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
AZAD INST. OF ENGG. AND TECHNOLOGY,
SUBMITTED BY :
NAME : Deepak kr Singh
ROLL NO. : 1005320022
TRAINING INCHARGE :
Mr. CH SATYNARAYAN
NTPC , SHAKTINAGAR
Department of Electrical Engineering
(Affiliated by GBTU)
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
Name of the student : Deepak Kr SINGH
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.
Training incharge (Internal)
Counter signed by
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
INTRODUCTION TO THERMAL POWER PLANT
SWITCHYARD AND ITS ELMENTS
FUTURE CAPACITY ADDITION IN NTPC , AWARDS , AND RANKING
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.
Installed Capacity (MW)
Date of Commissioning
NTPC ROLE IN DIFFERENT AREAS AND FUTURE SCOPE
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
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
Renewable energy projects
Owned By JVs
Coal & Gas
Regional Spread of Generating Facilities
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
Renewable Energy and Distributed Generation
Renewable 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.
NTPC plans to broad base generation mix by evaluating conventional and
alternate sources of energy to ensure long run competitiveness and mitigate
“To provide green power through locally available resources at affordable
price, promoting clean energy”
Portfolio of Renewable Power
NTPC 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 Projects
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
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
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
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.
Projects under Implementation (8 MW)
o 8 MW hydro energy based project at NTPC-Singrauli in Uttar
Projects under Consideration (3 MW)
o 3 MW hydro energy based project at NTPC-Rihand in Uttar
Tattapani Geothermal Project in Chhattisgarh: MoU Signed with Govt.
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.
India‟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
IEEMA Power award-2009 in the category of “Excellence in Distributed
NTPC Distributed Generation film “Energizing villages” has been
awarded in category “development venture” by Public Society of India,
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
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-ofthe-art pollution control systems / devices have been installed to control air
and water pollution:
Flue Gas Stacks
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
Upgradation & Retrofitting of Pollution Control Systems
Municipal Waste Management
Hazardous Waste Management
Bio-Medical Waste Management
Land Use / Bio-diversity
Reclamation of Abandoned Ash Green Belts, Afforestation & Energy Plantations
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 heatand-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 Plants
Thermal power plants are classified by the type of fuel and the type of prime mover
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-energydensity, 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.
temperatures 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
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 generated
The 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 Cycle
2. Oil Cycle
3. Air and Flue Gas Cycle
4. Steam Water Cycle
2.1.1 Coal Cycle
The 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 180200 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.HA&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 Cycle
In 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
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 Cycle
The 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
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
For the generation of steam De-mineralize water prepared removing minerals & impurities
to remove the minerals several chemicals are used.
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
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
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 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.
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 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
The function of condenser is to create suction at very low pressure to the exhaust
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 plant
1. Cooling tower
10. SteamControl valve
11. High pressuresteam
2. Cooling water pump
3. transmission line (3-phase) 12. Deaerator
4. Step-uptransformer (313. Feedwater heater
5. Electrical generator (314.Coal conveyor
6. Low pressure steam turbine 15.Coal hopper
7. Condensate pump
16. Coal pulverizer
8. Surface condenser
17. Boiler steam drum
pressure steam turbine
18. Bottom ash hopper
20. Forced draught (draft)fan
22.Combustion air intake
24. Air preheater
26. Induced draught
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
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:
Steam turbine couples with electric generator
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
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 mysteriousmotive-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 highpressurized, 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 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
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
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
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 “fielderected” 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
“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.
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
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
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.
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
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.
IN THE YEARS OF 1831–1832, MICHAEL FARADAY DISCOVERED THE OPERATING PRINCIPLE OF
ELECTROMAGNETIC GENERATORS . T HE 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 . W HILE CURRENT WAS INDUCED DIRECTLY UNDERNEATH THE
MAGNET, THE CURRENT WOULD CIRCULATE BACKWARDS IN REGIONS OUTSIDE THE INFLUENCE OF THE
MAGNETIC FIELD . T HIS 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. W IRE WINDINGS BECAME A BASIC FEATURE OF ALL SUBSEQUENT
GENERATOR DESIGNS .
DYNAMOS ARE NO LONGER USED FOR POWER GENERATION DUE TO THE SIZE AND COMPLEXITY OF THE
COMMUTATOR NEEDED FOR HIGH POWER APPLICATIONS. T HIS 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 . O N 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. N OW POWER GENERATION
DYNAMOS ARE MOSTLY A CURIOSITY .
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 . I N 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 .
ROTOR : THE ROTATING PART OF AN ELECTRICAL MACHINE
STATOR: THE STATIONARY PART OF AN ELECTRICAL MACHINE
ARMATURE: THE POWER-PRODUCING COMPONENT OF AN ELECTRICAL MACHINE . IN A GENERATOR,
ALTERNATOR, OR DYNAMO THE ARMATURE WINDINGS GENERATE THE ELECTRIC CURRENT . T HE 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. O NLY 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 .
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 .T HE FIELD COILS ARE CONNECTED IN SERIES OR PARALLEL
WITH THE ARMATURE WINDING. W HEN 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 .T HIS FLOWS THROUGH THE FIELD COILS , CREATING A
LARGER MAGNETIC FIELD WHICH GENERATES A LARGER ARMATURE CURRENT. T HIS "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. I N 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 .
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
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.
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 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 :
CAPACITOR VOTAGE TRANSFORMER
INTER CONNECTING 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
- 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 stepdown 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
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
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
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
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, 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. 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.
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
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
Leakage flux that escapes from the core and passes through one winding
only resulting in primary and secondary reactive impedance.
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 audiofrequency transformers in circuits that have a DC component flowing through the
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.
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
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.  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 halfcycle 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
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
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.
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
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).
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
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
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
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.
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 Eshaped 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 (noncrystalline) 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.
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. 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.
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
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
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.
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
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. 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.
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
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 firesuppression 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. 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.
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. 
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.
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.
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.
Rectifier phase-shift winding configuration: 2-winding, 6-pulse; 3-winding, 12-pulse; . . .
n-winding, [n-1]*6-pulse; polygon; etc..Types
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.
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
Audio transformer: Transformer used in audio equipment.
Output transformer: Transformer used to match the output of a valve amplifier to its
Instrument transformer: Potential or current transformer used to accurately and safely
represent voltage, current or phase position of high voltage or high power circuits.
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
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:
is the magnitude of the emf in volts and ΦB is the magnetic flux through the
circuit in webers.
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.
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
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.
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,