Lecture on Power Systems in engineering.pptx

Electrical Power Engineering
Dr. Saima Zafar
Department of Electrical Engineering
FAST-NU, Lahore
Modifications By: Mohsin Yousuf

1-2
Electrical Energy
 Energy exists in different forms in nature but the most important
form is the electrical energy. Electrical energy is superior to all other
forms of energy due to the following reasons :
 (i) Convenient form. It can be easily converted into other forms of
energy, heat, light, mechanical etc.
 (ii) Easy control. The electrically operated machines have simple and
convenient starting, control and operation.
 (iii) Greater flexibility. It can be easily transported from one place
to another with the help of conductors.
 (iv) Cheap.
 (v) Cleanliness. Electrical energy is not associated with smoke, fumes
or poisonous gases.
 (vi) High transmission efficiency. It can be transmitted conveniently
and efficiently from the centers of generation to the consumers with
the help of conductors known as transmission lines.
Power Generation

1-3
Generation of Electrical Energy
 The conversion of energy available in different forms
in nature into electrical energy is known as generation
of electrical energy.
 The electrical energy must be produced and transmitted
to the point of use at the instant it is needed.
 Energy is available in various forms from different
natural sources such as pressure head of water, chemical
energy of fuels, nuclear energy of radioactive
substances etc. All these forms of energy can be
converted into electrical energy by the use of suitable
arrangements.
Power Generation

1-4
 The arrangement essentially employs an alternator
coupled to a prime mover.
 The prime mover is driven by the energy obtained
from various sources.
 For example, chemical energy of
a fuel (e.g., coal) can be used to
Produce steam at high temperature
and pressure. The steam is fed to a
prime mover which may be a steam
engine or a steam turbine.
The turbine converts heat energy of steam into mechanical energy
which is further converted into electrical energy by the
alternator.
Power Generation
Generation of Electrical Energy

1-5
Sources of Energy
 The sources of energy for the generation of electrical energy are :
(i) The Sun: The heat energy by Sun can be focused over a small area
by means of reflectors. This heat can be used to raise steam and
electrical energy can be produced by turbine-alternator
combination.
(ii) The Wind: The wind energy is used to run the wind mill which
drives a small generator. In order to obtain the electrical energy
from a wind mill continuously, the generator is arranged to charge
the batteries. These batteries supply the energy when the wind
stops.
(iii) Water: Water stored at a suitable place, possesses potential
energy because of the head created. This water energy can be
converted into mechanical energy by water turbines which drives the
alternator which converts mechanical energy into electrical energy.
Power Generation

1-6
Sources of Energy
(iv) Fuels: Solid fuel(coal), liquid fuel (oil) and gas fuel(natural gas)
Heat energy of these fuels is converted into mechanical energy by
suitable prime movers such as steam engines, steam turbines, internal
combustion engines etc. The prime mover drives the alternator which
converts mechanical energy into electrical energy.
Although fuels continue to enjoy the place of chief source for the
generation of electrical energy, yet their reserves are diminishing
day by day. Therefore, the present trend is to harness water power
which is more or less a permanent source of power.
(v) Nuclear energy: Large amount of heat energy is liberated by the
fission of uranium and other fissionable materials. It is estimated
that heat produced by 1 kg of nuclear fuel is equal to that
produced by 4500 tones of coal. The heat produced due to nuclear
fission can be utilized to raise steam with suitable arrangements. The
steam can run the steam turbine which in turn can drive the
alternator to produce electrical energy.

1-7
Comparison of Energy Sources
Power Generation

1-8
Units of Energy
 The capacity of an agent to do work is known as its
energy.
 The most important forms of energy are
 Mechanical energy
 Electrical energy and
 Thermal energy
 Different units have been assigned to various forms
of energy.
 Since mechanical, electrical and thermal energies
are interchangeable, it is possible to assign the same
unit to them.
Power Generation

1-9
Mechanical Energy
 The unit of mechanical energy is newton-meter or joule
on the M.K.S. or SI system.
 The work done on a body is one newton-meter (or joule)
if a force of one newton moves it through a distance of
one meter i.e.,
Mechanical energy in joules = Force in newton × distance in meters
Power Generation

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Electrical Energy
 The unit of electrical energy is watt-sec or joule and is
defined as follows:
 One watt-second (or joule) energy is transferred
between two points if a p.d. of 1 volt exists between
them and 1 ampere current passes between them for 1
second i.e.,
Electrical energy in watt-sec (or joules) = voltage in volts ×
current in amperes × time in seconds
 1 watt-hour = 1 watt×1 hr = 1 watt×3600 sec = 3600
watt-sec
 1 kilowatt hour (kWh) = 1 kW × 1 hr = 1000 watt × 3600
sec = 36 x 105
watt-sec.
Power Generation

