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Vocational Training Report

of

Bakreswar Thermal Power Station

(5*210 MW)

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Vocational Training Report

Submitted By --

AVIJIT CHOWDHURY 3rd Year

Electrical Engineering Government College of Engineering & Textile

Technology, Berhampore

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ACKNOWLEDGEMENT

I, AVIJIT CHOWDHURY is a student of Government College of

Engineering and Textile Technology, Berhampore and I have completed

twenty-one days of vocational training in Bakreswar Thermal Power Station,

West Bengal Power Development Corporation Limited successfully and have

gathered detail regarding the mechanical and electrical aspect of the plant.

I therefore greatly acknowledge the valuable contributions and precise lectures

being arranged by the officials here. I pay my due respect to all those officials

who have helped me in implementing every minute detail in my project and

thus helping me a great deal in the successful completion of my project.

I pay my sincere regards to the respective DGM’s, HOD’s, Sr. Managers as well

as the operating personnel there.

I am also very thankful to my college for allowing and encouraging me to

complete the vocational training.

The sincere effort of everyone is worth noticeable.

Thank you.

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CONTENTS 1. Introduction ......................................................................................... 5 2. Technical Specification of Bakreswar Thermal Power Station................ 6 3. Overview of a Thermal power plant...................................................... 7 4. Mechanical operations

a. Coal Handling Plant........................................................................... 10 b. Raw Water System........................................................................... 12 c. De-Mineralization Plant.................................................................... 13 d. Boiler & Auxiliaries........................................................................... 14 e. ESP…………………….............................................................................. 17 f. Ash Handling Plant............................................................................ 18 g. Steam Turbine & Auxiliaries............................................................. 19 h. Cooling Water System...................................................................... 22 i. Chimney............................................................................................. 23

5. Electrical operations

a. Turbo Generator................................................................................. 24 b. Excitation System............................................................................... 27 c. Transformers....................................................................................... 29 d. Switch Yard......................................................................................... 30 e. Switch Gear……………………………………………………………………………………. 35 f. Protection System............................................................................... 36 g. Unit Auxiliary Power........................................................................... 40

h. DC Power System…………………………………………………………………………… 41 6. Pollution & Environment……………………………………………………………………… 42 7. Conclusion.............................................................................................. 47

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INTRODUCTION

Bakreswar Thermal Power Project, under The West Bengal Power

Development Corporation Limited, is one of the most reliable and prestigious

coal-fired power plants in West Bengal and in India as well. In two stages the

total capacity of the plant is (05 X 210) MW. Funded by the Overseas Economic

Co-Operation Fund(OECF) of Japan Govt. This Plant is situated in Birbhum

District just 10km away from Suri. The total power plant campus area is

surrounded by boundary walls and is basically divided into two major parts, first

the Power Plant area itself and the second is the Township area for the

residence and other facilities for Bk.T.P.S.’s employees.

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TECHNICAL SPECIFICATION

OF

BAKRESWAR THERMAR POWER STATION

INSTALLED CAPACITY: - 1) Total number of Units: - 5 X 210 MW each 2) Total Energy Generation: - 1050 MW 3) Source of Water: - Tilpara Barrage (on Mourakhi River), Bakreswar Dam (on Bakreswar River) 4) Sources of Coal: - Different Collieries situated in eastern India through Railway.

Stage Unit No. Capacity

(MW) Boiler

Manufacturer Turbine

Manufacturer

I 1 to 3 3 X 210 FUJI ELECTRIC BHEL

II 4 & 5 2 X 210 FUJI ELECTRIC BHEL

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OVERVIEW OF A THERMAL POWER PLANT

Simplified Diagram of a Power Plant:

The coal from mines is received at CHP (Coal Handling Plant) through

Railways. The unloaded coal (max. size 250 mm2) is scooped into conveyor and

is passed through suspended magnet, magnetic separators, and metal

detectors, to ensure that sized coal, free of foreign material is supplied. Then it

is sent to Crusher House for further crushing to 25 mm2 size. After crushing, the

coal again screened for elimination of extraneous materials, weighed and sent

to boiler bunkers. Excess coal, if any, is sent to coal yard for stacking. It then falls

through a weigher into the Bowl Mill where it is pulverized. The mill usually

consists of a round metal table on which large steel rollers or balls are

positioned. The table revolves, forcing the coal under the rollers or balls which

crush it.

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Air is drawn from the top of the boiler house by the FD Fan (Forced Draft

fan) and passed through the RAH (Regenerative Air Heater), and then send to

boiler for burning of coal. PA Fan (Primary Air fan) takes air from atmosphere

and distributes them into 2 parts one send to RAH for heating and other fed

directly to Mill blowing coal along pipes to boiler furnace.

The boiler consists of a large number of tubes extending the full height of

the structure and the heat produced raises the temperature of the water

circulating in them to form superheated steam which passes to the Boiler drum.

The steam is fed through the outlet valve to the HP Turbine (High Pressure

turbine) at around 540°C.After this, it is returned to the boiler and reheated

before being passed through the IP & LP Turbine (Intermediate and Low

Pressure Turbine). The water fed into boiler is DM water (De-Mineralized

water).

From the turbine the steam passes into Condenser to be turned back into

water. This is pumped through CEP (Condensate Extraction Pump) which sends

water through GSC (Gland Steam Cooler), LPH (Low Pressure Heater), and HPH

(High Pressure Heater) for further heating and BFP (Boiler Feed Pump) then

sends it to the Economizer where the temperature is raised sufficiently for the

condensate to be returned to the lower half of the steam drum of the boiler.

The flue gases produced in boiler are used to reheat the condensate in the

Economizer and then passes through the RAH to the ESP (Electrostatic

Precipitator) where ash is collected. Finally, they are drawn by the ID Fan

(Induced Draft fan) into the main flue and to the chimney.

From the boiler, a steam pipe conveys steam to the turbine through a stop

valve (which can be used to shut off steam in an emergency) and through

control valves that automatically regulate the supply of the steam to the

turbine. The turbine shaft usually rotates at 3000 RPM. This speed is

determined by the frequency of the electricity system and the number of poles

of machine (2- pole machine here).

