L25
02 March 2022
UNIT-3. INTRODUCTION TO ELECTRIC MACHINES &
POWER SYSTEM
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L26
04 March 2022
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The rotating magnetic field in a stator represented as moving north and south stator poles.
a) b)
c) d)
Depiction of rotating magnetic fields at a) 𝜔𝑡 = 0° b) 𝜔𝑡 = 30° b) 𝜔𝑡 = 60° a) 𝜔𝑡 = 90°
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A simple four-pole stator winding
The resulting stator magnetic poles.
Notice that there are moving poles of alternating polarity every 90° around the stator surface
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A winding diagram of the stator as seen from its inner surface, showing how the stator currents
produce north and south magnetic poles
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L27-28
05 March 2022
Construction of an Alternator
Types of Alternator
• In a synchronous generator, a rotor magnetic field is produced either by designing the rotor as a
permanent magnet or by applying a dc current to a rotor winding to create an electromagnet. The
rotor of the generator is then turned by a prime mover, producing a rotating magnetic field within
the machine.
• This rotating magnetic field induces a three-phase set of voltages within the stator windings of the
generator.
• Two terms commonly used to describe the windings on a machine are field windings and armature
windings.
• In general, the term field windings applies to the windings that produce the main magnetic field in a
machine, and the term armature windings applies to the windings where the main voltage is
induced.
• For synchronous machines, the field windings are on the rotor, so the terms rotor windings and
field windings are used interchangeably.
• Similarly, the terms stator windings and armature windings are used interchangeably.
Stator Construction
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Cross-sectional view of stator
Portion of stator
• The core of the stator is made of CRGO (cold rolled grain oriented) sheet steel or silicon steel. This
material has high permeability and low hysteresis loss.
• To minimize eddy-current loss, the core is made of laminations (about 0.5 mm thick), insulated from
each other by varnish or paper.
• The laminations are stamped out in complete rings (for smaller machines) or in segments (for larger
machines).
• These stampings have uniformly distributed open or semi-closed slots on its inner periphery to
accommodate the armature conductors.
• The slots in the laminated stator core of a synchronous machine are usually semi-enclosed, so as to
distribute the magnetic flux as uniformly as possible in the airgap, thereby minimizing the ripple
that would appear in the emf waveform if open slots were used.
• The open slots provide advantage when removal and replacement of defective coils is involved.
However, such slots have the disadvantage of making distribution of the air-gap flux non-
smooth. This would result in ripples in the induced emf wave.
• The whole structure is held in a cast iron frame.
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• The whole structure is held in a cast iron frame.
Actual stator from Siemens Energy
Rotor construction
The rotor of a synchronous generator is essentially a large electromagnet.
It is a cylindrical structure which can rotate inside the stator leaving a very small air gap. It houses the
windings to produce dc magnetic field.
Synchronous machines can be divided into two categories:
1. those with salient or projecting poles;
2. those with cylindrical rotors.
The magnetic poles on the rotor can be of either salient or nonsalient construction. The term salient
means "protruding" or "sticking out," and a salient pole is a magnetic pole that sticks out radially from the
shaft of the rotor.
A non-salient pole is a magnetic pole with windings embedded flush with the surface of the rotor.
Nonsalient-pole rotors are normally used for two- and four-pole rotors, while salient-pole rotors are
normally used for rotors with four or more poles.
Because the rotor is subjected to changing magnetic fields, it is constructed of thin laminations to reduce
eddy current losses.
• Salient Pole
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Cross-sectional view of salient pole rotor
• The salient-pole construction is used in comparatively small machines and machines driven at a
relatively low speed. For instance, if a 50 Hz synchronous machine is to operate at, say, 375 r/min,
then,
120𝑓 50
𝑃 = ⎯⎯⎯⎯⎯= 120 × ⎯⎯⎯ = 16
𝑁 375
The machine must have 16 poles; and to accommodate all these poles, the synchronous machine
must have a comparatively large diameter.
• Since the output of a machine is roughly proportional to its volume, such a synchronous machine
would have a small axial length.
