• The energy is neither be created nor be destroyed
but it can be converted from one form to another.
• The various energy sources
1. Burning coal oil
2. Natural gas
3. Water stored in dams
4. Diesel oil
5. Nuclear power
6. Other non conventional energy sources
• Depending upon the source of energy used, these
stations are called thermal power station,
hydroelectric power station, diesel power station,
nuclear power station etc.
• The generated electric power is to be supplied to the
• Generally the power stations are located too far away
from the town and cities where electrical energy is
• Hence there exists a large network of conductors
between the power stations and the consumers.
• The network is broadly classified into two parts
6. A Typical Transmission and Distribution
• The flow of electrical power from the generating
station to the consumer is called an electrical
power system or electrical supply system.
• It consists of the following important components
1. Generating station
2. Transmission network
3. Distribution network
16. S.No. Underground System Overhead System
1. Transmission is by using cables Transmission is by using the transmission
2. All the cables must be properly
insulated from each other
The appropriate spacing provided between
the conductors acts as an insulation. No
external insulation is necessary
3. The insulation cost is very high No transmission cost as air acts as an
4. Transmission over long distance is not
possible as laying of cables is difficult,
costly and complicated
Transmission over long distance is possible
with the help of transmission lines.
5. The voltage level used is below 66 kV
due to insulation difficulties
The voltage level used can as high as 400 kV
6. The maintenance cost is less The maintenance cost is high
7. The faults due to lightning, short circuit,
storms etc. are eliminated
The occurrence of faults due to lightning,
short circuits, storms and abnormal
weather conditions are possible.
8. It is very safe due to insulation used The conductors are bare without insulation
17. S.No. Underground System Overhead System
9. Maximum stress is on the insulation
between the conductors
Maximum stress is between conductor and
10. The beauty of the area, towns etc. is
The beauty of the area gets affected due
to overhead lines. Sometimes trees are
required to be cut
11. The size of the cables is high The size of the conductors is less
12. The voltage drop is less The voltage drop is more
19. Constants of a Transmission Line
20. Constants of a Transmission Line
• It is the opposition of line conductors to current
flow. The resistance is distributed uniformly along
the whole length of the line as shown in Fig. (i).
• However, the performance of a transmission line
can be analysed conveniently if distributed
resistance is considered as lumped as shown in
21. Constants of a Transmission Line
• When an alternating current flows through a
conductor, a changing flux is set up which links the
• Due to these flux linkages, the conductor
• Mathematically, inductance is defined as the flux
linkages per ampere i.e.,
22. Constants of a Transmission Line
• We know that any two conductors separated by an
insulating material constitute a capacitor.
• As any two conductors of an overhead
transmission line are separated by air which acts
as an insulation, therefore, capacitance exists
between any two overhead line conductors.
• The capacitance between the conductors is the
charge per unit potential difference i.e.,
• Capacitance, C = q/v farad
24. Factors to be considered while Designing
• Type and size of conductors
• Voltage level
• Line regulation and control of voltage
• Efficiency of transmission
• Corona loss
• Power flow capability and stability
• Requirement of compensation
• Levels of faults at various bus bars and requirement of
new circuit breakers
• Grounding needs
• Protection schemes for new lines
• Coordination of insulation
25. Factors to be considered while Designing
• Mechanical design aspects which include stress
and sag calculations, compensation of conductor,
spacing of conductor and configuration for
• Design of power system structure
• Economical aspects
26. Requirements of Transmission Line Conductors
• They should have low weight
• They should have high tensile strength and fatigue
• They must have high conductivity
• They should have low co-efficient of expansion,
low corona loss
• They should have less resistance and low cost
The conductors used in practice are made up from
the materials such as copper, aluminium and their
27. Aluminum Transmission Line Conductors
• The advantages of using aluminium conductors
over copper conductors are
1. They have low cost
2. Less resistance and corona loss
3. Less weight
• But aluminium has less tensile strength, high co-
efficient of expansion and large area which
restricts it use alone as a conductor.
28. Stranded Conductors
• Stranded conductors have three or more
strands of conductor material twisted
together to form a single conductor.
• Stranded can carry high currents and are
usually more flexible than solid conductors
29. Types of Aluminum Transmission Line
• AAC - All aluminium conductors
• AAAC - All aluminium alloy conductor (Silmalec-
0.5% of silicon & 0.5% of magnesium)
• ACSR - Aluminium conductors with steel
• ACAR - Aluminium conductors with alloy
42. SKIN EFFECT
• When a conductor is carrying steady direct
current (d.c.), this current is uniformly
distributed over the whole X-section of the
• However, an alternating current flowing through
the conductor does not distribute uniformly,
rather it has the tendency to concentrate near
the surface of the conductor as shown in Fig.
This is known as skin effect.
43. SKIN EFFECT
• The tendency of alternating current to
concentrate near the surface of a conductor is
known as skin effect.
44. SKIN EFFECT
• Due to skin effect, the effective area of cross-
section of the conductor through which current
flows is reduced.
• Consequently, the resistance of the conductor is
slightly increased when carrying an alternating
• The cause of skin effect can be easily explained.
A solid conductor may be thought to be
consisting of a large number of strands, each
carrying a small part of the current.
45. SKIN EFFECT
• The inductance of each strand will vary
according to its position. Thus, the strands near
the centre are surrounded by a greater magnetic
flux and hence have larger inductance than that
near the surface.
• The high reactance of inner strands causes the
alternating current to flow near the surface of
• This crowding of current near the conductor
surface is the skin effect.
46. SKIN EFFECT
• The skin effect depends upon the following
1. Nature of material
2. Diameter of wire − increases with the diameter
3. Frequency − increases with the increase in
4. Shape of wire − less for stranded conductor
than the solid conductor.
• It may be noted that skin effect is negligible
when the supply frequency is low (< 50 Hz) and
conductor diameter is small (< 1cm).
47. PROXIMITY EFFECT
• The current distribution may be non-uniform
because of another effect known as proximity
48. PROXIMITY EFFECT
• Let each of the line conductor is assumed to be
divided into 3 sections having equal cross sectional
area. Three parallel loops are formed by the pairs
xx', yy' and zz'.
• The flux linking loop xx' is least and it increases for
the remaining loops. Thus the inductance of inner
loop is less. Thus the current density is highest at
inner edges of the conductor.
• Due to this non-uniform distribution of current, the
effective conductor resistance increases. As the
distance between the conductors goes on reducing,
this distribution of current becomes more and more
49. PROXIMITY EFFECT
• For normal spacing of overhead lines this
proximity effect is negligible. For underground
cables this effect is significant as the conductors
are located close to each other.
• The proximity effect also depends on the same
factors as that of skin effect.