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Thermal Energy (Heat)
 Heat is a form of energy which produces the sensation of
warmth.
 The unit of heat is calorie. It is the amount of heat
required to raise the temperature of 1 gm of water
through 1ºC
1 calorie = 1 gm of water × 1ºC
1 kilo-calorie = 1 kg × 1ºC = 1000 gm × 1ºC = 1000 calories
Power Generation

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Relationship among Energy Units
 Electrical and Mechanical
1 kWh = 1 kW × 1 hr = 1000 watts × 3600 seconds
= 36×105
watt-sec (Joules)
∴ 1 kWh = 36 × 105
Joules
 Heat and Mechanical
1 calorie = 4·18 Joules
 Electrical and Heat
1 kWh = 1000 watts × 3600 seconds
= 36 × 105
Joules = 36 x 105
/4.18 calories = 860 × 103
calories
∴ 1 kWh = 860 × 103
calories or 860 kcal
Power Generation

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Efficiency
 When energy (from different natural sources) is converted into
electrical energy, in the process of conversion, some energy is
lost in the sense that it is converted to a form different from
electrical energy.
 Therefore, the output energy is less than the input energy.
 The output energy divided by the input energy is called energy
efficiency or efficiency of the system.
 Since Power is the rate of energy flow
Power Generation

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Calorific Value Fuels
 The amount of heat produced by the complete combustion
of a unit weight of fuel is known as its calorific value.
 Calorific value indicates the amount of heat available from
a fuel. The greater the calorific value of fuel, the larger is
its ability to produce heat.
 In case of solid fuels, the calorific value is expressed in
cal/gm or kcal/kg
 In case of liquid fuels, it is expressed in kcal/liter.
 In case of gaseous fuels, it is generally stated in kcal/m3
.
Power Generation

Power Generation 1-15
Advantages of Liquid Fuels over Solid Fuels
(i) The handling of liquid fuels is easier and they require less
storage space.
(ii) The combustion of liquid fuels is uniform.
(iii) The solid fuels have higher percentage of moisture and
consequently they burn with great difficulty. However,
liquid fuels can be burnt with a fair degree of ease and
attain high temperature very quickly compared to solid
fuels.
(iv) The waste product of solid fuels is a large quantity of
ash and its disposal becomes a problem. However, liquid
fuels leave no or very little ash after burning.
(v) The firing of liquid fuels can be easily controlled. This
permits to meet the variation in load demand easily.

(i) In case of liquid fuels, there is a danger of explosion.
(ii) Liquids fuels are costlier as compared to solid fuels.
(iii) Sometimes liquid fuels give unpleasant odour during
burning.
(iv) Liquid fuels require special types of burners for burning.
(v) Liquid fuels pose problems in cold climates since the oil
stored in the tanks is to be heated in order to avoid the
stoppage of oil flow.
Advantages of Solid fuels over Liquid Fuels

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Electric Power System (basic building blocks)
Power Generation

1-18
Electric Power System
 Electric power systems are real-time energy delivery
systems.
 The system starts with generation.
 High-voltage (HV) power lines in the transmission
portion of the electric power system efficiently
transport electrical energy over long distances to the
consumption locations.
 Finally, substations transform this HV electrical
energy into lower-voltage energy that is transmitted
over distribution power lines that are more suitable
for the distribution of electrical energy to its
destination, where it is again transformed for
residential, commercial, and industrial consumption.
Power Generation

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AC Voltage Generation
Two physical laws are used throughout the entire
electric power system from generation through
transmission, distribution, and consumption.
1. “A voltage is produced on any conductor in a
changing magnetic field.”
2. “A changing current flowing in a wire produces a
magnetic field around the wire.”
Power Generation

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 If one takes a coil of wire and puts it next to a
moving or rotating magnet, a measurable voltage will
be produced in that coil. This voltage is then
distributed throughout the electric power system.
 All generators in service today have coils of wire
mounted on stationary housings, called stators, where
voltage is produced due to the magnetic field
provided on the spinning rotor. The rotor is
sometimes called the field.
Power Generation

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 The amplitude of the generator’s output voltage can
be changed by changing the strength of rotor’s
magnetic field. Thus, the generator’s output voltage
can be lowered by reducing the rotor’s magnetic field
strength.
Power Generation

Alternator

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Single-Phase ac Voltage Generation
 Placing a coil of wire (i.e.,
conductor) in the presence
of a moving magnetic
field produces a voltage
 Changing the rotor’s speed
changes the frequency of
the sine wave.
 Increasing the number of
turns (loops) of conductor
or wire in the coil
increases the resulting
output voltage.
Power Generation

