1

ABOUT

UTTAR PRADESH RAJYA VIDYUT UTPADAN NIGAM LTD.

UP Rajya Vidyut Utpadan Nigam Ltd. was constituted on 25.08.1980

under the companies act 1956 for construction of new thermal power

projects in the state sector. On 14.1.2000, in accordance to up state

electricity reforms acts 1999, up state electricity board, till then

responsible for generation, transmission and distribution of power within

the state of Uttar Pradesh, was unbundled and operations of the state

sector thermal power stations was handed over to UPRVUNL.

At present 54.09% or 93918.38 MW (Data Source CEA, as on 31/03/2011) of total electricity production in India is from Coal Based Thermal Power Station. A coal based thermal power plant converts the chemical energy of the coal into electrical energy. This is achieved by raising the steam in the boilers, expanding it through the turbine and coupling the turbines to the generators .

Today it is looking after operations of five number thermal power plants located in different parts of UP, with a total installed generation capacity of 4683 MW with planting facility as follows.

NAMEOFTPS INSTALLED

CAPACITY

DERATED

CAPACITY

TOTAL

CAPACITY ANPARA TPS 3X210 MW 3X210 MW = 630 MW 1630 MW

2X500 MW 2X500 MW = 1000 MW

OBRA TPS 2X50 MW 2X50 MW = 100 MW 1288 MW

2X100 MW 2X94 MW = 18 MW 5X200 MW 5X200 MW = 1000 MW

PANKI TPS 2X110 MW 2XI05 MW = 210 MW 210MW

PARlCHHA TPS 2X110 MW 2X110 MW = 220 MW 1140 MW

HARDUAGANJ

TPS

1X60 MW 1X40 MW = 40 MW 415 MW

2X105 MW 2X55MW=110MW

2X250 MW 2X60 MW = 120 MW 1X110 MW 1XI05 MW = 105 MW

TOTAL

UPRVUNL

4683 MW

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HARDUAGANJ THERMAL POWER STATION

PLANT LOCATION

it is in district ALIGARH about 18 KM from ALIGARH railway station. ALIGARH is on DELHI-KANPUR road/rail rout.

ABOUT GENERATING UNITS AT HARDUAGANJ THERMAL POWER STATION

All units of this power station are coal fired thermal power plants,

having a total generating capacity of 415 mw. The power station

consists of following units -

STAGE UNITS INSTALLED DERATED DATE OF ORIGINAL

NO. CAPACITY CAPACITY COMMISSIONING EQUIPMENT

MANUFACTURER

i.

50 MW Deleted

ATPS ii. 50 MW Deleted

iii. 50 MW Deleted

01 50 MW Deleted 21.04.1968 USSR

02 50 MW Deleted 23.01.1969 USSR

BTPS 03 55 MW Deleted Mar.1972 M/S BHARAT HEAVY

ELECTRICALS LTD.

04 55 MW Deleted 18.09.1972

05 60 MW 60 MW 14.05.1977

CTPS 06 60 MW Deleted 26.10.1977

07 110 MW 105 MW Aug.1978

HTPS Extn.

08 250 MW 250 MW 01.02.2012

09 250 MW 250 MW 01.02.2012

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POWER PLANT DESCRIPTION

Rankine cycle

The Rankine cycle is a cycle that converts heat into work. The heat is

supplied externally to a closed loop, which usually uses water. This

cycle generates about 90% of all electric power used throughout the

world, including virtually all solar thermal, biomass, coal and

nuclearpower plants. It is named after William John MacquornRankine,

a Scottish polymath and Glasgow University professor. The Rankine

cycle is the fundamental thermodynamic underpinning of the steam

engine.

The four processes in the Rankine cycle

T-s diagram of a typical Rankine cycle operating between pressures of 0.06bar and

50bar.

There are four processes in the Rankine cycle. These states are identified by

numbers (in brown) in the above Ts diagram.

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Process 1-2: The working fluid is pumped from low to high pressure. As the

fluid is a liquid at this stage the pump requires little input energy.

Process 2-3: The high pressure liquid enters a boiler where it is heated at

constant pressure by an external heat source to become a dry saturated

vapour. The input energy required can be easily calculated using mollier

diagram or h-s chart or enthalpy-entropy chart also known as steam tables.

Process 3-4: The dry saturated vapour expands through a turbine, generating

power. This decreases the temperature and pressure of the vapour, and some

condensation may occur. Theoutput in this process can be easily calculated

using the Enthalpy-entropy chart or the steam tables.

Process 4-1: The wet vapour then enters a condenser where it is condensed

at a constant temperature to become a saturated liquid.

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump

and turbine would generate no entropy and hence maximize the net work output.

Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram

and more closely resemble that of the Carnot cycle. The Rankine cycle shown here

prevents the vapor ending up in the superheat region after the expansion in the

turbine, [1]

which reduces the energy removed by the condensers.

Rankine cycle with reheat

In this variation, two turbines work in series. The first accepts vapour from the

boiler at high pressure. After the vapour has passed through the first turbine, it re-

enters the boiler and is reheated before passing through a second, lower pressure

turbine. Among other advantages, this prevents the vapour from condensing during

its expansion which can seriously damage the turbine blades, and improves the

efficiency of the cycle, as more of the heat flow into the cycle occurs at higher

temperature.

Regenerative Rankine cycle

The regenerative Rankine cycle is so named because after emerging from the

condenser (possibly as a subcooled liquid) the working fluid is heated by steam

tapped from the hot portion of the cycle. On the diagram shown, the fluid at 2 is

mixed with the fluid at 4 (both at the same pressure) to end up with the saturated

liquid at 7. This is called "direct contact heating". The Regenerative Rankine cycle

(with minor variants) is commonly used in real power stations.

Another variation is where bleed steam from between turbine stages is sent to feed

water heaters to preheat the water on its way from the condenser to the boiler.

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These heaters do not mix the input steam and condensate, function as an ordinary

tubular heat exchanger, and are named "closed feed water heaters".

The regenerative features here effectively raise the nominal cycle heat input

temperature, by reducing the addition of heat from the boiler/fuel source at the

relatively low feed water temperatures that would exist without regenerative feed

water heating. This improves the efficiency of the cycle, as more of the heat flow

into the cycle occurs at higher temperature.

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POWER PLANT DESCRIPTION-(OVERVIEW)

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First of all we send DM water from hotwell to low pressure heater with the help of

condensate extraction pump. On the suction of L.P. heater the temp of water is

400C and on the exhaust the temperature of water we got 160

0C-170

0C. We take

steam for heating water in L.P. heater from different stages of turbine.After L.P.

heater this water goes into deaerator where dissolved gases available in water are

removed by steam. Now this water goes to high pressure heater through Boiler

feed pump. B.F.P. send the water into the H.P. heater.We use H.P. heater to

increase the temperature of water. At the exhaust of H.P. heater water temperature

becomes 2400C.After this, mixture of water and steam goes through economiser

where we use flue gases heat to increase the temperature of water. This water is

send into boiler drum.Boiler drum separates steam from water. From Boiler drum

water goes into the water wall. In this water wall water converts into saturated

steam. This saturated steam is send into super heater for becoming superheated

steam. The temperature of superheated steam raises upto and pressure. After this

superheated steam goes to turbine and strikes the turbine blades at a very high

speed. After rotating the turbine, steam temperature and pressure got down. This

low temperature steam send into the condenser for condensing. After condensing,

steam changed into water. We collect this water in hotwell and again we use this

water for making steam through above written process.

COAL TO ELECTRICITY

First of all we send pulverised coal in furnace where it burns and its chemical

energy converts into heat energy. By using this heat energy, water change into

steam form. This steam have high temperature and pressure. We send this steam

into turbine through steam line, where steams heat energy convert into turbine

mechanical energy. Turbine and generator are mounted on the shaft. Due to steam,

turbines are moving after that generator rotor also got rotary poles are available

and flux is cut by the rotor due to this electricity form.

Fuel:- fuel may be defined as a substance which contains hydrogen and carbon

which on burning with oxygen in atmospheric air produces a large amount of heat.

The amount of heat generated is known as calorific value of fuel.

