2. INTRODUCTION TO TRANSMISSION LINES/SUPPLY SYSTEM
In early days, there was a little demand for electrical energy so that small power
stations were built to supply lighting and heating loads. However, the
widespread use of electrical energy by modern civilization has necessitated to
produce bulk electrical energy economically and efficiently.
The increased demand of electrical energy can be met by building big power
stations at favorable places where fuel (coal or gas) or water energy is available in
abundance. This has shifted the site of power stations to places quite away from
the consumers. The electrical energy produced at the power stations has to be
supplied to the consumers.
There is a large network of conductors between the power station and the
consumers. This network can be broadly divided into two parts viz.,transmission
and distribution.
The conveyance of electric power from a power station to consumers’ premises is
known as electric supply system.
3. CONT’
An electric supply system consists of three principal components viz.,
1. the power station,
2. the transmission lines and
3. the distribution system.
Electric power is produced at the power stations which are located at favourable places, generally
quite away from the consumers. It is then transmitted over large distances to load centres with
the help of conductors known as transmission lines. Finally, it is distributed to a large number of
small and big consumers through a distribution network.
Electric power transmission is the bulk movement of electrical energy from a generating site, such
as a power plant, to an electrical substation. The interconnected lines which facilitate this movement
are known as a transmission network.
A transmission line is a specialized cable or other structure designed to conduct electromagnetic
waves in a contained manner.
Transmission lines are the conductors that serve as a path for transmitting (sending) electrical waves
(energy) through them. These basically forms a connection between transmitter and receiver in order
to permit signal transmission.
6. Types of transmission line
There are different kinds of transmission lines. These include
1. Coaxial cable: These lines are formed when a conducting wire is coaxially inserted inside another hollow
conductor. These are termed as coaxial as the 2 conductors share the same axis. These are widely used in
applications where high voltage levels are needed.
7. Cont’
2. Balanced two wire/ Open wire line: These are the conductors having 2 lines (wires), that are separated by
dielectric medium whose, one end connected to the source and other to the destination. These are low cost and
simplest form of transmission line. But, their installation cost is somewhat higher as well as its maintenance
sometimes becomes difficult due to the change in atmospheric conditions.
8. Cont’
3. Waveguides: This category of the transmission line is used for signal transmission at microwave frequencies.
These are basically hollow conducting tubes as they somewhat resemble like coaxial cable line but do not have
centre conductor as present in coaxial cables. The classic waveguide is a metal tube, usually with a rectangular
cross-section, which can range in length from a few centimeters to many meters .
9. Cont’
4. Micro strip: This is copper track running on a side of a PCB (Printed Circuit Board) while the other side is plain
ground side. A microstrip circuit uses a thin flat conductor which is parallel to a ground plane. Microstrip can be
made by having a strip of copper on one side of a printed circuit board (PCB) or ceramic substrate while the other
side is a continuous ground plane. The width of the strip, the thickness of the insulating layer (PCB or ceramic) and
the dielectric constant of the insulating layer determine the characteristic impedance. Microstrip is an open structure
whereas coaxial cable is a closed structure.
10. Cont’
5. Fiber Optic: This is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly
thicker than that of a human hair.[1] Optical fibers are used most often as a means to transmit light[a] between the two
ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer
distances and at higher bandwidths (data transfer rates) than electrical cables. Fibers are used instead of metal wires
because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a
problem from which metal wires suffer.
11. Classification of transmission line
They can be classified into
1. AC and DC transmission lines. (Based of the nature of the current)
AC transmission lines transmits the generated Alternating Current from the generation point to the distribution
substations. But DC transmission lines has a rectifier on the generation end and the power is converted back to AC
using an inverter at the consumer end.
13. Cont
2. High Voltage, Medium and Low Voltage transmission lines (Based on amount of voltage)
Here we have
High Voltage Transmission System – above 220 KV (Over Head Transmission Lines)
Medium Voltage Transmission system – less than 220 kV (OTHL and Underground Cables)
Low Voltage Distribution system – less than 1 kV – near the user (Underground Cables)
14. Cont
3. Overhead and Underground transmission system (Based on Structure used)
Overhead Transmission Line
An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large
distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-phase power) suspended by
towers or poles.
Since most of the insulation is provided by the surrounding air, overhead power lines are generally the least costly method of power
transmission for large quantities of electric energy
Overhead power transmission lines are classified in the electrical power industry by the range of voltages:
1. Low voltage (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.
2. Medium voltage (MV; distribution) – between 1000 volts (1 kV) and 69 kV, used for distribution in urban and rural areas.
3. High voltage (HV; subtransmission less than 100 kV; subtransmission or transmission at voltages such as 115 kV and 138 kV), used for
sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.
4. Extra high voltage (EHV; transmission) – from 345 kV, up to about 800 kV, used for long distance, very high power transmission.
