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THIS PROJECT IS A PART OF VOCATIONAL TRAINING PROGRAMME,AS DIRECTED BY THE RAJASTHAN TECHNICAL UNIVERSITY ,KOTA(Rajasthan) FOR IT’S SIXTH SEMESTER APPEAR BATCH.. I AM THANKFUL TO N.T.P.C. TRAINING & DEVELOPMENT CENTRE FOR IT’S KIND CO-OPERATION.I AM ALSO THANKFUL TO D.G.M.’s AND THEIR COLLEGES(SUPERITENDENTS,OPERATORS,LABOURS etc.)OF VARIOUSDEPARTMENTS(C&I,OPERATION, MECHANICAL MAINTENANCE,EMD) FOR THEIR OVERWHELMING RESPONSE. THIS PROJECT CONSISTS OF THE BRIEF NOTE ON VARIOUS DEPARTMENTS viz. TURBINE,BOILER,ASH HANDLING,D.M. PLANT,OPERATION etc. AND HOW THEY ACCOUNTS FOR PROPER FUNCTIONING OF THE THERMAL UNIT. THIS PROJECT HAS GIVEN ME GREAT PRACTICAL KNOWLEDGE ON THERMAL PLANT AND IT’S FUNCTION AND HOPE IT WOULD PROVE FRUITFUL FOR MY CAREER ASPECT. THANKING YOU. SWETA ANAND SEMESTER : 6TH SEM. BRANCH : ELECTRONICS & COMM ENGG. COLLEGE : SHRI BALA JI C. OF ENGG & TECHNOLOGY.

National Thermal Power PlantKahalgaon

Contents

1. INRODUCTION 1.1 National Thermal Power Corporation1.2 Installed Capacity1.3 NTPC Kahalgaon ( Overview )

2. NTPC 2.1 Introduction

2.2 National Thermal Power Corporation Layout

3. Major Areas in NTPC Kahalgaon3.1 Coal Handling Plant

3.2 Boiler and its Auxiliaries

3.3 Turbine Auxiliaries

3.4 Generator and its auxiliaries

3.5 Switchyard and Transmission Equipments

4. Transformers5. Turbine

6. Governor

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1. INRODUCTION

1.1. National Thermal Power Corporation

NTPC, the largest power Company in India, was setup in 1975 to accelerate power development in the country. It is among the world’s largest and most efficient power generation companies. In Forbes list of World’s 2000 Largest Companies for the year 2007, NTPC occupies 411th place.

NTPC has installed capacities of 29,394 MW. It has 15 coal based power stations (23,395 MW), 7 gas based power stations (3,955 MW) and 4 power stations in Joint Ventures (1,794 MW). The company has power generating facilities in all major regions of the country. It plans to be a 75,000 MW company by 2017.

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NTPC has gone beyond the thermal power generation. It has diversified into hydro power, coal mining, power equipment manufacturing, oil & gas exploration, power trading & distribution. NTPC is now in the entire power

value chain and is poised to become an Integrated Power Major.

NTPC's share on 31 Mar 2008 in the total installed capacity of the country was 19.1% and it contributed 28.50% of the total power generation of the country during 2007-08. NTPC has set new benchmarks for the power industry both in the area of power plant

construction and operations

with its experience and expertise in the power sector, NTPC is extending consultancy services to various organisations in the power business. It provides consultancy in the area of power plant constructions and power generation to companies in India and abroad.

In November 2004, NTPC came out with its Initial Public Offering (IPO) consisting of 5.25% as fresh issue and 5.25% as offer for sale by Government of India. NTPC thus became a listed company with Government holding 89.5% of the equity share capital and rest held by Institutional Investors and Public. The issue was a resounding success. NTPC is among the largest five companies in India in terms of market capitalization.

Recognising its excellent performance and vast potential, Government of the India has identified NTPC as one of the jewels of Public Sector 'Navratnas'- a potential global giant. Inspired by its glorious past and vibrant present, NTPC is well on its way to realise its vision of being "A world class integrated power major, powering India's growth, with increasing global presence".