• The field winding is usually an insulated copper strip wound on edge. The pole tips are well rounded
so as to make the flux distribution around the periphery nearer a sine wave and thus improve the
waveform of the generated emf.
Salient pole rotor
• Nonsalient Pole
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Cross-sectional view of cylindrical rotor
Cylindrical rotor with two-pole, eight slots and two conductors per slot
• Most synchronous machines are essentially high-speed machines. The centrifugal force on a high-
speed rotor is enormous. To withstand such a force the rotor is usually made of a solid steel forging
with longitudinal slots cut.
• In an actual rotor there are more slots and far more conductors per slot; and the winding is in the
form of insulated copper strip held securely in position by phosphor-bronze wedges.
• The regions forming the centers of the poles are usually left unslotted.
• In addition to its mechanical robustness, this cylindrical construction has the advantage that the flux
distribution around the periphery is nearer a sine wave than is the case with the salient-pole
machine. Consequently, a better emf waveform is obtained.
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machine. Consequently, a better emf waveform is obtained.
• Because of the smooth surface, this rotor has less winding loss compared to the salient pole type
rotor.
Cylindrical rotor
Excitation of field circuit
A dc current must be supplied to the field circuit on the rotor if it is an electromagnet. Since the rotor is
rotating, a special arrangement is required to get the dc power to its field windings.
There are two common approaches to supplying this dc power:
1. Supply the dc power from an external dc source to the rotor by means of slip rings and brushes.
2. Supply the dc power from a special dc power source mounted directly on the shaft of the
synchronous generator.
Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. One end of
the dc rotor winding is tied to each of the two slip rings on the shaft of the synchronous machine, and a
stationary brush rides on each slip ring. A "brush" is a block of graphite like carbon compound that
conducts electricity freely but has very low friction, so that it doesn't wear down the slip ring.
On larger generators and motors, brushless exciters are used to supply the dc field current to the
machine. A brushless exciter is a small ac generator with its field circuit mounted on the stator and its
armature circuit mounted on the rotor shaft.
Windings
There are two windings made of copper. These are placed in stator and rotor slots (or wound on
projecting poles). In synchronous machines the main field is created by the field poles (even in number)
and dc excited. The other winding which interchanges electric power with the external circuit and so
carries the load current is called the armature winding and in the seat of induced emf.
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Why Rotating Field machines are preferred?
Let us consider why synchronous machines, whether motors or generators, are usually constructed with
stationary armature windings and rotating poles. Suppose we have a 20 MVA, 11 kV, three-phase
synchronous machine; then
⎯⎯
𝑆 = √3𝑉 𝐼
⎯⎯
20 × 10 = √3 × 11 × 10 × 𝐼
∴ Line current = 𝐼 = 1050 𝐴
Hence, if the machine was constructed with stationary poles and a rotating three-phase winding, three
slip-rings would be required, each capable of dealing with 1050 A, and the insulation of each ring together
⎯⎯
with that of the brush would be subjected to a working voltage of 11/√3, namely 6.35 kV.
By using a stationary ac winding and a rotating field system, only two slip-rings are necessary and these
have to deal with the exciting current.
Assuming the power required for exciting the poles of the above machine to be 150 kW and the voltage
to be 400 V,
Exciting current = 150 × 10 /400 = 375 𝐴
In other words, the two slip-rings and brush gear would have to deal with only 375 A and be insulated for
merely 400 V. Hence, by using a stationary ac winding and rotating poles, the construction is considerably
simplified and the slip-ring losses are reduced.
Further advantages of this arrangement are:
1. The extra space available for the ac winding makes it possible to use more insulation and to enable
operating voltages of up to 33 kV.
2. With the simpler and more robust mechanical construction of the rotor, a high speed is possible, so
that a greater output is obtainable from a machine of given dimensions.
