A REPORT OF THE VOCATIONALTRAINING

FOR THE PERIOD OF THREE WEEKS FROM 16.06.13 TO 07.07.13

at

MEJIA THERMAL POWER STATIONP.O. MEJIA, DIST. BANKURA

WEST BENGAL-722183OF

DAMODAR VALLEY CORPORATION (D.V.C.)

by

SAGNIK CHOUDHURY

GURU NANAK INSTITUTE OF TECHNOLOGY

157/F, Nilgunj Road, Sodepur, Kolkata 700114

1

PREFACE

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.

2

ACKNOWLEDGEMENT

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 ThermalPower 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 Dubey2) Mr. Rupak Kumar Nag3) Mr. Malay Bal4) Mr. Bhaskar Dey

3

CONTENTS

Page No

1. Introduction ................................................................................................52.Technical Specification of Mejia thermal power plant.................................6 3.Overview of a Thermal power plant.............................................................74.Mechanical operation a.Coal handling Plant....................................................................................8 b.Water Treatment Plant................................................................................9 c.Water De-mineralization Plant...................................................................9 d.Boiler System.............................................................................................10 e.Ash handling plant......................................................................................13 f.ESP...............................................................................................................13 g.Boiler auxilliaries........................................................................................15 h.Steam Turbine..............................................................................................17 i.Cooling tower................................................................................................19 j.Chimney........................................................................................................195.Electrical operation a.Generator.......................................................................................................20 b.transformers..................................................................................................22 c.control room..................................................................................................26 d.Excitation system..........................................................................................27 e.Switchyard Section & its Components.........................................................27 f.Switchgear.....................................................................................................32 g.Switching Schemes.......................................................................................33 h.Protection......................................................................................35 i.Motors for thermal power plant.....................................................................38 j.Battery bank..................................................................................................386.Conclusion......................................................................................................397.Bibliography...................................................................................................40

4

INTRODUCTION

Damodar Valley Corporation was established on 7th July 1948.It is the most reputedcompany in the eastern zone of India. DVC in established on the Damodar River. It alsoconsists of the Durgapur Thermal Power Plant in Durgapur. The MTPS under the DVC isthe second largest thermal plant in West Bengal. It has the capacity of 2340MW with 4units of 210MW each, 2 units of 250MW each & 2 units of 500 MW each. With theintroduction of another two units of 500MW that is in construction it will be the largest inWest Bengal. Mejia Thermal Power Station also known as MTPS is located in the outskirtsof Raniganj in Bankura District. It is one of the 5 Thermal Power Stations of DamodarValley Corporation in the state of West Bengal. The total power plant campus area issurrounded by boundary walls and is basically divided into two major parts, first the PowerPlant area itself and the second is the Colony area for the residence and other facilities forMTPSs͛ employees.

5

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 MW3) Source of Water: - Damodar River4) 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 locations.

6

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 steamleaving 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.

Chimney Stack.

7

MECHANICAL OPERATION

COAL HANDLING PLANT Generally most of the thermal power plants uses low grades bituminous coal. Theconveyer 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 coalconverting it into fine granular dust by grinding process. The coal from the coalbunkers.Coal is the principal energy source because of its large deposits andavailability. 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 comesfrom the mine. The coal is delivered either through a conveyor belt system or by railor road transport. The bulk storage of coal at the power station is important for thecontinues supply of fuel. Usually the stockpiles are divided into three maincategories.

• 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

8

WATER TREATMENT PLANTRaw 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 thenthrough De-mineralised Plant to separate soluble impurities. Deaeration:In this process, the raw wateris sprayed over cascadeaerator in which water flowsdownwards over many stepsin the form of thin waterfalls.Cascading increases surfacearea of water to facilitateeasy separation of dissolvedundesirable gases (likehydrogen 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: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 (PAC).

Filtration: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.

Chlorination: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.

9

BOILER SYSTEM

BOILER: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 heatand latent heat. The sensible heat raises thetemperature and pressure of water as well assteam. The latent heat converts water intosteam (phase change). This conversion is alsoknown as boiling of water, which is dependenton pressure and corresponding temperature. Thermodynamically, boiling is a process ofheat addition to water at constant pressure & temperature. The quantity of latent heat decreases withincrease in pressure of water and it becomeszero at 221.06 bars. This pressure is termedas critical pressure. The steam generators aredesignated as sub-critical or super criticalbased on its working pressure as below criticalor above critical pressure. The steam, thusformed is dry & saturated. Further, additionof heat raises the temperature and pressure ofsteam, which is known as superheated steam.The differential specific weight betweensteam and water provides the driving forcefor natural circulation during the steam generation process. This driving forceconsiderably reduces at pressure around 175 Kg/cm2 and is not able to overcome thefrictional resistance of its flow path. For this, forced or assisted circulation isemployed at higher sub-critical pressure range due to the reason of economy.But, at supercritical pressures and above, circulation is forced one (such boiler iscalled once through boiler).

