SINGRAULI

SUMMER TRAINING REPORT

SUBMITTED BY: SUBMITTED TO:

Amit kumar singh Mr. Mona SubrahmanyamB.Tech :EEE(G) Galgotias college of engg. and tech. Gr. noida

CONTENT:

Introduction Operation Stage 1 & 2 Coal handling plant Switchyard Reference

ACKNOWLEDGEMENT:

With profound respect and gratitude, I take the opportunity to convey my thanks to complete training department of NTPC singrauli . I express gratitude to the Program Manager and other faculty members of Electrical & Electronics Engineering Department of Galgotias college of engineering and technology for providing this opportunity to undergo industrial training at National Thermal Power Corporation singrauli sonebhadra.

I am extremely grateful to all the technical staff of NTPC singrauli for their co-operation and guidance that helped me a lot during the course of training. I have learnt a lot working under them and I will always be indebted of them for this value addition in me

INTRODUCTION:

ABOUT NTPC

NTPC Limited, the largest thermal power generating company in India, was incepted in year 1975. It is a public sector company wholly owned by Government of India (GOI). In a span of 30 years, NTPC has emerged as a major power company of international repute and standard. NTPC’s core business includes engineering, construction and operation of power generating stations and providing consultancy to power utilities as well. Presently, the total installed capacity of NTPC stands at more than 27904 MW, which includes 18 coal and 8 gas/naphtha based power stations.. 

DETAILS OF POWER GENERATION:NTPC (COAL BASED) Singrauli Uttar Pradesh - 2000 Korba Chhattisgarh -2100 Ramagundam Andhra Pradesh- 2600 Farakka West Bengal -1600 Vindhyachal Madhya Pradesh- 3260 Rihand Uttar Pradesh -2000 Kahalgaon Bihar -2340 Dadri Uttar Pradesh -1330 Talchar Kaniha Orissa- 3000 Unchahar Uttar Pradesh -1050  Thermal Orissa -460 Tanda Uttar Pradesh -440 Simhadri Andhra Pradesh -1000 Badarpur Delhi -705 Sipat-II Chhattisgarh -1000

Total: 24885 MW

POWER GENERATION IN INDIA:

NTPC’s core business is engineering, construction and operation of power generating plants. It also provides consultancy in the area of power plant constructions and power generation to companies in India and abroad. As on date the installed capacity of NTPC is 27,904 MW through its 15 coal based (22,895 MW), 7 gas based (3,955 MW) and 4 Joint Venture Projects (1,054 MW). NTPC acquired 50% equity of the SAIL Power Supply Corporation Ltd. (SPSCL). This JV Company operates the captive power plants of Durgapur (120 MW), Rourkela (120 MW) and Bhilai (74 MW). NTPC also has 28.33% stake in Ratnagiri Gas & Power Private Limited (RGPPL) a joint venture company between NTPC, GAIL, Indian Financial Institutions and Maharashtra SEB Co Ltd. NTPC has set new benchmarks for the power industry both in the area of power plant construction and operation .Its providing power at the cheapest average tariff in the country..

NTPC has been operating its plants at high efficiency levels. Although the company has 19% of the total national capacity it contributes 29% of total power generation due to its focus on high efficiency. NTPC’s share at 31 Mar 2001 of the total installed capacity of the country was 24.51% and it generated 29.68% of the power of the country in 2008–09. Every fourth home in India is lit by NTPC. As at 31 Mar 2011 NTPC's share of the country's total installed capacity is 17.75% and it generated 27.4% of the power generation of the country in 2010–11. NTPC is lighting every third bulb in India. 170.88BU of electricity was produced by its stations in the financial year 2005–2006. The Net Profit after Tax on 31 March 2006 was  58.202 billion. Net profit after tax for the quarter ended 30 June 2006 was  15.528 billion, which is 18.65% more than that for the same quarter in the previous financial year. It is listed in Forbes Global 2000 for 2011 ranked it 348th I n the world.