Cylindrical Rotor from Siemens Energy
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Stator and Rotor from Siemens Energy
Synchronous Generator Assembly from Siemens Energy
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11 March 2022
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Slip Ring Induction motor Squirrel Cage Induction motor
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Stator and rotor of three phase induction motor
Principle of operation:
Three phase induction motor works on the principle of electromagnetic induction.
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• Consider a slip ring induction motor with rotor windings in open position, initially. (S is
open).
• The stator is supplied by a three phase supply with supply frequency '𝑓' and phase
voltage '𝑉'.
• The stator current set up rotating magnetic field in the air gap which runs at synchronous
speed inducing EMF in the stator winding.
• This EMF will be equal to the voltage applied (terminal voltage) if stator resistance and
leakage reactance are neglected.
• Rotating magnetic field also induces EMF in the rotor windings but no rotor current flows
as rotor is open circuited.
• The frequency of rotor EMF is '𝑓'.
→
• Since the rotor mmf 𝐹 = 0 (as rotor current = 0 ) no torque is developed and rotor
remains stationary.
• The machine acts merely as a transformer where the stator (primary) and rotor
(secondary) have emfs of the same frequency induced in them by the
rotating magnetic flux rather than by a stationary time-varying flux as in an ordinary
transformer.
• Let the rotor be now held stationary (blocked from rotation) and the rotor winding be
short-circuited.
→
• The rotor now carries 3-phase currents creating the mmf 𝐹 rotating in the same
direction and with the same speed as the stator field.
𝐹
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→
• 𝐹 causes reaction currents to flow into the stator from supply (just as in an ordinary
transformer) such that the flux/pole ϕ of the resultant flux density wave (rotating in the
air-gap at synchronous speed) remains the same as when the rotor was open-circuited.
→
• The interaction of ϕ and 𝐹 , which are stationary with respect to each other, creates
→
the torque tending to move the rotor in the direction of 𝜙 or the stator field 𝐹 .
Induced torque in an ac machine
○ In ac machines under normal operating conditions, there are two magnetic fields
present- a magnetic field from the rotor circuit and another magnetic field from
the stator circuit. The interaction of these two magnetic fields produces the torque
in the machine(just as two permanent magnets near each other will experience a
torque which causes them to line up).
A simplified ac machine with a sinusoidal stator flux distribution that peaks
in the upward direction and a single coil of wire mounted on the rotor.
○ The stator flux distribution in this machine is
𝐵 (𝛼) = 𝐵 sin 𝛼
○ The induced force on conductor 1 is
𝐅 = 𝑖(𝐥 × 𝐁) = 𝑖𝑙𝐵 sin 𝛼 (direction gven in figure)
𝑙 is length of coil.
The torque on the conductor is
𝛕 , = (𝐫 × 𝐅) = 𝑟𝑖𝑙𝐵 sin 𝛼 (Counterclockwise)
○ The induced force on conductor 2 is
𝐅 = 𝑖(𝐥 × 𝐁) = 𝑖𝑙𝐵 sin 𝛼 (direction gven in figure)
The torque on the conductor is
𝛕 , = (𝐫 × 𝐅) = 𝑟𝑖𝑙𝐵 sin 𝛼 (Counterclockwise)
○ Therefore, the torque on the rotor loop is
𝛕 = 2𝑟𝑖𝑙𝐵 sin 𝛼 (Counterclockwise)
• Let the short-circuited rotor be now permitted to rotate. It runs in the direction of the
stator field and acquires a steady speed of 𝑛.
• Obviously 𝑛 < 𝑛 , because if 𝑛 = 𝑛 , the relative speed between the stator field and
rotor winding will be zero and therefore the induced emfs and rotor currents will be zero
and hence no torque is developed.
• The rotor thus cannot reach the synchronous speed 𝑛 and hence cannot exceed 𝑛 .
• With the rotor running at 𝑛, the relative speed of the stator field with respect to rotor
conductors is (𝑛 – 𝑛) in the direction of 𝑛 .
