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Power Station Comprising
INTRODUCTION For the power generation with 2x110 MW and 3x210 MW of K.T.P.S. authorities are required to be operative to active full operation. The auxiliaries are basically operation either on L.T. System i.e. 415 V 3 Ø power supply is made available to the system after providing the station transformer of 3x50 MVA capacity with voltage 220 KV/ 7.2/7.2 KV & different service transformers of capacity 1.0 MVA , 1.5 MVA , 2.0 MVA , which are located near the load centre as the transformer having the voltage of 6.6 KV /415 V. The 6.6 KV power is distributed through 6.6 KV interconnected Bus System for all the five units with a control through DC of 220 V.
The 415 V power supply is done through a L.T. SWGR (Switchgear) which are located nearby the distribution transformer as well as the load centers. The all in -comers, which are breaker controlled , are having the control the L.T. SWGR are having the control system on 110/ 220 V AC. The 6.6 KV power supply which are either MOCB (Minimum Oil Circuit Breaker) of JYOTI MAKE or Air Circuit Breakers. The 6.6 KV power supply to various draining equipment’s i.e. more is made through breakers which are either MOCB of Jyoti make air circuit breaker which are either of voltage makers as well as SF 6 of NGEF make. The LT supply is also controlled through air break circuit breaker which are either L&T make or English Electric Company of India. The various H.T. motors are switched on / started through on direct ON line (DOL) in order to inverse the availability of equipment at full efficiency without time gap.
Further , the 6.6 KV system which is normally in delta configuration and terms as an unearthed system so also to keep the running motor complete in operating condition in case of any one .phase of motor winding is earthed due to any one reason. Earthling is detected by an protection system with alarm facility to take remedial measures immediately and at the same time to maintain the generation level in the same condition, prior to occurring the earth fault the single phase earth fault is detected in due course till the motor is not earthed to other or another phase.
“ PUBLIC ADDRESS SYSTEM” is available through in area of each unit which helps in fast communication for prompt remedial measure. Soot Blowers are there in the boiler area on the furnace side or Zone which helps in blowing the soot / ash deposition regularly of the furnace wall / economizer tubes to keep heat transfer at the required parameter. In April 1973, Central Electricity Authority prepared a Project Report for power station comprising of the two units of each of capacity 110 MW for RSEB subsequently in September., 1975 this was revised by the Consultant Thermal Design Organization , Central Electricity Authority for invention of 2x110 MW units being manufactured by BHEL, Hyderabad in 1st Stage.
The planning commission cleared the project report in Sept., 1976 for installation of two units each of 110 MW in first estimated cost of Rs. 143 Crores. K.T.P.S. IS DESISIGNED IN FOUR STAGES
v STAGE I - 2x110 MW v STAGE II - 2X210 MW v STAGE III - 1X210 MW v STAGE IV - 1X195 MW * Total Power Generation - 1045 MW * To be commissioned shortly in August, 2003.
LOCATION The Kota Thermal Power Station is ideally on the left bank of Chambal River at Up Stream of Kota Barrage . The large expanse of water reached by the barrage provides an efficient direct circulation of cooling system for the power station. The 220 KV GSS is within ½ Kms. from the power station.
LAND Land measuring approx. 250 hectares was required for the project in 1976, For disposal of ash tank very near to power station is acquired which the ash in slurry form is disposed off through ash and slurry disposal plants.
COAL Coal India limited owns and operates all the major coal fields in India through its coal producing subsidiary companies viz. Eastern Coal Fields Limited, Western Coal Fields Limited/ Coal India limited is supply coal from its coal mines of coal producing subsidiaries BCCL, SECL & ECL to Kota Thermal Power Station through railway wagons. The average distances of SECL, ECL & BCCL are 800, 950 and 1350 Kms. respectively.
WATER The source of water for power station is reservoir formed by Kota Barrage on the Chambal River. In case of large capacity plants huge quantities of coal and water is required. The cost of transporting coal and water is particularly high. Therefore, as far as possible, the plant must be located near the pit rather than at load centre for load above 200 MW and 375 MW . The transportation of electrical energy is more economical as compared to the transportation of coal.
Design features The satisfactory design consists of the flowing steps. Estimation of cost. Selection of site. Capacity of Power Station. Selection of Boiler & Turbine. Selection of Condensing Unit. Selection of Electrical Generator. Selection of Cooling System. Design of Control and instrumentation system.
