LECTURE NOTE FOR ELECTRICAL POWER I EEC122
1.0 POWER PLANTS
1.1 STEAM PLANT
The steam power plants convert the heat energy of coal into electrical energy. Figures 1A
and 1B below shows the schematic diagram and layout of steam plant respectively.
Figure 1A – Schematic Arrangement of Steam Power Plant
1
Figure 1B – Lay-out of Steam Plant
2
Coal is burnt in a boiler which converts water into steam. The expansion of steam in
turbine produces mechanical power which drives the alternator. Thus the main equipment
in a steam plant consists of boiler, steam turbine and alternator. To achieve efficient
conversion of heat energy into electric energy, a variety of auxiliary equipments are
needed. The auxiliary equipments in a steam plant are so much that they overshadow the
main equipment.
Figure 1 shows the lay-out of a steam plant. The coal handling plant supplies coal to the
boiler. The ash formed in the boiler is disposed off by the ash handling plant. Air taken
from the atmosphere by the action of forced or induced draft fan is heated in the preheater
(by the heat of flue gases) before being fed to the boiler. The flue gases pass through dust
collector, air pre-heater and economiser before being discharged to the atmosphere
through the chimney. The boiler vaporises water into steam, steam is further heated in the
super-heater and fed to the high pressure turbine. After expanding in high pressure
turbine, steam is heated again in a boiler and fed to the low pressure turbine. The exhaust
steam from the low pressure turbine is condensed by the condenser and the condensate,
along with make up water, is passed through economiser before being fed to the boiler.
1.2 DIESEL PLANT
In diesel power plant, diesel engine is used as the prime mover. Figure 2A and 2B below
shows the schematic diagram and layout of a diesel power plant.
Figure 2A – Schematic Arrangement of a Diesel Plant
3
Figure 2B – Layout of Diesel Power Plant
The diesel burns inside the engine and the products of this combustion act as the
―working fluid‖ to produce mechanical energy. . The diesel engine drives the alternator
which converts mechanical energy into electrical energy. As the generation cost is
considerable due to high price of diesel, therefore, such power plant are only used to
produce small power.
Although steam power plants and hydro-electric plants are invariably used to generate
bulk power at cheaper cost, yet diesel power plants are finding favour at places where
demand of power is less, sufficient quantity of coal and water is not available and the
transportation facilities are inadequate. These plants are also used as standby sets for
continuity of supply to important points such as hospitals, radio stations, cinema houses
and telephone exchanges.
1.3 GAS PLANT
A gas power plant can either be an open cycle or closed cycle gas power plant. The
figures 3A and 3B represent the diagrams of open and closed gas turbine plant
respectively while that of 3C shows the layout of gas turbine plant.
4
Figure 3A – Schematic Arrangement of an Open Cycle Gas Turbine Plant
Figure 3B – Schematic Arrangement of A Closed Cycle Gas Turbine Plant
5
A gas power plant consists of a compressor, combustion chamber, gas turbine and
alternator. In an open cycle gas turbine plant, the compressor takes in atmospheric air,
compresses it and supplies the pressurised air to the combustion chamber. Fuel is injected
into the combustion chamber and burnt in the streams of air supplied by the compressor.
The combustion raises the temperature of air and increases its volume under constant
pressure. The hot pressurised gas expands in the turbine, produces mechanical power and
turns the rotor of the turbine. Both the compressor and the alternator are coupled to the
turbine shaft. Due to the high temperature of the products of combustion, the turbine
output exceeds the input to the compressor. The turbine, therefore, drives the compressor
and the surplus power drives the alternator. The products of combustion, after expansion
though the turbine, are finally exhausted to the atmosphere.
Figure 3C – Layout of Gas Power Plant
6
Unlike in an open cycle plant where the fuel is mixed with air in the combustion chamber
and the heat rejection process occurs in the atmosphere as the turbine exhaust is
discharged into the atmosphere, the fuel is not mixed with working medium (which can
be air or any other gas) in closed cycle gas turbine. In closed cycle gas turbine, the heat
rejection process occurs in heat exchanger or recooler. Thus the same working medium is
circulated again and again through compressor, heater, turbine and recooler. The
intercooling, regeneration and reheating features can also be added. The function of the
recooler, in closed cycle gas turbine plant, is similar to that of condenser in a steam plant.
1.4 SOLAR AND WIND PLANT
1.4.1 SOLAR POWER PLANT
A solar power plant converts solar energy into electrical energy. It consists of the
following sub-systems which includes solar energy collection system, thermal energy
transfer system, thermal energy storage system and energy conversion system. The two
major types of solar power plant that have been found to be feasible are Solar Farm or
Distributed Collector System (DCS) and Solar Tower or Central Receiver System (CRS).
Figure 4A and 4B below shows the distributed collector system solar power plant and
central receiver system respectively.
Figure 4A – Distributed Collector System Solar Power Plant
7
In distributed collector system solar power plant, the solar energy collection system
collects the heat energy. This heat energy is transfer from the receiver to the turbine by
the thermal energy transfer system using heat transfer media such as water and gases.