• The frequency of induced emfs (and currents) in the rotor is therefore
𝑝 𝑛 𝑝 (𝑛 − 𝑛)
𝑓 = ⎯⎯⎯ (𝑛 − 𝑛) = ⎯⎯⎯ ⎯⎯⎯⎯⎯⎯⎯⎯
120 120 𝑛
= 𝑠𝑓
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𝑛 −𝑛
⇒ 𝑠 = ⎯⎯⎯⎯⎯⎯ (𝑠𝑙𝑖𝑝 𝑜𝑓 𝑟𝑜𝑡𝑜𝑟)
𝑛
• Since the rotor is running at a speed 𝑛 and the rotor field at (𝑛 – 𝑛) with respect to the
rotor in the same direction, the net speed of the rotor field as seen from the stator
(ground reference) is
𝑛 + (𝑛 – 𝑛) = 𝑛
i.e., the same as the stator field.
→
• Thus the reaction field 𝐹 of the rotor is always stationary with respect to the
→ →
stator field 𝐹 or the resultant field 𝐹 (with flux ϕ per pole).
→
• Since the rotor mmf 𝐹 is proportional to the rotor current 𝐼 and the resultant flux/pole
ϕ is fixed by terminal voltage independent of operating conditions,
the induction motor torque is given by,
𝑇 ∝ 𝐼 sin 𝛿
→ →
where 𝛿 is angle by which 𝐹 lags behind 𝐹
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15 March 2022
Basics of Power System
Simple Power System
• Generators supply the electrical energy. Transmission and distribution networks of overhead
lines and underground cables deliver the energy to consumers where all manner of electrical
loads, from motors to TVs, use it.
• Transformers initially raise the generated voltage for efficient transmission over long distances
and thereafter decrease the system voltage for local distribution and utilization.
• High voltage ensures less current for same magnitude of power transfer. Which lowers 𝐼 𝑅
loss and line voltage drop 𝐈𝐳.
• Electrical energy is most efficiently supplied by three-phase systems. The electrical loads are
arranged to ensure that the currents in each of the three individual phases are roughly equal.
In this condition, a power system is said to be balanced.
Single Line Diagram of Power System
Single line diagram of simple power system
• A single line diagram representation of power system in which the components are
represented by their symbols and interconnection between them is shown by a straight line
even though the system is three phase system.
• The rating and the impedances of the components are also marked on the single line diagram.
• The purpose of the single line diagram is to provide significant information about the system in
concise form.
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Single line diagram of WSCC 9 bus system
Elements of Power System
The main components of a power system are:
a. power plant
b. transformer
c. transmission line/ cables
d. substations
e. distribution line
f. distribution transformer
Schematic diagram depicting power system structure
• The power plant generates the power which is step-up or step-down through the transformer
for transmission.
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• The transmission line transfers the power to the various substations.
• Through substation, the power is transferred to the distribution transformer which step-down
the power to the appropriate value which is suitable for the consumers.
• Generating stations, transmission lines and the distribution systems are the main components
of an electric power system.
• Generating stations and a distribution system are connected through transmission lines, which
also connect one power system (grid, area) to another.
• A distribution system connects all the loads in a particular area to the transmission lines.
• Electric power is generated at a voltage of 11 to 25 kV which then is stepped up to the
transmission levels in the range of 66 to 765 kV (or higher).
• In India, several 400 kV lines are already in operation. Several 765 kV lines have been built so
far in India.
• The first stepdown of voltage from transmission level is at the bulk power substation, where
the reduction is to a range of 33 to 132 kV. Some industries may require power at these
voltage levels. This stepdown is from the transmission and grid level to sub-transmission level.
• The next stepdown in voltage is at the distribution substation. Normally, two distribution
voltage levels are employed.
○ The primary or feeder voltage (11 kV).
○ The secondary or consumer voltage (415 V three phase/230 V single phase).
• The distribution system, fed from the distribution transformer stations, supplies power to the
domestic or industrial and commercial consumers.
Generation
• In generation system the fuel (coal, water, nuclear energy, etc.) is converted into electrical
energy. The electrical power is generated in the range of 11kV to 25kV, which is step-up for
long distance transmission.
