UNESCO-NIGERIA TECHNICAL &
VOCATIONAL EDUCATION
REVITALISATION PROJECT-PHASE II
NATIONAL DIPLOMA IN
ELECTRICAL ENGINEERING TECHNOLOGY
ELECTRICAL POWER SYSTEM (I)
COURSE CODE: EEC 122
YEAR I- SEMESTER II
THEORY
Version 1: December 2008
TABLE OF CONTENTS
Week 1: Principle of Electrical Energy Generation ………………..1
1.1 Introduction……………………………………………………………1
1.2 Importance of Electrical Energy………………………………………2
1.3 Generation of Electrical Energy……………………………………….2
1.4 Sources of Energy……………………………………………………..2
1.5 Generating power Station……………………………………………...6
1.6 Steam power Station……………………………………………………8
Week 2: …………………………………………………………………………..10
1.7 Gas power Station……………………………………………………...10
1.8 Advantages…………………………………………………………….11
1.9 Disadvantages………………………………………………………….11
1.10 Main Component of Gas power Plant……………………………….12
Week 3:………………………………………………………………………….13
1.11Wind Energy Power Plant…………………………………………….13
1.12 Solar Energy Power Plant…………………………………………….13
1.13 Component of Solar Energy Plant…………………………………….14
1.14 Types of Photovoltaic (PV) Solar System…………………………….14
1.15 Solar Energy…………………………………………………………..15
1.16 Solar cells power Generation Unit…………………………………….16
Week 4: …………………………………………………………………………..17
1.17 Various Voltage Levels………………………………………………..17
1.18 Transmission Lines…………………………………………………….17
1.19 Classification of overhead Transmission lines………………………...18
Week 5:……………………………………………………………………………20
1.20 Principle of protection System and Devices……………………………20
1.21 Fuses……………………………………………………………………20
1.22 High Breaking Capacity………………………………………………..21
1.23 Semi enclose Rewirable Fuses…………………………………………21
Week 6:…………………………………………………………………………...22
1.24 Conductors……………………………………………………………..22
1.25 Commonly Used Conductors Materials……………………………….22
1.26 Types of Conductors…………………………………………………..23
Week 7:
Principle of distribution system.........................................................26
2.1
Introduction.........................................................................................26
2.2
Distribution system ...........................................................................26
2.3
Feeders...............................................................................................27
2.3
Distributors.........................................................................................27
2.4
Service mains......................................................................................28
2.4
Classification of Distribution System.................................................29
Week 8:....................................................................................................................31
2.5
Direct current (DC) Distribution.........................................................31
2.6
Methods of feeding a distributor.........................................................31
2.8
Uniformly loaded distribution ............................................................34
2.9
D.C distributor feed at one end...........................................................35
2.10 Distributor feed at both ends...............................................................36
2.11 Uniformly loaded fed at both ends......................................................37
Week 9:....................................................................................................................39
2.12 Alternating current distribution (AC) ................................................39
2.13 classification of A.C distribution system............................................39
2.14 Power factors referred to receiving end..............................................40
Week 10: principle of protections in power system................................................43
3.1
Fuses and if components....................................................................43
3.2 Current Rating....................................................................................45
3.2
fusing current......................................................................................45
3.4
fusing factor........................................................................................45
Week 11:.................................................................................................................48
3.5
Moulded case current breaker.............................................................48
3.6
current breakers..............................................................................48
3.7
Maintenance moulded case circuit breaker.....................................49
3.8
Circuit Breaker Ratings...................................................................49
Week 12:........................................................................................................51
3.9
Functions Circuit Breaker...............................................................51
3.10 Principle of operation......................................................................51
3.11 Arc Phenomenon................................................................................52
3.12 Principle of Arc Extinction.................................................................53
3.13 Methods of Arc Extinction..................................................................54
3.14 Resistance the arc Increase...................................................................54
Week 13:.................................................................................................................55
3.15 Isolators..............................................................................................55
3.16 different isolator & Circuit Breaker........................................................
Week 14:
Types of insulators & supports...........................................................57
3.7
Overhead & underground system insulators.......................................57
3.18 requirement of distribution system.....................................................58
Week 15:..................................................................................................................59
3.19 insulators.............................................................................................59
3.20 properties of insulators........................................................................59
3.21 Types of Insulators..............................................................................59
3.22 Advantages ........................................................................................61
Principle of Electrical Energy Generation
Week 1
1.1 INTRODUCTION
Energy is the basic necessity for the economic development of a country due to it importance
in human life. Most of our day to day activities make use of electrical energy because it makes the
activity much easier, simple within a limited time. It is practically impossible to estimate the actual
magnitude of the part that electrical energy has played in the building up of present day civilization.
With the advance or the availability of huge amount of energy in the modern times has resulted in a
shorter working day, higher agricultural and industrial production, a healthier and more balanced
diet and better transportation facilities. As a matter of fact, there is a close relationship between the
energy used per person and his standard of living. The greater the per capital consumption of energy
in a country, the higher is the standard of living of its people. An example of energy generation is
shown in figure 1.1
Figure 1.1
The energy exist in different forms in native but t he most important form is the
electrical energy. The modern society is so much dependent upon the use of electrical
energy in the sense that, it has become a part and parcel of our life.
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Principle of Electrical Energy Generation
Week 1
1.2 IMPORTANCE OF ELECTRICAL ENERGY
Energy may be needed as heat, light or as motive power etc. The present-day
advancement in science and technology has made it possible to convert electrical
energy into any desired form. This has given electrical energy a place of pride in the
modern hold. The survival of industrial undertakings and our social structures
depends primarily upon low cost and uninterrupted supply of electrical energy. In
fact, the advancement of a country is measured in terms of per capital consumption of
electrical energy. Electrical energy is higher to all other form of energy due to the fact
that electrical energy is;

Convenient in form

Easy control

Greater flexibility

Cheapness

Cleanliness and

Higher transmission efficiency.
1.3 GENERATION
GENERATION)
OF
ELECTRICAL
ENERGY
(ELECTRICITY
Electricity generation is the process where energy available in different forms in
nature is being converted into electrical energy. The electrical energy must be
produced and transmitted tot eh point of use at the instant it is needed. The entire
process takes only a fraction of a second. This instantaneous production of electrical
energy introduces technical and economical considerations unique to the electrical
power industry.
1.4 SOURCES OF ENERGY
Electrical energy is produced from energy available in various forms in nature. The
various sources of energy are:

Sun

Wind
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Principle of Electrical Energy Generation



Week 1
Water
Fuels
Nuclear energy.
SUN: The sun is the primary source of energy. The heat energy radiated by the sun
can focused over a small area by means of reflectors. This heat can be used to raise
steam and electrical energy can be produced. But this system has some limitations
such as:
 It requires a large area for the generation of even a small amount of electric
power
 It cannot be used in cloudy days or at night
 It is an uneconomical method.
Wind: This method can be used where wind flows for a considerable length of time.
Wind energy is used to run the wind mill continuously as shown in figure 1.2, the
generator is arranged to charge the batteries. This batteries supply t he energy when
the wind stops. This method has the advantages that maintenance and generation
costs are negligible. However, the draw backs or disadvantages of this method are (i)
variable output (ii) unreliable because of uncertainty about wind pressure and (iii)
power generated is quite small.
