A REPORT OF THE VOCATIONAL
FOR THE PERIOD OF THREE WEEKS FROM
16.06.13 TO 07.07.13
MEJIA THERMAL POWER STATION
P.O. MEJIA, DIST. BANKURA
DAMODAR VALLEY CORPORATION
GURU NANAK INSTITUTE OF TECHNOLOGY
157/F, Nilgunj Road, Sodepur, Kolkata 700114
Economic growth in India, being dependent on the power sector, has necessiated an
enormous growth in electricity demand over the last two decades. Electricity in bulk
quantities is produced in power plants, which can be of the following types: (a)
Thermal (b) Nuclear (c) Hydraulic, (d) Gas turbine and (e) Geothermal.
I have done my vocational training in MEJIA THERMAL POWER STATION under
DAMODAR VALLEY CORPORATION (D.V.C.) comprising 4 units of 210 MW
each, 2 units of 250 MW each and 2 units of 500 MW each. It is a modern thermal
power station having tilting burner corner fired combustion engineering USA design
boiler and KWU West Germany Design Reaction Turbine. Both these main
equipments have been designed, manufactured and supplied by Bharat Heavy
Electricals Limited, India. MTPS units have many special features such as Turbo
mill, DIPC (Direct Ignition of Pulverised Coal) system, HPLP bypass system,
Automatic Turbine Run up system, and Furnace Safeguard Supervisory System.
The dissertation has been prepared based on the vocational training
undergone in a highly esteemed organisation of Eastern region, a pioneer in
Generation Transmission & Distribution of power, one of the most technically
advanced & largest thermal power stations in West Bengal, the Mejia Thermal
Power Station (M.T.P.S), under DVC.
I would like to express my heartfelt gratitude to the authorities of Mejia
Thermal Power Station and Techno India for providing me such an
opportunity to undergo training in the thermal power plant of DVC, MTPS.
I would also like to thank the Engineers, highly experienced without whom
such type of concept building in respect of thermal power plant would not
have been possible. Some of them are:
1) Mr. Parimal Kumar Dubey
2) Mr. Rupak Kumar Nag
3) Mr. Malay Bal
4) Mr. Bhaskar Dey
1. Introduction ................................................................................................5
2.Technical Specification of Mejia thermal power plant.................................6
3.Overview of a Thermal power plant.............................................................7
a.Coal handling Plant....................................................................................8
b.Water Treatment Plant................................................................................9
c.Water De-mineralization Plant...................................................................9
e.Ash handling plant......................................................................................13
e.Switchyard Section & its Components.........................................................27
i.Motors for thermal power plant.....................................................................38
Damodar Valley Corporation was established on 7th July 1948.It is the most reputed
company in the eastern zone of India. DVC in established on the Damodar River. It also
consists of the Durgapur Thermal Power Plant in Durgapur. The MTPS under the DVC is
the second largest thermal plant in West Bengal. It has the capacity of 2340MW with 4
units of 210MW each, 2 units of 250MW each & 2 units of 500 MW each. With the
introduction of another two units of 500MW that is in construction it will be the largest in
West Bengal. Mejia Thermal Power Station also known as MTPS is located in the outskirts
of Raniganj in Bankura District. It is one of the 5 Thermal Power Stations of Damodar
Valley Corporation in the state of West Bengal. 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 Colony area for the residence and other facilities for
TECHNICAL SPECIFICATION OF MTPS:
INSTALLED CAPACITY: -
1) Total number of Units : - 4 X 210 MW(unit 1 to 4) with Brush Type Generators, 2 X 250
MW(unit 5 and 6) with Brush less Type Generators, 2*500 MW(unit 7 and 8) Generators.
2) Total Energy Generation: - 2340 MW
3) Source of Water: - Damodar River
4) Sources of Coal: - B.C.C.L and E.C.L, also imported from Indonesia
In a Thermal Power generating unit, combustion of fossil fuel (coal, oil or natural
gas) in Boiler or fissile element (uranium,plutonium) in Nuclear Reactor
generates heat energy. This heat energy transforms water into steam at high
pressure and temperature. This steam is utilised to generate mechanical energy
in a Turbine. This mechanical energy, in turn is converted into electrical
energy with thehelp of an Alternator coupled with the Turbine. The production
of electric energy utilising heat energy is known as thermal power generation.
The heat energy changes into mechanical energy following the principle of
Rankine reheat-regenerative cycle and this mechanical energy transforms into
electrical energy based on Faraday’s laws of electromagnetic induction. The
generated output of Alternator is electrical power of three-phase alternating current
(A.C.). A.C. supply has several advantages over direct current (D.C.) system and
hence , it is preferred in modern days. The voltage generated is of low
magnitude (14 to 21 KV for different generator rating) and is stepped up
suitably with the help of transformer for efficient and economical transmission
of electric power from generating stations to different load centres at distant
OVERVIEW OF A THERMAL POWER PLANT
A thermal power plant continuously converts the energy stored in the fossil
fuels(coal, oil, natural gas) into shaft work and ultimately into electricity. The
working fluid is water which is sometimes in liquid phase and sometimes in vapour
phase during its cycle of operation. Energy released by the burning of fuel is
transferred to water in the boiler to generate steam at high pressure and temperature,
which then expands in the turbine to a low pressure to produce shaft work. The steam
leaving the turbine is condensed into water in the condenser where cooling water
from a river or sea circulates carrying away the heat released during condensation.
