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NTPC VOCATIONALNTPC VOCATIONAL TRAININGTRAINING

NAME- DHIRAJ KUMAR

SUBJECT- Generation of Electricity from Coal

BRANCH- MECHANICAL & AUTOMATION ENGG. ENROLL NO-04214803610

DURATION- 08/06/16 – 09/07/16 (AT N.T.P.C KAHALGOAN, BHAGALPUR BIHAR)

COLLEGE/UNIVERSITY- MAHARAJA AGRASEN INSTITUTE OF TECHNOLOGY ROHINI, DELHI

(G.G.S.I.P.U UNIVERSITY DELHI)

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Acknowledgement

With profound respect and gratitude, I take the opportunity to convey my thanks to complete the training here.

I do extend my heartfelt thanks to Prof. R.P Yadav (TBMD,Head) ,Mr. Abhay Kr(Sr. Manager,BMD), Mr. Shazad Alam(Sr. Engineer Switch yard dept.) & Mr.S.P Dubey (DM Plant Head) for providing me this opportunity to be a part of this esteemed organization.

I am extremely grateful to all the technical staff of NTPC Khalgoan for their co-operation and guidance that helped me a lot during the course of training. I have learnt a lot working under them and I will always be indebted of them for this value addition in me.

I would also like to thank the training in charge of Maharaja Agrasen Institute of Technology, Rohini (Delhi) & all the faculty member of Mechanical & Automation Engineering department for their effort of constant cooperation. Which have been significant factor in the accomplishment of my industrial training.

(DHIRAJ KUMAR)

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OVERVIEW /INDEX

1. About the NTPC Kahalgoan2. Basics of Thermal power plant

2.1 Working Principal 3. Steam Turbine

3.1 Basics of Turbine3.2 Classification & Working of turbine

4. Steam Generation or Boiler 4.1 Basics of Steam generation process & boiler4.2 Boiler furnaces & Steam Drum4.3 Fuel Preparation system4.4 Fuel Firing system & Ignition system\4.5 Air Path

5. Boiler Auxiliary System 5.1 Fly Ash collection 5.2 Bottom Ash collection5.4 Boiler make-up water treatment plant & storage 5.4 DM Plants5.5 Condenser5.6 Feed water heater5.8 Super Heater 5.9 Deaerator5.9 Oil system5.10 Monitoring & Alarm system

6. Electric Generator6.1 Working Principle 6.2 Stator 6.3 Rotor6.4 Bearing 6.5 Auxiliary System

7. Switch Gear7.1 Principle of Switch gear7.2 Classification of switch gear7.3 Function7.4 Safety 7.5 High Tensile Switch gear

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8. Transformer 8.1 Basics of transformer8.2 Construction 8.3 Core8.4 Winding 8.5 Safety

9. Conclusion

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ABOUT THE NTPC KAHALGOAN

Address – Kahalgaon super thermal Power Station, P.O.- Kahalgaon STP ,Dist. Bhagalpur, Bihar- 813214, Telephone/Fax- (STD-06429) – 200333/ 200646

Approved capacity- Stage-I 840 MW + Stage-II 1500 MW

Installed Capacity- 2340 MWLocation- Bhagalpur, BiharCoal Source- Rajmahal Coal Fields of ECL.Water Source - Ganga RiverBeneficiary States - West Bengal, Bihar,Jharkhand, Orissa, SikkimApproved Investment -Stage-I Rs.2038.97 CroreStage-II Phase-I Rs. 4002.28 CroresStage-II Phase-II Rs. 1866.10 CroresUnit Sizes - 4X210 MW+3x500MWUnits Commissioned-Unit -I: 210 MW March 1992Unit -II: 210 MW March 1994Unit -III: 210 MW March 1995Unit -IV: 210 MW March 1996Unit -V: 500 MW March, 2007Unit -VI: 500 MW March, 2008Unit -VII: 500 MW June, 2009

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Basics of Thermal Power PlantThere are basically three main units of a thermal power plant:1. Steam Turbine2. Steam Generator or Boiler3. Electric Generator We have discussed about the processes of electrical generation further. A complete detailed-

“Typical diagram of a coal based Thermal Power Plant “

Description of the three units is given further-

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2.1Principle /Operation-Coal is conveyed (14) from an external stack and ground to a very fine powder by large metalSpheres in the pulverized fuel mill (16). There it is mixed with preheated air (24) driven by theForced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly ignites.