The design of steam power station requires wide experience as the subsequent operation and maintenance are greatly affected by its design. The most efficient design consist of properly sized component designed to operate safely and conveniently along with its auxiliaries and installation.
General Layout & Basic Idea A control system of station basically works on Rankin Cycle. Steam is produced in Boiler is exported in prime mover and is condensed in condenser to be fed into the boiler again. In practice of good number of modifications are affected so as to have heat economy and to increase the thermal efficiency of plant.
The Kota Thermal Power Station is divided into four main circuits :
v Fuel and Ash Circuit. v Air and Gas Circuit. v Feed water and Steam Circuit. v Cooling Water Circuit.
Fuel & Ash Circuit
Fuel from the storage is fed to the boiler through fuel handling device . The fuel used in KTPS is coal , which on combustion in the boiler produced the ash. The quantity of ash produced is approximately 35-40% of coal used. This ash is collected at the back of the boiler and removed to ash storage tank through ash disposal equipment.
Air and Gas Circuit
Air from the atmosphere is supplied to the combustion chamber of Boiler through the action of forced draft fan and induced draft fan. The flue gas gases are first pass around the boiler tubes and super heated tubes in the furnace, next through dust collector (ESP) & then economizer. Finally, they are exhausted to the atmosphere through fans. Feed Water and Steam Circuit
The condensate leaving the condenser is first heated in low pressure (LP) heaters through extracted steam from the lower pressure extraction of the turbine. Then its goes to dearator where extra air and non-condensable gases are removed from the hot water to avoid pitting / oxidation. From deaerator it goes to boiler feed pump which increases the pressure of the water. From the BFP it passes through the high pressure heaters. A small part of water and steam is lost while passing through different components therefore water is added in hot well. This water is called the make up water. Thereafter, feed water enters into the boiler drum through economizer. In boiler tubes water circulates because of density difference in lower and higher temperature section of the boiler. The wet steam passes through superheated. From superheated it goes into the HP turbine after expanding in the HP turbine. The low pressure steam called the cold reheat steam (CRH) goes to the reheater( boiler). From reheater it goes to IP turbine and then to the LP turbine and then exhausted through the condenser into hotwell.
Cooling Water Circuit
A large quantity of cooling water is required to condense the steam in condenser and marinating low pressure in it. The water is drawn from reservoir and after use it is drained into the river. ELECTRICITY GENERATOR AT K.T.P.S.
Thermal power station burns the fuel and use the resultant heat to raise the steam which drives the turbo-generator. The fuel may be “Fossil” ( Coal , Oil and Natural Gas) whichever fuel is used the object is same to convert the heat into mechanical energy to electrical energy by rotating a magnet inside the set of winding. In a coal fired thermal power station other raw materials are air and water. The coal is brought to station by train or other means travels from the coal handling system.
i) By conveyer belts to coal bunkers from where it is fed to pulverizing mills.
ii) Mills grind it fine as face powder.
iii) Then this powdered coal mixed with preheated air is blow into boiler by a fan known as primary air fan (PA fan).
iv) When it burns more like a gas as solid in conventional domestic or industrial grate with additional amount of air called secondary air supplied by “Forced Draft Fan”.
As the coal has been grinded so resultant ash is also as fine as powder. Some of its fine particles blinds together to form lumps which falls into the ash pit at the bottom of furnace.
v) The water quenched ash from the bottom of furnace is carried out boiler to pit for subsequent disposal.
vi) Most of ash still in fine particles form is carried out to electrostatic precipitators where it is trapped by electrode charged with high voltage electricity. The dust is then conveyed to the disposal area or to bunkers for sale. vii) Now after passing through ESP few gases are discharged upto chimney by “Induced Draft Fan”.
Meanwhile the heat reloaded from the coal has been absorbed by kilometers long tubes which lies in boiler walls inside the tubes “ Boiler Feed Water” which is transferred into turbine blades and makes them rotate. To the end of the turbine rotor of generator is coupled, so that when turbine rotates the rotor turns with it. The rotor is housed inside the stator having coil of copper bars in which electric is produced through the movement of magnetic field created by rotor.