5. Ultra high voltage (UHV) is often associated with ≥ ±800 kVDC and ≥ 1000 kVAC;
Since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed
to maintain safety. Adverse weather conditions, such as high winds and low temperatures, can lead to power outages.
15. Cont
Problem caused by overhead lines – This lead to the need of having underground lines.
16. Cont
Underground transmission lines
Electric power can also be transmitted by underground power cables instead of overhead power lines.
Underground cables take up less right-of-way than overhead lines, have lower visibility, and are less affected by
bad weather. However, costs of insulated cable and excavation are much higher than overhead construction. Faults
in buried transmission lines take longer to locate and repair.
In some metropolitan areas, underground transmission cables are enclosed by metal pipe and insulated with
dielectric fluid (usually an oil) that is either static or circulated via pumps.
17. Cont
4. Short, Medium and Long Transmission lines. (Based on the length)
a. Short Transmission Lines
If the line is not more than 80 Km or if the voltage is not over than 66 KV then the line is known as the short
transmission line. The capacitance of the line is governed by their length. The effect of capacitance on the short
transmission line is negligible, but for cable where the distance between the conductor is small, the effect of
capacitance cannot be ignored. While studying the performance of the short transmission line only resistance and
the inductance of the line is calculated. During analysis you use the arrangement below
18. Cont
b. Medium Transmission Lines
The line which is ranging from 80 to 240 km is termed as a medium transmission line. The capacitance of the medium
transmission line cannot be ignored. The capacitance of the medium transmission line is considered to be lumped at
one or more point of the lines. The effect of the line is more at high frequency, and their leakages inductance and
capacitance is considered to be neglected. The medium transmission line is sub-divided into Pi (π)– model and T –
model.
i) Pi Model of a Medium Transmission Line
In nominal Pi model, it is assumed that the half of the capacitance concentrate at the each end of the line.
ii) T – Model of a Medium Transmission Line
In T model, it is assumed that the capacitance is concentrated at the center of the line.
19. Cont
c. Long Transmission Line
The line having a length more than 240 km is considered a long transmission line. All the four parameters (resistance,
inductance, capacitance, and leakage conductance) are found to be equally distributed along the entire length of the
line.
20. Typical Elements of a Power Transmission System
1. Conductors, usually three for a single-circuit line and six for a double-circuit line. The usual material is aluminium
reinforced with steel.
2. Step-up and step-down transformers, at the sending and receiving ends respectively. The use of transformers
permits power to be transmitted at high efficiency.
3. Line insulators, which mechanically support the line conductors and isolate them electrically from the ground.
4. Support, which are generally steel towers and provide support to the conductors.
5. Protective devices, such as ground wires, lightning arrestors, circuit breakers, relays etc. They ensure the
satisfactory service of the transmission line.
6. Voltage regulating devices, which maintain the voltage at the receiving end within permissible limits. All these
elements will be discussed in detail in the subsequent chapters.
22. INTRODUCTION
Transmission of electric power is done by 3-phase, 3wire overhead lines.
An a.c. transmission line has resistance, inductance and capacitance uniformly distributed along its length. These
are known as constants or parameters of the line.
The performance of a transmission line depends to a considerable extent upon these constants. For instance, these
constants determine whether the efficiency and voltage regulation of the line will be good or poor.
Here we shall focus our attention on the methods of calculating these constants for a given transmission line. Out
of these three parameters of a transmission line, we shall pay greatest attention to inductance and capacitance.
Resistance is certainly of equal importance but requires less explanation since it is not a function of conductor
arrangement.
23. Constants of a transmission line
A transmission line has resistance, inductance and capacitance uniformly distributed along the whole length of the
line.
24. 1. Resistance. 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 analyzed
conveniently if distributed resistance is considered as lumped as shown in Fig.(ii). The distributed resistance of the
conductors is represented by a series resistor. Units is Ohm/unit length.
2. Inductance. When an alternating current flows through a conductor, a changing flux is set up which links the
conductor. Due to these flux linkages, the conductor possesses inductance. Mathematically, inductance is defined
as the flux linkages per ampere i.e.
The inductance is also uniformly distributed along the length of the line as show in Fig.(i). Again for the convenience of
analysis, it can be taken to be lumped as shown in Fig.(ii). Unit Henry per unit length.
3. Capacitance. 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.,
25.
26. Resistance of a Transmission Line
The resistance of transmission line conductors is the most important cause of power loss in a transmission line. The
resistance R of a line conductor having resistivity ρ, length l and area of cross section a is given by
R = ρ
l
a
The variation of resistance of metallic conductors with temperature is practically linear over the normal range of
operation. Suppose R1and R2 are the resistances of a conductor at t1ºC and t2ºC (t2 > t1) respectively. If 1 is the
temperature coefficient at t1ºC , then,
