INTRODUCTION

National Thermal Power Corporation Limited was formed in 1975 to plan, promote and organize an integrated and efficient development of Central Sector Power Stations.

The Singrauli Super Thermal Power Station was the first of the series of pithead power stations along with 400kV AC transmission line network. It is located on the banks of Govind Ballabh Pant Sagar (Rihand Reservoir), about 200km south of Varanasi in the Sonebhadra district of Uttar Pradesh.

For coal transportation, a captive railway system with rapid loading and unloading facility known as Merry-Go Round (MGR), continuously hauls coal from the Jayant block of Singrauli coalfields to the plant site. The rake consists of 30 wagons and will deliver 1800 MT of coal in each cycle. The average daily consumption of coal is 25,000 MT per day i.e. 8.0 million tonnes per annum considering average calorific value of 4000 kcal/kg and 7000 hrs of operation in an year for the ultimate capacity of the plant of 2000 MW having 5 units of 200 MW each and 2 units of 500 MW each.

The 5200MW generating units of Stage I are each equipped with coal-fired, regenerative, re-heat type steam generators with electrostatic precipitators, each generating 700 tonnes/hr of steam at 138 kg/cm pressure and 535C temperature. The steam generator feeds steam to a condensing, horizontal, tandem compound 3-cylinder re-heat type turbo generator rotating at 3000 rpm and each generates 200 MW. Three phase generator transformer of 250 MVA capacity steps up the generation voltage from 15.75 KV to 400 KV.

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Cooling water from the Rihand Reservoir is drawn through an approach channel. It is then pumped into concrete intake duct by vertical pumps of 15000 m /hr capacity each. From the ducts, the water is circulated through condensers and is then discharged into a duct from where it flows into an open channel. This open channel carries the water for a distance of 6 kms to affect sufficient cooling before it joins back into Rihand Reservoir.

The 2500MW generating units of Stage II are each equipped with coal-fired, regenerative, re-heat type steam generators with electrostatic precipitators, each generating 1725 tonnes/hr of steam at 178 kg/cm pressure and 540C temperature. The steam generator feeds steam to a condensing, horizontal, tandem compound 3-cylinder re-heat type turbo generator rotating at 3000 rpm and each generates 500 MW. Turbine is a single shaft machine with separate high pressure (HP), intermediate pressure (IP) and low pressure (LP) parts. The HP part being a single flow cylinder and the IP and LP parts double flow cylinders. The individual rotor generator is connected by rigid coupling. The generator is three-phase, horizontal, 2-pole cylindrical rotor type with a rated output of 588 MVA and terminal voltage of 21 KV and full load current of 16,200 A. Three single phase generator transformers of 200 MVA capacity each steps up the generation voltage from 21 KV to 400 KV.The circulating water system for cooling the steam in condensers is an open cycle system utilizing the water from Rihand Reservoir through 2.9 km long intake channel and pumped through underground RCC duct and the return water is discharged to the reservoir through 6 km long discharge canal. The intake channel and the discharge canal are common for both stage I and II units. For supplying cooling water 6 nos. of vertical pumps each of 27,000 m /hr capacity have been provided.

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To reduce the air pollution 220 m high multi-flue stack are there for better dispersion of the gases emitted by the boilers. There are total four stacks is SSTPS one for units 1, 2 & 3, second for units 4 & 5 and one each for units 6 & 7. Electrostatic precipitators are provided between the boiler and stack in each unit to precipitate the ash content of the flue gases and help in the reduction of air pollution. The ash so collected is dumped in the ash disposal yard in the slurry form.

The main sites in the thermal power plant are as follows: Steam generator Turbine Generator Switchyard Control & instrumentation

The various off-sites in the thermal power plant are as follows: Coal handling plant Water treatment plant Circulating water system Ash handling system Compressed air system Hydrogen generating plant

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operation

COAL TO STEAM

STEAM TO MECHANICAL POWER

MECHANICAL POWER TO ELECTRICITY

SWITCHING AND TRANSMISSION

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COAL TO STEAM

Coal from the coal wagons is unloaded in the coal handling plant. This coal is transported upto the raw coal bunkers with the help of belt conveyors. Coal is transported to bowl mills by coal feeders. The coal is pulverized in the bowl mill, where it is ground to a powder form. The mill consists of a round metallic table on which coal particles fall. This table is rotated with the help of a motor. There are three large steel rollers, which are spaced 120 apart. When there is no coal, these rollers do not rotate but when the coal is fed to the table it packs up between the roller and the table and this forces the roller to rotate. Coal is crushed by the crushing action between the rollers and the rotating table. This crushed coal is taken away to the furnace through coal pipes with the help of hot and cold air mixture from the primary air (P.A.) fan. The P.A. fan takes atmospheric air, a part of which is sent to the air preheaters for heating while a part goes directly to the mill for temperature control. Atmospheric air from forced draft (F.D.0 fan is heated in the air heaters and sent to the furnace as combustion air.

Water from the boiler feed pump passes through economiser and reaches the boiler drum. Water from the drum passes through down comers and goes to bottom ring header. Water from the bottom ring header is divided to all the four sides of the furnace. Due to heat and the density difference water rises up in the water wall tubes. Water is partly converted into steam as it rises up in the furnace. This steam and water mixture is again taken to the boiler drum where the steam is separated from water. Water follows the same path while steam is sent to the superheaters for

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superheating. The superheaters are located inside the furnace and the steam is superheated (540C) and finally goes to the turbine.

Flue gases from the furnace are extracted by the induced draft (I.D.) fan, which maintains a balanced draft in the furnace with F.D. fan. These flue gases emit their heat energy to various superheaters in the plant house and finally pass through the air preheaters and goes to the electrostatic precipitator where the ash particles are extracted. Electrostatic precipitators consist of metal plates, which are electrically charged. Ash particles are attracted to these plates, so that they do not pass through the chimney to pollute the atmosphere. Regular mechanical hammer blows cause the accumulation of ash to fall to the bottom of the precipitator where they are collected in a hopper for disposal. This ash is mixed with water to form slurry and is pumped to ash dyke.

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Fig. - Coal To Steam

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STEAM TO MECHANICAL POWER

From the boiler, a steam pipe conveys steam to the turbine through a stop valve (which can be used to shut off steam in an emergency) and through control valves that automatically regulate the supply of steam to the turbine. Stop valves and control valves are located in the steam chest and a governor, driven from the main turbine shaft, operates the control valves to regulate the amount of steam used (this depends upon the speed of the turbine and the amount of electricity required from the generator).

Steam from the control valves enters the high pressure cylinder of the turbine, where it passes through a ring of stationary blades fixed to the cylindrical wall. These act as nozzles and direct the steam into a second ring of moving blades mounted on a disc secured to the turbine shaft. This second ring turns the shafts as a result of the force of the steam. The stationary and moving blades together constitute a ‘stage’ of the turbine and in practice many stages are necessary, so that the cylinder contains a number of rings of stationary blades with rings of moving blades arranged between them. The steam passes through each stage in turn until it reaches the end of the high pressure cylinder and in its passage some of its heat energy is changed into mechanical energy.

The steam leaving the high pressure cylinder goes back to the boiler for reheating and returns by a further pipe to the intermediate pressure cylinder. Here it passes through another series of stationary and moving blades.

Finally, the steam is taken to the low pressure cylinders, each of which it enters at the center flowing outwards in

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opposite directions through the rows of turbine blades – an arrangement known as double flow – to the extremities of the cylinder. As the steam gives up its heat energy to drive the turbine, its temperature and pressure fall and it expands. Because of this expansion the blades are much larger and longer towards the low pressure end of the turbine.

The turbine shaft usually rotates at 3,000 rpm. This speed is determined by the frequency of the electrical system used in the country. In India, it is the speed at which a 2- pole generator is driven to generate alternating current at 50 Hz.

When as much energy as possible has been extracted from the steam it is exhausted directly to the condenser. This runs the length of the low pressure part of the turbine and may be beneath or on either side of it. The condenser consists of a large vessel containing some 20,000 tubes, each about 25 mm in diameter. Cold water from the water source i.e. the Rihand Reservoir is circulated through these tubes and as the steam from the turbine passes round them it is rapidly condensed into water condensate. Because water has a much smaller comparative volume than steam, a vacuum is created in the condenser. This allows the steam pressure to reduce down to pressure below that of the normal atmosphere and more energy can be utilized.