Since solar energy is not available at all time, heat energy storage becomes necessary.
This is achieved by thermal storage system which uses pressure vessel storing water and
steam at a temperature near the boiling point. Finally, the energy conversion system
converts heat energy to electrical energy. The solar heat from the receiver or storage
converts feed water into steam which is fed to turbine. The turbine drives the generator.
Steam is condensed in the condenser and water returns to the boiler for reuse as feed
water. The heat of the cooling water of the condenser may be used for subsidiary
purposes.
Figure 4B – Central Receiver System Solar Power Plant
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The central receiver system solar power system consists of a number of sub-systems
which includes heliostart or collection sub-system, receiver/tower sub-system, electric
power generation sub-system consisting of steam turbine and generator, cooling tower
sub-system for cooling the water used in condenser for condensing steam, master control
sub-system for controlling the whole system and storage sub-system which is needed to
maintain plant output at a constant level despite solar insulation variation. The operation
of this type of solar power system is similar to the one earlier described above.
1.4.2 WIND POWER PLANT
Wind power plants are turbines which use the energy in the motion of the wind to make
mechanical energy, which is then converted to electrical energy.
The components of a utility-scale "wind farm" include wind turbines, an underground
power transmission system, control and maintenance facilities, and a substation that
connects the farm with the utility power grid. Utility-scale wind turbines are classified by
size as follows: small (less than 50 kilowatts [kW]); intermediate (50 to 500 kW); and
large (above 500 kW). Small and intermediate turbines make up the bulk of the older
installed turbine base, but new turbines installed in the late 1990s are generally 600 kW
and larger.
In general, a wind farm is a group of wind turbines in the same location used for
production of electricity. A large wind farm may consist of several hundred individual
wind turbines distributed over an extended area, but the land between the turbines may be
used for agricultural or other purposes. A wind farm may also be located offshore.
Almost all large wind turbines have the same design — a horizontal axis wind turbine
having an upwind rotor with three blades, attached to a nacelle on top of a tall tubular
tower. In a wind farm, individual turbines are interconnected with a medium voltage
(often 34.5 kV), power collection system and communications network. At a substation,
this medium-voltage electric current is increased in voltage with a transformer for
connection to the high voltage electric power transmission system.
Utility-scale wind farms are generally located in areas with average annual wind speeds
of at least 13 miles per hour. Wind power is more available during certain seasons
because climatic conditions affect wind speed. Another application of wind is in
distributed use systems, which provide on-site power in either stand-alone or gridconnected configurations. Most such systems range in size from one to 25 kW.
Distributed wind systems are applicable to industry, water districts, rural residences,
agricultural use, and a wide variety of isolated power uses located in good wind resource
areas.
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1.5 GENERAL LAYOUT OF ELECTRICAL POWER SYSTEM
The figure 5A and 5B below shows the general layout of electrical power system (which
includes generation, transmission and distribution system) and PHCN grid system
respectively.
Figure 5A – General Layout of Electrical Power System
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Figure 5B – PHCN Grid System (Single Line Diagram of the Nigerian 330KV
Transmission Grid System)
1.5.1 REQUIREMENT OF A DISTRIBUTION SYSTEM
Some of the requirements of a good distribution system are:
1. Proper Voltage – Voltage variation at consumer’s terminal should be as low as
possible. The variations at consumer’s terminal should be within permissible limits which
are + or – 6% of rated value at consumer’s terminal.
2. Availability of Power on Demand – Power must be available to consumers in any
amount that they may require from time to time.
3. 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.
1.6 TRANSMISSION AND DISTRIBUTION SYSTEM
The transmission system is the large network of conductors that delivers bulk power from
power stations to the load centres and large industrial consumers beyond the economical
service range of the regular primary distribution lines. Furthermore, the distribution
system is that part of the electric power system which distributes electric power for local
use or to consumer for utilization. The difference between the two systems is that while
the transmission system delivers bulk power from power stations to load centres and
large industrial consumers, the distribution system distributes power for local use (i.e.
deliver power from substation to the local consumers).
1.7 VOLTAGE LEVELS BETWEEN THE GENERATING STATION AND THE
CONSUMER
The various voltage levels that exist between the generating station and the consumer can
be grouped under the primary transmission voltage, secondary transmission voltage,
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primary distribution voltages and secondary distribution voltages. In Nigeria, the primary
transmission voltage is 330KV while that of the secondary transmission voltage is 132
KV. The primary distribution voltage used are 11KV and 33KV while the secondary