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Coal Handling And Preparation

In a coal based power plant coal is transported from coal mines to the power plant by railway in wagons or in a merry-go-round system. Here 4 wagon tipplers are present, 3 in service and 1 in standby. Coal is unloaded from the wagons to a moving underground conveyor belt. This coal from the mines is of no uniform size. So it is taken to the Crusher house and crushed to a size of 20mm. From the crusher house the coal is either stored in dead storage( generally 40 days coal supply) which serves as coal supply in case of coal supply bottleneck or to the live storage(8 hours coal supply) in the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is supplied to the Coal Mills by a Raw Coal Feeder. The Coal Mills or pulverizer pulverizes the coal to 200 mesh size. The powdered coal from the coal mills is carried to the boiler in coal pipes by high pressure hot air. The pulverized coal air mixture is burnt in the boiler in thecombustionzones.Generally in modern boilers tangential firing system is used i.e. the coal nozzles/ guns form tangent to a circle. The temperature in fire ball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low Temperature Superheater, Platen Superheater and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.

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Principal

Coal based thermal power plant works on the principal of Modified Rankine Cycle.

i)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, so the 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.

Types of Pulverisers: Ball and Tube mills; Ring and Ball mills; MPS; Ball mill;

Demolition.

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ii)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.

iii)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.

WATER TREATEMENT PLANT

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Water used in power plant is taken from River. This water is passed

through gates where screens are placed which remove the floating and

heavy bodies from the water like wood and fishes.This water is stored in

pump house from where water is served for different purposes.Then Alum

is mixed with this water which precipitates the impurity in it and this water is

then send to clarifier which collect water in a pool.Then this water is passed

through 1 portable filter and 9 pressure filter.After that water is send to

Cation exchanger , where positive charged particles are neutralised from

water by adding Acidic solution to it from Acid storage tank. acid storage

tanks are present in WTP.Then this water is send to Anion exchanger to

neutralise negative charged particles from water by adding basic solution to

it from Alkali storage tank. Alkali storage tank are present in WTP.If then

also any charge particles left in the water it is neutralised in the mixed

beds.This water is send to degasifier tower for removing the CO2 generated

during the process of Anion and Cation removal.

After these processes DM water is send to the plant.

Quality of DM water:

Hardness: Nill

Ph value: 6.8 to 7.2

Silica SiO2: less than 0.08 ppm

Conductivity: 0.5 u mho/cm2

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I.D. FAN (Induced Draft Fan).

Each 250 MW boiler is provided with two 1.0. Fans o, axial KKK BHEL

make. Each fan is capable of delivering flue gases from the furnace for

subsequent evacuation through chimney. IG vane is used to shut the damper

of ID fan so flue gas will not pass through it. It is generally used during

maintainence.

Each fan consists:

1. Suction chamber

2. Inlet vane control

3. Impeller supported on two bearing

4. Outer guide vanes

5. Diffuser

6. Flexible coupling

7. Bearing self aligning roller type

8. Outlet damper

F.D. FAN (Forced Draft Fan)

Each 250 MW boiler is provided with two F.D. Fans,axial KKK BHEL

make. F.D. fans supplies air for combustion

process by taking directly atmospheric air. The secondary air after

pre heating in air heater mixes with the primary air and coal dust

in the manner to shape the flame and promote rapid mixing for

proper combustion.

Each fan consists:

1. Suction chamber

2. Inlet vane control

3. Impeller supported on two bearing

4. Outer guide vanes

5. Diffuser

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6. Flexible coupling

7. Bearing self aligning roller type

8. Outlet damper

P.A. FAN (Primary Air Fan)

Each 250 MW boiler is provided with two P.A. Fans of type NDF

22b, redial KKK BHEL make. This fan handles clean atmospheric air

which is then created in Ljungstrom Air Preheater the hot primary

air which comes out of the air preheater scavenges the bowl mill

and carries the coal particles to the burners. The bowl mills are

under pressure. A part of the primary cold air is used for regulating

temperature of hot air entering the bowl mill. A part of the primary

cold air is used for sealing R.C. Feeder, coal dust lines at the

discharge end of bowl mills and seal air ring header of mill.

Each fan consists:

1. Suction nozzle

2. Inlet guide vane unit, D-144

3. Intermediate piece

4. Impeller

5. Box-section scroll

6. Rotor

7. Bearing

8. Soft labyrinth seal

9. Coupling 10. Outlet damper

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Condenser

The condenser condenses the steam from the exhaust of the turbine into liquid to

allow it to be pumped. If the condenser can be made cooler, the pressure of the

exhaust steam is reduced and efficiency of the cycle increases. The functions of a

condenser are:-

1) To provide lowest economic heat rejection temperature for steam.

2) To convert exhaust steam to water for reserve thus saving on feed water

requirement.

3) To introduce make up water.

We normally use surface condenser although there is one direct contact condenser

as well. In direct contact type exhaust steam is mixed with directly with D.M

cooling water.

Low pressure heater & high pressure heater

From hotwell water is send to L.P. heater . Heat of steam ejected by turbine is

utilised to heat the water before sending it to boiler, so less fuel will be needed to

make superheated steam . After passing through L.P. heater temperature of

water becomes 160-1700 C at 135kg/cm2 pressure at exhaust. From L.P. heater

water is sent to dearetor then boiler feed pump.

After that, high pressure heaters are used to increase the temperature of water .

H.P. heaters are used since high pressure will be needed to supply water to the

boiler at the height of 52m. The mixture of water & steam is then send to

economiser which further increases its temperature. Water is not converted into

steam at such a high temperature because of the pressure which is maintained

from starting otherwise it will be difficult to increase the pressure of steam.

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Gland Steam Cooler

Steam which leak off from glands H.P.T. and I.P.T. is collected in GSC. If quantity

of steam is less, then it is send to condenser and if more then send to drip pump.

Deaerator

A constant pressure deaerator pegged at 7kg/cm2 abs is

envisaged in turbine cycle installed at 27m level in boiler side to

provide properly deaerator feed water for boiler, limiting gases

(mainly oxygen) to O.005ppm. It is a direct contact type heater

combined with feed storage tank of adequate capacity (87.9m3

2/3rd

height storage). The heating steam is normally supplied from

turbine extractors but during starting and low load operation from

auxiliary steam station.

Boiler Feed Pump

Boiler feed pump takes suction from deaerator and delivers to H.P. heaters.

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.

Boiler Fill Pump When boiler has to be checked or used for any other use, Boiler Fill Pump is used

to supply water as it consumes few kilo watts instead of running big auxiliaries like

Boiler Feed Pump which consume 4 Mega Watt.

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Turbine

The steam turbine is acondensingtendem compound, three

cylender (H.P., I.P., L.P.) horizontal, disc and diaphragm type with

nozzle governing and regenerative feed water heating. The L.P.

turbine is a double flow which incorporates a multi exhaust in

each flow.

The high pressure turbine (HPT) comprises of 12 stages, the first stage

being governing stage the steam flow in HPT being in

reverse direction, the blade are designed for anti-clock wise

rotation, when viewed in the direction of steam flow.

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19

HP and IP rotors are connected by rigid coupling and have a

common bearing. The low pressure turbine (LPT) is a double flow

consisting 4 stages in each.Baumann stage is and present before LPT, It

dumps majority of steam and majority steam goes to low pressure

turbine. Steam becomes moist after passing through stages of HPT and

IPT, so around 400 tonnes of steam is dumped out at Baumann stage.

Fig.LPT.

FOR 250MW UNIT LOAD CLASSIFICATION:-

I.P.T=49%=122.5MW

H.P.T=31%=77.5MW

L.P.T=20%=50MW

EFFICIENCY OF 250MW TURBINE=32.9%

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Principle of operation and design

An ideal steam turbine is considered to be an isentropic process, or constant

entropy process, in which the entropy of the steam entering the turbine is equal to

the entropy of the steam leaving the turbine. No steam turbine is truly isentropic,

however, with typical isentropic efficiencies ranging from 2090% based on the application of the turbine. The interior of a turbine comprises several sets of

blades, or buckets as they are more commonly referred to. One set of stationary

blades is connected to the casing and one set of rotating blades is connected to the

shaft. The sets intermesh with certain minimum clearances, with the size and

configuration of sets varying to efficiently exploit the expansion of steaTo

maximize turbine efficiency the steam is expanded, doing work, in a number of

stages. These stages are characterized by how the energy is extracted from them

and are known as either impulse or reaction turbines. Most steam turbines use a

mixture of the reaction and impulse designs: each stage behaves as either one or

the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.