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Cold water from OAC (Open Approach Channel) is circulated through the

condenser tubes and as the steam from the turbine passes round them it is

rapidly condensed into water. Water which gets heated up in condenser by

cooling steam is sent to Cooling tower and then left into OAC from where it can

be further used.

The electricity is produced in turbo generators and is fed through terminal

connections to Generator Transformer, those steps up the voltage to desired

level (400 KV, 220 KV, 33KV). From here conductors carry it to Switchyard from

where it is sent for use.

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MECHANICAL OPERATIONS

A. Coal Handling Plant (CHP)

FLOW CHART OF CHP SIDE

In Bk.T.P.S. coal comes through railway in BOBR (Bottom open box rake)

and NBOX type containers. BOBR type containers are easy to unload. Using

pressurised air, the bottom gates of BOBR containers are opened and the coal

then fell into the Track Hopper due to gravity. NBOX containers are bigger sized

container then BOBR but there is no gate available in the bottom of those

boxes. So this type of containers is rotated 155° with its track by Wagon Tippler.

Coal Mine RailwaysTrack Hopper / Wagon Tippler

Paddel FeederCrusher House

Stack YardCoal Bunker

Stack Yard

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Paddle Feeder (PF1/PF2, PF3/PF4) is used to feed the coal to conveyor belt

from Track Hopper. Then a network of Conveyor Belts (1A/1B, 2A/2B, 3A/3B)

carries the coal towards Crushers House. In each line two conveyor belts are

present (One Running, One Standby). If the amount of coal comes through rail is

excess, then some coal is stacked in uncrushed coal area.

In Bk.T.P.S. two Crushers House are present one for every stage. In each

Crushers House first the coal is drives towards Magnetic Separator which

separates metals from coal and send the coal to Reversible Belt Feeders (RBF1,

RBF2) [22KW each] drops the coal into Roller Screens (RS1, RS2, RS3) [30KW

each] separates the coal according to their size. Then bigger sized coal is sent to

Crushers to convert them into desired size (25 mm2). Then this coal is dropped

into Belt Feeders (BF1, BF2, BF3) [22KW each] which is now again return the

coal to another Conveyor Belt System (4A/4B upto 11A/11B) which leads the

coal to the Coal Bunker on the top of the Boiler or into the Stack Yard using

Stacker and Reclaimer.

This full system is controlled and monitored from CHP CONTROL ROOM.

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B. Raw Water System

The required water is taken from two places--

1) Tilpara Barrage- This Barrage is situated in Mourakhi River

approximately 12KM from BK.T.P.S. In a year 9 months the

plant full fills its water demand from Tilpara Intake Pump

House. It has 6 no. Intake Pump (each 1950 m3/hr capacity),

6 no. Dual Flow travelling water screen, 3 no. Screen Wash

Pump.

2) Bakreswar Dam- This water reservoir is specially made for this

plant on Bakreswar River approximately 6KM from BK.T.P.S.

Bakreswar Intake Pump House is used for 3 months in a year

to supply water in the plant. This pump house has 2 Intake

Pump (each 8000 m3/hr capacity).

The water comes from Intake Pump Houses are first stored into Raw Water

Pond which is situated near the plant. Here the dissolved solid in the water

precipitate due to gravity. If the TDS (TOTAL DISSOLVED SOLID) level is permit

able then it directly sends to Ash Plant or CHP otherwise Chlorine (Cl) is added

and Flux were added up and clarified the water. 65% of this clarified water is

now used in Cooling Tower Makeup Pump, Service Water Pump, Ash Makeup

Pump, etc. 35% of the clarified water is now filtrated using Gravity Sand Filter

Bed. After that this filtrated water is used in DM plant and Drinking Water for

plant and colony.

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C. De-mineralization Plant (DM Plant)

In De-mineralised Plant, the filter water of Water Treatment Plant is passed through the Pressure Sand Filter (PSF) to reduce turbidity and then through Activated Charcoal Filter (ACF) to adsorb the residual chlorine and iron in filter water. And then the water is passed through Cation and Anion Exchange Resin Beds which removes all unwanted ions from the water.

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D. Boiler & Auxiliaries

Boiler is device for generating steam for

power processing or heating purposes.

Boiler is designed to transmit heat from an

external combustion source contained

within the boiler itself.

The Boiler used in Bk.T.P.S. has

following characteristics:

1. The Boiler we have seen is basically a

Coal Fueled Boiler. But in time of light up

the Light Diesel Fuel is used for Ignition

purpose.

2. This Boiler is a Corner Fired Boiler that

means the coal intake nozzles are placed

to the Corner positions of the Furnace. We

have 6 no of Intake Positions in a Boiler.

3. This is a Water Tube Boiler that means the water and steam mixture is

flowing through the tubes and the Fire Ball is in the open place.

4. This Boiler is made to be used as a Dry Bottom Boiler which means the ash is

collected in a form of solid (from flue gas as well as from the bottom of the

boiler). Due to this the temperature inside the Furnace should be less than

1300°C otherwise the bottom ash should be converted into liquid.

5. The water steam mixture in the Boiler is Circulates Naturally due to pressure

difference.

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Boiler Accessories: -

Boiler Furnace: A boiler furnace is that space under or adjacent to a

boiler in which fuel is burned and from which the combustion products

pass into the boiler proper. It provides a chamber in which the combustion

reaction can be isolated and confined so that the reaction can be isolated

and confined so that the reaction remains a controlled force. It provides

support or enclosure for the firing equipment.

Boiler Drum: The function of steam drum is to separate the water

from the steam generated in the furnace walls and to reduce the resultant

solid contents of the steam to below the prescribed limit of 1ppm. The

drum is located on the upper front of the boiler.

Economizer: The purpose of the economizer is to preheat the boiler

feed water before it is introduced into the steel drum by recovering the

heat from the fuel gases leaving the boiler. The economizer in the boiler

rear gas passes below the rear horizontal super heater.

