Industrial training
Industrial training In Chandrapur Super Thermal Power Station (CSTPS)By-Aniket . S.NaphadePriyadarshini Coll. Of Engg., NagpurDeptt. Of Electrical Engg.(16/12/2013- to- 30/12/2013 )
Table of Contents1.OVERVIEW42.Chandrapur super thermal power station(CSTPS)63.Coal Handling Plant94.Power Plant Component and its Operation114.1 Boiler114.2 Turbine184.3 Condensor254.4 Generator28Exciter324.5 Generating Transformer365.WATER TREATMENT PLANT46Post-treatment of Water52Condensate polishing unit58Effluent Treatment plant (E.T.P)-596.Ash Handling Plant-60
1. OVERVIEWABOUT MAHAGENCOGovernment of Maharashtra founded Maharashtra state electricity board (MSEB) under Indian Companies Act 1956. In 2005 MSEB is trifurcated in 3 companies as follows- Mahanirmiti () or Mahagenco (Maharashtra State Power Generation Company Limited (MSPGCL)) Mahapareshan () or Mahatransco (Maharashtra State Electricity Transmission Company Limited (MSETCL)) Mahavitaran() orMahadiscom(Maharashtra State Electricity Distribution Company Limited (MSEDCL)) Mahagenco is second heighest electricity genrating company after NTPC. Mahagencohaving generation capacity of 10737 MW comprising 7480 MW thermal, 2585 MW hydel and 672 MW gas turbine.
2. Chandrapur super thermal power station(CSTPS)
Chandrapur Super Thermal Power Station(CSTPS) is athermal power plantlocated inChandrapur districtin the Indian state ofMaharashtra run by the MAHAGENCO. The power plant is one of the coal based power plants ofMAHAGENCO. The coal for the power plant is sourced from Durgapur and Padmapur Collieries ofWestern Coalfields Limited & Mahanadi coalfields limited.The plant was officially inaugurated by the then Prime MinisterIndira Gandhion 8 October 1984. Installed capacity of CSTPS is 2340MW.Water supplied to the plant from the Erai dam.
Unit NumberInstalled Capacity (MW)Date of Commissioning
12101983 August
22101984 July
32101985 May
42101986 March
55001991 March
65001992 March
75001997 October
Total unit 72340
Genration of electricity from thermal power station- Inthermal power station,steamdriven turbine is a prime mover of electrical generator . When water is heated in boiler, turns into steam and spins asteam turbine which drives anelectrical generator. After it passes through the turbine, the steam iscondensedin acondenserand recycled to where it was heated; this is known as aRankine cycle. Following energy conversion steps are used in the thermal power station-Chemical energy of coal
Heat energy of water due combustion of coal
Kinetic energy of steam in nozzle
Mechanical energy due to rotation of turbine
Electrical energy developed in generator armature due to rotation
3. Coal Handling Plant
Following is the function of the coal handling plant- Unloading of the coal received from coal mines. Carry coal to the bunker, & maintain bunker level. Storing of the coal in case of emergency.Coal for operation of boiler in CSTPS is provide from - Durgapur and Padmapur Collieries ofWestern Coalfields Limited . Mahanadi coalfields limited. Coal is also impetrated from Indonesia. From WCL mine, we get Bituminous coal. It is dark brown to black in colour. It is of higher quality thanlignite coal but of poorer quality thananthracite. Formation is usually the result of high pressure being exerted on lignite. It also called as soft coal. This contains high percentage of volatile substances which burnt with yellow flame. Following table shows the percentage of various elements in bituminous coalName of coalCarbon% Moisture%Ash%Calorific valuveKcal/kg Heat contentMj/Kg
Bituminous
60-80
2.2-15.9
3.3-11.74200 (for imported coal)3600 (for W.C.L.)
24-35
Amount coal required to produce one unit of electricity is called as coal factor.The coal factor of the coal from WCL mines is 0.6Kg.It is examined by the following analysis of coal- Proximate analysis Ultimate analysis Random samples are taken out from various mines and above analysis are done on that coal sample. According to it various grades are give to the coal. In WCL mines F. grade coal is seen.Unit generated in 24 Hr.-1unit=KwhCoal required for 3, 500MW & 4, 210MW plant is 39000MT.
Transportation of coal- Transportation
Road Railway Rope way
Coal from coal Collieries contains magnetic materials, stones, and other impurities. Magnetic materials are removed from coal by employing magnet above the conveyor belt, which carries coal for feeding boiler.Coal for 210MW, plant is brought by rope way. Capacity of rope way is 300 tons per hour.In following ways magnet are employed on conveyor belt- Suspended type magnet Rotary type magnetTo remove the non magnetic material skilled labor are stand besides the conveyor belt, remove impurities.