Impulse turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets.

These jets contain significant kinetic energy, which is converted into shaft rotation

by the bucket-like shaped rotor blades, as the steam jet changes direction. A

pressure drop occurs across only the stationary blades, with a net increase in

steam velocity across the stage. As the steamflows through the nozzle its pressure

falls from inlet pressure to the exit pressure (atmospheric pressure, or more

usually, the condenser vacuum). Due to this high ratio of expansion of steam, the

steam leaves the nozzle with a very high velocity. The steam leaving the moving

blades has a large portion of the maximum velocity of the steam when leaving the

nozzle. The loss of energy due to this higher exit velocity is commonly called the

carry over velocity or leaving loss.

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Reaction turbines

In the reaction turbine, the rotor blades themselves are arranged to form

convergent nozzles. This type of turbine makes use of the reaction force produced

as the steam accelerates through the nozzles formed by the rotor. Steam is directed

onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills

the entire circumference of the rotor. The steam then changes direction and

increases its speed relative to the speed of the blades. A pressure drop occurs

across both the stator and the rotor, with steam accelerating through the stator

and decelerating through the rotor, with no net change in steam velocity across the

stage but with a decrease in both pressure and temperature, reflecting the work

performed in the driving of the rotor.

Operation and maintenance

When warming up a steam turbine for use, the main steam stop valves (after the

boiler) have a bypass line to allow superheated steam to slowly bypass the valve

and proceed to heat up the lines in the system along with the steam turbine. Also, a

turning gear is engaged when there is no steam to the turbine to slowly rotate the

turbine to ensure even heating to prevent uneven expansion. After first rotating the

turbine by the turning gear, allowing time for the rotor to assume a straight plane

(no bowing), then the turning gear is disengaged and steam is admitted to the

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turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 1015 RPM (0.170.25 Hz) to slowly warm the turbine.

A modern steam turbine generator installation

Any imbalance of the rotor can lead to vibration, which in extreme cases can

lead to a blade breaking away from the rotor at high velocity and being ejected

directly through the casing. To minimize risk it is essential that the turbine be

very well balanced and turned with dry steam - that is, superheated steam with a

minimal liquid water content. If water gets into the steam and is blasted onto the

blades (moisture carry over), rapid impingement and erosion of the blades can

occur leading to imbalance and catastrophic failure. Also, water entering the

blades will result in the destruction of the thrust bearing for the turbine shaft. To

prevent this, along with controls and baffles in the boilers to ensure high quality

steam, condensate drains are installed in the steam piping leading to the turbine.

Turbine should not be run at critical speed.

4 critical speeds are specified b/w 1200 to 2400 rpm in these generators.

Speed is increased immediately if rotar rotates at these speeds.

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GOVERNING SYSTEM

Governing system is an important control system in the power plant as it regulates

the turbine speed, power and participates in the grid frequency regulation.

Need for governing system The control of a turbine with a governor is essential, as turbines need to be run up

slowly, to prevent damage while some applications (such as the generation of

alternating current electricity) require precise speed control. Uncontrolled

acceleration of the turbine rotor can lead to an over speed trip, which causes the

nozzle valves that control the flow of steam to the turbine to close. If this fails then

the turbine may continue accelerating until it breaks apart, often spectacularly.

Turbines are expensive to make, requiring precision manufacture and special

quality materials.

Basic scheme

Governing system controls the steam flow to the turbine in response to the control

signals like speed error, power error. It can also be configured to respond to

pressure error. It is a closed loop control system in which control action

goes on till the power mismatch is reduced to zero. As shown in the basic

scheme given in Fig. 1, the inlet steam flow is controlled by the control valve or the

governor valve. It is a regulating valve. The stop valve shown in the figure a head

of control valve is used for protection. It is either closed or open. In emergencies

steam flow is stopped by closing this valve by the protective devices. The electronic

part output is a voltage or current signal and is converted into a hydraulic

pressure or a piston position signal by the electro- hydraulic converter (EHC).

Some designs use high pressure servo valves. The control valves are finally

operated by hydraulic control valve servo motors.

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The steam flow through the control valve is proportional to the valve opening in

the operating range. So when valve position changes, turbine steam flow changes

and turbine power output also changes proportionally. Thus governing

system changes the turbine mechanical power output. In no load

unsynchronized condition, all the power is used to accelerate the rotor

only (after meeting rotational losses) and hence the speed changes. When the

turb ine genera tor uni t i s being s tar ted , governing sys tem

con trol s the speed precisely by regulating the steam flow. Once the

unit is synchronized to the power system grid, same control system is used to

load the machine. As the connected system has very large iner t ia( in f in i te

bus ) , one machine cannot change the f requency o f the g r id . But i t

can participate in the power system frequency regulation as part of a group of

generators that are used for automatic load frequency control. (ALFC).As shown

in the block diagram, the valve opening changes either by changing the

reference setting or by the change in speed (or frequency). This is called

Primary regulation.

. T h e reference setting can also be changed remotely by power system load

frequency control. This is called Secondary regulation.

. Only some generating units in a power system may be used for secondary

regulation.

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3 TYPES OF GOVERNING

1. Nozzle governing

2. By pass governing

3. Throttle governing

WE are using throttle governing in HADUAGANJ power plant, because in

this method there are less losses than others.

Ejectors

Ejectors are used to create vaccum in turbine condensor. First starting

ejector sucks air faster but create less vaccum of 500 mmHgcl. Then main

ejector is used, it sucks slowly but creates more vaccumupto 700 mmHgcl.

Vaccum is created because steam will not change its phase into water even

at 450C.

Also work done P1-P2 becomes more since in vaccum this formula become

additive,

i.e. P1-(-P2) = P1+P2

Another important factor is Latent heat of steam is less in vaccum to change

from steam to water.

Gland water cooler valves are filled with water so air would not pass

through it. GWC is used to supply air when vaccum is not needed in

condenser.

EMERGENCY STOP VALVE

Two emergency stop valve are used with turbine. ESV(Left) and

ESV(Right)are placed before turbine. When the turbine is in shut down

condition these valves are closed. Steam will be passed to turbine only after

opening both ESV(L) and ESV(R).

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Turbine Efficiency

HE = Sensible heat + latent heat + super heat

EfficiencyRankine = Output/Input = ( HE-HF )/(HE-Ha)

Ha is the sensible heat of condensate water at point A.

HE= enthalpy of superheated steam at point E in kCal/kg of steam.

HF = enthalpy of exhaust steam from L.P. turbine in kCal/kg.

Mollier chart

HE = hc + L + Cp(TE-TD)

Boiler heat rate:- Heat rate is a parameter which monitors the

performance of boiler/turbine/generator.

LOSSES:-Lickages, Drain, P.R.D.S. drain, surface losses(not proper insulation),

condenser, Regenerative system.

From above it reveals that heat rate of boiler increased performance of turbine

detoriates.

Auxiliary P.R.D.S System

This system is situated in boiler side . A part of the main steam is taken to

auxiliary steam station and after reducing pressure and temperature. The steam

is utilised in the following auxiliaries. Initially when unit is lighted up the auxiliary

P.R.D.S steam is taken from the other running unit by opening the interconnecting

valve of auxiliary P.R.D.S.

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Utilization of auxiliary P.R.D.S. system:-

(i) Feed water heating in feed water storage tank.

(ii) Deaerator heating and pressurisation.

(iii) Main ejectors and starting ejectors.

(iv) Gland steam cooler.

(v) Turbine seals.

(vi) Soot blower.

(vii) F.O. atomisation at burner level.

(viii) Steam fracing lines of F.O. lines.

(ix) Fuel oil heating.

(x) Steam coiled air pre heater.

STEAM COIL AIR PREHEATER

The steam coil air heaters on each at the outlet of F.D. fans are

used to control the air preheaters cold and temperature of

corrosion control. Steam coil air heater must be brought into service after

starting of F.D. fan but before light up of boiler. This can be out only after

cutting off oil burners from boilers. Air temperature is to be maintained

below 60-65cby regulating steam inlet to SCAPH.

BOWL MILL

The pulverised fuel system adopted for 250MW boilers employs a

bowl mill by the direct firing system utilizing hot air as dring cum

transporting medium. A tangential fire system admits the

pulverised cool together with the air required to combustion

(secondary air) to the furnace.

