Safety moduleOverview of safety hazard:What is hazard?

A hazard is any source of potential damage, harm or adverse health effects on something or someone under certain condition at work.How to behave inside the plant:

No loose clothing. Safe distance is to be maintained from rotating machine. Any electrical installation should not be touched. Safe distance should be kept from bare live terminals. Helmet is a must. Nobody should move in dark spaces. Nobody should move alone. Be aware from floor opening. Avoid moving on checker plate. Nobody should move over the trenches. Railing should not be leant upon. Moving into confined spaces (congested closed area with lower

concentration of oxygen, and containing mixture of toxic and explosive gases) for eg any flue duct, furnace, inside the condenser, cooling water duct, sewerage lines is to be avoided.

One should not move in regions where lifting work is being carried out. One should not go into a unit which is under maintenance. One must avoid going near welding and gas cutting sparks and must

use safety glasses.

UNSAFE ACT AND UNSAFE CONDITIONS SHOULD THEREFORE BE AVOIDED.CHEMICAL HAZARDS :

Hydrogen, chlorine, ammonia and carbon monoxide leakage-

HYDROGEN: This being highly explosive, use of mobile phones should be avoided and combustibles should not be kept near hydrogen cylinders.

CHLORINE: Chlorine is used in water treatment plants.Inhalation of chlorine causes its combination with water in mucus membrane to form Hydrochloric acid. It causes pitting in the membrane and respiratory problems start.

o Precautions:In case of chlorine leakage,one must go as up as possible because chlorine being 2.5 times heavier than air its density decreases with height.

STEL value : 3 ppm. AMMONIA : Ammonia is found near ESP and boiler. Inhalation of

ammonia may cause vomiting and head-ache. CARBON MONOXIDE: Carbon monoxide may be found in Feeder

floor, bunker floor etc. Inhalation causes it to mix with Haemoglobin in blood to form carboxy-haemoglobin that chokes the person. So it is often termed as “killer gas”.

o Precautions: It can be removed quickly from the body of the person if he is taken immediately into an open area.

STEL value: 400 ppm.

Basic understanding of thermal power generationIn a thermal power plant, the thermal energy of the superheated

steam is used to drive the generator, which in turn produces electricity. The

superheated steam is produced from any fossil fuel like coal, crude oil etc. Since coal is available in plenty and in cheap, we use coal as the primary fuel in the thermal power plants. The coal is crushed and pulverized before it is used in the boiler. The pulverization of coal increases the surface area of the coal, which helps in the complete combustion of the coal. The coal is feed to the boiler where it is fired to produce heat. Initially, a small quantity of crude oil is used for the purpose. The water is passed through the pipes making up the walls of the boiler. The water is heated to produce steam, which is further reheated to produce superheated steam. The superheated steam at an exceedingly high temp. Steam pressure drives the turbine. The turbine rotates following which the generator, which is coupled to the turbine, also rotates producing electricity.

Thus, the chemical energy of the coal is utilized to produce steam. The thermal energy of the steam is converted to the mechanical energy of the rotating turbines, which is converted to the electrical energy in the generator.

NTPC FARAKKA SUPER THERMAL POWER plantNTPC Farakka is a coal fired pit head based power plant. Lalmatia is the captive coal mine for this plant. Total capacity of NTPC Farakka is 2100MW.The plant consists of three stages. These are:

STAGE I : 3 units of 200MW

STAGE II : 2 units of 500MW

STAGE III : 1 unit of 500MW

Generation voltage level for Stage-I is 15.75 KV and for Stage-II and Stage-III is 21KV.The generated voltage is stepped up to 400KV for transmitting over a long distance.

DISTINCTION BETWEEN STAGE-III AND OTHER STAGES :

STAGE-I and STAGE-II : Cooling Water is taken from the feeder canal and after using in the condenser the hot water is returned to the feeder canal, hence it is an open cycle system. It causes threat to aquatic life.

STAGE III : Cooling tower is used in this system. After using the water in the condenser it is recirculated through cooling tower, where the hot water is cooled and reused. This is a closed cycle system and causes less harm to the environment.

SUPPLY OF RAW MATERIALS:

A)COAL: There are 3 sources of coal supply as: Lalmatia situated 85 Kms away from the main plant. Imported coal from Indonesia. Indian railways who supply coal from different mines across the

country.B) WATER:

Feedwater: Collected from feeder canal cut across the river Ganga.

