, any leakage flux that intercepts

nearby conductive material such as the transformers support structure will give rise to eddy

currents and be converted to heat. Cooling system Large power transformers may be equipped

with cooling fans, oil pumps or water-cooler heat exchangers design to remove heat. Power used

to operate the cooling system is typically considered part of the losses of the transformer

12.2 RATING OF TRANSFORMER

Manufactured by Bharat heavy electrical limited

No load voltage (hv) - 229 KV

No load Voltage (lv) -10.5 KV

Line current (hv) - 315.2 A

Line current (lv) - 873.2 A

Temp rise - 45 Celsius

Oil quantity -40180 lit

Total weight - 147725 Kg

Core & winding - 84325 Kg

Phase - 3

Frequency - 50 Hz

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13. CONTROL & INSTRUMENTATION (MAJOR ROLE OF ECE)

AUTOMATION AND CONTROL SYSTEM

13.1 AUTOMATION (Definition):

The word automation is widely used today in relation to various types of applications, such as

office automation, plant or process automation.

This subsection presents the application of a control system for the automation of a process plant,

such as power station. In this last application, the automation actively controls the plant during

the three main phases of operation: plant start-up, power generation in stable or shut- down.

During plant start-up & shut-down , sequence controllers as well as long range modulating

controllers in or out of operation every piece of the plant ,at the correct time & in coordinated

modes, taking modes, taking into account safety as well as overstressing limits.

During stable generation of power, the modulating portion of the automation system keeps the

actual generated power value within the limits of the desired load demand.

During major load changes, the automation system automatically redefines new set points &

switches ON or OFF process pieces, to automatically bring the individual processes in an

optimally coordinated way to the new desired load demand. This load transfer executed according

to pre-programmed adaptively controlled load gradients & in safe way.

13.2 AUTOMATION (Benefits):

The main benefit of plant automation is to increase overall plant availability and efficiency, the

increase of these two factors is achieved through a series of features summarized as follows:

i. Optimization of house load consumption during plant start–up, shut-down and operation

via:

o faster plant start-up through elimination of control errors creating delay

o Faster sequence of control actions compared to manual ones. figures 1 shows the

sequence of a rapid restart using automation for a typical coal-fired station even a

well-trained operator crew would probably not be able to bring the plant the full

load the same load without considerable risks

o Co-ordination of house load to generate power output.

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ii. Ensure and maintain plant operation even in case of disturbances in the control system

via:

o Coordinated ON/ OFF and modulating control switch over capability from a sub

process to a redundant one.

o Prevents sub process and process tripping chain reaction following a process

component.

iii. Reduce plant / process shutdown time for repair and maintenance as well as repair costs

via:

o Protection of individual components against overstress (in a stable or a stable plant

operation).

o Bringing process in a safe stage of operation, where process component are protected

against overstress.

14. PROCESS STRUCTURE

Analysis of processes in power station and industry advocates the advisability of dividing

complex overall process into individual sub-process having distinctly defined function, this

divisor of process in clearly defined groups termed as FUNCTIONAL GROUPS resulting a

hierarchical process structure. While the hierarchical structure is governed in the horizontal

direction by the number of tribes (motorized valves, fans, dampers, pumps, etc) in the words the

size of the process; in the vertical direction, there is a distinction made between fundamental

levels.

These being the:-

drive level

function group level

The above three levels are defined regards to the process and not form the control point of view.

14.1 CONTROL SYSTEM STUCTURE

The primary requirement to be fulfilled by any control system architecture is that it be capable of

being organized and implemented on true process-oriented lines. In order words, the control

system structure should map on the hierarchy process structure.

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SYSTEM OVERVEIW

The control and automation system used here is a micro based multiplexing system .this

system. Designed on a modular basis, allows tightening the scope of control hardware to the

particular control strategy and operating requirements of the process.

Regardless of the type and extent of process to control provides to control provides system

uniformity and integrity for:

Signal conditioning and transmission

Modulating control

15. CONTROL AND MONITORING MECHANISM

There are basically two types of problems faced in a power plant:

Metallurgical

Mechanical

Mechanical power related to the turbine that is the max speed permissible for a turbine is 3000

rpm, so speed should be monitored & maintained at that level.