From the condenser, the condensate is pumped through low pressure heaters by the extraction pump, after which its pressure is raised to boiler pressure by the boiler feed pump. It is further passed through feed heaters to the economiser and the boiler for reconversion into steam.

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The cooling water drawn from the reservoir is returned directly to the source after use.

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MECHANICAL POWER TO ELECTRICITY

The turbine shaft is mechanically coupled to the generator rotor shaft through thrust bearings. The steam rotates the turbine at 3000 rpm thus the rotor of the generator also rotates at 3000 rpm. This speed is necessary to generate electricity at a frequency of 50 Hz with a two pole turbo- generator.

The rotor carries the field winding over it. This field winding is excited by a DC excitation system. The supply to the excitation system is tapped from the unit auxiliary transformer. The flux generated by this field current cuts the armature coil. The armature coil is star- star connected and is induced with three phase emf. The emf is tapped with the help of slip rings and brushes. This emf is carried over to the generator transformer through a bus duct. The bus duct is voltage transformer grounded.

The generator transformer has delta connection in the primary side and star connection in the secondary side. The generator bus supplies electric power per phase to the three-phase transformer or bank of three single-phase transformers. These transformers transmit electric power to the switchyard for further transmission. These transformers also supply the unit auxiliary transformers required for the working of various electric motors, pumps and other equipments installed in the unit.

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SWITCHING AND TRANSMISSION

The electricity is usually produced in the stator windings of large modern generators and is fed through terminal connections to one side of a generator transformer that steps up the voltage to 400KV. From here conductors carry it to a series of three switches comprising of an isolator, a circuit breaker and another isolator.

The circuit breaker, which is a heavy- duty switch capable of operating in a fraction of second, is used to switch off the current flowing to the transmission lines. Once the current has been interrupted the isolators can be opened. These isolate the circuit breaker connected to its terminals. Here after the maintenance or repair work can be carried out safely.

From the circuit breakers the current is taken to the busbar conductors, which run the length of the switching compound – and then to another circuit breaker with its associated isolators, before being fed to the Grid. Each generator in a power station has its own transformer, circuit breaker and associated isolators but the electricity generated is fed into a common set of busbars.

Circuit breakers work like combined switches and fuses but they have certain special features and are very different from the domestic switch and fuse. When electrical current is switched off by separating two contacts, an arc is created between them. At the voltage use in homes, this arc is very small and lasts for a fraction of a second but at very high voltages used for transmission, the size and power of the arc is considerable and it must be quickly quenched to prevent damage.

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Three phase, four-wire system is used for large power transmission, as it is cheaper than the single-phase two-wire system that supplies the home. Also power is generated in a three-phase system.

The center of the power station is the control room. Here the engineers monitor the output of electricity, supervising and controlling the operation of generating plant and high voltage switchgear and directing power to the grid system as required. Instruments on the control panels show the output and the existing condition of the whole main plant and a miniature diagram indicates the precise state of the electrical system.

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DESIGN

COAL HANDLING PLANT

STEAM GENERATOR

TURBINE

TURBO- GENERATOR

SWITCHYARD

CONTROL & INSTRUMENTATION

AUXILIARY POWER DISTRIBUTION SYSTEM

WATER TREATMENT PLANT

CIRCULATING WATER SYSTEM

ASH HANDLING SYSTEM

HYDROGEN GENERATING PLANT

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COAL HANDLING PLANT

It is estimated that the coal required for a 2000MW Super Thermal Power Station is of the order of 8.4 million tonnes based on an average calorific value of 4000 kcal/kg and 7000 hours of operation per year.

Two coal handling systems one for 5200MW units and the second for 2500MW units have been provided. The capacity of each of the two conveying systems has been kept as 1200 tonnes/hour. Interconnection between the two coal handling systems has also been provided to transfer crushed coal from one crusher house to the other.

The Merry-go Round (MGR) system has been provided for loading the coal at the Jayant mines and unloading the coal into the track hopper automatically when the wagons are moving at a predetermined speed of 8km/hr. A closed loop of rail lines has been laid between the loading and unloading points and a rake of 30 bottom discharge type wagons of 60 tonnes capacity each transport coal of size 0 to 200mm from mines and unload it into track hopper in 10 minutes. Automatic bottom opening type wagons discharge coal into track hopper while the rake travels over the hopper, the discharge door being automatically opened by a line side tripping mechanism. The coal received at the track hopper is delivered to two parallel conveyors 1A &1B located on the sides through four nos. of rotary plough paddle feeders designed to handle maximum coal lump size of 200mm. Double stream of conveyor system carries the coal to the crusher house, having four nos. crushers and vibrating screens each having a capacity of 600T/hr. The coal after being crushed from 200mm to

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20mm size is conveyed to the boiler bunkers. The width of the conveyor belt is kept 1400mm. The crushed coal from the crusher house, if not required, is stacked in the open stock yard. Two nos. of stacker cum reclaimer are provided on rail track to handle maximum 20mm size coal lumps and have a capacity of 1200T/hr.

Paddle FeedersFour nos. of traveling paddle feeders are provided to collect coal from the track hopper. They travel along the entire length of the hopper and transfer the coal from the hopper, uniformly to the pair of underground conveyors 1A &1B. The paddle feeders move to and fro on the rail with the help of 4 nos. of wheel mounted on the supporting structures. The wheels are driven by electric motor of 415V supply.

Belt Conveyor SystemThe belting system is designed for conveyor capacity of 1200T/hr and belt speed of 2.6metre/sec. The belt is of cotton fabric with rubber covers of adequate strength having width of 1400mm.

Magnetic SeparatorsThis is an electromagnet placed above the conveyor to attract magnetic materials. Over this magnet there is one conveyor to transfer these materials to chute provided for dumping at ground level. Because of this, continuous removal is possible and it is also not necessary to stop the electric supply to the magnetic separators for removal of separated material.

Stacker and ReclaimerTwo nos. of traveling stacker/reclaimer each capable of both stacking and reclaiming are installed which operate on rail tracks running for adequate length to cover the entire coal storage yard. The belt of the stacker/reclaimer is mounted on a cantilever boom

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and has a capacity of 1200T/hr for both stacking and reclaiming. The boom can revolve about the center of the receiving hopper and discharge/reclaim materials on/from both sides of the track anywhere between 28 meters radius of the boom. These units work in conjunction with the conveyor 9A & 9B.

Crusher HouseThe plant has four nos. of crushers each capable of crushing coal of 200mm size at the rate of 600T/hr. the crusher with hammer tips is symmetrical in size and shape on either side. In case of wearing out of one side, the other can be used by turning over the tips. These crushers are placed in the crusher house, which have special strong foundations to bear the vibrations due to running of the crushers.

Vibrating FeederThe vibrating feeder is used for throwing the coal on the underground conveyor belt from where coal goes to the bunker. Coal from the stockyard, with the help of bulldozer, is taken to the vibrating feeder via reclaimer hopper and underground conveyor belt. In case the bunker requirement is more than the capacity of crusher or stacker reclaimer, then with the help of bulldozer the coal is sent to the bunker from the stockyard, through these feeders.

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STEAM GENERATOR

The steam generator used in Stage I of SSTPS has a primary steam flow of 700 tonnes/hr at 139 kg/cm pressure and 535ºC temperature at the superheater outlet. This boiler is tangentially fired, has balanced draft, natural circulation, radiant single reheat, dry bottom open door type and is direct fired with Indian bituminous pulverized coal. The steam generator used in Stage II of SSTPS has a primary steam flow of 1725 tonnes/hr at 178 kg/cm pressure and 540ºC temperature at the superheater outlet. This boiler is balanced draft, controlled circulation, dry bottom single drum type and is direct fired with Indian bituminous pulverized coal.