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1.2. Installed Capacity

An Overview

  No Of Plants Capacity MW

NTPC Owned

Coal 15 23395

Gas/Liquid Fuel 7 3955

Total 22 27350

Owned By JVs

Coal & Gas 4 2044

Total 26 29394

Regional Spread of Generating Facilities

Region Coal Gas Total

Northern 7035 2312 9347

Western 5860 1293 7153

Southern 3600 350 3950

Eastern 6900 - 6900

JVs 564 1480 2044

Tatal 23959 5435 29394

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Project Profile

Coal Based Power Stations

  Coal based State Commissioned

Capacity(MW)

1. Singrauli Uttar Pradesh 2,000

2. Korba Chattisgarh 2,100

3. Ramagundam Andhra Pradesh 2,600

4. Farakka West Bengal 1,600

5. Vindhyachal Madhya Pradesh 3,260

6. Rihand Uttar Pradesh 2,000

7. Kahalgaon Bihar 2,340

8. NTCPP Uttar Pradesh 840

9. Talcher Kaniha Orissa 3,000

10. Unchahar Uttar Pradesh 1,050

11. Talcher Thermal Orissa 460

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12. Simhadri Andhra Pradesh 1,000

13. Tanda Uttar Pradesh 440

14. Badarpur Delhi 705

15. Sipat Chattisgarh 500

Total (Coal) 23,395

Gas/Liq. Fuel Based Power Stations 

  Gas based State Commissioned

Capacity(MW)

16. Anta Rajasthan 413

17. Auraiya Uttar Pradesh 652

18. Kawas Gujarat 645

19. Dadri Uttar Pradesh 817

20. Jhanor-Gandhar Gujarat 648

21. Rajiv Gandhi CCPP

Kayamkulam Kerala 350

22. Faridabad Haryana 430

Total (Gas) 3,955

Power Plants with Joint Ventures

 Coal

Based State Fuel

Commissioned Capacity

(MW)

23. Durgapur West Bengal Coal 120

24. Rourkela Orissa Coal 120

25. Bhilai Chhattisgarh Coal 324

26. RGPPL Maharastra Naptha/LNG 1480

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Total(JV) 2044

Grand Total (Coal + Gas + JV) 29,394

1.3. NTPC Kahalgaon ( Overview )

The area is in Kahalgaon on the bank of River Ganga. In the state of Bihar. Kahalgaon is one of the major energy sources of India, the place, named after was once upon a time covered with dense and unnavigable forests and inhabited by wild animals. The place was considered very treacherous. Just two generations ago, small holders were tending their parcels of land here, and the original inhabitants were gathering honey and herbs in the forest. In the late fifties, a large scale dam banked up the water of the River Ganges. Later, rich coal deposits spread over an area of 2200 km² in the state of Jharkhand were discovered that could be used to generate electricity.

The population of Kahalgaon mainly consists of professionals and workers of these large industrial units and businessmen and employees of other organizations dealing with the power or coal industry, in addition to staff members of various government agencies

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Photo 1. National Thermal Power Corporation, Kahalgaon

Mining

All these Industrial developments have only been possible because Mother earth has blessed this part of the land with mineable reserves of Coal. Northern Coalfields Limited, the PSU coal major & the highest profit making subsidiary of Coal giant “Coal India Ltd” is the only feeder to these power, chemical & cement plants located within the vicinity of Kahalgaon Zone. With many billion tonnes of coal being mined for the last 35-40 yrs, it still sits on a geographical reserve of more than 8.7 billion tones, which can be mined for another 35-40 Yrs.

2. NTPC 2.1. Introduction

NTPC full form is National Thermal Power Corporation. The Place is now known as NTPC Kahalgaon. There is brief detail:-

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Approved Capacity 2340 MW

Installed Capacity 2340 MW

Location Kahalgaon, Bihar

Water Source River Ganga

Fuel Coal

Beneficiary StatesState & Union territories of NR, WR, ER, SR,Uttar Pradesh, Bihar

Fuel requirement4.1 million tonnes per year for stage I, 6.62 million per year for 2 units of stage II.3.67 million per year for 3rd unit of stage II

Sourse of fuelRajmahal, Murra, Chuperbita Coal field of eastern Coal field Ltd.

Total Area 3300 Acres, Approved

InvestmentRs. 1715 Crore (Stage I), 6330 Crore (Stage II)

Unit SizesStage - I: 4x 210 MWStage -II: 3x 500 MW

Units Commissioned

Unit -I 200 MW March 1992Unit -II 200 MW March 1994Unit -III 200 MW March 1995Unit -IV 200 MW November 1996

Address: National Thermal Power Corporation, Kahalgaon

2.2. National Thermal Power Corporation Layout

The general layout of thermal power plant consists of mainly four circuits as shown.