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Three-Phase ac Voltage Generation
 When three coils are placed in the presence of a
changing magnetic field, three voltages are
produced. When the coils are spaced 120 degrees
apart in a 360 degree circle, three-phase ac voltage
is produced.
 Three-phase generation can be viewed as three
separate single-phase generators, each of which are
displaced by 120 degrees.
Power Generation

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Three-phase ac Generator (AVR)
 Large power plant generators use electromagnets so that
the magnetic field can be varied.
 Varying the magnetic field strength of the rotor enables
generation control systems to adjust the output voltage
according to load demand and system losses.
 The operation of electromagnets is described by Physical
Law #2.
Power Generation

Real-time Generation
 Power plants produce electrical energy on a real-time
basis. Electric power systems do not store energy such
as most gas or water systems do.
 Electrical generation always produces electricity on an
“as needed” basis.

Generator Connections
 There are two ways to connect three windings that have a
total of six leads (the ends of the winding wires)
symmetrically.
 The two symmetrical connection configurations of a three-
phase generator (or motor) are called delta and wye.
 Generators usually have their stator windings connected
internally in either a delta or wye configuration.
 The generator nameplate specifies which winding
configuration is used on the stator.

Delta
 Delta configurations have all three windings connected
in series.
 The phase leads are connected to the three common
points where windings are joined.
Wye
 The wye configuration connects one lead from each
winding to form a common point called the neutral. The
other three phase leads are brought out of the
generator separately for external system connections.
The neutral is often grounded to the station ground
grid for voltage reference and stability.

 Electric power plant generators use either wye or delta
connections.
 The phase leads from the generator are connected to
the plant’s step-up transformer where the generator
output voltage is increased significantly to transmission
voltage levels for the efficient transportation of
electrical energy. (WHY?)

Power-plants and Prime Movers
Power generation plants produce the electrical energy
that is ultimately delivered to consumers through
transmission lines, substations, and distribution lines.
 Generation plants or power plants consist of
 three-phase generator(s),
 the prime mover (turbine),
 energy source,
 control room, and
 substation.
The generator portion has been discussed already. The
prime movers and their associated energy sources will
be discussed now.

 The mechanical means of turning the generator’s rotor is
called the prime mover.
The prime mover’s energy sources include the conversion
process of raw fuel, such as coal, to the end product—
steam—that will turn the turbine.
The bulk of electrical energy produced in today’s
interconnected power systems is normally produced
through a conversion process from coal, oil, natural gas,
nuclear, and hydro. To a lesser degree, electrical power is
produced from wind, solar, geothermal, and biomass energy
resources.

Depending upon the form of energy converted into electrical
energy, the generating stations are classified as under :
(i) Steam power stations
(ii) Hydroelectric power stations
(iii) Diesel power stations
(iv) Nuclear power stations

Steam Power Station (Thermal Station)
 A generating station which converts heat energy of coal combustion into
electrical energy is known as a steam power station.

Coal based Thermal Power Plant

 High-pressure and high-temperature steam is created in a
boiler, furnace and moved through a steam turbine generator
(STG) that converts the steam’s energy into rotational energy
that turns the generator shaft.
 The steam turbine’s rotating shaft is directly coupled to the
generator rotor. The STG shaft speed is tightly controlled for
it is directly related to the frequency of the electrical power
being produced.
 Temperatures on the order of 1,000°F and pressures on the
order of 2,000 pounds per square inch (psi) are commonly used
in large steam power plants.
 Steam at this pressure and temperature is called superheated
steam, sometimes referred to as dry steam.

Advantages
(i) The fuel (i.e., coal) used is quite cheap.
(ii) Less initial cost as compared to other generating stations.
(iii) It can be installed at any place. The coal can be transported to
the site of the plant by rail/road.
(iv) It requires less space as compared to the hydroelectric power
station.
(v) The cost of generation is lesser than that of the diesel power
station.
Disadvantages
(i) It pollutes the atmosphere due to the production of large amount
of smoke and fumes.
(ii) It is costlier in running cost as compared to hydroelectric plant.