The water is then fed back to the boiler by the pump and the cycle continues. The
figure below illustrates the basic components of a thermal power plant where
mechanical power of the turbine is utilised by the electric generator to produce
electricity and ultimately transmitted via the transmission lines.
1. Cooling tower. 2. Cooling water pump. 3. Transmission line (3-phase). 4. Unit transformer (3-
phase). 5. Electric generator (3-phase). 6. Low pressure turbine. 7. Condensate extraction pump.
8. Condenser. 9. Intermediate pressure turbine. 10. Steam governor valve. 11. High pressure
turbine. 12. De-aerator. 13. Feed heater. 14. Coal conveyor. 15. Coal hopper. 16. Pulverised fuel
mill. 17. Boiler drums. 18. Ash hopper. 19. Super heater. 20. Forced draught fan. 21. Re-heater.
22. Air intake. 23. Economiser. 24. Air pre heater. 25. Precipitator. 26. Induced draught fan. 27.
COAL HANDLING PLANT
Generally most of the thermal power plants uses low grades bituminous coal. The
conveyer belt system transports the coal from the coal storage area to the coal mill.
Now the FHP(Fuel Handling Plant) department is responsible for converting the coal
converting it into fine granular dust by grinding process. The coal from the coal
bunkers.Coal is the principal energy source because of its large deposits and
availability. Coal can be recovered from different mining techniques like
• shallow seams by removing the over burnt expose the coal seam
• underground mining.
The coal handling plant is used to store, transport and distribute coal which comes
from the mine. The coal is delivered either through a conveyor belt system or by rail
or road transport. The bulk storage of coal at the power station is important for the
continues supply of fuel. Usually the stockpiles are divided into three main
• live storage
• emergency storage
• long term compacted stockpile.
The figure below shows the schematic representation of the coal handling plant.
Firstly the coal gets deposited into the track hopper from the wagon and then via the
paddle feeder it goes to the conveyer belt#1A. Secondly via the transfer port the coal
goes to another conveyer belt#2B and then to the crusher house. The coal after being
crushed goes to the stacker via the conveyer belt#3 for being stacked or reclaimed
and finally to the desired unit. ILMS is the inline magnetic separator where all the
magnetic particles associated with coal get separated.
COAL HANDLING PLANT PROCEDURE
WATER TREATMENT PLANT
Raw water supply:
Raw water received at the thermal power plant is passed through Water Treatment Plant
to separate suspended impurities and dissolved gases including organic substance and then
through De-mineralised Plant to separate soluble impurities.
In this process, the raw water
is sprayed over cascade
aerator in which water flows
downwards over many steps
in the form of thin waterfalls.
Cascading increases surface
area of water to facilitate
easy separation of dissolved
undesirable gases (like
hydrogen sulphide, ammonia,
volatile organic compound etc.) or to help in oxygenation of mainly ferrous ions in
presence of atmospheri oxygen to ferric ions. These ferric ions promote to some extent
in coagulation process.
Coagulation takes place in clariflocculator. Coagulant destabilises suspended
solids and agglomerates them into heavier floc, which is separated out through
sedimentation. Prime chemicals used for coagulation are alum, poly-aluminium chloride
Filters remove coarse suspended matter and remaining floc or sludge after
coagulation and also reduce the chlorine demand of the water. Filter beds are developed
by placing gravel or coarse anthracite and sand in layers. These filter beds are regenerated by
backwashing and air blowing through it.
Neutral organic matter is very heterogeneous i.e. it contains many classes of high
molecular weight organic compounds. Humic substances constitute a major portion of
the dissolved organic carbon from surface waters. They are complex mixtures of organic
compounds with relatively unknown structures and chemical composition.
DM (Demineralised Water) 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.
Working principle of Boiler (Steam Generator):
In Boiler, steam is generated from de-
mineralized water by the addition of heat.
The heat added has two parts: sensible heat
and latent heat. The sensible heat raises the
temperature and pressure of water as well as
steam. The latent heat converts water into
steam (phase change). This conversion is also
known as boiling of water, which is dependent
on pressure and corresponding temperature.
Thermodynamically, boiling is a process of
heat addition to water at constant pressure &
The quantity of latent heat decreases with
increase in pressure of water and it becomes
zero at 221.06 bars. This pressure is termed
as critical pressure. The steam generators are
designated as sub-critical or super critical
based on its working pressure as below critical
or above critical pressure. The steam, thus
formed is dry & saturated. Further, addition
of heat raises the temperature and pressure of
steam, which is known as superheated steam.
The differential specific weight between
steam and water provides the driving force
for natural circulation during the steam generation process. This driving force
considerably reduces at pressure around 175 Kg/cm2
and is not able to overcome the
frictional resistance of its flow path. For this, forced or assisted circulation is
employed at higher sub-critical pressure range due to the reason of economy.
