POWER SYSTEMS-1 Complete notes examples

This Content shows entire syllabus of power systems-1.
The topics....
1. Power Generation Methods
2. Economics of Generation
3. AC Distribution & Insulated Cables
4. Electrical and Mechanical Design of Transmission Lines
5. Corona & Insulators

1. 1
Dr.G.Nageswara Rao
Professor , EEE Department
Lakireddy Bali Reddy College of Engineering
(LBRCE)

2. Course Outcomes: At the end of the course, the student will be able to:
CO1: Understand the operation of non-renewable electrical power generating
stations (Understand-L2)
CO2: Illustrate the economic aspects of power generation (Apply-L3)
CO3: Understand the a.c distribution system and performance of insulated
cables (Understand-L2)
CO4: Evaluate the electrical and mechanical parameters of transmission lines
(Apply-L3)
CO5: Analyze operation of overhead line insulators and phenomena of corona
(Understand-L2)

Course Educational Objective:
This course enables the student to learn different types of non-
renewable power generation methods, the economic aspects of
power generation, tariff methods and design aspects of
transmission lines.
2

3. 3
UNIT-I: POWER GENERATION METHODS
Introduction to typical layout of an electrical power system, present power
scenario in India, Generation of electric power: non-renewable sources
(Qualitative): Hydro station, Steam power plant, Nuclear power plant and
Gas turbine plant.
UNIT-II: ECONOMICS OF GENERATION
Introduction, definitions of connected load, maximum demand, demand
factor, load factor, diversity factor, Load duration curve, number and size
of generator units. Base load and peak load plants. Cost of electrical
energy-fixed cost, running cost, Tariff on charge to customer. UNIT-III: AC DISTRIBUTION & CABLES
AC Distribution: Introduction, AC distribution, Single phase, 3-phase-
3wire, 3 phase 4 wire system, bus bar arrangement, Selection of site and
layout of substation.
Insulated Cables: Introduction, insulation, insulating materials, extra
high voltage cables, grading of cables, insulation resistance of a cable,
capacitance of a single core and three core cables, overhead lines versus
underground cables, types of cables.

4. 4
Unit-IV: ELECTRICAL AND MECHANICAL DESIGN OF
TRANSMISSION LINES
Transmission line sag calculation: The catenary curve, sag tension
calculations, supports at different levels, stringing Chart, inductance and
capacitance calculations of transmission lines: line conductors,
inductance and capacitance of single phase and three phase lines with
symmetrical and unsymmetrical spacing, Composite conductors-
transposition, bundled conductors, and effect of earth on capacitance.
UNIT-V: CORONA& INSULATORS
Corona: Introduction, disruptive critical voltage, corona loss, Factors
affecting corona loss and methods of reducing corona loss, Disadvantages
of corona, interference between power and Communication lines,
Numerical problems.
Overhead Line Insulators: Introduction, types of insulators, Potential
distribution over a string of suspension insulators, Methods of equalizing
the potential, testing of insulators.

5. 5
TEXT BOOKS:
1. Soni, Gupta & Bahtnagar, Power Systems Engineering, Dhanpat Rai &
Sons, 2016.
2. C.L. Wadhwa, Electrical Power Systems, 6th Edition, New
AgeInternational,2009.

REFERENCE BOOKS:
1. M.V.Deshpande, Elements of Electrical Power Station Design, 3rd,
Wheeler Pub.1997.
2. C.L. Wadhwa, Generation, Distribution and Utilization of Electrical
Energy, 3rd Edition, New AgeInternational,2015. 3. V K Mehta & Rohit
Mehta, Principles of Power Systems (Multicolor Edition), 24/e, S.Chand
Publishing, 4th Edition ,2005.
W.D.Stevenson, Elements of Power System Analysis, 4th Edition,
McGraw Hill, 1982. https://www.slideshare.net/raoakhil/thermal-power-plants-237930541

6. 6

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Electricity sector in India is growing at a rapid pace.
The present peak demand is about 1,15,000 MW and the Installed
Capacity is 1,52,380 MW using generation from thermal (63%), hydro (25
%), Nuclear (9 %) and renewables (9 %)

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14. 14
Basic Principal of Steam Power Plant
The heat produced for burning of coal & with the help
of water steam is produced. This produced steam flow
towards turbine i.e. kinetic energy is converted into
mechanical energy. The input steam drives the prime
mover or turbine, simultaneously the generator also
start to rotate. At that time mechanical energy is
converted into electrical energy.
Thermal Power Plant

15. 15
Selection of Site for Thermal Power Plant
1. Supply of Fuel: The Steam power station should be located near the coal
mine so that transportation cost of fuel is minimum. If the land is not available
near to coal mines then provide adequate facilities for transportation of fuel.
2. Available of Water: A huge amount of water is required in boiler &
condenser, so that the plant should be located near the river, lake etc.
3. Transportation Facility: For steam power station provide better
transportation facility for the transportation of man, machinery etc.
4. Cost & Type of Land: The Steam Power Station should be located where
the cost of land is chief & also future extension is possible.
5. Near to Load Center: In order to reduce transmission & distribution losses
the plant should be located near to load center.
6. Distance from Populated Area: As the thermal power plant produces flue
gases, these gases will effect to live human being, so that the plant should be
located away from thickly populated area.
7. Disposal Facility Provided: As the thermal power plant produces ash, while
burning of coal. So, disposal of ash facility should be provided.
8. Availability of labour: Skilled and unskilled labour should be available
nearly.

16. General Layout of
Thermal Power Plant

17. 17
Flow Diagram of
Steam Thermal Power Plant

18. 18
The Basic Components
1.Boiler
(i) fire tube boiler and (ii) water tube boiler
1.Steam turbine
2.Generator
3.Condenser
4.Cooling towers
5.Circulating water pump
6.Boiler feed pump
7.Forced or induced draught fans
8.Ash precipitators

19. 19
Boiler
A boiler is a closed vessel in which the water or fluid is heated
Steam turbine
A steam turbine is a device which extracts thermal energy from the
pressurized steam. The energy must be used to organize mechanical
work on a rotating output shaft.
Generator
A generator is a device which is used to convert the mechanical
form of energy into the electrical energy.
Condenser
A condenser is a device used to converts the gaseous substance into
the liquid state substance with the help of cooling.
Cooling towers
A cooling tower is a heat rejection device, which discards the waste
heat into the atmosphere with help of the cooling water stream to a
lower temperature.

20. 20
Circulating water pump
Circulating pump is a special device used to circulate the liquids,
gases and slurries present in the closed circuit. The main purpose of
the circulating pump is circulating the water in a cooling system or
hydronic heating.
Boiler feed pump
A boiler feed pump is a specific type of pump which is used to feed
the water into the steam boiler. The condition of water supply
depends on the boiler produce the condensation of the steam.
Forced draught fans: Forced draught fans are used to provide a
positive pressure to a system.
Induced draught fans
Induced draught fans are used to provide a negative pressure or
vacuum in a slack or system
Ash precipitators: Precipitators are devices used to remove the fine
particles like smoke and dust. By using the force of induced
electrostatic charge minimally close the flow of gases through the
unit.

21. 21
Working Principle Of Thermal Power Plant
Water is used as the working fluid in the thermal power plant. We can see
coal based and nuclear power plants in this category. From the working of
the power plant energy, later from the fuel gets transferred into the form of
electricity. With the help of high pressure and high steams a steam turbine
in a thermal power plant is rotates, the rotation must be transfer to the
generator to produce power.
When turbine blades are rotated with the high pressure and high
temperature at that case the steam loses its energy. So it results in the low
pressure and low temperature at the outlet of the turbine. Steam must be
expanded upto the point where it reaches the saturation point. So from the
steam, there is no heat addition or removal that takes place. Entropy of the
steam remains same. So we can notice the change in the pressure and
volume and temperature along with the entropy diagrams. If the condition
comes to the low pressure and low temperature steam back to the original
state, from that we can produce continuous electricity.

22. 22
To compress the gaseous state liquids at that case large amount of
energy is required. So before the compression we need to convert the fluids
into liquid state. For this purpose condenser is required and heat is rejected
to the surroundings and converts the steam into liquid state. During this
process the temperature and volume of the fluid changes take place hardly,
so it turns into liquid state. And the fluid turns to the original state. To bring
the fluid to the original state external heat is added. To the heat exchanger
heat is added which is called as boiler. Then the pressure of the fluid must
remain same. In heat exchanger tubes it expands freely. Due to increase in
temperature the liquid state is transformed into the vapour state and the
temperature remains same. So know we complete the thermodynamic cycle
in the thermal power plant. It is known as Rankine cycle. By repeating the
cycle we can produce the power continuously.
With the help of boiler furnace heat is added to the boiler. Then the
fuel must reacts with the air and produces heat. The fuel must be either
nuclear or coal. In this process if we use coal as a fuel we can observe lot of
pollutants before ejects in to the air clean or removed the particles and send
into surroundings. The process is done in various steps. By using the electro
static precipitator the ash particles are removed. So with the help of the stack
clean exhaust must be send outside.

23. 23
Working Principle

24. 24
Advantages:
1.Cost of fuel: Fuel used in thermal power station (TPS) is cheaper
than cost of fuel used in diesel & nuclear power station.
2.Capital cost: Capital cost of TPS is less than hydro & nuclear
power station.
3.Near load center: TPS can be located near load center. The coal
can be transport from coal mines to power plant. As it is located load
centre it reduces transmission cost and losses in it.
4.Space required: Less space required as compared to hydro power
station.
5.Generating capacity: TPS build/construct of high generating
capacity, so used as a base load power plant.
6.Time required for completion of project: Time required for
completion of Thermal power project is very less as compare to
hydroelectric power station.

25. 25
Disadvantages:
1.Air pollution: It produces air pollution due to smoke and ash produced during combustion
of fuel.
2. Starting Time: TPP cannot be put into service immediately like hydroelectric power
plant. As thermal power plant required few hours (6-7 hour) to generate steam at high
pressure and high temperature.
3.Handling of fuel: Handling of coal and disposal of ash is quite difficult.
4. Fuel transportation cost: When power plant are located away from coal mines i.e. near
load centre at that time fuel transportation cost is more.
5. Preparation for fuel: There is more expenditure for preparation of coal (raw coal to
pulverized coal).
6.Space required: Large amount of space is required for storage of fuel and ash as compare
to Nuclear power plant.
7. Efficiency: It is less efficient power plant overall efficiency is maximum 30 %.
8. Stand by losses: Stand by losses is more as furnace is required to keep in operation even
when there is no load.
9.Maintenance cost: High maintenance and operating cost because number of axillaries
plant are required such as coal and ash handling plant, pulverizing plant, condensing plant
and water purification plant etc.
10. Availability of fuel: Less availability of high grade coal.
11. Simplicity and cleanness: Layout of thermal power plant is complicated than
hydroelectric power plant due to coal and ash.
12. Life: Life of thermal power plant is less than hydro power plant.
13. Cost per unit (cost of generation) is high

26. 26
Cooling Tower

27. In water tube boilers the water flows through tubes and hot combustion
gases flow over these tubes. Whereas in fire tube boilers the tubes are
surrounded by water and hot combustion gases flow through these tubes.

28. 28

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Surface Condenser

30. 30

31. 31
Principle And Working Of Surface Condenser
The Basic working principle of a surface condenser is the transfer of heat from a
higher-temperature body to a lower-temperature body. In this, the steam (high-
temperature body) liberates its heat to the cooling water tubes (low-temperature
body). In the process of heat transfer, the hot steam gets converted to water.
The steam enters from the exhaust Steam inlet and comes in contact with the water
carrying tubes. The water in the tubes has a circulating flow. As soon as the
exhaust steam comes in contact with the water-cooled tubes, the process of heat
transfer begins. The heat from the steam is removed and converted into a liquid
which Is known as condensate. This condensate is then removed from the
cylindrical vessel through a valve located at the bottom of the cylinder.
In thermal power stations, water is heated more than its boiling point to generate
steam which in turn is used to rotate the turbine. After passing through the turbine
the steam is fed into a surface condenser where it is converted into water and then
reused.

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Jet Condenser Surface Condenser
Both steam & cooling water are
mixed together
Both steam & cooling water are
not mixed together
Manufacturing cost is low Manufacturing cost is high
Occupies less area Occupies large area
The air pump requires large power The air pump requires less power
A small quantity of cooling water
is required
A large quantity of cooling water
is required

33. 33
Advantages
The following are the advantages of surface condenser
1.Its vacuum efficiency is high
2.They are mainly used in large plants area
3.It uses low-quality water
4.It also uses impure water for cooling purpose
5.The pressure ratio & steam are directly proportional.

Disadvantages
The following are the disadvantages of surface condenser
1.Water required is in the large amount
2.Complex in construction
3.High maintenance
4.It occupies a large area.