The arrangement of main boiler and its accessories is as follows:The boiler structural are divided into two parts:

Supporting Structures:Boiler supporting structure consists of a systematic arrangement of columns stiffened with horizontal beams and vertical diagonal bracings and comprise of low carbon steel material. It is composed of 18 main columns and 12 auxiliary columns. The main columns support the main boiler components viz. drum, water wall membrane, panels, superheaters, reheaters, economisers, air preheater, burners and galleries at various levels. The auxiliary columns support the boiler platforms and other ducts coming in that region. The total weight of supporting structures is about 970 M.T.

Galleries and Stairways:Galleries and stairways around the combustion and heat recovery areas are provided for proper approach to the boiler. Stairways on

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both the sides of the boiler are provided. All the floors are covered with the floor gratings of required depth for walkway and are welded to the structure. Their total weight is 900 M.T.

FurnaceA boiler furnace is that space under or adjacent to a boiler in which fuel is burned and from which the combustion products pass into the boiler proper. It provides a chamber in which combustion reaction can be isolated and confined so that the reaction remains a controlled force. In addition it provides support or enclosure for the firing equipment.

In stage I, fusion welded furnace is used and in stage II controlled circulation furnace is used.

Boiler DrumThe function of the boiler drum is to separate water from the steam generated in the furnace walls and to reduce the dissolved solid contents of the steam to below the prescribed limit of 1 ppm. The drum is located on the upper front of the boiler.

In stage I, the drum weighs about 127 MT is apporx.15.7 m. long and is placed at a height of 53340 mm. It is made of carbon steel. It is designed for maximum pressure of 176 kg/cm and maximum metal temperature of 354ºC. In stage II, the drum is 22.07 m. long and is placed at a height of 72 m. It is made of carbon steel. It is designed for maximum pressure of 204.9 kg/cm and maximum metal temperature of 366ºC.

SuperheaterSuperheater is meant to raise the temperature of saturated steam by absorbing the heat from flue gases and thus increases the cycle efficiency economically. There are three stages of super heater

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besides the sidewalls and the extended walls. The first stage consists of horizontal superheater of convection mixed flow type with upper and lower bank located above economiser assembly in the rear pass. The upper bank terminates into hanger tubes, which are connected to outlet header of the first stage superheater. The second stage superheater consists of pendant platen, which is of radiant parallel flow type. The third stage superheater pendant spaced is of convection parallel flow type.In Stage I, the primary steam flow is 700 tonnes/hr at 139 kg/cm pressure and 535ºC temperature at the superheater outlet. In Stage II, the primary steam flow is 1725 tonnes/hr at 178 kg/cm pressure and 540ºC temperature at the superheater outlet.

Attemperator Attemperation or desuperheating is the reduction or removal of superheat from steam to the extent required. The characteristic performance of a superheater, which receives its heat by convection from gases flowing over it, is raising temperature with increasing output. To obtain some degree of control, the superheater must be designed for full temperature at some partial load. As a result, there will be excessive surface, with corresponding excessive temperatures at higher loads. Attemperator is used to reduce the steam temperature.

Economiser The purpose of the economiser is to preheat the boiler feed water before it is introduced into the steam drum by recovering heat from the flue gases leaving the boiler. The economiser is located in the boiler rear gas pass below the rear horizontal superheater. The economiser is continuous loop type, without fins, and water flows in upward direction and gas in the downward direction.

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A single stage of economizer is used to absorb the heat from the flue gases and add this as sensible heat to the feed water before it enters into boiler drum. The economizer is non-steaming continuous plain tube type and of tubular construction.

Reheater A single reheat system is used to further increase the efficiency of the cycle by raising the temperature of already expanded steam. After passing through the high-pressure stage of the turbine, steam is returned to the reheated by two cold reheat lines. After being reheated to the designated temperature, the reheated system at 535 degree Celsius temperature and 24.5 kg/sq. cm pressure is returned to the intermediate pressure stage of the turbine via the hot reheat line.

BurnersIn stage I, there are total twenty four pulverized coal burners for corner fired boilers and twelve oil burners provided each in between two pulverized fuel burner. The pulverized coal burners are arranged in such a way that the six mills supply the coal burners at four corners of the furnace.

In stage II, there are total thirty-two pulverized coal burners for corner fired boilers and sixteen oil burners provided each in between two pulverized fuel burner. The pulverized coal burners are arranged in such a way that the eight mills supply the coal burners at four corners of the furnace.

Igniters There are twelve side igniters per boiler in stage I and sixteen in stage II. The atomizing air for the igniter is taken from the service air compressors. The burners are located at three elevations in stage I and four elevations in stage II. Each elevation has four oil

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burners and igniters. These elevations are normally referred to as AB elevation, CD elevation, EF elevation and GH elevation. Igniters are used for lighting the main oil gun. There are two igniter air fans to supply air for combustion of igniter oil.

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The arrangement of various boiler auxiliaries is as follows: Coal BunkerThese are in- process storage silos used for storing crushed coal from the coal handling system. Generally, are made up of welded steel plates. There are six such bunkers supplying coal to the corresponding mills in stage I and eight in stage II. These are located on top of the mills so as to aid in gravity feeding of coal.

Coal FeederEach mill is provided with a drag link gravimetric feeder to transport raw coal from the bunker to the inlet chute, leading to mill at desired rate. Coal feeders are essential as the mills do not have any storage provision therefore only that much coal should be sent to the mill that has to be directly sent to the furnace and this is decided by the load requirement.

Mills There are six mills in stage I, out of which five are required for operation at maximum load and one acts as standby. In stage II, there are eight mills and here six are required for operation and two act as standby. These are located adjacent to the furnace at ‘0’ m level. These mills pulverize the coal to the desired fineness to be fed to the furnace for combustion.

Primary Air FanPrimary air fan is used to supply primary air to transport the pulverized coal from the mills/bunkers to the furnace and to dry up the coal in the path. There are two PA fans per unit and are designed to handle atmospheric air upto a temperature of 50C. These fans are driven by a 6.6KV motor each. They are located at ‘0’ m level near the boiler.

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Air Preheater Air preheater transfers heat from the flue gases to the cold primary and/ or secondary air by means of rotating heating surface elements. Beneath these regenerative type air preheaters, there exists a steam coil air preheater. These are located in the secondary pass of the furnace at a height of around ‘16’ m level. Each unit has two such air preheaters.

Forced Draft FanThe FD fan is designed for handling secondary air for the boiler. These fans are located at ‘0’ m level near the PA fan. The fan is coupled with an 800W induction motor and is commissioned to drive the cold air through the air preheater.

Wind BoxThese act as distributing media for supplying secondary/ excess air to the furnace for combustion. These are generally located on the left and right sides of the furnace while facing the chimney.

Scanner Air FanScanner fans are installed in the boiler for supplying continuously cooling air to the flame scanner provided for flame supervision. Normally one fan remains in service while the other one remains available as standby.

Igniter Air FanIgniter fan provides necessary combustion air to all the igniters. Fan makes the suction from atmosphere directly and supplies air to the wind boxes of individual igniters at a fixed constant uncontrolled rate at ambient temperature.

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Electrostatic PrecipitatorTwo ESPs have been set up for each generating units to remove the major part of fly ash. Each ESP has 304 electrodes made of steel sheets. Between each pair of electrodes a unidirectional high voltage of 60kV is applied, connecting its negative polarity to emitting electrodes and positive to collecting electrodes. The flue gases that are normally neutral when pass between rows of these electrodes are ionized due to the emitting and the negative towards the collecting electrodes. Since dust particles have great affinity towards negative particles they get attached to them and are thus negatively charged. Thus the dust particles are deposited on the collecting electrodes and are dislodged from there by periodic rapping of electrodes and are drained to the ash disposal system through hoppers.

Induced Draught FanInduced draught fan is used to drive the waste flue gases out of the chimney after they have been deprived almost all of their heat energy. A 1300 kW induction motor is used to drive this fan. The major part of the energy transferred to the gas is the velocity energy after the impeller. The velocity energy is converted into pressure energy by the diffuser. Flow is controlled by changing the direction of gas entry to the impeller blades by providing adjustable guide vanes.

Chimney These are tall RCC structures with single or multiple flues. The height of these chimneys helps in natural draught of the flue gases to the atmosphere. There are four chimneys in SSTPS- one for units 1, 2 and 3, second for units 4 and 5 and one each for units 6 and 7.