Figure 1.2
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Principle of Electrical Energy Generation
Week 1
Water: When water is stored at a suitable place, it possesses potential energy because
of the head created. This water energy can be converted into mechanical energy with
the help of water turbines as shown in figure 1.3. The water turbines drive the
alternator which converts mechanical energy into electrical energy. This method or
generation of electrical energy has become very popular because it has low production
and maintenance costs.
Figure 1.3
Fuels: This can be further classifying into solid fuel (coal), liquid, fuel (oil) and gas
fuel (Gas). The heat energy of these fuels is converted into mechanical energy by
suitable prime movers such as steam engines, steam turbines, internal combustion
engine etc. the prime mover drives the alternator which converts mechanical energy
into electrical energy as can be seen in figure 1.4.
Figure 1.4
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Principle of Electrical Energy Generation
Week 1
Nuclear energy: It has been discovered that large amount of heat energy is liberated
by the fission of uranium and other fissionable materials. It is estimated that heat
produced by 1kg of nuclear fuel is equal to that produced by 4500 tones of coal. T he
heat produced due to nuclear fission can be utilized to raise steam with suitable
arrangements as shown in figure1.5. The steam can run the steam turbine which in
two can drive the alternator to produce electrical energy. The disadvantages of this
system are (a) High cost of nuclear plant (b) Disposal of radioactive waste and death
of
trained
personnel
to
handle
the
plant.
Figure 1.5
5
Principle of Electrical Energy Generation
Week 1
1.5 GENERATING STATION
Figure 1.6
Bulk electric power is produced by special plants known as generating stations or
called power plants as shown in figure 1.6.
A generating station essentially employs a prime mover coupled to an alternator for
the production of electric power. The prime mover (e.g. steam turbine, water turbine
etc) that converts energy from some other form into mechanical energy. The
alternator converts mechanical energy o f the prime mover into electrical energy. The
electrical energy produced by t he generating station is transmitted and distributed
with the help of conductors to various consumers.
But in modern generating station, several auxiliary equipment and instruments are
used, apart from prime mover – alternator combination, in order to ensure cheap,
reliable and continuous service.
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Principle of Electrical Energy Generation
Week 1
Depending upon the form of energy being converted into electrical energy, the
generating stations are classified as:
 Steam power plants
 Hydro power plant
 Diesel power plant/Gas power plant
 Nuclear power plants
 wind power plant
 Solar power plant
 MHD P-plants.
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Principle of Electrical Energy Generation
Week 1
1.6 STEAM POWER STATION (THERMAL STATION)
Figure 1.7a
A generating station which converts heat energy of coal combustion into electrical
energy is known as a steam power station. The steam is produced in the boiler by
utilizing the heat of coal combustion as shown in figure 1.7a & 17b. The steam is
then expanded in the prime mover (i.e steam turbine) and is condensed in a condenser
to be fed into the boiler again. The steam turbine drives the alternator which converts
mechanical energy of the turbine into electrical energy. This type of power station is
suitable where coal and water are available in abundance and a large amount of
electric power is to be generated.
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Principle of Electrical Energy Generation
Week 1
Figure 1.7b
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Generation of Electric Energy
Week 2
1.7 GAS TURBINE POWER PLANT
A generating station which employs gas turbine as the prime mover for the generation
of electrical energy is known as gas turbine power plant.
In a gas turbine power plant, air is used as the working fluid.
Figure 1.8
Figure 1.9
The air is combustion chamber where heat is added to air, thus raising its temperature.
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
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Generation of Electric Energy
Week 2
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 as shown in figure 1.9 & 1.10.
Figure 1.10
1.8 ADVANTAGES
 It is simple in design as compared to steam power station since no bitters and
auxiliaries are required.
 It is much smaller in sizes as compared to steam station of the same capacity.
 The initial and operating cost are much lower than that of equivalent steam
power station.
 It requires comparatively less water as no condenser is used
 The maintenance charges are quite small.
 Gas turbines are much simpler in construction and operation.
 It can be started quickly from cold conditions
 There are no standby losses.
1.9 DISADVANTAGES
 There is a problem for starting the unit. This is because before starting the
turbine, t he compressor has to be operated for which power is required from
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Generation of Electric Energy
Week 2
external source. But once it start,
the external power will be no longer
required as the turbine itself supplies the necessary power to the compressor.
 The overall efficiency of such plant is how (about 20%)
 The temperature of combustion chamber is quite high (300o)
1.10 THE MAIN COMPONENTS OF THE PLANT ARE:
(i)
(iii)
(v)
Compressor
Combustion chamber
Alternator
(ii)
(iv)
(vi)
Regenerator
Gas turbine
Starting motor
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Generation of Electric Energy
Week 3
1.11 WIND POWER PLANT
Figure 1.11
This method can be used where wind flows for a considerable length of time. Wind
energy is used to run the wind mill continuously as shown in figure 1.11, the
generator is arranged to charge the batteries. This batteries supply t he energy when
the wind stops. This method has the advantages that maintenance and generation
costs are negligible. However, the draw backs or disadvantages of this method are
 variable output
 Unreliable because of uncertainty about wind pressure and
 Power generated is quite small.
1.12 SOLAR ENERGY POWER PLANT
This is the energy receives from the sun as a result of the sun rays known as radiation
from the sun to the earth surface. This energy is trapped through the use of
photovoltaic cell and converted into DC power output and DC output can further be
converted into AC power, and this output can be use into many applications such as
water pumping for irrigation, lighting, and refrigeration of vaccine etc.
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Generation of Electric Energy
Week 3
These arrangements is shown in figure 1.12
1.13 THE BASIC COMPONENTS OF SOLAR ENERGY POWER PLANT
INCLUDE:
 The photovoltaic cell (PV Solar Panel)
 The Storage facilities (Batteries)
 The charger Controller
 The Inverter
 The load
Solar Cell Array
Subsystem
DC to AC
Conversion ( )
Battery for Storage
Control unit
Power Distribution
Unit
Loads
Figure 1.12
1.14 TYPES OF PV SOLAR SYSTEMS
There are various types of PV systems configurations used for different applications.
PV system could be use in the stand alone, integrated and grid connected mode. It
could also be used as directly connected systems without storage battery or with
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Generation of Electric Energy
Week 3
storage battery. The output from the system could be used as a DC or AC systems.
The different modes of usage provides for flexibility.
1.15 SOLAR ENERGY
The sun is a good source of energy. Heat energy in the suns
Rays can be used for a number of advantages particularly solar energy applications.
The solar energy is very versatile as it has limitless potential in transforming our lives.
Studies have shown that endowed with availability of this resource as well as it’s
viability for practical use.
Nigeria receives 5.80 x 106 MWh of electricity can be obtained from solar energy.
Solar energy technologies can be classified into two;Solar-thermal; and
Solar photovoltaic
Solar thermal; here the solar radiation is converted to thermal energy.
This heat energy can be directly used or indirectly by using the heat to boil water and
generate steam, which would in turn be used to generate steam turbine for electricity
generation. In other words, it may be used in application such as drying, cooking,
refrigeration and air conditioning.