Applications

The following are the applications of surface condenser
1.Refrigeration of vacuum
2.Evaporation of vacuum
3.Systems like
Desalination

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35. 35
Hydroelectric power plant has the following parts
Dam or weir: it contains the river water, forming a reservoir behind it and thus creating a
water drop that is used to produce energy. Dams can be made of earth or concrete.
Spillways: They release part of the impounded water without passing through the turbines;
water can then be used for irrigation purposes. They are located on the main wall of the
dam and can be at the top or at the bottom. Most of the water goes into a plunge pool at
the toe of the dam, to prevent scour damage by the falling water.
Water intakes: they let in the impounded water towards the turbines through a penstock.
Water intakes have gates to control the amount of water that reaches the turbines and grids
to filter out any debris such as trunks, branches, etc.
Powerhouse: it houses the hydraulic and electrical equipment (turbines, generators,
transformers) and the service area with control and testing rooms. It has inlet and outlet
gates to ensure the equipment area can be dry in case of repairs or disassembling
equipment.
Turbines: they harness the energy of the water that goes through them to rotate around a
shaft. There are three main types of turbines: Pelton, Francis and Kaplan turbines (propeller
type).
Transformers: electrical devices to increase or decrease the voltage in an alternating current
circuit.
Electrical power transmission lines: cables to transmit the electricity generated.

36. 36
GAS POWER PLANT LAYOUT

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Gas Turbine Power Plant
A generating station which employs a gas turbine as the prime mover for
the generation of electrical energy is known as a gas turbine power plant. In
a gas turbine power plant, air is used as the working fluid. The air is
compressed by the compressor and is led to the combustion chamber where
heat is added to the air, thus raising its temperature. We will understand the
gas turbine power plant layout and learn the diagram.

Heat is added to the compressed air either by burning fuel in the
chamber or by the use of air heaters. The hot and high-pressure air from the
combustion chamber is then passed to the gas turbine where it expands and
does the mechanical work. The gas turbine drives the alternator which
converts mechanical energy into electrical energy.

It may be mentioned here that compressor, gas turbine and the
alternator are mounted on the same shaft so that a part of the mechanical
power of the turbine can be utilised for the operation of the compressor.
Gas turbine power plants are being used as standby plants for hydro-electric
stations, as a starting plant for driving auxiliaries in power plants etc.

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The main components of the Gas Turbine Power Plant are :
(i) Compressor
(ii) Regenerator
(iii) Combustion chamber
(iv) Gas turbine
(v) Alternator
(vi) Starting motor
(i) Compressor: The compressor used in the plant is generally of rotatory
type. The air at atmospheric pressure is drawn by the compressor via the filter
which removes the dust from the air. The rotatory blades of the compressor
push the air between stationary blades to raise its pressure. Thus air at high
pressure is available at the output of the compressor.
(ii) Regenerator: A regenerator is a device which recovers heat from the
exhaust gases of the turbine. The exhaust is passed through the regenerator
before wasting to the atmosphere. A regenerator consists of a nest of tubes
contained in a shell as seen in the below power plant layout. The compressed
air from the compressor passes through the tubes on its way to the combustion
chamber. In this way, compressed air is heated by the hot exhaust gases.

39. 39
(iii) Combustion chamber: The air at high pressure from the compressor is
led to the combustion chamber via the regenerator. In the combustion
chamber, heat is added to the air by burning oil. The oil is injected through
the burner into the chamber at high pressure to ensure atomisation of oil and
its thorough mixing with air. The result is that the chamber attains a very high
temperature (about 3000 F). The combustion gases are suitably cooled to
1300F to 1500F and then delivered to the gas turbine.
iv) Gas turbine: The products of combustion consisting of a mixture of gases
at high temperature and pressure are passed to the gas turbine.These gases in
passing over the turbine blades expand and thus do the mechanical work. The
temperature of the exhaust gases from the turbine is about 900F. (v) Alternator: The gas turbine is coupled to the alternator as seen in the gas
turbine plant layout. The alternator converts mechanical energy of the turbine
into electrical energy. The output from the alternator is given to the bus-bars
through the transformer, circuit breakers and isolators. (vi) Starting motor: Before starting the turbine, the compressor has to be
started. For this purpose, an electric motor is mounted on the same shaft as
that of the turbine. The motor is energised by the batteries. Once the unit
starts, a part of the mechanical power of the turbine drives the compressor
and there is no need of motor now

40. 40
Gas turbine power plant Advantages:

(i) It is simple in design as compared to steam power station since no boilers and
their auxiliaries are required.
(ii) It is much smaller in size as compared to the steam power station of the same
capacity. This is expected since the gas turbine power plant does not require a
boiler, feed water arrangement etc.
(iii) The initial and operating costs are much lower than that of the equivalent steam
power station.
(iv) It requires comparatively less water as no condenser is used.
(v) The maintenance charges are quite small.
(vi) Gas turbines are much simpler in construction and operation than steam
turbines.
(vii) It can be started quickly form cold conditions.
(viii) There are no standby losses. However, in a steam power station, these losses
occur because the boiler is kept in operation even when the steam turbine is
supplying no load.

41. 41
Gas turbine power plant Disadvantages:

(i) There is a problem with starting the unit. It is because before starting the turbine,
the compressor has to be operated for which power is required from some external
source. However, once the unit starts, the external power is not needed as the turbine
itself supplies the necessary power to the compressor.

(ii) Since a greater part of power developed by the turbine is used in driving the
compressor, the net output is low.

(iii) The overall efficiency of such plants is low (about 20%) because the exhaust
gases from the turbine contain sufficient heat.

(iv) The temperature of the combustion chamber is quite high (3000F) so that its life
is comparatively reduced.

44. 44
1.Discuss the different sources of energy available in nature.
2.Draw the schematic diagram of a modern steam power station and explain its
operation
3.What is a steam power station ? Discuss its advantages and disadvantages.
4.What factors are taken into account while selecting the site for a steam
power station ?
5.Draw a neat schematic diagram of a hydro-electric plant and explain the
functions of various components.
6.Explain the functions of the following :
(i) dam (ii) spillways (iii) surge tank (iv) headworks (v) draft tube
7. Draw the schematic diagram of a nuclear power station and discuss its
operation
8. Explain with a neat sketch the various parts of a nuclear reactor
9. Discuss the factors for the choice of site for a nuclear power plant.
10. Explain the working of a gas turbine power plant with a schematic diagram.
11. Give the comparison of steam power plant, hydro-electric power plant, gas
power plant and nuclear power plant
12. Discuss the advantages and disadvantages of a gas power station.
CHAPTER REVIEW QUESTIONS

45. 45

46. 1
Hydro Electric Power Plant

47. Site Selection for Hydro Power Plant
2
01. Availability of water
Water is the main source of hydroelectric power plants. A huge amount of water should be
available so that the power plant can be built with a high head. The quantity of the water
available will be estimated on the basis of the measurement of streamflow over a certain
period or previous rainfall records.
02. Storage of water
There will be a wide variation of rainfall during the year. This makes it necessary to store
water for continuous generation of power throughout the year.
03. Head of water
The head of the water depends upon the topography of the area. If the head is more then
potential energy will be more.
04. Choice of the dam
The important consideration in the choice of the dam is safety and economics. Failure of the
dam may result in substantial loss of life and property. The dam must satisfy the stability
test for shock loads and unusual floods.
05. Distance from the power station to load center
The distance should be less between the power station and load center so that the cost of
transmission of power becomes less.
06. Accessibility of the site
The plant should be easily accessible by rain and load for transportation of plant equipment.

48. 3
Hydro Electric Power Plant

49. 4
01. Reservoir: The purpose of this reservoir is to store the water which will be further used to
generate electricity. The water will be stored during the rainy season. By storing water we get
potential energy.
02. Dam: dam will be constructed across the river or lakes to provide the head of the water.
These are classified based on their function, material, shape, and structural design.
03. Spillway: This spillway is the safety wall for the dam. It discharges the existing amount of
water from the reservoir into the rivers. That means spillway is required to reduce overtopping.
It keeps the reservoir level below the predetermined value.
04. Intake: Intake acts as a filter in Hydro Electric power plants. It removes unwanted material
from the water. In this stage, the potential energy will be converted into kinetic energy.
05. Penstock: This is the channel between the dam and turbine which helps to increase the
kinetic energy of the water. It is made up of stainless steel.
06. Surge tank: It acts as a pressure release wall for the water. It reduces the water hammer
effect. That means it holds the water whenever there is no requirement of load on the turbines,
and similarly, it discharges water whenever there is a requirement of load on the turbines. 07. Prime mover/turbine: For this reason, the kinetic energy will be converted into
mechanical energy, which is responsible for the rotation of the shaft of the turbines. Commonly
used turbines are Kaplan, Francis, Pelton, cross flow, etc.
08. Alternators / Generators: These are normally located near the foot of the dam. Water is
brought to alternators with the help of penstock. In this region, the mechanical energy is
converted to electrical energy. Thus final power will get in this stage.

50. 5
Working Principle
In a hydro electric power plant, water is stored in the dam reservoir
which has potential energy.
This potential energy is converted into kinetic energy when water from
the dam is allowed to flow through the pipes.
This kinetic energy is converted into mechanical energy allowing the
water flowing in pipe to drive the turbine.
At last, the mechanical energy by rotating the turbine is converted to
electrical energy in the generator which is coupled to the turbine.
The dam creates the head of the water from which water flows.
Penstock carries the water from the Dam to the turbine, and it provides
kinetic energy.
The fast flowing water through the penstock pushes turbine blades.
The water forces on turbine plates and rotates the generator rotor, which
in turn generates electricity.

51. 6
Advantages
1.Electricity can be produced at a constant rate once the dam is constructed
2.The gates of the dam can be shut down if electricity is not needed, which
stops electricity generation. Hence by doing this, we can save water for
further use in future when the demand for electricity is high.
3.One of the biggest advantages of hydroelectric power plants is that they are
designed to last many decades, and so they can contribute to the generation of
electricity for years.
4.Large dams often become tourist attractions because the lake that forms in the
reservoir area behind the dam can be used for leisure or water sports.
5.The water from the lake of the dam can be used for irrigation purposes in
farming.
6.Since the water is released to produce electricity, the build-up of water in the
dam is stored to produce extra energy until needed.
7.Hydroelectric energy generation does not pollute the atmosphere because the
hydroelectric power plant does not produce greenhouse gases.
8.Hydropower plants can be considered a reliable energy generation source.
Since hydropower totally depends on water present on this planet, this energy
source will remain inexhaustible because of the water cycle as it continuously
keeps on maintaining balance on the Earth.

52. 7
Disadvantages
1. It is not an easy task to assemble a hydropower plant because the dams are
extremely expensive to build, and they require extremely high standards and
calculations for their construction.
2. It becomes important that the hydropower plant must serve for many decades
because of its high cost of construction, and this totally depends on the availability
of water resources.
3. If flooding happens due to natural calamities or the failure of dams, it would
impact a large area of land, which means that the natural environment can be
destroyed.
4. People are forcibly removed from the particular area where a hydropower plant
is going to be assembled. This affects the day-to-day life of people living in that
area.
5. A serious geological damage can be caused due to the construction of large
dams.
6. To construct a hydro plant, it is important to block the running water source due
to which the fishes can’t arrive at their favourable place, and as the water stops
streaming, the areas along the riverside start to vanish out which eventually
influences the life of creatures that depend on fish for food.

53. 8
Different types of modern hydro power plants
1. Pumped storage hydropower plants
2. Reversible turbine pump hydropower plants
3. Underground hydropower plants
4. Tidal power plants
Types of Pumped Storage Plants
1.Daily, weekly or seasonal storage plants
2.High, medium or low head plants
3.According to the type of turbine used in the plant
4.Pure or mixed storage hydropower plants
5.Horizontal or vertical storage plants

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Pumped Storage Plants

55. Pumped Storage Hydropower Plants:
To supply the peak loads, hydropower plants has to have the installed capacity of high
loads of which remains idle during the off-peak hours.
The more the demands of variable power supply, it is necessary to devise some way to
achieve the economical loading of the power
plant by levelling up the load curve.
The following are some of the ways:
(a) Commercial Method: To sell electric current at a higher rate during peak hours than
during off-peak hours.
(b) Technical method: The following are the two methods:
(i) By installing special peak load power plants
(ii) By storing energy produced during off-peak hours. Such a system is known
as Pumped Storage Plants.
Purpose of Pumped Storage Hydropower Plants:
This type of plants combined with steam power stations reduces the power load
fluctuations to narrow limits.
In some cases, the storage plant consists of pump and motor with no turbines.
The pump increases the head in the feeder reservoir of a separate hydro-electric plant
while motor improves the power factor in the electric supply network.

56. 11
Advantages:
The pump storage plants entail the following advantages :
1.There is substantial increase in peak load capacity of the plant at
comparatively low capital cost.
2.Due to load comparable to rated load on the plant, the operating
efficiency of the plant is high.
3.There is an improvement in the load factor of the plant.
4.The energy available during peak load periods is higher than that of
during off peak periods so that in spite of losses incurred in pumping there
is over-all gain.
5. Load on the hydro-electric plant remains uniform.
6.The hydro-electric plant becomes partly independent of the stream flow
conditions.