 The schematic arrangement of a modern steam power station is shown in
Figure. The whole arrangement can be divided into the following stages
for the sake of simplicity (equipment is discussed separately):
1. Coal and ash handling arrangement
2. Steam generating plant
3. Steam turbine
4. Alternator
5. Feed water
6. Cooling arrangement
Schematic arrangement (Steam Power Station)

Choice of site for Steam Power Stations
 Supply of fuel
 Availability of water
 Transportation facilities
 Cost and type of land
 Nearness to load centers
 Distance from populated areas

 The most important constituents of a steam power station are:
1. Steam generating equipment
2. Condenser
3. Prime mover
4. Water treatment plant
5. Electrical equipment
Equipment of Steam Power Station

 Water treatment plant
Boilers require clean and soft water for longer life and better
efficiency. However, the source of boiler feed water is generally a
river or lake which may contain suspended and dissolved impurities,
dissolved gases etc. Therefore, it is very important that water is
first purified and softened by chemical treatment and then delivered
to the boiler.
The water from the source of supply is stored in storage tanks. The
suspended impurities are removed and dissolved gases are removed.
The water is then ‘softened’ by removing temporary and permanent
hardness through different chemical processes.
The pure and soft water thus available is fed to the boiler for steam
generation.

 Electrical equipment
A modern power station contains numerous electrical equipment.
However, the most important items are :
(i) Alternators. Each alternator is coupled to a steam turbine and
converts mechanical energy of the turbine into electrical energy. The
alternator may be hydrogen or air cooled. The necessary excitation is
provided by means of main and pilot exciters directly coupled to the
alternator shaft.
(ii) Transformers. A generating station has different types of
transformers, viz.,
(a) main step-up transformers which step-up the generation voltage
for transmission of power.
(b) station transformers which are used for general service (e.g.,
lighting) in the power station.
(c) auxiliary transformers which supply to individual unit-auxiliaries.
(iii) Switchgear. It houses such equipment which locates the fault on the
system and isolate the faulty part from the healthy section. It
contains circuit breakers, relays, switches and other control devices.

Efficiency of Steam Power Plant
 The overall efficiency of a steam power station is quite low (about
29%) due mainly to two reasons:
 Firstly, a huge amount of heat is lost in the condenser and secondly
heat losses occur at various stages of the plant.

Working of Steam Turbine

 Thermal Efficiency
The ratio of heat equivalent of mechanical energy transmitted to the
turbine shaft to the heat of combustion of coal is known as thermal
efficiency of steam power station.
The thermal efficiency of a modern steam power station is about 30%. It
means that if 100 calories of heat is supplied by coal combustion, then
mechanical energy equivalent of 30 calories will be available at the
turbine shaft and rest is lost. It may be important to note that more
than 50% of total heat of combustion is lost in the condenser. The other
heat losses occur in flue gases, radiation, ash etc.

 Overall Efficiency
The ratio of heat equivalent of electrical output to the heat of
combustion of coal is known as overall efficiency of steam power
station i.e.
The overall efficiency of a steam power station is about 29%. It may be
seen that overall efficiency is less than the thermal efficiency. This is
expected since some losses (about 1%) occur in the alternator. The
following relation exists among the various efficiencies.
Overall efficiency = Thermal efficiency X Electrical efficiency

Hydroelectric Power Station
A generating station which utilizes the potential energy of water at a high
level for the generation of electrical energy is known as a hydro-electric
power station.

HydroElectric Power
 Hydropower plants capture the energy of
falling water to generate electricity. A
turbine converts the kinetic energy of
falling water into mechanical energy. Then
a generator converts the mechanical
energy from the turbine into electrical
energy.

Animation of Hydro Power

Hydroelectric Power Plant

 Hydroelectric power stations are generally located in hilly areas
where dams can be built conveniently and large water reservoirs
can be obtained.
 Water head is created by constructing a dam across a river or
lake. From the dam, water is led to a water turbine.
 The water turbine captures the energy in the falling water and
changes the hydraulic energy (i.e., product of head and flow of
water) into mechanical energy at the turbine shaft.
 The turbine drives the alternator which converts mechanical
energy into electrical energy.
 Hydro-electric power stations are becoming very popular
because the reserves of fuels (i.e., coal and oil) are depleting
day by day.
 They have the added importance for flood control, storage of
water for irrigation and water for drinking purposes.

Advantages
(i) No fuel required as water is used for generation of electricity.
(ii) It is quite neat and clean as no smoke or ash is produced.
(iii) Very small running charges because water is available free.
(iv) Comparatively simple in construction and requires less maintenance.
(v) Not a long starting time/can be put into service instantly.
(vi) It is robust and has a longer life.
(vii) Serve many purposes. Also help in irrigation and controlling floods.
(viii) Highly skilled persons needed at the time of construction, yet for
operation, a few experienced persons may do the job well.
Disadvantages
(i) It involves high capital cost due to construction of dam.
(ii) There is uncertainty about the availability of huge amount of water
due to dependence on weather conditions.
(iii) Skilled and experienced hands are required to build the plant.
(iv) It requires high cost of transmission lines as the plant is located in
hilly areas which are quite away from the consumers.