4. Power Plant Component and its Operation
4.1 Boiler Boiler is a very important component of the thermal power plant. it is used to generate steam to drive the turbine. It is a closed vessel in which liquid is heated. This boiler is made up of steel. temperature of the boiler is in the range of 1400-1600 degree cel.Boilers are classified on different basis as follows According to circulation of steam. Following different method are used for circulation of steam Natural circulation- In circulation, difference in density of steam water is used to circulate steam. Forced circulation- force circulation method is used when operating pressure is approaches to critical pressure. According to firing type of boiler.In which portion of the boiler burner are place, according to which boiler are classified as follows- Front fire type Corner fire type
The source of the heat in boiler is combustion coal. In CSTPS water tube type boiler is used. Water tubes filled with water are arranged inside a furnace in a number of possible configurations: often the water tubes connect large drums, the lower ones containing water and the upper ones, steam and water; in other cases, such as a monotube boiler, water is circulated by a pump through a succession of coils. This type generally gives high steam production rates, but less storage capacity than the above. Water tube boilers can be designed to exploit any heat source and are generally preferred in high pressure applications since the high pressure water/steam is contained within small diameter pipes which can withstand the pressure with a thinner wall. Cool water at the bottom of the steam drum returns to the feed water drum via large-bore 'downcomer tubes', where it pre-heats the feed water supply. To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such watertube boilers inthermal power stationare also calledsteam generating units.In CSTPS , boiler is suspended from top, it is because of when water tubes are heated . due to heating ,water tubes expands. If bottom of the boiler is fixed then tubes will expand in upward direction, above the boiler the boiler draft is fitted. So due to heavy weight of boiler drum & tube expansion high stares are developed in boiler. So it may harmful & reduces boiler & its peripheries life. Water tube boiler are subjected to the high pressure, heat flux and temperature so circulation of steam and water gets important. A boiler equipped with a combustion chamber which has a strong current of air (draft) through the fuel bed will increase the rate of combustion, which is the efficient utilization of fuel with minimum waste of unused fuel. The stronger movement will also increase the heat transfer rate from the flue gases to the boiler, which improves efficiency and circulation. The combustion rate of the flue gases and the amount of heat transfer to the boiler are both dependent on the movement and motion of the flue gases. Hence, due to above reasons draught system holds high significance.The difference between atmospheric pressure and the pressure existing in the furnace or flue gas passage of a boiler is termed asdraft. The system, which maintain the draught in boiler is called as a draught system.Draught is produced by rising the level flue gases in stack.In CSTPS, Balanced draft system is employed. -5mm of water column pressure is maintaining in boiler. Forced draft is induces secondary air in boiler. secondary air is required for complete combustion of pulverized coal.
Generally, negative pressure is maintained inside the boiler. And, it is in the range of -5mm of water column. Negative pressure is maintain because of, when coal is burnt, positive pressure inside boiler cause the flame to reach to boiler wall, which causes damage to boiler wall & also reduces efficiency of boiler .hence draught system is employed to take out the flue gases from boiler.Following are the important periphery of boiler-Coal Mill- coal mill is used to pulverized a coal. Unburned coal is found in bottom ash if coal is not completely burned. Unburned coal is found 2-3% in bottom ash & about0.5% in fly ash.Major reason for incomplete combustion is size of coal. In coal mill coal is grind to obtain a coal particles of size about 70 micron.In CSTPS following type of coal mill is used- Ball & race type. Bowl type. Tube type.
Economizer- A feedwater economizer reduces steam boiler fuel requirements by transferring heat from the flue gas to incoming feedwater. Boiler flue gases are often rejected to the stack at temperatures higher than the temperature of the generated steam.Primary heater- Air preheater is used to heat the air which is required for combustion inside boiler , before it enter in boiler . it helps in improving rate of combustion of coal in boiler.Final superheater- It is a device in which the seam from the boiler is dried. It means that total moisture is removed from the steam.Platen superheater- It is a plate type heater.Electrostatic precipitator- Anelectrostatic precipitator(ESP), orelectrostatic air cleaneris aparticulatecollection device that removes particles from a flowing gas using the force of an inducedelectrostatic charge. Electrostatic precipitators are highly efficientfiltrationdevices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream.Stack- stack is vertical pipe through which the flue gases are exhausted to atmosphere from boiler.Fans in boiler- Following are the fans are used in boiler. Primary air fan (P.A. fan)- Function of primary air fan- Carry the pulverized coal to boiler from coal mills. Remove moisture from coal. Forced Draft Fan- Fans for boilers force ambient air into the boiler, typically through a preheater to increase overall boiler efficiency. Forced Draft (FD) fans purpose is to provide a positive pressure to a system. Induced draft Fan (I.D. Fan)- Induced Draft (ID) fans are used to create a vacuum or negative air pressure inboiler. Boiler ID Fans are often used in conjunction with FD fans to maintain system pressure which is slightly lower than ambient.
Cycles of boiler- Following air cycles are in the boiler- Primary air cycle- functions of primary air is follows- Carry the pulverized coal from the coal mill to the combustion chamber of boiler. Provide oxygen for combustion of coal particles.Primary air cycle is shown as follows-
Primary air Fan
Air pre heater
Coal mill
Boiler combustion chamber
Secondary air cycle- function of secondary air is provide oxygen for combustion of coal inside boiler. When pulverized coal particle burn, ash is deposits on the coal particle. Which causes incomplete combustion of coal particle. Hence efficiency of boiler lowered down. If coal burn near the burner head & boiler wall. It may damage it . this damage is avoided by forcing secondary air inside the boiler as it causes coal burn away from these peripheries.Secondary air removes this deposition of ash on coal particle.Secondary air cycle is as follows-F.D.Fan
Air pre heater
boiler
Flue gas cycle- Flue gases are formed after combustion of coal. Flue gases may contain carbo dioxide(co2),water vapors, sulpher dioxide(so2),and poolution causing gases like carbon mono-oxide(CO), particulate matter like soot, & fly ash.These gases are cleaned by using Electrostatic precipitator for removal of soot & fly ash.