Metallurgical problem can be view as the max inlet Temperature for turbine is 1060 C so

temperature should be below limit.

Monitoring of all the parameters is necessary for the safety of both:

Employees

Machines

So the parameters to be monitored are:

Speed

Temperature

Current

Voltage

Pressure

Eccentricity

Flow of gases

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Vacuum pressure

Valves

Level

Vibration

15.1 PRESSURE MONITORING

Pressure can be monitored by three types of basic mechanism:

Switches

Gauges

Transmitter type

For gauges we use bourdon tube: The bourdon tube is a non liquid pressure measurement device.

It is widely used in specifications where in expensive static pressure measurements are needed.

A typical bourdon tube contains a curved tube that is open to external pressure input on one end

& is coupled mechanically to an indicating needle on the other end, as shown below:

Transmitter types use transducers (electrical to electrical normally). They are used where the

continuous monitoring is required. Normally capacitive transducers are used.

For switches pressure switches are used & they can be used for digital means of monitoring as

switch being ON is referred as high & being OFF is as low.

All the monitored data is converted to either current or voltage parameter.

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Fig.13

The plant standard for current & voltage are as under:

Voltage: 0-10 Volts range

Current: 4-20 mA

We use 2mA the lower value so as to check for disturbances & wires breaks.

Accuracy of such systems is very high.

ACCURACY: +/-0.1%

The whole system used is SCADA based

We use DDCMIC control for this process.

Programmable Logic Circuits (PLCs) are used in the process as they are the heart of

instrumentation.

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15.1.1 CONTROL VALVES

A valve is a device that regulates the flow of substances (either gases, fluidized solids, slurries, or

by liquid) by opening, closing, or partially obstructing various passageways. Valves are

technically pipe fittings, but usually are discussed separately.

Valves are used in a variety of applications including industrial, military, commercial, residential,

transportation. Plumbing valves are the most obvious in everyday life, but many more are used.

Some valves are driven by pressure only, they are mainly used for safety purposes in steam

engines & domestic heating or cooking appliances. A typical bourdon tube contains a curved tube

that is open to external pressure input on one end & is coupled mechanically to an indicating

needle on the other end, as shown below Others are used in a controlled way like in Otto cycle

engines driven by a camshaft, where they play major role in engine cycle control.

Many valves are controlled manually with a handle attached to the valve stem. If the handle is

turned a quarter of a full turn (90) between operating positions, the valve is called a quarter-turn

valve. Butterfly valves, ball valves, & plug valves are often quarter turn valves. Valves can also

be controlled by devices called actuators attached to stem. They can be electromechanical

actuators such as an electric motor or solenoid, pneumatic actuators which are controlled by air

pressure, or hydraulic actuators which are controlled by the pressure of a liquid such as oil or

water.

So there are basically three types of valves that are used in power industries besides the handle

valves. They are:

Pneumatic Valves- They is air or gas controlled which is compressed to turn or move

them.

Hydraulic Valves- They utilizes oil in place of air as oil has better compression.

Motorized Valves- These valves are controlled by electric motors.

15.2 TEMPERATURE MONITORING

There are basically three methods of temperature monitoring. They are:

Electrical method

field system method

Metallic method

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15.2.1 ELECTRICAL METHOD OF TEMPERATURE MEASUREMENT

This method includes:

• RTD (PT 100, Cu 53)

• Thermocouple

Resistance Temperature Detectors (RTD)

Resistance of the element varies in proportionally to the temperature.

Constant current is fed and the with the varying temp voltage in the circuit varies and

the same can be converted into thermal through curve fitting equations

o PT 100- 100 ohm at 0 °C

o Range-200 to 850 °C

Thermocouple

Electrically conducting and thermoelectrically dissimilar material coupled at an interface

is called thermocouple. The legs are called thermo elements. See beck effect

• Type: B, E, J, K, N, R, S & T

• Type K :Ni-Cr Ni

• Range: -270 - 1350 °C

15.3 FLOW MEASUREMENT

• Differential Pressure- Orifice, Venture, Nozzle

• Variable Area Displacement

• Turbine Flow meters

• Ultrasonic

• Carioles Effect Mass Flow meter