• Conventional power plant of the generating substation is mainly classified into three types,
i.e., thermal power plant, hydropower plant and nuclear power plant.
• The generator and the transformer are the main components of the generating station.
• The generator converts the mechanical energy into electrical energy. The mechanical energy
comes from the burning of coal, gas and nuclear fuel, gas turbines, or occasionally the internal
combustion engine.
• The transformer transfers the power with very high efficiency from one level to another.
• The step-up transformer will reduce losses in the line which facilitates the transmission of
power over long distances.
Transmission
• The network that transmits and delivers power from the producers to the consumers is called
the transmission system.
• The transmission system constitutes the overhead lines which transfer the generated electrical
energy from generation to the distribution substations. It only supplies the large bulk of power
to bulk power substations or very big consumers.
• This energy can be transmitted in AC or DC form.
• Traditionally, AC has been used for years now, but HVDC (High Voltage DC) is rapidly gaining
popularity.
The transmission lines mainly perform the two functions:
1. It transports the energy from generating stations to bulk receiving stations.
2. It interconnects the two or more generating stations. The neighboring substations are also
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2. It interconnects the two or more generating stations. The neighboring substations are also
interconnected through the transmission lines.
• The transmission voltage is at more than 66 kV.
• The transmission line above 230 kV is usually referred to as extra high voltage (EHV).
• The high voltage line is terminated in substations which are called high voltage substations,
receiving substations or primary substations.
• In high voltage substation, the voltage is step-down to a suitable value for the next part of flow
toward the load.
• Very large industrial consumers may be served directly to the transmission system.
Sub-transmission System
• The portion of the transmission system that connects the high voltage substations through the
step-down transformer to the distribution substations is called the sub-transmission system.
• The sub-transmission voltage level ranges from 90 to 138KV. The sub-transmission system
directly serves some large industries.
Distribution
• The component of an electrical power system connecting all the consumers in an area to the
bulk power sources is called a distribution system.
• Substations are usually situated at convenient points near the load centers.
• The substations distribute the power to the domestic, commercial and relatively small
consumers.
Electric Power Generation
• Thermal (coal, oil, nuclear) and hydro generations are the main conventional sources of
electric energy.
• Some of the non-conventional sources being explored are solar, wind and tidal sources.
• A panoramic view of energy conversion to electrical form is presented.
Thermal Power Plant
• Thermal power station is the most conventional type of power plant.
• In these power plants, a fossil fuel such as coal is burned to produce heat. This heat is then
used to boil the water and convert it into the superheated steam. The superheated steam is
passed into a steam turbine. Blades of the turbine are rotated due to the pressure of the
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passed into a steam turbine. Blades of the turbine are rotated due to the pressure of the
steam. The steam turbine drives an alternator which is mechanically coupled to it. When rotor
of the alternator rotates, electricity is generated.
• These plants are sometimes called as steam power plants.
• Efficiency of thermal power plants is about 30%.
• The main disadvantage of a thermal power plant is efficiency and pollution.
• Coal: In a coal based thermal power plant, the coal from live storage is first crushed in small
particles and then taken into pulveriser to make it in powdered form. Fine powdered coal
undergoes complete combustion, and thus pulverised coal improves efficiency of the boiler.
• The ash produced after the combustion of coal is taken out of the boiler furnace and then
properly disposed.
• Boiler: The mixture of pulverized coal and air (usually preheated air) is taken into boiler and
then burnt in the combustion zone. On ignition of fuel a large fireball is formed at the center
of the boiler and large amount of heat energy is radiated from it.
• The heat energy is utilized to convert the water into steam at high temperature and pressure.
• Steel tubes run along the boiler walls in which water is converted in steam. The flue gases
from the boiler make their way through superheater, economizer, air preheater and finally get
exhausted to the atmosphere from the stack/chimney.
• Superheater: The superheater tubes are hanged at the hottest part of the boiler. The
saturated steam produced in the boiler tubes is superheated to about 540 °C in the
superheater. The superheated high-pressure steam is then fed to the steam turbine.