Flue gas cycle is as follows-
Boiler
Air heater
Electrostatic precipitator
Induced draft fan
stack
Boiler Maintenance- Blowdowns- Boiler blowdown is removal of water from the boiler. It is done in order to remove the amount of solids in thewater, and is performed as either bottom (sludge) blowdown, or continuous (surface) blowdown. The blowdown frequency and duration is primarily determined by the boiler water analysis. The water quality will vary greatlybased on boiler type and size, amount of condensate return, and boiler water treatment program.Water tube cleaning-after burning of coal two types of ash is formed, namely- Fly ash. Bottom ash. Fly ash is light in weight, and it flue with flue gases. It may deposite on the water tube surface inside the boiler. Due which less heat is absorbed by the water tube & hence efficiency of the boiler is lowered downHence it is necessary to clean the boiler water tube surface. To remove this deposition suit blowing method is used. in this method steam is forced to flow over water tube & boiler wall at a pressure of 150kg.following 2 type of suit blower are used- Short soot blower. It is used to clean to boiler walls & water tube surface near to boiler walls. Long soot blower Two soot blower of 10m are used from both side of boiler.
4.2 Turbine Turbine is a device which converts the kinetic energy of the steam in the mechanical rotational energy of rotor. Steam from the boiler is passed over the blade of the turbine, which tends to rotate the shaft of the turbine. Turbine shaft is mechanically coupled with generator shaft .when turbine shaft rotate generator shaft also rotate.Basically turbine is of following type- Impulse turbine. Reaction turbine.On 5 different basis turbine is classified ,those are as follows- Type of compounding. Type of blading. Division of flow. Type of steam flow. Type of exhausting condition.Turbine used in CSTPS has following features- turbine set consist of 3 turbine defined on the basis of the pressure inside it. These turbines with their type are as follows- High pressure turbine - impulse turbine. Intermediate pressure turbine Reaction turbine. Low pressure turbine Reaction turbine. Condensing type. Tandom compound. Tandom compounding means both impulse & reaction type turbine is used. Nozzle governing. Disc & diaphragm type. The Turbine contained one row of stationary blade & one row of rotating blades alternatively. The fixed blades are carefully shaped to direct the flow of steam against the moving blades at an angle and a velocity that will maximize the conversion of the steam's heat energy into the kinetic energy of rotary motion. Because the steam's temperature, pressure and volume change continuously as it progresses through the turbine, each row of blades has a slightly different length, and in certain parts of the turbine the twist of the blade is usually varied along the length of the blade, from root to tip. One set of stationary & rotating blade is called as stage. These no. of stages can b varied as per requirement. In CSTPS, no of stages for a given turbine is given in following tableType of turbineNo of stages
H.P. turbine12
I.P. turbine11
L.P. turbine4+4
The steam from the boiler is initially provided to high pressure turbine (H.P. turbine). Steam is expanded inside H.P. turbine .H.P. gives the starting torque & operating speed to the turbine rotor. The expanded steam from the H.P. turbine again brought into the boiler for heating. After rising the temperature of steam, it is fed to the intermediate pressure turbine (I.P.).then this steam is supplied to the low pressure turbine (L.P.). The function of I.P. turbine & L.P. turbine is to provide torque to the rotor. As steam transferred from I.P. turbine to L.P. turbine, steam pressure goes on the decreasing, and hence steam goes on expanding. In L.P turbine steam is expand in huge amount hence it requirement of space.To overcome this limitation steam is bifurcated in L.P. turbine. turbine is design so that steam enter at the middle part of the turbine & exit from the both end. The blades in each half face opposite ways, so that the axial forces negate each other but the tangential forces act together. This design of rotor is calledtwo-flow,double-axial-flow, ordouble-exhaust. This arrangement is common in low-pressure casings of a compound turbine.This arrangement has following advantages- Required space is less. Vibration cause is less. Steam can be expands up to require amount. Maintain the correct rotor position and balancing, this force must be counteracted by an opposing force.Following are the turbine peripheriesEmergency stop valve (E.S.V)- Hydraulically operated. Operate live steamLive steamissteamunder pressure, obtained by heating water in aboiler.Control valves ( C.V.)- Control valves used to control the flow of steam in boiler. during light load period, by controlling flow of steam we can generate limited electricity. Control valves are mounted on casing of H.P. turbine at the middle bearing side, it is common for H.P. & I.P turbine. There are total 4 no. of C.V in one turbine.Turbine rotor connectionRotor of I.P. & L.P. turbine is connected by semi flexible rod. 3 rotors are supplied with 5 bearings. H.P. & L.P. turbine is combined on radial bearing.Barring gear- barring gear is structure to support the rotor during sagging & hogging due to differential temperature between the top of the rotor shaft and the bottom. Once the rotor is cooled toambient temperature, unless there is a supplementary support structure for the rotor, it should be barred periodically to avoid sagging of the rotor under its own weight.