Super Heater: There are 3 stages of super heater besides the side

walls and extended side walls. The first stage consists of horizontal super

heater of convection mixed flow type with upper and lower banks located

above economizer assembly in the rear pass. The 2nd stage super heater

consists of pendant platen which is of radiant parallel flow type. The 3rd

stage super heater pendant spaced is of convection parallel flow type the

outlet temperature and pressure of the steam coming out form the super

heater is 540°C and 157 kg/cm2.

Preheater: The function of

preheater is to reheat the steam

coming out from high pressure

turbine to a temperature of 540°C.

Burners: there are total 24

pulverized coal burners for corner

fired C.E. type boilers and 12 oil

burners provided each in between 2

pulverized fuel burners.

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Induced draft fan (ID fan): Induced draft represents the system

where air or products of combustion are driven out after combustion at

boiler furnace by maintaining them at a progressively increasing sub

atmospheric pressure. This is achieved with the help of induced draft fan

and stack. Induced draft fan is forward curved centrifugal (radial) fan and

sucks the fly-ash laden gas of temperature around 125°C out of the

furnace to throw it into stack (chimney). The fan is connected with driving

motor through hydro-coupling or with variable frequency drive (VFD)

motor to keep desired fan speed.

Forced draft fan (FD fan): Forced draft represents flow of air or

products of combustion at a pressure above atmosphere. The air for

combustion is carried under forced draft conditions and the fan used for

this purpose is called Forced Draft (FD) fan. It is axial type fan and is used

to take air from atmosphere at ambient temperature to supply air for

combustion, which takes entry to boiler through wind box.

Primary air fan (PA fan) or Exhauster fan: The function of primary air

is to transport pulverized coal from coal mill to the furnace, to dry coal in

coal mill and also to attain requisite pulverized coal temperature for ready

combustion at furnace.

Coal mill or pulveriser: Most efficient way of utilizing coal for steam

generation is to burn it in pulverised form. The coal is pulverized in coal

mill or pulveriser to fineness such that 70-80% passes through a 200 mesh

sieve.

Fuel oil system: In a coal fired boiler, oil firing is adopted for the

purpose of warming up of the boiler or assisting initial ignition of coal

during introduction of coal mill or imparting stability to the coal flame

during low boiler load condition. Efficient or complete combustion of the

fuel oil is best achieved by atomizing oil by compressed air for light oil

(LDO).

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E. ESP (Electro-Static Precipitator)

It is a device which captures the dust particles from the flue gas thereby

reducing the chimney emission. Precipitators function by electrostatically

charging the dust particles in the gas stream. The charged particles are then

attracted to and deposited on plates or other collection devices. When enough

dust has accumulated, the collectors are shaken to dislodge the dust, causing it

to fall with the force of gravity to hoppers below. The dust is then removed by a

conveyor system for disposal or recycling.

Electrostatic precipitation removes particles from the exhaust gas stream

of Boiler combustion process. Six activities typically take place:

Ionization - Charging of particles

Migration - Transporting the charged particles to the collecting surfaces

Collection - Precipitation of the charged particles onto the collecting

surfaces

Charge Dissipation - Neutralizing the charged particles on the collecting

surfaces

Particle Dislodging - Removing the particles from the collecting surface

to the hopper

Particle Removal - Conveying the particles from the hopper to a

disposal point

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F. Ash Handling Plant

A large quantity of ash is, produced in steam power plants using coal.

Ash produced in about 10 to 20% of the total coal burnt in the furnace. Handling

of ash is a problem because ash coming out of the furnace is too hot, it is dusty

and irritating to handle and is accompanied by some poisonous gases. First this

ash is collected from ESP (Fly Ash, 80% of total Ash) and from the Bottom of

the Boiler (Bottom Ash, 20% of total Ash).

ASP processed the ash in two different ways.

1) It mixes the Ash with water and then send this mixture is sent to

Ash Pond which is situated 5Km away from the plant. Where due

to gravity the Ash Particles Precipitate due to gravity and the

clean surface water of the pond is taken back for reuse.

2) In the other way it sends the dry ash to the Ash Loading Plant

which loads the ash into Container Trucks and send them to

different industries like cement company, ash brick company etc.

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G. Steam Turbine & Auxiliaries

A Steam Turbine is a mechanical device that extracts thermal energy from

pressurized steam, and converts it into useful mechanical work.

In Bk.T.P.S. 210MW FUJI MAKE THREE CASING TANDEM COMPOUNDING

REHEAT REGENERATIVE CONDENSING REACTION TYPE TURBINE is used in each

unit as a prime mover of the alternator.

This turbine has 3 sections.

1) High Pressure Turbine (HPT) it has 23 stages.

2) Intermediate Pressure Turbine (IPT) it has 17 stages.

3) Low Pressure Turbine (LPT) it has 8 stages in each side.

Three turbines, HP turbine, IP turbine and LP turbine, are synchronized

together with a common shaft connected to the generator. HP and IP turbines

have single flow unit while LP turbine has double flow unit so as to

accommodate the increase in volume of steam due to the drop in pressure.

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First the steam enters into HP Turbine from the Boiler and after work

done the pressure and the temperature of the steam decreases. So, after HP

Turbine the steam goes to Reheater which is situated in the top of the Furnace.

From Reheater the steam directly enters into IP Turbine. After IP Turbine the

steam directly went to LP Turbine.

Power output is proportional to the steam pressure drop in the turbine In

each section of Turbine some extraction points are present. The goes to

different types of Heating Section.

Turbine Accessories: -

Regenerative Heaters- (a) The Regenerative Heaters are used to heat the Condensate water from the condenser to the boiler inlet. (b) This makes the mean temperature of heat addition in boiler high resulting in High Efficiency. (c) The heating is done by steam bled from different stages of HP, IP and LP turbines. (d) The heaters are non-mixing type.