1. Coal and Ash circuit2. Air and Gas circuit3. Feed Water and Steam circuit4. Cooling Water circuit

Coal and Ash Circuit:

In this circuit, the coal from the storage is fed to the boiler through coal handling equipment for the generation of steam. Ash produced due to combustion of coal is removed to ash storage through ash-handling system.Air and Gas Circuit:

Air is supplied to the combustion chamber of the boiler either through forced draught or induced draught fan or by using both. The dust from the air is removed before supplying to the combustion chamber. The exhaust gases carrying sufficient quantity of heat and ash are passed through the air-heater where the exhaust heat of the gases is given to the air and then it is passed

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through the dust collectors where most of the dust is removed before exhausting the gases to the atmosphere.

Feed Water and Steam Circuit: The steam generated in the boiler is fed to the steam prime mover to develop the power. The steam coming out of the prime mover is condensed in the condenser and then fed to the boiler with the help of pump. The condensate is heated in the feed-heaters using the steam tapped from different points of the turbine. The feed heaters may be of mixed type or indirect heating type. Some of the steam and water are lost passing through different components of the system; therefore, feed water is supplied from external source to compensate this loss. The feed water supplied from external source to compensate the loss. The feed water supplied from external source is passed through the purifying plant to reduce to reduce dissolve salts to an acceptable level. This purification is necessary to avoid the scaling of the boiler tubes. Purification done by DM plants.

Cooling Water Circuit:

The quantity of cooling water required to condense the steam is considerably high and it is taken from a lake, river or sea. At the Kahalgaon thermal power plant it is taken from an artificial lake created near the plant. The water is pumped in by means of pumps and the hot water after condensing the steam is cooled before sending back into the pond. This is a closed system where the water goes to the pond and is re circulated back into the power plant. Generally open systems like rivers are more economical than closed systems.

3. Major Areas in NTPC

3.1.Coal Handling Plant (CHP)3.1.1. Coal Handling plant layout & Locations of mines

3.1.2. Coal Handling Plant Working

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Coal source of plant are Rajmahal, Murra, Chuperbita Coal field of eastern Coal field Ltd. 14.3 million tonnes of coal are consumed by plant per year. Coal Reach to the CHP through train.Where the size of coal is Approx. Less than 200 mm. The coal was throw into Track Hopper (These self-cleaning, double rack and pinion style valves receive material from railroad cars or material reclaimed from outdoor storage piles by bulldozers. Their purpose is to shut off the material flow from the hoppers to material handling conveyors below.) From Track Hopper threw conveyer belts coal reaches to the Crusher House (it is a 6 store building where the coal is cursed into a size of less than 20mm) in between track hopper and crusher house “SUSPENDED MAGNETS”,”MAGNETIC DETECTOR” and “MAGNETIC SEPERATORS” are placed so that only coal pieces reach to the crusher house. Because if Heavy metal pieces reach to the crusher house they will damage it.Then if Plant unit need coal threw conveyer belts coal reaches to the coal bunkers of Unit.Coal Bunkers (they store the coal for a single unit and Supply coal to furnace threw coal mills when it needs) but when coal bunkers are full then the coal is stacked in the coal yard using the coal stacker and reclaimer machine (it is a huge machine used to stack/Reclaim the coal used in the plant they are always placed in coal yard where coal is stacked, In NTPC SSTPS there are 3 such type of machines are available two for stage-I and one for stage-II. )

In NTPC SSTPS Different types of Electrical machines are used in CHP which are:

S.No. Name Type Ratings

1. Conveyer motor

Squirrel cage induction motor 6.6 KV / 0.4 KV

2. Plaugh motor Squirrel cage induction motor 6.6 KV

3. Traverse drive motor

Squirrel cage induction motor 6.6 KV

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3.2. Boiler & its Auxiliaries

3.2.1. Boiler

“BOILER IS A CLOSED VESSEL IN WHICH STEAM IS GENERATED BY MEANS OF HEAT ENERGY, BOILER HAVING CAPACITY OF 22.75 LITERS COMES UNDER BOILER ACT.”

Type of boiler used in NTPC SSTPS in Stage-I and II are Water tube boilers

A water-tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Water-tube boilers are used for high-pressure boilers. Fuel is burned inside the furnace, creating hot gas which heats up water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will renter the furnace in through a super heater in order to become superheated. Superheated steam is used in driving turbines. Since water droplets can severely damage turbine blades, steam is superheated to 730°F (390°C) or higher in order to ensure that there is no water entrained in the steam.

