SINGRAULI SUPER THERMAL POWER PLANT SHAKTINAGAR

INDUSTRIAL TRAINING REPORT

   SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE

BACHELOR OF TECHNOLOGY  (Electrical Engineering)

 ATAZAD INST. OF ENGG. AND TECHNOLOGY,

LUCKNOW

 

SUBMITTED BY :

NAME : Deepak kr Singh ROLL NO. : 1005320022

TRAINING INCHARGE : Mr. CH SATYNARAYAN DGM (ELECTRICAL)

NTPC , SHAKTINAGAR

SESSION 2013-2014Department of Electrical Engineering

(Affiliated by GBTU)

DECLARATION

I, Mr. Deepak kr Singh ,hereby declare that this industrial training report is the record of authentic work carried out by me during the period from 10 june 2013 to 10 july 2013 in NTPC SHAKTINAGAR under the super vision of my training incharge Mr. CH Satynarayan (DGM , ELECTRICAL ,NTPC SHAKTINAGAR).

Signature

Name of the student : Deepak Kr SINGH

CERTIFICATE This is to certify that Mr.Deepak Kr Singh of

Azad Inst. OF Engg. And Technology has

successfully completed the training work

in partial fulfillment of requirement for the

completion of B.Tech course as prescribed by the AZAD INST.

OF ENGG. AND TECHNOLOGY. This training report is the record of

authentic work carried out by him during the period from 10 june 2013

to 10 july 2013

He has worked under my guidance.

Signature

Training incharge (Internal)

Counter signed by

 Acknowledgement  

I would like to express my deepest appreciation to all those who provided me the possibility to complete my industrial training. A special gratitude I give to our Training incharge , Mr. CH Satynarayan(DGM,Electrical,NTPC), whose contribution in stimulating suggestions and encouragement,  helped me to coordinate in my training period.

Furthermore I would also like to acknowledge with much appreciation the crucial role of the employee of Other sections who gave the permission to use all required  equipment and the necessary materials to complete the task . A special thanks goes to my team mate, who  help me to assemble the parts and gave suggestion about the task . . I have to appreciate the guidance given by other supervisor as well as the panels especially in our training period that has improved our presentation skills and knowledge.

A special thanks to Mr. H.K. Verma ( DGM, C & I ) For his guidance and care in NTPC.

Last but not least, many thanks to NTPC , who give me opportunity to complete my industrial training in such wonderful working environment,in achieving my goal.

Deepak Kr Singh

CONTENT

ABOUT NTPC INTRODUCTION TO THERMAL POWER PLANT BOILER THEORY

TURBINE THEORY GENERATOR THEORY GENERATOR TRANSFORMER SWITCHGEAR SWITCHYARD AND ITS ELMENTS FUTURE CAPACITY ADDITION IN NTPC , AWARDS , AND RANKING REFERENCES

ABOUT NTPC SHAKTINAGAR

SINGRAULI SUPER THERMAL POWER PLANT

Singrauli Super Thermal Power Plant is located at Shaktinagar in Sonebhadra district in

Indian state of Uttar Pradesh. The power plant is the first power plant of NTPC. It sources coal

from Jayant and Bina mines and water from Rihand Reservoir. The states benefitting from this

power plant are Uttar Pradesh, Uttranchal, Rajasthan, Punjab, Haryana, Jammu &

Kashmir andHimachal Pradesh and the Union Territories of Delhi and Chandigarh. An

investment worth  1,190.69 crore (US$200 million) has already been cleared. It even gets

international assistance from IDA.

The unit wise capacity and other details are as follows.

NTPC ROLE IN DIFFERENT AREAS AND FUTURE SCOPE

Power Generation 

StageUnit

NumberInstalled Capacity

(MW)Date of Commissioning

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

Presently, NTPC generates power from Coal and Gas. With an installed capacity of 41,184 MW, NTPC is the largest power generating major in the country. It has also diversified into hydro power, coal mining, power equipment manufacturing, oil & gas exploration, power trading & distribution. With an increasing presence in the power value

Installed Capacity

Present installed capacity of NTPC is 41,184 MW (including 5,364 MW through JVs) comprising of 23 NTPC Stations (16 Coal based stations, 7 combined cycle gas/liquid fuel based stations), 7 Joint Venture stations (6 coal based and one gas based) and 2 renewable energy projects. 

NO. OF PLANTS CAPACITY (MW)

NTPC OwnedCoal 16 31,855Gas/Liquid Fuel 7 3,955Renewable energy projects - 10Total 23 35,820Owned By JVsCoal & Gas 7 5,364Total 30 41,184

Regional Spread of Generating Facilities

REGION COAL GAS Renewable TOTAL

Northern 8,515 2,312 5 10,832Western 10,840 1,293 - 12,133Southern 4,600 350 5 4,955Eastern 7,900 - - 7,900JVs 3,424 1,940 - 4,864Total 35,279 5,895 10 41,184

 

Operations

In terms of operations, NTPC has always been considerably above the national average.

 

The table below shows the detailed operational performance of coal based stations over the years.

OPERATIONAL PERFORMANCE OF COAL BASED NTPC STATIONS

Generation(BU) PLF(%) Availability Factor(%)

2011-12 222.07 85.00 89.73

2010-11 220.54 88.29 91.62

2009-10 218.84 90.81 91.76

2008-09 206.94 91.14 92.47

2007-08 200.86 92.24 92.12

2006-07 188.67 89.43 90.09

2005-06 170.88 87.52 89.91

2004-05 159.11 87.51 91.20

2003-04 149.16 84.40 88.79

2002-03 140.86 83.57 88.70

2001-02 133.20 81.11 89.09

2000-01 130.10 81.80 88.54

1999-00 118.70 80.39 90.06

1998-99 109.50 76.60 89.36

Renewable Energy and Distributed Generation

 Renewable EnergyRenewable energy (RE) is being perceived as an alternative source of energy for “Energy Security” and subsequently “Energy Independence” by 2020. Renewable energy technologies provide not only electricity but offer an environmentally clean and low noise source of power.

Objectives

NTPC plans to broad base generation mix by evaluating conventional and alternate sources of energy to ensure long run competitiveness and mitigate fuel risks.

Vision“To provide green power through locally available resources at affordable price, promoting clean energy”

Portfolio of Renewable PowerNTPC has also formulated its business plan of capacity addition of about 1,000 MW thru renewable resources by 2017.In this endeavour, NTPC has already commissioned 10 MW Solar PV Projects and another 30 MW Solar PV and 8 MW Small Hydro Projects are under implementation. Further, 70 MW Solar Projects are under tendering.

Renewable Energy ProjectsSolar Energy:

Projects Commissioned (10 MW)o 5 MW Solar PV based project at NTPC-Dadri in Uttar Pradesh.o 5 MW Solar PV based project at Portblair in Andaman & Nicobar

Island. Projects under Implementation (35 MW)

o 10 MW (Phase-1) Solar PV based project at NTPC-Ramagundam in Andhra Pradesh.

o 10 MW Solar PV based project at NTPC-Unchahar in Uttar Pradesh.

o 10 MW Solar PV based project at NTPC-Talcher Kaniha in Orissa.o 5 MW Solar PV based project at NTPC-Faridabad in Haryana.

Projects under Tendering (65 MW)o 15 MW Solar PV based project at NTPC-Singrauli in Uttar Pradesh.

o 50 MW Solar PV based project at Rajgarh in Madhya Pradesh.

Wind Energy : Projects under Consideration (80 MW)

o 40 MW Wind energy projects in Karnataka.o 40 MW Wind energy projects in Maharashtra.

Hydro Energy: Projects under Implementation (8 MW)

o 8 MW hydro energy based project at NTPC-Singrauli in Uttar Pradesh.

Projects under Consideration (3 MW)o 3 MW hydro energy based project at NTPC-Rihand in Uttar

Pradesh.

Geothermal Energy: Tattapani Geothermal Project in Chhattisgarh: MoU Signed with Govt.

of Chhattisgarh.

Technology Development: Two Stage Gasifier: This gasifier is being developed in association with

SDC, TERI and Denmark Technical University (DTU).Prototype model testing completed at Gual Pahari, Gurgaon.

DPR under preparation for integration of Solar Energy with existing thermal projects of NTPC.

Distributed GenerationIndia’s ambitious growth plans require inclusion of all sectors, especially the rural sector where two third of our population lives. Such economic development cannot be achieved without availability of energy and subsequently efficient energy management which is crucial for rural development. As per census 2001, about 44% of the rural households do not have access to electricity. Some of the villages are located in remote & inaccessible areas where it would be either impossible or extremely expensive to extend the power transmission network. Total 16 DG projects commissioned by NTPC so far with total capacity of 340 kW and 2233 households electrified.

Awards: IEEMA Power award-2009 in the category of “Excellence in Distributed

Generation”. NTPC Distributed Generation film “Energizing villages” has been

awarded in category “development venture” by Public Society of India, Hyderabad

Environment 

While leading the nation’s power generation league, NTPC has remained committed to the environment. It continues to take various pro-active

measures for protection of the environment and ecology around its projects. NTPC was the first among power utilities in India to startEnvironment

Impact Assessment (EIA) studies and reinforced it with Periodic Environmental Audits and

Enviroment Policy & Management

Environment Policy & Environment Management System

For NTPC, the journey extends much beyond generating power. Right from its inception, the company had a well defined environment policy. More than just generating power, it is committed to sustainable growth of power.NTPC has evolved sound environment practices.

National Environment Policy

The Ministry of Environment and Forests and the Ministry of Power and NTPC were involved in preparing the draft Environment Policy (NEP) which was later approved by the Union Cabinet in May 2006.

NTPC Environment Policy

Since its inception NTPC has been at the forefront of Environment management. In November 1995, NTPC brought out a comprehensive document entitled ‘NTPC Environment Policy and Environment Management System. Amongst the guiding principles adopted in the document are the company's pro-active approach to environment, optimum utilisation of equipment, adoption of latest technologies and continual environment improvement. The policy also envisages efficient utilisation of resources, thereby minimising waste, maximising ash utilisation and ensuring a green belt all around the plant for maintaining ecological balance.

Environment Management, Occupational Health and Safety Systems

NTPC has actively gone for adoption of the best international practices on environment, occupational health and safety areas. The organisation has pursued the Environmental Management System (EMS) ISO 14001 and the Occupational Health and Safety Assessment System OHSAS 18001 at its different establishments. As a result of pursuing these practices, all NTPC power stations have been certified for ISO 14001 & OHSAS 18001 by reputed national and international certifying agencies.

Pollution Control Systems

While deciding the appropriate technology for its projects, NTPC integrates many environmental provisions into the plant design. In order to ensure that NTPC complies with all the stipulated environment norms, following state-of-the-art pollution control systems / devices have been installed to control air and water pollution:

Electrostatic Precipitators

Flue Gas Stacks

Low-NOX Burners

Neutralisation Pits

Coal Settling Pits / Oil Settling Pits

DE & DS Systems Cooling Tower

Ash Dykes & Ash Disposal Systems

Ash Water Recycling System

Dry Ash Extraction System (DAES)

Liquid Waste Treatment Plants & Management System

Sewage Treatment Plants & Facilities

Environmental Institutional Set-up

Following are the additional measures taken by NTPC in the area of Environment

Management:

Environment Management During Operation Phase

Monitoring of Environmental Parameters

On-Line Data Base Management

Environment Review

Upgradation & Retrofitting of Pollution Control Systems

Resources Conservation

Waste Management

Municipal Waste Management

Hazardous Waste Management

Bio-Medical Waste Management

Land Use / Bio-diversity

Reclamation of Abandoned Ash Green Belts, Afforestation & Energy Plantations

Introduction

A power station (also referred to as a generating station,power plant, or powerhouse) is an industrial facility for the generation ofelectric power.