More about turbine Anchor point- when steam is expand in turbine, heat is transferred from the steam to rotor & turbine casing. Due to heating elongation is produced in rotor & casing. It is subjected to axial thrust. To allow their controlled motion during operation and to prevent any eventuality between rotor and casing they are required to be anchored Rotors are anchored at Bearing no 2(between HP &IP) by means of thrust bearing. In some Turbines they are also anchored at free end Thrust bearing ( Anchor point) is always located near High temperature end to minimize the differential expansion.So anchor point is defined as casing which are connected by means of pedestals & keys. Anchor point is located near the high tepratureIn CSTPS, anchor point is located near front of L.P. turbine.
Cycles of turbine Cooling water cycle.- Two circulating water pump (C.W. pump)are circulating water, taken from sump. Water from the condenser water tube is cooled in cooling tower. 27000ton water per hour is cooled in one cooling tower. 14 C.T fans are employed in 210MW plant cooling tower. Temperature in cooling tower should be 28-30degree cel.
Following dia. shows the cooling water cycle-
Regenerative feed water cycle- Regenerative water is obtained from the condensate from the condenser. When steam in condenser is passed over the cool water tube, it condensate & collected in hot well. Hence regenerative feed water starts from hot well & end at boiler drum. 2 condensate extraction pumps are running & 1 pump is stand by. It is as follows
Condenser
Hot well
C.E.P - 3C.E.P - 2C.E.P - 1
Low pressure heater (L.P.)
De aerator
Feed storage tank (F.S.T.)
Feed water cycle- In feed water cycle 2 boiler feed pumps (B.F.P.) are running & 1 is stand by at a time. Feed water cycle is as follows-Feed water cycle (F.S.T)
B.F.P.-2B.F.P.-3B.F.P.-1
High pressure heater (H.P.)
economizer
Boiler drum
4.3 Condensor Condenser is used to condense the steam out from the L.P. turbine. Cooled water is circulating inside the water tube of condenser. Heat of the steam is transferred to cooled water inside water tube. Hence, due to this steam gets condense. Condensate is collected in hot well, which is just below the condenser. The heated water inside the water tube is sent to the cooling tower in which temperature of this water is decreased to the ambient temperature. Following are the important component of condenserShell- The shell is the condenser's outermost body and contains the heat exchanger tubes. The shell is fabricated fromcarbon steelplates and is stiffened as needed to provide rigidity for the shell.At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs, a hotwell is provided. Condensate is pumped from the outlet or the hotwell for reuse asboiler feedwater.For most water-cooled surface condensers, the shell is undervacuumduring normal operating conditions.Cooling water tubes- Generally the tubes are made ofstainless steel, copper alloys such as brass or bronze. Cooled water flows through it. Which extract heat from the steam. Due to absorption of heat tube gets elongate hence some clearance is provided for this expansion.Condensate extraction pump (C.E.P)- condensate extraction pump (C..P) is generally a centrifugal pump used to extract the condensate from the condenser. In CSTPS, there are total 3 C.E. pumps are installed per condenser unit in 210MW unit. Out of these 2 C.E. pumps are running &1 is stand by at a time.
More about condenser- Vacuum is maintained in condenser.It is because of the steam turbine itself is a device to convert theheatin steam to mechanicalpower. The difference between the heat of steam per unit mass at the inlet to the turbine and the heat of steam per unit mass at the outlet from the turbine represents the heat which is converted to mechanical power. Therefore, the more the conversion of heat perpoundorkilogramof steam to mechanical power in the turbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power. Most of the heat liberated due tocondensationof the exhaust steam is carried away by the cooling water.
Cooling Tower- Cooling towers are used to cool the water from the water tube of condenser. Water in water tube take heat from the steam from the turbine.
Heat from water is transferred by following method-Wet cooling method- In this method evaporation phenomenon is used to transfer heat.Dry cooling method- In this method convention of heat phenomenon is used to transfer of heat from water from water tube. Heat is transferred through a surface that separates the water from ambient air, such as in a heat exchanger.
4.4 Generator Generator is an electrical device which converts mechanical energy into electrical energy. This Energy conversion is based on the principal of the production of dynamically induced emf. Whenever conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faradays Laws of electromagnetic induction. This emf causes a current to flow if the conductor circuit is closed.Synchronous generator ( Alternator) is used to generate A.C electric power at a constant frequency 50Hz.In A.C. generator, following are constructional important parts- A stationary element called as stator, on which armature winding is mounted. A rotating part inside the stator called as rotor. this rotor is driven by the rotational mechanical energy from the turbine shaft at a constant speed. In CSTPS, 210MW unit 2 pole alternator is used which generate electricity at 3000rpm of frequency 50Hz.Stator Construction Stator is a heaviest component of the entire generator. It contains stator winding & stator body with a stator core. it is designed toWithstand high internal pressure, which may arise due to unlikely event of explosion of hydrogen air mixture without any residual deformations. Stator body is a totally enclosed gas tight fabricated structure made up of high quality mild steel and austenitic steel.Stator winding- Stator winding is made up of a hallow conducting pipe through which water is circulated for cooling hydrogen gas. The stator winding is placed in open rectangular slots of the stator core which are uniformly distributed on the circumference Bus bars are connected to bring out the three phases & six neutrals. This bus barware connected with terminal bushings. Both are water cooled, connection is made by brazing the two lugs properly. The insulation is highly resistant to high temperatures andtemperature changes. The composition of the insulation and synthetic resin permits the machine to be operated continuously under conditions corresponding to these for insulation class B temperature.END WINDING: In the end winding, the bars are arranged close to each other. Lower as well as upper layers of bars are braced with terelyne cord with binding ring as well as with adjacent bars. Bus bars are connected to bring out the three phases & six neutrals. These bus bars are connected with terminal bushings. Both are water cooled, connection is made by brazing the two lugs properly.TERMINAL BUSHING : Three phases and six neutral terminals are brought out from the stator frame through bushings, which are capable of withstanding high voltage, and provided with gastight joints. The bushings are bolted to the bottom plate of the terminal box, with their mounting flanges. The terminal box that is welded underneath the stator frame at exciter end is made of nonmagneticSteel to avoid admissible temperature rise. The conductor of the bushing is made of high conductivity copper tube. A copper pipe is connected to circulate water for cooling. The terminal bar conductor is housed in porcelain insulator which can be mounted on the terminal box by means of ring.The bushing is connected to terminal bus bar by means of flexible copper leads for making the electrical connections conveniently.