About NTPC Singrauli:

Singrauli Super Thermal Power Station

Singrauli Super Thermal Power Plant is located at Singrauli in Singrauli district in Indian state of Uttar Pradesh . The power plant is one of the coal based power plants of NTPC.The unit wise capacity and other details are as follows.

1. Stage2. Unit Number

3. Installed Capacity (MW)

4. Date of Commissio

ning

1st 1 200 1982 February

1st 2 200 1982 November

1st 3 200 1983 March

1st 4 200 1983 November

1st 5 200 1984 February

2nd 6 500 1986 December

2nd 7 500 1987 November

Total Seven2000

In stage 1 there are five units of 200 Mw 200*5=1000 MwIn stage 2 there are two units of 500 Mw500*2=1000 MwTotal installed capacity =1000+1000= 2000 Mw

OPERATION OF THERMAL POWER PLANT:

In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert the water into steam. The steam is used to turn a turbine, which is connected to a generator. When the turbine turns, electricity is generated and given as output by the generator, which is then supplied to the consumers through high-voltage power lines.

Detailed process of power generation in a thermal power plant:

1) Water intake:Firstly, water is taken into the boiler through a water source. If water is available in a plenty in the region, then the source is an open pond or river .If water is scarce, then it is recycled and the same water is used over and over again.

2) Boiler heating: The boiler is heated with the help of oil, coal or natural gas. A furnace is used to heat the fuel and supply the heat produced to the boiler. The increase in temperature helps in the transformation of water into steam.3) Steam Turbine: The steam generated in the boiler is sent through a steam turbine. The turbine has blades that rotate when high velocity steam flows across them. This rotation of turbine blades is used to generate electricity4) Generator: A generator is connected to the steam turbine. When the turbine rotates, the generator produces electricity which is then passed on to the power distribution systems.6) Ash collection system: There is a separate residue and ash collection system in place to collect all the waste materials from the combustion process and to prevent them from escaping into the atmosphere.

STAGE 1 & STAGE 2:

Stage 1 consist of five units of 200 Mw generatorsi.e. 200*5=1000 Mwstage 2 consist of two units of 500 Mw generatorsi.e. 500*2=1000 Mw stage 1 and stage 2 consist following sub sections

- Cooling water pump -Three-phase transmission line -Step up transformer- Electrical Generator -Low pressure steam -Boiler feed water pump -Surface condenser-Intermediate pressure steam turbine -Steam control valve -High pressure steam turbine -Deaerator Feed water heater -Coal conveyor -Coal hopper -Coal pulverizer -boiler steam drum -Bottom ash hoper -Super heater-Forced draught(draft) fan -Reheater- Combustion air intake -Economizer -Air preheater -Precipitator -Induced draught(draft) fan -Fuel gas stack

The description of some of the components written above is described as follows:

1. Cooling towers

Cooling Towers are evaporative coolers used for cooling water or other working medium to near the ambivalent web-bulb air temperature. Cooling tower use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries, Chemical plants, power plants and building cooling, for example. The tower vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 meters tall and 100 meters in diameter, or rectangular structure that can be over 40 meters tall and 80 meters long. Smaller towers are normally factory built, while larger ones are constructed on site.The primary use of large , industrial cooling tower system is to remove the heat absorbed in the circulating cooling water systems used in power plants , petroleum refineries, petrochemical and chemical plants, natural gas processing plants and other industrial facilities . The absorbed heat is rejected to the atmosphere by the evaporation of some of the cooling water in mechanical forced-draft or induced draft towers or in natural draft hyperbolic shaped cooling towers as seen at most nuclear power plants.