Almost all coal, nuclear,geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants are thermal. Natural gas is frequentlycombusted in gas turbines as well asboilers. The waste heat from a gas turbine can be used to raise steam, in a combined cycle plant that improves overall efficiency. Power plants burning coal, oil, or natural gas are often referred to

collectively as fossil-fuel power plants. Some biomass-fueled thermal power plants have appeared also. Non-nuclear thermal power plants, particularly fossil-fueled plants, which do not usecogeneration, are sometimes referred to as conventional power plants.

A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins asteam turbine which either drives anelectrical generator or does some other work, like ship propulsion. After it passes through the turbine, the steam is condensed in acondenser and recycled to where it was heated; this is known as aRankine cycle. The greatest variation in the design of thermal power stations is due to the different fuel sources. Some prefer to use the termenergy center because such facilities convert forms of heat energy into electrical energy.

In thermal power stations, mechanical power is produced by aheat engine that transforms thermal energy, often from combustion of afuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. Not all thermal energy can be transformed into mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as acogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses by-product heat for the desalination of water.

1.2 Classification of Thermal Power PlantsThermal power plants are classified by the type of fuel and the type of prime mover

installed.

 

1.2.1 By Fuel

Nuclear power plants use anuclear reactor's heat to operate a steam turbine generator. Fossil fuelled power plants may also use a steam turbine generator or in the case

of natural gas fired plants may use a combustion turbine. Geothermal power plants use steam extracted from hot underground rocks. Renewable energy plants may be fuelled by waste from sugar cane, municipal solid

waste, landfill methane, or other forms of biomass. In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-

density, fuel. Waste heat from industrial processes is occasionally concentrated enough to use for

power generation, usually in a steam boiler and turbine. Solar thermal electric plants use sunlight to boil water, which turns the generator.

1.2.2 By Prime Mover

Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.

Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated. This type was pioneered by the UK, Prince town being the world's first, commissioned in 1959.

Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new base load power plants are combined cycle plants fired by natural gas.

Internal combustionReciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.

Micro turbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.

1.3 Efficiencytemperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam. Historically, other working fluids such asmercury have been experimentally used in a mercury vapour turbine power plant, since these can attain higher temperatures than water at lower working pressures. However, the obvious hazards of toxicity, and poor heat transfer properties, have ruled out mercury as a working fluid.

 

THERMAL POWER PLANT

We are well aware that electricity is a form of energy. There are number of methods by which electricity can be produced, but most common method of production of electrical energy is to rotate a conductor in a magnetic field continuously cutting of magnetic lines will cause E.M.F. to be generated at the ends of conductor. If these terminals are connected through load then electricity will start flowing through that conductor.

        Now let us see what we are doing in Thermal Power Station for the purpose of production of Electricity. Actually speaking we are doing conversion of energies from form to another form, and our ultimate aim is to get Electrical energy.

 For this purpose the rotation movement is required to rotate the magnetic field so that it may cut the stationery conductors of the machine. To be more precise this rotational or mechanical energy is derived from a machine to which we call Turbine which is actually capable enough to convert heat energy to rotational energy.

 For obtaining heat energy we have to make use of the chemical energy, to which we call fossil fuel i.e. coal, oil, gas etc. This is achieved in a plant to which we call furnace or sometimes Boiler.

For transportation of heat energy from furnace to turbine inlet, we require a medium and we have chosen water as media. This water is converted into steam in furnace. Quality of steam is always monitored properly process of Electrical generation.

So we see that the rotational movement required to rotate the magnetic field of the electric generator is produced by the steam turbine. The power to the steam turbine is given by steam generator in the form of high pressure and high temperature steam.

The steam after doing work on the turbine shaft is condensed and condensate is pumped back into Boiler as high pressure and low temperature water, by means of Boiler feed pump. So if we represent whole process in a block diagram this will look like as given below.    

        

   

2.1 How Electricity is generatedThe complete and complex process of electricity generation in TPS can be divided into

four major cycles for the sake of simplicity. The main systems are discussed in these cycles in a step by step manner and some useful drawings are also enclosed. The four cycles are

1. Coal Cycle2. Oil Cycle3. Air and Flue Gas Cycle

4. Steam Water Cycle

2.1.1 Coal CycleThe simplest of the above four cycles is the coal cycle. In this cycle as explained earlier

crushed coal of about 20mm is transported by conveyor belts to the coal mill bunkers. From here the coal goes to coal mills through raw coal feeders. In the coal mills the coal is further pulverized (crushed) to powder form. The temperature of the coal mills are maintained at 180-200 degree centigrade by a suitable mixture of hot & cold air.

The air comes from Primary Air fans (P.A FANS) which are 2 in Nos. - A&B. The outlet duct after combining gets divided into two. One duct goes to the Air Heaters (A.H- A&B) where primary air is heated by the hot flue gases in a Heat Exchanger. This duct provides hot air & the other one provides cold primary air. A suitable mixture of this hot & cold air is fed to the coal mills to maintain their temperature. This is done to remove moisture of coal. More over this primary air is also used for transportation of powdered coal from coal mills to the four corners of the boiler by a set of four pipes. There are six coal mills – A, B, C, D, E&F   and their   outlets in the Boiler are   at different elevations. The high

Temperature of the primary air does not allow the air coal mixture to choke the duct from mill to boilers. A portion of the primary air is further pumped to high pressure and is known as seal air. It is used to protect certain parts of mills like bearings etc. where powered coal may pose certain problems in the functioning of the mill. When the air coal mixture enters the boiler it catches fire in the firing zone and some ash along with clinkers settles down. This is removed periodically by mixing it with water to make slurry.

2.1.2 Oil CycleIn the oil cycle the oil is pumped and enters the boiler from four corners at three

elevations. Oil guns are used which sprays the oil in atomized form along with steam so that it catches fire instantly. At each elevation and each corner there are separate igniters which ignite the fuel oil. There are flame sensors which sense the flame and send the information to the control roam.

2.1.3 Air & Flue Gas Cycle                 For the proper combustion to take place in the boiler right amount of Oxygen or air is needed in the boiler. The air is provided to the furnace in two ways - Primary Air & Secondary Air. Primary air is provided by P.A. fans and enters the boiler along with powdered coal from the mills. While the secondary air is pumped through Forced Draft fans better known as F.D Fans which are also two in numbers A&B. The outlet of F.D fans combine and are again divided into two which goes to Steam coiled Air pre heaters (S.C.A.P.H) A&B where its temperature is raised by utilizing the heat of waste steam. Then it goes to Air Pre heater-A&B where secondary air is heated further utilizing the heat of flue gases. The temperature of air is raised to improve the efficiency of the unit & for proper combustion in the furnace. Then this air is fed to the furnace.

From the combustion chamber the fuel gases travel to the upper portion of the boiler and give a portion of heat to the Platen Super Heater. Further up it comes in contact with the Reheater and heats the steam which is inside the tubes of reheater. Then it travels horizontally and comes in contact with Final Super Heater. After imparting the heat to the steam in super heater flue gases go downward to the Economizer to heat the cold water pumped by the Boiler Feed Pumps (B.F.P.) these all are enclosed in the furnace. After leaving the furnace the fuel gases go to the Air Heaters where more heat of the flue gases is extracted to heat primary and secondary air. Then it goes to the Electrostatic Precipitators (E.S.P.) Stage A&B where the suspended ash from the flue gases is removed by passing the fuel gas between charged plates. Then comes the induced draft fan (I.D Fan) which sucks air from E.S.P. and releases it to the atmosphere through chimney. The pressure inside the boiler is kept suitably below the atmospheric pressure with the help of 1.0. Fans so that the flame does not spread out of the openings of boiler and cause explosion. Further very low pressure in the boiler is also not desirable because it will lead to the quenching of flame.

2.1.4 Steam Water CycleThe most complex of all the cycles is the steam & water cycle. Steam is the working

substance in the turbines in all the thermal and nuclear power plants. As there is very high temperature and pressure inside the boiler, initially water has to be pumped to a very high pressure. Water has also to be heated to a suitably high temperature before putting it inside the boiler so that cold water does not cause any problem. Initially cold water is slightly heated in low pressure heaters. Then it is pumped to a very high pressure of about 200 Kg/Cm2 by boiler feed pumps A & B. After this it is further heated in high pressure heaters by taking the heat from the high pressure steam coming from various auxiliaries and / or turbines. Then this water goes to the economizer where its temperature is further raised by the flue gases.        This hot water then goes to the boiler drum. In the boiler drum there is very high temperature and pressure. It contains a saturated mixture of boiling water and steam which are in equilibrium. The water level in the boiler         is maintained between certain limit. From here relatively cold water goes down to the water header situated at the bottom, due to difference in density. Then this cold water rises gradually in the tubes of the boiler on being heated. The tubes are in the form of water walls. These tubes combine at the top in the hot water header. From here the hot water and steam mixture comes back to the boiler drum completing the small loop.

        From the boiler drum hot steam goes to platen super heater situated in the upper portion of the boiler. Here the temperature of the steam is increased.  Then it goes to final super heater. Here its temperature is further increased.

The turbine is a three cylinder machine with high pressure (H.P), intermediate pressure (I.P) & low pressure (L.P) casings taking efficiency into account the .The turbine speed is controlled by hydro dynamic governing system. The three turbines are on the same shaft which is coupled with generator. The generator is equipped with D.C excitation system. The steam from the final super heater comes by main steam line to the H.P turbine. After doing work in the H.P turbine its temperature is reduced. It is sent back to the boiler by cold reheat line to the reheater. Here its temperature is increased and is sent to the I.P turbine through hot reheat line. After doing work in the I.P turbine steam directly enters L.P turbine.

The pressure of L.P turbine is maintained very low in order to reduce the condensation point of steam. The outlet of L.P turbine is connected with condenser. In the condenser, arrangement is made to cool the steam to water. This is done by using cold water which is made to flow in tubes. This secondary water which is not very pure gains heat from steam & becomes hot. This secondary water is sent to the cooling towers to cool it down so that it may be reused for cooling. The water thus formed in the condenser is sucked by condensate water pumps (C.W. PUMPS) and is sent to deaerator. A suitable water level is maintained in the hot well of condenser.

Water or steam leakages from the system are compensated by the make up water, line from storage tanks which are connected to the condenser. The pressure in side condenser is automatically maintained less then atmospheric pressure and large volume of steam condense here to form small volume of water. In the deaerator the water is sprayed to small droplets & the air dissolved in it is removed so that it may not cause trouble at high temperatures in the Boiler. Moreover, the water level which is maintained constant in the deaerator also acts as a constant water head for the boiler feed pumps. Water from deaerator goes to the Boiler feed pumps after the heated by L.P. Heaters. Thus the water cycle in the boiler is completed and water is ready for another new cycle. This is a continuous and repetitive process.

2.2 Elements of Thermal Power Station

D.M. Plant

For the generation of steam De-mineralize water prepared removing minerals & impurities to remove the minerals several chemicals are used.