The electricity passes from the stator winding to the transformer which steps up the voltage so that it can be transmitted effectively over the power line of grid. The steam which has given up its heat energy in changed back into a condenser so that it is ready for reuse. The cold water continuously pumped in condenser. The steam passing around the tubes looses heat and rapidly change into water. But these two types of water ( boiler feed water and cooling water ) must never mix together. The cooling water is drawn from the river but the Boiler Feed Water must be pure than potable water ( DM Water). Now the question arises why do we bother to change steam from turbine to water when it is to be heated up again immediately ? Laws of Physics gives the answer which states that the boiling point of water is related to pressure. The lower the pressure lower the boiling point temperature. Turbine designer wants boiling point temperature as low as possible because it can only utilize the energy from steam when change back to water, he can get no more work out at it. So there is a condenser which by rapidly changing the steam into water a vacuum. The vacuum results in a must power at lower boiling points which in turn mean it can continue getting out of steam will below 1000C at which it would change into water.
To condense volume of cooling water is huge and continuous volume of cooling water is essential. In most of the power stations , the same water is to be used over and over again, so the heat which the water extract from the steam in the condenser is removed by pumping water out of cooling tower. The cooling tower is simple concrete shell acting of air. The water is sprayed out at top of tower and as it falls into pond beneath it cooled by the upward draft of air. The cold water in the pond is then re-circulated by pumps to condensers. Invariably however some of the water drawn upwards as vapor by the draft . SWITCH YARD
220 KV System
Two 220 KV bus bars have been provided in switch yard and are inter-connected through a bus coupler. Each of the two 110 MW generator is connected to this system through a step up of 125 MVA 240/ 11 KV yard generator transformer. There are two step down transformer each feeding 6.6 KV system ( Station Switchyard ) viz. BS-IS & SB-IB. Each station transformer has two windings one secondary side and is rated for 50/25/25 mva , 270/7/7.2 kva four feeder take off from 220 switch yard, two to SKATPURA ,GSS and other to HEERAPURA , Jaipur GSS. Each of four feeder are provided with bypass isolators which is connected across line breaker and breaker isolator. By closing bus coupler between 220 KV buses and putting line feeders whose breaker required maintenance of any one bus through by pass isolators and all other line feeders whose breaker is by passed is then transformed to bus coupler breaker. A brief description of equipments of 220 KV system is as follows.
CIRCUIT BREAKERS Each of generator transformer, station transformer, line feeder and bus coupler is provided with minimum oil circuit breaker of BHEL make. It is rated for 245 KW, 2500 A and 13400 MVA circuit breaker is used to break the circuit either in load condition or in no load condition.
ISOLATOR All the isolators are provided in 220KV switchyard and are motor operated. Triple pole double breaker type and power switch yard L&T make these and are rates for 245 KV and 1250 A. The four isolators are provided with earth switch.
CIRCUIT TRANSFORMER All the 220 KV current transformers are provided for measuring and protection. They are BHEL make, single phase, oil filled nitrogen sealed outdoor type. All the E.T.S. are multi-cored with each core having specification upon duty it is to perform. Feeder circuit have 5 cores.
1) Bus bar protection core I 1250/250/IA. 2) Distance protection core II 600-300/IA. 3) O/C and E/F protection core 600-300 /IA. 4) For metering and measuring 600-300/ IA.
POTENTIAL TRANSFORMER
Each of 220 KV buses is provided with three P.T.’S are core for each phase of BHEL make. There are single phase , oil filled outdoor. N2 sealed , elicitor magnetic type P.T. has two secondary windings on secondary side and selected for 220/53 KV, 10/53 KV. One secondary winding has O/P of 500 mva accuracy class .5 and is used for metering other secondary winding has O/) of 200 mva accuracy class 3 and used for protection.
LIGHTENING ARRESTOR For protection against lightening each of line feeder, generator transformer , station transformer has been provided with three L.A. (one for each phase). All the L.A. are 2 Ø outdoor type and are rated for 198 KV these are manufactured by W.S. insulator. The L.A. of generator transformer and station transformer are located near them.
It has larger value of capacitance and will change upto line voltage. If we have to do some work on line, first earth line through earthing isolator for discharging the line capacitance and then work.
220 KV MOCB Manufacturer : BHEL, Hyderabad. Total Nos. : 9 Type : HLR 245/2503 B-I. Rated Frequency. : 50 Hz. Nominal Current. : 2240 Amp. Type of operating mechanism. : Motor charging Spring Closed.
220 KV ISOLATORS Manufacturer : A&S Power SWGR LTD Number : 36 Type : Double break operated. Rated Current. : 1250 Amp. No. of Phase. : 3 Ø Rated Voltage. : 245 KV.