distribution voltages are 415V for three phase system and 240V for single phase system.
1.8 SHORT AND MEDIUM TRANSMISSION LINES
1.8.1 SHORT TRANSMISSION LINE
When the length of an overhead transmission line is upto about 50km and the line voltage
is comparatively low (< 20 kV), it is usually considered as a short transmission line. Due
to smaller length and lower 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.
1.8.2 MEDIUM TRANSMISSION LINE
When the length of an overhead transmission line is about 50 – 150km and the line
voltage is moderately high (>20 kV < 100 kV), 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 purpose of calculations, the distributed capacitance of the line is divided and
lumped in the form of condensers shunted across the line at one or more points.
1.8.3 DIFFERENCES BETWEEN SHORT AND MEDIUM TRANSMISSION
LINE
S/N
1
2
3
SHORT TRANSMISSION LINE
LENGTH OF LINE
It is upto about 50 km
LINE VOLTAGE
It is comparatively low (< 20 kV)
CAPACITANCE EFFECT
It is small and hence negligible
MEDIUM TRANSMISSION LINE
It is about 50 to 150 km
It is moderately high (>20 kV < 100 kV)
It is taken into account and hence not
negligible
1.9 PRINCIPLES OF PROTECTION SYSYTEM
Modern power systems are growing fast with more generators, transformers and large
network in the systems. For system operation a high degree of reliability is required. In
order to protect the system (line and equipment) from damage due to undue currents and
/or abnormal voltages caused by faults (such as short circuits) the need for reliable
protective devices, such as relay and circuit breaker arises. The most common electrical
hazard against which protection is required is the short-circuit. However, there are many
other abnormal conditions – e.g., overload, under-voltage and over-voltage, open-phase,
unbalanced-phase currents, reversal of power, under-frequency and over-frequency, overtemperature, power swings, instability – for which some protection is desired.
12
As a rule, on the occurrence of short circuits which may lead to heavy disturbances in
normal operation (damage to equipment, impermissible drop in voltage etc) the protective
scheme is designed to disconnect or isolate the faulty section from the system without
any delay. The protective scheme is designed to energise an alarm or signal whenever the
overloads and short –circuits do not present a direct danger to the faulted circuit element
and the entire installation, for an example, on occurrence of a single-phase fault to earth
in overhead circuit operated with an insulated neutral. In such cases it is possible for the
operating personnel to take the necessary measures for removal of the abnormality and
prevent any interruption in power supply to consumers.
The main functions of protective relaying are to detect the presence of faults, their
locations and initiate the action for quick removal from service of any element of a power
system when it suffers a short-circuit or when it starts to operate in any abnormal manner
that might cause damage or otherwise interfere with effective operation of the rest of the
system. The relaying equipment is added in this task by the circuit breakers that are cable
of disconnecting the faulty element when they are called upon to do so by the relaying
equipment.
Circuit breakers are generally located so that each generator, transformer, bus,
transmission line etc., can be completely disconnected from the rest of the system. The
circuit breakers must have sufficient capacity so that they can carry momentarily the
maximum short-circuit current that can flow through them and then interrupt this current;
they must also withstand closing in on such a short-circuit and then interrupting it
according to certain prescribed norms. Fusing is employed where protective relays and
circuit breakers are not economically justifiable.
In summary, protection system is an extremely important part of the power system as it is
provided to operate under abnormal conditions to prevent failure or isolate trouble and
limit its effect. Every protection system which isolates a faulty element is required to
satisfy four basic requirements: (i) reliability; (ii) selectivity; (iii) sensitivity; and (iv)
speed of operation. Without reliability and selectivity the protection would be rendered
largely ineffective and could even become liability.
1.10 TYPES AND SIZES OF CONDUCTORS
1.10.1 TYPES OF CONDUCTORS
Conductors can either be stranded or solid. The most commonly used conductor
materials for overhead lines are Copper, Aluminium, Steel-core Aluminium (ACSR),
Galvanised 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 overhead lines
are preferably stranded. Solid wires are only used when area of X-section is small. If
solid wires are used for large X-section and long spans, continuous vibration and
swinging would produce mechanical fatigue and they would fracture at the points of
support.
1.10.2 SIZES OF CONDUCTORS
The size of conductor in general simply refers to its nominal cross sectional area in mm 2
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1. COPPER - The nominal cross sectional area in mm 2 of copper conductors used for
overhead power transmission lines are 10, 16, 25, 35, 50, 70, 100, 125, 150 and 185.
2. ALUMINIUM - The nominal cross sectional area in mm 2 of aluminium conductors
used for overhead power transmission lines are 22, 35, 50, 60, 70, 100, 150, 200, 250,
300 and 400.
3. STEEL-CORE ALUMINIUM (ACSR) - The nominal cross sectional area in mm 2 of
steel-core aluminium conductors used for overhead power transmission lines are 25, 30,
35, 40, 50, 70, 95, 100, 120, 150, 175, 185, 200, 240 and 400.
1.11 CONSTRUCTION OF UNDERGROUND CABLES
The figure 6 below shows the general construction of a multi-core conductor