“Working diagram of Thermal Power Plant”

Water of a high purity flows vertically up the tube-lined walls of the boiler, where it turns into steam, and is passed to the boiler drum, where steam is separated from any remaining water. The steam passes through a manifold in the roof of the drum into the pendant super heater (19) where its temperature and pressure increase rapidly to around 200 bar and 540°C, sufficient to make the tube walls glow a dull red. The steam is piped to the High Pressure Turbine (11), the first of a three-stage turbine process. A steam governor valve (10) allows for both manual control of the turbine and automatic set-point following. The steam is exhausted from the high pressure turbine, and reduced in both pressure and temperature, is returned to the boiler re-heater (21). The reheated steam is then passed to the Intermediate Pressure Turbine (9), and from there passed directly to the Low Pressure Turbine set (6). The exiting steam, now a little above its boiling point, is brought into thermal contact with cold water (pumped in from the cooling tower) in the condenser (8), where it condenses rapidly back into water, creating near vacuum-like conditions inside the condenser chest. The condensed water is then passed by a Feed pump (7) through a deaerator (12), and pre-warmed, first in a feed heater (13) powered by Steam drawn from the high pressure set, and then in the economizer

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(23), before being returned to the boiler drum. The cooling water from the condenser is sprayed inside a cooling tower (1), creating a highly visible plume of water vapor, before being pumped back to the condenser (8) in cooling water cycle.The three turbine sets are sometimes coupled on the same shaft as the three-phase electricalGenerator (5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by the unit transformer (4) to a voltage more suitable for transmission (typically 250-500 kV) and is sent out onto the three-phase transmission system (3).Exhaust gas from the boiler is drawn by the induced draft fan (26) through an electrostaticPrecipitator (25) and is then vented through the chimney stack (27).

Steam Turbine3.1 Basics of Steam Turbine- A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. The output shaft of turbine coupled with generator shaft and start power transmission from to turbine shaft to rotor of generator. It is working on Rankine cycle. Steam turbines are used in all of our major coal fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam generated in 'Boilers' or 'Steam Generators' as they are sometimes called Energy in the steam after it leaves the boiler is converted into rotational energy as it passes through the turbine.

“Working diagram of Turbine at NTPC Khalgoan”

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3.2 Classification of Steam turbine -In a typical larger power stations, the steam turbines are split into three separate stages, the first being the High Pressure (HP), the second the Intermediate Pressure (IP) and the third the Low Pressure (LP) stage, where high, intermediate and low describe the pressure of the steam.

The turbine normally consists of several stages with each stage consisting of a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating blades convert the kinetic energy into forces, caused by pressure drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical energy. The rotational speed is 3000 rpm for Indian System (50 Hz) systems and 3600 for American (60 Hz) systems.]After the steam has passed through the HP stage, it is returned to the boiler to be re-heated to its original temperature although the pressure remains greatly reduced. The reheated steam then passes through the IP stage and finally to the LP stage of the turbine.

“A steam flow diagram in all stage of Turbine “

Steam turbine basically classified into two types according to design & application-Which are Reaction & Impulse Turbine.A distinction is made between "impulse" and "reaction" turbine designs based on the relative pressure drop across the stage. There are two measures for pressure drop, the pressure ratio and the percent reaction. Pressure ratio is the pressure at the stage exit divided by the pressure at the stage entrance. Reaction is the percentage isentropic enthalpy drop across the rotating

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blade or bucket compared to the total stage enthalpy drop. Some manufacturers utilize percent pressure drop across stage to define reaction. Either way, the principles are the same for all steam turbines. The configuration is decided by the use to which the steam turbine is put, co-generation or pure electricity production. For cogeneration, the steam pressure is highest when used as process steam and at a lower pressure when used for the secondary function of electricity production.Nozzles and Blades Steam enthalpy is converted into rotational energy as it passes through a turbine stage. A turbine stage consists of a stationary blade (or nozzle) and a rotating blade (or bucket). Stationary blades convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating blades convert the kinetic energy into impulse and reaction forces caused by pressure drop, which results in the rotation of the turbine shaft or rotor.