Deaerator

Deaerator is placed at the height of 26 m to provide the appropriate suction pressure for boiler feed pump. The main function of deaerator is:-

1. To remove the air bubbles from the water entered into boiler feed pump.2. To provide the suction head to the boiler feed pump.

Boiler feed pump

Boiler feed pump pumps the water coming from deaerator to the H.P. heater.  Boiler feed pump consists of a motor coupled with the pump through hydraulic coupling. On passing through the boiler feed pump the pressure of the water becomes about ten times of the suction pressure

Economizer

It consists of a large number of closely spaced parallel tubes of thin walls and smaller diameter. The feed water is passed through the economizer before supplying it to boiler. The heat of flue gases which would be lost is used to raise the temperature of the feed water due to which the efficiency of the boiler increases.

Air Pre-Heater

In the second path of flue gases, just below the economizer Air pre-heater is placed.  It  raise  the temperature of   the   atmospheric  air, coming   from   the  PA  and   FD fans  ,  for   the  dryness  of   the coal ,  which  confirms  the  proper   combustion  of  coal  used. To  raise   the  temperature of  the  air heat of  flue gases  is used , hence the efficiency of the  plant is  increased.

Boiler

 Boiler is used for the generation of steam from the feed water. After passing through economizer feed water enters into the boiler drum. From drum, with the help of down commers it enters into the water walls where the heat coming from the furnace converts water into the steam. 

            

Super heater

A number of super heaters are used to make a super- heat steam coming from the boiler drum. There are ten super heaters, one de-super heat one   Platon and a final super heater to convert the wet steam into the super heated steam. Heat of flue gases is used to dry the wet steam.

Turbine

Turbine converts the heat energy of the steam into mechanical energy. The  super heated  steam

works  on   the  blades of  the turbine  and  hence the blades starts  rotating  to produce the  mechanical

energy . The   mechanical energy then converted into the electrical energy with the help of generator. A series of three turbines is used to convert the heat energy into mechanical energy.

         1) High pressure turbine         2) Intermediate Pressure turbine         3) Low pressure turbine

Condensor The function of condenser is to create suction at very low pressure to the exhaust of

turbine thereby it permits the expansion of steam in primary to a very low pressure. The exhaust steam is condensed in the condenser and then again fed into the boiler.

Typical diagram of a coal-fired thermal power station

Elements of a coal fired thermal power plant1. Cooling tower 10. SteamControl valve 19.Superheater

2. Cooling water pump11. High pressuresteam turbine

20. Forced draught (draft)fan

3. transmission line (3-phase) 12. Deaerator 21. Reheater4. Step-uptransformer (3-phase)

13. Feedwater heater 22.Combustion air intake

5. Electrical generator (3-phase)

14.Coal conveyor 23.Economiser

6. Low pressure steam turbine 15.Coal hopper 24. Air preheater7. Condensate pump 16. Coal pulverizer 25.Precipitator

8. Surface condenser 17. Boiler steam drum26. Induced draught (draft)fan

9. Intermediate pressure steam turbine

18. Bottom ash hopper 27. Flue gas stack

PROCESS: COAL TO ELECTRICITY

We will see how the whole process of generation of electricity from the initial stage i.e. when coal burns. For burning the coal we require three T’s as shown in diagram

below. Unless until these three T’s are well in proportion fire or combustion of source of

chemical energy cannot take place. For providing a suitable atmosphere for combustion we take help of well designed furnace for given fuel in which after combustion of fuel heat is released. And this heat energy is transported through a medium i.e. steam.

The essential components of the plant are:

1. Boiler2. Steam turbine couples with electric generator3. The condenser4. The pump to send back condensed water to boiler

Now let us have close look of the working of each equipments of thermal power plant.

1. Feed water enters the boiler at the high pressure and low temperature and it is converted into high pressure and high temperature. Steam in the boiler. The heat required to convert feed water to steam is obtained from the heat released from the combustion of fuels burned in the furnace.

2. High pressure and high temperature steam from the boiler passes through the turbine blades and expands from boiler pressure, to the condenser pressure. The work performed in this process is transmitted through the shaft to the shaft of the electric generator, where the mechanical energy is converted to electrical energy.

3. The low pressure and low temperature exhaust steam from turbine is condensed into water in a condenser. The heat removal for condensation is done by cooling water through circulating water pumps.

4. The condensate from the condenser is pumped, by the boiler feed pump (B.F.P) as high pressure and low temperature water which is feed to boiler.

           And this cycle goes on.  The following medium for thermal power plant cycle is steam and before we go into the

details of the steam power cycle, we should know about steam.The use of steam can be traced back as far 56 AD when it provided the mysterious-

motive-power of Greek temple after the sacred fires had been lit. It may have been used even earlier for the same purpose by Egyptians but it was not until 1712 that any development of an industrial nature took place.

In those pioneer days of boiler development the life of an operator was not without dangers because explosions were frequent.

This led to the development of steam generators and also the establishment of the excellent codes of safety which we know today.

We used coals as fuel for the generation of heat energy. As the water in the Boiler evaporated due to the intense heat, it becomes high-pressurized steams.  

And the steams are passing through a conduit (there is a turbine at the other end of the tunnel), it forces its way through the Turbine, thus rotating the Turbine. (As the steams are high-pressurized, the Turbine will rotate very fast.) 

The Turbine is connected to a Generator via a coupler. As the Turbine is rotating (from the force of the steams), electrical energy is being produced. 

After the steams have passed through the turbine, it enters a Condenser. The Condenser has got a cooling agent (namely seawater) and the steam will go through the cooling agent via a pipe. The steam thus changes back to its liquid form and returns to the Boiler. 

And the whole process repeats.            

   

Diagram of the Basic Operation of a Thermal Power Station         

BOILER THEORY

Boiler systems are classified in a variety of ways. They can be classified according to the end use, such as foe heating, power generation or process requirements. Or they can be classified according to pressure, materials of construction, size tube contents (for example, waterside or fireside), firing, heat source or circulation. Boilers are also distinguished by their method of fabrication. Accordingly, a boiler can be pack aged or field erected. Sometimes boilers are classified by their heat source. For example, they are often referred to as oil-fired, gas-fired, coal-fired, or solid fuel –fired boilers.

Types of boilers: Fire tube boilers :

Fire tube boilers consist of a series of straight tubes that are housed inside a water-filled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As the hot gases flow through the tubes, they heat the water surrounding the tubes. The water is confined by the outer shell of boiler. To avoid the need for a thick outer shell fire tube boilers are used for lower pressure applications. Generally, the heat input capacities for fire tube boilers are limited to 50 mbtu per hour or less, but in recent years the size of firetube boilers has increased.

Most modern fire tube boilers have cylindrical outer shells with a small round combustion chamber located inside the bottom of the shell. Depending on the construction details, these boilers have tubes configured in either one, two, three, or four pass arrangements. Because the design of fire tube boilers is simple, they are easy to construct in a shop and can be shipped fully assembled as a package unit.

These boilers contain long steel tubes through which the hot gases from the furnace pass and around which the hot gases from the furnace pass and around which the water circulates. Fire tube boilers typically have a lower initial cost, are more fuel efficient and are easier to operate, but they are limited generally to capacities of 25 tonnes per hour and pressures of 17.5 kg per cm2.

Water tube boilers: 

               Water tube boilers are designed to circulate hot combustion gases around the outside of a large number of water filled tubes. The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums. In the older designs, the tubes were either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, water tube boilers can be fabricated in larger sizes and used for higher-pressure applications.Small water tube boilers, which have one and sometimes two burners, are generally fabricated and supplied as packaged units. Because of their size and weight, large water tube boilers are often fabricated in pieces and assembled in the field.                        In water tube or “water in tube” boilers, the conditions are reversed with the water passing through the tubes and the hot gases passing outside the tubes. These boilers can be of a single- or multiple-drum type. They can be built to any steam capacity and pressures, and have higher efficiencies than fire tube boilers.

Almost any solid, liquid or gaseous fuel can be burnt in a water tube boiler. The common fuels are coal, oil, natural gas, biomass and solid fuels such as municipal solid waste (MSW), tire-derived fuel (TDF) and RDF. Designs of water tube boilers that burn these fuels can be significantly different.

Coal-fired water tube boilers are classified into three major categories: stoker fired units, PC fired units and FBC boilers.

Package water tube boilers come in three basic designs: A, D and O type. The names are derived from the general shapes of the tube and drum arrangements. All have steam drums for the separation of the steam from the water, and one or more mud drums for the removal of sludge. Fuel oil-fired and natural gas-fired water tube package boilers are subdivided into three classes based on the geometry of the tubes.           

The “A” design has two small lower drums and a larger upper drum for steam-water separation. In the “D” design, which is the most common, the unit has two drums and a large-volume

combustion chamber. The orientation of the tubes in a “D” boiler creates either a left or right-handed configuration. For the “O” design, the boiler tube configuration exposes the least amount of tube surface to radiant heat. Rental units are often “O” boilers because their symmetry is a benefit in transportation

“D” Type boilers“This design has the most flexible design. They have a single

steam drum and a single mud drum, vertically aligned. The boiler tubes extend to one side of each drum. “D” type boilers generally have more tube surface exposed to the radiant heat than do other designs. “Package boilers” as opposed to “field-erected” units generally have significantly shorter fireboxes and frequently have very high heat transfer rates (250,000 btu per hour per sq foot). For this reason it is important to ensure high-quality boiler feedwater and to chemically treat the systems properly. Maintenance of burners and diffuser plates to minimize the potential for flame impingement is critical. 

   “A” type boilers:This design is more susceptible to tube starvation if bottom

blows are not performed properly because “A” type boilers have two mud drums symmetrically below the steam drum. Drums are each smaller than the single mud drums of the “D” or “O” type boilers. Bottom blows should not be undertaken at more than 80 per cent of the rated steam load in these boilers. Bottom blow refers to the required regular blow down from the boiler mud drums to remove sludge and suspended solids.

TURBINE THEORY

PRINCIPLE OF OPERATION AND DESIGN:-An ideal steam turbine is considered to be an isentropic process, or

constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly isentropic, however, with typical isentropic efficiencies ranging from 20–90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or buckets as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration

of sets varying to efficiently exploit the expansion of steam at each stage.

Impulse turbines:An impulse turbine has fixed nozzles that orient the steam flow into

high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss.       

  Reaction turbines:                 In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

 Operation and maintenance : When warming up a steam turbine for use, the main steam stop valves

(after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also, a turning gear is engaged when there is no steam

to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10–15 RPM (0.17–0.25 Hz) to slowly warm the turbine.

Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade breaking away from the rotor at high velocity and being ejected directly through the casing. To minimize risk it is essential that the turbine be very well balanced and turned with dry steam - that is, superheated steam with a minimal liquid water content. If water gets into the steam and is blasted onto the blades (moisture carry over), rapid impingement and erosion of the blades can occur leading to imbalance and catastrophic failure. Also, water entering the blades will result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine. Modern designs are sufficiently refined that problems with turbines are rare and maintenance requirements are relatively small

Speed regulation:          The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control.Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials.          During normal operation in synchronization with the electricity network, power plants are governed with a five percent droop speed control. This means the full load speed is 100% and the no-load speed is 105%. This is required for the stable operation of the network without hunting and drop-outs of power plants. Normally the changes in speed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication.