57. 12

58. 8-Feb-24 1
NUCLEAR
POWER PLANT
Dr.G.Nageswara Rao
Professor , EEE Department
Lakireddy Bali Reddy College of Engineering (LBRCE)

59. NUCLEAR BINDING ENERGY
Nuclei are made up of protons and neutron, but the mass of a
nucleus is always less than the sum of the individual masses of the
protons and neutrons which constitute it. The difference is a
measure of the nuclear binding energy which holds the nucleus
together. The enormity of the nuclear binding energy can perhaps be
better appreciated by comparing it to the binding energy of an
electron in an atom. The comparison of the alpha particle binding
energy with the binding energy of the electron in a hydrogen atom is
shown below. The nuclear binding energies are on the order of a
million times greater than the electron binding energies of atoms.

60. Fusion is the process where two light nuclei combine together releasing
vast amounts of energy.
Fission is the splitting of a heavy, unstable nucleus into two lighter
nuclei
Fission and Fusion
(Hydrogen Bomb) (Atom Bomb or Atomic Bomb)

61. 4
Fission Reaction Fusion Reaction
A fission reaction is splitting up of a large atom or a
molecule into two or more smaller ones.
Fusion is the process of combination of two or more
lighter atoms or molecules into larger ones.
Fission reaction doesn’t occur normally in nature. Fusion reaction process occurs in the stars, like in the sun,
etc.
This reaction produces highly radioactive substances. Few number of radioactive particles are developed by the
process of a fusion reaction.
Neutrons must be slowed down by moderation to increase
their capture probability in fission reactors.
This process requires high-temperature, high-density
environment.
This process consumes a very little amount of energy to
break up the atoms.
High amount of energy is consumed to combine protons
so that the nuclear forces can overcome the electrostatic
repulsion.
The energy released during the process of fission is much
larger than that of the released energy in other chemical
reactions.
The energy released by the process of fusion is around 3-4
times much greater than that of the energy liberated by the
process of fission.
Fission process is utilized in the nuclear power plant. Fusion process is one of the experimental technologies for
the production of power.
Uranium is one of the primary fuels used for the process
of fission in power plants.
The isotopes of hydrogen such as the Deuterium &
Tritium are some of the primary fuels used in the
experimental process of fusion power plants.
A fission bomb is one kind of nuclear weapon which is
also known as Atom Bomb or Atomic Bomb.
Hydrogen Bomb is one class of fusion bomb.

62. Differences between Fission and Fusion

63. 6
Nuclear Power Plant
Nuclear reactor is used to produce heat and heat exchanger performs to convert water into
steam by using the heat generated in nuclear reactor. This steam is fed into steam turbine
and condensed in condenser. Now steam turbine is turn to run an electric generator or
alternator which is coupled to steam turbine and thereby producing electric energy.
SELECTION OF SITE
1. Availability of water: At the power plant site an ample quantity of water should be
available for condenser cooling and made up water required for steam generation. Therefore
the site should be nearer to a river, reservoir or sea.
2. Distance from load center: The plant should be located near the load center. This will
minimize the power losses in transmission lines.
3. Distance from populated area: The power plant should be located far away
From populated area to avoid the radioactive hazard.
4. Accessibility to site: The power plant should have rail and road transportation facilities.
5. Waste disposal: The wastes of a nuclear power plant are radioactive and there should be
sufficient space near the plant site for the disposal of wastes.

65. 8
The working principle of nuclear power plant depends upon mainly four
components.
1.Nuclear Reactor
2.Heat Exchanger
3.Steam Turbine
4.Alternator
1. Nuclear Reactor:-
Nuclear reactor is the main component of nuclear power plant and nuclear fuel is
subjected to nuclear fission. Nuclear fission is a process where a heavy nucleus is
spitted into two or more smaller nuclei. . A heavy isotope generally uranium-235(U-
235) is used as a nuclear fuel in the nuclear reactor because it has the ability to control
the chain reaction in the nuclear reactor. Nuclear fission is done by bombarding
uranium nuclei with slow moving neutrons. The energy released by the fission of nuclei
is called nuclear fission energy or nuclear energy. By the braking of uranium atom,
tremendous amount of heat energy and radiation is formed in the reactor and the chain
reaction is continuously running until it is controlled by a reactor control chain reaction.
A large amount of fission neutrons are removed in this process, only small amount of
fission uranium is used to generate the electrical power.

66. The nuclear reactor is cylindrical type shape. Main body of reactor is enclosed by reactor core,
reflector and thermal shielding. It prevent reactor wall from getting heated. It is also used to protect
alpha ( α), bita (β) , gama (γ) rays and neutrons which are bounce back at the time of fission within the
reactor. Mainly Nuclear reactor consists, some fuel rods of uranium, moderator and control rods. Fuel
rods are made of the fission materials and released large number of energy at the time of bombarding
with slow moving neutrons. Moderator consists full of graphite which is enclosed by the fuel rods.
Moderator maintains the chain reaction by releasing the neutrons in a suitable manner before they
mixed with the fissile materials. Control rods are made of boron-10 and cadmium or hafnium which is
a highly neutron absorber and it is inserted into the nuclear reactor. When control rods are push down
into the reactor core, it absorbs most of fission neutrons and power of the reactor is reduced. But when
it is pulling out from the reactor, it releases the fission neutrons and power is increased. Real practice,
this arrangement depends upon according to the requirement of load. A coolant, basically sodium
metal is used to reduce the heat produce in the reactor and it carries the heat to the heat exchanger.
2. Heat Exchanger:-Coolant is used to raise the heat of the heat exchanger which is utilised in raising
the steam. After that, it goes back to the reactor.
3. Steam Turbine:-Steam is coming from the heat exchanger to fed into the steam turbine through the
valve. After that the steam is exhausted to the condenser. This condensed steam is fed to the heat
exchanger through feed water pump.
4. Alternator:-Steam turbine is coupled to an alternator which converts mechanical energy to
electrical energy. The output of alternator produces electrical energy to bus bars via major electrical
apparatus like transformer, circuit breakers, isolators etc.

67. BLOCK DIAGRAM OF NUCLEAR REACTOR

68. Main Components of a Nuclear Reactor
The Core: It contains all the fuel and generates the heat required for energy
production.
The Coolant: It passes through the core, absorbing the heat and transferring into
turbines.
The Turbine: Transfers energy into the mechanical form.
The Cooling Tower: It eliminates the excess heat that is not converted or
transferred.
Moderator: Moderators are used for reducing the speed of fast neutrons released
from the fission reaction and making them capable of sustaining a nuclear chain
reaction. Usually, water, solid graphite, and heavy water are used as a moderator in
nuclear reactors. Commonly-used moderators include regular (light) water (in
74.8% of the world’s reactors), solid graphite (20% of reactors), heavy water (5% of
reactors).
The Containment: The enveloping structure that separates the nuclear reactor from
the surrounding environment.
Neutron Poison: A neutron poison (also called a neutron absorber or a nuclear
poison) is a substance with a large neutron absorption cross-section.

69. 8-Feb-24 12
Monitoring Nuclear Fuel

70. 8-Feb-24 13

71. 8-Feb-24 14
Fuel assembly (fuel bundle, fuel element)

72. 8-Feb-24 15

73. 8-Feb-24 16
Fuel Assembly Manufacturing (Fuel rod to fuel assembly)
Materials
Pellet: UO2, UO2 containing gadolinia
Cladding: Zirconium alloy
Guide thimble tube: Zirconium alloy
Spacer: Zirconium alloy and inconel
Top/Bottom nozzle: Stainless steel
Type 14×14 15×15 17×17
10ft 12ft 12ft 12ft
Section size
(mm) 197 214 214

74. 8-Feb-24 17
(a) Schematic of nuclear fuel rod assembly 2 (b) Simplified schematic of a TN-32

75. 8-Feb-24 18

76. ADVANTAGES OF NUCLEAR POWER PLANTS
1.Since the requirement of fuel is very small, so the cost of fuel transportation,
storage etc. is small.
2.Nuclear power plant needs less space as compared to any other power station of
the same size. Example: A 100 MW nuclear power station needs 38 - 40 acres of
land whereas the same capacity coal based thermal power plant needs 120-130
acres of land.
3.This type of power plant is very economical to produce large electric power.
4.Nuclear power plant can be located near load centre because bulk amount of fuel
(like water, coal) is not required.
5.Nuclear power is most economical to generate large capacities of power like 100
MVA or more. It produces huge amount of energy in every nuclear fission process.
6.Using a small amount of fuel, this plant produces large electrical energy.
7.This plant is very reliable in operation.
8.Since, the large number of nuclear fuel is available in this world. So, a nuclear
power plant can generate electrical energy thousands of years continuously.
9.Nuclear Power Plant is very neat and clean as compared to a steam power plant.
10.The operating cost is low at this power plant but it is not affected for higher load
demand. Nuclear power plant always operates a base load plant and load factor
will not be less than 0.8.

77. DISADVANTAGES OF NUCLEAR POWER PLANTS

1.Initial installation cost is very high as compared to the other power
station.
2.Nuclear fuel is very much expensive and it is difficult to recover.
3.Capital cost is higher in respect of other power station.
4.Good technical knowledge is required to operate such type plant. So,
salary bill and other maintenance cost will be higher to operate such of
a plant.
5.There is a chance to spread of radioactive pollution from this type of
plant.
6.Nuclear Reactor does not response efficiently with the fluctuating load
demand. So, it is not suited for varying the load.
7.Cooling water requirement is twice than a coal based steam power
plant.

78. Types of Nuclear Reactors
Most nuclear reactors in the United States and in Europe use fuel composed of natural uranium
that is enriched with uranium 235, and ordinary water as a coolant. These reactors are known as
light-water reactors. There are two basic types: the pressurized water reactor and the boiling water
reactor.
Pressurized Water Reactor is the most common type of nuclear reactor used for the generation
of electricity. It uses ordinary water as both the moderator (to slow neutrons) and the coolant (to
transfer heat). It has two separate cooling circuits: one which flows through the core of the reactor
(the primary), and one which is used to drive the turbine (the secondary).

Boiling Water Reactor is similar in some ways to the more common pressurized water reactor.
This design also uses ordinary water as both the moderator (to slow neutrons) and the coolant (to
transfer heat). In the boiling water reactor, however, a single cooling circuit is used and the
cooling water boils inside the reactor.

CANDU (CANadian Deuterium Uranium), is also used to generate power. Developed by Canada,
this reactor uses only natural uranium as a fuel, but is moderated and cooled using heavy water.
Since the complex enrichment process can be skipped, this type is very popular in developing
nations. It is also known as a pressurized heavy water reactor.

80. 23

81. CANDU Reactor

82. 25
Nuclear Waste
Nuclear waste refers to any radioactive material produced by medical, research, nuclear
power facilities, or nuclear weapons programs. Nuclear waste can be grouped in two
categories: low-level and high-level. Low-level wastes are slightly contaminated materials.
A major source of low-level waste is mill-tailings from uranium ore processing. High-level
wastes are comprised mainly of spent fuel from nuclear reactors. A small amount of high-
level waste is very toxic.

83. 26
The major concern about nuclear waste is its disposal. Nuclear waste must be stored until the
radioactivity has dropped to safe levels without contaminating the surrounding environment. The
disposal of low-level waste is done by some form of shallow land burial. The disposal of high-level
waste is a more complex problem. The waste is highly toxic and must be stored for several centuries.
Currently there are no long-term storage facilities for high-level waste in the United States.

84. 27
Radioactive waste disposal
The dispose of nuclear waste in the Euopean Union

85. Dumping of Radioactive Materials at Sea

86. 8-Feb-24 29

87. 8-Feb-24 30

88. 8-Feb-24 31
Nuclear Explosion

89. 8-Feb-24 32

90. 8-Feb-24 33

91. 8-Feb-24 34
Nuclear Explosion

92. 35
YouTube Channel: GUDIPUDI FAMILY

93. https://www.slideshare.net/slideshows/power-generation-methodspower-generation-power-
plants/266214209

95. Introduction,definitionsofconnectedload,maximum
demand,demandfactor,loadfactor,diversityfactor,
Loaddurationcurve,numberandsizeofgenerator
units.Baseloadandpeakloadplants.Costofelectrical
energy-fixedcost,runningcost,Tariffonchargeto
customer.

96. 4
Important Terms and Factors
Connectedload:Itisthesumofcontinuousratingsofalltheequipment's
connectedtosupplysystem.
Maximumdemand:Itisthegreatestdemandofloadonthepowerstation
duringagivenperiod.
Demandfactor.Itistheratioofmaximumdemandonthepowerstationtoits
connectedloadi.e.,
Demandfactor=Maximumdemand/Connectedload
Thevalueofdemandfactorisusuallylessthan1.
Averageload:Theaverageofloadsoccurringonthepowerstationinagiven
period(dayormonthoryear)isknownasaverageloadoraveragedemand.
Dailyaverageload=No.ofunits(kWh)generatedinaday/24hours
Monthlyaverageload=No.ofunits(kWh)generatedinamonth/Numberof
hoursinamonth
Yearlyaverageload=No.ofunits(kWh)generatedinayear/8760hours

118. 19-Apr-24 26

120. 19-Apr-24 28

121. 19-Apr-24 29

122. 19-Apr-24 30

123. 19-Apr-24 31

124. 19-Apr-24 32

125. Residential Customers
Chittoor, Anantapur, Kurnool, YSR Kadapa, SPSR Nellore
districts -Southern Power Company.
Srikakulam, Vizianagaram, Visakhapatnam, East Godavari,
West Godavari districts -Eastern Power Company.
Krishna, Guntur and PrakasamDistricts -Central Power
Company.
https://aptransco.co.in

126. 34

127. 35

128. Grid Map

129. 37

133. TARIFF
The rate at which electrical energy is supplied to a consumer is known
as tariff
Objectivesoftariff.Likeothercommodities,electricalenergy
isalsosoldatsucharatesothatitnotonlyreturnsthecostbut
alsoearnsreasonableprofit.Therefore,atariffshouldinclude
thefollowingitems:
(i)Recoveryofcostofproducingelectricalenergyatthepower
station.
(ii)Recoveryofcostonthecapitalinvestmentintransmission
anddistributionsystems.
(iii)Recoveryofcostofoperationandmaintenanceofsupply
ofelectricalenergye.g.,meteringequipment,billingetc.
(iv)Asuitableprofitonthecapitalinvestment.