Steam Generation/Boiler4.1 Basics of Steam Generation & Boiler-The boiler is a rectangular furnace about 50 ft. (15 m) on a side and 130 ft. (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi (22.1MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine.

The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator. The generator includes the economizer, the steam drum, the chemical dosing equipment, and the furnace with its steam generating tubes and the super-heater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse) and the flue gas stack.

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“Schematic diagram of a coal-fired power plant steam generator”

Note – APH is the air preheater.

Note- For units over about 210 MW capacity, redundancy of key components is provided by installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers. On some units of about 60 MW, two boilers per unit may instead be provided.

4.2 Boiler Furnace and Steam Drum-Once water inside the boiler or steam generator, the process of adding the latent heat of vaporization or enthalpy is underway. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel. The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down the down comers to the lower inlet water wall headers. From the inlet headers the water rises through the water walls and is eventually turned into steam due to the heat being generated by the burners located on the front and rear water walls (typically). As the water is turned into steam/vapor in the water walls, the steam/vapor once again enters the steam drum.

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“External View of an Industrial Boiler at NTPC KHALGOAN Bhagalpur ,Bihar”

The steam/vapor is passed through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove the water droplets from the steam and the cycle through the water walls is repeated. This process is known as natural circulation.

4.3 Fuel Preparation System-In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder. Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour Point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100°C before being pumped through the furnace fuel oil spray Nozzles.

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“Boiler Side of the NTPC Khalgoan “

4.4 Fuel Firing System and Igniter System-From the pulverized coal bin, coal is blown by hot air through the furnace coal burners at anAngle which imparts a swirling motion to the powdered coal to enhance mixing of the coal powder with the incoming preheated combustion air and thus to enhance the combustion.To provide sufficient combustion temperature in the furnace before igniting the powdered coal,The furnace temperature is raised by first burning some light fuel oil or processed natural gas (by using auxiliary burners and igniters provide for that purpose).

4.5 Air Path-External fans are provided to give sufficient air for combustion. The Forced draft fan takes air from the atmosphere and, first warming it in the air preheater for better combustion, injects it via the air nozzles on the furnace wall.The Induced draft fan assists the FD fan by drawing out combustible gases from the furnace, maintaining a slightly negative pressure in the furnace to avoid backfiring through any opening.At the furnace outlet, and before the furnace gases are handled by the ID fan, fine dust carried by the outlet gases is removed to avoid atmospheric pollution. This is an environmental limitation prescribed by law, and additionally minimizes erosion of the ID fan.

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Boiler Auxiliary Systems5.1 Fly Ash CollectionFly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bagFilters (or sometimes both) located at the outlet of the furnace and before the induced draft fan.The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.5.2 Bottom Ash Collection and DisposalAt the bottom of every boiler, a hopper has been provided for collection of the bottom ash from the bottom of the furnace. This hopper is always filled with water to quench the ash and clinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site.

5.3 Boiler Make-up Water Treatment Plant and StorageSince there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow-down and leakages have to be made up for so as to maintain the desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. The impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to be removed from the water and that is done by a water

“Ash handling system at NTPC Khalgoan”5.5Demineralizing Treatment Plant (DM Plants).A DM plant generally consists of cation, anion and mixed bed exchangers. The final water from this process consists essentially of hydrogen ions and hydroxide ions which is the chemical composition of pure water. The DM water, being very pure, becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen absorption.The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input.However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive

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water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with atmospheric air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets de-aerated, with the dissolved gases being removed by the ejector of the condenser itself. 5.6 Condenser-A condenser is a heat-exchanger which is used to condense a substance from its gaseous state to liquid state by cooling it. The surface condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is cooled and converted to condensate (water) by flowing over the tubes as shown in the adjacent diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal of air and gases from the steam side to maintain vacuum.