But, at supercritical pressures and above, circulation is forced one (such boiler is
called once through boiler).
Important parts of Boiler & their functions:
Feed water enters into the boiler through economizer. Its function is to recover
residual heat of flue gas before leaving boiler to preheat feed water prior to its entry
into boiler drum. The drum water is passed through down-comers for
through the water wall for absorbing heat from furnace. The economizer
recirculation line connects down-comer with the economizer inlet header
an isolating valve and a non-return valve to protect economizer tubes from
overheating caused by steam entrapment and starvation. This is done to ensure
circulation of water through the tubes during initial lighting up of boiler, when
there is no feed water flow through economizer.
Boiler drum is located outside the furnace region or flue gas path. This stores
certain amount of water and separates steam from steam-water mixture. The
minimum drum water level is always maintained so as to prevent formation of
vortex and to protect water wall tubes (especially its corner tubes) from steam
entrapment / starvation due to higher circulation ratio of boiler.
The secondary stage consists of two opposite bank of closely spaced thin
corrugated sheets which direct the steam through a tortuous path and force the
remaining entrained water against the corrugated plates. Since, the velocity is
relatively low, this water does not get picked up again but runs down the
plates and off the second stage lips at the two steam outlets.
From the secondary separators, steam flows uniformly and with relatively low
velocity upward to the series of screen dryers (scrubbers), extending in layers
across the length of the drum. These screens perform the final stage of separation.
Superheaters (SH) are meant for elevating the steam temperature above the
saturation temperature in phases; so that maximum work can be extracted from
high energy (enthalpy) steam and after expansion in Turbine, the dryness fraction
does not reach below 80%, for avoiding Turbine blade erosion/damage and
attaining maximum Turbine internal efficiency. Steam from Boiler Drum passes
through primary superheater placed in the convective zone of the furnace, then
through platen superheater placed in the radiant zone of furnace and thereafter,
through final superheater placed in the convective zone. The superheated steam at
requisite pressure and temperature is taken out of boiler to rotate turbo-generator.
In order to improve the cycle efficiency, HP turbine exhaust steam is taken back to
boiler to increase temperature by reheating process. The steam is passed through
Reheater, placed in between final superheater bank of tubes & platen SH and
finally taken out of boiler to extract work out of it in the IP and LP turbine.
Though superheaters are designed to maintain requisite steam temperature, it is
necessary to use de-superheater to control steam temperature. Feed water,
generally taken before feed water control station, is used for de-superheating steam
to control its temperature at desired level.
Drain & Vent:
Major drains and vents of boiler are (i) Boiler bottom ring header drains, (ii)
Boiler drum drains & vents, (iii) Superheater & Reheater headers drains & vents,
(iv) Desuperheater header drains & vents etc. Drains facilitate draining or hot
of boiler, as and when required; while vents ensure blowing out of air from
boiler during initial lighting up as well as facilitate depressurizing of boiler.
The continuous blow down (CBD) valve facilitates reduction in contaminant
concentration in drum water and also complete draining of drum water. The
intermittent blow down (IBD) / emergency blow down (EBD) valve helps to
normalize the excess drum water level during emergency situation.
Technical data of the Boiler
Type Radiant, Reheat, Natural circulation, Single
Drum, Balanced drift, Dry bottom, Tilting
tangential, Coal and oil fired with DIPC (Direct
Ignition of Pulverized Coal) system.
Width 13868 mm.
Depth 10592 mm.
Volume 5240 m3
Fuel heat input per hour 106 kcal
Designed pressure 175.8 kg/cm2
Superheater Outlet pressure 155 kg/cm2
Low temperature SH (horizontally spaced) 2849 m2
(total heating surface area)
Platen SH (Pendant platen) 1097 m2
(total heating surface area)
Final superheater (vertically spaced) 1543 m2
(total heating surface area)
No. of stages One
Spray medium Feed water from boiler feed pump (BFP)
Type Vertical spaced
Total H.S. area 2819 m2
Control Burner tilt & excess air
Type Plain tube
Total H.S. area 6152 m2
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. It is desirable to
quench the ash before handling due to following reasons:
1. Quenching reduces the temperature of ash.
2. It reduces the corrosive action of ash.
3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will
4. Quenching reduces the dust accompanying the ash.
Flyash is collected with an electrostatic precipitator(ESP)
The principal components of an ESP are 2 sets of electrodes insulated from each
other. First set of rows are electrically grounded vertical plates called collecting
electrodes while the second set consists of wires called discharge electrodes.
The above figure shows the operation of an ESP. the negatively charged fly ash
particles are driven towards the collecting plate and the positive ions travel to the
negatively charged wire electrodes. Collected particulate matter is removed from the
collecting plates by a mechanical hammer scrapping system.
Technical data of the ESP (Electrostatic Precipitator)
Gas flow rate 339 m3
Dust concentration 62.95 gms/N-cubic meter
No. of rows of collecting electrode per field 49
No. of collecting electrode plate 294
Total no. of collecting plates per boiler 3528
Nominal height of collecting plate 13.5 m.