134. CharacteristicsofaTariff
(i)Properreturn:Thetariffshouldbesuchthatitensurestheproperreturnfrom
eachconsumer.Inotherwords,thetotalreceiptsfromtheconsumersmustbeequal
tothecostofproducingandsupplyingelectricalenergyplusreasonableprofit.
(ii)Fairness:Thetariffmustbefairsothatdifferenttypesofconsumersare
satisfiedwiththerateofchargeofelectricalenergy.Thusabigconsumershould
bechargedatalowerratethanasmallconsumer.Itisbecauseincreasedenergy
consumptionspreadsthefixedchargesoveragreaternumberofunits,thus
reducingtheoverallcostofproducingelectricalenergy.
(iii)Simplicity:Thetariffshouldbesimplesothatanordinaryconsumercan
easilyunderstandit.Acomplicatedtariffmaycauseanoppositionfromthepublic
whichisgenerallydistrustfulofsupplycompanies.
(iv)Reasonableprofit:Theprofitelementinthetariffshouldbereasonable.An
electricsupplycompanyisapublicutilitycompanyandgenerallyenjoysthe
benefitsofmonopoly.
(v)Attractive:Thetariffshouldbeattractivesothatalargenumberofconsumers
areencouragedtouseelectricalenergy.Effortsshouldbemadetofixthetariffin
suchawaysothatconsumerscanpayeasily.

135. Types of Tariff
1.Simpletariff.Whenthereisafixedrateperunitofenergyconsumed,itis
calledasimpletarifforuniformratetariff.Inthistypeoftariff,theprice
chargedperunitisconstanti.e.,itdoesnotvarywithincreaseordecreasein
numberofunitsconsumed.Theconsumptionofelectricalenergyatthe
consumer’sterminalsisrecordedbymeansofanenergymeter.Thisisthe
simplestofalltariffsandisreadilyunderstoodbytheconsumers.
Disadvantages
(i)Thereisnodiscriminationbetweendifferenttypesofconsumerssince
everyconsumerhastopayequitablyforthefixedcharges.
(ii)Thecostperunitdeliveredishigh.
(iii)Itdoesnotencouragetheuseofelectricity.
2.Flatratetariff.Whendifferenttypesofconsumersarechargedatdifferent
uniformperunitrates,itiscalledaflatratetariff.Inthistypeoftariff,the
consumersaregroupedintodifferentclassesandeachclassofconsumersis
chargedatadifferentuniformrate.Thedifferentclassesofconsumersaremade
takingintoaccounttheirdiversityandloadfactors.Theadvantageofsucha
tariffisthatitismorefairtodifferenttypesofconsumersandisquitesimplein
calculations.

136. Disadvantages
(i)Sincetheflatratetariffvariesaccordingtothewaythesupplyisused,separate
metersarerequiredforlightingload,powerloadetc.Thismakestheapplicationof
suchatariffexpensiveandcomplicated.
(ii)Aparticularclassofconsumersischargedatthesamerateirrespectiveofthe
magnitudeofenergyconsumed.However,abigconsumershouldbechargedata
lowerrateasinhiscasethefixedchargesperunitarereduced.
3.Blockratetariff.Whenagivenblockofenergyischargedataspecifiedrate
andthesucceedingblocksofenergyarechargedatprogressivelyreducedrates,itis
calledablockratetariff.Inblockratetariff,theenergyconsumptionisdividedinto
blocksandthepriceperunitisfixedineachblock.Thepriceperunitinthefirst
blockisthehighest**anditisprogressivelyreducedforthesucceedingblocksof
energy.Forexample,thefirst30unitsmaybechargedattherateof60paiseper
unit;thenext25unitsattherateof55paiseperunitandtheremainingadditional
unitsmaybechargedattherateof30paiseperunit.
Theadvantageofsuchatariffisthattheconsumergetsanincentivetoconsume
moreelectricalenergy.Thisincreasestheloadfactorofthesystemandhencethe
costofgenerationisreduced.However,itsprincipaldefectisthatitlacksameasure
oftheconsumer’sdemand.Thistypeoftariffisbeingusedformajorityof
residentialandsmallcommercialconsumers.

137. 4.Two-parttariff.Whentherateofelectricalenergyischargedonthebasis
ofmaximumdemandoftheconsumerandtheunitsconsumed,itiscalleda
two-parttariff.
Totalcharges=Rs(b×kW+c×kWh)
where,b=chargeperkWofmaximumdemand
c=chargeperkWhofenergyconsumed
Thistypeoftariffismostlyapplicabletoindustrialconsumerswhohave
appreciablemaximumdemand.
Advantages
(i)Itiseasilyunderstoodbytheconsumers.
(ii)Itrecoversthefixedchargeswhichdependuponthemaximumdemandof
theconsumerbutareindependentoftheunitsconsumed.
Disadvantages
(i)Theconsumerhastopaythefixedchargesirrespectiveofthefactwhether
thehasconsumedornotconsumedtheelectricalenergy.
(ii)Thereisalwayserrorinassessingthemaximumdemandoftheconsumer

138. 5.Maximumdemandtariff.Itissimilartotwo-parttariffwiththeonly
differencethatthemaximumdemandisactuallymeasuredbyinstalling
maximumdemandmeterinthepremisesoftheconsumer.Thisremoves
theobjectionoftwo-parttariffwherethemaximumdemandisassessed
merelyonthebasisoftherateablevalue.Thistypeoftariffismostly
appliedtobigconsumers.However,itisnotsuitableforasmallconsumer
(e.g.,residentialconsumer)asaseparatemaximumdemandmeteris
required.
6.Powerfactortariff.Thetariffinwhichpowerfactoroftheconsumer’s
loadistakenintoconsiderationisknownaspowerfactortariff.Inana.c.
system,powerfactorplaysanimportantrole.Alowpowerfactor
increasestheratingofstationequipmentandlinelosses.

139. 7.Three-parttariff.Whenthetotalchargetobemadefromthe
consumerissplitintothreepartsviz.,fixedcharge,semi-fixedcharge
andrunningcharge,itisknownasathree-parttariff.i.e.,
Totalcharge=Rs(a+b×kW+c×kWh)
wherea=fixedchargemadeduringeachbillingperiod.
Itincludesinterestanddepreciationonthecostofsecondary
distributionandlabourcostofcollectingrevenues,
b=chargeperkWofmaximumdemand,
c=chargeperkWhofenergyconsumed.

140. Cost of Electrical Energy
Thetotalcostofelectricalenergygeneratedcanbedividedintothreeparts,namely
(i)Fixedcost(ii)Semi-fixedcost(iii)Runningoroperatingcost
(i)Fixedcost.Itisthecostwhichisindependentofmaximumdemandandunits
generated.Thefixedcostisduetotheannualcostofcentralorganisation,interestoncapital
costoflandandsalariesofhighofficials.Theannualexpenditureonthecentralorganisation
andsalariesofhighofficialsisfixedsinceithastobemetwhethertheplanthashighorlow
maximumdemandoritgenerateslessormoreunits.Further,thecapitalinvestmentonthe
landisfixedandhencetheamountofinterestisalsofixed.
(ii)Semi-fixedcost.Itisthecostwhichdependsuponmaximumdemandbutis
independentofunitsgenerated.Thesemi-fixedcostisdirectlyproportionaltothemaximum
demandonpowerstationandisonaccountofannualinterestanddepreciationoncapital
investmentofbuildingandequipment,taxes,salariesofmanagementandclericalstaff.The
maximumdemandonthepowerstationdeterminesitssizeandcostofinstallation.The
greaterthemaximumdemandonapowerstation,thegreaterisitssizeandcostof
installation.
(iii)Runningcost.Itisthecostwhichdependsonlyuponthenumberofunits
generated.Therunningcostisonaccountofannualcostoffuel,lubricatingoil,
maintenance,repairsandsalariesofoperatingstaff.

143. 51

144. POWERSYSTEMS-I
Dr.G.NageswaraRao
Professor
LakireddyBaliReddyCollegeofEngineering

145. AC DISTRIBUTION & CABLES
03
AC DISTRIBUTION

149. BusBar
Theconductingmaterialoraconductorusedtocollectpower
fromtheinputterminalsofanelectricalsystemanddistribute
ittovariousoutputcircuitsisknownasanelectricalbusbar
orbussystem.Itactsasajunction,wheretheincoming
powerandoutgoingpowermeets.Itisusedtocollectallthe
electricalpowerinoneplace.Itisavailableintheformof
rectangularstrips,roundtubes,roundbars,andsquarebars
madeupofaluminium,copper,andbrass.

150. DistributionSystem
Thepartofpowersystemwhichdistributeselectricpowerforlocal
useisknownasdistributionsystem.

151. Distributionsystemgenerallyconsistsoffeeders,distributorsandtheservice mains.Fig
showsthesinglelinediagramofatypicallowtensiondistributionsystem.
(i)Feeders.Afeederisaconductorwhichconnectsthesub-station(orlocalisedgenerating
station)totheareawherepoweristobedistributed.Generally,notappingsaretakenfrom
thefeedersothatcurrentinitremainsthesamethroughout.Themainconsiderationinthe
designofafeederisthecurrentcarryingcapacity.
(ii)Distributor.Adistributorisaconductorfromwhichtappingsaretakenforsupplytothe
consumers.InFig.AB,BC,CDandDAarethedistributors.Thecurrentthrougha
distributorisnotconstantbecausetappingsaretakenatvariousplacesalongitslength.
Whiledesigningadistributor,voltagedropalongitslengthisthemainconsideration
sincethestatutorylimitofvoltagevariationsis±6%ofratedvalueattheconsumers’
terminals.
(iii)Servicemains.Aservicemainsisgenerallyasmallcablewhichconnectsthedistributorto
theconsumers’terminals.

152. ClassificationofDistributionSystems
(i)Natureofcurrent.Accordingtonatureofcurrent,distributionsystemmaybeclassifiedas
(a)d.c.distributionsystem(b)a.c.distributionsystem.
Now-a-days,a.c.systemisuniversallyadoptedfordistributionofelectricpowerasitissimpler
andmoreeconomicalthandirectcurrentmethod.
(ii)Typeofconstruction.Accordingtotypeofconstruction,distributionsystemmaybe
classifiedas
(a)overheadsystem(b)undergroundsystem.
Theoverheadsystemisgenerallyemployedfordistributionasitis5to10timescheaper
thantheequivalentundergroundsystem.Ingeneral,theundergroundsystemisusedatplaces
whereoverheadconstructionisimpracticableorprohibitedbythelocallaws.
(iii)Schemeofconnection.Accordingtoschemeofconnection,thedistributionsystemmaybe
classifiedas
(a) Radialsystem(b)ringmainsystem(c)inter-connectedsystem.
Eachschemehasitsownadvantagesanddisadvantages.

153. A.C. distribution calculations differ from those of d.c. distribution
in the following respects :
(i) In case of d.c. system, the voltage drop is due to resistance alone. However, in
a.c. system, the voltage drops are due to the combined effects of resistance,
inductance and capacitance.
(ii) In a d.c. system, additions and subtractions of currents or voltages are done
arithmetically but in case of a.c. system, these operations are done vectorially.
(iii) In an a.c. system, power factor (p.f.) has to be taken into account. Loads
tapped off form the distributor are generally at different power factors. There are
two ways of referring power factor viz
(a) It may be referred to supply or receiving end voltage which is regarded as the
reference vector.
(b) It may be referred to the voltage at the load point itself.

154. A.C.Distribution
Thea.c.distributionsystemisclassifiedinto
(i)primarydistributionsystemand(ii)secondarydistributionsystem
Primarydistributionsystem:Itisthatpartofa.c.distributionsystemwhich
operatesatvoltagessomewhathigherthangeneralutilisationandhandleslargeblocksof
electricalenergythantheaveragelow-voltageconsumeruses.Thevoltageusedfor
primarydistributiondependsupontheamountofpowertobeconveyedandthedistance
ofthesubstationrequiredtobefed.Themostcommonlyusedprimarydistributionvoltages
are11kV,6·6kVand3·3kV.Duetoeconomicconsiderations,primarydistributionis
carriedoutby3-phase,3-wiresystem.Fig.showsatypicalprimarydistributionsystem.
Electricpowerfromthegeneratingstationistransmittedathighvoltagetothesubstation
locatedinornearthecity.Atthissubstation,voltageissteppeddownto11kVwiththe
helpofstep-downtransformer.Powerissuppliedtovarioussubstationsfordistributionorto
bigconsumersatthisvoltage.Thisformsthehighvoltagedistributionorprimarydistribution.