“A Typical Water Cooled Condenser”For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam. Since the condenser temperature can almost always be kept significantly below 100.C where the vapor pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of non-condensable air into the closed loop must be prevented. Plants operating in hot climates may have to reduce output if their source of condenser cooling water becomes warmer; unfortunately this usually coincides with periods of high electrical demand for air conditioning.The condenser generally uses either circulating cooling water from a cooling tower to reject waste heat to the atmosphere, or once-through water from a river, lake or ocean.

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5.7 Feed-Water Heater-A Rankine cycle with a two-stage steam turbine and a single feed-water heater. In the case of a conventional steam-electric power plant utilizing a drum boiler, the surface condenser removes the latent heat of vaporization from the steam as it changes states from vapor to liquid. The heat content (Btu) in the steam is referred to as Enthalpy. The condensate pump then pumps the condensate water through a feed-water heater. The feed-water heating equipment then raises the temperature of the water by utilizing extraction steam from various stages of the turbine.

“Working diagram of Drum & Feed Water at NTPC Khalgoan”

A Rankine cycle with a two-stage steam turbine and a single feed-water heater preheating the feed-water reduces the irreversibility involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feed-water is introduced back into the steam cycle

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“A Rankine cycle with a 2-stage steam turbine and single feed-water heater “

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5.8 Super Heater-As the steam is conditioned by the drying equipment inside the drum, it is piped from the upper drum area into an elaborate set up of tubing in different areas of the boiler. The areas known as Super-heater and Re-heater. The steam vapor picks up energy and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves of the high pressure turbine.

“Working diagram of super heater or Re-heater in thermal power plant “

5.9Deaerator-A steam generating boiler requires that the boiler feed water should be devoid of air and other dissolved gases, particularly corrosive ones, in order to avoid corrosion of the metal.Generally, power stations use a deaerator to provide for the removal of air and other dissolved gases from the boiler Feed Water.A deaerator typically includes a vertical, domed De-aeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feed-water storage tank.Boiler Feed Water Deaerator (with vertical, domed aeration section and horizontal water storage section)There are many different designs for a deaerator and the designs will vary from one manufacturer to another. The adjacent diagram depicts a typical conventional trayed deaerator. If operated properly, most deaerator manufacturers will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm3/L).

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“Working diagram of Deaerator at NTPC Khalgoan”

“Boiler Feed Water Deaerator “

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5.10 Oil System-An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine generator . It supplies the hydraulic oil system required for steam turbine's main inlet steam stop valve, the governing control valves, the bearing and seal oil systems, the relevant hydraulic relays and other mechanisms.At a preset speed of the turbine during start-ups, a pump driven by the turbine main shaft takes over the functions of the auxiliary system.

5.11 Monitoring and Alarm system-Most of the power plant’s operational controls are automatic. However, at times, manual intervention may be required. Thus, the plant is provided with monitors and alarm systems that alert the plant operators when certain operating parameters are seriously deviating from their normal range.

Note-A central battery system consisting of lead acid cell units is provided to supply emergency electric power, when needed, to essential items such as the power plant's control systems, communication systems, turbine lube oil pumps, and emergency lighting. This is essential for a safe, damage-free shutdown of the units in an emergency situation.

Electric Generator 6.1 Basics Principle & Working -

The steam turbine-driven generators have auxiliary systems enabling them to work satisfactorily and safely. The steam turbine generator being rotating equipment generally has a heavy, large diameter shaft. The shaft therefore requires not only supports but also has to be kept in position while running. To minimize the frictional resistance to the rotation, the shaft has a number of bearings. The bearing shells, in which the shaft rotates, are lined with a low friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction between shaft and bearing surface and to limit the heat generated.