Nominal length of collecting electrodes per field
in the direction of gas field
Nominal width of collecting plate 750 mm.
Specific collecting area 206.4 m2
/cubic meter. sec-1
Type Spiral with hooks
Size Diameter—2.7 mm.
No. of electrodes in the frame forming one row 54 fields
No. of electrodes in the field 2592
Total no. of electrodes per boiler 31104
Total length of electrodes per field 14541 m.
Plate/wire spacing 150 mm.
Rectifier Rating 70 kV (peak), 80 mA (mean)
Type Silicon diode full wave bridge
located Mounted on the top of the
Type of control SCR (Silicon Controlled
Location In the control room at ground
Equipment controlled Geared motors of rapping
mechanism of collecting &
Location In the control room at the ground
Motors Quantity 24
Rating Geared motor 0.33 HP, 3 phase,
415 V, 50 Hz.
Location On root panels of the casing
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.
Technical data of the I.D.Fan (Induced Draught Fan) at Unit #1
No. of boiler 3
Type Radial, NDZV 31 Sidor
Medium handled Flue gas
Location Ground floor
Orientation Suction—Vertical/45 degree to Horizontal
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. In all units except
Durgapur TPS Unit #4, this fan also supplies hot /cold air to the coal mills. The
output of fan is controlled by inlet vane / blade pitch control system.
Technical data of the F.D.Fan (Forced Draught Fan) at Unit #1
No. of boiler 2
Type Radial, NDZV 28/Sidor
Medium handled Clean air
Location Ground floor
Orientation 45° horizontal, delivery-bottom horizontal.
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. In some units like Chandrapura TPS
unit 1, 2 & 3, the exhauster fan sucks pulverized coal and air mixture from coal
mill and sends it to the furnace.
Technical data of the P.A.Fan (Primary Air Fan) at Unit #1
No. of boilers 3
Type Radial, NDZV 20 Herakles
Medium handled Hot air
Location Ground floor
Orientation Suction—Vertical/45 degrees to Horizontal Delivery—Bottom Horizontal.
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. The factors that affect the operation of the mill or
reduce the mill output are:
Grindability of coal: Harder coal (i.e. coal having lower hard-grove index (H.G.I.))
reduces mill output and vice versa.
Moisture content of coal: More the moisture content in coal, lesser will be the
mill output & vice versa.
Fineness of output: Higher fineness of coal output reduces mill capacity.
Size of coal input: Larger size of raw coal fed to the mill reduces mill output.
Wear of grinding elements: More wear and tear of grinding elements reduces the
output from mill.
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 oi l (LDO) or by steam for heavy oil (HFO) in order to have proper turbulent
mixing of oil with combustion air. Use of HFO is beneficial with respect to LDO in
view of its lower cost and saving in foreign exchange.
The oil burners and igniters are the basic elements of oil system. Oil is supplied by
light oil pump or by heavy oil pump through oil heater. Steam heater reduces the
viscosity of heavy oil and aids flow ability as well as better atomization. The oil
burners are located in the compartmented corner of wind boxes, in the different
elevation of auxiliary air compartments, sandwiched between the coal burner nozzles. Each oil
burner is associated with an igniter, arranged at the side.
A steam turbine is a prime mover which continuously converts the energy of high
pressure, high temperature steam supplied by the boiler into shaft work with low
pressure, low temperature steam exhausted to a condenser.
500 MW(KWU) Steam turbine (Mejia TPS U #7&8)
Maker Bharat Heavy Electricals Limited
Type Reaction turbine
Type of governing Throttling
Number of cylinders 3
Speed (RPM) 3000
Rated output (kW) 210000(for unit1,2,3,4)
250000(for unit 5 & 6)
Steam pressure before emergency stop valve
Steam temperature before emergency stop valve 535°C (for unit1,2,3,4)
537°C (for unit 5 & 6)
Reheat temperature 535°C (for unit 1,2,3 &4)
537°C (for unit 5 & 6)
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. Wet cooling
towers could be mechanically draught or natural draught. In M.T.P.S the cooling towers are I.D.
type for units 1-6 and natural draught for units 7&8.
for units 7 and 8 natural draught
A chimney may be considered as a cylindrical hollow tower made of bricks or steel. In MTPS the
chimneys of eight 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 are made
The electrical operation of a power plant comprises of generation, transmission and
distribution of electrical energy. In a power station both distribution and transmission
operation can take place. When power is sent from power station to all other power
station in the grid, it is known as distribution of power. When power plant is driving
power from other power station it is known as transmission of power/electrical
In M.T.P.S. there are 6 electric generators for units 1 to 6. These are 3 phase turbo
generators, 2 pole cylindrical rotor type synchronous machines which are directly
coupled to the steam turbine. The generator consist of 2 parts mainly the stator and
Stator: The stator body is designed to withstand internal pressure of hydrogen-air
mixture without any residual deformation. The stator core is built up of segmental
punching of high permeability, low loss CRGOS steel and are in interleaved manner
on spring core bars to reduce heating and eddy current loss. The stator winding has 3
phase double layer short corded bar type lap winding having 2 parallel paths. The
winding bars are insulated with mica thermosetting insulation tape which consists of
flexible mica foil, fully saturated with a synthetic resin having excellent electrical
properties. Water cooled terminal bushings are housed in the lower part of the stator
on the slip ring side.