Crushed coal is fed to each pulveriser by its feeder at a rate to suit

the local dense and primary air is supplied from the primary air

fans. A portion of the primary air preheated in the air heater. The

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hot and cold primary air is proportionally mixed to admission to

the pulveriser to provide the required drying as indicated by the

pressure outlet temperature. The total primary air flow may

constitute from approximately to the total unit combustionair and

controlled such as to maintain the velocities required to

transport the coal through the pulveriser and coal piping. Pyrite Hopper

excretes out the coal which is left unpulverised.

FUEL

1. Coal Bituminous Coal

2. Fixed carbon 36.5%

3. Volatile matter 25.5%

4. Moisture 10%

5. Ash 28%

6. Hydrogen 34%

7. Grindability 50 Hard Groove Index (HGI)

8. High heating valve 4750 kcal/kg

Bituminous Coal 36.5%

FURNACE

Furnace of boiler forms the enclosure for combustion of fuel.This is where the latent heat of vaporisation of water is added.

29

BOILER

Boiler is a steam generator which generates the steam at the desired

rate, pressure and temperature to run prime mover such as Steam

Turbine in power stations, by burning fuel in the furnace and

converting chemical energy into thermal energy.Boiler operation is a

balance between heat flow from combustion of fuel and steam

generation, in the furnace. It is the most expensive equipment and has

complicated design, requiring great skill to install and operate.Boiler

Tube Failure is the main cause of forced outage in electric utility

steam generating boilers in the whole world.

The sub critical boiler designed for HARDUAGANJ 250 MW units are of conventional, single drum, natural circulation,radiantdry bottom, balanced draft, reheat type.

Classification of Boiler

1) CIRCULATION TYPE:

a) Natural Circulation Boiler:-Boilers operating below the critical pressure. The natural circulation is based on thermo-siphon

principle.

In this system, the flow of water and the water/steam mixture through

the boiler circuit is produced naturally by the force of gravity due to

density difference. The water density in the down comers provides the

driving force that pushes up the less dense water/steam mixture in the

risers.

As the pressure increases, the difference in density between water

and steam reduces thus the hydrostatic head available will not be

30

able to overcome the frictional resistance for a flow corresponding to

the minimum requirement of cooling of water wall tubes, Therefore

natural circulation is limited to boiler with drum operating pressure .

Circulation Number: - It is the ratio of quantity of water-steam mixture

flowing through the circuit to the quantity of steam produced in the

circuit. In other words, if 10 kg of water is circulated for each kg of

steam generated, the circulation number is 10.Higher circulation

number reduces the chance of scale formation in evaporating

surface. At lower operating pressure, circulation ratio increases

mainly because of the increased difference in density between water

and steam.

The general practice adopted for circulation number in various boilers

is-

1. Natural circulation industrial boiler 10-15

2. Natural circulation Utility boiler 7-9

3. Assisted circulation boiler 2-4

4. Forced circulation/once through boiler 1

b) Assisted circulation:-Beyond 175 kg/cm2 this circulation is also termed as Positive circulation in this system circulating water pumps are employed for force movement of water through different circuit of boiler. Generally

positive circulation is adopted above 182.7 kg/cm2.

In this type of boilers, since the pressure is high the circulation head caused

by density difference is too low to cause effective circulation.

c)Forced Circulation Boiler:- Boilers Operating above critical pressure are

called super critical Boilers. Above critical pressure, phase transformation is

absent and hence a once through system is adopted. These boilers are also

known as once through boilers. A typical operating pressure for such a

system is 260 kg/cm2. This type of forced circulation requires no boiler drum.

31

OPERATING PRESSURE

a) Sub Critical Boiler:-Boilers operating below the critical pressure.

b) Super Critical Boiler:-Boilers Operating above critical pressure. These

boilers are necessarily once through type.

2) FUEL USED:-

a) Stoker Fired Boiler. b) Pulverised Fuel Fired Boiler. c) Fluidised Bed Combustion Boiler.

3) DRAUGHT SYSTEM:-

a) Natural Draught:- The draught required to flow air and gas inside the

boiler is created by chimney.

b) Mechanised Draught

4) Boiler Setting:-

a) Single Pass Boilers:-All the pressure parts such as super heater, re-heater

and economiser are arranged as horizontal coils in a single pass of flue gas

which is the vertical extension of the furnace. These boilers are also known

as TOWER TYPE BOILERS.

b) Multi Pass Boilers:-In these boilers, the flue gas passes through more than

one pass and the pressure parts are distributed in all the passes. Many of the

electric utility boilers are of two pass.

Boiler Pressure Parts

In a steam generator, the parts through which the feed water and steam flows

above atmospheric pressure are termed as BOILER PRESSURE PARTS. Most

of the heat released by the fuel in the Boiler is transferred to pressure parts.

32

The following parts of the Boiler are listed under the category of Boiler pressure

parts-

1- Economiser

2- Boiler Drum

3- Water wall system

4- Super-heater

5- Re-heater

1- Economiser:--It is situated in 2nd pass of Boiler. Economiser is provided in the Boiler to improve the efficiency of the Boiler by extracting heat from flue

gases. Before water entering into the evaporating surface of the Boiler, heat

is added, either in the form of sensible heat or in the form of sensible as

well as latent heat by the economiser.

2- Drum and Drum Internals :-Boiler drum is situated on the top of the boiler. The main function of boiler drum is to separate steam from steam-

water mixture and store the water for evaporator.

3- Water wall system:-The water walls are tube panels. In water walls, water circulates for steam generation. In the water wall, heat is added to

evaporate the feed water in the form of saturated steam. The furnace is built

up of water wall panels for burning the fuel. The water wall panels cover all

the four sides as well as floor with a opening on only one side for the gases

to go out of the furnace.

4- Super Heater:-It is situated in IInd pass of boiler. Super heaters are provided in the boiler to raise the steam temperature above the saturation

temperature by absorbing heat from flue gases.

Temperature Control Methods of Super heater

I. Desuperheating spray control.

II. Burner tilting. III. Gas bypass dampers. IV. Gas recirculation.

5. Reheater:- Development of large capacity steam turbines with more number of stages posed a problem of retaining the steam within vapour

phase till the last stage. In Reheater, the temperature of steam is raised

after a part of energy is extracted by the steam turbine. This is called as

reheating of the steam which increases the cycle efficiency.

33

34

Differences Super Critical Boiler (SBC) & Sub Critical Boiler :-

Super Critical Boiler (SBC) Sub Critical Boiler

1. Boiler operates above critical

pressure.

Boiler operates below critical

pressure.

2. Drum is not provided in this type

of boilers only separator is used.

Drum is essential this type of

boiler for steam separation and

recirculation Water through

evaporator.

3. There is no provision of CBD. Heat is lost through CBD.

4. MS Temp. around 600 0C. MS Temp. around 540 0C.

5. Single phase Boiler. Two phase Boiler (water in water

wall & steam in superheater).

6. Once through Boiler. Natural circulation Boiler.

7. Water wall tube temperature rises

during operation.

Water wall tube temperature

constant during whole operation.

8. Water wall is used to raise the

temperature of steam. Thermal

Water wall is used to evaporate

the water.

9. Thermal stress is developed in

water wall tube due to high

temperature difference.

Boiler operates at saturation

temperature in water wall tubes,

all the circuit is at the same fluid

temperature.

10. Spiral wall is used to increase the

mass flow per tube by reducing

the number of tubes required to

envelop the furnace.

Spiral wall is not used. Vertical

water wall is used only.

11. Every water wall tube is the part

of all four walls.

Every water wall tube is not the

part of all four walls.

35

12. Riffle tube is used in water wall. Plan tube is used in water wall.

13. Its BFP requires more power

rating due to restrict flow inside

riffle tubes.

Lesser power of BFP is required.

14. Boiler efficiency is more (around

93%).

Boiler efficiency is lower (around

86%).

15. Heat rate is lesser. Heat rate is more in comprised to

SCB.

16. Boiler is costly. Boiler costs lesser than SCB.

Boiler Efficiency

Boiler efficiency is defined as the heat added to the working fluid expressed as a

percentage of heat in the fuel being burnt. The theoretical limit to boiler efficiency

is 100%.

Boiler efficiency depends solely on the boilers ability to burn the fuel and transfer the resulting heat to the water and steam.