Solar photovoltaic. Here the photovoltaic (pv) devices converts sunlight directly
into direct current (DC) electrical energy. This is done through the use of silicon solar
cells. Several solar cells are linked together to form a solar module. Because they are
modular in form, adding one or more cells can expand them. Or they can be
dismantled and used for other applications. The solar PV modules are light and easily
installed. They require small amount of maintenance. The modules produce DC
electricity which can be used directly or even stored in batteries to be used later. The
PV modules have been used for the following applications;Photo-voltaic pumps for pumping water
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Generation of Electric Energy
Week 3
Photo –voltaic refrigeration for preserving vaccines;
1.16 SOLAR CELL POWER GENERATION UNIT
For power generation the system consist of arrays, which are made up of photovoltaic
devices, the inverter to convert the DC into AC; the battery to store the energy during
daylight, as well as controller unit to manage the automatic operation of the system.
One of the main of solar photovoltaic electricity generation is the high cost of
module. A 12V module cost about #30,000 (at exchange rate of 1 US $)
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Generation of Electrical Energy
Week 4
1.17 VARIOUS VOLTAGE LEVELS
Generating voltages: 6.6KV, 11KV, 13.2KV or 33KV
High voltage transmission: 330KV, 132KV, 66KV, 6.6KV 3.3KV
Low voltage distribution: A.C 415/240V, 3 -, 4 wires
Standard frequency: Nigeria
:
50Hz + 1% and – 1%
1.18 TRANSMISSION LINES
Figure 1.13
Transmission Lines
The important consideration in the design and operation of a transmission line are the
determination of voltage drop, line losses and efficiency of transmission. These values
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Generation of Electrical Energy
Week 4
are greatly influenced by the lines constants R, L and C of transmission line as in
figure 1.13 above. For instance, the voltage drop in the line depends upon the values
of above three line constants. Similarly, the resistance of transmission line conductors
is the most important cause of power loss in the line and determines the transmission
efficiency. In this chapter, we shall develop formular by which we can calculate
voltage regulation, line losses and efficiency of transmission lines. These formular are
important for two principal reasons. Firstly, they provide an opportunity to understand
the effects of the line on bus voltages and the flow of power. Secondly, they help in
developing an overall understanding of what is occurring on electric power system.
1.19 CLASSIFICATION OF OVERHEAD TRANSMISSION LINES
A transmission line has three constant R, L and C distributed uniformly along the
whole length of the line. The resistance and inductance from the series impedance.
The capacitance existing between conductors for 1-phase line or from a conductor to
neutral for a 3-phase line forms a shunt path throughout the length of the line.
Therefore, capacitance effects introduce complications in transmission line
calculations. Depending upon the manner in which capacitance is taken into account;
the overhead transmission lines are classified as:
(i)
Short transmission lines: when the length of an overhead transmission line is
up to about 50km and the line voltage is corporately low (<20kV), it is usually
considered as a short transmission line. Due to smaller length and lower
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Generation of Electrical Energy
Week 4
voltage, the capacitance effects are small and hence can be neglected.
Therefore, while studying the performance of a short transmission line, only
resistance and inductance of the line are taken into account.
(ii)
Medium transmission lines: When the length of an overhead transmission line
is about 50-150km and the line voltage is moderately high (>20KV< 100kV), it
is considered as a medium transmission line. Due to sufficient length and
voltage of the line, the capacitance effects are taken into account. For purposes
of calculations, the distribution capacitance of the line is divided and lumped in
the form of condensers shunted across the line and at one or more points.
(iii)
Long transmission lines: When the length of an overhead transmission line
is more than 150 km and line voltage is very high (>100kV), it is considered
as a long transmission line. For the treatment of such a line, the line
constants are considered uniformly distributed over the whole length of the
line and rigorous methods are employed for solution.
It may be emphasized here that exact solution of any transmission line
must consider the fact that the constants of the line are not lumped but are
distributed uniformly throughout the length of the line. However, reasonable
accuracy can be obtained by considering these constants as lumped for short
3
Generation of Electrical Energy
Week 5
1.20 Principle of Protection system and Devices
Circuit protection would be unnecessary if overloads and short circuits could be
eliminated. Unfortunately, overloads and short circuits do occur. To protect a circuit against
these currents, a protective device must determine when a fault condition develops and
automatically disconnect the electrical equipment from the voltage source. An over current
protection device must be able to recognize the difference between over currents and short
circuits and respond in the proper way. Slight over currents can be allowed to continue for
some period of time, but as the current magnitude increases, the protection device must open
faster. Short circuits must be interrupted instantly. Several devices are available to
accomplish this.
1.21 Fuses
A fuse is a one-shot device (Figure1). The heat produced by overcurrent causes the
current carrying element to melt open, disconnecting the load from the source voltage. There
are three types of fuses, namely
 Semi-enclosed (Rewireable) fuse
 Cartridge fuses
 High Breaking Capacity(HBC)
Fuse Cap
Good Element
Glass or Ceramic
Body
Open
Element
Figure 1.15 Plug fuse
Figure 1.14 Cartridges
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Generation of Electrical Energy
Week 5
The cartridge type has fuses which look similar to those you would find in a standard
household plug. This type is improvement of the rewirable fuse type. It is main advantages,
is easy to replace, totally enclosed and its current rating is very accurate
1.22 HIGH BREAKING CAPACITY (HBC)
HBC stands for "high blow current (sometimes described as HRC = high rupture current).
HBC fuses are designed not to explode when failing
under currents many times their normal working current
(e.g.
1500 amps in a 10 amp circuit). They are therefore to be
preferred for the protection of main voltage circuits
where the power source may be capable of providing
very
high currents. HBC types can usually be recognized by
being sand filled though they may have a thick ceramic
body.
Figure1.16 A HBC fuse
1.23 SEMI-ENCLOSED (REWIREABLE) FUSES
As the name indicates, the rewireable type has a fuse wire held at
both ends by a small retaining screw. Once the fuse is blown, the
fuse wire is the only pieces to be replaced. It is cheap, but
replacing a wrong size of element can cause catastrophic
consequences.
Figure1.17 Rewireable fuses
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Generation of Electrical Energy
Week 6
1.24 CONDUCTORS
The conductor is one of the important items as most of the capital outlay is invested for it.
Therefore, proper choice of material and size of conductor is of considerable importance. The
conductor material used for transmission and distribution of electric power should have the
following properties:
(i) High electrical conductivity.
(ii) High tensile strength in order to withstand mechanical stresses.
(Iii) Low cost so that it can be used for long distances.
(iv) Cross arm which provide support to the insulators.
(v) Miscellaneous items such as phase plates, danger plate, lightning arrestors, anti-climbing
wires etc.
All above requirements are not found in a single material. Therefore while selecting a conductor
material for a particular case, a compromises made between the cost and the required electrical and
mechanical properties.
1.25 COMMONLY USED OF CONDUCTOR MATERIALS.
The most commonly used conductor materials for over head lines are copper, aluminum, steel-cored
aluminum, galvanized steel and cadmium copper. The choice of a particular material will depend
upon the cost, the required electrical and mechanical properties and the local conditions.