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(e) The drip formed is cascaded to lower heaters in the line and finally to Deaerator (for HP heaters) and Condenser (for LP heaters). (f) The heaters before BFP are called Low Pressure Heaters (LPH) and those after BFP are called High pressure heaters (HPH). (g) The Deaerator is the only mixing type heater in the power plant. This is used to separate the Dissolved Oxygen(O2) from water. (h) In some design drip pumps are used to pump the drip from LPHs to the main condensate line.

Pumps-

(a) BFP (Boiler Feed Pump) – 4MW, 400 M3/hr

(b) CEP (Condenser Extraction Pump) - 500 M3/hr

(c) CW & ACW Pumps

Turbine Gland Seal- Steam is used to seal the turbines so that the inner

steam should not come out as well as the outer air should not enter into the

turbine.

Condenser- Condenser is the place where the steam comes after the LP

Turbine and exchange its heat with water and itself change own phase and turn

into water. This water then stored into the Hotwell situated under the

Condenser.

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H. Cooling Water System

Cooling towers cool the warm water discharged from the condenser and

feed the cooled water back to the condenser. They thus reduce the cooling

water demand in the power plants.

In Bk.T.P.S. Induced Draft Cooling Towers are used. In this type of Cooling

Towers has one Induced Draft Fan is placed in the top of the tower which sucks

the air Horizontally through the Vertically falling Hot Water. In this time the

water releases its heat to the atmosphere and cool down.

ID FAN of a Cooling Tower

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I. Chimney

A Chimneys may be considered as a cylindrical hollow tower made of

bricks or steel. In Bk.T.P.S. the chimneys of five units are made of bricks.

Chimneys are used to release the exhaust gases (coming from the

furnace of the boiler) high up in the atmosphere. So, the height of the

chimneys is made high. In Bk.T.P.S. the height of the chimneys is

approximately 220m.

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ELECTRICAL OPERATIONS

A. TURBO GENERATOR

This Turbo Generator is basically a Synchronous Alternator. The stator

houses the Armature Windings and the rotor houses the Field Windings. DC

voltage is applied to the Field Windings through the Brushless Excitation

System. When the rotor is rotated the lines of Magnetic Flux cut through the

Stator Windings. This induces an e.m.f. in the Stator Windings.

The rotor is rotated at 3000 rpm by the Turbines to generate electricity at

a frequency of 50 Hz. Because this is a 2 pole machine. Due to this huge speed

the “TURBO” term is used.

Generator Components:

1. Rotor: The rotor is a cast steel ingot and it is further forged and

machined. The rotor is to be designed very accurately as it has to work on

speeds such as 3000 rpm. Also a fairly high current is to be carried by the rotor

windings to generate the necessary magnetic field.

2. 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. When rotating at high speeds

centrifugal force tries to lift the windings out of the slots, so they are screwed to

the rotor body. The two ends of the windings are connected to slip rings, usually

made of forged steel.

3. Stator core: The stator is the heaviest load to be transported. The

major part of this load is stator core. This comprises of 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 into compartments. The inner cage is usually fixed to the yoke

by an arrangement of springs to dampen the double frequency vibrations.

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4. 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. These bars are then placed in stator slots and held in

with wedges to form the complete winding. The end turns are rigidly braced and

packed with blocks to withstand the heavy forces.

Generator Cooling: -

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 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.

The conductors used in the rotor windings are hollow which is done to

have internal cooling of the rotor.

Hydrogen Cooling System: Hydrogen is used as a cooling medium

due to its high heat carrying capacity and low density. But it can also form an

explosive, or escape out of the generator casing which may result into many

catastrophic results. So the pressure of H2 should be maintained properly. The

filling in and Purging of H2 is to be done safely without bringing in contact with

air. To fill H2 inside generator first CO2 is filled through generator and then H2 is

passed since H2 has no reaction with CO2 and while taking H2 out of generator

first H2 is taken out then CO2 is passed through generator and then air is allowed

to enter.

To stop the hydrogen from escaping the generator casing oil sealing is done. The

shaft of the rotor has many blades connected at the end of the shaft. Each of

the blades rotates in the slot engraved in the generator casing. Now these slots

are filled with oil up to a certain level, so that the ends of the blades rotate in an

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oil medium which is separated among them. So when the H2 tries to escape and

comes near the end of the shaft it passes through the blade that is through the

oil in the slot, due to which a major part of it gets obstructed and the left out

gas proceeds to the next slot. Now at the end there is almost no or negligible

amount of gas leakage.

Stator Cooling System: The stator 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. The stator winding is

cooled in this system by circulating DM water through hollow conductors. The

DM water should be at 40°C. As it is a closed loop the water that comes out of

the generator is again cooled and demineralized. Water passes through lower

bars along the length to the other end returns through the upper bars of

another slot and drain into drain header.

RATINGS OF TURBO GENERATOR

Stage-I and II

Manufacturer: BHEL

R.P.M: 3000 rpm

Rated Power: 210 MW

Rated KVA: 247 MVA

Stator Voltage: 15.75 KV

Stator Current: 9054 A

Rotor Voltage: 270 V

Rotor Current: 2080 A

Frequency: 50 Hz

Connection: YY (DOUBLE -STAR)

Power Factor: 0.85 LAG

Rated H2 Pressure: 2 BAR

Class of Insulation: F

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B. Excitation System

Excitation System is used to send the Field Current in the Stator Winding

of an Alternator. Here we have Brushless Excitation System for this purpose.

In Brushless Excitation System two systems namely Pilot Exciter and Main

Exciter are connected to the common shaft of the TurboGenerator’S Prime

Mover.

Pilot Exciter is basically a 32 pole Permanent Magnet Generator where

the Permanent Magnets are placed in the Rotor. Now as the Prime Mover

rotates it generates an AC in its Stator. Using Automatic Voltage Regulator

(AVR) this AC is converted into Regulated DC which is now used to magnetise

the electro-magnets of the Main Exciter placed in its Stator Portion. Due to this

an AC voltage developed in the rotor conductors of the Main Exciter. Now Using

Drum Wheel which is also placed inside the Prime Mover Shaft this AC is

converted to DC and used as Field Current of the TURBOGENERATOR.