2.Three phase transmission line

Three phase electric power is a common method of electric power transmission. It is a type of polyphase system mainly used to power motors and many other devices. A Three phase system uses less conductor material to transmit electric power than equivalent single phase, two phase, or direct current system at the same voltage. In a three phase system, three circuits reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two current are delayed in time by one-third and two-third of one cycle of the electrical current. This delay between “phases” has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.At the power station, an electric generator converts mechanical power into a set of electric currents, one from each electromagnetic coil or winding of the generator. The current are sinusoidal functions of time, all at the same frequency but offset in time to give different phases. In a three phase system the phases are spaced equally, giving a phase separation of one-third one cycle. Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers: step-up” this voltage to one more suitable for transmission.After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (i.e. the “household” voltage).The power may already have been split into single phase at this point or it may still be three phase. Where the step-down is 3 phase, the output of this transformer is usually star connected with the standard mains voltage being the phase-neutral voltage. Another system commonly

seen in North America is to have a delta connected secondary with a center tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages( 120 V between two of the phases and neutral , 208 V between the third phase ( known as a wild leg) and neutral and 240 V between any two phase) to be available from the same supply.3.Electrical generator

An Electrical generator is a device that converts kinetic energy to electrical energy, generally using electromagnetic induction. The task of converting the electrical energy into mechanical energy is accomplished by using a motor. The source of mechanical energy may be a reciprocating or turbine steam engine, , water falling through the turbine are made in a variety of sizes ranging from small 1 hp (0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment , to 2,000,000 hp(1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines.Steam turbines are used in all of our major coal fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam generated in ‘Boilers’ or ‘steam generators’ as they are sometimes called.Electrical power station use large stem turbines driving electric generators to produce most (about 86%) of the world’s electricity. These centralized stations are of two types: fossil fuel power plants and nuclear power plants. The turbines used for electric power generation are most often directly coupled to their-generators .As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. Most large nuclear sets rotate at half those speeds, and have a 4-pole generator rather than the more common 2-pole one.

Energy in the steam after it leaves the boiler is converted into rotational energy as it passes through the turbine. The turbine normally consists of several stage with each stages consisting of a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the steam into kinetic energy into forces, caused by pressure drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical energy.STATOR:

The stator winding is made up of insulated copper conductor bars that are distributed around the inside diameter of the stator core, commonly called the stator bore, in equally spaced slots in the core to ensure symmetrical flux linkage with the field produced by the rotor. Each slot contains two conductor bars, one on top of the other. These are generally referred to as top and bottom bars. Top bars are the ones nearest the slot opening (just under the wedge) and the bottom bars are the ones at the slot bottom. The core area between slots is generally called a core tooth

ROTOR:

The rotor winding is installed in the slots machined in the forging main body and is distributed symmetrically around the rotor between the poles. The winding itself is made up of many turns of copper to form the entire series connected winding.

All of the turns associated with a single slot are generally called a coil. The coils are wound into the winding slots in the forging, concentrically in corresponding positions on opposite sides of a pole. The series connection essentially creates a single multi-turn coil overall, that develops the total ampere-turns of the rotor (which is the total current flowing in the rotor winding times the total number of turns)

Generator Cooling System

The 200/210 MW Generator is provided with an efficient cooling system to avoid excessive heating and consequent wear and tear of its main components during operation. This Chapter deals with the rotor-hydrogen cooling system and stator water cooling system along with the shaft sealing and bearing cooling systems.

Rotor Cooling System

The rotor is cooled by means of gap pick-up cooling, wherein the hydrogen gas in the air gap is sucked through the scoops on the rotor wedges and is directed to flow along the ventilating canals milled on the sides of the rotor coil, to the bottom of the slot where it takes a turn and comes out on the similar canal milled on the other side of the rotor coil to the hot zone of the rotor. Due to the rotation of the rotor, a positive suction as well as discharge is created due to which a certain quantity of gas flows and cools the rotor. This method of cooling gives uniform distribution of temperature. Also, this method has an inherent advantage of eliminating the deformation of copper due to varying temperatures.

Hydrogen Cooling System

Hydrogen is used as a cooling medium in large capacity generator in view of its high heat carrying capacity and low density. But in view of its forming an explosive mixture with oxygen, proper arrangement for filling, purging and maintaining its purity inside the generator have to be made. Also, in order to prevent escape of hydrogen from the generator casing, shaft sealing system is used to provide oil sealing.