All conductors used for over head lines are preferably stranded in order to increase flexibility. In
stranded conductors, there is generally one central wire and round this, successive layers of wires
containing 6, 12, 18, 24…… wires. Thus, if there are n layers, the total number of individual wires
is 3n (n+1) +1. In the manufacture of stranded conductors, the consecutive layers of wires are
twisted or spiraled in opposite direction so that layers are bound together.
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Generation of Electrical Energy
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TYPES OF CONDUCTORS
1.
Copper. Copper is an ideal material for over head lines owing to its high electrical
conductivity and greater tensile strength. It is always in the hard drawn form as stranded conductors.
Although hard drawing decrease the electrical conductivity slightly yet it increases the tensile
strength considerably.
Copper has high current density i.e., the current carrying capacity of copper per unit of Xsectional area id quite large. This leads to two advantages. Firstly, smaller X-sectional area of
conductor is required and secondly, the area offered by the conductor to wind load is reduced.
Moreover, this metal is quite homogeneous, durable and high scrap value.
There is hardly any doubt that copper is an ideal material for transmission and distribution of
electric power. However, due to its higher cost and non- availability, it is rarely used for these
purpose. Now– a – days the trend is to use aluminium in place of copper.
2.
Aluminium. Aluminium is cheap and light as compared to copper but it has much
smaller conductivity and tensile strength. The relative comparison of the two materials is
briefed below:
i
The conductivity of aluminium is 60% that of copper. The smaller conductivity of
aluminium means that for any particular transmission efficiency, the X - sectional area of
conductor must be lager in aluminium than in copper. For the same resistance, the diameter of
aluminium conductor is about 1.26 times the diameter of the copper conductor.
The increase X- section of Aluminium exposes a greater surface to wind pressure and,
therefore, supporting towers must be design for greater transverse strength. This often requires
the used of higher towers with consequence of greater sag.
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Generation of Electrical Energy
Week 6
ii The specific gravity of aluminium(2.71gm/cc) is lower than that of copper (8.9gm/cc).
Therefore, an aluminium conductor has a most one-half the weigh of equivalent copper
conductor. For this reason, the supporting strictures for aluminium need not be made so strong as
that of copper conductors.
iii Aluminum conductor being light, is liable to greater swings and hence larger cross- arms are
required.
iv Due to lower tensile strength and higher co - efficient of linear expansion of aluminium, the
sag is greater in aluminium conductors.
Considering the combined properties of cost, conductivity, tensile strength, weight etc., aluminium
has an edge over copper. Therefore, it is being used as a conductor material. It is particularly
profitable to use aluminium for heavy-current transmission where the conductors’ size is large and
its cost forms a major proportion of the total cost of complete installation.
3.
Steel cored aluminium. Due to low tensile strength, aluminium conductors produce greater
sag. This prohibit their used for larger span and makes them unsuitable for distance transmission. in
order to increase the tensile strength , the aluminium conductor is reinforced with a core of
galvanized steel wires. The composite conductor thus obtained is known as steel core aluminium
and is abbreviated as A.C.S.R.(aluminium conductor reinforced).
Steel – cored aluminium conductors consists of central core of galvanized steel wires
surrounded by a number of aluminium strands. Usually diameter of both steel and aluminium wires
is the same. The X- section of the two metal is generally in the ratio of 1:6but can be modified to
1:4in order to get more tensile strength for the conductor. Fig 8.1 shows steel cored aluminium
conductor having
one steel wire surrounded by six wires of aluminium. The result of this
composite conductors is that steel cored takes greater percentage of mechanical strength while the
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Generation of Electrical Energy
Week 6
aluminium strand carry the bulk of current. The steel cored aluminum conductors have the following
advantages:
(i) The reinforcement with steel increases the tensile strength but at the same time keeps the
composite conductors light. Therefore steel cored aluminium conductors produce smaller sag and
hence longer span can be used.
(ii) Due to smaller sag with steel cored aluminium conductors. Towers of smaller height can be
used.
4. Galvanized steel. Steel have high tensile strength. Therefore, galvanized steel conductors
can be used for extremely long span or short line section exposed to abnormally high stresses
due to climatic conditions. They have been found very suitable in rural areas where cheapness is
the main consideration. Due to poor conductivity and high resistance of steel, such conductors
are not suitable for transmitting high large power over a long distance. However, they can be
used to advantage for transmitting a small power over a small distance were the size of the
copper conductor desirable from economic considerations would be too small and thus
unsuitable for used because of poor mechanical strength.
5. Cadmium copper. The conductor material being employed in certain cases is copper alloyed with
cadmium. An addition of 1%or2% cadmium to copper increases the tensile strength by about 50%
and the conductivity is only reduced by 15% below that of pure copper. Therefore, cadmium copper
conductor can be used for exceptionally long spans. However, due to high cost of cadmium, such
conductors will be economical only for lines of small X- section i.e., where the cost of conductor
material is comparatively small compared with the cost of supports.
4
2.0 Principle of Distributions System
Week 7
2.1 INTRODUCTION
Electrical power is usually generated and transmitted in 3-phase. It is distributed in
three - phase or single – phase depending on the need of the consumer. The figure
2.1below shows a typical power distribution system. Power is supplied from the
generator (for example at 11KV) which is stepped – up by the step – up transformer
to a higher voltage (about 132KV). This high voltage is used to transmit electricity
over long distances so as to minimize power losses at a far end of the line. The
overhead high – voltage transmission line terminates in step – down transformers in a
substation where the voltage is stepped – down for distribution.
Fig.2. 1 & 2.2 A typical electric power Distribution system
2.2 DISTRIBUTION SYSTEM
The distribution system is that part of the power system which distributes electric
power for local use (to the consumer). It is the electric system between the substation
fed by the transmission system and the consumer’s meters. The distribution system
consists of feeders, distributors and the service mains.
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2.0 Principle of Distributions System
Week 7
Figure 2.3
2.3 FEEDERS:
Feeders are conductors which connect the source (the substation or localized
generating station) to the distributors serving a particular area. Current loading
on a feeder is the same throughout its entire length as no tapings are taken from
the feeder. A feeder is designed on the basis of its current carrying capacity.
The voltage drop of a feeder is relatively unimportant during design as it can be
compensated by means of voltage regulating equipment at the substation.
2.4 DISTRIBUTORS:
A distributor is a conductor that receives power directly from the feeder. It is a
conductor from which tapings are taken for supply to the consumer. It has
distributed loading which gives rise to variations of current along its entire
length. A distributor is designed from the point of view of the voltage drop in it.
2
2.0 Principle of Distributions System
Figure 2.4
Week 7
Figure 2.5
2.5 SERVICE MAINS:
The connecting wires or connecting link between the distributors and the
consumer’s terminal are the service mains.
Figure 2.6
Fig 2.23,2.24 & 2.25 above shows a typical A.C. distribution system. In the figure,
feeders connect the substation to the distributors. Power is tapped from the
distributors through the sub-distributors via the service mains to the consumer’s
premises.
3
2.0 Principle of Distributions System
Week 7
2.6 CLASSIFICATION OF DISTRIBUTION SYSTEMS
Distribution systems are classified based on three main aspects; nature of current, type
of construction and scheme of connection.
 NATURE OF CURRENT – Based on nature of current, distribution systems
are grouped into two – alternating current (A.C.) distribution system and direct
current (D.C.) distribution system.