Cooling Water: Collected from the same source.

C) AIR: atmospheric air is used.D) OIL: HFO is supplied by IOCL.

ASH DISPOSAL:

For the quality of coal used here, approximately 80% Fly Ash and 20% bottom ash is produced. Ash disposal is a major issue in coal fired power plants.Raw water is mixed with ash to form slurry and pumped out through series pumps( 3 for Fly ash and 2 for Bottom ash ) into ash dyke.Fly ash dyke is situated 12 Kms away and Bottom ash dyke MALANCHA is situated 5 Kms away from the main plant.

POWER EVACUATION:

The generated power is evacuated through different outgoing lines:

1)Farakka-Malda (2 lines)

2) Farakka-Kahalgaon (2 lines)

3) Farakka-Durgapur (2 lines)

4) Farakka-Lalmatia (1 line)

5)Farakka-Jeerat(2 lines)

Initially there were 9 lines. Few more lines were added after commissioning of Stage III.

DIFFERENT CYCLES INVOLVED IN THE ENTIRE OPERATION:

FUEL CIRCUIT : Coal is brought into CHP with the help of Wagon tipplers or track

hoppers and stored in the bunker. From bunker coal is sent to feeder via conveyer. The purpose of feeder is to vary the amount of coal as per load

requirement. From feeder coal is taken to mill where it is pulverized. Primary air is sucked from atmosphere through PA fan and takes

pulverized coal to the furnace.

Fig1: Coal circuit

2) AIR & FLUE GAS CIRCUIT: Secondary air is sucked from atm. with the help of FD fan(Forced

Draught) for combustion.A fire ball is generated inside the boiler.The temp. of fire ball is 1000-1100 deg. C.

Flue gas is generated due to combustion. Heat is extracted from flue gas to heat the steam in platen SH,

reheater,final SH, primary SH , economiser and to heat air in air preheater.

Fly ash particles are collected from flue gas at ESP(Electrostatic Precipitator) and flue gas is released to atm. with the help of ID fan(Induced Draught) which is situated at the base of chimney.

Fig2: Air & Flue gas circuit

3) STEAM, CONDENSATE & FEED WATER CYCLE: In 200MW unit of the plant the boiler drum is located at the

height of 54m. Inner wall of the boiler made up of water tubes which originate

from bottom ring header situated 7m from ground level. DM water picks up heat from fire ball and goes up as a mixture of

steam and water.Then it reaches drum .As steam being lighter it goes upward.Water comes down through DOWNCOMER .In 200MW unit no. of downcomers is 6 and natural circulation of steam and water takes place.But in 500MW unit the boiler drum being at a height of 72m forced circulation is employed. by through 10 downcomers.

Steam is separated from water in the boiler drum with the help of CYCLONE SEPARATOR & SECONDARY SEPARATOR(corrugated sheets) and it follows the circuit shown below.

Fig 3: Steam,Condensate & Feed Water cycle

4) COOLING WATER CIRCUIT: Raw water is taken from the feeder canal through CW pump. Raw water is passed through TRASH RACKS and TRAVELLING

WATER SCREENS to remove impurities (animal bodies, planktons etc) of different sizes.

Raw water is stored at the water box at the inlet of condenser and it flows through the innumerable number of condenser tubes. In stage I there are 13000 such tubes. The water comes at the outlet and is stored at the outlet water box.

The condensate is stored in the HOTWELL at a temperature of 45 deg C.

In STAGE-I and STAGE-II, after using in the condenser the hot water is returned to the feeder canal, hence it is an open cycle system.

In STAGE III, Cooling tower is used. After using the water in the condenser it is recirculated through cooling tower, where the hot water is cooled and reused. This is a closed cycle system and causes less harm to the environment.

Fig4: Cooling Water circuit

5) ASH CIRCUIT: The ash is collected in two forms as Bottom ash(20%) and Fly

ash(80%). Ash can be disposed in dry form or wet form. Fly ash particles are collected in ESP.In ESP,there are large

numbers of EMITTING ELECTRODE(supplied at 80 KV) and COLLECTING ELECTRODE(at ground potential).

Fly ash particles are deposited on the collecting electrodes and HAMMERED into ESP HOPPERS.