156. Secondarydistributionsystem.
Itisthatpartofa.c.distributionsystemwhichincludestherangeofvoltagesatwhichthe
ultimateconsumerutilisestheelectricalenergydeliveredtohim.Thesecondarydistribution
employs400/230V,3-phase,4-wiresystem.
Fig.showsatypicalsecondarydistributionsystem.Theprimarydistributioncircuitdelivers
powertovarioussubstations,calleddistributionsubstations.Thesubstationsaresituatednear
theconsumers’localitiesandcontainstepdowntransformers.
Ateachdistributionsubstation,thevoltageissteppeddownto400Vandpowerisdelivered
by3-phase,4-wirea.c.system.
Thevoltagebetweenanytwophasesis400Vandbetweenanyphaseandneutralis230V.
Thesinglephasedomesticloadsareconnectedbetweenanyonephaseandtheneutral,
whereas3-phase400Vmotorloadsareconnectedacross3-phaselinesdirectly.

158. OverheadVersusUndergroundSystem
(i)Publicsafety.Theundergroundsystemismoresafethanoverheadsystembecausealldistribution
wiringisplacedundergroundandtherearelittlechancesofanyhazard.
(ii)Initialcost.Theundergroundsystemismoreexpensiveduetothehighcostoftrenching,conduits,
cables,manholesandotherspecialequipment.Theinitialcostofanundergroundsystemmaybefiveto
tentimesthanthatofanoverheadsystem.
(iii)Flexibility.Theoverheadsystemismuchmoreflexiblethantheundergroundsystem.Inthelattercase,
manholes,ductlinesetc.,arepermanentlyplacedonceinstalledandtheloadexpansioncanonlybe
metbylayingnewlines.However,onanoverheadsystem,poles,wires,transformersetc.,canbeeasily
shiftedtomeetthechangesinloadconditions.
(iv)Faults.Thechancesoffaultsinundergroundsystemareveryrareasthecablesarelaid
undergroundandaregenerallyprovidedwithbetterinsulation.
(v)Appearance.Thegeneralappearanceofanundergroundsystemisbetterasallthedistributionlines
areinvisible.Thisfactorisexertingconsiderablepublicpressureonelectricsupplycompaniestoswitch
overtoundergroundsystem.
(vi)Faultlocationandrepairs.Ingeneral,therearelittlechancesoffaultsinanundergroundsystem.
However,ifafaultdoesoccur,itisdifficulttolocateandrepaironthissystem.Onanoverheadsystem,
theconductorsarevisibleandeasilyaccessiblesothatfaultlocationsandrepairscanbeeasilymade.

159. (vii)Currentcarryingcapacityandvoltagedrop.Anoverheaddistributionconductorhasa
considerablyhighercurrentcarryingcapacitythananundergroundcableconductorofthe
samematerialandcross-section.Ontheotherhand,undergroundcableconductorhasmuch
lowerinductivereactancethanthatofanoverheadconductorbecauseofcloserspacingof
conductors.
(viii)Usefullife.Theusefullifeofundergroundsystemismuchlongerthanthatofanoverhead
system.Anoverheadsystemmayhaveausefullifeof25years,whereasanunderground
systemmayhaveausefullifeofmorethan50years.
(ix)Maintenancecost.Themaintenancecostofundergroundsystemisverylowascompared
withthatofoverheadsystembecauseoflesschancesoffaultsandserviceinterruptionsfrom
wind,ice,lightningaswellasfromtraffichazards.
(x)Interferencewithcommunicationcircuits.Anoverheadsystemcauseselectromagnetic
interferencewiththetelephonelines.Thepowerlinecurrentsaresuperimposedonspeech
currents,resultinginthepotentialofthecommunicationchannelbeingraisedtoanundesirable
level.However,thereisnosuchinterferencewiththeundergroundsystem.

160. RequirementsofaDistributionSystem
(i)Propervoltage.Oneimportantrequirementofadistributionsystemisthatvoltagevariationsatconsumer’s
terminalsshouldbeaslowaspossible.Thechangesinvoltagearegenerallycausedduetothevariationofload
onthesystem.Lowvoltagecauseslossofrevenue,inefficientlightingandpossibleburningoutofmotors.High
voltagecauseslampstoburnoutpermanentlyandmaycausefailureofotherappliances.Therefore,agood
distributionsystemshouldensurethatthevoltagevariationsatconsumersterminalsarewithinpermissiblelimits.The
statutorylimitofvoltagevariationsis±6%oftheratedvalueattheconsumer’sterminals.Thus,ifthedeclared
voltageis230V,thenthehighestvoltageoftheconsumershouldnotexceed244Vwhilethelowestvoltageofthe
consumershouldnotbelessthan216V.
(ii)Availabilityofpowerondemand.Powermustbeavailabletotheconsumersinanyamountthattheymayrequire
fromtimetotime.Forexample,motorsmaybestartedorshutdown,lightsmaybeturnedonoroff,without
advancewarningtotheelectricsupplycompany.Aselectricalenergycannotbestored,therefore,thedistribution
systemmustbecapableofsupplyingloaddemandsoftheconsumers.Thisnecessitatesthatoperatingstaffmust
continuouslystudyloadpatternstopredictinadvancethosemajorloadchangesthatfollowtheknownschedules.
(iii)Reliability.Modernindustryisalmostdependentonelectricpowerforitsoperation.Homesandofficebuildings
arelighted,heated,cooledandventilatedbyelectricpower.Thiscallsforreliableservice.Unfortunately,electric
power,likeeverythingelsethatisman-made,canneverbeabsolutelyreliable.However,thereliabilitycanbe
improvedtoaconsiderableextentby(a)interconnectedsystem(b)reliableautomaticcontrolsystem(c)providing
additionalreservefacilities.

161. DesignConsiderationsinDistributionSystem
Goodvoltageregulationofadistributionnetworkisprobablythemost
importantfactorresponsiblefordeliveringgoodservicetotheconsumers.Forthis
purpose,designoffeedersanddistributorsrequirescarefulconsideration.
(i)Feeders.Afeederisdesignedfromthepointofviewofitscurrentcarrying
capacitywhilethevoltagedropconsiderationisrelativelyunimportant.Itis
becausevoltagedropinafeedercanbecompensatedbymeansofvoltage
regulatingequipmentatthesubstation.
(ii)Distributors.Adistributorisdesignedfromthepointofviewofthevoltage
dropinit.Itisbecauseadistributorsuppliespowertotheconsumersandthereis
astatutorylimitofvoltagevariationsattheconsumer’sterminals(±6%ofrated
value).Thesizeandlengthofthedistributorshouldbesuchthatvoltageatthe
consumer’sterminalsiswithinthepermissiblelimits.

162. ConnectionSchemesofDistributionSystem
Alldistributionofelectricalenergyisdonebyconstantvoltagesystem
(i)RadialSystem.Inthissystem,separatefeedersradiatefromasinglesubstationand
feedthedistributorsatoneendonly.Fig.1showsasinglelinediagramofaradialsystemfor
d.c.distributionwhereafeederOCsuppliesadistributorABatpointA.Obviously,the
distributorisfedatoneendonlyi.e.,pointAisthiscase.Fig.2showsasinglelinediagramof
radialsystemfora.c.distribution.Theradialsystemisemployedonlywhenpoweris
generatedatlowvoltageandthesubstationislocatedatthecentreoftheload.Thisisthe
simplestdistributioncircuitandhasthelowestinitialcost.However,itsuffersfrom
thefollowingdrawbacks:
(a)Theendofthedistributornearesttothefeedingpointwillbeheavilyloaded.
(b)Theconsumersaredependentonasinglefeederandsingledistributor.Therefore,anyfault
onthefeederordistributorcutsoffsupplytotheconsumerswhoareonthesideofthefault
awayfromthesubstation.
(c)Theconsumersat thedistant endof thedistributor would be subjectedto seriousvoltage
fluctuationswhentheloadonthedistributorchanges.Duetotheselimitations,thissystemis
usedforshortdistancesonly.

164. (ii)Ringmainsystem.Inthissystem,theprimariesofdistributiontransformersforma
loop.Theloopcircuitstartsfromthesubstationbus-bars,makesaloopthroughtheareatobe
served,andreturnstothesubstation.Fig.showsthesinglelinediagramofringmainsystem
fora.c.distributionwheresubstationsuppliestotheclosedfeederLMNOPQRS.
ThedistributorsaretappedfromdifferentpointsM,OandQofthefeeder
through distributiontransformers.Theringmainsystemhasthefollowingadvantages:
(a)Therearelessvoltagefluctuationsatconsumer’sterminals.
(b)Thesystemisveryreliableaseachdistributorisfedviatwofeeders.Intheeventoffault
onanysectionof thefeeder, thecontinuityof supplyismaintained. Forexample,suppose
thatfaultoccursatanypointFofsectionSLMofthefeeder.ThensectionSLMofthe
feedercanbeisolatedforrepairsandatthesametimecontinuityofsupplyismaintainedto
alltheconsumersviathefeederSRQPONM.

166. (iii)Interconnectedsystem.Whenthefeederringisenergisedbytwoormorethantwo
generatingstationsorsubstations,itiscalledinter-connectedsystem.Fig.showsthesingleline
diagramofinterconnectedsystemwheretheclosedfeederringABCDissuppliedbytwo
substationsS1andS2atpointsDandCrespectively.DistributorsareconnectedtopointsO,P,
QandRofthefeederringthroughdistributiontransformers.Theinterconnectedsystemhasthe
followingadvantages:
(a)Itincreasestheservicereliability.
(b)Anyareafedfromonegeneratingstationduringpeakloadhourscanbefedfromthe
other generating station.Thisreducesreservepower capacity and increasesefficiencyof the
system.

168. SELF-TEST
1.Fillintheblanksbyinsertingappropriatewords/figures.
(i)Theundergroundsystemhas.............initialcostthantheoverheadsystem.
(ii)Aringmainsystemofdistributionis.............reliablethantheradialsystem.
(iii)Thedistributiontransformerlinkstheprimaryand.............distributionsystems
(iv)Themostcommonsystemforsecondarydistributionis............3-phase,.............wiresystem.
(v)Thestatutorylimitforvoltagevariationsattheconsumer’sterminalsis.............%ofrated
value.
(vi)Theservicemainsconnectthe.............andthe.............
(vii)Theoverheadsystemis.............flexiblethanundergroundsystem.
ANSWERSTOSELF-TEST
(i)more(ii)more(iii)secondary(iv)400/230V,4(v)=6(vi)distributor,consumer
terminals(vii)more

169. CHAPTERUNITENDQUESTIONS
1.Whatdoyouunderstandbydistributionsystem?
2.Drawasinglelinediagramshowingatypicaldistributionsystem.
3.Defineandexplaintheterms:feeder,distributorandservicemains.
4.Discusstherelativemeritsanddemeritsofundergroundandoverheadsystems.
5.Explainthefollowingsystemsofdistribution:
(i)Radialsystem
(ii)Ringmainsystem
(iii)Interconnectedsystem
6.Discussbrieflythedesignconsiderationsindistributionsystem.
7.Withaneatdiagram,explainthecompletea.c.systemfordistributionof
electricalenergy.

170. Methods of Solving A.C. Distribution Problems
(i)w.r.t.receivingorsendingendvoltage
(ii)w.r.t.toloadvoltageitself.

171. (i)Power factors referred to receiving end voltage

182. Example:A3-phase,400VdistributorABisloadedasshowninFig.14.8.The3-phaseloadat
pointCtakes5Aperphaseatap.f.of0·8lagging.AtpointB,a3-phase,400Vinduction
motorisconnectedwhichhasanoutputof10H.P.withanefficiencyof90%andp.f.0·85
lagging.IfvoltageatpointBistobemaintainedat400V,whatshouldbethevoltageat
pointA?Theresistanceandreactanceofthelineare1Ωand0·5Ωperphaseperkilometre
respectively
Solution.Itisconvenienttoconsideronephaseonly.Figshowsthesinglelinediagramofthedistributor.
Impedanceofthedistributorperphaseperkilometre=(1+j0·5)

194. CHAPTER REVIEW TOPICS
1. How does a.c. distribution differ from d.c. distribution ?
2. What is the importance of load power factors in a.c. distribution ?
3. Describe briefly how will you solve a.c. distribution problems ?
4. Write short notes on the following :
(i) Difference between d.c. and a.c. distribution
(ii) Systems of a.c. distribution
5. Discuss about feeder, distributor and service main.