Types of Hydro Power Plant

 The schematic arrangement of a modern hydroelectric power station is
shown in Figure.
Schematic arrangement (Hydroelectric Power
Station)

Choice of site for Hydroelectric Power Stations
 Availability of water. Primary requirement is the availability of huge
quantity of water, a place where water is available at a good head.
 Storage of water. Wide variations in water supply from a river or
canal during the year. Store water by constructing a dam so site
selected for a hydro-electric plant should provide adequate facilities
for erecting a dam and storage of water.
 Cost and type of land. The land must be available at a reasonable
price. Tearing capacity of the ground should be adequate to
withstand the weight of heavy equipment to be installed.
 Transportation facilities. The site should be accessible by rail and
road so that necessary equipment and machinery could be easily
transported.
 It is clear from the above mentioned factors that ideal choice of site
for such a plant is near a river in hilly areas where dam can be
conveniently built and large reservoirs can be obtained.

 The most important constituents of a hydroelectric power station are:
1. Hydraulic structures
i. Dam
ii. Spillways
iii. Head-works
iv. Surge tank
v. Penstocks
2. Water turbines
i. Impulse turbines
ii. Reaction turbines
3. Electrical equipment
Equipment of Hydroelectric Power Station

Diesel Power Station
A generating station in which diesel engine is used as the
prime mover for the generation of electrical energy is
known as diesel power station.

 In a diesel power station, diesel engine is used as the prime mover.
The diesel burns inside the engine and the products of this combustion
act as the “working fluid” to produce mechanical energy.
 The diesel engine drives the alternator which converts mechanical
energy into electrical energy.
 The generation cost is more due to high price of diesel, therefore,
such power stations are only used to produce small power.
 Although steam power stations and hydro-electric plants are regularly
used to generate bulk power at cheaper cost, yet diesel power stations
are finding favor at places where demand of power is less, sufficient
quantity of coal and water is not available and the transportation
facilities are inadequate.
 These plants are also used as standby sets for continuity of supply to
important points such as hospitals, radio stations, cinema houses and
telephone exchanges.

Advantages
(i) The design and layout of the plant are quite simple.
(ii) It occupies less space as the number and size of equipment is small.
(iii) It can be located at any place.
(iv) It can be started quickly and can pick up load in a short time.
(v) There are no standby losses.
(vi) It requires less quantity of water for cooling.
(vii) Overall cost is less than steam power station of same capacity.
(viii) Thermal efficiency of plant is higher than steam power station.
(ix) It requires less operating staff.
Disadvantages
(i) The plant has high running charges as the fuel (diesel) used is costly.
(ii) Unsatisfactory under overload conditions for longer period.
(iii) The plant can only generate small power.
(iv) The cost of lubrication is generally high.
(v) The maintenance charges are generally high.

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 The schematic arrangement of a typical diesel power station.
Schematic arrangement (Diesel Power Station)

Nuclear Power Station
 A generating station in which nuclear energy is converted
into electrical energy is known as a nuclear power station.

 In nuclear power station a controlled nuclear reaction is used to make
heat to produce steam needed to drive a steam turbine generator.

 In nuclear power station, heavy elements such as Uranium (U235
)
or Thorium (Th232
) are subjected to nuclear fission in a special
apparatus known as a reactor. The heat energy thus released is
utilized in raising steam at high temperature and pressure. The
steam runs the steam turbine which converts steam energy into
mechanical energy. The turbine drives the alternator which
converts mechanical energy into electrical energy.
Nuclear Power Station (Principal)

 In the fission process, certain heavy elements, such as uranium,
are split when a neutron strikes them. When they split, they
release energy in the form of kinetic energy (heat) and
radiation.
 The process not only produces energy and radiation, it also
provides additional neutrons that can be used to fission other
uranium nuclei and, in essence, start a chain reaction.
 The controlled release of this nuclear energy using commercial-
grade fuels is the basis of electric power generation.
 The reactor is contained inside an obvious containment shell.
 Shutdown: Nuclear power plants also have an emergency backup
scheme of injecting boron into the reactor coolant. Boron is an
element that absorbs neutrons very readily.

 The most important feature of a nuclear power station is that huge
amount of electrical energy can be produced from a relatively small
amount of nuclear fuel as compared to other conventional types of
power stations.
 It has been found that complete fission of 1 kg of Uranium (U235
) can
produce as much energy as can be produced by the burning of 4,500
tons of high grade coal.
 Although the recovery of principal nuclear fuels (i.e., Uranium and
Thorium) is difficult and expensive, yet the total energy content of
the estimated world reserves of these fuels are considerably higher
than those of conventional fuels, viz., coal, oil and gas.
 At present, energy crisis is gripping us and, therefore, nuclear energy
can be successfully employed for producing low cost electrical energy
on a large scale to meet the growing commercial and industrial
demands.