Method of Boiler Efficiency Calculation

A. Direct Method. B. Indirect Method or Loss Method.

A. Direct Method :- In this method, the heat supplied to boiler and heat added to the steam in the boiler in given time.

Boiler Efficiency = (Enthalpy of stm-Enthalpy of feed water)xstm flow

Qty of coal x C.V.

= Ws[Cp(T-To)+L+(To-t)

Mf x C.V.

36

Where, Ws = Steam flow rate.

Cp = Specific heat of steam.

T = S.H. steam temperature.

To = Saturated temperature of steam.

L = Latent heat of conversion

t = Feed water inlet temperature.

Mf = Fuel burning rate.

C.V. = Calorific value of fuel.

The trouble with this method is that several of these quantities are difficult to

measure particularly coal quantity, the steam quantity and C.V. of coal.

B. Indirect Method or Loss Method :- The Boiler efficiency is calculated by Heat loss method as -

BOILER EFFICIENCY = 100 VARIOUS HEAT LOSS

Boiler Losses :-

1. Dry flue gas loss.

2. Wet flue gas loss (moisture in fuel & H2 in fuel).

3. Moisture in combustion air (in vapour form).

4. Un-burnt carbon loss (carbon in ash).

5. Un-burnt gas loss (incomplete combustion of carbon).

6. Radiation and unaccounted losses.

High Speed Diesel (Light Oil)

Light oil required for use with eddy plate oil side ignitors and warm-up oil guns.

Warming up guns are used during star up the boiler from cold condition when

steam is not available for heating heavy fuel oil. Worm up oil guns are in service

till the pressure is raised sufficiently to charge the auxiliary PRDS (Pressure

Reducing & De-superheating Station), when steam will be available for fuel oil

heating, tracing and fuel oil atomization.

Light oil has the advantage of being low viscous at ambient temperatures,

requires no heating and with low sulphur content. It has negligible carbon reside

so leaves no soot deposits on the colder heat transfer surfaces.

37

Ignitors

Application Star up and coal flame stabilisation.

No. Of 8 (4 per elevation)

Capacity 1.5 million cal/hr (6 million BTU/hr)

Oil firing rate 150 Kg/hr/ignitor.

Oil pressure at the control

cabinet

12.5 to 14 Kg/Cm2g.

Atomizing air quality L.O.

(light oil)

Service air, water free.

Heavy Fuel Oil System

In coal fired boiler oil firing is adopted for the purpose of warming up of the

boiler, imparting stability to the coal flame and low load operation. Efficient or

complete combustion of fuel oil is best achieved by finer atomizing of oil and

proper turbulent mixing with combustion air, supplying oil at appropriate

pressure and temperature are essential requirements for better atomization and

pumps and heaters are the main auxiliaries for the oil system.

Heavy fuel oil from storage tank enters the H.O. preheater where it is heated to

about 115 0C corresponding to an atomizing viscosity. The temperature of H.O.

from heatingunit is maintained at constant temperature by the automatic

temperature regulating valve, mounted on auxiliary steam supply line to heaters.

From the oil preheater oil is left to the boiler through oil delivery line and then

to each burner. Fuel oil from the oil preheater can be recirculated to return oil

line from near the boiler front oil heats up the oil lines. At the end of oil supply

line to burners, a recirculation line with valve for recirculation of fuel oil to the

return oil line is provided. Fuel oil of 115 0C temperature at boiler front and

burners can be obtained by recirculation valve. When oil is admitted to each oil

burner then the recirculation valve is closed. The recirculation valve after last

burner can be kept slightly open to circulate a small quantity of hot fuel oil to

prevent solidification of fuel oil at dead ends and to ensure uniform temperature

of fuel oil in piping. All heavy oil lines are steam traced with steam line to

38

prevent loss of heat and eventual solidification of heavy oil and to maintain the

temperature of heavy oil at the required value.

Technical Data

Heay Fuel Oil -

Used for Stabilising coal flame stabilisation

and low load carrying

Relevant standard I.S. 1593, 1960.

Flash point 66 0C.

Kinematic viscosity at 50 0C

max.

120 centistokes.

Sulphur, total by weight max. 4%

API gravity 12, SP gravity, 0.986 at 15 0C

Gross heating value 10270 Kcal/Kg.

Heay Fuel Oil Guns -

Application Stabilising coal flame at low load carrying

hot start up.

Type Tilting tangential steam atomized, internal

mixing

Capacity 25% of boiler MCR per elevation max. 25%

boiler MCR total in one or two elevations.

Oil firing rate 33380 kg/hr/gun

Oil pressure at the gun 8.5 Kg/Cm2g

Oil viscosity at the atomizing steam

gun quantity

16 to 18 centistokessuper heated by 10 15 0C

Steam consumption 310 kg/hr/gun during normal operation and

490 kg/hr/gun during scavenging.

Steam pressure at the gun 9.5 kg/cm(guage).

Differential pressure between oil

and steam

15 psi (1 kg/cm2) steam pressure to be more

than oil pressure.

Atomizer nozzle assembly designation mixing plate NB SH spray plate 60 NB.

39

Ignito Air Fan

The ignitor air system provides required air to the ignitors at all boiler

loads. Ignitors require approximately 350scfm of air per ignitor and

this air is ensured by keeping a 3 differential across the ignitor horn.

During the boiler start up condition this pressure will not be available

from the FD fan. For this purpose, system has been provided with

booster fans called ignitor fan. The ignitor air system is provided with

two fans in which one is working and the other is standby.

Air Preheater

Air preheater is an important boiler auxiliary, which primarily preheats the

combustion air for rapid and efficient combustion in the furnace. The air heater

recovers the waste heat from the outgoing flue gases of the boiler and transfers

the same to the combustion air. The flue gas temperature is 380 0C after leaving

the economizer, every 20 0C drop in flue gas temperature improves the boiler

efficiency about 1%.

Advantage ofAir Preheater:-

1. Boiler efficiency is increased. 2. More stable combustion in furnace. 3. The combustion is intensified with the use of hot air. 4. Lower grade coal can be burnt efficiently with hot air. 5. The use of hot air improves heat transfer rate and so less heat transfer area is

required.

6. Faster load variations are possible. 7. The coal is dried effectively for easy pulverisation and combustion.

40

Cooling Tower

Service water is cooled in cooling tower which is used to cool bearings of

motor/generator etc.

3 cooling tower pumps are present.

2 in service and 1 in reserve

The cooling tower is a semi-enclosed device for evaporative cooling of water by

contact with air. The hot water coming out from the condenser is fed to the tower

on the top and allowed to tickle in form of thin sheets or drops. The air flows

from bottom of the tower or perpendicular to the direction of water flow and then

exhausts to the atmosphere after effective cooling.

Cooling tower fan- maintain temperature of water upto 180C. No of fans varies

with season, maynot needed to run in winter.

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. ESP has 4 paths and each path contain 6 field i.e. total of

24 fields. The dust particles are ionized and attracted by charged electrodes. The

electrodes are maintained at 30KV.Hammering is done to the plates so that fly ash

comes down and collect at the bottom which is then forwarded through pipe with the help of compressed air. The fly ash is dry form is used in cement manufacture.

41

Ash slurry pump

ASP drags fly ash with pipes to the other side of mountain by the force of

compressed air where ash is collected in bunkers. These are then supplied to the

Jaypee Cement Factory.

If ash precipitates in the pipe, water is supplied in the pipes which flows the ash

with them.

We use 3 ASP in series of slurry tube. So according to slurry load and pressure

suitable number of ASP is used.

42

CHIMNEY

A chimney is a structure for venting hot flue gases or smoke from a boiler, stove,

furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or

as near as possible to vertical, to ensure that the gases flow smoothly, drawing air

into the combustion in what is known as the stack, or chimney, effect. The space inside a chimney is called a flue.

The height of chimneys plays a role in their ability to transfer flue gases using

stack effect, the dispersion of pollutants at higher altitude helps to ease down its

influence on surroundings. In the case of chemically aggressive output, the tall

chimney allows partial or complete self-neutralization of chemicals in the air

before they reach the ground. The dispersion of pollutants over greater area reduces their concentrations in compliance with regulatory limits.

Height of chimney is made according to the flue gases venting by it. If the gases are more harmful then the height of chimney is kept more.