220 KV Current Transformer Manufacturer. : BHEL, Trichy. Type : Outdoor, Oil filled. Rated Voltage. : 220 KV. Nominal : 220 KV. Max. : 245 KV. Rated Frequency. : 50 Hz. No. of Phase. : 1-Ø Class of Insulation : A. Rated Primary Voltage. : 2220/ 53 V. Secondary Voltage Wdg.I : 110/53 V. Wdg.II. : 110/53 V.
MAIN BUS BAR
Material : Electrolyte Aluminium. Type of Insulation. : Air. Maximum clearance B/W Phase.: 19.3 mm. Minimum clearance B/W Phase.: 15.3 mm.
CIRCUIT BREAKER
Make : L&T Circuit Breaker Ltd. Type : Air Circuit Breaker. Maximum Continuous Voltage : 500 V. for circuit breaker operation. No. of Phase. : 3-Ø Rated Voltage. : 415 V.
POWER CAPACITOR
Make : L&T Limited. Type. : ML1 ML2 ML3 ML4 ML8 ML12. No. of Poles. : 3. Rated Voltage for main Contacts. : 500 V.
220 KV LIGNTENING ARRESTOR
Manufacturer. : W-S Isolators India Ltd. Chennai. Type : Heavy Duty CPL II. No. of Phases. : 3-Ø Rated Voltage. : 198 KV. Nominal Discharge Current. : 10 KA. High Current Impulse. : 100 KA. Long Duration Rating. : 500 KA.
DC SYSTEM
INTRODUCTION
D.C. system play important role in all the major industries. We can call it as ‘back bone’ of industries. It can be utilized and stored during specific duration. Charging the battery initially to keep the batteries trickily charged and to mention the and load in normal time, a separate equipment called ‘charger’ is a must.
RECTIFIER
A.C. supply is converted to D.C. by this component only. It has two major classification. The basic components which are use now a days are diodes and SCRs.
CLASSIFICATION
Rectifiers are classified as follows : 1) Half wave rectifier. 2) Full wave rectifier. a) Uncontrolled. b) Half controlled. c) Fully controlled.
FILTER
Filters are used for smoothing the D.C. output voltage from rectifier unit. Chock input filter, and capacitor input filter is two type of filter in chock, input filters, the chock blocks A C ripples if any ripples get through chock, passes through capacitor ( very low xc) which appears to open for D C signals.
In capacitor input filter the capacitor following output wave and get charge to peak voltage Vp when rectifier conducts. One output voltage stands to reduce from peak voltage, the capacitor stands discharging and keep voltage almost constant.
BLOCKER DIODES
To get block the back feed from batteries to chargers, blockers diodes are connected in series with filter output.
Based on charges application, we call the chargers as float charges and boost charger. Float charger, keeps the battery in trickle charging condition and meant the load in normal condition. If there is any small interruption in A.C. supply batteries will meet the load. On resumption of supply, batteries will get charged. Boost charger is normally used to charge during initial charging and in the case of heavy discharging. It can also meet the loss but not directly. By making special tapping provision from batteries.
D.C.OUTPUT VARIATION
There are a number of methods to vary the charger output voltage to certain extent by making modification circuit. In the controlled rectifer bridge by having feed back system we can get the desired voltage by presenting the reference voltage in the un-controlled rectifier bridge, by varying the A.C. input voltage we can get desired output voltage.
In high rating charges , main transformer secondary is connected in series with another transformer secondary booster transformer primary can be varied by connecting the dim merstal. In this variation will be smooth. In low rating chargers , it is achieved by taking required number of tapping from secondary for A.C. voltage variation.
MODE OF OPERATIONS
Charger can be operated in two modes depending upon its design.
1) Constant Voltage Mode :
Here the charger output voltage is always maintained at constant voltage equal to reference voltage irrespective of charger output current. So some current limitation has to be provided in this mode. This mode will be ideal for keeping the batteries in floating condition and to meet the loads.
2) Constant Current Mode :
In this charger put current will be maintained at reference current sitting. It do not take care of voltage condition. This mode will be useful for calculating all rating of batteries charged.
PROTECTIONS
In A C side charges may be provided with overload protection to avoid overload, fuses and single phasing and phase fail protection. Sometime provided with AC under voltage and AC abnormal voltage protection. In DC side , Diodes and SCRs will be provided with semiconductor fuses for fast action on short cut faults. Output will be provided with HRC fuses converted output will be continuously monitored in each link to find the failure.