Rotor: Rotor is of cylindrical type shaft and body forged in one piece from chromium
nickel molybdenum and vanadium steel. Slots are machined on the outer surface to
incorporate windings. Winding consists of coil made from hand drawn silver copper
with bonded insulation. Generator casing is filled up with H2 gas with required
pressure, purity of gas is always maintained>97%. Propeller type fans are mounted
on either side of the rotor shaft for circulating the cooling gas inside the generators.
Technical Data of Turbogenerator
Rated kW capacity 210000 kW
Rated kVA capacity 247000 kVA
Rated terminal voltage 15750 V
Rated power factor 0.85 lag
Rated stator current 9050 amps
Rated speed 3000 RPM
Rated frequency 50 Hz
Efficiency at rated power output & power factor 98.55%
Power factor short circuit ratio 0.49
Class of insulation of generator windings Class 'B'
Temperature of cooling water (maximum) 37°C
Temperature of cooling Hydrogen (maximum) 44°C
Temperature of cooling distilate (maximum) 45°C
Maximum temperature of stator core 105°C
Maximum temperature of stator winding 75°C
Maximum temperature of rotor winding 115°C
Critical speed of rotor (calculated) 1370/3400 RPM
Fly wheel moment of rotor 21.1 T-M
Ratio of short circuit torque to full load torque 8
Quantity of oil required for cooling per bearing 300 litre/min.
Oil pressure for lubrication of bearings 0.3-0.5 kg/cm2
Quantity of oil required for both the shaft seals 7.7 litres/min.
Rated pressure of the shaft seal oil (gauge) 5 kg/cm2
Quantity of water required for gas coolers 350 m3
Maximum allowable water pressure in gas
Quantity of distillate for cooling stator winding 27 m3
Max. distillate pressure at inlet to stator winding 3.3 kg/cm2
Average qty of Hydrogen required for makeup 15 m3
% Purity of Hydrogen inside the machine 97% min
Max allowable moisture content inside the body 1.5 g/m3
Weights of different parts
Heaviest weight (weight of stator) (kg.) 170000
Bearing with brush rocker & foundation plate
Rotor (kg.) 42000
Gas cooler (kg.) 1415
Terminal bushing (kg.) 85
Total weight of generator (kg.) 239000
The electricity thus produced by the
generator then goes to the generating
transformer where the voltage is increased
for transmission of electricity with
minimized copper losses.
In general a transformer consists of
primary and secondary windings which are
insulated from each other by varnish. In
M.T.P.S. all are either oil cooled or air
cooled. Some of the transformer
accessories are: 1. Conservator tank 2.
Buccholz relay 3. Fans for cooling 4.
Lightning arrestors 5. Transformer
bushings 6. Breather and silica gel.
Generating transformer #1, 2,
MVA: 150/200/250 (H.V.) MVA: 150/200/250 (L.V.)
Volts at no load: 240000 (H.V.) Volts at no load: 15750 (L.V.)
Ampere line value: 361/482/602 (H.V.)
Ampere line value: 5505/7340/9175 (L.V.)
Phase-3 frequency: 50 Hz.
Mass of core and windings: 139000 kg.
Mass of oil: 38070 kg. Mass of heaviest package: 164000 kg.
Connection: YNd1 connection.
Generating transformer#5 and 6
MVA: 189/252/315 (H.V.) MVA: 189/252/315 (L.V.)
Volts at no load: 16.5kV (L.V.) Volts at no load: 240kV (H.V.)
Ampere line value: 757.57 (H.V.)
Ampere line value: 11022.14 (L.V.)
Phase-3 frequency: 50 Hz.
Mass of core and windings: 155000 kg.
Mass of oil: 53070 kg. Mass of heaviest package: 18000 kg.
Connection: YNd1 connection.
Specifications of Generator Transformer (GT) at Unit #7
Type of cooling ONAN/ONAF/OFAF
Rating HV (MVA) 120/160/200
Rating LV (MVA) 120/160/200
No load voltage HV (kV) 242.494
No load voltage LV (kV) 21
Line current HV (amps) 824.79
Line current LV (amps) 9523.8
Temperature rise oil (°C) 40 (Over ambient of 50°C)
Temperature rise winding (°C) 45 (Over ambient of 50°C)
Frequency (Hz) 50
Connection symbol YNd11
Impedance volt at 200 MVA Base
HV Position on 5/LV (nor tap) – 12% to 15%
HV Position on 1/LV (max tap) – 12% to 15%
HV Position on 9/LV (min tap) – 12% to 15%
Insulation level (HV) SL 1050 LI 1300 – AC 38
Insulation level (LV) LI 125 – AC 50
Core & Winding (kg) 153530
Weight of oil (kg) 48910
Total weight (kg) 257500
Oil quantity (litre) 56220
Transport weight (kg) 174900
Untanking weight (kg) 13790
Station Service Transformers
Normal source to the station auxiliaries and standby source to the unit auxiliaries during start up
and after tripping of the unit is station auxiliary transformer. Quantity of station service
transformers and their capacity depends upon the unit sizes and nos. Each station supply
transformer shall be one hundred percent standby of the other. Station service transformers
shall cater to the simultaneous load demand due to start up power requirements for the largest unit,
power requirement for the station auxiliaries required for running the station and power
requirement for the unit auxiliaries of a running unit in the event of outage of the unit
source of supply. The no. and approximate capacity of the SST depending upon the no. and
MW rating of the TG sets are indicated below.