  

GENERATOR THEORY

FARADAY'S LAW:

IN THE YEARS OF 1831–1832, MICHAEL FARADAY DISCOVERED THE OPERATING PRINCIPLE OF ELECTROMAGNETIC GENERATORS. THE PRINCIPLE, LATER CALLED FARADAY'S LAW, IS THAT AN ELECTROMOTIVE FORCE IS GENERATED IN AN ELECTRICAL CONDUCTOR THAT ENCIRCLES A VARYING MAGNETIC FLUX. HE ALSO BUILT THE FIRST ELECTROMAGNETIC GENERATOR, CALLED THE FARADAY DISK, A TYPE OF HOMO POLAR GENERATOR, USING A COPPER DISC ROTATING BETWEEN THE POLES OF A HORSESHOE MAGNET. IT PRODUCED A SMALL DC VOLTAGE.

THIS DESIGN WAS INEFFICIENT DUE TO SELF-CANCELLING COUNTER FLOWS OF CURRENT IN REGIONS NOT UNDER THE INFLUENCE OF THE MAGNETIC FIELD. WHILE CURRENT WAS INDUCED DIRECTLY UNDERNEATH THE MAGNET, THE CURRENT WOULD CIRCULATE BACKWARDS IN REGIONS OUTSIDE THE INFLUENCE OF THE MAGNETIC FIELD. THIS COUNTER FLOW LIMITS THE POWER OUTPUT TO THE PICKUP WIRES AND INDUCES WASTE HEATING OF THE COPPER DISC. LATER HOMO POLAR GENERATORS WOULD SOLVE THIS PROBLEM BY USING AN ARRAY OF MAGNETS ARRANGED AROUND THE DISC PERIMETER TO MAINTAIN A STEADY FIELD EFFECT IN ONE CURRENT-FLOW DIRECTION.

ANOTHER DISADVANTAGE WAS THAT THE OUTPUT VOLTAGE WAS VERY LOW, DUE TO THE SINGLE CURRENT PATH THROUGH THE MAGNETIC FLUX. EXPERIMENTERS FOUND THAT USING MULTIPLE TURNS OF WIRE IN A COIL COULD PRODUCE HIGHER MORE USEFUL VOLTAGES. SINCE THE OUTPUT VOLTAGE IS PROPORTIONAL TO THE NUMBER OF TURNS, GENERATORS COULD BE EASILY DESIGNED TO PRODUCE ANY DESIRED VOLTAGE BY VARYING THE NUMBER OF TURNS. WIRE WINDINGS BECAME A BASIC FEATURE OF ALL SUBSEQUENT GENERATOR DESIGNS.

DYNAMO:

DYNAMOS ARE NO LONGER USED FOR POWER GENERATION DUE TO THE SIZE AND COMPLEXITY OF THE COMMUTATOR NEEDED FOR HIGH POWER APPLICATIONS. THIS LARGE BELT-DRIVEN HIGH-CURRENT DYNAMO PRODUCED 310 AMPERES AT 7 VOLTS, OR 2,170 WATTS, WHEN SPINNING AT 1400 RPM.

THE DYNAMO WAS THE FIRST ELECTRICAL GENERATOR CAPABLE OF DELIVERING POWER FOR INDUSTRY. THE DYNAMO USES ELECTROMAGNETICPRINCIPLES TO CONVERT MECHANICAL ROTATION INTO PULSED DC THROUGH THE USE OF A COMMUTATOR. THE FIRST DYNAMO WAS BUILT BY HIPPOLYTE PIXII IN 1832.

THROUGH A SERIES OF ACCIDENTAL DISCOVERIES, THE DYNAMO BECAME THE SOURCE OF MANY LATER INVENTIONS,INCLUDING THE DCELECTRIC MOTOR, THE AC ALTERNATOR, THE AC SYNCHRONOUS MOTOR, AND THE ROTARY CONVERTER.

A DYNAMO MACHINE CONSISTS OF A STATIONARY STRUCTURE, WHICH PROVIDES A CONSTANT MAGNETIC FIELD, AND A SET OF ROTATING WINDINGS WHICH TURN WITHIN THAT FIELD. ON SMALL MACHINES THE CONSTANT MAGNETIC FIELD MAY BE PROVIDED BY ONE OR MORE PERMANENT MAGNETS; LARGER MACHINES HAVE THE CONSTANT MAGNETIC FIELD PROVIDED BY ONE OR MORE ELECTROMAGNETS, WHICH ARE USUALLY CALLED FIELD COILS.

LARGE POWER GENERATION DYNAMOS ARE NOW RARELY SEEN DUE TO THE NOW NEARLY UNIVERSAL USE OF ALTERNATING CURRENT FOR POWER DISTRIBUTION AND SOLID STATE ELECTRONIC AC TO DC POWER CONVERSION. BUT BEFORE THE PRINCIPLES OF AC WERE DISCOVERED, VERY LARGE DIRECT-CURRENT DYNAMOS WERE THE ONLY MEANS OF POWER GENERATION AND DISTRIBUTION. NOW POWER GENERATION DYNAMOS ARE MOSTLY A CURIOSITY.

ALTERNATOR:

WITHOUT A COMMUTATOR, A DYNAMO BECOMES AN ALTERNATOR, WHICH IS A SYNCHRONOUS SINGLY FED GENERATOR. WHEN USED TO FEED AN ELECTRIC POWER GRID, AN ALTERNATOR MUST ALWAYS OPERATE AT A CONSTANT SPEED THAT IS PRECISELY SYNCHRONIZED TO THE ELECTRICAL FREQUENCY OF THE POWER GRID. A DC GENERATOR CAN OPERATE AT ANY SPEED WITHIN MECHANICAL LIMITS, BUT ALWAYS OUTPUTS DIRECT CURRENT.

TYPICAL ALTERNATORS USE A ROTATING FIELD WINDING EXCITED WITH DIRECT CURRENT, AND A STATIONARY (STATOR) WINDING THAT PRODUCES ALTERNATING CURRENT. SINCE THE ROTOR FIELD ONLY REQUIRES A TINY FRACTION OF THE POWER GENERATED BY THE MACHINE, THE BRUSHES FOR THE FIELD CONTACT CAN BE RELATIVELY SMALL. IN THE CASE OF A BRUSHLESS EXCITER, NO BRUSHES ARE USED AT ALL AND THE ROTOR SHAFT CARRIES RECTIFIERS TO EXCITE THE MAIN FIELD WINDING.

THE TWO MAIN PARTS OF A GENERATOR OR MOTOR CAN BE DESCRIBED IN EITHER MECHANICAL OR ELECTRICAL TERMS.

MECHANICAL:

ROTOR : THE ROTATING PART OF AN ELECTRICAL MACHINE

STATOR: THE STATIONARY PART OF AN ELECTRICAL MACHINE

ELECTRICAL :

ARMATURE: THE POWER-PRODUCING COMPONENT OF AN ELECTRICAL MACHINE. IN A GENERATOR, ALTERNATOR, OR DYNAMO THE ARMATURE WINDINGS GENERATE THE ELECTRIC CURRENT. THE ARMATURE CAN BE ON EITHER THE ROTOR OR THE STATOR.

FIELD: THE MAGNETIC FIELD COMPONENT OF AN ELECTRICAL MACHINE. THE MAGNETIC FIELD OF THE DYNAMO OR ALTERNATOR CAN BE PROVIDED BY EITHER ELECTROMAGNETS OR PERMANENT MAGNETS MOUNTED ON EITHER THE ROTOR OR THE STATOR.

BECAUSE POWER TRANSFERRED INTO THE FIELD CIRCUIT IS MUCH LESS THAN IN THE ARMATURE CIRCUIT, AC GENERATORS NEARLY ALWAYS HAVE THE FIELD WINDING ON THE ROTOR AND THE STATOR AS THE ARMATURE WINDING. ONLY A SMALL AMOUNT OF FIELD CURRENT MUST BE TRANSFERRED TO THE MOVING ROTOR, USING SLIP RINGS.DIRECT CURRENT MACHINES (DYNAMOS) REQUIRE A COMMUTATOR ON THE ROTATING SHAFT TO CONVERT THE ALTERNATING CURRENT PRODUCED BY THE ARMATURE TODIRECT CURRENT,SO THE ARMATURE WINDING IS ON THE ROTOR OF THE MACHINE.

EXCITATION:

AN ELECTRIC GENERATOR OR ELECTRIC MOTOR THAT USES FIELD COILS RATHER THAN PERMANENT MAGNETS REQUIRES A CURRENT TO BE PRESENT IN THE FIELD COILS FOR THE DEVICE TO BE ABLE TO WORK. IF THE FIELD COILS ARE NOT POWERED, THE ROTOR IN A GENERATOR CAN SPIN WITHOUT PRODUCING ANY USABLE ELECTRICAL ENERGY, WHILE THE ROTOR OF A MOTOR MAY NOT SPIN AT ALL.

SMALLER GENERATORS ARE SOMETIMES SELF-EXCITED, WHICH MEANS THE FIELD COILS ARE POWERED BY THE CURRENT PRODUCED BY THE GENERATOR ITSELF.THE FIELD COILS ARE CONNECTED IN SERIES OR PARALLEL WITH THE ARMATURE WINDING. WHEN THE GENERATOR FIRST STARTS TO TURN, THE SMALL AMOUNT OF REMANENT MAGNETISM PRESENT IN THE IRON CORE PROVIDES A MAGNETIC FIELD TO GET IT STARTED, GENERATING A SMALL CURRENT IN THE ARMATURE.THIS FLOWS THROUGH THE FIELD COILS, CREATING A LARGER MAGNETIC FIELD WHICH GENERATES A LARGER ARMATURE CURRENT. THIS "BOOTSTRAP" PROCESS CONTINUES UNTIL THE MAGNETIC FIELD IN THE CORE LEVELS OFF DUE TOSATURATION AND THE GENERATOR REACHES A STEADY STATE POWER OUTPUT.

VERY LARGE POWER STATION GENERATORS OFTEN UTILIZE A SEPARATE SMALLER GENERATOR TO EXCITE THE FIELD COILS OF THE LARGER. IN THE EVENT OF A SEVERE WIDESPREAD POWER OUTAGE WHEREISLANDING OF POWER STATIONS HAS OCCURRED, THE STATIONS MAY NEED TO PERFORM A BLACK START TO EXCITE THE FIELDS OF THEIR LARGEST GENERATORS, IN ORDER TO RESTORE CUSTOMER POWER SERVICE.

30

SWITCHGEAR

In an electric power system, switchgear is the combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done

and to clear faultsdownstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply.

The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making opening manually operated switches too dangerous for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

High voltage switchgear was invented at the end of the 19th century for operating motors and other electric machines. The technology has been improved over time and can be used with voltages up to 1,100 kV.[2]

Typically, the switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear on the low voltage side of the transformers may be located in a building, with medium-voltage circuit breakers for distribution circuits, along with metering, control, and protection equipment. For industrial applications, a transformer and switchgear line-up may be combined in one housing, called a unitized substation or USS.

SWITHYARD

Swithyard is a pat of power palant , where generated votage comes from generator transformer.Switchyard system transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels.Switchyard include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.

ELEMENTS OF SWITCHYARD :

TRANSFORMERS CIRCUIT BREAKER ISOLATOR CURRENT TRANSFORMER CAPACITOR VOTAGE TRANSFORMER SHUNT REACTOR WAVE TRAP LIGHTNING ARRESTOR INTER CONNECTING TRANSFORMER BUSBAR

TRANSFORMER

A transformer is a static electrical device that transfers energy by inductive coupling between its winding circuits. A varying current in the primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic flux through the secondarywinding. This varying magnetic flux induces a varying electromotive force (emf) or voltage in the secondary winding.