TURBO GENERATOR
1. THEORY : TURBO GENERATOR manufactured by B.H.E.L. and incorporated with most modern design concepts and constructional features , which ensures reliability, with constructional & operational economy.
The generator stator is a tight construction, supporting & enclosing the stator windings , core and hydrogen coolers. Cooling medium hydrogen is contained within frame & circulated by fans mounted at either ends of rotor. The generator is driven by directly coupled steam turbine at a speed of 3000 r.p.m. the Generator is designed for continuous operation at the rated output . Temperature detectors and other devices installed or connected within then machine, permit the windings, teeth core & hydrogen temperature, pressure & purity in machine under the conditions. The source of excitation of rotor windings is thyrister controlled D.C. supply. The auxiliary equipment’s supplied with the machine suppresses and enables the control of hydrogen pressure and purity, shaft sealing lubricating oils. There is a provision for cooling water in order to maintain a constant temperature of coolant (hydrogen) which controls the temperature of windings.
2. STATOR FRAME : The stator frame of welded steel frame construction, which gives sufficient & necessary rigidity to minimize the vibrations and to withstand the thermal gas pressure. Heavy end shields enclose the ends of frame and form mounting of generator bearings and radial shaft seals. Ribs subdivide the frame and axial members to form duct from which the cooling gas to & fro radial ducts in the core and is re-circulated through internally mounted coolers. All the gas ducts are designed so as to secure the balanced flow of hydrogen to all parts of the core. The stator constructed in a single piece houses the core and windings. The horizontally mounted water cooled gas coolers being so arranged that it may be cleaned on the water side without opening the machine to atmosphere. All welded joints exposed to hydrogen are specially made to prevent leakage. The complete frame is subjected to hydraulic test at a pressure of 7 ATA. 3. STATOR CORE : - It is built up of special sheet laminations and whose assembly is supported by a special guide bass. The method of construction ensures that the core is firmly supported at a large number of points on its periphery. The laminations of high quality silicon steel which combines high permeability with low hystersis and eddy current losses. After stamping each lamination is varnished on both sides with two coats. The segment of insulating material is inserted at frequent intervals to provide additional insulation. The laminations are stamped out with accurately fine combination of ties. Laminations are assembled on guide bass of group separated by radial ducts to provide ventilation passage. The ventilation ducts are disposed so as to distribute the gas evenly over the core & in particularly to give adequate supports to the teeth. At frequent intervals during stacking the assembled laminations are passed together in powerful hydraulic press to ensure tight core which is finally kept between heavy clamping plates which are non-magnetic steel. Use of non-magnetic steel reduces considerably by heating of end iron clamping. The footed region of the core is provided by pressing figures of non-magnetic steel, which are welded to the inner periphery of the clamping plates. In order to reduce the losses in the ends packets special dampers are provided at either ends of core. Mostly dampers are provided to prevent hunting in ac machines.
4. STATOR BARS : Stator bars are manufactured as half bars. Each stator half coil is composed of double glass cover and bars of copper transposed in straight portion of “ Robill Method” so that each strip occupies every radial portion in the bar. For an equal length along the bar. They are made in strips to reduce skin effect. The winding overhead is in volute shape. The overhung portion of the bar is divided into four quadrants & insulated . The arrangement reduces additional losses due to damping currents which otherwise be present due to self-induced non-uniform flux distribution in the coil slots. The main distribution for the bar consists of resin rich mica loosed thermosetting epoxy. This has excellent mechanical and electrical properties & does not require any impregnation. Its moisture absorbing tendency is very low and behavior of mica is for superior than any other conventional tape insulation system. Semi-conductor coating is also applied to a part of overhung with a straight overlap of conductive coil in the sides to reduce eddy currents to minimum. Conductor material is electrolytic copper connections brazed with free coating silver alloy to obtain joints , which are both electrically & mechanically sound.
5. STATOR WINDINGS : Stator windings are double star layers , lap wound, three phase, short pitch type. The top & bottom are brazed and insulated at either end to form turns. Several such turns form a phase . Phases are connected to form a double star winding. The end of winding form involute shape ends, inclined towards machine axis by 20o, thus form a basket winding with total induced conical angle of 400 . Due to this stray load losses in the stator ends to zero. The arrangement of complete stator winding electrical circuit is viewed from turbine end of generator & rotor windings. Slot numbering is clockwise from turbine end. A thick line identifies the top bar in slot No.1 . End windings will be sealed against movement of short circuit by both axial & peripheral bracing. The later consists of hardened glass laminated blocks inserted between adjacent coil sides in coil overhangs, so that with the coils , they form a continuous rigid ring. Glass cord or top is used lashing the packing of blocks. The complete assembly is secured b y high tensile brass blots. The winding is designed to withstand short circuit stresses. The exposed portion of windings are finally coated. Insulation of individual bars & stator windings at various stress is tested with applied high voltages of AC of Hz.