Specifications of Station Service Transformer (SST) at Unit 7 and 8
Type of cooling ONAF/ONAN
Rating HV (MVA) 16/12.50
Rating LV (MVA) 16/12.50
No load voltage HV (kV) 11
No load voltage LV (kV) 3.45
Line current HV (amps) 839.78/656.08
Line current LV (amps) 2677.57/2091.85
Temperature rise oil (°C) 40
Temperature rise winding (°C) 45
Frequency (Hz) 50
Connection symbol Dyn1
Impedance volts % HV-LV 25%
Unit Auxiliary Transformer
The normal source of HV Power to unit auxiliaries is unit auxiliary transformer. The sizing of the
UAT is usually based on the total connected capacity of running unit auxiliaries i.e., excluding the
stand by drives. It is safe anddesirable to provide about 20% excess capacity than calculated. The
no. and recommended MVA rating of unit auxiliary transformers are as shown in the above
table: The UATs shall have Ddo(ungrounded system) or Dy1 (for grounded system) connection
with on load tap changer to provide +10 % variation in steps of 1.25 %. Usual cooling
arrangement to unit auxiliary transformers are ONAN. Radiators are usually divided in two
Unit auxiliary transformer #1,2,3
MVA: 12.5/16 Manufacturer: Atlanta Electricals
Volts at no load: 15750 (H.V.) Volts at no load: 6900 (L.V.)
Ampere line value: 458.2/586.5 (H.V.)
Ampere line value: 1045.9/1338.8 (L.V.)
Phase-3 frequency: 50 Hz.
Mass of core and windings: 14300kg.
Mass of oil: 8600kg. Mass of heaviest package: 25000kg.
Total weight: 30,500 kg.
Unit auxiliary transformer #5 & 6
Type of cooling: ONAN/ONAF (oil natural/ oil natural air force)
Rating (H.V.): 20/16 MVA Rating (L.V.): 20/16 MVA
No load voltage: 13.5 kV (H.V.) No load voltage: 6.9 kV (L.V.)
Line current: 1673.479/1336.783 amp.
Temperature rise of winding: 55*C
Insulation level: 931 KVI 38kV r.m.s (H.V.) 60kVI 20kV r.m.s (L.V.)
Specifications of Unit Auxiliary Transformer (UAT) at Unit #7
Type of cooling ONAN/ONAF
Rating HV (MVA) 45/36
Rating LV (MVA) 45/36
No load voltage HV (kV) 21
No load voltage LV (kV) 11.5
Line current HV (amps) 1238.64
Line current LV (amps) 2261.87
Temperature rise oil (°C) 40 (Over ambient of 50°C)
Temperature rise winding (°C) 45 (Over ambient of 50°C)
Frequency (Hz) 50
Connection symbol Dyn1
Impedance volt at 45 MVA Base
HV Position on 7/LV (nor tap) – 11.5%
HV Position on 1/LV (max tap) – 10% to 13%
HV Position on 17/LV (min tap) – 10% to 13%
Insulation level (high voltage) L1 125 – AC 50
Insulation level (low voltage) L1 75 – AC 28
Core & winding (kg) 40065
Weight of Oil (kg) 25765
Total weight (kg) 85265
Transport weight (kg) 50000
Untanking weight (kg) 41000
CONTROL ROOM UNIT:
The above figure shows the power line diagram in the control room. It clearly shows how the
electric power generated by the generator is transmitted through the generating transformers into the
bus and the distribution of power by the unit auxiliary transformers.
The purpose of excitation system is to continuously provide the appropriate amount of D.C. field
current to the generator field winding. The excitation system is required to function reliably under
the following conditions of the generator and the system to which it is connected.
Functional components of an excitation system :
A good excitation system consists of properly co-ordinated functional components
a) Excitation Power source
b) Semiconductor Rectifier
c) Voltage controller
d) Protective, limiting and switching equipments
e) Monitoring, Metering and indicating equipments and
f) Cooling system
Types of Excitation System :
In earlier days DC excitation system was in use. Increase in generator capacity in turn
raised the demand of excitation power which was notachievable by the DC exciters.
This led to the accelerated development of AC excitation system in pace with
generator capacity. With the maturing of solid state semiconductor technology AC
excitation system found to be superior technically as well as economically.