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In this method no Slip Ring or Brush Commutator system is used so this is

called Brushless Excitation System.

Exciter Portion of an 210MW TurboGenerator

Device Capacity

(KW)

Output

Voltage (V)

Output

Current (A)

Frequency

(Hz) Coolant

Pilot

Exciter 35 KW 220 V 105 A 400 Hz Air

Main

Exciter 1350 KW 420 V 3200 A DC 0Hz Air

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C. Transformers

The transformers used in a power station have its sides abbreviated as

Low Voltage (LV) and High Voltage (HV) rather than primary and secondary.

Major transformers in a power station:

1) Generator transformer (GT): The Generator is connected to this

Transformer by means of isolated bus duct. This transformer is used to step up

the generating voltage to grid voltage normally. This transformer is generally

provided with OFAF cooling.

2) Unit Auxiliary Transformer (UAT): The UAT draws its input from the

main bus duct connecting generator to the Generator Transformer. It is used

for the working of large devices such as BFP, CEP, FD FAN, ID FAN, CW & ACW

PUMPS etc.

3) Station Transformer (ST): The Station Transformer is used to feed the

power to the auxiliaries during the Start-ups as well as run the different parts of

the plant like CHP, AHP, Raw Water Plant, DM Plant, Fuel Oil Pump House etc.

4) Inter-Bus Transformer (IBT): IBT is a special type of Transformer having

Three Windings (Primary, Secondary, Tertiary). IBT is used to Share Power

between different Voltage BUSes. As well as they are used to manage the

Power Flow of the plant and it also helps to recover the plant in case of Power

Failure.

Rating of Transformers

NAME GT UAT IBT ST

Units/Nos 1 TO 5 1 TO 5 1 & 2 1 & 2 3 4 & 5

Capacity

(MVA) 250 N/A 315 40 50 50

Connections Ynd1 N/A YnYnd11 DYn1 Ynd11 YnYn0

Voltage Level

(KV)

15.75/400

&

15.75/220

15.75/6.6 400/220

/33 33/6.6 220/33 220/6.6

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D. Switch Yard

It is a switching station which has the following credits:

(i) Main link between Generating plant and Transmission system, which has a

large influence on the security of the supply.

(ii) Step-up and/or Step-down the voltage levels depending upon the Network

Node.

(iii) Switching ON/OFF Reactive Power Control devices, which has effect on

Quality of power.

In Bk.T.P.S. they have 3 different Switch Yard for Different Voltage Level.

1) 400 KV Switch Yard – Generator 1 and Generator 2 are connected to

this Switch Yard. The Generating Transformers of Unit 1 & 2 Step Up

the Generators Output Voltage (15.75KV) to 400 KV and send it to 400

KV Switch Yard. This Switch Yard operates in the concept of Two Main

Bus (MB1, MB2) and One Transfer Bus (TB). This Switch Yard is

connected to Two Outgoing Feeder a) Arambagh Feeder and b) Jeerat

Feeder. Both Outgoing Feeders are Single Circuit 3- Transmission

Line.

2) 220 KV Switch Yard – Generator 3, 4 & 5 are connected to this Switch

Yard. The Generating Transformers of Unit 3, 4 & 5 Step Up the

Generators Output Voltage (15.75KV) to 220 KV and send it to 220 KV

Switch Yard. This Switch Yard operates in the concept of Two Main Bus

(MB1, MB2) and One Transfer Bus (TB). This Switch Yard is connected

to Three Outgoing Feeder a) Bidhannagar Feeder, b) Satgachiya

Feeder, c) Gokorna Feeder. Both Outgoing Feeders are Double Circuit

3- Transmission Line.

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3) 33 KV Switch Yard – 33 KV Switch Yard is formed by taking power from

400 KV Switch Yard and 220 KV Switch Yard by Inter-Bus Transformers

(IBT 1, IBT 2). This Switch Yard operates in the concept of Three

Parallel Bus Formation (Section 1,2 & 3). IBT 1 and IBT 2 are used to

charge Section 1 and Section 2 respectively. Section 3 takes power

separately from 220 KV Switch Yard by Station Transformer 3

(ST3). Section 1 and Section 2 of 33 KV Switch Yard is used for Stage I

Reserve Power. Section 3 is Connected with Tilpara 1, Tilpara 2, Suri

and Dubrajpur Outgoing Feeder.

SWITCHYARD DIAGRAMS—

Internal Connections of 400KV & 220KV Switch Yard

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SWITCHYARD EQUIPMENTS—

Transformers: Transformer transforms the

voltage levels from higher to lower level or vice

versa, keeping the power constant. Inter Bus

Transformer (IBT) are used to connect 400KV,

220KV and 33KV switchyards.

Circuit breakers (52): Circuit breakers

makes or automatically breaks the electrical

circuits under loaded condition. In Bk.T.P.S. SF6

Circuit Breakers are Used.

Reactive Transformers: This type of transformers are used to minimize the

Ferranti Effect (Capacitive Effect) in Long Transmission Line (>150km).

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Isolators (29): Opens or closes the

electrical circuits under No-load conditions.

In Bk.T.P.S. Pantograph type and Centre-

Break type Isolators are used. Pantograph

type Isolators are used as Bus Side Isolator

and Centre-Break type Isolators are used as

Line Side Isolator.

Current Transformers (CT): Current

transformers are used to measure the

Current flowing through a line. CTs are single

phase oil immersed type. Secondary current

is generally 1A, but also 5A in certain cases.

CT is connected in series with the line.

Potential Transformers (PT): Potential Transformers are used to measure

the Voltage across any line. PTs are single phase oil immersed type. Secondary

Voltage is generally 110 Volts. CT is connected in parallel with the line. PT is only

useable below 220KV.

Capacitive Voltage Transformer (CVT): The CVTS are used to measure the

voltage across any line at 220KV & above.

Earth Switch: Earth switches are device which are normally used to earth a

particular system to avoid accident, which may happen due to induction on

account of live adjoining circuits. These don’t handle any appreciable current at

all.