Slurry is formed by mixing raw water with Fly ash and pumped out to Fly ash dyke with the help of FASPs(Fly ash slurry pumps)

Similarly, Bottom ash is collected in the BOTTOM ASH HOPPERS and disposed in slurry form in the Bottom ash dyke with the help of BASPs(Bottom ash slurry pumps).

Fig5:Ash disposal system

Basic Description of Flow of Coal:

(1)CHP (Coal Handling Plant): The input coal is unloaded at this portion, which passes it to the next block crusher house.

(2)Crusher house: crusher house crush the unloaded coal coming from the CHP in small size.

(3)Coal Bunker: Coalbunker stores the crushed coal and passes it to the Feeder.

(4)RC Feeder: It is the raw coal feeder. The coal, comes from the crusher house is still of big size. The feeder controls the quantity of the input coal to the boiler.

(5)Mill / pulveriser: Mill contains three rollers. It crushes it to powder finely. Primary air takes the pulverized coal from the mill to the boiler.

(6)Furnace : At furnace pulverized coal is combusted with the help of secondary air.

Brief description of system components:

Main components of the power plant are:

1) Boiler:

In Farakka NTPC, corner fired boilers are used. In stage I, there are 6 mills situated from a height of 18m to 27m. FIRING FLOORS of mill A is at a height of 18m and that of mill F is at 27m. Rest of the firing floors are situated in between. HFO OIL GUNS are situated at heights of AB, CD and EF.

Purpose of oil gun: a) during boiler start up , b) emergency purpose.

Fig 6: HFO Flow System

The working oil supplied by IOCL is stored in HFO tank 2. The oil in this tank is replenished by oil in tanks 1 and 3. The heat lost due to transportation is compensated by HEATER. HFO Circulation takes place all the time so that temperature of oil is maintained at 120 deg C when plant is running.

In case of shut down of any mill, the coal particles on the corresponding floor may not get sufficient ignition energy and thereby the fireball distorts and subsequently two separate balls are formed on either side of that mill. In that case HFO injection makes up that gap and stabilizes the fire ball.

The coal is composed of 4 parts – fixed carbon(FC),Volatile material(VM),Moisture & ash. The VM supplies the initial ignition energy for the complete combustion of FC. If the VM content in the coal used is less the coal particles have to travel a larger path to attain the required ignition energy and hence the fireball is formed at greater height.

Fig7: Isometric View of Boiler schematic

Fig8: Top view of firing floor corresponding to mill A

It is critical to maintain the level of water in boiler drum.

Case 1: level of water goes too high: water may go into the steam circuit. When water hits the turbine blades, PITTING may occur. (the high velocity water particles may produce indentations in the turbine blades)

Case 2: level of water goes low: If it is too low, the boiler drum base being close to the fire ball, its shape gets deformed. Also low water level causes water in the drum to get evaporated at a faster rate than the rate at which water is returned from hotwell to the drum

There are mainly three types of heater coils in boiler.

(1) Economiser (2) Re heater (3) Super heater

Economizer :

Economizer is a device in which the feed water is heated before it enters into a boiler. The heat is being taken from the waste flue gas of the boiler. It ensures economy of fuel. Hence it is called economizer. The economizer is placed at the second pass of boiler. The flue gases of the

boiler furnace, after working inside the boiler, flows through it before passing into the chimney.

Coal bunker-

These are in process storage silos used for storing crushed coal from the coal handling system. Generally, these are made up of welded steel plates. Normally there are six such bunkers supplying coal of the corresponding mills. These are located on the top of the mills so as to aid in gravity feeding of coal.

P. A. Fan -

The primary air fans (2 per unit-50% capacity each) are designed to suck primary air from atmosphere. Temperature of PA is increased to 300 deg C after passing through Air Pre-heater. These fans are located at ‘0’M level near the boiler.

Burners-

As evident from the name itself, these are used for burning pulverized coal. Every unit has a set of such burners located at different elevations of the furnace.

F. D. Fan-

These forced draught fans (2 per unit-50% capacity each) are designed for handling secondary air for the boiler. These fans are located at ‘0’M level near the P.A Fan.

Electrostatic Precipitator-

Air Pre-heater -

Air pre heater transfer the heat from the flue gas to cold primary and/or secondary air by extracting heat from waste flue gas. These are located in the secondary pass of the furnace at a height of around ‘16’M level. Each 200MW unit is provided with two such air pre-heaters.