Cool water at the bottom of the steam drum returns to the feed water drum via large-bore 'down comer tubes', where it helps pre-heat the feed water supply. (In 'large utility boilers', the feed water is supplied to the steam drum and the down comers supply water to the bottom of the water walls). To increase the economy of the boiler, the exhaust gasses are also used to pre-heat the air blown into the furnace and warm the feed water supply. Such water-tube boilers in thermal power station are also called steam generating units.

Properties of fuel

FLASH POINT -IT IS A MINIMUM TEMP AT WHICH THE FUEL IS HEATED TO GIVE OFF INFLAMABLE VAPOUR IN SUFFICIENT QUANTITY TO IGNITE WHEN BROUGHT IN CONTACT OF FLAME.

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POUR POINT -IT IS A MIN. TEMP AT WHICH OIL CAN HANDLE OR CAN FLOW EASILY IN PIPE LINE.

FIRE POINT - IT IS A MIN. TEMP OF FUEL AT WHICH IT STARTS BURNING WITHOUT EXTERNAL SUPPORT.

CALORIFIC VALUE -IT IS A HEAT ENERGY LIBERATED BY COMPLETE COMBUSTION OF UNIT MASS OF FUEL.

COAL BURNERS

EFFECTIVE UTILISATION OF PULVERISED COAL DEPENDS ON THE ABILITY OF BURNERS TO PRODUCE UNIFORM MIXING OF COAL AND AIR.

In NTPC SSTPS(Stage-I and Stage-II ) TANGENTIAL FIRING Method is used in which from corners tangentially the air and fuel are Passed inside furnace region where burning takes place as shown in figure.

3.2.2. Boiler Auxiliaries (Stage-I & II)

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Arrangement of Boiler Aux and various parts at diff. Elevation .

Role of Air Heaters

To recycle the heat of exit flue gas back in the combustion process Efficient coal combustion and pulverization depends on air heater

performance For every 20 deg drop in flue gas exit temp. the boiler efficiency

increased by about 1%.

Coal Mill

A ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or rods.

A tube mill is a revolving cylinder of up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material, mixed with water, is fed into the chamber from one end, and passes out the other end as slime.

ESP ( Electrostatic Precipitator )

An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream

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INDUCED DRAUGHT FAN

Made by: BHEL Type :3 phase Squirrel cage induction motor Stator connection : Y (star) Operated at Freq.: 50 Hz Voltage: 6600V Current: 138.5 A Power: 1300 KW P.f. : 0.85 Efficiency: 95.5 Rotor speed:744 rpm

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Weight: 13700 Kg Lubricant: Greave servogem-2 Max Wdg. Temp.:48°C

FORCED DARUGHT FAN

Made By: BHEL Type: 3 phase Squirrel cage induction motor Stator connection : Y (star) Operated at Freq.: 50 Hz Voltage: 6600V Current: 84.5 A Power: 800 KW P.f. : 0.875 Efficiency: 94.8 Rotor speed:1490 rpm Lubricant: Greave servogem-2 Max Wdg. Temp.:48°C

PRIMARY AIR FAN

Made By: BHEL Type: 3 phase Squirrel cage induction motor Stator connection : Y (star) Operated at Freq.: 50 Hz Voltage: 6600V Current: 84.5 A Power:1400 KW P.f. : 0.87 Efficiency: 96.3

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Rotor speed:1493 rpm Lubricant: Greave servogem-2 Max Wdg. Temp.:48°C

FLY ASH COMPOSITION(BY WEIGHT)

AL2O3- 20-25% CAO - 01.0% FE2O3+FE3O4- 0 5-8% K2O- 02.0% MGO - 00.7% MNO - 00.02% NA2O - 00.24% TIO2- 0.7-1.5% SILICA- 64.78%

Ash Utilization

Ash utilization is one of the key concerns at NTPC. The Ash Utilization Division, set up in 1991, strives to derive maximum usage from the vast quantities of ash produced at its coal-based stations. The division proactively formulates policy, plans and programme for ash utilization. It further monitors the progress in these areas and works at developing new fields of ash utilization.As the emphasis on gainful utilization of ash grew, the usage over the years also increased. From 0.3 million tonnes in 1991-1992, the level of utilization during 2006-07 stood at over 20.76 million tonnes.