The hydrogen cooling system mainly comprises of a gas control stand, a drier, an liquid level indicator, hydrogen control panel, gas purity measuring and indicating instruments,

4.Boiler feed water pump : A Boiler feed water pump is a specific type of pump used to pump water into a steam boiler. The water may be freshly supplied or retuning condensation of the steam produced by the boiler. These pumps are normally high pressure units that use suction from a condensate return system and can be of the centrifugal pump type or positive displacement type.

Construction and operationFeed water pumps range in size up to many horsepower and the electric motor is usually separated from the pump body by some form of mechanical coupling. Large industrial condensate pumps may also serve as the feed water pump. In either case, to force the water into the boiler; the pump must generate sufficient pressure to overcome the steam pressure developed by the boiler. This is usually accomplished through the use of a centrifugal pump.Feed water pumps usually run intermittently and are controlled by a float switch or other similar level-sensing device energizing the pump when it detects a lowered liquid level in the boiler is substantially increased. Some pumps contain a two-stage switch. As liquid lowers to the trigger point of the first stage, the pump is activated. I f the liquid continues to drop (perhaps because the pump has failed, its supply has been cut off or exhausted, or its discharge is blocked); the second stage will be triggered. This stage may switch off the boiler equipment (preventing the boiler from running dry and overheating), trigger an alarm, or both.

5 . Control valvesControl valves are valves used within industrial plants and elsewhere to control operating conditions such as temperature, pressure, flow, and liquid Level by fully partially opening or closing in response to signals received from controllers that compares a “set point” to a “process variable” whose value is provided by sensors that monitor changes in such conditions. The opening or closing of control valves is done by means of electrical, hydraulic or pneumatic systems

6. Deaerator

A Dearator is a device for air removal and used to remove dissolved gases (an alternate would be the use of water treatment chemicals) from boiler feed water to make it non-corrosive. A dearator typically includes a vertical domed deaeration section as the deaeration boiler feed water tank. A Steam generating boiler requires that the circulating steam, condensate, and feed

water should be devoid of dissolved gases, particularly corrosive ones and dissolved or suspended solids. The gases will give rise to corrosion of the metal. The solids will deposit on the heating surfaces giving rise to localized heating and tube ruptures due to overheating. Under some conditions it may give to stress corrosion cracking.Deaerator level and pressure must be controlled by adjusting control valves- the level by regulating condensate flow and the pressure by regulating steam flow. If operated properly, most deaerator vendors will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm3/L)

7. Feed water heater

A Feed water heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feed water reduces the irreversible involved in steam generation and therefore improves the thermodynamic efficiency of the system.[4] This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is introduces back into the steam cycle.In a steam power (usually modeled as a modified Ranking cycle), feed water heaters allow the feed water to be brought up to the saturation temperature very gradually. This minimizes the inevitable irreversibility’s associated with heat transfer to the working fluid (water). A belt conveyor consists of two pulleys, with a continuous loop of material- the conveyor Belt – that rotates about them. The pulleys are powered, moving the belt and the material on the belt forward. Conveyor belts are extensively used to transport industrial and agricultural material, such as grain, coal, ores etc.

8. Pulverizer

A pulverizer is a device for grinding coal for combustion in a furnace in a fossil fuel power plant.

9. Boiler Steam Drum

Steam Drums are a regular feature of water tube boilers. It is reservoir of water/steam at the top end of the water tubes in the water-tube boiler. They store the steam generated in the water tubes and act as a phase separator for the steam/water mixture. The difference in densities between hot and cold water helps in the accumulation of the “hotter”-water/and saturated –steam into steam drum. Made from high-grade steel (probably stainless) and its working involves temperatures 390’C and pressure well above 350psi (2.4MPa). The separated steam

is drawn out from the top section of the drum. Saturated steam is drawn off the top of the drum. The steam will re-enter the furnace in through a super heater, while the saturated water at the bottom of steam drum flows down to the mud-drum /feed water drum by down comer tubes accessories include a safety valve, water level indicator and fuse plug. A steam drum is used in the company of a mud-drum/feed water drum which is located at a lower level. So that it acts as a sump for the sludge or sediments which have a tendency to the bottom.