Lightning Arrestors (LA): station type “lightening arresters” are provided

at the terminals of the transformers for protection against lightening or any

surges developing in the system, the practice is also to install lightening

arresters at the incoming terminals of the line. Shielding of substation from

direct lightening stroke is provided through earth wires located at structures

‘peaks’. Recently masts are also used for the purpose of shielding substation.

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E. Switch Gear

Switchgears are basically the controlling circuits using which the used to

control any system from Remotely Situated Control Room.

HV Switchgears: Indoor metal clad draw out type switchgears with

associated protective and control equipments are employed. Air break, Air Blast

circuit breakers and Minimum Oil circuit breakers could still be found in some

very old stations. Present trend is to use SF6 or vacuum circuit breakers. SF6 and

vacuum circuit breakers require smaller size panels and thereby reasonable

amount of space is saved. The main bus bars of the switchgears are most

commonly made up of high conductivity aluminium or aluminium alloy with

rectangular cross section mounted inside the switchgear cubicle supported by

moulded epoxy, fibre glass or porcelain insulators. For higher current rating

copper bus bars are sometimes used in switchgears.

LV Switchgears: LV switchgears feed power supply to motors above

110KW and upto160KW rating and to Motor Control Centre’s (M.C.C). LV system

is also a grounded system where the neutral of transformers are solidly

connected to ground. The duty involves momentary loading, total load throws

off, direct on line starting of motors and under certain emergency condition

automatic transfer of loads from one source of supply to the other. The

switchgear consists of metal clad continuous line up of multi-tier draw out type

cubicles of simple and robust construction. Each feeder is provided with an

individual front access door. The main bus bars and connections shall be of high

grade aluminium or aluminium alloy sized for the specified current rating. The

circuit breakers used in the LV switchgear shall be air break 3 pole with stored

energy, trip free shunt trip mechanism. These are draw out type with three

distinct position namely, Service, Test and Isolated. Each position shall have

mechanical as well as electrical indication. Provision shall be there for local and

remote electrical operation of the breakers. Mechanical trip push button shall

be provided to trip manually in the event of failure of electrical trip circuit.

Safety interlocks shall be provided to prevent insertion and removal of closed

breaker from Service position to Test position and vice versa.

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F. Protection

The purpose of generator protection is to provide protection against

abnormal operating condition and during fault condition. In the first case the

machine and the associated circuit may be in order but the operating

parameters (load, frequency, temperature) and beyond the specified limits.

Such abnormal running condition would result in gradual deterioration and

ultimately lead to failure of the generator.

Protection under abnormal running conditions

a) Over current protection: The over current protection is used in

generator protection against external faults as back up protection. Normally

external short circuits are cleared by protection of the faulty section and are not

dangerous to the generator. If this protection fails, the short circuit current

contributed by the generator is normally higher than the rated current of the

generator and cause over heating of the stator, hence generators are provided

with back up over current protection which is usually definite time lag over

current relay.

b) Over load protection: Persistent over load in rotor and stator circuit

cause heating of winding and temperature rise of the machine. Permissible

duration of the stator and rotor overload depends upon the class of insulation,

thermal time constant, cooling of the machine and is usually recommended by

the manufacturer. Beyond these limits the running of the machine is not

recommended and overload protection thermal relays fed by current

transformer or thermal sensors are provided.

c) Over voltage protection: The over voltage at the generator terminals

may be caused by sudden drop of load and AVR malfunctioning. High voltage

surges in the system (switching surges or lightning) may also cause over voltage

at the generator terminals. Modern high speed voltage regulators adjust the

excitation current to take care against the high voltage due to load rejection.

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Lightning arresters connected across the generator transformer terminals take

care of the sudden high voltages due to external surges. As such no special

protection against generator high voltage may be needed. Further

protection provided against high magnetic flux takes care of dangerous increase

of voltage.

d) Unbalance loading protection: Unbalance loading is caused by single

phase short circuit outside the generator, opening of one of the contacts of the

generator circuit breaker, snapping of conductors in the switchyard or excessive

single phase load. Unbalance load produces –ve phase sequence current which

cause overheating of the rotor surface and mechanical vibration. Normally 10%

of unbalance is permitted provided phase currents do not exceed the rated

values. For –ve phase sequence currents above 5-10% of rated value dangerous

over heating of rotor is caused and protection against this is an essential

requirement.

e) Loss of prime mover protection: In the event of loss of prime mover the

generator operates as a motor and drives the prime mover itself. In some cases

this condition could be very harmful as in the case of steam turbine sets where

steam acts as coolant, maintaining the turbine blades at a constant temperature

and the failure of steam results in overheating due to friction and windage loss

with subsequent distortion of the turbine blade. This can be sensed by a power

relay with a directional characteristic and the machine can be taken out of bar

under this condition. Because of the same reason a continuous very low level of

output from thermal sets are not permissible.

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Protection under fault condition

a) Differential protection: The protection is used for detection of internal

faults in a specified zone defined by the CTs supplying the differential relay. For

a unit connected system separate differential relays are provided for generator,

generator transformer and unit auxiliary transformer in addition to the overall

differential protection. In order to restrict damage very high differential relay

sensitivity is demanded but sensitivity is limited by C.T errors, high inrush

current during external fault and transformer tap changer variations.

b) Back up impedance protection: This protection is basically designed as

back up protection for the part of the installation situated between the

generator and the associated generator and unit auxiliary transformers. A back

up protection in the form of minimum impedance measurement is used, in

which the current windings are connected to the CTs in the neutral connection

of the generator and its voltage windings through a P.T to the phase to phase

terminal voltage. The pick up impedance is set to such a value that it is only

energized by short circuits in the zone specified above and does not respond to

faults beyond the transformers.

c) Stator earth fault protection: The earth fault protection is the

protection of the generator against damages caused by the failure of insulation

to earth. Present practice of grounding the generator neutral is so designed that

the earth fault current is limited within 5 and 10 Amp. Fault current beyond this

limit may cause serious damage to the core laminations. This leads to very high

eddy current loss with resultant heating and melting of the core.

d) 95% stator earth fault protection: Inverse time voltage relay connected

across the secondary of the high impedance neutral grounding transformer

relay is used for protection of around 95% of the stator winding against earth

fault.