NTPC has adopted user friendly policy guidelines on ash utilisation. These include actions identified for:

(i) Ash Collection & Storage System

(ii) Facilities & Incentives to users

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(iii) Direct Department Activities

(iv) Administrative & Financial aspects.

3.3. Turbine Auxiliaries

Figure shown below gives the perfect representation of the arrangement of turbine auxiliaries. For proper function of turbine the auxiliaries are arranged at different location peeping the view of easy installation, proper operation and maintenance and technical requirement.

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Turb

ine

Auxi

liarie

s

Condensate system

This comprise of Condensate pumps – 3 per Unit of 50% capacity each located near

the condenser Hot well. L.P. Heater – Normally 4 in number with no. 1 located at the upper

part of the condenser and No. 2,3,and 4 around 4m level. Deaerator – one per unit located around 18’M’ level in CD bay.

Feed Water system

The main equipment coming under this system are Boiler feed pump - 3 per Unit of 50% capacity each located 0 ‘M’

level in the T.G. bay. High Pressure Heater – Normally three in number and located

near TG bay. Drip Pumps – Generally two in number of 100% capacity each

situated beneath the LP heater.

Turbine Lub. Oil System –

It consist of main oil pump (M.O.P) ,Starting Oil Pump (S.O.P.) AC stand by oil pumps and emergency DC oil pump and jacking oil pump (JOP) (one at each Unit).

Auxiliary Steam System

The main 16 ata header runs parallel to BC bay at the level of around 18 ‘M’.

3.4. Generator and its Auxiliaries

Constructional Features

Stator frame

1. Stator Components

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2. Stator Core3. Stator windings4. Bushings

1. Stator Frame

Totally enclosed gas tight fabricated structure made of high quality mild steel. and It houses the stator core and Hydrogen coolers

2. Stator Core

The entire core is laminated to minimize magnetic and eddy current losses

Each laminations is made up of cold rolled high quality silicon steel

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3. Stator Winding

The stator has a three phase, double layer, short pitched and bar type windings having two parallel paths.

Each slot accommodates two bars. Each bar consists of solid as well as hollow conductors with cooling

water passing through the hallow space. In the straight slot portion the strands are transposed by 360 0 to reduce

the eddy losses. Bar is taped with several layers of thermosetting epoxy mica tape. To prevent corona discharges between insulation and the wall of the

slot, the slot portion is coated with semi conducting varnish.

4. BushingsThree phases and six neutral terminals are brought out from the stator frame through bushings which are capable of withstanding high voltage and provided with gas tight joint.The conductor of the bushing is made of high conductivity copper tube on which silver plated terminal plates are brazed at both ends.

Rotor

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It comprises of the fallowing components Rotor shaft Rotor windings Retaining ring Fans Slip rings

Rotor Shaft

The main constituents are chromium, molybdenum, nickel and vanadiumOn 2/3 of its circumference approximately, the rotor body is provided with longitudinal slots to accommodate field windings

Rotor windings

The conductors are made of hard drawn silver bearing copper.Exhibits high creep resistance so that coil deformations due to thermal cycling.The individual turns are insulated from each other by layer of glass laminates.

Retaining Ring

The overhang portion of field winding is held by non magnetic steel forging of retaining ring against centrifugal forces.To reduce stray losses , the retaining rings are made of non magnetic, steel and cold worked, resulting high mechanical strength.

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Rotor Fan

The generator cooling gas is circulated by two single stage axial flow propeller type fans. These Fan hubs are made of alloy steel.

Slip rings

The slip rings consist of helically grooved alloy steel rings shrunk on rotor body shaft and insulated from it.The slip rings are provided with inclined holes for self ventilation

Generator Parameters

o Generator Volume : 56 cu mo CO2 reqd for expelling H2 at standstill : 120 cu mo CO2 reqd for expelling H2 under rolling : 160 cu mo H2 filling quantity at standstill : 300 cu mo H2 filling quantity under Standstill : 336cumo Nominal pressure of hydrogen : 3.0 ksc o Permissible variations : +/-0.2 ksc o Hydrogen purity : 99 %o Purity of H2 (minimum) : 97 %o Max temp of Cold gas : 44 deg c

Max moisture content in generator casing : 15 mg/m3 of H2 o Gas flow per Cooler : 7.5 m3/seco Heat Load :528 KWo Cooling water flow per cooler :87.5m3/seco No of H2 coolers :4o Pressure Drop through cooler :17.5mmwclo Minimum Temp of cooling water :13deg C

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to maintain min 20 deg c cold gas

Cooling of Generator

Hydrogen cooling: - Stator core, Rotor core and winding

Generator Hydrogen Cooling Circuit

Seal oil system

The locations where the rotor shaft passes through the stator casing are provided with radial seal rings. The gap between the seal ring and the shaft is sealed with seal oil.