 TYPE OF CONSTRUCTION – Based on type of construction, distribution
systems are divided into overhead systems or underground systems. In the
overhead system, bare aluminum or copper conductors are strung between
wooden, steel or concrete poles. These conductors are connected to the poles by
insulators and cross arms. The underground system uses insulated cables to
convey power from one system to the other. Underground cables have better
voltage regulation than overhead cables which is as a result of low inductance
and low inductive drops due to small spacing between the conductors.
Overhead conductors have considerably higher current carrying capacity than
underground conductors of the same material and cross-section.
 SCHEME OF CONNECTION – Under scheme of connection distribution
systems are further classified as radial system, ring main system or
interconnected system.
i) Radial System – In this system feeders branch out radially from a common
source and feed the distributors at one end only. In this type of system if a feeder fails
due to a fault, the supply to the consumer is interrupted until repairs are done. It is the
simplest distribution circuit and has the lowest initial cost but has some drawbacks
such as,
4
2.0 Principle of Distributions System
Week 7
 Any fault on the feeder or distributor cuts off supply to the consumers on the
side of the fault away from the substation as they are dependent on a single
feeder and distributor.

The consumer at the farthest end of the distributor would be subject to voltage
fluctuations when the load on the distributor changes.
ii) Ring Main System – In the ring main distribution system the feeder branches
out in the form of a loop or ring. The loop circuit starts from the substation bus bars
makes loop through the area to be served and returns to the substation. This makes a
complete loop and has isolating switches provided at the poles at strategic points for
isolating a particular section in case of a fault. Thus failure of one interconnecting
feeder does not interrupt the supply.
5
2.0 Principle of Distributions System
Week 8
2.7 DIRECT CURRENT (D.C.) DISTRIBUTION
Electrical power is mostly generated, transmitted and distributed as alternating
current. Direct current however is necessary for certain applications such as for the
operation of variable speed machinery, for electrochemical work and electric traction.
A.C. power is converted into D.C. power by use of mercury-arc rectifier, rotary
converters and motor generator sets. D.C. supply can be obtained as either 2-wire
system or 3-wire system for distribution.
1) 2-wire D.C. System – This system has 2 wires; the outgoing or positive
wire and the return or negative wire. Due to its low efficiency, it is not used for
transmission purposes but for distribution of D.C. power.
2) 3-wire D.C. System – This system has 3 wires, the middle wire which is the
neutral is earthed. The voltage between either of the outer wires and neutral is
half that between the negative and positive wire making two voltages available
at the consumer terminal.
2.8 METHODS OF FEEDING A DISTRIBUTOR
There are various methods of feeding a distributor
i) Distributor fed at one end
ii) Distributor fed at both ends
iii) Distributor fed at the center
iv) Ring mains
The nature of loading on the distributor also varies such as
a) Concentrated loading
b) Uniform loading
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2.0 Principle of Distributions System
Week 8
c) Combination of both concentrated and uniform loading
i) Distributor fed at one end:
A
C
I1
D
I2
E
B
I3
The distributor AB above is connected to supply at one end with loads I1, I2 and I3
taken at different points along its length. In this type of distributor when a fault occurs
on any section of the distributor, the whole distributor will have to be disconnected
from supply. Voltage across loads decreases away from feeding point (point E will
therefore have the lowest voltage). Current also decreases along various sections of
the distributor.
In fig 5 above is shown a distributor fed at one end. The voltage drop in the
distributor is
V = IACRAC + ICDRCD + IDERDE + …
IAC is the current in section AC of the distributor which is the sum of load currents I 1,
I2 , I3 …
IAC = I1 + I2 + I3 + …
ICD = I2 + I3 + …
IDE = I3 + …
ii) Distributor fed at both ends:
A
C
I1
D
I2
E
B
I3
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2.0 Principle of Distributions System
Week 8
The distributor AB in fig. 6 above is connected to the supply mains at both ends with
loads taken at different points. Voltage at the supply ends A and B may or may not be
equal. The load voltage decreases away from one feeding point reaches minimum
value then increases towards the other feeding point. In this type of distributor
continuity of supply is maintained in case of faults along the distributor as there are
two feeding points.
iii)
Distributor fed at the center:
A
C
I1
I2
B
I3
I4
In this type of feeding, the center of the distributor is connected to the supply making
it two singly fed distributors having a common feeding point.
iv)Ring Mains:
feeder
I1
I2
The distributor is in the form of a closed ring. It is the same as a straight distributor
fed at both ends with equal voltages and the two ends brought together to form a
closed ring.
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2.0 Principle of Distributions System
Week 8
Looking at the various types of loading (uniform loading, concentrated loading and a
combination of both) on the distributors above we have.
2.9 UNIFORMLY LOADED DISTRIBUTOR FED AT ONE END
A
B
i
i
A
C
x
i
B
dx
In fig. a above conductor AB is fed at one end A and uniformly loaded with i amperes
per unit length. Let,
i = current tapped off per unit length
l = total length of distributor
r = resistance per unit length of the distributor
Finding the voltage drop at a point C fig. b which is at a distance of x units from
feeding end A.
Current at point C = (il – ix) = i(l – x)
Consider a small section of length dx near point C
resistance = rdx
Voltage drop over length dx is
dv = i(l – x)(rdx) = (ilr – ixr)dx
Total drop up to point x is
=
V = ilrx – ½ irx2 = ir (lx – x2/2)
----- (1)
At point B voltage drop can be calculated by taking, x = l
ir (l2 – l2/2) = irl2/2 = ½ IR
----- (2)
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2.0 Principle of Distributions System
Week 8
Where I = il = total current entering at point A
R = rl = total resistance of distributor AB
Thus total drop in the distributor AB = ½ IR
Example 1: A 250m 2-wire D.C. distributor fed from one end is loaded uniformly at
the rate of 1.6A/meter. The resistance of each conductor is 0.0002Ω per meter. Find
the voltage necessary at feed point to maintain 250V (a) at the far end (b) at the
midpoint of the distributor.
Solution:
Current entering the distributor I = il = 1.6 x 250 = 400A
Resistance of distributor per meter run r = 2 x 0.0002 = 0.0004Ω
Total resistance of distributor R = r x l
R = 0.0004 x 250 = 0.1Ω
Voltage drop over entire distributor = ½ IR
= ½ x 400 x 0.1 = 20V
At feeding point voltage drop = 250 + 20 = 270V
At a distance x meters from feeding point the voltage drop is taken from eqn (1)
above.
V = ir (lx – x2/2)
At mid point x = l/2 = 250/2 = 125m
Voltage drop = 1.6 x 0.0004 ((250 x 125) - 1252/2) = 15V
At feeding point voltage = 250 + 15 = 265V
2.10 DC DISTRIBUTOR FED AT ONE END – CONCENTRATED LOADING
Example 2: A 2-wire dc distributor AB is 300m long. It is fed at point A with loads of
30A, 40A, 100A and 50A at distances of 40m, 100m, 150m and 250m from A. If the
5
2.0 Principle of Distributions System
Week 8
maximum permissible voltage drop is not to exceed 10V, find the cross-sectional area
of the distributor. Take ρ = 1.78 x 10-8Ωm.