ID Fans-

These are two induced draught fans per boiler located between the ESP & chimney. These fans are used for maintaining pressure inside the boiler by releasing flue gas through chimney.

Chimney-

These are tall RCC structures with single/multiple flue ducts (one flue duct per 200MW unit). The heights of these chimneys vary, depending upon the location considerations; anywhere between 150m to 220m.

Steam Circulation System

Re-heaters-

This is the part of the boiler which receives the steam back from the turbine after it has given up some of its heat energy in the high pressure section of the turbine. It raises the temperature of this steam, usually to its original value, for further expansion in the turbine. The purpose of this reheating is to add energy to the partially used steam. The construction & arrangement is similar to super heater. It has two sections- hot & cold Reheat sections. Due to resistance of flow through the reheat section, the hot reheat steam is at lower pressure compared to the cold reheat steam.

water treatment plant and storageUSE OF MAKE UP WATER:

Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow down and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system.

HARDNESS IN RAW WATER AND REMOVAL:

Impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. This is called SCALING. Thus, the salts have to be removed from the water, and that is done by a water demineralizing treatment plant (DM). A DM plant generally contains ION EXCHANGERS. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water.The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water. The piping and valves are generally of stainless steel.

Boiler feed pump-

It takes water input from the deaeretor at a very low pressure and pumps it to the boiler drum at a very high pressure (190Kg).

Condenser- Ejectors-

There are two 100%capacity ejectors of the steam ejecting type. 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.

Condensate Extraction Pump-

It extracts the condensate from the hot well of the condenser and pumps it to the de-aerator.

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 low pressure heaters in which last four extractions are used.L.P.Heater-1 has two parts LPH-1A and LPH-1B located in the upper parts of the condenser A & B respectively. These are of the horizontal type with shell and tube construction. L.P.H 2,3,4 are of similar construction and they are mounted in row at 5M level.

Deaerator-

This is used to remove oxygen. Due to the presence of certain gases like oxygen, carbon-di-oxide, ammonia, etc. in water then it is considered harmful because of their corrosive effect on metals, particularly at elevated temperatures. The boiler feed water should be free from all dissolved gases. This can be achieved by embodying into the boiler feed system a de-aerating system, whose function is to remove dissolved gases from the feed water by mechanical means.

H. P. Heaters-

These are regenerative feed water heaters operating at high pressures and located at the side of turbine. These are connected in series on feed-water side & by such arrangement feed-water enters the HP heaters. The steam is supplied to these heaters from the bleed point of the turbine through motor operated valves. These have a group bypass protection on the feed waterside. Here the feed water flows through the tube spirals & is heated by steam around the tubes in the shall of heaters. These heaters are cylindrical vessels with welded dished ends & with integrated, de-super heating, condensing, & & sub cooling sections. This design offers the

advantage to optimize the arrangement of piping & the location of the heaters at power station.

2) TURBINE:

Fig 9: Turbine cycle

Steam after working in the HP turbine temperature is reduced from 540 deg to 340 deg C. Pressure is also substantially reduced. The line of flow of steam from HP turbine is COLD REHEAT LINE or CRH LINE.

CRH line again goes to the boiler to be reheated at 540 deg C by the heat of the flue gas. This line is known as the HOT REHEAT LINE or HRH LINE. HRH Line goes to the IP Turbine. Exhaust steam from IP Turbine goes to the LP Turbine.

Exhaust of LP Turbine (80 deg C) goes to CONDENSER. COOLING WATER is circulated to cool down the steam and the condensate drains into the HOTWELL at 35 deg C.

From HOTWELL the condensate is sent back to boiler drum through different processes and reused.

The pressure of MS line is ~160 Kg/sqcm, that of HRH line is ~33-34 Kg/sq cm and CRH Line pressure is ~34 Kg/sqcm.

With the help of modulation of HP Control valves (HPCV) and IPCV alternator frequency is kept constant by controlling the amount of inlet steam.

STOP VALVE is used to stop the entry of steam into the turbine.

Turbine Section

Turbine Lub. Oil System

This consists of Main Oil Pump (MOP), Starting Oil Pump (SOP), AC stand by oil pumps and emergency DC oil pump and jacking oil pump (JOP) (one each per unit).