underground cable. The various parts are:
(i) Cores of conductor – A cable may have one or more than one core (conductor)
depending upon the type of service for which it is intended. For instance, the 3-conductor
cable shown in figure 6 is used for 3-phase service. The conductors are made of tinned
copper or aluminium and are usually stranded in order to provide flexibility to the cable.
Figure 6 – Construction of a Cable
(ii) Insulation – Each conductor is provided with a suitable thickness of insulation, the
thickness of layer depending upon the voltage to be withstood by the cable. The
commonly used materials of insulation are impregnated paper, varnished cambric or
rubber mineral compound.
(iii) Metallic Sheath - In order to protect the cable from moisture, gases or other
damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead
or aluminium is provided over the insulation as shown in figure 6.
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(iv) Bedding – Over the metallic sheath is applied a layer of bedding which consists of a
fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic
sheath against corrosion and from mechanical injury due to armouring.
(v) Armouring – Over the bedding, armouring is provided which consists of one or two
layers of galvanised steel wire or steel tape. Its purpose is to protect the cable from the
mechanical injury while laying it and during the course of handling. Armouring may not
be done in the case of some cables.
(vi) Serving – In order to protect armouring from atmospheric conditions, a layer of
fibrous material (like jute) similar to bedding is provided over the armouring. This is
known as serving.
It is important to mention here that bedding, armouring and serving are only applied to
the cables for the protection of conductor insulation and to protect the metallic sheath
from mechanical injury.
1.11.1 DIFFERENCES
SYSTEM
S/N
1.
2.
3.
4.
5.
6.
BETWEEN
OVERHEAD
OVERHEAD SYSTEM
FAULT
Chances of faults in overhead system are high
since the lines are expose to uncertain weather
conditions and other external interferences.
INITIAL COST
The initial cost is not very high
FLEXIBILITY
It is more flexible as poles and transformer can
be easily shifted to meet changes in load
conditions
FAULT LOCATION AND REPAIR
In overhead systems, conductors are visible thus
faults are easily traced and repaired.
CURRENT CARRYING CAPACITY &
VOLTAGE DROP
Overhead lines have high current carrying
capacity
MAINTENANCE COST
It is very high due to high occurrence of fault
AND
UNDERGROUND
UNDERGROUND SYSTEM
Chances of faults in underground system are
very rare as cables are laid underground and
have better insulation.
The initial cost is very high due to high cost of
trenching conduits, cables etc
It is not flexible
It is difficult to locate and repair fault in an
underground system.
Its current carrying capacity is not as high for the
same materials and cross section.
It is very low due to fewer chances of fault
1.12 WORKED EXAMPLES ON SHORT TRANSMISSION LINES
1.12.1 SHORT NOTE ON PERFORMANCE OF SINGLE PHASE SHORT
TRANSMISSION LINES
The effects of line capacitance are neglected for a short transmission line. Therefore,
while studying the performance of such a line, only resistance and inductance of the line
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are taken into account. The equivalent circuit of a single phase short transmission is
shown in figure 7 (i). Here, the total line resistance and inductance are shown as
concentrated or lumped instead of being distributed. The circuit is a simple a.c. series
circuit. The phasor diagram (Current I is taken as the reference phasor. OA represents the
receiving end voltage VR leading I by ΦR. AB represents the drop IR in phase with I. BC
represents the inductive drop IXL and leads I by 900. OC represents the sending end
voltage VS and leads I by ΦS) of the line for lagging load power is shown in figure 7 (ii).
Figure 7- Equivalent Circuit and Phasor Diagram
Let
I = Load Current
R= Loop Resistance i.e. Resistance of both conductors
XL= Loop Resistance
VR= Receiving End Voltage
Cos ΦR= Receiving End Power Factor (Lagging)
VS = Sending End Voltage
(i) Sending End Voltage (VS) = VR cos  R  IR   VR sin  R  IX L 
(ii) Percentage Voltage Regulation – The difference in voltage at the receiving end of a
transmission line between conditions of no load and full load is called voltage regulation
and is expressed as a percentage of the receiving end voltage.
V  VR
 100
% Voltage Regulation = S
VR
V cos  R  IR
(iii) Sending End Power Factor (p.f), cos ΦS = R
VS
(iv) Power Delivered = VR I R cos  R
(v) Line Losses = I 2 R
(vi) Power Sent Out = VR I R cos  R  I 2 R
(vii) Percentage Transmission Efficiency – The ratio of receiving end power to the
sending end power of a transmission line is known as the transmission efficiency of the
line i.e.
Re ceiving End Power
 100
% Transmission Efficiency, T 
Sending End Power
2
16
2
VR I R cos  R
 100
VS I S cos  s
OR
Power Delivered
=
 100
Power Sent Out
VR I R cos  R
=
 100
VR I R cos  R  I 2 R
=
1.12.2 WORKED EXAMPLE
1. What is the maximum length in km for a 1-phase transmission line having copper
conductor of 0.775cm2 cross sectional over which 200 kW at unity power factor and at
3300V.are to be delivered? The efficiency of transmission is 90%. Take specific
resistance as 1.725μ Ω cm.
SOLUTION
Receiving end power = 200kW = 200,000W
Transmission efficiency = 0.9
200,000
Sending end power =
 222,222 W
0.9
Therefore, Line losses = 222,222 – 200,000 = 22,222 W
200  10 3
Therefore, Line current, I =
 60.6 A
3,300  1
Let R Ω be the resistance of one conductor.
Line losses = 2I2R
222,222 = 2 (60.6)2 x R
22,222
Therefore,
R
 3.025 
2
2  60.6