10. Super Heater

A Super heater is a device in a steam engine that heats the steam generated by the boiler again increasing its thermal energy and decreasing the likelihood that it will condense inside the engine. Super heaters increase the efficiency of the steam engine, and were widely adopted. Steam which has been superheated is logically known as superheated steam; non-superheated steam is called saturated steam or wet steam; Super heaters were applied to steam locomotives in quantity from the early 20th century, to most steam vehicles, and so stationary steam engines including power stations.

11. EconomizersEconomizer, or in the UK economizer, are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, and heating, ventilating and air conditioning. In boilers, economizer are heat exchange devices that heat fluids , usually water, up to but not normally beyond the boiling point of the fluid. Economizers are so named because they can make use of the enthalpy and improving the boiler’s efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used the fill it (the feed water). Modern day boilers, such as those in cold fired power stations, are still fitted with economizer which is decedents of Green’s original design. In this context they are turbines before it is pumped to the boilers. A common application of economizer is steam power plants is to capture the waste hit from boiler stack gases (flue gas) and transfer thus it to the boiler feed water thus lowering the needed energy input , in turn reducing the firing rates to accomplish the rated boiler output . Economizer lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

12 Air Preheater

Air preheater is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler). The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the fuel gas. As a consequence, the flue gases are also sent to the flue gas

stack (or chimney) at a lower temperature allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack.

13. Precipitator

An Electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate 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, and can easily remove fine particulate matter such as dust and smoke from the air steam.ESP’s continue to be excellent devices for control of many industrial particulate emissions, including smoke from electricity-generating utilities (coal and oil fired), salt cake collection from black liquor boilers in pump mills, and catalyst collection from fluidized bed catalytic crackers from several hundred thousand ACFM in the largest coal-fired boiler application.

The original parallel plate-Weighted wire design (described above) has evolved as more efficient ( and robust) discharge electrode designs were developed, today focusing on rigid discharge electrodes to which many sharpened spikes are attached , maximizing corona production. Transformer –rectifier systems apply voltages of 50-100 Kilovolts at relatively high current densities. Modern controls minimize sparking and prevent arcing, avoiding damage to the components. Automatic rapping systems and hopper evacuation systems remove the collected particulate matter while on line allowing ESP’s to stay in operation for years at a time.

14. Fuel gas stack

A Fuel gas stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called fuel gases are exhausted to the outside air. Fuel gases are produced when coal, oil, natural gas, wood or any other large combustion device. Fuel gas is usually composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulates matter, carbon mono oxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a greater aria and thereby reduce the concentration of the pollutants to the levels required by governmental environmental policies and regulations.When the fuel gases exhausted from stoves, ovens, fireplaces or other small sources within residential abodes, restaurants , hotels or other stacks are referred to as chimneys.

15 Transformer:

A transformer is a device that transfers electrical energy from one circuit to another by magnetic coupling with out requiring relative motion between its parts. It usually comprises two or more coupled windings, and in most cases, a core to concentrate magnetic flux. An alternating voltage applied to one winding creates a time-varying magnetic flux in the core, which includes a voltage in the other windings. Varying the relative number of turns between primary and secondary windings determines the ratio of the input and output voltages, thus transforming the voltage by stepping it up or down between circuits. By transforming electrical power to a high-voltage,_low-current form and back again, the transformer greatly reduces energy losses and so enables the economic transmission of power over long distances. It has thus shape the electricity supply industry, permitting generation to be located remotely from point of demand. All but a fraction of the world’s electrical power has passed trough a series of transformer by the time it reaches the consumer.