Transformers range in size from thumbnail-sized used in microphones to units weighing hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in electronic and electric power applications. Transformers are essential for thetransmission, distribution, and utilization of electrical energy.

The ideal transformer

Ideal transformer circuit diagram

Consider the ideal, lossless, perfectly-coupled transformer shown in the circuit diagram at right having primary and secondary windings with NP and NS turns, respectively.

The ideal transformer induces secondary voltage ES =VS as a proportion of the primary voltage VP = EP and respective winding turns as given by the equation

,

where,

- VP/VS = EP/ES = a is the voltage ratio and NP/NS = a is the winding turns ratio,

the value of these ratios being respectively higher and lower than unity for step-

down and step-up transformers,[

- VP designates source impressed voltage,

- VS designates output voltage, and,

- EP & ES designate respective emf induced voltages.[c]

Any load impedance   connected to the ideal transformer's secondary winding causes current to flow without losses from primary to secondary circuits, the resulting input and output apparent power therefore being equal as given by the equation

.

Combining the two equations yields the following ideal transformer identity

.

This formula is a reasonable approximation for the typical commercial transformer, with voltage ratio and winding turns ratio both being inversely proportional to the corresponding current ratio.

The load impedance   is defined in terms of secondary circuit voltage and current as follows

.

The apparent impedance   of this secondary circuit load referred to the primary winding circuit is governed by a squared turns ratio multiplication factor relationship derived as follows[6][7]

.

Induction law

The transformer is based on two principles: first, that an electric current can produce a magnetic field and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

Referring to the two figures here, current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very highmagnetic permeability, usually iron,[d] so that most of the magnetic flux passes through both the primary and secondary coils. Any secondary winding connected load causes current and voltage induction from primary to secondary circuits in indicated directions.

Ideal transformer and induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where Vs = Es is the instantaneous voltage, Ns is the number of turns in the secondary coil, and dΦ/dt is the derivative [e]  of the magnetic flux Φ through one turn of the coil. If the turns of the coil are oriented perpendicularly to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer,[6] the instantaneous voltage across the primary winding equals

Taking the ratio of the above two equations gives the same voltage ratio and turns ratio relationship shown above, that is,

.

The changing magnetic field induces an emf across each winding. The primary emf, acting as it does in opposition to the primary voltage, is sometimes termed the counter emf.[9] This is in accordance with Lenz's law, which states that induction of emf always opposes development of any such change in magnetic field.

As still lossless and perfectly-coupled, the transformer still behaves as described above in the ideal transformer.

Polarity

Instrument transformer, with polarity dot and X1 markings on LV side terminal

A dot convention is often used in transformer circuit diagrams, nameplates or terminal markings to define the relative polarity of transformer windings. Positively-increasing instantaneous current entering the primary winding's dot end induces positive polarity voltage at the secondary winding's dot end.

The real transformer

Real transformer deviations from ideal

The ideal model neglects the following basic linear aspects in real transformers:

Core losses collectively called magnetizing current losses consisting of:

Hysteresis losses due to nonlinear application of the voltage applied in the

transformer core

Eddy current losses due to joule heating in core proportional to the square of

the transformer's applied voltage.

Whereas the ideal windings have no impedance, the windings in a real

transformer have finite non-zero impedances in the form of:

Joule losses due to resistance in the primary and secondary windings[15]

Leakage flux that escapes from the core and passes through one winding

only resulting in primary and secondary reactive impedance.

Leakage flux

Leakage flux of a transformer

The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings.Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings. Leakage flux results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss (see Stray losses below), but results in inferior voltage regulation, causing the secondary voltage to not be directly proportional to the primary voltage, particularly under heavy load.Transformers are therefore normally designed to have very low leakage inductance. Nevertheless, it is impossible to eliminate all leakage flux because it plays an essential part in the operation of the transformer. The combined effect of the leakage flux and the electric field around the windings is what transfers energy from the primary to the secondary.

In some applications increased leakage is desired, and long magnetic paths, air gaps, or magnetic bypass shunts may deliberately be introduced in a transformer design to limit the short-circuit current it will supply..Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon signs or for safely handling loads that become periodically short-circuited such as electric arc welders.

Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a DC component flowing through the windings.

Knowledge of leakage inductance is for example useful when transformers are operated in parallel. It can be shown that if the percent impedance (Z) and associated winding leakage reactance-to-resistance (X/R) ratio of two transformers were hypothetically exactly the same, the transformers would share power in proportion to their respective volt-ampere ratings (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger unit would carry twice the current). However, the impedance tolerances of commercial transformers are significant. Also, the Z impedance and X/R ratio of different capacity

transformers tends to vary, corresponding 1,000 kVA and 500 kVA units' values being, to illustrate, respectively, Z ~ 5.75%, X/R ~ 3.75 and Z ~ 5%, X/R ~ 4.75.

Equivalent circuit

Referring to the diagram, a practical transformer's physical behavior may be represented by an equivalent circuit model, which can incorporate an ideal transformer.Winding joule losses and leakage reactances are represented by the following series loop impedances of the model:

Primary winding: RP, XP

Secondary winding: RS, XS.

In normal course of circuit equivalence transformation, RS and XS are in practice usually referred to the primary side by multiplying these impedances by the turns ratio squared, (NP/NS) 2 = a2.

Real transformer equivalent circuit

Core loss and reactance is represented by the following shunt leg impedances of the model:

Core or iron losses: RC

Magnetizing reactance: XM.

RC and XM are collectively termed the magnetizing branch of the model.

Core losses are caused mostly by hysteresis and eddy current effects in the core and are proportional to the square of the core flux for operation at a given frequency.[23] The finite permeability core requires a magnetizing current IM to maintain mutual flux in the core. Magnetizing current is in phase with the flux, the relationship between the two being non-linear due to saturation effects. However, all impedances of the equivalent circuit shown are by definition linear and such non-linearity effects are not typically reflected in transformer equivalent circuits. With sinusoidal supply, core flux lags the

induced emf by 90°. With open-circuited secondary winding, magnetizing branch current I0 equals transformer no-load current. The resulting model, though sometimes termed 'exact' equivalent circuit based on linearity assumptions, retains a number of approximations. Analysis may be simplified by assuming that magnetizing branch impedance is relatively high and relocating the branch to the left of the primary impedances. This introduces error but allows combination of primary and referred secondary resistances and reactances by simple summation as two series impedances.

Transformer equivalent circuit impedance and transformer ratio parameters can be derived from the following tests: Open-circuit test, short-circuit test, winding resistance test, and transformer ratio test.

Basic transformer parameters and construction

Effect of frequency

Transformer universal emf equation

If the flux in the core is purely sinusoidal, the relationship for either winding between

its rmsvoltage Erms of the winding, and the supply frequency f, number of turns N, core

cross-sectional area a in m2 and peak magnetic flux density Bpeakin Wb/m2 or T (tesla) is

given by the universal emf equation:

If the flux does not contain even harmonics the following equation can be used for half-

cycle average voltage Eavg of any waveshape:

The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to time of the applied voltage. Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time. In practice, the flux rises to the point where magnetic saturation of the core occurs, causing a large increase in the magnetizing current and overheating the

transformer. All practical transformers must therefore operate with alternating (or pulsed direct) current.

The emf of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Conversely, frequencies used for some railway electrification systems were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50 – 60 Hz) for historical reasons concerned mainly with the limitations of early electric traction motors. As such, the transformers used to step-down the high over-head line voltages (e.g. 15 kV) were much heavier for the same power rating than those designed only for the higher frequencies.

Power transformer over-excitation condition caused by decreased frequency; flux

(green), iron core's magnetic characteristics (red) and magnetizing current (blue).

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with 'volts per hertz' over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

One example of state-of-the-art design is traction transformers used for electric multiple unit and high speed train service operating across the,country border and using different electrical standards, such transformers' being restricted to be positioned below the passenger compartment. The power supply to, and converter equipment being supply by, such traction transformers have to accommodate different input frequencies and voltage (ranging from as high as 50 Hz down to 16.7 Hz and rated up to 25 kV) while

being suitable for multiple AC asynchronous motor and DC converters & motors with varying harmonics mitigation filtering requirements.

Large power transformers are vulnerable to insulation failure due to transient voltages with high-frequency components, such as caused in switching or by lightning.

Energy losses

An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers, 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 98%.Experimental transformers using superconducting windings achieve efficiencies of 99.85%.[ The increase in efficiency can save considerable energy, and hence money, in a large heavily loaded transformer; the trade-off is in the additional initial and running cost of the superconducting design.

As transformer losses vary with load, it is often useful to express these losses in terms of no-load loss, full-load loss, half-load loss, and so on. Hysteresis and eddy current losses are constant at all loads and dominate overwhelmingly at no-load, variable winding joule losses dominating increasingly as load increases. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost. Designing transformers for lower loss requires a larger core, good-quality silicon steel, or even amorphous steel for the core and thicker wire, increasing initial cost so that there is a trade-off between initial cost and running cost (also see energy efficient transformer).[31]

Transformer losses arise from:

Winding joule losses

Current flowing through winding conductors causes joule heating. As frequency

increases, skin effect and proximity effect causes winding resistance and, hence, losses

to increase.

Core losses

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to

hysteresis within the core. According to Steinmetz's formula, the heat energy due to

hysteresis is given by

, and,

hysteresis loss is thus given bY

where, f is the frequency, η is the hysteresis coefficient

and βmax is the maximum flux density, the empirical exponent of which varies from

about 1.4 to 1 .8 but is often given as 1.6 for iron.

Eddy current losses

Ferromagnetic materials are also good conductors and a core made from such a

material also constitutes a single short-circuited turn throughout its entire length. Eddy

currents therefore circulate within the 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.

Eddy current losses can be reduced by making the core of a stack of plates electrically

insulated from each other, rather than a solid block; all transformers operating at low

frequencies use laminated or similar cores.

Magnetostriction related transformer hum

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

asmagnetostriction, the frictional energy of which produces an audible noise known

as mains hum or transformer hum. This transformer hum is especially objectionable in

transformers supplied at power frequencies and in high-frequency flyback

transformers associated with PAL system CRTs

Stray losses

Leakage inductance is by itself largely lossless, 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 materials such as the transformer's support structure will

give rise to eddy currents and be converted to heat.There are also radiative losses due

to the oscillating magnetic field but these are usually small.

Mechanical vibration and audible noise transmission

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces

between the primary and secondary windings. This energy incites vibration transmission

in interconnected metalwork, thus amplifying audible transformer hum.

Core form and shell form transformers

Core form = core type; shell form = shell type

Closed-core transformers are constructed in 'core form' or 'shell form'. When windings surround the core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form.Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 kV or 75 MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra high voltage and higher MVA applications because, though more labor intensive to manufacture, shell form transformers are characterized as having inherently better kVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage.

CONSTRUCTION

Cores

Laminated steel cores

Laminated core transformer showing edge of laminations at top of photo

Power transformer inrush current caused by residual flux at switching instant; flux

(green), iron core's magnetic characteristics (red) and magnetizing current (blue).

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive to construct. Thin laminations are generally used on high-frequency transformers, with some of very thin steel laminations able to operate up to 10 kHz.