Now R  
Therefore,

R



3.025  0.775
 1.36  10 6 cm  13.6 km
6
1.725  10
2. A single phase line is transmitting 1,100kW power to a factory at 11kV and at 0.8 pf
lagging. It has a total resistance of 2Ω and a loop reactance of 3Ω. Determine (i) the
voltage at the sending end, (ii) percentage regulation and (iii) transmission efficiency.
SOLUTION
Load Current, I 
P  1000 1,100  1000

 125 A
VR cos  R 11,000  0.8
(i) Voltage at sending end VS 

VR cos  R  IR2  VR sin  R  IX 2
11,000  0.8  125  22  11,000  0.6  125  32
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 9,050  6,975  11,426V
V  VR
11,426  11,000
 100 
 100  3.873%
(ii) Percentage regulation  S
VR
11,000
2
2
Line losses  I 2 R  125  2  31,250W  31.25kW
Power delivered
(iii) Transmission efficiency, T 
 100
Power delivered  line loss
2

1,100
 100  97.28%
1,100  31.25
2.0 DISTRIBUTORS AND FEEDERS
2.1 DISTRIBUTOR
A distributor is a conductor from which tappings are taken for supply to the consumer. In
figure 8 below, AB, BC, CD and DA are distributors. The current through a distributor is
not constant because tappings are taken at various places along its length. While
designing a distributor, voltage drop along its length is the main consideration since the
statutory limit of voltage variations is 6% of rated value at the consumer’s terminal.
Figure 8 – Diagram Showing Distributors and Feeder.
2.2 FEEDERS
A feeder is a conductor which connects the sub-station (or localised generating) to the
area where power is to be distributed (see figure 8 above). Generally, no tappings are
taken from the feeder so that current in it remains the same throughout. The main
consideration in the design of a feeder is the current carrying capacity.
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2.3 DIFFERENCES BETWEEN DISTRIBUTOR AND FEEDER
S/N
1
2
3
4
DISTRIBUTOR
FUNCTION
It supplies power to the service mains
from the feeder
CURRENT FLOWING
The current flow through is not
constant
TAPPINGS
Tappings are taken from it
MAIN DESIGN CONSIDERATION
The main design consideration is the
voltage drop across its length.
FEEDER
It supplies power to the distributor from
the distribution substation.
The current flowing through is constant
No tappings are taken from it
The main design consideration is the
current carrying capacity.
2.4 WORKED EXAMPLES ON VOLTAGE DROPS IN SIMPLE DISTRIBUTION
SYSTEM
1. A single phase a.c distributor AB 300 metres long is fed from end A and is loaded as
under:
(i) 100 Amps at 0.707 p.f. lagging 200 m from point A
(ii) 200 Amps at 0.8 p.f lagging 300 m from point A
The load resistance and reactance of the distributor is 0.2Ω and 0.1Ω per kilometre.
Calculate the total voltage drop in the distributor. The load power factors refer to the
voltage at the far end.
SOLUTION
Figure 9 shows the single line diagram of the distributor.
Impedance of the distributor/km = (0.2  j 0.1) 
Figure 9 – Single Line Diagram of Example 1

Impedance of section AC, Z AC  (0.2  j 0.1)  200 / 1000  (0.04  j 0.02)

Impedance of section CB, Z CB  (0.2  j 0.1)  100 / 1000  (0.02  j 0.01)
Taking voltage at the far end B as the reference vector, we have,

Load current at point B, I 2  I 2 (cos 2  j sin 2 )  200(0.8  j 0.6)  (160  j120) A
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
Load current at point C, I1  I1 (cos 1  j sin 1 )  100(0.707  j 0.707)  (70.7  j 70.7) A


Current in section CB, I CB  I 2  (160  j120) A

 
Current in section AC, I AC  I1  I 2  (70.7  j 70.7)  (160  j120)  (230.7  j190.7) A

 
Voltage drop in section CB, VCB  I CB Z CB  (160  j120)(0.02  j 0.01)
 (4.4  j 0.8)volts

 
Voltage drop in section AC, V AC  I AC Z AC  (230.7  j190.7)(0.04  j 0.02)
Voltage drop in the distributor
 13.04  j3.01volts


= V AC  VCB  13.04  j3.01  4.4  j 0.8
= 17.44  j3.81volts
Magnitude of the drop
=
17.442  3.812
 17.85V
2. A single phase distributor 2 kilometres long supplies a load of 120 Amps at 0.8 p.f.
lagging at its far end and a load of 80 Amps at 0.9 p.f. lagging at its mid-point. Both
power factors are referred to the voltage at the far end. The resistance and reactance per
km (go and return) are 0.05Ω and 0.1Ω respectively. If the sending end voltage at the far
end is maintained at 230V, calculate:
(i) The total voltage drop in the distributor.
(ii) Voltage at the sending end
(iii) Phase angle between voltages at the two ends
SOLUTION
Figure 10 shows the single line diagram of the distributor.
Impedance of the distributor/km = (0.05  j 0.1) 
Figure 10 – Single Line Diagram of Example 2

Impedance of section AC, Z AC  (0.05  j 0.1)  1000 / 1000  (0.05  j 0.1)

Impedance of section CB, Z CB  (0.05  j 0.1)  1000 / 1000  (0.05  j 0.01)
Let the voltage at point B be taken as the reference vector,

Then,
VB  230  j 0

(i) Load current at point B, I 2  120(0.8  j 0.6)  (96  j 72) A

Load current at point C, I1  80(0.9  j 0.436)  (72  j34.88) A
20


Current in section CB, I CB  I 2  (96  j 72) A

 
Current in section AC, I AC  I1  I 2  (72  j34.88)  (96  j 72)  (168  j106.88) A

 
Voltage drop in section CB, VCB  I CB Z CB  (96  j 72)(0.05  j 0.1)
 (12  j 6)volts

 
Voltage drop in section AC, V AC  I AC Z AC  (168  j106.88)(0.05  j 0.1)
Voltage drop in the distributor
 19.08  j11.45volts


= V AC  VCB  12  j 6  19.08  j11.45
= 31.08  j17.45volts
Magnitude of the drop
=
(ii) Sending end voltage,
Its magnitude is
31.082  17.452
 35.646V