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Cold Gas

Hot Gas

To ensure effective sealing seal oil pressure in the annular gap is

maintained at a higher level than the gas pressure within the generator casing.

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Seal Oil Operation

Seal oil supplied to the shaft seals is drained from both air & H2 sides. The air side seal oil is directly returned to SOT via a float valve. The oil drained from H2 side of the shaft seal is discharged into generator prechambers. The prechambers permit the escape of entrained gas bubbles and deforming of the oil. Oil from prechambers flow to IOT & a float valve maintains the oil level in IOT excess oil is returned back to SOST.IOT act as a gas barrier preventing the ingress of H2 to SOT.

Excitation System

Static excitation system is used in mainly 200MW Generator set. The AC power is trapped off from generator terminal. Stepped down and rectified by fully controlled thyristor bridge and then feed to generator field as excitation power. to control the generator output voltage. A high control speed is achieved by using an inertia free control and power electronics system. Any deviation in generator terminal voltage is sensed by error detector and causes the voltage regulator to advance or retard the firing angle of thyristor there by controlling the field excitation.

The Static Excitation System Consists of:

Rectifier Transformer Thyristor Converter Automatic voltage Regulator Field Flashing Circuit Field Breaker and Field Discharge Equipment

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2 Pole 3000 RPM Hydrogen / Hydrogen and Water cooled Turbo-generators

The Hydrogen and water cooled machines (THDF type) are offered with the stator winding directly water-cooled and the rotor winding directly cooled with Hydrogen. The stator core has a leaf spring suspension. The machines are with Micalastic system of impregnation and the bearings are mounted on end shields. The stator overhang is with a support ring. A magnetic shunt traps the end leakage flux. These machines are provided

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with multistage compressors and vertical coolers on turbine end. These can be offered with either brushless /static type of excitation systems

The (THW) type machines have the stator winding directly cooled by water whereas the rotor winding is directly cooled by Hydrogen by gap pick up method. The stator core is mounted resiliently on flexible core bars. Resin rich thermo-reactive insulation is used for the stator winding and top ripple springs are placed in the stator slots. These machines are also characterized by enclosed type slip rings with forced ventilation. Two axial fans are provided on rotor and four Hydrogen coolers are provided for systematic ventilation. These are offered with static excitation system

Generator Rating:

KW 500,000Power Factor 0.85 laggingKVA 588,000Stator Voltage 21,000Stator Ampere 16,200Rotor Voltage 340Rotor Ampere 4040Rpm 3000Hz 50Phase 3Connection Y YCoolant Water & HydrogenGas Pressure 4 BarInsulation Class BType TG-HH-0500-2Make BHEL- Haridwar

3.5. Switchyard & Transmission Equipments

An electric power system is composed of high –voltage transmission lines that feed power to a medium voltage (MV) network by means of substations.

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In NTPC Kahalgaon these MV networks generally operated at voltages between 15.7 KV to 22.0 KV. In turn they supply thousands of independent low voltage system that function between 220V to 600V.

Switchyard and transmission equipments are:

SubstationsSubstations are used throughout an electrical system. Starting with the generating station , a substation raises the medium-voltage generated by the Synchronous generator to the high-voltage needed to transmit the energy economically.The high transmission-line voltage is then reduced in those substations located close to the power consuming center. The electrical equipment in such distribution substation is similar to that found in substation associated with generating plant.