Advantages
(i) Less fuel required, thus no transportation cost.
(ii) Requires less space.
(iii) Low running charges as a small amount of fuel is used.
(iv) Very economical for producing bulk electric power.
(v) Can be located near the load centers .
(vi) Large deposits of nuclear fuels available for thousands of years.
(vii) Ensures reliability of operation.
Disadvantages
(i) The fuel used is expensive and is difficult to recover.
(ii) Capital cost on plant is very high as compared to other plants.
(iii) Formation and maintenance requires greater technical know-how.
(iv) Fission by-products may cause dangerous radioactive pollution.
(v) High maintenance charges, specially trained personnel required.
(vi) Not well suited for varying loads as the reactor does not
respond to the load fluctuations efficiently.
(vii) Disposal of radioactive by-products in a deep trench or in a sea
away from sea-shore.

 The schematic arrangement of a nuclear power station
is shown in Figure. The whole arrangement can be
divided into the following main stages:
1. Nuclear reactor
2. Heat exchanger
3. Steam turbine
4. Alternator
Schematic arrangement
(Nuclear Power Station)

Working of Nuclear Power Plant

Geothermal Power Plants
 Geothermal power plants
use hot water and/or steam
located underground to
produce electrical energy.
The hot water and/or steam
are brought to the surface
where heat exchangers are
used to produce clean steam
in a secondary system for
use with turbines.
 Availability of resource will
dry up with time.

Solar Reflective Power Plants
 Solar energy is reflected off mirrors and concentrated on a
centralized boiler system. The mirrors focus the sun’s energy
toward the receiver tubes in the collector area of the elevated
boiler. The receiver tubes contain a heat transfer fluid used in
the steam–boiler– turbine system.
 The collector area housing the receiver tubes absorbs the
focused sun energy to gain 30 to 100 times normal solar energy.
 The fluid in these tubes can reach operating temperatures in
excess of 400 degrees Celsius. The steam drives the turbine and
then goes through a condenser for conversion back to liquid
before being reheated in the boiler system.

Solar Reflective Power Plants

Combustion Turbine Generation Plants
 Combustion turbine (CT) power plants burn fuel in a jet engine and use
the exhaust gasses to spin a turbine generator.
 The air is compressed to a very high pressure.
 Fuel is then injected into the compressed air and ignited, producing
high-pressure and high-temperature exhaust gasses.
 The exhaust is moved though turbine blades much the same way steam
is moved through turbine blades in a steam power plant.

Combined-Cycle Power Plants
(Combustion and Steam)
 The combined-cycle power plant consists of two means of generation:
combustion turbine and steam turbine.
 Combustion turbine (CT) power plants burn fuel in a jet engine and use
the exhaust gasses to spin a turbine generator.
 The air is compressed to a very high pressure. Fuel is then injected
into the compressed air and ignited, producing high-pressure and high-
temperature exhaust gasses.
 The exhaust is moved though turbine blades much the same way steam
is moved through turbine blades in a steam power plant.

Wind Turbine Generators

Solar Direct Generation (Photovoltaic)
 Special materials convert sunlight into dc.
 Dc converted to utility ac by inverter.

Electric Supply System
The conveyance of electric power from a power
station to consumer’s premises is known as electric
supply system.
The electric supply system can be broadly classified as
 (i) d.c. or a.c. system
 (ii) overhead or underground system.
 3-phase, 3-wire a.c. system for generation and
transmission of electric power is an economical
proposition.
 Distribution of electric power by 3-phase, 4-wire
a.c. system.
Power Transmission 1-79

Typical ac Power Supply Scheme
The large network of conductors
between the power station and the
consumers can be broadly divided into
two parts:
 transmission system and
 distribution system
Each part can be further sub-divided
into:
 primary transmission and secondary
transmission
 primary distribution and secondary
distribution
1-80

Generating Station (G.S)
 Generating Station (GS) where electric power is
produced by 3-phase alternators operating in parallel.
 Usual generation voltage: 11 kV.
 For economy in transmission, 11 kV stepped up to 132 kV
(or more) with 3-phase transformers.
 Transmission of electric power at high voltages:
 saving of conductor material (Power 1/∞ voltage) and
 high transmission efficiency.
 But increase in transmission voltage introduces insulation
problems and cost of switchgear and transformer
equipment is increased.
 Generally primary transmission is carried at 66 kV,
132kV, 220 kV or 400 kV.

Electrical Power System

Primary Transmission
 The electric power at 132 kV is transmitted by 3-
phase, 3-wire overhead system to the outskirts
of the city.
 This forms the primary transmission.