V A  VB  VCB  V AC
 (230  j 0)  (12  j 6)  (19.08  j11.45)
= 261.08  j17.45
=
261.082  17.452
 261.67V
(iii) The phase difference θ between VA and VB is given by:
17.45
tan  
 0.0668
261.08
  tan 1 0.0668  3.82 0
3. A 2-wire dc distributor AB is 300 metres long. It is fed at point A. The various loads
and their positions are given below:
At Point
Distance from A in metres
C
50
D
140
E
200
F
240
If the maximum permissible voltage drop is not to
area of the distributor. Take ρ = 1.78 x 10-8Ωm.
Concentrated loads in Amperes
40
60
120
70
exceed 10V, find the cross-sectional
SOLUTION
The single line diagram of the distributor along with its tapped currents is shown below:
21
M1
Suppose the resistance AB 100 m length of the distributor is r ohms. Then the resistance
of the various sections of the distributor is:
RAC  0.5r ; RCD  0.9r ; RDE  0.6r ; R EF  0.4r
Referring to the above figure, the current in the various sections of the distributor are:
I AC  290 A ; I CD  250 A ; I DE  190 A ; I EF  70 A
Total voltage drop over the distributor equals
 I AC RAC  I CD RCD  I DE RDE  I EF REF
 290  0.5r  250  0.9r  190  0.6r  70  0.4r
 145r  225r  114r  28r
 512r
As the maximum permissible drop in the distributor is 10V,
10  512r
r  10
 0.01953
512
Cross-sectional area of conductor, α, is
l 1.78  10 8  100