Emergency stop valves and control valves

Turbine is equipped with emergency stop valves to cut off steam supply and with control valves regulate steam supply. Emergency stop valves (ESV) are provided in the mainsteam line and interceptor valves are provided in the hot reheat line. Emergency stop valves are actuated by servomotor controlled by the protection system. Control valves are actuated by the governing system through servomotors to regulate steam supply as required by the load.

Generator

The main shaft of the turbine is connected to the rotor of the main generator. The shaft rotates the rotor, which cuts the flux of the armature coil, and as a result an induced emf is produced.

Compressed Air System

There are two types of compressed air system namely Instrument air system or control air system & Station air system.

Air Compressor-

The station air compressor is a generally slow speed type & is arranged for belt drive. The cylinder heads and barrel are enclosed in a jacket which extends around the valve also. The inter cooler is provided between the low and high pressure cylinder which cools the air between stage and collects the moisture that condenses.

SWITCHYARDSwitch yard is the place adjacent to the power house where the transformers, circuit breakers, isolators, bus bars, CT & PTs, lightning arrestors, current limiting reactors and other equipments are installed. ACSR conductors are used in the line wires.

One and a half breaker scheme is used here. Three circuit breakers are used for controlling two circuits which are connected between two bus bars. Normally, both the bus bars are in service. A fault on any one of the bus bars is cleared by opening of the associated circuit breakers connected to the faulty bus bar without affecting continuity of supply. Similarly, any circuit breaker can be taken out for maintenance without causing interruption.

<| This is one and a half breaker scheme

Though unit 4 has double breaker scheme.

Reactor: A reactor is a coil having large inductive reactances in comparison to its ohmic resistance and is introduced in a circuit or system to limit the

short circuit currents to a safer value in order to protect the electrical installation. Oil immersed type reactors are employed here. Insulation and cooling arrangement employed are similar to those of an ordinary transformer.

Wave trap: This is used to block high frequency

component.

Circuit Breaker: Circuit Breaker acts like a switch when a failure occurs in transmission line.It is very

Costly equipment.

In one and a half scheme there is 3 breakers in one dia created by 2 main bay and one tie bay.

In one circle there is six breakers.

Isolator: Isolator isolates the breaker from the line for maintenance purpose or to save breaker in

Switching time as breaker is costly.

CT & PT: CT and PT is there for protection and metering purpose.

Lightning Arrester: It is an electrical device inserted in a power line to protect equipment from

Sudden fluctuations in current. It is also known as Surge Protector.

CVT: It senses any overvoltage of line. It helps in communication

between two sub stations also. As capacitor gives low impedence for high frequency so communication signal goes through it to PLCC (Power Linear Carrier Communication ) panel . It helps in Tele-protection, Tele –metering and Tele-communication. This is called carrier-aided protection system. The band of this signal is 30 kHz-500kHz, but to reduce noise it is kept as 50kHz to 500 kHz. It is trapped by Wave Trap before the transformer connection and maintain communication process between substations using supply line as communication line.

Bus Reactor: To balance the reactive power consumed by transmission line shunt reactor is there to make the line a lossless line.

TURBOGENERATORFSTPP has 5 units (stage-I : 3 x 200 and stage-II : 2 x 500)of 3-phase synchronous generators which convert mechanical energy to electrical energy.The generators are coupled to the steam turbine shaft.

Specification:

The following table shows the rating of the Generators of stage-I and stage-II.

Rated parameters Stage-I Stage-II

MAX. CONTINUOUS KVA RATING 247000 588000

MAX. CONTINUOUS KW RATING 210000 500000

RATED POWER FACTOR 0.83 lag 0.83 lag

STATOR Voltage 15750 21000

Current 9050 16200

ROTOR Voltage 310 240

Current 2600 4030

RATED SPEED 3000 3000

RATED FREQUENCY 50 50

PHASE CONNECTION YY YY

NO. OF TERMINALS BROUGHT OUT OF STATOR 9 9

COOLANT Water & Hydrogen Water & Hydrogen

GAS PRESSURE 3.5 kg/cm2 3.5 kg/cm2

INSULATION CLASS B B

MAKER’S NAME BHARAT HEAVY ELECTRICALS LTD.