Important parts of Boiler & their functions: Economizer:Feed water enters into the boiler through economizer. Its function is to recoverresidual 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 forcirculation through the water wall for absorbing heat from furnace. The economizer

10

recirculation line connects down-comer with the economizer inlet headerthrough an isolating valve and a non-return valve to protect economizer tubes from overheating caused by steam entrapment and starvation. This is done to ensurecirculation of water through the tubes during initial lighting up of boiler, whenthere is no feed water flow through economizer.

Drum: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 ofvortex 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 theremaining entrained water against the corrugated plates. Since, the velocity is relatively low, this water does not get picked up again but runs down theplates 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.

Superheater:Superheaters (SH) are meant for elevating the steam temperature above the saturation temperature in phases; so that maximum work can be extracted fromhigh 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 passesthrough primary superheater placed in the convective zone of the furnace, thenthrough 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.

Reheater: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 throughReheater, 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.

De-superheater (Attemperator):Though superheaters are designed to maintain requisite steam temperature, it isnecessary 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)

11

Boiler drum drains & vents, (iii) Superheater & Reheater headers drains & vents,(iv) Desuperheater header drains & vents etc. Drains facilitate draining or hotblow down of boiler, as and when required; while vents ensure blowing out of air fromboiler 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.

FurnaceWidth 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)

AttemperatorType Spray

No. of stages One

Spray medium Feed water from boiler feed pump (BFP)

ReheaterType Vertical spaced

Total H.S. area 2819 m2

Control Burner tilt & excess air

EconomiserType Plain tube

Total H.S. area 6152 m2

12

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 disintegrate.4. Quenching reduces the dust accompanying the ash. Flyash is collected with an electrostatic precipitator(ESP)

ELECTROSTAIC PRECIPITATOR 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/s

Temperature 142°C

Dust concentration 62.95 gms/N-cubic meterCollecting electrodesNo. of rows of collecting electrode per field 49

13

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 fieldin the direction of gas field

4.5 m.

Nominal width of collecting plate 750 mm.

Specific collecting area 206.4 m2/cubic meter. sec-1

Emitting electrodesType 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.

Electrical itemsRectifier Rating 70 kV (peak), 80 mA (mean)

Number 24

Type Silicon diode full wave bridge connection

located Mounted on the top of the precipitator

Rectifier control panel

Type of control SCR (Silicon Controlled Rectifier)

Number 24

Location In the control room at ground level

Auxiliary control panel

Number 2

Equipment controlled Geared motors of rapping mechanism of collecting & emitting electrodes.

Location In the control room at the groundlevel.

Motors Quantity 24

Rating Geared motor 0.33 HP, 3 phase, 415 V, 50 Hz.

Location On root panels of the casing

14

BOILER AUXILIARIES

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 #1No. of boiler 3

Type Radial, NDZV 31 Sidor

Medium handled Flue gas

Location Ground floor

Orientation Suction—Vertical/45 degree to Horizontal Delivery—Bottom 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 #1No. 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 #1No. 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.

15

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 oilburner is associated with an igniter, arranged at the side.

16

STEAM TURBINEA 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.

17

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 (abs)

150 kg/cm2

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)

18

Cooling tower

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.

Cooling towers

for units 7 and 8 natural draught

CHIMNEY 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 high.

19

ELECTRICAL OPERATION

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 energy.

ELECTRIC GENERATORIn 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 the rotor.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 chromiumnickel 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.

Turbogenerator

20

Technical Data of TurbogeneratorMain parametersRated 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 0.49Temperature ratingClass 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

Other particularsCritical 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/hr.

Maximum allowable water pressure in gas coolers

3 kg/cm2

Quantity of distillate for cooling stator winding 27 m3/hr.

Max. distillate pressure at inlet to stator winding 3.3 kg/cm2

Average qty of Hydrogen required for makeup 15 m3 per day

% Purity of Hydrogen inside the machine 97% min

Max allowable moisture content inside the body 1.5 g/m3

21

Weights of different partsHeaviest weight (weight of stator) (kg.) 170000

Bearing with brush rocker & foundation plate (kg)

9300

Rotor (kg.) 42000

Gas cooler (kg.) 1415

Terminal bushing (kg.) 85

Total weight of generator (kg.) 239000

TRANSFORMERS

The electricity thus produced by thegenerator then goes to the generatingtransformer where the voltage is increasedfor transmission of electricity withminimized 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, 2,3,4MVA: 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 6MVA: 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.