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e) 100% stator earth fault protection: Earth fault in the entire stator

circuits are detected by a selective earth fault protection covering 100% of the

stator windings. This 100% E/f relay monitors the whole stator winding by

means of a coded signal current continuously injected in the generator winding

through a coupling. Under normal running condition the signal current flows

only in the stray capacitances of the directly connected system circuit.

f) Rotor earth fault protection: Normally a single rotor earth fault is not so

dangerous as the rotor circuit is unearthed and current at fault point is zero. So

only alarm is provided on occurrence of 1st rotor earth fault. On occurrence of

the 2nd rotor earth fault between the points of fault the field winding gets short

circuited. The current in field circuit increases, resulting in heating of the field

circuit and the exciter. But the more dangerous is disturbed symmetry of

magnetic circuit due to partial short circuited coils leading to mechanical

unbalance.

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G. Unit Auxiliary Power

Unit Auxiliary Power is taken directly from the terminals of the Generator

using UAT (Unit Auxiliary Transformer). UAT steps down the Generator Output

(15.75 KV) to 6.6KV and form a BUS. There are 2 UAT’s are present in each unit.

This 6.6KV BUS directly supply power to FD FAN, ID FAN, BFP, CEP, CW & ACW

PUMPs of their own Unit. Some other BUSs are formed of 415V which supply

necessary power to different sides of the power plant as Cooling Tower, ESP and

Unit’s Own LV power.

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H. DC Power System

DC Power System is a very important part of a power plant. IN Bk.T.P.S.

the DC current majorly used at 220V. This DC supply is produced by converting

220V AC into DC using Rectifier.

Uses of DC Supply:

a) Basic Control System like Relay Coils, Breakers and

Instrumental System.

b) DC Drives like DC Jacking Oil Pump, DC Emergency Oil

Pump, DC Scanner Air Fan, DC Seal Oil Pump, DC AOP etc.

c) Charging Battery Bank.

d) DC Illumination.

Battery Bank: Battery Bank acts as the DC Power Source in case of Total

Power Failure. In Bk.T.P.S. Lead-Acid Batteries (+ve Plate = Pb,

-ve plate of PbO2 and H2SO4 as electrolyte) are used to store DC

Power. In Battery Bank 110 no of Cells of 2 Volt each are

present. The capacity of each Battery is 1395 A-hr.

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Pollution & Environment

Environmental issues in thermal power plant projects primarily include the

following:

• Air emissions

• Energy efficiency and Greenhouse Gas emissions

• Water consumption and aquatic habitat alteration

• Effluents

• Solid wastes

• Hazardous materials and oil

• Noise

Air Emissions:

The primary emissions to air from the combustion of fossil fuels or

biomass are Sulfur Dioxide (SO2), nitrogen oxides (NOX), particulate matter (PM),

carbon monoxide (CO), and greenhouse gases, such as carbon dioxide (CO2).

Depending on the fuel type and quality, mainly waste fuels or solid fuels, other

substances such as heavy metals (i.e., mercury, arsenic, cadmium, vanadium,

nickel, etc.), halide compounds (including hydrogen fluoride), unburned

hydrocarbons and other volatile organic compounds (VOCs) may be emitted in

smaller quantities, but may have a significant influence on the environment due

to their toxicity and/or persistence. Sulfur dioxide and nitrogen oxide are also

implicated in long-range and trans-boundary acid deposition. The amount and

nature of air emissions depends on factors such as the fuel (e.g., coal, fuel oil,

natural gas, or biomass), the type and design of the combustion unit (e.g.,

reciprocating engines, combustion turbines, or boilers), operating practices,

emission control measures (e.g., primary combustion control, secondary flue

gas treatment), and the overall system efficiency.

Energy Efficiency and GHG Emissions:

Carbon dioxide, one of the major greenhouse gases (GHGs) under the UN

Framework Convention on Climate Change, is emitted from the combustion of

fossil fuels. Recommendations to avoid, minimize, and offset emissions of

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carbon dioxide from new and existing thermal power plants include, among

others:

• Use of less carbon intensive fossil fuels

• Use of combined heat and power plants (CHP) where feasible;

• Use of higher energy conversion efficiency technology of the

same fuel type

• Consider efficiency-relevant trade-offs between capital and operating

costs involved in the use of different technologies.

• Use of high performance monitoring and process control techniques,

good design and maintenance of the combustion system so that initially

designed efficiency performance can be maintained.

• Where feasible, arrangement of emissions offsets (including the Kyoto

Protocol’s flexible mechanisms and the voluntary carbon market),

including reforestation, afforestation, or capture and storage of CO2 or

other currently experimental options.

• Where feasible, include transmission and distribution loss reduction and

demand side measures.

• Consider fuel cycle emissions and off-site factors.

Ta

Water Consumption and Aquatic Habitat Alteration:

Steam turbines used with boilers and heat recovery steam generators

(HRSG) used in combined cycle gas turbine units require a cooling system to

condense steam used to generate electricity. Typical cooling systems used in

thermal power plants include: (i) once-through cooling system where sufficient

cooling water and receiving surface water are available; (ii) closed circuit

wet cooling system; and (iii) closed circuit dry cooling system (e.g., air cooled

condensers). Combustion facilities using once-through cooling systems require

large quantities of water which are discharged back to receiving surface water

with elevated temperature. Water is also required for boiler makeup, auxiliary

station equipment, ash handling, and FGD systems. The withdrawal of such

large quantities of water has the potential to compete with other important

water uses such as agricultural irrigation or drinking water sources. Withdrawal

and discharge with elevated temperature and chemical contaminants such as

biocides or other additives, if used, may affect aquatic organisms, including

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phytoplankton, zooplankton, fish, crustaceans, shellfish, and many other forms

of aquatic life. Aquatic organisms drawn into cooling water intake structures are

either impinged on components of the cooling water intake structure or

entrained in the cooling water system itself. In the case of either impingement

or entrainment, aquatic organisms may be killed or subjected to significant

harm. In some cases, (e.g., sea turtles), organisms are entrapped in the intake

canals. There may be special concerns about the potential impacts of cooling

water intake structures located in or near habitat areas that support

threatened, endangered, or other protected species or where local fishery is

active. Conventional intake structures include traveling screens with relative

high through-screen velocities and no fish handling or return system. Measures

to prevent, minimize, and control environmental impacts associated with water

withdrawal should be established based on the results of a project EA,

considering the availability and use of water resources locally and the ecological

characteristics of the project affected area. Recommended management

measures to prevent or control impacts to water resources and aquatic habitats

include.