195. 1. Fill in the blanks by inserting appropriate words/figures.
(i) The most common system for secondary distribution is 400/..... V, 3-phase, ......... wire
system.
(ii) In a 3-phase, 4-wire a.c. system, if the loads are balanced, then current in the neutral wire
is .........
(iii) Distribution transformer links the ............ and ........... systems.
(iv) The 3-phase, 3-wire a.c. system of distribution is used for .......... loads.
(v) For combined power and lighting load, .............. system is used.
2. Pick up the correct words/figures from brackets and fill in the blanks.
(i) 3-phase, 4-wire a.c. system of distribution is used for .............. load. (balanced,
unbalanced)
(ii) In a.c. system, additions and subtractions of currents are done ..............
(vectorially, arithmetically)
(iii) The area of X-section of neutral is generally .............. that of any line conductor. (the
same, half)
(iv) For purely domestic loads, .............. a.c. system is employed for distribution

196. ASSIGNMENT QUESTIONS
i)Whatarethedifferentdistributionsystemadoptedinpowersystem?
ii)Whataretheadvantagesofringmaindistributionsystem?
iii)Whatarethetypesofdcdistributionsystemarethere?Explain.
iv)Asinglephasedistributor2km.longsuppliesaloadof120Aat0.8p.f.lagging
atitsfarendandaloadof80Aat0.9p.f.laggingatitsmidpoint.Bothpowerfactor
arereferredtothevoltageatthefarend.Theresistanceandreactanceperkm.go
andreturnare0.05Ωand0.1Ωrespectively.Thevoltageatthefarendismaintained
at230v,calculate
i)Voltageatthesendingend.
ii)Phaseanglebetweenvoltageatthetwoend.
ANSWERS TO SELF-TEST
1. (i) 230, 4 (ii) zero (iii) primary, secondary (iv) balanced (v) 3-phase 4-wire.
2. (i) unbalanced (ii) vectorially (iii) half (iv) single phase 2-wire.

197. Bus Bar Arrangements
Duringthedistributionofelectricalpowertovariousoutputcircuits,twoormorewires
areconnectedtoasinglewire.
Theimproperelectricalconnectiongetsopenedandtheinsulationofthewiremayget
damagedduetoheatgenerationinthewires.
Thisconditionmayleadtoanopencircuit,whichistoodangerousforthedistributionof
power.
Insuchcases,toavoidopen-circuitconditions,themultiplewiresareconnected
properlyusinganelectricbussystem.
Thebusbarisanelectricalcomponentusedinelectricaldistributionsystemstocollect
currentfromtheinputterminalsofanelectricalsystemanddistributesittovarious
outputcircuits.
Itisusedasajunctionbetweentheinputpowerandoutputpower.
Itdistributesthepowertovariousoutputcircuitswithmoreflexibility.

198. Busbar Arrangements in Substations

201. Main and Transfer Busbar Arrangement

204. Selection and location of site for substation
1.It should be located nearer or at the centerof the gravity of load.
2.It should provide safe and reliable arrangement
3.Maintenance of regulation clearances (deals with political issues)
4.Facilities for carrying out repairs and maintenance.
5.Immediate facilities against abnormalities such as possibility of
explosion or fire etc.
6.Good design and construction
7.Provision of suitable switchgear and protective gear etc.
8.Land cost
9.Number of incoming and out

205. 10.Transfer of power
11. Short-circuit levels
12. Types of substation (objective/function)
13. It should be away from airport and terrorist zones
14. Physical amenities should be available for engineers such as
transportation, schools, houses, hospitals, communication services,
availability of drinking water etc.
15.Drainage facility for rainwater
16.Should be easily operated and maintained
17.Should involve minimum capital cost
18.Provision for future expansion

206. Selection and rating of S/s equipment
• Surge arrester
• CT
• PT
• Isolator
• Circuit breaker
• Transformer
• Busbar
• Shunt capacitor
• Earth switch
• Relays
• Auxiliaries

209. UNIT-III
UNDERGROUND CABLES
Dr.G.Nageswara Rao
Professor
EEE Department
LBRCE

210. UndergroundCable:Consistsofoneormoreconductorscoveredwith
suitableinsulationandsurroundedbyaprotectingcover.
Requirements:
(i)Theconductorusedincablesshouldbetinnedstrandedcopperoraluminium
ofhighconductivity.Strandingisdonesothatconductormaybecome
flexibleandcarrymorecurrent.
(ii)Theconductorsizeshouldbesuchthatthecablecarriesthedesiredload
currentwithoutoverheatingandcausesvoltagedropwithinpermissible
limits.
(iii)Thecablemusthaveproperthicknessofinsulationinordertogivehigh
degreeofsafetyandreliabilityatthevoltageforwhichitisdesigned
(iv)Thecablemustbeprovidedwithsuitablemechanicalprotectionsothatit
maywithstandtheroughuseinlayingit.
(v)Thematerialsusedinthemanufactureofcablesshouldbesuchthatthereis
completechemicalandphysicalstabilitythroughout.

211. 1.Better general appearance
2.Less liable to damage through storms or lighting
3.Low maintenance cost
4.Less chances of faults
5.Small voltage drops
1.Greater installation cost
2.Insulation problems at high voltages compared with equivalent
overhead system
Advantages & Disadvantages

212. Requirements of the insulating materials used for cable are:
1.High insulation resistance.
2.High dielectric strength.
3.Good mechanical properties i.e., tenacity and elasticity.
4.It should not be affected by chemicals around it.
5.It should be non-hygroscopic because the dielectric strength of any
material goes very much down with moisture content
Vulcanized rubber insulated cables are used for wiring of houses, buildings and
factories for low power work.
There are two main groups of synthetic rubber material :
(i) general purpose synthetics which have rubber-like properties
(ii) special purpose synthetics which have better properties than the rubber
e.g. fire resisting and oil resisting properties.
The four main types are:
(i) butyl rubber, (ii) silicon rubber, (iii) neoprene, and (iv) styrene rubber.

213. Polyvinyl Chloride (PVC)
PVCmaterialhasmanygrades.
GeneralPurposeType:Itisusedbothforsheathingandasaninsulatingmaterial.Inthis
compoundmonomericplasticizersareused.ItistobenotedthataV.R.insulatedPVC
sheathedcableisnotgoodforuse.
HardGradePVC:Thesearemanufacturedwithlessamountofplasticizerascomparedwith
generalpurposetype.HardgradePVCareusedforhighertemperaturesforshortduration
oftimelikeinsolderingandarebetterthanthegeneralpurposetype.Hardgradecannotbe
usedforlowcontinuoustemperatures.
HeatResistingPVC:Becauseoftheuseofmonomericplasticizerwhichvolatilizesat
temperature80°C–100°C,generalpurposetypecompoundsbecomestiff.Byusing
polymericplasticizersitispossibletooperatethecablescontinuouslyaround100°C.
PVCcompoundsarenormallycostlierthantherubbercompoundsandthepolymeric
plasticizedcompoundsaremoreexpensivethanthemonomericplasticizedones.PVCis
inerttooxygen,oils,alkalisandacidsand,therefore,iftheenvironmentalconditionsare
suchthatthesethingsarepresentintheatmosphere,PVCismoreusefulthanrubber.

214. Impregnated Paper
Asuitablelayerofthepaperislappedontheconductordependingupontheoperating
voltage.Itisthendriedbythecombinedapplicationofheatandvacuum.Thisiscarried
outinahermeticallysealedsteamheatedchamber.Thetemperatureis120°–130°Cbefore
vacuumiscreated.
Protective Coverings
Acottonbraidisappliedovertheinsulatedconductorandisthenimpregnatedwitha
compound,whichiswaterandweatherproof.Therubberinsulatedcablesarecoveredwith
aleadalloysheathandisusedforfixedinstallationinsideoroutsidebuildingsinplaceof
braidedandcompoundfinishedcableinconduit.
Polythene
Thismaterialcanbeusedforhighfrequencycables.Thishasbeenusedtoalimitedextent
forpowercablesalso.Thethermaldissipationpropertiesarebetterthanthoseof
impregnatedpaperandtheimpulsestrengthcomparesfavorablywithanimpregnatedpaper-
insulatedcable.Themaximumoperatingtemperatureofthiscableundershortcircuitsis
100°C.

215. Construction

216. 1.CoresorConductors.Acablemayhaveoneormorethanonecore
(conductor)dependinguponthetypeofserviceforwhichitisintended.The
conductorsaremadeoftinnedcopperoraluminiumandareusuallystrandedin
ordertoprovideflexibilitytothecable.
2.Insulation.Eachcoreorconductorisprovidedwithasuitablethicknessof
insulation,thethicknessoflayerdependinguponthevoltagetobewithstoodby
thecable.Thecommonlyusedmaterialsforinsulationareimpregnatedpaper,
varnishedcambricorrubbermineralcompound.
3.Metallicsheath.Inordertoprotectthecablefrommoisture,gasesorother
damagingliquids(acidsoralkalies)inthesoilandatmosphere,ametallicsheath
ofleadoraluminiumisprovidedovertheinsulation.

217. 4.Bedding.Overthemetallicsheathisappliedalayerofbeddingwhich
consistsofafibrousmateriallikejuteorhessiantape.Thepurposeofbedding
istoprotectthemetallicsheathagainstcorrosionandfrommechanicalinjury
duetoarmouring.
5.Armouring.Overthebedding,armouringisprovidedwhichconsistsofone
ortwolayersofgalvanisedsteelwireorsteeltape.Itspurposeistoprotectthe
cablefrommechanicalinjurywhilelayingitandduringthecourseofhandling.
Armouringmaynotbedoneinthecaseofsomecables.
6.Serving.Inordertoprotectarmouringfromatmosphericconditions,alayer
offibrousmaterial(likejute)similartobeddingisprovidedoverthe
armouring.Thisisknownasserving.

218. Types of Cables
Classifiedintwowaysaccordingto
(i)Typeofinsulatingmaterialusedintheirmanufacture
(ii)Voltageforwhichtheyaremanufactured.
Lattermethodofclassificationis:
(i)Low-tension(L.T.)cables—upto1000V
(ii)High-tension(H.T.)cables—upto11,000V
(iii)Super-tension(S.T.)cables—from22kVto33kV
(iv)Extrahigh-tension(E.H.T.)cables—from33kVto66kV
(v)Extrasupervoltagecables—beyond132kV
Acablemayhaveoneormorethanonecoredependinguponthetypeofserviceforwhichit
isintended.Itmaybe(i)single-core(ii)two-core(iii)three-core(iv)four-coreetc.
Fora3-phaseservice,either3-single-corecablesorthree-corecablecanbeuseddepending
upontheoperatingvoltageandloaddemand

219. Cables for 3-Phase Service
1.Belted cables —upto 11 kV
2.Screened cables —from 22 kV to 66 kV
3.Pressure cables —beyond 66 kV

220. Belted cables

221. Screened cables

222. Pressure cables
Single core conductor channel oil filled cable
Single core sheath channel oil filled cable
Three core oil filled cable
Three core pressure cable

224. 
Unit-IV
ELECTRICAL ANDMECHANICAL DESIGNOF
TRANSMISSIONLINES
Transmission line sag calculation.
InductanceandCapacitancecalculationsoftransmission
lines:lineconductors,inductanceandcapacitanceofsingle
phaseandthreephaselineswithsymmetricaland
unsymmetrical spacing,Composite conductors
transposition,bundledconductors,andeffectofearthon
capacitance

225. Structure Types and Voltages

228. •Thedifferenceinlevelbetweenpointsof
supportsandthelowestpointontheconductoris
calledsag.

231. Sag Template
Thesagtemplateisusedforallocatingthepositionandheightof
thesupportscorrectlyontheprofile.Thesagtemplatedecidedthe
limitationsofverticalandwindload.Italsolimitstheminimum
clearanceanglebetweenthesagandthegroundforsafetypurpose.
Thesagtemplateisusuallymadeupoftransparentcelluloid,
perplex,orsometimescardboard.Thefollowingcurvesaremarked
onit.
1.Hot Template Curve or Hot Curve
2.Ground Clearance Curve
3.Support Foot or Tower Curve
4.Cold Curve or Uplift Curve

233. HotCurve–Thehotcurveisobtainedbyplottingthesagatmaximum
temperatureagainstspanlength.Itshowswherethesupportsmustbelocatedto
maintaintheprescribedgroundclearance.
GroundClearanceCurve–Theclearancecurveisbelowthehotcurve.Itis
drawnparalleltothehotcurveandataverticaldistanceequaltotheground
clearanceasprescribedbytheregulationforthegivenline.
SupportFootCurve–Thiscurveisdrawnforlocatingthepositionofthe
supportsfortowerlines.Itshowstheheightfromthebaseofthestandardsupport
tothepointofattachmentofthelowerconductor.Forwoodorconcreteline,pole
linethiscurveisnotrequiredtobedrawnsincetheycanbeputinanyconvenient
position.
ColdCurveorUpliftCurve–Upliftcurveisobtainedbyplottingthesagata
minimumtemperaturewithoutwindpriceagainstspanlength.Thiscurveisdrawn
todeterminewhetherupliftofconductoroccursonanysupport.Theuplift
conductormayoccuratlowtemperaturewhenonesupportismuchlowerthan
eitheroftheadjoiningones

234. (i)supportsareatequallevels
(ii)supportsareatunequallevels

242. P1:Atransmissionlinehasaspanof150mbetweenlevelsupports.
Theconductorhasacross-sectionalareaof2cm2.Thetensioninthe
conductoris2000kg.Ifthespecificgravityoftheconductormaterial
is9·9gm/cm3andwindpressureis1·5kg/mlength,calculatethesag.
Whatistheverticalsag?