43

250 MW TURBO GENERATOR:-

DESIGN AND CONSTRUCTIONAL FEATURES OF 250MW TURBO

GENERATOR

2.1General-The 250MW Turbo-generator incorporates the modern features of

direct cooling water and H2 and fast acting excitation system.

2.2Stator-The stator body is totally enclosed gas tight fabricated structure.

Hydrogen gas coolers are housed longitudinally inside the stator body. Stator core

is made up of segmental varnish insulated punching of cold rolled grain oriented

silicon steel assembled in an inter-leaved manner on core bars. The core consists

of several packets separated by steel spacers for radial cooling of the core by

hydrogen and is held in pressed condition by means of heavy non magnetic steel

press rings bolted to the ends of cold bars. The core bars are designed to provide

44

elastic suspension of the core in the stator body to isolate the magnetic vibrations

of stator core from foundation of generator.

Stator has a three phase double layer short chordate type windings having two

parallel paths. Each coil side consist of glass insulated soiled and hollow

conductors with cooling water passing through the later. The elementary

conductors are rebel transposed in the slot portion of winding to minimise eddy

losses. The overhang portion of the coils is securely lashed with glass chord to

bandage rings and special brackets of non magnetic steel which are fixed to core

press rings.

Ring type distillate headers of copper supported on insulators are provided

separately for distillate inlet and outlet in the stator on turbine side .The winding

ends are solidly soldered into the lugs. The three phase terminals and six neutral

terminals are brought out to facilitate external connections .

2.3Rotor-The cylindrical type rotor is forged in one piece (shaft and body) from

chromium nickel molybdenum steel. The rotor(field) windings is made from hard

drawn silver bearing copper and is held in position against centrifugal forces by

duralumin wedges in slot portion and by non magnetic steel regaining rings in the

over hangs portion.

The cantering rings are mounted at the end of retaining rings support and prevent

movement of rotor winding in axial direction due to thermal stresses.

2.4Hydrogen gas coolers-Four numbers gas coolers are mounted longitudinally

inside the generator stator body . The gas coolers consists of longitudinally placed

cooling tubes made out of admiralty brass with coiled copper wire wound outside

for increasing the cooling surface area. The cooling water flows through the tube

while the hydrogen comes into contact with external surface of cooling tubes.

Vent pipes are provided on the slip ring side to remove air from gas coolers while

filling them with water.

2.5 Ferquency variation The generator can be operated continuously at rated

output with a frequency variation (+_) 5% over the rated value. However the

45

performance of the generator with frequency variation is limited by the turbine

capability.

2.6 Temperature of the coolants Not more than 580c and not less than 200c

temperature of cold gas.

2.7 Overloading

2.8 Operation under unbalanced load

2.9 A-synchronous operation

3.0 Motoring action Motoring of the turbo-generator is allowed withih the limit

a ions turbine.

3.1Operation at reduced hydrogen pressure Continuous operation of the

generator with hydrogen pressure inside the stator body lower than the rated

value is not permitted.

4. STATIC EXCITATION SYSTEM OF 250 MW:-

Static excitation system has been used in 250 MW unit in order to furnish a

significant improvement in the overall system stability .This system controls the

generator voltage by direct variation of the excitation current. Instead of

conventional rotating converters,the system utilizes static silicon converters

(thermistors) which are highly efficient and can operate in wide temperature

range. Due to following advantages .this system has replaced the older ones and is

being widely used these days.

(i) Faster response time. (ii) Higher reliability. (iii) Main free performance.

(iv) no influence due to short circuit in the system.

46

Hydrogen cooling system:-

A hydrogen-cooled turbo generator is a turbo generator with gaseous hydrogen

as a coolant. Hydrogen-cooled turbo generators are designed to provide a low-

drag atmosphere and cooling for single-shaft and combined-cycle applications in

combination with steam turbines. Because of the high thermal conductivity and

other favourable properties of hydrogen gas this is the most common type in its field today.

The use of gaseous hydrogen as a coolant is based on its properties, namely low

density, high specific heat, and highest thermal conductivity at 0.168 W/(m.K) of

all gases; it is 7-10 times better coolant than air. Another advantage of hydrogen is

its easy detection by hydrogen sensors. A hydrogen-cooled generator can be

significantly smaller, and therefore less expensive, than an air-cooled one. For stator cooling, water can be used.

ALTERNATING CURRENT MOTORS

SERIES AC MOTOR A series ac motor is the same electrically as a dc series motor. Refer to figure 4-1

and use the left hand rule for the polarity of coils. You can see that the

instantaneous magnetic polarities of the armature and field oppose each other and

motor action results. Now, reverse the current by reversing the polarity of the

input. Note that the field magnetic polarity still opposes the armature magnetic

polarity. This is because the reversal effects of both the armature and the field. The

ac input causes these reversals to take place continuously. The construction of the

ac series motor differs slightly from the dc series motor. Special metals,

laminations, and windings are used. They reduce losses caused by eddy currents,

hysteresis, and high reactance. Dc power can be used to drive an ac series motor

efficiently, but the opposite is not true. The characteristics of a series ac motor are

similar to those of a series dc motor. It is a varying-speed machine. It has low

speeds for large loads and high speeds for light loads. The starting torque is very

high. Series motors are used for driving fans, electric drills, and other small

appliances. Since the series ac motor has the same general characteristics as the

series dc motor, a series motor has been designed that can operate both on ac and

dc. This ac/dc motor is called a universal motor. It finds wide use in small electric

47

appliances. Universal motors operate at lower efficiency than either the ac or dc

series motor.

They are built in small sizes only. Universal motors do not operate on poly phase

ac power.

SYNCHRONOUS MOTORS The construction of the synchronous motors is essentially the same as the

construction of the salient pole alternator. In fact, such an alternator may be run as

an ac motor. It is similar to the drawing in figure.Synchronous motors have the

characteristic of constant speed between no load and full load. They are capable of

correcting the low power factor of an inductive load when they are operated under

certain conditions. They are often used to drive dc generators. Synchronous motors

are designed in sizes up to thousands of horsepower. They may be designed as

either single-phase or multiphase machines. The discussion that follows is based

on a three-phase design.

Figure 4-6.Revolving-field synchronous motor.

Figure 4-7.Self-starting synchronous ac motor.

48

To start a practical synchronous motor, the stator is energized, but the dc supply to

the rotor field is not energized. The squirrel-cage windings bring the rotor to near

synchronous speed. At that point, the dc field is energized. This locks the rotor in

step with the rotating stator field. Full torque is developed, and the load is driven.

A mechanical switching device that operates on centrifugal force is often used to

apply dc to the rotor as synchronous speed is reached.

The practical synchronous motor has the disadvantage of requiring a dc exciter

voltage for the rotor. This voltage may be obtained either externally or internally,

depending on the design of the motor.

SYNCHRONOUS SPEED is the speed of stator field rotation. It is determined by

the number of poles and the frequency of the input voltage. Thus, for a given

motor, synchronous speed is constant.

SLIP is the difference between actual rotor speed and the synchronous speed in

induction motors. Slip must exist for there to be torque at the rotor shaft.

INDUCTION MOTORS

The induction motor is the most commonly used type of ac motor. Its simple,

rugged construction costs relatively little to manufacture. The induction motor has

a rotor that is not connected to an external source of voltage. The induction motor

derives its name from the fact that ac voltages are induced in the rotor circuit by

the rotating magnetic field of the stator. The stator construction of the three-phase

induction motor and the three-phase synchronous motor are almost identical.

However, their rotors are completely different (see fig. 4-8). The induction rotor is

made of a laminated cylinder with slots in its surface. The windings in these slots

are one of two types (shown in fig. 4-9). The most common is the squirrel-cage

winding. This entire winding is made up of heavy copper bars connected together

at each end by a metal ring made of copper or brass. No insulation is required

between the core and the bars. This is because of the very low voltages generated

in the rotor bars. The other type of winding contains actual coils placed in the rotor

slots. The rotor is then called a wound rotor.

49

Figure 4-8.Induction motor.

Figure 4-9.Types of ac induction motor rotors.

The rotating magnetic field generated in the stator induces a magnetic field in the

rotor. The two fields interact and cause the rotor to turn. To obtain maximum

interaction between the fields, the air gap between the rotor and stator is very

small.Lenzs law states that any induced emf tries to oppose the changing field that induces it.