Effluents:

Effluents from thermal power plants include thermal discharges,

wastewater effluents, and sanitary wastewater.

Thermal Discharges

As noted above, thermal power plants with steam-powered generators and

once-through cooling systems use significant volume of water to cool and

condense the steam for return to the boiler. The heated water is normally

discharged back to the source water (i.e., river, lake, estuary, or the ocean) or

the nearest surface water body. In general, thermal discharge should be

designed to ensure that discharge water temperature does not result in

exceeding relevant ambient water quality temperature standards outside a

scientifically established mixing zone. The mixing zone is typically defined as the

zone where initial dilution of a discharge takes place within which relevant

water quality temperature standards are allowed to exceed and takes into

account cumulative impact of seasonal variations, ambient water quality,

receiving water use, potential receptors and assimilative capacity among other

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considerations. Establishment of such a mixing zone is project specific and may

be established by local regulatory agencies and confirmed or updated through

the project's environmental assessment process. Where no regulatory standard

exists, the acceptable ambient water temperature change will be established

through the environmental assessment process.

Liquid Waste:

The wastewater streams in a thermal power plant include cooling tower

blowdown; ash handling wastewater; wet FGD system discharges; material

storage runoff; metal cleaning wastewater; and low-volume wastewater, such

as air heater and precipitator wash water, boiler blowdown, boiler chemical

cleaning waste, floor and yard drains and sumps, laboratory wastes, and

backflush from ion exchange boiler water purification units. All of these

wastewaters are usually present in plants burning coal or biomass; some of

these streams (e.g., ash handling wastewater) may be present in reduced

quantities or may not be present at all in oil-fired or gas-fired power plants. The

characteristics of the wastewaters generated depend on the ways in which the

water has been used. Contamination arises from demineralizers; lubricating and

auxiliary fuel oils; trace contaminants in the fuel (introduced through the ash-

handling wastewater and wet FGD system discharges); and chlorine, biocides,

and other chemicals used to manage the quality of water in cooling systems.

Cooling tower blowdown tends to be very high in total dissolved solids but is

generally classified as non-contact cooling water and, as such, is typically subject

to limits for pH, residual chlorine, and toxic chemicals that may be present in

cooling tower additives (including corrosion inhibiting chemicals containing

chromium and zinc whose use should be eliminated).

Solid Wastes:

Coal-fired and biomass-fired thermal power plants generate the greatest

amount of solid wastes due to the relatively high percentage of ash in the fuel.

The large-volume coal combustion wastes (CCW) are fly ash, bottom ash, boiler

slag, and FGD sludge. Biomass contains less sulfur; therefore, FGD may not be

necessary. Fluidized-bed combustion (FBC) boilers generate fly ash and bottom

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ash, which is called bed ash. Fly ash removed from exhaust gases makes up 60–

85% of the coal ash residue in pulverized-coal boilers and 20% in stoker boilers.

Bottom ash includes slag and particles that are coarser and heavier than fly ash.

Due to the presence of sorbent material, FBC wastes have a higher content of

calcium and sulphate and a lower content of silica and alumina than

conventional coal combustion wastes. Low-volume solid wastes from coal-fired

thermal power plants and other plants include coal mill rejects/pyrites, cooling

tower sludge, wastewater treatment sludge, and water treatment sludge.

Hazardous Materials and Oil:

Hazardous materials stored and used at combustion facilities include solid,

liquid, and gaseous waste-based fuels; air, water, and wastewater treatment

chemicals; and equipment and facility maintenance chemicals (e.g., paint

certain types of lubricants, and cleaners). Spill prevention and response

guidance is addressed in Sections 1.5 and 3.7 of the General EHS Guidelines.

In addition, recommended measures to prevent, minimize, and control hazards

associated with hazardous materials storage and handling at thermal power

plants include the use of double-walled, underground pressurized tanks for

storage of pure liquefied ammonia (e.g., for use as reagent for SCR) in quantities

over 100 m3; tanks of lesser capacity should be manufactured using

annealing processes (EC 2006).

Noise:

Principal sources of noise in thermal power plants include the turbine

generators and auxiliaries; boilers and auxiliaries, such as coal pulverisers;

reciprocating engines; fans and ductwork; pumps; compressors; condensers;

precipitators, including rappers and plate vibrators; piping and valves; motors;

transformers; circuit breakers; and cooling towers. Thermal power plants used

for base load operation may operate continually while smaller plants may

operate less frequently but still pose a significant source of noise if located in

urban areas. Noise impacts, control measures, and recommended ambient

noise levels are presented in Section 1.7 of the General EHS Guidelines.

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Conclusion

The practical experience that I have gathered during the overview training of Bakreswar Thermal Power Station having a capacity of 1050 MW for Unit# I to V in three weeks will be very useful as a stepping stone in building bright professional career in future life. It gave me large spectrum to utilize the theoretical knowledge and to put it into practice. The trouble shooting activities in operation and decision making in case of crisis made me more confident to work in the industrial atmosphere. Moreover, this overview training has also given a self-realization & hands-on experience in developing the personality, interpersonal relationship with the professional executives, staffs and to develop the leadership ability in industry dealing with workers of all categories. I would like to thank everybody who has been a part of this project, without whom this project would never be completed with such ease.