DIFFERENT PARTS

1. Stator - Stator Frame (Fabrication & Machining)2. Core Assembly - Stator Core, Core Suspension Arrangement3. End Shield4. Stator Winding Assembly - Stator Winding , Winding Assembly,Connecting Bus bar5. Rotor - Rotor Shaft, Rotor Wedges, Rotor Coils, Wound Rotor, Rotor Assembly6. Completing Assembly - Bearing Assembly, Shaft Seal Assembly, Oil Catchers,Insert Cover etc7. Exciter8. Auxiliary System

The machine usually consists of two main parts: 1. STATOR 1) FRAME 2) MAGNET CORE 3) WINDINGS

STATOR FRAME

It is a fabricated gas tight steelstructure suitably ribbed internally.It can withstand explosion pressure ofhydrogen air mixture without anyresidual deformation. H2 gas coolersare housed longitudinally inside statorbody.

STATOR CORE

Stator core is made up of insulated punchings of CRGO Si steel and is laminated to minimizeeddy current loss. It provides path for machines’ magnetic flux and has slots in which windings are assembled.Core bars are designed to provide elastic suspension of core in stator.

STATOR WINDINGS

The windings are three phase fractional pitched distributed in two layers of individual bars.

Generator voltage is induced in the stator windings and use of water cooling permits a high value of current density in the machine.The use of combination of solid and hollow conductors effectively reduces the depth of the slot which affect the losses in the winding, and better utilization of slots.

2. ROTOR

It is the rotating part and houses the field windings. It is a cylindrical type rotor.Rotor body is a high strength alloy steel single forging prepared by vacuum cast steel,containing slots for housing field windings and is supported on two bearings. The coils are held against centrifugal forces by means of wedges and by means of non-magnetic retaining ring on the overhang part of the winding.

FIELD WINDINGThese are made from hard drawn silver bearing copper.Gas(H2) pickup system is employed for complete cooling of rotor. Two propeller type fans are shaft mounted on either side of rotor body for circulating cooling gas inside generator. Special ducts (fins) are provided in the rotor body, through which the coolinggas flows to the rotor end windings. SLIP RINGS Helically grooved alloy steel rings are shrunk on rotor shafts and insulated from it.Slip rings are connected to field windings through semi flexible copper lead

Cooling system of the generatorGenerator auxiliary system are broadly classified into 3 parts:

1. STATOR WATER SYSTEM

Stator water cooling is a closed loop system. There are two full capacity single stage centrifugal pumps with change over facility. The pumps are driven by 3Ph. 415V A.C. motors. The stator water cooler is shell and tube type heat exchanger. DM water flows through the shell. There are two mechanical filters and one magnetic filters. Mechanical filters are of wire mesh type. Magnetic filter is having permanent magnet. The expansion tank is a hermetically sealed container made of S.S. Float valve is there in the expansion tank to maintain water level in the tank which act as suction storage tank for stator water pumps. Polishing unit (mixed bed ion exchanger) is there to maintain conductivity of stator water to desired level.

2)SEAL OIL SYSTEM

Generator shaft seals are supplied with pressurized seal oil to prevent hydrogen escape at the shaft

Oil pressure is kept higher than the gas pressure. There are one AC seal oil pp. and one DC seal oil pp. which feed oil to the seal through cooler and filter.

A vacuum pump is provided to maintain vacuum in seal oil tank The seal oil pressure to the seal is controlled by DPR which maintain specified DP between oil and hydrogen. There is provision for thrust oil to hold the seal ring in position against H2 pressure (0.5kg/cm2 more than seal oil pressure). There are 2 oil coolers to cool the hot oil.

BEARG

DRAINS

TG

1 2

EE

BEARING

TE

BEARING

GAS

EXHAUST

FROM GAS

SYSTEM

IOT

VACUUM

TANK

SOST

SOPSOP-3

DC

DP

SW

TS ES FR

FLOW M.

PS PG

2. GAS SYSTEM Generator gas system constitutes of hydrogen gas used to cool the rotor and certain parts of stator. H2-air mixture is explosive. So filling the generator with H2 by replacing air is dangerous. So initially air is replaced by CO2 and since CO2 is heavier than air CO2 is being filled from the bottom. Purging of air with CO2 is being done till the purity of CO2 inside casing reaches above 95%.

Now H2 is dried and then passed from the top to replace the CO2. Purging of H2 is continued till purity of H2 reaches 98%.

Why H2 is used as a coolant?

H2 is lightest gas with 0.09 gm / litre while air’s 1.3 i.e. 14.4 times & high thermal capacity

Thermal conductivity of H2 is 5 times that of air. Its specific heat is 3.42 at 0°C, as compared to 0.237 of air.