Basic principles

The principles of the transformer are illustrated by consideration of a hypothetical ideal transformer consisting of two windings of zero resistance around a core of negligible reluctance. A voltage applied to the primary winding causes a current, which develops a magneto motive force (MMF) in the core. The current required to create the MMF is termed the magnetizing current; in the ideal transformer it is considered to be negligible, although its presence is still required to drive flux around the magnetic circuit of the core. An electromotive force (MMF) is induced across each winding, an effect known as mutual inductance. In accordance with faraday’s law of induction, the EMFs are proportional to the rate of change of flux. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the back EMF”. Energy losses An ideal transformer would have no energy losses and would have no energy losses, and would therefore be 100% efficient. Despite the transformer being amongst the most efficient of electrical machines with ex the most efficient of electrical machines with experimental models using superconducting windings achieving efficiency of 99.85%, energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 95%. A small transformer such as plug-in “power brick” used for low-power consumer electronics may be less than 85% efficient. Transformer losses are attributable to several causes and may be differentiated between those originated in the windings, some times termed copper loss, and those arising from the magnetic circuit, sometimes termed iron loss. The losses vary with load current, and may furthermore be expressed as “no load” or “full load” loss, or at an intermediate loading. Winding resistance dominates load losses contribute to

over 99% of the no-load loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, and lending impetus to development of low-loss transformers. Losses in the transformer arise from: Winding resistance Current flowing trough the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses. Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected. Eddy current Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn trough out its entire length. Eddy currents therefore circulate with in a core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness. Magnetostriction Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and in turn causes losses due to frictional heating in susceptible cores. Mechanical losses In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic field between primary and secondary windings. These incite vibration with in near by metal work, adding to the buzzing noise, and consuming a small amount of power. Stray losses Leakage inductance is by itself loss less, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive material such as the transformers support structure will give rise to eddy currents and be converted to heat. Cooling system Large power transformers may be equipped with cooling fans, oil pumps or water-cooler heat exchangers design to remove heat. Power used to operate the

Types of transformer 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)

Classification Transformers are adapted to numerous engineering applications and may be classified in many ways:

• By power level: (from fraction of a volt-ampere(VA) to over a thousand MVA),

• By application: (power supply, impedance matching, circuit isolation), • By frequency range: (power, audio, radio frequency(RF)) • By voltage class: (a few volts to about 765 kilovolts

• By cooling type: (air cooled, oil filled, fan cooled, water cooled (Natural/ Forced) etc.) • By purpose: (distribution, rectifier, arc furnace, amplifier output, etc.). • By ratio of the number of turns in the coils

(Step-up The secondary has more turns than the primary. Step-down The secondary has fewer turns than the • By Connection : Single phase, Star / star, Star delta etc• By Construction :

(Core Type Shell Type )

Transformer Insulation

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 or petroleum crude. This has good dielectric strength. also a good cooling medium and absorbs heat from the windings in transformerThus 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

Transformer Oil

NAPTHANIC BASE OILS GENERALLY HAVE HIGHER RESISTIVITY VALUES WHEN COMPARED TO PARAFFINIC BASE OILS AND HAVE BETTER OXIDATION STABILITY. • EQUALLY GOOD PARAMETERS CAN BE ACHIEVED WITH PARAFFINIC

BASE OILS ALSO, WHEN PROPERLY REFINED.• OIL PARAMETERS ARE IMPORTANT. BASE OF OIL IS NOT

IMPORTANT(NONE OF THE STANDARDS SPECIFY THE BASE OF OIL)• SUGGESTED IS TO HAVE OIL WITH LOW VISCOSITY AS COMPARED TO

PRESENTLY BEING USED FOR BETTER COOLING AND FOR BETTER OIL FLOW.