Solution:
Total voltage drop over the distributor is
V = i1R1 + i2R2 + i3R3 + …
For 2-wire distributor voltage drop is
V = 2(i1R1 + i2R2 + i3R3 + …)
Cross-sectional area A =
Resistance of section AC, RAC = ρl/A = 1.78 x 10-8 x 40/A
RAD = 1.78 x 10-8 x 100/A
RAE = 1.78 x 10-8 x 150/A
RAF = 1.78 x 10-8 x 250/A
V = 10V
10 = 2 x 1.78 x 10-8/A [(40 x 30) + (100 x 40) + (150 x 100) + (250 x 50)]
A = 116.34 x 10-6 m2
2.11 DISTRIBUTOR FED AT BOTH ENDS – CONCENTRATED LOADING
Example 3: A 2-wire dc distributor AB is fed from both ends. At feeding point A, the
voltage is maintained at 230V and at B 235V. The total length of the distributor is
200m and loads of 25A, 50A, 30A and 40A are tapped at distances of 50m, 75m,
100m and 150m from A respectively. The resistance per kilometer of one conductor is
0.3Ω. Calculate (a) the currents in various sections of the distributor (b) minimum
voltage and the point at which it occurs.
Solution:
Resistance of 1000m length of distributor of both wires = 2 x 0.3 = 0.6Ω
At section AC resistance RAC is
RAC = 0.6 x 50/1000 = 0.03Ω
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2.0 Principle of Distributions System
Week 8
RCD = 0.6 x 25/1000 = 0.015Ω
RDE = 0.6 x 25/1000 = 0.015 Ω
REF = 0.6 x 50/1000 = 0.03 Ω
RFB = 0.6 x 50/1000 = 0.03 Ω
Voltage at B = voltage at A – voltage drop over AB
VB = VA – [IARAC + (IA – 25)RCD + (IA – 75)RDE + (IA – 105)REF + (IA – 145)RFB]
235 = 230 – [0.03IA + 0.015(IA – 25) + 0.015(IA – 75) + 0.03(IA – 105) + 0.03(IA –
145)]
235 = 230 – (0.12IA – 9)
IA = 33.33A
Current in AC, IAC = IA = 33.33A
ICD = IA – 25 = 8.33A
IDE = IA – 75 = - 41.67A ------ this shows that current flows in the opposite direction
that is E to D.
IEF = IA – 105 = - 71.67A
IFB = IA – 145 = - 111.67A
From the current at various sections calculated above, the currents are coming to load
point D from both sides making it the point of minimum potential.
VD = VA – (IACRAC + ICDRCD)
= 230 – [(33.33 x 0.03) + (8.33 x 0.015)] = 228.875V
2.12 UNIFORMLY LOADED DISTRIBUTOR FED AT BOTH ENDS
Example 4: (i) A uniformly loaded distributor is fed at the center. Show that
maximum voltage drop = IR/8 where I is the total current fed to the distributor and R
is the total resistance of the distributor. (ii) A 2-wire dc distributor 1000m long is fed
7
2.0 Principle of Distributions System
Week 8
at the center and is loaded uniformly at the rate of 1.25A/m. If the resistance of each
conductor is 0.05 Ω/km find the maximum voltage drop in the distributor.
Solution:
The distributor is fed at center C and uniformly loaded with loads of i A/m. Taking
the resistance per meter run of the distributor as r Ω. Maximum voltage drop occurs at
either end of the distributor.
Maximum voltage drop = voltage drop in half distributor
= ½ (il/2) (rl/2) = 1/8 (il) (rl) = 1/8 IR
Where I = il = total current fed to distributor
R = rl = total resistance of distributor
(ii) I = il = 1.25 x 1000 = 1250A
R = rl = 2 x 0.05 x 1 = 0.1 Ω
Maximum voltage drop = 1/8IR
= 1/8 (1250)0.1 = 15.62V
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2.0 Principle of Distributions System
Week 9
2.13 ALTERNATING CURRENT DISTRIBUTION (A.C)
Electricity was initially generated, transmitted and distributed as direct current (D.C.).
The main disadvantage of direct current system was that voltage levels could not be
easily changed. Now-a-days electrical energy is generated, transmitted and distributed
in the form of alternating current. One important reason for the widespread use of
alternating current in preference to D.C. is the fact that alternating voltage can be
conveniently changed in magnitude by means of a transformer. Transformer has made
it possible to transmit A.C. power at high voltage and utilize it at a safe potential.
2.14 CLASSIFICATION OF A.C. DISTRIBUTION SYSTEM
The A.C. distribution system is classified into two; primary distribution system and
secondary distribution system.
 PRIMARY DISTRIBUTION SYSTEM
The primary distribution system is that part of A.C. distribution system which
operates at voltages higher than general utilization and handles large blocks of
electrical energy than the average low-voltage consumer uses. Voltage used for
primary distribution depends upon the amount of power to be conveyed. The most
commonly used primary distribution voltages are 11KV, 6.6KV and 3.3KV. It is
carried out by 3-phase, 3-wire system.
 SECONDARY DISTRIBUTION SYSTEM
That part of A.C. distribution system which includes the range of voltages at which
the ultimate consumer utilizes the electrical energy delivered to him. It employs
400/230V, 3-phase, 4-wire system.
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2.0 Principle of Distributions System
Week 9
In A.C. systems, voltage drops are due to the combined effects of resistance,
inductance and capacitance. Power factor has to be taken into account as loads tapped
off from the distributor are generally at different power factors. The power factors of
load currents may be referred to receiving end voltage or to the respective load
voltages.
2.15 POWER FACTORS REFERRED TO RECEIVING END VOLTAGE
A
R1 + jX1
B
R2 + jX2
I1 cosØ1
I2 cosØ2
Consider an A.C. distributor AB (fig. a) with concentrated loads of I1 and I2 tapped off
at points C and B as shown above. Let the receiving end voltage VB be the reference
vector, lagging power factors at C and B be cosØ1 and cosØ2 with respect to VB. Let
R1, X1 and R2, X2 be the resistance and reactance of sections AC and CB of the
distributor. Then,
Impedance of section AC, Z AC = R1 + jX1
Impedance of section CB, Z CB = R2 + jX2
Load current at point C, I 1 = I1(cosØ1 – jsinØ1)
Load current at point B, I 2 = I2(cosØ2 – jsinØ2)
Current in section CB, I CB = I 2 = I2(cosØ2 – jsinØ2)
Current in section AC, I AC = I 1 + I 2
= I1(cosØ1 - jsin Ø1) + I2(cosØ2 - jsinØ2)
Voltage drop in section CB, V CB = I CB Z CB
= I2(cosØ2 - jsinØ2)(R2 + jX2)
Voltage drop in section AC, V AC = I AC Z AC = ( I 1 + I 2) Z AC
= [I1(cosØ1 - jsinØ1) + I2(cosØ2 - jsinØ2)](R1 + jX1)
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2.0 Principle of Distributions System
Week 9
Sending end voltage, V A = V B + V CB + V AC
Sending end current, I A = I 1 + I 2
Example:
1) A single phase A.C. distributor AB 300m long is fed from end A and is loaded as
under (i) 100A at 0.707 p.f. lagging 200m from point A. (ii) 200A at 0.8 p.f. lagging
300m from point A. The load resistance and reactance of the distributor is 0.2Ω and
0.1Ω per km. Calculate the total voltage drop in the distributor. The load power
factors refer to the voltage at the far end.