Laminating the core greatly reduces eddy-current losses

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of 'E-I transformer'. Such a design tends to exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap. They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

A steel core's remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remaining magnetism is reduced, usually after a few cycles

of the applied AC waveform.Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long, overhead power transmission lines, induced currents due togeomagnetic disturbances during solar storms can cause saturation of the core and operation of transformer protection devices.Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or amorphous (non-crystalline) metal alloy. The higher initial cost of the core material is offset over the life of the transformer by its lower losses at light load.

Solid cores

Powdered iron cores are used in circuits such as switch-mode power supplies that operate above mains frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic materials calledferrites are common.[46] Some radio-frequency transformers also have movable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Toroidal cores

Small toroidal core transformer

Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more

expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generatingelectromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited power capacity (see Classification parameters below). Because of the lack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher inrush current, compared to laminated E-I types.

Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to hundreds of megahertz, to reduce losses, physical size, and weight of inductive components. A drawback of toroidal transformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length of a coil winding through the core aperture each time a single turn is added to the coil. As a consequence, toroidal transformers rated more than a few kVA are uncommon. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.

Air cores

A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings near each other, an arrangement termed an 'air-core' transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material. The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution. They have however very high bandwidth, and are frequently employed in radio-frequency applications,] for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings. They're also used for resonant transformers such as Tesla coils where they can achieve reasonably low loss in spite of the high leakage inductance

Windings

Windings are usually arranged concentrically to minimize flux leakage.

The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound fromenamelled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.[53]

Cut view through transformer windings. White: insulator. Green spiral: Grain oriented

silicon steel. Black: Primary winding made of oxygen-free copper. Red: Secondary

winding. Top left: Toroidal transformer. Right: C-core, but E-core would be similar. The

black windings are made of film. Top: Equally low capacitance between all ends of both

windings. Since most cores are at least moderately conductive they also need

insulation. Bottom: Lowest capacitance for one end of the secondary winding needed

for low-power high-voltage transformers. Bottom left: Reduction ofleakage

inductance would lead to increase of capacitance.

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture. The windings of signal transformers minimize leakage inductance and stray capacitance to improve high-frequency response. Coils are split into sections, and those sections interleaved between the sections of the other winding.

Power-frequency transformers may have taps at intermediate points on the winding, usually on the higher voltage winding side, for voltage adjustment. Taps may be manually reconnected, or a manual or automatic switch may be provided for changing taps. Automatic on-load tap changers are used in electric power transmission or distribution, on equipment such as arc furnacetransformers, or for automatic voltage regulators for sensitive loads. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio

power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar.

Dry-type transformer winding insulation systems can be either of standard open-wound 'dip-and-bake' construction or of higher quality designs that include vacuum pressure impregnation (VPI), vacuum pressure encapsulation (VPE), and cast coil encapsulation processes.[54] In the VPI process, a combination of heat, vacuum and pressure is used to thoroughly seal, bind, and eliminate entrained air voids in the winding polyester resin insulation coat layer, thus increasing resistance to corona. VPE windings are similar to VPI windings but provide more protection against environmental effects, such as from water, dirt or corrosive ambients, by multiple dips including typically in terms of final epoxy coat.

Cooling

Cutaway view of liquid-immersed construction transformer. The conservator (reservoir)

at top provides liquid-to-atmosphere isolation as coolant level and temperature

changes. The walls and fins provide required heat dissipation balance.

To place the cooling problem in perspective, the accepted rule of thumb is that the life expectancy of insulation in all electric machines including all transformers is halved for about every 7°C to 10°C increase in operating temperature, this life expectancy halving rule holding more narrowly when the increase is between about 7°C to 8°C in the case of transformer winding cellulose insulation.

Small dry-type and liquid-immersed transformers are often self-cooled by natural convection and radiation heat dissipation. As power ratings increase, transformers are often cooled by forced-air cooling, forced-oil cooling, water-cooling, or combinations of these. Large transformers are filled with transformer oil that both cools and insulates the windings. Transformer oil is a highly refined mineral oil that cools the windings and insulation by circulating within the transformer tank. The mineral oil and paper insulation system has been extensively studied and used for more than 100 years. It is estimated that 50% of power transformers will survive 50 years of use, that the average age of failure of power transformers is about 10 to 15 years, and that about 30% of power transformer failures are due to insulation and overloading failures. Prolonged operation at elevated temperature degrades insulating properties of winding insulation and dielectric coolant, which not only shortens transformer life but can ultimately lead to catastrophic transformer failure. With a great body of empirical study as a guide, transformer oil testing including dissolved gas analysis provides valuable maintenance information. This can translate in a need to monitor, model, forecast and manage oil and winding conductor insulation temperature conditions under varying, possibly difficult, power loading conditions. Building regulations in many jurisdictions require indoor liquid-filled transformers to either use dielectric fluids that are less flammable than oil, or be installed in fire-resistant rooms. Air-cooled dry transformers can be more economical where they eliminate the cost of a fire-resistant transformer room.

The tank of liquid filled transformers often has radiators through which the liquid coolant circulates by natural convection or fins. Some large transformers employ electric fans for forced-air cooling, pumps for forced-liquid cooling, or have heat exchangers for water-cooling.An oil-immersed transformer may be equipped with a Buchholz relay, which, depending on severity of gas accumulation due to internal arcing, is used to either alarm or de-energize the transformer. Oil-immersed transformer installations usually include fire protection measures such as walls, oil containment, and fire-suppression sprinkler systems.

Polychlorinated biphenyls have properties that once favored their use as a dielectric coolant, though concerns over their environmental persistence led to a widespread ban on their use.[66] Today, non-toxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. PCBs for new equipment was banned in 1981 and in 2000 for use in existing equipment in United KingdomLegislation enacted in Canada between 1977 and 1985 essentially bans PCB use in transformers manufactured in or imported into the country after 1980, the maximum allowable level of PCB contamination in existing mineral oil transformers being 50 ppm .Some transformers,

instead of being liquid-filled, have their windings enclosed in sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.[67]

Experimental power transformers in the 500-to-1,000 kVA range have been built with liquid nitrogen or helium cooled superconducting windings, which, compared to usual transformer losses, eliminates winding losses without affecting core losses.[70][71]

Insulation drying

Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried of residual moisture before the oil is introduced. Drying is carried out at the factory, and may also be required as a field service. Drying may be done by circulating hot air around the core, or by vapor-phase drying (VPD) where an evaporated solvent transfers heat by condensation on the coil and core.

For small transformers, resistance heating by injection of current into the windings is used. The heating can be controlled very well, and it is energy efficient. The method is called low-frequency heating (LFH) since the current is injected at a much lower frequency than the nominal of the power grid, which is normally 50 or 60 Hz. A lower frequency reduces the effect of the inductance in the transformer, so the voltage needed to induce the current can be reduced.The LFH drying method is also used for service of older transformers.

Bushings

Larger transformers are provided with high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.

Classification parameter

Transformers can be classified in many ways, such as the following:

Power capacity: From a fraction of a volt-ampere (VA) to over a thousand MVA.

Duty of a transformer: Continuous, short-time, intermittent, periodic, varying

Frequency range: Power-frequency, audio-frequency, or radio-frequency.

Voltage class: From a few volts to hundreds of kilovolts.

Cooling type: Dry and liquid-immersed - self-cooled, forced air-cooled; liquid-immersed - forced oil-cooled, water-cooled.

Circuit application: Such as power supply, impedance matching, output voltage and current stabilizer or circuit isolation.

Utilization: Pulse, power, distribution, rectifier, arc furnace, amplifier output, etc..

Basic magnetic form: Core form, shell form.

Constant-potential transformer descriptor: Step-up, step-down, isolation.

General winding configuration: By EIC vector group - various possible two-winding combinations of the phase designations delta, wye or star, and zigzag or interconnected star;[j]other - autotransformer, Scott-T, zigzag grounding transformer winding.[75][76][77][78]

Rectifier phase-shift winding configuration: 2-winding, 6-pulse; 3-winding, 12-pulse; . . . n-winding, [n-1]*6-pulse; polygon; etc..Types[edit]

For more details, see Transformer types or specific main articles, as shown.

A wide variety of transformer designs are used for different applications, though they share several common features. Important common transformer types include:

Autotransformer: Transformer in which part of the winding is common to both primary and secondary circuits.

Capacitor voltage transformer: Transformer in which capacitor divider is used to reduce high voltage before application to the primary winding.

Distribution transformer, power transformer: International standards make a distinction in terms of distribution transformers being used to distribute energy from transmission lines and networks for local consumption and power transformers being used to transfer electric energy between the generator and distribution primary circuits.

Phase angle regulating transformer: A specialised transformer used to control the flow of real power on three-phase electricity transmission networks.

Scott-T transformer: Transformer used for phase transformation from three-phase to two-phase and vice versa.[79]

Polyphase transformer: Any transformer with more than one phase.

Grounding transformer: Transformer used for grounding three-phase circuits to create a neutral in a three wire system, using a wye-delta transformer, or more commonly, a zigzag grounding winding.

Leakage transformer: Transformer that has loosely coupled windings.

Resonant transformer: Transformer that uses resonance to generate a high secondary voltage.

Audio transformer: Transformer used in audio equipment.

Output transformer: Transformer used to match the output of a valve amplifier to its load.

Instrument transformer: Potential or current transformer used to accurately and safely represent voltage, current or phase position of high voltage or high power circuits.

Applications

An electrical substation in Melbourne, Australia showing 3 of 5 220kV/66kV

transformers, each with a capacity of 150 MVA.

Transformer at the Limestone Generating Station in Manitoba, Canada

Transformers are used to increase voltage before transmitting electrical energy over long distances through wires. Wires have resistance which loses energy through joule heating at a rate corresponding to square of the current. By transforming power to a higher voltage transformers enable economical transmission of power and distribution. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand.[All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.

Transformers are also used extensively in electronic products to step-down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers. Audio transformers allowed telephonecircuits to carry on a two-way conversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to a signal that has balanced voltages to ground, such as between external cables and internal circuits.

History

Discovery of induction phenomenon

Faraday's experiment with induction between coils of wire

The principle behind the operation of a transformer, electromagnetic induction, was discovered independently by Michael Faraday and Joseph Henry in 1831. However, Faraday was the first to publish the results of his experiments and thus receive credit for the discovery.The relationship between emf and magnetic flux is an equation now known as Faraday's law of induction:

.

where   is the magnitude of the emf in volts and ΦB is the magnetic flux through the circuit in webers.[86]

Faraday performed the first experiments on induction between coils of wire, including winding a pair of coils around an iron ring, thus creating the first toroidal closed-core transformer.However he only applied individual pulses of current to his transformer, and never discovered the relation between the turns ratio and emf in the windings.

Induction coils

Faraday's ring transformer

Induction coil, 1900, Bremerhavn, Germany

The first type of transformer to see wide use was the induction coil, invented by Rev. Nicholas Callan of Maynooth College, Ireland in 1836. He was one of the first researchers to realize the more turns the secondary winding has in relation to the primary winding, the larger the induced secondary emf will be. Induction coils evolved from scientists' and inventors' efforts to get higher voltages from batteries. Since batteries produce direct current (DC) rather than AC, induction coils relied upon vibrating electrical contacts that regularly interrupted the current in the primary to create the flux changes necessary for induction. Between the 1830s and the 1870s, efforts to build better induction coils, mostly by trial and error, slowly revealed the basic principles of transformers.

CIRCUIT BREAKER

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused byoverload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which operates once and then must 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.