Rotor Construction In rotor construction following are the important parts of rotor, which require very careful operation during its construction. Rotor winding. Rotor shaft.
Rotor winding Rotor winding is mounted on the rotor shaft. The field winding consists of several coils inserted into the longitudinal slots of the rotor body. The coils are wound around the poles so that one north magnetic pole and one south magnetic pole are obtained on shaft. Rotor winding is excited by the excitation voltage provided by excitation system. Rotor winding conductor are made up of hard drawn silver bearing copper. Apart from low electrical resistance this grade exhibits high creep resistance so that coil deformations due to thermal cycling due to start and stop operation are minimum. Insulation winding is made up of Layer of glass laminates insulates the Individual turns from each other. This laminate is built by glass prepped strips on the turn of copper and baked under pressure and temperature. Due to rotation & flow of current in winding heat is generated. if excessive heat is generated, it may damage the winding insulation & weaken the rotor. Hence winding cooling is necessary. It can be done by hydrogen cooling. Hydrogen gas can be rich up to deepest layer of winding hence efficient cooling is done.Rotor shaft- The rotor shaft is long forging measuring more than 9 meters in length and slightly more than one meter in diameter. The main constituents of the steel are chromium, molybdenum. Nickel, and vanadium. The shaft and body are forged integral to each other by drop forging process. On 2/3 of its circumference approximately, the rotor body is provided with longitudinal slots to accommodate field windings. The slots pitch is selected in such a way that two solid poles displaced by 180 are obtained. D.C exciter is also mounted on the generator shaft.Fans- Two single stage axial flow propeller type fans circulate the generator cooling gas. Fitted on either sides of rotor bodyBearings- The rotor shaft is supported on pedestal type of bearings which has spherical seating to allow self alignment. On the top of bearings pedestal a vent pipe emerges connecting bearing chamber to the atmosphere for venting out oil vapour or traces of Hydrogen. A current collector located just above the rotor shaft and touching it is also mounted on the bearing body to give, shaft voltage for rotor earth fault protection. To prevent the flow of shaft currents slip ring and bearing and connecting pipes are insulated from earth.Shaft Seal The locations where the rotor shaft passes through the stator casing, are provided with radial seal rings. The seal ring is guided in the seal body, which is bolted on to the end shield and insulated to prevent the flow of shaft currents. The seal ring is lined with babbit on the shaft journal side. The gap between the seal ring and the shaft is sealed with seal oil. The seal oil is supplied to the sealing gap from the seal body via radial holes and an annular groove in the seal ring. To ensure effective sealing, the seal oil pressure in the annular gap is maintained at a higher level than the gas pressure within the generator casing.
Following flow chart shows generated electricity route
Generator
Generating transformer
National grid
Cooling-
In generator heat is generated due to windage loss ,Cu losses, friction. The rise in temperature must be controlled to protect the insulating material. the temperature with which these insulating material can with stand is decided by the class of insulating material to which it belongs. For cooling generator components following cooling systems are used- Hydrogen cooling. Demineralised water (D.M.) cooling.
Hydrogen Cooling-
In CSTPS, 2 pole generator runs at 3000 rpm. So the rotor of large diameter is used. To minimize the size of generator air gap should be as small as possible & by increasing value of current density. But increasing in value of current density increases the losses in generator ultimately heat developed is also increased. If excess amount of heat is developed, it may damage the winding insulation. In Generator hydrogen gas may be leak from the bearings & casing. This hydrogen gas is highly explosive when it is in contact with air. This may cause fire to the generator. To avoid this leakage Generator seal oil system is employed. In order to prevent the escape of Hydrogen from the generator casing along the rotor shaft, shaft seals with oil under pressure are used. The shaft seals are radial thrust type and are mounted between the end shield and the bearing at either end of the stator. The seal oil supply system consists of an oil injector, two seal oil pumps, one cooler, two oil filters, differential pressure regulator, pressure oil regulator, damper tank, hydraulic seal, visual window, oil check pipe etc.
Demineralised (D.M.) Water Cooling- Stator conductor are made hallow . through this hallow conductor D.M. water is circulate . which absorb the heat from stator winding. D.M. Cooling water is admitted to upper bar and it returns through lower bar of next slot. The hot water then goes to expansion tank which is maintained under a vacuum of about 300 mm of Hg. This helps to remove gases from the water and thus corrosion is minimsed. From expansion tank, water is pumped by primary stator water pumps through DM/DM Coolers,filters back to stator conductors.