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Seal Air FanThese are used for supplying seal air to the mills to prevent ingress of coal dust into gearbox lubrication oil. There are two fans per boiler.

Soot BlowersThe soot blowers are used for efficient on-load cleaning of furnace, superheaters, reheaters and regenerative air heaters. There are three types of soot blowers provided in the plant in requisite numbers. They are:

1. Long retractable soot blowers 2. Wall blower 3. Air heater blower

Superheated steam is tapped from the superheater for the purpose of soot blowing. In stage I, there are 20 long retractable soot blowers, 56 wall blowers and 2 air heater blowers and in stage II, there are all together 104 soot blowers. All these soot blowers are operated together once in every eight hours for few minutes only.

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TURBINE

A steam turbine has two main parts viz. the cylinder and the rotor. The cylinder (stator) is a cast iron or steel housing usually divided at the horizontal centerline. Its halves are bolted together for easy access. The cylinder contains fixed blades, vanes and nozzles that direct steam into moving blades carried by the rotor. Each fixed blade set is mounted in diaphragms located in front of each disc on the rotor, or directly in the casing. A disc and diaphragm together make a turbine stage. Steam turbine can have many stages. The rotor is a rotating shaft that carries the moving blades on the outer edges of either drums or discs. The blades rotate as the rotor rotates. The rotor of a large steam turbine consists of high, intermediate and low pressure sections.

In a multiple stage turbine, steam at a high pressure and high temperature enters the first row of fixed blades or nozzles through an inlet valve or valves. As the steam passes through the fixed blades or nozzles it expands and its velocity increases. The high velocity jet of steam strikes the first set of moving blades. The kinetic energy of the steam changes into mechanical energy, causing the shaft to rotate. The steam then enters the next set of fixed blades and strikes the next row of moving blades.

As the steam flows through the turbine, its pressure and temperature decreases, while its volume increases. The decrease in pressure and temperature occurs as the steam transmits energy to the shaft and performs work. After passing through the last turbine stage, the steam exhausts into the condenser or process steam system.

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Large turbines use both impulse and reaction types. These combination turbines have impulse blades at the high pressure end and reaction blades at the low pressure end. The blade length and size increases throughout the turbine to use the expanding steam efficiently. Blade rows require seals to prevent steam leakage where the pressure drops. Seals for impulse blades are provided between the rotor and the diaphragm to stop leakage past the nozzle. Seals for reaction blades are provided at the tips of both the fixed and moving blades.

In stage I, condensing, tandem reheat, impulse type turbine is installed. The HP cylinder has 12 stages, IP cylinder has 11 stages and LP cylinder has 42 stages. The HP and IP parts are single flow cylinders and the LP part has double flow cylinder.

In stage II, 3 cylinder, reheat, reaction type turbine is installed. The HP cylinder has 18 stages, IP cylinder has 142 stages and LP cylinder has 62 stages. The HP part is a single- flow cylinder and the IP and LP parts are double flow cylinders.

The arrangement of various turbine auxiliaries is as follows:

Vacuum systemThis system comprises of condenser, ejector, CW pump and gland steam and gland steam coolers. The equipments under this system strive to maximize the work done of turbine by maintaining the rated vacuum limits.

Condenser: There are two condensers entered to the two exhausters of the LP turbine. These are surface type condensers with two-pass arrangement. Cooling water is pumped into each condenser by a vertical CW pump through the inlet pipe. Steam exhausted from the LP turbine by washing the outside of the

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condenser tubes losses its latent heat to the cooling water and is connected with water in the steam side of condenser. This condensate collects in the hot well, welded to the bottom of the condensers.Ejectors: there are two 100% capacity ejectors of the steam eject system. The purpose of the ejector is to evacuate air and other non-condensing gases from the condensers and thus maintain the vacuum in the condensers.C.W. Pumps: the pumps which supply the cooling water to the condensers are called the circulating water pumps. There are two such pumps in each unit. These pumps are normally vertical, wet-pit, mixed flow type, designed for continuous heavy duty.

Condensate System:The steam after condensing in the condenser known as condensate is extracted out of the condenser hot well by condensate pump and taken to the deaerator through ejectors, gland steam cooler and series of LP heaters. This comprises of condensate pumps, low pressure heaters and deaerator.

Condensate Pumps: the function of these pumps is to pump out the condensate to the deaerator through ejectors, gland steam coolers and LP heaters. These pumps have four stages and since the suction at a negative pressure, special arrangements have been made for providing sealing.L.P. Heaters: Turbine has been provided with non-controlled extractions which are utilized for heating the condensate, from turbine bleed steam. There are 4 LP heaters in which the last four extractions are used.Deaerator: The presence of certain gases, principally oxygen, carbon dioxide and ammonia, dissolved in water is generally considered harmful because of their corrosive attack on metals, particularly at high temperatures. The function of the deaerator

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unit is to remove dissolved gases from the boiler feed water by mechanical means.

Feed Water System:This system helps in the supply of feed water to the boiler at requisite pressure and steam/ water ratio. This system comprises of boiler feed pumps, high pressure heaters and drip pumps.

Boiler Feed Pump: The function of the boiler feed pump is that the water with the given operating temperature should flow continuously to the pump under a certain minimum pressure. It passes through the suction branch into the intake spiral and from there is directed to the first impeller. After leaving the impeller it passes through the distributing passages of the diffuser and thereby gets a certain pressure rise and at the same time it flows over to the guide vanes to the inlet of the next impeller. This will repeat from one stage to the other till it passes through the last impeller and the end diffuser. Thus the feed water reaching into the discharge space develops the necessary operating pressure. There are three boiler feed pumps in each unit in SSTPS. Two of these are turbine driven while one, which is a stand by is motor driven. The feed pump motor, which is the biggest motor in the whole plant, is a 9800 W induction motor.H.P. Heaters: These are regenerative feed water heaters operating at high pressure and located by the side of turbine. These are generally vertical type and turbine bleed steam pipes are connected to them. HP heaters are connected in series on feed water side and by such arrangement, the feed water, after feed pump enters the HP heaters.Drip Pump: The steam that bleeds from the turbine after condensation is termed as drip/ drain. Two numbers of sectional

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multistage centrifugal horizontal pumps per unit are provided. Out of these one will be running and other is stand- by. These are especially suited for the purpose of pumping from the space of high vacuum.

Turbine Lubricating Oil System:Turbine lub- oil system seeks to provide proper lubrication of turbo- generator bearings and operation of barring gear. This consists of main oil pump (MOP), starting oil pump (SOP), AC standby oil pumps and emergency DC oil pump and jacking oil pump (JOP).

Main Oil Pump: This is coupled with turbine rotor through a gear coupling. When the turbine is running at normal speed i.e. 3000 rpm or the turbine speed is more than 2800 rpm, then the desired quantity of oil to the governing system and to the lubrication system is supplied by this oil pump. Starting Oil Pump: It is a multistage centrifugal oil pump driven by AC electric motor. Starting oil pump is provided to meet the oil requirement of the turbo- set during starting and stopping. It also serves as stand by to main oil pump.AC Standby Oil Pump: This is a centrifugal pump driven by an AC electric motor. It runs for 10 minutes in the beginning to remove air from the governing system and fill the oil system with oil. This pump automatically takes over under interlock condition when the oil pressure falls below a certain standard level. Thus this pump meets the requirement of lubrication system under emergency conditions.Jack Oil Pump: This pump enables the main bearing of the complete rotor assembly to be raised or floated in the bearing during turbine generator start up and during shut down, thus preventing damage to the bearings when shaft speeds are too low

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for hydrodynamic lubrication to take place. This pump takes suction from the main oil tank and after lifting the bearing the drain is connected back to the main oil tank.Oil Coolers: The oil of the lubrication and the governing systems is cooled in the oil coolers. The cooling medium for these coolers is circulating water. The pressure of the cooling is kept lower than that of oil to avoid it’s mixing with oil if the tubes rupture. There are five oil coolers out of which four are for continuous operation and one remains as standby. All the oil coolers are arranged to operate in parallel. The cooling water temperature is not more than 36C.