In the case of an induction motor, the changing field is the motion of the resultant

stator field. A force is exerted on the rotor by the induced emf and the resultant

magnetic field. This force tends to cancel the relative motion between the rotor and

the stator field. The rotor, as a result, moves in the same direction as the rotating

stator field.

50

It is impossible for the rotor of an induction motor to turn at the same speed as the

rotating magnetic field. If the speeds were the same, there would be no relative

motion between the stator and rotor fields; without relative motion there would be

no induced voltage in the rotor. In order for relative motion to exist between the

two, the rotor must rotate at a speed slower than that of the rotating magnetic field.

The difference between the speed of the rotating stator field and the rotor speed is

called slip. The smaller the slip, the closer the rotor speed approaches the stator

field speed.

TRANSFORMER

A transformer is a static device that transfers electrical energy from one circuit to

another through inductively coupled conductorsthe transformer's coils without changing its frequency and power on the principle of induction. A varying current

in the first or primary winding creates a varying magnetic flux in the transformer's

core and thus a varying magnetic field through the secondary winding. This

varying magnetic field induces a varying electromotive force (EMF), or "voltage",

in the secondary winding. This effect is called inductive coupling.

If a load is connected to the secondary, current will flow in the secondary winding,

and electrical energy will be transferred from the primary circuit through the

transformer to the load. In an ideal transformer, the induced voltage in the

secondary winding (Vs) is in proportion to the primary voltage (Vp) and is given by

the ratio of the number of turns in the secondary (Ns) to the number of turns in the

primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus enables an

alternating current (AC) voltage to be "stepped up" by making Ns greater than Np,

or "stepped down" by making Ns less than Np. The windings are coils wound

around a ferromagnetic core, air-core transformers being a notable

exception.Transformers are essential for high-voltage electric power transmission,

which makes long-distance transmission economically practical.

51

Basic principles

An ideal transformer. The secondary current arises from the action of the

secondary EMF on the (not shown) load impedance. The transformer is based on

two principles: first, that an electric current can produce a magnetic field

(electromagnetism) and second that a changing magnetic field within a coil of wire

induces a voltage across the ends of the coil (electromagnetic induction). Changing

the current in the primary coil changes the magnetic flux that is developed. The

changing magnetic flux induces a voltage in the secondary coil.

Autotransformer

In an autotransformer portions of the same winding act as both the primary and

secondary. The winding has at least three taps where electrical connections are

made. An autotransformer can be smaller, lighter and cheaper than a standard dual-

winding transformer however the autotransformer does not provide electrical

isolation.

Autotransformers are often used to step up or down between voltages in the 110-

117-120 volt range and voltages in the 220-230-240 volt range, e.g., to output

either 110 or 120V (with taps) from 230V input, allowing equipment from a 100 or

120V region to be used in a 230V region.

52

A variable autotransformer is made by exposing part of the winding coils and

making the secondary connection through a sliding brush, giving a variable turns

ratio. Such a device is often referred to by the trademark name Variac.

Instrument transformers

Instrument transformers are used for measuring voltage and current in electrical

power systems, and for power system protection and control. Where a voltage or

current is too large to be conveniently used by an instrument, it can be scaled down

to a standardized low value. Instrument transformers isolate measurement,

protection and control circuitry from the high currents or voltages present on the

circuits being measured or controlled.

Current transformers, designed for placing around conductors

A current transformer is a transformer designed to provide a current in its

secondary coil proportional to the current flowing in its primary coil.

Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are

designed to have an accurately known transformation ratio in both magnitude and

phase, over a range of measuring circuit impedances. A voltage transformer is

intended to present a negligible load to the supply being measured. The low

secondary voltage allows protective relay equipment and measuring instruments to

be operated at a lower voltages.

53

Different Transformers in Power Station

Generating Transformer

Generating Transformer:

This is the main transformer of generating unit used for stepping up the

voltage from generating station for the transmission

In a generating plant for every generating unit one generating transformer is

required

Rated voltage on LV side corresponds to the rated generating voltage

Rated voltage on the HV side corresponds to rated voltage of the HV bus

Usually these transformers are outdoor type

LV terminals are connected to the generating terminals via isolated phase

bus systems

HV terminals are connected to the outdoor busbars by flexible ACSR

conductors via overhead flexible bus

Generator produces electricity which is stepped up to 400 kv before transmission.

Generating transformer steps up the voltage and decrease the current to reduce i2r

losses while transmitting supply over a large distance. Also along with losses

heavy conductors will be needed tocarry heavy current and to support these heavy

cables heavy towers will beneeded which will increase the overall cost.

So to minimise the losses and cost during transmission of supply GT is used.

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Lightning Arrester: Used for protection of transformer, if lightning falls from sky it will ground the flashover to earth.

PRV: If internal sparking caused inside GT, gases will expand and increase the pressure inside the transformer. The Pressure Regulating Valvewill operate and

reduces the pressure.

Unit auxiliary Transformer

The Purpose of Unit auxiliary Transformer is to feed power to generator

auxiliaries of that unit

These transformers are connected to generators and are used as stepping

down transformers. The HV side transformer voltage corresponds to the

voltage of the generating unit and the LV side voltage is stepped down

Rated KVA of Unit Auxiliary Transformers is approximately 15% of the

generating rating

Usually these transformers are outdoor transformers

One Unit auxiliary transformer is present for every generating unit.

Station Service Transformer

In general station service transformer is used for supplying power to

auxiliary equipment in the power plant when the plant is not generating any

power.

Rated HV voltage corresponds to the rated voltage of the outer busbars

Rated LV voltage corresponds to the auxiliary bus voltage

Rated KVA corresponds to the load of common auxiliaries of the station.

This corresponds to the 10% to 15% of the rating of the generating power.

These transformers are Outdoor type.

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Auxiliary Transformers

These transformers are located in power plant to step down voltage from

6.6KV to 415V.

The rating for this transformer corresponds to the rating of the auxiliary load

it should be bearing.

These transformers are indoor type and usually dry type transformers are

used.

INTERCONNECTING TRANSFORMER

ICT interconnect ATPS and BTPS switchyard. 220v supply bus from

ATPS is merged with 440v supply bus from BTPS. And another 6.6kv

line is taken from ICT to feed auxiliaries. If ATPS generator stop

working then also some emergency auxiliaries will be working. To feed

them, supply is taken from BTPS.Both terminals act as input. Power can

be transmitted from anyside.

i.e. ATPS to BTPS

or BTPS to ATPS

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Transformer Cooling Methods

Losses in the transformer are of the order of 1% of its full load kW rating. These

losses get converted in the heat thereby the temperature of the windings, core, oil

and the tank rises. The heat is dissipated from the transformer tank and the radiator

in to the atmosphere. Transformer cooling helps in maintaining the temperature

rise of various parts within permissible limits. In case of Transformer, Cooling is

provided by the circulation of the oil. Transformer Oil acts as both insulating

material and also cooling medium in the transformer. For small rating transformers

heat is removed from the transformer by natural thermal convection. For large

rating transformers this type of cooling is not sufficient, for such applications

forced cooling is used.

As size and rating of the transformer increases, the losses increase at a faster rate.

So oil is circulated in the transformer by means of oil pumps. Within the tank the

oil is made to flow through the space between the coils of the windings.

Several different combination of natural, forced, air, oil transformer cooling

methods are available. The choice of picking the right type of transformer cooling

method for particular appilcatiion depends on the factors such as rating, size, and

location.

Transformer Cooling Methods

Different Transformer Cooling methods are:

Air Cooling For Dry Type Transformers:

Air natural Type (A.N.)

Air Forced type (A.F.)

Cooling For Oil Immersed Transformers:

Oil Natural Air Natural Type (O.N.A.N.)

Oil Natural Air Forced Type (O.N.A.F.)

Oil Forced Air Natural Type (O.F.A.N.)

Oil Forced Air Forced Type (O.F.A.F.)

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Oil immersed Water Cooling:

Oil Natural Water Forced (O.N.W.F.)

Oil Forced Water Forced (O.F.W.F.)

Oil natural Air Forced Transformer Cooling:

In this method, air fans are mounted near the Transformer and the forced air is

directed on to the cooling tubes to increase the rate of cooling. The fans are

provided with automatic starting. When the temperature of the oil and windings

increases above a permissible value the thermostats switch on cooling fans. This

happens during heavy load condition and during higher ambient temperatures.