Secondary Transmission
 The primary transmission line terminates at the
receiving station (RS) which usually lies at the
outskirts of the city.
 At the receiving station, the voltage is reduced
to 33kV by step-down transformers.
 From this station, electric power is transmitted
at 33kV by 3-phase, 3-wire overhead system to
various sub-stations (SS) located at the
strategic points in the city.
 This forms the secondary transmission.

Primary Distribution
 The secondary transmission line terminates at
the sub-station (SS)
 where voltage is reduced from 33 kV to 11kV, 3-
phase, 3-wire.
 The 11 kV lines run along the important road sides
of the city.
 This forms the primary distribution.
 Big consumers (having demand more than 50 kW)
are generally supplied power at 11 kV for further
handling with their own sub-stations.

Secondary Distribution
 The electric power from primary distribution line
(11 kV) is delivered to Distribution Sub-stations
(DS).
 These sub-stations are located near the
consumers’ localities and step down the voltage
to 400 V, 3-phase, 4-wire for secondary
distribution.
 The voltage between any two phases is 400 V and
between any phase and neutral is 230 V.
 The single-phase residential lighting load is
connected between any one phase and neutral,
whereas 3-phase, 400 V motor load is connected
across 3-phase lines directly. Power Transmission 1-86

DC Transmission
 Transmission of electric power by d.c. has received active
consideration of engineers due to its numerous advantages.
 Advantages.
 It requires only two conductors
 There is no inductance, capacitance, phase displacement and surge
problems in d.c. transmission.
 Due to absence of inductance, voltage drop in a d.c. transmission
line is less than the a.c. line for the same load and sending end
voltage. Thus a d.c. transmission line has better voltage regulation.
 There is no skin effect in a d.c. system. Therefore, entire cross-
section of the line conductor is utilized.
 A d.c. line requires less insulation.
 A d.c. line has less corona loss (electric discharge due to ionization
of fluid surrounding the conductor) and reduced interference with
communication circuits. (results in waste of power)
 The high voltage d.c. transmission is free from the dielectric
losses (loss of energy in the form of heat), particularly in the case
of cables. Displacement of charges through dielectric during
charging and discharging of capacitors. 1-87

AC Transmission
 Now-a-days, electrical energy is almost exclusively
generated, transmitted and distributed in the form of a.c.
 Advantages
 Power can be generated at high voltages.
 Maintenance of a.c. sub-stations is easy and cheaper.
 The a.c. voltage can be stepped up or stepped down by
transformers with ease and efficiency. This permits to transmit
power at high voltages and distribute it at safe potentials.
 Disadvantages
 An a.c. line requires more copper than a d.c. line.
 The construction of a.c. transmission line is more complicated than
a d.c. transmission line.
 Due to skin effect in the a.c. system, the effective resistance of
the line is increased.
 An a.c. line has capacitance. Therefore, there is a continuous loss
of power due to charging current even when the line is open.
1-88

 The cost of conductor material is one of the most
important charges in a system. Obviously, the best
system for transmission of power is that for which the
volume of conductor material required is minimum.
 Therefore, the volume of conductor material required
forms the basis of comparison between different
systems.
Comparison of Conductor Material

Introduction
 It is often difficult to draw a line between the
transmission and distribution systems of a large power
system.
 It is impossible to distinguish the two merely by their
voltage because what was considered as a high voltage a
few years ago is now considered as a low voltage.
 In general, distribution system is that part of power
system which distributes power to the consumers for
utilization.
1-90

Distribution System
 That part of power system which distributes electric power for
local use is known as distribution system.
 In general, the distribution system is the electrical system
between the sub-station fed by the transmission system and the
consumers meters. It generally consists of feeders, distributors
and the service mains.
 Single line diagram of a typical low tension distribution system.
1-91

Distribution System
 (i) Feeders. A feeder is a conductor which connects the sub-
station (or localized generating station) to the area where power
is to be distributed. Generally, no tapings are taken from the
feeder so that current in it remains the same throughout. The
main consideration in the design of a feeder is the current
carrying capacity.
 (ii) Distributor. A distributor is a conductor from which tapings
are taken for supply to the consumers. In Figure, AB, BC, CD and
DA are the distributors. The current through a distributor is
not constant because tapings are taken at various places along
its length. While designing a distributor, voltage drop along its
length is the main consideration since the acceptable limit of
voltage variations is ± 6% of rated value at the consumers’
terminals.
 (iii) Service mains. A service mains is generally a small cable
which connects the distributor to the consumers’ terminals.
1-92