The basic function of the generator is to convert mechanical power, delivered from the shaft of the turbine, into electrical power. Therefore a generator is actually a rotating mechanical energy converter. The mechanical energy from the turbine is converted by means of a rotating magnetic field produced by direct current in the copper winding of the rotor or field, which generates three-phase alternating currents and voltages in the copper winding of the stator (armature). The stator winding is connected to terminals, which are in turn connected to the power system for delivery of the output power to the system.

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The class of generator under consideration is steam turbine-driven generators, commonly called turbo generators. These machines are generally used in nuclear and fossil fueled power plants, co-generation plants, and combustion turbine units. They range from relatively small machines of a few Megawatts (MW) to very large generators with ratings up to 1900 MW. The generators particular to this category are of the two- and four-pole design employing round-rotors, with rotational operating speeds of 3600 and 1800 rpm in North America, parts of Japan, and Asia (3000 and 1500 rpm in Europe, Africa, Australia, Asia, and South America).

At NTPC Khalgoan Power Station 3000 rpm, 50 Hz generators are used of capacities 500 MW and 210MW.

6.2 STATOR-The stator winding is made up of insulated copper conductor bars that are distributed around the inside diameter of the stator core, commonly called the stator bore, in equally spaced slots in the core to ensure symmetrical flux linkage with the field produced by the rotor. Each slot contains two conductor bars, one on top of the other. These are generally referred to as top and bottom bars. Top bars are the ones nearest the slot opening (just under the wedge) and the bottom bars are the ones at the slot bottom. The core area between slots is generally called a core tooth.

Stator of a Turbo GeneratorThe stator winding is then divided into three phases, which are almost always wye connected. Wye connection is done to allow a neural grounding point and for relay protection of the winding. The three phases are connected to create symmetry between them in the 360 degree

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arc of the stator bore. The distribution of the winding is done in such a way as to produce a 120 degree difference in voltage peaks from one phase to the other, hence the term “three-phase voltage.” Each of the three phases may have one or more parallel circuits within the phase. The parallels can be connected in series or parallel, or a combination of both if it is a four-pole generator. This will be discussed in the next section. The parallels in all of the phases are essentially equal on average, in their performance in the machine. Therefore, they each “see” equal voltage and current, magnitudes and phase angles, when averaged over one alternating cycle.The combination of the pitch and breadth create a “winding or distribution factor.” The distribution factor is used to minimize the harmonic content of the generated voltage. In the case of a two parallel path winding, these may be connected in series or parallel outside the stator bore, at the termination end of the generator. The connection type will depend on a number of other design issues regarding current-carrying ability of the copper in the winding.In a two-parallel path, three-phase winding, alternating voltage is created by the action of the rotor field as it moves past these windings. Since there is a plus and minus, or north and south, to the rotating magnetic field, opposite polarity currents flow on each side of the stator bore in the distributed winding.The currents normally flowing in large turbo generators can be in the order of thousands of amperes. Due to the very high currents, the conductor bars in a turbo generator have a large cross-sectional area. In addition they are usually one single turn per bar, as opposed to motors or small generators that have multiple turn bars or coils. These stator or conductor bars are also very rigid and do not bend unless significant force is exerted on them.

6.2 ROTOR-The rotor winding is installed in the slots machined in the forging main body and is distributed symmetrically around the rotor between the poles. The winding itself is made up of many turns of copper to form the entire series connected winding. All of the turns associated with a single slot are generally called a coil. The coils are wound into the winding slots in the forging, concentrically in corresponding positions on opposite sides of a pole.

The series connection essentially creates a single multi-turn coil overall, that develops the total ampere-turns of the rotor (which is the total current flowing in the rotor winding times the total number of turns).There are numerous copper-winding designs employed in generator rotors, but all rotor windings function basically in the same way. They are configured differently for different methods of heat removal during operation. In addition almost all large turbo generators have directly cooled copper windings by air or hydrogen cooling gas.