6. TERMINAL BUSHINGS : Six output leads (3 long,3 short) have been brought out of the coming on the exciter side. External connections are to be made to the three shorter terminals, which are phase terminals. The large terminals are of neutral & current transformer is inserted. The conductor of Generator terminal bushing having hollow copper tubes with Copper brazed at the ends to avoid leakage of hydrogen. Hollow portions enables bushings to be hydrogen cooled. Ends of bushings are Silver-plated : middle portion of the bushing is adequately insulated & has a circular flange for bolting the stator casing. Gaskets are provided between the Flange of terminal bushings and castings to make it absolutely gas tight.
7. BEARINGS : Generator bearings have electrical seats of consists of steel bodies with removable steel pads. The bearings are formed for forced lubrication of oil at a pressure of 2-3 ATM/ From the same pump that supplies oils to the turbine , bearings & governing gears. There is a provision to ensure & measure the rotor bearing temperature by inserting a resistance thermometer in the oil pockets.
8. VENTILATION SYSTEM : The machine is designed with ventilation system having 2 ATM rated hydrogen pressure. Two axial fans mounted on either side of the rotor to ensure circulation of hydrogen. The stator is designed for radial ventilation by stem. The end stator core packets & core clamping & plates are intensively cooled by Hydrogen through special ventilation system. Design of special ventilation is so as to ensure almost uniform temperature of rotor windings and stator core. Rated load operating temperature is well within the limits corresponding to the Class B operation. Embedded Resistance Temperature Detectors do continuous monitoring of Hydrogen temperature at active parts of Generator.
RESISTANCE TEMPERATURE DETECTORS (R.T.D.) : An R.T.D. is a point resistance element. Operation of R.T.D. depends on the principal that electrical resistance of metallic conductor varies linearly with temperature.
Ø APPLICATIONS : RTD & its associated equipments are designed for use with Generator to determine temperature at various parts & places. The equipment’s consists of two parts :- 1. Switch Board Equipment : is usually includes a temperature indicating meter, test resistor transfer switch & leads. 2. Machine Equipment : is usually includes temperature R.T.D. leads and terminal blocks with grounding connections.
Leads from RTD are brought out to the terminal board by cables through conduits to protect them from physical damage and from contact with high voltage coils. Some RTDs are in stator teeth with 7 spacers , 7 RTDs between the coil sides in stator slots with 7 spacers and 3 RTDs are there in the stator core with spacers. The location of RTDs is in three phases i.e. in the centre of machine , in each region of machine and midway between them. The detectors in the stator slots are distributed uniformly in all three phases. Measurement of temperature of Hydrogen cooling water for Hydrogen coolers & metals is as :
Six RTDs are provided at the inlets of each of six individual Hydrogen cooler elements for measurement of temperature of Hydrogen, similarly Six RTDs are provided at the outlets also. One RTD along-with one spacer is provided in the lower part of stator frame for measurement & signalization of hot Hydrogen. Six RTDs are provided at outlets of each of six individual Hydrogen Cooler elements for measurement of temperature of cooling water at the outlet.
MEASUREMENT OF BEARING TEMPERATURE
Two RTDs are provided in the shelves of Turbo-Generator for measurement of signalization of the bearing metal cap. All the terminals of RTDs are brought out to a common terminal board located on the stator frame.
HYDROGEN COOLERS
Three Hydrogen Coolers each comprising of two individual units are mounted inside the stator frame. The inlet and outlet of cooling water from both of machine i.e. from non-driving side as well as turbine side. The Clearing of the individual cooler element can be carried out from both ends of the Generator even during operation. The assembly of individual cooler elements in stator frame is however carried out only from the non-driving side.
ROTOR
Rotor shaft consists of single piece alloy steel forging of high mechanical and magnetic properties performance test includes :- 1. Tensile test on specimen piece. 2. Surface examination. 3. Sulfur prist tests. 4. Magnetic crack detection . 5. Visual examination of bore. 6. Ultrasonic examination.