• Economizer: An economizer is essentially a feed water heater which heats the water before
supplying to the boiler.
• Air pre-heater: The primary air fan takes air from the atmosphere, and it is then warmed in
the air pre-heater. Pre-heated air is injected with coal in the boiler.
• Superheater, economizer, air-preheater improve efficiency of plant.
• Steam turbine: High pressure super-heated steam is fed to the steam turbine which causes
turbine blades to rotate. Energy in the steam is converted into mechanical energy in the steam
turbine which acts as the prime mover.
• The pressure and temperature of the steam falls to a lower value and it expands in volume as
it passes through the turbine.
• The expanded low-pressure steam is exhausted in the condenser.
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• Condenser: The exhausted steam is condensed in the condenser by means of cold-water
circulation. Here, the steam loses its pressure as well as temperature and it is converted back
into water.
• Alternator: The steam turbine is coupled to an alternator. When the turbine rotates the
alternator, electrical energy is generated.
• This generated electrical voltage is then stepped up with the help of a transformer and then
transmitted where it is to be utilized.
• Feed water pump: The condensed water is again fed to the boiler by a feed water pump.
Some water may be lost during the cycle, which is suitably supplied from an external water
source.
• A practical thermal plant possesses more complicated design and multiple stages of turbine
such as High-Pressure Turbine (HPT), Intermediate-Pressure Turbine (IPT) and Low-Pressure
Turbine (LPT).
Advantages And Disadvantages Of A Thermal Power Plant
Advantages:
• Less initial cost as compared to other generating stations.
• It requires less land as compared to hydro power plant.
• The fuel (i.e., coal) is cheaper.
• The cost of generation is lesser than that of diesel power plants.
Disadvantages:
• It pollutes the atmosphere due to the production of large amount of smoke. This is one of the
causes of global warming.
• The overall efficiency of a thermal power station is low (around 30%).
Efficiency Of A Thermal Power Station
• A huge amount of heat is lost in various stages of the plant. Major part of heat is lost in the
condenser. That is why the efficiency of thermal plants is quite low.
Thermal Efficiency
.
Thermal Efficiency = ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
• Thermal efficiency of modern thermal power stations is about 30%. It means, if 100 calories of
heat are produced by coal combustion, the mechanical energy equivalent of 30 calories will be
available at the turbine shaft.
Overall Efficiency
Heat equivalent of Electrical output
Overall Efficiency = ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
Heat produced by coal combustion
• The overall efficiency of a thermal plant is about 29% (slightly less than the thermal efficiency).
Nuclear Power Plant
• Nuclear power stations utilize nuclear fission reactions to generate heat for steam generation.
Fission is the splitting of the nucleus of an atom into lighter nuclei producing gamma rays, free
neutrons and other subatomic particles. Fission of heavy elements releases large amounts of
energy as electromagnetic radiation and as kinetic energy of the fission products which heats
the power reactor vessel and the working fluid, usually water, which conducts heat away to a
steam generator. The available energy contained in nuclear fuel is millions of times greater
than that contained in a similar mass of a chemical fuel such as oil.
• This considerably reduces the transportation cost of fuel, which is a major advantage of
nuclear power plants.
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• Nuclear Reactor: Nuclear fission takes place in nuclear reactor. Since the nuclear fission is
radioactive, the reactor is covered by a protective shield. Splitting up of nuclei of heavy atoms
is called as nuclear fission, during which huge amount of energy is released. Nuclear fission is
done by bombarding slow moving neutrons on the nuclei of heavy element. As the nuclei
break up, it releases energy as well as more neutrons which further cause fission of
neighboring atoms. Hence, there is a chain reaction, which must be controlled, otherwise it
may result in explosion.
• A nuclear reactor consists of fuel rods, control rods and moderator.
• A fuel rod contains small round fuel pallets (uranium pallets).
• Control rods are of cadmium which absorb neutrons. They are inserted into reactor and can be
moved in or out to control the reaction.