243. P2:Atransmissionlinehasaspanof214metresbetweenlevelsupports.The
conductorshaveacross-sectionalareaof3·225cm2.Calculatethefactorof
safetyunderthefollowingconditions:
Verticalsag=2·35m;Windpressure=1·5kg/mrun
Breakingstress=2540kg/cm2;Wt.ofconductor=1·125kg/mrun

245. P3:Thetowersofheight30mand90mrespectivelysupportatransmission
lineconductoratwatercrossing.Thehorizontaldistancebetweenthe
towersis500m.Ifthetensionintheconductoris1600kg,findthe
minimumclearanceoftheconductorandwaterandclearancemid-way
betweenthesupports.Weightofconductoris1·5kg/m.Basesofthetowers
canbeconsideredtobeatwaterlevel.

246. Fig.showstheconductorsuspendedbetweentwosupports
AandBatdifferentlevelswithOasthelowestpointon
theconductor.
Here, l = 500 m ; w = 1·5 kg ; T = 1600 kg.
Difference in levels between supports,
h = 90 − 30 = 60 m.
Let the lowest point O of the conductor be at a distance x1
from the support at lower level (i.e., support A) and at a
distance x2 from the support at higher level (i.e., support
B).
Obviously,
x1 + x2 = 500 m ………………...(i)

248. P4:Anoverheadtransmissionlineatarivercrossingissupportedfromtwo
towersatheightsof40mand90mabovewaterlevel,thehorizontaldistance
betweenthetowersbeing400m.Ifthemaximumallowabletensionis2000kg,
findtheclearancebetweentheconductorandwateratapointmid-waybetween
thetowers.Weightofconductoris1kg/m.

251. STRINGING CHART
StringingchartisbasicallyagraphbetweenSag,Tensionwith
Temperature.AswewantlowTensionandminimumsaginour
conductorbutthatisnotpossibleassagisinverselyproportionalto
tension.Itisbecauselowsagmeansatightwireandhightension
whereasalowtensionmeansaloosewireandincreasedsag.
Therefore,wemakecompromisebetweentwobutifthecaseof
temperatureisconsideredandwedrawgraphthenthatgraphis
calledStringingchart.

252. Electrical Design of Overhead Lines
Ana.c.transmissionlinehasresistance,inductanceand
capacitanceuniformlydistributedalongitslength.
Theseareknownasconstantsorparametersoftheline.
Theperformanceofatransmissionlinedependstoaconsiderable
extentupontheseconstants.
Forinstance,theseconstantsdeterminewhethertheefficiency
andvoltageregulationofthelinewillbegoodorpoor.
Therefore,asoundconceptoftheseconstantsisnecessaryin
ordertomaketheelectricaldesignofatransmissionlinea
technicalsuccess.

253. Constants of a Transmission Line
Atransmissionlinehasresistance,inductanceandcapacitance
uniformlydistributedalongthewholelengthoftheline
(i)Resistance.Itistheoppositionoflineconductorstocurrentflow.
(i)Inductance.Whenanalternatingcurrentflowsthrougha
conductor,achangingfluxissetupwhichlinkstheconductor.
Duetothesefluxlinkages,theconductorpossessesinductance.
(iii)Capacitance.Asanytwoconductorsofanoverheadtransmission
lineareseparatedbyairwhichactsasaninsulation,therefore,
capacitanceexistsbetweenanytwooverheadlineconductors.The
capacitancebetweentheconductorsisthechargeper unit
potentialdifference

254. Skin Effect
Whenaconductoriscarryingsteadydirectcurrent(d.c.),thiscurrentisuniformly
distributedoverthewholeX-sectionoftheconductor.However,analternating
currentflowingthroughtheconductordoesnotdistributeuniformly,ratherithas
thetendencytoconcentratenearthesurfaceoftheconductor.Thisisknownas
skineffect.
Thetendencyofalternatingcurrenttoconcentratenearthesurfaceofaconductor
isknownasskineffect.
Duetoskineffect,theeffectiveareaofcross-sectionoftheconductorthrough
whichcurrentflowsisreduced.Consequently,theresistanceoftheconductoris
slightlyincreasedwhencarryinganalternatingcurrent.Thecauseofskineffect
canbeeasilyexplained.Asolidconductormaybethoughttobeconsistingofa
largenumberofstrands,eachcarryingasmallpartofthecurrent.The*inductance
ofeachstrandwillvaryaccordingtoitsposition.Thus,thestrandsnearthecentre
aresurroundedbyagreatermagneticfluxandhencehavelargerinductancethan
thatnearthesurface.Thehighreactanceofinnerstrandscausesthealternating
currenttoflownearthesurfaceofconductor.Thiscrowdingofcurrentnearthe
conductorsurfaceistheskineffect.

255. Theskineffectdependsuponthefollowingfactors:
(i)Natureofmaterial
(ii)Diameterofwire−increaseswiththediameterof
wire.
(iii)Frequency−increaseswiththeincreasein
frequency.
(iv)Shapeofwire−lessforstrandedconductorthan
thesolidconductor.
Itmaybenotedthatskineffectisnegligiblewhenthe
supplyfrequencyislow(<50Hz)andconductor
diameterissmall(<1cm).

290. DEFINITION:Theionizationofairsurroundingthehighvoltage
transmissionlinescausingtheconductorstoglow,producinga
hissingnoisewithvioletglowcolor,iscalledCoronaDischargeor
CoronaEffect.

291. CoronaEffectinTransmissionLines:
Thisphenomenon occurswhentheelectrostaticfieldacross
thetransmissionlineconductorsproducestheconditionof
potentialgradient.Theairgetsionizedwhenthepotential
gradientattheconductorsurfacereachesthevalueof
30kV/cmatnormalpressureandtemperature.
Intransmissionlines,conductorsaresurroundedbytheair.Air
actsasadielectricmedium.Whenthevoltageofair
surroundingtheconductorexceedsthevalueof30kV/cm,
thechargingcurrentstartstoflowthroughtheair,thatisair
hasbeenionized.Theionizedairactasavirtualconductor,
producingahissingsoundwithaluminousvioletglow.

292. Advantages & Disadvantages of Corona Effect
Advantages:
Themainadvantagesofcoronaeffectsare:
1.Duetocoronaacrosstheconductor,thesheathofair
surroundingtheconductorbecomesconductivewhichrisesthe
conductordiametervirtually.Thisvirtualincreaseinthe
conductordiameterreducesthemaximumpotentialgradientor
maximumelectrostaticstress.Thus,theprobabilityofflash-overis
reduced.
2.Effectsoftransientsproducedbylightningorelectrical
surgesarealsoreducedduetothecoronaeffect.As,the
chargesinducedonthelinebysurgeorothercauses,willbe
partiallydissipatedasacoronaloss.Inthisway,coronaprotects
thetransmissionlinesbyreducingtheeffectoftransientsthat
areproducedbyvoltagesurges.

293. Disadvantages:
1.Theglowappearacrosstheconductorwhichshowsthe
powerlossoccuronit.
2.Theaudionoiseoccursbecauseofthecoronaeffectwhich
causesthepowerlossontheconductor.
3.Thevibrationofconductoroccursbecauseofcoronaeffect.
4.Thecoronaeffectgeneratestheozonebecauseofwhichthe
conductorbecomescorrosive.
5.Thecoronaeffectproducesthenon-sinusoidalsignalthusthe
non-sinusoidalvoltagedropsoccurintheline.
6.Thecoronapowerlossreducestheefficencyoftheline.
7.TheradioandTVinterferenceoccursonthelinebecauseof
coronaeffect.

294. Factors Affecting Corona Discharge
1.SupplyVoltage:Astheelectricalcoronadischargemainlydependsupon
theelectricfieldintensityproducedbytheappliedsystemvoltage.Therefore,
iftheappliedvoltageishigh,thecoronadischargewillcauseexcessive
coronalossinthetransmissionlines.Oncontrary,thecoronaisnegligibleinthe
low-voltagetransmissionlines,duetotheinadequateamountofelectricfield
requiredforthebreakdownofair.
2.ConductorSurface:Thecoronaeffectdependsupontheshape,material
andconditionsoftheconductors.Theroughandirregularsurfacei.e.,
unevennessofthesurface,decreasesthevalueofbreakdownvoltage.This
decreaseinbreakdownvoltageduetoconcentratedelectricfieldatrough
spots,giverisetomorecoronaeffect.Theroughnessofconductorisusually
causedduetothedepositionofdirt,dustandscratching.Raindrops,snow,
fogandcondensationaccumulatedontheconductorsurfacearealso
sourcesofsurfaceirregularitiesthatcanincreasecorona.

295. 3.AirDensityFactor:Airdensityfactoralsodeterminesthecoronalossin
transmissionlines.Thecoronalossininverselyproportionaltoairdensityfactor.
PowerlossishighduetocoronainTransmissionlinesthatarepassingthrougha
hillyareabecauseinahillyareathedensityofairislow.
4.SpacingbetweenConductors:Ifthedistancebetweentwoconductorsis
verylargeascomparedtothediameterofconductor,thecoronaeffectmay
nothappen.Itisbecausethelargerdistancebetweenconductorsreducesthe
electro-staticstressattheconductorsurface,thusavoidingcoronaformation.
5.Atmosphere:Inthestormyweather,thenumberofionsismorethannormal
weather.Thedecreaseinthevalueofbreakdownvoltageisfollowedbythe
increaseinthenumberofions.Asaresultofit,coronaoccursatmuchless
voltageascomparedtothebreakdownvoltagevalueinfairweather.

296. How Corona Effect is Reduced:
Ithasbeenobservedthattheintensecoronaeffectsareobservedataworkingvoltage
of33kVorabove.Onthesub-stationsorbus-barsratedfor33kVandhighervoltages,
highlyionizedairmaycauseflash-overintheinsulatorsorbetweenthephases,causing
considerabledamagetotheequipment,ifcarefuldesigningisnotmadetoreducethe
coronaeffect.Thecoronaeffectcanbereducedbythefollowingmethods:
1.ByIncreasingConductorSize:Thevoltageatwhichcoronaoccurscanberaisedby
increasingconductorsize.Hence,thecoronaeffectmaybereduced.Thisisoneofthe
reasonsthatACSRconductorswhichhavealargercross-sectionalareaareusedin
transmissionlines.
2.ByIncreasingConductorSpacing:Thecoronaeffectcanbeeliminatedbyincreasing
thespacingbetweenconductors,whichraisesthevoltageatwhichcoronaoccurs.
However,increaseinconductorspacingislimitedduetothecostofsupportingstructure
asbiggercrossarmsandsupportstoaccompany theincreaseinconductorspacing,
increasesthecostoftransmissionsystem.
3.ByUsingCoronaRing:Theintensityofelectricfieldishighatthepointwherethe
conductorcurvatureissharp.Therefore,coronadischargeoccursfirstatthesharppoints,
edges,andcorners.Inorderto,mitigateelectricfield,coronaringsareemployedatthe
terminalsofveryhighvoltageequipment.

297. Coronaringsaremetallicringsoftoroidalshaped,whicharefixedat
theendofbushingsandinsulatorstrings.Thismetallicringdistributesthe
chargeacrossawiderareaduetoitssmoothroundshapewhich
significantlyreducesthepotentialgradientatthesurfaceofthe
conductorbelowthecriticaldisruptivevalueandthuspreventing
coronadischarge.
Important points:
Disruptivevoltageistheminimumvoltageatwhichthebreakdown
ofairoccursandcoronastarts.
Visualcriticalvoltageistheminimumvoltageatwhichvisible
coronabegins.

298. CORONA RING

300. Thank You

302. Insulators
Aninsulatorgivessupporttotheoverheadlineconductorsonthepolesto
preventthecurrentflowtowardearth.Inthetransmissionlines,itplaysan
essentialroleinitsoperation.
Thedesigningofaninsulatorcanbedoneusingdifferentmaterialslikerubber,
wood,plastic,mica,etc.
Thespecialmaterialsusedintheelectricalsystemareglass,ceramic,PVC,
steatite,polymer,etc.
Butthemostcommonmaterialusedintheinsulatorisporcelainandalso
specialcomposition,steatite,glassmaterialsarealsoused.

303. Insulators desirable properties :
(i)Highmechanicalstrengthinordertowithstandconductorload,windloadetc.
(ii)Highelectricalresistanceofinsulatormaterialinordertoavoidleakagecurrentsto
earth.
(iii)Highrelativepermittivityofinsulatormaterialinorderthatdielectricstrengthis
high.
(iv)Theinsulatormaterialshouldbenon-porous,freefromimpuritiesandcracks
otherwisethepermittivitywillbelowered.
(v)Highratioofpuncturestrengthtoflashover.
Themostcommonlyusedmaterialforinsulatorsofoverheadlineisporcelainbut
glass,steatiteandspecialcompositionmaterialsarealsousedtoalimitedextent.
Porcelainisproducedbyfiringatahightemperatureamixtureofkaolin,feldsparand
quartz.Itisstrongermechanicallythanglass,giveslesstroublefromleakageandis
lesseffectedbychangesoftemperature.