Substations Equipments

Transformers Circuit breakers Surge arresters Current Limiting Reactor Isolators Instrument transformers Relay and protection devices

Transformers

Generator Transformer:GT-(1,2,3,4,)

Rating

Maker: BHEL BhopalCooling method: OFWFVector group: YNd11

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KVA rating: 250KV (no load): HV: 400LV: 15.75Line Ampere:HV: 360.9LV: 9164.6

No. of phases: 3Frequency: 50HzIndependent volt: 14.41Top oil Temp. rise (above 50C ambient): 35C

Mean Wdgs Temp Rise:40 C

Generator Transformer:GT-(5)

Rating

Maker: ABBCooling method: OFAFVector group: YNd11Frequency: 50HzKVA rating: 250KV (no load): HV: 250LV: 250Line Ampere:HV: 343.66LV: 9164.28

No. of phases: 3Frequency: 50Hz

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Independent volt: 14.41

Top oil Temp. rise (above 50C ambient): 35C

Mean Wdgs Temp Rise:40 C

UST-3A

Maker: BBL BombayKVA – 1600Voltage at No loadHV: 6600VLV: 433VAmpereHV: 140ALV: 2133A

Phase: 3Frequency: 50Hz

UTA-2A

Maker: Hackbridge Hewittic andEasun Ltd.KVA – 1600Voltage at No loadHV: 6600VLV: 433VAmpereHV: 140ALV: 2133A

Phase: 3Frequency: 50Hz

Circuit breakers

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A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Circuit Breaker Operation

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically stored energy within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. The stored energy may be in the form of springs or compressed air. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

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When a current is interrupted, an arc is generated - this arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:

Lengthening of the arc Intensive cooling (in jet chambers)

Division into partial arcs

Zero point quenching

Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

In NTPC SSTPS Type of High-voltage circuit breakers used are given below:

Air blast Circuit Breaker SF6 Circuit Breaker

Air Blast circuit Breaker

These circuit breakers interrupt the circuit by blowing compressed air at supersonic speed across the opening contact. Compressed air is stored in reservoirs at a pressure of about 27-31 Kg/cm2 and is replenished by compressor located in the substation. The most powerful circuit breakers can typically open short-circuit current of 40KA at a line Voltage of 765KV in a matter of 3 to 6 cycles on a 60Hz line.

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Characteristics of SF 6 circuit breakers:

Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber

Autonomy provided by the puffer technique

The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers

Short break time of 2 to 2.5 cycles

High electrical endurance, allowing at least 25 years of operation without reconditioning

Possible compact solutions when used for GIS or hybrid switchgear

Integrated closing resistors or synchronized operations to reduce switching over-voltages

Reliability and availability

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Low noise levels.

The reduction in the number of interrupting chambers per pole has led to a considerable simplification of circuit breakers as well as the number of parts and seals required. As a direct consequence, the reliability of circuit breakers improved, as verified later on by CIGRE surveys.

Lightning Arrester

The purpose of lightning arrester is to limit the over voltage that may occur across transformers and other electrical apparatus due either to lightning or switching surges. The upper end of the arrester is connected to the line or terminal that has to be protected, while the lower end is solidly connected to ground.

Ideally a lightning arrester clips any voltage in excess of a specified maximum, by permitting a large current, if needed be, to be diverted to ground. In this way the arrestor absorbs energy from the incoming surge. So the E-I characteristics of an ideal surge arrester is therefore, a horizontal line whose level corresponds to the maximum permissible surge voltage. In practice, the E-I characteristics slopes upward but is still considered to be reasonably Flat.

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Current-Limiting Reactor

The MV bus in the NTPC SSTPS usually energized several feeder, which carry power to regional load centers surrounding the substation. If so happens that the output impedance of the MV bus is usually very low. Consequently, If the short circuit should occur on one of the feeders, The resulting short-Circuit current could be disastrous.

Isolators

Circuit-Isolator 

Circuit-Isolator provides three-pole, group-operated, visible-air-gap isolation in distribution substations. The Circuit-Isolator II can be used to interrupt low-level charging currents associated with substation buswork and circuit-breaker bushings, as well as other low-voltage currents commonly present in substations.

Benefits

Ideal for isolating transformers, circuit breakers, and other substation equipment for repair and maintenance. Each disconnect is factory-assembled and adjusted for easy installation. Can be custom engineered for mounting on almost any customer-supplied structure.

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Fig: Circuit Isolator

Instrument transformers

Current Transformer

A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary

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winding proportional to the alternating current flowing in its primary. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.

Capacitor Voltage Transformer

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors

connected in series. This results in a large voltage drop across the stack of

capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the secondary terminals.

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Transformer:

Photo: Transformer

TURBINEThere are three turbines for each generator. These are meant to extract maximum heat energy from the steam so as to achieve maximum efficiency. These turbines are named as:

a) High pressure turbine (HP Turbine)b) Intermediate pressure turbine (IP Turbine)c) Low pressure turbine (LP Turbine)

Steam comes into the HP turbine from boiler through the main steam line (MSL).the initial temperature and pressure of this steam is 540⁰C and 170 atm respectively. Each turbine consists of several stages and one stage is made up of one rotor blade and one stator blade.