Excitation system can be categorized and subdividedinto the following :
a) D.C. excitation system
i) Pilot Main Exciter excitation system
ii) Rotating Amplifier excitation system.
b) A.C. excitation system
i) Rotating High Frequency excitation system
ii) Static excitation system
iii) Brushless excitation system
A switchyard is essentially a hub for electrical power sources. For instance, a switchyard
will exist at a generating station to coordinate the exchange of power between the generators and
the transmission lines in the area. A switchyard will also exist when high voltage lines need to be
converted to lower voltage for distribution to consumers. Here in MTPS there is a big switch yard
section for the units one to six, and also for seven & eight there also a switch yard. Some of the
operation of the components of the switch yard is sometimes done from the control rooms of
respective units. That is the switch yard under each unit is sometimes control from the control
rooms of each unit respectively
Circuit Diagram: 220kV switchyard of M.T.P.S
A switchyard may be considered as a junction point where electrical power is comingin from
one or more sources and is going out through one or more circuits. Thisjunction point is in the form
of a high capacity conductor spread from one end to theother end of the yard. As the switchyard
handles large amount of power, it is necessarythat it remains secure and serviceable to supply the
outgoing transmission feederseven under conditions of major equipment or bus failure.
There are differentschemes available for bus bar and associated equipment connection to facilitate
switching operation. The important points which dictate the choice of bus switching
scheme are –
a. Operational flexibility, b. Ease of maintenance,
c. System security, d. Ease of sectionalizing,
e. Simplicity of protection scheme, f. Installation cost and land requirement.
g. Ease of extension in future.
The basic components of a switchyard are as follows:
1.Circuit breaker: A circuit breaker is an equipment that breaks a circuit either manually or
automatically under all conditions at no load, full load or short circuit. Oil circuit breakers, vacuum
circuit breakers and SF6 circuit breakers are a few types of circuit breakers.
2.Isolator: Isolators are switches which isolate the circuit at times and thus serve the purpose of
protection during off load operation.
3.Current Transformer: These transformers used serve the
purpose of protection and metering. Generallythe same transformer
can be used as a current or potential transformer depending on the
type of connection with the main circuit that is series or parallel
In electrical system it is necessary to
a) Read current and power factor
b) Meter power consumption.
c) Detect abnormalities and feed impulse to protectivedevices.
4.Potential transformers :
In any electrical power system it is
necessary to - Fig. C.T.
a) Monitor voltage and power factor,
b) Meter power consumption,
c) Feed power to control and indication circuit and
d) Detect abnormalities (i.e. under/over voltage, direction of
power flow etc) and feed impulse to protective device/alarm
circuit. Standard relay and metering equipments does not permit
them to be connected directly to the high voltage system.Potential
transformers therefore play a key role by performing the
a) Electrically isolating the instruments and relays from HV side.
b) By transferring voltage from higher values to proportional standardized lower values.
The use of power transformer in
a switchyard is to change the
voltage level. At the sending and
usually step up transformers
are used to evacuate power at
transmission voltage level. On
the other hand at the receiving
end step down transformers are
installed to match the voltage
to sub transmission or
distribution level. In many
are used widely for
interconnecting two switchyards
with different voltage level
(such as 132 and 220 KV)
33/11 KV Power Transformer in a switchyard
( 1-Main tank 2-Radiator 3-Reservoir tank 4-Bushing 5-WTI & OTI Index 6-Breather 7-Buccholz relay)
The live equipments are mounted over the steel structures or suspended from
gantries with sufficient insulation in between them. In outdoor use electrical porcelain
insulators are most widely used. Following two types of insulators are used in
a. Pedestal type b. Disc type
Pedestal type insulators are used on steel structures for rigid supporting of the pipe bus bars,
for holding the blade and the fixed contacts of theisolators.
The above figure shows a complete bay for 220kV switchyard.
Electric power is generated by the generator which is circulated to the main bus 1 or
2 and accordingly the respective isolator is closed. In case of any fault in the circuit
breaker the power from the generator goes via the transfer bus into the main bus by
means of the bus coupler. A bus tie represents the connection between the two main
buses. Two 80MVA transformers draw power from the main buses and transfer the
voltage to 33kV and the power goes to 33kV switchyard. A station service
transformer supplies power to the auxiliary load.
The above figure shows the power flow diagram of 33kV switchyard.
The electric power after voltage transformation to 33kV by 80MVA transformers goes to the main
bus of the 33kV switchyard from where power is fed to various industries and other nearbystations.
There are two earthing transformers in the yard. From the bus the power is fed to two 5MVA
transformers which step down the voltage level to 11kV and is thus distributed to the locality.
THE TYPE OF RELAYS USED IN MTPS FOR PROTECTION OF POWER
• Auxiliary relay for isolations
• Fail accept relay
• Directional over current relay
• Master trip relay
• Multi relay for generator function
• Supervision relay
• Instantaneous relay
• Bus bar trip relay
• Lock out relay
• Numerical LBB protection relay
• Transformer differential protection relay
• Circulating differential protection relay
• Contact multi-relay
• Auxiliary relay
• Trip circuit R-Phase relay
• EUS section relay
• DC fail accept relay
• Trip circuit R-phase super relay Y-phase B-phase
• LBB protection relay.