2 H2 + O2 mixture ignites on adiabatic compression at 526°C, 3H2 + O2 at 544°C. H2 and O2 combine slowly at 180°C or in bright sunlight. Explosion occurs with moist gases at 550 - 700°C. Ignition temp. of H2 in air is 538°C & Calorific value of H2 is 136 k Cal / gm.

For filling in TGs, 99.9% v/v purity gas is used. Traces of SO2 & NH3 shall not be detectable

Two propeller type fans are shaft mounted on either side of rotor body for circulating H2 in the generator.The H2 is itself cooled by DM water circulation.

TRANSFORMERSA static electromagnetic device with two or more windings ,which transforms a system of alternating voltage and current into another system of voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power.

TYPES OF TRANSFORMERS:

Power transformers

Used in transmission network of higher voltages, deployed for step-up and step down transformer application (400 kV, 200 kV, 110 kV, 66 kV, 33kV,22kV)

Distribution transformers

Used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V).

Transformer insulations

Minor insulation Like inter turn insulation, is achieved using cellulogic paper.

Major insulation Between primary and secondary, phase to phase and inner coil

to core. This is achieved by Bakelite, wooden blocks, cellulogic paper cylinders.

Transformer Oil Derivative of petroleum crude. This has good dielectric strength.

Also a good cooling medium and absorbs heat from the windings in transformer. The mineral oil has a flash point of 140°C and 160°C fire point. This also 'can Sustain the combustion with its own energy, once it catches fire. Thus this is unsuitable for the transformer located indoors. •The indoor transformers are filled with a synthetic liquid known as silicate liquid. This is fire assistant and has flash point well above 300°C.

Types of transformers in FSTPP:

Generator Transformer:

The generator is connected to this transformer by means of isolated bus ducts. This transformer is used to step up the generating voltage of around 15KV to grid voltage. This transformer is generally provided with OFAF cooling. It is also provided with off circuit/on load taps on the high voltage side. This transformer has elaborate cooling system consisting of number of oil pumps and cooling fans apart from various accessories. For 500 MW units there are 3 individual single phase transformers for each phase. The rated input voltage is 21KV.

RATED O/P 250MVA

RATED VOLT. (HV) 420KV

RATED VOLT. (LV) 15.75KV

RATED CURRENT(HV) 344A

RATED CURRENT(LV) 9175A

VECTOR GROUP Y n d11

Unit auxiliary transformer

The UAT draws its input from the main bus-duct connecting generator to the generator Transformer. The total KVA capacity of unit auxiliary transformer required can be determined by assuming 0.85 power

factor and 0.9 efficiency for total auxiliary motor load. It is safe and desirable to provide about 20% excess capacity than circulate so as to provide for miscellaneous auxiliaries and possible increase in auxiliary load. With higher unit ratings and higher steam conditions, the auxiliary power required also increases and limitations imposed by the switchgear voltages available, indicate the maximum size of unit auxiliary transformer which can be used.

Station transformer

The station transformer is required to feed power to the auxiliaries during start ups. This transformer is normally rated for the initial auxiliary load requirements of unit. In typical cases, this load is of the order of 60% of the load at full generating capacity. But in large stations where more than one units are operating, the station transformers should have sufficient capacity to start two units at a time in addition to feeding the common auxiliaries. It is also provided with on load tap changer to cater to the fluctuating voltage of the grid.RATED OUTPUT(LV):6.6KV RATED INPUT( HV) : 33KV

Excitation transformer

The excitation transformer is used in the static excitation system of the generator .The output of the generator is fed to the primary of the excitation transformer. The secondary is the input to 4 thyristor banks .In addition there are protective relays for the excitation transformer.

Auxiliary transformers

They are used to supply power to the LT auxiliary units like ESPs, lubricating

oil pumps, seal

oil pumps etc.

Tie / Auto transformer

Tie transformers are connected to the 400 KV bus. They are used to step down the voltage to 33 KV. Then the station transformers step down the voltage to 6.6 KV (station buses). The unit buses are connected to the station buses through circuit breakers and isolaters. When the generator trips, there

is no supply to the unit buses and the tie-transformers supply the station buses. Then the circuit breakers and the isolaters are closed. Thus the supply to the auxiliaries is maintained.

Instrument transformers

Potential Transformers step down values to safe levels for measurement. They are also called voltage transformers. Their standard output is 120V.