BHEL IS IN THE PROCESS OF FURTHER UPGRADING THE TRANSFORMER OIL PARAMETERS FOR HIGHER VOLTAGE CLASS TRANSFORMERS TO HAVE BETTER STABILITY OF OIL CHARACTERISTICS

Accessories & Auxiliaries Tap Changer(s)-(On load/Off load)

• Tank• Radiators• cooling fans, oil pumps, oil to water heat exchangers (Cooling ONAN / ONAF/ OFAF/

OFWF external coolers) • Bushings• Buchholz Relay/Oil Surge Relay• Temperature Indicators- WTI , OTI • Oil Level Indicators• Pressure Relief Device• Marshalling Box/Control cubicle• Oil Preservation Systems: Conservators (gas sealed, Bellows/membrane sealed)

BreathersThermo siphon Filters

Conservator:

Conservator With the variation of temperature there is corresponding variation in the oil volume. To account for this, an expansion vessel called conservator is added to the transformer with a connecting pipe to the main tank. In smaller transformers this vessel is open to atmosphere through dehydrating breathers (to keep the air dry). In larger transformers, an air bag is mounted inside the conservator with the inside of bag open to atmosphere through the breathers and the outside surface of the bag in contact with the oil surface.

SILICA GEL BREATHER

COAL HANDLING PLANT (C.H.P.)

Coal is delivered by highway truck, rail, barge or collier ship. Some plants are even built near coal mines and coal is delivered by conveyors.

A large coal train called a" unit train" may be a kilo metres (over a mile) long, containing 60 cars with 100tons of coal in each one, for a total load of 6,000 tons.

A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high.

A large thermal power plant such as the Badarpur Thermal Power Station, New Delhi stores several million tons of coal for use when there is no wagon supply.Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (50mm) in size. The coal is then transported from the storage yard to in-plant storage silos by rubberized conveyor belts at rates up to 4,000 tons/hour. In plants that burn pulverized coal, silos feed coal pulverisers (coal mill) that take the larger 2inch pieces grind them into the consistency of face powder, classify them, and mixes them with primary combustion air which transports the coal to the furnace and preheats the coal to drive off excess moisture content. In plants that do not burn pulverized coal, the larger 2 inch pieces may be directly fed into the silos which then feed the cyclone burners, a specific kind of combust or that can efficiently burn larger pieces of fuel

Run-of-Mine (ROM) Coal

The coal delivered from the mine that reports to the Coal Handling Plant is called Run-of-mine, or ROM, coal. This is the raw material for the CHP, and consists of coal, rocks, middlings, minerals and contamination. Contamination is usually Introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size.

Coal Handling

Coal needs to be stored at various stages of the preparation process, and conveyed around the CHP facilities. Coal handling is part of the larger field of bulk material handling, and is a complex and vital part of the CHP.

Stockpiles

Stockpiles provide surge capacity to various parts of the CHP. ROM coal is delivered with large variations in production rate of tonnes per hour (tph).A ROM stockpile is used to allow the wash plant to be fed coal at lower, constant rate.

Stacking

Travelling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles. Stackers are nominally rated in tph (tonnes per hour) for capacity and normally travel on a rail between stockpiles in the stockyard.A stacker can usually move in at least two directions typically: horizontally along the rail and vertically by luffing its boom. Luffing of the boom minimises dust by reducing the height that the coal needs to fall to the top of the stockpile. The boomis luffed upwards as the stockpile height grows.

Coal Sampling

Sampling of coal is an important part of the process control in the CHP.

A grab sample is a one off sample of the coal at a point in the process stream, and tends not to be very representative. A routine sample is taken at a set frequency, either over a period of time or per shipment.

Screening

Screens are used to group process particles into ranges by size. These size ranges are also called grades. Dewatering screens are used to remove water from the product. Screens can be static, or mechanically vibrated. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethylene

Magnetic Separation

Magnetic separators shall be used in coal conveying systems to separate tramp iron(including steel) from the coal. Basically, two types are available.

One type incorporates permanent or electromagnets into the head pulley of a belt conveyor. The tramp iron clings to the belt as it goes around the pulley drum and falls off into a collection hopper or trough after the point at which coal is charged from the belt .The other type consists of permanent or electromagnets incorporated into a belt conveyor that is suspended above a belt conveyor carrying coal. The tramp iron is pulled from the moving coal to the face of the separating conveyor, which in turn holds and carries the tramp iron to a collection hopper or trough. Magnetic separators shall be used just ahead of the coal crusher, if any, and/or just prior to coal discharge to the in-plant bunker or silo fill system.