Slots are milled on the rotor gorging to receive the rotor winding. Transverse slots machined in the pole faces of the rotor to equalize the moment of inertia in direct and quadrilateral axis of rotor with a view minimizing the double frequency.
VIBRATION OF ROTOR : The fully brazed rotor is dynamically balanced and subject to 120 % over speed test at the work balancing tunnel so as to ensure reliable operation. ROTOR WINDINGS : rotor winding is of direct coil type and consists of parallel strips of very high conductivity Silver Bearing Copper, bent on edge to form coil. The coils are placed in impregnated glass, laminated short shells; using glass strips inter turn insulation and will be brazed at the end to form continuous winding. The complete winging will be packed at high temperature and pressed to size by heavy steel damping rings. When the windings have cooled, heavy dove tail wedges of non-magnetic materials will seal the insulation at the top of slot portion. The cooling medium hydrogen gas will be brought in direct contact with copper by means of radial slots in embedded portion. Treated glass spacers inserted between the coils and solid ring prevent lateral movement of coil overhang. The formation and description of glass spacer is such as to leave ample space for ventilation.
BEARINGS
The bearings are self-aligned & consist of slip steel shells linked with special bearing metal having very low coefficient of friction. The bore is machined on an elliptical shape so as to increase the mechanical stability of the rotor. The bearing are pressure lubricated from the turbine oil supply. Special precautions are taken to prevent oil & oil vapor from shaft seals and bearing along the shaft. The circulation of shaft current is liable to damage. The bearing surface is protected by insulation so placed that the bearings, seals & necessary pipes are inclined from the frame.
SLIP RINGS
The slip rings are made of forged steel. They are located at either side of Generator Shaft. The slip ring towards the exciter side is given +ve polarity initially. They have helical grooves and skewed holes in the body for cooling purpose by air. Calibrated mica is first built up to required thickness on the shaft where slip rings are located. The slip rings are insulated from the rotor shaft. Excitation current is supplied to the rotor winding. Through the slip rings, which are connected to the winding. On one end and to the slip ring on the other end with insulated ( terminal) studs passing ‘though’ the radial holes in the rotor shaft. The terminal studs at both the ends of excitation leads are fitted gas cat seals to prevent leakage.
BUSH GEAR ASEMBLY
Generator bushes are made from the various compositions of natural graphite and binding material. They have a low coefficient of friction and are self lubricating. The brushes are provided with a double flexible copper or pigtails. A helical spring is mounted rapidly over each bush so that pressure is applied on the centerline of bush. A metal cap is riveted to the brass bead and is provided with a hole to maintain the position of the spring plug. Several brush holder, each carrying on brush in radial position are fixed to a silver plated copper studs mounted on the collecting arm concentric with each slip rings. The collecting arm is made out of a copper strip.
DRYING OF WINDING
Generator stator bars are insulated with mica insulation , which is homogeneous in nature and practically impervious to moisture, and reduce time required to draught. The insulation resistance of the stator phase winging against earth and with reference to other phases under hot condition shall not be less than the value obtained automatically.
Rin = µ/(s/100+1000) m 52 U = rated winding Voltage under test.
Rin = insulation resistance under hot conditions Rated o/p of turbo generator.
The insulation resistance of entire excitation system circuit. In hot condition must not fall below 0.5 m 52. The insulation resistance in calculated as per the formula :
Rin = Rv (U1 +U2) /( U-1) Rin = Insulation resistance of exciter () Rv = Internal resistance of voltmeter () U1 = Voltage measured btw. Slip ring & shaft/ earth(volts).
When starting the drying process, the winding insulation resistance will usually decrease when the drying process becomes effective; the insulation will gradually increase.
COOLING SYSTEM : a) GENERAL: In KTPS hydrogen cooling system is employed for generator cooling. Hydrogen is used for cooling medium primarily because of its superior cooling properties & low density. Thermal conductivity of hydrogen 7.3 times of air. It also has higher transfer co-efficient . Its ability to transfer heat through forced convection is about 75% better than air. Density of hydrogen is approx. 7/14 of the air at a given temperature and pressure. This reduces the windage losses in high speed machine like turbo-generator. Increasing the hydrogen pressure the machine improve its capacity to absorb & remote heat. Relative cooling properties of air and hydrogen are given below :-
1) Elimination of fire risk because hydrogen will not support combustion. 2) Corona discharge is not harmful to insula ? since oxidation is not possible. 3) Smooth operation of machine in view of vertical elimination of wind age noise & the use of heavy gas light enclosure and dirty proby casing.