ExciterAccording to Faradays Laws of electromagnetic induction, Whenever conductor cuts magnetic flux dynamically an induced emf is produced in it.Due to this induced emf current is flows in closed circuit. Need of excitation to A.C. generator- ac generators required a large magnetic field which cannot be provided by permanent magnets. so dc generators called auxillary generators are used to provide constant magnetic field.
More about exciter-Excitation can be given by both A.C. & D.C. source, but the A.C. excitation have Following limitations- A.C. excitation itself produce alternating flux, thus generator will give transformer action But it will not generate electricity as per mutual induction laws. If field is rotated by giving A.C. excitation, we will not get constant magnetic polls. Polls will change with respect to armature winding . so, it results in irregular waveform of generated emf.& there will be chances of burning of stator winding. In case of D.C. excitation , it gives constant magnetic field.
Generator are connected in parallel with another generator or infinite bus bars.An infinite bus means a large system whose voltage and frequencyremain constant independent of the power exchange between the synchronous generator and the bus, and independent of the excitation of the synchronous generator. The generator excitation which is controllable determines the flow of VARs into or out of the generator. Generally , D.C. generator as an exciter is mounted on the generator shaft itself. Objectives of excitation control :Besides maintaining the field current and steady state operating point, the excitationsystem is required to improve the natural damping behavior and to extend the stability limits.The operating conditions to be taken into consideration are : Good response in Voltage and reactive power control. Satisfactory steady state stability i.e. sufficient damping of electromagnetic and electromechanical transients. Transient stability for all stated conditions. Quick voltage recovery after fault clearance.Generator Protection- In CSTPS, near about 30 protection are given to the generator. Following points shows the requirement of protection Faults in generator reflect back on whole system. Generator faults may damage the generator winding which is so much costly. Also, this may damage the stator core, due to which severe mechanical torsion may produce. Fault current is flows through the winding even after generator is tripped; it is because of presence of trapped flux with in machine. There by increasing amount of fault damage. A fault produces high short circuit current.Following are the major protection are given to the generator- Mechanical protection. Protection against failure against prime mover. Failure of field. Over current. Over voltage. Over speed. Electrical protection. Unbalanced load. Stator winding.
Governing & Lubricating SystemTurbine in CSTPS is equipped with hydraulic mechanical governing system, which ensures smooth & safe operation of turbine & generator. This system is as follows-
Oil Tank
Bearings of turbine & generator4 control valveServomotor Operate PumpOil coolerLOPGovernor SOP DC ACMOP
4.5 Generating TransformerTransformer is a static device which step up or step down electrical energy. Function of transformer in power plant is to step up the voltage level to the transmission voltage level. Following table shows the transformer component and its function.componentfunction
coreProvide path for magnetic lines of flux.
Primary windingReceive electrical energy from energy source & creates magnetic field.
Secondary windingReceive electrical energy from primary winding & deliver to the load
enclosureProtect the above component from environmental attack
Core is made up of CRGO steel which have high magnetic properties due to which losses are reduced, &permeability is increaseTransformer bushing- transformer bushing is used to take out the terminal of the secondary winding to connect it to the transmission lines. Electrical power is the product of voltage and current, the insulation in a bushing must be capable of withstanding the voltage at which it is applied, and its current carrying conductor must be capable of carrying rated current without overheating the adjacent insulation. The bushing must also be able to withstand the various mechanical forces applied to it. Transformer insulation- In power transformer ,insulation is provided on two places in stator winding- Insulation between two winding turn in same layer. It is called as basic insulation. Insulation between two layer of winding . Basic insulation need not by excessive because the voltage difference between a winding turn and its neighboring turn is small in value and its the voltage across one winding turn. To insulate layer of winding from the other layer of winding paper insulation is used.Some of the salient features of our insulating materials are listed below: Excellent flexibility. Heat resistant. Moisture resistant. Brilliant electrical properties. Resistant to smoke or odour.
More about power transformer Paper insulation is used in between the two layer of winding it is because of the voltage difference between a winding turn and its neighboring winding turn one layer above or below it can be excessive and equals the voltage drop across a large number of turns in one windings layer. Therefore an extra insulation layer using paper strip is used between windings layers. Paper is used as a insulating material. it is because of cellulose paper have outstanding insulating properties.