 182.3  10 6 m 2  1.823cm 2
r
0.01953
2
2
3.0 COMPONENT PARTS OF A FUSE
Essentially, a fuse consist of a fusible element in the form of a metal conductor of
specially selected small cross sectional area, a case or cartridge to hold the fusible
element, and in some cases, provided with a means to aid arc extinction. The part which
actually melts and opens the circuit is known as the fuse element. It forms a series part of
the circuit to be protected against short-circuit or excessive overloads. There are three
major types of fuse as shown in figure 11. These are Cartridge fuse, Rewirable fuse and
High Breaking Capacity fuse.
22
Figure 11- Types of Fuse
3.1 PURPOSE OF FUSE
A fuse is a short piece of metal (e.g. copper, aluminium, silver, tin, zinc and lead)
inserted in the circuit, which melts when excessive current flows through it and thus
breaks the circuit. It could also be defined as a current interrupting device which breaks
or opens the circuit (in which it is inserted) by fusing the element when the current in the
23
circuit exceeds a certain value. The purpose of a fuse is to protect machines and
equipment from damage due to excessive currents.
3.2 ADVANTAGES OF FUSES
1. It is the cheapest form of protection available
2. It require no maintenance
3. Its operation is inherently completely automatic unlike a circuit breaker which require
an elaborate equipment for automatic action (the operation is quick and sure)
4. It can break heavy short circuit current without noise or smoke
5. The smaller size of fuse element impose a current a current limiting effect under short
circuit condition
6. It is simple and easy to install and replace.
3.3 DISADVANTAGES OF FUSES
1. Fuse element deteriorate in use
2. Considerable time is lost in rewiring or replacing fuse after operation
3. The current – time characteristics of a fuse cannot always be co-related with that of the
protected apparatus
4. Interlocking is not possible
4. On heavy short circuits, discrimination between fuse in series cannot be obtained
unless there is sufficient difference in the size of the fuse concerned. Discrimination
between two fuses is said to occur if on the occurrence of short circuit or over-current
fault, only the desired fuse operates.
3.4 STANDARD WIRE GUAGE AND CURRENT RATING
STANDARD WIRE GUAGE
0.2mm
0.35mm
0.50mm
CURRENT RATING
5A
10A
15A
3.5 DEFINITION OF KEY TERMS
3.5.1 FUSING CURRENT
Fusing current is the minimum current at which the fuse element melts and thus
disconnects the circuit protected by it.
3.5.2 CURRENT RATING OF FUSE ELEMENT (RATED CURRENT)
Current rating of fuse element is the current which the fuse element can normally carry
without overheating or melting. It could also be defined as the rms value of the current
which the fuse element or fuse wire can carry continuously without deterioration, and
with temperature rise within specified limits.
24
3.5.3 FUSING FACTOR
It is the ratio of minimum fusing current to the current rating of the fuse element i.e.
Minimum fusing current
Fusing Factor =
Current rating of fuse
3.5.4 FUSE LINK
Fuse link is that part of fuse which needs replacement when the fuse blown out.
3.5.5 FUSE ELEMENT OR FUSE WIRE
Fuse element or wire is that part of the fuse which actually melts when an excessive
current flows in the circuit and thus isolate the faulty device from supply.
3.5.6 RATED VOLTAGE
This is the voltage of the fuse which should be equal to or more than (i) voltage of the
circuit in a single phase a.c or 2 wire circuit (ii) line voltage in case of 3 phase a.c circuit
(iii) voltage between 2 outers in case of 3-wire d.c circuit.
3.6 MOULDED CASE OF A CIRCUIT BREAKER
Moulded case circuit breakers are intended for protecting electrical equipment against
overloading and short circuiting and sometimes also for switching. Figure 12 shows
picture of a moulded circuit breaker. A moulded case circuit breaker can also be said to
be a plastic that interrupts an electrical current if the current exceeds the trip rating of a
breaker. The plastic encloses the mechanism in the breaker box, and also separates
conductors from each other and from metal grounds.
Figure 12 – Pictures of Moulded Case Circuit Breaker
The model case of a circuit breaker provides the physical means of positioning the
breaker components and it protects the working parts from damage and contamination.
The model case also protects people from contact with energized components in the
breaker.
25
3.7 INTERRUPTION CAPACITY OF A CIRCUIT BREAKER
The interruption capacity of a circuit breaker is defined as the maximum fault current that
can be interrupted by a circuit breaker without failure of the circuit breaker.
3.8 PRINCIPLE OF OPERATION OF CIRCUIT BREAKER
A circuit breaker essentially consists of fixed and moving contacts called electrode.
Under normal operating conditions, these contacts remained closed and will not open
automatically until and unless the system becomes faulty. Of course, the contacts can be
opened manually or by remote control whenever desired.
When a fault occurs on any parts of the systems, the trip coils of the circuit breaker get
energized and the moving contacts are pulled apart by some mechanism, thus opening the
circuit. When the contacts of a circuit breaker are separated under full conditions, an arc
is struck between. The current is thus able to continue until the discharge cease.
The production of the 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.
3.9 COMPONENTS OF CIRCUIT BREAKER
The basic circuit breaker consists of a simple switch connected to either a bimetallic strip
or an electromagnet. Essential materials in CB are:
1. Stationary or moving contacts called electrode
2. Switch
3. Electromagnet
4. Bimetallic strips
3.10 ADVANTAGE OF FUSE OVER CIRCUIT BREAKER
1. It has a very small operating time (0.002 secs)
2. It performs both detection and interruption functions
3. Its operation is inherently completely automatic.
3.11 ADVANTAGE OF CIRCUIT BREAKER OVER FUSE
1. Circuit breaker needs no replacement after operation
2. Circuit breaker has a very large breaking capacity
3. It carries full load current continuously without over-heating or damages
4. It opens and closes the circuit on no load
5. It makes and breaks the normal operating current
6. It makes and breaks the short circuit current of the magnitude up to which it is
designed for.
26
3.12 CLASSES OF RELAY
1. Over-voltage, over-current and overpower relay
2. Under-voltage, under-current and under-power relay
3. Reverse current relay
4. Reverse power relay
5. Differential relay
3.13 OIL-LESS CIRCUIT BREAKER
1. Water circuit breaker
2. Air circuit breaker
3. Air-blast circuit breaker
4. Sulphur – hexafloride circuit breaker (SF6)
5. Vacuum circuit breaker
3.14 ISOLATOR
An isolator can be defined as a switch that isolates one portion of the circuit from another
and is not intended to be opened while current is flowing. Also, it can be defined as
simple pieces of equipment employed only for isolating circuit when the current has
already been interrupted.
3.15 DIFFERENCES BETWEEN CIRCUIT BREAKER AND ISOLATOR
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 under fault conditions
In summary, circuit breakers are mechanical devices designed to close or open contact
members, thus closing or opening of an electrical circuit under normal and abnormal
conditions.
On the other hand, an isolator can be defined as a switch that isolates one portion of the
circuit from another and is not intended to be opened while current is flowing. Also, it
can be defined as simple pieces of equipment employed only for isolating circuit when
the current has already been interrupted.
An isolator is an electrical protection device that can only be operated manually while a
circuit breaker is a piece of protection equipment that can make or break a circuit either
manually or by remote control under normal condition and break a circuit automatically
under fault conditions.
27
S/N
1
2
3
4
CIRCUIT BREAKER
It can operate under all conditions
(load and no load)
It is equipped with arc-quenching
devices
It has specified current breaking
capacity or current capacity