ORIGINS

An early form of circuit breaker was described by Thomas Edison in an 1879 patent application, although his commercial power distribution system used fuses. Its purpose was to protect lighting circuit wiring from accidental short-circuits and overloads. A modern miniature circuit breaker similar to the ones now in use was patented by Brown,

Boveri & Cie in 1924. Hugo Stotz, an engineer who had sold his company, to BBC, was credited as the inventor on DRP (Deutsches Reichspatent) 458329. Stotz's invention was the forerunner of the modern thermal-magnetic breaker commonly used in household load centers to this day.

Interconnection of multiple generator sources into an electrical grid required development of circuit breakers with increasing voltage ratings and increased ability to safely interrupt the increasing short circuit currents produced by networks. Simple air-break manual switches produced hazardous arcs when interrupting high currents; these gave way to oil-enclosed contacts, and various forms using directed flow of pressurized air, or of pressurized oil, to cool and interrupt the arc. By 1935, the specially constructed circuit breakers used at the Boulder Dam project use eight series breaks and pressurized oil flow to interrupt faults of up to 2500 MVA, in three cycles of the AC power frequency.

OPERATIONS

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 (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. 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 (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys, and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

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 the arc forms in. Different techniques are used to extinguish the arc including:

Lengthening / deflection of the arc Intensive cooling (in jet chambers) Division into partial arcs Zero point quenching (Contacts open at the zero current time crossing of the AC

waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz and 120 times per second for 60 Hz AC)

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.

ARC INTERRUPTION

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings have metal plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowoutcoils or permanent magnets deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc. Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device.

SHORT CIRCUIT CURRENT

Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt.

Under short-circuit conditions, a current many times greater than normal can exist (see maximum prospective short circuit current). When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue. This condition can create conductive ionized gases and molten or vaporized metal, which can cause further continuation of the arc, or creation of additional short circuits, potentially resulting in the explosion of the circuit breaker and the equipment that it is installed in. Therefore, circuit breakers must incorporate various features to divide and extinguish the arc.

In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulphur hexafluoride or have contacts immersed in oil to suppress the arc.

The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to explode when reset.

Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panelboard; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution-type circuit breakers.

TYPES OF CIRCUIT BREAKER

Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.

Low-voltage circuit breakersLow-voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, and include:

MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

There are three main types of MCBs: 1. Type B - trips between 3 and 5 times full load current; 2. Type C - trips between 5 and 10 times full load current; 3. Type D - trips

between 10 and 20 times full load current. In the UK all MCBs must be selected in accordance with BS 7671.

MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

Low-voltage power circuit breakers can be mounted in multi-tiers in low-voltage switchboards or switchgear cabinets.

The characteristics of low-voltage circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electric motor operators so they can trip (open) and close under remote control. These may form part of an automatic transfer switch system for standby power.

Low-voltage circuit breakers are also made for direct-current (DC) applications, such as DC for subway lines. Direct current requires special breakers because the arc is continuous—unlike an AC arc, which tends to go out on each half cycle. A direct current circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the arc. Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components:

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation.

2. Actuator mechanism - forces the contacts together or apart.3. Contacts - Allow current when touching and break the current when moved apart.4. Terminals5. Bimetallic strip.6. Calibration screw - allows the manufacturer to precisely adjust the trip current of

the device after assembly.7. Solenoid8. Arc divider/extinguisher

Magnetic circuit breakersMagnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases

the latch, which lets the contacts open by spring action. Some magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. A spring restrains the core until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker

Thermal magnetic circuit breakersThermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions. The thermal portion of the circuit breaker provides an "inverse time" response feature, which provides faster or slower response for larger or smaller over currents respectively.

Common trip breakers

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two-pole common trip breakers are common on 120/240-volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.

Two- and four-pole breakers are used when there is a need to disconnect multiple phase AC—or to disconnect the neutral wire to ensure that no current flows through the neutral wire from other loads connected to the same network when workers may touch the wires during maintenance. Separate circuit breakers must never be used for live and neutral, because if the neutral is disconnected while the live conductor stays connected, a dangerous condition arises: the circuit appears de-energized (appliances don't work), but wires remain live and RCDs don't trip if someone touches the live wire (because RCDs need power to trip). This is why only common trip breakers must be used when neutral wire switching is needed.

Medium-voltage circuit breakersMedium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components

installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:

Vacuum circuit breakers—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V,[7] which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

Air circuit breakers—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing breaker removal without disturbing power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure. Some Important manufacturer of VCB from 3.3 kV to 36 kV are ABB,Simens,C&S Electric Ltd.,Jyoti & BHEL.

High-voltage circuit breakersElectrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Bulk oil

Minimum oil Air blast Vacuum SF6

CO2

Some of the manufacturers are ABB, GE (General Electric), Tavrida Electric, Alstom, Mitsubishi Electric, Pennsylvania Breaker, Siemens, Toshiba, Končar HVS, BHEL, CGL, Square D (Schneider Electric), Becker/SMC (SMC Electrical Products).

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6gas to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1200kV breakers were launched by Siemens in November 2011, followed by ABB in April the following year. High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.

Disconnecting circuit breaker (DCB)The disconnecting circuit breaker (DCB) was introduced in 2000 and is a high-voltage circuit breaker modeled after the SF6-breaker. It presents a technical solution where the disconnecting function is integrated in the breaking chamber, eliminating the need for separate disconnectors. This increases theavailability, since open-air disconnecting switch main contacts need maintenance every 2–6 years, while modern circuit breakers have maintenance intervals of 15 years. Implementing a DCB solution also reduces the space requirements within the substation, and increases the reliability, due to the lack of separate disconnectors.

In order to further reduce the required space of substation, as well as simplifying the design and engineering of the substation, a Fiber optic current sensor (FOCS) can be integrated with the DCB. A 420 kV DCB with integrated FOCS can reduce a substation’s footprint with over 50 % compared to a conventional solution of live tank breakers withdisconnectors and current transformers, due to reduced material and no additional insulation medium.

Sulfur hexafluoride (SF6) high-voltage circuit-breakers

A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.

Carbon dioxide (CO2) high-voltage circuit breakerIn 2012 ABB presented a 72.5 kV high-voltage breaker that uses carbon dioxide as the medium to extinguish the arc. The carbon dioxide breaker works on the same principles as an SF6 breaker and can also be produced as a disconnecting circuit breaker. By switching from SF6 to CO2 it is possible to reduce the CO2 emissions by 10 tons during the product’s life cycle

ISOLATOR

An isolator is a two-port device that transmits microwave or radio frequency power in one direction only. It is used to shield equipment on its input side, from the effects of conditions on its output side; for example, to prevent a microwave source being detuned by a mismatched load.

NON RECIPORITY

from the ferrite or absorbed. An isolator is a non-reciprocal device, with a non-symmetric scattering matrix. An ideal isolator transmits all the power entering port 1 to port 2, while absorbing all the power entering port 2, so that to within a phase-factor its S-matrix is

To achieve non-reciprocity, an isolator must necessarily incorporate a non-reciprocal material. At microwave frequencies this material is invariably a ferrite which is biased by a static magnetic field. The ferrite is positioned within the isolator such that the microwave signal presents it with a rotating magnetic field, with the rotation axis aligned with the direction of the static bias field. The behaviour of the ferrite depends on the sense of rotation with respect to the bias field, and hence is different for microwave signals travelling in opposite directions. Depending on the exact operating conditions, the signal travelling in one direction may either be phase-shifted, displaced

TYPES:

Resonance absorptionIn this type the ferrite absorbs energy from the microwave signal travelling in one direction. A suitable rotating magnetic field is found in the TE10 mode of rectangular waveguide. The rotating field exists away from the centre-line of the broad wall, over the full height of the guide. However, to allow heat from the absorbed power to be conducted away, the ferrite does not usually extend from one broad-wall to the other, but is limited to a shallow strip on each face. For a given bias field, resonance absorption occurs over a fairly narrow frequency band, but since in practice the bias field is not perfectly uniform throughout the ferrite, the isolator functions over a somewhat wider band.

FIELD DISPLACEMENT

This type is superficially very similar to a resonance absorption isolator, but the magnetic biassing differs, and the energy from the backward travelling signal is absorbed in a resistive film or card on one face of the ferrite block rather than within the ferrite itself. The bias field is weaker than that necessary to cause resonance at the operating frequency, but is instead designed to give the ferrite zero permeability for one sense of rotation of the microwave signal field. The bias polarity is such that this special condition arises for the forward signal, while the backward signal sees the ferrite as an ordinary permeable material. Consequently the electromagnetic field of the forward signal tends to be excluded from the ferrite while the field of the backward wave is concentrated within it. This results in a null of the electric field of the forward signal on the surface of the ferrite where the resistive film is placed. Conversely for the backward signal, the electric field is strong over this surface and so its energy is dissipated in driving current through the film. In rectangular waveguide the ferrite block will typically occupy the full height from one broad-wall to the other, with the resistive film on the side facing the centre-line of the guide.

USING A CIRCULATOR

A circulator is a non-reciprocal three- or four-port device, in which power entering any port is transmitted to the next port in rotation (only). So to within a phase-factor, the scattering matrix for a three-port circulator is

A two-port isolator is obtained simply by terminating one of the three ports with a matched load, which absorbs all the power entering it. The biassed ferrite is part of the circulator and causes a differential phase-shift for signals travelling in different directions. The bias field is lower than that needed for resonance absorption, and so this type of isolator does not require such a heavy permanent magnet. Because the power is absorbed in an external load, cooling is less of a problem than with a resonance absorption isolator.

CURRENT TRANSFORMER

A current transformer (CT) is used for measurement of alternating electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

DESIGN

Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. The alternating current flowing in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary winding circuit. An essential objective of current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current.

The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed 'around' the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns. The primary winding may be a permanent part of the current transformer, with a heavy copper bar to carry current through the magnetic core.

Window-type current transformers (aka zero sequence current transformers, or ZSCT) are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. When conductors passing through a CT are not centered in the circular (or oval) opening, slight inaccuracies may occur.

Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical examples of low voltage single ratio metering current transformers are either ring type or plastic moulded case. High-voltage current transformers are mounted on porcelain bushings to insulate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window.

The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or burden, of the CT should be of low resistance. If the voltage time integral area is higher than the core's design rating, the core goes into saturation towards the end of each cycle, distorting the waveform and affecting accuracy.

USAGE

Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amps.

The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers and different characteristics (accuracy, overload performance) to be used for the devices.

SAFETY PRECAUTION

Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance up to its core saturation voltage. This may produce a high voltage across the open secondary into the range of several kilovolts, causing arcing, compromising operator with equipment safety, or permanently affect the accuracy of the transformer.

ACURACY

The accuracy of a CT is directly related to a number of factors including:

Burden Burden class/saturation class Rating factor Load External electromagnetic fields Temperature and Physical configuration. The selected tap, for multi-ratio CTs

For the IEC standard, accuracy classes for various types of measurement are set out in IEC 60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important especially in power measuring circuits, and each class has an allowable maximum phase error for a specified load impedance.

Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. A CT with a rating of 2.5L400 specifies with an output from its secondary winding of 20 times its rated secondary current (usually 5 A x 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent.

BURDEN

The secondary load of a current transformer is usually called the "burden" to distinguish it from the load of the circuit whose current is being measured.