• The moderator can be graphite rods or the coolant itself. Moderator slows down the neutrons
before they bombard on the fuel rods.
Two types of nuclear reactors that are widely used -
1. Pressurized Water Reactor (PWR):
This type of reactor uses regular water as coolant. The coolant (water) is kept at very high
pressure so that it does not boil. The heated water is transferred through heat exchanger
where water from secondary coolant loop is converted into steam.
Advantages of pressurized water reactors
○ The secondary loop is completely free from radioactive stuff.
○ In a PWR, the coolant water itself acts as a moderator.
2. Boiling Water Reactor (BWR):
In this type of reactor only one coolant loop is present. The water is allowed to boil in the
reactor. The steam is generated as it heads out of the reactor and then flows through the
steam turbine.
○ One major disadvantage of a BWR is that the coolant water comes in direct contact with
fuel rods as well as the turbine.
• Heat Exchanger: In the heat exchanger, the primary coolant transfers heat to the secondary
coolant (water). Thus, water from the secondary loop is converted into steam. The primary
system and secondary system are closed loop, and they are never allowed to mix up with each
other. Thus, heat exchanger helps in keeping secondary system free from radioactive stuff.
Heat exchanger is absent in boiling water reactors.
Concept of Green Energy
The term green or renewable energy describes the energy flows that recur naturally in the
environment, such as solar radiation, the wind, the tides and the waves. The origin of these sources
is either the sun (solar energy helps to shape the earth’s weather patterns) or the gravitational
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is either the sun (solar energy helps to shape the earth’s weather patterns) or the gravitational
effects of the moon and the sun. Such sources are sustainable energy sources since they are not
depleted by continued use. However, they are difficult to convert into more useful forms of energy,
particularly electrical energy.
Renewable energy sources also have a much smaller impact on the environment than fossil fuels,
which produce pollutants such as greenhouse gases as a by-product, contributing to climate change.
Gaining access to fossil fuels typically requires either mining or drilling deep into the earth, often in
ecologically sensitive locations. Green energy utilizes energy sources that are readily available all
over the world, including in rural and remote areas that don't otherwise have access to electricity.
Hydro:
Most hydroelectric power comes from the potential energy of water trapped behind a dam. The
water is released through the penstock, driving the water turbine and generator. The potential
energy stored is proportional to the volume of water stored and the head, the difference in height
between the reservoir surface and the outflow to the
turbine.
Wind:
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Wind is an inexhaustible resource and is currently the cheapest form of renewable energy. A wind
turbine obtains its power input by converting the force of the wind into a torque (turning force)
acting on the rotor blades. Most modern wind turbines are now broadly similar: the three-bladed,
horizontal-axis upwind design.
Solar photovoltaic (PV) energy conversion:
Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials
exhibit a property known as the photoelectric effect that causes them to absorb photons of light and
release electrons. When these free electrons are captured, an electric current results that can be
used as electricity.
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The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar
cells are made of the same kinds of semiconductor materials, such as silicon, used in the
microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an
electric field, positive on one side and negative on the other. When light energy strikes the solar cell,
electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors
are attached to the positive and negative sides, forming an electrical circuit, the electrons can be
captured in the form of an electric current -- that is, electricity. This electricity can then be used to
power a load, such as a light or a tool.
A number of solar cells electrically connected to each other and mounted in a support structure or
frame is called a photovoltaic module. Modules are designed to supply electricity at a certain
voltage, such as a common 12 volts system. The current produced is directly dependent on how
much light strikes the module.
Multiple modules can be wired together to form an array. In general, the larger the area of a module
or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-
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or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-
current (dc) electricity. They can be connected in both series and parallel electrical arrangements to
produce any required voltage and current combination.
Today's most common PV devices use a single junction, or interface, to create an electric field within
a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose energy is equal to
or greater than the band gap of the cell material can free an electron for an electric circuit. In other
words, the photovoltaic response of single-junction cells is limited to the portion of the sun's
spectrum whose energy is above the band gap of the absorbing material, and lower-energy photons
are not used.
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