306. SL Pin Insulator Post Insulator
1
It is generally used up to 33KV
system
It is suitable for lower voltage and also
for higher voltage
2It is single stag
It can be single stag as well as multiple
stags
3
Conductor is fixed on the top of the
insulator by binding
Conductor is fixed on the top of the
insulator with help of connector clamp
4
Two insulators cannot be fixed
together for higher voltage
application
Two or more insulators can be fixed
together one above other for higher
voltage application
4
Metallic fixing arrangement provided
only on bottom end of the insulator
Metallic fixing arrangement provided on
both top and bottom ends of the insulator

307. Pin Insulator
Thiskindofinsulatorisusedindistributionsystems.Thevoltagecapacityofthisinsulatoris
11kV.Itisdesignedwithahighmechanicalstrengthmaterial.Theseareconnectedin
verticalaswellashorizontalpositions.Theconstructionofthisinsulatorissimpleandneeds
lessmaintenanceascomparedwithothertypes.

308. Asthenamesuggests,thepintype
insulatorissecuredtothecross-
armonthepole.Thereisagroove
ontheupperendoftheinsulatorfor
housing theconductor.The
conductorpassesthroughthis
grooveandisboundbythe
annealedwireofthesamematerial
astheconductor.
Pintypeinsulatorsareusedfor
transmissionanddistributionof
electricpoweratvoltagesupto33
kV.Beyondoperatingvoltageof33
kV,thepintypeinsulatorsbecome
toobulkyandhenceuneconomical.
Causes of Insulator Failures
Safety factor of insulator =
Puncture strength / Flash -over voltage

309. Insulatorsarerequiredtowithstandbothmechanicalandelectricalstresses.Thelattertypeisprimarilydueto
linevoltageandmaycausethebreakdownoftheinsulator.Theelectricalbreakdownoftheinsulatorcan
occureitherbyflash-overorpuncture.
Inflashover,anarcoccursbetweenthelineconductorandinsulatorpin(i.e.,earth)andthe
dischargejumpsacrossthe*airgaps,followingshortestdistance.Figureshowsthearcing
distance(i.e.a+b+c)fortheinsulator.Incaseofflash-over,theinsulatorwillcontinuetoactin
itspropercapacityunlessextremeheatproducedbythearcdestroystheinsulator.
Incaseofpuncture,thedischargeoccursfromconductortopinthroughthebodyofthe
insulator.Whensuchbreakdownisinvolved,theinsulatorispermanentlydestroyeddueto
excessiveheat.Inpractice,sufficientthicknessofporcelainisprovidedintheinsulatortoavoid
puncturebythelinevoltage.Theratioofpuncturestrengthtoflashovervoltageisknownas
safetyfactori.e.
Post Insulator
PostinsulatorsaresimilartoPininsulators,butpostinsulatorsaremoresuitableforhigher
voltageapplications.
Postinsulatorshaveahighernumberofpetticoatsandagreatedheightcomparedtopin
insulators.Wecanmountthistypeofinsulatoronsupportingstructurehorizontallyaswellas
vertically.Theinsulatorismadeofonepieceofporcelainandithasclamparrangementarein
bothtopandbottomendforfixing.

310. Suspension type insulators
Thecostofpintypeinsulatorincreasesrapidlyastheworkingvoltageisincreased.
Therefore,thistypeofinsulatorisnoteconomicalbeyond33kV.Forhighvoltages(>33
kV),consistofanumberofporcelaindiscsconnectedinseriesbymetallinksintheform
ofastring.Theconductorissuspendedatthebottomendofthisstringwhiletheother
endofthestringissecuredtothecross-armofthetower.Eachunitordiscisdesigned
forlowvoltage,say11kV.Thenumberofdiscsinserieswouldobviouslydependupon
theworkingvoltage.Forinstance,iftheworkingvoltageis66kV,thensixdiscsinseries
willbeprovidedonthestring

311. Advantages
(i)Suspensiontypeinsulatorsarecheaperthanpintypeinsulatorsforvoltagesbeyond33kV.
(ii)Eachunitordiscofsuspensiontypeinsulatorisdesignedforlowvoltage,usually11kV.
Dependingupontheworkingvoltage,thedesirednumberofdiscscanbeconnectedinseries.
(iii)Ifanyonediscisdamaged,thewholestringdoesnotbecomeuselessbecausethedamaged
disccanbereplacedbythesoundone.
(iv)Thesuspensionarrangementprovidesgreaterflexibilitytotheline.Theconnectionatthe
crossarmissuchthatinsulatorstringisfreetoswinginanydirectionandcantakeuptheposition
wheremechanicalstressesareminimum.
(v)Incaseofincreaseddemandonthetransmissionline,itisfoundmoresatisfactorytosupply
thegreaterdemandbyraisingthelinevoltagethantoprovideanothersetofconductors.The
additionalinsulationrequiredfortheraisedvoltagecanbeeasilyobtainedinthesuspension
arrangementbyaddingthedesirednumberofdiscs.
(vi)Thesuspensiontypeinsulatorsaregenerallyusedwithsteeltowers.Astheconductorsrun
belowtheearthedcross-armofthetower,therefore,thisarrangementprovidespartialprotection
fromlightning.

312. STRAIN INSULATORS
Whenthereisadeadendofthelineorthereiscornerorsharpcurve,thelineissubjectedto
greatertension.Inordertorelievethelineofexcessivetension,straininsulatorsareused.Forlow
voltagelines(<11kV),shackleinsulatorsareusedasstraininsulators.However,forhighvoltage
transmissionlines,straininsulatorconsistsofanassemblyofsuspensioninsulatorsasshownin
Fig.Thediscsofstraininsulatorsareusedintheverticalplane.Whenthetensioninlinesis
exceedinglyhigh,asatlongriverspans,twoormorestringsareusedinparallel.

316. Shackle insulators
Inearlydays,theshackleinsulatorswereusedasstraininsulators.But
nowadays,theyarefrequentlyusedforlowvoltagedistributionlines.
Suchinsulatorscanbeusedeitherinahorizontalpositionorina
verticalposition.Theycanbedirectlyfixedtothepolewithaboltorto
thecrossarm.Figshowsashackleinsulatorfixedtothepole.The
conductorinthegrooveisfixedwithasoftbindingwire.

319. TheHighestPowerLineintheWorld:
Changji-GuquanPowerLine1100kV.TheChangji-Guquanhighvoltagedirectcurrent
transmissionlineofChinaisthehighestvoltagelevelproject,largesttransmission
capacityandlongesttransmissiondistanceintheworld.
https://www.youtube.com/watch?v=PqkSU464_yk
InIndia
765kVisthemaximumtransmissionvoltage.PowerGridCorporationbuildstheselines
forinterstatecommunicationthroughouttheentirenation.India'stransmissionvoltage
(highest)is765kVAC.ThehighestDCtransmissionvoltageinIndiais800kV.
InIndiathereisthe1200kVUltraHighVoltage(UHV)ACNationalTestStationofPowerGrid
CorporationofIndiaLimited(POWERGRID).Theworld'shighestvoltagelevel,1200kVUHVAC
sub-stationislocatedatBinainMadhyaPradesh.

321. Potential Distribution over Suspension Insulator String
Astringofsuspensioninsulatorsconsistsofanumberofporcelaindiscsconnectedinseries
throughmetalliclinks.Fig.shows3-discstringofsuspensioninsulators.Theporcelainportionof
eachdiscisinbetweentwometallinks.Therefore,eachdiscformsacapacitorCasshownin
Fig..Thisisknownasmutualcapacitanceorself-capacitance.
Ifthereweremutualcapacitancealone,thenchargingcurrentwouldhavebeenthesame
throughallthediscsandconsequentlyvoltageacrosseachunitwouldhavebeenthesamei.e.,
V/3asshowninFig.However,inactualpractice,capacitancealsoexistsbetweenmetalfitting
ofeachdiscandtowerorearth.ThisisknownasshuntcapacitanceC1.
Duetoshuntcapacitance,chargingcurrentisnotthesamethroughallthediscsofthestring
[SeeFig.Therefore,voltageacrosseachdiscwillbedifferent.Obviously,thediscnearesttothe
lineconductorwillhavethemaximum*voltage.ThusreferringtoFig.V3willbemuchmorethan
V2orV1.

323. The following points may be noted regarding the potential distribution over a string of
suspension insulators :
(i)Thevoltageimpressedonastringofsuspensioninsulatorsdoesnotdistributeitself
uniformlyacrosstheindividualdiscsduetothepresenceofshuntcapacitance.
(ii)Thediscnearesttotheconductorhasmaximumvoltageacrossit.Aswemovetowards
thecross-arm,thevoltageacrosseachdiscgoesondecreasing.
(iii)Theunitnearesttotheconductorisundermaximumelectricalstressandislikelytobe
punctured.Therefore,meansmustbeprovidedtoequalisethepotentialacrosseachunit.
(iv)Ifthevoltageimpressedacrossthestringwered.c.,thenvoltageacrosseachunitwould
bethesame.Itisbecauseinsulatorcapacitancesareineffectiveford.c.

324. String Efficiency
Thevoltageappliedacrossthestringofsuspensioninsulatorsisnotuniformlydistributedacross
variousunitsordiscs.Thediscnearesttotheconductorhasmuchhigherpotentialthantheotherdiscs.
Thisunequalpotentialdistributionisundesirableandisusuallyexpressedintermsofstringefficiency.
Stringefficiencyisanimportantconsiderationsinceitdecidesthepotentialdistributionalongthestring.
Thegreaterthestringefficiency,themoreuniformisthevoltagedistribution.Thus100%string
efficiencyisanidealcaseforwhichthevoltageacrosseachdiscwillbeexactlythesame.Althoughitis
impossibletoachieve100%stringefficiency,yeteffortsshouldbemadetoimproveitasclosetothis
valueaspossible
Theratioofvoltageacrossthewholestringtotheproductofnumberofdiscsandthe
voltageacrossthediscnearesttotheconductorisknownasstringefficiency

328. Methods of Improving String Efficiency
Ithasbeenseenabovethatpotentialdistributioninastringof
suspensioninsulatorsisnotuniform.Themaximumvoltageappears
acrosstheinsulatornearesttothelineconductoranddecreases
progressivelyasthecrossarmisapproached.Iftheinsulationofthe
higheststressedinsulator(i.e.nearesttoconductor)breaksdownor
flashovertakesplace,thebreakdownofotherunitswilltakeplacein
succession.Thisnecessitatestoequalisethepotentialacrossthe
variousunitsofthestringi.e.toimprovethestringefficiency.The
variousmethodsforthispurposeare:
(i)Byusinglongercross-arms.Thevalueofstringefficiency
dependsuponthevalueofKi.e.,ratioofshuntcapacitancetomutual
capacitance.ThelesserthevalueofK,thegreateristhestring
efficiencyandmoreuniformisthevoltagedistribution.ThevalueofK
canbedecreasedbyreducingtheshuntcapacitance.Inorderto
reduceshuntcapacitance,thedistanceofconductorfromtowermust
beincreasedi.e.,longercross-armsshouldbeused.However,
limitationsofcostandstrengthoftowerdonotallowtheuseofvery
longcross-arms.Inpractice,K=0·1isthelimitthatcanbeachieved
bythismethod.

329. (ii)Bygradingtheinsulators.Inthismethod,insulatorsofdifferent
dimensionsaresochosenthateachhasadifferentcapacitance.The
insulatorsarecapacitancegradedi.e.theyareassembledinthestringin
suchawaythatthetopunithastheminimumcapacitance,increasing
progressivelyasthebottomunit(i.e.,nearesttoconductor)isreached.
Sincevoltageisinverselyproportionaltocapacitance,thismethodtends
toequalisethepotentialdistributionacrosstheunitsinthestring.This
methodhasthedisadvantagethatalargenumberofdifferent-sized
insulatorsarerequired.However,goodresultscanbeobtainedbyusing
standardinsulatorsformostofthestringandlargerunitsforthatnearto
thelineconductor.
(iii)Byusingaguardring.Thepotentialacrosseachunitinastring
canbeequalisedbyusingaguardringwhichisametalringelectrically
connectedtotheconductorandsurroundingthebottominsulatoras
shownintheFig.Theguardringintroducescapacitancebetweenmetal
fittingsandthelineconductor.
Theguardringiscontouredinsuchawaythatshuntcapacitance
currentsi1,i2etc.areequaltometalfittinglinecapacitancecurrentsi1,
i2etc.TheresultisthatsamechargingcurrentIflowsthrougheachunit
ofstring.Consequently,therewillbeuniformpotentialdistributionacross
theunits.

330. Ina33kVoverheadline,therearethreeunitsinthestringofinsulators.Ifthecapacitancebetweeneach
insulatorpinandearthis11%ofself-capacitanceofeachinsulator,find(i)thedistributionofvoltageover3
insulatorsand(ii)stringefficiency