Auxiliary Steam System:Some of the thermal cycle equipments/ systems require steam for primary heating, actuation, sealing etc. This requirement is met by the auxiliary steam system.

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TURBO-GENERATOR

The turbo-generator essentially consists of a fixed stator and a revolving rotor. The stator core carries a three phase winding in which alternating emf is induced, the rotor carrying field magnets and coils which provide the magnetic flux of the machine, set up by exciting the generator field current.

The turbo-generator, one no. for each unit is of three phase, two-pole cylindrical rotor type which is directly driven by a steam turbine, at 3000RPM.

These generators have direct water-cooling for the stator winding and direct hydrogen cooling for the rotor winding. The stator frame consists of a cylindrical center section and two end shields, which are gas tight and pressure resistant. It accommodates the electrically active parts of the stator i.e. the stator core and stator winding. The stator winding consists of a double layer, short-pitched lap winding with 540 transposition. The rotor shaft is a single solid forging. On the forged round rotor, slots are milled out to insert and secure the conductors of the generator excitation windings. Rotor windings consist of two cooling ducts and L-shaped strips of laminated insulator for slot insulation.

The field current is supplied to the rotor winding through radial terminal bolts and two semi-circular conductors located in the hollow bores of the exciter and rotor shafts. The field current leads are connected to the exciter leads at the exciter coupling with Multi Kontakt plug-in contacts, which allow for unobstructed thermal expansion of the field current leads.

The nameplate specifications of the generators are as follows:

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Generator# 1 to 5:Make……………………………M/s BHELRated output……………………200 MW/ 235 MVAPower factor……………………0.85 lagFrequency………………………50 Hz Terminal voltage………………15.75 KV Speed………………………….3000 rpm Stator current………………….9050A Hydrogen pressure……………3.5 kg/cm Field current…………………..2600 ANo. of terminals brought out…. Six (6) Generator# 6&7:Make…………………………..M/s KWU, GermanySupplied by……………………M/s BHELType …………………………..THDF 115/59Rated output…………………..500MW/ 588MVAPower factor…………………..0.85(lag)Frequency……………………..50HzTerminal voltage………………21KV Speed………………………….3000RPM Stator current………………….16,200A Hydrogen pressure…………….4 kg/cm Short circuit ratio………………0.48 Field current……………………4040AClass & Type of insulation…….MICALASTIC(Similar to Class F) No. of terminals brought out…...six (6)

Excitation SystemFor Stage I:In 200 MW turbo- generator, static excitation system is used. This excitation system consists of an excitation transformer. This is a step down transformer. The input to this transformer is taken

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directly from the generator bus. From the excitation transformer the output goes to a thyristor bridge which acts as full- converter and rectifies ac to dc. The thyristor bridge gives a controlled dc output. The output of the rectifier bridge then energizes the rotor of the synchronous generator. This output of the rectifier bridge is fed to the rotor of the generator with the help of slip- rings and brushes.

For Stage II:In 500MW turbo-generator, brushless excitation system is used. Brushless exciter consists of a three-phase permanent magnet pilot exciter, whose output is rectified and controlled by the thyristor voltage regulator to provide a variable d.c. current for the revolving armature of the main exciter. The three-phase current is induced in the rotor of the main exciter and is rectified by the rotating diodes and fed to the field winding of the generator rotor. Since the rotating rectifier bridge is mounted on the rotor, slip rings are not required and the output of the rectifier is directly connected to the field winding through generator rotor shaft. A common shaft carries the rectifier wheels, the rotor of the main exciter and permanent magnet rotor of the pilot exciter. The main exciter is of 6-pole revolving armature type. The three phase pilot exciter is of 16-pole revolving field type.

De-excitation of the machine is effected by driving the thyristor to inverter mode of operation causing the thyristor to supply maximum reverse voltage to the field winding of the main exciter. Approximately 0.5 seconds after the de-excitation command is received two field suppression contactors connect field suppression resistors in parallel to the main exciter field winding and following this a trip command is transmitted to the field circuit breaker via its trip coil. In the event of failure of electronic

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de-excitation through inverter operation, de-excitation is effected with a delay of 0.5 seconds by the field suppression resistors.

Hydrogen Cooling SystemThe rotor winding is cooled by hydrogen flowing through the radial ventilating ducts. It is designed for hydrogen pressure upto 3kg/sq. cm gauge. Hydrogen is cooled by the gas coolers mounted on the stator body. The hydrogen cooler water is cooled by water heat exchanger situated outside the machine. The purity of hydrogen permitted is 97-99%. Hydrogen is preferred to air as the cooling media because of its lower density and better thermal properties. While filling the generator for the very first time with hydrogen, air inside it is purged by CO and CO is purged by hydrogen. It is done in order to have safe filling of hydrogen. The hydrogen inside the generator is maintained dry by continuously circulating the gas through suitable hydrogen dryer.

Temperature LimitsThe class B type insulation is provided on the generator windings. RTDs have been embedded in the windings for measurement of temperature. Winding and core temperature recorders have been set for tripping 105C for maximum temperature of stator winding. Rotor winding temperature is recorded by a special recorder, which functions on the principle of rotor winding resistance variation with temperature.

Generator Sealing Oil SystemTo prevent the leakage of hydrogen, used for cooling in the generator, along the generator shaft a seal oil system is applied. Shaft seals operate with flow of oil under pressure. A pressure regulating valve maintains a constant differential pressure of seal oil over hydrogen pressure in the generator. Vacuum treated oil is fed to the center of the seal ring assembly from the seal oil supply

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unit. From here the oil flows in both directions between the rings and the shaft, and thus a film is established in the constricted area, which prevents the leakage of hydrogen. The main seal oil pump is driven by a 440V ac motor. A dc motor supplied from the station battery drives an emergency oil pump, which starts automatically when seal oil pressure drops or ac motor trips due to any reason.

Generator Stator Water Cooling SystemA closed loop stator water cooling system is used to maintain a constant rate of flow of demineralised cooling water to the stator winding at requisite temperature. The stator water cooling system consists of two 100% primary water to water heat exchanger, two 100% duty ac motor driven demineralised water pumps, two 100% water filters, one 100% magnetic filter, an expansion tank, specific heat measuring instruments etc. suitable resistance temperature detectors are provided for measuring the temperature of stator water at the inlet and outlet of stator winding.

Generator Main BusGenerator main bus connections consist of natural air-cooled continuous enclosure type isolated phase buses. No power circuit breakers are interposed between the generator and the main generating transformers. However, disconnecting links are provided for isolating purposes. The bus duct enclosure is made of aluminium alloy sheet. Sealed openings are provided in the bus-duct-run near the insulators for inspection and maintenance. There is a main bus duct which is circular with diameter approximately 1000mm where as the tap-off duct is circular of diameter approximately 680mm.continuous current carrying capacity of main bus on nominal voltage of 15.75KV is 10000A.

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Temperature rise of conductors and for the enclosure (over the ambient of 50C) is 20C. The generator main bus has the three isolated phase buses connected in star connection. The neutral of the main bus is grounded through a Neutral Grounding Transformer (NGT) i.e. a common duct comes out from where the three isolated phase buses are joined at a common point and goes to the primary of the NGT.

Generator Transformer

For Stage I:For each unit in stage I one 250MVA, 15.75/ 400KV three phase outdoor transformer has been installed in the transformer yard. It is connected to the generator through isolated bus ducts. The LV winding is delta-connected and the HV winding is star-connected. The LV winding is delta-connected so that if by chance there is a grounding fault in the generator then that fault current will not pass on to the transmission line further as it will keep circulating in the delta circuit itself. The HV side is star-connected because the phase voltage in case of star-connection is 1/ 3 times the line voltage and as the bus system used consists of isolated phase buses it is more economical to use star-connection as the for lower voltage lesser insulation will be required. Oil forced and water forced (OFWF) cooling is provided to get continuous nominal rating of the transformer. It is equipped with all standard measuring and controlling fittings and accessories like Buccholz relay, on-load tap changer, oil temperature indicator etc. deluge system is also provided around the transformer for fire protection. The nameplate specifications of generator transformers in stage I are as follows:

Generator transformer # 1 to 5:

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Make………………………BHELManufacturing year……….1981Type of cooling……………OFWFRating……………………..250MVATemperature rise- Oil…….40C Water….60CKV at no-load- HV……400 LV……15.75Phase- HV……3 LV……3Frequency………………….50HzVector Group………………Ynd11% Impedence……………...14%Amperes- HV……360.9 LV……9184.9Insulation level- HV……1425KV LV……95KVCore & Winding weight…..140550 kgWeight of oil………………49430 kgTotal weight……………….237400 kgQuantity of oil……………..56820 liters

For Stage II:The dual purpose of these transformers are to step up the output of 500MW generators from generation voltage of 21 KV to 400KV voltage for power distribution and if required shall be back charged from 400KV side and used to step down for feeding loads through unit auxiliary transformers.