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For 500 MW, HP turbine consists of 6 stages. Final temperature and pressure after passing through all the stages of HP turbine becomes 345⁰C and 45 atm respectively.

Steam comes out of HP turbine and goes back to reheater in boiler through cold reheating line. Steam is then reheated in reheater and is fed to intermediate pressure turbine through hot reheating line (HRH). Initial temperature and pressure of steam in IP turbine is 540⁰C and 40 atm respectively.

Steam then passes through various stages and then the used steam is again fed back to boiler at reduced temperature and pressure.

Finally steam is fed to the LP turbine and steam is then passed to the condenser where it is converted back to water.

At the bottom of the condenser there is a hot well where water collects.

There are three condensate extract pumps (CEP) out of which two work at a time and extract the water from the hot well.

Steam coming out of the various outlets combine at one place in drain cooler where the temperature and pressure of various steams comes to a common level.

Now water need to be fed into the boiler but before that it needs some preheating.

There are three low pressure heaters (LP Heaters) and two high pressure heaters (HP Heaters)

The heating elements in these heaters is steam coming out of various stages of the IP Turbine and LP Turbine.

From HP Heater, the water goes to economizer.

From economizer heated water goes to the boiler drum. Boiler drum contains water and steam both.

Boiler drum is at a height of 90 meter. Steam being lighter collects at the top of the boiler drum and water flows to the bottom ring header.

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Because of high potential energy, water from boiler drum goes down in the boiler, gets heated, its temperature rises and attains enough kinetic energy to reach back to the boiler drum.

Steam from the boiler drum passes through superheater and attains a high temperature of 540⁰C at 170 atm pressure and goes to the HP turbine.

GovernorThe widespread use of electric clocks, the need for satisfactory operation of power stations running in parallel and the relation between system frequency and the speed of the motors has led to the requirement of close regulation of power system frequency. Control of system frequency and load depends upon the governors of the prime-movers. Fig.1 shows the basic characteristics of a governor. It is seen that with a given setting there is a definite relationship between turbine speed and the load being carried by the turbine. If the load carried by the turbine increases the speed decreases. In order to keep the speed same the governor setting by changing the spring tension in the fly-ball type of governor will be resorted to and the characteristics of the governor will be indicated by the dotted line. In practice the change in characteristics is obtained by remotely operating the governor control motor from the control room. A turbine can be adjusted to carry any given load at a desired speed. If constant frequency is required the operator can adjust the speed of the turbine by changing the governor characteristics as and when desired.

Speed governing system

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Fig. 2 shows the schematic diagram of a speed governing system which controls the real power flow in the power system. The speed governing system consists of the following parts:

1. Speed governor: this is a fly-ball type of speed governor and constitutes the heart of the system as it senses the change in speed or frequency. With the increase in speed the fly-balls move outwards and the point B on linkage mechanism moves downwards and vice-versa.

2. Linkage mechanism: ABC and CDE are the rigid links pivoted at B and D respectively. The mechanism provides a movement to the control valve in the proportion to change in speed. Link 4(l4) provides a feedback from the steam valve movement.

Fig.2 Turbine Speed governing system

3. Hydraulic Amplifier: This consists of the main piston and the pilot valve. Low power level pilot valve movement is converted into high power level piston valve movement which is necessary to open or close the steam valve against high pressure steam.

4. Speed changer: The speed changer provides a steady state power output setting for the turbine. The downward movement of the speed changer opens the upper pilot valve so that more steam is admitted to the turbine under steady condition. The reverse happens when the speed changer moves upward.

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BOILER Boiler is a long column where coal is fired to produce heat and to convert water into

superheated steam. In NTPC – Kahalgaon, the height of the boiler is 90 meter. In the boiler section there are ten elevations which are meant for coal input. Coal is

pulverized in the form of fine powders and fed into the boiler at different elevations. Oil is sprayed in between these elevations to ignite the coal powders and to allow the

heat to sustain for the maximum time in the boiler. There are numerous fine tubes inside the boiler through which water flows. This water

while flowing through the tubes extracts heat from the boiler and gets converted to steam at suitable temperature and pressure.

Maximum temperature is available at the topmost part of the flame. Due to this reason the boiler drum is located at the top (90 meter height).

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