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Rotor of a Turbo GeneratorCooling passages are provided within the conductors themselves to eliminate the temperature drop across the ground insulation and preserve the life of the insulation material.In an “axially” cooled winding, the gas passes through axial passages in the conductors, being fed from both ends, and exhausted to the air gap at the axial center of the rotor. In other designs, “radial” passages in the stack of conductors are fed from sub slots machined along the length of the rotor at the bottom of each slot. In the “air gap pickup” method, the cooling gas is picked up from the air gap, and cooling is accomplished over a relatively short length of the rotor, and then discharged back to the air gap. The cooling of the end-regions of the winding varies from design to design, as much as that of the slot section. In smaller turbine generators the indirect cooling method is used (similar to indirectly cooled stator windings), where the heat is removed by conduction through the ground insulation to the rotor body.The winding is held in place in the slots by wedges, in a similar manner as the stator windings.The difference is that the rotor winding loading on the wedges is far greater due to centrifugal forces at speed. The wedges therefore are subjected to a tremendous static load from these forces and bending stresses because of the rotation effects. The wedges in the rotor are not generally a tight fit in order to accommodate the axial thermal expansion of the rotor winding during operation.6.3 BEARINGS-All turbo generators require bearings to rotate freely with minimal friction and vibration. The main rotor body must be supported by a bearing at each end of the generator for this purpose. In some cases where the rotor shaft is very long at the excitation end of the machine to accommodate the slip/collector rings, a “steady” bearing is installed outboard of the slip collector rings. This ensures that the excitation end of the rotor shaft does not create a wobble that transmits through the shaft and stimulates excessive vibration in the overall generator rotor or the turbo generator line.There are generally two common types of bearings employed in large generators, “journal” and “tilting pad” bearings. Journal bearings are the most common. Both require lubricating and jacking oil systems. When installing the bearings, they must be aligned in terms of height and angle to ensure that the rotor “sits” in the bearing correctly.

6.4 AUXILIARY SYSTEMS-All large generators require auxiliary systems to handle such things as lubricating oil for the rotor bearings, hydrogen cooling apparatus, hydrogen sealing oil, de-mineralized water for stator winding cooling, and excitation systems for field-current application.

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There are five major auxiliary systems that may be used in a generator, which are-1. Lubricating Oil System2. Hydrogen Cooling System3. Seal Oil System4. Stator Cooling Water System5. Excitation System

SWITCHGEARBasics working & principle of Switch gear The term switchgear, used in association with the electric power system or grid, refers to the combination of electrical disconnects, fuses or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled.By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

7.2 Classifications of switchgear are-

1. According the current rating: 1.1By interrupting rating (maximum short circuit current that the device can safely interrupt) 1.2Circuit breakers can open and close on fault currents 1.3Load-break/Load-make switches can switch normal system load currents 1.4solators may only be operated while the circuit is dead, or the load current is very small.2. According voltage class: 2.1Low Tension (less than 440 volts AC) 2.2High Tension (more than 6.6 kV AC)3. According insulating medium: 3.1Air 3.2Gas (SF6 or mixtures) 3.3Oil 3.4Vacuum

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High Tension Switchgear at Thermal Power Plant

4. According IEC degree of internal separation: 4.1No Separation 4.2Bus bars separated from functional units 4.3Terminals for external conductors separated from bus bars 4.4Terminals for external conductors separated from functional units but not from each5. According interrupting device: 5.1Fuses 5.2Air Blast Circuit Breaker 5.3Minimum Oil Circuit Breaker 5.4Oil Circuit Breaker 5.5Vacuum Circuit Breaker 5.6Gas (SF6) Circuit breaker6. According operating method: 6.1Manually-operated 6.2Motor-operated 6.3Solenoid/stored energy operated7. According type of current: 7.1Alternating current 7.2Direct current8. According to the application: 8.1Transmission system 8.2Distribution.

A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.7.3 Functions:-One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear also is used to enhance system availability by allowing more than one source to feed a load.

7.4 Safety:-To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. James Harry Castell invented this technique in 1922. For example, if only one of two sources of supply is permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.

