training report on Mejia Thermal Power Station

19,377 views

Published on

Mejia Thermal Power Station is located at Durlovpur, Bankura, 35 km from Durgapur city in West Bengal. The power plant is one of the coal based power plants of DVC

Published in: Education, Technology, Business
1 Comment
28 Likes
Statistics
Notes
  • how to do summer vacation training at mejia thermal power station? pls tell me
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
No Downloads
Views
Total views
19,377
On SlideShare
0
From Embeds
0
Number of Embeds
6
Actions
Shares
0
Downloads
1,603
Comments
1
Likes
28
Embeds 0
No embeds

No notes for slide

training report on Mejia Thermal Power Station

  1. 1. A REPORT OF THE VOCATIONAL TRAINING FOR THE PERIOD OF THREE WEEKS FROM 16.06.13 TO 07.07.13 at MEJIA THERMAL POWER STATION P.O. MEJIA, DIST. BANKURA WEST BENGAL-722183 OF DAMODAR VALLEY CORPORATION (D.V.C.) by SAGNIK CHOUDHURY GURU NANAK INSTITUTE OF TECHNOLOGY 157/F, Nilgunj Road, Sodepur, Kolkata 700114 1
  2. 2. 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
  3. 3. 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 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 3
  4. 4. CONTENTS Page No 1. Introduction ................................................................................................5 2.Technical Specification of Mejia thermal power plant.................................6 3.Overview of a Thermal power plant.............................................................7 4.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........................................................................................................19 5.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..................................................................................................38 6.Conclusion......................................................................................................39 7.Bibliography...................................................................................................40 4
  5. 5. INTRODUCTION 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 MTPSs͛ employees. 5
  6. 6. 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 locations. 6
  7. 7. 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. Chimney Stack. 7
  8. 8. MECHANICAL OPERATION 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 categories. • 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
  9. 9. 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. Deaeration: 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: 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
  10. 10. 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 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 & temperature. 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:  Economizer: 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 circulation through the water wall for absorbing heat from furnace. The economizer 10
  11. 11. recirculation line connects down-comer with the economizer inlet header through 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.  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 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.  Superheater: 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.  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 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.  De-superheater (Attemperator): 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) 11
  12. 12. Boiler drum drains & vents, (iii) Superheater & Reheater headers drains & vents, (iv) Desuperheater header drains & vents etc. Drains facilitate draining or hot blow down 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. Furnace 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) Attemperator Type Spray No. of stages One Spray medium Feed water from boiler feed pump (BFP) Reheater Type Vertical spaced Total H.S. area 2819 m2 Control Burner tilt & excess air Economiser Type Plain tube Total H.S. area 6152 m2 12
  13. 13. 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 meter Collecting electrodes No. of rows of collecting electrode per field 49 13
  14. 14. 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 4.5 m. Nominal width of collecting plate 750 mm. Specific collecting area 206.4 m2 /cubic meter. sec-1 Emitting electrodes 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. Electrical items Rectifier 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 ground level. Motors Quantity 24 Rating Geared motor 0.33 HP, 3 phase, 415 V, 50 Hz. Location On root panels of the casing 14
  15. 15. 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 #1 No. 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 #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. 15
  16. 16.  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. 16
  17. 17. STEAM TURBINE 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. 17
  18. 18. 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
  19. 19. 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
  20. 20. 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 GENERATOR 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 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 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. Turbogenerator 20
  21. 21. Technical Data of Turbogenerator Main parameters 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 Temperature rating 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 Other particulars 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 /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
  22. 22. Weights of different parts Heaviest 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 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, 3,4 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. 22
  23. 23. 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 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 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
  24. 24. 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 #1,2,3 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. 24
  25. 25. 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) Phase 3 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 25
  26. 26. 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. 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
  27. 27. 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 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. 27
  28. 28. Circuit Diagram: 220kV switchyard of M.T.P.S 28
  29. 29. 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 respectively. 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 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 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 switchyards autotransformers 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 29
  30. 30. ( 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
  31. 31. 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 SYSTEM 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
  32. 32. 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
  33. 33. 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
  34. 34. 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
  35. 35. 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
  36. 36. 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
  37. 37. 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
  38. 38. 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
  39. 39. CONCLUSION 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. 39
  40. 40. BIBLIOGRAPHY ➢ 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

×