Coal Crusher  

Before the coal is sent to the plant it has to be ensured that the coal is of uniform size, and so it is passed through coal crushers.

Also power plants using pulverized coal specify a maximum coal size that can be fed into the pulverizer and so thecoal has to be crushed to the specified size using the coal crusher. Rotary crushers are very commonly used for this purpose as they can provide a continuous flow of coal to the pulverizer.

Pulverizer

Most commonly used pulverizer is the Boul Mill. The arrangement consists of 2stationary rollers and a power driven boul in which pulverization takes place as the coal passes through the sides of the rollers and the boul

A primary air induced draught fan draws a stream of heated air through the mill carrying the pulverizedcoal into a stationary classifier at the top of the pulverizer. The classifier separates the pulverized coal from the un pulverised coal

SWITCHYARD:

The current transformer:

These transformers are used with low range ammeters to measure currents in high voltage alternating current circuits. In addition to insulating the instrument from the high voltage line, they step-down the current in a known ratio. The current or series transformer has a primary coil of one or more turns of thick wire connected in series with the line whose current is to be measured. The secondary consists of large number of turns of line wire and connected across the ammeter terminals.   It should be noted that, since the ammeter resistance is very low, the current transformer normally works short-circuited. If for any reason the ammeter is taken out of the secondary winding, then this winding must be short-circuited. If this is not done, then due to the absence of counter amp-turns of the secondary, the unopposed primary m.m.f. will set up an abnormally high flux in the core, which will produce excessive core loss with subsequent heating and a high voltage across the secondary terminals. Hence the secondary of a current transformer should never be left open under any circumstances.

The voltage transformer:

The voltage transformer performs its task, similar to a current transformer. But unlike the former, it is used to step down the voltage. The voltage transformer is also used for the calibration purpose, and the same is also employed for the protection of various devices. These transformers are extremely accurate-ratio step-down transformers and are used in conjunction with standard low-range voltmeters whose deflection when divided by transformation ratio, gives the true voltage on the high-voltage side. In general, they are of the shell-type and do not differ much from the ordinary two-winding transformers. For safety, the secondary should be completely insulated from the high-voltage primary and should be, in addition, grounded for affording protection to the operator. 

Capacitor voltage transformers:

Voltage transformers can work effectively and properly up to a voltage of 33 kV. But above that voltage, insulation protection becomes a major factor of consideration, and therefore we apply a capacitor, as shown below. The arrangement thus illustrated is referred to as a capacitor voltage transformer (CVT). The same circuitry basically helps to step down the secondary voltage to a great extent, hence keeping the insulation level to a considerable safe level.

Lightning arresters:

Electrical equipments, which are situated in the switchyard, are open to the atmosphere, and hence may face situations of high voltages (also referred to as surge voltages), or lightning. This leads to a major problem, because the magnitude and intensity of the same is about 4-5 times the normal voltage, and hence is detrimental for the electrical circuitry situated there. Therefore, in order to prevent that, we have electrical equipments, referred to as lightning arresters, which are situated in the switchyard. Whenever such a dangerous situation arises, the same current is forced to flow through the lightning arresters and hence it goes to the ground. Hence, the lightning arresters play an important role in the protection of the electrical equipments, actually situated in the switchyard. They actually provide the least resistance path during a situation of enormous voltage flow.

Isolators:

The isolators serve a similar function as the circuit breaker. The only difference is that a circuit breaker is used to interrupt the fault current when the circuit is on load, or line-line. But the isolator is used to isolate the circuit, when the circuit is off load, or dead-line. The isolators are classified as:

1) Bus isolator(or pantograph isolator)2) Sequential isolator

Refrence www.wikipedia.com www.ntpc.co.in www.google.com Electrical machines by P.S. Bimbhra