 A distribution system may be classified according to
 Nature of current
 Type of construction
 Scheme of connection
 (i) Nature of current. According to nature of current,
distribution system may be classified as
 (a) d.c. distribution system
 (b) a.c. distribution system.
 Now-a-days, a.c. system is universally adopted for
distribution of electric power as it is simpler and more
economical than direct current method.
1-93
Classification of Distribution Systems

 (ii) Type of construction. According to type of construction,
distribution system may be classified as
 (a) overhead system
 (b) underground system.
 The overhead system is generally employed for distribution as it
is 5 to 10 times cheaper than the equivalent underground
system. In general, the underground system is used at places
where overhead construction is impracticable or prohibited by
the local laws.
 (iii) Scheme of connection. According to scheme of connection,
the distribution system may be classified as
 (a) radial system
 (b) ring main system
 (c) inter-connected system.
 Each scheme has its own advantages and disadvantages.
1-94
Classification of Distribution Systems

A.C. Distribution
 Now-a-days electrical energy is generated, transmitted
and distributed in the form of alternating current.
 One important reason for the widespread use of
alternating current in preference to direct current is the
fact that alternating voltage can be conveniently changed
in magnitude by means of a transformer.
 Transformer has made it possible to transmit a.c. power at
high voltage and utilize it at a safe potential. High
transmission and distribution voltages have greatly
reduced the current in the conductors and the resulting
line losses.
1-95

A.C. Distribution
 There is no definite line between transmission and
distribution according to voltage or bulk capacity. However,
in general, the a.c. distribution system is the electrical
system between the step-down substation fed by the
transmission system and the consumers’ meters.
 The a.c. distribution system is classified into
 (i) primary distribution system and
 (ii) secondary distribution system.
1-96

Primary Distribution System
 It is that part of a.c. distribution system which operates
at voltages somewhat higher than general utilization and
handles large blocks of electrical energy than the average
low-voltage consumer uses.
 The voltage used for primary distribution depends upon
the amount of power to be conveyed and the distance of
the substation required to be fed.
 The most commonly used primary distribution voltages are
11 kV, 6·6 kV and 3·3 kV.
 Due to economic considerations, primary distribution is
carried out by 3-phase, 3-wire system.
1-97

Primary Distribution System
Figure shows a typical
primary distribution system.
Electric power from the
generating station is
transmitted at high voltage
to the substation located in
or near the city. At this
substation, voltage is
stepped down to 11 kV with
the help of step-down
transformer. Power is
supplied to various
substations for distribution
or to big consumers at this
voltage. This forms the high
voltage distribution or
primary distribution.
1-98

Secondary Distribution System
It is that part of a.c.
distribution system which
includes the range of
voltages at which the
ultimate consumer utilizes
the electrical energy
delivered to him. The
secondary distribution
employs 400/230 V, 3-
phase, 4-wire system.
Figure shows a typical
secondary distribution
system. The primary
distribution circuit delivers
power to various
substations, called
distribution substations. 1-99

Secondary Distribution System
 The substations are situated near the consumers’ localities
and contain step-down transformers.
 At each distribution substation, the voltage is stepped
down to 400 V and power is delivered by 3-phase,4-wire
a.c. system.
 The voltage between any two phases is 400 V and between
any phase and neutral is 230 V.
 The single phase domestic loads are connected between
any one phase and the neutral, whereas 3-phase 400 V
motor loads are connected across 3- phase lines directly.
1-100

Overhead vs. Underground System
 Overhead lines are generally mounted on wooden, concrete or
steel poles which are arranged to carry distribution
transformers in addition to the conductors. The underground
system uses conduits, cables and manholes under the surface of
streets and sidewalks.
(i) Public safety
(ii) Initial cost
(iii) Flexibility
(iv) Faults
(v) Appearance
(vi) Fault location and repairs
(vii) Current carrying capacity and voltage drop
(viii) Useful life
(ix) Maintenance cost
(x) Interference with communication circuits
1-101

Design Considerations in Distribution System
Good voltage regulation of a distribution network is probably the
most important factor responsible for delivering good service to
the consumers. For this purpose, design of feeders and
distributors requires careful consideration.
 (i) Feeders: A feeder is designed from the point of view of its
current carrying capacity while the voltage drop consideration is
relatively unimportant. It is because voltage drop in a feeder can
be compensated by means of voltage regulating equipment at the
substation.
 (ii) Distributors: A distributor is designed from the point of
view of the voltage drop in it. It is because a distributor
supplies power to the consumers and there is a legitimate limit
of voltage variations at the consumer’s terminals (± 6% of rated
value). The size and length of the distributor should be such
that voltage at the consumer’s terminals is within the
permissible limits.
1-102

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