In higher rating transformers where the heat dissipation is difficult this type of

cooling is used. Fans are used to forced and air blast on radiators. Forced air

cooling increases the heat dissipation rate. In this type of cooling oil circulates by

natural convection and the blast of air is directed towards the better heat dissipation

rate.

Oil Natural and Air Forced Transformer Cooling

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Oil Forced Air Forced Transformer Cooling:

Transformers above 60 MVA employ a combination of Forced Oil and Forced Air

Cooling. Oil Natural Air Forced type of cooling is not adequate to remove the heat

caused by the losses which is approximately equal to 1% of the transformer rating

(0.6MW). In case of Forced Oil and Forced air cooling system a separate cooler is

mounted away from the transformer tank. This cooler is connected to the

transformer with pipes at the bottom and the top. The oil is circulated from the

transformer to the cooler through the pump. The cooler is provided with the fans

which blast air on the cooling tubes. This type of cooling is provided for the higher

rating transformers available at the Substations and Power Stations.

Oil Forced Air Forced Transformer Cooling

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59

SWITCHYARD & ITS EQUIPMENT

INTRODUCTION:-

The switchyard is a junction connecting the transmission &

Distribution system to the power plant.

Switchyard consist the air insulated aluminium bus type and of the

high voltage SF6 insulated dead tank circuit breakers arranged in a

ring bus configuration.

Each circuit breaker are equipped with a no load breaker, air insulated,

disconnect switch on each side.

An isolating disconnect switch are installed in each generator

transformer connection to the bus.

In switchyard a power transformer is used to step up or step down the

voltage.

Current and voltage transformer are located at points within the

switchyard to provide for the metering and relaying.

Control protection and monitoring for the switchyard will be located

in the switchyard relay room of the electrical building.

All protection and circuit breaker control will be powered from the

station battery tn 220v D.C system.

A grounding grid is provided to control step and touch potentials.

Lightning protection are provide by shield wires for any overhead

lines lightning arrestors.

Interface with SCADA system are provided the communication

between the facility switchyard and the control building is facilitated.

Revenue metering are provided on the outgoing lines recording net

power to or from the switchyard.

EQUIPMENTS USED IN SWITCHYARD

(1) BUS BAR

(2) INSULATOR

(3) LIGHTNING ARRESTOR

(4) EARTHING SYSTEM

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(5) CONTROL SYSTEM

(6) TRANSFORMER

(7) CIRCUIT BREAKRER

(8) ARCHING HORNS

(9) SCADA NODES

(10) ROW

TYPES OF BUS BAR

Rigid bus bars:-used for low,md,and high voltage.

Strain bus bar:-used for high voltages.

Insulated phase bus bars:-used for medium voltage.

Sulphuloride bus bar:-used for medium and high voltage system.

INSULATOR

Supported the poles and towers in such a way that currents from conductors do not

flow to earth through these supports.

CONTROL PANEL

Control S mostly consists of meters and protective relays. The meters include

ammeter, voltmeter, wattmeter, energy meter, etc. The relay include fuse failure

relay, auto reclose relay, check synchronising relay, auxiliary relay and transformer

relays like OLTC out of step winding temperature alarm, oil temperature alarm.

The trip indicators included are CB SF6 gas density low, CB Air pressure low, VT

fuse fail alarm, CB pole disc trip, carrier signal received back up protection, auto

reclose lock out control DC supply fails distance protection imperative carriers out

of service, distance protection trip etc.

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Circuit Breaker

A circuit breaker is an automatically operated electric switch designed to protect an

electrical circuit from damage cause by overload or short circuit. Its function is to

interrupt continuity to immediately discontinue electrical flaw.

SULPHUR HEXAFLUORIDE CIRCUIT BREAKERS

The SF6 is an electro-negative gas and has a strong tendency to absorb free

electrons .The contacts of the breakers are opened in a high pressure flow of SF6

gas and an arc is struck between them.The conducting free electrons in the arc are

rapidly captured by the gas to form relatively immobile ions.

The advantage of using SF6 circuit breakers :-

(i) Very short arcing time.

(ii) Can interrupt much larger currents.

(iii) Noiseless operation due to its closed gas circuit.

(iv) No moisture problem.

(v) Low maintenance cost.

(vi) No carbon deposits so that tracking and insulating problems are

eliminated.

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Air Blast Circuit Breaker Working

In the air blast circuit breakers the arc interruption takes place to direct a blast of

air, at high pressure and velocity, to the arc. Dry and fresh air of the air blast will

replace the ionized hot gases within the arc zone and the arc length is considerably

increased. Consequently the arc may be interrupted at the first natural current zero.

In air blast circuit breakers, the contacts are surrounded by compressed air. When

the contacts are opened the compressed air is released in forced blast through the

arc to the atmosphere extinguishing the arc in the process. A compressor plant is

necessary to maintain high air pressure in the receiver.

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In air blast circuit breaker high pressure air is forced on the arc through a nozzle at

the instant of contact separation. The ionized medium between the contacts is

blown away by the blast of the air. After the arc extinction the chamber is filled

with high pressure air, which prevents restrike. In some low capacity circuit

breakers, the isolator is an integral part of the circuit breaker. The circuit breaker

opens and immediately after that the isolator opens, to provide addition gap.

Advantages: How air blast circuit breaker is better than oil circuit breaker:-

1. The growth of dielectric strength is so rapid that final contact gap needed for arc

extinction is very small. this reduces the size of device.

2. The risk of fire is eliminated.

3. Due to lesser arc energy, air blast circuit breakers are very suitable for

conditions where frequent operation is required.

4. The arcing products are completely removed by the blast whereas the oil

deteriorates with successive operations; the expense of regular oil is replacement is

avoided.

5. The energy supplied for arc extinction is obtained from high pressure air and is

independent of the current to be interrupted.

6. The arcing time is very small due to the rapid build up of dielectric strength

between contacts. Therefore, the arc energy is only a fraction that in oil circuit

breakers, thus resulting in less burning of contacts.

Disadvantages: 1. Considerable maintenance is required for the compressor plant which supplies

the air blast.

2. Air blast circuit breakers are very sensitive to the variations in the rate of

restriking voltage.

3. Air blast circuit breakers are finding wide applications in high voltage

installations. Majority of circuit breakers for voltages beyond 110 kV are of this

type.

Aim of electric power supply

Supply of required amount of power to all consumers overthe entire geographical

area at all the time continuously.

Supply energy at lowest cost .

Maximum possible coverage of geographical area .

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Maximum security of supply and minimum fault duration.

Supply of the power with in the targeted limit of frequency(in case of ac supply).

Supply with in specified limits of voltages.

Why DC OVER A.C ?

Power transfer through an AC Transmission link is given by:-

Reactive power comes in play.

Power transfer through the A.C line cant be controlled easily, quickly and

accurately.

Losses are high as the reactive power is high .

Voltage drop will be more.

HVDC High voltage direct current(HVDC) bulk transmission of electrical power.

For long distance distribution HVDC system are less expensive and suffer lower

electrical losses.

For shorter distances, the higher cost of DC conversion equipment compared to an

AC system may be warranted where other benifits of direct current links are useful.

Economic consideration +ve to HVDC Lesser conductors are used as compare to AC

Lesser losser so better quality.

Long distance covered.

Simpler design of towers.

HV so more security.

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Advantages of coal based thermal Power Plant

They can respond to rapidly changing loads without difficulty A portion of the steam generated can be used as a process steam in

different industries Steam engines and turbines can work under 25 % of overload continuously Fuel used is cheaper Cheaper in production cost in comparison with that of diesel power stations

Disadvantages of coal based thermal Power Plant

Maintenance and operating costs are high Long time required for erection and putting into action A large quantity of water is required Great difficulty experienced in coal handling Presence of troubles due to smoke and heat in the plant Unavailability of good quality coal Maximum of heat energy lost Problem of ash removing

CONCLUSION

The electric power plant most importance role in daily life of common

people and industrial development of nation. Nations progress &

development is measured by per capita consumption of electricity. It

changes life style of the people. Thermal power plants supply around

75% of total power requirement of our country. Considering technical

feasibility and economical viability the higher capacity thermal power

plants are now being constructed with sophisticated and complex control

system.

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REFERENCE

Er. S.K.RANJAN (EE)

(EED-I, 2*250MW)

Er. PRADEEP KUMAR (AE)

(EED-I, 2*250MW)