At pressure 0.035 atm. of hydrogen heat carrying capacity is 1. But at 2.0atm. of hydrogen heat carrying capacity is 1.95 to overcome the serious possibility of hydrogen explosion with in the machine and to ensure the safety of operation purity of hydrogen on the generator. Casing must be maintained as high as possible. The purity of hydrogen should be 98% above but should not be less than 98% . In case of hydrogen purity drops below 98% an alarm is provided.
b) HYDROGEN DRYERS : Two nos. of dryers are provided to absorb the hydrogen in the Generator. Moisture in this gas is absorbed by silica gel in the dryer as the absorbed gas passes through it. The satural of silica gel is indicated by change in its color from blue to pink. The silica gel is reactivated by heating. By suitable change over from drier to the other on un-interrupted drying is achieved.
c) HYDROGEN FILLING SYSTEM : The filling operation is carried out in two steps. 1) Scavenging the air by CO2 with hydrogen.
Before filling the hydrogen at a pressure of 2 atm. In the machine it is necessary to store : - at least 18 cylinders of 20 Kg. CO2 & 48 cylinders of hydrogen.
EXCITATION SYSTEM
The electric power Generators require direct current excited magnets for its field system. The excitation system must be reliable, stable in operation and must response quickly to excitation current requirements. When excitation system response is controlled by fast acting regulators, it is chiefly dependent on exciter. Exciter supply is given from transformer and then rectified.
(A) Function of excitation system : The main function of excitation system is to supply required excitation current at rated load condition of turbo Generator. It should be able to adjust the field current of the Generator, either by normal controller automatic control so that for all operation & between no load and rated load. The terminal voltage of the system machine is maintained at its value. The excitation system makes contribution improving power system stability steady state condition. The excitation system that are commonly termed quick response system and have following principal feature :- Exciter of quick response & high voltage of not less than 1.4 times the rated filed voltage and nominal exciter response of minimum 0.5.
(B) Type of excitation system : There have been many developments in excitation system design. There has been continue reach among the design and the use alike from improving the excitation system performance. The ultimate is to achieve stability; accuracy etc. the modern excitation system adopted presently on BHEL make turbo-generator. I. Conventional DC excitation system . Brushes excitation system.
STATIC EXCITATION SYSTEM
In KTPS static excitation system is provided it mainly consists of the following :- 1) Rectifier transformer. 2) Nos. of thyristor converters. 3) An automatic voltage regulator(AVR). 4) Field suppression equipment. 5) Field flashing equipment.
GENERAL ARRANGEMENT : In the excitation system the power required for excitation of Generation are tapped from 11 KV bus ducts through a step down rectifier transformer. After rectification in thermistor, converter, the DC power is fed to the Generator field winding through a field breaker. The AVR control the o/p from thyristor converter by adjusting the firing angle depending upon Generator voltages. The field flashing system facilitates initial built up of the Generator voltage from the static AC or DC supply.
(I) RECTIFIER TRANSFORMER : This transformer steps down the bus voltage 11 KV to 640 V and has a rating of 1360 KVA. It is dry type, it is however provided with current relays and two temperature sensors. (II) A THYRISTOR CONVERTOR : The thyristor panel and are intended for controlled rectification of AC Input power. 6. Thyristor converter are connected in parallel each rates for continuous current o/p of 20 % of the rated capacity i.e. 20 % reserve. Each thyristor converter consists of 6 thyristor connected in 3-3 , full wave, 6-pulse bridge from and they are cooled by fans provided with a fuse for protection against short circuit. (III) AUTOMATIC VOLTAGE CONTROLS : The AVR is a transistorized thyristor controlled equipment with very fast response. The AVR is also having provision of stator and rotor currents limits and load angle limits for optimum utilization of lagging and leading reactive capacities of generator. (IV) FIELD SUPRESSION EQUIPMENT : The field equipment consists of a field breaker with discharge resistors. The field breakers have 4 main breaking contacts and two discharge contacts, which close before main contact break.
(a) A very fast response. (b) Extremely reliable in view of static components. (c) Low maintenance cost. (d) High efficiency. (e) Fast field suppression through field and discharge resistance as well as through thyristor bridge, feeding the Generator field.
OPERATION
After bringing the speed to operation speed say 3000 r.p.m. , the voltage is slowly built up with the help of excitation system. This action is taken for synchronizing the Generator. (A) SYNCHRONIZING : For synchronizing the Generator to the grid syst |