In power transformer, when transformer is loaded with load. Current is flows through the transformer winding which causes I2R losses in transformer. Due to I2R losses heat is produced in transformer. hence as load goes on increasing amount of heat developed also increases. Excess amount of heat is developed is main reason behind transformer winding insulation breakdown. It also adversely affects the life of the transformer.Hence cooling system is employed in transformer
Transformer cooling system-In generating transformer, heat developed is extremely high. Natural air cooling is not sufficient to maintain transformer temrature. Hence oil cooling system is employed in power transformer. Oil is used in transformer to insulate & cool the winding. Transformer core & both winding are immersed in oil filled up in oil tank. This transformer oil should have following properties-Transformer oil properties
Electrical Physical Chemical Dielectric strength. Interfacial water content. Specific resistance. Tension. Acidity. Dielectric dissipation viscosity sludge content.Factor. Flash point.To cool this hot oil water cooling arrangement is provided. Water is circulated through the tubes. This hot water is cooled naturally.Transformer accessories
Breather- Whenever electrical power transformer is loaded, the temperature of the insulating oil increases, consequently the volume of the oil is increased. As the volume of the oil is increased, the air, above the oil level in conservator, will come out. Again at low oil temperature the volume of the oil is decreased, which cause, the volume of the oil to be decreased which again causes air to enter into conservator tank. The natural air always consists of more or less moisture in it and this moisture can be mixed up with oil if it is allowed to be entered into the transformer. The air moisture should be resisted during entering the air into the transformer, because moisture is very harmful for transformer insulation. A silica gel breatheris most commonly used as a means of filtering air from moisture.Silica gel breather for transformeris connected with conservator tank by means of breathing pipe.
Conservator It is the important periphery of transformer. Conservator is a tank which conserves oil. When transformer is loaded, oil inside it is heated. Due to heating oil expand. To provide extra volume conservator is employed. When oil gets cooled it returns to transformer oil tank.Conservator tank is partially filled with oil. So, the vaccum space is filled up by the atmospheric air. A filtering device, called breatheris attached with the conservator to ensure that only dry and clean air can enter intothe transformer. So, a conservator ensures the safety operation of atransformer. High temperature of oil also leads togeneratesludge, which occurs in the presence of air. If somehowthe transformeris subjected to a major fault, then the temperature rise becomes quite high and this causes vaporization of a part of the oil. This oil vapour forms an explosive mixture with air and can ignite and cause huge damage. So, to prevent the contact of oil and moisture-enriched air, conservator and breather assembly is used.
Buchholz Relay- Principle of operation of buchholz relay is very simple. It is a mechanical phenomenon
Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down ofcore of transformer, core heating, thetransformer insulating oilwill be decomposed in different hydrocarbon gases, CO2and CO. The gases produced due to decomposition oftransformer insulating oilwill accumulate in the upper part the Buchholz Container which causes fall of oil level in it.
Fall of oil level means lowering the position of float and thereby tilting the mercury switch. The contacts of this mercury switch are closed and an alarm circuit energized. Sometime due to oil leakage on the main tank air bubbles may be accumulated in the upper part the Buchholz Container which may also cause fall of oil level in it and alarm circuit will be energized. By collecting the accumulated gases from the gas release pockets on the top of the relay and by analyzing them one can predict the type of fault in the transformer.
Pressure release valve (P.R.V.)- It is a oil pressure relief valve. When oil is heated in transformer tank, it expands & it exerts pressure. If this pressure crosses certain limit then this P.R.V. is operated. Pressure relief is very simple construction. It contain an aluminum foil of required thickness is fitted on the opening of valve. Whenever surge developed in oil. The aluminum foil is busted.
Circuit Breaker- Circuit breaker is nothing but the switch. Circuit breaker is enclosed in a close vessel. It is operated by external media. When relay senses faults. Then relay sends signal and then circuit breaker breaks the circuit, Due to high voltage arc is produced in it. To extinguish it following methods are used-Bulk oilMinimum oilAir blastVacuumSF6 ( sulfur hexafluoride) CO2Isolator- Isolator is a switch which isolates the circuit. it is very simple in construction, & mechanically operated switch. Circuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off thecircuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works.Other transformer used in CSTPS Besides generating transformer following transformer are also used. These transformers have their own significance in power plant. Following are those transformers- Unit auxiliary transformer. Station transformer.Unit auxiliary transformer- Unit auxiliary transformer is employed to provide electricity to the auxiliaries like B.F.P., C.E.P., cooling pumps, of power plant unit.Before starting power generation in plant, these auxiliaries must be started. To start these auxiliaries initially power is taken from station transformer. After generating 30MW electricity, this UAT is connected to primary side of generating transformer. This arrangement has following advantages- Electricity can be taken from generator itself as well as from external source.When electricity is taken from generator itself, then CSTPS does not need to pay for electricity required for power plant auxiliaries. Primary side voltage of UAT is equal to the generating voltage level. & secondary voltage is equal to rated voltage of auxiliaries.General layout of UAT is as follows
Station transformer- Station transformer is also called as start-up transformer. This station transformer provides initial power to the plant auxiliaries, when no power is available. Following are the function of station transformer-Provide electricity to auxiliaries of power plant, during start-up. During normal operation, power to the C.H.P. auxiliaries. it is because of working of all unit is depends upon coal supply from C.H.P. following auxiliaries are installed in C.H.P.-Motors of conveyor belts.Motors of crusher.Lightening systemProvide power to the boiler feed pump (B.F.P.). it is very important auxiliary of thermal power plant. If boiler drum level is not maintain, then many complications (like boiler over heating which damage boiler tubes & many other components) are arises. Hence power supply to B.F.P. must be maintained. To maintain this supply power supply is taken from station transformer. Station transformer is connected to the grid transmission lines, hence power supply reliability is maintained, even though generating units are tripped up.