Solution:
Impedance of distributor/km = (0.2 + j0.1) Ω
Impedance of section AC, Z AC = (0.2 + j0.1) x 2001000
Z AC = (0.04 + j0.02) Ω
Impedance of section CB, Z CB = (0.2 + j0.1) x 1001000
Z CB = (0.02 + j0.01) Ω
Taking voltage at the far end B as the reference vector,
Load current at point B, I 2 = I2(cosØ2 - jsinØ2)
= 200(0.8 – j0.6)
I 2 = (160 – j120) A
Load current at point C, I 1 = I1(cosØ1 - jsinØ1)
= 100(0.707 – j0.707)
I 1 = (70.7 – j70.7) A
Current in section CB, I CB = I 2 = (160 – j120) A
Current in section AC, I AC = I 1 + I 2 = (70.7 – j70.7) + (160 – j120)
3
2.0 Principle of Distributions System
I
AC
Week 9
= (230.7 – j190.7) A
Voltage drop in section CB, V CB = I CB Z CB = (160 – j120) (0.02 + j0.01)
V
CB
= (4.4 – j0.8) V
Voltage drop in section AC, V AC = I AC Z AC
= (230.7 – j190.7) (0.04 + j0.02)
V
AC
= (13.04 – j3.01) V
Voltage drop in the distributor = V AC + V CB
= (13.04 – j3.01) + (4.4 – j0.8) = (17.44 – j3.81) V
Magnitude of drop = (17.44)2 + (3.81)2
= 17.85 V
4
3.0 Principle of Protections in Power System
Week
10
3.1 FUSES AND ITS COMPONENTS
A fuse is defined in the I.E.E. Regulation as: “A device for opening a circuit by
means of a conductor designed to melt when an excessive current flows. The fuse
comprises all the parts that form the complete device”.
There are three types of fuses:
1.
Rewirable fuse
2.
Cartridge fuse
3.
High braking capacity (H.B.C) fuse, formerly termed the high
rupturing
capacity (H.R.C) fuse.
Rewirable Fuse: This consist (Fig. 6.14) of a porcelain bridge and base. The bridge
has two sets of copper contacts which fit into contacts in the base.
1
3.0 Principle of Protections in Power System
BRIDGE
Week
10
BASE
CABLE ENTRY
ASBESTOR TUBE
FUSE ELEMENT
FIXING HOLE
PORCELAIN
PROCELAIN
BRASS CONTACT WITH
CONNECTING TERMINAL
FIG. Rewirable Fuse
The fuse element, for example, tinned copper wire, is connected between the
terminals of the bridge. An asbestos tube, or pad, is generally fitted in the fuse tp
minimize the effect of arcing when the fuse element melts.
This type of fuse is termed a „semi-enclosed fuse‟ to distinguish it form the older type
of fuse which consisted simply of a piece of wire connected between two terminals.
That 3 of the I.E.E Regulations gives approximate sizes of tinned copper wire to be
used for elements in semi-enclosed fuse.
Example: 0.2, (standard wire gauge) – 5A current rating
0.35mm
- 10A current rating
0.50mm
- 15A current rating
2
3.0 Principle of Protections in Power System
Week
10
3.2 CURRENT RATING
is the current which the fuse element will carry continuously without deterioration.
3.3 FUSING CURRENT
is the current at which the fuse element will melt. This is approximately twice the
current rating of the fuse element (fusing factor = 2).
3.4 FUSING FACTOR
is the ratio
Advantages
1.
Cheap
2.
Easy to replace fuse element.
Disadvantages
1.
Fuse elements deteriorate in use
2.
Any size of fuse wire can be fitted, thus defeating the purpose of the fuse.
Note:
The fuse must be capable of protecting the smallest conductor in the
circuit.
3.
Lacking in discrimination. It is possible that a 15A fuse element may melt
before a 10A fuse element, depending largely on the condition of the wire.
Further, the rewritable fuse is not capable of discriminating between a
momentary high starting current and fault current.
3
3.0 Principle of Protections in Power System
4
Week
10
Easily damage, particularly with short-circuit currents.
Cartridge Fuse. This type has come into common use with
FUSE ELEMENT
END
CAP
END
CAP
PORCELAIN TUBE
FIG. Cartridge fuse
The fused 13A plug used on the domestic ring circuit. The diagram above shows the
construction of a cartridge fuse. The fuse element is contained in a porcelain tube
fitted with two connecting caps, and has a fusing factor of 1.5.
The colour rode for these fuses is as follows:
5A-WHITE
13A-BROWN
30A RED
60a PURPLE
15A-BLUE
High Breaking Capacity fuse. (H.B.C). This type of fuse is designed to protect
circuits against heavy overloads and is capable of opening a circuit under short-circuit
conditions without damaging surrounding equipment.
4
3.0 Principle of Protections in Power System
Week
10
The diagram below shows the construction of a high breaking capacity fuse. This
consists of the following:
1.
Porcelain tube
2.
Silver element
3.
Indicating element which ignites powder under the label to show when the
fuse element has opened.
INDICATING ELEMENT AND
INDICATING POWER
SILICON
PORCELAIN
BODY
CONNECTING LUG
SILVER
END CAP
ELEMENT
FIG. High breaking capacity fuse
4.
End caps
5.
Silica (fine sand) filling used to quench the arc.
NOTE. The fuse must always be placed in the phase or non-earthed conductor
of the installation, never in the neutral (earthed) conductor.
5
3.0 Principle of Protection in Power System
Week
11
3.5 MOLDED - CASE CIRCUITS BREAKER
The molded case of a CB provides the physical means of positioning the breaker
components, and it protects the working parts from damage and contamination. The
molded case also protects people from contact with energized components in the
breaker.
Molded - case circuit breakers can be used in any electrical circuits where protection
is required, including main service and feeders as well as branch circuits. They are
found in switchboards, panel boards, control centres and individual enclosures.
3.6 CIRCUIT BREAKERS
Figure 4.7: Miniature circuit breakers with different poles
The problem with fuses is they only work once. Every time you blow a fuse, you have to
replace it with a new one. A circuit breaker (Figure 4.7) does the same thing as a fuse .It
opens a circuit as soon as current climbs to unsafe levels, but you can use it over and over
again.
The basic circuit breaker consists of a simple switch,(see figure 4.8) connected to either a
bimetallic strip or an electromagnet. The diagram below shows a typical electromagnet
design.
1
3.0 Principle of Protection in Power System
Week
11
Figure4.8: Cut view of a miniature circuit breaker
3.7 MAINTENANCE OF MOLDED-CASE CIRCUIT BREAKER
Molded case breakers are relatively trouble - free devices, requiring little
maintenance. The only maintenance required is to see that all conductor terminals are
tight and free from corrosion, and that the breaker is dry and free from accumulated
dirt and dust.
3.8 CIRCUIT BREAKER RATINGS
A circuit’s breaker is selected for a particular duty taking the following factors into
consideration. :
i. The normal current it will have to carry
ii. The amount of current the supply system will feed into the circuit
Fault, which is the current the circuit breaker, will have to interrupt
Without damage to itself.