These transformers are installed in the transformer yard adjacent to the powerhouse building. The LV winding/ HV winding are delta/ star connected. The neutral terminal is solidly grounded. The vector group of these transformers when connected in a bank

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of three single- phase transformers is YNd11. These transformers are connected to respective generators through isolated bus ducts. No power circuit breakers or power switches are connected between generator and generator transformer. However, disconnecting links are provided at generating end for isolation purpose. These transformers are equipped with independent oil forced and water forced cooling system.

Generator transformer # 6 & 7:Make………………………BHELManufacturing year……….1985Type of cooling……………OFWFRating……………………..200MVATemperature rise- Oil…….50C Water….60CKV at no-load- HV……400/ LV……21Phase- HV……1 LV……1Frequency………………….50HzVector Group………………YNd11% Impedence……………...14%Amperes- HV……866.0 LV……9523.8Insulation level- HV……1050KV LV……125KVCore & Winding weight…..1230- 50 kgWeight of oil………………27500 kgTotal weight……………….179500 kgQuantity of oil……………..29540 liters

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400 KV SWITCHYARD AT SSTPS

Switchyard is located 350 meters south of main powerhouse building. 400kV switchyard is having two numbers of double main and transfer bus system. Approximately 2000MW of SSTPS power is transmitted through 400kV switchyard. It is consisting of 21 bays, which includes generator and interconnecting transformer (ICT) bays. 400kV is designed to limit the switching surge over voltages to 2.5 P.U. and sustain temporary over voltage to 1.5 P.U.The symmetrical fault current is 40kA (rms). The basic insulation level (B.I.L.) is 1425kV. The switching surge is 1050kV. Each bus comprises of three phase strung buses with four sub-conductors per phase. ACSR ‘MOOSE’ conductors are used for stringing on the gantries of the switchyard. It is tied up with double tension string assembly in twin/quadraple bundles with 450mm sub-conductor spacing. For connecting the breaker with isolators 4 IPS aluminium tubular buses in each bay are used. For intermediate supports, bus post insulators are provided. One double main transfer bus system having main buses 1 and 2 and transfer bus no. 1 caters for Bay no. 1 to Bay no. 12. Similarly, second double main transfer bus system having main bus no. 3 and 4 and transfer bus no. 2 caters for Bay no. 14 to Bay no. 21. Bay no. 13 interconnects 400kV main bus 1 to 3 and 2 to 4. There is no interconnection between transfer buses 1 and 2.

Bay-wise description of various feeders is as follows: -1. 400KV Lucknow Line + 63MVAR Reactor2. 200MW 15.75KV Generator-53. 400KV Allahabad Line-2 + 80MVAR Reactor4. 200MW 15.75KV Generator-45. 400KV Bus Coupler-16. 200MW 15.75KV Generator-3

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7. 400KV Anpara Line8. 200MW 15.75KV Generator-29. 400KV Allahabad Line-1 + 80MVAR Reactor 10. 200MW 15.75KV Generator-111. 400KV Transfer Bus Coupler -112. 400KV Side of 100MVA 400/132KV ICT-113. 400KVBus Section 1&214. 400KV Side of 100MVA 400/132KV ICT-215. 400KV Rihand Line-216. 400KV Transfer Bus Coupler-217. 400KV Vindhyachal Line18. 400KV Bus Coupler-219. 500MW 21KV Generator-620. 400KV Rihand Line-121. 500MW 21KV Generator-7

The bay width is 27.0m. Height of the gantry structure is 13.7m and intermediate gantry structure is 20.7m. Minimum ground clearance is 7.1m. Earthing mat is laid of 40mm diameter MS rounds throughout the switchyard and equipments grounding are done by 7512mm strips. Generator bays are connected to generator transformer secondary by overhead stringing.

In SSTPS, transfer bus coupler scheme is applied. This scheme is applied to transfer the load on one breaker to another breaker for maintenance of a breaker. Suppose if a generating unit is supplying power to main bus 1. Now the breaker of the bay of this generating unit has to be repaired then it is not a practical and economic solution to trip the unit for this purpose. Therefore a parallel path is created for the flow of power through the transfer bus using the transfer bus coupler bay breaker to the main bus 1. For this first the isolator in the generator bay connecting the feeder to the transfer bus is closed. Then, in the TBC the breaker

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and associated isolators to the main bus 1 are closed. After this the TBC bay breaker is closed and this creates a parallel path for the power. Now the circuit breaker, which is to be repaired, of the generator bay is opened and then the associated isolators are also opened.

The various bay layouts are as follows:

Long Line Bay with Shunt Reactor

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Generator Bay

Transfer Bus Coupler Bay

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The various equipments used in 400KV switchyard are as follows:Isolator An isolator is a switch, which can make or break an electric circuit when the circuit is to be switched on no-load. Isolators cannot operate unless the breaker is open. Bus 1 and 2 isolators cannot be closed simultaneously. No isolator can operate when corresponding earth switch is on. There are two types of isolators used in the switchyard, namely the sequential isolator and the pantograph isolator. The sequential isolator is a two-post type in which the moving contact moves through 90 on its axis. The pantograph isolator has two moving contact arms designed in scissor-like fashion, which move through only 20 on its axis.

Circuit BreakerA circuit breaker is a switch, which can make or break the circuit on load and even on faults. It is heavy-duty equipment mainly utilized for protection of various circuits and operation at load. It is installed accompanied by two isolators.The various types of circuit breakers used in the switchyard are:

1. Bulk oil circuit breaker2. Minimum oil circuit breaker3. Air blast circuit breaker4. Sulphur hexa-flouride circuit breaker

These circuit breakers have been classified on the basis of their quenching mechanism. The various operating mechanisms used for these circuit breakers are spring operation, solenoid operation and pressure or pneumatic operation.

Earth Switches These are devices which are normally used to earth a particular system to avoid accident, which may happen due to induction on

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account of live adjoining circuit. These switches do not handle any appreciable current at all. These are simple mechanically operated switches.

Lightening ArrestersLightening arresters are equipments which are connected at the transformer terminals and the incoming terminals of the line for protection against lightening or any surges developing in the system. In this plant, valve type lightening arresters are used. Such LAs consist of nonlinear resistors in series with spark- gaps. The spark- gap assembly acts as a fast switch, which gets ionized (conducting) at specified voltage. The entire assembly is placed in porcelain housing, properly sealed to keep out dust and moisture.

Wave TrapsWave traps are parallel resonant circuits having negligible impedance to power frequency currents but having very high impedance to carrier frequency currents. They are used to keep carrier signals in the desired channel so as to avoid interference with or from adjacent carrier current channels and also to avoid loss of carrier current signal in the adjoining power circuits.

Current TransformersA current transformer is a step down transformer which produces a replica of the high current flowing in the circuit for measurement purposes. It is intended to operate normally with rated current of the network flowing through the primary winding which is inserted in series in the network. The secondary winding of the CT is connected to measuring instruments and relays supplying a current which is proportional to and in phase with the current circulating in the primary except for the difference due to current error and phase displacement inherent in the design of the CT.

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Potential TransformerPotential transformers step down the system voltages to sufficiently low for indication of the system voltage conditions, metering of the supply of energy, relaying and synchronizing. In 400 KV switchyard capacitance voltage transformer is used. A set of CVT has been provided on each incoming/ outgoing line.