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7.5HIGH TENSION SWITCHGEAR-High voltage switchgear is any switchgear and switchgear assembly of rated voltage higher than1000 volts. High voltage switchgear is any switchgear used to connect or to disconnect a part of a high voltage power system.These switchgears are essential elements for the protection and for a safety operating mode without interruption of a high voltage power system. This type of equipment is really important because it is directly linked to the quality of the electricity supply. The high voltage is a voltage above 1000 V for alternating current and above 1500 V for direct current.

“High Tension Switchgear of a Thermal Power Plant”

History –The high voltage switchgear was invented at the end of the 19th century for operating the motors and others electric machines. It has been improved and it can be used in the whole range of high voltage until 1100 kV.

Classification of High Tension Switchgear:-

1. Disconnectors and Earthling Switches- They are above all safety devices used to open or to close a circuit when there is no current through them. They are used to isolate a part of a circuit, a machine, a part of an overhead-line or an underground line for the operating staff to access it without any danger. The opening of the line isolator or Bus-bar section isolator is necessary for the safety but it is not enough. Grounding must be done at the upstream sector and the downstream sector on the device which they want to intervene thanks to the earthling switches.

In principle, disconnecting switches do not have to interrupt currents, but some of them can interrupt currents (up to 1600 A under 10 to 300V) and some earthling switches must interrupt induced currents which are generated in a non-current-carrying line by inductive and capacitive coupling with nearby lines (up to 160 A under 20 kV) ).

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“A Vacuum Circuit Breaker (High Tension Switchgear)”

2. High-Current Switching Mechanism-They can open or close a circuit in normal load. Some of them can be used as a disconnecting switch. But if they can create a short-circuit current, they can’t interrupt it.

3. Contactor-Their functions are similar to the high-current switching mechanism, but they can be used at higher rates. They have a high electrical endurance and a high mechanical endurance. Contactors are used to frequently operate device like electric furnaces, high voltage motors. They cannot be used as a disconnecting switch. They are used only in the band 30 kV to 100 kV.

4. Fuses-The fuses can interrupt automatically a circuit with an overcurrent flowing in it for a fixed time.The current interrupting is got by the fusion of an electrical conductor which is graded.They are mainly used or protect against the short-circuits. They limit the peak value of the fault current.In three-phase electric power, they only eliminate the phases where the fault current is flowing,Which is a risk for the devices and the people. Against this trouble, the fuses can be associated with high-current switches or contactors.They are used only in the band 30 kV to 100 kV.

5. Circuit Breaker-A high voltage circuit breaker is capable of making, carrying and breaking currents under the rated

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voltage (the maximal voltage of the power system which it is protecting) : Under normal circuit conditions, for example to connect or disconnect a line in a power system; Under specified abnormal circuit conditions especially to eliminate a short circuit. From its characteristics, a circuit breaker is the protection device essential for a high voltage power system, because it is the only one able to interrupt a short circuit current and so to avoid the others devices to be damaged by this short circuit. The international standard IEC 62271-100 defines the demands linked to the characteristics of a high voltage circuit breaker.The circuit breaker can be equipped with electronic devices in order to know at any moment their states (wear, gas pressure…) and possibly to detect faults from characteristics derivatives and it can permit to plan maintenance operations and to avoid failures.To operate on long lines, the circuit breakers are equipped with a closing resistor to limit the over voltages.They can be equipped with devices to synchronize the closing and/or the opening to limit theOver voltages and the inrush currents from the lines, the unloaded transformers, the shunt Reactance and the capacitor banks.Some devices are designed to have the characteristics of the circuit breaker and the disconnector. But their use is limited.

TRANSFORMER8.1 Basics of Transformer- A transformer is a static device consisting of a winding, or two or more coupled windings, with or without a magnetic core, for inducing mutual coupling between circuits. When an alternating current flows in a conductor, a magnetic field exists around the conductor. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor, then a voltage is induced into the second conductor.

“A 220 kV Transformer at Power Plant”The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based.

ANSI/IEEE defines a transformer as a static electrical device, involving no continuously moving parts, used in electric power systems to transfer power between circuits through the use of electromagnetic induction.There are numerous types of transformers used in various applications including audio, radio, instrument, and power.