The burden, in a CT metering circuit is the (largely resistive) impedance presented to its secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2 Ω of impedance in the metering circuit before its secondary accuracy falls outside of an accuracy specification. These specification diagrams show accuracy parallelograms on a grid incorporating magnitude and phase angle error scales at the

CT's rated burden. Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate conductors. The most common source of excess burden is the conductor between the meter and the CT. When substation meters are located far from the meter cabinets, the excessive length of wire creates a large resistance. This problem can be reduced by using CTs with 1 ampere secondaries, which will produce less voltage drop between a CT and its metering devices.

KNEE POINT ORE SATURATION VOLTAGEThe knee-point voltage of a current transformer is the magnitude of the secondary voltage after which the output current ceases to linearly follow the input current within declared accuracy. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, up until the knee point. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. From the knee point upwards, the magnetizing current increases abruptly even with small increments in voltage across the secondary terminals. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to currents of 20 or 30 times rated current during faults

RATING FATOR

Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute maximum measurable primary current. Conversely, the minimum primary current a CT can accurately measure is "light load," or 10% of the nominal current (there are, however, special CTs designed to measure accurately currents as small as 2% of the nominal current). The rating factor of a CT is largely dependent upon ambient temperature. Most CTs have rating factors for 35 degrees Celsius and 55 degrees Celsius. It is important to be mindful of ambient temperatures and resultant rating factors when CTs are installed inside padmount transformers or poorly ventilated mechanical rooms. Recently, manufacturers have been moving towards lower nominal primary currents with greater rating factors. This is made possible by the development of more efficient ferrites and their corresponding hysteresis curves.

SPECIAL DESIGN

Specially constructed wideband current transformers are also used (usually with an oscilloscope) to measure waveforms of high frequency or pulsed currents within pulsed powersystems. One type of specially constructed wideband transformer

provides a voltage output that is proportional to the measured current. Another type (called a Rogowski coil) requires an external integrator in order to provide a voltage output that is proportional to the measured current. Unlike CTs used for power circuitry, wideband CTs are rated in output volts per ampere of primary current. CT RATIO.

HIGH VOLTAGE TYPE

Current transformers are used for protection, measurement and control in high voltage electrical substations and the electrical grid. Current transformers may be installed inside switchgear or in apparatus bushings, but very often free-standing outdoor current transformers are used. In a switchyard, live tank current transformers have a substantial part of their enclosure energized at the line voltage and must be mounted on insulators. Dead tank current transformers isolate the measured circuit from the enclosure. Live tank CTs are useful because the primary conductor is short, which gives better stability and a higher short-circuit current withstand rating. The primary of the winding can be evenly distributed around the magnetic core, which gives better performance for overloads and transients. Since the major insulation of a live-tank current transformer is not exposed to the heat of the primary conductors, insulation life and thermal stability is improved.

A high-voltage current transformer may contain several cores, each with a secondary winding, for different purposes (such as metering circuits, control, or protection)

CAPACITOR VOLTAGE TRANSFORMER 

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used inpower systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate aprotective relay. In its most basic form the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a transformer to isolate and further step down the voltage for the instrumentation or protective relay. The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices. The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect 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 wound primary voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency.  This forms a carrier communication network throughout the transmission network.

Variable Shunt Reactors 

Variable Shunt Reactors are used in high voltage energy transmission systems to stabilize the voltage during load variations. A traditional shunt reactor has a fixed rating and is either connected to the power line all the time or switched in and out depending on the load. Recently Variable Shunt Reactors (VSR) have been developed and introduced on the market. The rating of a VSR can be changed in steps, The maximum regulation range typically is a factor of two, e.g. from 100-200 Mvar. The regulation speed is normally in the order seconds per step and around a minute from max to min rating. VSRs are today available for voltages up to 550 kV. The largest three-phase VSRs in operation have a rating of 120-200 Mvar at 420 kV and single-phase variable shunt reactors banks rated 200-285 Mvar at 420 kV have been installed in Italy.

The variability brings several benefits compared to a traditional fixed shunt reactors. The VSR can continuously compensate reactive power as the load varies and thereby securing voltage stability. Other important benefits are:

reduced voltage jumps resulting from switching in and out of traditional fixed reactors

flexibility for future (today unknown) load and generation patterns improved interaction with other transmission equipment and/or systems such as

coarse tuning of SVC equipment limiting the foot print of a substation if parallel fixed shunt reactors can be replaced

with one VSR a VSR can be used as a flexible spare unit and be moved to other locations in the

power grid if needed

WAVE TRAP

Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals.Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU). This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals. 

The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation busbars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.

LIGHTNING ARRESTER

A  lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

In telegraphy and telephony, a lightning arrestor is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike.

If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-

produced extreme voltage spikes in incoming power lines can damage electrical home appliances

COMPONENT

A potential target for a lightning strike, such as a television antenna, is attached to the terminal labeled A in the photograph. Terminal E is attached to a long rod buried in the ground. Ordinarily no current will flow between the antenna and the ground because there is extremely high resistance between B and C, and also between C and D. The voltage of a lightning strike, however, is many times higher than that needed to move electrons through the two air gaps. The result is that electrons go through the lightning arrester rather than traveling on to the television set and destroying it.

A lightning arrester may be a spark gap or may have a block of a semiconducting material such as silicon carbide or zinc oxide. Some spark gaps are open to the air, but most modern varieties are filled with a precision gas mixture, and have a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or voltage-activated solid-state switches called varistors or MOVs.

Lightning arresters built for power substation use are impressive devices, consisting of a porcelain tube several feet long and several inches in diameter, typically filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder.

Lightning arresters are rated by the peak current they can withstand, the amount of energy they can absorb, and the breakover voltage that they require to begin conduction. They are applied as part of a lightning protection system, in combination with air terminals and bonding.

BUSBAR

In electrical power distribution, a busbar (also spelled bus bar, buss bar or bussbar, with the term bus being a contraction of the Latin omnibus - meaning for all) is a strip or bar of copper, brass or aluminium that conducts electricity within a switchboard,distribution board, substation, battery bank or other electrical apparatus. Its main purpose is to conduct electricity, not to function as a structural member.

The cross-sectional size of the busbar determines the maximum amount of current that can be safely carried. Busbars can have a cross-sectional area of as little as 10 mm2 but electrical substations may use metal tubes of 50 mm in diameter (20 cm2) or more as busbars. An aluminium smelter will have very large busbars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminium from molten salts.

DESIGN AND PLACEMENT

Busbars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their highsurface area to cross-sectional area ratio.

The skin effect makes 50–60 Hz AC busbars more than about 8 mm (1/3 in) thickness inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor switchyards.

A busbar may either be supported on insulators, or else insulation may completely surround it. Busbars are protected from accidental contact either by a metal earthed enclosure or by elevation out of normal reach. Power Neutral busbars may also be insulated. Earth (safety grounding) busbars are typically bare and bolted directly onto any metal chassis of their enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or busway, segregated-phase bus, or isolated-phase bus.

Busbars may be connected to each other and to electrical apparatus by bolted, clamp, or welded connections. Often joints between high-current bus sections have matching surfaces that are silver-plated to reduce the contact resistance. At extra-high voltages (more than 300 kV) in outdoor buses, corona around the connections becomes a source of radio-frequency interference and power loss, so connection fittings designed for these voltages are used.

Busbars are typically contained inside switchgear, panel boards, or busway. Distribution boards split the electrical supply into separate circuits at one location. Busways, or bus ducts, are long busbars with a protective cover. Rather than branching the main supply at one location, they allow new circuits to branch off anywhere along the route of the busway.

INTER CONNETTING TRANSFORMER

The function of the inter-connecting transformer is - as the name suggests - to inter-connect two systems at different voltages. Normally, they will be either 400kV/132kV or 220kV/110kV, of say about 100 MVA rating. They are bi-directional. During the plant start-up, they "import" power from the grid either at 400kV or 220kV and step down to 132kV or 110kV to supply the station auxiliaries. Once the plant is started and synchronized to the grid, the same transformer can now be used to "export" power to the grid. 

They are normally auto-transformers and they will have a delta connected tertiary winding of about 33kV voltage rating, for providing a circulating path for the zero-sequence currents. The spec would read: 400/132/33kV, 100MVA

Future Capacity Additions

NTPC has formulated a long term Corporate Plan to become a 1,28,000 MW company upto 2032. In line with the Corporate Plan, the capacity addition under implementation presently:

PROJECT STATE MW

Coal1. Bongaigaon Assam 7502. Barh-I Bihar 19803. Barh-II Bihar 13204. Lara-I Chhattisgarh 16005. Kudgi-I Karnataka 24006. Vindhyachal-V Madhya Pradesh 5007. Gadarwara-I Madhya Pradesh 16008. Mouda-II Maharashtra 1320

9. Solapur Maharashtra 132010. Rihand-III Uttar Pradesh 50011. Nabinagar, BRBCL Bihar 100012. Muzaffarpur Exp., KBUNL Bihar 39013. Nabinagar, NPGCPL Bihar 198014. Vallur-II, NTECL Tamil Nadu 50015. Meja, MUNPL Uttar Pradesh 1320Total 18,480Hydro1. Koldam HEPP ( 4 x 200) Himachal Pradesh 8002. Tapovan Vishnugad HEPP (4 x 130) Uttarakhand 5203. Singrauli CW Discharge(Small Hydro) Uttar Pradesh 84. Lata Tapovan Uttarakhand 171Total 1,499Solar1. Ramagundam Phase-I Andhra Pradesh 102. Talcher Kaniha Odisha 103. Unchahar Uttar Pradesh 10Total 30Grand Total (Coal + Hydro + Solar) 20,009

NTPC has a glorious record of excellence in every field of its activities ever since its inception in 1975. Leading the country’s power sector, we take pride in our people and their performance which has been acknowledged time and again at various national and international fora.

AWARDS GAIN BY NTPC

Environment Award

1. Earthcare Award - 2012 to NTPC for Climate Change Initiatives

2. Golden Peacock Environmental Management Award

3. CII Sustainability Award

4. 3rd Green Globe Foundation Awards

5. The Sunday Indian Special Mega Excellence – “India’s Best Environment Driven

Company Award – 2009.

Performance Awards

1. Life Time Achievement Award to GM NTPC -CenPEEP

2. SCOPE Excellence Award to NTPC

3. NTPC Finance Director bags GSBA- Top Rankers Excellence Award

4. Top Liner Maharatna Award to NTPC

5. SCOPE Excellence Award to Shri Arup Roy Choudhury, CMD, NTPC

6. PSU Excellence Award for NTPC

7. Enertia Awards for NTPC Projects & Shri D K Jain, Director (Technical), NTPC Ltd receives

award for Excellence in Nuclear, Thermal (Conventional Energy)

8. Vishwakarma Award for 12 NTPC Employees

9. Prime Minister’s Shram Award to NTPC’s Misri Lal Choudhary

10. The Best Performing CFO Award

11. India Pride Awards – Energy and Power Category

12. Enertia Award 2010

13. SAFA Best Presented Accounts Awards 2008

14. CII-EXIM Excellence Award, 2010

15.National Awards for Meritorious Performance

Quality Awards

1. International Gold Star Award for Quality 2009

2.

Company Rankings

1. Ranked 348th in Global ranking among ‘Global 2000’ list of companies compiled by Forbes in 2011.

2. Forbes' Global 2000 list of top listed firms

3. Platts Top 250 Global Energy Company Rankings – 2010

4. NTPC – the Most Respected Company in Power Sector

5. India’s Biggest News Makers Survey

6. Business Standard's "BS1000" companies

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