Shunt ReactorsShunt reactors are static capacitors, which are connected in parallel in the system, which produce reactive power in the power system. In long lines a shunt reactor is connected for reduction of line current, increase in voltage level at the load, reduction in system losses, increase in power factor of a source current and reduction in loading on source generators and circuits. They draw almost a fixed amount of leading current which is superimposed on the load current. This reduces the reactive component of the load current, thereby improving the power factor.

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CONTROL AND INSTRUMENTATION

The control and instrumentation systems installed in the plant are installed to provide a comprehensive intelligence feedback on the important parameters viz. temperature, pressure, level and flow. These systems are mostly based on state of art microprocessor technology. They monitor the following systems: - SG – C&I Systems:

1. Furnace safeguard supervisory system for purging, automatic firing, flame monitoring, sequential start- up and shut down of mills, etc.

2. Secondary air damper control system3. Auxiliary PRDS control system4. Soot blower control system5. Coal feeder controls6. Furnace temperature probes

TG – C&I Systems: 1. Electro- hydraulic governing control system2. Automatic turbine run up system3. HP- LP bypass control system4. Turbine stress control system5. Automatic turbine testing system6. Turbine protection system7. Generator auxiliaries control system

Steam & Water Analysis System This system does line analysis of various parameters like conductivity, pH, dissolved oxygen, residual hydrazine, silica, sodium, phosphates, chlorides, etc. at all critical points in condensate, feed water and steam cycle.

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The C&I systems employ the Distributed Control Monitoring & Information System (DDCMIS) and Computerized Data Acquisition System. The DDCMIS employs state of art microprocessors and is based on latest proven technology. It performs the functions of sequencing and modulating controls, plant start up/ shut down, in all regimes of plant operation including emergency conditions. The main purpose of DAS is to acquire sensor data and to produce useful output information for plant operators in the form of displays and hard copies. This system combines special hardware and software to facilitate interfacing between plant and operator. In addition, it also performs plant performance calculations and process monitoring.

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Auxiliary power distribution system

The auxiliary power distribution system distributes electrical power requirement to various loads, control circuits and other instrumentation circuits. The total load on auxiliaries in a power station is approximately 7% to 9% of the plant capacity or the actual power generated. This system is broadly divided into:-

1. Unit Auxiliary Power Distribution System Unit auxiliaries are those which are directly associated with the generating unit such as ID and FD fans, boiler feed pumps, coal mills, mill fans, circulating water pumps etc. The interruption of supply for the auxiliary motors connected on the unit bus should not be there. For supplying power to these unit auxiliaries the generator is connected to generator transformer through isolated phase bus duct and also through two nos. of unit auxiliary transformers which step down the voltage to 6.6 KV. The UATs are connected to the unit 6.6 KV bus system by 2500 A, 6.6 KV bus ducts. Each transformer is connected to unit buses A & B. Medium voltage MOCB switchgear is used for feeding power to motors rated above 200 KW. Facility is provided to transfer unit load to station in the event of tripping of unit through changeover system.

Unit Auxiliary Transformer # 1 to 5:Make………………………KA CKBRIDGE HEWITTIC AND EASUN

LTD.Manufacturing year……….1981

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Type of cooling……………ONAN (75%) ONAF(100%)Rating (KVA)…………….. 12000 16000Temperature rise- Oil…….40C Water….50CKV at no-load- HV……15.75 LV……6.9Phase- HV……3

LV……3Frequency………………….50HzVector Group………………DYn11Amperes- HV…… 439.8 586.53 LV…… 1004.1 1338.8Insulation level- HV……125 KV LV……60 KVCore & Winding weight…..19200 kgWeight of oil………………7100 kgTotal weight……………….37799 kgQuantity of oil……………..8250 liters

Unit Auxiliary Transformer # 6 to 7:Make………………………NGEFManufacturing year……….1986Type of cooling……………ONAN (75%) ONAF(100%)Rating (KVA)…………….. 17500 25000Temperature rise- Oil…….50C Water….55CKV at no-load- HV……21 LV……6.9Phase- HV……3 LV……3Frequency………………….50HzVector Group………………DYn1Amperes- HV…… 481.13 687.32

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LV…… 1664.3 2091.9Insulation level- HV……125 KV LV……60 KVCore & Winding weight…..21.6 TWeight of oil………………7.3 TTotal weight……………….42.2 TQuantity of oil……………..8296 liters2. Station Auxiliary Power Distribution System

Station auxiliaries are those which are required for general station services such as coal and ash handling system, lighting system, water purifying system etc. interruption of supply for the auxiliary motors for the station bus for a short duration can be tolerated. There are four station transformers in the plant. These transformers are supplied from the 132 KV yard. These transformers step down the voltage to 6.6 KV. The station transformers are resistance grounded.

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WATER TREATMENT PLANT

Water treatment plant is to produce such a quality of feed water from which there should not be any scale formation causing resistance to heat transfer and thus failure of tubes, no corrosion and no priming or foaming problems. This helps in giving trouble free, uninterrupted supply of clean steam. In this plant, raw water is fed from the Rihand reservoir, which consists of ionic and non-ionic, dissolved and undissolved solids and gaseous impurities. The process of removing is show in the flow diagram: -

Fig: -Block Diagram of water treatment Plant

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Pre-Treatment PlantThis plant has a capacity to produce 600cu. M/hr. of clarified water to meet the requirement of the DM plant. The various processes involved in the pretreatment of water are:

Chlorination Chlorine is dosed in raw water inlet to aerator and further in clarified water tank in order to remove bacteria and other microorganisms. It is also effective in oxidation of Fe, Mn and HS, removal of taste and odour producing compounds and

oxidation of organic compounds by forming chloroderivatives of these compounds.

Aeration

By aeration the water absorbs oxygen from the atmosphere, which helps in oxidation of organic matter present in water. The iron dissolved in water is precipitated as Fe O .

Coagulation After aeration, the water flows to the flash mixture where lime and alum are dozed by the pumps and then flows through a RCC channel by gravity. The added chemicals are thoroughly mixed with the raw water with the help of a stainless steel paddle fitted in the path. Chemical reaction takes place as under:

Al (SO ) . 18H O + 3Ca(OH) 3CaSO + 2Al(OH) + 18H O

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Flocculation & Clarification The water is subjected to slow spiral motion and fine precipitates agglomerate to look distinctly as flocks. The clear water enters through the bottom opening. The scrapper attached to the rotating bridge scraps the settled sludge. The clear water overflows from the top of rotating bridge. The clear water overflow from the top of clarifier and led to clarified water storage tank.

FiltrationThe clarified water is passed through four pressure filters in which graded anthracite coal is filled up. During this process suspended impurity and turbidity is filtered effectively. Now the water is passed through active carbon filter to remove residual chlorine and oil impurities. Then it is fed to ion-exchanger for removing mineral salts. In the cation exchanger cations such as Ca, Mg and Na react with strong cation exchange resin and stay with the reacted resin. Similar action takes place for removal of anions in the anion exchanger. In the end a mixed bed exchanger is kept which helps in removal of any left over anions or cations.

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CIRCULATING WATER SYSTEM

Condenser Cooling Water SystemThe CW system provides for pond cooling with Rihand Reservoir as the heat sink. Vertical wet pit type CW pumps draw water from the Rihand Reservoir through the approach canal and feed to the condensers. Hot water from the condensers is discharged back to the reservoir by means of discharge channel, which is common for stage I and stage II of the project. CW supply from CW pump house to the condensers and CW discharge from the condensers upto the discharge channel is through concrete ducts of horseshoe shape. Round the year there is a large variation in the water level of Rihand Reservoir. Minimum water level at the intake sump is 254 m and the maximum level is 271 m with operating floor level of the pump house being at 278.5 m total depth of the sump is 31 m. therefore there is large variation in the static lift of the CW pumps ranging from 7 m to 24 m. In SSTPS, the operating scheme used to overcome this problem is by varying the nos. of CW pumps in operation.

In stage I, total 13 nos. of pumps have been installed while 10 pumps are required. Requirement of cooling water of each unit is 27000 m /hr. The rated capacity of each pump is 15000 m /hr with a total head of 31.5 m. These pumps are designed for continuous operation with cooling water of maximum temperature of 36ºC.