It can isolate circuit when current is
still flowing
ISOLATOR
It is best operated under no load
condition
It is not equipped with arc-quenching
devices
It has no specified current breaking
capacity or current capacity
Isolates circuit when the current has
already been interrupted
4.0 INSULATORS AND THEIR APPLICATIONS
Insulators are materials that do not allow the flow of current through them. The insulators
provide necessary insulation between line conductors and supports and thus prevent any
leakage current from conductors to earth.
The different types of insulators available includes past cap type, pin type and the shackle
ring type.
4.1 CEMENTED-CAP TYPE SUSPENSION INSULATOR
Cemented-cap type suspension insulator shown in the figure 13 below is the most
commonly used type and consists of a single disc-shaped piece of porcelain grooved on
the under surface to increase the surface leakage path, and to a metal cap at the top, and
to a metal pin underneath. The cap is recessed so as to take the pin of another unit and
thus build up a string of any number of units. The cap is secured to the insulator by
means of cement.
Figure 13 – Cemented-Cap Type Suspension Insulator
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
28
secured to the cross-arm. Each disc is design for low voltage say 11KV. The number of
discs in series would depend upon the working voltage.
4.1.1 ADVANTAGE OF SUSPENSION TYPE INSULATOR
1. Suspension arrangement provide greater flexibility to the line
2. They are generally used with steel towers
3. Each unit or disc of suspension type insulator is designed for low voltage usually
11KV
4. They are cheaper than pin type insulators for voltage beyond 33KV
5. If anyone disc is damaged the whole string does not become useless because the
damaged disc can be replaced.
4.1.2 APPLICATION
They are used for high voltages above 33kV.
4.2 PIN TYPE INSULATOR
This type was amongst the earliest designs. The part section of a pin type insulator is
shown in figure 14 (i). The pin-type insulator is designed to be mounted on a pin which in
turn is secured to the cross-arm of the pole. The insulator is screwed on the pin and the
line conductor is placed in the groove at the top of the insulator and is tied down with a
soft copper or soft aluminium binding wire according to the conductor material [see
figure 14 (ii)].
Figure 14 – Pin Type Insulator
4.2.1 APPLICATION
Pin type insulators are used for transmission and distribution of electric power at voltage
upto 33KV. Beyond operating voltage of 33KV, the pin type insulators become too bulky
and hence uneconomical.
29
4.3 SHACKLE RING INSULATOR
The construction of shackle ring insulator is shown in figure 15 below. Every insulator is
coated with an extremely hard, smooth glaze that reduces accumulation of surface
deposits. The surface can be easily cleaned and it will not crack when subjected to
temperature changes.
Figure 15 – Shackle Type Insulator
The wet flash-over and dry flash-over voltages for shackle insulators are 10KV and
25KV respectively while the puncture voltage is about 35KV. Its operating voltage is
1,000V. Its weight, transverse mechanical load and total creepage distance are 0.5 kg,
1,150 kg and 63 mm respectively. The tapered hole in the shackle insulator distributes the
load more evenly and reduces the possibility of breakage when heavily loaded.
Shackle insulators may either be mounted horizontally or vertically, and the conductors
are fixed in the grooves by means of soft copper or aluminium binding wire according to
the conductor material. They can be directly fixed to the pole with a bolt or to the crossarm. The insulators are bellmounted to prevent water being held in contact with the
spindle. This type of insulator is used at all positions, intermediate, terminal or angle.
Where the angle exceeds 60* deviation they are generally used in conjunction with
shackle straps.
4.3.1 APPLICATION
In the early days, the shackle insulators were used as strain insulators. But nowadays,
they are frequently used for low voltage distribution lines.
4.4 INSULATING MATERIALS AND THEIR APPLICATIONS
The material most commonly used for overhead line insulators is porcelain but toughened
glass, steatite and special composition materials are also used to a limited extent.
30
4.4.1 PORCELAIN
Porcelain is produced by firing at a controlled temperature a mixture of kaolin, feldspar
and quartz. It is mechanically stronger than glass. It gives less trouble from leakage, and
is less susceptible to temperature variations and its surface is not affected by dirt deposits.
On the other hand, it is not as homogeneous as glass, owing to the fact that each
component shell of a porcelain insulator is glazed during manufacturing process and its
satisfactory performance in service depends to a considerable extent on the preservation
of this glaze which is only of the order of 25 microns in thickness.
Also fault cannot be detected easily as it is not transparent. In tension this material is
usually weak and does not withstand tensile stresses exceeding 5 kg/mm2. The dielectric
strength and comprehensive strength of a mechanically sound porcelain insulator are
about 6.5 kV/mm of its thickness and 700 kg/mm2 respectively. Normally, it is difficult
to manufacture homogeneous porcelain in the thickness required for some types of
insulators and, therefore, for a particular operating voltage, a two or more piece
construction is adopted in which each piece is fired and glazed separately and then they
are cemented together.
If this insulating material is manufactured at lower temperature, its mechanical properties
improve but the material remains porous and on putting it in service it may deteriorate. If
this material is manufactured at high temperature, its porosity is reduced but the material
becomes brittle. The porosity of this insulating material also reduces its dielectric
strength, also any impunity or air bubble left within the material results in a lower
dielectric strength. So a compromise is made between the mechanical strength and the
porosity of the material and a suitable temperature of the kiln is designed.
4.4.1.1 APPLICATION
They are used where high mechanical strength in insulator is a criterion.
4.4.2 GLASS
Glass is cheaper than porcelain in the simpler shapes and if properly toughened and
annealed gives resistivity and dielectric strength (14kv per mm of thickness of the
material). Owing to high dielectric strength, the glass insulators have simpler design and
even one piece design can be used.
Glass is quite homogenous material and can withstand higher compressive stresses as
compared to porcelain. It has also a lower coefficient of thermal expansion which
minimises the strains due to temperature changes and owing to its transparent nature
flaws in the material can be readily detected by visual examination. The main
disadvantage of glass is that moisture more readily condenses on its surface and
facilitates the accumulation of dirt deposits, thus giving a high surface leakage. Also in
large sizes the great mass of material combined with the irregular shape, may result in
internal strains after cooling.
4.4.2.1 APPLICATION
Glass insulator can be used upto 25kV under ordinary atmospheric conditions and well
up to 50kV in dry atmosphere.
31
4.4.3 STEATITE
Steatite is a naturally occurring magnesium silicate, usually found combined with oxides
in varying proportions. It has a much higher tensile and bending stress than porcelain.
4.4.3.1 APPLICATION
They are used at tension towers or where the transmission line takes a sharp turn.
4.4.4 SPECIAL ARTIFICIAL MATERIAL
The special artificial materials have an advantage that these can be easily moulded into
any shape without any internal stress. Metallic fittings can also be firmly embedded in the
material without any difficult. The disadvantage of insulators made from special artificial
material is that they deteriorate rapidly in bad climatic conditions and on being subjected
to flash-over their carbonised surface forms a conducting path.
4.4.4.1 APPLICATION
They are used in insulators for low voltages.
32