Current Transformers have standard output of 1 or 5 amps. They can produce high voltages if open circuited.

SWITCHGEAR“The apparatus used for Switching, Controlling and Protecting the Electrical Circuits and equipment” is known as Switchgear.

NEEDS OF SWITCHGEAR :

Switching during normal operating conditions for the purpose of Operation and Maintenance.

Switching during Faults and Abnormal conditions and interrupting the fault currents.

PARTS OF SWITCHGEAR :

Switching device:

Power circuit Control circuit

Measurement and display Protection

Power Circuit:

Circuit breakers / contactors Isolators

Earthing switch

Control Circuit :

Service / test /isolated position selectors Tripping and closing circuit

Spring charging, anti pumping arrangement Supply monitoring , space heaters , indications

Measurement and Protection:

Ammeter, voltmeter, energy meter Relays, CT, PT,

Classification of switchgears:

Method of arc quenching :

Bulk oil, Min. oil, Air Break, Air Blast, SF6 , Vacuum

Working voltage :

440V, 6.6 kV, 11 kV, 400 kV etc.

Indoor / out door

SOME INTERLOCKS :

Check synchronization for closing

Master relay contacts for trip and close1.) HV & LV Breaker interlocks 2.) Main / Reserve supply

change over

• The Auxiliary power system in a power plant must form a RELIABLE source of power to all unit and Station auxiliaries. The basic function of Switchgear is to control supply of electric power and to protect the equipment in the event of abnormal conditions. Hence the switchgears have to be RELIABLE, SAFE, and ADEQUATE.

•

Defining the reliability, safety aspects and adequacy aspects in terms of Quantitative parameters forms the essential part in “SPECIFICATIONS”

• 33KV, 11KV, 6.6KV and 3.3KV Switchgears

• Indoor, metal clad single front and fully Compartmentalized, with degree of protection IP42 and IP52 for metering compartments. For 33 KV the switchgears can be metal enclosed either.

•

Circuit Breakers are of either SF6 or Vacuum type. They shall comprise of three separate identical single pole interrupting units operated through a common shaft by a sturdy mechanism.

• Breakers are suitable for Switching transformers and motors at any load and also for starting 3.3 KV - Above 200 KW to1500 KW, 11 KV- above 1500 KW for 500MW units and 6.6 KV- above 200KW for 210MW units.

• Surge arresters are provided for all motor feeders to limit the over voltages. For Motors where frequent start/stop of motors is called for HRC fuse backed contactors are provided.

• Suitable Interlocks are provided to ensure that Breaker is off before opening the rear doors/covers.

Basic design features: Control and Safety• Circuit Breakers/contactors are being normally operated from remote through Distributed Digital Control & Management of Information System (DDCMIS)/ Programmable Logic Controller (PLC). The control Switch located on the Switchgear is normally used only for testing.

• All the logic for incomers, bus couplers, ties, transformer feeders and motor feeders is being generated in DDCMIS only. The reverse blocking schemes are still incorporated in Switchgear (hardwired).

ACKNOWLEDGEMENTWe the students of JADAVPUR UNIVERSITY,KOLKATA have undergone vocational training in NATIONAL THERMAL POWER STATION (FSTPS). We wish to acknowledge the support and helping hands extended by the entire members of the TRAINING DEPARTMENT and all those Engineers who helped & guided us on our visit to the various departments of the famous Thermal Power Station during the course of our training.

Any successful work is accompanied by the Helping & Co-operation of well-wisher. Whatever we have tried to present in our project cum training report would remains incomplete unless & until we extend our heartiest thanks to all the people who have spend their valuable time to help & explain us all that we wanted to know. May words will fall short to describe their importance to us, our gratefulness to them & also to their kind & co-operative attitude throughout the course of our training in NTPC (Farakka) .

No matter wherever we will stand in our life & career in the end, these glorious days of our short stay with all the people connected directly or indirectly to NTPC (Farakka) will never fade away from my mind.

It’s my honor to extended my gratitude & thanks to

Ms. Susmita Bhattacharya

Mr. D.Mohanti

Ms. Anwesha Mukherjee

Mr. Rohit Agarwal

Mr. S.K.SOM

This report summarizes the huge learning experience that we had in all the sections of this modern Power Plant.

DATE: 15.06.2013

PLACE: MURSHIDABAD