2
3.0 Principle of Protection in Power System
Week
11
Ratings of circuit breakers are specified according to certain standards and
recommendations. The ratings are the same for one pole, three pole, and four pole
circuit breakers.
3
Principle of Protection in Power System Week
Continued 12
3.9 FUNCTIONS CIRCUIT BREAKERS
A circuit breaker is a piece of equipment which can
(i)
Make or break a circuit either manually or by remote control
under normal conditions.
(ii)
Break a circuit automatically under fault conditions
(iii)
Make a circuit either manually (or by remote control) as
well automatic control for switching functions. The
latter control employs relays and operates only under
fault conditions. The mechanism of opening of the
circuit breaker under fault conditions has already been
briefed in the previous chapter.
OPERATING PRINCIPLE: A circuit breaker essentially
consists of fixed and moving contacts, called electrodes. Under
normal operating conditions, these contacts remain close and
will not open automatically until and unless the system becomes
faulty. Or course, the contacts can be opened manually or by
remote control whenever desired, when a fault occurs on any
Principle of Protection in Power System Week
Continued 12
part of the system, the trip coils of circuit breaker get energize
and the moving contacts are pulled apart by some mechanism,
thus opening the circuit.
When the contacts of a circuit breaker are separated under fault
conditions, an arc is struck between them, the current is thus
able to continue until the discharge ceases. The production of
arc not only delays the current interruption process but it also
generates enormous heat which may cause damage to the
system or to the circuit breaker itself. Therefore, the main
problem in a circuit breaker is to extinguish the arc within the
shortest possible time so that heat generated by it may not reach
a dangerous value.
2.0 Principle of Transmission, Distribution Week
and Protection
13
3.10 ISOLATORS
This is an electrical manual device used to protect electrical circuit in a
power system of a transmission and distribution network.
The operation of an isolator serves as a protection to the circuit, to and safe
guard the use of the circuit.
It also contents a lock that is used fixed or make contact between the two
terminals. And the terminals can be separated by removing the lock.
While A circuit breaker is a piece of equipment which can
(i)
Make or break a circuit either manually or by remote control under
normal conditions.
(ii)
Break a circuit automatically under fault conditions
(iii)
Make a circuit either manually (or by remote control) as well
automatic control for switching functions. The latter control
employs relays and operates only under fault conditions. The
mechanism of opening of the circuit breaker under fault
conditions has already been briefed in the previous chapter.
1
4.0 Types Insulators and Supports
Week
14
3.13 OVERHEAD AND UNDERGROUND SYSTEMS
Distribution systems can be either overhead or underground. Underground systems
use conduits, cables and manholes under the surface of streets while overhead systems
consist of lines mounted on wooden, concrete or steel poles. They are arranged to
carry distribution transformers as well as conductors. Some of the merits and demerits
of overhead and underground systems are:
1) FAULTS – Chances of faults in underground systems are very rare as
cables are laid underground and have better insulation.
2) INITIAL COST – Underground system is more expensive than overhead
system due to high cost of trenching, conduits, cables e.t.c.
3) FLEXIBILITY – Overhead system is more flexible than underground
system as poles and transformers can be easily shifted to meet changes in
load conditions.
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4.0 Types Insulators and Supports
Week
14
4) FAULT LOCATION AND REPAIRS – There are little chances of faults
in an underground system, but if they do occur it is difficult to locate and
repair. In overhead systems, conductors are visible thus faults are easily
traced and repaired.
5) CURRENT CARRYING CAPACITY & VOLTAGE DROP – Overhead
conductor has a higher current carrying capacity than underground
conductor of the same material and cross section.
6) MAINTENANCE COST – Maintenance cost of underground system is
very low compared with that of overhead system because of fewer
chances of faults.
3.14 REQUIREMENTS OF A DISTRIBUTION SYSTEM
Some of the requirements of a good distribution system are:
I.
PROPER VOLTAGE – Voltage variations at consumer’s terminals
should be as low as possible. Changes in voltage are generally caused
due to the variation of load on the system. A good distribution system
should ensure that the voltage variations at consumer’s terminals are
within permissible limits which are  6 % of rated value at consumer’s
terminals.
II.
AVAILABILITY OF POWER ON DEMAND – Power must be
available to consumers’ in any amount that they may require from time
to time.
III.
RELIABILITY – Modern industry is almost dependent on electric power
for its operation. Reliability of the system can be improved by
interconnected systems, providing additional reserve facilities.
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4.0 Types of Insulators and Supports
Week
15
3.15 INSULATORS
Insulators are materials that do not allow the flow of current through them. Overhead
line conductors should be supported on the poles in such a way that currents from
conductors do not flow to earth through supports that is line conductors must be
properly insulated from supports. This is achieved by securing line conductors to
supports with the help of insulators. The insulators provide necessary insulation
between line conductors and supports and thus prevent any leakage current from
conductors to earth.
3.16 PROPERTIES OF INSULATORS
i.
High mechanical strength in order to withstand conductor load, wind load.
ii.
High electrical resistance of insulator material in order to avoid leakage
currents to earth.
iii.
High relative permittivity of insulator material in order that dielectric strength
is high.
iv.
The insulator material should be non-porous; free from impurities and cracks
otherwise the permittivity will be lowered.
v.
High ratio of puncture strength to flashover.
3.17 TYPES OF INSULATORS
The most common used material for insulators of overhead lines is porcelain but
glass, steatite and special composition materials are also used to a limited extent. The
successful operation of an overhead line depends to a considerable extent upon the
proper selection of insulators. The most commonly used types of insulators are; pin
type, suspension, strain insulator and shackle insulator.
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4.0 Types of Insulators and Supports
Week
15
1) PIN TYPE INSULATORS:
The pin type insulator is secured to the cross arm on the pole. There is a groove on the
upper end of the insulator for housing the conductor. The conductor passes through
this groove and is bound by the annealed wire of the same material as the conductor.
Pin type insulators are used for transmission and distribution of electric power at
voltages up to 33KV. Beyond operating voltage of 33KV, the pin type insulators
become too bulky and hence uneconomical.
2) SUSPENSION TYPE INSULATORS:
Suspension type insulators are used for high voltages above 33KV. They consist of a
number of porcelain discs connected in series by metal links in the form of a string.
The conductor is suspended at the bottom end of this string while the other end of the
string is secured to the cross-arm. Each disc is designed for low voltage say 11KV.
The number of discs in series would depend upon the working voltage.
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4.0 Types of Insulators and Supports
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ADVANTAGES:
i. Suspension type insulators are cheaper than pin type insulators for
voltages beyond 33KV.
ii. If any one disc is damaged, the whole string does not become
useless because the damaged disc can be replaced.
iii. Suspension arrangement provides greater flexibility to the line.
iv. Suspension type insulators are generally used with steel towers.
v. Each unit or disc of suspension type insulator is designed for low
voltage usually 11KV.
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4.0 Types of Insulators and Supports
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3) SHACKLE INSULATORS
Shackle insulators are frequently used for low voltage distribution lines. They can be
used either in a horizontal position or in a vertical position. They can be directly fixed
to the pole with a bolt or to the cross arm.
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