SOME PUMP & MOTOR IS USED IN MTPS
• Service water pump -360Kw
• Primary air fan(PA fan) -800Kw
• Coal mill motor -2250Kw
• Condense extraction pump -500Kw
Boiler feed pump motor -3500Kw
ID fan motor -1500Kw
FD fan motor -1000Kw
CW pump motor -1200Kw
Indoor metal clad draw out type
switchgears with associated protective
and control equipments are employed
(fig. 2). 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
SF6or vacuum circuit breakers. SF6 and
vacuum circuit breakers requires
smaller size panels and thereby
reasonable amount of space is saved. Fig. 2: General arrangement of 6.6 KV
switchgearpanels The main bus bars of the switchgears are most commonly made
up of high conductivity aluminium or aluminium alloy with rectangular cross section
mounted in side the switchgear cubicle supported by moulded epoxy, fibre glass or
porcelain insulators. For higher current rating copper bus bars are sometimes used in
LV switchgears feed power supply to motors above 110 KW and upto160
KW rating and to Motor Control Centers (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 throw 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.
One Main Bus and Transfer Bus scheme
This scheme is used in switchyards up to 132 KV. Under normal condition all feeders arefed
through their respective circuit breakers from the main bus bar. During shutdown or outage
of any feeder breaker, that feeder can be transferred to transfer bus and diverted through bus
coupler breaker. In that case the protection shall be transferred to the bus coupler circuit
breaker by changing the position of the trip transfer switch located at the switchyard control
panel. This diversion of the feeder from its own circuit breaker to bus coupler circuit breaker
and the vice versa is possible even in live condition without any interruption of supply to
that feeder. In case of any main bus fault the entire switchyard will collapse. To avoid such
total collapse of the switchyard a bus section circuit breaker is provided in the middle
position of the main bus.
Two Main Bus and One transfer Bus scheme
In this scheme there is an arrangement for a duplicate main bus (MB).
All the feeders in the yard may be connected to either MB # 1 or MB # 2 or may
be divided in two groups and distributed in two buses. In case of outage of any
circuit breaker that feeder can be diverted through bus coupler breaker. Bus tie breaker is
used to tie up MB #1 & MB # 2.
One and Half Breaker Scheme
In one and half breaker scheme (Fig. 4) under normal condition all the circuit breakers will
remain closed. At the time of maintenance of feeder breaker, only that breaker would be
kept open and isolated. During maintenance of bus, all the breakers connected to that bus
would remain open to isolate the bus. At that time, the power supply may be maintained
through other bus. All equipments in the switchyard except the line side isolators can be
maintained without taking shut down of any feeder. This scheme has gained popularity in many
400 KV switchyards in our country.
The purpose of generator protection is to provide protectionagainst 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 currentof 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 b e
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. 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.
e) Unbalance loading protection: Unbalance loading is caused by single phase
short circuit outside the generator, opening of oneof 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.
g) 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 continuousvery low level of output from
thermal sets are not permissible.
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 an 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 faultsbeyond 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 leadsto 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.
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.
MOTORS FOR THERMAL POWER PLANT
All the motors in Thermal Power Stations shall be of the 3-ph. A.C. squirrel cage type
except for some auxiliaries, which are emergent in nature,for which DC motors shall be
used. For some small valves, single phase motors may be used. All A.C. motors
shall be suitable for direct on line starting.
Normally D.C. power is supplied by the float charger and the batteries are kept in float
condition at 2.15 V per cell to avoid discharging. The charger consists of silicon diode
or thyristor rectifiers preferably working on 3
ph. 415 V supply in conjunction with an
automatic voltage regulator. When there is a failure
in the A.C. supply the batteries will
come into operation and in this process the
batteries run down within few hours. After
normalization of A.C. power the batteries are
charged quickly by using the boost charger at 2.75
V per cell. During this time the float chargeris
isolated and load is connected through the tap off
point. After normalization of battery voltage these
are again put back into the float charging mode.
The output from the battery as well as the charger is connected to the D.C.
distribution board. From D.C. distribution board power supply is distributed to different
circuits. D.C. system being at the core of the protection and control mechanism very often
two 100% capacity boards with individual chargers and battery sets are used from
the consideration of the reliability and maintenance facility. These two boards are
interconnected by suitable tie lines.
The practical experience that I have gathered during the overview training of
large thermal power plant having a large capacity of 2340 MW for Unit# I to
VIII 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.
➢ Power Plant Engineering by P.K.Nag
➢ Engineering Thermodynamics by P.K.Nag
➢ Mejia Thermal Power Station – Technical Data & Operation Guide
➢ THERMAL POWER ENGINEERING by R.K.RAJPUT.
➢ THEORY & PERFORMANCE of ELECTRICAL MACHINE by J.B.GUPTA
➢ AC & DC MACHINE by B.L.THERAJA & A.K.THERAJA.
➢ A COURSE IN ELECRICAL POWER by J.B.GUPTA.