22

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)

Phase 3

Frequency (Hz) 50

Connection symbol YNd11

Impedance volt at 200 MVA BaseHV 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

Vector Diagram

AUXILIARY TRANSFORMRERS

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

23

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

Phase 3

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 equal halves.

Specification

Unit auxiliary transformer #1,2,3MVA: 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.

24

Unit auxiliary transformer #5 & 6Type of cooling: ONAN/ONAF (oil natural/ oil natural air force)Rating (H.V.): 20/16 MVA Rating (L.V.): 20/16 MVANo 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*CInsulation 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)

Phase 3

Frequency (Hz) 50

Connection symbol Dyn1

Impedance volt at 45 MVA BaseHV 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

25

CONTROL ROOM UNI T:

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 thebus and the distribution of power by the unit auxiliary transformers.

EXCITATION SYSTEM 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 which are 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

26

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

SWITCHYARD SECTION

A switchyard is essentially a hub for electrical power sources. For instance, a switchyardwill exist at a generating station to coordinate the exchange of power between the generators andthe transmission lines in the area. A switchyard will also exist when high voltage lines need to beconverted to lower voltage for distribution to consumers. Here in MTPS there is a big switch yardsection for the units one to six, and also for seven & eight there also a switch yard. Some of theoperation of the components of the switch yard is sometimes done from the control rooms ofrespective units. That is the switch yard under each unit is sometimes control from the controlrooms 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.

27

CircuitDiagram:

220kVswitchyard of

M.T.P.S

28

The basic components of a switchyard are as follows:1.Circuit breaker: A circuit breaker is an equipment that breaks a circuit either manually orautomatically under all conditions at no load, full load or short circuit. Oil circuit breakers, vacuumcircuit 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 ofprotection during off load operation.3.Current Transformer : These transformers used serve thepurpose of protection and metering. Generallythe same transformercan be used as a current or potential transformer depending on thetype of connection with the main circuit that is series or parallelrespectively.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 isnecessary 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 following functions. a) Electrically isolating the instruments and relays from HV side.

b) By transferring voltage from higher values to proportional standardized lower values.

5 . POWER TRANSFORMER: The use of power transformer ina switchyard is to change thevoltage level. At the sending andusually step up transformersare used to evacuate power at transmission voltage level. Onthe other hand at the receivingend step down transformers areinstalled to match the voltageto sub transmission ordistribution level. In manyswitchyards autotransformersare used widely forinterconnecting two switchyardswith different voltage level(such as 132 and 220 KV) 33/11 KV Power Transformer in a switchyard

29

( 1-Main tank 2-Radiator 3-Reservoir tank 4-Bushing 5-WTI & OTI Index 6-Breather 7-Buccholz relay)

6.Insulator :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 switchyard.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.

30

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 POWERSYSTEM COMPONENTS

• 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.

31

SOME PUMP & MOTOR IS USED IN MTPS

PUMP :-

• Service water pump -360Kw• Primary air fan(PA fan) -800Kw• Coal mill motor -2250Kw• Condense extraction pump -500Kw

MOTOR :-

Boiler feed pump motor -3500Kw ID fan motor -1500Kw FD fan motor -1000Kw

CW pump motor -1200Kw

SWITCHGEAR

HV Switchgears:

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

switchgears.

32

LV Switchgears:

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.

SWITCHING SCHEMES

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

33

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.

34

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.

GENERATOR PROTECTION

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.

35

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.

36

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. .

37

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.

Battery Bank

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.

38

CONCLUSION

The practical experience that I have gathered during the overview training oflarge thermal power plant having a large capacity of 2340 MW for Unit# I toVIII in three weeks will be very useful as a stepping stone in building brightprofessional career in future life. It gave me large spectrum to utilize thetheoretical knowledge and to put it into practice. The trouble shootingactivities in operation and decision making in case of crisis made me moreconfident to work in the industrial atmosphere.Moreover, this overview training has also given a self realization & hands-onexperience in developing the personality, interpersonal relationship with theprofessional executives, staffs and to develop the leadership ability in industrydealing 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.

39

BI BLIOGRAPHY

➢ 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.

➢ www.google.co.in

40