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In NTPC Khalgoan Thermal Power Station, we deal exclusively with power transformer applications involving the transmission and distribution of electrical power. The term power transformer is used to refer to those transformers used between the generator and the distribution circuits, and these are usually rated at 220 kVA and above. Power transformers must be used at each of these points where there is a transition between voltage levels. Power transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus.

8.2CONSTRUCTION-A power transformer is a device that changes (transforms) an alternating voltage and current from one level to another. Power transformers are used to “step up” (transform) the voltages that are produced at generation to levels that are suitable for transmission (higher voltage, lower current). Conversely, a transformer is used to “step down” (transform) the higher transmission voltages to levels that are suitable for use at various facilities (lower voltage, higher current).Electric power can undergo numerous transformations between the source and the final end use point.

1. Voltages must be stepped-up for transmission. Every conductor, no matter how large, will lose an appreciable amount of power (watts) to its resistance (R) when a current (T) passes through it. This loss is expressed as a function of the applied current (P=I2R). Because this loss is dependent on the current, and since the power to be transmitted is a function of the applied volts (E) times the amps (P=IE), significant savings can be obtained by stepping the voltage up to a higher voltage level, with the corresponding reduction of the current value.

2. If the transmission distance is long enough to produce 0.1 ohm of resistance across the transmission cable. This is where transformers play an important role.

Note-All power transformers have three basic parts, a primary winding, secondary winding, and a core. Even though little more than an air space is necessary to insulate an “ideal” transformer, when higher voltages and larger amounts of power are involved, the insulating material becomes an integral part of the transformer’s operation.

8.3CORE-The core, which provides the magnetic path to channel the flux, consists of thin strips of high grade steel, called laminations. Which are electrically separated by a thin coating of insulating

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material. The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled around the core sections. Core steel can be Hot or Cold rolled, grain-oriented or non-grain oriented, and even laser-scribed for additional performance. Thickness ranges from 0.23 mm to upwards of 0.36 mm.The core cross section can be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer.For power transformers the flux density is typically between 1.3 T and 1.8 T, with the saturation point for magnetic steel being around 2.03T to 2.05 T.

There are two basic types of core construction used in power transformers: 1. Core form construction -In core-form construction, there is a single path for the

magnetic circuit. For single-phase applications, the windings are typically divided on both core legs as shown. In three-phase applications, the windings of a particular phase are typically on the same core leg. Windings are constructed separate of the core and placed on their respective core legs during core assembly. Schematic Diagram of Shell-form Construction

2. Shell-form construction- the core provides multiple paths for the magnetic circuit. The core is typically stacked directly around the windings, which are usually “pancake”-type windings, although some applications are such that the core and windings are assembled similar to core form. Due to advantages in short-circuit and transient-voltage performance, shell forms tend to be used more frequently in the largest transformers, where conditions can be more severe. Variations of three-phase shell-form construction include five and seven legged cores, depending on size and application.

8.4 WINDING -The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported & cooled to withstand operational and test conditions. Copper and aluminum are the primary materials used as conductors in power-transformer windings. “Schematic Diagram of Shell-form Construction”While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable.

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In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength. The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors.

8.5 SAFETY-Safety is of primary concern when working around a transformer. The substation transformer is usually the highest voltage item in a facility’s electrical distribution system. The higher voltages found at the transformer deserve the respect and complete attention of anyone working in the area. A 6.6 kV system will arc to ground over 1.5 to 2.5 in. However, to extinguish that same arc will require a separation of 15 in. Therefore, working around energized conductors is not recommended for anyone but the qualified professional. The best way to ensure safety when working around high voltage apparatus is to make absolutely certain that it is de-energized.

CONCLUSION

All the minor major section in the Thermal power plant had been visited & also understand to the best of my knowledge. I believe that this training has made me well versed with the various process in the power plant. As far as I think there is a long way to go till we use our newest of ever improving technologies to increases the efficiency because the stock of coal are dwindling and they are not going to last forever. It’s imperative that we star shouldering the burden together to see shinning & sustainable future of India.

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