5. WATER TREATMENT PLANT Water treatment is important component of thermal power plant. water treatment plant working area are given below- Water treatment. Coal analysis. Oil tests. Flue gas analysis.Out of above mentioned area water treatment has prime importance. It is because of following reasons. Water contains different types of impurities, when these impurities enters inside boiler it will produce slice formation inside boiler tube due to which heat will not transfer properly hence tubes will melt. This impurities deposits inside the water tubes & block the water tubes. Presence of oxygen & carbon dioxide leads to corrosion of water tubes. In CSTPS, raw water is come from Erai Dam through pipelines. In Erai dam water is collected from the forest area near by it. So chemical impurities are not in raw water, therefore contamination of dam water is very less.but till following impurities are present in raw water- Turbidity- It is due to mud, minerals. Organic Matter- Organic present is due to the decomposition of dead animals, trees, leaves. Colour & odur- Colour & odur comes to water due to its organic matter contains. Dissolved gases- Gases like carbon monoxide, oxygen are dissolves in water. Living micro-organismWater is goes through following processes during treatment Pre-treatment. Post treatment.175000m^3 water is treated in one day in CSTPS.Pretreatment of water- Pretreatment is used to kill disease-causing organisms and help control taste and odor causing substances. A pretreatment chemical could be any number of oxidants or disinfectant. In water pre treatment plant the suspending particles are removed by screening the water. Following flow chart shows the water treatment process sequence-Water from Erai dam brought pipe lines
One day reserve tank
Flash mixture
Clariflocculator
Rapid gravity filter
Water sump
To post treatment plant
Function of each step is explain belowOne day reserve tank- One day reserve tank is used to store sufficient water storage, which will last long nearly one day. Sometimes water supply from dam is interrupted due to certain interrupts. Therefore to continue plant operation without any interrupts. Water from one day reserve is taken out. Water is stored in this tank, due these suspended particles, heavy impurities, silica particles settled down here. Besides this, one day reserve tank is used as a water source during any emergency.Flash mixture Alum & lime is added in water. Effect of adding alum & lime is explain as follows Alum not only settles certain hazardous chemicals and suspended solids but also bacterial colonies.Alum has a negative charge and tends to disperse in water very fast and very well .This causes it to join up with all of the offending particles and neutralize them. Now that the particles don't have any repelling charges, they tend to clump together into'flocs'.
Addition of lime softening can be achieved by addinglimein the form oflimewater,Ca(OH)2, which, in acarbonatationreaction withCO2, forms calcium carbonate precipitate, reacts next with multivalent cat ions to remove carbonate hardness, then reacts with anions to replace the non-carbonate hardness due to multivalent cations with non-carbonate hardness due to calcium. The process requires recarbonationthrough the addition of carbon dioxide to lower thepHwhich is raised during the initial softening process.[3]As lime is added to raw water, the pH is raised and the equilibrium ofcarbonatespecies in the water is shifted. Dissolvedcarbon dioxide(CO2) is changed intobicarbonate(HCO3-) and then carbonate (CO32-). This action causescalcium carbonateto precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated asmagnesium hydroxidein adouble displacementreaction. The byproduct from the reaction of water with lime produces large volumes of calcium carbonate and magnesium hydroxide sludge. This residual stream can substantially increase costs associated with the process. The sludge can be used as an agricultural soil amendment (it is analkalinitysupplement) and can also be used as a filler material in certain cementitious materials such as low strengthconcrete.Clariflocculator- Clariflocculator is a device in which flocculation & clarification reaction takes place simultaneously. Clariflocculator contains two concentric water tanks. The inner tank serves as a flocculation chamber. And the outer tank serve as a clarifier chamber. storror are used to During flocculation reaction flocs hydroxide is formed & then settled down. This floc is removed as a sludge by blowdown process.Mechanism of clariflocculator is as follows- These provide gentle agitation by slow moving paddles. This action serves to break up the mass rotation of liquid and promote mixing. The specially designed flocculation paddles enhance flocculation of the feed solids Agglomeration of the destabilized colloids is achieved as a result of particulate transport in the flocculation compartment Increased particle contact will promote floc growth and aids faster settling.There are 5 no. of clariflocculators are installed in CSTPS.
Rapid gravity Filter- Rapid sand filters contains relatively coarse sand and other granular media like bolder to remove particles and impurities that have been trapped in aflocthrough the use offlocculationchemicalstypically salts ofaluminiumoriron. Water and flocs flows through thefilter mediumunder gravity or under pumped pressure and the flocculated material is trapped in the sand matrix. Mixing, flocculation and sedimentation processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants, are often used in conjunction with the filtration system.
Rapid sand filters must be cleaned frequently, often several times a day, bybackwashing, which involves reversing the direction of the water and addingcompressed air. During backwashing, the bed isfluidizedand care must be taken not to wash away the media. Water sump- Water sump is a water storage tank in which pretreated water is stored. From thank water is sent for post treatment.Post treatment is different for both- Demineralized water (D.M. water) Soften waterDemineralized water- DM water is demineralized water . Dissolved impurities and dissolved gases in water. Sometimes water is hard, temporary hardness or permanent hardness and which is badly affects the boiler which helps formation of scales in sides the boiler. In D.M. water plant , water is passed through activated carbon filter for absorbing dissolved carbon dioxide. For removing temporary hardness and permanent hardness ion exchanging process used. By Ion - exchange process from which the minerals have been removed and get Demineralized water. Demineralized water have following qualities- Neutral P.H (6.5-7.5). Conductivity of water (