In stage II, 3 nos. of CW pumps have been installed for each 500MW unit. The rated capacity of each pump is 27000 m /hr at 31.5 m head. The operating speed is 375 rpm and the driver is a 3MW, 6.6KV, 16- pole, 50 Hz induction motor. In case of low water level, three pumps feed to one unit and in case of higher water levels two CW pumps are sufficient for each unit. For

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intermediate water levels five pumps are operated for both the units and the interconnecting butterfly valves are kept open.Ahead of the CW pumps single flow type traveling water screens have been provided with a clear opening size of 9.5 mm square which prevent the debris from entering into the CW system.

Equipment Cooling SystemThe equipment cooling system has been provided to remove the waste heat rejected from the various plant equipments and transfer it to the environment. The system is divided into two basic sub-systems:a) Primary circuit using DM water employed to pick up the heat

load from various auxiliary coolers and rejects the same to the plate type heat exchangers (PHE).

b) Secondary circuit using raw water employed to pick up the heat load from PHEs and reject the same to main circulating water discharge seal pit in transformer yard.

The ECW system is capable of operating continuously during all modes of plant operation. The ECW system meets the requirements of auxiliary coolers other than the main condensers. The following auxiliary coolers are cooled by primary water:

1. SG Package – Coal mill lubricating oil coolers, regenerative air pre-heaters - guide bearing and support bearing oil coolers, water cooled access door in furnace hopper zones, lubricating oil coolers for FD/ ID/ PA fans and motors, coolers for ID fan hydraulic couplings, sample coolers, ID fan motors stator coolers, air heaters fire se, coal mill journal hydraulic system cooler, boiler circulation pumps.

2. TG Package –

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Turbine lubricating oil coolers, turbine control fluid coolers, generator hydrogen coolers, exciter coolers, generator seal oil coolers – hydrogen and airside, primary water coolers.

3. BFP Package – Booster pump coolers, working oil coolers and lubricating oil coolers for MDBFP, BFP oil coolers, BFP motor coolers.

4. CEP Package – Motor and thrust bearing coolers

5. Oil coolers for generator transformers6. Auxiliary coolers for air compressors7. Primary sample coolers in SWAS panel room in control tower.

The ECW heat exchangers are of plate type located at ground floor of TG hall and sized to provide 38ºC (max.) DM water at design flow condition.

The pH of DM water in the closed loop is maintained around 9.5 by dosing sodium hydroxide. The sodium hydroxide solution is prepared in NaOH solution preparation tank and fed to the suction of DMCW pumps through 2×100% chemical dosing pumps.

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ASH HANDLING SYSTEM

The ash handling system is continuous hydro sluicing type. It envisages continuous removal of bottom ash and fly ash in slurry form from the different zones of bottom ash and fly ash collections in the steam generating units. The bottom ash slurry is led to a bottom ash slurry sump at the boiler bottom from where it is transferred by means of vertical slurry pumps to the main fly ash slurry trench in electrostatic precipitator (E.P.) area. The fly ash slurry flows through gravity channels and aided by high pressure jetting water is led to the slurry sump in the main ash slurry pump house. The combined bottom ash and fly ash slurry from the main slurry sump is pumped to the disposal area by means of slurry pumps and disposal lines. The HP and LP water required for slurry formation and transportation is supplied by HP and LP pumps installed in ash water pump house.

The entire ash handling system has been designed for removing and flushing the bottom ash and fly ash from both the units at the following rates:For Stage I:a) Fly ash – 162 tonnes/hr (for phase I) 108 tonnes/hr (for phase II)b) Bottom ash – 36 tonnes/hr (for phase I) 24 tonnes/hr (for phase II)For Stage II: a) Fly ash – 320 tonnes/hrb) Bottom ash – 72 tonnes/hr.

The ash slurry disposal system has been designed to pump the ash slurry continuously from the slurry pump to the disposal area through pipelines at a rate of 700 m /hr for each unit of stage I and 1500m /hr for each unit of stage II.

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Bottom Ash Removal SystemThe bottom ash resulting from the combustion of coal in the boiler falls into the ash hopper provided under the furnace bottom. Each hopper is divided into two sections and each section is provided with adequately sized opening with gates. The ash is spray- quenched in these hoppers and gets discharged into the water impounded slag baths provided under each section. Each slag bath is provided with a continuously moving scraper feeder for transferring the wet slag ash to the respective clinker grinder. The crushed ash through clinker grinder gets discharged into the slopping ash trenches provided beneath them and from there aided by high pressure water jets, the slurry is led to the bottom ash slurry sump provided adjacent to the boiler bottom. From the sump the slurry is transported to the main ash slurry trench in E.P. area by bottom ash slurry pumps for its further disposal to dump area by means of slurry disposal pumps located in main ash slurry pump house.

Fly Ash Removal System Fly ash removal system envisages removal of ash from each of the electrostatic precipitators, economiser, air preheater and stack hoppers continuously through suitable vertical pipe connections. Flushing equipments are provided below them. The slurry from the economiser and air preheater flushing equipments is conveyed to the bottom ash slurry sump from where it is pumped along with bottom ash to the main ash slurry trench in E.P. area.

The fly ash slurry from ESP and stack gets discharged into the sloping ash channel provided beneath them. The slurry aided by high pressure water jets flows down the sluice channel to the

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slurry sump in ash slurry pump house for its further disposal to dump area by means of slurry pumps.

Ash DykeThe ash dyke is provided for the disposal of fly ash and bottom ash in the form of slurry. Fly ash and bottom ash are collected in the slurry form in the sump of the ash slurry pump house. From there it is discharged through pipes into the ash dyke for the settlement of ash in the dyke. The ash free water is discharged into the Rihand Reservoir.

There are three main ash slurry sumps, one common for units 1, 2 & 3, second common for units 4 & 5 and third common for 6 & 7. Low pressure water is used for thorough mixing and high pressure water is used for sluicing. Each ash slurry sump is located in an ash slurry pump house. Each pump house has six vertical pumps for continuously conveying the ash slurry from the sump to the ash dyke.

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HYDROGEN GENERATION PLANT

Hydrogen gas is used for generator cooling. So supply of pure hydrogen in the power station is essential for generator filling and maintaining of hydrogen gas pressure inside the generator casing.

Hydrogen is prepared by electrolysis of pure demineralised water. When dc current is passed through water it decomposes the water into two elements, one volume of oxygen and two volumes of hydrogen. Pure distilled water is a bad conductor of electricity but if acid, alkali or salt is added it becomes a good conductor. To make economical use of electrolysis of water, a solution termed as electrolyte has be used which is prepared by adding NaOH or KOH with pure water. When current is passed through the electrolyte, hydrogen is given off at negative electrode, while oxygen is evolved at the positive electrode.

A.C. power at 400/440 V, 3 phase, is changed to dc power in a transformer and rectifier arrangement. Dc output of the plant is controlled by means of a regulator. Dc from the terminals of the rectifier is supplied to the cells through busbars. Gas production is directly proportional to dc current passing through the solution of caustic potash and DM water.

DM water of high purity is collected in a storage tank from where it is fed by gravity to the cell bank for make up. An automatic float valve is mounted in gas washing tank to provide a continuous supply of water in proportion to usage. The gases, after leaving the cells, pass upwardly to the collection headers and then through a water seal to atmosphere or to the gasholder as the case may be.

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Cooling water is supplied to the water seal, which regulates the pressure head against which the cells operate and also prevents any backward flow from the gasholder when the plant is not in operation. Valve is provided in between the gas washing tank and gasholder for directing the flow to atmosphere when desired.

Hydrogen gas flows from the gas washing tank to a low pressure wet seal gasholder. From the gasholder it flows to a compressor, which compresses it to rated pressure. After the compressor it flows through carbon filter and through a silica gel dryer. The dry hydrogen is then stored in storage cylinders from where it goes to the power station for use. Hydrogen gas is normally sent to HP compressors from the gasholder where it is compressed to a rated pressure. From the HP compressors, hydrogen flows through an after cooler, which has moisture separator columns, and then to a point filling station where it is filled in portable cylinders.

Oxygen produced in the process is let off to atmosphere.

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